Effect of size and shape of nanocrystalline TiO2 on photogenerated charges. An EPR study

摘要

The continuous wave and pulsed electron paramagnetic resonance study of the photogenerated charges in differently shaped anatase nano-objects dispersed in aqueous solution is presented. Excitation of TiO2 nano-objects (spherical, faceted, rods, and bricks) at temperatures <80 K results in the formation of paramagnetic species that correspond to electrons trapped at lattice titanium within the bulk, (Ti3+)(latt), and holes trapped at oxygen atoms at the surface of nano-objects, as confirmed by the electron spin echo envelope modulation technique. The distortion of the Ti-O bond, which is a result of reconstruction of the surface of nano-objects, causes variations in g-tensor values and spin relaxation processes of lattice-trapped electrons. On the other hand, size and shape do not affect spin-spin relaxation times of surface-trapped holes as it was found that relaxation is determined by dipolar coupling with H-1 nuclei from surrounding water.