BACKGROUND

Since their discovery in 1991, carbon nanotubes (CNTs) have remained one of the most attractive nanomaterials. The enduring interest of researchers and scientists towards CNTs over the past three decades relies on numerous findings that have proven the unique and superior physical properties of CNTs. For example, multi-walled CNTs possess an exceptionally high Young's modulus in the terapascal (TPa) range and a thermal conductivity of more than 3000 Watts per Kelvin per meter (WK−1m−1) at room temperature. Similarly, they exhibit metallic or semiconducting properties depending upon their diameter and helicity and can pass a high current density up to 109Amps per square centimeter (Acm−2) without any structural failure. Besides the intrinsic properties owned by CNTs, many strategies have been put forward to further enhance them with an aim to broaden their real-life applications. Among many, the filling of the CNT interior remains an attractive method to synergize the qualities of both the CNT and that of the filling material to bring about a novel nanohybrid, known as a filled CNT. This is possible due to the unique tubular structure of CNT that offers the possibility of filling its hollow core with a variety of inorganic and organic materials.

Related art methods for filling CNTs are ex situ (post-synthesis filling) methods. In addition, when it comes to real applications of metal-filled CNTs, it is important to separate metallic CNTs and semiconducting CNTs. Related art methods to attempt to do this include dielectrophoresis, gel-based adsorption, chemical treatment, and microwave irradiation.

BRIEF SUMMARY

Embodiments of the subject invention provide novel and advantageous filled carbon nanotubes (CNTs) and methods of synthesizing the same. In situ methods (e.g., chemical vapor deposition techniques) can be used to synthesize CNTs (e.g., multi-walled CNTs) filled with metal sulfide (e.g., nickel sulfide, iron sulfide, cobalt sulfide) nanowires. The CNTs can be completely (or nearly completely) and continuously (or nearly continuously) filled with the metal sulfide fillers (e.g., metal sulfide nanowires, such as single crystalline nanowires) up to several micrometers in length. The filled CNTs can be synthesized on a carbon substrate (e.g., a carbon cloth). The filled CNTs grown on the carbon substrate (e.g., carbon cloth) have excellent electrical conductivity, as well as high lithium storage and excellent cyclability when used as anode materials for lithium-ion batteries (LIBs). In addition, the filled CNTs grown on the carbon substrate (e.g., carbon cloth) can also be used as an anode materials for sodium-ion batteries (SIBS) and have great potential to develop high-performance SIBs. The allowed maximum electrical current in the filled CNTs is size-dependent so the filled CNTs can advantageously be used as nano breakers for electronic devices.

In an embodiment, a method of synthesizing filled CNTs can comprise: providing a carbon-based substrate in a reaction chamber; disposing a catalyst comprising a metal on the substrate; providing a sulfur-containing organic compound to the reaction chamber; and performing a chemical vapor deposition technique to simultaneously grow CNTs and fill the CNTs with a metal sulfide comprising the metal, thereby synthesizing the filled CNTs filled with the metal sulfide. The carbon-based substrate can be carbon cloth, graphene, graphite, graphene oxide, carbon block, carbon fiber threads, carbon nanotube films, a carbon-coated substrate, a graphite-coated substrate, or a carbon nanotube film-coated substrate. The metal can be, for example, nickel, cobalt, or iron. For example, the metal can be nickel and the metal sulfide can be Ni3S2. The sulfur-containing organic compound can be, for example, thiophene, dimethyl sulfide, or thiourea. The providing of the sulfur-containing organic compound to the reaction chamber can comprise: providing a supply of the sulfur-containing organic compound outside the reaction chamber; and sending a gas through the supply of the sulfur-containing organic compound and into the reaction chamber such that the sulfur-containing organic compound is bubbled into the reaction chamber with the gas. The method can further comprise: before providing the sulfur-containing organic compound to the reaction chamber, heating the reaction chamber having the catalyst disposed on the carbon-based substrate to a first predetermined temperature for a first predetermined amount of time while flowing an inert gas through the reaction chamber; passing hydrogen gas through the reaction chamber to reduce the catalyst; and after reducing the catalyst, stopping the flowing of the hydrogen gas through the reaction chamber while continuing to flow the inert gas through the reaction chamber while heating the reaction chamber to a second predetermined temperature. The method can further comprise: at the second predetermined temperature, flowing the inert gas at a first predetermined flow rate while flowing the hydrogen gas through a bubbler containing the sulfur-containing organic compound, for a second predetermined amount of time; and stopping the hydrogen gas, reducing the inert gas to a second predetermined flow rate lower than the first predetermined flow rate, and cooling down the reaction chamber naturally to room temperature to complete the synthesis of the filled CNTs. The method can further comprise collecting the filled CNTs via ultrasonication. The carbon-based substrate can be in pristine form immediately prior to the catalyst being disposed thereon, such that no pre-treatment process (e.g., heat pre-treatment process, chemical activation pre-treatment process, or plasma pre-treatment process) is performed on the carbon-based substrate prior to the catalyst being disposed thereon. The CNTs can have an average filling ratio (the (average) proportion of an individual CNT that is filled as a fraction of the total CNT) of the metal sulfide of, for example, at least 0.90. The CNTs can have an average filling rate (the proportion of CNTs that are filled as a fraction of all CNTs) of the metal sulfide of, for example, at least 0.90. The catalyst can be nanoparticles (NP) of the metal, and the method can further comprise: before disposing the catalyst on the substrate, dispersing the NPs in a solvent and putting the solvent in an ultrasonication bath to achieve a uniform suspension of the NPs in the solvent; immersing at least a portion of the carbon-based substrate in a container having the suspension of the NPs in the solvent and leaving the at least a portion of the carbon-based substrate dip coating in the container while stirring the container using a magnetic stirrer to achieve a uniform coating of the catalyst on the at least a portion of the carbon-based substrate; and transferring the at least a portion of the carbon-based substrate to a heating apparatus and heating the at least a portion of the carbon-based substrate to evaporate the solvent. The catalyst can be a salt of the metal, and the method further can further comprise: before providing the sulfur-containing organic compound to the reaction chamber, heating the reaction chamber to a first predetermined temperature for a first predetermined amount of time to oxidize the catalyst; after oxidizing the catalyst, flowing an inert gas through the reaction chamber and passing hydrogen gas through the reaction chamber to reduce the catalyst; and after reducing the catalyst, stopping the flowing of the hydrogen gas through the reaction chamber while continuing to flow the inert gas through the reaction chamber while heating the reaction chamber to a second predetermined temperature. The disposing of the catalyst on the carbon-based substrate can comprise: preparing a catalyst solution by mixing a powder of the salt of the metal in a solvent; providing the catalyst solution to the carbon-based substrate; and performing a two-stage spin coating to form a uniform coating of the catalyst on a surface of the carbon-based substrate.

In another embodiment, a compound can comprise: a carbon-based substrate; a CNT disposed on the carbon-based substrate; and a nanowire of a metal sulfide filled in the CNT. The carbon-based substrate can be carbon cloth, graphene, graphite, graphene oxide, carbon block, carbon fiber threads, carbon nanotube films, a carbon-coated substrate, a graphite-coated substrate, or a carbon nanotube film-coated substrate; the CNT filled with the nanowire can be straight from a first end thereof to a second end thereof; and/or each of the first end and the second end of the CNT filled with the nanowire can be closed with respective carbon shells. The metal sulfide can be, for example, nickel sulfide, cobalt sulfide, or iron sulfide. For example, the metal sulfide can be Ni3S2.

In another embodiment, a battery can comprise: a cathode (e.g., a cathode comprising lithium and/or sodium); an anode comprising a compound as described herein (e.g., a carbon-based substrate with at least one metal sulfide-filled CNT (e.g., Ni3S2-filled CNT(s)) disposed thereon, or an anode comprising a current collector comprising at least one metal sulfide filled CNT (e.g., Ni3S2-filled CNT(s))); and an electrolyte in contact (e.g., in direct physical contact) with the cathode and/or the anode (e.g., disposed between the anode and the cathode). The carbon-based substrate can be, for example, carbon cloth. The battery can be a lithium-ion battery or a sodium-ion battery.

