Environmental Engineering Reference
In-Depth Information
H
2
O + O
2
e
-
•OH + •O
2
H
Au NP
e
-
e
-
e
-
e
-
e
-
CB
Cu
2
O NW
VB
H
2
O
h
+
h
+
h
+
h
+
h
+
H
+
+ •OH
figure 3.13
Scheme of proposed ∙OH generation process on the surface of Cu
2
O-Au nanocomposites. Reproduced by permission from
Ref. [151]. © 2012, American Chemical Society.
Pan et al. [151] have developed a facile strategy for coupling Au NPs onto the surface of Cu
2
O nanowires. The presence of
Au NPs enhances the photodegradation efficiency of Cu
2
O nanowires. These Cu
2
O/Au nanocomposites display tunable optical
properties, and their photocatalytic properties are dependent on the coverage density of Au NPs. The enhanced photocatalytic
efficiency is believed to be due to the enhanced light absorption by surface plasmon resonance and the electron sink effect of
Au NPs. Charge separation prevents the recombination of electrons and holes (as shown in Fig. 3.13) and thus enhances the
photocatalytic activity of Cu
2
O nanowires. At the same time, the absorption of VL for semiconductors can be enhanced by
coupling with plasmonic metal nanostructures through a local field enhancement effect, leading to an enhanced photocatalytic
property.
Ag@Cu
2
O core-shell NPs exhibit photocatalytic activity over an extended wavelength range because of the presence of
localized surface plasmon resonance in the Ag core in comparison to pristine Cu
2
O NPs [151]. The photocatalysis action spectra
and transient absorption measurements show that the plasmonic energy is transferred from the metal to the semiconductor via
plasmon-induced resonant energy transfer and direct electron transfer simultaneously, which can generate electron-hole pairs
in the semiconductor. The localized surface plasmon resonance band of the Ag@Cu
2
O core-shell shows a red shift with an
increase in the Cu
2
O shell thickness, extending the light absorption of Ag@Cu
2
O heterostructures to longer wavelengths. As a
result, the photocatalytic activity of the Ag@Cu
2
O core-shell NPs is varied by the modulation of the shell thickness on the
nanometer scale. The Ag@Cu
2
O core-shell heterostructure is an efficient VL plasmonic photocatalyst, which allows for tunable
light absorption over the entire VL region by tailoring the shell thickness.
3.3.1.3.3 Cu
2
O/Polymer for Organics Degradation
Besides inorganic quantum dots [171], organic dye molecules
[172] have also been used as “photosensitizers.” Wang et al. [155] successfully synthesized Cu
2
O/(PANI) nanocomposites
by a simple, one-step hydrothermal method. The Cu
2
O NPs with a diameter of 40 nm disperse in the PANI. The
nanocomposites, for the first time, are used as a photocatalyst for the degradation of organic pollutants. The combination
of Cu
2
O NPs with PANI leads to high photocatalytic activity for the degradation of dyes under VL irradiation, which is
much higher than that of TiO
2
and pure Cu
2
O. A possible photocatalytic mechanism is proposed to explain the improvement
of the photocatalytic activity (Fig. 3.14). The significant enhancement of photocatalytic performance is attributed to the
synergistic effect between PANI and Cu
2
O. The band gap of Cu
2
O is 2.17 eV, and PANI has a band gap of 2.8 eV, which
indicates that both Cu
2
O and PANI can be excited by VL. After the Cu
2
O/PANI composites are synthesized, the two types
of materials closely combine together and form interfaces. The Cu
2
O in the photocatalyst absorbs photons and excites
electron and hole pairs when the system is irradiated with VL. The PANI also absorbs photons to induce π-π* transition,
transporting the excited electrons to the π*-orbital. Thus, the excited-state electrons produced by PANI are injected into
the CB of Cu
2
O. Subsequently, simultaneous holes on the VB of Cu
2
O migrate to the π-orbital of PANI because of the
enjoined electric fields of the two materials. The photoexcited electrons are effectively collected by Cu
2
O, and the holes
by PANI. The recombination process of the electron-hole pairs is hindered, and charge separation and stabilization are
achieved. Therefore, efficient electron-hole separation leads to significant enhancement of photocatalytic dye degradation
in the Cu
2
O/PANI composites. Based on this understanding, the role played by PANI can be illustrated by injecting
electrons into Cu
2
O CB under VL illumination and triggering the formation of a very reactive radical superoxide radial
