Uniform diameter multi-walled carbon nanotubes with a controlled wall number obtained by a simple chemical vapor deposition method Youqin Lin 1 , Anthony Dichiara 1 , Delong He, Paul Haghi-Ashtiani, Jinbo Bai ⇑ Laboratoire de Mécanique des Sols, Structures et Matériaux, CNRS UMR8579, Ecole Centrale Paris, Grande voie des Vignes, 92290 Châtenay Malabry, France article info Article history: Received 27 July 2012 In final form 9 October 2012 Available online 17 October 2012 abstract A simple method was proposed to prepare carbon nanotubes (CNTs) with uniform diameter and con- trolled wall number. Concentric graphene layers were homogeneously deposited on CNTs by aerosol- assisted chemical vapor deposition. The formation of new concentric graphitic layers on the pristine CNTs was linearly proportional with the deposition time, resulting from a competition between the dehydro- genation of hydrocarbons and the hydrogenation of the deposited carbon atoms on the CNT surface. This process homogeneously thickens the CNT diameter by increasing their wall number, allowing an accurate control of the CNT morphology, which is of great interest in a wide range of applications. Ó 2012 Elsevier B.V. All rights reserved. 1. Introduction Due to their unique anisotropic structure and their extraordi- nary properties, carbon nanotubes (CNTs) have spurred great inter- est in a wide range of applications, such as membrane filters [1], electron field emission devices [2] or conducting nanoprobes [3]. In these applications, the CNT inter-space (i.e. the volume between the CNTs) and aspect ratio are significant to ensure high perfor- mance devices. Hence, it is of paramount importance to make CNTs with controlled structure and density in large scales. Chemical va- por deposition (CVD) is believed to be one of the most suitable methods to fulfill these requirements. In general, the CNT diameter is adjusted by the catalyst nanoparticle size [4], through the con- trol of its chemical composition [5,6] and/or the use of specific templates [7,8], while the CNT length is monitored by varying the reaction time under specified CVD conditions [9]. However, such selective synthesis is not suitable for mass production. In addition, this method is limited to the growth of CNTs with few numbers of layers, like double, triple or quadruple-walled CNTs [5–10], and the variation of the CNT inter-space remains very dif- ficult to adjust. In another way, floating catalyst CVD methods al- low the continuous production of CNTs with tunable length in a very large scale [11]. Nevertheless, the synthesis of CNTs with spe- cific diameter remains one of the principal and challenging goals. For instance, such techniques usually induce CNTs with poor uni- formity [12,13], which requires the use of complex sorting pro- cesses, that can be a serious obstacle for further applications [14]. Up to now, no efficient methods are available to control inde- pendently the diameter and the length of the CNTs. In this Letter, we demonstrate a simple and reproducible thick- ening CVD growth method to precisely control the CNT number of layers from few layers up to several hundreds. Starting from CVD- grown CNT arrays with a given length and density, we show that the CNT wall numbers and inter-space can be precisely tailored, while the vertical-alignment and length are preserved. The mor- phology of the thickened CNTs was investigated by electron microscopies, while the deposited graphene structure was exam- ined by thermo-gravimetric analysis, selected area electron diffrac- tion, mass spectrometry and Raman spectroscopy. The involved mechanism of the CVD deposition of graphene on the CNT arrays was finally discussed. 2. Experimental details The graphene deposition was conducted by a CVD method on vertically-aligned CNT arrays. A quartz plate supporting the CNT arrays was put in a quartz tube CVD reactor (45 mm in diameter and 1200 mm in length). Then, it was heated up by an electrical resistance furnace to 800 or 850 °C under argon/hydrogen (H 2 , 30 vol.%) atmosphere. After 10 min for the system stabilization, the hydrocarbon, xylene C 6 H 4 (CH 3 ) 2 was injected at 0.2 mL/min in the form of spray, along with acetylene C 2 H 2 at 0.05 L/min. Here, the injection of liquid xylene and the gases were controlled by a mechanical syringe system fitted with a liquid flow meter (Razel Science, R99-E), and by digital mass flow meters (Bronkhorst), respectively. The total gas flow was kept to 1 L/min during the overall process. The deposition time was varied from 5 to 20 min. At the end, the reactor was cooled down to room temperature un- der argon atmosphere at 1 L/min. 0009-2614/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.cplett.2012.10.028 ⇑ Corresponding author. E-mail addresses: youqin.lin2009@gmail.com (Y. Lin), anthony.dichiara@ecp.fr (A. Dichiara), dlong.he@gmail.com (D. He), paul.haghi-ashtiani@ecp.fr (P. Haghi-Ashtiani), jinbo.bai@ecp.fr (J. Bai). 1 These authors equally contributed to this work. Chemical Physics Letters 554 (2012) 137–142 Contents lists available at SciVerse ScienceDirect Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett