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

Area of Science:

• Computational Chemistry
• Materials Science
• Quantum Mechanics

Background:

• Understanding excited state dynamics in titanium dioxide (TiO2) nanoparticles is crucial for photocatalysis and solar energy applications.
• Accurate theoretical descriptions of excited state relaxation are essential for predicting material properties and performance.

Purpose of the Study:

• To investigate and compare the performance of different Time-Dependent Density Functional Theory (TD-DFT) exchange-correlation (XC) potentials in describing excited state relaxation in TiO2 nanoparticles.
• To identify potential issues with specific XC potentials, such as TD-B3LYP, in modeling charge-transfer states during relaxation.

Main Methods:

• Employed Time-Dependent Density Functional Theory (TD-DFT) with three hybrid XC potentials: B3LYP, CAM-B3LYP, and BHLYP.
• Calculated excited state relaxation pathways for both naked and hydrated TiO2 nanoparticles.
• Validated results against coupled-cluster theory for small particle systems.

Main Results:

• TD-CAM-B3LYP and TD-BHLYP provided qualitatively similar and reliable results for excited state relaxation, consistent with coupled-cluster theory.
• TD-B3LYP showed significant discrepancies, predicting artificial conical intersections and inaccurately stabilizing charge-transfer (CT) states.
• The spurious stabilization of CT states by TD-B3LYP was hypothesized to lead to different optimized excited state geometries.

Conclusions:

• TD-CAM-B3LYP and TD-BHLYP are recommended for accurate studies of excited state relaxation in TiO2 nanoparticles.
• TD-B3LYP should be used with caution due to its tendency to misrepresent CT states and relaxation pathways.
• Excited state relaxation in small TiO2 nanoparticles is predicted to involve a substantial Stokes' shift, particularly when using TD-CAM-B3LYP and TD-BHLYP.