Synthesis,characterization and gas sensing performance of SnO<Subscript>2</Subscript> thin films prepared by spray pyrolysis

In this work, SnO₂ thin films were deposited onto alumina substrates at 350°C by spray pyrolysis technique. The films were studied after annealing in air at temperatures 550°C, 750°C and 950°C for 30 min. The films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and optical absorption spectroscopy technique. The grain size was observed to increase with the increase in annealing temperature. Absorbance spectra were taken to examine the optical properties and bandgap energy was observed to decrease with the increase in annealing temperature. These films were tested in various gases at different operating temperatures ranging from 50–450°C. The film showed maximum sensitivity to H ₂S gas. The H₂S sensing properties of the SnO₂ films were investigated with different annealing temperatures and H ₂S gas concentrations. It was found that the annealing temperature significantly affects the sensitivity of the SnO₂ to the H ₂S. The sensitivity was found to be maximum for the film annealed at temperature 950°C at an operating temperature of 100°C. The quick response and fast recovery are the main features of this film. The effect of annealing temperature on the optical, structural, morphological and gas sensing properties of the films were studied and discussed.     

UV-enhanced ethanol-sensing properties of TiO<Subscript>2</Subscript>-decorated ZnSnO<Subscript>3</Subscript> hollow microcubes at low temperature

This work demonstrates the combined effect of nano-TiO₂ decoration and UV radiation on the ethanol sensing of ZnSnO₃ hollow microcubes. TiO₂-decorated ZnSnO₃ samples were synthesized by a facile co-precipitation method, followed by the calcination treatment of precursor–dehydration. The morphology and structure of the as-prepared samples were further characterized by XRD, EDS, SEM and TG analysis. Obtained samples were fabricated to thick film gas sensors for ethanol detection. Sensing tests showed a strong influence of TiO₂ decoration on the sensing properties. Light excitation by means of a low-powered UV source (365 nm) has been used to reinforce the sensor performance especially at lower operating temperature. The optimal sensor based on TiO₂-decorated ZnSnO₃ showed promising performance towards ethanol, ensuring high response and fast dynamics at operating temperature as low as 80?°C.     

Structural,optical and excellent humidity sensing behaviour of ZnSnO<Subscript>3</Subscript> nanoparticles: effect of annealing

The chemi-resistive humidity sensing behaviour of as prepared and annealed fcc-ZnSnO₃ nanoparticles synthesized using wet chemical synthesis method is reported here. The effect of annealing on the evolution of varied nano-morphology of ZnSnO₃ is in accordance to Ostwald’s ripening law. The optical energy bandgap energy change from 4.64 to 3.84 eV for annealed samples confirms the role of annealing over improved sensing performance. At room temperature, an excellent humidity sensitivity of 4155% and response/recovery time of 19/22 s. is observed for 500 °C annealed ZnSnO₃ sample within 08–97% relative humidity range. The experimental data observed over the entire range of RH values well fitted with the Freundlich adsorption isotherm model, and revealing two distinct water adsorption regimes. This indicates that with an increase in annealing temperature the samples show improved adsorption capacity and strength. The excellent humidity sensitivity observed in the annealed nanostructures is attributed to Grotthuss mechanism considering the availability and distribution of available adsorption sites. This present result proposes utilization of low cost synthesis technique of ZnSnO₃ holds the promising capabilities as a potential candidate for the fabrication of next generation humidity sensors.     

Controllable synthesis of Ho-doped In<Subscript>2</Subscript>O<Subscript>3</Subscript> porous nanotubes by electrospinning and their application as an ethanol gas sensor

Pure and Ho-doped In₂O₃ nanotubes (NTs) and porous nanotubes (PNTs) were successfully synthesized by conventional electrospinning process and the following calcination at different temperatures. X-ray diffractometry (XRD), thermogravimetric analysis (TGA), Raman spectrometer, energy-dispersive spectroscopy, scanning and transmission electron microscopy were carefully used to investigate the morphologies, structures and chemical compositions of these samples. Their sensing properties toward ethanol gas were studied. Compared with pure In₂O₃ NTs (response value is 17), pure In₂O₃ PNTs (response value is 20) demonstrated enhanced sensing characteristics. What’s more, the response of Ho-doped In₂O₃ PNTs sensors to 100 ppm ethanol was up to 60 at 240 °C, which increased three times more than that of the pure In₂O₃ PNTs. Additionally, the minimum concentration for ethanol was 200 ppb (response value is 2). The increased gas-sensing ability was attributed not only to the hollow and porous structure, but to the Ho dopant. Furthermore, Ho-doped In₂O₃ PNTs enable sensor to discriminate between ethanol and the other gas distinctly, particularly acetone that is usually indistinguishable from ethanol. Also, by analyzing XRD, TGA and Raman spectrometer, a possible formation mechanism of porous nanotubes and sensing mechanism were put forward.     

Ethanol vapour sensing properties of screen printed WO<Subscript>3</Subscript> thick films

This paper presents ethanol vapour sensing properties of WO₃ thick films. In this work, the WO₃ thick films were prepared by standard screen-printing method. These films were characterized by X-ray diffraction (XRD) measurements and scanning electron microscopy (SEM). The ethanol vapour sensing properties of these thick films were investigated at different operating temperatures and ethanol vapour concentrations. The WO₃ thick films exhibit excellent ethanol vapour sensing properties with a maximum sensitivity of ∼1424.6% at 400°C in air atmosphere with fast response and recovery time.     

Ultrasonic disintegration of tungsten trioxide pseudomorphs after ammonium paratungstate as a route for stable aqueous sols of nanocrystalline WO<Subscript>3</Subscript>

A new facile method is proposed for the preparation of aqueous sols of highly crystalline tungsten trioxide with a particle size of 60–150 nm, containing no organic stabilizers/surfactants. These sols possess high sedimentation stability, which is quite unusual for inorganic colloidal systems containing relatively large particles, with a rather high density (ρ _c(WO₃) = 7.3 g/cm³). The method is based on the thermal decomposition of ammonium paratungstate, followed by dispersing the resulting powders in water under ultrasonic treatment. Thermal decomposition of ammonium paratungstate, and the composition and structure of the resulting tungsten trioxide and its aqueous dispersions, were investigated with thermal analysis combined with the mass spectrometry of gaseous thermolysis products, powder X-ray diffraction, scanning electron microscopy, low-temperature nitrogen adsorption, IR spectroscopy and dynamic light scattering. It has been demonstrated that the high sedimentation stability of WO₃ results from electrostatic stabilization, which might be caused by the formation of tungstic acid on the surface of WO₃ particles when they come into contact with water. The nanocrystalline WO₃ obtained can be used to produce gas sensors for ammonia.     

Optimized Sol–Gel Chemical Route Using Vacuum Suction for Fabrication of Densely Packed NiFe<Subscript>2</Subscript>O<Subscript>4</Subscript> Nanowires

Highly ordered nanowire arrays of NiFe₂O₄ spinel with a high aspect ratio were fabricated by sol–gel method associated with anodic aluminum oxide (AAO) templates. The preparation of nanowires was carried out by sol–gel method using nickel nitrate, ferric nitrate and citric acid. The molar proportion of nickel nitrate to ferric nitrate and citric acid was 1:2:3. The suction with 0.1 mbar vacuum was used to draw the gel into the AAO nanochannels. The results showed that the lowest annealing temperature is around 600?°C to obtain the single-phase nanostructured NiFe₂O₄. The NiFe₂O₄ nanowires were also uniform and parallel. TEM pictures determined the diameter size of the nanowires of about 100 nm. The magnetic results also showed that the wires have an easy axis of magnetization along their length and they are fully saturated in a field of 7 kOe. It seems that this material could be a good candidate for high perpendicular magnetic storage devices.     

Highly sensitive and rapidly responding room-temperature NO<Subscript>2</Subscript> gas sensors based on WO<Subscript>3</Subscript> nanorods/sulfonated graphene nanocomposites

Metal oxide/graphene nanocomposites are emerging as promising materials for developing room-temperature gas sensors. However, the unsatisfactory performances owing to the relatively low sensitivity, slow response, and recovery kinetics limit their applications. Herein, a highly sensitive and rapidly responding room-temperature NO₂ gas sensor based on WO₃ nanorods/sulfonated reduced graphene oxide (S-rGO) was prepared via a simple and cost-effective hydrothermal method. The optimal sensor response of the WO₃/S-rGO sensor toward 20 ppm NO₂ is 149% in 6 s, which is 4.7 times higher and 100 times faster than that of the corresponding WO₃/rGO sensors. In addition, the sensor exhibits excellent reproducibility, selectivity, and extremely fast recovery kinetics. The mechanism of the WO₃/S-rGO nanocomposite gas sensor is investigated in detail. In addition to the high transport capability of S-rGO as well as its excellent NO₂ adsorption ability, the superior sensing performance of the S-rGO/WO₃ sensor can be attributed to the favorable charge transfer occurring at the S-rGO/WO₃ interfaces. We believe that the strategy of compositing a metal oxide with functionalized graphene provides a new insight for the future development of room-temperature gas sensors.

Open image in new window

     

Structural,morphological, optical and gas sensing properties of pure and Ce doped SnO<Subscript>2</Subscript> thin films prepared by jet nebulizer spray pyrolysis (JNSP) technique

Pure and cerium (Ce) doped tin oxide (SnO₂) thin films are prepared on glass substrates by jet nebulizer spray pyrolysis technique at 450 °C. The synthesized films are characterized by X-ray diffraction (XRD), scanning electron microscopy, energy dispersive analysis X-ray, ultra violet visible spectrometer (UV–Vis) and stylus profilometer. Crystalline structure, crystallite size, lattice parameters, texture coefficient and stacking fault of the SnO₂ thin films have been determined using X-ray diffractometer. The XRD results indicate that the films are grown with (110) plane preferred orientation. The surface morphology, elemental analysis and film thickness of the SnO₂ films are analyzed and discussed. Optical band gap energy are calculated with transmittance data obtained from UV–Visible spectra. Optical characterization reveals that the band gap energy is found decreased from 3.49 to 2.68 eV. Pure and Ce doped SnO₂ thin film gas sensors are fabricated and their gas sensing properties are tested for various gases maintained at different temperature between 150 and 250 °C. The 10 wt% Ce doped SnO₂ sensor shows good selectivity towards ethanol (at operating temperature 250 °C). The influence of Ce concentration and operating temperature on the sensor performance is discussed. The better sensing ability for ethanol is observed compared with methanol, acetone, ammonia, and 2-methoxy ethanol gases.     

Study of YSZ-based electrochemical sensors with oxide electrodes for high temperature applications

Potentiometric sensors based on yttria stabilized zirconia (YSZ) with WO₃ as sensing electrode were fabricated using either Pt or Au electrodes. The sensors were studied in the temperature range 550–700°C in the presence of different concentrations (300-1000 ppm) of NO₂ and CO in air. The response to NO₂ was very stable with fast response time (20-40 s). The best sensitivity (18.8 mV/decade) using Pt electrodes was observed at 600°C. At the same temperature a cross-sensitivity (-15 mV/decade) to CO gas was also noticed. The response to CO was decreased (-4 mV/decade) using Au electrode. The role played by WO₃ on the sensing electrode was discussed.     

Phase stability,microstructure and high-temperature properties of NbSi<Subscript>2</Subscript>- and TaSi<Subscript>2</Subscript>-oxide conducting ceramic composites

NbSi₂- and TaSi₂-based electroconductive ceramic composites with the addition of 40–70 vol% Al₂O₃ and ZrO₂ particles were fabricated by high-temperature sintering (1400–1600 °C) under argon. Their phase stability, microstructural evolution, oxidation kinetics and electrical properties were studied at high temperatures. The densification of the composites was improved by increasing the oxide phase content and sintering temperature. The interaction of the starting metal disilicides with residual oxygen sources resulted in the formation of the hexagonal-structured 5–3 metal silicide (Nb₅Si₃ and Ta₅Si₃) phases. The increasing sintering temperature and volume percentage of the oxide phase reduced the pest oxidation, particularly for the silicide–alumina composites, which exhibited lower oxidation-induced mass changes than their dense monolithic metal silicides. Depending on the silicide–oxide volume percentage, their electrical conductivities ranged from 5.3 to 111.3 S/cm at 900 °C. Their phase stability, reduced oxidation rates and high electrical conductivities at high temperatures show promise for future high-temperature applications in advanced sensing.     

Hydrothermal synthesis of NaLa(WO<Subscript>4</Subscript>)<Subscript>2</Subscript>:Eu<Superscript>3+</Superscript> octahedrons and tunable luminescence by changing Eu<Superscript>3+</Superscript> concentration and excitation wavelength

NaLa(WO₄)₂:Eu³⁺ phosphors with different Eu³⁺ concentrations have been synthesized by a hydrothermal method. The phase is confirmed by XRD analysis, which shows a pure-phase NaLa(WO₄)₂ XRD pattern for all of NaLa(WO₄)₂:Eu³⁺ phosphors. The SEM and TEM images indicate that all of NaLa(WO₄)₂:Eu³⁺ phosphors have a octahedral morphology. These suggest that the Eu³⁺ doping has no influence on the structure and growth of NaLa(WO₄)₄ particles. By monitoring the emission of Eu³⁺ at 615 nm, NaLa(WO₄)₂:Eu³⁺ phosphors show excitation bands originating from both host and Eu³⁺ ions. Under the excitation at 271 nm corresponding to WO₄ ^2? groups, emission bands coming from the ¹A₁ → ³T₁ transition with the WO₄ ^2? groups and the ⁵D₀ → ⁷F_j (j = 0, 1, 2, 3 and 4) transitions of Eu³⁺ are observed. The emission intensity relating to WO₄ ^2? groups decreases with increasing Eu³⁺ concentration. But emission intensities of Eu³⁺ increase firstly and then decreases because of concentration quenching effect. Under the excitation at 395 nm corresponding to ⁷F₀ → ⁵L₆ transition of Eu³⁺, only characteristic Eu³⁺ emission bands can be observed. The results of this work suggest that tunable luminescence can be obtained for Eu³⁺ doped NaLa(WO₄)₂ phosphors by changing Eu³⁺ concentration and excitation wavelength.     

Synthesis of WO<Subscript>3</Subscript>/mesoporous ZrO<Subscript>2</Subscript> catalyst as a high-efficiency catalyst for catalytic oxidation of dibenzothiophene in diesel

The catalyst of WO₃ highly dispersed on mesoporous ZrO₂ was prepared by hydrothermal treatment and calcination. Ionic liquid templates with different cations and various calcination temperatures were taken into account to investigate factors that affect the structure of the catalysts. Wide-angle XRD, Raman, low-angle XRD, N₂ adsorption–desorption, SEM, EDS-mapping and TEM analyses were employed to characterize the composition, morphology and the dispersion of WO₃. It was found that [C₁₆H₃₃N(CH₃)₃]₄SiW₁₂O₄₀ and 700 °C were the optimal ionic liquid template and calcination temperature; the obtained catalyst, expressed as 700-C₁₆-WO₃/ZrO₂, exhibited the best catalytic activity in oxidation desulfurization. Dibenzothiophene (DBT) in the model oil could be oxidized to DBT sulfone (DBTO₂) completely, certified by GC-MS analysis, under optimum conditions. The oxidative desulfurization efficiencies of different substrates catalyze by 700-C₁₆-WO₃/ZrO₂, and the cycle performance on the oxidation of DBT were studied.     

The synthesis and characterization of Cu<Subscript>2</Subscript>ZnSnS<Subscript>4</Subscript> thin films from melt reactions using xanthate precursors

Kesterite, Cu₂ZnSnS₄ (CZTS), is a promising absorber layer for use in photovoltaic cells. We report the use of copper, zinc and tin xanthates in melt reactions to produce Cu₂ZnSnS₄ (CZTS) thin films. The phase of the as-produced CZTS is dependent on decomposition temperature. X-ray diffraction patterns and Raman spectra show that films annealed between 375 and 475 °C are tetragonal, while at temperatures <375 °C hexagonal material was obtained. The electrical parameters of the CZTS films have also been determined. The conduction of all films was p-type, while the other parameters differ for the hexagonal and tetragonal materials: resistivity (27.1 vs 1.23 Ω cm), carrier concentration (2.65 × 10⁺¹⁵ vs 4.55 × 10⁺¹⁷ cm^?3) and mobility (87.1 vs 11.1 cm² V^?1 s^?1). The Hall coefficients were 2.36 × 10³ versus 13.7 cm³ C^?1.     

Rietveld analysis of CaCu<Subscript>3</Subscript>Ti<Subscript>4</Subscript>O<Subscript>12</Subscript> thin films obtained by RF-sputtering

Calcium copper titanate, CaCu₃Ti₄O₁₂, CCTO, thin films with polycrystalline nature have been deposited by RF sputtering on Pt/Ti/SiO₂/Si (100) substrates at a room temperature followed by annealing at 600 °C for 2 h in a conventional furnace. The crystalline structure and the surface morphology of the films were markedly affected by the growth conditions. Rietveld analysis reveal a CCTO film with 100 % pure perovskite belonging to a space group Im3 and pseudo-cubic structure. The XPS spectroscopy reveal that the in a reducing N₂ atmosphere a lower Cu/Ca and Ti/Ca ratio were detected, while the O₂ treatment led to an excess of Cu, due to Cu segregation of the surface forming copper oxide crystals. The film present frequency -independent dielectric properties in the temperature range evaluated, which is similar to those properties obtained in single-crystal or epitaxial thin films. The room temperature dielectric constant of the 600-nm-thick CCTO films annealed at 600 °C at 1 kHz was found to be 70. The leakage current of the MFS capacitor structure was governed by the Schottky barrier conduction mechanism and the leakage current density was lower than 10^?7 A/cm² at a 1.0 V. The current–voltage measurements on MFS capacitors established good switching characteristics.     

Enhanced electrochemical reduction of hydrogen peroxide by Co<Subscript>3</Subscript>O<Subscript>4</Subscript> nanowire electrode

Crystalline Co₃O₄ nanowire arrays with different morphologies grown on Ni foam were investigated by varying the reaction temperature, the concentration of precursors, and reaction time. The Co₃O₄ nanowires synthesized under typical reaction condition had a diameter range of approximately 500–900 nm with a length of 17 µm. Electrochemical reduction of hydrogen peroxide (H₂O₂) of the optimized Co₃O₄ nanowire electrode was studied by cyclic voltammetry. A high current density of 101.8 mA cm^?2 was obtained at ?0.4 V in a solution of 0.4 M H₂O₂ and 3.0 M NaOH at room temperature compared to 85.8 mA cm^?2 at ?0.35 V of the Co₃O₄ nanoparticle electrode. Results clearly indicated that the Ni foam supported Co₃O₄ nanowire electrode exhibited superior catalytic activity and mass transport kinetics for H₂O₂ electrochemical reduction.     

Influence of Chromium Doping on Electrical and Magnetic Behavior of Nd<Subscript>0.5</Subscript>Sr<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> System

With a view to understand the influence of chromium doping at the Mn site on the electrical and magnetic behavior of the Nd_0.5Sr_0.5MnO₃ manganite system, a series of samples were prepared by the citrate sol–gel route method. The samples were characterized structurally by XRD. A systematic investigation of electrical resistivity over a temperature range 5–300 K was carried out mainly to understand the magneto-transport behavior in these materials. Studies on the variation of magnetization with temperature over a temperature range 80–330 K were undertaken. Investigation of magnetization at different magnetic fields at two different temperatures, viz. 80 and 300 K, was also carried out. The results show that chromium doping gave typical electrical and magnetic properties. It has been concluded that the coexistence of charge ordered and ferromagnetic phases induced by chromium doping plays an important role in the low-temperature behavior of the system.     

Towards understanding the morphological,magnetic, optical and electrical properties of MnO<Subscript>2</Subscript> nanowires for magneto-and optoelectronic applications

Mixed solutions of manganese sulfate and potassium permanganate were hydrothermally used to synthesis α-MnO₂ nanowires. Energy dispersive analysis of X-ray revealed the formation of stoichiometric MnO₂ and X-ray diffraction revealed the formation of single crystalline α-MnO₂. Examinations using both SEM and TEM yielded nanowires with diameters ranged from 40 to 50 nm and lengths ranged from 5 to 6 µm. The magnetic properties revealed that the α-MnO₂ NWs exhibit ferromagnetic/antiferromagnetic characteristics at liquid nitrogen and room temperatures. This kind of materials may be find usage in electromagnetic shielding applications. Direct optical band gap of 2.6 eV, that was blue shifted from the bulk value, was reported. Photoluminescence examinations displayed a strong emission peak around 395 nm and broad peak around 477 nm. The α-MnO₂ NWs showed a semiconducting behavior where the resistivity decreased with increasing temperature. Two activation energies were reported; the low temperature activation energy was 0.089 eV and the high temperature activation energy was 0.782 eV. Based on the observed high intensity UV emission peak, the obtained α-MnO₂ NWs may find applications in UV light emitting diodes.     

Rutile TiO<Subscript>2</Subscript> films as electron transport layer in inverted organic solar cell

Titanium dioxide (TiO₂) thin films were prepared by sol–gel spin coating method and deposited on ITO-coated glass substrates. The effects of different heat treatment annealing temperatures on the phase composition of TiO₂ films and its effect on the optical band gap, morphological, structural as well as using these layers in P3HT:PCBM-based organic solar cell were examined. The results show the presence of rutile phases in the TiO₂ films which were heat-treated for 2 h at different temperatures (200, 300, 400, 500 and 600 °C). The optical properties of the TiO₂ films have altered by temperature with a slight decrease in the transmittance intensity in the visible region with increasing the temperature. The optical band gap values were found to be in the range of 3.28–3.59 eV for the forbidden direct electronic transition and 3.40–3.79 eV for the allowed direct transition. TiO₂ layers were used as electron transport layer in inverted organic solar cells and resulted in a power conversion efficiency of 1.59% with short circuit current density of 6.64 mA cm^?2 for TiO₂ layer heat-treated at 600 °C.     

Fabrication and characterization of nanostructured (Sn–Ti)O<Subscript>2</Subscript> pellets and films for liquefied petroleum gas sensing

Present paper reports the synthesis of nanostructured (Sn–Ti)O₂ via physicochemical method, its characterization and performance as liquefied petroleum gas (LPG) sensor. The synthesized material was characterized using XRD that confirmed the formation of (Sn–Ti)O₂ nanocomposite. Minimum crystallite size was found as 7 nm. The material was also investigated through SEM, DSC, FTIR, PL and UV–Vis spectrophotometer. Further, the pellet, thick and thin films were fabricated for the sensing analysis. Pellets (9 mm diameter, 4 mm thickness) of (Sn–Ti)O₂ nanocomposite were made by hydraulic pressing machine by applying uniaxial pressure of 616 MPa, thick films (thickness ~2 µm) were made by screen printing technique and thin films were prepared using a Photo resist spinner unit. Further at room temperature, the pellet and films were exposed to LPG in a gas chamber under controlled conditions at room temperature and variations in resistance with the concentrations of LPG were observed. The maximum value of sensitivity of solid state pellet, thick and thin films based sensors were found 7, 9 and 39 for 5 vol% of LPG, respectively. Sensing characteristics were found to be reproducible, after 6 months of their fabrication, indicating the stability of the sensors.