The effects of annealing temperature on the thickness, morphology, band gap energy, and photocatalytic performance of ZIF-8-derived ZnO/TiO2 thin films

Abstract

Multi-oxide photocatalytic materials derived from metal organic frameworks (MOFs) are attracting widespread attention in recent years. The heterostructure originating from oxide–oxide junctions and nonmetal doping due to the combustion of organics from MOFs provide desirable properties to the photocatalytic material. In this study, the effects of annealing temperature on the material properties and photocatalytic activity of ZIF-8-derived ZnO/TiO2 thin films were investigated. XRD, XPS, and FTIR analyses confirmed the oxidation of ZIF-8 to ZnO and SEM analysis revealed the formation of a porous structure at the annealing temperature of 500 °C. UV–Vis analysis indicated that the band gap energy decreased from 3.23 to 2.04 eV with increasing annealing temperature. Photocatalytic methylene blue degradation experiments showed that the thin film annealed at 500 °C had higher activity than the films annealed at lower temperatures. The thin film annealed at 500 °C had more than two times the dye degradation efficiency of the film annealed at 400 °C (72% and 33%, respectively). This film was also active in the photocatalytic degradation of caffeic acid which is a colorless pollutant. The high photocatalytic activity of the thin film annealed at higher temperature was attributed to its narrower band gap energy, higher porosity, and more efficient electron–hole separation, which are due to the presence of heterostructure, nonmetal doping, and deficient zones formed by the oxidation of imidazole rings.