論文 / 著書情報
Article / Book Information
題目(和文)
Title(English)

Metallization and Functionalization of Flexible Silk Textile bySupercritical CO2 Promoted Electrochemical Techniques

著者(和文)

Chiu Wan-Ting

Author(English)

Wan-Ting Chiu

出典(和文)

学位:博士(工学),学位授与機関:東京工業大学,
報告番号:甲第10980号,
授与年月日:2018年9月20日,
学位の種別:課程博士,
審査員:曽根 正人,寺田 芳弘,木村 好里,細田 秀樹,三宮 工

Citation(English)

Degree:Doctor (Engineering),
Conferring organization: Tokyo Institute of Technology,
Report number:甲第10980号,
Conferred date:2018/9/20,
Degree Type:Course doctor,
Examiner:,,,,

学位種別(和文)

博士論文

Type(English)

Doctoral Thesis

Powered by T2R2 (Science Tokyo Research Repository)

Metallization and Functionalization of
Flexible Silk Textile by Supercritical CO2
Promoted Electrochemical Techniques

Doctor of Engineering

Department of Materials Science and Engineering
Tokyo Institute of Technology

Wan–Ting Chiu

September, 2018

Table of content

Table of content
Chapter 1 General introduction ................................................................................................. 1
1.1 Wearable electronic and medical device applications........................................................ 1
1.2 Methodology for the integration of materials for the applications ..................................... 2
1.2.1 Electroless plating ..................................................................................................... 2
1.2.2 Supercritical carbon dioxide promoted method .......................................................... 7
1.2.2.1 Supercritical carbon dioxide promoted catalyzation ............................................ 7
1.2.2.2 Supercritical carbon dioxide promoted metallization ........................................... 9
1.2.3 Cathodic deposition ................................................................................................ 11
1.3 Integration of textile, electrically conductive materials, and functional materials ............ 12
1.3.1 Metal thin film on silk textile .................................................................................. 13
1.3.1.1 Ni–P metallization ............................................................................................ 13
1.3.1.2 Pt metallization ................................................................................................. 17
1.3.1.3 Au metallization ............................................................................................... 17
1.3.2 Photocatalyst on metallized silk textile .................................................................... 19
1.3.2.1 ZnO Cathodic deposition .................................................................................. 21
1.3.2.2 Inclusion of TiO2 (P25) into Ni–P metallization ................................................ 24
1.4 Evaluations of fundamental properties ........................................................................... 27
Chapter 2 Methodology............................................................................................................. 1
2.1 Catalyzation for embedding the catalysts as active sites ................................................... 1
2.1.1 Conventional catalyzation ......................................................................................... 1
2.1.2 Sc–CO2 catalyzation ................................................................................................. 1
I

Table of content

2.2 Metallization to equip silk textile with electrical conductivity .......................................... 3
2.2.1 Conventional metallization ........................................................................................ 3
2.2.1.1 Ni–P conventional metallization ......................................................................... 3
2.2.1.2 Pt conventional metallization .............................................................................. 3
2.2.1.3 Au conventional metallization ............................................................................. 4
2.2.1.4 Ni–P/TiO2 conventional metallization ................................................................. 4
2.2.2 Sc–CO2 metallization ................................................................................................ 5
2.2.2.1 Pt sc–CO2 metallization ...................................................................................... 5
2.3 ZnO cathodic deposition to functionalize metallized silk textile ....................................... 6
2.4 Evaluations of the composite materials ............................................................................ 7
2.4.1 Surface morphology and crystal structure .................................................................. 7
2.4.2 Electrical conductivity .............................................................................................. 8
2.4.3 Adhesive firmness ..................................................................................................... 8
2.4.4 Corrosion resistance .................................................................................................. 9
2.4.5 Immersion test for evaluating biocompatibility ........................................................ 10
2.4.6 Photoelectrochemical measurement ......................................................................... 11
Chapter 3 Ni–P metallized silk textile via sc–CO2 promoted electroless plating to practiceelectrically conductive silk textile ........................................................................................... 13
3.1 Introduction ................................................................................................................... 13
3.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation............................................ 13
3.3 Morphology, composition and structure of silk/Ni–P ...................................................... 16
3.4 Electrical property and adhesive test of silk/Ni–P........................................................... 21
3.5 Corrosion resistance of silk/Ni–P ................................................................................... 26
3.6 Brief summary for silk/Ni–P .......................................................................................... 27IITable of content

Chapter 4 Pt metallized silk textile via sc–CO2 promoted electroless plating to improve thebiocompatibility of the composite ........................................................................................... 28
4.1 Introduction ................................................................................................................... 28
4.2 Activation of silk textile via sc–CO2 Pt(acac)2 catalyzation ............................................ 28
4.3 Morphology, composition and structure of silk/Pt .......................................................... 29
4.4 Electrical property of silk/Pt .......................................................................................... 32
4.5 Biocompatibility assessments of silk/Pt ......................................................................... 35
4.6 Brief summary for silk/Pt............................................................................................... 35
Chapter 5 Efficiency enhancement of platinum metallization on silk textile substrate by Pd(acac) 2
sc-CO2 catalyzation ................................................................................................................. 37
5.1 Introduction ................................................................................................................... 37
5.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation............................................ 37
5.3 Morphology, composition and structure of silk/Pt .......................................................... 39
5.4 Electrical property of silk/Pt .......................................................................................... 43
5.5 Corrosion resistance of silk/Pt........................................................................................ 47
5.6 Biocompatibility assessment of silk/Pt by immersion test ............................................... 50
5.7 Enhancement of the metallization efficiency .................................................................. 50
5.8 Brief summary for silk/Pt............................................................................................... 52
Chapter 6 Full sc-CO2 promoted platinum electroless plating for enhancing the electricalconductivity and reliability of the silk/Pt composite materials .................................................. 54
6.1 Introduction ................................................................................................................... 54
6.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation............................................ 55
6.3 Morphology, composition, and structure of sc–CO2 metallized silk/Pt ............................ 56
6.4 Electrical property of sc–CO2 metallized silk/Pt ............................................................. 63
III

Table of content

6.5 Corrosion resistance of sc–CO2 metallized silk/Pt .......................................................... 66
6.6 Biocompatibility assessment of sc–CO2 metallized silk/Pt by immersion test ................. 68
6.7 Brief summary for sc–CO2 metallized silk/Pt ................................................................. 68
Chapter 7 Functionalization of Au metallized silk textile by ZnO cathodic deposition.............. 70
7.1 Introduction ................................................................................................................... 70
7.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation............................................ 70
7.3 Morphology, composition and structure of silk/Au ......................................................... 72
7.4 Electrical property of silk/Au ......................................................................................... 74
7.5 Cathodic ZnO deposition ............................................................................................... 76
7.6 Photocurrent evaluation of silk/Au/ZnO ......................................................................... 82
7.7 Brief summary for silk/Au/ZnO ..................................................................................... 83
Chapter 8 Reliability and photocatalytic activity enhancement by Ni–P co–deposition............. 85
8.1 Introduction ................................................................................................................... 85
8.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation............................................ 85
8.3 Ni–P/TiO2 co–deposition ............................................................................................... 86
8.4 Electrical conductivity and adhesive tests ...................................................................... 93
8.5 Corrosion resistance ...................................................................................................... 95
8.6 Photocatalytic activity assessment ................................................................................. 97
8.7 Brief summary for silk/Ni–P/TiO2.................................................................................. 99
Chapter 9 General conclusions .............................................................................................. 101
Chapter 10 List of achievements and others ........................................................................... 107
10.1 Awards ...................................................................................................................... 107
10.2 Publications ............................................................................................................... 108
10.2.1 International Journal ........................................................................................... 108
IVTable of content

10.2.2 Domestic Journal ................................................................................................ 109
10.2.3 Invited Articles.................................................................................................... 109
10.2.4 Book ................................................................................................................... 109
10.2.5 International Conference ..................................................................................... 109
10.3 Research Experience.................................................................................................. 112
10.4 Acknowledgement ..................................................................................................... 115
10.5 Reference .................................................................................................................. 118

VFigure captions

Figure captions
Figure 1- 1 Various applications of wearable devices.......................................................... 1
Figure 1- 2 Electroless plating procedures for non–conductive substrates ........................... 3
Figure 1- 3 Catalytic activity of various metals (active sites) in different reducing agent
solution: (a)NaH2PO2, (b) HCHO, (c) NaBH4, (d) DMAB, and (e) NH2NH2 [10] ........ 7
Figure 1- 4 Pressure–Temperature phase diagram of carbon dioxide ................................... 9
Figure 1- 5 Comparison between (a) CONV catalyzation (without sc–CO2) and (b) sc–CO2
catalyzation and their characteristics .......................................................................... 9
Figure 1- 6 Comparison between CONV metallization and sc–CO2 metallization ............. 10
Figure 1- 7 Equipment setting of two electrode cathodic deposition ................................. 11
Figure 1- 8 Summary of the advantages and reasons of the chosen materials: (a) silk, (b)Metallization layer, (c) ZnO, and (d) TiO2 ................................................................ 12
Figure 1- 9 Schematic graphs of the effect of Cl− ions on the morphology of ZnOnanostructures under (a) low [OH −] and (b) high [OH−] [61]..................................... 23
Figure 1- 10 Pourbaix diagram of the system Zn/H2O, 10-4 M Zn2+ considering ZnO as solid
substance [62] .......................................................................................................... 24

Figure 2- 1 Illustration of CONV catalyzation in SnCl2/PdCl2 (HCl) solution ..................... 1
Figure 2- 2 Setup of sc–CO2 catalyzation reaction cell ....................................................... 2
Figure 2- 3 High pressure system used to generate and inject sc–CO2 into the reaction cell.
(a) CO2 gas tank, (b) CO2 liquidization unit, (c) liquidization unit, (d) high pressure
pump, (e) thermal bath, (f) reaction–cell (PEEK–lined SUS316L), (g) substrate, (h)cross stirrer, (i) catalyst, (j) back pressure regulator, and (k) trap................................. 2
Figure 2- 4 Illustration of CONV Ni–P metallization.......................................................... 3
VI

Figure captions

Figure 2- 5 Flow chart of the pre–agitation of Ni–P/TiO2 deposition .................................. 5
Figure 2- 6 Illustration of sc–CO2 assisted Pt metallization ................................................ 6
Figure 2- 7 Setup of cathodic ZnO deposition (deposition time: 2 hours, deposition
temperature: 70oC, deposition pressure: 1 atm) ........................................................... 7
Figure 2- 8 Illustration of electrical resistance measurement of silk/metal composites ........ 8
Figure 2- 9 Procedures of adhesive test for the composite materials.................................... 9
Figure 2- 10 Experimental setup of corrosion resistance measurement ............................. 10
Figure 2- 11 Flow chart of immersion test for evaluation of ion releasing rate .................. 11
Figure 2- 12 Experimental setup of photocurrent density measurement (evaluation
temperature: room temperature, electrolyte: 500 mL Na 2SO4, evaluation pressure: 1 atm,
without agitation) ..................................................................................................... 12

Figure 3- 1 (a) OM image and (b) X–ray diffraction pattern of the as–received silk, and (c)OM image and (d) X–ray diffraction pattern of the sc–CO2 catalyzed silk (triangle
symbols: Pd(acac)2 catalysts) ................................................................................... 14
Figure 3- 2 OM image of conventionally catalyzed silk at room temperature without agitation(red dashed circles: damaged silk substrate) ............................................................. 15
Figure 3- 3 Cross–section of backscattered electron SEM images of catalyzed silk at (a) 1hour and (b) 2 hours at 80 oC and 15 MPa ................................................................ 16
Figure 3- 4 X–ray diffraction patterns of (a) non–catalyzed pure silk, (b) Pd(acac)2 catalyzed
silk, (c) catalyzed/reduced silk, and (d) Ni–P metallized silk, respectively ................ 17
Figure 3- 5 OM images of the Ni–P metallized silk composites prepared by (a) 30 sec, (b) 1
min, (c) 2 min, (d) 4 min, (e) 5 min, and (f) 10 min of the metallization time ............ 19
Figure 3- 6 Elemental mapping of the silk/Ni–P composite prepared by 2 min of the Ni–P
VII

Figure captions

metallization time at 70oC and 1 atm with agitation, (a) SEM image, (b) nickel, (c)phosphorus, (d) carbon ............................................................................................. 20
Figure 3- 7 The Ni–P metallization layer thickness metallized on silk substrate as a functionof the metallization time ........................................................................................... 21
Figure 3- 8 Electrical resistance curves of the Ni–P metallized silk composites before andafter the adhesive test at room temperature and 1 atm (red circle: as–deposited; blue
upward triangle: first adhesive test; and green downward triangle: second adhesive test)(solid symbols: sc–CO2 method; hollow symbols: CONV method) ........................... 23
Figure 3- 9 SEM images showing surface condition of the silk/Ni–P composite prepared by(a) 1 min, (b) 2 min, and (c) 5 min of the metallization time ..................................... 24
Figure 3- 10 A summary of morphology and electrical resistance of the Ni–P metallized silk
as well as the correlations between each properties ................................................... 25
Figure 3- 11 (a) Tafel plots of the silk/Ni–P composite prepared with 4 min of themetallization time before and after the adhesive tests and (b) an OM image of thesilk/Ni–P composite after second adhesive test and the corrosion test ....................... 26

Figure 4- 1 OM images of (a) the non–catalyzed pure silk and (b) the catalyzed silk ........ 29
Figure 4- 2 X–ray diffraction patterns of (a) the non–catalyzed pure silk, (b) the catalyzed
silk, (c) the silk with reduced catalyst, and (d) the Pt metallized silk ......................... 31
Figure 4- 3 Pt metallization trend shown by SEM images at (a) 120 min, (b) 160 min, and (c)200 min of the Pt metallization time at 70 oC and 1 atm, respectively (black arrows in(c): Pt clusters and accumulations) ........................................................................... 31
Figure 4- 4 Pt metallization layer thickness as a function of the metallization time ........... 32
Figure 4- 5 Electrical resistance of the silk/Pt composite as a function of the metallization
VIII

Figure captions

time at room temperature ......................................................................................... 33
Figure 4- 6 A summary of morphology and electrical resistance of the Pt metallized silk inthree different stages as well as the correlations between each properties (i.e. coverage
and electrical resistance) .......................................................................................... 34

Figure 5- 1 OM images of (a) non–treated pure silk, (b) sc–CO2–assisted Pd(acac)2 catalyzed
silk, and (c) catalyzed silk with reduction treatment .................................................. 38
Figure 5- 2 X–ray diffraction patterns of (a) non–treated pure silk, (b) silk with sc–CO2
catalyzation treatment, and (c) catalyzed silk with the reduction treatment ................ 38
Figure 5- 3 Pt metallization trend demonstrated by SEM images at (a) 80 min, (b) 120 min,and (c) 180 min of the Pt metallization times ............................................................ 40
Figure 5- 4 Elemental mapping of Pt metallized silks at (a) 80 min and (b) 100 min of the Pt
metallization time (set at upper–right figures: carbon signals; set at bottom–rightfigures: carbon signals) ............................................................................................ 41
Figure 5- 5 X–ray diffraction pattern of Pt metallized silk substrate.................................. 42
Figure 5- 6 Pt layer thickness on silk substrate as a function of the metallization time ...... 42
Figure 5- 7 Electrical resistance curves as a function of metallization time of the silk/Pt
composites before and after the adhesion tests .......................................................... 43
Figure 5- 8 SEM images showing surface conditions of the silk/Pt composites prepared by(a) 80 min, (b) 120 min, and (c) 180 min of the metallization time (metallization
temperature: 70 oC; metallization pressure: 1 atm; with agitation) ............................. 45
Figure 5- 9 The relationship between Pt metallization morphology and electrical resistanceas well as the cross–section morphologies in three different stages ........................... 46
Figure 5- 10 (a) Polarization measurements of the silk/Pt composites before and after the
IX

Figure captions

adhesion tests in 3.5 wt.% NaCl solution at room temperature and 1 atm withoutagitation and (b) OM image of the silk/Pt composite after second adhesive test andpolarization measurement......................................................................................... 47
Figure 5- 11 (a) Polarization measurements of the silk/Pt composites before and after theadhesion tests in SBF solution at room temperature and 1 atm without agitation and (b)OM image of the silk/Pt composite after second adhesive test and polarization
measurement ............................................................................................................ 48
Figure 5- 12 Cross–sections of (a) Pt catalyzation followed by Pt metallization, (b) Pd
catalyzation followed by Pt metallization, and (c) illustration of the influence ofdifferent catalysts on the smoothness of Pt layer ....................................................... 52

Figure 6- 1 Experimental settings of the sc–CO2 assisted metallization step: (a) A thermalbath that is controlled by an isothermal furnace, (b) the reaction–cell (PEEK–lined
SUS316L), (c) Pt metallization electrolyte (80 vol.%), (d) sc–CO2 (19.8 vol.%), (e)non–ionic surfactant (0.2 vol.%), (f) silk textile, and (g) cross stirrer, and (h) sc–
CO2/surfactant micelles formed when the solution is stirred in the reaction cell ........ 55
Figure 6- 2 OM images of the (a) as–received and (b) sc–CO2 catalyzed silks .................. 56
Figure 6- 3 X–ray diffraction patterns of the (a) as–received, (b) sc–CO2–assisted Pd(acac)2
catalyzation treated, and (c) Pt metallized silks (blue diamond: Pd(acac)2 catalysts; redcircle: Pt) ................................................................................................................. 57
Figure 6- 4 X–ray diffraction patterns of Pt metallized silks by the (a) CONV at 1 atm, and(b) sc–CO2–assisted metallization methods............................................................... 58
Figure 6- 5 Illustration of influences of sc–CO2 in the metallization step: (a–c) CONV and(d–f) sc–CO2–assisted metallization methods ........................................................... 59
X

Figure captions

Figure 6- 6 SEM images of the Pt metallized silk with (a) 10 min, (b) 20 min, (c) 60 min, (d)100 min, and (e) 140 min of the metallization time, and (f) cross–section of the silk/Pt
by 140 min of the sc–CO2 metallization, and (g) and (f) cross–section of the silk/Pt by
CONV sc–CO2 metallization .................................................................................... 61
Figure 6- 7 Illustration of effects of sc–CO2 on morphologies of the Pt layer by the (a) CONVand (b) sc–CO2–assisted metallization ...................................................................... 62
Figure 6- 8 Thickness of the Pt layer metallized by the (a) CONV method at 100 min of
metallization time (5.3 Morphology, composition and structure of silk/Pt section) and(b) sc–CO2–assisted method as a function of the metallization time .......................... 63
Figure 6- 9 Electrical resistance–metallization time curves of silk/Pt composites before andafter the adhesive tests ............................................................................................. 65
Figure 6- 10 Polarization curves of the silk/Pt composites in (a) 3.5 wt.% NaCl and (b) SBF
solutions at room temperature and 1 atm .................................................................. 67

Figure 7- 1 (a) OM image and (b) X–ray diffraction pattern of the as–received silk as well as(c) OM image and (d) X–ray diffraction pattern of the catalyzed silk (diamond symbol:
Pd(acac)2 catalysts) .................................................................................................. 71
Figure 7- 2 OM images of the (a) Ni–P metallized silk and the Au metallization at (b) 3 min,(c) 20 min, and (d) 80 min ........................................................................................ 72
Figure 7- 3 X–ray diffraction patterns of the Au metallization at (a) 5 min, (b) 20 min, and(c) 80 min and (d) the composition revolution with the Au metallization time ........... 73
Figure 7- 4 Plot of the Au metallization thickness versus the metallization time (hollow
symbol: Ni–P metallization at 4 min and solid symbols: Au metallization thickness
versus Au metallization time) (OM image of Au metallized at 40 min of the
XI

Figure captions

metallization time is inserted into bottom–right) ....................................................... 74
Figure 7- 5 (a) Electrical resistances of the Au metallized silk at various metallization times
and SEM images of the Au metallized silk at (b) 20 min and (c) 80 min of themetallization time (hollow symbols: Au–free; solid symbols: Au metallized) ............ 76
Figure 7- 6 SEM images of the specimens deposited in (a) free of H2O2, (b) 0.1 wt.%, (c) 0.2
wt.%, and (d) 0.3 wt.% H2O2, and the OM images in (e) free of H 2O2, (f) 0.1 wt.%, (g)0.2 wt.%, and (h) 0.3 wt.% H2O2 .............................................................................. 78
Figure 7- 7 General scheme of the solute concentration profiles ....................................... 79
Figure 7- 8 (a) X–ray diffraction patterns and (b) voltage–time plot of the specimens
fabricated with electrolytes containing various H2O2 concentration........................... 81
Figure 7- 9 Photocurrent density of the silk/Au/ZnO hybrid materials with (a) non–corrected(an inserted figure shows the flexibility of the composite material) and (b) corrected by
the ZnO coverage ..................................................................................................... 83

Figure 8- 1(a) OM image and (b) XRD pattern of the as–received silk, and (c) OM imageand (d) XRD pattern of the sc–CO2 catalyzed silk (triangle symbols: Pd(acac)2). ...... 86
Figure 8- 2 SEM images and OM images (inserted into top–left) of Ni–P/TiO2 deposition
with various TiO2 concentration at (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 g/L,
respectively. ............................................................................................................. 87
Figure 8- 3 TiO2 particle size distribution with different amount of TiO2 in the electrolyte:(a) slight aggregation, (b) moderate aggregation, and (c) severe aggregation. ............ 88
Figure 8- 4 Mechanism of TiO2 inclusion with (a) TiO2 concentration less than criticalamount, (b) TiO2 concentration at critical amount, and (c) TiO2 concentration more thancritical amount. ........................................................................................................ 90
XII

Figure captions

Figure 8- 5 Elemental mapping of Ni–P/TiO2 composite layer deposited with 30 g/L TiO2.
................................................................................................................................ 91
Figure 8- 6 X–ray diffraction patterns of Ni–P/TiO2 deposition with various TiO2
concentrations at (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 g/L, respectively. ...... 92
Figure 8- 7 Relationship of Ni–P/TiO2 thickness and TiO2 concentration in the electrolyte.
................................................................................................................................ 93
Figure 8- 8 The influence of included TiO 2 concentration to the electrical resistance of thesilk/Ni–P/TiO2 composite material. .......................................................................... 94
Figure 8- 9 Polarization curves of pure Ni–P coating and Ni–P/TiO2 (3 g/L TiO2 in theelectrolyte) deposition on silk textile at (a) room temperature and (b) 37 oC (bodytemperature). ............................................................................................................ 96
Figure 8- 10 Photocurrent density of the silk/Ni–P/TiO2 incorporated materials illumined byperiodic white light in 0.5 M Na2SO4 electrolyte at room temperature. ...................... 98
Figure 8- 11 Photocurrent density of (a) the as–deposited and (b) second adhesive test–treatedsilk/Ni–P/TiO2 composite illumined by periodic white light in 0.5 M Na 2SO4 electrolyte
at room temperature. ................................................................................................ 99

XIII

Table captions

Table captions
Table 1- 1 Typical electroless plating bath components and operating parameters [4] ......... 4
Table 1- 2 Mechanism of catalyst works on various reducing agents [11] ........................... 6

Table 3- 1 Ni–P compositions in various metallization times ............................................ 20
Table 3- 2 Corrosion potentials and corrosion current densities of the silk/Ni–P composite
materials .................................................................................................................. 27

Table 5- 1 Corrosion potentials and corrosion currents of the silk/Pt composite materials in(a) 3.5 wt.% NaCl and (b) SBF solutions measuring at room temperature and 1 atm . 49

Table 6- 1 Corrosion potentials and corrosion current densities of the silk/Pt composites in(a) 3.5 wt.% NaCl and (b) SBF solutions at room temperature and 1 atm .................. 67

Table 8- 1 Composition of as–deposited Ni–P/TiO2 metallization layers on silk textiles. .. 88
Table 8- 2 Corrosion potentials and corrosion currents of the silk/Ni–P and silk/Ni–P/TiO2composites in 3.5 wt.% NaCl at (a) room temperature and (b) 37 oC (body temperature).
................................................................................................................................ 97

XIV

Chapter 1

Chapter 1 General introduction
1.1 Wearable electronic and medical device applications
Wearable electronic devices and medical devices were predicted to blossoming in thenear future [1, 2]. Wearable devices are the devices such as smart clothing and solar bag
which integrate electronic devices or functional materials and textile together to facilitate
the convenience of our daily life. Wearable medical devices are the gears that can detector support out health condition such as biosensor patch and peace maker. Figure 1- 1
shows various applications of wearable electronic and medical devices. There are some
wearable devices which are commercially available nowadays; however, most of thedevices are composed of rigid materials. Even small annoyance can irritate the active
users such as athlete. Therefore, a flexible, electronically conductive, and functional
wearable devices are now urgently demanded. This study focused on the exploitation of
the materials and the enhancement of reliabilities of the wearable devices.

Figure 1- 1 Various applications of wearable devices
1Chapter 1

1.2 Methodology for the integration of materials for the applications
1.2.1 Electroless plating
Due to the demands of flexibility and electrical conductivity of wearable devices,
combining the flexible and electrical conductive materials together are the greatest
challenge. Electroless plating is a promising method to integrate these two instinct
materials. Electroless plating is also known as chemical or autocatalytic plating [3].
Unlike conventional electroplating, no electrical current is required to carry out thedeposition. It is a technique which can be accomplished by simple procedures and facile
equipment. Processing temperature is relatively lower (<90 oC) than other deposition
techniques. The greatest difference between electroless plating and immersion plating iswhether it can be autocatalytic or not.
Electroless plating consists of three fundamental procedures, pretreatment is the firststep for cleaning and roughening the substrate’s surface, catalyzation is the second step
to inlay the catalyst into the substrates, and deposition is the last step to deposit thematerials on the substrate (Figure 1- 2). The basic component of deposition solution
includes (1) metal ions (i.e. Ni, Pt, and Au ion complexes), (2) reducing agent (i.e.
hydrazine, and hypophosphite), (3) complexing agent (i.e. ethylenediaminetetraacetic
acid (EDTA)), and (4) bath stabilizer (i.e. tungstate, sulphate, nitrate and chloride).
Complexing agent acts as buffer to help control pH and maintain control over the free
metal salt ions available to the solution, thus allowing solution stability. Stabilizer acts asa catalytic inhibitor, retarding potential spontaneous decomposition of the deposition bath.
The complexing agent and stabilizer determine the composition and brightness of thedeposit. Excessive use of stabilization materials can result in a depletion of plating rate
and bath life including poor metallurgical deposit properties. In the deposition step, the2Chapter 1

deposition properties can be controlled by many parameters such as (1) deposition
temperature, (2) solution pH, (3) metal ion concentration, and (4) reducing agent
concentration in the bath. Table 1- 1 show the typical plating bath components andoperating parameters [4].

Figure 1- 2 Electroless plating procedures for non–conductive substrates
It is widely accepted that electroless plating proceeds along the electrochemicalmechanism as the simultaneous reaction of cathodic metal deposition and anodic
oxidation of reductant [5]. The electrochemical conditions for electroless plating taking
place are, first, the oxidation potential of the reductant’s being less noble to the reversible
potential of the metal to be deposited. This would be the thermodynamic consideration.
Second, the metal is having enough catalytic activity for the anodic oxidation take placewith reasonable rate. While, this responses to the requirement of kinetics.
A reductant used in electroless plating have a special feature. A limited variety of
reductants is used in an electroless plating, i.e. hypophosphite, formaldehyde,borohydride, dialkylamine borane, and hydrazine, which have been also shown in Table
3

Chapter 1

1- 1. Electroless plating usually accompanies hydrogen evolution, the rate of which is notdirectly related to that of metal deposition. It was found that the hydrogen evolved during
electroless plating was originated mainly from the reducing agent [6, 7]. Electrolessplating proceeds mostly on certain metals that are known to be hydrogenation–
dehydrogenation catalysts (i.e. Pt, Pd, and Ni). It was reported that the position forhydrogenation catalysts such as thioura and mercaptobenzothiazole functions as stabilizer
in an electroless plating solution [8]. Usually the kinetic requirement is that the catalyst
has the ability to dissociate chemisorption to form adsorbed atomic hydrogen and anadsorbed anion radical. It is also proposed that the catalytic aspect in electroless plating
is originated from the catalytic property of the metal for of the metal [9].
Table 1- 1 Typical electroless plating bath components and operating parameters [4]As mentioned in the previous paragraph, the second step of electroless plating iscatalyzation, which is a critical step for the metallization. Whether metallization can
4Chapter 1

happen spontaneously depends on both thermo–dynamic and kinetic. The first
fundamental requirement is thermal dynamic (eq. (1. 1))∆𝑮 = −𝒏 × 𝑭 × ∆𝑼𝒆𝒒 = −𝒏 × 𝑭 × (𝑽𝑴/𝑴+ − 𝑽𝑹𝒆/𝑹𝒆∗ ) = −𝒏 × 𝑭 × ∆𝑽

(1. 1)

Where ΔG is Gibbs free energy change per mole of reaction, n is number of moles ofelectrons transferred in the reaction, F is Faraday constant = 96485 C/mol (charge permole of electrons), V is reduction potential of each species, M is metal, M + is metal ion,
Re is reducing agent, and Re* is oxidized reducing agent. Reactions happen
spontaneously when ΔG is negative (ΔG < 0). Thus, negative ΔV (ΔV < 0) is a criticalrequirement of a spontaneous reaction. An example is shown in the following:
𝑁𝑖 2+ + 2𝑒 − = 𝑁𝑖 … 𝑉𝑁𝑖 2+ /𝑁𝑖(1. 2)

𝑅𝑒 = 𝑅𝑒 2+ + 2𝑒 − … 𝑉𝑅𝑒/𝑅𝑒 2+(1. 3)

Total reaction:(1. 4)

𝑁𝑖 2+ + 𝑅𝑒 = 𝑁𝑖 + 𝑅𝑒 2+ … ∆𝑉
Where εtotal:(1. 5)∆𝑉 = 𝑉𝑁𝑖 2+/𝑁𝑖 + 𝑉𝑅𝑒/𝑅𝑒 2+

While ΔV < 0 is a requirement for the spontaneous reaction, 𝑉𝑁𝑖 2+/𝑁𝑖 + 𝑉𝑅𝑒/𝑅𝑒 2+ shouldbe negative. Thus, the math relationship of 𝑉𝑁𝑖 2+ /𝑁𝑖 < −𝑉𝑅𝑒/𝑅𝑒 2+ is established. Or theequation can be written as 𝑉𝑁𝑖 2+/𝑁𝑖 < 𝑉𝑅𝑒 2+/𝑅𝑒 . In the chemical aspect of view, it means
Ni2+ has higher ability to get the electrons than reducing agent. Therefore, Ni2+ is reduced
and reducing agent is oxidized in the solution. This is in accordance with the statementthat have been mentioned in the previous paragraph. This is the basic requirement in theview of thermodynamics. The requirement of kinetics will be discussed in the next
paragraph.
5

Chapter 1

Most of the literature claim that whether the catalyst is a proper catalyst for thedeposition system depends on the nature of the catalyst. To be specific, as mentioned
previously, deposition proceeds mostly on certain metals that are known to behydrogenation–dehydrogenation catalysts (i.e. Pt, Pd, and Ni). Whether the catalyst can
go on hydrogenation–dehydrogenation reaction or not is one of the kinetic requirements.
It has been reported that hydrogenation–dehydrogenation is the first and crucial step tostart the deposition [10, 11]. Without dehydrogenation reducing agents do provide
electrons to metal ions in the solution. The mechanisms of different reducing agents areshown in Table 1- 2 [11]. The catalytic of various metals are shown in Figure 1- 3 [10].
Table 1- 2 Mechanism of catalyst works on various reducing agents [11]6Chapter 1

Figure 1- 3 Catalytic activity of various metals (active sites) in different reducing agent
solution: (a)NaH2PO2, (b) HCHO, (c) NaBH4, (d) DMAB, and (e) NH2NH2 [10]Since electroless plating is less limited by the experimental apparatus, the substrate
can be non–electrical conductive and the shape of the substrate can be sophisticated.
Electroless plating provides a deposit that follows all contours of the substrate exactly,
without building up at the edges and corners. A sharp edge receives the same thickness ofdeposit as dose a blind hole. The advantages of electroless plating makes the integrationof textile and conductive material applicable. This study thus combined these two distinct
materials by electroless plating to fulfill the applications of wearable devices.
1.2.2 Supercritical carbon dioxide promoted method
1.2.2.1 Supercritical carbon dioxide promoted catalyzation
Supercritical carbon dioxide (sc–CO2) was introduced into the electroless plating inthis study for enhancing the characters of electroless plating. In the conventional (CONV)
catalyzation, SnCl2 and PdCl2 solutions were used as catalyzation solutions for inlaying
7

Chapter 1

Pd metals on the substrate [12-14]. By the treatments of immersion into SnCl2 followed
by PdCl2 solutions, the following reaction takes place:(1. 6)

𝑺𝒏𝟐+ + 𝑷𝒅𝟐+ = 𝑺𝒏𝟒+ + 𝑷𝒅

Pd metal can act as a catalyst in the next deposition step to activate the chemical reaction.
However, SnCl2 and PdCl2 are dissolved into HCl solvent, which is strongly acidic
solution. Textile substrate has high risk to be damaged by the CONV catalyzation [15].
In addition, Pd catalysts are only inlaid on the substrate surface and finally leads to poor
adhesion between textile substrate and conductive material layer.
Sc–CO2 was introduced into the catalyzation step to solve the CONV problems [16].
Carbon dioxide (CO2) turns into sc–CO2 when the pressure and temperature go beyond
its critical point (304 K and 7.4 MPa) [17, 18] (Figure 1- 4). Sc–CO2 owns the properties
between liquid and gas phases showing low viscosity, low surface tension, high self–
diffusivity, non–erosive to most of the polymers, and non–polarity [19]. Due to theaforementioned advantages, sc–CO2 can bring catalyst into the substrate since textile has
the affinity to non–polarity materials (Figure 1- 5). Materials in the following deposition
step can be deposited into the substrate showing the anchoring effect [20, 21] and enhance
the adhesive property between substrate and deposited material. In addition, due to theproperties of low viscosity and non–erosive to most of the polymers, textile remains
undamaged after the sc–CO2 catalyzation treatment. Moreover, chemical solutions can becompletely excluded in the sc–CO2 catalyzation, which can leads to environmental
friendly process. This study introduced sc–CO2 into the catalyzation step to solve thetraditional difficulties and enhance the adhesive property between textile substrate anddeposited materials.

8Chapter 1

Figure 1- 4 Pressure–Temperature phase diagram of carbon dioxide

Figure 1- 5 Comparison between (a) CONV catalyzation (without sc–CO2) and (b) sc–
CO2 catalyzation and their characteristics
1.2.2.2 Supercritical carbon dioxide promoted metallization
Since metal was deposited on the textile substrate in this study, here we use the termof metallization for the last step of electroless plating instead of deposition. Sc–CO2 was
9Chapter 1

not only introduced into catalyzation but also introduced into the metallization step toenhance the metallization properties. Hydrogen evolution takes place during most of themetallization steps, and the hydrogen bubbles leads to defects in the metallization layer
and finally results in poor properties such as low electrical conductivity and low
compactness of metallization layer. The introduction of sc–CO2 can solve this difficulty,
which is brought from the evolution of hydrogen. With accompany of surfactant, sc–CO2
and surfactant form the micelles in the aqueous solution since one side of the surfactant
is hydrophilic and the other side is hydrophobic [22]. Due to the aforementioned
advantages of sc–CO2 such as non–polarity and high self–diffusivity, it can dissolveshydrogen bubbles and take hydrogen bubbles away from the substrate surface. This
finally results in defect–free metallization layer. The comparison between CONV
metallization and sc–CO2 metallization was shown in Figure 1- 6. This study introduced
sc–CO2 into the metallization step to enhance the properties of metallization layer.

Figure 1- 6 Comparison between CONV metallization and sc–CO2 metallization
10

Chapter 1

1.2.3 Cathodic deposition
Wearable devises are not only limited in electronic devices but also has been
diversified into many applications such as combining photocatalyst into the composite
material. There are various synthetic methods to deposit photocatalytic metal oxides,
which include high–temperature vapor deposition [23], polymer–mediated chemical
reaction [24], hydrothermal–annealing process [25], and one–pot solution–based methods[26]. Cathodic deposition was used in this study due to its advantages such as relatively
low operation temperature (<90 oC), simple operation procedures, low cost, unnecessaryof evacuation system. In addition, patterning of the deposition can be carried out in thecathodic deposition while combing with lithography.
Typical cathodic deposition apparatus consist three electrodes (constant voltage
method), which are working electrode, counter electrode, and reference electrode.
However, two electrode method (constant current method) is widely used in industry due
to its simplicity. A two electrode cathodic deposition was thus used in this study tofabricate the metal oxide (i.e. ZnO) on the metallized textile (Figure 1- 7).

Figure 1- 7 Equipment setting of two electrode cathodic deposition
11

Chapter 1

1.3 Integration of textile, electrically conductive materials, and functional materials
In the materials point of view, flexibility, electrical conductivity, biocompatibility, andstitchability are basic requirements. It also should be a common cloth material. Based on
the aforementioned requirements, silk textile was chosen in this study to fulfill therequirements of flexibility, biocompatibility, and stitchability. In addition, silk is acommon cloth material. On the other hand, Ni–P, Pt, and Au were chosen as themetallization layer for achieving electrical conductivity. Ni–P was chosen for its low cost,suitable electrical conductivity, and high corrosion resistance. Pt and Au were further
chosen due to the biocompatibility and high electrical conductivity. In the function pointof view, ZnO was chosen for the functionality, ZnO plays a role of photocatalyst in thewearable devices. In addition, ZnO is also biocompatible material. Figure 1- 8 shows thesummary of the materials.

Figure 1- 8 Summary of the advantages and reasons of the chosen materials: (a) silk, (b)Metallization layer, (c) ZnO, and (d) TiO2
12

Chapter 1

1.3.1 Metal thin film on silk textile
1.3.1.1 Ni–P metallization
Catalytic surface is basic requirement for the metallization, in other word,
metallization only occurs on specific surfaces. Almost all metals of Group VIII (Fe, Co,
Ni, Rh, Pd, and Pt) is active for the reduction reaction. These metal are well known ashydrogenation–dehydrogenation catalyst. On the other hand, those metals, which are
more electropositive than Ni such as Al, are replaced by Ni firstly and Ni acts as an active
catalyst afterward (eq. (1. 7)).
2𝐴𝑙 + 3𝑁𝑖 2+ = 2𝐴𝑙 3+ + 3𝑁𝑖(1. 7)The very first step in the Al substrate is the displacement and the substrate shows catalytic
property after the displacement.
The most widely used engineering form of electroless plating is, by far, electrolessnickel plating [27]. Electroless nickel plating offers unique deposit properties including
uniformity of deposition in deep recesses, bores, and blind holes. Most commercial
deposition is done with an acid phosphorus bath owing to its unique physicalcharacteristics, including excellent corrosion, wear, and abrasion resistance, ductility,
lubricity, solderability, electrical properties, and high hardness. The chemical reducing
agent most commonly used is sodium hypophosphite (NaH2PO2), sodium borohydride(NaBH4), or aminoborane such as n–dimethylamine borane (DMBA; (CH3)2NHBH3).
Acid nickel–phosphorus deposits normally consist of 87–94% Ni and 6–13% P,
operating at 77–93 oC with a pH of 4.4–5.2. Low phosphorus electroless Ni baths contain
1–3% P and normally operate at 80–82 oC with a pH of 6.0–6.5. According to theconcentration of P in the Ni–P deposition, it can be classified into three classifications. 1–
13Chapter 1

3%P is low phosphorus, 6–10%P is medium phosphorus, and 11–13%P is highphosphorus. The pH of the solution is the controlling factor affecting the phosphorus
content of the deposit. The higher the pH, the lower the phosphorus content, resulting indeposit property changes. Lower phosphorus containing deposits typically have lesscorrosion resistance than 10% alloys. Deposits containing phosphorus in excess of 8%
are typically nonmagnetic.
The reducing agent is commonly NaH2PO2. Nickel deposition by hypophosphite was
firstly thought to be the following equations:𝑁𝑖 2+ + 𝐻2 𝑃𝑂2− + 𝐻2 𝑂 = 𝑁𝑖 + 𝐻2 𝑃𝑂3− + 2𝐻+
𝑐𝑎𝑡(1. 8)(1. 9)

𝐻2 𝑃𝑂2− + 𝐻2 𝑂 → 𝐻2 𝑃𝑂3− + 𝐻2
Overall:
𝑁𝑖 2+ + 2𝐻2 𝑃𝑂2− + 2𝐻2 𝑂 = 𝑁𝑖 + 2𝐻2 𝑃𝑂3− + 2𝐻+ + 𝐻2

(1. 10)However, eq. (1. 8), (1. 9), and (1. 10) failed to account for the co–deposition of Ni andphosphorus. More mechanisms, which will be shown in the following, were proposed
after the discovering of the Ni–P chemical metallization.
(1) Atomic hydrogen mechanism
Brenner and Riddell [28, 29] proposed the first mechanism of Ni–P metallization. They
postulated that the actual Ni reductant is atomic hydrogen which is generated by thereaction of water with hypophosphite:
𝐻2 𝑃𝑂2− + 𝐻2 𝑂 = 𝐻2 𝑃𝑂3− + 2𝐻𝑎𝑑

(1. 11)

𝑁𝑖 2+ + 2𝐻𝑎𝑑 → 𝑁𝑖 + 2𝐻 +(1. 12)The evolution of H2 gas, which always accompanies Ni–P metallization, was due to the14Chapter 1

recombination of two hydrogen atoms.
2𝐻𝑎𝑑 → 𝐻2(1. 13)

However, atomic hydrogen mechanism failed to account for the simultaneous reductionof Ni and H2, nor does it explain why the stoichiometric utilization of hypophosphite isalways less than 50%.
(2) Hydride transfer mechanism
Hersch [30] is the first one proposed the hydride transfer mechanism.
In acidic solution:cat

2𝐻2 𝑃𝑂2− + 2𝐻2 𝑂 → 2𝐻2 𝑃𝑂3− + 2𝐻+ + 2𝐻 −(1. 14)In alkaline solution:cat

2𝐻2 𝑃𝑂2− + 2𝑂𝐻 − → 2𝐻2 𝑃𝑂3− + 2𝐻 −(1. 15)

The reduction of nickel ion in this mechanism proceeds as follows:
𝑁𝑖 2+ + 2𝐻− → 𝑁𝑖 + 𝐻2

(1. 16)

Phosphorus co–deposition can be explain by:
𝐻2 𝑃𝑂2− + 𝐻 = 𝑃 + 𝑂𝐻− + 𝐻2 𝑂

(1. 17)

The hydride transfer mechanism presents a satisfactory explanation for the coupled
reduction of Ni and H2.
(3) Electrochemical mechanism
This mechanism was firstly proposed by Brenner and Riddell and modified by others
afterward.
𝐻2 𝑃𝑂2− + 𝐻2 𝑂 = 𝐻2 𝑃𝑂3− + 2𝐻+ + 2𝑒 −(1. 18)𝑁𝑖 2+ + 2𝑒 − → 𝑁𝑖

(1. 19)15

Chapter 1

2𝐻 + + 2𝑒 − → 𝐻2

(1. 20)𝐻2 𝑃𝑂2− + 2𝐻 + + 𝑒 − = 𝑃 + 2𝐻2 𝑂(1. 21)

According to the above equations, Ni ion should have a significant effect on the rate of
reduction; however, it is widely known that the rate has a zero order dependence on Ni
ion concentration.
(4) Coordination of hydroxyl ionsThis mechanism was proposed by Cavallotti and Salvago [31, 32] and was supported byRadin and Hintermann [33]. Ni firstly acts as a catalyst and results in the ionization ofwater:𝑐𝑎𝑡

𝐻2 𝑂 → 𝐻 + + 𝑂𝐻 −(1. 22)−𝑁𝑖(𝐻2 𝑂)2+
6 + 2𝑂𝐻 = (𝑂𝐻) − 𝑁𝑖𝑎𝑞 − (𝑂𝐻) + 6𝐻2 𝑂(1. 23)(𝑂𝐻) − 𝑁𝑖𝑎𝑞 − (𝑂𝐻) + 𝐻2 𝑃𝑂2− = 𝑁𝑖𝑂𝐻𝑎𝑑𝑠 +𝐻2 𝑃𝑂3− + 𝐻(1. 24)

𝑁𝑖𝑂𝐻𝑎𝑑𝑠 + 𝐻2 𝑃𝑂2− = 𝑁𝑖 +𝐻2 𝑃𝑂3− + 𝐻(1. 25)

Where NiOHads represents a hydrolyzed Ni+ species adsorbed at the catalytic surface. The
two hydrogen atoms can react and evolve as hydrogen gas:
𝐻 + 𝐻 = 𝐻2

(1. 26)The direct interaction of the catalytic Ni surface with H 2PO2- to give P co–deposition:
𝑁𝑖𝑐𝑎𝑡 + 𝐻2 𝑃𝑂2− = 𝑃 + 𝑁𝑖𝑂𝐻𝑎𝑑 + 𝑂𝐻−(1. 27)

The hydrolyzed Ni can react with water as follow:
𝑁𝑖𝑂𝐻𝑎𝑑𝑠 + 𝐻2 𝑂 = [𝑁𝑖(𝑂𝐻)2 ]𝑎𝑞 +𝐻(1. 28)Eq. (1. 25) and (1. 28) are seen to be competing reactions.
The reaction of hypophosphite ions with water must be included in the reaction scheme:
16Chapter 1

𝐻2 𝑃𝑂2− + 𝐻2 𝑂 = 𝐻2 𝑃𝑂3− + 𝐻2

(1. 29)Then the overall reaction would be:
𝑁𝑖 2+ + 4𝐻2 𝑃𝑂2− + 𝐻2 𝑂 = 𝑁𝑖 + 3𝐻2 𝑃𝑂3− + 𝐻 + + 𝑃 + 3/2𝐻2(1. 30)The mole ratio [Ni2+]/[H2PO2-] in eq. (1. 30) is observed to be 0.25.
1.3.1.2 Pt metallization
On the contrary of Ni–P electroless plating, it is known that the greatest challengeof Pt electoless plating is to control the deposition [34-36]. Therefore, there are only
limited literature concerning the Pt electroless plating [36, 37] . Since Pt owns theadvantages such as high biocompatibility [38], high electrical conductivity [39], highcorrosion resistance [40], and high thermal stability, Pt electroless was chosen in thisstudy. Electroless Pt plating has been plated mostly using hydrazine (N 2H4) orborohydride as the reducing agent. Among them, N2H4 is the most used reducing agent
for Pt electroless plating since there are no co–deposition in the Pt metallization film.
(NH3)2Pt(NO2)2 has been widely used as Pt ion source in the metallization solution. The
reaction equation is shown in eq. (1. 31):2(𝑁𝐻3 )2 𝑃𝑡(𝑁𝑂2 )2 + 𝑁2 𝐻4 ∙ 𝐻2 𝑂 = 2𝑃𝑡 + 5𝑁2 + 9𝐻2 𝑂(1. 31)

1.3.1.3 Au metallization
Gold is currently irreplaceable for the electronics industry, in spite of its high price,because of its unique physical properties such as highly electrical conductivity [41], highcorrosion resistance [42], and high thermal stability [43]. Although substitute materials
such as Pd and its alloys are now being used in certain applications, it is believed that Au
cannot be completely replaced by the other materials. In spite of the applicability of Au,
the present state of development of electroless Au plating is still far from being
17

Chapter 1

comparable to that of well–established methods of electroless Ni and Cu plating technique.
The mostly used Au ion source is Potassium dicyanoaurate(I) (KAu(CN) 2) along with the
reducing agent alkali metal borides (i.e. NaBH4, and KBH4) or an amine borane to makethe metallization solution an autocatalytic bath [44]. Most commercially available
electroless Au deposits are produced first by plating a thin deposit of immersion Au,
followed by electroless Au plating. There are a few true autocatalytic Au processes
available with 99.99% purity. Electroless Au can be successfully applied to Kovar, Ni, Ni
alloys, Cu, Cu alloys, and metallized ceramics. Electroless Au can be deposited onto
already present thin electrodeposited Au to give added strength [27]. The chemical
reactions involved in the borohydride process are rather unique because the reducing
agent that actually reacts with Au(CN)2 - is not BH4- itself but the species with the formula
BH3OH-, which is an intermediate formed during the stepwise hydrolysis of BH 4 -:𝐵𝐻4− + 𝐻2 𝑂 = 𝐵𝐻3 𝑂𝐻 − + 𝐻2

(1. 32)𝐵𝐻3 𝑂𝐻 − + 𝐻2 𝑂 = 𝐵𝑂2− + 3𝐻2(1. 33)−−−−
6𝐴𝑢(𝐶𝑁)−
2 + 𝐵𝐻3 𝑂𝐻 + 6𝑂𝐻 = 6𝐴𝑢 + 𝐵𝑂2 + 12𝐶𝑁 + 5𝐻2 𝑂(1. 34)All hydrogen gas produced during the metallization results from the hydrolysis reactions.
The metallization in the bath was shown to be very low because of the second step ofhydrolysis reaction proceed much more rapidly than the metallization reaction. Most of
the borohydride is lost by hydrolysis. Electroless Au plating in the borohydride systemtakes place on noble metals (i.e. Pd, Rh, Ag, and Au itself). It also takes place on Cu, Ni,
Co, Fe, and their alloys. Initial reactions on these two classes of metals are different. On
noble metals the reaction is catalytic from the very beginning, on the other hand, the gold
metallization on the other group is initiated by galvanic displacement.

18

Chapter 1

Hydrazine is also a promising reducing agent in the Au metallization. The
reactions take place on Ni substrate are shown in the following:2+
2𝐴𝑢(𝐶𝑁)−+ 4𝐶𝑁 −
2 + 𝑁𝑖 = 2𝐴𝑢 + 𝑁𝑖

(1. 35)(1. 36)

𝑅𝐶𝑂𝑂𝐻 + 𝑁2 𝐻4 = 𝑅𝐶𝑂𝑁𝐻𝑁𝐻2 + 𝐻2 𝑂−+
𝑅𝐶𝑂𝑁𝐻𝑁𝐻2 + 2𝐴𝑢(𝐶𝑁)−2 = 2𝐴𝑢 + 4𝐶𝑁 + 𝑁2 + 𝑅𝐶𝑂𝐻 + 2𝐻

(1. 37)+++−2𝐴𝑢(𝐶𝑁)−
2 + 𝑁2 𝐻5 + 2𝐻2 𝑂 = 2𝐴𝑢 + 2𝑁𝐻3 𝑂𝐻 + 𝐻 + 4𝐶𝑁(1. 38)

However, KAu(CN)2 was gradually replaced by non–cyanide bath such as Au(III)chloride complex, due to its toxicity to human body. Au(III) chloride complex (i.e.
KAuCl4) is very easily reducible to Au metal along with the ether–substituted tertiaryamine boranes reducing agent. This bath also shows autocatalytic property. On the other
hand, Au(I) sulfite is also another choice for replacing toxic KAu(CN)2 bath. This Au ion
source is accompany with various reducing agents including hypophosphite,formaldehyde (HCHO), hydrazine, borohydride, and dimethylamine borane ((CH 3)2NH ·BH3). All the bath shows autocatalytic property. These baths, which are without cyanide
species, even show better efficiency of Au metallization. Evidently, the catalytic activityof Au is poisoned by Au(CN)2- and/or CN- ions. Using hydrazine borane is also a choiceof the reducing agent for the cyanide–free baths. It is claimed that this bath containing
hydrazine borane as a reducing agent can be operated under relatively low temperature(58–60 oC). For avoiding the toxic metallization bath, an Au(III) chloride complex
metallization bath was used in this study.
1.3.2 Photocatalyst on metallized silk textile
Photocatalysis is a reaction of employing catalyst that is utilized for acceleratingchemical reactions, which requires or engages light. A photocatalyst is defined as a19

Chapter 1

material that is able to absorb light, produce electron/hole pair (e-/h+), which facilitatechemical reactions of the reactants and regenerate its chemical composition after eachcycle of such interactions [45-47]. There are two types of photocatalytic reactions (i.e.
homogeneous and heterogeneous photocatalysis).
The crucial features of the photocatalyst are the appropriate band gap, suitable
morphology, high surface area, stability and reusability [47, 48]. Metal oxides such asoxides of vanadium, chromium, titanium, zinc, tin, and cerium having thesecharacteristics go through similar primary photocatalytic processes, which induces lightabsorption and an electron–hole pair separation process with the generation of positively
charged holes that are able to oxidize reactants [47, 48]. In this process, a metal oxide isilluminated by either UV light, visible light, or a combination of both light sources, andphoto–excited electrons are excited from the valence band to the conduction band,
forming e-/h+. The photo–generated e-/h+ pair is capable of reducing and/or oxidizing areactant adsorbed on the photocatalyst surface. The photocatalytic activity of metal oxide
can be indicated by two sources in cathodic and anodic electrodes. The first one is thegeneration of OH radicals (OH•) by oxidation of OH-, and the second one is the formationof O2 radicals (O2•) by reduction of O2. The radicals can react with pollutants to degradeor transform them to less harmful products [48].
There are many catalysts reported in the literature for photocatalysis process.
These metal oxides (TiO2, ZnO, SnO2 and CeO2) have been widely used as photocatalysts,
especially as heterogeneous photocatalyst [47, 49]. This is due to their biocompatibility,
high stability in a variety of conditions, and capability to generate e-/h+ pairs whenilluminated by enough amount of light source. The proper characteristics of electronic
20

Chapter 1

configuration, light absorption properties, charge transport properties, and excited
lifetimes of metal oxides has made it applicable for photocatalyst [45-48, 50].
Heterogeneous photocatalysis utilizing metal oxides such as TiO2, ZnO, SnO2 and,CeO2 has proved its efficiency in degrading a wide range of pollutants into harmless CO 2
and water [51-56]. Many research has also indicated that metal oxides can be used as aphotocatalyst, photovoltaics, preventing fogging of glass, and splitting water intohydrogen and oxygen [45-48]. Hence, they are of great interests in fields of environmental
remediation, energy production and storage, and electronic industries [46-48]. Moreover,the heterogeneous photocatalysis has also been indicated as a promising self–cleaning,
antibacterial, and deodorization system [46-48].
The physicochemical properties of the metal oxides are crucial for the
performance of photocatalytic activity, which basically depends on size, shape,morphology, and composition [45-48]. The synthetic technique employed for the
preparation of materials that can control the size, shape, composition, and morphology ofthe materials, which can contribute to the development of certain properties of thephotoactive materials. This can facilitate the formation of metal oxides with the required
characteristics that improve the performance of the photocatalytic activity [55, 57].
Cathodic deposition has been point out that it is able to control the morphology, size, andshape of metal oxide by tuning electrolyte and/or controlling the potential and current
density. Thus cathodic deposition was chosen in this study for the formation of metaloxide and it is discussed in the 1.2.3 Cathodic deposition section.
1.3.2.1 ZnO Cathodic deposition
ZnO, which is a promising material for wearable devices, is a photocatalytic and21

Chapter 1

biocompatible material. There are many synthesis technique that have been mentioned in
the previous section. Cathodic deposition was chosen in this study for its low cost, simple
operation procedures, no need of vacuum system, mass–scale production, and relatively
low temperature (< 90 oC). Crystallinity is a critical requirement for photocatalyst [58,59], since the excited electrons might consumed by the defects. ZnO can show
crystallinity without annealing after synthesized by cathodic deposition. ZnO was thus
chosen in this study for wearable devices as a functional material.
In typical cathodic deposition, oxygen, nitrate, or hydrogen peroxide are used as thesource of oxygen. The reaction of nitrate reduction produces nitrite and ammonia, which
progressively accumulate in the deposition bath during the synthesis and leads todeposition instabilities. On the other hand, the use of dissolved oxygen does not produce
undesired reaction products, but its solubility is limited to a value lower than 10 -3 M in
hot water, and consequently ZnO electrodeposition cannot be performed rapidly with thisprecursor. H2O2 doesn’t give rise to undesired reaction products and its solubility in wateris high (up to 10 M). Thus, the cathodic deposition of ZnO usually combines threedifferent oxygen sources to reach the optimized deposition.
On the other hand, zinc chloride (ZnCl2) and zinc nitrate (Zn(NO3)2) are most used
Zn2+ sources. Similarly, NO3- in Zn(NO3)2 results in undesired products and the effect ofchloride ion (Cl-) from ZnCl2 will be discussed in the following. Potassium chloride (KCl)and sodium chloride (NaCl) are used as supporting electrolyte for enhancing the electricalconductivity. However, it was found that Cl- has influence on the morphology of ZnO.
Because of strong adhesion of Cl- , Cl- may play as a capping agent blocking the grain
growth on <0001> direction. In another word, the preferential adsorption of Cl- ions onto
22

Chapter 1(0001) ZnO surfaces as one of the responsible mechanism for the formation of platelet–
like ZnO nanocrystals. Chloride adsorption may result in the stabilization of (0001) ZnO
surface, therefore, hindering the growth along the corresponding direction (c axis) [60].
The mechanism is shown in Figure 1- 9.

Figure 1- 9 Schematic graphs of the effect of Cl− ions on the morphology of ZnOnanostructures under (a) low [OH−] and (b) high [OH−] [61]Electrons go through the working electrode reacting with either the oxygen
source and generate hydroxide (OH-). OH- reacts with Zn2+ in the solution and form zinchydroxide (Zn(OH)2). Zn(OH)2 decomposes into ZnO and water eventually due to theinstability of Zn(OH)2. An example of ZnO formation in the cathodic deposition wasshown in the eq. (1. 39), (1. 40), and (1. 41):
𝑁𝑂3− + 𝐻2 𝑂 + 2𝑒 − = 𝑁𝑂2− + 2𝑂𝐻−

(1. 39)𝐻2 O2 + 2𝑒 − = 2𝑂𝐻−

(1. 40)23

Chapter 1

𝑍𝑛2+ + 2𝑂𝐻− = 𝑍𝑛𝑂 + 𝐻2 𝑂

(1. 41)

The method consists in generating hydrogen ions at the surface of an electrode bycathodically reducing an oxygen precursor. The process leads to a pH increase in thevicinity of the electrode and to a local supersaturation for ZnO precipitation which
provoke the formation of a ZnO thin film on the electrode surface. As shown in Figure 110, only dose ZnO precipitate when the solution is alkaline.

Figure 1- 10 Pourbaix diagram of the system Zn/H 2O, 10-4 M Zn2+ considering ZnO as
solid substance [62]Owing to the demands of functional materials on the wearable devices and theaforementioned merits of cathodic deposition and ZnO, cathodic deposition was used inthis study to fabricate ZnO for functionalizing the textile.
1.3.2.2 Inclusion of TiO2 (P25) into Ni–P metallization
High corrosion resistance, better surface properties [63, 64], and functional
24

Chapter 1

characteristics of metallic materials are required for the development of modern
technology. Composites made up of fine particles of pure metals, ceramics, and polymers
in a metallic matrix have been the objective of investigation for some decades in academic
and industry to improve the aforementioned properties. The composites consist of
metallic matrix exhibit a broad range of enhanced properties, which satisfy the particularrequirements such as improved mechanical properties, optical characteristics, highresistance to oxidation, corrosion resistance, high wear resistance, and showing functional
characteristics. These materials are particularly useful for engineering and fortechnological utilizations in aerospace, automobile, photocatalyst, and photovoltaic
industries [65].
Several preparation techniques such as plasma spraying [66], chemical and physicalvapor deposition [67, 68], electro–codeposition and electroless–codeposition [65, 69],
powder metallurgy [70], and sol–gel [71] have been investigated for developing
composites materials with metallic matrix. Among which, the electro–codeposition andelectroless–codeposition method, which consist of electrolyte and particles, has theadvantages of uniform deposition on complexly shaped substrates, low cost, good
reproducibility, homogeneous distribution of particles, the reduction of waste, and low
processing temperature [69].
The advantages of Ni–P was mentioned in the 1.3.1.1 Ni–P metallization section. In
order to improve the surface properties, electro–codeposition of metallic matrix withvarious types of ceramics particles such as TiO2 [72], SiC [73], ZrO2 [74], CeO2 [75], andAl2O3 [76, 77] were studied. Among these supporting materials, TiO2 is one of the mostpromising metal oxides used for the material improvement and functionalization. TiO2
25

Chapter 1

performs a variety of applications such as self–cleaning device, wastewater treatment,
pollutant degradation, production of hydrogen fuel, dye–sensitized solar cell, monitoringof air contaminated by gases, and photocatalyst.
Eltoum et al. [78] synthesized Ni–TiO2 composite from gluconate electrolyte. It isshown that the inclusion of TiO2 nanoparticles (with mean diameter size of 50 nm TiO2)
has influence on the structure of Ni–matrix and improves the hardness and corrosion
resistance of the substrate. Thiemig et al. [69] fabricated Ni–TiO2 (with mean diametersize of 25 nm TiO2) composite materials by electro–codeposition from two different typesof Ni deposition baths: an acidic sulfamate bath and an alkaline pyrophosphate electrolyte.
It was reported that the mechanical properties of the Ni–TiO2 composite materials wereenhanced while comparing to pure Ni metallization. Parida et al. [72] investigated the Ni–
TiO2 composite films by direct current deposition technique on steel substrate from Watt‫׳‬sbath. The TiO2 incorporation (with mean diameter size around 30 nm TiO 2) in the films
resulted in a high micro–hardness and wear resistance compared to pure Ni metallization.
Benea et al. [63] worked in the electro–codeposition of Ni and TiO2. The results showed
high hardness and wear resistance. Momenzadeh et al. [64] studied the effect of TiO2
concentration in the electrolyte to the mechanical properties. They further introduced
sodium dodecyl sulfate (SDS) surfactant into the electrolyte to increase the TiO 2 inclusion
amount.
There are many studies working on the investigation of corrosion resistance,mechanical properties, or wear resistance; however, there are only very limited literatureworking on the photocatalytic activity of the composite materials of metal/oxide, and is
especially limited in the flexible substrates. This study thus worked on the improvement
26

Chapter 1of both corrosion resistance and photocatalytic activity by the inclusion of TiO 2.
Moreover, the adhesion of oxides to metallic matrix can be enhanced by the electroless–codeposition and thus the reliability can be improved comparing to cathodic deposition,
which was mentioned in 1.2.3 Cathodic deposition sections.
1.4 Evaluations of fundamental properties
Electrical conductivity is an essential property for the applications in wearable
devices. Hence electrical resistance of the silk/metal and silk/metal/oxide composite were
evaluated by an in–line four–point probe measurement in this study. On the other hand,
wearable devices are subjected to the external disturbance frequently while being
employed, assessments for the reliability thus is also a fundamental prerequisite. An
adhesive test to evaluate the adhesive firmness between the metallized layer and the silk
substrate was conducted in this study.
Corrosion resistance is a critical property for the application due to the exposure toliquids during the employment. Polarization tests were carried out in two different
solutions to evaluate the corrosion resistances of the silk/metal and silk/metal/oxide
composite materials in vivo and in vitro.
Furthermore, a critical issue concerning biocompatibility assessment must be
addressed for the targeting application in wearable and medical devises. According to J.C.
Wataha [79], allergy reactions would be triggered when there is metal ion released fromthe environment and enters human body. The metal ion released from the silk/metalcomposite becomes a criteria for evaluating the biocompatibility. For this reason, metal
ion–releasing rate was examined by immersing the silk/metal composite in a simulated
body fluid (SBF) (r–type) [80] for three months in body temperature (37oC) to assess the27Chapter 1

biocompatibility. Since the oxides composited materials are fabricated for the functional
materials such as photocatalytic and photovoltaic devices.
Photocurrent density, which is excited by the white light source is one of the mostimportant criteria for evaluating the photocatalytic activity. Photocatalytic activity
evaluations of the composite of silk/metal/oxide hybrid structure are demonstrated bymeasuring the photocurrent density in this study.

28

Chapter 2

Chapter 2 Methodology
2.1 Catalyzation for embedding the catalysts as active sites
2.1.1 Conventional catalyzation
A SnCl2/PdCl2 solution consisted of SnCl2 (10 mg/L), PdCl2 (100 mg/L), and HCl(87.5 g/L) (Okuno Chemical Industries Co., Ltd., Japan) was used for the CONV
catalyzation. Silk textile was immersed into the solution at room temperature and 1 atm
for 10 sec to activate the silk textile. An illustration of the CONV catalyzation is shownin Figure 2- 1.

Figure 2- 1 Illustration of CONV catalyzation in SnCl2/PdCl2 (HCl) solution
2.1.2 Sc–CO2 catalyzation
The as–received silk textile was hooked from top of the reaction cell. No pretreatment
was carried out before catalyzation. Twenty–five mg of Palladium(II) acetylacetonate(Pd(acac)2; 99%, Sigma–Aldrich, USA)) or Platinum(II) acetylacetonate (Pt(acac)2;>99.98%, Sigma–Aldrich, USA)) was introduced into the cell to maintain the catalyst
concentration at the saturation point throughout the catalyzation process. A magnetic stir
bar was enclosed into the reaction cell to provide the agitation. CO2 (99.99%, Nippon
1Chapter 2

Tansan Gas Co. Ltd., Japan) was introduced into the enclosed reaction cell. The
catalyzation conditions were set at 80 ± 1 oC and 15 ± 0.1 MPa in the reaction cell withagitation for 2 hours. No post treatment was carried out after the catalyzation. An
illustration of catalyzation details is shown in Figure 2- 2. High pressure system (Japan
Spectra Company, Japan) used in the catalyzation step was shown in Figure 2- 3.

Figure 2- 2 Setup of sc–CO2 catalyzation reaction cell

Figure 2- 3 High pressure system used to generate and inject sc–CO2 into the reaction
cell. (a) CO2 gas tank, (b) CO2 liquidization unit, (c) liquidization unit, (d) high pressure
pump, (e) thermal bath, (f) reaction–cell (PEEK–lined SUS316L), (g) substrate, (h)cross stirrer, (i) catalyst, (j) back pressure regulator, and (k) trap
2

Chapter 2

2.2 Metallization to equip silk textile with electrical conductivity
2.2.1 Conventional metallization
2.2.1.1 Ni–P conventional metallization
A commercially available acidic Ni–P electrolyte (Okuno Chemical Industries
Co., Ltd., Japan) consisted of nickel chloride (1.8 wt.%), sodium hypophosphite (2.4
wt.%), complexing agent (2.4 wt.%), and ion–exchanged water (93.4 wt.%) was used inthis study. The metallization was executed at an isothermal environment controlled at 70± 1 oC under ambient pressure with agitation. Various Ni–P metallization times (t = 0.5,
1, 2, 3, 4, 5, 10, and 20 min) were performed to examine the relationship between the Ni–
P metallization time and the metallization characteristics. No post treatment was
conducted after the Ni–P metallization. An illustration of CONV Ni–P metallization wasshown in Figure 2- 4.

Figure 2- 4 Illustration of CONV Ni–P metallization
2.2.1.2 Pt conventional metallization
The Pt metallization electrolyte consisted of 50 vol.% of a basic Pt solution(Pt1505281, Matex･Japan Co. Ltd., Japan), 2 vol.% of a N2H4 reducing agent solution(Pt1505282, Matex･Japan Co. Ltd., Japan). The pH value was tuned to 12 by ammonia
solution (01266–00, Kanto Chemical Co. Inc., Japan).The Pt metallization was carried
3

Chapter 2

out at 70 ± 1 oC under ambient pressure with agitation. Various Pt metallization times
were performed to examine the relationship between the metallization time and the silk/Pt
characteristics. No post–treatment was conducted after the metallization. Identicalexperimental setup was used as Figure 2- 4.
2.2.1.3 Au conventional metallization
The Au metallization electrolyte used in this study was a commercially available
electrolyte (MATEX JAPAN Co. Ltd., Japan) The Au electroless plating electrolyte wascomposed of ammonium gold sulfite (0.2 wt.%), reducing agent (0.3 wt.%), complexing
agent (0.3 wt.%), and ion–exchanged water (99.2 wt.%). A layer composed of a less noble
metal is needed to initiate metallization of the Au layer. Hence, Ni–P metallization was
conducted on the catalyzed silks at 70 oC and atmospheric pressure using the Ni–P
electroless plating electrolyte. Ni–P metallization time at 4 min was conducted to ensure
the full coverage. Likewise, Au metallization process was executed on the Ni–P
metallized silk for various lengths of metallization times (t = 3, 5, 10, 20, 40, 60, and 80
min). The Au metallization bath was set at 70 oC and atmospheric pressure. Identicalexperimental setup was used as Figure 2- 4. Specimens showing a full Au coverage wereselected to proceed with deposition of the ZnO layer (1.3.2 Photocatalyst on metallized
silk textile section).
2.2.1.4 Ni–P/TiO2 conventional metallization
A commercially available acidic Ni–P electrolyte (Okuno Chemical Industries Co.,
Ltd., Japan) consisted of nickel chloride (9.0 wt.%), sodium hypophosphite (12.0 wt.%),complexing agent (12.0 wt.%), and ion–exchanged water (67.0 wt.%) was used as Ni–P
metallization solution in this study. A commercially available TiO 2 (Degussa, Evonik
4Chapter 2

Industries, Germany) was used as included particles in the Ni–P metallization layer.
Different TiO2 amounts were introduced into 100 ml Ni–P metallization individually. The
concentrations of TiO2 in the Ni–P electrolyte were from 0, 10, 20, 30, 40, and 50 (gTiO2/L Ni–P electrolyte). Metallization time was at 10 min and the metallization was
conducted at 1 atm and 70 oC, which was controlled under water bath. TiO2 particles weremixed tempestuously in the electrolyte for 10 min before the metallization was carriedout. Electrolyte was agitated throughout the metallization process to establish suspended
TiO2 and well–mixed solution. A flow chart of pre–agitation is shown in Figure 2- 5.

Figure 2- 5 Flow chart of the pre–agitation of Ni–P/TiO2 deposition
2.2.2 Sc–CO2 metallization
2.2.2.1 Pt sc–CO2 metallization
Fifty ml of an acidic platinum–metallization electrolyte consisted of 0.06 M
chloroplatinic acid hexahydrate (≥37.50% Pt basis, Sigma–Aldrich, USA), 0.15 M N2H4
reducing agent (24–26%, Sigma–Aldrich, USA), and 0.2 vol.% non–ionic surfactant(polyoxyethylene lauryl ether, C12H25(OCH2CH2)15OH, 25%; Toshin Yuka Kogyo, Japan)
was added into the 50 ml reaction cell. CO2 was injected into the tightly sealed reaction
cell. Pt metallization was carried out in an isothermal furnace controlled at T = 70 ± 1 oC
5Chapter 2

and P = 15 ± 0.1 MPa with agitation. Various metallization time periods were performed
to examine the relationship between lengths of the metallization time and silk/Pt
characteristics. No post treatment was conducted after the metallization. An illustrationof the sc–CO2 metallization method is shown in Figure 2- 6. Same high pressure system(Figure 2- 3) was used in Pt sc–CO2 metallization.

Figure 2- 6 Illustration of sc–CO2 assisted Pt metallization
2.3 ZnO cathodic deposition to functionalize metallized silk textile
ZnO was deposited on the Au metallized silk (1.3.2 Photocatalyst on metallized silk
textile section). The cathodic deposition was run at a constant current density of 0.010
A/cm2 at 70 °C for 2 hours without agitation. Surface area of the working electrode, thatis the contact area between the Au layer and the electrolyte, was estimated to be 10 cm2,
which was calculated by estimating surface area of every bundle of the Au metallized silkin a piece of the specimen. A piece of Pt foil was used as the counter electrode. The
cathodic depositions were performed by using a Potentiostat/Galvanostat (1287A,
Solartron Analytical, UK) (Figure 2- 7). No post treatment was conducted after the ZnO
cathodic deposition step.
6

Chapter 2

Figure 2- 7 Setup of cathodic ZnO deposition (deposition time: 2 hours, deposition
temperature: 70oC, deposition pressure: 1 atm)
2.4 Evaluations of the composite materials
2.4.1 Surface morphology and crystal structure
An optical microscope (OM; VHV–5000, KEYENCE, Japan) and a scanningelectron microscope (SEM; JSM–7500, JEOL, Japan) were used to observe morphologiesof the composite materials. Thicknesses of the metallized layer were estimated byobserving the cross–sections of metallized silks via SEM and were analyzed andcalculated by ImageJ (National Institutes of Health, USA). Metallized silks were cut into
two pieces to unclose the cross–section of composite materials. One piece of thecomposite material was fixed perpendicularly to the SEM holder surface by carbon tape
7

Chapter 2

showing the cross–section to the electron beam. The composition and coverage of themetal layer were measured by energy–dispersive X–ray spectroscopy (EDX; EMAX
Evolution, HORIBA, Japan) and the inbuilt elemental mapping function. The phases andcrystal structures were identified by an X–ray diffractometer (XRD; Ultima IV, Rigaku,
Japan) with Cu kα radiation.
2.4.2 Electrical conductivity
An in–line four–point probe with 1.5 mm distance between each probes (MCP–
T370, Mitsubishi Chemical Analytech Co. Ltd., Japan) was carried out to evaluate theelectrical resistance with the samples dimension in 1 cm x 0.5 cm (length x width) at room
temperature. Ten times of the electrical resistance measurement were carried out to ensure
reliability of the values reported and build up the error bar. An illustration of electricalresistance measurement is shown Figure 2- 8.

Figure 2- 8 Illustration of electrical resistance measurement of silk/metal composites
2.4.3 Adhesive firmness
An adhesive test was conducted to evaluate reliability of the silk/metal composite(Figure 2- 9). For the adhesive test, a piece of 3M tape (810 Scotch® Magic™ Tape, 3M,
USA) was firmly stuck to the sample surface by applying a 1–kg load and peeled off
repeatedly. The adhesive test with one and two cycles of the sticking–peeling procedure
are named first adhesive test and second adhesive test. The electrical resistance was8Chapter 2

measured before and after the adhesive test to evaluate the durability and firmness of themetallized layer. Similarly, ten times of the electrical resistance measurements werecarried out to ensure reliability of the values reported and construct the error bar.

Figure 2- 9 Procedures of adhesive test for the composite materials
2.4.4 Corrosion resistanceThe corrosion resistance was measured by polarization measurements in 3.5 wt.%
NaCl or a SBF solutions (r–type) (Figure 2- 10). The 3.5 wt.% NaCl solution was used tosimulate the human sweat [81] and the SBF solution was used to simulate human body
fluid. These two solutions were thus chosen for the in vitro and in vivo simulation,
respectively. The potentiodynamic polarization curves were measured by using a
Potentiostat/Galvanostat (1287A, Solartron Analytical, UK). The silk/metal composite
with lowest electrical resistance, a Pt plate, and an Ag/AgCl reference electrode were usedas the working electrode, the counter electrode, and the reference electrode, respectively.
Total surface area of the working electrode , which is a piece of the 1 x 1 cm2 silk/metalcomposite, was calculated by considering surface area of every bundle of the metallized
silk textile, and the value was 9.5 cm2. The Pt counter electrode was cleaned ultrasonicallyin ethanol and pure water for 1 min, respectively, before the polarization measurements.
9

Chapter 2

The scan range started from –0.8 to + 0.4 V (for Ni–P), from –0.4 to + 0.6 V (for Pt) vs
Ag/AgCl, or from –0.8 to + 0.2 V (for Ni–P/TiO2) at a scan rate at 1 mV/s. Prior to thepotentiodynamic test, the three electrodes were immersed into the solution for 25 min tocertify the stability.

Figure 2- 10 Experimental setup of corrosion resistance measurement
2.4.5 Immersion test for evaluating biocompatibility
The metal ion releasing rate was evaluated by an immersion test (Figure 2- 11).
In the immersion test, a piece of the sample was immersed in the SBF in a 30 ml container
at 37 oC (body temperature) for three months. Then, ~15 ml of the SBF solution was
drawn out and centrifuged for 2 hours to remove undesired particles from the solution.
Released metal ion concentration was measured by inductively coupled plasma atomic
emission spectroscopy (ICP–AES; ICPS–8100, Shimadzu Co. Ltd., Japan). Ten times of
the ICP measurement were carried out to ensure reliability of the values reported.

10Chapter 2

Figure 2- 11 Flow chart of immersion test for evaluation of ion releasing rate
2.4.6 Photoelectrochemical measurement
The silk/metal/oxide composite (i.e. silk/Au/ZnO and silk/Ni–P/TiO2), a Pt plate,
and an Ag/AgCl reference electrode were used as the working electrode, the counterelectrode, and the reference electrode, respectively. The chronoamperometric I−t curves
were recorded at 0 V vs Ag/AgCl using the Potentiostat/Galvanostat under white lightillumination (500 W xenon lamp, with a light intensity of 100 mW/cm2) at room
temperature. A Na2SO4 (0.5 M) solution was used as the electrolyte. Since the composite
materials show cross–linked and sophisticated surface, the surface area of metal oxides
were processed and calculated by a built–in software of the OM. Similarly, ZnO coverageon the Au layer was also processed and calculated by a built–in software of the OM. The
setting for the photoelectrochemical measurement was illustrated by Figure 2- 12.

11

Chapter 2

Figure 2- 12 Experimental setup of photocurrent density measurement (evaluation
temperature: room temperature, electrolyte: 500 mL Na2SO4, evaluation pressure: 1 atm,
without agitation)

12

Chapter 3

Chapter 3 Ni–P metallized silk textile via sc–CO2
promoted electroless plating to practiceelectrically conductive silk textile
3.1 Introduction
Ni-P is consider as a promising material toward the applications of wearable
devices due to the well–developed Ni–P electroless plating technique, low cost, andtunable crystallinity. Ni–P was thus chosen in this study to practice the electricallyconductive textile for the applications of wearable devices.
3.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation
Figure 3- 1 shows OM images and X–ray diffraction patterns of the as–received
silks and the sc–CO2 catalyzed silks. The interwoven transparent bundles shown in Figure3- 1(a) indicate the silk textile and Figure 3- 1(b) shows a broad peak around 2θ = 20o,this broad peak reveals an amorphous structure of the silk textile. The light yellow flakes
around the silk bundles in Figure 3- 1(c) specify the sc–CO2 deposited Pd(acac)2 catalysts
and the diffraction peaks in Figure 3- 1(d) labelled by triangle symbols can be indexed toPd(acac)2 very well [82, 83], indicating the Pd(acac)2 catalysts were successfully
deposited on the silk substrates. As demonstrated in the Figure 3- 1(a) and (c), the silk
textile remained undamaged after the 2 hours sc–CO2 catalyzation treatment due to thelow surface tension, low viscosity, and non–corrosive properties to most of polymers of
sc–CO2 [15, 21, 84]. Deposited Pd(acac)2 catalysts were reduced to Pd metal particles by13

Chapter 3

reducing agent (N2H4 in this study) in the succeeding metallization electrolyte and act as
active sites for triggering the Ni–P metallization reactions.
The sc–CO2 catalyzation practiced a significant advancement when it is comparedto the CONV catalyzation process [15]. In the CONV catalyzation, on the contrary, silk
substrate was damaged by the acidic catalyzation solution (Figure 3- 2). CONV
catalyzation consists of SnCl2 and PdCl2 in the HCl electrolyte. However, HCl solution
is erosive to most of the polymers (i.e. silk). In addition, the catalysts were only depositedon the substrate surface. Although it was considered that increasing the immersion timein the catalyzation solution is helpful for the deposition of catalysts, silk substrate is also
damaged severely in the same time.

Figure 3- 1 (a) OM image and (b) X–ray diffraction pattern of the as–received silk, and(c) OM image and (d) X–ray diffraction pattern of the sc–CO2 catalyzed silk (triangle
symbols: Pd(acac)2 catalysts)
14

Chapter 3

Figure 3- 2 OM image of conventionally catalyzed silk at room temperature withoutagitation (red dashed circles: damaged silk substrate)
Figure 3- 3 shows the cross–sections of backscattered electron SEM images
treated by sc–CO2 catalyzation process at (a) 1 hour and (b) 2 hours. Dark gray phaseindicates epoxy, light gray phase suggests the cross–section of silk substrate, and thebright spots in nano–size are reduced Pd(acac)2 catalysts. In Figure 3- 3(a), Pd(acac)2
catalysts were embedded into the silk substrate around 190 nm. On the contrary, a greatamount of Pd(acac)2 catalysts were embedded into the silk substrate after 2 hours sc–CO2
catalyzation treatment (Figure 3- 3(b)). The catalysts were successfully embedded into
the substrate around 330 nm in the depth due to the non–polarity among sc–CO2,
Pd(acac)2 catalysts, and silk substrate and low surface tension of sc–CO2 [15, 16, 84, 85].
Structure of silk substrate still remained intact after the sc–CO2 catalyzation treatment.

15

Chapter 3

Figure 3- 3 Cross–section of backscattered electron SEM images of catalyzed silk at (a)1 hour and (b) 2 hours at 80 oC and 15 MPa
3.3 Morphology, composition and structure of silk/Ni–P
X–ray diffraction patterns of (a) the as–received silk, (b) the sc–CO2 catalyzed
silk, (c) the catalyzed/reducing agent treated silk, and (d) the catalyzed/Ni–P electrolyte
treated silk are shown in Figure 3- 4. In Figure 3- 4(a), a board peak at around 2θ = 20o
indicates amorphous structure of the silk. The characteristic peaks in the Figure 3- 4(b)correspond to the Pd(acac)2 well indicating the catalysts were successfully settled on thesubstrate. Due to the high intensity from Pd(acac) 2 catalysts, no silk characteristic peaks
were observed in Figure 3- 4(b). shows the reduced catalysts on the silk substrate,
Pd(acac)2 catalysts were reduced to metallic palladium particles after immersing thecatalyzed silk in a solution containing the reducing agent for 15 min at 70 oC. According16

Chapter 3

to Y.S. Cheng et al. [86] and C.R. Bhattacharjee et al. [87] the reduction reaction ofPd(acac)2 is believed to be the eq. (3. 1) :2𝑃𝑑(𝑎𝑐𝑎𝑐)2 + 2𝐻2 𝑃𝑂2− + 2𝐻2= 2𝑃𝑑 + 2𝐻 (𝑎𝑐𝑎𝑐 ) + 2𝐻𝑃𝑂32− + 𝐻2 + 2𝐻+(3. 1)

After the reduction, the catalyst–contained silk surface became active and allowed
initiation of the Ni–P metallization [88]. No Pd(acac)2 was found in Figure 3- 4(c)indicating that Pd(acac)2 was fully reduced to Pd metallic particles. Four diffraction peaks,
locate at 2θ = 40.1°, 46.6°, 68.1°, and 82.1° can be indexed to (111), (200), (220), and(311) planes of the FCC structure for palladium (JCPDS #89–4897). In Figure 3- 4(d), a
broad peak locates at 2θ = 20° can be indexed to the silk substrate, while the other broad
peak shows around 40–50° representing the amorphous Ni–P metallized layer. No Pd orPd(acac)2 characteristic peaks can be observed after the metallization indicating that thesmall amount of Pd metallic particles were thoroughly covered by the deposited Ni–P. No
impurities were found in the detection limitation.

Figure 3- 4 X–ray diffraction patterns of (a) non–catalyzed pure silk, (b) Pd(acac)2
catalyzed silk, (c) catalyzed/reduced silk, and (d) Ni–P metallized silk, respectively
17

Chapter 3

Morphologies of the metallized silks with various metallization times are shownin Figure 3- 5. Figure 3- 5(a) and (b) show the OM and SEM images of the Ni–P
metallized silk deposited for 30 sec and 1 min, respectively. As shown in Figure 3- 5(a),small amounts of the catalysts were still un–reduced (circled) and part of the silk was still
un–metallized showing the transparent color after 30 sec of the metallization time. No
SEM image is shown here due to the poor electrical conductivity of the specimen.
Similarly, in Figure 3- 5(b), the bright color in the inserted SEM image indicates silk
surfaces are still not fully covered by the Ni–P layer (pointed by arrows), and the Ni–P
layer is still not continuous resulting a high electrical resistance (results shown in the 3.4
Electrical property and adhesive test of silk/Ni–P section). Results obtained in this study
are classified into three stages according to the surface morphology and electricalconductivity. The specimens with non–continuous Ni–P are classified into the first stage,
which are specimens metallized at 30 sec and 1 min. On the other hand, in Figure 3- 5(c),Ni–P layer shows a continuous and a complete coverage at 3 min of the metallization time.
The least time needed to construct full coverage is defined as the minimum time. Silk
substrate metallized for 4 min is shown in Figure 3- 5(d) demonstrates the full coverage
and a consecutive growth of the Ni–P thickness. Silks metallized for 3 and 4 min of themetallization time both having a full coverage and smooth surface are classified into thesecond stage. Beyond 5 min of the Ni–P metallization time, rough surface were formed,
which resulted deteriorated electrically conductive properties (discussed in the 3.4
Electrical property and adhesive test of silk/Ni–P section). Meanwhile, redundant Ni–P
clusters (pointed by arrows) accumulated on the surface with an increase in the Ni–P
metallization time (Figure 3- 5(e) and (f)). The specimens with the metallization time
longer than 5 min are classified into the third stage.
18

Chapter 3

Figure 3- 5 OM images of the Ni–P metallized silk composites prepared by (a) 30 sec,(b) 1 min, (c) 2 min, (d) 4 min, (e) 5 min, and (f) 10 min of the metallization time
Elemental mapping results from the 4 min Ni–P metallization time are presented
in Figure 3- 6. Ni and P were homogeneously metallized on the silk substrate. Allcompositions of Ni–P layer in various metallization times were revealed in Table 3- 1.
Although carbon signal was found in the elemental mapping (Figure 3- 6). It is consideredthat the carbon signal was from silk substrate and contamination on the surface of thecomposite materials. Compositions in Table 3- 1 was not considered, only Ni and P
concentration are shown.
19

Chapter 3

Figure 3- 6 Elemental mapping of the silk/Ni–P composite prepared by 2 min of the Ni–
P metallization time at 70oC and 1 atm with agitation, (a) SEM image, (b) nickel, (c)phosphorus, (d) carbon
Table 3- 1 Ni–P compositions in various metallization times
Comp.

1 min

2 min

3 min

4 min

5 min

10 min20 minNi (at.%)

83.8

83.8

83.9

82.2

82.1

82.5

82.4

P (at.%)

16.2

16.2

16.1

17.8

17.9

17.5

17.6

The metallization time dependence of the Ni–P layer thickness is presented in
Figure 3- 7. Positive correlation in the metallization time and the thickness isdemonstrated. Thickness of the Ni–P layer with the longest metallization time (20 min)
was 0.7 μm. A decelerated growth rate can be observed when the metallization time islonger than 5 min, which can be attributed to the lowered Ni ion and reducing agent
concentrations in the bulk metallization solution. Ni ions and reducing agent in thesolution were consumed rapidly in the metallization step. At the beginning of themetallization, the concentrations were high and gave a Ni–P layer growth rate at
20

Chapter 3

approximately 0.08 μm/min in the initial 5 min of the metallization time. On the other
hand, the Ni ion and the reductant concentrations continued to drop and caused the growthrate to slow down to approximately 0.005 μm/min after 5 min of the metallization time.

Figure 3- 7 The Ni–P metallization layer thickness metallized on silk substrate as afunction of the metallization time
3.4 Electrical property and adhesive test of silk/Ni–P
Figure 3- 8 illustrates the electrical resistance as a function of the Ni–P
metallization time before and after the adhesive tests. The silk/Ni–P composites showed
high electrical resistance since the electrically conductive Ni–P metal layer was
discontinuous when the Ni–P metallization time was less than 2 min, which is referred as
the first stage (Figure 3- 5(a) and (b)).

21

Chapter 3

The lowest electrical resistance at 1.02 Ω was achieved as the Ni–P metallization
time was extended to 4 min. The metallization to reach the lowest electrical conductivity
is defined as optimized time. This result is in line with the coverage shown in Figure 3- 5
indicating that the coverage is one of the critical factors affecting the electrical propertyof the Ni–P composite materials. Time duration between the beginning of the fullcoverage and the time required to reach the minimum resistance is corresponded as thesecond stage.
Metallization time beyond 5 min is classified into the third stage, which theelectrical resistance is offset by the roughened Ni–P surface. In the third stage (Figure 35(e) and (f)), the growth of rough and loose Ni–P particles on the silk substrate surface
would bring unnecessary Joule heating and more electron scattering, which are the main
reasons for the deteriorated electrical resistance to the Ni–P composite materials [89]. In
addition, a direct evidence indicating the increase of the roughness with respect to themetallization time is revealed in Figure 3- 9. Figure 3- 9(a) shows surface morphology ofthe silk/Ni–P composite with 1 min of the metallization time. The surface metallized by
the Ni–P layer is smooth and charging effect is observed at the region, which was notcovered by the Ni–P layer. On the other hand, Figure 3- 9(b) and (c) demonstrate thesurface roughness increased when the Ni–P metallization time is increased to 2 and 5 min.
A proportional relationship between the electrical resistance and the roughness has been
indicated in a previous study [90].
Standard deviation of the electrical resistance decreases with the metallization
time indicating that the inhomogeneity can be mitigated as the metallization time extends
in the first two stages. However, the standard deviation increases after the second stage
22

Chapter 3

due to the increased surface roughness. A reliable electrical resistance cannot be obtained
owing to the poor conductivity when the metallization time is less than 2 min. Specimen
prepared by the CONV catalyzation and metallization at 4 min is also shown in Figure 38. Specimens fabricated by the CONV catalyzation show higher electrical resistance thanthose catalyzed by sc–CO2 method due to the deteriorated substrate surface. Roughsubstrate surface results in the coarse deposition of Ni–P and finally leads to the highelectrical resistance. Moreover, since the catalysts are only attach to the substrate surface,electrical resistance changed significantly after adhesive tests.

Figure 3- 8 Electrical resistance curves of the Ni–P metallized silk composites before
and after the adhesive test at room temperature and 1 atm (red circle: as–deposited; blue
upward triangle: first adhesive test; and green downward triangle: second adhesive test)(solid symbols: sc–CO2 method; hollow symbols: CONV method)23

Chapter 3

Figure 3- 9 SEM images showing surface condition of the silk/Ni–P composite preparedby (a) 1 min, (b) 2 min, and (c) 5 min of the metallization time
A summary of the influence of the morphology on the electrical resistance indifferent stages is illustrated in Figure 3- 10. In the first stage, the Ni–P layer is non–continuous resulting in poor electrical conductivity. When the metallization time isextended to the second stage, a continuous Ni–P layer is constructed, in other words, thesilk is fully covered. A full coverage, a smooth surface, and a sufficient Ni–P layer
thickness result in the lowest electrical resistance. In the last stage, the electricalconductivity is offset by the increased roughness.
24

Chapter 3

The electrical resistances after the adhesive tests are shown in Figure 3- 8. The
electrical resistances persist in the second and third stages even after second adhesive test.
In addition, there is no distinct change among these three measurements. However, theadhesive tests show serious impact on the samples in the first stage. Since the joint of thenon–metallized silk surface and the Ni–P layer can be the weak points of the composite
materials, thus, it was destructed more severely than the Ni–P fully covered samples. The
silk/Ni–P composite metallized with the optimized time shows perseverance in theadhesive tests indicating its feasibility in the practice of wearable devices even underadverse conditions.

Figure 3- 10 A summary of morphology and electrical resistance of the Ni–P metallized
silk as well as the correlations between each properties

25

Chapter 3

3.5 Corrosion resistance of silk/Ni–P
Corrosion behaviors of the silk/Ni–P composites in 3.5 wt.% NaCl are shown in
Figure 3- 11. Figure 3- 11(a) shows the Tafel plots of the as–deposited silk/Ni–P
composite and the silk/Ni–P composite after the adhesive tests. The silk/Ni–P composite
prepared by 4 min of the metallization time was chosen for the corrosion resistanceevaluations. No significant difference was found among the three curves indicating
excellent reliability of the Ni–P layer on the silk surface even after second adhesive test.
The corrosion potential (Ecorr) and the corrosion current density (Icorr) are summarized inTable 3- 2. The corrosion current densities are comparable to those of Ni–P layer on metal
substrate reported in other studies [91-94], which indicate the high corrosion resistanceof the Ni–P layer retained even when it is metallized on a non–metallic substrate.
Moreover, Figure 3- 11(b) shows the morphology after second adhesive tests and thecorrosion test, no obvious corrosion attack can be observed after the testing. The low
corrosion rate in a solution simulating the human sweat before and after the adhesive test
demonstrates the silk/Ni–P is practical in wearable devices.

Figure 3- 11 (a) Tafel plots of the silk/Ni–P composite prepared with 4 min of themetallization time before and after the adhesive tests and (b) an OM image of thesilk/Ni–P composite after second adhesive test and the corrosion test
26

Chapter 3

Table 3- 2 Corrosion potentials and corrosion current densities of the silk/Ni–P
composite materials
Samples

Ecorr (V)

Icorr (A/cm2)(a) As–deposited-0.32

5.89 x 10-7(b) First adhesive test

-0.33

6.00 x 10-7(c) Second adhesive test

-0.32

5.93 x 10-7

3.6 Brief summary for silk/Ni–P
Ni–P metallized silk with a high electrical conductivity and a high corrosion
resistance in a solution simulating the human sweat was successfully fabricated by the
sc–CO2–assisted electroless plating in this study. Reliability evaluations were conductedby the adhesive test to demonstrate the value in practical applications. In the catalyzation
step, the silk substrate was successfully catalyzed by Pd(acac) 2 catalyst without any
defects owning to usage of the sc–CO2. The coverage and the thickness of the Ni–P layer
increased as a function of the metallization time. The maximum thickness at 0.7 μm wasattained by 20 min of the metallization time. The deposition statuses are classified into
three stages. The composite material remains high electrical resistance in the first stage,
which can be attributed to the non–continuous Ni–P layer on the silk surface. On the other
hand, the lowest electrical resistance was achieved in the second stage owning to the fullcoverage and the smooth surface. Finally, the surface roughness increase in the third stage,
which lead to a deteriorated electrical conductivity. The lowest electrical resistance of
1.02 Ω was reached by 4 min of the metallization time. The Tafel plots show the silk/Ni–
P composite material possesses an excellent corrosion resistance and the adhesive tests
demonstrate the firmly adhesive property of the Ni–P layer on the silk.

27

Chapter 4

Chapter 4 Pt metallized silk textile via sc–CO2
promoted electroless plating to improve thebiocompatibility of the composite
4.1 Introduction
There are always dilemma between cost and quality. For example, Ni–P owns theadvantages of low cost and simple operation procedures, however, it might cause allergy
reactions to some of the users under some specific circumstances. On the other hand,
platinum owns high biocompatibility but it shows high cost and complicated procedures.
Since part of the wearable devices have contact to human body or are implanted into
human body directly, it is important to make the wearable devices biocompatibility. Pt is
widely considered as a biocompatible material. Along with the combination of silk, whichis also a biocompatible materials, the fully biocompatible composite materials can berealized in this study. Thus the metallization material was shifted to Pt catalyzation
followed by Pt metallization in this section to fulfill the fully biocompatible composite
materials for the applications of wearable devices.
4.2 Activation of silk textile via sc–CO2 Pt(acac)2 catalyzation
Figure 4- 1(a) and (b) show the uncatalyzed silk and the silk textile after thecatalyzation, respectively. The light yellow flakes correspond to the Pt(acac) 2 catalyst,
and the transparent thread bundles represent the silk. By means of conducting thecatalyzation with sc–CO2, the catalysts can be distributed into the silk textile without
damaging the silk structure. Although some of the catalysts aggregate in a small range,
28

Chapter 4

distribution of the catalysts in the substrate is nearly homogeneous.

Figure 4- 1 OM images of (a) the non–catalyzed pure silk and (b) the catalyzed silk
4.3 Morphology, composition and structure of silk/Pt
Figure 4- 2 demonstrates X–ray diffraction patterns of the (a) pure silk, (b)catalyzed silk, (c) silk with reduced catalyst, and (d) metallized silk, respectively. The
diffraction pattern of the pure silk in Figure 4- 2(a) shows the amorphous structure. On
the other hand, all the diffraction peaks in Figure 4- 2(b) correspond well to the Pt(acac)2.
In Figure 4- 2(c) and (d), five diffraction peaks located at 2θ = 40.2o, 46.7o, 67.9o, 81.7o,29Chapter 4

and 86.1o can be indexed to (111), (200), (220), (311), and (222) planes of the FCC
structure for Pt (JCPDS #87–0647). A broad peak located at around 2θ = 20o in Figure 42(c) is the contribution from the silk textile. No characteristic peak of unreduced catalyst
or other impurity phases were observed within the detection limit after the Pt metallization.
The coverage tendency at 120, 160, and 200 min of the Pt metallization time
durations is shown in Figure 4- 3. The bright phase corresponds to the silk, and the grayphase indicates the Pt metallization layer. In Figure 4- 3(a), the Pt metallization layer isstill non–continuous, which is referred as the first stage. While the metallization time isextended to 160 min in Figure 4- 3(b), the silk textile is fully metallized. Similarmorphology can be observed when the metallization time is extended to 140 min (notshown here). The silk/Pt composites with full coverage and smooth metallized surface areclassified into the second stage.
When the metallization times are extended to 180 and 200 min (third stage),complete coverage can be constructed along with Pt cluster accumulations (indicated byblack arrows) and rough surface (Figure 4- 3(c)). Figure 4- 3 shows that coverage of the
Pt metallized layer increases as a function of the metallization time length. However,rough morphology and undesired Pt clusters deposited on the surface are observed asshown in Figure 4- 3(c), pointed by the black arrows. The roughened surface would
inevitably deteriorate the electrical property (discussed below) in the third stage. On theother hand, the composition analysis results verified the as–deposited Pt layer to be purePt with trace impurity, which is in good agreement with the results from the X–ray
diffraction pattern. Electrical conductivity of the Pt metallized silk is directly related tothe coverage, which will be discussed in the 4.4 Electrical property of silk/Pt section.
30

Chapter 4

Figure 4- 2 X–ray diffraction patterns of (a) the non–catalyzed pure silk, (b) the
catalyzed silk, (c) the silk with reduced catalyst, and (d) the Pt metallized silk

Figure 4- 3 Pt metallization trend shown by SEM images at (a) 120 min, (b) 160 min,and (c) 200 min of the Pt metallization time at 70 oC and 1 atm, respectively (blackarrows in (c): Pt clusters and accumulations)
In the same way, the metallized Pt thickness grows with the metallization time
length. The dependence between the metallization time duration and the thickness ispresented in Figure 4- 4. The maximum Pt thickness deposited with the longest
metallization time at 200 min is 0.91 μm. The standard deviation decreases with themetallization time in the first two stages suggesting better deposit homogeneity. On thecontrary, the standard deviation increases as a function of the metallization time in the31Chapter 4

third stage indicating that the homogeneity decreases as the metallization time prolongs.
This can be attributed to the increased surface roughness. Reliable thickness cannot bemeasured when the metallization time was less than 140 min due to poor conductivity.

Figure 4- 4 Pt metallization layer thickness as a function of the metallization time
4.4 Electrical property of silk/Pt
The morphology (Figure 4- 3) also reflects on the electrical property. Figure 4- 5
demonstrates the electrical resistance as a function of the metallization time period. The
lowest electrical resistance at 86 mΩ is obtained as the metallization time is extended to
160 min with 100% Pt coverage and smooth surface in the second stage. This result is inline with the coverage shown in Figure 4- 3 indicating that the coverage acts as a criticalfactor to the electrical property. On the contrary, the silk/Pt composite still remains highelectrical resistance when the metallization time is less than 140 min (Figure 4- 3(a)) since
the Pt layer is discontinuous in the first stage. The electrical resistances rise up in the third
32

Chapter 4stage at 180 and 200 min of the metallization time durations, which can be attributed to
the rough surface. As shown in Figure 4- 3(c), the high surface roughness could bringunnecessary Joule heating and electron scattering, which are the main reasons for thedeteriorated electrical resistance [89]. Therefore, 160 min is the optimized metallization
time for this full Pt–contained electroless plating process.
Similarly, the standard deviation decreases with the Pt metallization time durationin the first two stages indicating that the inhomogeneity can be mitigated as the Pt
metallization time extends. On the contrary, the standard deviation increases in the third
stage due to the high surface roughness. Reliable electrical resistance cannot be obtained
from the samples with the metallization time less than 120 min owing to the poor
electrical conductivity of the silk/Pt composite materials.

Figure 4- 5 Electrical resistance of the silk/Pt composite as a function of themetallization time at room temperature
33

Chapter 4

A summary of the relationship between the morphologies of silk/Pt composite
materials and their electrical resistance in different metallization stages is shown in Figure
4- 6. In the first stage, parts of the silk textile are still non–metallized resulting in a highelectrical resistance. When the metallization time is extended to the second stage, the Pt
metallization layer becomes continuous along with smooth Pt metallization surface
leading to the lowest electrical resistance. However, in the third stage, when the Pt
metallization time is beyond the optimized time, the electrical resistance is deteriorated
due to the rough Pt metallization surface.

Figure 4- 6 A summary of morphology and electrical resistance of the Pt metallized silkin three different stages as well as the correlations between each properties (i.e.
coverage and electrical resistance)
34

Chapter 4

4.5 Biocompatibility assessments of silk/Pt
One of the key factors to trigger the allergy reaction is the ion released from themetal, and the Pt ion released from the silk/Pt composite would be the concern in thiscase. Therefore, examining the ion releasing rate in the SBF is a direct method to evaluate
the biocompatibility of the silk/Pt composite. The Pt ion concentration in human body
varies with race, environment, and diet. According to several literature based on different
living conditions, the average Pt input from diet is 1.44 μg/day, and the Pt output throughthe faeces can be up to 1.79 μg/day [95-98]. Regarding evaluation of the ion releasingrate of the silk/Pt composite prepared in this study, only 2.73 μg of Pt was found in the
SBF after 3 months of immersion at 37 oC, which is the human body temperature. The
average amount of Pt released from the silk/Pt composite per day would be around 0.03
μg, which is negligible when compared with the daily Pt input. Moreover, based on thedaily Pt output, no Pt would accumulate in human body. This result provides a direct
evidence that the silk/Pt composite material fabricated in the study is feasible forapplications in implantable medical and wearable devices.
4.6 Brief summary for silk/Pt
The applicable Pt metallized silk textile was successfully fabricated via sc–CO2–
assisted electroless plating. Sc–CO2 was introduced to enhance the electroless plating
characteristics. This was the first study that can practice the full Pt–contained electrolessplating, in other words, Pt catalyzation followed by Pt metallization. The Pt metallized
silk is thus feasible to the medical and wearable devices due to the high biocompatible
combination of Pt and silk. The silk textile structure persists after the catalyzation. The
coverage and thickness of the Pt metallized layer increase as a function of themetallization time. The maximum thickness at 0.91 μm can be attained after metallization
35

Chapter 4

for 200 minute. The metallization states can be classified into three stages according to
the coverage, the morphology, and the electrical resistance. The composite material with
non–continuous Pt coverage possesses high electrical resistance in the first stage. On theother hand, as the metallization time is extended to 160 min, a continuous metallized layerand smooth surface can be formed, which is within the second stage and leads to thelowest electrical resistance at 86 mΩ. Standard deviation of the electrical resistancedecreases with the metallization time revealing the homogeneity increases with themetallization time length in the first two stages. The electrical resistance and the standard
deviation increase again in the third stage because of the rough surface. The Pt ion
releasing rate can be neglected when comparing with the daily input and output of Pt
amount in human body indicating the feasibility for the applications in medical andwearable devices.

36

Chapter 5

Chapter 5 Efficiency enhancement of platinum
metallization on silk textile substrate by Pd(acac)2
sc-CO2 catalyzation
5.1 Introduction
To improve the efficiency of Pt metallization process and keep the composite
materials biocompatible, catalyzation process was changed to Pd(acac) 2 catalyst insteadof Pt(acac)2 (4.2 Activation of silk textile via sc–CO2 Pt(acac)2 catalyzation section).
Moreover, the process working with Pd(acac) 2 also show lower cost while compared to
the Pt(acac)2 one. Similar biocompatibility assessments were done in this section toevaluate the biocompatibility of the composite materials. Mechanisms of the influencesfrom the Pd(acac)2 catalysts in the metallization step were also discussed in the following
sections.
5.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation
Figure 5- 1 shows OM images of the non–treated pure silk, catalyzed silk, andcatalyzed silk with reduction treatment. Transparent thread bundles shown in Figure 51(a) indicate the silk textile, light yellow flakes shown in Figure 5- 1(b) correspond toPd(acac)2 catalysts, and the small particles with metallic luster shown in Figure 5- 1(c)are the reduced Pd metallic particles. As shown in Figure 5- 1(b), the Pd(acac)2 catalysts
are successfully settled on the substrate without damaging the structure of silk substrate.
After the reduction, small Pd metallic particles finely remained on the substrate shown in
Figure 5- 1(c).
37Chapter 5

Figure 5- 1 OM images of (a) non–treated pure silk, (b) sc–CO2–assisted Pd(acac)2
catalyzed silk, and (c) catalyzed silk with reduction treatment
X–ray diffraction patterns of the (a) pure silk, (b) catalyzed silk, and (c) catalyzed
silk with reduction treatment are shown in Figure 5- 2. The pure silk shows one broad
peak at around 2θ = 20o in Figure 5- 2(a). On the other hand, after the catalyzation, all the
X–ray diffraction characteristic peaks can be indexed to the contribution from Pd(acac) 2[82, 83], shown in Figure 5- 2(b). Figure 5- 2(c) shows X–ray diffraction pattern of the
reduced catalyst on silk. Four diffraction peaks at 2θ = 40.1o, 46.7o, 68.1o, and 82.1o canbe indexed to (111), (200), (220), and (311) planes of the FCC structure of palladium(JCPDS #89–4897).

Figure 5- 2 X–ray diffraction patterns of (a) non–treated pure silk, (b) silk with sc–CO2
catalyzation treatment, and (c) catalyzed silk with the reduction treatment
38

Chapter 5

The broad peak observed at 2θ = 20o in the X–ray diffraction pattern indicatesthe amorphous structure of silk textile as shown in Figure 5- 2. With the introduction of
sc–CO2, its nonpolar and low viscosity bring the catalysts into the substrate without
damaging the substrate structure. The OM image (Figure 3- 1(c)) along with thecharacteristic peaks in Figure 5- 2(b) reveal that the catalyst was successfully settled on
the substrate. Due to the high intensity from the Pd(acac) 2 catalysts, the broad peak fromsilk was suppressed and was not observed in Figure 5- 2(b). According to H. Vrubel et al.
[99] and D. A. P. Tanaka et al. [100], the reduction reaction of Pd(acac)2 is believed to be
the reactions listed in the following equations:Pd(𝐶5 𝐻7 𝑂2 )2 + 2𝑁2 𝐻4 = 𝑃𝑑 2+ + 2𝐶5 𝐻8 𝑁2 + 2𝑂𝐻− + 2𝐻2 𝑂(5. 1)

where Pd(C5H7O2)2 = Pd(acac)2
𝑃𝑑 2+ + 𝑁2 𝐻4 + 4𝑂𝐻 − = 2𝑃𝑑 + 𝑁2 + 4𝐻2 𝑂(5. 2)The broad peak at around 2θ = 20o in Figure 5- 2(c) is attributed to the amorphoussilk substrate and the sharp peaks indicate successful reduction of the Pd(acac) 2 to form
Pd metal. Results of Figure 5- 2(a–c) are in good agreement with Figure 3- 1(a) and (c).
The reduced catalysts can be used as the active sites for the deposition of Pt metals in thefollowing metallization step.
5.3 Morphology, composition and structure of silk/Pt
Three SEM images shown in Figure 5- 3 demonstrate a trend of coverage of the
Pt layer on the silk with various metallization times. Figure 5- 3(a) shows a SEM imageof the sample with 80 min of the metallization time, where the bright phase corresponds
to non–metallized silk while the gray phase indicates Pt metal. Since the electron cannot
be conveyed on the non–conductive silk, thus the electrons accumulate on the non–39

Chapter 5

metallized silk would show the charging effect. At 80 min of the metallization time, someparts of silk textile were non–metallized (pointed by arrows) showing a non–continuous
Pt layer and a smooth surface on the metallized part. Figure 5- 3(b) shows morphologyof the sample with 120 min of the metallization time, a continuous Pt metallized layer
was constructed showing gray color thoroughly and the surface remained smooth. Whenthe metallization time was extended to 180 min (Figure 5- 3(c)), the silk substrate wasalso completely covered. However, undesired Pt clusters accumulated on the surface, andthe metallized surface became rough (pointed by arrows). Similar phenomenon wasobserved in previous sections.

Figure 5- 3 Pt metallization trend demonstrated by SEM images at (a) 80 min, (b) 120
min, and (c) 180 min of the Pt metallization times
40Chapter 5

In the same way, elemental mapping of the Pt metallized silk (Figure 5- 4) shows
the critical time demarcation of full coverage, which lies between (a) 80 and (b) 100 minof the metallization time. The least time needed to construct full coverage is defined as
the minimum time, which is 100 min. The composition results show pure Pt layer withmerely trace amount of impurity.
Figure 5- 5 shows the X–ray diffraction pattern after the Pt metallization. Fivediffraction peaks located at 2θ = 40.2o, 46.8o, 68.4o, 82.4o, and 86.9o can be indexed to(111), (200), (220), (311), and (222) planes of the FCC structure of platinum (JCPDS#87–0647).

Figure 5- 4 Elemental mapping of Pt metallized silks at (a) 80 min and (b) 100 min of
the Pt metallization time (set at upper–right figures: carbon signals; set at bottom–rightfigures: carbon signals)
41

Chapter 5

Figure 5- 5 X–ray diffraction pattern of Pt metallized silk substrate
The metallization time dependence of the Pt layer thickness is presented in
Figure 5- 6. A positive correlation between the metallization time and the thickness isshown. A reliable thickness value could not be measured for the samples with ametallization time less than the minimum time at 100 min due to the poor electricalconductivity. Pt layer thickness at 0.90 μm was obtained with the longest metallization
time at 180 min, which the calculated average growth rate is 1.8 nm/min.

Figure 5- 6 Pt layer thickness on silk substrate as a function of the metallization time42

Chapter 5

5.4 Electrical property of silk/Pt
Figure 5- 7 illustrates the electrical resistance as a function of the metallization
time before and after the adhesion test. Again, a reliable electrical resistance cannot be
obtained owing to the poor electrical conductivity when the Pt metallization time is lessthan 80 min. The electrical resistance was high in the early stage of the Pt metallizationand then decreased with an increasing in the Pt metallization time. After reaching aminimum point at 120 min, the electrical resistance gradually increased and stabilized.
The electrical resistances of the silk/Pt composite materials after the adhesion test
are shown in Figure 5- 7. The adhesion tests showed serious impact on the samples in theearly stage. On the contrary, the electrical resistances of the composites persist after thesecond adhesion test in the last two stages. In addition, there is no distinct difference
between the first and the second adhesion test. The stages can be classified into threeaccording to the morphology and the electrical resistance, which will be discussed in thefollowing section.

Figure 5- 7 Electrical resistance curves as a function of metallization time of the silk/Pt
composites before and after the adhesion tests
43

Chapter 5

Based on the coverage, surface roughness, and electrical resistance, the Pt
metallization of silk substrates are classified into three stages. The partially metallized
silk with a discontinuous Pt layer at 80 min of the metallization is classified to the firststage (Figure 5- 3(a)). The partly metallized surface showed smooth morphology since
the metallization is still in the early stage. In this stage, the electrical resistance remained
high due to the discontinuous Pt layer. At 120 min of the metallization time (Figure 53(b)), the silk substrate was fully covered with Pt, which is classified into the second stage.
Electrical resistance was lowered since continuous Pt layer was constructed. The
metallized surface was smooth, and there was not much redundant clusters on the Pt layer.
Hence, the lowest electrical resistance at 55 mΩ was built up in the second stage at the
optimal metallization time of 120 min. At 180 min of the metallization time (Figure 53(c)), the silk was still fully covered; however, undesired Pt clusters and the roughmetallization resulted in the elevated electrical resistance. The rough Pt deposition on thesubstrate surface increased unnecessary Joule heating and caused formation of a relative
loose structure. SEM images revealing an increase in the roughness as the metallization
time increased are shown in Figure 5- 8. A proportional relationship between the electricalresistance and the roughness is indicated in the literature [90]. Therefore, coverage of the
Pt layer and the surface roughness both have significant influence on the electricalresistance.
The influence of morphology on the electrical resistance in different stages is
illustrated in Figure 5- 9. In the first stage, the silk was not fully metallized showing a
non–continuous Pt layer. It thus gave the highest electrical resistance. When themetallization time was prolonged to the second stage, full coverage was constructedshowing a continuous Pt layer. Along with the smooth surface, the electrical resistance44

Chapter 5

reached its minimum point at 120 min in the second stage. However, rough surface
emerged as the metallization time extended; the electrical conductivity was thus
deteriorated in the third stage.

Figure 5- 8 SEM images showing surface conditions of the silk/Pt composites preparedby (a) 80 min, (b) 120 min, and (c) 180 min of the metallization time (metallization
temperature: 70 oC; metallization pressure: 1 atm; with agitation)
45

Chapter 5

In the electrical resistance test, the electrical resistance is expected to be elevated
if the adhesion test successfully introduces defects into the Pt layer due to an increase insurface roughness of the Pt layer. The results shown in Figure 5- 7 indicate great impacts
in the first stage, on the contrary, the electrical resistance persists in the final two stages.
In the first stage, the Pt layer is not continuous and the edge of the Pt layer exposes to thetape during the adhesion tests. The Pt edge would be peeled off more easily than thesurface that is fully metallized since it is free from support in the edge. The silk metallizedwith the optimized time shows perseverance in the second adhesion test indicating its
feasibility in the practice of applications even when under adverse conditions.

Figure 5- 9 The relationship between Pt metallization morphology and electricalresistance as well as the cross–section morphologies in three different stages
46

Chapter 5

5.5 Corrosion resistance of silk/Pt
Figure 5- 10(a) shows results of the polarization measurements of the silk/Pt
composite materials before and after the adhesion test. The silk substrate with 120 min of
the metallization time was chosen for the corrosion resistance evaluation. No significant
difference was found among the three polarization curves indicating excellent corrosion
resistance of the Pt layer even after the second adhesion test. The Ecorr and Icorr are
summarized in Table 5- 1(a). Moreover, Figure 5- 10(b) shows the morphology after thesecond adhesion test and the polarization measurement. No obvious corrosion attack wasfound after the adhesion test and the polarization test in 3.5 wt.% NaCl solution. For
simulating the condition in the human body, similar polarization measurements werecarried out in the SBF and the results are shown in the following.

Figure 5- 10 (a) Polarization measurements of the silk/Pt composites before and after
the adhesion tests in 3.5 wt.% NaCl solution at room temperature and 1 atm withoutagitation and (b) OM image of the silk/Pt composite after second adhesive test andpolarization measurement
Figure 5- 11(a) shows the polarization curves of the silk–Pt composite materials
before and after the adhesion test. In the same way, metallization time at 120 min was
47

Chapter 5

chosen for its best suitability to this application. The corrosion resistance also persisted
after the second adhesion test, no significant difference within these three polarization
curves was observed. The Ecorr and Icorr are also summarized in Table 5- 1(b). The
morphology after the second adhesion test and polarization measurement is shown in
Figure 5- 11(b). Similarly, no obvious corrosion attack was inspected after adhesion test
and the corrosion test in the SBF solution.

Figure 5- 11 (a) Polarization measurements of the silk/Pt composites before and after
the adhesion tests in SBF solution at room temperature and 1 atm without agitation and(b) OM image of the silk/Pt composite after second adhesive test and polarization
measurement
In 3.5 wt.% NaCl solution, high corrosion resistance of the silk/Pt is shown in
the polarization measurements in Figure 5- 10(a), and limited corrosion attacks on thesample surface are revealed in Figure 5- 10(b). On the other hand, the corrosion current
densities of the composite materials in the SBF are all lower than those in 3.5 wt. % NaClsolution since 3.5 wt. % NaCl solution contains more Cl-, and Cl- has significant impact
on corrosion resistance of metals [95]. The corrosion potential of silk/Pt composite in the
SBF is also more positive than those in 3.5 wt. % NaCl solution indicating that the Pt
48

Chapter 5

layer is more inert in the SBF solution. Despite the fact that thin Pt layer was electrolessplated on the non–conductive substrates, the corrosion rates are comparable to those of
Pt metal, which indicates the well deposition of Pt layer on the silk. Overall, the silk/Pt
composites show high corrosion resistances in the two solutions. These results suggestthat the silk/Pt composite is not merely practical in wearable devices but also in
implantable devices.
Table 5- 1 Corrosion potentials and corrosion currents of the silk/Pt composite materialsin (a) 3.5 wt.% NaCl and (b) SBF solutions measuring at room temperature and 1 atm(a) Samples (in 3.5 wt.% NaCl)

Ecorr (V)

Icorr (A/cm2)

As–deposited-0.0903.58 x 10-7

First adhesive test

-0.0923.50 x 10-7

Second adhesive test

-0.093

3.56 x 10-7(b) Samples (in SBF)

Ecorr (V)

Icorr (A/cm2)

As–deposited

0.015

2.16 x 10-7

First adhesive test

0.018

2.02 x 10-7

Second adhesive test

0.019

2.02 x 10-7

The levels of damage severity from the adhesion test were evaluated by both
corrosion and electrical resistance measurements. Generally, defects and cracks would beformed after the adhesion test. Then the corrosion test solution can penetrate into thedefects and cracks to give an increase in the current during the polarization measurement.
Since the surface area is assumed to be constant, the current density is thus expected to
be increased after formation of the defects and cracks. However, no significant differencein the corrosion current density was found after the adhesion test indicating that the
samples stay intact after the adhesion test and hence shows the high adhesive property of
the Pt layer on the silk.
49

Chapter 5

5.6 Biocompatibility assessment of silk/Pt by immersion test
Releasing of the metal ions is an essential factor to provoke the allergy reactions[101], therefore, investigating released ion concentration in the SBF is a direct way toevaluate biocompatibility of the silk/Pt composite. After three months of the immersion
at human body temperature (37 oC) in the SBF, only 0.011 μg/day and 0.018 μg/day of
Pd and Pt were observed, respectively.
The metal ion concentration in human body varies with races, environments, anddiets. According to several literature based on different living conditions, average Pt input
from diet is 1.44 μg/day, and Pt output through the faeces can be up to 1.79 μg/day [9598]. On the other hand, the average Pd intake is 2 μg/day, and the Pd output can be up to
2.5 μg/day [102, 103]. From the immersion test, only 0.018 μg Pt/day and 0.011 μg Pd/day
were observed in the SBF solution. Both values are far less than the intake and have nochance to accumulate in the human body according to the literature and the ICP results.
In addition, the Pt ion releasing rate of the silk/Pt composite prepared in this study islower than those catalyzed by the Pt(acac) 2 (4.5 Biocompatibility assessments of silk/Pt
section). Since the samples catalyzed by Pd(acac) 2 shows relative smooth deposition andless defects (Figure 5- 12), it is thus corroded mildly in the immersion tests while
comparing to those catalyzed by Pt(acac)2.
5.7 Enhancement of the metallization efficiency
The electroless plating system catalyzed by Pd(acac) 2 demonstrates a more
efficient process and improved material properties than those catalyzed by Pt(acac) 2 (4.2
Activation of silk textile via sc–CO2 Pt(acac)2 catalyzation section). In this study, theoptimal metallization time needed to reach the lowest electrical resistance is at 120 min,50

Chapter 5

which shows 25% reduction in the time when comparing to the study using Pt(acac) 2 as
the catalyst. The improved efficiency is suggested to be attributed to the high reduction
efficiency of Pd(acac)2 to Pd metal. According to the literature [104], Pd(acac)2 owns alower decomposition temperature than that of Pt(acac) 2, it thus can be decomposed or
reduced to metal state more efficiently. After formation of the activation sites and gradualdeposition of the Pt layer on the silk surface, the catalysts would be fully covered by thePt and have no significant influence on the succeeding Pt deposition rate.
It is worth to mention that the silk/Pt composite materials fabricated in this study
showed electrical resistances 36% lower than those in 4.4 Electrical property of silk/Pt
section. When comparing with reduction of Pd(acac)2 to Pd metals, reduction of Pt(acac)2
to Pt metals is less efficient and takes longer time. The non–synchronize Pt(acac)2
reduction leads to uneven growth of the Pt activation sites and the Pt metallization later
on. The irregular sizes of Pt activation sites make the succeeding Pt deposition to growroughly and thus shows the high electrical resistance. The decomposition temperature ofPt(acac)2 is higher than those of Pd(acac)2 [104], which is an evident showing that thebonding between Pt and (C5H7O2)2 is stronger than those of Pd and (C5H7O2)2. It takes
time for Pt(acac)2 to be reduced to Pt metallic particles and therefore it shows non–synchronize reduction. The relationship between the metallization time and the Pt layerroughness is shown in Figure 5- 12. Rough Pt cross–section shown in Figure 5- 12(a) was
catalyzed by Pt(acac)2; while, the smooth one in Figure 5- 12(b) was catalyzed byPd(acac)2. By using Pd(acac)2, nucleation and growth of the activation site would be
uniform and lead to even growth of the Pt layer. Hence, the electrical resistance could belowered.

51

Chapter 5

Figure 5- 12 Cross–sections of (a) Pt catalyzation followed by Pt metallization, (b) Pd
catalyzation followed by Pt metallization, and (c) illustration of the influence ofdifferent catalysts on the smoothness of Pt layer
5.8 Brief summary for silk/Pt
The Pt metallized silk was successfully fabricated from the electroless platingpromoted by sc–CO2 in this study. In the catalyzation step, Pd(acac) 2 catalyst was
successfully settled on the silk substrate without damaging the structure due to theintroduction of sc–CO2. The coverage and thickness of the Pt layer increased as a functionof the metallization time. Silk textile was fully covered at 100 min of the metallization
52

Chapter 5

time, which is the minimum time to achieve full coverage. A maximum thickness of 0.90μm was obtained at 180 min of the metallization time, which the average growth rate is
1.8 nm/min. The Pt metallization is classified into three stages. In the first stage, non–continuous Pt layer on the silk remained high electrical resistance. On the other hand, thelowest electrical resistance and full coverage were achieved in the second stage. In theend, rough surface was deposited in the third stage resulting in deteriorated electricalconductivity. Pt metallized silk showed high corrosion resistance in both 3.5wt. % NaCland the SBF solutions. Average Pd and Pt ion releasing rates in the SBF were 0.011 μg/day
and 0.018 μg/day, respectively, which are negligible when comparing with the daily input
and output of Pd and Pt of human body. The lowest electrical resistance at 55 mΩ wasreached at the optimal metallization time of 120 min. Strong adhesion of the Pt layer on
the silk was revealed by both corrosion test and electrical resistance measurement after
the adhesion test. The results indicate the feasibility of this material for medical andwearable devices.

53

Chapter 6

Chapter 6 Full sc-CO2 promoted platinum
electroless plating for enhancing the electricalconductivity and reliability of the silk/Pt
composite materials
6.1 Introduction
There are two steps of electroless plating, the first step is catalyzation step to embedthe catalyst into the substrate making it active and the second step is the metallization tometallize the substrate equipping it with electrical conductivity. Sc–CO2 cannot only beapplied to the catalyzation step, but also can be applied to the metallization procedure. In
the previous three sections, sc-CO2 was only introduced into the catalyzation step. On theother hand, in this section, sc–CO2 is introduced into the metallization step to further
enhance the metallization properties. A non-ionic surfactant is introduced into the aqueous
metallization bath in the meanwhile (Figure 6- 1) to emulsify the aqueous electrolyte bathand form sc–CO2/surfactant micelles [105]. With the help of sc–CO2/surfactant micelles,
it can carry the hydrogen bubbles away from the substrate due to its non–polarity andhigh self–diffusivity. The metallization layer turns out to be compact and smooth byelimination of the hydrogen bubbles and the electrical conductivity was also enhanced
accordingly. Therefore, a sc–CO2–assisted method was introduced into the metallization
process in this section to enhance the properties of the metal film and solve the difficulties
in the CONV metallization.

54

Chapter 6

Figure 6- 1 Experimental settings of the sc–CO2 assisted metallization step: (a) A
thermal bath that is controlled by an isothermal furnace, (b) the reaction–cell (PEEK–
lined SUS316L), (c) Pt metallization electrolyte (80 vol.%), (d) sc–CO2 (19.8 vol.%),(e) non–ionic surfactant (0.2 vol.%), (f) silk textile, and (g) cross stirrer, and (h) sc–
CO2/surfactant micelles formed when the solution is stirred in the reaction cell
6.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation
Figure 6- 2 shows the (a) as–received silk and (b) sc–CO2 catalyzation–treatedsilk. Transparent thread bundles and light yellow flakes seen in the OM images indicate
the silk substrate and Pd(acac)2, respectively. With the help of sc–CO2 in the catalyzation
step, the Pd(acac)2 was successfully inlaid into the silk substrate without damaging thesilk structure (Figure 6- 2(b)).

55

Chapter 6

Figure 6- 2 OM images of the (a) as–received and (b) sc–CO2 catalyzed silks
6.3 Morphology, composition, and structure of sc–CO2 metallized silk/Pt
X–ray diffraction patterns of the (a) as–received silk, (b) sc–CO2 catalyzation–
treated silk, and (c) Pt metallized silk are shown in Figure 6- 3. Silk textiles in Figure 63(a) shows one broad peak at around 2θ = 20o indicating the amorphous structure of thesubstrate. After the catalyzation, all the diffraction peaks in Figure 6- 3(b) can be indexed
to Pd(acac)2 (diamond symbol) well suggesting the Pd(acac)2 was successfully deposited.
X–ray diffraction pattern of the Pt metallized silk is demonstrated in Figure 6- 3(c), fivediffraction peaks locating at 2θ = 40.2o, 46.8o, 68.4o, 82.4o, and 84.5o can be indexed to56

Chapter 6(111), (200), (220), (311), and (222) planes of the FCC structure for platinum (circle)(JCPDS #87–0647), respectively. This result indicates that metallic platinum phase wasdeposited with only trace amount of impurity, and no other phases were observed as wellwithin the X–ray detection limitation. Due to the small amount of reduced Pd metal
catalyst, no metallic Pd characteristic peaks were observed after the metallization step.

Figure 6- 3 X–ray diffraction patterns of the (a) as–received, (b) sc–CO2–assisted
Pd(acac)2 catalyzation treated, and (c) Pt metallized silks (blue diamond: Pd(acac)2
catalysts; red circle: Pt)
Figure 6- 4 shows diffraction patterns of the Pt metallized silk by the (a) CONV
method and (b) the sc–CO2–assisted metallization technique. In Figure 6- 4, full width at
half maximums of the diffraction peaks of the Pt layer fabricated by the sc–CO2–assisted
technique were clearly wider than those prepared by the CONV method indicating thatthe sc–CO2–assisted metallization brings a finer grain size.
57

Chapter 6

Figure 6- 4 X–ray diffraction patterns of Pt metallized silks by the (a) CONV at 1 atm,and (b) sc–CO2–assisted metallization methods
Influence of the sc–CO2 on the grain size is shown in Figure 6- 5. In the CONV
metallization, concentration of Pt complex around the electrode surface is similar to theconcentration in the bulk solution and Pt nucleated on the substrate surface in early stageof the metallization as shown in Figure 6- 5(a). As the metallization proceeds (Figure 65(b)), concentration of the Pt complex around the substrate surface gradually decreases,
which favors grain growth instead of nucleation. On the other hand, micelles composedof surfactant and sc–CO2 would repeatedly bounce on the substrate surface to promote
mixing of reactive species in the solution (Figure 6- 5(d)). In this case, transfer or
diffusion of Pt complex from the bulk solution to surface of the substrate is enhanced andleads to a higher Pt complex concentration near the substrate than that in the CONV
metallization as shown in Figure 6- 5(e). When concentration of the reactive species, thePt complex in this case, is high, nucleation is favored instead of grain growth, thus, highnuclei density and fine grain size are obtained Figure 6- 5(c) and (f)). In addition, high58

Chapter 6

nuclei density also brings about the smooth surface due to even growth rate of the Pt
metallization layer on entire surface of the substrate. Since no obvious difference in ratioof the X–ray diffraction peak intensities was found between the CONV and the sc–CO2–
assisted metallization method, the sc–CO2 is suggested to have no influence on preferredcrystal orientation of the Pt. Influence of the grain size on properties of the silk–Ptcomposites will be discussed in the following.

Figure 6- 5 Illustration of influences of sc–CO2 in the metallization step: (a–c) CONVand (d–f) sc–CO2–assisted metallization methods
59Chapter 6

Six SEM images of silk–Pt composites metallized at 10, 20, 60, 100, and 140
min are shown in Figure 6- 6(a–e) to demonstrate the Pt coverage trend. Figure 6- 6(a)reveals surface condition of the silk substrate metallized for only 10 min, and the darkphase is the Pt metallized region. Composition of the dark phase is pure Pt phase with
only limited impurities. The bright phase corresponds to the silk, since silk is a non–conductive material which shows charging effect under SEM observation. Some parts ofthe silk are still non–metallized (pointing by arrows) indicating the Pt metallization layer
is not continuous on the silk.
Depending on the coverage condition of Pt metallization layer, the metallization
results are classified into two stages. This partially metallized silk substrate is classified
into the first stage. Figure 6- 6(b) shows a SEM image of the sample metallized for 20
min. A continuous and fully covered Pt layer was achieved showing gray color on theentire surface, which are characterized into the second stage. As the Pt metallization timeextended to 60, 100, and 140 min, (Figure 6- 6(c–e)), the silk substrates were completely
covered.
It is worth to note that, the surface roughness increases as a function of themetallization time in the CONV metallization (Figure 3- 9). On the other hand, since sc–
CO2 gives brightener–like effect during the metallization [106], the surface becamesmoother than that metallized by the CONV metallization method when sc–CO2 isapplied. Moreover, hydrogen bubbles, which are generated as a side reaction during themetallization reaction, can be removed away from the substrate surface by the sc–CO2
micelles due to the non–polar and high self–diffusivity characteristics [107], and thiseventually leads to smooth Pt metallization layer as shown in Figure 6- 7.
60

Chapter 6

Figure 6- 6 SEM images of the Pt metallized silk with (a) 10 min, (b) 20 min, (c) 60
min, (d) 100 min, and (e) 140 min of the metallization time, and (f) cross–section of thesilk/Pt by 140 min of the sc–CO2 metallization, and (g) and (f) cross–section of thesilk/Pt by CONV sc–CO2 metallization
61

Chapter 6

Figure 6- 7 Illustration of effects of sc–CO2 on morphologies of the Pt layer by the (a)CONV and (b) sc–CO2–assisted metallization
The dependences between the Pt layer thicknesses with the metallization time
length are demonstrated in Figure 6- 8. Figure 6- 8(a) shows thickness of the Pt layer
metallized at 100 min by the CONV metallization (5.3 Morphology, composition andstructure of silk/Pt section). For the sc–CO2–assisted metallization, Figure 6- 8(b), the Pt
thicknesses show a positive correlation with the metallization time. The highest thickness
at 0.75 μm was constructed with the longest metallization time of 140 min. The average
growth rate was at 2.3 nm/min. This result is in accordance with the morphology shownin Figure 6- 6. No reliable thickness could be measured when the metallization time is62

Chapter 6

less than 10 min due to the poor electrical conductivity. The sc–CO2–assisted
metallization prepared specimens entered the second stage (full coverage stage) at 20 minof the metallization time having a thickness at 0.48 μm while the CONV samples entered
the second stage at 100 min of the metallization time along with 0.76 μm of the Pt
thickness. These results indicated that more Pt sources were used in growth of the Pt
thickness instead of covering the silk textile in the CONV case. The full coverage with athinner thickness can also be attributed to the high nuclei density in the sc–CO2–assisted
metallization (Figure 6- 5) [108]. Based on the aforementioned reasons, throughintroduction of sc–CO2, full Pt coverage was achieved with less metallization time andless Pt ion source indicating a more efficient metallization.

Figure 6- 8 Thickness of the Pt layer metallized by the (a) CONV method at 100 min of
metallization time (5.3 Morphology, composition and structure of silk/Pt section) and(b) sc–CO2–assisted method as a function of the metallization time
6.4 Electrical property of sc–CO2 metallized silk/Pt
Electrical resistances of the as–metallized and the adhesive test–treated silk/Pt
composites as a function of the metallization time are revealed in Figure 6- 9. Samples in
the first stage exhibited high electrical resistances, which are mostly due to the incomplete
63Chapter 6

Pt coverage (refer to Figure 6- 6(a)). On the other hand, at 60 min of the metallization
time, the electrical resistance reached 50 mΩ. The electrical resistance remained almost
unaffected as the metallization time extended beyond 60 min due to the same degree ofPt coverage and smooth surface. This result is in accordance with the coverage shown in
Figure 6- 6 indicating that the Pt coverage is one of the critical factors affecting theelectrical resistance. After a significant drop of the electrical resistance in the first stage,
the electrical resistance for the samples metalized at 20 to 140 min deliberately decreasedin the second stage (also refer to Figure 6- 6(b–f)).
In the CONV metallization, similar two stages were found; however, the electricalresistance was offset by the increasing roughness as the metallization time goes beyond
140 min (5.4 Electrical property of silk/Pt section). On the other hand, the sc–CO2–
assisted metallization gives compact and smooth Pt coatings on the silk (Figure 6- 6(f)).
The electrical resistance thus persisted after 60 min of the metallization time. The second
stage was prolonged by the sc–CO2–assisted metallization, and the region showing thedeteriorated electrical conductivity was not found in this study. No experiments wereconducted at the metallization time longer than 140 min since the electrical resistances
are expected to remain at the same level.
The grain size can affect the electrical resistance, and a finer grain size is expected
to result a higher electrical resistance [109]. In Figure 6- 4, finer grain size of the Pt layer
via the sc–CO2–assisted metallization was obtained while comparing to the CONV
metallization. However, electrical resistance of the silk/Pt synthesized by the sc–CO2–
assisted metallization method shows a 16% reduction while comparing to those without
sc–CO2 (5.4 Electrical property of silk/Pt section). Roughness of the Pt layer metallized
64Chapter 6

on the silk is suggested to be the cause of the difference in electrical resistance [109].
Cross–sections of the samples prepared by the CONV (Figure 6- 4(g) and (h)) and the sc–
CO2–assisted metallization methods (Figure 6- 6(f)) with the optimized metallization
conditions clearly demonstrated the difference in roughness of Pt layer metallized on silk.
Electrical resistances of the adhesive test–treated silk/Pt are shown in Figure 6- 9.
No adhesive tests were performed for the samples in the first stage due to the unfeasiblehigh electrical resistance for applications. Impact of the adhesive test on electricalresistance of the sc–CO2–assisted metallized samples was not obvious. 18% reduction in
the electrical resistance after second adhesive was observed in the sample prepared by the
CONV method (5.4 Electrical property of silk/Pt section). On the other hand, only 0.5%
reduction in the electrical resistance was observed after second adhesive test in this study.
The closely packed and smooth Pt coating prepared by the sc–CO2–assisted metallization
method allows the Pt layer to stay intact after second adhesive test and shows better
resistance against the adhesive tests than those prepared without utilizing sc–CO2.

Figure 6- 9 Electrical resistance–metallization time curves of silk/Pt composites before
and after the adhesive tests
65

Chapter 6

6.5 Corrosion resistance of sc–CO2 metallized silk/Pt
Figure 6- 10(a) shows polarization curves of the as–metallized and the adhesion
test–treated silk/Pt composites in 3.5 wt.% NaCl solution. Samples prepared with 140
min of the metallization time were chosen for the corrosion resistance evaluations. No
significant difference was found between the three polarization curves in Figure 6- 10(a)revealing excellent corrosion resistance of this Pt layer. The Ecorr and Icorr are summarizedin Table 6- 1. Specimens prepared by the sc–CO2–assisted metallization show higher
corrosion resistance than those by the CONV method (5.4 Electrical property of silk/Pt
section). In 3.5 wt.% NaCl solution, the corrosion current density after second adhesive
test decreased from 5.93 x 10-7 to 2.17 x 10-7 A/cm2 when sc–CO2 was introduced into
the metallization step. Decrease in the corrosion current density is suggested to be a resultof the smooth surface condition and compact Pt metallization layer when the sc–CO2–
assisted metallization step is applied.
Similarly, Figure 6- 10(b) shows polarization curves of the as–metallized andadhesion–treated samples in the SBF. In the same way, composites metallized at 140 min
were chosen as the working electrode. The Ecorr and Icorr are also listed in Table 6- 1, whichshows limited standard deviation around ± 0.005 (μA/cm2). The corrosion resistances are
similar to those of bulk Pt metal indicating high corrosion resistance of the Pt
metallization layer despite being deposited on a non–conductive substrate. In addition,
the corrosion current density after second test decreased from 2.02 x 10 -7 to 1.71 x 10-7
A/cm2 in the SBF while sc–CO2 was introduced (5.5 Corrosion resistance of silk/Pt
section). Silk/Pt composite shows high corrosion resistance in the corrosion test in both
3.5 wt.% NaCl and SBF. The results indicated that the silk/Pt composites fabricated by
sc–CO2–assisted metallization method is a promising materials for the wearable devices.
66

Chapter 6

Figure 6- 10 Polarization curves of the silk/Pt composites in (a) 3.5 wt.% NaCl and (b)SBF solutions at room temperature and 1 atm
Table 6- 1 Corrosion potentials and corrosion current densities of the silk/Pt compositesin (a) 3.5 wt.% NaCl and (b) SBF solutions at room temperature and 1 atm(a) Samples (in 3.5 wt.% NaCl)

Ecorr (V)

Icorr (μA/cm2) ± SD

As–deposited

0.16

0.217 ± 0.005

First adhesive test

0.16

0.220 ± 0.004

Second adhesive test

0.16

0.217 ± 0.004(b) Samples (in SBF)

Ecorr (V)

Icorr (μA/cm2) ± SD

As–deposited

0.25

0.175 ± 0.007

First adhesive test

0.26

0.167 ± 0.006

Second adhesive test

0.25

0.171 ± 0.005

67Chapter 6

6.6 Biocompatibility assessment of sc–CO2 metallized silk/Pt by immersion test
Allergy reaction is provoked when there is metal ion released from theenvironment and transferred into human body as reported by Wataha [101], therefore,
examining metal ion releasing rates in the SBF is a straightforward approach to assess thebiocompatibility of the silk/Pt composite. Metal ion concentrations in human body varywith races, environments, and diets. The input and output of Pd and Pt ions for humanbody has been already discussed in 5.6 Biocompatibility assessment of silk/Pt by
immersion test section. In this study, after 3 months of the immersion test in the SBF athuman body temperature (37 oC), the Pd and Pt releasing rates were only 0.037 μg/day
and 0.057 μg/day, respectively. The releasing rates are too small and can be neglected
when comparing to the daily intake and output rates. Despite of the small grain size, which
can promote release of metal ions, the corrosion resistance was balanced by the highcompactness of the Pt layer.
The silk/Pt composites prepared by sc–CO2–assisted catalyzation andmetallization methods showed smooth and compact surface conditions for the Pt
metallization layer, decent electrical conductivity, high reliability from the adhesive tests,
and high biocompatibility in both of the immersion and polarization measurements. The
results further demonstrated practicability of the Pt metallized silks in wearable andmedical devices.
6.7 Brief summary for sc–CO2 metallized silk/Pt
Robust and electrical conductive Pt metallized silks were successfully fabricated
via the sc–CO2–assisted eletroless plating technique in this study. In the catalyzation step,
the Pd(acac)2 was successfully inlaid into the silk textile without damaging the structure
68Chapter 6

with the help of sc–CO2. In the metallization step, a compact and smooth Pt layer was
metallized. The coverage and thickness of the Pt metallized layer increased as a functionof the metallization time. Full Pt coverage on the silk substrate was achieved in merely
20 min of the metallization time, which was shortened significantly while comparing to
the metallization method without sc–CO2. The electrical resistance reached 50 mΩ at 60
min of the metallization time and remained almost unchanged as the metallization timeextended to 140 min since the surface was smoothened by the introduction of sc–CO2.
The electrical resistance persisted after the adhesive tests, which also confirmed the greatenhancement in the reliability while comparing to those synthesized by the metallization
method without sc–CO2. The silk/Pt composite shows high corrosion resistance before
and after the adhesive test in both 3.5 wt.% NaCl and the SBF solutions. The Pd and Pt
ion releasing rates were both negligible comparing to the daily input and output of Pd and
Pt in human body.

69

Chapter 7

Chapter 7 Functionalization of Au metallized silk
textile by ZnO cathodic deposition
7.1 Introduction
The global market of wearable devices is predicted to keep prospering in the future.
In the meanwhile, wearable devices are diversified into different functions such as
biomedical devices, supercapacitors, and photocatalytic devices [110] due to diverse
requirements in the next–generation technology. The integration of functional and flexible
materials turns out to be the most critical step in fabrication of the component used inwearable devices. For example, integration of photocatalytic, electrical conductive, andflexible materials is needed to realize wearable devices capable of harvesting solar energy.
Regarding the photocatalytic functionality, there are various synthetic methods
to deposit photocatalytic metal oxides. Among the synthetic techniques, cathodic
deposition allows a simple and effective process for the synthesis of metal oxides (1.2.3
Cathodic deposition section). In addition, cathodic deposition can cooperate withlithography to allow further applications such as patterning. In this section, the flexible
cloth textile/metal/photocatalyst composite material was realized by sc–CO2 assisted
electroless plating and cathodic deposition for applications in functional wearable devices.
7.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation
Figure 7- 1 shows OM images and X–ray diffraction patterns of the as–received
silk and the catalyzed silk. The crossed–linked see–through bundles shown in Figure 71(a) imply the silk textile, and the yellow flakes in Figure 7- 1(c) specify the Pd(acac)270

Chapter 7

catalyst. A broad peak at 2θ = 20o can be observed in Figure 7- 1(b), which reveals an
amorphous structure of the silk textile. On the other hand, diffraction peaks labelled bydiamond symbols can be indexed to Pd(acac) 2 [82, 83], and the results indicated thecatalysts were successfully deposited on the substrate. As shown in the OM images, thesilk substrate remained intact after the catalyzation step due to the low surface tension,
low viscosity, and non–corrosive properties of sc–CO2. The sc–CO2 assisted catalyzation
step demonstrated a significant improvement while comparing to the CONV catalyzation
process which was shown in 3.2 Activation of silk textile via sc–CO2 Pd(acac)2
catalyzation section. Pd(acac)2 acts as an activation site after it was reduced to Pd metalby the reductant in the metallization solution. Pd metal can play the role of catalyst andactivate the following metallization.

Figure 7- 1 (a) OM image and (b) X–ray diffraction pattern of the as–received silk aswell as (c) OM image and (d) X–ray diffraction pattern of the catalyzed silk (diamondsymbol: Pd(acac)2 catalysts)
71

Chapter 7

7.3 Morphology, composition and structure of silk/Au
Development of the Au layer is shown in Figure 7- 2. Figure 7- 2(a) showsmorphology of the specimen with the Ni–P metallization time at 4 min before the Au
metallization. Figure 7- 2(b–d) demonstrate the Au metallization time at 3 min, 5 min,and 80 min, respectively. At 3 min of the Au metallization (Figure 7- 2(b)), part of theNi–P layer was still not enclosed showing a gray luster. As the Au metallization times was
extended to 5 min and longer (Figure 7- 2(c) and (d)), the Ni–P layer was completely
concealed by the Au layer.

Figure 7- 2 OM images of the (a) Ni–P metallized silk and the Au metallization at (b) 3min, (c) 20 min, and (d) 80 min
The X–ray diffraction patterns at various Au metallization time are shown in
Figure 7- 3(a–c). Five apparent diffraction peaks shown in Figure 7- 3(a–c) locate at 2θ
= 38.2o, 44.4o, 64.6o, 77.6o, and 81.7o can be indexed to (111), (200), (220), (311), and(222) planes, respectively, of the FCC structure for Au (JCPDS #65–2870). In Figure 73(a), at 3 min of the Au metallization time, only a thin layer of Au was metallized on the72Chapter 7

silk, and the broad diffraction peak of silk at around 20o was still detectable. Ni–P
diffraction peaks were not found in Figure 7- 3(a), because the Ni–P layer is designed as
the sacrifice layer for the Au metallization. In addition, it owns amorphous structure(Figure 3- 4), which the amorphous peaks might be overlapped by the high intensity Au
diffraction peaks. As the Au metallization proceeded, no diffraction peaks of the silk were
found in the X–ray diffraction patterns (Figure 7- 3(b–c)) since the Au layer is thickenough to suppress the diffraction peaks from the silk. Figure 7- 3(d) shows thecomposition change with the Au metallization time. In the early stage, due to theinsufficient thickness of Au, the Ni–P layer was detectable by the EDX. On the contrary,
the Ni–P was no longer detectable as the Au metallization time extended to 80 min.
Phosphorus was not detected owing to the insufficient amount (Table 3- 1).

Figure 7- 3 X–ray diffraction patterns of the Au metallization at (a) 5 min, (b) 20 min,and (c) 80 min and (d) the composition revolution with the Au metallization time73

Chapter 7

Figure 7- 4 illustrates growth of the Au layer thickness with the Au metallization
time. There is no obvious partition of the Ni–P and Au layer shown in the inserted SEM
image in Figure 7- 4. A positive correlation was found between the Au layer and the Au
thickness presenting the average growth rate at 0.48 μm/hr. The growth rate slowed downslightly from 60 min to 80 min of the Au metallization time due to the consumption andreduction in concentration of the metal ion and reducing agent as the reaction proceeded.

Figure 7- 4 Plot of the Au metallization thickness versus the metallization time (hollow
symbol: Ni–P metallization at 4 min and solid symbols: Au metallization thickness
versus Au metallization time) (OM image of Au metallized at 40 min of themetallization time is inserted into bottom–right)
7.4 Electrical property of silk/Au
The electrical resistances at various Au metallization times are shown in Figure
7- 5. The electrical resistance dropped down in the early stage and increased after reaching74

Chapter 7

a minimum point. Since the Ni–P layer is not fully covered by Au at 3 min of Au
metallization time (solid symbol) (Figure 7- 2(a) and Figure 7- 2(a)), it showed highelectrical resistance close to the Au–free specimen at 0 min of Au metallization (hollow
symbols) (Figure 7- 5(a)). An inserted graph in Figure 7- 5(a) shows results of the Au
metallization time between 0 to 5 min.
The electrical resistances obtained from 0 to 5 min (the dash line shown in Figure7- 5) of the metallization time, which merely showed a slight difference from the Au–freeone, are classified into the first stage. Full coverage was constructed as the Au
metallization time reached 5 min, and the electrical resistance was improved accordingly.
The electrical resistance reached its minimum at the Au metallization time of 20 min due
to the complete coverage, smooth surface, and sufficient Au thickness (Figure 7- 5(b)).
From the beginning of the full coverage to the region near the lowest electrical resistancecan be sorted to the second stage (from the dash line at 5 min to the other one at 40 min).
The electrical resistance increased slightly as the Au metallization time increased beyond
the minimum point. The surface grew coarser (pointed by the arrows in Figure 7- 5(c))
with the elongated metallization time, and the electrical resistances were deteriorated due
to more electron scattering [89]. The region of worsened electrical resistance and bumpy
surface can be categorized into the third stage.
The electrical resistances after the adhesive tests are also shown in Figure 7- 5(a).
No significant impact was observed in all stages since the silk was completely enclosedby the Ni–P and Au, and the adhesions between the silk and the Ni–P and between the
Ni–P and the Au both showed sturdy adhesive property.

75

Chapter 7

Figure 7- 5 (a) Electrical resistances of the Au metallized silk at various metallization
times and SEM images of the Au metallized silk at (b) 20 min and (c) 80 min of themetallization time (hollow symbols: Au–free; solid symbols: Au metallized)
7.5 Cathodic ZnO deposition
Figure 7- 6 shows SEM images (Figure 7- 6(a–d)) and OM images (Figure 7- 6(e–h)) of the pristine ZnO deposited with H2O2 concentration varied from 0 to 0.3 wt.%.
76

Chapter 7

Tower–like ZnO nanowires are observed in all of the SEM images (Figure 7- 6(a–d)).
ZnO coverage on the Au layer was processed and calculated by a built–in software of the
OM. The ZnO coverages were at 63%, 62%, 41%, and 32% for the specimens deposited
with free of H2O2 to 0.3 wt.% H2O2. Morphology of cathodically deposited ZnO is often
affected by Cl- in the solution. The Cl- ion can act as a capping agent adsorbing on the(0001) plan of ZnO to stabilize the surface and leads to formation of nano–rod likestructures. On the other hand, when concentration of the Cl- is low, tower–like structuresinstead of nano–rod structures are formed [61]. Furthermore, size of the ZnO particle
increased as the H2O2 concentration increased from 0 to 0.2 wt.% as shown in the SEM
images. Similar results on increase in the ZnO particle size as the H 2O2 concentration
increased were reported in a literature [111]. However, in Figure 7- 6(d), the particle size
decreased when the H2O2 concentration went beyond 0.2 wt.%. ZnO deposition equations
are shown in the following [112]:
𝑁𝑂3− + 𝐻2 𝑂 + 2𝑒 − = 𝑁𝑂2− + 2𝑂𝐻 −(7. 1)𝐻2 𝑂2 + 2𝑒 − = 2𝑂𝐻 −(7. 2)

𝑍𝑛2+ + 2𝑂𝐻− = 𝑍𝑛𝑂 + 𝐻2 𝑂

(7. 3)

OH- generated on the substrate from equation (1) and (2) are expected to react with Zn 2+
in the electrolyte to form ZnO on the substrate. As shown in equation (2), generation rateof the OH- would be increased with an increase in H2O2 concentration. Since thediffusivity of Zn2+ in aqueous solution is four times slower than that of H 2O2, diffusionof Zn2+ to the substrate surface cannot catch up with the generation of OH - as the H2O2
concentration increased [113]. When the amount of H2O2 was increased to 0.3 wt.%,
generation rate of the OH- could be too high and accumulate at the electrode surface.

77

Chapter 7

Figure 7- 6 SEM images of the specimens deposited in (a) free of H 2O2, (b) 0.1 wt.%,(c) 0.2 wt.%, and (d) 0.3 wt.% H2O2, and the OM images in (e) free of H2O2, (f) 0.1
wt.%, (g) 0.2 wt.%, and (h) 0.3 wt.% H2O2
78

Chapter 7

A general scheme of the solute concentration profiles in the electrolyte is shownin Figure 7- 7. OH- are generated on the substrate, hence the concentration is gradually
lowered toward the bulk solution direction. Zn2+ would diffuse from the bulk solution to
the substrate surface to react with the OH-, therefore, the concentration is lowered as it
approaches the substrate surface. Depends on the OH- generation rate, local OH concentration, or pH, on the substrate could vary. ZnO is formed at a pH range of ca. 8 to
12, and ZnO would dissolve away to form ZnO22- or HZnO2- when the pH is higher than
12 [62, 114]. The region suitable for deposition of ZnO is defined as region I, and theregion leads to dissolution of ZnO is denoted as region II as shown in Figure 7- 7.

Figure 7- 7 General scheme of the solute concentration profiles
79

Chapter 7

The region near surface of the substrate would be in region I when the H2O2
concentration was lower than 0.2 wt.%. Therefore, an increase in the H 2O2 concentration
led to an increase in the particle size (Figure 7- 6(a–b)) and the coverage (Figure 7- 6(e–f)). When the H2O2 concentration was increased to 0.3 wt.%, both the particle size (Figure
7- 6(d)) and the coverage (Figure 7- 6(h)) were decreased, which suggest the pH near thesubstrate surface could be higher than 12. The results suggest the region near the substratesurface is in region II when the H2O2 concentration is higher than 0.3 wt.%.
Figure 7- 8(a) shows X–ray diffraction patterns of the ZnO deposited withvarious H2O2 concentrations in the electrolyte. Only the diffraction peaks of Au (starsymbol) and ZnO (hexagon symbol) were observed. No secondary phase was found in
the X–ray diffraction patterns after the ZnO deposition. A relative crystallinity of ZnO on
the Au layer was roughly estimated by calculating ratio of the Au (111) intensity to theZnO (100) intensity with consideration of the ZnO coverage. The ratios were at 0.10, 0.11,
0.34, and 0.16 from free of H2O2 to 0.3 wt.% H2O2. The ratios indicated the electrolyte
containing 0.2 wt.% H2O2 produced ZnO with the best crystallinity, and the crystallinity
became worse when 0.3 wt.% H2O2 was used. Another explanation concerning
crystallinity of the ZnO is provided here. Crystallinity of the ZnO is reported to be
dependent on concentration of the Zn2+ [115], which defects and deteriorated crystallinity
are introduced to the ZnO when the Zn2+ concentration is low. In this study, when the
H2O2 concentration reached 0.3 wt.%, local concentration of OH - near the electrodesurface would be high, and higher diffusion rate of OH- to the bulk solution was built.
Then, pH far away from the substrate surface became suitable for formation of ZnO andconsumed great amount of the Zn2+. In this case, amount of the Zn2+ diffused to thesubstrate surface was lowered and resulted the deteriorated crystallinity.
80Chapter 7

Figure 7- 8(b) shows the voltage–time curves during the ZnO deposition. While there is
H2O2 in the electrolyte, the potential drop decreased as the H2O2 concentration increased.
The potential drop here refers to the potential difference between the potential at ca. 0 secof the reaction time and the potential as the reaction gradually stabilized with time. No
obvious difference was found in the specimens deposited with free of H 2O2 and 0.1 wt.%
H2O2, which are in good agreement with the ZnO coverage in Figure 7- 6(e) and Figure
7- 6(f) revealing that 0.1 wt.% H2O2 has limited influence on the ZnO deposition. The
potential fluctuation increased with an increase in the H 2O2 concentration, which is
because more N2 gas is emitted at high pH [116].

Figure 7- 8 (a) X–ray diffraction patterns and (b) voltage–time plot of the specimens
fabricated with electrolytes containing various H2O2 concentration81

Chapter 7

7.6 Photocurrent evaluation of silk/Au/ZnO
To assess the applicability as photocatalytic materials, photocurrent generated
from photoelectrochemical water splitting reaction using the as–deposited sample as thephotoanode was evaluated. Figure 7- 9 shows the chronoamperometric I−t curves of thesilk/Au/ZnO layered materials recorded in 0.5 M Na2SO4 electrolyte under chopped lightillumination. The current density was corrected by considering coverage of the ZnO on
the Au layer, which the ZnO coverages are provided in the beginning of this section.
Figure 7- 9(a) shows the as obtained current density, which is before thecorrection, while Figure 7- 9(b) shows the corrected values. All the specimens showed anincreased in the current density when the illumination is on, which imply the effective
generation, transfer and collection of the electrons during the water splitting process. The
ZnO deposited with 0.2 wt.% H2O2 showed the highest current density in both Figure 79(a) and Figure 7- 9(b) since it owns the highest crystallinity. It has been reported that thehigh crystallinity can assist in suppressing the possible photocorrosion effect [117]. In
addition, high crystalline ZnO leads to efficient transportation of the excited electrons.
Photocurrent density of the ZnO fabricated with 0.2 wt.% H2O2 increased 11.5 times
while comparing to the one deposited without H2O2. The greater photoelectrochemical
performance can be attributed to the facile carrier transport resulting from the oriented
crystallinity. In this study, high H2O2 concentration led to the high photocurrent owing to
the high crystallinity; however, excess OH - resulted in the dissolution of ZnO anddeteriorated crystallinity, which resulted deteriorated photocurrent density. A digital
photo is inserted into Figure 7- 9(a) to show the flexibility of the composite material.
Various evaluations have been done on the silk/Au/ZnO composite materials in82

Chapter 7

this study. The composite shows flexibility, high electrical conductivity, and highadhesive firmness. Photocurrent density was enhanced significantly by the optimized
ZnO deposition solution. Therefore, the results showed the potential of the materials in
the applications of flexible wearable devices.

Figure 7- 9 Photocurrent density of the silk/Au/ZnO hybrid materials with (a) non–corrected (an inserted figure shows the flexibility of the composite material) and (b)corrected by the ZnO coverage
7.7 Brief summary for silk/Au/ZnO
ZnO/Au layered structure was successfully fabricated on the silk textile by sc–
83

Chapter 7

CO2 assisted electroless deposition and cathodic deposition. Common problems
encountered in the conventional electroless plating were solved by the introduction of sc–
CO2. With the help of sc–CO2, silk was catalyzed without defects and the adhesive
property between the silk and the metallized layer was enhanced at the same time. Surface
coverage and thickness of the Au layer increased with the Au metallization time. The
lowest electrical resistance was achieved as the Au metallization time extended to 20 min.
On the other hand, ZnO was deposited on the Au metallized silk via the cathodic
deposition method. The coatings on the silk have been confirmed to be metallic Au phase
and ZnO wurtzite phase. ZnO deposited with 0.2 wt.% H2O2 showed the best crystallinity
and the highest photocurrent density. The ZnO modified specimen with 0.2 wt.% H2O2
performed 11.5 times enhancement in the photocurrent density while comparing to thatwithout H2O2.

84

Chapter 8

Chapter 8 Reliability and photocatalytic activity
enhancement by Ni–P co–deposition
8.1 Introduction
In this chapter, our objective is to enhance the various properties of thephotocatalytic composites such as adhesive property, corrosion resistance, andphotocatalytic activity. Co–deposition was utilized since the oxides and metallization
layer can be co–deposited and thus the adhesive property can be enhanced. On the other
hand, TiO2 was used for its inactive property, acting as a reinforcing phase, and highphotocatalytic activity.
8.2 Activation of silk textile via sc–CO2 Pd(acac)2 catalyzation
Figure 8- 1 shows OM images and XRD patterns of the as–received silk and thesc–CO2 catalyzed silk. The transparent interwoven bundles shown in Figure 8- 1(a)indicate the silk textile, and Figure 8- 1(b) shows a broad peak at around 2θ = 20o, thisbroad peak reveals an amorphous structure of the silk textile. The light yellow flakes
around the silk bundles in Figure 8- 1(c) specify the Pd(acac)2, and the diffraction peaks
in Figure 8- 1(d) labelled by triangle symbols can be indexed to Pd(acac) 2 very well [82,83], which indicated that the Pd(acac)2 organometallic compounds were successfully
deposited on the silk substrate. As demonstrated in the Figure 8- 1(a) and (c), the silk
textile remained undamaged after the 2 hours sc–CO2 catalyzation treatment due to thelow surface tension, low viscosity, and non–corrosive properties to most of polymers of
sc–CO2 [15, 21, 84]. The sc–CO2 catalyzation practiced a significant advancement when85

Chapter 8

it is compared to the conventional catalyzation process [15].

Figure 8- 1(a) OM image and (b) XRD pattern of the as–received silk, and (c) OM
image and (d) XRD pattern of the sc–CO2 catalyzed silk (triangle symbols: Pd(acac) 2).
8.3 Ni–P/TiO2 co–deposition
Surface conditions of the Ni–P/TiO2 decorated silk textiles prepared from theNi–P electrolytes containing various TiO2 concentrations (0, 10, 20, 30, 40, and 50 g/L)
can be observed from SEM and OM images (inserted into top–left of each SEM images)
shown in Figure 8- 2.

Figure 8- 2(a) shows morphology of the specimen deposited

without TiO2 particles in the Ni–P electrolyte. It shows smooth surface while comparing
to the other ones deposited with TiO2 in the electrolyte (Figure 8- 2(b–f)). The brightnano–sized protrusions shown in Figure 8- 2(b–f) indicate the deposition of TiO2 which
resulted in roughening of the surface. However, the surface roughness decreased when
the TiO2 concentration went beyond 30 g/L in Ni–P electrolyte.
86

Chapter 8

Figure 8- 2 SEM images and OM images (inserted into top–left) of Ni–P/TiO2
deposition with various TiO2 concentration at (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f)50 g/L, respectively.
The nano-scale TiO2 particles in the electrolyte would aggregate to form large
aggregate particles, and average size and size distribution of the aggregate particle are
related to concentration of the oxides particle in the solution [118-123]. In this study,
when the TiO2 concentration is low (10 and 20 g/L), there is only slight aggregation of
the TiO2 particles in the Ni–P electrolyte. Average size of the aggregate particle size issmall, and the distribution is narrow as illustrated in Figure 8- 3(a). Level of theaggregation gradually increases with an increase in concentration of the TiO2, and average87Chapter 8

size of the aggregate particle becomes larger with a wider size distribution as shown in
Figure 8- 3(b and c). Compositions of Ni–P/TiO2 depositions are shown in Table 8- 1.

Figure 8- 3 TiO2 particle size distribution with different amount of TiO 2 in theelectrolyte: (a) slight aggregation, (b) moderate aggregation, and (c) severe aggregation.
Table 8- 1 Composition of as–deposited Ni–P/TiO2 metallization layers on silk textiles.
TiO2 (g/L)
Atom

0

10

20

30

40

50

60

P (at.%)

22.11

18.82

19.85

19.89

19.17

18.37

20.09

Ti (at.%)

0.00

4.62

5.95

10.16

6.15

5.32

4.76

Ni (at.%)

77.89

76.56

74.20

69.95

74.68

76.31

75.15

Sum

100.00100.00100.00100.00100.00100.00100.0088

Chapter 8

The mechanism of the TiO2 inclusion is illustrated in Figure 8- 4. It is suggested
that Ni ions would adsorb on surface of the TiO2 aggregate particles suspending in theelectrolyte to form Ni2+–TiO2 complex particles, and the complex particles diffuse to thesubstrate then reduction of the Ni ions on surface of the complex particle occurred toengulf the TiO2 aggregate particle into the Ni–P layer [63, 124]. When the TiO2
concentration is low with a small aggregate particle size in the electrolyte, the TiO 2
aggregate particles are easily included into the Ni–P as shown in Figure 8- 3(a) and Figure
8- 4(a). Because of the small aggregate particle size, a relatively smooth surface isobtained as exampled by the specimen shown in Figure 8- 2(b and c). With an increase in
the TiO2 concentration, average size of aggregate particles included into the Ni–P
gradually increases and leads to a roughened surface condition as illustrated in Figure 84(b). Figure 8- 2(d) demonstrates the result of the rough surface caused by the large
aggregate particles in the moderate aggregate region (Figure 8- 3(b)). When size of the
TiO2 aggregate (or the Ni2+–TiO2 complex) particle reaches a critical size, a phenomenon
named steric obstruction takes place to affect inclusion of the particles [30-37]. Stericobstruction is unfavorable for the inclusion as shown in Figure 8- 4(c). At first, removalof already–adsorbed oxide particles and decrease in the number of new particles adheringto the substrate occur because of collisions between particles near the substrate [125].
Secondly, inclusion of the particles is dependent on reduction of Ni ions adsorbed on theaggregate TiO2 particle surface. Surface to volume ratio of the aggregate particle islowered with an increase in the aggregate TiO2 particle size. When the aggregate TiO2
particle size is larger than the critical size, shortage of Ni ions adsorbed on surface of theparticle makes it less likely to include the aggregate TiO2 particle into the Ni–P before
been removed away from the surface because of the collision [126]. In this study, average89Chapter 8

size of the aggregate TiO2 particle is suggested to be close to the critical size causing thesteric obstruction when the TiO2 concentration is at 30 g/L (Figure 8- 2(d) and Figure 84(b)), which is defined as the critical concentration resulting either maximum or
minimum characteristics in the composite.

Figure 8- 4 Mechanism of TiO2 inclusion with (a) TiO2 concentration less than criticalamount, (b) TiO2 concentration at critical amount, and (c) TiO2 concentration more thancritical amount.
90

Chapter 8

Figure 8- 5 shows elemental mapping results of the Ni–P/TiO2 composite
prepared with the electrolyte containing 30 g/L of TiO2. Figure 8- 5(a) shows the roughsurface condition of the composite layer deposited at the critical concentration. P, Ni, andTi elements were homogeneously distributed on the surface indicating well–mixing of theelectrolyte and homogeneous distribution of TiO2 in the composite layer as indicated byresults shown in Figure 8- 5(b) to (d).

Figure 8- 5 Elemental mapping of Ni–P/TiO2 composite layer deposited with 30 g/L
TiO2.
Figure 8- 6 shows XRD patterns of the Ni–P/TiO2 deposited silk textiles atvarious TiO2 concentrations in the electrolyte. A broad peak at around 2θ = 45o indicates
amorphous structure of the Ni–P phase while a sharp peak at around 2θ = 25o suggests
91

Chapter 8

anatase phase of the TiO2. All specimens show TiO2 diffraction peak at 2θ = 25o except
for the one without the introduction of TiO2 (0 g/L) in Figure 8- 6(a). No impurity or third
phase was found in the XRD pattern within the detection limitation. TiO 2 concentration
was approximately compared by the ratio between intensities of the Ni–P and the TiO2
major peaks to qualitatively estimate the TiO2 concentration in the Ni–P metallization
layer. The ratio increased as concentration of TiO2 in the electrolyte increased until itreached 30 g/L, which implies an increase in concentration of TiO2 in the composite layer.
Then the ratio decreased indicating a decrease in concentration of TiO 2 in the composite
layer (Figure 8- 6e and f). This observation confirmed 30 g/L of TiO2 in the electrolyte
being the critical concentration to give the highest TiO2 inclusion in the composite andthe effect of steric obstruction (Figure 8- 3 and Figure 8- 4).

Figure 8- 6 X–ray diffraction patterns of Ni–P/TiO2 deposition with various TiO2
concentrations at (a) 0, (b) 10, (c) 20, (d) 30, (e) 40, and (f) 50 g/L, respectively.
92

Chapter 8

Figure 8- 7 illustrates a positive correlation between the Ni–P/TiO2 composite
layer thickness and the TiO2 concentration in the electrolyte before reaching the criticalconcentration. A SEM image was inserted in the bottom–right corner of Figure 8- 7 toshow cross–section of the silk/Ni–P/TiO2 composite layer deposited with 50 g/L TiO2 inthe electrolyte. The average growth rate was at 3.4 μm/hr before reaching the criticalconcentration. On the other hand, the thickness decreased slightly in the 40 g/L and 50
g/L specimens since large TiO2 aggregate particles were formed to cause the stericobstruction (Figure 8- 3 and Figure 8- 4).

Figure 8- 7 Relationship of Ni–P/TiO2 thickness and TiO2 concentration in theelectrolyte.
8.4 Electrical conductivity and adhesive tests
Electrical resistances of composite layers prepared at various TiO 2
concentrations in the Ni–P electrolyte are shown in Figure 8- 8. Since TiO2 is asemiconductor, it owns lower electrical conductivity than Ni–P. TiO2 thus shows negative
93Chapter 8

influence on the electrical conductivity in the Ni–P metallization layer. Ni–P metallization
layer without TiO2 inclusion performed the lowest electrical resistance. The electricalresistance increased monotonously with TiO2 concentration in the electrolyte before
reaching the critical concentration of TiO2 (30 g/L) in the electrolyte. On the other hand,when the TiO2 concentration in the electrolyte went beyond the critical concentration, theelectrical resistance dropped owning to lower TiO2 concentration in the Ni–P
metallization layer. The inclusion trend of TiO2 revealed by the electrical resistances is inaccordance with the surface morphologies from SEM observations, the intensity ratiosfrom XRD patterns, and the layer thickness results.

Figure 8- 8 The influence of included TiO2 concentration to the electrical resistance ofthe silk/Ni–P/TiO2 composite material.
Adhesive test evaluations are also shown in Figure 8- 8. Square symbol
represents the as–deposited specimen, circle symbol depicts the first adhesive test, andthe triangle one indicates the second adhesive test. Here, first adhesive test indicates one
94Chapter 8cycle of the adhesive treatment is performed. Accordingly, second adhesive test means
two cycles of the test are conducted. The results show that the electrical resistances
persisted after the second adhesive test. Moreover, no significant difference was foundbetween the first and second adhesive tests, and thus no further adhesive tests wereconducted. These results demonstrate the high reliability and the robustness of thesilk/Ni–P/TiO2 composite materials and indicate that it is practicable for wearable devices.
8.5 Corrosion resistance
Figure 8- 9 shows the corrosion behaviors of the as–deposited silk/Ni–P
composite and the silk/Ni–P/TiO2 composite materials in 3.5 wt.% NaCl solution at bothroom temperature (Figure 8- 9(a)) and 37 oC (body temperature) (Figure 8- 9(b)). The
silk/Ni–P/TiO2 composite fabricated with 30 g/L of TiO2 in the electrolyte was chosen forthe corrosion resistance evaluations. The corrosion potential (E corr) and the corrosion
current density (Icorr) are summarized in Table 8- 2. These results show that the Ni–P/TiO2composite have excellent corrosion resistance when compared with the pure Ni–P coating
at both room temperature and 37oC. According to the results, TiO2 particles play animportant rule for the improvement of the corrosion resistance. TiO 2 particles act as inert
physical barriers to the initiation and development of defect corrosion and modify
microstructure of the Ni–P metallization layer to improve the corrosion resistance. On theother hand, the Ecorr of silk/Ni–P/TiO2 is more positive than the silk/Ni–P indicating thatthe silk/Ni–P/TiO2 composite is more inert than the silk/Ni–P due to the inclusion of inert
TiO2 particles. This result demonstrates that the composite material can persist in salty
human sweat at both room temperature and body temperature, which is promising for
applications in wearable devices.

95

Chapter 8

Figure 8- 9 Polarization curves of pure Ni–P coating and Ni–P/TiO2 (3 g/L TiO2 in theelectrolyte) deposition on silk textile at (a) room temperature and (b) 37 oC (bodytemperature).
96

Chapter 8

Table 8- 2 Corrosion potentials and corrosion currents of the silk/Ni–P and silk/Ni–
P/TiO2 composites in 3.5 wt.% NaCl at (a) room temperature and (b) 37 oC (bodytemperature).
(a) Room temperature

Ecorr (V)

Icorr (A/cm2)

Silk/Ni–P-0.53

6.00 x 10-7

Silk/Ni–P/TiO2 (3g/L)-0.37

1.56 x 10-7(b) 37 oC (body temperature)

Ecorr (V)

Icorr (A/cm2)

Silk/Ni–P-0.67

3.79 x 10-6

Silk/Ni–P/TiO2 (3g/L)-0.59

1.02 x 10-6

8.6 Photocatalytic activity assessment
To evaluate the practicability as photocatalytic materials, photocurrent generated
from photoelectrochemical water splitting reaction was evaluated. The as–deposited
sample was used as the photoanode. Figure 8- 10 shows the chronoamperometric I−t
curves of the silk/Ni–P/TiO2 composite materials recorded in 0.5 M Na2SO4 electrolyte
illuminated by periodic white light at room temperature. The current density wascorrected by considering overall surface area of the silk substrate. All the specimens
showed an increasing anodic current when the illumination was turned on, whichindicated the effective excitation, transfer, and collection of the electrons during the water
splitting reaction. TiO2 concentration in the metallization layer increased with TiO 2
concentration in the electrolyte, hence the current density increased monotonously from
0 g/L until the critical concentration (30 g/L). Because of the steric obstruction, less TiO 2
was included into the Ni–P metallization layer when the TiO2 concentration went beyond
the critical concentration, and the current density dropped. Again, the Ni–P/TiO2
deposited with 30 g/L TiO2 in the electrolyte showed the highest current density since itowns the greatest inclusion amount of TiO2 in the Ni–P metallization layer.
97

Chapter 8

Figure 8- 10 Photocurrent density of the silk/Ni–P/TiO2 incorporated materials
illumined by periodic white light in 0.5 M Na2SO4 electrolyte at room temperature.
Figure 8- 11 shows the photocurrent densities of the as–deposited and the second
adhesive test–treated specimens. The photocurrent densities persisted after the second
adhesive test showing its high reliability and robustness. These results reveal that theadhesive properties were enhanced significantly while compare to literature [127, 128]
due to the codeposition of Ni–P and TiO2.

98

Chapter 8

Figure 8- 11 Photocurrent density of (a) the as–deposited and (b) second adhesive test–
treated silk/Ni–P/TiO2 composite illumined by periodic white light in 0.5 M Na2SO4
electrolyte at room temperature.
Various evaluations were executed to reveal properties of the silk/Ni–P/TiO2composite materials in this study. The silk/Ni–P/TiO2 composite materials possessed
flexibility, electrical conductivity, high adhesive firmness, and high corrosion resistance.
Photocurrent density was enhanced significantly by the inclusion of TiO2 at its criticalconcentration in the electrolyte. Therefore, the results demonstrated potentials of thesilk/Ni–P/TiO2 composite materials for applications in flexible and functional wearable
devices.
8.7 Brief summary for silk/Ni–P/TiO2
Ni–P/TiO2 composite layer was successfully co–deposited on silk textile by sc–
99

Chapter 8

CO2 assisted electroless plating. Common difficulties encountered in the conventional
electroless plating were solved by the introduction of sc–CO2, and the Pd(acac)2organometallic compounds were embedded into the silk substrate at around 330 nm in thedepth. The composite layer on the silk were confirmed to be amorphous Ni–P phase andTiO2 anatese phase. The composite showed the highest electrical resistance when theconcentration of TiO2 in the electrolyte was 30 g/L (critical concentration) since thehighest TiO2 concentration was included into the Ni–P metallization layer. High adhesive
property of this silk/Ni–P/TiO2 composite was revealed by the adhesive tests, theelectrical resistance, and the photocatalytic activity evaluations. The Ni–P/TiO2 showed
higher corrosion resistance than that of Ni–P metallized silk without TiO2 particles at bothroom temperature and body temperature. The highest photocatalytic activity was obtainedin the composite with 30 g/L of TiO2 particles in the electrolyte.

100Chapter 9

Chapter 9 General conclusions
Chapter 3
In this chapter silk textile was metallized by thin film Ni–P and Pt to fulfill theelectrically conductive property, high corrosion resistance, and high wear resistance while
keeps the silk textile flexible. Ni–P was chosen for its electrical conductivity, high wear
resistance, high corrosion resistance, and low cost. Silk textile was firstly catalyzed byPd(acac)2, and the catalysts were successfully embedded into the silk textile around 200
nm in the depth without damaging the silk textile by the sc–CO2 catalyzation. On theother hand, silk was damaged and lost its original structure after the CONV catalyzationin the acidic catalyzation solution of SnCl2/PdCl2. Ni–P was successfully metallized on
the silk substrate and shows the homogeneous distribution of elemental composition. Ni–
P metallization thickness shows positive correlation to the metallization time.
Three stages were defined according to the Ni–P coverage on the silk textile andelectrical resistance, those Ni–P that is not fully covered was defined as the first stage
showing high electrical resistance. Smooth metallization and full coverage of Ni–P was
defined as second stage when the metallization time was extended. The lowest electricalresistance was observed in the second stage. Surface roughness increased withmetallization time was observed and the Ni–P metallization with high surface roughness
and full coverage was defined as third stage. The increased surface roughness was due tothe severe diffusion control when the metallization was further extended. The electricalresistance was deteriorated in the third stage due to the high surface roughness. Adhesive
101Chapter 9

properties of the silk/Ni–P was also carried out in this study. Hardly did the electricalresistance changed after the adhesive tests showing the firmly adhesive property of thesilk/Ni–P composite materials.
Corrosion resistance of silk/Ni–P was also evaluated. Corrosion resistance wasmeasured after as–deposition, first adhesive test, and second adhesive test, respectively.
No significant difference was observed after all the adhesive test and the corrosion
resistance was comparable to the bulk Ni–P indicating the high corrosion resistance ofthe silk/Ni–P deposited on the silk textile. The results from surface morphology, electricalresistance, and corrosion resistance suggested that the Ni–P is a proper material for theapplication of wearable devices.
Chapter 4
Ni–P is a promising material for the wearable devices owing to theaforementioned advantages. Due to the concern of biocompatibility, the metallization was
shifted to the Pt metallization to overcome the allergy reactions which would be triggeredby Ni. Pt(acac)2 catalyst was used to fulfill the full–Pt material. Similarly, Pt(acac)2
catalysts were successfully embedded into the silk textile without damaging the silk
structure by the sc–CO2 catalyzation. Similar three stages were also found in the Pt
metallization process and the thickness of Pt metallization also showed positive
correlation to Pt the metallization time.
Test of Pt ion releasing rate by immersion test in the SBF at body temperature
was carried out in this study to evaluate the biocompatibility of the silk/Pt composite
materials. Only trace amount of Pt ion was observed while comparing to the daily Pt input
and output to human body indicating the biocompatibility of the composite materials. The
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results from surface morphology, electrical resistance, and immersion test suggested thatthe silk/Pt is a proper composite material for the application of biocompatible wearable
devices and medical devices.
Chapter 5
For improving the metallization efficiency, the catalyst for the Pt metallization
was shifted to Pd(acac)2. Similarly, Pd(acac)2 catalysts were successfully embedded into
the silk textile without damaging the silk structure by the sc–CO2 assisted catalyzation.
Similar three stages were also found in the Pt metallization process and the thickness of
Pt metallization also showed positive correlation to the metallization time. Pt
metallization efficiency was improved due to the synchronization of the reduction ofPd(acac)2 catalysts.
Since the catalyst was changed, identical immersion test in SBF at bodytemperature was also carried out to evaluate the biocompatibility of the composite
materials. Only trace amount of Pd and Pt ions were observed while comparing to thedaily input and output of Pd and Pt to human body indicating the biocompatibility of thePd(acac)2 catalyzed silk/Pt composite materials. Thus, the metallization efficiency wasenhanced by the Pd(acac)2 catalyst while the composite materials still show it
biocompatibility. In addition, Pt metallization shows both high corrosion resistance in the
3.5 wt. NaCl solution and SBF. The results from surface morphology, electrical resistance,
immersion test, and corrosion resistance suggested that the Pt is a proper material for theapplications of biocompatible wearable devices and medical devices.
Chapter 6
Sc–CO2 was introduced into the metallization solution to improve the Pt
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metallization properties. Pd(acac)2 catalyst was chosen for its high metallization
efficiency. Similarly, Pd(acac)2 catalysts were successfully embedded into the silk textile
without damaging the silk structure by the sc–CO2 assisted catalyzation. Only two stages
were found in the sc–CO2 metallization since the diffusivity of metal ions was enhancedby the bouncing sc–CO2/surfactant micelles in the electrolyte.
Due to the smoothened Pt metallization, only two stages were defined according
to the surface morphology and electrical resistance. Since the surface roughness of Pt
metallization was suppressed by the introduction of sc–CO2 into the metallization
solution, third stage was not observed and thus no deteriorated electrical conductivity was
found. The Pt metallization properties was improved by the introduction of sc–CO2 to the
Pt metallization electrolyte.
Test of ion releasing rate by immersion test in the SBF was carried out to evaluate
the biocompatibility of the sc–CO2 assisted Pt metallization composite materials. Only
trace amount of Pt ion was observed while comparing to the daily Pt input and output tohuman body indicating the biocompatibility of the silk/Pt composite materials. The resultsfrom surface morphology, electrical resistance, and immersion test suggested that the sc–
CO2 assisted Pt metallization is a proper material for the applications of biocompatible
wearable devices and medical devices.
In chapter 3, Ni–P and Pt were metallized on the silk textile, respectively. Ni–P
owns the efficient metallization and low cost while Pt owns the biocompatibility and highcorrosion resistance. Both of the composite are promising materials for the wearable
devices according to the evaluations of electrical resistance, corrosion resistance,
immersion test, and adhesive tests.
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Chapter 9

Chapter 7
For functionalizing the silk textile, Au was metallized on the silk substrate andZnO was deposited on the Au metallized silk substrate. Pd(acac)2 catalyst was selected
for its low cost and high efficiency in the metallization. Similarly, Pt(acac)2 catalysts weresuccessfully embedded into the silk textile without damaging the silk structure by the sc–
CO2 assisted catalyzation. A thin Ni–P was deposited on the silk substrate for acting as adiffusion barrier layer between silk and Au metallization. Similar three stages were also
found in the CONV Au metallization process and the thickness of Au metallization also
showed positive correlation to the Au metallization time.
ZnO was successfully deposited on the Au metallized silk. The coverage of ZnO
decreased with increasing H2O2 concentration in the solution. Since the H2O2 was theOH– source of OH–, the substrate surface became more alkaline when there are more H2O2
in the electrolyte. Thus, ZnO dissolved back to the solution when the surface pH went
beyond 12 and the coverage of ZnO decreased accordingly. When he H2O2 at 0.2 wt.%,
ZnO shows the highest crystallinity, therefore, the silk/Au/ZnO composite shows thehighest photocurrent density in the photocatalytic activity test. The results from theelectrical resistance, adhesive test, and photocatalytic activity evaluations suggested thatthe silk/Au/ZnO composite material is a proper material for the application of functional
wearable devices.
Chapter 8
Ni–P/TiO2 composite layer was successfully co–deposited on silk textile by sc–
CO2 assisted electroless plating. Common difficulties encountered in the conventional
electroless plating were solved by the introduction of sc–CO2, and the Pd(acac)2
105

Chapter 9organometallic compounds were embedded into the silk substrate at around 330 nm in thedepth. The composite layer on the silk were confirmed to be amorphous Ni–P phase andTiO2 anatese phase. The composite showed the highest electrical resistance when theconcentration of TiO2 in the electrolyte was 30 g/L (critical concentration) since thehighest TiO2 concentration was included into the Ni–P metallization layer. High adhesive
property of this silk/Ni–P/TiO2 composite was revealed by the adhesive tests, theelectrical resistance, and the photocatalytic activity evaluations. The Ni–P/TiO2 showed
higher corrosion resistance than that of Ni–P metallized silk without TiO2 particles at bothroom temperature and body temperature. The highest photocatalytic activity was obtainedin the composite with 30 g/L of TiO2 particles in the electrolyte.

106Chapter 10

Chapter 10 List of achievements and others
10.1 Awards• Best poster award, Taiwan-Japan Joint Symposium & Nano Engineering andMicrosystem Technology Conference
May 2018• Best poster award, second place, The Minerals, Metals & Materials society (TMS)
2018
Mar. 2018• Thin Film Society (TSF) Special Award (Oral presentation)
Oct. 2017• Taiwan Association for Coating and Thin Film Technology (TACT) 2017 Gold Award
Oct. 2017• Research Fellow of Japan Society for the Promotion of Science, JSPS
Apr. 2017-Sept. 2018
• Best Paper Award, 15th Symposium on Development of Supercritical Fluid
Technology and Application, Taiwan Supercritical Fluid Association (TSCFA)
Sept. 2016
• Interchange Association international student scholarship, Japan–Taiwan Interchange
Association
Apr. 2016-now

107Chapter 10• Research assistantship, Tokyo Tech
Oct. 2015-now• 2015 Honors Scholarship, Japan Student Services Organization (JASSO)
Oct. 201510.2 Publications
10.2.1 International Journal
1. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, Y. Tahara, T. Hashimoto, H. Kurosu, and M.
Sone, “Platinum Coating on Silk by a Supercritical CO2 Promoted Metallization
Technique for Applications of Wearable Devices”, Surface & Coating Technology,
doi.org/10.1016/j.surfcoat.2018.02.070 (In press). SCI, IF: 2.589
2. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, Y. Tahara, T. Hashimoto, H. Kurosu, and M.
Sone, “Fabrication and Photocatalytic Performance of Au/ZnO Layered Structure on
Silk Textile for Flexible Device Applications”, Electrochimica Acta, 253(1) (2017)39-46. SCI, IF: 4.798
3. W.-T. Chiu, Y. Tahara, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M.
Sone, “Fundamental Property Assessments of Biocompatible Silk–Pt Composite
Prepared by Supercritical Carbon Dioxide Promoted Electroless Plating” ACS
Industrial & Engineering Chemistry Research, 56(31) (2017) 8864-8871. SCI, IF:
2.567
4. W.-T. Chiu, Y. Tahara, C.-Y. Chen, T.-F M. Chang, T. Hashimoto, H. Kurosu, and M.
Sone, “A Supercritical CO2 Promoted Electroless Ni-P Plating on Silk and Their
Fundamental Characteristics Investigations”, Journal of The Electrochemical Society,164(7) (2017) D406-D411. SCI, IF: 3.014
5. W.-T. Chiu, Y. Tahara, C.-Y. Chen, T.-F M. Chang, T. Hashimoto, H. Kurosu, and M.
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Sone, “Silk-Pt Composite Integration by Supercritical Carbon Dioxide Assisted
Electroless Plating for Medical Devices Application”, Microelectronic Engineering,175(C) (2017) 34-37. SCI, IF: 1.277

10.2.2 Domestic Journal
1. M. Sone, M. Sano, W.-T. Chiu, T.-F. M. Chang, H. Kurosu, and T. Hashimoto, “超臨界二酸化炭素を用いた生体適合性貴金属被覆高分子繊維の創成”, ケミカル
エンジニアリング, 6 (2016).
10.2.3 Invited Articles
1. W.-T. Chiu, M. Sano, C.-C. Wang, T.-F. M. Chang, C.-Y. Chen, H. Kurosu, M. Sone,“ 以 超 臨 界 二 氧 化 碳 無 電 鍍 達 成 應 用 於 穿 戴 式 裝 置 之 複 合 材 料 ”, ITRI
Materialsnet, October (2017).
10.2.4 Book
1. W.-T. Chiu, B.-H. Woo, M. Sano, T.-F. M. Chang, C.-Y. Chen, T. Hashimoto, H.
Kurosu, and M. Sone, “Highly Reliable Metallization on Polymer and Their
Fundamental Characteristics toward Wearable Devices Applications”, Advances in
Biosensors: Reviews, IFSA Publishing 2 (2018).
10.2.5 International Conference
1. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M. Sone,“Silk/Ni–P/TiO2 Composite Material Prepared by Supercritical CO2 Promoted
Electroless Plating for Flexible Photocatalytic Device Applications” 44th
International Conference on Micro and Nanoengineering, Copenhagen, Denmark,109

Chapter 10

September, 2018. (abstract accepted) *Oral presentation
2. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M. Sone,“Co-Deposition of Ni–P and TiO2 on Silk Textile by Supercritical CO2 Promoted
Electroless Plating for Gas Sensor Applications” The 2nd Taiwan-Japan Joint
Symposium, Taipei, Taiwan, May, 2018. *Best poster award
3. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M. Sone,“Co-deposition of Ni-P and P25 on Silk Textile by Supercritical CO2 Promoted
Electroless Plating for Flexible Photocatalyst Applications”, 22nd Topical Meeting of
the International Society of Electrochemistry, Tokyo, Japan, April, 2018.
4. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M. Sone,“Composite of ZnO/Au Hybrid Structure on Silk Textile for Flexible Photocatalyst
Application”, TMS 2018 147th Annual Meeting & Exhibition, Arizona, USA, March,2018. *Oral presentation
5. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M. Sone,“ZnO Synthesis on the Au Metallized Silk Textile for Flexible Photocatalyst
Application”, TMS 2018 147th Annual Meeting & Exhibition, Arizona, USA, March,2018. *Best poster award, second place
6. W-T Chiu, H. Tang, C.-Y. Chen, T.-F. M. Chang, H. Kurosu and M. Sone,“Realization of flexible and photocatalytic ZnO/Au/silk composite materials ascomponents in wearable devices”, 16th Symposium on Development of Supercritical
Fluid Technology and Application, Taichung, Taiwan, October, 2016.
7. W.-T. Chiu, Y. Tahara, C.-Y Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M.
Sone, “Highly Reliable Platinum Coating on Silk via an Efficient Supercritical CO 2
Promoted Electroless Plating for Biocompatible Wearable Devices”, TACT 2017
110

Chapter 10

International Thin Films Conference, Hualien, Taiwan, October, 2017. *Oral
presentation *Thin Film Society (TSF) Special Award (Oral presentation) * Taiwan
Association for Coating and Thin Film Technology (TACT) 2017 Gold Award
8. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M. Sone,“ZnO/Au Hybrid Layered Structure on Silk Textile Prepared by Supercritical Carbon
Dioxide Promoted Electrodeposition for Applications in Flexible Multifunction
Electronic Devices”, 43rd International Conference on Micro and Nano Engineering,
Braga, Portugal, September, 2017. *Final run in the poster competition
9. W.-T. Chiu, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M. Sone,“Metallization on Silk Utilizing Supercritical Carbon Dioxide Assisted Electroless
Plating for Wearable Device”, PRiME 2016/230th ECS Meeting, Hawaii, USA,
October, 2016.
10. W.-T. Chiu, Y. Tahara, C.-Y Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M.
Sone, “Fabrication of Pt Metallization on Silk via Supercritical Carbon OxideAssisted Electroless Plating for Wearable Medical Devices”, 2nd International
Conference and Expo on Separation Techniques, Valencia, Spain, September, 2016.
*Oral presentation
11. W.-T. Chiu, Y. Tahara, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M.
Sone, “Fabrication of Pt-Silk Composite Material by a Novel Catalyzation inSupercritical Carbon Dioxide for the Application of Medical Devices”, 42ndInternational Conference on Micro and Nano Engineering, Vienna, Austria,
September, 2016.
12. W.-T. Chiu, M. Sano, C.-Y. Chen, T.-F. M. Chang, T. Hashimoto, H. Kurosu, and M.
Sone, “Metallization of textiles by electroless plating with supercritical carbon
111

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dioxide for the applications in wearable devices”, 15th Symposium on Development ofSupercritical Fluid Technology and Application, Kaohsiung, Taiwan, September,2016. *Oral presentation *Best Paper Award, 15th Symposium on Development ofSupercritical Fluid Technology and Application, Taiwan Supercritical Fluid
Association (TSCFA)
13. T.-F. M. Chang, W.-T. Chiu, Y. Tahara, C.-Y. Chen, T. Hashimoto, H. Kurosu, and M.
Sone. “Fabrication of High Biocompatible Pt-Nylon and Pt-Silk Composite Materials
by Supercritical Carbon Dioxide-Assisted Metallization Method”, 4th International
Conference on Integrative Biology, Berlin, Germany, July, 2016.
14. W.-T. Chiu, Y. Tahara, C.-Y. Chen, T.-F. Mark Chang, T. Hashimoto, H. Kurosu, and
M. Sone, “Platinum Electroless Plating of Silk Utilizing Supercritical Carbon
Dioxide”, 19th Topical Meeting of the International Society of electrochemistry,
Auckland, New Zealand, April, 2016.
10.3 Research Experience
Tokyo Institute of Technology, Yokohama, Japan

Research assistant, Oct. 2015-now[Thesis project] Functionalization of silk textile via supercritical CO 2 promoted
electrochemical method(Granted by Grant-in-Aid for Scientific Research (JSPS KAKENHI Grant Number
17J07395) and the Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant
Number 26282013))
 Ni-P, Pt, and Au metallized silk textile via supercritical carbon dioxide promoted
electroless plating for wearable devices.
 Biocompatibility assessments of metallized silk composite materials by immersion
tests.
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 Electrical resistance, corrosion resistance, and adhesive strength evaluations of silkmetal materials for achieving highly reliable applications.
 ZnO fabrication on gold metallized silk by cathodic deposition and its photoelectrochemical evaluation for functional wearable devices.
[Project] Supercritical CO2 assisted Pt electroless plating on silk textile for wearable
device applications.
(Granted by Grant-in-Aid for Scientific Research (JSPS KAKENHI Grant Number
17J07395)
 Pt metallization on silk textile via supercritical CO2 promoted electroless plating
for the applications of wearable electronic and medical devices.
 Reliability assessments of the composite materials for fulfill the applications.
[Project] Construct the instrument for polymerization of polyaniline and atomic layergold insertion.
(Granted by Casio)
 Cooperation with Georgia Institute of Technology
 Constructed the devices for the polymerization of polyaniline and build up the flow
system for atomic gold layer insertion.
[Project] PET catalyzation under supercritical carbon dioxide for wearable device
applications.
(Granted by Industrial Technology Research Institute of Taiwan)
 Cooperation with Industrial Technology Research Institute of Taiwan
 Activated PET fiber by palladium(II) acetylacetonate using supercritical carbon
dioxide promoted catalyzation for the applications of wearable devices.
[Project] Investigate the chemical bonding between textile surface and catalyst by solid
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state NMR
(Granted by Grant-in-Aid for Scientific Research (B) (JSPS KAKENHI Grant Number
26282013))
 Cooperation with Nara Women's University
 Catalyzed silk and nylon 66 fabric via supercritical carbon dioxide catalyzation
technique and observe the catalyst-substrate interaction by NMR.
Georgia Institute of Technology, Atlanta, US

Visiting Research Student, Apr. 2017

 Polymerization of polyaniline by electrochemical method.
 Gold atomic layer deposition on the polyaniline and its performance in alcoholoxidation.

114

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10.4 Acknowledgement
Four years ago, I met Sone sensei in IUMRS international conference which was
hold in Taipei, Taiwan. That is the beginning of this three-year doctoral life. Before theentrance examination, I faced some difficulties, but Sone sensei was so friendly to me andgave me many assistances. Fortunately, I came to Sone lab after solving some troubles.
Sone sensei, as usual, is always kind to every students, and always has a good sense ineducating, communicating, and researching. Students in Sone lab usually have theopportunities to go to international conferences to explore our global prospect. I always
learnt a lot from the international conferences and expanded my academic contact all over
the world. I really appreciated these opportunities which I cannot have in other places.
Before graduating, I suffered from job hunting. Sone sensei gave me tons of assistances,and everything was settled down in the end. Without all the assistances from Sone sensei,
I wouldn't be able to survive from all the difficulties. There are still one billion of things
that I have to express my thanks to Sone sensei but one billion thanks still cannot convey
my gratitude to Sone sensei. Even though I am leaving Sone lab soon, I feel proud to bea member in Sone lab, and I hope Sone sensei can feel proud of me in the future.
Similarly, four years ago, I met Dr. Chang in IUMRS international conference.
Oh no, I think I met Dr. Chang earlier than the time I met Sone sensei. I met you when I
was a master student in Tsing Hua. Before entering Tokyo Tech, Dr. Chang already gave
me tons of assistances, they can be from life supports in Japan to academic issues. Without
your help, I cannot come to Sone lab that smoothly. After entering, every time I asked
you “do you have free time now? I would like to discuss something with you”, youranswers are always “YES”. You always gave me some good advices and helpfulcomments. You always have good sense in presentation and have a strong fundamental in
115

Chapter 10

electrochemistry and material sciences, I always benefit a lot from you. In daily life,
whenever there are any benefits, you always fight that for us such as international
conferences and some life supports. And Dr. Chen, you always have a good sense in sometrivial things which are always important but are always neglected by us. For example, I
cannot imagine that we go to the international conferences without you. You always
settled everybody down quickly and smoothly.
I would like to thank my previous advisors, who are Prof. Sinn-Wen Chen andProf. Yang-Yuan Chen in National Tsing Hua University and Academia Sinica. I reallyappreciated your understanding and the opportunity letting me be a member in your lab.
Without your helps, I would not be able to come to Japan and get my PhD degree.
I also want to express my appreciations to my dear friends in Sone lab and outof Sone lab. Tang and I always have some stupid conversations. It makes the daily life
funny and I always like to talk something stupid to you. I wish you have an even brighter
life than now in the future. My junior Teranishi-chan always supported me in learning
Japanese and always be on my side when I faced almost the greatest setback in my life
last year. My junior Ken Hashigata, without you, I might be in heaven right now, you area life saver lololol. I always like to talk to you, something serious and something stupid.
I will haunt you till I go to the tomb. And please remember to dress in pink when you
come to my funeral. All the master students in Sone lab, you always helped me intranslations. Every time I asked for your helps, I feel sorry to you since it was really time
consuming and troublesome. But you always helped my with the translation without any
complain. I really appreciate your kindness, especially Tei-kun, Igawa-kun, Nakajimakun, and Nitta-kun. It is really nice to have these cute juniors. My co-worker in Nakamoto
116

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lab PJ, oh well although I still cannot spell your name, but it is my pleasure to be yourfriend. You are a dedicated student, a good listener, and a good advice giver. I always liketo talk to you and I wish you a fruitful PhD life in the coming two years. Dr. Hsu, I always
learnt a lot of Japanese and the way living in Japan. It was always fun to have someconversions with you. And you always have a good sense in beauty, life, and foods.
Finally, I would like to express my gratitude to my family. Every time I went
back to Taiwan, you always let me be a piece of trash sleeping almost 16 hours per day
and feed me as a poultry. And thanks for not blocking me from being an international
student in Japan. Although I faced the greatest setback in Japan last year, I never regretthat I came to Japan. And I am sorry that I will not go back to Taiwan after my graduation.
But I believe that I will go back one day (probably…)

117

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10.5 Reference[1] K. Suganuma, Introduction,

Introduction to Printed Electronics, Springer2014, pp.

1-22.
[2] K. Hung, Y.-T. Zhang, B. Tai, Wearable medical devices for tele-home healthcare,Engineering in Medicine and Biology Society, 2004. IEMBS'04. 26th Annual
International Conference of the IEEE, IEEE, 2004, pp. 5384-5387.
[3] G.O. Mallory, J.B. Hajdu, Electroless plating: fundamentals and applications, William
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[4] J.R. Henry, Electroless (autocatalytic) plating, Metal finishing, 99 (2001) 424-435.
[5] M. Saito, Electrochemical Studies on Chemical Copper Plating, J. Metal. Finish. Soc.
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[6] K.M. Gorbunova, M.V. Ivanov, V.P. Moiseev, Electroless Deposition of Nickel‐
Boron Alloys Mechanism of Process, Structure, and Some Properties of Deposits, Journalof The Electrochemical Society, 120 (1973) 613-618.
[7] K. Holbrook, P. Twist, Hydrolysis of the borohydride ion catalysed by metal–boron
alloys, Journal of the Chemical Society A: Inorganic, Physical, Theoretical, (1971) 890894.
[8] F. Pearlstein, R. Weightman, Electroless Deposition of Ag Using Dimethylamine
Borane, Plating, 61 (1974) 154-157.
[9] Y. Okinaka, An Electrochemical Study of Electroless Gold‐Deposition Reaction,
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[10] I. Ohno, O. Wakabayashi, S. Haruyama, Anodic Oxidation of Reductants inElectroless Plating, Journal of The Electrochemical Society, 132 (1985) 2323-2330.
[11] J.E.A.M. Van Den Meerakker, On the mechanism of electroless plating. II. One
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mechanism for different reductants, Journal of Applied Electrochemistry, 11 (1981) 395400.
[12] C. De Minjer, P. vd Boom, The Nucleation with SnCl2‐PdCl2 Solutions of Glass
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[13] T. Osaka, H. Nagasaka, F. Goto, An electron diffraction study on mixed PdCl2/SnCl2
catalysts for electroless plating, Journal of The Electrochemical Society, 127 (1980) 23432346.
[14] T. Osaka, H. Takematsu, K. Nihei, A study on activation and acceleration by mixed
PdCl2/SnCl2 catalysts for electroless metal deposition, Journal of The Electrochemical
Society, 127 (1980) 1021-1029.
[15] M. Sano, Y. Tahara, C.-Y. Chen, T.-F.M. Chang, T. Hashimoto, H. Kurosu, T. Sato,
M. Sone, Application of supercritical carbon dioxide in catalyzation and Ni-P electrolessplating of nylon 6, 6 textile, Surface and Coatings Technology, 302 (2016) 336-343.
[16] W.-T. Chiu, Y. Tahara, C.-Y. Chen, T.-F.M. Chang, T. Hashimoto, H. Kurosu, M.
Sone, A Supercritical CO2 Promoted Electroless Ni-P Plating on Silk and Their
Fundamental Characteristics Investigations, Journal of The Electrochemical Society, 164(2017) D406-D411.
[17] M. McHugh, V. Krukonis, Supercritical fluid extraction: principles and practice,
Elsevier2013.
[18] S.P. Sawan, S.P. Sawan, Supercritical fluid cleaning: fundamentals, technology andapplications, Elsevier1998.
[19] G. Brunner, Gas extraction: an introduction to fundamentals of supercritical fluids
and the application to separation processes, Springer Science & Business Media2013.
119Chapter 10[20] H. Adachi, K. Taki, S. Nagamine, A. Yusa, M. Ohshima, Supercritical carbon dioxide
assisted electroless plating on thermoplastic polymers, The Journal of Supercritical Fluids,
49 (2009) 265-270.
[21] S. Tengsuwan, M. Ohshima, Electroless nickel plating on polypropylene via
hydrophilic modification and supercritical carbon dioxide Pd-complex infusion, The
Journal of Supercritical Fluids, 69 (2012) 117-123.
[22] T.-F.M. Chang, M. Sone, Function and mechanism of supercritical carbon dioxide
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