Picosecond Lifetime Hot Electrons in TiO2 Nanoparticles for High Catalytic Activity

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

doi:10.3390/catal10080916

Bochao Li, Toshihisa Tomie + Show 4 more

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https://doi.org/10.3390/catal10080916

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Abstract

A large number of studies have examined the origins of high-catalytic activities of nanoparticles, but very few have discussed the lifetime of high-energy electrons in nanoparticles. The lifetime is one of the factors determining electron transfer and thus catalytic activity. Much of the lifetime of electrons reported in the literature is too short for a high transfer-efficiency of photo-excited electrons from a catalyst to the attached molecules. We observed TiO2 nanoparticles using the femtosecond laser two-color pump-probe technique with photoemission electron microscopy having a 40 nm spatial resolution. A lifetime longer than 4 ps was observed together with a fast decay component of 100 fs time constant when excited by a 760 nm laser. The slow decay component was observed only when the electrons in an intermediate state pumped by the fundamental laser pulse were excited by the second harmonic pulse. The electronic structure for the asymmetry of the pump-probe signal and the origin of the two decay components are discussed based on the color center model of the oxygen vacancy.

Highlights

In chemical reactions, energetic electrons called “hot electrons” are transported over a potential barrier ∆E from one material to another [1,2,3,4,5,6,7]
We conclude that there is an excited state, Eint, in the conduction 1.63 eV above the 2t2g defect state which lies at 0.85 eV below the conduction band minimum (CBM), and that the electrons excited to the Eint are excited by the second harmonic wavelength (SH) pulse to the eg state, from which electrons are thermally ionized to be observed by PEEM
The result supports our claim that the essence of high catalytic activities of nanoparticles is in the creation of excited states in the conduction band with a long lifetime

Summary

Introduction

Energetic electrons called “hot electrons” are transported over a potential barrier ∆E from one material to another [1,2,3,4,5,6,7]. The chemical reaction rate or electron current, Ie , over the barrier is given by the Richardson–Dushman equation. KT is the thermal energy of the transferred electrons when the energy of electrons is high, kT is high or the potential barrier, ∆E, is low, and transfer current Ie is large. The efficient generation of hot electrons is the most important subject in nanomaterial science. The hot electron has been the focus of many reviews [1,2,5,15,16,17]

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