Surfaces of VO -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 VO polymorphs. Therefore, we investigated the properties of the surfaces of the most stable VO 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 VO are not stable and show a spontaneous phase transition to the monoclinic M phase. Doping with Mo stabilizes the surfaces with rutile structure even for small dopant concentrations (6.25 %). Both M 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 M phases. The electronic structure is only slightly changed upon doping.