Type of publicationArticle (peer-reviewed) AbstractThis progress review describes our work on the design of new TiO2 based photocatalysts. The key concept is the formation of composite structures through the modification of anatase and rutile TiO2 with molecular-sized nanoclusters of metals oxides. Our density functional theory (DFT) level simulations have been compared with experimental work synthesizing and characterizing surface modified TiO2. We use DFT to show that nanoclusters of metal oxides such as TiO2, SnO/SnO2, PbO/PbO2, ZnO and CuO are stable when adsorbed at rutile and anatase surfaces and can lead to a significant red shift in the absorption edge which will induce visible light absorption; this is the first requirement for a useful photocatalyst. We determine the origin of the red shift and the fate of excited electrons and holes. For p-block metal oxides the oxidation state of Sn and Pb can be used to modify the magnitude of the red shift and its mechanism. We describe comparisons of recent experimental studies of surface modified TiO2 that validate our DFT simulations. These nanocluster-modified TiO2 structures 2 form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with a correct choice of nanocluster modified can be applied to other reactions.

In this review we describe our work on new TiO2 based photocatalysts. The key concept in our work is to form new composite structures by the modification of rutile and anatase TiO2 with nanoclusters of metal oxides and our density functional theory (DFT) level simulations are validated by experimental work synthesizing and characterizing surface-modified TiO2. We use DFT to show that nanoclusters of different metal oxides, TiO2, SnO/SnO2, PbO/PbO2, NiO and CuO can be adsorbed at rutile and anatase surfaces and can induce red shifts in the absorption edge to enable visible light absorption which is the first key requirement for a practical photocatalyst. We furthermore determine the origin of the red shift and discuss the factors influencing this shift and the fate of excited electrons and holes. For p-block metal oxides we show how the oxidation state of Sn and Pb can be used to tune both the magnitude of the red shift and also its mechanism. Finally, aiming to make our models more realistic, we present some new results on the stability of water at rutile and anatase surfaces and the effect of water on oxygen vacancy formation and on nanocluster modification. These nanocluster-modified TiO2 structures form the basis of a new class of photocatalysts which will be useful in oxidation reactions and with the suitable choice of nanocluster modifier can be applied to CO2 reduction.

The doping of CeO2 with trivalent cations is a common technique for enhancing ionic conductivity in electrolytes for solid oxide fuel cell applications. However, the local defect structure in these materials is yet to be fully explored. Furthermore, many studies have overlooked the effect of the dopants on the reducibility of CeO2, which is important as electronic conductivity can short-circuit the fuel cell. Density functional theory (DFT)+U calculations have been performed on a series of CeO2 systems doped with trivalent cations. The most stable configuration and the relative attraction between dopant cations and oxygen vacancies were determined, and it was found that the defect structure is principally dependent on the ionic radius of the dopant cations. The reduction energy was found to be dependent on the structure around the dopants but did not vary significantly between dopants of similar ionic radii. From these results, it is possible to suggest which trivalent cations would be most suitable to enhance ionic conductivity without increasing electronic conductivity in solid oxide fuel cell electrolytes.

The photochemical fixation of CO2 to energy rich products for solar energy storage or feedstock chemicals is an attractive, albeit daunting, challenge. The overall feasibility of CO2 conversion is limited by the availability of efficient photo-active materials that meet the energetic requirements for CO2 reduction and are optically matched to the solar spectrum. Surface modification of TiO2 with earth abundant metal oxides presents one approach to develop visible active photocatalysts through band gap narrowing, while providing catalytic sites to lower the activation energy for CO2 reduction. In this work density functional theory was used to model the effect of surface modification of rutile and anatase using MnOx nanoclusters. The results indicate the formation of inter-band gap states following surface modification with MnOx, but surface water can change this. Oxygen vacancies are predicted to form in supported MnOx and the interaction with CO2 was investigated. MnOx-TiO2 was synthesized and characterised using surface analytical methods and photoelectrochemistry. The interaction of CO2 with the materials under irradiation was probed using in-situ FTIR to interrogate the role of oxygen vacancies in CO2 binding and reaction. These results provide insights into the requirements of a multifunctional catalyst for CO2 conversion

No abstract