Unusual melting transition of nitrogen physisorbed on carbon nanotube bundles

Abstract

The melting transition of nitrogen physisorbed on close-ended single-wall nanotube bundles was investigated using synchrotron X-ray diffraction measurements. The beta-nitrogen solid diffraction peak was observed above the coverage that corresponded to the monolayer and the average size of the nitrogen solid was approximately 30 angstrom. The diffraction peak was surprisingly maintained above the triple point of the bulk nitrogen solid. The crystal structure of N-2 changed from cubic N-2 (beta-phase) to hexagonal N-2 (beta-phase) at 35.61 K. The melting temperature of the nano-scale solid nitrogen in the experiment was between 80 K and 90 K, however, which is about 20 K higher than the melting temperature of normal bulk nitrogen. The observed extraordinary melting behavior of nitrogen might originate from a combination of two factors, i.e., the substrate field effect of the carbon nanotube surface (the interaction between the single walled carbon nanotubes and the adsorbates) and the capillary condensation. If the substrate field effect is especially prominent, the nitrogen molecules that were adsorbed mainly in the groove region would be under 1,100-Torr pressure from the nanotube bundles, compared to the corresponding melting temperature of the bulk beta-nitrogen solid under a high pressure.