Page 236 - Multifunctional Photocatalytic Materials for Energy

Photocatalysts for hydrogen generation and organic contaminants degradation 219

through the polymer chain. The charge flow mechanism in conjugated polymers is at-
tributed to intra-chain charge diffusion and inter-chain charge hopping, the latter being
the reason behind their low conductivities [5,6]. As there is no long-range order pres-
ent, organic semiconductors lack the high translational periodicity and symmetry of an
inorganic crystal lattice, and wave-functions cannot be approximated by Bloch func-
tions. The transport of charge carriers through the semi-crystalline polymeric chains
can be influenced by chemical defect/dopant or structural discontinuity. The typical
energy gaps lie between 1.5 and 3.5 eV. The thermal energy at room temperature is
about 25 meV; therefore such polymers possess no free carriers at room temperature
and favor flow of charges only when injected by some means.

10.1.3 Role of photocatalytic materials

The semiconducting materials enjoy a very special place in material engineering be-
cause of their tunable band gaps, which are also size-dependent. If charges (electrons
and holes) can be separated by some means within the material, then their chemical
energy can be used to carry out redox reactions. Photocatalytic materials work on
the principle of charge separation by absorbing solar radiation in consonance with
their characteristic band gaps. Semiconductors employed as photocatalytic materials
primarily have two responsibilities: carrying out charge separation and charge trans-
fer. Fig. 10.4A shows the solar-to-hydrogen (STH) conversion efficiency [9]. The
solar-to-hydrogen conversion efficiency is defined as the ratio of usable chemical en-
ergy from the produced hydrogen gas to total solar energy delivered to the system
[10,11]. Therefore, for higher STH efficiency, materials with lower band gaps are
preferred. Fig. 10.4B depicts some compound semiconducting metal oxides employed
as photocatalysts. The charge separation caused by photon absorption depends on the
optical band gap, absorption efficiency of the photocatalytic material, chemical purity,
crystallinity, stability, surface area, and activation range of solar spectrum [12].
In order to effect a redox reaction, the real challenge lies in creating a charge
separation and suppressing the recombination until the charge transfer process takes
place. The phenomena of charge generation, recombination, and transfer are prob-
abilistic and competitive events in nature. The timescales at which these processes
occur are tabulated in Table 10.2. Recombination is not a desirable process for pho-
tocatalysis. If the electrons in the conduction band can migrate to the surface, then
they can be suitably transferred to an electron-accepting species thereby carrying
out a reduction process. Similarly, if a hole from the valence band can be trans-
ferred to an electron-donating species, the hole can be transferred, and the species
can in turn be oxidized. It is quite clear that keeping the charges separated (~ms) for
12
about 10 times longer than their generation rate (~fs) is challenging. The band gap
and lifetimes of charge carriers depend on the morphology and dimensions of the
semiconductors, thereby providing a control on their generation/recombination. If
the size of the semiconducting nanoparticle is comparable to the diffusion lengths of
the charge carriers, the carriers can migrate to the surface. Sorption of appropriate
electron-donating or electron-accepting species can further promote the charge trans-
fer away from the semiconductor and contribute toward a redox reaction. The size of