Heterogeneous photocatalysis has attracted interest as a method for water treatment because it is very effective in both degrading
organic and inorganic chemical pollutants and destroying viruses and bacteria [1]. Additional applications include air purification,
self-cleaning and sterilizing surfaces, and water photolysis [2]. Anatase titanium dioxide is the most commonly employed
photocatalytic material but its efficiency is limited by an absorption spectrum confined to the ultraviolet (UV) range and a high
charge carrier recombination rate. These disadvantages have stimulated investigation of alternative photocatalyst materials such
as other transition metal oxides, main group element oxides, rare earth oxides and binary sulfides, as well as ternary and
quaternary compounds [3]. Until now, however, these efforts have met with relatively limited success.
Titanium dioxide exists in three naturally occurring phases: tetragonal rutile, tetragonal anatase, and orthorhombic brookite
(rutile is the most stable phase except in the case of very small crystal sizes where anatase and brookite become more stable).
The anatase phase has a band gap, Eg, of 3.2 eV and is thus able to absorb light only of wavelengths shorter than 388 nm. Its
(001) surface is highly photoreactive due to a high degree of coordinative unsaturation: only fivefold coordinated Ti and twofold
coordinated O atoms are present in the first layer [4]. However, less than 10% of the surface area of typical anatase crystals
consists of {001} facets, while a major part of the surface area is made up of stable {101} facets.
The application of surface science techniques has provided molecular-level insights into the basic mechanisms of photocatalytic
phenomena. Among the key topics of interest are photon absorption, charge carrier transport, trapping and recombination,
electron transfer dynamics, adsorption and the adsorbed state, photocatalytic reaction mechanisms, effects of inhibitors and
promoters, and the phase and form of the photocatalyst [5]. Studies of the relationships between crystal and surface structure,
surface chemistry, and electronic, electrochemical, and photoelectrochemical properties have advanced our understanding of
the behavior of photocatalytic materials, thereby enabling improvements in their performance [6]. Bulk and surface defects,
such as oxygen vacancies, exert a significant influence on the electronic properties and catalytic activity [7].
Absorption by the photocatalyst of photons of energy greater than that of the band gap leads to the formation of electron–
hole pairs that can initiate redox reactions at the semiconductor surface. However, this process is relatively inefficient because
only a small fraction of the incident photons take part in photocatalytic reactions [8]. The use of nanocrystalline materials has
been widely recognized as one way of improving the efficiency of the photocatalytic reaction [9]. Not only is the reactivity
increased due to the higher specific surface area of the nanomaterial but quantum confinement effects can also occur when the
Bohr radius approaches or becomes larger than the crystal dimensions. Quantization of the energy levels in the conduction and
valence bands and modification of the band gap result in an increased charge transfer rate, which may lead to improved photocatalytic
efficiency when the reaction rate is charge transfer limited [10].
The main focus of current work on methods for increasing photocatalytic activity is on the modification of the surface
chemistry and controlled growth of TiO2 nanocrystals. There has been much recent progress on the synthesis of nanocrystals
with a higher proportion of reactive facets, and theoretical modeling has assisted by providing an understanding of the influence
of surface energy effects in determining morphology as the crystal size is reduced [11]. Attempts have also been made to
increase the reaction rate by surface loading with noble metal clusters [12, 13], to enhance photocatalytic activity by doping
with metals to inhibit electron–hole recombination by charge trappping [14, 15], and to achieve visible light photocatalysis by
doping with various metals and nonmetals [16, 17].
There are many existing and potential applications of photocatalysis in drinking water purification and wastewater treatment,
and it is often claimed that the advantage of photocatalytic treatment with respect to chlorination is that it is able to degrade
organic pollutants without the production of toxic by-products. However, this is not entirely accurate as problems may still be
experienced due to the formation of long-lived toxic intermediates by complex photodegradation processes [18, 19]. It is therefore
essential to develop a more comprehensive knowledge of the reaction pathways in order to ensure the complete elimination
of any toxic compounds produced.
RICKERBY David;
2018-10-17
Wiley
JRC90642
9781118845530 (online),
https://onlinelibrary.wiley.com/doi/10.1002/9781118845530.ch10,
https://publications.jrc.ec.europa.eu/repository/handle/JRC90642,
10.1002/9781118845530.ch10 (online),
