Elucidating the Kinetic Root of the Evolution of the Oriented Nanoporous Metal from Reduction-Induced Decomposition

Kinetics dictates the evolution of many complex nanostructures. Among them is the bi-continuous nanoporous metal, the signature product of dealloying that has excelled in catalysis, sensing, and energy storage with its randomly oriented, tortuous pores. However, recent reports on liquid metal dealloying have suggested that a different structure, where pores align along the direction of dealloying, could evolve under the control of bulk diffusion. Here, we investigate the underlying mechanism of a similar observation in a dealloying-analogy, reduction-induced decomposition (RID). Using the RID of AgCl as a model system, we show that nanoporous Ag evolves with a structural orientation along the ingression of a concentrated NaBH4 reducing solution, in contrast to the typical random orientation when reduced with a low concentration of NaBH4. By in situ optical microscopy, we quantify a reaction rate >10 mu m/s associated with the formation of the oriented structure with 1 M NaBH4, which then drops proportionally to the square root of time. We further measure the activation energy, confirming diffusion-controlled kinetics. We thereby propose a mechanism based on the interplay between surface diffusion-controlled pore bifurcation and bulk diffusion-controlled interface smoothening. The mechanism guides the fabrication of one hundred-micron thick, fully oriented nanoporous Ag in an alkaline solution with slower surface diffusion and the fabrication of oriented nanoporous Cu from rapid RID of CuI. The work elucidates the link between reaction kinetics and the structural orientation of the nanoporous metal, paving the way for enabling rapid mass transports in the pores for functional applications.