Enhancement Effects on Visible-Light-Driven Water Oxidation by a Bifunctional Fe-Co-deposited SnO_<i>x</i> Catalyst Layer Deposited on an N-Doped CuWO₄ Photoanode

Efficient photoanodes for visible-light-driven water oxidation are eagerly desired to construct practical water splitting systems to produce O2 and H2 using solar energy, which is one of the most prospective approaches for sustainable H2 production. To further improve photoelectrochemical (PEC) water oxidation by a unique nitrogen-doped CuWO4 (N-CuWO4) photoanode, a Fe-co-deposited SnOx (Fe-SnOx) layer was formed on the photoanode surface by photoassisted electrodeposition. The incident photon-to-current conversion efficiency (IPCE) of the N-CuWO4 photoanode was improved by 1.9 times at 1.23 V vs a reverse hydrogen electrode (RHE) by the Fe-SnOx layer, which is contributed by the increased catalytic efficiency (ηcat) from 41.7 to 67.0%. Photoelectrochemical impedance spectroscopy (PEIS) measurement showed that the rate constant (kO2) of water oxidation at the surface increased from 5.5 to 10.2 s–1 and the rate constant (krec) of surface recombination of photogenerated carriers decreased a little from 8.9 to 8.6 s–1 by the Fe-SnOx layer. This indicates a bifunctional role of the Fe-SnOx catalyst layer in promoting the water oxidation reaction at the surface and suppressing the surface recombination of photogenerated carriers. For comparison with the case of a SnOx layer (no Fe-co-deposition), the kO2 value (6.7 s–1) of SnOx/N-CuWO4 electrodes was lower than that (10.2 s–1) for the Fe-SnOx/N-CuWO4 electrode but higher than that (5.5 s–1) for bare N-CuWO4. Both SnOx and Fe-SnOx layers on the N-CuWO4 surface improved kO2; however, the mechanism of improved kO2 is distinct between these layers: passivation effect of the SnOx layer to prevent electron tunneling at the interface of the N-CuWO4 surface/electrolyte and promotion effect of the Fe-SnOx layer on water oxidation at the surface. The krec value (6.9 s–1) for the SnOx/N-CuWO4 electrode was 1.3 times lower than that (8.9 s–1) of the bare N-CuWO4 electrode, which clearly shows the effect of the passivating SnOx layer on the suppression of the surface recombination of photogenerated carriers.