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 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 S, state of Qz lies below the conduction band edge of ZrO2 so electron injection from S, 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 similar to600 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.