- Figure 2: Semiconductor photocatalysis.
a, Photoelectrochemical cell design for water splitting (processes for an n-type semiconductor are shown). When illuminated with photons (hν) of energy exceeding the bandgap, excited charge carriers are formed in the semiconductor photoanode. The holes diffuse to the semiconductor surface and drive the oxygen-evolution half-reaction (2H2O + 4h+ O2 + 4H+). Electrons are collected and travel to the counter electrode where they drive the hydrogen-evolution half-reaction (2H+ + 2e− H2). b, Particle-based water-splitting photocatalyst. Excited charge carriers (both electrons and holes) diffuse to the particle surface where they drive the two half-reactions, usually at specially designed co-catalyst sites. VB, valence band; CB, conduction band. c, VB and CB for a range of semiconductors (data from refs 90 and 91) on a potential scale (V) versus the normal hydrogen electrode (NHE). Redox potentials for the water-splitting half-reactions versus the NHE are also indicated by dashed red lines. For the water-splitting reaction to be thermodynamically favourable, the bandgap should straddle these redox potentials, that is, the CB should have higher energy (more negative potential) than the hydrogen-evolution potential and the VB should be lower in energy than the oxygen-evolution potential.
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