Fabrication of CdO nanotubes via simple thermal evaporation H.B. Lu a , L. Liao a , H. Li b , Y. Tian a , D.F. Wang a , J.C. Li a, ⁎, Q. Fu a , B.P. Zhu a , Y. Wu a a Department of Physics and Key Laboratory of Acoustic and Photonic Materials and Devices, Ministry of Education, Wuhan University, Wuhan 430072, People's Republic of China b State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of Microsystem and Information Technology, Chinese Academy of Sciences, 865 Changning Road, Shanghai 200050, People's Republic of China ABSTRACT ARTICLE INFO Article history: Received 19 February 2008 Accepted 1 May 2008 Available online 16 May 2008 Keywords: CdO Nanomaterial Crystal Growth Optical materials and properties CdO nanotubes (NTs) with a mean diameter of 50 nm have been fabricated by the simple thermal evaporation of Cd powder without using any catalyst or template. The growth mechanism of CdO NTs is discussed. It is suggested that the CdO NTs are formed by the sublimation of the Cd cores of the Cd/CdO core/ shell nanocables. The direct and indirect band gaps of the CdO NTs are determined to be 2.85 eV and 2.05 eV, respectively. This type of high surface area structural CdO NTs could find promising applications in catalysis, gas sensors, nano-optics, and active material encapsulation. © 2008 Elsevier B.V. All rights reserved. 1. Introduction Since the pioneering work of the discovery of carbon nanotubes by Iijima [1], numerous works have contributed to the synthesis and characterization of the nanotubular structures. Compared with other structured morphologies, hollow tubular nanostructures exhibit enhanced or novel functionalities due to their higher surface area and the capability of forming composite structures by embedding specific particles in the interiors, and they thus may find potential applications in a wide range of areas, including catalysis, drug delivery, storage and release systems, bioencapsulation, nanoreactors, and templates for functional architectural composite materials [2–5]. Cadmium oxide (CdO), an important n-type semiconductor with a direct band gap of 2.5 eV and an indirect band gap of 1.98 eV [6,7], has promising applications in the wide fields such as solar cells, photo transistors, transparent electrodes, catalysts and gas sensors [8–11]. For these applications, particle size, porosity, and specific surface area are of major importance. Up to now, a number of CdO nanostructues have been synthesized in different interesting morphologies including nanowires [6,12], nanobelts [13] and nanoparticles [14]. However, to the best of our knowledge, synthesis of tubular CdO nanostructures has rarely been reported so far. This is mainly due to the fact that the tubular form is generally limited to layered materials such as carbon nanotubes which make it hard to obtain the tubular structure for the nonlayered CdO material. Only recently, Yu et al. prepared nanoporous CdO architectures via one-step thermal decomposition of cadmium carbonate microcrystals [15], and Zhang et al. synthesized hollow quasi-spherical and irregular bamboo-like CdO nanostructures via solvothermal treatment of the CdO 2 nanoparticles in ethanol and ethylene glycol, respectively [7]. In this letter, we report on the synthesis of crystalline CdO NTs by the simple thermal evaporation of pure Cd powder without using any catalyst or template at a temperature of 500 °C. The growth mechanism of CdO NTs will be discussed. 2. Experimental Cd powder (99.99% purity) was placed on a quartz boat that was inserted into the center of a horizontal tube furnace. Si(100) p-type substrates were etched by hydrofluoric acid and cleaned by the action of deionized water and ethanol. The cleaned Si substrates were located downstream at a position where the distance to the Cd source is about 7 cm. The system was first pumped to a base vapor pressure of about 3 Pa, and then argon was introduced into the system with a flow rate of 60 sccm (standard cubic centimeter per minute) as a carrier gas. Afterward, the tube was heated to 500 °C at a rate of 15 °C/min. When the temperature reached 500 °C, an air flow of 10 sccm was then added to the quartz tube. The synthesis process was carried out under the pressure of 15 Pa at 500 °C and lasted for about 30 min. The gas flows were cut off when the furnace was turned off, and the samples were then naturally cooled at 3 Pa. The morphology and crystalline structure of CdO NTs were character- ized by Sirion FEG scanning electron microscopy (SEM), D8 advanced X- ray diffraction (XRD). The high-resolution atom lattice images were observed by a JEOL JEM 2010 FEF high-resolution transmission electron microscope (HRTEM) with a point resolution of 0.19 nm. Optical absorption spectra were recorded using a UV–vis–NIR dual-beam Materials Letters 62 (2008) 3928–3930 ⁎ Corresponding author. Tel.: +86 27 6875 2567; fax: +86 27 6875 2569. E-mail address: jcli@acc-lab.whu.edu.cn (J.C. Li). 0167-577X/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2008.05.010 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet