Black ZnO Nanoparticles Synthesized by a Green Chemistry Process

In the present work, the standardization of the methodology to obtain black ZnO nanoparticles from Arabica coffee extract as a reducer agent and stabilizer of the reaction is presented for the first time through a scalable combustion green chemistry process without obtaining dangerous byproducts. The size distribution of the nanoparticles was found between 15 and 30 nm. High-resolution transmission electron microscopy shows distorted regions from the atomic column. Whereas, the estimated energy band gap measured by UV-vis spectroscopy is 2.22 eV, which is 30% value below the typical band gap for bulk ZnO. XPS measurements show a change in the binding energy of black ZnO compared to commercial ZnO. From experimental evidence, it is proposed that the black color of zinc oxide resulted from vacancies in the ZnO structure. The vacancies in the structure were theoretically modeled considering a variation in the Coulomb interaction between Zn-O atoms by applying the Hubbard + U DFT approximation. The theoretical electronic distribution of the influence of vacancies ZnO was compared with the experimental results obtained by Raman, FTIR and the experimental profile of the valence band region. These results open the exploration of green synthesized black zinc oxide nanoparticles to possible technological applications related to catalysis.