Surfaces of VO₂‐Polymorphs: Structure, Stability and the Effect of Doping

Abstract

Vanadium dioxide is an interesting and frequently applied material due to its metal‐insulator phase transition. However, there are only few studies of the catalytic activity and surface properties of different VO2 polymorphs. Therefore, we investigated the properties of the surfaces of the most stable VO2 phases theoretically at density‐functional theory level using a self‐consistent hybrid functional which has demonstrated its accuracy for the prediction of structural, electronic and energetic properties in a previous study. We found that the surfaces of the rutile R phase of VO2 are not stable and show a spontaneous phase transition to the monoclinic M1 phase. Doping with Mo stabilizes the surfaces with rutile structure even for small dopant concentrations (6.25 %). Both M1 and R surfaces strongly relax, with and without doping. In particular the metal‐metal distances in the uppermost layers change by up to 0.4 Å. Mo segregates in the topmost layer of both R and M1 phases. The electronic structure is only slightly changed upon doping.

Exploiting Phase Transitions in Catalysis: Adsorption of CO on doped VO2‐Polymorphs

“…Subsequently the surface properties of both VO 2 polymorphs were investigated. [13] A reconstruction of the R phase surfaces was found which has also been found experimentally. [14] Doping with Mo stabilizes the R phase and preserves the structure of both phases also at the surface.…”

“…The calculated pDOS of the surfaces are similar to the pDOS obtained with sc-PBE0 in a ChemPhysChem previous study. [13] The fundamental band gap calculated with M06-D3 is larger compared to the sc-PBE0 results, due to the higher amount of Fock-exchange (27 % compared to 12.7 %). CO adsorption leads to small changes of the surface pDOS, as expected for physisorption.…”

“…[20] The hybrid functional M06 with D3 dispersion correction [21,22] was applied instead of the previously established sc-PBE0 method. [12,13] This was advantageous since M06 showed higher numerical stability in the adsorption calculations. Additionally, optimized D3 parameters are available for M06 which is not the case for sc-PBE0.…”

“…The present basis sets are larger than those of our previous study. [13] This proved to be necessary to ensure stable spin states and to reduce the basis set superposition error in the adsorption calculations. The integral truncation tolerances were set to the standard values of (10 À 6 , 10 À 6 , 10 À 6 , 10 À 6 , 10 À 12 ) to reduce computational effort.…”

First principles studies of the electronic and structural properties of the rutile VO$_{2}$(110) surface and its oxygen-rich terminations

“…Furthermore, they find it necessary to include spin-polarization in order to avoid negative values for the surface free energies of oxygen-rich reconstructions. In a very recent study [17], the rutile and monoclinic VO 2 surfaces were studied with the hybrid sc-PBE0 functional with an adjusted Fock mixing parameter. Despite its good performance on both bulk phases, the hybrid sc-PBE0 functional surprisingly fails to describe the surface energies of rutile VO 2 which do not converge upon increasing the thickness of the slab and even yield negative values for thicker slabs.…”