In most studies describing the preparation of Cu2O crystals of various morphologies, the particle sizes are normally hundreds of nanometers to micrometers due to rapid particle growth, so they are not exactly nanocrystals. Here we report surfactant-free formation of sub-100 nm Cu2O nanocrystals with systematic shape evolution from cubic to octahedral structures by preparing an aqueous mixture of Cu(OAc)2, NaOH, and N2H4 solution. Adjustment of the hydrazine volume enables the particle shape control. Uniform nanocubes and octahedra were synthesized with edge lengths of 37 and 67 nm, respectively. Novel Cu2O octapods with an edge length of 135 nm were also produced by mixing CuCl2 solution, SDS surfactant, NaOH solution, and NH2OH · HCl reductant solution. All of them are nearly the smallest Cu2O nanocrystals of the same shapes ever reported. These small cubes, octahedra, and octapods were employed as catalysts in the direct synthesis of 1,2,3-triazoles from the reaction of alkynes, organic halides, and NaN3 at 55 °C. All of them displayed high product yields in short reaction times. The octahedra enclosed by the {111} facets are the best catalysts, and can catalyze this cycloaddition reaction with high yields in just 2 h when different alkynes were used to make diverse triazole products. Hence, the small Cu2O particles provide time-saving, energy-efficient, and high product yield benefits to organocatalysis.
Two
approaches have been employed to generate Cu2O nanoframes.
Novel edge-truncated cubic nanoframes with empty {110} edges can be
obtained directly by growing Cu2O nanocrystals in the presence
of HCl etchant. After 1 h of reaction for particle growth, introduction
of ethanol and sonication of the mixture effectively removes surface-adsorbed
sodium dodecyl sulfate (SDS) surfactant to facilitate HCl etching.
Crystal structures of the nanoframes have been examined. The entire
nanoframe formation process was captured by recording the complete
solution color changes. By injecting precise volumes of HCl solution
to a solution of presynthesized {100}-truncated and all-corner-truncated
Cu2O rhombic dodecahedra, nanoframes with etched {110}
faces and hollow interior were produced in 10 min. Observations made
on some partially etched particles suggest etching rapidly proceeds
from the surface {110} faces into the interior regions of the rhombic
dodecahedra. The strong light scattering feature observed for solid
rhombic dodecahedra extending from the visible to the near-infrared
regions largely disappears after their transformation into nanoframes.
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