Record high photocurrent density of In:Ag2S/ZnO heterojunction enabled by controllable doping for efficient solar hydrogen production

The high mobility of silver ions (Ag ⁺) in silver sulfide (Ag ₂S) promotes vacancy formation, which serves as the recombination center of the photoexcited charge carriers to severely limit the photoelectrochemical activity of Ag ₂S. To address this issue, trivalent indium (In ³⁺) is particularly introduced into Ag ₂S via low–temperature successive ionic layer adsorption and reaction (SILAR) technique, by which are the migration and the side reactions of Ag ⁺ to form the phase impurities largely suppressed. Herein, In ³⁺ is preferentially chosen due to its two more valence electrons with respect to that of Ag ⁺, effectively increasing the carrier concentration of In:Ag ₂S to not only improve its electrical conductivity but also uplift its Fermi level. The photoexcited electron transport is thereby accelerated to more efficiently separate from the geminate holes, which is further reinforced by depositing In:Ag ₂S on the zinc oxide nanorods (ZnO NRs) with a relatively low–lying Fermi level. The photogenerated holes are rectified by the built–in electric field at the In:Ag ₂S/ZnO heterojunction to flow in an opposite direction to that of the electrons, between which the recombination is thereby suppressed. Meanwhile, the light harvesting ability of In:Ag ₂S/ZnO NRs is also enhanced due to its reduced band gap by In ³⁺. Their synergistic effect renders In:Ag ₂S/ZnO NRs developed in the present contribution to deliver a record–high photocurrent density of 7.7 mA cm ⁻² under one–sun illumination, which far exceeds those of not only the undoped counterpart but also additional Ag ₂S–based photoelectrodes reported in the literature.