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Page 4 of 29 Teng et al. Microstructures 2023;3:2023019 https://dx.doi.org/10.20517/microstructures.2023.07

[36]
CNTs, their ends need to be opened first. Heat treatment in an oxidizing environment, such as H O , O ,
2
2
2
[37]
[40]
O , HNO , H SO , KMnO , or Br is mainly used opening approaches. To obtain a more pure CNT,
[39]
[38]
2
4
4
2
3
3
combined thermal treatment and concentrated acid treatment are necessary to remove the various
[41]
contaminants, such as amorphous carbon, graphite electrode particles, and catalyst .
Filling of CNTs from the liquid phase entails impregnating the opened tube with melts or solutions of target
compounds. Although many examples of liquid-phase filling of carbon nanotubes have been reported, the
mechanism has not been determined. We assume that liquid-phase filling of carbon nanotubes is a so-called
[30]
capillary wetting phenomenon , then the Young and Laplace equations can be used to describe the
mechanism :
[31]
where ΔP represents the pressure change, β represents the surface tension coefficient, and R and r represent
re
two different curvature radii of the surface, respectively. When the liquid level in the nanotube rises, it
saturates the tube wall and forms a meniscus at the top [Figure 1].
The meniscus is approximately regarded as a spherical shape, and the liquid is in contact with the capillary
wall at a certain angle θ, then the following formula is obtained :
[31]

where Δρ is the density difference between two phases.

According to the equation, when the contact Angle exceeds 90°, external forces are needed to drive the rise
of capillarity. When the contact angle is below 90°, spontaneous filling will occur. Surface tension β is the
force of liquid surface shrinkage. Liquids with low surface tension are more likely to spread through the
tube, which is manifested by the smaller contact angle and the reduced driving force required for
spontaneous filling. β of some common solvents and metal halides packed into carbon nanotubes at the
melting point has been tested or calculated as follows (unit: mN/m): water (72.8), methanol (22.07), acetone
(23.46), tetrachloromethane (26.43) , AgCl (113-173), AgBr (151), AgI (171), KCl (93), KI (70), NdCl
[42]
3
[43]
(102), ZnCl (1.3), PbO (132), V O (53), etc. The filling of salts such as metal halides and oxides is often
4
2
5
carried out in this method, such as KI, AgI, and Sb O filled by Sloan’s group . The following table [45-66] is a
[44]
3
2
list of various materials that have been reported in the literature for filling CNTs in recent years.
Vacuum and high temperatures are used to fill CNTs from the gas phase. An encapsulated material is
heated inside a sealed tube until it vaporizes (or sublimates), but keep the temperatures as low as possible to
[35]
prevent (or reduce) de-encapsulation . The vapor of the compound that is enclosed during CNTs
annealing enters the nanotube via capillary condensation and crystalizes during subsequent cooling. During
the gas phase filling process, no other substances are introduced; thus, there is no pollution to the
environment . The major defect of this approach, however, is also obvious. First, the reaction temperature
[67]
should be less than 1,000 °C to avoid destroying the CNTs or reacting with carbon to close the CNT’s
ends . Second, the filled substances are typically discrete in the hollow nanospace of CNTs, which makes it
[67]
[68]
difficult to control the filling yields . As a result, the gas phase filling approach is appropriate for molecule
crystals containing inorganic clusters, organic molecules, and complexes with low boiling or sublimation
temperatures. The filling systems for fullerenes and their derivatives are typical examples.
[69]
[61]