TiO2-based semiconductors for solar energy conversion with a focus on Ta-doped TiO2

Abstract

The demand for energy is growing with the development of the technology. At the same time, an increasing awareness of the impacts of climate change imposes the need to develop new technologies for energy sources that are environmentally clean, such as solar energy. One of the most common research strategies of solar energy conversion is light-induced water oxidation. The discovery of Fujishima and Honda [1] on the use of titanium dioxide, TiO2, photocatalyst in water oxidation paves the way for a new generation of solar materials, oxide semiconductors. TiO2 seems to be the promising photocatalyst owing to its high chemical stability in water and strong reactivity with water leading to its oxidation [1-3]. Since the discovery of TiO2 photoelectrode for its use in light induced water oxidation [1], intensive studies have been devoted aiming at processing TiO2-based materials with enhanced performance. TiO2 exhibits relatively large band gap (3 eV for rutile and 3.2 eV for anatase) which limits its sunlight absorption. Therefore, the common research strategy is reduction of the band gap to a lower value that is required for efficient light absorption [4]. Awareness is growing, however, that the solar-to-chemical energy conversion is influenced by a range of alternative key performance-related properties, KPPs, involving the population of surface active sites, charge transport and Fermi level, in addition to the band gap [5]. The common conceptual approach for processing an efficient TiO2 photocatalyst is based on the perception that the band gap width is controlling the performance. However, photocatalytic properties of TiO2 also depend on structure, chemical composition and surface properties as well as alternative phases attached. Awareness is growing, however, that proper understanding of TiO2 properties, involving its photocatalytic performance, requires recognition of the effect of the structural defects on its light-induced reactivity and the related solar energy conversion. Therefore, there is an increasingly urgent need to understand the effect of defect disorder on the properties of TiO2 and its photocatalytic performance. This is the objective of the present PhD project. The specific aim of this work was to understand the effect of donor type extrinsic defects, imposed by tantalum, on properties and photocatalytic performance of TiO2 and explanation of the effect of tantalum-induced defect disorder on bulk vs. surface properties of Ta-doped TiO2. This PhD considered several research components addressing specific objectives involving semiconducting properties and photocatalytic performance. Semiconducting Properties. Charge transport is one of the key performance related properties for efficient solar-to-chemical energy conversion. This property is closely related to the defect disorder of the material. It is very important to understand the transport of charge carriers from the site of their generation to the reaction sites, which is the solid/water interface. This work studied the semiconducting properties of polycrystalline Ta-doped TiO2 (0.39 at. %) in terms of the measurements of electrical properties (electrical conductivity and thermoelectric power) in the temperature ranges 1173 "" 1323 K in the gas phase of well-defined oxygen activity (10-12 "" 105 Pa). The effect of oxygen activity on the electrical properties was discussed in terms of defect disorder. It was shown that tantalum ions were incorporated into the titanium sublattice of TiO2 and formed donor-type levels. The results were used for derivation of defect disorder model. Photocatalytic Activity. This research component considered photocatalytic performance corresponding to light-induced partial water oxidation. The oxidation reaction takes place at the solid/water interface. Therefore, understanding of the reaction mechanism of TiO2 with water is required in the development of an efficient TiO2""based system. In this work the photocatalytic performance of Ta-doped TiO2 was tested by oxidation of methylene blue (MB). The obtained results were used for (i) understanding the effect of tantalum on the photocatalytic performance of TiO2, and (ii) deriving a theoretical model that could explain the mechanism of partial water oxidation. The research was conducted in partnership with (i) ANSTO providing its unique facility of proton-induced X-ray emission, PIXE, as well as (ii) RMIT University offering the surface analysis facility.

Date of Award	2018
Original language	English