Cu-Embedded SnSe2 with a High Figure of Merit at Ecofriendly Temperature.

There are many studies concentrated on high-temperature performance of SnSe2, but few studies were conducted on low-temperature properties of embedded SnSe2. In this work, a series of SnCu x Se2 (x = 0, 0.01, 0.02, and 0.05) layered structures have been successfully synthesized by a melt quenching, mechanical milling process, and spark plasma sintering (SPS) method. Meanwhile, the thermal and electrical transport properties of all synthesized samples are measured. These results suggest that the embedding of Cu into SnSe2 results in a high carrier concentration (1019/cm3). In addition, the enhancement of defect and interfacial phonon scattering caused by Cu embedding as well as the weak van der Waals force between layers makes a low thermal conductivity (0.81 W/mK) for the SnCu0.01Se2 at 300 K. Moreover, the maximum ZT is acquired up to 0.75 for the SnCu0.01Se2 sample at 300 K, which is about 2 orders of magnitude higher than the pristine sample (0.009). These features indicate that Cu-embedded SnSe2 can be a promising thermoelectric material at gentle temperature.

Introduction

Thermoelectric materials based on the Seebeck and Peltier effects can achieve the direct conversion of thermal and electric energy with the advantages of small size, light weight, no noise, environmentally friendly, and long life.[1,2] Thus, thermoelectricity is playing a very important role in sustainable energy development. The conversion efficiency of a thermoelectric material is determined by the dimensionless figure of merit (ZTeng), defined as where T, PF, and κ are the absolute temperature, power factor, and thermal conductivity, respectively.[3−5] It is clear and unambiguous that a high thermoelectric performance is obtained by two basic approaches: maximizing power factor (S2σ) and minimizing thermal conductivity. The Wiedemann–Franz law κe = LσT, Motts relation , and σ = neμ where L, σ, T, m*, kB, h, n, e, and μ are the Lorentz factor, electrical conductivity, absolute temperature, effective mass of the carriers, Boltzmann constant, Planck constant, electron charge, carrier concentration, and carrier mobility, respectively. The development of highly efficient thermoelectric materials is still a great challenge because S, σ, and κe are strongly coupled with each other.[6,7] In general, any methods and materials can decouple that the correlation between these parameters can significantly improve the efficiency of thermoelectric conversion.[8,9] Therefore, adjusting these parameters to improve the thermoelectric performance has become the goal of researchers in the field of thermoelectricity.[10]

Recently, thermoelectric materials have focused on well-known compositions such as Bi2Te3,[11] SnSe,[12] and SnSe2.[13,14] For example, Akshay’s research group reported that a ZT value of ∼0.78 could be finally achieved for the composition of Bi2Te3 at 300 K synthesized by the chemical method.[11] Besides, Wang’s study team also interestingly found that a maximum ZT of ∼0.34 at 350 K was achieved in a 1 at % Na-doped single-crystalline SnSe synthesized by the Bridgman method.[12] Moreover, a research group realized that ZTeng for SnCu0.005Se1.98Br0.02 is ≈0.25 between 773 and 300 K13. In addition, the thermoelectric figure of merit ZTmax of 0.63 is achieved for a 1.5 at % Cl-doped SnSe1.95 pellet at 673 K obtained by a vacuum-sealed high-temperature melting and SPS process claimed through Luo et al.[14] SnSe2 is a semiconductor material with an indirect broadband gap, high band degeneracy, and layered structure, which is mainly used in the field of photoelectricity and energy storage.[15] The atomic model of SnSe2 is shown in Figure , and each layer is covalently bonded by three atomic planes in the sequence of Se–Sn–Se. In addition, the interaction between layers was coupled by a weak van der Waals force, and the spacing was 0.62 nm. In this work, we have successfully synthesized a series of SnCuSe2 (x = 0, 0.01, 0.02, and 0.05) layered structures by a melt quenching, mechanical milling process, and spark plasma sintering (SPS) method. At the same time, we study the thermoelectric properties of the synthesized samples measured from 50 to 300 K. Interestingly, carrier mobility reduce a little with a small amount of Cu embedding compared to the raw sample, but its concentration has nearly increased 2 orders of magnitude than that of raw samples. Otherwise, embedding of Cu leads to enhancement of interfacial phonon scattering, defect phonon scattering, electron–phonon scattering, and the weak van der Waals force between the layers combined to achieve a low thermal conductivity (0.81 W/mK) for the SnCu0.01Se2 at 300 K. At the same time, the maximum ZT is achievable at 0.75 for the SnCu0.01Se2 sample at 300 K, which is about 2 orders of magnitude higher than the original sample (0.009) and is about 2 times greater than that of the previous work.[16−18] Therefore, Cu-embedded SnSe2 becomes a promising thermoelectric material applied widely in the industry at an ecofriendly temperature.

Figure 1

Crystal structural model of a layered SnSe2.

Results and Discussion

Figure shows representative XRD patterns of SnCuSe2 (x = 0, 0.01, 0.02, and 0.05) samples. The XRD spectrum can be well indexed as a SnSe2 phase without any secondary phase within the detection limit of the laboratory XRD, which is in good agreement with the standard data (PDF #23-0602). No extra peaks related to other crystalline phases are observed in the Cu-embedded samples, indicating that the crystal structure of the low concentration-embedded samples has not change. The lattice parameters are 3.81 × 3.81 × 6.14, and angles α, β, and γ are 90, 90, and 120°, respectively, which indicate that the crystal structure of the synthesized sample is an hexagonal crystal structure. At the same time, the half-peak width of all the samples is narrow, indicating that the crystallization is good. According to Bragg’s law nλ = 2d sinθ, the results show that the characteristic preferred orientation for the (00l ) Bragg reflection peaks is due to the lamellar structure. The cell dimension along the c axis of SnCu0.05Se2 increases slightly compared with SnSe2, whereas they have the same distance along the a and b axes, which is consistent with the measurement results of Zhou et al.[13]

Figure 2

XRD pattern of all the Cu-embedded SnSe2 samples.

The SEM image of the fresh fracture surfaces for the SnCu0.05Se2 nanocomposite is shown in Figure a. It is obvious from the picture that the synthesized sample has layered structures, and the Cu particles are embedded into the SnSe2 layer, which is consistent with the deeper observation by TEM in Figure b. Moreover, the element mapping diagram of the composition analysis of the SnCu0.05Se2 sample measured during TEM imaging shows clearly that Cu elements are uniformly dispersed in layered compound SnSe2. These extra grain boundaries and defects play an important role in the electrical transport and phonon scattering of samples. The high-resolution TEM (HRTEM) image of the SnCu0.05Se2 sample shows an interlayer spacing of 0.62 nm, which is the lattice spacing of the (001) plane (interlayer distance) SnSe2 (Figure c). This result further indicates that each Cu exists between the layers of SnSe2 without changing the lattice structure.

Figure 3

(a) SEM, (b) TEM, and (c) HRTEM images of the fresh fracture surfaces for SnCu0.05Se2 and elemental mapping spectra for Sn, Se, and Cu in the SnCu0.05Se2 during TEM imaging (from the highlighted portion of the TEM image).

To further determine the element composition of copper and clarify the chemical state of copper, the SnCu0.05Se2 sample is determined by X-ray photoelectron spectroscopy (XPS). Figure a shows the full scan of XPS spectra of sample SnCu0.05Se2, which shows that the sample is composed of Sn, Se, and Cu elements. All HRXPS data have been processed for peak splitting and fitting. At the same time, Figure b shows that the high-resolution peaks of Sn 3d3/2 and Sn 3d5/2 corresponding to their binding energies were 494.9 and 486.4 eV, respectively, with a splitting of 8.5 eV. Figure c shows that peaks due to Se 3d3/2 and Se 3d5/2 have appeared as a broad peak at 53.91 eV. In addition, the high-resolution XPS of Cu 2p is shown in Figure d, and there are two peaks at 931.1 and 951.1 eV corresponding to Cu 2p3/2 and Cu 2p1/2, respectively, indicating the presence of a simple copper in the SnSe2 samples.

Figure 4

Full scan of the (a) XPS spectrum and HRXPS spectra of (b) Sn 3d, (c) Se 3d, and (d) Cu 2p of the SnCu0.05Se2 sample.

Thermoelectric properties of the both embedded and pristine SnSe2 samples are measured as a function of temperature, ranging from 50 to 300 K. The carrier concentration of the embedded sample increases as the amount of embedding rises due to the effective electron injection of the simple copper in Figure . At 300 K, the carrier concentration of the SnCu0.05Se2 sample is 1 × 1019 cm–3, which is nearly 2 orders of magnitude higher than that of the pristine SnSe2 sample, which is 3.3 × 1017 cm–3. The carrier mobility with the changing temperature is shown in Figure . With the increase of temperature and the Cu-embedded amount, the mobility of all samples decreases to a small extent. The reason is due to electron–phonon scattering, which is where the electron mobility normally increases with decreasing temperature with the slope of μ ∼ T–3/2. At the same time, the enhancement of interfacial scattering leads to decrease the average free path of carriers. The decrease of average free path results in the reduction of carrier mobility.[20]

Figure 5

Carrier concentration of all the Cu-embedded SnSe2 samples.

Figure 6

Carrier mobility of all the Cu-embedded SnSe2 samples.

Figure shows the temperature dependence of electrical resistivity, the Seebeck coefficient, thermal conductivity, and power factor of SnCuSe2 (x = 0, 0.01, 0.02, and 0.05) samples. Figure a shows the electrical resistivity (ρ) of the samples as a function of temperature. The electrical resistivity of the samples decreases as temperature increases from 50 to 125 K, showing a semiconducting behavior, and then increases from 125 to 300 K, exhibiting a metal-like behavior. It can be seen from the spectra that the electrical resistivity of SnSe2 samples embedded with different concentration of Cu is much lower than that of the pristine sample. The sample of SnCu0.01Se2 (2.37 mΩcm) is reduced by more than 2 orders of magnitude compared with the pristine SnSe2 sample (506 mΩcm) at 300 K.

Figure 7

Temperature dependence of the thermal and electrical properties of SnCuSe2 (x = 0, 0.01, 0.02, and 0.05), (a) ρ (T) (mΩcm), (b) S (T) (μVK–1), (c) κ (T) (WK–1 m–1), and (d) PF (T) (mWK–2 m–1).

It can be seen from Figure b that the Seebeck coefficient values of all the samples are negative, indicating that the samples are n-type semiconductors, although Cu is embedded. n-Type semiconductors have free electrons as the main carriers. The results show that a certain amount of Cu embedding does not affect its conductive mechanism. The absolute values of the Seebeck coefficient for all samples show a similar temperature dependence at 50–100 K that increases with decreasing temperature, which might be due to the phonon drag effect.[19] Besides, the Seebeck coefficient of all embedded samples is significantly lower than that of pristine samples in the temperature range of 50–300 K. As the embedding amount of Cu increases, the corresponding absolute values of the Seebeck coefficient decrease from 375 to 136 μV/K at 300 K. The Seebeck coefficient is defined by the Motts relation, which shows that there is an inverse relationship between S and n2/37. Therefore, with the increase of the Cu-embedded amount, the absolute values of the Seebeck coefficient decrease due to increasing the carrier concentration.

The measured thermal conductivity of all samples of SnCuSe2 (x = 0, 0.01, 0.02, and 0.05) is shown in Figure c. The total thermal conductivity (κ) is usually composed of electronic (κe) and lattice thermal conductivity (κl) with κe and κl contributed by electrons and phonons, respectively. All the samples of SnCuSe2 (x = 0.01, 0.02, and 0.05) achieve a reduced thermal conductivity than those of Cu-free SnSe2 in the temperature range of 50–300 K. Especially, the samples of SnCuSe2 (x = 0.01 and 0.05) exhibit a significant reduction of thermal conductivity in the entire temperature range because of the increased interfaces, which cause a lattice phonon dissipation as well as carrier scattering at the same time.[20,21] The introduction of Cu particles into SnSe2 did not affect the morphology of the matrix shown in the SEM image. In addition, with the increase of Cu embedding, the phonon scattering from the interface and defects is enhanced, leading to a significant reduction in thermal conductivity.[22] At 300 K, it is sure that the SnCu0.01Se2 obtained the low thermal conductivity (0.81 W/mK).

Combining the Seebeck coefficient and electrical resistivity measured at different temperatures, the variation trend of power factor PF () with temperature can be obtained through formula calculation, as shown in Figure d. It can be seen clearly from the picture that the SnCu0.01Se2 sample obtained the high power factor PF = 1.96 mWK–2 m–1, which is nearly 2 orders of magnitude higher than the PF = 0.03 mWK–2 m–1 of pristine SnSe2 at 300 K.

Combining the results of the electrical and thermal transport properties, the temperature dependence of the ZT value is shown in Figure . Obviously, the highest ZT value of ∼0.75 is reached in the SnCu0.01Se2 sample at 300 K, which is 2 orders of magnitude higher than the pristine sample (0.009). One reason is because the electrical resistivity of SnCu0.01Se2 decreases significantly compared with pure SnSe2 (ρ0.01 ≈ 0.01ρ0). Moreover, SnCu0.01Se2 obtained the low thermal conductivity (0.81 W/mK). The SnCuSe2 materials achieve high thermoelectric conversion efficiency by optimizing electrical and thermal conductivity.

Figure 8

Figure of merit (ZT) of SnCuSe2 (x = 0, 0.01, 0.02, 0.05) as a function of temperature.

Conclusions

In summary, a series of the SnCuSe2 (x = 0, 0.01, 0.02, and 0.05) layered structure with uniform size distribution are synthesized by a melt quenching, mechanical milling process, and spark plasma sintering (SPS) method. The effect of SnCuSe2 (x = 0, 0.01, 0.02, and 0.05) on thermoelectric properties of nanocomposites is studied in the temperature range of 50–300 K. The results show that electrical resistivity is reduced by Cu embedding, which is found to have a maximum power factor of 1.96 mWK–2 m–1 in SnCu0.01Se2 at 300 K. Combining the small thermal conductivity (0.81 W/mK), the maximum ZT value obtained by the formula is 0.75, which is 2 orders of magnitude higher than the pristine sample (0.009). These results suggest that Cu-embedded layered compound SnSe2 can be considered as a promising environmentally friendly and economical thermoelectric material for collecting waste heat to convert into useful electrical energy at gentle temperature.

Experiment

Pure powders Sn, Se (5 N, Aladdin), and Cu (3 N, Aladdin) were used as raw materials and weighed according to an appropriate molar ratio of SnCuSe2 (x = 0, 0.01, 0.02, and 0.05), and the raw materials were all weighed in the vacuum glovebox. After weighing, the reaction materials were thoroughly mixed and put into a quartz tube, which was sealed by a vacuum. The materials in the tube were heated to 973 K, kept for 6 h, and cooled to room temperature. The obtained ingots were annealed at 773 K for 72 h and naturally cooled to room temperature in argon atmosphere. Afterward, samples were removed and placed in a ball mill for 9 h of milling at 350 r/min under argon atmosphere. The resulting powder was vacuum-sintered for 5 min under pressures of 773 K and 55 MPa by spark plasma sintering (SPS) equipment. After SPS, a cylindrical sample with a height of ≈2 mm and a diameter of 12.7 mm was obtained.

The phase of the as-synthesized products was studied by means of X-ray diffraction (XRD) (X’pert MRD-Philips diffractometer with Cu Kα radiation, λ = 0.154178 Å, a scanning speed of 0.01672°/s in the 2θ range from 10 to 65°, Philips, Holland). The morphology and the element distribution mapping of the powder samples were characterized by transmission electron microscopy (TEM) and scanning electron microscopy (SEM). An X-ray photoelectron spectrometer (AXIS ULTRA, London, Britain) was used to analyze the chemical valence and type of the elements of the sample. The disk samples were cut into 2 × 2 × 11 mm3 rectangular shape and polished and used as thermal and electrical transport property tests. In a physical property measurement system (PPMS-9 EverCool), the low-temperature electrical resistivity, thermal conductivity, and Seebeck coefficient of the samples were measured from 50-300 K with the thermal transport option (TTO) mode. According to the Stephen–Boltzmann law, the radiation loss was corrected for the entire measurement temperature range. At 50 K, the numerical error obtained by the system was within 5%. Hall measurements were carried out in the advanced electrical transport option (ETO) mode of the PPMS system from 50 to 300 K using a four-probe method with a magnetic field of up to ±1 T. In addition, the electrical resistivity was confirmed by the ETO model from PPMS.