SnO2 thin-films prepared by a spray-gel pyrolysis: Influence of sol properties on film morphologies

Nanostructured tin oxide films were prepared by depositing different sols using the so-called spray-gel pyrolysis process. SnO₂ suspensions (sols) were obtained from tin (IV) tert-amyloxide (Sn(t-OAm)₄) or tin (IV) chloride pentahydrate (SnCl₄·5H₂O) precursors, and stabilized with ammonia or tetraethylammonium hydroxide (TEA-OH). Xerogels from the different sols were obtained by solvent evaporation under controlled humidity.The Relative Gelling Volumes (RGV) of these sols strongly depended on the type of precursor. Xerogels obtained from inorganic salts gelled faster, while, as determined by thermal gravimetric analysis, occluding a significant amount of volatile compounds. Infrared spectroscopic analysis was performed on raw and annealed xerogels (300, 500 °C, 1 h). Annealing removed water and ammonium or alkyl ammonium chloride, increasing the number of Sn-O-Sn bonds.SnO₂ films were prepared by spraying the sols for 60 min onto glass and alumina substrates at 130 °C. The films obtained from all the sols were amorphous or displayed a very small grain size, and crystallized after annealing at 400 °C or 500 °C in air for 2 h. X-ray diffraction analysis showed the presence of the cassiterite structure and line broadening indicated a polycrystalline material with a grain size in the nanometer range. Results obtained from Scanning Electron Microscopy analysis demonstrated a strong dependence of the film morphology on the RGV of the sols. Films obtained from Sn(t-OAm)₄ showed a highly textured morphology based on fiber-shape bridges, whereas the films obtained from SnCl₄·5H₂O had a smoother surface formed by “O-ring” shaped domains.Lastly, the performance of these films as gas sensor devices was tested. The conductance (sensor) response for ethanol as a target analyte was of the same order of magnitude for the three kinds of films. However, the response of the highly textured films was more stable with shorter response times.     

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.     

Detection of DEMP vapors using SnO2-based gas sensors: understanding of the chemical reactional mechanism

Tin dioxide-based gas sensors make it possible to detect diethyl methyl phosphonate vapors (DEMP). However, the responses present drifts that can be observed vs. time. Such a problem raises the question of the reliability of these devices. The results presented in this article are two-fold: first, they are concerned with the study of the thermal degradation of DEMP by thermal degradation in an oxidizing atmosphere, and second, they are involved in the characterization of the chemical degradation on the tin dioxide-based gas sensors. Gas chromatography and infrared spectroscopy shows that, from 300 °C on, the thermal degradation of DEMP leads to the formation of ethylene (gas phase) and ethyl methyl phosphonic acid (condensed phase). Given that thermal degradation may occur at the sensor's surface, which can reach temperatures as high as 500 °C, these results showed that responses obtained using the DEMP vapors are principally due to ethylene and the drift in the responses vs. time is probably due to the adsorption of phosphorous compounds to the sensor's surface. The aim of this article is to describe the reactional mechanism of the DEMP interaction at the surfaces of tin dioxide-based sensors for temperatures ranging from 200 to 500 °C.     

On the correlation between morphology and gas sensing properties of RGTO SnO2 thin films

In this paper, we present the results of studies on optimalisation of morphology of the SnO₂ thin films grown by RGTO technique for application as gas sensor structures. The Sn thin films were grown on Si(111) wafer and Al₂O₃ ceramic plate heated in the range 235-295 °C and subsequently oxidized in dry oxygen atmosphere at high temperature, up to 700 °C. Our studies confirmed that the highest surface coverage of Sn droplets can be reached for the substrate temperature of about 265 °C leading to the highest surface-to-volume ratio of SnO₂ thin films. It was in a good correlation to the optimal gas sensor response and sensor sensitivity of RGTO SnO₂ thin films to nitrogen dioxide NO₂.     

Cathodoluminescence study of SnO2 powders aimed for gas sensor applications

In this paper we report on cathodoluminescence (CL) spectra of SnO₂ powders, synthesized using the wet chemical route. The analysis of influence of the modes of calcination (T_an-450–800 °C), and doping by both Pd and Pt (0.01–10.0 wt.%) on CL spectra was made. It was found that the measurement of CL spectra could be an effective research method of nanostructured metal oxides, aimed for gas sensor applications. It was established that in nanocrystalline SnO₂ the same system of energy levels, associated with radiative recombination, as in single crystalline and polycrystalline SnO₂, is retained. It was found that doping by both Pd and Pt modifies the structural properties of SnO₂ grains. Also, there is an optimum doping; near 0.1–0.2 wt.%, at which a maximum intensity of cathodoluminescence is reached. It was concluded that for low concentrations of both Pd and Pt additives in SnO₂ an improvement of the material's crystal structure is promoted, and is associated with a decrease in the non-radiating recombination rate.     

Gas-sensing properties of nanostructured SnO2-based sensor synthesized with different methods

Herein we report the preparation of SnO₂ nanomatierials by chemical precipitation, sol-gel and dissolution-pyrolysis. Furthermore, we studied their sensing properties. The composition, crystal structure and ceramic microstructure of the powders obtained are characterized by X-ray diffractometer (XRD) and scanning electron microscopy (SEM). The results show SnO₂ fabricated through the three methods has rutile structure and the sizes of spherical particles are below 30 nm. From the result, we can also know that the thick films deposited onto alumina substrates show different morphology, and which are fabricated by dissolution-pyrolysis has fibrous structure. We investigate the sensitivities, response and recovery times of the three sensors. The results of gas sensing measurement show that SnO₂-based sensor prepared by dissolution-pyrolysis method has high sensitivity, quick response and recovery behavior to the gases we studied. It also has wider range of working temperature that is from 25 to 400 °C compared with SnO₂-based sensor fabricated by the other two methods.     

Nanocrystalline SnO2 and In2O3 as materials for gas sensors: The relationship between microstructure and oxygen chemisorption

The correlations between microstructure of nanocrystalline TCO SnO₂ and In₂O₃ and parameters of oxygen chemisorption are analyzed. Nanocrystalline SnO₂ and In₂O₃ were prepared by wet chemical method. The sample's microstructure was characterized by TEM, XRD and low-temperature nitrogen adsorption. Electrical properties of TCO were studied at 200-400 °C depending on the oxygen partial pressure. Increase of TCO grain size leads to the increase of the fraction of atomic forms of chemisorbed oxygen at the fixed temperature. It could be due to the decrease of surface barrier resulting in the decrease of activation energy of dissociation of molecular ion O_2(ads)⁻.     

Hydrogen sensing properties of Pd-doped SnO2 sputtered films with columnar nanostructures

Pd-doped SnO₂ sputtered films with columnar nanostructures were deposited using reactive magnetron sputtering at the substrate temperature of 300 °C and the discharge gas pressures of 1.5, 12, and 24 Pa. Structural characterization by means of X-ray diffraction and scanning electron microscopy shows that the films composed of columnar nanograins have a tetragonal SnO₂ structure. The films become porous as the discharge gas pressure increases. Gas sensing measurements demonstrate that the films show reversible response to H₂ gas. The sensitivity increases as the discharge gas pressure increases, and the operating temperature at which the sensitivity shows a maximum is lowered. The highest sensitivity defined by (R_a − R_g) / R_g, where R_a and R_g are the resistances before and after exposure to H₂, 84.3 is obtained for the Pd-doped film deposited at 24 Pa and 300 °C upon exposure to 1000 ppm H₂ gas at the operating temperature of 200 °C. The improved gas sensing properties were attributed to the porosity of columnar nanostructures and catalytic activities of Pd doping.     

SnO2:Ga thin films as oxygen gas sensor

Ga-doped SnO₂ thin films deposited by spray pyrolysis were investigated as oxygen gas sensors. Gallium was added to the films to enhance the catalytic activity of the surface’s film to oxygen. Film resistance was studied in an environment of dry air loaded with oxygen in excess at partial pressures in the range from 0 to 8.78×10³ Pa. The best sensitivity lies close to partial pressures of 133.3 Pa. Film sensitivity reach a maximum at 350 °C. For this temperature and a doping concentration of 3 at.% of Ga in the starting solution, a sensitivity up to 2.1 was obtained.     

Influence of crystallinity on CO gas sensing for TiO2 films

In the present research, carbon monoxide (CO) gas sensing response was studied for TiO₂ thick films calcined and sintered between 700 and 900 °C. Crystalline phase, crystallite size, surface area, particle size, and amorphous content were measured for the calcined powder. Crystallinity of the powder was found to affect sensing response significantly towards CO. Anatase phase of TiO₂ thick film was stable up to 900 °C however, as calcination temperature increased from 700 to 900 °C, surface area and amorphous phase content decreased. Films calcined and sintered at 700 °C showed a lower response towards CO than those calcined at 800 °C. Upon increasing the calcination temperature further, particle growth and reduced surface area hindered the sensing response. A calcination temperature of 800 °C was necessary to achieve sufficient order in the crystal structure leading to more efficient adsorption and desorption of oxygen ions on the surface of TiO₂.     

Response to oxygen and chemical properties of SnO2 thin-film gas sensors

The measurements of the response—in terms of the conductance changes—to oxygen adsorption of tin dioxide (SnO₂) thin-film-based gas sensors were performed. The sensing SnO₂ layers were obtained by means of the rheotaxial growth and thermal oxidation (RGTO) method. The sensor responses were measured under a dry gas flow containing oxygen in nitrogen, within the range of temperature from 25 to 540 °C. For comparison, similar studies were performed for a commercial SnO₂ thick-film (TGS 812) gas sensor.The in-depth profiles of the chemical composition of the RGTO SnO₂ layers were determined from the scanning Auger microprobe experiment. The changes in concentration ratios [O]/[Sn] and [C]/[Sn] from the near-surface region towards the grain bulk were shown.     

SnO2-nanowires grown by catalytic oxidation of tin sputtered thin films for formaldehyde detection

The aim of the present work is to test the sensing behaviour of tin dioxide nanowires, which have been grown directly onto a sensing device. This device consists in an alumina substrate provided with platinum interdigitated microelectrodes and a Pt heater on the reverse side. The nanowire growing process based on a vapour-liquid-solid method consists of three steps: deposition of a tin thin film by DC sputtering, a 5 nm-thick Au layer deposition and an annealing treatment in the presence of oxygen for the growth of the SnO₂ nanowires.These samples have been tested under different concentrations of formaldehyde (HCHO), showing a high sensitivity and very short response and recovery times even at low operating temperatures (130 °C).     

Micro-structure of graphite-intercalated tin oxide and its influence on SnO2-based gas sensors

A nano-scaled graphite oxide (GO) was prepared with a micro-layer structure for intercalation. Graphite-intercalated SnO₂ was obtained at temperatures lower than 100°C. The morphology, microstructure, crystalline phases and thermal property of this intercalative composite were studied by atomic force microscopy (AFM), field-emission scanning electron microscopy (FE-SEM), X-ray diffractometry (XRD) and differential scanning calorimetry-thermogravimetry (DSC-TG) doped with a proper amount of graphite-intercalated composites (GITs), GIT-SnO₂ composite was obtained after heat treatment. This combined gas sensor reveals low resistance and high sensitivity to butane between 200°C and 300°C. Translated from Journal of Materials Science & Engineering, 2006, 24(4): 582–587 (in Chinese)     

CO and NH3 sensor properties and paramagnetic centers of nanocrystalline SnO2 modified by Pd and Ru

Nanocrystalline gas sensitive materials based on tin dioxide modified by Pd or Ru were synthesized via wet chemical route. The interaction of modified materials with CO and ammonia was studied by in situ DC-conductivity measurements and ex situ EPR spectroscopy. Modification by Pd yields the material highly sensitive to CO in low temperature region, while Ru-modified SnO₂ is very sensitive to NH₃ at raised temperature. We have detected that O₂⁻ and OH radicals are the main spin centers in unmodified nanocrystalline tin dioxide. The modification of tin dioxide by Pd and Ru is accompanied by formation of new spin centers in the samples: Pd^+ 3 and Ru^+ 3. The concentration of these paramagnetic species on the materials interacting with CO and ammonia gases decreased because of their transition to the diamagnetic state Pd^+ 2, Pd⁰ and Ru^+ 4, respectively.     

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.     

The effect of NO2 doping on the gas sensing properties of copper phthalocyanine thin film devices

In this work we report the effect on the NO₂ gas sensing properties of initially doping CuPc thin films with oxygen (in air) and NO₂ for room temperature operation. The pre-treatment with NO₂ is shown to improve the gas sensing properties by providing both an increase in the magnitude of the conductivity change for a given NO₂ concentration and a significant improvement in the recovery time. Data presented suggest that a simple time derivative of the change in current may provide a measure of concentration for real time gas sensing applications.     

Microwave-assisted synthesis and investigation of SnO2 nanoparticles

Microwave technique was adopted for preparation of tin dioxide nanoparticles with particles size ranging from 10 to 11 nm within 10 min. The formation of monocrystalline SnO₂ nanoparticles was confirmed by the XRD (X-Ray Diffraction) and TEM (Transmission Electron Microscopy) as well as with SAED (Selected Area Electron Diffraction) analysis. The structure of the SnO₂ crystal was found to be Cassiterite type tetragonal structure. The FT-IR results further supported the formation of tin dioxide from tin hydroxyl group without any post annealing. The samples were further characterized by thermo gravimetric analysis (TGA), electrical resistance measurements and photoluminescence spectrum.     

SnO_2/C复合材料的制备及NO_x气敏研究

为制备在室温条件下对NOx的检测具有低检测线、响应迅速、灵敏度高的材料,通过高压静电纺丝法制备了多孔SnO2/C复合材料,采用TEM、SEM、XRD等手段研究了复合材料形貌及结构.研究表明：SnO2/C复合材料表面存在丰富的微孔、介孔和大孔,且以10 nm左右的介孔居多.室温下（16～18℃）对SnO2/C复合材料NO...     

Fundamental studies on SnO2 by means of simultaneous work function change and conduction measurements

Simultaneous work function change and conduction measurements have been performed on undoped SnO₂ thick film sensors in order to investigate the surface reactions of CO under dry and humid conditions. On their basis, changes in the electron affinity have been observed for the reaction with water and the reaction with CO in the background of water. Additionally, by comparing the results for the changes in band bending, one can conclude that both water and CO compete for the same reaction sites, i.e. preadsorbed oxygen species. The kinetics of work function and band bending changes show that in the case of humid air more than one dipole species is involved in the reaction with CO.     

Synthesis and characterization of CdO-doped nanocrystalline ZnO:TiO2-based H2S gas sensor

This paper reports the preparation and gas-sensing characteristic of ZnO:TiO₂-based hydrogen sulfide (H₂S) gas sensor with different mol% of CdO by polymerized complex method. The structural and gas-sensing properties of ZnO:TiO₂ materials have been characterized using X-ray diffraction and gas-sensing measurement. The electrical resistance response of the sensor based on the materials was investigated at different operating temperatures and different gas concentrations. The sensor with 10 mol% CdO-doped ZnO:TiO₂ shows excellent electrical resistance response toward H₂S gas. The cross sensitivity was also checked for reducing gases like CH₄, CO and H₂ gas. The selectivity and sensitivity of ZnO:TiO₂-based H₂S gas sensor were improved by the addition of 10 mol% of CdO at an operating temperature of 250 °C.