PCCP
PAPER

Cite this: Phys. Chem. Chem. Phys.,2015, 17, 32511

X-ray diﬀraction and high-resolution TEM
observations of biopolymer nanoskin-covered
metallic copper fine particles: preparative
conditions and surface oxidation states
Tetsu Yonezawa,* Yoshiki Uchida and Hiroki Tsukamoto
Metallic copper fine particles used for electro conductive pastes were prepared by the chemical
reduction of cupric oxide microparticles in the presence of gelatin. After reduction, the fine particles

Received 10th October 2015,
Accepted 11th November 2015

were collected by decantation with pH control and washing, followed by drying at a moderate

DOI: 10.1039/c5cp06107e

varied by altering the pH of the particle dispersion, as shown by X-ray diﬀraction and high-resolution

temperature. The surface oxidation state of the obtained copper fine particles could be considerablytransmission electron microscopy. Our results strongly indicate that decantation under a nitrogenwww.rsc.org/pccp

atmosphere can prevent the oxidation of copper fine particles but a slight oxidation was found.

Introduction
Owing to their unique properties, metal nanoparticles andfine particles have great potential to be used in developing
new products.1–4 Nanotechnology is currently used in diverse
applications, including construction and composite materials,5catalysts,6,7 antibacterial coatings,8 biosensing applications,9
plasmonic photo devices,10 electrical devices,11–13 and someconsumer products. The steady increase in the use of nanoparticles and fine particles has resulted in their increased
production and the development of handling and processing
techniques for such materials.
Additionally, the recent and remarkable progress in printed
electronics is based on the development of useful materials,
including nanomaterials.12–16 In particular, conductive inks
and pastes consisting of metal nanoparticles and fine particles
are key materials in printed electronics because they can beused in low-cost wet printing processes.12–15 Printing processes
using these modern technologies are expected to facilitate theinnovation of new manufacturing processes that will increase
the productivity and reduce the required energy and resource
consumption compared with the conventional processes. Indeed,
to produce flexible, lightweight, cost-effective and eco-friendly
electronic devices, such as smart labels using radiofrequency
identification (RFID), organic light-emitting diodes (OLEDs),
and flexible-displays, printed electronic technologies will befrequently used. Noble metals, especially silver nanoparticle
Division of Materials Science and Engineering, Faculty of Engineering,
Hokkaido University, Kita 13 Nishi 8, Kita-ku, Sapporo, 060-8628, Japan.
E-mail: tetsu@eng.hokudai.ac.jp

This journal is © the Owner Societies 2015

inks, are highly stable and commonly used in conducting
wires.17,18 However, high material costs limit their usage inlarge-scale printed electronic applications. The migration of silveris also an important issue that can degrade device performance.
Conducting polymers may represent another possibility, but inmany cases, their conductivity is not sufficient. In contrast,
copper nanoparticles and fine particles are very attractive fromthe cost and anti-migratory perspectives. Copper is an ideal
material for industrial-scale printed electronics. Therefore, inrecent years, the synthesis of copper nanoparticles and fineparticles has attracted substantial interest.19–22 However, they
can be oxidized, which limits their wide use. Additionally, in
printed electronic materials, the presence of copper oxides
on the particle surface is undesirable, increasing the required
sintering temperature and reducing the electro-conductivity.
To overcome the issue of copper nanoparticles and fineparticle oxidation, many attempts have been made to improve
the properties of the materials themselves and further develop
appropriate sintering technologies. One promising approach toprevent the oxidation of the particle surface is the applicationof a protective coating19 of thin layers of polymers,14,23–25 metaloxides,26 or noble metals.27 For conductive inks and pastes,
copper particles coated with organic polymers or molecules arehighly useful because they can be readily dispersed in a suitablemedium. The reported organic protective coating reagents, suchas PVP,23 CTAB,28 oleic acids,29 and alkane thiols, etc.,19 form anonconductive shell around the surface. Polymers are uniquecandidates for protective layers of copper fine particles because
polymer molecules attach to the particle surfaces at multiple
points. Therefore, because of multiple interactions between
the polymer molecules and the metal surface, these polymer

Phys. Chem. Chem. Phys., 2015, 17, 32511--32516 | 32511

Papermolecules cannot be easily detached from the surface. We
previously described one interesting example of such particles,
using the poly-ol reduction of copper ions to produce poly(ethylene
glycol)-stabilized copper nanoparticles.30 Many chemical reduction
processes have been performed using cupric salts, such as CuCl2
and CuSO4. However, for applications in electronics, the materials
should not be contaminated by halogens and sulfur. Alternatively,organic copper compounds, such as copper acetylacetonate andthe copper–AOT complex,19 can be used, although they are
relatively expensive. Because copper is a low-cost metal, using
expensive metal sources is inappropriate in industrial processes.
In our previous study, we proposed the use of gelatin as astabilizer and an antioxidizing reagent for copper nano- andfine particles prepared via the chemical reduction of cupricoxide (CuO) and a copper salt.11,24,31,32 CuO is a solid copper
source but contains only oxygen as the counter anion. Thus, interms of contamination, CuO is an optimal candidate metalsource for copper fine particles in conductive inks and pastes.
Furthermore, this process can be achieved with a copper
concentration of 1 mol dm 3 (64 g of Cu dm 3),24 which ismuch higher than that required in typical chemical reduction
processes. Using the produced gelatin-stabilized copper fineparticles, conductive pastes were prepared and used to generate theinner electrodes of multi-layered ceramic capacitors (MLCCs).11
Below, we describe the relationship between the surface
oxidation states of the gelatin-stabilized copper fine particles
and their preparative conditions. Following the reduction of
CuO by hydrazine under an ambient atmosphere, the particles
were collected by decantation and drying after adjusting thepH of the particle dispersion. We then controlled the experimental atmosphere to obtain metallic copper fine particles anddetermined their surface oxidation states by X-ray diﬀraction(XRD) and high-resolution transmission electron microscopy(HR-TEM).

PCCP

Fig. 1 Preparation procedures to obtain gelatin covered copper fineparticles from CuO microparticles by chemical reduction with hydrazine.
After the reduction, three processes were performed separately. Process A:
the pH of the obtained dispersion was decreased to 10 by adding saturated
aqueous citric acid under ambient conditions. Then, the collected particles
were washed with water and ethanol under ambient conditions. The washedparticles were dried at 60 1C under the nitrogen atmosphere. Process B: thepH of the obtained dispersion was decreased to 8.5 by addition of saturated
aqueous citric acid under ambient conditions. Then, the collected particles
were washed with water and ethanol under ambient conditions. The washedparticles were dried at 60 1C under the nitrogen atmosphere. Process C: thepH of the obtained dispersion was decreased to 8.5 by adding saturated
aqueous citric acid in a plastic bag under nitrogen flow. Then, the collectedparticles were washed with water and ethanol under nitrogen flow. The
washed particles were dried at 60 1C under the nitrogen atmosphere.

The metal source, CuO microparticles, was supplied by Nisshin
Chemco (Japan), and the average diameter of the microparticles
was approximately 4 mm. Hydrazine monohydrate, which wasused as a reducing reagent, and aqueous ammonia (28%),
which was used to adjust pH, were purchased from Wako or
Junsei Chemicals (Japan). Gelatin (bovine origin) was obtained
from Nitta Gelatin Inc. (Osaka, Japan). Water was purified
using an Organo/ELGA Purelab system (418 MO cm).

by hydrazine monohydrate (120 cm3) was carried out underambient conditions. Subsequently, the fine particles werecollected by decantation after adding saturated aqueous citricacid to reduce the pH of the dispersion. Decantation was performed under ambient conditions at pH = 10 (Process A) and 8.5(Process B) or under a nitrogen atmosphere at pH = 8.5 (Process C).
The solvents used for decantation in Process C were treated withnitrogen gas by bubbling (2 dm3 min 1) for 5 min before use.
The purification process was easily carried out in a large plastic
bag under nitrogen (B99.5%) flow. Then, the collected particles
were dried at 60 1C under the nitrogen atmosphere.

Preparation of copper fine particles

Characterisation

The detailed preparative procedure is described in Fig. 1. The
copper fine particles were prepared using a procedure similar
to that described previously.31,32 First, 32 g of gelatin was
dissolved in hot water (950 cm3). CuO microparticles (80 g) wereintroduced into the gelatin solution. Then, aqueous ammonia(28%) was introduced to adjust the pH of the dispersion at
approximately 11. The reduction of CuO (80 g) to metallic copper

Scanning electron microscopy (SEM) images of the particles
were collected using a JEOL JSM-6701F with an acceleration
voltage of 15 kV. XRD patterns were obtained using a Philips
PANalytical Pro (Cu Ka). TEM images were collected using
a Hitachi H-9500 with an acceleration voltage of 300 kV, and
HR-TEM images were obtained using a JEOL JEM-2010F with anacceleration voltage of 200 kV. Before XRD measurements andExperimental
Materials

32512 | Phys. Chem. Chem. Phys., 2015, 17, 32511--32516

This journal is © the Owner Societies 2015

PCCP

Paper

TEM observations, we kept our samples under nitrogen in order
to avoid oxidation in that period.

Results and discussion
Preparation of gelatin nanoskin-covered copper fine particles
The copper fine particles were prepared by the chemical
reduction of CuO microparticles by hydrazine, as previously
reported.31,32 In the presence of gelatin, the reduction proceedssmoothly by the addition of hydrazine monohydrate underalkaline conditions. The injection of hydrazine monohydrate
into the dispersion of CuO microparticles and gelatin instantly
changed the color of the dispersion from black to red-brown,
indicating that the reduction by hydrazine had occurred. As we
added NH4OH in the preparation dispersion, copper(II) ions are
extracted as copper–ammonium complexes into the solution.
Then, these complexes are reduced to metallic copper by hydrazine.
The prepared metallic copper particles were collected by decantation at pH = 10 (Process A) or 8.5 (Processes B and C) after the
reduction. No filtration or centrifugation was required to collectthe particles. Filtration and centrifugation are highly timeconsuming processes in the preparation of nanoparticles and fineparticles. The particles could be obtained readily and reproducibly.
In all cases, red-brown copper fine particles were obtained using
these preparation procedures. Following the reaction, the obtained
copper fine particles were collected by decantation after adjusting
the pH by adding saturated aqueous citric acid solution. The
reaction was carried out at pH = 11 to facilitate hydrazine
decomposition. Then, we adjusted the pH of the dispersion tocollect the particles by decantation. The isoelectric point ofgelatin (prepared under alkaline conditions) has been frequently
reported to be 4.7, and the second critical point gelatin occurs at
pH 7.7.33,34 The critical point at pH 7.7 is considered to be theisoelectric point of the sol forms of gelatin. When the pH value ofthe copper particle dispersion is lowered by adding saturated
aqueous citric acid, the electrostatic repulsion forces between
the particles decrease because of the ionic state of the gelatinmolecules and the increasing ionic strength of the dispersion(the salting-out effect). Then, at a lower pH, the copper fineparticles can aggregate and be readily collected by simple
decantation. In these experiments, copper particles could be
easily obtained by decantation at both pH = 10 and 8.5. Therefore, we used these two conditions in further experiments.

Fig. 2 Scanning electron microscopy (SEM) images and size distributionsof the obtained copper fine particles stabilized by a gelatin layer. (a)Process A: decantation was carried out at pH = 10 under an ambient
atmosphere. (b) Process B: decantation was carried out at pH = 8.5 under
an ambient atmosphere. (c) Process C: decantation was carried out at
pH = 8.5 under a nitrogen atmosphere.

Characterization of gelatin-layer covered copper fine particles

particle sizes, and the size distributions are similar to normal
distributions. The particle sizes did not change dramaticallyas the particle-collecting conditions after reduction werevaried. The average particle sizes obtained in this study wereca. 120–130 nm.
TEM images of the surfaces of the copper fine particles
prepared under diﬀerent conditions are presented in Fig. 3.
Thin smooth gelatin layers are readily evident on the particles’
surfaces. Comparison of Fig. 3(a) (Process A, decantation at
pH = 10) and Fig. 3(b) and (c) (Processes B and C, respectively:
decantation at pH = 8.5,) reveals that the thickness of thegelatin layer in Fig. 3(a) is clearly smaller than those in
Fig. 3(b) and (c). Adjusting the pH of the gelatin solution changes
the thickness of the gelatin layer: approximately 2.7 nm (ProcessA), 5.0 nm (Process B), and 6.5 nm (Process C). These findings
revealed that the thickness of the gelatin layer on the particle
surface can be well controlled by adjusting the pH at decantation. Because the isoelectric point of gelatin in sol is ca. pH = 7.7,
the gelatin molecules are almost neutral or slightly negative at
pH = 8.5 and are more negatively charged at pH = 10. Therefore,
the particles collected at pH = 8.5 are covered in a more dense
and thicker layer of gelatin than those collected at pH = 10. TGA
measurements of the copper particles obtained by Process C
indicated that the amount of gelatin covering on the particle
surface is about 2 wt% (data not shown).
Fig. 4 presents the XRD patterns of the copper fine particles
obtained via these three preparation procedures. Strong sharp
peaks corresponding to fcc metallic copper can be clearlyobserved in all patterns.35 However, in Fig. 4(a) (Process A),

No obvious oxidation of the particles was noted, and no color
change of the dry particles was observed for several weeks. CuO
and cuprous oxide (Cu2O) are usually black and red, respectively. Fig. 2 presents the SEM images and size distributionsof the obtained copper fine particles. The copper fine particles
are not perfectly spherical but, rather, polyhedral, as reported
previously.24,31,32 The particle surface was smoothly covered bygelatin and no obvious spots are observed on the particles’
surfaces. This suggests that no obvious oxidation occurred in
the ambient atmosphere. All samples exhibit fairly uniform

Fig. 3 Transmission electron microscopy (TEM) images of the surfaces ofthe obtained copper fine particles stabilized by a gelatin layer: (a) preparedby Process A, (b) prepared by Process B, and (c) prepared by Process C.

This journal is © the Owner Societies 2015

Phys. Chem. Chem. Phys., 2015, 17, 32511--32516 | 32513

Papertwo small peaks at 35.61 and 38.91, which correspond tothe (002) (35.4951) and (111) (38.7261) surfaces of CuO,36
respectively, are present. In the TEM image of the particles
prepared by Process A (Fig. 3(a)), two small spots are evident on
the particle surface (arrowed). In our previous studies, oxidative
annealing of the gelatin-stabilized copper fine particles resulted
in Cu2O spots on the surface.31,32 Cu2O is gradually oxidized toCuO under ambient conditions. The XRD pattern (Fig. 4(a)) andthe TEM image (Fig. 3(a)) of the copper fine particles preparedby Process A strongly suggest that CuO crystalline layers are
generated on the particle surface during the collection of theparticles. In contrast, the XRD patterns of the copper fine particles
collected at pH = 8.5 under ambient conditions (Fig. 4(b)) or thenitrogen atmosphere (Fig. 4(c)) do not show any peaks corresponding to copper oxides. Therefore, according to these XRD
patterns, the surfaces of the copper particles obtained at
pH = 10 are partially oxidized, whereas the surfaces of theparticles obtained at pH = 8.5 under both air nitrogen do notappear to have been oxidized to CuO.
However, if the oxidized grains are very small, the XRD peaksof the copper oxides become very broad and are diﬃcult to
identify. Therefore, other analytical methods should be applied
to reveal the existence of such small oxidized grains. For thispurpose, we chose HR-TEM to observe the surface of the copper
fine particles. Alternatively, selected area electron diﬀraction(SAED) could be a good technique to confirm copper oxidationon the surface. However, a careful observation of lattice fringes
can also provide useful information about oxidation, and theoxidized region can be directly identified in the images. The
HR-TEM images of the obtained copper fine particles arepresented in Fig. 5. The lattice fringes can be clearly observedin the images, indicating that all particles were well crystallized.
Their surfaces were covered by gelatin, as seen in the images,
especially in Fig. 5(a). Based on these images, we determined

Fig. 4 X-ray diﬀraction (XRD) patterns of the obtained copper fine particles stabilized by a gelatin layer: (a) prepared by Process A, (b) prepared byProcess B, and (c) prepared by Process C. Arrows on the pattern (a)indicate the peaks corresponding to CuO(002) and CuO(111).

32514 | Phys. Chem. Chem. Phys., 2015, 17, 32511--32516

PCCP

Fig. 5 High-resolution transmission electron microscopic (HR-TEM)
images of the surfaces of the gelatin-stabilized copper fine particles. (a)Prepared by Process A, (b) prepared by Process B, and (c) prepared byProcess C. Circles A–E indicate the area where the distances of the fringe
lattices were measured.

the distance between the two fringe lines, which corresponds tothe lattice parameters of the stable surfaces of the samples. In
the inner area of Fig. 5(a) (circle A), the average distance
between the two fringe lines is 2.13  0.11 Å. This distancecorresponds to a Cu(11% 1) lattice (2.087 Å)35 and shows that theparticles have metallic cores. In contrast, in the surface regionof circle B, the average distance between the two fringe lines is
2.52  0.11 Å, which corresponds to CuO(002) (2.53 Å) or
CuO(11% 1) (2.526 Å) lattices.36 Based on the XRD pattern shownin Fig. 4(a) and the HR-TEM image (Fig. 5(a)) some areas on thesurface of the particles obtained by decantation at pH = 10
under ambient conditions clearly exhibit oxidation.
We also obtained gelatin-stabilized copper fine particles bydecantation under ambient conditions after decreasing the pHof the dispersion to 8.5. This pH value is near the isoelectric
point of gelatin. The XRD pattern of these particles does notshow clear peaks indicating CuO and Cu2O,37 as observed inFig. 4(b). The oxide peaks were not observed after a couple ofmonths keeping under an ambient atmosphere as a powderform. Fig. 5(b) presents the HR-TEM image of the copper fineparticles collected by decantation at pH = 8.5. The gelatin layeron the surface of the particles is denser and thicker than thaton the particles obtained at pH = 10. The surface of the layer isalso smoother. Because the prepared copper fine particles werecollected at pH = 8.5, the gelatin molecules on the particle
surfaces exhibited stronger interactions with each other. Therefore, more gelatin molecules were adsorbed on the particle
surface, forming a thicker gelatin layer. The lattice fringes ofthis sample were also evaluated. Inside area of Fig. 5(b) (circle C),
the average distance between the two lattice fringe lines is
2.13  0.04 Å, corresponding to a Cu(111) lattice (2.087 Å),30
indicating that the particles have metallic cores. The distance
between the lattice fringes in three areas in the surface region
was also determined. The average distances between the two
fringes in circles D and F are 2.48  0.07 Å and 2.55  0.07 Å,
respectively. These values also correspond to CuO(002) (2.53 Å)This journal is © the Owner Societies 2015

PCCP

or CuO(11% 1) (2.526 Å) lattices.36 However, the average distanceof the two lattice fringes in circle E is 2.13  0.02 Å, whichcorresponds to a Cu(111) lattice.35 Interestingly, the surface ofthe copper particles is perfectly metallic, although this area issimply covered by a polymer layer. The lattice fringes in the
HR-TEM image strongly suggest that even at pH = 8.5, theoxidation of copper particles proceeded but was much slower
than that at pH = 10 because of the diﬀerence in the thicknessof the gelatin layer on the particle surface and also probably
because of its density.
During the reduction step in the preparation process, thedispersion, which contained hydrazine, was under reductive
conditions. CuO micro particles were reduced, resulting in
metallic copper fine particles. Hydrazine produces nitrogenand hydrogen gases during the reduction reaction. Therefore,
the surface of the copper metallic particles must not be oxidized
after the formation of the copper fine particles. However, after
the reduction, oxygen from the atmosphere began to dissolve inthe dispersion, and the gelatin molecules began to swell. Therefore, the surface of the obtained copper fine particles can be
gradually oxidized during decantation. The oxidation rate can bevaried by adjusting the density and the thickness of the gelatin
layer on the particle surface. Therefore, to prevent the surface
oxidation of the copper fine particles, we investigated theeﬀect of the atmosphere in contact with the dispersion during
decantation. We added saturated aqueous citric acid to theparticle dispersion and stirred the mixture under the nitrogen
atmosphere. The subsequent decantation was also performed
under nitrogen flow. For this purpose, the entire preparation
system was covered with a plastic bag, and nitrogen gas was
blown in from the top. Then, the obtained powder was dried
under the nitrogen atmosphere at 60 1C.
The XRD pattern of the obtained copper fine particles isshown in Fig. 4(c). Clear peaks corresponding to metallic
copper can be observed, but no peaks corresponding to copperoxides are present. This pattern suggests that the surface iscovered by metallic copper. The HR-TEM image of these particles
is shown in Fig. 5(c). The average distance between the two
fringes in circle G in the inner region is 2.13  0.09 Å, whichsuggests a Cu(111) lattice.35 In contrast, that in circle H in theparticle’s outermost region is 2.23  0.07 Å. This value is slightly
larger than that of a Cu(111) lattice but much smaller thanthose of CuO(002) (2.53 Å), CuO(11% 1) (2.526 Å),36 or Cu2O(111)(2.413 Å).37 Other areas in the surface region also show small
fringe distances. After a careful search on the literature it was
found that the spacing of the Cu64O(404) face is 2.215 Å.38 Thus,
it can be concluded that the particles obtained by decantation,
washing, and drying under the nitrogen atmosphere have metallic
structures or very slightly oxidized areas (Fig. 6). Additionally, it
can be noted that further surface oxidation can be prevented by
the dry gelatin layers.
Two important results can be extracted from these experimental findings. First, surface oxidation occurs after reductionin the aqueous dispersion, likely because of the swelling of thegelatin layer in the aqueous solution. Second, the gelatin layeron the copper particles’ surfaces strongly prevents the surface

This journal is © the Owner Societies 2015

Paper

Fig. 6 Schematic illustration of the relationship between the purificationand collection procedure and surface oxidation of the particles. Whencollection of the particles was performed by decantation at a higher
pH (10), the surface gelatin layer was thinner than that on the particles
collected by decantation at a lower pH (8.5). The surface oxidation wasobserved on the particles collected under an ambient atmosphere. The
particles collected at a lower pH showed partial surface oxidation. Slightsurface oxidation was observed when the particles were collected under anitrogen atmosphere.

oxidation of the dry particles. We previously reported that gelatincovered copper fine particles are quite stable and not easily
oxidized in air.24 However, in this study, we found that nonoxidized copper particles can be obtained, as shown in Fig. 5(c),even if they are kept under ambient conditions, if they are dried.
Gelatin is a cost-eﬀective and very useful polymer for nanoparticle synthesis. It is a polypeptide with amino groups,
mercaptans, and carboxylic acid groups in the side chains.
These functional groups can eﬀectively coordinate with theparticle surface. Therefore, the gold number of gelatin isextremely high, similar to that of poly(vinylpyrrolidone) (PVP),
which is frequently used for nanoparticle stabilization.2,23
The inter molecular interactions between the gelatin polymer
chains are also very strong because of the peptide groups in themain chain. Thus, the gelatin molecules should be densely
adsorbed on the copper particle surface. This unique propertyof gelatin is indispensable for its anti-oxidant protection of dry
copper fine particles under ambient conditions.

Conclusions
In conclusion, we prepared gelatin-coated copper fine particles
via three procedures after reduction. After reduction, decantation, washing, and drying steps were performed. The copper fineparticles obtained by decantation at pH = 10 under ambientconditions exhibit small CuO peaks in their XRD patterns,
indicating that the particle surface was oxidized. In contrast,
according to the lattice fringes observed by HR-TEM, decantation at pH = 8.5, washing, and drying under the nitrogen atmosphere resulted in copper fine particles stabilized by a gelatin
layer with non-oxidized or slightly oxidized to Cu64O surfaces.

Phys. Chem. Chem. Phys., 2015, 17, 32511--32516 | 32515

Paper

The obtained particles were not readily further oxidized underambient conditions in dry form. These copper fine particles are
suitable for printed electronics, for which the surface oxidation
state of the particles is very important. Our results also indicatethat XRD analyses cannot perfectly reveal the surface oxidationof copper fine particles.

Acknowledgements
This work is partially supported by NEDO and by KAKENHI(to TY, 24241041) from JSPS, Shinsei Foundation, and Murata
Foundation. Authors thank Prof. N. Sakaguchi (Hokkaido Univ.)
for his kind assistance in HR-TEM observation and Nitta Gelatin
Inc. for gelatin supply.

Notes and references
1 Nanoparticles, ed. G. Schmid, Wiley-VCH, Weinheim, 2011.
2 N. Toshima and T. Yonezawa, New J. Chem., 1998, 22, 1179–1201.
3 N. Li, P. Zhao and D. Astruc, Angew. Chem., Int. Ed., 2014, 53,
1756–1789.
4 T. Yonezawa, Kobunshi Ronbunshu, 2013, 70, 684–692.
5 S. Kango, S. Kalia, A. Celli, J. Njuguna, Y. Habibi and
R. Kumar, Prog. Polym. Sci., 2013, 38, 1232–1261.
6 N. Yan, C. Xiao and Y. Kou, Coord. Chem. Rev., 2010, 254,
1179–1218.
7 F. Viñes, J. R. B. Gomes and F. Illas, Chem. Soc. Rev., 2014,
43, 4922–4939.
8 S. Chernousova and M. Epple, Angew. Chem., Int. Ed., 2013,
52, 1636–1653.
9 K. Saha, S. S. Agasti, C. Kim and X. Li, Chem. Rev., 2012, 112,
2379–2779.
10 P. K. Jain, X. Huang, I. H. El-Sayed and M. A. El-Sayed,
Plasmonics, 2007, 2, 107–118.
11 T. Yonezawa, S. Takeoka, H. Kishi, K. Ida and M. Tomonari,
Nanotechnology, 2008, 19, 145706.
12 J. Perelaer, P. J. Smith, D. Mager, D. Soltman, S. K. Volkman,
V. Subramanian, J. G. Korvink and U. S. Schubert, J. Mater.
Chem., 2010, 20, 8446–8453.
13 A. C. Arias, J. D. MacKenzie, I. MacCulloch, J. Rivnay and
A. Salleo, Chem. Rev., 2010, 110, 3–24.
14 Y. Zhang, P. Zhu, G. Li, T. Zhao, X. Fu, R. Sun, F. Zhou andC.-p. Wong, ACS Appl. Mater. Interfaces, 2014, 6, 560–567.
15 D. Deng, Y. Jin, Y. Cheng, T. Qi and F. Xiao, ACS Appl. Mater.
Interfaces, 2013, 5, 3839–3846.

32516 | Phys. Chem. Chem. Phys., 2015, 17, 32511--32516

PCCP

16 Y.-C. Liao and Z.-K. Kao, ACS Appl. Mater. Interfaces, 2012, 4,
5109–5113.
17 J. A. Lewis and B. Y. Ahn, Nature, 2015, 518, 42–43.
18 W. Shen, X. Zhang, Q. Huang and Q. Xu, Nanoscale, 2014, 6,
1622–1628.
19 S. Magdassi, M. Grouchko and A. Kamyshny, Materials,
2010, 3, 4626–4638.
20 Y. Yong, T. Yonezawa, M. Matsubara and H. Tsukamoto,
J. Mater. Chem. C, 2015, 3, 5890–5895.
21 J. Xiong, Y. Wang, Q. Xue and X. Wu, Green Chem., 2011, 13,
900–904.
22 D. Mott, J. Galkowski, L. Wang, J. Luo and C.-J. Zhong,Langmuir, 2007, 23, 5740–5745.
23 Y. Lee, J.-r. Choi, K. J. Lee, N. E. Stott and D. Kim, Nanotechnology, 2008, 19, 415604.
24 M. Tomonari, K. Ida, T. Yamashita and T. Yonezawa,
J. Nanosci. Nanotechnol., 2008, 8, 2468.
25 T. Yonezawa, N. Nishida and A. Hyono, Chem. Lett., 2010,
39, 548–549.
26 Y. Kobayashi and T. Sakuraba, Colloids Surf., A, 2008, 317,
756–759.
27 M. Grouchko, A. Kamyshny and S. Magdassi, J. Mater.
Chem., 2009, 19, 3057–3062.
28 X.-F. Tang, Z.-G. Yang and W.-J. Wang, Colloids Surf., A,
2010, 360, 99–104.
29 G.-J. Lee, C. K. Kim, M. K. Lee and C. K. Rhee, Powder
Technol., 2014, 261, 143–146.
30 H. Kawasaki, Y. Kosaka, Y. Myojin, T. Narushima,
T. Yonezawa and R. Arakawa, Chem. Commun., 2011, 47,
7740–7742.
31 M. Matsubara, T. Yonezawa and H. Tsukamoto, Bull. Chem.
Soc. Jpn., DOI: 10.1246/bcsj.20150305.
32 T. Yonezawa, H. Tsukamoto and M. Matsubara, RSC Adv.,
2015, 5, 61290–61297.
33 J. H. Highberger, J. Am. Chem. Soc., 1939, 61, 2302–2303.
34 B. Gaihre, M. S. Khil, D. R. Lee and H. K. Kim, Int.
J. Pharmacol., 2009, 365, 180–189.
35 R. P. van Ingen, R. H. J. Fastenau and E. J. Mittemeijer,
J. Appl. Phys., 1994, 76, 1871–1883.
36 S. Åsbrink and A. Waskowska, J. Phys.: Condens. Matter,
1991, 3, 8173–8180.
37 A. Werner and H. D. Hochheimer, Phys. Rev. B: Condens.
Matter Mater. Phys., 1982, 25, 5929–5934.
38 R. Guan, H. Hashimoto and K. H. Kuo, Acta Crystallogr.,
Sect. B: Struct. Sci., 1985, 41, 219–225.

This journal is © the Owner Societies 2015