Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 52

couple, which leads to the spontaneous corrosion of the photocatalyst, and therefore, will
not be suitable for the photocatalytic applications in water or air. Although WO 3 satisfies
the conditions (a) and (b), the cost for its synthesis can be the major concern due to the
extreme chemical inertness of W, which fails to meet the criterion (c). Due to its
excellent thermodynamic properties and low synthesis cost, TiO 2 has been recognized as
one of the most suitable materials for photon harvesting application. Among various
types of TiO 2 particles currently available, Degussa P25 is the most widely investigated
in water purification applications due to it high photoactivity and commercial
availability. However, the biggest challenge of TiO 2, as a photocatalyst, is its wide
bandgap, which limits its photoactivity under visible light.
3.3.1 Thermodynamic Aspects of TiO 2 /Water Interface

To evaluate the performance of a semiconductor photocatalyst, it is extremely
important to understand the thermodynamic aspects at the semiconductor-liquid
interface. The thermodynamics in such system can be explained in terms of energy.
The characteristic energy positions (e.g., Fermi energy E F , conduction band edge, E C ,
and valance band edge E V ) of a semiconductor material are most commonly expressed in
absolute value scale (i.e., AVS). However, in electrochemistry, it usually positions the
redox couple energy of a specific element with respect to the NHE. The AVS energy is
reported to be offset by -4.5 eV from the redox energy with respect to NHE at
temperature of 25 o C. Figure 3.6 is schematic energy diagram of semiconductor TiO 2
and electrolyte interface. From this figure, it is clear that the conduction band edge
bends upward from its flat band potential due to the presence of depletion region
(Schottky barrier), which in turn, gives a plausible effect if a reduction reaction is
desired, but a hindering effect when oxidation reaction is wanted instead. For the
oxidation scheme in a particulate semiconductor-liquid system, the oxidation potential is
governed by the energy difference between the valance band edge and the chemical
potential of the redox species (E F o redox ) , i.e., φ O . However, for the reduction reaction
scheme, the reduction potential is the energy differences between the conduction band
edge and the chemical potential of the redox species (E F o redox ), φ R .

The rate constant for an interfacial electron transfer is typically found to be
greater than 5 x 10 10 /s (Linsebigler et al., 1995). The reduction potential of the acceptor
redox couple (ΔE = E C - E F o redox ) or φ R is normally considered as the driving force for the
heterogeneous electron transfer in a solid-liquid system. This is due to several reasons.
First, the electron flow is strictly confined by energy position of E C , E V , and E F in the
semiconductor colloidal system. Thus, E C , E V , and E F can only be controlled by pH or
the ionic strength of the solution. Second, the oxygen molecules are the major electron
scavengers in both gas and liquid phases. Without significant modification in the
electronic structure of TiO 2 , the reduction potential of the acceptor redox couple will not
be expanded.