Amorphous Phosphorus-Doped Cobalt Sulfide Modified on Silicon Pyramids for Efficient Solar Water Reduction

Cobalt sulfide (CoS_x) functioned as a co-catalyst to accelerate the kinetics of photogenerated electrons on Si photocathode, leading to the enhancement of solar hydrogen evolution efficiency. By doping phosphorus heteroatoms, CoS_x materials showed an improved catalytic activity because of superior surface area and quantity of active sites. Furthermore, increased vacancies in unoccupied electronic states were observed, as more phosphorus atoms doped into CoS_x co-catalysts. Although these vacant sites improved the capability to accept photoinduced electrons from Si photoabsorber, chemisorption energy of atomic hydrogen on catalysts was the dominant factor affecting in photoelectrochemical performance. We suggested that P-doped CoS_x with appropriate doping quantities showed thermoneutral hydrogen adsorption. Excess phosphorus dopants in CoS_x contributed to excessively strong adsorption with H atoms, causing the poor consecutive desorption ability of photocatalytic reaction. The optimal P-doped CoS_x-decorated Si photocathode showed a photocurrent of -20.6 mA cm^-2 at 0 V. Moreover, a TiO₂ thin film was deposited on the Si photocathode as a passivation layer for improving the durability. The current density of 10 nm TiO₂-modified photocathode remained at approximately -13.3 mA cm^-2 after 1 h of chronoamperometry.