X-ray Photoelectron Spectroscopy Study of Counterion Incorporation in Poly(3,4-ethylenedioxythiophene) (PEDOT) 2: Polyanion Effect, Toluenesulfonate, and Small Anions

Poly(3,4-ethylenedioxythiophene) (PEDOT) is widely used in organic electronics and biomaterial coatings because of its outstanding electrical properties and chemical stability. In our previous research, X-ray photoelectron spectroscopy (XPS) was used to investigate the incorporation of counterions into PEDOT. In this research, XPS was applied to systematically investigate the chemical composition of electrochemically polymerized PEDOT films to determine counterion affinity. The counterions probed here included polyanions such as poly(sodium 4-styrenesulfonate) (PSSNa) and poly(acrylic acid) (PAA), and small anions including lithium perchlorate, lithium bromide, sodium nitrate, sodium thiosulfate, sodium chloride, sodium phosphate monohydrate, sodium phosphate dibasic hepthydrate, sodium acetate trihydrate, sodium p-toluenesulfonate (TosNa), calcium carbonate, and sodium carbonate. In addition to these counterions, an ion mixture, phosphate-buffered solution (PBS), was also examined. In counterions with ion mixtures containing PSSNa during PEDOT electrochemical polymerization, PSSNa was found to be the dominant counterion incorporated into PEDOT due to its polymeric nature over the monomeric-like version TosNa, though bromide anions were found to act as PEDOT counterions even in the presence of PSSNa (or PAA). The overall qualitative PEDOT counterion affinity for anions with one negative charge observed in the XPS spectra was the following: ClO4- and Br- over smaller contributions of Cl-, Tos(-), and COO- (acetate), with no phosphate or NO3- contributions. As for the anions with two negative charges, S2O32- dominated over both carbonate and phosphate anions. The trends found in counterion affinity did follow the general trend for anionic hydration suggested by the Hofmeister series. Systematic investigations on counterion incorporation into PEDOT can greatly improve the basic understanding of the chemical composition of various PEDOT films when different counterions or ion mixtures are used in the electrochemical polymerization process, aiding in the design of optimized PEDOT films with tailored properties.