NO2 Gas Response Research Articles

Negatively-Doped Single-Walled Carbon Nanotubes Decorated with Carbon Dots for Highly Selective NO2 Detection.

In this study, we demonstrated a highly selective chemiresistive-type NO2 gas sensor using facilely prepared carbon dot (CD)-decorated single-walled carbon nanotubes (SWCNTs). The CD-decorated SWCNT suspension was characterized using transmission electron microscopy (TEM), X-ray diffraction (XRD), and UV-visible spectroscopy, and then spread onto an SiO2/Si substrate by a simple and cost-effective spray-printing method. Interestingly, the resistance of our sensor increased upon exposure to NO2 gas, which was contrary to findings previously reported for SWCNT-based NO2 gas sensors. This is because SWCNTs are strongly doped by the electron-rich CDs to change the polarity from p-type to n-type. In addition, the CDs to SWCNTs ratio in the active suspension was critical in determining the response values of gas sensors; here, the 2:1 device showed the highest value of 42.0% in a sensing test using 4.5 ppm NO2 gas. Furthermore, the sensor selectively responded to NO2 gas (response ~15%), and to other gases very faintly (NO, response ~1%) or not at all (CO, C6H6, and C7H8). We propose a reasonable mechanism of the CD-decorated SWCNT-based sensor for NO2 sensing, and expect that our results can be combined with those of other researches to improve various device performances, as well as for NO2 sensor applications.

Unveiling the impact of interfacially engineered selective V2O5 nanobelt bundles with flake-like ZnO and Co–ZnO thin films for multifunctional visible-light water splitting and toxic gas sensing

We report on novel interfacial assembly behavior of V2O5 nanobelt bundles (V2O5 NBBs) paired with flake-like ZnO (V2O5/ZnO) and Co (1.8 at.%)-ZnO (V2O5/Co–ZnO) thin films for the first time. This work accomplished highly stable visible-light-driven photoelectrochemical (PEC) water splitting and toxic NO2 gas sensing. Mainly, charge carrier acceleration ability is associated with the interfacial diffusion behavior between nanobelt and flake-like morphologies. Structural studies revealed strong network formation between predominant ZnO (002) and V2O5 (001). Because of nanobelt/flake-like intimate interaction, we fairly established the flake-like ZnO and Co–ZnO morphological features on V2O5 NBBs. Accordingly, chemical analysis of V2O5/Co–ZnO illustrated a shift in the O 1s peak position towards lower binding energy, which is ascribed to the impressive combined oxygen bonding formation at the interface of Co–ZnO and V2O5. Taking advantage of the interfacial interaction and abundant hydrophilicity (63°) of V2O5/Co–ZnO, we controllably achieved stable water splitting activity about 21.0 min. In an effort, V2O5/Co–ZnO composite outperformed the photocurrent generation with 1.45 × 10−4 A/cm2, which is 42.0 times higher than ZnO. Also, V2O5/Co–ZnO attained NO2 gas response (130.0%) at 50 ppm. Comprehensively, nanobelt/flake-like morphological integration serves as a highly stable photoanode in the PEC cell as well as a toxic gas sensor.

Fabrication of enhanced sensitive and selective porous indium oxide nanocube sensor for NO2 detection

Porous nanoarchitectures of metal oxide semiconductors (MOS) are promising advanced materials due to their relatively higher surface area, more active sites and are beneficial for variety of applications. Herein, we report a simple method for making porous Indium oxide nanocubes (PINC) using a novel surfactant based biogenic hydrothermal method followed by a nanoscale Kirkendall effect. The nucleation and growth of PINCs were optimized by controlling amount of Cetyl Trimethylammonium Bromide (CTAB), which helps to control the size of nanocubes. The formation of porosity on the nanocubes through the nanoscale Kirkendall effect was proposed. Furthermore, the PINC based films were employed to fabricate solid-state chemiresistive gas sensor and sensing properties were studied. The PINC based film sensor was found to respond towards Carbon dioxide (CO2), Acetone ((CH3)2CO), Ammonia (NH3), and Nitrogen dioxide (NO2) gases. However, It exhibited highest gas response to NO2 gas (response S = 3450) with a shorter response (Rs = 14s) and recovery time (Rc = 115s) at 100 °C operating temperature. Besides, the sensing properties such as linearity, selectivity and real-time sensing analysis were also studied systematically. Herein, we demonstrated the effect of porosity and size of nanocubes on the sensing properties of PINC based sensor device.

Plasmon expedited response time and enhanced response in gold nanoparticles-decorated zinc oxide nanowire-based nitrogen dioxide gas sensor at room temperature

A highly sensitive and rapidly responsive nitrogen dioxide (NO2) gas sensor based on gold (Au) nanoparticles (NPs)-decorated zinc oxide (ZnO) nanowires (NWs) is presented. The Au NPs decoration was conducted onto ZnO NWs with and without a (3-aminopropyl)triethoxysilane (APTES) layer on their surface by using the electrostatic force. The samples without the APTES layer exhibited high NO2 gas sensitivity (i.e. expedited response time and enhanced gas response) due to localized surface plasmon resonance (LSPR) of the Au NPs; in particular, the NO2 gas response and the response time were increased by three times and shortened by 86%, respectively, compared with the undecorated ZnO NWs. The presence of the APTES layer improved the Au NPs attachment, but hindering the gas adsorption on the ZnO NWs surface, as proven by the observed photocurrent and gas response. Our findings imply that the response time of semiconductor gas sensors can be remarkably expedited by the LSPR effect, which is useful for developing practical gas sensors.

Morphology-dependent room temperature NO2 detection of CuO nanostructure/rGO composites

To study the effect of morphologies on the gas sensing behaviour, herein, we have synthesized three different CuO nanostructures, namely CuO nanobrick, CuO hierarchical flower-like and CuO nanochain. Hydrothermal method has been used to synthesize the CuO nanobrick and CuO hierarchical flower-like morphologies, whereas wet chemical method for CuO nanochain-like morphology. The present study reveals that all the CuO nanostructures-reduced graphene oxide (rGO) composite sensors show maximum response for NO2 gas at room temperature and intermediate humidity level (~ 50%RH). Furthermore, among all studied sensors, the CuO hierarchical flower-like/rGO sensor exhibits maximum NO2 gas-sensing response. At RT, the CuO hierarchical flower-like/rGO sensor shows response of ~ 58.1% in 30 s for 20 ppm NO2 which is ~ 1.7 times of CuO nanobrick/rGO sensor and almost twice of CuO nanochain/rGO composite sensor. Moreover, the gas sensing behaviours of composite samples synthesized with different weight (wt) ratios of rGO to CuO hierarchical flower-like for NO2 gas have also been studied to find optimum weight ratio of rGO and CuO hierarchical flower-like for which best sensing performance is observed. Consequently, CuO hierarchical flower-like with 25 wt% of rGO exhibits the highest response. All sensors show good reproducibility when tested for five successive cycles of 20 ppm NO2 and outstanding selectivity for NO2 gas. Furthermore, the sensing mechanism has been discussed in this paper by considering the role of morphology. Our work suggests that morphology of CuO and adding appropriate amount of rGO are crucial factors and influence the sensing behaviour of NO2 sensor.

A synergistic combination of zinc oxide nanowires array with dual-functional zeolitic imidazolate framework-8 for hybrid nanomaterials-based gas sensors

The complementary hybridization of nanomaterials enables to realize newly synergistic properties, which is a central concept in next-generation widespread applications. In particular, it is well-established that the hybridization of ZnO nanostructure with zeolitic imidazolate framework-8 (ZIF-8) allows the improvement of gas selectivity owing to molecular sieving capability caused by inherently size-defined porous structures of the ZIF-8. Here, we focused on a different viewpoint related with the preconcentration effect of ZIF-8 hybridized with ZnO nanowires (NWs) array on NO2, NH3, and H2 gas response. Hydrothermally-grown, structurally optimized ZnO NWs array are rationally employed for the nucleation sites of ZIF-8 nanocrystals as well as gas sensing channel. The chemical conversion of the surface of ZnO NWs to ZIF-8 was systematically implemented for the formation of core-shell hybrid structure, where morphological features of the ZIF-8 nanocrystals hybridized with the ZnO NWs array were optimized by simply adjusting synthetic conditions. The thickness of encapsulated ZIF-8 could be effectively manipulated by altering the concentration of 2-methylimidazole (HmIM), which strongly affects to determining gas response of ZIF-8@ZnO NWs-based gas sensors because of a variation in gas-loading capabilities related with the preconcentration effect. For the gas sensor based on ZIF-8@ZnO NWs synthesized using 8 mM HmIM solution, the gas response of NO2, NH3, and H2 gases were improved compared to those of the pristine ZnO NWs-based gas sensor. In addition, we consolidated the molecular sieving effect of the ZIF-8 by measuring the gas response of CH4 and C3H8 for suggesting the dual functionality of the hybrid system.

Growth of Fe doped ZnO nanoellipsoids for selective NO2 gas sensing application

In this work, Undoped ZnO and Fe-doped ZnO nanostructures were successfully synthesized by using the simple hydrothermal method and their structural, morphological by various characterization techniques and studied the gas sensing properties. The Field emission electron microscopy (FE-SEM) observed that the nanorods assembled together to form the flower like in undoped ZnO and after Fe doping it turns into nanoellipsoid like morphology. NO2 gas response analysis revealed that Fe-doped ZnO nanoellipsoids displayed a higher selectivity compared to undoped ZnO. Among the different doping concentration 8 wt% exhibited the highest response of 209.8 when exposed to 100 ppm concentration of NO2.

Two-dimensional MoSe2 nanosheets via liquid-phase exfoliation for high-performance room temperature NO2 gas sensors

Molybdenum selenide (MoSe2) has drawn significant interest due to its typical semiconductor properties. MoSe2 is a relatively novel material in the field of gas sensors especially at room temperature. Herein, we utilize a facile and efficient synthetic method of liquid-phase exfoliation to exfoliate bulk MoSe2 into nanosheets. Anhydrous ethanol is used as dispersant, so the low boiling point makes it easy to be removed from MoSe2 nanosheets, which does not affect the subsequent sensing performance. The exfoliated few-layered MoSe2 nanosheets shows significantly enhanced NO2 gas response (1500% to 10 ppm NO2 which is 18 times greater than pristine bulk MoSe2), a low detection concentration (50 ppb), an outstanding repeatability, a remarkable selectivity, and a reliable long-term device durability (more than 60 d) at room temperature (25 °C). The reason of the significant improvement in gas sensing performance can be attributed mainly to the higher surface-to-volume ratio of exfoliated MoSe2 nanosheets. It promotes the adsorption of gas molecules on the surface of the material, thereby facilitating the charge transfer process. The superior performance of this gas sensor makes MoSe2 nanosheets a potential candidate for room temperature NO2 detection.

Ag decorated WO3 sensor for the detection of sub-ppm level NO2 concentration in air

The WO3 has been broadly investigated for its application in the field of gas sensing. WO3 material was successfully synthesized by sol-gel method and the Ag nanoparticles decorated on WO3 sensor using drop-cast method. The morphological, structural and chemical properties of pure WO3 and Ag decorated WO3 (Ag-WO3) films were investigated using FESEM, AFM, EDX, HRXRD, XPS and TEM. The fabricated sensors were investigated at various operating temperatures (50–300 °C) and NO2 gas concentrations (0.1–3 ppm). The gas sensing study reveals that the 0.5%Ag-WO3 sensor shows higher NO2 sensing response at an operating temperature of 200 °C. The sensor has high selectivity for NO2 gas in the presence of different interfering gases such as NH3, acetone, SO2, methanol, CO2, NO and NO2. The sensor showed excellent reproducibility for NO2 gas response.

ZnO nanorod based highly selective visible blind ultra-violet photodetector and highly sensitive NO2 gas sensor

By using 1D ZnO nanorod, obtained from a simple two step chemical method as the sensor material, we report the fabrication of highly selective metal-semiconductor-metal (MSM) ultraviolet (UV) photodetector and NO2 gas sensor. The fabricated MSM UV photodetector and gas sensor possesses simplicity in material synthesis and sensor design. The effect of morphological evolution and structural intactness on the sensing properties of UV photodetector and NO2 gas sensor are deeply studied. The well aligned 1 dimensional (1D) ZnO nanorod provides high surface area for the sensing mechanism. The UV photodetector possesses high responsivity in the UV A region (364.81 A/W) with fast photoswitching characteristics at 5 V bias. It shows formation of good ohmic contact between metal semiconductor junctions. NO2 sensing measurement was carried out at relatively lower temperature of about 175 °C. The ZnO nanorods are highly selectivity towards NO2 gas and maximum gas response observed is 35 at 40 ppm of NO2. The NO2 gas sensor shows good repeatability at lower gas concentration of 2 ppm. The present synthesis technique provides cost effective route for obtaining highly ordered ZnO nanorods with simple sensor technology for multifunctional ZnO nanorod sensor.

EFFECT OF ANNEALED TEMPERATURE ON NO2 GAS-SENSING PERFORMANCES OF SnO2 NANOWIRE SENSORS

NO2 gas is a highly toxic gas and emitted from vehicles such as motocycles and internal engine transportations. It is one of the major reasons which cause air pollution in recent years, especially in big cities. Therefore, the development of NO2 gas sensor for environmental monitoring has been gained research attention in the global. In this work, we report a simple and effective method to prepare SnO2 nanowire gas sensors. The SnO2 nanowires were directly grown on the unplosihed Al2O3 substrate equipped with a pair of Pt-electrodes. The morphology and microstructure of as-grown nanowires have been investigated via X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM) and transimission electron microscopy (TEM). The results indicated that the diameters and lengths of nanowires are about 60 – 100 nm and tens of μm, respectively. The gas-sensing performance of the SnO2 nanowires sensors annealed at different temperatures have also investigated. The results revealed that the annealed temperatures of 400 C and 500 oC do not affected on gas-sensing performance of SnO2 nanowires sensors, while the annealed temperature of 600oC results in strong decrease in NO2 gas response as compared with as-grown SnO2 nanowires sensor.

Self-Formed Channel Devices Based on Vertically Grown 2D Materials with Large-Surface-Area and Their Potential for Chemical Sensor Applications.

2D layered materials with sensitive surfaces are promising materials for use in chemical sensing devices, owing to their extremely large surface-to-volume ratios. However, most chemical sensors based on 2D materials are used in the form of laterally defined active channels, in which the active area is limited to the actual device dimensions. Therefore, a novel approach for fabricating self-formed active-channel devices is proposed based on 2D semiconductor materials with very large surface areas, and their potential gas sensing ability is examined. First, the vertical growth phenomenon of SnS2 nanocrystals is investigated with large surface area via metal-assisted growth using prepatterned metal electrodes, and then self-formed active-channel devices are suggested without additional pattering through the selective synthesis of SnS2 nanosheets on prepatterned metal electrodes. The self-formed active-channel device exhibits extremely high response values (>2000% at 10 ppm) for NO2 along with excellent NO2 selectivity. Moreover, the NO2 gas response of the gas sensing device with vertically self-formed SnS2 nanosheets is more than two orders of magnitude higher than that of a similar exfoliated SnS2 -based device. These results indicate that the facile device fabrication method would be applicable to various systems in which surface area plays an important role.

Optimization of Zn2SnO4 thin film by post oxidation of thermally evaporated alternate Sn and Zn metallic multi-layers

An attempt to optimize Zn2SnO4 (ZTO) thin film has been done by alternate thermal evaporation of ‘Sn’ and ‘Zn’ in the sequence Sn10/Zn15/Sn15/Zn15/Sn5 onto corning glass substrates and subsequent post oxidation process. The XRD patterns of as-deposited metallic thin films changed over from metallic to oxidized ZTO phase with cubic spinel structure along with ZnO phase. The film thickness and roughness increased apparently due to diffusion of oxygen into the metallic inter-layers. The binding energies estimated from the XPS data also indicated oxidation of the metallic multi-layers. Post-annealing of the film developed transparency in visible region to around 40%. The direct band gap of ZTO and ZnO were estimated to be 3.46 eV and 3.10 eV respectively. The annealed ZTO thin film was measured for electrical transport parameters by Hall Effect measurement. Raman spectrum observed at 668 cm−1 is due to stretching vibration of short metal-oxide bonds in the MO6 octahedron. The photoluminescence spectrum showed blue emission centered around 436 nm predominantly due to oxygen vacancies in addition to weak orange emission centered at 625 nm. Gas sensor using thin films of as-deposited and ZTO formed by post-annealing process showed significant sensing response for NO2 gas. These results reveal that the ZTO thin film deposited by alternate thermal evaporation and subsequent post oxidation is worth exploring as transparent conducting electrode and also as a gas sensing material.

Effect of solution concentration on physicochemical and NO2 gas sensing properties of sprayed MoO3 nanobelts

In our previous report, we have reported the work on NO2 gas sensing properties of spray deposited molybdenum trioxide (MoO3) nanobelts at various substrate temperatures ranging from 300 °C to 450 °C with an interval of 50 °C. In the present work, MoO3 nanobelts have been spray deposited at optimized substrate temperature of 400 °C with different solution concentrations ranging from 10 mM to 50 mM at an interval of 10 mM solution concentration. X-ray diffraction study shows that the spray deposited MoO3 films exhibit an orthorhombic crystal structure. The formation of orthorhombic phase of MoO3 is confirmed using Raman spectroscopic study. Field emission-scanning electron microscopy images show the nanobelts-like morphology. Atomic force microscopy micrographs reveals that the root mean square (RMS) surface roughness of nanobelts increases from 12 nm to 17 nm with increase in solution concentration from 10 mM to 40 mM and then decreases to 8 nm for 50 mM solution concentration. Optical properties show that the band gap energy decreases to 3.25 eV for 50 mM solution concentration. For 40 mM solution concentration, 40.3% NO2 gas response at 200 °C is obtained for 100 ppm concentration with response and recovery times of 17 s and 153 s, respectively. Finally, energy band bending model based on chemisorption of O2 and NO2 gas on the surface of MoO3 nanobelts is discussed.

Palladium (Pd) sensitized molybdenum trioxide (MoO3) nanobelts for nitrogen dioxide (NO2) gas detection

The MoO3 nanobelts have been grown onto the glass substrates using chemical spray pyrolysis (CSP) deposition technique at optimized substrate temperature of 400°C. XRD study shows that the film is polycrystalline in nature and possesses an orthorhombic crystal structure. The FE-SEM micrographs show the formation of nanobelts-like morphology of MoO3. The presence of Pd and its oxidation states in Pd-sensitized MoO3 film is confirmed using EDAX and XPS study, respectively. The percentage gas response is defined as |Rg-Ra|Ra×100% where, Ra and Rg are the film resistances in presence of air and analyte gas, respectively. Before Pd sensitization, MoO3 nanobelts show NO2 gas response of 68% for 100ppm concentration at operating temperature of 200°C with response and recovery times of 15s and 150s, respectively. Selectivity coefficient study shows that the Pd-sensitized MoO3 nanobelts are more sensitive and selective towards NO2 gas among various gases such as NH3, H2S, CO, CO2 and SO2. The Pd-sensitized MoO3 nanobelts shows the enhanced response of 95.3% towards 100ppm NO2 gas concentration with response and recovery times of 74s and 297s, respectively. The lower detection limit is found to be 5ppm which is four times less than immediately dangerous to life or health (IDLH) value of 20ppm. Finally, the proposed NO2 gas sensing mechanism based on chemisorption model is discussed.

Gas sensing properties of (MoO3)0.4(V2O5)0.6 microsheets: Effect of Pd sensitization

The (MoO3)0.4(V2O5)0.6 microsheets have been successfully prepared onto the glass substrates using chemical spray pyrolysis (CSP) deposition method at optimized substrate temperature of 400 °C. The structural characterization using XRD confirm that both MoO3 and V2O5 exhibit an orthorhombic crystal structure. The two-dimensional FE-SEM micrograph shows the more porous distribution due to the random orientation of microsheets on the film surface. The EDAX analysis of Pd-sensitized (MoO3)0.4(V2O5)0.6 microsheets confirm the presence of Pd, Mo, V and O on the surface of microsheets. Selectivity study show the sensitive and selective response to NO2 gas than NH3, H2S, CO, CO2, SO2 gases at operating temperature of 200 °C. Before Pd-sensitization (MoO3)0.4(V2O5)0.6 microsheets show the NO2 gas response of 80% whereas Pd-sensitized (MoO3)0.4(V2O5)0.6 microsheets exhibit an enhanced response of 115% at 200 °C for 100 ppm NO2 gas concentration. The spill-over gas sensing mechanism reactions based on the chemisorption process at different operating temperatures is discussed to explain how Pd affect on the NO2 gas sensing properties of (MoO3)0.4(V2O5)0.6 microsheets.

Synthesis of flower shaped ZnO thin films for resistive sensing of NO2 gas

A soft chemical synthetic method is used to obtain highly crystalline ZnO flower thin films. Hexagonal wurtzite ZnO structure is observed from X-ray diffraction studies. A large-area jasmine flower-shaped morphology of ZnO is uniformly maintained at all deposition temperatures. The films were characterized by energy dispersive X-ray spectroscopy, Raman spectroscopy and field emission scanning electron microscopy. Elemental analysis showed the presence of Zn and O elements without any other impurity. Raman spectroscopy spectroscopy in combination with elemental and resistivity analysis indicated wurtzite ZnO structure and the presence of oxygen vacancies. The films deposited at 338 K were studied with respect to their gas sensing capability at operating temperatures of 473 K. They show a fast response (65 s) and recovery time (54 s) for NO2 gas at a concentration as low as 10 ppm.

Effect of film thickness on NO2 gas sensing properties of sprayed orthorhombic nanocrystalline V2O5 thin films

The nanocrystalline V2O5 thin films with different thicknesses have been grown onto the glass substrates using chemical spray pyrolysis (CSP) deposition method. The XRD study shows that the films exhibit an orthorhombic crystal structure. The narrow scan X-ray photoelectron spectrum of V-2p core level doublet gives the binding energy difference of 7.3eV, indicating that the V5+ oxidation state of vanadium. The FE-SEM micrographs show the formation of nanorods-like morphology. The AFM micrographs show the high surface area to volume ratio of nanocrystalline V2O5 thin films. The optical study gives the band gap energy values of 2.41eV, 2.44eV, 2.47eV and 2.38eV for V2O5 thin films deposited with the thicknesses of 423nm, 559nm, 694nm and 730nm, respectively. The V2O5 film of thickness 559nm shows the NO2 gas response of 41% for 100ppm concentration at operating temperature of 200°C with response and recovery times of 20s and 150s, respectively. Further, it shows the rapid response and reproducibility towards 10ppm NO2 gas concentration at 200°C. Finally, NO2 gas sensing mechanism based on chemisorption process is discussed.

Orthorhombic MoO3 nanobelts based NO2 gas sensor

Molybdenum trioxide (MoO3) nanobelts have been deposited onto the glass substrates using chemical spray pyrolysis (CSP) deposition method. The XRD patterns reveal that films are polycrystalline having an orthorhombic crystal structure. Raman spectra confirm that the films are orthorhombic in phase. The XPS study shows the presence of two well resolved spectral lines of Mo-3d core levels appearing at the binding energy values of 232.82eV and 235.95eV corresponding to Mo-3d5/2 and Mo-3d3/2, respectively. These binding energy values are assigned to Mo6+ oxidation state of fully oxidized MoO3. The FE-SEM micrographs show the formation of nanobelts-like morphology. The AFM micrographs reveal that the RMS surface roughness increases from 16.5nm to 17.5nm with increase in film thickness from 470nm to 612nm and then decreases to 16nm for 633nm film thickness. The band gap energy is found to be decreased from 3.40eV to 3.38eV. To understand the electronic transport phenomenon in MoO3 thin films, dielectric properties are studied. For 612nm film thickness, the highest NO2 gas response of 68% is obtained at an operating temperature of 200°C for 100ppm concentration with response and recovery times of 15s and 150s, respectively. The lower detection limit is found to be 10ppm which is half of the immediately dangerous to life or health (IDLH) value of 20ppm. Finally, NO2 gas sensing mechanism in an orthorhombic MoO3 crystal structure is discussed in detail.

Effect of Nb Doping on Chemical Sensing Performance of Two-Dimensional Layered MoSe2.

Here, we report that Nb doping of two-dimensional (2D) MoSe2 layered nanomaterials is a promising approach to improve their gas sensing performance. In this study, Nb atoms were incorporated into a 2D MoSe2 host matrix, and the Nb doping concentration could be precisely controlled by varying the number of Nb2O5 deposition cycles in the plasma enhanced atomic layer deposition process. At relatively low Nb dopant concentrations, MoSe2 showed enhanced device durability as well as NO2 gas response, attributed to its small grains and stabilized grain boundaries. Meanwhile, an increase in the Nb doping concentration deteriorated the NO2 gas response. This might be attributed to a considerable increase in the number of metallic NbSe2 regions, which do not respond to gas molecules. This novel method of doping 2D transition metal dichalcogenide-based nanomaterials with metal atoms is a promising approach to improve the performance such as stability and gas response of 2D gas sensors.