Binary Phase of Layered Nanotubes
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Binary Phase of Layered Nanotubes R.L.D. Whitby,1 W.K. Hsu,1 C.B. Boothroyd,2 P.K. Fearon,1 H.W. Kroto1 and D.R.M. Walton1 1
School of Chemistry, Physics and Environmental Science, University of Sussex, Brighton,
BN1 9QJ, UK. 2
Department of Materials Science & Metallurgy, University of Cambridge, Pembroke Street,
Cambridge, CB2 3QZ, UK.
ABSTRACT A binary phase of layered nanotubes, where MWCNs (Multi-walled carbon nanotubes) are coated by WS2, are generated by pyrolysing WO3-coated MWCNs in an H2S/N2 atmosphere at 900 oC. TGA and TEM show that WS2 acts as an antioxidant with respect to the MWCN core.
INTRODUCTION It has been demonstrated that MWCNs can act as templates for the generation of other types of nanotubes,[1, 2] despite problems associated with their high surface tension.[3] Previous methods for coating MWCNs with WO3 involved surface oxidation, which generated –COOH or –OH groups[2, 4] and the MWCNs were often found uncapped and peeled.[4] Here we describe a technique for depositing WO3 on the surface of MWCNs and conversion the WO3 into WS2 without affecting the carbon template.
EXPERIMENT H2WO4 (250 mg) and MWCNs (50 mg) were mixed at –78 oC in liquid NH3 (~20 cm3) and the mixture was set aside in order to attain room temperature. The solid residue was then heated at 350 oC for 15 min in an air-flow (100 cm3 min-1) to convert H2WO4 into WO3 and then further heated at 900 oC for 15 min in a mixture of H2S and N2 (ca. 1:3 ratio, total 50 cm3 min-1) to convert WO3 into WS2. The products were ultrasonically dispersed in acetone and mounted on lacey carbon films (Cu support grids) for TEM analysis. The following equipment was employed as appropriate: high-resolution transmission electron microscopy (HRTEM, JEOL 4000 EX II operated at 400 kV, Philips CM 200 operated at 200 kV) equipped with an energy dispersive Xray probe and line mapping (EDX), and thermogravametric analysis (Perkin Elmer TGA-7, heating rate 10 K min-1).
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DISCUSSION HRTEM showed that 60 % of the MWCNs were either partly or fully coated with dark material, typically with 1 to 3 layers (Figure 1). A number of features are distinguishable in the HRTEM images of the sample: 1) the d-spacing of MWCN layers is maintained after the experiment at ca. 3.4 Å; 2) the d-spacing of the dark layers is ca. 6.2 Å, consistent with separate WS2 nanotubes; 3) the distance between the centres of the outermost C layer and the innermost WS2 layer is ca. 4.4 Å; 4) the residual amorphous layer coating both the partly and fully coated MWCN is WO3-x, resulting from the incomplete conversion of WO3 into WS2.
Figure 1. HRTEM image of MWCNs and polyhedral carbon particle coated with WS2. EDX analysis was carried out on both partly and fully coated MWCNs. Only a C signal was detected when the probe was focused on the uncoated sections of a partly coated MWCN. C, W and S, together with a trace of O, were detected when the EDX probe was focused on the coated section (Figure 2). Quantitative analysis showed the atomic ratio of W to S was 1:2 (± 0.1), imply
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