Surface degradation of α-naphthalene sulfonate-doped polypyrrole during XPS characterization

The surface compositions of non-aging chemically synthesized polypyrrole samples doped with alpha-naphthalene sulfonate (PPy/alpha-NS-) were investigated by X-ray photoelectron spectroscopy (XPS). The power of the X-ray source used was 300 W. Employing liquid nitrogen cooling of the sample holder, the spectra in the regions of C1s, O1s, N1s and S2p were found to be reproducible with a gradual change in S2p spectrum when the accumulated X-ray exposure time was less than 1050 min. Beyond this accumulated X-ray exposure time, dramatic changes were observed in all spectral regions. The decreases in the amount of dopant and HSO4- or SO42- co-dopant species correspond to the decrease in the amount of polaron charge carrier species of PPy (-NHcircle+-). We found increases in the amount of the imine-like nitrogen (=N-) and hydrocarbon, and the presence of new sulfur-containing species. These results suggested the deprotonation of the polaron by the dopant and the desulfonation of the protonated dopant. After desulfonation, these new sulfur-containing species were removed whereas the carbons of the naphthalene rings were still adhering to the surface of the PPy pellet. This degradation became more severe when the liquid nitrogen cooling system of the spectrometer was not used. The results indicate that the degradation was mainly induced by heat from the X-ray beam. (C) 2002 Elsevier Science B.V. All rights reserved.