One-pot fabrication and thermoelectric properties of Ag nanoparticles–polyaniline hybrid nanocomposites

Abstract

In this context, a one-pot and in situ strategy for fabrication of AgNPs (Ag nanoparticles)/PANI (polyaniline) nanocomposites in a micellar solution of dodecylbenzene sulfonic acid (DBSA, anionic surfactant) is introduced. Guided by this strategy, AgNPs were directly synthesized from silver nitrate. AgNPs/PANI hybrid nanocomposites with AgNPs were consolidated via spark plasma sintering (SPS). The phase structure and microstructure of the as-prepared composites were evaluated by several characterizations, and the growth mechanism of AgNPs was speculated. The thermoelectric properties of the samples with increasing silver nitrate content were systematically investigated. Compared with pure bulk PANI, the thermoelectric performance of AgNPs/PANI hybrid nanocomposites exhibits a distinct enhancement on the addition of AgNPs. The Seebeck coefficient (S) decreased slightly while the electric conductivity (σ) was found to increase remarkably. However, thermal conductivity (κ) remained unchanged with increasing silver nitrate content, which resulted in an obvious enhancement in the figure of merit (ZT) of the composites. Consequently, the maximum ZT of the AgNPs/PANI hybrid nanocomposites amazingly reached 5.73 × 10⁻⁵, which is about 3.8 times of the ZT of the pure PANI (1.503 × 10⁻⁵). This study suggests that the hybridization of organic/low-dimensional metal particles is promising to effectively improve the thermoelectric properties of conducting polymers.

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Introduction

It is clear that fossil fuel energy has come to the edge of exhaustion, and thus searching and developing new, clean, effective and reproducible energy has become an urgent concern. Thermoelectric (TE) materials can transfer energy between heat and electricity without a mechanical device, making them reliable and simple. These materials are now attracting increasing attention from the research community.¹ Furthermore, numerous advantages make it extensively applicable in various fields such as waste heat recovery from automobile hot exhaust stream, thermoelectric refrigeration, etc.² The performance of thermoelectric materials is determined by its dimensionless figure of merit ZT. A good thermoelectric material should possess high σ, large S and low κ, where high Seebeck coefficient provides high voltage in thermal power generators, large electrical conductivity minimizes Joule heating and low thermal conductivity reduces heat losses.^3–5 In addition, the power factor (S²σ), which determines the electrical performance of thermoelectric materials, can also reflect the pros and cons of the thermoelectric properties.

Currently, numerous TE materials including inorganic semiconductors, such as PbTe, Bi₂Te₃, CoSb₃ and their alloys, have been applied practically. However, most inorganic thermoelectric materials are prepared by melt growth (arc melting method, zone melting, melt-annealing) and powder metallurgy that involves high temperature, long-term and high-cost fabrication processes. Compared with inorganic thermoelectric materials, organic thermoelectric materials, which have been widely considered as a potential candidate for TE materials, have intrinsically low thermal conductivity, low toxicity, mechanical flexibility and inexpensive processability.^6–10,59

Usually, organic conducting polymers, such as polyaniline (PANI), poly(3,4-ethylenedioxythiophene), poly (styrenesulfonate) (PEDOT:PSS), polythiophene (PTH), polycarbazoles (PC), polypyrrole (PPY) and polyacetylene (PA), are available to TE devices.^11–20 Due to its low cost, structural diversification, unique doping/dedoping progress, low thermal conductivity, and the ease of synthesis, PANI is regarded as one of the most potential effective and suitable TE material among conducting polymers. However, low electrical conductivity and Seebeck coefficient lead to the serious lag of its large-scale application. Practice application has employed different methods to improve the TE properties of PANI, among which one approach is to prepare an organic/inorganic hybrid material to adjust the carrier concentration and achieving high ZT performance. In the last decade, metal oxides, metals and carbon materials have been introduced into PANI matrix to enhance its thermoelectric properties.^20–22 Wu et al. prepared (PANI)_XV₂O₅·nH₂O, which exhibited room-temperature conductivity in the range of 10⁻⁴ to 10⁻¹ S cm⁻¹, and observed that its thermoelectric power factor was from −30 to −200 μV k⁻¹.²³ Anno et al. fabricated polyaniline (PANI)/bi-nanoparticles composites by a planetary ball-milling technique.²⁴ Yao et al. prepared single-walled nanotubes/PANI nanocomposites through in situ polymerization.²⁵ Segregated-network carbon nanotube (CNT)–polymer composites were prepared by Yu et al.²⁶ Polyaniline/NaFe₄P₁₂ whisker and polyaniline/NaFe₄P₁₂ nanowire composites were prepared by an in situ compounding method.²⁷ Chatterjee et al. synthesised structure-ordered cable-like polyaniline–bismuth telluride nanocomposites with a ZT value of 0.0043.²⁸ A Ag₂Te/PANI core–shell thermoelectric nanostructure was reported by Wang et al.²⁹ Chen et al. reported the thermoelectric performance of ATT/TiO₂/PANI nanocomposites doped with different acids.³⁰ Graphite oxide (GO)/ordered-polyaniline (PANI) composites were prepared through in situ polymerization with maximum thermoelectric figure of merit of up to 4.86 × 10⁻⁴, which is 2 orders of magnitude higher than that of pure PANI.³¹ Recently, we reported the synthesis of multi-walled carbon nanotube/polyaniline (MWCNT/PANI) hybrid nanocomposites by cryogenic grinding (CG) and spark plasma sintering (SPS).³² It has been proved that the electrical properties of MWCNT/PANI composites are far better than those of pure PANI. Although their electrical conductivity reaches up to 1.59 × 10² S m⁻¹, MWCNT content increases up to 30 wt% because CNTs exhibit a rich variety of attractive electronic properties such as metallic and semiconducting behaviour.

Metal silver is considered to have the best electrical and thermal conductivity among metals.³³ Due to its high electric conductivity, nano-sized silver has attracted the attention of the scientific community working on TE materials, making it a favourite in practical application such as conductive inks, thick film pastes, catalysis, sensing devices and dielectric material.^34–39 Recently, the fabrication of Ag/PANI composites has become very active because the incorporation of Ag into PANI can result in new composite materials with enhanced electronic properties. For example, Stejskal. J. proposed four basic strategies for the preparation of the composites of conducting polymers and silver.⁴⁰ Nadagouda M. N. et al. have prepared Ag–polyaniline core–shell particles.⁴¹ Silver–polyaniline nanocomposites were also fabricated by gamma radiolysis method.⁴² The oxidation of aniline with silver nitrate to polyaniline–silver composites was studied by Blinova N. V. et al.^43,44

In this study, we prepared AgNPs/PANI hybrid nanocomposites via a one-pot method using AgNO₃ as a precursor, DBSA as a dopant, and APS as an oxidizing agent. It is the first time that this composite was treated as a thermoelectric material. AgNPs were directly reduced without any assistance of a reducing agent. The as-prepared composite powder was consolidated by spark plasma sintering (SPS). Field-emission scanning electron microscopy (FESEM) and transmittance electron microscopy (TEM) images show that AgNPs with an average size of 20–150 nm are well distributed in the PANI matrix. The thermoelectric properties of the as-prepared AgNPs/PANI hybrid nanocomposites samples were investigated, and the maximum ZT of the AgNPs/PANI hybrid nanocomposites was found to be 3.8 times higher than that of the pure PANI.

Experimental section

Materials synthesis

Aniline (99.9%, monomer), silver nitrate (AgNO₃, 99.9% Aldrich grade) and ammonium peroxydisulfate (APS, initiator) was obtained from Sinopharm Chemical Reagent Co., Ltd, and DBSA (AR grade) was received from Kanto. Aniline can not be used until it is distilled and purified. All the solutions were prepared using deionized water during the synthesis process.

Solution A: 11 g of DBSA was added into distilled water (600 ml) in a round bottom flask under constant vigorous stirring for one hour to obtain aqueous micellar. Solution B: 3 g of pre-cooled solution of aniline monomer was dissolved in 50 ml deionized water. After the solutions A and B were mixed together, the mixture was stirred for half an hour. Silver nitrate (0, 0.001 M, 0.002 M, 0.003 M, 0.004 M) was separately dissolved in 50 ml of deionized water, and then added to the above mentioned solution. Silver nitrate (0, 0.001 M, 0.002 M, 0.003 M, 0.004 M) was dissolved in 50 ml of deionized water, and then put into the mixed solution mentioned above. 7.5 g ammonium peroxydisulfate (APS) was dissolved in a 250 ml beaker with 100 ml deionized water as solution C. Then, solution C was transferred to a round bottom flask and was frozen at 0 °C for 8 h. The resulting product was demulsificated with a large amount of methyl alcohol and then filtered with deionized water by washing several times. Finally, it was freeze-dryed at −80 °C in a refrigerant air drier for 48 h. The as-prepared composite powder was consolidated at room temperature for a dwell time of 10 min by spark plasma sintering (SPS, Dr Sinter 725, Sumitomo Coal Mining Co., Tokyo, Japan).

Characterization

The phase purity of all the AgNPs/PANI hybrid nanocomposites was examined by X-ray powder diffraction (XRD, Rigaku D/Max-2550 PC, Japan) using Cu Kα radiation at 40 kV and at 200 mA. The structure of polyaniline and AgNPs/PANI hybrid nanocomposites samples was characterized by a Nicolet 8700 FTIR spectrometer (FTIR, Thermo Fisher/Nicolet 6700, USA). The spectra were collected by averaging 32 scans ranging from 500 to 4000 cm⁻¹. Finally, samples were placed into the ultraviolet and visible spectrophotometer (UV-Vis, PerkinElmer/Lambda A35, USA) at room temperature to record the absorption spectra. Field-emission scanning electron microscopy (FESEM, HITACHI/S-4800, Japan) and transmission electron microscopy (TEM, JEOL/JEM-2100F, Japan) were employed to investigate the morphology and microstructure of the sample.

The thermoelectric properties of the as-prepared bulk samples were measured at 300–380 K, and the electric resistance and Seebeck coefficient were investigated by a Seebeck Coefficient/Electric Conductivity Measuring System (ZEM-3, ULVAC-RIKO, Japan).

The thermal diffusivity was investigated by a laser-flash method on a disk using a commercial system (Netzsch Instruments/LAF-457, Germany). The density of the composites was measured by the Archimedes method (METTLER TOLEDO/AL104, Switzerland). Heat capacities (C_p) were measured on a Differential Scanning Calorimetry (DSC, Netzsch/STA409PC, Germany). The values of the thermal conductivity are calculated according to:

κ = λρC_p (1)

where λ is the thermal diffusivity, ρ is the density and C_p is the heat capacity.

Results discussion

Morphology characterizations

Fig. 1 shows the X-ray diffraction patterns of AgNPs/PANI hybrid nanocomposites with different silver nitrate contents. The broad peak at 2θ = 19.578°, 25.402°, which suggests the existence of PANI can be attributed to the periodicity parallel to the amorphous polymer chain.⁴⁵ The sharp peaks at 2θ values of 38.179°, 44.340°, 64.50°, 77.40°, 81.561°, corresponding to (1 1 1), (2 0 0), (3 1 1) and (2 2 2), respectively, can be regarded as the pure phase of Ag (JCPDS file no. 04-0783).⁴⁶ In summary, the information mentioned above clearly indicates that AgNPs exist in the composites in their crystalline form.

Fig. 1 X-ray diffractograms of AgNPs/PANI hybrid nanocomposites with different contents of silver nitrate.

For the reduction of AgNO₃, the amine nitrogen is selected to act as sites for reducing the Ag⁺. The sonication-derived AgNPs/PANI hybrid nanocomposites in N-methylpyrrolidone were studied by UV-vis spectroscopy, as shown in Fig. 2. The peak at ∼336 nm corresponds to surface plasmon resonance (SPR) of the AgNPs embedded in the polymer matrix.^39,47,48 The maximum absorption peak was at approximately 665 nm, corresponding to the π–π* transition of quinoneimine rings.⁴³

Fig. 2 UV-vis spectra of AgNPs/PANI hybrid nanocomposites with 0.004 M silver nitrate.

Fig. 3 shows the FTIR spectra of AgNPs/PANI hybrid nanocomposites with different silver nitrate contents. The characteristic band centred at 665 cm⁻¹ corresponds to the C–S stretching of the benzenoid ring of DBSA.⁴⁵ The peaks at 1297 cm⁻¹ and 1241 cm⁻¹ are assigned to C–N stretching of the second amine of PANI backbone and the conducting PANI emeraldine salt (ES) form, respectively. The band at 1130 cm⁻¹ indicates an in-plane bending vibration of C–H (mode of N = quinoid = N, quinoid = N + H − B, and B − N + H − B), which is formed during protonation. We assign the peak ranging from 2700 to 3000 cm⁻¹ to aliphatic C–H stretching mode, depending on the long alkyl tail of DBSA.⁴⁹ The bands near 1485 and 1570 cm⁻¹ correspond to the C=C stretching of the benzenoid and quinoid rings, respectively.⁵⁰ We observe that there is a shift in the peaks associated with C=N and C=C stretching of quinoid ring compared to pure PANI, which has been reported in the literature,³⁵ and no appreciable change in peak position is detected for benzenoid ring. Therefore, we conclude that Ag⁺ resides close to the imine nitrogen of the PANI and be reduced to AgNPs.⁴⁸

Fig. 3 FTIR of AgNPs/PANI hybrid nanocomposites with different contents of silver.

Typical low and high magnification FESEM micrographs of AgNPs/PANI hybrid nanocomposites with 0.004 M silver nitrate can be observed in Fig. 4. The images show that the AgNPs are well dispersed in PANI matrix and the diameter of AgNPs is about 40–150 nm. Meanwhile, some small AgNPs can also be recognized and are identified by a red arrow in Fig. 4(e–g). The EDS spectra (Fig. 4(d)) and elemental composition (Table 1) show the presence of Ag, O and S, which indicate the presence of AgNPs in the nanocomposites and O and S in PANI matrix.

Fig. 4 FESEM image (a–c) and elemental analysis (d) of AgNPs/PANI hybrid nanocomposites with 0.004 M silver nitrate.

Table 1 Elemental composite of AgNPs/PANI hybrid nanocomposites with 0.004 M silver nitrate

El	AN	Series	Unn. [wt%]	C atom. [at.%]
Ag	47	L-series	85.41	51.06
C	6	K-series	4.28	22.95
O	8	K-series	5.16	20.78
S	16	K-series	2.59	5.20

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Fig. 5 shows a typical TEM image (Fig. 5(a–c)) of AgNPs/PANI hybrid nanocomposites with 0.004 M silver nitrate and the AgNPs with particle size of approximately 20–30 nm, which are well dispersed in PANI matrix. It is clear that these small AgNPs match well with those observed by FESEM (pointed with red arrows). In fact, the size distribution of AgNPs is broad and about 20–160 nm in the present work, as shown in Fig. 4 and 5. Due to the impact of labile factors in the synthesis process, the results showed a big difference in the size of the AgNPs. In this process, the silver ions were absorbed onto the imine nitrogen of the PANI and then reduced to AgNPs. We did not use surfactants to prevent AgNPs from unrestricted growth, which results in a different size distribution of AgNPs. SEM images show that the diameter of AgNPs is about 60–150 nm; however, some much smaller AgNPs could be found in PANI matrix from Fig. 4, thus the size of AgNPs in Fig. 5(c) and (d) is about 20–30 nm. The clear and uniform lattice fingers spacings (Fig. 5(d)) 0.24 nm agree with the interplanar distance of the (1 1 1) lattice planes of Ag.

Fig. 5 TEM image (a–c) and elemental analysis (d) of AgNPs/PANI hybrid nanocomposites with 0.004 M silver nitrate.

Thermoelectric properties

The thermoelectric properties of the AgNPs/PANI hybrid nanocomposites were measured in the temperature range of 300 K to 380 K, as displayed in Fig. 6. It can be observed that the electrical conductivity of the AgNPs/PANI hybrid nanocomposites also increases dramatically with increasing silver nitrate content and finally reaches 2.10 × 10² S m⁻¹ with the silver nitrate content of 0.004 M at 380 K, which is 2.75 times larger than that of pure PANI. The enhancement of the electrical conductivity (σ = neμ, where n is the carrier concentration, e is the electron charge and μ is the carrier mobility) can be attributed to the introduction of AgNPs in the PANI matrix, to provide the silver islands which lower the carrier hopping barriers and increase the charge transfer channel and carrier concentration of the nanocomposites. The Seebeck coefficient decreases with increasing silver nitrate content. The Seebeck coefficient can be written as

Fig. 6 The electrical conductivity (a), the Seebeck coefficient (b), the power factor (c), (d) the thermal conductivity values and the ZT value (e) of AgNPs/PANI hybrid nanocomposites with different contents silver.

For a composite with high impurity content, the effective thermopower would be dramatically decreased because of the small-thermopower term on the right of eqn (2).⁵⁸ The power factor (S²σ) of the nanocomposites changed from 1.03 × 10⁻⁸ W m⁻¹ K⁻² to 2.91 × 10⁻⁸ W m⁻¹ K⁻² at 380 K with increasing silver nitrate contents. Fig. 6(d) shows that there is no obvious change in the thermal conductivity value, which remains low in the range of 0.150–0.305 W m⁻¹ K⁻¹. The total thermal conductivity can be calculated by κ_total = κ_e + κ_l, where κ_e is the electronic contribution and κ_l is the lattice contribution. κ_l can be reduced by the selective scattering of phonons through the formation of nanoscale inclusions in nanostructure.^28,51–56 In the AgNPs/PANI hybrid nanocomposites, the PANI and Ag nanoparticles nanostructure provides numerous nano-interfaces to selectively scatter phonons, decreasing κ compared with bulk silver (the thermal conductivity of metal silver is 430 W m⁻¹ K⁻¹ (ref. 54)) and not increasing significantly compared with pure PANI. The highest ZT value is 5.73 × 10⁻⁵ at 380 K for AgNPs/PANI hybrid nanocomposites with 0.003 M silver nitrate. Although the ZT of the AgNPs/PANI is not competitive with other PANI-based composites, Ag wt% is just 6.4% in our composites. In other PANI-based composites, for example in MWCNTs/PANI, the MWCNTs wt% was more than 30%.³² In GO/PANI, the GO wt% was more than 40%.³¹ In SWNT/PANI, the SWNT wt% was more than 40%.²⁵ Meanwhile, we can speculate from Fig. 6(a, c and e) that the changing ZT trends of the AgNPs/PANI would increase with increasing silver nitrate content.

Fabrication and conductive mechanism

The increase of electrical conductivity in the AgNPs/PANI hybrid nanocomposites is attributed to the incorporation of AgNPs. A possible mechanism for the enhancement of the conductivity can be explained by the information in Fig. 7. First, the aniline monomer reacts with DBSA and the resulting product forms anilinium cation with a long alkyl tail, as shown in the Fig. 7(a). As the reaction proceeds, anilinium cations diffuse into DBSA to form anilinium cation–DBSA micelle. Then, homodispersed silver ions attach to the imine nitrogen of the micelle by absorption. The reaction product anilinium cation–DBSA micelle then serves as a template for silver ions, as shown in the Fig. 7(b). After that, the silver ions were directly reduced to AgNPs. Meanwhile, aniline was oxidized with AgNO₃ in DBSA but this process was slow and required months.⁴³ After APS was added to the solution, anilinium cation–DBSA micelles were rapidly transformed into PANI through a series of chain reactions. It is clear from the detection results that these in situ synthesized AgNPs were distributed uniformly in PANI matrix, which can be confirmed by FESEM (Fig. 4) and TEM image (Fig. 5). Because silver has excellent electrical conductivity, we deduced that AgNPs increased the carrier transport path as a conducting bridge not only in the intramolecular but also in the intermolecular chains, as shown in the Fig. 7(c). Usually, the carrier transport in pure PANI is principally controlled by the interchain and intrachain hopping processes, and the transport behavior follows the variable range hopping (VRH) model.⁵² Adding AgNPs into the PANI matrix formed new conductive channels, enhanced the carrier mobility, shortened carrier hopping distance and improved the efficiency of the interchains of the carriers. Therefore, AgNPs can effectively improve the conductivity of the AgNPs/PANI hybrid nanocomposites. The conduction mechanism is similar to the one reported previously.^48,57

Fig. 7 (a–c). Schematic representation of AgNPs/PANI hybrid nanocomposites.

Conclusions

This study described the fabrication and characterization of AgNPs/PANI hybrid nanocomposites prepared via a one-pot method in detail. XRD, FESEM and TEM results show that the AgNPs was synthesized successfully and was embedded uniformly in the PANI matrix. As the silver nitrate content increased from 0.001 M to 0.004 M, the electrical conductivity of AgNPs/PANI hybrid nanocomposites increased from 0.416 × 10² S m⁻¹ to 2.10 × 10² S m⁻¹, the thermal conductivity still maintained low values at about 0.25 W m⁻¹ K⁻¹ and the maximum ZT reached 5.73 × 10⁻⁵. Meanwhile, we speculated the mechanism of AgNPs/PANI hybrid nanocomposites. These materials have experimentally demonstrated that the existence of AgNPs truly improves the thermoelectric properties of PANI.

Acknowledgements

This work was funded by the Natural Science Foundation of China (no. 51374078), Shanghai Committee of Science and Technology (no. 13JC1400100), “Shu Guang” project supported by Shanghai Municipal Education Commission and Shanghai Education Development Foundation (no. 11SG34), Shanghai Rising-Star Program (no. 12QH1400100), PCSIRT (no. IRT1221), the Fundamental Research Funds for the Central Universities, and DHU Distinguished Young Professor Program in University.

Notes and references

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