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Polypyrrole/Graphene/Polyaniline Ternary Nanocomposite with High Thermoelectric Power Factor Yihan Wang, Jie Yang, Lingyu Wang, Kai Du, Qiang Yin, and QinJian Yin ACS Appl. Mater. Interfaces, Just Accepted Manuscript • Publication Date (Web): 22 May 2017 Downloaded from http://pubs.acs.org on May 23, 2017
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Polypyrrole/Graphene/Polyaniline Ternary Nanocomposite with High Thermoelectric Power Factor †
†
†
‡，*
‡
Yihan Wang , Jie Yang , Lingyu Wang , Kai Du , Qiang Yin
, Qinjian Yin
†，*
† College of Chemistry, Sichuan University, Chengdu 610065, China. ‡ Research Center of Laser Fusion, China Academy of Engineering Physics, P.O. Box 919-987 KEYWORDS: thermoelectric, conducting polymer, polypyrrole, graphene, polyaniline, ternary nanocomposite
ABSTRACT: Polypyrrole/Graphene/Polyaniline (PPy/GNs/PANi) ternary nanocomposite with high thermoelectric power factor has been successfully prepared through the combination of in situ polymerization and solution process. FTIR, Raman spectra, XRD and SEM analyses show the strong π-π interactions existed among PPy, GNs and PANi, leading to the formation of more ordered regions in the composite. Both the in situ polymerization and solution process can enhance the dispersion homogeneity of graphene in the polymer matrix, bringing about increased nano-interfaces in the PPy/GNs/PANi composite. The thermoelectric properties of Polypyrrole/Graphene (PPy/GNs), Polyaniline/Graphene (PANi/GNs) and PPy/GNs/PANi composites are measured at different temperatures after being cold pressed. Consequently, the PPy/GNs/PANi composite with 32 wt% graphene demonstrates optimal electrical conductivity , Seebeck coefficient and extremely high power factor of up to 52.5 μ W m-1 K-2, which is almost 1.6×103 times, 1.4×103 times, 2.7 times and 3.6 times higher than those of the pure PANi, pure PPy, PPy/GNs composite, and PANi/GNs composite, respectively.
Introduction Thermoelectric (TE) materials have drawn much attention due to their good ability to convert heat energy directly into useable electricity.1 They have been widely applied in solid state cooling/heating and power generation, as well as energy harvesting.2 The efficiency of TE materials can be evaluated by the thermoelectric figure of merit ZT = S2σT/κ, where S (μ V K-1), σ (S m-1), T (K)and K (W m1 -1 K ) are the Seebeck coefficient, the electrical conductivity, the absolute temperature, and the thermal conductivity, respectively. Thermoelectric performance also can be weighted with a parameter called power factor (PF) as PF = S2σ. Up to now, TE materials with high performance are mostly based on the inorganic compounds with excellent ZT value or high power factor, such as BiSbTe, Bi2Te3 and SiGe alloys.3 Nevertheless, the high prices and the complicated processing of raw materials cause the inorganic compounds less attraction. To date, abundant researches have demonstrated that filling nanomaterials based on carbon, such as carbon nanotubes (CNTs) and GNs, into polymer substrate provide a promising way to increase the power factor.4 To be specific, graphene with a 2D geometry have been extensively used as a nano-filler to prepare composite materials because of its large aspect ratio, high surface area as well as other superior properties such as mechanical behavior and electron transport ability.5 One of the most common GNs-based nanocomposites is GNs-based polymer nanocomposite, which shows the superior thermal, electrical,
electrochemical and thermoelectric properties compared with the neat polymer or graphene.6-8 For the application as TE materials, conducting polymer-based nanocomposite and their derivatives have exhibited many excellent performances.9-11 Among the conducting polymers, PPy has been considered as a promising candidate for TE materials. However, related studies in the past several years imply the poor thermoelectric properties of TE materials based on PPy.12-18 For instance, PPy/GNs shows the TE power factor of 10.24 μ W m-1 K-2, in which PPy grows along GNs nanosheets via templatedirected in situ polymerization.14 Likewise, Liang et al. prepared PPy/SWCNT composite with 40 wt% SWCNT content, yielding a power factor of 19.7±0.8 μ W m-1 K-2. 15 Besides, Zhang et al. and Liang et al. found that PPy/rGO and PPy/SWCNT composites displayed a maximum power factor of 8.56±0.76 μ W m-1 K-2 and 21.7±0.8 μ W m-1 K-2, respectively, and the maximum value is 3.9 μ W m-1 K-2 of pure PPy. 16,18,20 It has been demonstrated that composite materials may exhibit superior properties by integrating the merits of constituents.21-24 For examples, the combination of PPy with PANi would remarkably reduce the charge transfer resistance of PANi. Similarly, the doping of PANi can enhance the capacity performance of PPy.21-22,25 Such combination can lead to improved performance of the composites due to the synergistic effects among each constituent.26 . So it is possible that the combination of PANi and PPy will lead to a synergy effect and help improve the
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properties of TE materials, PPy/GNs/PANi composite would have excellent thermoelectric performance. Herein, PPy/GNs composite and pure PANi nano-rods were synthesized as the precursors to synthesize PPy/GNs/PANi ternary nanocomposite. First, PPy/GNs composite were fabricated through the in situ polymerization to acquire the even coating of PPy nanoparticles on graphene. Then, the uniform polyaniline nano-rods were obtained by rapidly mixing oxidant with monomer solution.27 Eventually, the PPy/GNs/PANi ternary composite was formed by mixing pure PANi and PPy/GNs composite in ethanol under ultrasonic. The GNs can be dispersed more homogeneously in PPy matrix via the in situ polymerization, resulting in increased nano-interfaces in PPy/GNs accordingly. Also, due to the homogeneous dispersion of GNs in PPy, the molecular conformation of PANi can be expanded through the solution process, leading to increased mobility of carriers. The electrical conductivity and Seebeck coefficient of the PPy/GNs/PANi composite can be greatly improved because of the synergetic effects between the two processes.9 Consequently, the PPy/GNs/PANi composite demonstrates the highest power factor among the reported TE materials based on PPy, which is up to 52.5 μ W m-1 K-2. Results and discussion Structural characterization FTIR spectra, Raman spectra and X-ray diffractometer were conducted to investigate the chemical composition and specific interactions between the polymeric matrix and graphene. Figure 1 shows the typical FTIR spectra of pure GNs, PANi, PPy, PPy/GNs, and PPy/GNs/PANi nanocomposites prepared with the same GNs contents. For the pure PANi, several absorption peaks are observed, including 1570 cm-1 (C=C stretching of the quinoid rings), 1485 cm-1 (C=C stretching of the benzenoid rings), 1300 cm-1 (C-N stretching modes), 1245 cm−1 (C-N+• stretching modes), 1143 cm-1 (in-plane C-H bending of quinoid structure) and 816 cm-1 (C-H out-plane flexural vibration), respectively.28-30 The main absorption bands of PPy appear at 1529 cm-1 and 1445 cm-1, which can be ascribed to the C-C and C-N stretching vibration. The bands located at 1293 cm-1 and 1031 cm-1 are assigned to the C-H in-plane deformation vibration, and the bands at 1151 cm-1 are attributed to the C-N stretching vibration.31 It is worth noting that the characteristic absorption band of PPy at 1529 cm-1 blue shifts to 1540 cm-1 after adding GNs into the PPy matrix. Such shifting may be caused by the interaction between GNs and PPy backbone, such as π-π stacking. Furthermore, the intensity ratio (I1529/I1445) of IR peak at 1529 cm-1 and 1445 cm-1 was also increased from 1.52 in PPy to 1.86 in the PPy/GNs composite. According to previous reports in the literature, the figure of I1529/I1445 is proportional to the delocalization degree.32 In other words, the larger intensity ratio brings about greater degree of conjugation of the polymer chain, so PPy chain in the PPy/GNs complexes has a greater degree of conjugation than pure PPy. From this analysis, it is supposed that the
Figure 1. FTIR spectra of pure GNs, PANi, PPy, PPy/GNs-1 composite, PPy/GNs composite and PPy/GNs/PANi composite
π-bonded surface of PPy/GNs would form stable π-π stacking with PANi in PPy/GNs/PANi composite. This hypothesis is proved by the infrared spectra of PPy/GNs/PANi, where the typical peaks at 1570 cm-1, 1485 cm-1, 1300 cm-1, 1245 cm-1, and 1143 cm-1 in PANi blue shift to 1573 cm-1, 1497 cm-1, 1304 cm-1 , 1248 cm-1, and 1153 cm-1, respectively. Moreover, the peak situated at 1036 cm-1 in PPy/GNs also blue shift to 1044 cm-1 in PPy/GNs/PANi, indicating strong π-π interactions between PPy and PANi as well as the hydrogen bonding between N atoms and H atoms exist in the PPy/GNs/PANi composite, which can also be proved by Raman spectra as shown in Figure 2. In the Raman spectra, for PPy/GNs composite, the characteristic peak at 1351 cm-1 is attributed to the D band (C-N stretching vibration), while the strong peak at 1571 cm-1 is attributed to the G band (C=C stretching vibration).33,34 In addition to the existed D band and G band of GNs, two new peaks at 1045 cm-1 and 968 cm-1 are observed due to the presence of PPy with the polaron (NH+) and bipolaron structures (N+).35 The spectrum of pure PANi shows several strong characteristic peaks, including 1582 cm-1, 1487 cm-1, 1222 cm-1, and 1162 cm-1, which are correspond to the C-C stretching of the benzenoid rings, C=N stretching of the quinoid rings, C-N stretching, and C-H bending vibration of the quinoid/benzenoid rings, respectively.36 The characteristic peaks of PPy and PANi emerge in the Raman spectra of PPy/GNs/PANi composites at the same time. Nevertheless, the peak at 1487 cm-1 of PPy/GNs/PANi composite obviously shifts to the low frequency than the pure PANi, indicating the site selection effect between quinoid rings of PANi and aromatic pyrrole rings of PPy/GNs causes the Raman shift. Previous researches on the Raman characterization of multi-walled carbon nanotubes/polyaniline (MWCNTs/PANi) composite indicate that the site selective interaction (i.e., π-π conjugation) between CNTs and the quinoid rings of PANi is the main cause of the intensity reduction at 1164 cm-1 and 1483 cm-1. Such site selection interaction could promote the chemical transformation of quinoid rings to benzenoid rings and lead to a more ordered molecular chain structure of the PANi in the composite material, 37,38 which is more conducive to the transmission of carriers.
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Figure 2. Raman spectra of pure GNs, PANi, PPy/GNs composite and PPy/GNs/PANi composite
Thus, this charge transfer interaction with site selective effect was believed to exist between PANi quinoid rings and PPy/GNs composite.36 Moreover, we found that in PPy/GNs/PANi composite, each characteristic peak of PANi and PPy appears to lower wavenumber displacement in varying degree, which indicates that strong interactions exist between PANi and PPy/GNs, such as π-π conjugated stacking and hydrogen bonding.39,40 Figure 3 depicted the XRD patterns of pure GNs, PANi, PPy, PPy/GNs composite, PANi/GNs composite and PPy/GNs/PANi composite. Three typical peaks at 26.2 °, 44.1 ° and 54.8 °correspond to the graphite-like structure of pure GNs. Based on the Prague equation 2dsinθ=nλ, where d is the inter-planar spacing, θ is the diffraction half-angle, n is the diffraction series and λ is the wavelength of the X rays, with the increase of the diffraction angle, the inter-planar distance decreases. No obvious characteristic diffraction peaks of the graphene appear in PPy/GNs and PPy/GNs/PANi composites, implying the complete interaction between GNs and PPy.14,31 Compared with the XRD pattern of pure PPy, the main diffraction of PPy/GNs becomes narrower and shifts from 2θ=25.90 ° to 26.47 ° with introducing the GNs content to the composite. The main peak of PANi/GNs also becomes narrower and shifts from 2θ=25.16 ° to 26.22 ° compared with pure PANi. The periodic mono-distribution among the polymer backbone chains usually associates with the peak sharpening, which suggests that the molecules of PPy in the PPy/GNs composite and PANi in the PANi/GNs can arrange more ordered and neat closely than in pure polymers.41 Such ordered molecular arrangement may facilitate carrier transport and increase electrical conductivity. The appearance of characteristic peaks of graphene of PANi/GNs composite in the XRD pattern illustrates that direct mixing process causes the poor dispersion of PANi in graphene and hence results in serious aggregation due to increased grain boundary defects, which can be observed in the following SEM images. Therefore, the X-ray diffractometer, together with the FTIR spectra and Raman spectra analysis, indicate that there are strong interactions between PANi and PPy/GNs
Figure 3. XRD patterns of pure GNs, PANi, PPy, PPy/GNs composite, PANi/GNs composite and PPy/GNs/PANi composite
Scheme 1. The diagram PPy/GNs/PANi composite
of
interactions
in
in PPy/GNs/PANi. As shown in Scheme 1, it is hypothesized that the possible combining mode of PPy/GNs/PANi composite includes π-π stacking and hydrogen bonding. Morphologies of electroactive polymers and their composites Morphologies of electroactive polymers and their composites are presented in Figure 4. The surface of pristine GNs sheet (Figure 4a) looks very smooth, and shows a typical curved sheet-like structure. The pure PPy presents an irregular spherical structure, and the diameter of these spheres is about 300 nm (Figure S1). In the image of PPy/GNs composite (Figure 4b), it can be seen that the thickness of graphene sheets increase distinctly due to the coverage of PPy on the surface of graphene. Such coverage could hinder the agglomeration of GNs because of weakened van der Waals force between GNs sheets, and hence form the PPy/GNs composite with homogeneous structure. In the magnified image of Figure 4b (Figure 4c), PPy particles (marked with pink arrow) and rods (marked with black arrow) can be both found on the surface of GNs. The diameter of PPy rods is about 100-150 nm. In addition, the burr at the edge of the graphene sheet is also formed by the accumulation of the PPy particles. PANi nano-rods with the diameter of 100 nm can be ob-
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Figure 4. SEM images of (a) GNs, (b) PPy/GNs composite, (c) magnified image of PPy/GNs composite, (d) PANi, (e) PANi/GNs composite and (f) PPy/GNs/PANi composite with 32wt% GNs
served distinctly in Figure 4d. It was clearly shown in Figure 4e that the coating of PANi on GNs sheets is nonuniform and the coated particles are found as agglomerations in PANi/GNs composite. But directly mixing PANi with graphene covered by PPy nanoparticles in solution, agglomeration can be significantly prevented, and PANi nano-rods disperse more evenly on the surface of PPy/GNs composite (Figure 4f). The formation mechanism of PPy/GNs/PANi ternary composite with good distribution of PANi is proposed as below. When the PANi was mixed directly with GNs, there is only π-π stacking between PANi and GNs, the force is not strong enough to overcome the interactions between PANi chain, giving rise to serious reunion in PANi/GNs. In contrast, through in situ polymerization process, the pyrrole could arrange along the surface of GNs firstly due to the strong π-π conjugation and then polymerize. Consequently, the PPy chains pack more ordered to form the well-proportioned structure of PPy/GNs composite, which can be confirmed by the XRD pattern. While the charge transfer interaction with site selective effect was believed to exist between PANi quinoid rings and PPy/GNs composite proved by Raman spectra, when PANi was introduced to PPy/GNs, π-π interactions besides hydrogen bonding between the two polymers with similar electrical properties and structures bring about the good compatibility of PANi and PPy in the PPy/GNs/PANi composite,39 which is very beneficial to improve the dispersive performance of the materials. Thermoelectric properties of nanocomposites The electrical conductivity and Seebeck coefficient of pure PPy, PANi, PPy/GNs, PANi/GNs and PPy/GNs/PANi with 32wt% graphene content at different temperatures were measured. As described in Figure 5, the increase of conductivities of pure PPy and PANi/GNs composites is
accompanied by the temperature rising in the measured range. This is because elevated temperature aggravates thermal motion of molecule and increases the carrier concentration, so the conductivity of the sample is enhanced, showing the nature of the semiconductor. However, the carrier concentration is mainly determined by the doping concentration, which is less affected by the temperature, so the tendency of the conductivity with temperature is not very remarkable. The conductivity of pure PANi reduces with the increase of temperature from432 S m-1 at 20 ℃ to 298 S m-1 at 90℃, showing the conductive characteristics of metal, namely dσ/dT
