Dr. Wu, H.-L.; Assist Prof. Sato, R.; Mr. Kimura, M.; Prof. Teranishi, T.
(Advanced Inorganic Synthesis, Division of Synthetic Chemistry)

Mr.Yamaguchi,A.; Assist Prof. Haruta, M.; Prof. Kurata, H.
(Electron Microscopy and Crystal Chemistry, Advanced Research Center for Beam Science)

Ionic nanocrystals (NCs) have been widely used as photo-functional materials such as photocatalysts and photo-electric conversion materials, which are determined by the constituent elements, morphologies, and crystal structures. Because the stable crystal structures of the ionic crystals follow their phase diagrams, it has been difficult to chemically synthesize the high-temperature stable phases.
We investigated the crystal structures of the pseudomorphic nanocages, which were formed by the anion exchange (O ^2– → S^2–) of hexahedral Cu₂O NCs enclosed with {100} planes and rhombic dodecahedral Cu₂O NCs enclosed with {110} planes at ambient conditions. It was discovered that the shape-dependent anionic framework (surface anion sublattice and stacking pattern) of Cu₂O NCs determined the crystal system of anion-exchanged products (Figure). In other words, the cubic system can transform into not only the cubic system but also the triclinic and hexagonal systems. Furthermore, we found that the resulting nanocages have the multiply-twinned structure. A high-temperature stable phase such as wurtzite ZnS was also obtained with this method at ambient conditions. We envisage that this pseudomorphic transformation method could be applicable to a number of other ionic NCs and thin films at ambient temperatures to find the novel properties and functions.

Figure.  (Upper) The hexahedral Cu_1.8S nanocages with a cubic phase are formed by the anion exchange (O^2– → S^2–) and the subsequent etching of hexahedral Cu₂O NCs enclosed with {100} planes at ambient conditions. Further cation exchange (Cu⁺ → Cd²⁺, Zn²⁺) gives the hexahedral CdS or ZnS with a cubic phase. (Lower) The rhombic dodecahedral Cu_1.75S nanocages with a triclinic phase are formed by the anion exchange (O^2– → S^2–) and the subsequent etching of rhombic dodecahedral Cu₂O NCs enclosed with {110} planes at ambient conditions. Further cation exchange (Cu⁺ → Cd²⁺, Zn²⁺) gives the hexahedral CdS or ZnS with a hexagonal phase. 

Wu, H.-L.; Sato, R.; Yamaguchi, A.; Kimura, M.; Haruta, M.; Kurata, H.; Teranishi, T., Formation of pseudomorphic nanocages from Cu₂O nanocrystals through anion exchange reactions, Science,DOI: 10.1126/science.aad5520 (2016).