Investigating Polaron Formation in Anatase and Brookite TiO2 by Density Functional Theory with Hybrid-Functional and DFT + U Methods

TL;DR

This study compares polaron formation in anatase and brookite TiO2 using hybrid DFT and DFT+U methods, revealing differences in localization and energetics that influence photocatalytic properties.

Contribution

It provides the first detailed comparison of electron and hole polaron geometries and energetics in anatase and brookite TiO2 using hybrid functional and Hubbard U methods.

Findings

Brookite forms both electron and hole polarons, unlike anatase.

Brookite polarons are more energetically favorable than in anatase.

Polaron localization varies with the Hubbard U parameter.

Abstract

Anatase and brookite are robust materials with enhanced photocatalytic properties. In this study, we used density functional theory (DFT) with a hybrid functional and the Hubbard on-site potential methods to determine electron and hole polaron geometries for anatase and brookite and their energetics. Localized electron and hole polarons were predicted not to form in anatase using DFT with hybrid functionals. In contrast, brookite formed both electron and hole polarons. The brookite electron-polaronic solution exhibits coexisting localized and delocalized states, with hole polarons mainly dispersed on two-coordinated oxygen ions. The Hubbard on-site potential testing over the wide range of 4 eV to 10 eV revealed that brookite polarons are formed at 6 eV, while anatase polarons are formed at 8 eV. The brookite electron polaron was always localized on a single titanium ion under the Hubbard model, whereas the hole polaron was dispersed over four oxygen atoms, consistent with the hybrid DFT studies. The anatase electron polarons were dispersed at lower on-site potentials but were more localized at higher potentials. Both methods predict that brookite has a higher driving force for the formation of polarons than anatase.