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Nanomaterials 2022, 12, 3528 2 of 13 limitations [30] associated with small metal domains. Compounds that do not catalyze the growth of CNTs are hard to be filled in situ in the nanotubes [13]. In addition, in situ filling methods remain unfeasible for filling single-walled carbon nanotubes (SWCNTs) and double-walled carbon nanotubes (DWCNTs) [31–33]. Ex situ filling methods have relatively simple operation processes and are suitable for a large range of filling species, thus mitigating the limitations of in situ filling methods [13,30]. There are two common ex situ filling methods. The first is a physical method called ‘capillary-induced filling’ developed by Ajayan and Iijjima [34], which achieves filling by annealing a precursor at a temperature above its melting point. The second is a wet chemical method developed by Tsang et al. [35], which utilizes acidic treatment to open the caps of CNTs, allowing entry to a precursor solution, which may then be transformed chemically by heating or reduction [35,36]. Of these methods described above, the wet chemical method is the most widely used in scientific research due to its relatively simple operation and due to it having the largest filling material range [13,14]. Unfortunately, the wet chemical methods often exhibit low filling efficiency. For exam- ple, D. Ugarte et al. point out that although the opening procedures have been optimized, the metal nitrate filling efficiency into their opened long tubes is rather low (2–3%) [36]. Most of the filling materials exist as particles or short rods and distribute periodically far away from the next one along the nanotube [24,25,37], indicating that the filling efficiency is low. As a kind of ex situ method, filling CNTs using the wet chemical method generally involves two steps: (1) opening the closed end caps, and (2) filling the CNTs [30]. Con- ventional wisdom holds the opinion that a good opening is the precondition of solutions entering CNTs, and many methods to open CNTs have been developed, such as gas-phase oxidation [38,39] and liquid-phase oxidation [36,40,41]. However, these methods still often exhibit very low filling efficiencies, providing CNTs that can be partially or even completely empty [13,42]. To date, the CNT-filling efficiency for a large ion range using wet chemical methods cannot be satisfactory based on these current opening strategies [36,37,39]. In addition to end caps, blockages from the mid-sections of CNTs can occur, which are more difficult to remove. Removing blockages is a significant problem that needs to be resolved, but it has not been discussed in detail in previous works [13,14,30]. CNT blockages can be residual catalyst materials from CNT fabrication [43], amorphous graphite generated during the treatment of CNTs, internal defects, or accumulated filling materials [42]. These blockages hinder the movement of solutions inside CNTs, and result in part of the CNTs among the blockages being empty. It stands to reason that a blockage in the mid-section of a CNT becomes increasingly difficult to remove completely as the length of the CNT increases. Thus, shortening CNTs and then removing blockages is a simple but very useful strategy to fabricate unobstructed CNTs and can further enhance the filling capacity of CNTs. Accordingly, in the present work, ultra-short CNTs were prepared by ball milling, and the blockages inside the CNTs were efficiently removed using acid pickling. Ball milling has been reported to be a high-efficiency method to modify the length, particle size distribution, hydrogen adsorption, specific surface area and dispersion capacity of CNTs [43–45]. Acid pickling is a common method used to efficiently remove amorphous graphite, metals and other impurities inside CNTs [13,14,39]. Our previous work demonstrated a useful wet chemical method to fill CNTs, where ions can accumulate inside wide CNTs immersed in very dilute salt solutions [42]. Therefore, the ion-enrichment capacity of ultra-short CNTs soaked in dilute salt solutions was investigated using transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). 2. Materials and Methods 2.1. Preparation of Ultra-Short CNTs Raw long multi-walled carbon nanotubes (MWCNTs) with an average diameter of ~6 nm and an average length of 20 μm, and another kind of MWCNTs with diameters of 20–50 nm and an average length of 20 μm were purchased from Beijing DK Nano Technology Co., Ltd. These two kinds of MWCNTs were sonicated in a 3:1 (v/v) mixturePDF Image | Ion Enrichment inside Ultra-Short Carbon Nanotubes
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