Silicon microwire arrays decorated with amorphous heterometal-doped molybdenum sulfide for water photoelectrolysis

Silicon is a promising photocathode material for solar hydrogen evolution because of its small band gap, negative conduction band position, and ideal theoretical current density. In this study, p-type Si microwire (p-Si MW) arrays were prepared as photocathodes because of the large surface area and high light-harvesting capability. However, Si MWs suffered from low photocatalytic activity because of slow photo-induced carriers during driving of water-splitting reaction. Therefore, molybdenum sulfide (MoS₂) with appropriate band alignment with p-Si material was employed for surface modification to function as a co-catalyst for collecting photo-generated minority carriers and reducing recombination possibility. The onset potential and current density at 0 V versus reversible hydrogen electrode (RHE) of Si@MoS₂ MWs were +0.122 V and −8.41 mA cm⁻². Heterometal atoms were employed to dope MoS₂ co-catalyst and expose more sulfur-terminated active sites to further boost photoelectrochemical performance. Optimal activity of Si@MMoS_x (M = Fe, Co, Ni) was achieved by doping Co heteroatoms, and its turn-on voltage and photocurrent density at 0 V (vs. RHE) were respectively increased to +0.192 V and −17.2 mA cm⁻². X-ray absorption spectroscopy was applied to demonstrate that Fe ions of FeMoS_x were dichalcogenide materials, forming a composite with MoS₂ and contributing better photoelectrolytic efficiency. By contrast, two-valent heteroatoms of CoMoS_x and NiMoS_x substituted the Mo⁴⁺ ions in MoS₂. For charge compensation, more defects and edges were revealed as active sites of solar hydrogen production by adding Co or Ni dopants in MoS₂ co-catalyst, which led to lower overpotential.