Effect of sol pH on microstructures, optical and magnetic properties of (Co,Fe)-codoped ZnO films synthesized by sol-gel method

Cobalt-iron-codoped ZnO films were synthesized with different precursor sol pHs by a sol-gel method. All the samples possess hexagonal wurtzite structure, with no evidence of secondary phases. The average crystallite size of the samples was in the range of 10-40 nm, accompanying significant differences with the increase of the H+ ion level. The Co/Fe ions are indeed substituted at the Zn2+ site into the ZnO lattice, as shown by XRD and XPS. Strong ultraviolet emission, blue double emission and weak green emission peaks are observed in the photoluminescence spectra of all samples at room temperature. Further studies reveal that the bandgap energy of the ZnO films increases from 3.15 eV (pH = 5) to 3.24 eV (pH = 9). Enhanced room-temperature ferromagnetism is observed for the (Co, Fe)-codoped ZnO films at sol pHs lower than 8. Our work clearly establishes that the origin of room-temperature ferromagnetism in the (Co, Fe)-codoped ZnO films is linked to O vacancies, tunable by the sol pH. (C) 2015 Elsevier B.V. All rights reserved.