Page 65 - Multifunctional Photocatalytic Materials for Energy

54 Multifunctional Photocatalytic Materials for Energy

4.3 Principle of photocatalysis for solar fuel generation

4.3.1 Basic principle of photocatalysis
A detailed explanation of photocatalysis can be found in many excellent published
books and papers [24–30]. Briefly, semiconductor photocatalysis drives heteroge-
neous photochemical catalytic reactions on the surface of a solid-state semiconductor.
A complete photocatalysis process is shown in Fig. 4.4. There are two basic configura-
tions for photocatalytic reactions: the particulate system (a) and the photoelectrochem-
ical (PEC) system (b) [6]. They share the same basic principles. When a semiconductor
absorbs photons with energy equal to or greater than its band gap, electrons will be
excited from the VB to the CB, leaving holes in the VB. Then the electrons and holes
migrate to the semiconductor/electrolyte interface for chemical reactions (electron for
reduction and hole for oxidation). This is, however, an ideal process. Many competing
processes consume the charge carriers, as discussed in Section 4.2.3. The possible
photoinduced chemical and physical processes involved inside and on the surface of a
semiconductor photocatalyst are shown in Fig. 4.4, including several critical steps: (i)
light absorption, (ii) charge separation, (iii) charge migration, (iv) charge recombina-
tion, and (v) redox reactions [6,29,30]. The overall solar energy conversion efficiency
is determined by three major processes: (i) light harvesting, (ii–iv) transport of photo-
generated carriers, and (v) carrier injection at semiconductor’s surface. Understanding
light absorption and charge carrier behaviors at the semiconductor/electrolyte inter-
face is of paramount importance in the efficient design of photocatalysts for solar fuel
generation.

Fig. 4.4 Scheme illustration of (A) a particulate photocatalyst, and (B) a photoelectrochemical
(PEC) cell with an n-type photoelectrode. Note: (I) light absorption, (II) charge separation,
(III) charge migration, (IV) charge recombination, and (V) red-ox reactions. CB, conduction
band; VB, valence band; E g , band gap; A, acceptor; D, donor.
Reproduced with permission from J. Li, N. Wu, Semiconductor-based photocatalysts and
photoelectrochemical cells for solar fuel generation: a review, Cat. Sci. Technol. 5 (2015)
1360–1384. Copyright © Royal Society of Chemistry.