Titania catalysts : crystal chemical insights and tailored functionalities
Lim, Suo Hon
Date of Issue2009
School of Materials Science and Engineering
The functionality of titania in photocatalytic, photovoltaic and sensing applications is controlled by the morphology, microstructure, and phase assemblage of this semiconducting oxide. Many approaches have been reported to synthesize high purity titania nanomaterials, but soft chemical methods are favoured as crystalline products can be easily obtained after heat treatment. However, defective and amorphous titania variants have been less investigated, as the transition from the gel to the crystalline phase is frequently assumed to be straightforward. While it is possible, in principle, to determine the phase proportions by powder X-ray diffraction (XRD), the reliability of weight percentages extracted by Rietveld analysis can be compromised by microabsorption. Spectroscopic methods also provide valuable insights but may be surface sensitive, rather than representative of the bulk. This thesis describes the quantitative determination of titania phase assemblages, using X-ray, neutron and electron diffraction methods, for a range of catalysts of prescribed composition and architectures. In this manner, key linkages between crystal chemistry, microstructure and catalytic activity are established, leading to innovative tailored synthesis and morphological designs. While the conversion of the titania dimorphs − anatase to rutile − is often implied to be a direct process obeying first-order kinetics, the transition must in a strict crystallographic sense must be reconstructive, rather than displacive. Moreover, refined neutron diffraction data reveals that anatase can be grossly nonstoichiometric and contain up to 10% titanium (Ti) vacancies when calcined at < 600ºC. In addition, sol-gel synthesized titania is shown to contain a persistent amorphous phase (aperiodic titania) that coexists with rutile to relatively high temperature (1000˚C). While neutrons are highly penetrating, diffracted X-ray intensities will be attenuated due to microabsorption. Studying the degree of attenuation as a function of energy by multiple wavelength X-ray diffraction, allows the evolution of the titania microstructure from the gel to be monitored. It is found that anatase disassociates to aperiodic titania before rearranging as rutile, with this amorphous phase enveloping the crystalline material and modifying functionality. The findings are supported by X-ray absorption spectroscopy (XAS) where Ti coordination and order correlation are consistent with the appearance of aperiodic materials prior to rutile crystallization. The complete chain of transformations can be summarized as: amorphous gel → nonstoichiometric anatase → aperiodic titania → rutile.