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Nanomaterials 2022, 12, 3528 7 of 13 Accordingly, we reasoned that the unobstructed ultra-short CNTs would show a reduced incidence of blockage by salt aggregation (Figures 5a and S2) or other blocking materials (Figure 3a–c). Figure 5c shows an HAADF-STEM image of an ultra-short MWCNT that was soaked in 0.14 M KCl solution. Salt aggregations filled nearly the entire ultra-short CNT. Elemental mappings showing the distribution of the elements C, O, K and Cl of the salt aggregation in the ultra-short CNT are also illustrated. The signal for C was the same shape as the CNT. The signals for K and Cl were high-intensity and located in the middle of the range of the C signal, and their widths were similar to the inner diameter of the CNT. Furthermore, the O signal, mainly from water inside the tube and functional groups on the tube, had a dispersive width equal to that of the C signal and was a little higher in the middle. Thus, the EDS mapping of the ultra-short CNT demonstrated that the tube was filled with high-concentration KCl solution. Figure 5d shows the EDS results of the aggregations demonstrating that C, O, K and Cl were present. Furthermore, 16 salt aggregations in ultra-short CNTs were randomly selected and their K/O atomic ratios are shown in Figure 5e, indicating that the concentrations of these salt aggregations might be at least 1–2 orders of magnitude higher than that in the solution outside the CNTs. The K/C mass ratios of the aggregations were also tens of times higher than that in the solution outside the CNTs (see more details in Figure S3). These results demonstrate that the ultra-short CNTs could enrich ions in a dilute solution. The high concentrations of the salt solutions inside the short CNTs should be attributed to the hydrated-cation–π interactions between the hydrated cations and the π electrons in the aromatic rings of the CNTs [42]. Most carbon-based nanochannel surfaces, such as CNTs, comprise aromatic hexagonal carbon rings rich in π electrons [47]. The non-covalent interactions between a cation and a π-electron-rich carbon-based structure are referred to as cation–π interactions [48]. The polycyclic aromatic ring structure of CNTs is very rich in π electrons, so the cation–π interactions inside are strong enough to cause adsorption of hydrated cations [42]. The confine effect of CNT channels to the solution or water [42,49,50] kept the enriched salts inside the CNTs when the nanotubes left the dilute solutions. The formation of salt blockages further enhanced the stability of these enriched salts inside the CNTs [42]. This indicated that ion adsorption inside the CNTs had certain advantages over that of the outside, due to the fact that the outside surface of raw CNTs without any functional groups usually exhibits weak ion adsorption [51]. We hold the opinion that when the CNTs were moved out from the dilute solutions, water flowed away from the hydrophobic outside surface of the CNTs [52] and took away adsorbed ions outside the nanotubes. The filling capacity of these ultra-short CNTs can be described in terms of the filling length to total CNT length ratio, i.e., the length of the salt aggregation divided by the length of the corresponding CNT. The length ratios for 16 random ultra-short CNTs were analyzed, and the results are shown in Figure 5f. The highest value was 0.67 and the average for the 16 samples was ~0.49, which is much better than the value of 0.03 for the long CNTs (see more details in Figure S4). Thus, the ultra-short CNTs had a much better filling capacity than that of the long CNTs, as commonly evidenced by the fact that long CNTs filled with guest materials can be partially or even completely empty (Figure 5b) [13,36,37,42]. This highly improved filling capacity of the ultra-short CNTs was ascribed to the decreased incidence of blockage. Figure 6 shows a schematic comparing the filling capacity of raw long CNTs and ultra-short CNTs. The orange and light blue regions in the schematic represent positions that cannot be filled and that can be filled with ion aggregations, respectively. The long CNT is blocked by residual catalyst, amorphous graphite and internal defects, while the ultra-short CNTs have a higher orifice density, shorter length and unobstructed channels. Here, the orifice density of a CNT can be defined as the number of openings per unit mass of CNTs.PDF Image | Ion Enrichment inside Ultra-Short Carbon Nanotubes
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