Single-walled carbon nanotubes (SWCNTs) have concave and convex graphene surfaces. Theoretical studies predict that the hydrogen physisorption energy is considerably enhanced on such concave surfaces. In this study, we have investigated the adsorption energy of H2 and D2 on SWCNTs with and without defects by thermal desorption spectroscopy (TDS) under an ultra-high-vacuum (UHV) condition.
SWCNTs were synthesized by an arc discharge method with Fe and Ni catalysts. After purification of the samples, we confirmed that the residual Fe and Ni catalysts were less than 0.25wt% by ICP-OES (Inductively coupled plasma - optical emission spectroscopy) and that the samples contained more than 80wt% SWCNTs in the soot by TGA (thermogravimetric analysis). We derived the diameter of SWCNTs to be 1.5 nm from the frequency of the radial-breathing mode measured by Raman scattering spectroscopy. In order to analyze the adsorption site of hydrogen molecules, two different samples were prepared: one is non-heated, and the other is heated at 723 K in air. TEM measurements revealed that some defects were introduced in heated-SWCNTs. After the samples were exposed to H2 at a sample temperature of 13K, we measured TDS spectra from the two SWCNTs. TDS spectra from the non-heated SWCNTs revealed a sharp peak at 19.4 K. On the other hand, two peaks appeared at 20.5 and 26.8 K in the TDS spectra from the heated-SWCNTs. The lower-temperature peak was broadened compared to the non-heated sample's one. On the basis of the Redheadfs formula, we estimated the adsorption energy of H2 corresponding to the three desorption peaks to be 51.6, 54.6 and 72.0 meV. Since introduction of defects to SWCNTs allow molecules access to the inner part of the tubes, the high-temperature peak is attributed to either inner-wall or interstitial sites of SWCNTs. The sharp low-temperature peak observed for the non-heated sample, on the other hand, is unambiguously assigned to the groove site of the SWCNT bundles. We furthermore attribute the broadening of the peak around 20K to the effect of defects such as surface reconstruction.
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