Environmental Engineering Reference
In-Depth Information
temperature. Figure 3.9 displays SeMic and TeMic images of the Cu
2
O nanoframes and nanocages. each truncated rhombic
dodecahedral particle contains 12 hexagonal {110} faces and 6 {100} faces. Type I nanoframes are constructed from the
hexagonal {110} skeleton. They are 300-350 nm in diameter. The {100} faces are formed in the nanocages so that they have a
truncated rhombic dodecahedral morphology. Nanocages have larger diameters (350-400 nm) and thicker walls than type I
nanoframes. The added HCl promotes the etching process. When ethanol is added and sonication of the solution is carried out,
the adsorption of SdS molecules on the nanocage surfaces may be temporarily disrupted. Removal of SdS facilitates the
reaction of Cu
2
O and HCl to form HCuCl
2
. The faster etching rate on the {110} faces than on the {100} faces transforms the
nanocages into type II nanoframes with thinner walls and smaller particle sizes. Type II nanoframes are 200-250 nm in diameter.
The nanocages are not stable under acidic conditions and collapse after the solution is aged for 6 h.
3.3
Cu
2
O-Based NaNOCOmpOsites fOr eNvirONmeNtal prOteCtiON
Semiconductor photocatalysis has been one of potential solutions to energy shortage and environmental pollution worldwide in
recent years. The ability of this advanced oxidation technology has been widely demonstrated to remove persistent organic
compounds and microorganisms from water, and produce hydrogen via water splitting, which is not only low cost but environ-
mentally friendly. In this section, we will first discuss Cu
2
O-based nanocomposites for environmental protection through the
degradation of organic pollutants, focusing on the relationship between structure and photocatalytic activity and photocatalytic
mechanism. Then, Cu
2
O-based nanocomposites for disinfection and hydrogen evolution will be introduced.
3.3.1 Cu
2
O-Based Nanocomposites for Organics degradation
This section begins with an introduction to facet-dependent photocatalytic activity of Cu
2
O crystals. Then, some examples of
organics degradation over Cu
2
O without sharp crystals is described. Finally, Cu
2
O-based nanocomposites for organics deg-
radation is presented. Here, the Cu
2
O/TiO
2
nanocomposite, the most extensively investigated Cu
2
O-based cocatalyst, will be
highlighted by introducing related mechanisms for organics degradation. Particularly, the mechanism for Cu
2
O working as a
photo-oxygen cathode will be described in detail for photocatalysis due to the formation of photogenerated Fenton reagents in
the presence of Fe
2+
.
3.3.1.1 Facet-Dependent Photocatalytic activity of Cu
2
O Crystals
As we know, on the one hand, crystallography of a
semiconductor photocatalyst determines the semiconductor's electric and energy band structure, which is crucial to its physical
and chemical properties. On the other hand, the morphology of the photocatalyst is also important for its photocatalytic prop-
erties. Over the past few decades, a lot of work has been done on the chemical properties of definite crystal planes. It is gener-
ally considered that the chemical reactivity of crystals can be significantly affected by their shapes, due to surface atom
arrangement, bonding, surface energy, and so on. Inorganic single crystals with highly reactive surfaces are therefore desired.
Unfortunately, surfaces with high reactivity usually possess high surface energy, and such surfaces tend to shrink rapidly during
the crystal growth process to minimize total surface energy. Thus, crystals with a dominant high reactivity surface are of great
interest, yet challenging. The facet-dependent photocatalytic activities of Cu
2
O nanocrystals have been studied recently.
The special O-Cu-O 180° linear coordination makes the crystalline surfaces of {111} and {100} possess distinctive chemical
activities.
Xu et al. [126] used the synthesized Cu
2
O nanocrystals with well-defined structures and sharp faces to investigate their
comparative photocatalytic activity. They found that the octahedral Cu
2
O with exposed {111} crystal surfaces show improved
ability on adsorption and photodegradation of methyl orange than Cu
2
O cubes with exposed {100} crystal surfaces. Two years
later, Kuo et al. [127] reported similar results that octahedral Cu
2
O crystals with entirely {111} facets are photocatalytically
more active than truncated cubic crystals with mostly {100} facets. The {111} planes of Cu
2
O are also found to show higher
catalytic activity (two times at 30°C) than that of {100} facets on the N-arylation reaction of iodobenzene with imidazole [128].
A crystal model analysis shows that the (100) planes contain oxygen atoms as they do in the unit cell. However, a cut of the unit
cell over one of the (111) planes reveals the presence of surface Cu atoms with dangling bonds. This simple comparison indi-
cates that the {111} faces have higher surface energy and are expected to be more catalytically active than the {100} facets.
Furthermore, Cu
2
O crystals bounded by the {111} facets contain positively charged copper atoms at the surfaces, whereas those
bounded by the {100} facets such as the cubes are electrically neutral. This observation suggests that Cu
2
O bounded by the
{111} facets should interact more strongly with negatively charged molecules, and the corresponding photodegradation of these
molecules is more effective. That is why methyl orange, a negatively charged molecule, was first chosen for the photodecom-
position experiments. So, Cu
2
O has a poor performance with regard to the degradation of a positively charged molecule. Kuo
