We investigated the influence of the pH value, precursor concentration (C), growth time and temperature on the morphology of zinc oxide (ZnO) nanostructures. The pH of the starting solution was varied from 1.8 to 12.5. It was found that the final pH reaches an inherent value of 6.6 independently of the initial pH solution. Various ZnO structures of nanotetrapod-like, flower-like, and urchin-like morphology were obtained at alkaline pH (8 to 12.5) whereas for pH solution lower than 8 rod-like nanostructures occurred. Moreover, we observed the erosion of the nanorods for a pH value less than 4.6. By changing the concentrations the density and size were also varied. On going from a high (C>400 mM) to lower (C<25 mM)C, the resulted ZnO nanostructures change from a film to nanorods (NRs) and finally nanowires (NWs). It was also found that the length and diameter of ZnO NRs follow a linear relation with time up to 10 hours, above which no further increase was observed. Finally the effect of growth temperature was seen as an influence on the aspect ratio.

ZnO nanotubes and nanorods grown on gold thin film were used to create pH sensor devices. The developed ZnO nanotube and nanorod pH sensors display good reproducibility, repeatability and long-term stability and exhibit a pH-dependent electrochemical potential difference versus an Ag/AgCl reference electrode over a large dynamic pH range. We found the ZnO nanotubes provide sensitivity as high as twice that of the ZnO nanorods, which can be ascribed to the fact that small dimensional ZnO nanotubes have a higher level of surface and subsurface oxygen vacancies and provide a larger effective surface area with higher surface-to-volume ratio as compared to ZnO nanorods, thus affording the ZnO nanotube pH sensor a higher sensitivity. Experimental results indicate ZnO nanotubes can be used in pH sensor applications with improved performance. Moreover, the ZnO nanotube arrays may find potential application as a novel material for measurements of intracellular biochemical species within single living cells.

A two-step chemical bath deposition was utilized to synthesize ZnO nanorod arrays (ZNRAs) on metals, poly(3,4-ethylenedioxythiophene)/poly(strenesulfonate) (PEDOT/PSS) coated flexible plastic foils, and copper oxides coated glass substrates. The whole synthesis procedure was carried out at a low temperature of 50 °C, without any other substrate treatments. The low growth temperature showed improved influence on both the ZNRAs structural and optical properties. Scanning electron microscopy (SEM) images revealed well-aligned ZNRAs with large aspect ratios, and X-ray diffraction (XRD) analysis indicated that single crystalline ZNRAs were achieved with high c-axial orientation tendency. Room temperature photoluminescence (PL) measurements demonstrated excellent optical properties of the as-grown ZNRAs with very low defect concentration contrary to what was believed to be achieved when lowering the growth temperature. The impact of the low deposition temperature on the ZNRAs structure is discussed in connection to the thermodynamics constraints, while the temperature effect on the defects formation and density in the as-deposited ZNRAs is elaborated and compared with recent theoretical calculations that appeared in the literature.

We demonstrate hydrothermal synthesis of coral-like CuO nanostructures by selective growth on ZnO nanorods (NR) at low temperatures. During the hydrothermal processing the resultant hydroxylated and eroded surface of ZnO NR becomes favorable for the CuO nanostructures growth via oriented attachments. Heterojunction p-n diodes fabricated from the CuO/ZnO nanocorals (NC) reveal stable and high rectification diode properties with a turn-on voltage ~1.52 V and negligible reverse current. The humidity sensing characteristics of the CuO/ZnO NC diodes exhibit a remarkable linear (in a semilogarithmic scale) decrease in the DC resistance by more than three orders when the relative humidity is changed from 30 -90 %. The NC humidity sensor is also found to reveal the highest sensitivity factor ~6045 among available data for the constituent material's and a response and recovery time of 6 s and 7 s, respectively.

An efficient catalytic effect of petals and flowers like CuO nanostructures (NSs) on the degradation of two organic dyes, methylene blue (MB) and rhodamine B (RB) were investigated.The highest degradation of 95% in CuO petals and 72 % in flowers for MB is observed in 24 h.For RB, the degradation was 85 % and 80 % in petals and flowers, respectively for 5 h. It was observed that CuO petals appeared to be more active than flowers for degradation of both dyes associated to high specific surface area. The petals and flower like CuO NSs were synthesized using the chemical bath method at 90 o C. The grown CuO NSs were characterized using scanning electron microscopy (SEM), high resolution transmission electron microscopy (HRTEM), and X-ray diffraction (XRD).