Morphological Structure Characterization of PAH/NiTsPc Multilayer Nanostructured Films

doi:10.4236/msa.2011.211221

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Materials Science s a nd Applications, 2011, 2, 1661-1666

doi:10.4236/msa.2011.211221 Published Online November 2011 (http://www.SciRP.org/journal/msa)

1661

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: *[email protected]

Received August 15th, 2011; revised September 30th, 2011; accepted October 11th, 2011.

ABSTRACT

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

1662

diluted in 100 mL of ultrapure Milli-Q water (resistivity

of 18 M.cm). 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-

ditions.

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 (



max)

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

equation.

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

Morphological Structure Characterization of PAH/NiTsPc Multilayer Nanostructured Films

1663

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

1664

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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