Remarkable Facet Selective Reduction of 4-Nitrophenol by Morphologically Tailored (111) Faceted Cu₂O Nanocatalyst

In this work, we have disclosed the facile syntheses of morphologically diverse Cu₂O nanoparticles using our laboratory designed modified hydrothermal reactor employing low-cost copper (II) acetate precursor compounds. The reaction conditions dovetail the effect of ethylene glycol (EG) and glucose to exclusively evolve the morphology tuned Cu₂O nanomaterial at different pHs. The morphology tuning produces octahedron (Oh), dwarf hexapod (DHP), and elongated hexapod (EHP) Cu₂O structures only with the optimized reagent concentrations. Interestingly, all of them were bestowed with a (111) facet, a superlative facet for facile nitroarene reduction. Thus, the morphology reliant catalytic reaction becomes evident. However, when used individually, EG and glucose evolve ill-defined CuO/Cu₂O and Cu₂O structures, respectively. We have observed that a change in pH of the medium at the onset of the reaction is obligatory for the evolution of tailor-made morphologically diverse Cu₂O nanoparticles. However, preformed Cu₂O particles do not suffer further structure/morphology changes under deliberate pH (6.0-9.0) change. With the as-obtained Oh, DHP, and EHP Cu₂O structures, we further delve into the realm of catalysis to understand the splendor of the nanocatalyst, morphology and surface area dependence, facet selective reactivity, and other factors affecting the catalytic efficiency. The remarkable rate of catalysis of 4-nitrophenol (4-NP), evident from the catalyst activity parameter (k _a = 123.6 g^-1 s^-1), to produce 4-aminophenol in the presence of a reducing agent like sodium borohydride (NaBH₄) of the as-prepared catalysts is evidence of the collaborative effects of the effective surface area, surface positive charge, and active (111) facet of the Cu₂O nanocatalyst. We have also studied the effect of other common anions, namely, Cl^-, NO₂ ^-, NO₃ ^-, CO₃ ^2-, and SO₄ ^2- on the reduction process. To obtain a general consensus about facets, we compared (100) and (111) faceted Cu₂O nanocatalysts not only for 4-NP reduction but also for the reduction of toxic chromium Cr(VI) in the presence of formic acid to further emphasize the importance of facet selectivity in catalysis and the versatility of the morphology tuned as-prepared Cu₂O.