In Situ Electrochemical Quartz Crystal Microbalance Measurement of Au Deposition and Dissolution in Room-Temperature Ionic Liquid Containing Chloride Ion

Abstract

In-site gold electro-deposition and electro-dissolution have been examined using polycrystalline gold-coated quartz crystal electrode in a room-temperature ionic liquid of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4). In EMIBF4 media containing 1-ethyl-3-methylimidazolium chloride, the species of AuI (probably [AuICl2]−) resulting from the electrochemical oxidation of the Au surface was very stable and a reversible voltammetric response of the AuI/Au0 couple was observed at 0.25 V vs. Ag wire quasi-reference electrode (Ag(QRE)). In situ electrochemical quartz crystal microbalance (EQCM) technique combined with cyclic voltammetry, double potential-step chronoamperometry and double potential-step chronocoulometry has been successfully applied to investigate the cathodic deposition of AuI ion to Au metal and the anodic dissolution of Au from the Au electrode surface. It was found that the frequency change of quartz crystal electrode during the electrolysis can be interpreted in terms of rigid mass changes based on the Sauerbrey equation. The EQCM analysis of the electro-dissolution and electro-deposition of Au in the potential range of −0.60 to 0.50 V vs. Ag(QRE) gave the molecular mass equivalent of Au (197 g mol−1) corresponding to a one-electron reduction and oxidation of the AuI/Au0 redox couple.

Time-differential frequency response of electrochemical quartz crystal electrode during the dissolution and deposition processes of Au in H2SO4 solution containing chloride ion

“…Therefore, the estimation of the so-called apparent equivalent molar mass (k) per second (calculated from the time-differential frequency response) [1,2] will be carried out to elucidate the whole gravimetric behavior of the electrochemical deposition and dissolution of Au at each applied potential. The k is defined by the following Eq.…”

“…The EQCM measurement system is composed of a 6 MHz AT-cut quartz crystal electrode (QCE) (working electrode area: 1.45 cm 2 ) which is driven at its resonant frequency by a feedback oscillator, with a stability of the observed frequency being the nearest 0.2 Hz. For a 6 MHz crystal operating in the fundamental mode, the approximate sensitivity of the EQCM is 12.3 ng cm À2 Hz À1 , which is an absolute measurement of mass per unit area [1,3,21]. Time-differentiation of the frequency data and the amount of charge was accomplished using the Excel slope function of Microsoft Ò according to the previously reported procedure [1,2].…”

“…For a 6 MHz crystal operating in the fundamental mode, the approximate sensitivity of the EQCM is 12.3 ng cm À2 Hz À1 , which is an absolute measurement of mass per unit area [1,3,21]. Time-differentiation of the frequency data and the amount of charge was accomplished using the Excel slope function of Microsoft Ò according to the previously reported procedure [1,2]. In order to examine the surface roughness effect upon the resonant frequency at the surface of Au-or Pt-QCE during potential scanning or holding, the full width at half height (Df whh ) in the admittance spectra of the quartz crystal oscillator, which can be interpreted on the basis of the solution viscosity change induced by the roughness change at the interface between the electrode and the solution, were measured with 4194 A impedance/gain-phase analyzer (Hewlett Packard Co. Ltd.) [2,4].…”

“…Generally, in aqueous and common organic solutions, except for room temperature ionic liquids (RTILs) [1][2][3][4] in which the stability of [AuCl 2 ] À species can be obtained, it has been well-accepted that the overall anodic dissolution of an Au electrode occurs as 3-electron oxidation [5,6], which has been recently supported with electrochemical quartz crystal microbalance (EQCM) [7][8][9][10][11][12] reported by Conway et al [13,14]. By an analysis using the Sauerbrey equation (that is, the slope for responses of the frequency vs. charge (Q) calculated from the amount of charge flowing during the redox reactions), they have reported that the oxidation wave at ca.…”

“…Previously, in situ EQCM technique based on estimation (the differential frequency response) [1,2] of the so-called apparent equivalent molar mass (k) per second has been proved to be very powerful in elucidating electrode reaction processes accompanying a mass change during potential-scan electrolysis in aqueous, organic media and RTILs [3,[4][5][6][7][8]. Here, the size and morphology of Au particles and relative ratio of its crystalline orientation domains on Pt, Au, and carbon electrodes have been found to largely depend on the electrolysis potential used in the electro-deposition, that is, on whether [AuCl 2 ] À species is present or not [11,12].…”