Environmental Engineering Reference
In-Depth Information
E abs
(eV)
E NHE
(eV)
Cu 2 O
TiO 2
Electrolyte
-2
-3
-1
-4
H 2 O/H 2
2.0 eV
0
1.23 eV
-5
O 2 /H 2 O
+1
-6
3.0 eV
+2
-7
+3
-8
figure 3.4 energy band diagram for the Cu 2 O/TiO 2 heterostructure. The TiO 2 layer is very thin and considered fully depleted with
negligible band bending at the electrolyte interface compared to the Cu 2 O/TiO 2 interface. Reproduced by permission from Ref. [56]. © 2002,
elsevier Science B.V.
electrons excited to the CB of Cu 2 O can transfer to TiO 2 , whereas the holes generated in the VB in TiO 2 prefer opposite transfer
to Cu 2 O. Charge carriers separated in different semiconductors effectively reduce the chance of electron-hole pair recombination
and also prolong their lifetime, thereby increasing quantum efficiencies. In addition, the working wavelength range extends to
a visible region due to the absorption of visible light (VL) by Cu 2 O, which further enhances the efficiency of solar energy
transition. Synergy between these effects enables the Cu 2 O/TiO 2 system to exhibit great potential for solar cell and photocataly-
sis applications. Additionally, the stability of Cu 2 O and heterojunctions thereof are significantly affected by the material that
Cu 2 O is coupled with. For example, Cu 2 O heterojunctions with In 2 O 3 , SnO 2 , and ZnO were reported to be unstable, while
Cu 2 O-CdO junctions were relatively stable [84, 85]. And a thin layer of TiO 2 deposited on p-type Cu 2 O successfully protects
the Cu 2 O film against photocorrosion, suggesting that TiO 2 may be one of the best candidates to couple with CuO and/or Cu 2 O
to enhance stability [56].
3.2.2.3 Morphology Control Because of the anisotropy of crystals, people found different crystal faces usually exhibit
different properties, such as stability and activity. In the past few decades, great achievements have been made by investigating
the chemical properties of definite crystal faces by employing bulk single crystals. Many photocatalysts have been observed to
show different photoactivities on different faces; for example, the {001} facets of anatase TiO 2 show higher activity than the
{101} facets [86], the {001} facets of ZnO display high activity than that of other facets [87], the {110} facets of Ag 3 PO 4 exhibit
higher surface energy than that of {100} planes [88], and the tetrahexahedral Pt and Au nanocrystals enclosed by 24 {037} or
{122} facets possess excellent electro-oxidation activity [89]. The study of the properties of definite crystal facets not only
helps us to precisely control their structures and shapes, but also offers an opportunity for discovering multifunctional materials
with potentially exciting and unique properties. Hence, it is necessary to develop effective and facile methods for the control-
lable synthesis of Cu 2 O as well as to investigate corresponding growth mechanisms. Cu 2 O nanocrystals are relatively easy to
prepare and low in cost because of abundant copper sources, simple preparation process, and low energy consumption. They
can also form a wide variety of morphologies. Various interesting Cu 2 O nanostructures such as rods [90], wires [91, 92], spheres
[93-95], flowers [96], towers [97], flower leaves [98], dendritic structures [99], cacti [100], whiskers [101], bipyramids [102],
eight-pod cubes, six-armed star-like structures [103], different polyhedral structures (such as cubes [104-106], octahedra [107-109],
50-facet microcrystals [110], truncated octahedrons [111], triangular nanoplates [112]), and different hollow structures (such
as spheres [113-116], cubes, [117] truncated rhombic dodecahedral Cu 2 O nanocages and nanoframes [118]) have been
synthesized.
Here, we pay more attention to the synthesis of Cu 2 O nanostructures with well-defined morphologies with sharp facets.
These nanostructures possess well-defined facets and sharp edges and are ideal for facet-dependent property studies. Hollow
Cu 2 O nanostructures are also reviewed here. These are usually synthesized directly without the use of templates or through the
dissolution of preformed solid Cu 2 O nanocrystals. For this reason, it is possible that they have well-defined morphologies such
as cubic, octahedral, and other complex but symmetrical shapes.
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