Research Article Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
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Significant Optimization of Electron−Phonon Transport of n‑Type Bi2O2Se by Mechanical Manipulation of Se Vacancies via Shear Exfoliation Lin Pan,*,†,‡ Lei Zhao,† Xiaoxuan Zhang,§ Changchun Chen,† Pingping Yao,∥ Changlong Jiang,⊥ Xiaodong Shen,†,‡ Yinong Lyu,†,‡ Chunhua Lu,†,‡ Li-Dong Zhao,*,§ and Yifeng Wang*,†,‡
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†
College of Materials Science and Engineering, and ‡Jiangsu Collaborative Innovation Center for Advanced Inorganic Function Composites, Nanjing Tech University, Nanjing 210009, China § School of Materials Science and Engineering, Beihang University, Beijing 100191, China ∥ Anhui Institute of Optics and Fine Mechanics and ⊥Institute of Intelligent Machines, Chinese Academy of Sciences, Hefei 230031, China S Supporting Information *
ABSTRACT: Very recently, a novel layered oxyselenide Bi2O2Se has attracted much attention as a promising n-type eco-friendly thermoelectric material, especially for the n-type counterpart of p-type BiCuSeO. However, very poor electrical conductivity of intrinsic polycrystalline Bi2O2Se prohibits the further development of its thermoelectric performance. In the present work, a novel and facile method using a kitchen blender was developed for large-scale production of Bi2O2Se nanosheets. The electrical transport behavior of the resultant bulk Bi2O2Se via shear exfoliation changes from semiconductivity to metallic, electrical conductivity, which is greatly improved by more than 3 orders of magnitude from 0.1 to 470 S cm−1 at room temperature. Besides, thermal conductivity had been reduced to 0.93 W K−1 m−1 at 773 K. This synergistical promotion of electron−phonon transport could mainly come from increased interfacial defects of shear-exfoliated Bi2O2Se to introduce a large amount of electrons by Se vacancies and induce the intensive scattering of phonons by vacancies and interfaces. A high ZTpeak of 0.5 at 793 K had been achieved for Bi2O2Se with shear exfoliation for 60 min, which is 1.5 times larger than the ZT record of Bi2O2Se-based thermoelectrics. In addition, the figure of merit for the thermoelectric module based on p-type BiCuSeO and n-type Bi2O2Se has been evaluated to be around 0.8 at 793 K, making BiCuSeO−Bi2O2Se module a very promising candidate for mid-temperature thermoelectric applications. KEYWORDS: thermoelectric, Bi2O2Se, shear exfoliation, 2D materials, ZT
1. INTRODUCTION
and good chemical and thermal stability has become paramount. Recently, eco-friendly p-type BiCuSeO-based materials with an iron-based superconductor LaFeAsO structure (Figure S1a) have attracted much attention for their promising TE performances in the medium temperature range.6−12 Unfortunately, a fatal factor prohibiting the practical application of BiCuSeO TEs is that no n-type BiCuSeO counterpart with reasonable performance has been developed so far.13,14 Thus, finding appropriate TEs as an n-type counterpart of BiCuSeO becomes the top priority for practical application of BiCuSeO TEs. Very recently, a new versatile 2D material Bi2O2Se nanosheet composed of buckled Bi−O layers (which consist
Thermoelectric (TE) effect enables direct conversion between thermal and electrical energy without any mechanical moving parts, providing an alternative route for eco-friendly power generation and refrigeration.1,2 The TE conversion efficiency of TE materials is evaluated by a dimensionless figure of merit ZT = S2σT/κ, where S, σ, κ, and T are the Seebeck coefficient, the electrical conductivity, the thermal conductivity, and the absolute temperature, respectively.3,4 Therefore, efficient TE materials require a perfect combination of high S and σ with low κ. In the half past century, the most successful commercial applications of the TE module are still based on Bi2Te3− Sb2Te3 (300−400 K), PbTe (600−850 K), and Si−Ge alloys (>1000 K).5 However, these materials all contain toxic (Pb) or noble elements (Ge, Te), which become the bottleneck for TE widespread applications. Thus, the finding of novel ecofriendly TE materials with low toxicity, affordable elements, © XXXX American Chemical Society
Received: March 27, 2019 Accepted: May 28, 2019
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DOI: 10.1021/acsami.9b05470 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
2. MATERIALS AND METHODS
of edge-sharing OBi4 tetrahedra) alternately stacked with planar Se layers, which has quite similar crystal structure to BiCuSeO, was found to exhibit ultrahigh electron mobility (>20 000 cm2 V−1 s−1) in fabricated transistors.15,16 Subsequent studies showed that the Bi2O2Se thin film exhibits good properties in highly sensitive infrared (IR) photodetectors17 and magnetoresistance devices,18 which were predicted to be promising candidates for realizing novel quantum phenomena, future logic devices, and flexible electronic, ferroelectric, and TE materials.15,19−22 In particular, Bi2O2Se has been expected to be a perfect n-type counterpart of BiCuSeO (Figure S1).19,23 The predicted promising TE properties of Bi2O2Se are attributed to its intrinsic high Seebeck coefficient and low lattice thermal conductivity, which originate from quantum confinement of carriers and interfacial scattering of phonons with weak atomic bonding between layers in its natural superlattice crystal structure.24,25 However, high TE performance of bulk Bi2O2Se has not been realized experimentally so far because of its low electrical conductivity no matter what kind of methods adopted, including doping, solid solution, defects engineering or composites.23,26−31 In the present work, a novel and facile method using a kitchen blender was developed for large-scale production of Bi2O2Se nanosheets (Figure 1). Moreover, benefited from synergistical
Bi2O2Se powders were synthesized using high-temperature solid-state reaction. Stoichiometric mixtures of Bi2O3 (2N), Bi (4N), and Se (4N) powders were ground by hand using an agate mortar and then heated at 873 K for 24 h in silica tubes sealed under argon. The obtained samples were ground to powders and dispersed in deionized water before shear exfoliation by a kitchen blender (Westinghouse HS0450) with a set time (0, 10, 30, and 60 min). To avoid overheating of both motor and liquid, we applied a duty cycle of 3 min on/3 min off with a rotating speed of 30 000 rpm. After filtering and drying, the gathered powders were subsequently densified by using a SPS (LABOX-110H, Sinter Land) system at 873 K with a holding time of 5 min in a ø = 10 mm graphite mold under 50 MPa in an argon atmosphere, resulting in column-shaped samples of ø = 10 mm × 12 mm for the Bi2O2Se series which were denoted as 0min, 10min, 30min, and 60min, respectively. Room-temperature (RT) X-ray diffraction (XRD) characterization was performed using an ARL X’TRA diffractometer (SmartLab3, RIGAKU, Japan) with Cu Kα radiation. Diffuse reflectance spectra were measured on powders at RT using a conventional spectrophotometer (Bruker VERTEX 70v). The spectroscopic analyses of samples were performed by an IR instrument (Thermo-Fisher Nicolet iS10 FTIR spectrometer) on a KBr disk. Scanning electron microscopy (SEM) studies were performed using a field emission scanning electron microscope (FEI Nova NanoSEM450). The morphology and thickness of the sample 60min were investigated by transmission electron microscopy (TEM, JEOL 2010) and atomic force microscopy (AFM, Bruker, Dimension Edge), respectively. The electrical conductivity and Seebeck coefficient were measured simultaneously by a commercial system (LSR-3, Linseis). All electrical characterizations were performed on bars cut from in-plane direction. Hall effects of all samples were measured with a van der Pauw configuration under vacuum by the ResiTest8300 system (Toyo Tech. Co., Japan). In-plane thermal diffusivity (D) was measured by a laser flash method with a typical pellet of 10 mm × 10 mm × 1.5 mm cut from the SPS samples. Specific heat (Cp) was estimated by the Dulong−Petit law; density (d) was determined by the Archimedes method, and the total thermal conductivity was calculated from the relationship κ = DCpd. ZT values were obtained by integrating the measured S, σ, and κ. The temperature-averaged errors of the measurements are 6% for S, 8% for σ, and 11% for κ, which result in a total error less than 20% for ZT.
3. RESULTS AND DISCUSSION Chemical characterization of the samples 0min and 60min was carried out using IR spectroscopy as shown in Figure S3. Generally, the IR spectrum of the shear-exfoliated sample shows nearly the similar characteristic peaks to the sample without shear exfoliation. It indicates that shear exfoliation does not introduce obvious functional groups to Bi2O2Se, which also exhibits good chemical stability of Bi2O2Se. XRD patterns at RT for all the samples are shown in Figure 2a. It can be seen that for all samples, the main diffraction peaks correspond to the tetragonal phase of Bi2O2Se (space group: I4/mmm, no. 139, PDF no. 73-1316). The peaks are sharp, which indicate a good crystallinity of the samples. In addition, the relative intensities of the (00l) peaks become much stronger with the increase of treatment time, which indicates a better preferential orientation of the grains. The orientation degree can be estimated by the Lotgering method.32 The calculated Lotgering factor based on XRD data increased from 0.01 to 0.03, 0.05, and 0.12 for samples 0min, 10min, 30min, and 60min, respectively. These results demonstrate that the whole treatment does not introduce any impurity phase to Bi2O2Se and furthermore makes these layered samples preferentially oriented with their a, b plane
Figure 1. Scheme for the shear exfoliation of Bi2O2Se, leading to the decrease of the number of layers (a), cleavage of the Se layer (b), and the increase of Se vacancies (c) by using a kitchen blender.
optimization of electron−phonon transport, the TE performance of the resultant bulk Bi2O2Se from spark plasma sintering (SPS) of Bi2O2Se nanosheets has been greatly improved (ZT ≈ 0.5@793 K). In addition, the ZT of the TE module based on p-type BiCuSeO and n-type Bi2O2Se has been evaluated to be around 0.8 at 793 K, which paves the way for practical application of the eco-friendly TE module at mid-temperature. On the other hand, the method suggested in this study opens new avenues to be explored to optimize electron−phonon transport properties of other 2D materials. B
DOI: 10.1021/acsami.9b05470 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 2. (a) Bulk XRD patterns of samples 0min, 10min, 30min, and 60min. The inset is enlarged typical peaks for all samples. (b) TEM images of the shear-exfoliated sample 60mim. (c) AFM of the shear-exfoliated sample 60mim. (d) Optical absorption spectra of all samples with the band gap illustrated in the inset.
gap decreases from ∼0.62 eV of sample 0min to ∼0.52 eV of sample 60min. The narrower band gap of the shear-exfoliated sample might originate from Se defects.35 The decrease of Eg could make some contributions to the improvements of electron−phonon transport behavior, which will be discussed later. The temperature dependences of σ and S for all samples in the range of 322−793 K are shown in Figure 3. The increase of σ with increasing temperature in the whole temperature range and the negative values of S indicate a nondegenerate n-type semiconductor behavior for Bi2O2Se without shear exfoliation. Besides, σ of the sample is quite low (0.1 S cm−1 at RT and 7.8 S cm−1 at 774 K), which is similar to the previous reports.23 In contrast, it is amazing that σ of all samples with shear exfoliation greatly improves to hundreds of S cm−1 at RT and decreases with increasing temperature in the whole temperature range, which shows a metallic behavior. Typically, σ of 60min reaches 470 S cm−1 at RT, which is nearly 3 times larger than the previous record (around 180 S cm−1) of La-doped Bi2O2Se.30 Moreover, σ increases monotonically with increasing shear exfoliation time. It is well-known that there is a tradeoff between the Seebeck coefficient and the electrical conductivity based on the semiclassical Boltzmann transport theory.36,37 Thus, the absolute Seebeck coefficient |S| decreased with increasing shear exfoliation time. For instance, at around 780 K, σ increases from 7.8 S cm−1 for 0min to 56, 104, and 217 S cm−1 for 10min, 30min, and 60min, respectively. In contrast, |S| decreases from 343 μV K−1 for 0min to 245, 199, and 165 μV K−1 for 10min, 30min, and 60min, respectively. Assuming one carrier type, σ is dominated by carrier concentration n and mobility μ, that is, σ = neμ. Thus, the improved σ might come from the increase of n or μ or both. In order to get a better understanding of the electron−phonon transport behavior, Hall effect measurements were performed at RT (Table S1). The results indicate that the carrier concentration had been remarkably increased from ∼8 × 1015
perpendicular to the pressing direction. This estimation is consistent with the SEM observations. As shown in the fracture cross-sectional SEM (Figures S4 and S5), the obtained samples are highly dense with low porosity. In addition, in contrast to sample 0min, the bar-shaped grains of 60min with the size of around 1 × 4 μm are layered slightly perpendicular to the pressing direction. It means that in our samples, the electrical and thermal conductivity in the in-plane direction would be better. Herein, in this work, we choose this direction for studying the electrical−thermal transport of shear-exfoliated Bi2O2Se. Otherwise, it is noteworthy that the peaks of XRD patterns for the shear-exfoliated samples slightly shift to low angle. As shown in the inset of Figure 2a, in the enlarged (103) and (110) peaks for all specimens, we can clearly find that both peaks shift to low angle with increasing shear exfoliation time, which may result from the expansion of crystal lattice. In order to examine the variation of the crystal lattice, Rietveld refinements have been performed on the XRD patterns, as shown in Figure S6. The results indicate that both lattice constants a and c monotonically increase with increasing shear exfoliation time (Figure S7). The previous calculation research manifested that the optimized lattice constant of the Bi2O2Se nanosheet is slightly larger than that of the bulk Bi2O2Se.19 Thus, some low-dimensional nanosheet might be exfoliated from bulk Bi2O2Se during the process of blender treatment as confirmed by TEM (Figure 2b) with AFM (Figure 2c). The TEM image clearly shows that the lamellar objects with a length of ∼500 nm and a width roughly half the length are consistent with what is usually observed for exfoliated layered compounds.33,34 Herein, the shear-exfoliated Bi2O2Se nanosheet has a mean thickness of 20−30 layers and thus results in great improvement in the electron−phonon transport properties of the resultant bulk Bi2O2Se. Besides, the optical absorption spectrum of all samples is exhibited in Figure 2d, where the absorption edge apparently moves a little to the lower energy. Accordingly, the fitted band C
DOI: 10.1021/acsami.9b05470 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Meanwhile, it is worthy to point that the mobility μ has also been improved in the samples 30min and 60min (Table S1). In particular, the mobility μ of the sample 60min reaches 172.8 cm2 V−1 s−1 at RT, which is nearly double the value of the sample 30min. It is well-known that texturation in 2D materials could be beneficial for intensifying the mobility of electrons. As shown before, the Lotgering factor increased from 0.01 to 0.03, 0.05, and 0.12 for samples 0min, 10min, 30min, and 60min, respectively. Hence, it could be easy to speculate that the higher mobility should be partly attributed to the preferential orientation of exfoliated Bi2O2Se. In addition, let us recall that the mobility could reach 20 000 cm2 V−1 s−1 in Bi2O2Se nanoflakes. Thus, the improved mobility could also be estimated from the decrease of the number of layers of shear-exfoliated Bi2O2Se. Besides, according to Yan’s point of view, the electron conduction channels of Bi2O2Se distribute away from ionized donor defects (Se vacancies) in different layers. Such a spatial separation (self-modulation doping) can strongly inhibit the scattering caused by donor sites and lead to the increase of electron mobility.16 Otherwise, it is well-known that the smaller band gap would lower the energy barriers for the charge carriers to jump over, thus giving rise to higher carrier concentration and consequently higher electrical conductivity. Herein, the great promotion of electrical conductivity mainly comes from the increase of both carrier concentration and mobility of exfoliated Bi2O2Se. Combining the electrical conductivity and Seebeck coefficient, the calculated power factor (PF = S2σ) in the range of 322−793 K is shown in Figure 3c. In general, the PF of all samples increases with temperature except for the sample 10min with the temperature above 700 K. Moreover, the PF of all shear-exfoliated samples is much larger than the sample without shear exfoliation in the whole temperature range. Typically, the PF of 60min is around 0.6 mW m−1 K−2 at 791 K, which is about 1.6 times larger than the previous record value of Bi2O2Se-based materials.30 Similar to previous reports, the total thermal conductivity κ for all Bi2O2Se samples exhibits low values in the whole temperature range, with a monotonous decrease when the temperature increases (Figure 4a). The intrinsically low κ of Bi2O2Se might originate from its natural superlattice structure and low contribution from electronic thermal conductivity. Compared with the sample without shear exfoliation (0min), κ of shear-exfoliated Bi2O2Se (10min, 30min, and 60min) has been elevated remarkably in the whole temperature range and raised monotonically with increasing shear exfoliation time. For a better insight into the origin of the variation of κ, lattice thermal conductivity κl has been obtained by subtracting the electronic thermal conductivity κe from κ in the way, which can be found in other reports (Figures S8 and S9).30,42 One can see that κl of all samples is nearly proportional to T−1, which indicates an Umklapp phonon−phonon scattering mechanism in the whole temperature range (Figure 4b). Besides, in contrast to κ, κl of shear-exfoliated Bi2O2Se (10min, 30min, and 60min) has been slightly suppressed at higher temperature, with the lowest κl of 0.63 W m−1 K−1 achieved at 793 K for the sample 60min, which is lower than the previous lowest value (above 0.7 W m−1 K−1) in the previous report.30 The origin of reduction of κl could be mainly ascribed to the formation of large amounts of Se vacancies by shear exfoliation and induce the intensive scattering of phonons by vacancies and interfaces.
Figure 3. Temperature dependences of (a) electrical conductivity, (b) Seebeck coefficient, and (c) PF for all samples from 322 to 793 K.
cm−3 for sample 0min to 1.2 × 1019, 1.3 × 1019, and 1.7 × 1019 cm−3 for 10min, 30min, and 60min, respectively. The increase of carrier concentration of Bi2O2Se might originate from native point defects (i.e., Se vacancies) as manifested by defect chemistry reaction: •• Se×Se → V Se + 2e′ + Se, where one Se atom defect will introduce two extra electrons to Bi2O2Se. In the process of kitchen blender treatment, multiple fluid dynamics events such as shear, turbulence, collisions, and their cooperative effects could make some contributions to exfoliate Bi2O2Se, which is similar to the exfoliation of graphene and MoS2.33,38−40 Because of the weak interaction between Bi2O2Se and Se layers, the cleavage most probably occurs on the Se plane, leaving 50% Se atoms and meanwhile 50% Se vacancies attached to each Bi2O2Se plane, as required by the charge neutral and symmetry of structure requirement.41 The hypothesis was supported by Chen’s study. They found that there are more than ∼50% defects (Se vacancies) on the surface of Bi2O2Se.41 It is well-known that there are more surfaces in few-layer 2D samples than in original bulk materials. Thus, there will be much more Se vacancies in our shear-exfoliated Bi2O2Se, which will definitely be beneficial to greatly improve the electrical properties of resultant Bi2O2Se pellets. D
DOI: 10.1021/acsami.9b05470 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces
Figure 5. Temperature dependence of ZT for shear-exfoliated Bi2O2Se (a) and the TE module based on p-type BiCuSeO and ntype Bi2O2Se (b) from 322 to 793 K. The left inset shows the schematic diagram of the TE module based on p-type BiCuSeO (left) and n-type Bi2O2Se (right). The right inset shows the temperature dependence of β.
Figure 4. (a) Total thermal conductivity κ as a function of temperature. (b) Lattice thermal conductivity κl as a function of reciprocal of temperature for all samples from 322 to 793 K.
The figure of merit ZT resulting from the combination of the electrical and thermal transport properties over the range of 322−793 K is shown in Figure 5a. ZT monotonically increases with temperature. A high ZTpeak of around 0.5 at 793 K was obtained for 60min because of much larger PF and relatively lower thermal conductivity as compared to the sample without shear exfoliation. This result also makes Bi2O2Se the state-ofthe-art n-type oxygen-containing TE material. On the basis of our previous study on BiCuSeO and the present work, we can now evaluate the figure of merit for the TE module based on p-type BiCuSeO and n-type Bi2O2Se. The figure of merit for one couple TE module can be expressed as Z = [(Za1/2 ± βZb1/2)/(1 + β)]2, where Za and Zb are the figures of merit for single leg of thermocouple, σκ respectively, β = σa κb , which reflects the matching degree of
contrast, the thermal conductivity had been reduced to 0.93 W K−1 m−1 at 773 K. A high ZTpeak of around 0.5 at 793 K had been achieved for Bi2O2Se with blender treatment of 60 min, which is 1.5 times larger than the ZT record of Bi2O2Se-based TEs. In addition, the figure of merit for the TE module based on p-type BiCuSeO and n-type Bi2O2Se has been evaluated to be around 0.8 at 793 K, making BiCuSeO−Bi2O2Se TE module a very promising candidate for mid-temperature TE applications.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.9b05470. Schematic diagram of TE module based on p-type BiCuSeO and n-type Bi2O2Se, typical sample used in this study, IR spectral data, Rietveld refinement results, shear exfoliation time dependence of lattice constants, SEM photographs, temperature dependences of Lorentz constants and electrical and thermal conductivity, and carrier concentration and mobility (PDF)
b a
electron−phonon transport properties of couple legs, and ± exhibits the positive (negative) value when the sign of Seebeck coefficient of couple legs is different (same).43 As shown in Figure 5b, the ZT of the TE module monotonically increases with temperature and reaches a peak of 0.8 at 793 K because of a better matching of electron−phonon transport properties between p-type BiCuSeO and n-type Bi2O2Se at higher temperature. The result indicates that the BiCuSeO−Bi2O2Se TE module could be a very promising candidate for midtemperature TE applications.
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AUTHOR INFORMATION
Corresponding Authors
*E-mail:
[email protected] (L.P.). *E-mail:
[email protected] (L.Z.). *E-mail:
[email protected] (Y.W.).
4. CONCLUSIONS In this work, a facile method using a kitchen blender was developed for large-scale production of Bi2O2Se nanosheets and for improving their bulk TE properties. The results indicated that the electrical conductivity had been greatly improved by more than 3 orders to 470 S cm−1 at RT, which is nearly 3 times larger than the record of doped Bi2O2Se. In
ORCID
Lin Pan: 0000-0001-9436-0186 Changlong Jiang: 0000-0003-3650-3702 Li-Dong Zhao: 0000-0003-1247-4345 E
DOI: 10.1021/acsami.9b05470 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
Research Article
ACS Applied Materials & Interfaces Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China under grant nos. 51272103, 51672127, and 51772012. This work was also supported by the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD).
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DOI: 10.1021/acsami.9b05470 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX
