Direct synthesis of ZnO nanoparticles by a solution-free mechanochemical reaction

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Abstract
This paper presents a study on the synthesis and characterization of ZnO nanoparticles by a two-step synthesis procedure. The first step is the solution-free mechanochemical synthesis of ZnC2O4·2H2O nanoparticles by grinding a powder mixture of Zn(CH3COO)2 and H2C2O4·2H2O at room temperature for a certain time. The second step is the thermal decomposition of ZnC2O4·2H2O nanoparticles at 450 °C to form ZnO nanoparticles. The obtained ZnO nanoparticles have been structurally characterized by x-ray diffraction, indicating a wurtzite structure (hexagonal phase, space group P63mc) with high crystallinity. The average grain size of the ZnO nanoparticles is in the range of 24–40 nm, depending on the mechanical grinding time. The UV–vis diffuse reflectance spectrum of the ZnO nanoparticles indicates a band gap of about 3.3 eV with an onset of absorption at about 3.1 eV, showing a weak red shift compared to 3.37 eV for the bulk ZnO. X-ray photoelectron spectroscopy and Raman spectra showed a nonadsorption on the surface and a very low concentration of oxygen vacancies, respectively, indicating a high quality of the prepared ZnO nanoparticles. The formation process of the ZnC2O4·2H2O nanoparticles by the solution-free mechanochemical reaction is discussed.