Nanoassemblies of sulfonated polyaniline multilayers - ARPAL

Nanotechnology 11 (2000) 30–36. Printed in the UK
PII: S0957-4484(00)07559-0
Nanoassemblies of sulfonated
polyaniline multilayers
Nabin Sarkar†§‖, Manoj Ku Ram†§, Anjana Sarkar†,
Riccardo Narizzano‡, Sergio Paddeu† and Claudio Nicolini‡
† Polo Nazionale Bioelettronica, Piazza Colombo 3/5 16121 Genoa, Italy
‡ Department of Science and Technology of Biophysics, Medicine and Odontostomatology
(DI.S.T.BI.M.O.), University of Genoa, Via Corso Europa 30, 16132 Genoa, Italy
§ Fondazione EL.B.A, Via del Babuino 181, 00187, Rome, Italy
E-mail: sarkar@ibf.unige.it
Received 7 September 1999
Abstract. A self-assembly layer-by-layer (LBL) technique was used for the sequential
adsorption of polycation, poly(diallyldimethylammonium chloride) (PDDA) and polyanion,
sulfonated polyaniline (SPANI) on glass, indium–tin-oxide (ITO) coated glass plates,
polystyrene sulfonate (PSS)/glass and PSS/ITO surfaces, respectively. The building up of
such multilayers was characterized by the increment of the adsorbed amount through
UV–visible spectroscopy and cyclic voltammetry. The atomic force microscopic study
showed the granular surface topology of such self-assembled films of sulfonated polyaniline.
PDDA/SPANI LBL films were electrically active and detailed electrochemical parameters
were investigated.
1. Introduction
The fabrication of ultra-thin supramolecular films of
conjugated polymers is a challenging task because the highly
conducting form of various materials is insoluble, infusible
and intractable [1, 2]. In the recent past emphasis has
been put on understanding the fabrication of conjugated
polymer film by various techniques, with special regard to
the possibility of controlling the coordinates of polymer
molecules within a nanoscopic scale, and building up welldefined
molecular structures [3–5]. Recently, Decher and
co-workers extended the pioneering work of Iler et al to a
new preparative method of organized thin films by layer-bylayer
(LBL) adsorption of linear polyions [6–8]. Alternate
adsorption of a polycation and a polyanion is readily achieved
by excessive adsorption of polyelectrolytes on oppositely
charged surfaces. The coulombic attraction between opposite
charges of polyanion and polycation is the driving force in the
multilayer build-up resulting in total coverage of the substrate
independent of the substrate size and surface topography [9].
This technique has several advantages: (1) the preparative
procedure is simple and an elaborate apparatus is not
required, (2) a large variety of water soluble polyions can
be used, (3) the individual layer has molecular thickness, and
(4) any charged surface can be used. Recently, LBL films
of charged polyelectrolytes (proteins, conducting polymers,
zirconium phosphate, optical dyes, alumino-silicates, and
clay), bola-amphiphiles and colloidal particles have been
fabricated [10–12]. Rubner and Skotheim [20] extended this
technique to manufacture thin films of conjugated polymers,
‖ Author for correspondence.
and since then the molecular-level fabrication of LBL films of
polypyrrole, polyaniline (PANI), poly(phenylene vinylene)
and poly(o-anisidine) conducting polymers are found in the
literature [13–18].
PANI has attracted considerable attention since
MacDiarmid et al reinvestigated this material, due to its
good environmental stability, adequate level of electrical
conductivity, and interesting electrochemical and optical
properties [19]. The excellent stability of PANI films in
both air and water, including the ease of transition between
the insulating and the conducting states with voltage, has
prompted researchers to use them in electrochromic display,
gas sensor, photovoltaic, battery, EMI shielding and other
microelectronic devices [19, 20]. Sulfonated polyaniline
(SPANI), a derivative of PANI, is of interest because of
its unusual physical properties, improved processibility and
potential industrial applications [21–23]. SPANI is the
first self-doped water-soluble conducting PANI derivative
and a prime model for dopant and secondary dopant
induced processibility in parent PANI-doped with HCl [24].
Recently, SPANI has been used in the fabrication of
multilayer heterostructure in light-emitting diode and the
electrochemical control of electrolyte acidity [25].
The objective of this work was the fabrication of SPANI
LBL film using polycation, poly(diallyldimethylammonium
chloride) (PDDA) and SPANI as polyanion. Further, an
attempt has been made at the deposition of LBL layers of
SPANI, used as polyanion as well as polycation at different
pH of the solution. The films were characterized by use of
UV–visible spectroscopy, cyclic voltammetry, atomic force
microscopy and electrical conductivity measurements. The
0957-4484/00/010030+07$30.00 © 2000 IOP Publishing Ltd

Nanoassemblies of sulfonated polyaniline multilayers
surface morphology of such bilayer films was investigated
using atomic force microscopy. The electrochemical kinetics
of such LBL films was also investigated by electrochemical
surveying.
2. Experimental details
2.1. Solution preparation and deposition
The emeraldine base form of PANI was synthesized as
reported in [26]. The emeraldine base powder was dried, and
sulfonated by dissolving in fuming sulfuric acid at 4–5 ◦ C
with constant stirring for 2 h. Later, this solution was added
drop-wise to methanol for the precipitation of the product
and the temperature was maintained between 10 and 20 ◦ C.
Precipitation was completed by the addition of acetone. The
green powder was collected on a Buchner funnel, and washed
repeatedly with methanol until the filtrate showed a value of
pH 7. The precipitate was dried under vacuum for 72 h [27].
Microscopic glass, indium–tin-oxide (ITO) coated
glass plates were used as substrate for the fabrication of
PDDA/SPANI multilayer films. Substrates were activated
following a procedure reported previously [16]. The first
layer of activated surface was deposited by sulfonated
polystyrene (PSS, M w = 70 000) solution for 15 min,
prepared by using 2 mg ml −1 of PSS in water, which provided
the charges necessary to adsorb the first layer of PDDA.
SPANI (0.1 g) was dissolved in 10 ml of 0.1 N NaOH solution
and diluted to 30 ml by distilled water. Then, this solution was
filtered to remove any trace of undissolved SPANI particles.
Later, this solution was adjusted to pH = 6 by the dropwise
addition of 1 M HCl solution. The polyelectrolyte
PDDA was purchased from Aldrich with M w = 200 000–
350 000, and used in an aqueous solution (2 mg ml −1 ). The
multilayer structure was fabricated by alternate dipping of
treated substrates in the PDDA and SPANI solution for 10 min
each by rigorous washing in a solution of pH 2, and dried
by nitrogen gas. The alternating layers of PDDA and SPANI
were also deposited onto various substrates, prior to one layer
deposition of PSS, polyanion. Such treated substrate was
used to build multilayers structures of SPANI and PDDA.
Later, SPANI solutions at pH = 5 as polycation, and at
pH = 10 as polyanion were used for the deposition of SPANI
LBL films on PSS-deposited glass or ITO-coated glass plates,
respectively. SPANI (0.1 g) was dissolved in 10 ml of 0.1 N
NaOH solution, and diluted to 30 ml by distilled water. Then,
this solution was filtered to remove any trace of undissolved
SPANI particles. This solution was divided into two parts,
one was adjusted to pH = 5 and other to pH = 10 by slow
addition of 1 M HCl. The pH 5 SPANI solution acted as
polycation and the pH 10 solution acted as polyanion during
deposition of multilayer films. Alternating layers of SPANI
(pH = 5) and SPANI (pH = 10) were deposited onto the
glass and ITO-coated glass plates, prior to the deposition of
one layer of PSS solution, by alternating submersions of the
film samples in the electrolyte solutions to build a multibilayer
structure. Between each deposition, the films were
washed with a solution of pH 2 using HCl acid and dried by
blowing nitrogen gas.
2.2. Optical measurements
The UV–visible spectra of LBL films deposited on the optical
glass substrates were recorded by using the UV–visible
spectrophotometer (Jasco model 7800).
2.3. Electrical and electrochemical measurements
The electrical characterization was performed using an
electrometer (Keithley model 6517). Current–voltage (I–
V ) characteristics were obtained by an applied potential
(step of 0.05 V). Similar interdigitated electrodes were used
for the electrical measurements [16]. The electrochemical
measurements on LBL deposited SPANI films on ITO-coated
glass plates were made by Potentiostat/Galvanostat (EG &
G PARC, model 263A) with a supplied software (M270).
A standard three-electrode configuration was used, where
PDDA/SPANI and SPANI (deposited at pH 5 and 10) films
on PSS/ITO-coated glass plates acted as a working electrode,
with platinum as a counter and Ag/AgCl as a reference
electrode.
2.4. Atomic force microscopy
The surface morphology of the SPANI/PDDA LBL films was
investigated by an atomic force microscope (AFM), which
was a home-built instrument (Polo Nazionale Bioelettronica),
working in contact mode in air at a constant contact force.
Our AFM was operated in air, at constant deflection (i.e.
vertical contact force) with triangular shaped gold-coated
Si 3 N 4 . The tips of the microlevers had standard aspect ratio
(about 1:1) and the levers had nominal force constant of
0.03 N m −1 . The constant force set point was about 0.1 nN,
while the images acquired were 256×256 pixel maps. During
the acquisition the row scanning frequency was set to 4 Hz,
i.e. a physical tip–sample motion speed of 8, 4, 2 µm s −1 in
the 2, 1, 0.5 µm scan size images, respectively. Some images
presented features that were saturated in the post-processing
redistribution of the available grey levels. Henceforth, it was
possible to observe the finest structure of the samples. The
images shown in this paper are representative of the samples,
as similar looking images appeared in four different regions
of the analysed samples, positioned at the vertices ofa4mm
side square, centred on the specimen [16, 28].
3. Results and discussion
3.1. UV–visible
Figure 1(a) shows the optical absorption spectra of
PDDA/SPANI deposited on a PSS/glass slide as a function
of the number of bilayers. As PDDA is not absorbing in
the considered spectral region, the UV–visible absorption
can only be emanating due to the SPANI layers. A typical
UV–visible absorption spectrum of SPANI has three distinct
absorption bands in the regions 340, 430–450 and 800–
900 nm. These spectra depict the features characteristic
of the SPANI forms, implying that the polymer is in the
protonated form. The band at 340 is attributed to the
π–π ∗ band-gap absorption and 430–450 and 800–900 are
due to the protonation of SPANI. The constant absorbance
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N Sarkar et al
Figure 2. Plot of absorbance at 340 nm for SPANI in
PDDA/SPANI LBL films as a function of layers. Plot of
absorbance at 340 nm for SPANI deposited on glass substrates as
polyanion and as polycation.
Figure 1. (a) The optical spectra of PDDA/SPANI films as a
function of number of bilayers made in 6 pH and washed by a
solution of pH 2 using HCl, namely (1) 2 bilayer, (2) 5 bilayers
(3) 10 bilayers, (4) 5 bilayers, (5) 20 bilayers. (b) Plot of
UV–visible absorbance at 340 nm versus time of deposition for the
SPANI deposited on PDDA surface at pH 6, and washed by using
a solution of pH 2.
magnitude change for each layer is indicative of the uniform
deposition of SPANI/PDDA. The adsorption time on PDDA
is an important parameter in obtaining a complete layer of
SPANI on PDDA surface. Figure 1(b) shows the absorption
magnitude at 340 nm of SPANI on PDDA surface as a
function of time. It reveals a deposition of SPANI on the
PDDA/PSS surface by simply immersing in the solution, but
a complete layer of SPANI required an immersion period of
5–6 min. A small increase in the absorption magnitude was
noticed for a period of 10 min of immersion, while saturation
in the absorption was obtained at an exposure time of 60 min.
In the light of these results each layer of sulfonated SPANI
was deposited for an exposure time of 5 min in the resulting
solution.
The deposition of PDDA/SPANI bilayers was monitored
by the change in absorption magnitude at 340 nm as shown
in figure 2 (curve 1). The linear variation in absorbance
versus the number of bilayers indicates a regular deposition
of PDDA/SPANI LBL films. The deposited bilayers films
were undoped using 0.1 M NaOH solution, and the UV–
visible spectra were registered. A linearity in absorption
magnitude at 330 nm versus the number of bilayers was
observed (figure not shown). The UV–visible spectra of
SPANI bilayers deposited when used as polyanion and
polycation were performed. Figure 2 (curve 2) reveals a
linearity for the absorption magnitude at 340 nm versus the
number of bilayers but shows a smaller optical absorption
magnitude than for the PDDA/SPANI LBL films. Such
variation in absorption magnitude could be due to a smaller
transfer of the material for the SPANI used as polycation and
polyanion. The UV–visible studies suggested that films could
(a)
(b)
Figure 3. AFM images of PDDA/SPANI LBL films deposited on
glass substrates as a function of bilayers, namely: (a) 5 bilayers
and (b) 20 bilayers of 250 nm × 250 nm in size.
be better formed by selection of polycation (PDDA) for LBL
deposition of SPANI. So, further studies were performed in
PDDA/SPANI bilayers films.
3.2. AFM observations
The morphology of multilayer films of SPANI/PDDA on
glass surface was investigated by AFM as shown in figure 3.
32

Nanoassemblies of sulfonated polyaniline multilayers
to that in figure 3(a). The height of the 5 bilayers of
PDDA/SPANI LBL film was observed to vary between 20
to 10 nm.
3.3. Electrical and electrochemical properties
(a)
(b)
Figure 4. Three-dimensional images of AFM of 5 bilayers,
PDDA/SPANI LBL, films deposited on glass substrate: (a) The
image dimension is 2 × 2 µm 2 in size. (b) The image dimension is
500 × 500 nm 2 in size.
In order to evaluate the morphological evolution of a SPANI,
we undertook the AFM measurements on PDDA/SPANI
structure as a function of bilayers. The AFM images in
figure 3 present the surface topography of 250 × 250 nm 2
dimension of 5 and 20 bilayers of PDDA/SPANI LBL
films. These pictures reveal that the topological surfaces
of PDDA/SPANI bilayer films are covered by particles size
varying from 30 to 62.5 nm in diameter. The horizontal and
vertical radii measured for the marked grain in figure 3(a)
are measured as 32.5 and 33.2 nm, whereas figure 3(b)
shows horizontal and vertical radii as 30 and 16 nm,
respectively. The RMS values of each AFM image of
5 bilayers and 20 bilayers have been calculated as 7.4 and
5.1 nm. Interestingly, variation in size can be observed in 5-
bilayer PDDA/SPANI films, whereas 20-bilayer films show
nearly equal grain size and compact film. The obtained
thickness and size of the grains are greater than those
attributed to true PANI bilayers [16]. The distribution of
the grain size decreases with increase in number of bilayers
in figure 3 which is in contrary to the results of Langmuir–
Blodgett or LBL films of PANI [29]. The size of the grains
depend upon the nature of SPANI molecules in LBL films.
Figure 4 shows two three-dimensional images of 5-bilayer
PDDA/SPANI LBL film of 2 × 2 µm 2 and 500 × 500 nm 2
in size. These images show a variation in the particle similar
The electrical conductivity of PDDA/SPANI was measured
to be 10 −3 Scm −1 . The films undoped in 0.1 M NaOH
solution showed a decrease in conductivity value to 0.75 ×
10 −9 Scm −1 (figure not shown).
The cyclic voltammogram of PDDA/SPANI LBL films
in 0.1 M HCl electrolyte solution are shown as a function of
bilayers in figure 5(a). The peaks of cyclic voltammograms
are associated to redox processes of SPANI films. The inset
in figure 5(a) reveals the linearity in the peak current as
a function of bilayers. The oxidation peak potentials are
observed at 664.2, 455.3, 179.3 mV, whereas the reduction
peak potentials are attributed at 580.8, 430.3 and 31.38 mV,
respectively. The variation in the separation between any two
redox potentials could be the consequence of the steric effect
associated with the bulky sulfonic substituents and a higher
S/N ratio for SPANI as compared to the emeraldine base
form of PANI. An alternative interpretation could be that the
sulfonic acid group attached to phenyl rings lowers the energy
level of the valence band or the highest occupied molecular
orbital in the polymer molecule increasing the band gap
between the conduction and valence bands. Therefore,
more energy is needed to remove an electron from the
polymer chain and consequently the half-width potential
(E 1/2 ) increases in PDDA/SPANI LBL films. The cyclic
voltammogram of SPANI is similar to PANI [16, 28]. The
reduced state of SPANI contains fixed negative charges or
is compensated by mobile cations when oxidation of the
negative charges are used to compensate the formed radical
cations where mobile cations are expelled. Figure 5(b)
shows the cyclic voltammogram of 5-bilayer PDDA/SPANI
LBL films on ITO glass plates at different scan rates. The
PDDA/SPANI multilayers films show a diffusion-controlled
system similar to PANI [16]. Other interesting features
such as the electrochromic effect (colour from faded yellow
to violet) of the films was also observed by sweeping the
potential from −0.2 to 0.8 V for PDDA/SPANI films.
The redox kinetics process in PDDA/SPANI films was
studied in HCl, H 2 SO 4 to CH 3 COOH media as shown
in figure 6(a). The redox changes are associated to
variations in electronic resonant structure of the polymer
backbone caused by oxidation and protonation forms of
SPANI conducting polymer. The overall process involves
loss of two electrons and deprotonation for SPANI films. The
redox processes and lower switching of SPANI conducting
polymer may be dependent on the redox ionic conductivity of
the polymer matrix. It was shown that HCl ions show a faster
electrochromic effect (switching half-time H 1/2
f
= 200 ms)
than that found in H 2 SO 4 (H 1/2
f
= 220 ms) and CH 3 COOH
(H 1/2
f
= 420 ms). Table 1 shows the electrochemical
parameters of the LBL films in different acid media. The
redox processes in CH 3 COOH acid are slower than that of
H 2 SO 4 or HCl acids. The weak acid also has longer diffusion
time. Later, the lifetime of the SPANI films were estimated
by applying the pulse continuously at 100 mV s −1 . The CV
33

N Sarkar et al
Figure 5. (a) Cyclic voltammogram of PDDA/SPANI LBL films
on glass ITO coated plate in 0.1 M HCl acid medium as a function
of bilayers deposited, namely: (1) 5 layers, (2) 10 layers,
(3) 15 layers and (4) 20 layers. Inset shows the variation of peaks
potential (180 mV) versus number of bilayers. (b) Cyclic
voltammograms of 5 bilayers PDDA/SPANI LBL films on ITO
glass plates at different scan rate: (1) 2 mV s −1 ,(2)5mVs −1 ,
(3) 10 mV s −1 , (4) 20 mV s −1 , (5) 50 mV s −1 , (6) 100 mV s −1 .
was recorded in figure 6(b) to understand the mechanism
and lifetime of SPANI films. There is a change in the peak
potential after 10 4 as well as 10 5 cycles but the shape of the
CV always follows the Nernst equation (closed and reversible
to 3×10 5 cycles). It closely follows the shape of CV after 10 5
cycles, which diffuses ions and maintains the electroactivity
in close comparison to electrochemical films. The lifetime of
sulfonated films was observed to be greater than 10 5 cycles.
Figure 7 shows the redox current versus time for
15 bilayers of PDDA/SPANI film. It reveals to nearly equal
oxidation and reduction system, when the potential is swept
from −0.2 to 0.9 V. The redox changes are associated with
the electronic resonant structure of the polymer backbone
caused by the oxidation and reduction processes of the
SPANI films. The redox switching of the SPANI conducting
polymer may be dependent on the redox ionic conductivity
of the polymer matrix. Figure 7 (inset) shows the plot of
oxidation current versus time and reduction current versus
time plot for 15 bilayers SPANI films. The slope obtained for
SPANI shows that redox processes are diffusion controlled.
The value of the slope was obtained to be 0.64 and 0.57.
To investigate the ion diffusion process on switching time,
we considered the current transient for the redox switching
process. The diffusion coefficient was estimated by using the
Bulfer–Volmer equation, where two electrons participated in
the reaction mechanism of the PANI system. The diffusion
coefficient for the redox process was estimated to be 1.2 ×
10 −8 cm 2 s −1 . The oxidation and reduction response time
Figure 6. (a) Plot of square of scan rate versus current for the
5 bilayers, PDDA/SPANI LBL, films in (1) 0.1 M H 2 SO 4 ,
(2) 0.1 M HCl and (3) 0.1 M CH 3 COOH media. (b) CVs of
15 bilayers of PDDA/SPANI LBL films in 0.1 M HCl acid at
50 mV s −1 as a function of scanning cycle, namely: (1) 1st cycle,
(2) 10 2 cycles, (3) 10 3 cycles, (4) 10 4 cycles, (5) 10 5 cycles and
(6) 10 6 cycles.
Figure 7. Redox current response of 15 bilayers of PDDA/SPANI
LBL films in 0.1 M HCl acid medium. Inset shows the plot of
fitting for the oxidation and reduction current.
was found to be 200 ms. Similar studies of oxidation
and reduction were performed for each PANI system, and
the response time is given in table 2. Interestingly, the
response time of SPANI was found to be greater than the
other substituted PANI [28].
4. Conclusion
LBL films of PDDA/SPANI and SPANI (pH 5)/SPANI
(pH 10) were fabricated. The LBL multilayers of conducting
34

Table 1. Electrochemical parameters of PDDA/SPANI films.
Diffusion
Oxidation Reduction coefficients D 0
Acidic media potential (mV) potential (mV) Slope in cm 2 s −1
HCl 664.2, 455.3, 179.3 664.2, 455.3, 179.3 5.53 1.2 × 10 −8
H 2 SO 4 630, 228.1 563.6, 89.3 5.125 0.92 × 10 −8
CH 3 COOH 381.9, 124 328.6, 15.45, 2.7 4.061 0.75 × 10 −9
Nanoassemblies of sulfonated polyaniline multilayers
Table 2. CV of 15 bilayers of PDDA/SPANI films.
After number of cycles Oxidation potential (mV) Reduction potential (mV)
2nd 689.7, 161.3 588.3, 8.33
2 × 10 2 677.9, 494.9, 186.9 565.1, 17.66
10 3 438.5, 315 359.2, 162.5
10 4 369.9, 28.09 334.5, 133.5
10 5 414.0 87.8, −37.24
10 6 246.4 −32.67
polymer were monitored by UV–visible spectroscopy. The
linear variation in the PDDA/SPANI revealed a uniform
deposition process, whereas the SPANI used as polyanion and
polycation did not give a uniform film because the dipping
solution was found to change the structure of polyanion
or polycation SPANI. The nucleation and growth of the
deposited films were investigated by AFM, showing a surface
topology of PDDA/SPANI bilayers films covering particle
size varying from 30 to 62.5 nm in diameter. The distribution
of the grain size decreases with increase in the number
of bilayers which is contrary to the results of Langmuir–
Blodgett or LBL films of PANI. The redox processes in
CH 3 COOH medium are slower than in H 2 SO 4 or HCl media.
The weak acid takes longer diffusion time in the SPANI
films. The diffusion coefficient (D 0 ) was calculated to
be 1.2 × 10 −8 cm 2 s −1 . Typical multilayer films reveal
a conductivity range from 10 −3 to 0.75 × 10 −9 Scm −1 .
This investigation allowed us to fabricate thin films of
SPANI in a controlled way and we look forward to use
such a structure in photovoltaic and light-emitting diode
devices. We are also investigating the structural properties of
bilayer PDDA/SPANI LBL films to address the technological
application of conjugated polymers.
Acknowledgments
We are grateful to M Panza for his help in carrying out the experiments.
Financial support from Fondazione Elba (Chapter
2102 of Ministry of Universities and Research of Italy) and a
Murst-PST contract on Neural Net Work with Polo Nazionale
Bioelettronica is gratefully acknowledged. This paper was
presented as a poster in the Elba Foresight Conference on
Nanotechnology in Rome on April 14–17, 1999.
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