Liquid flame spray fabrication of WO3-reduced graphene oxide nanocomposites for enhanced O3-sensing performances

WO₃ is one of the inspiring sensing materials that show high response to O₃; an efficient fabrication of WO₃ film with incorporation of complementary additives is essential for enhanced sensitivity. Here we report film deposition by liquid flame spraying, characterization of nanostructured WO₃-reduced graphene oxide (rGO) composites and their gas-sensing activities to O₃. The starting feedstock was prepared from WC_l6 and rGO for pyrolysis synthesis by flame spraying. Nano-porous WO₃-rGO films were successfully fabricated and characterized by transmission electron microscopy, field emission scanning electron microscopy, Raman spectrometry, thermal analyses and X-ray diffraction. Nanosized WO₃ grains exhibited oriented nucleation on rGO flakes whereas rGO retained intact its nano-structural features after spraying. Constrained grain growth of WO₃ of 60–70 nm in size was realized in the rGO-containing films with as compared to ~220 nm in the pure WO₃ film. The WO₃-rGO film sensors showed quicker response to O₃ and faster recovery than rGO-free WO₃ film sensors. Addition of rGO in 1.0 wt% or 3.0 wt% in the films caused a significantly reduced effective working temperature of the film sensors from ~ 250 °C to ~ 150 °C.     

Robust SiO2 gate dielectric thin films prepared through plasma-enhanced atomic layer deposition involving di-sopropylamino silane (DIPAS) and oxygen plasma: Application to amorphous oxide thin film transistors

DIPAS (di-isopropylamino silane, H₃Si[N(C₃H₇)₂]) and O₂ plasma were employed, using plasma-enhanced atomic layer deposition (PEALD), to deposit silicon oxide to function as the gate dielectric at low temperature, i.e., below 200 °C. The superior amorphous SiO₂ thin films were deposited through the self-limiting reactions of atomic layer deposition with a deposition rate of 0.135 nm/cycle between 125 and 200 °C. PEALD-based SiO₂ thin layer films were applied to amorphous oxide thin film transistors constructed from amorphous In-Ga-Zn-O (IGZO) oxide layers, which functioned as channel layers in the bottom-gated thin film transistor (TFT) structure, with the aim of fabricating transparent electronics. The SiO₂ gate dielectric exhibited the highest TFT performance through the fabrication of heavily doped n-type Si substrates, with a saturation mobility of 16.42 cm²/V·s, threshold voltage of 2.95 V and large on/off current ratio of 3.69 × 10⁸. Ultimately, the highly doped Si was combined with the ALD-based SiO₂ gate dielectric layers, leading to a saturation mobility of 16.42 cm²/V·s, threshold voltage of 2.95 V, S-slope of 0.1944, and on/off current ratio of 3.69 × 10⁸. Semi-transparent and transparent TFTs were fabricated and provided saturation mobilities of 22.18 and 24.29 cm²/V·s, threshold voltages of 4.18 and 2.17 V, S-slopes of 0.1944 and 0.1945, and on/off current ratios of 9.63 × 10⁸ and 1.03 × 10⁷, respectively.     

Xanthate sensing properties of Pt-functionalized WO3 microspheres synthesized by one-pot hydrothermal method

WO₃ microspheres in a hierarchical nanorod-assembled architecture were prepared by using a facile one-pot hydrothermal method. The morphology and structure of pure, and 1, 3, and 5 mol% Pt-functionalized WO₃ microspheres were characterized by means of SEM, TEM, XRD, XPS and FTIR measurements. Structural characterizations demonstrated that these WO₃ microspheres assembled by numerous one-dimensional WO₃ nanorods were approximately 2–5 µm in diameter. The nanorods with their diameters in the range of 70–90 nm showed a single crystal hexagonal structure. Gas sensors based on pure and Pt-functionalized WO₃ microspheres showed reversible response and outstanding selectivity to xanthate gas at the operating temperature range of 75–175 °C. The sensor response increased with the increase of xanthate gas concentration. The highest response of 102.7 was obtained for the sensor based on 3 mol% Pt-functionalized WO₃ microspheres to 100 ppm xanthate gas at an operating temperature of 100 °C, which could be ascribed to the large effective surface area and high porosity of WO₃ microspheres as well as the catalytic effect of Pt nanoparticles.     

Microstructural characterization and crystallization behaviour of (1 − x)TeO2–xWO3 (x = 0.15, 0.25, 0.3 mol) glasses

DTA, XRD and SEM investigations were conducted on the (1 − x)TeO₂–xWO₃ glasses (where x = 0.15, 0.25 and 0.3). Whereas the 0.75TeO₂–0.25WO₃ and 0.7TeO₂–0.3WO₃ glasses show no exothermic peaks, an indication of no crystallization in their glassy matrices, two crystallization peaks were observed on the DTA plot of the 0.85TeO₂–0.15WO₃ glass. On the basis of the XRD measurements of the 0.85TeO₂–0.15WO₃ glass samples heated to 510 °C and 550 °C (above the peak crystallization temperatures), α-TeO₂ (paratellurite), γ-TeO₂ and WO₃ phases were detected in the sample heated to 510 °C and the α-TeO₂ and WO₃ phases were present in the sample heated to 550 °C. SEM micrographs taken from the 0.85TeO₂–0.15WO₃ glass heated to 510 °C showed that centrosymmetrical crystals were formed as a result of surface crystallization and were between 3 μm and 15 μm in width and 12 μm and 30 μm in length. On the other hand, SEM investigations of the 0.85TeO₂–0.15WO₃ glass heated to 550 °C revealed the evidence of bulk massive crystallization resulting in lamellar crystals between 1 μm and 3 μm in width and 5 μm and 30 μm in length. DTA analyses were carried out at different heating rates and the Avrami constants for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C were calculated as 1.2 and 3.9, respectively. Using the modified Kissinger equation, activation energies for crystallization were determined as 265.5 kJ/mol and 258.6 kJ/mol for the 0.85TeO₂–0.15WO₃ glass heated to 510 °C and 550 °C, respectively.     

Micro-structure of ITO ceramics sintered at different temperatures and its effect on the properties of deposited ITO films

Structural characterizations of two ITO ceramics that were respectively sintered at 1560 °C and 1600 °C were focused on and the results indicate that the lower sintering temperature is good for ITO ceramics to have the triangle fine grains, larger elemental concentration gradients of indium and tin and more content of In₄Sn₃O₁₂ phase which displays the stronger grain orientation growth along the crystallographic direction of [0-11]. ITO films with 100 nm thickness deposited at 25 °C–230 °C were used to investigate the effect of micro-structure on the film properties. Grain orientation growth of In₄Sn₃O₁₂ phase is conductive to form ITO films of columnar structure. Otherwise, uniform micro-structure and higher solubility of SnO₂ in In₂O₃ main phase contribute to deposit ITO films of higher sheet resistance, less thickness uniformity and higher transmittance at 25 °C, smaller etching angle and lower etching rate at 230 °C.     

Decomposition of nonionic surfactant on a nitrogen-doped photocatalyst under visible-light irradiation

A visible-light-active N-containing TiO₂ photocatalysts were prepared from crude amorphous titanium dioxide by heating amorphous TiO₂ in gaseous NH₃ atmosphere. The calcination temperatures ranged from 200 to 1000 °C, respectively. UV–vis/DR spectra indicated that the N-doped catalysts prepared at temperatures <400 °C absorbed only UV light (E_g = 3.3 eV), whereas samples prepared at temperatures ≥400 °C absorbed both, UV (E_g = 3.10–3.31 eV) and vis (E_g = 2.54–2.66 eV) light. The chemical structure of the modified photocatalysts was investigated using FT-IR/DRS spectroscopy. All the spectra exhibited bands indicating nitrogen presence in the catalysts structure. The photocatalytic activity of the investigated catalysts was determined on a basis of a decomposition rate of nonionic surfactant (polyoxyethylenenonylphenol ether, Rokafenol N9). The most photoactive catalysts were those calcinated at 300, 500 and 600 °C. For the catalysts heated at temperatures of 500 and 600 °C Rokafenol N9 removal was equal to 61 and 60%, whereas TOC removal amounted to 40 and 35%, respectively. In case of the catalyst calcinated at 300 °C surfactant was degraded by 54% and TOC was removed by 35%. The phase composition of the most active photocatalysts was as follows: (a) catalyst calcinated at 300 °C—49.1% of amorphous TiO₂, 47.4% of anatase and 3.5% of rutile; (b) catalyst calcinated at 500 °C—7.1% of amorphous TiO₂, 89.4% of anatase and 3.5% of rutile; (c) catalyst calcinated at 600 °C—94.2% of anatase and 5.8% of rutile.     

Microwave dielectric properties of the BaTi5O11 thin film grown on the poly-Si substrate using rf magnetron sputtering

BaTi₅O₁₁ thin films were grown on the poly-Si/SiO₂/Si substrate using rf magnetron sputtering. The BaO-TiO₂ thin film deposited on the poly-Si substrate had an amorphous phase even though the growth temperature was high at 550 °C. The amorphous film was crystallized into the BaTi₅O₁₁ phase when the film was post annealed above 800 °C. The post annealing temperature is one of the most important factors for the formation of the crystalline BaTi₅O₁₁ thin film. The homogeneous BaTi₅O₁₁ thin film was obtained when the film was grown at 550 °C and rapid thermal annealed (RTA) at 900 °C for 3 min. The dielectric constant (ɛ_r) of the BaTi₅O₁₁ film measured at 100 kHz was about 35 and the dissipation factors of all the films were smaller than 4.0%. The dielectric properties of the BaTi₅O₁₁ thin film were also measured at microwave frequencies. For the BaTi₅O₁₁ thin film grown at 550 °C and RTA at 900 °C for 3 min, the ɛ_r of 34–30 and dielectric loss of 0.025 ± 0.005 were obtained at 1–6 GHz.     

Characterization of nickel manganite NTC thermistor films prepared by aerosol deposition at room temperature

NiMn₂O_4+δ thermistor thick films have been successfully deposited by the so-called Aerosol Deposition Method (ADM) at room temperature on alumina substrates, Si-wafers, as well as on special planar four-wire interdigital electrode structures for high-precision electrical characterization. The NTCR films are homogeneous, completely dense and scratch resistant. Both as-deposited and tempered, the NTCR films exhibit a cubic spinel structure. Between 25 °C and 90 °C, the NTCR film resistance behaves as it is typical for variable range hopping (VRH) with parabolic density of states. As a result of moderate film tempering, the thermistor constant B and the specific resistance at 25 °C (ρ₂₅) decrease from 4250 K to 4020 K and 65 Ω·m to 40 Ω·m respectively, and are close to bulk values. In combination with the excellent reproducibility of the ρ₂₅ and B values, AD processing of films appears to be a promising alternative for classical ceramic bulk processes.     

Growth of highly c-axis oriented LaNiO3 films with improved surface morphology on Si substrate using chemical solution deposition and rapid heat treatment process

LNO (LaNiO₃) thin films were directly deposited onto Si substrates with a thin layer of amorphous natural oxide (SiO₂) using three different precursor solutions. Effects of the constitution of precursor solution and the annealing heating rate on the surface morphology and the orientation were investigated. The LNO film derived from the mixture of a methanol solvent and an acetylacetone chelating agent had the flat surface with no cracks and pinholes. The heating rate of rapid annealing process had a critical effect on the oriented growth of the LNO film, and its c-axis orientation degree increased with the annealing heating rate. The LNO film with the heating rate of 40 ℃/s exhibited the highest degree of c-axis orientation (99.57%) and the lowest resistivity (9.35 × 10^?4 Ω cm). It would be a potential bottom electrode and/or seed layer to integrate perovskite-type films on it for functional devices.     

Photoelectrochemical properties of iron (III)-doped TiO2 nanorods

Fe (iron)-doped TiO₂ nanorods were grown on fluorine doped tin oxide (FTO) substrates with various Fe doping concentrations using modified chemical bath deposition (M-CBD). We investigated the effects of Fe doping concentration on the morphological, structural, optical, and photoelectrochemical (PEC) properties of the TiO₂ nanorods. From this study, it was found that the PEC properties were mainly dependent on the morphological and optical properties of the Fe-doped TiO₂ nanorods. At low Fe doping concentration, the PEC properties were highly affected by the optical properties. On the other hand, the PEC properties were significantly affected by the morphological properties at high doping concentration. We observed a maximum photocurrent density of 0.48 mA/cm² at a Fe doping concentration of 2 at% from this study. In addition, the donor density and flat-band potential of the Fe doping concentration from the Mott–Schottky plot were analyzed.     

Electrochromic Nb-doped WO3 films: Effects of post annealing

The Nb-doped WO₃ films were deposited by e-beam co-evaporation method using ceramic WO₃ targets and metal Nb slugs. The films were analyzed by glancing incident angle X-ray diffraction (GIAXRD), UV/visible spectrophotometer, electrochemical cyclic voltammetry, X-ray photoelectron spectroscopy (XPS). The as-prepared film is brown and amorphous in structure. The film has low transmission in optical visible region. The XPS results indicate that the as-deposited film is non-stoichiometric. By applying a negative potential, the as-deposited film does not show obvious electrochromic effect. However, the electrochromic properties of Nb-doped WO₃ films are improved by post annealing treatment at 350, 400, and 450 °C in oxygen atmosphere. The Nb-doped WO₃ films transform into crystalline structure and become transparent after post annealing treatment. The energy band gap, optical modulation, and color efficiency increase with annealing temperature.     

Sintering behaviour and microwave dielectric properties of BaAl2−2x(ZnSi)xSi2O8 ceramics

BaAl_2?2x(ZnSi)_xSi₂O₈ (x = 0.2–1.0) ceramics were prepared using the conventional solid-state reaction method. The sintering behaviour, phase composition and microwave dielectric properties of the prepared compositions were then investigated. All compositions showed a single phase except for x = 0.8. By substituting (Zn_0.5Si_0.5)³⁺ for Al³⁺ ions, the optimal sintering temperatures of the compositions decreased from 1475 °C (x = 0) to 1000 °C (x = 0.8), which then slightly increased to 1100 °C (x = 1.0). Moreover, the phase stability of BaAl₂Si₂O₈ was improved. A novel BaZnSi₃O₈ microwave dielectric ceramic was obtained at the sintering temperature of 1100 °C. This ceramic possesses good microwave dielectric properties with ε_r = 6.60, Q × f = 52401 GHz (at 15.4 GHz) and τ_f = ?24.5 ppm/°C.     

Facile growth of 1‐D nanowire‐based WO3 thin films with enhanced photoelectrochemical performance

A flame reactor embedded with a constant tungsten wire feeding system to prepare one‐dimensional (1‐D) nanostructured tungsten oxide thin film for photoelectrochemical (PEC) water splitting was developed. Photoactive vertically‐aligned nanowire‐based WO₃ thin films could be obtained with a controlled thickness via a flame vapor deposition process followed by air‐annealing. The PEC performances of WO₃ photoelectrodes for different thin film thicknesses were examined. The optimum thickness of WO₃ thin film was found to be about 7.2 μm for PEC water splitting based on incident photon‐to‐current efficiency plots and I–V curves. The WO₃ prepared with optimum thickness showed better PEC performance than those of recently reported nanostructured WO₃ photoanodes. © 2015 American Institute of Chemical Engineers AIChE J, 62: 421–428, 2016     

Electrical conductivity of Ca-substituted lanthanum manganites

The physico-chemical properties of substituted perovskites materials have been analyzed with the aim of studying the relationships between structure and properties in this class of materials. Investigations were carried out into the effect of substitution in lanthanum manganites La_1–xCa_xMnO₃ (x = 0; 0.05; 0.1; 0.15) materials obtained through sol-gel method followed by heat treatment at low temperatures and X-ray diffraction, into the surface area thereof, as well as into transmission electron microscopy for purposes of morpho-structural characterization. The results indicated a well-crystallized Pm-3m perovskite-type structure, and 20 nm average crystallite sizes for all samples. By means of complex impedance measurements in the 20 Hz–2 MHz frequency range, the electrical conductivity was determined at temperatures between 30 °C and 120 °C; the results showed that the conductivity obeys a Jonscher's universal law. It was found that below 10 kHz, the dc component of the conductivity increases with temperature for all samples, indicating that electrical conduction processes are activated thermally, in agreement with Mott's variable-range hopping (VRH) model; the model parameters (hopping distance and hopping energy) were also determined. In the high frequency range (f > 200 kHz) the ac conductivity is attributed to charge carriers hopping between the nearest neighboring states, in agreement with the correlated barrier hopping (CBH) model. Using this model, the energy band gap values: 0.364 eV, 0.372 eV and 0.424 eV of the substituted samples were found.     

Controlling microstructure and film growth of relaxor-ferroelectric thin films for high break-down strength and energy-storage performance

The relaxor ferroelectric Pb_0.9La_0.1(Zr_0.52Ti_0.48)O₃ (PLZT) thin films were deposited using pulsed laser deposition, and their microstructures, break-down field strengths and energy storage performances were investigated as a function of the buffer layer and electrode. A large recoverable energy-storage density (U_reco) of 23.2 J/cm³ and high energy-storage efficiency (η) of 91.6% obtained in the epitaxial PLZT film grown on SrRuO₃/SrTiO₃/Si are much higher than those in the textured PLZT film (U_reco = 21.9 J/cm³, η = 87.8%) on SrRuO₃/Ca₂Nb₃O₁₀-nanosheet/Si and the polycrystalline PLZT film (U_reco = 17.6 J/cm³, η = 82.6%) on Pt/Ti/SiO₂/Si, under the same condition of 1500 kV/cm and 1 kHz, due to the slim polarization loop and significant antiferroelectric-like behavior. Owing to the high break-down strength (BDS) of 2500 kV/cm, a giant U_reco value of 40.2 J/cm³ was obtained for the epitaxial PLZT film, in which U_reco values of 28.4 J/cm³ (at BDS of 2000 kV/cm) and 20.2 J/cm³ (at BDS of 1700 kV/cm), respectively, were obtained in the textured and polycrystalline PLZT films. The excellent fatigue-free properties and high thermal stability were also observed in these films.     

Fabrication of porous Al2O3-based ceramics using ball-shaped powders by preceramic polymer process in N2 atmosphere

Porous Al₂O₃-based ceramics were successfully fabricated using ball-shaped powders by preceramic polymer process in N₂ atmosphere. These results showed that the amorphous Si-O-C ceramics were formed on the surface of ball-shaped Al₂O₃ particles by the pyrolysis of the silicone resin during sintering in N₂ atmosphere, which played a role in connecting the Al₂O₃ particles by forming the sintering necks. When the sintering temperatures increased from 1100 °C to 1600 °C, the formed Si-O-C ceramics still existed in the amorphous state and had no crystallization. Interestingly, the amorphous β-SiC formed at 1300 °C and its amount gradually increased with further increasing temperatures. The linear shrinkage rate of the samples varied from 0.49% to 0.73% and the weight loss rate increased from 2.01% to 10.77%. The apparent porosity remarkably varied with the range of 24.9% and 34.5%, as the bulk-density varied from 2.66 to 2.47 g/cm³. The bending strength gradually increased from 9.36 to 22.51 MPa with increasing temperatures from 1100 °C to 1500 °C, however, the bending strength remarkably decreased at 1600 °C, which was attributed to the comprehensive function of the high porosity, broken Al₂O₃ particles and weak connection between Al₂O₃ particles in the samples.     

Effects of oxidation curing and sintering temperature on the microstructure formation and heat transfer performance of freestanding polymer-derived SiC films for high-power LEDs

Effects of oxidation cross-linking and sintering temperature on the microstructure evolution, thermal conductivity and electrical resistivity of continuous freestanding polymer-derived SiC films were investigated. The as-received films consisting of β-SiC nanocrystals embedded in amorphous SiO_xC_y and free carbon nanosheets were fabricated via melt spinning of polycarbosilane (PCS) precursors and cured for 3 h/10 h followed by pyrolysis from 900 °C to 1200 °C. Results reveal that nanoscale structure (β-SiC/SiO_xC_y/C_free) provides an ingenious strategy for constructing highly thermal conductive, highly insulating and highly flexible complexes. In particular, the 3 h-cured films sintered at 1200 °C with satisfying thermal conductivity (46.8 W m^?1 K^?1) and electrical resistivity (2.1 × 10⁸ Ω m) are suitable for the realization of high-performance substrates. A remarkable synergistic effect (lattice vibration of β-SiC nanocrystals and close-packed SiO_xC_y, free-electron heat conduction of β-SiC and free carbon, and supporting role of oxygen vacancy) contributing to thermal conductivity improvement is proposed based on the analysis of microstructure, intrinsic properties and simulations. Eventually, the SiC films without additional dielectric layers are directly silk-screen printed with high-temperature silver paste and used as heat dissipation substrates for high-power LED devices via chip-on-board (COB) package. The final devices can emit bright light with low-junction temperature (52.6 °C) and good flexibility owing to the mono-layer SiC substrate with low thermal resistance and desirable mechanical properties. This work offers an effective approach to design and fabricate flexible heat dissipation ceramic substrates for thermal management in advanced electronic packaging fields.     

Sunlight assisted photoelectrocatalytic degradation of benzoic acid using stratified WO3/TiO2 thin films

The stratified WO₃/TiO₂ thin films have been deposited onto glass and FTO coated glass substrates using simple chemical a spray pyrolysis method. The structural, morphological, compositional and photoelectrocatalytic properties of the stratified WO₃/TiO₂ thin films are studied. The photoelectrochemical (PEC) study shows that, both short circuit current (I_sc) and open circuit voltage (V_oc) are (I_sc =1.192 mA and V_oc =0.925 V) relatively high at 50 ml spraying quantity of TiO₂ solution on pre-deposited WO₃. XRD analysis confirms that films are polycrystalline with monoclinic and tetragonal crystal structures for WO₃ and TiO₂ respectively. Specific surface area of 72.14 m² g⁻¹ is measured by Brunauer-Emmett-Teller (BET) technique. Photoelectrocatalytic degradation of benzoic acid (BA) dye in aqueous solutions is studied. The end result shows that the degradation percentage of benzoic acid (BA) using stratified WO₃/TiO₂ photoelectrode has reached 66% under sunlight illumination after 320 min. The amount of degradation is confirmed by COD analysis.     

Structural,optical and methanol sensing properties of sprayed In2O3 nanoparticle thin films

Indium oxide (In₂O₃) nanoparticle thin films were grown on cleaned glass substrates by the chemical spray pyrolysis technique using the precursor solution of indium nitrate (In (NO₃)₃). The XRD studies confirm that the films are polycrystalline In₂O₃, possessing cubic structure with lattice parameters, a = b = c = 10.17 Å. The optical studies show a direct optical band gap of 3.32 eV and an indirect band gap of 2.6 eV in the prepared films. The films exhibit high optical transparency >80% in the visible region, reaching a maximum of 85% at 684 nm wavelength. Further, the gas sensing properties of the films have been investigated for various concentrations of methanol in air at different operating temperatures. At 300 °C the film exhibits a very high response 99% to methanol vapor at a concentration of 40 ppm in air, which is ideal to be used as a methanol sensor. The film shows fast response and recovery to methanol vapor at higher operating temperatures. A possible methanol sensing mechanism has been proposed.     

Superior high-temperature oxidation resistance of magnetron sputtered Hf–B–Si–C–N film

A hard and optically transparent amorphous Hf₇B₂₃Si₁₇C₄N₄₅ film with a contamination level less than 4 at%, prepared by reactive pulsed dc magnetron sputtering, was subjected to systematic investigation of high-temperature oxidation behavior in air up to 1700 °C. We focus on thermogravimetric analysis of the film in air and on the evolution of the film structure, microstructure and elemental composition with an annealing temperature ranging from 1100 °C to 1700 °C. The film exhibits a superior oxidation resistance up to 1600 °C due to a formation of a nanocomposite protective oxide layer on the surface above 1000 °C. The layer consists of monoclinic and tetragonal (or orthorhombic) HfO₂ nanocrystallites surrounded by a SiO₂-based amorphous matrix, most probably containing boron. The HfO₂ nanocrystallites exhibit a gradient in size with a dense population of small (a couple of nm) crystallites next to the interface and larger but dispersed crystallites close to the surface.