polypyrrole nanotubes prepared by different...

12. - 14. 10. 2010, Olomouc, Czech Republic, EU

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POLYPYRROLE NANOTUBES PREPARED BY DIFFERENT AZO-DYES

Jitka ŠKODOVÁ, Dušan KOPECKÝ

Department of Physics and Measurements, Institute of Chemical Technology in Prague, Technická 5,166 28 Praque 6,Czech Republic, [email protected]

Abstract

This contribution deals with synthesis and characterization of conductive nanotubes suitable for preparation

of active layers of chemical gas sensors. Polypyrrole nanotubes were synthesized by chemical

polymerization using soft-template method. Soft-templates were created by various azo dyes (methyl orange,

methyl red, congo red, Acid RED 1). The aim was to clarify the mechanism of nanotubes formation using

chemically and structurally similar molecules of azo dyes. The effect of different synthesis conditions

(temperature, reaction time) on geometry of nanotubes was also investigated. Morphology of prepared

structured and unstructured polypyrrole (PPY) was observed by scanning electron microscope (SEM). There

were also determined geometric dimensions of prepared nanotubes by means of image analysis (width ~

hundreds of nanometers, length ~ units of micrometers).

Keywords: Polypyrrole, Nanotubes, Soft-template method, Azo dyes

1. INTRODUCTION

Conductive polymers (CP) are very popular for many scientific teams for more than 35 years, since their

rediscovery in the 1970s. The reason is their high electrical conductivity (up to 105 S·cm-1 [1]), which is

typical mainly for inorganic substances, combined with mechanical properties and flexibility of organic

materials. Conductive polymers have been used in many experimental and practically usable applications,

such as: biosensors [2], sensors [3], packaging [4], organic transistors [5], supercapacitors [6], batteries [7],

surface coating [8] or anti-static envelope [9].

An innovation in CP is finding that under certain conditions are able to create uniform structured shapes with

nanometers sizes (nanostructures). In particular, nanowires, nanorods or nanofibers are interesting. They

are often collectively called 1-D structures of conductive polymers. The description is based on the widely

used simplification that assumes that structural properties of 1-D polymer structure are predominantly

determined by the longest dimension (which is always orders of magnitude larger than the other two). The

fundamental advantage of these structures is their high specific surface. In the context of sensor technology,

on which this research is focused, the 1-D structured CP are expected to offer mainly one advantage. We

expect that polymer sensors would respond faster to the detected gas because of its easier diffusion into the

polymer material volume and higher number of active centers for the production of donor-acceptor complex

between the reference gas and material. Higher sensor sensitivity would thereby achieved.

Noticeable advantage of 1-D structured CP over their unstructured counterparts is also a higher electrical

conductivity and the fact that preparation of CP with 1-D structures, using advanced methods of synthesis is

relatively easy.

Synthesis of 1-D structured polypyrrole (PPY) may be performed using template or special template free

procedures.

There are two means of template synthesis, so-called hard and soft template method. The hard-template

method uses zeolites and membranes as a hard template. After the synthesis, this form cannot be easily


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removed without damaging the 1-D structure and so hard template stays in the solution. Methods using hard

template are not the subject of this contribution. Their detailed description may be found in [10]. Much

information about template free synthesis such as interfacial polymerization, radiolytic synthesis or

electrochemical approach can be found in [11].

Soft-template method is currently being intensively studied. It is used in the experimental part of this

contribution. This method involves the creation of support structures made of auxiliary materials that are

used to route the procedure of polymerization in the synthesis of CP. Soft-template method, unlike previous

methods, has certain advantages: (i) change in temperature of synthesis, time of synthesis or molar ratio

of reactants may influence the geometric dimension of the prepared 1-D structures, (ii) it is effective, cheap

and simple method, (iii) template autonomously degrades after the reaction is over and can therefore be

easily removed from the solution [12].

Basically there are two methods of preparation of 1-D structured CP with the soft template. The first method

uses surfactant as the soft template. Surfactant is a substance characterized by a non-polar long-chain

ending in a polar group. In a non-polar environment in the presence of an oxidant, surfactant molecules

create reverse micelles (hydrophobic part of the molecule is oriented to the non-polar environment and

hydrophilic part is hidden inside the micelles). Reverse micelles create hollow cylindrical shapes in this

environment. After adding the monomer to the solution with soft template, polymerization begins inside this

soft template. The polymer nanotubes are created. When preparing the 1-D polymer nanotubes, the most

often used surfactant is bis(2-ethylhexyl)sulfosuccinate (AOT) (Fig.1.) [2,13].

Fig. 1. Structure of AOT

The second and newer method uses wires of complex created by azo-dye and oxidant as soft-template [12].

One example of these azo-dyes able to form complex is methyl orange [12] (Fig.2a). Azo-dye-oxidant

complexes create fibrous structures. After addition of the monomer, 1-D structure of the polymer form on the

surface of the fibers, which gradually degrade during reaction. Therefore, azo-dye-oxidant complex soft-

template is sometimes called self-degraded method. 1-D polymer structures are created on the surface

of soft template. After polymerization remaining degraded soft template must be removed from 1-D polymer

structures by long-washing, suitable is Soxhlet extraction.

The aim of this contribution was: to explore this new way in the synthesis of 1-D structures of polypyrrole

(PPY), to determine the dependence of geometric dimensions of prepared nanotubes at a temperature of

synthesis and test four different azo-dyes for preparation of 1-D structured PPY – Methyl Orange, Methyl

Red, Congo Red, Acid RED 1 (Fig. 2a,b,c,d).


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Methyl orange

Methyl red

Acid RED 1

Kongo red

Fig. 2. Structure of a) methyl orange, b) methyl red, c) Acid RED 1, d) kongo red

2. EXPERIMENTAL

Pyrrole, FeCl3, Acid RED 1, Methyl Orange, Methyl Red were purchased from Sigma-Aldrich and were used

without any modifications.

Typical reaction proceeded as follows: in 1.62 g (10 mM) FeCl3 was dissolved in 200 ml of 5 mM solution of

azo-dye and deionized water. Azo-dyes, which were used, are on the figure 2. Molar ratio of reactant of

reactants monomer : oxidant : azo-dye was 10:10:1 for all reactions. 0.7 ml pyrrole was added dropwise in

the first two hours of synthesis to the tempered solution. The mixture was stirring during synthesis of a

constant speed. Information about synthesis are given in Tab. 1. Prepared PPY 1-D structures were washed

with deionized water and ethanol. Due to complex structure of prepared PPY restraining the remnants of the

template was necessary to use long-washing. The best washing method seemed Soxhlet extraction. The

mixture was extracted with methanol until extraction reagent was colorless. Prepared PPY structures were

dried at 22 °C.

For comparison, there was prepared unstructured PPY from pyrrole (1 mM) and oxidant FeCl3 (1mM) in

aqueous environment.

Table 1

Conditions of synthesis

Reaction Azo-dye Temperature of synthesis (°C) Time of synthesis (h)

A Methyl red 5 24

B Congo red 5 24

C Acid RED 1 5 24

D Methyl orange 5 24

E Methyl orange 45 24

F Methyl orange 90 8

c) a) b)


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Prepared PPY 1-D structures were dispersed in small amount of deionized water using ultrasound. A small

amount of each solution was placed on a microscopic covering glass and then observed using the scanning

electron microscope (SEM) JEOL model JSM-7500F.

3. RESULT AND DISSCUSION

Figure 3 shows SEM image of unstructured PPY which was prepared by standard chemical polymerization.

The structure of this PPY has characteristic fruticose formations that create highly disordered system.

Fig. 3. SEM image of unstructured polypyrrole

Figure 4 shows SEM images of structured PPY prepared by the template made up of different azo-dyes.

Fig. 4. SEM images of structured polypyrrole prepared from a) methyl orange, b) methyl red, c) congo red, d) Acid RED 1

It is apparent that used azo-days in all cases significantly affect the structure of the prepared PPY, but only

in two cases, 1-D structure was formed. Best result was achieved using methyl orange at low temperature of

synthesis of up to 50 °C (Fig. 4a). Nanowires are relatively uniform without significant defects; their sizes are

approximately 80 nm diameter and hundreds of nm length. 1-D structure also appeared in the volume

of PPY synthesized in the presence of Acid RED 1. Nanowires are often deformed and without significant

transitions in 1-D structures.


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Another character has PPY synthesized in the presence of methyl red and congo red. Using methyl red,

round fruticose units were formed, their structure is similar to PPY on the figure 3. Using congo red, solid

PPY material creates thin scales.

The two last mentioned azo-dyes are thus proved to be unsuitable for preparation of 1-D structure of PPY.

The reason can be incapability of the two azo-dyes create a sufficiently large oligomeric structures when is

used given concentration, which would create usable template.

The synthesis using methyl orange showed the best results. Therefore, it has been tested for the effect of

temperature on the synthesis nanowires generated. Literatures suggest that 1-D polymer structures formed

at lower temperatures [14,15]. It was found that 1-D structures formed at higher temperature too. Figure 5

shows nanowires which were prepared at 45 °C. With increasing temperature of synthesis, however, occurs

formation of defects. At high temperatures around 90 °C structures completely disappear and there is

compact porous mass.

Fig. 5. SEM images of PPY prepared at a) 5, b) 45, c) 90 °C

4. CONCLUSIONS

The contribution deals with preparation of 1-D structured PPY by chemical synthesis in presence of azo-dyes

which form soft matrix (methyl orange, methyl red, congo red, Acid RED 1). It was found that under similar

conditions synthesis using methyl orange has the best results. It is necessary to carry out additional research

for the other azo-dyes which would see the limit concentration of these substances necessary for creating a

sufficiently large functional nanostructures template. The research of the temperature dependence showed

that 1-D structures can be prepared at elevated temperatures up to 50 °C.

ACKNOWLEDGEMENTS

This work was supported from specific university research MSMT no. 21/2010 and by the Ministry of

Education of the Czech Republic – project MSM 604 613 7306.

LITERATURE

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Interface Science, 2010, nr. 341, page 1


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page C655.

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