High Crystalline Quality Conductive Polypyrrole Film Prepared by Interface Chemical Oxidation Polymerization Method

Abstract

The authors report that polypyrrole (PPy) films with large area and high crystalline quality have been achieved using an interfacial chemical oxidation method. By dissolving different reactants in two immiscible solvents, the PPy is synthetized at the interface region of the two solutions. The PPy films have sharp XRD diffraction peaks, indicating that the molecular chains in the film are arranged in a high degree of order and that they reflect high crystalline quality. High crystal quality is also conducive to improving electrical conductivity. The conductivity of the as prepared PPy film is about 0.3 S/cm, and the carrier mobility is about 5 cm²/(Vs). In addition, the biggest advantage of this method is that the prepared PPy film has a large area and is easy to transfer to other substrates. This will confidently broaden the application of PPy in the future.

1. Introduction

Since the invention of conductive polyacetylene in the 1970s [1], conductive polymers (CPs) have attracted great interest due to their good mechanical, optical and electrical properties. Nowadays, CPs have been widely used in optoelectronic devices [2,3,4,5], chemical sensors [6,7,8,9], capacitor electrodes [10,11,12,13,14,15] and other fields [16,17,18,19]. In the past few decades, many different types of CP have been synthesized using different methods, such as polypyrrole [20,21], polythiophene [22,23], polyaniline [24,25] and so on [26]. Among various materials, polypyrrole (PPy) and its composite materials have attracted much attention due to the ease of manufacturing it, its variable conductivity and its environmental stability.

PPy films can be prepared via chemical and electrochemical polymerization. The films prepared by electrochemical methods can usually achieve a uniform surface and high conductivity. However, the parameter control during the reaction process is more complicated, and any parameter changes will cause changes in the properties of the PPy film. So far, there has not been a comprehensive analysis of the influencing factors of the reaction process. In contrast, the chemical polymerization method is relatively simple in operation and can produce large amounts of thin films. However, chemical methods usually produce black PPy powders [20], which need to be pressed into a thin film which leads to poor electrical conductivity and mechanical properties. Therefore, in order to obtain high performance films, chemical preparation methods need to be improved.

In this paper, an interfacial chemical oxidation method was used to prepare a PPy film with a smooth surface and regular interlayer arrangement. The oxidant and pyrrole monomers were dissolved in different solvents which are immiscible with each other. Then, the polymerization reaction occurs at the interface of the two solutions. This method can control the reaction to form a continuous PPy film at the interface and avoid the formation of dispersed PPy particles. The results showed that the surface of PPy film was continuous and flat and the surface roughness can reach 1 nm. More importantly, the XRD results show that the film had obvious diffraction peaks, indicating that the PPy film had a very high degree of order in the molecular chain arrangement. As far as we know, the sharp XRD diffraction peaks of PPy films have not been reported in previous papers. The conductivity of the film is about 0.3 S/cm, which can be changed by changing the synthesis conditions. The size and thickness of the film also can be changed by controlling the interface area and reaction time, respectively.

2. Materials and Methods

The PPy film is synthesized by chemical oxidative polymerization method at the interface of two different solutions, and the synthesis process is shown in Figure 1. During the experiment, ammonium persulfate ((NH)₄S₂O₄) was used as the oxidant, and hydrochloric acid (HCl) was used as the protic acid dopant. (NH)₄S₂O₄ (>99.99%) and pyrrole (>99.7%) were purchased from Shanghai Aladdin Biochemical Technology Co., Ltd., Shanghai, China. HCl and chloroform were purchased from Tianjin Xintong Fine Chemical Co., Ltd., Tianjin, China. and they were of analytical grade and used without further purification. Firstly, 0.1 mol/L dilute hydrochloric acid solution with a volume of 50 mL was prepared and then add 1.141 g of ammonium persulfate to form a uniform solution with 0.1 mol/L. Secondly, 0.35 mL pyrrole monomer was dissolved in 50 mL chloroform to produce a 0.1 mol/L solution. The prepared solutions were cooled down to 0 °C together in a refrigerator, and then the protic acid solution containing the oxidant was poured into the chloroform solution containing the pyrrole monomer, and the reaction was carried out at 0 °C. As the reaction processes, PPy film with a light grey color can be observed at the interface of the two solutions. Finally, the clean quartz glass was used as the substrate to remove the PPy film and the film was washed with deionized water for 3 to 4 times and then dried at 60 °C in a drying oven for the following test.

The morphology of the PPy films was investigated via scanning electron microscope (SEM) and atomic force microscope (AFM). The molecular structure of PPy film was studied by X-ray diffraction (XRD), Raman spectroscopy and Fourier infrared spectroscopy (FTIR). The electrical characteristics of the PPy films were measured by Keithley 4200 and Hall effect measurement system.

3. Results and Discussion

Figure 2 shows the SEM image and AFM image of PPy film. It can be seen from Figure 2a that the surface of the film is continuous and flat without any holes or wrinkles.

The AFM image shown in Figure 2b shows that the surface of the film is very smooth with a root square roughness of 1 nm. Usually, when the PPy film is prepared by chemical method, the oxidant and the pyrrole monomer are dissolved in the same solvent. Therefore, the reaction process is random, and the PPy powder is generated and dispersed in the solution without forming a thin film. In this work, in order to control the progress of the reaction, the oxidant and the pyrrole monomer were dissolved in different immiscible solvents. Once the reactants are in contact at the interface, rapid and random nucleation of PPy was initiated in the interface of the two solutions. Due to the hydrophobic nature, the generated PPy was restricted to the interface to form a dense film [27,28]. By controlling the reaction speed, a film with a flat surface can be obtained.

Figure 3a shows the XRD pattern of the PPy film. For solid materials, the crystal structure can be divided into crystalline and amorphous. For semiconductor materials, the crystalline state with long-range ordering of atoms can reduce the barrier effects on free carriers which were caused by grain boundaries, thus improving the conductivity of the materials.

The crystallinity can be determined by the X-ray diffraction method. PPy is a common conductive polymer material. It has a conjugated long chain structure. The π electrons in the conjugated double bond (C=C) which are similar to the free electron in metals can move in the molecular chain to make it conductive. In addition, the π electrons can also transfer between adjacent chains. Therefore, the longer the molecular chain, the larger the number of π electrons, and the more regular the molecular chain is arranged, which is more conducive to the transmission of electrons. From the XRD pattern, three sharp diffraction peaks can be found. In order to facilitate the test, the PPy film is placed on the substrate. The substrate material is quartz glass and it has no diffraction peak. In order to determine whether the diffraction peaks belong to PPy, we referred to the related literature. However, the XRD results in the literature are broad bulges and no sharp diffraction peaks appear [9,11,29], indicating that this is the first time we have reported the diffraction peaks of PPy films. According to the molecular structure of the pyrrole monomer as shown in Figure 3d, it is known that there are two types of carbon atoms [30] (α-C and β-C), indicating that there are three possible connection modes for polymerization process, namely α-α connection, α-β connection and β-β connection, as shown in Figure 3d. The α-α connection can form an ordered planar structure, while other connections can cause disorder in the polymer chain. PPy belongs to conjugated polymer and its electrical conductivity is mainly affected by carrier concentration and carrier mobility. The carrier mobility is related to the chain spacing and the effective conjugation length of the molecular chain. High α-α connection ratio indicates a high degree of order, which is conducive to carrier migration. The appearance of XRD diffraction peaks in Figure 3a indicates that the film is composed of a plane structure, indicating that its molecular chains are mainly α-α linkages. As a comparison, the XRD result (red line) of PPy prepared under similar conditions using only water as solvent using the chemical oxidation method is also given in Figure 3a. Figure 3b shows the Raman spectrum of the PPy film. The scatting peaks correspond to different vibrations. The diffraction peak at 1575 corresponds to the stretching vibration of the C=C, the diffraction peak at 1047 corresponds to the bending vibration in the C-H symmetry plane, and the Raman peaks at 935 and 986 correspond to the deformation vibration of the pyrrole ring [3,5]. Calculating the intensity ratio between the stretching vibration peak and the skeleton vibration peak can give a qualitative measurement of the conjugation length. To further investigate the structure of PPy film, the FTIR spectral in the range of 600 to 4000 was recorded as shown in Figure 3c. The broad band at 3000 to 3500 cm⁻¹ corresponds to N-H and C-H stretching vibrations of PPy. The peaks at 1555 and 1470 cm⁻¹ can be attributed to the asymmetric and symmetric stretching vibrations of Py rings, respectively. The peak observed at 1314 cm⁻¹ is due to the C-N stretching in the Py ring of the PPy backbones. The peak at 1194 cm⁻¹ corresponds to the C-H stretching of the Py ring while the peaks at 1044 and 920 cm⁻¹ were attributed to C-H in-plane deformation vibration and out-of-plane deformation vibration, respectively. The peak at 790 cm⁻¹ might be C-H and N-H out of plane deformation vibration [14,15,20]. From the XRD results and Raman data, it can be concluded that the prepared PPy film is mainly composed of longer conjugated molecular chains and forms a relatively regular planar configuration, which is conducive to the transfer of carriers within and between the molecular chains.

Figure 4a shows the optical transmission spectrum of PPy film in the range of 200–800 nm. It can be seen that the transmittance of the PPy film in the visible light is above 80% and there is no obvious absorption in the ultraviolet region. This can also be seen in the downright inset of Figure 4a. In order to determine the conductivity of the PPy film, I-V and Hall tests were carried out, as shown in Figure 4b. It can be seen from the figure that a larger current can be obtained at a lower voltage, indicating that the PPy film has excellent conductivity. Hall results show that the PPy film shows p-type conductivity. The conductivity and carrier concentration of the prepared PPy film is 1 S/cm and 1 × 10¹⁸ cm⁻³, respectively. Meanwhile, the carrier mobility is 20 cm/Vs. As mentioned above, the high conductivity can be attributed to the longer conjugated molecular chain and the regular structure of the molecular chains. In order to determine that the conductivity of the PPy film can vary with the preparation conditions, we also prepared samples with different pyrrole monomer and oxide ratios and tested their conductivity. The results are shown in

Table 1. The conductivity values of different oxide and monomer ratios.

Samples	Molar Ratio ((NH4)2S2O8/Py)	Conductivity (S/cm)
S1	5:1	3.8 × 10−4
S2	2:1	2.8 × 10−4
S3	1:1	7.0 × 10−3
S4	1:2	1.45
S5	1:5	2.71

. In the experiment, we fixed the concentration of pyrrole monomer and changed the concentration of oxidant to prepare a series of samples. It can be seen from the results that sample S2 exhibits the lowest conductivity, and sample S5 exhibits the highest conductivity. The conductivity range is 3.8 × 10⁻⁴~2.7 S/cm, which shows that it can be adjusted in a larger range under different preparation conditions.

4. Conclusions

In summary, we reported the preparation of large-area and high crystalline quality conductive PPy film by the interface chemical oxidation method. The polypyrrole film showed expected XRD diffraction peaks, indicating a relatively long and highly ordered arrangement of molecular chains. The film also shows high conductivity and carrier mobility. Its conductivity can be changed in a wide range by changing the growth conditions. This work can be used as a reference for other researchers and may promote the development of polymer-based optoelectronic devices or supercapacitor electrode materials for energy storage applications.