Design of p-n heterojunction between CoWO4 and Zn-defective Zn0.3Cd0.7S for efficient photocatalytic H2 evolution

Abstract

To enhance the efficiency of photocatalytic H₂ evolution, numerous methods are employed by increasing the utilization of photogenerated charge carriers (PCCs), including catalyst design, defect regulation, and selection of suitable H⁺ resources. Using self-assembly method, CoWO₄/Zn_xCd_1-xS with p-n heterojunction was synthesized. Although CoWO₄ (CW) cannot produce H₂ under visible light irradiation, it can provide photogenerated electrons (e^-) to Zn_0.3Cd_0.7S (ZCS), and largely increase the photocatalytic activity of ZCS. The optimal CW/ZCS composite can reach 15.58 mmol·g^-1·h^-1, which is 45.8 and 24.3 times higher than the values of the pure CdS and ZCS, respectively. The largely enhanced photocatalytic H₂ production is attributed to the Zn vacancies (V_Zn), p-n heterojunction, and p-chlorobenzyl alcohol (Cl-PhCH₂OH) as the H⁺ source of H₂ production. V_Zn on the ZCS surface as the capture center of photogenerated holes (h⁺), can regulate the carrier distribution, which results in more photogenerated e^- and less generated h⁺. The combination of p-n heterojunction and V_Zn can enhance the separation and transfer efficiency of PCCs, and effectively inhibit the recombination of charge carriers. To further improve the utilization rate of PCCs, the photocatalytic H₂ evolution is proceeded by Cl-PhCH₂OH oxidation in N,N-dimethylformamide solution, with 4-chlorobenzaldehyde (Cl-PhCHO) generated. The separated photogenerated e^- and h⁺ both participated in the redox reaction of H⁺ reduction and Cl-PhCH₂OH oxidation, considering that the amount of H₂ and Cl-PhCHO products are close to 1:1. This work not only facilitates the separation and transfer of PCCs, but also provides directions for the design of efficient photocatalysts and H₂ evolution in the organic phase.

Keywords: Photogenerated charge carriers; Zn vacancies; p-Chlorobenzyl alcohol; p-n Heterojunction.