Construction of Hollow Nanocoils via Controlled Interfacial Reaction in Colloidal Solution

Conversion chemistry has attracted much attention owing to its simplicity and versatility. Especially, interfacial reactions caused by electrochemical spontaneity in colloidal solutions induce the creation of unique morphologies. Elaborately designed reaction solutions generate unequal diffusion rates of different atomic species at the solid-liquid interface, as known as the Kirkendall effect, allowing nanomaterials to establish various morphologies such as hollow and frame shapes [1].<br/>The coil structure, one of the topologies that exist in many natural materials, exhibits unique properties that do not appear in existing simple structures [2]. In particular, nanoscale artificial coil fabrication is pioneering new areas that have not been previously shown in applications such as bio and energy fields. Despite considerable research efforts, however, realizing and applying the nanoscale coil shape is complex, and has limited the accessible compositions and shapes.<br/>In this study, we developed a strategy to control the interfacial reaction at a solid-liquid interface that allows the shape-regulated transition from a metal (CoFe) nanocoil to a single-component hollow nanocoil (CrPO₄) and a multicomponent hollow nanocoil (MnO_x+Mn(PO₄)_y) [3]. Based on the Pourbaix diagram, we controlled the final state of the metal cations, which were reduced by CoFe via galvanic exchange at the interface between the CoFe nanocoil and the aqueous reaction solution. HCrO₄^- was reduced to the single state of Cr³⁺, which then reacted with a phosphate ion, thereby forming a single-component HNC with a smooth surface. In contrast, MnO₄^- was simultaneously reduced to MnO_x(s) and Mn²⁺_(aq), and therefore a multicomponent HNC with a rough surface consisting of MnO_x and Mn(PO₄)_y was formed. To demonstrate the great potential of the synthesized hollow nanocoils, multicomponent HNCs with rough surfaces act as a substrate that can increase the catalytically active area of 4 nm-sized Mn₃O₄ nanoparticles, increasing the catalytic efficiency for the oxygen evolution reaction (OER) catalyst relative to catalysts consisting of only nanoparticles at neutral pH.<br/><b>Reference</b><br/>[1] Y. Xia <i>et al., Adv. Mater</i>, 25, 6313 (2013)<br/>[2] N. A. Kotov <i>et al., Adv. Mater</i>, 32, 1906738 (2020)<br/>[3] J. H. Moon <i>et al., Small</i>, 2103575 (2021)