Black ZnO nanoparticles synthesized by a green chemistry process

Abstract The green chemistry synthesis has acquired importance in recent years because of the avoidance of hazardous byproducts. In the present work, black ZnO nanoparticles were synthesized by a combustion green chemistry process using coffee extract as a reducer agent and stabilizer of the reaction. The preponderance size distribution of the black ZnO nanoparticles was found between 15 and 30 nm. High-resolution transmission electron microscopy shows distorted regions from the atomic column, while the estimated energy band gap measured by UV-Vis spectroscopy is 2.22 eV, which is a 30% value below the typical band gap for bulk ZnO. XPS measurements display a shift in the binding energy of the black ZnO compared against commercial ZnO. From the experimental evidence, it is proposed that the black color of the zinc oxide was a consequence of vacancies in the ZnO structure. Vacancies in the structure were modeled theoretically by considering a variation in the Coulomb interaction between Zn – O atoms by applying the Hubbard + U DFT approximation. The theoretical electronic distribution of the vacant ZnO was compared with 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 many technological applications.