High-sensitivity Gas Research Articles

Three-dimensional TiO2/SiO2 composite aerogel films via atomic layer deposition with enhanced H2S gas sensing performance

The SiO2 aerogel films with three dimensional (3D) networks were chosen as the templates to deposit TiO2 nanoparticles by atomic layer deposition (ALD). The microstructure, morphology and gas sensing properties of the composite aerogel films were investigated. The composite aerogel films kept the mesoporous structures with TiO2 nanoparticles uniformly coated on. After the 500℃ calcination, the TiO2 nanoparticles changed from amorphous to anatase phase. The calcined composite aerogel films were used to fabricate the gas sensor to detect H2S gas. The sensor had an excellent gas sensitivity of 1.34 with the response and recovery time of 53 and 74s, respectively, at 250℃ when the H2S gas concentration was as extremely low as 0.5ppm. The sensor still had a high gas sensitivity with the H2S concentration of 10ppm even when the operating temperature was 100℃. Compared to other H2S gas sensors based on TiO2 materials, the enhanced sensing performance could be due to the larger specific surface area of the aerogel template，the smaller grain size and the lattice mismatch between TiO2 nanoparticles and SiO2 aerogels, which made the composite aerogel films had more active sites and lattice defects, resulting in the increase of oxygen vacancy and enhancement of the gas sensitivity.

Plasmonic Crystal-Based Gas Sensor Toward an Optical Nose Design

This paper presents a high-sensitivity gas sensor based on plasmonic crystal incorporating a thin layer of graphene oxide (GO). The plasmonic crystal consists of a periodic array of polymeric nanoposts with gold (Au) disks at the top and nanoholes in an Au thin film at the bottom. The thin GO layer coated atop the plasmonic crystal is the gas absorbent material for the sensor. Gas adsorption of GO modifies the refractive index of the plasmonic structure and returns a shift in the resonance wavelength of the surface plasmon polariton excited at the GO coated Au surface. The differences in target gas species and their adsorption in GO lead to a difference in the resonance shift. To identify the gas species in a complex gas mixture, a sensor array is designed with different sensing elements coated with different thicknesses of GO. The optical responses of the different sensing elements to a gas mixture are analyzed using principal component analysis (PCA)-based pattern recognition algorithm. The PCA separates the sensor responses to different species of gases, providing specificity of the device. The proposed sensor demonstrates a refractive index sensitivity of 449.63 nm/RIU. In addition, volatile organic compounds, such as ethylene, methanol that serve as plant health indicators and ammonia that plays a key role in the ecosystem, are detected by the sensor. A change in a gas concentration results in a differing amount of adsorbate and correlated shift in the resonance wavelength of the device. The GO coated sensor exhibits sensitivities of 0.6 pm/ppm to gaseous ethylene, 3.2 pm/ppm to methanol, and 12.84 pm/ppm to ammonia. The integration of plasmonic sensing element arrays with varying GO thicknesses to modulate the gas adsorption, offers a promising approach to detect different gas species in a single test.

Spray deposited ruthenium incorporated CdO thin films for opto-electronic and gas sensing applications

The ruthenium (Ru) doped CdO thin films were deposited by spray pyrolysis method on glass substrate, with various Ru concentrations (0.1, 0.125, 0.150, 0.175 and 0.2 wt%). The effect of Ru doping concentration on the electrical, optical, structural, microstructural and gas sensing properties was studied by Hall measurement, UV–vis–NIR spectroscopy, X-ray diffraction, X-ray photoelectron spectroscopy, field emission scanning electron microscopy, micro-Raman and chemi-resistivity method. The results obtained show that the carrier mobility, visible-NIR transparency, surface morphology and gas response are effectively tuned by varying Ru doping concentration. The 0.1 wt% Ru doped CdO thin film shows relatively high optical band gap (2.32 eV) and maximum transmittance (70%) in the range between 600 and 900 nm. The high electrical properties (mobility 110 cm2/V⋅s and carrier concentration 1.77 × 1020 cm−3) are obtained for 0.150 wt% Ru doped CdO thin film. The high formaldehyde gas sensitivity (35%), fast response and recovery time are observed for 0.150 wt% Ru doped CdO thin film.

Tungsten oxide thin films: detection and trapping of hazardous gases

Synthesis of tungsten (W) and tungsten tri-oxide (WO3) thin films on alumina substrate by a peculiar Red-ox reaction route using hot-filament chemical vapor deposition technique is described. The resulting tungsten and tungsten oxide films were characterized using various techniques such as x-ray diffraction (XRD), Raman spectroscopy and Scanning electron microscopy (SEM). XRD results revealed the complete conversion of cubic phase of pure tungsten into monoclinic phase of tungsten oxide. Raman spectroscopic analysis also confirmed the formation of WO3. SEM images show considerable alteration in morphology from well faceted particles to wafers when pure W-film was converted to WO3 film. The wafer like morphology of WO3 films is found to be suitable for gas sensing towards hazardous gases such as NO2 and NH3. The WO3 films showcased their highly responsive nature towards NO2 gas with exceptionally high gas sensitivity ~32. WO3 film demonstrated longer recovery time towards NO2 gas unlike NH3 gas making them attractive for their utilization in ‘Newer application’ such as a catalyst support material in catalytic converter devices which are potential representatives to arrest pollutant gases (NO2) getting flown into the living environment.

Correlation Between High Gas Sensitivity and Dopant Structure in W-doped ZnO

The proverbial ``canary in a coal mine'' is truly a thing of the past; nowadays zinc oxide is a key material in solid-state gas sensors, yet the mechanism by which doping can improve its performance is not fully understood. The authors use two-dimensional photoelectron diffraction to study thin films of tungsten-doped ZnO, and find clear evidence for substitution of segregated W atoms into Zn sites in the subsurface layer, which drastically improves the sensing response. This insight into the surface's structure-function relationship suggests how best to prepare these films for this important application.

Hollow-Core Fiber-Based High Finesse Resonating Cavity for High Sensitivity Gas Detection

High finesse hollow-core photonic bandgap fiber (HC-PBF) resonating Fabry–Perot gas cells are presented. These gas cells are made with a piece of HC-PBF sandwiched by two single mode fibers with mirrored ends. A HC-PBF cavity made with 6.75-cm-long HC-1550-06 fiber achieved a cavity finesse of 128, corresponding to an effective optical path length of ∼5.5 m. Experiment with a 9.4-cm-long Fabry–Perot gas cell with a finesse of 68 demonstrated a detection limit of 7 ppm acetylene. Compared with a single-path nonresonating HC-PBF, the use of a high finesse resonating HC-PBF cavity can reduce significantly the effect of modal interference on gas detection and improve the detection sensitivity. The cavity-enhanced HC-PBF gas cells enable stronger light–gas interaction and can be used to develop all-fiber gas sensors with high sensitivity and fast response.

High Sensitivity Ammonia Gas Sensor Based on a Silica-Gel-Coated Microfiber Coupler

In this paper, a high-sensitivity ammonia gas sensor is proposed based on a silica-gel-coated microfiber coupler (MFC). The MFC structure is formed by the two tapered fibers with 3 μm waist diameter each, which were fabricated by using a customized microheater brushing technique. Silica gel coating was prepared by a sol-gel technique and applied on the surface of the MFC as a thin layer. The spectral characteristics of the proposed sensor were studied under various ammonia gas concentrations. The experimental results show that the coating thickness strongly affected the sensitivity of the MFC-based sensor to ammonia gas concentration. For the sensor with a 90 nm silica gel coating thickness, the highest measurement sensitivity is 2.23 nm/ppm for ammonia gas concentration, and the resolution is as good as 5 ppb, while the measured response and recovery times are ∼50 and 35 s, respectively. Finally, it is demonstrated that the proposed sensor offers good repeatability and selectivity to ammonia gas.

Templated diamond growth on porous carbon foam decorated with polyvinyl alcohol-nanodiamond composite

Porous and self-standing structures are highly promising as membranes, electrodes, high sensitivity gas sensors, and supercapacitors due to their high surface area and tuneable surface properties. Composition of such materials with a polycrystalline diamond film may provide new functionalities for biological and electrochemical applications. However, the diamond growth on foreign substrates as a pinhole-free continuous thin film is still a not a trivial task. Here, we present a successful templated deposition process of polycrystalline diamond film on reticulated vitreous carbon foam (C-foam). Samples of polyvinyl alcohol (PVA) polymer-nanodiamond (ND) mixtures with various ND concentrations were prepared and characterized by means of optical spectroscopy, Raman spectroscopy, dynamic light scattering, zeta potential and pH measurements. The ND/PVA mixtures are employed for the preparation of diamond film by microwave plasma enhanced chemical vapour deposition process, and the effect of their composition (NDs/PVA ratio) on the nucleation (seed density, clustering) and diamond growth (substrate etching, polymer transformation) is analysed. The nucleation, early-stage of diamond growth and the resulting diamond-coated C-foams are characterized by scanning electron microscopy, atomic force microscopy and Raman spectroscopy. The obtained results underline the importance to tailor the NDs/PVA ratio for a well-defined three-dimensional C-foam coverage with the continuous diamond film.

Non-proximate mass spectrometry using a heated 1-m long PTFE tube and an air-tight APCI ion source

Direct and rapid trace-level gas analysis is highly needed in various fields such as safety and security, quality control, food analysis, and forensic medicine. In many cases, the real samples are bulky and are not accessible to the space-limited ion source of the mass spectrometer. In order to circumvent this problem, we developed an airtight atmospheric-pressure chemical ionization (APCI) ion source equipped with a flexible 1-m-long, 2-mm-i.d. PTFE sniffing tube. The ambient air bearing sample gas was sucked into the heated PTFE tube (130 °C) and was transported to the air-tight ion source without using any extra pumping system or a Venturi device. Analytes were ionized by an ac corona discharge located at 1.5 mm from the inlet of the mass spectrometer. By using the airtight ion source, all the ionized gas in the ion source was introduced into the vacuum of the mass spectrometer via only the evacuation of the mass spectrometer (1.6 l min−1). Sub-pg limits of detection were obtained for carbaryl and trinitrotoluene. Owing to its flexibility and high sensitivity, the sniffing tube coupled with a mass spectrometer can be used as the stethoscope for the high-sensitive gas analysis. The experimental results obtained for drugs, hydrogen peroxide and small alkanes were discussed by DFT calculations.

Polymer Sensitive Elements for Gas Sensors of Ammonia

The influence of ammonia on the optical absorption spectra of thin films of polyaniline (PANI), poly-3,4-ethylenedioxythiophene (PEDOT), mixtures thereof, and the bilayer film structures based on PANI and PEDOT obtained by electrochemical method were studied. The kinetics of changes in the optical absorption of these structures under the action of ammonia was explored in the spectral range of 350 ... 900 nm. We proved that the thin-film structure of PANI / PEDOT significantly extends the spectral range of sensitivity to ammonia of individual PANI n PEDOT films by superposition of high gas sensitivity of PANI and PEDOT films in the range of wavelength of 500 ... 700 nm and 350... 550 nm respectively. It was shown that the area under the spectra of change of the optical absorption of thin-film structure of PANI / PEDOT is in 1.45 times larger than the area of the same for individual components of the layers, which is the key to increasing the sensitivity of the gas sensors. The results can be used to optimize ammonia gas sensors for monitoring biotechnological processes in food industry and the environment.

Synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles

Herein, we report the simple synthesis, characterization and acetone gas sensing applications of Ag-doped ZnO nanoneedles prepared by facile hydrothermal method. The synthesized nanoneedles were characterized through different characterization techniques to examine its crystallinity, phase structure, morphological, compositional, optical, vibrational and gas sensing properties. The detailed morphological studies revealed that the Ag-doped ZnO nanoneedles are assembled into non-homogeneously distributed flower-shaped structures which are grown in high density. Further characterizations confirmed that the synthesized nanoneedles are pure, possessing well-crystallinity and exhibiting good optical and vibrational properties. The synthesized Ag-doped ZnO nanoneedles were used as functional material to fabricate high sensitive acetone gas sensors. The effect of operating temperature and concentration of the acetone were analyzed for detailed sensing performance of synthesized nanoneedles. By detailed sensing experiments, the response and recovery times of 10s and 21s, respectively were calculated at acetone concentration of 100ppm at an optimized operating temperature of 370°C. A maximum sensitivity of 30.233 was recorded at 370°C operating temperature for 200ppm of acetone for the fabricated acetone sensor based on Ag-doped ZnO nanoneedles.

Synthesis and Characterization of CuO Nanodisks for High-Sensitive and Selective Ethanol Gas Sensor Applications.

Herein, the fabrication and characterization of highly sensitive and selective ethanol gas sensor based on CuO nanodisks is reported. The CuO nanodisks were synthesized by facile hydrothermal process and detailed characterization revealed the well-crystallinity, high-purity and high density growth of the prepared material. To fabricate the ethanol gas sensor, the prepared nanodisks were coated on alumina substrate. The fabricated sensor exhibited high-sensitivity and the recorded gas response (resistance-ratio), response time (τ res) and recovery time (τ recov) were 6.2, 119 and 35 s, respectively for 100 ppm of C₂H₅OH at 300 °C. Further, the fabricated sensor shows high selectivity towards ethanol gas compared to H₂ and CO gases.

Sensing properties of MWCNTs layers electrodecorated with metal nanoparticles for detection of aromatic hydrocarbon compounds

ABSTRACTAn electrophoretic process is proposed to deposit electrochemically-preformed Au or Pd NPs, with controlled size, directly on MWCNTs-based chemiresistors to improve the detection of aromatic pollutants, compared to pristine ones.The sensing properties of pristine and functionalized MWCNTs were evaluated at an operating temperature of 40°C towards various concentrations of one aromatic pollutant, belonging to the dangerous BTEX class of compounds,m-Xylene. The sensing performance was related to the metal used in the functionalization process. Metal-doped MWCNTs sensors exhibited a very high gas sensitivity to m-Xylene even at low (80 ppb) concentration at low operating temperature (40°C), good reversibility and repeatability, with the sensing properties controlled by the type of deposited metal catalyst.The results indicate that Metal-modified MWCNT-based chemiresistive gas sensors has good potential in practical applications, due to its remarkable performance, low power consumption, and facile synthesized methods.

Synthesis of uniform porous NiO nanotetrahedra and their excellent gas-sensing performance toward formaldehyde

Porous NiO nanotetrahedrons and elongated irregular nanoparticles were synthesized, exhibiting superior gas-sensing behavior toward formaldehyde with high gas sensitivity and stability.

Solution combustion synthesis and enhanced gas sensing properties of porous In2O3/ZnO heterostructures

Self-assembled porous In2O3/ZnO heterostructures are synthesized via a low temperature solution combustion method. An extremely high gas sensitivity can be reached when exposed to Cl2 at 370 °C.

Three-Dimensional Graphene Interconnected Structure, Fabrication Methods and Applications: Review

Enormous amounts of research and development are currently underway in the arena of improving threedimensional graphene and graphene oxide networks for use as foams, sponges and aerogels for a wide range of applications. The underlying reason for the interest in 3D grapheme can be attributed to the splendid intrinsic and extrinsic properties brought as a result of the 3D porous structures which has given rise to high specific surface areas, good mechanical strength, fast mass and electron movement. Therefore a review about the present modes of fabrication, material properties and the applications of 3D grapheme with strong emphasis on highly efficient graphene sponges and graphene foams is expounded. Following on from this, a concrete understanding of the fabrication of functionalized graphene aerogel for use in oil water separation will be addressed. To add on, the superiority of threedimensional (3D) graphene based macro porous architectures, as an absorbent in comparison to various sorbent materials will be scrutinized. Furthermore, it was concluded that although large amounts of research and developments have been achieved thus far, there is still some room for improvement. In fact the benefits of these improvements will be applicable in a wide range of applications ranging from: high sensitivity gas detectors, super capacitors, lithium ion batteries, oil-water separators to name a few.

Graphene-Based Long-Period Fiber Grating Surface Plasmon Resonance Sensor for High-Sensitivity Gas Sensing.

A graphene-based long-period fiber grating (LPFG) surface plasmon resonance (SPR) sensor is proposed. A monolayer of graphene is coated onto the Ag film surface of the LPFG SPR sensor, which increases the intensity of the evanescent field on the surface of the fiber and thereby enhances the interaction between the SPR wave and molecules. Such features significantly improve the sensitivity of the sensor. The experimental results demonstrate that the sensitivity of the graphene-based LPFG SPR sensor can reach 0.344 nm%−1 for methane, which is improved 2.96 and 1.31 times with respect to the traditional LPFG sensor and Ag-coated LPFG SPR sensor, respectively. Meanwhile, the graphene-based LPFG SPR sensor exhibits excellent response characteristics and repeatability. Such a SPR sensing scheme offers a promising platform to achieve high sensitivity for gas-sensing applications.

Room-temperature high-performance ammonia gas sensor based on layer-by-layer self-assembled molybdenum disulfide/zinc oxide nanocomposite film

A high-sensitivity ammonia gas sensor based on molybdenum disulfide/zinc oxide (MoS2/ZnO) nanocomposite film was reported in this paper. The multilayered film of MoS2/ZnO was fabricated on a PCB substrate with interdigital electrodes (IDE) via layer-by-layer self-assembly technique. The surface morphology, nanostructure and elemental composition of the MoS2/ZnO nanocomposite were described by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). The results showed that ZnO and MoS2 had good contact in terms of electric and structure. Ammonia-sensing properties of the sensor were investigated by exposing it to various concentrations of ammonia at room temperature. The experimental results revealed that the MoS2/ZnO nanocomposite film sensor exhibited ultra-high sensitivity, good repeatability, excellent stability, outstanding selectivity and fast response/recovery characteristics. Moreover, the sensing device exhibited significantly enhancement in ammonia gas sensing compared to the drop-casted pure ZnO and layer-by-layer self-assembled MoS2/polymer sensors. These results render the MoS2/ZnO nanocomposite a promising candidate for high-performance ammonia sensing at room temperature. The underlying sensing mechanisms of the MoS2/ZnO sensing device towards ammonia gas were ascribed to the excellent electrochemical properties and special nanostructure of the nanocomposite.

Porous LaFeO3/SnO2 nanocomposite film for CO2 detection with high sensitivity

Perovskite LaFeO3 nanocrystalline powder, synthesized by sol–gel method, was composited with SnO2 nanopowders to fabricate porous LaFeO3/SnO2 thick film sensors. The as-prepared sensors exhibit high gas sensitivity, fast response and low working temperature for CO2 detection. The optimal sensing performance of LaFeO3/SnO2 thick film sensors is obtained with the molar ratio of La/Sn = 1:1. The observed response is nearly 2 times higher than that of bare LaFeO3 at 4000 ppm CO2 and the response time is less than 20 s at 250 °C. CO2 sensing mechanism of the as-prepared nanocomposites porous film is addressed. The results demonstrate that the as-formed P-N junctions can greatly enhance the carrier transfer efficiency which is attributed to the improved sensing performance. Moreover, the hydroxyl species absorbed on the surface of the nanocomposites are also involved in the sensing process.

High sensitive and fast formaldehyde gas sensor based on Ag-doped LaFeO3 nanofibers

Ag-doped LaFeO3 nanofibers are synthesized by electrospinning method. The chemical and structural characterizations of the composite nanofibers are performed with various techniques such as X-ray diffraction (XRD), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Meanwhile, their formaldehyde (HCHO) sensing properties are investigated in detail. The experimental results indicate that the sensor based on the sample Ag-LaFeO3 shows excellent gas sensing properties to formaldehyde. The response to 5 ppm formaldehyde is 4.8 at the operating temperature and the sensors have good selectivity. The response and recovery time to formaldehyde gas is 2 s and 4 s, respectively.