Effect of growth temperature on the optical properties of ZnO nanostructures grown by simple hydrothermal method

Here we report an easy and rapid synthesis technique of wurtzite ZnO nanostructures in the form of flowers, nano-rods and nano-tubes that are achieved by a facile hydrothermal method. A growth mechanism is proposed based on a series of temperature dependent experiments keeping other parameters during the synthesis in the aqueous medium at optimized levels. Pure ZnO results in nano-rods while Sr doped ZnO material forms flower and tube like structures. The XRD and TEM investigations show that ZnO nanostructures possess good crystalline structures with a growth direction along the c-axis of the crystal plane. Raman spectra confirm five phonon vibration modes for ZnO nanostructures at 99, 333, 382, 438 and 582 cm(-1) and one more defect induced low intensity peak at 663 cm(-1) for Sr doped ZnO. Ultraviolet-visible (UV-vis) spectroscopy shows the band gap energy of ZnO nanostructures decreases from 3.24 to 3.22 eV with the substitution of Sr into the ZnO lattice. Photoluminescence spectra reveal the existence of several defect states in all of the samples. Defect intensity seems negligibly affected by the variation of growth temperature, whereas, Sr doping plays a major role in controlling oxygen and Zn related defects. I-V characteristics of the ZnO and Sr doped ZnO show rectification behaviour of the Schottky diodes.