{"paper_id":"00345797-c33e-4a55-bd6f-9a0c9a4de1b7","body_text":"Enhanced thermoelectric performance of polycrystalline SnSe by compositing with layered Ti3C2Tx | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Enhanced thermoelectric performance of polycrystalline SnSe by compositing with layered Ti 3 C 2 T x Yi Qin, Xiaohan Li, Ting Zhao, Jianfeng Zhu, Yanling Yang, Meiqian Xie This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-607248/v1 This work is licensed under a CC BY 4.0 License Status: Posted Version 1 posted You are reading this latest preprint version Abstract Thermoelectric materials convert thermal energy into electricity directly. Constructing nanostructured composite architectures can be an effective strategy to develop thermoelectric performance. SnSe/Ti 3 C 2 T x composite materials were synthesized through the electrostatic self-assembly method followed by spark plasma sintering. The interfaces introduced by Ti 3 C 2 T x can scatter carriers effectively, thus increasing the Seebeck coefficients ( S ), finally, a high absolute S value of ~ 296.2 µV K − 1 was obtained at 773 K. At the same time, the high-density interfaces of SnSe/Ti 3 C 2 T x composites enhance the phonon scattering, a low lattice thermal conductivity k la t of 0.54 W m − 1 K − 1 was obtained in sample ω = 0.1%. Benefit from the elevated Seebeck coefficient and decreased thermal conductivity, a ZT of 0.1 was obtained in sample ω = 0.1% at 773 K along the pressing direction, compared with the pure SnSe, the thermoelectric performance improved by 68%. This research will provide a new way for the development of the thermoelectric properties of polycrystalline SnSe. Ceramics polycrystalline SnSe Ti3C2Tx electrostatic self-assembly thermoelectric properties Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 Figure 6 Figure 7 1. Introduction Thermoelectric materials convert thermal energy into electricity directly while they are independent of moving parts and extremely reliable [1], so they will play a key role in power generation. The property of thermoelectric materials is manifested by the dimensionless figure of merit ( ZT ), ZT = (S 2 σ/к) T , where S , σ , S 2 σ , к , and T are the Seebeck coefficient, electrical conductivity, power factor, thermal conductivity, and absolute temperature, respectively [2]. Many efforts have been taken for developing the ZT value [3–5], the ameliorating methods are divided into two classes, one is using doping to enhance the electrical performance of the thermoelectric materials [6–8], and the other is reducing the lattice thermal conductivity by amplifying phonon scattering [9,10]. But because of the coupling relationship between thermal conductivity and electrical conductivity, it is essential to find materials that have intrinsically low thermal conductivity and adjustable electrical conductivity. SnSe is a thermoelectric material that combines these two advantages. SnSe has a layered orthorhombic crystal structure [11], the strong anharmonicity in Sn-Se bonding gives rise to high Grüneisen parameters [12], leading to intrinsically low thermal conductivity. Also, a phase transition from Pnma to Cmcm in SnSe results in the divergence of two overlapped conduction bands, which lessens the band mass and obtains high carrier mobility, thus ZT of 2.8 has been achieved in n-type Br-doped SnSe single crystals [13], which is the highest ZT value reached so far. However, SnSe single crystals are easier to disintegrate and harder to prepare than polycrystalline SnSe, so researchers want to use polycrystalline SnSe in place of SnSe single crystals [14]. Polycrystalline SnSe has poorer thermoelectric performance compared with SnSe single crystals [15], so it is necessary that take measures to improve polycrystalline SnSe thermoelectric performance, now many methods have been adopted to raise its thermoelectric properties, such as carrier concentration control [16], band structure adjustment [17], vacancy engineering [18]. Besides, it’s also an effective strategy to introduce a suitable nanoscale second phase into the tin selenide matrix [19]. MXenes are new 2D materials that were firstly prepared by selectively etching MAX phases in 2011 [20]. They exhibit metal-like electrical properties and have graphene-like structures [21]. The excellent electrical properties can improve the electrical performance of the composites and the dimensional graphene-like nanostructure can enhance the phonon scattering and further reduce the thermal conductivity of the composites [22]. MXenes have been used in Bi 0.4 Sb 1.6 Te 3 to enhance thermoelectric properties, because Bi 0.4 Sb 1.6 Te 3 and MXenes have different work functions, they will cause band bending and scatter low-energy carriers. In addition, the heterogeneous interfaces between Bi 0.4 Sb 1.6 Te 3 and MXenes can scatter phonons efficiently to reduced thermal conductivity. Finally, a high average ZT of ≈1.23 was achieved in the temperature ranging from 300 to 475 K [5]. Hence, MXenes can be expected to improve the thermoelectric performance of polycrystalline SnSe. In this study, we prepared SnSe powders and Ti 3 C 2 T x (T x represents O, OH, and F groups). Ti 3 C 2 T x is representative of MXenes materials. Because the surface of Ti 3 C 2 T x is electronegative [23] and the surface of SnSe powder can be positively charged by adjusting the pH value of SnSe powder dispersion [24], we decided to adopt the electrostatic self-assembly method to prepare SnSe/Ti 3 C 2 T x composites powders. Then the powders were sintered by SPS at 773 K under 50 MPa for 5 min. After that, the thermoelectric properties of the sintered pellets were tested. Due to the addition of Ti 3 C 2 T x , the thermoelectric properties of the composites are better than those of the pure SnSe, the thermoelectric performance improved by 68% when ω = 0.1%. This research will provide a new way for the development of the thermoelectric properties of polycrystalline SnSe. 2. Materials And Methods SnSe/Ti 3 C 2 T x -ω (ω = 0, 0.05%, 0.1%, 0.2%) composites, or SnSe-T-ω were synthesized according to the process in Fig. 1. The Ti 3 C 2 T x nanosheets were synthesized by etching Ti 3 AlC 2 (11 Technology, 99%) in a mixture solution of 1 g LiF (Aladdin, 99%) and 20 ml hydrochloric acid (9 M) for 24 h at 35 °C. Then the products were washed by several centrifugation cycles of deionized water (DI), followed by an ultrasonic concussion for 60 min. Then the dark green supernatant was carefully collected after centrifugation for 60 min at 3500 rpm. The SnSe powders were synthesized by a simple hydrothermal method. SnCl 2 (Aladdin, 98.0-103.0%), Se powders (Aladdin, 99.99%), and NaOH (Aladdin, 97%) were dissolved in 50ml deionized water under ultrasonic for 20 min, then the mixture was transferred to a reaction kettle (100 ml) and heated at 463 K for 12 h in an oven. After cooling to room temperature, the black precipitate was collected by centrifugation and thoroughly washed with deionized water several times. Finally, the product was dried at 60 °C for 12 h and collected as powder. Ti 3 C 2 T x and SnSe powders were composited by electrostatic self-assembly. SnSe powders were dispersed into deionized water and the pH value of the solution was adjusted to 2 by adding hydrochloric acid (HCl). The dispersion of Ti 3 C 2 T x was added into the SnSe solution with slow stirring, then washing the mixed solution with water and freeze-drying. SPS was done using SPS-2T-2-MIN (Chen Hua, China) instrument. The dried powders were sintered at 773 K under the pressure of 50 MPa for 5 min. The obtained SnSe/Ti 3 C 2 T x bulks were cut along the parallel directions to the pressing for electrical transport property measurements and thermal transport property measurement. The Seebeck coefficients ( S ) and electrical conductivities ( σ ) were measured by a Cryoall CTA-3S instrument system under a helium atmosphere from 300 to 773 K. σ and S were measured in parallel to the pressing direction. The Hall coefficient ( R H ) was measured by the Van der Pauw technique with a Hall measurement system (MMR H5000) under ± 0.9T. Hall carrier density ( n H ) was calculated through n H = 1/( eR H ) and Hall carrier mobility ( µ H ) was acquired through µ H = R H / ρ . The thermal conductivity (κ) was calculated through the equation: κ = DC p ρ , where D , C p , ρ are thermal diffusivity, specific heat capacity, and density, respectively. D was measured with Netzsch LFA 467, C p was taken from the ref [15], ρ was measured by Archimedes method. Crystalline phases were identified by X-ray diffraction analysis (XRD, D/MAX-RA, Rigaku, Japan) with Cu K α radiation and scanning from 10º to 60º. The sampling interval was 0.02º 2θ. The microstructure of samples was investigated by field emission scanning electron microscopy (FEI Verios 460) and HRTEM (FEI Tecnai G2 F20 S-TWIN). 3. Results And Discussion We firstly prepared Ti 3 C 2 T x nanosheets by etching Ti 3 AlC 2 [25]. The XRD pattern of the Ti 3 C 2 T x nanosheet is shown in Fig. 2(a). The (104) peak at ≈38° in Ti 3 AlC 2 indicates the existence of the Al layer in Ti 3 AlC 2 . After etching, the (104) peak was disappeared and the (002) peak shifted to a small angle about 6°, which confirm the elimination of the Al layer and the formation of Ti 3 C 2 T x with a large interlayer distance. The scanning electron microscope (SEM) and Atomic Force Microscope (AFM) were used to verify the successful preparation of Ti 3 C 2 T x nanosheets. Fig. 2(b) shows that the exfoliated Ti 3 C 2 T x nanosheets have a large transverse dimension of about one hundred micrometers, and the nanosheets so thin and flexible that form folds on the surface. Fig. 2(c) shows the AFM pictures of Ti 3 C 2 T x nanosheets. The height profile indicates the average thickness of 4 nm, which proves that the prepared Ti 3 C 2 T x is two-dimension material. Then SnSe powders were prepared by the conventional hydrothermal method. The XRD pattern of SnSe is shown in Fig. 3(a). The results are indexed with orthorhombic SnSe (space group Pnma, PDF#48-1224), and no peaks indicative of impurities are present. We can know that the synthesized SnSe powders have a particle size of ≈400 nm, and there are a few flake-like powders of several micrometers from Fig. 3(b) and (c). The results of Transmission Electron Microscopy (TEM) are shown in Fig. 3(d) and (e). The lattice fringes can be seen clearly from the High-resolution TEM image, the crystal plane spacing between two apparent planes is estimated to be 0.29 nm, which corresponds to the (111) planes of SnSe [26]. The SnSe sheets were mixed with Ti 3 C 2 T x nanosheets by an electrostatic self-assembly method. Ti 3 C 2 T x nanosheets surface is electronegative due to the preparation process [23], and SnSe sheets can be positively charged by adjusting the pH value of the solution [24]. As shown in Fig. 3(f), the zeta potential values of SnSe and Ti 3 C 2 T x show different changing rules with the change of pH. The zeta potential of SnSe changes from positive to negative as the pH changes from 2 to 10, but the zeta potential of Ti 3 C 2 T x is negative all the time. When the pH value is adjusted to 2, the zeta potential of SnSe sheets and Ti 3 C 2 T x nanosheets are +27 and −12, respectively. Therefore, the two different materials can be adsorbed together by electrostatic force when drop adding Ti 3 C 2 T x dispersion into the SnSe aqueous dispersion. Fig. 4(a) shows the XRD patterns of Ti 3 C 2 T x and SnSe/Ti 3 C 2 T x composites. The XRD pattern of Ti 3 C 2 T x includes one peak at 2θ = 6°, which can be attributable to the (002) reflection with an interlayer distance of 0.5 nm. The diffraction peaks of SnSe/Ti 3 C 2 T x composites prove that the powders of synthesized material are orthorhombic SnSe (space group Pnma, PDF#48-1224), and no peaks indicative of impurities are present. It is a pity that the diffraction peak of Ti 3 C 2 T x can’t be observed, even in the SnSe/Ti 3 C 2 T x -0.2% sample, which means that the amount of Ti 3 C 2 T x in the composites is so low that the diffractometer could not pick it up. To determine the presence of Ti 3 C 2 T x in the composites, SEM and TEM are adopted. Fig. 4(b) shows the microstructure of SnSe/Ti 3 C 2 T x -0.2% composites, where the thin Ti 3 C 2 T x nanosheets are distributed on the surface of the SnSe sheets and SnSe plates are wrapped by the ultrathin Ti 3 C 2 T x nanosheets. TEM image Fig. 4(c) shows that Ti 3 C 2 T x nanosheets wrap around the SnSe particles. HRTEM image Fig. 4(d) exhibits the lattice fringes with a spacing of 0.3 nm for the particle which corresponds to the (011) planes of SnSe. Fig. 4(e) gives the corresponding elemental maps of Sn, Se, Ti, and C, which shows that Sn and Se elements are evenly distributed in the wafer, Ti and C are distributed around the wafer. This confirms that the sheet is SnSe and Ti 3 C 2 T x is wrapped on top of the SnSe. The as-synthesized SnSe/Ti 3 C 2 T x composites powders are sintered by the SPS. The sintered samples all have a good density, which can be confirmed by Table 1. Table 1 Density of the SnSe/Ti 3 C 2 T x composites. The theoretical density of SnSe is 6.18 g/cm 3 . SnSe/Ti 3 C 2 T x -ω Measured density (g/cm 3 ) Relative density (%) ω = 0 6.12 99.03 ω = 0.05% 6.08 98.38 ω = 0.1% 6.08 98.38 ω = 0.2% 6.06 98.06 Fig. 5 shows the XRD patterns of the sintered pellets along with both the parallel and perpendicular to the pressing direction. The diffraction peaks correspond to orthorhombic SnSe. However, the intensities of the (400) peak are a sharp increase in the XRD pattern collected perpendicular to the pressing direction (Fig. 5(b)), indicating that strong preferred { h00 } orientation occurs in the SnSe pellets [27]. Fig. 5(c) and (d) are SEM images of SnSe/Ti 3 C 2 T x -0.1% pellets, fractured from different directions. The microstructures of the pellets are composed of microplates, indicating that SnSe powders grew further during the high-temperature SPS process. It can be seen that the sample exhibits a layered structure along the parallel direction and a flat plane structure along the perpendicular direction, indicating an obvious anisotropy in the sintered pellets, which fit well with the XRD patterns in Fig. 5(a) and (b). Since the performance of SnSe is anisotropic, it is easier to obtain low thermal conductivity in the direction of parallel pressure [28], so the test directions in this paper are all parallel to the pressing direction. The graph of the Seebeck coefficients as a function of temperature is shown in Fig. 6(a), in which the Seebeck coefficients decrease (positive) firstly then increase (negative) as long as the temperature increase. During the low-temperature stage (< 500 K), the main carriers in the material are holes, so S is positive. Then the temperature continues to increase, the bipolar transport will begin (500~600 K), in this case, the S should be explained by , p and n-type carriers contribute to S together. Because the n-type carriers are going to provide a negative potential, the Seebeck coefficients will decrease sharply, before the main carriers change from p to n-type, about at 600 K, the drop rate of S goes up, when the contribution of p and n carriers to S is equal, S will be zero and the type of the main carrier changes. And then S will continue to increase in the negative direction as the temperature increases, finally, a high absolute S value of~296.2 μV K -1 at 773 K is observed(ω = 0.2%). The S of composite Ti 3 C 2 T x samples is higher than the pure sample, indicating that interfaces in composites scatter carriers, thus increasing S . The temperature dependences of σ for SnSe/Ti 3 C 2 T x composites are shown in Fig. 6(b), the σ of all samples increases with increasing temperature. When the temperature is below 600 K, the σ growth is slow, this is because there is no significant increase in carrier concentration at this stage. Then, σ increases rapidly at temperatures above 600 K due to the thermal excitation of electrons [29]. This tendency of σ with temperature is common in polycrystalline SnSe [30]. At room temperature, the σ of all samples compositing Ti 3 C 2 T x is lower than pure SnSe, because the interfaces between SnSe and Ti3C2Tx enhance the scattering of carriers to decreased carrier mobility, which can be evidenced from room temperature Hall measurement in Fig. 6(d). The electron intrinsic excitation becomes the main source of carriers during the high-temperature stage, but the electrical conductivity of the composites still does not exceed pure SnSe, indicating that the interface in the composites still has a strong scattering effect on the carriers so that decrease σ and enhance S. Fig. 6(c) shows the temperature dependences of PF for SnSe/Ti 3 C 2 T x composites. PF ( S 2 σ ) is the result of σ and S 's calculations. During the low-temperature stage (<600 K), even if there is a change in the main carrier type, there is little effect on the PF due to the low conductivity. Then, as the temperature rises, the conductivity increases sharply, as a result, an optimized PF of 0.74 μW cm -1 K -2 at 773 K can be observed when ω = 0.2%. The total thermal conductivity k tot and lattice thermal conductivity k lat of all samples in the temperature range from 300 K to 773 K are shown in Fig. 7(b) and (c). We estimate the lattice thermal conductivity through the Wiedemann-Franz Law ( κ ele = LσT ). Because L will not change the total thermal conductivity and final ZT , an estimation of L can be made using the SPB model with acoustic phonon scattering, the L is calculated from: where k B is the Boltzmann constant, e is the electric charge, r is the scattering rate, and η refers to the reduced Fermi energy, which can be derived from the measured Seebeck coefficients with consideration of acoustic phonon dominated scattering ( r = -1/2 ) via the following equation: where F x (η) is Fermi integral: Because the electrical conductivity is so low at a temperature below 600 K, the electronic contribution to the thermal conductivity becomes obvious only above 600 K, which leads to no significant difference between the total thermal conductivity k tot and lattice thermal conductivity k lat . It is noteworthy that the min k lat is obtained in sample ω = 0.1% at 773 K (0.54 W m -1 K -1 ), which is comparable to the values of single-crystal SnSe reported by Wei [31]. This indicates that the interfaces between the SnSe matrix and Ti 3 C 2 T x can scatter phonon effectively. But the thermal conductivity of sample ω = 0.2% is not lower than that of the pure SnSe, this can be explained for two reasons, one is the Ti 3 C 2 T x has a higher thermal conductivity than SnSe [32], the other is the uneven distribution of Ti 3 C 2 T x in SnSe matrix results in a reduced scattering interface. Fig. 7(d) presents the temperature dependence of the figure-of-merit ZT in the temperature range of 300-773 K. The interfaces introduced by Ti 3 C 2 T x can scatter carriers effectively, thus increasing the Seebeck coefficients ( S ), finally, a high absolute S value of ~296.2 μV K -1 was obtained at 773 K. At the same time, the high-density interfaces of SnSe/Ti 3 C 2 T x composites enhance the phonon scattering, a low lattice thermal conductivity k la t of 0.54 W m -1 K -1 was obtained in sample ω = 0.1%. Because of the lowest k la t in sample ω = 0.1%, the peak ZT value of 0.1 presents at 773 K in sample ω = 0.1%. Compared with the pure sample, the performance of the composite sample is indeed improved, but the ZT value is still very low, so measures still need to be taken to improve the conductivity, especially the mobility. 4. Conclusions In this work, we successfully synthesize SnSe/Ti 3 C 2 T x composites materials by the electrostatic self-assembly method and SPS. Because low energy carriers were filtered by interfaces, a high absolute S value of ~296.2 μV K -1 was obtained at 773 K. The low k lat of 0.54 W m -1 K -1 was obtained at 773 K due to the scattering of phonons at the interfaces, therefore, a ZT of 0.1 was obtained in sample ω = 0.1% at 773 K along the pressing direction, which improves by 68% compared with the pure SnSe in our research. It’s a pity that Ti 3 C 2 T x can increase the carrier concentration of SnSe but reduce the mobility, which ultimately fails to achieve the purpose of improving the electrical conductivity, therefore, to obtain a higher ZT value, additional measures should be taken to increase mobility. Declarations Acknowledgments This work was supported by the General Project in Industrial Area of Shaanxi Province (Grant No. 2020GY-281), the Shaanxi Provincial Education Department serves Local Scientific Research Plan (Grant No. 20JC008), the National Natural Science Foundation of China (Grant No. 51702193, 51502165), the Natural Science Foundation of Shaanxi Provincial Department of Education (Grant No. 20JK0525) and the Scientific Research Fund of Shaanxi University of Science & Technology (Grant No. BJ16-20, BJ16-21). References [1] Bell L E. Cooling, Heating, Generating Heat with and Recovering Waste Thermoelectric. Science 2008, 321 : 1457–1461. [2] Tan G, Zhao L D, Kanatzidis M G. 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[31] Wei P C, Bhattacharya S, Liu Y F, et al . Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe. ACS Omega 2019, 4 : 5442–5450. [32] Gholivand H, Fuladi S, Hemmat Z, et al . Effect of surface termination on the lattice thermal conductivity of monolayer Ti 3 C 2 T z MXenes. J Appl Phys 2019, 126 : 125-129. Cite Share Download PDF Status: Posted Version 1 posted You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. Our growing team is made up of researchers and industry professionals working together to solve the most critical problems facing scientific publishing. Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {\"props\":{\"pageProps\":{\"initialData\":{\"identity\":\"rs-607248\",\"acceptedTermsAndConditions\":true,\"allowDirectSubmit\":true,\"archivedVersions\":[],\"articleType\":\"Research Article\",\"associatedPublications\":[],\"authors\":[{\"id\":32192571,\"identity\":\"95c8c567-9f99-42f5-93b1-60f9088b44f9\",\"order_by\":0,\"name\":\"Yi Qin\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Shaanxi University of Science and Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Yi\",\"middleName\":\"\",\"lastName\":\"Qin\",\"suffix\":\"\"},{\"id\":32192572,\"identity\":\"921eef05-f30f-4d2f-a4e4-d7441ec9135d\",\"order_by\":1,\"name\":\"Xiaohan Li\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Shaanxi University of Science and Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Xiaohan\",\"middleName\":\"\",\"lastName\":\"Li\",\"suffix\":\"\"},{\"id\":32192573,\"identity\":\"10bb4ae9-1e24-4038-8797-8bb147b9aa8a\",\"order_by\":2,\"name\":\"Ting Zhao\",\"email\":\"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAAA5ElEQVRIiWNgGAWjYDACCRBhwMDAD+UzNhCl5QBQiyRI6QHitYAsOkCsFvnZzQ8ffyi4Y7f5/Bnjzx8YbGQ3HGB+9gCfFsY5x4wNDhg8S952I8dM4gBDmvGGA2zmBvi0MEskAFUaHE42u8FjBnTY4cQNB3jYJPBpYZNI/wbWYtx/xvjDAYb/hLXwSOSAbbEzYMgxADrsAGEtEhI5xQZnDA4nSNxIK5M4Y5BsPPMwmxleLfIz0jc+qPhz2J6///DmDxUVdrJ9x5uf4dUCA4kNYAoUVMzEqAcCeyLVjYJRMApGwUgEAKtfTMLOkrNuAAAAAElFTkSuQmCC\",\"orcid\":\"\",\"institution\":\"Shaanxi University of Science and Technology\",\"correspondingAuthor\":true,\"prefix\":\"\",\"firstName\":\"Ting\",\"middleName\":\"\",\"lastName\":\"Zhao\",\"suffix\":\"\"},{\"id\":32192574,\"identity\":\"40a08a2f-365f-4c5d-bb39-0c513f16f295\",\"order_by\":3,\"name\":\"Jianfeng Zhu\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Shaanxi University of Science and Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Jianfeng\",\"middleName\":\"\",\"lastName\":\"Zhu\",\"suffix\":\"\"},{\"id\":32192575,\"identity\":\"f9c98756-83f5-4893-ac07-88d721a59b95\",\"order_by\":4,\"name\":\"Yanling Yang\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Shaanxi University of Science and Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Yanling\",\"middleName\":\"\",\"lastName\":\"Yang\",\"suffix\":\"\"},{\"id\":32192576,\"identity\":\"557f7f2a-643c-4e02-8748-686a2bd909ea\",\"order_by\":5,\"name\":\"Meiqian Xie\",\"email\":\"\",\"orcid\":\"\",\"institution\":\"Shaanxi University of Science and Technology\",\"correspondingAuthor\":false,\"prefix\":\"\",\"firstName\":\"Meiqian\",\"middleName\":\"\",\"lastName\":\"Xie\",\"suffix\":\"\"}],\"badges\":[],\"createdAt\":\"2021-06-09 22:34:40\",\"currentVersionCode\":1,\"declarations\":\"\",\"doi\":\"10.21203/rs.3.rs-607248/v1\",\"doiUrl\":\"https://doi.org/10.21203/rs.3.rs-607248/v1\",\"draftVersion\":[],\"editorialEvents\":[],\"editorialNote\":\"\",\"failedWorkflow\":false,\"files\":[{\"id\":10270564,\"identity\":\"29795b84-785b-42a4-a477-0928ffad881d\",\"added_by\":\"auto\",\"created_at\":\"2021-06-11 18:22:57\",\"extension\":\"jpg\",\"order_by\":1,\"title\":\"Figure 1\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":30272,\"visible\":true,\"origin\":\"\",\"legend\":\"Schematic illustration of preparation of SnSe/Ti3C2Tx-ω (ω = 0, 0.05%, 0.1%, 0.2%) composites.\",\"description\":\"\",\"filename\":\"1.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-607248/v1/48b733cc19ede197674d21fd.jpg\"},{\"id\":10271189,\"identity\":\"b890ff26-f150-42d8-a64d-589c252c6e03\",\"added_by\":\"auto\",\"created_at\":\"2021-06-11 18:31:57\",\"extension\":\"jpg\",\"order_by\":2,\"title\":\"Figure 2\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":78403,\"visible\":true,\"origin\":\"\",\"legend\":\"(a) XRD patterns of Ti3C2Tx nanosheets. (b) SEM image of Ti3C2Tx nanosheets\\n(c) AFM image with a height profile of a Ti3C2Tx nanosheet.\\n\",\"description\":\"\",\"filename\":\"2.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-607248/v1/31e23936902978de55dba45e.jpg\"},{\"id\":10270951,\"identity\":\"e08f6ecb-09f5-4989-bda5-8d1a9f6dcab0\",\"added_by\":\"auto\",\"created_at\":\"2021-06-11 18:25:57\",\"extension\":\"jpg\",\"order_by\":3,\"title\":\"Figure 3\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":66388,\"visible\":true,\"origin\":\"\",\"legend\":\"(a) XRD patterns of SnSe. (b) SEM image of SnSe. (c) particle size distribution diagram of SnSe in Fig. 3(b). (d) low-magnification and (e) high-magnification TEM images of SnSe. (f) Zeta potential of Ti3C2Tx and SnSe as a function of pH.\",\"description\":\"\",\"filename\":\"3.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-607248/v1/b3a8a2872a0610f6527a3d74.jpg\"},{\"id\":10271011,\"identity\":\"324ab1c2-2965-4b45-8db2-72253c2cbc81\",\"added_by\":\"auto\",\"created_at\":\"2021-06-11 18:28:57\",\"extension\":\"jpg\",\"order_by\":4,\"title\":\"Figure 4\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":141868,\"visible\":true,\"origin\":\"\",\"legend\":\"(a) XRD patterns of Ti3C2Tx and SnSe/Ti3C2Tx composites. (b) SEM image of SnSe/ Ti3C2Tx-0.2%. (c) low-magnification and (d) high-magnification TEM images of SnSe/ Ti3C2Tx-0.2%. (e) the corresponding elemental maps of Sn, Se, Ti, and C.\",\"description\":\"\",\"filename\":\"4.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-607248/v1/45c332db2536ba114b728b38.jpg\"},{\"id\":10271013,\"identity\":\"4330556b-437c-4ec2-9a8b-11a194e4245f\",\"added_by\":\"auto\",\"created_at\":\"2021-06-11 18:28:57\",\"extension\":\"jpg\",\"order_by\":5,\"title\":\"Figure 5\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":91466,\"visible\":true,\"origin\":\"\",\"legend\":\"XRD patterns of SnSe/Ti3C2Tx composites sintered pellets (a) parallel and (b) perpendicular to the pressing direction. SEM images of SnSe/Ti3C2Tx-0.1% pellet fractured from (c) parallel and (d) perpendicular to the pressing direction.\",\"description\":\"\",\"filename\":\"5.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-607248/v1/0a92765d09bd7ee51f5483d7.jpg\"},{\"id\":10271010,\"identity\":\"69ed932d-f0bb-4de9-87c7-54679faf6a81\",\"added_by\":\"auto\",\"created_at\":\"2021-06-11 18:28:57\",\"extension\":\"jpg\",\"order_by\":6,\"title\":\"Figure 6\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":71807,\"visible\":true,\"origin\":\"\",\"legend\":\"Temperature-dependent electrical properties of SnSe/Ti3C2Tx composites measured parallel to the pressing direction. (a) Seebeck coefficient. (b) electrical conductivity. (c) power factor. (d) Hall carrier concentration and carrier mobility as a function of Ti3C2Tx mass fraction at room temperature.\",\"description\":\"\",\"filename\":\"6.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-607248/v1/917fc2440394c98ae68ca3b8.jpg\"},{\"id\":10270955,\"identity\":\"607f6dc2-5a65-47cd-9381-05aa76725c97\",\"added_by\":\"auto\",\"created_at\":\"2021-06-11 18:25:57\",\"extension\":\"jpg\",\"order_by\":7,\"title\":\"Figure 7\",\"display\":\"\",\"copyAsset\":false,\"role\":\"figure\",\"size\":83777,\"visible\":true,\"origin\":\"\",\"legend\":\"Temperature-dependent (a) thermal diffusion coefficient. (b) total thermal conductivity. (c) lattice thermal conductivity. (d) thermoelectric figure of merit (ZT) of SnSe/Ti3C2Tx composites measured parallel to the pressing direction.\",\"description\":\"\",\"filename\":\"7.jpg\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-607248/v1/e5805b754f2d4525ea6df129.jpg\"},{\"id\":15673343,\"identity\":\"6b434477-a50a-4f0f-a852-b0e22dfe7696\",\"added_by\":\"auto\",\"created_at\":\"2021-11-18 14:17:30\",\"extension\":\"pdf\",\"order_by\":0,\"title\":\"\",\"display\":\"\",\"copyAsset\":false,\"role\":\"manuscript-pdf\",\"size\":583820,\"visible\":true,\"origin\":\"\",\"legend\":\"\",\"description\":\"\",\"filename\":\"manuscript.pdf\",\"url\":\"https://assets-eu.researchsquare.com/files/rs-607248/v1/87895240-ddc2-4199-bd76-c05765b89d0f.pdf\"}],\"financialInterests\":\"\",\"formattedTitle\":\"\\u003cp\\u003eEnhanced thermoelectric performance of polycrystalline SnSe by compositing with layered Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e\\u003c/p\\u003e\",\"fulltext\":[{\"header\":\"1. Introduction\",\"content\":\"\\u003cp\\u003eThermoelectric materials convert thermal energy into electricity directly while they are independent of moving parts and extremely reliable [1], so they will play a key role in power generation. The property of thermoelectric materials is manifested by the dimensionless figure of merit (\\u003cem\\u003eZT\\u003c/em\\u003e), \\u003cem\\u003eZT = (S\\u003csup\\u003e2\\u003c/sup\\u003e\\u0026sigma;/к) T\\u003c/em\\u003e, where \\u003cem\\u003eS\\u003c/em\\u003e, \\u003cem\\u003e\\u0026sigma;\\u003c/em\\u003e, \\u003cem\\u003eS\\u003csup\\u003e2\\u003c/sup\\u003e\\u0026sigma;\\u003c/em\\u003e, \\u003cem\\u003eк\\u003c/em\\u003e, and \\u003cem\\u003eT\\u003c/em\\u003e are the Seebeck coefficient, electrical conductivity, power factor, thermal conductivity, and absolute temperature, respectively [2]. Many efforts have been taken for developing\\u0026nbsp;the \\u003cem\\u003eZT\\u003c/em\\u003e value [3\\u0026ndash;5], the ameliorating methods are divided into two classes, one is using doping to enhance the electrical performance of the thermoelectric materials [6\\u0026ndash;8], and the other is reducing the lattice thermal conductivity by amplifying phonon scattering [9,10]. But because of the coupling relationship between thermal conductivity and electrical conductivity, it is essential to find materials that have intrinsically low thermal conductivity and adjustable electrical conductivity. SnSe is a thermoelectric material that combines these two advantages.\\u003c/p\\u003e\\n\\u003cp\\u003eSnSe has\\u0026nbsp;a layered orthorhombic crystal structure\\u0026nbsp;[11], the strong anharmonicity in Sn-Se bonding gives rise to high Gr\\u0026uuml;neisen parameters [12], leading to intrinsically low thermal conductivity. Also, a phase transition from \\u003cem\\u003ePnma\\u003c/em\\u003e to \\u003cem\\u003eCmcm\\u003c/em\\u003e in SnSe results in the divergence of two overlapped conduction bands, which lessens the band mass and obtains high carrier mobility, thus \\u003cem\\u003eZT\\u003c/em\\u003e of 2.8 has been achieved in n-type Br-doped SnSe single crystals [13], which is the highest \\u003cem\\u003eZT\\u003c/em\\u003e value reached so far. However, SnSe single crystals are easier to disintegrate and harder to prepare than polycrystalline SnSe, so researchers want to use polycrystalline SnSe in place of SnSe single crystals [14]. Polycrystalline SnSe has poorer thermoelectric performance compared with SnSe single crystals [15], so it is necessary that take measures to improve polycrystalline SnSe thermoelectric performance, now many methods have been adopted to raise its thermoelectric properties, such as carrier concentration control [16], band structure adjustment [17], vacancy engineering [18]. Besides, it\\u0026rsquo;s also an effective strategy to introduce a suitable nanoscale second phase into the tin selenide matrix [19].\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eMXenes are new 2D materials that were firstly prepared by selectively etching MAX phases in 2011 [20]. They exhibit metal-like electrical properties and have graphene-like structures [21]. The\\u0026nbsp;excellent electrical properties can improve the electrical performance of the composites and the dimensional graphene-like nanostructure can enhance the phonon scattering and further reduce the thermal conductivity of the composites [22]. MXenes have been used in Bi\\u003csub\\u003e0.4\\u003c/sub\\u003eSb\\u003csub\\u003e1.6\\u003c/sub\\u003eTe\\u003csub\\u003e3\\u003c/sub\\u003e to enhance thermoelectric properties, because Bi\\u003csub\\u003e0.4\\u003c/sub\\u003eSb\\u003csub\\u003e1.6\\u003c/sub\\u003eTe\\u003csub\\u003e3\\u003c/sub\\u003e and MXenes have different work functions, they will cause band bending and scatter low-energy carriers. In addition, the heterogeneous interfaces between Bi\\u003csub\\u003e0.4\\u003c/sub\\u003eSb\\u003csub\\u003e1.6\\u003c/sub\\u003eTe\\u003csub\\u003e3\\u003c/sub\\u003e and MXenes can scatter phonons efficiently to reduced thermal conductivity. Finally, a high average ZT of \\u0026asymp;1.23 was achieved in the temperature ranging from 300 to 475 K [5].\\u0026nbsp;Hence, MXenes can be expected to improve the thermoelectric performance of polycrystalline SnSe.\\u003c/p\\u003e\\n\\u003cp\\u003eIn this study, we prepared SnSe powders and Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e (T\\u003csub\\u003ex\\u003c/sub\\u003e represents O, OH, and F groups). Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e is representative of MXenes materials. Because the surface of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e is electronegative [23] and\\u0026nbsp;the surface of SnSe powder can be positively charged by adjusting the pH value of SnSe powder\\u0026nbsp;dispersion [24], we decided to adopt the electrostatic self-assembly method to prepare SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composites powders. Then the powders were sintered by SPS at 773 K under 50 MPa for 5 min. After that, the thermoelectric properties of the sintered pellets were tested. Due to the addition of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e, the thermoelectric properties of the composites are better than those of the pure SnSe, the thermoelectric performance improved by 68% when \\u0026omega; = 0.1%. This research will provide a new way for the development of the thermoelectric properties of polycrystalline SnSe.\\u003c/p\\u003e\"},{\"header\":\"2. Materials And Methods\",\"content\":\"\\u003cp\\u003eSnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e-\\u0026omega; (\\u0026omega; = 0, 0.05%, 0.1%, 0.2%) composites, or SnSe-T-\\u0026omega; were synthesized according to the process in Fig. 1.\\u0026nbsp;The Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets were synthesized by etching Ti\\u003csub\\u003e3\\u003c/sub\\u003eAlC\\u003csub\\u003e2\\u003c/sub\\u003e (11 Technology, 99%) in a mixture solution of 1 g LiF (Aladdin, 99%) and 20 ml hydrochloric acid (9 M) for 24 h at 35 \\u0026deg;C. Then the products were washed by several centrifugation cycles of deionized water (DI), followed by an ultrasonic concussion for 60 min. Then the dark green supernatant was carefully collected after centrifugation for 60 min at 3500 rpm. The SnSe powders were synthesized by a simple hydrothermal method. SnCl\\u003csub\\u003e2\\u0026nbsp;\\u003c/sub\\u003e(Aladdin, 98.0-103.0%), Se powders (Aladdin, 99.99%), and NaOH (Aladdin, 97%) were dissolved in 50ml deionized water under ultrasonic for 20 min, then the mixture was transferred to a\\u0026nbsp;reaction kettle (100 ml) and heated at 463 K for 12 h in an oven. After cooling to room temperature, the black precipitate was collected by centrifugation and thoroughly washed with deionized water several times. Finally, the product was dried at 60 \\u0026deg;C for 12 h and collected as powder.\\u003c/p\\u003e\\n\\u003cp\\u003eTi\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e and SnSe powders were composited by electrostatic self-assembly. SnSe powders were dispersed into deionized water and the pH value of the solution was adjusted to 2 by adding hydrochloric acid (HCl). The dispersion of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e was added into the SnSe solution with slow stirring, then washing the mixed solution with water and freeze-drying.\\u003c/p\\u003e\\n\\u003cp\\u003eSPS was done using SPS-2T-2-MIN (Chen Hua, China) instrument. The dried powders were sintered at 773 K under the pressure of 50 MPa for 5 min. The obtained SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e bulks were cut along the parallel directions to the pressing for electrical transport property measurements and thermal transport property measurement.\\u003c/p\\u003e\\n\\u003cp\\u003eThe Seebeck coefficients (\\u003cem\\u003eS\\u003c/em\\u003e) and electrical conductivities (\\u003cem\\u003e\\u0026sigma;\\u003c/em\\u003e) were measured by a Cryoall CTA-3S instrument system under a helium atmosphere from 300 to 773 K.\\u003cem\\u003e\\u0026nbsp;\\u0026sigma;\\u0026nbsp;\\u003c/em\\u003eand \\u003cem\\u003eS\\u0026nbsp;\\u003c/em\\u003ewere measured in parallel to the pressing direction. The Hall coefficient (\\u003cem\\u003eR\\u003csub\\u003eH\\u003c/sub\\u003e\\u003c/em\\u003e) was measured by the Van der Pauw technique with a Hall measurement system (MMR H5000) under \\u0026plusmn; 0.9T. Hall carrier density (\\u003cem\\u003en\\u003csub\\u003eH\\u003c/sub\\u003e\\u003c/em\\u003e) was calculated through \\u003cem\\u003en\\u003csub\\u003eH\\u003c/sub\\u003e\\u003c/em\\u003e = 1/(\\u003cem\\u003eeR\\u003csub\\u003eH\\u003c/sub\\u003e\\u003c/em\\u003e) and Hall carrier mobility (\\u003cem\\u003e\\u0026micro;\\u003csub\\u003eH\\u003c/sub\\u003e\\u003c/em\\u003e) was acquired through \\u003cem\\u003e\\u0026micro;\\u003csub\\u003eH\\u003c/sub\\u003e\\u0026nbsp;\\u003c/em\\u003e= \\u003cem\\u003eR\\u003csub\\u003eH\\u003c/sub\\u003e\\u003c/em\\u003e/\\u003cem\\u003e\\u0026rho;\\u003c/em\\u003e. The thermal conductivity (\\u0026kappa;) was calculated through the equation: \\u003cem\\u003e\\u0026kappa; = DC\\u003csub\\u003ep\\u003c/sub\\u003e\\u0026rho;\\u003c/em\\u003e, where \\u003cem\\u003eD\\u003c/em\\u003e, \\u003cem\\u003eC\\u003csub\\u003ep\\u003c/sub\\u003e\\u003c/em\\u003e, \\u003cem\\u003e\\u0026rho;\\u003c/em\\u003e are thermal diffusivity, specific heat capacity, and density, respectively. \\u003cem\\u003eD\\u003c/em\\u003e was measured with Netzsch LFA 467, \\u003cem\\u003eC\\u003csub\\u003ep\\u003c/sub\\u003e\\u003c/em\\u003e was taken from the ref [15], \\u003cem\\u003e\\u0026rho;\\u0026nbsp;\\u003c/em\\u003ewas measured by Archimedes method.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eCrystalline phases were identified by X-ray diffraction analysis (XRD, D/MAX-RA, Rigaku, Japan) with Cu \\u003cem\\u003eK\\u003c/em\\u003e\\u0026alpha; radiation and scanning from 10\\u0026ordm; to 60\\u0026ordm;. The sampling interval was 0.02\\u0026ordm; 2\\u0026theta;. The microstructure of samples was investigated by field emission scanning electron microscopy (FEI Verios 460) and HRTEM (FEI Tecnai G2 F20 S-TWIN).\\u003c/p\\u003e\"},{\"header\":\"3. Results And Discussion\",\"content\":\"\\u003cp\\u003eWe firstly prepared Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets by etching Ti\\u003csub\\u003e3\\u003c/sub\\u003eAlC\\u003csub\\u003e2\\u003c/sub\\u003e [25]. The XRD pattern of the Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheet is shown in Fig. 2(a). The (104) peak at\\u0026nbsp;\\u0026asymp;38\\u0026deg;\\u0026nbsp;in Ti\\u003csub\\u003e3\\u003c/sub\\u003eAlC\\u003csub\\u003e2\\u003c/sub\\u003e indicates the existence of the Al layer in Ti\\u003csub\\u003e3\\u003c/sub\\u003eAlC\\u003csub\\u003e2\\u003c/sub\\u003e. After etching, the (104) peak was disappeared and the (002) peak shifted to a small angle about 6\\u0026deg;, which confirm the elimination of the Al layer and the formation of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e with a large interlayer distance. The scanning electron microscope (SEM) and Atomic Force Microscope (AFM) were used to verify the successful preparation of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets. Fig. 2(b) shows that the exfoliated Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets have a large transverse dimension of about one hundred micrometers, and the nanosheets so thin and flexible that form folds on the surface. Fig. 2(c) shows the AFM pictures of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets. The height profile indicates the average thickness of 4 nm, which proves that the prepared Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e is two-dimension material.\\u003c/p\\u003e\\n\\u003cp\\u003eThen SnSe powders were prepared by the conventional hydrothermal method. The XRD pattern of SnSe is shown in Fig. 3(a). The results are indexed with orthorhombic SnSe (space group \\u003cem\\u003ePnma,\\u0026nbsp;\\u003c/em\\u003ePDF#48-1224), and no peaks indicative of impurities are present. We can know that the synthesized SnSe powders have a particle size of \\u0026asymp;400 nm, and there are a few flake-like powders of several micrometers from Fig. 3(b) and (c). The results of Transmission Electron Microscopy (TEM) are shown in Fig. 3(d) and (e). The lattice fringes can be seen clearly from the High-resolution TEM image, the crystal plane spacing between two apparent planes is estimated to be 0.29 nm, which corresponds to the (111) planes of SnSe [26].\\u003c/p\\u003e\\u003cp\\u003eThe SnSe sheets were mixed with Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets by an electrostatic self-assembly method. Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets surface is electronegative due to the preparation process\\u0026nbsp;[23], and SnSe sheets can be positively charged by adjusting the pH value of the solution [24]. As shown in Fig. 3(f), the zeta potential values of SnSe and Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u0026nbsp;\\u003c/sub\\u003eshow different changing rules with the change of pH. The zeta potential of SnSe changes from positive to negative as the pH changes from 2 to 10, but the zeta potential of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e is negative all the time. When the pH value is adjusted to 2, the zeta potential of SnSe sheets and Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets are +27\\u0026nbsp;and \\u0026minus;12, respectively. Therefore, the two different materials can be\\u0026nbsp;adsorbed together by electrostatic force when drop adding Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e dispersion into the SnSe aqueous dispersion.\\u003c/p\\u003e\\n\\u003cp\\u003eFig. 4(a) shows the XRD patterns of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e and SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composites. The XRD pattern of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e includes one peak at 2\\u0026theta; = 6\\u0026deg;,\\u0026nbsp;which can be attributable to the\\u0026nbsp;(002) reflection with an interlayer distance of 0.5 nm. The diffraction peaks of SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e\\u003cem\\u003e\\u0026nbsp;\\u003c/em\\u003ecomposites prove that the powders of synthesized material are orthorhombic SnSe (space group \\u003cem\\u003ePnma,\\u0026nbsp;\\u003c/em\\u003ePDF#48-1224), and no peaks indicative of impurities are present. It is a pity that the diffraction peak of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e can\\u0026rsquo;t be observed, even in the SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e-0.2% sample, which means that the amount of\\u0026nbsp;Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e in the composites is so low that the diffractometer could not pick it up. To determine the presence of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e in the composites, SEM and TEM are adopted.\\u003c/p\\u003e\\n\\u003cp\\u003eFig. 4(b) shows the microstructure of SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e-0.2% composites, where the thin Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets are distributed on the surface of the SnSe sheets and SnSe plates are wrapped\\u0026nbsp;by the ultrathin Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets.\\u0026nbsp;TEM image Fig. 4(c) shows that Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e nanosheets wrap around the SnSe particles. HRTEM image Fig. 4(d) exhibits the lattice fringes with a spacing of 0.3 nm for the particle which corresponds to the (011) planes of\\u003c/p\\u003e\\n\\u003cp\\u003eSnSe.\\u0026nbsp;Fig. 4(e) gives the corresponding elemental maps of Sn, Se, Ti, and C, which shows that Sn and Se elements are evenly distributed in the wafer, Ti and C are distributed around the wafer. This confirms that the sheet is SnSe and Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e is wrapped on top of the SnSe.\\u0026nbsp;\\u003c/p\\u003e\\u003cp\\u003eThe as-synthesized SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composites powders are sintered by the SPS. The sintered samples all have a good density, which can be confirmed by Table 1.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp style='margin-top:0in;margin-right:0in;margin-bottom:0in;margin-left:0in;text-align:center;font-size:16px;font-family:\\\"Times\\\",serif;line-height:200%;'\\u003e\\u003cstrong\\u003e\\u003cspan style=\\\"font-size: 15px; line-height: 200%; font-family: Calibri, sans-serif;\\\"\\u003eTable 1\\u0026nbsp;\\u003c/span\\u003e\\u003c/strong\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003eDensity of the SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e\\u003cem\\u003e\\u0026nbsp;\\u003c/em\\u003ecomposites. The theoretical density of SnSe is 6.18 g/cm\\u003csup\\u003e3\\u003c/sup\\u003e.\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n\\u003ctable style=\\\"border-collapse:collapse;border:none;\\\"\\u003e\\n \\u003ctbody\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border-top: 1pt solid windowtext;border-left: none;border-bottom: 1pt solid windowtext;border-right: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003eSnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e-\\u0026omega;\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border-top: 1pt solid windowtext;border-left: none;border-bottom: 1pt solid windowtext;border-right: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:0in;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003eMeasured density (g/cm\\u003csup\\u003e3\\u003c/sup\\u003e)\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.9pt;border-top: 1pt solid windowtext;border-left: none;border-bottom: 1pt solid windowtext;border-right: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003eRelative density (%)\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e\\u0026omega;\\u0026nbsp;= 0\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e6.12\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.9pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e99.03\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e\\u0026omega;\\u0026nbsp;= 0.05%\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e6.08\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.9pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e98.38\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e\\u0026omega;\\u0026nbsp;= 0.1%\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e6.08\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.9pt;border: none;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e98.38\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003ctr\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid windowtext;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e\\u0026omega;\\u0026nbsp;= 0.2%\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.85pt;border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid windowtext;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e6.06\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003ctd style=\\\"width: 143.9pt;border-top: none;border-right: none;border-left: none;border-image: initial;border-bottom: 1pt solid windowtext;padding: 0in 5.4pt;vertical-align: top;\\\"\\u003e\\n \\u003cp style='margin:0in;text-align:center;text-indent:10.1pt;line-height:200%;font-size:16px;font-family:\\\"Times\\\",serif;'\\u003e\\u003cspan style=\\\"font-size: 15px;\\\"\\u003e\\u003cspan style=\\\"font-family: Calibri, sans-serif;\\\"\\u003e\\u003cspan style=\\\"line-height: 200%;\\\"\\u003e98.06\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/span\\u003e\\u003c/p\\u003e\\n \\u003c/td\\u003e\\n \\u003c/tr\\u003e\\n \\u003c/tbody\\u003e\\n\\u003c/table\\u003e\\n\\u003cp\\u003e\\u003cbr\\u003e\\u003c/p\\u003e\\u003cp\\u003eFig. 5 shows the XRD patterns of the sintered pellets along with both the parallel and perpendicular to the pressing direction. The diffraction peaks correspond to orthorhombic SnSe. However, the intensities of the (400) peak are a sharp increase in the XRD pattern\\u003c/p\\u003e\\n\\u003cp\\u003ecollected perpendicular to the pressing direction (Fig. 5(b)), indicating that strong preferred {\\u003cem\\u003eh00\\u003c/em\\u003e} orientation occurs in the SnSe pellets [27]. Fig. 5(c) and (d) are\\u0026nbsp;SEM images of SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e-0.1% pellets, fractured from different directions. The microstructures of the pellets are composed of microplates, indicating that SnSe powders grew further during the high-temperature SPS process. It can be seen that the sample exhibits a layered structure along the parallel direction and a flat plane structure along the\\u0026nbsp;perpendicular direction,\\u0026nbsp;indicating an obvious anisotropy in the sintered pellets, which fit well with the XRD patterns in Fig. 5(a) and (b).\\u003c/p\\u003e\\n\\u003cp\\u003eSince the performance of SnSe is anisotropic, it is easier to obtain low thermal conductivity in the direction of parallel pressure [28], so the test directions in this paper are all parallel to the pressing direction. The graph of the Seebeck coefficients as a function of temperature is shown in Fig. 6(a), in which the Seebeck coefficients decrease (positive) firstly then increase (negative) as long as the temperature increase.\\u0026nbsp;\\u003c/p\\u003e\\n\\u003cp\\u003eDuring the low-temperature stage (\\u0026lt; 500 K), the main carriers in the material are holes, so \\u003cem\\u003eS\\u003c/em\\u003e is positive. Then the temperature continues to increase, the bipolar transport will begin (500~600 K), in this case, the \\u003cem\\u003eS\\u003c/em\\u003e should be explained by , p and n-type carriers contribute to \\u003cem\\u003eS\\u003c/em\\u003e together. Because the n-type carriers are going to provide a negative potential, the Seebeck coefficients will decrease sharply, before the main carriers change from p to n-type, about at 600 K, the drop rate of \\u003cem\\u003eS\\u003c/em\\u003e goes up, when the contribution of p and n carriers to \\u003cem\\u003eS\\u003c/em\\u003e is equal, \\u003cem\\u003eS\\u003c/em\\u003e will be zero and the type of the main carrier changes. And then \\u003cem\\u003eS\\u003c/em\\u003e will continue to increase in the negative direction as the temperature increases, finally, a high absolute \\u003cem\\u003eS\\u0026nbsp;\\u003c/em\\u003evalue of~296.2 \\u0026mu;V K\\u003csup\\u003e-1\\u003c/sup\\u003e at 773 K is observed(\\u0026omega; = 0.2%).\\u003c/p\\u003e\\u003cp\\u003eThe \\u003cem\\u003eS\\u003c/em\\u003e of composite Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e samples is higher than the pure sample, indicating that interfaces in composites scatter carriers, thus increasing \\u003cem\\u003eS\\u003c/em\\u003e.\\u003c/p\\u003e\\n\\u003cp\\u003eThe temperature dependences of \\u0026sigma; for SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composites are shown in Fig. 6(b), the \\u0026sigma; of all samples increases with increasing temperature. When the temperature is below 600 K, the \\u0026sigma; growth is slow, this is because there is no significant increase in carrier concentration at this stage. Then, \\u0026sigma; increases rapidly at temperatures above 600 K due to the thermal excitation of electrons [29]. This tendency of \\u0026sigma; with temperature is common in polycrystalline SnSe [30]. At room temperature, the \\u0026sigma; of all samples compositing Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003eis lower than pure SnSe, because the interfaces between SnSe and Ti3C2Tx enhance the scattering of carriers to decreased carrier mobility, which can be evidenced from room temperature Hall measurement in Fig. 6(d). The electron intrinsic excitation becomes the main source of carriers during the high-temperature stage, but the electrical conductivity of the composites still does not exceed pure SnSe, indicating that the interface in the composites still has a strong scattering effect on the carriers so that decrease σ and enhance S.\\u003c/p\\u003e\\u003cp\\u003eFig. 6(c) shows the temperature dependences of PF for SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composites. PF (\\u003cem\\u003eS\\u003csup\\u003e2\\u003c/sup\\u003e\\u003c/em\\u003e\\u003cem\\u003e\\u0026sigma;\\u003c/em\\u003e) is the result of \\u0026sigma; and \\u003cem\\u003eS\\u003c/em\\u003e\\u0026apos;s calculations. During the low-temperature stage (\\u0026lt;600 K), even if there is a change in the main carrier type, there is little effect on the PF due to the low conductivity. Then, as the temperature rises, the conductivity increases sharply, as a result, an optimized PF\\u0026nbsp;of 0.74 \\u0026mu;W cm\\u003csup\\u003e-1\\u003c/sup\\u003e K\\u003csup\\u003e-2\\u003c/sup\\u003e at 773 K can be observed when \\u0026omega;\\u003cem\\u003e\\u0026nbsp;\\u003c/em\\u003e= 0.2%.\\u003c/p\\u003e\\n\\u003cp\\u003eThe total thermal conductivity\\u003cem\\u003e\\u0026nbsp;k\\u003csub\\u003etot\\u003c/sub\\u003e\\u003c/em\\u003e and lattice thermal conductivity \\u003cem\\u003ek\\u003csub\\u003elat\\u003c/sub\\u003e\\u003c/em\\u003e of all samples in the temperature range from 300 K to 773 K are shown in Fig. 7(b) and (c).\\u0026nbsp;We estimate the lattice thermal conductivity through the Wiedemann-Franz Law (\\u003cem\\u003e\\u0026kappa;\\u003csub\\u003eele\\u003c/sub\\u003e\\u0026nbsp;\\u003c/em\\u003e=\\u003cem\\u003e\\u0026nbsp;L\\u0026sigma;T\\u003c/em\\u003e). Because \\u003cem\\u003eL\\u003c/em\\u003e will not change the total thermal conductivity and final \\u003cem\\u003eZT\\u003c/em\\u003e, an estimation of \\u003cem\\u003eL\\u003c/em\\u003e can be made using the SPB model with acoustic phonon scattering, the \\u003cem\\u003eL\\u003c/em\\u003e is calculated from:\\u003cem\\u003e\\u0026nbsp;\\u003c/em\\u003e\\u003c/p\\u003e\\u003cp\\u003e\\u003cimg src=\\\"https://myfiles.space/user_files/58653_1b1c6aeb34a62c68/58653_custom_files/img1623409302.jpg\\\"\\u003e\\u003c/p\\u003e\\u003cp\\u003ewhere \\u003cem\\u003ek\\u003csub\\u003eB\\u003c/sub\\u003e\\u003c/em\\u003e\\u003csub\\u003e\\u0026nbsp;\\u003c/sub\\u003eis the Boltzmann constant, \\u003cem\\u003ee\\u003c/em\\u003e is the electric charge, \\u003cem\\u003er\\u003c/em\\u003e is the scattering rate, and \\u003cem\\u003e\\u0026eta;\\u003c/em\\u003e refers to the reduced Fermi energy, which can be derived from the measured Seebeck coefficients with consideration of acoustic phonon dominated scattering (\\u003cem\\u003er = -1/2\\u003c/em\\u003e) via the following equation:\\u003c/p\\u003e\\u003cp\\u003e\\u003cimg src=\\\"https://myfiles.space/user_files/58653_1b1c6aeb34a62c68/58653_custom_files/img1623409322.jpg\\\"\\u003e\\u003c/p\\u003e\\u003cp\\u003ewhere\\u003cem\\u003e\\u0026nbsp;F\\u003csub\\u003ex\\u003c/sub\\u003e(\\u0026eta;)\\u0026nbsp;\\u003c/em\\u003eis Fermi integral:\\u003c/p\\u003e\\u003cp\\u003e\\u003cimg src=\\\"https://myfiles.space/user_files/58653_1b1c6aeb34a62c68/58653_custom_files/img1623409350.jpg\\\"\\u003e\\u003c/p\\u003e\\u003cp\\u003eBecause the electrical conductivity is so low at a temperature below 600 K, the electronic contribution to the thermal conductivity becomes obvious only above 600 K, which leads to no significant difference between the total thermal conductivity\\u003cem\\u003e\\u0026nbsp;k\\u003csub\\u003etot\\u003c/sub\\u003e\\u003c/em\\u003e and lattice thermal conductivity \\u003cem\\u003ek\\u003csub\\u003elat\\u003c/sub\\u003e\\u003c/em\\u003e.\\u0026nbsp;It is noteworthy that the min \\u003cem\\u003ek\\u003csub\\u003elat\\u003c/sub\\u003e\\u0026nbsp;\\u003c/em\\u003eis obtained in sample \\u0026omega; = 0.1% at 773 K (0.54 W m\\u003csup\\u003e-1\\u003c/sup\\u003e K\\u003csup\\u003e-1\\u003c/sup\\u003e), which is comparable to the values of single-crystal SnSe reported by Wei [31]. This indicates that the interfaces between the SnSe matrix and Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u0026nbsp;\\u003c/sub\\u003ecan scatter phonon effectively. But the thermal conductivity of sample \\u0026omega; = 0.2% is not lower than that of the pure SnSe, this can be explained for two reasons, one is the Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e has a higher thermal conductivity than SnSe [32], the other is the uneven distribution of Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e in SnSe matrix results in a reduced scattering interface.\\u003c/p\\u003e\\n\\u003cp\\u003eFig. 7(d) presents the temperature dependence of the figure-of-merit \\u003cem\\u003eZT\\u003c/em\\u003e in the temperature range of 300-773 K. The interfaces introduced by Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e can scatter carriers effectively, thus increasing the Seebeck coefficients (\\u003cem\\u003eS\\u003c/em\\u003e), finally, a high absolute \\u003cem\\u003eS\\u0026nbsp;\\u003c/em\\u003evalue of ~296.2 \\u0026mu;V K\\u003csup\\u003e-1\\u003c/sup\\u003e\\u003csub\\u003e\\u0026nbsp;\\u003c/sub\\u003ewas obtained at 773 K. At\\u0026nbsp;the same time, the high-density interfaces of SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composites enhance the phonon scattering, a low lattice thermal conductivity \\u003cem\\u003ek\\u003csub\\u003ela\\u003c/sub\\u003e\\u003c/em\\u003e\\u003csub\\u003et\\u003c/sub\\u003e of 0.54 W m\\u003csup\\u003e-1\\u003c/sup\\u003e K\\u003csup\\u003e-1\\u003c/sup\\u003e was obtained in sample \\u0026omega; = 0.1%.\\u0026nbsp;Because of the lowest\\u003cem\\u003e\\u0026nbsp;k\\u003csub\\u003ela\\u003c/sub\\u003e\\u003c/em\\u003e\\u003csub\\u003et\\u003c/sub\\u003e in sample \\u0026omega; = 0.1%, the peak \\u003cem\\u003eZT\\u003c/em\\u003e value of 0.1 presents at 773 K in sample \\u0026omega; = 0.1%. Compared with the pure sample, the performance of the composite sample is indeed improved, but the \\u003cem\\u003eZT\\u003c/em\\u003e value is still very low, so measures still need to be taken to improve the conductivity, especially the mobility.\\u003c/p\\u003e\"},{\"header\":\"4. Conclusions\",\"content\":\"\\u003cp\\u003eIn this work, we successfully synthesize SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composites materials by the electrostatic self-assembly method and SPS. Because low energy carriers were filtered by interfaces, a high absolute \\u003cem\\u003eS\\u0026nbsp;\\u003c/em\\u003evalue of ~296.2 \\u0026mu;V K\\u003csup\\u003e-1\\u003c/sup\\u003e\\u003csub\\u003e\\u0026nbsp;\\u003c/sub\\u003ewas obtained at 773 K. The low \\u003cem\\u003ek\\u003csub\\u003elat\\u003c/sub\\u003e\\u003c/em\\u003e of 0.54 W m\\u003csup\\u003e-1\\u003c/sup\\u003e K\\u003csup\\u003e-1\\u003c/sup\\u003e was obtained at 773 K due to the scattering of phonons at the interfaces, therefore, a \\u003cem\\u003eZT\\u003c/em\\u003e of 0.1 was obtained in sample \\u0026omega; = 0.1% at 773 K along the pressing direction, which improves by 68% compared with the pure SnSe in our research. It\\u0026rsquo;s a pity that Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e can increase the carrier concentration of SnSe but reduce the mobility, which ultimately fails to achieve the purpose of improving the electrical conductivity, therefore, to obtain a higher ZT value, additional measures should be taken to increase mobility.\\u003c/p\\u003e\\n\\u003cp\\u003e\\u003cbr\\u003e\\u003c/p\\u003e\"},{\"header\":\"Declarations\",\"content\":\"\\u003cp\\u003e\\u003cstrong\\u003eAcknowledgments\\u003c/strong\\u003e\\u003c/p\\u003e\\n\\u003cp\\u003eThis work was supported by the General Project in Industrial Area of Shaanxi Province (Grant No. 2020GY-281), the Shaanxi Provincial Education Department serves Local Scientific Research Plan (Grant No. 20JC008), the National Natural Science Foundation of China (Grant No. 51702193, 51502165), the Natural Science Foundation of Shaanxi Provincial Department of Education (Grant No. 20JK0525) and the Scientific Research Fund of Shaanxi University of Science \\u0026amp; Technology (Grant No. BJ16-20, BJ16-21).\\u003c/p\\u003e\"},{\"header\":\"References\",\"content\":\"\\u003cp\\u003e[1] Bell L E. 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N-Type Ultrathin Few-layer Nanosheets of Bi Doped SnSe: Synthesis and Thermoelectric Properties. \\u003cem\\u003eACS Energy Lett\\u003c/em\\u003e 2018, \\u003cstrong\\u003e3\\u003c/strong\\u003e: 1153\\u0026ndash;1158.\\u003c/p\\u003e\\n\\u003cp\\u003e[31] Wei P C, Bhattacharya S, Liu Y F,\\u003cem\\u003e\\u0026nbsp;et al\\u003c/em\\u003e.\\u0026nbsp;Thermoelectric Figure-of-Merit of Fully Dense Single-Crystalline SnSe. \\u003cem\\u003eACS Omega\\u003c/em\\u003e 2019, \\u003cstrong\\u003e4\\u003c/strong\\u003e: 5442\\u0026ndash;5450.\\u003c/p\\u003e\\n\\u003cp\\u003e[32] Gholivand H, Fuladi S, Hemmat Z,\\u003cem\\u003e\\u0026nbsp;et al\\u003c/em\\u003e.\\u0026nbsp;Effect of surface termination on the lattice thermal conductivity of monolayer Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ez\\u003c/sub\\u003e MXenes. \\u003cem\\u003eJ Appl Phys\\u003c/em\\u003e 2019, \\u003cstrong\\u003e126\\u003c/strong\\u003e: 125-129.\\u003c/p\\u003e\"}],\"fulltextSource\":\"\",\"fullText\":\"\",\"funders\":[],\"hasAdminPriorityOnWorkflow\":false,\"hasManuscriptDocX\":true,\"hasOptedInToPreprint\":true,\"hasPassedJournalQc\":\"\",\"hasAnyPriority\":false,\"hideJournal\":true,\"highlight\":\"\",\"institution\":\"\",\"isAcceptedByJournal\":false,\"isAuthorSuppliedPdf\":false,\"isDeskRejected\":\"\",\"isHiddenFromSearch\":false,\"isInQc\":false,\"isInWorkflow\":false,\"isPdf\":false,\"isPdfUpToDate\":true,\"isWithdrawnOrRetracted\":false,\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true},\"keywords\":\"polycrystalline SnSe, Ti3C2Tx, electrostatic self-assembly, thermoelectric properties\",\"lastPublishedDoi\":\"10.21203/rs.3.rs-607248/v1\",\"lastPublishedDoiUrl\":\"https://doi.org/10.21203/rs.3.rs-607248/v1\",\"license\":{\"name\":\"CC BY 4.0\",\"url\":\"https://creativecommons.org/licenses/by/4.0/\"},\"manuscriptAbstract\":\"\\u003cp\\u003eThermoelectric materials convert thermal energy into electricity directly. Constructing nanostructured composite architectures can be an effective strategy to develop thermoelectric performance. SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composite materials were synthesized through the electrostatic self-assembly method followed by spark plasma sintering. The interfaces introduced by Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e can scatter carriers effectively, thus increasing the Seebeck coefficients (\\u003cem\\u003eS\\u003c/em\\u003e), finally, a high absolute \\u003cem\\u003eS\\u003c/em\\u003e value of ~\\u0026thinsp;296.2 \\u0026micro;V K\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e was obtained at 773 K. At the same time, the high-density interfaces of SnSe/Ti\\u003csub\\u003e3\\u003c/sub\\u003eC\\u003csub\\u003e2\\u003c/sub\\u003eT\\u003csub\\u003ex\\u003c/sub\\u003e composites enhance the phonon scattering, a low lattice thermal conductivity \\u003cem\\u003ek\\u003c/em\\u003e\\u003csub\\u003e\\u003cem\\u003ela\\u003c/em\\u003et\\u003c/sub\\u003e of 0.54 W m\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e K\\u003csup\\u003e\\u0026minus;\\u0026thinsp;1\\u003c/sup\\u003e was obtained in sample ω\\u0026thinsp;=\\u0026thinsp;0.1%. Benefit from the elevated Seebeck coefficient and decreased thermal conductivity, a \\u003cem\\u003eZT\\u003c/em\\u003e of 0.1 was obtained in sample ω\\u0026thinsp;=\\u0026thinsp;0.1% at 773 K along the pressing direction, compared with the pure SnSe, the thermoelectric performance improved by 68%. This research will provide a new way for the development of the thermoelectric properties of polycrystalline SnSe.\\u003c/p\\u003e\",\"manuscriptTitle\":\"Enhanced thermoelectric performance of polycrystalline SnSe by compositing with layered Ti3C2Tx\",\"msid\":\"\",\"msnumber\":\"\",\"nonDraftVersions\":[{\"code\":1,\"date\":\"2021-06-11 18:22:55\",\"doi\":\"10.21203/rs.3.rs-607248/v1\",\"editorialEvents\":[{\"type\":\"communityComments\",\"content\":0}],\"status\":\"published\",\"journal\":{\"display\":true,\"email\":\"info@researchsquare.com\",\"identity\":\"researchsquare\",\"isNatureJournal\":false,\"hasQc\":true,\"allowDirectSubmit\":true,\"externalIdentity\":\"\",\"sideBox\":\"\",\"snPcode\":\"\",\"submissionUrl\":\"/submission\",\"title\":\"Research Square\",\"twitterHandle\":\"researchsquare\",\"acdcEnabled\":true,\"dfaEnabled\":false,\"editorialSystem\":\"\",\"reportingPortfolio\":\"\",\"inReviewEnabled\":false,\"inReviewRevisionsEnabled\":true}}],\"origin\":\"\",\"ownerIdentity\":\"c06ab054-d1ce-409c-9974-c1a5023734ab\",\"owner\":[],\"postedDate\":\"June 11th, 2021\",\"published\":true,\"recentEditorialEvents\":[],\"rejectedJournal\":[],\"revision\":\"\",\"amendment\":\"\",\"status\":\"posted\",\"subjectAreas\":[{\"id\":4959303,\"name\":\"Ceramics\"}],\"tags\":[],\"updatedAt\":\"2021-07-21T19:14:12+00:00\",\"versionOfRecord\":[],\"versionCreatedAt\":\"2021-06-11 18:22:55\",\"video\":\"\",\"vorDoi\":\"\",\"vorDoiUrl\":\"\",\"workflowStages\":[]},\"version\":\"v1\",\"identity\":\"rs-607248\",\"journalConfig\":\"researchsquare\"},\"__N_SSP\":true},\"page\":\"/article/[identity]/[[...version]]\",\"query\":{\"redirect\":\"/article/rs-607248\",\"identity\":\"rs-607248\",\"version\":[\"v1\"]},\"buildId\":\"WvIrzKhiLBfengagbw6Ux\",\"isFallback\":false,\"isExperimentalCompile\":false,\"dynamicIds\":[84888],\"gssp\":true,\"scriptLoader\":[]}","source_license":"CC-BY-4.0","license_restricted":false}