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^[1-4]. Generally, conventional gas sensor materials are constructed by semiconducting metal oxides, conducting polymers, and carbon nanotubes^{[5, 6]}. Among these, metal oxides are the most successfully commercialized sensing material owing to their ease of fabrication, high sensitivity and economical cost^{[7, 8]}. However, their drawbacks including high-temperature operation, large power consumption, and low selectivity are also significant that render them inadequacy for the next-generation wearable sensor application. The conducting polymers-based sensors could be conducted at room temperature without extra power requirement but suffered from the degraded property in air, specifically in humidity^[9]. The usage of carbon nanotubes could greatly lower the operating temperature of the sensor bringing in superb sensitivity, while their long response and recovery time as well as complex process hinder the wide application^[10]. Thus, it is of great importance to develop room-temperature workable sensor materials with excellent sensing performance for next-generation sensor application.

Since the first discovery of graphene, the two dimensional (2D) structured nanomaterials have attracted extensive research interest worldwide^[11-18]. Benefited from their tremendous surface-volume ratio, atomical thickness as well as excellent conducting or semiconducting property, 2D structured materials have also exhibited extraordinary potential in the gas detection field^[19-22]. Specifically, their unique 2D structure exposes most atoms that could interact with environmental gas molecules and output enormous signal. Moreover, the capability of 2D nanomaterials to identify gas analytes at room temperature as well as their inherent flexible property render them to be a promising candidate for constructing flexible and wearable gas sensor integrated on a low Young's modulus substrates^[23]. Even so, numerous challenges including selectivity, sensitivity, response time, recovery time, and stability have to be addressed before the practical application of 2D nanomaterials in gas detection field^[24]. Taking graphene as an example, the atomic surface of graphene is chemically inert resulting in a weak adsorption of gas molecules. Strategies including surface functionalization, foreign atoms doping, defect engineering, and ligand conjugation are generally adapted to improve the sensing performance of intrinsic graphene^{[25, 26]}. In contrast, 2D transition metal dichalcogenides (TMDCs), analogs of graphene, present multiple band structures, versatile physical and chemical properties, layer-dependent band gaps, and excellent catalytic property, that are much more adaptive in the design of practical gas sensing devices^{[27, 28]}. However, sensor fabricated from 2D TMDCs may suffer from the sluggish response and recovery owing to the strong interaction between molecules and TMDCs surface. The incomplete recovery may gradually degrade the sensing performance and long-term stability.

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^[28-30]. Gas diffusion and adsorption in 2D nanomaterials were greatly facilitated owing to the low dimensional morphologies and catalytic effect of the additive components, respectively. More recently, research interests have turned to developing graphene-like 2D/2D nanocomposites, i.e. hybridizing 2D nanomaterial with other 2D nanomaterials^{[31, 32]}. Numerous benefits contributed by geometrical, electronic and chemical effects could be expected.

Although several recent reviews have involved in the gas sensing studies of 2D nanomaterials and 2D-based nanocomposites^[33-35], no review has covered graphene-like 2D/2D nanocomposites. Herein we comprehensively review the literature of 2D/2D nanocomposites to understand the fundamental mechanisms and current progress in the field of gas detection. In this review, we summarize the reported gas sensing mechanisms of 2D nanomaterials for the basic understanding. The unique gas sensing characteristics of selected 2D/2D nanocomposites are then discussed. Lastly, the challenges and future directions to develop 2D structured nanocomposites-based sensors will be addressed. This critical review will provide great insight into the evolution of 2D structured nanocomposites to reveal the enhancement effect of hybridization.

The working principle of conventional metal oxide-based gas sensors is based on the so-called surface adsorbed oxygen ions mechanism^[29], while that of 2D nanomaterials-type gas sensors is mainly based on the charge-transfer processes between gas molecules and the surface of sensing materials^[34]. The gas molecules act as charge acceptors or donors depending on their electron affinity upon physical or chemical adsorption on the 2D surface, resulting in the change of the overall electrical conduction. Taking MoS₂ as an example, the adsorption of oxidizing gas molecules (e.g. NO₂, SO₂, O₂, Cl₂, etc.) tend to withdraw electrons from the conduction band of MoS₂ resulting in a decreased electrical conduction, while the reducing gas molecules (e.g. NH₃, H₂, H₂S, CH₄, etc.) incline to increase the electrical conduction. Once the 2D nanomaterials are re-exposed to the air or other environmental gases, desorption of gas molecules takes place with a speed depending on the adsorption energy and diffusion condition, and the electrical conduction approach its initial value.

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)^{[36, 37]}. Chemiresistors are considered to be the most commercialized sensor type due to their simplicity of fabrication and operation, high sensitivity, and long-term stability^{[38, 39]}. In this kind of gas sensor, the sensing layer is generally deposited between two interdigitated metal electrodes on an insulating substrate. The electrical conduction property of the sensing layer would vary depending on the concentration and species of adsorbed gas. Through monitoring the resistance/current of the device before and after adsorption, the information of the target gas could be obtained. Another kind of widely used gas sensor is based on the field-effect transistor (FET)^{[40, 41]}. In a typical FET scheme, sensing layer is deposited between two electrodes (source and drain), forming the conductive channel. A gate electrode is covered on one side of the conductive channel through a thin dielectric layer. The conductivity of channel can be modulated by a voltage applied to the gate electrode. The target gas can be detected by observing the transfer characteristics and the output characteristics of the device.

Generally, the experimental results of 2D-based sensors are often lower than their theoretical values^[25]. Such discrepancy could be partially attributed to the restacking or aggregating of 2D nanosheets owing to the interlayer Van de Waals force. Thus, strategy able to prevent restacking and further improve the sensing performance should be devised. Compared to the utilization of single 2D nanomaterials, the integration of multiple 2D nanomaterials may solve several challenges of existing sensors including sensitivity, selectivity, and response (or recovery) speed issues. The collective benefits of 2D/2D hybridization can be divided into three general aspects: geometrical effects, electronic effects, and chemical effects as follows.

(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.

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.

Ma et al. fabricated a gas sensor based on defective graphene (DGr)/pristine graphene (Gr) hybrid layer^[42]. The defect density of defective graphene was controlled by the fluence of Si⁺ implantation, and the defect size was tuned by an H₂ etching process. The defective graphene was transferred onto pristine graphene with a PMMA assisted wet transfer method to finally obtain Gr/DGr hybrid-based gas sensor device. As shown in Figs. 1(a)–1(c), both the defect density and defect size could influence the response performance significantly. The DGr prepared with a 5 × 10¹³ cm^–2 Si⁺ ion bombardment and 120 min H₂ etching exhibited the best sensing performance with a maximum response value over 248% at room temperature. The dynamic response and reproducibility properties as shown in Figs. 1(d) and 1(e), respectively, indicated the excellent sensing performance of DGr/Gr hybrid. Furthermore, the defect engineering endows the Gr strong adsorption with NO₂ gas molecules. As shown in Fig. 1(f), the typical response of DGr/Gr hybrid to NO₂ was at least 65 times higher than that to the other target gases, indicating that the excellent selectivity property of DGr/Gr based gas sensor.

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^{[43, 44]}. In this combination, the graphene usually plays the role of conductive layer, and the MoS₂ acts as the analyte acceptor. Cho et al. reported an atomically thin graphene/MoS₂ heterostructure-based gas sensor, where the patterned graphene as electrodes was synthesized via chemical vapor deposition (CVD) and MoS₂ was mechanically exfoliated^[45]. As shown in Figs. 2(a) and 2(b), the limit detection concentration of this device was as low as 1.2 ppm for NO₂ gas and 5 ppm for NH₃ at 150 °C. Furthermore, the device can be constructed on a polyimide substrate and the gas response characteristic of the flexible device was well maintained at 150 °C, even after 5000 bending cycle tests, as displayed in Figs. 2(c) and 2(d). Zhang et al. fabricated graphene/MoS₂ nanocomposite through a liquid-phase co-exfoliation method and constructed a thin film gas sensor for methanol detection^[46]. On the interface of the exfoliated graphene and MoS₂, the MoS₂ acts as an electron donor which results in a slight n-type doping effect to graphene. The methanol as a typical reducing gas enhanced the n-type doping level of MoS₂, as shown in Figs. 3(a) and 3(b). Results showed that the sensitivity of graphene/MoS₂ nanocomposite gas sensor was almost double to that of pure MoS₂ sensor. The response time and recovery time were also shortened. The remarkable improvement of sensing performance was ascribed to the synergetic effect of graphene and MoS₂ nanoflakes, in which MoS₂ acted as an excellent analytes acceptor and graphene acted as a charge highway, as shown in Fig. 3(c). The graphene/MoS₂ nanocomposite sensor also presented excellent stability (Fig. 3(d)).

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^[47]. As seen in Figs. 4(a) and 4(b), the limit of detection toward NO₂ was below 50 ppb (14 ppb) at room temperature (200 °C). The average time of response and recovery was 21.6 s and 29.4 s at 200 °C against different NO₂ concentrations, from 50 ppb to 1 ppm (Fig. 4(c)). Fig. 4(d) was a plot of the selectivity of the MoS₂/GA sensor compared to GA alone at 200 °C, and the MoS₂/GA sensor exhibited excellent selectivity than GA sensor. The good NO₂ detection performance was attributed to a novel structure of 3D hybrid aerogel, in which the high electrical and thermal conductivity graphene serves as a scaffold, and single to few-layer MoS₂ covered on the scaffold serving as selective and sensitive layer for NO₂ detection. Meanwhile, the 3D graphene scaffold provided the high specific area, which availed the carrier transport. Hiroshi Tabata et al. investigated the NO₂ sensing characteristics of a graphene/MoS₂ heterojunction (GMH) utilizing a passivation technique with gas barrier layers^[48]. As shown in Figs. 5(a) and 5(b), the passivation technique guaranteed the response was originated from the GMH area. For the 2D layered semiconductors rarely have the dangling bonds on their surface, the graphene/MoS₂ interface has a weaker Fermi-level pinning effect (Fig. 5(c)), which enabled more effective modulation of the Schottky barrier height in the graphene/MoS₂ interface. The GMH device demonstrated a significant change in resistance, by a factor of greater than 10³, upon exposure to 1 ppm NO₂ under a reverse-bias condition, while it dramatically decreased when measured at a large forward bias, as displayed in Fig. 5(d). Fig. 5(e) plotted the time-dependent sensor responses of GMH under different gate voltages. Fig. 5(f) displayed the corresponding drain current against drain voltage curves before and after NO₂ exposure. It was found when a reverse bias or a large negative back-gate voltage was applied, the sensor responded over 10³ upon exposure to 1 ppm NO₂. This is because the drain current was determined by the NO₂-induced modulation in the GMH and the barrier height at the counter Schottky diode of the MoS₂/Ti contact was not reflected in the sensor response in both cases. Conversely, the response was decreased due to the fact that the barrier height at the counter Schottky diode of the MoS₂/Ti contact covered the response of NO₂-induced modulation.

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^[49]. Sun et al. synthesized rGO/MoS₂ composites by a hydrothermal method, using 2D rGO as template^[50]. The effect of the ratio of MoS₂ in rGO/MoS₂ composites for sensing hydrogen peroxide (H₂O₂) vapor was studied. The average response and response/recovery time towards 50 ppm of H₂O₂, C₃H₆O, C₂H₆O vapors at room temperature were shown in Figs. 6(a)–6(c). Results show that rGO/MoS₂ composites gas sensors had excellent selectivity toward H₂O₂ and the response/recovery time were both less than 20 s. Comparing with pure rGO sensor, the response toward H₂O₂ vapor achieved an increase of about 12 times. Kumar and his co-workers synthesized rGO/MoS₂ hybrid material with microwave-assistant method to fabricate gas sensor on polymer substrate^[51]. The effect of thickness of sensitive layer on sensing performance had been investigated. Results show that the rGO/MoS₂ layer had the best response toward NH₃ and lowest sensitivity towards N₂. When the thickness of rGO/MoS₂ layer was 2 mm, the ratio of response between NH₃ and N₂ differed three orders of magnitude. The response of the 10 μm thickness rGO/MoS₂ layer was about two orders of magnitude higher than that of the 2 mm thickness layer, indicating that only the top layer effectively attended the reaction with the gas molecules. Jung et al. fabricated highly transparent and flexible NO₂ gas sensor film based on MoS₂/rGO composites through soft lithographic patterning method^[52]. The transmittance of the thin film MoS₂/rGO composites gas sensor on PET substrate reached 93% and the sensing characteristics were still maintained with a bending radius of 14 mm as shown in Figs. 7(a) and 7(b). Moreover, the transparent and flexible gas sensor could detect a concentration as low as 0.15 ppm of NO₂. The sensitivity of the MoS₂/rGO composites thin film gas sensor was four times higher than that of pure rGO thin gas sensor under a bias voltage of 0.1 V at 90 °C. The sensing response also showed a composition dependent property as shown in Fig. 7(c). Results revealed that the MoS₂/rGO composite with a ratio of 1 : 2.5 presented the best response toward NO₂ which was four times higher than that of pure rGO gas sensor.

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^[53]. Fig. 8 shows the sensing performance of pure rGO sensor and MoS₂/rGO composite sensor, and both sensors exhibited p-type characters. As shown in Figs. 8(a) and 8(b), rGO/MoS₂ composite sensor exhibited a sensing response towards 2 ppm NO₂ approximately twofold to that of pure rGO sensor. Figs. 8(d)–8(f) depicted the I–V characteristics of rGO-Au, MoS₂-Au and rGO/MoS₂-Au contacts, respectively. An ohmic contact was observed for rGO-Au, MoS₂-Au contacts. With respect to the rGO/MoS₂-Au contact, obvious rectifying behavior was observed indicating the existence of p–n junctions at interface of rGO and MoS₂. The surface-absorbed NO₂ withdrew electrons from both rGO and MoS₂ and extended the hole accumulation region (HAR) on rGO surface and electron-shell depletion region (EDR) on MoS₂ surface. As the resistance change of MoS₂ was more reluctant than that of rGO, the HAR extension exerted more effect on total resistance alteration than EDR, which led to the resistance decreased. Li et al. fabricated a hybrid film composed of rGO and MoS₂ for formaldehyde detection by a simple layer-by-layer self-assembly method^[54]. The rGO/MoS₂ hybrid film exhibit fast response/recovery and high reproducibility than that of the MoS₂ film as shown in Figs. 9(a)–9(f). The enhanced sensing performance can be illustrated with Fig. 8(g), electrons donated by formaldehyde firstly transferred to conduction band of MoS₂ mediated by the surface adsorbed oxygen, then further transferred to the rGO substrate, leading to fast conductive change of the hybrid films. Such a two-step efficient electron transfer could enhance the electron transfer from analytes to rGO, thereby increasing the sensitivity of the hybrid films^{[55, 56]}.

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))^[57]. Two kinds of MoS₂ were prepared by either hydrothermal synthesis (HT) or chemical exfoliation (CE) method. The results revealed that the surface defects on MoS₂ enhanced the sensitivity of rGO/MoS₂ sensor, for the MoS₂ produced by HT process had more defects than that of CE method. Moreover, Fig. 10(e) shows the bending property of rGO/MoS₂-HT sensor. Only small decline of sensing response was observed suggesting the robustness of the flexible devices. The authors believed that the rGO in the rGO/MoS₂ composite film acted as a conductive network that bridges the electrodes and the continuous conducting channel resulting in a robust bending resistive sensing layer. The result of density functional theory calculation, as depicted in Fig. 10(c), shows that the MoS₂ nanosheets in the hybrid film acted as HCHO adsorbent and electron acceptors while the rGO served as a conducting network, and thus resulted in a p-type response. The introduction of MoS₂ enhanced the adsorption of HCHO molecules^[58] and decreased the energy barriers in electron transfer process^{[59, 60]}.

Paolucci et al. reported a NO₂ gas sensor fabricated from WS₂-decorated rGO composite^[56]. Fig. 11 shows the responses of a single rGO film and a WS₂-decorated rGO film to dry air at 25 and 50 °C. The substantial contribution of the WS₂ semiconductor to the overall resistance response was confirmed by comparing the baseline values. The elevated operating temperature at 50 °C resulted in an enhanced response. However, the drift of resistance baseline indicated the serious drawback of WS₂ decorated rGO hybrid as gas sensor material. Fig. 11(b) presents a schematic illustration of the sensing model of WS₂-decorated rGO film, in which NO₂ as electron acceptor captured electrons form n-type WS₂ surface. As a consequence, electron-depleted n-type WS₂ flakes drained electrons from the underlying p-type rGO resulting in the increase of hole concentration in p-type rGO and decrease of the overall resistance of the p-type WS₂-decorated rGO. The rapid electron transport from the highly conducting rGO to the less-conducting WS₂ contributed much to the final sensing performance in which the rGO flakes serve as highly conductive channels bridging the distant electrodes.

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^[61]. The atomic ratio of Mo and W was well controlled and a series of MoS₂/WS₂ heterostructures were synthesized. Figs. 12(a)–12(c) show the response value and response/recovery time of those MoS₂/WS₂ heterostructure sensors upon exposure to 0.01–50 ppm NO₂. The results show there was an optimized atomic ratio of Mo and W which exhibits best response, quickest response/recovery time and lowest detection limit. The optimized heterostructure sensor exhibited very quick response (6 s) and recovery (16 s) at room temperature (Figs. 12(b)) and 12(c)). The MoS₂/WS₂ heterostructure sensors presented p-type sensing behavior as the resistance of the sensor decreased abruptly upon exposure to NO₂, as shown in Fig. 12(e). Impressively, the MoS₂/WS₂ heterostructure sensor with optimized composition achieved the lowest detection limit of 0.01 ppm, much lower than that of pure MoS₂ (Fig. 12(d)). Figs. 12(e) and 12(f) demonstrated the excellent reproducibility and high selectivity of the MoS₂/WS₂ heterojunction based NO₂ sensor. This work suggests that the integration of two TMDCs could not only improve the sensing response but also address the mail challenges existing in single TMDC-based gas sensor, i.e. slow response speed and low selectivity. Sun et al.^[55] had studied the geometry, electronic structures, and electron transport properties of the MoS₂/WS₂ heterojunction with first principles calculations. The adsorption of CO, H₂O, NH₃, NO, and NO₂ gas molecules on the MoS₂/WS₂ heterojunction had also been studied. Results show that NH₃ performed as electron donor and all other gases performed as electron acceptor. The gas molecule adsorption significantly affected the electronic transport properties of the heterojunction. Both the rectification behavior and the value of the passing current can be altered by gas adsorption.

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^[62]. As shown in Fig. 13, the gas sensor based on rGO-MoS₂-CdS films, with the higher specific surface area, more adsorption sites and lots of heterojunctions, showed a largely enhanced sensor response of 27.4% toward 0.2 ppm NO₂, about 7 times higher than the value of binary rGO-MoS₂ 2D nanocomposite based gas sensor. Moreover, the gas sensor presented an outstanding selectivity toward NO₂ gas against the other gases.

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^[63]. Fig. 14(a) displays the device configuration where the top-gate of BP with a superior gas adsorption capability serves as the sensing material, while the conduction channel of MoS₂ is isolated from ambient environment by the coverage of the BN dielectric layer. The adsorption of the gas analyte on the gate material led to the charge transfer from the gas analyte to the gate material, which modulated the Fermi level of both the gate and conduction channel materials, and thus the resistance of the conduction channel. Figs. 14(b) and 14(c) plot the band structure of the 2D material FET before and after NO₂ adsorption. When the device was exposed to the NO₂ gas, the Fermi level of p-doped BP shifted towards the valence band, leading to the shift of MoS₂ towards the valence band. The movement of Fermi level of MoS₂ resulting in the increase of channel resistance at fixed gated and source-drain biases. The kinetic information of gas adsorption on the device was then able to obtain. Fig. 14(f) shows the real-time response of the device to NO₂ with concentrations ranging from 10–100 ppb. The device failed to fully recover back to the baseline upon in N₂ for 15 min, which may be due to the strong adsorption energy between BP and NO₂. The sensitivity of the ternary 2D FET device for NO₂ achieved 1.19% resistance change per ppb, and the detection limit to NO₂ was 3.3 ppb. As shown in Figs. 14(g)–14(i), FET device was also tested with NH₃ and various volatile organic compounds. The device resistance increased upon exposure to DCM, while decreasing upon exposure to NH₃, hexane, acetone and DMF.

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.

This work was supported by Zhejiang Provincial Natural Science Foundation of China (No. LY18F010009) and Ningbo Natural Science Foundation (No. 2018A610002).