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Journal of Alloys and Compounds 446­447 (2007) 389­392

Neutron spectroscopy study of single-walled carbon nanotubes hydrogenated under high pressure
A.I. Kolesnikov
a b

a,

, I.O. Bashkin b , V.E. Antonov b , D. Colognesi c , J. Mayers d , A.P. Moravsky e

c

IPNS, Argonne National Laboratory, 9700 S. Cass Avenue, Argonne, IL 60439, USA Institute of Solid State Physics RAS, 142432 Chernogolovka, Moscow District, Russia Consiglio Nazionale delle Ricerche, Istituto di Fisica Applicata 'Nello Carrara', via Madonna del Piano, 50019 Sesto Fiorentino (FI), Italy d ISIS Facility, Rutherford Appleton Laboratory, Chilton, Didcot, Oxon OX11 0QX, UK e MER Corporation, 7960, South Kolb Road, Tucson, AZ 85706, USA Received 29 September 2006; received in revised form 21 November 2006; accepted 28 November 2006 Available online 2 January 2007

Abstract Single-walled carbon nanotubes (SWNT) were loaded with 5.2 wt% hydrogen at a hydrogen pressure of 3 GPa and T = 620 K, quenched to 80 K and studied at ambient pressure and 15 K by inelastic neutron scattering (INS) in the range of energy transfers 3­400 meV. An analysis of the measured INS spectra showed that the quenched SWNT & H sample contained hydrogen in two different forms, as H atoms covalently bound to the carbon atoms (4.7 wt%) and as H2 molecules (0.5 wt%) exhibiting nearly free rotational behavior. Annealing the sample in vacuum at 332 K removed about 65% of the H2 molecules and annealing at 623 K removed all of them. This demonstrates that H2 molecules were kept in this sample more tightly than in earlier studied SWNT & H samples that were hydrogenated at lower pressures and temperatures and lost all molecular hydrogen on heating in vacuum to room temperature. © 2006 Elsevier B.V. All rights reserved.
Keywords: Nanostructured materials; High-pressure; Inelastic neutron scattering

1. Introduction Single-walled carbon nanotubes (SWNT) reversibly absorb and desorb considerable amounts of hydrogen at moderate hydrogen pressures below 20 MPa and room or lower temperatures [1]. The dominating mechanism of hydrogen sorption at these P­T conditions is physisorption of H2 molecules on the graphene layers of SWNT [2]. The hydrogenation of SWNT at P =9GPa and T = 450 C with following quenching under pressure down to -140 C produced a new C H compound containing 6.8 wt% H [3]. On heating this compound in vacuum, about 0.5 wt% H left the sample by room temperature whereas the intense release of the main quantity of hydrogen began only above 500 C.

An effective means for studying the hydrogen bonding in carbon nanomaterials is using inelastic neutron scattering (INS). The INS technique has already been successfully applied for the investigation of low-pressure hydrogenated SWNT [4­12] and fullerenes [13], and also for the C60 fullerenes hydrogenated under high pressure [14­17]. The present paper describes results of an INS investigation of SWNT and, for comparison, of C60 hydrogenated at 3 GPa and T = 620 K. To analyze the nature of hydrogen bonding in these high-pressure compounds, the spectra were measured for the SWNT & H and C60 & H samples in the quenched state and after partial removal of hydrogen by vacuum annealing.
2. Experimental details
The SWNT was prepared by direct current arc vaporization of graphite/metal composite rods (contained Co/Ni catalyst in a 3:1 mixture). The raw SWNT material was purified by leaching out the metal catalyst with hydrochloric acid followed by oxidation of nontube carbon components by air at 300­600 C,



Corresponding author. Tel.: +1 630 252 3555; fax: +1 630 252 4163. E-mail address: akolesnikov@anl.gov (A.I. Kolesnikov).

0925-8388/$ ­ see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2006.11.207


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similar to the known procedure [18]. The purification procedure yields nanotubes ° about 14 ± 1 A in diameter and about 10 m in length as revealed by electron microscopy observation using HRTEM, SEM, X-ray and neutron diffraction. The C60 sample of 99.99 wt% purity was prepared as described previously [19] and sublimed in a vacuum better than 10-5 Torr at 870 K. The SWNT (weight of 0.6 g) and fullerenes C60 (0.5 g) were hydrogenated at a hydrogen pressure of 3 GPa and 620 K, quenched to 80 K and then studied by INS. INS data were measured using the TOSCA instrument at the ISIS pulsed neutron source [20]. TOSCA is an inverse geometry time-of-flight spectrometer with fixed energy for registered neutrons, Er 4 meV. The data were recorded over a wide range of energy transfer, 3­400 meV, with a resolution of 2% of the energy transferred. The time-of-flight spectra were converted into dynamical structure factor S(Q, E) versus energy transfer using the standard ISIS data analysis programs. The background spectrum for the empty container was measured under the same conditions and subtracted from the original data. INS spectra were measured first on the quenched hydrogenated samples and then on the same samples annealed in vacuum (10 Torr) at 332 K (for 40 min) and then at 623 K (60 min) for SWNT and at 300 K for C60 (90 min). All INS data were collected at 15 K in order to minimize the multiphonon neutron scattering and the Debye­Waller factors. The hydrogen content in the heated C60 Hx was x = 38 (determined by burning the sample in an oxygen flow), and H/C atomic ratio for SWNT & H sample heated at 623 K was 0.56 (or 4.7 wt%) as estimated from the ratio of total intensities of INS spectra for the C60 H38 and the annealed SWNT & H.

3. Results and discussion The measured INS spectra for hydrogenated SWNT and C60 samples are shown in Figs. 1a and 2a. The peak at 355 meV can be assigned to stretching C H modes [16,17], revealing the existence of C H covalent bonds in the quenched and annealed samples. The widths of the peaks are smaller (FWHM 12 meV) for the SWNT & H spectra than for C60 & H (FWHM 20 meV). According to [21], the C H stretching peak should be expected around 360­365 meV for hydrogen connected to sp3 hybrid atom and around 375­380 meV for sp2 hybridization. The observed values for the stretching C H peaks clearly indicate the sp2 sp3 rehybridization in SWNT and C60 during hydrogenation. The bands between 125 and 180 meV are due to bending C H modes [14­17]. There is a clear splitting of the band for two peaks at 146 meV (FWHM 27 meV) and 166 meV (FWHM 9 meV) in the INS spectra for SWNT & H sample (Fig. 1a), and only one broad peak is observed for hydrogenated C60 at 155 meV (FWHM = 25 meV). The clear splitting of the bending mode peak for SWNT & H is due to strong anisotropy of the potential well for covalently bound H, presumably, in the directions along and perpendicular to the SWNT axis. Fig. 3 shows the calculated multiphonon neutron scattering contributions for the INS spectra of the annealed samples. A harmonic isotropic approximation was used with assumption that all hydrogen atoms are similar. A good agreement of the calculated and experimental data in the range of the two-phonon band between 280 and 340 meV in the INS spectrum of SWNT & H indicates that two peaks in the bending mode range (at 146 and 166 meV) originate from neutron scattering on the same hydrogen atoms. In the case that the origin of the two bending peaks arises from hydrogen atoms of different type (placed on different symmetry positions), there would be two peaks at 292 and 332 meV. Thus, we can conclude that covalently bound hydrogen atoms occupy equivalent positions in SWNT & H, presumably

Fig. 1. (a) INS spectra at 15 K of SWNT & H measured for the following sample states: 1-quenched, 2-annealed at 332 K, 3-annealed at 623 K. (b) INS spectra of the molecular H2 in SWNT & H obtained as difference between the spectra for the sample states: 1-quenched and annealed at 623 K, 2-quenched and annealed at 332 K, 3-annealed at 332 and 623 K.

on the outer surface of the tubes, thus forming exohydrogenated SWNT & H. The difference spectra between the quenched and annealed hydrogenated SWNT samples are shown in Fig. 1b. A narrow peak at 14.5 meV in the difference spectra is a direct proof of the presence of molecular hydrogen in the quenched SWNT & H sample as well as in the sample after annealing at 332 K. This peak is close to the energy 14.7 meV of the J =0 J =1 transition for a free H2 rotor, which is the transition of parahydrogen, p-H2 , to ortho-hydrogen, o-H2 . Rotational states for a quantum rotor are characterized by quantum number J and its z-component m. The rotational energy for a free rotor can be described by EJ = BJ(J + 1), where B = 7.35 meV is the rotational constant for hydrogen molecule; and the z-component is not included because the levels are degenerate for a free rotor. Other peaks in the difference spectra for hydrogenated SWNT have small intensity and low statistics to be unambiguously assigned. The content of molecular hydrogen in quenched hydro-


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Fig. 4. INS spectra of the molecular H2 in SWNT & H (curve 1) and in C60 H38 (curve 2) obtained as difference between the INS spectra for the quenched and annealed sample states.

Fig. 2. (a) INS spectra at T = 15 K of the quenched C60 H38 and H2 (curve 1) and that after annealing at 300 K (curve 2). (b) INS spectrum of molecular H2 in C60 H38 obtained as difference between the above curves.

genated SWNT estimated from intensity change in INS spectra after annealing was about 11 wt% compared to covalently bound hydrogen, or 0.5 wt% of initial SWNT. As seen from Fig. 2b, the spectrum of H2 in C60 H38 (the difference between the spectra for the quenched and annealed hydrogenated fullerenes) has two broad peaks with maxima at about 10 and 16 meV. The splitting of the J =0 J = 1 peak for

H2 in hydrofullerenes indicates that the H2 molecules occupy the sites in the C60 H38 unit cell which strongly affect the rotational behavior of hydrogen. Assuming an orientationally dependent potential for H2 in the form V( )= V0 (1 - cos(2 )) [22] and using the shifts of rotational energy levels calculated in [22],we can estimate the strength of the orientational potential for H2 in C60 H38 as being about V0 = 15 meV. In addition, the H2 spectrum in C60 H38 shows a broad intense peak at 31 meV which can be attributed to the J =1 J = 2 transition due to the presence of oH2 (the corresponding value for free H2 rotor is 29.4 meV). The absence of a clear peak at 29.4 meV (see Figs. 1b and 4) in the difference spectra for SWNT indicates a negligible amount of o-H2 in the quenched SWNT & H sample. This is probably due to the presence of residual catalytic particles in SWNT, which accelerates conversion of o-H2 to p-H2 at low temperature. From the 14.5 meV peak intensity change in the INS spectrum of SWNT & H (Fig. 1) we can conclude that about 65% of H2 molecules left the sample after the first annealing (at 332 K), but the remaining H2 molecules (35%) were still contained in SWNT & H. After the second annealing at 623 K there were nearly no H2 molecules left in the sample. The presence of the peak at 14.5 meV after annealing of SWNT & H at 332 K is a direct proof of the existence of molecular hydrogen in SWNT & H sample at temperatures above room temperature (59 C). In the case of hydrogen adsorption in SWNT at lower pressures (<15 MPa) and temperatures (<300 K), hydrogen molecules completely leave the sample after heating to room temperature in vacuum. 4. Conclusions Our measurements show that hydrogenated SWNT and fullerenes quenched to 80 K under high hydrogen pressure (P = 3 GPa) consist of SWNT & H or C60 H38 molecules with covalently bound hydrogen, and interstitial molecular hydrogen that leaves the sample on heating. The molecular hydrogen is

Fig. 3. INS spectra for the annealed states of SWNT & H (a) and C60 H38 (b) samples; the calculated multiphonon neutron scattering contributions are shown by dashed curves.


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almost rotationally free in SWNT & H, but it exhibits a highly perturbed rotational behavior in the hydrofullerenes. Acknowledgements The work performed at the IPNS was supported by the Office of Basic Energy Sciences, Division of Materials Sciences, US Department of Energy, under Contract No. W-31-109-ENG-38; and the work done at ISSP RAS was supported by the Russian Foundation for Fundamental Research, grant No. 06-02-17426, and by the Russian federal program "Research and development in the priority directions of science and engineering", contract No. 661-05 "Development of hydrogenation technique for fullerenes and carbon nanotubes". We would like to thank the ISIS at Rutherford Appleton Laboratory for the use of neutron beam time. References
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