In situ scanning tunneling microscopy (STM), atomic force microscopy (AFM) and quartz crystal microbalance (EQCM) studies of the electrochemical deposition of tantalum in two different ionic liquids with the 1-butyl-1-methylpyrrolidinium cation

The electrochemical reduction of 0.1 M TaF₅ in two hydrophobic ionic liquids (1-butyl-1-methylpyrrolidinium tris(pentafluoroethyl) trifluorophosphate ([Py₁,₄]FAP) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl) amide ([Py1,4]TFSA) is probed using three in situ techniques: scanning tunneling microscopy (STM), atomic force microscopy (AFM), and electrochemical quartz crystal microbalance (EQCM). These techniques reveal that under similar conditions TaF₅ is more easily reduced in the liquids with [TFSA]⁻ than [FAP]-anions. Increasing the temperature reduced the viscosity and density of the ionic liquids which facilitates TaF₅ electroreduction, in particular, in [Py₁,₄]TFSA. A herringbone reconstruction of the Au electrode was observed by STM for both ionic liquids with and without TaF₅. Ta deposition was proved by STM and EQCM in [Py₁,₄]TFSA. Cracked layers, with ionic liquid trapped inside, were obtained by direct plating from the [TFSA]⁻ ionic liquid. No Ta containing deposits could be obtained in the liquid with the [FAP]⁻ anion.