Application of modifiled tio2 nanotube arrays in determination and removal of environmental pollutants

TiO2 nanotube arrays, which were synthesized for the first time in 2001, have been used in many fields, including photocatalysis and sensor due to their highly ordered orientation, unifo

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博士学位论文 改性二氧化钛纳米管在环境污染物检 测和处理方面的应用 (英文版) 学位申请人姓名 TRAN THI THANH THUY （陈氏青翠） 培 养 单 位 化学化工学院

导师姓名及职称 蔡青云教授

学 科 专 业 分析化学

研 究 方 向 生命科学中新分析技术

论 文 提 交 日 期 2013 年 04 月

学校代号：10532

湖南大学博士学位论文

改 性 二 氧 化 钛 纳 米 管 在 环 境 污 染 物

检测和处理方面的应用 (英文版) 学位申请人姓名： TRAN THI THANH THUY（陈氏青翠） 导师姓名及职称： 蔡青云教授

培 养 单 位： 化学化工学院

专 业 名 称： 分析化学

论 文 提 交 日 期： 2013 年 04 月

论 文 答 辩 日 期： 2013 年 05 月

答 辩 委 员 会 主 席 ： 王玉枝教授

Removal of Environmental Pollutants

By TRAN THI THANH THUY B.S (Vietnam National University-Ho Chi Minh City

University of Natural Sciences) 2002 M.S (Vietnam National University-Ho Chi Minh City

University of Natural Sciences) 2009

A dissertation submitted in partial satisfaction of the

Requirements for the degree of Doctor of Science

in Analytical Chemistry

in the Graduate School

of Hunan University

Supervisors Professor Cai Qingyun

May, 2013

Abstract

Nowadays, with outstanding development of industry, environment becomes seriously contaminated, which directly affects people’s health Determination and removal of organic pollutants, therefore, draw a great deal of attentions We can’t solve these entirely, of course, but we can hope to take a small part for general scence In this thesis, some basic researchs have been carried out based on using the modified titanium dioxide nanotube arrays for analysis and removal of some organic pollutants Base on the optical/electrical activity of the titanium dioxide nanotube arrays, new titanium dioxide nano-composite materials were fabricated by modification of the titanium dioxide nanotube arrays for detetermination and removal of organic pollutants in environment

TiO2 nanotube arrays, which were synthesized for the first time in 2001, have been used in many fields, including photocatalysis and sensor due to their highly ordered orientation, uniform surface morphology, adjustable pore size, length; and special electrical, and optical properties On the other hand, TiO2 nanotube arrays were greatly used due to its low cost, widespread availability, non-toxicity, high photocatalytic activity, and excellent chemical stability However, as is commonly known, photocatalytic application of TiO2 is limited by its relatively large bandgap of 3.0 eV for rutile and 3.2 eV for the anatase phases, which limits photoactivity to the ultraviolet region Moreover, the recombination of the photogenerated electron-hole pairs can lead

to reduce the photoconversion efficiency With the modification of TiO2, the functional nano-TiO2 composite materials would be with enhanced absorption in visible light and photoconversion efficiency Those are our research goals and the contents are as belows:

To enhance conductivity and also reduce recombination of photogenerated electron-hole pairs within TiO2, we have chosen appropriate narrow-band semiconductor materials such as ZnSe and Cu-Zn-S to decorate TiO2 The decorated TiO2semiconductor can be excited under solar light When the conduction band of a selected narrow-band semiconductor is more negative than that of TiO2, the photogenerated electrons of the narrow-band semiconductor will transfer to the conduction band of TiO2

or vice versa, and then participate in reduction Whereas, the photogenerated holes of TiO2 formed at the valence band may transfer to the narrow-band semiconductor and take part in oxidation This is why functionaized nano-TiO composite materials can

promote the photo-carriers separation and absorb visible light, leading to enhanced optical properties of the composite materials Based on this theory, we fabricated novel functional nano-TiO2 composite materials by decoration of TiO2 nanotubes with narrow-band binary or ternary semiconductor heterostructures (ZnSe/TiO2 and Cu-Zn-S/TiO2) ZnSe nanoparticles-sensitized TiO2 nanotube arrays show higher photocatalytic ability by 35% compared with that of non-sensitized TiO2 nanotube arrays for photocatalytic degradation of pentachlorophenol Moreover, they exhibit a significantly increased capability for photocatalytic degradation of this compound with support of the photo-Fenton system which is investigated as oxidants Cu-Zn-S ternary semiconductor sensitized TiO2 nanotube arrays exhibits high photoelectrocatalytic degradation capability to 2,4-dichlorophenoxyacetic acid and anthracene-9-carboxylic acid This photoelectrocatalytic degradation capability of Cu-Zn-S/TiO2 toward 2,4-dichlorophenoxyacetic acid and anthracene-9-carboxylic acid is higher than that of the non- sensitized TiO2 nanotube arrays by 48.2% and 31.5%, respectively

The good photoelectrical, chemical, and physical properties of TiO2 nanotubes offer a good chance for using the TiO2 nanotubes as the substrate of photoelectrical sensors Photoelectrochemical sensors were therefore prepared by modifying molecularly imprinted polymer on the TiO2 nanotube arrays (MIP@TiO2 NTAs) The proposed sensor is highly sensitive to perfluorooctane sulfonate (PFOS) in water samples with a limit of detection of 86ng/mL Moreover, the PFOS MIP@TiO2 NTA photoelectrochemical sensor exhibits outstanding selectivity These results affirm that semiconductor is a good choice for application in the analytical field, especially in determination of organic pollutants

In conclusion, we have fabricated, modified, and applied the composite nano-materials based on TiO2 nanotube arrays These researches have been continuous developed the photoelectrocatalytic and photoelectrochemical analysis based on semiconductor materials

Keywords: Semiconductor; TiO2 nanotube; molecularly imprinted polymer; photocatalytic; sensor; organic pollutants

TABLE OF CONTENTS

学位论文原创性声明 I Abstract II

摘 要 IV LIST OF FIGURES IX LIST OF SCHEMES……….X LIST OF TABLES XI

CHAPTER 1 INTRODUCTION 1

1.1 Semiconductor nanostructure materials 1

1.1.1 Preparation of semiconductor nano structure materials 2

1.1.2 Properties of semiconductor nano structure materials 3

1.2.3 Application of semiconductor nanomaterials 5

1.2 TiO2 nanotube arrays 5

1.2.1 Anodization preparation of TiO2 NTAs 6

1.2.2 Properties of TiO2 NTAs 8

1.2.3 Modiffication of TiO2 NTAs 10

1.2.4 Application of TiO2 NTAs and its modification 12

1.3 Research Objectives 14

CHAPTER 2 PREPARATION AND PROPERTIES OF TiO 2 NANOTUBE…… 16

2.1 Introduction 16

2.2 Experimental procedures 16

2.2.1 Materials 16

2.2.2 Methods 16

2.3 Results and Discussions 17

2.3.1 Characterization of the TiO2 NTAs 17

2.3.2 Photoelectrochemical properties of the TiO2 NTAs 19

2.4 Section summary 20

CHAPTER 3 SYNTHESIS AND PHOTOCATALYTIC APPLICATIONS OF TERNARY Cu–Zn–S NANOPARTICLE-SENSITIZED TiO 2 NANOTUBE …….21

3.1 Introduction 21

3.2 Experimental procedures 22

3.2.1 Materials 22

3.2.2 Methods 22

3.3 Results and Discussions 23

3.3.1 Characterization of the Cu–Zn–S ternary-sensitized TiO2 NTAs…………23

3.3.2 Photoelectrochemical properties of the Cu–Zn–S sensitized TiO2 NTAs…25 3.3.3 Photocatalytic degradation of organic pollutants 26

3.3.4 Cu-Zn-S sensitized TiO2 NTAs Stability 31

3.4 Section summary 32

CHAPTER 4 PHOTOCATALYTIC DEGRADATION OF PENTACHLOROPHENOL ON ZnSe/TiO 2 NTAs SUPPORTED BY PHOTO-FENTON SYSTEM 33

4.1 Introduction 33

4.2 Experimental procedures 34

4.2.1 Materials 34

4.2.2 Methods 34

4.3 Results and discussions 36

4.3.1 Characterization of the ZnSe/TiO2 NTAs 36

4.3.2 Photoelectrochemical properties of the ZnSe/TiO2 NTAs 39

4.3.3 Photocatalytic degradation of PCP 40

4.3.4 Stability of the ZnSe/TiO2 NTAs 48

4.4 Section summary 49

CHAPTER 5 MOLECULARLY IMPRINTED POLYMER MODIFIED TiO 2 NANOTUBE ARRAYS FOR PHOTOELECTROCHEMICAL DETERMINATION OF PERFLUOROOCTANE SULFONATE (PFOS)… 50

5.1 Introduction 50

5.2 Experimental procedure 51

5.2.1 Materials 51

5.2.2 Fabrication of the PFOS MIP@TiO2 NTA photoelectrochemical sensor…51 5.2.3 Characterization and photoelectrochemical measurements of the PFOS photoelectrochemical sensor……… 52

5.3 Results and discussions 53

5.3.1 Fabrication and characterization of the PFOS MIP@TiO2 NTA photoelectrochemical sensor………53

5.3.2 Determination of PFOS 58

5.4 Section summary 62

CONCLUSIONS 63

REFERENCES 64

APPENDIX: PUBLICATIONS 85 ACKNOWLEDGMENT 86

LIST OF FIGURES

Figure 1.1 Groups of semiconductors…….……… 2

Figure 1.2 pn-junction of semiconductors……… 2

Figure 1.3 Photogeneration of electron-hole pair of n-type semiconductor… 3

Figure 1.4 Current-potential curves of a n-type semiconductor……… 4

Figure 1.5 Fundamental principle of semiconductor-based photocatalysis…… 5

Figure 1.6 The formation of TiO2 nanotube array at constant anodization voltage 8

Figure 2.1 FESEM top-surface images of TiO2 nanotube arrays ……… 18

Figure 2.2 PL, UV-vis DRS, and XRD spectrums of TiO2 NTAs…….… 19

Figure 2.3 Photocurrent responses of TiO2 NTAs…….…… ……….… 20

Figure 3.1 FESEM top-surface, TEM images of Cu–Zn–S/TiO2 NTAs………… 24

Figure 3.2 UV–vis diffuse reflectance spectra of Cu–Zn–S/TiO2 NTAs.……… 25

Figure 3.3 Photocurrent response of Cu–Zn–S/TiO2 NTAs.……… 26

Figure 3.4 The effect of pH and initial concentration on the photoelectrocatalytic degradation of 2,4-D ……… 28

Figure 3.5 UV-Vis determination of photoelectrocatalytic 2,4-D……… 29

Figure 3.6 UV-Vis determination of photoelectrocatalytic 9-AnCOOH……… 31

Figure 3.7 Photoelectrocatalytic stability of Cu–Zn–S/TiO2 NTAs……… 32

Figure 4.1 FESEM top-surface images and EDS spectrum of ZnSe/TiO2 NTAs 37

Figure 4.2 UV–vis and PL spectrums of ZnSe/TiO2 NTAs……… 38

Figure 4.3 Photocurrent response and EIS analysis of ZnSe/TiO2 NTAs……… 39

Figure 4.4 The effect of HA, Fe3+, H2O2 and initial concentrations of PCP on the removal of PCP……… 42

Figure 4.5 UV–Vis determination of photocatalytic degradation of PCP…… 44

Figure 4.6 Photocatalytic stability of ZnSe/TiO2 NTAs……… ….…… 48

Figure 5.1 FESEM top-surface images of TiO2 NTAs and MIP@ TiO2 NTAs… 55

Figure 5.2 PL and FT–IR spectrums of MIP@ TiO2 NTAs……… 56

Figure 5.3 Photocurrent responses of MIP@ TiO2 NTAs on PFOS…… …… 58

Figure 5.4 Photocurrent responses and linear calibration curve of MIP@TiO2 NTAs in different PFOS concentraions……… 59

Figure 5.5 Sensivity and Selectivity of MIP@TiO2 NTAs to PFOS……… 61

LIST OF SCHEMES

Scheme 1.1 The scenario of methods for fabrication of TiO2 nanotubes……… 6 Scheme 1.2 The photocatalytic mechanism of TiO2……… 12 Scheme 1.3 The charge-transfer processes involved in modified TiO2 by a second

semiconductor 13 Scheme 3.1 Mechanism of photocatalysis degradation of Cu–Zn–S/TiO2

NTAs…… 27 Scheme 4.1 Mechanism of photocatalysis degradation of ZnSe/TiO2 NTAs… 47 Scheme 5.1 The fabrication of the PFOS photoelectrochemical sensor and the

molecularly imprinted polymer process……… 54

condition 46 Table 5.1 Recovery study for PFOS using MIP@TiO2 NTAs photoelectrochemical

sensor with various water samples……… 60

CHAPTER 1 INTRODUCTION

1.1 Semiconductor nanostructure materials

Semiconductors were observed in the early 19th century and developed greatly in the first half of the 20th Century The word semiconductor is composed of semi and conductor Semi means not completely while the conductor means something, which can conduct electricity That means a semiconductor is a material that has intermediate conductivity between a conductor and an insulator Semiconductor nanomaterials may

be in the range of 1 to 100 nm size with various shapes and properties may change according to size and/or shape [1,2] General; semiconductor materials have electrical resistivity in the range of 10-2-109 Ωcm and energy gap for electronic excitations lies between zero and about 4 electron volts (eV) Metals or semimetals are materials with zero bandgap, while those with an energy gap larger than 4 eV are usually known as insulators [3]

Semiconductors are normally classified into two types: intrinsic and extrinsic Intrinsic semiconductor has equal numbers of negative carriers (electrons) and positive carriers (holes)， thus it possesses a very pure chemical property and poor conductivity) Whereas extrinsic semiconductor is improved intrinsic semiconductor with a small amount of impurities added by a process, named as doping or decoration, this changes the electrical properties of the semiconductor and improves its conductivity Current conduction in a semiconductor occurs via free electrons and holes, known as charge carriers Doping impurities to a semiconductor greatly increases the number of charge carriers within it [3]

Doping or decoration process produces two groups of semiconductors: the negative charge conductor (n-type) which is doped excess free electrons; and the positive charge conductor (p-type) which contains excess free holes, (Figure 1.1)

Figure 1.1 Groups of semiconductors.

When p-type and n-type semiconductors are contacted together, a pn-junction is formed as follow [4]:

Figure 1.2 pn-junction of semiconductors

The following is several types of the known semiconductors: elemental semiconductors, binary compounds, oxides, layered semiconductors, organic semiconductors, magnetic semiconductors, other miscellaneous semiconductors [3] 1.1.1 Preparation of semiconductor nano structure material

Nowaday, there are many different technologies for synthesizing semiconductor materials They were classified in two groups: top-down and bottom-up technologies [5] The top-down technology relies on dimensional reduction through selective etching and various nanoimprinting techniques The bottom-up technology starts with individual atoms and molecules to build up the desired nanostructures In particular, versatile bottom-up technologies based on chemistry have attracted considerable attention

because of their relatively low cost and high throughput [6] Bottom-up technologies refer to the buildup of a material from the bottom: atom-by-atom, moleculeby-molecule,

or cluster-by-cluster Growth species such as atoms, ions, and molecules, after impinging on the growth surface, assemble into crystal structures one after another In recent years, a number of techniques, including coprecipitation, sol-gel processes, microemulsions, freeze drying, hydrothermal processes, laser pyrolysis, ultrasound and microwave irradiation, templates, and chemical vapor deposition, electro-deposition,…have been developed to control the size, morphology, and uniformity of nanostructures [7, 8] Among various media for crystal growth, the solution based method offers significant advantages, including (1) low reaction temperatures, (2) size-selective growth, (3) morphological control, and (4) large-scale production [9, 10] 1.1.2 Properties of semiconductor nano structure materials

1.1.2.1 Light absorption and carrier generation

The optical bangap of the semiconductor is an important parameter in defining its light absorption behavior When the semiconductor absorbs a photon of energy, an electron-hole pair is generated in itself Optical excitation, consequently, results in the band to band transition, which delocalized electron in the CB leaving behind a delocalized hole in the VB [11] (Figure1.3) However, this electron-hole pair can be recombined that limited the optical properties of semiconductors Optical transition in semiconductor can also involve localized states in the band gap These become particularly important for semiconductor in nanocrystalline form

Figure 1.3 Photogeneration of electron-hole pair of n-type semiconductor [11]

1.1.2.2 Photocurrent- potential behavior

The current-voltage characteristics of an illuminated semiconductor electrode on contact with a redox electrolyte can be obtained by simply adding together the majorit y and minority current component As shown in Figure 1.4, in the dark, the surface concentration of electrons is denoted in the equilibrium situation, thus the semiconductor-electrolyte interface is not perturbed by an external (bias) potential and the current is zero [11] Under band gap illumination, the interface is driven a way from equilibrium; the current is dependent on potential When the surface concentration of electrons is higher than that of the equilibrium situation, a reduction current (cathodic current) should flow across the interface such that the oxidized redox species are converted to reduced species On the other hand, when the surface concentration of electrons is lower than that of the equilibrium situation, the current flow direction is reversed and an anodic current should flow [12]

Figure 1.4 Current-potential curves of an n-type semiconductor in the dark and under band gap

illumination [11]

1.1.2.3 Dynamics of photoinduced charge transfer

The important processes in a dynamic of photoinduced charge transfer across the semiconductor-electrolyte interface as follows:

Carrier generation within the semiconductor

Diffusion of minority carriers from the fied- free region to the space charge layer edge and transit through this layer

Charge transfer across the interface and carrier recombination via surface states or via traps in the space charge layer [3, 4]

1.1.2.4 Photocatalysis

The aim of semiconductor photocatalysis is effectively a removal of organic pollutants UV or visible light is used to generate electron-hole pairs in the semiconductor When a photon with an energy of hv matches the band gap energy of the semiconductor, an electron in the valence band (VB) is excited into the conduction band (CB), leaving a positive hole in VB The electrons then react with oxygen in the sample

to form O2- and holes react with surface hydroxyl groups to form HO• radicals as Figure 1.5 The radical species then attack the organic pollutants [12,13]

Figure 1.5 Fundamental principle of semiconductor-based photocatalysis [14]

1.2.3 Application of semiconductor nanomaterials

As it is known, the very large surface-to-volume ratio has major influence on the optical and surface properties of semiconductor nanomaterials Therefore, they have attracted significant attention in research and application in many fields including energy conversion, sensing, electronics, photonics, and biomedicine Parameters such as size, shape, and surface characteristics can be varied to control their properties for different applications of interest [2,15]

Over recently decades, nanostrcutcture materials derived from TiO2 have extensively been investigated for vast applications due to its low cost, widespread availability, non-toxicity, environmentally friendly, high photocatalytic activity, corrosion-resistant material, and excellent chemical stability [14] It is a UV light responsible semiconductor; electron and hole pair is generated by the UV irradiation,

resulting chemical reactions at the surface From the 1950s, there are many studies utilize TiO2 as a photocatalyst [16], an electrode of dye-sensitized solar cell [17], a gas sensor [18], and so on TiO2 exists in nature in three different polymorphs: rutile, anatase, and brookite It appears as a variety of morphologies of nanostructured including nanoparticles, nanorods, nanowires, nanostructured films, or coatings, nanotubes, and mesoporous/nanoporous structures [19]

Nanotubes are of great interest due to their high surface volume ratios and size dependent properties TiO2 based nanotubes with high specific surface area, ion-changeable ability, and photocatalytic ability have been considered for extensive applications [20] Some recent studies have indicated that TiO2 nanotubes have improved properties compared to any other form of TiO2 for application in photocatalysis [21,22] , sensing [23-25] , photoelectrolysis [26-28], and photovoltaics [29-31] Currently developed methods of fabricating TiO2 nanotube arrays, including assisted–template method [32,33], the sol–gel process [34,35], seeded growth [36], hydrothermal processes [37,38] and electrochemical anodic oxidation [39,40] The scenario of methods for fabrication of TiO2

nanotubes is presented in Scheme 1.1

Scheme 1.1 The scenario of methods for fabrication of TiO2 nanotubes

Anodic oxidation preparation of TiO2 nanotube arrays by using anodic aluminum oxide (AAO) nanoporous membrane often encounters difficulties of prefabrication and post-removal of the templates and usually results in impurities [20] Year of 2001, Grimes and co-workers [41,42] first reported the fabrication of TiO2 nanotube arrays via anodic oxidation of titanium foil in a fluoride-based solution [20] The anodizing approach can build a porous TiO2 nanotube arrays film of controllable pore size, good uniformity, and conformability over large areas at low cost [41] Further studies focused

on precise control and extension of the nanotube morphology [43], length and pore size

[44]

, and wall thickness [45]

Electrolyte composition decides both the rate of nanotube array formation as well

as the rate of dissolution of the resultant oxide One of the electrolytes has been used for fabrication of TiO2 nanotube is 0.5 wt% HF aqueous solution at room temperature [41] Fluoride solution can help to dissolve TiO2 by forming (TiF6)2− anions However, too strong acidity of HF solution results in a too fast dissolution of the formed TiO2

nanotubes Thickness of TiO2 nanotube films can be significantly increased when a KF

or NaF solution is used as an electrolyte [46] The acidity of the electrolyte might be tuned to adjust the balance of dissociation of TiO2 at the electrolyte/oxide interface and oxidation of Ti at the oxide/metal interface [47] In all cases, the better electrolyte is probably a NH4F based solution for nanotube array formation In a solution containing

of fluoride ion, (1) oxide growth at the surface of the metal occurs due to an interaction

of Ti with O2− or OH− anions; (2) Ti4+ cations migrate from the oxide/metal interface to the electrolyte/oxide interface and are ejected into solution by an electric field; (3) field assisted dissolution of the oxide at the electrolyte/oxide interface [48] The principal chemical reaction at the hydroxide/metal interface should be: Ti + xOH− → Ti(OH)x + 4e− (1-1) and Ti(OH)x then decomposes to form TiO2 at the oxide/hydroxide interface With the anodization, a thin layer of oxide forms on the Ti surface (Figure 1.6a) Dissolution of the oxide at the electrolyte/oxide interface forms small pits, making the barrier layer at the bottom of the pits relatively thin which act as pore forming centers (Figure 1.6b) [42, 49] as:

TiO2 + H2O + F-→ ([TiF6])2- + O2−+ OH− + H+ (1-2) Then, these pits convert into bigger pores and the pore density increases (Figure 1.6c) Simultaneously, with the formation of pores the voids are also formed between the pores undergoes oxidation and field assisted dissolution (Figure 1.6d) Thereafter, both voids and tubes grow in equilibrium.The pores in the TiO2 nanotubes are developed from pits on the foil surface and continue their growth based on a balance of the oxidation of Ti metal at the oxide/metal interface and dissolution of oxide at the electrolyte/oxide interface [42,50] (Figure 1.6e)

Figure 1.6 The formation of TiO 2 nanotube array at constant anodization voltage: (a) oxide layer formation, (b) pit formation on the oxide layer, (c) growth of the pit into shaped pores, (d) metallic part between the pores undergoes oxidation and field assisted dissolution, and (e) fully developed

TiO 2 nanotube array with a top view

1.2.2.1 Crystal structure

TiO2 exists mainly in three phases: anatase, rutile, and brookite Among the different polymorphs, rutile is generally considered to be the thermodynamically most stable bulk phase, while at the anatase is considered to be stable crystals phase [51,52] TiO2 nanotubes which formed by electrochemical anodic oxidation method are amorphous and present nanocrystallites in the tube wall [53,54] Moreover, by annealing, amorphous material converts into crystalline material TiO2 nanotubes amorphous material can be converted into anatase (300-500oC) or rutile (>550oC) by a thermal treatment in air In case of anatase crystals, the surface energy is lower than for rutile

[14]

1.2.2.2 Optical and electrical properties

The optical properties of TiO2 nanotubes were investigated mainly in photoelectrochemical arrangements in an electrolyte For amorphous nanotubes layers, a photocurrent behavior is often obtained with a band gap of approximately 3.1-3.3 eV [55] With anatase phase, the photocurrent strongly increases and a gap of 3.2 eV results In a photoelectrochemical configuration, a voltage is applied at relatively moderate bias, total carrier depletion of the walls is reached This desmonstrates that the photocurrent dependence of the potential the photocurrent [56] The electrical conductivity of self-organized TiO2 nanotubes has been estimated via resistivity after TiO2 nanotubes were annealed at different temperatures [57] For low temperatures (<200 oC), the resistivity increases with temperatures a result of evaporation of surface water At about

300oC, conversion of the amorphous material to anatase occurs, and a significantly higher conductivity is obtained At temperatures higher than 500oC, the anatase material

is increasingly converted into more resistive rutile, which leads to a considerably lower conductivity [14]

1.2.2.3 Reactivity

TiO2 surfaces have been extensively studied regarding gas-phase adsorption and catalytic effects on various reactions such as CO oxidation [58,59], selective reduction of NOx [60] and O2 and water decomposition [61,62] They investigated CO, CO2, H2, O2, and alkane activity on different crystal structures of TiO2 nanotubes In solution, the most important reactive features of TiO2 are: 1) its solubility in some complexing agents (HF, organic acids), 2) the possibility of modifying the surface with organic monolayers by surface hydroxy group reactions its electrochemical properties

TiO2 nanotubes have some beneficial effects as a substrate for noble metal catalysts

in electrocatalysis For example, it has been shown that for TiO2 nanotubes decorated with gold, a more facile O2 reduction reaction can be observed [63], or that a highly accelerating effect for methanol oxidation catalysts can be obtained [64,65] Such electrocatalytic effects of TiO2 nanotubes have also been explored for glucose sensing

[66]

All of these applications rely on the fact that owing to carrier depletion conditions

in the TiO2 substrate, a high overpotential for the oxygen evolution reaction is provided

1.2.3 Modiffication of TiO2 NTAs

Modification of the TiO2 nanotubes is mainly carried out by three ways: 1) heat treatments; 2) introducing other elements; or 3) by tube-wall decoration The modification makes the materials suitable for various applications that base on specific electrical, optical, or chemical properties With electronic properties, annealing changes the conductivity and lifetime of charge carriers of materials, whereas introducing other elements decreasing the optical bandgap, thus enabling a visible-light photoresponse 1.2.3.1Annealing

Temperature effects to electrical conductivity properties of TiO2 nanotubes When TiO2 nanotubes annealed at temperatures above 450oC, some cracks can occur in the tube walls which were considered to slow down electron transport [67] Furthermore, during annealing of TiO2 in atmospheres, thin rutile layers are formed at the nanotube bottoms, which can unprofitably affect various applications that use electrode configurations Annealing in a vacuum usually leads to loss of O2 from the material and formation of Ti3+ This shows visible light absorption and enhanced conductivity

1.2.3.2Doping

The bandgap of 3.2 eV of TiO2 nanotubes allows only it to absorb light only in the

UV range with about 7% of the solar spectrum can be absorbed Doping a secondary electronically active species into the nanotubes enhances absorption light in the visible region For this reason, a variety of transition metals, such as V, Cr, Mn, and Fe [68-71]were investigated which successful in activating a response to visible light There are many methods to prepare doped TiO2 nanotubes: 1) growing TiO2 nanomaterials in a solution of the doping species [72-74]; 2) synthesis in gas atmospheres of the doping species [75,76]; 3) production of the nanomaterials by sputtering in an atmosphere of doping species [77]; 4) high energy ion implantation [78,79]; and 5) the incorporation of active electrolyte species for TiO2 structures that grow from the metals by electrochemical oxidation

1.2.3.3Conversion of tubes

Titanate of TiO2 nanotubes into their perovskite oxide as PbTiO3, BaTiO3, SrTiO3, and ZrTiO3 shows a variety of interesting piezoelectric or ferroelectric properties [80-83] The heat treatment was used to form these perovskite oxides after anodization of an

appropriate alloy [84-86] These perovskite oxides have a high potential for oxygen evolution, therefore, they are suitable for a wide range of electrochemical applications, and other applications that require high electron conductivity Moreover, TiO2 nanotubes can be converted into protonated titanates which are also promising for applications in catalysis, photocatalysis, electrocatalysis, lithium batteries, hydrogen storage, and solar-cell technologies [87-89]

1.2.3.4 Filling and decoration

Filling or decoration method is the modification of TiO2 nanotube surfaces with nanoparticles as metals, semiconductors, polymers By this approach, the material changes into beneficial properties as follows: 1) heterojunction formation that either changes the band bending or provides suitable energy levels for charge injection; 2) catalytic effects for charge-transfer reactions; 3) surface plasmon effects, leading for example to field enhancement in the vicinity of metal particles and thus allowing more efficient charge transfer [14]

There are several approaches for decoration or filling of TiO2 nanotubes with different materials Electrodeposition reactions into TiO2 nanotubes provide a very general-purpose way to decorate or fill oxide nanotubes TiO2 is an n-type semiconductor, therefore, a cathodic potential needed for electrodeposition of metals represents a forward bias; as a result, the tube walls have such a high conductivity that deposition occurs favourably on top of the layers rather than within the tubes [90] TiO2nanotubes can be decorated by narrow-bandgap semiconductors to enhance their photocatalytic activity; such as CdS, CdSe, PbS, quantum dots [91-93] These quantum dots can be deposited on the nanotube wall by sequential chemical bath deposition, chemical treatment or by electrochemical deposition methods At the same time, TiO2nanotubes can be decorated by ternary chalcopyrite semiconductors which are also important in solar energy Ternary chalcopyrite semiconductors, for example, as I-III-VI compounds (I = Cu, Ag, Au; III = In, Tl; V = S, Se, Te); II-IV-V compounds (II = Zn, Cd, Hg; IV= Ge, Sn; V = As, Sb) [94] or II–III–VI compounds (II=Mg, Hg,Cd, Zn; III=Al, In,Ga;VI=O, S,Se,Te) [95] Moreover, decoration or filling of TiO2 nanotubes by noble metal nanoparticles as Au, Ag, and Pt can also be carried out to enhance their photocatalytic activity [64, 96] Further, TiO2 nanotubes can be decorated by oxide nanoparticles, for example WO3 [97] (hydrolysis of precursors); NiO (precipitation reaction of Ni(OH)2 followed by a suitable thermal treatment [98] These oxide

nanoparticles inject charges to the conduction band of TiO2 result in enhancing significant photoelectrochemical activity under visible light

1.2.3.5 Monolayers

TiO2 nanotube surfaces can be modified by covalent attachment of organic monolayers [99-101] These organic monolayers are attached onto TiO2 surfaces mostly because of the following purposes: 1) to change the surface wettability; 2) to modify the biocompatibility; 3) to obtain chemical or biochemical sensors; or 4) to attach an electron injection system Modification of TiO2 nanotubes by covalent attachment of organic monolayers combined with a photocatalytic reaction was used to create surfaces that could be adjusted to have desired wettability property [102,103]

1.2.4.1 Photocatalysis

TiO2 is the most photocatalytically active semiconductor for the decomposition of organic pollutants [16,104-106] The band-edge positions relative to typical environments is the reason for this high activity The photocatalytic mechanism can be expressed in two ways as shown in following Scheme 1.2 [12]

Scheme 1.2 The photocatalytic mechanism of TiO 2

Under photon irradiation electrons of TiO2 were excited from the valence band (VB)

to the conduction band (CB) Hydroxyl radicals and peroxo ions are generated by charge exchange at the valence band (H2O + h+ HO•) and at the conduction band (O2 + e- 

O2-) These radicals and peroxo ions are able to practically oxidize all organic pollutants

to CO2 and H2O [12] The modification of TiO2 tubes with different semiconductor systems can result in improved photocatalytic activity because of the charge-transfer processes involved in coupled semiconductor systems The electrons photoinduced on the conduction band of a higher level semiconductor can be injected into the lower conduction band of TiO2 tubes As a result, more efficient charge-carrier separation can

1.2.4.3 Electrochromic devices

TiO2 was used as an excellent host lattice for ion intercalation devices These devices rely on uptake and release of small ions such as H+ and Li+ which are frequently combined with a change in the redox state of TiO2 and a resulting change in the electronic and optical properties of TiO2 For example, lithium ion intercalation into TiO2, accompanied with reduction of Ti4+ at the lattice to Ti3+, changes the apparent bandgap of the TiO2 from the UV to the visible range, which leads to a blue coloration

of the material [109-112]

1.2.4.4 Cell interaction and biomedical coatings

A very important application of modified TiO2 nanotube surfaces is in biomedical applications The size of TiO2 nanotube surfaces is ideal with living matter or biologically relevant species Moreover, nanotube layers can be coated easily by the self-organizing nature, entire, even complex shaped surfaces [113, 114]

1.2.4.5 Gas sensing

TiO2 is a gas sensing which high sensitivity to CO, H2, and NOx gases, in particular,

as nanoparticulated films [115-117] Doping gold nanoparticles on TiO2 nanotube layers can make a strongly enhanced reaction rate with O2 in aqueous solution [63]

1.3 Research Objectives

Environmental pollution nowadays is one of the most concerned problems Purifying the contaminated environment is an important task in whole the world As the semiconductor nanomaterials are with advantage properties, such as electric, optic, and catalytic properties as well as their applications in the treatment of the environmental pollutants, the first objective of this thesis is to prepare TiO2 nanotube array-based semiconductor nanomaterials focusing on the modification with semiconductor nanoparticles The second objective of this thesis is to apply the prepared materials in photocatalytic degradation of organic pollutants and photoelectrochemical sensing of persistent organic pollutants The specific objectives of this thesis are as follows:

(1) Studying on the self-organized growth short of TiO2 nanotube arrays (NTAs) by anodic oxidization of a pure titanium sheet in electrolyte solutions containing sodium fluoride and sodium hydrogen sulfate with constant potential in suitable time with focus

on the study of the electric and optic properties of the TiO2 NTA films (in chapter 2)

(2) Synthesis of ternary Cu-Zn-S nanoparticle-sensitized TiO2 nanotube arrays by pulse-electrodeposition of ternary Cu-Zn-S nanoparticles onto the surface of TiO2nanotube array films using a three-electrode electrochemical cell, and photocatalytic applications of the Cu-Zn-S/TiO2 NTAs materials in the photoelectrocatalytic

degradation of 2,4-dichlorophenoxyacetic acid and anthracene-9-carboxylic acid (in chapter 3)

(3) Photocatalytic degradation of pentachlorophenol on ZnSe/TiO2 supported by the photo-Fenton system with focus on the investigation of photo-Fenton system was as

oxidants and the photocatalytic degradation process (in chapter 4)

(4) Fabrication of photoelectrochemical sensor by surface modification of molecularly imprinted polymer (MIP) onto TiO2 NTAs, and application of the MIP@TiO2 NTA photoelectrochemical sensor in the determination of perfluorooctane

sulfonate in water, with focus on the investigation on the selectivity and sensitivity (in chapter 5)

Functional nano-TiO2 composite semiconductors are potential nanomaterials for greatly applications, especially treatment of environmental pollutants, due to their optical/electrical catalytic performance Base on these researchs, we hope to contribute a small part in studying of current environmental cleanup

CHAPTER 2 PREPARATION AND PROPERTIES

2.1 Introduction

In recently years, TiO2 nanotubes have attracted great interest due to their high surface ratios and size-dependent properties The TiO2 nanotubes show improved properties compared to other forms of TiO2 for applications in photocatalysis, photoelectrolysis, sensing, and photo-voltaics TiO2 nanotubes have been farbicated by a variety of methods, including deposition into a nanoporous alumina template, sol-gel transcription using organo-gelators as templates, and seeded growth However, anodization of titanium in electrolyte-based baths achieves the remark properties of highly ordered nanotube arrays with controllable dimensions [118-120]

In this work, short highly ordered and vertically aligned TiO2 nanotube arrays were fabricated by anodization of titanium in NaF and NaHSO4 based baths [41] The surface morphologies and photoelectrochemical properties are also carried out to express the properties of TiO2 nanotube

2.2 Experimental procedures

2.2.1 Materials

Titanium foil (99.8 % purity, 0.25 mm thick) was purchased from Aldrich (Milwaukee,WI) NaF, NaHSO4 were obtained from Shanghai Chemical Corporation of China All other reagents of analytical grade were obtained from commercial sources and used as received Deionized water was used for preparation of all aqueous solutions 2.2.2 Methods

Titanium foil samples were cut into 3.5 cm x 1.0 cm pieces The Ti foil samples were first ultrasonically cleaned in acetone and ethanol, each for 5 min, and then cleaned in deionized water The cleaned titanium pieces were anodized at a constant potential of 20 V in an electrolyte containing 0.1 M NaF and 0.5 M NaHSO4 at room temperature for 2 h using a platinum cathode [42, 121] The TiO2 NTAs formed on the Ti

substrate was then annealed in air at 500oC for 3 h for crystallization in the anatase phase

Field emission scanning electron microscope (FESEM, Hitachi S-4800) was used for studying the morphologies of TiO2 NTAs The crystal structure of the TiO2 NTAs was characterized by an X-ray diffractometer (XRD, M21X, MAC Science Ltd.,Japan) with Cu Ka radiation (λ= 1.54178Å) UV–Vis diffuse reflectance absorption spectrum was determined using a Cary 300 Conc UV–visible spectrophotometer with an integrating sphere Photoluminescence (PL) spectra were recorded using Hitachi F-4600 fluorescence spectrophotometer at an excitation wavelength of 270 nm

Photoelectrochemical measurements were conducted using an electrochemical workstation (CHI660C, Shanghai Chenhua Instrument Co Ltd.) in a standard three-electrode configuration with a TiO2 NTA sample, 3.0 cm2 in area, as the working electrode, a Pt wire counter electrode, and a SCE reference electrode A 300 W xenon lamp (CHF-XQ-300W, Beijing Changtuo Co., Ltd.) was used as the light source, filtered

to 100 mWcm-2 AM1.5G as determined by a radiometer (OPHIR, Newport, USA) Photoelectrochemical properties were measured in 0.1 M KOH, 0.05 M Na2SO4, and 0.1

M PBS (pH 7) containing of 0.1 M KCl aqueous solution

2.3 Results and Discussions

Figure 2.1 shows the surface morphologies of TiO2 NTAs obtained by FESEM The TiO2 NTAs have an average length of 500 nm and an inner pore diameter ranging from

70 to 110 nm and wall thickness of about 15 nm The formation mechanism of the TiO2NTAs is well documented [46, 122-125]

Figure 2.1 FESEM top-surface images of TiO 2 nanotube arrays The left inset shows the cross

sectional view of the NTAs

Figure 2.2a shows the photoluminescence emission spectrum of TiO2 NTAs with the excitation wavelength at 270 nm The photoluminescence emission spectrum has been widely used to investigate the efficiency of charge carrier trapping, immigration, and transfer, and to understand the fate of electron-hole pairs in semiconductor particles

[126]

As shown in Figure 2.2a the broad photoluminescence emission background exhibits from 350 to 500 nm which is defined as the direct solid fluorescence spectrum Figure 2.2b shows the UV–Vis diffuse reflectance spectrum of TiO2 NTAs There are three characteristic absorption peaks of TiO2 at 390 nm, which results from the absorption of the trapped holes, and the other two at 475 nm, and 630 nm, respectively, which result from the absorption of the trapped electrons [91]

Figure 2.2c shows XRD spectrum of the TiO2 NTA phases before and after annealing at 500 oC in air atmosphere As shown in Figure 2.2c, the annealed TiO2 NTA film is a mixture of anatase and rutile types The present of new two peaks at 2 = 25.5o, 27.5o appropriate to anatase TiO2 (JCPDS No 21-1272) and rutile TiO2 (JCPDS No 04-0551), respectively At the same time, the as-prepared TiO2 NTA film shows as an amorphous phase via the diffraction peaks (2 = 38.4o, 40.2o, 53.0o, and 70.6o), which are indexed to the metal titanium phase (JCPDS No 44-1294) [127]

Figure 2.3 shows the photocurrent response of TiO2 NTAs in different electrolytes including 0.1 M KOH, 0.05 M Na2SO4, and 0.1 M PBS (pH 7) solution containing 0.1 M KCl under AM 1.5G illumination The measured photocurrent densities are 0.300 mA

cm–2, 0.247 mA cm–2, and 0.136 mA cm–2 in 0.1 M KOH, 0.05 M Na2SO4, and 0.1 M PBS (pH 7), respectively The dark photocurrents are near zero Maximum photocurrent density is achieved in a 0.1 M KOH solution with of a 0 V anodic bias Under AM 1.5G illumination, TiO2 is excited to produce electrons and holes; most of them then recombine again, and the rest can induce redox reactions At the cathode, the excited electrons promote the reduction of H2O through an external circuit, E.q (2-2) At the

anode, the holes which stay at the valence band transfer to the interface of the electrode/electrolyte, and react with OH- to form hydroxyl radicals, E.q (2-3) This transfer circle can be shuttled freely along the oriented TiO2 NTAs and KOH electrolyte leading to the current in solution [128,129] In KOH solution, with existing of free OH- ions, the formation of hydroxyl radicals is advanced, thus the photocurrent density as well as increased.

(c)

Figure 2.3 Photocurrent responses of TiO 2 NTAs in 0.1 M KOH solution (a), 0.05 M Na 2 SO 4

solution (b), and 0.1 M PBS (pH 7) solution containing of 0.1 M KCl (c)

2.4 Section summary

TiO2 NTAs were prepared by anodiation of titanium foils to form on the Ti surface The formed TiO2 NTAs are highly ordered and vertically aligned on the surface of Ti The photocurrent under AM 1.5G illumination is dependent on the electrolytes with the highest photocurrent achieved in KOH electrolyte The as prepared TiO2 NTAs will be used in the following photoelectrochemical applications

CHAPTER 3 SYNTHESIS AND PHOTOCATALYTIC

APPLICATIONS OF TERNARY Cu–Zn–S

3.1 Introduction

2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most widely used systemic pesticide/herbicides while anthracene-9-carboxylic acid (9-AnCOOH) is toxic to the epithelium, inhibiting transport, that in turn increases the permeability of the paracellular pathway Unfortunately, due to their excellent chemical stability it is difficult to remove 2,4-D and 9-AnCOOH from contaminated wastewater [130-133] Our interest is in the photocatalytic degradation of these agents Among photocatalytic materials, nano-architectured TiO2 is one of the most useful due to its low cost, widespread availability, non-toxicity, high photocatalytic activity, and excellent chemical stability [14, 134-136] However as is commonly known, photocatalytic application of TiO2 is limited by its relatively large bandgap of 3.0 eV for rutile and 3.2

eV for the anatase phases, which limits photoactivity to the ultraviolet region [137,138] Many studies have focused on shifting the optical response of titania by doping with transition and/or noble metals [139,140], nonmetals [141-143], or semiconductors [126,144,145] Sensitization of TiO2 with binary [146,147] and ternary [144,148] low bandgap semiconductors has attracted considerable, particularly since the photocorrosion stability can be improved by doping or shelling one material with another Further, the bandgap can be tuned by elemental doping For example, ternary metal sulfides have excellent photocorrosion stability and greater photocatalytic activity than binary metal sulfides [149,150] As a ternary metal sulfide, Cu-Zn-S has a direct band gap of 2.3 eV making it well suited for solar applications [151-154]

In this work, semiconducting low bandgap ternary Cu-Zn-S nanoparticles are used

to sensitize highly-oriented TiO2 nanotube arrays (NTAs) Pulse electrodeposition [155] is used for deposition of the Cu-Zn-S nanoparticles onto the TiO2 NTAs Compared with other deposition methods, such as SILAR and solution growth techniques [152-154], electrodeposition allows for facile control of the size and composition of the semiconductor nanoparticles The Cu-Zn-S nanoparticles sensitized TiO2 nanotube

arrays were then applied to photocatalytic degradation of 2,4-D and 9-AnCOOH

3.2 Experimental procedures

3.2.1 Materials

Titanium foil (99.8 % purity, 0.25 mm thick) was purchased from Aldrich (Milwaukee,WI) 2,4-D was obtained from Shanghai Chemical Corporation of China 9-AnCOOH was obtained from Sigma-Aldrich Chemie InC All other reagents of analytical grade were obtained from commercial sources and used as received Deionized water was used for preparation of all aqueous solutions

3.2.2 Methods

TiO2 NTAs were prepared by anodization of titanium foils as in section 2.2.2 [42, 121] Ternary Cu-Zn-S nanoparticles were pulse electrodeposited onto TiO2 NTA samples using a three-electrode electrochemical cell with the TiO2 NTAs resting upon a Ti substrate as the working electrode, a Pt wire as the counter electrode, and a saturated calomel electrode (SCE) as reference in a pH 2.5 electrolyte solution containing CuC12

(2 mM), ZnCl2 (5 mM), and Na2S2O3 (20 mM); solution pH was adjusted by addition of HCl solution An ‘on’ pulse potential of -2.0 V (vs.SCE) was applied to the cathode for 0.2s, followed by an ‘off’ potential of 0 V (vs.SCE) for 1.0 s After deposition, the Cu-Zn-S/TiO2 NTA electrodes were repeatedly rinsed with deionized water A field emission scanning electron microscope (FESEM, Hitachi S–4800), and transmission electron microscope operating at 200kV (TEM-2100F; JEOL, Tokyo, Japan) was used for studying the Cu–Zn–S/TiO2 NTA morphologies Elemental analyses were studied with an energy dispersive X-ray spectrometer (EDX) The UV-Vis diffuse reflectance absorption spectrum was determined using a Cary 300 Conc UV–visible spectrophotometer with an integrating sphere

Photoelectrochemical measurements were conducted using an electrochemical workstation (CHI660C, Shanghai Chenhua Instrument Co Ltd.) in a standard three-electrode configuration with a Cu-Zn-S/TiO2 NTA sample, or non-sensitized TiO2NTA sample, 3.0 cm2 in area, as the working electrode, a Pt wire counter electrode, and

a SCE reference electrode A 500 W xenon lamp (CHF-XQ-500W, Beijing Changtuo Co., Ltd.) was used as the light source, filtered to 100 mWcm-2 AM1.5G as determined by a radiometer (NOVA Oriel 70260) Photocatalytic activity of the Cu-Zn-S/TiO2 NTAs was

evaluated by degradation of 2,4-D or 9-AnCOOH in a quartz cell, the reaction solution was 20 mL, with tests carried out under stirring

Photoelectrochemical properties were measured in 0.05 M Na2SO4 or 0.1 M KOH aqueous solution Tests on the degradation of organic pollutants were performed in 20

mL 0.05 M Na2SO4 aqueous solution containing 20 mgL−1 2,4-D or 9-AnCOOH; solution pH was adjusted by addition of H2SO4 or NaOH 2,4-D and 9-AnCOOH concentrations, in aqueous solutions, were measured based on their maximum absorption at 227 nm and 253 nm, respectively, using an UV-Vis Cary 300 spectrophotometer (Varian, USA) The absorbance A0 measured after stirring for 0.5 h in the dark was taken as the initial concentration C0 of the solution The absorbance At

measured after variable periods of illumination was taken as corresponding to the residual concentration Ct The degree of organic compound degradation was calculated by:

Removal efficiency = (C0-C t)

C0 ´100% =

(A0- A t)

A0 ´100%

3.3 Results and Discussions

Figure 3.1 shows the surface morphologies of non-sensitized TiO2 NTAs and Cu-Zn-S nanoparticle sensitized TiO2 NTAs obtained by FESEM The TiO2 NTAs have an inner pore diameter ranging from 70 to 110 nm and wall thickness of about 15 nm (Figure 3.1a) The formation mechanism of the TiO2 NTAs is well documented [46,122] Pulse-potential deposition results in the formation of Cu-Zn-S nanoparticles on the surface of the TiO2 NTAs as shown in Figure 3.1b and c The particle density increases with increasing number of deposition cycles TEM analysis, Figure 3.1d, shows the particle size is around 30 nm As shown in Figure 3.1e, the Cu-Zn-S nanoparticles appear primarily distributed on the top surface of the TiO2 NTAs Composition was determined by EDX spectroscopy, Figure 3.1f; the calculated molar percentages of Cu,

Zn, and S are about 0.048 %, 0.047 %, and 0.099 %, respectively, corresponding to the molar ratio of 1:1:2

Figure 3.1 FESEM top-surface images of: (a) non-sensitized TiO 2 nanotube arrays; (b) TiO 2

nanotube arrays sensitized with 10 Cu-Zn-S deposition cycles; (c) TiO 2 nanotube arrays sensitized with 300 Cu-Zn-S deposition cycles; (d, e) TEM images and (f) EDS spectrum of Cu-Zn-S

sensitized TiO 2 nanotube arrays

UV–Vis diffuse reflectance spectra is shown in Figure 3.2; characteristic absorption peaks of non-sensitized TiO2 NTAs present in UV region which result from the absorption of the trapped holes [91], while sensitization of TiO2 NTAs samples with Cu-Zn-S nanoparticles results in a red shift of the absorption peaks The narrow band-gap of Cu-Zn-S ternary is responsible for the improved absorption capability of TiO2 NTAs in the visible-light region

0.5 0.7 0.9 1.1 1.3 1.5 1.7

(d) 10 cycles (c) 100 cycles (b) 300 cycles (a) 0 cycle 380nm

470nm

605nm

Figure 3.2 UV–vis diffuse reflectance spectra of: (a) non-sensitized TiO 2 nanotube arrays film; nanotube array film sensitized with (b) 300, (c) 100, and (d) 10 Cu-Zn-S deposition cycles

Figure 3.3A shows the photocurrent response of TiO2 NTAs and Cu-Zn-S/TiO2NTAs samples in a 0.05 M Na2SO4 solution under AM1.5G illumination Samples Cu-Zn-S sensitized with (a) 10 cycles, (b) 100 cycles, (c) 5 cycles, and (d) 300 deposition cycles show, respectively, measured photocurrent densities of 1.65 mAcm–2, 1.56 mAcm–2, 1.47 mAcm–2, and 1.27 mAcm–2, while that of the non-sensitized TiO2NTAs sample (e) is 0.81 mAcm–2 Dark photocurrents of all samples are near zero Maximum photocurrent density is achieved with 10 deposition cycles; it appears too little Cu-Zn-S deposition results in poor light absorption, while too much Cu-Zn-S deposition blocks the nanotube array pores hindering separation of the photogenerated charge

Photocurrent density-voltage (J–V) characteristics of the samples were investigated

in 0.1M KOH electrolyte to further examine photoelectrochemical properties, Figure 3.3B Sample photocurrents gradually increase with increasing applied potential, with increasing potential promoting separation of the photo-generated charges [156,157] Under 0.5 V bias (vs.SCE) the photocurrent density of the 10 deposition cycle Cu-Zn-S/TiO2NTAs electrode is 2.6 times that of the non-sensitized TiO2 NTAs sample A more negative zero-current potential represents superior separation efficiency of the

photogenerated electrons and holes [158]; the zero-current potential of the 10 cycle sample is -0.92 V, compared to that of both the 100 cycle and 300 cycle samples with a zero-current potential of -0.88 V

(A)

0 0.5 1 1.5 2 2.5

(e)

(B)

Figure 3.3 (A) Photocurrent response, measured in 0.05 M Na 2 SO 4 solution, of: TiO 2 nanotube arrays sensitized with Cu-Zn-S through (a) 10, (b) 100, (c) 5, and (d) 300 deposition cycles; (e) non-sensitized TiO 2 nanotube arrays (B) Current-voltage characteristics measured in 0.1 M KOH solution of: (a, b) non-sensitized TiO 2 nanotubes arrays in the dark, and under illumination, respectively; Cu-Zn-S sensitized TiO 2 nanotube arrays of (c) 300, (d) 100, and (e) 10 deposition cycles

3.3.3 Photocatalytic degradation of organic pollutants

Figure 3.4A shows the effect of pH on the degradation of a 20 mgL−1 2,4-D solution after 2.5 h irradiation with the (10-cycles deposited) Cu-Zn-S/TiO2 NTAs as the catalyst The degradation efficiency of 2,4-D is of 53.3 %, 72.9 %, 100 %, 62.3 %, 55.2%, and 32.6 % at pH = 1, 2, 3, 5, 7, 10, respectively Following the works of Serpone [159-162] we hypothesize the following photocatalytic degradation mechanism as illustrated in Scheme 3.1

+ hν