Exploring\nthe Solution-Based Metathesis Reaction Pathway\nToward Cu<sub>3</sub>Si Formation

Copper silicide, Cu<sub>3</sub>Si, has a wide range of\napplications,\nincluding catalysis, photovoltaics, and energy storage. The complexity\nof the Cu–Si phase diagram makes synthesizing one phase with\ncontrol over stoichiometry and high purity challenging. The specific\nCu<sub>3</sub>Si phase described herein is more typically made by\nusing traditional solid-state methods. We demonstrate that Cu<sub>3</sub>Si@Si/SiO<sub>2</sub> matrix particles can be successfully\nsynthesized by combining colloidal solution and metathesis methods.\nThe reaction pathway is surprisingly complex and nuanced. Using powder\nX-ray diffraction, transmission electron microscopy, scanning election\nmicroscopy, and energy dispersive X-ray spectroscopy (EDS), it was\nfound that rather than Mg<sub>2</sub>Si and CuCl<sub>2</sub> in oleylamine\n(OLA) proceeding directly to Cu<sub>3</sub>Si@Si/SiO<sub>2</sub> matrix\nparticles (as you might expect in the solid state), it proceeds through\na two-step process. In the first step, Mg<sub>2</sub>Si quickly reduces\nthe Cu–OLA complex to Cu<sup>0</sup>, resulting in MgCl<sub>2</sub> and destabilized, amorphous Si matrices, which are capped\nwith OLA. Next, OLA aids in shuttling Cu to the Si matrix, and Cu\ndiffuses into the destabilized, amorphous structure to form Cu<sub>3</sub>Si particles embedded in a Si/SiO<sub>2</sub> matrix. We show\nthat the solvent is critical for controlling this reaction. Finally,\nthe matrix encasing the Cu<sub>3</sub>Si particles was selectively\nanalyzed by scanning transmission electron microscopy/EDS, electrochemical\ncycling, and X-ray photoelectron spectroscopy. This revealed that\nthe matrix contains active Si with minimal amounts of Mg, and the\nmatrix readily oxidizes, mainly to SiO<sub>2</sub>. This unique synthesis\nof Cu<sub>3</sub>Si@Si/SiO<sub>2</sub> matrix particles, although\nstill diffusion-limited, combines solution and metathesis methods\nto lower the high formation energy barrier commonly observed in solid-state\nmethods.