Preparation and Gas Sensitive Properties of Nanometer-Sized SnO_(2)

Nanometer-sized SnO_(2) particles were prepared by a sol-gel method using inorganic salt as a precursor material. Its crystallization was investigated by means of TG-DTA,IR absorption spectra, X-ray diffractometry and TEM as well as its resistivity change and the gas sensitivity varied with temperature were measured in various reducing gas. The results indicate that well-crystallized nano-sized SnO_(2)with size around 15nm can be obtained at annealing temperature 600℃. The activation energy for the growth of nano-SnO_(2) was calculated to be 26.55kJ.mol^(-1) when the annealing temperature was higher than 500℃. The measurements also show that there is a peculiar resistance change varied with temperature for nano SnO_(2).It has relevance to the increase in surface adsorbed oxygen. The selective detectivities to C_(4)H_(10) and petrol can be increased when ruthenium ion was doped in nano SnO_(2)as a catalyst and so do the gas sensitivity to CO,CH_(4),H_(2) etc. when rhodium ion was doped in.The detection to the several reducing gas can be realized when the temperature ranged from 260℃ to 400℃.

Studies on Gas Sensitive Materials of γ-Fe_2O_3 Doped with Rare Earth Oxides

Fine powders of γ-Fe_2O_3,doped with Y_2O_3,CeO_2,Eu_2O_3 or Tb_2O_3 have been prepared by the chemical co-precipitation method.The sensitivity of gas sensation has been measured with respect to the relative resist- ance change in the ceramic matrix upon introduction of inflammable gases.The structure of the materials has been studied with X-ray diffraction spectroscopy(XRD),electron diffraction spectroscopy( ED) and transmis- sion electron microscopy(TEM).The addition of rare earth oxides,which improves ceramic microstructure of γ-Fe_2O_3,improves gas sensitivity of γ-Fe_2O_3.The stability can be increased because of the increase of phase transition temperature.In addition,the selectivity of gas sensation of γ-Fe_2O_3 can be improved because of the variation of rare earth oxides.

The Photovoltage-gas Sensitive Properties of 7,7,8,8-Tetracyanoquinodimethane(TCNQ)

In this paper, surface photovoltage technique (SPV) was applied to the study of photovoltaic and gas sensitive properties of TCNQ polycrystal. It was found that SPV shows two peaks at 390 nm(P1) and 480 nm (P2) in the ultraviolet-visible range. And they are opposite in phase. The results of gaseous adsorption confirm that P, shows the acceptor characteristics, while PI shows that of donor. During adsorption, donor gas interacts with conjugated II * orbital, acceptor gas with the terminal group C=N .

Thin Film Nanocomposites for Gas Sensors

Nanocomposites on the base of thin films SnO_2 with additives of metal oxides such as SiO_2,ZrO_2,MnO_2 and others,were prepared by reactive ion-beam sputtering of metal target in the ambient of O_2+At,and then thermal treated at 500℃.Atomic composition and morphology of thin film nanocomposites were investigated.Electrical properties and gas sensitivity of nanocomposites were studied.It Was determined that the grain size of polycrystals depends on the film composition and can run down to 10 nm.It was found that temperature of maximal gas sensitivity of the films depends on their composition as well and it can be decreased by 100℃～150℃lower than temperature of maximal sensitivity of undoped films SnO_2.

Gas Sensitivity of Poly (3, 4-ethylene dioxythiophene) Prepared by a Modified LB Film Method

An arachidic acid/poly （3, 4-ethylene dioxythiophene） （AA/PEDOT） multilayer Langmuir-Blodgett （LB） film was prepared by a modified LB film method. The theories were utilized to explain the effects between HCl molecule and LB film. The gas sensitivity mechanism of poly （3, 4-ethylene dioxythiophene） （PEDOT） multilayer film can be explained by the charge transfer between p system of PEDOT and oxidization HCl system. The gas sensitivity of PEDOT LB film deposited interdigital electrode to HCl was tested. The results showed that film thickness, treating temperature, deposition speed had different influence on film gas sensitivity. The AA/PEDOT film deposited device exhibited nonlinear behavior to HCl gas at lower concentration （20-60 ppm） and linear response behavior at higher gas concentration was observed. The time of the compound LB film of the AA/PEDOT responding to the 30 ppm HCl gas is about 20 seconds, which is far quicker than the time of the film to the PEDOTPRESS film（about 80 seconds）. It is not higher film press to better film. When the film press attains 45 mNs/m, the sensitivity of the AA/PEDOT film on the contrary descends.

The Study on the Gas Sensitivity of the One-Dimensional ZnO

One-dimensional (1D) ZnO was prepared through hydrothermal synthesis by cetyltrimethylammonium bromide (CTAB) controling its morphology.The products were characterized by X-ray diffraction (XRD),transmission electron microscopy (TEM) and scanning electron microscopy (SEM).The sintered gas sensor was prepared using ZnO nanorods and its gas sensitivity was measured in static gas atmosphere.The result showed that the ZnO nanorods had lower working temperature than granulated ZnO and had very good sensitivity to 10μg/g trimethylamine (TMA),methanol,ammonia, acetone and ethanol at the working temperature of 170℃.SEM picture revealed that there were many multi-pores in the surface of the sensors.These multi-pores might help to absorb gases and improve the gas sensitivity.

Shape Controlled Preparation,Characterization and Gas Sensitivities of Shuttle-like Cr-doped α-Fe_2O_3 Nanoparticles

α-Fe2O3 nanoparticles doped with various molar fractions of Cr^3＋ were synthesized by a forced hydrolysis route and were characterized by X-ray diffraction（XRD）,scanning electronic microscopy（SEM）,X-ray photoelectron spectroscopy（XPS） and inductive coupled plasma（ICP） techniques.The particles reserve shuttle-like shape in the presence of Cr^3＋.The crystallite sizes of Fe2O3 become smaller with the increased Cr^3＋ concentration in solution.The responses of Cr doped α-Fe2O3 sensors were studied towards reducing gases such as ethanol,methanol,acetone,gasoline and n-hexane.Gas sensors based on these materials have higher sensitivities and rapid response/recovery time to alcohol than to hydrocarbon.

Electronic Nose with an Air Sensor Matrix for Detecting Beef Freshness

China is one of the largest meat producing countries in the wodd. With the growing concern for food safety more attention has been paid to meat quality. The application of conventional test methods for meat quality is limited by many factors, and subjectiveness, such as longer time to prepare samples and to test. A sensor matrix was constructed with several separate air sensors, and tests were conducted to detect the freshness of the beef. The results show that the air sensors TGS2610, TGS2600, TGS2611, TGS2620 and TGS2602 made by Tianjin Figaro Electronic Co, Ltd could be used to determine the degree of freshness but TGS2442 is not suitable. This study provides a foundation for designing and making an economical and practical detector for beef freshness.

Design and Availability Research of a Flammable and Explosive Volatiles Monitoring and Early Warning System (FEVMEW) for the Bus Crowded Places

In order to reduce the arson or accidental fire losses, we developed a gas sensitive detector used for the rapid detection and early warning of flammables in crowded places such as buses. A MEMS (Micro-Electro-Mechanical System) based thin film semiconductor was fabricated as the gas sensor. To obtain the target gas selective response, the surface of the sensitive film was modified with highly active metal catalytic nano-particles. Thus the anti-interference ability was improved and the false alarm rate was effectively reduced. Furthermore, the modular embedded system for information acquisition and transmission was developed. Supported by the Airflow Precision control system (APs), the rapid warning of volatile gas of flammable substances was realized. Experiments showed that RAs has satisfied selectivity to volatiles of usual flammable liquid, such as the output voltage reaches 3 V (0 - 3.3 V). With simulation about the actual installation state in bus, MWs sounds an alarm at 2 minutes after splashing 50 mL 92# petrol to the floor. For the last two years, FEVMEW has been integrated into more than 4000 buses in Hefei. This design has been proved feasible according to the actual operation.

CO_2-triggered gelation for mobility control and channeling blocking during CO_2 flooding processes

CO2 flooding is regarded as an important method for enhanced oil recovery （EOR） and greenhouse gas control. However, the heterogeneity prevalently dis- tributed in reservoirs inhibits the performance of this technology. The sweep efficiency can be significantly reduced especially in the presence of ＂thief zones＂. Hence, gas channeling blocking and mobility control are important technical issues for the success of CO2 injection. Normally, crosslinked gels have the potential to block gas channels, but the gelation time control poses challenges to this method. In this study, a new method for selectively blocking CO2 channeling is proposed, which is based on a type of CO2-sensitive gel system （modified polyacry- lamide-methenamine-resorcinol gel system） to form gel in situ. A CO2-sensitive gel system is when gelation or solidification will be triggered by CO2 in the reservoir to block gas channels. The CO2-sensitivity of the gel system was demonstrated in parallel bottle tests of gel in N2 and CO2 atmospheres. Sand pack flow experiments were con- ducted to investigate the shutoff capacity of the gel system under different conditions. The injectivity of the gel system was studied via viscosity measurements. The results indi- cate that this gel system was sensitive to CO2 and had good performance of channeling blocking in porous media. Advantageous viscosity-temperature characteristics were achieved in this work. The effectiveness for EOR in heterogeneous formations based on this gel system was demonstrated using displacement tests conducted in double sand packs. The experimental results can provide guideli- nes for the deployment of theCO2-sensitive gel system for field applications.

Microwave-assisted hydrothermal synthesis and gas sensitivity of nanostructured SnO_2

Precursors for nanostructured SnO2 were synthesized via a microwave-assisted hydrothermal method under different conditions, using SnCI2.2H2 O, urea and citric acid as reactants. After calcination of the pre-cursors at 700 ℃ for 2 h, nanostructured SnO2 with different morphologies were obtained, and were then characterized using X-ray powder diffraction （XRD）, and field-emission scanning electron microscopy （FESEM）. The results show that synthesis temperature and time play an important role in the formation of the 3D hierarchical morphology of the nanostructured SnO2. Gas sensing experiments demonstrate that the synthesized SnO2 materials, especially those with a 3D network structure, exhibit superb sensitivity to alcohol vaoors at 240 ℃.

Gas Sensitivity of In0.3Ga0.7As Surface QDs Coupled to Multilayer Buried QDs

A detailed analysis of the electrical response of In0.3Ga0.7As surface quantum dots(SQDs)coupled to 5-layer buried quantum dots(BQDs)is carried out as a function of ethanol and acetone concentration while temperature-dependent photoluminescence(PL)spectra are also analyzed.The coupling structure is grown by solid source molecular beam epitaxy.Carrier transport from BQDs to SQDs is confirmed by the temperature-dependent PL spectra.The importance of the surface states for the sensing application is once more highlighted.The results show that not only the exposure to the target gas but also the illumination affect the electrical response of the coupling sample strongly.In the ethanol atmosphere and under the illumination,the sheet resistance of the coupling structure decays by 50%while it remains nearly constant for the reference structure with only the 5-layer BQDs but not the SQDs.The strong dependence of the electrical response on the gas concentration makes SQDs very suitable for the development of integrated micrometer-sized gas sensor devices.

The Gas Sensitivity of Polypyrrole Film Fabricated by in Situ Polymerization

1 Results Polypyrrole (PPy) is a typical conducting polymer with high conductivity and good stability,and has been widely used for detection of gases including ammonia and nitroxide[1].However the reversiblity and selectivity of the response to these gases are always not satisfying.In the paper,conductive PPy film was deposited on the interdigital electrode modified by one bilayer of poly(diallyldimethylammonium chloride) and poly(sodium styrenesulfonate),by in-situ polymerization with FeCl3 as oxidizin...

Highly sensitive hybrid nanofiber-based room-temperature CO sensors： Experiments and density functional theory simulations

Chemical sensors （CSs） are an emerging area in nanoscience research, which focuses on the highly sensitive detection of toxic and hazardous gases and disease- related volatile organics. While the field has advanced rapidly in recent years, it lacks the theoretical support required for the rational design of innovative materials with tunable measurement responses. Herein, we present a one-dimensional （1D） hybrid nanofiber decorated with ultrafine NiO nanoparticles （NiO NPs） as an efficient active component for CSs. Highly dispersed （110）-facet NiO NPs with a high percentage of Ni2~ active sites with unsaturated coordination were confined in a TiO2 nanofiber （TiO2 NF） matrix that is favorable for surface catalytic reactions. The CSs constructed using the 1D heterostructure NiO/TiO2 nanofibers （NiOdrio2 HNFs） exhibited a highly selective response to trace CO gas molecules （1 ppm） with high sensitivity （AR/Ro = 1.02）, ultrafast response/ recovery time （T 〈 20 s）, and remarkable reproducibility at room tem- perature. The density functional theory （DFT） simulations and experimental results confirmed that the selective response could be attributed to the high molecular adsorption energy of the NiO nanoparticles with （110） facets and abundant interfaces, which act synergistically to promote CO adsorption and facilitate charge transfer.

A First-principles Study on Gas Sensitivity of Pd or Pt Loaded(5,5)Carbon Nanotube with a Di-vacancy Defect to CO and NO

Binding energies and geometrical and electronic structures for adsorptions of CO and NO on metal M(Pd or Pt)loaded or M and di-vacancy co-decorated(5,5)single-walled carbon nanotubes(M-CNTs or M-V2-CNTs)are studied using a GGA-PBE method in the work.The calculated results show that the di-vacancy defect in a perfect(5,5)tube opens the band gap,makes the(5,5)tube transform from a conductor into a semiconductor,and strengthens the adsorption of metal M on the(5,5)tube.For the adsorptions of CO and NO on M-CNT and M-V2-CNT,the CO and NO molecules can be both chemically adsorbed on loaded Pd or Pt atoms due to their active adsorption sites.NO is easily adsorbed on M-V2-CNT because of its electron configuration with a high 2p energy level and its adsorption significantly changes the band gap of M-V2-CNT and makes M-V2-CNT transform from a semiconductor to a conductor.However,the adsorption of CO can not cause the conductivity of M-V2-CNT change.M-V2-CNT has a good sensitivity to the NO gas,suitable as a sensor for detecting the NO gas molecule.In addition,the existence of di-vacancy defect decreases the interaction between CO or NO and Pt-CNT,which will contribute to the desorption of CO and NO gases.The work is expected to provide a theoretical basis for designing NO sensing devices.

Synthesis of organic semiconductor hexadecachloro zinc phthalocyanine and its gas sensitivity

Due to the difficulty in synthesizing perhalogenated metallophthalocyanine, the method of ammonium molybdate solid phase catalysis was introduced, and by using tetrachlorophthalic anhydride and urea as the raw materials, hexadecachloro zinc phthalocyanine （ZnPcCl16） was synthesized. Components of the composite were analyzed by energy spectrum, and its functional group structures and absorption peaks were characterized by IR and UV-vis spectroscopy. The thin films of gas sensors were prepared in a vacuum evaporation system and evaporated onto SiO2 substrates, where sensing electrodes were made by MEMS micromachining. The optimal conditions for the films are： substrate temperature of 150 ℃ evaporation current of 95 A and film thickness of 50 nm. The result showed that the sensors were ideally sensitive to Cl2 gas and could detect the minimum concentration of 0.3 ppm.

Hydrothermal growth of symmetrical ZnO nanorod arrays on nanosheets for gas sensing applications

The hierarchical ZnO nanostructures with 2-fold symmetrical nanorod arrays on zinc aluminum carbonate （ZnAl-CO3） nanosheets have been successfully synthesized through a two-step hydrothermal process. The primary nanosheets, which serve as the lattice-matched substrate for the self-assembly nanorod arrays at the second-step of the hydrothermal route, have been synthesized by using a template of anodic aluminum oxide （AAO）. The as-prepared samples were characterized by XRD, FESEM, TEM and SAED. The nanorods have a diameter of about 100 nm and a length of about 2μ m. A growth mechanism was proposed according to the experimental results. The gas sensor fabricated from ZnO nanorod arrays showed a high sensitivity to ethanol at 230℃. In addition, the response mechanism of the sensors has also been discussed according to the transient response of the gas sensors.

Synthesis of ZnO Nanoparticles onto Sepiolite Needles and Determination of Their Sensitivity toward Humidity, NO_2 and H_2

This work investigated the sensitivity toward humidity, NO2 and H2 of ZnO modified sepiolite （Si12Mg8O30（OH）4.（H2O）4.8H2O）. To this aim, sepiolite powder was first modified by leaching magnesium ions in HCI then by precipitating nano-sized Zn-based compounds under basic conditions. A subse- quent thermal treatment at 550 ℃ for 1 h was performed. The powders were characterized by X-ray diffraction （XRD）, specific surface area measurements, thermogravimetric and differential thermal anal- ysis and field emission scanning electron microscopy, as well as high resolution transmission electron microscopy. The XRD patterns showed that all leached heat treated samples were made of anhydrous sepiolite and of ZnO. Sensors were then obtained by screen printing these materials onto commercial alumina substrates with Pt electrodes. All the investigated compositions were capable of detecting NO2 down to ppm level and 20 ppm H2, at an optimal working temperature of 300°C. These detection limits are in line with the current best results reported in literature.