Cody A. Friesen

Electrochemical Stability of Nanometer-Scale Pt Particles in Acidic Environments

Understanding and controlling the electrochemical stability or corrosion behavior of nanometer-scale solids is vitally important in a variety of applications such as nanoscale electronics, sensing, and catalysis. For many applications, the increased surface to volume ratio achieved by particle size reduction leads to lower materials cost and higher efficiency, but there are questions as to whether the intrinsic stability of materials also decreases with particle size. An important example of this relates to the stability of Pt catalysts in, for example, proton exchange fuel cells. In this Article, we use electrochemical scanning tunneling microscopy to, for the first time, directly examine the stability of individual Pt nanoparticles as a function of applied potential. We combine this experimental study with ab initio computations to determine the stability, passivation, and dissolution behavior of Pt as a function of particle size and potential. Both approaches clearly show that smaller Pt particles dissolve well below the bulk dissolution potential and through a different mechanism. Pt dissolution from a nanoparticle occurs by direct electro-oxidation of Pt to soluble Pt(2+) cations, unlike bulk Pt, which dissolves from the oxide. These results have important implications for understanding the stability of Pt and Pt alloy catalysts in fuel cell architectures, and for the stability of nanoparticles in general.

Reversible stress changes at all stages of Volmer–Weber film growth

Stresses caused by Volmer–Weber growth of polycrystalline Cu films have been measured in situ during: Island nucleation and growth, island coalescence, and post-coalescence film thickening. Growth interruptions followed by resumption of growth resulted in the observation of reversible stress changes in all regimes. Reversible stress changes in the pre-coalescence and post-coalescence regimes are similar in that: The stress evolves in the tensile direction during growth interruptions, the initial rate of stress evolution is significantly faster when growth is resumed than when growth is first interrupted, and the magnitude of the reversible stress change increases with increasing pre-interruption deposition rate. It is argued that reversible stress changes are associated with changes in adatom and other surface defect concentrations, corresponding with changes in the growth flux. It is shown that the change in stress-thickness product with changing film thickness (the instantaneous stress) can be related t...

Electrochemical Stability of Elemental Metal Nanoparticles

The corrosion behavior of nanometer-scale solids is important in applications ranging from sensing to catalysis. Here we present a general thermodynamic analysis of this for the case of elemental metals and use the analysis to demonstrate the construction of a particle-size-dependent potential-pH diagram for the case of platinum. We discuss the data set required for the construction of such diagrams in general and describe how some parameters are accessible via experiment while others can only be reliably determined from first-principles-based electronic structure calculations. In the case of Pt, our analysis predicts that particles of diameter less than approximately 4 nm dissolve via the direct electrochemical dissolution pathway, Pt --> Pt(2+) + 2e(-), while larger particles form an oxide. As an extension of previously published work by our group, electrochemical scanning tunneling microscopy is used to examine the stability of individual Pt-black particles with diameters ranging from 1 to 10 nm. Our experimental results confirm the thermodynamic predictions, suggesting that our analysis provides a general framework for the assessment of the electrochemical stability of nanoscale elemental metals.

Direct Observation of Bifunctional Electrocatalysis during CO Oxidation at Ruθ=0.37/Pt{111} Surfaces via Surface Stress Measurements

The surface stress response during the electrooxidation of CO at Pt{111}, Ru{0001}, and Ru(theta=0.37)/Pt{111} textured electrodes was studied in 0.1 M HClO(4) electrolytes. The surface stress signal resolves for the first time the adsorption of OH(-) at the CO-covered Ru{0001} surface prior to significant CO oxidation, a phenomenon that is not discernible in the voltammetry. The surface stress signal shows that significant tensile surface stress occurs upon oxidation of the adsorbed CO and occurs at nearly the same potential on Ru{0001} and Ru/Pt{111} surfaces. These observations demonstrate that the mechanism of bifunctionality is the OH(ads) provided to the Pt surface sites via Ru sites.

Superhard Nanobuttons: Constraining Crystal Plasticity and Dealing with Extrinsic Effects at the Nanoscale

The compressive plastic strength of nanosized single-crystal metallic pillars is known to depend on their diameter D. Herein, the role of pillar height h is analyzed instead, and the suppression of the generalized crystal plasticity below a critical value h(CR) is observed. Novel in situ compression tests on regular pillars as well as nanobuttons, that is, pillars with h < h(CR), show that the latter are much harder, withstanding stresses >2 GPa. A statistical model that holds for both pillars and buttons is formulated. Owing to their superhard nature, the nanobuttons examined here underline with unprecedented resolution the extrinsic effects-often overlooked-that naturally arise during testing when the Saint-Venant assumption ceases to be accurate. The bias related to such effects is identified in the test data and removed when possible. Finally, continuous hardening is observed to occur under increasing stress level, in analogy to reports on nanoparticles. From a metrological standpoint the results expose some difficulties in nanoscale testing related to current methodology and technology. The implications of the analysis of extrinsic effects go beyond nanobuttons and extend to nano-/microelectromechanical system design and nanomechanics in general.

The Influence of Platinum Surface Morphology on the Electrooxidation of Methanol in Alkaline Solutions

In this work we compare methanol oxidation characteristics in pH 0 and pH 14 electrolytes, and examine the effect of methanol concentration and platinum surface condition (e.g. H-UPD, OH adsorption, Pt oxidation/reduction, planar, nanoporous) on the oxidation current. We observe that in rotating disc electrode (RDE) experiments the oxidation currents on smooth platinum decrease with increasing rotation rate. We show that this decrease is associated with increasing the rate at which oxidation intermediates are swept into the bulk electrolyte before being fully oxidized. To increase the dwell time of intermediates a platinum electrode with 3 nm length scale surface porosity was synthesized by dealloying a Pt-Cu alloy. We then perform the same RDE experiments and show that in this case the methanol oxidation current increases with increasing convection. We attribute this behavior to the trapping of reaction intermediates that otherwise were swept away on the polished sample.

Stumbling on extrinsic effects in super-hard nanobuttons

The compressive plastic strength of nanosized single crystal metallic pillars is known to depend on the diameter D, but little attention has been given to the pillar height h. The important role of h is analyzed here, observing the suppression of generalized crystal plasticity below a critical value hCR that can be estimated a priori. Novel in-situ compression tests on regular pillars (D = 300-900 nm) as well as nanobuttons (i.e. very short pillars with h less than hCR, such as D = 200 nm and h < 120 nm in this case) show that the latter ones are exceedingly harder than ordinary Ni pillars, withstanding stresses greater than 2 GPa. This h-controlled transition in the plastic behaviour is accompanied by extrinsic plastic effects in the harder nanobuttons. Such effects normally arise as Saint-Venants assumption ceases to be accurate. Some bias related to those effects is identified and removed from test data. Our results underline that nanoscale testing is challenging when current methodology and technology are pushed to the limit.