CVD Synthesis of Highly Graphitized Single-walled Carbon Nanotubes Using Nitrogen-pretreated Fe–Mo/MgO Catalyst

Abstract

Single-walled carbon nanotubes (SWNTs) with highly graphitized structure were synthesized by thermal chemical vapor deposition using an improved nitrogen-pretreated FeMo/ MgO catalyst. The effects of nitrogen pretreatment of FeMo/ MgO on the structure and properties of SWNTs were studied by TEM, Raman spectroscopy, and TGA. The investigations revealed that the nitrogen pretreatment of the catalyst promoted the growth of SWNTs. It also enhanced the structural features and thermal properties of SWNTs.Since their discovery in 1991, 1 carbon nanotubes (CNTs) have been ruling the regime of nanoresearch for their remarkable structural, mechanical, thermal, electronic, and phonon properties.2 It is because of these excellent properties, CNTs find their use in several applications such as high strength composites, catalytic reactions, hydrogen storage, field-emission displays, sensors, and so on.3 Depending on the requirement of a particular application, CNTs are modified suitably by a postsynthesis functionalization or during the synthesis itself by controlling the process conditions. In the later case, CNTs are preferably synthesized by chemical vapor deposition (CVD). Among all methods of CNT synthesis, CVD is very amenable due to its flexibility in manipulating reaction parameters. 4 In a conventional CVD process, a transition-metal-based catalyst is activated in inert atmosphere to synthesize CNTs from a gaseous hydrocarbon. Several CVD methods have been reported for the synthesis of CNTs, including those at varying catalyst formulation and reaction parameters.5 Chiral selective growth of single-walled CNTs (SWNTs) can be achieved during nucleation by changing the catalyst composition or structure and controlling the kinetic parameters.6 CNTs doped with N 2 by introducing N 2 gas flow in the flow of reactant gases have also been studied for structural changes, mechanical applications, and catalytic reactions.7 In this letter, we report the synthesis of highly graphitized SWNTs by employing improved nitrogen-pretreated FeMo/MgO catalyst nanoparticles in a thermal CVD and study the effects of nitrogen pretreatment of the catalyst on the structure and properties of SWNTs. We demonstrate herein that simple nitrogen pretreatment can enhance the catalytic activity, which in turn can promote the CNT growth mechanism to form long length, highly graphitized CNTs.The FeMo/MgO catalyst containing 5 wt % of FeMo in 9:1 ratio was prepared from Fe(NO 3 ) 3 ¢9H 2 O (98+%, SigmaAldrich), (NH 4 ) 6 Mo 7 O 24 ¢4H 2 O (99.98%, Sigma-Aldrich), and MgO nanopowder (Strem Chemicals). In a typical procedure, 0.5 g of MgO was taken in 10 mL of methanol and ultrasonicated for 20 min to obtain a suspension. In another beaker, 0.1628 g of Fe(NO 3 ) 3 ¢9H 2 O and 0.0046 g of (NH 4 ) 6 Mo 7 O 24 ¢4H 2 O were mixed in 10 mL of methanol and ultrasonicated for 20 min to obtain a solution. This freshly prepared solution was added dropwise to the suspension of MgO under magnetic stirring at room temperature. The stirring was continued until methanol...