Carbon nanotubes: functionalisation and their application in chemical sensors

Abstract

Carbon nanotubes (CNTs) have been recognised as a promising material in a wide range of applications, from safety to energy-related devices. However, poor solubility in aqueous and organic solvents has hindered the utilisation and applications of carbon nanotubes. As studies progressed, the methodology for CNTs dispersion was established. The current state of research in CNTs either single wall or multiwall/polymer nanocomposites has been reviewed in context with the various types of functionalisation presently employed. Functionalised CNTs have been playing an increasingly central role in the research, development, and application of carbon nanotube-based nanomaterials and systems. The extremely high surface-to-volume ratio, geometry, and hollow structure of nanomaterials are ideal for the adsorption of gas molecules. This offers great potential applications, such as in gas sensor devices working at room temperature. Particularly, the advent of CNTs has fuelled the invention of CNT-based gas sensors which are very sensitive to the surrounding environment. The presence of O₂, NH₃, NO₂ gases and many other chemicals and molecules can either donate or accept electrons, resulting in an alteration of the overall conductivity. Such properties make CNTs ideal for nano-scale gas-sensing materials. Conductive-based devices have already been demonstrated as gas sensors. However, CNTs still have certain limitations for gas sensor application, such as a long recovery time, limited gas detection, and weakness to humidity and other gases. Therefore, the nanocomposites of interest consisting of polymer and CNTs have received a great deal of attention for gas-sensing application due to higher sensitivity over a wide range of gas concentrations at room temperature compared to only using CNTs and the polymer of interest separately.

Fig. 1. Nanostructures of SWCNTs and MWCNTs, fullerenes and a carbon nano-onion (CNO). Reproduced with permission from ref. 60, copyright 2008, RSC.

Fig. 2. (a) Structure of a multi-walled carbon nanotube made up of three shells of hexagonal lattice sheet of different chirality, (b) roll-up of a graphene sheet that leads to three different types of CNTs and (c) image of purified MWNTs with carboxylation functionalisation under TEM. Reproduced with permission from ref. 55, copyright 2011, Composites Part A: Applied Science and Manufacturing.

Fig. 3. The covalent functionalisation phenomena at the side and end-cap of CNT structure. Adapted from ref. 77, copyright 2015, Elsevier.

Fig. 4. Comparison between covalent and non-covalent functionalisation of CNTs. Adapted from ref. 36, copyright 2015, Elsevier. And adapted from ref. 37, copyright 2015, Elsevier.

Fig. 5. Number of publications with the specific keywords from the year 2010 to 2020.

Fig. 6. Functionalisation of CNT through the oxidation process using H₂SO₄/HNO₃. Adapted from ref. 43, copyright 2015, Elsevier.

Fig. 7. AFM image (a) as-received MWCNTs and the effect of the oxidation process after: (b) HNO₃ treatment; (c) HNO₃/H₂SO₄ 2 hour, (d) HNO₃/H₂SO₄ 4 hour, (e) HNO₃/H₂SO₄ 6 hour, and (f) UV/Vis spectra for sample (a) to (e). Reproduced with permission from ref. 92, copyright 2011, Diamond and Related Materials.

Fig. 8. The route of (a) covalent functionalisation of CNTs (A: direct sidewall functionalisation) and (b) (B: defect functionalisation). Reproduced with permission from ref. 65, copyright 2011, Diamond and Related Materials.

Fig. 9. Non-covalent functionalisation routes. Adapted from ref. 129, copyright 2015, Elsevier. And adapted from ref. 136, copyright 2015, Elsevier.

Fig. 10. Schematic diagram of decorating MWCNTs with CdS NPs with in situ polymerised PTh acting as an inter-linker. Reproduced with permission from ref. 148, copyright 2015, Elsevier.

Fig. 11. Schematic diagram of the latex technology of MWCNTs dispersed in NR latex. Reproduced with permission from ref. 150, copyright 2015, Elsevier.

Fig. 12. (a) Schematic diagram of carboxylated functionalised of MWCNTs, and (b) proposed mechanism for ethanol (alcohol) vapour detection using conductive polymer–MWCNTs–OOH sensors. Adapted from ref. 176, copyright 2015, Elsevier.

Fig. 13. Sensitivity of PPy/MWCNT with (a) different NH₃ concentration and (b) different MWCNT loadings towards the NH₃ at 2000 ppm. Reproduced from ref. 191, copyright 2015, Elsevier.

Fig. 14. Gas sensing sensitivity of PPy/MWCNT (4 wt%) sensor towards 2000 ppm of NH₃. Reproduced from ref. 191, copyright 2015, Elsevier.

Fig. 15. (a) Response of the CNT/PMMA composite to analytes and sensor response, and (b) response of the f-CNT/PMMA composite to analytes and sensor response towards dichloromethane, chloroform and acetone vapours. Reproduced from ref. 194, copyright 2015, Elsevier.

Fig. 16. (a) Selectively sensing response to 10 ppm of NH₃ and other analytes for the PANI/MWCNT nanocomposite film and (b) gas sensing response of the PANI/MWCNT nanocomposite film in bending and extending states to 1 ppm of NH₃. Reproduced from ref. 198, copyright 2017, Elsevier.

Fig. 17. Suggested mechanism of CHCl₃ molecules with PANI/c-MWCNT nanocomposite. Reproduced from ref. 199, copyright 2015, Elsevier.

Fig. 18. Variation in sensor response toward CHCl₃ vapour as a function of c-MWCNT concentration in PANI/c-MWCNT nanocomposite. Reproduced from ref. 199, copyright 2015, Elsevier.

Fig. 19. Schematic diagram of PANI coated with SWCNT–COOH on interdigitated electrodes. Adapted from ref. 200, copyright 2015, Elsevier.

Fig. 20. (a) SEM images of a bare SWCNT before electropolymerisation, (b) SEM image of coated SWCNT–COOH after electropolymerisation, and (c) NH₃ sensitivity of PANI–SWCNTs–COOH and unfunctionalised SWCNTs (SWCNT–COOH, without PANI). Reproduced from ref. 200, copyright 2015, Elsevier.

Fig. 21. The chemical structure of P3OT and P3CT, and their responses of the P3CT/SCNT, P3OT/CNT sensor, and the non-functionalised CNT sensor to 20 s vapour exposures of various compounds (1% of saturated vapour) and 32 ppb of NMPEA. Reproduced from ref. 201, copyright 2018, Elsevier.

Fig. 22. Shows the proposed mechanism of the interaction of NH₃ with PTh/MWCNTs nanocomposite. Reproduced from ref. 202, copyright 2020, Elsevier.