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Publicly Available Published by De Gruyter June 18, 2020

Combination of nanoparticles and carbon nanotubes for organic hybrid thermoelectrics

  • Naoki Toshima EMAIL logo and Yukihide Shiraishi

Abstract

Carbon nanotubes (CNTs) are usually very expensive, but inexpensive CNTs have been mass-produced by a super-growth (SG) method. The SG-CNTs, however, have many defects resulting in a low conductivity, which is a disadvantage of the SG-CNTs. We discovered that even the defective SG-CNTs can provide a good thermoelectric performance by forming ternary hybrid films made of the SG-CNTs, nanoparticles (NPs) of a conducting polymer complex, poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT) and poly(vinyl chloride) (PVC). The good thermoelectric performance of the ternary film (PETT-NP/SG-CNT/PVC) was possibly attributed to the defect repair effect in addition to the bridging effect of the PETT-NPs among the CNTs. In order to confirm this new concept, we attempted the deposition of metal NPs at the defects of the SG-CNTs. We initially made a physical mixture of palladium (Pd) NPs and the SG-CNTs in dispersions to cover the SG-CNT defects with the Pd-NPs. The obtained films showed only a slight improvement in electrical conductivity. Chemical reduction of the Pd ions in the dispersion of the SG-CNTs, on the other hand, provided hybrids with an enhanced electrical conductivity, thus, use as thermoelectric materials. The thermoelectric figure-of-merit was estimated to be ∼0.3, which is a relatively high value for organic hybrid materials.

Introduction

Carbon nanotubes (CNTs) are recognized as one of the special nanomaterials having exceptional mechanical, electrical, thermal, and optical properties. These properties depend on the chirality and number of walls of the CNTs. From the viewpoint of a high electrical conductivity, single-walled (SW) and double-walled (DW) CNTs have received much attention among the CNTs. Many kinds of potential applications have been reported for the CNTs [1], [2], [3], [4]. As for energy conversion, the CNTs, especially the SW-CNTs and DW-CNTs have been used for organic hybrid thermoelectric materials [5], [6], [7], [8], [9], [10], [11], [12], [13], [14]. In the history of thermoelectrics, only inorganic materials have been the target of the research to convert heat at high temperature to electricity because of the high heat resistivity [15], [16], [17]. Based on the growing interest in global warming, however, the energy conversion of waste heat at a relatively low temperature, which has been abandoned without use, to electricity, is now attracting researchers’ attention. The organic hybrid thermoelectric materials are expected the candidate materials for such energy conversion because they have the advantages of low cost of the raw materials and device processing over inorganic materials [13], [18], [19]. Based on this background, the organic hybrid thermoelectric materials containing SW- or DW-CNTs are being advocated and now attracting a great deal of attention [5], [6], [7], [8], [10], [11], [12], [13], [14].

However, in order to apply the organic hybrid thermoelectric materials containing the CNTs to practical thermoelectric devices for energy conversion, there is a big problem to be solved before any practical applications. That is the high cost of the raw materials of the SW- or DW-CNTs in general. It is mainly because the SW- or DW-CNTs can be typically obtained after separation and purification processes. For example, the price of the SW-CNTs prepared by chemical vapor deposition (Timesnano) is about 1000 US$/g, and that by arc discharge (Sigma-Aldrich) is about 400 US$/g. A new method called the “Super Growth (SG)” method was proposed, in which the SW-CNTs with a big aspect ratio were produced by improved chemical vapor deposition by an addition of quite small amount of water [20]. Recently, the Nippon Zeon Corporation provided the mass-produced SW-CNTs based on the SG method at a rather low cost (Zeonano’s SW-CNT (Sigma-Aldrich): about 80 US$/g).

Although the SG-CNTs are inexpensive, they have the disadvantage of a low electrical conductivity due to many defects on the surface of the tubes. In fact, the electrical conductivity of the bulk film prepared by suction filtration of the dispersed SG-CNTs (so-called Buckypaper) was found to be 335 ± 15 S/cm, while that of the arc-discharge CNTs was 690 ± 16 S/cm. Since the thermoelectric performance of the materials can be evaluated by the thermoelectric figure-of-merit (ZT = (S 2 σ/κ)▪T, where S, σ, κ, and T are the Seebeck coefficient, electrical conductivity, thermal conductivity, and absolute temperature, respectively) or power factor (PF = S 2 σ), the low electrical conductivity of the film of the SG-CNTs is really a weak point of the SG-CNTs as raw materials for thermoelectric applications.

We now present the application of the hybridization technique by using nanoparticles (NPs) along with the inexpensive but defective SG-CNTs. Metal NPs have already been applied to the hybridization with conducting polymers [21], [22], [23], [24] and CNTs [25] to improve the thermoelectric performance. In general, the metal NPs are considered to work as a bridging connector among the conducting polymers or CNTs, which results in an improved electrical conductivity. The NPs may sometimes change the density of states (DOS) of the films of the conducting polymers or CNTs, which possibly results in a change in the Seebeck coefficient.

We have used two kinds of NPs for the hybridization with the defective SG-CNTs; one is the NPs of an electro-conducting metal complex, while the other is the NPs of metallic palladium. The former NPs may strongly interact with the defective SG-CNTs to form a homogeneous dispersion which could increase the electrical conductivity of the SG-CNT films mainly by bridging. The latter NPs may interact with the functional groups of the SG-CNTs, which could repair the defects of the SG-CNTs, resulting in an improved electrical conductivity of the defective SG-CNTs, in addition to the bridging effect.

Results and discussions

Nanoparticles of poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT)

In 2012, Y. Sun and coauthors reported the thermoelectric properties of poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT) and showed the potential application of the PETT as organic thermoelectric materials [26]. The pressed pellet of the PETT was conductive and the electrical conductivity was as high as 8.31 S/cm. However, the obtained PETT was an insoluble solid and decomposed at 190 °C.

We were interested in the PETT, and tried to prepare the PETT-NPs which could be dispersed in an organic solvent. Thus, a solution of sodium methoxide in methanol containing a surfactant, dodecyltrimethylammonium bromide (DTAB), was used instead of the pristine sodium methoxide solution in methanol for the synthesis of a dispersion of the PETT-NPs [27], [28]. The experimental procedures are briefly written in Experimental Section. The reaction processes are illustrated in Fig. 1.

Fig. 1: 
Preparation of poly(nickel 1,1,2,2-ethenetetrathiolate) nanoparticles (PETT-NPs) from 1,3,4,6-tetrathiapentalene-2,5-dione in presence of DTAB (dodecyltrimethylammonium bromide).
Fig. 1:

Preparation of poly(nickel 1,1,2,2-ethenetetrathiolate) nanoparticles (PETT-NPs) from 1,3,4,6-tetrathiapentalene-2,5-dione in presence of DTAB (dodecyltrimethylammonium bromide).

The obtained powder of PETT-NPs can be well dispersed in various polar organic solvents. A picture of the well-dispersed solution of the PETT-NPs in NMP (N-methylpyrrolidone) shown in Fig. 2a indicates a good dispersion. From the TEM (transmission electron microscopy) photograph of the PETT-NP samples taken from the methanol dispersion (Fig. 2b), the average diameter of the PETT-NPs was found to be 38 ± 12 nm. The NPs are stable against heat below the decomposition temperature of 189 °C. The electrical conductivity of the cold-pressed pellet of the PETT-NPs was 0.01 S/cm, which was lower than that of the pellet of the PETT alone (8.31 S/cm), but still in the electoconducting region.

Fig. 2: 
Dispersions of PETT-NPs; (a) A photograph of the dispersion of PETT-NPs in NMP, and (b) TEM image of PETT NPs dispersed in methanol: cited from Figure 20.3 in [28].
Fig. 2:

Dispersions of PETT-NPs; (a) A photograph of the dispersion of PETT-NPs in NMP, and (b) TEM image of PETT NPs dispersed in methanol: cited from Figure 20.3 in [28].

Hybrid films of the SG-CNTs and the NPs of poly(nickel 1,1,2,2-ethenetetrathiolate)

The PETT-NPs were used to prepare an organic hybrid thermoelectric material by using the arc-discharged SW-CNTs (Arc-CNTs) and poly(vinyl chloride) (PVC) [29]. The ternary CNT-containing organic hybrid thermoelectric materials, after a treatment with methanol, showed the reasonably high apparent thermoelectric figure-of-merit at 340 K, i. e., ZT a = 0.28 (S = 30.3 ± 0.4 μV/K, σ = 630 ± 23 S/cm, κ = 0.07 W/(m۰K)) [29].

The PETT-NPs are not only well dispersed by themselves but also make the CNTs well dispersed in NMP. The excellent ability to disperse the CNTs was observed not only in the case of the Arc-CNTs but also in the case of the SG-CNTs. Photographs of the dispersions of (a) the SG-CNTs only in NMP, (b) the SG-CNTs with PVC in NMP, and (c) the ternary dispersion of the SG-CNTs, PVC, and PETT-NPs in NMP are shown in Fig. 3 [28]. The SG-CNTs form aggregates in the dispersion by themselves and by the addition of PVC. By the addition of the PETT-NPs, however, no aggregates can be visually recognized. We can easily understand the good dispersion ability of the PETT-NPs.

Fig. 3: 
Photographs of the dispersions of the SG-CNTs alone (a), SG-CNTs with PVC (b), and SG-CNTs with PVC and PETT-NPs (c), placed between two glass plates immediately after a 5-min strong sonication: cited from Figure 20.6 in [28].
Fig. 3:

Photographs of the dispersions of the SG-CNTs alone (a), SG-CNTs with PVC (b), and SG-CNTs with PVC and PETT-NPs (c), placed between two glass plates immediately after a 5-min strong sonication: cited from Figure 20.6 in [28].

Since the Arc-CNTs were expensive, we tried to prepare similar ternary CNT-containing organic hybrid thermoelectric films by using the inexpensive but defective SG-CNTs instead of the Arc-CNTs [26]. The obtained ternary films in the SEM photograph (Fig. 4) were similar to those of the ternary films prepared by using the Arc-CNTs. Although the electrical conductivity of the SG-CNT Buckypaper (335 ± 15 S/cm) was lower than that of the Arc-CNT one (690 ± 16 S/cm), the conductivities of the ternary hybrid films were 548 ± 89 and 630 ± 23 S/cm, respectively, for the SG-CNTs and Arc-CNTs. This revealed that the electrical conductivity of the SG-CNT-containing ternary films was well improved by utilizing the PETT-NPs as an additive for the ternary films. The thermoelectric figure-of-merit ZT of the SG-CNT-containing ternary hybrid film after methanol treatment, calculated by the thermal conductivity through the plane, was as high as 0.29 at 340 K. The thermoelectric parameters were found to be 40.0 ± 1.8 μV/K, 548 ± 89 S/cm, 86.6 ± 6.5 μW/(m·K2), 0.10 W/(m⋅K) for the Seebeck coefficient (S), electrical conductivity (σ), power factor (PF), and thermal conductivity through the plane (κ), respectively, at 340 K. These data can be compared to those of the corresponding ternary hybrid film containing Arc-CNTs described in the previous section as well as some data reported [6], [9], [11], [12], [29], [30] in Table 1. The comparison of the data of the ternary hybrid films reveals that the slightly lower electrical conductivity of the SG-CNT-containing ternary hybrid film than that of the Arc-CNT-containing system can be compensated by a slightly higher Seebeck coefficient, resulting in the similar thermoelectric figure-of-merit. This result may suggest the possibility that the PETT-NPs may improve the electrical conductivity of the SG-CNT-containing system not only by the bridging effect between the CNTs at the connecting sites, but also by the effect of the NPs repairing the defective sites on the surface of the single SG-CNT. Table 1 also reveals that our systems using the defective SG-CNTs have the relatively good thermoelectric properties similar to those of the reported ones using the expensive SW-CNTs.

Fig. 4: 
SEM photograph of the ternary film made from the SG-CNTs, PVC, and PETT-NPs after the methanol treatment.
Fig. 4:

SEM photograph of the ternary film made from the SG-CNTs, PVC, and PETT-NPs after the methanol treatment.

Table 1:

Comparison of thermoelectric properties of some carbon-nanotube hybrid materials.*

Sample σ(S/cm) S(μV/K) PF(μW/(m۰K2)) κ(W/(m۰K)) ZT(−) Year Ref. No.
SW-CNT/PANI 125 40.0 20 2010 [6]
SW-CNT/PANI 769 65.0 176 0.43 0.12 2014 [30]
SW-CNT/PETT-NP/PVC 630 30.5 58.6 0.07 0.31 2015 [29]
SG-CNT/TPM-CB 497 −59.0 172 0.004 2017 [11]
Bi0.8Sb0.2/MW-CNT(0.3 wt%) 3.5 −92.5 299 0.42 2017 [9]
SW-CNT/PEDOT(40 layers) 34 144.0 72 2020 [12]
SG-CNT/PETT-NP/PVC 548 40.0 86.6 0.10 0.29 This Work
SG-CNT/Pd-NP 164 60.0 59 0.07 0.3 This Work
  1. *

    SW-CNT: single-walled carbon nanotube, PANI: polyaniline, PETT: poly(nickel 1,1,2,2-ethenetetrathiolate), NP: nanoparticle, PVC: poly(vinyl chloride), SG-CNT: super-growth carbon nanotube, TPM-CB: triphenylmethane carbinol base, MW-CNT: multi-walled carbon nanotube, PEDOT: poly(3,4-ethylenedioxythiophene), σ: electrical conductivity, S: Seebeck coefficient, PF: power factor, κ: thermal conductivity, ZT: thermoelectric figure-of-merit.

Physical hybridization of the SG-CNTs and the Pd NPs

In the previous sections, we have used the special NPs made of the conducting polymeric metal complex (PETT-NPs). The NPs may form bridges between the nanotubes, which may improve the carrier mobility, and thus, the electrical conductivity of the film. This kind of interpretation was used in past reports about conducting inorganic oxide and/or metal NPs [25], [31]. This strategy was sometimes effective, and in the other case, it had no effect or negative effect.

In the case of the combination of the PETT-NPs and SG-CNTs, the PETT-NPs have the possibility to repair the defects on the surface of the SG-CNTs. Because the inexpensive SG-CNTs have a high surface area and many defective sites on the surface, the electrical conductivity of the SG-CNT pristine Buckypaper is relatively low compared to that of the Buckypaper of the expensive CNTs, which is a disadvantage of the SG-CNTs. Thus, if the defects of the SG-CNTs can be repaired by covering with the NPs, the carrier mobility on the surface of the SG-CNTs can be improved, which could result in an increased electrical conductivity.

Based on this consideration, the hybrid films were prepared by a physical mixture of the SG-CNTs and commercially-available Pd-NPs (palladium black) or separately-prepared Pd-NPs. [32] The Pd black and separately-prepared Pd-NPs were found to have the average diameters of 6.2 ± 3.4 and 2.2 ± 0.7 nm, respectively, based on TEM measurements. The experimental procedures are briefly written in Experimental section. The preparation process of the hybrid films is shown in Fig. 5.

Fig. 5: 
Schematic illustration of the preparation of Pd-NP/SG-CNT films by a physical mixture method.
Fig. 5:

Schematic illustration of the preparation of Pd-NP/SG-CNT films by a physical mixture method.

The Seebeck coefficient of the SG-CNT films was observed to decrease with the addition of the Pd black, while the decline was less in the case of the separately-prepared Pd-NPs. The electrical conductivity (σ) of the hybrid film of the SG-CNTs slightly increased by the addition of a small amount of the Pd black, then decreased with an increase in the Pd content. The Pd content dependence of the electrical conductivity at 345 K is shown in Fig. 6. The highest conductivity was observed to be 101.5 S/cm at the concentration of 8.4 wt.% of Pd, while it was 92.3 S/cm for the pristine SG-CNT film. When the separately-prepared Pd NPs were used instead of Pd black, a better electrical conductivity (115.5 S/cm at the Pd content of 2.6 wt.%) was observed. This is possibly due to the smaller size of the Pd-NPs than that of the Pd black.

Fig. 6: 
Palladium content dependence of the electrical conductivity (σ) of the hybrid film of the SG-CNTs decorated with commercially available Pd-NPs (Pd black) at 345 K.
Fig. 6:

Palladium content dependence of the electrical conductivity (σ) of the hybrid film of the SG-CNTs decorated with commercially available Pd-NPs (Pd black) at 345 K.

Hybrid films of the SG-CNTs and Pd-NPs prepared by accumulated chemical reduction

In the previous section, we tried the physical mixing of the Pd-NPs and the SG-CNTs in dispersions to cover the defects of the SG-CNTs with Pd-NPs. The obtained hybrid films showed only a slight improvement in the electrical conductivity. In general, however, stable dispersions of the metal NPs in water and/or alcohols can be prepared by reduction of the corresponding metal ions in solution in the presence of functional polymers like poly(N-vinyl-2-pyrrolidone) (PVP) [33], [34], [35]. Carbon nanotubes are polymeric one-dimensional nanomaterials composed of carbon. Thus, the chemical reduction of Pd ions in the dispersion of the SG-CNTs may provide the hybrids of the SG-CNTs decorated with Pd-NPs at the defective sites of the SG-CNTs. The experimental procedures are briefly written in Experimental Section. The preparation processes are shown in Fig. 7.

Fig. 7: 
Schematic illustration of the preparation of Pd-NP@SG-CNT films by an accumulated chemical reduction method.
Fig. 7:

Schematic illustration of the preparation of Pd-NP@SG-CNT films by an accumulated chemical reduction method.

The hybrids were characterized by HREM, SEM, IPC, Hall measurement, Raman spectra, and XPS, which resulted in the possible location of the Pd-NPs at the exact defective sites of the SGCNTs. The process of the defect repair of the SG-CNTs with Pd-NPs can be understood to be similar to that of the formation of polymer-protected metal NPs in solution via macromolecular complexes [34], [35]. The symbol of Pd@SG-CNT refers to this hybrid film.

The electrical conductivity of Pd@SG-CNT increased with the increase in the Pd content, resulting in 164.1 S/cm for the Pd content of 10.8 wt.%. The palladium content dependence of the electrical conductivity of Pd@SG-CNT and that of the SG-CNT films decorated with Pd black are compared in Fig. 8. The hybrid Pd@SG-CNT films provide the enhanced electrical conductivity, and a slight increase in the Seebeck coefficient (not shown here). Thus, the apparent thermoelectric figure-of-merit was estimated to be ∼0.3, which is a relatively high value for organic hybrid materials. The thermoelectric properties are compared to those of some reported hybrid films containing CNTs in Table 1.

Fig. 8: 
Palladium content dependence of the electrical conductivity (σ) of the hybrid film of the SG-CNTs prepared by the accumulated chemical reduction method (Pd@SG-CNT, red circle) at 345 K. Palladium content dependence of the electrical conductivity of the hybrid film of the SG-CNTs decorated with commercially available Pd-NPs (Pd black) (triangle) are also revealed for comparison.
Fig. 8:

Palladium content dependence of the electrical conductivity (σ) of the hybrid film of the SG-CNTs prepared by the accumulated chemical reduction method (Pd@SG-CNT, red circle) at 345 K. Palladium content dependence of the electrical conductivity of the hybrid film of the SG-CNTs decorated with commercially available Pd-NPs (Pd black) (triangle) are also revealed for comparison.

The Hall measurement of Pd@SG-CNT revealed that the carrier concentration did not change very much (3.2 × 1020 cm−3 and 2.4 × 1020 cm−3 for the pristine SG-CNT film and Pd@SG-CNT, respectively), while the carrier mobility of the SG-CNT films increased from 2.0 cm2/(V۰s) to 5.5 cm2/(V۰s) by the chemical hybridization with the Pd-NPs. This result may suggest that the presence of Pd-NPs at the defects of the SG-CNTs, in other words, the repair of the defects with the Pd-NPs deposited by the chemical reduction results in enhancement of the carrier mobility.

Experimental section

Preparation of the NPs of poly(nickel 1,1,2,2-ethenetetrathiolate) (PETT)

A solution of sodium methoxide in methanol containing a surfactant, DTAB, was used instead of the pristine sodium methoxide solution in methanol for the synthesis of a dispersion of the PETT-NPs. The reaction of 1,3,4,6-tetrathiapentalene-2,5-dione with a DTAB-containing sodium methoxide solution, then the treatment of the mixtures with nickel(Ⅱ) chloride provided the PETT-NPs as a powder after filtration and drying, as described in reference [27], [28].

Fabrication of the ternary hybrid films of the SG-CNTs and the PETT-NPs

The ternary hybrid films were prepared by drop-casting the mixed dispersions of the SG-CNTs, PETT-NPs, and PVC in NMP at the mass ratio of 8:10:3 on a polyimode substrate. The films were slowly dried in air on a hot plate at about 60 °C for 12 h. The film thickness was typically about 20 μm. The methanol treatment was carried out by dipping the film in methanol, and dried in air at 80 °C for 30 min.

Palladium nanoparticles (Pd-NPs)

Palladium black, purchased from Kojima Chemicals Co., Ltd., Saitama, Japan, was used as the commercially-available Pd-NPs. Dispersions of the separately-prepared Pd NPs were prepared by heating a solution of palladium acetate in NMP (1.4 mmol/L in a round-bottom flask) at 100 °C for 45 min and then cooling it rapidly to 0 °C with an iced water. The NMP is used as a reductant for the palladium ions in this reaction.

Fabrication of the hybrid films of the SG-CNTs and the Pd-NPs

The hybrid films of the SG-CNTs and the Pd-NPs were prepared by a physical mixing and chemical reduction. Both processes are illustrated in Fig. 5 and Fig. 7. The hybrid films were prepared by a suction filtration of the dispersions of SG-CNTs and commercially-available Pd black or separately-prepared Pd-NPs in NMP at charged mass ratios of 9:1, 8:2, 7:3, and 5:5 with a membrane filter, and the produced films on the membrane filters were washed with methanol, and dried in air at room temperature at first, then under vacuum at 40 °C over night.

The hybrid films were also prepared by a chemical reduction, followed by a suction filtration and dry process. The chemical reduction was carried by the following way; the dispersions of the SG-CNTs and palladium acetate (1.4 mmol/L) in NMP at charged mass ratios of CNT:Pd = 9:1, 8:2, 7:3, and 5:5 were treated by heat at 100 °C for 45 min, then cooling down with an iced water. The hybrid films were fabricated from the dispersions in the similar way to the case of the physical mixtures.

Measurements of the thermoelectric properties

The Seebeck coefficient S and electrical conductivity σ were measured with an ADVANCE RIKO, Inc. ZEM-3 and ZEM-3HR thermoelectric evaluation system in an in-plane direction. The average thickness of the CNT sheet samples (4 × 16 mm) was measured with Mitsutoyo Ltd. contact-type micrometer. The thermal diffusivity α was measured with a NETZSCH LFA447 NanoFlash® Xenon flash analyzer in a through-plane direction. The specific heat C p was measured with a NETZSCH DSC 204 F1Phenix differential scanning calorimeter. The density ρ was measured by an Archimedes method. The thermal conductivity in a through-plane direction κ was calculated by the equation κ  = C p۰α۰ρ. The thermal conductivity in an in-plane direction κ ǁ was estimated by dividing κ with eight according to the reference [36].

Summary and conclusions

Single-walled carbon nanotubes (SW-CNTs), which are polymeric one-dimensional nanomaterials composed of carbon having excellent electrical conductivities, are promising nanomaterials for the development of organic hybrid thermoelectric films for energy harvesting from waste heat, but they are usually very expensive. Recently, inexpensive SW-CNTs have been mass-produced using an SG method by the Nippon ZEON Corp. However, the SG-CNTs have many defects resulting in a low conductivity, which is a disadvantage of the SG-CNTs. We discovered that Arc-CNTs can provide a good thermoelectric performance by forming ternary hybrid films made of the Arc-CNTs, NPs of the conducting polymer complex PETT, and the PVC [29]. Instead of the expensive Arc-CNTs, the inexpensive but defective SG-CNTs were used to prepare the ternary films (PETT-NP/SG-CNT/PVC). They showed good thermoelectric properties at the same level as those of the ternary films of PETT-NP/Arc-CNT/PVC [28]. It may possibly be attributed to the defect repair effect of the PETT-NPs on the SG-CNTs in addition to the bridging effect. In order to confirm this new concept of “defect repair,” we tried the deposition of metal NPs at the defective sites of the inexpensive SGCNTs for improved thermoelectric energy conversion. The physical mixing of the Pd-NPs and the SG-CNTs in dispersions is a simple method. Because the defective sites could be activated by functional groups like –OH and –COOH, the Pd-NPs might be expected to approach the active sites, which may result in covering the defects of the SG-CNTs with Pd-NPs. The hybrid films obtained by the physical mixture showed only a slight improvement in the electrical conductivity. Thus, an accumulated chemical reduction method was developed to prepare the hybrid films. It is well known in the literature that a good dispersion of Pd-NPs can be easily prepared by heat treatment of the alcohol solution of Pd ions with the addition of a functionalized polymer like PVP [33], [34], [35]. In this process, the Pd ions interact with the functional groups of PVP, then the reduction of the Pd ions occurs to produce Pd atoms which are still in contact at the same place in the polymer. The continuous reduction and deposition provide the Pd-NPs which are still in contact with the polymer. In this way, the Pd-NPs are stably protected by the polymer. Thus, if the accumulated chemical reduction of Pd ions occurs in the dispersion of the SG-CNTs, the Pd-NPs are expected to interact with the active sites of the SG-CNTs. In other words, the Pd-NPs can cover the defects of the SG-CNTs. In fact, the hybrids of Pd-NP/SG-CNT prepared by the chemical reduction revealed an enhanced performance in electrical conductivity, thus, the thermoelectrics. The thermoelectric figure-of-merit was estimated to be ∼0.3, which is a relatively high value for organic hybrid materials. This fact suggests that the deposition of the Pd-NPs occurs at the defective sites of the SG-CNTs. In other words, the “defect repair” was successfully performed by the accumulated chemical reduction method using the defective SG-CNTs and Pd ions. Fig. 9 illustrates the carrier transport in the Pd-NP@SG-CNT hybrid system in which the NPs play the roles of both defect repair and bridging. This defect repair technique can be applied to various other fields like biochemistry and electronics, and we expect that even the defective CNTs, repaired by metal NPs, will be used for practical purposes like electrodes of solar cells and lithium ion batteries.

Fig. 9: 
Schematic illustration of the carrier transport in the Pd@SG-CNT hybrid system, where the Pd-NPs deposited on the SG-CNTs play the role of the defect repair as well as the bridging.
Fig. 9:

Schematic illustration of the carrier transport in the Pd@SG-CNT hybrid system, where the Pd-NPs deposited on the SG-CNTs play the role of the defect repair as well as the bridging.


Corresponding author: Naoki Toshima, Sanyo-Onoda City University (formerly Tokyo University of Science Yamaguchi), Daigaku-dori, Sanyo-Onoda, Yamaguchi, 756-0884, Japan, e-mail:

Article note: A collection of papers from the 18th IUPAC International Symposium Macromolecular-Metal Complexes (MMC-18), held at the Lomonosov Moscow State University, 10–13 June 2019.


Funding source: MEXT Japan

Funding source: JSPS Japan

Funding source: NEDO Japan

Funding source: Nippon ZEON Corporation

Acknowledgments

The authors thank Dr. Keisuke Oshima for his excellent experiments.

  1. Funding: The financial supports by MEXT Japan, JSPS Japan, NEDO Japan, and the Nippon ZEON Corporation are also acknowledged.

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Published Online: 2020-06-18
Published in Print: 2020-06-25

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