Comparative study of structural and optical properties of ZnO nanostructures prepared by three different aqueous solution methods

• ZnO nanostructures were produced via three different aqueous solution methods.
• Temperature of crystallization for sol–gel samples was as low as 350 °C.
• Nanorods, cylindrical and spherical nanoparticles were achieved.
• Sol–gel samples presented intense UV emission.
• Red, green and yellow emission was observed for different calcination temperatures.

In the present work, zinc oxide (ZnO) nanoparticles with different morphologies and sizes were successfully synthesized via three different aqueous solution routes named proteic sol–gel, PVA-assisted sol–gel and microwave-assisted hydrothermal method. Sol–gel samples were crystallized into hexagonal structure after calcination at 350 °C, presenting uniform growth and predominantly spherical particles. On the other hand, the sample produced via hydrothermal method assumed nanorod morphology, probably due to the adsorption of ammonium on the surface of ZnO nuclei, which affect the growth orientation of the crystals. All the samples exhibited a sharp UV emission peak, related to excitonic recombination, and a broad emission band in the visible region, attributed to internal transitions in color centers. Sol–gel samples calcined at the lowest temperatures presented an UV emission intensity that was 44 and 89 times higher than the visible emission, which can be related to the passivation of the defects by hydrogen ions. As-prepared hydrothermal sample presented a broad emission band centered at approximately 596 nm, which is possibly related to OH groups adsorbed on the particle surfaces. Nevertheless, the emission band of samples calcined above 800 °C was shifted to 540 nm, which is probably related to oxygen vacancy according to the results from chemical analysis.