Preparation of (Ga0.2Cr0.2Mn0.2Ni0.2Zn0.2)3O4 high-entropy oxide with narrow bandgap for photocatalytic CO2 reduction with water vapor

High-entropy oxides (HEOs) have become very promising materials due to their excellent properties. In this work, inspired by the abundant surface defects and narrow band gaps of high-entropy oxides, a high-entropy oxide (Ga0.2Cr0.2Mn0.2Ni0.2Zn0.2)3O4 with the Fd-3m spinel structure was designed and synthesized by sol–gel method for photocatalytic CO2 conversion. The results showed that the calcination temperature had a very important effects on the formation of high-entropy oxide, structural defects and the element dispersion. Meanwhile, (Ga0.2Cr0.2Mn0.2Ni0.2Zn0.2)3O4 HEO with appropriate calcination temperature exhibited excellent photocatalytic activity and stability for CO2 reduction with water vapor without any co-catalyst under visible light due to its ultra-narrow bandgap (1.6 eV), abundant structural defects, uniform dispersion of metal elements and suitable band structure.