TiO2 photocatalysis and related surface phenomena
Introduction
Photocatalysis is generally thought of as the catalysis of a photochemical reaction at a solid surface, usually a semiconductor [1], [2], [3], [4], [5], [6], [7], [8], [9], [10], [11], [12], [13], [14], [15], [16]. This simple definition, while correct and useful, however, conceals the fact that there must be at least two reactions occurring simultaneously, the first involving oxidation, from photogenerated holes, and the second involving reduction, from photogenerated electrons. Both processes must be balanced precisely in order for the photocatalyst itself not to undergo change, which is, after all, one of the basic requirements for a catalyst.
It will be seen in this review of the fundamentals and selected applications of photocatalysis, principally on titanium dioxide, that there is a host of possible photochemical, chemical and electrochemical reactions that can occur on the photocatalyst surface. The types of reactions occurring, their extent and their rates depend upon a host of factors that are still in the process of being unraveled. Furthermore, there can indeed be changes that occur, involving the surface and bulk structure and even decomposition of the photocatalyst, a fact that appears to stretch the definition of the term.
This topic started its early history as mostly a nuisance involving the chalking of titania-based paints [17], [18] and then gradually transformed into a highly useful approach to the remediation of water and air and then into an approach to maintain surfaces clean and sterile. Along the way, it has also transformed into an approach to photolytically split water into hydrogen and oxygen [19], [20], [21] and also an approach to perform selective oxidation reactions in organic chemistry [22].
Clearly, with so many varied aspects, photocatalysis is nearly impossible to review comprehensively. In the present review, we have tried to put together an overview of some of the more fundamental aspects, which are in their own right extremely scientifically interesting and which also need to be better understood in order to make significant progress with applications.
The review will be divided into several sections: 2. Historical overview; 3. Properties of TiO2 materials; 4. Fundamentals of photocatalysis; 5. Fundamentals of the photo-induced hydrophilic effect; 6. Brief review of applications; 7. Summary, and Appendix (film preparation methods).
Section snippets
Historical overview
We will give a brief overview of the early history of photocatalysis, which will be based just on papers that we have been able to access, which means that we will almost certainly be ignoring some important papers. The earliest work that we have been able to find is that of Renz, at the University of Lugano (Switzerland), who reported in 1921 [17] that titania is partially reduced during illumination with sunlight in the presence of an organic compound such as glycerol, the oxide turning from
Crystal structures
As often described, there are three main types of TiO2 structures: rutile, anatase and brookite. The size dependence of the stability of various TiO2 phases has recently been reported [77], [78]. Rutile is the most stable phase for particles above 35 nm in size [77]. Anatase is the most stable phase for nanoparticles below 11 nm. Brookite has been found to be the most stable for nanoparticles in the 11–35 nm range, although the Grätzel group finds that anatase is the only phase obtained for
Photoelectrochemical basis of photocatalysis
As described in the Historical Overview, it became recognized by several researchers that photocatalysis is based on “back-to-back” or short-circuited photoelectrochemical and electrochemical reactions, involving electrogenerated electrons and holes. At the most global level, these can be written: Reactions (4.2), (4.3) can be designated as the oxygen photoevolution reaction (OPER) and the oxygen reduction reaction (ORR), respectively. Of course,
Overview
Our group first reported, along with co-workers from the TOTO Corp., in 1997 on the phenomenon that we termed the “light-induced amphiphilic surface” [496]. When a titania film was illuminated with UV light, the contact angle for water decreased to near 0∘, and the same occurred also with organic liquids. We expected that there would be several applications for this new effect, including self-cleaning surfaces and anti-fogging mirrors. With friction force microscopy, it was observed that there
Self-cleaning surfaces
The TiO2 surface can decompose organic contamination with the aid of ultraviolet light. This observation suggests the application of TiO2 photocatalysis to a novel “self-cleaning” technique, i.e., a surface coated with TiO2 can maintain itself clean under ultraviolet illumination (Fig. 6.1) [5], [9], [10], [392]. This technique is obviously of great value, since it can utilize freely available solar light or waste ultraviolet emission from fluorescent lamps, save maintenance costs, and reduce
Summary
We have tried to provide an overview of the field of photocatalysis from its very beginning in 1921 [17] through its developments in fundamental studies, both experimental and theoretical, which have been strongly tied to applications. Of course, it is impossible to do justice to this vast field in a review of even this length, and new work is emerging every day. This is the nature of the field. We have also tried to focus on some of the aspects of the field that would not be treated in a
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