Abstract
We have investigated the suitability of Time-Dependent Density Functional Theory (TD-DFT) to describe vertical low-energy excitations in naked and hydrated titanium dioxide nanoparticles. Specifically, we compared TD-DFT results obtained using different exchange-correlation (XC) potentials with those calculated using Equation-of-Motion Coupled Cluster (EOM-CC) quantum chemistry methods. We demonstrate that TD-DFT calculations with commonly used XC potentials (e.g., B3LYP) and EOM-CC methods give qualitatively similar results for most TiO2 nanoparticles investigated. More importantly, however, we also show that, for a significant subset of structures, TD-DFT gives qualitatively different results depending upon the XC potential used and that only TD-CAM-B3LYP and TD-BHLYP calculations yield results that are consistent with those obtained using EOM-CC theory. Moreover, we demonstrate that the discrepancies for such structures originate from a particular combination of defects that give rise to charge-transfer excitations, which are poorly described by XC potentials that do not contain sufficient Hartree–Fock like exchange. Finally, we consider that such defects are readily healed in the presence of ubiquitously present water and that, as a result, the description of vertical low-energy excitations for hydrated TiO2 nanoparticles is nonproblematic.
Highlights
Titanium dioxide (TiO2) nanostructures have attracted great interest in the past few decades due to their low cost, environmental compatibility, and experimentally proven potential for photocatalytic[1−5] and photovoltaic[6] applications
We will investigate how the choice of the XC potential affects the shape of the Time-Dependent Density Functional Theory (TD-Density Functional Theory (DFT)) optical spectrum, and in particular we compare the spectra of the (TiO2)n clusters that show different trends in their excitations
We have studied the suitability of TD-DFT to describe low-energy excitations in TiO2 nanoparticles through a comparison with EOM-Coupled Cluster (CC) quantum chemistry calculations
Summary
Titanium dioxide (TiO2) nanostructures have attracted great interest in the past few decades due to their low cost, environmental compatibility, and experimentally proven potential for photocatalytic[1−5] and photovoltaic[6] applications. To understand the physics and chemistry underlying the application of TiO2 nanostructures in photocatalysis and photovoltaics from a theoretical point of view, TiO2 nanostructures and extended systems have been computationally extensively studied using a variety of methods.[16−31] Most of these studies employ either ground state Density Functional Theory (DFT) or its excited state variant Time-Dependent Density Functional Theory (TD-DFT). We will show that for an important subset of structures TD-DFT can give qualitatively different results depending on the XC potential used and that, in this case, only TD-CAM-B3LYP and TD-BHLYP calculations yield results that are qualitatively consistent with those obtained using EOM-CC theory. TiO2 nanoparticles the qualitative discrepancies between the different methods, observed for naked particles, disappear
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