Lithium electrochemistry of SiO2 thin film electrode for lithium-ion batteries

The lithium electrochemistry of SiO₂ thin film prepared by reactive radio frequency sputtering has been investigated for the first time. The reversible discharge capacities of SiO₂/Li cells cycled between 0.01 and 3.0 V are found in the range from 416 to 510 mAh/g during the first 100 cycles. By using ex situ transmission electron microscopy, selected-area electron diffraction and X-ray photo-electron spectroscopy measurements, both Li-Si alloying process and the reversible conversion reaction of SiO₂ into Li₂Si₂O₅ are proposed in the lithium electrochemical reaction of SiO₂. SiO₂ film electrode with high-reversible capacity and good cycle performance exhibits it potential anode material for future lithium-ion batteries.     

Nanostructured Ni3Sn2 thin film as anodes for thin film rechargeable lithium batteries

Thin film Ni₃Sn₂ anodes were deposited on a Cu substrate by e-beam evaporator at room temperature. The deposited films were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM). They were tested as anodes for thin film rechargeable lithium batteries. These film electrodes exhibited an excellent cycle performance over 500 cycles. Ni₃Sn₂ films remained without undergoing any crystallographic phase change during cycling.     

Porous WO3 from anodized sputtered tungsten thin films for NO2 detection

In this paper, porous WO₃ films were prepared by anodic oxidation of metallic tungsten (W) films deposited on alumina substrates. The structural and morphological properties of the porous WO₃ films were investigated using field emission scanning electron microscope (FESEM) and X-ray diffraction (XRD). A large number of cracks appeared on the surface of films after anodization, which makes the films porous. The porous WO₃ sensors achieved their maximum response values to NO₂ at a low operating temperature of 150 °C. The porous WO₃ sensors showed high response values, great stability and fast response-recovery characteristics to different concentration of NO₂ gas due to the high specific surface area and special structural and morphological properties.     

Structure, electrochromic and optical properties of WO3 film prepared by dip coating-pyrolysis

The tungsten oxide (WO₃) film was grown by dip coating-pyrolysis method with the PEG-400 as the structure-directing agent. Microstructure of the WO₃ film was characterized by TG-DSC, XRD and SEM techniques. It was found that the film annealed at 350 °C for 2 h comprised cubic WO₃ and orthorhombic WO₃. The measurements of the cyclic voltammetry (CV) and UV-vis spectrum suggested that the WO₃ film had a good electrochromic reversibility performance. The film possessed excellent modulation to the visible light and the maximal average transmittance modulation reached 70.06%.     

Ab-initio density functional theory study of a WO3 NH3-sensing mechanism

WO 3 bulk and various surfaces are studied by an ab-initio density functional theory technique.The band structures and electronic density states of WO 3 bulk are investigated.The surface energies of different WO 3 surfaces are compared and then the (002) surface with minimum energy is computed for its NH 3 sensing mechanism which explains the results in the experiments.Three adsorption sites are considered.According to the comparisons of the energy and the charge change between before and after adsorption in the optimal adsorption site O 1c,the NH 3 sensing mechanism is obtained.     

Nanoporous silicon flakes as anode active material for lithium-ion batteries

Nanoporous-silicon (np-Si) flakes were prepared using a combination of an electrochemical etching process and an ultra-sonication treatment and the electrochemical properties were studied as an anode active material for rechargeable lithium-ion batteries (LIBs). This fabrication method is a simple, reproducible, and cost effective way to make high-performance Si-based anode active materials in LIBs. The anode based on np-Si flakes exhibited a higher performances (lower capacity fade rate, stability and excellent rate capability at high C-rate) than the anode based on Si nanowires. The excellent performance of the np-Si flake anode was attributed to the hollowness (nanoporous structure) of the anode active material, which allowed it to accommodate a large volume change during cycling.     

Carbon-coated silicon/crumpled graphene composite as anode material for lithium-ion batteries

Silicon is a promising anode material for high-capacity Li-ion batteries (LIBs). However, its insulating property and large volume change during the lithiation/delithiation process result in poor cycling stability and in pulverization of Si. In this work, glucose-derived carbon-coated Si nanoparticles (C–Si NPs) are in conjunction with crumpled graphene (cGr) particles by a spray-drying method to prepare a novel composite (C–Si/cGr) material. The prepared C–Si NPs are uniformly embedded in the ridges of the cGr particles. The carbon layer of C–Si can make a good contact with the graphene sheet, resulting in enhanced electrical conductivity and fast charge transfer. In addition, the unique crumpled structure of the cGr can buffer the large volume change upon cycling process and facilitate the diffusion of electrolyte into the composite material. When employed as an anode electrode of LIBs, the C–Si/cGr composites deliver enhanced electrochemical performance, including stable cycling with a discharge capacity of 790 mAh·g⁻¹ after 100 cycles and a rate capability of 654 mAh·g⁻¹ at 2C. The synergistic effect of the carbon layer coating of Si NPs and the crumpled structure of the cGr particles results in a composite with improved the electrochemical performance, which is likely related to its high electrical conductivity and good mechanical stability of composite material.     

Preparation of WO3 nanoparticles and application to NO2 sensor

WO₃ nanoparticles were prepared by evaporating tungsten filament under a low pressure of oxygen gas, namely, by a gas evaporation method. The crystal structure, morphology, and NO₂ gas sensing properties of WO₃ nanoparticles deposited under various oxygen pressures and annealed at different temperatures were investigated. The particles obtained were identified as monoclinic WO₃. The particle size increased with increasing oxygen pressure and with increasing annealing temperature. The sensitivity increased with decreasing particle size, irrespective of the oxygen pressure during deposition and annealing temperature. The highest sensitivity of 4700 to NO₂ at 1 ppm observed in this study was measured at a relatively low operating temperature of 50 °C; this sensitivity was observed for a sensor made of particles as small as 36 nm.     

NbTi0.5Ni0.5O4 as anode compound material for SOFCs

NbTi_0.5Ni_0.5O₄ (NTNO) has been prepared using solid state synthesis and investigated as a potential anode material. The oxide form of NTNO has single phase rutile-type structure with tetragonal (P42/mnm) space group. The reduced form is a composite of nano-scaled particles of metallic Ni and Nb_1.33Ti_0.67O₄ phase. Reduced NTNO showed high electronic conductivity up to 280 S.cm^− 1 at 900 °C in reducing atmosphere, but suffers from low CTE equal to 3.78 10^− 6 K^− 1. Studies of NTNO as anode material were carried out in a three electrode - electrochemical half cell configuration under pure humidified H₂ at 900 °C using a 2 mm thick zirconia electrolyte and without any additional current collector material. The results show a reasonable series resistance (R_s) equal to 2.7 Ωcm² (about 50% higher than for metallic gold layers) indicating a good current collection performance for a 10 μm layer of material. The polarization resistance (R_p) was equal to 33 Ωcm² and is attributed to a poor density of three phase boundaries (TPB) and shortage of oxide ion conduction in the anode layer. The results show the potential of NTNO as an anode material, especially after optimization of the microstructure towards the increase of TPB length.     

Pulsed laser deposited iron fluoride thin films for lithium-ion batteries

Iron fluoride thin films were successfully grown by Pulsed Laser Deposition (PLD), and their physico-chemical properties and electrochemical behaviours were examined by adjusting the deposition conditions, such as the target nature (FeF₂ or FeF₃), the substrate temperature (T_s ≤ 600 °C), the gas pressure (under vacuum or in oxygen atmosphere) and the repetition rates (2 and 10 Hz). Irrespective of the FeF₂ or FeF₃ target nature, iron fluoride thin films, deposited at 600 °C under vacuum, showed X-ray diffraction (XRD) patterns corresponding to the FeF₂ phase. On the other hand, iron fluoride thin films deposited at room temperature (RT) from FeF₂ target were amorphous, whereas the thin films deposited from FeF₃ target consisted of a two-phase mixture of FeF₃ and FeF₂ showing sharp and broad diffraction peaks by XRD, respectively. Their electrochemical behaviour in rechargeable lithium cells was investigated in the 0.05-3.60 V voltage window. Despite a large irreversible capacity on the first discharge, good cycling life was observed up to 30 cycles. Finally, their electrochemical properties were compared to the ones of iron oxide thin films.     

Structural, electrical and optical properties of TiO2 doped WO3 thin films

TiO₂ doped WO₃ thin films were deposited onto glass substrates and fluorine doped tin oxide (FTO) coated conducting glass substrates, maintained at 500 °C by pyrolytic decomposition of adequate precursor solution. Equimolar ammonium tungstate ((NH₄)₂WO₄) and titanyl acetyl acetonate (TiAcAc) solutions were mixed together at pH 9 in volume proportions and used as a precursor solution for the deposition of TiO₂ doped WO₃ thin films. Doping concentrations were varied between 4 and 38%. The effect of TiO₂ doping concentration on structural, electrical and optical properties of TiO₂ doped WO₃ thin films were studied. Values of room temperature electrical resistivity, thermoelectric power and band gap energy (E_g) were estimated. The films with 38% TiO₂ doping in WO₃ exhibited lowest resistivity, n-type electrical conductivity and improved electrochromic performance among all the samples. The values of thermoelectric power (TEP) were in the range of 23-56 μV/K and the direct band gap energy varied between 2.72 and 2.86 eV.     

Effect of WO3 on structural and optical properties of CeO2-PbO-B2O3 glasses

Pure and WO₃ doped CeO₂-PbO-B₂O₃ glasses are prepared by the melt-quench technique. The structural and optical analyses of glasses are carried out by XRD, FTIR, density and UV-vis spectroscopic measurement techniques. FTIR analysis indicates the transformation of structural units of BO₃ into BO₄ with W-O-W vibration and the presence of WO₄ and WO₆ units observed with increase in WO₃ contents. Decrease in band gap for CeO₂-PbO-B₂O₃ glasses from 2.89 to 2.30 eV and for WO₃ doped glasses from 2.89 to 1.95 eV has been observed and discussed. This decrease in band gap with WO₃ doping approaches to semiconductor behavior. It shows that the presence of WO₃ in the glass samples causes more compaction of the borate network due to the formation of BO₄ groups and the presence of WO₄ and WO₆ groups, which result in a decrease in the optical band gap energy and increase in the density.     

Blue luminescence in Tm-doped KGd(WO4)2 single crystals

Up-conversion blue emissions of trivalent thulium ions in monoclinic KGd(WO₄)₂ single crystals at 454 and 479 nm are reported for a single pump laser source at 688 nm. We grew thulium-doped KGd(WO₄)₂ single crystals at several concentrations from 0.1% to 10%. We recorded a polarized optical absorption spectrum for the ³F₂+³F₃ energy levels of thulium at room temperature and low temperature (6 K). From the low temperature emission spectra we determined the splitting of the ³H₆ ground state. The blue emissions are characterized as a function of the dopant concentration and temperature from 10 K to room temperature. To our knowledge, this is the first time that sequential two-photon excitation process (STEP) generated blue emissions in thulium-doped single crystals with a single excitation wavelength.     

Correlation between lithium storage and diffusion properties and electrochromic characteristics of WO3 thin films

As essential electrochromic(EC) materials are related to energy savings in fenestration technology,tungsten oxide(WO3) films have been intensively studied recently.In order to achieve better understanding of the mechanism of EC properties,and thus facilitate optimization of device performance,clarification of the correlation between cation storage and transfer properties and the coloration performance is needed.In this study,transparent polycrystalline and amorphous WO3 thin films were deposited on SnO2:F-coated glass substrates by the pulsed laser deposition technique.Investigation into optical transmittance in a wavelength range of 400-800 nm measured at a current density of 130 μA·cm-2 with the applied potential ranging from 3.2 to 2.2 V indicates that polycrystalline films have a larger optical modulation of ～ 30% at 600 nm and a larger coloration switch time of 95 s in the whole wavelength range compared with amorphous films(～ 24% and 50 s).Meanwhile,under the same conditions,polycrystalline films show a larger lithium storage capacity corresponding to a Li/W ratio of 0.5,a smaller lithium diffusion coefficient(2×10-12cm2·s-1 for Li/W=0.24) compared with the amorphous ones,which have a Li/W ratio of 0.29 and a coefficient of ～2.5×10-11cm2·s-1 as Li/W=0.24.These results demonstrate that the large optical modulation relates to the large lithium storage capacity,and the fast coloration transition is associated with fast lithium diffusion.     

LaCaMnO3 thin film laser energy/power meter

A prototype laser energy/power meter was designed based on the anisotropic Seebeck effect of LaCaMnO₃ thin film grown on vicinal cut LaAlO₃ substrate. The optical response of this device to the pulsed laser and the chopped CW laser was measured from infrared to ultraviolet. The peak voltage of the measured signal shows a good linear relation to the laser energy and power in the measured range. It was shown that at 1064 nm wavelength, this prototype device demonstrates higher sensitivity than that of commercial device.     

溶胶凝胶法制备Pt/WO3氢气敏感材料的研究

采用溶胶凝胶法制备纳米级WO₃,掺杂不同含量的氯铂酸并混合搅拌均匀,再进行热处理,将所得粉体均匀涂覆在光纤光栅周围,制备出具有氢敏特性的光纤光栅传感器.实验中,通过改变氯铂酸掺杂量和热处理温度并进行XRD物相分析得:随着Pt:W的降低以及热处理温度的升高,WO₃的结晶度不断提高;通入不同浓度的氢气对传感器进行氢敏性能测试发现,经过300℃热处理,Pt:W为1:9时,对4%浓度的氢气能达到15 s的响应速度,最高有140 pm的中心波长变化,多次重复通氢气,重复性良好;当热处理温度达到500℃时,材料对氢气已经不敏感.     

Oxygen vacancy in LiTiPO5 and LiTi2(PO4)3: A first-principles study

The structures of LiTiPO₅ and LiTi₂(PO₄)₃, as well as the possibility of oxygen vacancies formation in the systems are studied by first-principles calculations. It is found that oxygen vacancies can be formed in LiTiPO₅ and LiTi₂(PO₄)₃ under oxygen poor condition. The formation of oxygen vacancies introduce a defect band within their band gaps, which is expected to improve the electronic conductivity of LiTiPO₅ and LiTi₂(PO₄)₃ significantly. Meanwhile, a great concentration of oxygen vacancies may increase the discharge voltage of LiTiPO₅ and LiTi₂(PO₄)₃.     

Correlation between crystallinity and resistive switching behavior of sputtered WO3 thin films

The as-deposited WO₃ thin films were post-annealed at different temperatures (300 °C and 600 °C) in air to investigate a correlation between crystallinity and switching behavior of WO₃ thin films. Associating the results of XRD, FTIR, XPS and FESEM measurements, the annealing-caused crystallinity change contributes to the variation of the switching behaviors of the WO₃ thin films. The as-deposited WO₃ films with low crystalline structure are preferred for random Ag conducting path, resulting in large switching ratio but fluctuating I–V hysteresis, whereas the annealed WO₃ films with crystallized compact structure limits Ag conducting path, favoring the stable I–V hysteresis but small switching ratio. It is therefore concluded that electrochemical redox reaction-controlled resistance switching depends not only on electrode materials (inert and reactive electrodes) but also on crystallinity of host oxide.     

Unconventional photoluminescence effect of SrTiO3 thin film under UV-illumination

We find that PL intensity I(t) of SrTiO₃ thin film measured under UHV condition increases with UV-laser illumination over long time scale of ∼ 2 h. The intensity increase takes place at lower sample temperature as well, 200, 100 K, and 20 K. When O₂ and N₂ gas are introduced into the sample chamber the PL intensity decreases with the UV-illumination time, opposite to the UHV-case. We consider a quantitative thermal energy flow model of the laser-power and heat absorption by the sample, but find that temperature change of the sample is not large enough to account for the time dependent I(t). We propose photo-catalysis effect on STO surface as possible scenario of the PL intensity change.     

Surface electronic structure–catalytic activity relationship of partially reduced WO3 bulk or deposited on TiO2

Catalytic reactions of n-pentane and 1-pentene were carried out as a function of the reduction process of WO₃ as bulk or supported on TiO₂. In situ characterization by XPS–UPS techniques of the different chemical species formed following the reduction processes was performed. At reduction temperatures up to 773 K, two distinct W⁵⁺ and W⁴⁺ states were obtained. The acid functions associated with W⁵⁺, mainly of Brönsted type, enabled to isomerize only 1-pentene to unsaturated products. Dual metal–acid functions (bifunctional) of the W⁴⁺ state perform the isomerization processes of 1-pentene and n-pentane to isopentane in similar way to highly dispersed platinum on alumina catalysts. A conversion of 56% of n-pentane and a selectivity of 87% to isopentane were obtained at 623 K reaction temperature.