Light-induced charge separation and redox chemistry at the surface of TiO2/host-guest hybrid nanoparticles

The photoinduced charge transfer between guest molecules and hybrid TiO2/cyclodextrin nanoparticles was studied using low-temperature EPR and cyclic voltammetry. The photoexcitation of TiO2 at 4.6 K yields to the localization of valence band holes at carboxyl groups of surface-conjugated cyclodextrin and conduction band electrons at lattice Ti atoms. The presence of I-adamantanol in the cyclodextrin cavity does not affect charge separation and trapping because of its unfavorable oxidation potential. However, when ferrocenemethanol (E-ox = 0.52 V vs NHE) was used as the guest molecule the formation of ferrocenium cation was observed, revealing electron transfer from guest molecules to TiO2 nanoparticles. This results in dissociation of the host-guest assembly because of repulsion of the charged ion from the hydrophobic cavity of cyclodextrin into the bulk of the aqueous solution and consequently leads to efficient charge separation and redox chemistry.