129Xe NMR study of Xe adsorption on multiwall carbon nanotubes

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Abstract

129Xe NMR spectroscopy has been used to study the adsorption of Xe on multi-wall carbon nanotubes (MWCNT). The results obtained have shown the 129Xe NMR ability to probe the intercrystalline (aggregate) and the inner porosity of CNT. In particular, the effects on porosity of tubes openings by hydrogen exposure and of ball milling were examined. Dramatic changes observed in the 129Xe NMR spectra after moderate ball milling of MWCNTs were attributed to the destruction of the initial intercrystalline pore structure and to the Xe access inside the nanotubes. To examine the exchange dynamics the mixture of as-made and milled MWCNTs was studied with one- and two-dimensional (1D and 2D) 129Xe NMR. The exchange between the interior of milled nanotubes and the aggregate pores of as-made MWCNTs was fast on the NMR acquisition time scale. The Xenon exchange between the interior of the as-made MWCNTs and the large aggregate pores occurred on a longer time scale of 10 ms, as was established by 2D 129Xe NMR exchange spectroscopy. Variable temperature 129Xe NMR data were also discussed and analyzed in terms of the fast exchange approximation.

Introduction

Carbon nanotubes (CNT) are attractive for fundamental materials science investigations and industrial applications. Electrical and thermal conductivity, high durability, gas storage capacity, adsorption and molecular sieving are properties of CNT currently being investigated. A great number of publications have appeared since the discovery of CNT in 1991 [1]. However, the CNT family has not been extensively examined by 129Xe NMR [2], [3], a well-established technique for the characterization of porous silicates [4]. In fact, an extension of 129Xe NMR to the field of carbon nanomaterials could have many useful applications, as it has for numerous silica-based materials.

The first problem to solve when studying adsorption on CNT is the precise identification of adsorption sites. Generally two principal surface sites are inherent in CNT: (1) intercrystalline or aggregate pores corresponding to the external surface; (2) 1-D nanoscale cavities in the central core of the nanotubes. The important role of intercrystalline (aggregate) pores in adsorption, capillarity or other physico-chemical properties has been demonstrated in Ref. [5]. This type of pore appears to have a huge adsorption capacity responsible for 78.5% of the total adsorbed amount [5]. Therefore an understanding of the intercrystalline pore structure in relation to the aggregation mechanisms is necessary for the development of efficient gas-storage materials. In a study of single-wall CNT bundles [6], volumetric measurements depending on the sorbate molecule size were used to identify the interstitial channels, the grooves and the remaining outer parts as adsorption sites.

From the theoretical point of view, the CNT family is fascinating as an ideal geometrical model of cylindrical pores [7]. However, analysis of the inner cavity filling from usual adsorption measurements is quite complicated [8]. As discussed previously [9] the porosity of as-made CNT can hardly be ascertained using N2 adsorption. Adsorption data seem to be more informative with regard to the surface characteristics than to the porosity. Several studies report the native nanotubes as being closed [9], [10]. The interior of CNT becomes accessible to guest molecules after various treatments: uniform burning-out of the tips by mild oxidation [11], ball milling [12] or thermal activation [13]. The purification procedures also may result in removing of the tips from both ends of the CNT [14].

The research reported here focuses on using the 129Xe NMR chemical shift for the direct characterization of inter- and intracrystalline CNT porosity.

Section snippets

Experimental

Two multi-wall CNT samples (MWCNT) were obtained by the catalytic chemical vapor deposition (CCVD) technique described in the Ref. [15]. The only forms of carbon visible by TEM (not shown) are isolated and bundled thin multi-wall CNT of average inner/outer diameters of 4/15 nm. The average nanotubes length was 5 μm. These samples differ in the time of exposure of the supported catalyst (Co–Fe/Al2O3) to the conditions for MWCNTs growth (C2H4 at 700 °C), which were 360 and 100 min, resulting in

Results and discussion

Fig. 1, Fig. 2 display the 129Xe NMR spectra of Xe adsorbed on the as-made and ball milled MWCNTs at a pressure 100 kPa. Several lines contribute to 129Xe NMR spectra of both as-made CNT. Two lines are well resolved in the spectrum of Open-CNT: at 10±2 ppm (a1) and at 43±2 ppm (b1). A third line at 30±2 ppm (b0) unambiguously resulted from the line shape decomposition. The integral intensities ratio was 100/24/34 for a1, b0 and b1, respectively. A narrow signal near zero corresponds to gaseous Xe.

Conclusion

Generally, our results are similar to those obtained by 129Xe NMR for mesoporous silica. This similarity is based on the fast exchange approximation valid for pure mesoporous materials. Specifically, 129Xe NMR can probe the texture of MWCNT, nonuniform aggregate porosity and inner channels. Despite the use of enriched xenon (129Xe) low NMR sensitivity was the main drawback of this study.

Acknowledgment

The French embassy in Moscow, Russian Foundation of Basic Research (Grant RFBR 04-03-33070) and NANOCYL S.A. are acknowledged for financial support. We also thank Dr. M.-A. Springuel-Huet for helpful assistance.

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