14 - Fundamentals and Applications of the Photocatalytic Water Splitting Reaction

Abstract

The overall water splitting reaction on a semiconductor photocatalyst occurs in three steps, which are as follows: the photocatalyst absorbs photon energy greater than the band-gap of the semiconductor material and generates photo-excited electron–hole pairs in the bulk; the photo-excited electrons and holes separate and migrate to the surface with minimum recombination; adsorbed species are reduced and oxidized by the photo-generated electrons and holes to produce H2 and O2, respectively, at different surface reaction sites. Different types of experimental setup have been used for water splitting reactions. The two major reaction cells used are the inner and outer irradiation types. The inner irradiation type consists of the photocatalytic reactant solution placed in the annular cylindrical zone of a quartz reactor with the light source placed at the centre. In the outer irradiation type, the photocatalytic reactant solution is placed in a cylindrical or cubical quartz cell at a distance from the light source. Thus, the inner irradiation type is much more efficient than the outer irradiation one. This is because in the inner irradiation type the light reaches the catalyst from all directions and more surface of the photocatalyst is exposed to it. Photocatalytic water splitting is an attractive and challenging theme in chemistry. Moreover, powdered photocatalyst systems will be advantageous for large-scale application of solar water splitting because of their simplicity.