Dynamics of interfacial electron transfer from photoexcited quinizarin (Qz) into the conduction band of TiO₂ and surface states of ZrO₂ nanoparticles

Abstract

Electron injection and back-electron-transfer (BET) dynamics of quinizarin (Qz) adsorbed on TiO₂ and ZrO₂ 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 TiO₂ or ZrO₂ nanoparticles surface. Following photoexcitation of Qz/TiO₂ and Qz/ZrO₂ 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 TiO₂ and ZrO₂ 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 S₁ state of Qz lies below the conduction band edge of ZrO₂ so electron injection from S₁ state into the nanoparticle is not thermodynamically possible. However, the detection of Qz^•+ as well as injected electrons in the case of Qz/ZrO₂ system confirms that electron injection also takes place in ZrO₂. We have attributed this to the injection into surface states of ZrO₂ 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/TiO₂ and Qz/ZrO₂ 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.