PVAc/PEDOT:PSS/graphene–iron oxide nanocomposite (GINC): an efficient thermoelectric material

Abstract

A green method for the synthesis of a graphene–iron oxide nanocomposite (GINC) and its PVAc based polymer nanocomposites was reported in an earlier communication. The fabricated PVAc–GINC film exhibited a conductivity of 2.18 × 10⁴ S m⁻¹ with a Seebeck coefficient of 38.8 μV K⁻¹. Hence, the power factor (PF) reached a value of 32.90 μW m⁻¹ K⁻² which is 27 fold higher than a thermoelectric material based on a PVAc–graphene composite as reported in the contemporary literature. In continuation of the above mentioned study, PEDOT:PSS was used to further enhance the power factor (PT) and figure of merit (ZT) of the system. During evaluation, a PEDOT:PSS/GINC composite (5 : 95) showed a remarkable increase in various thermoelectric properties like electrical conductivity (8.0 × 10⁴ S m⁻¹) with a Seebeck coefficient of 25.42 μV K⁻¹ and thermal conductivity 0.90 W m⁻¹ K⁻¹. Hence PF and ZT reach up to 51.93 μW m⁻¹ K⁻² and 0.017, respectively. To improve the mechanical strength of the polymer composite, cellulose fibre was also employed. By the addition of cellulose fibre, though the mechanical strength of the composite increases the PF reaches 5.6, which is 10 times lower than the PEDOT:PSS/GINC composite.

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Utilizing a thermoelectric (TE) material, a potential difference can be developed by exploiting a temperature difference, or a temperature difference can be created by the application of voltage.¹ The TE performance of a material is expressed by a dimensionless figure of merit, i.e. ZT, which is defined as S²σT/κ where S denotes thermopower, i.e. the Seebeck coefficient, σ denotes electrical conductivity, κ is thermal conductivity and T is absolute temperature.⁸ Because of the low κ value of conductive polymers compared to commonly utilized TE inorganic materials, such as Bi₂Te₃-based materials, conductive polymers have become prospective candidates for tailoring the properties of TE materials. Additionally, conductive polymers possess some other beneficial features like low density, low cost and less toxicity, and are relatively straightforward to synthesize and easily processed into versatile forms.²

Several researchers across the globe have been trying to dig out effective properties from graphene since its discovery in 2004.¹ The unique properties of graphene have attracted it widespread attention for its high carrier mobility,² room temperature quantum effect and ambipolar electric field effect. If the number of layers present in graphite is restricted to a few, i.e. 10 or less, the resultant entity is known as graphene. Graphene shows exceptional properties which are reduced drastically by increasing the number of graphene layers until it reaches it’s 3D form,^3–5 i.e. graphite. Due to such distinct properties, graphene has become an interesting material for electronic composites and advance mechanical resonators.^6,7 Additionally, graphene shows excellent electrical, optical and thermal properties.¹ A high Seebeck coefficient has been predicted in graphene-based nanostructures⁹ and the electrical conductance of graphene is comparable to that of copper.¹⁰ The large scale production of graphene sheets has been reported in the literature.¹¹ These factors makes graphene a front runner for futuristic thermoelectric applications. However, the ability of graphene to conduct heat is an order of magnitude larger than that of copper.¹² Therefore, it is mandatory to restrain its thermal conductance for its TE application. The high thermal conductance of graphene is mainly due to the contribution of the lattice, whereas the electronic contribution to thermal conductance can be ignored.^12,13 Therefore, suitable engineering of phonon transport properties makes it possible to diminish the total thermal conductance without considerable reduction of the electrical conductance and the power factor. Computational studies performed on the thermal conductivity of graphene-based structures has revealed that boundaries and edge irregularity can strongly affect thermal conductance.^14,15 Further, considerable effects on thermal conductance have been observed due to defects, vacancies, isotope doping, and hydrogen passivation.^15–17

Higher power factors can be assimilated by two mechanisms in polymer based composites; polymer doping,^18–20 and blending with different conducting nanofillers^21–23 like CNTs^24,25 and graphene.^26–29 The TE properties of these polymer composites can be upgraded to be comparable to those of chalcogenides. However, their performance is still inferior for many reasons.^18,21,22 High intrinsic electrical conductivity makes poly(3-hexylthiophene) (P3HT),³¹ polyaniline (PANI),^18,32–34 and (3,4-ethylenedioxythiophene):poly (styrenesulfonate) (PEDOT:PSS),³⁰ useful. The electrical properties of polymers can simply be enhanced without disturbing the thermal conductivity and mechanical flexibility.³⁵ Percolation theory predicts a drastic increase in electrical conductivity after reaching a percolation threshold.^36,37

A simple preparation method of GINC involves a solvothermal reaction.³⁸ We have recently prepared GINC using a novel method. The novelty is due to its simplicity, economical nature and eco-friendliness. A comprehensive study of thermoelectric properties (electrical conductivity, Seebeck coefficient, thermal conductivity, power factor and figure of merit) of PVAc, PEDOT:PSS and PVAc/PEDOT:PSS polymer with different fillers like GINC and graphene has been evaluated and presented. Additionally, a cellulose based polymer composite have been formulated and evaluated for thermoelectric properties.

In spite of high thermal conductivity, such materials could be engineered in a simple way to enhance thermoelectric properties of the synthesized polymer nanocomposites. PVAc was chosen because of its good adhesive nature and binding capability with a lower thermal conductivity. Such properties help to increase the filler loading and efficiency of the thermoelectric material. To increase electrical conductivity efficiently, graphene has been used as a substrate. It is known that metal oxides such as iron oxide, nickel oxide, cadmium oxide and doped zirconium oxide have shown impressive TE properties for various applications.⁴⁰ Amongst these oxides, the thermoelectric properties of Fe₂O₃ have been studied well in the literature. It has been observed that Fe₂O₃ can be a promising transition metal oxide for TE applications as it exhibits a high thermoelectric power factor at room temperature as well as at elevated temperatures. Fe₂O₃ thin films have shown a peak Seebeck coefficient of 1650 μV K⁻¹ in a temperature range of 270–290 K. A peak electrical conductivity of 5.5 × 10³ S m⁻¹ has been reported in the same temperature range, resulting in a large PF of 1.5 × 10⁴ μW m⁻¹ K². Nano-Fe₂O₃ was decorated over graphene sheets during the exfoliation of GINC. After decoration, the staking nature of graphene sheet was supposed to reduce drastically which ultimately reduces the thermal conductivity as well as tendency of graphene to transform to graphite. PEDOT:PSS has been incorporated as a conducting polymer at different concentrations and optimization has also been carried out to enhance the TE efficiency.

Conducting polymers help to modulate the junction which makes the electrical network intact but obstruct the thermal network. Further, cellulose fibres were used to try to enhance the mechanical properties of the nanocomposite.

Experimental

Synthesis of PVAc/PEDOT:PSS–GINC composites

Nano-iron oxide and nanographene were prepared separately during the preparation of the GINC nanocomposite (graphene : iron oxide: 1 : 1). The detailed synthesis procedure can be found in our earlier article.^39,48 In the present study, we have made polymer composites based on PEDOT:PSS (conductive grade, 1.3 wt% in H₂O, σ: 1 S cm⁻¹ make: Sigma Aldrich), PVAc/PEDOT:PSS and GINC. The detailed synthetic methodology, characterization and applications are emphasized in Scheme 1.

Scheme 1 Synthesis procedure of PVAc/PEDOT:PSS–GINC composites, their evaluation and possible applications.

To improve mechanical strength, cellulose fibres were employed. The detailed synthetic procedure and characterization are highlighted in Scheme 2. The optimised composition, based on 5% PEDOT:PSS and 95% GINC, was used in ethanol solution. To achieve better dispersion, mechanical stirring and ultrasonication were carried out. Then, the dispersed solution was passed through a Whatman filter paper with the help of Buchner funnel. During suction, filler particles were adsorbed on the filter paper, making a conducting network for electrical conduction. After drying, the formed composite samples had been prepared with the required dimensions and their thermoelectric properties were evaluated.

Scheme 2 Synthesis procedure of the cellulose based PVAc/PEDOT:PSS–GINC composites.

Characterization of polymer GINC composites

Environmental scanning electron microscopy (ESEM) images with different magnification (800× and inset, 3000×) are presented in Fig. 1. Fig. 1 consists of different micrographs of various cellulose polymer GINC nanocomposites with varying concentrations of polymer GINC nanocomposites. The micrographs show that cellulose fibres and pores were filled with the polymer–GINC composites. It shows continuous dispersion of PEDOT:PSS/GINC over the cellulosic film. The insets of the micrographs highlight continuous network formation between cellulosic fibres and polymer GINC composites. As the concentration increases, the coating ability of polymer GINC over cellulose fibres is enhanced drastically. Hence, an effective conductive network has been formed, and efficiency is enhanced. Fig. 1j represents a simple cellulosic film. Small pores are visible in this micrograph.

Fig. 1 Environmental scanning electron micrographs at 800×, and 3000× magnification (insets) of cellulose polymer GINC based composites with (a) 10% PEDOT:PSS solution, (b) 20% PEDOT:PSS solution, (c) 30% PEDOT:PSS solution, (d) 40% PEDOT:PSS solution, (e) 50% PEDOT:PSS solution, (f) 60% PEDOT:PSS solution, (g) 70% PEDOT:PSS solution, (h) 80% PEDOT:PSS solution, (i) 90% PEDOT:PSS solution and (j) cellulosic film.

Band gap measurement

To examine the optical energy gaps of the synthesized compounds, optical diffuse reflectance measurements were performed on finely grounded powders at room temperature. Spectra were recorded in the range of 200 nm to 800 nm using a Cary 5000 UV-vis spectrometer. Absorption (α/Λ) data were calculated from reflectance data using the Kubelka–Munk equation α/Λ = (1 − R)²/(2R), where R is the reflectance and α and Λ are the absorption and scattering coefficients, respectively. Finally, the energy band gaps were derived from α/Λ vs. E (eV) plots. A detailed graphical representation is depicted in Fig. 2. The detailed assignments of these are highlighted in Table 1.

Fig. 2 (i) Graphical representation of energy band gaps derived from α/Λ vs. E (eV) plots. Table 1: tabulated representation of band gaps for various compositions.

Table 1 Optical band gaps for various compositions^a

Sr. No.	Sample name	Band gap (eV)
a P:P= PEDOT:PSS, Gr = graphene, GINC = graphene iron oxide nanocomposite.
1	10P:P + 90Gr	3.04
2	30P:P + 70Gr	3.06
3	5P:P + 95Gr	3.12
4	10PVAc + 10P:P + 80GINC	3.13
5	20P:P + 80Gr	3.20
6	15PVAc + 5P:P + 80GINC	3.11
7	30P:P + 70GINC	3.07
8	20P:P + 80GINC	3.25
9	40P:P + 60GINC	3.31
10	10P:P + 90GINC	3.27
11	5P:P + 95GINC	3.26
12	5PVAc + 15P:P + 80GINC	3.32
13	40P:P + 60Gr	3.02

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In general, electrical conductivity and Seebeck coefficient increase with a decrease in band gap. In the above table, mild variations are observed in band gap calculation. Hence, it is very difficult to correlate any relationship between band gap and electrical conductivity.

Thermoelectric application of polymer based GINC and cellulosic polymer based GINC

Thermoelectric properties are parameters, viz. Seebeck coefficient or thermopower, electrical conductivity and thermal conductivity, which help to measure PF and ZT.

Thermoelectric power/Seebeck coefficient (S) measurements

To calculate the thermopower as a function of temperature, samples of the polymer nanocomposite film as well as cellulosic polymer nanocomposite with dimensions of 30 mm × 6 mm × 1 mm were cut and placed on thermally insulating fibre glass. A Peltier heater was placed at one end of the sample with a thermally conductive epoxy (electrically insulating 2763 Stycast), while at the other end a piece of copper (for drainage of heat) was placed to make a contact with the Peltier cooling module. The temperature gradient and voltage drop along the film was measured with thermocouples arranged in series (electrically insulated from the sample with 2763 Stycast) with two copper wires. To make sure that the thermal gradient and the voltage drop were being measured at the same place, two small Cu films were attached to the polymer–GINC film with thermally/electrically conducting silver epoxy (Dupont 4929N). The thermocouple and the voltage wires were attached to these Cu films. The thermoelectric voltages were scrutinized with respect to temperature difference using a Keithley 2182A nanovoltmeter. The base temperature was altered with a Peltier cooling module. Two independent methods were adopted to determine the thermoelectric power: (1) after reaching a stable state through an applied current to the heater and (2) by fitting the linear V vs. ΔT response to a heating pulse. The unconventionality between both methods and different experimentation was always lower than 5%. A highly sensitive IR camera was used to measure the temperature gradient along the sample.

Electrical resistivity measurements

Due to the highly electrically conductive nature of the composite, delta mode four probe methods were used to measure the electrical resistivity. The smallest possible current (100 mA) was obtained by a Keithley 6220 and voltage was monitored using a Keithley 2182A nanovoltmeter. To avoid heating of the sample at low temperature, the smallest possible current was used. A polymer nanocomposite sample with dimensions of 8 mm × 3 mm × 1 mm was prepared and subjected to testing to measure electrical conductivity. The sample was tested several times within a one month interval. The properties were found to be consistent. This reflects the sample stability under room temperature and atmosphere.

A remarkable increase in electrical conductivity (6.7 × 10⁴ S m⁻¹) of a PEDOT:PSS/GINC composite has been achieved (Fig. 3d). Thermal conductivity and Seebeck coefficient have been affected by a small degree. Raw PVAc has an electrical conductivity of 10⁻¹³ S m⁻¹ and the electrical conductivity was calculated under ambient conditions. Results were reproduced even after two months, indicating a good stability of the nanocomposite over a period of time. The Seebeck coefficient (see Fig. 3b) also exhibits an interesting trend with an initial decrease and then a final increase to reach a maximum with 20 wt% PEDOT:PSS concentration. Fig. 3c exhibits the variation of power factor (PF) as a function of filler concentration. According to Fig. 3c, PF increases and reaches a very high value, 34.17 μW m⁻¹ K⁻² at 20 wt% filler concentration. In the same way, the thermoelectric figure of merit, i.e. ZT, was found to be maximum, i.e. 0.003.

Fig. 3 (a) Thermal conductivity, (b) Seebeck coefficient, (c) power factor, (d) electrical conductivity and (e) ZT as a function of PVAc:PEDOT:PSS concentration at room temperature (300 K). CP1: 15% PVAc + 5% PEDOT:PSS solution + 80% GINC, CP2: 10% PVAc + 10% PEDOT:PSS solution + 80% GINC, CP3: 5% PVAc + 15% PEDOT:PSS solution + 80% GINC, and CP4: 20% PEDOT:PSS solution + 80% GINC.

In Fig. 4, five different compositions (CP1–CP5) containing different concentration of PEDOT:PSS and filler, i.e. GINC/graphene (detailed compositions and a data table are given in the ESI, Tables S2 and S3†) are evaluated in terms of their thermal conductivity, electrical conductivity, Seebeck coefficient, power factor and ZT. In Fig. 4a, the thermal conductivities of PEDOT:PSS–graphene composites are much higher compared to those of the PEDOT:PSS–GINC composites, which is the main drawback of the graphene based composites. On the other hand, the Seebeck coefficients are higher for the PEDOT:PSS–GINC composites compared to the PEDOT:PSS graphene composites. In the same way, the electrical conductivities of the PEDOT:PSS–GINC composites were found to be much higher than the PEDOT:PSS graphene composites. Hence, higher values of PF and ZT were achieved in the case of the PEDOT:PSS–GINC composites. Basically, in the presence of iron oxide nanoparticles, the thermal conducting network was destructed but electrical conductivity remained unchanged. The reason is that during composite formation electron transport remains intact but phonon transport is disturbed. During the optimization of PEDOT:PSS and GINC concentrations in the composite, the composition with 5 wt% PEDOT:PSS solution and 95 wt% graphene/GINC shows very high PF and ZT values, i.e. 51.93 μW m⁻¹ K⁻² and 0.017, respectively. This value is found to be the highest ever reported in the literature for a PEDOT:PSS based system. The improvement of electrical conductivity follows the percolation law for composites which predicts an enhancement of electrical conductivity up to a critical concentration level of filler. These phenomena come into play, when two dissimilar materials with a large difference in electrical conductivity are mixed.

Fig. 4 (a) Thermal conductivity, (b) Seebeck coefficient, (c) power factor, (d) electrical conductivity and (e) ZT as a function of PEDOT:PSS concentration at room temperature (300 K). CP1: 5% PEDOT:PSS solution + 95% graphene/GINC, CP2: 10% PEDOT:PSS solution + 90% graphene/GINC, CP3: 20% PEDOT:PSS solution + 80% graphene/GINC, CP4: 30% PEDOT:PSS solution + 70% graphene/GINC, and CP5: 40% PEDOT:PSS solution + 60% graphene/GINC.

Fig. 5 shows the graphical representation of electrical conductivity, Seebeck coefficient and power factor with various concentrations of PEDOT:PSS/GINC composites in a cellulose matrix. Though mechanical strength increases, power factor values are found to be much less compared to the bare composite. During the study of thermoelectric properties, PEDOT:PSS/GINC composites showed at least 50 fold increases in ZT, and a fourfold increase in power factor were observed compared to the PEDOT:PSS–graphene composite with equal filler loading (95 wt%). The detailed data are given in Table S4 in the ESI.† This is one of the novel findings from this study. In GINC, nano-iron oxides were decorated over 2D graphene sheets. The presence of nano-iron oxide particles helps to destroy the phonon transport network but the electron transport network remain intact. When GINC is employed as conducting filler, it not only decouples σ and S, but also enhances both parameters simultaneously. Further, PEDOT:PSS helps to modulate the junction by forming thin layer of coatings. These junction helps to transport electrons only, however, the enhancement of Seebeck coefficient is marginal with respect to electrical conductivity in the case of PEDOT:PSS/GINC composites. The electrically conductive nature of PEDOT:PSS assists in enhancing electrical conductivity and reduces the thermal conductivity of the matrix. During the study, we have understood an unusual mechanism of PEDOT:PSS in the presence of GINC. PEDOT:PSS is a polar conducting polymer, and is highly compatible with GINC. During the preparation of polymer nanocomposites, PEDOT:PSS is easily coated over GINC. Hence, the interlayer junctions were modulated in such a way that reduces thermal conductivity but increases electrical conductivity and Seebeck coefficient, and hence increases the power factor. In addition, phonons are responsible for thermal conductivity. Phonons get scattered during conduction, which hence reduces the thermal conductivity. In PVAc, though its thermal conductivity is comparatively low, the efficiency of decreasing the thermal conductivity is relatively poor. The reason is, PVAc is a non conducting polymer and it is not sufficiently compatible with GINC. Hence the modulation of interlayer junctions becomes difficult. Besides this, phonon scattering is also not effective for PVAc compared to PEDOT:PSS.

Fig. 5 (a) Electrical conductivities, (b) Seebeck coefficients and (c) power factors of PEDOT:PSS (5 wt%)/GINC (95 wt%) composites. Concentration at room temperature (300 K). CP1: cellulose paper + PEDOT:PSS/GINC composite (10 wt%), CP2: cellulose paper + PEDOT:PSS/GINC composite (15 wt%), CP3: cellulose paper + PEDOT:PSS/GINC composite (20 wt%), CP4: cellulose paper + PEDOT:PSS/GINC composite (30 wt%), and CP5: cellulose paper + PEDOT:PSS/GINC composite (40 wt%).

A comparative summary of the latest results based on polymer matrix (see Table 2) and other composites of inorganic and organic materials are highlighted in Table S5 in the ESI.† The corresponding references are given in S6 in the ESI.†

Table 2 Summary of the thermoelectric properties of various PVAc based carbon material composites

Sample^49	σ, S m^−1	S, μV K^−1	κ, W m^−1 K^−1	Calculated PF (S^2σ) μW m^−1 K^−2
PVAc + CNT (20%) [ref. 42]	4800 (300 K)	40–50 (300 K)	0.18–0.34 at 300 K	PF = 7.8–12
PVAc + SWCNT (40%) [ref. 43]	900	40	0.25	PF = 1.44
PVAc + SWCNT (3 wt%) + GA [ref. 44]	22–49	39–42	0.22–0.25	PF = 0.033
PVAc + Au + CNT [ref. 45]	10^5	—	Unaffected	Unaffected
PVAc + DOC + MWCNT (7–12%)	32–63	5–10	0.13–0.17	PF = 0.34–0.50
PVAc + TCPP + MWCNT (7–12%)	10–100	22–26	0.14	PF = 0.079–0.34
PVAc + DOC + DWCNT (7–12%)	—	50–70	0.15	PF = 0.045–0.096
PVAc + TCPP + DWCNT (7–12%) [ref. 46]	—	70–82	0.155–0.16	PF = 0–0.204
PVAc + polyethyleneimine (10 wt%) + CNT with 99% purity (20 wt%) + SDBS (20–60 wt%)	420–1250	−66 to −75	—	PF = 1.89–7.03
PVAc + CNT with 99% purity (20 wt%) + SDBS (20 wt%) + PEI (0–40 wt%)	320–430	−65 to −80	—	PF = 1.35–2.752
PVAc + CNT with 99% purity (20 wt%) + SDBS (40 wt%) + PEI (0–40 wt%), composition IX [ref. 47]	440–920	−110 to 110	—	PF = 5.32–11.13
PVAc + Au deposited CNT (0–20 wt%) + PEDOT:PSS (15% vol. replacement by Au) [ref. 45]	6 × 10^5	2.5	—	PF = 3.75
PEDOT:PSS + PVAc + CNT (35–75%) [ref. 41]	5 × 10^4 to 1.35 × 10^5	19–34	0.2–0.4	PF = 30–110
PVAc + GINC (80 wt%)	2.18 × 10^4	38.8	—	PF = 32.90
PVAc + graphene (95%) [ref. 39]	2.89 × 10^3	20.7	—	PF = 1.24

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Conclusions

In this work, the conducting polymer PEDOT:PSS was used and its concentration optimized to further improve the properties of polymer–GINC composites by means of tailoring Seebeck coefficient, electrical conductivity and thermal conductivity. During evaluation, PEDOT:PSS/GINC composites (5 : 95) show a remarkable increase in various thermoelectric properties like electrical conductivity (8.0 × 10⁴ S m⁻¹), with a Seebeck coefficient of 25.42 μV K⁻¹ and thermal conductivity of 0.90 W m⁻¹ K⁻¹. Hence, PF and ZT reach up to 51.93 μW m⁻¹ K⁻² and 0.017, respectively. Thermal conductivity was measured using the Laser Flash technique, which is based on measuring the thermal transient of the rear surface of the sample when a pulsed laser illuminates the front; in this way it is possible to avoid interference between the thermal sensor and the heat source. The physical model of the Laser Flash measurement is supposed to have a single pulsed heat source (delta like), for example a laser shot, on the sample front surface. The study of the thermal transient of the rear surface provides the desired thermal information. To improve the mechanical strength of the polymer composites, cellulose fibres were employed. By the addition of cellulose fibres, though mechanical strength of the composite increases, PF reaches 5.6, which is 10 times lower than the PEDOT:PSS/GINC composites.

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Footnote

† Electronic supplementary information (ESI) available: Detailed data table of various compositions, reported values of other organic/inorganic composites and their references. See DOI: 10.1039/c5ra26138d

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