Novel Approaches to Energy Level Tuning of ZnO:Fe/SnO₂ Nanocomposites for Photocatalytic Applications

New approaches to the band gap engineering of ZnO:Fe/SnO₂ nanocomposites are developed in this study. The main idea is to control the band structure of the material by varying the modifier (iron) content during the preparation of the film-forming sol and the co-evolution of its components. Experimental X-ray diffraction (XRD), Scanning Electron Microscopy (SEM), and Energy-Dispersive X-ray Spectroscopy (EDX) data demonstrate that materials with controlled crystallite size and hierarchical morphology can be obtained with Fe contents ranging from 1 to 6 at.%. For the first time, a combined analysis of XPS data and Tauc plot band gap measurements demonstrates preferential doping of zinc oxide within the composite studied. It is also found that the formation of type II heterostructures at Fe concentrations of up to 4 at.% takes place. The authors suggest that the formation of a Z-scheme heterostructure occurs at iron contents of 5 and 6 at.%, and X-ray amorphous phases act as charge transfer mediators. It is established that the photocatalytic properties of the material are determined by the interrelations between the band structure of the material and the spectral characteristics of the radiation sources, while optical power is not a dominant factor. Using the ZnO/SnO₂ nanocomposite ensures 98.12% decomposition of Brilliant Green in 120 min under short-wave UV radiation, while using the photocatalytic material containing 6 at.% iron ensures 95.84% degradation of the dye in the same time under soft UV radiation. Employing a low-power lamp simulating the solar spectrum in the 380–780 nm range enables 24.91% decomposition of the organic pollutant when using ZnO:Fe(5 at.%)/SnO₂ as a photocatalyst.