Dose-dependent isotherm of Kr adsorption on heterogeneous bundles of closed single-walled carbon nanotubes

TL;DR

This study investigates how krypton adsorption on heterogeneous bundles of closed single-walled carbon nanotubes varies with dose protocols, revealing metastable states and phase formation influenced by microstructure and dosing methods.

Contribution

It provides detailed insights into dose-dependent adsorption behaviors and phase formation of krypton in heterogeneous SWCNT bundles, highlighting the impact of microstructure and dosing protocols.

Findings

Adsorption isotherms depend on dosing protocol (IAD vs CAD).

Substeps in isotherms indicate phase formation of confined Kr.

Site population varies with adsorption dose and protocol.

Abstract

We present 77 K isotherms of krypton adsorption on bundles of closed highly-pure HiPco single-walled carbon nanotubes (SWCNTs). Two volumetric adsorption protocols were used, one with an increasing Kr dose per injection (IAD), one with a constant dose (CAD). Detailed microstructural examination showed that the SWCNTs combine into small bundles (of 25-30 SWCNTs) which are heterogeneous in diameter with a consequential range of interstitial channel (IC) shapes and sizes. The IC-sites are the subnanoscaled pores with alternating enlargements and constrictions along the tube axes. This results in adsorption dosing (AD) dependent characteristics of the low-pressure region of the isotherm. In the IAD protocol the switch-back behavior of the isotherm stemmed from metastable adsorption. Using the CAD protocol, different branches are observed. Well-pronounced substeps were established which we interpret as corresponding to the formation of various phases of confined Kr with different atoms arrangement. The height of a given substep obtained in different measurements depends on the AD value which can strongly influence the population of the site. Some substeps existing only for certain values of AD suggests the existence of a certain selectivity or of a preferential phase formation according to this value.