Improving Photoelectrochemical Activity of a Silver Sulfide-Based Photoelectrode by Bismuth Doping for Hydrogen Evolution from Industrial Waste Water

Abstract

Trivalent bismuth (Bi3+) is put forward for the first time as a multifunctional dopant to tailor the optoelectronic property of silver sulfide (Ag2S), which is known as a promising photoelectrode candidate in view of its narrow band gap of ∼1.1 eV comparable to that of its toxic PbS counterpart. The introduction of Bi3+ into Ag2S gives rise to Bi:Ag2S to harvest light with a photon energy even lower than 1.1 eV via providing additional energy levels within its band gap. The electrical conductivity of Bi:Ag2S is also improved via offering extra free electrons to increase the carrier concentration to facilitate the transport of photoexcited electron–hole pairs. As an additional result of such increment in electron density is the Fermi level of Bi:Ag2S closer to its conduction band edge, leading to its potential difference with respect to that of the sulfide (S2–) and sulfite (SO3 2–) redox couples present in large quantity in industrial wastewater markedly amplified. More importantly, the degree of surface band bending is thereby well-strengthened to promote the separation of the photogenerated electron–hole pairs, which is further reinforced by depositing Bi:Ag2S on the zinc oxide nanorods (ZnO NRs) to form a Bi:Ag2S/ZnO NR heterojunction. The synergistic effect of the aforesaid enhancements renders the hydrogen evolution rate over Bi:Ag2S/ZnO NRs largely accelerated, as evidently manifested in its photocurrent density achieving 7 mA cm–2, far exceeding those reported for additional Ag2S-based photoelectrodes in the literature, of which the great promise is in view of such outperformance well-corroborated.