Optimization of chemical and electrochemical parameters for the preparation of n-type Bi2Te2.7Se0.3 thin films by electrodeposition

A 2^3–1 fractional factorial design comprising four runs and three centre points was applied in order to optimize the electrodeposition process to find a compound with the best stoichiometry leading to a Bi₂Te_2.7Se_0.3 thin film suitable for thermoelectric applications. The key factors considered were the deposition potential, the percentage of bismuth and the percentage of selenium in the solution. The Bi^III, Se^IV, Te^IV electrolyte mixtures in 1 M HNO₃ (pH 0), allowed deposition of ternary alloys to be achieved at room temperature on stainless steel substrates. The deposition mechanism was investigated by linear voltammetry. The films were characterized by micropobe analysis, X-ray diffraction, scanning electron microscopy and atomic force microscopy. The XRD patterns of the film show that the as-deposited are polycrystalline and isostructural to Bi₂Te₃. The SEM study shows that the film is covered by crystallites while the AFM image reveals a low level of roughness.     

Substrate temperature-dependent thermoelectric figure of merit of nanocrystalline Bi2Te3 and Bi2Te2.7Se0.3 prepared using pulsed laser deposition supported by DFT study

This work reports the pulsed laser deposition of n-type selenium (Se) doped bismuth telluride (Bi₂Te_2.7Se_0.3) and n-type bismuth telluride (Bi₂Te₃) nanostructures under varying substrate temperatures. The influence of the substrate temperature during deposition on the structural, morphological and thermoelectric properties for each phase was investigated. Density functional theory (DFT) simulations were employed to study the electronic structures of the unit-cells of the compounds as well as their corresponding partial and total densities of states. Surface and structural characterization results revealed highly crystalline nanostructures with abundant grain boundaries. Systematic comparative analysis to determine the effect of Se inclusion into the Bi₂Te₃ matrix on the thermoelectric properties is highlighted. The dependence of the thermoelectric figure of merit (ZT) of the nanostructures on the substrate temperatures during deposition was demonstrated. The remarkable room temperature thermoelectric power factor (PF) of 2765 μW/mK² and 3179 μW/mK² for pure and Se-doped Bi₂Te₃ compounds respectively, signifies their potential of being useful in cooling and power generation purposes. The room temperature ZT values of the Se-doped Bi₂Te₃ was found to be 0.92, about 30% enhancement as compared with the pure phase, which evidently results from the suppressed thermal conductivity in the doped species caused by phonon scattering at the interfaces.     

Enhanced thermal stability of Bi2Te3-based alloys via interface engineering with atomic layer deposition

The ease of Te sublimation from Bi₂Te₃-based alloys significantly deteriorates thermoelectric and mechanical properties via the formation of voids. We propose a novel strategy based on atomic layer deposition (ALD) to improve the thermal stability of Bi₂Te₃-based alloys via the encapsulation of grains with a ZnO layer. Only a few cycles of ZnO ALD over the Bi₂Te_2.7Se_0.3 powders resulted in significant suppression of the generation of pores in Bi₂Te_2.7Se_0.3 extrudates and increased the density even after post-annealing at 500 °C. This is attributed to the suppression of Te sublimation from the extrudates. The ALD coating also enhanced grain refinement in Bi₂Te_2.7Se_0.3 extrudates. Consequently, their mechanical properties were significantly improved by the encapsulation approach. Furthermore, the ALD approach yields a substantial improvement in the figure-of-merit after the post-annealing. Therefore, we believe the proposed approach using ALD will be useful for enhancing the mechanical properties of Bi₂Te₃-based alloys without sacrificing thermoelectric performance.     

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.     

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.     

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.     

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.     

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.     

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.     

High-throughput optimization and fabrication of Bi2Te2.7Se0.3-based artificially tilted multilayer thermoelectric devices

Artificially tilted multilayer thermoelectric devices (ATMTDs) have attracted growing attention due to their ease in miniaturization and high flexibility in device design. However, most of these devices are inefficient due to the lack of effective strategy to optimize their material matching and geometrical configurations. Herein, a high-throughput optimization approach is employed to screen high-performance Bi₂Te_2.7Se_0.3-based ATMTDs from a material genome database covering 230 kinds of candidates. 14 kinds of ATMTDs are found to have ZT_zx,max values exceeding 0.3 and tilt angles greater than 15°. Bi_0.1Sb_1.9Te₃/Bi₂Te_2.7Se_0.3 ATMTD is screened out and fabricated because of its excellent transverse figure of merit, large tilt angle, and good interface compatibility. Consequently, transverse figure of merit over 0.3, thermal sensitivity greater than 0.11 mV·K^?1, and power density up to 1.1 kW·m^?2 are recorded in Bi_0.1Sb_1.9Te₃/Bi₂Te_2.7Se_0.3 ATMTD. This indicates that ATMTDs have great potential for application in the fields of temperature detection and power generation.     

Enhancements of thermoelectric performance in n-type Bi2Te3-based nanocomposites through incorporating 2D Mxenes

Bi₂Te_2.7Se_0.3 compound has been considered as an efficient n-type room-temperature thermoelectric (TE) material. However, the large-scale applications for low-quality energy harvesting were limited due to its low energy-conversion efficiency. We demonstrate that TE performance of Bi₂Te_2.7Se_0.3 system is optimized by 2D Ti₃C₂T_x additive. Here, a 43% reduction of electrical resistivity is obtained for the nanocomposites at 380 K, originating from the increased carrier concentration. Consequently, the g = 0.1 sample shows a maximum power factor of 1.49 Wmm^?1K^?2. Meanwhile, the lattice thermal conductivity for nanocomposite samples is reduced from 0.77 to 0.41 Wm^?1K^?1 at 380 K, due to the enhanced phonon scattering induced by the interfaces between Ti₃C₂T_x nanosheets and Bi₂Te_2.7Se_0.3 matrix. Therefore, a peak ZT of 0.68 is achieved at 380 K for Bi₂Te_2.7Se_0.3/0.1 wt% Ti₃C₂T_x, which is enhanced by 48% compared with pristine sample. This work provides a new route for optimizing TE performance of Bi₂Te_2.7Se_0.3 materials.     

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.     

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.     

A mechanistic study of electrodeposition of bismuth telluride on stainless steel substrates

Miniaturization of Bi₂Te₃ compounds is of great interest in semiconductor industries due to their distinct anisotropic thermoelectric properties at room temperature. The aim of the present work was to investigate the mechanism of the electrodeposition of Bi₂Te₃ compounds on stainless steel substrates and relate the morphology and composition of the resulting deposits to experimental parameters. Cyclic voltammetry (CV) experiments in acidic solutions containing Bi³⁺ and/or HTeO₂⁺ ions show that the deposition potential for the Bi₂Te₃ compound is more positive than either of the single elements alone. A detailed mechanism of the co-deposition was obtained by varying the concentrations of the two elements and evaluating the corresponding morphological and compositional changes of the deposits. The results show that the deposition of Te is kinetically hindered and that Bi deposition plays a major role during the co-deposition.     

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.     

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.     

Synergistically optimizing electrical and thermal transport properties of Bi2O2Se ceramics by Te‐substitution

Bi₂O₂Se oxyselenides, characterized with intrinsically low lattice thermal conductivity and large Seebeck coefficient, are potential n‐type thermoelectric material in the mediate temperature range. Given the low carrier concentration of ~10¹⁵ cm^?3 at 300 K, the intrinsically low electrical conductivity actually hinders further enhancement of their thermoelectric performance. In this work, the isovalent Te‐substitution of Se plays an effective role in narrowing the band gap, which notably increases the carrier concentration to ~10¹⁸ cm^?3 at 300 K and the electron conduction activation energy has been lowered significantly from 0.33 to 0.14 eV. As a consequence, the power factor has been improved from 104 μW·K^?2·m^?1 for pristine Bi₂O₂Se to 297 μW·K^?2·m^?1 for Bi₂O₂Se_0.96Te_0.04 at 823 K. Meanwhile, the suppressed lattice thermal conductivity derives from the introduced point defects by heavier Te atoms. The gradually decreased phonon mean free path reflects the increasingly intense phonon scattering. Ultimately, the ZT value attains 0.28 for Bi₂O₂Se_0.96Te_0.04 at 823 K, an enhancement by a factor of ~2 as compared to that of pristine Bi₂O₂Se. This study has demonstrated that Te‐substitution of Se could synergistically optimize the electrical and thermal properties thus effectively enhancing the thermoelectric performance of Bi₂O₂Se.     

Preparation and thermoelectric properties of Bi2SexTe3−x bulk materials

Bi₂Te₃ and Bi₂Se₃ nanoplates were synthesized by a microwave-assisted wet chemical method, and Bi₂Se_xTe_3−x (x=1, 2, 3) bulk nanocomposites were then prepared by hot pressing the Bi₂Te₃ and Bi₂Se₃ nanoplates at 80 MPa and 723 K in vacuum. The phase composition and microstructures of the bulk samples were characterized by powder X-ray diffraction and field-emission scanning electron microscopy, respectively. The electrical conductivity of the Bi₂Se_xTe_3−x bulk nanocomposites increases with increasing Se content, and the Seebeck coefficient value is negative, showing n-type conduction. The absolute Seebeck coefficient value decreases with increasing Se content. A highest power factor, 24.5 µWcm⁻¹ K⁻², is achieved from the sample of x=1 at 369 K among the studied samples.     

Grain boundary engineering strategy for simultaneously reducing the electron concentration and lattice thermal conductivity in n-type Bi2Te2.7Se0.3-based thermoelectric materials

This study demonstrates atomic layer deposition (ALD) of an extremely thin Al₂O₃ layer over n-type Bi₂Te_2.7Se_0.3 to alleviate the adverse effects of multiple boundaries on their thermoelectric performance. Multiple boundaries reduce thermal conductivity (κ), but generate electrons, deviating from the optimum carrier concentration. Only one Al₂O₃ ALD cycle effectively suppresses Te volatilization at the grain boundaries, resulting in a decrease from 5.8 × 10¹⁹/cm³ to 3.6 × 10¹⁹/cm³ in the electron concentration. Concurrently, the one-cycle-Al₂O₃ coating produces fine grains, thus inducing numerous boundaries, ultimately suppressing the lattice κ from 0.64 to 0.33 W/m·K. A further increase in the number of Al₂O₃ cycles leads in a significant rise in the resistance, resulting in degradation of thermoelectric performance. Consequently, the ZT value is increased by 51 % as a result of Al₂O₃ coating with a single ALD cycle. Our approach offers new insights into the simultaneous reduction of the κ and electron concentration in n-type Bi₂Te₃-based materials.     

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.