In the current study describes the synthesis, characterization, and photocatalytic activity (PCA) of ZnO nanoparticles (NPs) synthesized via the polymeric precursor technique in the first step, following annealing ranging from 500 to 900 °C in a second step. X-ray diffraction data analysis confirms the formation of the wurtzite crystal structure in the as-prepared and thermally annealed samples. To adjust the crystallite mean size and physical properties, thermal annealing procedures in atmospheric air are used. The UV–Vis spectroscopy studies show that the optical band gap of the as-fabricated ZnO NPs decreases monotonically, from ~ 3.4 eV to ~ 3.1 eV, as the annealing temperature increases. The PCA of the ZnO NPs against methylene blue dye tests revealed an increase in efficiency as the annealing temperature increases from 500 °C, reaching a maximum performance for the sample annealed at 700 °C. Above that annealing temperature, the efficiency tends to slightly decrease. Our findings suggest that the PCA tests of the ZnO NPs do not show a straightforward dependence on the NPs’ size, but it seems to be correlated with the density of defects, such as oxygen vacancies, which are modulated by the annealing temperature. In addition, thermoluminescence (TL) glow curves exhibit a well-defined peak located at ~ 622 K for the as-prepared ZnO NPs. The TL peak intensity shows a decreasing trend with the annealing temperature up to ~ 700 °C. This result was correlated with the modulation of charge traps as the annealing temperature is increased. It means, the TL peak intensity is directly related to the amount of trapped charges (electrons and holes); therefore, a more intense TL peak suggests less photogenerated charge available for the catalytic activity, leading to a marked reduction of the PCA. Graphical abstract: [Figure not available: see fulltext.].
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