Enhanced thermoelectric performance of n-type PbTe through the introduction of low-dimensional C60 nanodots

Journal Pre-proof Enhanced thermoelectric performance of n-type PbTe through the introduction of lowdimensional C60 nanodots Huan He, Wenbin Qiu, Zhengshang Wang, Xudong Cui, Yan Zhang, Zhengguo Wang, Longqing Chen, Hao Deng, Yixiang Sun, Liuwei Zhao, Xiaochong Liang, Jun Tang PII:

S0925-8388(20)30226-7

DOI:

https://doi.org/10.1016/j.jallcom.2020.153863

Reference:

JALCOM 153863

To appear in:

Journal of Alloys and Compounds

Received Date: 6 November 2019 Revised Date:

31 December 2019

Accepted Date: 14 January 2020

Please cite this article as: H. He, W. Qiu, Z. Wang, X. Cui, Y. Zhang, Z. Wang, L. Chen, H. Deng, Y. Sun, L. Zhao, X. Liang, J. Tang, Enhanced thermoelectric performance of n-type PbTe through the introduction of low-dimensional C60 nanodots, Journal of Alloys and Compounds (2020), doi: https:// doi.org/10.1016/j.jallcom.2020.153863. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2020 Published by Elsevier B.V.

Enhanced thermoelectric performance of n-type PbTe through the introduction of low-dimensional C60 nanodots Huan He1, †, Wenbin Qiu2, †, Zhengshang Wang3, Xudong Cui3, Yan Zhang4, Zhengguo Wang4, Longqing Chen1, 2, Hao Deng2, Yixiang Sun2, Liuwei Zhao1, Xiaochong Liang1, Jun Tang1, 2, * 1

College of Physics, Sichuan University, Chengdu 610065, China Key Laboratory of Radiation Physics and Technology of Ministry of Education, Institute of Nuclear Science and Technology, Sichuan University, Chengdu 610064, China 3 Sichuan Research Center of New Materials, Institute of Chemical Materials, China Academy of Engineering Physics, 596 Yinhe Road, Shuangliu, Chengdu 610200, China 4 International Center for Quantum Materials, School of Physics, Peking University, Beijing 100871, China 2

Abstract Lead telluride (PbTe) has long been considered as an ideal p-type thermoelectric material at an intermediate temperature range. However, the relatively low thermoelectric performance of n-type PbTe largely limits the commercial applications of integral PbTe devices. In current work, we report that a significant enhancement of the ZT value of ≈ 1.3 can be achieved at 823 K in PbTe0.998I0.002-0.5%C60 by adding low-dimensional C60 nanodots. This remarkable improvement in thermoelectric performance is attributed to the incorporation of C60 in n-type PbTe matrix, which creates dense nanodots that can simultaneously manipulate electron and phonon transport. On one hand, the dispersion of C60 nanodots in n-type PbTe matrix leads to highly depressed lattice thermal conductivity (κlat) (~ 52%) due to the refinement of grains and the extra phonon scattering centers. On the other hand, the introduction of C60 nanodots increases the scattering parameter rx, and brings about the overall improvement of Seebeck coefficient S, especially at room temperature. This work demonstrates the great potential of low-dimensional dopant in optimizing PbTe thermoelectric materials, which should be equally applicable in improving the performance of other thermoelectric materials.

†

Equal contribution * Correspondence: [email protected] Keywords: Thermoelectric materials, n-type PbTe; Fullerene; Nanodots 1. Introduction Thermoelectric (TE) materials are well known for enabling a direct and reverse

conversion of heat to electricity without unnecessary vibration or waste emission[1-4]. Unfortunately, a wide application of this eco-friendly energy technology is limited by their low conversion efficiency[5-7]. The conversion efficiency is determined by the dimensionless figure of merit, ZT = σS2T/(κele+κlat), where σ is the electrical conductivity; S is the Seebeck coefficient; T is the absolute temperature in Kelvin; κele and κlat are the lattice and electronic components of the thermal conductivity, respectively[8-11]. These thermoelectric parameters are internally coupled, thus it is hard to optimize a certain property independently without affecting other parameters[12, 13]. Lead telluride (PbTe), as one of the most promising thermoelectric materials, possesses the outstanding ZT values over the intermediate temperature range (500– 800 K)[14]. In the past few years, numerous achievements in p-type PbTe realized by band convergence[15], energy barrier filtering[16], and all-scale hierarchical architecturing[17]. Unfortunately, the thermoelectric performance of n-type PbTe materials is relatively lower than their p-type counterparts, which hinders the large-scale

application

of

PbTe[18,

19].

Therefore,

the

development

of

high-performance n-type PbTe materials is highly focused on[20]. Various strategies, including

introducing

point

defects[21],

nanostructuring[22],

and

reducing

dimensional[23], have been applied to improve the thermoelectric performance of n-type PbTe. Most of these approaches aim at maintaining a higher power factor PF or a very low lattice thermal conductivity κlat. However, it is rarely reported about the synergistic manipulation of electron and phonon transport through a simple dopant[24, 25]. Fullerene molecules (C60), as a zero-dimensional nanodots, have very high elastic modulus[26], indicating that C60-decorated grain boundaries would scatter phonon

sharply and reduce the lattice thermal conductivity κlat. X. Shi[27] et al. reported that the maximum ZT = 1.3 at 850 K can be obtained in the BayCo4Sb12-based composites with the incorporation of 0.43% C60, which was 53% higher than that of C60-free sample. Similar C60-induced improvement in thermoelectric performance was also found by Zhao[28] et al. in their C60-contained Bi0.5Sb1.5Te3 alloy with a ZT value of 1.47 at 358 K, which is originated from a half-cut thermal conductivity. Since C60 nanodots can scatter phonon to decrease κlat, it would also modify the transport of electron. This motivates us to seek for a possible avenue to enhancing ZT values by nanodots engineering. In this work, we report the remarkable improvement of ZT by adding low-dimensional C60 nanodots into the n-type PbTe0.998I0.002 samples, due to their simultaneous modulation of electron and phonon. The C60 nanodots not only provide an extra phonon scattering center coupled with the refinement of grains, leading to the apparent reduction of κlat, but also promotes S by increasing the scattering parameter rx. Eventually, the maximum ZT value reaches ~ 1.3 at 823 K for PbTe0.998I0.002-0.5%C60. The average ZTave reaches ~ 0.7, and the calculated conversion efficiency η reaches 11.5% in the temperature range of 323-823 K. 2. Experiment Section In this work, n-type PbTe0.998I0.002-C60 composites were synthesized by a combination of the convention solid-state reaction and ball milling techniques. Pb (99.99%, 1-3 mm), Te particles (99.99%, 0.5-2 mm), PbI2 (99.9%) and fullerene (C60, 99.9%) were firstly mixed based on stoichiometric ratios and sealed in evacuated quartz tubes. The tubes were placed in a muffle furnace, slowly heated up to 1223 K, dwelled for 7 hours, and air quenched to room temperature. Afterwards, the samples are sealed again in evacuated quartz tubes and post-annealed at 923 K for another 3 days. The obtained PbTe0.998I0.002 ingots were hand-pulverized and then fully ball-milled (QM-DY2, 400 rpm, 5 hours) with certain amount of C60 under argon atmosphere. Subsequently, the as-milled powders were sieved, poured into a graphite die (φ = 12.7 mm), and consolidated by spark plasma sintering (SPS, LABOX-325, Japan) at 873 K for 10 min under 50 MPa. As a result, highly dense samples with a

theoretical density higher than 96% could be obtained. The detailed specifications of as-prepared samples including mass density are given in supplementary information. Powder X-ray diffraction (PXRD) measurements with Cu-Kα radiation were performed on an X-ray diffractometer (DX-2700) to identify structural information. The Raman spectra were measured by using a LabRAM HR spectrometer, with the 633 nm wavelength He-Ne laser as the excitation source. The surface morphologies were characterized by a scanning electron microscope (SEM). The temperature dependence of electrical conductivity σ and Seebeck coefficient S were measured by using a CTA pro instrument (Beijing Cryoall Science and Technology Co. Ltd). The thermal diffusivity D was acquired by the laser flash method (LFA 457, Netzsch). The heat capacity Cp for the lead chalcogenide composites was determined based on the measurements of Blachnik by Cp(kB/atom) = 3.07 + 0.00047(T/K - 300)[29]. The Hall coefficient RH and carrier concentration n were determined by Hall measurements conducted in a Physical Property Measurement System (PPMS, Quantum Design). For investigating the electronic information near Fermi level, we carried out photoemission spectroscopy (PES) measurements by utilizing an analyzer (model DA30L) with a helium discharging lamp. The phonon energy, pass energy and energy resolution were 21.2 eV, 10 eV and 15 meV, respectively. The entire system was kept in ultra-high vacuum state better than 8×10-11 mbar. The surface of polycrystalline specimens for PES was carefully treated by mechanically polishing superficial layers by a wobble stick. As a result, a clean surface was able to be exposed to the evacuated chamber for subsequent PES measurements.

3. Results and discussion

Figure 1. (a) PXRD patterns of PbTe0.998I0.002-x%C60 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) composites. The right panel shows no peak shift of (200) diffraction peaks for all samples, but gradual broaden is observed with the increase of C60 content. (b) Raman patterns of C60 molecules, PbTe0.998I0.002, and PbTe0.998I0.002-0.5%C60, respectively.

To unveil structural properties, powder X-ray diffraction (PXRD) measurements were performed upon grinded bulk samples after SPS process, as shown in Figure 1a. All the diffraction peaks were in good agreement with face-centre-cubic PbTe (PDF#38-1435) without any distinct peak of C60 due to its low content. Furthermore,

the 2θ values of diffraction peaks were almost identical regardless of the amount of C60 additions, indicating that C60 did not enter the PbTe lattice but act as nanodots instead. Moreover, it was noted that all characteristic peaks gradually broadened as the C60 content increased (inset of Figure 1a), which should result from the refinement of crystal grains. In addition, the Raman spectra for C60 molecules, PbTe0.998I0.002, and PbTe0.998I0.002-0.5%C60 samples are illustrated in Figure 1b. The characteristic

peaks

of

C60

were

clearly

observed

in

the

sample

of

PbTe0.998I0.002-0.5%C60 while absent in the pristine PbTe matrix. Hereto, the combined results of PXRD and Raman spectra demonstrated the existence of C60 in the samples and the unchanged band structure.

Figure 2. SEM images of the fracture of the n-type PbTe0.998I0.002-x%C60 bulk samples: (a) x = 0; (b) x = 0.1; (c) x = 0.2; (d) x = 0.3; (e) x = 0.4; (f) x = 0.5.

Microstructural and compositional information for the fractured surface of PbTe0.998I0.002-x% C60 (x = 0, 0.1, 0.2, 0.3, 0.4, 0.5) are shown in Figure 2a~f. Obviously, the grains got gradually refined upon the increasing of C60 content. The average grain sizes decreased from approximately 3 µm in pristine PbTe matrix down to 60 nm in the one with 0.5% of C60 addition (Figure S1). This phenomenon is highly consistent with the work of Gothard[30] et al. that C60 effectively promotes the grain refinement of Bi2Te3 alloys even the amount is small. Hence, we consider that C60 nanodots could impede the aggregation of PbTe grains and further smash the

particles during ball milling process. As a result, grain refinement provides more grain boundaries, which is expected to enhance phonon scattering and thereby reduce κlat.

Figure 3. Thermal transport properties of PbTe0.998I0.002-x%C60. Temperature dependence of (a) total thermal conductivity κtot; (b) electrical thermal conductivity κele; (c) lattice thermal conductivity κlat. The dashed represents the minimal κlat, min. (d) κlat as a function of temperature for the samples of x = 0 and x = 0.5. The dashed line and dotted line represent the theoretical results for x = 0 and x = 0.5 based on Debye model, respectively.

To investigate the effect of C60 nanodots on the thermal properties of composites, the thermal parameters were measured. The detailed heat capacity (Cp), thermal diffusivity (D), and Lorenz numbers L are shown in supporting information (Figure S2a-2c). Apparently, the total thermal conductivity (κtot) reveals a significant decrease with increasing C60 fraction, as shown in Figure 3a. The κlat and κele subtracted from κtot are depicted in Figure 3b and 3c. Apparently, the κlat decreases with the increase

of C60 content and the decrease of grain sizes, which accords well with the PXRD and SEM results. Especially for sample with x = 0.5, the κlat value of ~0.40 W m-1 K-1 at 823 K almost reaches the theoretical minimum thermal conductivity κlat, min[19] value of ∼0.36 W m-1 K-1. The extremely low κlat motivates us to explore the origination of the remarkable κlat reduction since grain boundary scattering itself is not significant enough to suppress κlat down to amorphous limit. Therefore, a well-accepted Debye model is applied to unveil the essential origin of the extraordinary κlat reduction:

κ lat =

k B  k BT    2π 2ν  h 

3

∫

θ D /T

0

τc ( x)

x 4e x

( e x − 1)

2

(1)

dx

where kB is the Boltzmann constant, ћ is Plank’s constant, ν is the average sound velocity, θD is the Debye temperature, τc is combined relaxation time, and x is defined as x = ћω/kBT. Apparently, as shown in Figure 3d, the experimental values for x = 0 match well with the theoretical results (dashed line), which considers the Umklapp process,

normal

process,

and

point

defects

(UN+PD)

as

the

dominant

phonon-scattering sources, verifying the accuracy of our simulation. However, further calculations demonstrate that the experimental values for x = 0.5 are obviously lower than the simulated results (dotted line, UN+PD+B) even after we take additional grain boundary scattering (B) into consideration. Consequently, we speculate that the extra phonon scattering centers are provided by the barriers between the low-dimensional C60 nanodots and the PbTe matrix, which leads to the conspicuous reduction of κlat. Our findings confirm that C60 nanodots enable the apparent κlat reduction via the synergistic effect of grain refinement and extra phonon-scattering mechanisms, which represents an effective and important step toward high-performance thermoelectric materials.

Figure 4. (a) The carrier concentration n as a function of the C60 content at room temperature (RT) 300 K. (b) Schematic of the transfer of electrons from the C60 nanodots to the PbTe matrix; χ is the electron affinity of PbTe. W1 and W2 are the work functions of PbTe and C60, EF1 and EF2 are the Fermi levels of PbTe and C60, respectively. (c) PES spectra of PbTe0.998I0.002-x%C60 (x = 0 and 0.5) at 300 K; (d) Enlarged PES spectra near Fermi level for x = 0, 0.5. The inset demonstrates the raise of Fermi level after C60 incorporation.

Since C60 nanodots provide more scattering mechanisms for phonon, they would also affect electron transport. In order to reveal the influence of C60 nanodots on electron scattering, we firstly measured the carrier concentration n as shown in Figure 4a. The carrier concentration n increases slightly at room temperature (RT) with the rising content of C60. According to the theory of semiconductor contact[31], a charge transfer process is occurring from composites with small work function to those with

a large one. Hence, as plotted in Figure 4b, we can deduce that the electrons transport from C60 to PbTe matrix, considering that the former’s work function (W2, ~4.53 eV)[32] is lower than the latter’s (W1). To clarify, the value of W1 is calculated by W1 = χ +(CB-EF1), where χ (~4.60 eV) is the electron affinity of PbTe[33], and CB-EF1 is the energy difference between the bottom of conduction band and Fermi level, which is positive in the case of n-type PbTe composite[5]. To further explore the electronic structure upon C60 addition, we performed angle-integrated photoemission spectroscopy (PES) at room temperature for PbTe0.998I0.002-x%C60 (x = 0, 0.5). This technique has been successfully employed in studying the impurity levels of Tl-doped PbTe due to the capability of roughly reflecting valence-band density of states (DOS)[34]. As shown in Figure 4c, the peak around E-EF = -1 eV represents the DOS maximum of valence band, denoting the approximate position of the valence band. Notably, we find an energy shift ~ 0.3 eV between the positions of two valance bands[35]. Considering the band structure is not changed as C60 does not enter the lattice of PbTe, it is the ascent of Fermi level (E-EF = 0) by ~ 0.3 eV that leads to the energy shift, rather than the position movement of either valance band or conduction band. The schematic is illustrated in the inset of Figure 4d. In addition, from Figure 4d, the sample of x = 0.5 shows a raised intensity at the position of Fermi level, representing that the position of the conduction band is in the vicinity of Fermi level. This phenomenon is another solid proof that evidences the raise of Fermi level. As a result, it reflects that more electron transfer to the conduction band, thus increases the carrier concentration n.

Figure 5. Temperature dependence of (a) electrical conductivity σ; (b) carrier mobility µ. (c) Seebeck coefficient S; (d) Seebeck coefficient S and scattering parameter rx as a function of the C60 content at 323 K.

The electrical properties of the samples are discussed in Figure 5, including electrical conductivity σ, carrier mobility µ, and Seebeck coefficients S. Evidently, the introduction of C60 nanodots substantially reduces σ, especially at room temperature, as shown in Figure 5a. As µ dramatically decreases (Figure 5b) and n slightly increases (Figure 4a) at room temperature upon the increment of C60 content, we can thus conclude that the decrease of σ is derived from the decline of µ based on the equation of σ = neµ. Note the µ for sample with x = 0 shows a temperature dependence of T-1.5, which indicates that acoustic phonon dominates carrier scattering. However, the µ of C60-contained composites gradually deviated from T-1.5 at a relatively high C60 content, suggesting the existence of other scattering sources such as grain boundaries or barriers created by C60 nanodots. Figure 5c shows the temperature dependence of Seebeck coefficients S in this work. Distinctly higher S

values are observed in C60-contained samples compared with the pristine PbTe matrix. As discussed before, C60 neither enters into the lattice nor alters the band structure of PbTe. Instead, they act as nanodots that would modify the electronic transport behavior. Hence, the additional scattering mechanism induced by C60 nanodots leads to the variation in the scattering parameter (rx) of C60-contained samples without affecting the effective mass (m*). Since acoustic phonon scattering is the dominant carrier scattering mechanism, the scattering parameter (r0) of the matrix should be -1/2 at room temperature. Actually, rx can be obtained according to the equation[36]: S = 8π 2 k B2

( 3eh ) m * T π ( 3n ) ( r 23

2

x

+ 1)

(2)

Here, the values of S and n are measured at room temperature, and kB is the Boltzmann constant, h is the Planck constant. Therefore, the ratio of the scattering parameter rx+1 for PbTe0.998I0.002-x%C60 to r0+1 for the matrix can be expressed as[37]:

rx + 1 S x  nx  =   r0 + 1 S0  n0 

23

(3)

where S0 (Sx) and n0 (nx) are the S and n values of the sample of PbTe0.998I0.002-x%C60 measured at room temperature. As shown in Figure 5d, the value of rx increases gradually as the C60 content increases from 0 to 0.5%. The calculated results are listed in Table S1. It is known that n, which has a negative correlation with S, increases after C60 incorporation. Therefore, the enhancement in S is attributed to the enlarged rx by adding C60 nanodots into the sample of PbTe0.998I0.002.

Figure 6. (a) The ratio of quality factor (B/B0) in C60-contained samples to that in x = 0. B0 denotes the quality factor for the sample with x = 0. Temperature dependence of (b) ZT values in this work; (c) ZT values of PbTe-La[38], PbTe-1%MgTe[39], Pb0.995Sn0.005Te0.847Se0.15I0.003[40], and x = 0.5 (this work). (d) The average ZTave and the calculated conversion efficiency η with different C60 contents.

Since the introduction of C60 nanodots induces both positive (increasing S, meanwhile decreasing κlat) and negative (reducing µ) effects, it is essential to find out the optimum content for the best thermoelectric performance. For all concerned samples in this work, we calculate the dimensionless quality factor (B) which was first proposed by Chasmar and Stratton[41]. The B value is defined as: 32  m*  k  2e ( k BT ) µ0 m * B= B  = 4.3223 ×10−6 µ0   32 3 κ lat  e  ( 2π ) h  me  2

52

32

T5 2

κ lat

(4)

Here, µ0 is the carrier mobility, m* is the effective mass, me is the electronic mass, and the κlat is the lattice thermal conductivities. In Figure 6a, B0 denotes the quality factor for the sample with x = 0, and the ratio of quality factor (B/B0) is plotted. Obviously,

the quality factor has a positive correlation with C60 content. PbTe0.998I0.002-0.5%C60 possesses the highest B value, implying a larger possibility of a high ZT value. As expected, it is consistent with the calculated ZT results in Figure 6b. For PbTe0.998I0.002-0.5%C60, the maximum ZT reaches ~1.3 at 823 K, exhibiting an enhancement of 62.5% compared to the inherent ZT value (~0.8). Such high performance for x = 0.5 sample is superior to many other PbTe systems[38-40] (Figure 6c). This enhancement is attributed to the speculation that C60 nanodots simultaneously manipulate thermal and electric properties, leading to the significant reduction of κlat and the overall increment of S throughout the entire temperature range. In addition, we conducted a repetitive test on the three different PbTe0.998I0.002-0.5%C60 samples, as shown as in Figure S3. Good experimental repeatability was confirmed for PbTe0.998I0.002-0.5%C60 sample. It should be noticed that the average ZTave and the thermoelectric conversion efficiency η are closely related to the thermoelectric applications. Hence, we calculated ZTave in the temperature range from 323 to 823 K, as well as the efficiency η, as shown in Figure 6d. The average ZTave is ~ 0.7 and η is ~11.5% for the high-performance PbTe0.998I0.002-0.5%C60. 4. Conclusions In conclusion, the incorporation of C60 nanodots into PbTe not only refines the grain sizes but also triggers additional scattering mechanisms to decrease the lattice thermal conductivity. Meanwhile, n is enhanced due to charge transferring from C60 nanodots to PbTe. Furthermore, C60 nanodots increase the scattering parameter rx, which leads to the enhancement of S. As a consequence, we achieve a significant improvement of ZT value ~1.3 at 823 K for PbTe0.998I0.002 -0.5%C60, as well as the average ZTave of ~ 0.7 and calculated conversion efficiency η of ~11.5% in a wide temperature range of 323-823 K. Our work demonstrates the synergistic optimizing of electric and thermal properties by C60 nanodots, which paves the way for exploring high-performance thermoelectric materials. Acknowledgements This work was supported by the National Science Foundation of China (NSFC)

[grant numbers 91421107, 11574004]. The authors thank Dr. Qingshuang Ma from Tianjin University for performing Hall measurements.

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Highlights 1. C60 nanodots can simultaneously manipulate electron and phonon transport. 2. C60 nanodots refines the grains to decrease the κlat. 3. C60 nanodots triggers additional scattering mechanisms to further reduce the κlat. 4. The introduction of C60 nanodots enhances the S by increasing the rx. 5. The C60-contained sample has a 45% enhancement in ZT maximum.

Credit author statement Huan He: Conceptualization, Methodology, Software Priya Singh, Data curation, Writing- Original draft preparation. Wenbin Qiu: Formal analysis, Visualization, Investigation. Zhengshang Wang: Formal analysis, Review. Xudong Cui: Supervision. Yan zhang: Resources, Funding acquisition. Zhengguo Wang: Formal analysis, Supervision. Longqing Chen: Investigation. Hao Deng: Investigation. Yixiang Sun: Experiment. Liuwei Zhao: Experiment. Xiaochong Liang: Software. Jun Tang: Review, Resources, Supervision, Funding acquisition.

Declaration of interests1 ☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. ☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: