Boosting light harvesting and charge separation of WO₃ via coupling with Cu₂O/CuO towards highly efficient tandem...

Photoanodes based on semiconductor WO₃ have been attractive due to its good electron mobility, long hole-diffusion length, and suitable valence band potential for water oxidation. However, the semiconductor displays disadvantages including a relatively wide bandgap, poor charge separation and transfer, and quick electron-hole recombination at the interface with the electrolyte. Here we present a significantly improved photoanode with a tandem structure of ITO/WO₃/Cu₂O/CuO, which is prepared first by hydrothermally growing a layer of WO₃ on the ITO surface, then by electrodepositing an additional layer of Cu₂O, and finally by heat-treating in the air to form an exterior layer of CuO. Photocurrent measurements reveal that the prepared photoanode produces a maximum current density of 4.7 mA cm^-2, which is, in comparison, about 1.4 and 5.5 times the measured values for ITO/WO₃/Cu₂O and ITO/WO₃ ones, respectively. These enhancements are attributed to (1) harvested UV, visible, and NIR light of the solar spectrum, (2) accelerated charge separation at the heterojunction between WO₃ and Cu₂O/CuO, (3) better electrocatalytic activity of formed Cu O than pure Cu₂O, (4) formation of a protective layer of CuO. This study thus may lead to a promising way to make high-performance and low-cost photoanodes for solar energy harvesting.