Publication date: 15th December 2014
Despite promising developments in photoelectrochemical water splitting, the efficiencies of most metal oxide semiconductors are still far away from the theoretical efficiencies. The mismatch between carrier diffusion length and optical absorption depth remains a key issue. Nanostructuring does not always solve this, since recent work by several groups has shown that surface recombination is an important loss mechanism for several important oxides. We find that this is also the case for BiVO4. We performed an intensity-modulated photocurrent spectroscopy (IMPS) study on BiVO4 modified with cobalt phosphate. The results suggest that the CoPi strongly reduces the surface recombination rate, but that it also suppresses the kinetics of hole injection into the electrolyte. The implications of this surprising observation will be discussed. One possible approach to avoid the need for nanostructuring is to concentrate the optical absorption in a smaller region of the material. Towards this end, we have explored the surface modification of BiVO4 photoanodes with Ag@SiO2 core-shell nanoparticles [1]. Localized surface plasmon resonances in the Ag core are found to enhance the optical absorption in the BiVO4, mainly due to far-field effects (scattering). Intriguingly, we find that the improvement in photocurrent (x2.5) is much larger than the increase in optical absorption (x1.3). We tentatively attribute this to a plasmon-induced enhancement of the water oxidation kinetics. The next steps towards a stand-alone water splitting device are to combine the metal oxide with a small-bandgap absorber in a tandem device [2,3]. Efficiencies of over 5% have been recently achieved [4], and we will briefly discuss to what extent further improvements are possible. Moreover, we will show some results on BiVO4 films deposited with magnetron sputtering, which is one of the few practical techniques that allows scale-up to large areas. We will conclude by showing some initial results for Fe2WO6, a ternary oxide with an indirect bandgap of 1.6 eV, and critically discuss whether or not this material merits further study.
[1] Abdi et al., Phys. Chem. Chem. Phys. 16 (2014) 15272; [2] Abdi et al., Nat. Commun. 4:2195 (2013); [3] Bornoz et al., J. Phys. Chem. C 118 (2014) 16959; [4] Han et al., ChemSusChem 7 (2014) 2832
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