Emerging polymorphism in nanostructured TiO₂: Quantum chemical comparison of anatase, rutile, and brookite clusters

Abstract

Density functional theory (DFT) and time-dependent DFT calculations have been performed on a set of 34 titanium dioxide clusters ((TiO2) n with n 125) to investigate structural and electronic properties of nanostructured TiO 2 (nano-TiO 2 ) materials. The investigated clusters include models of the three low-energy polymorphic forms of TiO 2 anatase, rutile, and brookite. A systematic comparison of clusters of increasing size show clear trends for emerging bulk properties in the investigated systems as the surface-to-bulk ratio changes from small clusters dominated by undercoordinated surface atoms to more realistic model nano-crystals with significant bulk components. Differences and similarities in terms of atomic coordination, structural stability, and electronic properties for the three different polymorphic forms of nano-TiO 2 are discussed. The calculations provide evidence for emerging polymorphism with increasing cluster sizes so that the different TiO 2 forms can be clearly distinguished based on structural characteristics associated with the local bonding environment of the constituent atoms.

Iron sensitizer converts light to electrons with 92% yield

“…Continued development could further prolong MLCT lifetimes 36,37 or lead to strategies for slowing down electron-cation recombination.…”

“…This is consistent with QC calculations, which show an increase in the energy of the first transition from 505 to 427 nm as a result of deprotonation of the carboxylic acid group upon surface binding (Supplementary Data 3). Structure 2 is believed to be energetically well-suited for the injection of electrons into the TiO 2 conduction band, based on a comparison of its LUMO energy level with that of a (TiO 2 ) 92 cluster model 37 . This is corroborated by the corresponding calculations for RuN3 (ref.…”

“…As a reference, 2 in MeCN and 2 on Al 2 O 3 were also studied in order to distinguish the effects of COOH functionalization and surface immobilization from electron injection. Global analysis (GA) was performed on all data sets to identify the spectral features of the photogenerated species 36,37 . The measured spectra and kinetics are shown in Fig.…”

“…All energies were calculated with the PBE0 49,50 functional in conjunction with standard Gaussian type orbital basis sets of triple-zeta quality, 6-311G(d,p) for all atoms except Ru where an SDD effective core potential was employed, and a complete MeCN polarizable continuum model. Geometry optimizations for 1, 2 and RuN3 were performed at the same level of theory as the energies, and the optimized anatase-(TiO 2 ) 92 cluster was taken from the literature 37 . All complex calculations were run with a total charge without counterions, and no symmetry constraints were applied, allowing for possible Jahn-Teller effects.…”

Modeling Excited States in TiO2 Nanoparticles: On the Accuracy of a TD-DFT Based Description

“…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). In the former case, optical excitations are generally assumed to map onto excitations from occupied to unoccupied Kohn–Sham orbitals, which is theoretically somewhat difficult to justify.…”