with the semiconductor indirectly influences the energetics and interfacial charge
transfer processes in a favorable way. The deposition of a noble metal on semiconductor nanoparticles is an essential factor for maximizing the efficiency of
photocatalytic reactions [173]. It is normally assumed that the noble metal (e.g., Pt)
acts as a sink for photoinduced charge carriers and promotes interfacial charge
transfer processes. However, recent studies have presented a more complex reaction mechanism dictating the redox processes in semiconductor–metal systems
[174].
19.6.1
Improving the Efficiency of Photocatalytic Transformations
It has been shown that the photocatalytic electron transfer processes at the semiconductor interface can be greatly enhanced by depositing a noble metal on the
semiconductor particle [175–179]. The photogenerated holes are capable of oxidizing thiocyanate ions at the semiconductor interface [180]. The thiocyanate radicals
((SCN) 2
À . ) generated in the photocatalytic oxidation can be conveniently monitored
from its absorbance at 480 nm [180–183].
By employing a 337 nm pulsed laser as the excitation source for exciting TiO 2
colloids and initiating the redox reactions at the interface one can monitor the
interfacial hole transfer process (reactions (1) and (2)).
TiO 2 þ hn ! TiO 2 ðe þ hÞ
ð 1Þ
TiO 2 ðhÞ þ SCN
À ! TiO 2 þ SCN
.
aðSCN
À Þ ! ðSCNÞ 2
À.
ð2Þ
Since the formation of (SCN) 2
À. radicals immediately after the laser pulse excitation represents the quantitative estimate of the hole oxidation process, we monitored maximum absorbance at 480 nm at different TiO 2 :Au ratios. (The TiO 2 :Au
ratio was varied by varying the gold concentration during sample preparation.)
Figure 19.13 shows the dependence of (SCN) 2
À . yield on the gold shell concentration. In the absence of gold capping, TiO 2 colloids generate (SCN 2 )
À . radicals with
a quantum yield of 0.09. At low concentrations of gold we see an increase in the
efficiency of the oxidation process. For a [Au]:[TiO 2 ] ratio of 0.17 we see more than
40% enhancement in the oxidation efficiency (F(SCN) 2
À. ¼ 0:13). As we further
increase the Au concentration the efficiency of thiocyanate oxidation at gold-capped
TiO 2 nanoparticles decreases. Inability of the photogenerated holes to reach the
electrolyte interface as well as increased absorption by the gold are the likely reasons for observing lower (SCN) 2
À . yield at higher capping concentrations of gold.
On the other hand, when low concentrations of metal are used to cap the semiconductor core we can expect the outer layer to be discontinuous. Such a configuration of core shell particles (i.e. small metal islands deposited on the TiO 2 core)
provides a favorable geometry for facilitating the interfacial charge transfer under
UV irradiation. It should be noted that, a new band appears at 390 nm in the case
of TiO 2 /Au nanoparticles as the transient absorption corresponding to (SCN) 2
À .
19.6 Semiconductor–Metal Nanocomposites 633
transfer processes in a favorable way. The deposition of a noble metal on semiconductor nanoparticles is an essential factor for maximizing the efficiency of
photocatalytic reactions [173]. It is normally assumed that the noble metal (e.g., Pt)
acts as a sink for photoinduced charge carriers and promotes interfacial charge
transfer processes. However, recent studies have presented a more complex reaction mechanism dictating the redox processes in semiconductor–metal systems
[174].
19.6.1
Improving the Efficiency of Photocatalytic Transformations
It has been shown that the photocatalytic electron transfer processes at the semiconductor interface can be greatly enhanced by depositing a noble metal on the
semiconductor particle [175–179]. The photogenerated holes are capable of oxidizing thiocyanate ions at the semiconductor interface [180]. The thiocyanate radicals
((SCN) 2
À . ) generated in the photocatalytic oxidation can be conveniently monitored
from its absorbance at 480 nm [180–183].
By employing a 337 nm pulsed laser as the excitation source for exciting TiO 2
colloids and initiating the redox reactions at the interface one can monitor the
interfacial hole transfer process (reactions (1) and (2)).
TiO 2 þ hn ! TiO 2 ðe þ hÞ
ð 1Þ
TiO 2 ðhÞ þ SCN
À ! TiO 2 þ SCN
.
aðSCN
À Þ ! ðSCNÞ 2
À.
ð2Þ
Since the formation of (SCN) 2
À. radicals immediately after the laser pulse excitation represents the quantitative estimate of the hole oxidation process, we monitored maximum absorbance at 480 nm at different TiO 2 :Au ratios. (The TiO 2 :Au
ratio was varied by varying the gold concentration during sample preparation.)
Figure 19.13 shows the dependence of (SCN) 2
À . yield on the gold shell concentration. In the absence of gold capping, TiO 2 colloids generate (SCN 2 )
À . radicals with
a quantum yield of 0.09. At low concentrations of gold we see an increase in the
efficiency of the oxidation process. For a [Au]:[TiO 2 ] ratio of 0.17 we see more than
40% enhancement in the oxidation efficiency (F(SCN) 2
À. ¼ 0:13). As we further
increase the Au concentration the efficiency of thiocyanate oxidation at gold-capped
TiO 2 nanoparticles decreases. Inability of the photogenerated holes to reach the
electrolyte interface as well as increased absorption by the gold are the likely reasons for observing lower (SCN) 2
À . yield at higher capping concentrations of gold.
On the other hand, when low concentrations of metal are used to cap the semiconductor core we can expect the outer layer to be discontinuous. Such a configuration of core shell particles (i.e. small metal islands deposited on the TiO 2 core)
provides a favorable geometry for facilitating the interfacial charge transfer under
UV irradiation. It should be noted that, a new band appears at 390 nm in the case
of TiO 2 /Au nanoparticles as the transient absorption corresponding to (SCN) 2
À .
19.6 Semiconductor–Metal Nanocomposites 633