MS-10-O-3344 Electron tomography investigation of a novel etching process yielding hollow structures in Cu_2-xSe(core)/Cu_2-xS(shell) nanocrystals exposed to oxidizing environments

Electron tomography has proven to be a unique tool for the elucidation of nanoscale mechanisms that cannot be clarified by simple two-dimensional imaging. In this contribution a novel etching process, induced by exposure of colloidal Cu_2-xSe(core)/Cu_2-xS(shell) bullet-in-rod nanocrystals (NCs) to oxidative environments, is analyzed via a combination of TEM-related techniques, with a key role played by electron tomography. The progressive addition of an oxidizing agent (namely, cupric chloride, CuCl₂, in solution with methanol) leads to extraction of electrons and Cu⁺ ions from the NCs suspended in toluene. The process has been followed by exposure of the NCs to increasing amounts of CuCl₂ (η = molar ratio of CuCl₂ to Cu⁺ ions in the NCs). Surprisingly, the NC region initially dismantled is the Cu_2-xSe core rather than the relatively thick (about 3 nm) outer Cu_2-xS shell (Fig. 1 a-c). HAADF-STEM tomography-based volume reconstruction of the weakly etched NCs (η=2) evidences in most cases a hollow morphology (Fig. 2i). Only a small fraction of the NCs exhibits empty channels connecting the central void to the shell surface at the same etching stage (Fig. 2ii). This demonstrates that Cu⁺ ions extracted from the NCs into the solution must have diffused from the core through into the shell. This mechanism is substantially different from the classical Kirkendall effect [1], in which the formation of hollow structures is due to the different diffusion rates of two species across an interface. In this case the out-diffusion of a single species (Cu⁺ ions) is demonstrated.
Further addition of CuCl₂ to the Cu_2-xSe/Cu_2-xS NCs leads to dismantling of the shell, after the core region has been emptied (Fig. 2iii-iv). A central porous region is left with CuS_xSe_1-x composition. Selected area electron diffraction (SAED) patterns and HRTEM analyses show an evolution of the shell material from monoclinic Cu₂S (α-chalcocite) to a covellite (hexagonal CuS)-like structure, which shows higher stability by surviving till later stages of the oxidation process (Fig. 1 d-h).
The etching mechanism presented here is triggered both by the stronger tendency of Cu_2-xSe over Cu_2-xS to oxidation and by the fast Cu⁺ ion diffusion in copper chalcogenides. This novel process for the fabrication of copper chalcogenide hollow crystals can be employed in the production of nanostructured crystals for applications in catalysis, energy storage, plasmonics and medicine.

[1] Y. Yin et al., Science 304, 711 (2004)