Page 211 - Multifunctional Photocatalytic Materials for Energy

196 Multifunctional Photocatalytic Materials for Energy

Type I e e − − E cb1 Type II e − E cb1

∆E cb
E cb2 e e − − ∆E cb

∆E g1 ∆E g1
∆E g2 E cb2 e −
E vb2 h h − −
h h − − E vb1 ∆E vb ∆E g2 h − E vb1
E vb2 h − ∆E vb

Type III e −
E cb1
∆E g1 ∆E cb
h − E vb1
E cb2 e −

∆E vb
∆E g2
E vb2 h −
Fig. 9.7 Three different heterojunctions between two semiconductors.

Type-III (broken-gap): The band gaps do not overlap at all, and the situation for
carrier transfer is like that in type-II, as shown in Fig. 9.7.
The three types of semiconductor composites can be divided into two morpholo-
gies: the core/shell structure and the Janus-type structure (Fig. 9.8).
In a core/shell morphology, one semiconductor is completely covered by the
second one. Consequently, only the charge carriers injected in the external mate-
rial can undergo redox reactions at the surface. On the contrary, in the Janus-type
structure, both semiconductors are exposed to the aqueous media. In this case,
both electrons and holes are available for oxidative or reductive transformations.
Among the previous types of heterostructures, The Janus type-II photocatalytic
n-p nanocomposite is the most studied, because when it is irradiated, the electric
field generated by contact of the two semiconductors reduces the likelihood of re-
combination of photogenerated electron-hole pairs. Consequently, the separation
produces charge carries with longer lifetime which can undergo redox process on
the surface [51].
A considerable number of studies on the preparation of n-p nanoheterostructured
metal-based photocatalysts for different applications have been carried out [52–57].
In Table 9.1 some n-p heterojunction metal-based nano-photocatalytic systems re-
cently prepared and used for solar-driven hydrogen production by water photosplit-
ting are reported [23,51,75].