Experimental and First Principles DFT Studies on the Band Gap Behaviours of Microsized and Nanosized Zn(1-x)MnxO Materials
In this study, nano- and microsized zinc oxide (ZnO) materials were doped with different manganese (Mn) contents (1–5 mol %) via a simple sol-gel method. The structural, morphological, optical and chemical environments of the materials were investigated using X-ray diffraction (XRD), field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDX), UV-Visible spectroscopy (UV-vis) and X-ray photoelectron spectroscopy (XPS). XRD results revealed that all synthesised materials were pure and single phased with a hexagonal wurtzite structure of ZnO. However, at the low annealing temperature, a nanorod-like shape can be obtained for all Zn(1-x)MnxO materials.In addition,EDX spectra confirmed the presence of Mn in ZnO lattice and the atomic percentage was nearly equal to the calculated stoichiometry. UV-Vis spectroscopy further revealed that materials in nano size exhibited band gap widening with the increase of Mn content in ZnO lattice. In contrast, micron state materials exhibited band gap narrowing with increasing Mn content up to 3% and then begin to widen when Mn˃ 3%. This is because band gaps of these materials are affected by the dimensions of the crystals and the Mn content in the materials. Furthermore, XPS results revealed the existence of multiple states of Mn in all synthesised materials. By combining the information obtained from UV-Vis and XPS valence band, the shifting in valence band maximum (VBM) and conduction band minimum (CBM) were observed. Based on XPS results, the calculation of density functionaltheory studies revealed that the presence of Mn2+, Mn3+, and Mn4+ ions in the materials influences the band gap changes.It was also revealed that the nanosized of Zn0.99Mn0.01O exhibited the highest photocatalytic activity than that of the other samples for degrading methylene blue (MB) dyes, owing to their smallest crystallite size.