Variation of Ni and Mg dopants in spinel ZnIn₂(O,S)₄ microflower to generate Ni_In⁻ and Mg_In⁻ substitutional defects for a high-rate visible-light-induced hydrogen generation

Carbon-free based fuels are a promising solution to overcome the world energy crisis and environmental pollution. Therefore, hydrogen fuel is a critical and sustainable energy source to replace the limited carbon-based fossil fuels, which can be generated using a simple photocatalysis method. Herein, we demonstrated that Ni- and Mg-doped ZnIn₂(O,S)₄ (NMZI) photocatalysts prepared with a hydrothermal process performed an unprecedented HER rate without a noble-metal cocatalyst under visible-light illumination. The amounts of Ni and Mg dopants in NMZI were varied and the as-prepared NMZI were characterized with SEM, TEM, XRD, Raman, FTIR, XPS, EPR, DRS, PL, EIS, TPC, CV, and tested for photocatalytic HER. It was found that the Ni and Mg co-dopants induced substitutional surface defects of Zn_In⁻, Ni_In⁻, and Mg_In⁻. As a result, the formation of oxygen vacancy (V_O²⁺) compensated the induced defect charges, which are crucial in the HER mechanism. EDS, ICP, and XPS analyses suggest a non-stoichiometric Zn(Zn,Ni,Mg,In)₂(O,S)_3.5 with Zn, Ni, and Mg occupying In sites in the structure. The NMZI catalyst could achieve a maximum HER rate of 9.4 mmol/g․h under visible-light illumination. It is believed that the HER performance improvement is due to the defect formation on NMZI catalyst that subsequently enhances the optical, electrochemical, and photocatalytic properties. The photocatalytic HER mechanism has been elucidated and discussed thoroughly in this work.