Xinjian Xie

A class of monolayer metal halogenides MX2: Electronic structures and band alignments

With systematic first principles calculations, a class of monolayer metal halogenides MX2 (M = Mg, Ca, Zn, Cd, Ge, Pb; M = Cl, Br, I) has been proposed. Our study indicates that these monolayer materials are semiconductors with the band gaps ranging from 2.03 eV of ZnI2 to 6.08 eV of MgCl2. Overall, the band gap increases with the increase of the electronegativity of the X atom or the atomic number of the metal M. Meanwhile, the band gaps of monolayer MgX2 (X = Cl, Br) are direct while those of other monolayers are indirect. Based on the band edge curvatures, the derived electron (me) and hole (mh) effective masses of MX2 monolayers are close to their corresponding bulk values except that the me of CdI2 is three times larger and the mh for PbI2 is twice larger. Finally, the band alignments of all the studied MX2 monolayers are provided using the vacuum level as energy reference. These theoretical results may not only introduce the monolayer metal halogenides family MX2 into the emerging two-dimensional ma...

Elucidating dz2 orbital selective catalytic activity in brownmillerite Ca2Mn2O5

3d-orbital filling in transition metal oxide is crucial to govern the catalytic activity in oxygen evolution reduction, nevertheless, it is not fundamentally accessible why specific orbital occupation produces a highest catalytic performance. Here, we utilize brownmillerite Ca2Mn2O5 to clarify the orbital selective catalytic behavior due to the crystal field splitting and on-site coulomb interactions. Within density functional theory plus dynamical mean field theory, Ca2Mn2O5 shows a paramagnetic Mott insulating behavior at room temperature, consistent with optical adsorption spectra and magnetic susceptibility. As the center of the dz2 orbital locates in the lower Hubbard sub-band, the unit occupation on dz2 orbital provides a moderate bonding with external O* species to cause a high catalytic activity of Ca2Mn2O5 with a square pyramid crystal field. Such concept of unit occupation of dz2 near Fermi level could be extended to other crystal fields for future design of oxide catalysts.