2. Interfacial back electron transfer and transient molecular probe for TiO2 surface property.
We have developed a transient molecular probe for characterization of the surface properties of TiO2 nanoparticles in colloidal solution. The probe molecule is all-trans-retinoic acid (ATRA) adsorbed at the TiO2 nanoparticle surface. After photoexcitation, the photoinduced interfacial charge recombination would generate ATRA triplet state (ATRAT) with a substantial quantum yield only at the interface. We have confirmed the validity of this mechanism by both femto-second and nanosecond time-resolved spectroscopy. It is found that the triplet-triplet absorption spectrum of ATRA adsorbed molecule is sensitive to its binding form with the surface Ti atom through the carboxylic group, the polarity of the medium, as well as the hydrophobicity/hydrophylicity of the TiO2 nanoparticles. The ATRAT monolayer adsorbed at the surface can be used as a transient molecular probe for the surface binding forms, coordination state of the surface Ti atoms and the light-induced wettability change of the TiO2 nanoparticle.
Schematic diagram showing how molecular probe senses the light induced wettability conversion at an individual TiO2 nanoparticle.
Charge recombination rate as an indicator for the wettability change on an individual TiO2 nanoparticle in solution.