Fabrication of self-assembled polyaniline films by doping-induced deposition

Thin Solid Films 360 (2000) 24±27 www.elsevier.com/locate/tsf

Fabrication of self-assembled polyaniline ®lms by doping-induced deposition Dan Li a,*, Yadong Jiang b, Zhiming Wu b, Xiangdong Chen b, Yanrong Li b a

Materials Chemistry Laboratory, Department of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094, People's Republic of China b Department of Materials Science and Engineering, University of Electronic Science and Technology, Chengdu, 610054, People's Republic of China Received 8 November 1998; received in revised form 9 November 1999; accepted 9 November 1999

Abstract The ultrathin ®lms of polyaniline (PAni)/poly (styrenesulfonic acid)(PSSA) were fabricated via a novel self-assembling process by alternately immersing the substrates into dilute PAni solution in N-methylpyrrolidinone (NMP) and the aqueous solution of PSSA. The process was characterized by UV±Vis absorption spectroscopy. It was found that the oxidation state of polyaniline in single monolayers was dependent on the thickness of the ®lm. The self-assembling mechanism was based on the acid-base reaction between PAni and PSSA. The thickness of the ®lms can be easily manipulated at nanometer scale by controlling the solution chemistry and recycling times. The resulting ®lms are uniform and adhere strongly to the substrates. q 2000 Elsevier Science S.A. All rights reserved. Keywords: Polyaniline; Poly(styrenesulfonic acid); Self-assembly; Conducting polymer

1. Introduction Conducting polymers are an important and interesting class of organic conductors. Among the members of this family, polyaniline is one of the most technologically promising due to its low cost, versatile processability, and relatively stable electrical conductivity. Ultrathin ®lms of polyaniline have received great interest in recent years due to their potential applications in chemical sensors, transparent electrodes for light-emitting devices and other molecular electronic devices [1±6]. Recently, the ultrathin selfassembled layer of polyaniline has been used to control charge injection and electroluminescence ef®ciency in polymer light-emitting diodes [5]. There are two widely used techniques to fabricate ultrathin ®lms: the Langmuir±Blodgett (LB) technique and the self-assembly method. Compared with LB technique, the latter method has at least three advantages: (1) the substrate can take any form, (2) deposition time is independent of the substrate area, and (3) the method can be used in the laboratory without special equipment such as LB thoughs [6]. The layer-by-layer self-assembling technique has been used to prepare various multilayered ®lms in recent years, including * Corresponding author. E-mail address: [email protected] (D. Li)

polyelectrolytes, poly(p-phenylene vinylene), conducting polymers, organic dyes, inorganic semiconductors, and even fullerenes [7]. Recently, Rubner and co-workers [3± 4] fabricated a series of polyaniline multilayered ®lms at molecular level by a self-assembling process based on electrostatic interaction or hydrogen bonding, using a polyaniline solution containing hydrochloric acid as the assembling solution. However, the doped PAni is susceptible to precipitation in aqueous solutions and the stability of the electrical conductivity of the ®lm is relatively poor because small molecular dopants tend to migrate out from the ®lm [8]. In the present paper, we report a new self-assembling process to fabricate the polyaniline ®lms based on the acidbase reaction between PAni and PSSA.

2. Experimental Polyaniline, in its emeraldine base form, was synthesized chemically by direct oxidation of aniline using the procedure similar to Ref. [9]. The solution of PAni base was prepared by dissolving the polymer powder in NMP. The concentration of the solution was 0.1 wt.% unless speci®cally stated. Poly(styrenesulfonic acid) was obtained by perfusing the solution (1 wt.%) of poly(sodium styrenenesulfonate)(Aldrich, Mw,70,000) through the cation-

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved. PII: S00 40-6090(99)0094 8-7

D. Li et al. / Thin Solid Films 360 (2000) 24±27

exchange resin column. NMP(99% purity) and poly(diallyldimethylammoniumchloride)(P 1)(Aldrich, high molecular weight) were used as received. Glass and quartz slides, and silicon wafers were used as substrates for different measurement. The substrates were cleaned in a hot H2SO4/H2O2 (7:3) bath for 1 h, extensively rinsed with pure water, and sonicated in pure water 20 min. Then, the cleaned substrates were immersed in 1 wt.% P 1 aqueous solution for 30 min. Kotov et al. [10] have found that a monolayer of P 1 as thick as several nanometers can be strongly absorbed onto these substrate surfaces. As a result, the surface of the substrates was positively charged. This surface was subsequently immersed into PSSA solution for several minutes, and a monolayer of PSSA was absorbed onto it to make the surface become acidic as a result of the electrostatic self-assembly [7]. A small absorption peak at 228 nm in the UV±Vis absorption spectrum of the sample, characteristic of PSSA, con®rmed the presence of PSSA on the surface. The assembly process involved the following steps. First, the substrate with an acidic surface was immersed into the PAni solution for 10 min, rinsed with N, Ndimethylformamide (DMF) to remove loosely-absorbed materials, and dried with compressed air (step 1). The substrate was subsequently immersed into the PSSA solution, rinsed with pure water, and dried with compressed air (step 2). Multilayered PAni/PSSA ®lms were obtained by repeating the two steps. The relative absorbed amount of PAni on the surface and the ®lm thickness were investigated by UV±Vis absorption spectroscopy. The spectra were recorded using a Beijing Eraic UV1100 spectrophotometer. The electrical conductivity was measured using the standard four-probe method and a Model D41-5/ZM electrical conductivity analyzer. The morphology of the ®lms was examined using scanning electron microscopy (SEM) in a Hitachi S-450 electron microscope.

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3. Results and discussions A schematic diagram of the self-assembling process is shown in Fig. 1. When the substrate with an acidic surface was immersed in the PAni base solution (step 1), a monolayer of PAni was absorbed onto the surface as a result of the acid-base reaction of the sulfonic acid groups with imine in the PAni base. The study of the relationship between the absorbed amount and the immersing time shows that the absorbing process is fast. The amount absorbed after a 30 s immersion is almost equal to that obtained after a 30 min immersion. Some comparative experiments were carried out to investigate the absorption mechanism in this assembly process. The substrate only treated by the P 1 solution was immersed into the PAni solution, rinsed with DMF, and dried. Its UV± Vis spectrum showed that the absorbance was unchanged compared with that of the pristine substrate, indicating that the surface covered with P 1 has a poor ability to absorb PAni molecules. It can be concluded that the acidi®cation of the surface is crucial to the absorption of PAni molecules and that the acid-base reaction between PSSA and PAni base is the driving force of the deposition process. The selective absorption behavior of PAni on different surfaces implies that the assembling process may be used to fabricate patterned circuit for microelectronic applications if the surface is previously patterned by different functional groups [11]. The concentration of the polyaniline solution is another important factor in¯uencing the absorbed amount. Fig. 2 shows the UV±Vis absorption spectra of the ®lms that resulted from polyaniline solutions of different concentrations. This indicates that the absorbed amount of PAni increased along with the increase of PAni concentration. The spectra are similar to that of polyaniline base, indicating that the deposited polyaniline was not fully protonated. One possibility for this phenomenon is that the amount of the

Fig. 1. Schematic diagram of layer-by-layer self-assembling of PAni/PSSA ®lm.

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D. Li et al. / Thin Solid Films 360 (2000) 24±27

absorbed PSSA is very small and its molecular chains are of poor mobility due to the fact they have been ®xed on the surface as a result of the electrostatic interaction between PSSA and P 1. Based on the relationship of thickness and absorbance of the polyaniline ®lm reported in the Ref. [12] and our optical data, we estimate that thickness of the PAni monolayer is approximately 2.5, 5 and 10 nm, respectively for the three PAni concentrations using Beer's law. From Fig. 2, one can see that the location of the absorption band at ,600 nm in the UV±Vis spectra is dependent on thickness. The absorption peak of the ®lm as thick as 10 nm is located at 620 nm, which is exactly in agreement with that of the ®lm prepared by the casting method [13]. However, the peak is shifted to 583 and 550 nm respectively, when the thickness is reduced to 2.5 and 5 nm. The blue shift of the peaks at ,600 nm implies that polyaniline is in a higher oxidation state [13]. Such phenomenon was only observed in the unprotonated polyaniline (i.e. polyaniline base) ®lms. When the ®lm was protonated by PSSA, no apparent peak shift appeared. This indicates that the oxidation state of polyaniline base in air is dependent on thickness when the thickness is less than 10 nm. The absorbed PAni in step 1 was further protonated when it was immersed in PSSA solution (step 2). Its UV±Vis spectrum (Fig. 3) is similar to that of the polyaniline salt, indicating the accomplishment of the protonation process. The protonation process in step 2 did not only lead to the transformation of the absorbed PAni from base-type to salttype, but also led to the deposition of another monolayer of PSSA. This resulted in a surface rich in sulfonic acid groups, which induced another monolayer of PAni to be absorbed on the surface. Thus, multilayered PAni/PSSA ®lms were obtained by repeating the steps 1 and 2. Fig. 3 shows the UV±Vis spectra of the multilayered ®lms with various

Fig. 2. UV±Vis spectra of single monolayers of PAni prepared by selfassembling process from PAni solution with various concentrations: (a) 0.2 (b) 0.1 and (c) 0.05 wt.%.

Fig. 3. UV±Vis spectra of PAni/PSSA multilayered ®lms with various number of bilayers (each bilayer was always terminated with PSSA and the concentration of the PAni solution was 0.1 wt.%). The inset shows how the absorbance at 320 nm increases with the number of bilayers for various PAni solutions with different concentrations.

numbers of PAni/PSSA bilayers. The absorbance increases almost linearly with the increase of bilayers, indicating that each deposited layer of PAni contributes an equal amount of material to the ®lm. The thickness of the ®lm can be easily manipulated through choosing appropriate concentration of PAni solution and repetitions. We have successfully fabricated the polyaniline ®lms to a thickness of 2,200 nm using this technique. The scanning electron microscopy (SEM) of the 20 bilayered ®lm is shown in Fig. 4. Our comparative study shows that the surface is much smoother and more uniform than the ®lms prepared by in-situ polymerization deposition. Another remarkable property of the ®lm is that the ®lm adheres well to the substrate. The polyaniline ®lm prepared by the casting method can be easily peeled off from the substrate when it is immersed in water [14]. However, the

Fig. 4. SEM microscopy of the self-assembled ®lm.

D. Li et al. / Thin Solid Films 360 (2000) 24±27

®lm, which was fabricated by the self-assembling technique, adheres so strongly to the substrate that it can resist extensive rinsing by water and DMF. Such strong adhesion results from the chemical bonding between the ®lm and the surface of the substrate. In addition, the polymeric acid, PSSA, does not only play a role as a dopant to PAni, but also as a `glue' between PAni layers to make the ®lm very solid. The electrical conductivity of the resulting ®lms with 10 bilayers of PAni/PSSA is up to ,1 S/cm. A preliminary study showed that the conductivity was sensitive to humidity, NH3 and Cl2. Further research is in progress to investigate the sensitive properties of the ®lms. In addition, the ®lms have a high optical transitivity in visible spectrum region. This is advantageous to light-emitting devices or other optical coatings. 4. Conclusion Monolayered and multilayered ®lms of polyaniline were fabricated by a new self-assembling technique based on an acid-base reaction mechanism. The thickness of the ®lms can be easily controlled by the assembling solution chemistry and number of deposition cycles. The oxidation state of base-type polyaniline ®lm was found dependent on thickness when the thickness was less than 10 nm. The ®lms were uniform and adhered strongly to substrates. Also, the ®lms may ®nd applications in light-emitting devices and chemical sensors.

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Acknowledgements The authors acknowledge the National Science Foundation of China under award number 69771025 and the Doctoral Foundation of National Education Committee of China for ®nancial support.

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