Unveiling complex magnetic behaviour and effective photo-Fenton catalytic activity in the mixed phases of spinel and wurtzite structures of Fe-incorporated ZnO nanocrystals

Abstract

A comprehensive exploration was conducted on the structural features, optical bandgap, magnetic characteristics and photo-Fenton catalytic activity of chemically synthesized Fe-substituted ZnO nanocrystals. Previous studies have demonstrated that undoped ZnO adopts a hexagonal wurtzite structure. X-ray powder diffraction analysis disclosed the persistence of the wurtzite hexagonal structure in 1 % Fe-incorporated ZnO, while the presence of a secondary cubic phase of spinel type structures (ZnFe₂O₄) emerged in the ZnO lattice with Fe content ≥ 3 %. Remarkably, the proportion of the secondary phase is systematically increased from 2.75 % to 17.90 % as the Fe content was raised from 3 % to 10 %. Microscopy analysis unveiled hexagonal, spherical, and rod-like structures across all nanocrystals. Selected area electron diffraction patterns further confirmed the coexistence of cubic and hexagonal phases. Raman spectroscopy indicated a decline in crystalline quality and the introduction of defects and disorder in the host lattice due to Fe integration. The absorption spectra were taken to assess the impact of Fe substitution on the optical properties and revealed a decreasing trend in the optical bandgap from 3.23 to 3.21 eV and 2.21–2.05 eV with rising Fe content. The first band gap is related to the ZnO and the latter is the optical band gap of ZnFe₂O₄. The photoluminescence plots displayed near band edge emissions as well as visible emissions, which intensified with increased Fe-doped nanocrystal concentrations. X-ray photoelectron spectroscopy analysis affirmed the integration of Fe²⁺ and Fe³⁺ cations into the ZnO matrix. Thorough magnetic investigation uncovered complex magnetic behaviour attributed to the emergence of a secondary spin glass-like phase within the weak ferromagnetic ZnO host. The co-existence of two phases containing Fe²⁺ and Fe³⁺ ions enhanced the photo-Fenton catalytic activity, leading to the complete decomposition of various organic pollutants such as methylene blue and crystal violet. The photocatalytic test also showed the ZnO co-hosting ZnFe₂O₄ had a decolouration efficiency of 80.62 % and 77.88 % for methyl orange and thymol blue dyes. The coexistence of two phases in Fe-incorporated ZnO nanocrystals reveals complex magnetic behaviour and enhanced photo-Fenton catalytic activity. This finding holds potential for designing innovative materials applicable in futuristic spintronics devices and waste water treatment methodologies.