Electronic band structure engineering and enhanced thermoelectric

Article Cite This: Chem. Mater. 2018, 30, 1085−1094

pubs.acs.org/cm

Electronic Band Structure Engineering and Enhanced Thermoelectric Transport Properties in Pb-Doped BiCuOS Oxysulfide Jean-Baptiste Labégorre,† Rabih Al Rahal Al Orabi,*,‡,§ Agathe Virfeu,‡ Jacinthe Gamon,∥,⊥ Philippe Barboux,⊥ Lauriane Pautrot-d’Alençon,∥ Thierry Le Mercier,∥ David Berthebaud,† Antoine Maignan,† and Emmanuel Guilmeau*,† †

Laboratoire CRISMAT, UMR-CNRS 6508, ENSICAEN, UNICAEN, Normandie Université, 6 Boulevard du Maréchal Juin, 14050 Caen Cedex 04, France ‡ Solvay, Design and Development of Functional Materials Department, Axel’One, 87 avenue des Frères Perret, 69192 Saint Fons Cedex, France § Department of Physics, Central Michigan University, Mt. Pleasant, Michigan 48879, United States ∥ Centre de Recherches d’Aubervilliers, Solvay, 52 rue de la Haie-Coq, 93308 Aubervilliers Cedex, France ⊥ Chimie ParisTech, PSL Research University, CNRS, Institut de Recherche de Chimie de Paris, 11 rue Pierre et Marie Curie, 75005 Paris, France S Supporting Information *

ABSTRACT: In this paper, Bi1−xPbxCuOS samples (0 ≤ x ≤ 0.05) have been synthesized with a simple and scalable ballmilling process, followed by a reactive Spark Plasma Sintering. Our results highlight that, Pb for Bi substitution increases the charge carriers concentration by more than 2 orders of magnitude from 1.4 × 1017 cm−3 to 2.6 × 1019 cm−3 for x = 0 and x = 0.05, respectively. As a result, the electrical resistivity is divided by more than 50 at room temperature and the Seebeck coefficient drops from 707 μV K−1 to 265 μV K−1 where our experimental results are supported with density functional theory (DFT) calculations. Electronic structure calculations show that, just below the top of the valence band, several other bands are present and may contribute to the transport properties with appropriate tuning of the heavy-light valence band and the position of the Fermi level. Pb doping increases the number of holes pockets and several band degeneracies appear around the Fermi level, leading to a drastic enhancement of the power factor up to 0.2 mW m−1 K−2 at 700 K. This is 5 times higher than the value of the pristine compound. The intrinsically low thermal conductivity of 0.7 W m−1 K−1 at 700 K is interpreted on the basis of vibrational properties calculations within the Density Functional Perturbation Theory (DFPT) approach. It indicates that soft acoustic modes along the Γ−Z direction suggest weak interatomic bonding between the layers and possible strong anharmonicity. The power factor being enhanced with a minimal impact on the thermal conductivity, the figure of merit ZT reaches 0.2 at 700 K for x = 0.05. To the best of our knowledge, it is considered the best reported value among the family of oxysulfides

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aforementioned issues.2−5 A significant step has been especially crossed with the discovery of relatively low thermal conductivity and high electrical performances (p-type), that exists inside the natural superlattices of NaxCoO26 and Ca3Co4O9.7,8 Nevertheless, these oxides are still not considered to have good performance compared to other thermoelectric materials because of their relatively high electrical resistivity and large thermal conductivity. In 2010, another layered material BiCuOSe attracted tremendous attention due to its promising ZT in the medium temperature range (0.32 at 673 K).9 This

INTRODUCTION Thermoelectric devices are promising candidates to reduce the CO2 emissions and fight against the energy crisis due to their ability to achieve direct and reversible energy conversion between heat and electricity. The conversion efficiency of a thermoelectric material is commonly described by the dimensionless figure of merit ZT (ZT = S2T/ρκ), where T is the absolute temperature (K), S is the Seebeck coefficient (μV K−1), ρ is the electrical resistivity (Ω m), and κ is the thermal conductivity (W m−1 K−1).1 However, current thermoelectric modules are confined to niche markets due to elements scarcity, high costs, toxicity and low thermal stability under air. During the last two decades, efforts have been devoted to develop thermoelectric oxides that are capable of addressing the © 2018 American Chemical Society

Received: November 30, 2017 Revised: December 20, 2017 Published: January 8, 2018 1085

DOI: 10.1021/acs.chemmater.7b04989 Chem. Mater. 2018, 30, 1085−1094

Article

Chemistry of Materials

Pb doping has been well-investigated in p-type BiCuOSe for its role in the significant improvement of the power factor compared to those of pristine BiCuOSe. However, to the best of our knowledge, electrical transport properties were solely investigated for Pb-doped BiCuOS.27−29 The similarities of the band and crystal structure between BiCuOS and BiCuOSe gave a hint that Pb-doping could modify the electronic properties and enhance the power factor. In this paper, we present the effect of Pb doping on the band structure and electronic properties of BiCuOS, using both theoretical and experimental studies for the first time. We prove that Pb doping in BiCuOS improves the electronic properties by activating several hole pockets with high effective masses, resulting in an enhanced power factor. This yields to a high figure of merit ZT of 0.2 at 700 K for a Pb concentration of 5%. To the best of our knowledge, it is the best ever achieved figure of merit value for an oxysulfide compound.

oxychalcogenide superlattice crystallizes in the tetragonal P4/ nmm space group and is built of two different layers that are stacked along the c axis alternately (Figure 1).10 The covalent

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EXPERIMENTAL DETAILS

Bi1−xPbxCuOS (x = 0, 0.025, 0.0375, 0.05) powders were synthesized from stoichiometric mixtures of Bi2O3 (>99.5%, Alfa Aesar), Bi (>99.99%, Alfa Aesar), PbO (>99%, Merck), Cu (>99%, < 625 mesh, Alfa Aesar) and S (>99%, < 325 mesh, Alfa Aesar). The precursors were weighed and 5.5 g of each composition were set inside a 45 mL tungsten carbide jar with 7 tungsten carbide balls (Ø 10 mm). The jar was closed in argon-filled glovebox to ensure an inert atmosphere during the process. The mechanosynthesis was held in a planetary ball mill (Fritsch Pulverisette 7 Premium line) during 6 cycles of 30 min at 600 rpm. Finally, the resulting powder were ground, placed in 10 mm graphite dies and densified by Spark Plasma Sintering (SPS) under a uniaxial pressure of 64 MPa under vacuum at 748 K for 25 min (heating and cooling rates of 100 K/min). The relative densities of the samples were respectively 90.6% (x = 0), 94.5% (x = 0.025), 94.9% (x = 0.0375), and 94.1% (x = 0.05) of the theoretical values. Powder X-ray diffraction (PXRD) patterns were collected at room temperature using a PANalytical X-Pert Pro diffractometer equipped with a Cu Kα X-ray tube. The scanning angle 2θ was varied between 20 and 100°. PXRD patterns were refined by the Rietveld method with FullProf and WinPLOTR software packages.30,31 Details of the refinements are listed in Table S1. Microstructures and grain sizes of sintered samples were examined by scanning electron microscopy (SEM) on a ZEISS Supra 55. The electrical resistivity (ρ) and the Seebeck coefficient (S) were simultaneously measured on bar-shaped samples with typical size of 3.0 × 3.0 × 8.0 mm3 using the standard four-probe method (ULVAC-RIKO ZEM-3). The measurements were performed from room temperature to 673 K under partial helium pressure. Heating and cooling cycles were carried out to assess the thermal stability during the measurement and gave repeatable data. The thermal conductivity (κ) was calculated from the product of the geometrical density, heat capacity (Dulong−Petit), and thermal diffusivity obtained by laser flash diffusivity method on a Netzsch LFA 457 (with 6.0 × 6.0 × 1.0 mm3 samples) measured between 300 and 673 K. Finally, Hall effect measurements by Van der Pauw method were carried out in a PPMS (Physical Properties Measurement System) system from Quantum Design to determine the carrier concentrations (nH = 1/eRH where e is the electronic charge) and mobilities at room temperature. Samples with dimensions of 3.0 × 3.0 × 0.3 mm3 were used with indium as contacts. The Hall coefficients were calculated from the linear fits of the Hall resistivities versus magnetic fields between −7 and 7 T. It is noteworthy that the electrical and thermal properties were measured along the direction perpendicular to the compression applied during the sintering process. The estimated measurements uncertainties are 6% for the Seebeck coefficient, 8% for the electrical resistivity, 11% for the thermal conductivity and 16% for the final figure of merit, ZT.32

Figure 1. Crystal structure of 2 × 2 × 2 supercells of BiCuOS (left) and Bi0.937Pb0.063CuOS (right) used for the computations.

antifluorite-like layer (Cu2Se2)2−, build up from edge-sharing CuSe4 tetrahedra, is the conductive pathway for charge carriers transport. The ionic (Bi2O2)2+ block is insulating and acts as a charge reservoir. The succession of these two weakly bonded layers behave like phonon scattering interfaces and give it a very low lattice thermal conductivity (∼0.5 W m−1 K−1 at 773 K).11 On the other hand, the high electrical resistivity problem should be solved in order to enhance the merit figure of this material further. This has been achieved by increasing the carrier concentration with monovalent or divalent cations (Na+, K+, Mg2+, Ca2+, Sr2+, Ba2+, or Pb2+)9,12−18 substitution inside the insulating (Bi2O2)2+ layer. Furthermore, other strategies have been used, such as the creation of copper vacancies in the conductive (Cu2Se2)2− layer,19 as well as limiting the decrease of the charge carriers mobility via modulation doping.20 Electronic band structure engineering through the isoelectronic substitution of Se by Te has also been developed successfully.21 These studies led to a substantial improvement of the thermoelectric performances to record ZT values of 1.4 (at 923 K) and 1.5 (at 873 K) for doped and textured samples22 and for dually doped samples, respectively.23 Thus, this material became a viable alternative to the use of PbTe and nearly competes with the high performance of multiple-filled skutterudites (ZT = 1.7 at 850 K).24 Despite the achievements reached in the BiCuOSe system in the past years, there is a lack of studies on the isostructural sulfur counterpart due to its lower thermoelectric performances, i.e., a ZT ∼ 0.07 at 650 K.25 The low value of ZT is mainly attributed to (1) a lower carrier concentration (1017 cm−3 against ∼1019 cm−3 in BiCuOSe), (2) a wider band gap (1.1 eV against 0.75 eV in BiCuOSe) which is much higher than the optimal value predicted by the Goldsmid−Sharp formula Eg = 2eSmaxTmax of 0.35 eV for thermoelectric materials,26 and (3) a higher thermal conductivity in BiCuOS than in BiCuOSe (S is much lighter than Se). Nevertheless, sulfur being significantly less toxic and more abundant than selenium, there would be undeniable benefits in replacing Se by S, whereas maintaining relatively thermoelectric performances. 1086


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FIRST-PRINCIPLE CALCULATIONS Density functional theory (DFT) geometry optimizations were carried out with the CASTEP8.0 code33 using a set of ultrasoft pseudopotentials34,35 with the PBEsol exchange−correlation functional.36 Cell parameters and atomic positions were both relaxed. During the geometry optimizations, a convergence threshold of 0.02 eV Å−1 was used for the residual forces and 0.1 kbar for the pressure. The cutoff energy for plane-waves was set to 700 eV. For the electronic band structures we used the full-potential linearized augmented plane wave (FLAPW) approach, as implemented in the WIEN2K code.37 A plane-wave cutoff corresponding to RMTKmax= 7 was used in all calculations. The radial wave functions inside the nonoverlapping muffin-tin spheres were expanded up to lmax = 12. The charge density was Fourier expanded up to Gmax = 16 Å−1. Total energy convergence was achieved with respect to the Brillouin zone (BZ) integration mesh with 500k-points. Because GGA exchange-correlation functionals are known to underestimate experimental band gaps, we used the modified Becke-Johnson (mBJ) functional which leads to excellent agreement with the experimental values for the energy separation between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO).38 Since the Bi atoms in oxychalcogenide compounds have fairly high atomic number and mass, the relativistic effect can not be neglected, and we have included the effect of spin−orbit coupling to elucidate realistic electronic structure. We used 5000 k-points in the BZ to compute the band derivatives and the density of states. The doping effect was examined by constructing a 2 × 2 × 2 (64 atoms) supercell in which one of the Bi atoms was replaced by a Pb atom and the corresponding Pb doping concentration is 6.3 at. %. The band structure of both undoped and Bi0.937Pb0.063CuOS was calculated using the 2 × 2 × 2 supercell under the same calculation conditions. Vibrational properties have been obtained by using Density Functional Perturbation Theory (DFPT)39 as implemented in the Quantum ESPRESSO (QE) package.40 The calculation has been performed by using a set of ultrasoft pseudopotentials35 with the PBEsol exchange-correlation functional.36 The kinetic energy cutoff and the charge density cutoff are set to be 40 and 480 Ry, respectively. An 8 × 8 x 4 k-point mesh is used to sample the Brillouin zone. The Grüneisen’ parameter can be defined as γqs = − d ln(ωqs) d ln(V ) and characterizes the relationship between phonon frequency and volume change. In the quasiharmonic DFT phonon calculations, the system volume is isotropically expanded by +4% from the DFT relaxed volume.

Figure 2. (a) PXRD patterns of the Bi1−xPbxCuOS (0 ≤ x ≤ 0.05) samples processed by SPS. The green stars indicate the main diffraction peak (012) of metallic bismuth (R-3m) observed in the samples x = 0.0375 and x = 0.05. Rietveld refinements of PXRD patterns of (b) x = 0 and (c) x = 0.025 sintered samples.

as highlighted in Figure S1. During SPS, the as-synthesized powders react and high purity bulk polycrystalline samples are formed when x ≤ 0.025. This short process enables a reduction of the synthesis time as compared to sealed tube (>10 h)27 or hydrothermal (>55 h)43 synthesis. The refined unit cell parameters of BiCuOS, a = 3.870(1) Å and c = 8.561(1) Å are in good agreement with the values reported for single crystals, i.e. a = 3.8705(5) Å and c = 8.561(1) Å.10 Moreover, the Rietveld refinements of PXRD patterns (see for Figure 2b, c, for x = 0 and x = 0.025 respectively; refinement parameters are reported in Table S1) evidenced a copper site occupancy ranging between 0.95 and 0.99 for all the samples. This is considered consistent with the ability of MCuOCh (M = La, Bi ; Ch = S, Se, Te) system to easily form copper vacancies, leading to nonintentional p-type doping in pristine samples.27,29,44,45 As shown in Figure 3, the cell parameters a and c, which are determined from Rietveld refinements, increase monotonically with the Pb fraction from x = 0 to 0.0375. The linear increase of both lattice parameters

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RESULTS AND DISCUSSION Crystal Structure. Figure 2a displays the PXRD patterns of Bi1−xPbxCuOS (0 ≤ x ≤ 0.05) sintered samples. All the major Bragg peaks can be indexed in the ZrSiCuAs structure with the tetragonal P4/nmm space group.41 The samples with x ≤ 0.025 are single phase, whereas the main peak (012) of metallic bismuth (Bi) is discernible at ∼ 27.1° for the two highest lead contents, indicating 1 in Ba Heavily Doped BiCuSeO Oxyselenides. Energy Environ. Sci. 2012, 5, 8543−8547. (18) Luu, S. D. N.; Vaqueiro, P. Synthesis, Structural Characterisation and Thermoelectric Properties of Bi1-xPbxOCuSe. J. Mater. Chem. A 2013, 1, 12270−12275. (19) Liu, Y.; Zhao, L.; Liu, Y.; Lan, J.; Xu, W.; Li, F.; Zhang, B.; Berardan, D.; Dragoe, N.; Lin, Y.-H.; et al. Remarkable Enhancement in Thermoelectric Performance of BiCuSeO by Cu Deficiencies. J. Am. Chem. Soc. 2011, 133, 20112−20115. (20) Pei, Y.-L.; Wu, H.; Wu, D.; Zheng, F.; He, J. High Thermoelectric Performance Realized in a BiCuSeO System by Improving Carrier Mobility through 3D Modulation Doping. J. Am. Chem. Soc. 2014, 136 (39), 13902−13908. (21) Liu, Y.; Lan, J.; Xu, W.; Liu, Y.; Pei, Y.-L.; Cheng, B.; Liu, D.-B.; Lin, Y.-H.; Zhao, L.-D. Enhanced Thermoelectric Performance of a BiCuSeO System via Band Gap Tuning. Chem. Commun. 2013, 49, 8075−8077. (22) Sui, J.; Li, J.; He, J.; Pei, Y.-L.; Berardan, D.; Wu, H.; Dragoe, N.; Cai, W.; Zhao, L.-D. Texturation Boosts the Thermoelectric

thereby, becomes the best ever achieved for an oxysulfide compound.

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CONCLUSION The thermoelectric properties of the Bi1−xPbxCuOS series (0 ≤ x ≤ 0.05) have been studied both experimentally and theoretically. The Pb for Bi substitution increases the charge carriers concentration and enables to increase substantially the electrical conductivity while moderately decreasing the Seebeck coefficient. This leads to a maximum power factor for x = 0.05 sample. The intrinsically low thermal conductivity provided by weak interatomic bonding between the layers and possible strong anharmonicity, is imperceptibly increased by the cationic substitution. Finally, the combination of low thermal conductivity with a power factor of 0.2 mW m−1 K−2 at 700 K increased the figure of merit ZT up to 0.2 in 5 atom % Pbdoped BiCuOS sample. This value is 5 times higher with respect to BiCuOS. To the best of our knowledge, it is considered as the best ever reported value among the family of oxysulfides. Despite the obtained low ZT value, the approaches developed to optimize the electrical and thermal properties of BiCuOS appears as an interesting playground, which can be extended to other oxychalcogenides and materials in the future.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.7b04989. Figures S1−S4 and Table S1 (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. Tel: +33 231 45 13 67. *E-mail: [email protected]. Tel: +33 428 27 10 80. ORCID

Rabih Al Rahal Al Orabi: 0000-0001-5880-5838 Jacinthe Gamon: 0000-0002-0888-4248 Emmanuel Guilmeau: 0000-0001-7439-088X Notes

The authors declare no competing financial interest.

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