Ion-Exchangeable One-Dimensional Lepidocrocite Titanate—Atomic Structure and Quantum Confinement Effects

Nanoscale TiO₂-based materials, with their distinctive structural and functional characteristics, have gained considerable interest in both industrial and academic sectors owing to their potential applications across diverse fields. Titanate structures have traditionally been synthesized through hydrothermal and sol-gel methods. However, these often yield relatively large structures due to crystal growth during high-temperature treatments. 1D titania-based nanofilaments (NFs) have been previously synthesized by Badr et al. [1] using various Ti-containing, earth-abundant, and non-toxic powder precursors. We conduct an atomic-level study to determine the structure of nanofilaments, employing scanning transmission electron microscopy (STEM) and electron energy-loss spectroscopy (EELS) at both room temperature and cryogenic conditions (96 K).
The synthesized titania-based nanofilaments have cross-sectional dimensions of 5 × 7 Ų and extend to tens of nanometers in length. STEM imaging indicates that the basic structural unit remains composed of two Ti atoms wide nanofilaments with a lepidocrocite crystal structure [2]. The space between the NFs is eminently ion-exchangeable. Based on density functional theory (DFT) calculations, the thickness of a single nanofilament (NF) along the b-axis is approximately 5 Å, and the difference from the measured spacing suggests that the interfilamentous space is filled with monovalent (H₃O⁺, Li⁺, Na⁺) or divalent (Mg²⁺,Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Zn²⁺) cations.
The intercalated transition metals are expected to bond with the oxygen atoms present in adjacent layers of the lepidocrocite titanate. These metal-oxygen interactions help to stabilize the layered structure without disrupting the original Ti-O framework, effectively enhancing the material's stability and ion exchange properties. Our atomic-scale characterization reveals an ABA-type stacking arrangement for lepidocrocite nanofilament bundles intercalated with transition metal cations, eliminating odd-numbered (0k0) and even-numbered (0k1) diffraction spots. The Ti–Ti spacing along the c-axis shows significant variation from various STEM images, which may indicate changes in bond lengths resulting from ion intercalation. These variations could influence the structural stability and electronic properties of the material.
To better understand the influence of different cations on the properties of the 1D lepidocrocite titanate, we also analyze the oxidation states of the intercalated cations. By utilizing EELS, we present a comprehensive study of the oxidation states of various intercalated cations, providing insight into their role in stabilizing the structure and modulating material properties. This detailed oxidation state analysis is crucial to establish structure-property relationships, enabling a deeper understanding of how different intercalants can be used to tailor the functional properties of lepidocrocite titanate.
The extreme confinement of these 1DL structures, with their remarkably small size, suggests a quantum size effect contributing to the exceptionally high measured band gap energy titania material. Among titanate nanostructures, the 1DL system uniquely exhibits quantum confinement that directly impacts its band gap. In this study, we measure the band gap for structures intercalated with different cations to explore this behavior further.


1.Hussein. O. Badr, Jacob. Cope, Takayuki. Kono, et al., “Titanium Oxide-based 1D Nanofilaments, 2D Sheets, and Mesoporous Particles - Synthesis, Characterization, and Ion Intercalation”, Matter (2023).
2.Francisco Lagunas, David Bugallo, Fatemeh Karimi, et al., "Ion-Exchange Effects in One-Dimensional Lepidocrocite TiO₂: A Cryogenic Scanning Transmission Electron Microscopy and Density Functional Theory Study," Chemistry of Materials (2024).
3.This work is supported by a grant from the National Science Foundation (DMR-2309396)