Ultrafast plasmon induced electron injection mechanism in gold–TiO2 nanoparticle system

To study plasmon-induced charge transfer mechanism between an excited gold nanoparticle (NP) and a TiO2 NP, which can be applied to solar cell and photocatalyst technologies, ultrafast femtosecond visible-pump/infrared-probe transient absorption spectroscopy was utilized to explore charge separation and recombination dynamics in gold–TiO2 NP systems. In this review, our recent works are summarized. TiO2 NPs of different diameters were chosen as electron acceptors for a gold NP donor with a 10-nm diameter. Electron transfer from gold NPs 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 NPs. By using a reference of Ru-complex dye sensitized TiO2 film, the occurrence of ultrafast electron injection from gold NPs to TiO2 NP film was clearly proved. It was found that electron injection was completed within 50 fs and the electron injection yield reached 20–50% under 550 nm excitation. The excitation wavelength dependence between 400 and 680 nm suggested that there were two pathways for the injection: one was through direct electron-hole generation, and the other seemed to relate to enhanced electric field by plasmon. The charge recombination decay within 1.5 ns was nonexponential and 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 TiO2 particles. Also, two-photon absorption cross-section of the 10-nm gold NP at a near-infrared wavelength (1200 nm) was estimated to be as large as 108 GM (1 GM = 10−50 cm4 s photon−1 molecule−1).

► Plasmon-induced charge transfer in gold–TiO2 nanoparticle systems was revealed.
► Electron injection into TiO2 was within 50 fs with the efficiency of 20–100%.
► The charge-separated state has a lifetime of the 100 ps timescale.
► A very large two-photon absorption cross-section for NIR light was also evaluated.
► This might be useful for photodynamic therapy.