EPR Signals of Oxide Semiconductors

Starting from old fundamental overviews [26-28], a lot of papers have been published concerning the EPR spectra observed in the oxide semiconductor lattice, first of all for Ti02 [29-38], Zr02 [39-43] and ln203 [44]. 

After photolysis at room temperature of the degassed aqueous solutions of colloidal Ti02 (anatase, 2R = 10-15 run) in the presence of poly(vinyl alcohol) (PVA), EPR spectra recorded at 77 K showed existence of several types of paramagnetic centers in the system 

Indeed, irradiation of Ti02 particles with light of energy higher than the band gap (A. 390 nm) results in generation of electron-hole pairs  

These electrons and holes can be trapped both at the interior sites and on the surface of colloidal particles. Then, electrons can be located in the conduction band (e cb) or on Ti4+ ions, at the surface (Ti3+)surf and in the bulk lattice (Ti3+)Uttice. It follows from Table 8.1 that surface and lattice Ti3+ centers can be distinguished by difference it their EPR parameters. 

Formation of Ti3+ centers in Ti02 from trapped electrons is usually connected with generation of various radicals from trapped holes, but such reactions and species will be discussed in section 8.3. 

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