Elsevier

Surface Science Reports

Volume 63, Issue 12, 15 December 2008, Pages 515-582
Surface Science Reports

TiO2 photocatalysis and related surface phenomena

https://doi.org/10.1016/j.surfrep.2008.10.001Get rights and content

Abstract

The field of photocatalysis can be traced back more than 80 years to early observations of the chalking of titania-based paints and to studies of the darkening of metal oxides in contact with organic compounds in sunlight. During the past 20 years, it has become an extremely well researched field due to practical interest in air and water remediation, self-cleaning surfaces, and self-sterilizing surfaces. During the same period, there has also been a strong effort to use photocatalysis for light-assisted production of hydrogen. The fundamental aspects of photocatalysis on the most studied photocatalyst, titania, are still being actively researched and have recently become quite well understood. The mechanisms by which certain types of organic compounds are decomposed completely to carbon dioxide and water, for example, have been delineated. However, certain aspects, such as the photo-induced wetting phenomenon, remain controversial, with some groups maintaining that the effect is a simple one in which organic contaminants are decomposed, while other groups maintain that there are additional effects in which the intrinsic surface properties are modified by light. During the past several years, powerful tools such as surface spectroscopic techniques and scanning probe techniques performed on single crystals in ultra-high vacuum, and ultrafast pulsed laser spectroscopic techniques have been brought to bear on these problems, and new insights have become possible. Quantum chemical calculations have also provided new insights. New materials have recently been developed based on titania, and the sensitivity to visible light has improved. The new information available is staggering, but we hope to offer an overview of some of the recent highlights, as well as to review some of the origins and indicate some possible new directions.

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: hνeCB+hVB+2H2O+4hVB+O2+4H+O2+4H++4eCB2H2O. 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

References (637)

  • A. Mills et al.

    J. Photochem. Photobiol. A: Chem.

    (1997)
  • A. Fujishima et al.

    J. Photochem. Photobiol. C

    (2000)
  • D.F. Ollis

    C. R. Acad. Sci. Paris, Serie IIC, Chim.

    (2000)
  • D.A. Tryk et al.

    Electrochim. Acta

    (2000)
  • A. Fujishima et al.

    Internat. J. Hydrogen Energy

    (2007)
  • M.C. Markham et al.

    J. Catal.

    (1965)
  • E. Pelizzetti et al.

    Chemosphere

    (1988)
  • R.I. Bickley et al.

    J. Solid State Chem.

    (1991)
  • J. Peral et al.

    J. Catal.

    (1992)
  • L.A. Phillips et al.

    J. Molec. Catal.

    (1992)
  • G.B. Raupp et al.

    Appl. Surf. Sci.

    (1993)
  • R. Hengerer et al.

    Surf. Sci.

    (2000)
  • N. Ruzycki et al.

    Surf. Sci.

    (2003)
  • L.A. Bursill et al.

    Prog. Solid State Chem.

    (1972)
  • Y. Le Page et al.

    J. Solid State Chem.

    (1983)
  • Y. Le Page et al.

    J. Solid State Chem.

    (1982)
  • M. Hirasawa et al.

    Appl. Surf. Sci.

    (2002)
  • P.V. Kamat

    Chem. Rev.

    (1993)
  • A. Heller

    Acc. Chem. Res.

    (1995)
  • M.R. Hoffmann et al.

    Chem. Rev.

    (1995)
  • J. Peral et al.

    J. Chem. Tech. Biotech.

    (1997)
  • A. Fujishima et al.

    TiO2 Photocatalysis: Fundamentals and Applications

    (1999)
  • A. Fujishima et al.
  • K. Hashimoto et al.

    Japan. J. Appl. Phys.

    (2005)
  • A. Fujishima et al.

    C. R. Chimie

    (2006)
  • C. Renz

    Helv. Chim. Acta

    (1921)
  • A.E. Jacobsen

    Ind. Eng. Chem.

    (1949)
  • A. Fujishima et al.

    Nature

    (1972)
  • T. Watanabe, A. Fujishima, K. Honda, in: T. Ohta (Ed.), Solar-Hydrogen Energy Systems, Oxford, 1979, pp....
  • A. Fujishima, Innovative Hydrogen Production from Water (UNESCO),...
  • M.A. Fox et al.

    Chem. Rev.

    (1993)
  • E. Baur et al.

    Helv. Chim. Acta

    (1924)
  • E. Baur et al.

    Helv. Chim. Acta

    (1927)
  • C. Renz

    Helv. Chim. Acta

    (1932)
  • C.F. Goodeve et al.

    Trans. Faraday Soc.

    (1938)
  • C.F. Goodeve et al.

    Trans. Faraday Soc.

    (1938)
  • M.C. Markham et al.

    J. Phys. Chem.

    (1953)
  • T.R. Rubin et al.

    J. Am. Chem. Soc.

    (1953)
  • M.C. Markham et al.

    J. Am. Chem. Soc.

    (1954)
  • M.C. Markham et al.

    J. Am. Chem. Soc.

    (1958)
  • R.E. Stephens et al.

    J. Phys. Chem.

    (1955)
  • J.G. Calvert et al.

    J. Am. Chem. Soc.

    (1954)
  • C.B. Vail et al.

    J. Am. Chem. Soc.

    (1954)
  • M.G. Markham et al.

    J. Phys. Chem.

    (1957)
  • J.C. Kuriacose et al.

    J. Phys. Chem.

    (1961)
  • M.C. Markham et al.

    J. Phys. Chem.

    (1962)
  • A. Nergararian et al.

    J. Phys. Chem.

    (1963)
  • Cited by (0)

    View full text