Thermoelectric,Properties,of,n-type,Poly,(nickel,1,1,2,2-ethenetetrathiolate),Prepared,by,a,New,One-step,Solvothermal,Method

WANG Yijiang, YAO Qin, QU Sanyin, CHEN Lidong,3

(1. State Key Laboratory of High Performance Ceramics and Superfine Microstructure, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China; 2. Uniνersity of Chinese Academy of Sciences, Beijing 100049, China; 3. CAS Key Laboratory of Materials for Energy Conνersion, Shanghai Institute of Ceramics, Chinese Academy of Science, Shanghai 200050, China; 4. School of Physical Science and Technology, Shanghai Tech Uniνersity, Shanghai 201210, China)

Abstract: Poly(nickel 1,1,2,2-ethenetetrathiolate) (poly[Nax(Ni-ett)]) is one of the most promising n-type organic thermoelectric materials which can be used in wearable devices. However, the conventional solution method is time-consuming and the prepared poly[Nax(Ni-ett)] usually has poor crystallinity, which does not benefit for achieving high thermoelectric performance. Here, a new one-step solvothermal method under the high reaction temperature and high vapor pressure was developed to prepare poly[Nax(Ni-ett)] with a quite short period. The experimental results show crystallinity and electrical conductivity are greatly enhanced as compared with those prepared by conventional solution method. As a result, a maximum ZT value of 0.04 was achieved at 440 K, which is about four times of the polymer prepared by the conventional solution method. This study may provide a new route to enhance the TE properties of n-type organic thermoelectric materials.

Key words: thermoelectric; organic materials; n-type poly[Nax(Ni-ett); solvothermal method

Thermoelectric technology can realize the direct conversion between heat and electricity. Recently, organic thermoelectric (OTE) materials have become a hot topic in thermoelectric community because of their advantages such as lightweight, low-cost, flexible and nontoxic. The performance of an OTE material is characterized by its thermoelectric figure of merit

ZT

(=

S

σT

/

κ

), where

T

is the absolute temperature,

S

is the Seebeck coefficient,

σ

is the electrical conductivity, and

κ

is the thermal conductivity. Currently, most researches on OTE materials are focusing on the p-type conducting polymers while the development of n-type OTE materials is very slow. To make an organic thermoelectric device with high energy conversion efficiency, both high performance p-type and n-type OTE materials are required.However, the high performance n-type organic thermoelectric materials are hard to be prepared owing to their sharply performance degradation in air. In 2012, poly(nickel 1,1,2,2-ethenetetrathiolate) (poly[-Na(Ni-ett)]) was reported by Sun

et al

as a new n-type organic thermoelectric material. Poly[Na(Ni-ett)] shows high stability in air, which is more preferable for the real applications such as wearable devices as compared with other n-type OTE materials reported before. The solution method is the conventional method to prepare poly[Na(Ni-ett)], which includes a two-step oxidative polymerization reaction. The whole reaction process of this solution method is presented in the Fig.1.

Fig.1 The reaction mechanism of solution method

In this solution method, the first step is to fabricate the intermediate product bis(1,3-dithiol-2-one-4,5-dithiolate)nickel ([Ni(dmid)]) by the reactions of 1,3,4,6-tetrathiapentalene-2,5-dione (TPD), methoxide anion (OMe) and Niions in methanol under an inert gas. The second step is to transform the [Ni(dmid)]into the final product poly[A(Ni-ett)] by the continuous polymerization. In this step, the purpose of the oxidation process is to weaken the coulomb repulsion between the [Ni(dmid)]andOMe and accelerate the cleave reaction. However, because this solution method proceeds at room temperature and atmospheric pressure, the total reaction rate is very low and the synthesis period needs about one day. In addition, the products usually have poor crystallinity which is not benefit for achieving high electrical conductivity and excellent thermoelectric performance. Thus, a more convenient method should be developed to fabricate the high performance poly[Na(Ni-ett)].Herein, we proposed a more efficient one-step solvothermal method to directly prepare poly[Na(Ni-ett)]. The raw materials, 1,3,4,6-tetrathiapentalene-2,5-dione (TPD), methoxide anion (OMe), and Niions, are located in a closed hydrothermal reactor. Via raising the temperature of the reactor, the condition of high temperature and high pressure can be achieved. In this condition, the reaction among these raw materials might be greatly accelerated because the intermediate product [Ni(dmid)]could be directly oxidized into nearly electric neutrality to ensure the proceeding of the following reaction process. The whole process of this one step method could be illustrated by Fig.2.

Fig.2 The reaction mechanism of solvothermal method

The phase composition, microstructure, and thermoelectric properties of the prepared poly[Na(Ni-ett)] were systematically examined and then compared with those of the sample prepared by the conventional solution method. A thermoelectric figure of merit of 0.04 was achieved at 440 K for the poly[Na(Ni-ett)] prepared by the one-step solvothermal method, which was almost four times of that prepared by the solution method.

2.1 Synthesis of poly[Nax(Ni-ett)]

2.1.1 Materials

1,3,4,6-tetrathiapentalene-2,5-dione (TPD, Alfa, ＞98%), Sodium methoxide solution (Alfa，98%), NiCl·6HO (Alfa, 97%) and Methanol (Sinopharm, AR) were used. All materials were further purified prior to use.

2.1.2 Solution method

Solution method was synthesized by the reported procedure. The raw materials, 1,3,4,6-tetrathiapentalene-2,5-dione (TPD; 1 g), sodium methoxide (1.26 g) were added in to a flask. Evacuate and refill the flask with nitrogen for three times, then 35 mL methanol was added and mixed. After the flask was refluxed at 70 ℃ for 12 h, a solution of NiCl·6HO (1.14 g NiCl·6HO dissolved in 15 mL methanol) was added. The mixtures were reacted for another 12 h, and then was placed in the air for 4 hours to let the polymer to participate. The solution was filtered and washed with water, methanol, and alcohol successively and then dried under vacuum at 40 ℃ for 12 h. Finally, the obtained powder was purified by the Soxhlet extraction system. This sample was named SU-poly[Na(Ni-ett)].

2.1.3 Solvothermal method

1,3,4,6-tetrathiapentalene-2,5-dione (TPD, 1 g), sodium methoxide (1.26 g) and NiCl·6HO (1.14 g) were added into 100 mL polytetrafluoroethylene liner. Then, 50 mL methanol was added. The mixed solution was stirred on magnetic stirring for 2 h. After that, the polytetrafluoroethylene liner was put into a hydrothermal reactor and was heated in the oven at 100 ℃ for 12 h. The product was filtered and washed with water, methanol, and alcohol successively and then dried under vacuum at 40 ℃ for 12 h. Finally, the obtained powder was purified by the Soxhlet extraction system. This sample was named SV-poly[Na(Ni-ett)].

2.2 Characterization

The morphology of poly[Na(Ni-ett)] was characterized by scanning electron microscopy (SEM;FEI Magellan 400, Germany). The functional groups were measured by a Raman spectrometer (XploRA One-532, Horiba, Japan) and infrared spectrometer (Spotlight400, PE, USA). The structure of poly[Na(Ni-ett)] was investigated by X-ray diffraction (XRD; D/max 2550 V, Rigaku, Japan) and X-ray photoelectron spectroscopy spectra (XPS; ESCALAB 250, Thermo Fisher Scientific, Britain).

The polymers were made into compressed cuboid for the thermopower related tests. The electrical conductivity and Seebeck coefficient were measured by ZEM-3 (ULVAC Co. Ltd). The thermal diffusivity was measured in an argon atmosphere using a laser flash method (LFA 457, Netzch Co. Ltd). The heat capacity was measured by the DSC (DSC-8000, PerkinElmer) in nitrogen gas. The density was measured using the Archimedes method. Thermal conductivity was calculated by multiplying the measured values of the thermal diffusivity, the sample density, and the heat capacity.

The molecular structure and component of poly[Na(Ni-ett)] prepared by solvothermal method (SV-poly[Na(Ni-ett)]) were characterized by X-ray photoelectron spectroscopy (XPS) and Raman spectrum, and IR spectrum respectively, as shown in Figs.3-5.

In the range of 850 - 860 eV of the XPS spectra, two bonding energies of Ni(2P) peaks located at 853.3 and 855.6 eV could be discerned (Fig.3), indicating that there are two kinds of Ni atoms with different chemical environments in the polymer. The dominant peak located at 853.3 eV could be assigned to the center metal Ni in the backbone. The very weak shake-up satellite peak (around 860 eV) suggests that the center Ni atom is in a low-spin state, which indicates that the polymer has a square-planar structure. In addition, the peak located at 855.6 eV proves that the presence of the other kind of Ni atom which acts as the counter cations.

Fig.3 X-Ray photoelectron spectroscopy (XPS) fingerprint of SVpoly[Nax(Ni-ett)]

Then the functional groups of the polymer could be affirmed unambiguously according to the Raman and IR spectroscopy. In the Fig.4(a), the bands located at 362.9 and 495 cmare assigned as the typical Ni-S bond and C-S bond. In the Fig.4(b), the frequencies at 488, 963.9, and 1 611.1 cmare attributed to the ν Ni-S, ν C-S, and ν C=O of poly[Na(Ni-ett)]. These results confirmed that the poly[Na(Ni-ett) had been successfully prepared by the solvothermal method.

Fig.4 (a) Raman spectrum and (b) IR spectrum of SVpoly[Nax(Ni-ett)]

Fig.5 shows the scanning electron microscope (SEM) images of the poly[Na(Ni-ett)] polymers prepared by solution and solvothermal methods, respectively. The energy dispersive spectrometer (EDS) analyses indicate that the atomic ratio of Ni and S is almost to 1:4 in both SV-poly[Na(Ni-ett)] and SU-poly[-Na(Ni-ett)] samples, being consistent with the ideal elemental ratio of poly[Na(Ni-ett)]. Comparing with the two-step solution method, the SV-poly[Na(Ni-ett)] sample synthesized by the one-step solvothermal method exhibits a larger-size plate-like structure.

Fig.5 SEM images and EDS analyses: (a)-(c) for SU-poly[Nax(Niett)]; (b)-(d) for SV-poly[Nax(Ni-ett)]

Fig.6 is X-ray diffraction patterns of poly[Na(Niett)] synthesized by the two different methods. Four broad peaks could be observed at 2

θ

=14.7°, 27.2°, 38.9°, and 51°, which agree well with the previous report. The peaks located at 2

θ

=14.7° and 27.2° are assigned to the periodicity parallel and perpendicular to the polymer backbone chain, respectively. It is noted that the SV-poly[Na(Ni-ett)] shows a higher degree of crystallinity than that of SU-poly[Na(Ni-ett)]. The peaks at 2

θ

=14.7° and 27.2° for polymer prepared by solvothermal method are more sharp than the polymer prepared by conventional solution method. The full widths at half-maximum (FWHMs) of the main peaks at 2

θ

= 14.7° and 27.2° decrease from 4.32° to 3.99° and 5.31° to 5.02°, respectively. The peak sharpening usually relates to the monodistribution of the periodicity between the polymer backbone chains, suggesting that the molecular arrangement of SV-poly[Na(Ni-ett)] is more ordered than that of SU-poly[Na(Ni-ett)]. These results further prove that the one-step solvothermal method can effectively improve the crystallinity of the products because the reaction was carried out under the high temperature and high pressure environment.

Fig.6 X-ray diffraction patterns of poly[Nax(Ni-ett)] synthesized by solution and solvothermal method respectively

The electrical conductivity, Seebeck coefficient, and thermal conductivity for the poly[Na(Ni-ett)] samples prepared by different methods were measured and are shown in Fig.7. Both the electrical conductivity and Seebeck coefficient increase with the increase of temperature. The natural logarithm of conductivity of the polymer decreases with the

T

linearly, which indicates that the transport of the carriers in the poly[Na(Ni-ett)] follows the three dimensional variable range hopping (3D-VRH) model. The Seebeck coefficient of the samples prepared by different methods are similar, while the electrical conductivity of SV-poly[Na(Ni-ett)] is remarkably higher than that SU-poly[Na(Ni-ett)]. The higher ordering degree of SV-poly[Na(Ni-ett)] molecules could decrease the inter-chain and intra-chain hopping defects, and then increase the electrical conductivity by increasing the carrier mobility. However, this higher ordering degree has little effect on the Seebeck coefficient, which may be attribute to the similar carrier concentration and band structure. Consequently, the thermoelectric Fig.of merit for SV-poly[Na(Ni-ett)] is remarkably improved. The maximum ZT value is up to 0.04 at 440 K, which is almost four times of SU-poly[Na(Ni-ett)].

Fig.7 Temperature dependence of (a) electrical conductivity, (b) natural logarithm of conductivity,(c) Seebeck coefficient, (d) power factor, (e) thermal conductivity and (f) ZT values for poly[Nax(Ni-ett)] synthesized by solution and solvothermal method, respectively

Overall, a new one-step solvothermal method was designed to synthesize the n-type poly[Na(Ni-ett)]. The X-ray photoelectron spectroscopy, Raman spectrum and IR spectrum analysis confirmed that the obtained poly[Na(Ni-ett)] powder showed a larger plate-like structure and a higher degree of crystallinity, which is attributed to the high reaction temperature and high pressure environment. As a result, the electrical conductivity was greatly enhanced, and then the maximum

ZT

value of the poly[Na(Ni-ett)] prepared by the solvothermal method is up to 0.04 at 440 K, about four times of the polymer prepared by conventional solution method. Moreover, this reaction time of solvothermal method was shortened to 14 h, about half of that required for the solution method. Our research provides a convenient method to prepare n-type organic thermoelectric materials.