Photocatalysis of Titanium Dioxide for Water Disinfection: Challenges and Future Perspectives

The performance of metal oxides such as titanium dioxide (TiO2), in the conversion of solar energy into chemical energy, is determined by semiconducting properties. The conversion process is closely related to the light-induced reactivity between oxide semiconductors and water, which may lead to partial water oxidation and consequently water disinfection. Key performance-related properties are considered here, including light absorption, light-induced ionisation over the band gap, charge separation, charge transport, charge transfer, and the chemical reactions taking place at anodic and cathodic sites. Optimisation of these interconnected performance-related properties is discussed, along with the photocatalytic application in water disinfection. 1. Introduction Over the last decade research activities which aimed at the development of photocatalysts for water purification by photosensitive oxide materials, such as , have intensified [1–5]. The underlying concept of water purification involves the utilisation of solar energy to oxidise water molecules for the production of reactive oxygen species and other oxidising radicals, which are toxic to microorganisms in water. has been the focus of intensive research activities due to its high photocatalytic activity under the photon energy of ultraviolet and possibly visible light, chemical and thermal stability, resistance to chemical breakdown, and strong mechanical properties. Its application in water disinfection is enhanced by the ability of to completely destroy organic pollutants and microorganisms [6–11]. It appears, however, that most of the reported experimental data on photocatalytic water purification are not comparable, even for the same chemical systems, due to lack of reproducibility. Therefore, there is an urgent need to assess the reasons for this incompatibility. Generation of meaningful antimicrobial data may lead to derivation of theoretical models. Such models could then be used to compare photocatalytic systems and predict the effect of basic properties, such as chemical composition, structure, and semiconducting properties, on performance. As is known to the present, oxide materials are well defined in terms of reproducibility when they are in thermodynamic equilibrium with the gas phase of controlled oxygen activity. In the case of , its properties are controlled by the conditions of the equilibrium [12]. At lower temperatures (below equilibrium) they are profoundly influenced by cooling procedures, such as cooling rate and the associated gas phase composition. In many instances,