Improved Gas Response Research Articles

Mechanism of Interfacial Molecular Interactions Reveals the Intrinsic Factors for the Highly Enhanced Sensing Performance of Ag-Loaded Co3O4.

The noble metal-loaded strategy can effectively improve the gas-sensing performances of metal oxide sensors. However, the gas-solid interfacial interactions between noble metal-loaded sensing materials and gaseous species remain unclear, posing a significant challenge in correlating the physical and chemical processes during gas sensing. In this study, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) and in situ Raman spectroscopy were conducted to collaboratively investigate the interfacial interactions involved in the ethanol gas-sensing processes over Co3O4 and Ag-loaded Co3O4 sensors. In situ DRIFTS revealed differences in the compositions and quantities of sensing reaction products, as well as in the adsorption-desorption interactions of surface species, among Co3O4 and Ag-loaded Co3O4 materials. In parallel, in situ Raman spectroscopy demonstrated that the ethanol atmosphere can modulate the electron scattering of Ag-loaded Co3O4 materials but not of raw Co3O4. In situ experimental results revealed the intrinsic reason for the highly enhanced sensing performances of the Ag-loaded Co3O4 sensors toward ethanol gas, including a decreased optimal working temperature (from 250 to 150 °C), an improved gas response level (from 24 to 257), and accelerated gas recovery dynamics. This work provides an effective platform to investigate the interfacial interactions of sensing processes at the molecular level and further advances the development of high-performance gas sensors.

Highly Dispersed Pt-Incorporated Mesoporous Fe2O3 for Low-Level Sensing of Formaldehyde Gas.

Highly dispersed Pt-incorporated mesoporous Fe2O3 (Pt/m-Fe2O3) of 4 μm size is prepared through a simple hydrothermal reaction and thermal decomposition procedures. Furthermore, the formaldehyde gas-sensing properties of Pt/m-Fe2O3 are investigated. Compared with our previous mesoporous Fe2O3-based gas sensors, a gas sensor based on 0.2% Pt/m-Fe2O3 shows improved gas response by over 90% in detecting low-level formaldehyde gas at 50 ppb concentration, an enhanced selectivity of formaldehyde gas, and a lower degradation of sensing performance in high-humidity environments. Additionally, the gas sensor exhibits similar properties as the previous sensor, such as operating temperature (275 °C) and long-term stability. The enhancement in formaldehyde gas-sensing performance is attributed to the attractive catalytic chemical sensitization of highly dispersed Pt nanoparticles in the mesoporous Fe2O3 microcube architecture.

H2S gas sensing properties in polysilicon control-gate FET-type gas sensor

H2S gas sensing characteristics of a polysilicon control-gate (CG) FET-type gas sensor are investigated. The 15 nm thick In2O3 deposited using a radio frequency magnetron sputtering method is used as the sensing material. The metal CG is modified to poly-silicon CG, and the gas sensor with polysilicon control-gate operates well without any problems. The modified FET-type gas sensor has two CG terminals (VCG1 and VCG2), and the gas sensor with opened VCG2 shows better gas sensing characteristics by means of higher transconductance (gm) than gas sensor with grounded VCG2. H2S gas sensing characteristics are examined while varying the operating temperature, the concentration of the H2S gas, and the pre-bias voltage (Vpre). The sensing mechanism for detecting the H2S gas in an FET-type gas sensor is examined. The polysilicon control-gate FET-type gas sensor is able to detect up to 5 ppm of H2S gas. The FET-type gas sensor shows significantly improved gas response by using the pre-bias scheme.

The enhanced practicability of ZnSnO3 sensors based on Co, Ni co-doping and N719 dye sensitization

In this paper, Co, Ni co-doping and N719 dye sensitization were combined for the first time to improve the performance and practicability of the ZnSnO3 sensor. The doping ratio of Co2+ and Ni2+ has a great impact on the response to 500 ppm ethanol of ZnSnO3. The ZC0.3N0.1SO sensor has a response of 289.74 at 15% relative humidity and 220 °C, which is the maximum among the doped sensors. Moreover, the ZC0.3N0.1SO sample is further decorated with TiO2 and sensitized by N719 dye. The operating temperature (OT) of the N719-TiO2 @ZC0.3N0.1SO sensor can be reduced from 220 °C to 160 °C under visible light, and the maximum response of 290.47 is obtained at 160 °C. The comprehensive evaluation indicates that the N719-TiO2 @ZC0.3N0.1SO gas sensor possesses high practicability. Finally, the reasons for the improved gas response based on Co, Ni co-doping, the low temperature gas-sensing mechanism of N719-TiO2 @ZC0.3N0.1SO under visible light were analyzed and discussed.

Two-Dimensional calcium silicate nanosheets for trapping atmospheric water molecules in humidity-immune gas sensors

In humid conditions, water vapor can easily neutralize the surface active sites of metal oxide sensors, leading to a lowering in the sensitivity of the gas sensor and a resultant inaccurate signal in practical applications. Herein, we present a new hybrid sensor by introducing a two-dimensional calcium silicate (CS) nanosheet as a water-trapping layer in SnO2 nanowires. Unlike the heavily wrinkled and aggregated morphology of conventional CS nanosheets, our nanosheet in the hybrid material is ultrathin and flat. Moreover, it was grown in the empty spaces between the spider-web-like networks of SnO2 nanowires without covering the nanowire surface. These two morphological features improve moisture trapping with minimal reduction in the active sensing area. Consequently, stable and sensitive gas detection under humid conditions was achieved in this hybrid sensor. The superior humidity-independent sensing is ascribed to the preferential adsorption of water molecules on hydroscopic CS nanosheets through the hydrogen bond. Based on density functional theory calculations, we determined that the improved gas response is driven by the additional formation of oxygen vacancy in SnO2 due to the diffusion of aliovalent Ca ions from the CS nanosheet.

NH3 Sensor Based on 3D Hierarchical Flower-Shaped n-ZnO/p-NiO Heterostructures Yields Outstanding Sensing Capabilities at ppb Level.

Hierarchical three-dimensional (3D) flower-like n-ZnO/p-NiO heterostructures with various ZnxNiy molar ratios (Zn5Ni1, Zn2Ni1, Zn1Ni1, Zn1Ni2 and Zn1Ni5) were synthesized by a facile hydrothermal method. Their crystal phase, surface morphology, elemental composition and chemical state were comprehensively investigated by XRD, SEM, EDS, TEM and XPS techniques. Gas sensing measurements were conducted on all the as-developed ZnxNiy-based sensors toward ammonia (NH3) detection under various working temperatures from 160 to 340 °C. In particular, the as-prepared Zn1Ni2 sensor exhibited superior NH3 sensing performance under optimum working temperature (280 °C) including high response (25 toward 100 ppm), fast response/recovery time (16 s/7 s), low detection limit (50 ppb), good selectivity and long-term stability. The enhanced NH3 sensing capabilities of Zn1Ni2 sensor could be attributed to both the specific hierarchical structure which facilitates the adsorption of NH3 molecules and produces much more contact sites, and the improved gas response characteristics of p-n heterojunctions. The obtained results clear demonstrated that the optimum n-ZnO/p-NiO heterostructure is indeed very promising sensing material toward NH3 detection for different applications.

Improved stability of gas sensor by inclusion of Sb in nanostructured SnO2 thin films grown on sodalime

In the present work an attempt has been made to improve the stability in the oxygen gas sensing property of SnO2 with Sb doping using low cost sol-gel spin coating techniques. Un-doped and Sb (0, 0.5, 1.0, 1.5 and 2 at. %) doped SnO2 thin films were deposited on sodalime glass substrate by spin coating technique at 2500 rpm and annealed at 400 °C. The properties of thin films were investigated by their structural, morphological, optical and electrical analysis for oxygen gas sensing at low operating temperature. The crystalline phase and crystallite size were measured by X-ray diffraction pattern. It was observed that peaks show the polycrystalline nature with the tetragonal crystal structure. Scanning electron microscopic images revealed that all thin films have spherical type morphology. The transmittance spectra were recorded in the wavelength range 200–800 nm and optical band gap was varied from 3.56 eV to 3.92 eV with increasing doping concertation. The I–V measurements were recorded from −5 V to 5 V by two probe method. The highest conductivity was observed for 1 at. % Sb doped SnO2 thin film sample. The gas sensing properties of above thin films were measured as a function of operating temperature in the temperature range from 30 °C to 125 °C and the highest response was obtained for 1 at. % Sb doped SnO2 thin film sample. It has been observed from the experimental results that Sb doped SnO2 thin films exhibit the improved gas response, low operating temperature and good selectivity.

A formaldehyde gas sensor with improved gas response and sub-ppm level detection limit based on NiO/NiFe2O4 composite nanotetrahedrons

Formaldehyde is a hazardous indoor volatile organic pollutant, thus selective and precise detection at a low concentration level is crucial. In this paper, NiO/NiFe2O4 nanocomposites were prepared through a simple two-step hydrothermal route and their gas sensing performances were investigated. SEM and TEM results showed that the composite products were NiO nanotetrahedrons decorated with numerous separated NiFe2O4 nanoparticles on the outer layer surface. XPS characterization confirmed that the as formed NiFe2O4 exhibited a p-type conductivity due to the hole transfer between Ni3+ and Ni2+ and therefore p-p isotype heterojunctions formed in the composite products. The element ratio of Fe to Ni was optimized to gain higher gas-sensing performance through adjusting the feeding amount of Fe3+ in the synthesis process. As a consequence, the gas sensor based on the sample NiFe-0.008 showed the highest response of 33.3–200 ppm formaldehyde vapor at 240 °C and a low detection limit of 200 ppb. The composite sensor also showed short response/recovery time and good long term stability. The enhanced gas sensing properties to formaldehyde can be attributed to the unique tetrahedral morphology and p-p heterojunction structure, which can provide more active sites and significantly enlarge resistance variation. This work will provide a new perspective of p-p heterojunction for the practical application in formaldehyde gas sensor.

SO2 gas sensing characteristics of FET- and resistor-type gas sensors having WO3 as sensing material

Sensing characteristics of the SO2 gas are investigated using a horizontal floating-gate FET-type gas sensor. SO2 gas sensing characteristics of the resistor-type gas sensor, a conventional sensor, fabricated on the same wafer are also investigated and compared with the FET-type gas sensor. The 18-nm-thick WO3 deposited using a radio frequency magnetron sputtering method is used as the sensing material. The SO2 gas sensing characteristics are examined while varying the operating temperature, the concentration of the SO2 gas, and the pre-bias (Vpre). The sensing mechanism for detecting the SO2 gas in an FET-type gas sensor is examined. Both sensors are able to detect up to 10 ppm of SO2 gas, and the FET-type sensor shows significantly improved gas response by using the pre-bias scheme.

Synthesis and gas sensing properties of WS2 nanocrystallites assembled hierarchical WS2 fibers by electrospinning

A gas sensor based on a hierarchical WS2 structure embedded with vertically aligned WS2 nanocrystallites was demonstrated. The three-dimensional (3D) hierarchical structure provides many edge sites of nanocrystallites and an extremely large gas contact volume, resulting in a high gas response. The decreased contact resistance between the 3D hierarchical WS2 fibers and sensor electrode resulted in improved NO2 response. We fabricated a one-dimensional (1D) conductive WS2 fiber using a two-step annealing process under sulfur flow (sulfurization). It delivers a continuous and conductive carrier path and lowers the potential barrier at the interface of the WS2 nanocrystallites (top) and electrospun WS2 fiber (bottom), resulting in an improved gas response. We developed 3D hierarchical WS2 fibers embedded with vertically aligned WS2 nanocrystallites to increase the gas adsorption site in comparison with that of 1D WS2 fibers without WS2 flakes. Vertically aligned WS2 nanocrystallites were formed after a two-step annealing treatment. Sensors based on the 3D hierarchical WS2 fibers embedded with WS2 flakes, showed higher response to NO2 gas in comparison to that of pure WS2 fibers without WS2 flakes.

Ethanol sensing characteristics of BaTiO3/LaFeO3 nanocomposite

BaTiO3/LaFeO3 nanocomposite with BaTiO3 and LaFeO3 nanoparticles in the mole ratio of 1:2 was prepared by a citric sol-gel method and used as sensing materials to fabricate nanoparticle-based sensors. TEM, BET and XPS results suggested that the capability of absorbing oxygen species from air played the more important role than the grain size or specific surface area in the determination of gas sensing performance for nanoparticles. BaTiO3/LaFeO3 sensor showed improved gas response towards ethanol with respect to the LaFeO3 sensor at the expense of deteriorated dynamic response. Owing to the higher content of adsorbed oxygen species and ratio of Fe4+/Fe on the surface of the sensing material, better and stable sensing performance to ethanol was achieved by the BaTiO3/LaFeO3 sensor at the prime working temperature of 128 °C, with a high gas response (102.7) to 100 ppm ethanol.

Electrical and Ammonia Gas Sensing Properties of PQT-12/CdSe Quantum Dots Composite-Based Organic Thin Film Transistors

The electrical and ammonia gas sensing properties of poly (3, 3”’-dialkylquaterthiophene) (i.e., PQT-12) and CdSe quantum dots (QDs) composite-based back-gated organic thin film transistors (OTFTs) fabricated on SiO2 coated heavily doped p-Si substrates by solution method have been investigated in this paper. The properties of pristine PQT-12 and PQT-12/CdSe QDs composite films have been investigated by transmission electron microscopy and atomic force microscopy. To investigate the effect of colloidal CdSe QDs on the characteristics of PQT-12-based OTFTs, the performance of pristine PQT-12-based OTFT has been compared with that of the PQT-12/CdSe QDs composite-based OTFT. The field effect mobility, threshold voltage, on/off current ratio, and subthreshold swing of the PQT-12/CdSe QDs composite-based OTFT are $4.2\times 10^{-3}$ cm2/Vs, −14.4 V, $1.9\times 10^{3}$ , and 5.2 V/dec while the corresponding values for pristine PQT-12-based OTFT are $1.4\times 10^{-3}$ cm2/Vs, −22.1 V, $3.6\times 10^{2}$ , and 7.9 V/dec, respectively. When the devices are exposed to ammonia gas, an estimated gas response of ~51% is obtained for PQT-12/CdSe QDs composite-based OTFT whereas the gas response of ~41% is achieved for the pristine PQT-12 OTFT sensor. The improved gas response in the PQT-12/CdSe QDs composite-based OTFT is attributed to enhanced charge transfer due to the CdSe QDs as compared with the pristine PQT-12-based OTFT.

Improved NO2 sensing performance of electrospun WO3 nanofibers with silver doping

This work reports the enhancement of NO2 gas sensing response which owing to the incorporation of Ag nanoparticles into WO3 nanofiber matrices. Ag-doped WO3 nanofibers were fabricated by electrospinning. The Ag dopant levels ranged from 1 to 10 mol%. The microstructures and chemical compositions were examined using a series of material characterization methods including TEM, SEM, XRD, and XPS. Porous mats of nanofibers comprising monoclinic WO3 nanoparticles (28–39nm) incorporated with small Ag particles were observed. The NO2 gas sensing performance was investigated at various gas concentrations (0.5–5ppm) and operating temperatures (200–275°C). For 5ppm NO2 gas detection measured at 225°C, the 3 mol% Ag-doped WO3 nanofiber gas sensor exhibited the highest gas response (Rg/Ra) of 90.3. This was approximately 9 times higher than that of the undoped sample. The response and recovery time were found to be in the range of 6–18min. The response analysis obtained from CH4, NH3, SO2, H2S, and NO2 gas measurements suggests that the sensor has excellent selectivity toward NO2 gas. The improved gas response of Ag-doped WO3 nanofibers was attributed to both electronic and chemical sensitization mechanisms provided by Ag nanoparticles. Significant decreases in gas responses were observed in the samples with higher Ag dopant levels (5 and 10 mol%), where the crystallite size of Ag nanoparticles became too large and hindered their catalytic effects.

On-chip growth of semiconductor metal oxide nanowires for gas sensors: A review

Abstract Semiconductor metal oxide nanowires (SMO-NWs) show great potential for novel gas sensor applications because of their distinct properties, such as a high surface area to volume aspect ratio, high crystallinity and perfect pathway for electron transfer (length of NW). SMO-NW sensors can be configured as resistors or field-effect transistors for gas detection and different configurations, such as a single NW, multiple NWs, and networked NW films, have been established. Surface-functionalizing NWs with catalyst elements and self-heating NWs provide additional advantages for highly selective and low-power consumption gas sensors. However, an appropriate design of SMO-NWs is of practical importance in enhancing the gas-sensing performance of SMO-NW sensors. The on-chip growth of SMO-NWs possesses many advantages which can thus be effectively used for the large-scale fabrication of SMO-NW sensors with improved gas response and stability. This review aims to provide up-to-date information on the on-chip fabrication of SnO2, ZnO, WO3, CuO, and other SMO-NW sensors. It also discusses a variety of promising approaches that help advance the on-chip fabrication of SMO-NW-based gas sensors and other NW-based devices.

Synthesis and Characterisation of Nanocomposites of TiO₂ and MgAl₂O₄ for Gas Sensing Applications.

In this study, a conventional mixed oxide method was used to prepare nanocomposites of titanium dioxide and magnesium aluminate samples. The synthesis process of a low concentration of posttransition metal oxide like TiO2 with pre-transition metal oxides like MgO and Al₂O3 and its gas sensing behaviour were investigated. The present work focuses on applying different nanocomposite samples of (TiO2) x and MgAl₂O₄ (at x = 0 magnesium aluminate namely MA; x = 0°25 and 0.75 N namely MAT0.25 and MAT0.75 at 4 and 10 wt% of TiO2 in MgAl₂O₄ respectively and TiO₂ namely T) for gas sensing applications (O₂, CO and H₂ gases). The composite samples were characterized by their X-ray diffraction pattern, Fourier transform infrared spectroscopy, a particle size analyzer, X-ray fluorescent spectroscopy, scanning electron microscopy, ultraviolet visible spectroscopy, and Brunauer–Emmett–Teller methods. The response to changes in gas pressure (from 0.5 to 2 bar) was quantitatively studied in all samples (MA, MAT0.25, MAT0.75 and T) at different operating temperatures from 300 to 600 K. All samples showed a fast and improved gas response at different operating temperatures. Moreover, it was observed that the gas response of the composite sample, MAT0.75 increased by 11% more than the pure titanium sample at an operating temperature of 360 K, on the passage of O₂ gas.

Facile Synthesis of Hierarchical CuO Microspheres and their Gas Sensing Properties for NOx at Room Temperature

In this research, hierarchical CuO microspheres have been successfully synthesised by a facile reflux method. Scanning electron microscopy results clearly revealed that the hierarchical CuO microspheres were composed of two-dimensional nanosheets. The morphology of the prepared products could be tailored by changing the precursor concentration. The CuO-2 sample shows a higher NOx gas sensing performance with a low detection limit of 0.97 ppm, high gas response of 64.93 %, and short response time of 5.33 s to 97.0 ppm NOx at room temperature. The CuO-2 sensor also presents good selectivity and stability. The significantly improved gas response was concluded to be related to the well aligned microstructures and the improved conductivity of the CuO-2 sample. The unique hierarchical structure allows effective and rapid gas diffusion towards the sensing surfaces. In addition, the sensing mechanism based on the hierarchical CuO microspheres is discussed.

Synthesis of multi-dimensional ZnO nanostructures in aqueous medium for the application of gas sensor

Novel cacti-like structure and nanoneedles of zinc oxide (ZnO) were grown onto glass substrate using chemical route at comparatively low temperature (90°C), and employed for the application of gas sensor. The grown nanostructures were characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FE-SEM) and Transmission Electron Microscope (TEM) and photoluminescence (PL) spectroscopy. FE-SEM and TEM images showed that vertically aligned ZnO nanoneedles were formed on substrate and secondary branches emanated from primary aligned nanoneedles. PL spectra showed distinctively different peaks for nanoneedles and cacti-like structure where the peak intensities for cacti structures are as high as the one compared for aligned nanoneedles. Also, the intense visible peak detected for cacti structure confirmed the presence of defects due to oxygen vacancy in the grown nanostructures. Further, gas sensing behaviors were studied for these two different nanostructures against nitrogen dioxide (NO2) gas, and their selectivities toward reducing gases such as hydrogen disulfide, ethanol and liquefied petroleum gas were compared. It was found that cacti-like structure exhibited high gas response at 200°C and is selective for NO2 gas as compared with that for nanoneedles. The improved gas response is due to high surface area of cacti structure and presence of oxygen vacancies. Moreover, the contact between two adjacent cacti branches creates barrier potential at junction, which controls its resistance of sensors resulting in high gas sensing. Therefore, novel cacti-like nanostructure demonstrated to be the best candidate as NO2 gas sensor at low cost and temperature.

Pd sensitized porous silicon hydrogen sensor—Influence of ZnO thin film

The Pd sensitized porous silicon (PS) hydrogen sensor with a thin zinc oxide (ZnO) over layer demonstrated the improved gas response behaviour at the higher temperature. Porous silicon was produced by the electrochemical anodization of the p-type monocrystalline silicon substrate of the resistivity, 2–5 Ω cm and (1 0 0) orientation, using a mixture of HF and ethanol as the electrolyte. The ZnO thin film was coated on the unmodified and on the Pd modified PS surfaces by the sol–gel dip coating method. The catalytic electrode to ZnO was the Pd–Ag (26 wt%) alloy and the contact electrode to the p-silicon substrate was Al. The effect of the Pd modified ZnO on the hydrogen sensing was also studied. The detail gas sensor study with the Pd–Ag/Pd:ZnO/Pd:PS/Si/Al structure indicates that the Pd sensitized ZnO on the Pd modified PS exhibits the minimum fluctuations in the current and the consistent hydrogen sensing performance including an improved gas response (86%), the response time (28 s) and the long-term stability. However, the long recovery time (∼290 s) is due to the slower desorption kinetics after the hydrogen cut off, for using nitrogen carrier gas instead of air that helps in avoiding the cross-sensitivity from the oxygen in air.

Improved Gas Response at Room Temperature of Activated Nanocrystalline WO3 Films

Advanced reactive gas deposition was used to produce pure and Auactivated nanocrystalline WO3 films for gas-sensing studies. Many different methods – such as X-ray diffraction, X-ray photoelectron spectroscopy, energy dispersive spectroscopy, scanning electron microscopy, and atomic force microscopy – were used to characterize structural properties of the films. The WO3 particles in the films had the high-temperature tetragonal crystal structure after deposition, and the average crystallite size was about 10nm. The effect of sintering on structural, electrical, and gassensing properties of both pure and Au-activated WO3 films was also studied. Gas response experiments with films on alumina substrate were done at different operation temperatures, from room temperature up to about 450°C, at exposure to different concentrations of H2S and H2 in dry synthetic air.