A novel SnO(2)-based gas sensor

A novel 'two-terminal' semiconductor gas sensor was developed based on a heavily Sb-doped SnO(2) film prepared by cathodic sputtering. The sensor is heated at its operational temperature by the gas sensitive film itself. A device for detecting the leakage of flammable gases, some noxious or hazardous gases can be made in this way.     

Particle formation in the hydrolysis of tetraethyl orthosilicate in pH buffer solution

Particle formation in the hydrolysis and condensation of tetraethyl orthosilicate (TEOS) was studied by varying pH (9.5-11) with the basic catalysts NH3, methylamine (MA), and dimethylamine (DMA) in the presence of 5 mol/m3 CH3COOH, which was chosen to suppress time variations of pH and ionic strength during the reaction. Spherical particles were formed for MA and DMA at catalyst concentrations of 0.02-0.2 kmol/m3 and for NH3 at catalyst concentrations of 0.1-1.5 kmol/m3. In a common range of catalyst concentrations for spherical particle formation, average particle size was largest for DMA and smallest for NH3. Hydrolysis rate of TEOS could be quantified by the use of buffer systems as a function of TEOS and OH- concentrations. A specific relation was not found between the hydrolysis and the particle size. The zeta potential of silica particles measured in the reaction solvent was in the order DMA < MA < NH3, and ionic strength, estimated from pH in the reactions, was in the order DMA approximately equal to MA > NH3. This suggested that the particle sizes were controlled by electrostatic particle interactions.     

Unveiling the elusive role of tetraethyl orthosilicate hydrolysis in ionic-liquid-templated zeolite synthesis

Despite previous reports showing that crystallization kinetics affects zeolite phase selectivity because zeolites are metastable species in their synthesis solution rather than thermodynamic end points, the critical kinetics-controlling parameter is yet to be determined. This work elucidated the effect of tetraethyl orthosilicate (TEOS) hydrolysis before hydrothermal treatment on the final zeolite phase selectivity in the ionic liquid-templated synthesis of 10-membered ring zeolites (MFI- or TON-type zeolites). The results showed that the dissolved silica concentration in the synthesis solution, which is controlled by varying the TEOS hydrolysis temperature and addition rate, induced heterogeneous nuclear growth. Specifically, in 1-butyl-3-methylimidazolium ([BMIM]Br)-directed syntheses, the high and low concentrations of dissolved silica species led to MFI and TON zeolite formation, respectively. The experimental results are supported by silica polymerization modeling using the Reaction Ensemble Monte Carlo method and theoretical calculations on composite building unit formation. The results are valuable for understanding the nucleation mechanism in zeolite crystallization.     

Seed polymerization of tetraethyl orthosilicate in the presence of colloidal silica spheres

Kinetic analyses were made for the seed polymerization of tetraethyl orthosilicate (TEOS) in the presence of colloidal silica sphere seeds by turbidity and dynamic light scattering (DLS) measurements. Transmission electron microscopy (TEM) of the spheres formed was also used. TEOS is polymerized exclusively on the surfaces of the seed spheres, their sizes ranging from 29 to 184 nm in diameter. The sphere size versus time and the cube root of the absorbance versus time from DLS and turbidity measurements agree well, especially in the beginning of the reaction. The seed polymerization starts immediately on the addition of seed spheres, though the polymerization in the absence of the seeds proceeds after a certain induction time ranging several tens of seconds to several minutes. The polymerization rates of the reaction increase when the size and/or the concentration of the seed spheres increases. The thickness of the TEOS layers formed on the seed surfaces increases as the seed size increases; this is confirmed by the TEM pictures. These results are consistent with the polymerization mechanism of the formation of small preliminary particles followed by their coalescence on the surfaces of seeds to the final large spheres coated with silica layers. Received: 25 January 2001 Accepted: 30 May 2001     

Single-crystalline Sb-doped SnO2 nanowires: synthesis and gas sensor application

The synthesis of semiconducting Sb-doped SnO2 nanowires in mass production by an in situ doping approach are reported, and the ethanol sensing results demonstrated that Sb-doped SnO2 nanowires have a promising application for the fabrication of gas sensors with low resistance, and quick response and recovery times.     

Stability of semiconductor sensors based on nanosized SnO2 and Pd/SnO2

Adsorption semiconductor hydrogen sensors were created from nanosized SnO₂ and Pd/SnO₂ materials by the sol-gel method. The sensors were shown to be stable over a long time of operation.     

Kinetics of autocatalytic hydrolysis of tetraethyl orthosilicate in the presence of aluminium nitrate

Kinetics of autocatalytic hydrolysis of tetraethyl orthosilicate in the presence of aluminium nitrate has been studied experimentally. According to the suggested reaction mechanism and the proposed mathematical model, kinetic constants have been determined.

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Microwave-assisted hydrothermal synthesis of Pt/SnO2 gas sensor for CO detection

In this paper, the Pt/SnO₂ nanostructures were prepared via a facile one-step microwave assisted hydrothermal route. The structure of the introduced Pt/SnO₂ and its gas-sensing properties toward CO were investigated. The results from the TEM test reveal that Pt grows on the SnO₂ nanostructure, which was not found for bulk in this situ method, constructing Pt/SnO₂. The results indicated that the sensor using 3.0 wt% Pt/SnO₂ to 100 ppm carbon monoxide performed a superior sensing properties compared to 1.5 wt% and 4.5 wt% Pt/SnO₂ at 225 °C. The response time of 3.0 wt% sensor is 16 s to 100 ppm CO at 225 °C. Such enhanced gas sensing performances could be attributed to the chemical and electrical factors. In view of chemical factors, the presence of Pt facilitates the surface reaction, which will improve the gas sensing properties. With respect to the electrical factors, the Pt/SnO₂ plays roles in increasing the sensor’s response due to its characteristic configuration. In addition, the one-step in situ microwave assisted process provides a promising and versatile choice for the preparation of gas sensing materials.     

Seed polymerization of tetraethyl orthosilicate in the presence of anionic and cationic polystyrene colloidal spheres

Kinetic analyses are made for the seed polymerization of tetraethyl orthosilicate (TEOS) in the presence of anionic and cationic polystyrene colloidal sphere seeds by turbidity and dynamic light-scattering measurements. Transmission-electron microscopy pictures of the spheres formed are also used. The seed polymerization of TEOS is difficult to take place on the surface of anionic polystyrene spheres (44–212 nm in diameter). On the other hand, the reaction proceeds easily on the cationic polystyrene spheres. Hairy and soft surfaces of polystyrene spheres will disturb the seed polymerization. Furthermore, the electrostatic attraction between the anionic hydrolytic products of TEOS molecules and cationic polystyrene spheres plays an important role for the seed polymerization.     

A new approach to selective detection of gas by an SnO2 solid-state sensor

Tin dioxide (SnO₂) is sensitized for different gaseous compounds by heating at 500 °C in an SO₂—air mixtures. Such treatment induces strong modifications of the electrical properties of SnO₂ and constitutes an attempt to solve the problem of selectivity for chemical sensors. According to the nature of the surrounding gas, the electrical conductance curves as a function of the temperature present a maximum at different temperatures: 400 °C with C₆H₆ and 100 °C with H₂S. These maxima, whose values are related to the gas concentration, can be used for selective gas detection.A benzene detector device using two sensors heated to 400 and 500 °C respectively selectivity for a large number of gaseous compounds.     

Qualitative and quantitative analysis of organophosphorus pesticide residues using temperature modulated SnO(2) gas sensor

Qualitative and quantitative analysis of organophosphorus pesticide residues (acephate and trichlorphon) using temperature modulated SnO₂ gas sensor were studied. The testing method employed only a single SnO₂-based gas sensor in a rectangular temperature mode to perform the qualitative analysis of pure pesticide vapor and a binary vapor mixture in the air. Experimental results showed that in the range 250-300 °C and at the modulating frequency of 20 mHz the high selectivity of the sensor could be achieved. The quantitative analysis of the pure pesticide vapor and their mixture were performed by fast Fourier transformation (FFT). The higher harmonics of the FFT characterized the non-linear properties of the response at the sensor surface. The amplitudes of the higher harmonics exhibited characteristic variations that depend on the concentration and the kinetics of pesticide species on the sensor surface.     

Dynamic determination of domestic liquefied petroleum gas down to several ppm levels using a Sr-doped SnO2 thick film gas sensor

Sr-doped SnO₂ thick film gas sensors were prepared for domestic liquefied petroleum gas (LPG) determination down to several ppm levels using the screen-printing technique. Characterization of Sr-doped SnO₂ thick film was investigated by XRD, XPS and DTA-TGA analyses. The sensitivity, selectivity, sintering temperature, and static and dynamic measurement were investigated. The results showed that the Sr-doped SnO₂ thick film sensor is suitable for several ppm levels LPG determination because of the high sensitivity (S = 12.7 to 10 ppm LPG). The dynamic measurements showed that the sensor exhibited high sensitivity and selectivity to domestic LPG at 210–300 °C.     

Self-assembly of a silylated steroid-based organogelator and its use as template for the in situ sol-gel polymerization of tetraethyl orthosilicate

In this paper we report the synthesis of a new steroid-based low-molecular weight organogelator, the analysis of the self-assembled fibrilar network (SAFIN) and its use as template for the preparation of SiO₂ nanotubes. This novel steroidal organogelator has a unique structure among the well known family of steroid-based organogelators, the most important characteristic of this molecule is the presence of a silyl ether group at C-3 together with a 6β,19-oxo bridge. It was capable to gelate hydrocarbons and tetraethyl orthosilicate at very low concentrations (<1 wt %). An insight into the aggregation mechanism is provided revealing that complementary interaction between an α-oriented hydrogen bond donor and a β-oriented acceptor on the steroid skeleton is the driving force for the primary 1D self-assembly. The SAFIN was successfully used as template to grow silica nanotubes (external diameter: 40-60 nm, internal diameter: 7 nm and several micrometers length) through a catalyst-free in situ co-assembly polymerization process. Hydrogen bond or electrostatic interactions between the anionic silicate intermediate species and the SAFIN are proposed to be the driving force for templating.     

Combined semiconductor gas sensor system for detection of specific substances in particular environments

A system for characterization of a particular gaseous substance in a particular gaseous environment is developed. Five semiconductor gas sensors and a personal computer are used. The data sets obtained for three samples (acetone, acetic acid and ethanol) at four concentrations in 12 different environments were evaluated statistically. The accuracy of characterization was 85% to 96%. Although semiconductor gas sensors require 10–20 min to recover their initial conditions after individual measurements, this system could be used for continuous monitoring of a particular substance in the presence of different concentrations of the other substances.     

Detection of liquid petroleum gas using mixed nanosized tungsten oxide-based thick-film semiconductor sensor

The thick-film semiconductor sensor for liquid petroleum gas (LPG) detection was fabricated using a mixed WO₃-based sensor. We present the characterization of both their structural properties by means of XRD measurements and the electrical characteristics by using gas-sensing properties. The sensing characteristics such as sensitivity, working range, cross-sensitivity and response time were studied by using nanosized WO₃-based mixed with different metal oxides (SnO₂, TiO₂ and In₂O₃) and doped with noble metals (Au, Pd and Pt). The WO₃-based mixed with 5 wt.% In₂O₃ and 0.5 wt.% Pd showed the higher sensing characteristic at low concentration of LPG sensor at an operating temperature 225 °C.     

Thermodynamic study of alcohol on SnO2 (100)-based gas nano-sensor

Alcohol dehydrogenation and condensation reactions are involved in chain growth pathways of SnO₂. These pathways lead to the formation of acetaldehyde and other products with high selectivity. It is recognised that together with the atmospheric oxygen, the presence of humidity greatly influences gas detection. Accordingly, it is important to understand the role of alcohol vapours in the sensing mechanism. Interaction between alcohol molecules and SnO₂ is investigated using MNDO method by semi-empirical calculations. We study the structural, total energy, thermodynamic properties of absorption of CH₃OH and C₂H₅OH on SnO₂ at 298?K. When exposed to ethanol, the SnO₂-based sensors showed oxidation products consisting of acetaldehyde, ethyl acetate and CH₄?+?CO. All the geometry optimisation structures were carried out using the Gaussian program package. Density functional theory optimised intermediates and transient states. The results show a sensitivity enhancement in resistance and capacitance when ethanol is near the surface, so converted into different products.     

Optimization and modeling of Zn2SnO4 sensitivity as gas sensor for detection benzene in the air by using the response surface methodology

In this paper, the performance of the benzene gas detection sensor in the air is optimized by an experimental design method. So in this work, Nanostructured thin films of ZnO and Zn₂SnO₄ were prepared in wurtzite form via a facile atmospheric pressure chemical vapor deposition (CVD) method, using metallic zinc and tin precursors. Characterization of the gas sensor was performed by using Powder X-ray diffraction (PXRD), scanning electron microscopy (SEM) and surface area analysis (using BET method). The results show that Zn₂SnO₄ nanowire network exhibited good sensitivity at 299 °C temperature to low concentrations (100 ppb) of Benzene which can be potentially used as a resistive gas sensor. Ultimately modeling and optimization of Zn₂SnO₄ sensor performance to detect benzene by surface response method in design expert11 software has been done. Also, the effect of each parameter on the sensitivity of the sensor was analyzed by analysis of variance (ANOVA). Moreover, the performance efficiency of the Zn₂SnO₄ sensor is estimated with the reliable correlations obtained in the modeling. The two parameters selected to optimize the performance of the gas sensor include the operating temperature of the sensor and the concentration of the sensor. Comparison of the modeling results and the predicted values for the sensor sensitivity to benzene shows 97.60% excellent agreement.     

Supramolecular sensor based on SnO2 electrode modified withoctadecylsilyl monolayer having molecular binding sites

Octadecylsilyl (ODS) monolayer covalently bound onto SnO2 was found to aquire molecular binding sites via extracting out co-implanted inert guest(n-hexadecane). Intense electrochemical response of ODS/SnO₂ was found toward vit-K₁, K₂ and E adsorbed to the molecular binding sites.