Sputter-Coated TiO₂ Films as Passivation and Hole Transfer Layers for Improved Energy Conversion with Solar Fuel WO₃/CuWO₄ Photoanodes

Abstract

Atomic-layer-deposited (ALD) “leaky” TiO 2 has gained interest as a charge-selective protection layer for semiconductor solar fuel electrodes. Here, the use of sputter-deposited TiO 2 layers as hole-selective contacts for WO 3 /CuWO 4 type-2 heterojunction water oxidation photoanodes is demonstrated for the first time. TiO 2 protection layers with varying thicknesses (2 to 128 nm) were deposited by using the radio frequency (RF) magnetron sputtering technique. The resulting TiO 2 films are amorphous as evidenced by Raman spectroscopy and powder X-ray diffraction (XRD). Photoelectrochemical scans and vibrating Kelvin probe photovoltage spectroscopy show that 2–8 nm TiO 2 layers nearly double the photocurrent to 0.97 mA cm –2 under AM1.5 illumination (19% AQE at 350 nm), increase the surface photovoltage signal by 25%, and increase the WO 3 /CuWO 4 effective band gap. These outcomes can be attributed to the selectivity of TiO 2 for photoholes. Additionally, SPV data suggest that TiO 2 overlayers suppress copper-based surface recombination defects. Reduced photocurrents and photovoltages are measured in thicker TiO 2 films (16 to 128 nm) as a result of an increasing hole transfer resistance and because of light shading effects according to photoaction spectra. The TiO 2 films also improve the stability of the WO 3 /CuWO 4 photoelectrodes, allowing nearly constant O 2 evolution over 3 h after an initial 20–35% loss. Overall, this work establishes RF magnetron sputtering as a useful method to install amorphous TiO 2 passivation layers for improved WO 3 /CuWO 4 solar fuel photoelectrodes. Furthermore, we show how the combination of PEC and SPV measurements provides insight into the function of the TiO 2 coatings.