Defect Chemistry, Surface Structures, and Lithium Insertion in Anatase TiO₂

Abstract

Atomistic simulation techniques are used to investigate the defect properties of anatase TiO 2 and Li x TiO 2 both in the bulk and at the surfaces. Interatomic potential parameters are derived that reproduce the lattice constants of anatase, and the energies of bulk defects and surface structures are calculated. Reduction of anatase involving interstitial Ti is found to be the most favorable defect reaction in the bulk, with a lower energy than either Frenkel or Schottky reactions. The binding energies of selected defect clusters are also presented: for the Ti 3+ -Li + defect cluster, the binding energy is found to be approximately 0.5 eV, suggesting that intercalated Li ions stabilize conduction band electrons. The Li ion migration path is found to run between octahedral sites, with an activation energy of 0.45-0.65 eV for mole fractions of lithium in Li x TiO 2 of x e 0.1. The calculated surface energies are used to predict the crystal morphology, which is found to be a truncated bipyramid in which only the (101) and (001) surfaces are expressed, in accord with the available microscopy data. Calculations of defect energies at the (101) surface suggest that single Ti 3+ defects and neutral Ti 3+ -Li + pairs tend to segregate to the surface.

Voltage enhancement in dye-sensitized solar cell using (001)-oriented anatase TiO2 nanosheets

“…The remaining face on the anatase crystal is (001) which is consistent with the conclusion that a truncated bipyramid is the corresponding crystal morphology [12]. Only rarely, the rhombic-shaped crystals are found, exposing the (010) face [13].…”

GGA+Udescription of lithium intercalation into anataseTiO2

“…This preference for an off-center Li position was given support by data from molecular dynamics using the partial charge COMPASS force field. 78 In contrast, atomistic simulation by Olson et al 27 with formal-charge Buckingham potentials found the stable intercalation site to be at the octahedron center. Recent molecular-dynamics simulations performed by Kerisit et al 79 with a polarizable modification of the partialcharge Matsui-Akaogi potential reported hopping between pairs of intraoctahedral sites separated by 0.8 Å along the c direction with a calculated activation energy of 17 meV.…”

“…84 Defects modify the distribution of trap energies and in the case of deep traps are thought to act as recombination centers. 27 To calculate the binding energies of electrons interacting with interstitial lithium we performed additional calculations with the Li-intercalated anatase system in +1 and −1 charge states. This was done by changing the number of electrons included in the calculation with a corresponding background charge included so that the Coulomb energy converges with periodic boundary conditions.…”

“…The interactions between interstitial lithium and electrons trapped at Ti centers have previously been studied by Olson et al with atomistic simulations. 27 They reported a binding energy of 0.54 eV for a ͓Li i…”

“…26 Although surface polarization is likely to play a large role, 25 given the ease with which Li intercalates into anatase TiO 2 some interstitial Li is expected to be present. It has been suggested that interstitial lithium modifies the distribution of charge traps within the TiO 2 substrate, [26][27][28] with a consequential effect on the dynamics of charge carriers diffusing through the electrode. This interaction between intercalated lithium and electrons resident within the Ti sublattice has theoretical support from interatomic forcefield calculations of Olson et al 27 who predicted strong binding between interstitial Li and a neighboring Ti in a +3 oxidation state.…”