Properties of carbon black-PEDOT composite prepared via in-situ chemical oxidative polymerization

2.1 Materials

3,4-Ethylenedioxythiophene (EDOT) of 99.9% purity was obtained from Shanghai Herochem Co., Ltd. Carbon Black (CB) with average particle size of 18 nm (diameter) was obtained from Hangzhou Juy New Materials Technology Co., Ltd. The oil absorption of di-n-butyl phthalate (DBP) is about 170 mL/100 g and its pH is 7.5. Ammonium persulfate (APS) of analytical purity was obtained from Shanghai Vita Chemical Reagent Co., Ltd. Iron trichloride (FeCl₃) was purchased from Guangdong Weng Jiang Reagent Co., Ltd, and ethanol was purchased from Tianjin Damao Chemical Reagent Factory.

2.2 Nanocomposite preparation

The preparation process of CB-PEDOT composite is shown in Figure 1. CB was dispersed in a mixed solution of DI-water and ethanol (20 vol% of DI-water), and then the mixture was sonicated for 20 min to form a suspension. Subsequently, the EDOT monomer was added into the suspension and sonicated for 20 min to achieve a uniform dispersed system. The dispersed system was bubbled with nitrogen gas for 10 min to remove dissolved oxygen to avoid over-oxidation of EDOT monomer. Specific amounts of aqueous APS solution and aqueous ferric chloride solution were then added dropwise into the aforementioned dispersed system as in-situ polymerization initiators. Subsequently, the dispersed system was magnetically stirred in a water bath at 30°C for 12 h under closed conditions. The system changed from milky white to clear at first, and then gradually turned black after 20 min of the polymerization reaction, which indicated the formation of CB-PEDOT. The dispersed system was subjected to suction filtration with water and absolute ethanol several times to remove the excess APS and the unreacted EDOT monomer. The final polymerization product was dried in a vacuum oven at 60°C for 12 h.

Figure 1

Experimental procedure for preparation of CB-PEDOT composite.

For comparison, pure PEDOT was also synthesized using the same method. The structures of pure PEDOT and CB-PEDOT were characterized by FTIR and TGA.

2.3 Composite characterization

The morphologies of PEDOT and CB-PEDOT composite were observed using scanning electron microscope (SEM) (Quanta-200, FEI, USA). The difference in the morphology of pure PEDOT and CB-PEDOT composite was observed by transmission electron microscopy (TEM) (JEM-2100F, JEOL, Japan). Using the potassium bromide (KBr) disc technique, the FTIR spectra of the samples with the same amount were obtained using FTIR spectrometer (Nicolet, 380, Thermo, USA). Spectra were obtained between 4000 and 400 cm^-1. Specific spectral peaks were identified and assigned to functional groups. The thermal properties were examined by TGA (Q50, TA Instruments, USA), where the samples were heated from room temperature to 600°C in N₂. The electrical conductivity was measured by using four-probe tester (RTS-8, Probes Tech, China).

3.1 Morphology of CB-PEDOT

The morphology of CB-PEDOT was observed via scanning electron microscopy (Figure 2). As can be seen from Figure 2a, pure PEDOT displayed an aggregate morphology. The aggregated PEDOT cannot be easily reprocessed, which restricts its application in many fields. When the nanoscale CB was added into the EDOT monomer, a porous and spongy structure was achieved. Figure 2b-d shows the morphology of the composite CB-PEDOT. It can be clearly seen that the composite structure had a coarse morphology with many pores. Based on the difference between the morphologies in Figure 2a and Figure 2b-d, it can be concluded that nanoscale CB hindered the agglomeration of PEDOT through polymerization. As a result, the processability of PEDOT was improved significantly. This deduction was confirmed by the EDS analysis. Figure 3 and Table 1 are the EDS analysis results of CB-PEDOT. As shown in Figure 3 and Table 1, element C was 93.84% in weight, which came from carbon black nanoparticles. This indicated that most of the carbon black nanoparticles were adhered to the surface of PEDOT. Obviously, the addition of carbon black nanoparticles increased the

Figure 2

SEM morphologies of (a) pure PEDOT; and (b, c, d) CB-PEDOT.

Figure 3

EDS analysis results of the surface of CB-PEDOT composite (CB:PEDOT=2:1).

spaces between PEDOT molecules. Consequently, the agglomeration of PEDOT molecules decreased and the processability was improved. At the same time, the porous and spongy structure increased the specific surface area, which could be helpful to enhance the energy storage density of the material.

The above results were also confirmed by the results of transmission electron microscopy (Figure 4). Figure 4a shows that pure PEDOT was easily agglomerated through polymerization. After doping with nanoscale carbon black, the molecular chains of PEDOT were extended more freely (Figure 4b). Consequently, the new CB-PEDOT composite was easily dispersed in solutions. It can be coated on and adhere to the surface of other suitable materials.

Figure 4

TEM morphologies of (a) pure PEDOT; and (b) CB-PEDOT.

3.2 Structural analysis of CB-PEDOT

The chemical structures of carbon black nanoparticles, PEDOT and CB-PEDOT were examined by FTIR spectroscopy (Figure 5). In the FTIR spectrum of CB, there was a strong absorption peak near 3440 cm⁻¹ which corresponded to the stretching vibration of -OH. The absorption peak at 1629 cm⁻¹ was assigned to the bending vibration of C-OH, and the peak at 1110 cm⁻¹ was attributed to the vibrational absorption of C-O-C. These characteristic peaks indicated that carbon black contained a large number of oxygen-containing groups, which is consistent with previous reports (37).

Figure 5

FTIR spectra of pure PEDOT, CB and CB-PEDOT (over the range of 400 - 4000 cm^-1).

In the FTIR spectrum of CB-PEDOT, the absorption bands at 1484 and 1396 cm⁻¹ were assigned to C=C and C-C stretching vibrations of the thiophene ring, respectively (14). The characteristic peaks in the spectrum at 984 cm⁻¹, 844 cm⁻¹ and 693 cm⁻¹ corresponded to C-S stretching mode (38). The absorption peak at 1215 cm⁻¹ was attributed to the C-O-C stretching vibration. The absorption peak at 2850-2980 cm⁻¹ was due to the stretching vibration of -CH₂ in the thiophene heterocycle. These peaks shifted toward higher wavenumber (so-called blue shift) by about 4~6 cm⁻¹ compared to the bands in pure PEDOT. The shift indicated that a certain interfacial interaction existed between the molecular chains of PEDOT and CB. This interaction stabilized the composite and the energy required for vibration became higher. Thus, the groups tended to be more stable, which caused blue shift of the absorption bands (39,40). However, it is difficult to accurately examine the interfacial interaction between the outer CB layer and the inner PEDOT in the composites by FTIR analysis alone (41,42).

According to the above analysis, EDOT underwent oxidative polymerization in water/ ethanol to form oligomeric free radicals under the action of initiator (APS) and oxidant (FeCl₃) (Figure 6). It is believed that during the PEDOT polymerization process, a small amount of carbon black nanoparticles were encapsulated by PEDOT. However, the CB nanoparticles covered most of the surface of PEDOT, which effectively hindered the agglomeration of PEDOT during polymerization. Thereby, the dispersibility of PEDOT in solution was increased. Figure 6 shows that the prepared CB-PEDOT composite can be stably dispersed in a water/ alcohol solution.

Figure 6

Schematic of polymerization and structure of the CB-PEDOT composite.

3.3 Thermal stability of CB-PEDOT

The thermal stability of a hybrid is critical for its potential applications. Therefore, the thermal stability of CB-PEDOT was investigated by thermogravimetric analysis (TGA). The thermal stability and the mass loss

of CB-PEDOT nanocomposite were examined by TGA at a heating rate of 5°C/min from room temperature to 600°C under a nitrogen atmosphere. Figure 7 shows the TGA results of pure PEDOT and CB-PEDOT. As can be seen from Figure 7, there was a slight mass loss before 100°C which could be ascribed to the adsorbed moisture in the material. Due to the decomposition of oxygen functional groups on the PEDOT surface, the second mass loss was observed at 200 to 400°C. This mass loss was followed by a slight continuous mass loss up to ∼600°C, which can be attributed to the decomposition of residual polymer components. With the increase in proportion of carbon black, the mass loss decreased. The TGA curve of pure PEDOT showed a significant drop at 200°C due to the burning and elimination of organic polymer components (43). However, when the ratio of carbon black to PEDOT was 3:5, the TGA curve of the composite material showed a significant decrease from about 230°C. This indicated that the thermal stability of composite material increased due to the addition of carbon black.

Figure 7

TGA analysis of pure PEDOT and CB-PEDOT (a) thermal weight loss ratio, (b) derivative change rate.

Based on the TGA results, it is evident that the CB-PEDOT nanocomposite was successfully synthesized, and the thermal stability of CB-PEDOT was improved compared to pure PEDOT.

3.4 Electrical properties of CB-PEDOT

The CB-PEDOT composite was tableted to obtain a disc with a diameter of about 10 mm and a thickness of 500±100 μm (Figure 8) (44). The electrical conductivity of CB-PEDOT was tested using a Four-probe tester. As can be seen from Figure 9, conductive carbon black exerted a significant influence on the electrical conductivity of the composite. When the ratio of carbon black to PEDOT was below 2:5, the electrical conductivity was no more than 0.03 S/cm. With further increase in the ratio, the electrical conductivity increased in a linear manner. When the ratio was 5:5, the electrical conductivity was as high as 1.33 S/cm.

Figure 8

Discs having a diameter of about 10 mm and a thickness of 500±100 μm using the method of infrared compression.

Figure 9

The electrical conductivity of CB-PEDOT at different ratios.

The electrical conductivity of the new CB-PEDOT was significantly higher than that of PEDOT (Laboratory Preparation, 6.25e^-4 S/cm). In addition, according to the conductivity of the commercialized PEDOT:PSS product reported in literature (Clevios ^TM P AI 4083, 10^-5∼10^-6 S/m) (45), the effect of carbon black on conductivity can be further confirmed. When more carbon black was added, the electrical conductivity was higher. However, when the ratio between carbon black and PEDOT was higher than 5:5, there was less bonding between carbon black and PEDOT. It can be seen in Figure 3a, superfluous CB would exist at a form of isolated particles which is detrimental to the stability of the composite CB-PEDOT. It is also unfavorable for forming a coating structure. According to the EDS analysis of Figure 3, most of the carbon black and PEDOT were adhered to the surface of PEDOT. At the same time, some small carbon black particles are isolated in composites. It is believed that excess addition of CB may hinder the polymerization of EDOT, which would be detrimental to the application of PEDOT. However, Carbon black can also form an effective thermal barrier around the center PEDOT, which improves the stability of the composites to a certain extent.