Synthesis of multi-walled carbon nanotubes by catalytic chemical vapor deposition using Cr 2 - x Fe x O 3 as catalyst M.D. Lima ⁎ , R. Bonadiman, M.J. de Andrade, J. Toniolo, C.P. Bergmann Av. Osvaldo Aranha, 99, sala 705-c, CEP 90035-190, Porto Alegre, Rio Grande do Sul, Brazil Ceramic Materials Laboratory, LACER, Materials Department, Federal University of Rio Grande do Sul, Brazil Received 31 July 2005; received in revised form 28 January 2006; accepted 17 February 2006 Available online 2 May 2006 Abstract Chromium oxide and iron oxide solid solution was used as a catalyst for multi-walled carbon nanotubes synthesis by the catalytic chemical vapor deposition technique. The catalyst was prepared by the solution combustion synthesis method. Natural gas (NG) was employed as a carbon source for the carbon nanotube growth instead of methane, which is typically used. The carbon nanotube synthesis was carried out under H 2 /NG and Ar/NG atmospheres at 950 °C. The Cr 2 - x Fe x O 3 catalyst was capable to produce carbon nanotubes only in H 2 /NG atmospheres. Partial elimination of the catalyst after the synthesis was possible using a concentrated solution of HNO 3 . © 2006 Elsevier B.V. All rights reserved. Keywords: Carbon nanotubes; Cr 2 O 3 ; Natural gas 1. Introduction Carbon nanotubes (CNTs) are a new class of materials discovered in 1991 by S. Iijima [1]. They have attracted much interest from the scientific community because of their extraordinary mechanical, electrical and thermal properties. Several different processes for their synthesis have already been developed. Among the main ones are laser ablation technique, graphite electrodes discharge and catalyzed chemical vapor deposition (CCVD). The last one has the highest potential for the mass production on CNTs. The synthesis of CNTs by CCVD is dependent on the catalyst metallic particle formation. The growth of the CNTs occurs by deposition of carbon onto the catalyst particle, followed by its saturation with carbon and the precipitation of the carbon in the form of CNTs [2]. Eventually, the effect of the catalyst particle will cease because of excessive carbon deposition. This fact usually limits the efficiency of the catalyst. Despite the fact that CNTs have been discovered more than 13 years ago, and the intense research that has been done, a low- cost and high selectivity method for their synthesis was not developed yet, especially for single-walled carbon nanotubes (SWCNT). However, using the CCVD technique, several authors successfully synthesized CNTs employing different catalysts under diverse atmospheres and temperatures [3–11] with good efficiency. The synthesis effectiveness is measured by the ratio between the produced carbon during the synthesis and the amount of catalyst used in the process. In the case of multi-walled carbon nanotubes (MWCNT), a significant progress has been made concerning the controlled and continuous growth processes. Charanjeet et al. [3] achieved aligned MWCNT yields of 1.6 mg/cm 2 h by the injection of a solution of ferrocene and toluene in a CVD reactor. Li et al. [4], using a non-continuous method and a Ni–Mo–MgO catalyst, achieved a very high yield (3000% in 30 min) in the MWCNT production. Some authors used the solution combustion synthesis (SCS) as a technique for the preparation of catalysts for carbon nanotubes production [12–17]. Using this technique, it is possible to produce oxides or oxide mixtures in a very quick manner, via an exothermic combustion reaction. Inorganic salts (usually nitrates) of the metals that will form the oxide catalysts are mixed in an aqueous solution. This solution is heated up until the water almost completely evaporates; after that, the temperature of the remaining material rises and it ignites [18]. In Diamond & Related Materials 15 (2006) 1708 – 1713 www.elsevier.com/locate/diamond ⁎ Corresponding author. E-mail address: marciodiaslima@yahoo.com.br (M.D. Lima). 0925-9635/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2006.02.009