Abstract
1. Introduction
Gas sensor is an electronic device that can qualitatively or quantificationally detect the specific gases, and has been widely used in many fields, such as indoor/outdoor gas monitoring, industrial control, agricultural production, medical diagnosis, and military and public safety[
Since the first discovery of graphene, the two dimensional (2D) structured nanomaterials have attracted extensive research interest worldwide[
Hybridizing functional elements, e.g. novel metals or metal oxides, is a feasible and controllable way to tailor the sensing performance of intrinsic 2D nanomaterials. Numerous studies have demonstrated the improvement effect of hybridization on the gas sensing performance of 2D nanomaterials[
Although several recent reviews have involved in the gas sensing studies of 2D nanomaterials and 2D-based nanocomposites[
2. Working mechanism of gas sensors
The working principle of conventional metal oxide-based gas sensors is based on the so-called surface adsorbed oxygen ions mechanism[
Gas sensors with different structures work in different ways. Here, we mainly introduce two main types of gas sensing devices including chemiresistors and field-effect transistor (FET)[
3. Features of 2D/2D nanocomposites as sensing materials
Generally, the experimental results of 2D-based sensors are often lower than their theoretical values[
(1) Geometrical effects
(i) Owing to the heterogenous nature, the hybridization of different 2D nanomaterials would prevent homogeneous restacking and enlarge the active surface area.
(ii) The heterogenous hybridization would result in porous structure which exposes a large number of active sites leading to a higher sensing response, as well as facilitate the gas diffusion leading to accelerated response and recovery.
(iii) The combination of 2D nanomaterials with unique 2D structure tends to form a close contact between the components, facilitates the preservation of the intrinsic mechanical and flexible property.
(2) Electronic effects
(i) The combination of 2D nanomaterials with different semiconducting properties would form either an n/p or n/n or p/p-type heterojunction at the interface which induces heterojunction effect.
(ii) The potential energy barrier at the heterojunction hinders the electron transmission enhancing the response towards the gas with low electron affinity, and thus improving the sensor selectivity.
(iii) The heterojunction could facilitate the charge separation, avoid the charge accumulation during the adsorption/desorption process, resulting in an increased sensitivity and response speed.
(3) Chemical effects
(i) The integration of certain 2D nanomaterials with excellent catalytic properties would decrease the activation energy required in the gas adsorption/desorption process and shorten the response and recovery time.
(ii) The catalytic 2D nanomaterials could improve selective adsorption of analytes.
4. Gas sensing properties of 2D/2D nanocomposites
Motivated by these above-mentioned synergistic effects, numerous 2D/2D nanocomposites have been developed, but not many works have been done regarding gas sensing properties of 2D/2D nanocomposites as summarized in Table 1. In this section, we introduce the recent progress on the graphene-like 2D/2D nanocomposites-based gas sensors. Relative works involving different substrates including flexible polymer and rigid silica as well as different working temperatures ranging from room temperature (RT) to 150 °C have been included for a comprehensive understanding of the recent trend. The cases have been classified with hybrid types, since the mechanism and gas sensing performance of 2D/2D nanocomposites are ineluctably influenced by the choice of material with their innate properties.
4.1. Graphene + graphene
Ma et al. fabricated a gas sensor based on defective graphene (DGr)/pristine graphene (Gr) hybrid layer[
Figure 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[
4.2. Graphene + MoS2
The recent studies on the integration of graphene with 2D layered semiconductors have emerged for different application, in which the graphene/MoS2 nanocomposites have been researched mostly[
Figure 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[
Figure 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[
Performance improvement of 2D gas sensor can also be realized by ingenious structure design. Through synthesizing MoS2/graphene hybrid aerogel (GA), Long et al. integrated the novel 3D hybrid aerogel on a low power microheater platform, realizing an excellent NO2 detection device[
Figure 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[
Figure 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 (
4.3. rGO + MoS2
In case of sensor application, reduced graphene oxide (rGO) presents superiority compared to intrinsic graphene and graphene oxide (GO) on the account of its rich functional groups and partly regained conductivity[
Figure 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[
Figure 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[
Zhou et al. prepared MoS2/rGO composite films by a combination of hydrothermal method and air brush technology and investigated their NO2 sensing response at 60 °C[
Figure 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.
Figure 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[
In another work, authors fabricated rGO/MoS2 hybrid films on flexible polyethylene naphthalate (PEN) substrates by a simple self-assembly method and studied their sensing performance towards 2.5–15 ppm HCHO at room temperature (Fig. 10(a))[
Figure 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[
4.4. rGO + other TMDCs
Paolucci et al. reported a NO2 gas sensor fabricated from WS2-decorated rGO composite[
Figure 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[
4.5. TMDCs + TMDCs
The integration of multiple TMDCs is also a favorable way to improve the sensing performance of the single TMDCs since their rich semiconducting and chemical catalytic properties make it possible to form numerous distinct functional heterostructure. Ikram et al. synthesized a heterojunction of few-layer MoS2 with multilayer WS2 via a simple one-pot hydrothermal process[
Figure 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[
4.6. Ternary 2D nanocomposites
Previously we discussed the merits of binary 2D/2D nanocomposites as gas sensor material. Recent studies show that the sensing property of binary 2D/2D nanocomposites could be further enhanced by adding the third component, i.e. the ternary 2D/2D nanocomposites. Shao et al. found that CdS nanocones could be grown on the 2D layered rGO-MoS2 substrate by a facile solvothermal treatment process, to form rGO-MoS2-CdS nanocomposite films[
Figure 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[
Other than the chemical integration, the 2D nanomaterials could also be geometrically integrated to form a functional device. Sigang Shi et al. reported a gas sensor based on a ternary 2D nanomaterial-based FET device in which few-layer black phosphorus (BP), boron nitride (BN) and molybdenum disulfide (MoS2) were used as the top-gate, dielectric layer and conduction channel, respectively[
Figure 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[
5. Conclusions and outlook
In this review, we comprehensively summarize the achievements in recent studies on the gas sensor application of 2D/2D nanocomposites. The sensing mechanism of 2D nanomaterials was briefly introduced. The collective benefits and mechanisms of 2D/2D nanocomposites were discussed in three aspects: geometrical effects, electronic effects, and chemical effects. The reported experimental results demonstrated the promising gas sensor performances, such as high sensitivity and selectivity, improved response and recovery speed, and long-term stability of 2D/2D nanocomposites-based sensors even at room temperature. Part of the sensor devices were fabricated on polymer substrates presenting excellent flexible property. Such achievements offer great potential for practical implementation as next-generation flexible and wearable gas sensors.
Although significant progress has been demonstrated, there are still remaining questions as follows:
(1) The sensing mechanism of single 2D nanomaterials has been developed much, while the underlying sensing mechanisms of 2D/2D nanocomposites are still vague. The role of synergistic effect and hybridized effect on gas sensing performance of 2D/2D nanocomposites should be determined.
(2) Several teams reported the improved selectivity after hybridizing different 2D nanomaterials. However, scientific comprehension on such improvement has not been achieved. More simulation works and experimental results are required to clarify this issue.
(3) Most of the reported literatures in this field have not involved in the effect of humidity on the sensing performance of 2D/2D nanocomposites, while recent works have found numerous 2D TMDCs are very sensitive to water molecules. Thus the cross influence of humidity and target gases should be carefully investigated.
(4) Numerous studies have shown that the sensing performance of 2D/2D nanocomposites are greatly dependent on the ratio of the components. Considering the complexity and economy, however, current researches on 2D/2D nanocomposites generally involved in several compositions. Systematical study on the optimization of the composition ratio is required.
In terms of future prospective, we believe that there are still tremendous opportunities in the field of 2D/2D nanocomposites-based gas sensors. Our comment on the future development trends of 2D/2D nanocomposites-based gas sensors could be divided into three sections. First, hundreds of new semiconducting 2D nanomaterials have been discovered in last decade. These analogs of graphene presenting distinct electronic and chemical properties with tremendous specific surface are ideal materials for gas sensor application. Thus there are abundance of possibilities to combine these 2D nanomaterials to construct 2D/2D nanocomposites as new potential sensor material. Second, the synergetic effect arise from 2D/2D composition could be further enhanced by elaborate design. For example, the formation of II type heterostructure using semiconducting 2D nanomaterials could facilitate the charge separation that improve the gas molecule adsorption and accelerate the reaction. Hybridizing photosensitive TMDCs with gas sensitive materials could improve the adsorption and desorption of gas molecules upon light irradiation owing to the illumination induced high-concentration charge carriers. Third, reported literatures have demonstrated that the sensing properties of well-designed ternary 2D nanocomposites surpass that of binary 2D nanocomposites. This is because adding new component could provide a new dimension for sensing modulation. Thus, developing multielement 2D nanocomposites would be an efficient approach to exploit next-generation high-performance, flexible, and low power consumption sensor devices.
Acknowledgments
This work was supported by Zhejiang Provincial Natural Science Foundation of China (No. LY18F010009) and Ningbo Natural Science Foundation (No. 2018A610002).
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