Characterization and Reversible Migration of Subsurface Hydrogen on Rutile TiO₂(110) by Atomic Force Microscopy at 78 K

In this study, we have systematically characterized and reversibly manipulated the subsurface hydrogen (H_sub) on rutile TiO₂(110)-(1 × 1) by a combination of noncontact atomic force microscopy, scanning tunneling microscopy, and Kelvin probe force microscopy at 78 K. Four different configurations of the H_sub, including the monomer, dimer, trimer, and tetramer, are clearly characterized and discriminated by simultaneous atomic force microscopy and scanning tunneling microscopy measurements. Specifically, by using Kelvin probe force microscopy, the local contact potential difference of the subsurface hydrogen is mapped with atomic resolution. In addition to multichannel characterization, we unprecedentedly demonstrate ability to reversibly migrate the H_sub between the subsurface and surface layers, which is electrically activated by switching the polarities of voltage pulse. The dominant underlying mechanism of the reversible migration process of the H_sub is tentatively explained by the inelastic electron tunneling effect and the local electric field in the tunneling junction. Our study opens up an unprecedented playground for the systematic investigation and deliberate manipulation of the subsurface defects, and may potentially revolutionize the investigation of the catalytic reactions based on transition metal oxides.