Preparation and characterization of Bi2Se3 nanowires by electrodeposition

The electrodeposition of Bi₂Se₃ nanowires on an anodic aluminum oxide template was investigated by cyclic voltammetry in a tartaric acid aqueous solution. The electrochemical behavior of the Bi₂Se₃ nanowires in the electrolytic solution was also investigated using cyclic voltammetry, and the underpotential deposition mechanism of the Bi₂Se₃ nanowires was determined. According to the cyclic voltammetric curves, −0.20 V vs. SCE (saturated calomel electrode) was chosen as the deposition potential of the Bi₂Se₃ nanowires. The ratio of Bi to Se is nearly 2:3, verified by energy-dispersive X-ray spectroscopy and with the addition of surfactant. X-ray diffraction, scanning electron microscopy, selected-area electron diffraction and high-resolution transmission electron microscopy indicate that annealing can improve the crystallinity and chemical composition of Bi₂Se₃ nanowires. Surfactant can also improve the surface morphology and composition of the Bi₂Se₃ nanowires.     

Orientation-controlled synthesis and characterization of Bi2Te3 nanofilms, and nanowires via electrochemical co-deposition

An electrochemical deposition technique based on co-deposition was used to deposit preferentially oriented Bi₂Te₃ nanostructures (nanofilm, and nanowire). The shared underpotential deposition (UPD) potentials for both Bi and Te co-deposition were determined by cyclic voltammetric measurements. The scanning probe microscopy (scanning tunneling microscopy (STM) and atomic force microscopy (AFM)) and the X-ray diffraction (XRD) data indicated that the electrodeposition of Bi₂Te₃ results in nanofilm-structured deposits with a preferential orientation at (0 1 5) and nanowired-structured deposits with a preferential orientation at (1 1 0) in acidic and basic (in the presence of ethylenediaminetetraacetic acid (EDTA)) medium, respectively. The results show that the nucleation and growth mechanism follows 3D mode in acidic solutions and 2D mode in basic solution containing EDTA additive. The optical characterization performed by reflection absorption Fourier transform infrared (RA-FTIR) spectroscopy showed that the band gap energy of Bi₂Te₃ nanostructures depends on the thickness, size, and shape of the nanostructures and the band gap increases as the deposition time decreases. Moreover, the quantum confinement is strengthened in the wire-like deposits relative to the film-like deposits. Energy dispersive X-ray spectroscopy (EDS) analysis demonstrated that Bi₂Te₃ nanostructures were always in 2:3 stoichiometry, and they were made up of only pure Bi and Te.     

Electrochemical aspects of depositing Sb2Te3 compound on Au substrate by ECALE

Deposition of Sb₂Te₃ thin films on polycrystalline Au substrates by electrochemical atomic layer epitaxy (ECALE) is described in this paper. Electrochemical aspects were characterized by means of cyclic voltammetry, anodic potentiodynamic scanning and coulometry. A steady ECALE deposition for Sb₂Te₃ compound could be attained after negatively adjusting the underpotential deposition (UPD) potentials of Sb and Te on Au in steps over the initial 40 cycles, and the potentials could be kept constant for the following deposition. A 400 cycle deposit, which was grown with the steady deposition potentials, was proved to be a single phase Sb₂Te₃ compound by X-ray diffraction analysis and SEM observation shows the deposit consisted of nanoscale particles with average size about 100 nm. The 2:3 stoichiometric ratio of the deposit was further verified by energy dispersive X-ray (EDX) quantitative analysis.     

Composition-dependent characterization and optimal control of electrodeposited Bi2Te3 films for thermoelectric application

A method to control composition of Bi₂Te₃ films by mass transfer manipulation has been developed. The film composition can be varied by a diffusion-controlled method, which is related to the change of Bi³⁺/HTeO₂⁺ ratios in a controlled diffusion layer. A homogeneous and dense film with precise chemical composition could thus be obtained under constant electrode polarization. Meanwhile, the solo dependence of film properties on composition change of both Te-rich and Bi-rich films were investigated. Firstly, the studies of XRD and FE-SEM showed that different Te contents in deposit would lead to different dimensions of unit cell and grain sizes. The Seebeck coefficient increased apparently when the Te content was over 60 at.% Te. Te-rich films had higher carrier concentration but slower mobility than Bi-rich films. Inverse relations were observed between carrier concentration and carrier mobility and between Seebeck coefficient and conductivity. Therefore, an optimal power factor of 7 × 10⁻⁴ W/m K² was realized near the stoichiometric Bi₂Te₃.     

Effect of potential on bismuth telluride thin film growth by electrochemical atomic layer epitaxy

In the present study, bismuth telluride compound thin film was grown by means of electrochemical atomic layer epitaxy (ECALE) with an automated thin layer flow cell deposition system. The dependence of the Bi and Te deposition potentials on Pt electrode was studied. Because developing a contact potential between the substrate and the growing semiconductor, the deposition potential adjustment is necessary for the first 30 or more cycles of each component. The dependence of the deposit as a function of the deposition potential adjustment slope has been investigated. The results show that an excess elemental Bi existed at a slope of −2 mV/p (p indicates per cycle), indicating that this is a lack of deposition at the potential. Single-phase Bi₂Te₃ compound could be obtained between −4 and −6 mV/p. Bi₂Te₃ and Bi₄Te₃ coexistence is observed at a slope of −10 mV/p. The EDS data indicates that the stoichiometry of compound is consistent with XRD result. SEM studies show that the deposits are inhomogeneous and have an micron sized particles morphology.     

Development of growth cycle for antimony telluride film on Au (1 1 1) disk by electrochemical atomic layer epitaxy

Electrochemical deposition of Sb₂Te₃ thin film on Au (1 1 1) disk via the route of electrochemical atomic layer epitaxy (ECALE) is described in this paper. Electrochemical aspects of Te and Sb on Au, Te on Sb-covered Au, and Sb on Te-covered Au were studied by means of cyclic voltammetry and coulometry. The apparent variation of coverage for Te or Sb on hetero-covered substrate is explained by considering the thermodynamic process of compound formation. A steady ECALE deposition for Sb₂Te₃ compound could be attained after negatively adjusting the underpotential deposition (UPD) potentials of Sb and Te on Au in steps over the initial 40 cycles, and the potentials could be kept constant for the following deposition. A 200-cycle deposit, which was grown with the steady deposition potentials, was proved to be a single phase Sb₂Te₃ compound by X-ray diffraction analysis. The 2:3 stoichiometric ratio of the deposit was further verified by energy dispersive X-ray (EDX) quantitative analysis. The p-type semiconductive property was demonstrated by measurements of the Seebeck coefficient and the electrical resistivity with a value of 145 μV/K and 9.37 μΩm, respectively. The morphologies of deposits with various growth cycle numbers were observed with FE-SEM. The evolvement mechanism of the morphology was investigated. The results show that the morphology of deposit has changed after initial potential adjustment and numberless thin sheets appeared and grew uprightly during the continuous cycle process. Fourier transform infrared spectroscopy (FTIR) absorption measurements suggested a band gap of 0.26 eV in very good agreement with literature reports for Sb₂Te₃ single crystals.     

One step synthesis of lamellar R-3m LiCoO2 thin films by an electrochemical–hydrothermal method

In this study, we report the synthesis of lamellar R-3m LiCoO₂ thin films electrodes for lithium rechargeable batteries by a single step method based on an electrochemical–hydrothermal synthesis in a concentrated LiOH solution with a cobalt salt. This process combines the effect of temperature (between 150 °C and 200 °C), pressure and galvanostatic current. The obtained films were not annealed after the electrochemical–hydrothermal synthesis.For the first time, the theoretical study of the potential–pH diagram of cobalt was carried out at high temperature and high concentration. These calculations show that a pH value higher than 12 is necessary to avoid the direct precipitation of cobalt hydroxide Co(OH)₂ inside the solution. An improvement of the soluble species stability with an increase of the temperature and a decrease of the cobalt concentration is predicted. The influence of the deposition conditions (temperature and concentration) at a constant current density was experimentally studied. X-ray diffraction (XRD) shows the formation of well-crystallized LiCoO₂ thin films. Raman spectroscopy confirmed the achievement of the electrochemically active R-3m LiCoO₂ phase without any trace of the Fd3m phase at temperatures as low as 150 °C. Electrochemical measurements demonstrate good performances of the material synthesized between 150 °C and 200 °C with better capacity retention at higher temperature.     

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.     

The effect of Bi2O3 compensation during thermal treatment on the crystalline and electrical characteristics of bismuth titanate thin films

Bismuth titanate thin films are deposited on ITO/glass substrates by rf magnetron sputtering at room temperature using a Bi₄Ti₃O₁₂ ceramic target. The deposited Bi₄Ti₃O₁₂ films are annealed in a conventional furnace in ambient air for 10 min at temperatures ranging from 550 to 640 °C. One specimen is annealed in a crucible containing additional Bi₂O₃ compensation powder, while the other specimen is annealed in ambient air. XRD analysis shows that the crystal phases of films annealed with Bi₂O₃ powder are better than those of films annealed without Bi₂O₃ powder. Furthermore, the EDS results reveal that the bismuth weight percentage of the former is higher than that of the latter. SIMS analysis shows that the bismuth decreases near the surface of Bi₄Ti₃O₁₂ film annealed without Bi₂O₃ powder, but reveals a stable distribution throughout the film annealed with Bi₂O₃ powder. These results imply that bismuth is readily evaporated during the thermal treatment process, particularly from the region near the film surface. Finally, the dielectric and polarization properties of the thin films annealed with Bi₂O₃ powder are found to be superior to those of the films annealed in ambient air.     

Quantum confinement in PbSe thin films electrodeposited by electrochemical atomic layer epitaxy (EC-ALE)

PbSe is a IV-VI compound semiconductor with a narrow band gap (0.26 eV), used in photodetectors, photoresistors and photoemitters in the infrared (IR). This paper reports the first instance of PbSe formation by electrochemical atomic layer epitaxy (EC-ALE). The film’s composition and structure were characterized using electron probe microanalysis (EPMA) and X-ray diffraction (XRD), respectively. The optical properties were studied via infrared absorption measurements. Films were stoichiometric via EPMA, a Pb/Se ratio of one. XRD indicated the expected rock salt structure for PbSe, and a preferred (2 0 0) orientation. IR adsorption studies, of films grown with 10-50 cycles, showed strong blue shifts of the fundamental absorption edge, which was believed to result from quantum confinement in the thin films.     

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.     

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.     

Electrodeposition and electrochemical characterisation of thick and thin coatings of Sb and Sb/Sb2O3 particles for Li-ion battery anodes

The possibilities to electrodeposit thick coatings composed of nanoparticles of Sb and Sb₂O₃ for use as high-capacity anode materials in Li-ion batteries have been investigated. It is demonstrated that the stability of the coatings depends on their Sb₂O₃ concentrations as well as microstructure. The electrodeposition reactions in electrolytes with different pH and buffer capacities were studied using chronopotentiometry and electrochemical quartz crystal microbalance measurements. The obtained deposits, which were characterised with XRD and SEM, were also tested as anode materials in Li-ion batteries. The influence of the pH and buffer capacity of the deposition solution on the composition and particle size of the deposits were studied and it is concluded that depositions from a poorly buffered solution of antimony-tartrate give rise to good anode materials due to the inclusion of precipitated Sb₂O₃ nanoparticles in the Sb coatings. Depositions under conditions yielding pure Sb coatings give rise to deposits composed of large crystalline particles with poor anode stabilities. The presence of a plateau at about 0.8 V versus Li⁺/Li due to SEI forming reactions and the origin of another plateau at about 0.4 V versus Li⁺/Li seen during the lithiation of thin Sb coatings are also discussed. It is demonstrated that the 0.4 V plateau is present for Sb coatings for which the (0 1 2) peak is the main peak in the XRD diffractogram.     

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.     

Electrodeposition and characterization of Fe80Ga20 alloy films

Due to its high magnetostriction and good mechanical properties Fe₈₀Ga₂₀ is interesting for magnetostrictive microactuators and sensors. Here we use electrodeposition to grow Fe–Ga films onto Au and Pt coated Si substrates by potentiostatic and pulse potential deposition. Composition, microstructure and structure are analysed. The desired composition of Fe₈₀Ga₂₀ was obtained at −1.4 V_SCE and −1.5 V_SCE, respectively. The origin of low reproducibility and high oxygen content up to 50 at.% is investigated. Optimum deposition conditions to achieve dense, homogeneous films with low oxygen content are identified. In these films the saturation magnetization reaches a maximum value of 1.7 T confirming the high quality of electrodeposited films.     

Electrodeposition of indium onto Mo/Cu for the deposition of Cu(In,Ga)Se2 thin films

A study of the electrodeposition and the oxidation process of indium on Mo/Cu substrates from a bath containing 0.008 M InCl₃, 0.7 M LiCl at pH 3 is described in this work. The voltamperometric study showed a reduction process which corresponds to the conversion of In³⁺ to In⁰ and an oxidation process which takes place in different steps. Utilizing the chronoamperometric technique the total efficiency of process, the number of monolayers, the film thickness and the diffusion coefficient were evaluated. The analysis of current transients, using theoretical growth model, showed that the electrodeposition of indium adjusts to a three-dimensional growth under instantaneous nucleation limited by diffusion. The kinetic growth parameters were evaluated through a non-linear fit. The films were characterized by X-ray diffraction and scanning electron microscopy techniques. These studies showed that the films were of crystalline in nature with compact and uniform surface, even for the film with a deposition time of 1 min.     

One-step electrochemical preparation of the ternary (BixSb1−x)2Te3 thin films on Au(1 1 1): Composition-dependent growth and characterization studies

This study reports on the synthesis of ternary semiconductor (Bi_xSb_1−x)₂Te₃ thin films on Au(1 1 1) using a practical electrochemical method, based on the simultaneous underpotential deposition (UPD) of Bi, Sb and Te from the same solution containing Bi³⁺, SbO⁺, and HTeO₂⁺ at a constant potential. The thin films are characterized by X-ray diffraction (XRD), atomic force microscopy (AFM), energy dispersive spectroscopy (EDS) and reflection absorption-FTIR (RA-FTIR) to determine structural, morphological, compositional and optic properties. The ternary thin films of (Bi_xSb_1−x)₂Te₃ with various compositions (0.0 ≤ x ≤ 1.0) are highly crystalline and have a kinetically preferred orientation at (0 1 5) for hexagonal crystal structure. AFM images show uniform morphology with hexagonal-shaped crystals deposited over the entire gold substrate. The structure and composition analyses reveal that the thin films are pure phase with corresponding atomic ratios. The optical studies show that the band gap of (Bi_xSb_1−x)₂Te₃ thin films could be tuned from 0.17 eV to 0.29 eV as a function of composition.     

Long-term degradation of Ta2O5-doped Bi2O3 systems

Bismuth oxide in δ-phase is a well-known high oxygen ion conductor and can be used as an electrolyte for intermediate temperature solid oxide fuel cells (IT-SOFCs). 5-10 mol% Ta₂O₅ are doped into Bi₂O₃ to stabilize δ-phase by solid state reaction process. One Bi₂O₃ sample (7.5TSB) was stabilized by 7.5 mol% Ta₂O₅ and exhibited single phase δ-Bi₂O₃-like (type I) phase. Thermo-mechanical analyzer (TMA), X-ray diffractometry (XRD), AC impedance and high-resolution transmission electron microscopy (HRTEM) were used to characterize the properties. The results showed that holding at 800-850 °C for 1 h was the appropriate sintering conditions to get dense samples. Obvious conductivity degradation phenomenon was obtained by 1000 h long-term treatment at 650 °C due to the formation of α-Bi₂O₃ phase and Bi₃TaO₇, and 〈1 1 1〉 vacancy ordering in Bi₃TaO₇ structure.     

Low-temperature atomic layer deposited Al2O3 thin film on layer structure cathode for enhanced cycleability in lithium-ion batteries

The deposition of Al₂O₃ on LiCoO₂ electrodes using a low-temperature atomic layer deposition has been investigated. Scanning electron microscopy confirms that Al₂O₃ films can be homogeneously deposited on LiCoO₂ particles of porous electrodes at 120 °C. The results of X-ray photoelectron spectroscopy show that the Al₂O₃ preferentially deposits on the LiCoO₂. Furthermore, the results of cycling stability tests show that the cells with Al₂O₃-coated LiCoO₂ electrodes have enhanced performance.     

Structure and electrochemical properties of nanocarbon-coated Li3V2(PO4)3 prepared by sol-gel method

A liquid-based sol-gel method was developed to synthesize nanocarbon-coated Li₃V₂(PO₄)₃. The products were characterized by XRD, SEM and electrochemical measurements. The results of Rietveld refinement analysis indicate that single-phase Li₃V₂(PO₄)₃ with monoclinic structure can be obtained in our experimental process. The discharge capacity of carbon-coated Li₃V₂(PO₄)₃ was 152.6 mAh/g at the 50th cycle under 1C rate, with 95.4% retention rate of initial capacity. A high discharge capacity of 184.1 mAh/g can be obtained under 0.12C rate, and a capacity of 140.0 mAh/g can still be held at 3C rate. The cyclic voltammetric measurements indicate that the electrode reaction reversibility is enhanced due to the carbon-coating. SEM images show that the reduced particle size and well-dispersed carbon-coating can be responsible for the good electrochemical performance obtained in our experiments.