In Situ Scanning Tunneling Microscopy of Polycrystalline Platinum Electrodes under Potential Control Copper Electrodeposition and Pyrrole Electropolymerization

The surface changes of Pt electrodes immersed in solution under potential control were studied by scanning tunneling microscopy. Various structures were observed on a polished, flame-annealed-quenched polycrystalline electrode. Extended cycling of the electrode in 1M H2SO4 between the regions where an adsorbed oxide film forms and is removed causes significant roughening of the electrode surface. The electrodeposition of copper on polycrystalline Pt and of polypyrrole on a Pt film on mica was also investigated. The scanning tunneling microscope (STM) allows very high resolution (atomic or near-atomic) examination of the surfaces of conductive or semiconductive substrates (1, 2). Although originally mainly applied to samples in a vacuum or a gaseous environment, this technique has been used to investigate various substrates immersed in aqueous and nonaqueous solutions (3-9). We have further demonstrated the in s i tu study of anodic dissolution of nickel electrodes in sulfuric acid solution under external polarization conditions and open-circuit corrosion of stainless steel in aqueous chloride media (10). It has long been recognized that electrochemical oxidation-reduction cycling can modify the structure of Pt electrode surfaces (11-13). Vazquez et al. first used the STM to investigate ex s i tu (in air) Pt electrode surface roughening by fast, repetitive oxidation-reduction cycles (14). Later, Itaya et al. (15a) performed an in s i tu STM measurement on polycrystalline Pt before and after moderate electrochemical activation without potentiostatic control of substrate and tip. We have also examined the topography of different Pt surfaces after different pretreatments in air or in liquids (15b). These latter STM experiments, utilizing a simple two-electrode (substrate and tip) STM configuration, were able to provide structural images as long as the superimposed faradaic currents remained constant over the time scale of the experiment. However, the results are expected to be potential dependent due to faradaic processes, such as dissolution/redeposition, which can change the morphology of the substrate and tip or affect the interfacial tunneling properties. Thus, appropriate control of the electrochemical reactions, both on the substrate and on the tip, by potentiostatic control is important for maintaining a well-defined electrochemical environment for STM measurements. Lustenberger et al. (16) and Wiechers et al. (17) have recently reported in s i tu STM measurements under potentiostatic control on highly oriented pyrolytic graphite (HOPG) and Au( l l l ) single crystals, respectively. Reports from this laboratory have also demonstrated in s i tu STM study ofa HOPG surface and its anodic oxidation under potential control in 0.1M H2SO4 (18). An important aspect of in s i tu monitoring of a substrate under potential control is that it allows one to examine the same small area (e.g., 600 • 600A) of the electrode surface as the electrode is subjected to different treatments. A difficulty with studying surfaces by STM is that only a tiny area is examined in any one scan, and it is difficult to establish what is an example of a representative portion of the surface. This difficulty is compounded by small changes in the tip that can occur with time. Thus, large displacements of th e tip between treatments or removal of the sample from the environment (as in e x s i tu studies) prevents one from noting progressive changes of exactly the same area of the electrode by STM. Film growth by electrodeposition or electropolymerization on various substrates is a convenient and techni*Electrochemical Society Active Member. cally important method for modifying electrode surfaces, e.g., for electrocatalysis. Definition of surface structure of these overlayers is crucial to the understanding of the specific physical and chemical properties of the modified electrode. However, there are still only a few in s i tu techniques that are suitable for the study of various growth modes of thin films on electrodes, especially for subor monolayer coverage. The unique capability of STM for obtaining structural information at the atomic or near-atomic level makes it ideally suited for the in s i tu studies of these kinds of electrochemical processes on the electrode surface. There have been some demonstrations of the applicability of STM for in s i tu monitoring of electrodeposition of metals on HOPG (19-21) and Au (7a). We report here in s i tu STM monitoring of surface modification of Pt electrodes under potential control by electrochemical oxidationreduction cycles, electrodeposition of copper, and electropolymerization of pyrrole in aqueous and nonaqueous solutions.