Carbon 39 (2001) 1709–1715 Synthesis of multi branched carbon nanotubes in porous anodic aluminum oxide template * ´ ´ ´ Y.C. Sui , J.A. Gonzalez-Leon, A. Bermudez, J.M. Saniger ´ ´ Laboratorio de Materiales y Sensores, Centro de Instrumentos, Apdo Postal 70-186, Universidad Nacional Autonoma de Mexico, Mexico D.F. 04510, Mexico Received 25 February 2000; accepted 4 November 2000 Abstract Porous anodic aluminum oxide (AAO) templates, obtained by anodization of aluminum plates at relatively high voltages, were used for growing multi-branched carbon nanotubes which copy the three-dimensional branching structure of the nanochannels in the template. The formation of these branched nanochannels is characteristic of AAO templates grown at 60 V. When AAO templates are grown at 40 V or lower, straight channels and carbon nanotubes (CNTs) are obtained. At 60 V some ramifications grow for a short time from the main axis of the pore, following the direction of film growth. The repetition of this process along the main channel induces a kind of three-dimensional pore structure which is copied when the CNTs are grown in the template. In this way it is possible to get bundles of branched carbon nanotubes grown into AAO templates with a tailored ramified channel structure.  2001 Elsevier Science Ltd. All rights reserved. Keywords: A. Carbon nanotubes; B. Electrochemical treatment; Pyrolysis; C. Electron microscopy; Atomic force microscopy (AFM) 1. Introduction some polymeric materials such as polyacrylonitrile or propylene and by the catalytic pyrolysis of acetylene There has been increasing interest in carbon nanotubes within the pores [15,16]. Recent work shows that Y- (CNTs) due to the many potential applications such as junction carbon nanotubes can be grown within Y-shaped electron emitters [1], probes for electrochemistry devices AAO nanochannels when the voltage conditions are ad- and scanning probe microscopy [2,3], single electron justed in a specific way along a double step template transistors [4], quantum wires [5], hydrogen storage media growth process [17]. Compared with the complex branch- [6], chemical sensors [7], nanotweezers [8], and so on. For ing phenomena observed in CNTs produced under specific these purposes both the development of new manipulation arc-discharge condition [18], the Y-shaped CNTs produced techniques and alternative synthetic routes must be found. by AAO template are well-aligned and similar in shape Several manipulation methods have been adopted to and size. In this paper, multi-branched CNTs were pro- connect CNTs with electrodes, to control the position of duced in AAO template in which a branched porous CNTs [9–12], etc. In relation with the synthetic routes, structure was previously grown following a single step and special attention is being paid to the production of aligned fast anodizing process by applying a relatively high arrays of CNTs confined in a template. The growth in anodization voltage. anodic aluminum oxide (AAO) is one of the more promis- ing approaches owing to the possibility of tailoring the size and density of their pores [13,14]. 2. Experimental methods Templates of porous AAO are relatively easy to produce and there is no practical limitation to their size. CNTs Ultra-pure Al (99.999%) rod was hot rolled to plates confined in AAO have been obtained by the pyrolysis of with thickness of 0.5 mm, which were then annealed at 5008C for 1 h and mechanically polished. An electro- chemical polishing was then carried out in 5:1 v / v solution *Corresponding author. Tel.: 152-5-622-8616; fax: 152-5- of EtOH / HClO in order to have a smooth stress-free 622-8620. 4 E-mail address: yunchengsui@yahoo.com (Y.C. Sui). aluminum surface. For the anodization of the samples the 0008-6223 / 01 / $ – see front matter  2001 Elsevier Science Ltd. All rights reserved. PII: S0008-6223(00)00297-9