Cu2O Nanocrystals: Surfactant-Free Room-Temperature Morphology-Modulated Synthesis and Shape-Dependent Heterogeneous Organic Catalytic Activities

Abstract

A facile room-temperature surfactant-free solution–chemical route has been developed to fabricate Cu 2 O nanocubes, octahedrons, spheres, plates, and polyhedrons by varying the reaction atmosphere (air or Ar) and reducing agent. It is believed that the oxygen adsorption and reaction on the surface of Cu 2 O may be able to slow down the reduction of Cu(OH) 2 and the nucleation of Cu 2 O and further exert a noticeable influence on the following growth. We find that reducing agents are also important in determining the samples’ final morphology, structure, and composition. It has been demonstrated that the size of Cu 2 O nanocrystals can be modulated by changing the pH value of the reacting solution or the initial concentration of reacting agents or by adding solvents with high viscosity. In addition, the organic catalytic activity has been demonstrated to be susceptible to the shapes of the synthesized Cu 2 O materials. {111} planes of Cu 2 O are found to show higher catalytic activity (two times at 30 °C) than that of {100} facets on the N-arylation reaction of iodobenzene with imidazole. The difference among various crystal planes in catalytic activity may be related to crystal facets’ densities of surface Cu atoms, surface energies, turnover frequency, and electronic surface properties. Furthermore, O 2 -assisted selective etching is proposed to improve the activity of {100} facets by increasing the active sites. This fundamental understanding shows that morphological control of transition-metal oxides allows selective exposure of catalytically active planes and will most probably be applicable in the development of the next generation of highly efficient heterogeneous catalysts. The selective etching of crystallographic planes may be developed to a general technique to improve catalytic activity of noble metal, oxide nanocrystals, as well as porous catalysts.