Page 26 - Multifunctional Photocatalytic Materials for Energy

Metal oxide powder photocatalysts 15

chlorine, and 1.5-fold stronger than ozone, and the TiO 2 process is much safer and
more efficient than other processes [14,15].

2.5.5 Organic synthesis

Photocatalytic organic synthesis using metal oxides are well known, and research in
that area is progressing. Choi et al. recently reported the photocatalytic synthesis of
2-hydroxyterephthalic acid (HTPA) using terephthalic acid (TPA). In this this synthe-
sis, HTPA was treated with novel photocatalysts ZnO and ZnS under visible light irra-
•
diation for 6 h in-situ, which generated OH radicals and reacted with HTPA, leading
to the formation of TPA [6,14,15,17].

2.5.6 Energy
Global demands for sustainable energy sources, production, and storage continue to
increase, but these demands may be eased through recent developments in the use
of novel nanomaterials that are more energy- and cost-efficient than other materials
currently in use. Because of recent advancements in their print-like manufacturing
process and because they can be made into flexible rolls rather than discrete panels,
nanostructured solar cells such as TiO 2 and ZnO nanotube-based, dye-sensitized solar
cells have the potential to be less expensive and easier to install than their predecessors
[31,32]. Because of the abundance of water resources and sunlight as energy, it is also
inevitable that hydrogen derived from water will be a future source of clean energy.
There is expanding research on ways to improve the efficiency of hydrogen production
and light harvesting using different metal oxide nanostructures such as TiO 2 , ZnO, Ce/
TiO 2 , and CeO 2 -graphene. [1–5,33–35] This means that these materials will become a
major, sustainable source of clean energy for future generations.

2.6 Future perspectives

This chapter presents an overview of the recent studies related to heterogeneous pho-
tocatalysis using metal oxides as photocatalysts. These studies provide an important
basis for the understanding of this topic and related areas of concern; however, we
believe that many questions remain unanswered. These questions can be answered
through a combined effort of critical field studies, thorough laboratory studies, and
modeling using novel tools. Some of the future directions for study include
i. Metal oxide-based photocatalytic reactions carried out under atmospherically relevant con-
ditions of gas phase concentrations, relative humidity, solar flux, and so on;
ii. Metal oxide-based photocatalytic studies that probe in-situ both surface and gas phase spe-
cies formed under pertinent environmental conditions, as these are best suited for mecha-
nistic studies of photocatalytic reactions;
iii. Investigation of the effects of the physicochemical properties of metal oxides on heteroge-
neous photocatalysis that include shape, size, coatings, and also use as nanocomposites;