Holger Wolfschmidt

Enhanced Reactivity for Hydrogen Reactions at Pt Nanoislands on Au(111)

We report high exchange current densities exceeding 1 A cm(-2) at Pt nanostructures on Au(111) for hydrogen-related reactions. Such activity is found at Pt nanoparticles with a coverage of less than 10 % of a monolayer on Au(111) and on single Pt particles deposited on Au(111). Potential pulse technique as well as micropolarization curves with overpotentials of +/-10 mV were used in the case of extended nanostructured surfaces to determine the activity. Single Pt particles were investigated in an in situ electrochemical scanning tunneling microscope setup using the STM tip as local sensor. The reactivity obtained on Pt nanostructured Au(111) towards hydrogen reactions were subsidized by single particle reactivity measurements. The specific activity of platinum is enhanced by more than a factor of 1000 as compared to a Pt(111) single crystal. Aspects that may explain this enhancement such as an involvement of the substrate, highly reactive defect sites and enhanced mass transport are discussed.

Charge transfer reactions at nanostructured Au(111) surfaces: influence of the substrate material on electrocatalytic activity

Nanostructured electrodes can be used as model catalysts in order to gain a basic understanding of electrocatalytic properties. In particular, the influence of particle size and particle dispersion of noble metal catalysts and a possible influence of the support material can be studied in detail. Electrocatalytic reactions such as the hydrogen oxidation reaction (HOR), the hydrogen evolution reaction (HER) and the oxygen reduction reaction (ORR) are important for technical applications. Hence, palladium and platinum as typical catalysts were investigated on Au(111) substrates regarding the HOR, HER and ORR. A significant increase in catalytic activity was found for Pd and Pt deposited on Au(111) where, with a decreasing amount of deposited metal, an increase of specific activity is observed which is contrary to expectations. A different behaviour was found for the ORR, where, according to expectations, the reactivity increases with increasing amounts of Pt. Parameters influencing the electrocatalytic activity of nanostructured surfaces, such as strain of the overlayers induced by the support and a possible direct involvement of the Au(111) surface in the mechanism of HER, are discussed.

STM, SECPM, AFM and Electrochemistry on Single Crystalline Surfaces

Scanning probe microscopy (SPM) techniques have had a great impact on research fields of surface science and nanotechnology during the last decades. They are used to investigate surfaces with scanning ranges between several 100 μm down to atomic resolution. Depending on experimental conditions, and the interaction forces between probe and sample, different SPM techniques allow mapping of different surface properties. In this work, scanning tunneling microscopy (STM) in air and under electrochemical conditions (EC-STM), atomic force microscopy (AFM) in air and scanning electrochemical potential microscopy (SECPM) under electrochemical conditions, were used to study different single crystalline surfaces in electrochemistry. Especially SECPM offers potentially new insights into the solid-liquid interface by providing the possibility to image the potential distribution of the surface, with a resolution that is comparable to STM. In electrocatalysis, nanostructured catalysts supported on different electrode materials often show behavior different from their bulk electrodes. This was experimentally and theoretically shown for several combinations and recently on Pt on Au(111) towards fuel cell relevant reactions. For these investigations single crystals often provide accurate and well defined reference and support systems. We will show heteroepitaxially grown Ru, Ir and Rh single crystalline surface films and bulk Au single crystals with different orientations under electrochemical conditions. Image studies from all three different SPM methods will be presented and compared to electrochemical data obtained by cyclic voltammetry in acidic media. The quality of the single crystalline supports will be verified by the SPM images and the cyclic voltammograms. Furthermore, an outlook will be presented on how such supports can be used in electrocatalytic studies.