Texture effect on the electrochemical properties of LiCoO2 thin films prepared by PLD

LiCoO₂ thin films were prepared by pulsed laser deposition (PLD) on Pt/Ti/SiO₂/Si (Pt) and Au/MgO/Si (Au) substrates, respectively. Crystal structures and surface morphologies of thin films were investigated by X-ray diffraction (XRD) and field emission scanning electron microscopy (FESEM). The LiCoO₂ thin films deposited on the Pt substrates exhibited a preferred (0 0 3) texture with smooth surfaces while the LiCoO₂ thin films deposited on the Au substrates exhibited a preferred (1 0 4) texture with rough surfaces. The electrochemical properties of the LiCoO₂ films with different textures were compared with charge-discharge, dQ/dV, and Li diffusion measurements (PITT). Compared with the (1 0 4)-textured LiCoO₂ thin films, the (0 0 3)-textured thin films exhibited relatively lower electrochemical activity. However, the advantage of the (1 0 4)-textured film only remained for a small number of cycles due to the relatively faster capacity fade. Li diffusion measurements showed that the Li diffusivity in the (0 0 3)-textured film is one order of magnitude lower than that in the (1 0 4)-textured film. As discussed in this paper, we believe that Li diffusion through grain boundaries is comparable to or even faster than Li diffusion through the grains.     

The investigation on electrochemical reaction mechanism of CuF2 thin film with lithium

Crystalline CuF₂ thin films were prepared by pulsed laser deposition under room temperature. The physical and electrochemical properties of the as-deposited thin films have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), galvanostatic cycling and cyclic voltammetry (CV). Reversible capacity of 544 mAh g⁻¹ was achieved in the potential range of 1.0–4.0 V. A reversible couple of redox peaks at 3.0 V and 3.7 V was firstly observed. By using ex situ XRD and TEM techniques, an insertion process followed by a fully conversion reaction to Cu and LiF was revealed in the lithium electrochemical reaction of CuF₂ thin film electrode. The reversible insertion reaction above 2.8 V could provide a capacity of about 125 mAh g⁻¹, which makes CuF₂ a potential cathode material for rechargeable lithium batteries.     

Electrochemical performance of LiFePO4 thin films with different morphology and crystallinity

LiFePO₄ thin films have been prepared by pulsed laser deposition method on titanium substrates. The influence of the deposition parameters, e.g. substrate temperature, ambient argon pressure, and post-annealing on the crystallinity and morphology of as-deposited thin films are investigated. Well-crystallized pure olivine-phase is obtained under optimized deposition condition (20–30 Pa, 500 °C). It shows a high electrochemical activity (83% theoretical capacity) at low current density (0.33 μA cm⁻², 1/20 C) and elevated testing temperature (45 °C). Moderate post-annealing treatment can enhance the utilization of the films further. The deposition of the film at a too high temperature or post-annealing for too long time could introduce Fe³⁺ impurities, i.e., Li₃Fe₂(PO₄)₃ and Fe₄(P₂O₇)₃, which can be easily detected by extending the electrochemical test voltage down to 2.5 V.     

EPR studies on NO interaction with MoOx/t-ZrO2 catalysts obtained by slurry deposition

Variable temperature adsorption of nitric oxide on MoO₃ supported on tetragonal zirconia (MoO_x/t-ZrO₂), obtained by slurry deposition, was investigated by EPR spectroscopy. The influence of molybdenum loading and co-adsorbed oxygen on the adsorption process of NO was elucidated. Particular attention was devoted to redox character of NO activation. Another important aspect concerned is the nature of surface nitrosyl complexes of molybdenum and their thermal stability. The role of oxygen in NO transformation over catalyst surface was also discussed.     

Electrodeposition and characterization of Bi2Se3 thin films by electrochemical atomic layer epitaxy (ECALE)

Bismuth selenide thin films were grown on Pt substrate via the route of electrochemical atomic layer epitaxy (ECALE) in this work for the first time. The electrochemical behaviors of Bi and Se on bare Pt, Se on Bi-covered Pt, and Bi on Se-covered Pt were studied by cyclic voltammetry and coulometry. A steady deposition of Bi₂Se₃ could be attained after negatively stepped adjusting of underpotential deposition (UPD) potentials of Bi and Se on Pt in the first 40 deposition cycles. X-ray diffraction (XRD) analysis indicated that the films were single phase Bi₂Se₃ compound with orthorhombic structure, and the 2:3 stoichiometric ratio of Bi to Se was verified by EDX quantitative analysis. The optical band gap of the as-deposited Bi₂Se₃ film was determined as 0.35 eV by Fourier transform infrared spectroscopy (FTIR), which is consistent with that of bulk Bi₂Se₃ compound.     

Highly oriented diamond growth on SixGe1−x (100) thin films

Highly oriented (100) diamond films have been successfully grown on Si_xGe_1−x (100) thin films by bias enhanced nucleation (BEN) in microwave plasma chemical vapor deposition (MPCVD) system. Raman spectra show the 1332 cm⁻¹ peak which proves the formation of diamond. Diamond nucleation density on Si_xGe_1−x substrate estimated by scanning electron microscopy is higher than 10⁹ cm⁻². The interface between diamond and Si_xGe_1−x substrate was characterized by transmission electron microscopy (TEM). About 20 nm decrease in thickness of the Si_xGe_1−x film was observed after bias enhanced nucleation step. TEM shows the existence of silicon carbide and heteroepitaxial diamond grains grown on Si_xGe_1−x substrate. Characterization from high-resolution TEM on the specimen of short time deposition reveals that a number of epitaxial diamond grains were directly nucleated on Si_xGe_1−x with {111} interplanar spacing ratio of diamond and Si_xGe_1−x of 2:3. The diamond nucleation is found to be preferred on the ridge position of the rough substrate surface. Diamond {100} facets were quickly developed in the early stage of growth.     

Fabrication and lithium electrochemistry of Ag2Se thin film anode

Ag₂Se thin film has successfully been fabricated by pulsed-laser deposition method. X-ray diffraction (XRD) measurement indicated that the as-deposited Ag₂Se thin film consisted mainly of orthorhombic β-Ag₂Se. A reversible electrochemical reaction of Li/Ag₂Se cell has been investigated by discharge-charge and cyclic voltammetry (CV) measurements for the first time. The reversible capacity of Ag₂Se electrode exhibited more than 5.6 Li per Ag₂Se at 10 μA/cm². Both alloying/de-alloying processes and selenidation/reduction processes of Ag were revealed during the lithium electrochemical reaction of Ag₂Se thin film electrode. Ag₂Se was found to be one novel candidate of anode materials for rechargeable lithium batteries.     

Preparation and characterization of Bi-doped antimony selenide thin films by electrodeposition

Bi-doped antimony selenide (Sb_2−xBi_xSe₃) thin films have been prepared by potentiostatical electrodeposition and post annealing treatment. Cyclic voltammetry (CV) was used to investigate the electrochemical behaviors of electrodeposition. The suitable deposition potential for film preparation was determined to be about −0.40 V vs. SCE combining with CV, energy dispersive X-ray spectroscopy (EDS), environmental scanning electron microscope (ESEM) studies. After annealing, film shows improved crystallinity and a basic orthorhombic Sb₂Se₃ structure but having a larger d-spacing due to the substitution of Bi for Sb in Sb₂Se₃ lattice. The annealed film exhibits an absorption coefficient of larger than 10⁵ cm⁻¹ in the visible region, an direct optical band gap of 1.12 ± 0.01 eV, the n-type conductivity, an carrier concentration of 1.1 × 10¹⁹ cm⁻³ and an flat band potential of −0.40 ± 0.03 V vs. SCE.     

Cyclic voltammetry study of electrodeposition of Cu(In,Ga)Se2 thin films

The electrodeposition of Cu(In,Ga)Se₂ has been investigated by cyclic voltammetry (CV) in a DMF-aqueous solution that contained citrate as a complexing agent. The effects of the citrate ion on the reduction potentials of Cu²⁺, In³⁺, Ga³⁺ and H₂SeO₃ were examined for a unitary system. Furthermore, a cyclic voltammetry study was performed in a ternary Cu-In-Se system, a quaternary Cu-In-Ga-Se system, and binary Cu-Se, In-Se and Ga-Se systems. The insertion of In and Ga into the solid phase may proceed by an underpotential deposition mechanism, which involves two different routes: In³⁺ and Ga³⁺ reduction by a surface-induced effect from Cu₃Se₂ and/or reaction with H₂Se.     

Co3O4 thin film prepared by a chemical bath deposition for electrochemical capacitors

Co₃O₄ thin film is synthesized on ITO by a chemical bath deposition. The prepared Co₃O₄ thin film is characterized by X-ray diffraction, and scanning electron microscopy. Electrochemical capacitive behavior of synthesized Co₃O₄ thin film is investigated by cyclic voltammetry, constant current charge/discharge and electrochemical impedance spectroscopy. Scanning electron microscopy images show that Co₃O₄ thin film is composed of spherical-like coarse particles, together with some pores among particles. Electrochemical studies reveal that capacitive characteristic of Co₃O₄ thin film mainly results from pseudocapacitance. Co₃O₄ thin film exhibits a maximum specific capacitance of 227 F g⁻¹ at the specific current of 0.2 A g⁻¹. The specific capacitance reduces to 152 F g⁻¹ when the specific current increases to 1.4 A g⁻¹. The specific capacitance retention ratio is 67% at the specific current range from 0.2 to 1.4 A g⁻¹.     

Operando FTIR study of the photocatalytic oxidation of acetone in air over TiO2–ZrO2 thin films

Operando FTIR spectroscopy has been used to study the photocatalytic oxidation of acetone vapors over semiconductors films containing TiO₂ and ZrO₂. Preparation of these coatings was carried out by dipping a silicon wafer in stable sols containing particles of TiO₂, Ti_1−xZr_xO₂, or a mixture of ZrO₂ and TiO₂. These differences in chemical composition and phase homogeneity were selected in order to determine their effect on the photocatalytic performance. A transmission cell specifically designed for in situ studies of photocatalytic coatings was utilized for the FTIR experiments under reaction conditions. In contrast with investigations with powdered photocatalysts, the use of thin films guarantees that the whole semiconductor is irradiated, and for that reason purely photochemical reactions are monitored. Acetone adsorption takes place molecularly and is higher on the Ti_1−xZr_xO₂ coating. This fact is very likely related to the higher specific surface of the samples containing Zr. However, the maximum photocatalytic rate for acetone degradation corresponds to the films composed by a binary mixture of TiO₂ and ZrO₂. On the other hand, remarkable differences on the type and concentration of intermediates appearing as a result of the photocatalytic oxidation of acetone are found for the coatings studied. A simple kinetic model was applied to analyze the evolution of both gas phase and surface species. The parameters obtained indicate that each specific surface process is affected in a different way by the variation in the composition of the photoactive films.     

A composite CdS thin film/TiO2 nanotube structure by ultrafast successive electrochemical deposition toward photovoltaic application

Fabricating functional compounds on substrates with complicated morphology has been an important topic in material science and technology, which remains a challenging issue to simultaneously achieve a high growth rate for a complex nanostructure with simple controlling factors. Here, we present a novel simple and successive method based on chemical reactions in an open reaction system manipulated by an electric field. A uniform CdS/TiO₂ composite tubular structure has been fabricated in highly ordered TiO₂ nanotube arrays in a very short time period (~90 s) under room temperature (RT). The content of CdS in the resultant and its crystalline structure was tuned by the form and magnitude of external voltage. The as-formed structure has shown a quite broad and bulk-like light absorption spectrum with the absorption of photon energy even below that of the bulk CdS. The as-fabricated-sensitized solar cell based on this composite structure has achieved an efficiency of 1.43% without any chemical doping or co-sensitizing, 210% higher than quantum dot-sensitized solar cell (QDSSC) under a similar condition. Hopefully, this method can also easily grow nanostructures based on a wide range of compound materials for energy science and electronic technologies, especially for fast-deploying devices.     

As-deposited LiCoO2 thin film cathodes prepared by rf magnetron sputtering

LiCoO₂ thin films were deposited using radio frequency (rf) magnetron sputtering system on stainless steel substrates. Different rf powers, up to 150 W, were applied during deposition. The as-deposited films exhibited (1 0 1) and (1 0 4) preferred orientation and the nanocrystalline film structure was enhanced with increasing rf power. The film crystallinity was examined using X-ray diffraction, Raman scattering spectroscopy and transmission electron microscopy. The compositions of the films were determined by inductively coupled plasma-mass spectroscopy. The average discharge capacity of as-deposited films is about 59 μAh/(cm² μm) for cut-off voltage range of 4.2 and 3.0 V. From the electrochemical cycling data, it is suggested that as-deposited LiCoO₂ films with a nanocrystalline structure and a favorable preferred orientation, e.g. (1 0 1) or (1 0 4) texture, can be used without post-annealing at high temperatures for solid-state thin film batteries.     

High temperature phase stabilities and electrochemical properties of InBaCo4−xZnxO7 cathodes for intermediate temperature solid oxide fuel cells

InBaCo_4−xZn_xO₇ oxides have been synthesized and characterized as cathode materials for intermediate temperature solid oxide fuel cells (IT-SOFC). The effect of Zn substitution for Co on the structure, phase stability, thermal expansion, and electrochemical properties of the InBaCo_4−xZn_xO₇ has been investigated. The increase in the Zn content from x = 1 to 1.5 improves the high temperature phase stability at 600 °C and 700 °C for 100 h, and chemical stability against a Gd_0.2Ce_0.8O_1.9 (GDC) electrolyte. Thermal expansion coefficient (TEC) values of the InBaCo_4−xZn_xO₇ (x = 1, 1.5, 2) specimens were determined to be 8.6 × 10⁻⁶ to 9.6 × 10⁻⁶/°C in the range of 80–900 °C, which provides good thermal expansion compatibility with the standard SOFC electrolyte materials. The InBaCo_4−xZn_xO₇ + GDC (50:50 wt.%) composite cathodes exhibit improved cathode performances compared to those obtained from the simple InBaCo_4−xZn_xO₇ cathodes due to the extended triple-phase boundary (TPB) and enhanced oxide-ion conductivity through the GDC portion in the composites.     

Scanning electrochemical microscopy studies of micropatterned copper sulfide (CuxS) thin films fabricated by a wet chemistry method

Patterned copper sulfide (Cu_xS) microstructures on Si (1 1 1) wafers were successfully fabricated by a relatively simple solution growth method using copper sulfate, ethylenediaminetetraacetate and sodium thiosulfate aqueous solutions as precursors. The Cu_xS particles were selectively deposited on a patterned self-assembled monolayer of 3-aminopropyltriethoxysilane regions created by photolithography. To obtain high quality Cu_xS films, preparative conditions such as concentration, proportion, pH and temperature of the precursor solutions were optimized. Various techniques such as optical microscopy, atomic force microscopy (AFM), X-ray diffraction, optical absorption and scanning electrochemical microscopy (SECM) were employed to examine the topography and properties of the micro-patterned Cu_xS films. Optical microscopy and AFM results indicated that the Cu_xS micro-pattern possessed high selectivity and clear edge resolution. From combined X-ray diffraction analysis and optical band gap calculations we conclude that Cu₉S₅ (digenite) was the main phase within the resultant Cu_xS film. Both SECM image and cyclic voltammograms confirmed that the Cu_xS film had good electrical conductivity. Moreover, from SECM approach curve analysis, the apparent electron-transfer rate constant (k) in the micro-pattern of Cu_xS dominated surface was estimated as 0.04 cm/s. The SECM current map showed high edge acuity of the micro-patterned Cu_xS.     

Wetting angles and photocatalytic activities of illuminated TiO2 thin films

TiO₂ thin films have been prepared by physical vapour deposition (PVD) and plasma enhanced chemical vapour deposition (PECVD) to study the UV-induced photo-activity of this material. Wetting angle variations and photo-catalytic activity for the degradation of dyes upon UV illumination have been compared for thin films with different crystalline structure (amorphous, rutile and anatase), microstructure (columnar, compact, etc.) and porosities as estimated from the values of their refraction indices and their direct assessment with a quartz crystal monitor. The surface of the thin films became superhydrophilic upon UV light irradiation and then it recovered its original state by keeping the samples in the dark. Wetting angle decays follow very similar kinetics for amorphous and crystalline films, independently of their actual porosities. By contrast the photo-catalytic activity was very dependent on the crystalline structure of the films (anatase > rutile > amorphous) and on their porosities. The different behaviour depicted by the films with regard to these two properties suggests that they respond to different though related mechanisms and that they cannot be considered as equivalent when trying to prove the photo-activity of TiO₂.     

Synthesis and electrochemical characterization of LiCoxMn1−xPO4/C nanocomposites

LiCo_xMn_1−xPO₄/C nanocomposites (0 ≤ x ≤ 1.0) were prepared by a combination of spray pyrolysis at 300 °C and wet ball-milling followed by heat treatment at 500 °C for 4 h in 3% H₂ + N₂ atmosphere. X-ray diffraction analysis indicated that all samples had the single phase olivine structures indexed by orthorhombic Pmna. The lattice parameters linearly decreased with increasing cobalt content, which confirmed the existence of solid solutions. It was clearly seen from the scanning electron microscopy observation that the LiCo_xMn_1−xPO₄/C samples were agglomerates with approximately 100 nm primary particles. The LiCo_xMn_1−xPO₄/C nanocomposites were used as cathode materials for lithium batteries, and electrochemical performance was comparatively investigated with cyclic voltammetry and galvanostatic charge–discharge test using the Li?1 M LiPF₆ in EC:DMC = 1:1?LiCo_xMn_1−xPO₄/C cells at room temperature. The cells at 0.05 C charge–discharge rate delivered first discharge capacities of 165 mAh g⁻¹ (96% of theoretical capacity) at x = 0, 136 mAh g⁻¹ at x = 0.2, 132 mAh g⁻¹ at x = 0.5, 125 mAh g⁻¹ at x = 0.8 and 132 mAh g⁻¹ (79% of theoretical capacity) at x = 1.0, respectively. While the first discharge capacity increased with the cobalt content at high charge–discharge rates more than 0.5 C due to higher electronic conductivity of LiCoPO₄ in comparison with LiMnPO₄, the cycleability of cell became worse with increasing the amount of cobalt. The existence of Mn²⁺ seemed to enhance the cycleability of LiCo_xMn_1−xPO₄/C nanocomposite cathode.     

“Deactivation of copper electrode” in electrochemical reduction of CO2

Metallic Cu electrode can electrochemically reduce CO₂ to CH₄, C₂H₄ and alcohols with high yields as revealed by the present authors. Many workers reported that formation of CH₄ and C₂H₄ rapidly diminishes during electrolysis of CO₂ reduction. This paper shows that such deactivation of Cu electrode is reproduced with electrolyte solutions prepared from reagents used by these workers. Deactivated Cu electrodes recovered the electrocatalytic activity for CO₂ reduction by anodic polarization at −0.05 V versus she in agreement with the previous reports. Features of the deactivation depend greatly on the individual chemical reagents. Purification of the electrolyte solution by preelectrolysis with a Pt black electrode effectively prevents the deactivation of Cu electrode. Anode stripping voltammetry of Cu electrodes, which were deactivated during electrolysis of CO₂ reduction, showed anodic oxidation peaks at ca. −0.1 or −0.56 V versus she. The severer the deactivation of the Cu electrode was, the higher electric charge of the anodic peak was observed. It is presumed that some impurity heavy metal, originally contained in the electrolyte, is deposited on the Cu electrode during the CO₂ reduction, poisoning the electrocatalytic activity. On the basis of the potential of the anodic peaks, Fe²⁺ and Zn²⁺ are assumed to be the major contaminants, which cause the deactivation of the Cu electrode. Deliberate addition of Fe²⁺ or Zn²⁺ to the electrolyte solutions purified by preelectrolysis exactly reproduced the deactivation of a Cu electrode in CO₂ reduction. The amount of the deposited Fe or Zn on the electrode was below the monolayer coverage. Electrothermal atomic absorption spectrometry (etaas) showed that Fe originally contained in the electrolyte solution is effectively removed by the preelectrolysis of the solution. Mechanistic difference is discussed between Fe and Zn in the deterioration of the electrocatalytic property of Cu electrode in the CO₂ reduction. The concentration of the impurity substances originally contained in the chemical reagents as Fe or Zn is estimated to be far below the standard of the impurity levels guaranteed by the manufacturers. Presence of trimethylamine in the electrolyte solution also severely poisons a Cu electrode in the CO₂ reduction. It was concluded that the deactivation of Cu electrode in CO₂ reduction is not caused by adsorption of the products or the intermediates produced in CO₂ reduction.     

Structure and magnetic properties of Fe1−xCox nanowires in self-assembled arrays

Fe_1−xCo_x nanowires in self-assembled arrays with varying compositions were produced by the template-assisted pulsed electrochemical deposition method. The structural and magnetic properties of the arrays were investigated using several experimental techniques. TEM analyses indicated that the nanowires were regular, uniform, 8 μm in length and 50 nm in diameter. The results of X-ray diffraction indicated that the body-centered-cubic (bcc) (α), face-centered-cubic (fcc) (γ), and hexagonal-close-packed (hcp) () Fe–Co phases appeared in different compositions. Magnetic measurements showed that the coercivity and squareness of the hysteresis loops of the Fe_1−xCo_x changed with their compositions, which may be attributable to shape anisotropy. The room temperature ⁵⁷Fe Mössbauer spectra of the arrays of the Fe_1−xCo_x nanowires revealed strong shape anisotropy.     

In situ Raman microspectrometry investigation of electrochemical lithium intercalation into sputtered crystalline V2O5 thin films

We report here the first in situ Raman microspectrometry study of the electrochemical lithium insertion and de-insertion reaction into crystalline sputtered Li_xV₂O₅ thin films (0 ≤ x ≤ 0.94) in liquid electrolyte. We show that the orthorhombic Pmmn symmetry of the pristine material is kept upon lithium intercalation in the Li_xV₂O₅ film (0 ≤ x ≤  0.94). In fact, a subsequent and unexpected solid solution behaviour is evidenced, leading to the typical Raman fingerprint of the -LiV₂O₅ phase for the Li₀.₉₄V₂O₅ composition. After the charge, a complete recovery of the local structure is found, in good accord with the excellent electrochemical reversibility exhibited by these thin films. Such limited structural changes differ from that usually observed for the bulk material, which emphasizes the key role of the microstructure and morphology on the nature and magnitude of the structural rearrangements induced by the lithium insertion process.