An integrated cobalt disulfide (CoS2) co-catalyst passivation layer on silicon microwires for photoelectrochemical hydrogen evolution

Abstract

An integrated cobalt disulfide (CoS₂) co-catalyst passivation layer on Si microwires (MWs) was used as a photocathode for solar hydrogen evolution. Si MWs were prepared by photolithography and dry etching techniques. The CoS₂–Si photocathodes were subsequently prepared by chemical deposition and thermal sulfidation of the Co(OH)₂ outer shell. The optimized onset potential and photocurrent of the CoS₂–Si electrode were 0.248 V and −3.22 mA cm⁻² (at 0 V), respectively. The best photocatalytic activity of the CoS₂–Si electrode resulted from lower charge transfer resistances among the photoabsorber, co-catalyst, and redox couples in the electrolyte. X-ray absorption near edge structure was conducted to investigate the unoccupied electronic states of the CoS₂ layer. We propose that more vacancies in the S-3p unoccupied states of the CoS₂–Si electrode were present with a lower negative charge of S₂²⁻ to form weaker S–H bond strength, promoting water splitting efficiency. Moreover, the CoS₂ co-catalyst that completely covered underlying Si MWs served as a passivation layer to prevent oxidation and reduce degradation during photoelectrochemical measurements. Therefore, the optimal CoS₂–Si electrode maintained the photocurrent at about −3 mA cm⁻² (at 0 V) for 9 h, and its hydrogen generation rate was approximately 0.833 μmol min⁻¹.