Chemical Modification of Polaronic States in Anatase TiO₂(101)

Abstract

Two polymorphs of TiO 2 , 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 Ti 3+ polaronic states of anatase TiO 2 (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 TiO 2 (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.

TiO2 Polarons in the Time Domain: Implications for Photocatalysis

“… 36 Intriguingly, it has also been shown that anatase polarons couple strongly with a Ti 3+ conduction band state 2.0 eV above E F , giving rise to a 2.8 eV resonant photoexcitation scheme. 37 This is less than the anatase band gap of 3.2 eV, which is in line with reports that Ti 3+ self-doping of anatase gives rise to an extended absorption spectrum. 20 Calculations suggest that like rutile, the initial and excited state in this scheme are t 2 g in character.…”

“…The initial occupied state shifts 0.3 eV higher in BE, which can be excited into states 3.0 eV above E F . 37 Moreover, because these states are now located at the surface, the 2PPE yield is vastly increased (see Figure 3b). The capability to tune polaron−light resonances could be key in photocatalytic design, which gives rise to numerous potential avenues of investigation.…”

“… 30 In the anatase TiO 2 (101) case, polaron photoexcitation is enhanced when the electric field vector is perpendicular to the [010] azimuth. 30 , 37 …”

“…The polarization of light is shown in the panel legend, and the [010] azimuth is vertical. Adapted with permission from ref ( 37 ). Copyright 2021 American Chemical Society.…”

“…30 In the anatase TiO 2 (101) case, polaron photoexcitation is enhanced when the electric field vector is perpendicular to the [010] azimuth. 30,37 The polarization dependence of 2PPE intensity is proportional to the transition dipole moment (μ) from an initial state, |i⟩, to an intermediate state, |j⟩, and from |j⟩ to a final state, |p⟩, above the vacuum level. 38 This proportionality is given by…”