Photo-physics of surface-treated titanium dioxides

The photoelectric properties of commercially used TiO2 pigments were studied by transient photo-electromotive-force (PEMF) measurements. The influence of the crystal structure (anatase or rutile) and the pigment coating (and other) on the PEMF parameters of TiO2 was investigated. Pure anatase type pigments show in the microsecond time range a creation of a positive PEMF signal. In the millisecond time range the PEMF decay starts with a positive sign. This means that anatase pigments behave as typical n-type photoconductors. However, pure rutile type pigments show in the microsecond time range a negative PEMF signal or a signal starting with negative sign. This means that rutile pigments act as typical p-type photoconductors during the first few microseconds. Coating of rutile or anatase pigments decrease the PEMF maximum voltages U-max in comparison to untreated pigments. However, in some cases coating creates an increase of the PEMF decay rate which is explained by an increase of the charge carrier recombination. Rutile pigments epitaxially coated with Al2O3 and SiO2 may show very complex PEMF signals with two negative maximum voltages. Such types of signals may indicate heterojunctions between the TiO2 core and the coating shell. In this way the PEMF parameters are an excellent tool to characterise TiO2 pigments. (C) 2002 Published by Elsevier Science B.V.