Environmental Engineering Reference
In-Depth Information
11.1.2 Charge Transfer at Semiconductor Nanomaterial Interface
Compared with bulk semiconductors, nanosized semiconductors exhibit several advan-
tages in photocatalytic reactions: (i) the magnitude of band bending, (ii) possible quan-
tum size effect, and (iii) increased surface area. In the dark, for large particles (
r
0
≫
D
,
where
r
0
and
D
are the radius of the semiconductor particle and the depletion layer length,
respectively), the major carrier (e.g.,
e
−
in an n-type semiconductor) density at the surface is
small owing to the depletion layer beneath the particle surface. In contrast, no space charge
region is formed in a much smaller particle (
r
0
≪
D
). Upon light excitation, as shown in
Figure 11.2, some minor carriers (e.g.,
h
+
in an n-type semiconductor) in large particles
are transferred to the electron donor in the solution, which alleviates the positive space
charge, and subsequently causes an upward band bending in the particle surface. With a
much smaller particle, the photogenerated charge carriers can easily reach the surface and
react with the binding species, which signiicantly weaken the band-bending effect.
In the case of nanoparticle systems, charge separation occurs via diffusion since the
band bending is small. The average transit time (τ
D
) from the interior to the surface can be
expressed by Fick's diffusion model:
r
D
2
0
τ
=
(11.1)
D
2
π
dc
where
D
dc
is the diffusion coeficient. In general, τ
D
is a few picoseconds. For TiO
2
with
a radius of 6 nm, the τ
D
of the electrons is 3 ps, which ensures that most charge carriers
can reach the surface before recombination. It should be noted that Fick's diffusion model
breaks down for particles exhibiting a quantum size effect, in which the wave function
of the photogenerated charge carriers spreads over the whole semiconductor nanocluster.
Considering the reaction of photogenerated charge carriers with an acceptor molecule
in solution, this semiconductor-solution interface reaction can be characterized by the rate
D
Solution
Solution
-
-
CB
CB
r
0
r
0
E
F
+
+
VB
VB
r
0
>
D
r
0
<<
D
FIGURE 11.2
Schematic diagram of charge carrier transfer in large and small particles in a solution redox system. (Adapted
with permission from Hoffmann, M. R., Martin, S. T., Choi, W. Y., Bahnemann, D. W.,
Chem. Rev
. 95, 69. Copyright
1995, American Chemical Society.)
