ྫ࿪ 4 ೧ౕ ദ࢞࿨ชPreparation of TiO2-based thin films with improved
photocatalytic activity and super-hydrophobicity by plasmaenhanced chemical vapor depositionʤϕϧθϜ CVD ๑ͶΓΖ߶͏ޭৰഖ‫׈‬੓ͳ௔૆ਭ੓Ν༙ͤΖ
TiO2 മຜ͹ࡠ੣ʥࢨ಍‫گ‬ҽ: ౣీ ָ ‫گ‬दSupervisor: Manabu Shimada

Graduate School of Engineering Hiroshima University
Department of Chemical Engineering

Lang Jianghua
2022 ⵳ 9 㘇

Abstract
The plasma enhanced chemical vapor deposition (PECVD) process is a practicalmethod in film deposition thanks to its simple operation step and relatively low cost.
TiO2 films with photocatalytic activity have been successfully deposited by PECVD
method. In addition, different functional materials can be deposited with TiO2
simultaneously by PECVD method. By introducing foreign materials, the TiO2-based
composite exhibits improved and new properties. In this dissertation, efforts have been
made to improve the photocatalytic activity of TiO2 thin film. In addition, a TiO2
composite film with super-hydrophobic surface was fabricated by introducing a
polymer.
Chapter 1 demonstrated the background, motivation, review of previous researchand objectives of this research.
In Chapter 2, a TiO2-CNT-Ag ternary composite film was successfully synthesizedusing the PECVD by simultaneously feeding a carbon nanotube (CNT)/Ag nanoparticle
suspension and titanium tetraisopropoxide (TTIP) gas. Here, Ag nanoparticles wereprepared from AgNO3 solution by a co-precipitant in advance. During deposition,
CNT/Ag suspension was spray-dried to generate those nanoparticles. The obtained
CNT/Ag nanoparticles were fed with TTIP simultaneously. The as-prepared film wasdetermined by scanning electron microscopy (SEM), transmission electron microscopy(TEM),

energy-dispersive

X-ray

spectroscopy(EDS),

X-ray

photoelectron

spectroscopy (XPS), X-ray diffraction (XRD), and ultraviolet (UV)-visible
spectroscopy. Moreover, the Ag/Ti ratio of the film was confirmed using an inductivity
coupled plasma optical emission spectrometer. The performance of the TiO2-composite
film for the degradation of Rhodamine 6G under simulated solar light irradiation was
evaluated. The ternary structure that CNT and Ag nanoparticles were embedded intoI

TiO2 matrix was confirmed. The rate constant of the prepared TiO2-CNT-Ag for
Rhodamine 6G degradation was approximately 1.8 times that of prepared TiO2. This
result indicates that the addition of CNT and Ag significantly improved thephotocatalytic activity of the prepared films.
In Chapter 3, a TiO2-Ag nanoparticle composite films by PECVD using a mixtureof aerosolized droplets of AgNO3 solution, which was used as an Ag nanoparticle
precursor, and TTIP, which acted as the TiO2 precursor. Notably, the use of PECVD
eliminated the need of pre-preparing Ag nanoparticles, thus increasing the process
efficiency. The structures and morphologies of the deposited films were characterized
by UV-visible spectroscopy, XRD, XPS, SEM, TEM, and EDS, and the effects of the
raw material concentration on the photocatalytic activity of the deposited films were
determined by assessing the degradation of methylene blue under UV-light irradiation.
The Ag ions were successfully reduced to metallic nanoparticles and embedded in the
TiO2 film, and the best photocatalytic activity was achieved for a TiO2 composite film
prepared by 1wt% AgNO3. Importantly, the photocatalytic activity was 1.75 times thatof pristine TiO2.
Chapter4

presents

thedepositionofasuper-hydrophobic

hybridTiO2/Polydimethylsiloxane (PDMS) film by supplying PDMS and TTIP vapor via
PECVD method. Two different experimental conditions are performed to fabricate thecomposite film. It is worth noting that no curing agent was used in this work to
immobilize the polymer. The prepared composite films were characterized by SEM,
TEM-EDS, FTIR, XPS, and XRD. The hydrophobicity was estimated by recording thewater droplets staying on the film surface using a digital microscope. The photocatalytic
activity of the composite films was determined by MB degradation under UV lightirradiation. The TiO2/PDMS composite films prepared under one condition showsIIalmost identical photocatalytic activity with TiO2 film, about 29% MB was degradedafter 6 hours. However, many products lost their super-hydrophobicity. Under anothercondition, a TiO2/PDMS composite film with durable super-hydrophobicity was
obtained. And the composite film degrades 16% MB under UV light irradiation.
Chapter 5 summarizes the content of this dissertation. Suggestions for future
research in film fabrication by PECVD method are also proposed.

III

IV

Contents
Contents ........................................................................................ 澳
1. Introduction ............................................................................ 1澳
1.1 Introduction of photocatalysts.................................................................................. 1澳
1.2 TiO2-based photocatalysts........................................................................................ 2澳
1.2.1 Photocatalysts ............................................................................................... 2澳
1.2.2 TiO2-based photocatalysts............................................................................. 3澳
1.2.3 TiO2-based photocatalysts with super-hydrophobicity ................................. 6澳
1.3 Film deposition method ........................................................................................... 7澳
1.3.1 Solution-based chemistry method ................................................................. 8澳
1.3.2 Physical vapor deposition (PVD) method................................................... 10澳
1.3.3 CVD method ............................................................................................... 12澳
1.3.4 Advantages and limitations of film deposition method .............................. 13澳
1.4 Reviews of materials reported in the dissertation .................................................. 15澳
1.4.1 Preparation of TiO2-CNT-Ag ternary composite films ............................... 15澳
1.4.2 Preparation of TiO2-Ag from AgNO3 reduction ......................................... 16澳
1.4.3 Preparation of super-hydrophobic TiO2/PDMS composites. ...................... 17澳
1.5 Objectives and outline of the dissertation .............................................................. 19澳
References .................................................................................................................... 21澳

2. Preparation of TiO2-CNT-Ag Ternary Composite Film with
Improved Photocatalytic Activity via Plasma-Enhanced
Chemical Vapor Deposition .................................................... 31澳
2.1 Introduction ............................................................................................................ 31澳
V

2.2. Experimental Section ............................................................................................ 32澳
2.2.1 Preparation of CNT and Ag suspensions .................................................... 32澳
2.2.2 Experimental setup...................................................................................... 33澳
2.2.3 Preparation of P25 film ............................................................................... 35澳
2.2.4 Film characterization .................................................................................. 35澳
2.2.5 Evaluation of photocatalytic activity .......................................................... 36澳
2.3. Results and discussion .......................................................................................... 37澳
2.3.1 Morphology................................................................................................. 37澳
2.3.2 Elements states ............................................................................................ 44澳
2.3.3 Crystalline ................................................................................................... 45澳
2.3.4 Optical properties ........................................................................................ 47澳
2.3.5 Elements analysis ........................................................................................ 48澳
2.3.6 Photocatalytic activity of the films ............................................................. 49澳
2.4 Conclusions ............................................................................................................ 53澳
References .................................................................................................................... 54澳

3. Ag-Doped TiO2 Composite Films Prepared Using AerosolAssisted, Plasma-Enhanced Chemical Vapor Deposition .... 57澳
3.1 Introduction ............................................................................................................ 57澳
3.2 Experimental section .............................................................................................. 58澳
3.2.1 Preparation method ..................................................................................... 58澳
3.3.2 Film characterization .................................................................................. 58澳
3.2.3 Evaluation of photocatalytic activity .......................................................... 58澳
3.3 Results and Discussion .......................................................................................... 59澳
3.2.1 Crystalline ................................................................................................... 59澳
3.3.2 Morphology................................................................................................. 60澳
VI

3.3.2 Elements analysis ........................................................................................ 62澳
3.2.4 Optical properties ........................................................................................ 64澳
3.3.5 Photocatalytic activity ................................................................................. 66澳
3.4 Conclusions ............................................................................................................ 68澳
References .................................................................................................................... 69澳

4. Preparation of Super-hydrophobic and Photocatalytic
TiO2/PDMS Films by Plasma-Enhanced Chemical Vapor
Deposition ................................................................................. 71澳
4.1 Introduction ............................................................................................................ 71澳
4.2 Experimental Section ............................................................................................. 72澳
4.2.1 Experimental setup...................................................................................... 72澳
4.2.2 Characterization .......................................................................................... 74澳
4.2.3 Evaluation of film properties. ..................................................................... 74澳
4.3 Results and discussion ........................................................................................... 74澳
4.3.1 Film prepared under Condition A ............................................................... 74澳
4.3.1 Film prepared under condition B ................................................................ 83澳
4.4 Conclusions ............................................................................................................ 88澳
References .................................................................................................................... 89澳

5. Summary ............................................................................... 91澳
5.1 Conclusions ............................................................................................................ 91澳
5.2 Suggestions for further study ................................................................................. 92澳

Appendix Ⅰ˖˖List of Figures ................................................... 95澳
Appendix Ⅱ˖List of Tables .................................................... 99澳
VIIAppendix Ⅲ˖
˖List of Publications ...................................... 100澳Presentation in Conferences ................................................. 101澳

VIII

Chapter 1
Introduction
1.1 Introduction of photocatalysts
The progress of technology drives the progress of human society, improving theability to utilization of natural resources, and enabling human life to become more andmore convenient. However, human activities have also caused severe environmental
pollutants. Especially, water pollution has become an urgent problem to be solved.
Numerous organic pollutants are being released from manufacturers such as textile,
paint, and leather into water bodies. The aquatic systems and human health may beseriously affected by these organic effluents, which are extremely difficult to be
degraded or eliminated by nature. Sustainable Development Goal (SDG) 6 is to ensure
the availability of water and sustainable management of water and sanitation for all. To
achieve this goal, environmentally friendly, affordable, reliable, sustainable, andmodern water treatment technologies are required to be developed. Photocatalysis,
which can utilize solar energy to degrade pollutant, is one of the promised technologies.
The term “photocatalysis” is the acceleration of a photoreaction in the presence of acatalyst (Spasiano et al., 2015).
A wide range of photocatalysts was developed. Transition-metal oxide and theircomposites, like TiO2 (Yan et al., 2012), ZnO (Chen et al., 2017; Tian et al., 2012),
WO3 (Fu et al., 2015; Loka et al., 2021; Song et al., 2010), and SnO2 (Chu et al., 2011;
Huang et al., 2014; Lin et al., 2012; Yuan et al., 2010) are able to be active under solar
irradiation and produce photoinduced electron. This ability was regarded as the key
factor during the photocatalytic process. Among the above-mentioned transition-metaloxide materials, TiO2 is notable because of its chemical stability, nontoxicity, and high1

photocatalytic activity (Li et al., 2019; Sobczyk-Guzenda et al., 2015).

1.2 TiO2-based photocatalysts
1.2.1 Photocatalysts
Fig. 1.1 is the common opinion of the schematic that is used for explaining
photocatalysis in literature (Chong et al., 2010). Photocatalytic activity was initiatedwhen light irradiation energy is matching or greater than the bandgap (Eg) ofsemiconductor. The electrons (e-) on valence band (VB) are excited to conduction band(CB) and leaving positive holes (h+). The holes and electrons promote the reduction andoxidation, respectively, of different substance (absorbed O2 and H2O on TiO2 surface).
This process results in the formation of powerful oxidizing agents, like ∙OH and O2−.
These radicals play a crucial role in organic decomposition.

Fig. 1.1 Photo-induced mechanism of electron-hole pair in TiO2 particle withpowerful oxidizing agents.

However, its photocatalytic activity is limited by the following factors. First, due
2

to the large band gap (3.2 eV) of TiO2 (anatase phase), its photocatalytic activity can beinitiated only under ultraviolet light irradiation (3%–5% of the total solar spectrum)(Rapsomanikis et al., 2014; Varshney et al., 2016). Second, the fast recombination ofelectron–hole pairs results in low efficiency for photocatalytic activity (Lee et al., 2005).
These two challenges attract considerable attention and promote a broad range of
researchers to develop heterojunction materials. Various heterojunction materials havedemonstrated advantages properties such as fast charge separation, favorable electrontransfer (Zhu et al., 2022), hindering of electron-hole recombination (Hieu et al., 2021),
and a high surface area (Hao et al., 2016). Hence, TiO2-based heterojunction materials
have attracted considerable attention for their improved photocatalytic activity.
Besides the research on photocatalytic activity improvement of TiO2, efforts havebeen made on different applications of TiO2. The wetting properties of droplets on TiO2formed surfaces can be switched under light inducing (Xu et al., 2013). It becomeshydrophilic after UV light irradiation and reversible after storage in dark or heat
treatment. Thus, the fabrication of TiO2 hybrid surfaces with stable hydrophobicity
gained increasing interest in recent years. These surfaces have been used in a wide range
of applications, like building materials, oil-water separation, and metal protector.

1.2.2 TiO2-based photocatalysts
Researchers have already successfully improved the TiO2 photocatalytic activities
by utilizing various materials. Especially noble metals, such as Ag, Au, Cu and nonmetals such as nitrogen (N), carbon nanotubes (CNTs) are popular and common
materials to collaborate with TiO2, which can increase its capacity for visible lightabsorption or its reactivity in the UV spectrum (Basavarajappa et al., 2020).

3

Fig. 1.2 The possible schematic of electron-hole pairs generated in TiO2-Ag
composite: (a) under UV light irradiation; (b) under visible light irradiation.

The optical properties of TiO2 can be significantly influenced by using noble metal
nanoparticles. The noble metal nanoparticles (such as Ag, Au) exhibit the localized
surface plasmon resonance (LSPR) effect, which can contribute to visible lightabsorption after being coupled with TiO2 (Al-Hajji et al., 2020; Herderick et al., 2007;
Jiang et al., 2012; Yang et al., 2015). Among all the noble metals, Ag was a reliablecandidate in terms of its relative stability, low cost, resource abundance, and nontoxic
compared to other noble metals (Wang et al., 2016). Researchers believe Ag
nanoparticles can produce electron-hole pairs under visible light due to LSPR. The
strong electron oscillating collectively on the LSPR excitation can overcome the
Schottky barrier (formed on the interface between TiO2 and Ag), thus allowing theelectrons transfer from noble metal to CB of TiO2 (Liu et al., 2015). Moreover, TiO2
can be excited under UV light irradiation and form electrons and holes on VB and CB,
respectively. The CB energy level of TiO2 is still higher than the fermi level of metal,
so that electrons on CB can transfer to metal. The formation of the Schottky barrier on
the interface of TiO2 and Ag prevents the recombination of electron-hole pairs, thereby
4enhancing the photocatalytic activity. Fig. 1.2 demonstrate the possible schematic ofelectron-hole pairs generated in TiO2-Ag composite under different light source.
By forming an intermediate energy level between the CB of dopants and the CB
of TiO2, using non-metal, such as CNTs and nitrogen is another viable approach to
improve the photocatalytic activity of TiO2. Especially, CNTs is well-known for its
unique structure, electrical, thermal and chemical properties (Koo et al., 2014). It isnoticed that CNTs were found to have synergistic effects in organic degradation whencollaborating with TiO2 (Cao et al., 2013), which helps to enhance electron-hole
separation (An et al., 2007). Nevertheless, different photocatalysts mechanisms of
TiO2/CNT were reported (Woan et al., 2009). The common idea believes CNTs serveas the electron scavenger when the excited electrons transfer from the conduction bandof TiO2 to CNTs, as shown in Fig. 1.3 (a). Some researchers argued that CNTs can act
as photosensitizer in composite. In this model, CNTs donate the electrons to CB of TiO2,
instead of accepting the electrons from CB of TiO2. After that, positively charged CNT
attracts the electron from the VB of TiO2, as shown in Fig. 1.3 (b). Furthermore,
researchers also confirmed the existence of Ti-O-C bonds, which allows the lightabsorption towards to longer wavelengths, herein improved the photocatalytic activityof TiO2 (Pyrgiotakis et al., 2005), as shown in Fig. 1.3 (c).

5Fig. 1.3 The proposed mechanism of CNT driven improvement of photocatalytic
activity: (a) CNTs act as an electron acceptor, prevent the electron-hole pair
combination; (b) CNTs were regarded as a photosensitizer, generate the electronhole pair; (c) The formation of Ti-O-C bonds.

1.2.3 TiO2-based photocatalysts with super-hydrophobicity
The super-hydrophobic phenomenon is common in nature, which allows the water
droplets rolling off the surface and take away the contaminants. However, this property
becomes invalid when dealing with some organic pollutants. Photocatalysts candegrade pollutants and promise the recovery of super-hydrophobic properties.
The surface interacts with water is evaluated by measuring the contact angle
6

between them, as shown in Fig. 1.4. Generally, super-hydrophobic surfaces are defined
as water contact angle above 150°. According to the literatures, the key factors informing a super-hydrophobic surface are concluded as low surface energy andmultiscale structure (Wang et al., 2021; Wang et al., 2019). Thus, the common strategies
to fabricate super-hydrophobic surfaces are preparing a rough structure by low surface
energy materials directly or preparing a rough structure using low surface energy as amodification. Alkyl and fluorinated alkyl groups, like CH3 and CF3 are typical groups
that have a low energy of interaction with water (Crick et al., 2010). The hydrophobic
properties of polydimethylsiloxane (PDMS) have been reported and its chemical
inertness is crucial for the resulting material (Wang et al., 2019).

Fig. 1.4 The schematic of contact angle between the droplet and substrate.

1.3 Film deposition method
A catalyst film sometimes overcomes some drawbacks encountered with powder.
For example, the form of film (1) eliminates the need for catalyst separation andfiltration, (2) enables application in continuous flow systems, and (3) reduces
agglomeration at various catalyst loadings (Arabatzis et al., 2003; Lang et al., 2022).
Thus, the technologies for immobilizing the photocatalyst into the form of film draw
considerable attention. Various film deposition methods have been developed which
7can be classified as physical vapor deposition (PVD), Chemical vapor deposition (CVD)and solution-based chemistry. Some common preparation methods are illustrated in Fig.
1.5 (Ukoba et al., 2018).

Fig. 1.5 Simple classification of some common thin film preparation method.

1.3.1 Solution-based chemistry method
1.3.1.1. Sol-gel Method
Sol-gel method was also named soft chemistry. During the process, a sol or a gel
as an intermediate reactive was produced by hydrolysis and condensation from thesolution precursor. Then, evaporation and heat treatment methods enable the dense
ceramic to be obtained from a gel (Znaidi, 2010). Utilizing the matter transformation inthis process, the sol can be immobilized on the substrate by spin coating or dip coating,resulting in a thin film.
Nanostructured titania thin films were prepared by sol-gel combinate with dip
coating method (Simonenko et al., 2016). A titania precursor was fabricated by using8

titanium butoxide, acetylacetone, n-butanol and ethanol. Then, the film was formed byapplying the precursor to the silica substrate via dip-coating method. The hydrolysis
and polycondensation were proceeded for 60 min under air to generate the threedimension network. A crystallized titania film was obtained after heat treatment.

1.3.1.2 Hydrothermal Method
Hydrothermal method has been widely used in recent years, which is simulated
the formation of some minerals and rocks in nature. A steel pressure vessel, also called
autoclave, is always used as the reactor, where the reaction process can be performed
under controlled temperature and/or pressure (Yang et al., 2019). In addition, stable
crystalline phase films can be formed by hydrothermal treatment of a substrate solution
impregnated with the substrate.
Using the hydrothermal method, Wu et al. (2015) grew a high-quality ZnO thinfilm on sapphire. In this process, ZnO buffer layer on a c-sapphire substrate is preparedby radio-frequency molecular beam epitaxy. The obtained samples were submerged in
an aqueous solution of Zn(NO3)2 and HMTA in a Teflon holder. Hydrothermal process
is carried out under 75 ℃ for 4 h. As a result, a large-area film with smooth surface wassynthesized on a sapphire substrate.

1.3.1.3 Spray pyrolysis
Spray pyrolysis is becoming popular in thin film deposition. Spray pyrolysis
entails spraying target solutions on a heated surface to form the film. The constituentsof the film then react to generate a chemical compound. The chemical reactants arechosen so that undesirable products breakdown pyrolytically at the deposition
temperature. In addition, ultrasonic, electrostatic and air blast are always selected as the9atomizer for the spay method.
Deyu et al. (2019) achieved the Al-doped SnO2 film deposited by ultrasonic spraypyrolysis. An SnCl4·5H2O/methanol solution was employed as the precursor. In
addition, different aluminum precursor was also added in a certain concentration.
Assisted by a homemade ultrasonic spray pyrolysis deposition system, the different
precursor was deposited on substrates. The process was carried out under 500 ℃. The
aluminum doped SnO2 thin films with small grain size were obtained.

1.3.2 Physical vapor deposition (PVD) method
Physical vapor deposition (PVD) has been widely used in thin film deposition.
Every PVD process involves three basic steps: (1) the vapor phase species creation, (2)vapor phase species transportation and (3) film growth or deposition on a substrate. In
this method, the conversion of materials from solid-phase to vapor-phase can beachieved by thermal evaporation, sputtering (Abegunde et al., 2019).

1.3.2.1 Thermal Evaporation
In an evaporation process, a vapor is created by heating the target materials to very
high temperature, following transport of the vapor to the substrate in a vacuum. A thinfilm is obtained after the vapor is condensed on the substrate.
Luo et al. (2016) reported a PVD system for CdTe film deposition. The systempressure was maintained at about 0.9 kPa, the temperature for vaporizing CdTe powder
was about 700 ℃, and the temperature of substrate was 460 ℃. Since the different
temperatures between the substrate and CdTe vapor, the recrystallization occurred on
the substrate. Consequently, a polycrystalline with a cubic structure film was achieved.

101.3.2.1 Sputtering
In a sputtering process, a vapor source was created by high energy ion
bombardment on the target. Because of the potential difference between the negatively
biased target (cathode) and a node, inert gas, most often argon gas, was charged andaccelerated. The contact of the ions with the surface of the target causes the ejection of
atoms that condense on a substrate to form a film (Sarakinos et al., 2010).
Gao et al. (2004) introduced a ZnO thin films deposited in a magnetron sputtering.
The deposition process was carried out in approximately 2×10−6 Torr. Argon was
introduced into the chamber. Sputtering was performed with direct current power of
0.25 A for direct deposition form ZnO target. The resulting films have a high crystal
quality and a big grain size.

1.3.2.3 Pulsed Laser deposition
Primarily, pulsed laser deposition involves focusing a high-powered laser beam ona revolving target within a vacuum chamber. Because of the high energy of the focus
beam, the chemical bonds in the target were broken, which caused the particles (ions,electrons, atoms, radicals, or clusters) to "escape" from the target. After making contact
with the substrate, these particles become adhered to it. Both diffusion and aggregationof particles can lead to the formation of these films (Vanalakar et al., 2015).
A NiO thin film was generated using pulsed laser deposition on Si substrate(Fasaki et al., 2010). The reactive pulsed laser deposition experiments were carried out
in a stainless-steel vacuum chamber. Pulses with certain laser fluence was focused on
the surface of Ni targets. The films grow on the Si substrates with a thermally oxidized
layer of Si substrate. A film with constant thickness and less grain boundaries was
deposited by controlling the substrate temperature.
11

1.3.3 CVD method
CVD method is a powerful technique for film deposition. It basically starts fromthe evaporation and activation of raw materials. Then one of two types of reactions canoccur: homogeneous gas-phase reactions, which take place in the gas phase, andheterogeneous chemical reactions, which take place on/near a heated surface. After that,a film is generated on a suitable substrate (Munoz et al., 2013). Thermal, laser, andplasma is common energy source for chemical reactions.

1.3.3.1 Thermal CVD
Thermal CVD (TCVD) requests the substrate to be heated to high temperature,
therefore meeting the energy needed for reaction. The mixture gases including reactants
and carrier gas were fed into the reaction chamber, followed by the reaction taking place
and the deposition started.
Tang et al. (2014) reported a TCVD method for the preparation of TiO2 coating.
The TiCl4 vapor was fed into the reactor with carrier gas and reactant. The film was
generated with the substrate temperature upper 650 ℃. The resulting films consist ofsmall particles and island structures.

1.3.3.2 Atomic layer deposition
Atomic layer deposition (ALD) has emerged as a widely used surface technology,
which can also be regarded as pulsed CVD. Most ALD is based on the sequential
alternating pulses of precursor that react with the substrate (Johnson et al., 2014). For
one cycle, only a finite number of reactants are allowed to be deposited on the substrate,
which restricts each cycle deposit a single layer. Hence, the different cycles can be
performed sequentially to deposit thickness-desired thin film.
12

Aaltonen et al. (2011) reported a Li2O-Al2O3 thin film deposited by ALD. Two
different alternate pulsing of precursor, trimethyl aluminum and O3, LiOtBu and H2O
were supplied sequentially. The Al2O3 thin layer was generated as the base layer for
Li2O-Al2O3 composite film.

1.3.3.3 Plasma enhanced CVD
Plasma enhanced CVD (PECVD) is a popular technique since it inherits thebenefits of CVD technology and enables the film deposition at lower temperatures. In
PECVD, the temperature of a chemical initiator was instead by the free plasma radicals.
These plasma radicals in the reactor chamber can decompose the precursor even at a
low temperature and leads to the deposition of thin film.
Borras et al. (2009) reported a PECVD method for the growth of TiO2 thin films.
The Ar/O2 gas was employed to generate the plasma with a power of 400 W. Titanium
isopropoxide (TTIP) was supplied as a precursor. A TiO2 film in the form of anatase
was prepared by PECVD. The growth of the film can be described in four stages:
nucleation, coalescence, column formation and column development, which is also
named as Kolmogorov model.

1.3.4 Advantages and limitations of film deposition method
The preparation method and processing conditions play an essential role inmorphologies and properties of the thin film. Thus, a proper method is necessary tomeet the requirement. Here, the advantages and limitations of the mentioned method
are summarized.
The sol-gel method is a controllable method that allows the deposition of largearea films with good reproducibility. However, the requirement of a long period and13relatively high temperature limits its application in film preparation. The hydrothermal
method can produce a film with high crystallinity. However, the processes are difficult
to control since it happens in an autoclave. The spray pyrolysis is a simple, costeffective method. However, it is a challenge to deposit the film with uniform thickness.
The thermal evaporation PVD method is suitable for any metal materials, but thedisadvantages such as high cost cannot be ignored. The sputtering deposition ensures
the high quality of the film. However, the effect of ionic bombardment on the substrateduring deposition must be considered. PLD is another simple method to deposit thinmetal films. But the emission of particulates from target is the main drawback of thismethod.
TCVD method has some advantages such as producing a film with desired
morphology and high durability. However, the requirement for high deposition
temperature needs to be addressed. ALD method is exceptionally effective at depositing
multilayer films with excellent adhesion. However, ALD is facing challenges like highmaterials waste rate. PECVD is suitable for deposition on multicomponent film in ahigh deposition rate and relatively low cost. However, it also faces the high materials
waste rate.

14

1.4 Reviews of materials reported in the dissertation
1.4.1 Preparation of TiO2-CNT-Ag ternary composite films
As mentioned in 1.2.2, CNT and Ag are reliable materials to improve thephotocatalytic activity of TiO2. Researchers found that a ternary structure utilizing CNT,
Ag, TiO2, has been proved to be more efficient than not only pristine TiO2 but also
TiO2-based (Ag or CNT) binary structure. It is noticed that different processes always
lead to different ternary structures.
Zhao et al. (2020) fabricated the CNTs-Ag-TiO2 (CAT) ternary structurecomposite using AgNO3, TiO2 precursor and CNTs via sol-gel method. The ternary
structure was obtained from three steps: functionalization and activation of CNTs; Ag
nanoparticles are coated on treated CNTs by reduction of AgNO3 solution; and TiO2
nanoparticles are fabricated from precursor by sol-gel method and consequently
wrapped on binary composite. The structure can be described as follows: CNT was
wrapped by TiO2 nanoparticles and Ag nanoparticles. Compared to pristine TiO2 andCNTs-TiO2, CAT ternary structure composite exhibits a better photocatalytic activity
under visible light.
The ternary structure that CNTs decorated by TiO2/Ag composite particles has
been reported several times (Koo et al., 2014; Mohammad et al., 2019). In thesemethods, Ag doped TiO2 nanocomposites were first prepared by reducing AgNO3
where TiO2 or TiO2 precursor existed. Then ternary composite was fabricated byattaching this nanocomposite to activated CNTs. Moreover, Ag@TiO2 core shell
nanoparticles attached to CNTs are reported by Yang et al. (2014). The Ag@TiO2 coreshell can be achieved by adding Ag nanoparticles into TiO2 precursor. Different ternary
structures exhibit unique properties. However, the photocatalytic activity depends on
many factors. Thus, it is a challenge to judge a composite by structure only.
15

It is noticed that the mentioned ternary structure can be concluded as CNTssupported nanocomposite powders. A catalyst film sometimes overcomes somedrawbacks encountered with powder. Therefore, it is valuable to fabricate a ternarycomposite film by utilizing TiO2, CNT, Ag. In addition, the choice of a suitabledeposition method is important for film deposition. PECVD method is suitable for the
preparation of multicomponent film in a simple operation. Thus, the TiO2-CNT-Ag
ternary composite film can be deposited by PECVD method.

1.4.2 Preparation of TiO2-Ag from AgNO3 reduction
Several methods have been applied for preparing Ag/TiO2 photocatalysts by using
pure Ag or Ag precursors. Pure Ag is generally utilized in PVD method. Kim et al.
(2015) fabricated TiO2/Ag/TiO2 multilayer thin films using pure Ag target via
sputtering method. Kusdianto et al. (2017) deposited a TiO2-Ag nanocomposite film byvaporizing the Ag granules in a PVD-PECVD system. Sliver precursor, generally silver
nitrate (AgNO3) is a more common Ag source of TiO2-Ag composite. It has been widely
utilized in chemistry methods. In these methods, Ag nanoparticles can be obtained fromthe Ag+ by reducing agents, like sodium citrate, ascorbate and borohydride.
Nevertheless, plasma reduction method has been reported to reduce the metal ions
without using any reducing agent. Mariotti et al. (2012) synthesized Au from HAuCl4
aqueous by an atmospheric pressure plasma-liquid interactions system. In this system,
He gas plasma was generated above the aqueous and then formed a plasma-liquidinterface. The electrons can be injected into HAuCl4 aqueous from plasma. In thisprocess, injected electrons are considered as the reduction species to produce themetallic Au.
Gallingani et al. (2020) recently obtained the metallic Ag by plasma conversion of
16

AgNO3 liquid aerosol droplets. The AgNO3 aqueous solution was carried into plasma
reactor by Argon gas. The precursor reacts with plasma species inside the droplets,
which causes the conversion of AgNO3 to metallic Ag. Since that, preparation of TiO2Ag composite films by supplying AgNO3 aqueous directly via PECVD process are
considered to be feasible.

1.4.3 Preparation of super-hydrophobic TiO2/PDMS composites.
TiO2 is known as a photo-induced hydrophilic material when exposed to UV light.
The vacancies are formed when the oxidation reactions happen on TiO2 surface. These
vacancies will absorb the OH groups, leading to a super-hydrophilic behavior of TiO2
surface (Neves et al., 2020). However, the super-hydrophobic property is morefavorable in some applications such as water-oil separation, self-cleaning and surface
protection. Table 1.1 illustrates some specific applications of TiO2-based hydrophobic
material.

Table 1.1 Application of TiO2-based composite with super-hydrophobicity
Substrate
Filter paper

Preparation process

Application

Reference

The super-hydrophobic paper was Oil-water

(Gao

et

obtained by adhering the bonded separational., 2015)

TiO2 particles with coating low
energy materials.
DionysosandA sol prepared by using TEOS, TTIP Self-cleaning (Kapridaki

Thasos and PDMS was brushed on a EtOH marble

et

al.,

marbles

impregnated marble surface.

2013)

BricksA sol was obtained by mixing TiO2, Building(Alfieri et

17

SiO2 and Dynasylan® F 8815, before preservational., 2017)brushed on the bricks.
Cotton fabric PDMS, N-doped TiO2 particles were Self-cleaning (Pakdel etmixed and coated on cotton fabric by textile
methoddip-coatingal., 2021)

withultrasonication.
Steel

A suspension contains polyaniline Surface(Huang. et

TiO2 hierarchical composite particles anticorrosional., 2020)

were added dropwise on steel anddried at room temperature.
MarbleA TiO2 powder was dispersed in Cultural(La Russa

differentethydrophobicorganic heritage

material. The mixture was applied on protection

al.,

2016)

the Carrara marble.

Different materials have been utilized in hybrid composites with hydrophobic
properties. Among them, silicones with methyl groups are industrial compatible
materials for their chemically inert and resistant to harsh environments. Different
processes have been developed to fabricate the TiO2/PDMS composite films. Tavares
et al. (2014) reported a TiO2/PDMS nanocomposite film prepared by mixing the TiO2
with a PDMS-polymerizing agent-hexane mixture, before spraying on glass slides. Liuet al. (2019) reported a PMDS-grafted TiO2 prepared in two steps, including preparing
TiO2 film by spin coating and grafting the PDMS on the TiO2 surface under UV
illumination. The mentioned methods usually rely on multi-step and time-consuming
processes necessary for nanoparticle synthesis or functionalization. Thus, develop a18

simple method is available for the fabrication of TiO2/PDMS thin film.

1.5 Objectives and outline of the dissertation
In the study of this dissertation, different raw materials such as solid particles,aqueous solution, and polymer, were employed to be co-deposited with TiO2 by plasmaenhanced chemical vapor deposition (PECVD). The main objectives of this dissertation
are: (1) Use CNT/Ag solid nanoparticles to fabricate TiO2-CNT-Ag ternary composite
films with improved photocatalytic activity. (2) Fabricate TiO2-Ag binary composite
films in a simple process by using AgNO3 aqueous directly. (3) Fabricate TiO2/PDMS
composite films with super-hydrophobic and photocatalytic activity. This dissertation
was divided into 5 chapters as shown in Fig. 1.6. The brief objectives in Chapters 2-5
are shown below.

Fig. 1.6 Diagrammatic illustration of the brief objective of dissertation.

In Chapter 2, the TiO2-CNT-Ag ternary composite films were fabricated by
PECVD method. Researchers have already improved the photocatalytic activity of TiO2
19

by using CNT and Ag. However, CNTs are always used as the support material forcoating with TiO2 and Ag nanoparticles. Few researchers have focused on preparing
TiO2-based composite films, utilizing TiO2, Ag and CNTs. Among the film deposition
method, PECVD has demonstrated superiority in the preparation of multicomponent
films. Thus, the PECVD method is employed to fabricate TiO2-CNT-Ag ternarycomposite films. In the preparation process, aqueous suspensions of CNTs and Ag
nanoparticles were spray-dried to obtain aerosol particles of these solid materials. The
particles were supplied into a PECVD reactor, where a TiO2 film is prepared fromtitanium tetraisopropoxide vapor. The ternary structure will be described bymorphology, chemical compounds, and crystal. The photocatalytic activity of as
prepared composite films was evaluated.
Chapter 3 introduces a TiO2-Ag composite film by reducing Ag+ in PECVD.
Previous research introduced that the aerosol droplets of AgNO3 aqueous solution canbe reduced to Ag nanoparticles in plasma. Thus, using AgNO3 aqueous solution directly
without pre-preparing the Ag nanoparticle is considered to be feasible. During theprocess, AgNO3 aqueous solution was aerosolized and supplied to the PECVD reactor.
In the plasma, Ag nanoparticles were obtained by the reduction of AgNO3, and thesewere deposited with TiO2 simultaneously. The morphology and chemical species aswell as the photocatalytic activity of TiO2-Ag was carried out.
The TiO2/PDMS composite film with photocatalytic activity and superhydrophobicity was investigated in Chapter 4. TiO2 is known as a photo-inducedhydrophilic material when exposed to UV light. However, researchers found that
TiO2/PDMS composite with hydrophobic properties is favorable in some applications,like self-cleaning surface. The TiO2/PDMS composite has been generated in various
processes. However, the common strategies in fabricating TiO2/PDMS are the liquid20phase method. It is always related to complex operation steps. Recently, Thongrom etal. (2018) obtained PDMS film in a one-step PECVD method. Thus, it is feasible tofabricate the TiO2/PDMS composite film in PECVD method. According to different
experimental ideas, two different experimental conditions were designed to fabricate
TiO2/PDMS composite. The morphology and molecular structure of these composites
and their super-hydrophobicity and photocatalytic activity were determined.
Chapter 5 highlights the summary of each chapter. Suggestions for further researchon thin film preparation are also proposed.

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30

Chapter 2
Preparation of TiO2-CNT-Ag Ternary Composite Film withimproved Photocatalytic Activity via Plasma-Enhanced
Chemical Vapor Deposition

2.1 Introduction
Carbon nanotube (CNTs) and Ag nanoparticles has been reported to improve thephotocatalytic activity of TiO2. To fabricate a TiO2/CNT/Ag ternary structure, various
processes have been developed. Wang et al. (2009) reported the creation of an AgCNT/TiO2 ternary composite using the sol-gel and photoreduction techniques. TiO2 andAg nanoparticles were employed to decorate CNTs to create the ternary structure. AgCNT/TiO2 had a photocatalytic activity that was roughly twice as high as P25. Koo etal. (2014) reported the creation of an Ag-TiO2-CNT composite through photochemical
reduction. The TiO2/Ag nanoparticles were evenly packed on the CNT surface. The
photocatalytic activity of Ag-TiO2-CNT ternary composites is better than that of AgTiO2. Notably, CNTs are common support material to decorate with or coated by TiO2
and Ag nanoparticles. The preparation of TiO2-based composite films, utilizing TiO2,
Ag, and CNTs received few concentrations. Various film deposition methods have beendeveloped to prepare composite films, as described in Chapter 1. Among these methods,
PECVD is believed as the most versatile method that allows multiple materials to bedeposited simultaneously. It also can produce the uniform films with adequate coverage.
In the existence literature on TiO2/CNT/Ag ternary composites, the fabrication
process always started form preparation of binary composite. However, I believed it ispossible to fabricate ternary films by supply different materials simultaneously without
31

preparing a binary composite. Kubo et al. (2017) prepared TiO2 films embedded with
SiO2 nanoparticles by PECVD, where the SiO2 solid suspensions and vaporized TTIP
were supplied simultaneously. Therefore, by supplying CNTs and Ag solid mixtures,
we have shown that producing TiO2-CNT-Ag ternary composite films is feasible.
In this chapter, TiO2-CNT-Ag ternary composite films were fabricated using
PECVD. During deposition process, the CNT/Ag solid suspensions were sprayed witha nozzle and heated to obtain an aerosol of CNTs and Ag nanoparticles. These particles
and aerosolized TTIP were simultaneously supplied into a plasma reactor, where the
TiO2-CNT-Ag films were deposited on a substrate. The morphology, components andoptical properties of the obtained composite films was confirmed. The degradation of
Rhodamine 6G by the films under simulated solar light irradiation was recorded.

2.2. Experimental Section
2.2.1 Preparation of CNT and Ag suspensions
The CNT suspensions (0.5 wt%) were formulated using 0.5 g of CNTs (SigmaAldrich, St. Louis, U. S., diameter: 110–170 nm and length: 5–9 μm) dispersed in 95
mL of deionized water with Triton X-100 as the dispersant.
The Ag nanoparticle suspensions (0.1 wt%) were fabricated by co-precipitation
method (Šileikaitė et al., 2006). Subsequently, AgNO3 aqueous solution was preparedby dissolving 0.18 g of AgNO3 (Nacalai Tesque, Inc., Tokyo, Japan) into 98 mL
deionized water, and trisodium citric acid aqueous solution (2 mL, 0.01 g/mL) (Wako
Pure Chemical Industries, Ltd., Osaka, Japan) was added. The mixture was then stirred
at 100 °C for 1 h and cooled to ~25 °C.
A solid suspension for supplying was obtained by mixing the CNT suspension, theAg nanoparticle suspension and water with a certain concentration.
32

2.2.2 Experimental setup
Fig. 2.1 depicts the experimental setup consisting of different feeding systems(Kubo et al., 2017). 10mL CNT/Ag mixtures suspension was supplied through a syringe
pump (YSP-301, YMC. Co., Ltd., Kyoto, Japan) equipped with a capillary tube. A twofluid nozzle was employed to spray the solid suspension, with He gas flowing at a rateof 1000 sccm. A ribbon heater was used to preheat the transport tube to dry the solid
suspension. The obtained CNT/Ag aerosol particles were supplied to the plasma reactor.
Meanwhile, A He gas with flow rate of 50 sccm was used to carry the vaporized TTIP
(Tokyo Chemical Industry Co., Ltd., Tokyo, Japan) in a bubbler (45 °C). In addition,
the vaporized TTIP was transferred by O2 gas at a flow rate of 50 sccm and He gas aat
a flow rate of 400 sccm. The quartz tube (inner diameter: 25.4 mm; length: 300 mm)
act as the reactor and was supported by a cavity. Furthermore, a quartz mesh was placedon a recessed part in the middle of the quartz tube and hold a glass substrate, on which
the films could be fabricated. The plasma power was setup to 190 W (2.45 GHz). The
vacuum pressure in the equipment was maintained by a vacuum pump (Pascal 2025C1,
Pfeiffer Vacuum, Inc., Asslar, Germany) and ~6 kPa pressure was kept during thematerials feeding. The films were generated on a glass substrate (1 × 1 cm) by thesimultaneous feeding of the TTIP and CNT/Ag solid mixtures for 15 min. The
fabricated films were calcined at 600 °C and 700 °C with a N2 flow rate at 200 sccm.

33

Fig. 2.1 Experimental setup for PECVD.

In addition, the mass of Ti in raw materials during deposition were estimated asfollow steps:
The vapor pressure of TTIP (PTTIP) was estimated by the equation (Siefering et al.,1990):
 ͳͲ   ൌͻǤͺ͵͹Ǧ͵ͳͻ͵Ǥ͹

2.1



The Flow rate of TTIP (FTTIP sccm) was estimated by the equation:‫ܨ‬୘୘୍୔ ൌଵଷଷǤଷ௉౐౐౅ౌ
௉ా౫ౘౘౢ౛౨

2.2‫ܨ‬୆୳ୠୠ୪ୣ୰

The total volume of TTIP during deposition (VTTIP) was obtained from FTTIP. In
addition, the volume of TTIP in 20 °C (V澺TTIP) was estimated by the equation:ଶଽଷ௉௏ా౫ౘౘౢ౛౨ ౐౐౅ౌ
ᇱ
ൌ
ܸ୘୘୍୔
ଵ଴ଵǤଷଶହ்

2.3

ా౫ౘౘౢ౛౨

The density of TTIP in 20 °C is 0.96g/mL. Thus, the mass of Ti during deposition(mTi) was calculated by:34ᇱ݉୘୧ ൌ ͲǤͻ͸ܸ୘୘୍୔

ସ଻Ǥଽ

2.4

ଶ଼ସǤଶ

where the pressure of bubbler (PBubbler) was regarded same as that in the reactor
has the unit of mmHg. The TBubbler represent the temperature of bubbler and the FBubbler
represent the flow rate of bubbler.

2.2.3 Preparation of P25 film
A self-made P25 film was used as the reference. The P25 film was prepared bygrinding P25 powder (commercial TiO2 powder, Sigma-Aldrich, St. Louis, U. S.),
HNO3, and deionized water, and then spin-coated on the quartz glass substrate (Azmar
et al., 2019). The obtained film was annealed at 400 °C for 2 h.

2.2.4 Film characterization
The microstructure of the films was observed by SEM (S-5200, Hitachi HighTechnologies, Tokyo, Japan) and TEM (JEM-2010, JEOL, Tokyo, Japan), in
combination with EDS (JED-2300T, JEOL, Tokyo, Japan). The elemental analysis wasinvestigated by XPS (ESCA-3400, SHIMADAZU, Kyoto, Japan). The crystal
structures of the films were determined by XRD (RINT-2100, Rigaku, Tokyo, Japan),using Cu-Kα radiation (λ = 1.5406 Å). The UV–vis spectra of the films were performed
using a UV–Vis spectrophotometer V-650 (JASCO, Tokyo, Japan).
Further, an ICP-OES instrument (SPS3000, Hitachi High Technologies, Tokyo,
Japan) was employed to demonstrate the elemental content of the films. The obtained
films were put into a polypropylene sample tube and immersed by 1 mL of ethanol. A
micro grinder equipped with an electroplated diamond burr was used to scrape the film
off the substrate. Ethanol was used as a lubricant and collector for the scraped film
powder. The powder was dried before dissolved in HF. H3BO4 was utilized to neutralize
35

excess HF, before solvents were filtered and used for the ICP-OES analysis.

2.2.5 Evaluation of photocatalytic activity
The degradation of Rhodamine 6G by the films under simulated solar-lightirradiation was evaluated to study the photocatalytic activities of the films. A spectro
multi-channel photo detector (MCPD-3000, Otsuka Electronics, Osaka, Japan) wasused to record the spectrum of the light which is used to activate the film, the result isshown in Fig. 2.2. To reach adsorption equilibrium, a cuvette cell, in which the films
were immersed in 3 mL of a Rhodamine 6G aqueous solution (5 mg/L), was left in thedark for 30 min. Subsequently, the catalytic process was initiated by irradiation withsimulated solar light (300–800 nm). Based on the relation between the absorbance andconcentration of Rhodamine 6G, the concentrations of Rhodamine 6G was converted
from the maximum absorbances of Rhodamine 6G. The absorption spectrum before andafter irradiation were recorded through UV–Vis spectrophotometry (V-650, Jasco)
every 10 min. Furthermore, the degradation efficiency was carried out by Ct/C0, and therate constant k was determined by follows Eq. 2.5:஼ బ ൌ ݇‫ݐ‬

2.5

஼೟

where C0 and Ct represent the concentrations of Rhodamine 6G before and after
irradiation, respectively, and t is the irradiation time.
The photocatalytic activity of the commercial photocatalyst P25 film preparedthrough spin coating was tested under the same conditions.

36Fig 2.2 Light spectrum used to activate the films.

2.3. Results and discussion
2.3.1 Morphology
The TiO2, TiO2-CNT, TiO2-Ag, and TiO2-CNT-Ag films were prepared bysupplying only TTIP, a TTIP and 0.25 wt% CNT suspension, a TTIP and 0.1 wt% Ag
suspension, and TTIP and CNT/Ag mixed suspensions (0.25 wt%/0.1 wt%),
respectively. The corresponding products are marked as T, T-C, T-A, and T-C-A0,
respectively. Fig. 2.3 (a) shows the morphology of the CNTs. The morphology of the
prepared T-C films containing cracks was shown in Fig. 2.3 (b). The cracks sometimes
appear on the periphery of the annealed film surface, which could be attributed to theheat treatment. The exposed CNTs in these cracks demonstrate that they weresuccessfully incorporated into the film.

37

Fig. 2.3 SEM images of: (a) CNTs, (b) and (c) prepared T-C film containing cracks.

Further, Fig 2.4 shows the SEM images of the deposited T, T-C, T-A, and T-C-A
films. The cross-sectional morphologies of the films were shown in Fig. 2.4 (a)–(d),which appears a dense layer with a relative uniform thickness of around 1 μm. The film
thickness was not changed by the supplying of CNTs and Ag. The T films shows arelative smooth surface, while the structure of CNTs observed on surface, which areattached to the surface of T-C and T-C-A0 films (Fig. 2.4 (b) and (d)). Only few amountsof CNTs appeared, which can be attributed to a well-dispersed CNT suspension during
deposition process. However, it is a challenge to confirm the presence of Ag
nanoparticles in the T-A and T-C-A0 (Fig. 2.4 (c) and (d)). The observed big columnar
grains in cross-sectional images of the TiO2 film may leads to the formation of a relative
regular surface, as observed on TiO2 surface (Fig. 2.4 (e)). However, the addition of
CNTs and Ag suspensions caused in some irregular structure on the film surface, likethe strip structure in Fig. 2.4 (f), large aggregate particles in Fig. 2.4 (g), and bothphenomena in Fig. 2.4 (e). These irregular structures may be attributed to the formationof core-shell structure of the additives.

38

Fig. 2.4 SEM images of the prepared films. Cross-sectional morphologies of (a) T, (b)T-C, (c) T-A, and (d) T-C-A0. Surface morphologies of (e) T, (f) T-C, (g) T-A,
and (h) T-C-A0.

In addition, the elements mapping of the irregular structure that appeared in the39other samples prepared under the same conditions was investigated, as shown in Fig.
2.5. The existence and distribution of Ti, O, Ag and C was confirmed. The aggregatedparticles contain Ag and TiO2, and the strip structures contain CNT and TiO2. Thus,
these results confirmed the ternary structure that CNTs and Ag nanoparticles wasembedded into TiO2.

Fig. 2.5 Elemental mapping of T-A and T-C-A.
40

Fig. 2.6 SEM images of T-C-A films deposited using different concentrations of
CNT/Ag mixed suspensions: (a) T-C-A0 (0.25 wt%/0.1 wt%), (b) T-C-A1 (0.25
wt%/0.05 wt%), (c) T-C-A2 (0.25 wt%/0.01 wt%), (d) T-C-A3 (0.01 wt%/0.1wt%), and (e) T-C-A4 (0.5 wt%/0.1 wt%).

Furthermore, the effect of different concentrations of CNT/Ag mixed suspensions,such as 0.25 wt%/0.05 wt%, 0.25 wt%/0.01 wt%, 0.01 wt%/0.1 wt%, and 0.5 wt%/0.1
wt% concentrations, on the T-C-A films surface was investigated. The products were41

remarked as T-C-A1, T-C-A2, T-C-A3, and T-C-A4, respectively. Fig. 2.6 shows theSEM images of these films. By observing the surface structure, the concentration of themixed suspension seems to have a significant impact on the amount of CNTs on thefilm surface. However, referring to Fig. 2.3, it is impossible to determine the exact
amount of CNTs based on the surface morphology. Additionally, due to the embedded
structure and the small size of the Ag particles, it was difficult to determine themorphology of the Ag particles in all of the T-C-A films.

Fig. 2.7 SEM images of P25: (a) pristine P25 powder, (b) film surface, and (c) film
cross-section.

Fig. 2.7 shows the SEM images of the pristine P25 powder and prepared P25 films.
Fig. 2.7 (a) shows the SEM images of the pristine P25 powder stuck on carbon tape,which exhibits a random distribution. Fig. 2.7 (b) shows that the morphology of the P25
42

film deposited on the substrate obtained by spin coating is porous structure. Fig. 2.7 (c)shows the cross section of the P25 film, which contains a gap between the film andsubstrate. The porosity of the structure and the gap between the P25 film and substrateindicates that it is vulnerable to destruction.

Fig. 2.8 TEM images of the prepared T-C-A0 film: (a) CNTs with Ag and TiO2
particles attached, (b) magnified version of the area in the yellow square in (a),and EDS results corresponding to the black square in (b).

The prepared T-C-A0 film, which was chosen as a representative sample, was thensubjected to TEM-EDS to confirm the presence of Ag. The original structure was notpreserved since the observed sample was scraped from the films and collected by amicrogrid. Fig. 2.8 (a) depicts the TiO2 and Ag particles appeared to be attached to the
CNT surface. However, as seen in Fig. 2.1, the CNTs were incorporated into the TiO2
film. The yellow square in Fig. 2.8 (a) is shown magnified in Fig. 2.8 (b), demonstrating
the aggregated TiO2 and Ag nanoparticles. Further evidence of the presence of Ag in
the film is provided by the EDS analysis of the region in the red square in Fig. 2.8 (b).

43

2.3.2 Elements states
The composite compounds and states in the T-C-A0 film were examined using
XPS; the results are shown in Fig. 2.9. The survey spectrum of this film shows the peaks
for Ti 2p, Ag 3d, C1s, and O 1s (Fig. 2.9 (a)). A typical Ti4+ spectrum with Ti 2p3/2 at459.3 eV and Ti 2p1/2 at 465.0 eV was observed in Fig. 2.9(b). The presence of metallic
Ag was demonstrated by Ag 3d5/2 at 368.1 eV and Ag 3d3/2 at 374.1 eV in Fig. 2.9 (c).
In addition, the two decomposed peaks from the O 1s profile correspond to the latticeoxygen in TiO2 (530.5 eV) and hydroxyl-like group (532.1 eV), respectively.

Fig. 2.9 X-ray photoelectron spectra of the T-C-A0 film: (a) survey, (b) Ag 3d, (c) Ti
2p, and (d) O 1s spectra.

44

2.3.3 Crystalline
The XRD patterns of the T, T-C, T-A, and T-C-A0 films are shown in Fig. 2.10 (a).
The diffraction peaks marked with “A” correspond to the (101), (004), (112), (200), and(211) planes of anatase, indicating that the developed products existed in an anatase
phase. The absence of an Ag peak in the patterns of the T-C-A0 and T-A films may be
due to the low Ag content in these films. As a result, the main characteristic peak of Ag
at 38.2° (111) coincides with the characteristic peaks of TiO2 (Koo et al., 2014). The
same phenomenon occurs for the CNTs. The CNTs peak at 26.1q was shielded byanatase peak at 25.9q, suggesting that the film contained a relative low amount of CNTs
compared to TiO2 (Askari et al., 2017).
The XRD analysis of the films annealed at various temperatures is shown in Fig.
2.10 (b), JCPDS cards of Ag, SiO2, anatase and rutile were used as the reference.
Additionally provided is the P25 film used in this study. Both the rutile and anatase
phases can be seen in the P25 pattern. Notably, the T-C-A0 film annealed at 600 °C
only exhibits the anatase phase, while annealing at 700 °C results in the presence of therutile phase. This result is consistent with the previous observations that the phasetransfer occurs between 600 and 700 °C (Choudhury et al., 2013).

45

Fig. 2.10 XRD patterns of (a) T and T-composite films and (b) P25 and T-C-A0 films
annealed at different temperatures.

46

2.3.4 Optical properties
The absorption spectra for the T, T-C, T-A, and T-C-A0 films are shown in Fig.
2.11 (a). For all the films, the light absorption below 400 nm consistent with a typicalintrinsic band gap absorption of TiO2 (Yu et al., 2006). Compared to the T film, theabsorption thresholds of the T-C, T-A, and T-C-A films extend to longer wavelength
range. Zhao et al. (2020) attributed this behavior to the photosensitizing effect of CNTs
and the local surface plasmon resonance (LSPR) effect of the Ag particles. Furthermore,
the addition of CNTs and Ag to the composite film is thought to have aligned the energy
level, which is attributed to the charge transmission between the TiO2, Ag, and CNTs.
(Aazam, 2014). The band gap energy can be estimated by employing the Tauc relation:αhυ = B(hυ-Eg)1/2, where hυ is the photon energy, α is the absorption coefficient, Eg is
the optical band gap, and B is the absorption constants for direct transitions. The plotof (αhυ)2 versus photoenergy (hυ) was determined and the bandgap was estimated by
the intercept of the tangent to the plot, as shown in Fig. 2.11 (b). The bandgap of the T
film is 3.47 eV; however, it decreases to 3.39 eV, 3.31 eV, and 3.22 eV for the T-C film,
T-A film, and T-C-A film, respectively. The reduction of the band gap of T-C-A
nanocomposite may be attributed to the interaction between TiO2 and CNT as a resultof the C–O–Ti bond formation and the SPR effect of Ag nanoparticles (Chaudhary etal., 2017). These results indicate the capability of the T-C-A samples for capturingvisible light, comparatively.

47

.

Fig. 2.11 Optical properties of the T and T-composite film: (a) absorbance spectra; (b)(αhυ)2 versus photon energy (hυ) plot of the prepared films.

2.3.5 Elements analysis
The ratio of Ag/Ti content in the film was analyzed by ICP-OES. The results arepresented in Table 2.1. The units of Ag/Ti (g / g) refer to the raw materials (theoreticalvalues) and correspond to the conditions when films were deposited. Ag/Ti (mg / mg)
refers to the results of the ICP-OES analysis and corresponds to the content of the film
(absolute values). Ag elements were present in all the T-C-A films, proving the presence48

of Ag. However, decreasing Ag concentration results in a decrease of Ag/Ti ratio. This
result suggests that the concentration of the raw material had a direct impact on the Ag
content in the films.

Table 2.1 Ratio of Ag/Ti content in the T-C-A film
Ag and Ti in the raw materials

Ag and Ti in the films

SamplesContent (g/g)

Ratio

Content (mg/mg)

Ratio

T-C-A00.01/0.0914

0.109

0.0027/0.149

0.0182

T-C-A10.005/0.0914

0.05470.0027/0.194

0.0139

T-C-A20.001/0.0914

0.01090.0028/0.237

0.01182.3.6 Photocatalytic activity of the films
The photocatalytic activities of the samples were evaluated based on thedecomposition of aqueous Rhodamine 6G by different films under simulated solar light.
The films employed in this section were annealed at 600 °C. Fig. 2.12 (a) presents theplot of Ct/C0 versus the correspond irradiation time (t) for different samples,
Furthermore, we prepared the ln (Ct/C0) versus t plot (Fig. 2.12 (b)) to obtain thecorresponding rate constant k (Fig. 2.12(c)) from the slope of the fitting curve. The
results confirmed the photocatalytic activity of the film decreased in the following order:
T-C-A0 > T-A > T-C > T. Furthermore, the k of T-C-A0 is almost 1.8 times higher thanthat of T.

49

Fig. 2.12 Photocatalytic activity for the degradation of Rhodamine 6G by different
films under simulated solar light: (a) degradation with respect to time, (b) ln(Ct/C0) versus to time plot, and (c) rate constants degradation for various
samples.

50

In addition, the photocatalytic activity of the self-prepared P25 film and T-C-A0
film annealed at different temperature was determined under the same conditions. Fig.
2.13 (a) presents the plot of Ct/C0 versus the correspond irradiation time (t) for different
samples, Furthermore, we prepared the ln (Ct/C0) versus t plot (Fig. 2.13 (b)) to obtainthe corresponding rate constant k (Fig. 2.13(c)) from the slope of the fitting curve. The
higher annealing temperature resulted in the increased photocatalytic activity, one of
the reasons should be the phase transfer of the crystalline phase at different annealing
temperatures. Moreover, the T-C-A0 film annealed at 700 °C shows higher
photocatalytic activity than the self-made P25 film. The rate constant k of T-C-A0
annealed at 700 °C is approximately 1.3 times higher than that of P25 film.

51

Fig. 2.13 Photocatalytic degradation of Rhodamine 6G by P25 and T-C-A films
annealed at different temperatures: (a) degradation with respect to time, (b) ln(Ct/C0) versus to time plot, and (c) corresponding rate constants of degradation
for samples.
To the best of my knowledge, other researchers have also estimated thephotocatalytic activity based on the rate constant. Soltanieh et al. (2021) prepared a
TiO2/Ag0@MWCNTs composite with rate constant of 0.07675 min-1. Wang et al. (2009)
illustrated an Ag-CNT/TiO2 composite with a rate constant 0.03052 min-1. Numerous
factors affect the photocatalytic activity, such as the concentration of dyes, power of theirradiation light, and morphology of the catalyst. Consequently, it is challenging tocompare the activity of catalysts produced in different experiments. In this study,
however, the photocatalytic activity of TiO2 was successfully improved. Therefore, thedeveloped method is useful to improve the performance of photocatalysts.
These results demonstrate that the incorporation of CNTs and Ag particles into theTiO2 film considerably increases the photocatalytic activity of TiO2. The mechanism ofphotocatalytic reaction for ternary structure is complex. The Ag particles can be
activated by light irradiation due to the LSPR effect and produce photoinduced
electrons. These photoinduced electrons can be transferred to the conduction band of52

TiO2. In addition, since the band gap of TiO2 in the ternary composite has narrowed, it
is hypothesized that TiO2 can also function as a sensitizer to create electrons(Chaudhary et al., 2017; de la Flor et al., 2021). In addition, CNTs act as electron
acceptors is a common idea, which accepts the photoinduced electrons from TiO2 andAg, thus reducing electron-hole recombination (Wang et al., 2019). In this case, thepossible mechanism should be: the electrons were excited to the conduction band of
TiO2 since the narrowing band gap of ternary composite, then the activated electrons
can easily be transferred to the CNTs and Ag, the recombination of electron–hole pairs
was reduced, as shown in Fig. 2.14.

Fig. 2.14 Possible schematic explaining the photocatalytic activity of the T-C-A
ternary film.

2.4 Conclusions
In this study, a T-C-A ternary composite film was successfully prepared using anovel and simple process with relatively good coverage. The characterization results
confirmed the structure that CNT and Ag particles were embedded into TiO2 matrix,53

and the thickness was approximately 1 Pm. The surface structures and optical propertiesof the films were altered by the addition of CNT and Ag. The crystal structure of thefilm was influenced by annealing temperature. The addition of CNT and Ag cansignificantly improve the performance of TiO2, where the photocatalytic activity of TC-A was approximately 1.8 times higher than that of T. In addition, the photocatalytic
activity of T-C-A with mixed-phase was 1.3 times higher than that of self-made P25
film. The investigated process shows the immense potential for improving thephotocatalytic activity of TiO2.

References
Aazam, E. S. Visible light photocatalytic degradation of thiophene using AgTiO2/multi-walled carbon nanotubes nanocomposite. Ceramics International,
40(5), 6705-6711. (2014).
Askari, M. B., Banizi, Z. T., Seifi, M., Dehaghi, S. B., & Veisi, P. Synthesis of TiO 2nanoparticles and decorated multi-wall carbon nanotube (MWCNT) with anatase
TiO2 nanoparticles and study of optical properties and structural characterizationof TiO2/MWCNT nanocomposite. Optik, 149, 447-454. (2017).
Azmar, A., Subban, R. H. Y., & Winie, T. Improved long-term stability of dye-sensitizedsolar cell employing PMA/PVAc based gel polymer electrolyte. Optical Materials,
96, 109349. (2019).
Chaudhary, D., Singh, S., Vankar, V. D., & Khare, N. A ternary Ag/TiO2/CNT
photoanode for efficient photoelectrochemical water splitting under visible lightirradiation. International Journal of Hydrogen Energy, 42(12), 7826-7835. (2017).
Choudhury, B., & Choudhury, A. Local structure modification and phasetransformation of TiO2 nanoparticles initiated by oxygen defects, grain size, and54

annealing temperature. International Nano Letters, 3(1), 55. (2013).
de la Flor, M. P., Camarillo, R., Martínez, F., Jiménez, C., Quiles, R., & Rincón, J.
Synthesis and characterization of TiO2/CNT/Pd: An effective sunlight
photocatalyst for neonicotinoids degradation. Journal of Environmental Chemical
Engineering, 9(5), 106278. (2021).
Koo, Y., Littlejohn, G., Collins, B., Yun, Y., Shanov, V. N., Schulz, M., Pai, D., & Sankar,
J. Synthesis and characterization of Ag–TiO2–CNT nanoparticle composites withhigh photocatalytic activity under artificial light. Composites Part B: Engineering,57, 105-111. (2014).
Kubo, M., Taguchi, T., & Shimada, M. Preparation of nanoparticle-embedded thin films
by simultaneous feeding of gaseous and solid raw materials in plasma-enhanced
chemical vapor deposition process. Thin Solid Films, 632, 55-65. (2017).
Siefering, K. L., & Griffin, G. L. Growth Kinetics of CVD TiO2: Influence of CarrierGas. Journal of The Electrochemical Society, 137(4), 1206-1208. (1990).
Šileikaitė, A., Prosyčevas, I., Puišo, J., Juraitis, A., & Guobienė, A. Analysis of silver
nanoparticles produced by chemical reduction of silver salt solution. Journal of
Materials Science, 12(4), 1392-1320. (2006).
Soltanieh, A. M., Khavar, A. H. C., Ganjidoust, H., Mahjoub, A. R., & Khazaee, Z.
Plasmon-induced charge separation by Ag nanoparticles between titanium dioxideand MWCNTs for natural sunlight-driven photocatalysis. Journal of the IranianChemical Society. (2021).
Wang, S., Gong, Q., Zhu, Y., & Liang, J. Preparation and photocatalytic properties ofsilver nanoparticles loaded on CNTs/TiO2 composite. Applied Surface Science,
255(18), 8063-8066. (2009).
Wang, T., Tang, T., Gao, Y., Chen, Q., Zhang, Z., & Bian, H. Hydrothermal preparation55

of Ag-TiO2-reduced graphene oxide ternary microspheres structure composite forenhancing photocatalytic activity. Physica E: Low-dimensional Systems andNanostructures, 112, 128-136. (2019).
Yu, J. G., Yu, H. G., Cheng, B., Zhou, M. H., & Zhao, X. J. Enhanced photocatalytic
activity of TiO2 powder (P25) by hydrothermal treatment. Journal of Molecular
Catalysis a-Chemical, 253(1-2), 112-118. (2006).
Zhao, C., Guo, J., Yu, C. L., Zhang, Z. J., Sun, Z., & Piao, X. Q. Fabrication of CNTsAg-TiO2 ternary structure for enhancing visible light photocatalytic degradationof organic dye pollutant. Materials Chemistry and Physics, 248, 122873. (2020).

56

Chapter 3
Ag-Doped TiO2 Composite Films Prepared Using AerosolAssisted, Plasma-Enhanced Chemical Vapor Deposition

3.1 Introduction
In Chapter 2, TiO2 composite films were fabricated by the addition of an Ag
nanoparticle suspension, this process requires the pre-preparation of the nanoparticle
mixture. Especially, Ag nanoparticles need to be prepared in advance by coprecipitation from AgNO3 aqueous solution. Thus, the direct supply of silver precursor
is considered to be a more straightforward method.
Generally, the reduction of AgNO3 requires the reducing agent. However, Zou etal. (2006) found that metal ions can be reduced to their metallic state by plasma
treatment. Recently, Gallingani et al. (2020) confirmed the aerosol droplets of AgNO3
solution can be reduced by plasma. Therefore, in this study, to increase the process
efficiency, AgNO3 was supplied directly. During the process, AgNO3 aqueous solutions
were aerosolized and supplied to the PECVD reactor. In the plasma, AgNO3 aerosol
droplets were reduced to obtain Ag nanoparticles, and these particles were deposited on
the substrate. Simultaneously, the species cracked from TTIP vapor was introduced by
He and O2 to form a TiO2 film. Notably, the effect of AgNO3 concentration on themorphology and photocatalytic activity of the TiO2-Ag composite films wasinvestigated.

57

3.2 Experimental section
3.2.1 Preparation method
The experimental setup for fabricating TiO2-Ag has been described in Chapter 2.
However, different raw material, AgNO3 aqueous solutions instead of solid suspensions,
was employed. As a precursor of Ag+, AgNO3 aqueous solutions with concentrations
ranging from 0 to 4 wt% were supplied by a syringe pump. Assisted by a two-fluid
nozzle, the aerosol droplets of AgNO3 aqueous solutions were supplied into plasmachamber. The other experimental conditions are followed with Chapter 2. The annealing
process is carried out under N2 atmosphere at 600 °C for 12 hours.

3.3.2 Film characterization
The crystalline of the films was investigated by XRD. The morphologies of thefilms were observed by SEM and TEM -EDS. The chemical species and their energy
states and ratio of contents were confirmed by XPS. The optical properties of the films
were investigated using a V-650 spectrophotometer.

3.2.3 Evaluation of photocatalytic activity
The photodegradation of methylene blue (MB) aqueous solution was carried out
under UV irradiation. The measurement was conducted in a thermostatically controlled
system maintained at 25 °C. The prepared films with MB solution were held in a cuvetteand left in the dark for 30 min to reach adsorption equilibrium. A UV light (365 nm,
1407 μW/cm) was employed to initiate the photocatalytic reaction. The absorption
spectrum of the samples was recorded every 30 min, and the absorption maximum of
MB was converted to the correspond concentration using the Beer-Lambert law. The
58

percent degradation of MB was calculated as [(C0−Ct)/C0] × 100%, where C0 and Ct
are the concentrations of MB at time t = 0 and after irradiation for time t, respectively.
Further, the reaction rate constant k was determined by Eq. 3.1.
஼ బ ൌ ݇‫ݐ‬

3.1

஼೟

3.3 Results and Discussion
3.2.1 Crystalline
The effect of AgNO3 concentration on the crystal structure of the prepared films
was investigated by XRD, as shown in Fig. 3.1. JCPDS cards of TiO2, Ag and SiO2
were employed as reference. Although different concentrations of AgNO3 aqueous
solution were supplied, only anatase phase peaks were observed in all films. The peaks
located at 25.82°, 37.53°, 38.34°, 39.16°, 48.56°, 54.44°, and 55.56° correspond to the(101), (103), (004), (112), (200), (105), and (211) planes, respectively. However, it is achallenge to define the Ag peaks in the pattern, which could be attributed to the lowcontent of Ag loading or coincidence with anatase peaks (Shi et al., 2018). In addition,
the peak observed at 23° may correspond to the substrate. There are still two weak peaks(35° and 43°) that cannot be identified, which may be attributed to unknown
contamination.

59

Fig. 3.1 XRD patterns of the pristine TiO2 film and TiO2-Ag composite films
deposited using different concentrations of AgNO3.

3.3.2 Morphology
The surface and cross section morphologies of the TiO2-Ag composite films
deposited using different AgNO3 concentrations were shown in Fig. 3.2. Fig. 3.2(a)shows a dense surface structure in pristine TiO2 film. The secondary spherical particles
that consist of the aggregation of tiny particles was observed in TiO2-Ag composite
films; these secondary particles show a wide range of sizes. The addition of additives
can leads to the formation of agglomerates during process (Chen et al., 2000). In
addition, the increasing concentration of AgNO3 resulted in a more severe
agglomeration on the surface of the films, as reported previously (Singh et al., 2017;
Tomas et al., 2009). Moreover, the tightly packed aggregated particles indicating a good60

coverage film was produced by PECVD method. Cross section images were used todetermine the layer thickness. The thickness of all the films is roughly 0.7 μm. These
results indicate the reproducibility of this process in film deposition.

Fig. 3.2. SEM images on the surface view and cross section of (a) pristine TiO2 film
and TiO2-Ag composite films deposited using different AgNO3 concentrations:(b–f) 0.1, 0.5, 1, 2, and 4 wt.%, respectively.

The samples for TEM-EDS are scraped from the surface of the film using a
microgrid. The results reflect only a very small part of the surface of the film. The
61

Fig.3.3 demonstrates that some very tiny TiO2 particles were attached to a spherical Ag
nanoparticle with 40–60 nm. The large size of the Ag nanoparticles may be the reason
for the aggregation on the surface of the film as shown in the SEM images. A highresolution TEM (HRTEM) images of the region enclosed in a yellow square was shownin Fig. 3.3 (b). The interplanar spacings of 0.35 and 0.24 nm correspond to the (101)plane of anatase and (111) plane of Ag, respectively. In addition,the presence of Ag in
the film was also confirmed by EDS analysis.

Fig. 3.3. (a) TEM image, (b) HTREM images and the corresponding EDS profile of
the region in the yellow square of the TiO2-Ag (1 wt%) film.

3.3.2 Elements analysis
Next, the chemical species and their oxidation states of the prepared 1 wt%
AgNO3-supplied TiO2-Ag film were performed by XPS measurements. Fig. 3.4 (a)illustrates that the XPS survey profile contains four peaks corresponding to Ti 2p, O 1s,
C1s, and Ag 3d. Fig. 3.4 (b) demonstrates the typical metallic Ag spectrum that contains
Ag 3d5/2 (368.2 eV) and Ag 3d3/2 (374.1 eV) peaks; The doublet separation between theTi 2p1/2 (464.7 eV) and 2p3/2 (459 eV) is 5.7 eV, which is consistent with binding energy
62

of Ti4+ in TiO2, as shown in Fig. 3.4 (c). The O 1s spectrum was deconvoluted into threepeaks at 530.2, 532.2, and 534.3 eV (Fig. 3.4 (d)), corresponding to the lattice oxygenin TiO2, dissociated oxygen or hydroxyl-like group and absorbed water (Demirci et al.,2016; Yu et al., 2003), respectively.

Fig. 3.4 XPS profiles of the TiO2-Ag (1 wt.%) film: (a) Survey, (b) Ag 3d, (c) Ti 2p,
and (d) O 1s spectra.

In addition, the ratio of Ag/Ti in raw materials was estimated by the method
mentioned in Chapter 2. The ratio of Ag/Ti in the films was estimated by the peak areaof Ag 3d and Ti 2p, the results were shown in Table 3.1. It is mentioned that the accuracyof the results deserves further study, since the thickness of the film interferes with theaccuracy of XPS measurement. The results show that increasing AgNO3 concentration
results in an increase of Ag/Ti ratio. This result suggests that the concentration of the
raw material had a direct impact on the Ag content in the films.

63

Table 3.1 Ratio of Ag/Ti content in the T-C-A film
Film

Ag/Ti in the raw materials

Ag/Ti in the films

TiO2-Ag (0.1 wt%)

0.06940.0290TiO2-Ag (0.5 wt%)

0.347

0.0423

TiO2-Ag (1 wt%)

0.694

0.0872

TiO2-Ag (2 wt%)

1.389

0.104

TiO2-Ag (4 wt%)

2.777

0.255

3.2.4 Optical properties
Fig. 3.5 (a) demonstrate the UV-vis spectra of the pristine TiO2 film and the TiO2Ag composite films prepared using different concentrations of AgNO3 (0.1–4 wt%).
The threshold wavelength of pristine TiO2 film is approximately 358 nm, which only
fall in the range of UV light. The increase of the concentration of AgNO3 result in aredshift of the absorption edges of the TiO2-Ag composite films; that is, they shift
toward the visible region. It suggests that the appearance of Ag can significantly
improve the visible-light absorption of TiO2. The bandgap energy was calculated as
described in Chapter 2. The plots of (αhυ)2 versus hυ plots for different samples areshown in Fig. 3.5 (b). The bandgap values were estimated by the tintercept of thetangent to the plot, and the results are listed in Table 3.2. The bandgap of pristine TiO2
was 3.54 eV, while those of the TiO2-Ag composite films decreased from 3.21 to 2.61
eV with increasing concentration of AgNO3. The difference in the bandgaps of thesesamples probably originates from the different surface microstructures andcompositions (Yu et al., 2005).

64

Fig. 3.5 (a) Absorbance spectra of the pristine TiO2 film (T) and TiO2-Ag composite
films deposited using AgNO3 concentrations of 0.1–4 wt.%. (b) Plot of (αhυ)2versus photon energy (hυ) of the prepared films.

65

Table 3.2 Bandgap energies (Eg) of the films.
Film

Threshold wavelength (nm)Eg (eV)

Pristine TiO2

350

3.54

TiO2-Ag (0.1 wt%)

386

3.21

TiO2-Ag (0.5 wt%)

389

3.18

TiO2-Ag (1 wt%)

410

3.02

TiO2-Ag (2 wt%)

442

2.80

TiO2-Ag (4 wt%)

457

2.61

3.3.5 Photocatalytic activity
The photocatalytic activities of the samples were evaluated based on thedecomposition of MB under UV irradiation. Fig. 3.6 (a) shows the degradation of MB(Ct/C0) with respect to time (t) for the different films under UV-light irradiation. All thefilms exhibited photocatalytic activities under the same conditions. However, thepresence of Ag nanoparticles increased the photocatalytic activities of the TiO2-Ag
composite films, comparing to that of the pristine TiO2 film. A possible mechanism has
been introduced in Chapter 1. The photodegradation efficiency of MB degradation in 3
h was calculated using the formula [(C0-C180)/C0] × 100%, where C0 and C180 are theinitial concentrations and concentrations after 3 hours irradiation, respectively. The
TiO2-Ag (1 wt%) composite film demonstrated the best photocatalytic activity,
resulting in an MB degradation of around 41%, while the pure TiO2 film displayed an
activity of just about 11%. Furthermore, we prepared the ln (Ct/C0) versus t plot (Fig.
3.6 (b)) to obtain the corresponding rate constant k (Fig. 3.6 (c)) from the slope of thefitting curve. The resulting k values further demonstrate that the TiO2-Ag (1 wt%)
composite film exhibits the maximum photocatalytic activity, which was almost 1.7
66

times that of pristine TiO2. However, an excessive AgNO3 concentration resulted inlower activities. Researchers believe that: (1) the excess addition of Ag may increase
the reflection of UV irradiation; (2) the negatively charged Ag can attract the positive
holes. The high content of Ag increased the probability of trapping holes, thus making
Ag a new recombination center and affecting photocatalytic ability. (Ko et al., 2011).

67

Fig. 3.6 Photocatalytic activity for the degradation of MB over different films: (a)degradation with respect to time, (b) ln (Ct/C0) versus to time plot, and (c) rate
constants degradation for various samples.
Furthermore, the photocatalytic activity of the films was also carried out by
degradation of R6G under visible light. However, there is no significant difference inphotocatalytic activity between films. Therefore, the photocatalytic ability of thin films
on different dyes needs further study. In addition, comparing to the photocatalytic
activity of T-C-A in chapter 1, T-A films in this chapter exhibit better photocatalytic
activities under UV light irradiation. However, T-C-A film in chapter 1 exhibits better
photocatalytic activity under simulated solar light irradiation.

3.4 Conclusions
In this work, TiO2-Ag composite films were successfully prepared using AgNO3
as raw material. XRD patterns revealed the typical anatase phase of the film annealed
at 600 瀽C. The existence of metallic Ag in the composite film was confirmed. The
presence of Ag nanoparticles significantly altered the morphology and bandgap of the
TiO2-Ag composite films. The TiO2-Ag composite films exhibit improved
photocatalytic activities as compared to pristine TiO2. Among them, the film preparedusing 1 wt% AgNO3 exhibits a best photocatalytic activity. Overall, the results
68demonstrate a successful process for preparing TiO2-Ag composite film. Compared to
Chapter 2, supplying AgNO3 aqueous solution instead of Ag nanoparticles ensures asimple procedure.

References
Chen, C. Y., & Tuan, W. H. Effect of silver on the sintering and grain-growth behaviorof barium titanate. Journal of the American Ceramic Society, 83(12), 2988-2992.
(2000).
Demirci, S., Dikici, T., Yurddaskal, M., Gultekin, S., Toparli, M., & Celik, E. Synthesis
and characterization of Ag doped TiO2 heterojunction films and theirphotocatalytic performances. Applied Surface Science, 390, 591-601. (2016).
Gallingani, T., Abuyazid, N. H., Colombo, V., Gherardi, M., & Sankaran, R. M. Online
ion mobility spectrometry of nanoparticle formation by non-thermal plasma
conversion of metal salts in liquid aerosol droplets. Journal of Aerosol Science,
150, 105631. (2020).
Ko, S., Banerjee, C. K., & Sankar, J. Photochemical synthesis and photocatalytic
activity in simulated solar light of nanosized Ag doped TiO2 nanoparticle
composite. Composites, Part B, 42(3), 579-583. (2011).
Shi, H., Yu, Y., Zhang, Y., Feng, X., Zhao, X., Tan, H., Khan, S. U., Li, Y., & Wang, E.
Polyoxometalate/TiO2/Ag composite nanofibers with enhanced photocatalytic
performance under visible light. Applied Catalysis B: Environmental, 221, 280289. (2018).
Singh, J., Sahu, K., Pandey, A., Kumar, M., Ghosh, T., Satpati, B., Som, T., Varma, S.,
Avasthi, D. K., & Mohapatra, S. Atom beam sputtered Ag-TiO2 plasmonic
nanocomposite thin films for photocatalytic applications. Applied Surface Science,
411, 347-354. (2017).
69

Tomas, S. A., Luna-Resendis, A., Cortes-Cuautli, L. C., & Jacinto, D. Optical andmorphological characterization of photocatalytic TiO2 thin films doped with silver.
Thin Solid Films, 518(4), 1337-1340. (2009).
Yu, J.-G., Yu, H.-G., Cheng, B., Zhao, X.-J., Yu, J. C., & Ho, W.-K. The effect of
calcination temperature on the surface microstructure and photocatalytic activityof TiO2 thin films prepared by liquid phase deposition. The Journal of Physical
Chemistry B, 107(50), 13871-13879. (2003).
Yu, J. G., Xiong, J. F., Cheng, B., & Liu, S. W. Fabrication and characterization of AgTiO2 multiphase nanocomposite thin films with enhanced photocatalytic activity.
Applied Catalysis B-Environmental, 60(3-4), 211-221. (2005).
Zou, J.-J., Zhang, Y.-p., & Liu, C.-J. Reduction of supported noble-metal ions using
glow discharge plasma. Langmuir, 22(26), 11388-11394. (2006).

70

Chapter 4
Preparation of Super-hydrophobic and Photocatalytic
TiO2/PDMS Films by Plasma-Enhanced Chemical Vapor
Deposition

4.1 Introduction
Super-hydrophobic surface has attracted considerable attention in recent years for
its wide application, such as self-cleaning, oil-water separation, anticorrosion. Superhydrophobic surface can deal with some contaminants in a wash process. The common
strategies to form super-hydrophobic surfaces are create rough surface by low surface
energy material; or combinate the low surface energy materials to rough surface.
Polydimethylsiloxane (PDMS) with typical hydrophobicity, chemical stability and lowdensity surface property has been widely used in super-hydrophobic surface. However,super-hydrophobic coatings with long-term stability and sufficiently resistant tooutdoor contamination needs to be developed (Xu et al., 2018). Recently, scientists
have developed the super-hydrophobic surfaces that consists of photocatalysis. These
composites maintain good performance even in harsh environments. Titanium dioxide(TiO2) is the most studied photocatalysis for its unique physicochemical properties,
commercial availability and inexpensiveness.
TiO2/PDMS composites for super-hydrophobic surface were prepared by different
process. However, most of the literatures focused on the liquid phase method. Thesemethods always related to a complex step. Thus, develop a simple method is available
for fabrication of TiO2/PDMS thin film. PECVD method is a relatively simple methodthat allows different materials to deposit simultaneously. To my knowledge, only few
71reports prepare the TiO2/PDMS composite in PECVD method. However, PECVD
method has been reported to produce the PDMS films. Some researchers prepared the
PDMS film by using hexamethyldisiloxane as the precursor, followed by apolymerization in plasma reactor (Pryce et al., 2000). Lee et al. (2009) described a SiOx
thin film deposited from PDMS/curing agent in a O2/H2 plasma. Thongrom et al. (2018)
demonstrate a simpler film deposition process by PECVD which supply the PDMS
directly.
In this study, TiO2/PDMS composite films was prepared by supplying PDMS
directly via PECVD method. It is worth mentioning that no curing agent is used in thisprocess. The vaporized PDMS and TTIP were fed into a plasma reactor. The
morphology and component of the films was confirmed. The obtained films exhibited
both super-hydrophobicity and photocatalytic activity.

4.2 Experimental Section
4.2.1 Experimental setup
The experimental setup for fabricate TiO2/PDMS include two material supplying
system as shown in Fig. 4.1. The PDMS (Sigma-Aldrich, St. Louis, U. S., average Mn550~) was vaporized in a steel bubbler under 40° and carried by He gas. The other parts
are same as described in Chapter 2 and 3. TiO2/PDMS composite was deposited on asilicon substrate under different conditions.
Condition A: The conditions for preparation TiO2 film have been confirmed in
Chapter 2 and 3. Thus, TiO2/PDMS is prepared based on this condition. The composite
film was obtained by supplying PDMS (20 sccm) and TTIP (50 sccm) simultaneously.
The effect of additional PDMS on the structure and properties of TiO2 thin films wasinvestigated.
72Condition B: The PDMS films with super-hydrophobicity were deposited in acertain condition. The TiO2/PDMS is prepared based on this condition. During theprocess, different TTIP flow rate (50 and 200 sccm) was supplied with PDMS
simultaneously. The effect of additional TiO2 on the structure and properties of PDMS
films was investigated. The detail of this condition was listed in Table 4.1.

Fig. 4.1 Experimental setup of PECVD for preparing TiO2/PDMS composite.

Table 4.1 The detail of Condition B
Experiment factor

ķ

ĸ

Ĺ

ĺ

Ļ

Gas flow rate (sccm)

200

400

50

50/200700/850

Heating temperature (°C)

40

80

80

45

80

Plasma power (W)140

Deposition time (min)273

4.2.2 Characterization
The morphology of prepared film was determined by SEM and TEM-EDX. The
chemical bonds of the film were defined by XPS and FTIR. The crystal structure of thefilm was determined by XRD.

4.2.3 Evaluation of film properties.
The photocatalytic activity was performed by degradation of MB under UV light.
A same procedure as described in Chapter 2 and 3 was performed for photocatalytic
activity.
The super-hydrophobicity was determined by water contact angle. A 2 μL water
droplet was kept on the film surface and recorded by a digital microscope (ViewTer500UV, 3R Group. Co. Ltd, Fukuoka, Japan). The water contact angle was estimatedby using software ImageJ. The water contact angle of the film was recorded before andafter photocatalytic test.

4.3 Results and discussion
4.3.1 Film prepared under Condition A
4.3.2.1 Morphology
Fig. 4.2 (a) and (c) show the surface structure of the as-prepared films. Fig. 4.2 (b)and (d) show the cross-section morphology of the as-prepared films. It is obvious thatboth the surface and cross-section structure between TiO2 and TiO2-PDMS composite
are totally different. The TiO2 film consists of nanosized columnar grains and exhibitsa relative flat surface, which has been reported in previous study (Kubo et al., 2017).
However, when introduced with PDMS, the columnar structure disappeared and74replaced by an impressive rough surface. The rough surface consists of huge secondary
particles formed by the aggregation of tiny particles, as shown in Fig 4.2 (c). The shapeof these secondary particles is corresponding to the “mountains-liked” morphology, as
observed in cross-section morphology (Fig. 4.2 (d)). The hierarchical structure
increased the roughness of surface, which is beneficial for hydrophobicity. The TiO2
layer thickness is approximately 1μm and the TiO2-PDMS films shows an increased
thick layer approximately 8 μm.

Fig 4.2 SEM images of the prepared films. Surface morphologies of (a) TiO2 and (c)TiO2-PDMS. Cross sectional morphologies of (b) TiO2 and (d) TiO2-PDMS.

TEM-EDS is performed to analyze the composite structure in more detail. A TEM
grid was employed to collect some powder form the film surface, which was
subsequently investigated by TEM-EDS. Both Fig. 4.3 (a) and Fig 43 (c) are collectedfrom the same TiO2-PDMS composite. In Fig. 4.3 (a), only the powder with aggregationof tiny particles was observed. These particles were analyzed by EDS (Fig. 4.3 (b)), and75

the results revealed the existence of a large amount of Si and a small amount of Ti. Fig.
4.3 (c) contains two different component structure, as shown in Fig. 4.3 (d) and (e). Fig
4.3 (d) depicts the aggregation of tiny particles which are almost the same components
with the Fig. 4.3 (a). However, the columnar structure is mainly consisting of Ti, as
shown in Fig. 4.3 (e).

Fig. 4.3 TEM images of the prepared TiO2/PDMS composite films. The structure of
aggregation of tiny particles (a) and the corresponding EDS profile (b). The
structure consists of aggregation tiny particles and columnar particles (c); Thehigh magnification images (d) and the corresponding EDS profile (e); The highmagnification images (f) and the corresponding EDS profile (g).

76

4.3.2.2 Molecular structure
The molecular structure of as-prepared TiO2 film and as-prepared TiO2-PDMS
composite film was investigated by FTIR. The PDMS raw material was also performed
as a reference. The obtained results were demonstrated in Fig. 4.4. The pattern of TiO2PDMS was compared with that of PDMS raw material. The peaks at 3446 cm-1 and
1637 cm-1 correspond to the stretching vibration of the surface-absorbed watermolecules and O-H groups of samples respectively (Shao et al., 2015)The absorption
peak around 2964 cm-1 corresponds to vibration mode of the C-H stretching; Thedeformation of the CH3 group leads to the absorption peak at 1398 cm-1. The absorption
peak at 1261 cm-1 is attributed to the vibration mode of Si-CH3. The two adjacent peaks
between 1100 cm-1 and 1000 cm-1 present the vibration mode of the Si-O-Si stretching(Tavares et al., 2014). Another peak that corresponding to vibration mode of theasymmetric Si-O-Si stretching is located at 804 cm-1. Compared to the pattern of PDMS
raw material, the characteristic peak at 945 cm-1 presents the Si-OH or Si-O-Ti, which
may suggests the formation of a three-dimensional structure with interconnected
inorganic and organic component (Novotná et al., 2008). In addition, the adjacent peaks
between 1100 cm-1 and 1000 cm-1 transferred to a single peak at 1095 cm-1, whichindicates that the structure of Si-O-Si cm-1 network was formed. This particular
spectrum suggests that the Si atoms may be bonded with more than two O atoms thatinvolve in a three-dimensional structure (Liu et al., 2015). Moreover, the relative
intensity of the Si-C peak at 1261 cm-1 and CH3 cm-1 peaks at 2964 cm-1 were decreased
significantly, which may be attributed to the removal of methyl side groups in plasma(Pryce et al., 2000) before the formation of Si-O-Si crosslink network. These results
may indicate a complex crosslink structure was formed between PDMS and TiO2. In
addition, the methyl group plays an important role in super-hydrophobicity. Thus, the77structure containing the methyl group is desired.

Fig. 4.4 FTIR spectrum of TiO2, raw PDMS and TiO2/PDMS composite films.

4.3.2.2 Elements states
The surface composition and chemical structures of as-prepared TiO2/PDMS composite
film were examined by XPS, and the results are presented in Fig.4.5. The peaks
corresponding to Ti 2p, O 1s, and Si 2p are observed in the survey spectrum of TiO2
and TiO2/PDMS films, as shown in Fig. 4.5 (a). The Si 2p existence in TiO2 film may
be attributed to the substrate. Comparted to TiO2 film, it is obviously that the relative
intensity of Ti 2p in composite films decreased a lot. The possible reasons may be theaddition of PDMS diluted the concentration of Ti during deposition, or too thick layer
influence the XPS results since the XPS can only detect the approximately 1 um of thecomposite film. The high-resolution spectra of the Ti 2p and O 1s was shown in Fig.
4.5 (b-e). The doublet separation between the Ti 2p1/2 and 2p3/2 is 5.7 eV, which isconsistent with the standard binding energy of Ti4+ in TiO2, as shown in Fig. 4.5 (b) and78

(d). In addition, the doublet peaks centered at 459.1 and 464.8 shifted to 459.6 and465.3, respectively. It may be caused by the formation of Ti-O-Si (Ding et al., 2021).
The O 1s peak in TiO2 spectrum consist of three components: Ti-O (530.5 eV), hydroxyl
groups OH (531.8 eV), and absorbed water (532.6 eV) as shown in Fig. 4.5 (c). The O
1s peak in TiO2/PDMS spectrum can be resolved into four peaks, Ti-O (529.6 eV), TiO-Si (531.4 eV), Si-O-Si (532.0 eV), absorbed water (533 eV) (Lung et al., 2015), as
shown in Fig. 4.5 (e). These results confirmed the formation of Ti-O-Si. And indicatesthe composite film with crosslink structure.

Fig. 4.5 XPS profiles of the TiO2 and TiO2/PDMS film: (a) Survey, (b) and (d) Ti 2p,(c) and (e) O 1s spectra.
794.3.2.4 Crystalline

Fig. 4.6 XRD patterns of as prepared and after annealed TiO2 film and TiO2/PDMS
composite films.

The crystalline phases of as prepared TiO2 and TiO2/PDMS composite films andthe films annealed at 300 ℃ were determined by XRD as shown in Fig. 4.6. The asprepared TiO2 pattern exhibits a weak peak which is attributed to the anatase phase.
However, it is observed that, the supplying of PDMS may decrease the growth andcrystallization of TiO2 particles during deposition. Li et al considered the addition of Si
into TiO2 films can hinder the formation of stable nuclei larger than the critical size.
During the position process, silica atoms can prevent the diffusion of Ti atoms therefore
delay the nucleation-growth of Ti crystallites (Li et al., 2016). Generally, the TiO2 film
fabricated in out method is amorphous and it exhibits enhanced crystallinity after
annealing. In this study, after the films annealed at 300℃, the sharper peaks appearedin TiO2 pattern indicates that the annealing at this temperature is effective for80crystallization. However, TiO2/PDMS composite are still amorphous. Novotná et al.
(2008) believe that the highly dispersed SiO2 which from PDMS decomposition
inhibited the crystallization of TiO2. Neves et al. (2020) also reported the same
phenomenon when calcinate the TiO2/PDMS mixture at a temperature beyond thethermal stability of PDMS. Therefore, the thermal stability of products needs to be
carried out in my future work.

4.3.2.6 Water contact angle
A Digital Microscope was employed to record the water droplet contacts with thefilm surface. The water contact angle was estimated by the captured picture. The results
are shown in Fig. 4.7. TiO2 film shows a typical hydrophilic property (Fig. 4.7(a)),
while TiO2/PDMS shows the super-hydrophobic property with water contact angle
approximately 157° (Fig. 4.7(b)).

Fig. 4.7 Water contact angle of TiO2 and TiO2/PDMS film before photocatalytic
activity test.

4.3.2.5 Photocatalytic activity
The photocatalytic activities of the samples were evaluated based on the81decomposition of MB under UV irradiation. Fig. 4.8 shows the degradation of MB(Ct/C0) with respect to time (t) for the different films under UV-light irradiation. All thefilms exhibited photocatalytic activities under the same conditions. TiO2/PDMS
composite film shows almost same photocatalytic activity with TiO2 film within 6 hours.
In addition, the TiO2/PDMS composite film yielding an MB degradation of
approximately 29.3 %, whereas the pristine TiO2 film showed an activity of
approximately 29.0%.

Fig. 4.8 Photocatalytic activity for the degradation of MB (C0-Ct) with respect to time
t over different films.

After photocatalytic test, the water contact angle was measured again.
Unfortunately, the some of the composite films lost their super-hydrophobicity. In orderto understand this phenomenon, the morphology and molecular structure of the film
after photocatalytic activity test needs to be analysis again.

82

4.3.1 Film prepared under condition B
4.3.1.1 Morphology
Fig 4.9 shows the fabricated PDMS film with water contact angle approximatelyof 160°. This result proved the PDMS can be deposited as a super-hydrophobic film.
The morphology of TiO2/PDMS composite film was demonstrated in Fig. 4.10. The
composite film with a relative rough surface was confirmed and the structure barely
affected by varying flow rates of TTIP, as shown in Fig. 4.10 (a) and (b). Fig. 4.10 (cf) shows the results of TEM-EDS analysis. The appeared peaks corresponding to Si andO are originated form PDMS. However, there are no peaks corresponding to Ti when
TTIP flow rate is 50 sccm. When the TTIP flow rate increased from 50 to 200 sccm,
the Ti was observed in the EDS spectrum. In addition, the thickness of the TiO2/PDMS
composite film prepared by 200 sccm TTIP is approximately 3.5 μm. These results
confirmed the composition of TiO2/PDMS film and indicates a different structurecompared to Fig. 4.2.

Fig. 4.9 The surface morphology of PDMS film and image of water contact angle.

83

Fig. 4.10 The SEM, TEM and EDS of TiO2/PDMS fabricated by different TTIP flow
rate: 50 sccm (a) (c) and (e); 200sccm (b), (d) and (f).

4.3.1.2 Elements states
XPS analysis was employed to determine the chemical species and chemical statesin TiO2/PDMS composite prepared by TTIP with flow rate 200 sccm. The results wereshown in Fig. 4.11. The survey spectrum (Fig. 4.11 (a)) illustrates contains the peaks
corresponding to O 1s, Ti 2p, C1s, Si 2s and Si 2p. Among them, Fig. 4.11 (b)demonstrates the typical Ti 2p spectrum, with the 5.7 eV between two peaks, which84

indicates the existence of TiO2 in the composite film. The O 1s spectrum could be
deconvoluted into three peaks at 529.9, 532.0, and 533.3 eV (Fig. 4.11 (c)),
corresponding to the Ti-O band, Si-O-Si and absorbed water, respectively. Additionally,
the absence of Ti-O-Si, which confirmed a different molecular structure comparing to
the film prepared under Condition A.

Fig. 4.11 The XPS spectrum of TiO2/PDMS composite film: (a) survey spectrum, (b)Ti 2p spectrum and (c) O 1s spectrum.

4.3.2.3 Crystalline
The XRD pattern of as-prepared TiO2/PDMS prepared by TTIP with 200 sccm isshow in Fig. 4.12. The broaden peak at around 23° which is attributed to the amorphous
SiO2, and the peaks appeared at around 28°, 31° and 47 ° may be attributed to the Si.
85

There are no peaks corresponding to TiO2, which may be attributed to the low content
of TiO2 or TiO2 exists in an amorphous phase.

Fig. 4.12 The XRD pattern of as-prepared TiO2/PDMS composite film.

4.3.1.3 Photocatalytic activity.
The photocatalytic activities of the samples were evaluated based on thedecomposition of MB under UV irradiation. Fig. 4.13 shows the degradation of MB(Ct/C0) with respect to time (t) for the different films under UV-light irradiation. PDMS
film and TiO2/PDMS prepared by TTIP with flow rate 50 sccm cannot degrade the MB,
which is attributed to the absence of TiO2. When increase the TTIP flow rate to 200
sccm, the obtained film exhibits the photocatalytic activity. The percent MB
degradation by the different films after 6 h of irradiation was calculated using theformula [(C0-Ct)/C0] × 100%, where C0 and Ct are the concentrations of MB at time t =0 and after irradiation for time t, respectively. The TiO2/PDMS composite film prepared86

by TTIP with 200 sccm flow rate shows a photocatalytic activity which degrades 16%
MB after 6 hours irradiation.

Fig. 4.13 Photocatalytic activity for the degradation of MB (C0-Ct) with respect totime t over PDMS film, TiO2/PDMS composite prepared by different TTIP flow
rare: 50 sccm (TP 50); 200 sccm (TP 200).

4.3.2.7 Water contact angle
The water contact angle of the different film before and after photocatalytic test
was estimated, the results are shown in Fig. 4.14. PDMS and TiO2/PDMS film possessa super-hydrophobicity and this property is kept after photocatalytic activity test, thatis, the film resistant to UV irradiation. The

87

Fig. 4.14 Water contact angle of TiO2 and TiO2/PDMS film before photocatalytic test(a) and (c); and after photocatalytic test (b) and (d).

4.4 Conclusions
In this work, the TiO2/PDMS composite films were prepared by PECVD method
in two different experimental conditions. Under condition A, the addition of PDMS
changed the morphology of the TiO2 entirely and inhibit the crystal growth of TiO2
heavily. The formation of Si-O-Ti indicates an interconnection between inorganic andorganic component. The photocatalytic activity of the composite film decomposesapproximately 29% MB, which is almost same with TiO2 film. However, most of thesamples lost the super-hydrophobicity after photocatalytic test. The potential reason
will be studied in my future work. Under condition B, the presence of TiO2 in composite
film was confirmed, and it helps the film decomposing 16% MB under UV lightirradiation. In addition, the durable super-hydrophobicity after UV-irradiation of thecomposite film was proved. Thus, the films obtained under condition B are more
compliant with the expectation.
88

References
Ding, X., Liu, R., Zhao, J., Hu, J., Guan, H., & Tong, Y. Photocatalytic TiO2/PDMS
coating to drive self-cleaning: a facile approach for anti-stain silicone rubber
surfaces. Polymer Bulletin. (2021).
Kubo, M., Taguchi, T., & Shimada, M. Preparation of nanoparticle-embedded thin films
by simultaneous feeding of gaseous and solid raw materials in plasma-enhanced
chemical vapor deposition process. Thin Solid Films, 632, 55-65. (2017).
Lee, J., H., Kim, Y. S., Oh, J. S., Kyung, S. J., Lim, J. T., & Yeom, G. Y. Characteristicsof SiOx thin film deposited by atmospheric pressure plasma-enhanced chemical
vapor deposition using PDMS/O2/He. Journal of The Electrochemical Society,156(7), D248. (2009).
Li, D., Elisabeth, S., Granier, A., Carette, M., Goullet, A., & Landesman, J. P. Structuraland optical properties of PECVD TiO2–SiO2 mixed oxide films for opticalapplications. Plasma Processes and Polymers, 13(9), 918-928. (2016).
Liu, G., Hung, W.-S., Shen, J., Li, Q., Huang, Y.-H., Jin, W., Lee, K.-R., & Lai, J.-Y.
Mixed matrix membranes with molecular-interaction-driven tunable free volumes
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(2015).
Lung, C. Y. K., Heinonen, M., Kukk, E., & Matinlinna, J. P. Surface modification oftitanium with thermally treated polydimethylsiloxane coating and the effect on
resin to titanium adhesion. Surface and Interface Analysis, 47(1), 105-112. (2015).
Neves, J. C., Mohallem, N. D. S., & Viana, M. M. Polydimethylsiloxanes-modified
TiO2 coatings: The role of structural, morphological and optical characteristics ina self-cleaning surface. Ceramics International, 46(8, Part B), 11606-11616.
(2020).
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Novotná, P., Zita, J., Krýsa, J., Kalousek, V., & Rathouský, J. Two-component
transparent TiO/SiO2 and TiO2/PDMS films as efficient photocatalysts forenvironmental cleaning. Applied Catalysis B: Environmental, 79(2), 179-185.
(2008).
Pryce, L., G., H., Edell, D. J., & Gleason, K. K. Pulsed-PECVD Films from
Hexamethylcyclotrisiloxane for Use as Insulating Biomaterials. Chemistry ofMaterials, 12(11), 3488-3494. (2000).
Shao, P., Tian, J., Zhao, Z., Shi, W., Gao, S., & Cui, F. Amorphous TiO2 doped withcarbon for visible light photodegradation of rhodamine B and 4-chlorophenol.
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Tavares, M. T. S., Santos, A. S. F., Santos, I. M. G., Silva, M. R. S., Bomio, M. R. D.,
Longo, E., Paskocimas, C. A., & Motta, F. V. TiO2/PDMS nanocomposites for use
on self-cleaning surfaces. Surface and Coatings Technology, 239, 16-19. (2014).
Thongrom, S., Tirawanichakul, Y., Munsit, N., & Deangngam, C. One-step microwave
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Xu, F., Wang, T., Bohling, J., Maurice, A. M., Chen, H., Wu, L., & Zhou, S. Extendedhydrophobicity and self-cleaning performance of waterborne PDMS/TiO2
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Chapter 5
Summary

5.1 Conclusions
In this thesis, different materials were employed to fabricate TiO2-based composite
films by PECVD method. By introducing these foreign materials, the TiO2-based
composite exhibit the expected improvement of photocatalytic activity and superhydrophobicity. The conclusion of this dissertation can be described as following.
The TiO2-CNT-Ag ternary films were successfully prepared utilizing CNTs andAg solid mixture via PECVD method. During the process, solid nanoparticles were
deposited simultaneously with TiO2 onto the substrate. This process resulted in thenanoparticle-embedded structure, which was proved by the morphology observation.
The addition of CNTs and Ag does not affect the film thickness nor the crystal structure.
However, these particles influenced the surface roughness and band gap energy of theTiO2. Furthermore, the addition of CNT and Ag significantly improved thephotocatalytic activity of the prepared films.
To simplify the operation process, AgNO3 aqueous solutions was supplied directly
instead of being prepared as silver particles. The influence of the AgNO3 concentrationon the structure and photocatalytic activity of the TiO2-Ag composite films wasinvestigated. Results present that the addition of AgNO3 aqueous solution during
preparation process caused a secondary particle on the film surface, which can be
attributed to the formation of aggravated Ag nanoparticles. The addition of Ag
nanoparticles significantly altered the morphology of the films owing to particle
aggregation. Notably, the appearance of the Ag nanoparticles significantly decreased
91the bandgap energy of the composite. In addition, appropriate AgNO3 concentration(1wt% in this dissertation) leads to a higher photocatalytic activity of the films.
The TiO2/PDMS composite film with photocatalytic activity and superhydrophobicity was fabricated by PECVD method. Two different experimental
conditions were applied for generating composite films. The different conditions
resulted in different film structures and properties. The film obtained under condition
A exhibits a relatively better photocatalytic activity, but unsatisfied superhydrophobicity. The condition B yields a film with durable super-hydrophobicity, andit also exhibits the photocatalytic activity. Compared to condition A, the condition B is
more conducive to the synthesis of films with super-hydrophobicity and photocatalytic
activity.
Overall, the investigated process exhibited immense potential for use in the
preparation of films with different materials.

5.2 Suggestions for further study
TiO2-based composite films were successfully prepared by utilizing different raw
materials via PECVD method. In response to the shortcomings of the work, thesuggestion for future work can be conclude as follows:
The particle size of Ag in preparation of TiO2-CNT-Ag ternary composite film islack of investigation. Co-precipitation method may produce different size of Ag
nanoparticles. The particle size can also significantly influence the structure andphotocatalytic activity of TiO2 film. Thus, prepare different size of Ag nanoparticle may
be helpful to fabricate films with better photocatalytic activity. The use of different
carbon nanotubes also deserves to be considered, as we all know the different properties
between multi-walled CNT and single walled CNT.
92

The detail of reduction of AgNO3 in plasma should be classified in future works.
A controllable process for fabrication Ag nanoparticle is useful for many applications.
This also facilitates the application of other metallic materials. The photocatalytic
activity of composite film under visible light needs to be investigated.
The reason for the absence of hydrophobicity of the films after photocatalysis
needs to be investigated, which may be helpful in preparing films with better properties.
The effects of experiment factor on film structure and properties need to be explore in
more detail. In addition, the composite film shows a unique structure, which may leadto different properties.
The other nanoparticles like graphene, copper as well as some ionic solution canbe employed as raw material. In addition, not only TiO2 but also other support materials
like ZnO, SnO2, WO3 films can be deposited by PECVD method. Overall, in
accordance with certain requirements, different raw material can be applied to codeposition by PECVD.

93

94

Appendix Ⅰ˖˖List of Figures
Fig. 1.1 Photo-induced mechanism of electron-hole pair in TiO2 particle with powerfuloxidizing agents. ............................................................................................................ 2澳
Fig. 1.2 The possible schematic of electron-hole pairs generated in TiO2-Ag composite:(a) under UV light irradiation; (b) under visible light irradiation. ................................. 4澳
Fig. 1.3 The proposed mechanism of CNT driven improvement of photocatalytic
activity: (a) CNTs act as an electron acceptor, prevent the electron-hole pair
combination; (b) CNTs were regarded as a photosensitizer, generate the electron-hole
pair; (c) The formation of Ti-O-C bonds. ...................................................................... 6澳
Fig. 1.4 The schematic of contact angle between the droplet and substrate. ................. 7澳
Fig. 1.5 Simple classification of some common thin film preparation method. ............ 8澳
Fig. 1.6 Diagrammatic illustration of the brief objective of dissertation. .................... 19澳
Fig. 2.1 Experimental setup for PECVD. .................................................................... 34澳
Fig 2.2 Light spectrum used to activate the films. ....................................................... 37澳
Fig. 2.3 SEM images of: (a) CNTs, (b) and (c) prepared T-C film containing cracks. 38澳
Fig. 2.4 SEM images of the prepared films. Cross-sectional morphologies of (a) T, (b)T-C, (c) T-A, and (d) T-C-A0. Surface morphologies of (e) T, (f) T-C, (g) T-A, and (h)T-C-A0. ........................................................................................................................ 39澳
Fig. 2.5 Elemental mapping of T-A and T-C-A. ........................................................... 40澳
Fig. 2.6 SEM images of T-C-A films deposited using different concentrations of
CNT/Ag mixed suspensions: (a) T-C-A0 (0.25 wt%/0.1 wt%), (b) T-C-A1 (0.25
wt%/0.05 wt%), (c) T-C-A2 (0.25 wt%/0.01 wt%), (d) T-C-A3 (0.01 wt%/0.1 wt%),and (e) T-C-A4 (0.5 wt%/0.1 wt%). ............................................................................. 41澳
Fig. 2.7 SEM images of P25: (a) pristine P25 powder, (b) film surface, and (c) film
cross-section. ................................................................................................................ 42澳
95

Fig. 2.8 TEM images of the prepared T-C-A0 film: (a) CNTs with Ag and TiO2 particles
attached, (b) magnified version of the area in the yellow square in (a), and EDS resultscorresponding to the black square in (b). ..................................................................... 43澳
Fig. 2.9 X-ray photoelectron spectra of the T-C-A0 film: (a) survey, (b) Ag 3d, (c) Ti
2p, and (d) O 1s spectra. .............................................................................................. 44澳
Fig. 2.10 XRD patterns of (a) T and T-composite films and (b) P25 and T-C-A0 films
annealed at different temperatures. .............................................................................. 46澳
Fig. 2.11 Optical properties of the T and T-composite film: (a) absorbance spectra; (b)(αhυ)2 versus photon energy (hυ) plot of the prepared films. ...................................... 48澳
Fig. 2.12 Photocatalytic activity for the degradation of Rhodamine 6G by different films
under simulated solar light: (a) degradation with respect to time, (b) ln (Ct/C0) versus
to time plot, and (c) rate constants degradation for various samples. .......................... 50澳
Fig. 2.13 Photocatalytic degradation of Rhodamine 6G by P25 and T-C-A films
annealed at different temperatures: (a) degradation with respect to time, (b) ln (Ct/C0)
versus to time plot, and (c) corresponding rate constants of degradation for samples.52澳
Fig. 2.14 Possible schematic explaining the photocatalytic activity of the T-C-A ternaryfilm. .............................................................................................................................. 53澳
Fig. 3.1 XRD patterns of the pristine TiO2 film and TiO2-Ag composite films depositedusing different concentrations of AgNO3. .................................................................... 60澳
Fig. 3.2. SEM images on the surface view and cross section of (a) pristine TiO2 film
and TiO2-Ag composite films deposited using different AgNO3 concentrations: (b–f)
0.1, 0.5, 1, 2, and 4 wt.%, respectively. ....................................................................... 61澳
Fig. 3.3. (a) TEM image, (b) HTREM images and the corresponding EDS profile of theregion in the yellow square of the TiO2-Ag (1 wt%) film. .......................................... 62澳
Fig. 3.4 XPS profiles of the TiO2-Ag (1 wt.%) film: (a) Survey, (b) Ag 3d, (c) Ti 2p,
96and (d) O 1s spectra. .................................................................................................... 63澳
Fig. 3.5 (a) Absorbance spectra of the pristine TiO2 film (T) and TiO2-Ag composite
films deposited using AgNO3 concentrations of 0.1–4 wt.%. (b) Plot of (αhυ)2 versus
photon energy (hυ) of the prepared films. .................................................................... 65澳
Fig. 3.6 Photocatalytic activity for the degradation of MB over different films: (a)degradation with respect to time, (b) ln (Ct/C0) versus to time plot, and (c) rate constantsdegradation for various samples. ................................................................................. 68澳
Fig. 4.1 Experimental setup of PECVD for preparing TiO2/PDMS composite. .......... 73澳
Fig 4.2 SEM images of the prepared films. Surface morphologies of (a) TiO2 and (c)TiO2-PDMS. Cross sectional morphologies of (b) TiO2 and (d) TiO2-PDMS............. 75澳
Fig. 4.3 TEM images of the prepared TiO2/PDMS composite films. The structure of
aggregation of tiny particles (a) and the corresponding EDS profile (b). The structure
consists of aggregation tiny particles and columnar particles (c); The high magnification
images (d) and the corresponding EDS profile (e); The high magnification images (f)and the corresponding EDS profile (g). ....................................................................... 76澳
Fig. 4.4 FTIR spectrum of TiO2, raw PDMS and TiO2/PDMS composite films. ........ 78澳
Fig. 4.5 XPS profiles of the TiO2 and TiO2/PDMS film: (a) Survey, (b) and (d) Ti 2p,(c) and (e) O 1s spectra. ............................................................................................... 79澳
Fig. 4.6 XRD patterns of as prepared and after annealed TiO2 film and TiO2/PDMS
composite films. ........................................................................................................... 80澳
Fig. 4.7 Water contact angle of TiO2 and TiO2/PDMS film before photocatalytic activity
test. ............................................................................................................................... 81澳
Fig. 4.8 Photocatalytic activity for the degradation of MB (C0-Ct) with respect to time
t over different films. ................................................................................................... 82澳
Fig. 4.9 The surface morphology of PDMS film and image of water contact angle. .. 83澳
97

Fig. 4.10 The SEM, TEM and EDS of TiO2/PDMS fabricated by different TTIP flow
rate: 50 sccm (a) (c) and (e); 200sccm (b), (d) and (f). ................................................ 84澳
Fig. 4.11 The XPS spectrum of TiO2/PDMS composite film: (a) survey spectrum, (b)Ti 2p spectrum and (c) O 1s spectrum. ........................................................................ 85澳
Fig. 4.12 The XRD pattern of as-prepared TiO2/PDMS composite film. .................... 86澳
Fig. 4.13 Photocatalytic activity for the degradation of MB (C0-Ct) with respect to time
t over PDMS film, TiO2/PDMS composite prepared by different TTIP flow rare: 50
sccm (TP 50); 200 sccm (TP 200)................................................................................ 87澳
Fig. 4.14 Water contact angle of TiO2 and TiO2/PDMS film before photocatalytic test(a) and (c); and after photocatalytic test (b) and (d). ................................................... 88澳

98

Appendix Ⅱ˖˖List of Tables
Table 1.1 Application of TiO2-based composite with super-hydrophobicity ............... 17澳
Table 2.1 Ratio of Ag/Ti content in the T-C-A film ..................................................... 49澳
Table 3.1 Ratio of Ag/Ti content in the T-C-A film ..................................................... 64澳
Table 3.2 Bandgap energies (Eg) of the films. ............................................................. 66澳
Table 4.1 The detail of Condition B ............................................................................. 73澳

99

Appendix Ⅲ˖
˖List of Publications
The dissertation was partly written based on the following publications.
1. Lang, J.; Takahashi, K.; Kubo, M.; Shimada, M. Ag-Doped TiO2 Composite Films
Prepared Using Aerosol-Assisted, Plasma-Enhanced Chemical Vapor Deposition.
Catalysts 2022, 12, 365.
2. Lang, J.; Takahashi, K.; Kubo, M.; Shimada, M. Preparation of TiO2-CNT-Ag
Ternary Composite Film with Enhanced Photocatalytic Activity via PlasmaEnhanced Chemical Vapor Deposition. Catalysts 2022, 12, 508.

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Presentation in Conferences
1. Lang, J.; Takahashi, K.; Kubo, M.; Shimada, M. Photocatalytic activity andpreparation of TiO2-CNT-Ag ternary thin films via aerosol-assisted and plasmaenhanced chemical vapor deposition. European Aerosol Conference (EAC 2021),
EAC2021_AT P3-9_131_Lang.pdf, virtual conference. Aug. 30–Sep. 3, 2021.
2. Lang, J.; Takahashi, K.; Kubo, M.; Shimada, M. Preparation of Ag-embedded TiO2
film by reducing metal ions via plasma-enhanced chemical vapor deposition. SCEJ(The society of Chemical Engineers, Japan) 87th Annual Meeting, L321, Mar. 16–18,
2022.
3. Lang, J.; Takahashi, K.; Kubo, M.; Shimada, M. Ag-Doped TiO2 Composite films
prepared using Aerosol-assisted, plasma-enhanced chemical vapor deposition. 12th
Asian Aerosol Conference (AAC) 2022, fb1930, June 12–16, 2022.

101