The Role of Cation-Vacancies for the Electronic and Optical Properties of Aluminosilicate Imogolite Nanotubes: A Non-local, Linear-Response TDDFT Study

Abstract

We report a combined non-local (PBE-TC-LRC) Density Functional Theory (DFT) and linear-response time-dependent DFT (LR-TDDFT) study of the structural, electronic, and optical properties of the cation-vacancy based defects in aluminosilicate (AlSi) imogolite nanotubes (Imo-NTs) that have been recently proposed on the basis of Nuclear Magnetic Resonance (NMR) experiments. Following numerical determination of the smallest AlSi Imo-NT model capable of accommodating the defect-induced relaxation with negligible finite-size errors, we analyse the defect-induced structural deformations in the NTs and ensuing changes in the NTs' electronic structure. The NMR-derived defects are found to introduce both shallow and deep occupied states in the pristine NTs' band gap (BG). These BG states are found to be highly localized at the defect site. No empty defect-state is modeled for any of the considered systems. LR-TDDFT simulation of the defects reveal increased low-energy optical absorbance for all but one defects, with the appearance of optically active excitations at energies lower than for the defect-free NT. These results enable interpretation of the low-energy tail in the experimental UV-vis spectra for AlSi NTs as being due to the defects. Finally, the PBE-TC-LRC-approximated exciton binding energy for the defects' optical transitions is found to be substantially lower (up to 0.8 eV) than for the pristine defect-free NT's excitations (1.1 eV).

Role of initial precursors on the liquid-crystalline phase behavior of synthetic aluminogermanate imogolite nanotubes

“…Finally, based on Levard et al and Yucelen et al publications (Levard et al, 2010;Yucelen et al, 2012a), the presence of defects on the internal part of the nanotubes walls, made of GeO4 tetrahedra, cannot be ruled out. On the grounds of the recent simulation investigations of Poli and co-workers (Poli et al, 2019), showing that structural vacancies in Si-SWINTs impact the low-energy tail in UV-vis spectra, we have thus measured the UV-vis spectra of all samples in liquid phase (Fig. 4b).…”

“…It can be inferred from the modifications of the spectra in the UV part that Ge-DWINTs present a larger amount of defects when synthesized from aluminum sources other than perchlorate. The resulting changes in the nanotubes electronic structure and in electrostatic interactions between nanotubes (Poli et al, 2019;Teobaldi et al, 2009) should lead to significant changes on the liquid-crystalline phase behavior expected for anisometric Ge-DWINTs.…”

Metal Chalcogenides Based Heterojunctions and Novel Nanostructures for Photocatalytic Hydrogen Evolution

“…Lastly, imogolite hollow cylinders are introduced in brief, and their application for photocatalysis is proposed. Interestingly, the presence of a second nanotube (double-walled structure) or the role of defects may induce strong changes in the wall polarization [98]. These differences further illustrate the potential of controlling the structure of INTs for finely tuning the electrostatics inside the nanotubes' cavities.…”

“…Although the computed VB and CB edges and the corresponding energy drive toward photoreduction (or photooxydation) appear to depend weakly on the nanotube diameter and the curvature, the separation of charges in methyl-modified INTs seems to be related to the nature of isomorphic substitution with respect to the inner cavity (Figure 15a,b) [73]. Interestingly, the presence of a second nanotube (double-walled structure) or the role of defects may induce strong changes in the wall polarization [98]. These differences further illustrate the potential of controlling the structure of INTs for finely tuning the electrostatics inside the nanotubes' cavities.…”

“…More importantly, INTs present a real-space separation of the valence band and the conduction band edges, which may be beneficial for electron-hole (e − /h + ) separation via optical charge-transfer excitations across the nanotube walls [95]. The forward-looking improvement of e − /h + separation can therefore be effective in sustaining photocatalytic reactivity directly by the nanotube itself [96][97][98]. Although the computed VB and CB edges and the corresponding energy drive toward photoreduction (or photooxydation) appear to depend weakly on the nanotube diameter and the curvature, the separation of charges in methyl-modified INTs seems to be related to the nature of isomorphic substitution with respect to the inner cavity (Figure 15a,b) [73].…”