Design for a longer photoinduced charge separation and improved visible-light-driven H2 generation through structure reversal and oxygen vacancies via Ni substitution into ZnFe2O4 spinel

Abstract The objective of this study was to replace some of the metal ions in the n-type zinc ferrite spinel lattice with Ni2+ ions to evaluate its catalytic behavior during water splitting. XPS and Raman spectroscopy revealed that the substituted Ni2+ ions were preferentially located at the octahedral (Oh) sites in the zinc ferrite spinel lattice, rather than the tetrahedral (Td) sites. When the amount of substituted Ni2+ was >0.6 mol, the crystal structure was converted to an inverse spinel structure. Ni2+ substitution resulted in the formation of oxygen vacancies in the spinel lattice. With Ni2+ substitution, the bandgap state changes from direct to indirect. The TRPL, IMVS, and IMPS analyses revealed the carrier diffusion length, e−/h+ recombination time, and charge transport rate for the Zn1-xNixFe2O4 inverse spinel. The electrons excited from the valence bands (VBs) to the CBs (O 2p → Fe3+ 3d) moved to Ni2+ 3d (Fe3+ 3d → Ni2+ 3d) and, subsequently, the longer carrier diffusion length suppressed e−/h+ recombination. Eventually, an optimum hydrogen production of 19.2 μmol g−1 was achieved using the Zn0.4Ni0.6Fe2.0O4 catalyst for a 10 h-hydrogen evolution reaction in neutral water splitting, without any sacrificial agent under visible light; Zn0.4Ni0.6Fe2.0O4 exhibited a four-fold improvement compared to ZnFe2O4. The material did not significantly deteriorate the catalytic performance, even after six regeneration experiments, and the catalyst material was separated using a magnet after the reaction. Thus, the goal of this study was to develop a simple, eco-friendly, and efficient catalyst via partial cation substitution in the spinel structure.