Low Valence Cation Doping of Bulk Cr₂O₃: Charge Compensation and Oxygen Vacancy Formation

Abstract

ABSTRACT:The different oxidation states of chromium allow its bulk oxide form to be reducible, facilitating the oxygen vacancy formation process, which is a key property in applications such as catalysis. Similar to other useful oxides such as TiO 2 , and CeO 2 , the effect of substitutional metal dopants in bulk Cr 2 O 3 and its effect on the electronic structure and oxygen vacancy formation are of interest, particularly in enhancing the latter. In this paper, density functional theory (DFT) calculations with a Hubbard +U correction (DFT+U) applied to the Cr 3d and O 2p states, are carried out on pure and metal doped bulk Cr 2 O 3 to examine the effect of doping on the electronic and geometric structure. The role of dopants in enhancing the reducibility of Cr 2 O 3 is examined to promote oxygen vacancy formation. The dopants are Mg, Cu, Ni, and Zn, which have a formal +2 oxidation state in their bulk oxides. Given this difference in host and dopant oxidation states, we show that to predict the correct ground state two metal dopants charge compensated with an oxygen vacancy are required. The second oxygen atom removed is termed 'the active' oxygen vacancy and it is the energy required to remove this atom that is related to the reduction process. In all cases, we find that substitutional doping improves the oxygen vacancy formation of bulk Cr 2 O 3 by lowering the energy cost.

First-Principles Investigation of Native Interstitial Diffusion in Cr2O3

“…Under such conditions, Cr i defects have lower formation energy compared to V Cr and both Cr i and V O have formation energies approximately equal to 1.5 eV, making Cr i and V O the dominant defects. Similarly, bulk Cr 2 O 3 samples can be strongly p-type doped with E F close to VBM, when divalent ions such as Mg are present as impurities [40]. Under such conditions, our calculations suggest that Cr i could be the dominant defect in bulk Cr 2 We have studied the diffusion of Cr and O interstitials in charge states ranging from [0, 3] and [-2, 0], respectively.…”

“…It is noteworthy, that Cr 2 O 3 passivation films on Ni alloy substrate could be p-type doped due to trace amounts of Ni substitution [40]. For Ni doping, Fermi level in Cr 2 O 3 stabilizes at 0.4 eV above VBM [40]. Under such conditions, Cr i defects have lower formation energy compared to V Cr and both Cr i and V O have formation energies approximately equal to 1.5 eV, making Cr i and V O the dominant defects.…”

Activation of CO2 at chromia-nanocluster-modified rutile and anatase TiO2

“…A Monkhorst-Pack (2×1×1) k-point sampling grid is used. For consistency with previous work we apply the DFT+U approach [58] to describe the Ti 3d and Cr 3d states, with U = 4.5 eV for the Ti 3d states and 3.5 eV for the Cr 3d states [59,60]. Convergence criteria for electronic and ionic relaxations are 0.0001 eV and 0.02 eV/ Å.…”

Surface Modification of Perfect and Hydroxylated TiO2 Rutile (110) and Anatase (101) with Chromium Oxide Nanoclusters

“…In addition, in previous studies, we found that this correction is necessary to describe O 2p states that result after valence band hole formation. 30,31,69 Finally, the partially filled 3d states in Cr also require a DFT + U correction to be consistently described, and here, we use U (Cr 3d) = 3.5 eV, 60 consistent with values used in other studies.…”