DFT Study on Ce-Doped Anatase TiO₂: Nature of Ce³⁺ and Ti³⁺ Centers Triggered by Oxygen Vacancy Formation

A systematic study of TiO₂ anatase, Ce-doped TiO₂ anatase with 2.8 and 5.6% dopant concentration and of the systems resulting from oxygen vacancy formation has been carried out by means of periodic density functional theory based calculations using PBE, PBE+U, and hybrid functionals. For each approach, several situations are considered for the oxygen vacancy formation, differing on the position of the removed oxygen or on the resulting electronic structure. The hybrid B3LYP functional and PBE+<i>U</i> approaches provide a physically meaningful description of localized <i>d</i> and <i>f</i> electrons in Ti³⁺ and Ce³⁺ species, respectively. Nevertheless, quasi-degenerate solutions were encountered featuring either fully localized spin (simple and split) or partially localized spin. Although standard PBE calculations result always in fully (unphysical) delocalized solutions, the most stable geometry thus predicted, in which Ce is six-coordinated and V_O folded by 3­[TiO₅], is in agreement with the B3LYP and PBE+<i>U</i> results. The present work provides compelling evidence that the remarkable catalytic properties of these systems partially arise from the facilitated oxygen vacancy (V_O) formation triggered by the Ce dopant, which is further enhanced when dopant concentration is increased

Symmetry-Driven Band Gap Engineering in Hydrogen Functionalized Graphene

Band gap engineering in hydrogen functionalized graphene is demonstrated by changing the symmetry of the functionalization structures. Small differences in hydrogen adsorbate binding energies on graphene on Ir(111) allow tailoring of highly periodic functionalization structures favoring one distinct region of the moiré supercell. Scanning tunneling microscopy and X-ray photoelectron spectroscopy measurements show that a highly periodic hydrogen functionalized graphene sheet can thus be prepared by controlling the sample temperature (<i>T</i>_s) during hydrogen functionalization. At deposition temperatures of <i>T</i>_s = 645 K and above, hydrogen adsorbs exclusively on the HCP regions of the graphene/Ir(111) moiré structure. This finding is rationalized in terms of a slight preference for hydrogen clusters in the HCP regions over the FCC regions, as found by density functional theory calculations. Angle-resolved photoemission spectroscopy measurements demonstrate that the preferential functionalization of just one region of the moiré supercell results in a band gap opening with very limited associated band broadening. Thus, hydrogenation at elevated sample temperatures provides a pathway to efficient band gap engineering in graphene <i>via</i> the selective functionalization of specific regions of the moiré structure