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

Atomic-layer-deposited (ALD) "leaky" TiO₂ has gained interest as a charge-selective protection layer for semiconductor solar fuel electrodes. Here, the use of sputter-deposited TiO₂ layers as hole-selective contacts for WO₃/CuWO₄ type-2 heterojunction water oxidation photoanodes is demonstrated for the first time. TiO₂ protection layers with varying thicknesses (2 to 128 nm) were deposited by using the radio frequency (RF) magnetron sputtering technique. The resulting TiO₂ 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 TiO₂ 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 WO₃/CuWO₄ effective band gap. These outcomes can be attributed to the selectivity of TiO₂ for photoholes. Additionally, SPV data suggest that TiO₂ overlayers suppress copper-based surface recombination defects. Reduced photocurrents and photovoltages are measured in thicker TiO₂ 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 TiO₂ films also improve the stability of the WO₃/CuWO₄ photoelectrodes, allowing nearly constant O₂ evolution over 3 h after an initial 20-35% loss. Overall, this work establishes RF magnetron sputtering as a useful method to install amorphous TiO₂ passivation layers for improved WO₃/CuWO₄ solar fuel photoelectrodes. Furthermore, we show how the combination of PEC and SPV measurements provides insight into the function of the TiO₂ coatings.