A group of researchers recently published a paper in the journal Carbon that demonstrated a scalable method to synthesize single-walled carbon nanotube (SWCNT)-based coaxial cables that can be effectively used to develop novel one dimensional (1D) van der Waals (vdW) heterojunctions.
Study: Space-confined synthesis of SWNT bundles wrapped by MoS2 crystalline layers as flexible sensors and detectors. Image Credit: ustas7777777/Shutterstock.com
Significance of CNTs for Creating 1D Heterojunctions
1D nanostructures such as CNTs and semiconducting nanowires are considered suitable for the fabrication and design of different functional vdW and core-shell heterojunctions that have significant potential in various applications in catalysts, conductors, batteries, and nanoelectronics.
Among the 1D nanostructures, CNTs can be assembled into macroscopic configurations, such as sponges and films, and then merged with other materials to prepare coaxial structures. For instance, polymers or metals are wrapped or deposited on CNT fibers to improve the conductivity and strength of the fibers and fabricate functional devices such as wearable and high-sensitivity strain sensors.
Following a similar approach, a porous CNT sponge can be coated with various transitional metal dichalcogenides, such as tin sulfide (SnS2) and molybdenum disulfide (MoS2), through the in situ hydrothermal methods to synthesize a sequence of coaxial heterostructures that can serve lithium-ion battery electrodes and strain sensors.
Limitations of SWCNTs in the Fabrication of 1D Heterojunctions
SWCNTs represent the ideal template for the construction of 1D heterojunctions owing to their small diameter of fewer than 2 nanometers and exceptional electrical and mechanical properties such as tunable conducting behavior, excellent flexibility, and ultrahigh modulus and strength. However, uniformly wrapping the two-dimensional (2D) nanosheets or crystalline layers on SWCNTs is still very challenging owing to the large curvature and small diameter of SWCNTs.
Currently, other materials are typically attached to SWCNTs in place of forming a coaxial structure owing to these challenges. However, a number of SWCNT-based 1D vdW heterojunctions were recently fabricated successfully. For instance, SWCNT@ boron nitride nanotube (BNNT)@MoS2 coaxial nanotubes were prepared by directly growing BNNT and MoS2 in a sequence using the chemical vapor deposition (CVD) method. However, the yield of SWCNT@BNNT@MoS2 obtained by the method was very poor. Thus, scalable and reliable synthesis methods must be developed to fabricate 1D heterojunctions with greater versatility and in large numbers.
Synthesis of SWCNT Bundles and SWCNT@MoS2 Films
In this study, researchers synthesized SWCNT@MoS2 films by wrapping MoS2 crystalline layers around SWCNT bundles through the hydrothermal method. The SWCNT bundles were synthesized through the space-confined method. Hydrothermal deposition represents a scalable and simple method to synthesize a large number of highly crystalline MoS2-coated SWCNT bundles. The tunable MoS2 sheets can be used to coat an extensive range of SWCNT bundle sizes.
Ethanol, hydrochloric acid, ammonium molybdate, sulfuric acid, thiourea, ferrocene, 1,2-dichlorobenzene, sublimed sulfur, and xylene were used as the starting materials.
Initially, the floating catalyst CVD method was used to synthesize the SWCNT membrane at 1200o Celsius. Sulfur/ ferrocene was used as catalysts and xylene was utilized as the precursor during the SWCNT synthesis. The atmospheric pressure CVD method was used to synthesize the multi-walled CNT (MWCNT) sponge at 860o Celsius. Ferrocene and 1, 2-dichlorobenzene were utilized as the catalyst and carbon precursor, respectively.
Two MWCNT porous blocks were fabricated by cutting an MWCNT sponge and then bounding them with SWCNT fibers to prevent dispersion. Subsequently, the SWCNT film was sandwiched between the MWCNT porous blocks and the sandwiched SWCNT film was submerged in 100 mg/mL ammonium molybdate solution for 30 minutes. The sandwiched film was freeze-dried and again immersed in a melt thiourea bath for 30 minutes at 200o Celsius.
Eventually, the sample was placed in a Teflon-line autoclave for 12 hours at 120o Celsius to obtain a wrapped amorphous MoS2 layer. The final sandwiched sample was annealed for four hours at 800o Celsius under the protection of argon flow in order to improve the crystallinity of the MoS2 before collecting the SWCNT@MoS2 film. MoS2 loadings from in SWCNT@MoS2 film were controlled by varying the concentrations of the molybdenum precursor.
Characterization and Evaluation of the Synthesized Samples
X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), thermogravimetric analysis, and energy dispersive X-ray spectroscopy (EDX) were performed to characterize the synthesized samples. Researchers also performed photoresponse tests, mechanical measurements, and gas sensing measurements to evaluate the samples.
Significance of the Study
SWCNT bundles and flexible, ultrathin, and large area SWCNT@MoS2 films were synthesized successfully through the porous MWCNT sponge-enabled space-confined method and hydrothermal method, respectively. The crystalline multi-walled MoS2 sheets were formed continuously and uniformly around the SWCNT bundles. Tunable MoS2 layers wrapped around the SWCNT bundles with sizes ranging from several to hundreds of nanometers.
The co-axial SWCNT@MoS2 heterojunctions synthesized in this study were completely different from the previously fabricated multi-walled nanotube or SWCNT-based 1D vdW structures as these SWCNT@MoS2 films retained their original structural flexibility.
The SWCNT@MoS2 samples displayed significantly enhanced gas sensitivity and light response compared to pristine SWCNT films due to the combination of electrical and optical properties of both MoS2 and SWCNT in the films, which make them suitable and effective as flexible photodetectors and toxic gas sensors. The formation of 1D heterojunctions and uniform MoS2 coating were collectively responsible for the changes in current due to gas absorption or light illumination.
To summarize, the findings of the study demonstrated that the method used to synthesize the SWCNT@MoS2 films in this study can also be used to effectively synthesize different 1D heterojunction nanotube structures using more transition metal dichalcogenides and other materials and to assemble flexible large-area optoelectronic and electronic devices.
Li, H., Li, M., Li, Y. et al. (2022) Space-confined synthesis of SWCNT bundles wrapped by MoS2 crystalline layers as flexible sensors and detectors. Carbon. https://www.sciencedirect.com/science/article/pii/S000862232200272X?via%3Dihub