Hydrogen gas-sensing with bilayer structures of WO3 and Pd in SAW and electric systems

A bi-layer sensor structure of WO₃ (~ 100 nm) with a very thin film of palladium (Pd~ 18 nm) on the top, has been studied for hydrogen gas-sensing application at ~ 80 °C and ~ 120 °C and low hydrogen concentrations (0.025-1%). The structures were obtained by vacuum deposition (first the WO₃ and then the Pd film) onto a LiNbO₃ Y-cut Z-propagating substrate making use of the Surface Acoustic Wave method and additionally (in this same technological processes) onto a glass substrate with a planar microelectrode array for simultaneous monitoring of planar resistance of the structure. A very good correlation has been observed between these two methods — frequency changes in SAW method correlate very well with decreases in the bi-layer structure resistance. The SAW method is faster at the lower interaction temperature such as 80 °C, whereas at an elevated temperature of 120 °C, the electrical planar method is also fast and has a lower limit of detection.     

Size reduction of WO3 nanoparticles by ultrasound irradiation and its applications in structural nanocomposites

The porous WO₃ (pore size 2–5 nm) nanoparticles were synthesized using a high intensity ultrasound irradiation of commercially available WO₃ nanoparticles (80 nm) in ethanol. The high resolution transmission electron microscopic (HRTEM) and X-ray studies indicated that the 2–5 nm uniform pores have been created in commercially available WO₃ nanoparticles without much changing the initial WO₃ nanoparticles (80 nm) sizes. The nanocomposites of WO₃/SC-15 epoxy were prepared by infusion of 1 wt.%, 2 wt.% and 3 wt.% of porous WO₃ nanoparticles into SC-15 epoxy resin by using a non-contact (Thinky) mixing technique. Finally the neat epoxy and nanocomposites were cured at room temperature for about 24 h in a plastic rectangular mold. The cured epoxy samples were removed and precisely cut into required dimensions and tested for their thermal and mechanical properties. The HRTEM and SEM studies indicated that the sonochemically modified porous WO₃ nanoparticles dispersed more uniformly over the entire volume of the epoxy (without any settlement or agglomeration) as compared to the unmodified WO₃/epoxy nanocomposites.     

Effects of oxygen stoichiometry on electrochromic properties in amorphous tungsten oxide films

We have investigated the electrochromic properties of amorphous granular tungsten oxide (WO_3 + δ) thin films with over-stoichiometric oxygen content (δ), using LiClO₄ with propylene carbonate as an electrolyte. Different optical and electrochromic characteristics are observed with increasing δ. All the devices are electrochemically stable for more than 5000 color/bleach cycles without apparent degradation, and they have a faster response to coloration than to bleaching. WO_3 + δ films with an optimized δ value show an optical modulation of 86% at a wavelength of 630 nm and the highest coloration efficiency ever reported of ~ 213 cm²/C. The δ-dependent coloration mechanism is discussed using the site saturation model. It is proposed that WO_3 + δ films with the optimal δ value have favorable thickness and stoichiometry for the generation of Li⁺W⁺⁵ states.     

Preparation and characterization of α-Fe2O3 polyhedral nanocrystals via annealing technique

Polyhedral nanocrystals of α-Fe₂O₃ are successfully synthesized by annealing FeCl₃ on silicon substrate at 1000 °C in the presence of H₂ gas diluted with argon (Ar). Uniformly shaped polyhedral nanoparticles (diameter ~ 50-100 nm) are observed at 1000 °C and gases flow rate such as; Ar = 200 ml/min and H₂ = 150 ml/min. Non-uniform shaped nanoparticles (diameter ~ 20-70 nm) are also observed at an annealing temperature of 950 °C with lower gases flow rate (Ar = 100 ml/min and H₂ = 75 ml/min). Nanoparticles are characterized in detail by field-emission electron microscopy (FE-SEM), energy dispersive X-ray (EDX) and high resolution transmission electron microscopy (HRTEM) techniques. HRTEM study shows well resolved (110) fringes corresponding to α-Fe₂O₃, and selected area diffraction pattern (SADP) confirms the crystalline nature of α-Fe₂O₃ polyhedral nanoparticles. It is observed that polyhedral formation of α-Fe₂O₃ nanocrystals depends upon annealing temperature and the surface morphology highly rely on the gas flow rate inside the reaction chamber.     

NH4Cl-assisted low temperature synthesis of anatase TiO2 nanostructures from Ti powder

A simple, low temperature and low cost method, which was based on heating the mixture of Ti and NH₄Cl powders in air at 300 °C, has been developed for the controlled synthesis of anatase TiO₂ nanostructures including irregular nanoparticle aggregates, curved nanowires built up by the oriented attachment of nanoparticles, and nanoplates constructed with nanoparticles. The characterization results from X-ray diffraction and Raman spectra indicated that the as-obtained products were anatase TiO₂. Field emission scanning electron microscope images revealed that the products obtained for 3, 10 and 16 h comprised, in turn, irregular nanoparticle aggregates (8-55 nm), curved nanowires built up by the oriented attachment of nanoparticles (~ 9 nm), and nanoplates constructed with nanoparticles (~ 8 nm).     

One-step synthesis of Fe-phthalocyanine/Fe3O4 hybrid microspheres

Fe-phthalocyanine/Fe₃O₄ hybrid microspheres were synthesized from bis-phthalonitrile and FeCl₃·6H₂O through a simple and effective solvent-thermal route. The hybrids were monodispersed solid microspheres with diameter of ~ 400 nm. The ferromagnetic signature emerged with the saturated magnetization of ~ 55.7 emu g⁻¹, and the coercive force of ~ 93.7 Oe at 300 k. The addition of bis-phthalonitrile oligomer brought Fe₃O₄ nanoparticles novel dielectric property: a new dielectric loss peak appeared at ~ 8 GHz. Considering the microwave magnetic loss properties, two microwave magnetic loss peaks were presented at ~ 1.5 GHz and ~ 10 GHz, the former peak was attributed to the natural properties of the Fe₃O₄, and the latter originated from the interface effects between the bis-phthalonitrile oligomer and Fe₃O₄.     

Electrochromic properties of N-doped tungsten oxide thin films prepared by reactive DC-pulsed sputtering

In depositing nitrogen doped tungsten oxide thin films by using reactive DC-pulsed magnetron sputtering process, nitrous oxide gas (N₂O) was employed instead of nitrogen (N₂) as the nitrogen dopant source. The nitrogen doping effect on the structural and electrochromic properties of WO₃ thin films was investigated. X-ray diffraction (XRD) results show that the films are amorphous. Morphological images reveal that the films are characterized by a hybrid structure comprising nanoparticles embedded in amorphous matrix and open channels between the agglomerated nanoparticles, which promotes rapid charge transport through the film. Increasing the nitrogen doping concentration is found to decrease the nanoparticle size and the band gap energy. The electrochromic properties were studied using cyclic voltammetric and spectroeletrochemical techniques. The film with N content of ~ 5 at.% exhibits higher optical modulation and coloration efficiency as well as faster ion transport kinetics. The results reveal that electrochromic and lithium ion transport properties are moderately enhanced relative to the un-doped tungsten oxide thin films by appropriate content of dopant, due to the effects of nitrogen doping.     

Investigations of thin film structures of WO3 and WO3 with Pd for hydrogen detection in a surface acoustic wave sensor system

A single thin film sensor structure of WO₃ (∼ 50 nm) and bilayer sensor structure of WO₃ (∼ 50 nm) with a very thin film of palladium (Pd ∼ 18 nm) on the top, have been studied for hydrogen gas-sensing application at ∼ 30 °C and ∼ 50 °C. The structures were obtained by vacuum deposition (first the WO₃ and than the Pd film) onto a LiNbO₃ Y-cut Z-propagating substrate making use of the surface acoustic wave method and additionally (in this same technological processes) onto a glass substrate with a planar microelectrode array for simultaneously monitoring of the planar resistance of the structure. In the case of a bilayer structure a very good correlation has been observed between these two methods — frequency changes in SAW method correlate very well with decreases of the bilayer structure resistance. These frequency changes are on the level of 2.4 kHz to 4% of hydrogen concentration in dry air, whereas in the case of a single WO₃ structure almost no frequency shift is observed.     

Influence of spray pyrolysis deposition parameters on the optoelectronic properties of WO3 thin films

The electrochromic (EC) properties of tungsten oxide (WO₃), such as coloration efficiency, cyclic durability and reversibility strongly depend on the structural and morphological properties, which are influenced by the deposition method and parameters.This paper presents the steps for optimizing the deposition parameters (substrate temperature, air flow pressure and precursor solution molarity) for improving the optical and electrical properties of WO₃ thin films for EC applications. WO₃ thin films were deposited by spray pyrolysis using tungsten hexachloride (WCl₆) dissolved in ethanol as precursor solution. The EC properties of optimized films were tested in two different electrolytes (H₂SO₄ 1 M and acetic acid/sodium acetate buffer with pH = 4) and changes in structure, composition and morphology of the films after coloration/bleaching cycles were discussed.The deposition temperature, carrier gas pressure and solution molarity were optimized at 250 °C, 120 kPa and 0.14 M respectively. Under these condition a dense, uniform film, with homogenous distribution of particles, good adhesion to the substrate, low roughness (9.02 nm), high transparency (> 70% in the 500-1100 nm range) and conductivity was obtained. Transmission modulation is higher for the sample cycled in H₂SO₄ 1 M (64% at 630 nm) compared to that cycled in the buffer (21% at 630 nm), whereas opposite results were obtained for coloration efficiencies 28 cm² C^− 1 (at 630 nm) and 35 cm² C^− 1 (at 630 nm), respectively. Changes in surface chemistry and morphology of the optimized sample were observed after cycling in H₂SO₄.     

Nanoparticle formation by the debris produced by femtosecond laser ablation of silicon in ambient air

The debris produced by femtosecond laser ablation (180 fs, 775 nm, 1 kHz) of Si in ambient air is deposited around the ablated craters in a circular zone with diameters between ~ 40 and 300 μm for laser fluences (F) in the region F = 0.2-8 J/cm². The debris consists of nanoparticles. The mean height of the nanoparticles increases with laser fluence (from ~ 70 to 500 nm for fluences in the range F = 0.25-4.38 J/cm²) but at high fluences (F = 8 J/cm²) becomes equal to ~ 170 nm. The average horizontal dimension of the nanoparticles increases with laser fluence. Their average vertical dimension increases in proportion to their average horizontal dimension, but at high fluences becomes much smaller than their corresponding average horizontal dimension. The nanoparticles were found to be single crystals with d spacing of 1.71 ± 0.08 Å (corresponding to {311}).     

Wet chemical route to transparent BiFeO3 films on SiO2 substrates

BiFeO₃ nanoparticles were prepared by a wet chemical synthesis method. Transparent films were deposited on glass and quartz substrates by dip and spin coating processes from the synthesized sol. We obtained thicker films (~ 2 µm) by dip coating process and thinner films (~ 200 nm) by spin coating process. Transmission electron microscopy images confirmed that the particles are nanocrystalline in size. From the optical transmittance spectra the band gap of the BiFeO₃ nanoparticles was determined in the range of ~ 3.03-2.88 eV (~ 410-430 nm). Electrical resistivity, polarization, zero-field-cooled and field-cooled magnetizations versus temperature characteristics were also studied for these films.     

Textured surface boron-doped ZnO transparent conductive oxides on polyethylene terephthalate substrates for Si-based thin film solar cells

Textured surface boron-doped zinc oxide (ZnO:B) thin films were directly grown via low pressure metal organic chemical vapor deposition (LP-MOCVD) on polyethylene terephthalate (PET) flexible substrates at low temperatures and high-efficiency flexible polymer silicon (Si) based thin film solar cells were obtained. High purity diethylzinc and water vapors were used as source materials, and diborane was used as an n-type dopant gas. P-i-n silicon layers were fabricated at ~ 398 K by plasma enhanced chemical vapor deposition. These textured surface ZnO:B thin films on PET substrates (PET/ZnO:B) exhibit rough pyramid-like morphology with high transparencies (T ~ 80%) and excellent electrical properties (Rs ~ 10 Ω at d ~ 1500 nm). Finally, the PET/ZnO:B thin films were applied in flexible p-i-n type silicon thin film solar cells (device structure: PET/ZnO:B/p-i-n a-Si:H/Al) with a high conversion efficiency of 6.32% (short-circuit current density J_SC = 10.62 mA/cm², open-circuit voltage V_OC = 0.93 V and fill factor = 64%).     

MoO3 nanoparticles distributed uniformly in carbon matrix for supercapacitor applications

A highly uniform nanocomposite of MoO₃ and carbon with a weight ratio of 1:1 is prepared by employing a simple procedure of ball milling. Such composite as electrochemical pseudocapacitor materials for potential energy storage applications exhibits a high specific capacitance of ~ 179 F/g at a charge and discharge current density of 50 mA/g with excellent cycling ability over 1000 cycles. Compared with the capacitance of pure milled graphite (~ 22 F/g) and MoO₃ (< 10 F/g), an enhanced electrochemical performance of the composite with a weight ratio of 1:1 is attributed to its unique structure, in which MoO₃ nanoparticles (with a size range of 1-180 nm) are uniformly dispersed in an electrically conductive carbon host.     

Photocatalytic properties of BiFeO3 nanoparticles with different sizes

BiFeO₃ nanoparticles with different average grain sizes have been prepared through a polyacrylamide gel route. In the present synthesis route, the grain size is tailored by varying the ratio of bis-acrylamide to acrylamide. The photocatalytic activity of the as-prepared BiFeO₃ nanoparticles has been investigated by the degradation of methyl orange (MO), a typical azo dye. It is revealed that the products exhibit a pronounced photocatalytic activity under ultraviolet as well as visible-light irradiation. With decrease in particle size, the photocatalytic activity exhibits a rising trend. The influences of catalyst dosage and initial dye concentration on the photocatalytic efficiency have been also investigated. In the present experiments, the optimum loading of BiFeO₃ nanoparticles and initial concentration of MO are obtained to be ~ 2.5 g L⁻¹ and ~ 10 mg L⁻¹, respectively.     

HWCVD MoO3 nanoparticles and a-Si for next generation Li-ion anodes

We have employed hot wire chemical vapor deposition (HWCVD) for the generation of MoO₃ nanostructures at high density. Furthermore, the morphology of the nanoparticles is easily tailored by altering the HWCVD synthesis conditions. The MoO₃ nanoparticles have been demonstrated as high-capacity Li-ion battery anodes for next-generation electric vehicles. Specifically, the MoO₃ anodes have been shown to have approximately three times the Li-ion capacity of commercially employed graphite anodes in thick electrodes suitable for vehicular applications. However because the materials are high volume expansion materials (≥ 100%), conformal Al₂O₃ coatings deposited with atomic layer deposition (ALD) were required before high rate capability was demonstrated. Recently, NREL is exploring high capacity Si anode materials that have a volume expansion of ~ 400%. It is assumed that new ALD coatings will need to be developed in order to stabilize Si as an anode material. Silicon is a superior choice for an anode material to the metal oxide structures due to both a higher capacity and a significantly lower hysteresis in the voltage vs. Li/Li⁺ for the charge/discharge profiles.     

Polycrystalline films of tungsten-doped indium oxide prepared by d.c. magnetron sputtering

Electrical and optical properties of polycrystalline films of W-doped indium oxide (IWO) were investigated. These films were deposited on glass substrate at 300 °C by d.c. magnetron sputtering using ceramic targets. The W-doping in the sputter-deposited indium oxide film effectively increased the carrier density and the mobility and decreased the resistivity. A minimum resistivity of 1.8 × 10^− 4 Ω cm was obtained at 3.3 at.% W-doping using the In₂O₃ ceramic targets containing 7.0 wt.% WO₃. The 2.2 at.% W-doped films obtained from the targets containing 5.0 wt.% WO₃, showed the high Hall mobility of 73 cm² V^− 1 s^− 1 and relatively low carrier density of 2.9 × 10²⁰ cm^− 3. Such properties resulted in novel characteristics of both low resistivity (3.0 × 10^− 4 Ω cm) and high transmittance in the near-infrared region.     

Thin film deposition and characterization of pure and iron-doped electron-beam evaporated tungsten oxide for gas sensors

Pure tungsten oxide (WO₃) and iron-doped (10 at.%) tungsten oxide (WO₃:Fe) nanostructured thin films were prepared using a dual crucible Electron Beam Evaporation (EBE) technique. The films were deposited at room temperature under high vacuum onto glass as well as alumina substrates and post-heat treated at 300 °C for 1 h. Using Raman spectroscopy the as-deposited WO₃ and WO₃:Fe films were found to be amorphous, however their crystallinity increased after annealing. The estimated surface roughness of the films was similar (of the order of 3 nm) to that determined using Atomic Force Microscopy (AFM). As observed by AFM, the WO₃:Fe film appeared to have a more compact surface as compared to the more porous WO₃ film. X-ray photoelectron spectroscopy analysis showed that the elemental stoichiometry of the tungsten oxide films was consistent with WO₃. A slight difference in optical band gap energies was found between the as-deposited WO₃ (3.22 eV) and WO₃:Fe (3.12 eV) films. The differences in the band gap energies of the annealed films were significantly higher, having values of 3.12 eV and 2.61 eV for the WO₃ and WO₃:Fe films respectively. The heat treated films were investigated for gas sensing applications using noise spectroscopy. It was found that doping of Fe to WO₃ produced gas selectivity but a reduced gas sensitivity as compared to the WO₃ sensor.     

One-pot solvothermal synthesis of sandwich-like graphene nanosheets/Fe3O4 hybrid material and its microwave electromagnetic properties

A novel sandwich-like graphene nanosheets (GNs)/Fe₃O₄ hybrid material was synthesized through a facile one-pot solvothermal method using FeCl₃ as iron source, ethylene glycol as the reducing agent and graphene nanosheets as templates. The Fe₃O₄ nanoparticles, with the average diameters of ca. 40 nm, were self-assembled on the graphene nanosheets through electrostatic attraction and formed sandwich-like nanostructure. The ferromagnetic signature emerged with the saturated magnetization of ~ 72.3 emu g^− 1, and the coercive force of ~ 196.1 Oe at 300 K. The magnetic loss was caused mainly by natural resonance which is in agreement with the Kittel equation. The novel electromagnetic hybrid material is believed to have potential applications in the microwave absorbing performances.     

Enhanced conductivity of pulsed laser deposited n-InGaZn6O9 films and its rectifying characteristics with p-SiC

Wide band gap InGaZn₆O₉ films of thickness ~ 350 nm were deposited on sapphire (0001) at room temperature by using the pulsed laser deposition technique. The transparent films showed the optical transmission of > 80% with the room temperature Hall mobility of ~ 10 cm²/V s and conductivity of 4 × 10² S/cm at a carrier density > 10²⁰ cm^− 3. The electrical properties as a function of deposition temperatures revealed that the conductivity and mobility almost retained up to the deposition temperature of 200 °C. The films annealed in different atmospheres suggested oxygen vacancy plays an important role in determining the electrical conductivity of the compound. Room temperature grown heterostructure of n-InGaZn₆O₉/p-SiC showed a good rectifying behavior with a leakage current density of less than 10^− 9 A/cm², current rectifying ratio of 10⁵ with a forward turn on voltage ~ 3 V, and a breakdown voltage greater than 32 V.     

Improved dielectric properties of lead lanthanum zirconate titanate thin films on copper substrates

Thin films of lead lanthanum zirconate titanate (PLZT) were directly deposited on copper substrates by chemical solution deposition and crystallized at temperatures of ~ 650 °C under low oxygen partial pressure (pO₂) to create film-on-foil capacitor sheets. The dielectric properties of the capacitors formed have much improved dielectric properties compared to those reported previously. The key to the enhanced properties is a reduction in the time that the film is exposed to lower pO₂ by employing a direct insertion strategy to crystallize the films together with the solution chemistry employed. Films exhibited well-saturated hysteresis loops with remanent polarization of ~ 20 μC/cm², dielectric constant of > 1100, and dielectric loss of < 0.07. Energy densities of ~ 32 J/cm³ were obtained at a field of ~ 1.9 MV/cm on a ~ 1 μm thick film with 250 μm Pt electrodes.