Ultrafast electron transfer dynamics in sensitised TiO₂ nanoparticles

Abstract

We have studied electron transfer dynamics between TiO₂ nanoparticles and molecular adsorbates using femtosecond mid-infrared spectroscopy. We have demonstrated that dynamics of the injected electrons in TiO₂ could be directly monitored through their mid-IR absorption and those of the adsorbates could be measured by their vibrational spectral change. Ru(dcbpy)₂(NCS)₂ (dcbpy=2,2'-bypyridine-4,4'-dicarboxylate) sensitized TiO₂ nanocrystalline films were studied as a model system for ultrafast electron injection from the excited state of the sensitizer to nanoparticles. Optical excitation of the MLCT band at 400 nm promotes an electron from a filled Ru d orbital to the π* orbital of the dcbpy ligand. The subsequent electron injection to TiO₂ was found to occur with a time constant of ca 50 fs by directly measuring the transient IR absorption signal of the injected electrons in TiO₂. These injection dynamics are as fast as, if not fast than, the electronic or vibrational relaxation within the excited states. Back electron transfer from nanoparticles to the adsorbates was studied in interfacial charge transfer complexes formed by Fe(II)(CN)₆^4- and TiO₂ colloidal nanoparticles. Optical excitation at 400 nm directly promotes an electron from Fe(Il)(CN)₆^4- to TiO₂ as indicated by the measured instrument-response—function limited appearance time of transient IR signal. The back electron transfer time from TiO₂ to Fe(III)(CN)₆^3- was measured by the bleach recovery of CN stretching mode. A highly non-single-exponential recombination process was observed and was tentatively attributed to different recombination rates for injected electrons trapped at different sites in TiO₂. The measured decay of the IR absorption of electrons can be attributed to back electron transfer and electron trapping. Since the back electron transfer kinetics can be measured independently, the trapping dynamics can be determined. Electron trapping dynamics in a bulk crystal and nanocrystalline thin films were found to be similar in the first nanosecond, showing a »1 ns decay time. Trapping dynamics are much faster in the colloidal nanoparticles, indicating a much higher trap state density.