Page 21 - Multifunctional Photocatalytic Materials for Energy

10 Multifunctional Photocatalytic Materials for Energy

[1,5,6,20–22], TiO 2 has been examined widely as an efficient photocatalyst for purifi-
cation of water and degradation of dyes, pesticides, and so on. A significant diversity of
semiconducting materials, mainly metal oxides and chalcogenides, have been explored
with respect to their photocatalytic behavior, but only a few of them are considered to
be effective photocatalysts. Metal oxide (ZnO, TiO 2 , SnO 2 , CeO 2 , Fe 2 O 3 , and V 2 O 3 )
nanomaterials have attracted considerable interest in several areas of materials science,
physics, and chemistry owing to their fascinating properties. In general, wide band gap
metal oxides, such as TiO 2 , have proved to be better photocatalysts than low band gap
materials, such as cadmium sulfide (CdS), mainly because of the higher free energy of
photogenerated charge carriers of the former and the inherently low chemical and pho-
tochemical stability of the latter [3,4]. However, low band gap metal oxides are altered
better by the solar spectrum, thus they pose significant potential for the utilization of a
continuous and readily available power supply, i.e., sunlight. In recent years, substantial
work has been done on the growth of more efficient photocatalysts, based on improved
harvesting of light as well as increased quantum efficiency. In this way, favorable out-
comes have been obtained with the use of several approaches targeted at the optical
and/or modification of electronic properties of different metal oxides, including metal
deposition, dye sensitization, doping with transition metals or nonmetallic elements,
use of composite semiconductor photocatalysts, and so on [4,7,13].
The potential uses of heterogeneous photocatalysis depend mainly on the prog-
ress in scaled-up reactor designs, light harvesting abilities, and reduced recombina-
tion efficiency of electron-hole pairs. The main task in the design of a photocatalytic
reactor is optimization in the mass transfer and efficient light harvesting ability of the
catalyst, especially in liquid phase reactions. Mass transfer restrictions can be dealt
with by using monolithic reactors, spinning disc reactors, and microreactors, which
have proved to be much more efficient than conventional reactors. Photon transfer
can be optimized using light-emitting diodes (LEDs) and optical fibers; however, key
advances in this field are still lacking. Thus the artificial formation of photons needed
for photocatalytic reactions is the most significant basis in terms of operating costs
in practical applications, and a considerable amount of research has been done in the
development of solar photoreactors. Among the various types of solar reactor config-
urations evaluated so far, compound parabolic collectors are the most promising ones
and have been successfully scaled up for applications related to wastewater treatment
and water cleaning and disinfection [22–26]. Metal oxide-based photocatalysis is cur-
rently one of the most active interdisciplinary research areas, and it has been examined
from the standpoint of catalysis, photochemistry, electrochemistry, organic, inorganic,
physical, polymer, and environmental chemistry. Because of the amount of research
in these core areas, the fundamental processes of photocatalysis are now understood
much better. The applicability of photocatalysis has been shown in laboratory-scale
conditions for many different processes, such as air cleaning, water treatment, disin-
fection applications, production of fuels from water and atmospheric gases, selective
organic synthesis, and metal recovery [4,5,14]. However, industrial applications re-
main restricted and incomplete. The present lack of extensive industrial applications is
due mainly to the low photocatalytic efficiency of metal oxide photocatalysts and the
lack of efficient and large-scale photoreactor setups.