Preparation and CO gas-sensing behavior of Au-doped SnO2 sensors

A study on the low-temperature CO gas sensors based on Au/SnO₂ thick film was reported. Au/SnO₂ powders, with different Au loading from 0.36 to 3.57 wt%, were prepared by a deposition-precipitation method. Thick films were fabricated from Au/SnO₂ powders. The Au/SnO₂ thick-film sensors exhibited high sensitivity to CO gas at relatively low operating temperature (83-210 °C). We also reported the effect of the Au loading in Au/SnO₂ on the CO gas sensing behavior. The optimal Au loading in as-prepared Au/SnO₂ was 2.86 wt%.     

Fast response detection of H2S by CuO-doped SnO2 films prepared by electrodeposition and oxidization at low temperature

Fast response detection of H₂S by CuO-doped SnO₂ films prepared was prepared by a simple two-step process: electrodeposition from aqueous solutions of SnCl₂ and CuCl₂, and oxidization at 600 °C. The phase constitution and morphology of the CuO-doped SnO₂ films were characterized by X-ray diffraction and scanning electron microscopy. In all cases, a polycrystalline porous film of SnO₂ was the product, with the CuO deposited on the individual SnO₂ particles. Two types of CuO-doped SnO₂ films with different microstructures were obtained via control of oxidation time: nanosized CuO dotted island doped SnO₂ and ultra-uniform, porous, and thin CuO film coated SnO₂. The sensor response of the CuO doped SnO₂ films to H₂S gas at 50–300 ppm was investigated within the temperature range of 25–125 °C. Both of the CuO-doped SnO₂ films show fast response and recovery properties. The response time of the ultra-uniform, porous, and thin CuO coated SnO₂ to H₂S gas at 50 ppm was 34 s at 100 °C, and its corresponding recovery time was about 1/3 of the response time.     

One-pot synthesis of intestine-like SnO2/TiO2 hollow nanostructures

In the present study the intestine-like binary SnO₂/TiO₂ hollow nanostructures are one-pot synthesized in aqueous phase at room temperature via a colloid seeded deposition process in which the intestine-like hollow SnO₂ spheres and Ti(SO₄)₂ are used as colloid seeds and Ti-source, respectively. The novel core (SnO₂ hollow sphere)-shell (TiO₂) nanostructures possess a large surface area of 122 m²/g (calcined at 350 °C) and a high exposure of TiO₂ surface. The structural change of TiO₂ shell at different temperatures was investigated by means of X-ray diffraction and Raman spectroscopy. It was observed that the rutile TiO₂ could form even at room temperature due to the presence of SnO₂ core and the unique core-shell interaction.     

The gas sensitivity of nano TiO2-SnO2 film doped with Cd(II) ion

The nanocomposite oxide (0.2TiO₂-0.8SnO₂) doped with Cd²⁺ powder have been prepared and characterized by XRD and their gas-sensing sensitivity were characterized using gas sensing measurement. Experimental results show that, bicomponent nano anatase TiO₂ and rutile SnO₂ particulate thick film doped with Cd²⁺ behaves with good sensitivity to formaldehyde gas of 200 ppm in the air, and the optimum sensing temperature was reduced from 360 °C to 320 °C compared with the undoped Cd²⁺ thick film. The gas sensing thick films doped with Cd²⁺ also show good selectivity to formaldehyde among benzene, toluene, xylene and ammonia as disturbed gas and could be effectively used as an indoor formaldehyde sensor.     

DRIFT studies of thick film un-doped and Pd-doped SnO2 sensors: temperature changes effect and CO detection mechanism in the presence of water vapour

Diffuse reflectance infrared Fourier transform measurements were performed on tin oxide based thick film gas sensors operated in normal working conditions. We characterised SnO₂ sensors at different temperatures between room temperature and 300 °C. The results show the presence of different surface OH groups as well as coordinated water on the SnO₂ sensor surface. Their intensity changes with temperature. During the temperature cycles the bands’ peak positions are reversibly changed but their intensity is not. CO measurements were performed at 300 °C at different humidity levels (0 and 50% r.h.) on un-doped and Pd-doped sensors. In the presence of CO we observed in the spectra: a decrease of the OH groups on the SnO₂ surfaces, the appearance of gaseous CO₂ and CO in the pores of the sensitive layer and an increase of hydrated protons and of the free charge concentration. The effects are dramatically influenced by the water vapour concentration, temperature, dopands (Pd) and can be correlated with simultaneously performed sensor resistance measurements.     

SnO2:Cu films doped during spray pyrolysis deposition: The reasons of the gas sensing properties change

The influence of Cu doping on electrophysical, structural and gas sensing properties of the SnO₂ films deposited by spray pyrolysis was considered in this paper. It was shown that the addition of Cu in SnO₂ even in small concentrations was accompanied by strong changes in the SnO₂-based gas sensors performances. The reasons of observed changes were discussed. The conclusion was made that the decrease of response of heavy doped SnO₂:Cu-based gas sensors was mainly connected with both structural disordering of heavy doped SnO₂:Cu metal oxide, and the appearance of the fine dispersed phase formed in the SnO₂ matrix.     

Influence of effective surface area on gas sensing properties of WO3 sputtered thin films

WO₃ thin films having different effective surface areas were deposited under various discharge gas pressures at room temperature by using reactive magnetron sputtering. The microstructure of WO₃ thin films was investigated by X-ray diffraction, scanning electron microscopy, and by the measurement of physical adsorption isotherms. The effective surface area and pore volume of WO₃ thin films increase with increasing discharge gas pressure from 0.4 to 12 Pa. Gas sensors based on WO₃ thin films show reversible response to NO₂ gas and H₂ gas at an operating temperature of 50-300 °C. The peak sensitivity is found at 200 °C for NO₂ gas and the peak sensitivity appears at 300 °C for H₂ gas. For both kinds of detected gases, the sensor sensitivity increases linearly with an increase of effective surface area of WO₃ thin films. The results demonstrate the importance of achieving high effective surface area on improving the gas sensing performance.     

TiO2-SnO2 system for gas sensing—Photodegradation of organic contaminants

Catalytic photodegradation of organic contaminants by means of UV light has been demonstrated for gas sensors based on composites of TiO₂-SnO₂. Thin film resistive-type gas sensors of TiO₂-SnO₂ have been deposited at 350 °C by RF sputtering from a Ti-SnO₂ target with varying surface ratio of SnO₂/Ti. Photodegradation experiments of bromothymol blue by UV light have been performed by means of the optical spectrophotometry over the wavelength range extending from 300 nm to 600 nm. The influence of the UV illumination on the sensor response to 100-20,000 ppm of H₂ has been investigated in situ on motor oil contaminated sensors. It has been found that sequential exposures to UV light lead to a partial recovery of the sensor signal to hydrogen.     

Role of surface properties of MoO3-doped SnO2 thin films on NO2 gas sensing

The role of surface morphology of MoO₃-doped SnO₂ thin film on the gas sensing properties is analyzed. SnO₂ thin films doped with 1, 3, 5 and 10 wt% MoO₃ are prepared by sol-gel spin coating process. Structural and morphological properties are studied using glancing angle X-ray diffractometer, atomic force microscopy, transmission electron microscopy and high resolution transmission electron microscopy. Energy dispersive X-ray analysis and X-ray photoelectron spectroscopy studies are used for chemical analysis. A good correlation is found between the characteristics of the surface and gas sensing properties of these films. MoO₃ addition is found responsible for increase in acidic nature of films which in turn increases their sensitivity and selectivity towards NO₂ gas.     

X-ray absorption near-edge spectroscopy investigation of the oxidation state of Pd species in nanoporous SnO2 gas sensors for methane detection

We report the correlation of the aging of Pd-doped SnO₂ methane sensors with the change of the oxidation state of Pd. Mesoporous SnO₂ doped with palladium species was prepared and exposed to different gas mixtures at high temperature (600 °C) to simulate long term usage. After each exposure step a fraction of the sample was cooled down to “freeze” the current oxidation state of Pd which was then analyzed by X-ray Absorption Near-Edge Spectroscopy (XANES) using the 'white line' (i.e. the absorption peak corresponding to the transition from the 2p_3/2 core level to unoccupied 4 d states) intensity of the L(III) edge as a probe for the oxidation state. The Pd oxidation state correlates with the response of the resistive SnO₂ sensor to methane gas, as determined by measuring the gas response to different concentrations of methane. Samples treated with 5000 ppm methane in air show a significant reduction of Pd(II) to Pd(0), depending clearly on the carrier gas (synthetic air, pure nitrogen) and on the temperature (600 °C vs. 300 °C).     

Characterizations of SnO2 and SnO2:Sb thin films prepared by PECVD

Structural characterizations of tin oxide (SnO₂) thin films, deposited by plasma-enhanced chemical vapor deposition (PECVD), were investigated with scanning electron microscope (SEM), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The results show that the films are porous, the crystalline structure transforms from crystalline to amorphous phase as deposition temperature changes from 500°C to 200°C, and the chemical component is non-stoichiometric (Sn:O is 1.0716 prepared at 450°C with a value of O₂ flow 3.5 l/min). Sheet resistance of the thin films decreases with increasing of deposition temperature. Whereas, sheet resistance increases with increasing of oxygen flow. Tin oxide doped with antimony (SnO₂:Sb) thin films prepared by same method have a better selectivity to alcohol than to carbon monoxide; the maximum sensitivity is about 220%. The gas-sensing mechanism of SnO₂ thin films is commentated.     

Characterizing individual SnO2 nanobelt field-effect transistors and their intrinsic responses to hydrogen and ambient gases

The intrinsic electrical properties of individual single-crystalline tin dioxide nanobelts, synthesized via catalyst-free physical vapor deposition, were studied and correlated to the surface oxygen deficiency with the presence of various ambient gases, especially hydrogen. Four-terminal field-effect transistor (FET) devices based on individual SnO₂ nanobelts were fabricated with SiO₂/Si as back gate and RuO₂/Au as contacts. Four-probe I–V measurements verify channel-limited transistor characteristics and ensure that the hydrogen gas sensing reflect electrical modification of the nanobelt channel. The demonstrated results of the intrinsic SnO₂ nanobelt based hydrogen sensor operating at room temperature provide useful information on the synthesis of room temperature chemo-resistive gas sensors with good sensitivity and stability. To evaluate the impact of surface gas composition on the electrical properties of SnO₂ nanobelts, their temperature-dependent resistivity (ρ), effective carrier mobility (μ_eff) and effective carrier concentration (n_e) were determined under different oxygen concentrations.     

Silicon doped SnO2 films for liquid petroleum gas sensor

Silicon doped SnO₂ films were synthesized by sputtering SnO₂ layer onto glass substrates with appropriate amount of silicon sputtered onto them. The bilayer structures were subjected to rapid thermal annealing for the incorporation of Si in SnO₂ matrix. The films thus obtained were characterized by measuring optical and microstructural properties. Liquid petroleum gas (LPG) sensing properties were also investigated. FTIR and Raman studies were also carried out on these films, both, before and after LPG exposure.     

CO sensing with SnO2 thick film sensors: role of oxygen and water vapour

The paper investigates the gas response of nanocrystalline SnO₂ based thick film sensors upon exposure to carbon monoxide (CO) in changing water vapour (H₂O) and oxygen (O₂) backgrounds. The sensing materials were undoped, Pt- and Pd-doped SnO₂. We found that in the absence of oxygen, the sensor signal (defined as the ratio between the resistance in the background gas, R₀ and the resistance in the presence of the target gases, R, namely R₀/R) have the highest values. These values are higher for doped materials than for the undoped ones. The presence of humidity increases dramatically the sensor signal of the doped materials. In the presence of oxygen, the sensor signal decreases significantly for all sensor materials. The results indicate that there is a competitive adsorption between O₂ and H₂O related surface species and, as a result, different sensing mechanisms can be observed for CO.     

Y2O3-SnO2常温气敏薄膜的Sol-Gel制备及性能研究

以无机盐ＳnCl2.2H2O,Y(NO3)3.6H2O为原料,无水乙醇为溶剂,采用溶胶－凝胶工艺制备了Ｙ２Ｏ３掺杂的ＳnO２薄膜,采用差热－失重分析研究了Ｙ２Ｏ３掺杂的ＳnO2干凝胶粉末的热分解、晶体过程。研究了Ｙ２Ｏ３－ＳnO2薄膜的电学和气敏性能,从实验中得到了Ｙ２Ｏ３掺杂份量对ＳnO2薄膜电学及气敏性能的影响。实验表明Ｙ２Ｏ３掺杂的ＳnO2薄膜在常温下对ＮＯx具有较好的灵敏度的选择性,并具有较好的响应恢复性能,在常温下对Ｈ２Ｓ气体也具有一定的灵敏度。     

The effect of Au and Pt nanoclusters on the structural and hydrogen sensing properties of SnO2 thin films

Au and Pt nanoparticle modified SnO₂ thin films were prepared by the sol-gel method on glass substrates targeting sensing applications. Structural and morphological properties of these films were studied using X-ray Diffraction and Scanning Electron Microscopy. It was proved that the films crystallized in tetragonal rutile SnO₂ crystalline structure. Scanning Electron Microscopy observations showed that the metallic clusters' dimensions and geometry depend on the kind of the metal (Au or Pt) while SnO₂ films surface remains almost the same: nanostructured granular very smooth. Optical properties of the films were studied using UV-visible spectroscopy. The modified SnO₂ films were tested as hydrogen sensors. The response of SnO₂, SnO₂-Au and SnO₂-Pt thin films against hydrogen was investigated at different operating temperatures and for different gas concentrations. The addition of metal nanoparticles was found to decrease the detection limit and the operating temperature (from 180 °C to 85 °C), while increasing the sensing response signal.     

Gas sensing mechanism of TiO2-based thin films

Two classes of thin film gas sensors have been studied: TiO₂ doped with Cr or Nb and TiO₂-SnO₂ mixed systems. Thin films have been prepared by the reactive sputtering from mosaic targets. It is demonstrated that titanium dioxide doped with Nb and Cr should be considered as a bulk sensor. Its performance is governed by the diffusion of point defects, i.e. very slow diffusion of Ti vacancies for TiO₂: 9.5 at% of Nb and fast diffusion of oxygen vacancies in the case of TiO₂: 2.5 at% Cr sensor. The corresponding response times are 55 min for TiO₂: 9.5 at% of Nb and 20 s for TiO₂: 2.5 at% Cr. In turn, sensors based on TiO₂-SnO₂, particularly those of the SnO₂-rich composition, belong to the group of surface sensors.     

CO2 and H2 detection with a CHx/porous silicon-based sensor

There has been great interest in the last years in gas sensors based on porous silicon (PS). Recently, a gas sensing device based on a hydrocarbon CH_x/porous silicon structure has been fabricated. The porous samples were coated with hydrocarbon groups deposited in a methane argon plasma. We have experimentally demonstrated that the structure can be used for detecting a low concentration of ethylene, ethane and propane gases [Gabouze N, Belhousse S, Cheraga H. Phy State Solidi (C), in press].In this paper, the CH_x/PS/Si structure has been used as a sensing material to detect CO₂ and H₂ gases. The sensitivity of the devices, response time and impedance response to different gas exposures (CO₂, H₂) have been investigated.The results show that current-voltage and impedance-voltage characteristics are modified by the gas reactivity on the PS/CH_x surface and the sensor shows a rapid and reversible response to low concentrations of the gases studied at room temperature.     

Influence of surface Pd doping on gas sensing characteristics of SnO2 thin films deposited by spray pirolysis

In this article the analysis of steady state and transient gas sensing characteristics of undoped and Pd surface doped SnO₂ films, deposited by spray pyrolysis, is described. The influence of parameters such as air humidity (2-50% RH), operation temperature (25-500 °C) and Pd surface concentration (0-1% ML Pd) on gas response to CO and H₂ (0.1-0.5%), response time, shape of sensitivity S(T) curves and activation energy of τ(1/kT) dependencies are discussed. A mechanism based on a chemisorption model is proposed to explain how Pd influences the gas sensing characteristics of SnO₂ films.     

Self-catalytic formation and characterization of Zn2SnO4 nanowires

Mass production of transparent semiconducting ternary oxide Zn₂SnO₄ nanowires is successfully synthesized by the thermal evaporation method without any catalyst. The as-synthesized products are characterized with field-emission scanning electron microscope (FE-SEM), X-ray powder diffraction (XRD), energy-dispersive spectroscopy (EDS), high-resolution transmission electron microscope (HR-TEM) and selected area electron diffraction (SEAD). A formation of Zn₂SnO₄ nanowires based on a self-catalytic VLS growth mechanism is discussed. The photoluminescence spectrum (PL) of the nanowires shows a broad blue-green emission around the 300-600 nm wavelengths with a maximum center at 580 nm under room temperature.