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

## TL;DR

This study explores the atomic structure and stability of one-dimensional lepidocrocite titania nanofilaments in water using computational methods.

## Contribution

The paper identifies the minimal stable width of 1D lepidocrocite titania and explains its water stability through hydrogen bonding at ribbon edges.

## Key findings

- The minimal stable width of 1DL is one lattice constant along [001] due to hydrogen bonding at ribbon edges.
- Quantum confinement effects are observed through bandgap and Raman peak shifts as the cross-sectional width varies.
- 1DL shows exceptional water stability compared to 2DL due to water-induced terminations.

## Abstract

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.