Photoinduced Charge Recombination at Dye-Sensitized Individual TiO2 Nanoparticles and Its Application in Probe for the Local Polarity Change around the Nanoparticle in Solution

Abstract

Application of the Marcus relation to the interfacial charge recombination at the TiO2 film is often limited by interparticle electron transport known as carrier diffusion in the dye-sensitized sintered TiO2 nanocrystalline films, where the nanoparticles are interconnected. Dye-sensitized disconnected individual TiO2 nanoparticles may provide a suitable case for inspecting the Marcus relation in interfacial charge recombination. In this paper we investigated the photoinduced interfacial charge recombination process occurring at all-trans-retinoic acid (ATRA)-sensitized individual TiO2 nanoparticles in hexanol and methanol solutions (containing a small amount of acidified water for suspension of TiO2 nanoparticles) by 355 nm pulsed-laser photolysis. It was found that in the nanosecond to microsecond temporal domain, charge recombination kinetics is monoexponential, and does not depend on the laser excitation power in a range from 0.45 to 6.0 mJ/(cm2 pulse). Temperature dependence study of the observed charge recombination rate shows that it follows the Marcus relation. The experimentally determined corresponding reorganization energy for the photoinduced charge recombination of ATRA/TiO2 in hexanol/H2O (v:v ≈ 40:1) and methanol/H2O (v:v ≈ 9:1) is 0.44 ± 0.10 and 0.48 ± 0.20 eV, respectively. By tuning the relative ratio of water in methanol hence the reorganization energy of the mixed solvent, the observed reorganization energy dependent charge recombination rate also agrees with that predicted from the Marcus relation. On the basis of experimental results, it can be concluded that the injected electron is immobilized on the individual TiO2 nanoparticles and the charge recombination seems more like a germinate pair recombination. This is different from what has been observed for the dye-sensitized TiO2 crystalline film. Furthermore, oscillations in the measured interfacial charge recombination rate have been observed when ATRA/TiO2 hexanolic solution was subjected to prolonged laser irradiation. The oscillation was interpreted as the change of the reorganization energy in response to the photoinduced local polarity change around the TiO2 nanoparticle, which arises from the photoinduced wettability change occurring at the TiO2 nanoparticle surface. This provides a unique example of a molecular probe for detecting the photoinduced wettability change of the TiO2 nanoparticles in solution.