Exploring the Solution-Based Metathesis Reaction Pathway Toward Cu3Si Formation

Copper silicide, Cu₃Si, has a wide range of applications, including catalysis, photovoltaics, and energy storage. The complexity of the Cu–Si phase diagram makes synthesizing one phase with control over stoichiometry and high purity challenging. The specific Cu₃Si phase described herein is more typically made by using traditional solid-state methods. We demonstrate that Cu₃Si@Si/SiO₂ matrix particles can be successfully synthesized by combining colloidal solution and metathesis methods. The reaction pathway is surprisingly complex and nuanced. Using powder X-ray diffraction, transmission electron microscopy, scanning election microscopy, and energy dispersive X-ray spectroscopy (EDS), it was found that rather than Mg₂Si and CuCl₂ in oleylamine (OLA) proceeding directly to Cu₃Si@Si/SiO₂ matrix particles (as you might expect in the solid state), it proceeds through a two-step process. In the first step, Mg₂Si quickly reduces the Cu–OLA complex to Cu⁰, resulting in MgCl₂ and destabilized, amorphous Si matrices, which are capped with OLA. Next, OLA aids in shuttling Cu to the Si matrix, and Cu diffuses into the destabilized, amorphous structure to form Cu₃Si particles embedded in a Si/SiO₂ matrix. We show that the solvent is critical for controlling this reaction. Finally, the matrix encasing the Cu₃Si particles was selectively analyzed by scanning transmission electron microscopy/EDS, electrochemical cycling, and X-ray photoelectron spectroscopy. This revealed that the matrix contains active Si with minimal amounts of Mg, and the matrix readily oxidizes, mainly to SiO₂. This unique synthesis of Cu₃Si@Si/SiO₂ matrix particles, although still diffusion-limited, combines solution and metathesis methods to lower the high formation energy barrier commonly observed in solid-state methods.