Studies On The Preparation And Properties Of ZnO Nanorod Thin Films By Solution Chemical Method

Increasing concerns on energy crisis and pollution problem arouse considerable attention on nanocrystalline photoelectrochemical cells (NPC) for their potentials in simple process, low cost and large scale production. In this paper, developments on NPC cells and its anode were reviewed. Solution chemical deposition of ZnO nanorod thin films without and with electrical field and the working mechanism were investigated.Equiareal ZnO naorods with high orientation and a high aspect ratio on ZnO-coated seed substrates were fabricated from the Zn(NO3)2/NaOH supersaturated solutions; the mechanisms without and with electrical field were discussed based on the phase stability diagram for solid ZnO and the oxidation of OHˉ. The phase composition, crystalline morphology and optical transmittance of ZnO nanorod thin films were analyzed by SEM, XRD, TEM, HRTEM and UV-VIS.It was proposed that two processes are required for crystallization in a supersaturated solution: heterogeneous nucleation on a specific surface and subsequent crystal growth. The prepared ZnO single crystal grows layer by layer along c-axis. The electrical field enhances ion adsorption to substrate and lowers the nucleation energy barrier by increasing charge intensity; meanwhile it produces H+ through oxidation of OHˉand increases properly the degree of solutionsupersaturation near the substrate surface. Both aspects do favor to rod growth. The factors affecting the rod growth such as pretreatment of substrates, potential, precursor concentration, deposition temperature and growth time were elucidated. The experimental results showed that the average diameter of the nanorods is ~25 nm, with a length of 600~800 nm after growing for 1.5 h. The nanorods have wurtzite structure without electrical field assistance and are primarily of zincite structure under electrical field. The texture of the ZnO seed surface has great effects on morphology of resultant nanorods. The seed surface prepared by heating at 300~400oC, fast cooling, drawing at lower speed or proper PEG adding has better orientation and few surface defects, which leads to higher density of nuclei on the seed surface and thus the preferred crystal growth of ZnO rods perpendicular onto substrates. The solution condition also plays a great role in rod size. With the Zn2+ ion concentration increasing, the average diameter of ZnO nanorods increases slightly, while the growth...