Effects of supercritical-CO2 treatment on the pore structure and H2 adsorptivity of single-walled carbon nanohorns

We investigated the effects of supercritical-CO₂ treatment on the pore structure and consequent H₂ adsorption behavior of single-walled carbon nanohorns (SWCNHs) and SWCNH aggregates. High-resolution transmission electron microscopy and adsorption characterization techniques were employed to elucidate the alterations in the SWCNH morphology and aggregate pore characteristics induced by supercritical-CO₂ treatment. Our results confirm that supercritical-CO₂ treatment reduces the interstitial pore surface area and volume of SWCNH aggregates, notably affecting the adsorption of N₂ (77 K), CO₂ (273 K), and H₂ (77 K) gasses. The interstitial porosity strongly depends on the supercritical-CO₂ pressure. Supercritical-CO₂ treatment softens the individual SWCNHs and opens the core of SWCNH aggregates, producing a partially orientated structure with interstitial ultramicropores. These nanopores are formed by the diffusion and intercalation of CO₂ molecules during treatment. An increase in the amount of H₂ adsorbed per interstitial micropore of the supercritically modified SWCNHs was observed. Moreover, the increase in the number and volume of ultramicropores enable the selective adsorption of H₂ and CO₂ molecules. This study reveals that supercritical-CO₂ treatment can modulate the pore structure of SWCNH aggregates and provides an effective strategy for tailoring the H₂ adsorption properties of nanomaterials.