Sensor Performance of Nanostructured TiO<Subscript>2</Subscript>–SnO<Subscript>2</Subscript> Films Prepared by an Aqueous Sol–Gel Process

Nanostructured TiO₂–SnO₂ thin films and powders were prepared by a facile aqueous particulate sol–gel route. The prepared sols showed a narrow particle size distribution with hydrodynamic diameter in the range 17.2–19.3 nm. Moreover, the sols were stable over 5 months, since the constant zeta potential was measured during this period. The effect of Sn:Ti molar ratio was studied on the crystallisation behaviour of the products. X-ray diffraction analysis revealed that the powders were crystallised at the low temperature of 400 °C containing anatase-TiO₂, rutile-TiO₂ and cassiterite-SnO₂ phases, depending on annealing temperature and Sn:Ti molar ratio. Furthermore, it was found that SnO₂ retarded the anatase to rutile transformation up to 800 °C. The activation energy of crystallite growth was calculated in the range 0.96–6.87 kJ/mol. Transmission electron microscope image showed that one of the smallest crystallite sizes was obtained for TiO₂–SnO₂ binary mixed oxide, being 3 nm at 600 °C. Field emission scanning electron microscope analysis revealed that the deposited thin films had nanostructured morphology with the average grain size in the range 20–40 nm at 600 °C. Thin films produced under optimized conditions showed excellent microstructural properties for gas sensing applications. They exhibited a remarkable response towards low concentrations of CO gas at low operating temperature of 200 °C, resulting in increased thermal stability of sensing films as well as a decrease in their power consumption.     

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.     

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.     

Snx.Ti1‐xO2 solid solution catalysts for nitrogen oxide selective catalytic reduction by propene in presence of oxygen

A series of SnO₂‐TiO₂ binary oxide catalysts prepared by co‐current precipitation method was found to be a novel and good system for the selective catalytic reduction of NO by propene in the presence of oxygen with high activity and good selectivity to N₂. The NO conversion to N₂ over SnO₂‐TiO₂ oxide catalysts varied with SnO₂ content and attainted a maximum at 65% over the catalyst with SnO₂ content at 40wt% for a feed with 1186 ppm NO, 948 ppm propene and 2.23% O₂, in He and a space velocity of 15000 h^?1 at 350°C. The SnO₂TiO₂ catalysts could sustain moderate activity in the presence of 10% steam. Because of the identical valence and the similar radius of Sn⁴⁺ and Ti⁴⁺, SnO₂‐TiO₂ binary oxides can form solid solution in three different phases as proved by XRD, electron diffraction and TPR. Sn⁴⁺ is the main active species in the SnO₂‐TiO₂ catalysts, and it is enriched on the solid solution surface as tested by XPS analysis. H₂‐TPR, NH₃‐TPD and BET tests show that SnO₂‐TiO₂ solid solution can dilute SnO₂ and suppress the activity of propene complete oxidation over SnO₂. This may be beneficial to the reactivity enhancement of NO conversion.     

Preparation and characterization of transparent TiO2 thin films coated on fused-silica substrates

The transparent TiO₂ thin films coated on fused-SiO₂ substrates were prepared by the sol–gel method and spin-coating technique. Effects of calcination temperature on crystal structure, grain size, surface texture, and light transmittance of the films were investigated. After calcining at 600–1,200 °C, the thicknesses of the TiO₂ films were all around 80 nm and the molecular structures of the films were anatase, even at 1,200 °C. The calcined TiO₂ films had the ultraviolet light (wavelength 200–400 nm) transmittances of ≤29% and the visible light (wavelength 400–800 nm) transmittance of ≥72%. By photocatalytically decomposing the methylene blue (MB) in water, the photocatalytic activities of the TiO₂ thin films were measured and represented using the characteristic time constant (τ) for the MB degradation. While the films prepared at 1,000 and 1,200 °C photodecomposed about 54 mol% of the MB in water (the corresponding τ ≈ 14.8 h) after exposing to 365-nm UV light for 12 h, the films prepared at 600 and 800 °C had smaller τ (≈9.0 h) and photodecomposed about 74 mol% of the MB in water at the same testing conditions.     

Synthesis of hierarchical shell-core SnO2 microspheres and their gas sensing properties

Using SnSO₄, d-glucose, urea and water, hierarchical shell-core SnO₂ microspheres were successfully synthesized via a simple hydrothermal method. The characterization results showed that the sizes of as-prepared SnO₂ microspheres were 0.6–1 μm, with shell thicknesses of 40−60 nm. The shell and large core of the SnO₂ microspheres were all comprised of the same basic rice-like nanoparticles with diameters of 16−25 nm and lengths of 16−45 nm. Further investigaton showed that the glucose and urea served as structural guiding agents, and urea facilitated the formation of the hierarchical structure. The as-prepared SnO₂ nanomaterials were used to fabricate a gas sensor with an electrode blade used for the gas sensitivity tests. The hierarchical shell-core SnO₂ microspheres exhibited high sensitivity and selectivity toward ethanol, with a responsivity of 63.8 for 50 ppm ethanol at 250 °C, while the response and recovery time were 7 s and 28 s respectively. Moreover, the responsivity of the materials showed good linearity at ethanol concentrations from 500 ppb to 10 ppm. The simple synthetic method, environmentally-friendly raw materials, and excellent gas sensitivity demonstrate that the as-prepared SnO₂ nanomaterial has great potential applications for the sensing of ethanol gas.     

Concurrent synthesis of SnO/SnO<Subscript>2</Subscript> nanocomposites and their enhanced photocatalytic activity

The SnO/SnO₂ nanocomposites were synthesized using semisolvothermal reaction technique. These nanocomposites were prepared using different combination of solvents viz., ethanol, water, and ethylene glycol at 180 °C for 24 h. The synthesized nanocomposites were analyzed with various characterization techniques. Structural analysis indicates the formation of tetragonal phase of SnO₂ for the sample prepared in ethanol, whereas for other solvent combinations, the mixture of SnO and SnO₂ having tetragonal crystal structures were observed. The optical study shows enhanced absorbance in the visible region for all the prepared SnO/SnO₂ nanocomposites. The observed band gap was found to be in the range of 3.0 to 3.25 eV. Microstructural determinations confirm the formation of nanostructures having spherical as well as rod-like morphology. The size of nanoparticles in ethanol-mediated solvent was found to be in the range of 5 to 7 nm. Thermogravimetric analysis indicate the weight gain around 1.3 wt% confirming the conversion of SnO to SnO₂ material. The photocatalytic activity of synthesized nanocomposites was evaluated by following the aqueous methylene blue (MB) degradation. The sample prepared in ethylene glycol-mediated solvent showed highest photoactivity having apparent rate constant (K_app) 0.62 × 10^?2 min^?1.     

Resistive-type sensors based on few-layer MXene and SnO2 hollow spheres heterojunctions: Facile synthesis,ethanol sensing performances

High-performance and low-cost gas sensors are highly desirable and involved in industrial production and environmental detection. The combination of highly conductive MXene and metal oxide materials is a promising strategy to further improve the sensing performances. In this study, the hollow SnO₂ nanospheres and few-layer MXene are assembled rationally via facile electrostatic synthesis processes, then the SnO₂/Ti₃C₂T_x nanocomposites were obtained. Compared with that based on either pure SnO₂ nanoparticles or hollow nanospheres of SnO₂, the SnO₂/Ti₃C₂T_x composite-based sensor exhibits much better sensing performances such as higher response (36.979), faster response time (5 s), and much improved selectivity as well as stability (15 days) to 100 ppm C₂H₅OH at low working temperature (200 °C). The improved sensing performances are mainly attributed to the large specific surface area and significantly increased oxygen vacancy concentration, which provides a large number of active sites for gas adsorption and surface catalytic reaction. In addition, the heterostructure interfaces between SnO₂ hollow spheres and MXene layers are beneficial to gas sensing behaviors due to the synergistic effect.     

Preparation and characterization of highly photoactive nanocrystalline TiO<Subscript>2</Subscript> powders by solvent evaporationinduced crystallization method

Highly photoactive bi-phase nanocrystalline TiO₂ photocatalyst was prepared by a solvent evaporation-induced crystallization (SEIC) method, and calcined at different temperatures. The obtained TiO₂ photocatalyst was characterized with X-ray diffraction (XRD), transmission electron microscopy (TEM) and BET surface areas. The photocatalytic activity was evaluated by the photocatalytic oxidation of acetone in air. The results show that solvent evaporation can promote the crystallization and phase transformation of TiO₂ at 100°C. When calcination temperatures are below 600°C, the prepared TiO₂ powders show bimodal pore size distributions in the mesoporous region. At 700°C, the pore size distributions exhibit monomodal distribution of the inter-aggregated pores due to the collapse of the intra-aggregated pores. At 100°C, the obtained TiO₂ photocatalyst by this method shows good photocatalytic activity, and at 400°C, its photocatalytic activity exceeds that of Degussa P25. This may be attributed to the fact that the prepared TiO₂ photocatalyst has higher specific surface areas, smaller crystallite size and bimodal pore size distribution.     

Photocatalytic Activity for Water Decomposition to Hydrogen over Nitrogen‐doped TiO2 Nanoparticle

Nitrogen-doped TiO2 nanoparticle photocatalysts were obtained by an annealing method with gaseous ammonia and nitrogen. The influence of dopant N on the crystal structure was characterized by XRD, XPS, BET, TEM and UV-Vis spectra. The results of XRD indicate that, the crystal phase transforms from anatase to rutile structure gradually with increase of annealing temperature from 300 to 700 ℃. XPS studies indicate that the nitrogen atom enters the TiO2 lattice and occupies the position of oxygen atom. Agglomeration of particles is found in TEM images after annealing. BET results show that the specific surface areas of N-doped samples from 44.61 to 38.27 m2/g are smaller than that of Degussa TiO2. UV-Vis spectra indicate that the absorption threshold shifts gradually with increase of annealing temperature, which shows absorption in the visible region. The influence of annealing condition on the photocatalytic property has been researched over water decomposition to hydrogen, indicating that nitrogen raises the photocatalytic activity for hydrogen evolution, and the modified TiO2 annealed for 2 h at 400 ℃ under gas of NH3/N2 (V/V＝1/2) mixture shows better efficiency of hydrogen evolution. Furthermore, the N-doped TiO2 nanoparticle catalysts have obvious visible light activity, evidenced by hydrogen evolution under visible light (λ＞400 nm) irradiation. However, the catalytic activity under visible light irradiation is absent for Degussa as reference and the N-doped TiO2 annealed at 700 ℃.     

Metal oxide semiconductors based on tin dioxide: Gas-sensitivity to methane in a wide range of temperatures,concentrations and humidities of the gas phase

The response of metal oxide semiconductors (MOSs) made by the thick-film technology on the basis of SnO₂ with various catalytic additives was studied in dry gaseous media containing 200 ppm CH₄ in the temperature range 100–600° C. Concentration dependence was studied for four sensors (on the basis of pure SnO₂ and with three catalytic additives, 3% Pd, 1% Sb₂O₅ + 3% La₂O₃, and 1% Pt + 3% Pd) in the concentration range 1–20600 ppm CH₄ at the humidity of the gas phase varied from 0 to 100%. It was found out that, in the strength of all performance and technical characteristics, the structure SnO₂ + 1% Pt + 3% Pd working at 400°C and consuming ～180 mW was the best for recording CH₄.     

Al2O3/SnO2 Co-Nanoparticle Modified Grafted Collagen for Improving Thermal Stability and Infrared Emissivity

Al2O3/SnO2 co-nanoparticles were prepared with a modified sol-gel technique followed by a thermal treatment process. With these co-nanoparticles the grafted collagen-Al2O3/SnO2 nanocomposites were obtained using a supersonic dispersion method. X-ray diffraction, FT-IR analysis, transmission electron microscopy, TGA/DTA and infrared emissivity test were performed to characterize the resulting nanoparticles and nanocomposites, respectively. The Al2O3/SnO2 co-nanoparticles showed a narrow distribution of size between 20-40 nm and could be uniformly absorbed on the tri-helix scaffolds of the grafted collagen without any aggregation. The nanocomposites possessed better thermal stability and substantially lower infrared emissivity than the grafted collagen and Al2O3/SnO2 co-nanoparticles with an increase of degradation temperature from 39 to 210 ℃ and a decrease of infrared emissivity from 0.850 of the grafted collagen and 0.708 of the Al2O3/SnO2 co-nanoparticles to 0.424, which provided a potential application of the nanocomposites to areas such as photoelectronics.     

Crystal growth of SnO2 and other MO2 (M = Ti,Zr, Hf) oxides by flux of B2O3?V2O5 system

Single crystals of SnO₂ and MO₂ (M = Ti, Zr, Hf) oxide were grown from flux of B₂O₃? V₂O₅ system. Mixtures of the flux and the starting powder of Zn₂SnO₄, TiO₂, ZrO₂, or HfO₂ were soaked at a temperature of 1030–1340°C for 10–72 hr and then were cooled down to 900°C at a rate of 5°C/hr. Grown crystals of SnO₂ were pale brown needles. An increase in V₂O₅ content of the flux (up to V₂O₅/B₂O₃ ratio equal to 2) or in the soaking temperature increases the crystal size. A largest crystal with the size of 15.0 × 0.4 × 0.4 mm was obtained in the case of V₂O₅/B₂O₃ = 2. Crystals of TiO₂ were black needles or platelets, and those of ZrO₂ and HfO₂ were yellowish, transparent needles or blocks. The maximum size of TiO₂, ZrO₂ or HfO₂ crystal was 12.0 × 0.1 × 0.1 mm, 4.0 × 0.3 × 0.3 mm or 11.0 × 0.6 × 0.6 mm, respectively. The long axis of the crystals was all C-axis and main faces on the crystals were of {100} and/or {110} families. All these crystals were found to include the impurities of boron and vanadium. The electrical resistivities of SnO₂ and TiO₂ crystals were measured to be 1.4 × 10⁶ and 5.6 × 10⁴ Ω · cm at 25°C, respectively.     

Synthesis and characterization of high surface area TiO2/SiO2 mesostructured nanocomposite

Recently titania synthesis was reported using various structuration procedures, leading to the production of solid presenting high surface area but exhibiting moderate thermal stability. The study presents the synthesis of TiO₂/SiO₂ nanocomposites, a solid that can advantageously replace bulk titania samples as catalyst support. The silica host support used for the synthesis of the nanocomposite is a SBA-15 type silica, having a well-defined 2D hexagonal pore structure and a large pore size. The control of the impregnation media is important to obtain dispersed titania crystals into the porosity, the best results have been obtained using an impregnation in an excess of solvent. After calcination at low temperature (400 °C), nanocomposites having titania nanodomains (～2–3 nm) located inside the pores and no external aggregates visible are obtained. This nanocomposite exhibits high specific surface area (close to that of the silica host support, even with a titania loading of 55 wt.%) and a narrow pore size distribution. Surprisingly, the increase in calcination temperature up to 800 °C does not allow to detect the anatase to rutile transition. Even at 800 °C, the hexagonal mesoporous structure of the silica support is maintained, and the anatase crystal domain size is evaluated at ～10 nm, a size close to that of the silica host support porosity (8.4 nm). Comparison of their physical properties with the results presented in literature for bulk samples evidenced that these TiO₂/SiO₂ solids are promising in term of thermal stability.     

Schottky barrier and catalytic effect induced high gas sensing of one-step synthesized Pd–SnO2 nanorods

Uniformly loaded Pd–SnO₂ nanorods are synthesized via a simple one-step hydrothermal route. The gas sensors fabricated from Pd–SnO₂ nanorods exhibit high sensitivity and fast response. The sensor response at 300 °C is up to 9.9, 36.8, 55.6, 89.1 and 168.2 upon exposure to 100, 200, 300, 500 and 1000 ppm ethanol, respectively. And the work temperature can be lowered down to 200 °C. Such behaviors can be attributed to Schottky barrier at Pd/SnO₂ interface and catalytic effect of Pd nanoparticles. Our results open a way for uniform modification of SnO₂ nanorods with Pd nanoparticles and enhancing their gas sensing performance.     

Tailoring of morphology and crystal structure of nanomaterials in MgO–TiO2 system by controlling Mg:Ti molar ratio

The morphological manipulation and structural characterisation of TiO₂?CMgO binary system by an aqueous particulate sol?Cgel route were reported. Different crystal structures including pure MgTiO₃, mixtures of MgTiO₃ and TiO₂ and mixtures of MgTiO₃ and Mg₂TiO₄ were tailored by controlling Mg:Ti molar ratio and annealing temperatures as the processing parameters. X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) revealed that all compounds crystallised at the low temperature of 500?°C. Furthermore, it was found that the average crystallite size of the compounds depends upon the Mg:Ti molar ratio as well as the annealing temperature, being in the range 3?C5?nm at 500?°C and around 6?nm at 700?°C. Field emission scanning electron microscope (FE-SEM) analysis revealed that the deposited thin films had nanocrystalline structure with the average grain size of 25?C30?nm at 500?°C depending upon the Mg:Ti molar ratio. Moreover, atomic force microscope (AFM) images presented that the thin films had a hill-valley like morphology made up of small grains.     

Sensitivity of semiconductor metal oxides (SnO2, WO3, ZnO) to hydrogen sulfide in dry and humid gas media

The sensitivity of semiconductor sensors based on tin (SnO₂), tungsten (WO₃), and zinc (ZnO) oxides and SnO₂ with catalytic admixtures of La₂O₃ and CuO to hydrogen sulfide is studied at H₂S concentration 50 ppm in dry air in the temperature range 100–600°C. Concentration dependences for oxides are studied in the temperature range 350–450°C and H₂S concentration range 0.5–100 ppm at the humidity of gas media 0–80 rel. %. It is shown that, under the specified conditions, the resistance and of sensors to H₂S in air weakly depends on humidity. It was found that sensors based on SnO₂ with an admixture of 3% La₂O₃ working at 350°C are the best for the registration of H₂S by the set of performance and operation characteristics. A presumable mechanism of H₂S interaction with oxide surfaces is considered, according to which each H₂S molecule releases seven electrons to the conductivity zone of the oxide and molecules of metal oxides in the surface layer are, possibly, partially replaced by sulfide molecules.     

Synthesis of high surface area nanocrystalline anatase-TiO2 powders derived from particulate sol-gel route by tailoring processing parameters

Stabilised titania sols were prepared using an additive free particulate sol-gel route, via electrostatic stabilisation mechanism, with various processing parameters. Peptisation temperature, 50°C and 70°C, and TiO₂ concentration, 0.1, 0.2 and 0.4 molar, were chosen as processing parameters during sol preparation. Results from TiO₂ particle size and zeta potential of sols revealed that the smallest titania hydrodynamic diameter (13 nm) and the highest zeta potential (47.7 mV) were obtained for the sol produced at the lower peptisation temperature of 50°C and lower TiO₂ concentration of 0.1 M. On the other hand, between the sols prepared at 70°C, smaller titania particles (20 nm) and higher zeta potential (46.3 mV) were achieved with increasing TiO₂ concentration up to 0.4 M. X-ray diffraction (XRD) and Brunauer-Emmett-Teller (BET) results of produced powders annealed at different temperatures showed that the 300°C annealed powder made from 0.1 M sol prepared at 50°C was a mixture of anatase and brookite, corresponding to a major phase of anatase (∼95% estimated), with the smallest average crystallite size of 1.3 nm and the highest specific surface area (SSA) of 193 m²/g. Furthermore, increasing TiO₂ concentration up to 0.4 molar for the sols prepared at 70°C resulted in decreasing the average crystallite size (1.9 nm at 300°C) and increasing SSA (116 m²/g at 300°C) of the powders annealed at different temperatures. Anatase-to-rutile phase transformation temperature was increased with decreasing peptisation temperature down to 50°C, whereas TiO₂ concentration had no effect on this transition. Anatase percentage increased with decreasing both peptisation temperature and TiO₂ concentration. Such prepared powders can be used in many applications in areas from photo catalysts to gas sensors.     

Synthesis of mesoporous SnO2 nanomaterials with selective gas-sensing properties

Using SBA-15/KIT-5/KIT-6 as the hard templates, the mesoporous SnO₂ nanomaterials with different structures were synthesized by nanocasting. X-ray diffraction, transmission electron microscopy, and nitrogen adsorption isotherms were used to testify their structure characteristics. These mesoporous SnO₂ nanomaterials showed high specific surface areas (57–96 m² g^?1) and pore volume (0.17–0.27 cm³ g^?1). The nanopore of these templates makes the nanosize particle of the final mesoporous SnO₂ nanomaterials (4–9 nm) at last. The sensing properties of acetone, ethyl alcohol and methyl alcohol were investigated. The response of SnO₂-15, SnO₂-5, and SnO₂-6 are 17.0, 19.5, and 16.1, respectively as the concentration of ethyl alcohol on 200 ppm. The sensitivity of SnO₂-5 is 28.2 as the concentration of acetone was increased to 200 ppm. With the large surface area, high pore volume, and nanosized particles (close to 2 L = 6 nm of SnO₂), the SnO₂-5 show four fold enhancement in sensitivity compared to commercial SnO₂ powder and low detection limit (even at 200 ppb). The surface area and particle size play a significant party in the gas response. With the large surface area and smallest particle size, SnO₂-5 shows the highest sensitivity of all. These mesoporous nanomaterials show well potential application on the gas response.     

The effect of TiO2 additives on the structural stability and thermal properties of yttria fully-stabilized zirconia

TiO₂(0–20 mol%)-8 mol% YSZ (8YSZ) ceramics were synthesized by a traditional solid-state reaction method. A cubic single phase was observed for 8YSZ, 4 mol% TiO₂-8YSZ and 8 mol% TiO₂-8YSZ. Tetragonal and cubic mixed phases were observed for 12–20 mol% TiO₂-8YSZ ceramics. The sintering temperature was 1,700 °C for 8YSZ and 4 mol% TiO₂-8YSZ ceramics, whereas it was 1,500 °C for 8–20 mol% TiO₂-8YSZ. The thermal conductivity at room temperature decreased in proportion to increasing TiO₂ content, from 3.0 to 2.3 W/m K. The specific heat of TiO₂-8YSZ ceramics was unaltered as the TiO₂ content changed.