Evidence of progressive Fe²⁺ to Fe³⁺oxidation in Fe²⁺-doped ZnO nanoparticles

Oxide-diluted magnetic semiconductors have received considerable attention in diverse scientific and technological fields because they combine the optoelectronic properties of the hosting semiconductor with the magnetic properties of the metal dopant. In this report, the role of Fe doping on the structural, vibrational, optical, hyperfine, and magnetic properties of Fe-doped ZnO nanoparticles (Zn_1−xFe_xO) synthesized via a polymeric precursor method is presented. Our findings display that the crystallite size decreases from ∼23 nm (x = 0.000) to ∼8 nm (x = 0.200) as the Fe-content (x) is increased. From the XRD data analysis, our results suggest an isovalent solid solution between Fe²⁺ and Zn²⁺ ions for lower Fe-content (up to 0.075) and aliovalent solution (Fe³⁺ and Zn²⁺ ions) for higher Fe-content. Elliot's theory was used to assess the band gap energy of E_g ∼ 3.4 eV, and an exciton binding energy of E_b ∼ 66 meV for the undoped sample. The excitonic peak exhibits a broadening trend with increasing Fe-content, suggesting disorder enhancement in the ZnO matrix. Besides, FTIR data analysis suggests that the Zn-O bond length increases with Fe-content up to 0.075 and decreases above this value. The intensity ratio of the O-H and Zn-O modes shows a discontinuity as the Fe-content is increased. Room temperature Mössbauer spectra carried out for samples with x = 0.050, 0.075, and 0.200 show that the isomer shift and quadrupole splitting increase with the Fe-content, in agreement with the structural properties. Magnetic measurements suggest that the iron ions stabilize as Fe²⁺ in samples with low Fe-content and then as Fe³⁺ in samples with high Fe-content. Besides, the occurrence of short-range antiferromagnetic interactions was determined, which becomes stronger as the Fe-content is increased.