The Sieradzki Research Group

Research


Electrochemical Surface Science

ECSTM images of the low density phase obtained under different bias voltages; STM tip potentials. Image sizes 6 x 6 nm. (a) sulfate in the (root 3 x root 3) structure obtained with a tip bias of 30 mV. (b) copper honeycomb structure with a tip bias of -30 mV. (c) ECSTM image demonstrating the reproducibility of the images as the bias is switched (as indicated) during imaging.

We use electrochemical scanning tunneling microscopy (ECSTM) to examine fundamental issues with respect to the behavior of solid elemental metal and alloy surfaces in electrolytes. We have used this technique to examine dealloying, underpotential deposition (upd), elemental deposition (so-called defect-mediated growth; an analogue of mbe growth in electrolytes), surface reconstructions and the stability of 2-10 nanometer diameter metallic particles. 

The ECSTM results above show both the adsorbed Cu and SO42- in the upd honeycomb phase at 2/3 ML coverage. To our knowledge our group is the only group that has successfully imaged both adsorbed sulfate and copper in this adlayer.





Electrochemical Stability of Elemental and Alloy Nanoparticles


ECSTM showing a potential – time sequence of 5 Pt particles dissolving in 0.1M H2SO4; J. Am. Chem. Soc., 596, 11723, 2010. Right: Particle-size dependent potential – pH diagram for Pt/10-6 MPt2+; J. Am. Chem. Soc., 132, 11723, 2010.

Our most recent endeavor is aimed at developing an understanding of the factors that control the stability of nanometer scale metal particles such as that used in catalytic structures. For example, it is well known that the stability of platinum and platinum alloy catalysts is an important issue in the operation of PEM fuel cells. Today many Pt alloy particles for this application are fabricated by a process involving dealloying.

We have recently developed techniques and conducted experiments to examine the stability of individual 2-5 nm diameter platinum particles in an electrochemical STM. Future work will be aimed at understanding dealloying of alloy nanoparticles and the stability of these particles.





Dealloying and Nanoporous Solids



Kinetic Monte Carlo (KMC) simulation of nanoporous gold formation; Nature, 410, 450 (2001). On the right is an analysis of the results. The data points are the KMC results and the lines correspond to a theoretical thermodynamic analysis for the critical potential; Nature Materials, 5, 946 (2006).

Our group has been engaged in dealloying research and nanoporous solids for over 25 years. We "invented" monolithic nanoporous gold and performed the first set of mechanical and electrochemical characterization of such solids.

Additionally, these porous nanostructures which always evolve during dealloying of single phase alloys are believed to serve as a "trigger" for the film-induced cleavage mechanism of stress-corrosion cracking. Recent work has focused on a thermodynamic description of the so-called critical potential for dealloying.





Sample Size Effects in the Plastic Behavior of Solids



Pillar arrays and stress-strain curves. (A) Example of FIB-machined pillar array. (B) Compression stress-strain curves for two sets of Ni pillars: 24 pillars with diameter of 160 nm (red) and 15 Pillars with diameter of 270 nm (blue). Acta Materialia 56, 511 (2008).

Currently there is a considerable effort underway to understand dislocation-mediated plasticity in crystalline solids. Our own work which has focused on the behavior of FIB-machined Ni nanoscale pillars shows evidence for power-law scaling of the flow stress with the reciprocal of the pillar diameter and that the exponent in the scaling relation derives statistical expectations that have been used to understand the size-dependent strength of brittle solids such as glass.





Dealloying and Stress Corrosion Cracking



This sample was dealloyed to form a 1 mm thick porous layer and then removed from the electrolyte, immersed in DI water for 300 s and finally loaded to fracture in air. The grain boundary crack in the figure penetrated a distance of ~4 x the thickness of the dealloyed layer.

One of our major interests in SCC is to understand the mechanics of crack injection from a brittle layer into a ductile substrate, Recent work has focused on experiments of AgAu alloys using focused ion beam machining cross-sectioning to examine such crack injections. Briefly a prescribed thickness nanoporous gold layer is fabricated by dealloying the surface and the sample is removed from the electrolyte. The sample is then washed and/or dried with DI water and loaded. The figure shows evidence of crack injection into an un-dealloyed region of grain boundary.