nanosheets for high performance Flexible n-type Thermoelectric Films

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1 Supporting Information Chemical Welding on Semimetallic TiS 2 nanosheets for high performance Flexible n-type Thermoelectric Films Yuan Zhou, 1, Juanyong Wan, 1, Qi Li 2, Lei Chen, 3 Jiyang Zhou, 1 Heao Wang, 1 Dunren He, 1 Xiaorui Li, 1 Yaocheng Yang 1 and Huihui Huang 1,* 1 Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education, School of Physics and Electronics, Hunan University, Changsha , China 2 Physical Science Division, IBM Thomas J. Watson Center, Yorktown Heights, NY 10598, U.S.A. 3 Department of Chemistry, Stony Brook University, Stony Brook, NY , U.S.A. These authors contributed equally to this work. * Corresponding Authors. s: huangh@hnu.edu.cn S-1

2 1. Fabrication process of the chemically exfoliated TiS 2 nanosheets. Figure S1. (a-c) Schematic diagram of the fabrication process of TiS 2 nanosheets through organolithium chemical exfoliation method. The TiS 2 nanosheets were prepared by liquid-phase chemical exfoliation of bulk TiS 2 powder (see Figure S1(a-c)). Firstly, after refluxing the TiS 2 powder in n-butyllithium, the lithium ions were randomly absorbed on the sulfur atoms of TiS 2 forming Li x TiS 2 powder (see Figure S1(b)), the inserted Li ions would also expand the interlamellar space of TiS 2 and weaken the Van der Waals forces between the layers. Secondly, the monolayer TiS 2 nanosheets were fabricated by reacting the Li x TiS 2 powder with water (see Figure S1(c)), during such process, the hydrogen ions replaced the lithium ions, generating the monolayer hydrogen incorporated TiS 2 nanosheets aqueous dispersion. Strictly speaking, the TiS 2 nanosheets synthesized in this work should be H x TiS 2 nanosheets, however, we use TiS 2 nanosheets to simplify the description. S-2

3 2. Stability of TiS 2 nanosheets prepared by ultrasonic exfoliation. Figure S2. (a) Image of the TiS 2 nanosheets dispersion prepared by ultrasonic exfoliation. (b) Image of the TiS 2 nanosheets dispersion after aging for 1 month. (c) TEM image of the as-prepared TiS 2 nanosheets. To prove that the mechanism of the easily oxidizable TiS 2 nanosheets is originated from the changed surface states of TiS 2 nanosheets during the lithium exfoliation process, we prepared TiS 2 nanosheets through a relatively much softer ultrasonic exfoliation process for comparison [1] (see Figure S2). During the ultrasonic exfoliation process, the TiS 2 bulk powder was added into to a mixed solution composed of N-Methyl-2-pyrrolidone (NMP) and 1-Octyl-2-pyrrolidone (N8P). The mixture was then sonicated for 1h and centrifuged to remove the non-exfoliated materials. Figure S2(a) shows the image of the TiS 2 nanosheets dispersion, the size of the TiS 2 nanosheets is much smaller than that of the lithium exfoliation prepared TiS 2 nanosheets (see Figure S2(c)). After aging the dispersion for one month, no precipitates or color changes can be observed from the dispersion (see Figure S2(a-b)), which indicates the TiS 2 nanosheets prepared from ultrasonic exfoliation are much more stable than the nanosheets prepared from the lithium exfoliation method. By comparing the two exfoliation processes, it can also be concluded that the oxidizable property of TiS 2 nanosheets prepared from the lithium exfoliation process should be originated from the electron injection from Li to Ti 4+ during the lithium process. It s worth noting that the ultrasonic exfoliated TiS 2 nanosheets are not suitable for fabricating flexible thermoelectric films due to its small size and the high boiling point solvent. S-3

4 3. Detailed calculation process of the equivalent resistance circuit model of the TiS 2 nanosheets and the Al:[TiS 2 ns] nanosheets. Figure S3 (a-b) The simplified model of the TiS 2 nanosheets and the Al:[TiS 2 ns] nanosheets. (c-d) The equivalent resistance circuit model of the TiS 2 nanosheets and the Al:[TiS 2 ns] nanosheets. To understand the improved electrical conductivity of the Al:[TiS 2 ns] restacked film, simplified equivalent resistance circuit models of the TiS 2 nanosheet restacked film and the Al:[TiS 2 ns] restacked film are shown in Figure S3. We use R 1 +R 2 and R 3 +R 4 to represent the resistances of two adjacent TiS 2 nanosheets. Therefore, the TiS 2 nanosheet restacked film can be regarded as a parallel connection of two adjacent TiS 2 nanosheets (see Figure S3(a) and S3(c)). As for the Al:[TiS 2 ns] restacked film, the Al 3+ ion acts as a bridge that connects the nearby TiS 2 nanosheets (see Figure S3(b)). To illustrate its effect, a resistor R A is added in the circuit and the simplified equivalent resistance circuit model of the Al:[TiS 2 ns] restacked film could be considered as a Huygens bridge circuit (see Figure S3(d)). Consequently, the overall resistance of the Al:[TiS 2 ns] restacked film should be much smaller than that of the S-4

5 TiS 2 nanosheet restacked film as long as R 1 /R 2 R 3 /R 4, detailed calculation process is showed as follows. Calculation of numerator: 2 0 So, After the R A was added to the circuit, the total resistance was decreased, which signify that after the Al 3+ ions welding the TiS 2 nanosheets, the total resistance was decreased which well matched with our experimental measurement. 4. Band structure calculation of the pristine monolayer TiS 2. S-5

6 Figure S4. Crystal structure (a) and the GW approximation and Wannier function interpolated band structure (b) of the pristine monolayer TiS 2. The pristine monolayer of TiS 2 has D 3d point-group symmetry with 3 atoms per unit cell. We used GGA+G 0 W 0 calculations to describe the quasiparticle energies and obtained a more accurate description of the electronic structure of TiS 2 monolayer. The band structure is plotted by using the Wannier function interpolation within the Wannier90 code. Our calculation shows that TiS 2 monolayer is an indirect semiconductor with a band gap of 1.37 ev. 5. Full XRD spectra of TiS 2 nanosheets restacked film and Al:[TiS 2 ns] restacked film after aging for 4 days. Figure S5. Full XRD spectra of Al:[TiS 2 ns] restacked film (a) and TiS 2 nanosheets restacked film (b) after aging for 4 days. 6. Table S1. Comparison of the room temperature thermoelectric performances of the Al:[TiS 2 ns] restacked film in this work and several selected state-of-art flexible n-type thermoelectric materials. Materials Temperat ure (K) Conduct ion type Seebeck Coefficient Electrical Conductivity Power Factor (µw m -1 K -2 ) Year [Ref] S-6

7 (µv K -1 ) (10 4 S m -1 ) Al:[TiS 2 ns] restacked film 290 n This work Cu 0.1 Bi 2 Se 3 nanoplate/pvdf 290 n [2] composite TiS 2 [(HA) 0.08 (H 2 O) 0.22(DMSO) 0.03 ] 308 n [3] CoCp2@SWNTs 320 n [4] NDINE/SWCNT RT n ~135 [5] Nanocarbon 310 n [6] Poly(Ni-ett) 300 n [7] Ni/PVDF 300 n [8] composite As shown in the above table, the room temperature power factor of the Al:[TiS 2 ns] restacked film is comparable to the state-of-art flexible n-type thermoelectric materials including carbon nanotubes, polymer and nanocarbon. Considering other parameters like flexibility, feasibility and preparation costs, the semimetallic TiS 2 nanosheets based n-type thermoelectric films are a very promising candidate for the next-generation flexible thermoelectric materials. References [1]. Zhou, Y.; Huang, H. Efficient Exfoliation of TiS 2 Through a Mixed-solvent Strategy. Preparing. [2]. Dun, C.; Hewitt, C. A.; Huang, H.; Xu, J.; Zhou, C.; Huang, W.; Cui, Y.; Zhou, W.; Jiang, Q.; Carroll, D. L., Flexible n-type Thermoelectric Films based on S-7

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