Morphology-Controlled ZnO Nanomaterials for Enhanced Photoelectrochemical Performance

We report and compare the photoelectrochemical properties of ZnO nanowalls and nanowires electrochemically deposited on an indium-tin-oxide coated glass substrate in an aqueous zinc nitrate electrolyte at 70 degrees C with and without post-annealing at 400 degrees C. Post-annealing at 400 degrees C produced a drastic change in the morphology of ZnO nanowalls but not in that of ZnO nanowires. X-ray diffraction and X-ray photoelectron spectroscopy studies provide detailed characterization of the respective changes in crystallinity and composition profile of the as-deposited nanostructures before and after post-annealing. Furthermore, the optical band gap is found to be smaller for the post-annealed nanowalls and nanowires. The Mott-Schottky measurements show a negative shift in the flat-band potential for the post-annealed samples. Upon 100 mW/cm(2) (AM 1.5) light illumination, the photocurrent density for the as-deposited (post-annealed) nanowalls and nanowires were measured to be 0.54 mA/cm(2) (1.56 mA/cm(2)) and 0.12 mA/cm(2) (1.03 mA/cm(2)), respectively, at an applied potential of 0.5 V versus Ag/AgCl. The significant improvement in the photoelectrochemical properties from the post-annealed ZnO nanostructures is believed to be due to increases in crystallinity and oxygen vacancy defects, and to reduction in the band gap with the corresponding increase in light absorption at higher wavelength. This study suggests that further improvement lies in synthesizing mesoporous and/or hydrid ZnO nanostructures with semiconductor materials with a smaller band gap.