Coverage-dependent adsorption and desorption of oxygen on Pd(100)

Catalysis and Surface Chemistr

Understanding the Voltammetry of Bulk CO Electrooxidation in Neutral Media through Combined SECM Measurements

Recently, the bulk electrooxidation of CO on gold or platinum has been usedto detect CO produced during CO2reduction in neutral media. The CO bulk oxidationvoltammetry may show two distinct peaks depending on the reaction conditions, which up tonow have not been understood. We have used scanning electrochemical microscopy (SECM)to probe CO oxidation and pH in the diffusion layer during CO2reduction. Our results showthat the two different peaks are due to diffusion limitation by two different species, namely,CO and OH−.Wefind that between pH 7 and 11, CO oxidation by water and OH−gives riseto thefirst and second peak observed in the voltammetry, respectively. Additional rotating discexperiments showed that specifically in this pH range the current of the second peak isdiffusion limited by the OH−concentration, since it is lower than the CO concentration

The dualism between adatom- and vacancy-based single crystal growth models

In homoepitaxial crystal growth, four basic growth morphologies (idealized growth modes) have been established that describe the deposition of atoms on single crystal surfaces: step-flow, layer-by-layer, mound formation, and random/self-affine growth. Mound formation leads to nano-scale surface patterning. However, the formation of (nano)-islands, patterns, and roughness occurs also during ion bombardment, electrochemical etching and oxidation/reduction cycling. Here we show, in analogy to many particle/anti-particle formalisms in physics, the existence of the dualism between individual adatom and single vacancy growth modes. We predict that all standard adatom growth modes do exist also in their counter, vacancy version. For the particular case of mound formation, we derive the theoretical equations and show the inverse similarity of the solution. We furthermore treat simultaneous growth by adatoms and vacancies, and derive the analytical solution of the growth shape evolution of the mounds. Finally, we present an experimental verification, in which both adatom and vacancy mound formation are active. The theoretically predicted mound shape nicely fits the experimental observation

Atomic-Scale Identification of the Electrochemical Roughening of Platinum

Electrode degradation under oxidizing conditions is a major drawback for large-scale applications of platinumelectrocatalysts. Subjecting Pt(111) to oxidation−reduction cycles is known to lead to the growth of nanoislands. We study thisphenomenon using a combination of simultaneous in situ electrochemical scanning tunneling microscopy and cyclicvoltammetry. Here, we present a detailed analysis of the formed islands, deriving the (evolution of the) average island growthshape. From the island shapes, we determine the densities of atomic-scale defect sites, e.g., steps and facets, which show anexcellent correlation with the different voltammetric hydrogen adsorption peaks. Based on this combination of electrochemicalscanning tunneling microscopy (EC-STM) and CV data, we derive a detailed atomistic picture of the nanoisland evolutionduring potential cycling, delivering new insights into the initial stages of platinum electrode degradation

Mediator-Free SECM for Probing the Diffusion Layer pH with Functionalized Gold Ultramicroelectrodes

Probing pH gradients during electrochemical reactions is important to better understand reaction mechanisms and to separate the influence of pH and pH gradients from intrinsic electrolyte effects. Here, we develop a pH sensor to measure pH changes in the diffusion layer during hydrogen evolution. The probe was synthesized by functionalizing a gold ultramicroelectrode with a self-assembled monolayer of 4-nitrothiophenol (4-NTP) and further converting it to form a hydroxylaminothiophenol (4-HATP)/4-nitrosothiophenol (4-NSTP) redox couple. The pH sensing is realized by recording the tip cyclic voltammetry and monitoring the Nernstian shift of the midpeak potential. We employ a capacitive approach technique in our home-built Scanning Electrochemical Microscope (SECM) setup in which an AC potential is applied to the sample and the capacitive current generated at the tip is recorded as a function of distance. This method allows for an approach of the tip to the electrode that is electrolyte-free and consequently also mediator-free. Hydrogen evolution on gold in a neutral electrolyte was studied as a model system. The pH was measured with the probe at a constant distance from the electrode (ca. 75 μm), while the electrode potential was varied in time. In the nonbuffered electrolyte used (0.1 M Li2SO4), even at relatively low current densities, a pH difference of three units is measured between the location of the probe and the bulk electrolyte. The time scale of the diffusion layer transient is captured, due to the high time resolution that can be achieved with this probe. The sensor has high sensitivity, measuring differences of more than 8 pH units with a resolution better than 0.1 pH unit

Voltammetric scanning electrochemical cell microscopy : dynamic imaging of hydrazine electro-oxidation on platinum electrodes

Voltammetric scanning electrochemical cell microscopy (SECCM) incorporates cyclic voltammetry measurements in the SECCM imaging protocol, by recording electrochemical currents in a wide potential window at each pixel in a map. This provides much more information compared to traditional fixed potential imaging. Data can be represented as movies (hundreds of frames) of current (over a surface region) at a series of potentials and are highly revealing of subtle variations in electrode activity. Furthermore, by combining SECCM data with other forms of microscopy, e.g. scanning electron microscopy and electron backscatter diffraction data, it is possible to directly relate the current–voltage characteristics to spatial position and surface structure. In this work we use a “hopping mode”, where the SECCM pipet probe is translated toward the surface at a series of positions until meniscus contact. Small amounts of residue left on the surface, upon probe retraction, demark the precise area of each measurement. We use these techniques to study hydrazine oxidation on a polycrystalline platinum substrate both in air and in a deaerated environment. In both cases, the detected faradaic current shows a structural dependence on the surface crystallographic orientation. Significantly, in the presence of oxygen (aerated solution) the electrochemical current decreases strongly for almost all grains (crystallographic orientations). The results highlight the flexibility of voltammetric SECCM for electrochemical imaging and present important implications for hydrazine electroanalysis

Electrochemical oxidation of Pt(111) beyond the place-exchange model

Oxide formation plays an important role in the degradation of Pt electrocatalysts. However, the exact oxide structure and reaction mechanism are not fully understood. Here, we used in situ surface X-ray diffraction experiments to resolve the oxide formation at a Pt(111) model electrode at potentials near the onset of the oxygen evolution reaction. Fast experiments are possible by using X-ray photons with a high kinetic energy in combination with a large 2D detector. By employing very low potential sweep rates we obtain a more ordered oxidized surface compared to literature data from potential step experiments. This demonstrates that the oxidation process is strongly governed by the reaction kinetics. The increased surface order enables us to disentangle two subsequent oxidation process; initially the place-exchange process, followed by the formation of a partially disordered oxide in which still 50% of the surface atoms reside on sites commensurate to the Pt(111) surface. The reduction experiments indicate that the place-exchange process is structurally reversible, whereas the disordered oxide causes the surface roughening observed during potential cycling. Despite the increased surface order, oxide superstructures are not observed. These results provide important insights in the oxidation and degradation process of Pt(111), which are valuable for the design of improved electrocatalysts and they rationalize operating procedures

Magnetite (Fe3−O4) homoepitaxy observed by X-ray intensity growth oscillations

Processes on the Fe3−O4 (001) surface like oxidative regrowth, (partial)lifting of the subsurface cation vacancy reconstruction and theelement-specific incorporation of adatoms demonstrate the sensitiverelation of oxygen pressure, cation transport and structure in the nearsurfaceregion of Fe3−O4 influencing the performance of catalysts anddevices [1,2,3]. We exemplarily studied the homoepitaxial growth ofFe3−O4 (001) in dependence of the O2 pressure and iron flux. Xrayintensity growth oscillations proved ordered growth of Fe3−O4for all probed conditions while atomic force microscopy revealed newlyformed micrometre-sized surface structures exceeding the amount ofdeposited material [4]. Our results indicate the presence of multipleparallel processes during reactive Fe3−O4 homoepitaxy suggestingsimilar processes to occur also in other applications of Fe3−O4.[1] Nie et al., J. Am. Chem. Soc. 135, 10091 (2013), [2] Arndt, B. etal. PCCP 22, 8336 (2020), [3] Mirabella et al., Electrochimica Acta,389, 138638 (2021), [4] van der Vegt et al., Phys. Rev. Lett. 68, 3335(1992