Gas Sensors Based on Single-Wall Carbon Nanotubes

Abstract

Single-wall carbon nanotubes (SWCNTs) have a high aspect ratio, large surface area, good stability and unique metallic or semiconducting electrical conductivity, they are therefore considered a promising candidate for the fabrication of flexible gas sensors that are expected to be used in the Internet of Things and various portable and wearable electronics. In this review, we first introduce the sensing mechanism of SWCNTs and the typical structure and key parameters of SWCNT-based gas sensors. We then summarize research progress on the design, fabrication, and performance of SWCNT-based gas sensors. Finally, the principles and possible approaches to further improving the performance of SWCNT-based gas sensors are discussed.

Keywords: gas sensor; single-wall carbon nanotubes.

Figure 1

Schematic of SWCNTs rolled from graphene sheets using different chiral angles, Reproduced with permission from Royal Society of Chemistry [9].

Figure 2

Logical structure of a gas sensor. Adapted with permission from American Chemical Society https://doi.org/10.1021/acs.chemrev.6b00361 (accessed on 4 August 2022) [28]. Analytes interact with the sensing material (CNTs or functional active sites on CNTs) changing some of its physical properties (e.g., temperature, ΔT; conductivity, Δσ; work function, ΔΦ; and permittivity, Δε). Transduction converts one of these physical quantities into a change in an electrical parameter (capacitance, inductance, and resistance are mentioned). Finally, the circuit connected to the sensor gives rise to a signal, usually a current or voltage change that can be measured.

Figure 3

Graphical representation of several important performance parameters in a sensor exposed to increasing concentrations of analyte gas. Reprinted with permission from [30], Copyright 2019, American Chemical Society.

Figure 4

(A) Schematic of sensing mechanisms in SWCNT network-based resistive sensors; (B) at the interface between the metallic electrode and the SWCNT (Schottky barrier); (C) at the sidewall or along the length of the SWCNT (intra-SWCNT); (D) at the SWCNT−SWCNT interface (inter-SWCNT). Reproduced with permission from Ref. [37]. Copyright 2016 John Wiley and Sons.

Figure 5

Hypothetical transfer (I_SD−V_G) curves before (black) and after (red) gas adsorption for three different sensing mechanisms. Insets illustrate the corresponding changes in the band diagrams: (a) Schottky barrier modulation corresponds to a change of barrier height, the work function difference between metal and SWCNT; (b) N-doping of the CNT induces a shift of the I−V curve to more negative voltages; (c) Change in Mobility induced by factors that reduce the conductivity, such as the addition of resistive elements or carrier scattering. Adapted with permission from Ref. [47]. Copyright 2008, American Chemical Society.

Figure 6

Microstructures of small-bundle [74] SWCNTs (a–c) and isolated [75] SWCNTs (d–f) with carbon-welded joints. (a,d) Typical SEM images. (b,e) Typical TEM images. (c,f) Statistical data of the numbers of isolated and bundled SWCNTs in the network. Reproduced with permission from Ref. [74]. Copyright 2018 Elsevier, and Reproduced with permission from Ref. [75]. Copyright 2018, American Association for the Advancement of Sciences.

Figure 7

(a) Optical images of a hydrogen gas sensor constructed using SWCNT films with a transparency of 85%. Responses of (b) m-SWCNT and (c) s-SWCNT films with different transparencies upon exposure to H₂ with a concentration of 5% (vol.). Reproduced with permission from Ref. [20]. Copyright 2019 Elsevier.

Figure 8

(a) Oxidation of SWCNTs. (b) Addition of base to SWCNT−COOH solution. (c) Addition of acid to SWCNT−COOH solution. (d,e) Sensor responses (ΔR/R₀) of acid- or base-pretreated SWCNT−COOH samples to (d) NH₃ and (e) CO₂. Data for each bar is averaged from three different gas exposures. Reprinted with permission from Ref. [95]. Copyright 2019, American Chemical Society.

Figure 9

(a) Schematic of the covalent functionalization of CNTs by iodonium salts. Reprinted with permission from [106], Copyright 2016, American Chemical Society. (b) Bio-inspired carbon monoxide sensor. Schematic of a FET substrate with source-drain electrodes, a SiO₂ dielectric layer and a Si gate electrode. Chemical structures of a pyridyl-functionalized SWCNT and iron porphyrin (Fe−(tpp) ClO₄), describing the coordination of the pyridyl group to the iron center of the porphyrin. (c) Comparison of the responses to CO in both N₂ and air to the responses to CO₂ and O₂. Reproduced with permission from Ref. [42]. Copyright 2017 John Wiley and Sons.

Figure 10

Diagram of device fabrication and sensing element composition. (a) Sensor fabrication including spray coating of an SWCNT−P4VP network, Pt coordination and anion exchange (b,c). Proposed surface speciation of (b) SWCNT−P4VP−Pt and (c) SWCNT−P4VP−Pt−POM. Reproduced with permission from Ref. [43]. Copyright 2021 National Academy of Sciences.

Figure 11

(a) Schematic of a H₂ sensor based on a SWCNT film decorated with Pd. (b) Transfer curves of the sensors based on s-SWCNTs (left) and unsorted-SWCNTs (right) before (blue) and after (red) 311 ppm of H₂ exposure for 200 s. (c) Real-time response to different H₂ concentrations at room temperature. Inset: magnified plot at low concentrations. Reprinted with permission from Ref. [44]. Copyright 2018, American Chemical Society.

Figure 12

(a) SEM image of an s-SWCNT device with random Pd decorations all along its length. (b) Response of the device to pulses of H₂ in air, before (blue) and after (red) the Pd deposition. (c) Atomic force topography image of a second SWCNT with a production of a point defect and the selective decoration of these sites with Pd. (d) Response of the device with a point defect, before (blue) and after (red) Pd deposition, showing the nearly thousand-fold better response. Reprinted with permission from Ref. [142]. Copyright 2010, American Chemical Society.