Tungsten Oxide Layer Research Articles

High Rate Capability of Tungsten Oxide Coated Graphite Anode for Lithium-Ion Battery

On increased demands for electric vehicles (EV), Lithium-Ion Batteries (LIBs) face new challenge in terms of fast charging. Graphite, successfully implemented as anode material in current LIBs, shows prominent limitation during fast charging process. As resolving such a problem, many researchers have been suggesting the alternative of graphite materials. In this presentation, we present fast charging capability of graphite anode by coating tungsten oxide layer by means of sol-gel reaction. The tungsten oxide coated graphite showed high performance at a high rate without compromising reversible capacity. The tungsten oxide coated graphite anode showed 90.3% of capacity retention at a rate of 5 C (1800 mA g-1) compared to reversible capacity obtained at a rate of 0.2 C (72 mA g-1). This result showed that the tungsten oxide coated graphite can provide a way to overcome the limitation in term of rate capability. More detailed microstructure and electrochemical properties of tungsten oxide coated graphite material will be discussed in the presentation.

Ion conductor barrier height for the switching time improvement of all-thin-film inorganic complementary electrochromic devices

In this study, an all-thin-film inorganic complementary electrochromic device (ECD) with the lithium doping zirconium dioxide (ZrO2:Li) ion conductor layer has demonstrated for the lower cost ECD applications. Even though the ZrO2:Li has demonstrated a lower lithium (Li) ion conductivity of 1.62 × 10−8 S/cm and ion mobility of 8.67 × 10−12 cm2/Vs, the switching characteristic of the ECDs has been improved. According to the refractive index measurement, the ion conductor ZrO2:Li with the higher refractive index value is also indicated a higher energy bandgap material. As a result, the possible mechanism is due to the electron energy barrier height 1.2 eV between the interface of electrochromic layer tungsten oxide (WO3) and ion conductor layer ZrO2:Li. Besides, the induction of an electric field between the WO3 and high bandgap ZrO2:Li material interface also accelerates the Li ion transported through the interface. Furthermore, the ZrO2:Li with the lower ion conductivity and mobility is due to the dense material structure, which is verified by the cross-sectional field emission scanning electron micrograph profile. Additionally, the dense material structure has also provided better surface morphology for thin-film device structures with the layer-by-layer deposition process.

Application of tungsten oxide thin film in the photonic crystal cavity for hydrogen sulfide gas sensing

In this work, we report the design and simulation of a high sensitivity hydrogen sulfide (H2S) gas sensor based on the Ln slot photonic crystal cavity. To improve the sensitivity of the sensor, a layer of tungsten oxide (WO3) is coated on the inner walls of the slot. Change in concentration of H2S varies the refractive index of WO3 and leads a more wavelength shift in the output transmittance spectrum of the sensor which increases the sensitivity. We showed that the proposed sensor possesses an ultra-high sensitivity up to 2.3× 105 nmRIU−1, and a high Q factor of 3 ×103.

Tungsten passivation layer (WO3) formation mechanisms during chemical mechanical planarization in the presence of oxidizers

Effects of single and mixed oxidants of Fe(NO3)3 and H2O2 containing acidic silica slurries were studied to investigate the mechanism of tungsten (W) chemical mechanical planarization (CMP). The W polishing rate obtained from the CMP test depicted high W polishing rate in the presence of mixed oxidants of Fe(NO3)3 and H2O2 as compared to a single oxidant of either H2O2 or Fe(NO3)3. The formation of a passive layer of tungsten oxide (WO3) and W dissolution could be the reason for these results as confirmed by XPS. Further investigation revealed that the generation of much stronger oxidants of hydroxyl radicals (OH) was solely responsible for WO3 layer formation. Quantitative evaluation of OH generation was estimated using a UV–visible spectrophotometer and confirmed that in-situ generation of hydroxyl radicals (OH) could be a main driving force for the high W polishing rate by converting a hard W film into a soft passive film of WO3. WO3 film formation was further confirmed using potentiodynamic polarization studies, which showed a smaller value of corrosion current density (Icorr) in mixed oxidants of Fe(NO3)3 and H2O2 as compared to the large values of Icorr observed for H2O2 alone. This study revealed that a single oxidizer of either Fe(NO3)3 or H2O2 was not capable of achieving a high W removal rate. Rather, only mixed oxidants of Fe(NO3)3 and H2O2 could cause a high W polishing rate due to excessive in-situ generation of OH radicals during the W CMP process.

Facile low-temperature synthesis of nickel oxide by an internal combustion reaction for applications in electrochromic devices

Electrochromic nickel oxide ion storage anodes compatible to tungsten oxide as cathode layer are prepared by a combustion reaction with urea as fuel and nickel nitrate as oxidizer at temperatures not higher than 230 °C to stay well within the temperature range in which thermally sensitive intercalated tungsten oxide layers are still stable. The precursors for nickel oxide are well available materials and the preparation of the layers can be performed at low energy input by spin-coating onto the substrate and moderate heating on a hot-plate. Ethanol and acetone where used as solvents and reveal large differences in the resulting film morphologies, electric and electrochromic characteristics. Cyclic voltammetry measurements in contact to lithium perchlorate (LiClO4) in propylene carbonate show remarkable colouration efficiencies at 550 nm of 47 cm2 C−1 for NiO prepared from ethanol solution and 92 cm2 C−1 for NiO prepared from acetone solution, which are, to the best of our knowledge, among the highest reported values for Li+-intercalation into NiO prepared at moderate substrate temperature reported so far.Graphic abstract

Improving Photoelectrochemical Properties of Anodic WO3 Layers by Optimizing Electrosynthesis Conditions.

Although anodic tungsten oxide has attracted increasing attention in recent years, there is still a lack of detailed studies on the photoelectrochemical (PEC) properties of such kind of materials grown in different electrolytes under various sets of conditions. In addition, the morphology of photoanode is not a single factor responsible for its PEC performance. Therefore, the attempt was to correlate different anodizing conditions (especially electrolyte composition) with the surface morphology, oxide thickness, semiconducting, and photoelectrochemical properties of anodized oxide layers. As expected, the surface morphology of WO3 depends strongly on anodizing conditions. Annealing of as-synthesized tungsten oxide layers at 500 °C for 2 h leads to obtaining a monoclinic WO3 phase in all cases. From the Mott-Schottky analysis, it has been confirmed that all as prepared anodic oxide samples are n-type semiconductors. Band gap energy values estimated from incident photon−to−current efficiency (IPCE) measurements neither differ significantly for as−synthesized WO3 layers nor depend on anodizing conditions such as electrolyte composition, time and applied potential. Although the estimated band gaps are similar, photoelectrochemical properties are different because of many different reasons, including the layer morphology (homogeneity, porosity, pore size, active surface area), oxide layer thickness, and semiconducting properties of the material, which depend on the electrolyte composition used for anodization.

Tuning the Photo-Luminescence Properties of WO3 Layers by the Adjustment of Layer Formation Conditions.

In this research we have applied sol-gel synthesis for the deposition of tungsten (VI) oxide (WO3) layers using two different reductants (ethanol and propanol) and applying different dipping times. WO3 samples were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier Transform Infrared spectroscopy (FTIR), photoluminescence (PL) and time-resolved photoluminescence decay methods. Photoelectrochemical (PEC) behaviour of synthesized coatings was investigated using cyclic voltammetry in the dark and under illumination. Formation of different structures in differently prepared samples was revealed and significant differences in the PL spectra and PEC performance of the samples were observed. The results showed that reductant used in the synthesis and dipping time strongly influenced photo-electrochemical properties of the coatings. Correlation between the morphology, PL and PEC behaviour has been explained.

Activity of sol-gel derived nanocrystalline WO3 films in photoelectrochemical generation of reactive chlorine species

Conventional chlor-alkali method used for production of chlorine gas and chlorine-based disinfectants is among the most energy-intensive processes in chemical industry, therefore, more sustainable alternatives with lower carbon footprint are sought. Photoelectrochemical (PEC) generation of reactive chlorine species (Cl2, HClO, ClO−) is a promising technology in the area of water disinfection and purification, as it combines the advantages of (i) using the renewable solar energy; (ii) possibility to produce disinfectants on-site and on-demand and (iii) eliminates the need for sophisticated infrastructure for storage and handling of chlorine species.In the present study nanocrystalline tungsten (VI) oxide layers were formed on conducting glass substrate using simple sol-gel synthesis technique and polytethylene glycol as a structure-directing agent. It is shown that addition of PEG in moderate amounts favours formation of the structure of interconnected nanocrystalline particles, which ensures effective separation and transport of photogenerated charge carriers and, therefore, is crucial for the photoelectrochemical activity of the films. This is corroborated by the analysis of SEM and XRD data, revealing the influence of polytethylene glycol from the initial stages of WO3 phase crystallization. Faradaic efficiency of photoelectrochemical hypochlorite formation is shown to be about 30%, and antimicrobial effect of PEC chlorination with WO3 photoanodes is demonstrated on Gram-positive Bacillus sp. and Gram-negative E.coli C41(DE3) bacteria.

Gate-Modulated Ultrasensitive Visible and Near-Infrared Photodetection of Oxygen Plasma-Treated WSe2 Lateral pn-Homojunctions.

We investigate the development of gate-modulated tungsten diselenide (WSe2)-based lateral pn-homojunctions for visible and near-infrared photodetector applications via an effective oxygen (O2) plasma treatment. O2 plasma acts to induce the p-type WSe2 for the otherwise n-type WSe2 by forming a tungsten oxide (WOx) layer upon O2 plasma treatment. The WSe2 lateral pn-homojunctions displayed an enhanced photoresponse and resulted in open-circuit voltage (VOC) and short-circuit current (ISC) originating from the pn-junction formed after O2 plasma treatment. We further notice that the amplitude of the photocurrent can be modulated by different gate biases. The fabricated WSe2 pn-homojunctions exhibit greater photoresponse with photoresponsivities (ratio of the photocurrent and incident laser power) of 250 and 2000 mA/W, high external quantum efficiency values (%, total number of charge carriers generated for the number of incident photons on photodetectors) of 97 and 420%, and superior detectivity values (magnitude of detector sensitivity) of 7.7 × 109 and 7.2 × 1010 Jones upon illumination with visible (520 nm) and near-infrared lasers (852 nm), respectively, at low bias (Vg = 0 V and Vd = 1 V) at room temperature, demonstrating very high-performance in the IR region superior to the contending two-dimensional material-based photonic devices. These superior optoelectronic properties are attributed to the junctions induced by O2 plasma doping, which facilitate the effective carrier generation and separation of photocarriers with applied external drain bias upon strong light absorption.

Scalable photoelectrochromic glass of high performance powered by ligand attached TiO2 photoactive layer

A scalable photoelectrochromic (PEC) glass which can be darkened on exposure to sunlight and return to its original clear state in the absence of sunlight without any expensive materials like ruthenium dye or transparent conductive oxide was fabricated. The PEC glass is composed of light absorbing layer of ligand attached titanium oxide, color changing layer of tungsten oxide and the electrolyte containing redox mediator. This work reports, for the first time, on an application of salicylic acid attached TiO2 photoactive layer for PEC devices. The applicability of various salicylic acid derivatives attached TiO2 films in PEC device is individually investigated in dye solar cells. Among various salicylic acid derivatives studied, 5-methylsalicylic acid showed the best performance in terms of charge generation for coloration of WO3. By using 5-methylsalicylic acid instead of ruthenium dye, lower cost, higher stability, and simpler electrolyte composition have been realized. The changes in transmittance for coloration (ΔTC) depend on the thickness of WO3 and TiO2 in this device. Thus, we changed the thickness of WO3 and TiO2 to get the highest variation of transmittance for coloration. The preliminary optimization of PEC device exhibits ΔTC of 40.8% at 550 nm, the wavelength most sensitive to the human eye, and of 64.3% at 700 nm.

Multi-functional anodes boost the transient power and durability of proton exchange membrane fuel cells

Proton exchange membrane fuel cells have been regarded as the most promising candidate for fuel cell vehicles and tools. Their broader adaption, however, has been impeded by cost and lifetime. By integrating a thin layer of tungsten oxide within the anode, which serves as a rapid-response hydrogen reservoir, oxygen scavenger, sensor for power demand, and regulator for hydrogen-disassociation reaction, we herein report proton exchange membrane fuel cells with significantly enhanced power performance for transient operation and low humidified conditions, as well as improved durability against adverse operating conditions. Meanwhile, the enhanced power performance minimizes the use of auxiliary energy-storage systems and reduces costs. Scale fabrication of such devices can be readily achieved based on the current fabrication techniques with negligible extra expense. This work provides proton exchange membrane fuel cells with enhanced power performance, improved durability, prolonged lifetime, and reduced cost for automotive and other applications.

Towards full-colour tunability of inorganic electrochromic devices using ultracompact fabry-perot nanocavities

Intercalation-based inorganic materials that change their colours upon ion insertion/extraction lay an important foundation for existing electrochromic technology. However, using only such inorganic electrochromic materials, it is very difficult to achieve the utmost goal of full-colour tunability for future electrochromic technology mainly due to the absence of structural flexibility. Herein, we demonstrate an ultracompact asymmetric Fabry-Perot (F-P) nanocavity-type electrochromic device formed by using partially reflective metal tungsten as the current collector and reflector layer simultaneously; this approach enables fairly close matching of the reflections at both interfaces of the WO3 thin layer in device form, inducing a strong interference. Such an interference-enhanced device that is optically manipulated at the nanoscale displays various structural colours before coloration and, further, can change to other colours including blue, red, and yellow by changing the optical indexes (n, k) of the tungsten oxide layer through ion insertion.

Evaluating crystal-orientation-dependent properties on polycrystalline tungsten: Example oxidation

Various material properties, like oxidation, depend on the surface orientation of crystals. Most evaluations are focused on a few crystal orientations or using single crystals. A software tool written in python is introduced to correlate the thickness of the tungsten (W) oxide layer, i.e., the oxidation rate, and the individual single crystal grains with their surface orientation of polycrystalline W. The results are visualized in an inverse pole figure, which represent the oxidation rates for all crystal orientations. The use of polycrystalline W samples allows to analyze many different crystal orientations in a single experiment. Recrystallized, polished and polycrystalline W samples were pre-characterized using electron backscatter diffraction in a scanning electron microscope (SEM). Subsequently, the samples were oxidized in a thermobalance. A confocal laser scanning microscope was used for measuring the height of the oxide layer, which is scaled to thickness of the oxide layer by using SEM images from focused ion beam prepared cross-sections. Previous data of oxidation of W in the temperature range of 720 K to 870 K were reanalyzed with the new analysis tool. At grain boundaries, the oxidation is influenced by the neighboring grains. To evaluate the magnitude of this effect on the oxidation, samples with different textures were oxidized and evaluated. The results for the different textures agree within the estimated error bars, demonstrating the effectiveness of the automated analysis method. W grains with 〈100〉 surface orientation showed the highest oxidation rate.

Sodium ion storage performance of magnetron sputtered WO3 thin films

WO3 thin film electrodes were successfully prepared by magnetron sputtering (MS) deposition under an oblique angle configuration (OAD). Intercalation of Na ions in the tungsten oxide layers has been studied using electrochemical techniques. Sample characterization before and after sodium intercalation has been carried out by Raman, XPS and XRD measurements. ToF-SIMS analysis has been also performed in order to analyze the element depth profiles along the electrode thickness. Electron microscopy evaluation of the cross section confirms the porous structure of the coatings. Batteries integrating these WO3 electrodes have a discharge capacity of 120 mA h g−1 at the initial cycles and show an adequate capacity retention upon 300 cycles. The WO3-OAD thin-films are proposed as promising electrodes for Na-ion batteries.

Methanol oxidation on pure and platinum-doped tungsten oxide supported by activated carbon

Abstract Thin films of pristine and platinum-doped tungsten oxide were examined as potential catalysts for efficient partial oxidation of methanol. The tungsten oxide layers with different thicknesses were deposited by the means of reactive magnetron sputtering on oxidized silicon wafer and silicon coated with carbon activated via ion etching. Morphology and chemical analysis were checked before and after the reaction by X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). It was found that while pure tungsten oxide layers on silicon exhibit relatively small hydrogen production rates and high ratio of carbon oxides, both etched carbon interlayer (as long as the oxide thickness is low enough) and platinum doping can lead to significant improvement of reactivity, increasing hydrogen yield and decreasing fraction of undesired carbon monoxide in the product mix. It is suggested that it is due to the strong electronic interaction between WOx and Pt or active C, increasing the acidity of the tungsten oxide reaction sites and thus altering its reactivity preference from methanol dehydration towards dehydrogenation. If both species are present in a very thin oxide, however, the competing electronic interactions lead to partial reduction of their positive effect. As prepared sputtered thin films are chemically and morphologically unstable under reducing environment of methanol oxidation reaction, but the presence of dispersed Pt in the layer substantially stabilizes its physical structure.

Electrochromic devices based on tungsten oxide layers modified with polyethylene glycol

For the synthesis of tungsten-oxide-based electrochromic layers, the effect of the addition of polyethylene glycol (PEG) in different concentrations into peroxopolytungstic acid sol followed by thermal treatment at different temperatures on the performance of electrochromic devices (ECDs) is studied. The modification of the tungsten-oxide-layer sol-gel synthesis under optimal conditions (PEG: WO3 ratio 1∶2, temperature 450°C) provides a significant improvement of the ECD optical contrast and coloration efficiency due to the formation of a highly developed surface of the tungsten oxide layer, enhancing its contact with electrolyte while retaining continuity of the electrochromic layer.

Electrochromic devices based on tungsten oxide layers modified with polyethylene glycol

Electrochromic devices based on tungsten oxide layers modified with polyethylene glycol

1D tungsten oxide nanostructures on a Cu(1 1 0) surface

Thin epitaxial layers of tungsten oxide on metal substrates are suitable as model systems for investigation of chemical reactivity and catalytic properties. However, the ability to prepare epitaxial tungsten oxide model system in situ is quite rare. Here we present a method to prepare highly ordered tungsten oxide thin film on a Cu(1 1 0) single crystal substrate using physical vapor deposition in a reactive atmosphere of atomic oxygen. The oxygen induced reconstruction of the copper substrate gives rise to unique self-organized 1D structures of tungsten oxide parallel with the Cu[1 −1 0] crystallographic direction. Utilizing a combination of photoemission spectroscopy and density functional theory calculations we reveal emergent physicochemical properties related to the low-dimensionality of the system. Specifically, we observe a support mediated charge redistribution at the interface and a momentum dependent modulation of the valence-band electronic structure. The unusual character of the 1D oxide nanostructures on Cu(1 1 0) surface opens up a unique avenue for preparation of tungsten oxide-based functionalized nanostructures and provides options for further investigation of the fundamental properties of tungsten oxide.

The effect of anodization conditions on the morphology of porous tungsten oxide layers formed in aqueous solution

Nanostructured WO3 layers were obtained by one-step anodic oxidation of tungsten in an aqueous solution containing fluoride ions. A detailed investigation of the growth of anodic films on metallic substrate was focused on the study of the influence of anodization conditions, such as anodization potential, duration of the process, electrolyte composition, applied temperature, and electrolyte agitation on morphological features of as-received materials. Such kind of comprehensive studies has been performed for the first time. It was found that the optimal conditions allowing synthesis of nanostructures with well-defined pores and the thickest possible oxide layer are: anodization potential of 50 V, anodization time of 4 h, the electrolyte containing 1 M ammonium sulfate and 75 mM ammonium fluoride, temperature of 20 °C, and agitation of electrolyte of 250 rpm. A post-treatment procedure was developed to effectively remove the precipitates formed on the surface of anodic layers during anodization. The obtained materials were also examined as photoanodes in photoelectrochemical (PEC) water splitting experiments. It was confirmed, that PEC performance of nanostructured WO3 photoanodes can be significantly improved (higher photocurrents by 24%) after ultrasonic treatment in a 20 wt% HF solution for 10 s and ethanol for 5 s.

Solid Electrolyte Interface Formation on a Thin Film Electrode: An in Situ Neutron Reflectometry Study

Characterization of the solid electrolyte interface (SEI) in Li-ion batteries presents a challenge. Removing an electrode from a cell for ex situ measurements on the SEI risks its reaction with the atmosphere and also loss of material during washing. In situ characterization of the SEI is thus of great value. Neutron reflectometry (NR) enables such measurements with relatively high sensitivity to low atomic number elements, which may constitute a significant fraction of the SEI. Fitting of layer models to NR data yields a scattering length density (SLD) profile in the surface normal direction, from which a composition-depth profile may be inferred based upon the elements present in the cell. In this work, the interfacial structure of magnetron sputtered tungsten thin film electrodes in contact with a LiPF6 solution in an ethylene carbonate/diethyl carbonate (EC/DEC) solvent was characterized with NR. Tungsten was selected as a model electrode for its relatively low SLD contrast with the silicon substrate (relative to earlier measurements with a copper electrode1) and because it is not expected to alloy with lithium. NR was first measured for the bare electrode in an inert atmosphere, then at open circuit in contact with solution, and at +0.25 V, +2.65 V, and +0.25 V vs Li/Li+ after potential cycling. The bare electrode data were best fit with a four-layer model, including silicon oxide, an interfacial silicon oxide-tungsten layer, tungsten, and tungsten oxide layers. X-ray reflectivity data also supported this model. NR data collected at open circuit (about +2.5 V to 2.6 V) indicated a similar structure for the buried layers, but the tungsten oxide surface layer was no longer visible. The oxide layer may still have been present but more difficult to detect as a discrete layer because its SLD was intermediate between that of tungsten and the solution and because it was relatively thin and its interface width was comparable to the layer thickness. NR data collected when the electrode was polarized to and held at +0.25 V vs Li/Li+ were best fit a with a model containing two SEI layers: an inner low SLD layer and an outer layer with a SLD close to that of solution. The inner layer may be inferred to consist of Li2O or LiOH, given its very low SLD (the SLDs of other candidate species were too high to account for this layer). The inner layer SLD did not differ greatly in isotopic labelling measurements in which both EC and DEC were deuterated and where only DEC was deuterated. This indicated the inner SEI layer did not incorporate significant quantities of hydrogen atoms from EC, since the SLD of LiOD is significantly greater than that of LiOH. Since deuterated DEC can be a source of deuterons but not of protons, this suggests the layer was primarily Li2O, barring any adventitious sources of protons. This could have been produced at least in part by reduction of the native tungsten oxide to Li2O and metallic tungsten. The SLD of the outer layer did depend upon solvent deuteration, which indicates that it had solution-filled porosity or had EC-derived constituents (or both). When the electrode was polarized to +2.65 V, the inner low SLD surface layer was removed, although an outer SEI layer of SLD close to that of the solution was still present. The electrode potential was then cycled between +2.65 V and +0.25 V ten times and then held at +0.25 V. NR data measured at this potential indicated a low SLD layer formed at the surface again, though its SLD was not as low as in the first +0.25 V hold. The two SEI layers were less readily discerned as independent layers than in the first hold at +0.25 V, suggesting roughening of the SEI. These data show that the SEI is not immutable once formed, confirm thicknesses measured in earlier NR measurements, and show that the lower contrast of the thin film tungsten electrode may improve sensitivity of NR measurements to surface layers. Surface oxides of electrode materials, although very thin, can play an instrumental role in the formation and composition of the SEI and may lead to significant variations with differing electrode materials. In future work, we plan to use this platform to characterize the effects of electrolyte additives on SEI structure. 1. Owejan, J. E.; Owejan, J. P.; DeCaluwe, S. C.; Dura, J. A., Chem. Mater. 2012, 24 (11), 2133-2140.