Electronic structure of point defects in controlled self-doping of theTiO2(110) surface: Combined photoemission spectroscopy and density functional theory study

Abstract

Point defects in metal oxides such as TiO 2 are key to their applications in numerous technologies. The investigation of thermally induced nonstoichiometry in TiO 2 is complicated by the difficulties in preparing and determining a desired degree of nonstoichiometry. We study controlled self-doping of TiO 2 by adsorption of 1/8 and 1/16 monolayer Ti at the ͑110͒ surface using a combination of experimental and computational approaches to unravel the details of the adsorption process and the oxidation state of Ti. Upon adsorption of Ti, x-ray and ultraviolet photoemission spectroscopy ͑XPS and UPS͒ show formation of reduced Ti. Comparison of pure density functional theory ͑DFT͒ with experiment shows that pure DFT provides an inconsistent description of the electronic structure. To surmount this difficulty, we apply DFT corrected for on-site Coulomb interaction ͑DFT+ U͒ to describe reduced Ti ions. The optimal value of U is 3 eV, determined from comparison of the computed Ti 3d electronic density of states with the UPS data. DFT+ U and UPS show the appearance of a Ti 3d adsorbate-induced state at 1.3 eV above the valence band and 1.0 eV below the conduction band. The computations show that the adsorbed Ti atom is oxidized to Ti 2+ and a fivefold coordinated surface Ti atom is reduced to Ti 3+ , while the remaining electron is distributed among other surface Ti atoms. The UPS data are best fitted with reduced Ti 2+ and Ti 3+ ions. These results demonstrate that the complexity of doped metal oxides is best understood with a combination of experiment and appropriate computations.

Charge compensation in trivalent cation doped bulk rutile TiO2

“…DFT corrected for on-site Coulomb interactions, DFT+U [48][49][50], in which a Hubbard U term is added to the DFT energy expression to describe localised electronic states, has been widely used to provide a reasonable and consistent description of the systems describe above [36][37][38][39][40][41][42][43][44][45][46][47]. However, there are many issues with DFT+U, not least of which is the empirical nature of the U parameter and the dependence of material properties on the value of U [51].…”

Surface and interstitial Ti diffusion at the rutile TiO2(110) surface

“…We studied self-doped titania films, characterising the electronic properties of surface adsorbed Ti. 20 W ef o u n dt h a ta na p p r o a c hs u c ha sD F T+U is indeed needed to interpret the experimental results and consistently describe the Ti 3+ ions present in this system. The best agreement with the spectroscopy of the gap states induced by the adsorbed adatom is obtained with U = 3 eV.…”

“…These latter calculations share important similarities with B3LYP results 22 which also localise the Ti 3d states. In this paper we employ experimentally benchmarked calculations of static structures 20 to consider the dynamical aspects of Ti adatom and interstitial mobility in the rutile (110) surface. We find key barriers and transition pathways in the surface and subsurface regions and explore their electronic structure.…”

“…20,21 To investigate the diffusion pathways between the various adatom and interstitial sites, of which there are many possibilities, we first employ atomistic calculations using the charge equilibration (QEq) methodology. 17,[23][24][25][26][27][28][29][30][31] In order to provide a realistic set of minimum energy pathways (MEP) for full investigation with DFT + U, we consider the effectiveness of the QEq approach in describing the energy landscape of the defects, and find that some modification to the original model is required.…”

TinO2n−1Magnéli phases studied using density functional theory

“…Despite the existence of many studies about defects in rutile TiO 2 , both experimental [20][21][22][23][24][25][26][27][28] and theoretical [21,25], there are few works on the intrinsically defective Magnéli phases [29][30][31][32][33][34][35]. Electronic structure calculations have been used to understand mainly the Ti 2 O 3 [32] and Ti 4 O 7 [29][30][31].…”