pH-induced modification on the structural, optical and morphological properties of Zn_0.94Ni_0.04Mn_0.02O nanopowders

Zn_0.94Mn_0.02Ni_0.04O nanopowders were synthesized with different pH values from 4 to 10 by sol–gel method. The X-ray diffraction pattern confirmed that all the samples had hexagonal wurtzite structure. In the acid environment (pH = 4), the reaction rate was very slow and provided the poor crystal quality (D = 16.2 nm). Average crystal size was maximum (24.2 nm) with elliptical shape at pH = 8 which is attributed to the availability of much (OH)⁻ ions in the base solution. The observed constant c/a ratio revealed that there was no change in hexagonal wurtzite structure by pH. The energy dispersive X-ray spectra confirmed the presence of Ni and Mn in Zn–O lattice. The optical absorption spectra showed that the absorption was increased up to pH = 8 due to de-generated defect states and vacancies. The observed high intensity defect related green band around 488 nm at pH = 8 was due to the improved crystal size in addition to the presence of defects related phases. A weak absorption band around 391 nm observed only at pH = 10 was originated from the intrinsic defects and the interstitial position of Zn²⁺ ion. The higher transmittance (≈90 %) noticed at pH = 10 showed the way to the industrial applications especially as transparent electrode. The continuous blue shift in band gap from 3.61 (pH = 4) to 3.68 eV (pH = 10) is explained by Burstein–Moss effect. Presence of chemical bonding was confirmed by Fourier transform infrared spectra.