Fig. 1. (Color online) The sensing response of DGr/Gr hybrid fabricated with different (a) irradiation fluence and (b) H₂ etching time to 100 ppm NO₂ at room temperature. (c) The response-recovery curve of the gas sensor based on DGr/Gr. (d) Dynamic response of the DGr/Gr based sensor to different concentrations of NO₂ at room temperature. (e) Cycled response to 100 ppm NO₂ at room temperature. (f) Responses of the DGr/Gr based gas sensor toward different gas species at room temperature^[34].

Fig. 2. (Color online) (a) Transient response of graphene/MoS₂ sensor to NO₂ gas molecules (1.2 to 5 ppm). (b) Transient response of graphene/MoS₂ sensor to NH₃ gas molecules (5 to 100 ppm). All gas-sensing tests were performed at an operating temperature of 150 °C. (c) Optical image of a graphene/MoS₂ heterostructured device on a bent polyimide substrate, inset displays the semitransparent sensing device placed on a paper with the KIMS logo. (d) Comparison of the gas response characteristics of the flexible heterostructured device before/after the bending cycle test, inset is the 3D schematic images showing the bending test condition. No serious performance degradation was observed, even after performing 5000 bending cycle tests^[45].

Fig. 3. (Color online) Typical sensing response of (a) the exfoliated MoS₂ based thin film sensor and (b) the co-exfoliated MoS₂/graphene-based thin film sensor to 10, 20, and 50 ppm methanol. (c) The synergetic effect of the MoS₂/graphene nanocomposite as methanol gas sensor. (d) Repeated sensing response of the co-exfoliated MoS₂/graphene thin film sensor to 50 ppm methanol^[46].

Fig. 4. (Color online) (a) Real time response of the MoS₂/graphene hybrid aerogel (MGA) sensor at room temperature toward different NO₂ concentrations. (b) Real time resistance change of the MGA sensor with the microheater temperature of 200 °C. (c) MGA sensor response to 0.5 ppm NO₂ at various microheater temperatures, displaying improvement in response and recovery time. (d) Selectivity peroperties of the MGA sensor compared to that of GA alone at microheater temperature of 200 °C^[47].

Fig. 5. (Color online) (a) Schematic and (b) optical microscope images of the graphene/MoS₂ heterojunction (GMH) device with a gas barrier layer. (c) Metal–semiconductor–metal diode model for n-type MoS₂ with graphene and Ti asymmetric contacts and its band diagram. (d) Time-dependent sensor responses of GMH under different bias conditions (V_DS = −1, 1, and 3 V) in linear scale (top) and semilogarithmic scale (bottom). (e) Time-dependent sensor responses of GMH under different gate voltages (V_BG = 0 and 40 V) in linear scale (top) and semilogarithmic scale (bottom). (f) Transfer curves of the GMH device measured at V_DS = 1 V in linear (top) and in semilogarithmic scales (bottom)^[48].

Fig. 6. (Color online) Statistical graph of (a) average response, (b) response time and (c) recovery time of rGO/MoS₂ composites gas sensors. (d) Plots of the fitting of response vs. concentration. (e) Dynamic response of MoS₂/rGO sensor to different concentrations of H₂O₂ vapor^[50].

Fig. 7. (Color online) (a) The UV–vis transmittance spectra of patterned MoS₂/rGO layer. (b) Resistivity of the MoS₂/rGO layer on PET as the function of bending radius. (c) Dynamic response of (i) rGO, (ii) MoS₂/rGO (1 : 10), (iii) MoS₂/rGO (1 : 5) and (iv) MoS₂/rGO (1 : 2.5) thin film gas sensor with different concentrations of NO₂^[52].

Fig. 8. (Color online) (a) Sensing responses of rGO sensor and rGO/MoS₂ sensor toward various concentrations of NO₂. (b) Histogram analysis obtained from (a). (c) Schematic illustration of resistance configuration of interdigital electrode sensors. I–V relationships of (d) rGO-Au, (e) MoS₂-Au and (f) rGO/MoS₂-Au contacts^[53].

Fig. 9. (Color online) (a) Dynamic response curves and (b) summarized response values of the devices based on MoS₂, rGO and rGO/MoS₂ hybrids film toward HCHO at room temperature. (c) Comparison of the response time of the three devices to HCHO. (d) Reproducibility and (e) stability properties of the rGO and rGO/MoS₂ hybrid films toward 2.5 ppm HCHO. Schematic illustrations of (f) the fabricated sensing device and (g) the energy diagram of rGO, MoS₂ and formaldehyde^[54].

Fig. 10. (Color online) (a) Photo image of the flexible device based on rGO/MoS₂ hybrid film in the bending state. (b) Adsorption model of HCHO molecule on rGO/MoS₂ hybrid film. (c) Schematic illustration of HCHO sensing mechanism of rGO/MoS₂ hybrid film. (d) Real-time sensing response curves of the rGO/MoS₂-HT and rGO/MoS₂-CE sensors to 2.5–15 ppm HCHO. (e) Real-time sensing response curves of the rGO/MoS₂-HT sensor to 2.5–15 ppm HCHO upon different bending angles. (f) Long-term stability of rGO/MoS₂-HT sensor^[57].

Fig. 11. (Color online) (a) Sensing responses of single rGO (red line) and WS₂-decorated rGO films (blue line) in dry air and 2–10 ppm NO₂ operated at (left) 25 °C and (right) 50 °C. (b) Schematic illustration of the proposed sensing mechanism of WS₂-decorated rGO hybrid upon NO₂ exposure^[56].

Fig. 12. (Color online) (a) Response of four types of MoS₂/WS₂ heterojunction toward different concentrations of NO₂ at room temperature. (b and c) Response time and recovery time of the sensors, respectively. (d) Response, and response/recovery time of optimized MoS₂/WS₂ heterojunction as the functions of gas concentrations. (e) Reproducibility of MoS₂/WS₂ heterojunction sensor toward 10 ppm NO₂ at room temperature. (f) Selective response of MoS₂/WS₂ heterojunction sensor^[61].

Fig. 13. (Color online) (a) Sensing response values of rGO-MoS₂-CdS nanocomposite film to 0.2 ppm of different target gases at 75 °C. (b) Normalized responses of rGO-MoS₂-CdS nanocomposite gas sensor as a function of NO₂ gas concentrations under different operation temperatures: (a) 25 °C, (b) 50 °C, (c) 75 °C, and (d) 100 °C. (c) Dynamic responses of three types of rGO-MoS₂-CdS nanocomposite sensors toward different concentration of NO₂ at 75 °C. (d) Cyclic response of three types of rGO-MoS₂-CdS nanocomposite sensors toward 0.2 ppm of NO₂^[62].

Fig. 14. (Color online) (a) Schematic of the ternary 2D nanomaterial-based FET device. (b) Band structure of the FET device before and (c) after NO₂ adsorption. (d) Transfer and (e) output curves of the FET before and after exposure to 100 ppb NO₂ for 10 min. (f) Real-time sensing response of the FET to NO₂. (g) Real-time sensing response of the FET device to NH₃. (h) Real-time sensing response of the FET device to DCM, hexane, acetone and DMF. (i) Relative resistance change as a function of the square root of the gas concentrations^[63].

Table 1. Literature study on gas sensor performance of 2D/2D nanocomposites-based gas sensors.