Reactive conversion of polycrystalline SnO 2 into single-crystal nanofiber arrays at low oxygen partial pressure
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Ye Cai and Sehoon Yoo School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332
Kenneth H. Sandhageb) School of Materials Science and Engineering, School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332 (Received 8 February 2008; accepted 4 June 2008)
Single-crystal SnO2 nanofibers have been formed from SnO2 polycrystals via reaction at low oxygen partial pressures. Polycrystalline SnO2 disks coated with Au nanoparticles were exposed to humid H2/N2 at 700 to 800 °C. Single-crystal SnO2 nanofibers formed beneath Au nanoparticles, with the nanofiber length oriented parallel to the [100] crystallographic direction of SnO2. Because this simple process does not require either a separate source of a Sn–O-bearing vapor species located upstream of the substrate or a temperature gradient, single-crystal nanofibers may be formed on large area SnO2-bearing substrates.
I. INTRODUCTION
Stannic oxide (SnO2) based compositions possess attractive electrical, optical, and chemical properties for a number of device applications (e.g., as electrodes in solar cells,1 light emitting diodes,2 flat panel displays,3 or batteries4; as catalysts5,6; as sensors6–8 for CO, fuel gas, NH3, or H2O detection). Significant activity is underway to synthesize tin oxide based nanorods, nanowires, nanobelts, or nanotubes with high aspect ratios and wellcontrolled crystallinity for enhanced performance in such devices.9–14 A number of authors have used vapor phase processes, which are typically conducted by generating a vapor precursor that is transported via a carrier gas (such as Ar or N2) to a substrate, whereupon a single-crystal tin oxide based nanostructure is formed by deposition and growth.9–21 The localized and sustained growth of high aspect ratio nanostructures on the substrate is accomplished through the use of a temperature gradient, metal catalyst particles (such as gold) deposited on the substrate, or both. The purpose of the present paper is to demonstrate a simple reactive process for inducing the growth of oriented single-crystal nanofibers on a SnO2-based substrate at low oxygen partial pressures at or below 800 °C. Unlike previously reported methods of tin oxide nanofiber Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/JMR.2008.0321 J. Mater. Res., Vol. 23, No. 10, Oct 2008
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or nanobelt synthesis employing vapor deposition techniques, the substrate itself acts as the vapor source in the present work, so that the need for a separate upstream source of vapor is obviated. Moreover, a temperature gradient is not required to induce the localized formation of nanofibers. By avoiding the need to establish a carefully controlled temperature difference between an upstream vapor source and the downstream substrate to induce the formation of tin oxide nanofibers, the simple, modest-temperature reaction process of the present work
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