Conformal SnO_{: X} heterojunction coatings for stabilized photoelectrochemical water oxidation using arrays of silicon microcones

The efficiency of photoelectrodes towards fuel-forming reactions is strongly affected by surface-based charge recombination, charge-transfer losses, and parasitic light absorption by electrocatalysts. We report a protective tin oxide (SnO_x) layer formed by atomic-layer deposition that limits surface recombination at n-Si/SnO_x heterojunctions and produces ∼620 mV of photovoltage on planar n-Si photoanodes. The SnO_x layer can be deposited conformally on high aspect-ratio three-dimensional structures such as Si microcone arrays. Atomic-level control of the SnO_x thickness enabled highly conductive contacts to electrolytes, allowing the direct electrodeposition of NiFeOOH, CoO_x, and IrO_x electrocatalysts for photoelectrochemical water oxidation with minimal parasitic absorption losses. SnO_x-coated n-Si microcone arrays coupled to electrodeposited catalysts exhibited photocurrent densities of ∼42 mA cm^-2 and a photovoltage of ∼490 mV under 100 mW cm^-2 of simulated solar illumination. The SnO_x layer can be integrated with amorphous TiO₂ to form a protective SnO_x/TiO₂ bilayer that exhibits the beneficial properties of both materials. Photoanodes coated with SnO_x/TiO₂ exhibited a similar photovoltage to that of SnO_x-coated photoanodes, and showed >480 h of stable photocurrent for planar photoelectrodes and >140 h of stable photocurrent for n-Si microcone arrays under continuous simulated solar illumination in alkaline electrolytes.