Classification of Photoreactor on the Basis of the State of the Catalyst

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Morphologies before Reaction

Since the morphologies of these thin films have been well characterised in Chapter 4 (and ref.[250]), only the results relevant to further discussion will be presented here. For all morphologies, slight variations in film thickness were noticed within the different batches; these are summarised below (there was also significant change in crystal surface smoothness for morphology S2-CG only). Fig. 5.1 shows the cross-sectional views of SEM images produced from the different preparation methods before carrying out the reaction. For films grown from solution 1 (S1, Fig. 5.1A and 5.1B), surface morphologies for template and clean glass growth display different growth patterns, but similar-sized ZnO deposits. For both S1-MS and S1-CG (Fig. 5.1A and 5.1B), growth is characterised by long ZnO rods (producing a film thickness of approximately 4-6µm for S1-MS and 2-3µm for S1-CG). The only major difference is that for S1-MS, the ZnO structure was well aligned and uniformly arrayed, with single crystals growing up from the magnetron sputtered template (Fig. 5.1A).

Morphologies after reaction under oxygen-limited conditions

Figs. 5.2E to 5.2H show the cross-sectional views of the surface structure for the four different morphologies (S1-MS, S1-CG, S2-MS and S2-CG) after reaction under oxygenlimited conditions. Experiments without additional oxygen being supplied test the ZnO thin film catalyst under the toughest reaction regime (i.e; reaction under oxygen limited or without the supply of any other source of oxidant other than the surrounding air so that re reaction is limited in oxidant species because of mass transfer limitations), where overall reaction rates, and thus the formation of reaction intermediates and products, could be mass transfer limited due to lack of oxidant. Each morphology was photocatalytically active (see Section 5.3) and reaction rates were comparable to those under oxygen-rich conditions. As in Chapter 4 (and ref [250]) the ZnO surface morphology shows significant degradation from all reactions conducted compared to the initial structures (Figs. 5.1A-D) for this new batch of films. After reaction, the ZnO columnar structures have rougher surfaces and are even hollowed out by the reaction (Fig. 5.2E is an example of this) in spite of the fact that the pH of the reaction fluid remains nearly the same (7.10 ± 0.11) before and after the reaction.

Impact of Dopant on Surface Morphologies

The nature and type of morphology, summarised in previous chapters (see Sections 4.3.1 and 5.2), plays an important role in photocatalytic activity and reaction mechanism. Therefore and in order to preserve this parameter, both anionic and cationic dopant [160-164] (also see Appendix I) were used to obtain a morphology as close as possible to undoped ZnO thin films. Dopant usually reduces the band gap (hence metal oxide is expected to be more active at low UV energy or at higher UV irradiation wavelength) [179] and the delocalisation of the d-states in the band gap of metal oxide, which helps in reducing the recombination chances of UV-induced electron-hole pairs [266]. The extent of dopant, both type and concentration, on both surface morphology and band gap, is summarised in the following sections.

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N:ZnO thin film morphologies obtained by using N2 gas as nitrogen source

A top surface view of the SEM image of the nitrogen-doped morphology, obtained from the mixture of solutions (N:S1) on a magnetron sputtered coated ZnO glass template, (N:S1-MS) is shown in Fig. 6.1A. The morphology (N:S1-MS) was nearly same as undoped S1-MS. No significant differences in crystal structure, size and smoothness were noticed except in the population and the orientation of crystals. However, no clear indication of N2 incorporation was depicted by EDX analysis (Fig.6.1B). Most of the crystals are positioned at a greater inclination than that for undoped nanostructured ZnO thin films (Fig. 4.1A). The band gap was approximately the same as for the undoped: another indication of inefficient dopant incorporation in the ZnO crystal.

Morphologies obtained at high concentration of TEA with pH control

Figs.6.5-6.8 shows three different morphologies, N1:S1-MS, N1:S1-CG and N2:S2-MS, obtained from a high concentration of TEA (37.5%), on MS or CG glass substrate by using two different mixtures of solutions N′:S1 and N′:S2, with pH control 5 or 7.5. N:ZnO morphologies obtained by using TEA as an N2 dopant source also showed huge variations in morphologies compared to undoped ZnO thin films (see Fig. 4.1). Morphology N1:S1-MS (Fig. 6.5A) obtained, at pH 5, had both solid and X-shaped, horizontally aligned, hollow crystal structures. Again, no solid evidence of nitrogen incorporation as dopant was found (see Fig. 6.5B: EDX analysis at position P1 (X-shaped hollow crystal) and Fig. 6.5C: EDX analysis at position P2 (crystal growth near to MS layer).

Table of Contents :

  • Abstract
  • Acknowledgements
  • List of Figures
  • List of Tables
  • Nomenclature
  • 1 Introduction
    • 1.1 Project Objectives
    • 1.2 Outline of Thesis
  • 2 Literature Review
    • 2.1 Wastewater Treatment and Advanced Oxidation
    • 2.2 Photocatalysis
    • 2.2.1 Introduction to Photocatalysis
    • 2.2.2 Advantages of Photocatalysis
    • 2.2.3 Disadvantages of Photocatalysis
    • 2.3 Homogeneous vs. Heterogeneous Photocatalysis
    • 2.3.1 Homogeneous Photocatalysis
    • 2.3.2 Heterogeneous Photocatalysis
    • 2.4 The Conventional Mechanism of Photocatalysis
    • 2.5 Photocatalytic Properties of Semiconductors
    • 2.6 ZnO as Photocatalyst
    • 2.7 Factors Affecting Photocatalysis
    • 2.7.1 Suspended Versus Supported (Immobilised) Photocatalysts
    • 2.7.2 Catalyst Concentration/Catalyst Loading
    • 2.7.3 Effect of pH
    • 2.7.4 Effect of UV Light Intensity and Wavelength
    • Effect of light intensity
    • Effect of UV wavelength
    • Overall implications of intensity and wavelength on photocatalytic activity/reaction rate
    • 2.7.5 The Effect of Photocatalyst Type: Preparation Technique/Method
    • 2.7.6 The effect of Surface Morphology
    • ZnO (Powder) as photocatalyst
    • ZnO thin films
    • 2.7.7 Effect of Crystallinity
    • 2.7.8 Effect of Dissolved Oxidant
    • Effect of dissolved oxygen
    • Effect of H2O
    • Effect of ozone
    • Overall effect of oxidant
    • 2.7.9 The Effect of Mass Transfer
    • 2.8 Photocatalytic Reactors
    • 2.8.1 Classification of Photoreactor on the Basis of the State of the Catalyst
    • Slurry reactor
    • Immobilised reactor
    • 2.8.2 Classification of Photoreactor on the Basis of Type of Illumination
    • 2.8.3 Classification of Photoreactors on the Basis of Position of the Irradiatio Source
    • 2.9 Increasing Photocatalytic Activity – the Effect of Doping
    • 2.9.1 Types of Dopant:
    • Cationic doping
    • Anionic doping
    • 2.9.2 Impact of Dopant on the Semiconductors
    • 2.9.3 Doped ZnO Photocatalytic Materials
    • 2.9.4 Nitrogen-doped ZnO (N:ZnO)
    • Nitrogen doping methods
    • Effect of N2 dopant on band gap and photocatalysis
    • Effect of doped nitrogen concentration on photocatalysis
    • Effect of N2 doping on morphology
    • Effect of N2 doping on crystallinity
    • 2.9.5 Cobalt-doped ZnO (Co:ZnO)
    • Methods used for Co:ZnO
    • Effect of Co dopant on band gap and photocatalysis
    • Effect of Co dopant type and concentration on Co:ZnO
    • Effect of Co dopant on surface morphology
    • Effect of Co doping on crystallinity
    • 2.10 Kinetic Modelling
    • 2.11 Implications of the Literature
  • 3 Materials and Methods
    • 3.1 Materials
    • 3.2 Photocatalytic Experiments
    • 3.2.1 Photocatalytic Reaction Vessel
    • 3.2.2 Reaction Conditions for Initial Trial Experiments
    • 3.2.3 Reaction Conditions for Second, Third, and Last Phase of Experiments
    • Reaction conditions for experiments under both oxygen-limited and rich conditions
    • Reaction under oxygen-limited conditions
    • Reaction under oxygen-rich conditions
    • 3.3 Preparation of Undoped and Doped ZnO Nanostructured Thin Films
    • 3.3.1 Preparation of Undoped Nanostructured Zno Thin Films
    • 3.3.2 Nitrogen-doped ZnO (N:ZnO) Thin Films
    • N:ZnO thin films preparation using dopant N2 gas
    • N:ZnO thin films preparation using TEA as nitrogen source
    • 3.3.3 Preparation of Cobalt-Doped Nano Structure Zinc Oxide Thin Films (Co:ZnO)
    • 3.4 Analytical Techniques
    • 3.4.1 UV-Visible Spectrophotometer (UV-Vis)
    • 3.4.2 High Performance Liquid Chromatography (HPLC)
    • 3.4.3 Liquid Chromatography and Mass Spectroscopy (LC-MS)
    • 3.4.4 Scanning Electron Microscopy
    • 3.4.5 X-ray Diffractometer
    • 3.4.6 Atomic Absorption Spectroscopy
  • 4 The Effect of Morphology on Undoped ZnO Photocatalysed Reaction Rate, Film Stability and Implications to the Photocatalytic Mechanism
  • 5 A More Detailed Investigation of Conventional versus Lattice Photocatalysed Reactions at both 254nm and 340nm
  • 6 Doped Nanostructured ZnO Thin Films: Impact Of Dopant On Photocatalytic Activity, Reaction Mechanism and Stability

Characterisation of Semi-Conductor Zinc Oxide (ZnO) thin films as Photocatalysts

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