ABSTRACT
Figure 7.1 Scheme of water splitting with the help of a solid catalyst. But this semiconductor must perform another important function for the photosystem: namely, it must be a suitable redox catalyst to assist the formation of O2 without going through one-electron intermediates. Then the reaction of water splitting runs in a way as 2H2O(liquid) Æ O2(gas) + 4H+(aqua) + 4e-(E°ox = 1.23 V) (7.2) The same applies for water reduction to H2-one needs a strong reductant and redox catalysts to reduce protons to H2. A photogenerated electron loses its energy via thermolization and eventually occurs at the energy level as the bottom of the conduction
band of the semiconductor playing the role of the reductant and the semiconductor again performs its catalytic function. The reaction then runs as 2H+(aq) + 2e-Æ H2(gas) (E°(red) = 0.00 V) (7.3) Figure 7.1 shows a schematic diagram of water splitting into H2and O2 under the action of light with the use of a semiconductor photocatalyst. Photocatalysis on semiconductor particles passes through three main steps: (i) absorption of photons with energies exceeding the semiconductor bandgap width, which results in the generation of electron (e−)–hole (h+) pairs in the semiconductor particles; (ii) charge separation followed by migration of these photogenerated carriers in the semiconductor particles; (iii) surface chemical reactions between these carriers with various compounds (e.g., H2O) [173]. Obviously, some electrons and holes recombine with each other without participating in chemical reactions. 7.2 Requirements for Semiconductor CatalystsFrom simple thermodynamic consideration it is obvious that a semiconductor photocatalyst used for water splitting must have the bottom of the conduction band more negative than the reduction potential of water to produce H2, and the top of the valence band more positive than the oxidation potential of water to produce O2, as it is shown in Fig. 7.1. This requirement makes serious restrictions in the list of semiconductors suitable for water splitting. For this reason, the position of the valence and the conduction bands in energy scale, relative to reduction potential of water to produce O2and H2 is a very important parameter for being an effective catalyst for water splitting. The positions of electron bands in energy scale for a series of semiconductors is shown in Fig. 7.2. However it is difficult to develop an oxide semiconductor photocatalyst that possesses both a sufficiently negative conduction band for H2 production and a sufficiently narrow band gap (i.e., <3.0 eV) for visible light absorption, hence the O 2p orbital forms the highly positive valence band (at ca. +3.0 V vs. NHE) [174]. From Fig. 7.2 one can easily understand that many visible light-absorbing oxide photocatalysts, such as WO3, SnO2, Fe2O3, or BiVO4 [175-181],
cannot produce H2 from water due to their conduction bands being too low for water reduction. Meanwhile, there is another important restriction for this function: the photocatalyst must be stable in aqueous solutions against photocorrosion under photoirradiation. Oxide semiconductors are generally highly stable against photocorrosion and have thus been extensively used as heterogeneous photocatalysts. Some non-oxide (e.g., sulfides [182-187] and nitrides [188-190]) semiconductors possess appropriate band levels for water splitting under visible light, but they are generally unstable and readily become deactivated through photocorrosion or self-oxidation.
