A van der Waals Density Functional Study of MoO₃ and Its Oxygen Vacancies

Abstract

The electronic structure of layered molybdenum trioxide MoO 3 is highly sensitive to changes in oxygen stoichiometry as Mo 6+ has an empty 4d shell. Applications of MoO 3 are responsive to small changes in vacancy concentration, with some functions relying on a narrow window of oxygen non-stoichiometry. Difficulties in analyzing the energetics of oxygen vacancies by computational methods stem from the inability to accurately model the layered structure of MoO 3 . One unit cell parameter is governed by long range forces across the structural gaps and these dispersed interactions are not well described by conventional density functional theory (DFT) methods. With the exchange functional vdW-DF2 we accurately model the structure, in good agreement with experimental data. This basis allows exploration of the effect of oxygen non-stoichiometry on the electronic structure and properties of the oxygen deficient material.The layered structure efficiently screens the structural perturbations caused by oxygen vacancies.The enthalpies of formation are calculated for oxygen vacancies at the three symmetry inequivalent oxygen sites. The oxygen deficiency in MoO 3 gives rise to Mo 4d gap states with energy levels dependent on the type of oxygen vacancy.

Systematic Study of Oxygen Vacancy Tunable Transport Properties of Few‐Layer MoO3−x Enabled by Vapor‐Based Synthesis

“…This result is important because it is in excellent agreement with experimental values for bulk MoO 3 and thus confirms that the 2D nanostructuring of MoO 3 does not significantly alter the bandgap magnitude as highlighted by the present DFT calculations. It also confirms recent theoretical findings showing that the introduction of vacancies in MoO 3 does not alter the width of the bandgap but gives rise to gap states . The analysis of the core‐loss and low‐loss spectra thus highlights that electron irradiation at low‐dose of few‐layer MoO 3 leads to the creation of oxygen vacancies in the MoO 3 network.…”

“…DFT + U calculations were performed to give insight into the electronic structure of the monolayer form (Figure e) compared to the bulk form (Figure b). The bulk case shows an indirect bandgap of 1.96 eV, similar to other DFT + U studies of MoO 3 . The DFT‐calculated electronic structure of an MoO 3 monolayer shows a slightly larger indirect bandgap of 2.03 eV.…”

Structural and electronic properties of bulk and ultrathin layers of V2O5 and MoO3

“…The first DFT+U (U = 6. 52 Finally, Inzani et al 21 have checked many possible formulations of Van der Waals dispersive energy contributions, along with U corrections on Mo(d). They concluded that even though the geometry was better described with vdW-DF2 approach, almost no effect on the band-gap resulted from a variation of U on Mo(d) from 2 to 8 eV.…”

“…They have concluded that the geometry was better described with a vdW-D2 functional, and that there is almost no effect on the band-gap varying U on Mo(d) from 2 to 8 eV. 21 The scope of this paper is twofold. On one side we will try to define a computational setup which is able to describe with a similar accuracy not only the structural parameters and the band gap of the material, which are the usual targets of the DFT calculations, but also of the dielectric constant and of the formation energy of the bulk material.…”

Functionalizing New Intercalation Chemistry for Sub‐Nanometer‐Scaled Interlayer Engineering of 2D Transition Metal Oxides and Chalcogenides

“…As a representative of 2D TMOs, orthorhombic MoO 3 (α‐MoO 3 ), unlike other phases of molybdenum trioxide (e.g., monoclinic β, ε, and hexagonal h phases), possesses typical van der Waals force‐binded layered structures with layer thickness of ≈0.7 nm . As shown in Figure a, each MoO 3 monolayer comprises two sublayers of distorted MoO 6 octahedra, edge‐sharing along the c ‐axis, and corner‐sharing along the a ‐axis . Oxygen atoms locate at three unique positions: the terminal oxygen (O t ) only double‐bonding to one Mo atom with a distance of 1.67 Å, the asymmetric bridge oxygen atom (O a ) bonding to two Mo atoms with length of 1.74 and 2.25 Å, and the symmetrical bridge oxygen (O s ) atom bonding to three Mo atoms with two horizontal bonds of distance of 1.95 Å and one vertical bond of distance of 2.33 Å .…”