A New Technique of Electro-Synthesis of Polypyrrole Nanoparticles

Abstract

We report here a novel two phase electro-oxidation technique of synthesis of submicron size polypyrrole (ppy) from pyrrole (py) onto a steel electrode.  Pyrrole forms a hydrogen bonded red colour complex with picric acid which is almost insoluble in water but soluble in di-chloromethane (DCM). Again, water and DCM forms an immiscible solvent pair. We demonstrated that nano sized polypyrrole is deposited on to a steel anode coupled with a Pt cathode by passing a small current of 0.25 mAcm^-2 for a period of 600 seconds when electro-oxidation is carried out in aqueous saturated picric acid phase placed over  DCM phase containing a solution of pyrrole-picrate complex.  TEM picture shows formation of nano sized polymers. This is probably due to slow diffusion of pyrrole- picrate complex from organic phase to aqueous phase and its extremely low concentration at the electrode surface probably control the chain growth kinetics.

Key words:  Electro-oxidation, Nanosized polypyrrole, Pyrrole-picric acid complex 

.Introduction

Polypyrrole is a promising conducting polymer for wide variety of electronic, opto-electronic and electro-chemical devices including electrode materials of aqueous and non-aqueous batteries. It is easily deposited on to metallic and carbon substrates by electro- polymerization of pyrrole in a suitable organic solvent. Major problems of electro-oxidation of pyrrole in aqueous medium are its poor solubility in water and corrosions of non-noble metal anode in acidic solution. Previously, we had reported electro-polymerization of pyrrole on to steel substrate using aqueous picric acid as dopant.  We found polypyrrole films so deposited were brittle. Polymer chain growth kinetics depends on concentration of monomer as well as radical, this can be controlled taking advantage of the fact that both pyrrole and pyrrole-picrate complex are more soluble in DCM but less soluble in water, hence a two  phase electro-oxidation as reported  here is expected to produce submicron size polymer. To the best of our knowledge, this technique of two phase electro-oxidation of preparation of nanosized polymer has not been reported earlier.

Material and Method

Chemicals

Pyrrole (Fluka), Picric acid (Merck) and Dichloromethane (Merck) were used as obtained. Triple distilled water was prepared using a all glass setup.

Pyrrole (1ml) was taken in a 50ml  cylindrical beaker to which 1ml saturated aqueous picric acid (picric acid is only slightly soluble in water) was added when  reddish globules of Pyrrole-picric acid (py-pic)complex was formed. To this solution 8ml of DCM was added when py-pic complex dissolved forming a yellow solution. Ten ml of saturated picric acid was then poured over this DCM solution of py-pic complex to form a two phase system. A working electrode of thin steel sheet (4cm^-2) and a counter Pt electrode of same dimensions were placed in the aqueous phase just over the phase boundary , a current of 0.25 mA cm^-2  under galvanostatic condition was passed for 600 seconds when a blue-black deposition of poly-pyrrole was  formed on to working  electrode .The deposition was taken out with a sharp knife for TEM analysis. For CV of pyrrole-picrate complex in DCM the solution was prepared as stated above using 0.01M KClO3 as electrolyte. All electro-chemical measurements were taken by PAR VarsaStat (II )Potentiostat/Galvanostat.

Results and Discussions

A simple experimental arrangement was used in this study. It may be mentioned that the electrodes were kept just above the phase boundary in order to avoid large migration distant of the py-pic complex. Pyrrole forms a complex via hydrogen bonding with trinitro-benzene derivatives including picric acid which is soluble in DCM. From CV of py-pic it is known pyrrole oxidation potential is lowered due its hydrogen bonding with –NO₂ group of picric acid. We propose py-pic complex will easily deprotonated to form pyrrole anion which will form pyrrole radical upon anodic-oxidation which is prerequisite for polymerization

 It is reported polypyrrole film depositions on to different substrate material are best obtained under galvanostatic condition at a current density of 25mA cm^-2, we have therefore used this optimised condition in our study. With deposition of ppy film, the resistance of steel electrode surface increases as indicated in potential –time deposition curve.

Now the major advantage of two phase electro-oxidation in this case is that the ppy-pic complex is soluble in DCM and it slowly diffuse to aqueous phase resulting a low concentration of monomer in the vicinity of the electrode surface which retards the chain growth kinetics of polypyrrole formation. TEM picture shows these polymers are of almost spherical in shape with diameters of 250-300 nm.

We concluded that nano size polypyrrole are easily synthesized by a two phase electro-deposition technique using DCM and water as two immiscible solvent pair and picric acid as complexing agent. 

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