Synthesis and characterization of hierarchically nanostructured ZnO for photocatalytic and optoelectronic applications

Abstract

Synthesis of ZnO nanostructures is currently attracting intense worldwide interest due to their significant potential and extensive applications. Numerous ZnO nanostructures have been demonstrated, for example, nanowires, nanotubes, nanobelts, nanorods, nanopropellers. However, few studies on hierarchical nanostructures have appeared, not to mention their applications. This work aims at preparing and characterizing hierarchical ZnO nanostructures and their suitable applications. Two methodologies were employed to synthesize hierarchical ZnO nanostructures. One is alkaline aqueous precipitation. The other is a topotactic reaction between Zn5(OH)6(CO3)2 and ZnO. According to the specific hierarchical nanostructures and properties, the suitable applications of these prepared ZnO in dye-sensitized solar cell, Li-ion cell and photocatalysts were studied. Monodisperse spherelike ZnO particles were synthesized via alkaline aqueous precipitation. The spherelike particles are aggregates of many nanoparticles about 20 nm in diameter. The size of ZnO spheres can be easily tunable in the range of 0.1-2 m by changing some parameters, such as the concentration of zinc source, solvent, and additive. The measurements of photovoltaic properties of dye-sensitized solar cells constructed by different-size ZnO spheres demonstrate that secondary colloidal spheres 300-600 nm in diameter perform the highest short-circuit current density and overall efficiency, as compared to other diameters. This result confirms the predicted resonant scattering existing in spheres of diameter within magnitude of visible wavelength. Flowerlike ZnO films were fabricated by the combination of sol-gel and alkaline aqueous precipitation routes. The flowerlike clusters are 2-4 m in diameter and each flowerlike cluster is an assembly of nanoparticles of 20-30 nm in diameter. The electrochemical properties, phase and composition of the ZnO film before and after cyclic voltammetric measurement were investigated. The results reveal that reaction mechanism of lithium with as-prepared ZnO is probably a reversible process. The proposed reversible reactions would potentially improve capacity retention and cycling, which might break the reported negative conclusions such as very poor cycling on lithium storage in ZnO. The nanosheet-assembled quasi spherelike ZnO, synthesized by alkaline aqueous precipitation, has a high concentration of surface defects, while several hierarchical ZnO nanostructures, synthesized by the topotactic reaction, have large specific surface area. These three-dimensional ZnO nanostrutures as photocatalysts application were investigated. The findings demonstrate that they all perform some degree of improved photocatalytic activity relative to ZnO powder. Furthermore, these hierarchically nanostructured ZnO catalysts were reused for six times without any loss of activity, revealing excellent stability and recyclability. In addition, the comparative experiments suggest that the photocatalytic activity is influenced by surface defects, specific surface area, surface orientation of hierarchically nanostructured ZnO. This work develops the preparation of tunable hierarchically nanostructured ZnO and gives a different-aspect attempt in application. It will contribute to the improvement of nano-photocatalysis, nano-optoelectronics, nano-Li-batteries.

Date of Award	17 Feb 2010
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Guei Gu Kenneth SIU (Supervisor) & Paul Kim Ho CHU (Co-supervisor)