Morphology Controlled Solution-Based Synthesis of Cu₂O Crystals for the Facets-Dependent Catalytic Reduction of Highly Toxic Aqueous Cr(VI)

Abstract

In this study, we demonstrate the systematic shape evolution of Cu2O crystals from the octahedron, through truncated octahedron, cube, and finally to truncated cube by varying the reaction temperature with an optimum precursor concentration of 25 mM Cu­(NO3)2·3H2O and 1 g of polyvinylpyrrolidone (PVP) as the shape controlling reagent. The average size of these crystals increased with temperature from ∼70 nm (at 40 °C) to ∼1 μm (at 100 °C). With a much lower (6 mM) and higher (250 mM) precursor concentration, nanoparticles and polyhedron-shaped crystals are respectively formed in the studied temperature region (40–120 °C). The role of precursor concentration, PVP quantity, reaction medium, and reaction temperature in the formation of diverse Cu2O crystals morphologies are demonstrated and discussed. Furthermore, the catalytic activity of the as-synthesized Cu2O crystals is tested for the reduction of Cr­(VI) at room temperature. The toxic Cr­(VI) is found to be rapidly reduced to nontoxic Cr­(III) in a short span of 4 min in the presence of Cu2O cubes in the acidic medium. The repeat catalytic measurements of Cr­(VI) reduction for 20 cycles confirm higher stability of cube-shaped Cu2O crystals with {100} exposed facets as compared to octahedrons with {111} exposed facets, a classic example of facets-dependent catalytic properties of crystals