pH study of zinc oxide nanorods grown on indium tin oxide coated substrate

Controlled growth of ZnO nanorods on various substrates is of great interest in photonic and electronic device applications. Also of interest is increasing the optical activity of zinc oxide nanorods in the visible spectrum. In this study, we report pH dependence for the morphology and photoluminescence of aligned ZnO nanorods grown on an indium tin oxide (ITO) coated glass substrate deposited by a wet chemical bath deposition method. The ZnO nanorods were grown by a chemical bath deposition technique using equimolar ratios of zinc (II) nitrate and hexamethylenetetramine in solution at 95 ^oC. The pH of the reaction solution prior to oven heating was varied from pH 5 to 10.6. Surface properties of the ZnO nanorods on ITO substrates were studied using scanning electron microscopy and photoluminescence spectroscopy. We also compared the use of NaOH to adjust the pH with the use of NH₄OH, the latter necessary at higher pH due to the relative insolubility of zinc. It was found that the size of the nanorods can vary twofold with pH and the choice of base. Uniformity of coverage is also significantly dependent upon these variables and will be discussed as it relates to solubility and crystal growth. It was also found that the intensity of the photoluminescence in the visible range is pH dependent. For example, the intensity of luminescence at 550 nm for ZnO nanorods grown at pH 7 using NH₄OH is 532% of the corresponding emission for a sample prepared at pH 5. (author)