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

Bochao Li,Toshihisa Tomie,Boyu Ji,Hao Li,Jingquan Lin,Chang Yang

doi:10.1063/1.5120378

Bochao Li, Toshihisa Tomie + Show 4 more

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https://doi.org/10.1063/1.5120378

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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.

Highlights

Titanium dioxide (TiO2) is one of the most important materials and its properties were studied by many people
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
We propose that high catalysis of TiO2 NPs originates in a long lifetime of hot electrons which is realized by the generation of the new excited states in the conduction band

Summary

Introduction

Titanium dioxide (TiO2) is one of the most important materials and its properties were studied by many people. The optical absorption of TiO2 increasing from 600 nm to the maximum at about 2 μm is identified as transitions from Ti3d defect states in the bandgap.. Yamada and Kanemitsu reported a lifetime of 18 ns for photoluminescence (PL), 24 ns for photoconductivity (PC), and 48 ns for transient absorption (TA). They attributed the PC decay to the electron decay and TA of 1.2 eV light to the optical absorption of holes excited by a 3.5 eV laser. Zhang et al. reported a lifetime of 80 ps observed at 780 nm illumination They attributed the TA to the d-d transition of electrons trapped in the mid-gap states. Du et al. studied the lifetime of electrons transferred from Au nanoparticle (NP)

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