Excited states in the conduction band and long-lifetime hot electrons in TiO2 nanoparticles observed with photoemission electron microscopy

Abstract

The authors propose that high catalysis of TiO2 nanoparticles (NPs) originates in the generation of excited states in the conduction band, which realize a long lifetime of hot electrons. This paper confirms their claim by observing individual NPs in the femtosecond laser pump-And-probe technique using photoemission-electron-microscopy. The low slope of the power dependence of the brightness of NPs is attributed to thermionic electron emission from an excited state below the vacuum level. An intermediate excited state is required to explain the asymmetrical delay dependence of the brightness enhancement, and the energy position is assigned to 0.78 eV above the conduction band minimum from the laser wavelength for the longest decay time of the brightness enhancement. A lifetime as long as 4 ps for hot electrons of 0.8 eV kinetic energy is observed when resonantly excited by a 760 nm light. The observed lifetime in TiO2 NPs is more than 10 times longer than those in previous papers using single crystals, which supports the authors' claim.