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Water splitting by using sunlight for the production of hydrogen yields a storable product, which can be used as a fuel.[1, 2] There is considerable research into H2 generation, namely the reduction of protons to H2 in aqueous solution using semiconductor photocathodes.[3, 4] To maximize the photoelectrochemical (PEC) performance, the selection of the active materials and device configurations should be carefully considered. First, the short-circuit current density (Jsc) should be maximized by choosing materials with high optical absorption coefficients and low carrier recombination rates,[5] both in the bulk and at the surface. The reflectance should be minimized by using surface nanotexturing to further improve light absorption.[6–8] The onset potential (Eos) of the PEC device versus the reversible H+/H2 redox potential should be maximized. Finally, the surface energy needs to be controlled to minimize the accumulation of gas bubbles on the surface of the photoelectrode. Light absorbers with band gaps in the range of 1.1–1.7 eV provide both a good match to the terrestrial solar spectrum and a significant fraction of the 1.23 eV free energy required to split water.

Overpotentials associated with the electron transfer to (solvated) protons in aqueous solution should be minimized by improving carrier transport from semiconductor to electrolyte by decorating the semiconductor with cocatalysts, tuning band edges, and decreasing contact resistance. p-Type Si has been extensively investigated as a photocathode for photochemical hydrogen production. Planar Si has relatively low short-circuit current densities under AM1. 5G illumination, approximately …