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In this paper, we report on the synthesis and structure of hierarchical zinc oxide nanotubes. Hierarchical nanotubes grown by physical vaporization of zinc in the presence of a catalyst were decorated with many secondary zinc oxide nanorods on the outer surface. The axis of these nanotubes with an average diameter of 65 nm was aligned along the c axis of wurtzite zinc oxide. The hierarchical zinc oxide nanotubes, many of which were single crystals, were transparent or opaque, depending on whether they had a zinc layer inside. The opaque nanotubes showed an abrupt change in electronic transmittance during investigation with transmission electron microscopy. The unique structure of the hierarchical ZnO nanotubes and the quantum effect resulting from the reduced dimension will modify the original properties of ZnO, leading to novel applications. I. INTRODUCTION

It has been reported that many materials with layered crystal structures form nano-sized tubular structures. Graphite, BN, NiCl2, or dichalcogenides such as MoS2 and WS2 can roll up the layers into single-wall or multiwalled nanotubes.1–5 On the other hand, some materials with no layered crystal structure also showed the capability to have similar tubular structures. Pure titanium foil can produce amorphous titanium oxide nanotube arrays through anodic oxidation, and gold can form crystalline nanotubes using silver nanowires as a template.6,7 ZnO is known to form various shapes of nanoparticles.8–10 We report here a new nanostructure of ZnO synthesized in the presence of a catalyst, which is a hierarchical nanotube. Wurtzite ZnO is known to be appropriate for diverse electronic or optoelectronic applications, a photocatalyst like TiO2, and a protective ceramic coating.11–15 The hierarchical ZnO nanotubes may advance the conventional properties of the bulk ZnO because of the unique structure of the hierarchical ZnO nanotubes and the quantum effect due to the reduced dimension. For example, greatly increased surface area and highly stressed surface due to their nano-sized structure will enhance capabilities of ZnO as a catalyst and a gas sensor. II. EXPERIMENTAL

Zinc powder (60 mg) was inserted separately with nickel powder (30 mg) into a quartz boat. The quartz boat loaded with the zinc and nickel powder was put into a

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Address all correspondence to this author. 225 Rhines Hall, Department of Materials Science and Engineering, University of Florida, Gainesville, Florida 32611. e-mail: [email protected] J. Mater. Res., Vol. 18, No. 12, Dec 2003

tube furnace, heated up to 900 °C at the heating speed of 15 °C/min in an inert atmosphere (argon), and held for 15 min before cooling to room temperature. The gray deposition was formed on the inner walls of the furnace tube. It was collected and characterized in a JEOL JSM6335F field-emission scanning electron microscope (SEM; 15 kV), a Philips 420EM transmission electron microscope (TEM; 120 kV), and a JEOL JEM-2010F high-resolution field emission transmission electron microscope (HRTEM; 200 kV). III. RESULTS AND