Preparation and Thermoelectric Properties of Graphite / poly ( 3 , 4-ethyenedioxythiophene ) Nanocomposites

Graphite/poly(3,4-ethyenedioxythiophene) (PEDOT) nanocomposites were prepared by an in-situ oxidative polymerization process. The electrical conductivity and Seebeck coefficient of the graphite/PEDOT nanocomposites with different content of graphite were measured in the temperature range from 300 K to 380 K. The results show that as the content of graphite increased from 0 to 37.2 wt %, the electrical conductivity of the nanocomposites increased sharply from 3.6 S/cm to 80.1 S/cm, while the Seebeck coefficient kept almost the same value (in the range between 12.0 μV/K to 15.1 μV/K) at 300 K, which lead to an increased power factor. The Seebeck coefficient of the nanocomposites increased from 300 K to 380 K, while the electrical conductivity did not substantially depend on the measurement temperature. As a result, a power factor of 3.2 μWm−1 K−2 at 380 K was obtained for the nanocomposites with 37.2 wt % graphite.

These examples from the background research show that the TE properties of PEDOT and its derivatives can be greatly improved when using carbon materials, such as carbon black, carbon nanotubes, or graphene, as fillers in composites with a PEDOT or PEDOT-derivative matrix.Compared to carbon nanotubes and graphene, graphite is much cheaper, motivating the need for studies of thermoelectric properties of graphite/PEDOT nanocomposites.In this work, graphite/PEDOT nanocomposites were prepared by an in-situ oxidative polymerization method and their composition, microstructure, and TE properties were investigated.

Preparation of Graphite/PEDOT Nanocomposites
A designed amount of graphite (to obtain 8.9, 18.1, 29.8 and 37.2 wt % with respect to the sum of the weight of graphite and monomer) was added into 100 mL CHCl 3 , ultrasonicated for 1 h and then stirred for 30 min (solution A).Next, 0.85 mL EDOT monomer was dissolved in 50 mL CHCl 3 and then stirred for 30 min (solution B).Solution B was mixed with solution A, ultrasonicated for 10 min and then stirred for 30 min (solution C). 8.65 g FeCl 3 •6H 2 O as oxidant was dissolved in 50 mL CHCl 3 to form solution D. Solution D was dropped into solution C, stirred for 24 h, and then washed with deionized water and anhydrous ethanol for several times until the filtrate became colorless.Finally, the products were dried at 60 • C under vacuum for 8 h.The prepared graphite/PEDOT nanocomposite powders were pressed into thin pellets with a diameter of 15 mm and thickness of about 1.5 mm by compacting at a room temperature under the pressure of 30 MPa for 30 min.The pure PEDOT particles and pellets were prepared using the same procedure without addition of graphite.

Sample Characterization
The graphite/PEDOT nanocomposite powders were characterized by X-ray power diffraction (XRD; TD3500, Dandong, China), and scanning electron microscopy (SEM; FEI Quanta200 FEG) and X-ray photoelectron spectroscopy (XPS; PHI 5000 VersaProbe (ULVAC-PHI)), using Al Kα radiation from an X-ray tube operated at 15 kV and 40 W. The spot size was 200 µm.The base pressure for the measurement was about 6 × 10 −7 Pa.The samples were not plasma-etched before measurement.The electrical conductivity and Seebeck coefficient of the graphite/PEDOT nanocomposite bulk materials were measured simultaneously from 300 K to 380 K on an MRS-3L thin-film TE test instrument system in a vacuum atmosphere (Wuhan Giant Instrument Technology Co., Ltd., Wuhan, China).

Results and Discussion
Figure 1 is a schematic illustration of the fabrication process of the graphite/PEDOT nanocomposites and bulk materials.When the EDOT monomer was added into the graphite dispersion, the EDOT monomer was adsorbed on the surface of graphite due to electrostatic attraction [29].After the oxidant of FeCl 3 •6H 2 O was dropped into the solution, the EDOT monomer was in-situ polymerized on the surface of graphite, and then formed graphite/PEDOT nanocomposites.Figure 2a shows XRD results of PEDOT, graphite and graphite/PEDOT nanocomposites with 37.2 wt % graphite.It can be seen that the as-prepared PEDOT shows an amorphous XRD feature as expected [30], while the graphite shows a strong peak at about 26.48 • , which is attributed to diffraction from the (002) plane of graphite [31].The graphite/PEDOT nanocomposite also shows this characteristic peak of graphite combined with the amorphous feature from PEDOT. Figure 2b shows XPS survey spectra of graphite and graphite/PEDOT nanocomposites with 37.2 wt % graphite.It can be seen from Figure 2b that the pure graphite mainly contains carbon, with a small amount of oxygen from surface contaminations detected.In contrast, for the graphite/PEDOT nanocomposites, S and Cl were detected.The Cl originates from the FeCl 3 •6H 2 O used in the synthesis process [32,33].Sulphur is a characteristic element of PEDOT.The binding energy at around 164.2 eV is attributed to the S 2p band in PEDOT [34].Since XPS is a highly surface-sensitive technique, this indicates that that PEDOT was coated on the surfaces of graphite.
an x-ray tube operated at 15 kV and 40 W. The spot size was 200 µm.The base pressure fo rement was about 6 × 10 −7 Pa.The samples were not plasma-etched before measurement ical conductivity and Seebeck coefficient of the graphite/PEDOT nanocomposite bulk mate measured simultaneously from 300 K to 380 K on an MRS-3L thin-film TE test instrum in a vacuum atmosphere (Wuhan Giant Instrument Technology Co., Ltd., Wuhan, China ults and Discussion igure 1 is a schematic illustration of the fabrication process of the graphite/PE omposites and bulk materials.When the EDOT monomer was added into the grap rsion, the EDOT monomer was adsorbed on the surface of graphite due to electros tion [29].After the oxidant of FeCl3•6H2O was dropped into the solution, the EDOT mono in-situ polymerized on the surface of graphite, and then formed graphite/PE omposites.Figure 2a shows XRD results of PEDOT, graphite and graphite/PE omposites with 37.2 wt % graphite.It can be seen that the as-prepared PEDOT show hous XRD feature as expected [30], while the graphite shows a strong peak at about 26 is attributed to diffraction from the (002) plane of graphite [31].The graphite/PE omposite also shows this characteristic peak of graphite combined with the amorphous fea PEDOT.Figure 2b shows XPS survey spectra of graphite and graphite/PEDOT nanocompo 7.2 wt % graphite.It can be seen from Figure 2b that the pure graphite mainly contains car a small amount of oxygen from surface contaminations detected.In contrast, for ite/PEDOT nanocomposites, S and Cl were detected.The Cl originates from the FeCl3•6 in the synthesis process [32,33].Sulphur is a characteristic element of PEDOT.The bin y at around 164.2 eV is attributed to the S2p band in PEDOT [34].Since XPS is a highly sur ive technique, this indicates that that PEDOT was coated on the surfaces of graphite.Figure 3a-f show SEM images of graphite (a & b) and graphite/PEDOT nanocomposites with 8.9 wt % (c), 18.1 wt % (d), 29.8 wt % (e), and 37.2 wt % (f) graphite.The surfaces of pure graphite are much smoother than those of graphite/PEDOT nanocomposites, indicating that PEDOT is homogeneously coated on the graphite surfaces.Figure 3g,h show an SEM image and corresponding energy dispersive spectrometer (EDS) map of sulphur for the graphite/PEDOT nanocomposites with 37.2 wt % graphite.As indicated by the EDS map, the sulphur is homogeneously dispersed in the graphite/PEDOT nanocomposites (Figure 3h).
The electrical conductivity, Seebeck coefficient, and power factor at 300 K for the graphite/PEDOT nanocomposites with graphite content from 0 to 37.2 wt % are shown in Figure 4a.The electrical conductivity and Seebeck coefficient of pure PEDOT are in the range of 3.5 S/cm-6.0S/cm and 12.0 µV/K-16.0µV/K, respectively.With increasing graphite content from 8.9 wt % to 37.2 wt %, the electrical conductivity of the composites significantly increases from 30.5 S/cm to 80.1 S/cm, in all cases an order of magnitude higher than that of pure PEDOT.The reason for this increase is in part because the electrical conductivity of graphite is much higher than that of PEDOT [35].It may also be affected by the π-π interactions between PEDOT and graphite, which causes PEDOT to grow along the graphite surface.As a result, a more ordered PEDOT molecular chain may be formed, which is beneficial to decrease the carrier hopping barrier and enhance the carrier mobility [29,36].
The Seebeck coefficient of all the composites retains almost the same value (14.1 µV/K-15.1 µV/K), which is close to that of pure PEDOT.As the graphite content is increased from 8.9 wt % to 37.2 wt %, the power factor of the graphite/PEDOT composites increases from ~0.7 µW/mK 2 to 1.6 µW/mK 2 at 300 K.This was mainly due to the increased electrical conductivity and almost unchanged Seebeck coefficient of all the nanocomposites.wt % (c), 18.1 wt % (d), 29.8 wt % (e), and 37.2 wt % (f) graphite.The surfaces of pure graphite are much smoother than those of graphite/PEDOT nanocomposites, indicating that PEDOT is homogeneously coated on the graphite surfaces.Figure 3g,h show an SEM image and corresponding energy dispersive spectrometer (EDS) map of sulphur for the graphite/PEDOT nanocomposites with 37.2 wt % graphite.As indicated by the EDS map, the sulphur is homogeneously dispersed in the graphite/PEDOT nanocomposites (Figure 3h).In order to investigate the influence of temperature on the TE properties of graphite/PEDOT composites, the electrical conductivity, Seebeck coefficient, and power factor of graphite/PEDOT, composites with different graphite content were measured in the temperature range from 300 K to 380 K (Figure 4b-d).As the measurement temperature was increased from 300 K to 380 K, the electrical conductivity of the composites was nearly constant.In contrast, the Seebeck coefficient increased, e.g., from 15.1 µV/K to 24.7 µV/K, and from 14.1 µV/K to 21.7 µV/K, for the nanocomposites with Energies 2018, 11, 2849 6 of 9 8.9 wt % and 37.2 wt % graphite, respectively.As a result, the power factor also increased as the temperature increased, and a maximum power factor of 3.2 µWm −1 K −2 at 380 K was obtained for the nanocomposites with 37.2 wt % graphite.This is about 30 times higher than that of pure PEDOT (0.1 µW/mK 2 ) prepared by the same procedure, and also much higher than those of a CB/PEDOT nanocomposite (0.96 µWm −1 K −2 with 2.52 wt % CB at 300 K) [16] and a graphite-PEDOT: PSS coated polyester fabric (0.025 µWm −1 K −2 with 15 wt % graphite content at 398 K) [20].However, this value is lower when compared to the nanocomposites using graphene or carbon nanotube as fillers, for example, a graphene/PEDOT:PSS nanocomposite (11.09 µWm −1 K −2 with 2 wt % graphene at 300 K) [17], a rGO/PEDOT composite (5.2 µWm −1 K −2 with 16 wt% rGO at 300 K) [18], a MWCNT/PEDOT composite (25.9 µWm −1 K −2 with 26.5 wt % MWCNT at 300 K) [19], a PEDOT:PSS/graphene-iron oxide nanocomposite (51.93 µWm −1 K −2 with 95 wt % GINC at 300 K) [28], a graphene/PEDOT:PSS composite (53.3 µWm −1 K −2 with 3 wt % graphene at room temperature) [23], a SWCNT/PEDOT:PSS composite (300 µWm −1 K −2 with 74 wt % SWCNTs at room temperature) [24], and a CNT/PEDOT composite (1050 µWm −1 K −2 with 10.7 wt % CNTs at room temperature) [25].The possible potential applications of our graphite/PEDOT nanocomposites can be used in the following aspects: wrist watches, remote wireless sensors, biomedical devices, etc.

Conclusions
Graphite/PEDOT nanocomposites were prepared by an in-situ polymerization method with different content of graphite.The electrical conductivity of these nanocomposites was greatly enhanced with the graphite content increased from 0 to 37.2 wt %, while the Seebeck coefficient stayed nearly constant at 300 K.As the measured temperature increased from 300 K to 380 K, the power factor of the nanocomposites increased, mainly because of the similar trend in the Seebeck coefficient.A power factor of 3.2 µW/mK 2 at 380 K was obtained for the nanocomposites with 37.2 wt % graphite, which is about 30 times higher than that of the pure PEDOT, though lower than nanocomposites using graphene or carbon nanotubes as filler.Nonetheless, graphite is economically advantageous compared to carbon nanotubes and graphene.Furthermore, the present study shows that using graphite as filler is an effective method to enhance the TE properties of PEDOT, and also

Conclusions
Graphite/PEDOT nanocomposites were prepared by an in-situ polymerization method with different content of graphite.The electrical conductivity of these nanocomposites was greatly enhanced with the graphite content increased from 0 to 37.2 wt %, while the Seebeck coefficient stayed nearly constant at 300 K.As the measured temperature increased from 300 K to 380 K, the power factor of the nanocomposites increased, mainly because of the similar trend in the Seebeck coefficient.A power factor of 3.2 µW/mK 2 at 380 K was obtained for the nanocomposites with 37.2 wt % graphite, which is about 30 times higher than that of the pure PEDOT, though lower than nanocomposites using graphene or carbon nanotubes as filler.Nonetheless, graphite is economically advantageous compared to carbon nanotubes and graphene.Furthermore, the present study shows that using graphite as filler is an effective method to enhance the TE properties of PEDOT, and also indicates that graphite could be used in other conducting polymer systems.

8 Figure 4 .
Figure 4. (a) electrical conductivity, Seebeck coefficient, and power factor of graphite/PEDOT nanocomposites with different content of graphite from 0 to 37.2 wt % at 300 K. Temperature dependency of (b) electrical conductivity, (c) Seebeck coefficient, and (d) power factor of graphite/PEDOT nanocomposites with different content of graphite from 8.9 to 37.2 wt %.

Figure 4 .
Figure 4. (a) electrical conductivity, Seebeck coefficient, and power factor of graphite/PEDOT nanocomposites with different content of graphite from 0 to 37.2 wt % at 300 K. Temperature dependency of (b) electrical conductivity, (c) Seebeck coefficient, and (d) power factor of graphite/PEDOT nanocomposites with different content of graphite from 8.9 to 37.2 wt %.