Thin alumina-coated polyoxometalates on gold-titania half-shell array photoanode for sustainable plasmonic water oxidation

In solar water splitting, the poor chemical and mechanical durability of photoanode materials under oxidative environments has been raised as a crucial issue. Despite promising water oxidation activity, the stable immobilization of polyoxometalates (POMs) onto photoanodes is very challenging. Here we report sustainable photoelectrochemical water oxidation through the deposition of catalytic POMs onto a plasmonic Au/TiO2 photoanode, followed by the atomic layer deposition of a thin Al2O3 layer. Vacuum deposition techniques were used with polystyrene nanospheres as sacrificial templates to fabricate an Au/TiO2 half-shell array as a photoanode with strong plasmonic absorption and excellent stability in aqueous media. The thin Al2O3 layer serves as a protective layer for [Co4(H2O)2(PW9O34)2]10- (Co4POMs) attached to Au/TiO2 half-shells functionalized with cysteamines. The Al2O3 layer increased the photocurrent and delayed current attenuation by preventing the dissociation of Co4POMs from the surface during the water oxidation. A thicker Al2O3 layer exhibited a higher protection effect but reduced the catalytic activity of Co4POMs in the early stage of water oxidation due to the limited exposure of Co4POMs to electrolytes. Furthermore, regardless of the POMs, the Al2O3 layer also passivated the TiO2 surface, improving electron transfer through the POMs/Au half-shell structure. This work suggests that the catalyst/plasmonic photoelectrode with a thin protection layer is a promising alternative to conventional semiconductor-based photoelectrodes for sustainable and efficient water oxidation.