EC-AFM investigation of reversible volume changes with electrode potential in polyaniline

The volume changes in electrochemically deposited polyaniline (PANI) on highly oriented pyrolytic graphite (HOPG) in SO42-and Cl− medium along with PANI microtubules and nanowires in SO42- medium were investigated by electrochemical atomic force microscopy (EC-AFM). Volume expansion was observed in the oxidized state with contraction in the reduced state due to the movement of solvated anions inside the polymer matrix during the oxidation process and its expulsion during the reduction process. Reversible volume changes with changing electrochemical potential were observed in case of microtubules and nanowires. To observe the effect of large counter anion, EC-AFM studies of volume changes with electrode potential in polyaniline modified with poly(styrene sulfonate) (PANI–PSS) nanowires were performed in SO42- medium. The changes were found to be more than 33% and in the reverse direction in this case due to presence of larger immobile PSS anion in the polymer matrix with volume changes predominantly due to movement of solvated H+ ions inside the polymer matrix during the reduction process. To further support our observations we performed the in situ electrochemical synthesis of PANI nanoparticles on HOPG inside AFM electrochemical cell and monitored its volume changes with electrode potential using EC-AFM. The volume changes in these nanoparticles could be clearly observed with variation in percentage changes between 17.5% and 89.8%.