Nanocrystalline ZnO samples with different average crystallite sizes in the range 12–89 nm and varying concentrations of native point defects are synthesized by the decomposition of carbonate precursor in air ambience at different temperatures, viz., 250, 450, 650 and 850 \(^\circ\) C. Analysis of Zn 2p and O 1s XPS spectra reveal the presence of elemental Zn, oxygen vacancies and adsorbed oxygen containing species in all samples. The optical band gap of the samples is in the range of 3.17–3.20 eV. Photoluminescence spectra of samples with smaller average crystallite sizes are dominated defect induced emission in the visible region while that of sample with larger average crystallite size is dominated by the band edge emission in the UV region. PL emission spectra reveals the presence of native point defects, V \(_{\text{O}}^{+}\) , V \(_{\text{O}}^{++}\) , Zn \(_{i}\) and O \(_{i}\) , the relative contribution of which vary with the synthesis temperature. Photocurrent response increases with an increase in average crystallite size. The sample with largest average crystallite size (89 nm) exhibits better photocatalytic activity among the samples with better efficiency (40 in minutes), rate constant ( \(\text{k} = 10 \times 10^{-2}\,\text{min}^{-1}\) ) and reusability (93% degradation in 5 cycles). The degradation kinetics under superoxide and hydroxyl scavengers reveals that the degradation mechanism is dominated by the reaction of hydroxyl radicals. Larger average crystallite size leads to better photonic absorption and charge carrier separation while trapping of free electrons in the defect induced energy levels in the forbidden gap leads to longer recombination time thereby improving the catalytic activity.
