Sputter-Coated TiO Films as Passivation and Hole Transfer Layers for Improved Energy Conversion with Solar Fuel WO/CuWO Photoanodes.

Atomic-layer-deposited (ALD) "leaky" TiO has gained interest as a charge-selective protection layer for semiconductor solar fuel electrodes. Here, the use of sputter-deposited TiO layers as hole-selective contacts for WO/CuWO type-2 heterojunction water oxidation photoanodes is demonstrated for the first time. TiO protection layers with varying thicknesses (2 to 128 nm) were deposited by using the radio frequency (RF) magnetron sputtering technique. The resulting TiO 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 layers nearly double the photocurrent to 0.97 mA cm under AM1.5 illumination (19% AQE at 350 nm), increase the surface photovoltage signal by 25%, and increase the WO/CuWO effective band gap. These outcomes can be attributed to the selectivity of TiO for photoholes. Additionally, SPV data suggest that TiO overlayers suppress copper-based surface recombination defects. Reduced photocurrents and photovoltages are measured in thicker TiO 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 films also improve the stability of the WO/CuWO photoelectrodes, allowing nearly constant O 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 passivation layers for improved WO/CuWO solar fuel photoelectrodes. Furthermore, we show how the combination of PEC and SPV measurements provides insight into the function of the TiO coatings.