Chemical Welding on Semimetallic TiS2 Nanosheets for High

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Chemical Welding on Semimetallic TiS2 nanosheets for high performance Flexible n-type Thermoelectric Films Yuan Zhou, Juanyong Wan, Qi Li, Lei Chen, Jiyang Zhou, Heao Wang, Dunren He, Xiaorui Li, Yaocheng Yang, and Huihui Huang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b15026 • Publication Date (Web): 28 Nov 2017 Downloaded from http://pubs.acs.org on November 29, 2017

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ACS Applied Materials & Interfaces

Chemical Welding on Semimetallic TiS2 nanosheets for high performance Flexible n-type Thermoelectric Films Yuan Zhou,1,▽Juanyong Wan,1,▽ Qi Li2,▽Lei Chen,3 Jiyang Zhou,

1

Heao Wang,

1

Dunren He, 1 Xiaorui Li, 1 Yaocheng Yang1 and Huihui Huang1,* 1

Key Laboratory for Micro-/Nano-Optoelectronic Devices of Ministry of Education,

School of Physics and Electronics, Hunan University, Changsha 410082, 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 11794-3400,

U.S.A. ▽

These authors contributed equally to this work.

*

Corresponding Authors. E-mails: [email protected]

KEYWORDS: Chemical Welding; Semimetallic; 2D Materials; Thermoelectric; Flexible; TiS2

1

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Abstract

Solution-based processing of two-dimensional materials provides the possibility to allow these materials to be incorporated into large-area thin films, which can translate the interesting fundamental properties of two-dimensional materials into available devices. Here, we report for the first time a novel chemical welding method to achieve high performance flexible n-type thermoelectric films using two-dimensional semimetallic TiS2 nanosheets. We employ chemically exfoliated TiS2 nanosheets bridged with multivalent cationic metal Al3+ to crosslink the nearby sheets during the film deposition process. We find such treatment can greatly enhance the stability of the film and can improve the power factor by simultaneously increasing the Seebeck coefficient and the electrical conductivity. The resulting TiS2 nanosheets based flexible film shows a room temperature power factor of ~216.7 µW m-1K-2, which is among the highest chemically exfoliated two-dimensional transition metal dichalcogenide nanosheets based films and comparable to the best flexible n-type thermoelectric films to our knowledge, indicating its potential applications in wearable electronics.

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Introduction

Nowadays, the worldwide problem of the energy crisis has become more and more serious, it is necessary to transform the schema of energy consumption and develop alternative approaches for energy harvesting. Under the effort of scientists, novel environmentally friendly energy sources like solar energy, wind energy, and thermoelectric energy, etc. have emerged during the past several decades. Among these new-type energies, the thermoelectric energy brings a promising chance which has attracted great interest due to its unique ability of directly converting wasted heat to useful electricity.1 Furthermore, once affiliated with a flexible property, the thermoelectric materials are readily compatible with wearable devices, which have the strong requirement of getting rid of the regular recharging or replacement batteries and using sustainable environmental energies especially the body heat as their power sources.2-6

The efficiency of a thermoelectric material depends on the figure of merit (ZT) of the thermoelectric material given by the formula ZT= α2σT/κ, where α is the Seebeck coefficient, σ is the electrical conductivity, T is the average absolute temperature, and κ is the thermal conductivity. In terms of application, factors like stability, cost and preparation complexity are also considered as important criteria of the performance of a thermoelectric material. Over the past years, numerous studies had been done in the 3



interest of increasing the performance of flexible thermoelectric materials. Great progress has been made on p-type flexible thermoelectric materials. However, there is still a major challenge because the n-type thermoelectric material with high flexibility and comparable performance has not been successfully synthesized yet, which immensely hindered the development of flexible thermoelectric devices. For instance, high performance flexible p-type thermoelectric polymer materials with the highest thermoelectric performance have been achieved on poly(3,4-ethylenedioxythio-phene) polystyrene sulfonate (PEDOT:PSS) with the reported room temperature ZT value of 0.42.7 However, the n-type thermoelectric polymer materials show a degraded performance owing to their low electron affinities, which result in the poor stability of the n-type polymer materials. Therefore, it is urgent to search for an n-type flexible thermoelectric material with high performance and reliable stability at the same time.5

Recently, researchers have paid growing attention to the two-dimensional (2D) transition metal dichalcogenides (TMDCs) because of the potentially suitable flexible thermoelectric materials with high thermoelectric performance, low material cost, controllable fabrication cost and high mechanical ﬂexibility based on such materials.6, 8-9

Among the methods to obtain atomic-thin 2D TMDCs, liquid-phase chemical

exfoliation method has been proved to be an effective and scalable approach to produce high-yield atomic-thin 2D nanosheets.12-13 For example, recently the previous work on semimetallic 1T phase MoS2 nanosheets in our group using the 4


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organolithium chemical exfoliation method achieved a much higher room temperature power factor of 73.1 µWm-1K-2 than the pristine graphene or single wall carbon nanotubes can yield.6 However, the 1T phase 2D MoS2 nanosheets are still p-type flexible thermoelectric material. To find the idea n-type flexible thermoelectric counterpart, Xie’s work on organolithium chemically exfoliated TiS2 nanosheets inspired us.14 The TiS2 nanosheet restacked film shows high flexibility and extremely high electrical conductivity which is an ideal n-type flexible TE material. From the same work, we also learned that the high electrical conductivity of the 2D TiS2 nanosheets originates from the lithiation exfoliation process due to extra electrons provided by active Li atoms injected into Ti atoms leading Ti4+ reduced to Ti3+. Therefore, we presume that the TiS2 nanosheets exfoliated by the organolithium chemical method might be easily oxidized which is not mentioned in Ref14.

Here, we demonstrate a facile chemical welding method for the first time to achieve high performance flexible n-type thermoelectric films based on lithium exfoliated TiS2 nanosheets. The utilization of multivalent cationic metal Al3+ to bridge the TiS2 nanosheets has several advantages. First, the stability of the TiS2 nanosheets assembled film is significantly improved thanks to the surface charge balance of the lithium exfoliated TiS2 nanosheets and the much more compact layered structure of the film. Second, the Seebeck coefficient and the electrical conductivity of the film are simultaneously enhanced that is unusual for common thermoelectric materials, 5



and the physical insights are discussed in the following text. Third, the whole fabrication process is simple and requires only basic equipment.

Experimental Section

Materials. TiS2 bulk powder, n-butyllithium (1.6 M in hexane) and hexane were purchased from Sigma Aldrich.

Preparation of ultrathin TiS2 nanosheets.The TiS2 nanosheets were prepared according to Ref.14 with some modifications. The LixTiS2 was synthesized by the lithium intercalation using 0.6 g TiS2 powder with 6 ml n-butyllithium diluted with 24 ml hexane. After refluxing at 60oC under argon for 12 h, the mixture was filtered and washed with hexane several times to remove the excess butyllithium. The as-prepared LixTiS2 precursor was then exfoliated in water at 1.5 mg/mL, sonicated for 1h and centrifuged to further remove the lithium cations as well as the non-exfoliated materials, this process was repeated for several times to ensure the removal of lithium cations. A high-quality dispersion of ultrathin TiS2 nanosheets was obtained.

Assembly of TiS2 nanosheets into flexible thin films. The exfoliated ultrathin TiS2 nanosheets dispersions were vacuum filtrated over mixed cellulose ester membranes with 25 nm pore size to form homogeneous thin films, of which the thickness can be controlled by the amount of the dispersion. To carry out the chemical welding strategy, 0.1 M AlCl3 aqueous solution was added during the filtration process (see Figure

6


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1(a)), the volume ratio of AlCl3 aqueous solution to TiS2 nanosheets dispersion was controlled as 1:10. The as-prepared thin films were vacuum annealed at 50oC to remove the residual water. It is worth noting that it is crucial not to prepare a mixed solution of AlCl3 and TiS2 nanosheets before the filtration process because the AlCl3 aqueous solution will cause the aggregation and precipitation of TiS2 nanosheets.

Characterizations. The synthesized TiS2 thin films were analyzed by X-ray diffraction (XRD) using Cu Kα radiation (Bruker D2 Phaser diffractometer). The cross sectional morphology was measured by the Scanning Electron Microscope (SEM, JEOL, JSM-6330F). Transmission Electron Microscope (TEM) techniques were performed using a JEM-2100 electron microscope. The thermoelectric properties of the semimetallic TiS2 thin films were measured using a custom built apparatus. A typical 4-probe technique was used to measure the electrical conductivity. The standard correction term was introduced here due to the finite dimensions of the probes and boundaries of the sample. The Seebeck coefficient was measured by heating one copper block and simultaneously measuring ∆T and the thermoelectric voltage generated.

Calculations. In this work, our density functional theory (DFT) calculations were carried out using the QUANTUM ESPRESSO (QE) package15 with norm-conserving pseudopotentials

and

generalized

gradient

approximation

(GGA)

exchange-correlation functionals parameterized by Perdew-Burke-Enzerhof (PBE).16 7



A self-consistency convergence criterion of 10-8 eV was used for all calculations. All the structures were fully relaxed until force components on any atoms were less than 10-3 eV/Å. The number of plane-waves was determined by a kinetic energy cutoff of 500 eV. The Monkhorst-Pack grids with a maximum separation of 0.05 Å-1 between k-points were used for sampling the Brillouin zone.17 This sampling density was checked with respect to the convergence of the bulk TiS2 total energy, corresponding to an 18×18×4 k-point grid for the reciprocal space of a 3 atom KCaI3 primitive cell. The TiS2 monolayer was sampled by using a 1×1×4 sized supercell with 23.2 Å interlayer spacing. The H0.5TiS2 monolayer was simulated in a 2×2×4 supercell to achieve lower total system energy compared to the configuration with all hydrogen atoms adsorbed on the same side of the TiS2 monolayer. To provide an accurate description of the electronic structure, we carried out GW approximation (GGA+G0W0) calculations to calculate the quasiparticle effects. For the H-adsorbed TiS2 monolayers we included 1024 bands in total (> 95% are unoccupied) to achieve convergence of the screened interactions. In addition, the position of the Ti-3d orbitals was corrected within the rotationally invariant GGA+U approach,18 the U parameters were determined by the constrained random phase approximation described in Ref19. The band structure was calculated by the maximally-localized Wannier functions (MLWFs) in the Wannier90 code.20

Results and Discussion 8


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Figure 1. (a-b) The fabrication process of TiS2 nanosheets assembled flexible thin film of using Al3+ to crosslink the nearby TiS2 nanosheets. (c) An image of the as-prepared flexible Al:[TiS2 ns] restacked film. (d-e) Typical cross-sectional SEM images of the as-prepared TiS2 nanosheet restacked film and the Al:[TiS2 ns] restacked film, respectively. (f) XRD patterns of the as-prepared TiS2 nanosheet restacked film and the Al:[TiS2 ns] restacked film. (g-h) Schematic diagrams of TiS2 nanosheet restacked film and the Al:[TiS2 ns] restacked film with the interlamellar distance d1 and d2 (d1＞d2), respectively.

9



The chemical welded TiS2 nanosheets assembled films were fabricated by simply adding the Al3+ aqueous solution during the standard vacuum filtration filming process (see Figure 1(a-b)). The resulting film shows high flexibility (see Figure 1(c)) and is named as Al:[TiS2 ns] restacked film distinguished from the TiS2 nanosheet restacked film in the following text. The cross-sectional SEM image of the Al:[TiS2 ns] restacked film shows a clear layered structure with an average thickness of ~800 nm which is similar with that of the TiS2 nanosheet restacked film (see Figure 1(d-e)). In order to verify the effects of the Al3+ on the compactness of the film, XRD patterns of the as-prepared Al:[TiS2 ns] restacked film and TiS2 nanosheet restacked film were compared, shows in Figure 1(f). The (001) peaks of TiS2 dominate both patterns, which are attributed to the layered structures of the restacked films. It is distinct that the (001) peak of Al:[TiS2 ns] restacked film is right-shifted against the TiS2 nanosheet restacked film, indicating the average interlamellar distance between TiS2 nanosheets reduces upon the appearance of Al3+. To explain this effect, schematic diagram of the two restacked films are showed in Figure 1(g-h), in which the interlamellar distances of the TiS2 nanosheet restacked film and the Al:[TiS2 ns] restacked film are marked as d1 and d2, respectively. The value of d1 and d2 can be calculated as: d1=5.889Å, d2=5.658Å, based on the Bragg equation: 2dsinθ=nλ, where d is the crystal plane distance, θ is the angle between the incident X-ray and the corresponding crystal face, n is the integral number of wavelengths, and λ is the X-ray

10


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wavelength. The smaller interlamellar distance of the Al:[TiS2 ns] restacked film (d2

＜d1) reveals that the Al3+ acts as the welding flux of TiS2 nanosheets.

Table 1. Comparison of the room temperature thermoelectric performances of the chemical exfoliated 2D TMDC nanosheets assembled films and other state-of-art TiS2 based thermoelectric materials.

Conduction

Seebeck Coefficient

Type

(µV K-1)

Materials

Electrical

Power

Conductivity

Factor (µW

(104 S m-1)

m-1K-2)

6.0

216.7

Ref

This Al : [TiS2 ns]

n

-60.1

work This

1T-TiS2 n

-42.5

2.0

36.1 work

nanosheets 1T-MoS2 p

+85.6

0.99

73.1

6

n

-72

0.1

5-7

9

p

+13

15

26-34

9

n

-276

1.19

910

10

nanosheets 1T-WS2 nanosheets 1T-NbSe2 nanosheets TiS2 bulk

11



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compound TiS2[(HA)0.08(H2O n

-78

7.9

480.6

11

)0.22(DMSO)0.03]

Figure 2. Temperature dependent (40 K – 290K) Seebeck coefficient (a), electrical conductivity (b) and power factor (c) of a typical as-prepared Al:[TiS2 ns] restacked film.

Room temperature thermoelectric properties including the Seebeck coefficients and the electrical conductivities of the as-prepared TiS2 nanosheet restacked film and the Al:[TiS2 ns] restacked film were measured using a custom built apparatus, the results are shown in Table 1. Compared with the TiS2 nanosheet restacked film, the Seebeck coefficient α and the electrical conductivity σ have been simultaneously enhanced for the Al:[TiS2 ns] restacked film. This phenomenon is unusual compared with most of the thermoelectric material system. For a single-phase thermoelectric material, the α and σ are usually strongly coupled, for example, a high α prefers a rapid variation in the density of states of the material, which is opposite to the direction of increasing σ.21 The physical insights of the decoupled α and σ will be discussed in the following text. The resulting room temperature power factor of the Al:[TiS2 ns] restacked film is ~216.7 µW m-1K-2, which is much higher than the power factors of the chemically 12


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exfoliated MoS2, WS2 and NbSe2 nanosheets based films (see Table 1) and is comparable to the best flexible n-type thermoelectric films (see Table S1 in the supporting

information).

Although

the

TiS2

bulk

compound

and

the

TiS2[(HA)0.08(H2O)0.22(DMSO)0.03] have a larger room temperature thermoelectric performance, but their limited flexibility restricts their applications in wearable electronics. Temperature dependent thermoelectric properties are important indices of probing the electronic structures of thermoelectric materials. As for the Al:[TiS2 ns] restacked film,

temperature dependent Seebeck coefficients and electrical

conductivities in the range of 40 K – 290 K were measured (see Figure 2). The Seebeck coefficient decreases as decreasing the temperatures, and the electrical conductivity increases as decreasing the temperatures, showing a typical metal-like behavior. The overall power factor of the Al:[TiS2 ns] restacked film has a large temperature window of ~210 µW m-1K-2 in the range of 100 K – 300 K (see Figure 2(c)), indicating their potential applications in the low-temperature region.

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Figure 3. (a) A typical TEM image of the TiS2 nanosheet. (b) Images of the same TiS2 nanosheets dispersion of just prepared (left) and after placing 48 hours under ambient (right). (c-e) The possible mechanism of the easily oxidized property of TiS2 nanosheets. (f) Compared aging tests of the as-prepared TiS2 nanosheet restacked film and the Al:[TiS2 ns] restacked film by directly immersing the films in water. (g) Schematic diagram of one Al3+ ion welds nearby TiS2 nanosheets.

The stability property of the chemically exfoliated 2D TMDC nanosheets is an important factor but lack of systematic studies. Figure 3(a) shows the TEM image of a typical exfoliated monolayer TiS2 nanosheet with the size around 800 nm, which is 14


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taken from the as-prepared TiS2 nanosheets dispersion (see the left image of Figure 3(b)). After placing the as-prepared TiS2 nanosheets aqueous dispersion in atmospheric environment for 48 hours (see Figure 3(b)), the color of the dispersion changed from black to white, which signifies the oxidization of the TiS2 nanosheets. The underlying mechanism could be explained as follows. Firstly, during the refluxing process, the TiS2 powder reacted with n-butyllithium, leading part of the Ti4+ reduced to Ti3+ through a simple electron transferring process (see Figure 3(c)). Secondly, after reacting with water, the hydrogen ions replaced the lithium ions generating HxTiS2 nanosheets. However, the HxTiS2 nanosheets will easily react with water forming negatively charged TiS2 nanosheets and hydronium (see Figure 3(d-e)). This hypothetical model well explained two consequences. 1) the negatively charged surface of TiS2 nanosheets explains the stable dispersion of TiS2 nanosheets in water. 2) the extra electrons make TiS2 nanosheets unstable and oxidizable which matches with our experimental result closely. Therefore, it’s reasonable to predict the unstable properties of the TiS2 nanosheets assembled films, which is not described in the previous literature14. To study the effects of Al3+ on the stability of the TiS2 nanosheet restacked films, compared aging tests were performed by directly immersing the Al:[TiS2 ns] restacked film and the TiS2 nanosheet restacked film in water (see Figure 3(f)). The TiS2 nanosheet restacked film fractured after one day aging and became completely white after four days aging, demonstrating its unstable property which is consistent with the dispersion results. However, the apperance of the 15



Al:[TiS2 ns] restacked film almost stayed unchanged after 4 days of aging, indicating the significantly improved stability compared to the plain TiS2 nanosheet restacked film. Adding multivalent Al3+ has two major effects: 1) equilibrate the negative charges on the surface of TiS2 nanosheets, 2) weld the nearby TiS2 nanosheets as illustrated in Figure 3(g). Therefore, the compacted films can effectively repel the penetration of water molecules which is important to the improved stability property and electrical conductivity of the Al:[TiS2 ns] restacked film.

Figure 4. (a-c) Compared thermoelectric properties of the Al:[TiS2 ns] restacked film and the TiS2 nanosheet restacked film versus aging time. (a) Electrical conductivities. (b) Seebeck coefficients. (c) Thermoelectric power factors. (d) Compared XRD patterns of the Al:[TiS2 ns] restacked film

16


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and the TiS2 nanosheet restacked film after 96 h aging in high humidity atmosphere with relative humidity of ~70%.

To further study the stability properties of the Al:[TiS2 ns] restacked film, we performed aging tests by comparing the thermoelectric properties of the as-prepared TiS2 nanosheet restacked film and the Al:[TiS2 ns] restacked film at different aging time (see Figure 4(a-c)). The aging tests were performed by exposing the as-prepared samples in a high humidity atmosphere with relative humidity of ~70%. After aging for 20 h, both Seebeck coefficients of the TiS2 nanosheet restacked film and the Al:[TiS2 ns] restacked film have been improved (see Figure 4(a)), however, the enhanced Seebeck coefficients originate from the decreased electrical conductivities (see Figure 4(b)). The electrical conductivity of the Al:[TiS2 ns] restacked film has been decreased from 6.03×104 S/m to 3.39×104 S/m (see Figure 4(b)), resulting in a 31.0% decrease of the thermoelectric power factor (see Figure 4(c)), whereas the electrical conductivity of the TiS2 nanosheet restacked film has been remarkably reduced from 2.01×104 S/m to 7.60×102 S/m (see Figure 4(b)), which results in a 94.1% decrease of its thermoelectric power factor (see Figure 4(c)), indicating the complete failure of the TiS2 nanosheet restacked film as a thermoelectric material. The above results reveal the significantly improved stability of the Al:[TiS2 ns] restacked film. After aging for 96 h, the Seebeck coefficient of the Al:[TiS2 ns] restacked film has been further increased to -69.9 µV/K (see Figure 4(a)), while its electrical conductivity has been decreased to 1.28×104 S/m (see Figure 4(b)), resulting in a thermoelectric power factor of 62.7 µWm-1K-2 (see Figure 4(c)), indicating that there is still a large space to improve the thermoelectric stability of the lithium exfoliated TiS2 nanosheets based thin films. We think one of the reason is that 17



the addition of Al cation can’t replace all H-S bonds with Al-S bonds, which leads to the degradation of the films. Strategies like fine control of the amount of Al cations and adding other cations or even mixed cations in the system will be studied to further improve the stability. Figure 4(d) shows the compared XRD patterns of the Al:[TiS2 ns] restacked film and the TiS2 nanosheet restacked film after 96 h aging, the XRD pattern of the Al:[TiS2 ns] restacked film still shows a typical pattern of the TiS2 nanosheet restacked film (see Figure 4(d)), whereas no XRD peaks have been observed for the TiS2 nanosheet restacked film, further proving the significantly improved thermoelectric stability of the Al:[TiS2 ns] restacked film.

Figure 5. (a) Crystal structure of the H0.5TiS2 monolayer used in our simulation. (b) PBE+G0W0 calculated band structure of H0.5TiS2 monolayer. (c) Illustration of the density of states in a general semimetal. (d) Schematic diagram of the charge carrier transport between nearby TiS2 18


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nanosheets. (e) Schematic energy band diagram of the TiS2-Al3+-TiS2 structure, the CB and VB represent the conduction band bottom and valence band top of the semimetallic TiS2, respectively.

Basically, for an n-type semiconductor, the electrical transport can be described by Boltzmann transport theory,

α=

8π 2 k B 2 *  π  mT  3eh 2  3n 

2/3

σ = neµ where

kB

is the Boltzmann constant,

e

is the carrier charge, h is the Planck’s

constant, m* is the effective mass of the charge carrier,

n

is the carrier concentration,

and µ is the carrier mobility. According to this theory, the α and σ are strongly interrelated. Therefore, an increase in the σ would probably lead to a decrease in the α.5 However, in this work, the α is simultaneously enhanced as improving the σ for the Al:[TiS2 ns] restacked film, which indicates the decoupling of α and σ by using the Al3+ welds the lithium exfoliated TiS2 nanosheets.

We presumed that the band structure of the partially reduced TiS2 nanosheets and the carrier transport property of the TiS2-Al3+-TiS2 structure would result in the decoupling effect of α and σ in the Al:[TiS2 ns] restacked film. The TiS2 nanosheets synthesized by the organolithium exfoliated method are actually HxTiS2 nanosheets with some Ti4+ partially reduced to Ti3+ as discussed above, and the parameter x in the HxTiS2 nanosheets should be around 0.5 according to the previous literature.14 The 19



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band structure of H0.5TiS2 monolayer is plotted in Figure 5(b) with detailed calculation process showed in the experimental section. In this band structure, the Fermi level crosses both positive and negative curvature parts of the Brillouin zone, suggesting both electron and hole conduction in this material, which suggests the semimetal nature of H0.5TiS2 monolayer. For comparison, the pristine TiS2 monolayer is an indirect semiconductor with a band gap of 1.37 eV based on the same calculation method (see Figure S4 in the supporting information). Our calculated band structure of TiS2 monolayer is qualitatively similar to previous theoretical investigations with a larger band gap,22-23 which is reasonable given that the previous calculations have used traditional DFT which generally underestimates the band gap by neglecting the quasiparticle effects. The Ti-3d orbitals are completely empty and construct the lower part of the conduction band in the pristine TiS2 monolayer, suggesting ionic-bond like charge transfer from Ti-3d to S-3p orbitals. However, upon hydrogen adsorption, the H-S bonds also contributes to the completed S-3p shell, and the Ti-3d orbitals become partially filled as indicated in Figure 5(b). As a result, unlike the semiconductor nature of pristine TiS2 monolayer, the H0.5TiS2 monolayer can be classified as a semimetal, whose general shape of the density of states is illustrated in Figure 5(c). Therefore, both the electrons and holes exist near the Fermi level of the H0.5TiS2 monolayer, but they contribute negative and positive Seebeck coefficients under a temperature gradient, respectively. This property inevitably reduces the eventual Seebeck coefficient of the H0.5TiS2 monolayer ( α

= αe +αh

20


with different signs). In

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the Al:[TiS2 ns] restacked film, thanks to the existence of Al3+ between the nearby TiS2 nanosheets (see Figure 5(d)), the charge carrier transportation is different from one single TiS2 layer. The schematic energy band diagram of the TiS2-Al3+-TiS2 structure is shown in Figure 5(e), the Al3+ between the nearby TiS2 nanosheets would cause the band bending of the semimetallic TiS2, which could not only block the minority holes crossing the interfaces but also trap the low energy electrons (see Figure 5(e)), both effects should result in a larger negative Seebeck coefficient.5 Therefore, the α and σ of the Al:[TiS2 ns] restacked films have been successfully decoupled due to the Al3+ ions which can incorporate two positive effects as discussed.

CONCLUSION

In conclusion, we have successfully fabricated flexible n-type thermoelectric thin films with a high room temperature power factor of above 200 µW m-1K-2 based on the Al3+ welded TiS2 nanosheets that were synthesized by the organolithium chemical method. We find that this strategy can significantly improve the stability of the TiS2 nanosheet restacked films. Moreover, our method can boost the thermoelectric properties of the TiS2 nanosheets by decoupling the α and σ thanks to the multifunctional Al3+ ion in altering the electronic structures of the welded nanosheets. Following these results, we believe that this study opens up new opportunities to

21



improve the thermoelectric properties of nanomaterials based films, especially the 2D TMDCs that were synthesized by the organolithium chemical method.

ASSOCIATED CONTENT

Supporting Information

Illustrated fabrication process of the chemically exfoliated TiS2 nanosheets, compared stability performance of the TiS2 nanosheets prepared by ultrasonic exfoliation, detailed calculation process of the equivalent resistance circuit model of the TiS2 nanosheets and the Al:[TiS2 ns] nanosheets, band structure calculation of the pristine monolayer TiS2, full XRD spectra of TiS2 nanosheets restacked film and Al:[TiS2 ns] restacked film, comparison of the room temperature thermoelectric performances of the Al:[TiS2 ns] restacked film in this work and several selected state-of-art flexible n-type thermoelectric materials.

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]

Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS 22


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This study was conducted under support from the National Natural Science Foundation of China (Grant NO. 61704053), the Natural Science Foundation of Hunan Province (Grant NO. 2017JJ3032) and Fundamental Research Funds for the Central Universities of China.

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Chemical Welding on Semimetallic TiS2 Nanosheets for High

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