Surface Modification of TiO2 Nanoparticles with Carotenoids. EPR Study

Abstract

EPR measurements demonstrate efficient charge separation on carotenoid-modified titanium dioxide nanoparticles (7 nm). Strong complexation of carotenoids containing terminal carboxy groups (−CO2H) with the TiO2 surface leads to electron transfer from the adsorbed carotenoid molecule to the surface trapping site. For these systems, EPR signals of the carotenoid radical cations Car•+ and the electrons trapped on the TiO2 are observed before irradiation (77 K). Their UV−visible spectra show an absorption band with a maximum near 650 nm that is characteristic of the trapped electrons. Surface modification of the TiO2 by other carotenoids results in the formation of a complex with an optical absorption band near 545 nm. These systems form charge-separated pairs [Car•+···TiO2(e-tr)surf. TiO2(e-tr)latt.] only upon 365−600 nm illumination at 77 K. Complexation of the TiO2 colloids with carotenoids enhances spatial charge separation, shifts the absorption threshold into the visible region, and thus greatly improves the reducing ability of the semiconductor. Photoreduction of acceptor molecules such as 2,5-dichloro-1,4-benzoquinone, nitrobenzene, and oxygen is demonstrated.