7.3.5 Hydrogen Production Using Alternative Processes:
Photoelectrochemical (PEC) Hydrogen Generation
Honda and Fujishima suggested the possibility of water splitting using TiO 2 as
electrode (Fujishima and Honda 1972). Since then, numerous efforts have been
made to improve the performance of TiO 2 nanomaterials for hydrogen generation
(Chen et al. 2013; Huang et al. 2015; Liu et al. 2014a; Zhao et al. 2014; Zhang and
Yang 2013). In general, a predominant PEC cell relies on two factors: the efficient
usage of solar energy and the instant transportation/separation of charges (Bard and
Fox 1995). Hence, the development of nanosized photoactive semiconductors to
satisfy the requirements has been a long-standing objective in the research of PEC
cells, especially for one-dimensional TiO 2 due to its superior charge transport
property (Lou and Chen 2014).
A PEC process is based on the excitation of the semiconductor by energetic
photons (hυ ! E g ) which generate (e
À
/h
+ ) pairs that could be used in the
photoredox reactions. A great deal of attention has been paid to hydrogen as a
clean fuel with a high-energy density. The stable oxides have a valence band
(VB) made up of O 2
À
: 2p orbital, while the conduction band (CB) derives from
metal parentage, thus resulting in a large gap. Such a handicap can be overcome by
using a configuration that has the band edges properly matched to the H 2 O/H 2
level. A PEC cell is composed of an electrolytic cell: three electrodes (Pt as
auxiliary electrode, a saturated calomel electrode (SCE), and the working electrode). When the PEC cell is exposed to the light, a charge transport within the PEC
occurs, and there is an evolution of gases at the photoanode or photocathode. The
oxygen energy level (O 2 /H 2 O) should be above the VB of the photoanode to allow
electron transfer. For the same reason, the hydrogen energy level (H
+ /H 2 ) should
also be below the Fermi level of the metal cathode (or the CB of a semiconducting
photocathode, see Fig. 7.11).
Generally, photoelectrode is a n- or a p-type semiconductor electrode, which
conducts the electrons generated by the light-induced chemical reactions initiated at
the semiconductor surface. The band-gap energy of the semiconductor must be at
least 1.5 eV, higher than the effective redox potential of water (1.23–1.40 eV)
(Minggu et al. 2010; Arifin et al. 2013).
Many TiO 2 nanostructures, including nanoparticle films, nanotube, and
nanorods, have been used as the photoanodes in PEC for water splitting. Some
reports have shown that the control of the composition of the material and their
morphology greatly enhances the PEC performance under irradiation. Thus, the
effect of the morphology on the performance of the TiO 2 photoanodes in the form
of nanotubes, nanoparticles, and nanofilms has been studied by Leng et al. (2010).
For TiO 2 -based PEC system, a rapid recombination between photogenerated
electrons and holes is the major factor in decreasing the photoreactivity. When
the surface of TiO 2 nanorods is loaded with metal oxide, the highly dispersed metal
nanoparticles not only facilitate the excitation of electrons and generation and
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
237
Photoelectrochemical (PEC) Hydrogen Generation
Honda and Fujishima suggested the possibility of water splitting using TiO 2 as
electrode (Fujishima and Honda 1972). Since then, numerous efforts have been
made to improve the performance of TiO 2 nanomaterials for hydrogen generation
(Chen et al. 2013; Huang et al. 2015; Liu et al. 2014a; Zhao et al. 2014; Zhang and
Yang 2013). In general, a predominant PEC cell relies on two factors: the efficient
usage of solar energy and the instant transportation/separation of charges (Bard and
Fox 1995). Hence, the development of nanosized photoactive semiconductors to
satisfy the requirements has been a long-standing objective in the research of PEC
cells, especially for one-dimensional TiO 2 due to its superior charge transport
property (Lou and Chen 2014).
A PEC process is based on the excitation of the semiconductor by energetic
photons (hυ ! E g ) which generate (e
À
/h
+ ) pairs that could be used in the
photoredox reactions. A great deal of attention has been paid to hydrogen as a
clean fuel with a high-energy density. The stable oxides have a valence band
(VB) made up of O 2
À
: 2p orbital, while the conduction band (CB) derives from
metal parentage, thus resulting in a large gap. Such a handicap can be overcome by
using a configuration that has the band edges properly matched to the H 2 O/H 2
level. A PEC cell is composed of an electrolytic cell: three electrodes (Pt as
auxiliary electrode, a saturated calomel electrode (SCE), and the working electrode). When the PEC cell is exposed to the light, a charge transport within the PEC
occurs, and there is an evolution of gases at the photoanode or photocathode. The
oxygen energy level (O 2 /H 2 O) should be above the VB of the photoanode to allow
electron transfer. For the same reason, the hydrogen energy level (H
+ /H 2 ) should
also be below the Fermi level of the metal cathode (or the CB of a semiconducting
photocathode, see Fig. 7.11).
Generally, photoelectrode is a n- or a p-type semiconductor electrode, which
conducts the electrons generated by the light-induced chemical reactions initiated at
the semiconductor surface. The band-gap energy of the semiconductor must be at
least 1.5 eV, higher than the effective redox potential of water (1.23–1.40 eV)
(Minggu et al. 2010; Arifin et al. 2013).
Many TiO 2 nanostructures, including nanoparticle films, nanotube, and
nanorods, have been used as the photoanodes in PEC for water splitting. Some
reports have shown that the control of the composition of the material and their
morphology greatly enhances the PEC performance under irradiation. Thus, the
effect of the morphology on the performance of the TiO 2 photoanodes in the form
of nanotubes, nanoparticles, and nanofilms has been studied by Leng et al. (2010).
For TiO 2 -based PEC system, a rapid recombination between photogenerated
electrons and holes is the major factor in decreasing the photoreactivity. When
the surface of TiO 2 nanorods is loaded with metal oxide, the highly dispersed metal
nanoparticles not only facilitate the excitation of electrons and generation and
7 Titanium Oxide-Based Nanomaterials with Photocatalytic Applications. . .
237