Recent Progress on Functionalized Graphene Quantum Dots and Their Nanocomposites for Enhanced Gas Sensing Applications

Abstract

Gas-sensing technology has witnessed significant advancements that have been driven by the emergence of graphene quantum dots (GQDs) and their tailored nanocomposites. This comprehensive review surveys the recent progress made in the construction methods and applications of functionalized GQDs and GQD-based nanocomposites for gas sensing. The gas-sensing mechanisms, based on the Fermi-level control and charge carrier depletion layer theory, are briefly explained through the formation of heterojunctions and the adsorption/desorption principle. Furthermore, this review explores the enhancements achieved through the incorporation of GQDs into nanocomposites with diverse matrices, including polymers, metal oxides, and 2D materials. We also provide an overview of the key progress in various hazardous gas sensing applications using functionalized GQDs and GQD-based nanocomposites, focusing on key detection parameters such as sensitivity, selectivity, stability, response and recovery time, repeatability, and limit of detection (LOD). According to the most recent data, the normally reported values for the LOD of various toxic gases using GQD-based sensors are in the range of 1-10 ppm. Remarkably, some GQD-based sensors exhibit extremely low detection limits, such as N-GQDs/SnO₂ (0.01 ppb for formaldehyde) and GQD@SnO₂ (0.10 ppb for NO₂). This review provides an up-to-date perspective on the evolving landscape of functionalized GQDs and their nanocomposites as pivotal components in the development of advanced gas sensors.

Keywords: GQDs based nanocomposite; functionalized GQDs; gas sensing mechanism; improved sensing.

Figure 1

Synthetic schematic diagram of N-GQDs@SnO₂ nanocomposites [66]. I–IV represent the steps of the process order.

Figure 2

Fabrication process for a GQD@SnO₂ nanodome based gas sensor [61].

Figure 3

(a) Responses of ZnO and N-GQDs@ZnO (G-Z-2) to 5 ppm of different gases at 100 °C; (b) Long-time stability of sensors to 5 ppm NO₂ [67].

Figure 4

Schematic illustration of the heterojunctions formed by GQD-metal oxides (G/M). (a) Metal oxides work function, Φ_M, and Fermi energy, E_F; (b) GQDs’ work function, Φ_G; (c) Idealized equilibrium band diagram for the G/M junction. Φ_i is the energy barrier to the flow of electrons (black dots) from the GQDs to the metal oxides, while Φ_B is the Schottky barrier height for the electron flow in the opposite direction. w is the extension of the depletion layer and corresponds to the bent part of the energy bands Reproduced from [99].

Figure 5

Schematic illustration of (a) TiO₂@NGQDs’ hybrid formation; (b) O₂ adsorption and conversion to the oxygen ion species on TiO₂; (c) the most stable configuration of NO gas adsorbed on to the TiO₂ surface; schematic illustration of the energy and structures of the TiO₂@NGQDs p−n junction and the electron transfer in the nanocomposite; (d) TiO₂ and NGQDs before contact; (e) TiO₂@NGQDs nanocomposite in air; and (f) exposure to NO. EC, EV, and EF are the conduction band, valence band, and Fermi energy, respectively [86].

Figure 6

(a) Improved gas response of the NGQDs/PANI composite toward 100 ppm ethanol; (b) Gas response of PANI and NGQDs/PANI film sensors toward 50–150 ppm of ethanol at 26 °C in 45% RH; (c) real-time resistance change as a function of time for the NGQDs/PANI film sensor toward ethanol gas [65].

Figure 7

Response and recovery curves of MoS₂/rGO and MoS₂/rGO/GQD-based sensors exposed to 30 and 50 ppm NO₂ [62].

Figure 8

Schematic illustration of (a) the initial potential barrier formation for the SnO₂ nanodomes structure; (b) the sensing mechanism of GQD@SnO₂ nanodomes which shows enhanced NO₂ adsorption due to the GQDs; (c) the formation of an electron depletion layer with its electronic band structure [61].

Figure 9

(a) The SEM image of pure ZnFe₂O₄; (b) the SEM image of ZnFe₂O₄–GQDs; (c) the TEM images of ZnFe₂O₄–GQDs; (d) the HRTEM images of ZnFe2O4–GQDs; (e) The response of the ZnFe₂O₄–GQDs composite to acetone (1000, 500, 250, 100, 10 and 5 ppm) at room temperature; (f) Energy band diagram of the GQDs/SiNW heterojunction. Figure 9a–e were adapted from [90] and Figure 9f was adapted from [89].

Figure 10

Response curves to different concentrations of the sensors’ NO₂ based on (a) graphene and (b) SnO₂–Gr–2; (c) the response and recovery times of the sensors [121] and resistance curves for 5 ppm NO₂ as a function of operating temperature for (d) pristine SnO₂ nanodomes and (e) a GQD@SnO₂ nanodome-based gas sensor [61].

Figure 11

Schematic sensing mechanism of (a) B–GQD and (b) B–GQDs–AL [95].

Figure 12

Response vs. time plots for (a) PFLGr and (b) BFLGr for 16 to 256 ppm of NH₃; (c) response and recovery plot for the BFLGr sensor for 32 ppm of NH₃; (d) repeatability plot for BFLGr for 256 ppm of NH₃ [123]; (e) the response curves of the PANI polymer matrix, 20 wt% PANI/hollow In₂O₃ nanofiber, and 20 wt% PANI/GQD/hollow In₂O₃ nanofiber composites with an exposure of 1 ppm NH₃ at room temperature [124].