Silicon (Si) is a promising semiconductor material in photoelectrochemical (PEC) H2 evolution due to its advantages of Earth-abundant element, non-toxicity, broad absorption of the solar spectrum, high saturated-current and industrial...

Since the water oxidation half‐reaction requires the transfer of multi‐electrons and the formation of O−O bond, it's crucial to investigate the catalytic behaviours of semiconductor photoanodes. In this work, a bio‐inspired copper‐bipyridine catalyst of Cu(dcbpy) is decorated on the nanoporous Si photoanode (black Si, b‐Si). Under AM1.5G illumination, the b‐Si/Cu(dcbpy) photoanode exhibits a high photocurrent density of 6.31 mA cm−2 at 1.5 VRHE at pH 11.0, which is dramatically improved from the b‐Si photoanode (1.03 mA cm−2) and f‐Si photoanode (0.0087 mA cm−2). Mechanism studies demonstrate that b‐Si/Cu(dcbpy) has improved light‐harvesting, interfacial charge‐transfer, and surface area for water splitting. More interestingly, b‐Si/Cu(dcbpy) exhibits a pH‐dependent water oxidation behaviour with a minimum Tafel slope of 241 mV/dec and the lowest overpotential of 0.19 V at pH 11.0, which is due to the monomer/dimer equilibrium of copper catalyst. At pH ∼11, the formation of dimeric hydroxyl‐complex could form O−O bond through a redox isomerization (RI) mechanism, which decreases the required potential for water oxidation. This in‐depth understanding of pH‐dependent water oxidation catalyst brings insights into the design of dimer water oxidation catalysts and efficient photoanodes for solar energy conversion.

Silicon (Si) is a commercially available semiconductor for photoelectrochemical hydrogen production with advantages of its abundance, low environmental hazard, wide range of solar energy absorption spectrum, high current density and...