Page 62 - Multifunctional Photocatalytic Materials for Energy

Energy band engineering of metal oxide for enhanced visible light absorption 51

the maximum energy level of the VB and the minimum level of the CB is called the
band gap (E g ) of a semiconductor, which is equal to the forbidden band. Electrons
cannot move along the filled VB, and there are no intrinsic electrons to move in the
CB [16]. An external energy must be applied to promote an electron from the VB to
the CB, which depends on and varies with semiconductors, and in reverse determines
the physical and chemical properties of a semiconductor, such as electrical, optical,
magnetic, among others.

4.2.2 Light absorption of a semiconductor
In this section, we start with the relationship between the light absorption and the
electronic energy band structure, and thereby highlight the optoelectronic proper-
ties of semiconductors which are highly related to their photocatalytic process and
performances.
As we previously noted, there is a forbidden energy region in which energy states
cannot exist. To move an electron from the VB to the CB, an extra energy must be
provided, such as thermal or light energy. The light absorption of a semiconductor is
therefore determined by its band gap. Only photons with energy greater than or equal
to the band gap can excite the semiconductor and generate the hole-electron pairs. The
absorption cutting edge can be calculated by the photon energy
hc 1240
E = » (4.1)
g l l
where E g is the band gap (eV) of the semiconductor, h is the Planck's constant
−1
4.13566751691 × 10 −15 eV s, c is the speed of light 299.79 m s , and λ is the wave-
length (nm). To enable the visible light absorption >400 nm, the maximum E g for a
semiconductor is around 3.1 eV.
The light absorption coefficient is a property of a material that defines the amount
−1
of light absorbed by it. The inverse of the absorption coefficient, α , is the average
distance traveled by a photon before it gets absorbed, that is, the penetration depth.
The relationship between absorption coefficient and light intensity can be described
by the Lambert-Baer Law, as [17]

I x () = I e -a x i (4.2)
0
where α is the absorption coefficient, I and I 0 are the transmitted and the incident
light intensities, respectively, and x is the distance to the surface. Therefore the light
intensity decays exponentially when passing through the semiconductor with the rule
1 I
a = ln 0 (4.3)
d I
where d is the thickness of the sample. The wavelength-dependent value of α deter-
mines how far the light enters the semiconductor [18–20].
Here we distinguish between direct and indirect band gap semiconductors, which
are the result of different electronic energy band structures. The apparent difference