Amorphous Tungsten Oxide Research Articles

Charge transport between localized states in lithium-intercalated amorphous tungsten oxide

We report on electron transport in amorphous tungsten oxide films, prepared by dc magnetron sputtering and subsequently electrochemically intercalated with lithium. The ion–electron intercalation process allows us to change the position of the Fermi level in a controlled way and to determine the density-of-states in the conduction band. We present a new method to determine the localization length of the electron states in disordered materials. The method is based on measurements of the electronic density-of-states together with electrical resistance in the variable range hopping regime. We find that the electronic states of amorphous tungsten oxide are localized up to about 1.3eV into the conduction band, where an insulator-metal transition occurs. The localization length was determined on the insulating side of the transition and the estimated scaling exponent is consistent with the scaling theory of localization.

Infrared absorption in Li-intercalated tungsten oxide

Thin films of amorphous and polycrystalline tungsten oxide were produced by reactive dc magnetron sputtering and nanocrystalline films were deposited by advanced gas evaporation. The films were submitted to electrochemical intercalation of Li ions before infrared reflectance measurements were carried out. For crystalline films, the reflectance in the wavelength region 10–30 μm increases upon intercalation, indicating an increasing free-electron contribution. On the other hand, all the films display an increased absorption at wavelengths less than 10 μm when intercalated. The thermal emittance could be varied from about 0.5 to 0.7–0.75 by intercalation in films with thicknesses in excess of 1 μm. Both absorption and interference contribute to the emittance contrast.

Reactive Sputtering Deposition of a-WO3-z on Flexible PET/ITO Substrates for Electrochromic Devices

The deposition of electrochromic thin films of a-WO3-z (amorphous tungsten oxide) on flexible PET (polyethylene terephthalate)/ITO (indium tin oxide) substrates by a reactive radio frequency (r.f.) sputtering method was investigated. The method used argon and oxygen at room temperature (23 °C). Cyclic voltammetry measurements showed that only low driving voltages −1 and +1 V were needed to provide reversible Li+ ion insertion and extraction. Cyclic potentiostatic switching and visible–UV spectroscopy showed that thin films with oxygen deficiency z of 0.22 can offer excellent light modulation, with up to 62% transmittance variation and color efficiency of 61.3 cm2 · C−1.

Confocal spectromicroscopy of amorphous and nanocrystalline tungsten oxide films

A Raman confocal spectromicroscopic system was used to study in situ phase composition and surface morphology in amorphous and nanocrystalline tungsten oxide and tungstate thin films, prepared on silicon and glass substrates by dc magnetron co-sputtering technique. The possible use of these films for the phase-change optical recording was demonstrated using 442 nm He–Cd laser with a variable power of up to 50 mW. The formation of nanocrystalline tungsten trioxide or tungstate phases was observed under the laser irradiation. These nanocrystalline phases show relatively strong Raman activity, which can be used for information reading purposes. A multilayer structure composed of several tungstate films with different chemical composition is proposed as potential write-once optical recording media.

Optical and kinetic properties of cathodically deposited amorphous tungsten oxide films

Optical properties and the coloration–decoloration kinetics of electrochromic films of amorphous tungsten oxide (a-WO 3), produced by cathodic deposition from a sodium tungstate based aqueous peroxide electrolyte, have been investigated. As films color in 1 N H 2SO 4, sequential appearance of bands with maxima at ∼1 eV, 1.6 eV, 2 eV, and 2.4 eV is observed in their optical absorption and electrosorption spectra, is the same as in the case of reduction of nanosized hydrated-WO 3 colloids with a gradual decrease in their size to that of 12-tungsten polyanions with Keggin structure, indicating the presence of such polytungstates in cathodically deposited a-WO 3, too. When polytungstate is reduced by one electron, an absorption band with a maximum at ∼1.6 eV appears in the optical spectrum of the film. This band corresponds to the optical excitation of charge transfer of the W 5+ → W 6+ type between two adjacent tungsten atoms. The reduction of polytungstate by a second electron with potential shift towards more negative values is accompanied by the appearance of an analogous band with a maximum at ∼2 eV. The reduction of such polytungstates involves participation of the bulk of injected electrons, indicating their dominate role in the nanostructure of the films investigated. The effective co-diffusion coefficient of electrons and protons in cathodically deposited a-WO 3 exhibits a potential dependence with a maximum at 0.1 V against a silver-chloride electrode, where its value is ∼10 −8 cm 2/s. It has been shown that the decrease in this coefficient at potential values of over 0.1 V is caused by a decrease in electron mobility.

A comparison of electrochromic properties of sol–gel derived amorphous and nanocrystalline tungsten oxide films

A pristine acetylated peroxotungstate sol with and without 4 wt% of oxalic acid dihydrate (OAD) yielded nanocrystalline and amorphous tungsten oxide (WO3) films respectively by dip coating technique. Contrary to the expected trend, whereby, the nanostructured 4% OAD film with a triclinic modification should have shown superior electrochromic efficiency, its amorphous 0% OAD counterpart exhibits higher optical modulation and coloration efficiency at photopic wavelengths. This anomalous behavior of the amorphous 0% OAD film was correlated to a higher W5+ content in its colored state. The colored nanocrystalline 4% OAD film contained a lower proportion of the W5+ color centers. Under the same level of lithiation, while the 4% OAD film was also constituted by W4+ states, its 0% OAD counterpart did not contain any 4+ states of tungsten. X-ray photoelectron spectroscopic (XPS) investigations also confirmed that the single peak at ∼1.4 eV in the absorption coefficient–wavelength spectrum of the colored 0% OAD film arises from the small polaronic transitions between the W6+ and W5+ states of tungsten whereas the W6+–W5+ and the W5+–W4+ charge transitions produce two distinct peaks at 1.2 and 1.6 eV in the α–λ spectrum of the colored 4% OAD film. The microstructure of the 4% OAD film, characterized by an interconnected network of nanocrystallites and pores promotes rapid ion insertion and extraction. Therefore, the larger magnitudes of NIR reflectance modulation, diffusion coefficient for lithium, electrochemical activity and faster color–bleach kinetics observed for the 4% OAD film, are a direct consequence of its structure. Band gap widening upon lithium insertion observed for both films, is a repercussion of Burstein–Moss effect and structural changes that occur upon coloration.

Sputtering Deposition Parameters and Their Interactions in Amorphous Tungsten Oxide Thin Film as Electrochromic Electrode

Amorphous tungsten oxide (a-WO3) thin films were deposited as electrochromic electrodes on ITO coated glass substrates via reactive DC magnetron sputtering from metallic tungsten target. A two-level full-factorial design-of-experiment (DoE) was carried out with the following variables: 1) sputter target power density, 2) total pressure, 3) oxygen to argon flow ratio and 4) deposition time. Characterizations via lithium intercalation in potentiostatic mode were conducted in a wet electrochemical cell. Our work shows that parameters interaction should be considered in optimizing WO3 thin film for electrochromic applications. Work based on a factorial design reduces the number of experiments and allows reaching near-optimized region with small amount of effort.

A case study of optical properties and structure of sol–gel derived nanocrystalline electrochromic WO3 films

Sol–gel derived, as-deposited amorphous tungsten oxide (WO3) films become nanocrystalline with a pseudocubic triclinic structure upon annealing at 600 °C. The annealed films are constituted of nanorods along with interconnected nanoparticles and pores. While the high transmission modulation and colouration efficiency in the visible region and the fast bleaching kinetics as shown by the nanostructured film were typical of amorphous WO3, the absorption coefficient spread in the 300–2000 nm wavelength range, the reflectance modulation and the colouration efficiency peak in the NIR region were reminiscent of crystalline WO3. The larger magnitude of the indirect band gap shift to blue than the direct gap shift for the same level of lithium intercalation (18 mC cm−2) and the irreversibility of the direct gap shift upon bleaching are characteristic of the nanocrystalline structure of the film. The moderate decline in the anodic peak current maximum, the diffusion coefficient for lithium and the optical modulation at 632.8 nm upon repetitive switching between the coloured and bleached states ratified that nanostructured films sustain 1000 cycles without much deterioration.

Electrochromic Nanostructured Tungsten Oxide Films by Sol-gel: Structure and Intercalation Properties

As-deposited sol-gel derived amorphous tungsten oxide films transform into nanostructured films with an interconnected framework of grains and pores and a dominant triclinic crystalline phase upon annealing at . Transmission electron microscopy and scanning electron microscopy images clearly reveal the annealing-induced microstructural evolution for the film. Subsequent to lithium intercalation, the film annealed at shows quasi-reversible structural changes, as ascertained by X-ray diffraction and Fourier transform infrared spectral data. Dynamic transmission modulation for film revealed a high optical modulation of 72% and a coloration efficiency maximum of at under a lithium intercalation level of . X-ray photoelectron spectroscopy of the W 4f core levels demonstrated a progressive increase in the content at the expense of proportion as the insertion coefficient was raised from 0 to 0.25, with 0.20 as the threshold value above which the content exceeds the proportion. A new state also appears which acts to lower the coloration efficiency for . The presence of charged oxygen interstitials in the vicinity of electrochemically active tungsten sites is also responsible for the coloration efficiency decline at high ion insertion levels.

Evaluation of the Structure of Amorphous Tungsten Oxide W28O72 by the Combination of Electron-, X-Ray- and Neutron-Diffraction (Three-Beam Experiment)

From the combination of quantitative electron-diffraction data with X-ray- and neutron-diffraction data (so-called three-beam experiment) the partial structure factors and pair correlation functions of amorphous sputter deposited W28O72 were determined. On the basis of the experimental atomic distances and coordination numbers, and by comparison with crystalline WO3, a structural model was developed, which consists of twisted WO6 octahedra. Reverse Monte Carlo simulation in accordance with the experimental data was performed to verify the results

Structural effect of tungsten oxides on selective oxidation of cyclopentene to glutaraldehyde

Tungsten oxides with different chemical structures, such as hydrated tungsten oxide (WO 3 · H 2O or WO 3 · 0.33H 2O), tungstic acid hydrate (H 2WO 4 · H 2O), and amorphous tungsten oxide, were prepared starting from ammonium tungstate and using nitric, hydrochloric, acetic, and sulfuric acids as precipitating reagents, respectively. Amorphous and less hydrated tungsten oxides exhibited higher catalytic activities than tungstic acid hydrate and tungsten oxide monohydrate did in the catalytic oxidation of cyclopentene to glutaraldehyde with aqueous H 2O 2. The weak interaction between H 2O 2 and amorphous or less hydrated tungsten oxide was beneficial to the formation of glutaraldehyde.

Optical charge transfer absorption in lithium-intercalated tungsten oxide thin films

Amorphous tungsten oxide exhibits electrochromism when intercalated with protons or lithium ions. Thin films of the material were prepared by dc magnetron sputtering and subsequently electrochemically intercalated with lithium. The optical absorption in the wavelength range 300to2500nm was measured for a number of lithium concentrations. All the spectra can be fitted by a superposition of three Gaussian peaks, representing the three possible electronic transitions between W6+, W5+, and W4+ sites. The variation of the peak strength with lithium concentration is consistent with the predictions of a statistical theory.

Sputtering Deposition Parameters and Their Interactions in Amorphous Tungsten Oxide Thin Film as Electrochromic Electrode

not Available.

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.

Oxygen-Sputtered Tungsten Oxide Thin Films for Enhanced Electrochromic Properties

This paper reports an enhanced coloration in amorphous stoichiometric tungsten oxide (a-WO 3 ) thin films prepared by pulsed dc magnetron sputtering of metallic tungsten with oxygen as sputter gas. The films prepared at higher oxygen pressure (5.2 x 10 -2 mbar) have high coloration efficiency (182 cm 2 /C), large optical modulation (∼83%) and faster switching speed (t c ∼ 32 s at X = 633 nm). The work function of the virgin and the colored tungsten oxide thin films measured by Kelvin probe are 4.91 and 4.26 eV, respectively. The coloration process conforms to the recent theory based on an extended site saturation model.

Amorphous indium tungsten oxide films prepared by DC magnetron sputtering

Indium-tungsten-oxide (IWO) films were prepared by dc magnetron sputtering at ambient substrate temperature (Ts). Characteristics of the films were compared with those of In2O3–SnO2 (ITO) thin films prepared under the same condition. The sputter-deposited IWO films have entirely amorphous structure with an average transmittance of over 85% in the visible range and exhibit a minimum resistivity of 3.2 × 10−4Ωcm at W content [W/(In + W)] of 0.57 at.%. An in-situ heating X-ray diffraction measurement have shown that the crystallization temperature of IWO films is higher than those of ITO films (150–160∘C) and increases with increasing W content. This enabled a smooth amorphous surface of IWO films as compared with a rough surface of partially crystallized ITO films as revealed by an atomic force microscopy. IWO films are useful for transparent electrode of organic light emitting diode and polymer LCDs because of the low resistivity, high transparency and smooth surface obtainable by the conventional dc magnetron sputtering at room temperature.

Tungsten oxide nanoparticles synthesised by laser assisted homogeneous gas-phase nucleation

Tungsten oxide nanoparticles were generated by excimer (ArF) laser assisted chemical vapor deposition from WF 6/H 2/O 2/Ar gas mixtures. The deposited particles were characterized by X-ray diffraction, X-ray photoelectron spectroscopy, and transmission electron microscopy. The deposition rate as a function of the partial pressures of the reactants and of the laser fluence was measured by X-ray fluorescence spectroscopy. The mean diameter of the deposited tungsten oxide particles varied with the experimental parameters and was typically 23 nm. Particles with a higher degree of crystallinity were observed at a laser fluence exceeding 130 mJ/cm 2, and X-ray amorphous particles were obtained below 110 mJ/cm 2. The amorphous tungsten oxide had a stoichiometry ranging from WO 2.7 to WO 3. Deposits were formed only when hydrogen was present in the gas mixture.

Flexible foils with electrochromic coatings: science, technology and applications

This paper covers a number of aspects of a novel flexible electrochromic foil capable of varying its optical transmittance. The foil includes thin films of tungsten oxide and nickel oxide and an intervening polymer electrolyte serving as lamination material. Concerning scientific aspects, we discuss the prevalent defects in amorphous tungsten oxide and how they lead to a consistent picture of the optical properties of tungsten oxide films as a function of non-stoichiometry and ion intercalation. We also present a refined model for the coloration/bleaching due to proton extraction/insertion in surface sheaths of nano-crystallites of nickel oxide. We then turn to aspects of technology and treat ways to enhance the bleached-state transmittance by mixing the nickel oxide with another oxide having a wide band gap, pre-assembly charge insertion/extraction by facile gas treatments of the films and practical device manufacturing. The final part covers some applications with emphasis on architectural “smart windows” capable of achieving improved indoor comfort together with significant energy savings due to lowered demands for space cooling. We also touch upon applications concerning eyewear.

Esterification of palmitic acid with methanol over tungsten oxide supported on zirconia solid acid catalysts: effect of method of preparation of the catalyst on its structural stability and reactivity

Esterification of palmitic acid with methanol was studied on zirconia supported tungsten oxide solid acid catalysts prepared by both impregnation and co-precipitation methods; the tungsten loading and calcination temperature were varied. These catalysts were characterized by X-ray diffraction for structural elucidation and by temperature-programmed desorption of ammonia for their surface acidity. Whereas the impregnated catalysts exhibited both tetragonal and monoclinic phases of zirconia, at calcination temperatures beyond 500 °C their co-precipitated analogues showed stability towards the tetragonal phase. The formation of monoclinic phases of zirconia and crystalline tungsten oxide decreased the esterification activity, as a consequence of decreased surface acidity. Coexistence of tetragonal zirconia with the amorphous tungsten oxide offered maximum esterification activity.

Comment on “Infrared spectra and second-harmonic generation in barium strontium titanate and lead zirconate titanate thin films: ‘Polaron’ artifacts” [J. Appl. Phys. 94, 3333 (2003)

Scott et al. [Scott et al., J. Appl. Phys. 94, 3333 (2003)] have questioned our measurements of the optical absorption of amorphous tungsten oxide, and have suggested that the polaron absorption might be an artifact due to interference effects. In this comment we point out that we obtained the absorption coefficient from measurements of both the transmittance and the reflectance of thin films deposited on transparent substrates. By this method the “true” absorption is easily obtained.