Fabrication of self-assembled polyaniline ®lms bydoping-induced deposition

Dan Lia,*, Yadong Jiangb, Zhiming Wub, Xiangdong Chenb, Yanrong Lib

aMaterials Chemistry Laboratory, Department of Chemical Engineering, Nanjing University of Science and Technology, Nanjing, 210094,

People's Republic of ChinabDepartment 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, transpar-

ent electrodes for light-emitting devices and other molecular

electronic devices [1±6]. Recently, the ultrathin self-

assembled layer of polyaniline has been used to control

charge injection and electroluminescence ef®ciency in poly-

mer light-emitting diodes [5].

There are two widely used techniques to fabricate ultra-

thin ®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 labora-

tory 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

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 elec-

trostatic interaction or hydrogen bonding, using a polyani-

line solution containing hydrochloric acid as the assembling

solution. However, the doped PAni is susceptible to preci-

pitation in aqueous solutions and the stability of the elec-

trical 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 acid-

base reaction between PAni and PSSA.

2. Experimental

Polyaniline, in its emeraldine base form, was synthesized

chemically by direct oxidation of aniline using the proce-

dure 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 styrenene-

sulfonate)(Aldrich, Mw,70,000) through the cation-

Thin Solid Films 360 (2000) 24±27

0040-6090/00/$ - see front matter q 2000 Elsevier Science S.A. All rights reserved.

PII: S0040-6090(99)00948-7

www.elsevier.com/locate/tsf

* Corresponding author.

E-mail address: danli_uestc@263.net (D. Li)

exchange resin column. NMP(99% purity) and poly(diallyl-

dimethylammoniumchloride)(P1)(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.% P1

aqueous solution for 30 min. Kotov et al. [10] have found that

a monolayer of P1 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, N-

dimethylformamide (DMF) to remove loosely-absorbed

materials, and dried with compressed air (step 1). The

substrate was subsequently immersed into the PSSA solu-

tion, 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 conductiv-

ity 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 elec-

tron microscopy (SEM) in a Hitachi S-450 electron micro-

scope.

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 mono-

layer 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 inves-

tigate the absorption mechanism in this assembly process.

The substrate only treated by the P1 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 P1 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 concentra-

tions. 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

D. Li et al. / Thin Solid Films 360 (2000) 24±27 25

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

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 P1.

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 absorp-

tion 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 salt-

type, 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

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 fabri-

cated 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

D. Li et al. / Thin Solid Films 360 (2000) 24±2726

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.

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

Fig. 2. UV±Vis spectra of single monolayers of PAni prepared by self-

assembling process from PAni solution with various concentrations: (a) 0.2

(b) 0.1 and (c) 0.05 wt.%.

®lm, which was fabricated by the self-assembling techni-

que, 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 humid-

ity, NH3 and Cl2. Further research is in progress to investi-

gate 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 chem-

istry and number of deposition cycles. The oxidation state of

base-type polyaniline ®lm was found dependent on thick-

ness 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.

Acknowledgements

The authors acknowledge the National Science Founda-

tion of China under award number 69771025 and the

Doctoral Foundation of National Education Committee of

China for ®nancial support.

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