Insertion of H, Li, Na and K into thin films prepared from silicotungstic acid—a photoelectron spectroscopy study

Electrochemical insertion by a set of different ions (H⁺, Li⁺, Na⁺ and K⁺) into a tungsten oxide thin film was studied by photoelectron spectroscopy. The tungsten oxide thin film incorporating Si atoms was produced from a silicotungstic acid (SiWA) solution. The insertion compounds were measured by core level photoelectron spectroscopy (W 4f) and the contributions from ions of different oxidation states could be monitored simultaneously. SiWA films having a W⁶⁺/W_tot ratio of 0.7 could be prepared for all cations investigated. At this ratio the W 4f core level electronic structure for H⁺ inserted SiWA films was found to be very similar to that of H⁺ inserted into crystalline monoclinic WO₃ in that both films show the presence of W⁴⁺, W⁵⁺ and W⁶⁺. The measurements on Li⁺ inserted SiWA films indicate an electronic structure very similar to that of the smaller (H⁺) ion. The K⁺ inserted film displays a similar behaviour although the existence of W⁴⁺ was difficult to ascertain. Interestingly, a different behaviour was observed for the Na⁺ inserted compound. In this case, the binding energy shift of the W 4f peak upon reduction is clearly different from that obtained for the other insertion materials.     

Effects of oxygen stoichiometry on electrochromic properties in amorphous tungsten oxide films

We have investigated the electrochromic properties of amorphous granular tungsten oxide (WO_3 + δ) thin films with over-stoichiometric oxygen content (δ), using LiClO₄ with propylene carbonate as an electrolyte. Different optical and electrochromic characteristics are observed with increasing δ. All the devices are electrochemically stable for more than 5000 color/bleach cycles without apparent degradation, and they have a faster response to coloration than to bleaching. WO_3 + δ films with an optimized δ value show an optical modulation of 86% at a wavelength of 630 nm and the highest coloration efficiency ever reported of ~ 213 cm²/C. The δ-dependent coloration mechanism is discussed using the site saturation model. It is proposed that WO_3 + δ films with the optimal δ value have favorable thickness and stoichiometry for the generation of Li⁺W⁺⁵ states.     

Structural properties of TiO2–WO3 thin films prepared by r.f. sputtering

TiO₂–WO₃ thin films were prepared by radio frequency (r.f.) reactive sputtering from metallic target. Structural and morphological properties of the thin films have been studied through X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and scanning electron microscopy (SEM). The influence of the annealing on the phase composition TiO₂–WO₃ system was studied. The binding energies of titanium and tungsten are characteristic for Ti⁴⁺ and W⁶⁺. The influence of tungsten on anatase–rutile phase transition in TiO₂ was observed. The structural modeling has been performed to account the preferred orientation in tungsten doped titanium oxide.     

Preparation and characterizations of tungsten oxide electrochromic nanomaterials

Nanoscaled tungsten oxide thin films were fabricated by galvanostatic electrodeposition. The effect of preparation parameters such as tungsten ions concentration, pH, current density and annealing on the properties and performance of WO₃ thin films electrochromic materials was investigated. XRD, SEM–EDS, TEM, FTIR, UV–VIS spectrophotometry, and electrochemical measurements were utilized to characterize the structural and compositional properties as well as the electrochromic behaviour of the prepared thin films. Triclinic WO₃ structure was prepared at 0.1 M W⁺ and current density of 0.5 mA cm⁻², while at 0.2 M W⁺ and 1 mA cm⁻², orthorhombic structure was revealed. High energy gap of 3.5 eV with diffusion coefficient of 6.81 × 10⁻¹¹ cm² S⁻¹ and coloration efficiency of 62.68 cm² C⁻¹ were obtained for the films prepared at pH 2, 1 mA cm⁻², and 0.1 M W⁺.     

Current-voltage characteristics and grain growth of Li2CO3-doped tungsten trioxide ceramics

Ceramics samples of tungsten trioxide doped with lithium carbonate from 0.5 to 5 mol% were prepared by conventional electroceramic technique. The current-voltage characteristics of these ceramics were measured under various ambient temperatures. All of the I-V curves showed non-ohmic electrical properties with obvious negative-resistance characteristic at room temperature. It is found that there exists a direct correlation between the negative-resistance phenomenon in the I-V curves and the electrical history of these samples. The suitability of some models regarding the negative-resistance characteristics is discussed. X-ray diffraction (XRD) revealed coexistence of two phases of tungsten trioxide, which depends on the amount of lithium. Scanning electron microscope (SEM) showed great differences for both grain shape and size between the Li-doped and undoped WO₃ ceramics, and this indicates that Li₂CO₃ doped into WO₃ influences strongly the growing of WO₃ during sintering process.     

Thermal Analysis of the Ternary System Li<Subscript>2</Subscript>WO<Subscript>4</Subscript>-WO<Subscript>3</Subscript>-Li<Subscript>2</Subscript>B<Subscript>4</Subscript>O<Subscript>7</Subscript>

Thermal analysis results indicate that the liquidus surface of the Li₂WO₄-WO₃-Li₂B₄O₇ system consists of the primary phase fields of Li₂WO₄, Li₂B₄O₇, WO₃, Li₂WO₄ · WO₃ (congruent melting), 3Li₂WO₄ · 2Li₂B₄O₇ (congruent melting), and Li₂WO₄ · 3WO₃ (incongruent melting). Low-melting-point compositions are selected that are potentially attractive for the low-temperature synthesis of lithium tungsten bronze powders.     

Comparative study of gasochromic and electrochromic effect in thermally evaporated tungsten oxide thin films

Substoichiometric tungsten oxide films (WO_3 − y, 0.49 ≥ y ≥ 0.15) were prepared by non-reactive thermal evaporation of WO₃ powder in vacuum. The thin film composition, structure and optical properties were investigated with the purpose to establish their dependence on the deposition conditions and to prove a possible correlation between electrochromic and gasochromic colouration. An analogy in the dependencies of the maximum achievable optical density on the thin film oxygen content for gasochromically and electrochromically coloured films was observed.In-situ performed XPS measurements suggested that the main mechanism of gasochromic colouration is charge transfer between W⁶⁺ and W⁵⁺ states, i.e., similar to the electrochromic effect.     

Structure and ionic conductivity of Lix(MeO)4Me30O90 (Me = NbV, WVI)

The compound Li_x(MeO)₄Me₃₀O₉₀ (Me = Nb^V, W^VI) has been prepared and investigated by X-ray powder and electron diffraction techniques as well as by measurements of complex impedance. The structure is of the tripled tetragonal tungsten bronze type with one third of the pentagonal tunnels filled with equal numbers of Me and 0 atoms. The conductivity at 300°C is 0.20 and 0.25 ohm^?1m^?1 for sintered samples with x equal to 2 and 4, respectively. The location and probable routes of transport of the lithium ions are discussed.     

Electrochemical lithium insertion in the solid solution Bi2WO6-Sb2WO6 with Aurivillius framework

Following the structural evolution of the Aurivillius crystalline framework in the solid solution Bi₂WO₆-Sb₂WO₆ we have carried out an electrochemical lithium insertion study in this system. A slight loss of the specific capacity of the electrochemical cell was observed as amount of Sb was increased. In general, the different compositions within solid solution Bi_2−xSb_xWO₆ (0.25 ≤ x ≤ 0.75) exhibited a similar behaviour featured mainly by two semiconstant potential regions located at 1.7 and 0.8 V versus Li⁺/Li^o. The oxide Sb₂WO₆ with Autivillius structure but without Bi was tested as cathode too. The maximum amount of lithium inserted, 13.5 lithium atoms per formula, is the same amount inserted in its homologous bismuth oxide Bi₂WO₆.     

Interactions of gamma rays with tungsten-doped lead phosphate glasses

Undoped lead phosphate glass of the composition PbO 50 mol%, P₂O₅ 50 mol% together with samples of the same ratio doped with various WO₃ contents were prepared. UV–Visible spectroscopic studies were measured out in the range 200–1100 nm before and after successive gamma irradiation. Infrared and Raman spectroscopic measurements were carried out for the undoped and WO₃-doped samples. All the prepared samples are observed to absorb strongly in the UV region due to the combined contributions of absorption from trace iron impurities and sharing of lead Pb²⁺ ions. The bluish WO₃-doped lead phosphate samples reveal visible absorption bands which are attributed to the existence of pentavalent W⁵⁺ ions. ESR measurements support this assumption. Infrared and Raman spectra indicate the presence of metaphosphate chains as the structural main building units and the possible presence of appreciable pentavalent (W⁵⁺O₃) of W⁵⁺ units together with hexavalent WO₄ units. Gamma irradiation reveal the shielding behaviour of the studied tungsten-doped lead phosphate glasses due to the combined presence of heavy Pb²⁺ ions and tungsten ions.     

Seed Growth of Tungsten Diselenide Nanotubes from Tungsten Oxides

We report growth of tungsten diselenide (WSe₂) nanotubes by chemical vapor deposition with a two‐zone furnace. WO₃ nanowires were first grown by annealing tungsten thin films under argon ambient. WSe₂ nanotubes were then grown at the tips of WO₃ nanowires through selenization via two steps: (i) formation of tubular WSe₂ structures on the outside of WO₃ nanowires, resulting in core (WO₃)–shell (WSe₂) and (ii) growth of WSe₂ nanotubes at the tips of WO₃ nanowires. The observed seed growth is markedly different from existing substitutional growth of WSe₂ nanotubes, where oxygen atoms are replaced by selenium atoms in WO₃ nanowires to form WSe₂ nanotubes. Another advantage of our growth is that WSe₂ film was grown by simply supplying hydrogen gas, where the native oxides were reduced to thin film instead of forming oxide nanowires. Our findings will contribute to engineer other transition metal dichacogenide growth such as MoS₂, WS₂, and MoSe₂.     

Achieving Ultrahigh-Rate and High-Safety Li+ Storage Based on Interconnected Tunnel Structure in Micro-Size Niobium Tungsten Oxides

Developing advanced high-rate electrode materials has been a crucial aspect for next-generation lithium ion batteries (LIBs). A conventional nanoarchitecturing strategy is suggested to improve the rate performance of materials but inevitably brings about compromise in volumetric energy density, cost, safety, and so on. Here, micro-size Nb₁₄W₃O₄₄ is synthesized as a durable high-rate anode material based on a facile and scalable solution combustion method. Aberration-corrected scanning transmission electron microscopy reveals the existence of open and interconnected tunnels in the highly crystalline Nb₁₄W₃O₄₄, which ensures facile Li⁺ diffusion even within micro-size particles. In situ high-energy synchrotron XRD and XANES combined with Raman spectroscopy and computational simulations clearly reveal a single-phase solid-solution reaction with reversible cationic redox process occurring in the NWO framework due to the low-barrier Li⁺ intercalation. Therefore, the micro-size Nb₁₄W₃O₄₄ exhibits durable and ultrahigh rate capability, i.e., ≈130 mAh g⁻¹ at 10 C, after 4000 cycles. Most importantly, the micro-size Nb₁₄W₃O₄₄ anode proves its highest practical applicability by the fabrication of a full cell incorporating with a high-safety LiFePO₄ cathode. Such a battery shows a long calendar life of over 1000 cycles and an enhanced thermal stability, which is superior than the current commercial anodes such as Li₄Ti₅O₁₂.     

Comparative microscopic and spectroscopic analysis of temperature-dependent growth of WO3 and W0.95Ti0.05O3 thin films

We present a comparative microscopic and spectroscopic study of the morphology and composition of WO₃ and W_0.95Ti_0.05O₃ thin films, grown by radio-frequency magnetron reactive sputtering at substrate temperatures varied from room temperature to 500 °C, using atomic force microscopy (AFM), Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS). With increasing growth temperature, the AFM results show increase in the average crystallite size and in the surface roughness for both undoped and doped samples. The AFM data, along with the Raman results, clearly indicate that for the given set of experimental conditions, higher growth temperatures are required to obtain crystalline Ti-doped WO₃ films than for WO₃ films. Also, the Raman results suggest a potential phase transformation from a monoclinic WO₃ structure to an orthorhombic, but more probably a tetragonal, configuration in the W_0.95Ti_0.05O₃ thin films. This remark is based on the observed shifting, with Ti doping, to lower frequencies of the Raman peaks corresponding to W–O–W stretching modes of WO₃ at 806 and 711 cm⁻¹ to 793 and 690 cm⁻¹, respectively. XPS data indicate that the doped material has a reduced WO_3−x stoichiometry at the surface, with the presence of W⁶⁺ and W⁵⁺ oxidation states; this observation could also be related to the existence of a different structural phase of this material, corroborating with the Raman measurements.     

Gamma rays interactions with WO3-doped lead borate glasses

Optical and FT infrared spectral properties of tungsten ions in a host lead borate glass with composition PbO 55%, B₂O₃ 45% (wt%) were studied. The same spectral properties were re-measured after subjecting the samples to successive gamma irradiation. The work was undertaken to justify the state of tungsten ions in such glass system by combined spectral investigations. Optical and FTIR spectral studies were confirmed by investigating electron spin resonance (ESR) of the undoped and WO₃-doped samples before and after gamma irradiation. The optical spectrum of the undoped glass exhibits strong and wide UV absorption bands, which are related to the combined UV spectra of trace iron impurities (Fe³⁺ ions) and that from divalent lead (Pb²⁺) ions. Optical studies of WO₃-doped sample indicate the presence of tungsten ions mostly in the hexavalent W⁶⁺ state. The presence of tungsten ions as structural groups was obtained by comparing the FTIR spectra of the undoped and WO₃-doped samples. ESR spectra confirm the optical and FTIR spectral studies. The studied host lead borate glass has been found to show obvious shielding behavior towards successive gamma irradiation as revealed by the constancy of optical absorption spectral curves.     

Preliminary characterization of electrodeposited W2C coatings for wear applications

The recent development of hard adherent W₂C coatings by the simultaneous electrochemical reduction of tungstate (WO₄^2-) and carbonate (CO₃^2-) ions in a molten fluoride bath is described. The elements tungsten and carbon thus formed on the cathode combine in a subsequent chemical step to form the carbide. The structure of the deposit depends on the length of the plating time as well as melt composition and electrochemical parameters such as current and voltage. In pin-on-disk-type wear tests some fragmentation of the coating and abrasive wear were observed.     

X-ray photoelectron spectroscopy: A powerful tool for a better characterization of thin film materials

X-ray photoelectron spectroscopy (XPS) is one of the most powerful tools to characterize thin films materials. To illustrate the use of XPS, some examples will be given on materials used as positive electrode in microbatteries. Further analyses of the film to understand the redox process are quite difficult with conventional methods due to the amorphous nature of the cathode. Here surface methods like XPS are very useful. Two main kinds of information can be obtained from XPS analysis: the oxidation states, and the determination of atomic environments. Different kinds of positive electrode materials were studied, titanium and molybdenum oxysulfides (MO _y S _z , M=Ti, Mo) and lithium cobalt oxide (Li _x CoO_2+y ) and have been illustrated in the present work. In light of the binding energies obtained for the reference compounds, several types of environments and different formal oxidation states have been found for the transition elements. XPS is also very useful for folllowing the oxydo-reduction mechanisms occurring during the intercalation and the de-intercalation of lithium, corresponding respectively to the discharge and the charge of the battery. After strict identification of each species, the evolution of their binding energies could be followed very easily. The XPS analyses of oxysulfides thin films at different stages of their cycling process have shown apparently good efficiency of the oxygen-rich compositions. During the redox process, the results obtained have clearly shown the important contribution of the sulfur atoms beside the transition metal atom.     

Hot-wire chemical vapor deposition of WO3−x thin films of various oxygen contents

We present the synthesis of tungsten oxide (WO_3−x) thin films consisting of layers of varying oxygen content. Configurations of layered thin films comprised of W, W/WO_3−x, WO₃/W and WO₃/W/WO_3−x are obtained in a single continuous hot-wire chemical vapor deposition process using only ambient air and hydrogen. The air oxidizes resistively heated tungsten filaments and produces the tungsten oxide species, which deposit on a substrate and are subsequently reduced by the hydrogen. The reduction of tungsten oxides to oxides of lower oxygen content (suboxides) depends on the local water vapor pressure and temperature. In this work, the substrate temperature is either below 250 °C or is kept at 750 °C. A number of films are synthesized using a combined air/hydrogen flow at various total process pressures. Rutherford backscattering spectrometry is employed to measure the number of tungsten and oxygen atoms deposited, revealing the average atomic compositions and the oxygen profiles of the films. High-resolution scanning electron microscopy is performed to measure the physical thicknesses and display the internal morphologies of the films. The chemical structure and crystallinity are investigated with Raman spectroscopy and X-ray diffraction, respectively.     

Hybrid of spinel zinc-cobalt oxide nanospheres combined with nitrogen-containing carbon nanofibers as advanced electrocatalyst for redox reaction in lithium/polysulfides batteries

The polysulfides shuttle effect and torpid kinetic are of the crucial barriers for lithium/sulfur batteries. Herein, nitrogen-containing carbon nanofibers (NCFs) combined with spinel zinc-cobalt oxide (ZCO) nanospheres hybrid (denoted as ZCONCFs) were designed as membrane electrode containing Li₂S₆ catholyte for lithium/polysulfides batteries, which promote electrochemical performance by suppressing the shuttle effect and enhancing the redox kinetics of lithium polysulfides. The conductive NCFs provide fast electronic transport and anchored ZCO nanospheres possess a strong affinity to sulfur species, which could effectively anchor lithium polysulfides, boost their redox reaction catalytically-accelerate the reversible soluble/insoluble phases conversion process, and greatly improve the utilization of active material. The results show that ZCONCFs membrane electrode with 5 mg sulfur loading exhibited stability cycling capacity and improved reaction kinetics, which delivered a high initial capacity of 1160 mAh g^?1 at 0.2C and sustain a capacity of 830 mAh g^?1 after 300 cycles. The cell with ZCONCFs exhibits 8.22 mAh under the sulfur loading of 10 mg and the capacity decay rate is 0.11% per cycle after 150 cycles. This effective method could significantly improve the application capacity of lithium/sulfur batteries.     

Surface tuning for promoted charge transfer in hematite nanorod arrays as water-splitting photoanodes

Hematite (α-Fe₂O₃) nanorod films with their surface tuned by W⁶⁺ doping have been investigated as oxygen-evolving photoanodes in photoelectrochemical cells. X-ray diffraction, field emission scanning electron microscopy, UV-visible absorption spectroscopy, and photoelectrochemical (PEC) measurements have been performed on the undoped and W⁶⁺-doped α-Fe₂O₃ nanorod films. W⁶⁺ doping is found to primarily affect the photoluminescence properties of α-Fe₂O₃ nanorod films. Comparisons are drawn between undoped and W⁶⁺-doped α-Fe₂O₃ nanorod films, WO₃ films, and α-Fe₂O₃-modified WO₃ composite electrodes. A close correlation between dopant concentration, photoluminescence intensity, and anodic photocurrent was observed. It is suggested that W⁶⁺ surface doping promotes charge transfer in α-Fe₂O₃ nanorods, giving rise to the enhanced PEC performance. These results suggest surface tuning via ion doping should represent a viable strategy to further improve the efficiency of α-Fe₂O₃ photoanodes.      

In Situ Electrochemically Derived Amorphous‐Li2S for High Performance Li2S/Graphite Full Cell

High‐capacity Li₂S cathode (1166 mAh g^?1) is regarded as a promising candidate for the next‐generation lithium ion batteries. However, its high potential barrier upon the initial activation process leads to a low utilization of Li₂S. In this work, a Li₂S/graphite full cell with the zero activation potential barrier is achieved through an in situ electrochemical conversion of Li₂S₈ catholyte into the amorphous Li₂S. Theoretical calculations indicate that the zero activation potential for amorphous Li₂S can be ascribed to its lower Li extraction energy than that of the crystalline Li₂S. The constructed Li₂S/graphite full cell delivers a high discharge capacity of 1006 mAh g^?1, indicating a high utilization of the amorphous Li₂S as a cathode. Moreover, a long cycle life with 500 cycles for this Li₂S/graphite full cell is realized. This in situ electrochemical conversion strategy designed here is inspired for developing high energy Li₂S‐based full cells in future.