Amorphous Cobalt Oxide Nanowalls as Catalyst and Protection Layers on n‑Type Silicon for Efficient Photoelectrochemical Water Oxidation

Silicon for photoelectrochemical (PEC) water splitting suffers from severe photocorrosion in the electrolyte and high overpotential at the interface with the electrolyte, although it is an earth-abundant material with a narrow band gap for the absorption of a wide range of the solar spectrum. Highly active transition-metal/metal oxides are considered promising materials for protecting the silicon surface, as they exhibit efficient catalytic properties which can reduce overpotential, thus improving the water splitting efficiency. In this study, electrodeposition, a simple and inexpensive method in comparison to vacuum processes such as atomic layer deposition and sputtering, was used to produce efficient Co-based catalysts on silicon substrates to fabricate photoanodes for the oxidation of water to generate oxygen. Distinctive morphologies of the Co-based catalysts were obtained by changing the types of additives in the precursor solution. The PEC performances of n-Si photoanodes with amorphous CoO_x nanowalls (NWs) and Co nanoparticles (NPs) depended on the thicknesses and coverages of the catalysts. A fully covered CoO_x NWs/n-Si photoanode exhibited stability higher than that of a partially covered Co NPs/n-Si photoanode. The optimized CoO_x NWs/n-Si photoanode exhibited an onset potential of 1.06 ± 0.01 V vs the reversible hydrogen electrode (RHE) and a photocurrent density of 23.3 ± 0.8 mA/cm² at 1.23 V vs RHE. A charge injection efficiency of almost 100% at 1.4 V vs RHE and high external quantum efficiency of 90% at 700 nm were achieved. These results show that morphology-controlled transition-metal-based catalysts fabricated by the facile electrodeposition method can be one of the promising pathways for the design of high-performance Si-based photoanodes.