Assigning the (1x2) surface reconstruction on reduced rutile by first-principles energetics

The first systematic survey of the surface structure of reduced rutile TiO2-x by first-principles calculations is reported. Motivated by the reconstructions observed in experiment, rows of ions are considered on the (110) surface so as to satisfy (1x2) periodicity. The surface is reduced by the removal of one O anion per (1x2) cell and over 100 reconstructed surfaces are computed with cations at both bulk and interstitial sites. These spin-polarized density-functional calculations reveal that the most stable reconstruction is the added row +Ti2O3 structure of Onishi and Iwasawa [Surf. Sci. 313, L783 (1994)], at a surface energy of 3.29+/-0.08 J m(-2). We describe this as a twinned dislocation, stabilized by relaxation of the cations away from the surface. All other reconstructions are computed to be at least 0.3 J m(-2) less stable; many of these feature rutilelike Ti-O half ribbons and rocksaltlike dislocations. Some of these may account for less stable (1x2) surface phases.