Interfacial charge transfer dynamics in small molecule-modified TiO 2 nanoparticles

Abstract

Direct molecule-semiconductor interfacial charge transfer interactions have received considerable research attention for their applications in various fields. In this study, the dynamics of molecule-TiO 2 interfacial charge transfer complexes is monitored with femtosecond fluorescence upconversion and transient absorption. Small molecules (catechol, dopamine, benzhydroxamic acid, acetyl acetonate and salicylate)-modified TiO 2 nanoparticles are prepared and the complexation is followed with optical absorption measurements. Although little visible luminescence is observed from these molecule- TiO 2 nanoparticles, ultrafast emission in broad range of wavelengths is detected with fluorescence upconversion which is ascribed to the interfacial charge transfer emission. The charge transfer emission arose out of the radiative recombination of the electrons in the conduction band of TiO 2 with holes in the molecule. Femtosecond fluorescence anisotropy measurements have shown that the interfacial charge-transfer excitation is mostly a localized one for catechol, dopamine and benzhydroxamate modified TiO 2 nanoparticles. However, the possibility of delocalized charge-transfer excitations is observed for salicylate and acetyl acetonate-TiO 2 nanoparticles. The decay of the charge transfer emission is ascribed to the relaxation of the localized states to delocalized states in the TiO 2 conduction band. Transient absorption measurements have shown long-lived charge separation in the case of surface-modified TiO 2 nanoparticles. Further measurements on the influence of charge-transfer excitations on the interfacial electron transfer in surface-modified TiO 2 nanoparticles are being carried out.