Atomic Structure, Stability, Raman Modes, and Electronic Properties of Quantum-Confined One-Dimensional Lepidocrocite Titanate and Water: A First-Principles Study.

Quantum-confined, one-dimensional, 1D, lepidocrocite (1DL) titania nanofilaments are a recently discovered polymorph of TiO2 that holds great promise for various applications, including photocatalysis, water purification, dye degradation, and energy storage. These exceptional functionalities originate from 1DL's unique atomic structure and diverse self-assembling morphologies, which are still under active investigation. Current understanding focuses on the atomic structure along the 1DL [100] growth direction, indicating that it shares a backbone atomic structure typical of two-dimensional lepidocrocite, 2DL, titania but exhibits significantly greater length along [100]. What has remained elusive is what the minimal achievable width along the [001] direction and why the 1DLs, despite their very small dimensions, are exceptionally water stable. In this work, the atomic structure and thermodynamic and dynamic stability of 1DL unit cells with varying [001] widths are investigated under a pH-neutral aqueous environment using first-principles calculations and ab initio molecular dynamics simulations based on density functional theory. We attribute the remarkable water stability to terminations induced by water molecules at the ribbon edges that tend to form hydrogen bonds between them when the number of water terminations is 4 per unit cell. Consequently, the theoretically minimal stable width of 1DL is found to be as small as only one lattice constant (2 TiO6 octahedra) of 2DL along the [001] direction. Additionally, the effects of cross-sectional width variation on the bandgap and Raman peak shifts are systematically studied and compared with those of 2DL to reveal quantum confinement effects induced by dimensionality reduction.