Orthogonal deposition of Au on different facets of Ag cuboctahedra for the fabrication of nanoboxes with complementary surfaces

We report the fabrication of Ag-Au cuboctahedral nanoboxes enclosed by {100} and {111} facets, respectively, through the orthogonal deposition of Au on two different facets of Ag cuboctahedra. Specifically, we titrate aqueous HAuCl₄ into an aqueous mixture containing Ag cuboctahedra, ascorbic acid, and NaOH (under basic conditions), in the presence of poly(vinylpyrrolidone) (PVP) and cetyltrimethylammonium chloride (CTAC), respectively. In the case of PVP, the oxidation of Ag was initiated from the {111} facets of the cuboctahedra through the galvanic replacement reaction between Au(iii) and Ag, accompanied by the deposition of Au onto the {100} facets. Because the dissolved Ag(i) ions could react with NaOH to form Ag₂O on the {111} facets and thus terminate the galvanic reaction, the Au(iii) ions would be further reduced by the ascorbate monoanion (HAsc^-) to generate Au atoms for their continuing deposition on the {100} facets, converting Ag cuboctahedra to Ag@Au_{{100}} cuboctahedra. Upon the etching of Ag from the core, we obtained Ag-Au cuboctahedral nanoboxes enclosed by {100} facets. In contrast, when CTAC was present, the oxidation of Ag through a galvanic reaction could continuously proceed on {100} facets as the dissolved Ag(i) ions would react with the excessive amount of Cl^- ions derived from CTAC to produce soluble AgCl₂^- ions rather than insoluble Ag₂O. As a result, the dissolved Ag(i) and Au(iii) ions would be co-reduced by HAsc^- for the generation of Ag and Au atoms, followed by their co-deposition onto {111} facets for the generation of Ag@Au_{{111}} concave cuboctahedra. After the removal of Ag from the core by etching, we obtained Ag-Au_{{111}} cuboctahedral nanoboxes enclosed by {111} facets. Both samples of cuboctahedral nanoboxes exhibited strong optical absorption in the infrared region. Interestingly, the cuboctahedral nanoboxes enclosed by {111} facets showed significantly enhanced catalytic activity toward the reduction of 4-nitrophenol by NaBH₄ relative to their counterparts encased by {100} facets.