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

the Section "INTRODUCTION" of the review. The investigation of the morphological and electronic
structure of filled CNTs heterostructures using transmission electron microscopy and spectroscopic
techniques forms the focus of the Section "FILLING METHOD AND MECHANISM" of the review. The
S e c t i o n " M O R P H O L O G I C A L A N D E L E C T R O N I C S T R U C T U R E O F F I L L E D C N T S
HETEROSTRUCTURES" of the manuscript reviews potential applications of filled CNTs heterostructures
in a different field. Finally, we put forward a view on the possible opportunities and challenges of filled
CNTs heterostructures.

FILLING METHOD AND MECHANISM
Successful filling of gas, liquid, and solid materials into the hollow nanospace of the CNT is possible, and
[27]
the filling methods are categorized as in situ (filling of CNT during their growth) and ex-situ (filling of
pre-formed CNT) . The mechanism of filling materials inside CNTs is mainly due to the unique tubular
[28]
structure of CNTs, which provides a sealed space for encapsulating materials inside. The diameter of CNTs
can be controlled to match the size of guest molecules, thereby effectively trapping them inside. In addition,
the strong van der Waals force between guest molecules and the inner wall of CNTs also helps to limit the
effect and prevent molecules from diffusing outward. Depending on the properties of the filled materials,
the vast majority of filled CNTs heterostructures are made using the ex-situ method, which primarily
includes encapsulation from the gas or liquid phases, as well as sequential transformations in the cavity after
pre-encapsulation. According to some studies, the adsorption effect of the lumens of carbon nanotubes is
[29]
the main reason for filling the gas molecule . The principle of filling liquids and solids is mainly based on
capillary and wetting effects . According to Young’s equation and Laplace equation theory, the force
[30]
[31]
between the liquid and the inner surface of the carbon tube must be large enough to allow it to infiltrate .
During the filling process, it is inevitable that the CNTs will have indirect contact or a surface coating with
the guest material, and necessary post-processing, such as cleaning with appropriate solvents, is required to
maximize the elimination of residual substances outside the tube that may affect the system.

In-situ filling
In-situ filling is filling CNTs directly with foreign material while they are being synthesized. As a result, the
carbon nanotubes can be kept intact with a high fill rate, effectively isolating the encapsulation material
from the surrounding environment. However, the in situ filling method yields a low filling yield, and some
impurity elements will enter the interior of the carbon nanotubes during the filling process . In situ filling
[32]
mainly includes arc discharge and molten salt electrolysis .
[33]
[34]
The arc discharge method is also called the graphite arc method. The vacuum reactor is filled with inert gas
or hydrogen. Graphite rods of different sizes are used as electrodes. During the growth and filling process,
the graphite rods at one end are gradually consumed, and the carbon nanotubes grow at the other end. This
method was used by Guerret-Piécourt et al. to create element-filled CNTs in situ .
[33]

Molten salt electrolysis is a method in which an inorganic salt is heated to melt into a liquid electrolyte in a
carbon crucible for electrolysis to fill CNTs, which was first proposed by Hsu et al. They used this method
to fill CNTs with Li C , LiCl, Sn, Pb, Bi, and Sn-Pb alloy [34,35] .
2 2
Ex-situ filling
Ex-situ filling refers to the process of filling guest substances into the pre-synthesized carbon nanotubes,
which is the most widely used approach to filled nanotubes preparation. According to the physical
properties of the filled materials, they are introduced into the cavity of CNTs in the form of liquid or steam,
corresponding to the liquid phase method and the gas phase method , respectively. In order to fill the
[28]