Doping is an efficient strategy to narrow the bandgap of wide-bandgap metal oxides, in order to harvest more light. Let's consider the mechanism by which doping reduces the bandgap of a metal oxide through the introduction of doping levels above the valence band maximum (VBM) of the metal oxide. This mechanism can give rise to two different types of doping levels:
- Localized doping levels that do not merge with the VBM, as shown in Fig. 1a
- Delocalized doping levels that merge apparently with the VBM, as shown in Fig. 1b
What are the possible advantages of the second type of doping levels as compared to the first one for photocatalytic applications? The one advantage I can think of is that the localized doping levels can act as charge recombination centers, what are other possible advantages?