Pure Tin Dioxide Research Articles

Formation of multilayer films for gas sensing by in situ thermophoretic deposition of nanoparticles from aerosol phase

Dry aerosol synthesis applying the flame spray pyrolysis was used to manufacture and directly (in situ) deposit tin dioxide nanoparticles on sensor substrates. For the first time this technique was used to synthesize a combination of two porous layers for gas-sensor fabrication. Two different sensing layers were deposited on ceramic substrates, i.e., pure tin dioxide and palladium-doped tin dioxide. The top layer was a palladium-doped alumina as a filter. The fabricated sensors were tested with methane, CO, and ethanol. In the case of CH4, the pure tin dioxide sensor with the Pd/Al2O3 filter showed higher sensor signals and improved selectivity with respect to water vapor compared to single tin dioxide films. At temperatures up to 250 °C the Pd doping of the tin dioxide strongly increased the sensitivity to all gases. At higher temperatures the sensor signal significantly decreased for the Pd/SnO2 sensor with a Pd/Al2O3 filter, indicating high catalytic activity.

Bridged Polystannoxane: A New Route toward Nanoporous Tin Dioxide

Hydrolysis and condensation under acidic conditions of bis(triprop-1-ynylstannyl)p-phenylene 1, p-xylene 2, or butylene 3 precursors yielded bridged polystannoxanes in the form of xerogels after being dried under reduced pressure. The gels were amorphous and nonporous with very low specific surface areas. Thermal treatment of the xerogels in air at 400−500 °C resulted in nanocrystalline nanoporous cassiterite tin dioxide materials, which have been thoroughly characterized by elemental analysis, FTIR, TGA−MS, powder XRD, and N2 adsorption porosimetry. The calcined materials consisted of a porous network of aggregated nanoparticles with a nanoporosity stemming from the interparticle space. A careful tuning of the precursor nature and the calcination conditions enabled the synthesis of materials with Brunauer−Emmet−Teller surface areas ranging from 40 to 150 m2 g-1, a mean pore size between 3.5 and 12 nm, and an average particle size of 5 to 25 nm. Starting from precursor 1 appeared to be the best strategy to obtain pure nanocrystalline tin dioxide materials with good textural properties for applications, with the p-phenylene bridge being easily removed by calcination.

Formation of Highly Porous Gas-sensing Films by In-situ Thermophoretic Deposition of Nanoparticles from Aerosol Phase

AbstractGas sensors based on tin dioxide nanoparticles show high sensitivity to reducing and oxidizing gases. Dry aerosol synthesis applying the flame spray pyrolysis was used for manufacture and directly (in-situ) deposit nanoparticles on sensor substrates. For the first time this technique has been used to synthesize a combination of two stacked porous layers for gas sensor fabrication. Compared to state-of-the-art techniques, aerosol technology provides a direct and versatile method to produce homogeneous nanoparticle films. Two different sensing layers were deposited directly on interdigital ceramic substrates. These porous bottom layers consisted either of pure tin dioxide or palladium doped tin dioxide. The top layer was a palladium doped alumina nanoparticle film which served as a chemical filter. The fabricated gas sensors were tested with methane, CO and ethanol. In case of CH4 detection, the pure tin dioxide sensor with the Pd/Al2O3 filter layer showed higher sensor signals and significantly improved analyte selectivity with respect to water vapor compared to single tin dioxide films. At temperatures up to 250°C the Pd-doping of the tin dioxide strongly increased the sensitivity to all gases. At higher temperatures the sensor signal significantly decreased for the Pd/SnO2 sensor with a Pd/Al2O3 filter on top indicating high catalytic activity.

Direct formation of highly porous gas-sensing films by in situ thermophoretic deposition of flame-made Pt/SnO 2 nanoparticles

Flame spray pyrolysis (FSP) was used to make pure and Pt-doped tin dioxide nanoparticles in one-step. The aerosol generated by the dry FSP method was directly, in situ thermophoretically deposited onto interdigitated Pt-electrodes to form a porous, thick film of controlled thickness within the active sensor area. Tin oxide grain size (10 nm) and a high film porosity (98%) were preserved for all film thicknesses from 9 to 40 μm using different deposition times. The dependence of the film thickness on deposition time was theoretically estimated to enable precise control of the deposition process. Platinum doping did not affect the SnO 2 grain size, crystallinity, or the porous film structure. These sensors exhibited high carbon monoxide (CO) sensor signals (8 for 50 ppm CO in dry air at 350 °C), good reproducibility, high analytical sensitivity and a remarkably low detection limit (1 ppm CO in dry air at 350 °C). The in situ platinum doping enhanced the overall sensor performance. Increasing the film thickness increased the sensor resistance and can be used to tune sensor performance.

Methane sensitivity of Fe-doped SnO 2 thick films

It has been observed that iron as a solid solution in SnO 2 remains in Fe 3+ state and acts as a p-type dopant. This results in an appreciable increase in the resistance of iron doped tin dioxide thick films, although above 350 °C, such increase in resistance is not significant with respect to that of pure tin dioxide. Methane sensitivity of such iron doped tin dioxide films showed marginally inferior behaviour to that of pure tin dioxide. However, above 350 °C, in contrast to pure tin dioxide, the methane sensitivity of iron doped tin dioxide films has been found to be very poor.

Microstructural and morphological analysis of pure and Ce-doped tin dioxide nanoparticles

Structural and morphological studies in pure and Ce-doped tin dioxide nanoparticles with high stability against particle growth were performed in samples, obtained using the polymeric precursor method and prepared at different annealing temperatures. A Ce-rich surface layer was used to control the particle size and stabilize SnO 2 against particle growth. The formation of this segregated layer can contribute to a decreased surface energy, acting in the driving force, or reducing the surface mobility. Only the cassiterite SnO 2 phase was observed below 1000 °C and a secondary phase (CeO 2 ) was observed for the Ce-doped SnO 2 at temperatures higher than 1000 °C, when de-mixing process occurs. The evolution of crystallite size, microstrain and morphology of the nanoparticles with annealing temperatures was investigated by X-ray diffraction (XRD), associated to Rietveld refinements, X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM).

Novel synthesis route to conductive antimony-doped tin dioxide and micro-fabrication method

A novel method for the fabrication of directly photopatternable pure and antimony-doped organo tin polymers via free-radical polymerization is presented. With all synthesized materials, 3 μm feature sizes can be photopatterned by using a mercury I-line UV-lamp. Photopolymerized organo tin films are crystallized by thermal annealing to obtain conductive pure or antimony-doped tin dioxide thin films. The results indicate that the direct photopatterning process increases the conductivity of fabricated films with all Sb-doping concentrations in comparison with the films fabricated without direct photopatterning. The smallest obtained resistivity for the single layer structure was 0.08 Ω cm.

A new metal oxide catalyst in alcohol condensation

The characteristics of a new ternary catalyst obtained from tin dioxide, cerium dioxide and rhodium oxide (Sn–Ce–Rh–O) are presented. The material is also used as a chemical gas sensor. The properties of this catalyst have been compared to the properties of pure tin dioxide. The Sn–Ce–Rh–O catalyst shows high activity and selectivity in the ketonisation of n-butanol. At the temperature of 350°C and 1 h −1 load, dipropylketone has been obtained with 89% selectivity at 88% n-butanol conversion. The catalytic material has a crystal structure of a rutile type. It is built from very small crystallites of 5 nm size on average. The distribution of particular components of the catalyst is homogeneous. The specific area of the Sn–Ce–Rh–O catalyst is 70 m 2/g.

Copper on-top-sputtering induced modification of tin dioxide thin film gas sensors

Thin film gas sensors based on pure tin dioxide were modified by sputtering of copper on the surface of the films. A gradual increase of an amount of the on-top-Cu produced increasing doping with uniform distribution of copper throughout entire tin oxide film. An increase of an amount of Cu caused continuous variation of the clean air resistance and the resistance response to CO and H 2 gases at various temperatures (290–550 K). For certain Cu amounts, samples show a large sensitivity towards one gas component while the cross-sensitivity remains small. The Cu-effect is discussed considering the variation of the Fermi level induced by doping and the clusters of copper on the surfaces of the grains.

Sol gel deposition of pure and antimony doped tin dioxide thin films by non alkoxide precursors

Pure and antimony doped tin oxide films have been deposited by the alcoholic sol gel method using non alkoxide precursor-SnCl 2 · 2H 2O as host and SbCl 3 as the dopant precursor. Using the dip coating method, thin films of thickness up to 300 nm have been uniformly deposited on coming 7059, KBr and Silicon substrates. The influence of various parameters such as viscosity and pH of the sol; pulling speed and sintering temperature on thickness of the coatings and their microstructure have been investigated. Pure as deposited films have a visible transmission of (83%) but very high resistivity (7.1 × 10 −2 Ω-m). Doping and annealing treatments bring it down to about 5 × 10 −6 Ω-m. It has been possible to obtain antimony doped tin oxided films having a sheet resistance of 5 Ω/□ and a visible transmission of 80% after annealing treatment by hydrogen plasma.

Synthesis and characterization of tin dioxide powders for the realization of thick-film gas sensors

Semiconductor gas sensors produced with screen-printing techniques and based on home-made tin dioxide inks are presented. The ink consists of home-made tin dioxide powder added to a polymer solution to make it screen printable on 96% alumina substrates. The major work is performed on the preparation and the characterization of pure undoped tin dioxide powder produced by two different synthetic pathways. Inks prepared with powders from each method are consecutively handled in an identical way to obtain gas sensors. The sensor response towards different gases is measured and compared for both types of starting materials.

Metal stannates and their role as potential gas-sensing elements

A selective gas sensor, sensitive to the presence of carbon monoxide in preference to the lower hydrocarbons, can be fabricated from a mixture of bismuth oxide and tin dioxide when sintered at 800 °C. At temperatures above ca. 650 °C a solid-state reaction takes place in which bismuth stannate (Bi2Sn2O7) is formed and in the above sensor all of the Bi2O3is converted to the stannate. This material is one of a group of mixed oxide stannates which possess a pyrochlore structure and have the general formulae M2Sn2O7. Several of these materials can be produced by heating an intimate mixture of lanthanum metal oxides (M2O3 where M = La, Nd, Sm, Gd, Yb, Dy, Tm and Ho) and tin dioxide at temperatures of 1500 °C. Sensors were produced containing these materials in an attempt to reproduce the behaviour of the original device and further understand the chemical, physical and topographical features responsible for conferring selectivity. However, none of the new sensors produce results consistent with those observed for the tin–bismuth system. It has subsequently been shown that when SnO2 is subjected to heat treatment at 1500 °C, it can exhibit both resistance increases and decreases upon exposure to the same gas, depending on the operating conditions. In order to reduce these interfering effects the sintering temperature was lowered to 1350 °C and sensors fabricated from pure tin dioxide fired at this temperature respond in a conventional manner to all reducing gases tested. A series of sensors produced from some of the SnO2/M2O3 materials listed above exhibit a general trend of increasing carbon monoxide and hydrogen sensitivity with decreasing M3+ ionic radius.

A potentially selective methane sensor based on the differential conductivity responses of Pd- and Pt-doped tin oxide thick layers

Derived from a concept suggested by E. M. Logothetis et al., Proc. 2nd Int. Meet. Chemical Sensors, Bordeaux, France, 1986, p. 175, two differential sensing devices, based on the different catalytic properties of palladium and platinum, are proposed. The first one consists in covering two screen-printed layers of pure tin dioxide by Pd and Pt-doped filters. No major difference is observed between these sensors, probably because of the lack of thickness of the screen-printed filters. In the second device, tin dioxide is directly doped with palladium for one sensor and with platinum for the other. The difference in the response to methane is now sufficient to consider the feasibility of a differential device. However, such sensors have no long-term stability which can only be achieved by covering the layers with inert filters. These filtered SnO 2:Pd and SnO 2:Pt sensors, now satisfactory as far as differential sensitivity and long-term stability are concerned, also exhibit an important differential response to ethanol, but fortunately reverse to that to methane. The latter result enables a sensing device to be constructed which is selective to methane with respect to ethanol, and insensitive to changes in relative humidity and/or gas flow.