Boosting light harvesting and charge separation of WO 3 via coupling with Cu 2 O/CuO towards highly efficient tandem photoanodes.

Photoanodes based on semiconductor WO 3 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/WO 3 /Cu 2 O/CuO, which is prepared first by hydrothermally growing a layer of WO 3 on the ITO surface, then by electrodepositing an additional layer of Cu 2 O, 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/WO 3 /Cu 2 O and ITO/WO 3 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 WO 3 and Cu 2 O/CuO, (3) better electrocatalytic activity of formed Cu x O than pure Cu 2 O, (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.