Co-solvent medium volume ratio effect on the properties of refluxed sol-gel synthesized ZnO nanopowder

Nanocrystalline ZnO were synthesized using refluxed sol-gel method by varying the co-solvent volume ratio of methanol to water (MeOH:Water) from 0:1 to 1:0 with step of 0.2, respectively. It is known that solvent type and ratio significantly affect the morphology, structural and luminescence properties of ZnO. All the produced ZnO samples have nanoneedle, rod, sheet, spherical and nanoplate like mixed morphology and pure highly crystalline hexagonal wurtzite structure. The crystallite size varies from 22.2 to 26.7 nm with an increase in the amount of methanol in a co-solvent from 0:1 to 1:0 and the average lattice parameters are calculated as a = b = 3.246 A and c = 5.202 A. The room temperature PL measurement results showed that both near band gap emission (NBE) at around 380 nm and deep level emission (DLE) in the yellow-green region centered at 530 nm are displayed with an excitation NeCu laser source of wavelength 248.6 nm. The ratio of the intensity of NBE to DLE decreases as the amount of the methanol increases in the solvent. The NBE emission band of the prepared ZnO samples were blue shifted and the emission energy center increased from 3.237 to 3.263 eV as methanol amount increased in the co-solvent. The free and donor-bound excitonic emissions were also studied using low temperature PL measurements and quantum the quenching effects are observed from the results. Primary and secondary longitudinal (LO) phonon replica of the bound excitons also observed during low temperature PL measurements. The optical band gaps of the produced samples determined from UV–Vis reflectance measurements decreased as methanol volume increased in the co-solvent.