localized order-disorder transitions induced by Li segregation in amorphous TiO2 nanoparticles

Atomistic Insights into Lithium Storage Mechanisms in Anatase, Rutile, and Amorphous TiO2 Electrodes

Density functional theory calculations were used to investigate the phase transformations of Li_xTiO₂ (at 0 ≤ x ≤ 1), solid-state Li⁺ diffusion, and interfacial charge-transfer reactions in both crystalline and amorphous forms of TiO₂. It is shown that in contrast to crystalline TiO₂ polymorphs, the energy barrier to Li⁺ diffusion in amorphous TiO₂ decreases with increasing mole fraction of Li⁺ due to the changes of chemical species pair interactions following the progressive filling of low-energy Li⁺ trapping sites. Sites with longer Li-Ti and Li-O interactions exhibit lower Li⁺ insertion energies and higher migration energy barriers. Due to its disordered atomic arrangement and increasing Li⁺ diffusivity at higher mole fractions, amorphous TiO₂ exhibits both surface and bulk storage mechanisms. The results suggest that nanostructuring of crystalline TiO₂ can increase both the rate and capacity because the capacity dependence on the bulk storage mechanism is minimized and replaced with the surface storage mechanism. These insights into Li⁺ storage mechanisms in different forms of TiO₂ can guide the fabrication of TiO₂ electrodes to maximize the capacity and rate performance in the future.

The structural and electronic properties of reduced amorphous titania

Reduced amorphous titania has been modeled by removing oxygen atoms to clarify the properties of these materials.

A density functional tight binding study of acetic acid adsorption on crystalline and amorphous surfaces of titania

We present a comparative density functional tight binding study of an organic molecule attachment to TiO2 via a carboxylic group, with the example of acetic acid. For the first time, binding to low-energy surfaces of crystalline anatase (101), rutile (110) and (B)-TiO2 (001), as well as to the surface of amorphous (a-) TiO2 is compared with the same computational setup. On all surfaces, bidentate configurations are identified as providing the strongest adsorption energy, Eads = -1.93, -2.49 and -1.09 eV for anatase, rutile and (B)-TiO2, respectively. For monodentate configurations, the strongest Eads = -1.06, -1.11 and -0.86 eV for anatase, rutile and (B)-TiO2, respectively. Multiple monodentate and bidentate configurations are identified on a-TiO2 with a distribution of adsorption energies and with the lowest energy configuration having stronger bonding than that of the crystalline counterparts, with Eads up to -4.92 eV for bidentate and -1.83 eV for monodentate adsorption. Amorphous TiO2 can therefore be used to achieve strong anchoring of organic molecules, such as dyes, that bind via a -COOH group. While the presence of the surface leads to a contraction of the band gap vs. the bulk, molecular adsorption caused no appreciable effect on the band structure around the gap in any of the systems.