Study of dynamic processes at the electrochemical interface by in situ high speed STM : surface diffusion and adsorbate interactions
The knowledge about surface diffusion and adsorbate-adsorbate interactions is important for deep understanding of various processes on surfaces and interfaces, such as crystal growth, phase transitions, self-assembly or catalysis. Up to now, the direct microscopic measurements of adsorbate-adsorbate interactions were done in a number of selected systems under ultra-high vacuum condition, but no such studies were performed at solid-liquid interfaces. This work is a continuation of the recent in situ high-speed STM (scanning tunneling microscopy) study of tracer diffusion of sulphur adsorbates on Cu(100) in aqueous solution of 0.01 M HCl performed by T. Tansel . It revealed a linear dependence of diffusion barriers on the electrode potential, which originates from electrostatic interaction of the adsorbate dipole moment with the electric field in the double layer. In the present work, development of software for image recognition and further quantitative analysis allowed the extraction of adsorbate-adsorbate interactions and diffusion barriers of adsorbates in the vicinity of neighboring adsorbates from experimental video sequences. For measurements of interactions between adsorbates, two different methods— dynamic and equilibrium analysis—were applied, providing consistent results, obtained for the first time at an electrochemical interface. The measured pairwise interatomic potential for Sad showed anisotropic behavior, with pronounced repulsion at nearest neighbor sites and attraction at next-nearest neighbor sites of the c(2 × 2) adlattice. The Sad diffusion barriers were considerably decreased in the vicinity of neighboring Sad, as compared to the tracer diffusion barrier of isolated Sad. In further STM measurements, the model system was modified by introducing species possessing a different (cationic) nature, such as Pb adsorbates. The results of tracer diffusion for Pbad revealed a similar potential dependence, as in the case of Sad. This behaviour was not expected because of different adsorbate dipole moments of these species, which are opposite in direction. The observed trend was rationalized by the dominant contribution of neighboring Clad forming the c(2 × 2) lattice to the total surface dipole moment during a single jump event. In addition, attractive pair interactions on the order of 20 meV were found between Pbad at nearest and next-nearest neighbor sites of the c(2 × 2) lattice. In the potential regime corresponding to Pb underpotential deposition on Cu(100), a Pb surface alloy phase nucleated at steps and continued by growing along the upper terraces. With our high temporal and spatial resolution STM we could study the fast dynamic processes associated with surface phase transitions during Pb alloying/dealloying. The formation of a novel transient (4×3) alloy phase with 0.25 ML Pb coverage and its continuous transformation into a c(4 × 4) Pb alloy phase with a coverage of 0.375 ML was observed. By changing the potential sweep in positive direction, Pb dissolution started, resulting in formation of a novel transient c(4 × 4) Pb alloy phase with lower coverage. Further Pb dissolution lead to the re-adsorption of Clad forming c(2 × 2) domains and development of closed-loop Pb ribbon structures. Based on analysis of high resolution STM images we have suggested a new model for this structure.
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