The present study describes the synthesis, characterization, and photocatalytic activity (PCA) evaluation of ZnO nanoparticles (NPs) prepared via the polymeric precursor method in a first step, following annealing in the range of 500 to 900 oC in a second step. The formation of the wurtzite crystal structure is confirmed via x-ray diffraction data analysis. Thermal annealing treatments in atmospheric air are carried out to control the crystallite mean size and physical properties. The UV-Vis spectroscopy measurements show a decreasing trend of the optical band gap of the as-fabricated ZnO NPs, from ~3.4 eV to ~3.1 eV, as the annealing temperature increases. The PCA of the ZnO NPs against methylene blue dye revealed an increase in efficiency as the annealing temperature increases, reaching with a maximum performance for the sample annealed at ~700 °C. Our findings suggest that the PCA of the ZnO NPs does 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 glow curves exhibit a well-defined peak located at ~622 K for the as-prepared ZnO nanopowder. The peak intensity shows a decreasing trend with the annealing temperature up to ~700 °C. This result is correlated with the modulation of traps as the annealing temperature increases, which seems to be directly related to the amount of trapped charges (electrons and holes), leading to a marked reduction of the PCA.