Wide Range pH-Tolerable Silicon@Pyrite Cobalt Dichalcogenide Microwire Array Photoelectrodes for Solar Hydrogen Evolution

This study employed silicon@cobalt dichalcogenide microwires (MWs) as wide range pH-tolerable photocathode material for solar water splitting. Silicon microwire arrays were fabricated through lithography and dry etching technologies. Si@Co(OH)₂ MWs were utilized as precursors to synthesize Si@CoX₂ (X = S or Se) photocathodes. Si@CoS₂ and Si@CoSe₂ MWs were subsequently prepared by thermal sulfidation and hydrothermal selenization reaction of Si@Co(OH)₂, respectively. The CoX₂ outer shell served as cocatalyst to accelerate the kinetics of photogenerated electrons from the underlying Si MWs and reduce the recombination. Moreover, the CoX₂ layer completely deposited on the Si surface functioned as a passivation layer by decreasing the oxide formation on Si MWs during solar hydrogen evolution. Si@CoS₂ photocathode showed a photocurrent density of -3.22 mA cm^-2 at 0 V (vs RHE) in 0.5 M sulfuric acid electrolyte, and Si@CoSe₂ MWs revealed moderate photocurrent density of -2.55 mA cm^-2. However, Si@CoSe₂ presented high charge transfer efficiency in neutral and alkaline electrolytes. Continuous chronoamperometry in acid, neutral, and alkaline solutions was conducted at 0 V (vs RHE) to evaluate the photoelectrochemical durability of Si@CoX₂ MWs. Si@CoS₂ electrode showed no photoresponse after the chronoamperometry test because it was etched through the electrolyte. By contrast, the photocurrent density of Si@CoSe₂ MWs gradually increased to -5 mA cm^-2 after chronoamperometry characterization owing to the amorphous structure generation.