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Functional Nanostructured Materials (including low-D carbon)

The Thermoelectric Performance of Sb2Te3-based Alloy is Improved by Introducing PN Junctions Xiaoyu Wang, Huijuan Wang, Bo Xiang, Liang-Wei Fu, Hao Zhu, Dong Chai, Bin Zhu, Yuan Yu, Na Gao, Zhong-Yue Huang, and Fang-Qiu Zu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b01719 • Publication Date (Web): 19 Jun 2018 Downloaded from http://pubs.acs.org on June 20, 2018

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ACS Applied Materials & Interfaces

The Thermoelectric Performance of Sb2Te3-based Alloy is Improved by Introducing PN Junctions

Xiao-Yu Wanga, Hui-Juan Wangb, Bo Xiangc, Liang-Wei Fud, Hao Zhue, Dong Chaia, Bin Zhua, Yuan Yua,f, Na Gaoa, Zhong-Yue Huanga and Fang-Qiu Zua,*

a

Liquid/Solid Metal Processing Institute, School of Materials Science & Engineering, Hefei

University of Technology, Hefei 230009, China *Corresponding E-mail: [email protected] b

Experimental Center of Engineering and Material Sciences, University of Science and Technology

of China, Hefei 230027, China c

Key Laboratory of Advanced Functional Materials and Devices of Anhui Province, Hefei 230009,

China d

Department of physics, Southern University of Science and Technology, Shenzhen 518055, China

e

Department of Chemistry, School of Chemistry & Materials Science, University of Science and

Technology of China, Hefei 230026, China f

I. Physikalisches Institute (IA), RWTH Aachen, 52074, Aachen, Germany

ABSTRACT：Interface engineering has been demonstrated to be an effective strategy for enhancing the thermoelectric (TE) performance of materials. However, a very typical interface in semiconductors, i.e., the PN junction (PNJ), is scarcely adopted by the thermoelectrical community because of the coexistence of holes and electrons. Interestingly, our explorative results provide a definitively positive case that appropriate PNJs are able to enhance the TE performance of p-type 1

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Sb2Te3-based alloys. Specifically, owing to the formation of the charge-depletion layer and built-in electric field, the carrier concentration and transport can be optimized, and thus the power factor is improved and the electronic thermal conductivity is decreased. Meanwhile, PNJs provide scattering centers for phonons, leading to a reduced lattice thermal conductivity. Consequently, the p-type (Bi2Te3)0.15-(Sb2Te3)0.85 composites comprising PNJs achieve a ~131 % improvement of the ZT value compared with the pure Sb2Te3. The increased ZT demonstrates the feasibility of improving the TE properties by introducing PNJs, which will open a new and effective avenue for designing TE alloys with high-performance. KEYWORDS: thermoelectric material, interface engineering, PN junction, p-type Sb2Te3, thermoelectric performance

1. INTRODUCTION Thermoelectric (TE) materials have attracted ever-increasing attention since their promising applications in the portfolios of low-grade power generation and solid-state refrigeration.1,

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Generally, the dimensionless figure of merit (ZT) is used to assess the TE performance, which is described as ZT=S2σκ-1T, where S, σ, κ and T are the Seebeck coefficient, electrical conductivity, total thermal conductivity and absolute temperature, respectively.1, 3 Hence, an effective TE material should have a high power factor (PF=S2σ) and low κ.4, 5 In the past few decades, intensive efforts have been devoted to improving the ZT of TE materials, in which interface engineering has been confirmed to be an effective strategy via decreasing κ and/or boosting PF.6 For instance, the introduction of nanostructured PbS into PbTe matrix diminished κ, leading to an improved ZT.7 By constructing Bi2Te3-Bi2Se3 heterojunctions, PF of the n-type composites was also significantly

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enhanced from 8.9 µW K-2 cm-1 to 12 µW K-2 cm-1.8 Other nanoparticles, such as Cu9, Ag10, SiC11, C6012 and Cu2Se13 were also applied in forming heterostructures. However, the associated interfaces are mostly PP/NN junctions or interfaces between metals and semiconductors. Reports on the effect of PN junctions (PNJs) on TE properties are scarce, and the underlying mechanism remains unclear. Generally, it is supposed that PNJs in the matrix are detrimental to the ZT enhancement for the following two reasons. On one hand, compounds with PNJs have a low carrier concentration (n) and mobility (µ) owing to the combinations of electrons and holes during the formation of the charge-depletion layer and the impeded transport of carriers by the built-in electric field, which will decrease σ.14 On the other hand, if the two types of carriers coexist in the matrix passing through independent transport channels, a fatal effect would occur on S because the electrons and holes have opposite Seebeck values.15 Consequently, PNJs are seldom introduced into TE materials. However, it is hard to state that PNJs are definitely disadvantageous for TE performance if the distribution and density can be reasonably controlled. Referring to alloys with a high n, especially pure Sb2Te316, SnTe17-20 and GeTe21, 22, PNJs are expected to adjust their n to an optimal range.23 Moreover, carrier transports through PNJs are forbidden by the built-in electric field, i.e., PNJs have a strong energy filtering effect for carriers, which is conducive to promoting S.6, 24, 25 Additionally, it should be noteworthy that the enhanced scattering of phonons by PNJs (heterojunctions) can lead to a decreased lattice thermal conductivity (κlat).26 Hence, boosting ZT by introducing proper PNJs is potentially feasible for materials with a high n. As two classical binary telluride-based TE alloys, n-type Bi2Te3 and p-type Sb2Te3 are both single parabolic band semiconductors. They have the same crystal structure (hexagonal) in the space group of R 3ത m and similar lattice constants.27,

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However, the high n restricts the further

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improvement of ZT for both alloys.29-31 Herein, to provide a positive confirmation of this strategy, we took (Bi2Te3)x-(Sb2Te3)1-x composites as examples to investigate the effect of PNJs on their microstructures and electrical and thermal transport properties. Intriguingly, our results showed that the microstructures were effectively regulated, as well as the simultaneous optimization of PF and κ by introducing PNJs. Eventually, a large improvement of ZT was obtained for the specimen with 15 % Bi2Te3, which was nearly 131 % greater than that of the pure Sb2Te332. This work provides an incentive to explore the effect of PNJs on TE properties and proposes a novel design strategy for TE materials. 2. EXPERIMENTAL SECTION 2.1 Synthesis In this work, highly pure elemental pellets of Bi (4N), Sb (4N) and Te (4N) were weighed according to the nominal compositions of Bi2Te3 and Sb2Te3, and then separately melted at 1023 K for 4 h. After cooling to room temperature, the as-cast ingots of Bi2Te3 and Sb2Te3 were separately subjected to ball milling in a vacuum agate pot at 450 rpm for 3 h. Subsequently, the Bi2Te3 and Sb2Te3 powders weighed according to the stoichiometry of (Bi2Te3)x-(Sb2Te3)1-x (x=0.10, 0.15, 0.20, 0.25) were mixed using a magnetic stirrer in deionized water. Then the mixed powders were dried at 363 K under vacuum. Before sintering into bulks, the mixtures were annealed in a pure H2 atmosphere at 573 K for 3 h. Finally, the annealed powders were sintered by spark plasma sintering (SPS) in a Φ20 mm graphite mold at 673 K for 5 min under an axial pressure of 40 MPa. To confirm the electrical transport behavior of Bi2Te3, pure Bi2Te3 bulk with a size of Φ20×8 mm2 was prepared with the same sintering process. A bar with the size of 2×2×15 mm3 was cut from the bulk. The electrical transport behavior of Bi2Te3 was measured, and the result is shown in

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Figure S1. The negative S indicates that pure Bi2Te3 has an n-type electrical transport behavior.33, 34 Additionally, to confirm the PNJ between Bi2Te3 and Sb2Te3, a sample, of which the upper part was n-type Bi2Te3 and the lower part was p-type Sb2Te3 along the SPS pressure direction, was prepared in the same sintering process. The sample is presented in Figure S2. The detailed fabrication and testing processes can be found in the supporting information (see Section 2, Figure S3). Figure S4 shows the unilateral conduction for the PNJ formed by p-type Sb2Te3 and n-type Bi2Te3. It is clearly observed that the breakdown reverse voltage is 71.85 µV. Hence, the interface consisting of p-type Sb2Te3 and n-type Bi2Te3 is a PN junction. 2.2 Materials characterization The crystal structures of the as-mixed powders and as-sintered bulks were characterized by X-ray diffraction (XRD, D/MAX2500V, Rigaku, Japan) with Cu Kα radiation (λ=0.15418nm). Morphology and elemental ratios of powders and bulks were characterized by field-emission scanning electron microscope (FE-SEM, SU8020, Hitachi, Japan), field-emission transmission electron microscope (FE-TEM, JEM-2100F, Japan) and energy dispersive spectrometer (EDS, Inca, Oxford instruments). 2.3 Thermoelectric properties measurements S and σ were measured from 300 K to 550 K in He atmosphere on a commercial LRS-3 (Linseis, Germany) with a measurement uncertainty of less than ±7% for S and ±3% for σ. Thermal conductivity was calculated by the relationship κ=dCpD, where the density d was measured by the Archimedes method (in Figure S5) and the thermal diffusivity D (in Figure S6) was measured by a laser flash method (LFA-457, Netzsch, Germany, with a measurement uncertainty of less than ± 5%). The calculated specific heat Cp is shown in Figure S7.35 n and µ were estimated from the hall

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resistivity, which was measured in the van der Pauw configuration (Lake Shore 8400, Korea). They were calculated by the equations of n=1/eRH and µ=σ/ne. 3. RESULTS AND DISCUSSIONS 3.1 Structural and chemical characterizations The XRD patterns of the as-mixed powders are shown in Figure S8. All of the peaks are consistent with the Sb2Te3 (JCPDS#15-0874) and Bi2Te3 (JCPDS#15-0863) phases without any peak shift, indicating no solid solution or oxidation after the mixing, drying and annealing processes. To identify the phase configurations of bulk samples, XRD and scanning transmission electron microscope (STEM) detections were carried out. In Figure 1(a), it is easy to observe two sets of peaks in these patterns, which can be indexed to Sb2Te3 (JCPDS#15-0874) and Bi2Te3 (JCPDS#15-0863). That is, the main phase compositions are still p-type Sb2Te3 and n-type Bi2Te3. Figure 1(b) and Figure S9 are STEM images of the bulk composite with x=0.15, in which the dark and light contrasts are observed in different regions. To investigate the composition distribution near these interfaces between the dark zones and light ones, an EDS line scan in a selected interface from grain A to B was carried out, as shown in Figure 1(b). An evident composition contrast is observed, as shown in Figure 1(c). Specifically, the Bi or Sb signals primarily exist on each side of the interface, indicating that grain A is Bi2Te3 and grain B is Sb2Te3. The coexistence of Bi and Sb near the interface may suggest the formation of BiySb2-yTe3 alloys during the SPS process. As is well known, BiySb2-yTe3 alloys can be n-type or p-type with different stoichiometry.36 Combining the measurement results of the unilateral conductivity for the PNJ (presented in Figure S4), we conclude that diffusion has no influence on the formation of PNJs in the matrix. To verify the compositions of bulk composites, EDS analysis was performed. Table S1 shows

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the EDS results of all bulk samples, which demonstrate that the compositions are in accordance with the nominal ones, suggesting that there is no component loss during the fabrication processes.

Figure1. (a) XRD patterns of as-sintered bulks of (Bi2Te3)x-(Sb2Te3)1-x (x=0.10, 0.15, 0.20, 0.25), along with the standard patterns for Bi2Te3 (JCPDS#15-0863, orange lines) and Sb2Te3 (JCPDS#15-0874, dark cyan lines). (b) STEM image showing the interfaces for the sample (Bi2Te3)0.15-(Sb2Te3)0.85 with PNJs. (c) Corresponding EDS profiles for Sb and Bi taken across the Bi2Te3/Sb2Te3 interface from A to B in Figure 1(b). To determine the effect of PNJs on the microstructure of sintered bulks, a SEM investigation was conducted. Figure 2 shows SEM images of the fracture surfaces for the as-sintered composites, in which a dense morphology with typical fine cleavage step patterns is observed. Interestingly, clusters of fine grains are observed among coarse grains. Moreover, the fine grain regions grow in number and size with the increase in the Bi2Te3 content. Figure S10 shows an enlarged SEM image of the fracture surface for bulk composite with x=0.15, which reveals fine grains of several hundred nanometers in size. To unravel this interesting phenomenon, the morphology and main element distribution of the as-mixed powders were characterized by SEM and EDS mapping. In Figure S11, it is clearly observed that powders of all samples have the same morphology, with size distributions spanning from several hundred nanometers to several micrometers. Curiously, grains larger than five micrometers are not observed in Figure S11, but are observed in Figure 2. Namely, the growth of

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grains occurred during the sintering process. As is known, element diffusion in homojunction is easier than that in heterojunction.11, 12 Consequently, the mixed Bi2Te3 in Sb2Te3 (as demonstrated in Figure S12) suppress grain growth during SPS, resulting in broadened fine grain regions.

Figure 2. (a-d) SEM images of the free fracture surface of the as-sintered composites (Bi2Te3)x-(Sb2Te3)1-x (x=0.10, 0.15, 0.20, 0.25 for figures a, b, c and d, respectively). 3.2 Electrical transport properties The temperature dependence of σ from 300 K to 550 K for the bulk samples is illustrated in Figure 3(a). With the increase of Bi2Te3 content, σ significantly decreases over the whole temperature range. Typically, the room temperature σ is ~221×103 S m-1 for the sample with x=0.0032, but only ~105×103 S m-1 and ~82×103 S m-1 for composites with x=0.10 and 0.15. The composite of (Bi2Te3)0.25-(Sb2Te3)0.75 exhibits the lowest σ, ranging from 10.5×103 S m-1 to 22.9×103 S m-1 in the whole temperature range. By striking contrast, a single-phase p-type Bi0.5Sb1.5Te3 9, 30, 37, 38 with the same nominal composition as the composite with x=0.25 has a much higher value of σ (~100×103 S m-1), which demonstrates the strong influence of PNJs on σ.

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Figure 3(b) shows the temperature dependence of S. A positive S of all samples is indicative of a p-type electrical transport behavior. Contrary to the variation trend of σ with x, the value of S increases with the content of Bi2Te3 until x=0.20. The peak S of 226.4 µV K-1 at 328 K is achieved in the bulk with x=0.20. It should be noted that S of the sample with x=0.25 is only 45.3 µV K-1 at 308 K. This phenomenon is rather paradoxical to the general understanding that Bi0.5Sb1.5Te3 has a high S (~200 µV K-1) near room temperature.9, 30, 37, 38 The novel variation of S with the content of Bi2Te3 suggests the unusual effect from PNJs again. As a result, due to the evident increase in S, the PF for the sample with x=0.15 is largely enhanced, reaching the highest value of 26.3 µW K-2 cm-1 at 328 K, as shown in Figure 3(c). Although the composite of (Bi2Te3)0.20-(Sb2Te3)0.80 has the largest S, its σ is relatively poor. Hence, the PF of the sample with x=0.20 is not optimal among these samples.

Figure 3. Temperature dependence of the electrical transport properties of as-sintered samples (Bi2Te3)x-(Sb2Te3)1-x (x=0.0032, 0.10, 0.15, 0.20, 0.25). (a) Electrical conductivity. (b) Seebeck coefficient. (c) Power factor. To understand the variation in electrical transport properties with x, a Hall measurement was carried out, as shown in Table 1. It is observed that n and µ decrease simultaneously with an increase in Bi2Te3. In principle, the reduced n in composites with PNJs is mainly attributed to the combination of holes and electrons during the formation of the carrier-depletion layers,14 as suggested in Figure 4(a) and (b). The decreased µ originates from the forbidden transport of electrons and holes by the

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built-in electric field. Figure 4(c) depicts the band alignment at the interface between the p-type Sb2Te3 and n-type Bi2Te3.39-41 Obviously, an ultrahigh interface potential exists in the PNJ14 and the transport of carriers can only occur along homogeneous junctions, as explained in Figure 4(d). Accordingly, the decreased σ is attributed to the reduced n and µ. Additionally, the rapidly increased S with the increasing Bi2Te3 content mainly results from two aspects. On one hand, since S is negatively related to n, the decrease of n is favorable for the improvement of S. On the other hand, PNJs with an ultrahigh interface potential enhance the energy filtering effect for carriers, which can be confirmed by the increased effective mass m*.11, 42 Figure S13 shows the Pisarenko relation43, 44 of pure Sb2Te332 and samples with PNJs at 300 K. For samples with x=0.10, 0.15 and 0.20, the value of S lies between the theoretical Pisarenko line of m*=0.9m0 and m*=1.2m0, while the S of pure Sb2Te332 can be roughly fitted with a Pisarenko line of m*=0.8m0. Consequently, the rapidly increased S with the increasing Bi2Te3 content mainly results from the reduced n and enhanced energy filtering effect. Exceptionally, the sample with x=0.25 has the lowest n but an exceedingly low S, which is the result of the two types of carriers taking part in the electrical transport, in which S can be expressed as15: S=(σnSn+σpSp)/(σn+σp)

(1)

where Sp and Sn represent contributions of holes and electrons on S, respectively, which have opposite effects on S. σn is the electron conductivity, and σp is the hole conductivity. Since the content of Bi2Te3 is high enough in the sample with x=0.25, there may be two connected regions composed of p-type Sb2Te3 and n-type Bi2Te3, where holes and electrons can separately pass through these regions, as schematically shown in Figure S14(b). Hence, S seriously degenerates for (Bi2Te3)0.25-(Sb2Te3)0.75. By contrast, for other samples, the Bi2Te3 grains are separated by Sb2Te3 particles and the electrons

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are limited in a cage constructed by PNJs (as explained in Figure 4(d)). There are only passageways for holes in the connected regions of Sb2Te3, as presented in Figure S14(a). Table 1.

Characteristic parameters for the as-sintered samples (Bi2Te3)x-(Sb2Te3)1-x (x=0.0032, 0.10,

0.15, 0.20, 0.25) at 308K. Samples

n (×1019 cm-3)

µ (cm2 V-1 s-1)

S (µV K-1)

σ (×103 S m-1)

x=0.0032 x=0.10 x=0.15 x=0.20 x=0.25

7.02 4.01 3.31 1.77 0.86

197.6 163.1 155.5 141.7 76.3

100.61 145.42 177.82 215.32 45.32

220.95 104.57 82.41 25.20 10.49

Figure 4. (a) Distribution of carriers before the formation of PNJ: the cyan dotted line represents the interface of the two phases with different types of electrical transport behavior. (b) Distribution of carriers after the formation of PNJ: the cyan dotted line frame is the charge-depletion layer, and the direction of arrows shows the direction of the built-in electric field. (c) Band alignment at the interface between p-type Sb2Te3 and n-type Bi2Te3. (d) Cross-sectional schematics of the transports for electrons and holes in the case of the left side with a high potential. 11



3.3 Thermal transport properties Figure 5(a) shows the κ of all samples. Compared with the sample of x=0.0032 (~2.495 W m-1 K-1 at 300 K), the κ of the bulk composites with PNJs significantly is decreased. As can be seen, the sample with x=0.25 exhibits the lowest κ, ranging from 0.81 to 1.02 W m-1 K-1 in the entire measurement temperature range. In general, κ contains electronic thermal conductivity (κele), which is determined by the Wiedemann-Franz law45, 46 (κele=LσT, where L is the Lorenz number; the detail calculation is described in the supporting information, Section 12), κlat and the bipolar effect (κbip) 39-41

. Figure 5(b) and (c) present the calculated results of κele and κlat+κbip (κlat+κbip=κ-κele). Ascribed

to the decrease of n and µ with the increase of Bi2Te3, κele of the composites decreases rapidly with the increasing Bi2Te3 content. Compared with the sample of x=0.0032, composites with PNJs have a large number of fine grain regions, resulting in an enhanced phonon scattering from boundaries.47 Hence, κlat+κbip for samples with PNJs significantly decreases compared to that of x=0.0032. Sample (Bi2Te3)0.15-(Sb2Te3)0.85 has the lowest κlat+κbip, with a minimum value of 0.51 W m-1 K-1 at 401 K, which is even lower than that of compound Sb2Te3-PVP (~ 0.97 W m-1 K-1 at the same temperature).48

Figure 5. (a) Temperature dependence of the total thermal conductivity for as-sintered samples (Bi2Te3)x-(Sb2Te3)1-x (x=0.0032, 0.10, 0.15, 0.20, 0.25). (b) Temperature dependence of electronic thermal conductivity calculated by the Wiedemann-Franz law. (c) Temperature dependence of the

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lattice thermal conductivity calculated by κlat+κbip=κ-κele. To further understand the origin of the low κlat+κbip of the bulk composite of (Bi2Te3)0.15-(Sb2Te3)0.85, the microstructure was investigated by TEM. Low magnification TEM images are shown in Figure 6(a) and Figure S16. These images suggest that the matrix is composed of grains with sizes ranging from several hundred nanometers to several micrometers, which is in accordance with the results presented in Figure 2. Figure 6(b) and Figure S17 present two typical types of boundaries in bismuth telluride alloys: the normal grain boundaries and twin boundaries (confirmed by the selected area electron diffraction (SAED) pattern). In Figure 6(c), the SEAD pattern along the [110] zone axis from the twin boundary (boundary 2 in Figure 6(b)) clearly reveals the crystallographic relations between the twin grains: the unsplit reflection [00l] corresponds to the twin plane; the split reflections along the [00l] direction are induced by the deviated planes from two twin grains.31 Figure 6(d) presents a high resolution TEM (HRTEM) image for the normal high-angle (~50°) grain boundary marked by the blue frame in Figure 6(b), and the thickness of the boundary is approximately 1.744 nm. The remarkable twin boundary with a misorientation angle of 60° is presented in Figure 6(e), and the five-layer atomic structure of Te2-Sb-Te1-Sb-Te2 is shown in the inset image. Figure S18 presents a typical low-angle grain boundary with a misorientation angle of ~9.16°. Furthermore, this grain boundary is also a semi-coherent interface that extends to both grains along the (015) plane. The dislocation array is clearly observed in the IFFT image of the region around the boundary. According to our previous works,49,

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twin boundaries and

semi-coherent interfaces are beneficial for the scattering of phonons, while with little sacrifice in electrical conductivity. Figure 6(f) shows an HRTEM image for the Sb2Te3 matrix in the composite of (Bi2Te3)0.15-(Sb2Te3)0.85. The inverse fast Fourier transform (IFFT) image of the yellow dotted line

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region in Figure 6(f) clearly demonstrates the lattice defects. Statistically, lattice dislocations, demarcated by the yellow line ovals in Figure 6(g), are observed. In all, the characteristic length scales of these microstructures are comparable to the estimated wavelength of heat-carrying phonons,51 allowing multi-scale phonons to be effectively scattered. Hence, a low κlat+κbip could be obtained in the sample with x=0.15.

Figure. 6. (a) Low magnification TEM images of the bulk sample (Bi2Te3)0.15-(Sb2Te3)0.85. (b) Low magnification TEM images with two typical types of boundaries in bismuth telluride alloys. Boundary 1 is a normal grain boundary, and boundary 2 is a twin boundary. (c) The selected area electron diffraction pattern along the [110] zone axis from the twin boundary. (d) HRTEM image of the normal grain boundary with a misorientation angle of 50° marked by the blue frame in Figure 6(b). (e) HRTEM image of the twin boundary marked by the red frame in Figure 6(b), where the 14


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inset is an enlarged image showing the five-layer atomic structure. (f) HRTEM image of the p-type Sb2Te3 matrix in the composite of (Bi2Te3)0.15-(Sb2Te3)0.85. (g) IFFT image of the zone characterized by the yellow dotted line box in Figure 6(f). 3.4 Dimensionless figure of merit: ZT According to the measurement of the electrical and thermal transport properties, ZT of the bulk is obtained from room temperature to 550 K, as shown in Figure 7(a). Due to the simultaneous optimization in electrical and thermal transport properties, a high ZT of ~1.03 at 401 K is obtained in (Bi2Te3)0.15-(Sb2Te3)0.85, which is almost a 131 % improvement compared with the value of Sb2Te332. Due to the serious degradation of the electrical transport properties, the maximum ZT value of (Bi2Te3)0.25-(Sb2Te3)0.75 is only ~0.06 at 550 K. Furthermore, there is also a significant improvement for the average ZT (ZTave) in the whole measuring temperature range, as shown in Figure S19. The ZTave value for the sample with x=0.15 is 0.89, an approximately 129 % improvement compared with that of the x=0.00 sample32. Figure 7(b) presents ZTmax of p-type Sb2Te3-based alloys fabricated by different methods. It is observed that our method has a high ZTmax in these methods. Due to the large grain size in the Sb2Te3 ingots fabricated by zoon melting (ZM), the κ is relatively high and ZTmax is only 0.4 at 547K.32 Further improvement of ZTmax (~0.92) is achieved for the reduced κ by In-Ag co-alloying with subsequent ball milling (BM) and hot pressing (HP).32 Although Sb2Te3-based nanoparticles can be prepared via a surfactant-assisted reflux (SAR) method48, microwave-assisted (MA) method52, 53, hydrothermal method28 or solvothermal method54, and be kept in bulks by cold compaction with sintering (CCS) 48, 53 or SPS28, 52, 54, the degenerated electrical transport properties restrict the further improvement of ZT. In this work, the optimization of PF and κ is easily obtained via introducing

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proper PNJs, resulting in enhanced ZT.

Figure 7. (a) Temperature dependence of ZT for as-synthetized samples (Bi2Te3)x-(Sb2Te3)1-x (x=0.0032, 0.10, 0.15, 0.20, 0.25). (b) State-of-the-art ZT values for p-type Sb2Te3-based alloys prepared by different methods28, 32, 48, 52-54. 4. CONCLUSIONS AND OUTLOOK In this study, we provided a positive confirming case that introducing PNJs can significantly improve the TE performance of Sb2Te3-based alloys and revealed the mechanism of PNJs on the TE properties. Via introducing proper PNJs into the matrix, an evidently enhanced ZT (1.03 at 401 K) and ZTave (0.89 between 300 K and 550 K) are achieved in the composite of (Bi2Te3)0.15-(Sb2Te3)0.85, whose ZT and ZTave are, respectively, nearly 131 % and 129 % higher than those of the pure Sb2Te332. The significantly enhanced ZT originates from the improved S and reduced κ. Specifically, the increased S occurs due to two reasons, as follows: a) the optimized n via the charge-depletion layer; b) the enhanced energy filtering effect from the built-in electric field. Alternatively, the reduced κ is mainly caused by the decreased κele and κlat+κbip. Ascribed to the reduction in n and µ, κele significantly decreases. Meanwhile, the decreased κlat+κbip is attributed to the enhanced scattering of the phonons from PNJs and other scattering cores, including nanoscale grains, boundaries and atomic-scale lattice dislocations. The results of this study strongly demonstrate the possibility of 16


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improving TE properties by introducing PNJs and indicate a new way to enhance the performance of TE materials.

ACKNOWLEDGEMENTS This work was supported by the National Natural Science Foundation of China (Grant No. 51371073), the National Key Basic Research Program of China (2012CB825702) and the Natural Science Foundation of Anhui Province (Grant No.1808085ME108). Dr. Xiao-Lan Xue from Nanjing University was greatly acknowledged for her proof reading the article.

ASSOCIATED CONTENT Supporting Information Data of the electrical transport properties for the pure Bi2Te3, the preparation and measurement of the unilateral conductivity for the Bi2Te3-Sb2Te3 bulk, the measurement of the density and thermal diffusivity for the bulk samples, the calculation of the specific heat for the bulk samples, the XRD patterns for the as-mixed powders, the additional STEM/SEM images and the EDS results of the bulk and powders samples, the Pisarenko relation of pure Sb2Te3 and samples with PNJs at 300K, the scheme of passageways of carriers in the matrix, the calculation of the Lorenz number, the additional TEM images of the bulk sample (Bi2Te3)0.15-(Sb2Te3)0.85 and the calculation of ZTave. This material is available free of charge via the Internet at http://pubs.acs.org.

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