Monitoring of the voltage-induced microstructure of C12mimBr ionic liquids on a HOPG surface using in situ XAFS

Abstract

The voltage-induced microstructure of 1-dodecyl-3-methylimidazolium bromide ionic liquids (C₁₂mimBr ILs) on a highly oriented pyrolytic graphite (HOPG) surface was studied using in situ X-ray absorption fine structure (XAFS) in combination with in situ XPS and molecular dynamic (MD) simulations. Under no voltage conditions, C₁₂mimBr on the HOPG surface showed increased electron density and reduced first shell distance around Br compared with C₁₂mimBr in the bulk, indicating the existence of the charge and inductive effect between the ILs and HOPG. These behaviours led to the formation of an IL mixed layer structure on the HOPG surface. When a positive voltage of 0.1 V was applied, the coulombic interactions between Br and HOPG mainly induced C₁₂mimBr IL structural transformation into a ‘co-adsorbed overscreening’ configuration with a Br anion near the IL/HOPG interface. When a positive voltage of 5.0 V was applied, the enhanced coulomb interactions triggered IL structural transformation into an ‘alternating anion and cation’ configuration with many Br anions near the IL/HOPG interface. MD simulations also verified the structural transformation with the increasing voltage as revealed using XAFS. In situ XPS further confirmed that the Br atoms were closer to the HOPG surface compared with the cations. These results give an important insight into the behaviour of ILs for their electrochemical applications.