Topochemical reactions are chemical reactions of solids that enable transforming their chemical compositions and crystal structures, with short movements of the constitutive atoms. When applied to nano-objects, these approaches enable to reach complex compositions, heterostructures and shapes.[1] Topochemical reactions of nano-objects however rely on colloidal syntheses below 300 °C in usual solvents, which limits their use to ionic or metallic objects. Post-modifications of more covalent materials would pave the way to specific properties, related to (electro)catalysis, magnetism, and hardness, for instance, but they require higher temperatures.[2-4] In this work, we will show how to trigger and control topochemical reactions of covalent nanoparticles by using liquids stable at relatively high temperatures, thereby giving access to new compositions. We will focus on molten salts as thermally stable liquids, and focus on topochemical reactions involving both cation exchange and galvanic replacement.

Cation exchange in indium pnictide nanocrystals dispersed in molten KGaI₄ and KAlI₄ has been very recently unveiled to tune the optical properties of these III-V nanocrystals.[1,2] With the aim of targeting more covalent materials and possibly modulating (electro)catalytic properties versus CO₂ and CO electroreduction (CO₂R and COR), we have focused on cation exchange on CuSi₂P_3. This compound crystallizes in a distorted zinc blende structure composed of corner-sharing [CuP₄] and [SiP₄] tetrahedra.[5] The material encompasses Cu⁺ cations in a covalent [Si₂P₃]^- framework. It shows high performances as electrocatalyst for COR into acetate. We hypothetize that partial exchange of Cu⁺ and Si by Zn²⁺ and Ga^3+/2+/+ would enable adjusting the catalytic selectivity and activity, also probably impacting optoelectronic properties. The existence of zinc blende-related Zn_1.5Si_1.5P₃, and Ga_1.5Si_1.5P₃ [6,8] with [MP₄] (M=Zn or Ga) and [SiP₄] tetrahedra hints at the possibility to exchange Cu and Si in CuSi₂P₃ nanoparticles.

In this talk, we will demonstrate successful topochemical reactions of covalent silicophosphide nanoparticles in molten salts.  By using synchrotron radiation-based in situ X-ray diffraction, during the reactions in molten salts, we unveiled minute scale transformations of CuSi₂P₃ into Zn_1.5Si_1.5P₃ or into a bimetallic copper zinc silico-phosphide (Cu_0.9Zn_0.2Si_2.0 P_2.4) by reaction with Zn²⁺ precursors. The selectivity of the reaction can be tune by adjusting the amount of Zn reagent. On the other hand, we succeeded to react CuSi₂P₃ nanoparticles with different gallium precursors (GaCl₃, GaI₃ and Ga₂I₃ [2]) to yield bimetallic copper gallium silicophosphides Cu_0.8Ga_0.2Si_2.0P_2.3, Cu_0.6Ga_0.4Si_2.0P_1.9 and Cu_0.7Ga_0.4Si_2.0P_2.2 via a similar process. Partial substitution of Cu by Zn or Ga into the silicophosphide is accompanied by a phase transition from a cubic structure to a tetragonal one. TEM confirmed that the morphology and size of the particles did not evolve significantly, while TEM-EDS mapping confirmed the homogenous distribution of the elements at the particle scale. EXAFS at the Cu-K edge showed the preservation of [CuP₄] tetrahedra, while EXAFS at the Ga-K and Zn-K edges indicated the formation of [ZnP₄] and [GaP₄] tetrahedra, as expected for an exchange reaction, which was confirmed by solid-state ³¹P, ⁷¹Ga and ²⁹Si NMR. XANES demonstrated that cationic exchange with Zn or Ga is accompanied by galvanic reactivity, initial Cu⁺ gets oxidized to an average oxidation state Cu^1.5+ meanwhile Ga and Zn species are reduced during the topochemical reaction.The evaluation of the electrocatalytic properties of these new bimetallic silicophosphides for CO reduction is under way.