Page 22 - Multifunctional Photocatalytic Materials for Energy

Metal oxide powder photocatalysts 11

2.5 Applications of powdered metal oxides
photocatalysts

Over the past few decades, metal oxide nanomaterials, such as TiO 2 , ZnO, SnO 2 , WO 3 ,
Fe 2 O 3 , CeO 2 , and so on, have been largely studied for their photocatalytic properties
[1,4,15–22]. Metal oxide nanomaterials used as photocatalysts showed excellent deg-
radation of organic and toxic pollutants because of the materials' high reactivity at
low concentrations, low toxicity, and high stability. Metal oxide photocatalysis has
attracted significant attention because of its promising applications in various fields,
such as environmental remediation by the photodecomposition of hazardous dyes
in polluted water, industrial effluents, and solar energy conversion. Therefore metal
oxides, such as TiO 2 , ZnO, SnO 2 , and CeO 2 , have been the prime choice for basic
research and practical applications owing to their high activity, good stability, easy
availability, low cost, nontoxicity, and chemical inertness.
In recent years, emerging alarms about energy and environmental problems have
encouraged extensive research on solar energy utilization. Dyes are used widely in a
range of fields, but their discharge into water can cause environmental pollution. In
addition, most dyes are toxic, carcinogenic, and harmful, resulting in adverse impacts
on human and animal health. Dyes are used widely in several industries, including
textile, plastic, rubber, paper, concrete, and medicine, with the textile industry as the
main user. Unfortunately, approximately 10% of dyes used in industry are discharged
directly into the environment as a harmful pollutant, which is environmentally unsafe
and aesthetically unacceptable. Heterogeneous photocatalysis involves the application
of metal oxide catalysts (e.g., TiO 2 , ZnO, SnO 2 , WO 3 , and CeO 2 ) irradiated with light
of an appropriate wavelength to generate highly reactive transitory oxidative species
−
•
•
(i.e., OH, and O 2 ) for the mineralization of organic contaminants, impurities, and
pollutants [5,6]. Therefore a range of approaches have been explored for the photo-
catalytic degradation of organic dyes using semiconductor photocatalysts [4,7,13–19].
Generally, semiconductor catalysts show relatively low quantum degradation effi-
+
−
ciency because of the high recombination rate of light-induced e /h pairs at or near
the surface of the photocatalysts, which is considered one of the major limitations to
hindering the photocatalytic efficiency [7,14].
The chemistry that occurs at the surfaces of metal oxides has attracted consid-
erable attention for a range of industrial applications (Fig. 2.3), including catalysis,
photocatalysis, water purification, deodorization, air purification, self-cleaning, self-
sterilizing, antifogging surfaces in optical display technology, chemical synthesis,
solar energy devices, antibacterial activities, batteries, and energy production and
storage [4,10,14–30].

2.5.1 Water purification

Water is a well-recognized necessity, because without it, the world would be de-
void of life as we know it. Water cleansing and treatment have become worldwide
problems, particularly in industrialized countries where wastewater normally con-
tains organic pollutants, such as organic dyes from the textile industries and organic