In this paper, we have investigated the initial interfacial electron transfer dynamics in butylamine-capped CdSe quantum dot(QD)-sensitized nanocrystalline TiO2
films by ultrafast spectroscopy. Different with previous reports, the experiment results indicate that after surface modification of CdSe QDs, a two-phase electron injection mechanism(hot and cold electron injection) is validated, namely, electrons transfer from high vibration energy levels in conduction band and conduction band bottom of CdSe quantum dots into conduction band of TiO2
, respectively. This mechanism depicts a picture on the detailed charge transfer processes at the nano interfaces. We further find that the electron coupling strength (3.6±0.1 meV) of hot electron injection is two orders of magnitude larger than that of relaxed ground-state electron injection, which gives a value of ～50 μeV accompanied with reorganization energy 0.083 eV on the basis of Marcus theory.