This work confirms the presence of a large facet-dependent photocatalytic activity of Cu2 O crystals through sparse deposition of gold particles on Cu2 O cubes, octahedra, and rhombic dodecahedra. Au-decorated Cu2 O rhombic dodecahedra and octahedra showed greatly enhanced photodegradation rates of methyl orange resulting from a better separation of the photogenerated electrons and holes, with the rhombic dodecahedra giving the best efficiency. Au-Cu2 O core-shell rhombic dodecahedra also displayed a better photocatalytic activity than pristine rhombic dodecahedra. However, Au-deposited Cu2 O cubes, pristine cubes, and Au-deposited small nanocubes bound by entirely {100} facets are all photocatalytically inactive. X-ray photoelectron spectra (XPS) showed identical copper peak positions for these Au-decorated crystals. Remarkably, electron paramagnetic resonance (EPR) measurements indicated a higher production of hydroxyl radicals for the photoirradiated Cu2 O rhombic dodecahedra than for the octahedra, but no radicals were produced from photoirradiated Cu2 O cubes. The Cu2 O {100} face may present a high energy barrier through its large band edge bending and/or electrostatic repulsion, preventing charge carriers from reaching to this surface. The conventional photocatalysis model fails in this case. The facet-dependent photocatalytic differences should be observable in other semiconductor systems whenever a photoinduced charge-transfer process occurs across an interface.
Size-tunable small to ultrasmall Cu2 O nanocubes and octahedra are synthesized in aqueous solution without the introduction of any surfactant. These nanocrystals provide strong evidence of the existence of facet-dependent optical absorption properties of Cu2 O nanoparticles, showing nanocubes always have a more redshifted absorption band than that of octahedra having a similar volume by about 15 nm.
Copper
nanocubes with tunable edge lengths over the range from
49 to 136 nm and ultrasmall octahedra with opposite corner distances
of 45, 51, and 58 nm have been synthesized in aqueous solutions by
reducing CuCl2 or copper acetate with ascorbic acid in
the presence of octahedral gold nanocrystal cores and hexadecylamine
(HDA) at 100 °C for 45 min to 1.5 h. Addition of HDA increases
the solution pH and acts as a coordinating ligand to the copper ions
to facilitate controlled copper shell growth. Due to ultralarge lattice
mismatch between Au and Cu, nonuniform copper deposition yields cubes
and octahedra with noncentrally located gold cores. The Au–Cu
octahedra show little shift in the plasmonic band with increasing
particle size. For Au–Cu nanocubes, the degree of absorption
band red-shift gets smaller as cube size increases. The Au–Cu
nanocubes have shown reasonable reactivity toward 4-nitrophenol reduction
at 40 °C.
Au-Pd core-shell nanocrystals with tetrahexahedral (THH), cubic, and octahedral shapes and comparable sizes were synthesized. Similar-sized Au and Pd cubes and octahedra were also prepared. These nanocrystals were used for the hydrogen-evolution reaction (HER) from ammonia borane. Light irradiation can enhance the reaction rate for all the catalysts. In particular, Au-Pd THH exposing {730} facets showed the highest turnover frequency for hydrogen evolution under light with 3-fold rate enhancement benefiting from lattice strain, modified surface electronic state, and a broader range of light absorption. Finite-difference time-domain (FDTD) simulations show a stronger electric field enhancement on Au-Pd core-shell THH than those on other Pd-containing nanocrystals. Light-assisted nitro reduction by ammonia borane on Au-Pd THH was also demonstrated. Au-Pd tetrahexahedra supported on activated carbon can act as a superior recyclable plasmonic photocatalyst for hydrogen evolution.
Cu2O cubes, octahedra, and rhombic dodecahedra can be
pseudomorphically converted to Cu crystals of the corresponding morphologies
through the addition of ammonia borane. Nitroarene can be completely
reduced during the compositional transformation with four equivalents
of ammonia borane at 30 °C in 25 min. All the obtained polyhedral
Cu crystals can give 100% nitroaniline conversion to p-phenylenediamine exclusively, but commercial Cu2O powder
shows a comparatively lower 4-bromonitrobenzene conversion and yields
a mixture of products. Use of sodium borohydride as a reducing agent
resulted in the formation of deformed Cu particles and a low nitroaniline
conversion percentage. Cu2O cubes cannot be converted to
Cu particles with the addition of hydrazine, and nitroaniline conversion
did not occur. Nitro group reduction is successful with high yields
for diverse nitroarene molecules giving only a single product starting
from a solution of the nitroarene compound, Cu2O cubes
and ammonia borane.
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