Environmental Engineering Reference
In-Depth Information
and Huang [129] mentioned in their review paper that when methylene blue, a positively charged molecule, was used for
photodegradation experiments, neither the cubes nor the octahedra caused any photodegradation. On the other hand, Cu
2
O crys-
tals bounded by the {100} are less sensitive to the charge of the adsorbed molecules and are not photocatalytically active. Pang
et al. [130] reported that the efficiency and selectivity of bacteriostatic effects vary with Cu
2
O morphology. Octahedral Cu
2
O
with {111} crystal surfaces exhibits high selectivity and better bacteriostatic effect for five different bacteria than cubic Cu
2
O
with {100} crystal surfaces. These results are further confirmed by Ren et al. [131], that is, they observed that Cu
2
O octahedral
crystals exhibited higher activity in inactivating
E. coli
than Cu
2
O cubic crystals. They suggested the difference in the antibac-
terial activity of Cu
2
O crystals can be ascribed to the atomic arrangements of different exposed surfaces.
Cu
2
O with high-index facets may exhibit higher chemical activities that are of great importance in practical applications. In
2010, Leng et al. [110] reported the synthesis of 50-facet Cu
2
O polyhedral microparticles partially enclosed by {311} high-
index planes in a water-ethanol mixed alkaline solvent using d-(+)-glucose as a reductant at 60°C. The microcrystals prepared
in this manner show a higher specific catalytic rate toward CO oxidation. The observed enhanced specific catalytic rate toward
CO oxidation over the polyhedral 50-facet Cu
2
O microcrystals is ascribed to the presence of these high-index facets. In order
to study the shape-dependent catalytic activity of Cu
2
O with high-index planes, Leng et al. [132] selected five of the typical
morphologies of Cu
2
O, including cubes, octahedra, 18-facet polyhedra, small rhombicuboctahedral, and a 50-facet polyhedral.
They found that among the three kinds of particles enclosed by low-index planes, for example, (111), (110), or (100) planes,
the octahedral particles show the highest specific catalytic activity followed by rhombic dodecahedral and then cubic particles.
The much higher specific catalytic activity observed for the 50-facet polyhedral particles (big 50-facet polyhedra) prepared
using d-(+)-glucose as the reductant can be attributed to the presence of (311) high-index planes possessing surface defects or
steps, and kink atoms with low coordination numbers. Sun et al. [133] also systematically investigated the crystal-facet-depen-
dent effect of polyhedral Cu
2
O microcrystals on photodegradation of MO. The sequence of photodegradation of the MO
aqueous solution under UV irradiation in the presence of different polyhedral Cu
2
O microcrystals is as follows: 50-facet poly-
hedral Cu
2
O with high-index {522} facets greater than 50-facet polyhedral Cu
2
O with high-index {211} facets greater than
26-facet Cu
2
O cubes greater than 26-facet polyhedral Cu
2
O octahedra. The photocatalytic superiority of the novel 50-facet
polyhedral architectures can be attributed to the introduction of highly active components of high-index surfaces, which can
offer a higher number of unsaturated Cu dangling bonds and surface oxygen vacancies, and accelerate the formation of highly
oxidative ∙OH radicals, leading to enhanced decomposition of MO dye. This study not only provides convincing evidence that
the performance of catalysts can be enhanced by crystal-facet tailoring, but also promotes the synthesis and application of other
polyhedral crystals with high-index surfaces in the catalytic field.
Additionally, the catalytic activity of Cu
2
O with high-index surfaces was also investigated by Wang et al. [123]. They
synthesized truncated concave octahedral Cu
2
O microcrystals mainly enclosed by {
hhl
} high-index facets using an aqueous
solution of Cu(CH
3
COO)
2
, SdS surfactant, NaOH, and d-(+)-glucose reductant. In this system, SdS was found to be crucial
for the formation of concave Cu
2
O microcrystals, and the crystal growth rate also affected the morphology of Cu
2
O microcrys-
tals. In the catalytic oxidation of CO, truncated concave octahedral Cu
2
O enclosed mainly by {332} high-index facets exhibited
enhanced catalytic activity in comparison with that of low-index {111} and {100} facets, which can be due to the presence of
high-density steps on {332} high-index facets. The order of CO catalytic activities of the crystal facets was found to follow the
sequence {332} > {111} > {100}. These results indicate that we can make use of surface-engineering strategies to improve the
catalytic activity of materials due to the different activities of different crystal facets.
3.3.1.2 Cu
2
O without Sharp Crystals for Organics Degradation
Cu
2
O without sharp crystals is also important for applica-
tion in photocatalysis. The photocatalytic activity of materials is closely correlated not only to the crystal plane structure, but
also to the surface state and microstructure. It is demonstrated that the Cu
2
O nanowhiskers composed of quantum dots have
good photocatalytic activity under VL [134]. Tang et al. [135] reported that the highest decolorization ratio of 90% was observed
for the Cu
2
O sample prepared by the addition of 0.05 g/l CTAB. Zheng et al. [53] compared the efficiency of MO degradation
between Cu
2
O microcrystals and N-doped TiO
2
(1 g/l). Although the Cu
2
O microcrystals prepared thus have lower surface
areas, they show much higher photocatalytic activities for MO bleaching. After being irradiated under VL for 60 min, the MO
dye was completely bleached over Cu
2
O in the first run, while only 40% of MO dye was bleached over N-doped TiO
2
. However,
as the irradiation time increases, the photocatalytic activity of Cu
2
O gradually decreases. After 16 runs of MO bleaching, Cu
2
O
microcrystals completely transformed into nanosheets, and the photocatalytic activity of nanosheets was only half of that from
microcrystals in the first run. However, it still shows higher activity than N-doped TiO
2
. Shoeib et al. [136] reported that the
decolorization efficiencies of MO reached 98.7% in 30 min without any further decrease in photocatalytic efficiency with
increase in the irradiation time to 120 min. Zhang et al. [137] reported that the photocatalytic degradation ratio of methyl orange
over Cu
2
O microcubes reached 98.1% under natural solar light with constant stirring for 3 h at room temperature. Besides MO,
other organics or toxic gases have also been reported as models for the studies on Cu
2
O photocatalytic activity. Leng et al. [110]
