NO2 Gas Response Research Articles

Synthesis and Characterization of TiO2 Nanotubes for High-Performance Gas Sensor Applications

In this study, we investigated the fabrication, properties, and sensing applications of TiO2 nanotubes. A pure titanium metal sheet was used to demonstrate how titanium dioxide nanotubes can be used for gas-sensing applications through the electrochemical anodization method. Subsequently, X-ray diffraction indicated the crystallization of the titanium dioxide layer. Scanning electron microscopy and transmission electron microscopy then revealed the average diameter of the TiO2 nanotubes to be approximately 100 nm, with tube lengths ranging between 3 and 9 µm and the thickness of the nanotube walls being about 25 nm. This type of TiO2 nanotube was found to be suitable for NO2 gas sensor applications. With an oxidation time of 15 min, its detection of NO2 gas showed a good result at 250 °C, especially when exposed to a NO2 gas flow of 100 ppm, where a maximum NO2 gas response of 96% was obtained. The NO2 sensors based on the TiO2 nanotube arrays all exhibited a high level of stability, good reproducibility, and high sensitivity.

Polar tri-glycol side chain for enhanced detection to low concentrations of NO2 at room temperature

It has been widely studied the impact of polar side-chain on the modification of conjugated polymers and the thermoelectric properties of composites, but relatively little research has been carried out in the gas sensing field. In this study, two π-conjugated polymers PFTC8 and PFTO with similar backbone but different polar-side chains were synthesized. The separation efficiency of the polymers for semiconducting single-walled (sc-SWCNTs) and the NO2 sensing performance were also investigated. It was found that the introduction of the polar side chains not only enhanced the sorting efficiency of the polymer for sc-SWCNTs (greater than 99 %), but also further improved the responsive performance of the sensor to NO2. After the introduction of polar side chains, the response value of the composite PFTO@sc-SWCNTs towards 1 ppm NO2 was achieved to be 21.7 % at room temperature. Furthermore, the response time (Tres = 23 s) and recovery time (Trec = 134 s) of PFTO@sc-SWCNTs to 1 ppm NO2 were also greatly shortened. The strategy for improving the NO2 gas response performance by introducing polar side chains is proposed.

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.

A room-temperature ppb-level detection limit of NO2 sensor based on Cellulose/SnO2 heterojunction under UV illumination

In light of growing concerns over air pollution, particularly NO2 emissions from vehicular and industrial sources, there is a critical demand for advanced gas sensors. Traditional metal oxide semiconductors, while widely used, face challenges such as poor selectivity and high operating temperatures. In this work, we have achieved a high selectivity and room temperature NO2 gas sensor by composite of sponge-like porous cellulose with SnO2 nanoparticle. As a result, the gas sensor exhibits excellent performance in terms of high response (260.4 at 20 ppm), excellent selectivity and low detection limit (100 ppb) for NO2 gas, due to the unique sponge-like porous structure of cellulose with its high specific surface area facilitating the uniform distribution of SnO2 particles. This study not only highlights the superior sensing capabilities of cellulose-based sensors but also lays the foundation for further advancements in environmentally-friendly gas sensing applications.

Enhanced NO2 gas response of ZnO–Ti3C2Tx MXene nanocomposites by microwave irradiation

ZnO–Ti3C2Tx MXene nanocomposites with varying MXene contents (0.5, 1, 2, and 5 wt%) were synthesized for NO2 detection, and the impact of microwave (MW) irradiation time (1–8 min) on NO2 response was explored. It was observed that the sensor containing 2 wt% Ti3C2Tx MXene, irradiated for 5 min, indicated the best response of 42.65 to NO2 (10 ppm) at 300°C. Additionally, the optimal gas sensor demonstrated long-term stability (over six months), and reproducibility. The boosted NO2 response was ascribed to the creation of ZnO–MXene Schottky barriers, an increase in oxygen vacancies due to MW irradiation, a large surface area of the nanocomposite sensor, and the presence of surface groups on MXene. We confirmed the promising effects of MW irradiation in enhancing gas sensing, showcasing it as a cost-effective and readily available technique.

NO2 gas response and recovery properties of ambipolar CNT-FETs with various CNT/CNT junctions

Gas sensors based on ambipolar carbon nanotube (CNT) field-effect transistors with various amounts of CNTs were fabricated by dielectrophoretic assembly. The nitrogen dioxide (NO2) gas response and recovery properties of the transistors were measured to investigate the effect of CNT amount on gas response. For the device with a small amount of CNTs, responses from the CNT bulk and CNT/electrode contacts were observed. For devices with a large amount of CNTs, in which a network-like structure of CNTs was observed near the electrodes, an increased current in both electron and hole conduction regions was observed compared with that for the device with a small amount of CNTs. The increased current in the electron conduction region rapidly decreased during recovery. This response is consistent with that of CNT/CNT X-type contacts, which have a high resistance before NO2 adsorption. Equivalent circuits of CNT channels with CNT/CNT contacts were developed, allowing the transistor behavior to be qualitatively discussed. Evaluation of time constants revealed that CNT/electrode contacts and CNT/CNT X-type contacts exhibited high NO2 adsorption and desorption rates, respectively.

High-response room-temperature NO2 gas sensor fabricated with thermally reduced graphene oxide-coated commercial cotton fabric

Electronic textile-based gas sensors with a high response for NO2 gas were fabricated using reduced graphene oxide (rGO)-coated commercial cotton fabric (rGOC). Graphene oxide (GO) was coated on cotton fabric by simply dipping the cotton into a GO solution. To investigate the relationship between the degree of reduction and the sensing response, the GO-coated fabrics were thermally reduced at various temperatures (190, 200, 300, and 400 °C). The change in the amount of oxygen functional groups on the rGOCs was observed by x-ray photoelectron spectroscopy, Raman spectroscopy, and x-ray diffraction patterns. The maximum sensing response of 45.90 % at 10 ppm of NO2 gas at room temperature was exhibited by the rGOC treated at 190 °C, which was the lowest heat-treatment temperature. The high response comes from the greater amount of oxygen functional groups compared to other rGOC samples, and the tubular structure of the cotton.

Ce-doped ZnO nanostructures: A promising platform for NO2 gas sensing

In this comprehensive study, Ce-doped ZnO nanostructures were hydrothermally synthesized with varying Ce concentrations (0.5%, 1.0%, 1.5%, and 2.0%) to explore their gas-sensing capabilities, particularly towards NO2. Structural characterization revealed that as Ce doping increased, crystal size exhibited a slight increment while band gap energies decreased. Notably, the 0.5% Ce-doped ZnO nanostructure demonstrated the highest NO2 gas response of 8.6, underscoring the significance of a delicate balance between crystal size and band gap energy for optimal sensing performance. The selectivity of the 0.5% Ce-doped ZnO nanostructures to NO2 over other gases like H2, acetone, NH3, and CO at a concentration of 100 ppm and an optimized temperature of 250 °C was exceptional, highlighting its discriminatory prowess even in the presence of potential interfering gases. Furthermore, the sensor displayed reliability and reversibility during five consecutive tests, showcasing consistent performance. Long-term stability testing over 30 days revealed that the gas response remained almost constant, indicating the sensor's remarkable durability. In addition to its robustness against humidity variations, maintaining effectiveness even at 41% humidity, the sensor exhibited impressive response and recovery times. While the response time was swift at 11.8 s, the recovery time was slightly prolonged at 56.3 s due to the strong adsorption of NO2 molecules onto the sensing material hindering the desorption process. The study revealed the intricate connection between Ce-doping levels, structure, and gas-sensing. It highlighted the 0.5% Ce-doped ZnO nanostructure as a highly selective, reliable, and durable NO2 gas sensor, with implications for future environmental monitoring and safety.

Multi-Functional Applications of H-Glass Embedded with Stable Plasmonic Gold Nanoislands.

Metal nanoparticles (MNPs) are synthesized using various techniques on diverse substrates that significantly impact their properties. However, among the substrate materials investigated, the major challenge is the stability of MNPs due to their poor adhesion to the substrate. Herein,it isdemonstrated how a newly developed H-glass can concurrently stabilize plasmonic gold nanoislands (GNIs) and offer multifunctional applications. The GNIs on the H-glassaresynthesized using a simple yet, robust thermal dewetting process. The H-glass embedded with GNIs demonstrates versatility in its applications, such as i) acting as a room temperature chemiresistive gas sensor (70% response for NO2 gas); ii) serving as substrates for surface-enhanced Raman spectroscopy for the identifications of Nile blue (dye) and picric acid (explosive) analytes down to nanomolar concentrations with enhancement factors of 4.8 × 106 and 6.1 × 105 , respectively; and iii) functioning as a nonlinear optical saturable absorber with a saturation intensity of 18.36 × 1015 Wm-2 at 600nm, and the performance characteristics are on par with those of materials reported in the existing literature. This work establishes a facile strategy to develop advanced materials by depositing metal nanoislands on glass for various functional applications.

Titanium-based metal-organic-framework-coated SnO2 nanowires with enhanced NO2 gas sensing capability in humid environment

Herein, the NO2 gas sensing features of SnO2 nanowires (NWs) and titanium-based metal–organic framework 901 (MOF-901)-coated SnO2 NWs were investigated. Both gas sensors showed excellent selectivity to NO2. At 200 °C, the responses of SnO2 NWs and MOF-901-coated SnO2 NWs to NO2 (10 ppm) were 9.7 and 8.2, respectively. However, in a humid atmosphere, the latter nanowires showed better performance, mainly because of their hydrophobic nature and the metal–organic framework layer. At relative humidities (RHs) of 20% and higher, the MOF-901-coated sensor showed higher performance than the pristine sensor. In particular, at 80% RH, responses of 6.6 and 4.0 were obtained for these two sensors to 10 ppm NO2, respectively. Furthermore, the MOF-901-coated SnO2 NWs showed better selectivity than the other sensor. We showed that MOF-901 behaved an important role in reducing the effects of water vapor and that it can be used as a molecular sieve for enhancing the NO2 gas response of SnO2 NWs in humid environments.

Utilization of two-dimensional multicomponent Quasicrystal for NO2 gas detection

The two-dimensional (2D) materials provide an effective ultra-sensitive response for sensing due to the large surface area available for the analyte. Al70Co10Fe5Ni10Cu5 (Quasicrystal-QC) alloy can be effectively exfoliated into a 2D form consisting of the decagonal quasicrystalline phase. The ultrathin thickness of a few nm (∼6 nm) was achieved, exhibiting anisotropic electron-transport properties. The 2D-QC shows pseudo-conducting behavior in the planar region, which endowed the material in nanosheets to work as ultrasensitive NO2 gas sensors (1–100 ppm) at 100 °C. The fabricated device exhibits a NO2 gas response (ΔR/Ra %) of 46% compared to its bulk counterpart. In addition, the mechanism of gas sensing was further corroborated using density functional theory (DFT) simulations and strongly support the interpretation of the experimental data.

Optimization of synthesis conditions and sensing performance of electrospun NiFe2O4 nanofibers for H2S and NO2 detection

We evaluated the combined impact of process parameters, including electrospun time, applied voltages, and needle tip-to-collector distances, on the gas-sensing characteristics of on-chip sensors based on NiFe2O4 nanofibers fabricated using a conventional electrospinning approach. A porous structure and many nanograins were present in NiFe2O4 nanofibers with diameters of 30–100 nm. The sensor response for both H2S and NO2 gas reached its peak after 30 min of electrospinning. Stronger applied voltages and shorter spinning distances resulted in a higher sensor response to reducing H2S gas, whereas weaker applied voltages and longer spinning distances led to a higher sensor response to oxidizing NO2 gas. The NiFe2O4 nanofiber sensor showed a high response to 1 ppm H2S and 10 ppm NO2 of around 4.3 with quick response time (11 and 27 s) and slow recovery time (278 and 303 s), respectively, at the operating temperature of 350 °C. The NiFe2O4 nanofiber sensor also displayed outstanding stability and high selectivity. These gas-sensing results pointed to NiFe2O4 nanofibers as a potentially effective substitute for creating the perfect platform for H2S and NO2 detection.

Enhancement of selective NO2 gas sensing via Xenon ion irradiation of ZnO nanoparticles

Thanks to their impact on gas sensing enhancement, irradiation techniques are being progressively employed in gas-sensing applications. Effects of Xe irradiation doses on the NO2-sensing features of ZnO nanoparticle (NP) was investigated in this study. In this regard, ZnO NPs were irradiated with Xe ions at doses of 1 × 1014, 1 × 1015, and 1 × 1016 ions/cm2. The NO2 gas response of irradiated ZnO NPs was superior to that of the pristine one at the optimized sensing temperature of 200 ℃. Furthermore, irradiation with a dose of 1 × 1015 ions/cm2 brought about the highest response to NO2 gas, with a response of approximately 88.5 toward 10 ppm-NO2. Additionally, the optimized sensor exhibited outstanding NO2 gas selectivity. The presence of high amounts of oxygen vacancies owing to irradiation at the optimized dosage mainly resulted in the high response. This study can be used as guidelines to conduct similar studies on other materials to enhance their gas-sensing characteristics.

Au sputter-deposited ZnO nanofibers with enhanced NO2 gas response

A selective NO2 gas sensor was fabricated using Au-decorated ZnO nanofibers. A layer of Au was sputter-deposited on the synthesized ZnO nanofibers. The thicknesses of the Au layers were varied as 3, 6, 9, 12, and 15 nm for optimization. Annealing of the samples at a high temperature converted the Au layer to isolated Au nanoparticle (NPs) dispersed on the ZnO nanofibers. Different characterizations including structural, chemical, and morphological confirmed that the composition and morphology of the Au-decorated ZnO nanofibers were appropriate for gas sensing. The sensor with an Au layer thickness of 9 nm demonstrated an optimal response at 350 °C. The enhanced sensing response stemmed from the optimal amount of Au, generation of Au–ZnO Schottky junctions, and catalytic effect of Au NPs. This study highlights the need for the optimization of the amount of Au NPs to achieve improved response to a target gas.

Improved toxic NO2 gas sensing response of Cu-doped ZnO thin-film sensors derived by simple co-precipitation route

The application of ZnO nanomaterial in the gas sensing field has been extensively studied. The simple co-precipitation method was used to synthesize 0–4 at.% Cu-doped ZnO (ZC0-ZC4) nanomaterials and the drop-casting method was used to prepare ZC0-ZC4 thin-film sensors. XRD, FESEM and EDX, HRTEM, FTIR, UV–Visible spectroscopy and XPS techniques were used to examine the structural, morphological and spectral properties of pure ZnO and Cu-doped ZnO samples. The prepared ZC0-ZC4 thin-film sensors were investigated at various operating temperatures (120–280 °C) and NO2 gas concentrations (5–100 ppm). The gas sensing study reveals that the 3 at.% Cu-doped ZnO (ZC3) thin-film sensor shows a maximum 326% NO2 gas sensing response at 100 ppm concentration and 200 °C working temperature. The ZC3 sensor has high selectivity for the NO2 gas in the presence of different interfering gases such as Cl2, H2S, SO2 and NO2. The ZC3 sensor showed excellent reproducibility and response/recovery time for the NO2 gas response. The probable NO2 gas sensing mechanism of Cu-doped ZnO sensors is studied and discussed.

Superior sensitivity and low detection limit of NO2 sensor based on layered tungsten oxide

In this study, well-designed layer-like WO3 was synthesized by a facile hydrothermal route. Different characterization techniques have been conducted to comprehensively investigate the properties of morphology and structure of WO3-based sensing materials. The resultant consequences demonstrated that the layer-like WO3 with abundant mesoporous structure displayed higher specific surface area with respect to the pristine WO3 nanoplates. For the first time, the excellent gas sensing device based on layer-like WO3 was constructed successfully, which achieved an admirable gas-sensing response of 106 towards 5 ppm nitrogen dioxide gas at the working temperature of 160 ℃, with a favorable selectivity, and the lowest detection limit was reached to 0.243 ppb. Moreover, the layer-like WO3 sensor possessed considerable reproducibility and stability even in aging tests for 30 days. Those results can be ascribed to this promising structure optimization, which not only provides more reaction active sites but also promoted the gas sensing reaction procedure. Undoubtedly, this work is believed to be useful to offer a feasible strategy to enhance NO2 gas response in practical application.

Room temperature NO2 sensing performance of Ag nanoparticles modified VO2 nanorods

Ag-modified VO2 nanorods were prepared by a two-step method to improve the NO2 sensing performance at room temperature (25 °C). VO2 nanorods were synthesized by hydrothermal method, and Ag nanoparticles were decorated on the surface of VO2 nanorods by annealing. The effects of annealing temperature on the microstructure and room temperature NO2 sensing properties were studied. The morphology and crystal structure of the samples were characterized by field emission scanning electron microscope and X-ray diffraction. The results showed that Ag nanoparticles with a diameter of about 45 nm were tightly decorated on the surface of VO2 nanorods when the annealing temperature was 450 °C. The room temperature gas sensing measurement of sensor to NO2 with concentration spanned from 1 to 5 ppm was carried out. The gas sensing test results showed that the sample annealed at 450 °C had the maximum sensitivity, and the response value was up to 2.54–5 ppm NO2. The response value gradually decreases as the relative humidity increases. The sensor also showed good selectivity and reproducibility for detection of NO2 gas. The good sensing performance could be attributed to the catalytic effect of Ag nanoparticles and the changing of the energy band gap. At the same time, the mechanism of Ag-modified VO2 nanorods to NO2 gas response is further discussed. These results are significant to prompt the applications of VO2 in room temperature NO2 gas detection.

Selective, sensitive, and stable NO2 gas sensor based on porous ZnO nanosheets

In this study, we synthesized porous (porosity: ~16%, average pore size: ~60 nm) ZnO nanosheets (thickness: ~80 nm) using a conventional solvothermal method to investigate NO2 gas sensing properties. Porous ZnO nanosheets triggered the detection of NO2 gas with high sensitivity. Responses of 2.93 – 0.5 ppm and 74.68 – 10 ppm NO2 gas at 200 °C were observed in the porous ZnO nanosheet-based gas sensor. In addition, improved sensing properties with high selectivity to NO2 gas, reasonable stability, and high response even in the presence of water vapor molecules were obtained. We found that the enhanced NO2 gas response of the porous ZnO nanosheet-based gas sensor was due to the synergetic effects of the high surface area, ZnO/ZnO homojunctions, and structural defects. We developed a highly sensitive NO2 gas sensor with improved reliability using morphologically engineered ZnO, which was prepared via a simple and scalable chemical-synthesis route.

Room temperature ppt-level NO2 gas sensor based on SnO x /SnS nanostructures with rich oxygen vacancies

In this paper, tin oxidation (SnO x )/tin-sulfide (SnS) heterostructures are synthesized by the post-oxidation of liquid-phase exfoliated SnS nanosheets in air. We comparatively analyzed the NO2 gas response of samples with different oxidation levels to study the gas sensing mechanisms. The results show that the samples oxidized at 325 °C are the most sensitive to NO2 gas molecules, followed by the samples oxidated at 350 °C, 400 °C and 450 °C. The repeatabilities of 350 °C samples are better than that of 325 °C, and there is almost no shift in the baseline. Thus this work systematically analyzed the gas sensing performance of SnO x /SnS-based sensor oxidized at 350 °C. It exhibits a high response of 171% towards 1 ppb NO2, a wide detecting range (from 1 ppb to 1 ppm), and an ultra-low theoretical detection limit of 5 ppt, and excellent repeatability at room temperature. The sensor also shows superior gas selectivity to NO2 in comparison to several other gas molecules, such as NO, H2, SO2, CO, NH3, and H2O. After x-ray diffraction, x-ray photoelectron spectroscopy, scanning electronmicroscopy, transmission electron microscope, and electron paramagnetic resonance characterizations combining first principle analysis, it is found that the outstanding NO2 sensing behavior may be attributed to three factors: the Schottky contact between electrodes and SnO x /SnS; active charge transfer in the surface and the interface layer of SnO x /SnS heterostructures; and numerous oxygen vacancies generated during the post-oxidation process, which provides more adsorption sites and superior bandgap modulation. Such a heterostructure-based room-temperature sensor can be fabricated in miniaturized size with low cost, making it possible for large-scale applications.

Tunable NH4F-Assisted Synthesis of 3D Porous In2O3 Microcubes for Outstanding NO2 Gas-Sensing Performance: Fast Equilibrium at High Temperature and Resistant to Humidity at Room Temperature

NO2 gas sensors based on metal oxides under wild conditions are highly demanded yet an incomplete surface reaction and humidity interference on the gas-sensing performance limit their applications. Herein, we report three-dimensional (3D) porous In2O3 microcubes via a simple hydrothermal strategy to produce outstanding NO2 gas-sensing performance: fast equilibrium of the surface reaction at 150 °C and negligible humidity dependence on the NO2 gas sensing at room temperature. The 3D porous In2O3 microcubes with high surface areas, suitable pore sizes, rich oxygen vacancies, and high conductivity are testified. The underlying structural transformation mechanism for 3D porous In2O3 is investigated in detail. The as-made 3D porous In2O3 microcubic gas sensors present excellent gas-sensing performance to 50 ppm NO2 at 150 °C, including a high response value (2329), fast response/recovery time (10/9 s), a low detection limit (10 ppb), long-term stability (60 days), and strong selectivity. Furthermore, they exhibit relatively stable NO2 gas response under humidity variation (20-80%). The NO2 gas mechanism under the interference of water is also clarified.