Selective Etching of Copper Sulfide Nanoparticles and Heterostructures through Sulfur Abstraction: Phase Transformations and Optical Properties

Abstract

Integrating top-down methods, such as chemical etching, for the precise removal of excess material in nanostructures with the bottom-up size and shape control of colloidal nanoparticle synthesis could greatly expand the range of accessible nanoparticle morphologies. We present mechanistic insights into an unusual reaction in which trialkylphosphines ("phosphines"), which are commonly used to protect nanoparticle surfaces as a surfactant ligand, chemically etch copper sulfide, Cu2-xS, nanostructures in the presence of oxygen. Furthermore, Cu2-xS is removed highly selectively from zinc sulfide-Cu2-xS heterostructures. Structural and optical characterizations show that the addition of phosphine destabilizes the highly Cu-deficient roxbyite phase and injects Cu into the interiors of the nanoparticles, even at room temperature. Analysis of the etching products confirms that chalcogens are removed in the form of phosphine chalcogenides and shows that the removed copper is solubilized as Cu2+. The morphology of etched Cu2-xS particles changes dramatically as the concentration of phosphine is reduced, producing anisotropically etched particles indicative of facet-selective surface chemical reactions. Additionally, ceric ammonium nitrate, another oxidizing agent, can be used to control the etching reaction; the use of this redox agent affords strictly isotropically etched particles. These results demonstrate the highly pliable structural and chemical properties of nanocrystalline Cu2-xS and raise the possibility of using surface-active ligands formerly thought to be passivating to dramatically reshape as-synthesized colloidal nanostructures into more functional forms.