Sn cation valency dependence in cation exchange reactions involving Cu_2-xSe nanocrystals

We studied cation exchange reactions in colloidal Cu_2-xSe nanocrystals (NCs) involving the replacement of Cu⁺ cations with either Sn²⁺ or Sn⁴⁺ cations. This is a model system in several aspects: first, the +2 and +4 oxidation states for tin are relatively stable; in addition, the phase of the Cu_2-xSe NCs remains cubic regardless of the degree of copper deficiency (that is, "x") in the NC lattice. Also, Sn⁴⁺ ions are comparable in size to the Cu⁺ ions, while Sn²⁺ ones are much larger. We show here that the valency of the entering Sn ions dictates the structure and composition not only of the final products but also of the intermediate steps of the exchange. When Sn⁴⁺ cations are used, alloyed Cu_2-4ySn_ySe NCs (with y ≥ 0.33) are formed as intermediates, with almost no distortion of the anion framework, apart from a small contraction. In this exchange reaction the final stoichiometry of the NCs cannot go beyond Cu_0.66Sn_0.33Se (that is Cu₂SnSe₃), as any further replacement of Cu⁺ cations with Sn⁴⁺ cations would require a drastic reorganization of the anion framework, which is not possible at the reaction conditions of the experiments. When instead Sn²⁺ cations are employed, SnSe NCs are formed, mostly in the orthorhombic phase, with significant, albeit not drastic, distortion of the anion framework. Intermediate steps in this exchange reaction are represented by Janus-type Cu_2-xSe/SnSe heterostructures, with no Cu-Sn-Se alloys.