Plasmon-Induced Charge Separation and Recombination Dynamics in Gold-TiO2 Nanoparticle Systems: Dependence on TiO2 Particle Size

To study plasmon-induced charge-transfer mechanisms between excited gold nanoparticles and TiO2, ultrafast visible-pump/infrared-probe femtosecond transient absorption spectroscopy was used and charge separation and recombination dynamics in gold-TiO2 nanoparticle systems were explored. TiO2 nanoparticles of different diameters (9, 20, 30, and 50 nm) were chosen as electron acceptors for gold nanoparticles with a 10 nm diameter. Electron transfer from gold nanoparticles to the conduction band of TiO2 was observed by the transient absorption of electrons in the conduction band of TiO2 at 3440 nm after optical excitation of the surface plasmon band of gold nanoparticles at 550 nm. It was found that electron injection was completed within 50 fs and the electron injection yield reached 20-50%. The charge recombination decay within 1.5 ns was nonexponential and was strongly dependent on the particle diameter of TiO2. Larger TiO2 particles resulted in longer charge recombination times because of the longer diffusion length of electrons in those TiO2 particles.