Spark Plasma Sintered Bulk Nanocomposites of Bi2Te2.7Se0.3

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Spark Plasma Sintered Bulk Nanocomposites of Bi2Te2.7Se0.3 Nanoplates Incorporated Ni Nanoparticles with Enhanced Thermoelectric Performance Bingsheng Du,† Xiaofang Lai,† Qiulin Liu,‡ Haitao Liu,†,¶ Jing Wu,† Jiao Liu,† Zhihua Zhang,⊥ Yanzhong Pei,§ Huaizhou Zhao,*,‡ and Jikang Jian*,† †

Physics and Optoelectronic Engineering College, Guangdong University of Technology, Guangzhou 510006, China Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China § Key Laboratory of Advanced Civil Engineering Materials of Ministry of Education, School of Materials Science and Engineering, Tongji University, 4800 Caoan Road, Shanghai 201804, China ⊥ Liaoning Key Materials Laboratory for Railway, School of Materials Science and Engineering, Dalian Jiaotong University, Dalian 116028, China ¶ School of Physical Science and Technology, Xinjiang University, Urumqi 830046, China

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‡

S Supporting Information *

ABSTRACT: Bi2Te3-based compounds are important near room temperature thermoelectric materials with commercial applications in thermoelectric modules. However, new routes leading to improved thermoelectric performance are highly desirable. Incorporation of superparamagnetic nanoparticles was recently proposed as a means to promote the thermoelectric properties of materials, but its feasibility has rarely been examined in mainstream thermoelectric materials. In this study, high quality single-crystalline Bi2Te2.7Se0.3 nanoplates and Ni nanoparticles were successfully synthesized by solvothermal and thermal decomposition methods, respectively. Bulk nanocomposites consisting of Bi2Te2.7Se0.3 nanoplates and superparamagnetic Ni nanoparticles were prepared by spark plasma sintering. It was found that incorporation of Ni nanoparticles simultaneously increased the carrier concentration and provided additional scattering centers, which resulted in enlarged electric conductivities and Seebeck coefficients. The greatly improved ZT was achieved due to the increase in power factor. Spark plasma sintered bulk nanocomposites of Bi2Te2.7Se0.3 nanoplates incorporated by 0.4 mol %Ni nanoparticles (in molar ratio) showed a figure-ofmerit ZT of 0.66 at 425 K, equivalent to 43% increase when compared to pure Bi2Te2.7Se0.3 nanoplates. The results revealed that incorporation of magnetic nanoparticles could be an effective approach for promoting the thermoelectric performance of conventional semiconductors. KEYWORDS: Bi2Te2.7Se0.3 nanoplates, Ni nanoparticles, thermoelectric, nanocomposites, solvothermal method coupled together.11,12 In past decades, several means have been developed to decouple the three parameters and optimize the thermoelectric performance of materials. Among these, bottom-up or top-down nanostructuring is an effective route to increase ZT values of thermoelectric materials through reduction in thermal conductivity without detriment in electric transport.13−16 For instance, Cao et al. prepared bismuth telluride-based thermoelectric material by hydrothermal method combined with hot pressing technology with superior

1. INTRODUCTION With the increasing global energy crisis, more attention is being paid to thermoelectric conversion materials for direct conversion between heat and electricity.1−3 Bismuth telluride is a traditional thermoelectric material with excellent thermoelectric properties near room temperature.4−6 The performance of thermoelectric materials is defined by the dimensionless figure of merit ZT = (S2Tσ/Κ),7,8 where S is the Seebeck coefficient, σ is electrical conductivity, and Κ is thermal conductivity. High-performance thermoelectric materials require low Κ, high S, and elevated σ.9,10 However, single modulation of one parameter cannot effectively improve the thermoelectric performance because all three parameters are © XXXX American Chemical Society

Received: May 14, 2019 Accepted: August 12, 2019

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DOI: 10.1021/acsami.9b08392 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 1. (a) XRD patterns and (b) SEM image of the as-synthesis BTS nanoplates. (c) TEM image and EDS mappings of Bi, Te, and Se of a typical BTS nanoplate. (d) SAED pattern and (e) HRTEM image. (f) TEM-EDS spectrum and the corresponding quantitative results of one BTS nanoplate. The signals of Cu and Cr originated from the specimen holder.

ZT value of 1.47 at 420 K.17 Xie and co-workers synthesized (Bi, Sb)2Te3 bulk material by melt spinning combined with spark plasma sintering (SPS) and achieved a high ZT of 1.50 at 300 K.18 Recently, Zhao et al. developed a new strategy to improve the thermoelectric performances of Ba0.3In0.3Co4Sb12 by introducing magnetic nanoparticles to simultaneously optimize both the electrical and thermal conductivities of obtained nanocomposites.19,20 Despite the great potential, the feasibility of the new strategy has not yet been testified in other thermoelectric materials. To this end, incorporation of magnetic nanoparticles into bismuth telluride-based materials was tested in this work as an example for enhancing the thermoelectric properties of traditional thermoelectrical materials. Herein, the electron and phonon regulating effects for enhanced thermoelectric performance n-type Bi2Te2.7Se0.3 nanoplates were explored by incorporation of Ni nanoparticles. Bi2Te2.7Se0.3(BTS)/xNi nanocomposites at various molar

percentages (x = 0, 0.2%, 0.4%, and 0.6%) were prepared by solvothermal method followed by SPS. The results indicated that the thermoelectric performance of n-type Bi2Te3-based thermoelectric materials could be improved by adding appropriate amounts of Ni nanoparticles. Such improvements were mainly due to increased power factor when Ni nanoparticles were incorporated in the composite.

2. RESULTS AND DISCUSSION 2.1. Structure, Morphology, and Magnetic Characterization. The optimal composition for n-type Se-doped Bi2Te3 thermoelectrical materials has been determined as Bi2Te2.7Se0.3 (BTS).21 Hence, nanocomposites consisting of BTS nanoplates and superparamagnetic Ni nanoparticles were investigated here. In this account, BTS nanoplates were prepared by the solvothermal method.21,22 The crystal phase, morphology, B

DOI: 10.1021/acsami.9b08392 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 2. (a) TEM image, (b) HRTEM image, and (c) TEM-EDS mapping of BTS/0.4%Ni nanocomposite.

Figure 3. (a) XPS of Bi 4f7/2 and 4f5/2 core levels of BTS/x%Ni specimen prepared at X = 0 (black) and X = 0.4 (red); (b) XRD patterns of bulk BTS/x%Ni specimens.

As presented in the SAED pattern and HRTEM image (Figure S1c), the nanoparticles were well crystallized. The BTS/xNi nanocomposites were prepared by ultrasonic dispersion of the as-prepared BTS nanoplates and Ni nanoparticles in ethylene glycol. The morphology and microstructure of BST/0.4%Ni nanocomposite are presented in Figure 2. The TEM morphology depicted well-adhered Ni nanoparticles on BTS nanoplates (Figure 2a). The lattices of Bi2Te3 and Ni can be well identified by the HRTEM image in Figure 2b. Note that lattice space of BTS was very close to that of Bi2Te3. The EDS mapping showed the existence of the elements Bi, Te, Se, and Ni. The morphology, microstructure, and composition characterizations all confirmed the successful incorporation of Ni nanoparticles into BTS nanoplates. To identify the electron transfer between the matrix and Ni nanoparticles, the magnetic properties of Ni nanoparticles and X-ray photoelectron spectroscopy (XPS) of the nanocomposites were examined, and the results are shown in Figure S2 and Figure 3a. The presence of zero-field cooling and fieldcooling M−T junction indicated that Ni nanoparticles were subjected to magnetic transition from ferromagnetism to superparamagnetism at about 315 K. At temperatures above 315 K, the Ni nanoparticles became superparamagnetic. The M−H test results at different temperatures also confirmed that Ni nanoparticles underwent superparamagnetic transition near room temperature. In the magnified M−H diagram, Ni nanoparticles clearly exhibit very small coercivity at 350 K, confirming their superparamagnetic states. The matrix (BTS)

and microstructure of BTS nanoplates were examined, and the results are gathered in Figure 1. The X-ray diffraction (XRD) pattern of BTS specimen could exclusively be indexed as the rhombohedral Bi2Te3 phase (ICDD-PDF #15−0863).23 In addition, no impurity diffraction peaks were identified (Figure 1a). The morphology of BTS consisted of well-faceted hexagonal nanoplates with lateral size around 0.5 μm to 1 μm (Figure 1b). The transmission electron microscopy (TEM) morphology of a single nanoplate and energy-dispersive X-ray spectroscopy mapping are depicted in Figure 1c. All the elements Se, Te, and Bi looked uniformly distributed in the nanoplate, which indicated that Se was successfully doped into the Bi2Te3 matrix. Figure 1d and e illustrate the selected area electron diffraction (SAED) pattern and corresponding highresolution (HR)TEM image along the [001] zone axis, respectively. The nanoplates showed high crystallinity and quality. Figure 1f presents the X-ray energy-dispersive spectroscopy (EDS) pattern, identifying the Se, Te, and Bi signals, in which, Cu and Cr peaks originated from the specimen holder. The atomic ratio of Se/Te/Bi was estimated to 5.89:53.57:40.54, corresponding to a chemical formula Bi2Te2.7Se0.3. The structures and sizes of Ni nanoparticles were examined by XRD and TEM. As shown in Figure S1a of the Supporting Information, the XRD pattern of Ni nanoparticles can be indexed by a cubic Ni phase (ICDD-PDF #04−0850).24 The TEM morphology of Ni nanoparticles is displayed in Figure S1b. The sizes of the nanoparticles ranged from 10 to 20 nm. C

DOI: 10.1021/acsami.9b08392 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 4. Temperature dependence of (a) in-plane electrical conductivity, (b) Seebeck coefficient, (c) power factor, (d) carrier concentration, and (e) Hall mobility of hot pressed BTS/xNi bulk specimen. The in-plane direction is perpendicular to the press direction.

throughout the specimens without segregation (Figure S3), which indicated that SPS treatment did not alter the phase and compositional distribution of bulk specimens. The crosssection scanning electron microscopy (SEM) images of SPSsintered bulk specimens exhibited layered structures (Figure S4), which can be counted by their nanoplate units. 2.2. Electrical Transport Properties. Figure 4a presents the temperature dependence of in-plane electrical conductivity of bulk specimens. The in-plane direction was perpendicular to press direction and parallel to the specimen surface. The electrical conductivities of all four specimens showed similar downward trends with the increase in temperature. This can be attributed to enhancement in scattering with temperature. Compared to pure BTS, all the composites revealed high electrical conductivities, mainly associated with increased carrier concentration caused by incorporation of Ni nanoparticles. As discussed in the Supporting Information, the work function of the Ni nanoparticles is smaller than that of the BTS matrix, resulting in the electron transfer from Ni nanoparticles to the BTS matrix, which is schematically depicted by Figure S6. Meanwhile, as suggested by the literature,20 the interface barrier between the Ni nanoparticles and the BTS matrix and the superparamagnetism of the Ni nanoparticles introduced the additional scattering of electrons, resulting in the decrease of carrier mobility. Note that the electrical conductivity of BTS/0.4% Ni was lower than that of BTS/0.2% Ni, suggesting a great decrease in carrier mobility. The changes in electrical conductivities of specimens were further examined by Hall tests (Figure 4d,e). As the amount of Ni nanoparticles increased, the carrier concentrations rose as well, while mobilities of the nanocomposites decreased. This confirmed simultaneous additional electrons and scattering when Ni nanoparticles were incorporated in the matrix.

shows the weak ferromagnetism at 50 and 350 K (Figure S2d), which is consistent with the previous observation on Bi2Te3 nanostructures,25 and can be attributed to the nanostructurerelated surface defects.26 At 50 K, both the remnant magnetization and coercivity of BTS/0.4%Ni composite sample are obviously larger than the BTS matrix owing to the incorporation of strong ferromagnetism Ni nanoparticles (Figure S2e,f). The remnant magnetization and coercivity of BTS/0.4%Ni sample decreased at 350 K due to the superparamagnetism transition of Ni nanoparticles at 350 K. Interestingly, both the matrix and BTS/0.4% Ni exhibit a negative differential magnetization under high magnetic field, which is similar to the result of Zhao et al.19 and can be attributed to field-induced antiferromagnetic coupling effect. According to the metal−semiconductor contact theory,27 the superparamagnetic Ni nanoparticles could transfer extra electrons to the semiconductor matrix, and the mechanism of the charge transfer between the Ni nanoparticles and BTS nanoplates is revealed in Figure S6. This would increase the carrier concentrations, enhancing the electrical conductivity of the nanocomposite.20 In the XPS spectrum (Figure 3a), Bi 4f core levels of BTS/0.4%Ni showed lower binding energy than that of pure BTS, confirming changes in potential environment by the Ni nanoparticles incorporated in the matrix.28 The as-prepared BTS/Ni nanocomposites were pressed to bulk materials by SPS in graphite die for further characterizations. The crystal phases of the bulk specimens were then examined by XRD, and the results are shown in Figure 3b. All XRD patterns of bulk nanocomposites matched well the peaks of BTS. The diffraction peaks of Ni were absent in BTS/Ni nanocomposites due to their low concentrations. In addition, the elemental distribution of bulk BTS specimens confirmed the presence of Bi, Te, and Se homogeneously distributed D

DOI: 10.1021/acsami.9b08392 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Figure 4b shows the temperature dependence of Seebeck coefficients. As temperature increased, the Seebeck coefficients first enhanced and then declined. In addition, the values were all negative, which indicated n-type electrical conductivity. The Seebeck coefficients of BTS/0.2%Ni and BTS/0.4%Ni specimens increased significantly over the entire temperature range when compared to BTS. Furthermore, BTS/0.4%Ni showed the largest Seebeck coefficient of −165 μVK−1 at 425 K. For heavy doped semiconductors (i.e., highly degenerated), the Seebeck coefficient can be calculated by the formula: S = [π2kB2T/(3eh2)]8m*[π/(3n)]2/3(1 + Rx),20,29 where kB, h, m*, and Rx are the Boltzmann constants, Planck constant, effective mass, and scattering parameters, respectively. The incorporated Ni should not affect the energy band structure of BTS. Hence, m* should not be altered, and the Seebeck coefficient should mainly be influenced by the carrier concentration and Rx. The increase in carrier concentrations of the composites would decrease the Seebeck coefficient. Meanwhile, Rx was enhanced with the incorporation of Ni nanoparticles (Table S2), which can benefit the increase in Seebeck coefficient. Hence, the optimized Seebeck coefficient of the composites should be obtained by balancing the two opposing factors. BTS/0.4%Ni specimen showed the highest Seebeck coefficient, while BTS/ 0.6%Ni possessed the lowest Seebeck coefficient. Moreover, the Pisarenko curves were plotted to acquire the m* = 1.3m0 at 300 K (Figure S7), which is close to the reported m* = 1.2m0 for Bi2Te2.7Se0.3.30 The experimental data match well with the fitted S−n plots shown in Figure S7, also indicating that the increase on the scattering of electrons for the composite samples is due to the incorporation of Ni nanoparticles. As Ni content rose, the electrical conductivity and Seebeck coefficient for the specimens displayed different trends, resulting in various temperature dependent power factor curves (Figure 4c). The BTS/0.4%Ni specimen had the highest power factor of 7.1× 10−4 Wm−1 K−2 at 375 K. Zhao and co-workers reported a similar phenomenon when Co nanoparticles were incorporated in the Ba0.3In0.3Co4Sb12 matrix.20 2.3. Anisotropic Thermoelectric Properties and Figure of Merit ZT. Bi2Te3 is a layer-structured material with strong anisotropic thermal and electrical transport properties.31,32 To gain a better understanding of the anisotropic features and obtain ZT values, the thermoelectric properties of BST and BTS/0.4% Ni specimens were examined along in-plane (perpendicular to press direction) and out-plane (parallel to press direction) directions. Figure 5 shows the thermoelectric parameters of BST and BTS/0.4%Ni specimens in both in-plane and out-plane direction in comparison with previous data of BTS nanoplates.33 Figure 5a and b display the anisotropic temperature dependence of electrical conductivity and Seebeck coefficient. The electrical conductivity for BST and BTS/0.4%Ni specimens along both directions decreased with temperature, exhibiting metallic transport behaviors. BTS/0.4%Ni displayed higher electrical conductivity than BTS, associated with the elevated carrier concentration caused by incorporated Ni nanoparticles in the BTS matrix. The distinct microstructure between in-plane and out-plane directions resulted in such anisotropic transport properties. In other words, the electrical conductivity along out-plane direction was lower than that along in-plane direction due to the interlayer scattering caused by the intrinsic layered structure. Compared to previously reported BTS data,33 BTS and BTS/0.4%Ni specimens

Figure 5. Thermoelectric characteristics of BTS and BTS/0.4Ni specimens along in-plane and out-plane directions as a function of temperature: (a) electrical resistivity, (b) Seebeck coefficient, (c) total thermal conductivity, and (d) and ZT. The related data for BTS plates reported in ref 33 are added for comparison.

exhibited relatively small electrical conductivities, probably linked to low carrier mobilities. Figure 5b indicates that the inplane Seebeck coefficients of both specimens were slightly larger than those obtained along the out-plane direction, and similar to previously reported data below 400 K. At temperatures above 400 K, the Seebeck coefficient of BTS became slightly lower than previous data.33 However, the Seebeck coefficients of BTS/04%Ni along both directions were much larger than the reported literature.33 BTS/0.4%Ni showed a maximum Seebeck coefficient of −165 μV K−1 at 425 K along in-plane direction. Figure 5c depicts the temperature dependence of total thermal conductivity along in-plane and out-plane directions. The thermal conductivity increased as temperature rose, which should be ascribed to increased carrier concentrations. Both BTS and BTS/0.4%Ni showed much lower thermal conductivities than those in the literature,33 which should be caused by interface scattering and defect scattering. Moreover, our SPS specimens possessed a low density of 80% relative to the theoretical density, implying the existence of numerous pores. The porosity of each specimen can be estimated by SEM observation (Table S1). The data basically coincided with the density values. The pores should provide additional scattering centers for electrons and phonons, leading to lower thermal and electrical conductivities. The lattice thermal conductivity for a porous material can be calculated by the formula: κL = (1 − P)3/2κL,F, where κL,F and P (P = 1 − r, r is the relative density) are lattice thermal conductivity of an ideal fully dense material and porosity, respectively.34 Hence, pores in materials can greatly reduce the lattice thermal conductivity. For instance, it has been reported that lattice thermal conductivity of low-density Bi2Te2.56Se0.44 decreased from 0.21 Wm−1 K−1 to 0.14 Wm−1 K−1.34 Similar to direction dependence for electrical conductivity, both specimens showed smaller thermal conductivities along the outplane direction than in-plane direction. The out-plane thermal E

DOI: 10.1021/acsami.9b08392 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Figure 6. Electron thermal conductivity κe (a) and phonon thermal conductivity κL (b) as a function of temperature for BTS and BTS/0.4% Ni specimens, respectively.

3. CONCLUSIONS In summary, well-crystallized Bi2Te2.7Se0.3 nanoplates and Ni nanoparticles were synthesized by the solvothermal method and thermal decomposition, respectively. Thermoelectric bulk nanocomposites consisting of Bi2Te2.7Se0.3 nanoplates and Ni nanoparticles were then prepared by SPS technique. The electrical conductivities and Seebeck coefficients of the nanocomposites greatly increased due to incorporated Ni nanoparticles, which significantly improved the thermoelectric performances. A maximum ZT of 0.66 was obtained at 425 K with BTS/0.4%Ni specimen, which was 43% higher than that of BTS specimen. This study demonstrates that promoting the performance of conventional thermoelectric materials would be feasible by embedding magnetic nanoparticles.

conductivity of BTS exhibited the lowest value among all specimens. Figure 5d presents the change in ZT as a function of temperature. The in-plane ZT values of both specimens were larger than the out-plane values, and BTS/0.4%Ni composite presented much higher ZT along both directions than BTS and the literature.33 Because of enhanced electrical conductivity and Seebeck coefficient, BTS/0.4%Ni achieved a maximum in-plane ZT value of 0.66 at 425 K, which was 43% higher than that of BTS matrix, and 35% larger than values reported in the literature.33 The total thermal conductivity of a material consists of electron thermal conductivity κe and phonon thermal conductivity κL. To further explore the phonons transport features in the specimens, κe and κL were calculated, and the results are displayed in Figure 6. The κe was calculated according to Wiedeman-Franz law: κe = LσT, where L is the Lorenz number, σ is the electrical conductivity, and T is absolute temperature. The Lorenz number L can be obtained according to the formula:35 L = 1.5 + exp ( − |S| ). Figure S5

4. MATERIALS AND METHODS 4.1. Chemicals. All chemicals, including bismuth oxide powder (Bi2O3, 99.99%), tellurium oxide powder (TeO2, 99.99%), selenium oxide powder (SeO2, 99.99%), polyvinylpyrrolidone (PVP, K30), ethylene glycol (Eg, 98%), sodium hydroxide solution (NaOH, 5 M), nickel(II) acetylacetonate (Ni(acac)2, 98%), oleylamine (OAm, 70%), trioctylphosphine (TOP, 97%), and hexane (≥99%), were purchased from Alfa Aesar unless otherwise stated. All chemicals were used as received without further treatment. 4.2. Synthesis of Bi2Te2.7Se0.3 (BTS) Nanoplates. Bi2Te2.7Se0.3 nanoplates were synthesized by means of the solvothermal method.20,21 Briefly, polyvinylpyrrolidone (0.3 g), Bi2O3 (0.5 mmol), TeO2 (1.35 mmol), SeO2(0.15 mmol) and 2 mL ,of NaOH solution (5 M) were added into 18 mL of ethylene glycol. The resulting suspension was transferred to an autoclave (50 mL) and kept at 200 °C for 8 h. After cooling to room temperature, the product was collected by centrifugation, washed several times with deionized water and absolute ethanol, and then dried at 60 °C in vacuum oven. 4.3. Synthesis of Ni Nanoparticles. The monodispersed Ni nanoparticles were synthesized by a thermal decomposition method.37 Typically, Ni(acac)2 (2 g), TOP (8 mL), and OAm (25.5 mL) were placed in a four-neck flask and magnetically stirred under N2 flow. The resulting solution was degassed at 100 °C for 30 min and then kept at 230 °C for 8 h. After cooling to room temperature, the product was collected by centrifugation, washed several times with a mixture of hexane and ethanol, and then dried in vacuum oven at room temperature. 4.4. Synthesis of BTS/Ni Nanocomposites. BTS/xNi nanocomposites with three molar ratios (x = 0.2%, 0.4%, and 0.6%) were prepared in this study. In a typical synthesis, appropriate amounts of BTS nanoplates and Ni nanoparticles were dispersed in ethylene glycol. The formed mixture was then sonicated for 1 h and left under

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shows the temperature dependence of L for BTS and BTS/ 0.4%Ni samples. Obviously, the temperature dependence of Lorenz number L exhibits similar trend with that of Seebeck coefficients because the Lorenz number was only determined by Seebeck coefficients in the above formula. The temperature dependence of κe is illustrated in Figure 6a. κe increased with temperature, consistent with the temperature dependence of carrier concentration shown in Figure 4d. κe of BTS/0.4% Ni was higher than that of BTS over the entire temperature range, mainly attributed to the large carrier concentration. The outplane κe was reasonably lower than the in-plane κe due to the layered microstructure. In Figure 6b, κL first decreased and then increased with temperature. The composite showed larger κL than BTS and presented different behaviors when compared to the literature.20 The increase in κL at high temperature should be attributed to the bipolar effect,30 and larger κL for nanocomposites was probably due to noncomplete deduction of electronic thermal conductivity from the total thermal conductivity.36 Despite the increase in thermal conductivity, promotion of electrical transport properties in the nanocomposite can overcome the detrimental effect of thermal conductivity, achieving enhanced thermoelectric performances. F

DOI: 10.1021/acsami.9b08392 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces constant stirring at 80 °C for 1 h. The obtained BTS/xNi was collected by centrifugation, washed several times with absolute ethanol, and then dried at 60 °C in vacuum oven. 4.5. Characterization. The crystal structures of the as-prepared products were examined by powder X-ray diffraction (XRD, PANalytical PW3040/60, Cu Ka radiation). Scanning electron microscopy (SEM, Hitach, S-3400N−II) and transmission electron microscopy (TEM, JEOL JEM-2100F) were employed to observe the morphologies and microstructures of the specimens. An energy dispersive spectrometer coupled to scanning and transmission electron microscopes was used to identify the chemical compositions. X-ray photoelectron spectroscopy (XPS) was performed on a Thermo Scientific Escslab 250Xi. The magnetic properties were measured by VSM-Versalab. 4.6. Thermoelectric Transport Measurements. To evaluate the thermoelectric properties, the as-prepared powders were first pressed into bulk specimens at 523 K for 5 min under 50 MPa pressure in vacuum atmosphere. The resulting hot-pressed pellets showed a density of 80% relative to the theoretical density. The thermal conductivity (κ) was calculated using the formula: κ = DCpd, where D, Cp, and d are thermal diffusivity, heat capacity, and density, respectively. Linseis XFA 500 was employed to measure the thermal diffusivity (D) under high purity helium atmosphere. To this end, the specimens were cut into thin discs (diameter 12.7 mm and thickness 2 mm). The density (d) was measured by the Archimedes drainage method. Heat capacity (Cp) was obtained from Dulong-Petit law. The Hall values were measured using the van der Pauw technique under a reversible magnetic field of 1.5 T. σ and S were simultaneously collected on a LSR-3 (Linseis) under high purity helium atmosphere with deviation measurement around ±5%.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b08392. XRD patterns, TEM image, HRTEM image, and SAED patterns of Ni nanoparticles; measured magnetic properties; SEM-EDS elemental mappings of BTS bulk materials pressed by BTS nanoplates; cross-sectional morphologies of different specimens; Lorenz number of BTS and BTS/0.4Ni specimens; porosity of different component specimens; illustration of charge transfer between Ni nanoparticles and BTS nanoplates; roomtemperature charge-transport properties and scattering parameters; room-temperature S-n plots (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. ORCID

Yanzhong Pei: 0000-0003-1612-3294 Jikang Jian: 0000-0001-5752-7282 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Grant Nos. U1601213, 51472052, and 51702058).

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DOI: 10.1021/acsami.9b08392 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX