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Interface tuning charge transport and enhanced thermoelectric properties in flower-like SnSe2 hierarchical nanostructures
Applied Surface Science 510 (2020) 145478
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Full Length Article
Interface tuning charge transport and enhanced thermoelectric properties in flower-like SnSe2 hierarchical nanostructures ⁎
Jun Suna, Shuai Liub, , Chen Wanga, Yu Baic, Guanjun Chena, Qiaomei Luoa, Fei Maa,
T
⁎
a
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China College of Sciences, Xi'an Shiyou University, Xi'an, Shannxi 710065, China c Xi'an Jiaotong University Suzhou Institute, Suzhou, Jiangsu 215123, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: SnSe2 Solvothermal Nanostructure Thermoelectric properties
Thermoelectric properties could be well improved in hierarchical nanostructures due to the selective scattering on electrons and phonons by interfaces. In this paper, flower-like SnSe2 nanostructure is synthesized by solutionbased method and, the nanostructure is sintered into pellets by spark plasma sintering (SPS) to evaluate thermoelectric properties. It is demonstrated that the flower-like SnSe2 nanostructure exhibits the ultralow thermal conductivity of 0.44 Wm−1 K−1 due to the strong phonon scattering by high-density of interface and grain boundaries, which have been confirmed by both experiments and simulation. Besides, the electrical transport of the flower-like SnSe2 is optimized synergistically owing to the moderate interfacial potential barrier. The highest power factor of 43 μWm−1 K−2 and competitive ZT value are measured at 550 K. The thermoelectric performance of flower-like SnSe2 is better that that of nanoplate and bulk counterparts. It provides an efficient method to improve the thermoelectric properties of SnSe2 based materials.
1. Introduction With the aggravation of energy crisis and environmental pollution, thermoelectric (TE) materials which can directly convert waste heat into electricity make it possible to greatly improve the utilization efficiency of energy and generate power in an environment-friendly way [1–4] The thermoelectric conversion efficiency is evaluated by a dimensionless figure of merit ZT = [(S 2σ )/ κ ] T , in which S , σ , κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. For high thermoelectric figure of merit, an excellent electrical transport properties (power factor, S 2σ ) and a low thermal conductivity are required, but they often follow unfavourably opposing trends [5] In nanostructured materials, such as, superlattices [6], quantum dots [7], nanowires, and nanocomposites [8], the lattice thermal conductivity might be lowered substantially due to quantum confinement effect, size effect and grain boundary scattering, and thus the thermoelectric properties could be improved significantly [9–12]. Most of the traditional thermoelectric materials with high ZT values contain metal elements, such as, tellurium, lead and antimony, with the toxic, expensive or volatile features. Selenides are expected to replace tellurides, both economically and environmentally [13]. Particularly, Tin selenide (SnSe) has been proved to be an excellent thermoelectric
⁎
materials with an ultralow thermal conductivity. The ZT value of singlecrystal SnSe is the highest one of 2.6 at 923 K [14]. However, the synthesis of single-crystal SnSe is challenging, and the thermoelectric performance of polycrystalline SnSe is much lower than that of singlecrystal one. This restricts the large-scale application of SnSe [15]. SnSe2, as a similar semiconductor [16], with a typical Cdl2-type structure, is also predicted to be a promising n-type TE material. The ZT value of SnSe2 along the a-axis could reach 2.95 at 800 K with a carrier concentration of 1020 cm−3 [17]. Experimentally, mechanical alloying is commonly adopted prepare polycrystal SnSe2. Kanatzidis et al. prepared SnSe2 nanoplates with Se vacancies and Cl doping introduced, leading to increased ZT value of 0.63 [18]. Fu Li at al. prepared Sn0.99Ag0.01Se2 nanoplates to enhance the thermal stability by Ag doping and, the ZT value of SnSe2 is increased up to ~0.4 at 773 K [19]. In general, the measured thermoelectric performance is much lower than the theoretical prediction. Exploring the relationship between nanostructures and thermoelectric properties and regulating nanostructures to improve thermoelectric performance are worth doing. In this study, a simple solution method is proposed to synthesize SnSe2 nanoflower, and the optimized hydrothermal conditions and growth mechanism are discussed. The electrical conductivity and Seebeck coefficient is synergistically optimized due to the energy filter effect in SnSe2 nanoflowers. The thermal conductivity of SnSe2
Corresponding authors. E-mail addresses: [email protected] (S. Liu), [email protected] (F. Ma).
https://doi.org/10.1016/j.apsusc.2020.145478 Received 8 December 2019; Received in revised form 13 January 2020; Accepted 19 January 2020 Available online 22 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Structural and compositional characterization of SnSe2 nanoflowers. (a-c) SEM images at different magnifications. (d) XRD pattern of as-synthesized SnSe2 nanoflowers.
Fig. 2. Microstructural characterization of SnSe2. (a-b) Bright-field TEM image of SnSe2 nanoflower. (c) High-resolution TEM (HRTEM) image of layers on a nanoflower. (d) The selected area electron diffraction (SAED) pattern.
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Fig. 3. FE-SEM images of the samples prepared for (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 3 h, (e,f) 4 h, (g) 6 h and (h) 24 h.
were heated at 180 °C for 24 h. The resulting sediment was collected by centrifugation, washed using an ethanol and toluene mixed solution (2:1) four times, and dried under vacuum. SnSe2 nanoplates were also prepared for comparison. The preparation details [20] and characterization results can be found in the Supporting Information (Figs. S1–S3).
nanoflowers is greatly reduced owing to the strong phonon scattering by high-density interface and grain boundaries, which have been confirmed by both measurement and simulation of mean free path of phonons. The results provide a new scheme to design high-performance TE materials.
2. Experimental section 2.2. Structural characterization 2.1. Synthesis of nanostructures The phase structures were characterized by X-ray diffraction with a D/max-RB diffractometer (XRD-700, Shimadzu, Japan) using CuKα radiation at a scanning rate of 4°/min. The microscopic morphology of the samples was observed by field emission scanning electron microscopy (FE-SEM, Quanta 600 FEG, FEI, Holland), with an energy-dispersive X-ray spectrometer (EDS). The microstructure was observed by scanning transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCLAB
In a typical synthesis process of SnSe2 nanoflowers, 68 ml oleylamine (OAm) was heated up to 70 °C, and adding 2.4 mmol SeO2 to dissolve completely, the transparent solution was obtained for 10 min. Then 2.4 mmol SnCl2 and 2.5 ml 1-dodecanethiol (NDM) were added, and the mixture turned dark red immediately. After that, the mixture was stirred for 15 min to get a clear yellowish solution. Finally the mixture was added in a 100 ml Teflon lined autoclave and the reactants 3
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Fig. 1(a) displays an overview of the SnSe2 powders which are flowerlike with an average size of 2 μm. Fig. 1(b, c) shows the high-resolution SEM images of SnSe2 nanoflowers, the SnSe2 nanoflowers are built of well-ordered nanoplates about 10 nm in thickness. As shown in Fig. 1(d), the XRD peaks at 2θ = 14.44, 30.75, 40.13, and 47.77° can be indexed to the (0 0 1), (0 1 1), (0 1 2) and (1 1 0) planes of hexagonal SnSe2 (P3¯ml space group, JCPDS NO. 01-089-2939). Fig. 2(a) shows the bright-field TEM image of a single SnSe2 nanoflower with a hexagonal outline. Fig. 2(b) shows the layer structure at the edge of the nanoflower, and the symmetrical contrasts of the TEM images suggests uniform thickness. Fig. 2(c) shows a representative HRTEM image at the lamellae edge of a single nanoflower, the lamellae is single crystalline with an interplanar spacing of about 0.29 nm, corresponding to the (1 0 1) plane. Fig. 2(d) shows the SAED pattern of another lamellae and, the sharp and clear diffraction spots confirm the single-crystalline feature. Fig. 2(e) exhibits the EDS mapping of Se and Sn, they are uniformly distributed with the Sn/Se ratio of 1:2, which is in good agreement with the stoichiometry of SnSe2. Fig. 3(a-h) shows the SEM images of the samples prepared for different hydrothermal durations, and Fig. 4 displays the XRD patterns. When the solvothermal reaction proceeds for 0.5 h, the products are mainly composed of fusiform selenium rods with a diameter of about 1.3 μm (Fig. 3(a)). If the reaction time is elongated to 1 h, the fusiform selenium rods disperse into thinner and shorter cylindrical nanorods with a diameter of about 0.5 μm (Fig. 3(b)). At the reaction time of 2 h, almost all of them become tiny nanorods, and the SnSe2 nucleates on the nanorods (Fig. 3(c)). If the reaction time reaches at 3 h, the irregular flakes on the nanorods will gradually evolve into hexagonal ones (Fig. 3d). As the reaction time is extended to 4 h, the nanorods disappear and the hexagonal flakes are completely dispersed, with a diameter of 1–4 μm (Fig. 3f), most of the nanoflakes have spiral steps on the surface (Fig. 3(e)). After 6 h, most of them become the hierarchical nest structures. The upper and lower surfaces are stacked with thin layers (Fig. 3g). At the reaction time of 24 h, nanoflowers about 2 μm in diameter are obtained (Fig. 3h). The formation of SnSe2 nanoflowers could be due to screw dislocation mechanism [21,22]. Fig. 5 shows the top view of the multi-layer nanoplate growing with the screw morphology, including clockwise (Fig. 5a) and counter-clockwise (Fig. 5b) spiral direction. If the crystal growth rate at the center of the spiral is slightly faster than that on the periphery of the screw, the newly generated steps from the spiral core will catch those at the outer edge, and a step is piled up, leading to small aspect ratio nanostructure (Fig. 5(c)) [23]. For the larger and more complex nanoplates, multiple spiral cores are evidenced (Fig. 5e) and many steps appear from the edge (Fig. 5d) [24]. The steps will contact and overlap with each other for longer growth time, and they commonly become “bird nest” with small width-
Fig. 4. XRD patterns of the samples of different reaction time.
Xi+) was used to analyze the elemental components as well as the chemical valences of Sn and Se. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Mettler Toledo, TGA/ DSC3+) were measured to evaluate the thermal stability of the samples within Ar flow at 300–1000 K. 2.3. Measurement of TE properties The SnSe2 powers were heated at 450 °C under Ar flow in a tube furnace for 60 min to decompose the remaining organic ligands. Then the powders were loaded in a graphite mold 12.7 mm in diameter and densified by Spark Plasma Sintering (SPS, LABOX-325S) under an axial pressure of 35 MPa at 723 K in vacuum for 5 min. Afterward, diskshaped dense bulk samples (Ф13mm × 3 mm) were produced. The densities of the SPS-sintered samples were measured by the Archimedes method. The Seebeck coefficient and electrical conductivity were measured simultaneously in the temperature range of 300–600 K using a TE measurement system (ZEM-2, Ulvac Riko, Japan). To this end, the bulk samples were cut into rectangular shapes and polished mechanically. The thermal conductivity is measured by a laser thermal conductivity meter (LFA 457, NETZSCH, Germany). 3. Results and discussion 3.1. Microstructure and morphology of the samples Fig. 1(a-c) show the SEM images of the as-prepared nanostructures.
Fig. 5. SEM images of spiral growth structure. 4
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vol NDM is added. The XRD patterns in Fig. 7(d) indicate that the amount of NDM affects the composition of the products. Therefore, NDM, as a surfactant, can be adopted to change the growth rate of crystals in different directions, so that the morphology of SnSe2 changes from nanoplate to nanoflower. Fig. 8(a) shows the TGA curve of the SnSe2 nanoflower, it can be found that the weight loss of the powder is less than 1% at 300–550 K, at which the thermoelectric performances are measured in this work. Moreover, almost no absorption/exothermic peak could be identified from the DSC curve at 300–550 K, indicating no phase change or decomposition of SnSe2 nanoflowers (Fig. 8(b)). Accordingly, the SnSe2 nanoflowers exhibit good thermal stability at 300–550 K.
3.2. Thermoelectric properties of SnSe2 nanostructure The SnSe2 nanostructures are sintered into pellets by SPS. The relative densities of the SPS-sintered samples are above 90% of the theoretical density of SnSe2. The phases and microstructure of sintered pellets were characterized from both perpendicular and parallel to the pressing direction. The anisotropy of the microstructures is not considerable because of randomly distributed nanoflowers with randomly oriented nanosheets even after SPS sintering (Fig. S4). The XRD patterns of the as-prepared samples and the pellets. Almost all the XRD peaks of the pellet match well with the standard pattern of hexagonal SnSe2, that is, the phase of SnSe2 nanoflower is maintained after SPS sintering (Fig. S5). XRD patterns are also measured on both directions (Fig. S6). Similar phases of hexagonal SnSe2 are indexed on both the two directions, indicating almost isotropic phase [25]. Therefore, the anisotropic action of SPS sintering should have little influence on the thermoelectric properties of SnSe2 pellets. So the Seebeck coefficient and electrical conductivity are measured perpendicular to the pressing direction and the thermal conductivity is measured parallel to the pressing direction. The electrical conductivity (σ), Seebeck coefficient (S) and power factor (σS2) are measured at temperature ranging from 300 K to 550 K, and the results are plotted in Fig. 9. As shown in Fig. 9(a), the electrical conductivity of SnSe2 nanoflower is increased from ~100 Sm−1 at 300 K to ~900 Sm−1 at 550 K, characteristic of a
Fig. 6. Schematic evolution of the polymorph morphology.
to-thickness ratios. Fig. 6 schematically shows the evolution of SnSe2 nanoflower morphology. (1) SeO2 is reduced by oleylamine into selenium with a fusiform rod-like structure. (2) Selenium and Sn2+ undergo oxidationreduction reaction to form SnSe2, and the SnSe2 nanoplates are adsorbed on the surface of selenium. (3) As the selenium is reacted, the hexagonal SnSe2 nanoplates are assembled into a column and begin to disperse. (4) Multiple spiral steps appear and grow from the edge of the nanoplates through the screw dislocation growth mechanism. With the rapid growth and interaction of the spiral steps, the nanoplates grow along the normal direction. (5) The growth steps from the edge of the nanoplates result in faster overall growth at the edges than that at the center, which leads to the formation of nanoflowers. The formation of SnSe2 nanoflower is indeed related to 1-dodecanethiol (NDM), indicating that the structure is controllable. By adjusting the amount of NDM, the shape of products is changed from sheet to flower, as shown in Fig. 7. As shown in Fig. 7(a), the SnSe2 lamella with a very large width-to-thickness ratio will be formed if no NDM is added. But the sample becomes a mixture of flakes and flowers if 1.76%
Fig. 7. SEM image and XRD patterns of the samples with different amounts of 1-dodecanethiol. 5
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transport property. The increased σ and S of flower-like SnSe2 leads to sharp increase in σS2, reaching 43 μWm−1 K−2 at 550 K, as shown in Fig. 9(c). The σS2 might continue to rise with temperature, but the σS2 of flake SnSe2 decreases from 48 μWm−1 K−2 at 300 K down to 26 μWm−1 K−2 at 550 K. The Hall carrier concentration (n) and Hall mobility (μ) are measured by the Van Der Pauw method, and the results are listed in Table 1. The carrier concentration of SnSe2 nanoflowers at 300 K is 5.04 × 1019 cm−3, which is higher than that of nanoplates (2.7 × 1018 cm−3). The higher carrier concentration of nanoflowers is related with both the component and defects. The atom ratio in SnSe2 nanoflowers after sintering is Sn : Se = 37.28 : 62.72, slightly lack of Se, which can be ascribed to the evaporation of a small amount of Se atoms during sintering process (Fig. S7). As a result, redundant Sn atoms occupy the Se vacancies (VSe) and induce more free carriers as following [28]:
Sni + VSe → SnSe 2 + + 2e−. Fig. 8. TGA (a) and DSC (b) results of flower-like SnSe2 powder. The yellow shadow region highlights the temperature range at which the TE performances are measured.
(1)
As compared to nanoplates, the nanoflowers have larger specific surface area, and Se atoms are more likely to volatilize during the heating process and more Se vacancies are generated. Therefore, the more SnSe and higher carrier concentration are obtained in the flowerlike SnSe2. The carrier mobility of SnSe2 nanoflowers at 300 K is 0.14 cm2 V−1s−1, which is lower than that of nanoplates (6.69 cm2 V−1s−1). Based on the degenerate parabolic band semiconductor model, the carrier effective mass (m*) is calculated by
nondegenerate semiconductor. The electrical conductivity of SnSe2 nanoplates is about 500 Sm−1 at the temperature range, which is higher than the reported results [18,19,26,27]. As shown in Fig. 9(b), the measured S of SnSe2 nanostructure is negative, indicating n-type conducting behavior. The absolute value of S of flower-like SnSe2 is increased from 190 μVK−1 at 300 K to 220 μVK−1 at 550 K by 16%, which is slightly lower than that of the flake SnSe2 and the reported values [16,19,26,27]. The Seebeck coefficient and the electrical resistivity during the cooling process are different from those in the heating process only a little, indicating good reversibility of electrical
S=
8π 2kB2 ∗ π 2/3 m T⎛ ⎞ 3eh2 3 ⎝ n⎠
(2)
in which kB , h, e and n are Boltzmann constant, Planck constant, carrier charge and carrier concentration, respectively [29]. The m* value of the nanoflowers is 1.25 m0 at 300 K, which is much higher than that of
Fig. 9. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, and (d) thermal conductivity for SnSe2 pellets. 6
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optimization of conductivity and Seebeck coefficient is realized in the nanoflower structure. Fig. 9(d) shows the temperature dependent thermal conductivity. The flower-like SnSe2 exhibits much lower thermal conductivity of 0.44 Wm−1 K−1 which is 43% lower than that of flake SnSe2 at room temperature. Commonly, the thermal conductivity κ is composed of the electronic thermal conductivity κ e and the lattice thermal conductivity κl , and can be described by κ = κ e + κl . The κ e of the samples can be estimated by the Wiedemann-Franz law κ e = LσT [33–35], in which σ is the electrical conductivity, T is the absolute temperature, and L is the Lorenz number and estimated using a single parabolic band model [18], the results are listed in Table 1. The κ e of the SnSe2 samples are very low and the intrinsic κl is close to the κ . The κl value of flower-like SnSe2 is lower than that of most of reported bulk SnSe2 [18,19,26], primarily due to the scattering of phonons by interface [36] and the grain-boundaries. To further understand the ultralow thermal conductivity of SnSe2 nanoflower, theoretical calculation of the lattice thermal conductivity of SnSe2 is performed by ShengBTE software package which is based on the full iterative solution to the Boltzmann equation [37]. The computation details and comparison of the lattice constants are shown in the Supporting information. Fig. 10(a) displays the phonon dispersion relations of SnSe2 at its equilibrium volume along the high symmetric A-Γ-M-K-Γ directions. The hexagonal SnSe2 has three atoms in its primitive unit cell and therefore nine phonon modes in the dispersion relations. The density of state in Fig. 10(b) indicates that the phonons are mainly in the region of medium frequency. Fig. 10(c) shows the cumulative κl of SnSe2 with respect to mean free paths (MFPs) of phonon. The cumulative κl increases rapidly with MFPs at the range of 1−10 nm, suggesting the weak scattering to mid-wavelength phonons in SnSe2 crystal. As shown in Fig. 10(d), κl along the interlayer direction of hexagonal SnSe2 is much lower than that of interlayer, indicating the highly anisotropic transport performance and strong scattering between
Table 1 Carrier concentration (n), carrier mobility (μ), carrier effective mass (m*), the Lorenz constant (L) and energy barrier height (Eb) of SnSe2 nanoflower and nanoplate pellets at 300 K. Samples
n (1019 cm−3)
μ (cm2 V−1 s−1)
m*/m0
L (10−8 V2 K−2)
Eb (meV)
Nanoflower Nanoplate
5.04 0.27
0.14 6.69
1.25 0.29
1.65 1.53
131 69
nanoplates (0.29 m0). Physically, the effective mass of carriers is related to the curvature of a given band and will increase with the density of states at the Fermi level [11], which is strongly dependent on the material dimension [30]. The nanoflowers are indeed composed of thin nanosheets 10 nm in thickness with 2D characteristics, while the nanoplates 200 nm in thickness are characteristic of 3D features. Therefore, as for the nanoflower structures, the density of states near the Fermi level is higher, which leads to larger carrier effective mass and lower carrier mobility. Given that the grain-boundary thickness is smaller than the grain size D, the energy barrier is calculated as [31], 1/2
1 ⎞ μ = De ⎛ ∗ ⎝ 2πm kB T ⎠ ⎜
⎟
E exp ⎛− b ⎞ ⎝ kB T ⎠ ⎜
⎟
(3)
in which D is the grain size calculated from the XRD peak of (0 1 1) plane according to the Scherrer equation. The Eb value of SnSe2 nanoflowers is about 131 meV, which is much higher than that of SnSe2 nanoplate (69 meV), that is, the nanoflower structure elevates the boundary potential barrier significantly by introducing massive interfaces, which will filter the low-energy charge carriers and increase the average carrier energy [6,32]. As shown in Fig. 9(a) and (b), as the temperature is elevated, the electrical conductivity is increased, but the Seebeck coefficient does not decrease. Therefore, the synergistic
Fig. 10. (a) Phonon dispersion and (b) phonon densities of states of SnSe2. (c) Cumulative lattice thermal conductivity of SnSe2 as a function of phonon mean free path (d) and temperature. 7
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Supervision, Funding acquisition. Declaration of Competing Interest 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. Acknowledgements This work was jointly supported by National Natural Science Foundation of China (Grant Nos. 51771144, 51901177), Natural Science Foundation of Shaanxi Province (Nos. 2019TD-020, 2019JLM30, 2017JZ015), 111 Project 2.0 by China (BP2018008), Natural Science Foundation of Jiangsu Province (No. BK20190221), the fund of the Shaanxi Key Laboratory of Surface Engineering and Remanufacturing (tywl2019-01), Fundamental Research Funds for the Central Universities. This work was carried out using the HPCC Platform at the Xian Jiaotong University.
Fig. 11. Temperature dependence of ZT value for SnSe2 pellets.
interface and phonons in SnSe2. As a result, the total thermal conductivity of SnSe2 nanoflowers is higher than κl along the interlayer direction of SnSe2, but much lower than that of interlayer direction and nanoplates. Therefore, the ultralow thermal conductivity could be ascribed to the scattering of interfaces and grain boundaries on the phonons of all the wavelength [8]. The thermal conductivity of TE material is mainly contributed by phonons, dependent on the nanostructure of sample. Since there is no obvious phase change and decomposition at the measurement temperature range of 300–550 K, the thermal conductivity of flower-like SnSe2 should exhibit good reversibility in the test temperature range. As shown in Fig. 11, the ZT value increases substantially with the temperature as a result of the sharp increase of power factor and ultralow thermal conductivity. The highest ZT value of the SnSe2 nanoflower pellet is obtained at 550 K, which is three times that of SnSe2 nanoplate and several times that of pure SnSe2 bulks reported previously [18,19,26]. The carrier energy filtering effect of the interface and grain boundaries in nanoflower structure leads to an optimization of electrical and thermal transport. The nanoflower structures are proved to be beneficial to improved thermoelectric properties and can be adopted for the design of other materials.
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4. Conclusion In summary, flower-liked SnSe2 nanostructure was synthesized via a facile solution method. The SnSe2 nanoflowers were germinated from nanoplates by screw dislocation mechanism. The products were sintered into pellets by SPS to evaluate thermoelectric properties. As compared to SnSe2 nanoplates, the higher power factor of 43 μWm−1 K−2 is obtained at 550 K in SnSe2 nanoflowers as a result of synergistically optimization on the electrical conductivity and Seebeck coefficient by a moderate interfacial potential barrier. Furthermore, ultralow thermal conductivity of 0.44 Wm−1 K−1 was attained in SnSe2 nanoflowers due to the enhanced phonons scattering by high density interface and grain boundaries. Finally, a maximum ZT value was obtained at 550 K from the SnSe2 nanoflowers pellet. Therefore, the route to construct thermoelectric materials with hierarchical nanostructure turns out to be effective and feasible. CRediT authorship contribution statement Jun Sun: Methodology, Investigation, Writing - original draft. Shuai Liu: Conceptualization, Methodology, Formal analysis, Writing review & editing. Chen Wang: Software, Formal analysis. Yu Bai: Supervision, Project administration. Guanjun Chen: Validation. Qiaomei Luo: Resources. Fei Ma: Writing - review & editing, 8
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[35] Q. Zhang, X. Ai, L. Wang, Y. Chang, W. Luo, W. Jiang, L. Chen, Adv. Funct. Mater. 25 (2015) 966–976. [36] L. Yang, Z.-G. Chen, G. Han, M. Hong, Y. Zou, J. Zou, Nano Energy 16 (2015) 367–374. [37] W. Li, J. Carrete, N.A. Katcho, N. Mingo, Comput. Phys. Commun. 185 (2014) 1747–1758.
[31] S. Liu, N. Peng, Y. Bai, H. Xu, D.Y. Ma, F. Ma, K. Xu, Dalton Trans. 46 (2017) 4174–4181. [32] J. Martin, L. Wang, L. Chen, G.S. Nolas, Phys. Rev. B 79 (2009) 115311. [33] Y. Luo, J. Yang, G. Li, M. Liu, Y. Xiao, L. Fu, W. Li, P. Zhu, J. Peng, S. Gao, J. Zhang, Adv. Energy Mater. 4 (2014) 1300599. [34] Y. Pan, T.-R. Wei, Q. Cao, J.-F. Li, Mater. Sci. Eng., B 197 (2015) 75–81.
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Full Length Article
Interface tuning charge transport and enhanced thermoelectric properties in flower-like SnSe2 hierarchical nanostructures ⁎
Jun Suna, Shuai Liub, , Chen Wanga, Yu Baic, Guanjun Chena, Qiaomei Luoa, Fei Maa,
T
⁎
a
State Key Laboratory for Mechanical Behavior of Materials, Xi'an Jiaotong University, Xi'an 710049, China College of Sciences, Xi'an Shiyou University, Xi'an, Shannxi 710065, China c Xi'an Jiaotong University Suzhou Institute, Suzhou, Jiangsu 215123, China b
A R T I C LE I N FO
A B S T R A C T
Keywords: SnSe2 Solvothermal Nanostructure Thermoelectric properties
Thermoelectric properties could be well improved in hierarchical nanostructures due to the selective scattering on electrons and phonons by interfaces. In this paper, flower-like SnSe2 nanostructure is synthesized by solutionbased method and, the nanostructure is sintered into pellets by spark plasma sintering (SPS) to evaluate thermoelectric properties. It is demonstrated that the flower-like SnSe2 nanostructure exhibits the ultralow thermal conductivity of 0.44 Wm−1 K−1 due to the strong phonon scattering by high-density of interface and grain boundaries, which have been confirmed by both experiments and simulation. Besides, the electrical transport of the flower-like SnSe2 is optimized synergistically owing to the moderate interfacial potential barrier. The highest power factor of 43 μWm−1 K−2 and competitive ZT value are measured at 550 K. The thermoelectric performance of flower-like SnSe2 is better that that of nanoplate and bulk counterparts. It provides an efficient method to improve the thermoelectric properties of SnSe2 based materials.
1. Introduction With the aggravation of energy crisis and environmental pollution, thermoelectric (TE) materials which can directly convert waste heat into electricity make it possible to greatly improve the utilization efficiency of energy and generate power in an environment-friendly way [1–4] The thermoelectric conversion efficiency is evaluated by a dimensionless figure of merit ZT = [(S 2σ )/ κ ] T , in which S , σ , κ and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively. For high thermoelectric figure of merit, an excellent electrical transport properties (power factor, S 2σ ) and a low thermal conductivity are required, but they often follow unfavourably opposing trends [5] In nanostructured materials, such as, superlattices [6], quantum dots [7], nanowires, and nanocomposites [8], the lattice thermal conductivity might be lowered substantially due to quantum confinement effect, size effect and grain boundary scattering, and thus the thermoelectric properties could be improved significantly [9–12]. Most of the traditional thermoelectric materials with high ZT values contain metal elements, such as, tellurium, lead and antimony, with the toxic, expensive or volatile features. Selenides are expected to replace tellurides, both economically and environmentally [13]. Particularly, Tin selenide (SnSe) has been proved to be an excellent thermoelectric
⁎
materials with an ultralow thermal conductivity. The ZT value of singlecrystal SnSe is the highest one of 2.6 at 923 K [14]. However, the synthesis of single-crystal SnSe is challenging, and the thermoelectric performance of polycrystalline SnSe is much lower than that of singlecrystal one. This restricts the large-scale application of SnSe [15]. SnSe2, as a similar semiconductor [16], with a typical Cdl2-type structure, is also predicted to be a promising n-type TE material. The ZT value of SnSe2 along the a-axis could reach 2.95 at 800 K with a carrier concentration of 1020 cm−3 [17]. Experimentally, mechanical alloying is commonly adopted prepare polycrystal SnSe2. Kanatzidis et al. prepared SnSe2 nanoplates with Se vacancies and Cl doping introduced, leading to increased ZT value of 0.63 [18]. Fu Li at al. prepared Sn0.99Ag0.01Se2 nanoplates to enhance the thermal stability by Ag doping and, the ZT value of SnSe2 is increased up to ~0.4 at 773 K [19]. In general, the measured thermoelectric performance is much lower than the theoretical prediction. Exploring the relationship between nanostructures and thermoelectric properties and regulating nanostructures to improve thermoelectric performance are worth doing. In this study, a simple solution method is proposed to synthesize SnSe2 nanoflower, and the optimized hydrothermal conditions and growth mechanism are discussed. The electrical conductivity and Seebeck coefficient is synergistically optimized due to the energy filter effect in SnSe2 nanoflowers. The thermal conductivity of SnSe2
Corresponding authors. E-mail addresses: [email protected] (S. Liu), [email protected] (F. Ma).
https://doi.org/10.1016/j.apsusc.2020.145478 Received 8 December 2019; Received in revised form 13 January 2020; Accepted 19 January 2020 Available online 22 January 2020 0169-4332/ © 2020 Elsevier B.V. All rights reserved.
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Fig. 1. Structural and compositional characterization of SnSe2 nanoflowers. (a-c) SEM images at different magnifications. (d) XRD pattern of as-synthesized SnSe2 nanoflowers.
Fig. 2. Microstructural characterization of SnSe2. (a-b) Bright-field TEM image of SnSe2 nanoflower. (c) High-resolution TEM (HRTEM) image of layers on a nanoflower. (d) The selected area electron diffraction (SAED) pattern.
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Fig. 3. FE-SEM images of the samples prepared for (a) 0.5 h, (b) 1 h, (c) 2 h, (d) 3 h, (e,f) 4 h, (g) 6 h and (h) 24 h.
were heated at 180 °C for 24 h. The resulting sediment was collected by centrifugation, washed using an ethanol and toluene mixed solution (2:1) four times, and dried under vacuum. SnSe2 nanoplates were also prepared for comparison. The preparation details [20] and characterization results can be found in the Supporting Information (Figs. S1–S3).
nanoflowers is greatly reduced owing to the strong phonon scattering by high-density interface and grain boundaries, which have been confirmed by both measurement and simulation of mean free path of phonons. The results provide a new scheme to design high-performance TE materials.
2. Experimental section 2.2. Structural characterization 2.1. Synthesis of nanostructures The phase structures were characterized by X-ray diffraction with a D/max-RB diffractometer (XRD-700, Shimadzu, Japan) using CuKα radiation at a scanning rate of 4°/min. The microscopic morphology of the samples was observed by field emission scanning electron microscopy (FE-SEM, Quanta 600 FEG, FEI, Holland), with an energy-dispersive X-ray spectrometer (EDS). The microstructure was observed by scanning transmission electron microscopy (TEM, JEM-2100F, JEOL, Japan). X-ray photoelectron spectroscopy (XPS, Thermo Fisher ESCLAB
In a typical synthesis process of SnSe2 nanoflowers, 68 ml oleylamine (OAm) was heated up to 70 °C, and adding 2.4 mmol SeO2 to dissolve completely, the transparent solution was obtained for 10 min. Then 2.4 mmol SnCl2 and 2.5 ml 1-dodecanethiol (NDM) were added, and the mixture turned dark red immediately. After that, the mixture was stirred for 15 min to get a clear yellowish solution. Finally the mixture was added in a 100 ml Teflon lined autoclave and the reactants 3
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Fig. 1(a) displays an overview of the SnSe2 powders which are flowerlike with an average size of 2 μm. Fig. 1(b, c) shows the high-resolution SEM images of SnSe2 nanoflowers, the SnSe2 nanoflowers are built of well-ordered nanoplates about 10 nm in thickness. As shown in Fig. 1(d), the XRD peaks at 2θ = 14.44, 30.75, 40.13, and 47.77° can be indexed to the (0 0 1), (0 1 1), (0 1 2) and (1 1 0) planes of hexagonal SnSe2 (P3¯ml space group, JCPDS NO. 01-089-2939). Fig. 2(a) shows the bright-field TEM image of a single SnSe2 nanoflower with a hexagonal outline. Fig. 2(b) shows the layer structure at the edge of the nanoflower, and the symmetrical contrasts of the TEM images suggests uniform thickness. Fig. 2(c) shows a representative HRTEM image at the lamellae edge of a single nanoflower, the lamellae is single crystalline with an interplanar spacing of about 0.29 nm, corresponding to the (1 0 1) plane. Fig. 2(d) shows the SAED pattern of another lamellae and, the sharp and clear diffraction spots confirm the single-crystalline feature. Fig. 2(e) exhibits the EDS mapping of Se and Sn, they are uniformly distributed with the Sn/Se ratio of 1:2, which is in good agreement with the stoichiometry of SnSe2. Fig. 3(a-h) shows the SEM images of the samples prepared for different hydrothermal durations, and Fig. 4 displays the XRD patterns. When the solvothermal reaction proceeds for 0.5 h, the products are mainly composed of fusiform selenium rods with a diameter of about 1.3 μm (Fig. 3(a)). If the reaction time is elongated to 1 h, the fusiform selenium rods disperse into thinner and shorter cylindrical nanorods with a diameter of about 0.5 μm (Fig. 3(b)). At the reaction time of 2 h, almost all of them become tiny nanorods, and the SnSe2 nucleates on the nanorods (Fig. 3(c)). If the reaction time reaches at 3 h, the irregular flakes on the nanorods will gradually evolve into hexagonal ones (Fig. 3d). As the reaction time is extended to 4 h, the nanorods disappear and the hexagonal flakes are completely dispersed, with a diameter of 1–4 μm (Fig. 3f), most of the nanoflakes have spiral steps on the surface (Fig. 3(e)). After 6 h, most of them become the hierarchical nest structures. The upper and lower surfaces are stacked with thin layers (Fig. 3g). At the reaction time of 24 h, nanoflowers about 2 μm in diameter are obtained (Fig. 3h). The formation of SnSe2 nanoflowers could be due to screw dislocation mechanism [21,22]. Fig. 5 shows the top view of the multi-layer nanoplate growing with the screw morphology, including clockwise (Fig. 5a) and counter-clockwise (Fig. 5b) spiral direction. If the crystal growth rate at the center of the spiral is slightly faster than that on the periphery of the screw, the newly generated steps from the spiral core will catch those at the outer edge, and a step is piled up, leading to small aspect ratio nanostructure (Fig. 5(c)) [23]. For the larger and more complex nanoplates, multiple spiral cores are evidenced (Fig. 5e) and many steps appear from the edge (Fig. 5d) [24]. The steps will contact and overlap with each other for longer growth time, and they commonly become “bird nest” with small width-
Fig. 4. XRD patterns of the samples of different reaction time.
Xi+) was used to analyze the elemental components as well as the chemical valences of Sn and Se. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) (Mettler Toledo, TGA/ DSC3+) were measured to evaluate the thermal stability of the samples within Ar flow at 300–1000 K. 2.3. Measurement of TE properties The SnSe2 powers were heated at 450 °C under Ar flow in a tube furnace for 60 min to decompose the remaining organic ligands. Then the powders were loaded in a graphite mold 12.7 mm in diameter and densified by Spark Plasma Sintering (SPS, LABOX-325S) under an axial pressure of 35 MPa at 723 K in vacuum for 5 min. Afterward, diskshaped dense bulk samples (Ф13mm × 3 mm) were produced. The densities of the SPS-sintered samples were measured by the Archimedes method. The Seebeck coefficient and electrical conductivity were measured simultaneously in the temperature range of 300–600 K using a TE measurement system (ZEM-2, Ulvac Riko, Japan). To this end, the bulk samples were cut into rectangular shapes and polished mechanically. The thermal conductivity is measured by a laser thermal conductivity meter (LFA 457, NETZSCH, Germany). 3. Results and discussion 3.1. Microstructure and morphology of the samples Fig. 1(a-c) show the SEM images of the as-prepared nanostructures.
Fig. 5. SEM images of spiral growth structure. 4
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vol NDM is added. The XRD patterns in Fig. 7(d) indicate that the amount of NDM affects the composition of the products. Therefore, NDM, as a surfactant, can be adopted to change the growth rate of crystals in different directions, so that the morphology of SnSe2 changes from nanoplate to nanoflower. Fig. 8(a) shows the TGA curve of the SnSe2 nanoflower, it can be found that the weight loss of the powder is less than 1% at 300–550 K, at which the thermoelectric performances are measured in this work. Moreover, almost no absorption/exothermic peak could be identified from the DSC curve at 300–550 K, indicating no phase change or decomposition of SnSe2 nanoflowers (Fig. 8(b)). Accordingly, the SnSe2 nanoflowers exhibit good thermal stability at 300–550 K.
3.2. Thermoelectric properties of SnSe2 nanostructure The SnSe2 nanostructures are sintered into pellets by SPS. The relative densities of the SPS-sintered samples are above 90% of the theoretical density of SnSe2. The phases and microstructure of sintered pellets were characterized from both perpendicular and parallel to the pressing direction. The anisotropy of the microstructures is not considerable because of randomly distributed nanoflowers with randomly oriented nanosheets even after SPS sintering (Fig. S4). The XRD patterns of the as-prepared samples and the pellets. Almost all the XRD peaks of the pellet match well with the standard pattern of hexagonal SnSe2, that is, the phase of SnSe2 nanoflower is maintained after SPS sintering (Fig. S5). XRD patterns are also measured on both directions (Fig. S6). Similar phases of hexagonal SnSe2 are indexed on both the two directions, indicating almost isotropic phase [25]. Therefore, the anisotropic action of SPS sintering should have little influence on the thermoelectric properties of SnSe2 pellets. So the Seebeck coefficient and electrical conductivity are measured perpendicular to the pressing direction and the thermal conductivity is measured parallel to the pressing direction. The electrical conductivity (σ), Seebeck coefficient (S) and power factor (σS2) are measured at temperature ranging from 300 K to 550 K, and the results are plotted in Fig. 9. As shown in Fig. 9(a), the electrical conductivity of SnSe2 nanoflower is increased from ~100 Sm−1 at 300 K to ~900 Sm−1 at 550 K, characteristic of a
Fig. 6. Schematic evolution of the polymorph morphology.
to-thickness ratios. Fig. 6 schematically shows the evolution of SnSe2 nanoflower morphology. (1) SeO2 is reduced by oleylamine into selenium with a fusiform rod-like structure. (2) Selenium and Sn2+ undergo oxidationreduction reaction to form SnSe2, and the SnSe2 nanoplates are adsorbed on the surface of selenium. (3) As the selenium is reacted, the hexagonal SnSe2 nanoplates are assembled into a column and begin to disperse. (4) Multiple spiral steps appear and grow from the edge of the nanoplates through the screw dislocation growth mechanism. With the rapid growth and interaction of the spiral steps, the nanoplates grow along the normal direction. (5) The growth steps from the edge of the nanoplates result in faster overall growth at the edges than that at the center, which leads to the formation of nanoflowers. The formation of SnSe2 nanoflower is indeed related to 1-dodecanethiol (NDM), indicating that the structure is controllable. By adjusting the amount of NDM, the shape of products is changed from sheet to flower, as shown in Fig. 7. As shown in Fig. 7(a), the SnSe2 lamella with a very large width-to-thickness ratio will be formed if no NDM is added. But the sample becomes a mixture of flakes and flowers if 1.76%
Fig. 7. SEM image and XRD patterns of the samples with different amounts of 1-dodecanethiol. 5
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transport property. The increased σ and S of flower-like SnSe2 leads to sharp increase in σS2, reaching 43 μWm−1 K−2 at 550 K, as shown in Fig. 9(c). The σS2 might continue to rise with temperature, but the σS2 of flake SnSe2 decreases from 48 μWm−1 K−2 at 300 K down to 26 μWm−1 K−2 at 550 K. The Hall carrier concentration (n) and Hall mobility (μ) are measured by the Van Der Pauw method, and the results are listed in Table 1. The carrier concentration of SnSe2 nanoflowers at 300 K is 5.04 × 1019 cm−3, which is higher than that of nanoplates (2.7 × 1018 cm−3). The higher carrier concentration of nanoflowers is related with both the component and defects. The atom ratio in SnSe2 nanoflowers after sintering is Sn : Se = 37.28 : 62.72, slightly lack of Se, which can be ascribed to the evaporation of a small amount of Se atoms during sintering process (Fig. S7). As a result, redundant Sn atoms occupy the Se vacancies (VSe) and induce more free carriers as following [28]:
Sni + VSe → SnSe 2 + + 2e−. Fig. 8. TGA (a) and DSC (b) results of flower-like SnSe2 powder. The yellow shadow region highlights the temperature range at which the TE performances are measured.
(1)
As compared to nanoplates, the nanoflowers have larger specific surface area, and Se atoms are more likely to volatilize during the heating process and more Se vacancies are generated. Therefore, the more SnSe and higher carrier concentration are obtained in the flowerlike SnSe2. The carrier mobility of SnSe2 nanoflowers at 300 K is 0.14 cm2 V−1s−1, which is lower than that of nanoplates (6.69 cm2 V−1s−1). Based on the degenerate parabolic band semiconductor model, the carrier effective mass (m*) is calculated by
nondegenerate semiconductor. The electrical conductivity of SnSe2 nanoplates is about 500 Sm−1 at the temperature range, which is higher than the reported results [18,19,26,27]. As shown in Fig. 9(b), the measured S of SnSe2 nanostructure is negative, indicating n-type conducting behavior. The absolute value of S of flower-like SnSe2 is increased from 190 μVK−1 at 300 K to 220 μVK−1 at 550 K by 16%, which is slightly lower than that of the flake SnSe2 and the reported values [16,19,26,27]. The Seebeck coefficient and the electrical resistivity during the cooling process are different from those in the heating process only a little, indicating good reversibility of electrical
S=
8π 2kB2 ∗ π 2/3 m T⎛ ⎞ 3eh2 3 ⎝ n⎠
(2)
in which kB , h, e and n are Boltzmann constant, Planck constant, carrier charge and carrier concentration, respectively [29]. The m* value of the nanoflowers is 1.25 m0 at 300 K, which is much higher than that of
Fig. 9. Temperature dependence of (a) electrical conductivity, (b) Seebeck coefficient, (c) power factor, and (d) thermal conductivity for SnSe2 pellets. 6
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optimization of conductivity and Seebeck coefficient is realized in the nanoflower structure. Fig. 9(d) shows the temperature dependent thermal conductivity. The flower-like SnSe2 exhibits much lower thermal conductivity of 0.44 Wm−1 K−1 which is 43% lower than that of flake SnSe2 at room temperature. Commonly, the thermal conductivity κ is composed of the electronic thermal conductivity κ e and the lattice thermal conductivity κl , and can be described by κ = κ e + κl . The κ e of the samples can be estimated by the Wiedemann-Franz law κ e = LσT [33–35], in which σ is the electrical conductivity, T is the absolute temperature, and L is the Lorenz number and estimated using a single parabolic band model [18], the results are listed in Table 1. The κ e of the SnSe2 samples are very low and the intrinsic κl is close to the κ . The κl value of flower-like SnSe2 is lower than that of most of reported bulk SnSe2 [18,19,26], primarily due to the scattering of phonons by interface [36] and the grain-boundaries. To further understand the ultralow thermal conductivity of SnSe2 nanoflower, theoretical calculation of the lattice thermal conductivity of SnSe2 is performed by ShengBTE software package which is based on the full iterative solution to the Boltzmann equation [37]. The computation details and comparison of the lattice constants are shown in the Supporting information. Fig. 10(a) displays the phonon dispersion relations of SnSe2 at its equilibrium volume along the high symmetric A-Γ-M-K-Γ directions. The hexagonal SnSe2 has three atoms in its primitive unit cell and therefore nine phonon modes in the dispersion relations. The density of state in Fig. 10(b) indicates that the phonons are mainly in the region of medium frequency. Fig. 10(c) shows the cumulative κl of SnSe2 with respect to mean free paths (MFPs) of phonon. The cumulative κl increases rapidly with MFPs at the range of 1−10 nm, suggesting the weak scattering to mid-wavelength phonons in SnSe2 crystal. As shown in Fig. 10(d), κl along the interlayer direction of hexagonal SnSe2 is much lower than that of interlayer, indicating the highly anisotropic transport performance and strong scattering between
Table 1 Carrier concentration (n), carrier mobility (μ), carrier effective mass (m*), the Lorenz constant (L) and energy barrier height (Eb) of SnSe2 nanoflower and nanoplate pellets at 300 K. Samples
n (1019 cm−3)
μ (cm2 V−1 s−1)
m*/m0
L (10−8 V2 K−2)
Eb (meV)
Nanoflower Nanoplate
5.04 0.27
0.14 6.69
1.25 0.29
1.65 1.53
131 69
nanoplates (0.29 m0). Physically, the effective mass of carriers is related to the curvature of a given band and will increase with the density of states at the Fermi level [11], which is strongly dependent on the material dimension [30]. The nanoflowers are indeed composed of thin nanosheets 10 nm in thickness with 2D characteristics, while the nanoplates 200 nm in thickness are characteristic of 3D features. Therefore, as for the nanoflower structures, the density of states near the Fermi level is higher, which leads to larger carrier effective mass and lower carrier mobility. Given that the grain-boundary thickness is smaller than the grain size D, the energy barrier is calculated as [31], 1/2
1 ⎞ μ = De ⎛ ∗ ⎝ 2πm kB T ⎠ ⎜
⎟
E exp ⎛− b ⎞ ⎝ kB T ⎠ ⎜
⎟
(3)
in which D is the grain size calculated from the XRD peak of (0 1 1) plane according to the Scherrer equation. The Eb value of SnSe2 nanoflowers is about 131 meV, which is much higher than that of SnSe2 nanoplate (69 meV), that is, the nanoflower structure elevates the boundary potential barrier significantly by introducing massive interfaces, which will filter the low-energy charge carriers and increase the average carrier energy [6,32]. As shown in Fig. 9(a) and (b), as the temperature is elevated, the electrical conductivity is increased, but the Seebeck coefficient does not decrease. Therefore, the synergistic
Fig. 10. (a) Phonon dispersion and (b) phonon densities of states of SnSe2. (c) Cumulative lattice thermal conductivity of SnSe2 as a function of phonon mean free path (d) and temperature. 7
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Supervision, Funding acquisition. Declaration of Competing Interest 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. Acknowledgements This work was jointly supported by National Natural Science Foundation of China (Grant Nos. 51771144, 51901177), Natural Science Foundation of Shaanxi Province (Nos. 2019TD-020, 2019JLM30, 2017JZ015), 111 Project 2.0 by China (BP2018008), Natural Science Foundation of Jiangsu Province (No. BK20190221), the fund of the Shaanxi Key Laboratory of Surface Engineering and Remanufacturing (tywl2019-01), Fundamental Research Funds for the Central Universities. This work was carried out using the HPCC Platform at the Xian Jiaotong University.
Fig. 11. Temperature dependence of ZT value for SnSe2 pellets.
interface and phonons in SnSe2. As a result, the total thermal conductivity of SnSe2 nanoflowers is higher than κl along the interlayer direction of SnSe2, but much lower than that of interlayer direction and nanoplates. Therefore, the ultralow thermal conductivity could be ascribed to the scattering of interfaces and grain boundaries on the phonons of all the wavelength [8]. The thermal conductivity of TE material is mainly contributed by phonons, dependent on the nanostructure of sample. Since there is no obvious phase change and decomposition at the measurement temperature range of 300–550 K, the thermal conductivity of flower-like SnSe2 should exhibit good reversibility in the test temperature range. As shown in Fig. 11, the ZT value increases substantially with the temperature as a result of the sharp increase of power factor and ultralow thermal conductivity. The highest ZT value of the SnSe2 nanoflower pellet is obtained at 550 K, which is three times that of SnSe2 nanoplate and several times that of pure SnSe2 bulks reported previously [18,19,26]. The carrier energy filtering effect of the interface and grain boundaries in nanoflower structure leads to an optimization of electrical and thermal transport. The nanoflower structures are proved to be beneficial to improved thermoelectric properties and can be adopted for the design of other materials.
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4. Conclusion In summary, flower-liked SnSe2 nanostructure was synthesized via a facile solution method. The SnSe2 nanoflowers were germinated from nanoplates by screw dislocation mechanism. The products were sintered into pellets by SPS to evaluate thermoelectric properties. As compared to SnSe2 nanoplates, the higher power factor of 43 μWm−1 K−2 is obtained at 550 K in SnSe2 nanoflowers as a result of synergistically optimization on the electrical conductivity and Seebeck coefficient by a moderate interfacial potential barrier. Furthermore, ultralow thermal conductivity of 0.44 Wm−1 K−1 was attained in SnSe2 nanoflowers due to the enhanced phonons scattering by high density interface and grain boundaries. Finally, a maximum ZT value was obtained at 550 K from the SnSe2 nanoflowers pellet. Therefore, the route to construct thermoelectric materials with hierarchical nanostructure turns out to be effective and feasible. CRediT authorship contribution statement Jun Sun: Methodology, Investigation, Writing - original draft. Shuai Liu: Conceptualization, Methodology, Formal analysis, Writing review & editing. Chen Wang: Software, Formal analysis. Yu Bai: Supervision, Project administration. Guanjun Chen: Validation. Qiaomei Luo: Resources. Fei Ma: Writing - review & editing, 8
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