Size-controlled interparticle charge transfer between TiO₂ and quantized capacitors

The charge-transfer quenching of photoexcited TiO₂ nanoparticles by hexanethiolate monolayer protected gold clusters (MPCs) with core diameters of 1.1-4.9 nm is reported. The quenching efficiency of Au MPCs was compared using Stern-Volmer plots. The quenching constants (KQ) taken from slopes of the plots reveal that KQ increases from 5.6 × 10⁵ to 4.09 × 10⁷ by more than 70-fold as the core diameter increases from 1.1 to 4.9 nm. The origin of the size dependence is considered. K_Q was found to linearly correlate with the MPC capacitance. This suggests that the quenching process is dominantly controlled by the capacitance of the electron acceptor (Au MPC). K_Q for 1.1 nm diameter Au MPC was found to be significantly smaller than that predicted by the K_Q-capacitance correlation. This may reflect the size-dependent opening of an energy gap at the Fermi level for molecule-like MPCs.