Ultrafast electron transfer dynamics in sensitised TiO₂ nanoparticles

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)64- and TiO2 colloidal nanoparticles. Optical excitation at 400 nm directly promotes an electron from Fe(Il)(CN)64- 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)63- 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