Environmental Engineering Reference
In-Depth Information
CB
O 2p
Cu 3d-4s
τ
2
τ
1
2.17 eV
τ
3
Cu 3d
VB
Cu 3d-4s
O 2p
figure 3.2
Schematic diagram of the energy band of bulk Cu
2
O semiconductor. Reproduced by permission from Ref. [45]. © 2010,
American Institute of Physics.
the center of the Brillouin zone (Γ point) [42]. The experimental band gap is about 2.0 eV [33, 50]. Theoretical
calculations using local density functional theory (LdFT) yielded values between 0.45 and 0.8 eV [51], while Hartree-
Fock (H-F) calculations reported a value of 9.7 eV [48]. It is well known that LdFT underestimate band gaps while H-F
calculations overestimate them.
The special crystal structure of Cu
2
O results in an energy band configuration different from other common metal oxide
semiconductors. The top of the VB of common metal oxide semiconductors is usually composed of an O2p orbit with a local
area distribution charge so that its band gap can only be narrowed by doping with other elements. As for Cu
2
O, the Cu
+
ion
electronic structure ends up with 3d
10
4s
0
, in which the energy for 4s orbitals is only slightly higher than that for the 3d levels.
So, the band gap of Cu
2
O can be widened by reducing the interaction of Cu d-d of the upper VB, and the band gap of Cu
2
O can
also be adjusted by doping with different elements to change Cu-Cu interaction [52].
3.2.2
modification
Though Cu
2
O has been reported to be used as a stable photocatalyst with exposing (111) facets [53], Cu(I) in Cu
2
O is
unstable because it is easily oxidized to Cu(II) by photogenerated holes or another oxidant, or it might be reduced to
Cu(0) by photogenerated electrons. The thin-layer CuO on the surface of Cu
2
O can prevent further photocorrosion of
Cu
2
O to form a stable Cu
2
O/CuO core/shell structure [36], but it also reduces the photocatalytic activity of Cu
2
O because
CuO has a less narrow band gap (1.4 eV [46]), which suggests a weak redox ability and high combination rate of photo-
generated electrons and holes. Additionally, though Cu(I) in Cu
2
O may be reduced to Cu(0) by photogenerated electrons,
it cannot easily happen since photogenerated electrons are dominantly captured by oxygen in the process of photocatalysis
[40]. In the contrary, Cu(0) is usually oxidized to Cu(I) to form a passivation layer of Cu
2
O if Cu(0) is exposed to air [54].
Thus, in order to improve stability of Cu
2
O, it is very important to prevent Cu
2
O from photocorrosion by holes. doping
[55] and hybridization [56] have been proven to be effective in the improvement of photocatalytic activity and stability of
narrow semiconductors. Our theoretical calculations and experimental results show that Zn-doped CdS [55] indeed had
much higher photocatalytic activity and stability than pristine CdS. So, in order to improve photocatalytic activity and
stability for Cu
2
O, it is promising to modify Cu
2
O by doping it with other elements, coating with other semiconductors,
and morphology control.
3.2.2.1 Doping
It is known that impurities in semiconductors play a fundamental role in determining the electronic quality
of a semiconductor. even a very small concentration of impurities can significantly alter the properties of a semiconductor, and
therefore a systematic study of the most common impurities and their effects as a function of concentration is necessary. The
existence of Cu-Cu interactions in the cubic Cu
2
O structure plays a key role in the unusual behavior of Cu
2
O. Through doping,
these interactions can be disrupted and the band gap for Cu
2
O may be changed.
