Controlled synthesis, properties, and applications of ultralong carbon nanotubes

Abstract

Carbon nanotubes (CNTs) are typical one-dimensional nanomaterials which have been widely studied for more than three decades since 1991 because of their excellent mechanical, electrical, thermal, and optical properties. Among various types of CNTs, the ultralong CNTs which have lengths over centimeters and defect-free structures exhibit superior advantages for fabricating superstrong CNT fibers, CNT-based chips, transparent conductive films, and high-performance cables. The length, orientation, alignment, defects, cleanliness, and other microscopic characteristics of CNTs have significant impacts on their fundamental physical properties. Therefore, the controlled synthesis and mass production of high-quality ultralong CNTs is the key to fully exploiting their extraordinary properties. Despite significant progress made in the study of ultralong CNTs during the past three decades, the precise structural control and mass production of ultralong CNTs remain a great challenge. In this review, we systematically summarize the growth mechanism and controlled synthesis strategies of ultralong CNTs. We also introduce the progress in the applications of ultralong CNTs. Additionally, we summarize the scientific and technological challenges facing the mass production of ultralong CNTs and provide an outlook and in-depth discussion on the future development direction.

Fig. 1. Growth mechanism, controlled synthesis, and excellent properties of ultralong CNTs. Reproduced from ref. with permission from Wiley-VCH, copyright 2023. Reproduced from ref. with permission from American Chemical Society, copyright 2023. Reproduced from ref. with permission from American Chemical Society, copyright 2024.

Fig. 2. (a) Schematic diagram of the VLS and VSS growth mechanisms of CNTs. (b) Diagram of base-growth and tip-growth modes of CNTs. Reproduced from ref. with permission from Wiley-VCH, copyright 2019.

Fig. 3. Timeline of major milestones in the controlled synthesis of ultralong CNTs. Reproduced from ref. with permission from Royal Society of Chemistry, copyright 2017. Reproduced from ref. with permission from Wiley-VCH, copyright 2022. Reproduced from ref. with permission from American Chemical Society, copyright 2023. Reproduced from ref. with permission from Springer Nature, copyright 2004. Reproduced from ref. with permission from Springer Nature, copyright 2017. Reproduced from ref. with permission from Springer Nature, copyright 2018. Reproduced from ref. with permission from Springer Nature, copyright 2019. Reproduced from ref. with permission from American Association for the Advancement of Science, copyright 2020. Reproduced from ref. with permission from American Chemical Society, copyright 2021.

Fig. 4. (a) Schematic illustration of the tip-growth mechanism of ultralong CNTs. (b) SEM image of SWCNT arrays grown across microtrenches. Reproduced from ref. with permission from American Chemical Society, copyright 2007. (c) Illustration of tip-growth of ultralong CNTs based on Schulz–Flory distribution. (d) Atomic force microscopy image of a CNT with a catalyst nanoparticle on its tip. (e) Theoretical percentage and (f) theoretical number density of ultralong CNTs. Reproduced from ref. with permission from American Chemical Society, copyright 2013.

Fig. 5. (a) Schematic diagram of the formation mechanism of thermal buoyancy. (b) Diameter distribution of SWCNTs by AFM. The inset is a typical AFM image of an individual SWCNT. Reproduced from ref. with permission from American Chemical Society, copyright 2010. (c) Schematic illustration of the heat balance of the substrates with different emissivity. (d) Relationship between substrate temperature and substrate emissivity. (e) Thermophoretic force and drag force applied on a CNT in a non-uniform temperature field. (f) Distribution of the ratio of thermophoretic force to drag force in the vertical direction. (g) Distribution of the dominant regions of thermophoresis and drag. Reproduced from ref. with permission from Wiley-VCH, copyright 2023.

Fig. 6. (a) Schematic of the growth method of the 18.5 cm long CNTs. (b) The optical images of an ultralong CNT array. (c) The SEM images of the strip and the beginning, middle, and end of the ultralong CNT array from left to right. Reproduced from ref. with permission from American Chemical Society, copyright 2009. (d) Optical picture of the 20 cm long Si substrate. (e) Optical picture of the substrate connected by Si (dark substrates) and SiO₂ (white substrates) substrates. (f) Raman spectra and (g) TEM pictures of CNTs. (h) Effect of water concentration on the growth rate of CNTs. Reproduced from ref. with permission from American Chemical Society, copyright 2010. (i) SEM image of the 550 mm long CNTs. (j) Number of CNTs at different lengths on the substrate. Inset: TEM images of as-grown CNTs. (k) Raman spectrum of as-grown CNTs. (l) Mechanical properties of as-grown CNTs. Reproduced from ref. with permission from American Chemical Society, copyright 2013.

Fig. 7. (a) Inhibition of catalyst particle aggregation and growth of high-density CNT horizontal arrays by silicon dioxide nanospheres. Reproduced from ref. with permission from Elsevier Ltd., copyright 2013. (b) Graphene layer-loaded catalyst particles for growing horizontal arrays of CNTs. Reproduced from ref. with permission from Royal Society of Chemistry, copyright 2014. (c) Schematic diagram of catalyst pre-deposition process. (d) SEM image of multiple growth of ultralong CNTs using a single-deposited catalyst. Reproduced from ref. with permission from Elsevier Ltd., copyright 2016. (e) Synthesis of high-density ultralong CNT arrays by SIDS method. Reproduced from ref. with permission from American Chemical Society, copyright 2023.

Fig. 8. (a) Cerium oxide as a catalyst particle carrier for the preparation of highly selective semiconductor-type CNT horizontal arrays. Reproduced from ref. with permission from American Chemical Society, copyright 2014. (b) Effect of temperature on wall number and diameter of CNTs. Reproduced from ref. with permission from Wiley-VCH, copyright 2010. (c) Different carbon sources (up: CO gas, down: CH₄ gas) lead to the growth of SWCNTs with different diameter distributions. Reproduced from ref. with permission from Royal Society of Chemistry, copyright 2012. (d) Diameter distributions of SWCNTs grown at different ethane concentrations at 900 °C (up to down: 140 ppm, 1600 ppm, and 14 400 ppm, respectively). Reproduced from ref. with permission from American Chemical Society, copyright 2006. (e) The fitted diameter diagrams of the SWCNT samples as a function of CO₂ concentration. Reproduced from ref. with permission from Elsevier Ltd., copyright 2011.

Fig. 9. (a) Schematic illustration of ultralong CNTBs composed of continuous CNTs. (b) Schematic illustration of the in situ fabrication of CNTBs by the gas-flow focusing method. (c) TEM images of CNTBs with different component numbers. Reproduced from ref. with permission from Springer Nature, copyright 2018. (d) Optical visualization diagram, (e) optical picture, and (f) SEM picture of a single SCNTN. (g) Optical microscopy images of SCNTNs with different areal densities. Reproduced from ref. with permission from Wiley-VCH, copyright 2022. (h) Synthesis and (i) morphologies of SCNT-CNs. Reproduced from ref. with permission from American Chemical Society, copyright 2024.

Fig. 10. (a) Stress-strain curves for single CNTs and CNTBs with gauge lengths of about 1.5 mm. (b) The relationship between the tensile strength of CNTBs and the CNTB component numbers. Reproduced from ref. with permission from Springer Nature, copyright 2018. (c) Fatigue behavior of CNTs at different temperatures. (d) Comparison of mechanical performance between CNTs and some high-performance materials. Reproduced from ref. with permission from American Association for the Advancement of Science, copyright 2020. (e) Illustrations showing inner-shell sliding processes for ultralong DWCNTs. (f) Schematic of an inner shell being pulled out of a DWCNT and intershell friction of three DWCNTs with different outer diameters. Reproduced from ref. with permission from Springer Nature, copyright 2013.

Fig. 11. (a) FET devices based on ultralong CNT arrays. (b) Transfer characteristics of the width normalized transistor for a single channel plotted in both linear (blue, left axis) and logarithmic (green, right axis) scales with applied V_DS of −0.1 V. Reproduced from ref. with permission from Springer Nature, copyright 2019. (c) A 3D sketch of the device for high-frequency measurement. (d) The high-frequency response of the photodetector at 0 V. Reproduced from ref. with permission from AIP Publishing LLC, copyright 2014. (e) Schematic illustration of the torsion balance unit. (f) Torque versus incident photon force. Reproduced from ref. with permission from American Association for the Advancement of Science, copyright 2021. (g) Schematic illustration of the inter and intramode for the testing of SCNT-CNs-based airflow sensors. (h) Relative resistance variation of SCNT-CNs with different areal densities under different gas velocities. (i) Performance comparison between SCNT-CNs-based airflow sensors and previously reported airflow sensors. Reproduced from ref. with permission from American Chemical Society, copyright 2024.