Electron injection into the surface states of ZrO₂ nanoparticles from photoexcited quinizarin and its derivatives: effect of surface modification

Abstract

The effect of surface modification on interfacial electron transfer (IET) dynamics into the surface states of ZrO₂ nanoparticles sensitized by quinizarin (Qz) and its derivatives has been carried out using time-resolved emission spectroscopy. The surface of ZrO₂ nanoparticles has been modified by sodium dodecyl benzyl sulfonate . We have observed that Qz's can form a strong charge-transfer (CT) complex with both unmodified and surface-modified (SM) ZrO₂ nanoparticles. We have confirmed electron injection into the surface states of ZrO₂ nanoparticles from the photoexcited Qz molecule in our earlier work (J. Phys. Chem. B 2004, 108, 4775; Langmuir 2004, 20, 7342). In the present investigation, we have observed electron injection from photoexcited Qz derivatives into the surface states of both unmodified and SM ZrO₂ nanoparticles and also detected CT emission. Monitoring CT emission, we have determined back electron transfer (BET) dynamics of the dye−nanoparticle systems. We have found that the BET rate for the QZs/ZrO₂ systems decreases as the relative driving force increases following Marcus inverted region kinetic behavior for an IET process. BET dynamics was found to be faster on SM ZrO₂ nanoparticles as compared to that of the unmodified (bare) one. Our time-resolved emission data indicates that upon surface modification the majority of the deeper trap states of ZrO2 nanoparticles can be removed with the formation of some new shallower trap states in the band gap region.