Page 17 - Multifunctional Photocatalytic Materials for Energy

6 Multifunctional Photocatalytic Materials for Energy

syntheses [6]. In heterogeneous photocatalysis, the reaction pathway involves the forma-
tion of an interface among photocatalysts and fluids containing the reactants and prod-
ucts. Processes involving irradiation on adsorbate-metal-oxide interfaces are generally
categorized in the branch of photochemistry. Therefore the term “heterogeneous photo-
catalysis” is mainly used in the cases where a light-absorbing metal oxide photocatalyst
is utilized, which is in contact with either a liquid or a gas phase.
A material is considered to be a catalyst when it accelerates a chemical reaction with-
out being consumed in the reaction and lowers the free activation enthalpy of the reaction.
Photocatalysis is defined as the acceleration of a reaction in the presence of light and a
suitable catalyst. A Photocatalyst can be defined as a material that accelerates a chemical
reaction in the presence of appropriate light and a suitable catalyst without being utilized
and that lowers the free activation enthalpy of a chemical reaction [3]. Photocatalysis is a
rapidly emerging area with a high possibility of an extensive range of industrial applica-
tions, such as disinfection of water and air, mineralization of organic pollutants, produc-
tion of renewable fuels, and organic syntheses [3–6]. In this chapter, the discussion will
be restricted to the description of metal oxide-mediated photocatalysis.

2.2 Semiconductors and photocatalysis

Semiconductors are mainly useful as photocatalysts because of their promising com-
bination of light absorption properties, electronic structure, charge transport charac-
teristics, and excited-state lifetimes. A semiconductor is nonconductive in its undoped
ground state because an energy gap, i.e., a wide band gap, exists between the top of
the filled valence band (VB) and the bottom of the vacant conduction band (CB).
Accordingly, electron transport between these bands must occur only with consider-
able energy change. In semiconductor photocatalysis, excitation of an electron from
the VB to the CB is achieved by absorption of a photon of energy equal to or higher
than the band gap energy of the semiconductor. This light-induced formation of an
electron-hole pair is a prerequisite step in all semiconductor-assisted photocatalytic
reactions. The formed photogenerated species tend to recombine and dissipate energy
as heat of photons, because the kinetic barrier for the electron-hole recombination step
is low. However, CB electrons and VB holes can be separated well in the presence of
an electric field, such as the one generated spontaneously in the space charge layer
of a semiconductor-metal or a semiconductor-fluid interface. Therefore the lifetimes
of photogenerated carriers increase, and the possibility is offered to these species to
exchange charges with substrates adsorbed on the photocatalyst surface and induce
primary and secondary chemical reactions.
Transfer of an electron to or from a substrate adsorbed onto a light-activated semicon-
ductor (i.e., interfacial electron transfer) is probably the most critical step in photocata-
lytic reactions, and its efficiency controls to a large extent the ability of the semiconductor
to serve as a photocatalyst for a given redox reaction. The efficiency of electron transfer
reactions is, in turn, a function of the position of a semiconductor's VB- and CB-edges
relative to the redox potentials of the adsorbed substrates. For an anticipated electron
transfer step to occur, the potential of the electron donor species should be located above