Page 241 - Multifunctional Photocatalytic Materials for Energy

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in A with the hole generated in B, thus completing the photo-excitation cycle in which
the excited state of solid C has been achieved via a lower-energy, two-photon process.
As such, photocatalytic activity of the wide band gap material will be similar to the
higher-energy, one-photon process. Photo-excitation of components A and B will be
very efficient because the two solids are activated through their fundamental absorp-
tion band [16].

10.2 Hydrogen economy and photocatalytic splitting
of water

Hydrogen is one of the most important clean, renewable, and sustainable energy
sources and currently is produced by steam reforming. Moreover, photocatalytic
water splitting into oxygen and hydrogen is the most effective way of H 2 produc-
tion, an important step in realizing a H 2 economy. As discussed in Section 10.1.3, a
semiconductor that absorbs in the visible range is desired, but most semiconductors
have band gaps ranging up to the near-UV absorption range. Because of its nontox-
icity, abundance, and low cost, TiO 2 has long been a promising material for water
splitting, as first reported by Fujishima and Honda in 1972 under UV light [18,19].
However, TiO 2 has performed poorly because of its fast carrier recombination and
absorption of solar radiation in the ultraviolet region because of a large band gap.
Efforts to overcome such issues by doping/forming composites are now employed. In
order to warrant a good electron-hole transfer, the bottom of the conduction band and
the top of the valence band of the semiconductor must have higher energy than those
of TiO 2 . Single-component semiconductors have shown poor photocatalytic activity
because of their fast recombination kinetics. Co-catalysts have been used in addition to
semiconductors to enhance the conversion efficiency by improving the charge transfer
through forming heterojunction interfaces [20]. Fig. 10.8 shows co-catalysts attached
to a semiconducting nanoparticle utilized for hydrogen and oxygen evolution [21].
Polymers with extended π-conjugated electron systems have attracted considerable
attention recently because of their absorption coefficients in the visible region and
high conductivity, allowing high mobility in charge carriers. Among conductive poly-
mers, polyaniline (PANI) has been widely used to improve electronic conductivity as
well as solar energy transfer and photocatalytic activity of TiO 2 , because of PANI’s
easiness of preparation, comparatively low cost, and excellent environmental stability.
There are some examples of PANI combined with other semiconductors for H 2 gener-
ation, such as the case of PANI-CdS composite nanoparticles synthesized by He et al.
2-
[22] for direct H 2 evolution in the presence of an SO 3 2- /S sacrificial reactant. A two-
step reaction came up where, first, a nitrogen atom of PANI formed an initial complex
with Cd(CH 3 COO) 2 through an interaction of the π-conjugated chain. Second, the
PANI-Cd(CH 3 COO) 2 intermediate reacted with thioacetamide to form CdS. These
hybrid photocatalysts were used to generate hydrogen with a 5 h experiment of the
as-prepared photocatalysts, as shown in Fig. 10.9A. As the diagram shows, the activity
of hydrogen production of the PANI-CdS composite catalyst gradually decreased as
the PANI content increased. PANI-CdS-1 showed the highest activity for hydrogen