Epitaxial growth of WOx nanorod array on W(001)

Nanorods of substoichiometric tungsten oxide (WO_x) were grown on W(001) substrates. Two methods for the growth of nanorods were used: oxidation of the substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen. The grown nanorods were observed using a scanning electron microscope and an atomic force microscope. The diameters of the nanorods were 5–20 nm. The nanorods were slightly inclined from the directions parallel or normal to the surface. The inclination of nanorods was explained in terms of the epitaxial relationship between WO₃ crystals and the W(001) substrate. The WO₃ crystals formed at the initial stage of growth act as the nuclei of WO_x nanorods. We observed selective enhancement of the growth in a certain epitaxial direction depending on the method of growth, and an array of WO_x nanorods was produced on the W(001) substrate.     

Epitaxial growth of WOx nanorod array on W(001)

Nanorods of substoichiometric tungsten oxide (WO_x) were grown on W(001) substrates. Two methods for the growth of nanorods were used: oxidation of the substrate under appropriate conditions and the deposition of tungsten oxide from a tungsten foil heated in the presence of oxygen. The grown nanorods were observed using a scanning electron microscope and an atomic force microscope. The diameters of the nanorods were 5–20 nm. The nanorods were slightly inclined from the directions parallel or normal to the surface. The inclination of nanorods was explained in terms of the epitaxial relationship between WO₃ crystals and the W(001) substrate. The WO₃ crystals formed at the initial stage of growth act as the nuclei of WO_x nanorods. We observed selective enhancement of the growth in a certain epitaxial direction depending on the method of growth, and an array of WO_x nanorods was produced on the W(001) substrate.     

Nanostructured tungsten and tungsten trioxide films prepared by glancing angle deposition

Nanostructured tungsten (W) and tungsten trioxide (WO₃) films were prepared by glancing angle deposition using pulsed direct current magnetron sputtering at room temperature with continuous substrate rotation. The chemical compositions of the nanostructured films were characterized by X-ray photoelectron spectroscopy, and the film structures and morphologies were investigated using X-ray diffraction and high resolution scanning electron microscopy. Both as-deposited and air annealed tungsten trioxide films exhibit nanostructured morphologies with an extremely high surface area, which may potentially increase the sensitivity of chemiresistive WO₃ gas sensors. Metallic W nanorods formed by sputtering in a pure Ar plasma at room temperature crystallized into a predominantly simple cubic β-phase with <100> texture although evidence was found for other random grain orientations near the film/substrate interface. Subsequent annealing at 500 °C in air transformed the nanorods into polycrystalline triclinic/monoclinic WO₃ structure and the nanorod morphology was retained. Substoichiometric WO₃ films grown in an Ar/O₂ plasma at room temperature had an amorphous structure and also exhibited nanorod morphology. Post-deposition annealing at 500 °C in air induced crystallization to a polycrystalline triclinic/monoclinic WO₃ phase and also caused a morphological change from nanorods into a nanoporous network.     

Synthesis and characterization of novel nanorod superstructures and twin octahedral morphologies of WO3 by hydrothermal treatment

Controlled WO₃ morphologies, such as nanorods and octahedral structures, were synthesized by the hydrothermal technique using sulfate salts based structure directing agents (SDAs). The role of the sulfate salts’ cation in controlling the shape, size and phase of WO₃ nanomaterials was investigated by choosing sulfate salts whose cations are from d-bloc elements (FeSO₄, (NH₄)₂Fe(SO₄)₂, CoSO₄, CuSO₄, ZnSO₄), an alkaline earth metal (MgSO₄) and a non-metal ((NH₄)₂SO₄). In addition chloride (MnCl₂) and acetate (Zn(CH₃CO₂)₂) anion based SDAs were also used in order to clarify the role of sulfate ions in the growth of WO₃ nanostructures. We controlled the pH of the reaction medium with oxalic acid. The obtained WO₃ samples were investigated by SEM, micro-Raman, and XRD. At pH = 1, the WO₃ samples exhibit novel superstructures consisting of aligned hexagonal nanorods, whereas at pH = 5.25, novel twin octahedral morphology with a cubic structure is obtained. The results demonstrate that the phase and morphology change is influenced by the pH and both the anion and the cation of the SDA. A growth mechanism for the obtained novel WO₃ morphologies is presented.     

The structure and composition of oxidized and reduced tungsten oxide thin films

The structure, morphology and composition of pure WO₃ thin films deposited onto vacuum-cleaved NaCl(001) single crystals have been studied at different substrate temperatures up to 580 K and under different oxidative and reductive treatments in the temperature range 373-873 K by Transmission Electron Microscopy, Selected-Area Electron Diffraction and X-ray Photoelectron Spectroscopy (XPS). A transition from an amorphous structure obtained after deposition at 298 K to a more porous structure with small crystallites at the highest substrate temperatures has been observed. XPS spectra reveal the presence of W⁶⁺ irrespective of the preparation procedure. Significant changes in the film structure were only observed after an oxidative treatment in 1 bar O₂ at 673 K, which induces crystallization of a monoclinic WO₃ structure. After raising the oxidation temperature to 773 K, the film shows additional reconstruction and a hexagonal WO₃ structure becomes predominant. This hexagonal structure persists at least up to 873 K oxidation temperature. However, these structural transformations observed upon oxidation were almost completely suppressed by mixing the WO₃ thin film with a second oxide, e.g. Ga₂O₃. Reduction of the WO₃ films in 1 bar H₂ at 723-773 K eventually induced the formation of the β-W metal structure, as evidenced by electron diffraction and XPS.     

The formation of WO3 nanorods using the surfactant-assisted hydrothermal reaction

Monoclinic tungsten oxide (WO₃) nanorods were grown using the hydrothermal method on a seeded W foil. The seed layer was formed by thermal oxidation of W foil at 400°C for 30 min. Cetyltrimethylammonium bromide (CTAB) or hexamethylamine (HMT) was used in the reactive hydrothermal bath, along with sodium tungstate dihydrate (Na₂WO₄.2H₂O) and hydrochloric acid (HCl). The concentration of CTAB was varied from 0.01 M to 0.07 M and the concentration of HMT was varied from 0.01 M and 0.05 M. The result showed that CTAB-assisted hydrothermal reaction produced WO₃ nanorods with 4–7 nm diameter, and provided that CTAB concentration was less than 0.07 M. WO₃ nanorods could not be obtained when CTAB concentration was 0.07 M. Columnar structured WO₃ was produced with the presence of HMT in the hydrothermal bath. This was due to decomposition of HMT to form hydroxyl ions (OH^?) that inhibited the growth of nanorods. Cyclic voltammetry (CV) analysis showed better electrochromic property of WO₃ nanorods compared to columnar structured WO₃.     

Nanoarchitectonics of a Au nanoprism array on WO3 film for synergistic optoelectronic response

Abstract

A layered photoelectrode consisting of a conductive indium tin oxide substrate, a WO₃ nanocrystalline film and an array of Au nanoprisms was fabricated via a multistep process. Scanning electron microscopy and atomic force microscopy showed that the Au nanoprisms had a uniform size and shape and formed periodic hexagonal patterns on the WO₃ film. The optical absorption of the photoelectrode combined the intrinsic absorption of WO₃ and plasmonic absorption of Au. Using this photoelectrode, we investigated the effect of the Au nanoprism array on the optoelectronic conversion performance of the WO₃ film. Photoelectrochemical measurement indicated that the array substantially enhanced the photocurrent in the WO₃ film. Electrochemical impedance measurements revealed that the Schottky junctions formed between Au and WO₃ can facilitate the separation of photogenerated carriers as well as the interfacial carrier transfer. In this study, we demonstrate that covering a semiconductor with plasmonic noble metal nanoparticles can improve its optoelectronic conversion efficiency.     

Hydrothermal in-situ growth of Tris(8-hydroxyquinolate) aluminum nanorods thin films

Tris(8-hydroxyquinolate) aluminum(Alq₃) thin films assembled with large-scaled nanorods have been fabricated on Al substrates through hydrothermal in-situ growth method assisted by the surfactant of sodium dodecylbenzenesulfonate. The obtained Alq₃ thin film is composed of uniformly sized (500-800nm × 4-10 μm) nanorods with regular hexagonal cross section, which are assembled to form dense nanorod arrays perpendicularly to the Al substrate. X-ray diffraction revealed that the prepared Alq₃ nanorods were the α-phase. Photoluminescence spectra showed that the Alq₃ nanorods thin film possessed a spectral blue-shift (10 nm) compared with the Alq₃ solution. The hydrothermal growth mechanism of nanorods was studied, which implied that the hydrothermal in-situ growth process on the metal substrate played an important role in the formation of the Alq₃ nanorods thin film. This simple hydrothermal method provides a convenient fabrication approach for nanocrystalline functional organic/metal interface.     

Thermic investigations of hexagonal hydrogen bronze,HxWO3

The thermic-kinetic behaviour of hydrogen tungsten bronze, H_xWO₃, was investigated by differential scanning calorimetry and X-ray analysis. It is shown that the thermal decomposition of bronze into hydrogen and WO₃, is exothermic. Further, it is shown that the hexagonal WO₃ system is exothermally transformed (irreversibly) to the monoclinic WO₃ system. Activation energies, frequency factors and rate constants of the corresponding processes are determined. It is shown that the hexagonal WO₃ system adsorbs hydrogen at approximately 480 K, and desorbs at room temperature in air or an argon-nitrogen atmosphere. It is also shown that H_xWO₃ is unstable and that it decomposes with time, forming brown WO₃ which gives no yellow modification during the WO₃ phase transformation. The newly formed modification adsorbs hydrogen without changing colour.     

Direct synthesis of highly crystalline single-phase hexagonal tungsten oxide nanorods by spray pyrolysis

The metastable state hexagonal-tungsten oxide (h-WO₃) has been attracting attention over the past decade because of its high reactivity that arises from the hexagonal channels in its crystal structure. Simplification of the process used to synthesize h-WO₃ is an important step to facilitate the industrial applications of this material. In this study, we addressed this challenge by developing a spray pyrolysis process to synthesize highly crystalline h-WO₃. The ratio of the monoclinic to the hexagonal phase was controlled by adjusting the segregation time. Single-phase h-WO₃ nanorods were synthesized using a carrier gas flow rate of 1?L/min, which was equivalent to a segregation time of 18.4?s. The ability of the h-WO₃ nanorods to adsorb nitrogen and carbon dioxide was evaluated to confirm the presence of hexagonal channels in the crystal structure.     

Effect of hydrothermal duration on synthesis of WO<Subscript>3</Subscript> nanorods

Among different type of transition metal oxides, tungsten trioxide (WO₃) is a suitable candidate for electronic device fabrication due to its n-type property and wide band gap. Herein, one-dimensional tungsten trioxide (WO₃) nanorods were achieved from an aqueous solution of sodium tungstate dihydrate (Na₂WO₄·2H₂O) and sodium chloride (NaCl) in an acidic media by a time-optimized hydrothermal synthesis in autoclave at 180°C or different synthesis durations. For studying morphology and size of obtained powder, X-ray diffraction (XRD), scanning electron microscope (SEM), and high resolution transmission electron microscope (HRTEM) were applied. Finally, WO₃ nanorods of about 2–3 μm in length and 100–200 nm in diameter were obtained during 3 h hydrothermal process.     

Structural and chemical characterization of BaTiO3 nanorods

An electron-microscopy investigation was performed on BaTiO₃ nanorods that were processed by sol-gel electrophoretic deposition (EPD) into anodic aluminium oxide (AAO) membranes. The BaTiO₃ nanorods grown within the template membranes had diameters ranging from 150 to 200 nm, with an average length of 10-50 μm. By using various electron-microscopy techniques we showed that the processed BaTiO₃ nanorods were homogeneous in their chemical composition. The BaTiO₃ nanorods were always polycrystalline and were composed of well-crystallized, defect-free, pseudo-cubic BaTiO₃ grains, ranging from 10 to 30 nm. No intergranular phases were observed between the BaTiO₃ grains. A low-temperature hexagonal polymorph that is coherently intergrown with the BaTiO₃ perovskite matrix was also observed as a minor phase. When annealing the AAO templates containing the BaTiO₃ sol in an oxygen atmosphere the presence of the hexagonal polymorph was diminished.     

Luminescent properties of GdPO4:Eu nanorods

Highly luminescent GdPO₄:Eu³⁺ nanorods with hexagonal and monoclinic structures were successfully synthesized by a hydrothermal method. Results are shown that the prepared GdPO₄:Eu has a hexagonal or monoclinic structure under different synthetical process. The properties of the Eu³⁺-doped GdPO₄ nanorods were characterized by XRD, SEM and UV–vis spectroscopy. A study of the photoluminescence of Eu³⁺-doped GdPO₄ has revealed that the optical properties of these nanophosphors are strongly dependent on their crystal structures and morphologies.     

Phase transitions of low-temperature sintering tungsten-doped ZnO-TiO<Subscript>2</Subscript> ceramics

WO₃-doped zinc titanate ceramics were prepared by conventional mixed-oxide method combined with a chemical processing. The effects of WO₃ addition on the low-temperature sintering behavior, phase transition and dielectric properties of zinc titanate ceramics were investigated. The results show that the densification temperature of ZnTiO₃ ceramics can be reduced from 1150 to 900 °C with WO₃ addition and chemical processing. Small amount of WO₃ (<1.00wt %) accelerated the decomposition of hexagonal ZnTiO₃ phase to cubic Zn₂TiO₄ phase, while excessive addition (for example, 3.00wt %) restrained the decomposition. At the same time, the phase transition temperature from hexagonal ZnTiO₃ phase to cubic Zn₂TiO₄ is lowered by adding WO₃. WO₃ addition affects the dielectric properties of ceramics. The dielectric properties of WO₃-doped zinc titanate ceramics were measured at different frequencies. The results showed the decreasing tendency with the increasing measuring frequencies for both the dielectric constants and the loss tangents, and there existed the best dielectric properties for 1.0% WO₃-doped ceramics.     

Catalytic synthesis of large-scale GaN nanorods

A novel rare earth metal seed was employed as the catalyst for the growth of GaN nanorods. Large-scale GaN nanorods were synthesized successfully through ammoniating Ga₂O₃/Tb films sputtered on Si(1 1 1) substrates. Scanning electron microscopy, X-ray diffraction, transmission electron microscopy, high-resolution transmission electron microscopy, and X-ray photoelectron spectroscopy were used to characterize the structure, morphology, and composition of the samples. The results demonstrate that the nanorods are high-quality single-crystal GaN with hexagonal wurtzite structure. The growth mechanism of GaN nanorods is also discussed.     

Sol-gel derived tungsten oxide films with pseudocubic triclinic nanorods and nanoparticles

Sol-gel derived tungsten oxide (WO₃) films were deposited using acetylated peroxotungstic acid as the precursor material adopting sol-gel dip coating route, followed by thermal treatment at 500 °C in air for 3 h. Structure-microstructure investigations of these films using X-ray diffraction and transmission electron microscopy techniques revealed the films to exhibit nanorods as the major microstructural feature with combined nanowires and nanoparticles as a minor constituent. The kinetic mechanisms responsible for the formation of nanorods have also been elucidated. Both the nanorods and the particles exhibited a pseudocubic triclinic crystal structure.     

Synthesis of highly crystalline hexagonal cesium tungsten bronze nanoparticles by flame-assisted spray pyrolysis

Highly crystalline and hexagonal single-phase cesium tungsten bronze (Cs_0.32WO₃) nanoparticles were successfully synthesized by a flame-assisted spray pyrolysis followed by annealing under a reducing gas atmosphere. The resulting Cs_0.32WO₃ nanoparticles featured a pure hexagonal Cs_0.32WO₃ phase with a high crystallinity and homogeneous chemical composition. Unlike conventional methods, the proposed process in this paper has several advantages, including a short reaction time and the ability to yield products with high purity and good energy efficiency. Furthermore, the Cs_0.32WO₃ nanoparticles produced in this research showed a remarkable near-infrared shielding ability with a 97.7% cut-off at 1500?nm.     

Study of the Superconducting Cs-doped WO<Subscript>3</Subscript> Crystal Surface by Electron Backscattered Diffraction

The crystallographic surface structure of thermal vapor grown Cs _X WO₃ 0.005≤x≤0.3 crystals was investigated locally by electron backscattered diffraction in the environmental scanning electron microscope. Monoclinic to hexagonal phase transformation was shown to take place upon Cs doping to nominal concentrations of x=0.005 and x=0.05, while monoclinic to trigonal phase transition was observed at a concentration of x=0.3. In particular, the 2D superconducting crystals, of x=0.005 nominal concentration, were of inhomogeneous crystallographic phase according to the local Cs doping. The superconducting Cs-doped regions of the hexagonal phase were shown to be epitaxially grown on the WO₃ monoclinic crystal surface, the (0001) of this phase being parallel to the (001) plane of the WO₃ crystal. Our results support previous observations in these 2D superconducting Cs _X WO₃ crystals.     

Electronic Structure Engineering of Pt Species over Pt/WO3 toward Highly Efficient Electrocatalytic Hydrogen Evolution

Pt-based supported materials, a widely used electrocatalyst for hydrogen evolution reaction (HER), often experience unavoidable electron loss, resulting in a mismatching of electronic structure and HER behavior. Here, a Pt/WO₃ catalyst consisting of Pt species strongly coupled with defective WO₃ polycrystalline nanorods is rationally designed. The electronic structure engineering of Pt sites on WO₃ can be systematically regulated, and so that the optimal electron-rich Pt sites on Pt/WO₃-600 present an excellent HER activity with only 8 mV overpotential at 10 mA cm⁻². Particularly, the mass activity reaches 7015 mA mg⁻¹ at the overpotential of 50 mV, up to 26-fold higher than that of the commercial Pt/C. The combination of experimental and theoretical results demonstrates that the O vacancies of WO₃ effectively mitigate the tendency of electron transfer from Pt sites to WO₃, so that the d-band center could reach an appropriate level relative to Fermi level, endowing it with a suitable $\Delta G_{{\mathrm{H}}^{\ast }}$. This work identifies the influence of the electronic structure on catalytic activity.     

Solution precursor plasma sprayed tungsten oxide particles and coatings

Solution precursor plasma spray process was used to deposit single particles and coatings of tungsten oxide (WO₃), and the microstructures of single particles and coatings were characterized by field emission scanning electron microscopy. The effects of substrate temperature and spraying distance on the microstructure of single particles and coatings were studied. In the case of WO₃ particles, the particle spheroidization degree became better as the increase in substrate temperature. When the substrate temperature increased up to 200°C, bubble-like morphologies appeared. For the deposited WO₃ coatings, a highly porous structure was obtained when a 100?mm spraying distance was used. Besides, the grain size of coatings decreased through increasing the spraying distance from 60 to 100?mm.