Abstract. The main focus of this thesis, photocatalysis, has attracted enormous attention to tackle the global energy and environmental issues. A design and synthesis of photocatalysts with high photocatalytic activities have increased the scientific interest worldwide in order to develop a green and environmentally friendly technology. Heterogeneous photocatalysis on titanium dioxide, TiO2, is a potential solution to the issues we are facing. However, the photocatalytic efficiency of TiO2 is greatly limited by the large band gap energy and the high recombination rate of the photogenerated electrons and holes. Therefore, attempts are made to, not only doping TiO2 with various elements, but also loading TiO2 surface with appropriate cocatalysts. A coupling of TiO2 with another semiconductor with a suitable band gap and a band edge position can enhance visible light adsorption by reducing the band gap and inhibit recombination phenomena of the charge carriers.

In this thesis, hematite (\alpha-Fe2O3) clusters on anatase TiO2 (101) surface and further the effect of oxygen vacancy in the combined system were investigated. We used powerful density functional based calculations to model our systems. The simulations provided insight to phenomena occurring at the atomic level in the investigated systems and thus better understanding on photocatalytic processes on theoretical level. We found energetically favorable interaction between TiO2 and hematite clusters, thus enabling the modification of the surface properties. Due to the adsorption, impurity states arise, narrowing the band gap of TiO2. A notable charge transfer from the cluster to the surface was observed, which may play crucial role in the photocatalytic reactions. The band alignment of TiO2 and hematite showed a potential formation of a heterojunction that can promote the observed charge transfer and suppress the recombination rate of TiO2. A large Fe2O3 cluster size has been proposed to hinder the photocatalytic performance of TiO2. Our results did not completely confirm or refute this claim. They suggest that the larger cluster size can have an influence on the electronic properties and thus the photocatalytic performance.

An introduction of oxygen vacancies, both at the TiO2 and at the cluster, further altered the properties of the combined system. Oxygen vacancies can further decrease the band gap of TiO2, thus expanding the visible light response of the system, and lead to formation of a different types of heterojunction between TiO2 and hematite. The effect of oxygen vacancy located at the cluster is not observed to be as significant in order to decrease the band gap. In the presence of oxygen vacancy at TiO2 we observed the direction of the charge transfer to reverse, occurring from the surface to the cluster and thus promoting the oxidization ability of TiO2. Overall, the results indicate that hematite clusters and further oxygen vacancy can modify the surface properties of TiO2 and the photocatalytic mechanism in the combined system. They also suggest that hematite as a cocatalyst can improve the photocatalytic performance of pristine TiO2 surface.