Jonah Erlebacher

Atomic origins of the high catalytic activity of nanoporous gold

Distinct from inert bulk gold, nanoparticulate gold has been found to possess remarkable catalytic activity towards oxidation reactions. The catalytic performance of nanoparticulate gold strongly depends on size and support, and catalytic activity usually cannot be observed at characteristic sizes larger than 5 nm. Interestingly, significant catalytic activity can be retained in dealloyed nanoporous gold (NPG) even when its feature lengths are larger than 30 nm. Here we report atomic insights of the NPG catalysis, characterized by spherical-aberration-corrected transmission electron microscopy (TEM) and environmental TEM. A high density of atomic steps and kinks is observed on the curved surfaces of NPG, comparable to 3-5 nm nanoparticles, which are stabilized by hyperboloid-like gold ligaments. In situ TEM observations provide compelling evidence that the surface defects are active sites for the catalytic oxidation of CO and residual Ag stabilizes the atomic steps by suppressing {111} faceting kinetics.

An Atomistic Description of Dealloying Porosity Evolution, the Critical Potential, and Rate-Limiting Behavior

We describe the microscopic details of porosity formation during dealloying as illuminated by a kinetic Monte Carlo model incorporating site coordination-dependent surface diffusion of all alloy components, and site coordination-dependent dissolution of the less-noble atoms. Our simulation model reproduces the entire range of phenomena associated with selective dissolution. These phenomena include composition and geometric restrictions on dealloying (parting limits), a composition-dependent critical potential, a passivation regime, a regime of steady-state dissolution flux, and porosity formation. We find that an intrinsic critical potential exists as a well-defined threshold potential separating surface passivation and porosity formation behaviors, but this intrinsic critical potential typically sits at values well below the experimental measurements of the empirical critical potential. Finally, by detailed examination of the temperature dependence of the dissolution flux, and also of the types of surface sites contributing to the dissolution flux, we predict that the binding energy of a terrace atom may be straightforwardly extracted from analysis of experimental polarization curves.

Oxygen reduction in nanoporous metal–ionic liquid composite electrocatalysts

The improvement of catalysts for the four-electron oxygen-reduction reaction (ORR; O(2) + 4H(+) + 4e(-) → 2H(2)O) remains a critical challenge for fuel cells and other electrochemical-energy technologies. Recent attention in this area has centred on the development of metal alloys with nanostructured compositional gradients (for example, core-shell structure) that exhibit higher activity than supported Pt nanoparticles (Pt-C; refs 1-7). For instance, with a Pt outer surface and Ni-rich second atomic layer, Pt(3)Ni(111) is one of the most active surfaces for the ORR (ref. 8), owing to a shift in the d-band centre of the surface Pt atoms that results in a weakened interaction between Pt and intermediate oxide species, freeing more active sites for O(2) adsorption. However, enhancements due solely to alloy structure and composition may not be sufficient to reduce the mass activity enough to satisfy the requirements for fuel-cell commercialization, especially as the high activity of particular crystal surface facets may not easily translate to polyfaceted particles. Here we show that a tailored geometric and chemical materials architecture can further improve ORR catalysis by demonstrating that a composite nanoporous Ni-Pt alloy impregnated with a hydrophobic, high-oxygen-solubility and protic ionic liquid has extremely high mass activity. The results are consistent with an engineered chemical bias within a catalytically active nanoporous framework that pushes the ORR towards completion.

Structure/Processing/Properties Relationships in Nanoporous Nanoparticles As Applied to Catalysis of the Cathodic Oxygen Reduction Reaction

We present a comprehensive experimental study of the formation and activity of dealloyed nanoporous Ni/Pt alloy nanoparticles for the cathodic oxygen reduction reaction. By addressing the kinetics of nucleation during solvothermal synthesis we developed a method to control the size and composition of Ni/Pt alloy nanoparticles over a broad range while maintaining an adequate size distribution. Electrochemical dealloying of these size-controlled nanoparticles was used to explore conditions in which hierarchical nanoporosity within nanoparticles can evolve. Our results show that in order to evolve fully formed porosity, particles must have a minimum diameter of ∼15 nm, a result consistent with the surface kinetic processes occurring during dealloying. Nanoporous nanoparticles possess ligaments and voids with diameters of approximately 2 nm, high surface area/mass ratios usually associated with much smaller particles, and a composition consistent with a Pt-skeleton covering a Ni/Pt alloy core. Electrochemical measurements show that the mass activity for the oxygen reduction reaction using carbon-supported nanoporous Ni/Pt nanoparticles is nearly four times that of commercial Pt/C catalyst and even exceeds that of comparable nonporous Pt-skeleton Ni/Pt alloy nanoparticles.

Three-dimensional morphology of nanoporous gold

We report transmission electron tomography of nanoporous gold fabricated by chemically dealloying Au35Ag65 films. A number of algorithms were employed to quantitatively characterize the complex three-dimensional nanoporous structure. It was found that gold ligaments and nanopore channels are topologically and morphologically equivalent, i.e., they are inverses of each other in three-dimensional space. Statistical analysis reveals that this bicontinuous nanostructured material is actually quasiperiodic and has, on average, a near zero surface curvature. These quantitative measurements will help in understanding the structural stability of nanoporous gold and in modeling its physical and chemical performances.

Pattern formation during dealloying

Dealloying is a process during which an element is selectively dissolved from an alloy. If the starting material is single phase and within certain compositional limits dealloying results in the formation of bicontinuous structures on nanometer length scales. We introduce the topic of dealloying and discuss its phenomenology. Various models of dealloying and porous metal formation are presented. � 2003 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Size dependence of effective Young's modulus of nanoporous gold

Nanoporous gold (NPG) is a brittle, three-dimensional, random structure of Au with nanometer scale open porosity that is made by dealloying Au∕Ag alloys in acid. In this work, Young’s modulus of NPG with controlled porosity variation between 3 and 40nm is determined by mechanical testing of ∼100nm thick, free standing, large-grained, stress-free films of NPG using a buckling-based method [C. Stafford et al., Nat. Mater. 3, 545 (2005)]. Results showing a dramatic rise in the effective Young’s modulus of NPG with decreasing ligament size, especially below 10nm are presented, and possible reasons for this behavior are discussed.

Dealloying Silver/Gold Alloys in Neutral Silver Nitrate Solution: Porosity Evolution, Surface Composition, and Surface Oxides

The electrochemistry of dealloying silver/gold alloys in neutral pH silver nitrate solution to form nanoporous gold (NPG) is discussed. At pH 7, porosity evolution was observed to occur at high potentials, above that required for oxygen evolution, and within the nominal domain of the Pourbaix diagram where silver would be expected to form a passivating oxide. Electron microscopy shows that a small pore (∼5 nm) NPG is formed over a potential regime of 1.3-2.0 V vs normal hydrogen electrode, but electrochemical measurements show that the specific capacitance of samples over the same voltage range rises nearly threefold. The observations are explained in terms of residual surface oxides passivating the pores behind the dissolution front, which is itself acidified (and thus corrosive) due to an accumulation of protons associated with oxide formation and water dissociation. A model is proposed that is consistent with the electrochemical and microscopy results. This method of fabricating NPG has advantages of simplicity and safety, and the porosity formation mechanism may be extended to other systems.

Nonlinear amplitude evolution during spontaneous patterning of ion-bombarded Si(001)

The time evolution of the amplitude of periodic nanoscale ripple patterns formed on Ar+ sputtered Si(OOl ) surfaces was examined using a recently developed in situ spectroscopic technique. At sufficiently long times, we find that the amplitude does not continue to grow exponentially as predicted by the standard Bradley-Harper sputter rippling model. In accounting for this discrepancy, we rule out effects related to the concentration of mobile species, high surface curvature, surface energy anisotropy, and ion-surface interactions. We observe that for all wavelengths the amplitude ceases to grow when the width of the topmost terrace of the ripples is reduced to approximately 25 nm. This observation suggests that a short circuit relaxation mechanism limits amplitude . growth. A strategy for influencing the ultimate ripple amplitude is discussed.

Dealloying of Noble-Metal Alloy Nanoparticles

Dealloying is currently used to tailor the morphology and composition of nanoparticles and bulk solids for a variety of applications including catalysis, energy storage, sensing, actuation, supercapacitors, and radiation damage resistant materials. The known morphologies, which evolve on dealloying of nanoparticles, include core-shell, hollow core-shell, and porous nanoparticles. Here we present results examining the fixed voltage dealloying of AgAu alloy particles in the size range of 2-6 and 20-55 nm. High-angle annular dark-field scanning transmission electron microcopy, energy dispersive, and electron energy loss spectroscopy are used to characterize the size, morphology, and composition of the dealloyed nanoparticles. Our results demonstrate that above the potential corresponding to Ag(+)/Ag equilibrium only core-shell structures evolve in the 2-6 nm diameter particles. Dealloying of the 20-55 nm particles results and in the formation of porous structures analogous to the behavior observed for the corresponding bulk alloy. A statistical analysis that includes the composition and particle size distributions characterizing the larger particles demonstrates that the formation of porous nanoparticles occurs at a well-defined thermodynamic critical potential.