Effect of Bimetallic Pd/Pt Clusters on the Sensing Properties of Nanocrystalline SnO<Subscript>2</Subscript> in the Detection of CO

Nanocrystalline tin dioxide modified by Pd and Pt clusters or by bimetallic PdPt nanoparticles was synthesized. Distribution of the modifers on the SnO₂ surface was studied by high-resolution transmission electron microscopy and energy dispersive X-ray microanalysis with element distribution mapping. It was shown that the Pd/Pt ratio in bimetallic particles varies over a broad range and does not depend on the particle diameter. The effect of platinum metals on the reducibility of nanocrystalline SnO₂ by hydrogen was determined. The sensing properties of the resulting materials towards 6.7 ppm CO in air were estimated in situ by electrical conductivity measurements. The sensor response of SnO₂ modified with bimetallic PdPt particles was a superposition of the signals of samples with Pt and Pd clusters.     

Sensor properties of hybrid SnO2-polysilazane materials

New hybrid materials based on nanocrystalline tin dioxide and two types of surface-immobilized polymer organosilicon structures with hydrocarbon substitutes were synthesized for gas sensors application. The sensing responses of pure SnO₂ and hybrid samples were determined in the presence of NO₂ (ppb range), CO (ppm range) and different humidity (RH = 15 – 95 %). Also the influence of water presence on sensor signal towards NO₂ and CO was analyzed. Strong influence of nature of hydrocarbon substitutes on sensor response value towards NO₂ and H₂O was discovered.     

Sensors based on SnO<Subscript>2</Subscript> + In<Subscript>2</Subscript>O<Subscript>3</Subscript> composite films for detecting CO in air

The sensor properties of nanostructured films of SnO₂, In₂O₃, and their combinations for detecting CO in air in the temperature range of 330–520°C were investigated. It was found that SnO₂ films show the least sensitivity to CO. Sensitivity grows as the concentration of In₂O₃ in SnO₂ increases, and it reaches its maximum value in pure In₂O₃. At the same time, the maximum of sensitivity to CO in air shifts towards low temperatures. Sensor response time was found to be about 1 s for the studied SnO₂ and In₂O₃ films, and about 0.5 s for the composite film. The mechanism of sensor sensitivity for the studied metal oxide films in detecting CO in air is discussed.     

Role of surface hydroxyl groups in promoting room temperature CO sensing by Pd-modified nanocrystalline SnO2

SnO₂/Pd nanocomposites were synthesized via sol-gel method followed by variable processing procedures. The materials are sensitive to CO gas in the concentration range 2-100 ppm at room operating temperature. It was shown that modification of nanocrystalline tin dioxide by Pd changes the temperature dependence of sensor response, decreasing the temperature of maximal signal. To understand the mechanism of room temperature CO sensitivity, a number of SnO₂/Pd materials were characterized by XRD, TEM, BET, XPS and TPR techniques. From the results of FTIR, impedance and sensing measurements under variable ambient conditions it was concluded that improvement in CO sensitivity for Pd-modified SnO₂ is due to alteration of CO oxidation pathway. The reaction of CO with surface OH-groups at room temperature was proposed, the latter being more reactive than oxygen species due to the possible chain character of the reactions. It was proposed that Pd additive may initiate chain processes at room temperature.     

Detection of Carbon Monoxide in Humid Air with Double-Layer Structures Based on Semiconducting Metal Oxides and Silicalite

Double-layer structures based on gas-sensitive semiconducting metal oxides and silicalite-1 were tested in detection of carbon monoxide in humid air. Pure tin dioxide and that modified with antimony and palladium served as materials of the sensitive layer. Upon deposition of a silicalite-1 layer on SnO₂ and SnO₂/PdO_x, the signal for CO in dry air at room measurement temperature (T = 25°C) grows, but an increase in the air humidity results in that the sensor sensitivity fully disappears. Raising the measurement temperature to 100°C makes weaker the adverse effect of the humidity. The double-layer structure containing the SnO₂(Sb)/PdO_x nanocomposite is characterized by the most stable sensor signal that is independent of the air humidity within the range RH = 4?65%.     

Influence of La(III) on the reactivity and sensor properties of nanocrystalline SnO<Subscript>2</Subscript>

Synthesis of Sn(IV) and La(III) based nanocomposites has been effectuated. According to the data obtained by XRD and TPR-H₂ methods, it is supposed that La in nanocompites is located in amorphous La₂Sn₂O₇ segregation. The effect of La(III) on the adsorption properties of SnO₂ surface and concentration of chemisorbed oxygen was determined. Sensor properties of obtained materials towards 10 ppm of CO in air were studied by in situ DC conductance measurements. It is shown that La introduction allows to increase sensor response of SnO₂ during CO detection in air.     

Template-free synthesis of hierarchical porous SnO<Subscript>2</Subscript>

Hierarchical porous tin dioxide has been successfully prepared through a fast one-pot template-free synthesis route. The boiling of the mixture of alcohol and glycerol can be utilized to generate nanopores in the SnO₂ monolith. Polycrystalline hierarchical SnO₂ with well proportioned composition has also been obtained in the pore walls of tin dioxide.     

Synthesis and study of solid acid materials SnO<Subscript>2</Subscript>/B<Subscript>2</Subscript>O<Subscript>3</Subscript>

SnO₂/B₂O₃ samples were produced by a reaction between SnCl₄, H₃BO₃, and (NH₂)₂CO in a boiling aqueous solution. The Sn: B molar ratio in these samples was 1: 1, 1: 2, and 1: 3. The phase composition and degree of crystallinity of these materials was studied. The surface acidity of the samples was analyzed by the method based on a temperature-programmed reaction of dehydration of 2-methyl-3-butyn-2-ol. Thermal transformations of SnO₂/B₂O₃ samples were examined by means of differential-thermal analysis.     

Features of behavior of solid-state CO<Subscript>2</Subscript> sensors at low temperatures

The dependence of the impedance of low-temperature sensors for carbon dioxide based on the solid-state electrochemical cells Na_0.5WO₃/Na₅GdSi₄O₁₂/SnO₂(Sb₂O₄) on the concentration of carbon dioxide in the air was studied. The reversible change in the sensor resistance was shown to be due to adsorption processes at intergrain boundaries of the solid electrolyte. The composition of the products of the electrochemical processes occurring in the sensors was established. Electronic Publication     

The influence of structural factors on the gas-sensitive properties of Fe<Subscript>2</Subscript>O<Subscript>3</Subscript>-SnO<Subscript>2</Subscript>

The influence of the dispersity and structural and phase state of oxide materials based on Fe₂O₃ and SnO₂ on the gas-sensitive properties of these materials used as sensitive layers in chemical sensors was considered. It was found that high-dispersity Fe₂O₃-SnO₂ (Fe: Sn = 9: 1) ceramic layers possessed high sensitivity to ethanol vapor in both dry and humid atmosphere and low sensitivity to CO and CH₄. The maximum response to ethanol vapor in humid atmosphere was characteristic of layers with structures of substitution-interstitial solid solutions of Sn⁴⁺ in α-Fe₂O₃.     

Dissociative adsorption of molecular hydrogen onto Pt<Subscript>6</Subscript> and Pt<Subscript>19</Subscript> platinum clusters located on the tin dioxide surface: Quantum-chemical modeling

It is suggested that, for the operation of platinum catalysts based on tin dioxide in air hydrogen fuel cells, hydrogen spillover (migration) leading to a change in the electron and proton contributions of the catalyst conductivity is of crucial importance. The hydrogen adsorption, dissociation, and migration in the platinum-tin dioxide-hydrogen system surface have been modeled by the density functional theory method within the generalized gradient approximation (GGA) under periodic conditions using a projector-augmented plane-wave (PAW) basis set with a pseudopotential. It has been demonstrated that the adsorption energy of a hydrogen molecule onto a platinum cluster increases from 1.6 to 2.4 eV as the distance to the SnO₂ substrate decreases. The calculated Pt-H bond length for adsorbed structures is 1.58–1.78 Å. The computer modeling has demonstrated that: (1) the hydrogen adsorption energy on clusters is higher than on the perfect platinum surface; (2) dissociative chemisorption onto Pt _n clusters can occur without a barrier and depends on the adsorption site and the cluster structure; (3) the adsorption energy of hydrogen onto the SnO₂ surface is higher than the adsorption energy onto the platinum cluster surface: (4) multiple H₂ dissociation on the tin dioxide surface occurs with a barrier; (5) the dissociation adsorption of hydrogen molecules onto the platinum cluster surface followed by atom migration (spillover) is energetically favorable.     

Tin Acetylacetonate as a Precursor for Producing Gas-Sensing SnO<Subscript>2</Subscript> Thin Films

To expand the range of precursors used in the sol–gel technology for applying nanostructured SnO₂ thin films promising as components of semiconductor chemical gas sensors, the efficiency of using tin acetylacetonate solutions with various precursor concentrations was demonstrated. It was determined that finely divided SnO₂ with a crystallite size of 3–4 nm (cassiterite) can be obtained by hydrolysis by atmospheric moisture in the course of solvent evaporation at room temperature. Using tin acetylacetonate solutions with various precursor concentrations for applying SnO₂ thin films by dip coating to the surface of rough ceramic Al₂O₃-based substrates with platinum interdigital electrodes and a microheater resulted in significant differences in microstructure, continuity, thickness, and porosity of the produced coatings. In a lower-concentration (0.13 mol/L) tin acetylacetonate solution, a multilayer dense continuous SnO₂ coating was applied, whereas in a higher-concentration (0.25 mol/L) solution, the formed layer comprised aggregated nanoparticles 30–60 nm in size and had much more defects and higher porosity. The sensitivity of the obtained thin-film nanostructures to the most practically important gaseous analytes: CO, H₂, CH₄, CO₂, and NO₂. The produced two-dimensional nanomaterials were shown to be promising for detecting carbon monoxide at 200–300°C in dry air.     

Physicochemical and functional properties of modified tin dioxide

Specimens of tin dioxide with modifying Sb and Pt additives are synthesized. Their physicochemical properties (specific surface area, porosity, and conductivity), chemisorption and catalytic activity in the model reaction of CO oxidation are studied. A considerable chemisorption of CO on SnO₂ and SnO₂-SbO _x is observed at 150–180°C. The oxidation of CO in the flow of gases starts in the same temperature range. An addition of platinum leads to a significant increase in the rate of CO oxidation, the reaction starts at 80°C. It is proposed that the process proceeds at the SnO₂/Pt interface.     

Dopants in nanocrystalline tin dioxide

The review surveys studies aimed at constructing new materials for gas sensors based on nanocrystalline tin dioxide. The influence of doping with various impurities (Pt, Pd, Ru, Rh, Cu, Ni, or Fe) on the composition, microstructure, and electrophysical and sensor properties of nanocrystalline SnO₂ was discussed. The conditions for the preparation of powders and thick and thin SnO₂ films by the wet chemical method and aerosol pyrolysis of organometallic compounds are reported. The mechanism of interaction of pure and doped nanocrystalline SnO₂ with a gas phase was analyzed based on the data from Mossbauer, Auger electron, and X-ray photoelectron spectroscopy and the results of in situ Raman spectroscopy, XANES, and conductivity measurements.     

Electrical properties of polymeric composites based on nanocrystalline tin dioxide modified with copper iodide

Electrical properties of synthesized nanostructured materials based on nanocrystalline tin dioxide modified with copper iodide and polychlorotrifl uoroethylene were studied. It was shown that the complex dielectric permittivity in the microwave range and the low-frequency electrical conductivity of the polymeric composites depend on the copper iodide concentration on the surface of tin dioxide and reach the maximum values at a volume fraction of CuI of about 0.5. The percolation thresholds were determined for the three-component system. Their values increase from 0.04 to 0.05 vol. fraction as the content of copper iodide in the CuI/SnO₂ system is raised from 0.04 to 0.6 vol. fraction.     

XPS and XANES studies of SnO<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript> nanolayers

This paper presents the results of our XPS (X-ray photoelectron spectroscopy) and XANES (X-ray absorption near edge structure) studies of tin oxide nanolayers obtained by magnetron spraying of the metal and its further oxidation in air at different temperatures. It was shown that at 240°C (annealing temperature), tin monoxide was dominant in the surface layer of the samples. When the temperature was increased to 450°C, the phase composition corresponded to tin dioxide. Increased sorption ability was found for the samples oxidized at 450°C. The band structure model of SnO _x nanolayers obtained by superposition of the XANES and XPS data revealed cross transitions with energy ～3.7 eV in the presence of the SnO and SnO₂ phases. Surface doping of nanolayers with palladium gave the Pd, PdO, and PdO₂ components, among which PdO was most intense. Alternate treatments with O₂ and H₂ gases led to the disappearance of palladium dioxide and the reduction of PdO to the Pd metal. After the volume doping of nanoplayers with palladium, the surface layer contained PdO and PdO₂; the latter was represented by two types of particles with different sizes.     

Study on the service life and deactivation mechanism of Ti/SnO<Subscript>2</Subscript>-Sb electrode by physical and electrochemical methods

The Sb doped tin dioxide electrode (Sb-doped SnO₂) inter-layer was prepared using electroposition layer-by-layer onto a titanium plate, and the Sb-doped SnO₂ surface catalytic layer (Ti/SnO₂-Sb) was prepared using thermo-decomposition method. Accelerated service life tests were carried out in 0.5 M H₂SO₄ solution and 1.0 M NaOH solution, respectively. The deactivation mechanism of the electrodes is studied using oxygen evolution reaction (OER) as the reaction mode. Cyclic voltammetry test showed that the electrodes after accelerated life tests had no catalytic-oxidizing activity upon phenol. Electrochemical impedance spectroscopy (EIS) analysis exhibited that the membrane resistance of the deactivated electrode increases obviously in 0.5 M H₂SO₄ solution and 1.0 M NaOH solution, with the values of 1231 and 90.6 Ω, respectively. The structure, morphology and the content of the fresh and deactivated electrode were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray detector (EDX). This suggested that the Ti content on the electrode surface increases after deactivation, and TiO₂ membrane with poor conductivity is grown on the electrode surface.     

Solution,Solid-State Two Step Synthesis and Optical Properties of ZnO and SnO<Subscript>2</Subscript> Nanoparticles and Their Nanocomposites with SiO<Subscript>2</Subscript>

Nanostructure luminescent ZnO and SnO₂ materials are prepared by a two-step solid-state method based on the solution preparation of the macromolecular precursors ZnCl₂·Chitosan and SnCl₂·Chitosan having different ratios (1:1, 1:5 and 1:10), their pyrolysis under air at 800 °C. The pyrolytic ZnO and SnO₂ nanomaterials show a dependence of the particle size, morphology and luminescent properties with the ratio [metal/polymer] in the MCl₂·Chitosan precursors. Thus, ZnO semiconductor materials exhibit luminescence spectra with several emission at 440 nm corresponds to a radiative transition of an electron from the shallow donor level of oxygen vacancies, and the zinc interstitial, to the valence band. On the other hand, the photoluminescence spectrum of the nanostructured SnO₂ shows an intense blue luminescence at a wavelength of 420 nm which may be attributed to oxygen-related defects that have been introduced during the growth process of the nanoparticles. Additionally, whereas SnO₂ was successfully incorporated into SiO₂ structure (SnO₂//SiO₂) by pyrolysis of solid-state mixtures of the precursors SnCl₂·Chitosan in the presence of SiO₂, the same reaction carried out with ZnCl₂·Chitosan precursors led to a mixture of Zn₂SiO₄ and SiO₂. Thus, this new methodology yields nanostructured semiconductor materials, ZnO and SnO₂, suitable for optoelectronic and sensor solid-state devices.     

Room temperature LPG resistive sensor based on the use of a few-layer graphene/SnO<Subscript>2</Subscript> nanocomposite

A nanocomposite consisting of a few layers of graphene (FLG) and tin dioxide (SnO₂) was prepared by ultrasound-assisted synthesis. The uniform SnO₂ nanoparticles (NPs) on the FLG were characterized by X-ray diffraction in terms of lattice and phase structure. The functional groups present in the composite were analyzed by FTIR. Electron microscopy (HR-TEM and FE-SEM) was used to study the morphology. The effect of the fraction of FLG present in the nanocomposite was investigated. Sensitivity, selectivity and reproducibility towards resistive sensing of liquid propane gas (LPG) was characterized by the I-V method. The sensor with 1% of FLG on SnO₂ operated at a typical voltage of 1 V performs best in giving a rapid and sensitive response even at 27 °C. This proves that the operating temperature of such sensors can be drastically decreased which is in contrast to conventional metal oxide LPG sensors.

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Graphical abstract Schematic of a room temperature gas sensor for liquefied petroleum gas (LPG). It is based on the use of a few-layered graphene (1 wt%)/SnO₂ nanocomposite that was deposited on an interdigitated electrode (IDEs). A sensing mechanism for LPG detection has been established.

     

SnO<Subscript>2</Subscript> thin films from an aqueous citrato peroxo Sn(IV) precursor

In this work, tin(II) oxalate was studied as a novel chloride-free starting material for the preparation of a stable Sn-containing precursor solution. This precursor was applied for the chemical solution deposition (CSD) of transparent conducting coatings of SnO₂ on Si/SiO₂ substrates. An influence of synthesis parameters, such as pH, complexing agent to metal ion ratio on the stability of the citrato peroxo Sn(IV) precursor has been investigated in this study. Insights into the precursor chemistry and its thermal decomposition based on TG-DSC analysis are also presented. The obtained SnO₂ films were characterized by high temperature X-ray diffraction (HT-XRD) and scanning electron microscopy (SEM) to evaluate phase purity and film thickness, respectively.