Geometric and electronic properties of 3,4-ethylenedioxythiophene (EDOT), styrene sulfonate (SS), and EDOT: SS oligomers up to 10 repeating units were studied by the self-consistent charge density functional tight-binding (SCC-DFTB) method. An application of PEDOT:PSS for ammonia (NH3) detection was highlighted and investigated both experimentally and theoretically. The results showed an important role of H-bonds in EDOT:SS oligomers complex conformation. Electrical conductivity of EDOT increased with increasing oligomers and doping SS due to enhancement of π conjugation. Printed PEDOT:PSS gas sensor exhibited relatively high response and selectivity to NH3. The SCC-DFTB calculation suggested domination of direct charge transfer process in changing of PEDOT:PSS conductivity upon NH3 exposure at room temperature. The NH3 molecules preferred to bind with PEDOT:PSS via physisorption. The most favorable adsorption site for PEDOT:PSS-NH3 interaction was found to be at the nitrogen atom of NH3 and hydrogen atoms of SS with an average optimal binding distance of 2.00 Å.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-017-1878-2) contains supplementary material, which is available to authorized users.

In this paper, we have reported a theoretical study of the geometric and electronic structures of EDOT:SS oligomers based on semi-empirical Austin model1 (AM1) method and density functional theory at B3LYP/3-21G* level. The effects of polymer chain length of both EDOT and SS on structural and electronic properties including bond length, bond angle, binding distance, charge, the highest occupied orbital (HOMO), the lowest unoccupied molecular orbital (LUMO), and energy gap have been studied from the optimized oligomers which were built by varying repeating unit of monomer as n = 1, 2, 3 and 4. The results show that AM1 is not appropriate for geometry optimization of EDOT:SS system comparing to B3LYP/3-21G* level. The binding distance between H atom on EDOT and O atom on SS tends to close together with the average distance of 2.21 Å. The most positive charges locate at sulfur atoms on EDOT and EDOT:SS. The electrical conductivity of EDOT, SS and EDOT:SS increases when polymer chain is extended.

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