High-performance NO2 Gas Sensor Research Articles

Highly responsive gas sensors based on grain boundary–rich polycrystalline few-layer MoS2 films for NO2 detection

Abstract This study addresses the issues of insufficient sensitivity and poor reversibility for NO2 detection by successfully fabricating a sensor based on uniform and high-quality few-layer MoS2 polycrystalline material using chemical vapor deposition. This approach aims to improve the response of the sensor by exploiting the abundance of grain boundary (GB) defects in polycrystalline MoS2 membranes. Comprehensive surface morphology analysis of the few-layer MoS2 polycrystalline films was conducted using microscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy to characterize their chemical composition and properties. Subsequently, evaluation of 1–100-ppm NO2 was conducted at room temperature (25 °C). The results show excellent performance of the sensor, with a response range of 11–82.24. Notably, under ultraviolet excitation at room temperature, this sensor exhibits a response time of only 41 s to 50 ppm of NO2 with complete recovery and improved sensitivity, maintaining reliable stability over eight weeks. Furthermore, the findings reveal that the sensor demonstrates high selectivity toward NO2 gas with limit of detection and limit of qualification values of 10 and 34 ppb, respectively. Owing to the abundant adsorption sites provided by GB defects in polycrystalline thin films, the response performance of the sensor is effectively enhanced. This study provides valuable insights into the future design and development of high-performance NO2 gas sensors.

N-n type In2O3@-WO3 heterojunction nanowires: enhanced NO2 gas sensing characteristics for environmental monitoring.

Solvothermal synthesis of 1D n-In2O3@n-WO3 heterojunction nanowires (HNWs) and their NO2 gas sensing characteristics arereported. The n-In2O3@n-WO3 HNWs have been well-characterised using XRD, Raman spectroscopy, XPS, SEM and HRTEManalyses. The NO2 sensing performance of n-In2O3@n-WO3 HNWs showed superior performance compared withpristine WO3 NWs. Due to the distinctive configuration of WO3-In2O3 heterojunctions, the n-In2O3@n-WO3 HNWs demonstrated remarkable sensitivity reaching 182% in response towards 500ppb of NO2 gas at operating temperature of 200°Cwhich is nearly 3.5 times greater than the response observed with pristine WO3 (50%). Moreover, the n-In2O3@n-WO3 HNWs also exhibited fast response (8-13s)/recovery (54-62s) time characteristics. A plausible sensing mechanism has been discussed. The enhancement in sensor characteristics shows that n-In2O3@n-WO3 HNWs could serve as a promising material for high-performance NO2 gas sensors for real-time environmentalmonitoringapplications. This work could provide new understandings of the sensing mechanism of n-In2O3@n-WO3-based heterojunction nanowires, which can be applied to the design of novel n-n type MOS heterojunction materials for the application of low-temperature real-time high-performance NO2 sensors.

High-performance NO2 gas sensor based on reduced graphene oxide/ZrO2 hybrids

The increasing concern over environmental pollution, particularly from nitrogen dioxide (NO2) emissions, necessitates the development of efficient NO2 detection sensors. This study introduces reduced graphene oxide (rGO)/ZrO2 hybrids for enhanced NO2 gas sensing. Utilizing a modified Hummer’s method, graphene oxide (GO) flakes were synthesized and subsequently sputtered with 10 nm ZrO2 film, followed by thermal annealing to produce rGO/ZrO2 hybrids. The hybrids were characterized using various techniques including SEM, TEM, AFM, Raman spectroscopy, and XRD, confirming successful synthesis and reduction of GO, as well as the formation of ZrO2 nanoparticles. Gas sensing tests revealed superior sensitivity to NO2 in the hybrids due to efficient electron transfer between rGO and ZrO2, resulting in increased hole concentration in rGO and enhanced conductivity. The cyclic performance of the hybrids showed stable response and recovery to NO2, while selectivity tests demonstrated high sensitivity to NO2 over other gases including NH3, ethanol, and oxygen. This study highlights the potential of rGO/ZrO2 hybrids as high-performance NO2 gas sensors, offering promising prospects for environmental monitoring and public health protection.

SnO2 Nanowire/MoS2 Nanosheet Composite Gas Sensor in Self-Heating Mode for Selective and ppb-Level Detection of NO2 Gas

The development of low-cost and low-power gas sensors for reliable NO2 gas detection is important due to the highly toxic nature of NO2 gas. Herein, initially, SnO2 nanowires (NWs) were synthesized through a simple vapor–liquid–solid growth mechanism. Subsequently, different amounts of SnO2 NWs were composited with MoS2 nanosheets (NSs) to fabricate SnO2 NWs/MoS2 NS nanocomposite gas sensors for NO2 gas sensing. The operation of the sensors in self-heating mode at 1–3.5 V showed that the sensor with 20 wt.% SnO2 (SM-20 nanocomposite) had the highest response of 13 to 1000 ppb NO2 under 3.2 V applied voltage. Furthermore, the SM-20 nanocomposite gas sensor exhibited high selectivity and excellent long-term stability. The enhanced NO2 gas response was ascribed to the formation of n-n heterojunctions between SnO2 NWs and MoS2, high surface area, and the presence of some voids in the SM-20 composite gas sensor due to having different morphologies of SnO2 NWs and MoS2 NSs. It is believed that the present strategy combining MoS2 and SnO2 with different morphologies and different sensing properties is a good approach to realize high-performance NO2 gas sensors with merits such as simple synthesis and fabrication procedures, low cost, and low power consumption, which are currently in demand in the gas sensor market.

High-performance NO2 gas sensor enabled by Fe, N co-doped GQDs modification and pulse-driven temperature modulation

Nitrogen dioxide (NO2) is a typical reactive nitrogen species harmful to health and the environment. However, NO2 sensors still suffer from low sensitivity and poor response/recovery rates. Herein, 3D In2O3 assembled by stacked nanosheets were prepared and modified with N element-doped GQDs (N-GQDs) and Fe, N co-doped GQDs (Fe,N-GQDs), respectively. The results of NO2-sensing performance indicated that the optimal sample (0.3 wt% Fe,N-GQDs/In2O3) showed a high response of 414.5 to 1 ppm NO2 and could detect as low as 10 ppb at 50℃, which was 1.3 and 4.0 times higher than that of 0.3 wt% N-GQDs/In2O3 and In2O3, respectively. The gas-sensing kinetics were analyzed by establishing gas adsorption/desorption isotherms. The enhanced NO2 sensing properties of 0.3 wt% Fe,N-GQDs/In2O3 were mainly due to the enhanced active site accessibility of stacked nanosheets with open layer spaces, the heterointerface between Fe,N-GQDs and In2O3, and electrical modulation of Fe,N-GQDs. Additionally, A pulse-driving circuit was designed for a pulse temperature modulation (PTM) strategy to further boost NO2 detection. The sensor response is further enhanced by 1.7 times, and the response/recovery time is reduced by 42.5 %/60.1 %. The NO2 concentration in vehicle exhaust was determined using PTM mode with good spike recoveries. It validated the reliability of the sensor for the quantitative detection of NO2 in the real environment. This work provides insights into sensor design and has the potential to contribute to the development of novel sensing platforms for other gas pollutants.

Fabrication of NiO based thin film for high-performance NO2 gas sensors at low concentrations

Recent industrial and technical developments continuously result in the release of dangerous and toxic gases that have adverse effects on human life and the environment. Metal oxide nanomaterials attract attention for extremely sensitive gas sensors owing to their high surface-to-volume ratio, thermal stability, availability, and simplicity of production. Currently, p-type NiO based gas sensors are very promising, due to their outstanding repeatability, high sensitivity, low cost, and environmentally friendly characteristics. In this work, NiO prepared by low-cost hydrothermal method and its gas sensing behavior towards NO2 gas has been studied in detail. The as-prepared structure of NiO has been analyzed by using various characterization techniques like XRD, FESEM, FTIR, BET and UV–visible absorption for the study of structural, morphological and optical properties. The NiO thin film deposited on a glass substrate was analyzed under varying concentrations of NO2 at different working temperatures (100–250 °C). The sensor demonstrated a remarkable sensitivity of 45 % to 10 ppm of NO2 at low operating temperature of 200 °C. Moreover, its consistent repeatability further underscores its reliability, emphasizing its suitability for accurate and dependable NO2 detection in diverse scenarios.

Extrinsic oxygen defects in SnO/SnO2 heterostructure for efficient NO2 gas detection

High selectivity and fast response in the detection of NO2 are key factors in protecting human health in life-threatening environments. The p-n SnO/SnO2 heterostructure is a promising material as a metal oxide for effective chemical reactions with a target gas. It has a synergistic effect on the intrinsic semiconducting properties and the formation of an interfacial electric field in the heterojunction. The extrinsic oxygen vacancies in the SnO/SnO2 structure enhance the chemical reaction in the sensing property. In this study, we developed a defect-SnO/SnO2 heterostructure-based NO2 gas sensor. It shows excellent sensing performances with a high response of 100.86, fast response time of 5.83 s, and superior selectivity (approximately 23 times higher than those of other target gases) in the detection of 50 ppm NO2 at 150 °C. Raman and electron paramagnetic resonance (EPR) results supported the presence of oxygen vacancies in the defect-rich SnO/SnO2 crystal and X-ray photoelectron spectroscopy (XPS) and time-of flight secondary ion mass spectrometry (ToF-SIMS) analyses described the role of SnO/SnO2 heterostructure with oxygen defects as an effective electron donor for the adsorbed oxygen molecules and NO2 target gas. We believe that the direct formation of oxygen vacancies in the SnO/SnO2 heterostructures is a significant reference for the development of high-performance NO2 gas sensors.

NO2 Adsorption Sensitivity Adjustment of As/Sb Lateral Heterojunctions through Strain: First Principles Calculations

Strain engineering is an effective way to adjust the sensing properties of two-dimensional materials. In this paper, lateral heterojunctions (LHSs) based on arsenic and antimony have been designed along the armchair (AC) or zigzag (ZZ) edges. The adsorption and sensing characteristics of As/Sb LHSs to NO2 before and after applying different types of strain are calculated by first principles. The band gaps of all As/Sb heterostructures are contributed by As-p and Sb-p orbitals. In addition, the adsorption energy of As/Sb ZZ-LHS with −4% compression strain is the largest. Furthermore, its work function changes significantly before and after the adsorption of NO2. Meanwhile, strong orbital hybridizations near the Fermi level are observed and a new state is yielded after applying compressive strain. These results indicate that the As/Sb LHS with ZZ interface under −4% compression strain possesses the best sensing properties to NO2. This work lays the foundation for the fabrication of high-performance NO2 gas sensors. High-performance gas sensors can be used to track and regulate NO2 exposure and emission, as well as to track NO2 concentrations in the atmosphere and support the assessment of air quality.

High-performance selective NO2 gas sensor based on In2O3–graphene–Cu nanocomposites

The control of atmosphere content and concentration of specific gases are important tasks in many industrial processes, agriculture, environmental and medical applications. Thus there is a high demand to develop new advanced materials with enhanced gas sensing characteristics including high gas selectivity. Herein we report the result of a study on the synthesis, characterization, and investigation of gas sensing properties of In2O3–graphene–Cu composite nanomaterials for sensing elements of single-electrode semiconductor gas sensors. The nanocomposite has a closely interconnected and highly defective structure, which is characterized by high sensitivity to various oxidizing and reducing gases and selectivity to NO2. The In2O3-based materials were obtained by sol–gel method, by adding 0–6 wt% of pre-synthesized graphene–Cu powder into In-containing gel before xerogel formation. The graphene–Cu flakes played the role of centers for In2O3 nucleation and then crystal growth terminators. This led to the formation of structural defects, influencing the surface energy state and concentration of free electrons. The concentration of defects increases with the increase of graphene–Cu content from 1 to 4 wt%, which also affects the gas-sensing properties of the nanocomposites. The sensors show a high sensing response to both oxidizing (NO2) and reducing (acetone, ethanol, methane) gases at an optimal working heating current of 91–161 mA (280–510 °C). The sensor with nanocomposite with 4 wt% of graphene–Cu additive showed the highest sensitivity to NO2 (46 ppm) in comparison with other tested gases with an absolute value of sensing response of (− ) 225 mV at a heating current of 131 mA (430 °C) and linear dependence of sensing response to NO2 concentration.

S-doped SnO2 derived from SnS nanoparticles for highly sensitive NO2 detection at room temperature

As the main pollutant in the air, NO2 is hugely harmful to the environment and health. It is crucial to develop a room temperature NO2 gas sensor with high response and good selectivity. In this work, S-doped SnO2 was successfully prepared by annealing with SnS nanoparticles as the precursor. S-doped SnO2 showed excellent response to 5 ppm NO2 (10.7), with a fast response (27 s) and recovery times (32 s), and good selectivity at room temperature (RT). The excellent NO2 sensitivity of S-doped SnO2 is attributed to its large specific surface area and large charge transfer between the substrate and NO2. This work can be extended to prepare high-performance NO2 gas sensors by annealing other transition metal dichalcogenides.

Synthesis of rGO-SnO2 nanocomposites using GO as an alkali-resistant substrate for high-performance detection of NO2

GO was used as a substrate to controllable synthesize rGO-SnO2 nanocomposites for preparing high-performance NO2 gas sensors. However, no product was obtained in the absence of GO substrates in a strongly alkaline environment at pH of 13.12. GO makes up for the shortcomings of alkali resistance of ordinary substrates. The flower-like hierarchical rGO-SnO2 nanocomposites were self-assembled from French fries-like SnO2 nanorods and rGO nanosheets. And each flower was composed of several French fries-like SnO2 nanorods with a length of about 800 nm and a square-shaped side length of approximately 80 nm. These SnO2 nanorods exposed profusely {200} crystal faces with high surface energy. The rGO-SnO2 nanocomposites based sensor showed a high response of 1809–1 ppm NO2 with fast response/recovery times of 251 s/14 s at 125 °C as well as good selectivity, high reproducibility, and long-term stability. The fast recovery speed of the sensor is due to the stable heating-assisted recovery method. The use of alkali-resistant GO as a substrate for nanoscale SnO2 growth effectively solves the problem of preparing SnO2 in a strongly alkaline liquid phase environment, which also contributes to the application of GO. This work provides a basis for developing high-performance NO2 gas sensors.

Visible light enhanced NO2 sensing performance of Au nanoparticles modified SnS2 hierarchical structure at room temperature

High-performance NO2 gas sensor at room temperature is vital in daily life. Herein, the performance of SnS2 hierarchical structure modified by Au nanoparticles was explored as a NO2 sensing material at room temperature. The samples were synthesized by solvothermal method and subsequent in-situ reduction method which could be confirmed by the analysis results of X-ray powder diffractometer (XRD), field-emission scanning electronic microscope (FESEM), transmission electron microscopy (TEM) and X-ray photoelectron spectra (XPS). Through a series of gas sensing measurements, the sensor based on Au-SnS2 possesses outstanding NO2 sensing properties under visible light at room temperature, including high response at trace concentration (14.24/5 ppm), excellent response/recovery time (29/148 s), low detection limit (36 ppb), satisfactory selectivity and repeatability. Combined with the first-principles calculations, the enhanced sensing performance can be contributed to the modification of Au with rich charge transfer (−0.5153 e) and the local surface plasmon resonance (LSPR) between Au and visible light with the enhanced optical absorption capacity and carrier density of the material. This work proffers opportunities for real-time NO2 detection in daily life.

Controllable synthesis of W18O49 nanoneedles for high-performance NO2 gas sensors

Controlling the morphology of W18O49 nanostructures is an effective way to achieve high NO2-sensing performance. Herein, we describe the synthesis of W18O49 nanostructures via a facile solvothermal process by reacting a WCl6 precursor with a reducing agent. Several well-designed experiments were performed to realize the morphology-controlled synthesis of W18O49 nanostructures by adjusting solvent types, WCl6 dosages, solvothermal temperatures and times. One-dimensional (1D) nanoneedles composed of monoclinic W18O49 with preferential growth along the (010) direction, which expose more surface oxygen vacancies and demonstrate a significant benefit for improved gas sensing performance, were obtained under optimized synthetic conditions. Subsequently, the W18O49 nanoneedles displayed a high response to NO2 with excellent dynamic response-recovery characteristics at the optimal operating temperature of 160 °C. Moreover, the W18O49 sensor detected NO2 in a wide range of concentrations (0.2–500 ppm) and showed good selectivity and long-term stability. The gas sensing ability of W18O49 nanoneedles toward NO2 is discussed, and rich oxygen vacancies, 1D needle-shaped nanostructures, and high BET surface area contributed to their outstanding gas sensing performance.

Recent developments in 2D MXene-based materials for next generation room temperature NO2 gas sensors.

MXenes with distinctive structures, good electrical conductivity and abundant functional groups have shown great potential in the fabrication of high performance gas sensors. Since the sensing mechanism of MXene-based gas sensors often involves a surface-dominant process, they can work at room temperature. In this regard, a significant amount of research has been carried out on MXene-based room temperature gas sensors and they can be viewed as one of the possible materials for NO2 sensing applications in the future. In this review, we focus on the most recent research and improvements in pure MXenes and their nanocomposites for NO2 gas sensing applications. First, we have explored the mechanisms involved in MXenes for NO2 gas sensing. Following that, other ways to tune the MXene sensing performance are investigated, including nanocomposite formation with metal oxides, polymers, and other 2D materials. A comparative analysis of the RT NO2 sensor performance based on MXenes and their hybrids is provided. We also discuss the major challenges of using MXene-related materials and the areas that can further advance in the future for the development of high-performance room temperature NO2 gas sensors.

Adsorption kinetics of NO2 gas on oxyfluorinated graphene film.

We report the fabrication of high-performance NO2 gas sensors based on oxyfluorinated graphene (OFG) layers. At room temperature, the times of adsorption/desorption of NO2 on/from the surface of thin OFG films are less than 1200 s and can be reduced by increasing the operation temperature. The sensors are capable of detecting NO2 molecules at sub-ppm level with a sensitivity of 0.15 ppm-1 at 348 K. The temperature dependence of the rate constants shows that the simultaneous presence of a large number of fluorine- and oxygen-containing groups on the graphene surface provides the formation of low-energy sites (ΔHa < 0.1 eV) for NO2 adsorption. The combination of the high sensitivity of the sensor and a reasonable adsorption/desorption time of the analyte is promising for on-line monitoring.

High-Performance Room-Temperature NO2 Gas Sensor Based on Au-Loaded SnO2 Nanowires under UV Light Activation.

Optical excitation is widely acknowledged as one of the most effective means of balancing sensor responses and response/recovery properties at room temperature (RT, 25 °C). Moreover, noble metals have been proven to be suitable as photosensitizers for optical excitation. Localized surface plasmon resonance (LSPR) determines the liberalization of quasi-free electrons in noble metals under light irradiation, and numerous injected electrons in semiconductors will greatly promote the generation of chemisorbed oxygen, thus elevating the sensor response. In this study, pure SnO2 and Au/SnO2 nanowires (NWs) were successfully synthesized through the electrospinning method and validated using XRD, EDS, HRTEM, and XPS. Although a Schottky barrier led to a much higher initial resistance of the Au/SnO2 composite compared with pure SnO2 at RT in the dark, the photoinduced resistance of the Au/SnO2 composite became lower than that of pure SnO2 under UV irradiation with the same intensity, which confirmed the effect of LSPR. Furthermore, when used as sensing materials, a detailed comparison between the sensing properties of pure SnO2 and Au/SnO2 composite toward NO2 in the dark and under UV irradiation highlighted the crucial role of the LSPR effects. In particular, the response of Au/SnO2 NWs toward 5 ppm NO2 could reach 65 at RT under UV irradiation, and the response/recovery time was only 82/42 s, which far exceeded those under Au modification-only or optical excitation-only. Finally, the gas-sensing mechanism corresponding to the change in sensor performance in each case was systematically proposed.

Urchin-like Bi2S3 nanostructures with rich sulfur vacancies for ppb-level NO2 sensing

The unique features of Bi2S3 enable it to be a promising candidate for preparing high-performance NO2 gas sensors. However, pristine Bi2S3 is usually inert for effective detection of ppb-level NO2 at room temperature owing to the lack of active sites. Herein, urchin-like Bi2S3 nanostructures with rich-defects were designed and prepared by a one-step hydrothermal method to realize ppb-level NO2 detection at room temperature. The optimal sensor based on urchin-like Bi2S3 with sulfur defects demonstrated an ultrahigh sensing response of 21.7 (almost 13 times higher than that of pristine Bi2S3) and a short response/recovery time of 42/36 s to 1 ppm NO2. In addition, an ultralow theoretical detection limit of 3 ppb was achieved for the sensor. The improved sensing performance could be ascribed to the numerous sulfur vacancies and the distinctive hierarchical structures, which promote the adsorption of NO2 molecules and boost charge transfer between NO2 and Bi2S3. This study would provide a new perspective on nanostructured materials for NO2 sensing at room temperature.

Impedimetric-type NO2 sensor based on the p-NiO/n-NiNb2O6 heterojunction sensing electrode

Development of high-performance NO2 gas sensors with good sensitivity and long-term stability at high temperatures is expected for exhaust gas monitoring. The sensor performance mainly depends on sensing material. In this work, p-NiO/n-NiNb2O6 heterojunction material is successfully synthesized and an impedimetric NO2 sensor was assembled using the above material as sensing electrode (SE) and gadolinia-doped ceria (CGO) with a dense/porous bilayer structure as a solid electrolyte. Introduction of SE to the CGO porous layer by screen printing followed by cold isostatic pressing treatment before calcination allows SE to successfully enter inside the porous layer and greatly increases the area of the three-phase boundary (TPB) and improves the long-term stability of the sensor. Using NiO/NiNb2O6 heterojunction SE can effectively improve the modulus (|Z|) response value (S|Z| = (|Z|air - |Z|gas)/|Z|air) of the sensor. Among the sensors, 90NiO/10NiNb2O6 (the mass ratio of NiO to Nb2O5 is 9:1) based NO2 sensor exhibits the best response performance and the highest sensitivity (0.34/ppm at 500 °C). The sensor also shows an excellent anti-interference property. Introduction of 300 ppm CO2, CH4, NO, CO, or H2 into 300 ppm NO2 makes response signal change below 3 % and 300 ppm NH3 only leads to signal reduction by 8.9 %, which is relatively low in a similar sensing system. The sensor response toward NO2 is slightly affected by O2 concentration change in 5–21 %. The superior performance of the sensor can be principally attributed to the heterojunction effect, which promotes the charge transfer and the adsorption capacity for NO2.

Three-Dimensional MoS2/Reduced Graphene Oxide Nanosheets/Graphene Quantum Dots Hybrids for High-Performance Room-Temperature NO2 Gas Sensors.

This study presents three-dimensional (3D) MoS2/reduced graphene oxide (rGO)/graphene quantum dots (GQDs) hybrids with improved gas sensing performance for NO2 sensors. GQDs were introduced to prevent the agglomeration of nanosheets during mixing of rGO and MoS2. The resultant MoS2/rGO/GQDs hybrids exhibit a well-defined 3D nanostructure, with a firm connection among components. The prepared MoS2/rGO/GQDs-based sensor exhibits a response of 23.2% toward 50 ppm NO2 at room temperature. Furthermore, when exposed to NO2 gas with a concentration as low as 5 ppm, the prepared sensor retains a response of 15.2%. Compared with the MoS2/rGO nanocomposites, the addition of GQDs improves the sensitivity to 21.1% and 23.2% when the sensor is exposed to 30 and 50 ppm NO2 gas, respectively. Additionally, the MoS2/rGO/GQDs-based sensor exhibits outstanding repeatability and gas selectivity. When exposed to certain typical interference gases, the MoS2/rGO/GQDs-based sensor has over 10 times higher sensitivity toward NO2 than the other gases. This study indicates that MoS2/rGO/GQDs hybrids are potential candidates for the development of NO2 sensors with excellent gas sensitivity.

Porous reduced graphene oxide for ultrasensitive detection of nitrogen dioxide

The defect engineering in graphene plays a significant role for the application of gas sensors. In this work, we proposed an efficient method to prepare ultrasensitive gas sensors based on the porous reduced graphene oxide (PRGO). Photo-Fenton etching was carried out on GO nanosheets in a controlled manner to enrich their vacancy defects. The resulting porous graphene oxide (PGO) was then drop-coated on interdigital electrodes and hydrothermal reduced at 180 °C. Controllable reduction was achieved by varying the water amount. The gas sensor based on PRGO-5 min-6 h exhibited superior sensing and selective performance toward nitrogen dioxide (NO2), with an exceptional high sensitivity up to 12 ppm−1. The theoretical limit of detection is down to 0.66 ppb. The excellent performance could be mainly attributed to the typical vacancy defects of PRGO. Some residue carboxylic groups on the edges could also facilitate the adsorption of polar molecules. The process has a great potential for scalable fabrication of high-performance NO2 gas sensors.