SIZE EFFECTS ON THE PHOTOPHYSICAL PROPERTIES OF COLLOIDAL ANATASE TIO2 PARTICLES - SIZE QUANTIZATION OR DIRECT TRANSITIONS IN THIS INDIRECT SEMICONDUCTOR

Colloidal TiO2 sols with average particle size 2R(p) = 2.1 nm (specimen A), 13.3 nm (specimen B), and 26.7 nm (specimen C) were prepared and characterized by absorption and photoluminescence spectroscopies, Electron and X-ray diffraction examination showed the specimens contain anatase as the only crystalline phase (similar to 20-30%) and a significant amorphous component (similar to 70-80%). The three specimens gave identical steady-state absorption features at high loadings. Luminescence around 340-350 nm (at the absorption edge at the lower loadings) was more intense in the 13.3 nm TiO2 particles; several weaker bands were seen at longer wavelengths. Congruence of the absorption onsets for the three specimens at 15 g L(-1) or at the lower loadings (0.015 and 0.30 g L(-1)) argues against size quantization effects for particles with R(p) greater than or equal to 1.0 nm, Spectra at the lower loadings exhibited absorption thresholds at considerably higher energy (relative to bandgap); they are attributed to direct transitions in an otherwise indirect bandgap TiO2 semiconductor, Considerations of the effective mass model to determine particle size from spectral blue shifts of absorption edges and to ascertain size quantization in semiconductor colloidal particles suggest that it is rather premature to reach any conclusion on size quantization manifestations in the absence of a precise knowledge of the effective masses of charge carriers in small nanosized particles, and more so for anatase colloids for which inferences of effective masses of electrons and holes have been taken from rutile bulk crystals.