Kinetics of coloration of electrochromic tungsten oxide films produced by cathodic electrodeposition

The dependence of the kinetic characteristics of electrochromic indicator devices based on films of amorphous tungsten oxide α-WO₃ on the speed of reversible coloration of their working electrode with a film of this kind was studied.     

The microstructure and properties of thin WO3 films modified by gold

Direct current magnetron sputtering of a metallic W + Au target or high-frequency magnetron sputtering of WO₃ + Au oxide targets was used to prepare thin (about 100 nm) nanocrystalline WO₃ films with the addition of gold (disperse layers of catalytic gold were additionally deposited on the surface of films). The composition and micromorphology of the surface of films and the electrical and gas sensitive characteristics of nitrogen dioxide sensors were studied to determine the mechanism of the influence of gold on the properties of WO₃ films. The films were shown to contain the β-WO₃ orthorhombic and γ-WO_2.72 monoclinic phases and gold particles. The presence of the nonstoichiometric γ-WO_2.72 phase was shown to increase the concentration of oxygen vacancies in films and decrease the resistance of sensors to 1–2 MΩ. Gold nanoparticles 9–15 nm in size segregated on the surface of semiconductor crystallites and increased the response of sensors to NO₂. The conclusion was drawn that deposited catalytic gold layers increased the response to traces of nitrogen dioxide.     

X-Ray Analysis of Multi-Films for Electrochromic Device Application

Multilayer films based on tungsten oxide (WO₃), ITO (indium tin oxide) and CdS were deposited mainly by reactive dc magnetron sputtering onto glass substrates for electrochromic application. The thin films were analyzed by means of XPS (X-ray photoelectron spectroscopy), GIXD (grazing incidence X-ray diffraction) and XRD (X-ray diffraction). XRD and XPS results confirmed that the films were WO₃, CdS and ITO, respectively. The surface and interface of the CdS/ITO bi-layered film was studied by GIXD in different incidence angles. Detailed results about the amorphous characterization of the films during room temperature growth and post annealing are given.     

Structural, Vibrational, and Gasochromic Properties of Porous WO3 Films Templated with a Sol-Gel Organic–Inorganic Hybrid

 The structure and the gasochromic properties of sol-gel-derived WO₃ films with a monoclinic structure (m-WO₃) were studied by focusing attention on the size of the monoclinic grains. The size of the m-WO₃ grains is modified by the addition of an organic–inorganic hybrid to the initial peroxopolytungstic acid (W-PTA) sols which are based on chemically bonded poly-(propylene glycol) to triethoxysilane end-capping groups (ICS-PPG). The results obtained with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the heat treatment (500°C) of WO₃/ICS-PPG (0.5, 1, 2, 5, and 10 mol%) composite films results in a change of their morphology, and nanodimensional pores are formed between the grains. High-resolution TEM (HRTEM) analysis revealed the presence of an amorphous phase on the outside of the m-WO₃ grains, whereas energy-dispersive X-ray spectra (EDXS) showed that this amorphous phase contained W and Si. Impregnation of the WO₃/ICS-PPG film with H₂PtCl₆/i-propanol solution followed by heat treatment at 380°C gave the films their gasochromic properties. Infrared and Raman spectroscopic studies of the WO₃/ICS-PPG film confirmed the results of the corresponding HRTEM and EDXS analysis. In situ UV/Vis and in situ IR spectra of the films were measured in hydrogen and in air, and colouring/bleaching changes and the corresponding kinetics were assessed. The IR spectra of gasochromically coloured films showed that the mesoporous WO₃/ICS-PPG (1 mol%) film transforms to tetragonal H _x WO₃ bronze. The IR spectra of the H _x WO₃ bronze are discussed with the aim to establish the existence of the metal-OH vibrations of gasochromically formed oxyhydroxide tungsten bronze.     

Microstructural evolution of protective La–Cr–O films studied by transmission electron microscopy

Transmission Electron Microscopy (TEM) and Electron Energy Loss Spectroscopy (EELS) were performed to study the microstructural evolution of La–Cr–O thin films deposited by radio frequency (RF)-magnetron sputtering on stainless steel substrates. Chromium L edges and oxygen K edges are analyzed to determine the valence states of the chromium and elucidate the phase evolution of the thin film. The as-deposited amorphous thin film crystallized to LaCrO₄ and finally transformed to the LaCrO₃ stable phase during annealing at 800°C. An intermediate Cr/Mn oxide layer was formed in all annealed samples. The thickness of this oxide layer stabilizes after 700°C, which indicates that the LaCrO₃ thin film plays a role in inhibiting the growth of an oxide layer on the metal surface.     

Formation and dielectric properties of anodic oxide films on Zr–Al alloys

Zr–Al alloys containing up to 26 at.% aluminum, prepared by magnetron sputtering, have been anodized in 0.1 mol dm⁻³ ammonium pentaborate electrolyte, and the structure and dielectric properties of the resultant anodic oxide films have been examined by grazing incidence X-ray diffraction, transmission electron microscopy, Rutherford backscattering spectroscopy, and AC impedance spectroscopy. The anodic oxide film formed on zirconium consists of monoclinic and tetragonal ZrO₂ with the former being a major phase. Two-layered anodic oxide films, comprising an outer thin amorphous layer and an inner main layer of crystalline tetragonal ZrO₂ phase, are formed on the Zr–Al alloys containing 5 to 16 at.% aluminum. Further increase in the aluminum content to 26 at.% results in the formation of amorphous oxide layer throughout the thickness. The anodic oxide films become thin with increasing aluminum content, while the relative permittivity of anodic oxide shows a maximum at the aluminum content of 11 at.%. Due to major contribution of permittivity enhancement, the maximum capacitance of the anodic oxide films is obtained on the Zr–11 at.% Al alloy, being 1.7 times than on zirconium at the formation voltage of 100 V.     

Structural,Vibrational, and Gasochromic Properties of Porous WO<Subscript>3</Subscript> Films Templated with a Sol-Gel Organic–Inorganic Hybrid

Summary.  The structure and the gasochromic properties of sol-gel-derived WO₃ films with a monoclinic structure (m-WO₃) were studied by focusing attention on the size of the monoclinic grains. The size of the m-WO₃ grains is modified by the addition of an organic–inorganic hybrid to the initial peroxopolytungstic acid (W-PTA) sols which are based on chemically bonded poly-(propylene glycol) to triethoxysilane end-capping groups (ICS-PPG). The results obtained with scanning electron microscopy (SEM) and transmission electron microscopy (TEM) show that the heat treatment (500°C) of WO₃/ICS-PPG (0.5, 1, 2, 5, and 10 mol%) composite films results in a change of their morphology, and nanodimensional pores are formed between the grains. High-resolution TEM (HRTEM) analysis revealed the presence of an amorphous phase on the outside of the m-WO₃ grains, whereas energy-dispersive X-ray spectra (EDXS) showed that this amorphous phase contained W and Si. Impregnation of the WO₃/ICS-PPG film with H₂PtCl₆/i-propanol solution followed by heat treatment at 380°C gave the films their gasochromic properties. Infrared and Raman spectroscopic studies of the WO₃/ICS-PPG film confirmed the results of the corresponding HRTEM and EDXS analysis. In situ UV/Vis and in situ IR spectra of the films were measured in hydrogen and in air, and colouring/bleaching changes and the corresponding kinetics were assessed. The IR spectra of gasochromically coloured films showed that the mesoporous WO₃/ICS-PPG (1 mol%) film transforms to tetragonal H _x WO₃ bronze. The IR spectra of the H _x WO₃ bronze are discussed with the aim to establish the existence of the metal-OH vibrations of gasochromically formed oxyhydroxide tungsten bronze. Received October 4, 2001. Accepted (revised) November 19, 2001     

AFM,ellipsometry, XPS and TEM on ultra-thin oxide/polymer nanocomposite layers in organic thin film transistors

Here we report on the fabrication and characterization of ultra-thin nanocomposite layers used as gate dielectric in low-voltage and high-performance flexible organic thin film transistors (oTFTs). Reactive sputtered zirconia layers were deposited with low thermal exposure of the substrate and the resulting porous oxide films with high leakage currents were spin-coated with an additional layer of poly-α-methylstyrene (PαMS). After this treatment a strong improvement of the oTFT performance could be observed; leakage currents could be eliminated almost completely. In ellipsometric studies a higher refractive index of the ZrO₂/PαMS layers compared to the “as sputtered” zirconia films could be detected without a significant enhancement of the film thickness. Atomic force microscopy (AFM) measurements of the surface topography clearly showed a surface smoothing after the PαMS coating. Further studies with X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) also indicated that the polymer definitely did not form an extra layer. The polymer chains rather (self-)assemble in the nano-scaled interspaces of the porous oxide film giving an oxide–polymer “nanocomposite” with a high oxide filling grade resulting in high dielectric constants larger than 15. The dielectric strength of more than 1 MV cm⁻¹ is in good accordance with the polymer-filled interspaces.     

In situ PM‐IRRAS studies of organothiols and organosilane monolayers–ZnO interfaces at high water activities

In situ photoelastic‐modulated Fourier transform infrared reflection absorption spectroscopy has been applied for the investigation of interfacial stability of organothiol and organosilane monolayer films on nanocrystalline zinc oxide thin films. It has been shown that for octadecyltriethoxysilane films, exposure to high water activities results in physisorption of water in the cross‐linked film. This high water activity at the interface leads to a reversible wet de‐adhesion of the interfacial silanol groups from the ZnO surface. However, the organothiol seems to form a denser monolayer and a stable by S–Zn bond that is resistant to the competition with adsorbed water. The reversible attachment for cross‐linked organosilanol films has been demonstrated for the first time by means of an in situ spectroscopic method on model ZnO surfaces. Copyright © 2016 John Wiley & Sons, Ltd.     

Synthesis and evaluation of bis-β-diketonate dioxotungsten(VI) complexes as precursors for the photodeposition of WO3 films

In this paper non-stoichiometric tungsten oxide thin films have been successfully prepared by direct UV irradiation of bis-β-diketonate dioxotungsten(VI) precursor complexes spin-coated Si(1 0 0) substrates. Photodeposited films were characterized by Fourier transform-infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and the surface morphology examined by Atomic Force Microscopy (AFM). The results of XRD analysis showed that the as-photodeposited WO_3−x films are amorphous and have a rougher surface than thermally treated films. Post-annealing of the films in air at 500 °C transforms the sub-oxides to a monoclinic WO₃ phase.     

Enhanced Property of Thin Cuprous Oxide Film Prepared through Green Synthetic Route

Thin cuprous oxide films have been prepared by chemical vapor deposition (pulsed spray evaporation-chemical vapor deposition) method without post-treatment. The synthesis ofcuprous oxide was produced by applying a water strategy effect. Then, the effect of water on the morphology, topology, structure, optical properties and surface composition of the obtained films has been comprehensively investigated. The results reveal that a pure phase of Cu₂O was obtained. The introduction of a small quantity of water in the liquid feedstock lowers the band gap energy from 2.16 eV to 2.04 eV. This finding was mainly related to the decrease of crystallite size due to the effect of water. The topology analyses, by using atomic force microscope, also revealed that surface roughness decreases with water addition, namely more uniform covered surface. Moreover, theoretical calculations based on density functional theory method were performed to understand the adsorption and reaction behaviors of water and ethanol on the Cu₂O thin film surface. Formation mechanism of the Cu₂O thin film was also suggested and discussed.     

Sol-gel synthesis of TeO<Subscript>2</Subscript>-based materials using citric acid as hydrolysis modifier

Sol-gel processing of tellurium oxide has been investigated in the tellurium isopropoxide/citric acid/isopropanol/water system. As evidenced by Fourier transformed infrared spectroscopy (FTIR), citric acid has been found to be a relevant chemical modifier to control hydrolysis-condensation reactions of highly reactive tellurium isopropoxide Te(OCH(CH₃)₂)₄. Thus, depending on the main synthesis chemical parameters such as alkoxide concentration, water and modifier ratios, colloidal sols and gels have been successfully synthesised. The thermal behaviour of the dried gels has been investigated by X-ray diffraction, differential scanning calorimetry coupled with thermogravimetry and also FTIR spectroscopy. On the one hand, the crystallisation of the non-centrosymmetric γ-TeO₂ polymorph as well as the α-TeO₂ phase which the crystallite size ranges from a few ten nanometers (∼50 nm) to a few microns as a function of heat treatment, and, on the other hand, the synthesis of homogeneous sols which can be handled in air and so particularly suitable for the elaboration of thin films provide new opportunities for making tellurite based materials and thin film devices for practical applications.     

Toward a quantitative description of the anodic oxide films on aluminum

A method to quantify the composition of anodic oxide films on aluminum using Infrared Spectroscopic Ellipsometry (IRSE) is proposed. It consists of obtaining the absorption coefficient of the film as a function of wavelength. Using values of the absorption coefficients for the pure components of the film, the percentages (mole or wt%) of each component in the sample can be calculated.The method is demonstrated in a study of the structure of the oxide film on electropolished aluminum and the anodically formed barrier layer film. Both surface oxides were found to be initially a form of amorphous Al₂O₃. While the barrier film is essentially free of water as prepared, the film on electropolished aluminum contained about 25 wt% water. Hydration of both types of films by immersion in boiling water results in the formation of pseudoboehmite (AlOOH). The technique may have more general applicability to the quantitative determination of the composition of corrosion films and other surface layers on metals.     

An Efficient Route to Nanostructured Tungsten Oxide Films with Improved Electrochromic Properties

An effective chemical route to nanostructured tungsten oxide films derived from a peroxopolytungstic acid (PTA)/thiourea precursor solution is demonstrated. The conventional procedure of preparing the precursor needs more than 24 h for well‐mixing and refluxing the PTA‐based solution, while the thiourea‐assisted approach takes less than 1 h to prepare the precursor solution because the excess hydrogen peroxide can be efficiently eliminated by oxidation of thiourea. With the precursor solution, tungsten oxide films are deposited by spin coating followed by high temperature annealing. The film annealed at 400 °C possesses a porous nanostructure of nanocrystalline tungsten oxide embedded in an amorphous tungsten oxide matrix, which arises from the gaseous species released through decomposition of thiourea oxides during annealing. The 400 °C‐annealed, thiourea‐assisted tungsten oxide film exhibits electrochromic (EC) properties superior to those of the film prepared without thiourea, including large transmittance modulation and coloration efficiency, fast response time and adequate reliability. When increasing the annealing temperature to 450 °C, the thiourea‐assisted tungsten oxide film is also porous but well‐crystallized and shows inferior EC properties. Electrochemical impedance spectroscopy analysis indicates that, in addition to the porous structure, a fast charge‐transport rate within the solid portion of the 400 °C‐annealed nanostructured film plays a crucial role in enhancing EC performances of the thiourea‐assisted tungsten oxide film.     

Influence of PEG additive and annealing temperature on structural and electrochromic properties of sol–gel derived WO<Subscript>3</Subscript> films

Sol–gel derived tungsten oxide (WO₃) films have been deposited by spin coating route using acetylated peroxotungstic acid (APTA) or a mixture of APTA and polyethylene glycol (PEG) dissolved in ethanol as the precursor solution, followed by thermal treatment in air. The influence of PEG additive and annealing temperature on the structural and electrochromic (EC) behavior of the films have been investigated. For films annealed at 300 °C, a porous nanocrystalline/amorphous microstructure was obtained in the WO₃-PEG film, while monoclinic microstructure was formed in the pure WO₃ film. Moreover, for the WO₃-PEG films, the film microstructure was found to depend on the annealing temperature. Electrochemical studies indicate that the WO₃-PEG film annealed at 300 °C (WP-300) exhibits superior EC properties, which produces faster switching speed (t _c = 19 s, t _b = 3 s),better reversibility (K = 0.97) as well as higher optical modulation (ΔT = 32% at 550 nm) and coloration efficiency (η = 22 cm²/C at 550 nm). Our results suggest that PEG addition in combination with an appropriate annealing treatment can benefit the EC properties, arising from the ease of ion diffusion within the EC material, as evident from the nanocrystallines embedded into the amorphous matrix with a porous character.     

Optical Characteristics and Structure of Electrochromic Amorphous Tungsten Trioxide Films Electrodeposited from Aqueous Electrolytes

The structure of electrochromic amorphous tungsten trioxide films electrodeposited from an aqueous tungsten-containing electrolyte and specific features of the optical and electrochemical characteristics of these films in different stages of their reversible coloration in a 1 N H₂SO₄ solution were studied.     

Effective charge propagation and storage in hybrid films of tungsten oxide and poly(3,4-ethylenedioxythiophene)

Hybrid (composite) electroactive films consisting of such an organic conducting polymer as poly(3,4-ethylenedioxythiophene), PEDOT, and such a polynuclear inorganic compound as amorphous tungsten oxide, WO₃/H _x WO₃ were fabricated on carbon electrodes through electrodeposition by voltammetric potential in acid solution containing EDOT monomer and sodium tungstate. Electrostatic interactions between the negatively charged tungstic units (existing within WO₃) and the oxidized positively charged conductive polymer (oxidized PEDOT) sites create a robust hybrid structure which cannot be considered as a simple mixture of the organic and inorganic components. It is apparent from scanning electron microscopy that hybrid structures are granular but fairly dense. Because PEDOT and mixed-valence tungsten oxides are electronically conducting, the resulting hybrid films are capable of fast propagation. The reversible and fast redox reactions of tungsten oxide component lie in the potential range where PEDOT matrix is conductive. Furthermore, the hybrid films exhibit good mediating capabilities towards electron transfers between model redox couples such as cationic iron(III,II) and anionic hexacyanoferrate(III,II). Since the films accumulate effectively charge and show high current densities at electrochemical interfaces, they could be of importance to electrocatalysis and to construction of redox capacitors.     

Interaction of acetic acid with solid water

The interaction of acetic acid (AA, CH(3)COOH), with solid water, deposited on metals, tungsten and gold, at 80 K, was investigated. We have prepared acid/water interfaces at 80 K, namely, acid layers on thin films of solid water and H(2)O adlayers on thin acid films; they were annealed between 80 and 200 K. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy UPS(HeII) were utilized to obtain information on the electronic structure of the outermost surface from the study of the electron emission from the weakest bound MOs of the acids, and of the molecular water. Temperature-programmed desorption (TPD) provided information on the desorption kinetics, and Fourier-transformed infrared spectroscopy (FTIR) provided information on the identification of the adsorbed species as well as on the water and acid crystallization. The results are compatible with the finding of ref 1 (preceding paper), made on the basis of DFT calculations, that AA adsorbs on ice as cyclic dimers. Above 120 K, a rearrangement of the AA dimers is suggested by a sharpening of the spectral features in the IR spectra and by spectral changes in MIES and UPS; this is attributed to the glass transition in AA around 130 K. Above 150 K the spectra transform into those characteristic for polycrystalline polymer chains. This structure is stable up to about 180 K; desorption of water takes place from underneath the AA film, and practically all water has desorbed through the AA film before AA desorption starts. There is no indication of water-induced deprotonation of the acid molecules. For the interaction of H(2)O molecules adsorbed on amorphous AA films, the comparison of MIES with the DFT results of ref 1 shows that the initial phase of exposure does not lead to the formation of a top-adsorbed closed water film at 80 K. Rather, the H(2)O molecules become attached to or incorporated into the preexisting AA network by H bonding; no water network is formed in the initial stage of the water adsorption. Also under these conditions no deprotonation of the acid can be detected.     

Solution thermolysed tungsten oxide-based electrochromic devices: thickness-dependent step potential analysis

The characterisation of electrochemical behaviour of electrochromic (EC) devices based on solution thermolysed (ST) tungsten oxide (WO₃) thin films was carried out using the step potential excitation method. The method, based on generating plots of current density (J) as a function of passed charge (ΔQ), has been applied for the characterisation of EC-WO₃ thin films in proton-containing aqueous electrolyte. EC devices have been fabricated by employing WO₃ thin films with variable thickness (T) ranging from 0.04 to 0.52 μm. The J vs time (t) responses (chronoamperometry) of these devices were recorded at a fixed applied potential (±0.7 V vs S.C.E.) and values of total passed H⁺ charges (ΔQ) into the WO₃ host lattice during the coloration process are calculated. The J-ΔQ curves corresponding to films of different thickness were plotted as a function of the passed charge volume density, ΔQ /T, and an intercalatable film thickness is calculated to be 0.13 μm. The modulation in optical transmittance after coloration and bleaching was studied in the wavelength range between 350 and 850 nm and an optical efficiency (ξ_λ) is calculated at λ=700 nm. It is found that the ξ_λ wanes with increasing intercalation. Electronic Publication     

Tribological performance of fluoroalkylsilane modification of sol–gel TiO<Subscript>2</Subscript> coating

This paper explores the possibility of making coatings with super friction-reducing and wear protection properties by using both sol–gel and self-assembling techniques. The thin film of TiO₂ was firstly prepared on glass substrates via a sol–gel method, followed by sintering at 480°C. The self-assembled monolayer of Fluoroalkylsilane (FAS) were then prepared on TiO₂ thin film to obtain TiO₂–FAS dual-layer film. The contact angle measurement and X-ray photoelectron spectroscopy were used to determine the wetting behavior and chemical structure of films, respectively. The friction behavior of films sliding against a steel ball was examined on a macro friction and wear tester. It is shown that FAS is strongly adsorbed on sol–gel derived TiO₂ thin film, making it strongly hydrophobic. Good friction-reducing and wear protection behavior is observed for the glass substrate after duplex surface-modification with sol–gel TiO₂ and top layer of FAS.