Plasmon induced electron transfer at gold-TiO2 interface under femtosecond near-IR two-photon excitation

Charge-transfer process from photoexcited gold nanoparticles to TiO2 is significant due to the very strong surface plasmon absorption band of gold nanoparticles and the very high photocatalytic activity of TiO2. Bioapplication of gold-TiO2 systems utilizing near-infrared (NIR) light based on the two photon excitation of the surface plasmon band will be interesting. In this study, TiO2 particles with a 30 nm average diameter loaded with 10 nm gold spherical nanoparticles were used to explore the electron injection kinetics by observing electrons in TiO2 using femtosecond IR-probe transient absorption (TA) spectroscopy. Optical excitation of the gold-TiO2 system using ∼ 150 fs light at 1200 nm led to very fast (< 240 fs) electron generation in the conduction band of TiO2. Excitation intensity dependence of the initial TA signals gave a second order nonlinear relation, confirming the occurrence of two photon induced electron injection. Two photon absorption cross section of gold nanoparticle at 1200 nm was estimated to be very high, around 108 GM (1 GM = 10-50 cm4 s photon-1 molecule-1).