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

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 S₂^2- 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.