A combined G0W0/BSE scheme of characterizing photoexcitations in hydroxylated rutile TiO2(110)

Speaker
Affiliation
Cnr Nano
Date
2024-11-07
Time
14:30
Venue
ON-SITE S3 Seminar Room, 3rd Floor, Physics Building ONLINE https://tinyurl.com/NanoColloquia
Host
Andrea Ferretti

In reduced TiO2, electronic transitions arising from polaronic excess electrons in surface-localized band-gap states (BGS) are known to contribute to both photoabsorption and the photocatalytic activity of TiO2 in the visible spectrum. Recent state-selective studies using two-photon photoemission (2PPE) spectroscopy have also identified an alternative photoexcitation mechanism contributing to the photoabsorption of the reduced rutile TiO2(110) surface. This process involves highly anisotropic d→d excitations from BGS, originating  from surface and subsurface defects, including bridging hydroxyl groups and oxygen vacancies[1–3]. Density Functional Theory (DFT) has been used to characterize the electronic excited states involved in t2g→t2g transitions[1, 2]. However, the accuracy of DFT is inherently limited by its theoretical framework, as in principle it is exact for ground state properties but fails to account for the interactions between photogenerated electron-hole pairs, i.e., excitons. This has highlighted the need to move beyond DFT to a many-body perturbation theory (MBPT) framework, where excitations are treated as a collection of effective particles propagating through the system. In this seminar, I will show how a combination of the G0W0 method with the Bethe-Salpeter equation (BSE) offers a powerful first-principles approach for characterizing the optical excitations from BGS, as revealed by 2PPE experiments on hydroxylated rutile TiO2(110). Using a BSE spectrum projection tool onto defect states, as the coupling between defect-state transitions and those from valence states was found to be negligible, we identified distinct classes of configurations where polaronic excitations exhibit similar spectral profiles across energies relevant for interpreting 2PPE spectra. No optical coupling has been found between the polarons since they behave as isolated quasi-particles influenced by their chemical environment. Furthermore, the transitions contributing to peaks within the studied energy range involve both t2g→t2g and t2g→eg excitations, depending on the polaron’s symmetry and its localization within the slab. The high-energy d-states involved in these transitions are predominantly localized along the Ti5c surface line, highlighting significant implications for the photocatalytic properties of rutile TiO2(110).

 

References:
[1] Z. Wang, et al., J. Am. Chem. Soc. 137, 9146 (2015)
[2] A. Tanner, et al., Phys. Rev. B 103, L121402 (2021)
[3] T. Waang, et al., J. Phys. Chem. Lett. 12, 10515 (2021)

 

Seminar realized in the framework of the funded projects:
-MaX - Materials design at the eXascale - GA No. 101093374