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

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.