Environmental Engineering Reference
In-Depth Information
(a)
(b)
1000
(ii)
6
SP states
H
2
O
-2
CB
4
e
-
e
-
-1
2
(iii)
hv
100
(i)
0
Au
TiO
2
0
E
0
(H
+
/H
2
)
E
f
-2
h
+
1
-4
E
0
(O
2
/H
2
O)
10
2
-6
-8
-1
-10
3
VB
Plasmonic metal
1
-5
0
X
(nm)
5
10
4
Semiconductor
FIGURE 11.13
(See color insert.)
(a) Schematic diagram of plasmon-induced charge separation and associated photochemis-
try at the metal-semiconductor heterojunction. (b) Optical simulations showing SPR-enhanced electric ields
owing to photoexcited Au particles, permeating into a neighboring TiO
2
structure. (a: Adapted with permis-
sion from Linic, S., Christopher, P., Ingram, D.B.
Nat. Mater
., 10, 911. Copyright 2011, Nature Publishing Group.
b: Adapted with permission from Liu, Z., Hou, W., Pavaskar, P., Aykol, M., Cronin, S.B.
Nano Lett
., 11, 1111.
Copyright 2011, American Chemical Society.)
splitting due to the SPR, in which the plasmonic electrons in the metal can inject into the TiO
2
CB and then participate in the H
2
evolution reaction, leaving holes in the Au nanoparticle,
which produce O
2
.
53
It is important to note that only energetic electrons in metal nanopar-
ticles can transfer to semiconductors and possess suficient energy to execute half-reactions
(e.g., H
2
evolution reaction). These results suggest that the plasmonic metal-semiconductor
systems can not only enhance visible light absorption, but also greatly promote the electron-
hole pair separation in the plasmonic metal nanoparticle by the electronic heterojunction.
Apart from electron transfer between the metal and semiconductor, SPR can also induce
enhancement of the semiconductor photoreactivity even if the semiconductor and plasmonic
metal are separated by thin nonconductive spacers. This indicates the radiative energy
transfer can take place through a near-ield electromagnetic mechanism. It is reported that
the electron-hole formation rate in the semiconductor is signiicantly proportional to the
electromagnetic ield intensity if the semiconductor encounters the photoexcited plasmonic
nanostructure. Owing to the spatial nonhomogeneity of plasmonic electromagnetic ield, the
highest enhancement of the SPR-induced electron-hole formation rate appears at the regions
of the semiconductor closest to the plasmonic nanostructure. This mechanism is also sup-
ported by the enhancement of the photocatalytic water splitting under the visible region
by coupling strongly plasmonic Au nanoparticles with TiO
2
.
55
Electromagnetic simulation
of the Au/TiO
2
composite ilm (Figure 11.13b) demonstrated that the plasmonic nanopar-
ticles coupled the visible light effectively from the far ield to the near ield at the TiO
2
sur-
face. Subsequently, most of the photogenerated charges excited by the SPR ield contribute
to the photocatalytic reaction. The near-ield electromagnetic mechanism in the plasmonic
metal-semiconductor nanostructure is further supported by the results of SPR-coupled
wavelength-dependent enhancement of the semiconductor photoluminescence emission.
56
It is important to note that the structure, size, and shape of the building blocks for the
plasmonic metal-semiconductor all have signiicant impact on its practical performance. By
tuning the relative geometric arrangement, the energy transfer mechanisms of the excited
