Electrochimica Acta 82 (2012) 570–575 Contents lists available at SciVerse ScienceDirect Electrochimica Acta j ourna l ho me pag e: www.elsevier.com/locate/electacta Study of electrode surface dynamics using coherent surface X-ray scattering Hoydoo You a,∗ , Michael Pierce a,b , Vladimir Komanicky a,c , Andi Barbour a , Chenhui Zhu a a Materials Science Division, Argonne National Laboratory, Argonne, IL 60439, United States b Rochester Institute of Technology, Department of Physics, Rochester, NY 14623, United States c Safarik University, Faculty of Sciences, Kosice 04154, Slovakia a r t i c l e i n f o Article history: Received 5 December 2011 Received in revised form 14 March 2012 Accepted 15 March 2012 Available online 23 March 2012 Keywords: Surface reconstruction Coherent X-ray scattering Surface X-ray scattering Surface dynamics X-ray speckles a b s t r a c t We present a coherent X-ray scattering technique that allows in situ measurements of electrode surface dynamics. This technique can measure the transient dynamics of phase relaxation from one phase to another upon changing electrochemical conditions or temperature. It can also measure the dynamics of microstates even if the electrode is apparently static in macroscopic states. We will discuss the underlying physics of the X-ray speckle correlation spectroscopy and present our recent study of Au (1 0 0) surface in vacuum, water, and electrolytes. © 2012 Elsevier Ltd. All rights reserved. 1. Introduction Structure of the electrode surface is important to our funda- mental understanding of many electrochemical processes such as electrodeposition, electrocatalysis, and corrosion stability. X-ray techniques are, in general, a powerful tool for studying structures of electrode surfaces in situ. In particular, surface X-ray scatter- ing (SXS) [1] has been widely used to study structural details of the electrode interfaces such as under potential deposition (UPD) layers [2], metal surface reconstruction [3], halide layers on metal surfaces [4], incipient-stage corrosion [5], and surface processes on conducting oxides [6]. While most of these studies were limited to static or time-averaged surface structures of electrodes, it is of great interest to study dynamics of the electrode reaction processes and mechanisms. Coherent X-ray scattering has a significantly higher sensitivity to the structural and temporal details than ordinary incoherent X-ray scattering, and coherent surface X-ray scattering (CSXS) is more sensitive to spatial and temporal details of surface/interface than ordinary SXS. A coherent diffraction pattern retains much of the spatial information that would have been averaged out in ordinary X-ray diffraction. Because of the spatial sensitivity, the coherent X-ray diffraction images can be inverted to a real space image using lensless oversampling techniques [7,8] or using X-ray reflection interface microscopy [9]. However, the imaging ∗ Corresponding author. E-mail address: hyou@anl.gov (H. You). from interfaces is yet at a limited resolution compared to other microscopy techniques, such as scanning probe microscopy or vis- ible light microscopy [10], and will not be covered in this paper. While imaging with CSXS is currently not possible, CSXS can be used to study dynamic behavior of surfaces, This was recently demonstrated in our vacuum surface studies [11,12]. Because detailed microscale spatial information is retained in the diffrac- tion pattern, the diffraction patterns are often complex and random in appearance. This kind of intensity distribution is known as a speckle pattern. Since the speckle pattern is sensitive to micro- scopic structures, any change in the surface microstate will be reflected sensitively to the distribution of the speckles. There- fore, it is possible to follow the time evolution of the surface or interface structures even if we do not invert the diffraction pat- tern to a real-space image. This can be done by analyzing the intensity–intensity autocorrelation of the diffraction patterns. The technique for systematic autocorrelation analyses is known as X- ray photon correlation spectroscopy (XPCS). We will focus on the speckles from Au (0 0 1) surfaces in various conditions and analyze the autocorrelation functions. Dynamics of the well-known hex reconstruction of Au (0 0 1) was studied in vacuum condition and published earlier [11]. In there, the CSXS technique revealed the previously unknown dynamics transition of the hex reconstructed Au (0 0 1) surface at high temperatures. Similarly, in situ CSXS measurements of the Au (0 0 1) surface in 0.l M HClO 4 solution have proven successful and were com- pared to scanning tunneling microscopy (STM) images at similar conditions [13]. It was shown that the real space image indeed evolves in the time scales consistent with those measured with the 0013-4686/$ – see front matter © 2012 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2012.03.069