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Metal oxide powder photocatalysts 2

Mohammad M. Khan
Universiti Brunei Darussalam, Gadong, Brunei Darussalam

2.1 Historical developments and introduction

In 1972 Fujishima and Honda first reported photocatalysis by the splitting of water
under UV irradiation, and since then research in this field has expanded considerably
[1]. Photocatalysis has drawn extra attention from researchers because of its use in a
variety of products across a broad range of areas, mainly environmental- and energy-
related fields. Until recently, metal oxides, such as TiO 2 , ZnO, SnO 2 , and so on, were
the main choices for most studies in basic research and practical applications because
of their high activity, low cost, high stability, nontoxicity, and chemical inertness,
which made them suitable for applications in water and air purification, sterilization,
hydrogen evolution, and so on [2–5].
Development of the term “photocatalysis” indicated the improvement of some vital
concepts of photochemistry. The turning point enabling photochemistry to become
a science on its own occurred when it was distinguished from thermal chemistry. In
fact, until the beginning of the 20th century, many scientists thought that irradiation or
illumination was one of the several existing ways to catalyze a reaction (i.e., to make
it faster), such as treating with chemicals, heating, and so on. Giacomo Ciamician was
the first scientist to make a systematic effort to understand the chemical effect of light
in ascertaining whether “light and light alone” caused the reactions to accelerate, and
not, for instance, heat [2]. He properly described these reactions as “photochemical
reactions,” whereas “photocatalytic-tagged reactions” were described as being accel-
erated by light, while maintaining the same course as the thermal reactions. Later,
researchers recognized that photochemical reactions involved electronically excited
states, “electronic isomers” of ground states that have a reactivity (and thermodynam-
ics) of their own. However, in 1914 Bodenstein observed it, and it became a common
concept after several years [2,3].
The word photocatalysis is of Greek origin and comprises two parts: prefix “photo”
derived from phos, meaning light; and “catalysis” derived from katalyo, meaning break
apart or decompose. Although there is no agreement in the scientific community on a
proper definition of photocatalysis, it is generally described as a process in which light
is used to activate a substance, the photocatalyst, that modifies the rate of a chemical
reaction without being involved itself in the chemical reaction. Thus the main difference
between a conventional thermal catalyst and a photocatalyst is that the latter is activated
by photons of appropriate energy, whereas the former is activated by heat. Photocatalytic
reactions may take place heterogeneously or homogeneously. However, heterogeneous
photocatalysis has been more deeply studied in recent years because of its potential
use in a range of environmental- and energy-related applications as well as in organic

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