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Cite This: Chem. Mater. 2018, 30, 5539−5543

Observation of High Seebeck Coefficient and Low Thermal Conductivity in [SrO]-Intercalated CuSbSe2 Compound Kejun Bu,†,‡ Jian Huang,† Mengjia Luo,†,‡ Mengjia Guan,†,‡ Chong Zheng,# Jie Pan,† Xian Zhang,§ Sishun Wang,† Wei Zhao,† Xun Shi,† Li Xu,∥ and Fuqiang Huang*,†,§

Chem. Mater. 2018.30:5539-5543. Downloaded from pubs.acs.org by UNIV OF SOUTH DAKOTA on 08/29/18. For personal use only.

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State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, China ‡ University of Chinese Academy of Sciences, Beijing 100049, China § State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China # Department of Chemistry and Biochemistry, Northern Illinois University, DeKalb, Illinois 60115, United States ∥ Material Laboratory of State Grid Corporation of China, State Key laboratory of Advanced Transmission Technology, Global Energy Interconnection Research Institute Co., Ltd., Beijing 102209, China S Supporting Information *

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hermoelectric (TE) materials offer a potential for applications as power generators because of capability of creating electricity from waste heat sources.1,2 The efficiency of a TE material is characterized by a dimensionless figure of merit ZT = S2σT/k, where σ, S, k, and T are electrical conductivity, Seebeck coefficient, total thermal conductivity, and absolute temperature, respectively.3 Most recent efforts in TE materials have focused on modified traditional TE materials. Since then, there has been few new materials with enhanced ZT because of rare combination of interdependent electrical and thermal transport properties required to be present in a single material.4 Another alternative strategy to achieve high TE efficiency is to enhance S and lower k. In pursuit of this goal, we report a family of multicomponent materials which are based on different structural/functional units and have a potential to be excellent TE materials with high S and low k.5 Some layered and quasi-layered materials in this family feature separated electronic conduction units (ECU) and electronic insulation units (EIU) (or act as phonon scattering units (PSU)). These materials, including SnSe,6 Bi2Te3,7 Cu2CdSnSe4,5 CsM2Bi3Te7 (M = Pb, Sn),8 CuSbSe2,9 and BiCuSeO (Figure 1),10 show outstanding TE performance. The discovery of them motivates us to design new multicomponent TE compounds with unique ECU and EIU in order for us to explore their potential TE properties. The chalcostibite CuSbSe2 (D16 2h, Pnma) of the Cu/Sb/Se (CASe) system has a typical 2D layered structure. It is a promising p-type TE material with CuSbSe2 layers and van der Waals (VDW) gaps in between, as illustrated in Figure 1. It exhibits an intrinsically very low k (0.38 W·m−1·K−1 at 625 K) and an extremely high S (535 μV·K−1 at 625 K), making it a potential TE material for middle temperature applications.9 In CuSbSe2, the 5s2 lone pair electrons of Sb3+ are responsible for the soft phonon modes and strong vibrational anharmonicity in the lattice, which can lead to a low k.11,12 More interestingly, the layers in CuSbSe2 confine the charge carriers in 2D and result in a high S about 535 μV·K−1 at 625 K.9 We consider the VDW gap of CuSbSe2 as an important key to achieve a high ZT value. With this thought, we designed a new compound by © 2018 American Chemical Society

Figure 1. Top: Schematics of ECU and EIU in two well-known TE materials, BiCuSeO, and Cu2CdSnSe4. Bottom: The van der Waals gap between two [CuSbSe2] layers of CuSbSe2 compound and the M2+O2− layer in M2+O2−CuSbSe2.

intercalating an ionic layer into the VDW gap of CuSbSe2. The ionic layer consists of alkaline earth metals (M2+, M = Mg, Ca, Sr, Ba) and oxygen ions. The new M2+O2−component is expected to not only enhance S by acting as insulating layer but also lower k by serving as a thermal dispersive layer (Figure 1). For the reasons stated, we synthesized a new quasi-2D compound, SrOCuSbSe2, the structure of which resembles that of many excellent layered TE materials. It shows a clear separation between the ECU (CuSbSe2 layers) and EIU (SrO layers) units. The as-synthesized SrOCuSbSe2 compound possesses an anisotropic TE properties with very low k (k = 0.38 W·m−1·K−1 at 850 K) and extremely high S (S = 522 μV· K−1 at 850 K). As a result, its maximum ZT is 0.22 at 850 K. Received: June 25, 2018 Revised: August 6, 2018 Published: August 6, 2018 5539

DOI: 10.1021/acs.chemmater.8b02651 Chem. Mater. 2018, 30, 5539−5543

Communication

Chemistry of Materials

Figure 2. (a) 2D 2∞[CuSbSe2O]2− layers of SrOCuSbSe2 along the b direction, the Sr2+ cations are located between the layers. (b) High-resolution TEM image of SrOCuSbSe2 sample. (Inset) Select area electron diffraction from the FIB region. (c) The atomic model based on the spherical aberration corrected TEM. (d) Experimental PXRD pattern (black line) and simulated pattern (red line) of SrOCuSbSe2 compound. (e) The solidstate UV−vis-NIR absorption spectra (inset) and energy band gap of SrOCuSbSe2. (f) Temperature variation of the resistivity for SrOCuSbSe2. (Insets) Plots of ln(ρ) vs T−1/4 (VRH model) and ln(ρ/T) vs 1/T (SPH model).

S3). Furthermore, the carrier concentration (nh) and hall mobility (μh) are 5.4 × 1018 cm−3 and 0.235 cm2·V−1·s−1 at room temperature, respectively. The TG-DTA results show that SrOCuSbSe2 has slight (1.16%) and heavy (8.85%) weight losses around 100 and 787 °C, respectively. These losses are considered as the process of water evaporation and decomposition of the compound (Figure S5). PXRD data on the residue after the thermal analysis measurements are shown in Figure S6. Consequently, compared to the CuSbSe2, thermal stability of the compound is highly enhanced by intercalation of SrO layers. The TE properties as a function of temperature for SrOCuSbSe2 are shown in Figure 3. The in-plane σ (perpendicular to the SPS pressure direction) is higher than that in the c direction (Figure 3a), showing a typical concept behavior of the ECU/EIU layered compound that confines the migration of high charge carriers in the ab plane. With the increase of temperature, the σ of SrOCuSbSe2 increases by 3 orders of magnitude from 0.1234 S·m−1 (300 K) to 366.24 S· m−1 (850 K) (Figure 3a). The increase of σ may be owing to the creation of holes at high temperature, which is generated from the hybridization of the Cu-3d with the Se-4p orbitals near the valence band maximum (VBM). Similar phenomena have also been reported in other compounds, such as Cudoped Cu2CdSnSe4,5 Cu3SbSe4,13 and Cu22Fe8Ge4S32.14 As shown in Figure 3b, SrOCuSbSe2 behaves as a p-type semiconductor, verified by the positive value of S. At 600 K, the sample offers ultrahigh S of 718.18 μV·K−1 along the c direction (parallel to the SPS pressure direction), higher than those of some of well-known TE materials (Table S3). The extremely high S with anisotropy may be related to this multicomponent structure wherein insulating SrO layers and conductive CuSbSe2 layers alternate forming a natural superlattice with two-dimensional confinement of charge carriers.15 The multicomponent structure of SrO/CuSbSe2 is

SrOCuSbSe2 single crystals were synthesized via traditional melting salt method at 800 °C. The SEM image and EDS of well-defined SrOCuSbSe2 crystals are presented in Figure S4. The Sr/Cu/Sb/Se distributes uniformly and the atomic ratio is close to 1:1:1:1:2 (Table S1). The measured PXRD pattern of the compound corresponds well to the simulated one obtained from single-crystal data, implying a high degree of phase purity (Figure 2d). The SrOCuSbSe2 compound crystallizes in the D22h, P21/m (No. 11) space group of the monoclinic system (Table S4), it has a quasi-2D layered structure. The structure features 2∞[CuSbSe2O]2− anionic layers, which are separated by Sr2+ ions (Figure 2a). The 2∞[CuSbSe2O]2− layer can be constructed by interconnect anionic infinite 1∞[CuSe4]7− chains by square-pyramidal [SbSe4O]7− units via one edge sharing and two vertices sharing (Figure S1c−e). SrOCuSbSe2 can be regarded as the combination of the insulating SrO and distorted conductive CuSbSe2 structural layers. To understand more details about above-mentioned structure, the compound was examined by using TEM. The distance of the layer along the c-axis is about 0.86 nm (Figure 2b), which is consistent with the distance of crystal structure (Figure S2d). Importantly, the atomic model based on the spherical aberration corrected TEM clearly presents separation between SrO layers and CuSbSe2 layers along the [-1 0 0] direction and matches well with crystal structure (Figure 2c). Detailed descriptions of crystal structure and transmission electron images are shown in the SI. Accompanied by the onset wavelength around 900 nm, the optical absorption spectroscopy of the compound reveals an indirect band gap of 1.56 eV (Figure 2c), in accord with the black colors. This value is so quite large than those of the wide bandgap TE materials, such as CuSbSe2 (1.1 eV),9 BiCuSeO (0.8 eV),10 and Cu2CdSnSe4 (0.98 eV).5 With the temperature ranges from 140 to 300 K, the resistance can be both fitted SPH model and VRH model (Figure 2d and Equations S2 and 5540

DOI: 10.1021/acs.chemmater.8b02651 Chem. Mater. 2018, 30, 5539−5543

Communication

Chemistry of Materials

3d). The calculated kL of the sample follows the relationship as kL ∝ T−1, which indicates that the dominant scattering in phonon transport is the phonon−phonon Umklapp scattering (Figure 3d inset).9 Because of the low kL, it is worth it to compare it with the calculated minimum lattice thermal conductivity kLmin for SrOCuSbSe2. According to Equations S9 and S10, the kLmin is about 0.522 W·m−1·K−1 at 300 K, which is much lower than the value of 0.987 W·m−1·K−1 obtained in experiment, suggesting that it is possible to reduce kL of SrOCuSbSe2 further via means such as defect scattering through doping. Based on the measured transport properties, the temperature dependence of ZT for SrOCuSbSe2 reaches a maximum value of 0.22 at 850 K, which is comparable to some well-known TE materials (Table S3 and Figure 3f). Significantly, ZT increases rapidly owing to the increasing PF, decreasing k and high thermal stability. Thus, due to the intercalation of SrO layers, SrOCuSbSe2 has a potential TE performance using in the high temperature applications. Density functional theory (DFT) calculations were performed to understand the band structure of SrOCuSbSe2. The SrOCuSbSe2 compound has semiconductor characters with an indirect bandgap of 1.51 eV (A→Γ) (Figure S7 and S8), which matches well with the UV−vis-NIR results (Figure 2c). The band structure of SrOCuSbSe2 possesses a very flat valence band (Γ→A and Γ→B), which is also consistent with the obtained large hole carrier mass (0.40 m0), low carrier mobility (0.235 cm2·V−1·s−1), and a high S. The total density of states (DOS) of Sr2+ ions (Figure S9) make a minor contribution to the VBM. The Cu-3d, Se-4p, Sb-5p, and Sb-5s states contribute much more to the VBM, whereas the conductive band minimum (CBM) mainly consists of the Sb5p, Se-4p, and Se-4s states with minor contribution from the O-2p states (Figure S10). The bonding nature of SrOCuSbSe2 is also analyzed through the calculated electron localization function (ELF) (Figure S3a). The isosurface of lobe-like charge extension around the Sb atoms is the classic signature of the 5s2 lone-pair electrons, which is responsible for soft phonon modes and strong vibrational anharmonicity in the lattice.3,12 The ELF details are shown in SI, which explain the relationship between low k and lone pair effects. To better understand the ultralow k of SrOCuSbSe2, the lattice dynamical properties were investigated using DFT phonon calculation based on finite displacement methods.19,20 No imaginary eigenvalues were noted, confirming that the obtained crystal is thermodynamically stable (Figure 4a). The mechanical stability of the compound is also ensured by the Born criterion as the principal minor determinants of elastic constants matrix are all positive. The acoustic modes are suppressed by the optical branches only along the Γ-Z and X-Γ direction (z and x axis), corresponding to the lone-pair electrons of Sb atoms along the specific orientation. The partial density of state of Sb dominates low-frequency phonons, further indicating the mainly contribution from lone-pair effect of Sb (Figure 4b). When the vibration DOS is projected to different orientations (Figure S11), obvious anisotropy can be seen for different atoms except Cu. Sb atoms vibrate more in the x axis (nearly Sb−Se bonding direction), whereas Sr and Se atoms vibrate mainly along the z direction, playing as insulating unit via phonon scattering. The atomic displacement parameter (ADP) is generally correlated to the bonding strength. As shown in Figure 4d, the hierarchy of chemical bonds is consistent with the anisotropy of atomic vibration as discussed. Unlike the CuSbSe 2

Figure 3. Temperature dependent on TE properties of the SPSed polycrystalline SrOCuSbSe2 (Per/Par: the perpendicular and the parallel direction of the SPS pressure). (a) Electrical conductivity (σ). (b) Seebeck coefficient (S). (c) Thermal conductivity (k) and carrier thermal conductivity (ke). (d) Lattice thermal conductivity (kL). Inset: the relationship of kL ∝ T−1. (e) Power factor (PF). (f) Thermoelectric merit (ZT) of the SrOCuSbSe2 and some well-known TE materials.

similar to the two-dimensional hole conducting structures of BiCuSeO,10 CuSbSe2, and Cu2CdSnSe4,5,9 which contain artificial insulating/conductive units and exhibit high S. In the meantime, a large effective mass (m*) will result in a large S. Thus, m* is estimated according to Equations S4−6 using the observed carrier concentration (nh) and Seebeck coefficient (S) values.15,16 The calculation provides an effective mass (m*) of 0.40 m0, which is comparable to those of PbTe (0.24 m0),17 PbS (0.40 m0),17 SnSe (0.75 m0),18 and BiCuSeO (0.60 m0) at room temperature.10 With this effective mass, high S is not unexpected. More importantly, compared to its parent structure CuSbSe2 (535 μV·K−1 at 625 K),9 the higher S at 600 K may be due to the insulating SrO layers which can create in stronger 2D confinement of the charge carriers. Consequently, temperature dependence of the power factor (PF), presented in Figure 3e, shows that the sample has a value with a maximum around 100 μW·m−1·K−2 at 850 K. This value remains lower than the ones reported for CuSbSe2 (140 μW· m−1·K−2) and other CASe compounds due to the low σ.9,13 The total k in the entire temperature range of measurements are shown in Figure 3c. As expected, due to the 2D crystal structure and intrinsically low k in the chalcostibite compounds, the k value of SrOCuSbSe2 drops to 0.38 W· m−1·K−1 at 850 K. Different from σ, k shows the minor anisotropy that the perpendicular direction is slightly larger than the parallel direction, indicating SrO layers mainly act as EIU and secondarily act as PSU. In order to further reveal the influence of k, the carrier thermal conductivity (ke), and the lattice thermal conductivity (kL) values were estimated (Equations S7 and S8). Because sample has a negligible ke (ke < 15%k) (Figure 3c), the kL dominates the total k (Figure 5541

DOI: 10.1021/acs.chemmater.8b02651 Chem. Mater. 2018, 30, 5539−5543

Chemistry of Materials

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Communication

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.8b02651. Data for CuOSbSe2, Sr (CIF)

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Experimental details, crystal structure details, TG-DTA, TEM, Supplementary equations, ELF calculation, DFT calculation of SrOCuSbSe2 (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (Fuqiang Huang).

Figure 4. Theoretically calculated phonon properties of SrOCuSbSe2. (a) Phonon dispersion. The TA, TA′, and LA modes are marked as red, green, and blue lines, respectively. (b) Grüneisen dispersion. (c) Total and projected phonon density of states of different atoms. (d) Atomic displacement parameter (ADP) for different atoms along the x, y, and z directions.

ORCID

Fuqiang Huang: 0000-0001-7727-0488 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research project was financially supported by National key research and development program (Grant 2016YFB0901600), CAS Center for Excellence in Superconducting Electronics, the Key Research Program of Chinese Academy of Sciences (Grants QYZDJ-SSW-JSC013 and KGZD-EW-T06), Science and Technology Commission of Shanghai (Grants16JC1401700 and 16ZR1440500), and the Science and Technology Projects of State Grid (SGRIDGKJ[2015]452).

compound, the weak bonding of Cu atoms in SrOCuSbSe2 is observed not only in z direction but also in x and y directions, meaning that the SrO EIU intercalation gives more freedom to the Cu atoms, which have a contribution to lower the kL. Hence, the kL of the SrOCuSbSe2 compound can act as a “transitional” state between CuSbSe2 and Cu3SbSe3, because it possesses the 5s2 lone pair electrons similar to CuSbSe2 and slight random diffusion of the Cu atoms similar to Cu3SbSe3.11 Besides the ADP analysis from harmonic approximation, Grüneisen parameters provide us the information about the lattice anharmonicity (Figure 4c). The Grüneisen parameters calculated for SrOCuSbSe2 are much bigger than those of CuSbSe2 and comparable to the value of BiCuSeO (Table S8).11,21 Thus, the ultralow kL of SrOCuSbSe2 can be expected due to the strong anharmonicity, which has been verified in our experiments (Table S3). Higher values of the Grüneisen parameters in SrOCuSbSe2 are also found along the Γ-Z and X-Γ dispersion corresponding to the two directions of the 5s2 lone-pair electrons at the Sb atoms. Given the above, the dual effects of SrO EIU intercalation and lone-pair electrons of Sb atoms play an important role in the low kL. In summary, we have successfully synthesized a new quasi2D compound SrOCuSbSe2 through a traditional melting salt method. The compound represents a new structure type in which the layers of SrO are intercalated into the parent CuSbSe2 lattice. Such a structure feature shows clear separation between the regions of ECU (CuSbSe2 layers) and EIU (SrO layers). The SrO EIU layers restrict the movement of the charge carriers, which result in an extremely high S. According to the DFT phonon calculations, such a low kL is mainly attributed to the effect of lone-pair electrons of Sb atoms and secondarily owing to SrO EIU layers intercalation. As a result, its maximum ZT is 0.22 at 850 K. Although SrOCuSbSe2 has a large band gap, it also shows a good TE performance. Our data suggest that SrOCuSbSe2 is a promising candidate for hightemperature p-type TE applications. Further improvement in the ZT may be achieved by increasing the σ via heavy hole doping, and related work is underway.

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REFERENCES

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DOI: 10.1021/acs.chemmater.8b02651 Chem. Mater. 2018, 30, 5539−5543