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.     

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.     

Galvanic displacement of BixTey thin films from sacrificial iron group thin films

Bi_xTe_y thin films synthesized by galvanic displacement were systematically investigated by observing open circuit potential (OCP), surface morphology, microstructure and film composition. Surface morphologies and crystal structures of synthesized Bi_xTe_y thin films were strongly depended on the type of the sacrificial materials (i.e., nickel (Ni), cobalt (Co) and iron (Fe)). Galvanically deposited Bi_xTe_y thin films from the sacrificial Ni and Co thin films exhibited Bi₂Te₃ intermetallic compounds and hierarchical structures with backbones and sub-branches. A linear relationship of deposited Bi content in Bi_xTe_y thin films as a function of [Bi³⁺]/[HTeO₂⁺] ratio (within a range of less than 0.8) in the electrolyte was also observed. Surface morphologies of Bi_xTe_y thin films were altered with the film composition.     

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.     

Novel mixed method for the electrochemical deposition of thick layers of Bi2+xTe3−x with controlled stoichiometry

A new method for the electrochemical deposition of Bi_2+xTe_3−x is presented, which combines voltage-controlled deposition pulses with current-controlled resting pulses. This method is based on results of a comprehensive electrochemical investigation including cyclic voltammetry, chronoamperometry and chronopotentiometry, which has been performed on the system Bi and Te on Pt in 2 M HNO₃. The influence of electrolyte composition, deposition potential and deposition pulse duration on morphology and stoichiometry of the deposited material as well as the variation of the composition over the thickness of the layer has been investigated by means of SEM and EDX. The crystal structure was examined with XRD. Layers deposited with the new method show a constant and reproducible stoichiometry over their entire thickness. Layers of up to 800 μm thickness deposited with deposition rates of up to 50 μm/h have been achieved. The composition and hence the thermoelectric behavior may be adjusted via electrolyte composition or the deposition potential. Fabrication of n-type and, for the first time, p-type Bi_2+xTe_3−x is demonstrated and verified by measurements of the Seebeck coefficients. The suitability of the proposed method for low-cost fabrication of micro-thermoelectric devices is shown. The advantages of this method may also apply for electrochemical deposition of other binary or ternary compounds.     

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.     

Sodium Doping Effects on Layered Cobaltate Bi2Sr2Co2Oy Thin Films

Highly c‐axis oriented Bi_2?xNa_xSr₂Co₂O_y (x = 0, 0.1, 0.2, 0.3) thin films were successfully deposited onto LaAlO₃ (0 0 1) single crystal substrates by the chemical solution deposition method. The Na‐doping effects on microstructures as well as transport and thermoelectric properties were investigated. The crystallite size normal to the thin film surface was decreased with Na doping, and the carrier concentration and mobility was enhanced and decreased, respectively. As for the transport properties, it is suggested that Na‐doping‐induced disorders play a more important role at low temperatures, while at the medium temperature the doping play a trivial role, at higher temperatures Na‐doping weakens the electron correlation. Combined with the transport properties and Seebeck coefficient results, it is suggested that the thermoelectric properties are mainly controlled by carrier concentration due to the hole doping in blocking layers. Power factor at 300 K was enhanced about 22% for the Bi_1.7Na_0.3Sr₂Co₂O_y thin film as compared with that of the undoped thin films, suggesting that Na doping at blocking layer in Bi₂Sr₂Co₂O_y was an effective route to enhance the thermoelectric power factor.     

Recent progress in electrodeposition of thermoelectric thin films and nanostructures

Thermoelectric power generators and coolers have many advantages over conventional refrigerators and power generators such as solid-state operation, compact design, vast scalability, zero-emissions and long operating lifetime with no maintenance. However, the applications of thermoelectric devices are limited to where their unique advantages outweigh their low efficiency. Despite this practical confine, there has been a reinvigorated interest in the field of thermoelectrics through identification of classical and quantum mechanical size effects, which provide additional ways to enhance energy conversion efficiencies in nanostructured materials. Although, there are a few reports which demonstrated the improvement of efficiency through nanoengineering, the successful application of these nanostructures will be determined by a cost-effective and high through-put fabrication method. Electrodeposition is the method of choice to synthesize nanoengineered thermoelectric materials because of low operating and capital cost, high deposition rates, near room temperature operation, and the ability to tailor the properties of materials by adjusting deposition conditions. In this paper, we reviewed the recent progress of the electrodeposition of thermoelectric thin films and nanostructures including Bi, Bi_1−xSb_x, Bi₂Te₃, Sb₂Te₃, (Bi_1−xSb_x)₂Te₃, Bi₂Se₃, Bi₂Te_3−ySe_y, PbTe, PbSe, PbSe_1−xTe_x and CoSb₃.     

Enhancement of thermoelectric performance of N-type Bi2Te3 based thin films via in situ annealing during magnetron sputtering

In this work, n-type Bi₂Te₃ based thin films were prepared in 300 °C via DC magnetron sputtering, and influences of sputtering power and annealing time on thermoelectric properties of films were investigated. The raise of sputtering power brings about the improvement of deposited rate and enhancement of grain size. Taking the consideration that the large-sized grains are to phonon scattering, we determine the medial power of 30 W as the basic technical parameters for the purpose of further optimizing performance through an in situ annealing process. Subsequently, thin-film treated by in situ annealing process acts out an obvious reduction in electrical conductivity attributed to the decrease in carrier concentration. Especially, the film annealed for 40 min shows an enhancement in the Seebeck coefficient and leads to a maximum power factor 0.82 m W m⁻¹ K⁻² at 543 K.     

Effect of real working environment/formation of oxide phase on thermoelectric properties of flexible Sb2Te3 films

Flexible Sb₂Te₃ thin films, for thermoelectric generator applications, were deposited by DC magnetron sputtering. As-deposited films were annealed in air to simulated a realistic operating environment. The oxidation behavior of the films was studied by monitoring their phase change on exposure to air at different temperatures between 50 and 300 °C for annealing times from 1 to 15 h. Oxidation of Sb and Te formed Sb₂Te₄ and TeO₂ phases when annealing above 100 °C and Sb₂Te₃ decomposed into oxide phases at an annealing temperature of 250 °C for 15 h. The thermoelectric performance decreased as the content of Sb₂O₄ and TeO₂ phases increased. These findings show the limitations of Sb₂Te₃ films operating in air without vacuum or a protective environment. We propose that the kinetic growth of oxide formation on the Sb₂Te₃ thin films depend on chemical activation energy and oxygen diffusion through the oxide barrier by the variation of annealing temperature and annealing time, respectively.     

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.     

Temperature dependence of piezoelectric properties on Nd and V co-substituted Bi4Ti3O12 ceramics for ceramic resonator applications

The temperature dependences of the piezoelectric properties of (Bi_4−yNd_y)_1−(x/12)(Ti_3−xV_x)O₁₂ [BNTV-x, y (x = 0.01, y = 0.00–1.00)] were investigated for environmentally friendly lead-free piezoelectric ceramic resonators with low-temperature coefficients of resonance frequency, TC-f. The |TC-f| in the (33) mode improved with increasing concentration of modified Nd ions, y, and exhibited the smallest |TC-f| value of 77.4 ppm/°C at y = 0.75 (BNTV-0.75). The |TC-f| in the other vibration mode (t), was also investigated for the BNTV-0.75 ceramic, and a smaller value of 42 ppm/°C was obtained. The (t) mode of the BNTV-0.75 ceramic showed excellent piezoelectric properties: Q_m = 4200, Q_e max = 31 and TC-f = −49.8 ppm/°C. These properties are very similar to those of commercialized hard PZT ceramics for resonator applications. The BNTV-0.75 ceramic seems to be a superior candidate material for lead-free piezoelectric applications of ceramic resonators.     

Cd1−xZnxTe thin films formed by non-aqueous electrochemical route

Polycrystalline thin films of graded band gap ternary cadmium zinc telluride (Cd_1−xZn_xTe) have been electrodeposited in a non-aqueous bath onto an indium tin oxide (ITO) coated glass cathode. Ethylene glycol was used as the non-aqueous medium. The cathodic electrodeposition of the ternary semiconductor Cd_1−xZn_xTe was studied using cyclic voltammetry in conjunction with photovoltammetry, optical, compositional, structural measurements and surface morphology. It is shown that the band gap of this alloy can be tuned from 1.42 to 2.21 eV by controlling the Cd:Zn mole fractions. X-ray diffraction (XRD) and energy dispersive X-ray analysis (EDAX) measurements showed formation of Cd_1−xZn_xTe, where x varied between 0 and 1. It was found that the observed XRD reflections of all the samples index to the cubic phase of the Cd_1−xZn_xTe. The direction of the thermoemf developed in Cd_1−xZn_xTe films has also been shown to be opposite to the thermoemf for binary CdTe films. The resistivity rises with increase in the band gap indicating formation of a continuous solid solution of CdTe and ZnTe in the ternary phase Cd_1−xZn_xTe. The electrodeposited films have also been shown to possess polycrystalline pyramidal grains with compact and void free morphology.     

Single step electrosynthesis of Cu2ZnSnS4 (CZTS) thin films for solar cell application

The Cu₂ZnSnS₄ (CZTS) thin films have been electrodeposited onto the Mo coated and ITO glass substrates, in potentiostatic mode at room temperature. The deposition mechanism of the CZTS thin film has been studied using electrochemical techniques like cyclic voltammetery. For the synthesis of these CZTS films, tri-sodium citrate and tartaric acid were used as complexing agents in precursor solution. The structural, morphological, compositional, and optical properties of the CZTS thin films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX and optical absorption techniques respectively. These properties are found to be strongly dependent on the post-annealing treatment. The polycrystalline CZTS thin films with kieserite crystal structure have been obtained after annealing as-deposited thin films at 550 in Ar atmosphere for 1 h. The electrosynthesized CZTS film exhibits a quite smooth, uniform and dense topography. EDAX study reveals that the deposited thin films are nearly stoichiometric. The direct band gap energy for the CZTS thin films is found to be about 1.50 eV. The photoelectrochemical (PEC) characterization showed that the annealed CZTS thin films are photoactive.     

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.     

Lanthanum-doped Bi4Ti3O12 prepared by the soft chemical method: Rietveld analysis and piezoelectric properties

Bi_4−xLa_xTi₃O₁₂ (BLT) thin films and powders with x ranging from 0 to 0.75 were prepared by the polymeric precursor solution. The effect of lanthanum on the structure of BIT powders was investigated by Rietveld Method. The increase of lanthanum content does not lead to any secondary phases. Orthorhombicity of the bismuth titanate (BIT) crystal lattice decreased with the increase of lanthanum content due the reduction of a/b ratio. The BLT films show piezoelectric coefficients of 45, 19, 16 and 10 pm/V for x = 0, 0.25, 0.50 and 0.75, respectively. The piezoelectric response is strongly reduced by the amount of lanthanum added to the system.     

Amorphous LiCoO2 thin films on Li1+x+yAlxTi2−xSiyP3−yO12 prepared by radio frequency magnetron sputtering for all-solid-state Li-ion batteries

Amorphous LiCoO₂ thin films were deposited on the NASICON-type glass ceramics, Li_1+x+yAl_xTi_2−xSi_yP_3−yO₁₂ (LATSP), by radio frequency (RF) magnetron sputtering below 180 °C. The as-deposited LiCoO₂ thin films were characterized by X-ray diffraction, scanning electron microscopy and atomic force microscope. All-solid-state Li/PEO₁₈-Li (CF₃SO₂)₂N/LATSP/LiCoO₂/Au cells were fabricated using the amorphous film. The electrochemical performance of the cells was investigated by galvanostatic cycling, cyclic voltammetry, potentiostatic intermittent titration technique and electrochemical impedance spectroscopy. It was found that the amorphous LiCoO₂ thin film shows a promising electrochemical performance, making it a potential application in microbatteries for microelectronic devices.     

The electrochemistry of Pd-coated MgySc(1 − y) thin film electrodes: A thermodynamic and kinetic study

The thermodynamic and kinetic properties of Pd-coated Mg_ySc_(1 − y) thin film electrodes are investigated. These thin film electrodes can be described as a two-layer structure, in which the Pd and Mg_ySc_(1 − y) layer contribute to the overall electrochemical response. In order to identify the response of the Pd layer in the two-layer system, thin films consisting of solely Pd, with identical thickness and orientation, were measured. Based on the fact that the chemical potentials of the individual layers of the Pd-coated Mg_ySc_(1 − y) thin films are equal at equilibrium, the exact hydrogen concentration in each layer could be determined. It is shown that during the major part of the hydrogen extraction process of the Mg_ySc_(1 − y) thin films, the composition of the Pd topcoat is close to PdH_0.001. The kinetics of the surface reactions was investigated using electrochemical impedance spectroscopy and showed that, when cross-correlating the results of Mg_ySc_(1 − y) thin films (y = 0.65 − 0.85) and pure Pd films, the surface kinetics are completely dominated by the Pd topcoat. Additionally, it was shown that the charge transfer reaction, and not the absorption reaction is the rate-determining step. The impedance response, dominating the overall kinetic impedance at the hydrogen-depleted state, could be linked to the transfer of hydrogen across the Pd/Mg_ySc_(1 − y) interface in the two-layer thin film electrode.     

Laser ablation of Ga-Sb-Te thin films monitored with quadrupole ion trap time-of-flight mass spectrometry

Laser ablation of Ga-Sb-Te chalcogenide thin films prepared by radiofrequency magnetron co-sputtering was monitored with quadrupole ion trap time-of-flight mass spectrometry (QIT-TOF-MS). The mass spectra of 11 thin films of various compositions (Ga: 0–53.1, Sb: 0–52.0, and Te: 0–100.0 at. %) were recorded. Several series of unary (Ga_x, Sb_y, and Te_z) binary (Ga_xSb_y, Ga_xTe_z, and Sb_yTe_z), and ternary Ga_xSb_yTe_z clusters were identified in both positive and negative ion modes. Stoichiometry of observed clusters was determined. Up to 18 binary clusters (positively and negatively charged) were detected for thin film with low Sb content of 6.5 at. %. The highest number (4) of ternary clusters was observed for thin film with high Te content of 66.7 at. %. The number of generated clusters and their peaks intensity varied according to the chemical composition of thin films. Altogether, 41 clusters were detected. The laser ablation monitoring shows laser-induced fragmentation of thin film structure. The relation of clusters stoichiometries to the chemical composition of thin films is discussed. The fragmentation can be diminished by covering a surface of thin films with paraffin's, glycerol, or trehalose sugar thin layers. The stoichiometry of generated clusters shows partial structural characterization of thin films.