Surface defect-rich ZnO nanostructures with high yellow-orange luminescence

ZnO nanostructures such as nanoislands, nanosheets, microflowers and nanoparticles were obtained successfully using a facile and rapid solution route employing low temperatures and reduced molar concentrations from precursor without using any surfactant agent or additional procedures. In this route, we use ZnCl2 as precursor ranging from 0.05 to 0.4 M and two hydrothermal times (2 and 4 h) to obtain different ZnO morphologies. According to SEM micrographs, the morphology of ZnO nanostructures showed a strong dependence on the ZnCl2 concentration and hydrothermal synthesis time. ZnO microflowers were obtained at 0.05 M during 2 h, while ZnO nanoislands, nanoparticles and nanosheets were observed at 0.05, 0.2 and 0.4 M, respectively, in 4 h. X-Ray diffraction and Raman results showed the wurtzite-type ZnO structure and good crystallinity in all cases. In addition, both techniques confirmed the best crystalline quality in ZnO nanoparticles followed by ZnO nanosheets. Finally, according to photoluminescence measurements, all samples exhibit a high yellow-orange emission, associated to a large number of surface defect states, such as oxygen and zinc interstitials. The largest amount of zinc interstitials and zinc vacancies were observed in ZnO nanoislands, while ZnO microflowers have the highest visible emission (followed by ZnO nanoparticles) which in turn, offered the lowest excitonic recombination from the conduction band to the valence band, which may result in a higher photocatalytic efficiency.

Automated summary

ZnO nanostructures with various morphologies were successfully synthesized using a simple solution route. The morphology of the nanostructures was found to depend strongly on the concentration of ZnCl2 and the hydrothermal synthesis time. The synthesized samples exhibited good crystallinity and high yellow-orange emission, indicating the presence of surface defect states.