Magnetically Controllable Flowerlike, Polyhedral Ag-Cu-Co₃O₄ for Surface-Enhanced Raman Scattering

Syntheses of Cu-, Ag-, and Ag-Cu-Co₃O₄ nanomaterials are of interest for a wide range of applications including electrochemistry, thermal catalysis, energy storage, and electronics. However, Co₃O₄-based nanomaterials have not been explored for surface-enhanced Raman scattering (SERS). Here, we present Cu-, Ag-, and Ag-Cu-Co₃O₄ nanomaterials of a hierarchical flower shape comprising two separate phases: a pure Cu or Ag core and multiple Co₃O₄ branches, in which the optical properties of the core and the magnetic properties of the branches are integrated. In addition, a series of nonmagnetic Cu-dominant Cu-Co-O polyhedra without Co₃O₄ branches were derived from Cu-Co₃O₄. The polyhedron morphology can be controlled and transformed among cubes, cuboctahedra, and truncated octahedra by tuning the amounts of ligands and additives to vary the potential energy and growth rate of specific crystal facets. The flowerlike Cu-, Ag-, and Ag-Cu-Co₃O₄ were characterized for SERS enhancement, showing that Ag-Cu-Co₃O₄ does not enhance SERS from 4-mercaptobenzoic acid (4-MBA) but dramatically and selectively does so for adsorbed rhodamine 6G. Obviously, the synergy of Ag and Cu within the Co₃O₄ flower constraint promotes the SERS activity. This type of spinel with not only excellent SERS activity but also ferromagnetism could be of great potential in tandem SERS detection/magnetic separation and related applications.