Carbon Ion Engineering of Silicon Cathodes for Enhanced Photoelectrochemical Hydrogen Evolution

Silicon is an attractive photoelectrode material for solar-driven hydrogen production due to its abundance and mature fabrication technologies, yet its narrow band gap, high carrier recombination, and instability in aqueous media limit its photoelectrochemical (PEC) performance. Here, we report a single-step scalable surface modification strategy using low-energy carbon ion implantation to engineer a nanostructured Si/SiC/C heterointerface on p-type silicon. The implantation process forms a composite surface layer comprising silicon, silicon carbide, and graphitic carbon, as confirmed by X-ray photoelectron and optical spectroscopies. This heterostructure exhibits enhanced light absorption, reduced recombination, and improved charge transport. Without any p-n junction formation, metal cocatalyst deposition, or multistep processing, the one-step engineered p-Si electrode exhibits an impressive cathodic current density of 10.7 mA/cm² at -1.15 V vs RHE under acidic conditions (0.5 M H₂SO₄), which is substantially higher than that of pristine p-Si. Electrochemical impedance spectroscopy reveals reduced charge transfer resistance, while the formation of built-in electric fields at the c-Si/α-Si and α-Si/SiC junctions promotes efficient charge separation. The carbon-implanted SiC phase also provides a tunable band gap with superior optoelectronic properties compared to pure SiC. This work highlights carbon ion implantation as a powerful and practical approach for advancing silicon-based photoelectrodes, offering opportunities for durable, high-efficiency PEC systems in solar fuel generation.