Investigation of the Effects of Polyhedral Gold Nanocrystal Morphology and Facets on the Formation of Au–Cu<sub>2</sub>O Core–Shell Heterostructures

Rhombic dodecahedral and highly edge- and corner-truncated octahedral gold nanocrystal cores with entirely or significant {110} faces were employed to investigate the core–shell orientation relationship of Au–Cu<sub>2</sub>O core–shell heterostructures. By increasing the volume of reductant added, we synthesized Au–Cu<sub>2</sub>O face-raised cubes, cuboctahedra, and octahedra. TEM characterization indicates a fixed core–shell orientation relationship. The {100}, {110}, and {111} facets of the gold cores align parallel to the corresponding faces of the Cu<sub>2</sub>O shells. Structural requirements of the gold cores for the preparation of Au–Cu<sub>2</sub>O stellated icosahedra indicate that icosahedral gold nanocrystals are good candidates. Use of trisoctahedral nanocrystal cores cannot generate stellated icosahedra, but unusual face-raised octahedra with V-shaped {111} edges can be synthesized. Core–shell cubes, face-raised cubes, core–shell octahedra, and face-raised octahedra were examined for their comparative photocatalytic activity toward the photodegradation of methyl orange. Remarkably, all the cubes were found to be photocatalytically inactive because they are bounded by essentially {100} facets. Face-raised octahedra with more {111} facets showed significantly better photocatalytic performance than regular Au–Cu<sub>2</sub>O core–shell octahedra. The results reveal that gold nanocrystal-enhanced photocatalysis can be achieved only with Cu<sub>2</sub>O shells exposing proper facets.