Photogenerated charges in TiO2 nanostructures

Alexander Riss 1Thomas Berger 1Johannes Bernardi 2Oliver Diwald 1Erich Knoezinger 1

1. Institute of Materials Chemistry, Vienna University of Technology (TU), Veterinärplatz 1, Wien 1210, Austria
2. University Service Centre for Transmission Electron Microscopy (USTEM), Wiedner Hauptstraße 8-10/138, Wien 1040, Austria

Abstract

Metal oxide nanocrystals, and in particular TiO2, are employed for many technologically relevant surface applications. Because chemical and photochemical reactivity critically depends on the size and the shape of the crystals, control over these properties represents a key issue in materials' synthesis. Morphologically well-defined nanostructures will be introduced as model systems to establish a correlation between chemical and photochemical reactivity, on one hand, and the abundance of specific surface sites, on the other.

TiO2 absorbs photons with energies greater than 3.2 eV which leads to the generation of excited electrons and holes. These can either (a) recombine under photoluminescence emission or heat generation, (b) become persistently trapped or (c) undergo redox reactions with molecules at the particle surface. The particle surface largely determines the efficiency of these processes. For this reason, photoexcitation processes have been investigated on different TiO2 based model systems: TiO2 anatase nanoparticles produced via chemical vapour deposition[1], titanate nanotubes[2] and TiO2 nanorods. Persistent charge trapping and oxygen radical formation were studied by electron paramagnetic resonance. The presence of O2, an electron scavenger, results in charge transfer to the oxygen molecule to generate adsorbed O2- radicals.

Quantification in terms of trapped charges per particle was carried out and the influence of the surface structure and properties on the obtained figures will be discussed. Tracking UV-induced reactions on nanomaterials of different size and shape in comparison with studies on the standard TiO2 P25 represents a valuable approach towards the identification of active sites on nanocrystalline samples.

[1] T. Berger et al.; ChemPhysChem 6 (2005) 2104
[2] T. Kasuga et al.; Langmuir 14 (1998) 3160

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Presentation: Oral at E-MRS Fall Meeting 2006, Symposium B, by Alexander Riss
See On-line Journal of E-MRS Fall Meeting 2006

Submitted: 2006-05-15 12:34
Revised:   2009-06-07 00:44
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