In
this study, we demonstrate the systematic shape evolution of
Cu<sub>2</sub>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<sub>3</sub>)<sub>2</sub>·3H<sub>2</sub>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<sub>2</sub>O crystals morphologies are demonstrated
and discussed. Furthermore, the catalytic activity of the as-synthesized
Cu<sub>2</sub>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<sub>2</sub>O cubes in the acidic medium. The repeat catalytic measurements
of Cr(VI) reduction for 20 cycles confirm higher stability of cube-shaped
Cu<sub>2</sub>O crystals with {100} exposed facets as compared to
octahedrons with {111} exposed facets, a classic example of facets-dependent
catalytic properties of crystals
