Published: February 22, 2011 r2011 American Chemical Society 3772 dx.doi.org/10.1021/ja1115748 | J. Am. Chem. Soc. 2011, 133, 3772–3775 COMMUNICATION pubs.acs.org/JACS High-Speed in situ Surface X-ray Diffraction Studies of the Electrochemical Dissolution of Au(001) Frederik Golks, †,‡ Klaus Krug, †,§ Yvonne Gr€ under, †,# J€ org Zegenhagen, ‡ Jochim Stettner, † and Olaf M. Magnussen* ,† † Institut f€ ur Experimentelle und Angewandte Physik, Universit € at Kiel, Leibnizstrasse 19, 24098 Kiel, Germany ‡ European Synchrotron Radiation Facility, 6 Rue Jules Horowitz, 38000 Grenoble, France ABSTRACT: We present in situ X-ray surface diffraction studies of interface processes with data acquisition rates in the millisecond regime, using the electrochemical dissolu- tion of Au(001) in Cl-containing solution as an example. This progress in time resolution permits monitoring of atomic-scale growth and etching processes at solid-liquid interfaces at technologically relevant rates. Au etching was found to proceed via a layer-by-layer mechanism in the entire active dissolution regime up to rates of ∼20 ML/s. Furthermore, we demonstrate that information on the lateral surface morphology and in-plane lattice strain during the electrochemical process can be obtained. H igh-resolution in situ studies of solid-liquid interfaces by structure-sensitive methods, such as scanning probe micro- scopy and synchrotron-based techniques, have become an indis- pensable tool in modern interface science. Specifically, they have contributed enormously to the in-depth understanding of the complex interface processes in crystal growth and dissolution by revealing in detail the mechanisms of nucleation and growth on the atomic scale. For example, in the field of electrochemical deposition or dissolution, these studies clarified the potential- dependent growth behavior and resulting nanoscale morphology in the early stages of deposition or dissolution. They revealed the influence of surface defects and heterogeneities on the initial nucleation in the submonolayer regime. Furthermore, they showed how the substrate as well as anionic or organic adsorbates affect the homogeneous nucleation densities and the shape of monolayer or vacancy islands, and allowed even the propagation of individual kinks to be followed along steps during growth or dissolution. 1,2 Despite the unquestionable importance of these results for nanoscale electrochemistry, a common general problem of all of these studies is the low growth rates, which are at most a couple of monolayers (ML) per minute, but typically much lower. In contrast, the minimum current densities typically employed in technological electroplating, e.g., in the copper damascene process used in ULSI manufacturing, 3 are in the range of 10 mA 3 cm -2 , corresponding to local growth rates of g10 ML/s. Hence, like in heterogeneous catalysis, where industrial processes and scientific model studies under ultra-high vacuum-conditions are separated by a huge “pressure gap”, a substantial “current density gap” exists between atomic-resolution in situ studies and applications. Since the deposition rate (i.e., the flux of particles to the crystal surface) is a central parameter in kinetic growth theory and influences the growth behavior as well as the resulting deposit morphology, 4-6 this is a serious drawback in clarifying the mechanisms of real-world electrodeposition processes. in situ studies at high growth rates by high-resolution scanning probe microscopy are difficult due to the inherently low image acquisition rates and the influence of the scanning tip. The former can be somewhat mitigated by progress in increasing the time resolution. 7 The latter is a fundamental problem, leading to significant shielding of the scanned area and consequently to orders of magnitude lower local growth rates. 8,9 In contrast, studies by photon-based methods, such as X-ray diffraction, are in principle not plagued by such interference by the experimental probe. However, before now most studies have been performed in thin-layer electrochemical cells, where the sample surface is covered by an electrolyte layer of only a few tens micrometers thickness. This results not only in a large ohmic drop but also in strongly hindered transport to the electrode surface. Thus, true in situ studies during electrochemical growth processes are not possible in this way. As we recently demonstrated for the case of Au electrochem- ical deposition and dissolution in Cl-containing solution, 10-12 the growth behavior and interface structure can be directly followed by surface X-ray scattering (SXS) using synchrotron radiation. By employing a transmission geometry, electrochemical cells can be realized that — contrary to thin-layer SXS cells — feature unrestricted mass transport and time constants compar- able to those achieved with conventional electrochemical cells. in situ measurements during growth or dissolution at moderate rates (a few seconds per ML) were possible with such cells, with the local microscopic rates being almost identical to the macro- scopic rates derived from the current density. These studies revealed a layer-by-layer dissolution mechanism for the electro- chemical dissolution of Au(111) up to the onset of passivation 12 and a complex, potential-dependent growth behavior, featuring step-flow, layer-by-layer, 3D, and re-entrant layer-by-layer growth, for the homoepitaxial electrodeposition of Au on Au(001). 10,11 Here we demonstrate for the case of electrochemical dissolu- tion of Au(001) that, even at rates approaching those employed in technological deposition and dissolution processes, atomic- scale data can be obtained by in situ surface X-ray diffraction. Key to this improvement was the implementation of a fast 1D X-ray detector (Dectris Mythen 1K). It permits a much higher data Received: December 23, 2010