Evidence for the Predominance of Subsurface Defects on Reduced Anatase ${\mathrm{TiO}}_2(101)$

Scanning tunneling microscopy (STM) images taken on a freshly cleaved anatase ${\mathrm{TiO}}_2(101)$ sample show an almost perfect surface with very few subsurface impurities and adsorbates. Surface oxygen vacancies are not typically present but can be induced by electron bombardment. In contrast, a reduced anatase (101) crystal shows isolated as well as ordered intrinsic subsurface defects in STM, consistent with density functional theory (DFT) calculations which predict that O vacancies ($V_{\mathrm{O}}$’s) at subsurface and bulk sites are significantly more stable than on the surface.

Figure 1

Models of anatase ${\mathrm{TiO}}_2(101)$ in side (left) and top (right) view (red balls: O; light blue balls: Ti). The purple shading indicates the contrast of atomically-resolved STM images, ovals extending across undercoordinated surface ${\mathrm{O}}_{2c}$ and ${\mathrm{Ti}}_{5c}$ on undisturbed surface and circles above subsurface defects. (a) Various on- and subsurface O vacancies (left) and their corresponding formation energies (right); only part of the six-layer-thick slab used for the calculations is shown. (b), (c) Illustration of formation of ordered subsurface defects invoking Frenkel hops. Formation of a subsurface O vacancy (yellow) initiates the migration of a neighboring Ti atom to an interstitial site (${\mathrm{Ti}}_i$, black), leaving behind a Ti vacancy ($V_{\mathrm{Ti}}$, black square). This process is repeated along open channels parallel to the crystallographic (b) [010] and (c) $[1\overline{1}\overline{1}]$ directions.

Figure 2

STM images from a freshly cleaved anatase (101) surface. (a) Large-scale overview ($300\times 300{\AA }^2$; $V_{\mathrm{sample}}=+1.3\mathrm{V}$, $I_{\mathrm{tunnel}}=1.9\mathrm{nA}$). (b) High-resolution image ($70\times 70{\AA }^2$; $+1.6\mathrm{V}$, 1.6 nA). The dotted circle indicates a subsurface impurity, the arrow an adsorbate, probably water. (c) After irradiation with 300 eV electrons ($70\times 70{\AA }^2$; $+1.5\mathrm{V}$, 2 nA), which creates surface O vacancies (marked with arrows).

Figure 3

Comparison of XPS Ti $2p$ lines of a freshly cleaved (a) and a reduced (b) anatase (101) surface.

Figure 4

STM images of the reduced anatase (101) surface. (a) Large-scale overview ($300\times 300{\AA }^2$; $+1.2\mathrm{V}$, 0.9 nA). The dark spots are attributed to subsurface defects. As shown in the inset, they exhibit tendency to line up along [010], $[1\overline{1}\overline{1}]$, and $[11\overline{1}]$ directions. (b) High-resolution image ($100\times 100{\AA }^2$; $+1.2\mathrm{V}$, 0.1 nA) with subsurface defects. Marked with colored circles are half ovals that are darker (green), same brightness (red), or brighter (blue) than regular ovals; red dashes mark extra-bright ovals. Features marked by dotted ellipses and black squares are tentatively assigned to adsorbates and surface impurities, respectively.