Cu₂O-directed in situ growth of Au nanoparticles inside HKUST-1 nanocages

Controllable integration of metal nanoparticles (MNPs) and metal-organic frameworks (MOFs) is attracting considerable attention as the obtained composite materials always show synergistic effects in applications of catalysis, delivery, as well as sensing. Herein, a Cu₂O-directed in situ growth strategy was developed to integrate Au nanoparticles and HKUST-1. In this strategy, Cu₂O@HKUST-1 core-shell heterostructures, HKUST-1 nanocages, Cu₂O@Au@HKUST-1 sandwich core-shell heterostructures and Au@HKUST-1 balls-in-cage heterostructures were successfully synthesized. Cu₂O@HKUST-1 core-shell heterostructures were synthesized by soaking Cu₂O nanocrystals in benzene-1,3,5-tricarboxylic acid solution. The well-defined Cu₂O@HKUST-1 core-shell heterostructures were demonstrated to be dominated by the ratio of Cu²⁺ cations to btc^3- ligands in solution during the period of HKUST-1 formation. Cu₂O@Au@HKUST-1 sandwich core-shell or Au@HKUST-1 balls-in-cage heterostructures were obtained by impregnating HAuCl₄ into Cu₂O@HKUST-1 core-shell heterostructures. Due to the porosity of HKUST-1 and reducibility of Cu₂O, HAuCl₄ could pass through the HKUST-1 shell and be reduced by the Cu₂O core in situ forming Au nanoparticles. Finally, CO oxidation reaction at high temperatures was carried out to assess the catalytic functionality of the obtained composite heterostructures. This strategy can circumvent some drawbacks of the existing approaches for integrating MNPs and MOFs, such as nonselective deposition of MNPs at the outer surface of the MOF matrices, extreme treatment conditions and additional surface modifications.