Study of the synthesis and Cl2 gas-sensing properties of nanometer HNO3-doped SnO2

We focused on the effects of the inorganic acid HNO₃ on the gas-sensing properties of nanometer SnO₂ and prepared the powders via a dissolution-pyrolysis method. Furthermore, the powders were characterized by means of X-ray diffraction (XRD), scanning electron microscope (SEM), field emission scanning electron microscopy (FESEM) and energy-dispersive X-ray spectra (EDS). Several aspects were surveyed, including the calcining temperature, concentration of nitric acid and the working temperature. The results showed that the gas response of 3 wt% HNO₃-doped SnO₂ powders (calcined at 500 °C) to 10 ppm Cl₂ reached 316.5, at the working temperature 175 °C. Compared with pure SnO₂, appropriate HNO₃ could increase the gas sensitivity to Cl₂ gas more significantly.     

Detection of hydrogen with SnO2-coated ZnO nanorods

SnO₂-coated ZnO nanorods on c-plane sapphire substrates were synthesized by pulsed laser deposition. The thickness of the polycrystalline SnO₂ was ∼10 nm, as determined by high-resolution transmission electron microscopy, while the diameter of the ZnO nanorods was ∼30 nm. The sensitivity of the SnO₂/ZnO structures to hydrogen was tested by depositing Ti/Au Ohmic contacts on a random array of the nanorods and measuring the current at fixed voltage. There was no response to 500 ppm H₂ in N₂ at room temperature, but we obtained a sensitivity of ∼70% at 400 °C. The SnO₂/ZnO structures exhibit drift in their recovery characteristics and for sequential detection of hydrogen, as generally reported for SnO₂ thin film sensors.     

Effects of deposition temperature on the structural and morphological properties of SnO2 films fabricated by pulsed laser deposition

Tin oxide (SnO₂) thin films were grown on Si (1 0 0) substrates using pulsed laser deposition (PLD) in O₂ gas ambient (10 Pa) and at different substrate temperatures (RT, 150, 300 and 400 °C). The influence of the substrate temperature on the structural and morphological properties of the films was investigated using X-ray diffraction (XRD), atomic force microscopy (AFM) and scanning electron microscopy (SEM). XRD measurements showed that the almost amorphous microstructure transformed into a polycrystalline SnO₂ phase. The film deposited at 400 °C has the best crystalline properties, i.e. optimum growth conditions. However, the film grown at 300 °C has minimum average root mean square (RMS) roughness of 3.1 nm with average grain size of 6.958 nm. The thickness of the thin films determined by the ellipsometer data is also presented and discussed.     

Characteristics of electron beam evaporated nanocrystalline SnO2 thin films annealed in air

Tin oxide (SnO₂) thin films (about 200 nm thick) have been deposited by electron beam evaporation followed by annealing in air at 350-550 °C for two hours. Optical, electrical and structural properties were studied as a function of annealing temperature. The as-deposited film is amorphous, while all other annealed films are crystalline (having tetragonal structure). XRD suggest that the films are composed of nanoparticles of 5-10 nm. Raman analysis and optical measurements suggest quantum confinement effects that are enhanced with annealing temperature. For instance, Raman peaks of the as-deposited films are blue-shifted as compared to those for bulk SnO₂. Blue shift becomes more pronounced with annealing temperature. Optical band gap energy of amorphous SnO₂ film is 3.61 eV, which increases to about 4.22 eV after crystallization. Two orders of magnitude decrease in resistivity is observed after annealing at 350-400 °C due to structural ordering and crystallization. The resistivity, however, increases slightly with annealing temperature above 400 °C, possibly due to improvement in stoichiometry and associated decrease in charge carrier density.     

CO sensor derived from mesostructured Au-doped SnO2 thin film

Pure and Au-doped mesostructured SnO₂ thin films were successfully prepared by using non-ionic surfactant Brij-58 (polyoxyethylene acyl ether) as organic template and tin tetrachloride and hydrogen tetrachloroaurate(III) trihydrate as inorganic precursor. Thin films were deposited onto the glass substrates at 450 °C by simple spray pyrolysis technique. The novel mesostructured tin oxide thin films with different Au concentration exhibit highly selective response towards CO. The correlation of the Au incorporation in the mesostructure with particular morphology and gas sensing behavior is discussed using scanning electron microscopy (SEM), X-ray diffraction (XRD), BET surface area and transmission electron microscopy (TEM) studies.     

Structure and photoluminescence properties of epitaxial SnO2 films grown on α-Al2O3 (0 1 2) by MOCVD

SnO₂ thin films have been successfully deposited on α-Al₂O₃ (0 1 2) substrates by metalorganic chemical vapor deposition (MOCVD) in the temperature range 500-700 °C. The films were epitaxially grown in the tetragonal SnO₂ phase and were (1 0 1) oriented. In-plane orientation relationship [0 1 0]SnO₂||[1 0 0]Al₂O₃ and [1 0 1?]SnO₂||[1? 2? 1]Al₂O₃ was determined between the film and substrate. Photoluminescence (PL) spectra measured at room temperature revealed that the film grown at 700 °C showed an intense ultra-violet (UV) PL peak at 333 nm, which was a band-edge emission peak in SnO₂ films. At a temperature of 13 K, a new broad PL band centered at about 480 nm was observed. The corresponding PL mechanisms are discussed in detail.     

Fabrication of porous BaSnO3 hollow architectures using BaCO3@SnO2 core-shell nanorods as precursors

We present a strategy to synthesize porous BaSnO₃ hollow architectures with that were 150-300 nm in diameter and 1.5-5 μm in length using precursor of BaCO₃@SnO₂ nanorods prepared by hydrothermal treatment. BaCO₃@SnO₂ nanorods, consisting of a BaCO₃ core and a SnO₂ shell, could be used effectively for the solid-state synthesis of polycrystalline BaSnO₃ powder at 800 °C (lower than convention for BaCO₃ and SnO₂ mixtures). The core/shell structure of the precursor could play a role as a structural directing template for preparing BaSnO₃ hollow architectures during the calcination process. The X-ray diffractometer (XRD), scanning electron microscope (SEM), and transmission electron microscope (TEM) are employed to characterize the structures and morphologies. When applied to DSSC, the porous BaSnO₃ hollow architectures exhibit distinct photovoltaic effect.     

Microwave-assisted synthesis of SrFe12O19 hexaferrites

Ultra-fine and homogeneous SrFe₁₂O₁₉ hexaferrites were synthesized by a microwave-assisted calcination route. The calcined precursors were prepared by a sol-gel auto-combustion method using Fe(NO₃)₃·9H₂O, Sr(NO₃)₂ and citric acid as starting materials. The structures, powder morphology and magnetic properties of the products were characterized by X-ray diffraction, scanning electron microscope and vibrating sample magnetometer. The results showed that microwaves are helpful to reduce the calcination temperature and shorten the calcination time. The ferrites with saturation magnetization, remanence and intrinsic coercivity of 54.80 emu/g, 29.52 emu/g and 5261 Oe, respectively, were obtained in samples calcined at 800 °C for 80 min.     

Structural and optical properties of SrCu2O2 films deposited on sapphire substrates by pulsed laser deposition

We deposited SrCu₂O₂ (SCO) films on sapphire (Al₂O₃) (0 0 0 1) substrates by pulsed laser deposition. The crystallographic orientation of the SCO thin film showed clear dependence on the growth temperature. X-ray diffraction (XRD) and transmission electron microscopy (TEM) analysis showed that the film deposited at 400 °C was mainly oriented in the SCO [2 0 0] direction, whereas when the growth temperature was increased to 600 °C, the SCO film showed a dominant orientation of SCO [1 1 2]. The SCO film deposited at 500 °C was obvious polycrystalline, showing multi peaks from (2 0 0), (1 1 2), and (2 1 1) diffraction in the XRD spectrum. The SCO film deposited at 600 °C showed a band gap energy of 3.3 eV and transparency up to 80% around 500 nm. The photoluminescence (PL) spectra of the SCO films grown at 500 °C and 600 °C mainly showed blue-green emission, which was attributed to the intra-band transition of the isolated Cu⁺ and Cu⁺–Cu⁺ pairs according to the temperature dependent-PL analysis.     

Au/TiO2/Ru(0 0 0 1) model catalysts and their interaction with CO

Au/TiO₂/Ru(0 0 0 1) model catalysts and their interaction with CO were investigated by scanning tunneling microscopy and different surface spectroscopies. Thin titanium oxide films were prepared by Ti deposition on Ru(0 0 0 1) in an O₂ atmosphere and subsequent annealing in O₂. By optimizing the conditions for deposition and post-treatment, smooth films were obtained either as fully oxidized TiO₂ or as partly reduced TiO_x, depending on the preparation conditions. CO adsorbed molecularly on both oxidized and reduced TiO₂, with slightly stronger bonding on the reduced films. Model catalyst surfaces were prepared by depositing submonolayer quantities of Au on the films and characterized by X-ray photoelectron spectroscopy and scanning tunneling microscopy. From X-ray photoelectron spectroscopy, a weak interaction between the Au and the TiO₂ substrate was found. At 100 K CO adsorption occurred on both the TiO₂ film and on the Au nanoparticles. CO desorbed from the Au particles with activation energies between 53 and 65 kJ/mol, depending on the Au coverage. If the Au deposit was annealed to 770 K prior to CO exposure, the CO adsorption energy decreased significantly. STM measurements revealed that the Au particles grow upon annealing, but are not encapsulated by TiO_x suboxides. The higher CO adsorption energy observed for smaller Au coverages and before annealing is attributed to a significantly stronger interaction of CO with mono- and bilayer Au islands, while for higher particles, the adsorption energy becomes more bulk-like. The implications of these effects on the known particle size effects in CO oxidation over supported Au/TiO₂ catalysts are discussed.     

In-situ bridging of SnO2 nanowires between the electrodes and their NO2 gas sensing characteristics

A simple and efficient way of making highly sensitive SnO₂ nanowire-based gas sensors without an individual lithography process was studied. The SnO₂ nanowires network was floated upon the Si substrate by separating the Au catalyst layer from the substrate. As the electric current is transported along the networks of the nanowires, not along the surface layer on the substrate, the gas sensitivities could be maximized in this networked and floated structures. The sensitivity was 5-30 when the NO₂ concentration was 1-10 ppm. The response time was ca. 20-60 s.     

Synthesis of SnO2/ZnO composite nanofibers by electrospinning method and study of its ethanol sensing properties

In this paper, we report the synthesis of SnO₂/ZnO composite nanofibers via electrospinning method. Polyvinyl alcohol (PVA)/zinc acetate/stannous chloride nanofibers were electrospun using a solution containing PVA, zinc acetate and stannous chloride in distilled-water followed by calcination at 650 °C for 2 h, obtaining the related composite nanofibers. The nanofibers were characterized by simultaneous thermal analysis (STA), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDX) and Fourier Transform Infrared spectroscopy (FTIR). Ethanol sensing of the nanofibers showed a good and desirable sensing behavior such as high sensitivity, quick response and recovery times.     

Fabrication of textured SnO2:F thin films by spray pyrolysis

Transparent conductive SnO₂:F thin films with textured surfaces were fabricated on soda-lime-silica glass substrates by spray pyrolysis. Structure, morphology, optical and electrical properties of the films were investigated. Results show that the film structure, morphology, haze, transmittance and sheet resistance are dependent on the substrate temperature and film thickness. An optimal 810 nm-thick SnO₂:F film with textured surface deposited at 520 °C exhibits polycrystalline rutile tetragonal structure with a (2 0 0) orientation. The sheet resistance, average transmittance in visible region, and haze of this film were 8 Ω/□, 80.04% and 11.07%, respectively, which are suitable for the electrode used in the hydrogenated amorphous silicon solar cells.     

Anatase TiO2 films based CO gas sensor: Film thickness, substrate and temperature effects

Electrical response of anatase TiO₂ films is discussed for different concentrations of CO gas (20-100 ppm) in a nitrogen gas ambient. We investigated temperature (100-300 °C) and film thickness (100-1000 nm) effects for films deposited on glass, sapphire (0 0 0 1) and, Si(1 0 0) substrates. In general, there is a drop in resistance of the device when exposed to CO gas. Films deposited on sapphire showed a larger decrease in the resistance at 300 °C compared to those deposited on glass and silicon substrates. However, films grown on glass and silicon substrates showed a larger decrease in the resistance values for temperature around 200 °C when CO (ppm) values are greater than 40. The change in resistance of the films varies as square root of the CO gas concentration at 200 °C for films deposited on all the three substrates. In general, the decrease in resistance for thicker films is large compared to thinner ones, indicating participation of the bulk in the detection of CO gas. In the presence of O₂, film loses its sensitivity to CO gas when the concentration level of O₂ approaches 1800 ppm.     

Hydrothermal preparation and photoluminescent properties of MgAl2O4: Eu spinel nanocrystals

Nanoplates of the MgAl₂O₄ spinel doped with Eu³⁺ ions were prepared by a microwave assisted hydrothermal method. Structural properties of the precursor calcined at 700 and 1000 ^oC powders were characterized by X-ray diffraction (XRD) and transmission electron microscopy (TEM). According to the obtained XRD patterns the formation of single-phase spinels after calcination was confirmed. The average spinel particle size was determined to be 11 nm after calcination at 700 °C and it increased up to 14 nm after calcination at 1000 °C. The photoluminescent properties of prepared powders with different Eu³⁺ ion concentrations (0-5% mol) were investigated using excitation and emission spectroscopy at room and low temperatures (77 K).     

Rapid growth of nanocrystalline CuInS2 thin films in alkaline medium at room temperature

Layer-by-layer (LbL) deposition of CuInS₂ (CIS) thin films at room temperature (25 °C) from alkaline CuSO₄ + In₂(SO₄)₃ and Na₂S precursor solutions was reported. The method allowed self-limited growth of CIS films with nanocrystalline structure and composed of densely packed nanometer-sized grains. The as-deposited CIS film was 250 nm thick and composed of closely packed particles of 20-30 nm in diameter. The alkaline cationic precursor solution was obtained by dissolving CuSO₄ and InSO₄ in deionized water with a appropriate amount of hydrazine monohydrate (H-H) and 2,2′,2″-nitrilotriethanol (TEA). CIS films were annealed at 200 °C for 2 h and effect of annealing on structural, optical, and surface morphological properties was thoroughly investigated by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-vis spectrometer, C-V, and water contact angle techniques, respectively.     

ZrB2-based Ohmic contacts to p-GaN

The annealing temperature dependence of contact resistance and layer stability of ZrB₂/Ti/Au and Ni/Au/ZrB₂/Ti/Au Ohmic contacts on p-GaN is reported. The as-deposited contacts are rectifying and transition to Ohmic behavior for annealing at ≥750 °C, a significant improvement in thermal stability compared to the conventional Ni/Au Ohmic contact on p-GaN, which is stable only to <600 °C. A minimum specific contact resistance of ∼2 × 10⁻³ Ω cm⁻² was obtained for the ZrB₂/Ti/Au after annealing at 800 °C while for Ni/Au/ZrB₂/Ti/Au the minimum value was 10⁻⁴ Ω cm⁻² at 900 °C. Auger Electron Spectroscopy profiling showed significant Ti, Ni and Zr out diffusion at 750 °C in the Ni/Au/ZrB₂/Ti/Au while the Ti and Zr intermix at 900 °C in the ZrB₂/Ti/Au. These boride-based contacts show promise for contacts to p-GaN in high temperature applications.     

High-pressure study on the adsorption and oxidation of CO on gold/titania model catalysts

The results of an IR study on the interaction of CO/O₂ gas mixtures with planar Au/TiO₂ model catalysts at elevated pressures and at room temperature are presented. The model catalysts were prepared by deposition of a flat titania film on a Ru(0 0 0 1) substrate and subsequent evaporation of gold on the titania film. In the presence of the gas mixtures, an IR band in the CO stretching region was formed, pointing to linearly adsorbed CO. The position of this band is nearly independent of the Au coverage employed. Compared to pure CO, the IR band is shifted to higher wave numbers when CO/O₂ gas mixtures are used. Although the production of CO₂ was detected in the CO oxidation reaction on the model catalysts, the formation of other IR bands, revealing the build-up of carbonates or other side-products which is usually observed for Au/TiO₂ real powder catalysts, was very weak.     

Effect of growth temperature on the morphology and bonded states of SnO2 nanobaskets

The nanobaskets of SnO₂ were grown on in-house fabricated anodized aluminum oxide pores of 80 nm diameter using plasma enhanced chemical vapor deposition at an RF power of 60 W. Hydrated stannic chloride was used as a precursor and O₂ (20 sccm) as a reactant gas. The deposition was carried out from 350 to 500 °C at a pressure of 0.2 Torr for 15 min each. Deposition at 450 °C results in highly crystalline film with basket like (nanosized) structure. Further increase in the growth temperature (500 °C) results in the deterioration of the basket like structure and collapse of the alumina pores. The grown film is of tetragonal rutile structure grown along the [1 1 0] direction. The change in the film composition and bonded states with growth temperature was evident by the changes in the photoelectron peak intensities of the various constituents. In case of the film grown at 450 °C, Sn 3d_5/2 is found built up of Sn⁴⁺ and O-Sn⁴⁺ and the peaks corresponding to Sn²⁺ and O-Sn²⁺ were not detected.     

EPR study of nitrogen-doped mesoporous TiO2 powders

Mesoporous titanium dioxide powders were synthesized by the sol-gel route either directly or using the P123 templating agent and nitrogen doping from thiourea. X-ray diffraction studies show that doping and structuring give rise to rutile growth at a relatively low temperature of 400 °C compared to the transition point of pure TiO₂, which is close to 900 °C. Systematic EPR experiments were performed in order to monitor the involved paramagnetic centres with respect to calcination treatments and also to probe the nitrogen doping efficiency in the host media. Several paramagnetic species were found in these mesostructured samples mainly with oxygen radicals for calcinations at 400 °C, while nitrogen centres appear for calcination temperature as high as 500 °C. The effect of material microstructure on the effective incorporation of nitrogen in the TiO₂ lattice is also examined and compared in representative samples.