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1.4 CHEMICAL SCIENCES
This thesis presents a study of the effect of chemical modifiers and dopants on both the anatase to rutile transformation and also the photocatalytic efficiency of semiconductor nanomaterials. The main focus of the work is based on the crystallisation and phase transformation of the widely investigated semiconductor metal oxide, titanium dioxide (TiO2) Of the three polymorphs associated with titanium dioxide, anatase is widely regarded as the most effective photocatalyst. Typically anatase will transform to rutile in the temperature range 600 – 700 °C however, modification of a titanium precursor with a chelating agent can result in extended transformation temperature. The effect of employing various concentrations of formic acid and water on phase transition is systematically studied by XRD, FTIR and Raman spectroscopy (Chapter 3). Retention of anatase (41%) at increased temperatures (800 °C) and 10% at 900 °C is achieved at optimum conditions. On comparison, a control sample prepared without modification with formic acid show rutile formation at 600 °C. FTIR and Raman studies indicate that a bridging titanium-formate structure is formed upon addition of titanium isopropoxide to formic acid. It is proposed that under different reaction conditions, a syn-syn or synanti bridging structure is favoured. It is concluded that the syn-anti bridging structure hinders cross linking of the metal oxide oligomer network, resulting in a weakened structure, facilitating low temperature rutile formation when compared with the syn-syn binding mode which forms a more ordered network, stabilising anatase at increased temperatures. Using the optimum formic acid, titanium isopropoxide molar ratios (TTIP:FA:H2O, 1:4:4) obtained in chapter 3, silver (1, 3 and 5 mol %) was added to the system and the effects are reported (chapter 4). Through XRD it is shown that silver lowers the anatase to rutile transformation temperature. Samples calcined at 700 °C show that 5 mol % Ag-TiO2 contains both anatase (46%) and rutile (54%), whereas the undoped TiO2 consists primarily of anatase (95%). At 800 °C all silver containing samples transform to rutile but the undoped sample consists of both anatase (55%) and rutile (45%). XPS, FTIR and Raman spectroscopy show that incorporation of silver causes a reduction in the intensity of the COO- stretches associated with the titanium formate bridging structure indicating that the metal oxide complex is weakened in the presence of silver. XPS analysis show that Ag0 and Ag2O had formed on the titania-formate surface prior to calcination (> 100 °C) indicating that photo-oxidation of silver has occurred. It is concluded that the presence of silver (Ag0 and Ag2O) hindered bridging ligands, resulting in a weakened gel network. This structurally weak network could easily collapse upon calcination to form rutile. Titanium dioxide doped with aluminium also shows extended transformation temperatures. In chapter five, aluminium, silver, nitrogen, and sulfur are used to dope titanium dioxide powders. Addition of 1 mol % aluminium resulted in the retention of anatase (87%) at 900 °C and even at 1000 °C, anatase (20%) is still present as seen from XRD. Nitrogen doped titanium dioxide is synthesised using 1, 3-diaminopropane as a nitrogen source. Visible light absorption of nitrogen doped titanium dioxide is seen through diffuse reflectance spectroscopy and it is shown that increased amounts of nitrogen do not result in greater visible light absorbance. Addition of silver to N-TiO2 result in increased photocatalytic activity for methylene blue degradation, 9.9 x 10-2 min-1 compared to N-TiO2 without silver (9.6 x 10-2 min-1) which is accredited to a reduction in recombination. Dimethysulfoxide and sodium thiosulfate are investigated as sources for sulfur doping of TiO2. Sodium thiosulfate produces S-TiO2 that is highly photoactive (43.1 x 10-2 min-1) compared to S-TiO2 synthesised from dimethylsulfoxide (8.4 x 10-2 min-1). A combination of sodium thiosulfate and 1, 3 diaminopropane for N, S co-doped TiO2 resulted in the formation of sodium titanates. Through FTIR and Raman spectroscopy it is shown that both nitrogen atoms from 1, 3-diaminopropane chelate to a single titanium metal centre. 1, 3-diaminopropane is also found to cause retention of sodium during the sol-gel synthesis leading to the formation sodium titanate. TiO2/ZnO hybrid materials show enhanced photocatalytic efficiency through the formation of heterojunctions where excited electrons migrate from TiO2 to ZnO, delaying recombination. Chapter six describes a sol-gel synthesis of ZnO/TiO2 hybrid. A number of molar ratios of zinc precursor to titanium precursor are investigated. Anatase, rutile and zinc oxide are formed from the sol-gel synthesis but it is found that zinc titanate phases dominated the material structures. Through XRD it is shown that when the titanium precursor is in excess over the zinc precursor, high percentages of anatase (at initial calcination temperatures 400 – 700 °C) and rutile are formed. Zinc metatitanate forms at ~ 600 °C. The samples with excess titanium precursor show high percentages of zinc metatitanate at temperatures 700 – 900 °C and at 1000 °C there is an almost 1:1 conversion of zinc metatitanate to zinc orthotitanate. For samples where zinc precursor is in excess, zinc oxide is the dominant phase at low calcination temperatures (400 – 600 °C) but at increased temperatures (> 700 °C), Zn2Ti3O8 is formed. The Zn2Ti3O8 undergoes ~1:1 phase transformation to form zinc orthotitanate at 1000 °C. Raman and FTIR show the formation of titanium oxalate and zinc oxalate coordinated structures. These are believed to form infinite chains that may combine during crystallisation to form zinc titanate structures. In summary, the results shown in the current study have shown how strongly the synthesis conditions can affect the crystalline behaviour of metal oxide materials. A greater understanding of the synthesis of these materials can contribute to the wide area of semiconductor research.
Nolan, N.T. (2010). Sol-Gel Synthesis and Characterisation of Novel Metal Oxide Nanomaterials for Photocatalytic Applications. Doctoral Thesis. Technological University Dublin. doi:10.21427/D7SW22