Ppm NO2 Research Articles

The Effect of Doping rGO with Nanosized MnO2 on Its Gas Sensing Properties

Manganese dioxide (MnO2) has drawn attention as a sensitiser to be incorporated in graphene-based chemoresistive sensors thanks to its promising properties. In this regard, a rGO@MnO2 sensing material was prepared and deposited on two different substrates (silicon and Kapton). The effect of the substrate nature on the morphology and sensing behaviour of the rGO@MnO2 material was thoroughly analysed and reported. These sensors were exposed to different dilutions of NO2 ranging from 200 ppb to 1000 ppb under dry and humid conditions (25% RH and 70% RH) at room temperature. rGO@MnO2 deposited on Kapton showed the highest response of 6.6% towards 1 ppm of NO2 under dry conditions at RT. Other gases or vapours such as NH3, CO, ethanol, H2 and benzene were also tested. FESEM, HRTEM, Raman, XRD and ATR-IR were used to characterise the prepared sensors. The experimental results showed that the incorporation of nanosized MnO2 in the rGO material enhanced its response towards NO2. Moreover, this material also showed very good responses toward NH3 both under dry and humid conditions, with the rGO@MnO2 sensor on silicon showing the highest response of 18.5% towards 50 ppm of NH3 under 50% RH at RT. Finally, the synthetised layers showed no cross-responsiveness towards other toxic gases.

A Study on Gas‑sensing Behavior of Reduced Graphene Oxide (rGO) Green Method, Enhansing the Performance of Green-synthesized Reduced Graphene Oxide

This paper presents a comparative study on gas-sensing behavior of reduced graphene oxide (rGO) synthesized by chemical and green synthesis route. GO is synthesized by Hummers method and then reduced employing two reducing agents hydrazine hydrate [represented by (rGO)1] and L-citrulline [represented by (rGO)2]. Synthesized products were then obtained in the form of thin films, and tested for 10 ppm NO2 and CO at operating temperatures of 50, 100, and 150 °C. The green-synthesized reduced graphene oxide (rGO)2 exhibits higher relative response of 254.7% as compared to conventionally synthesized (rGO)1 (93.9%) and GO (22.7%) for 10 ppm NO2 at operating temperature of 150 °C. Furthermore, a switching of conductivity from usual p-type behavior to n-type on exposure of NO2 is observed at all operating temperatures (50, 100, and 150 °C) for GO, (rGO)1, and (rGO)2. The XRD, FTIR, and Raman confirm the oxidation and reduction process. (rGO)2 shows high thermal stability as observed through TGA. FESEM and TEM images show wrinkled sheet structure for GO as well (rGO)1 and (rGO)2. The data observed from the characterization of resultant products have made it possible to explain better reduction of GO through green-reducing agent and enhanced gas-sensing performance of green-synthesized reduced graphene oxide.

Waste loofah sponge-derived graphitic carbon/ZnO tubules enabling ultrahigh response to NO2 gas at near room temperature

The fabrication of low-temperature and ultrahigh-response ZnO-based sensor is very important for real-time and effective monitoring of harmful NO2, but the less association has been reported to date. Herein, based on the concept of biomimetic manufacturing and waste reutilization, two biomorphic ZnO tubules were controllably prepared by calcining zinc nitrate solution immersed precursors in air using loofah sponge as template. Among them, the GC/ZnO tubules obtained from at 425 °C are composited of 5.26 wt% graphitic carbon (GC) and uniform nanoparticles, featuring small-size mesopores, large specific surface area, and abundant oxygen vacancies. Such advantageous features facilitate the rapid diffusion of target gases, increase the conductivity of sensing material, readily expose more active sensing units and promote surface chemical reactions, thereby effectively boosting the sensitive and selective detection of oxidizing NO2 gas. At near room temperature of 50 °C, GC/ZnO sensor achieves response value of 912 toward 5 ppm NO2 gas, which is 3.14 times higher than that of pure ZnO-500, and is the largest value among reported carbon-doped ZnO-based sensors. Additionally, it also exhibits other good comprehensive performance. Therefore, the cost-effective GC/ZnO tubules can serve as candidate material for accurately detecting NO2 gas in the atmospheric environment.

Enhancing reactive oxygen release-replenishment and Lewis acid acidity by Nb doping in NbmCeOx for simultaneous elimination of NOx and toluene

Multi-pollutant control (MPC) is a promising technology for the simultaneous removal of nitrogen oxidation (NOx) and volatile organic compounds (VOCs) in fossil and biomass fuel combustion. This study uses Nb doping CeO2 bimetallic oxide catalysts to meet low-temperature NH3-SCR de-NOx, catalytic toluene oxidation and simultaneous elimination of NOx and toluene. X-ray diffraction (XRD), Raman and transmission electron microscopy (TEM) are applied to confirm the Nb doping into the lattice of CeO2 for Nb0.034CeOx and Nb0.36CeOx, but excess Nb in polymeric state NbOx present in Nb0.36CeOx. The effects of Nb doping on surface adsorbed oxygen, surface lattice oxygen and bulk phase lattice oxygen were investigated by XPS, EPR, O2-TPD, H2-TPR and DFT calculations. Using in-situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS), it is investigated that the surface reactive oxygen species (including surface adsorbed oxygen and surface lattice oxygen) participates in NO oxidation, NH3 activation and toluene oxidation process. The reaction mechanism of NH3-SCR de-NOx, toluene oxidation and multi-pollutant co-elimination processes, as well as interactions between multiple reactants in the MPC reaction, were also studied with in-situ DRIFTS. It is found that Nb doping contributes to the mobility of lattice oxygen, which favors the oxidation of NH3 to NH2 and the release and rapid replenishment of reactive oxygen species at temperature ≥ 210 °C. The strong Lewis acid sites (Nb5+) promote the adsorption of NH3 species and their stability. These facilitate the NH3-SCR de-NOx in a wide temperature range and achieve the elimination of multiple pollutants on Nb0.034CeOx. So, Nb0.034CeOx exhibits matching activity temperature window in 170–390 °C for MPC (100 ppm toluene and 1000 ppm NO) over 90 % NO and toluene conversion in wet condition. Polymeric NbOx inhibits the activity of surface adsorbed oxygen and surface lattice oxygen and weakens the acidity of Lewis acid, therefore leading Nb0.36CeOx to poor performance of toluene oxidation and NH3-SCR at low-temperature.

MOF-derived La-doped ZnO dodecahedron nanostructures for efficient detection of NO2 gas

NO2 is a toxic gas that poses a significant threat to the environment and human health. In this work, we studied the ultra-high NO2 sensitivity of lanthanum (La)-doped ZnO porous nanocages. La doping into ZnO via a simple one-step co-precipitation MOF encapsulation-calcination route resulted in a significant improvement in NO2 sensing performance. This study focused on doping ZnO with 2 %, 4 %, and 8 % molar ratios of lanthanide sources. It was revealed that 2 % La-doped ZnO was the best possible parameter for NO2 gas sensitization. The La-doped ZnO sensor outperforms the pure MOF-derived ZnO sensor with a high response of 118–50 ppm NO2, short response time of 28 seconds, and a detection limit of 5.68 ppb. The material exhibits good reproducibility and some humidity resistance and has good selectivity for NO2. The excellent NO2 sensitivity can be attributed to the nanoporous structure providing abundant adsorption sites and the synergistic effect generated by the formation of a heterojunction between La-doped lanthanum oxide and zinc oxide. These findings can be generalized to the design of porous doped ZnO for high-performance gas sensors and other applications.

Solid-State Gas Sensors with Ni-Based Sensing Materials for Highly Selective Detecting NOx.

Precise monitoring of NOx concentrations in nitric oxide delivery systems is crucial to ensure the safety and well-being of patients undergoing inhaled nitric oxide (iNO) therapy for pulmonary arterial hypertension. Currently, NOx sensing in commercialized iNO instruments predominantly relies on chemiluminescence sensors, which not only drives up costs but also limits their portability. Herein, we developed solid-state gas sensors utilizing Ni-based sensing materials for effectively tracking the levels of NO and NO2 in the NO delivery system. These sensors comprised of NiO-SE or (NiFe2O4 + 30 wt.% Fe2O3)-SE vs. Mn-based RE demonstrated high selectivity toward 100 ppm NO under the interference of 10 ppm NO2 or 3 ppm NO2 under the interference of 100 ppm NO, respectively. Meanwhile, excellent stability, repeatability, and humidity resistance were also verified for the proposed sensors. Sensing mechanisms were thoroughly investigated through assessments of adsorption capabilities and electrochemical reactivity. It turns out that the superior electrochemical reactivity of NiO toward NO, alongside the NO2 favorable adsorption characteristics of (NiFe2O4 + 30 wt.% Fe2O3), is the primary reason for the high selectivity to NOx. These findings indicate a bright future for the application of these NOx sensors in innovative iNO treatment technologies.

Improving the NO2 Gas Sensing Performances at Room Temperature Based on TiO2 NTs/rGO Heterojunction Nanocomposites.

The development of energy-efficient, sensitive, and reliable gas sensors for monitoring NO2 concentrations has garnered considerable attention in recent years. In this manuscript, TiO2 nanotube arrays/reduced graphene oxide nanocomposites with varying rGO contents (TiO2 NTs/rGO) were synthesized via a two-step method for room temperature NO2 gas detection. From SEM and TEM images, it is evident that the rGO sheets not only partially surround the TiO2 nanotubes but also establish interconnection bridges between adjacent nanotubes, which is anticipated to enhance electron-hole separation by facilitating electron transfer. The optimized TiO2 NTs/rGO sensor demonstrated a sensitive response of 19.1 to 1 ppm of NO2, a 5.26-fold improvement over the undoped TiO2 sensor. Additionally, rGO doping significantly enhanced the sensor's response/recovery times, reducing them from 24 s/42 s to 18 s/33 s with just 1 wt.% rGO. These enhancements are attributed to the increased specific surface area, higher concentration of chemisorbed oxygen species, and the formation of p-n heterojunctions between TiO2 and rGO within the nanocomposites. This study provides valuable insights for the development of TiO2/graphene-based gas sensors for detecting oxidizing gases at room temperature.

Plasmonic Au-enabled photoenhanced room temperature NO2 sensing of 2D SnS2-heterostructure via single-step mechanochemical synthesis

The development of high-performance gas sensors for NO2 detection is critical due to its harmful effects on human health and environment. Tin disulfide (SnS2), with its layered structure akin to transition metal dichalcogenides (TMDs), shows potential for NO2 sensing. However, SnS2-based sensors typically exhibit weak response and incomplete recovery at room temperature. In this study, we propose 2D SnS2 nanosheets modified with gold nanoparticles (Au-NPs) by a single-step mechanochemical synthesis, in which photoelectric sensors depend on the light absorption properties of the sensitive material, thus Au-NPs with LSPR effect coupled with SnS2 can enhance the absorption capacity of the composite in the UV light band. The higher light absorption efficiency means that more electrons and holes can be produced under light excitation, and the holes can react with the NO2 adsorbed on the surface of the material, speeding up the desorption of NO2. As compared to the unmodified SnS2 sensor, the response values of Au/SnS2 sensor to 10 ppm NO2 at room temperature increased from 7.74 to 16.01, while the response/recovery time reduced from 169/201 s to 98/154 s. Moreover, the Au/SnS2 sensor can detect low concentrations of NO2 (50 ppb) with excellent repeatability, selectivity, and long-term stability at room temperature. This study demonstrates that integrating Au-NPs modification with UV photo-assistance is a viable strategy for developing high-performance room temperature gas sensors.

Discharge fault detection of dry air switchgear based on ZnO-MoS2 gas sensor

Abstract When discharge faults occur in dry air switchgear, the air decomposes to produce diverse gases, with NO2 reaching the highest levels. Detecting the NO2 level can reflect the operation status of the equipment. This paper proposes to combine ZnO cluster with MoS2 to improve the gas-sensitive properties of the monolayer. Based on the Density Functional Theory (DFT), the effect of (ZnO)n size on the behavior of MoS2 is considered. Key parameters such as adsorption energy and band gap of (ZnO)n-MoS2/NO2 system were calculated. The ZnO-MoS2 heterojunction was successfully synthesized by a hydrothermal method. The gas sensor exhibits a remarkable response and a fast response-recovery time to 100 ppm NO2. In addition, it demonstrates excellent selectivity, long-term stability and a low detection limit. This work confirms the potential of the ZnO-MoS2 composite structure as a highly effective gas sensor for NO2 detection, which provides valuable theoretical and experimental insights for fault detection in dry air switchgear.

A room-temperature NO2 gas sensor based on Zn2+ doped Cu2O/CuO composites with ultra-high response

Metal oxide semiconductors (MOS) have attracted increasing interest in gas sensors recently due to their exceptional design flexibility and superior gas sensing capabilities. However, the high operating temperatures and reliance on light activation in MOS-based gas sensors limit their potential applications. In this work, the Zn2+ doped Cu2O/CuO composites were successfully fabricated by one-step hydrothermal method for the detection of NO2 gas at room temperature. The experimental results show that the optimal Zn-Cu2O/CuO composite presents responses that are 2.9, 3.4, 3.1 times higher than those of the pure Cu2O/CuO composite to 10, 5 and 1 ppm NO2. Additionally, the response and recovery times are shortened by factors of 3.1 and 1.4, respectively. Notably, the detection limit of the Zn-Cu2O/CuO composite is 2 ppb at room temperature, significantly below China's annual average NO2 concentration limit of ∼18 ppm. Furthermore, the Zn-Cu2O/CuO based sensor demonstrates the great selectivity, repeatability and long-term stability for NO2 detection. The gas sensitivity enhancement mechanism is thoroughly discussed. This result provides a new approach for the effective detection of ppb-level NO2 by CuO-based gas sensing materials at room temperature.

Ultrathin Two-Dimensional Titanium Oxysulfide for Enhanced Sensitivity and Stability of Room Temperature NO2 Sensing

Metal oxysulfide as a novel type of group VI-A semiconductor materials presents great potential for gas sensing due to its fascinating properties of tunable electronic structure and excellent stability against oxidation. Here, we realize the preparation of two-dimensional titanium oxysulfide (2D TiS1.4O0.6) via incorporating the oxygen atoms into the sulfur vacancy sites of TiS2 during the ultrasonic exfoliation process. 2D TiS1.4O0.6 exhibits slightly broadened band gap and upshifted Fermi level compared with pure 2D TiS2. As a result, the sensor based on TiS1.4O0.6 shows a room-temperature reversible gas response with a response factor of 39.07 towards 10 ppm NO2, which is more than two times compared with that of TiS2. TiS1.4O0.6 based gas sensor also exhibits a reversible response of 2.39, 8.73, 17.39, 31.91, and 39.12 towards the NO2 concentration of 0.5, 1, 2, 5, and 10 ppm at room temperature without any external stimulation. Moreover, the sensor maintains a stable gas response magnitude of 39.2, 38.91, 32.64, and 25.5 after one week, one month, two months, and three months. The excellent repeatability, anti-humidity ability, and selectivity are also demonstrated. This work provides an effective route for preparing 2D titanium oxysulfide, and demonstrates the feasibility of oxysulfide in high sensitivity and long-term gas sensing.

ZnO/SnO2 based composite heterostructure for NO2 gas sensing properties

In recent years, interest in heterostructures and research on them has been increasing day by day. While semiconductors mainly exhibit different characteristics, they can exhibit different characteristics when they come together to form a heterostructure, and investigating the reasons for these is of great interest. For this purpose, in this study, such a hetero structure (ZnO/SnO2) was obtained by the Sol-Gel Dip Coating (SGDC) combined method and its effects on its characteristics against NO2 gas were tried to be examined.In this study, SnO2/ZnO composite heterostructures were grown by SGDC method. In this sense, this study is a first in the literature in terms of enlarging this structure with the SGDC technique. The number of SnO2 cycles was kept constant at 1 cycles and the ZnO layers were at different cycles as 1, 2, 3 and 5. The effect of ZnO cycles number on structural, morphological and most importantly NO2 gas detection properties was investigated in the formed heterostructures. In the XRD results, both ZnO and SnO2 peaks were observed and it was determined that these peaks belonged to the hexagonal wurtzite phase for ZnO and to the tetragonal rutile phase for SnO2. The lattice strain and crystallite size were calculated using Williamson-Hall and Scherrer method. The SEM images of ZnO-SnO2 manifest the change in the microstructure with increasing ZnO layer. The microstructure consisted of thin crystalline plates and small round-shape particles. The nano plates and walls shape and size changed with increasing ZnO layers. The operating temperature was defined as 165 °C for all sensors from NO2 gas sensing measurements. The responses of 1 ppm NO2 gas concentrations were calculated as 20 %, 201 %, 1078 % and 676 % for Z1S1, Z2S1, Z3S1 and Z5S1 heterosturucture respectively.

Effect of substrate temperature on the physical and sensing properties of nanostructured Fe2O3 thin films

Using Chemical Bath Deposition (CBD) Method and various substrate temperatures, Fe2O3 films were successfully deposited. The produced film thickness was around (320 nm). Using X-ray diffraction, researchers may examine the polycrystalline structure of Fe2O3 thin films. These nanofilms contain strong peaks at 2θ =32.21, suggesting a preferred orientation along the (110) plane, and the grain size increases with substrate temperature, according to XRD tests. When the base temperature was raised from 350 to 450 o C, the strain parameter decreased from 31.35 to 28.43. AFM testing of the surface morphology of the deposition of material yields excellent homogenous coatings. The findings show that the average particle size of the nanoparticles ranges from (69.8 to 32.7) nm. SEM images show Fe2O3 films at (350, 400, 450) °C. Increased temperature reduces grain size, influencing morphology variations. The absorbance increases with substrate temperatures and decreases rapidly at short wavelengths, which correspond to the energy gap. The transmittance increases with increasing wavelength range. It decreases with rising substrate temperatures. The band gap values vary from 2.17 eV to 2.06 eV by increasing the substrate temperatures from 350 to 450 o C. It was discovered that the band gap reduces as the temperature of the Fe2O3 substrate increases. In addition, the optical constants for all films, including the absorption coefficient, the refractive index, and the extinction coefficient, were computed. Fe2O3 film's resistance over time at 350, 400, and 450°C for 300 ppm NO2 demonstrates oxidation effect and temperature sensitivity. Sensitivity decreases with higher base temperature due to charge carrier recombination, affecting NO2 response.

Selective passivation of 2D MoS2 nanosheets surface defects by spontaneous growth of Au nanoparticles for full response-recovery NO2 detection at room temperature

Full recovery at room temperature (RT) is quite challengng for two dimensional transition metal dichalcogenides chemiresistive gas sensors. The main reason for the incomplete recovery is the strong bonding of gas molecules onto sensing layer defects. Here, we show that the recovery rate of MoS2 chemiresistive NO2 sensors can be improved by selective passivation of MoS2 nanosheets (NSs) defects with Au nanoparticles (NPs) via a simple spontaneous reduction method. MoS2 NSs were produced by liquid shear exfoliation. Pristine and Au NPs functionalized MoS2 were characterized using ς-potential, UV–Visible spectroscopy, TEM, SEM, AFM, EDS, XRD, XPS and Raman spectroscopy. The analyses confirm the presence of Au NPs on the edges of MoS2 NSs at defective sites with NP sizes of 1–4 nm and 5–30 nm. Both pristine MoS2 and Au-decorated MoS2 NSs were employed to fabricate NO2 chemiresistive devices. Au-decorated MoS2 sensors showed an improved performance (for 1 ppm NO2, Au-MoS2 sensor exhibited a response of 5.6 % instead of 2.2 % for MoS2 sensor), and gave faster response and better recovery time. Concurrently, the functionalization is independent of the adsorption and desorption of NO2. Most importantly, the functionalization of MoS2 NSs helps to full response-recovery within one hour, without either thermal or UV irradiation treatment.

Single walled carbon nanotubes/ZnO nanowires heterostructure array for NO2 gas sensing at low ppm levels

A layer-by-layer ZnO nanowire (NW)-Single Walled Carbon Nanotube (CNT) hybrid gas sensor has been proposed for effective detection of NO2 gas under a low exposure limit of 5 ppm. The fabricated hybrid sensor displayed good sensor response (S %), good selectivity and fast response time towards the target gas. S % value of 36.4 % under 5 ppm NO2 level at 250 °C and good selectivity response towards different interfering gases like SO2, CO and H2 have been observed. Meanwhile, fast response time values of 44.08 s (response) and 62.11 s (recovery) were observed at 300 °C under 5 ppm NO2 level. The sensor baseline resistance of the hybrid heterostructure (HS) was also found to be lower when compared to that of a bare ZnO NW sensor. In addition, the presence of Single Walled CNT in the HS was also shown to reduce the operational temperature of the sensor. Temperature played a significant role by controlling the adsorption and desorption kinetics of gas molecules on the surface. Moreover, the formation of p-n interface between Single Walled CNT (p-type) and ZnO NW (n-type) might have modulated the potential charge barrier on interacting with the adsorbed gases which inturn might have impacted the response of the hybrid sensor. In addition, the large surface areas offered by both NWs and nanotubes might have improved the gas adsorption capability thereby enhancing the overall hybrid sensor’s response.

Effect Of The Composition Of The Etching System MF-HCl (M = Li+, Na+, NH4+) on the gas-sensitive properties of Ti3C2Tх/Tioх nanocomposites

The influence of the nature of MF-HCl etching systems (M = Li+, Na+, NH4+) on the process of synthesis of Ti3C2Tx MXenes on the basis of Ti3AlC2 MAX-phase, microstructure, phase purity, interlayer distance, composition of functional surface groups, thermal behavior and yield of the obtained products has been studied. The room temperature sensing properties of Ti3C2Tx receptor layers deposited by microplotter printing were studied with respect to a wide range of gas analytes (H2, CO, NH3, NO2, NO2, O2, benzene, acetone, methane and ethanol). Increased sensitivity to ammonia was revealed for the MXenes obtained by exposure to hydrochloric acid solutions of sodium and ammonium fluorides and to carbon monoxide for the sample synthesized using the LiF-HCl system. High responses (~20–30% to 100 ppm NO2) were observed for all three receptor materials, but sensor recovery processes were significantly hampered. To improve the sensing characteristics, Ti3C2Tx sensing layers were subjected to relatively low-temperature heat treatment in an air atmosphere to form Ti3C2Tx/TiOx nanocomposites. It was found that a high and selective oxygen response at very low operating temperatures (125-175°C) was observed for the MXenes partially oxidized, which is particularly characteristic of the material produced using the HCl-NaF system.

Anchoring Ni atomic clusters on yolk-shell In2O3 microspheres for highly sensitive, ultra-rapid responding/recovering ppb-level NO2 detections

Highly sensitive, sophisticated and rapidly responding/recovering NO2 sensors are of a pressing demand across various modern applications. However, traditional metal oxide semiconductors (MOSs), such as In2O3, often grapple with issues such as diminished sensitivity and tardy responding/recovering. To address these challenges, we present a novel, simultaneous enhancement strategy centered around Ni atomic clusters (NiAC) integrated into In2O3 yolk-shell microspheres. This innovative structural and compositional design not only enhances NO2 adsorption thermodynamics but also improves kinetics reacting rate. The optimized heterostructured NiAC-In2O3 exhibits exceptional NO2 sensing performances with high response (454.62 to 10 ppm NO2), instantaneous responding/recovering dynamics (1/3 s), and an impressively ultra-low detection limit (1 ppb) at 200 ℃. Reaction kinetics calculation, finite element method (FEM) simulations and further validation through In-situ diffuse reflection infrared Fourier transform spectroscopy (In-situ DRIFTS) and temperature-programmed desorption (TPD) analyses confirms the accelerated NO2 adsorption/desorption dynamics, enhanced NO2 dissociation and trace-concentration NO2 molecule capturing capability attributed to NiAC. Considering these merits, The fabricated NiAC-In2O3 ensures high-precise, instantaneous, ppb-level NO2 detections.

Synthesis of copper-doped nanocrystalline tin stannate by thermal decomposition of a precursor

Zinc hydroxostannate ZnSn(OH)6, containing 5 mol.% Cu, was obtained from hydrochloric acid solutions of tin(IV), zinc and copper by adding sodium hydroxide to pH = 8–9. The thermolysis process of the obtained sample and the phase composition of the decomposition products were studied using thermal analysis, X-ray diffraction, and scanning electron microscopy. It was shown that the main stages of dehydration are completed at a temperature of about 350 °C and, as a result of thermolysis, an X-ray amorphous product is formed, from which a solid solution of copper in zinc stannate is obtained at an annealing temperature above 650 °C. At an annealing temperature of 800 °C, a mixture of nanocrystalline tin dioxide SnO2 with a cassiterite structure and copper-doped zinc orthostannate Zn2SnO4 with a spinel structure is formed. The room temperature sensors based on ZnSnO3 and carbon nanofibers as a conductive additive showed high response to NO2 (–6.1% to 2 ppm NO2).

TiO2@ZrO2 p-n heterostructured nanocomposites for enhanced NO2 gas sensing

Hydrothermal approach was used to synthesize pure TiO2, ZrO2 and 2.5–10 % TiO2-ZrO2 p-n type nanocomposite heterojunctions. Morphological and structural features of heterostructures were established by XRD, TEM/HRTEM and SEM investigations. Band gap energy calculations revealed a reduction in band structure from 4.96 eV of pure ZrO2 to 3.50 eV of 7.5 % TiO2-ZrO2 heterostructures. BET surface area characterization displayed large surface area of the nanocomposites which performed an essential part in the enhanced sensing activity of the nanocomposite heterostructures. 7.5 % TiO2-ZrO2 sensor exhibited enhanced response to NO2 gas possessing excellent response and recovery times. The maximum sensitivity of 991 was shown by 7.5 % TiO2-ZrO2 sensor to 100 ppm NO2 gas that was nearly four folds compared to pristine ZrO2 nanospheres (263). The improved NO2 gas sensing of the nanocomposites was stemmed to the construction of p-n heterostructures that resulted in the emancipation of more reactive sites and thus led to the better sensing result of the nanocomposites. The gas sensing mechanism and improved sensing characteristics have been also elaborated. The strategy thus suggested in this report may offer considerably to the understanding of more sensitive NO2 gas sensors.

Chemiresistive flexible gas sensor for NO2 sensing at room-temperature using in situ constructed Au@In2S3/In2O3 hybrid microflowers

The construction of a novel Au nanoparticles loaded In2S3/In2O3 hybrid microflowers composite was conducted through a well-designed hydrothermal and partial oxidation method and used for NO2 sensing at room-temperature (RT). The characterization data confirmed the hybrid structures and demonstrated that the ratio of adsorbed oxygen species on the surface increased after partial oxidation. The sensing performance of flexible gas sensors based on In2S3/In2O3, In2O3, AuxIn2S3/In2O3 (x = 0.5, 1 and 2 wt%) were tested and compared at RT. Heterostructure construction and Au NPs loading significantly improved the sensing properties at RT. Peculiarly, the response value of 1 wt% Au NPs loaded In2S3/In2O3 (Au1In2S3/In2O3) sensor for 100 ppm NO2 at RT was 20.7, which was 9.2 times higher than that of In2O3 sensor. Meanwhile, it also possessed excellent selectivity, fast response/recovery time (12/27 s), good repeatability, long-term and mechanical stability. The outstanding sensing performance for NO2 at RT mainly associated to the heterojunction between In2S3 and In2O3 and the catalytic effect of Au NPs. Finally, density functional theory (DFT) was used to calculate and analyze the gas adsorption and charger transfer, which related to the enhanced sensing performance.