Methane sensor based on SnO<Subscript>2</Subscript>/In<Subscript>2</Subscript>O<Subscript>3</Subscript>/TiO<Subscript>2</Subscript> nanostructure

Two sets of samples of SnO₂/In₂O₃/TiO₂ system have been fabricated with different concentrations of component materials. In the first set TiO₂ with rutile structure was used, while in the second set it has the structure of anatase. Thin films (up to 50 nm) of obtained mixtures were deposited. Their sensitivity and selectivity with respect to methane (CH4) were studied. Nanostructure on the basis of 70%SnO₂ — 10%In₂O₃ — 20%TiO₂(anatase) exhibits sufficient sensitivity to methane.     

Preparation and investigation of composite SiO<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> films

SiO₂-TiO₂ films [Si:Ti = 1:(0.06–2.3)] are obtained by the sol-gel method. The structural and photoluminescent properties of the films and powders heat-treated at different temperatures are studied. It is shown that after 700°C the composite consists of TiO₂ crystallites that are structurally similar to anatase and distributed in an amorphous SiO₂ matrix. The photoluminescence spectra have maxima at 450–500 nm. The photoluminescence intensity depends on the treatment temperature and TiO₂ content. __________ Translated from Zhurnal Prikladnoi Spektroskopii Vol. 74, No. 3, pp. 357–361, May–June, 2007.     

Refractometry of mechanically compressed (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of uniaxial mechanical pressure σ _m ≤ 150 bar on the spectral (300–800 nm) dependence of the birefringerence Δn _i and refractive indices n _i of (NH₄)₂SO₄ crystals has been investigated. It is shown that the dispersion of n _i (λ) and Δn _i (λ) is normal and sharply increases with approach to the absorption edge. It is established that uniaxial pressure does not change the character of the dispersions dn _i /dλ and dΔn _i /dλ and only affects their magnitudes. It is shown that the increase in the refractive indices under uniaxial stress is mainly due to the increase in the refraction caused by the increase in the band gap and long-wavelength shift of the UV absorption band maximum.     

Conventional wet impregnation versus microwave-assisted synthesis of SnO<Subscript>2</Subscript>/CNT composites

Carbon nanotubes decorated with SnO₂ nanoparticles were prepared by conventional and microwave-assisted wet impregnation. The composites were thoroughly characterized by X-ray diffraction, Raman spectroscopy, BET-surface area measurement, Scanning and transmission electron microscopy. The XRD studies revealed the formation of tetragonal phase of SnO₂. The microwave method produced CNTs heavily decorated with SnO₂ nanoparticles with average size 5 nm in a total reaction time of 10 min because of the rapid volumetric heating. DC conductivity increased significantly for the nanocomposite samples when compared with the pure CNTs. In electrical conductivity properties, sample prepared by microwave method was found to be superior to the one prepared by conventional procedure due to homogeneous distribution of nanoparticles.     

Impedance spectroscopy analysis of Li<Subscript>2</Subscript>SnO<Subscript>3</Subscript>

In this work, Li₂SnO₃ has been synthesized by the sol–gel method using acetates of lithium and tin. Thermogravimetric analysis (TGA) has been applied to the precursor of Li₂SnO₃ to determine the suitable calcination temperature. The formation of the compound calcined at 800 °C for 9 h has been confirmed by X-ray diffraction (XRD) analysis. The Li₂SnO₃ is then pelletized and electrically characterized by using electrochemical impedance spectroscopy (EIS) in the frequency range from 50 Hz to 1 MHz. The complex impedance spectra clearly show the dominating presence of the grain boundary effect on electrical properties whereas the complex modulus plots reveal two semicircles which are due to the grain (bulk) and grain boundary. The spectra of imaginary parts of both impedance and modulus versus frequency show the existence of peaks with the modulus plots exhibiting two peaks that are ascribed to the grain and grain boundary of the material. The peak maximum shifts to higher frequency with an increase in temperature and the broad nature of the peaks indicates the non-Debye nature of Li₂SnO₃. The activation energy associated with the dielectric relaxation obtained from the electrical impedance spectra is 0.67 eV. From the electric modulus spectra, the activation energies related to conductivity relaxation in the grain and grain boundary of Li₂SnO₃ are 0.59 and 0.69 eV, respectively. The conductivity–temperature relationship is thermally assisted and obeys the Arrhenius rule with the activation energy of 0.66 eV. The conduction mechanism of Li₂SnO₃ is via hopping.     

Superproton phase transition in a K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystal

The conductivity and heat capacity of tripotassium hydrogen trisulfate single crystals were studied. It was shown that these single crystals undergo a solid-solid phase transformation upon heating with the formation of new phases, and that their properties change considerably. Anomalies related to the sorption and desorption of water vapor were observed in the formed multiphase system. Such anomalies were observed for the first time in measurements of the true heat capacity.     

Red photoluminescence mechanism in SnO<Subscript>2</Subscript> nanostructures

Tin dioxide nanoribbons were fabricated for clarifying the origin of the red photoluminescence band. It is found that the red band abruptly decreases its intensity after annealing the nanoribbons in O₂. The time-resolved photoluminescence decay curve shows that the red band has a luminescence lifetime of ns. The electron spin resonance spectrum discloses that the red band is related to a kind of combined oxygen-vacancy (V _o⁺ and V _o⁺⁺) centers. Spectral analysis and theoretical calculation confirm that the red band arises from a transition between the combined oxygen-vacancy defect states in the band gap.     

Biotemplate fabrication of SnO<Subscript>2</Subscript> nanotubular materials by a sonochemical method for gas sensors

A sonochemical method is developed to fabricate SnO₂ nanotubular materials from biological substances (here, it is cotton). The cotton fibers in SnCl₂ solution were first treated with ultrasonic waves in air, followed by calcinations to give nanotubular materials that faithfully retain the initial cotton morphology. The microstructure and morphology of the obtained SnO₂ nanotubules were characterized by the combination of field-emission scanning electron microscope (FE-SEM), transmission electron microscopy (TEM), Fourier transform infrared (FT-IR), X-ray diffraction (XRD), and N₂ adsorption/desorption measurements. The thermal behavior and crystalline properties were examined in the temperature range of 450–700 °C. The nanocrystals composing of SnO₂ nanotubules were estimated about 8.5, 13.2, and 14.2 nm corresponding to calcination temperatures of 450, 550, and 700 °C, respectively. The sensor performance of biomorphic SnO₂ nanotubules calcined at 700 °C was investigated in the atmosphere of ethanol, formaldehyde, carbinol, carbon monoxide, hydrogen, ammonia, and acetone, respectively, which exhibited a good selectivity for acetone at a working temperature of 350 °C. The sensitivity to 20 ppm acetone, S, was 6.4 at 350 °C with rapid response and recovery (around 10–9 s). These behaviors were well explained in relation to the morphology of the nanotubules thus produced.     

Cold SO<Subscript>2</Subscript> molecules by Stark deceleration

We produce SO₂ molecules with a centre of mass velocity near zero using a Stark decelerator. Since the initial kinetic energy of the supersonic SO₂ molecular beam is high, and the removed kinetic energy per stage is small, 326 deceleration stages are necessary to bring SO₂ to a complete standstill, significantly more than in other experiments. We show that in such a decelerator possible loss due to coupling between the motional degrees of freedom must be considered. Experimental results are compared with 3D Monte-Carlo simulations and the quantum state selectivity of the Stark decelerator is demonstrated.     

Structural and electrical properties of pure and H<Subscript>2</Subscript>SO<Subscript>4</Subscript> doped (PVA)<Subscript>0.7</Subscript>(NaI)<Subscript>0.3</Subscript> solid polymer electrolyte

Solid polymer electrolytes (SPE) based on poly-(vinyl alcohol) (PVA)_0.7 and sodium iodide (NaI)_0.3 complexed with sulfuric acid (SA) at different concentrations were prepared using solution casting technique. The structural properties of these electrolyte films were examined by X-ray diffraction (XRD) studies. The XRD data revealed that sulfuric acid disrupt the semi-crystalline nature of (PVA)_0.7(NaI)_0.3 and convert it into an amorphous phase. The proton conductivity and impedance of the electrolyte were studied with changing sulfuric acid concentration from 0 to 5.1 mol/liter (M). The highest conductivity of (PVA)_0.7(NaI)_0.3 matrix at room temperature was 10⁻⁵ S cm⁻¹ and this increased to 10⁻³ S cm⁻¹ with doping by 5.1 M sulfuric acid. The electrical conductivity (σ) and dielectric permittivity (ε′) of the solid polymer electrolyte in frequency range (500 Hz–1 MHz) and temperature range (300–400) K were carried out. The electrolyte with the highest electrical conductivity was used in the fabrication of a sodium battery with the configuration Na/SPE/MnO₂. The fabricated cells give open circuit voltage of 3.34 V and have an internal resistance of 4.5 kΩ.     

Structural transformations of K<Subscript>3</Subscript>H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals with variations in temperature

Single crystals of the K₃H(SO₄)₂ compound are investigated using X-ray diffraction on Xcalibur S and Bruker diffractometers. The structure of the low-temperature monoclinic phase is refined (space group C2/c, z = 4, a = 14.698(1) Å, b = 5.683(1) Å, c = 9.783(1) Å, β = 103.01(1)°, T = 293 K, Bruker diffractometer), the structural phase transition is revealed, and the structure of the high-temperature trigonal phase is determined (space group R \(\bar 3\) m, z = 3, a = 5.73(1) Å,c = 21.51(1) Å,T = 458 K, Xcalibur diffractometer).     

Electrical properties of Na<Subscript>2</Subscript>SO<Subscript>4</Subscript>-based composite systems

Composite electrolytes are well-known multiphase systems and exhibit maxima in the conductivity at certain second-phase concentration. An attempt has been made to investigate a number of sodium sulfate (Na₂SO₄)-based composite systems. The dispersoids that have been used are MgO, Al₂O₃, and SiO₂. The samples have been characterized using impedance spectroscopy, X-ray diffraction, and differential scanning calorimetry. The maximum conductivity has been observed for MgO dispersed system, and the percolation threshold has been observed at 30-mol% dispersoid, MgO concentration. Interestingly, two maxima have been observed in case of the Na₂SO₄–SiO₂ and Na₂SO₄–Al₂O₃ composite systems. In the Na₂SO₄–SiO₂ system, the first maximum occurs at lower concentration, i.e., in the range between 10 and 20 mol%, whereas the second occurs at the 40-mol% dispersoid concentration. For the Na₂SO₄–Al₂O₃ system, although slightly indistinguishable, two peaks in the conductivity vs composition plot have been observed around 12- and 30-mol% Al₂O₃ concentrations.     

Optical properties of mechanically compressed K<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The effect of a uniaxial mechanical compression (σ_m ≤ 100 bar) on the spectral dependences (300–800 nm) of the birefringence Δn _i and refractive indices n _i of K₂SO₄ crystals is studied. The electronic polarizabilities, refractions, and parameters (λ_0i , B _1i ) of ultraviolet oscillators of mechanically compressed crystals are calculated. It is shown that the dispersions of Δn _i(λ) and n _i(λ) are normal and sharply increase near the absorption edge. It is found that the uniaxial compression changes the value of the dispersions dΔn _i/dλ and dn _i/dλ rather than their character. It is ascertained that the simultaneous action of the compressions σ_x and σ_z, as well as of σ_y and σ_z, leads to the appearance of new isotropic states in the K₂SO₄ crystal, which manifests itself in the equality of corresponding birefringences. It is shown that the baric dependences n _i(σ) are determined by the change in the density of oscillators (～30%), by the shift of the absorption edge and effective band maximum and by the change in the oscillator strength (～70%).     

Preparation of RE<Subscript>2</Subscript>O<Subscript>2</Subscript>SO<Subscript>4</Subscript> (RE=La,Pr-Lu) microspheres from rare-earth-based infinite coordination polymers

Rare-earth-based infinite coordination polymer (RE-ICP) spheres with diameters ranging from 50 nm to 2 μm have been prepared using meso-2,3-dimercaptosuccinic acid (DMSA) as ligand under hydrothermal conditions. RE₂O₂SO₄ microspheres with similar morphology were obtained by calcining the corresponding RE-ICP spheres. However, as for Ce-ICP and Sc-ICP, CeO₂ and Sc₂O₃ were obtained. The products were characterized using X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, infrared spectroscopy, laser Raman spectrometry, and energy-dispersive X-ray spectrum. Elemental analysis and inductive coupled plasma atomic emission spectrometer were adopted to study the composition of the Eu-ICP. To explore their potential applications, several samples of the products were selected and their properties were investigated. The Eu-ICP and Eu₂O₂SO₄ microspheres give strong red emissions when excited with a 394-nm ultraviolet light. Furthermore, the Eu-ICP displays a high selectivity for Fe(III). The obtained CeO₂ has a strong absorption in the UV region and the Gd₂O₂SO₄ microspheres show paramagnetic behavior.

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Graphical abstract A series of RE₂O₂SO₄ microspheres were prepared using a coordination polymer precursor method.

     

Effect of uniaxial pressure on the infrared spectra of (NH<Subscript>4</Subscript>)<Subscript>2</Subscript>SO<Subscript>4</Subscript> crystals

The infrared reflectance spectra of a mechanically free or uniaxial-pressure-confined (NH₄)₂SO₄ crystal were studied for the first time in the spectral range 800–1700 cm^?1 in three crystallographic directions. Using the Kramers-Kronig relations, the dispersion and pressure dependences of the following quantities are obtained: the index of refraction n, the real (?₁) and imaginary (?₂) parts of the permittivity, the frequencies of longitudinal (ω_LO) and transverse (ω_TO) optical vibrations, the damping constant γ, and the oscillator strength f of the mechanically free or clamped (NH₄)₂SO₄ crystal. A considerable change in the main reflection bands with pressure was observed, which is due to the effect of uniaxial pressure on the NH₄ and SO₄ tetrahedral frames.     

Synthesis of nanocrystalline SnO<Subscript>2</Subscript> in supercritical water

Nanocrystalline SnO₂ was synthesized in supercritical water at 385–415°C and 30 MPa (38–106 s residence time) in a tubular flow reactor from an aqueous solution of 0.1–0.4 M SnCl₄. The conversion rate was between 53 and 81%, but increased to 97.8% when 0.1 M NaOH was added. Nanoparticles were analyzed by a series of independent analytical techniques, including TEM, Raman, XRD, SEM, EDX and FT-IR. The initial size of the particles was about 3.7 nm. After calcination at 450°C for 2 h, the particle size increased to 4 nm. The particles were of low crystallinity, as indicated by the weak Raman and XRD signals. All particles were composed of Sn and O, as verified by the EDX spectra. The crystals were tetragonal, as confirmed by the weak XRD spectrum. After calcination at 600°C for 10 h, the particle size increased to 9 nm, while high crystallinity was confirmed by Raman and XRD analyses. All the crystals had the same structure, as indicated by TEM electron diffraction patterns. Using this one-step supercritical water process, nanoparticles of SnO₂ can be conveniently produced continuously in a flow reactor in less than 2 min.     

Anomalous behavior of the velocity of hypersonic acoustic phonons in (NH<Subscript>4</Subscript>)<Subscript>3</Subscript> H(SO<Subscript>4</Subscript>)<Subscript>2</Subscript> crystals

The Gross-Pitaevski equation modified through the inclusion of a term accounting for the nonlocality of interatomic interaction was used to demonstrate the occurrence of extremely narrow two-and three-dimensional solitonic states in atomic Bose-Einstein condensates. The estimates of the sizes of these states gave a value of ～ 20–60 nm (atomic “needles” and “bullets”) for lithium atoms. The soliton lifetimes caused by two-and three-particle collisions were estimated. The limiting possibilities of the formation of nanostructures using needles and bullets were compared with the possibilities of other nanolithographic methods.     

Antiferromagnetic interactions in Er-doped SnO<Subscript>2</Subscript> DMS nanoparticles

Diluted magnetic semiconductor (DMS) nanoparticles of Sn_1−x Er _x O₂ (x = 0.0, 0.02, 0.04, and 0.1) were prepared by sol–gel method. The X-ray diffraction patterns showed SnO₂ rutile structure for all samples with no impurity peaks. The decrease in crystallite size with Er concentration was confirmed from TEM measurements (from 12 to 4 nm). The UV–Visible absorption spectra of Er-doped SnO₂ nanoparticles showed blue shift in band gap compared to undoped SnO₂. The electron spin resonance analysis of Er-doped SnO₂ nanoparticles indicate Er³⁺ in a rutile lattice and also decrease in intensity with Er concentration above x = 0.02. Temperature-dependent magnetization studies and the inverse susceptibility curves indicated increased antiferromagnetic interaction with Er concentration.     

Ferromagnetism in Rh-doped SnO<Subscript>2</Subscript> from first-principles calculation

The electronic structures and magnetic properties for Rh-doped SnO₂ crystals have been investigated by density functional theory. The results demonstrate a magnetic moment, which mainly arises from d orbital of Rhodium, of 1.0 μ _B per Rhodium with a little contribution from the Oxygen atoms surrounding it. The Rh-doped SnO₂ system exhibits half-metallic ferromagnetism with high Curie temperature. Several doped configurations calculations show that there are some robust ferromagnetic couplings between these local magnetic moments. The p–d hybridization mechanism is responsible for the predicted ferromagnetism. These results suggest a recipe obtaining promising dilute magnetic semiconductor by doping nonmagnetic elements in SnO₂ matrix.     

Synthesis and characterization of a polyaniline-modified SnO<Subscript>2</Subscript> nanocomposite

Polyaniline-modified tin oxide and tin oxide nanoparticles were synthesized using a solution route technique. The obtained pristine products were characterized with X-ray diffraction, thermogravimetric analysis, scanning electron microscopy, and optical absorption spectroscopy. Thermogravimetric analysis results showed that the polyaniline-modified SnO₂ nanoparticles exhibit higher thermal stability than the SnO₂ nanoparticles. Scanning electron microscopy analysis on the as-synthesized powders showed spherical particle in the range of 50–100 nm.