Dynamics of Interfacial Electron Transfer from Photoexcited Quinizarin (Qz) into the Conduction Band of TiO2 and Surface States of ZrO2 Nanoparticles

Electron injection and back-electron-transfer (BET) dynamics of quinizarin (Qz) adsorbed on TiO 2 and ZrO 2 nanoparticles has been studied by femtosecond transient absorption spectroscopy in the visible and near-IR region. A good fraction of Qz forms a charge-transfer (CT) complex while being adsorbed on the TiO 2 or ZrO 2 nanoparticles surface. Following photoexcitation of Qz/TiO 2 and Qz/ZrO 2 systems, electron injection into the nanoparticles has been confirmed for both the systems by direct detection of electron in the nanoparticle and cation radical of Qz (Qz .+ ). The dynamics of BET from TiO 2 and ZrO 2 to the parent cation has been measured by monitoring the decay kinetics of Qz •+ and electron in the nanoparticles and it is found to be multiexponential. As S 1 state of Qz lies below the conduction band edge of ZrO 2 so electron injection from S 1 state into the nanoparticle is not thermodynamically possible. However, the detection of Qz •+ as well as injected electrons in the case of Qz/ZrO 2 system confirms that electron injection also takes place in ZrO 2 . We have attributed this to the injection into surface states of ZrO 2 nanoparticles. It has been observed that electron injection takes place in <50 fs and the majority of the injected electrons come back to the parent cation with a time constant of <1 ps for both the systems. We have observed multiphasic recombination dynamics with time constants ranging from ∼600 fs to the pico-, nano-, and microsecond time scale for both Qz/TiO 2 and Qz/ZrO 2 systems. Our investigation has revealed that electron injection into the surface states of nanoparticles is possible or facilitated when the adsorbed sensitizer molecule forms a strong CT complex with the semiconductor nanoparticles.