Introduction

Because of its high stability and its good electrical conductivity polypyrrole (PPy) can be used in many technical applications, e.g., in the field of microchips and microstructured electronic devices. Polypyrrole can also be used as a material in sensor technology. The interaction of different gases with the polymer structure can be detected by the paramagnetic resonance [1] or electrical conductivity measurements [2]. By producing PPy films, electrical conductivities up to 150 S/cm can be obtained. Electropolymerized PPy films differ in their molecular structure according to polymerization conditions such as the electrochemical parameters of the polymerization. At low current densities (l.c.d.) below 3 mA/cm2 one-dimensional polypyrrole chain structures are mainly produced [3]. Higher current densities predominantly lead to two-dimensional molecular polymer structures. The electronic state of such PPy films produced with high current density (h.c.d.) has been investigated by several solid-state spectroscopic methods such as ultraviolet and X-ray photoelectron spectroscopy (UPS and XPS), as well as temperature-dependent electrical conductivity measurements [4-6],

In this paper we focus on electrochemically prepared PPy produced at low current densities. The growth and the dependence of the film morphology on electrode potential after deposition are of special interest. Due to the relatively high conductivity of the doped polypyrrole films they can be investigated by both atomic force microscopy (AFM) and scanning tunneling microscopy (STM) techniques. Li and Wang did ex-situ STM and lateral force microscopy (LFM) measurements [7] at intrinsic conducting polymers differing in some respects from the results of in-situ STM and AFM investigations presented here. They observed island structures of PPy films produced on different electrode surfaces. Naoi et al. have discussed a nodular surface PPy structure obtained at charge densities of 0.5 mC/cm2 and ordered crystalline structures at charge densities of 0.2 mC/cm2 [8]. Several other groups have reported on helical and superhelical PPy structures found by STM methods [9 - 12]. The advantage of in situ STM is that the initial stages of the formation of nucleation centers on the electrode surface can be studied in respect of their time and local distribution in the electrochemical environment with possible scan sizes of several nanometers up to 0.7 pm. To avoid pH-induced polypyrrole film deposition caused by self-protonation, pH buffers are applied. AFM was applied to check the validity of the STM results. In these AFM investigations even larger areas (up to 125 x 125 pm2) can be scanned in-situ at the electrode surface.

Both methods were used in the electrochemical studies with the usual three-electrode arrangement with HOPG as working electrode. Both the initial stages of the polymer and the influence of the electrode potential on the surface morphology of an entire PPy film were studied in-situ.

2 Experimental

The supporting electrolyte used in all electropolymerization techniques was toluene-4-sulfonic acid (tosylate) (0.1 - 0.2 M). Solutions were made with twice-distilled water. Pyrrole distilledunder vacuum and stored in a refrigerator has been used in concentrations in the range 0.04 - and 0.1 M. For slowing down the polymer formation rate by protonation, pH buffers were applied to the aqueous solutions. Phosphate buffer was used for the range pH 5 - 7. The NanoScope HI (Digital Instruments, Santa Barbara, CA, USA) was used for the SPM studies. For electrochemical studies, electrochemical AFM and electrochemical STM equipment was applied. The scanning heads used allow atomic resolution and 0.7 x 0.7 pm2 scan areas (for electrochemical STM and electrochemical AFM) and large-scale scanning of 0.5 x 0.5 pm2 and 125 x 125 pm2 for electrochemical AFM. Electrochemically etched Pt/Ir (80:20) tips with a wire diameter of 0.25 mm and a length of about 12 mm were prepared for a use in in situ electrochemical STM after insulating the tip in molten apiezon wax. To obtain an uncovered end of the tip just this very end (approx. 250 pm2 surface area [13]) of the tunneling tip was exposed to an electrolyte solution. For electrochemical AFM, commercial Si3N4 cantilevers (gold-coated NanoProbes; Digital In-struments) were used. The working electrode (WE) was freshly cleaved HOPG (Advanced Ceramics, Cleveland, OH, USA). As counter electrode (CE) an oxide-free Pt wire (flame-reduced) was used for electrochemical AFM. For electrochemical STM a Pt ring electrode (treated in the same manner) arrangement was possible due to the cell geometry. An Ag/AgCl reference electrode (RE) was used for both electrochemical SPM methods. The potentiostat was driven by the NanoScope IH electrochemical software, version 3.0. The data were processed with the same software. The polypyrrole films were produced both potentiostatically and potentiodynamically. For the potentiostatic mode a voltage ramp from the open cell potential (approx. 20 mV) to +500 mV was created. Potentiodynamic PPy formation was performed by cycling the electrode potential between -200 and +500 mV. The sweep rate was varied between 20 mV/s and 500 mV/s. After deposition, the PPy films were washed with distilled water and dried overnight in a desiccator. Afterwards the PPy electrodes were replaced in the electrochemical in-situ cells of the AFM and STM and studied in monomer-free tosylate solutions.

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