Sputter-Coated\nTiO<sub>2</sub> Films as Passivation\nand Hole Transfer Layers for Improved Energy Conversion with Solar\nFuel WO<sub>3</sub>/CuWO<sub>4</sub> Photoanodes

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