Gold-Based Cubic Nanoboxes with Well-Defined Openings at the Corners and Ultrathin Walls Less Than Two Nanometers Thick

We report a facile synthesis of Au-based cubic nanoboxes as small as 20 nm for the outer edge length, together with well-defined openings at the corners and walls fewer than 10 atomic layers (or <2 nm) in thickness. The success relies on the selective formation of Ag₂O at the corners of Ag nanocubes, followed by the conformal deposition of Au on the side faces in a layer-by-layer fashion. When six atomic layers of Au are formed on the side faces to generate Ag@Au_6L core-shell nanocubes, we can selectively remove the Ag₂O patches at the corner sites using a weak acid, making it possible to further remove the Ag core by H₂O₂ etching without breaking the ultrathin Au shell. This synthetic approach works well for Ag nanocubes of 38 and 18 nm in edge length, and the wall thickness of the nanoboxes can be controlled down to 2 nm. The resultant Au nanoboxes exhibit strong plasmonic absorption in the near-infrared region, consistent with computational simulations.