Electron Transfer Dynamics in Quantum Dot/Titanium Dioxide Composites Formed by in Situ Chemical Bath Deposition

We present experimental transient absorption (TA) results on the dynamics of electron relaxation and electron transfer in cadmium sulfide quantum dots (QDs) grown by chemical bath deposition techniques on nanocrystalline oxide substrates. The quantum dots are prepared in situ in nanocrystalline titanium dioxide (TiO₂) and zirconium oxide (ZrO₂) films. The conduction band offset between the CdS QDs and TiO₂ allows for efficient electron injection from the photoexcited QDs into the conduction band of TiO₂. An unprecedented peak is seen in the transient absorption spectrum, which may be used to track the spectral response of the QD/TiO ₂: composites. Dynamic measurements in the visible and mid-IR are used to evaluate the time scale of the electron injection process. The TA dynamics for these systems are found to be multiexponential. A comparison of the TA dynamics for CdS/TiO₂ and CdS/ZrO₂ composites in the visible and mid-IR region indicates that electron transfer occurs on the time scale of ∼10-50 ps.