Materials Science s a nd Applications, 2011, 2, 1661-1666
doi:10.4236/msa.2011.211221 Published Online November 2011 (
Copyright © 2011 SciRes. MSA
Morphological Structure Characterization of
PAH/NiTsPc Multilayer Nanostructured Films
Josmary R. Silva1, Jac keli ne B. B rito1, Sonia T. Tanimoto2, Nara C. de Souza1*
1Grupo de Materiais Nanoestruturados, Universidade Federal de Mato Grosso, Barra do Garças, Brazil; 2Centro de Ciências Naturais
e Humanas, Universidade Federal do ABC, Santo André, Brazil.
Email: *
Received August 15th, 2011; revised September 30th, 2011; accepted October 11th, 2011.
Morphological structure and growth process of LbL nanostructured films from nickel tetrasulfonated phthalocyanine
(NiTsPc) alternated with polyallylamine hydrochloride (PAH) were investigated. The experimental results of UV-visible
adsorption kinetics (modeled by Johnson-Mehl-Avrami functions) and AFM images analyzes suggested that the surface
morphology structure of films is formed by rod-shaped aggregates produced by a two-step growth process: nucleation
and diffusion-limited growth.
Keywords: Phthalocyanine, Layer-by-Layer Films, Growth Processes, Adsorption Kinetics
1. Introduction
Phthalocyanines are interesting due to their properties
and possible applications [1-3]. As this type of com-
pound is hi ghly conjugated [4], it has interesting electric
and photoelectric properties. Neutral phthalocyanines are
insoluble in polar solvents; however, this can be changed
by substituting the benzene rings with polar compounds.
Several techniques have been used to produce phthalo-
cyanine films, such as evaporation deposition [5-7], mo-
lecular beam deposition [8], Langmuir-Blodgett [9-13],
and self-assembly layer-by-layer (LbL) [14-18]. The lat-
ter one is possible because the sulfonation of the ring of
phthalocyanine resulting in tetrasulfonated phthalocya-
nines, which are more soluble in polar solvents, such as
water, and thus allows their application in the production
of films by the LbL technique.
The research on LbL films has been performed focu-
sing on the effects from char ge density o f the molec ules in
solution [16-20], drying procedure, solution ionic strength,
solvent nature, and dipping time on the growth charac-
teristics of the multilayers [17]. With regard to the struc-
ture of the films, important findings were the interpene-
tration of successive polyelectrolyte layers [21], and the
preferential orientation of polymer chains perpendicu-
larly to the interfaces [22]. In particular, the dc conduc-
tion processes in LbL films from phthalocyanines was
investigated in the early days due to its important role in
technological applications, such as light emission diode.
For instance, Promnimit et al. [19] investigated the cur-
rent-voltage characteristics in the forward and reverse bias
conditions demonstrated rectifying behaviors i n the onset
of co nd ucti o n vol ta ge , which make s t he se fil ms a ttr ac ti ve
for future electronic devices. The surface morphology of
films is another interesting property to be investigated
because the film/electrode interfaces are found in all
devices and play an important role in their operation [20].
The surface morphology structure of LbL films is de-
terminated by the processes responsible for the growth of
the layers and is strongly dependent on the preparation
conditions [21].
Although there are several studies on LbL films from
NiTsPc [22-25], only a few reports on the surface mor-
phology structure have been carried out [26-28]. In this
work, we have prepared LbL films from NiTsPc alter-
nated with PAH and investigated their surface morpholo-
gy structure and the mechanisms o f the adsorptio n pr o c e ss
associated to it. From adsorption kinetics experiments
and AFM analyses, we have found that films are formed
by rod-shaped aggregates promoted by nucleation and
diffusion-limited growth.
2. Materials and Methods
Nickel tetrasulfonated phthalocyanine (NiTsPc) and poly
(allylamine hydrochloride) (PAH) were acquired from Al-
drich and used as supplied. NiTsPc and PAH solutions
were prepared as follows: 0.05 g of NiTsPc or PAH were
Morphological Structure Characterization of PAH/NiTsPc Multilayer Nanostructured Films
diluted in 100 mL of ultrapure Milli-Q water (resistivity
of 18 The mixtures were stirred for 1 h and the
solutions were completely clear. The pH was adjusted to
7.5 by adding appropriate amounts of 1 M NH4OH. The
experimental procedures for film fabrication were essen-
tially the same as those described by Decher et al. [29].
To build multilayers, PAH (cationic solution) was alter-
nated with NiTsPc (anionic solution). The films were
rinsed with an aqueous solution with pH adjusted to 7.5
with 1 M NH4OH. After that, they were adsorbed onto
BK7 optical glass (36 mm × 14 mm × 1 mm) rendered
hydrophilic in 3:7 hydrogen peroxide (H2O2)/concen-
trated sulfuric acid (H2SO4). The slides were then rinsed
with pure water and further cleaned in a solution con-
taining 5:1:1 (v:v:v) ultrapure water, H2O2, and am-
monium hydroxide (NH4OH). BK7 glass was chosen
due to its negligible absorbance in the visible region
and its nicely polished surface. NiTsPc adsorption was
monitored by measuring the UV-Vis spectra with a
double-beam spectrophotometer (Hitachi U2001). Sur-
face morphology was investigated using a atomic force
microscope (AFM) from Topometrix. 512 512-pixel
images were obtained in tapping mode in ambient con-
3. Results and Discussion
In order to investigate the morphological structure of sur-
face and the growth processes of self-assembled PAH/
NiTsPc films, we have performed adsorption kinetics
experiments with bilayers, which use distinct substrates.
It is well kno wn that any study of adsorption kinetic s by
UV-Vis spectrocopy requires the knowledge of the ab-
sorbance peak of material. As reported else-where [30]
NiTsPc in solution displays two bands in its UV-Vis
spectrum, peaking at 620 nm (Q-band) and 330 nm (Soret
band) [30]. The peak around 620 nm is associated with
dimers and the shoulder in the 620 - 645 nm region is
associated with the NiTsPc monomers [31]. Both adsorp-
tions are attributed to -* electronic transitions. These
bands are also observed in our LbL films from PAH/
NiTsPc with absorbance maximum shifts to higher ener-
gy values. In our work, the absorbance at 615 nm (
was used to monitor the building of multilayers, since
PAH does not adsorb in this wavelength [32]. The expe-
rimental procedures to obtain the films are essentially the
same as those found in our latest work [32].
The amount of mass adsorbed per area unit can be es-
timated using the Beer-Lambert la w from the absorbances
and taking into consideration the two faces of the substrate
[32]. The immersion time of the substrate in the NiTsPc
solution varied during the films growth from 5 to 100 s on
PAH layers (3 minutes for all film layers). Figure 1 dis-
plays the adsorbed mass amount of NiTsPc per bi-layer
0 20406080100
Adsorbed amount per bilayer (mg/m2)
Immersion time (s)
Figure 1. Adsorbed mass amount of NiTsPc per bilayer as a
function of the immersion time. The points were obtained
from the growth curve slope of the films for variable im-
mersion times. The full curve is the fitting with the JMA
versus the accumulated immersion time. It is noted that
the adsorbed amount of mass increases with the increase
in time and exhibited a saturation plateau.
The experimental curve showed in Figure 1 was fitte d
by using the Johnson-Mehl-Avrami-type (JMA) second-
order function:
AK eKe
 , (1)
where A is the absorbance (taken as proportional to the
amount of adsorbed material) [33], K1, K2,
1, and
2 are
constants, and n is the Avrami exponent. This model
gives a phenomenological description of the adsorption
kinetics without revealing molecular details; it has been
used to explain gro wth processes due to its mathematical
simplicity and proper description of the experimental
data of polymeric systems [34]. The results of the phe-
nome no log ica l a nal ysi s of t he gro wth o f t he P AH/ NiT sP c
LbL films employing the JMA model were summarized
in Table 1. The proportionality constants K1 and K2 co-
rrespond to the adsorbed mass amount of NiTsPc per
area unit (K1 + K2).
From the JMA analysis, we can say that the process of
formation of the films (growth) results from two mecha-
nisms: one first extremely fast , with constant time
1, that
Table 1. Parameters obtained from the JMA fitting of
adsorbed mass amount pe r bilayer.
1 (s) K2
2 (s) n
PAH/NiTsPc 11 35 2 5 1 31 31
Copyright © 2011 SciRes. MSA
Morphological Structure Characterization of PAH/NiTsPc Multilayer Nanostructured Films
Copyright © 2011 SciRes. MSA
may be associated with a nucleation, and a slow one with
time constant
2, associated with a diffusion-controlled
growth. A t wo-stage a d sorp tion proc ess was also pro posed
by Raposo [34] and Tsukruk [35]. It is possible to use the
Avrami parameter n to distinguish between growth on the
interface or by diffusion. The parameter value of Avrami,
n = 1, indicates needle-shaped or rod-shaped structures
(1D adsorption) produced by diffusion controlled growth
and without the formation of new nuclei during adsorp-
tion [21,36,37].
Due to the sensibility of the parameter n, it is difficult
to identify the process without additional information on
the film growth [33], which can be obtained from AFM
image analysis. To corroborate the adsorption kinetics
results, morphological investigation of the 10-bilayer PAH/
NiTsPc films were carried out with layers of NiTsPc ad-
sorbed for 3 min. Figure 2 shows the image of the sur-
face of PAH/NiTsPc film (NiTsPc on top), which ex-
hibits rod-shaped aggregates and a schematic model pro-
posed to its structure. This finding is consistent with the
previous adsorption kinetics results [38,39]. We can see
the film forming units, which initially seem small “lying”
rods that can be due to aggregation of adjacent NiTsPc
rings in the films, which are driving by intermolecular for-
ces [40,41].
Figure 3 (log-log) shows that the heights of aggre-
gates increases when their diameters decreases. The data
were taken at distinct cross sections of the images and
the error bars included in the inset are average values of
several scans. The inset shows three differents height
profile. As noted, there is a preferential vertical growth,
unlike from films prepared with the polydisperse POMA
[21] and PANI [42], which show globular aggregates
with no preferential growth direction. This kind of struc-
ture had already been identified by Nishida et al. [43] for
phthal ocyani ne fi lms o nto gol d surfac es b y STM dat a. In
fact, our result corroborates the aggregate growth as co-
lumns suggested by the previous results of adsorption
kinetics and parameters of Avrami.
The growth process of NiTsPc in LbL films starts with
nuclei formed in the beginning of the adsorption. The
adsorption process is the result of compromise between
the energy factors, such as electrostatic forces, Van der
Waals forces, hydrogen bonds, solvent quality, and en-
tropic factors. The adsorption of polyelectrolytes on char-
ged surface can be described by a rapid adsorption fo-
llowed by a relatively long period in which the chains are
being rearranged. In the first stage, the molecules adsorb
on the surface while keeping its conformation in solution.
When substrates are immersed in the solution of NiTsPc,
immediately some molecules are adsorbed (according to
the results of kinetics). The interaction of chains by hy-
drogen bonds favors the formation of aggregates. These
aggregates tend to carry their connection with the surface
increasing the number of segments attached to the sub-
strate, i.e. a rea r r an gemen t of m olecu les.
4. Conclusions
Nickel tetrasulfonated phthalocyanine has been used to-
gether with PAH to produce LbL films. The morpho-
Figure 2. AFM image of a 10-bilayer PAH/NiTsPc film whose layers were adsorbed for 3 min using a NiTsPc solution. The
scanning window s ize is 2 m × 2 m. The scheme show s the NiTsP c molecule which forms the aggregates.
Morphological Structure Characterization of PAH/NiTsPc Multilayer Nanostructured Films
Figure 3. Dia meter vers us height of ag greg ates of t he PA H/Ni TsP c fil m. The inset s s how the full cu rve d raw n on t he i mage o f
the surface of the film, which identifies the rod-shaped structures and the height profile of the films in three distinct
cross-sections of the image (the arrow indicates a column correspo nding to a surface forming unit).
logical structure of the films was investigated by using a
combined method of the adsorption kinetics curves, mo-
deled with JMA functions, and the behavior of diameter
versus height of aggregates obtained from AFM images.
This approach indicated that the aggregates are rod-sha-
ped, which are produced by an adsorption process with
two mechanisms: nucleation in the first sta ge (
1 5 - 10
s) and diffusion-limited growth in another. Since NiTsPc
is a semiconductor organic with applications in sensors,
our results may be used as a starting point to future re-
search on devices and bring out new insights on the na-
nostructured films from thi s kind of mate rial.
5. Acknowledgements
This work was supported by CNPq and Capes (Brazil).
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