Environmental Engineering Reference
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Cu 2 O
H 2 O
e -
reduction
-1.35
WO 3
-0.61
H 2 /H 2 O
e -
2.0 eV
-0.01
+0.57
Visible light
H 2 O/O 2
h +
2.6 eV
+2.54
h +
H 2 O
Visible light
oxidation
figure 3.18 Reaction scheme of photoinduced water splitting over coupled p-type Cu 2 O and n-type WO 3 photocatalysts in a suspension
system. Reprinted with permission from Ref. [202]. © 2008, elsevier B.V.
it can be concluded that the redox potentials for H 2 and O 2 evolution and the oxidation of Cu 2 O to CuO are all within the band
gap, and, therefore, all of these processes are possible in principle. However, the driving force for water oxidation is minimal,
while the oxidation of Cu 2 O is thermodynamically favorable. In the presence of light, morphological instabilities due to the
oxidation and reduction of Cu 2 O induced by photogenerated electrons have been observed, and the photocorrosion of Cu 2 O has
been recognized as a reason for the decay of its photocatalytic property.
In order to be a viable material to be used in direct H 2 production, particularly under solar energy, Cu 2 O was used to couple
with other semiconductors such as TiO 2 [21, 200, 201], WO 3 [202], and Cu/TiO 2 [203]. Our previous studies found a Cu 2 O/TiO 2
bilayer film [200] and Cu 2 O/TiO 2 [201] nanocomposites as photocatalysts for water splitting, while Cu 2 O film and Cu 2 O NPs
prepared by the chemical deposition method were completely nonactive. Similar results were also reported by Yasomanee'
group [201]. In our study, neither Cu 2 O nor TiO 2 could produce H 2 from water splitting, but the combined systems could
produce H 2 from water splitting. The mechanism of photocatalytic reaction is proposed based on energy band theory and exper-
imental results. The photogenerated electrons from Cu 2 O were captured by Ti 4+ ions in TiO 2 , and Ti 4+ ions were further reduced
to Ti 3+ ions. Thus, the photogenerated electrons were stored in Ti 3+ ions as a form of energy. These electrons trapped in Ti 3+ can
be released if a suitable electron acceptor is present, whereas holes stay in the VB of Cu 2 O to form hole centers. This hampers
the recombination of the photogenerated electrons with the holes. Thus, electrons trapped in Ti 3+ ions may have a long lifetime
and can be transferred to the interface between the composite and solution. These electrons can be directly combined with H+
to produce H 2 . Additionally, holes are accumulated in the VB of Cu 2 O to form hole centers, which can be consumed by weak
oxidation. Senevirathna and Pitigala [21] deposited quantum-sized Cu 2 O on TiO 2 NPs to produce the catalyst TiO 2 /Cu 2 O and
found that this composite system can photogenerate hydrogen from water under sacrificial conditions.
Hu et al. [202] coupled Cu 2 O with WO 3 to form a heterocontact for the improvement of photoinduced charge separation in
Cu 2 O. Prior to the experiments of the Cu 2 O/WO 3 suspension system, pristine Cu 2 O was detected and no trace of H 2 was evolved
from Cu 2 O/pure water suspensions even in the presence of methanol (a sacrificial hole scavenger). The results indicate that the
photoinduced carriers within Cu 2 O are lost to back reactions due to inefficient charge transport. This is generally encountered
for narrow-gap semiconductors. For the Cu 2 O/WO 3 suspension system, charge transfer between p- and n-type semiconductors
through a mechanism denoted as “interparticle electron transfer” is essential for improving charge separation. The n-type semi-
conductor employed should have a VB edge more positive than that of the p-type Cu 2 O. The working scheme is demonstrated
in Figure 3.18. In this p-n coupling, two photons have to be absorbed to produce one net electron-hole pair for redox reactions
at the solid/liquid interface. This electron-hole pair consists of the minority hole and minority electron from the n- and p-type
semiconductors, respectively. On the other hand, the majority electrons and holes of the respective semiconductors combine
during the instant particle contact. Through this coupling, the number of photoinduced charges lost to back reactions in Cu 2 O
can be reduced. In this p-n coupling, WO 3 was chosen as the n-type semiconductor to couple with the p-type Cu 2 O since it has
a VB edge (about +2.54V versus Ag/AgCl at pH = 7), much more positive than the water oxidation potential and its CB edge.
Additionally, its CB edge is not negative enough to reduce H 2 O to H 2 (Fig. 3.18).
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