Employment of Low-Temperature Concentrated Solar Heat to Synthesize Zinc Oxide Nanostructures

The use of solar heat as energy input for the synthesis of some types of nanomaterials with great demand provides an alternative to the utilization of energy from the grid or other sources, which often come from fossil fuels. Thus, it can be seen as a valid approach to green chemistry. In this work, the synthesis of zinc oxide (ZnO) through a wet chemical method, using low-temperature solar heat is investigated. The concept of compound parabolic concentrator was employed to construct a solar collector which also served as a synthesis reactor. Solar radiation was concentrated on the outer surface of a tubular receiver, while the synthesis reaction occurred inside. The natural fluctuations of solar radiation caused the synthesis temperature to vary between 50 and 70 degrees C. Three solar syntheses were done at these conditions. For comparison, three other syntheses were done at 50, 60, and 70 degrees C of constant temperature, in an electric hot plate, under controlled laboratory conditions and using electricity from the grid. Crystalline clusters of nanoflakes with sizes between 40.4 and 55.7 nm of pure ZnO were obtained using solar heat, remarkably similar in shape and size to those made in the laboratory at 50 degrees C. However, a slightly lower bandgap energy of 3.27 eV was attained. The band gap energies of the ZnO grown at 50, 60, and 70 degrees C were 3.28, 3.31, and 3.29 eV, respectively. In the ultraviolet range, nearly 90% of absorbance was observed for all the ZnO manufactured, regardless of the synthesis temperature. Therefore, these results may help demonstrate that it is feasible to produce high-quality ZnO with low-temperature solar heat, establishing a new green approach for the synthesis of nanomaterials.