Nanoassemblies of sulfonated polyaniline multilayers - ARPAL

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
31