2023 Volume 64 Issue 10 Pages 2503-2507
The electrodeposition of bismuth–tellurium thermoelectric material was investigated by galvanostatic and potentiostatic electrolysis in ethylene glycol (EG)-BiCl3–TeCl4 non-aqueous solutions at 393 K. Controlling the molar ratio of BiCl3 to TeCl4 in the bath and the electrolysis conditions, the Bi–Te alloys with various compositions were electrodeposited. The composition of the electrodeposit obtained in the 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath with the BiCl3 to TeCl4 molar ratio of 50:1 at the current density of 20 A·m−2 was 40.14 mol%Bi–59.86 mol%Te and close to that of Bi2Te3. This electrodeposit exhibited a n-type thermoelectric conversion for the given temperature difference and its Seebeck coefficient was −145 µV K−1.
Fig. 7 Thermoelectric forces of 0.01 mol%Bi–99.99 mol%Te, 18.75 mol%Bi–81.25 mol%Te, and 40.14 mol%Bi–59.86 mol%Te alloy films obtained in the present study.
The energy harvesting techniques that extract electrical energy from ambient energy such as solar power, thermal energy, kinetic energy, and so on are attracting attention.1) Although the obtained electrical energy by energy harvesting technique is small, it can be used for the power sources of wireless and autonomous devices and sensors. The thermoelectric conversion using the thermoelectric materials is one of the energy harvesting techniques. Thermoelectric materials can convert thermal energy from a temperature gradient directly into electrical energy. An intermetallic compound in the bismuth–tellurium binary system, Bi2Te3, is known as a fundamental material widely used in thermoelectric devices operating at room temperature. In order to reduce thermal conductivity, the solid solution of Bi2Te3 and Sb2Te3 (Bi2−xSbxTe3), and Bi2Te3 and Bi2Se3 (Bi2Te3−xSex), have been used as a p-type and a n-type thermoelectric material, respectively.2,3) The Bi2Te3 based materials are excellent thermoelectric materials for applications near room temperature due to their high Seebeck coefficient, low electrical resistivity, and low thermal conductivity.4,5) The thermoelectric devices usually consist of pairs of p-type and n-type materials,6) and preparing the thermoelectric materials as films has the advantage in producing lightweight and high-capacity devices. The electrodeposition is a simple and fast film forming method, and suitable for the mass production.
Several studies about the electrodeposition of Bi–Te based thermoelectric materials in aqueous solutions were reported.7–12) Matsuoka et al. reported the electrodeposition of the Bi–Te binary alloy films from hydrochloric aqueous solutions containing Bi(NO3)3 and TeO2.9) Takahashi et al. reported the electric and thermoelectric properties of electrodeposited Bi2Te3 from nitric acid aqueous solutions containing Bi-EDTA complex and TeO2.11) In general, electrolysis in aqueous solutions, especially in acidic solutions, is often accompanied by hydrogen evolution, which causes a decrease in the coulomb efficiency and degrades the adhesiveness and surface morphology of the film. There are a few reports on electrodeposition of Bi–Te alloy from non-aqueous solutions. Eba et al. reported the electrodeposition of Bi–Te alloy from AlCl3–NaCl–KCl molten salts containing BiCl3 and TeCl4 at 423 K.13) In our previous studies, the electrodeposition of thermoelectric materials of Fe–Al alloy in AlCl3–NaCl–KCl–FeCl2 molten salts at 413 K,16) and Zn–Sb and Co–Sb alloys in ethylene glycol (EG) based non-aqueous solutions at 393 K17–19) were investigated. Although EG is a divalent alcohol and not an electrolyte, we have used EG as a solvent for non-aqueous solutions to electrodeposit metals and alloys because it is inexpensive and easy to handle, and a relatively large amount of metal chlorides dissolve in EG. As for the electrodeposition of Bi–Te alloy using EG non-aqueous solutions, Nguyen et al. reported the electrodeposition of Bi2Te3 at 323 K from non-aqueous solution using ethylene glycol (EG) containing Bi(NO3)3·5H2O and TeCl4 as sources of metal ions, and Li(NO3) as an additive to improve electrolyte conductivity.14) They also reported the electrodeposition of bismuth telluride at 323 K from chloride-free EG based solutions containing Bi(NO3)3, Te(NO3)4, and LiNO3 and investigated the effects of chloride ions on the electrodeposition of Bi and Te.15) In the present study, the electrodeposition of Bi–Te alloy in EG-BiCl3–TeCl4 non-aqueous solution without additives at 393 K was investigated to prepare the thermoelectric conversion material of Bi2Te3. In order to obtain Bi2Te3 with thermoelectric conversion property, it is important to control the composition of Bi–Te alloy. The relationships between the composition of the electrodeposited Bi–Te alloy and the electrolysis conditions including the bath composition of the EG-BiCl3–TeCl4 non-aqueous solution were clarified in this paper.
The electrolytic baths used for the Bi–Te electrodepositions were composed of ethylene glycol (EG, purity: >99.5%, water content: <0.2%, Kanto Chemical Co., Inc.), bismuth chloride (BiCl3, purity: 99.9%, Kojundo Chemical Lab. Co., Ltd.) as the Bi ion source, and tellurium chloride (TeCl4, purity: 99.9%, Kojundo Chemical Lab. Co., Ltd.) as the Te ion source. The mass of 20.00 g of EG was used for the bath, and the total concentration of metal chlorides (BiCl3 and TeCl4) in the bath was 2.50 mol%. The molar ratios of BiCl3 to TeCl4 in the bath were 10:1, 25:1, and 50:1. The chemicals were heated to 333 K and dehydrated for 24 h under vacuum using a rotary pump to prepare the baths. The baths were heated to 393 K to use for the electrodeposition.
2.2 Electrodeposition of Bi–Te alloyThe electrolysis was conducted by galvanostatic or potentiostatic electrolysis at a temperature of 393 K using a platinum plate for the cathode (purity: 99.95%, 0.2 mm × 30 mm × 10 mm, Nilaco Co.), and a carbon rod for the anode (purity: 99.99%, ϕ6.0 mm, Nilaco Co.). The reference electrode used for the potentiostatic electrolysis and electrochemical measurement was a Zn wire immersed in a glass tube containing 85.00 mol%EG-15.00 mol%ZnCl2 bath, with the bottom separated by fritted glass.20) The potential concerning the electrolysis and electrochemical experiments in the present study was referenced to this electrode. The Bi–Te alloy films were prepared by galvanostatic or potentiostatic electrolysis in EG-BiCl3–TeCl4 baths at 393 K for 1.0 × 105 C m−2. The area of the electrodeposited Bi–Te alloys was 10 mm × 10 mm on both sides of the Pt substrate. After the electrolysis, the electrodes were rinsed with ethanol and water, and then dried.
The cathodic polarization curves were measured to study the reduction behavior of Bi and Te ionic species in the EG-BiCl3–TeCl4 baths. The electrodes mentioned above were immersed in the bath and connected to an automatic polarization system (HSV-110, HOKUTO DENKO Corp.), and the potential was swept from the rest potential to negative at a scan rate of 0.05 V s−1 to measure the cathodic polarization curves.
The preparation of bath and the electrolysis were carried out in an Ar-gas-filled glove box (MIWA MFG, DB0-1-W).
The crystal structures of the electrodeposits were identified by using an X-ray diffractometer (XRD, UltimaIV, Rigaku Corp.). The composition of the electrodeposit was analyzed by using an electron probe microanalyzer (EPMA, JXA-8530FPlus, JEOL Ltd.). The compositions were measured at least 10 spots over an entire electrodeposit, and the average of them was adapted.
2.3 Measurement of thermoelectric forceThe thermoelectric forces generated by the temperature differences on the Bi–Te alloy film obtained in the present study were measured at room temperature. The temperature difference was generated by heating the electrodeposited end of the Pt substrate and cooling the other (non-electrodeposited) end of the Pt substrate. The details were described in our previous paper.16)
At most 2.605 g of BiCl3 can be dissolved in 20.00 g of EG at 393 K, corresponding to the concentration of 97.50 mol%EG-2.50 mol%BiCl3. On the other hand, at least 2.50 mol% of TeCl4 can be dissolved in EG. Before the electrodeposition of Bi–Te alloy, the reduction behaviors of the Bi and Te ionic species in EG based baths were investigated by measuring the cathodic polarization curves.
Figure 1 shows the cathodic polarization curves measured in (a) 97.50 mol%EG-2.50 mol%BiCl3, (b) 99.00 mol%EG-1.00 mol%TeCl4, (c) 99.80 mol%EG-0.20 mol%TeCl4, and (d) 99.96 mol%EG–0.04 mol%TeCl4 baths at 393 K with the scan rate of 0.05 V s−1. As shown in Fig. 1(a), the cathodic current was initially observed at 0.80 V and increased as the potential moved in the negative direction, corresponding to the reduction of Bi ionic species to Bi in EG-BiCl3 bath. In EG-TeCl4 baths, the cathodic currents caused by the reduction of Te ionic species to Te were observed at around 1.25 V. The gradients of cathodic currents became lower as the concentration of TeCl4 in bath decreased. In the bath (d), the cathodic current started to increase at 1.20 V, showed plateau at around 0 V, and increased again at potentials more negative than −0.50 V. The difference between the potentials at which the cathodic current started to increase (1.20 V) and −0.50 V is 1.70 V. The standard electrode potentials for Te4+/Te and Te/Te2− in the aqueous solutions at 298 K are 0.57 and −1.14 V vs. SHE,21) respectively, and the difference between them is 1.71 V. This potential difference is in good agreement with the one measured in the present study. Therefore, the cathodic current observed at potentials more negative than −0.50 V is due to the reduction of Te to Te2−. From the results shown in Fig. 1, the potential for the electrodeposition of Te is higher than that of Bi in the EG based bath. For the electrodeposition of Bi–Te alloy, it is necessary to increase the concentration of BiCl3 and decrease that of TeCl4 in the bath to electrodeposit Bi and Te at the same potential and current density. Therefore, the 97.50 mol%EG-2.50 mol%BiCl3 bath, which is almost saturated with BiCl3 in EG, was used as the fundamental bath, and the electrodepositions of Bi–Te alloy were conducted in the baths prepared by substituting a part of BiCl3 with TeCl4 so that the concentration of TeCl4 in the bath was about 0.04–0.20 mol%.
Cathodic polarization curves measured in (a) 97.50 mol%EG-2.50 mol%BiCl3, (b) 99.00 mol%EG-1.00 mol%TeCl4, (c) 99.80 mol%EG-0.20 mol%TeCl4, and (d) 99.96 mol%EG–0.04 mol%TeCl4 baths at 393 K with the scan rate of 0.05 V s−1.
Figure 2 shows the Te content in the electrodeposits obtained by galvanostatic electrolysis in (a) 97.50 mol%EG-2.27 mol%BiCl3–0.23 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 10:1, (b) 97.50 mol%EG-2.40 mol%BiCl3–0.10 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 25:1, and (c) 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 50:1 as a function of applied current density. The values in this figure represent as the Te contents in the Bi–Te binary alloy electrodeposits. The Te contents in the electrodeposits obtained in the bath (a) were 99.99% at 5 to 25 A m−2 and 99.59 mol% at 30 A m−2. As can be seen from Fig. 1, this was due to the preferential electrodeposition of Te in the bath with high concentration of TeCl4. As the concentration of TeCl4 in the bath decreased, the Te content tended to decrease. The Te contents in the electrodeposits obtained in the baths (b) and (c) at 20 A m−2 were 81.25 and 59.86 mol%, respectively, and decreased significantly at 30 A m−2 due to the increase in amount of co-electrodeposition of Bi and the reduction of Te to Te2−. It was found that the electrodeposits with compositions close to the target stoichiometric composition of Bi2Te3 were obtained at 5 to 20 A m−2 in the bath (c).
Te content in electrodeposits obtained in (a) 97.50 mol%EG-2.27 mol%BiCl3–0.23 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 10:1, (b) 97.50 mol%EG-2.40 mol%BiCl3–0.10 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 25:1, and (c) 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 50:1 as a function of applied current density.
Figure 3 shows the X-ray diffraction patterns of the electrodeposits obtained at 20 A m−2 in the baths with BiCl3 to TeCl4 molar ratios of (a) 10:1, (b) 25:1, and (c) 50:1. The XRD peaks corresponding to Te, Te and Bi2Te3, and Bi2Te3 except for Pt used as the substrate were detected in the electrodeposits obtained in the bath (a), bath (b), and bath (c), respectively, satisfying the relationship between the composition and the equilibrium phase in the Bi–Te binary system.22)
X-ray diffraction patterns of the electrodeposits obtained at the current density of 20 A m−2 for 1.0 × 105 C m−2 at 393 K in (a) 97.50 mol%EG-2.27 mol%BiCl3–0.23 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 10:1, (b) 97.50 mol%EG-2.40 mol%BiCl3–0.10 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 25:1, and (c) 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 50:1.
Figure 4 shows SEM images of the surfaces of the electrodeposits obtained at 20 A m−2 in the baths mentioned above. The surface morphology changed by the Te content in the electrodeposits. The fine grains of approximately 1 µm in size were observed in the electrodeposit obtained in the bath (a) (Te content: 99.99 mol%). The electrodeposits obtained in the bath (b) and bath (c) were composed of rough and needle-like grains of 3–5 µm in size. It is considered that the needle-shape electrodeposits are derived from Bi2Te3 since the XRD peaks for Bi2Te3 are detected in these electrodeposits.
SEM images of the electrodeposits obtained at the current density of 20 A m−2 for 1.0 × 105 C m−2 at 393 K in (a) 97.50 mol%EG-2.27 mol%BiCl3–0.23 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 10:1, (b) 97.50 mol%EG-2.40 mol%BiCl3–0.10 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 25:1, and (c) 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 50:1.
The potentiostatic electrolysis was also conducted in the 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath with BiCl3 to TeCl4 mole ratio of 50:1. The Te content in the electrodeposits as a function of applied potential are shown in Fig. 5. The Te content in the electrodeposits was almost constant in the potential range from 0.50 to 0 V, became slightly higher at −0.10 and −0.25 V, and decreased significantly at −0.40 V. This decrease in Te content was caused by the reduction of Te to Te2−, as can be seen in Fig. 1(d). The composition of the electrodeposits obtained at 0.50 to 0 V were close to that of Bi2Te3. The current efficiency of the electrodeposit obtained at 0.50 V was 99.2%, decreasing slightly to 96.4% at −0.10 V, and 92.5% at −0.25 V. For further investigation of the potentiostatic electrodeposition of Bi–Te alloy, the partial current densities of Bi and Te were calculated by Faraday’s law using the average values of cathodic currents measured during the electrolysis, the current efficiency, and the compositions of Bi–Te electrodeposits, assuming the reductions of Bi(III) ionic species to Bi and Te(IV) ionic species to Te in the potential range from 0.50 to −0.25 V.
Te content in the electrodeposits obtained by potentiostatic electrolysis in the 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath as a function of applied potential.
Figure 6 shows the partial current densities of Bi and Te for potentiostatic electrolysis in the 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath as a function of applied potential. The partial current densities of Bi (iBi) and that of Te (iTe) increased monotonically as the potential became more negative. As can be seen from the results shown in Fig. 5, the ratio of the partial current density of Te to that of Bi (iTe/iBi) was almost 2.0 in the potential range from 0.50 to 0 V and was 2.5 or higher at the potentials more negative than 0 V. The ratio of the reduction current of Bi ionic species in the 97.50 mol%EG-2.50 mol%BiCl3 bath shown in Fig. 1(a) to that of Te ionic species in the 99.96 mol%EG-0.04 mol%TeCl4 bath shown in Fig. 1(d) is not necessarily 2.0 in the potential range from 0.50 to 0 V. It is considered that the interaction between Bi and Te ionic species in the EG-BiCl3–TeCl4 ternary baths results in the (iTe/iBi) ratio of 2.0.
The partial current densities of Bi and Te for the potentiostatic electrolysis in the 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath as a function of applied potential.
From the results mentioned above, the relationships between the electrolysis conditions and the composition of the electrodeposits were clarified.
3.3 Thermoelectric effect of Bi–Te electrodepositFigure 7 shows the thermoelectric forces of 0.01 mol%Bi–99.99 mol%Te, 18.75 mol%Bi–81.25 mol%Te, and 40.14 mol%Bi–59.86 mol%Te alloy films prepared by galvanostatic electrolysis at 20 A m−2 in the present study. Thermoelectric forces were generated by and increased with the temperature difference. Negative thermoelectric forces were generated from the 40.14 mol%Bi–59.86 mol%Te alloy film (close to stoichiometric composition of Bi2Te3), indicating that this film had an n-type thermoelectric conversion function. The Seebeck coefficient was evaluated as the slope of the linear function of the temperature difference and thermoelectric force passing through the origin. Its Seebeck coefficient was −145 µV K−1. The Seebeck coefficients of electrodeposited Bi–Te alloy films prepared by Matsuoka et al.9) and Wu et al.15) are −90.6 µV K−1 for the composition of 34 mol%Bi–66 mol%Te and −120 µV K−1 for the composition close to the stoichiometric Bi2Te3, respectively. The Seebeck coefficient of 40.14 mol%Bi–59.86 mol%Te alloy film prepared in the present study is close to that prepared by Wu et al.15) On the other hand, positive thermoelectric forces were generated from 18.75 mol%Bi–81.25 mol%Te and 0.01 mol%Bi–99.99 mol%Te alloy films, indicating that these films had a p-type thermoelectric conversion function. The Seebeck coefficients of 18.75 mol%Bi–81.25 mol%Te and 0.01 mol%Bi–99.99 mol%Te alloy films were 73 and 104 µV K−1, respectively. The Seebeck coefficient changed to positive as the Te content in film increased. In the film exhibiting the positive Seebeck coefficient, Te was detected by XRD as shown in Fig. 3(a) and Fig. 3(b). It is considered that the electronic energy band structure of Bi2Te3 was changed by the addition of Te in the film with high Te content.9,23)
Thermoelectric forces of 0.01 mol%Bi–99.99 mol%Te, 18.75 mol%Bi–81.25 mol%Te, and 40.14 mol%Bi–59.86 mol%Te alloy films obtained in the present study.
The electrodeposition of Bi–Te film with a thermoelectric conversion function in ethylene glycol (EG)-BiCl3–TeCl4 non-aqueous solutions was investigated by galvanostatic and potentiostatic electrolysis. The Bi–Te alloys with various compositions were obtained by controlling the molar ratio of BiCl3 to TeCl4 in the bath. The compositions of the electrodeposits were close to the stoichiometric composition of Bi2Te3 when the galvanostatic electrolysis at 5–20 A m−2 or the potentiostatic electrolysis at 0.50 to 0 V vs. Zn(II)/Zn were conducted in the bath with the molar ratio of BiCl3 to TeCl4 in the bath was adjusted to 50:1. The electrodeposit obtained in the 97.50 mol%EG-2.45 mol%BiCl3–0.05 mol%TeCl4 bath with the BiCl3 to TeCl4 molar ratio of 50:1 at the current density of 20 A m−2 was 40.14 mol%Bi–59.86 mol%Te and exhibited a n-type thermoelectric conversion for the given temperature difference and its Seebeck coefficient was −145 µV K−1.
