Morphological and structural characterization of WO3 and Cr-WO3 thin films synthesized by sol-gel process

WO₃ thin films were prepared by spin-coating methanol solutions of a tungsten chloromethoxide, and easily modified with Cr by the addition of Cr 2-ethylhexanoate. The films were heat-treated up to 700 °C, and characterized by X-ray diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy and Electron Energy Loss spectroscopy. The film morphology was rough and porous, not depending on Cr presence, while their structure was constituted by packed spheroidal or elongated dense structures, giving rise to the peculiar film surface morphology. Cr was distributed in the film structure without phase separations, up to as high as 5% Cr atomic concentration.     

Novel electrochromic devices based on complementary nanocrystalline TiO2 and WO3 thin films

Electrochromic devices were elaborated based on two complementary electrodes made of a nanocrystalline metal oxide thin film deposited on conducting glass. The first electrode holds a 5 μm thick nanocrystalline TiO₂ film derivatized by a monolayer of a phosphonated triarylamine which can be rapidly oxidized by electron transfer to the conducting support followed by charge percolation inside the monolayer. The oxidation in accompanied by a blue coloration due to the absorption band at 730 nm of the stable triarylamminum radical cation. The second electrode bears a 0.2 μm thick nanocrystalline WO₃ film which turns from colorless to blue by reduction and lithium ion insertion. The former electrode reaches an absorbance of at least 3 between 700 and 730 nm after full oxidation (16 mC/cm²) at 1.0 V vs. NHE while for the second, complete reduction at −1.3 V (74 mC/cm²) leads to A=2.4 at 774 nm. An electrochromic device comprising both electrodes separated by an electrolytic solution of 0.1 Li⁺ in 4,7-dioxaoctanitrile reaches an absorbance of 2.2 at 700 nm, 4 s after a voltage step to 1.5 V. The system was shown to sustain at least 14400 coloration-discoloration cycles without degradation.     

Hydrogen activity tuning of Pt-doped WO3 photonic crystal

A photonic crystal responsive to H₂ has been fabricated by replicating polystyrene colloidal crystal template with WO₃ sol-gel, followed with subsequent Pt sputter-deposition. Under H₂ stimulation, the reflection peak of the photonic crystal rapidly shifted to short wavelength and its intensity declined accordingly. The reflection peak returned to its original location after exposure to oxygen atmosphere. The Pt-WO₃ photonic crystal also exhibited excellent, tunable optical performance with high sensitivity to hydrogen in the range of 100% to 0.1%.     

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.     

Ta3N5 photoanodes for water splitting prepared by sputtering

Ta₃N₅ thin-film photoelectrodes were prepared using a reactive sputtering technique, and their properties for photoelectrochemical water splitting under visible light were investigated. The crystal phases of the films were dependent on the sputtering conditions, such as the N₂/O₂ ratio of the sputtering atmosphere and the substrate temperature (T_s). Single-phase Ta₃N₅ films were obtained by sputtering at N₂/O₂ = 30 and T_s = 1013 K with post-annealing in an NH₃ flow. The Ta₃N₅ photoelectrodes had an anodic photoresponse in water photoelectrolysis, although the photocurrent rapidly decreased because of self-oxidation of the photoanode by photogenerated holes. However, modification of the NH₃-treated Ta₃N₅ films with IrO₂ promoted the oxidation of water and suppressed the self-oxidation of Ta₃N₅.     

Synthesis and characterization of WO3 nanostructures prepared by an aged-hydrothermal method

Nanostructures of tungsten trioxide (WO₃) have been successfully synthesized by using an aged route at low temperature (60 °C) followed by a hydrothermal method at 200 °C for 48 h under well controlled conditions. The material was studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Energy Dispersive Spectroscopy (EDS), transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM), Raman spectroscopy and X-ray photoelectron spectroscopy (XPS). Specific Surface Area (S_BET) were measured by using the BET method. The lengths of the WO₃ nanostructures obtained are between 30 and 200 nm and their diameters are from 20 to 70 nm. The growth direction of the tungsten oxide nanostructures was determined along [010] axis with an inter-planar distance of 0.38 nm.     

Changes in atomic and electronic structures of amorphous WO3 films due to electrochemical ion insertion

The structural changes in amorphous WO₃ films were investigated both on the atomic and electronic levels, and the experimental findings were interpreted using molecular orbital calculations. Electrochemical fast intercalation resulted in the splitting of a peak in the valence band region of the X-ray photoelectron spectrum. This splitting could be attributed to the formation of non-bridging oxygen. Decomposition of WO₆ units into WO₄ units could also be inferred from the data. This decomposition was, however, not responsible for the split of the photoelectron peak. From the population analyses it was found that the average bond strength decreased due to the intercalation, while select WO bonds increased in strength. It was expected that these changes in the chemical bonding character lead to localization of electrons and distortion of WO₆ units, which was consistent with the theoretical interpretations of electrochromism, the intervalence charge transfer model and the small polaron absorption theory.     

Response kinetics of a fiber-optic gas sensor using Pt/WO3 thin film to hydrogen

Response kinetics of a fiber-optic hydrogen gas sensor in air- and inert-atmosphere were characterized. The sensor is mainly based on the evanescent field interaction in hydrogen sensitive cladding which is used Platinum-supported tungsten trioxide (Pt/WO₃). When the sensor was exposed to 1 vol.% H₂/air and H₂/N₂ gas, the changes in optical power propagating through the fiber were about 30% and 50%, respectively. The detection limit was about 0.1 vol.% in air-atmosphere. The humidity dependence of the response kinetics was also evaluated. While the response speed in N₂-atmosphere was accelerated, the speed in air-atmosphere was suppressed by the humidity.     

Real-time investigation on photocatalytic oxidation of gaseous methanol with nanocrystalline WO3-TiO2 composite films

Here we report on our investigation on photocatalytic oxidation (PCO) of gaseous methanol with WO₃-TiO₂ composite films. WO₃ and WO₃-TiO₂ composite thin films were prepared by drop casting method. PCO of gaseous methanol and hydrogen generation process on platinum loaded WO₃-TiO₂ composite thin films in high vacuum were investigated using a home-made reactor with a six-channel quadrupole mass spectrometer at real-time scale under UVA (300-400 nm) light illumination.In the case of Pt loaded WO₃ thin films, PCO of gaseous methanol proceeds through intermediates viz. formaldehyde, CO and finally to CO₂ and H₂. PCO of gaseous methanol occurs via direct hole transfer over Pt loaded WO₃ thin films. On the other hand, PCO of gaseous methanol over Pt loaded WO₃-TiO₂ composite thin films proceeds with competitive direct and indirect hole transfer reactions. Our real-time analysis of gas phase photocatalysis realized the identification of direct and indirect hole transfer processes and the reaction intermediates thereof.     

Facile fabrication of three-dimensional interconnected nanoporous N-TiO2 for efficient photoelectrochemical water splitting

Three-dimensional (3D) interconnected porous architectures are expected to perform well in photoelectrochemical (PEC) water splitting due to their high specific surface area as well as favourable porous properties and interconnections. In this work, we demonstrated the facile fabrication of 3D interconnected nanoporous N-doped TiO₂ (N-TiO₂ network) by annealing the anodized 3D interconnected nanoporous TiO₂ (TiO₂ network) in ammonia atmosphere. The obtained N-TiO₂ network exhibited broadened light absorption, and abundant, interconnected pores for improving charge separation, which was supported by the reduced charge transfer resistance. With these merits, a remarkably high photocurrent density at 1.23 V vs. reversible hydrogen electrode (RHE) was realized for the N-TiO₂ network without any co-catalysts or sacrificial reagents, and the photostability can be assured after long term illumination. In view of its simplicity and efficiency, this structure promises for perspective PEC applications.     

Preparation and photocatalytic activity of WO3/TiO2 nanocomposite particles

A novel photocatalyst WO₃/TiO₂ nanocomposite was prepared through a hydrothermal method by using cetyltrimethylammonium bromide (CTAB) as surfactant. The obtained WO₃/TiO₂ was characterized by X-ray diffraction (XRD), field emission scanning electron microscope (FESEM), transmission electron microscope (TEM) and diffused reflectance spectroscopy (DRS). Photocatalytic experiments indicate that the nanocomposites show much higher photoactivity than that of pure TiO₂ in the photodegradation reaction of Rhodamine B (RhB). The increased photoactivity of WO₃/TO₂ may be attributed to the improvement of the light absorption properties and the slow down of the recombination between the photoexcited electrons and holes during the photoreaction.     

Novel MoO3 and WO3 hollow nanospheres assembled with polymeric micelles

Novel β-MoO₃ and WO₃ hollow nanospheres were synthesized using a soft template of polymeric micelle with core-shell-corona architecture. Poly(styrene-b-[3-(methacryloylamino)propyl] trimethylammonium chloride-b-ethylene oxide) micelles (PS-PMAPTAC-PEO) with cationic shell block effectively produce core/shell composite particles through electrostatic interaction with anionic precursors WO₄²⁻ and MoO₄²⁻. Transmission electron microscope (TEM) images of β-MoO₃ and WO₃ have confirmed the hollow structure with average outer diameter of 42 ± 2 and 46 ± 2 nm, respectively; the hollow cavity diameters were found to be 16  ± 1 nm and 14 ± 1 nm for β-MoO₃ and WO₃, respectively. The combination of nitrogen adsorption/desorption analyses and TEM observation confirmed the presence of disordered mesopores in the shell domain of β-MoO₃ and WO₃ hollow particles.     

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.     

Optical properties and aging of gasochromic WO3

The limits of WO₃ as an optical gas sensor were derived by establishing the change in optical constants induced by 2% H₂ in Ar. Using Langmuir's adsorption equation, it was found that at low H₂ concentrations a high sensitivity is predicted, but the coloration could saturate at 57.9% of the material's maximum ion adsorption. The air poisoning problem observed in these devices was shown to be remedied by coating with a permeable polydimethylsiloxane layer, thus eliminating common atmospheric gases as the possible poisoning agents.     

Nanostructured WO3 deposited by modified thermal evaporation for gas-sensing applications

In this work, we present a simple method, based on a modified thermal evaporation technique, to obtain films of nanostructured WO₃ with high surface roughness. This method consists on sublimation from a metallic tungsten wire followed by oxidation in low vacuum conditions and reactive atmosphere (p_O2 = 0.22 mbar), with substrates heated at high temperature (600 °C). Electron microscopy (SEM, TEM) and atomic force microscopy (AFM) analysis revealed that the deposited films are composed of agglomerates with nanometric size and present high surface roughness and large effective area suitable for gas-sensing applications. Sensing measurements highlighted promising performances, particularly at the working temperature of 100 °C: high responses towards sub-ppm concentrations of NO₂ have been observed compared to the lower ones observed for NH₃ and CO. NO₂ tests performed with sensors based on sputtered thin films highlighted that sensors obtained by this thermal evaporation like method exhibit improved performances.     

The effect of heat treatment on physical properties of nanograined (WO3)1-x-(Fe2O3)x thin films

Thin films of (WO₃)_1-x-(Fe₂O₃)_x composition were deposited by thermal evaporation on glass substrates and then all samples were annealed at 200-500 °C in air. Optical properties such as transmittance, reflectance, optical bangap energy, and the optical constants of the “as deposited” and the annealed films were studied using ultraviolet-visible spectrophotometry. It was shown that the annealing process changes the film optical properties which were related to Fe₂O₃ concentration. Moreover, using X-ray photoelectron spectroscopy, we have indicated that WO₃ is stoichiometric, while iron oxide was in both FeO and Fe₂O₃ compositions so that the FeO composition converted to Fe₂O₃ after the annealing process. Using atomic force microscopy, it was observed that surface of the “as deposited” films were smooth with a nanometric grain size. The film surface remained unchanged after annealing up to 300 °C. Surface roughness and the grain size of the films with x = 0, 0.05, and 0.75 highly increased at higher annealing temperatures (400 and 500 °C), but were nearly unchanged for medium x-values (0.3 and 0.4).     

Pulsed laser deposition of (WO3)1 − x(Nb2O5)x thin films: Characterization and gasochromic studies

A series of (WO₃)_{1 − x}(Nb₂O₅)_x (x = 0, 0.05, 0.1 and 0.15) mixed oxide films were fabricated by pulsed laser deposition (PLD) at 27 Pa oxygen partial pressure on ITO glass substrates. The thickness of the (WO₃)_{1 − x}(Nb₂O₅)_x thin films is about 350 ± 30 nm and their surface has a uniform morphology. A layer of platinum (Pt) was then sputtered onto the surface of the film. The hydrogen gas sensing performance of Pt catalyst activated (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated. The cycling of the coloration was obtained from UV–vis spectra. Gasochromic coloration of (WO₃)_{1 − x}(Nb₂O₅)_x thin films were investigated at room temperature in H₂–N₂ mixtures containing 1–100 mol% of H₂. The results show that the shortest response time of (WO₃)_{1 − x}(Nb₂O₅)_x/Pt hydrogen sensor is within 30 s and the highest transmittance change (ΔT) varies from 20% to 30%.     

A simple route to large-scale production of tungsten trioxide nanoparticles

Tungsten trioxide particles in high yield were prepared via a simple solid evaporation route with ammonium paratungstate hydrate as precursor and Ar gas as carrier gas. Detailed characterization by scanning electron microscopy has shown that increasing carrier gas flow rate promotes morphological evolution from large and irregular semi-spherical particles to non-agglomerated quasi-spherical particles, and finally to single-crystalline nanoparticles with an average diameter of 60 nm. The adsorption activity of the tungsten trioxide particles is size-dependent and increased with decreasing particle size. The present method could readily produce large-scale tungsten trioxide nanoparticles with ideal adsorption performance, and can be utilized to fabrication of various semiconductor oxides with advanced properties.     

Electrochromic and morphological investigation of dry-lithiated nanostructured tungsten trioxide thin films

Electrochromic properties of porous nanostructured thin films of tungsten trioxide were investigated. Films were fabricated at normal and high vapor incidence angles with the technique of glancing angle deposition in a thermal evaporation chamber. A dry lithiation method was subsequently used to intercalate films with lithium atoms. Coloration in both visible and near-infrared regions was observed with lithium insertion. We report on the morphology, porosity, and optical properties of as-deposited and lithiated films, and discuss the role of substrate tilt in comparing the coloration efficiency of these films.     

Preparation of hexagonal WO3 from hexagonal ammonium tungsten bronze for sensing NH3

Hexagonal tungsten oxide (h-WO₃) was prepared by annealing hexagonal ammonium tungsten bronze, (NH₄)_0.07(NH₃)_0.04(H₂O)_0.09WO_2.95. The structure, composition and morphology of h-WO₃ were studied by XRD, XPS, Raman, ¹H MAS (magic angle spinning) NMR, scanning electron microscopy (SEM), and BET-N₂ specific surface area measurement, while its thermal stability was investigated by in situ XRD. The h-WO₃ sample was built up by 50-100 nm particles, had an average specific surface area of 8.3 m²/g and was thermally stable up to 450 °C. Gas sensing tests showed that h-WO₃ was sensitive to various levels (10-50 ppm) of NH₃, with the shortest response and recovery times (1.3 and 3.8 min, respectively) to 50 ppm NH₃. To this NH₃ concentration, the sensor had significantly higher sensitivity than h-WO₃ samples prepared by wet chemical methods.