Highly efficient photocatalytic hydrogen evolution and simultaneous formaldehyde degradation over Z-scheme ZnIn₂S₄-NiO/BiVO₄ hierarchical heterojunction under visible light irradiation

In this work, we successfully constructed a highly efficient Z-scheme ZnIn₂S₄-NiO/BiVO₄ photocatalytic system and reported a breakthrough study that has uncovered the photocatalytic reaction mechanism and pathway of hydrogen (H₂) evolution and simultaneous formaldehyde (HCHO) degradation over the as-prepared Z-scheme photocatalytic system. By exploring the effects of oxygen (O₂) and pH on the photocatalytic activity of the Z-scheme photocatalytic system, it was found that O₂ and alkalinity are necessary for photocatalytic HCHO degradation and H₂ production. O₂ can boost the oxidation conversion of HCHO into formic acid (HCOOH) and restrain the HCHO polymerization reaction, therefore accelerate the formation of H₂ in the reaction system. With the increase of pH value from 4 to 13, the reducibility of HCHO and the electronegativity of catalyst increase, which makes the activity of catalyst system increase. The results provide a new route for the optimization of reaction conditions for simultaneous photocatalytic H₂ production and HCHO degradation. Under the optimal conditions (80% of O₂, 13 of pH and 1.5 mol·L⁻¹ of HCHO initial concentration), the best H₂ production activity and HCHO degradation activity are 2724.89 μmol·h⁻¹ and 17.00 mmol·h⁻¹, respectively. On this basis, the possible pathway and mechanism of photocatalytic H₂ production and simultaneous HCHO degradation were proposed, which shed lights on elucidating the reaction path and underlying mechanism for photocatalytic H₂ production and pollutant degradation.