Imaging of the Hydrogen Subsurface Site in Rutile ${\mathrm{TiO}}_2$

From an interplay between simultaneously recorded noncontact atomic force microscopy and scanning tunneling microscopy images and simulations based on density functional theory, we reveal the location of single hydrogen species in the surface and subsurface layers of rutile ${\mathrm{TiO}}_2$. Subsurface hydrogen atoms (${\mathrm{H}}_{\mathrm{sub}}$) are found to reside in a stable interstitial site as subsurface OH groups detectable in scanning tunneling microscopy as a characteristic electronic state but imperceptible to atomic force microscopy. The combined atomic force microscopy, scanning tunneling microscopy, and density functional theory study demonstrates a general scheme to reveal near surface defects and interstitials in poorly conducting materials.

Figure 1

(a),(c) nc-AFM topography images of the ${\mathrm{TiO}}_2(110)$ surface recorded with a time lapse of 4 min. $\mathrm{\text{Size}}=10\times 10{\mathrm{nm}}^2$, $\Delta f=-46.9\mathrm{Hz}$, $U_{\mathrm{bias}}=0.97\mathrm{V}$, and $A_{p-p}\sim 25\mathrm{nm}$. (b),(d) Simultaneously recorded tunneling current (STM) images ($I_t$) of (a) and (c), respectively. Circles: Bridging OH; squares: subsurface OH. (e) Overlapped line scans showing the AFM and STM corrugation before (blue) and after (red) a H atom was moved to the subsurface site.

Figure 2

(a),(c) Enlarged AFM ($Z$) images illustrating the surface before and after conversion of a sOH to a ${\mathrm{OH}}_{\mathrm{sub}}$. (b),(d) The corresponding enlarged STM ($I_t$) images. The superimposed grid illustrates the position of the bridging oxygen sublattice of ${\mathrm{TiO}}_2(110)$.

Figure 3

(a)–(c) $\mathrm{LDA}+U$ minimum energy path calculations showing the energy diagram for (a) the sOH hydrogen to move to subsurface site 3, (b) H rotation around a O(3c) site, and (c) H diffusion from site 3 to 4. (d) Ball model of the ${\mathrm{TiO}}_2(110)$ surface where we sketch the hydrogen path between the marked sites.

Figure 4

Simulated $I_t$ images with $U_{\mathrm{bias}}=0.5\mathrm{V}$ for ${\mathrm{TiO}}_2(110)$ with surface OH (a) and ${\mathrm{OH}}_{\mathrm{sub}}$ (b). The ${\mathrm{TiO}}_2(110)$ lattice is displayed beneath each image where the position of H is indicated by a blue cross. (c),(d) $I_t$ shows cross sections along the green line from (a) and (b) together with a side view of the structure. O: red, Ti: blue, and H: gray.