Synthesis, characterization and applications of nanomaterials in the field of photocatalysis

Table of Contents

ABSTRACT

CHAPTER 1 INTRODUCTION
1.1 NANOTECHNOLOGY
1.2 BACKGROUND OF TIO2
1.2.2 PHOTOCHEMICAL MECHANISM
1.2.1 POLYMORPHIC FORMS OF TIO2
1.2.3 APPLICATIONS OF TIO2
1.3 MODIFICATIONS OF TIO2 PROPERTIES
1.3.1 MODIFICATIONS OF TIO2 BY COUPLING OF SEMICONDUCTOR
1.3.2 MODIFICATIONS OF TIO2 BY DOPING
1.3.3 MODIFICATIONS OF TIO2 BY BISMUTH OXIDE
1.3.4 MODIFICATIONS OF TIO2 BY COPPER OXIDE
1.3.5 MESOPOROUS MATERIALS
1.3.6 HIERARCHICALLY POROUS NANOARCHITECTURES
1.4 PREPARATION METHODS
1.4.1 SOLGEL PROCESS
1.4.2 HYDROTHERMAL PROCESS
1.4.3 THE EVAPORATION INDUCED SELF ASSEMBLY METHOD
1.5 BISMUTH OXYCHLORIDES
1.6 BISMUTH OXIDE
1.7 SURFACE MODIFICATIONS
1.8 SPECIFIC OBJECTIVES OF WORK

CHAPTER 2 SYNTHESIS AND CHARACTERIZATION OF MESOPOROUS TIO2 WITH ENHANCED PHOTOCATALYTIC ACTIVITY FOR THE DEGRADATION OF CHLOROPHENOL
2. 1. INTRODUCTION
2.2.1 MATERIALS AND APPARATUS
2.2.2 CATALYST PREPARATION
2.2.3 CHARACTERIZATION
2.2.4 MEASUREMENTS OF PHOTOCATALYTIC ACTIVITIES OF CHLOROPHENOL
2. 3 RESULTS AND DISCUSSION
2.3.1 XRAY DIFFRACTION SPECTROSCOPY
2.3.2 N2 ADSORPTIONDESORPTION
2.3.3 SCANNING ELECTRON MICROSCOPY
2.3.4 TRANSMISSION ELECTRON MICROSCOPY
2.3.5 CHEMICAL REACTIONS MECHANISM FOR THE FORMATION OF POROUS STRUCTURE
2.3.6 PHOTOCATALYTIC DEGRADATION OF CHLOROPHENOL
2.3.7 MECHANISM OF PHOTODEGRADATION
2.4 CONCLUSIONS

CHAPTER 3 MESOPOROUS TITANIA WITH HIGH CRYSTALLINITY DURING SYNTHESIS BY DUAL TEMPLATE SYSTEM AS AN EFFICIENT PHOTOCATALYST
3.1 INTRODUCTION
3.2 EXPERIMENTAL SECTION
3.2.1 MATERIALS AND APPARATUS
3.2.2 SYNTHESIS OF MESOPOROUS TIO2
3.2.3 CHARACTERIZATION
3.2.4 PHOTOCATALYTIC ACTIVITY OF PHENOL
3.3 RESULTS AND DISCUSSION
3.3.1 THERMOGRAVIMETRIC ANALYSIS
3.3.2 XRAY DIFFRACTION
3.3.3 THERMAL STABILITY
3.3.4 N2 SORPTION DATA
3.3.5 TRANSMISSION ELECTRON MICROSCOPE
3.3.6 PHOTOCATALYTIC ACTIVITIES
3.3.7 MECHANISM FOR PHENOL DEGRADATION
3.4 CONCLUSIONS

CHAPTER 4 STUDY ON HIGHLY VISIBLE LIGHT ACTIVE BI2O3 LOADED ORDERED MESOPOROUS TITANIA
4.1 INTRODUCTION
4.2. EXPERIMENTAL SECTION
4.2.1 MATERIALS AND APPARATUS
4.2.2 SYNTHESIS OF MESOPOROUS TIO2 WITH BISMUTH OXIDE IMPREGNATION
4.2.3 CHARACTERIZATION
4.2.4 MEASUREMENTS OF PHOTOCATALYTIC ACTIVITIES
4. 3 RESULTS AND DISCUSSION
4.3.1 THERMOGRAVIMETRIC ANALYSIS
4.3.2 XRAY DIFFRACTION
4.3.3 RAMAN SPECTRA
4.3.4 TRANSMISSION ELECTRON MICROSCOPY
4.3.5 UVVIS ABSORPTION SPECTRA
4.3.6 PHOTOLUMINESCENCE SPECTRA
4.3.7 FOURIER TRANSFORM INFRARED SPECTRA
4.3.8 N2 ADSORPTIONDESORPTION
4.3.9 XRAY PHOTOELECTRON SPECTROSCOPY
4.3.10 MECHANISM OF THE FORMATION OF MESOPOROUS TIO2 WITH WELLORDERED PORE MORPHOLOGY AND BI2O3 ASSISTED PHOTOCATALYTIC PROCESS
4.3.11 PHOTOCATALYTIC ACTIVITY
4.3.12 KINETICS OF REACTION
4.4 CONCLUSIONS

CHAPTER 5 BISMUTHDOPED ORDERED MESOPOROUS TIO2: VISIBLELIGHT CATALYST FOR SIMULTANEOUS DEGRADATION OF PHENOL AND CHROMIUM
5.1 INTRODUCTION
5.2 EXPERIMENTAL SECTION
5.2.1 MATERIALS AND APPARATUS
5.2.2 CATALYST PREPARATION
5.2.3 CHARACTERIZATION
5.2.4 PHOTOCATALYTIC ACTIVITY
5.3 RESULTS AND DISCUSSION
5.3.1 XRAY DIFFRACTION
5.3.2 UVVIS ABSORPTION SPECTRA
5.3.3 RAMAN SPECTRA
5.3.4 PHOTOLUMINESCENCE SPECTRA
5.3.5 TRANSMISSION ELECTRON MICROSCOPY
5.3.6 N2 ADSORPTIONDESORPTION
5.3.7 XRAY PHOTOELECTRON SPECTROSCOPY
5.3.8 PHOTOCATALYTIC ACTIVITY
5.3.9 MECHANISM OF PHOTODEGRADATION
5.4 CONCLUSION

CHAPTER 6 IONIC LIQUID ASSISTED MESOPOROUS TITANIA DOPED WITH COPPER AS A VISIBLE LIGHT PHOTOCATALYST
6.1 INTRODUCTION
6.2 EXPERIMENTAL SECTION
6.2.1 MATERIALS AND APPARATUS
6.2.2 CATALYST PREPARATION
6.2.3 CHARACTERIZATION
6.3 RESULTS AND DISCUSSION
6.3.1 XRAY DIFFRACTION SPECTROSCOPY
6.3.2 UVVISIBLE DIFFUSE REFLECTANCE SPECTRA
6.3.3 RAMAN SPECTRA
6.3.4 TRANSMISSION ELECTRON MICROSCOPY
6.3.5 FOURIER TRANSFORM INFRARED
6.3.6 NITROGEN SORPTION DATA
6.3.7 ENERGY DISPERSIVE XRAY SPECTROSCOPY
6.3.8 XRAY PHOTOELECTRON SPECTROSCOPY
6.3.9 EPR SPECTROSCOPY
6.3.10 MEASUREMENT OF PHOTOCATALYTIC ACTIVITY
6.3.11 MECHANISM OF PHOTODEGRADATION
6.4 CONCLUSIONS

CHAPTER 7 WO3/BIOCL A NOVEL HETEROJUNCTION AS VISIBLE LIGHT PHOTOCATALYST
7.1 INTRODUCTION
7.2 EXPERIMENTAL
7.2.1 MATERIALS AND APPARATUS
7.2.2 CATALYST PREPARATION
7.2.3 CATALYST CHARACTERIZATION
7.2.4 PHOTOCATALYTIC ACTIVITY
7.3 RESULTS AND DISCUSSION
7.3.1 XRAY DIFFRACTION SPECTROSCOPY
7.3.2 UVVISIBLE DIFFUSE REFLECTANCE SPECTRA
7.3.3 RAMAN SPECTRA
7.3.4 SCANNING ELECTRON MICROSCOPY
7.3.5 TRANSMISSION ELECTRON MICROSCOPY
7.3.6 ENERGY DISPERSIVE XRAY SPECTROSCOPY
7.3.7 NITROGEN SORPTION DATA
7.3.8 THERMOGRAVIMETRIC ANALYSIS
7.3.9 MEASUREMENT OF PHOTOCATALYTIC ACTIVITY
7.3.10 MECHANISM FOR THE DEGRADATION
7.4 CONCLUSIONS

CHAPTER 8 SUMMARY AND CONCLUSIONS

REFERENCES

LIST OF ABBREVIATIONS

ACKNOWLEDGEMENT

LIST OF PUBLICATION

Synthesis, characterization and applications of nanomaterials in the field of photocatalysis

Abstract

Considerable effort has been made to design, fabricate, and manipulate nanostructured materials by innovative approaches. The precise control of nanoscale structures will pave the way not only for elucidating unique size/shape dependent physicochemical properties but also for realizing new applications in science and technology. Nanotechnology offers unprecedented opportunities for improving our daily lives and the environment in which we live.

This thesis mainly describes recent progress in the design, fabrication, and modification of nanostructured semiconductor materials for environmental applications. The scope of this thesis covers TiO2, Bi2O3 and BiOCl materials, focusing particularly on TiO2-based nanostructures (e.g., pure, doped, coupled, mesoporous, hierarchically porous, and ordered mesoporous TiO2).

Mesoporous titania is of particular interest since this class of materials possesses well-defined porosity and large specific surface areas. For photocatalytic degradation of organics, these desirable properties are anticipated to improve the efficiency. So in the first part of work, I have synthesized the mesoporous titania by using poly ethylene glycol as a template in dilute acetic acid aqueous solution by hydrothermal process and investigated the effect of PEG molecular weights and thermal treatment on the resultant structure and photocatalytic activity. When the molecular weights of PEG vary from 600 to 20,000, the particle sizes of mesoporous-TiO2 structure decrease from 15.1 to 13.3 nm and mean pore sizes increase from 6.9-10.6 nm. The activities of these mesoporous-TiO2 photocatalysts prepared by using PEG are evaluated and compared with Degussa P-25 using chloro-phenol as a testing compound.

However, it is quite difficult to fabricate mesoporous TiO2 with an anatase crystalline wall during the synthesis process with thermal stability. In order to get the highly crystalline mesoporous TiO2 during the synthesis process, I have used a new combination of pluronic P123 and poly ethylene glycol templates with acetic acid as the hydrolytic retardant. The highly crystalline mesoporous TiO2 was achieved during synthesis process. The anatase-rutile phase transformation of the mesoporous TiO2 was not observed up to 800 oC. The mesoscale order of mesoporous-TiO2 was retained after thermal treatment up to 450 oC. It maintained its pore properties at high temperature of 700 oC with a significant pore size 10.4 nm and pore volume 0.11 cm3/g. Mesoporous-TiO2 has biporous mesostructure with the small mesopore of 5.5 nm and the large mesopore of 8.7 nm in mean pore diameters. The mesoporous-TiO2 showed higher photocatalytic activity and mineralization for phenol degradation than mesoporous titania prepared by using single templates and standard system of Degussa P25. The remarkably high catalytic activity of the mesoporous-TiO2 is attributed to its high crystallinity and biporous mesostructure.

As the previous part of this work was based on random mesoporosity but mesoporous titania materials with tunable pore structure and tailored framework composition are of great interest for broad applications ranging from adsorbent materials, separations, catalysis, energy storage, and biological conversions. The ordered mesoporous channels offer larger surface area and enhanced accessibility. In this part of work, an efficient method of evaporation-induced self-assembly process is adopted to prepare well-defined two-dimensional hexagonal mesoporous nanocrystalline anatase TiO2. Because the formation is kinetically controlled and greatly influenced by the atmospheric conditions of the laboratory, the reproducibility in obtaining high-quality mesoporous titania is still a challenge. The well-defined two-dimensional hexagonal mesoporous nanocrystalline anatase TiO2 was synthesized by the EISA technique of non-ionic amphiphilic triblock-copolymer template P 123, titanium tetrachloride and tetrabutyl titanate.

The utilization of mesoporous titania as a highly active photocatalyst under visible light thus remains challenging. TiO2 doped with C, N, or B has brought new perspectives because it renders the TiO2 responsive to visible-light irradiation. Investigations on alternative materials like bismuth are rare. So these ordered mesoporous TiO2 were loaded with different % of Bi2O3 using the wet impregnation method. For comparison Degussa P25 impregnated with Bi2O3 was also prepared. These samples were composed of anatase and showed an extension of light absorption into the visible region and reduced the electron-hole recombination rate. The photo oxidation efficiency was evaluated by methyl orange and 2,4-dichlorophenol degradation under visible illumination. The samples loaded with different % of Bi2O3 showed higher photocatalytic activity than mesoporous-TiO2, P25 and Bi2O3 loaded P25. The catalyst exhibited high activity due to the Bi2O3-photosensitization and well-ordered 2D pore structure.

In order to make the EISA process more controllable and reproducible, high concentration of HCl was employed, which simultaneously lowers the condensation and polymerization rates of Titanium isopropoxide, thus renders it a controllable and reproducible method for producing highly ordered mesostructures. The synthesis of highly ordered mesoporous Bi-doped TiO2 was a big challenge so far. By this EISA process the bismuth doped ordered mesoporous-TiO2 was successfully obtained. Analyses reveal that the well-ordered mesostructure was doped with Bi, which exists as Bi3+ and Bi(3+x+). The Bi doped mesoporous TiO2 samples exhibited improved photocatalytic activities for simultaneous phenol oxidation and chromium reduction in aqueous suspension under visible and UV light over the pure ms-TiO2, P-25, and conventional Bi-doped TiO2. The high catalytic activity was due to both the unique structural characteristics and the Bi doping.

For the preparation of mesoporous TiO2 materials, the templates were replaced by water immiscible room-temperature ionic liquid [C4mim]+BF4-, as template and an effective additional solvent through the sol-gel method at low temperature. Mesoporous TiO2 was designed as an efficient photocatalyst sensitive to visible light by copper impregnation in different weight ratios. The samples presented anatase as the nanocrystalline phase and extended the absorption edge to the visible region. Cu was doped into the lattice of mesoporous titania as illustrated by TEM and HRTEM micrographs. XPS analysis indicate the mixed valent of Cu1+ and Cu2+ states with more reduced Cu1+. The beneficial effect of Cu can be explained by considering the formation of Cu+1species by means of a transfer of photo-generated electrons from TiO2 to Cu+2, which reduce the electron hole recombination effectively. The catalytic activity depends on the substitutional Cu1+/Cu2+ sites as interfacial Ti-O-Cu linkages, reduced band gap energy, the mesoporosity, efficient charge separation and optimal copper doping.

So far, the major strategies in the previous part of work focus on the preparation of mesoporous titania with different ways and to develop it as a visible light photocatalyst. Next I have worked on BiOCl compounds with different strategies to use them as visible light photocatalyst.

First the bismuth oxychloride nanostructure is prepared by a new low temperature route and a novel heterojunction is developed between BiOCl and tungsten oxide to make it an efficient visible light photocatalyst. WO3/BiOCl heterojunction system extends the absorption edge to the visible region efficiently. BiOCl works as a main photocatalyst while WO3 acts as the photosensitizer absorbing visible light in the WO3/BiOCl composite. The individual BiOCl and WO3 show very low photocatalytic efficiency under visible light irradiation but their heterojunction provides unexpectedly high efficiency in decomposing rhodamine B as compared to Degussa P25, pure BiOCl and WO3

Keywords: mesoporous TiO2, bismuth doping, copper doping, BiOCl, hierarchical macro/mesoporous structure, photocatalysis

Chapter 1 Introduction

1.1 Nanotechnology

In the last decades, a little word attracted enormous attention, interest and investigation from all over the world: “nano”. What it presents in terms of science and technology, which are also called nanoscience and nanotechnology, is much, much more than just a word describing a specific length scale. It has dramatically changed every aspect of the way that we think in science and technology and will definitely bring more and more surprises into our daily life as well as into the world in the future [[1]].

What is actually so exciting about “nano”? “Nano” means one billionth (10-9), so 1 nanometer refers to 10-9meter and is expressed as 1 nm. 1 nm is so small that things smaller than it can only be molecules, clusters of atoms or particles in the quantum world. Nanometer is a special point in the overall length scale because nanometer scale is the junction where the smallest manufacturable objects “meet” the largest molecules in nature. The structures, devices and systems having at least one dimension in nanometer scale are not only smaller than anything that we’ve ever made before, but also possibly the smallest solid materials that we are able to produce. Besides, in nanometer scale, the properties of materials that we are familiar with in our daily life, such as color, melting point, electronic, catalytic or magnetic properties [[2]], will change dramatically or be replaced by completely novel properties due to what is usually called size effect [[3]]. “At this size scale, everything, regardless of what it is, has new properties. And that is, where a lot of the scientific interest is. All these make “nano” so fascinating.

The environment and energy are the biggest challenges of the 21st century. Ironically, the solution to these large problems may lie in something very small. Nanomaterials, with attractive chemical and physical properties, are being explored for potential uses in energy and environmental applications. During the past decade, rapid advances in materials science have led to significant progress in environmental remediation and renewable energy technologies such as photocatalytic oxidation, adsorption/separation processing, solar cells, fuel cells, and biofuels. The design and creation of new materials and substances chemically modified from the molecular and atomic levels to sizes on the nanoscale promise significantly enhanced functions for environmental applications. Meanwhile, the development of advanced characterization techniques has facilitated a fundamental molecular-level understanding of structure-performance relationships, which are strongly related to grain size and size distribution, shape, chemical composition, presence of interfaces (grain boundaries and free surfaces), and interactions between the constituent domains. This knowledge, together with effective synthesis strategies, has inspired the design and fabrication of novel nanostructured materials for a wide variety of applications.

Recently, a number of excellent reviews and reports on the preparation, modification, assembly, characterization, properties, engineering, and applications of nanostructured materials have been published [[4]]. This thesis work represents recent progress in the design, fabrication, and modification of semiconductor nanostructured materials. It also highlights their environmental applications. The synthesis of nanostructured materials is a very active research field [[5]]. The ability to fabricate and process nanostructured materials lies at the heart of nanotechnology, paving the way for understanding novel properties and realizing their potential applications. To date, many technologies have been explored to synthesize nanostructured materials. These technical approaches can be essentially grouped in two paradigms: top-down and bottom-up [[6]]. In particular, versatile bottom-up methods based on chemistry have attracted considerable attention because of their relatively low cost and high throughput [[7]]. Bottom-up approaches refer to the buildup of a material from the bottom: atom-by-atom, molecule by-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, have been developed to control the size, morphology, and uniformity of nanostructures simultaneously [[8],9]. The successful implementation of the bottom-up strategy requires, in the end, the controlled growth of nanostructures. Among various media for crystal growth, the solution based method offers significant advantages, including

- low reaction temperatures,
- size-selective growth,
- morphological control, and
- large-scale production.

The liquid-phase approach to the synthesis of inorganic nanostructures has been recently reviewed [[10],11]. This approach is also the core idea that has guided the work presented in this thesis.

1.2 Background of TiO2

In 1972, Fujishima and Honda achieved ultraviolet (UV) light induced water cleavage using a titanium dioxide photo anode in combination with a platinum counter electrode soaked in an electrolyte aqueous solution [[12]]. Since then, semiconductor photocatalysis has attracted considerable attention because of its promising applications in environmental purification as well as solar energy conversion. Semiconductors (e.g., TiO2, ZnO, Fe2O3, WO3, and CdS) can act as photocatalysts for light-induced chemical transformations because of their unique electronic structure, which is characterized by a filled valence band and an empty conduction band. When a photon with an energy of hν matches or exceeds the band gap energy (Eg) of the semiconductor, an electron in the valence band (VB) is excited into the conduction band (CB), leaving a positive hole (h+) in VB. The as photogenerated CB electrons and VB holes in the excited states can recombine and dissipate the input energy as heat, become trapped in metastable surface states, or react with electron donors and electron acceptors adsorbed on the semiconductor surface. During the past decade, research on nanostructured semiconductors as the new building blocks to construct light-energy-harvesting assemblies has grown rapidly and has drawn from a number of scientific disciplines [[13]].

Of the various semiconductors tested to date, TiO2 is the most promising photocatalyst because of its appropriate electronic band structure, photostability, chemical inertness, and commercial availability. A variety of morphologies of nanostructured TiO2 including nanoparticles, nanorods, nanowires, nanostructured films or coatings, nanotubes, and mesoporous/nanoporous structures have been reported, and many TiO2-based composites have also been prepared. Significant progress has been made in a variety of areas ranging from photovoltaics and photocatalysis to photo/electrochromics and sensors. The effective utilization of clean, safe, and abundant solar energy by the TiO2 photocatalyst will lead to promising solutions not only for the energy crisis but also for serious environmental challenges.

Heterogeneous photocatalysis became the next application of TiO2 and it is the most promising method for the water and air pollutant removal. The main advantages of this method when the so-called first generationTiO2 is used are:

- chemical stability of TiO2 in the aqueous media in the large range of pH (1 ≤pH ≤14)
- low production cost
- no additives required- only oxygen in the air
- system applicable under both high and low contaminant concentration
- total mineralization of wide group of organic pollutants or oxidation to non toxic compounds
- Great deposition capacity for noble metal recovery

1.2.1 Polymorphic forms of TiO2

TiO2 occurs in nature in three different polymorphs, namely, rutile, anatase, and brookite (in order of abundance) (Figure 1.1). For the catalytic purpose anatase is the most favorable form. The unit cell structures of the rutile and anatase TiO2. These two structures can be described in terms of chains of TiO6 octahedra, where each Ti4+ ion is surrounded by an octahedron of six O2- ions. The two crystal structures differ in the distortion of each octahedron and by the assembly pattern of the octahedra chains. In rutile, the octahedron shows a slight orthorhombic distortion; in anatase, the octahedron is significantly distorted so that its symmetry is lower than orthorhombic. The Ti-Ti distances in anatase are larger, whereas the Ti-O distances are shorter than those in rutile. In the rutile structure, each octahedron is in contact with 10 neighbor octahedrons (two sharing edge oxygen pairs and eight sharing corner oxygen atoms), while, in the anatase structure, each octahedron is in contact with eight neighbors (four sharing an edge and four sharing a corner). These differences in lattice structures cause different mass densities and electronic band structures between the two forms of TiO2.

- Anatase - Stable at low temperatures and used as photocatalyst in wastewater treatment.
- Rutile - Stable at high temperatures and used in industrial products such as paint.
- Brookite - Found in minerals and belong to orthorhombic crystal system.

Anatase and rutile TiO2 are more common as they are easy to synthesis. Anatase TiO2 has higher photocatalytic activity than rutile type due to the difference in band gap energy (Eg). For anatase TiO2, Eg is 3.2eV which corresponds to wavelength of 387nm while Eg is 3.0eV for rutile TiO2 which corresponds to wavelength of 413nm. Anatase and rutile TiO2 have similar valence band (VB) energies that produce strong oxidizing power holes, therefore, 0.2eV difference in Eg between these two types of TiO2 is due to the difference in conduction band (CB) energies. The CB energy for rutile is close to the potential required to electrolytically reduce water to hydrogen gas. But the CB energy for anatase is 0.2eV higher which means that it has higher reducing power to drive the very important reaction involving the electrolytic reduction of dissolved oxygen to superoxide radical anion, O2--[12].

Anatase type is thermodynamically less stable than rutile TiO2. However, its formation is kinetically favoured at lower temperature (usually less than 600oC). The low temperature contributes to the formation of the higher surface area for anatase TiO2, and higher surface density of active sites for adsorption and photocatalysis [[14]].

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Figure 1.1 The different polymorphs of TiO2.

1.2.2 Photochemical mechanism

As shown in Figure 1.2, TiO2 in the anatase crystal form has a band gap of 3.2 eV. Under light illumination, the photogenerated electrons and holes can initiate redox reactions with chemical species adsorbed on the surface or interface of the photocatalyst. The first step in photocatalysis processes is an interaction of the semiconductor with the light. A photon with an energy (hν) equal or higher than the Eg of the semiconductor is absorbed, then an electron (e-) is promoted into the CB from the VB leaving a hole (h-) behind (Fig. 1.2). This process can be expressed by the following quasi- reaction:

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The generated charges diffuse to the surface where:

- in a photocatalysis process the holes and the electrons react with solution or gas derived species adsorbed on the photocatalyst surface (Fig. 1.2) [[15]].

This photoexcitation process is always followed by the deexcitation events. While the (c) and (d) pathways are the desirable deexcitation through the redox reaction of the electrons and holes with acceptor or donor type of species. The pathways marked as (a) and (b) illustrate the unwanted recombination of the charges both in the volume (a) and on the surface (b) [[16]]. According to Table 1.1 there are two main processes which determine the overall quantum efficiency:

- competition between charge carrier recombination and trapping
- competition between trapped carrier recombination and interfacial charge transfer

An improvement of the quantum efficiency and through this an improvement of the photoactivity can be realised by diminishing of the recombination rate of the charge carriers and increasing the interfacial charge transfer [[15]].

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Figure 1.2 Schematic photoexcitation in a semiconductor photocatalyst followed by the deexcitation pathways, after [15,16].

Table 1.1 Time characteristics of processes involved in the photochemical mechanism in the TiO2 applied as a photocatalyst[15].

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A very reactive free hydroxy radical is formed during redox reaction which is responsible for organic pollutant degradation. Pathways for OH- formation upon heterogeneous photocatalysis can be expressed by the following sequential reactions. Holes trapped at the surface of semiconductors are reacted with surface adsorbed water molecules and hydroxyl surface groups to form OH-:

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Electron donors (organic pollutants) that adsorbed on semiconductor surface are directly oxidized by OH- [[17]].

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Alternatively, the formation of additional OH- can come from superoxide radical anion (O2--) produced from electron uptake of dissolved oxygen [[18]].

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It may be observed from Equations 1.8 to 1.10 that more OH• are formed once the oxygen is adsorbed onto the photocatalyst and oxidized by the surface trapped electron.

1.2.3 Applications of TiO2

Titanium dioxide is one of the most widely applied metal oxides, thanks to its unique properties. Due to its high refractive index it is used as a pigment in paint industry. Its non-toxicity and stability makes it possible to apply it in the pure form ass a food additive, in pharmaceuticals, and cosmetic products. TiO2 plays also an important role as a biocompatible material for bone implants etc. Schematic representation of some titania applications is given below in Figure 1.3. TiO2 nanomaterials normally have electronic band gaps larger than 3.0 eV and high absorption in the UV region. TiO2 nanomaterials are very stable, nontoxic, and cheap. Their optical and biologically benign properties allow them to be suitable for UV protection applications.

The catalysed photolysis is mostly related to dyes photodecomposition under the solar light irradiation[19-21] . The so-called photogenerated catalysis is applied not only for the decomposition of dyes but also for mineralization of many groups of organic pollutants (Table 1.2). The number of new TiO2 applications based on its photoelectrochemical properties towards environmental usage is increasing. The photocatalytic water purification is discussed in details in the following chapters. The organic pollutants used in this thesis work are bold mark in the Table 1.2.

Table 1.2 General group classification of organic pollutants decomposed by photocatalysis systematized by Herrmann in 1999[14].

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Figure 1.3 Representing the basic applications of TiO2

1.3 Modifications of TiO2 properties

The widespread technological use of TiO2 is to some extent constrained by its wide band gap (3.2 eV), which requires ultraviolet irradiation for photocatalytic activation. The width of the band gap determines the light absorption and results in a low efficiency of solar energy conversion (Fig 1.4). Because UV light accounts for only a small fraction (5%) of the sun’s energy compared to visible light (45%), the shift in the optical response of TiO2 from the UV to the visible spectral range will have a profound positive effect on the practical applications of the material. One possible approach to overcome this issue is to make TiO2 photocatalytically active beyond its absorption threshold of 400 nm by creating energy levels within the band gap or to adequately shift the conduction band (CB) and/or the valence band (VB) so that photons of lower energy are able to excite electrons [[22]-25]. There are several ways to achieve this goal. First, doping TiO2 nanomaterials with other elements can narrow the electronic properties and, thus, alter the optical properties of TiO2 nanomaterials. Second, sensitizing TiO2 with other colorful inorganic or organic compounds can improve its optical activity in the visible light region. Third, coupling collective oscillations of the electrons in the conduction band of metal nanoparticle surfaces to those in the conduction band of TiO2 nanomaterials in metal-TiO2 nanocomposites can improve the performance. In addition, the modification of the TiO2 nanomaterials surface with other semiconductors can alter the charge-transfer properties between TiO2 and the surrounding environment, thus improving the performance of TiO2 nanomaterials-based devices. Additionally, new physical and chemical properties emerge when the size of the material becomes smaller down to nanometer scale. Well crystalline anatase particles are most suitable for photocatalytic applications.

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Figure 1.4 The solar irradiance vs. wavelength for AM 1.526and UV-Vis range of solar spectrum with indicated absorption range of TiO2.

1.3.1 Modifications of TiO2 by coupling of semiconductor

The coupling of different semiconductor systems may result in improved photocatalytic activity. Figure 1.5 shows the scheme for the charge-transfer processes involved in coupled semiconductor systems. The electrons photoinduced on the conduction band of a higher level can be injected into the lower conduction band of the second semiconductor. As a result, more efficient charge-carrier separation can be achieved.

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Figure 1.5 Schematic illustration of charge transfer in a coupled semiconductor system.

1.3.2 Modifications of TiO2 by doping

The optical response of any material is largely determined by its underlying electronic structure. The electronic properties of a material are closely related to its chemical composition (chemical nature of the bonds between the atoms or ions), its atomic arrangement, and its physical dimension (confinement of carriers) for nanometer-sized materials. The chemical composition of TiO2 can be altered by doping. Specifically, the metal (titanium) or the nonmetal (oxygen) component can be replaced in order to alter the material’s optical properties. It is desirable to maintain the integrity of the crystal structure of the photocatalytic host material and to produce favorable changes in electronic structure. It appears easier to substitute the Ti4+ cation in TiO2 with other transition metals, and it is more difficult to replace the O2- anion with other anions due to differences in charge states and ionic radii. The small size of the nanoparticle is beneficial for the modification of the chemical composition of TiO2 due to the higher tolerance of the structural distortion than that of bulk materials induced by the inherent lattice strain in nanomaterials [[23],27].

1.3.3 Modifications of TiO2 by bismuth oxide

Bi2O3 is a p-type semiconductor with conduction and valence band edges +0.33 and +3.13 V relative to NHE, respectively. Recently, a number of Bi-based photocatalysts, such as NaBiO3 [[28]], Bi3O4Cl [[29]], Bi4Ti3O12 30 and Bi2WO4 31 were synthesized; these materials showed high photocatalytic activities even under visible light irradiation. It seems that bismuth maybe a proper candidate dopant element to extend the TiO2 spectral response and thus to improve its photoreactivity. Bian et al. reported the synthesis of an active Bi2O3/TiO2 visible photocatalyst with an ordered mesoporous structure [[32]]. Wang et al. prepared bismuth- and sulfur-codoped TiO2 by a simple solgel method; its high photocatalytic performance is associated with the existence of numerous oxygen vacancies, the acidic sites on the surface of TiO2, and its high specific surface area [[33]]. Rengaraj and co-workers synthesized a Bi doped TiO2 nanocatalyst, and their degradation experiments demonstrated that the presence of Bi3+ in the TiO2 catalysts substantially enhanced the photocatalytic degradation of methyl parathion under UV light [[34]]. However, the synthesis of highly ordered mesoporous Bi-doped TiO2 has never been reported before. Bi-doped TiO2 photocatalysts have also been found to be active for the photodegradation of pollutants in aqueous solution [[35]-40]. Yao et al. [[41],42] and Thanabodeekij et al. [[43]] reported that bismuth titanate compounds BixTiyOz were catalytically active for the photodecolorization of methyl orange and photodegradation of 4-nitrophenol. They found that the Bi_O polyhedra in bismuth titanate compounds were photocatalytic centers. It was also suggested that the Bi3+ species in doped TiO2 photocatalysts are active centers in the photoinduced degradation, reduction, or decolorization of pollutants.

1.3.4 Modifications of TiO2 by copper oxide

In order to solve low quantum yield and selectivity problems many researchers had modified TiO2 by doping it with metal impurities. The influence of various transition metal ions on the photoactivity of pure TiO2 for many reactions has also been studied with the aim of improving the efficiency of the photocatalytic process. It has been hypothesized that the addition of transition metals to titania increases the rate of photocatalytic oxidation, due to the electron scavenging by the metal ions at the semiconductor surface through the following reaction:

illustration not visible in this excerpt

where M n + represents transition metal ions[[44]]. This reaction prevents electron-hole recombination and results in an increased rate of formation of .OH radical. Yamashita et al. [[45]] reported that addition of copper(II) into the TiO2 matrix could improve the efficiency and selectivity of CO2 photoreduction to produce methanol. Tseng et al. [[46]] also observed that the formation of methanol was found to be much more effective on Cu-titania catalysts. Gombac et al. [[47]] synthesize CuO x -TiO2 photocatalysts for H2 production from ethanol and glycerol solutions. Hiroshi et al.[[48]] prepared an efficient visible light sensitive photocatalysts by grafting Cu(ll) ions onto TiO2 and WO3 photocatalysts. Cao et al.[[49]] prepared hierarchical meso-macroporous titania-supported by CuO nanocatalysts for CO oxidation. Other investigators also noted that addition of copper could improve the photocatalytic activity.

1.3.5 Mesoporous materials

Since the time when nanoscience became popular, people have been always thinking how to bring the fascinating properties of individual nanoscale objects into the macroscopical world. In contrast to direct uses of nanomaterials as powders or dispersions, manufacturing highly complex devices and systems with novel properties by precise control and utilization of nanomaterials as building blocks is very challenging and promising to maximize their potential applications in nanotechnology. However, great difficulties to handle the nanomaterials make the conventional manufacturing technology inapplicable in nanoscale domain.

Mesoporous titania is of particular interest since this class of materials possesses well-defined porosity and large specific surface areas. For photocatalytic degradation of organics, these desirable properties are anticipated to improve the photocatalytic activity. However, it is quite difficult to fabricate mesoporous titania with an anatase crystalline wall during the synthesis process with thermal stability. The relatively low thermal stability of the titania based mesoporous materials is often attributed to their phase transformation and pore shrinkage derived from elimination of the structure-directing agents and the wall is too thin to retain the mesoporous structure during crystallization. From a viewpoint of catalytic application, it is very significant to synthesize the mesoporous titania with high crystallinity and thermal stability.

The purpose of this work is to synthesize mesoporous titania with highly crystalline structure to enhance photocatalytic activity. For this purpose I have used the PEG and P123 templates singly and in a dual template system to get the highly crystalline anatase TiO2 during the synthesis.

A number of interesting porous transition-metal oxides especially mesoporous structures, have been reported (as pioneered by Stucky in synthesizing porous silica, niobia, and titania) [50-52] . Despite these advances, the development of new methods to fabricate stable porous transition-metal oxides materials remains a challenge. In general, several factors, including the hydrolysis and condensation of transition-metal oxides precursors, structural integrity collapsing during the redox reactions, possible phase transitions, and crystallization processes, affect the quality and function of porous transition-metal oxides materials.

1.3.6 Hierarchically porous nanoarchitectures

There has been rapidly growing interest in recent years in the construction of functional materials with complex hierarchical structures. Materials with multimodal or multiscale pores are desirable in catalysis and separation processes, where optimization of the diffusion and confinement regimes is required [[50]]. The host-guest interactions can be promoted because micropores and mesopores can provide size and shape selectivity for the guest molecules. The presence of macrochannels should also make the guest molecules more accessible to the active sites and avoid pore blockage. Theoretical calculations and simulations also show that catalytic processes occur more efficiently in materials with hierarchical micropores or mesopores.

The fabrication of such structures with control on size of macroporosity has remained an experimental challenge. In this part of work, a new and simple approach without any template was adopted for producing hierarchical materials with highly monodisperse macropores with interconnecting mesoporous channels.

The porosity, such as pore size and specific surface area, is one of the most relevant properties of porous materials. On the basis of the pore diameter, an official classification was proposed by the International Union of Pure and Applied Chemistry (IUPAC) [[53],54], which is given in Table 1.3 below.

Table 1.3 Classification of pores according to their diameter or width

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By measuring the gas adsorption isotherms of numerous porous materials, six types of isotherms have been summarized (shown in Figure 1.6) [[55],56]. The relative pressure is expressed by p/p0, where p is the pressure of the vapour and p0is the saturation vapour of the adsorptive.

In Figure 1.6, the isotherm of Type I is considered as the isotherm of microporous materials. Type II and Type III are associated with nonporous and macroporous materials, respectively. Type IV and Type V are all the exhibitions of mesoporosity. Type IV c is of theoretical interest and relatively rare in practice. Various types of isotherms shown in Figure 1.6 are all related to the different behaviors of the interactions between the adsorbents, the solids, and the adsorbates, the adsorbed gas such as nitrogen, in microporous, mesoporous or macroporous materials. Therefore, the isotherm of one substance can be utilized to determine its pore size qualitatively.

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Figure 1.6 Classification of gas adsorption isotherms.

The specific surface area is another important aspect of porosity. Usually, the specific surface area is expressed by the following equation:

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where A is the specific surface area, m2/g; a is the total surface area of a measured sample, m2; m is the mass of the measured sample, g. Currently, there are two main methods to evaluate the specific surface area from the gas adsorption data: the Brunauer-Emmett-Teller (BET) method and the comparative method [[55],56]. The basic idea of BET method is to evaluate the specific surface area by fitting the adsorption data to the BET model and equation [[55]]. The same purpose as BET method is realized by comparing the adsorption isotherm for a given porous material with the adsorption isotherm for a suitable reference adsorbent of known specific surface area.

1.4 Preparation methods

The basic methods used in this thesis are sol-gel, hydrothermal and evaporation induced self assembly process. The general description related to these is given here but their use in experimental procedures with modifications to get the highly desirable properties is given in next chapters.

1.4.1 Sol-gel process

The sol-gel process is a versatile solution process for making ceramic and glass materials. In general, the sol-gel process involves the transition of a system from a liquid "sol" (mostly colloidal) into a solid "gel" phase. Applying this process, it is possible to fabricate ceramic or glass materials in a wide variety of forms: ultra-fine or spherical shaped powders, thin film coatings, ceramic fibers, micro porous inorganic membranes, monolithic ceramics and glasses, or extremely porous aerogel materials. An overview of the sol-gel process is presented in a simple graphic work below (Fig. 1.7).

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Figure 1.7 Schematic illustration of sol-gel formation mechanism.

Simply saying, sol-gel process is the procedure that molecular precursors, e.g. metal chlorides or metal alkoxides, react with certain solvent, e.g. H2O or organic solvents, and form 3D metal oxide network via inorganic polymerization including hydrolysis/solvolysis and condensation reactions. Conventionally, aqueous sol-gel process, in which water is the solvent, is broadly used for the synthesis of metal oxide bulk materials as well as nanoparticles. The reaction mechanisms of hydrolysis and condensation processes are shown in the following. Firstly, the metal alkoxide or metal chloride is hydrolyzed and an M-OH species is generated:

Hydrolysis

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In the second step, the hydroxy groups react with each other or other metal alkoxide/chloride and a 3D M-O-M network is then formed upon the propagation of the condensation reaction and results in the elimination of ROH, water or HCl.

Condensation

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The reaction mechanism of aqueous sol-gel process is rather simple. However, resulting from the high reactivity of the precursor towards hydrolysis, it has several disadvantages. For instance, the resulting products are often amorphous, which means that post thermal treatments are not avoidable to get crystalline material; the reaction parameters, such as temperature, pH, concentration of anions and even the method of mixing, have to be carefully controlled to achieve the desired products and reproducibility.

The chemical reactivity of metal alkoxides toward hydrolysis and condensation mainly depends on the electronegativity of the metal atom and its ability to increase its coordination number “N”, i.e. on its size (Table 1.4). Silicon alkoxides are rather stable while titanium alkoxides are very sensitive to moisture. The hydrolysis rate of [Abbildung in dieser Leseprobe nicht enthalten] is 4 about five orders of magnitude greater than that of [Abbildung in dieser Leseprobe nicht enthalten]. Gelation times of silicon alkoxides are of the order of hours whereas titanium alkoxides have gel times of the order of seconds or minutes. Most alkoxides are very sensitive to moisture and must be handled with care under a dry atmosphere otherwise precipitation occurs as soon as water is present. Alkoxides of highly electronegative elements such as PO(OEt)3 cannot be hydrolyzed under ambient conditions, whereas the corresponding vanadium derivatives VO(OEt)3 are readily hydrolyzed into vanadium pentoxide gels.

Table 1.4 Hydrolysis rate of metal alkoxides as a function of the electronegativity "c", ionic radius "r" and maximum coordination number "N" of the metal[57].

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The chemical reactivity of metal alkoxides is also related to the alkyl group R; the larger the R, the slower is the hydrolysis because of steric hindrance.

Chemical precursors play a key role in the sol-gel coating and affect directly the porosity, refractive index, hardness and other performance characteristics of the resultant coatings and thin films. In addition, the type of precursor is frequently a decisive factor for production in terms of cost and usability.

1.4.2 Hydrothermal process

Hydrothermal synthesis is normally conducted in steel pressure vessels called autoclaves with or without Teflon liners under controlled temperature and/or pressure with the reaction in aqueous solutions. The temperature can be elevated above the boiling point of water, reaching the pressure of vapor saturation. The temperature and the amount of solution added to the autoclave largely determine the internal pressure produced. It is a method that is widely used for the production of small particles in the ceramics industry.

Possible advantages of the hydrothermal method over other types of crystal growth include the ability to create crystalline phases which are not stable at the melting point. Also, materials which have a high vapour pressure near their melting points can also be grown by the hydrothermal method. The method is also particularly suitable for the growth of large good-quality crystals while maintaining good control over their composition. Disadvantages of the method include the need of expensive autoclaves, and the impossibility of observing the crystal as it grows. There are three different methods of hydrothermal process.

1. Temperature-difference method

The most extensively used method in hydrothermal synthesis and crystal growing. The super saturation is achieved by reducing the temperature in the crystal growth zone. The nutrient is placed in the lower part of the autoclave filled with a specific amount of solvent. The autoclave is heated in order to create two temperature zones. The nutrient dissolves in the hotter zone and the saturated aqueous solution in the lower part is transported to the upper part by convective motion of the solution. The cooler and denser solution in the upper part of the autoclave descends while the counter flow of solution ascends. The solution becomes supersaturated in the upper part as the result of the reduction in temperature and crystallization sets in.

2. Temperature-reduction technique

In this technique crystallization takes place without a temperature gradient between the growth and dissolution zones. The super saturation is achieved by a gradual reduction in temperature of the solution in the autoclave. The disadvantage of this technique is the difficulty in controlling the growth process and introducing seed crystals. For these reasons, this technique is very seldom used.

3. Metastable-phase technique

This technique is based on the difference in solubility between the phase to be grown and that serving as the starting material. The nutrient consists of compounds that are thermodynamically unstable under the growth conditions. The solubility of the metastable phase exceeds that of the stable phase, and the latter crystallize due to the dissolution of the metastable phase. This technique is usually combined with one of the other two techniques above.

1.4.3 The evaporation induced self assembly method

In the EISA approach, the templating agent and the inorganic precursors are gathered in the same solution, for which chemical conditions (i.e., composition, stoichiometry, addition of polymerization catalyst or inhibiting agent, aging time, etc.) are adjusted so as to favor homogeneous dispersion of both parts. The latter solution is then cast on the substrate through conventional chemical solution deposition by spin, dip, meniscus, or spray coating processes. The self-assembly is triggered during evaporation of the latter deposited solution layer just after deposition on the substrate and is thus governed by the progressive departure of the volatile components. This is a complex dynamic step that involves at least four simultaneous or subsequent mechanisms that may be governed by totally different parameters [[58],59]. These are:

(1) the fast evaporation of the solvent;
(2) the film water content equilibration with the atmosphere;
(3) the formation and stabilization of the template/inorganic biphasic homogeneous layer;
(4) the consolidation of the inorganic network through consolidation.

In addition, these steps are either thermodynamically or kinetically governed and do not East China University of Science and Technology - 19 - necessarily take place in the given precise order with possible overlapping along the process of thin film deposition. This overall complex transformation can be simply seen as a straight polycondensation of the inorganic precursors around the organic micelles (or mesophases) freezing the latter liquid crystal mesostructure (see Figure 1.8). The only prerequisite to a homogeneous highly ordered stable hybrid material is that step (3) must be completed before step (4) is too advanced. Also, one must keep in mind that evaporation takes place at the atmosphere/wet film interface, creating concentration gradients that are responsible for potential nonhomogeneities. Indeed, the structuration can be assimilated to a frontier of transformation progressing from the atmosphere interface toward the substrate interface, and step (3) is completed when this frontier has reached the solid surface [[60]].

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Figure 1.8 Scheme illustrating the templating approach combined with the various steps involved in the evaporation-induced self-assembly process during thin film formation by liquid deposition techniques[61].

In the viewpoint of stability, photosensitization of TiO2 with inorganic substrates is superior over that with organic substrates since the organic compounds usually suffer from degradation during photocatalysis. Bi2O3 is an important photosentisizer with a direct band gap of 2.8 eV [[62]]. As a result, the Bi2O3/TiO2 could be easily activated by visible lights owing to the photosensitization by Bi2O3 [[37],38]. Design of TiO2 with well-defined mesoporous structure is a promising way to achieve high photocatalytic activity since the ordered mesopore channels facilitate fast intraparticle molecular transfer. While the large surface area may enhance the light harvesting, the adsorption for reactant molecules, and even the dispersion of Bi2O3 nanoparticles. Meanwhile, a high crystallization degree of photocatalysts is favorable for rapid transfer of photocharges from bulk to surface, which could inhibit the recombination between photoelectrons and holes, leading to enhanced quantum efficiency. However, preparation of semiconductor oxides with both the ordered mesoporous structure and highly crystalline pore wall is usually a challenging task.

1.5 Bismuth oxychlorides

Layered bismuth oxychlorides have demonstrated not only good catalytic activities and selectivities in the oxidative coupling of methane [[63]], oxidative dehydrogenation of ethane [64,65] and n-butane [[66]], but also outstanding photocatalytic activities [[67]-69] and introduce a new family of promising photocatalysts. Bismuth oxychloride is a wide bandgap semiconductor (Eg =3.46 eV) within the ultraviolet range, with a tetragonal PbFCl-type structure (space group P 4/ nmm; No. 129). BiOCl crystallizes into unique layered structures consisting of [Cl-Bi-O-Bi-Cl] sheets stacked together by the nonbonding interaction through the Cl atoms along the c -axis. In each [Cl-Bi-OBi-Cl] layer, a bismuth center is surrounded by four oxygen and four chlorine atoms in asymmetric decahedral geometry (Figure 1.9). The strong intralayer bonding and the weak interlayer van der Waals interaction gives rise to highly anisotropic structural, electrical, optical, and mechanical properties, which have made BiOCl attractive in applications such as cosmetics, pharmaceuticals [[70]], battery cathodes, photocatalysis [[67]], and photo electrochemical devices [[71]].

illustration not visible in this excerpt

Figure 1.9 Schematic drawing of the crystal structure of A OCl (A =Bi, La, Sm) of a tetragonal PbFCl-type structure with space group of P 4/ nmm 66.

Thus far, the major strategies for developing a visible light photocatalyst are modification of the TiO2 band gap by doping [[72]-74], or development of new semiconductor materials capable of absorbing visible light. In this part of, a heterojunction structure between BiOCl and WO3 was developed as an efficient photocatalyst under visible light irradiation, even though individual BiOCl and WO3 show very low photocatalytic efficiency. Recently, Zhang et al. reported that nanoparticular BiOCl can be an efficient photocatalyst in decomposing methyl orange in UV light [[67]]. This suggests that under UV light irradiation BiOCl is a potential photocatalyst which may compete with TiO2, even though its band gap is considerably larger than that of the anatase TiO2 (E g = 3 . 2 eV). WO3, with a band gap 2.6-2.8 eV, can absorb some portion of visible light ([Abbildung in dieser Leseprobe nicht enthalten] < 440 nm), but alone, its photocatalytic activity is very low. For the first time it was found in this work that the heterojunction structure between BiOCl and WO3 induces complete decomposition of RhB under visible light irradiation. Moreover, this new material is not harmful to environment, and the synthetic procedure is very simple with low production cost.

1.6 Bismuth oxide

In the last decades the research in semiconductor photocatalysis was focused on visible light activity, mainly for the purification of water and air and the cleavage of water. Since semiconductor materials absorbing in the visible like CdS suffer from photocorrosion[[75]] or low activity (e. g. WO3, Fe2O3)[[76]], recent research was concentrated on the visible light sensitization of UV-active titanium dioxide which is photostable and highly active. However, up to now, examples of undoped metal oxide semiconductor powders of high visible light activity are rare. Bismuth oxide (Figure 1.10) has primarily attracted attention in materials science because of its high oxide ion conductivity and non-linear optical properties [[77]-79]. It has the absorption edge at 2.8 eV with suitable band edge potentials for water oxidation (Ecb = 0.33, Evb = 3.13 eV) [[80]], high refractive index, dielectric permittivity and thermal stability. Furthermore, Bi2O3 is rather inert in neutral water, which is a fundamental prerequisite for suitable application as photocatalyst for water purification. The attractive feature of Bi2O3 is the significantly acceptable activity for the mineralization of a large variety of molecules. This can be of great potential interest as in fact polluted water contains often a concoction of various classes of compounds which must be simultaneously degraded. In fact, the use of a single photocatalyst with some activity towards a broad range of compounds might be more flexible than the use of multi step systems with high activity towards selective components. In an effort to explore the effect of various surface modifications on mechanistic aspects of charge carriers transfer process and Bi2O3 phase stability. There are some recent reports for NiO-ZnO [[81]] and Bi2O3-NiO [[82]] nanocomposite systems that show superior performances with respect to the single components.

The first attempts in visible light photocatalysis using ternary bismuth oxide were reported by Tang et al. [[83]] This group investigated the photocatalytic activity of CaBi2O4 in acetaldehyde and methylene blue degradation at λ ≥ 440 nm. Photocatalysis by binary bismuth oxide was established by Zhang et al., who prepared nanocrystalline α-Bi2O3 by sonochemical synthesis and applied the powder for photodegradation of methyl orange with visible light (λ > 400 nm). However, this synthesis requires a surfactant and high energy ultrasound [[84]].

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Figure 1.10 Schematic drawing of the crystal structure of Bi2O3.

In this thesis, preparation of highly efficient visible light active Bi2O3 without any impregnation or doping process have been done by following a simple route.

1.7 Surface modifications

Chemistry that occurs at the surfaces of metal oxides is critical in a variety of industrial applications including catalysis, optical display technology, solar energy devices, and corrosion prevention. Defects such as oxygen vacancies and step edges are the most reactive sites on the surfaces of metal oxides. Understanding metal oxide reactivity thus requires an understanding of the nature of surface defects. The defect sites on a particle surface and distortions at an interface, in part, control the adsorptive affinity and reactivity for compounds that react with photogenerated charges. Defect sites have been shown to either inhibit or enhance recombination depending on the specific defect [[85]]. Consequently efficient surface traps are essential to reduce the extent of charge carrier recombination for enhanced photocatalytic activity and better performance. Currently most of the existing active photocatalysts such as TiO2, ZnO, WO3 and Bi2O3 are suffering from the limitation of high photogenerated electron-hole recombination rate resulting low photon conversion efficiencies [86-89] . However, beside the exploration of new active photocatalysts, efforts are underway to enhance the efficiency of the existing photocatalysts by introducing suitable traps at the surface. The most common approach, based on the transfer of photogenerated electrons to the surface states, is the impregnation of metal or non-metal ions at the surface of the base/bulk photocatalyst with the formation of heterostructures [[90]-92].

Much of the surface chemistry of metal oxides is believed to be related to the defects [[93]]. Understanding the origin and role of the defect states is important for improving or expanding the scope of a system for specific applications. So far there are number of reports related to defect studies of titania, zinc oxide and cerium oxide[[94]-98]. The investigations on the alternative materials are rare. In this respect, bismuth oxide (Bi2O3) is an attractive material. In this phd work a new route was adopted to create the surface defects, which have the competing effects like doping.

1.8 Specific objectives of work

My PhD work was done in different parts. Specific objectives of the work are as follows: which are briefly described here and their detail is given in next chapters.

1. The design of photocatalytic nanoparticles systems which have higher photocatalytic efficiency (both surface properties and quantum yield) with UV (wavelength of 350 nm), or visible light irradiation.
2. The determination of photocatalytic efficiencies in organic dye degradation and toxic pollutants using Degussa P25 as a standard to compare with formed efficient systems.
3. The preparation of visible light photocatalysts (composite and doped systems) in different with mesoporous structures to overcome the band gap potential of titania and its utilization for environmental cleaning.
4. The preparation of pure mesoporous titania nanoparticles by a combination of sol gel and hydrothermal processes using different combinations of templates with highly crystalline anatase structure and used these for degradation of phenol and chloro-phenol.
5. The preparation of ordered mesoporous TiO2 via nonaqueous EISA process and its impregnation with bismuth oxide to used in the degradation of MO and 2-4 dichloro-phenol under visible light.
6. The synthesis of bismuth doped ordered mesoporous titania via nonaqueous EISA process for simultaneous degradation and conversion of phenol and chromium. So far there was no report about bismuth doping with ordered mesoporous titania.
7. The synthesis of mesoporous titania by using ionic liquid as template and doped with copper in order to use it under visible light irradiations.
8. The preparation of hierarchical macro/mesoporous titania with control on mesopores by a simple and cost effective route for photocatalysis.
9. The preparation of bismuth oxychloride nanoparticles by a new sol gel method and use these in visible light by WO3 impregnation.

Chapter 2 Synthesis and characterization of mesoporous TiO2 with enhanced photocatalytic activity for the degradation of chloro-phenol

2. 1. Introduction

Metal oxide functional materials, especially titanium dioxide have been a hot topic of interest to researchers in many different fields due to enormous potential applications of such materials as membranes, solar energy conversion, chemical sensors and catalysis [[99]-105]. Among these, mesoporous titanium dioxide is of great interest because this class of materials possesses well-defined porous sizes, porosity and large specific surface areas [106,107]. When these are used as photocatalysts for degradation of organics, these desirable properties are anticipated to improve the activity because the performances of such photocatalysts rely on these attributes [[101],102,106,107]. During the past two decades, many synthetic methods have been proposed to obtain mesoporous titanium dioxide including sol-gel, template assisted, hydrothermal, solvothermal, ultrasound-induced, ion liquid and evaporation-induced self-assembly [[108]-114]. Among these methods, the sol-gel is probably the simplest method but formation of aggregated nanoparticles during the precipitation and post-calcination processes often lead to poorly defined mesoporous structures [[115]]. In order to obtain mesoporous structures and better control on porous size and porosity of the resultant material, various structural directing reagents have been introduced to sol-gel processes. Among all the used surfactants, polyethylene glycol is a nondegradable and hydrophilic polymer that can be crosslinked into hydrogels through various chemistry methods. PEG-based hydrogels were developed by introducing terminal acrylate functional groups which could take part in photopolymerization reactions[[116],117]. PEG has been frequently used as a structure-directing reagent to obtain nanoporous SiO2 [106,118,119], aluminosilicate [[120],121] and other composite materials [113]. However, only a limited number of investigations have been reported to produce mesoporous-TiO2 by employing PEG as the structural directing reagent [107,122-124].

In the traditional sol–gel method for TiO2 preparation, it is often difficult to control the rate of hydrolysis because titanium precursors are highly reactive towards water. As a result, the physicochemical properties of TiO2 have been practically uncontrollable. Several attempts have been made to solve this problem. In practice, strong acids (i.e. HCl and HNO3) and complex reagents (i.e. acetylacetone, oxalate and citrate) are commonly employed to reduce the rapidity of the hydrolysis process [125,126]. When a strong acid is employed, the pH of the reaction solution changed rapidly with time which may lead to formation of less porous and larger particle-sized TiO2 [[127]]. Weak acid such as acetic acid (HAc) has been used to replace strong acids because it control the hydrolysis process of titanium sources due to the chelating effect of acetic anions and formation of pH buffer [[110],123]. In this preparation method, it is possible to control the physical properties of the resulting materials including surface area, particle size and pore structure by adjusting preparation variables.

In this part of thesis work, we report the preparation of mesoporous-TiO2 by employing PEG with different molecular weights and acetic acid, as the structural directing reagent and hydrolytic retardants, respectively. The procedure involves a post hydrothermal treatment to improve the quality of the resultant mesoporous-TiO2. The effects of molecular weight of PEG, hydrothermal treatment and calcination temperatures are studied in comparison to control the physical properties of the resulting materials including surface area, particle size and pore structure (structural characteristics) of the resultant mesoporous-TiO2. The mechanism of chemical reactions involved in the synthesis of mesoporous-TiO2 and degradation of chloro-phenol have also been proposed.

2.2 Experimental section

2.2.1 Materials and apparatus

The materials, reagents, apparatus and instruments used in this part of thesis work are given in Table 2.1 and 2.2.

Table 2.1 The reagents and materials used in experiment.

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Table 2.2 The apparatus used in experiment.

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2.2.2 Catalyst preparation

Mesoporous-TiO2 was prepared using the hydrothermal-assisted sol-gel method. In a typical synthesis procedure 5.0 g of TBT was added drop-by-drop to 30 mL of acetic acid aqueous solution (20%, v/v) under vigorous stirring. The mixed solution was sealed and stirring was continued for 4 h to obtain solution A. In a separate beaker, 3.0 g of PEG with the molecular weight of 600 (designated as PEG 600) was dissolved in 20 mL ethanol under vigorous stirring to obtain solution B. Solution B was then added drop-by-drop to solution A. The final mixed solution was sealed and stirred for 24 h at room temperature. The resultant solution was then transferred into a Teflon sealed container for hydrothermal treatment under a constant temperature of 140 oC for 48 h. The precipitations were then collected and dried overnight in air at 80 oC. The as-prepared sample was then subjected to a thermal treatment process at 450 oC for 4 h. To examine the effect of calcination temperature, the as-prepared sample was also calcined at different temperatures. Furthermore, in order to evaluate the effect of the molecular weight of PEGs, we replaced the PEG 600 by 2000, 10000 and 20000, respectively.

2.2.3 Characterization

The prepared samples were characterized by these techniques. Powder X-ray diffraction is used to study the crystal structures of polycrystalline materials. X-ray diffraction patterns of all samples were collected in the range 20-80º [Abbildung in dieser Leseprobe nicht enthalten] using a Rigaku D/MAX 2550 diffractometer [Abbildung in dieser Leseprobe nicht enthalten], operated at 40 kV and 100-200 mA. The crystallite size was estimated by applying the Scherrer equation to the full width at half-maximum of the (101) peak of anatase. D = [Abbildung in dieser Leseprobe nicht enthalten] where [Abbildung in dieser Leseprobe nicht enthalten] is the half-height width of the diffraction peak of anatase, K=0.89 is a coefficient, [Abbildung in dieser Leseprobe nicht enthalten] is the diffraction angle and λ is the X-ray wavelength corresponding to the CuKα irradiation.

The morphology was studied using scanning electron microscopy and transmission electron microscopy. Transmission electron microscopy was performed on a JEOL JEM2010 instrument at an acceleration voltage of 120 kV. The samples usually were prepared by dispersing a small amount of powder in ethanol. Next, one drop of the dispersion was put on a carbon-coated copper grid and left to dry under ambient conditions before insertion into the device.

A JEOL, JSM-6360LV instrument was used for obtaining SEM images of the samples. The samples were loaded on carbon-coated stubs and sputter coated with Au/Pd alloy prior to analysis.

Nitrogen adsorption and desorption isotherms were obtained at 77 K with a Micromeritics ASAP 2010 system. All the samples were degassed at 473 K before the measurement. For the determination of the surface area, the BET method was used.

Fourier transform infrared spectra were carried out by employing a Nicolet 740 FT-IR spectrometer equipped with a TGS detector and a KBr beam splitter.

Chloro-phenol degradation intermediates were determined by the HPLC series 1100(Agilent) equipped with a reverse-phase C18 analytical column (Zorbax SB-C18, USA) of 150mm x 2.1 mm and 3.5 μm particle diameter. Column temperature was maintained at [Abbildung in dieser Leseprobe nicht enthalten]. The mobile phase used for eluting chloro-phenol from the HPLC columns consisted of methanol and water (50:50,v/v)at a flow-rate of 1.0mL min_1.

2.2.4 Measurements of photocatalytic activities of chloro-phenol

Chloro-phenol was chosen as a model pollutant to evaluate the photocatalytic activities. The photocatalytic reactions were carried out at 30 °C using a home-made reactor. A high-pressure Hg lamp of 300-W having the strongest emission wavelength of 365 nm, was used as a UV light source (the average light intensity was about 1230 μW/cm2). It was mounted 10 cm away from the reaction solution. During the reaction, a water-cooling system cooled the water-jacketed photochemical reactor to maintain the solution at room temperature. The photocatalyst (1.5gL-1) was added into a 100 mL quartz photoreactor containing 50 mL of a 50 mgL-1 aqueous solution of chloro-phenol. The mixture was sonicated for 20 min and stirred for 30 min in the dark in order to reach the adsorption-desorption equilibrium. Under UV irradiation and vigorous stir, each reaction was lasted for 5 h. Preliminary studies indicated a linear light absorbance verse chloro-phenol concentration and that the decomposition of chloro-phenol in the absence of photocatalyst or UV irradiation could be neglected.

To analyze the concentration of chloro-phenol and degradation products, the suspension was first centrifuged and filtered through 0.22 μm Millipore membrane filters to remove the catalyst. The membrane filters are made of mixed cellulose esters and had no effect on chloro-phenol concentration. The concentrations of chloro-phenol were measured with a UV-vis spectrophotometer (Varian Cary 100) with UV absorbance in the range of 200-400 nm and the UV λmax value of chloro-phenol is 280 nm corresponded to the maximal adsorption of chloro-phenol. The concentrations of chloro-phenol were calculated from the height of peak by using calibration curve. The measurements were repeated for the catalyst and the experimental error was found to be within ±3%.

Chlorinated phenols widely used as pesticides, herbicides and wood preservatives, are among the top priority pollutants and found in aqueous ecosystems as byproducts of chlorinated water. Moreover, chlorinated phenols are chemical precursors of the more toxic polychlorinated dibenzo-p-dioxins . Due to this, chloro-phenol was employed as an environmentally relevant model pollutant. The chloro-phenol was degraded in an illuminated suspension of TiO2 according to the following stoichiometry

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Several methods to degrade these pollutants have been investigated, namely the (photo)-Fenton technique [129], H2O2 activated by metal complexes of N-containing macrocyclic ligands [130], and photocatalysis by semiconductor metal oxides and polyoxometalates [131]. When evaluating the efficiency of a method for the decomposition of chlorophenols, besides the degradation rate, three points must be considered:

- dechlorination of the substrate, a key step in the degradation of chlorophenols since the dechlorinated products usually lose the toxicity of their parent molecules;
- mineralization of the benzene ring to form harmless CO2 and HCl; and
- if complete mineralization is not achievable, toxicity of the degraded products must then be considered.

2. 3 Results and discussion

Mesoporous-TiO2 was synthesized by using PEG as template and acetic acid a weak acid for the formation of smaller and uniform titanium hydrate. Fine mesoporous-TiO2 particles during the hydrolysis process were obtained due to the better control of the hydrolysis process of titanium source, chelating effect of acetic anions and formation of pH buffer [[132],133]. The post hydrothermal treatment was performed to increase the crystallinity of mesoporous-TiO2 . Mesoporous-TiO2 materials were found to have a high crystallinity with a nanocrystalline anatase structure. The addition of PEG with higher molecular weight enlarged the mesopore size and widened the mesopore size distribution of the material.

2.3.1 X-ray diffraction spectroscopy

The wide angle XRD patterns of mesoporous-TiO2 prepared with and without hydrothermal treatment are presented in Fig. 2.1. The samples exhibit characteristic anatase peaks which can be indexed to 25.3 (101), 37.2 (004), 48.9 (200), 54.0 (105), 55.3 (211), 62.4 (204) and 68.7° (116) (JCPDS no. 21-1272) and suggest that anatase is the highly crystalline phase [[103],135]. The mean crystal sizes of the resultant mesoporous-TiO2 were calculated from the broadening of the (101) XRD peak of anatase phase according to the Scherrer formula [[132]]. The mean crystal sizes of 15.0 and 16.4 nm were obtained from samples with and without hydrothermal treatment, respectively.

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Figure 2.1 XRD patterns obtained from samples (a) without hydrothermal treatment; (b) with hydrothermal treatment prepared by PEG 600 and calcined at 450 oC.

The effect of molecular weights of PEG on the mean crystalline size of the hydrothermally treated samples was investigated as shown in Fig. 2.2. These samples were calcined at 450 oC for 4 h. It was found that the mean crystalline sizes decreased from 15.0 to 13.3 nm when the molecular weight increased from 600 to 20,000 (see Table 2.3). These results are in agreement with the report [[103]]. The wide angle XRD patterns of hydrothermally treated mesoporous-TiO2 calcined at different temperatures were recorded and given in Fig. 2.3.

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