Growth of ZnO Nanostructures by the Reaction of ZnO Powder with Carbon Monoxide

Abstract

Wurtzite-type zinc oxide (ZnO) is a promising semiconductor material with a wide bandgap of 3.3 eV at room temperature. Recently, ZnO nanostructures have attracted great research interest because of their simplicity in fabrication, variety in morphology, and diversity of application. 1,2Until now, the methodologies known for growth of ZnO nanostructures have included metal organic chemical vapor deposition, 3,4 solution-based process, 5 and vapor phase transport (VPT).6-11 The VPT process, in which Zn, 6,7 ZnO, 8and a mixture of ZnO and carbon 9-14 are used as sources, has been the most frequently used for synthesizing a variety of ZnO nanostructures. For the carbothermal reduction using a mixture of ZnO and carbon powders, Zn vapors formed in the temperature range between 800 and 1200 o C are transported via a carrier gas onto a substrate and deposited as a thin film or nanostructures of ZnO on the substrate at lower temperatures.9-14 Despite of the many studies on the carbothermal reduction, doubt remains as to how Zn vapor is oxidized to ZnO vapor during its transportation to the substrate. Two oxidants, oxygen (O 2 ) and carbon monoxide (CO), will be considered in the conversion of Zn to ZnO. The CO gas can be generated by the reaction of carbon with O 2 .In this work, we fabricated ZnO nanostructures on the substrate by the VPT process in which ZnO powder was heated under a flow of CO and demonstrated the plausibility of the oxidation of Zn vapor to ZnO vapor by CO. To the best of our knowledge, this is the first report of the fabrication of ZnO nanostructures by the reaction of ZnO powder with CO. Our previous paper showed that β-Ga 2 O 3 nanobelts were obtained by the reaction of β-Ga 2 O 3 powder and CO. 15ZnO powder in an alumina crucible was heated at 750 o C for 3 h in a gas mixture of nitrogen (N 2 ) and 1 vol % CO (hereafter referred to as 1 vol % CO/N 2 ). The reaction temperature used in this study was lower than that 9-14 in the carbothermal reduction method. Zn vapor generated by the reduction of ZnO powder by CO at a hot zone is oxidized to ZnO vapor and then deposited on the substrate away from the crucible:(2) ZnO (g) → ZnO nanostructures on a substrateThe substrate used in this study was fluorine-doped tin oxide (FTO) glass applicable to dye-sensitized solar cells. Photoelectrode films with nanostructured ZnO are known to significantly enhance solar-cell performance.16 As shown in Figure 1(a), the colors of the as-grown products on the FTO glass gradually changed from black to greyish with increasing distance from the glass to the source. The deposited glass was classified into three zones according to the color. The temperature of each zone at the glass decreased along the gas flow direction. The morphologies of the ZnO nanostructures were determined by the distance, as done in other studies.7,14 The product deposited at zone A above 530 o C consisted of irregularly shaped ZnO particles. Nanobelts and nanowires were grown at zone B, while hexagonal nanorods with a diameter of ca. 100 nm and...