Achieving highly-efficient H2S gas sensor by flower-like SnO_(2)–SnO/porous GaN heterojunction

A flower-like SnO_(2)–SnO/porous Ga N(FSS/PGaN) heterojunction was fabricated for the first time via a facile spraying process, and the whole process also involved hydrothermal preparation of FSS and electrochemical wet etching of GaN,and SnO_(2)–SnO composites with p–n junctions were loaded onto PGaN surface directly applied to H_(2)S sensor. Meanwhile,the excellent transport capability of heterojunction between FSS and PGaN facilitates electron transfer, that is, a response time as short as 65 s and a release time up to 27 s can be achieved merely at 150℃ under 50 ppm H_(2)S concentration, which has laid a reasonable theoretical and experimental foundation for the subsequent PGaN-based heterojunction gas sensor.The lowering working temperature and high sensitivity(23.5 at 200 ppm H2S) are attributed to the structure of PGaN itself and the heterojunction between SnO_(2)–SnO and PGaN. In addition, the as-obtained sensor showed ultra-high test stability.The simple design strategy of FSS/PGaN-based H_(2)S sensor highlights its potential in various applications.

Room-Temperature Gas Sensors Under Photoactivation:From Metal Oxides to 2D Materials

Room-temperature gas sensors have aroused great attention in current gas sensor technology because of deemed demand of cheap,low power consumption and portable sensors for rapidly growing Internet of things applications.As an important approach,light illumination has been exploited for room-temperature operation with improving gas sensor's attributes including sensitivity,speed and selectivity.This review provides an overview of the utilization of photoactivated nanomaterials in gas sensing field.First,recent advances in gas sensing of some exciting different nanostructures and hybrids of metal oxide semiconductors under light illumination are highlighted.Later,excellent gas sensing performance of emerging two-dimensional materialsbased sensors under light illumination is discussed in details with proposed gas sensing mechanism.Originated impressive features from the interaction of photons with sensing materials are elucidated in the context of modulating sensing characteristics.Finally,the review concludes with key and constructive insights into current and future perspectives in the light-activated nanomaterials for optoelectronic gas sensor applications.

Recent progress on gas sensors based on graphene-like 2D/2D nanocomposites

Two-dimensional(2D)nanomaterials have demonstrated great potential in the field of flexible gas sensing due to their inherent high specific surface areas,unique electronic properties and flexibility property.However,numerous challenges including sensitivity,selectivity,response time,recovery time,and stability have to be addressed before their practical application in gas detection field.Development of graphene-like 2D/2D nanocomposites as an efficient strategy to achieve high-performance 2D gas sensor has been reported recently.This review aims to discuss the latest advancements in the 2D/2D nanocomposites for gas sensors.We first elaborate the gas-sensing mechanisms and the collective benefits of 2D/2D hybridization as sensor materials.Then,we systematically present the current gas-sensing applications based on different categories of 2D/2D nanocomposites.Finally,we conclude the future prospect of 2D/2D nanocomposites in gas sensing applications.

Gas sensors based on TiO_(2) nanostructured materials for the detection of hazardous gases: A review

Hazardous gases have been strongly associated with being a detriment to human life within the environment The development of a reliable gas sensor with high response and selectivity is of great signifcance for detecting different hazardous gases.TiO_(2) nanomaterials are promising candidates with great potential and excellent per-formance in gas sensor applications,such as hydrogen,acetone,ammonia,and ethanol detection.This review begins with a detailed discussion of the di ferent dimensional morphologies of TiO_(2),whitch affect the gas sensing performance of TiO_(2) sensors.The diverse morphologies of TiO_(2) can easily be tuned by regulating the manufacturing conditions.Meanwhile,they exhibit unique characteristics for detecting gases,including large specific suface area,superior elecron tr ansport rates,extraordinary pemmeability,and active reaction sites,which offer new opportunities to improve the gas sensing properties.In addition,a variety of efforts have been made to functional TiO_(2) nanomaterials to further enhance sensing properties,including TiO_(2)-based composites and light-assisted gas sensors.The enhanced gas sensing mechanisms of multi-component composite nano-materials based on TiO_(2) include loaded noble metals,doped elements,constructed heterojunctions,and com-pounded with other functional materials.Finally,several studies have been summarized to demonstate the compar ative sensing properties of TiO_(2)-based gas sensors.

SnO_(2)nanostructured materials used as gas sensors for the detection of hazardous and flammable gases:A review

SnO_(2)has been extensively used in the detection of various gases.As a gas sensing material,SnO_(2)has excellent physical-chemical properties,high reliability,and short adsorption-desorption time.The application of the traditional SnO_(2)gas sensor is limited due to its higher work-temperature,low gas response,and poor selectivity.Nanomaterials can significantly impact gas-sensitive properties due to the quantum size,surface,and small size effects of nanomaterials.By applying nanotechnology to the preparation of SnO_(2),the SnO_(2)nanomaterial-based sensors could show better performance,which greatly expands the application of SnO_(2)gas sensors.In this review,the preparation method of the SnO_(2)nanostructure,the types of gas detected,and the improvements of SnO_(2)gas-sensing performances via elemental modification are introduced as well as the future development of SnO_(2)is discussed.

Zinc Oxide Nanostructure Thick Films as H₂S Gas Sensors at Room Temperature

The ZnO nanostructures have been synthesized and studied as the sensing element for the detection of H2S. The ZnO nanostructures were synthesized by hydrothermal method followed by sonication for different interval of time i.e. 30, 60, 90 and 120 min. By using screen printing method, thick films of synthesized ZnO nanostructure were deposited on glass substrate. Gas sensing properties of ZnO nanostructure thick films were studied for low concentration H2S gas at room temperature. The effects of morphology of synthesized ZnO nanostructure on gas sensing properties were studied and discussed. ZnO nanostructure synthesized by this method can be used as a promising material for semiconductor gas sensor to detect poisonous gas like H2S at room temperature with high sensitivity and selectivity.

Improvement of the Sensitivity of ZnGa_2O_4 Gas Sensors by Electron Beam Irradiation

In the present paper,the electron beam irradiation was used to improve gas sensing properties of ZnGa_2O_4 gas sensors.The effects of electron beam irradiation on the performance of ZnGa_2O_4 gas sensors were reported.Results show that the sensitivity of ZnGa_2O_4 gas sensors to various gases increased after electron beam irradiation,and the optimal working temperature decreased.The effect of irradiation dose and the reaction mechanism were discussed.

In-situ fabrication of ZnO nanoparticles sensors based on gas-sensing electrode for ppb-level H_(2)S detection at room temperature

The zinc oxide(ZnO)nanoparticles(NPs)sensors were prepared in-situ on the gas-sensing electrodes by a one-step simple sol-gel method for the detection of hydrogen sulfide(H_(2)S)gas.The sphere-like ZnO NPs were characterized by scanning electron microscopy(SEM),transmission electron microscopy(TEM),x-ray diffraction(XRD),energy dispersive x-ray analysis(EDX),and their H_(2)S sensing performance were measured at room temperature.Testing results indicate that the ZnO NPs exhibit excellent response to H_(2)S gas at room temperature.The response value of the optimal sample to750 ppb H_(2)S is 73.3%,the detection limit reaches to 30 ppb,and the response value is 7.5%.Furthermore,the effects of the calcining time and thickness of the film on the gas-sensing performance were investigated.Both calcining time and film thickness show a negative correlation with the H_(2)S sensing performance.The corresponding reaction mechanism of H_(2)S detection was also discussed.

Strategy and Future Prospects to Develop Room-Temperature-Recoverable NO2 Gas Sensor Based on Two-Dimensional Molybdenum Disulfide

Nitrogen dioxide(NO2),a hazardous gas with acidic nature,is continuously being liberated in the atmosphere due to human activity.The NO2 sensors based on traditional materials have limitations of high-temperature requirements,slow recovery,and performance degradation under harsh environmental conditions.These limitations of traditional materials are forcing the scientific community to discover future alternative NO2 sensitive materials.Molybdenum disulfide(MoS2)has emerged as a potential candidate for developing next-generation NO2 gas sensors.MoS2 has a large surface area for NO2 molecules adsorption with controllable morphologies,facile integration with other materials and compatibility with internet of things(IoT)devices.The aim of this review is to provide a detailed overview of the fabrication of MoS2 chemiresistance sensors in terms of devices(resistor and transistor),layer thickness,morphology control,defect tailoring,heterostructure,metal nanoparticle doping,and through light illumination.Moreover,the experimental and theoretical aspects used in designing MoS2-based NO2 sensors are also discussed extensively.Finally,the review concludes the challenges and future perspectives to further enhance the gas-sensing performance of MoS2.Understanding and addressing these issues are expected to yield the development of highly reliable and industry standard chemiresistance NO2 gas sensors for environmental monitoring.

Effect of Nanoparticle Size on Gas-sensing Properties of Tin Dioxide Sensors

Sn（OH）4 was prepared by the conventional solution precipitate method,followed by supercritical CO2 drying.The resultant Sn（OH）4 was divided into three aliquots and calcined at 400,600 and 800 °C,respectively,thus SnO2 nanoparticles with average crystallite sizes of 5,10 and 25 nm were obtained.Furthermore,three SnO2 thick film gas sensors（denoted as sensors S-400,S-600 and S-800） were fabricated from the above SnO2 nanoparticles.The adhesion of sensing materials on the surface of alumina tube is good.Compared to the sensors S-600 and S-800,sensor S-400 showed a much higher sensitivity to 1000 μL/L ethanol.On the other hand,sensor S-800 showed a much lower intrinsic resistance and improved selectivity to ethanol than sensors S-400 and S-600.X-Ray diffraction（XRD）,transmission electron microscopy（TEM） and selective area electron diffraction（SAED） measurements were used to characterize the SnO2 nanoparticles calcined at different temperatures.The differences in the gas sensing performance of these sensors were analyzed on the basis of scanning electron microscopy（SEM）.

Enhancement of H_2 Sensing Properties of In_2O_3-based Gas Sensor by Chemical Modification with SiO_2

The sensitivity and selectivity to H_2 of a new In_2O_3-based gas sensor were improvedsignificantly by surface chemical modification. A dense layer of SiO_2 near the surface of the porousIn_2O_3 bead was formed by chemical vapor deposition(CVD)of diethoxydimethysilane(DEMS).The dense layer functioned as a molecular sieve, thereby the diffusion of gases with large moleculardiameters,except for H_2, was effectively controlled, resulting in a prominent selectivity and highsensitivity for H_2. The working mechanism of the sensor was also presented.

One-step Growth of Hierarchical Nanotreelike TiO2 on ITO without Template and Its Application in Gas Sensor

In this work,a distinctive hierarchical tree-like rutile TiO2 architecture growing directly on the conductive surface of indium tin oxide(ITO)conductive glass substrates was successfully prepared via a facile one-step hydrothermal process,where titanium butoxide used as Ti source and HCl as an acidic medium solution.The as-obtained products were characterized by X-ray diffraction(XRD),field emission scanning electron microscopy(FE-SEM),transmission electron microscopy(TEM),selected area electron diffraction(SAED)and applied in gas sensor.The characterization of FE-SEM indicates that the morphology of the products can be controlled by regulating solution acidity,the amount of titanium butoxide,reaction time,addition agents,and so on.The gas sensing test shows that the sensor fabricated with 3D nanotree-like rutile TiO2 has higher gas response towards CH3 COCH3 gas than those with 1 D rod-like TiO2 or common TiO2 precipitate,indicating that the 3D nanotree-like architectures may be promising gas sensitive materials.

Hydrogen and Ozone SAW Based Gas Sensors with RF Magnetron Sputtered InO_x and Electron Beam Evaporated SiN_y

Layered Surface Acoustic Wave (SAW) devices with an InO_x/SiN_u/36°YX LiTaO_3 structure were investigated for sensing low concentrations of hydrogen (H_2) and ozone (O_3) at different operating temperatures.The sensor consists of a 1μm thick silicon nitride (SiN_y) intermediate layer deposited by electron beam evaporation on a 36°Y-cut X-propagating piezoelectric lithium tantalate (LiTaO_3) substrate and a 100 nm thin indium oxide (InO_x) sensing layer deposited by R.F.magnetron sputtering.The device fabrication is described and the performance of the sensor is analyzed in terms of response magnitude as a function of operating temperature.Large frequency shifts of 360 kHz for 600μg/g of H_2 and 92 kHz for 40 ng/g O_3 were recorded.In addition,the surface morphology of the deposited films were investigated by Atomic Force Microscopy (AFM) and the chemical composition by X-Ray Photoelectron Spectroscopy (XPS) to correlate gas-sensing behavior to structural characteristics of the thin film.

SnO_(2)/Co_(3)O_(4)nanofibers using double jets electrospinning as low operating temperature gas sensor

SnO_(2)/Co_(3)O_(4)nanofibers(NFs)are synthesized by using a homopolar electrospinning system with double jets of positive polarity electric fields.The morphology and structure of SnO_(2)/Co_(3)O_(4)hetero-nanofibers are characterized by using field emission scanning electron microscope(FE-SEM),transmission electron microscope(TEM),x-ray diffraction(XRD),and x-ray photoelectron spectrometer(XPS).The analyses of SnO_(2)/Co_(3)O_(4)NFs by EDS and HRTEM show that the cobalt and tin exist on one nanofiber,which is related to the homopolar electrospinning and the crystallization during sintering.As a typical n-type semiconductor,Sn O_(2)has the disadvantages of high optimal operating temperature and poor reproducibility.Comparing with Sn O_(2),the optimal operating temperature of SnO_(2)/Co_(3)O_(4)NFs is reduced from 350℃to 250℃,which may be related to the catalysis of Co_(2)O_(2).The response of SnO_(2)/Co_(3)O_(4)to 100-ppm ethanol at 250℃is 50.9,9 times higher than that of pure Sn O_(2),which may be attributed to the p–n heterojunction between the n-type Sn O_(2)crystalline grain and the p-type Co_(2)O_(2)crystalline grain.The nanoscale p–n heterojunction promotes the electron migration and forms an interface barrier.The synergy effects between Sn O_(2)and Co_(2)O_(2),the crystalline grain p–n heterojunction,the existence of nanofibers and the large specific surface area all jointly contribute to the improved gas sensing performance.

Oscillation in Response and Recovery Properties of SnO_2 Gas Sensor

Six kinds of Sn(OH)_4 precipitates were prepared by the chemical precipitation method (liquid-phase),of which the pH values are 3.0 (sample 1),4.5 (sample 2),6.0 (sample 3),7.5 (sample 4),9.0 (sample 5) and 10.5 (sample 6),respectively. After washed and dried,the Sn(OH)_4 precipitates were first calcined into tin oxide powder.Six kinds of gas sensors were made from the obtained SnO_2 powders.The XRD patterns of all the samples were recorded.The sensing properties of all samples to C_2H_5OH and H_2 gases were measured in different gas concentrations and temperatures,including the sensitivities of all samples,their response and recovery properties.The recovery properties of all samples to C_2H_5OH exhibit the sinusoidal oscillation character.

Fabrication and Characterization of NASICON-Based CO_2 Gas Sensor

Tube-type CO_2 gas sensors based on NASICON (Na Super Ion Conductor) material were fabricated.The material was synthesized by conventional sol-gel method,and the resulted powders were characterized by XRD.The tube-type CO_2 sensor was prepared with the sensing electrode Li_2CO_3-BaCO_3 binary carbonates in molar ratio 1:1.5.The concentration of CO_2 range from 300μg/g to 3000μg/g,the sensitivity of the sensor was 62.3 mV/decade.The response and recovery time (90%)corresponding to the switching change between 300μg/g and 1000μg/g CO_2 were 20 s and 2 min,respectively.If the sensing electrode was modified with binary oxides,the steady time of the sensor could be greatly reduced from 30 min to 5 min and the stability and humid-resistance of the sensor were improved.

H_2S Gas Sensor Based on Nanocrystalline ZnO Synthesized by Electrochemical-Deposition

ZnO nanocrystals were prepared by a direct current electrochemical deposition process under 3.0V working voltage and 30A/m^2 current density using zinc sulfate as raw materials.The nanocrystals were characterized by X-ray diffraction (XRD)and transmission electron microscopy(TEM).The results indicated that the nanocrystals are hexagonal wurtzite ZnO with particle size range of 25nm～40nm without any treating.Gas sensing properties of the sensors were tested by mixing a gas in air at static state;the tested results showed that the sensors based on nanocrystalline ZnO had satisfied gas sensing properties to H_2S gas at rather low temperature.

Pure SnO₂Gas Sensor with High Sensitivity and Selectivity towards C₂H₅OH

To observation, poisonous gases in the environment, Sensors with high selectivity, high response and low operating temperature are required. In this work, pure SnO2 nanoparticles was prepared by using a simple and inexpensive technique (hydrothermal method) without a template. Various confirmatory tests were performed to characterize SnO2 nanoparticles such as energy dispersive X-ray spectroscopy (EDX), X-ray diffraction (XRD), Scanning Electron Microscopy (SEM) and Transition Electron Microscopy (TEM), during the detection of the gas, we found that pure SnO2 nanoparticles has a high selectivity for ethanol to 100 ppm at a low temperature (180°C) and a high response (about 27 s) and a low detection limit of 5 ppm, also it have response/recovery times about (4 s, 2 s) respectively. The distinctive sensing properties of SnO2 sensor make it a promising candidate for ethanol detection. Furthermore, the gas-sensing mechanism have been examined.

High-sensitivity formaldehyde gas sensor based on Ce-doped urchin-like SnO_(2) nanowires derived from calcination of Sn-MOFs

Metal-organic frameworks(MOFs)have attracted widespread attention due to their regular structures,multiple material centers,and various ligands.They are always considered as one kind of ideal templates for developing highly sensitive and selective gas sensors.In this study,the advantages of MOFs with the high specific surface area(71.9891 m^(2)·g^(-1))and uniform morphology were fully utilized,and urchin-like SnO_(2) nanowires were obtained by the hydrothermal method followed by the calcination using Sn-MOFs consisting of the ligand of C_(9)H_(6)O_(6)(H_(3)BTC)and Sn/Ce center ions as sacrificial templates.This unique urchin-like nanowire structure facilitated gas diffusion and adsorption,resulting in superior gas sensitivity.A series of Ce-doped SnO_(2) nanowires with different doping ratios were synthesized,and their gas sensing properties towards formaldehyde were studied.The resulted Ce-SnO_(2) was revealed to have high sensitivity(201.2 at 250℃),rapid response(4 s),long-term stability,and good repeatability for formaldehyde sensing,and the gas sensing mechanism of Ce-SnO_(2) exposed to formaldehyde was also systematically discussed.

Air-stable,all-dry transferred ReS_(2)/GaSe heterostructure-based NO_(2)gas sensor

Two-dimensional(2D)materials have gained considerable attention in chemical sensing owing to their naturally high surface-to-volume ratio.However,the poor response time and incomplete recovery re-strict their application in practical,high performance gas sensors.In this work,we fabricated air-stable ReS_(2)/GaSe heterostructure-based NO_(2)gas sensors with excellent gas sensing response,recovery,selectiv-ity and a low limit of detection(LOD)toward nitrogen dioxide(NO_(2)).The ReS_(2)/GaSe heterostructure was prepared via mechanical exfoliation and an all-dry transfer method.Before the sensing measurements,temperature-dependant transport measurements were carried out.The Schottky Barrier Height(SBH)of the ReS_(2)/GaSe heterostructure was calculated and the corresponding transport mechanisms were dis-cussed.The fabricated gas sensors showed a significant response enhancement with full reversibility to-ward ppm-level NO_(2)(response of∼17%at 3 ppm,a LOD of∼556 ppb)at an operating temperature of(33°C).In particular,the total response and recovery time of the ReS_(2)/GaSe was revealed to be less than 4 min(∼38 s and∼174 s,respectively)for the 250 ppm concentration,which is one of the best response and recovery time toward ppm-level NO_(2).The excellent sensing performances and recovery characteris-tics of the ReS_(2)/GaSe structure are attributed to its efficient charge separation,unique interlayer coupling and desirable band alignments.This atomically thin,ultrasensitive gas sensor that operates at room tem-perature is a strong technological contender to conventional metal oxide gas sensors,which often require elevated temperatures.