The widespread use of fossil fuels has exacerbated global warming and brought about extreme climate change effects that have prompted researchers to explore ecologically friendly and renewable energy sources. Hydrogen is a potential substitute for fossil fuels because of its abundance on the Earth, zero carbon emission, ad high energy conversion. Photoelectrochemical (PEC) water splitting is a sustainable method of converting solar energy into hydrogen. A photocathode (a p-type semiconductor) and a photoanode (an n-type semiconductor) make up a tandem PEC cell. Transition metal-based semiconductors such as TiO2, SrTiO3, Ta2O5 offer excellent photoanode materials due to their low cost and simple synthesis processes. But obtaining p-type metal oxides with high hole mobility is difficult due to the considerable localization of holes around the metal oxides' valence band edge because of their ionicity. As a potential cure, covalency might be introduced to the metal-oxygen bonding to promote the formation of an enlarged valence-band structure. However, the synthesised p-type (Cu2O, CaFe2O4) semiconductors either exhibit photo corrosion or have extremely limited charge mobility. If n-type semiconductors could serve as both photocathodes and photoanodes, or if an n-type photocathode combined with a p-type semiconductor is used in a PEC cell, then the use of Pt in photoelectrochemical cells could be completely eliminated and the cost of the PEC cell could be significantly decreased. We propose synthesis of n-type semiconductors (Mn/Fe-TNT@Ti-500, N-doped SrTiO3) and their application as photocathodes for water reduction.
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