Elsevier

Surface Science

Volume 603, Issues 10–12, 1 June 2009, Pages 1605-1612
Surface Science

Photochemistry on TiO2: Mechanisms behind the surface chemistry

https://doi.org/10.1016/j.susc.2008.11.052Get rights and content

Abstract

Photochemistry from TiO2 surfaces is described for two cases: The UV-induced photodesorption of O2 from TiO2(1 1 0) – 1 × 1; and the hydrophilic effect caused by UV irradiation on TiO2. In both cases fundamental information about how these processes occur has been found. In the case of the O2 photodesorption kinetics, it has been found that the rate of the process is proportional to the square root of the UV flux, showing that second-order electron–hole pair recombination is dominant in governing the photodesorption rate. In addition these measurements provide an estimate of the concentration of hole traps in the TiO2 crystal. In other measurements of the UV-induced hydrophilicity, starting with the atomically-clean TiO2 surface, it has been shown that the effect occurs suddenly at a critical point during irradiation as a result of photooxidation of a monolayer of hydrocarbon (n-hexane) at equilibrium with ppm concentration of n-hexane in O2 at 1 atmosphere pressure.

Introduction

The winning of the Nobel Prize in Chemistry in 2007 by Gerhard Ertl represents a singular recognition of the importance of the field of surface chemistry as practiced in the latter half of the twentieth century. The Prize recognized an exciting and significant area of scientific research, as well as the work of a highly admired scientist who has led the way in the field. The majority of the work done in this period by Ertl and others [1], has dealt with the type of surface chemistry which is thermally activated, and indeed, the thermal activation of surface processes currently drives the majority of technological applications of surface chemistry.

There is another mode of surface species’ activation which is driven by electronic excitation. Here, either the electronic activation of surface species, or the electronic activation of the substrate, on which the surface species reside, is the first step in causing new surface chemistry to occur [2]. The exploration of the electronic activation of surface processes now occurs at a very active research frontier and will in the future grow significantly as interest in harnessing sunlight to produce electricity and to cause new surface reactions increases. Indeed the ability to initiate surface chemistry by electronic excitation opens new vistas for research and applications which have in the past mainly been recognized by the DIET (Desorption Induced by Electronic Transitions) Conferences [3] as well as by several Surface Science Reports [4], and Chemical Reviews [2], [5], [6].

This short review summarizes work in the photoactivation of surface chemistry on semiconductor TiO2 surfaces. It is partly based on earlier reviews of this topic [2], [5], [6] by ourselves, as well as on recent work which has been done. In 1972, Fujishima and Honda discovered the photosplitting of water on TiO2 electrodes [7], offering the potential for H2(g) and O2(g) production from sunlight. This was followed by the development of a sunlight-driven photovoltaic cell which employs dye-modified TiO2 electrodes, the Graetzel cell [8], [9], [10]. These two important developments were accompanied by much research and engineering in a third area, leading to the use of TiO2 as a photochemical substrate for photooxidation reactions, a major application area. A wide range of new methods for “slow-but-sure” solar-driven environmental remediation of contamination by organic matter in the atmosphere and in water medium has resulted from this effort. Prime examples of this include self-cleaning windows coated with TiO2 films [11] and TiO2-based paints and films [11] which clean themselves in sunlight leaving white surfaces after extensive exposure to dirty atmospheres, followed by washing by rain. In addition, photochemically induced hydrophilicity [12] and photoinduced antimicrobial [11], [13] properties of TiO2 films have recently been discovered and these ideas are now employed for new photochemically activated cleaning technologies driven by sunlight, or even by the small ultraviolet component of fluorescent lighting inside buildings.

Section snippets

Photoexcitation on semiconductor surfaces-basic principles

Fig. 1 shows a schematic of the photoexcitation of a semiconductor solid particle by exposure to radiation with energy above the bandgap energy [5]. An exciton, produced by the absorption of a photon is shown by the star symbol. This is followed by charge separation – the production of an electron–hole pair. Charge transport to the particle surface by processes C and D lead respectively to desirable reduction and oxidation reactions at the surface. Processes A and B represent electron–hole pair

Using surface chemical photokinetics to observe charge carrier recombination and the presence of hole traps

The photodesorption of O2, adsorbed on TiO2 (1 1 0), provides a relatively simple surface process with easily-measured kinetics which can be used to directly observe hole trapping by its kinetic effect. Fig. 4 shows the apparatus used in these measurements. A filtered Hg UV source, of measured intensity, emitting radiation selected within 10 nm wide spectral regions, is focused on the crystal containing adsorbed O2, and a shutter controls the exposure to light.

The rate of photodesorption of

Application of surface science methods for the understanding of the mechanism of photoinduced hydrophilicity on TiO2 surfaces

The photoinduced hydrophilic effect was first reported by Wang et al. [12] on TiO2 films and the effect is shown in Fig. 8. UV irradiation in air causes water droplets to wet the TiO2 film surface, resulting in a lowering of the contact angle over time.

The anatase TiO2 film was deposited on a glass surface followed by annealing to 773 K [26]. The experiment was carried out in the ambient atmosphere, and this study and many others have shown that under these conditions, the contact angle

Summary and look to the future

This review has concerned two surface science studies from our own laboratory designed to reveal the underlying mechanisms of photochemistry on TiO2 surfaces. Both studies employ careful control of surface conditions and care in experimental design. In the case of O2 photodesorption from TiO2(1 1 0), the discovery of the dependence of the rate on the ½-power of the incident UV flux clearly shows that charge carrier recombination governs the efficiency and that methods to reduce recombination

Acknowledgements

I acknowledge, with thanks, the support of the Army Research Office for a DARPA-MURI grant as well as direct support for this work. I also thank Dr. Sunhee Kim for help with the figures and references in the article.

References (29)

  • R.D. Ramsier et al.

    Surf. Sci. Rep.

    (1991)
    X.L. Zhou et al.

    Surf. Sci. Rep.

    (1991)
  • A.L. Linsebigler et al.

    Chem. Rev.

    (1995)
  • A. Hagfeldt et al.

    Chem. Rev.

    (1995)
  • T.L. Thompson et al.

    Chem. Phys. Lett.

    (2004)
  • C.J.G. Cornu et al.

    J. Phys. Chem. B

    (2001)
  • G. Ertl

    Angew. Chem. Int. Ed.

    (2008)
  • J.T. Yates et al.

    Chem. Rev.

    (2006)
  • N.A. Tolk, M.M. Traum, J.C. Tully, T.E. Madey, in: Proceedings of Conference Series on Desorption Induced by Electronic...
  • T.L. Thompson et al.

    Chem. Rev.

    (2006)
  • A. Fujishima et al.

    Nature

    (1972)
  • M. Grätzel

    Nature

    (2001)
  • M. Grätzel

    J. Photochem. Photobiol. C

    (2003)
  • A. Fujishima et al.

    TiO2 Photocatalysis: Fundamentals and Applications

    (1999)
  • R. Wang et al.

    Nature

    (1997)
    R. Wang et al.

    Adv. Mater.

    (1998)
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