Nanoparticles of ZnO2 were synthesized through sol–gel technique using zinc acetate and hydrogen peroxide in aqueous solution exposed to UV irradiation and dried at 100 °C. Using electron microscope techniques, X-ray diffraction, Fourier transform infrared spectroscopy and thermogravimetric analysis we have studied in detail the structure and morphology of the obtained powders. With annealing treatment of the as-synthesized nanoparticles it was possible to achieve different crystalline structures from pure ZnO2 to pure ZnO nanoparticles. Three different anoparticles were selected: the as-synthesized nanoparticles obtained at 100 °C constituted by ZnO2, the sample annealed at 160 °C composed by ZnO2–ZnO mixture and the annealed sample at 220 °C constituted by pure ZnO. Pseudomonas aeruginosa was employed to evaluate the antimicrobial activity of three selected nanoparticle samples. The inhibition zone was largest for ZnO2 nanoparticles.