EQCM study of the [Au(III)Cl4](-)-[Au(I)Cl2](-)-Au(0) redox system in 1-ethyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquid.

The electrochemical behavior of the Au(III)Cl(4)-Au(I)Cl(2)-Au(0) redox system in room temperature ionic liquid (RTIL) of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF(4)) has been investigated quantitatively using an in situ electrochemical quartz crystal microbalance (EQCM) technique based on a Pt film-coated quartz crystal electrode (Pt-QCE). A series of two-electron (2e) and one-electron (1e) reductions of the Au(III)Cl(4) to Au(I)Cl(2) and Au(I)Cl(2) to Au metal were recognized at the Pt surface. Besides, the disproportionation reaction of Au(I)Cl(2) (i.e., the 2e-reduction product of Au(III)Cl(4)) to Au(III)Cl(4) and Au metal was also observed. Electro-dissolution of the Au deposited on the Pt electrode through a 1e-oxidation reaction in the presence of chloride ions was also confirmed using the Pt-QCE based EQCM technique. A 2e-oxidation reaction of Au(I)Cl(2) (i.e., the dissolved product) to Au(III)Cl(4) along with the oxidation of Cl(-) ion on the Pt surface was also realized at high anodic potential. The results demonstrate that in situ EQCM technique is applicable and powerful in elucidating electrochemical surface phenomena accompanying a mass change in RTIL.