Page 268 - Multifunctional Photocatalytic Materials for Energy

Multidimensional TiO 2 nanostructured catalysts for sustainable H 2 generation 251

into chemical fuels through photoelectrochemical WS is attracting increasing interest
as one of the most important solutions to the global energy crisis [139–141]. Since WS
using a TiO 2 photoelectrode was first reported, more and more efforts have been made
to prepare efficient photoelectrodes. In fact, many semiconductor materials have been
explored as light absorbers and electrode materials. As we all known that semiconduc-
tors (such as SiC) produce photocorrosion, they also provide suitable band structures,
nontoxicity, strong redox potential, and chemical stability at a low cost; and TiO 2 -
based materials are still the most widely used and ideal photocatalysts for hydrogen
production and decomposition of pollutants [142–145].
It is known that the WS reaction consists of two thermodynamic half-reactions:
the hydrogen evolution reaction (HER) (Eq. 11.2) and the oxygen evolution reaction
− 1
(OER) (Eq. 11.3). The free energy ∆G (∆G ¼ + 237.2 kJ mol ) (Eq. 11.4) and the
standard redox potential ∆E (∆E ¼ 1.23 V) (Eq. 11.3) of the multielectron WS reac-
tion. More important, OER has proved to be the essential component of light-driven
WS systems in environmental remediation applications because it also provides holes
for the degradation of pollution.

-
TiO + hv ® e + h + (11.1)
2
+
Reduction :2 H + e 2 - ® H DE = 0 V (11.2)
2
+
+
Oxidation : 2 H O 4 h ® O + 4 H + DE = . 1 23 V (11.3)
2 2
-
1
Overall reaction : 2 H O ® 2 H + O ( DG =+ 237 .2 kJmol ) (11.4)
2
2
2
Different mechanisms, such as physical mechanisms, metal catalytic effect, and
quantum tunneling effect, have been proposed for plasmonic WS, including (i) di-
rect hot-electron transfer, (ii) local electromagnetic field enhancement, (iii) transfer of
plasmon resonance energy, (iv) plasmon-heating effect, (v) far-field light scattering,
and (vi) dipole-dipole coupling reaction.
There are two primary routes for generating H 2 by a plasmonic reaction: the pho-
tocatalytic approaches and the photoelectrochemical methods. The fundamental prin-
ciple of solar-induced WS for semiconductor photocatalysts (shown Fig. 11.8B) is
that the electrons can be excited from the valence band (VB) to the conduction band
(CB) under light irradiation of suitable wavelengths, thus leaving holes in the VB. As
a result, the photoinduced charges and holes are efficiently separated, and then these
electrons and holes can transfer to the surfaces of the photocatalysts and promote the
HER and OER, respectively. Based on the laws of thermodynamics, the bottom of the
+
CB must be located at a more negative potential than the reduction potential of H
to H 2 (− 0.41 V vs normal hydrogen electrode (NHE)), and the top of the VB must
exceed the oxidation potential of H 2 O to O 2 (0.82 V vs NHE). As a result, to convert
the energy into H 2 and O 2 , the minimum band gap energy (Eg) of the photocatalyst
should be 1.23 eV (1000 nm) with respect to NHE. Additionally, because of the loss
of energy, kinetic overpotentials are needed to drive the HER (E CB in Fig. 11.8B) and
the OER (E VB in Fig. 11.8B); thus the Eg of single semiconductor materials should