Basic Research Needs for Solar Energy Utilization

VB

• CB

+

A

A-

D

D+

Cat 1 Cat^2

VB

• CB

+

A

A-

D

D+

Cat 1 Cat^2

Figure 12 Schematic diagram of

photoredox processes at a

semiconductor particle

Nocera 2001). All water oxidation catalysts examined to date are based on transition metals that
have oxidation states accessible in the 1–1.5 V range. Standing alone among these catalysts is the
water oxidation enzyme of PSII in green plants. In its OEC, the enzyme contains a Mn cluster,
which is capable of turning over about 10^3 O 2 molecules/s (see Photosystem II, Using Light to
Split Water sidebar) (Ferreira et al. 2004; Rüettinger and Dismukes 1997). While several
transition-metal complexes have been shown to catalyze water oxidation, turnover frequencies
remain disappointingly low.

Heterogeneous Semiconductor-based Photocatalysis. In 1972, Fujishima and Honda
reported the first complete water photoelectrolysis system (Fujishima and Honda 1972). It was
based on an n-TiO 2 anode with a small electrical bias to compensate for the insufficient reducing
power of electrons in the conduction band of TiO 2 to drive the cathodic water reduction reaction.
The requirements for an effective photocatalytic semiconductor are dictated by the positioning of
the valence and conduction bands and by chemical stability during the catalytic cycle. In the case
of reactions to produce H 2 and O 2 , the conduction band must be above 0 V vs. normal hydrogen
electrode (NHE) to produce H 2 , and the valence band must be below +1.2 V vs. NHE to produce
O 2 (Grätzel 1983). Soon after, n-SrTiO 3 , for which the conduction band is higher in energy than
that of TiO 2 , was used without an electrical bias (Wrighton et al. 1976). In 1979,
photoelectrocatalytic reduction of CO 2 in aqueous suspensions of semiconductor powders was
first accomplished (Inoue et al. 1979).

Heterogeneous systems based on photo-driven redox
reactions at semiconductor interfaces remain among the
most successful approaches to photocatalysis, promising
a means of solar energy harvesting and water pollutant
degradation. The general strategy involved in these
systems relies on the formation of an electron-hole pair
upon absorption of a photon by a semiconductor
sensitizer (Figure 12). The electron (conduction band) is
used to reduce an acceptor in the surrounding medium,
while the hole (valence band) is used to oxidize a donor.
In addition, catalysts (Pt group metals for reductions and
Pt group metal oxides for oxidations) are often added to
the semiconductor to facilitate the redox processes after
electron-hole formation. Bulk semiconductors offer poor quantum conversion efficiencies
(0.1–0.01%) because of low surface areas and significant charge recombination (Grätzel 1983).
The development of high-surface-area semiconductor dispersions and porous thin films has
greatly improved the capabilities of semiconductor-based photocatalysts. Semiconductor
catalytic dispersions have been made with both solid (e.g., polymers) and liquid (e.g., colloids)
supports and can be used to catalyze transformations on both liquid and gaseous substrates.

Semiconductor particle photocatalysis has been the most studied heterogeneous approach to
H 2 production (Linsebigler et al. 1995; Kudo et al. 2004). Colloids or nanoparticles offer several
advantages over bulk catalysts by having high electron-hole pair separation efficiencies
(~100%), large interfacial surface areas, and short electron-hole diffusion lengths to the
interface; in addition, they are readily studied by using spectroscopic techniques (Grätzel 1983).