Dealloying at High Homologous Temperature: Morphology Diagrams

Abstract

Dealloying under conditions of high homologous temperature, T H , (or high intrinsic diffusivity of the more electrochemically reactive component) is considerably more complicated than at low T H since solid-state mass transport is available to support this process. At low T H the only mechanism available for dealloying a solid is percolation dissolution, which results in a bicontinuous solid-void morphology for which nanoporous gold serves as the prototypical example. At high T H , there is a rich set of morphologies that can evolve depending on alloy composition and the imposed electrochemical conditions, including negative or void dendrites, Kirkendall voids and bi-continuous porous structures. We report on a study of morphology evolution upon delithiation of Li-Sn alloys, produced by the electrochemical lithiation of Sn sheets. Electrochemical titration and time of flight measurements were performed in order to determine the intrinsic diffusivity of Li,D Li , as a function of alloy composition, which ranged from ∼5 × 10 −8 -4 × 10 −12 cm 2 s −1 . The activation energy forD Li was measured in the temperature range 30-140 • C and found to be 37.4, 37.9 and 22.5 kJ/mole, respectively for the phases Li 2 Sn 5 , LiSn and Li 7 Sn 3 . Morphology evolution was studied under conditions of fixed dealloying potential and fixed current density and our results are summarized by the introduction of dealloying morphology diagrams that reveal the electrochemical conditions for the evolution of the various morphologies. Today electrochemical dealloying processes are used to design nanostructures for a variety of functions encompassing electrocatalysis 1-3 and biosensors 4-7 with additional applications being explored such as actuation [8][9][10][11] and structural composites. 12-14 To date, the targeted nanoscale morphologies include bi-continuous structures such as nanoporous gold (NPG) formed by a percolation dissolution mechanism 15-18 and so-called skin or core-shell nanoparticle structures, formed by a passivation-like process.2 Several of these architectures are employed in the fabrication of Pt-based alloy nanoparticle electrocatalysts for fuel cell applications, 1,3,19 Since the solid-state transport processes available to support selective dissolution are temperature dependent one should expect the resulting morphologies to be similarly dependent on temperature. For example, in a noble metal alloy such as Ag-Au at 300 K (i.e., low homologous temperature) the diffusivity of each of the components is of order 10 −32 cm 2 s −120 which is about 20 orders of magnitude too low to contribute to dealloying processes over technologically relevant time scales. Consequently, percolation dissolution is the only mechanism available for dealloying resulting in the well-studied morphology of NPG. However, at higher homologous temperature, T H , we should expect additional sets of dealloying morphologies to evolve. Here our use of the term high T H is meant to designate that the solid-state mobility of the component that is selecti...

Dealloyed nanoporous materials with interface-controlled behavior

“…17,18 Figure 2 shows a dealloying morphology diagram for Ag-Au alloys, which conveniently summarizes the above issues visually.…”

Revealing 3D Morphological and Chemical Evolution Mechanisms of Metals in Molten Salt by Multimodal Microscopy

“…However, as the temperature increases or in other alloy systems with lower melting points, the bulk diffusion may start contributing to the process. Under such conditions, “inverse dendrite” formation as proposed by Sieradzki et al may also occur in molten salt dealloying systems, via a vacancy-mediated lattice diffusion mechanism.…”

Dissimilar Metal Solution from Solid Alloys as Observed for Steels and Nickel‐Based Alloys in the Presence of Lead‐Based Liquid Alloys or Liquid Tin

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