An in-situ electrochemical quartz crystal microbalance study of the underpotential deposition of copper on Au(111) electrodes

Abstract

The combination of the quartz crystal microbalance (QCM) with electrochemical methods has made possi- ble the in situ measurement of minute mass changes that may accompany electrode processes, such as elec- trodeposition, film growth, oxide formation, ion update into polymer films, and ionic adsorption [ll. While the electrochemical QCM provides valuable coverage in- formation which is often difficult to obtain by other means and complements other surface-sensitive in situ probes of smooth electrodes such as infrared (IR) spectroelectrochemistry, X-ray absorption and scatter- ing, scanning tunneling microscopy @TM), and non- linear optical spectroscopy [2], a significant limitation has been the inability to apply the QCM to ordered single crystal surfaces. We report here the first preparation of a highly ordered Au(lll) electrode on a quartz crystal. We also report results obtained using this electrode in an elec- trochemical QCM in conjunction with cyclic voltamme- try and coulometry to investigate the underpotential deposition (UPD) of copper onto the Au(ll1) surface. Even though copper UPD on gold has been studied using a QCM on polycrystalline surfaces [3,4] and by many other in situ interfacial techniques such as sur- face extended X-ray absorption fine structure (SEXAFS) [5], STM [6,7], and IR spectroelectrochem- istry [S], etc. on both single crystal and polycrystalline surfaces, there is still controversy as to the structure and composition of the adlayer at various stages in the UPD process. The QCMs consisted of 2.5 cm diameter AT-cut quartz crystal disks (Valpey-Fisher), which were pol- ished to an optical finish on both sides. Gold elec- trodes were vapor deposited by a modification of, the procedure described previously [3]. First, ca. 50 A of chromium metal (99.99%, Aldrich) was evaporated onto one side of the crystal to enhance the adhesion of gold. Then 2000 A of gold (99.999%, Aldrich) was evapo- rated onto the Tame side. These evaporations were done at room temperature in a vacuum ff 2