CHAPTER 2: Understanding the Chemistry of Photocatalytic Processes

Much effort has been devoted to the understanding of the fundamental and the engineering aspects of semiconductor photocatalysis with the goal of improving its efficiency. Remarkably, the quantum yields of the photocatalytic reactions are still very low. On the one hand, all charge carriers generated upon light absorption have to escape recombination and reach the semiconductor surface. Here they are either trapped or directly transferred to preadsorbed substrate molecules. Finally, a diffusion controlled charge transfer process to suitable reductants or oxidants needs to compete with the surface recombination processes of the trapped charge carriers. Since, on the other hand, many one-electron transfer steps are required to fully oxidize organic substrates into CO₂, various chemical intermediates will be formed that subsequently need to be oxidized before leaving the photocatalyst surface. Otherwise, undesired products can be generated in such complex photocatalytic reactions, making control of the overall processes very difficult.