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.     

Effect of citric acid on photoelectrochemical properties of tungsten trioxide films prepared by the polymeric precursor method

Effect of citric acid (CA) on microstructure and photoelectrochemical properties of WO₃ films prepared by the polymeric precursor method was investigated. The obtained materials were characterized by means of X-ray diffraction (XRD), field-emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (HRTEM). The results showed that samples prepared with adding different amounts of citric acid had a pure phase of cubic. The addition of citric acid could significantly increase the particle size and change the surface of WO₃ films. The photoelectrochemical measurements were performed using a standard three-electrode system cell. The films prepared from mass ratios of CA/PEG (R = 0, 0.2, 0.4, 0.6 and 1) showed 1.0, 1.4, 1.7, 2.1 and 0.9 mA cm⁻² at 1.2 V under illumination with a 500 W xenon lamp (I₀ = 100 mW/cm²), respectively.     

Effect of ultrasonic irradiation on structure and electrochemical properties of LiMn2O4 thin films cathode material for Li-ion micro-batteries

Two kinds of spinel LiMn₂O₄ thin film for lithium ion micro-batteries were successfully prepared on polycrystal Pt substrates by spin coating methods, which were carried out under ultrasonic irradiation (USG) and magnetic stirring (MSG), respectively. The microstructures and electrochemical performance of LiMn₂O₄ thin films were characterized by thermogravimetry analysis (TGA), X-ray diffraction (XRD), scanning electron microscopy (SEM), and galvanostatic charge-discharge measurements. It was found that the crystalline structure of USG samples grew better than that of the MSG samples. At the same time, higher discharge capacity and better cycle stability were obtained for the LiMn₂O₄ thin films of USG at the current density of 50 μAh/cm² between 3.0 and 4.3 V. The 1st discharge capacity was 57.8 μAh/cm²-μm for USG thin films and 51.7 μAh/cm²-μm for MSG thin films. After 50 cycles, 91.4% and 69% of discharge capacity could be retained respectively, indicating that ultrasonic irradiation condition during spin coating was more suitable for preparing spinel LiMn₂O₄ thin films with better electrode performance for lithium ion micro-batteries.     

Microwave synthesis, characterization, and electrochemical properties of α-Fe2O3 nanoparticles

Ultrafine α-Fe₂O₃ nanoparticles with an extremely narrow distribution were synthesized by microwave heating. Transmission electron microscopy (TEM) images showed that most primary particles have ellipsoid shapes, and the average diameter of the primary particles was less than 10 nm. The electron diffraction pattern and fringes in some particles in TEM images showed that these nanoparticles were single crystals. The BET surface area of the freeze-dried product was 217 m²/g. The initial discharge capacity of the α-Fe₂O₃ nanoparticles exceeded 1007 mA/g (cut-off voltage: 0.5 V). This large capacity corresponds to that calculated by assuming the reduction of Fe³⁺ to Fe⁰. The α-Fe₂O₃ nanoparticles also work as a rechargeable electrode material. The charge-discharge test between 4 V and 1.5 V gave a good rechargeable capacity of about 150 mAh/g.     

A structural analysis of W-Sb mixed oxide catalyst

An investigation on the structure of W-Sb mixed oxide catalyst, W₁₂Sb_xO_y (x = 1, 3, 5), is proposed. The W-Sb mixed oxide powders were prepared by the calcination of aqueous precursors, antimony tartrate and ammoniummetatungstate, and characterized with scanning electron microscope, X-ray diffractometer, and transmission electron microscope. At low content of Sb (x = 1), the W-Sb mixed oxide powder consisted of polyhedral particles, and their crystal structure was triclinic WO₃. At higher content (x = 3, 5), majority of the oxide powders were bar-shaped particles, consisting of triclinic WO₃ and tetragonal WO₃. With electron diffraction pattern and simulation, Sb incorporation into the cuboctahedral sites of perovskite-like WO₃ was proved and its effect on the phase transition from triclinic to tetragonal was discussed.     

Porous LiNi0.75Co0.25O2 microspheres prepared via a hydrothermal process as cathode material for lithium ion batteries

Porous LiNi_0.75Co_0.25O₂ microspheres are successfully prepared by a simple hydrothermal process by using H[Ni_0.75Co_0.25OOH]₃ and LiOH as starting materials in the presence of urea for the first time. The synthesized samples are characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), specific surface area (S_BET), and electrochemical performance. The synthesized LiNi_0.75Co_0.25O₂ has a good electrochemical performance with an initial discharge capacity of 169.3 mA g⁻¹ and good capacity retention of 96.7% after 50 cycles at 0.2 C (25 mA g⁻¹). The electrochemical lithium ion insertion/extraction process is quite reversible even at 5 C. Furthermore, the structure in the charge-discharge process is stable and the impedance increased slowly during cycling.     

Electrochemical behaviour of sputtered c-V2O5 and LiCoO2 thin films for solid state lithium microbatteries

Here are reported for the first time electrochemical data on all-solid-state lithium microbatteries using crystalline sputtered V₂O₅ thin films as cathode materials and LiPON as solid electrolyte. The stable specific capacity of 30 µAh/cm² found with a 2.4 µm thick film competes very well with the best values obtained for solid state microbatteries using amorphous films. With the challenge of decreasing the temperature of heat treatment for sputtered LiCoO₂ thin films, we show that a temperature of 500 °C combined with an optimized bias sputtering (-50 V) allows to get highly crystalline deposits, to minimize the presence of Co₃O₄ and to suppress any trace of the cubic phase. At the same time the theoretical specific capacity is reached in the 4.2 V-3 V range and a good cycling behaviour is achieved with a high capacity of 50 µAh/cm²/µm after 140 cycles at 10 µA.cm².     

VO2-WO3 nanocomposite thin films synthesized by pulsed laser deposition technique

Pure VO₂ and VO₂-WO₃ composite thin films were grown on quartz substrate by pulsed laser deposition (PLD) technique. The influence of varying WO₃ molar concentration in the range from x = 0.0 to x = 0.4 on structural, electrical and optical properties of VO₂-WO₃ nanocomposite thin films has been systematically investigated. X-ray diffraction studies reveal the single crystalline monoclinic VO₂ phase (m-VO₂) up to 10% of WO₃ content whereas both m-VO₂ as well as h-WO₃ (hexagonal WO₃) phases were present at higher WO₃ content (0.2 ≤ x ≤ 0.4). Optical transmittance spectra of the films showed blue shift in the absorption edge with increase in WO₃ content. Temperature dependence of resistivity (R-T) measurements indicates significant variation in metal-insulator transition temperature, width of the hysteresis, and shape of the hysteresis curve. Cyclic Voltammetry measurements were performed on VO₂-WO₃ thin films. A direct correlation between V/W ratio and structure-property relationship was established. The present investigations reveal that doping of WO₃ in VO₂ is effective to increase the optical transmittance and to reduce the semiconductor to metal phase transition temperature close to room temperature.     

Facile synthesis of Nb2O5 nanorod array films and their electrochemical properties

Nb₂O₅ nanorod array films were synthesized by a facile hydrothermal process using niobium metal foil and NH₄F as precursors. The Nb₂O₅ nanorods stand on the niobium metal foil substrate and are less than 100 nm in diameter and about 1 μm in length. X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM) characterizations indicate that these nanorods have orthorhombic structure and grew longitudinally along 〈0 0 1〉 direction. The nanorod growth mechanism was discussed. Thermal annealing at a temperature below 500 °C did not change the microstructure of nanorods but improve the crystallinity. The Nb₂O₅ nanorod array films have been tested as cathode material for lithium battery, which showed a good specific capacity up to 380 mAh g⁻¹ even after 50 charge/discharge cycles.     

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.     

XPS, XRD and SEM characterization of a thin ceria layer deposited onto graphite electrode for application in lithium-ion batteries

Thin ceria layer deposited by electro-precipitation onto graphite was synthesised and characterized by X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The electro-precipitated ceria has a cubic structure with nanocrystallites of about 6 nm. The SEM analyses shows that the ceria layer reflects the morphology of the graphite electrode, exhibits small cracks usually found on the electro-precipitated films but covers almost completely the surface of the graphite. The ceria layer is composed of 75% Ce(IV) and 25% Ce(III) oxides as indicated by the XPS analyses. Cyclic voltammetry and galvanostatic charge-discharge tests in ethylene carbonate/dimethyl carbonate (1/1) (wt/wt) in the presence of 1 M LiPF₆ show that reversible lithium insertion and deinsertion occurs in the graphite/ceria electrode and that the ceria layer on the graphite electrode prevents from the loss of capacity during the first four cycles. The reduction of the electrolyte occurs at about 0.7 V vs Li/Li⁺ on both electrodes but XPS and SEM analyses show that the SEI layer is thin and not as homogenous on the graphite as on the graphite/ceria electrode. The composition of the SEI layer on the graphite/ceria electrode, mainly composed of Li₂CO₃, ROCO₂Li, R-CH₂OLi and LiF, is different than those obtained on the graphite.     

Combustion synthesis and luminescence properties of NaY1−xEux(WO4)2 phosphors

The red phosphors NaY_1−xEu_x(WO₄)₂ with different concentrations of Eu³⁺ were synthesized via the combustion synthesis method. As a comparison, NaEu(WO₄)₂ was prepared by the solid-state reaction method. The phase composition and optical properties of as-synthesized samples were studied by X-ray powder diffraction and photoluminescence spectra. The results show that the red light emission intensity of the combustion synthesized samples under 394 nm excitation increases with increase in Eu³⁺ concentrations and calcination temperatures. Without Y ions doping, the emission spectra intensity of the NaEu(WO₄)₂ phosphor prepared by the combustion method fired at 900 °C is higher than that prepared by the solid-state reaction at 1100 °C. NaEu(WO₄)₂ phosphor synthesized by the combustion method at 1100 °C exhibits the strongest red emission under 394 nm excitation and appropriate CIE chromaticity coordinates (x=0.64, y=0.33) close to the NTSC standard value. Thus, its excellent luminescence properties make it a promising phosphor for near UV InGaN chip-based red-emitting LED application.     

Hexagonal nanorods of tungsten trioxide: Synthesis, structure, electrochemical properties and activity as supporting material in electrocatalysis

Tungsten trioxide, unhydrated with hexagonal structure (h-WO₃), has been prepared by hydrothermal method at a temperature of 180 °C in acidified sodium tungstate solution. Thus prepared h-WO₃ has been characterized by X-ray diffraction (XRD) method and using electrochemical techniques. The morphology has been examined by scanning and transmission electron microscopies (SEM and TEM) and it is consistent with existence of nanorods of 50-70 nm diameter and up to 5 μm length. Cyclic voltammetric characterization of thin films of h-WO₃ nanorods has revealed reversible redox behaviour with charge-discharge cycling corresponding to the reversible lithium intercalation/deintercalation into the crystal lattice of the h-WO₃ nanorods. In propylene carbonate containing LiClO₄, two successive redox processes of hexagonal WO₃ nanorods are observed at the scan rate of 50 mV/s. Such behaviour shall be attributed to the presence of at least two W atoms of different surroundings in the lattice structure of h-WO₃ nanorods. On the other hand, in aqueous LiClO₄ solution, only one redox process is observed at the scan rate of 10 mV/s. The above observations can be explained in terms of differences in the diffusion of ions inside two types of channel cavities existing in the structure of the h-WO₃ nanorods. Moreover, the material can be applied as active support for the catalytic bi-metallic Pt-Ru nanoparticles during electrooxidation of ethanol in acid medium (0.5 mol dm⁻³ H₂SO₄).     

Structural and electrochemical properties of Nd-doped LiFePO4/C prepared without using inert gas

To further improve the electrochemical performance of LiFePO₄/C, Nd doping has been adopted for cathode material of the lithium ion batteries. The Nd-doped LiFePO₄/C cathode was synthesized by a novel solid-state reaction method at 750 °C without using inert gas. The Li_0.99Nd_0.01FePO₄/C composite has been systematically characterized by X-ray diffraction, EDS, SEM, TEM, charge/discharge test, electrochemical impedance spectroscopy and cyclic stability. The results indicate that the prepared sample has olivine structure and the Nd³⁺ and carbon modification do not affect the structure of the sample but improve its kinetics in terms of discharge capacity and rate capability. The Li_0.99Nd_0.01FePO₄/C powder exhibited a specific initial discharge capacity of about 161 mAh g^− 1 at 0.1 C rate, as compared to 143 mAh g^− 1 of LiFePO₄/C. At a high rate of 2 C, the discharge capacity of Li_0.99Nd_0.01FePO₄/C still attained to 115 mAh g^− 1 at the end of 20 cycles. EIS results indicate that the charge transfer resistance of LiFePO₄/C decreases greatly after Nd doping.     

Synthesis process and the luminescence properties of rare earth doped NaLa(WO4)2 nanoparticles

Rare earth doped NaLa(WO₄)₂ nanoparticles have been prepared by a simply hydrothermal synthesis procedure. The X-ray diffraction (XRD) pattern shows that the Eu³⁺-doped NaLa(WO₄)₂ nanoparticles with an average size of 10-30 nm can be obtained via hydrothermal treatment for different time at 180 °C. The luminescence intensity of Eu³⁺-doped NaLa(WO₄)₂ nanoparticles depended on the size of the nanoparticles. The bright upconversion luminescence of the 2 mol% Er³⁺ and 20 mol% Yb³⁺ codoped NaLa(WO₄)₂ nanoparticles under 980 nm excitation could also be observed. The Yb³⁺-Er³⁺ codoped NaLa(WO₄)₂ nanoparticles prepared by the hydrothermal treatment at 180 °C and then heated at 600 °C shows a 20 times stronger upconversion luminescence than those prepared by hydrothermal treatment at 180 °C or by hydrothermal treatment at 180 °C and then heated at 400 °C.     

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.     

Annealing effects on the structural and electrical transport properties of n-type Bi2Te2.7Se0.3 thin films deposited by flash evaporation

N-type Bi₂Te_2.7Se_0.3 thermoelectric thin films with thickness 800 nm have been deposited on glass substrates by flash evaporation method at 473 K. Annealing effects on the thermoelectric properties of Bi₂Te_2.7Se_0.3 thin films were examined in the temperature range 373-573 K. The structures, morphology and chemical composition of the thin films were characterized by X-ray diffraction, field emission scanning electron microscope and energy dispersive X-ray spectroscopy, respectively. Thermoelectric properties of the thin films have been evaluated by measurements of the electrical resistivity and Seebeck coefficient at 300 K. The Hall coefficients were measured at room temperature by the Van der Pauw method. The carrier concentration and mobility were calculated from the Hall coefficient. The films thickness of the annealed samples was measured by ellipsometer. When annealed at 473 K, the electrical resistivity and Seebeck coefficient are 2.7 mΩ cm and −180 μV/K, respectively. The maximum of thermoelectric power factor is enhanced to 12 μW/cm K².     

Chemical synthesis of spinel nickel ferrite (NiFe2O4) nano-sheets

The present investigation is related to the deposition of single-phase nano-sheets spinel nickel ferrite (NiFe₂O₄) thin films onto glass substrates using a chemical method. Nano-sheets nickel ferrite films were deposited from an alkaline bath containing Ni²⁺ and Fe²⁺ ions. The films were characterized for their structural, surface morphological and electrical properties by means of X-ray diffraction (XRD), transmission electron microscope (TEM), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), and two-point probe electrical resistivity techniques. The X-ray diffraction pattern showed that NiFe₂O₄ nano-sheets are oriented along (3 1 1) plane. The FT-IR spectra of NiFe₂O₄ films showed strong absorption peaks around 600 and 400 cm⁻¹ which are typical for cubic spinel crystal structure. Microstructural study of NiFe₂O₄ film revealed nano-sheet like morphology with average sheet thickness of 30 nm. The room temperature electrical resistivity of the NiFe₂O₄ nano-sheets was 10⁷ Ω cm.     

Formation of ⊥c texture of tungsten disulfide thin films with nickel

The formation of ⊥c texture of WS₂ thin films by solid state reaction between the spray deposited WO₃ and gaseous sulfur vapours with Ni interfacial layer has been reported. X-ray diffraction technique has been used to measure the degree of preferred orientation and texture of WS₂ films. Scanning electron microscopy, transmission electron microscopy and atomic force microscopy have been used to characterize the microstructure and morphology. The electronic structure and chemical composition was studied using X-ray photoelectron spectroscopy. The WS₂ films comprise single crystalline quality hexagonal crystallites of 15 μm × 15 μm size with their basal planes parallel to the substrate. The film consists of turbostratic stacking sequence of 2H and 3R polytypes of WS₂. The tungsten-to-sulfur ratio was estimated to be 1:1.8. The various qualitative models used to explain promotional effects are briefly outlined and the plausible underlying mechanism of formation of ⊥c texture with nickel, in this study, is given.     

Microstructures and multiferroic properties of textured Bi0.8La0.2FeO3 thin films

La-substituted BiFeO₃, Bi_0.8La_0.2FeO₃, thin films were fabricated on Pt/Ti/SiO₂/Si substrates by pulsed laser deposition. X-ray diffraction and high-resolution transmission electron microscope were used to analyze the structures of the films. The results show the films fabricated under optimized growth condition are (0 1 2) textured. X-ray photoemission spectroscopy results indicate that the oxidation state of Fe ion is Fe³⁺ in the films without detectable Fe²⁺. The films show low leakage current and excellent dielectric characters. Multiferroic properties with a remnant ferroelectric polarization of 5.2 μC/cm² and a remanent magnetization of 0.02 μ_B/Fe were established. These results have some implications for further research.