Electrochemically deposited thermoelectric n-type Bi2Te3 thin films

Electrochemically deposited n-type BiTe alloy thin films were grown from nitric acid baths on sputtered Bi_xTe_y/SiO/Si substrates. The film compositions, which varied from 57 to 63 at.% Te were strongly dependent on the deposition conditions. Surface morphologies varied from needle-like to granular structures depending on deposited Te content. Electrical and thermoelectric properties of these electrodeposited Bi_xTe_y thin films were measured before and after annealing and compared to those of bulk Bi₂Te₃. Annealing at 250 °C in reducing H₂ atmosphere enhanced thermoelectric properties by reducing film defects. In-plane electrical resistivity was highly dependent on composition and microstructure. In-plane Hall mobility decreased with increasing carrier concentration, while the magnitude of the Seebeck coefficient increased with increasing electrical conductivity to a maximum of −188.5 μV/K. Overall, the thermoelectric properties of electrodeposited n-type BiTe thin films after annealing were comparable to those of bulk BiTe films.     

Electrodeposition of P-type BixSb2−xTey thermoelectric film from dimethyl sulfoxide solution

The electrochemical behaviors of Bi(III), Te(IV), Sb(III) and their mixtures in DMSO solutions were investigated using cyclic voltammetry and linear sweep voltammetry measurements. On this basis, Bi_xSb_2−xTe_y film thermoelectric materials were prepared by potentiodynamic electrodeposition technique from mixed DMSO solution, and the compositions, structures, morphologies as well as the thermoelectric properties of the deposited films were also analyzed. The results show that Bi_xSb_2−xTe_y compound can be prepared in a very wide potential range by potentiodynamic electrodeposition technique in the mixed DMSO solutions. After anneal treatment, the deposited film prepared in the potential range of −200 to −400 mV shows the highest Seebeck coefficient (185 μV/K), the lowest resistivity (3.34 × 10⁻⁵ Ω m), the smoothest surface, the most compact structure and processes the stoichiometry (Bi_0.49Sb_1.53Te_2.98) approaching to the Bi_0.5Sb_1.5Te₃ ideal material most. This Bi_0.49Sb_1.53Te_2.98 film is a kind of nanocrystalline material and (0 1 5) crystal plane is its preferred orientation.     

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.     

Electrodeposition of bismuth telluride films from a nonaqueous solvent

The author examines Bi₂Te₃ deposition from a DMSO solution containing TeCl₄ and Bi(NO₃)₃ × 5H₂O by means of cyclic voltammetry and electrochemical quartz crystal microbalance (EQCM). Accumulated charges and related mass changes for Bi₂Te₃ deposition on working electrodes are measured in situ. The deposit composition is more dependent on Te⁴⁺ concentrations in DMSO solution than on the potential. In a DMSO solution containing 0.01 M Te⁴⁺ and 0.0075 M Bi³⁺, Bi₂Te₃ deposits were obtained in the potential range between −0.2 and −0.8 V vs. Ag/AgCl. In a DMSO solution containing 0.05 M Te⁴⁺ and 0.0375 M Bi³⁺, Te-rich deposits were formed from −0.2 to −0.8 V vs. Ag/AgCl.     

Investigations on the electrodeposition behaviors of Bi0.5Sb1.5Te3 thin film from nitric acid baths

The electrochemical reduction process of Bi³⁺, HTeO₂⁺, Sb^III and their mixtures in nitric acid medium was investigated by means of cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements. The reduction products electrodeposited at various potentials were examined using X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The results show that cathodic process in the nitric acid solution containing Bi³⁺, HTeO₂⁺ and Sb^III involves the following reduction reactions in different polarizing potential ranges: In low polarizing potential ranges, Te⁰ is formed firstly on the electrode surface through the electrochemical reduction of HTeO₂⁺; with the negative shift of the cathodic polarizing potential, the reduction reaction of Bi³⁺ with Te⁰ to form Bi₂Te₃ takes place; when the cathodic polarizing potential is negative enough, Bi³⁺ and Sb^III react with Te⁰ to form Bi_0.5Sb_1.5Te₃. The results indicate that Bi_0.5Sb_1.5Te₃ films can be fabricated by controlling the electrodepositing potential in a proper high potential ranges.     

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.     

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.     

Fabrication and electrical properties of Bi2-xSbxTe3 ternary nanopillars array films

Highly oriented Bi_2-xSb_xTe₃ (x?=?0, 0.7, 1.1, 1.5, 2) ternary nanocrystalline films were fabricated using vacuum thermal evaporation method. Microstructures and morphologies indicate that Bi_2-xSb_xTe₃ films have pure rhombohedral phase with well-ordered nanopillars array. Bi, Sb and Te atoms uniformly distributed throughtout films with no precipitation. Electrical conductivity of Bi_2-xSb_xTe₃ films transforms from n-type to p-type when x?>?1.1. Metal-insulator transition was observed due to the incorporation of Sb in Bi₂Te₃. Bi_2-xSb_xTe₃ film with x?=?1.5 exhibits optimized electrical properties with maximum electrical conductivity σ of 2.95?×?10⁵ S?m^?1 at T?=?300?K, which is approximately ten times higher than that of the undoped Bi₂Te₃ film, and three times higher than previous report for Bi_0.5Sb_1.5Te₃ films and bulk materials. The maximum power factor PF of Bi_0.5Sb_1.5Te₃ nanopillars array film is about 3.83?μW?cm^?1 K^?2 at T?=?475?K. Highly oriented (Bi,Sb)₂Te₃ nanocrystalline films with tuned electronic transport properties have potentials in thermoelectric devices.     

Electrodeposition of Sb, Bi, Te, and their alloys in AlCl3-NaCl-KCl molten salt

The Electrochemistry of Sb, Bi, and Te in AlCl₃-NaCl-KCl molten salt containing SbCl₃, BiCl₃, and/or TeCl₄ at 423 K was investigated by voltammetry, and electrodeposition of the three metals was performed under constant potential control in the melt. The voltammogram on a glassy carbon (GC) electrode in a melt containing 0.025 mol dm⁻³ [M] SbCl₃ showed a couple of redox peak corresponding to the Sb/Sb(III) redox reaction, and a stable layer of pure Sb was deposited under the constant potential control. The voltammograms in the melt containing 0.025 M BiCl₃ or 0.025 M TeCl₄ showed several redox couples. Stable deposit layers of pure Bi and Te were not obtained under the constant potential control, as the deposited layers detached from the electrode and immediately dissolved into the molten salt. Binary alloy deposition was possible in a melt containing BiCl₃ and SbCl₃, and also with BiCl₃ and TeCl₄. A stable Bi-Sb alloy deposit of metallic Sb and Bi-Sb solid solution was obtained at 0.8 and 0.9 V versus Al/Al(III) in the melt containing BiCl₃ and SbCl₃. The atomic ratio of Bi in the deposit was 37% at 0.9 V and 57% at 0.8 V. A stable Bi-Te alloy deposit was also obtained with the molten salt containing BiCl₃ and TeCl₄. The deposited Bi-Te alloy consisted of a mixture of Bi₂Te₃, BiTe, and Bi₂Te. The alloy deposit had good crystallinity and the preferential orientation was the (1 1 0) plane.     

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.     

The underpotential deposition of Bi2Te3−ySey thin films by an electrochemical co-deposition method

Bi₂Te_3−ySe_y thin films were grown on Au(1 1 1) substrates using an electrochemical co-deposition method at 25 °C. The appropriate co-deposition potentials based on the underpotential deposition (upd) potentials of Bi, Te and Se have been determined by the cyclic voltammetric studies. The films were grown from an electrolyte of 2.5 mM Bi(NO₃)₃, 2 mM TeO₂, and 0.3 mM SeO₂ in 0.1 M HNO₃ at a potential of −0.02 V vs. Ag|AgCl (3 M NaCl). X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive spectroscopy (EDS) were employed to characterize the thin films. XRD and EDS results revealed that the films are single phase with approximate composition of Bi₂Te_2.7Se_0.3. SEM studies showed that the films are homogeneous and have micronsized granular crystallites.     

Enhanced thermoelectric properties induced by chemical pressure in Ca3Co4O9

We report the investigation of boron substitution on structural, electrical, thermal, and thermoelectric properties of Ca_3−xB_xCo₄O₉ (x=0, 0.5, 0.75, and 1) in the temperature range between 300 K and 5 K. X-ray diffraction studies show that the Ca₃Co₄O₉ phase is successfully preserved as the majority phase in the x=0.5 sample despite the small size of boron ions. Electrical transport measurements confirm that B³⁺ substitution for Ca²⁺ causes an increase in resistivity due to the decrease in carrier concentration. x=0.5 sample is found to have a Seebeck coefficient of 181 μV/K at room temperature which is ~1.5 times higher than that of the pure Ca₃Co₄O₉. Our results indicate that the chemical pressure due to the large ionic radii difference between B³⁺ (0.27 Å) and Ca²⁺ (1 Å) enhances the thermoelectric properties as long as the unique crystal structure of Ca₃Co₄O₉ is preserved.     

High thermoelectric performance achieved in Bi0.4Sb1.6Te3 films with high (00l) orientation via magnetron sputtering

To obtain p-type Bi–Sb–Te-based thin films with excellent thermoelectric performance, the Bi_0.4Sb_1.6Te₃ target is prepared by combining mechanical alloying with the spark plasma sintering technique. Afterward, Bi_0.4Sb_1.6Te₃ thin films are deposited via magnetron sputtering at variable working pressures. With an increasing working pressure, the frequency of collisions between the argon ions and sputtered atoms gradually increases, the preferred orientation of (00l) increases, and the sputtering rate decreases. The Seebeck coefficient increases from ∼140 μV/K to ∼220 μV/K as the carrier concentration decreases along with an increasing working pressure. Furthermore, the decrease in carrier concentration and acceleration of carrier mobility also affect the change in electrical conductivity. The maximum power factor of the p-type Bi_0.4Sb_1.6Te₃ thin film deposited at 4.0 Pa and at room temperature exceeds 20.0 μW/cm K² and is higher than that of most p-type Bi–Sb–Te-based films.     

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.     

Synthesis and characterization of highly stable optically passive CeO2-ZrO2 counter electrode

Transparent and adherent CeO₂-ZrO₂ thin films having film thicknesses ∼543-598 nm were spray deposited onto the conducting (fluorine doped tin oxide coated glass) substrates from a blend of equimolar concentrations of cerium nitrate hexahydrate and zirconium nitrate having different volumetric proportions (0-6 vol.% of Zr) in methanol. CeO₂-ZrO₂ films were polycrystalline with cubic fluorite crystal structure and the crystallinity was improved with increasing ZrO₂ content. Films were highly transparent (T ∼ 92%), showing decrease in band gap energy from 3.45 eV for pristine CeO₂ to 3.08-3.14 eV for CeO₂-ZrO₂ films. The different morphological features of the film obtained at various CeO₂-ZrO₂ compositions had pronounced effect on the ion storage capacity and electrochemical stability. CeO₂-ZrO₂ film prepared at 5 vol.% Zr concentration exhibited higher ion storage capacity of 24 mC cm⁻² and electrochemical stability of 10,000 cycles in 0.5 M LiClO₄ + PC electrolyte due to its film thickness (584 nm) coupled with relatively larger porosity (8%). The optically passive behavior of such CeO₂-ZrO₂ film (with 5 vol.% Zr) is affirmed by its negligible transmission modulation irrespective of repeated Li⁺ and electron insertion/extraction. The coloration efficiency of spray deposited WO₃ thin film is found to enhance from 47 to 107 cm² C⁻¹ when CeO₂-ZrO₂ is coupled as a counter electrode with WO₃ in an electrochromic device (ECD). These films can be used as stable ‘passive’ counter electrodes in electrochromic smart windows as they retain full transparency in both the oxidized and reduced states and ever-reported longevity.     

Li diffusion in LiNi0.5Mn0.5O2 thin film electrodes prepared by pulsed laser deposition

Kinetic and transport parameters of Li ion during its extraction/insertion into thin film LiNi_0.5Mn_0.5O₂ free of binder and conductive additive were provided in this work. LiNi_0.5Mn_0.5O₂ thin film electrodes were grown on Au substrates by pulsed laser deposition (PLD) and post-annealed. The annealed films exhibit a pure layered phase with a high degree of crystallinity. Surface morphology and thin film thickness were investigated by field emission scanning electron microscopy (FESEM). The charge/discharge behavior and rate capability of the thin film electrodes were investigated on Li/LiNi_0.5Mn_0.5O₂ cells at different current densities. The kinetics of Li diffusion in these thin film electrodes were investigated by cyclic voltammetry (CV) and galvanostatic intermittent titration technique (GITT). CV was measured between 2.5 and 4.5 V at different scan rates from 0.1 to 2 mV/s. The apparent chemical diffusion coefficients of Li in the thin film electrode were calculated to be 3.13 × 10⁻¹³ cm²/s for Li intercalation and 7.44 × 10⁻¹⁴ cm²/s for Li deintercalation. The chemical diffusion coefficients of Li in the thin film electrode were determined to be in the range of 10⁻¹²-10⁻¹⁶ cm²/s at different cell potentials by GITT. It is found that the Li diffusivity is highly dependent on the cell potential.     

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.     

Synthesis of Bi2Te3 nanopowders by vacuum arc plasma evaporation

Starting from elemental bismuth and tellurium, bismuth telluride (Bi₂Te₃) nanopowders were successfully prepared by vacuum arc plasma evaporation (VAPE) technique for the first time. The phase composition in the obtained nanopowders is closely related with the Bi:Te atomic ratio in starting precursor. The microstructure and morphology of the samples were characterized via X-ray diffraction (XRD), transmission electron microscopy (TEM) and selected area electron diffraction (SAED). Compositional analysis was also carried out by energy dispersive analysis of X-rays (EDAX). The as-synthesized Bi₂Te₃ nanopowders have a rhombohedral crystal structure with lattice parameters a = 4.381 Å and c = 30.310 Å. The average particle size is about 35 nm obtained from TEM and confirmed from XRD results.     

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.     

Cyclic durable high-capacity Sn/Cu6Sn5 composite thin film anodes for lithium ion batteries prepared by electron-beam evaporation deposition

Sn/Cu₆Sn₅ alloy composite thin films were directly prepared by electron-beam deposition for anodes of lithium ion batteries. The thin film was comprised of micro/sub-microcrystalline Sn and Cu₆Sn₅, where the polyhedral micro-sized Sn grains were uniformly dispersed in the loose Cu₆Sn₅ matrix. Lithiation reaction kinetics were confirmed to be controlled by a diffusion step and the diffusion coefficient of Li⁺ in the thin film anode was determined to be 1.91 × 10⁻⁷ cm²/s. The galvanostatic cycling behavior of Sn/Cu₆Sn₅ composite thin film anodes was studied under different conditions. Stable capacities of more than 370 mAh/g were obtained by discharging from 1.25 to 0.1 V. Structure changes and fading mechanism of the thin film electrodes was discussed based on SEM, XRD and EDX investigations. The present results demonstrated that the multi-phase composite structure can improve electrochemical performance of the Cu-Sn alloy thin film electrodes.