Pt與In2O3之修飾對於TiO2奈米線之光催化性質的影響

國 立 交 通 大 學

材料科學與工程學系

碩 士 論 文

Pt與In

O

之修飾對於TiO

奈米線之光催化性質的影響

Effects of Pt and In2O3 Decorations on the Photocatalytic Properties of

Anatase TiO2 Nanowires

研 究 生：陳宇志

指導教授：徐雍鎣 博士

I

Pt 與 In

O

之修飾對於

TiO

奈米線之光催化性質的影響

Abstract (in Chinese)

透過結合不同能帶結構的半導體奈米晶體能提供許多的優點，如:更 寬廣的吸收範圍、降低光漂白現象、能夠調控的能階和改善光電的轉 換效率。在此論文中，我們透過沉積-鍛燒的方法成功合成出氧化銦 修飾的銳鈦礦二氧化鈦奈米線 (In2O3-TiO2 NWs)。由於氧化銦和二氧 化鈦能帶結構的差異，使此複合材料能產生載子轉移，造成電子電洞 對的分離。In2O3-TiO2 奈米線內的載子分離特性，也展現在其降解染 料的光催化的特性上。相較於商用的P-25 TiO2粉末和利用物理方式

混合的In2O3-loaded TiO2 NWs，In2O3-TiO2 NWs 在紫外光的照射下，

展現出更佳的光催化特性。為了更進一步提升其光催化活性，適量的 Pt 奈米粒子也被用來修飾在 In2O3-TiO2 NWs 上。這樣的提升是來自

於在修飾Pt 到二氧化鈦表面上之後，增加了電子由 In2O3傳遞到TiO2

II

接收陽光中的能量進行高效率的光催化反應。電子在In2O3,TiO2和

Pt 的轉移現象，我們也透過其相對的光電流量測和時間解析螢光光光 譜來探討分析。

III

Effects of Pt and In2O3 Decorations on the Photocatalytic Properties of

Anatase TiO2 Nanowires

Student： Yu-Chih Chen Advisors：Dr. Yung-Jung Hsu National Chiao Tung University

Department of Materials Science and Engineering

Abstract (in English)

Combination of semiconductor nanocrystal (NCs) with different band structures has been extensively explored since a lot of advantages may be offered by this composite system, for example, the wide absorption profile, the enhanced resistance to photobleaching, the continuous tunability of energetic levels, and the improved photoelectric conversion efficiency. In this thesis, we successfully synthesized In2O3-decorated anatase TiO2

nanowires (In2O3-TiO2 NWs) through a simple precipitation-annealing

process. The difference in band structures between In2O3 and TiO2

provided charge carriers with another preferential route for charge transfer, resulting in the separation of photoexcited electrons and holes.

IV

The charge carrier separation in In2O3-TiO2 NWs was revealed with their

photocatalytic decomposition toward an organic dye. As compared to the commercial P-25 TiO2 powder and In2O3-loaded TiO2 NW samples, the

as-synthesized In2O3-TiO2 NWs exhibited superior photocatalytic

efficiency under UV illumination, demonstrating their potential as an efficient photocatalyst in relevant redox reactions. A further enhancement in the photocatalytic activity of In2O3-TiO2 NWs can be achieved when Pt

nanoparticles of suitable amount were deposited on the surface of TiO2.

This improvement is due to the increasing driving force of electron transfer from In2O3 to TiO2 achieved by the decoration of Pt on TiO2

surfaces. Furthermore, the result of performance evaluation under natural sunlight shows that the present In2O3-TiO2-Pt NWs can be used as highly

efficient photocatalysts which may practically harvest energy from sunlight. The electron transfer event among In2O3, TiO2 and Pt was also

characterized with the corresponding photocurrent measurement and time-resolved PL spectroscopy.

V

致謝

Acknowledgements

VI

Table of Content

Abstract (in Chinese)

I

Abstract (in English)

III

Acknowledgements V

Figure Captions

VIII

Chapter 1. Introduction

1

1.1 Introduction of TiO2 ... 1 

1.2 One-dimentional TiO2 nanostructures ... 2 

1.2.1 Previous Studies of 1-D TiO2 nanostructures ... 4 

1.2.2 Recently Studies of 1-D TiO2 Nanostructures ... 7 

1.3 Semiconductor-Metal Composite System ... 9 

1.4 Semiconductor-Semiconductor Composite System ... 11 

Chapter 2. Experimental Section

13

2.2 Instruments and Principles ... 13 

2.3 Preparation of Anatase TiO2 Nanowires ... 16 

2.4 Preparation of Pt-decorated TiO2 NWs ... 17 

VII

2.6 Preparation of Pt and In2O3 co-decorated TiO2 NWs ... 18 

2.7 Photocurrent Measurement ... 19 

2.8 Photocatalytic Activity Study ... 19 

2.9 Photoluminescence Lifetime Measurement ... 20 

2.10 Characterization ... 21 

Chapter 3. Results and Discussion

22

3.2 Decoration of Pt on TiO2 NWs ... 23 

3.3 Photocatalytic Properties of Pt-decorated TiO2 NWs ... 27 

3.4 Effects of Various Metal Decorations and Recycling Test .... 30 

3.5 Decoration of In2O3 on TiO2 NWs ... 33 

3.6 Photocatalytic Properties of In2O3-decorated TiO2 NWs ... 38 

3.7 Further Decoration of Pt on In2O3-TiO2 NWs ... 42 

3.8 Photocatalytic Properties of In2O3-TiO2-Pt NWs ... 44 

3.9 Photocurrent Measurement for In2O3-TiO2-Pt NWs ... 48 

3.10 Time-Resolved Photoluminescence Spectra ... 52 

Chapter 4. Conclusion

58

VIII

Figure Captions

Figure 1.1 List of bandgaps of common semiconductors. ... 2  Figure 1.2.1 Comparison of photocatalytic efficiency for TiO2 nanorods

with different aspect ratios. ... 3  Figure 1.2.2 Morphological phase diagram of nanostructures of H2Ti3O7

after 20 h of hydrothermal treatment ... 5  Figure 1.2.3 TEM images of H2Ti3O7 with (A) nanoparticle/nanotube

mixture, (B) nanotubes, (C) nanotube/nanoribbon mixture, (D) and nanoribbons ... 6  Figure 1.2.4 Schematic illustration for the formation mechanism of

H2Ti3O7 nanotubes ... 6 

Figure 1.2.5 (a) SEM image of H2Ti3O7 NWs. (b,c,d) TEM and ED

results of the anatase TiO2 nanoshuttles. (e,f) TEM and the

corresponding HRTEM images for a single shuttle ... 8  Figure 1.2.6 TEM images of the TiO2 products obtained with different

hydrothermal reaction times: (a) 3 h, (b) 6 h. ... 8  Figure 1.3.1 Fermi level equilibration in a semiconductor-metal

IX

Figure 1.3.2 (a) Typical TEM image, (b) ED pattern and (c) HRTEM image of Au-decorated NaxH2-xTi3O7 NBs ... 10 

Figure 1.3.3 Photodegradation of thionine under visible irradiation by using Au-decorated NaxH2-xTi3O7 NBs with various Au

contents. ... 11  Figure 1.4.1 Charge crrier dynamis in a coupled semiconductor system.

... 12  Figure 3.1.1 SEM images of (a) H2Ti3O7 and (b) anatase TiO2 NWs. (c)

The corresponding XRD patterns. ... 23  Figure 3.2.1 (a) Typical TEM, (b) HRTEM and (c) EDS analyses of

Pt-decorated TiO2 NWs. ... 26 

Figure 3.2.2 TEM images of Pt-decorated TiO2 NWs with the Pt contents of (a) 1.0, (b) 2.0, (c) 3.0, (d) 5.0 and (e) 10 wt %. ... 26  Figure 3.3.1 Absorption spectra of MB solutions under different UV

irradiation times by using TiO2 NWs. ... 29 

Figure 3.3.2 C/Co versus irradiation time plots for MB photodegradation

under UV illumination by using Pt-decorated TiO2 NWs with

X

Figure 3.4.1 The MB photodegradation results by using different photocatalysts including Pt-loaded, Pt-decorated, Ag-decorated, Au-decorated TiO2 NWs, and P-25 TiO2

powder ... 32  Figure 3.4.2 Recycling test on Pt-decorated TiO2 NWs with Pt content of

3 wt %. ... 32  Figure 3.5.1 SEM images of (a) In2O3-decorated TiO2 NWs (molar ratio

of In2O3 to TiO2 =1:1), and (b) pure In2O3 nanoparticles ... 35 

Figure 3.5.2 SEM image of In(OH)3-decorated TiO2 NWs. ... 36 

Figure 3.5.3 HRTEM image taken on an individual In2O3-decorated TiO2

NW ... 36  Figure 3.5.4 In2O3-decorated TiO2 NWs with various In2O3/TiO2 molar

ratios of (a) 0.25:1, (b) 0.5:1, (c) 1:1, (d)1.5:1, (e) 2:1, (f) 2.5:1 (g) 5:1 ... 37  Figure 3.6.1 Absorption spectra of RB solutions under different UV

XI

Figure 3.6.2 C/Co versus irradiation time plots for RB photodegradation

under UV illumination by using In2O3-decorated TiO2 NWs

with various In2O3/TiO2 molar ratios ... 40 

Figure 3.6.3 The RB photodegradation results under UV light irradiation by using different photocatalysts including In2O3-loaded,

In2O3-decorated, TiO2 NWs and commercial P-25 TiO2

powders ... 41  Figure 3.6.4 Recycling test on In2O3-decorated TiO2 NWs with the

In2O3/TiO2 molar ratio of 1:1. ... 42 

Figure 3.7.1 TEM image and the correspding EDS spectra for

In2O3-TiO2-Pt NWs. ... 44 

Figure 3.8.1 Band structures and charge transfer event in In2O3-TiO2-Pt

NWs. ... 46  Figure 3.8.2 MB photodegradation results under UV light irradiation by

using In2O3-TiO2-Pt NWs with various Pt contents. ... 46 

Figure 3.8.3 Absorption spectra of MB solutions after exposure of 1 h of daytime sunlight (a) without any catalyst and (b) in the

XII

Figure 3.9.1 Photocurrent response of pure TiO2 NWs, In2O3-TiO2 NWs

and In2O3-TiO2-Pt NWs with differernt Pt contents. ... 50 

Figure 3.9.2 Illustration of the apparent Fermi level potential of TiO2 in

(a) In2O3-TiO2 and (b) In2O3-TiO2-Pt systems. ... 51 

Figure 3.10.1 Charge separation scheme for Pt/In2O3/TiO2 in present of

RB dye. ... 53  Figure 3.10.2 Quenching of photoluminescence emission of RB Dye

(doner) with different samples (acceptors). ... 53  Figure 3.10.3 Time-resolved PL spectra of RB solutions containing In2O3,

In2O3-TiO2 and In2O3-TiO2-Pt samples. ... 56 

Table 1 Kinetic analysis of emission decay of RB in the presence of different samples. ... 57 

1

Chapter 1. Introduction

1.1 Introduction of TiO

Titanium dioxide (TiO2) has been widely employed as photocatalytic

materials in the purification and treatment of contaminated air and water owing to its low cost, nontoxicity, and chemical stability [1]. Recently, one-dimensional nanostructures of TiO2 such as nanotubes and nanowires

have attracted intensive research interests because of their dimensionality-dependent physicochemical properties and potential applications in the fields of solar-energy conversion, lithium batteries, and supercapacitors. TiO2 exists in nature as well-known minerals of rutile,

anatase and brookite. Anatase is metastable with respect to rutile and may transform spontaneously into rutile at temperature above 800 ºC [2]. As shown in Figure 1.1, anatase TiO2 possesses relatively negative

conduction band potential that ensures promising activity in many photocatalytic processes. However, its wide bandgap energy (3.2 eV) allows only ultraviolet (UV) absorption. As a result, an effort to modulate the band structure of TiO2 to enable photocatalysis under visible light is

2

Figure 1.1 List of bandgaps of common semiconductors [3].

1.2 One-dimentional TiO

nanostructures

One-dimensional (1-D) nanostructures including nanowires, nanorods, nanoribbons and nanotubes have been the subject of intensive research due to their potential use as active components or interconnects in nanoscale electronic and electromechanical devices [4]. The space charge region in 1-D nanostructured crystals is well constructed along the longitudinal direction, enabling charge transfer along single direction and thus the facilitation of carrier collections. Furthermore, the pronounced delocalization of charge carriers in 1-D crystals can significantly lower

3

the probability of e--h+ recombination [5]. Consequently, improved photoelectric conversion efficiency such as the photocatalytic activity was regularly observed in 1-D nanostructures when compared with their spherical or particulate counterparts. As shown in Figure 1.2.1, TiO2

nanorods with higher aspect ratios were found to exhibit superior photocatalytic performance as compared to the nanoparticles and short nanorods [5].

Figure 1.2.1 Comparison of photocatalytic efficiency for TiO2 nanorods

4

1.2.1 Previous Studies of 1-D TiO

nanostructures

There are many synthetic methods to obtain TiO2 in 1-D forms, such

as sol-gel method [6, 7], hydrothermal method [8],solvothermal method [9], direct oxidation process [10], chemical vapor deposition [11], electrochemical deposition [12], and sonochemical method [13]. As one of the Ti-O series products, hydrogen titanate (H2Ti3O7) has draw much

research attention due to its capability of feasible transformation into 1-D TiO2 upon heat treatment. Since the pioneering work by Kasuga et al [14,

15], alkaline hydrothermal approach has been widely employed to produce H2Ti3O7. By using P-25 TiO2 powder as the precursor in

concentrated NaOH solution, various 1-D H2Ti3O7 including nanotubes,

nanosheets, nanowires and nanoribbons can be obtained in high yield. As revealed in Figure 1.2.2, the morphology of H2Ti3O7 in the alkaline

hydrothermal process can be readily controlled by modulating the concentration of NaOH and the reaction temperature. Figure 1.2.3 shows the TEM images of the as-obtained H2Ti3O7 products by using the

alkaline hydrothermal method. A formation mechanism for these 1-D nanostructures of H2Ti3O7 was proposed and illustrated in Figure 1.2.4

5

partially dissolved in NaOH, leaving terraces on their surface. The dissolved atoms recrystallized in sheet-like structures, which further curved up into nanoloops with single-spiral, multiple-spiral, or onion-like cross section. The rest of the bulk crystal was then transformed into nanotubes by oriented crystal growth, supplied with TiO6 building blocks

via the alkaline dismantling of the anatase raw material of TiO2.

Figure 1.2.2 Morphological phase diagram of nanostructures of H2Ti3O7

6

Figure 1.2.3 TEM images of H2Ti3O7 with (A) nanoparticle/nanotube

mixture, (B) nanotubes, (C) nanotube/nanoribbon mixture, (D) and

nanoribbons. Conditions of reaction: (A) 5 mol dm-3 of NaOH at 140 °C, (B) 9 mol dm-3 of NaOH at 160 °C, (C) 5 mol dm-3 of NaOH at 220 °C, and (D) 7.5 mol dm-3 of NaOH at 220 °C [16].

Figure 1.2.4 Schematic illustration for the formation mechanism of H2Ti3O7 nanotubes [17].

7

1.2.2 Recently Studies of 1-D TiO

Nanostructures

With the open-ended tabulate and layered structures, H2Ti3O7 has

been shown promising in the preparation of diverse 1-D TiO2

nanostructures. Recently, Wang et al successfully synthesized anatase TiO2 nanoshuttles through the hydrothermal treatment on the

first-prepared H2Ti3O7 NWs [18]. Figure 1.2.5 displays the TEM images

of TiO2 nanoshuttles obtained by controlling the pH value of the reaction

solution. Due to the minimization of total surface energies, TiO2

nanoshuttles were grown with the thermodynamically stable facets of (101) and (001) exposed at their surfaces. The morphology of the final products can be further modulated by controlling the hydrothermal reaction time. As shown in Figure 1.2.6, fishbone-like composite structures can be obtained by prolonging the reaction time to 3 and 6 h. These delicate structures may find practical applications in photocatalysts.

8

Figure 1.2.5 (a) SEM image of H2Ti3O7 NWs. (b,c,d) TEM and ED

results of the anatase TiO2 nanoshuttles. (e,f) TEM and the corresponding

HRTEM images for a single shuttle [18].

Figure 1.2.6 TEM images of the TiO2 products obtained with different

9

1.3 Semiconductor-Metal Composite System

Semiconductor-metal nanocomposites have been widely employed in photocatalysis [19-21]. The overall photocatalytic efficiency of semiconductors can be enhanced by contacting with metals. When semiconductor and metal nanoparticles are in contact, the photoexcited charge carriers re-distribute between the semiconductor and metal domains [22]. This phenomenon promotes the separation of electron-hole pairs. Figure 1.3.1 displays the Fermi level equilibration in a semiconductor-metal nanocomposite system. Within this system, metal may play an important role of storing and shuttling electrons. The photogenerated electrons of semiconductor would thus preferentially transfer to metal domain, leaving positively charged holes in semiconductor to achieve charge carrier separation [23]. Pu et al recently reported a semiconductor-metal composite system of Au-decorated NaxH2-xTi3O7 nanobelts (NBs). The morphological investigation was

shown in Figure 1.3.2 [24]. Due to the band offsets between NaxH2-xTi3O7

and Au, a notable charge separation was expected to take place at the interface of NaxH2-xTi3O7/Au, leading to the enhanced photocatalytic

10

Figure 1.3.1 Fermi level equilibration in a semiconductor-metal nanocomposite system [22]

Figure 1.3.2 (a) Typical TEM image, (b) ED pattern and (c) HRTEM image of Au-decorated NaxH2-xTi3O7 NBs [24].

11

Figure 1.3.3 Photodegradation of thionine under visible irradiation by using Au-decorated NaxH2-xTi3O7 NBs with various Au contents. [24]

1.4 Semiconductor-Semiconductor Composite System

Besides the semiconductor-metal composite system, combination of two semiconductors with appropriate band offsets (type-II heterogeneity) may also improve the charge carrier separation. Kamat et al studied the photoelectrochemical behavior of CdSe/TiO2 composite films and found

an increased photocurrent over the electrode [25]. As shown in Figure 1.4.1, upon light illumination, the photogenerated electrons quickly transferred from CdSe to TiO2 due to the lower conduction band potential

12

interface of CdSe/solution, allowing for oxidation reaction with the redox couple of solution. Combining two different semiconductors has two main advantages: (1) extension of light absorption in a wider range, and (2) retardation of charge carrier recombination.

Figure 1.4.1 Charge crrier dynamis in a coupled semiconductor system [25].

13

Chapter 2. Experimental Section

2.1 Chemicals

All chemicals were analytic grade reagents and used without further purification.

1. P-25 (TiO2), Degussa.

2. Sodium hydroxide (NaOH), 99.995%, Aldrich. 3. Ethanol (CH3CH2OH), 99.5%, Aldrich.

4. Chloropatinic acid hydrate (H2PtCl6),99.995%, Aldrich.

5. Indium (III) chloride (InCl3), 99.99%, Aldrich.

6. Ammonium hydroxide solution (NH4OH), 33%, Aldrich.

7. Chloroauric acid (HAuCl4), 99.99%, Aldrich.

8. Silver nitrate (AgNO3), 99.99%, J.T Baker.

9. Methylene blue, (C16H18N3SCl．3H2O), Aldrich.

10. Rhodamine B, (C28H31ClN2O3), Aldrich.

2.2 Instruments and Principles

1. Scanning Electron Microscope (SEM): The kinetic energy of

accelerated electrons in SEM is dissipated as many different signals when hitting the sample surfaces. These signals mainly include

14

secondary and backscattered electrons. Secondary and backscattered electrons are then collected and used for imaging the morphology and compositional contrast of samples, respectively.

2. Transmission Electron Microscope (TEM): In TEM, the electrons are focused with electromagnetic lenses and transmitted through the sample to image and analyze the microstructure. The electron beams are basically accelerated at several hundred kV, producing wavelength much smaller than that of light. For example, 200 kV of acceleration voltage produces electron beam with a wavelength of 0.025Å. The resolution of TEM is however limited by aberrations inherent in electromagnetic lenses, which is about 1-2 Å.

3. Energy-Dispersive X-Ray Spectroscopy (EDS): EDS is a typical facility of SEM and TEM. It includes a sensitive X-ray detector, a liquid nitrogen cooling system, and software to analyze the energy spectra. An EDS detector contains a crystal that absorbs X-ray by ionization, yielding free electrons in the crystal to produce an electrical charge bias. The signals are then converted into the characteristic X-ray of element to identify the composition of samples.

15

produced through the interaction of the incident X-ray with the sample under the regime of Bragg's Law (nλ = 2d sin θ). This law correlates the wavelength of X-ray (λ) with the diffraction angle (2θ) and the lattice spacing of crystal (d) of the sample. By scanning the sample through a wide 2θ range to collect primary diffraction peaks, one may identify the crystal structure of sample by referring to the standard reference patterns.

5. UV-Visible Spectrophotometer: Absorption of incident radiation by the electrons in materials usually leads to a high frequency, i.e. low wavelength, absorption band that can be observed in the range of 200 to 800 nm. For a solution containing an absorbing substance, the absorptivity ratio at a fixed wavelength is defined as Io/I, which is logarithmically related to the concentration of solute (c) and the optical path length of sample cell (b) according to the Beer Lambert law: Absorbance (A) = log10 (Io/I) = α b c, where α is a constant named

absorption coefficient.

6. Photoluminescence Spectroscope: When materials are exposed to X-ray, γ- ray or light with sufficiently high energy, ionization of atoms takes place to remove the electrons from their orbits, rendering

16

the electronic structure of the atoms unstable. The electrons in higher orbit may then "fall" into the lower one to fill the hole left behind. During the falling process, energy with an equal amount to the energetic difference between the two orbits will be released in the form of photon to emit radiation.

2.3 Preparation of Anatase TiO

Nanowires

Commercial TiO2 powder (Degussa P-25) was used as the starting

material. In a typical process, 1.0 g of Degussa P-25 was first dispersed in 10 mL of absolute ethanol, and then mixed with 10 mL of 10 M NaOH aqueous solution under vigorous stirring for 10 min. The mixed solution was transferred into a Teflon-line stainless-steel autoclave. Subsequently, the autocalve was heated and maintained at 200oC for 24 h, and then cooled to room temperature naturally. The resultant white slurry was washed with deionized water and HCl aqueous solution (pH=1) until the pH value is below 7. A complete removal of Na constituent can be achieved through this acid-washing process [26], which resulted in the formation of H2Ti3O7 NWs. These NWs were then annealed in air at 700 o

17

2.4 Preparation of Pt-decorated TiO

NWs

To decorate NWs with Pt nanoparticles, 5 mg of anatase TiO2 NWs

were dispersed in 20 mL absolute ethanol, followed by the addition of 50 μL H2PtCl6 (5 mM) with vigorous stirring. The mixed solutionin quartz

tube was irradiated by UV light (with the wavelength of 365 nm) for 30 min to generate electros for reduction of Pt ions. Note that ethanol was used as the sacrificial reagent for the photogenerated holes. The product (TiO2-Pt NWs with the Pt content of 1.0 wt %) was centrifuged, washed

with deionized water and ethanol, and then dried at 60oC in air. In this work, two other novel metals (Ag and Au) were also deposited on NW surfaces, with which we may learn the effects of various metal decorations on the photocatalytic properties of TiO2. For the synthesis of

Au-decorated anatase TiO2 NWs, HAuCl4 was used as the Au source,

followed by the same procedure performed in the preparation of Pt-decorated sample. As to the fabrication of Ag-decorated anatase TiO2

NWs, AgNO3 was used as the Ag source, followed by the same procedure

18

2.5 Preparation of In

O

-decorated TiO

NWs

To prepare In2O3-decorated TiO2 NWs, 221.18 mg of InCl3 powder

was first dissolve in 5 mL deionized water, followed by the addition of 500 µL NH4OH solution. After stirring for 10 min, TiO2 NWs of 40 mg

were added into the above solution. The mixed solution was then stirred for 12 h at room temperature. The resultant white precipitation was collected and washed with deionized water for several times. After dried in air at 60oC, the white powder was further annealed in air at 700 oC for 2 h to produce In2O3-decorted TiO2 NWs (In2O3-TiO2).

2.6 Preparation of Pt and In

O

co-decorated TiO

NWs

To prepare the Pt and In2O3 co-decorated TiO2 NWs, 5 mg of

In2O3-TiO2 NWs were dispersed in 20 mL absolute ethanol in a quartz

tube first. H2PtCl6 of a suitable amount (5 mM) was then added into the

quartz tubes and irradiated by UV light (with the wavelength of 365 nm) for 30 min to grow Pt nanoparticles on the surfaces of TiO2. The product

was centrifuged, washed with deionized water and ethanol, and then dried at 60oC in air.

19

2.7 Photocurrent Measurement

Photocurrent measurement was conducted in a photoelectrochemical system under white light irradiation (xenon lamp, 500 W, with a light intensity of 175 mW/cm2). NW film, well adhered to FTO substrates, was prepared by dripping NW suspensions of sufficient amount onto FTO glasses, followed by a heat treatment to remove the solvent. The NW film on FTO substrate was used as the photoanode in the three-electrode cell which consisted of Pt counter electrode, Ag/AgCl reference electrode, and 0.01 M NaOH redox couple.

2.8 Photocatalytic Activity Study

Photodegradation of ethylene blue (MB) and rhoamine B (RB) were monitored to test the photocatalytic activity of the NW samples. Irradiation was provided by UV lamps (8W) with a wavelength of 365 nm. In a typical experiment, 5.0 mg of photocatalyst was added into 20 mL of MB (or RB) solution in the photoreactor vessel. The concentrations of MB and RB in the presence of NWs under illumination were both 1.0×10-5 M. At certain time intervals of irradiation, 1.5 mL of the reaction solution was withdrawn and centrifuged to remove the photocatalyst particles. The filtrates were

20

analyzed with a UV-Visible spectrophotometer to measure the concentration variation of dye through recording the corresponding absorbance of the characteristic peak. To investigate the recyclability and stability of photocatalysts, three cycles of photocatalytic reactions were conducted by using In2O3-TiO2-PtNWs with the Pt content of 5.0 wt % as

the representative sample. The photodegradation of MB (1.0×10-5 M) under natural sunlight was also examined.

2.9 Photoluminescence Lifetime Measurement

Time-resolved photoluminescence (PL) spectra were measured using a home-built single photon counting system. Diode laser (375 nm) with the pulse duration of 50 ps was used as the excitation source. The signals collected at the excitonic emission of RB (λ = 581 nm) were dispersed with a grating spectrometer, detected by a high-speed photomultiplier tube, and then correlated using a single photon counting card. The emission decay data were analyzed with the biexponential kinetics in which two decay components were derived. The lifetimes (τ1 and τ2), pre-exponential factors

21

summarized for RB (1μM), RB/In2O3 (1mg in 1μM RB), RB/In2O3-TiO2

(2mg in 1μM RB) and RB/ In2O3-TiO2-Pt (2mg in 1μM RB).

2.10 Characterization

The morphology and dimensions of the products were examined with a field-emission SEM (FESEM, JEOL, JSM-6500F). The crystallographic structure of the samples was investigated with XRD (Burker, D2 phaser) and a high-resolution TEM (HRTEM, JEOL, JEM-2010) operated at 200 kV. The compositional information was obtained with an EDS, an accessory of the TEM (JEM-2010). UV-Visible absorption spectra were obtained using a Hitachi U-3900H at 25oC under ambient atmosphere. For PL spectroscopy, a Hitachi F-4500 equipped with a xenon lamp (150W) was used. Photocurrent measurement for the TiO2 NWs film was measured

by using an electrochemical workstation (Jiehan 5600, Taiwan) under a xenon lamp (500W, 175mW/cm2) illumination.

22

Chapter 3. Results and Discussion

3.1 Structural Investigation on TiO

NWs

Figure 3.1.1 shows the SEM images and XRD patterns of H2Ti3O7

and anatase TiO2 NWs. The as-prepared H2Ti3O7 NWs have a typical

diameter of 70-150 nm and length up to a few μm. After calcinated at 700oC for 4h, a dehydration reaction occurred to H2Ti3O7, resulting in the

formation of anatase TiO2. This phase transformation can be confirmed

by the corresponding XRD analyses shown in Figure 3.1.1(c) [27]. Note that a similar morphology to that of the H2Ti3O7 NWs was found for the

anatase TiO2 which preserved the NW structures with the diameter of

23

Figure 3.1.1 SEM images of (a) H2Ti3O7 and (b) anatase TiO2 NWs. (c)

The corresponding XRD patterns.

3.2 Decoration of Pt on TiO

NWs

To extract the photoexcited electrons from TiO2 NWs for

participation in photocatalytic reduction reaction, we introduced an electron-acceptor, Pt, at the surfaces of NWs. We used the photodeposition method to deposit Pt nanoparticles onto TiO2 NWs. As

24

presented on NW surfaces after the photodeposition process. The HRTEM image, shown in Figure 3.2.1 (b), clear reveals an interlayer spacing of 1.89 Å in the NW region, in good agreement with the d spacing of the (200) lattice planes of anatase TiO2 [28]. In the particle

region, an interlayer spacing of 1.93 Å was obtained, complying with the lattice spacing of (200) planes of the fcc Pt [29]. With increasing the amount of H2PtCl6, the density of Pt nanoparticles grown at the surfaces of

NWs was gradually increased (Figure 3.2.2). These Pt nanoparticles however have irregular shape and would aggregate once the concentration of H2PtCl6 becomes relatively high. It should be noted that no

free-standing Pt nanoparticles were observed in the products, demonstrating the advantage of the current approach to obtain metal nanoparticle-decorated NWs. A growth mechanism for the formation of Pt nanoparticles was proposed as follows (equations 1 to 5) [30-32]. Irradiation for PtCl62- ions under UV light would transform them into

PtCl42- and then Pt0 (equation 1). The photodeposition process of Pt on

NWs mainly followed the three steps: (1) transformation of PtCl62-into

PtCl42-, (2) dissociation of Cl from PtCl42-, followed by the reduction of

25

association of Pt0-Pt0 [32]. Here ethanol plays an important role of accepting photogenerated holes of NWs and facilitating the formation of PtCl42- (equations 2 to 5) [31]. PtCl6 2- → PtCl42- → Pt0 (Eq. 1) PtCl62- + RCH2OH + hν → PtCl63- + ‧RCH2O + H+ (Eq. 2) PtCl63- ↹ PtCl52- + Cl- (Eq. 3) PtCl52- + ‧RCH2O → PtCl42- + RCH2＝O + HCl (Eq. 4) 2PtCl52- ↹ PtCl62- + PtCl42- (Eq. 5)

Based on the above interpretation, we believed that the present photodeposition approach can be extended to obtain other novel metal-decorated NWs such as Ag- and Au-decorated samples [33-35]. With these samples, we may learn the effects of various metal decorations on the photocatalytic properties of TiO2 NWs.

26

Figure 3.2.1 (a) Typical TEM, (b) HRTEM and (c) EDS analyses of Pt-decorated TiO2 NWs.

Figure 3.2.2 TEM images of Pt-decorated TiO2 NWs with the Pt contents

27

3.3 Photocatalytic Properties of Pt-decorated TiO

NWs

MB, a typical textile dye, was used as the model contaminant for photocatalysis here. Since MB is a cationic dye that may accept electrons from photocatalysts upon light illumination, we may realize the electron transfer event from TiO2 to Pt for the present Pt-decorated TiO2 NWs.

Note that ethanol was used as the sacrificial holes scavenger for photocatalyst to facilitate the further utilization of photoexcited electrons. Figures 3.3.1 represents the time-depended UV-Visible spectra of MB solutions under UV illumination in the presence of pristine TiO2 NWs. It

can be seen that the intensity of the characteristic absorption peak decreased with the irradiation time [36]. The bleaching of the absorption at 654 nm implies the reduction of MB and thus the operation of photocatalysis by using NWs. We then analyzed the normalized concentration of MB (C/Co) as a function of irradiation time in order to

understand the progress of MB photodegradation (Figure 3.3.2). As compared to pure NWs, Pt-decorated samples exhibit superior photocatalytic efficiency toward MB photodegradation. This improvement mainly resulted from the deposited Pt that can promote charge separation by attracting the photoexcited electrons from NWs, thus providing more

28

electrons for the decomposition of MB. As increasing the amount of Pt from 1 to 3 wt%, an enhanced photocatalytic activity was found for TiO2

NWs correspondingly. However, an obvious decrease in the photocatalytic activity was observed when Pt content was increased from 5 to 10 wt %. This outcome demonstrated an optimal content of Pt decoration for improving the photocatalytic performance of TiO2 NWs. It has been

widely documented that the excess content of metal may cover a large part of photocatalyst surfaces, which in turn decreases the number of active sites for photocatalysis as well as retards the access of light irradiation to photocatalyst surfaces [37]. Both of the above two situations may account for the depressed photocatalytic efficiency as observed here.

29

Figure 3.3.1 Absorption spectra of MB solutions under different UV irradiation times by using TiO2 NWs.

Figure 3.3.2 C/Co versus irradiation time plots for MB photodegradation

under UV illumination by using Pt-decorated TiO2 NWs with various Pt

30

3.4 Effects of Various Metal Decorations and Recycling Test

To further demonstrate the remarkable photocataytic properties for the present Pt-decorated samples, five kinds of photocatalysts including Pt-loaded, Au-decorated, Ag-decorated, Pt-decorated TiO2 NW, and P-25

TiO2 powder were compared in MB photodegradation under the same

conditions. Note that Pt-loaded sample was prepared by simply mixing NWs with Pt colloids, which led to the random distribution of Pt around TiO2. As to the preparation of Au- and Ag-decorated NWs, a similar

approach to the synthesis of Pt-decorated sample was used. The comparative results were shown in Figure 3.4.1, from which several points can be observed. First, Pt-loaded sample did not perform as better as Pt-decorated NWs did, presumably owing to the less contact of Pt with NWs in Pt-loaded sample. Because of the limited contact between Pt and TiO2 NWs, a retarded charge separation was expected for Pt-loaded NWs

sample, thus leading to the lower photocatalytic activity. Second, as compared to the Ag- and Au-decorated samples, Pt-decorated NBs exhibited superior photocatalytic efficiency toward MB photodegradation, which can be explained by the difference of work function among the deposited metals. Among the three metals, Pt has the largest work

31

function [38], which means that the difference between the conduction band of TiO2 and the Fermi level of metal was the largest in Pt-decorated

case. It had been pointed out that the driving force of electron transfer from semiconductor to metal is proportional to such energetic difference. In the current case, Pt-decorated TiO2 NWs may possess higher driving

force for charge separation and consequently showed better photocatalytic efficiency as compared to Ag- and Au-decorated samples. Third, as compared to the commercial product like P-25 TiO2 powder,

Pt-decorated TiO2 NWs showed better photocatalytic performance,

demonstrating their potential as an efficient photocatalyst in relevant redox reactions. We performed a recycling test by using Pt-decorated TiO2 NWs

with the Pt content of 3 wt % to investigate their recyclability and stability. As shown in Figure 3.4.2, no obvious decay of photocatalytic efficiency was found for Pt-decorated NWs after being repeatedly used for three times in MB photodegradation. This result shows that the present Pt-decorated TiO2 NWs had excellent stability and could be

32

Figure 3.4.1 The MB photodegradation results by using different photocatalysts including Pt-loaded, Pt-decorated, Ag-decorated,

Au-decorated TiO2 NWs, and P-25 TiO2 powder. Content of metal = 3.0

wt %. The insets were the corresponding TEM images.

Figure 3.4.2 Recycling test on Pt-decorated TiO2 NWs with Pt content of

33

3.5 Decoration of In

O

on TiO

NWs

On the other hand, we may provide TiO2 NWs with an appropriate

hole-acceptor to scavenge the photogenerated holes of TiO2 for

photocatalytic oxidation purpose. Through a precipitation-annealing process, we could decorate TiO2 NWs with In2O3 nanoparticles which had

suitable band structures to allow hole transfer from TiO2 to In2O3 (Ecb=

－0.62V, Evb= 2.18V vs. NHE for In2O3; Ecb= －0.29V, Evb= 2.91V vs.

NHE for anatase TiO2) [39]. As shown in Figure 3.5.1 (a), the diameter of

In2O3 nanoparticles grown at the surfaces of TiO2 NWs was about 20~50

nm. The corresponding XRD analysis shown in Figure 3.5.1 (c) confirmed the crystal structure of In2O3 nanoparticles. Note that pure

In2O3 nanoparticles can also be obtained by using the same procedure

without the addition of NWs. Figure 3.5.1 (b) displays the morphology of the single-phase In2O3 nanoparticles. The dimension and shape of the

pure In2O3 nanoparticles were similar to those deposited at the surfaces of

NWs, with which we may make a fair comparison of their photocatalytic properties.

The decoration of TiO2 NWs with In2O3 nanoparticles was achieved

34

NH4OH solution, white precipitation of an abundant amount rapidly

formed within the solution. The corresponding XRD analysis show that the precipitated particles were made of In(OH)3, which resulted from the

reaction of InCl3 with NH4OH. Subsequently, TiO2 NWs of a given

amount were added into the above solution and the mixed solution was stirred for 12 h to allow the attachment of In(OH)3 nanoparticles onto the

surfaces of TiO2 NWs. SEM investigation shown in Figure 3.5.2 clearly

reveals the decoration of NWs with a lot of nanoparticles of In(OH)3.

During the stirring process, it was expected that In–OH bonds of In(OH)3

nanoparticles would bind to the unsaturated oxygen at TiO2 surfaces to

form Ti–O–In–OH linkage, which further led to the attachment of In(OH)3 onto TiO2 NWs [40]. These In(OH)3-decorated NWs were then

annealed at 700oC in air for 2 h to transform In(OH)3 into In2O3 through a

dehydration process. As a result, In2O3-decorated TiO2 NWs were

obtained. The HRTEM image taken at the interface of NW and nanoparticle regions of In2O3-TiO2 NWs clearly reveals two distinct sets

of lattice fringes (Figure 3.5.3). An interlayer spacing of 0.18 nm was observed in the NW region, in good agreement with the d spacing of the (200) lattice planes of TiO2 [28]. In the particle region, interlayer spacings

35

of 0.29 nm and 0.41nm were obtained, complying with the lattice spacing of (222) and (211) planes of the bcc In2O3, respectively [41]. Figures

3.5.4 reveals the gradual increase in In2O3 particle density for

In2O3-decorated TiO2 NWs, which was achieved by employing InCl3 of

increasing amounts. As increasing the amount of InCl3, the resultant

surface-attached In2O3 particles became prevalent and aggregated on NW

surfaces.

Figure 3.5.1 SEM images of (a) In2O3-decorated TiO2 NWs (molar ratio

of In2O3 to TiO2 =1:1), and (b) pure In2O3 nanoparticles. The

36

Figure 3.5.2 SEM image of In(OH)3-decorated TiO2 NWs.

Figure 3.5.3 HRTEM image taken on an individual In2O3-decorated TiO2

NW. The interface between TiO2 and In2O3 was highlighted with a

37

Figure 3.5.4 In2O3-decorated TiO2 NWs with various In2O3/TiO2 molar

38

3.6 Photocatalytic Properties of In

O

-decorated TiO

NWs

To investigate the photocatalytic properties of In2O3-decoarted TiO2

NWs, RB was used as the test pollutant since it could react with photogenerated holes to get decomposed through a photocatalytic oxidation process. Note that the oxygen dissolved in water may act as the sacrificial electron consumer for photocatalyst to facilitate the further utilization of photogenerated holes. The time-dependent UV-visible spectra of RB solutions under UV illumination in the presence of TiO2

NWs were first shown in Figure 3.6.1. It can be seen that the intensity of the characteristic absorption peak at 554 nm [42] decreased dramatically with the irradiation time, implying the successful degradation of RB by the photocatalysis of TiO2. Figure 3.6.2 displays the photodegradation

results by using In2O3-decorated TiO2 NWs with different In2O3/TiO2

molar ratios. The photocatalytic efficiency of TiO2 NWs was enhanced as

increasing the molar ratio of In2O3/TiO2 from 0.25 to 1.0. This

enhancement mainly came from the deposited In2O3 that can promote

charge separation by attracting more photogenerated holes from TiO2 NWs,

thus providing more holes for the decomposition of RB. When the molar ratio of In2O3/TiO2 was further increased to 2.5 and 5.0, a decrease in the

39

photocatalytic activity was however observed for TiO2 NWs. This

depression might result from the excess In2O3 nanoparticles which may

cover a large part of NW surfaces to block the light absorption of TiO2

[43]. As a result, an unfavorable effect, analogous to that of excess metal loading, would emerge to hinder the resulting photocatalytic performance of TiO2. Although In2O3 itself may still absorb light to generate charge

carriers, the efficiency for these charge carriers to participate in photocatalysis was actually poor, as can be clearly seen in Figure 3.6.2.

Figure 3.6.1 Absorption spectra of RB solutions under different UV irradiation times by using TiO2 NWs.

40

Figure 3.6.2 C/Co versus irradiation time plots for RB photodegradation

under UV illumination by using In2O3-decorated TiO2 NWs with various

In2O3/TiO2 molar ratios. The results by using pure TiO2 NWs and pure

In2O3 nanoparticles were also included for comparison.

To demonstrate the remarkable photocatalytic properties for In2O3-decorated TiO2 NWs, further comparative experiments were

conducted. In Figure 3.6.3, three different photocatalysts including In2O3-loaded, In2O3-decorated TiO2 NWs, and P-25 TiO2 powder were

used and compared. Note that In2O3-loaded NWs were prepared by simply

mixing NWs with In2O3 nanopartilces, resulting in a random distribution of

In2O3 nanoparticles around NWs. The as-prepared In2O3-decroated NWs

exhibited superior photocatalytic performance as compared to the other two samples. This is due to the pronounced charge separation which was

41

believed to occur at the interface of In2O3/TiO2 in In2O3-decorated TiO2

NWs. Furthermore, these In2O3-decorated NWs were used in the

recycling photocatalytic operation to test their recyclability and stability. As shown in Figure 3.6.4, after being repeatedly used for three times in RB photodegradation, no obvious decay of photocatalytic efficiency was observed. This result demonstrates that the present In2O3-decorated TiO2

NWs could be promisingly utilized in the long-term course of photocatalysis.

Figure 3.6.3 The RB photodegradation results under UV light irradiation by using different photocatalysts including In2O3-loaded, In2O3-decorated,

TiO2 NWs and commercial P-25 TiO2 powders. Molar ratio of In2O3/TiO2

42

Figure 3.6.4 Recycling test on In2O3-decorated TiO2 NWs with the

In2O3/TiO2 molar ratio of 1:1.

3.7 Further Decoration of Pt on In

O

-TiO

NWs

To further promote the photocatalytic efficiency of TiO2, we applied

both Pt and In2O3 decorations for TiO2 NWs. Due to the difference in

band structures between In2O3 and TiO2, the photogenerated holes in TiO2

would preferentially transfer to In2O3, leaving photoexcited electrons in

TiO2 domain to achieve charge separation. We can then utilize this

characteristic to deposit Pt at the TiO2 region of In2O3-TiO2 NWs by

using the photodeposition method described in section 3.2. Figure 3.7.1 displays the TEM images of In2O3-decorated TiO2 NWs after they were

43

treated with the Pt photodeposition process. The TEM-EDS results shown in Figure 3.7.1 clearly reveal the deposition of Pt nanoparticles at TiO2

region of In2O3-TiO2 NWs. This outcome reaffirms our proposition that

In2O3 can attract photogenerated holes of TiO2 to leave photoexcited

electrons in TiO2. Upon UV light irradiation, the photoexcited electrons

in TiO2 region of In2O3-TiO2 NWs can then react with the surrouding

PtCl62- ions to grow Pt nanoparticles, resulting in the deposition of Pt at

44

Figure 3.7.1 TEM image and the correspding EDS spectra for In2O3-TiO2-Pt NWs.

3.8 Photocatalytic Properties of In

O

-TiO

-Pt NWs

To demonstrate the even superior photocatalytic performance for In2O3-TiO2-Pt NWs, MB which can serve as an electron-acceptor was

45

again used as the test pollutant here. The band structures shown in Figure 3.8.1 illustrate the charge transfer event that occurred in In2O3-TiO2-Pt

NWs and the subsequent photocatalysis to decompose MB. We expected that much more photoexcited electrons can be extracted from In2O3-TiO2-Pt NWs, which may further lead to the enhanced

photocatalytic efficiency toward MB photodegradation. As shown in Figure 3.8.2, an even better performance of photocatalysis was observed for In2O3-TiO2 NWs once they were decorated with Pt. There again

existed an optimal content of Pt (5.0 wt% here) for improving the photocatalytic efficiency of In2O3-TiO2 NWs. Above this optimal value,

the photocatalytic efficiency of NWs was depressed due to the excess coverage of Pt on TiO2 surfaces.

46

Figure 3.8.1 Band structures and charge transfer event in In2O3-TiO2-Pt

NWs.

Figure 3.8.2 The MB photodegradation results under UV light irradiation by using In2O3-TiO2-Pt NWs with various Pt contents.

47

Since In2O3 has a direct bandgap energy of 2.8 eV, the current

In2O3-TiO2-Pt NWs may be sensitized by In2O3 to absorb visible light and

perform photocatalysis. To explore the applicability of these NWs under visible light illumination, their phtocatalytic performance under natural sunlight was evaluated. As illustrated in Figure 3.8.3, after exposure to 1 h of daytime sunlight, MB was totally degraded by using In2O3-TiO2-Pt

NWs, accompanied with an obvious decoloration of the resultant solution. This result shows that the current In2O3-TiO2-Pt NWs can be used as

highly efficient photocatalysts which may practically harvest energy from sunlight.

48

Figure 3.8.3 Absorption spectra of MB solutions after exposure of 1 h of daytime sunlight (a) without any catalyst and (b) in the presence of

In2O3-TiO2-Pt NWs. Content of Pt = 5.0 wt %. Insets show the

corresponding solution color.

3.9 Photocurrent Measurement for In

O

-TiO

-Pt NWs

To further elucidate the effect of Pt on the charge separation of In2O3-decorated TiO2 NWs, we compared the photocurrent response of

pure TiO2 NW, In2O3-TiO2 NW, and In2O3-TiO2-Pt NW electrodes by

inserting them in a photoelectrochemical cell. Figure 3.9.1 depicts the photocurrent generation for the three kinds of NW electrodes subjected to the white light irradiation. There were several points to be observed from the figure. First, all the electrodes showed prompt response to the on/off

49

cycles of light illumination, demonstrating the effective charge transfer and successful electron collection for the samples within the photoelectrochemical cell. Second, the photocurrent of TiO2 NWs was

significantly enhanced if they were decorated with In2O3. The significant

retardation of charge recombination in In2O3-TiO2 NWs accounted for

such a drastic photocurrent enhancement [44]. Third, a much higher photocurrent was produced when In2O3-TiO2 NWs were further decorated

with Pt of 1.0 wt%. This outcome arose from the increasing driving force of electron transfer from In2O3 to TiO2 achieved by the decoration of Pt

on TiO2 surfaces. Note that semiconductor-metal heterostructures would

undergo a Fermi level equilibration due to the accumulation of photexcited electrons in metal [45]. The equilibrated Fermi level of such composite system appeared to be lower in potential (less negative) than that of the corresponding pure semiconductor. As illustrated in Figure 3.9.2, the apparent Fermi level of TiO2 in In2O3-TiO2-Pt NWs (EF**) was

presumably lower in potential than that of TiO2 in In2O3-TiO2 NWs (EF*).

A larger energetic difference in Fermi levels between In2O3 and TiO2 was

thus expected for In2O3-TiO2-Pt NW system, which in turn promote the

50

an enhancement in the resulting photocurrent production was observed for In2O3-TiO2-Pt NWs. The photocurrent generated from In2O3-TiO2-Pt

NWs was however depressed as the content of Pt was increased to 5.0 and 10 wt%. We supposed that too much Pt deposited on TiO2 of

In2O3-TiO2 NWs would trap the photoexcited electrons in a considerably

high amount, thus leading to a decreased number of electrons that were collected by the external voltammeter.

Figure 3.9.1 Photocurrent response of pure TiO2 NWs, In2O3-TiO2 NWs

51

Figure 3.9.2 Illustration of the apparent Fermi level potential of TiO2 in

52

3.10 Time-Resolved Photoluminescence Spectra

If the observed photocurrent enhancement with the decoration of Pt (1.0 wt%) on In2O3-TiO2 NWs indeed involved the increasing driving

force of electron transfer from In2O3 to TiO2, we should be able to reveal

this event in the corresponding emission decay profiles. Since RB has the band structure that can well fit to In2O3 to allow charge transfer, it was

used as the sensitizer to monitor the charge transfer event among In2O3,

TiO2 and Pt (Figure 3.10.1). As shown in Figure 3.10.2, a significant

quenching in the PL emission of RB was observed for In2O3 as compared

to pure RB. Such quenching became more noticeable in In2O3-TiO2 and

even more significant in In2O3-TiO2-Pt sample, implying the successful

53

Figure 3.10.1 Charge separation scheme for Pt/In2O3/TiO2 in present of

RB dye.

Figure 3.10.2 Quenching of photoluminescence emission of RB Dye (donor) with different samples (acceptor).

54

Figure 3.10.3 shows the time-resolved PL spectra for the three RB solutions relatively containing In2O3, In2O3-TiO2, and In2O3-TiO2-Pt NWs.

The emission decay data were analyzed with biexponential kinetics in which two decay components were derived. The lifetimes (τ1 and τ2),

pre-exponential factors (A1 and A2), and intensity-average lifetime (<τ>)

were determined and summarized in Table 1. The average lifetimes of In2O3, In2O3-TiO2 and In2O3-TiO2-Pt were 1.2171, 1.0472, and 0.6770 ns,

respectively. Note that the difference in the average emission lifetime between In2O3-TiO2 (<τ> = 1.0472 ns) and In2O3 (<τ> = 1.2171 ns)

indicates the emergence of a nonradiative pathway from the interaction between In2O3 and TiO2. This proposition can be confirmed by the more

pronounced emission quenching of RB observed for In2O3-TiO2 sample.

Such difference became more noticeable as In2O3-TiO2 NWs were further

decorated with Pt (<τ> = 0.6770 ns for In2O3-TiO2-Pt versus <τ> = 1.2171

ns for In2O3), inferring a much more significant electronic interaction

between In2O3 and TiO2. If electron transfer was the predominant process

that dictated the emission quenching of RB, we can then estimate the electron-transfer rate constants (ket) of In2O3-TiO2 and In2O3-TiO2-Pt

55

)

(

1

)

(

1

In

O

TiO

In

O

k



















)

(

1

)

(

1

In

O

TiO

Pt

In

O

TiO

k























Using the lifetime values listed in Table 1, we obtained the electron-transfer rate constants from In2O3 to TiO2 (ket,1) and from TiO2 to

Pt (ket,2) as 0.13×109 and 0.52×109 s-1, respectively. In addition, we

noticed that the rate constant of ket,1′ (0.65× 109) was larger than ket,1(0.13

× 109), which means that the electron transfer from In2O3 to TiO2 was

faster in the In2O3-TiO2-Pt system. This result, together with those of

photocatalysis, photocurrent measurement and PL quenching, confirms our suggestion that an increasing driving force of electron transfer from In2O3 to TiO2 can be achieved by the decoration of Pt on the TiO2 region

56

Figure 3.10.3 Time-resolved PL spectra of RB solutions containing In2O3,

57

Table 1Kinetic analysis of emission decay of RB in the presence of different samples. system A1 τ1 (ns) A2 τ2 (ns) <τ> (ns) ket (s -1₎ RB/In2O3 3213.4 1.4048 6318 0.1224 1.2171 ket,1=0.13 × 109 RB/In2O3-TiO2 1995.1 1.4917 9259 0.1627 1.0472 RB/In2O3 3213.4 1.4048 6318 0.1224 1.2171 ket,1′=0.65× 109 RB/In2O3-TiO2 -Pt 846 1.4454 11031 0.1665 0.6776 RB/In2O3-TiO2 1995.1 1.4917 9259 0.1627 1.0472 ket,2=0.52× 109 RB/In2O3-TiO2 -Pt 846 1.4454 11031 0.1665 0.6776

58

Chapter 4. Conclusion

In this thesis, we have successfully synthesized In2O3-decorated

anatase TiO2 NWs via a simple precipitation-annealing process. The

difference in band structures between In2O3 and TiO2 provided charge

carriers with another preferential route for charge transfer, resulting in the separation of photoexcited electrons and holes. The charge carrier separation in In2O3-TiO2 NWs was revealed with the photocatalytic

decomposition of RB. As compared to the commercial P-25 TiO2 powder

and In2O3-loaded TiO2 NW samples, the as-synthesized In2O3-TiO2 NWs

exhibited superior photocatalytic efficiency under UV illumination, demonstrating their potential as an efficient photocatalyst in relevant redox reactions. A further enhancement in the photocatalytic activity of In2O3-TiO2 NWs can be achieved when Pt nanoparticles of suitable

amount were deposited on the surface of TiO2. This improvement is due

to the increasing driving force of electron transfer from In2O3 to TiO2

achieved by the decoration of Pt on TiO2 surfaces. Time-resolved PL data

showed that a higher electron-transfer rate constant from In2O3 to TiO2

was observed for In2O3-TiO2-Pt NWs, consistent with the results of

59

NWs may find promising photocatalytic applications, especially in the photocatalytic decomposition of various organic pollutants. The present study also gives rise to a new class of highly efficient metal/semiconductor hybrid photocatalysts which may effectively utilize the solar power.

60

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JCPDS 89-4595.

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