Describing Excited State Relaxation and Localization in TiO2 Nanoparticles Using TD-DFT

• Published in: Journal of chemical theory and computation Vol. 10; no. 12; pp. 5538 - 5548
• Main Authors: Berardo, Enrico, Hu, Han-Shi, van Dam, Hubertus J. J, Shevlin, Stephen A, Woodley, Scott M, Kowalski, Karol, Zwijnenburg, Martijn A
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 09-12-2014
• ISSN: 1549-9618; 1549-9626
• DOI: 10.1021/ct500787x

We have investigated the description of excited state relaxation in naked and hydrated TiO2 nanoparticles using Time-Dependent Density Functional Theory (TD-DFT) with three common hybrid exchange-correlation (XC) potentials: B3LYP, CAM-B3LYP and BHLYP. Use of TD-CAM-B3LYP and TD-BHLYP yields qualitatively similar results for all structures, which are also consistent with predictions of coupled-cluster theory for small particles. TD-B3LYP, in contrast, is found to make rather different predictions; including apparent conical intersections for certain particles that are not observed with TD-CAM-B3LYP nor with TD-BHLYP. In line with our previous observations for vertical excitations, the issue with TD-B3LYP appears to be the inherent tendency of TD-B3LYP, and other XC potentials with no or a low percentage of Hartree–Fock like exchange, to spuriously stabilize the energy of charge-transfer (CT) states. Even in the case of hydrated particles, for which vertical excitations are generally well described with all XC potentials, the use of TD-B3LYP appears to result in CT problems during excited state relaxation for certain particles. We hypothesize that the spurious stabilization of CT states by TD-B3LYP even may drive the excited state optimizations to different excited state geometries from those obtained using TD-CAM-B3LYP or TD-BHLYP. Finally, focusing on the TD-CAM-B3LYP and TD-BHLYP results, excited state relaxation in small naked and hydrated TiO2 nanoparticles is predicted to be associated with a large Stokes’ shift.