The 0.9 eV Absorption Band of Polyaniline: A Morphologically Sensitive Electronic Absorption
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THE 0.9 eV ABSORPTION BAND OF POLYANILINE: A MORPHOLOGICALLY SENSITIVE ELECTRONIC ABSORPTION D. ZHANG, J.-H. HWANG, S. C. YANG Department of Chemistry, University of Rhode Island, Kingston, RI 02881
INTRODUCTION The optical absorption spectra of polyaniline in the visible and UV spectral regions have been characterized before. The electrically conductive form has a An band gap absorption at 3.8 eV and polaron absorptions at 2.9 and 1.5 eV [1,2] . additional electronic absorption band at 0.9 eV was found to be also related to the existence of polaron. The assignment of this transition has yet to be made. Epstein and co-workers [31 have found a photo-induced absorption band at 0.9 eV by photo-exciting emeraldine base (insulator) with 2.0 eV light. It is not known yet whether these two 0.9 eV transitions belong to the same set of electronic energy levels of polaron, or they belong to two different electronic systems but are coincidentally have the same transition energy. In this communication we present a comparative study of the 0.9 eV band and the 2.9 and 1.5 eV bands in polyaniline samples of different morphology. We will show that although the 0.9 eV band is, like the other two bands, due to the existence of polarons, it is far more sensitive to the micro-environment of Using template molecules to control polyaniline than the other polaron bands. the morphology, we are able to alter the micro-environment around polyaniline molecule, we found the 0.9 eV band splits into two bands in samples in which the molecular aggregation of polyaniline were likely to have been altered, while the other two bands are unaffected by these changes. We propose that the experimentally observed morphological sensitivity of the 0.9 eV band may be explained by assigning the 0.9 eV band as an inter-molecular charge transfer absorption involving a charge transfer complex of two neighboring polyaniline molecules. RESULTS AND DISCUSSION The 0.9 eV band is sensitive to the electrode used for electrochemical polymerization Thin polyaniline films with thickness ranging from 100 to 300 nm were prepared by anodic oxidation on a tin oxide coated glass in 1.5 M aniline dissolved in 3 M HCI solution. The electrolysis was potentiostatically controlled at +0.65 V vs saturated calomel electrode (S.C.E.). Platinum was used as counter electrode. Two types of tin oxide coated glass were used as working electrode. Glass coated with fluoride doped tin oxide (SnO2:F) was made by PPG Industries. This transparent electrode is trademarked as NESA and has approximately 25 The glass coated with antimony doped tin oxide ohm/square surface resistivity. (Sn02:Sb) was purchased from Delta Technology. This glass has about 100 Examining by scanning electron microscope, the ohm/square surface resistivity. bare electrode surface of SnO2:F contains micro-crystalites of 100 to 200 nm dimension and the surface appears to be rocky at a magnification of x50 K. The Sn02:Sb surface appears to be smooth under the same scale of magnification. When thin films (50 nm) of polyanili
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