Synthesis of Boron Nitride Nanotubes by a Template-Assisted Polymer Thermolysis Process Mikhael Bechelany, ² Samuel Bernard,* ,² Arnaud Brioude, ² David Cornu, ² Pierre Stadelmann, ‡ Catherine Charcosset, § Koffi Fiaty, § and Philippe Miele ² Laboratoire des Multimateriaux et Interfaces (UMR CNRS 5615), UniVersite ´ Lyon1, UniVersite ´ de Lyon, 43 bd du 11 NoVembre 1918, 69622 Villeurbanne Cedex, France, Interdepartemental Centre of Electron Microscopy, CIME, Swiss Federal Institute of Technology, EPFL, CH-1015 Lausanne, Switzerland, and Laboratoire d’Automatique et de Ge ´ nie des proce ´ de ´ s (UMR CNRS 5007), CPE - UniVersite ´ Lyon1, UniVersite ´ de Lyon, 43 bd du 11 NoVembre 1918, 69622 Villeurbanne Cedex, France ReceiVed: May 30, 2007; In Final Form: July 5, 2007 Highly ordered arrays of boron nitride nanotubes (BN-NTs) were prepared for the first time by combining a polymer thermolysis route and a template process. The strategy involves four steps, i.e., synthesis of a liquid polymeric borazine, liquid-phase infiltration of an alumina membrane, thermolysis at 1200 °C under nitrogen of the polymer confined in the 200 nm ordered nanochannels, and template etching to generate BN-NTs arrays supported by a BN sheet. Their crystallinity was controlled through further high-temperature treatments. The thin multiwall BN-NTs displaying dimensions corresponding to the pore size were characterized by electron microscopies and electron energy loss spectroscopy. It was demonstrated that the formation of nanotubes was governed by evaporation of low molecular weight species during the initial steps of the thermolysis, causing loss of nanowire integrity, while a thin film remained and covered the pore walls as a result of the high surface energy of the alumina mold. Introduction The past decade has seen an increasing demand for the preparation of one-dimensional (1D) inorganic nanostructures including nanotubes and nanowires in various phases and compositions. 1-5 Such nanostructured materials displaying novel physical and chemical properties have great potential in a wide variety of application going from optoelectronic nanodevices to composite materials. 6-12 Among such materials, boron nitride nanotubes (BN-NTs) are paid special attention as alternatives to conventional carbon nanotubes (CNTs) as a result of the structural similarity between h-BN and graphite. BN nanotubes are even thought to be more amenable than carbon nanotubes for a number of applications including gas adsorption, electron transport, field emission measurements, and composite structural material. In particular, BN-NTs are predicted to have interesting electronic properties insensitive to tube diameters, and their resistance toward oxidation up to 700 °C in air represents a significant advantage over carbon nanotubes which readily oxidize at 400 °C. 13 There are currently numerous routes for the synthesis of BN nanotubes like chemical vapor deposition, arc-discharge, and laser-ablation methods, 14-18 but only a few techniques allow production of aligned BN NTs with a narrow size distribution despite their potential applications in engineering materials as, for instance, in nanocomposites. The possibility to assemble individual nanotubes into a special arrangement for preparing ordered arrays of nanotubes with controlled diameter, length, and density could arise from the membrane-assisted template process. The templating process historically introduced by Possin et al. 19 helps to prepare many kinds of highly ordered arrays of 1D nanoscale objects including organic polymers, carbon, metals, and metal oxide systems when combining with chemical approaches such as electrodeposition, chemical vapor deposition (CVD), and sol-gel techniques. 20-27 In the case of the membrane-assisted template process, the technique involves nanoporous anodic aluminum oxide (AAO) membranes as inorganic templates containing a large number of straight cylindrical pores with a narrow size distribution. Arrays of monodispersed and highly ordered nanowires or nanotubes are shaped by filling the nanochannels with the desired material in the gas or liquid phase. The diameter, length, and density of 1D nanostructures are ideally determined by the size, depth, and interval of pores in AAO templates. To the best of our knowledge, only a membrane-assisted template process coupled with a CVD approach was used to prepare ordered arrays of BN nanotubes. 28 However, these processes are usually very complex and require heavy equipment which drastically increase the cost of the resulting nanostructures in the case of industrial application. In addition, the density and quality of the as-obtained nanotubes closely depend on the efficiency of the gas-filling process and usually require a relatively long infiltration time. In our work, the strategy used to prepare BN-NTs can overcome these limitations to a certain extent. It is based on a liquid-phase infiltration (LPI) process through the combination of the templating process and the polymer thermolysis route. Using the general polymer thermolysis route, 29-31 a large variety of net-shaped non-oxide ceramics, i.e., polymer-derived ceramics (PDCs), can be built up from molecular units and shaped by controlling the structure of the molecular units as well as the polymerization and thermolysis procedures. Interestingly, the * To whom correspondence should be addressed. E-mail: Samuel.Bernard@univ-lyon1.fr. Tel.: +33 472 433 612. Fax: +33 472 440 618. ² Laboratoire des Multimateriaux et Interfaces, Universite ´ Lyon1. ‡ Swiss Federal Institute of Technology. § Laboratoire d’Automatique et de Ge ´nie des proce ´de ´s, Universite ´ Lyon1. 13378 J. Phys. Chem. C 2007, 111, 13378-13384 10.1021/jp074178k CCC: $37.00 © 2007 American Chemical Society Published on Web 08/21/2007