A deep understanding of defects is essential for the optimization of materials for solar energy conversion. This is particularly true for metal oxide photo(electro)catalysts, which typically feature high concentrations of charged point defects that are electronically active. In photovoltaic materials, except for selected dopants, defects are considered detrimental and should be eliminated to minimize charge recombination. However, photocatalysis is a more complex process in which defects can have an active role, such as in stabilizing charge separation and in mediating rate-limiting catalytic steps. In this Review, we examine the behaviour of electronic defects in metal oxides, paying special attention to the principles that underpin the formation and function of trapped charges in the form of polarons. We focus on how defects alter the electronic structure of metal oxides, statically or transiently upon illumination, and discuss the implications of such changes in light-driven catalytic reactions. Finally, we compare oxide defect chemistry with that of new photocatalysts based on carbon nitrides, polymers and metal halide perovskites.
Bibliographical noteFunding Information:
The authors thank L. Harnett for sharing his data and insights on WO. E.P. acknowledges support from grant IJC2018-037384-I, funded by MCIN/AEI /10.13039/501100011033. S.S. and J.R.D. acknowledge funding from the European Union’s Horizon 2020 Research And Innovation Programme under grant agreements 884444-SUN2CHEM and 732840-A-LEAF. M.S. thanks the UK EPSRC for a Doctoral Prize Fellowship. A.A.B. is a Royal Society University Research Fellow. This work was partially funded by CEX2019-000910-S (MCIN/AEI/10.13039/501100011033), Fundació Cellex, Fundació Mir-Puig and Generalitat de Catalunya through CERCA. 3
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