Chemical Modification of Polaronic States in Anatase TiO2(101).

Alex J Tanner,Helen H Fielding,Geoff Thornton,Robin Kerr

doi:10.1021/acs.jpcc.1c03684

Alex J Tanner, Helen H Fielding + Show 2 more

Open Access

https://doi.org/10.1021/acs.jpcc.1c03684

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Abstract

Two polymorphs of TiO2, anatase and rutile, are employed in photocatalytic applications. It is broadly accepted that anatase is the more catalytically active and subsequently finds wider commercial use. In this work, we focus on the Ti3+ polaronic states of anatase TiO2(101), which lie at ∼1.0 eV binding energy and are known to increase catalytic performance. Using UV-photoemission and two-photon photoemission spectroscopies, we demonstrate the capability to tune the excited state resonance of polarons by controlling the chemical environment. Anatase TiO2(101) contains subsurface polarons which undergo sub-band-gap photoexcitation to states ∼2.0 eV above the Fermi level. Formic acid adsorption dramatically influences the polaronic states, increasing the binding energy by ∼0.3 eV. Moreover, the photoexcitation oscillator strength changes significantly, resonating with states ∼3.0 eV above the Fermi level. We show that this behavior is likely due to the surface migration of subsurface oxygen vacancies.

Highlights

The polaronic Ti3+ states of TiO2 have long been a source of technological interest
Do these excess electrons give rise to the conductivity that permits many surface studies, they facilitate a wide range of redox chemistries such as water splitting and hydrogen generation.[1−3] In recent years, it has been shown that these polaronic states, commonly referred to as the band gap states c(eBsGseSs),4−o6fpoTteiOnt2i,allcyanconatlrsiobutuinngdetrogothephcaottaoleyxticcitpathiootnoypierlod
Two features become apparent at higher hν, labeled feature 1 and feature 2, which have a different electron energy dependence when varying the photon energy

Summary

Introduction

The polaronic Ti3+ states of TiO2 have long been a source of technological interest They arise from the reduction of TiO2 and result in n-type semiconductor properties in the material. Of the two predominant TiO2 polymorphs, anatase demonstrates higher catalytic performance.[7] Despite this, the polaronic states of anatase remain poorly understood. In part, this is because, unlike in rutile TiO2, anatase polarons rarely exist at the surface in ultrahigh vacuum (UHV) conditions, resulting in fewer microscopy studies and weaker spectroscopic signals.[8,9]

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