Morphology Controlled Solution-Based Synthesis of Cu ₂ O Crystals for the Facets-Dependent Catalytic Reduction of Highly Toxic Aqueous Cr(VI)

In this study, we demonstrate the systematic shape evolution of Cu 2 O 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(NO 3 ) 2 ·3H 2 O 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 Cu 2 O crystals morphologies are demonstrated and discussed. Furthermore, the catalytic activity of the as-synthesized Cu 2 O 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 Cu 2 O cubes in the acidic medium. The repeat catalytic measurements of Cr(VI) reduction for 20 cycles confirm higher stability of cube-shaped Cu 2 O crystals with {100} exposed facets as compared to octahedrons with {111} exposed facets, a classic example of facets-dependent catalytic properties of crystals.