Electrocompression of the Au(111) Surface Layer during Au Electrodeposition A. H. Ayyad, J. Stettner, and O. M. Magnussen Institut fu ¨r Experimentelle und Angewandte Physik, Universita ¨t Kiel, Olshausenstrasse 40, 24098 Kiel, Germany (Received 3 September 2004; published 18 February 2005) In situ grazing-incidence x-ray diffraction studies of reconstructed Au(111) electrodes in aqueous electrolyte solutions are presented, which reveal a signiﬁcantly increased compression of the Au surface layer during Au electrodeposition as compared to Au(111) surfaces under ultrahigh vacuum conditions or in the Au-free electrolyte. The compression increases towards more negative potentials, reaching 5.3% at the most negative potentials studied. It may be explained within a simple thermodynamic model by a release of potential-induced surface stress. DOI: 10.1103/PhysRevLett.94.066106 PACS numbers: 68.08.–p, 61.10.–i, 68.35.Bs, 82.45.Qr The structure of solid surfaces during chemical reactions or surface processes, such as deposition or etching, is of considerable technological interest. It has been shown that under these conditions the surface structure can con- siderably deviate from that found in conventional studies of the clean surface under reaction-free conditions. For example, homoepitaxial growth on Pt(111) in ultrahigh vacuum (UHV) induces the formation of a surface recon- struction, usually found only at high temperatures [1]. This was attributed to the presence of a supersaturated Pt gas phase during deposition and leads to a reentrant layer-by- layer growth in this system. Similar phenomena may occur during electrochemical processes at the solid-liquid inter- face, as will be shown here for the case of Au homoepi- taxial electrodeposition on Au(111) electrodes in acidic electrolytes. Au(111) surfaces under UHV conditions exhibit at room temperature the well-known herringbone reconstruction, where the topmost Au layer is 4.3% uniaxially compressed, resulting in a dislocation network structure [2,3]. The same type of reconstruction is stable at the Au(111)-electrolyte interface negative of a critical (electrolyte-dependent) po- tential [4 –6]. The compression of the reconstructed sur- face layer, formed in an electrochemical environment, varies with the potential (in particular close to the critical potential), but approaches a limiting value of 4.3% at sufﬁciently negative potentials, i.e., is identical to that found under UHV conditions. However, an even higher compression might in principle be expected at electro- chemical interfaces, since both surface stress and surface energy, which contribute to the driving force of the Au(111) reconstruction [7–10], strongly depend on the potential [8,10]. Here we will demonstrate that during Au electrodeposition indeed a signiﬁcantly increased com- pression of the reconstructed layer can be obtained and discuss this effect by considering previous data on the potential-dependent surface stress and energy of Au(111) electrodes. The Au surface structure was monitored in situ during the growth process by grazing-incidence x-ray diffraction (GID), performed at beam line ID 32 of the European Synchrotron Radiation Facility (ESRF). A Au(111) crystal (4 mm diameter, Mateck, 99.999%) with a miscut of <0:1  and a mosaic spread of 0:16  was used, which was pre- pared prior to the experiments by ﬂame annealing. The experiments were performed in solutions prepared from suprapure HCl, KCl, and H 2 SO 4 (Merck), KAuCl 4 (Johnson Matthey), and Milli-Q water. Since the (electrolyte-dependent) stability range of the Au(111) re- construction is 0:7V negative of the Au=AuCl  4 equi- librium potential (0.82 V in 0:1MHCl  50 M HAuCl 4 ), the deposition rate is completely determined by AuCl  4 transport in the electrolyte, i.e., by the concentration of the metal species in solution, its (temperature-dependent) dif- fusion coefﬁcient in the electrolyte, and the hydrodynamic conditions. It approaches a ﬁxed value after an initial period where a steady-state diffusion proﬁle evolves in front of the surface. Deposition rates that are low enough to maintain a sufﬁciently smooth interface (see below), i.e., of 1ML= min, can only be obtained at very low AuCl  4 concentrations ( 200 M). This prohibits the use of thin- layer electrochemical cells, commonly employed in in situ electrochemical GID experiments [5], where transport to the surface is severely limited. Instead, a hanging meniscus cell similar to that described in Ref. [11] was used, con- sisting of a glass capillary ﬁlled with electrolyte and in- cluding a Pt counterelectrode and a salt bridge to a Ag=AgCl (3 M KCl) reference electrode, which establishes contact with the Au(111) electrode surface. The x-ray beam (  0:626  A) passes through the freestanding me- niscus, i.e., is scattered only by the sample and the elec- trolyte solution. The meniscus, which is surrounded by an N 2 atmosphere to keep the electrolyte oxygen free, is stable for up to 5 h and can be maintained during the exchange of the electrolyte. As veriﬁed by cyclic voltammetry, this geometry does not inhibit AuCl  4 transport to the electrode surface and allows one to obtain high-quality electrochem- ical data, characteristic for a Au(111) single crystal parallel to the diffraction experiments. The hexagonal coordinate system [2,3,5] of the Au(111) substrate was used, where PRL 94, 066106 (2005) PHYSICAL REVIEW LETTERS week ending 18 FEBRUARY 2005 0031-9007= 05=94(6)=066106(4)$23.00 066106-1  2005 The American Physical Society