Room-temperature Gas Sensor Research Articles

Design of p–p heterojunctions based on CuO decorated WS2 nanosheets for sensitive NH3 gas sensing at room temperature

Tungsten disulfide (WS2) nanosheets (NSs) have become a promising room-temperature gas sensor candidate due to their inherent high surface-to-volume ratio, tunable electrical properties, and high on-state current density. For further practical applications of WS2-based gas sensors, it is still necessary to overcome the insensitive response and incomplete recovery at room temperature. In this work, we controllably synthesized high-performance ammonia (NH3) gas sensor based on CuO decorated WS2 NSs. The optimized p–p WS2/CuO heterojunctions improve the surface catalytic effect, thereby enhancing the gas-sensing performance. The pure WS2 NSs-based gas sensors showed a low response and an incomplete recovery in the case of NH3 sensing. After the functionalization of CuO nanoparticles, the WS2/CuO heterostructure-based gas sensor exhibits an improved response value of 40.5% to 5 ppm NH3 and full recoverability without any external assistance. Density functional theory calculations illustrate that the adsorption of CuO for NH3 is much superior to WS2. The p–p heterojunctions strategy demonstrated in this work has great potential in the design of sensitive materials for gas sensors, and provides useful guidance for enhancing the room-temperature sensitivity and recoverability.

Flexible MXene/rGO/CuO hybrid aerogels for high performance acetone sensing at room temperature

The design and construction of high-performance room temperature gas-sensing materials have attracted great attention in the field of current gas sensor technology. Herein, we report an efficient approach for the production of ultrafine CuO nanoparticles-decorated three-dimensional (3D) hybrid aerogel by employing MXene (Ti3C2Tx) and reduced graphene oxide (rGO) nanosheets as cobuilding blocks. Benefiting from the synergistic effects of highly interconnected porous networks, large specific surface areas, homogeneous CuO nanoparticles dispersion, and good electron conductivity, the resulting 3D MXene/rGO/CuO aerogel expresses high acetone-sensing performance at room temperature (RT). The sensor response toward 100 ppm acetone is 52.09 % at RT with the fast response time (∼6.5 s) and recovery time (∼7.5 s), excellent reproducibility and selectivity. The strategy of compositing 3D graphene/MXene architectures with metal oxide semiconductor provides a novel pathway for the future development of RT gas sensors.

Recognition of binary mixture of NO2 and NO gases using a chemiresistive sensors array combined with principal component analysis

Novel room-temperature type chemiresistive gas sensor arrays consisted of two, three or four metal oxide semiconductor (MOS) gas sensors that were constructed using iron oxide/multi-walled carbon nanotubes/tungsten oxide (Fe2O3/MWCNTs/WO3), surface Au-, Pd- and Ag- modified films as sensing materials. Principal component analysis (PCA) coupled with various MOS chemiresistive gas sensor arrays were used to analyze the obtained data under testing conditions of binary mixture gases containing various composition of NO and NO2 at room-temperature. The careful pattern recognition of sensing data obtained with a chemiresistive gas sensor array consisting of three MOS sensors made it possible to qualitatively differentiate the variation in the composition of gas species in a binary mixture of NO2 and NO gases with related to their concentrations.

Graphene-Ag2S hybrid nanostructures: A hybrid gas sensor for room temperature hydrogen sensing application

Herein, we aimed to synthesize an efficient room-temperature gas sensor using biomass-derived graphene nanosheets-Ag2S based hybrid nanostructure by a microwave-assisted hydrothermal method. The electron micrographs of the synthesized Ag2S-GNS nanostructure depicts rice-shaped Ag2S nanoparticles impregnated onto graphene surfaces. Interestingly, the bandgap of Ag2S and Ag2S-GNS based hybrid nanostructures were identified to be 2.04 eV and 1.75 eV, respectively. The Ag2S-GNS based hybrid nanostructure showed better hydrogen sensing efficiency than the pure Ag2S nanoparticles. The maximum response of 45.5 % for 150 ppm exposure of hydrogen gas was obtained for Ag2S-GNS based hybrid nanostructure at room temperature. The sensing response and recovery of Ag2S-GNS hybrid nanostructures towards hydrogen gas were found to be 19 sec and 31 sec, respectively. The high gas sensitivity and fast response time of the hybrid sensor were due to assorted attributes and synergetic effect between Ag2S and GNS hybrids.

Conformal Self-Assembly of Nanospheres for Light-Enhanced Airtightness Monitoring and Room-Temperature Gas Sensing.

The fabrication of conformal nanostructures on microarchitectures is of great significance for diverse applications. Here a facile and universal method was developed for conformal self-assembly of nanospheres on various substrates including convex bumps and concave holes. Hydrophobic microarchitectures could be transferred into superhydrophilic ones using plasma treatment due to the formation of numerous hydroxyl groups. Because of superhydrophilicity, the nanosphere suspension spread on the microarchitectures quickly and conformal self-assembly of nanospheres can be realized. Besides, the feature size of the conformal nanospheres on the substrates could be further regulated by plasma treatment. After transferring two-dimensional tungsten disulfide sheets onto the conformal nanospheres, the periodic nanosphere array was demonstrated to be able to enhance the light harvesting of WS2. Based on this, a light-enhanced room-temperature gas sensor with a fast recovery speed (<35 s) and low detecting limit (500 ppb) was achieved. Moreover, the WS2-covered nanospheres on the microarchitectures were very sensitive to the changes in air pressure due to the formation of suspended sheets on the convex bumps and concave holes. A sensitive photoelectronic pressure sensor that was capable of detecting the airtightness of vacuum devices was developed using the WS2-decorated hierarchical architectures. This work provides a simple method for the fabrication of conformal nanospheres on arbitrary substrates, which is promising for three-dimensional microfabrication of multifunctional hierarchical microarchitectures for diverse applications, such as biomimetic compound eyes, smart wetting surfaces and photonic crystals.

Adsorption, sensing and optical properties of molecules on BC3 monolayer: First-principles calculations

Since acetone gas sensors play a key role in air monitoring and non-invasive diagnosis of diabetes, the realization of high-performance acetone gas sensors is highly important. In this study, the adsorption behaviors and optical properties of CH3COCH3, H2, N2, O2, CO2, H2O, HCN, HCOH, and SO2 molecules on the BC3 monolayer were investigated using first-principles calculations to exploit the possibilities of the BC3 monolayer as a room-temperature acetone sensor. Our results show that the BC3 monolayer as a room-temperature acetone gas sensor has high selectivity and sensitivity, and rapid response and recovery time. Importantly, the adsorption of CH3COCH3 modifies remarkably the optical properties of the BC3 monolayer, whereas the other adsorption has infinitesimal influence.

Two-Dimensional Nanomaterials for Wearable Breath Sensors

There has been considerable effort to develop wearable electronics from life-supporting devices for solders to fashion accessories such as smartwatches. The research of gas sensor has also attempted to adapt wearables with the power of nanotechnology. On the wearable platform, miniature gas sensors will provide real-time information about the atmosphere to protect each personnel from possible hazardous chemical attacks. In addition, wearable gas sensor can be facilitated to monitor human’s breath as medical applications. [1] One of the important factors to develop wearable gas sensors is to operate at room temperature with excellent sensing performance. Recently, two-dimensional (2D) nanostructured materials have been highlighted as new sensing materials due to their working ability at low temperatures. Unlike conventional metal oxides, 2D materials such as graphene are able to work at room temperature with comparable gas response to NO2 gas. However, insufficient sensing performance, such as sluggish recovery and low gas response, need to be developed. [2] As one attempt to enhance performance, 2D material was hybridized with metal oxide. By incorporating two nanostructured materials, synergistic effect, which comes from benefits of each material, can improve sensing performance. [3] Another novel innovation is the introduction of newly discovered 2D materials. 2D transition metal carbides and/or carbonitrides (called MXenes) are a new family of 2D materials. The vast diversity in a combination of constituent elements and ordered structure in MXenes offers future possibilities to find out a new generation of sensing materials with their superior performance. [4, 5]In this study, different types of 2D nanostructured materials and their hybrids were investigated for the breath analysis of various volatile compounds. Graphene oxide (GO) was incorporated with TiO2 nanoparticles, and the composite was photo-reduced under UV irradiation. Room temperature gas sensing was carried out against various VOC gases, and sensing performance was evaluated by comparing with pure GO. With the tailored hetero-junction at the interfaces of GO and TiO2, the composite can identify ethanol, methanol, and acetone, and its gas response was enhanced. In addition, we introduced new 2D nanostructured materials, MXenes including Ti3C2Tx and V2CTx. 2D V2CTx gas sensors showed excellent gas-sensing performance in terms of high response toward non-polar gases. Compared to the sensing properties of Ti3C2Tx MXene, 2D V2CTx sensor showed higher selectivity and long-term stability. The comparative results suggest that the modification of ordered structure and constituent elements of MXenes play a pivot role in the interaction between analyte and MXenes leading to an extensive change designed to detect individual compounds. [5, 6]References[1] Patel, S., Park, H., Bonato, P., Chan, L., & Rodgers, M. (2012). A review of wearable sensors and systems with application in rehabilitation. Journal of Neuroengineering and Rehabilitation, 9(1), 21.[2] Choi, S. J., & Kim, I. D. (2018). Recent Developments in 2D Nanomaterials for Chemiresistive-Type Gas Sensors. Electronic Materials Letters, 1-40.[3] Meng, F. L., Guo, Z., & Huang, X. J. (2015). Graphene-based hybrids for chemiresistive gas sensors. TrAC Trends in Analytical Chemistry, 68, 37-47.[4] Yu, X. F., Li, Y. C., Cheng, J. B., Liu, Z. B., Li, Q. Z., Li, W. Z., ... & Xiao, B. (2015). Monolayer Ti2CO2: a promising candidate for NH3 sensor or capturer with high sensitivity and selectivity. ACS Applied Materials & Interfaces, 7(24), 13707-13713.[5] Lee, E., VahidMohammadi, A., Prorok, B. C., Yoon, Y. S., Beidaghi, M., & Kim, D. J. (2017). Room Temperature Gas Sensing of Two-Dimensional Titanium Carbide (MXene). ACS Applied Materials & Interfaces, 9(42), 37184-37190.[6] Lee, E., VahidMohammadi, A., Yoon, Y. S., Beidaghi, M., & Kim, D. J. (2019). Two-Dimensional Vanadium Carbide MXene for Gas Sensors with Ultrahigh Sensitivity Toward Nonpolar Gases. ACS Sensors, 4, 1603-1611.

Synergistic Effects of Au and SnO2 Nanoparticles Decorated on WS2 Nanosheets for Flexible, Room-Temperature CO Gas Sensing

Two-dimensional (2D) materials such as graphene and graphene oxide are desirable for sensing applications owing to their high surface-to-volume ratio. Recently another class of 2D materials namely transition metal dichalcogenides (TMDs) with semiconducting properties have gained extensive attention for gas sensing studies due to their high surface area and excellent electronic and catalytic properties. TMDs form layered structures, where adjacent layers are held together by weak van der Waals forces. Accordingly, they can be easily exfoliated into individual 2D layers with high surface area which is a merit for sensing applications.WS2 is a promising 2D TMD for sensing applications thanks to high surface area and excellent electrical properties. However, for sufficient sensing applications its sensitivity needs to be improved more. A good strategy for that purpose is to functionalize with noble metal and metal oxide nanoparticles (NPs). NPs can make additional charge depletion zone in WS2 nanosheets, thereby amplifying their resistance modulation during interaction with gas molecules. Furthermore, NPs can provide a spillover effect; facilitating adsorption, dissociation, and transfer of target gases to WS2 nanosheets surface. This can lead to improved sensitivity towards a specific gas.In this study, WS2 nanosheets that are bare or decorated with Au, SnO2, or a combination of Au and SnO2 were realized on flexible polyamide substrates. The fabricated sensors were operated for CO gas sensing in applied-voltage-induced self-heating mode at room temperature (24 ºC). Not only optimal applied voltage was varied for the sensing of different gases, but also that the sensors behaved uniquely in response to CO gas. Therefore, the optimized applied voltage for the fabricated sensors and their room temperature sensing characteristics were investigated. Furthermore, we investigated that tilting, bending and stretching conditions to the optimized gas sensor. Also, we suggested that the enhanced response was related to the formation of Au-WS2 and Au-SnO2 Schottky junctions, n-SnO2-n-WS2 heterojunctions, and the catalytic performance of Au NPs towards CO gas. The results obtained in this study provide new avenues towards fabrication of flexible, low power gas sensors using metal chalcogenides.

The Room-Temperature Planar Type Gas Sensor Based on DPA-Ph-Dbpzdcn/TPA-Dcpp for NH3 Monitoring Application

IntroductionAmmonia (NH3) gas sensor is the indispensable element in safety, air quality monitoring and respiratory analysis fields. The chemoresistor sensors are mainly focused in the investigation of NH3 sensors attributing to the high stability, simple sensing structure, low cost and portable device. However, solving the high working temperature issue and achieving reproducible and stable gas sensing properties by designing novel sensitive material is highly desirable for widespread development of ammonia sensor [1]. Here, the planar room-temperature NH3 sensors are fabricated through spin-coating DPA-Ph-DBPzDCN/TPA-DCPP organic semiconductor materials onto the Al2O3 substrate with Au interdigital electrodes. The two sensors exhibit favorable sensing response as 72.73%/76.4% to 100 ppm NH3 under 98% RH, respectively, also demonstrates the low detection limits, good reproducibility, selectivity and long-term stability. The DFT calculation results further demonstrate that the excellent NH3 sensing characteristics are attributed to the enhanced absorption between NH3 and the N groups of organic molecules, which formed the H2O pre-adsorption under the high RH. This work is expected it can offer some guidance and inspiration for the rational design of sensing materials and further development of room temperature gas sensor. Results and Conclusions The synthesis process of DPA-Ph-DBPzDCN/TPA-DCPP were detailed reported previously [2, 3]. The sensor devices were successful fabricated via spin-coated sensitive materials onto the Al2O3 substrate with Au interdigital electrodes and illustrated in Figure 1. It was illustrated from scanning electron microscopy (SEM) image that the organic sensing films were formed uniformly and tightly. Then, the response transients of sensors in different NH3 concentration under 98% and 25% RH were monitored (Figure 2). The response value improved alone with the increasing of NH3 concentration and performed the highest response value as 72.73%/76.4% to 100 ppm NH3 under the 98% RH, respectively. Furthermore, there had a decreasing trend of the lowest detectable NH3 concentration along with the increasing RH, especially, the TPA-DCPP sensor exhibited the lowest detection limit as 500 ppb under 98% RH.Accordingly, the density functional theory (DFT) calculations were performed to gain further insight into the interaction between gas molecules and the sensitive materials, also the enhanced ammonia sensing properties under the high RH. The three most likely ammonia adsorbing sites (A: trihpenylamine-ortho C on the benzene ring; B: pyrazine-N; C: cyano-N) were selected due to the negative charge was mainly located. The configuration and the charge density difference after NH3 adsorption (Figure 3) demonstrated that the charge transfer occurred through the NH3 given electrons to organic compounds, thereby the conductivity of organic semiconductors increased until to an equilibrium state. Simultaneously, the NH3, H2O, NH3-H2O adsorption energies of the corresponding adsorbing sites were given in Table 1. It was shown that the NH3 adsorbing to organic compounds after H2O pre-adsorption were more capable compared with the NH3 adsorption only. Meanwhile, the NH3 adsorption energy before and after H2O pre-adsorption of the pyrazine-N on TPA-DCPP (-0.1809 eV, -0.4011 eV) was obviously higher than the DPA-Ph-DBPZdCN molecule (-0.14362 eV, -0.15679 eV), resulting in the response value of TPA-DCPP sensor was apparently improved compared to the DPA-Ph-DBPZdCN sensor under the low/high RH, that were attributed to the increasing of the negative charge on pyrazine-N owing to the directly link between and the pyrazine the cyano group, the change of molecular structure leading to a significantly stronger coulombic effect between pyrazine-N with NH3. Taken together, the excellent NH3 sensing characteristics were attributed to the enhanced absorption between NH3 and H2O pre-adsorption organic molecules under the high RH [4]. The DPA-Ph-DBPZdCN/TPA-DCPP could be the promising NH3 organic sensitive materials for the room temperature sensor, meanwhile, the discussion of sensing mechanism bring the further understanding of the organic gas sensors, providing the new concept of organic materials which has potential application value in gas monitoring of production and daily life at room temperature.

Facile hydrothermal synthesis of Ti3C2Tx-TiO2 nanocomposites for gaseous volatile organic compounds detection at room temperature

The gaseous volatile organic compounds (VOCs) sensors with high-selectivity and low-power consumption have been expected for practical applications in environmental monitoring and disease diagnosis. Herein, we demonstrate a room-temperature VOCs gas sensor with enhanced performance based on Ti3C2Tx-TiO2 nanocomposites. The Ti3C2Tx-TiO2 nanocomposites with regular morphology are successfully synthesized via a facile one-step hydrothermal synthesis strategy by using Ti3C2Tx itself as titanium source. Attributed to the formation of interfacial heterojunctions and the modulation of carrier density, the Ti3C2Tx-TiO2 sensor exhibits about 1.5–12.6 times enhanced responses for the detection of various VOCs at room temperature than pure MXene sensor. Moreover, the nanocomposite sensor has better response to hexanal, both an air pollutant and a typical lung cancer biomarker. The gas response of the Ti3C2Tx-TiO2 sensor towards 10 ppm hexanal is about 3.4%. The hexanal gas sensing results display that the nanocomposite sensor maintains a high signal-to-noise ratio and the lower detection limit to hexanal gas is as low as 217 ppb. Due to the low power consumption and easy fabrication process, the Ti3C2Tx-TiO2 nanocomposite sensor is promising for application in IoT environmental monitoring as well as real-time health monitoring.

In2O3 nanocubes/Ti3C2Tx MXene composites for enhanced methanol gas sensing properties at room temperature

Highly active two-dimensional (2D) nanocomposites, integrating the unique merits of individual components and synergistic effects of composites, have been recently receiving attention for gas sensing. In this work, In2O3 nanocubes/Ti3C2Tx MXene nanocomposites were synthesized using In2O3 nanocubes and layered Ti3C2Tx MXene via a facile hydrothermal self-assembly method. Characterization results indicated that the In2O3 nanocubes with sizes approximately 20–130 nm in width were well dispersed on the surface of layered Ti3C2Tx MXene to form numerous heterostructure interfaces. Based on the synergistic effects of electronic properties and gas-adsorption capabilities, In2O3 nanocubes/Ti3C2Tx MXene nanocomposites exhibited high response (29.6%–5 ppm) and prominent selectivity to methanol at room temperature. Meanwhile, the low detection concentration could be reduced to ppm-level, the response/recovery times are shortened to 6.5/3.5 s, excellent linearity and outstanding repeatability. The strategy of compositing layered MXene with metal oxide semiconductor provides a novel pathway for the future development of room temperature gas sensors.

Performance Improvement of n-ZnO/p-rGO Heterojunction Based Room Temperature Hydrogen Gas Sensor

This letter reports a room-temperature hydrogen gas sensor based on the nanohybrid structure of ZnO nanorods (NRs) and reduced graphene oxide (rGO) with better sensitivity. In this nanohybrid structure, ZnO NRs were grown up by the chemical bath deposition technique followed by the electrochemical exfoliation of rGO layer. The collaborative hybridization of these two dissimilar sensing elements revealed much better response at room temperature, which could hardly be achieved by the individual ones. The sensing was carried out at room temperature at ~40% RH, with hydrogen as a target gas with fastest response and recovery times (17.02 and 27.06 s), which have not been achieved earlier. The physics behind it is the high carrier mobility of rGO. A very high response magnitude (586.93% at 100 ppm) was found due to the increased number of gas-interaction sites and availability of free surface energy provided by ZnO NRs and rGO. A comparative discussion elucidating the role of ZnO NRs-rGO junctions has also been presented correlating the experimental findings.

Multi-walled carbon nanotubes/polyaniline on the ethylenediamine modified polyethylene terephthalate fibers for a flexible room temperature ammonia gas sensor with high responses

Abstract The flexible room-temperature gas sensors are highly desirable to apply in wearable and portable equipment owing to their wearability. The research mainly focuses on the gas-sensing materials and few studies paid attention to the conjunction between flexible substrates and sensing materials. In this work, the polyethylene terephthalate (PET) fibers substrate is modified by ethylenediamine (EDA) to expose the amino group on the surface (PET-NH2). This method is simple, cost-efficient, and easy to operate. The positively charged amino group could interact with negatively charged carboxyl (−COOH) of multi-walled carbon nanotubes (MWCNTs) by electrostatic attraction. The EDA treatment on PET effectively strengthens the connection between the flexible PET substrate and gas sensing materials with carboxyl as evidenced by SEM images. After the in situ polymerization process, the polyaniline (PANI) is successfully decorated on the MWCNTs. The fabricated PET-NH2-MWCNTs/PANI sensor exhibits strongly enhanced ammonia (NH3) sensing performance as compared to the PET-MWCNTs/PANI sensor without the EDA treatment. The results suggest that the EDA treatment is an effective method to conjunct between flexible substrates and sensing materials, which shows great potential in the field of flexible gas sensors.

Room Temperature Gas Sensor Application of Carbon Nanowalls using Electrical Resistance Change by Surface Adsorption of Toxic Gases

Carbon is not only a readily available material, but also has appropriate properties through various structural modifications. Carbon nanowalls (CNWs), based on carbon with vertical porous nanostructure, offer broad sensing surface area. Using carbon nanowalls, we demonstrated it function as gas sensing layer in our work. Carbon nanowalls synthesized via plasma enhanced chemical vapor deposition (PECVD) was in form numerous nanoflakes. In addition, various defect on its surface with high D band and low G band in Raman spectra was observed. When ammonia (NH3) and nitrogen dioxide (NO2) that may cause environmental pollution as an analyte gas were adsorbed on carbon nanowalls during exposing to them, difference in electrical resistance over 300 percentage between ammonia and nitrogen dioxide appeared. In various characteristics such as response time, sensitivity, saturation-recovery and selectivity, carbon nanowalls-based gas sensor exhibited better performance in nitrogen dioxide than ammonia. The reason for these performance is that the charge transfer of nitrogen dioxide greater than that of ammonia. In addition, the electron affinities of ammonia and nitrogen dioxide are 0.55 ± 0.10 and 2.11 ± 0.18 eV, respectively, which may sufficiently affect the each reaction between two analyte gases. Through this work, we offer several results of the proof-of-concept study for the gas sensor application of carbon nanowalls.

Graphene oxide nanocomposites based room temperature gas sensors: A review

Over the last few decades, various volatile organic compounds (VOCs) have been widely used in the processing of building materials and this practice adversely affected the environment i.e. both indoor and outdoor air quality. A cost-effective solution for detecting a wide range of VOCs by sensing approaches includes chemiresistive, optical and electrochemical techniques. Room temperature (RT) chemiresistive gas sensors are next-generation technologies desirable for self-powered or battery-powered instruments utilized in monitoring emissions that are associated with indoor/outdoor air pollution and industrial processes. In this review, a state-of-the-art overview of chemiresistive gas sensors is provided based on their attractive analytical characteristics such as high sensitivity, selectivity, reproducibility, rapid assay time and low fabrication cost. The review mainly discusses the recent advancement and advantages of graphene oxide (GO) nanocomposites-based chemiresistive gas sensors and various factors affecting their sensing performance at RT. Besides, the sensing mechanisms of GO nanocomposites-based chemiresistive gas sensors derived using metals, transition metal oxides (TMOs) and polymers were discussed. Finally, the challenges and future perspectives of GO nanocomposites-based RT chemiresistive gas sensors are addressed.

Adsorption, Gas-Sensing, and Optical Properties of Molecules on a Diazine Monolayer: A First-Principles Study.

Using first-principles calculations, the structural, electronic, and optical properties of CO2, CO, N2O, CH4, H2, N2, O2, NH3, acetone, and ethanol molecules adsorbed on a diazine monolayer were studied to develop the application potential of the diazine monolayer as a room-temperature gas sensor for detecting acetone, ethanol, and NH3. We found that these molecules are all physically adsorbed on the diazine monolayer with weak adsorption strength and charge transfer between the molecules and the monolayer, but the physisorption of only NH3, acetone, and ethanol remarkably modified the electronic properties of the diazine monolayer, especially for the obvious change in electric conductivity, showing that the diazine monolayer is highly sensitive to acetone, NH3, and ethanol. Further, the adsorption of NH3, acetone, and ethanol molecules remarkably modifies, in varying degrees, the optical properties of the diazine monolayer, such as work function, absorption coefficient, and the reflectivity, whereas adsorption of other molecules has infinitesimal influence. The different adsorption behaviors and influences of the electronic and optical properties of molecules on the monolayer show that the diazine monolayer has high selectivity to NH3, acetone, and ethanol. The recovery time of NH3, acetone, and ethanol molecules is, respectively, 1.2 μs, 7.7 μs, and 0.11 ms at 300 K. Thus, the diazine monolayer has a high application potential as a room-temperature acetone, ethanol, and NH3 sensor with high performance (high selectivity and sensitivity, and rapid recovery time).

Highly sensitive and selective room-temperature NO2 gas sensor based on novel Fe2O3 nanorings/reduced graphene oxide heterojunction nanocomposites

Novel Fe2O3 nanorings/reduced graphene oxide (Fe2O3 NRs/rGO) heterojunction nanocomposites were synthesized by a simple hydrothermal method. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier Transform infrared spectrometer and transmission electron microscope (TEM) were used to analysis the microstructure and morphology of Fe2O3 NRs/rGO nanocomposites. Gas sensing results indicated that the optimal NO2 gas sensor based on Fe2O3 NRs/rGO nanocomposites exhibited a room temperature response of 23.8 toward NO2 concentration of 5 ppm, which was 12 times higher than that of α-Fe2O3 nanorings. The HR-TEM results showed that there was a close interface between α-Fe2O3 and rGO, which proved the formation of heterojunction nanocomposite structure. In addition, the sensor based on Fe2O3 NRs/rGO nanocomposites has a short response time of 15 s and excellent NO2 selectivity. The detection limit of NO2 is as low as 1 ppm at room temperature. The enhancement of NO2 sensing performances can be attributed to the improved electronic conductivity, much higher specific surface area, more adsorption sites and the formation of the heterojunction between Fe2O3 nanorings and reduced graphene oxide.

Gas emission from human skin positions detected by vertical-channel organic semiconductor sensor

The gas emitted from the human skin is collected and measured by the highly sensitive room-temperature organic semiconductor gas sensor with vertical electrical channel. Among various positions, palms give the highest signal and heads follow. The signals are collected from 4 healthy men and 4 healthy women. The response is defined as the percent of electric current change at 5 V in 30 s after the measurement. By comparing the responses from sensors made by three different organic materials, the dominant component from the palm has different pattern from ammonia. Gas chromatography study suggests aldehydes in the palm gas. Different people show consistent results but slightly depending on sex. The body skin measurement can be extended to patients in order to establish the connection between skin gas and the diseases situations.

Significantly enhanced NO2 gas-sensing performance of nanojunction-networked SnO2 nanowires by pulsed UV-radiation

A unique combination of high response and fast response-recovery is still a challenge in the development of room-temperature gas sensors. Herein, we demonstrated the on-chip growth of nanojunction-networked SnO2 NW sensors to work under UV-radiation at room temperature. The morphological, compositional, and structural properties of synthesized SnO2 nanowires were examined using field emission electron microscopy, energy dispersive spectroscopy, X-ray diffraction, and high-resolution transmission electron microscopy, respectively. The results presented the SnO2 NWs with smooth surfaces were entangled between the Pt electrode. Besides, the internal properties showed the SnO2 NWs were crystallized as the tetragonal rutile structure of SnO2. The use of UV-radiation with the optimum intensity of 50 μW/cm2 increased the gas response to 5 ppm NO2 up to 7-fold, while response and recovery times decreased about 8- and 4-fold, respectively. Moreover, alternative use of pulsed UV-radiation (provided only during the air recovery phase) can enhance significant gas response as compared with continuous UV-radiation. The enhancement of gas response could be attributed to the photo-adsorption and -desorption of NO2 molecule due to the photogeneration of electron-hole pairs. The combination of NW-NW nanojunctions and pulsed UV-radiation is expected to be a novel strategy for high-performance room temperature gas sensors.

Thin-layered MoS2 nanoflakes vertically grown on SnO2 nanotubes as highly effective room-temperature NO2 gas sensor

The unique properties of heterostructure materials make them become a promising candidate for high-performance room-temperature (RT) NO2 sensing. Herein, a p-n heterojunction consisting of two-dimensional (2D) MoS2 nanoflakes vertically grown on one-dimensional (1D) SnO2 nanotubes (NTs) was fabricated via electrospinning and subsequent hydrothermal route. The sulfur edge active sites are fully exposed in the MoS2@SnO2 heterostructure due to the vertically oriented thin-layered morphology features. Moreover, the interface of p-n heterojunction provides an electronic transfer channel from SnO2 to MoS2, which enables MoS2 act as the generous electron donor involved in NO2 gas senor detection. As a result, the optimized MoS2@SnO2-2 heterostructure presents an impressive sensitivity and selectivity for NO2 gas detection at RT. The response value is 34.67 (Ra/Rg) to 100 ppm, which is 26.5 times to that of pure SnO2. It also exhibits a fast response and recovery time (2.2 s, 10.54 s), as well as a low detection limit (10 ppb) and as long as 20 weeks of stability. This simple fabrication of high-performance sensing materials may facilitate the large-scale production of RT NO2 gas sensors.