Tungsten Doping Research Articles

Improvement of the specific capacitance of V2O5 nanobelts as supercapacitor electrode by tungsten doping

Tungsten doped vanadium pentoxide (W-doped V2O5) nanobelts were successfully synthesized by a facile hydrothermal route and combination of calcination. The results revealed that W atoms were successfully doped into the crystal lattice of V2O5 matrix, indicating that the homogeneous solid-solutions of W-doped V2O5 nanobelts were obtained. The electrochemical properties of W-doped V2O5 nanobelts as supercapacitor electrode were investigated by cyclic voltammetry (CV) and galvanostatic charge discharge (GCD) methods. W-doped V2O5 nanobelts exhibit the excellent capacity and good rate capability. Their specific capacitance are 407, 381, 350, 328, 295 and 273 F g−1 at the current density of 0.5, 1, 2, 5, 10 and 20 A g−1, respectively. W-doped V2O5 nanobelts also show excellent energy densities of 246, 217, 212, 199, 178 and 165 W h kg−1 at a power densities of 0.99, 1.98, 3.96, 9.90, 19.80 and 39.60 kW kg−1. The specific capacitance of W-doped V2O5 nanobelts is much higher than the previous values of V2O5 materials, achieving the aim of improving the specific capacitance of V2O5 nanobelts.

Thermal kinetic analysis of metal–insulator transition mechanism in W-doped VO2

Tungsten (W) doping can decrease the phase transition temperature of VO2, but underlining reasons are not clear. In this study, differential scanning calorimetry was employed to investigate the kinetics of the solid–solid transition in hydrothermal-synthesized W (X = 0.50, 1.09, 2.58 and 3.81 %)-doped VO2 nanoparticles. We firstly revealed the W-doping mechanism by combining the classical nucleation kinetics model with isoconversional kinetic analysis and applied them on the solid–solid transition taking place in doping. The experimentally observed large lag in the cooling stage and asymmetry effects of the decreasing temperature on insulator–metal transition and metal–insulator transition can be rationally explained. In the heating stage, W doping decreases free energy barrier (ΔG*) for homogenous nucleation and reduces geometrical factor (f(Θ)) and both factors promote the transition and thus lower the phase transition temperature quickly. However, in cooling stage, the free energy barrier (ΔG het * ) for heterogeneous nucleation was much larger than that of heating stage due to lacking of proper nucleation sites. The effect of decreasing geometrical factor was accompanied with the effect of increasing free energy barrier for homogenous nucleation by doping W. Such a competition mechanism slows down the trend of reducing temperature. It is important to unravel interaction mechanisms of doped W on different VO2 phases, which is helpful to further tailor kinetic properties of VO2 phase transition.

Photodegradation of methylene blue in the visible spectrum: An efficient W6+ ion doped anatase titania photocatalyst via a solvothermal method

Solvothermal prepared tungsten (VI) doped titania were evaluated as photocatalysts for methylene blue (MB) degradation under the visible spectrum. Incorporation of hexavalent tungsten alters both the surface and bulk properties. Addition of tungsten into the titania lattice changes the lattice parameter, morphology and photocatalytic performance. X-ray photoelectron spectroscopy and x-ray diffraction suggested that W6+ does not change the chemical environment of Ti4+ but contracts the lattice of titania. By adding 5 wt% tungsten, the resulting sample shows high surface area (130.4 m2/g), a contracted lattice and high efficiency for photodegradation (120 min bleaching with a remaining concentration of 21%). However, doping of tungsten beyond 5 wt% creates more recombination center for photogenerated electron–hole pairs, which deteriorates the photocatalytic performance. The W6+ ions function as electron receivers and avoid the recombination of electron–hole pairs. It was shown that OH. radicals are responsible for the ring opening reaction of aromatic MB.

Enhanced photoelectrochemical water oxidation of bismuth vanadate via a combined strategy of W doping and surface RGO modification.

Nanoporous bismuth vanadate is modified simultaneously via tungsten doping and graphene surface modification for use as an efficient photoanode. The modified films were prepared on a FTO substrate by a drop-cast method followed by photoreduction of graphene oxide. SEM, XRD, Raman and XPS characterization was conducted to confirm the incorporation of tungsten and reduced graphene oxide (RGO), and to look into their influences on the structure and performance of BiVO4. Electrochemical impedance spectroscopy analysis clearly revealed enhanced carrier density and improved electronic conductivity, which are beneficial for the enhancement of PEC performance in comparison to either individually doped or RGO modified BiVO4. Our results indicated that the enhanced PEC performance can be attributed to the synergistic effect of bulk doping and surface modification that facilitates electron and hole transport and transfer in the bulk and at the semiconductor-electrolyte interface.

Amount of tungsten dopant influencing the photocatalytic water oxidation activity of LaTiO2N crystals grown directly by an NH3-assisted flux method

The effect of tungsten dopant on the photocatalytic water oxidation activity of LaTiO2N crystals grown by an NH3-assisted flux method was studied.

Atomic Origins of Monoclinic-Tetragonal (Rutile) Phase Transition in Doped VO2 Nanowires.

There has been long-standing interest in tuning the metal-insulator phase transition in vanadium dioxide (VO2) via the addition of chemical dopants. However, the underlying mechanisms by which doping elements regulate the phase transition in VO2 are poorly understood. Taking advantage of aberration-corrected scanning transmission electron microscopy, we reveal the atomistic origins by which tungsten (W) dopants influence the phase transition in single crystalline WxV1-xO2 nanowires. Our atomically resolved strain maps clearly show the localized strain normal to the (122̅) lattice planes of the low W-doped monoclinic structure (insulator). These strain maps demonstrate how anisotropic localized stress created by dopants in the monoclinic structure accelerates the phase transition and lead to relaxation of structure in tetragonal form. In contrast, the strain distribution in the high W-doped VO2 structure is relatively uniform as a result of transition to tetragonal (metallic) phase. The directional strain gradients are furthermore corroborated by density functional theory calculations that show the energetic consequences of distortions to the local structure. These findings pave the roadmap for lattice-stress engineering of the MIT behavior in strongly correlated materials for specific applications such as ultrafast electronic switches and electro-optical sensors.

Synthesis of sub-10 nm VO2 nanoparticles films with plasma-treated glass slides by aqueous sol–gel method

This paper describes an aqueous sol–gel synthesis of thermochromic thin films consisted of vanadium dioxide nanoparticles (VNPs) on glass slides. The glass slides were treated by argon/oxygen plasma to generate dispersedly negative charge sites on the surface to attract VO2+ from a sol–gel solution. After heat treatment in a low-pressure carbon monoxide/carbon dioxide (CO/CO2) atmosphere, the VNPs could be generated in sub-10nm of particle size on the surface. Various levels of doping were achieved by adding small quantities of a water-soluble tungsten compound to the sol; however, the particle size increased slightly with the tungsten doping levels. The change in electrical conductivity with temperature for VNP films were measured and compared to VO2 crystalline films. VNP films exhibited the lower transition temperature of the semiconductor to metal phase change; at a doping level of 4wt% the transition temperature was measured at 32.2±1.2 and 24.1±1.2°C for the VO2 and VNP films, respectively. The VNP films showed excellent visible transparency and a large change in transmittance at near-infrared (NIR) wavelengths before and after the metal–insulator phase transition (MIT). The current method is a landmark in the development of nanostructured material toward applications in energy-saving smart windows.

Novel tungsten stabilizing SrCo1−xWxO3−δ membranes for oxygen production

Mixed conducting perovskite membranes have the potentials to replace the conventional expensive cryogenic method to separate oxygen from air and improve the viabilities of these clean energy technologies which need pure oxygen as the feed gas. In this paper, a series of SrCo1−xWxO3−δ (x=0.05–0.2) perovskite membranes were synthesized and characterized for their potential in oxygen separation. The crystal structure and oxygen permeation flux of the membranes were systematically investigated. Due to the enhancement of phase stability by tungsten doping, the preferred cubic perovskite structure at lower temperature is successfully obtained and maintained at the doping concentration from 5% to 10%. However, the further doping increase leads to the formation of other crystal phase unfavorable for oxygen permeation. The tungsten doping concentration also exerts significant effects on the oxygen permeability of the resultant membranes. SrCo0.9Wx0.1O3−δ exhibited the highest oxygen permeation flux among the tested samples, and reached a flux value up to 2.6mlcm−2min−1 at 900°C under an air/helium oxygen gradient for a 1mm thick membrane, comparing favorably with the best membrane ever-reported. Long-term operation test also signals the excellent permeation stability under operating conditions.

Effects of Tungsten Doping Contents on Tribological Behaviors of Tungsten-Doped Diamond-Like Carbon Coatings Lubricated by MoDTC

A systematic study was carried out to assess the influence of tungsten content on the tribological properties of tungsten-doped diamond-like carbon (W-DLC) coatings lubricated by molybdenum dithiocarbamate (MoDTC). Four W-DLC coatings with W doping content of 0.0 % (pure DLC), 3.08, 10.73 and 27.7 % were fabricated, respectively, following the microstructures, and mechanical properties of these W-DLC coatings were observed, and next, the friction and wear tests were conducted under boundary lubrication with MoDTC; finally, the valence states of tribofilms were identified by X-ray photoelectron spectroscope. The results showed that W-DLC coatings present lower coefficients of friction and higher wear rates under MoDTC lubrication comparing those under PAO lubrication. It also revealed that W content has little influence on anti-friction behaviors, while the wear rates decreased firstly and then increased with higher W content. It was proposed that the tribological properties of W-DLC coatings mainly depend on the ratio of hardness to Young’s modulus (H/E), the degree of graphitization and the formation of tribofilms containing MoS2 and MoO x .

Simple preparation of tungsten supported carbon nanoreactors for specific applications: Adsorption, catalysis and electrochemical activity

Porous carbon supported tungsten carbide nanoreactors, two sizes (∼25 and 170nm), were designed using economical petroleum pitch residue followed by tungsten (W) doping. X-ray diffractions showed both carbon tungsten composites (CTC-25 and CTC-170) contained tungsten subcarbide (W2C) and monocarbide (WC) as the major and minor crystalline phases, respectively. The present study provides a multiple perspective of carbon tungsten composites (CTCs) for methanol oxidation (as an electrode), adsorption (as an adsorbent) and degradation (as a solid catalyst) of methylene blue (MB). The operational electrodes were designed from both CTCs and used as a catalyst in an electrocatalysis process. The electrocatalysts exhibited high and stable catalytic performance (CTCE-25>CTCE-170) in methanol electro-oxidation. The newly synthesized W-doped carbon nanoreactors were used successfully as an adsorbent for MB and a heterogeneous catalyst for MB oxidation. Ordered CTC-25 and CTC-170 exhibited dynamic MB adsorption within 15min and complete oxidation of MB in 25–40min. A synergetic effect between tungsten carbide and the carbon cage framework was noted.

회분식 반응기에서 TiO<sub>2</sub> 광촉매의 MEK 분해특성-금속담지영향

In photocatalytic reaction, the doping of metal matter can alter the titania surface properties. As such the metal matter can increase the rate of the reaction. The influence of metal doping and calcination condition of TiO2 photocatalyst was investigated at the batch-type photoreactor. Several metal matters were doped to the TiO2 catalyst to improve photodegradation efficiency. During the experiments, water content was 3wt%, and reactor temperature was 40℃. Palladium-doped TiO2 was found to be the best, where as platinum or tungsten-added also showed good results. Additional doping of platinum or tungsten on Pd/TiO2 had no increase on the removal efficiency. To obtain proper calcination condition, various experiments about calcination temperature and time were carried out. As a result, the optimum calcination condition was temperature of 400℃, time of 1 hour.

Intrinsic evolutions of dielectric function and electronic transition in tungsten doping Ge2Sb2Te5 phase change films discovered by ellipsometry at elevated temperatures

Tungsten (W) doping effects on Ge2Sb2Te5 (GSTW) phase change films with different concentrations (3.2, 7.1, and 10.8%) have been investigated by variable-temperature spectroscopic ellipsometry. The dielectric functions from 210 K to 660 K have been evaluated with the aid of Tauc-Lorentz and Drude dispersion models. The analysis of Tauc gap energy (Eg) and partial spectral weight integral reveal the correlation between optical properties and local structural change. The order degree increment and chemical bond change from covalent to resonant should be responsible for band gap narrowing and electronic transition enhancement during the phase change process. It is found that the elevated crystalline temperature for GSTW can be related to improved disorder degree. Furthermore, the shrinkage of Eg for GSTW should be attributed to the enhanced metallicity compared with undoped GST.

Structure and physical properties of PZT-type ceramics with cadmium and tungsten dopants

Ceramics of Pb (Zr0.56Ti0.44)O3 with 2.0 at% cadmium and 2.0 at% tungsten dopants (PZT-CW) were created using hot-pressing method. Next, the basic properties of PZT-CW ceramics were investigated. An analysis of the crystallographic structure, tests of dielectric and piezoelectric parameters of the ceramics, as well as internal friction measurements were performed. The PZT-CW ceramics obtained using the hot-pressing method have the microstructure of regularly crystallized grain. For this reason, it holds a set of electrophysical parameters illustrating the possibilities of its application in electrical engineering.

Tungsten concentration influence on the structural, electronic, optical and photocatalytic properties of tungsten–silver codoped titanium dioxide

To activate the wide functionalities of TiO2 in photoelectrochemical applications, tungsten–silver codoped TiO2 with different tungsten doping concentrations were synthesized using hydrothermal method. The codoped TiO2 displayed pure anatase phase with strong absorption in visible light region describing the possible modification in electronic band structure due to codoping. Transmission electron microscopy observation shows that nano-particles have spherical morphology with uniform particle size distribution. X-ray photoelectron spectroscopy (XPS) verified the existence of tungsten and silver in the TiO2 network, responsible for tuning the absorption edge and improving the photoactivity. The photoactivity of synthesized nano-materials was tested by photodegradation of methylene blue under visible light irradiations. All the synthesized codoped samples exhibit enhanced photo-response compared to that of undoped TiO2. Among all the codoped samples, the one with tungsten doping concentration of 3.5at% possesses the best photocatalytic activity attributed to the synergistic effect between the dopants and optimal doping concentration.

Effect of Lubricant Additives on the WDLC Coating Structure When Tested in Boundary Lubrication Regime

Improvements in coating deposition technology enable the mass production of high-quality diamond-like carbon (DLC) coatings at an industrial scale and also increase their use in lubricated contacts. However, the understanding of the interactions of different lubricant additives with this material is not yet fully developed. This study focuses on several fundamental aspects of the tungsten-doped DLC coating (denoted as WDLC) behaviour under boundary lubrication conditions with model lubricants. The effect of lubricant additives on the coating structure change is discussed in terms of carbon structure and the tungsten dopant. Electron energy-loss spectroscopy, XPS and Raman spectroscopy characterization for the upper carbon layers indicate that the WDLC coating interacts chemically with selected lubricant additives. The study provides information on both coating and additive optimization under boundary lubrication.

Plasma induced tungsten doping of TiO2 particles for enhancement of photocatalysis under visible light.

Here we report a novel method for modifying commercially available TiO2 nanoparticles by a microwave-induced plasma technique. After the plasma treatment TiO2 nanoparticles showed enhanced visible absorption due to the doped W atoms, and the photocatalytic methylene blue degradation above 440 nm was successfully improved.

Single-step electrochemical anodization for synthesis of hierarchical WO3–TiO2 nanotube arrays on titanium foil as a good photoanode for water splitting with visible light

Hierarchical WO3–TiO2 nanotube (WTNs) composite structures were obtained by anodization of titanium in a single-step process using sodium tungstate as the tungsten source. The resulting WTNs showed tubes with diameters in rang of 80–110nm, wall thickness of 20–40nm and tube lengths in the range of 7–8μm. Diffuse reflectance spectra showed an increase in the visible absorption relative to pure TiO2 nanotubes (TNs). The photo-electrochemical performance was examined under simulated sunlight irradiation in 1M NaOH electrolyte. Photo-electrochemical characterization shows that tungsten doping efficiently enhances the photo-catalytic water splitting performance of WTNs composite. The sample (WTNs-2) exhibited better photo-catalytic activity than the TNs and WTNs fabricated using other W concentrations. This can mainly be attributed to better charge carrier separation and transportation in photoelectrochemical water splitting by providing an effective way to address recombination losses in these composite materials.

Tungsten doping of Ta3N5-nanotubes for band gap narrowing and enhanced photoelectrochemical water splitting efficiency

Ordered W-doped Ta2O5 nanotube arrays were grown by self-organizing electrochemical anodization of TaW alloys with different tungsten concentrations and by a suitable high temperature ammonia treatment, fully converted to W:Ta3N5 tubular structures. A main effect found is that W doping can decrease the band gap from 2eV (bare Ta3N5) down to 1.75eV. Ta3N5 nanotubes grown on 0.5 at.% W alloy and modified with Co(OH)x as co-catalyst show ~33% higher photocurrents in photoelectrochemical (PEC) water splitting than pure Ta3N5.

Rotatable serial co-sputtering of doped titania

Serial co-sputtering is an extension of conventional magnetron sputtering: it utilizes cylindrical primary rotating sputter sources within a metallic or reactive sputtering process. With one or more auxiliary sputter sources the surface of the primary sputter target is simultaneously coated with additional elements. In the primary sputtering process, these elements get implanted and mixed into the primary target material leading to production of multi-component films.In this paper we use an installation of this technology called “Megatron™”, which involves an effective gas separation between primary and auxiliary chamber volumes. With this setup, we fabricated TiO2:W and TiO2:Nb layers by using a rotating Titania target as primary source and planar W and Nb targets as auxiliary sources, respectively.In both cases the so called sputter yield amplification effect (SYA) was demonstrated: Within the TiO2 matrix the heavier element e. g. tungsten keeps the sputter cascade close to the target surface and thus significantly enhances the sputter rate. Additionally, by niobium doping in combination with a post-deposition annealing it is possible to get TiO2:Nb layers with tailored Nb composition as conductive transparent oxides (TCO).Due to the independent power control of the secondary target the tungsten and niobium concentration can easily be optimized. We present doping series for optimizing either the SYA effect for TiO2:W or the effect of a transparent conductive TCO for TiO2:Nb. With tungsten doping we achieved an enhancement of more than twice the DC sputter rate. With niobium doping and a post-deposition annealing step (350 °C) we reach resistivity values of about 1.2 × 10−3 Ωcm.

금속물질로 개질된 광촉매를 이용한 톨루엔 광분해특성 연구

In photocatalysis, the addition of metal matter to TiO2 can alter the surface properties. As such, the metal can increase the rate of the photodegradation reaction. In this study, a range of modified TiO2 photocatalysts were prepared and tested to improve the activity of photodegradation at a batch-typed photoreactor. To obtain a good sol solution of the TiO2 photocatalyst, several types of dispersion agents and stabilizers were investigated. The photocatalyst solutions were modified with isoproply alcohol as the dispersion agent and sodium silicate as the stabilizer. The effects of various metallic elements on enhancing the photocatalytic activity of TiO2 on the degradation of toluene were examined. Palladium-added TiO2 was found to be the best, whereas copper or tungsten-added also showed good results. In the case of palladium addition, the increase in removal efficiency was 25%. On the other hand, Fe-added TiO2 showed a notable decrease in photocatalytic degradation. Additional doping of copper or tungsten on the Pd/TiO2 had no positive effect on the photodegradation activity.