Up-scaling the synthesis of Cu₂O submicron particles with controlled morphologies for solar H₂ evolution from water

The synthesis of Cu2O was studied to examine the effects of up-scaling on the size and morphology of the resultant particles. As a result, a successful protocol employing an automated laboratory reactor was developed for large-scale synthesis of phase-pure Cu2O colloids with specific sizes in the submicron to micrometer range (0.2-2.6 μm). The as-synthesized products have been studied by means of powder X-ray diffraction, Raman spectroscopy, X-ray photoelectron spectroscopy, UV-Vis-NIR spectroscopy, scanning electron microscopy, and photoelectrochemical measurements. A broad range of morphologies, both equilibrium (stellated octahedrons, cubes, cuboctahedrons, truncated octahedrons, truncated cuboctahedrons) and metastable (cage-like hierarchical structures, microspheres with flower-like texture), with uniform sizes have been selectively prepared either by careful tuning of synthesis conditions. Recrystallization of primary aggregates through Ostwald ripening is proposed as the formation mechanism for these Cu2O structures. As a photocathode for photoelectrochemical H2 evolution, Cu2O submicron cubes with exposed {001} facets exhibit a high open-circuit potential of ca. 0.9 V vs. the RHE at pH 1.