Crystallinity-Driven Formation of Highly Arrayed ZnO Nanorods Via Low-Temperature Chemical Bath Deposition on Transparent Conductive Oxide Substrates

Abstract

The wide-band gap semiconductor zinc oxide (ZnO) nanostructures have attracted much interest in extensive applications such as gas sensors, photoelectrodes in dye sensitized solar cells and catalysts, etc. Among the various synthesis methods of ZnO nanostructures, chemical bath deposition (CBD) method is a low cost and simple technique to synthesize nanostructures. However, there are still remaining issues on growth of well- arrayed ZnO nanostructures with high uniformity and high crystallinity. In this research, we conducted a comprehensive investigation into various parameters, including seed layers, seed-layer thickness, precursor concentration, and reaction time during CBD process. Three kinds of transparent conductive oxide (TCO) films, specifically AZO, GZO, and ITO films, each with a thickness of 300nm, were individually deposited on glass substrates using radio frequency magnetron sputtering and DC magnetron sputtering. These obtained TCO films served as seed layers for the fabrication of ZnO nanorods in CBD process. The solution was prepared with Zn(NO3)2•6H2O and hexathylenetetramine (HMTA) diluted into deionized water, which was heated to 95 °C during CBD process. As the results: 1) Effect of different seed layers: ZnO nanorods exhibited vertically aligned hexagonal structures with uniform morphology when grown on AZO and GZO thin films. The alignment was found to depend on the crystal growth direction of the underlying TCO seed layers. Higher crystallinity of the seed layer contributed to superior alignment, with the crystal mismatch ratio between ZnO nanorods and GZO/AZO films being significantly lower than that with ITO film. 2) The thickness effect investigation was using AZO films varied from 100nm to 300nm. It was found that the thickness influenced the morphology of ZnO nanorods, correlating with the grain sizes of the seed layer thin films. Increasing film thickness led to larger average grain sizes, contributing to the diameter increase of ZnO nanorods in the CBD process, resulting in the improvement of crystallinity of ZnO nanorods. 3) The precursor ratio between concentration ratio of Zn(NO3)2•6H2O and HMTA was set as 2:0.25, 2:0.5, 2:1 and 2:2 individually for ZnO nanorods growth on AZO film. Increasing HMTA concentration initially led to an increase in diameter and length of ZnO nanorods, followed by a decrease at higher ratio of 2:2. The highest crystallinity of ZnO nanorods was obtained at the optimized ratio at 2:1. 4) The growth reaction time was investigated on ZnO nanorods grown on AZO film with concentration ratio of Zn(NO3)2•6H2O and HMTA at 2:1 with varied reacting time from 1 to 5 hours in CBD process. It was found that the reaction time significantly influenced on the growth and crystallinity of well-aligned ZnO nanorods on the as-deposited AZO seed layer. The crystallinity of ZnO nanorods in (0001) growth orientation was improved when the reaction time was increased from 1 hour to 5 hours during the CBD growth. The highest crystallinity was obtained from the ZnO nanorods grown with 5 hours reaction time. When the growth reaction time was increased, more Zn2+ ions were generated and strongly bonded with oxygen. The formed ZnO could be stacked along the c-axis (0001) growth direction and resulted in the length increasing during the CBD process. All fabricated ZnO nanorods showed a high transmittance of over 70% in the visible region. The ZnO nanorods synthesized on AZO substrates with optimized fabrication conditions will be in high potential for optoelectronic applications.