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

Abstract

In this study, we have systematically characterized and reversibly manipulated the subsurface hydrogen (H sub ) on rutile TiO 2 ( 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.

Single hydrogen atom manipulation for reversible deprotonation of water on a rutile TiO2 (110) surface

“…The two hydrogen atoms form a water configuration including O ad 2− and the adjacent O s atoms. The characteristics of a net positively charged hydrogen atom were previously well accepted by various experimental techniques 1 – 23 , 26 , 30 , 34 , 44 . Two black spots that correspond to bistable O s H defects can be observed in Fig.…”

“…Finally, we demonstrate the capability of negative bias KPFS for the lateral manipulation of O s H defect on a rutile TiO 2 surface using an individual single O s H defect, because this O s H defect is also the most fundamental atomic feature on this surface 1 and known to provide a critical role in photocatalysis at tremendous condition 1 – 23 . Figure 6(a) shows an atomically resolved AFM image of a rutile TiO 2 (110) surface partially exposed to oxygen at room temperature obtained using a hole mode tip 9 , 30 , 34 .…”

“…The hydrogen atom detection on a rutile TiO 2 surface is an important topic owing to its intriguing chemical and physical properties related to atomistic water splitting and hydroxyl production on this surface 1 – 8 . Detail understanding of this adsorbate and its control at atomistic level is essential for fully elucidating the nature of a deprotonated water reaction on a TiO 2 surface 1 – 23 . Moreover, oxidation of hydrogen atoms by oxygen molecule on the rutile TiO 2 surface results in the reactive oxygen species (ROS), such as reaction intermediates of water species H 2 O, HO 2 , H 2 O 2 , and H 3 O 2 24 , 25 .…”

“…Especially on the metallic surface, controllable on-surface lateral manipulation of the single molecule 45 – 47 has been well established and the adsorbate can also be activated 48 by means of electric field 49 , 50 . Moreover, on the rutile TiO 2 surface, the vertical desorption of hydrogen atom 9 , 10 , 16 , 18 , 21 , reversible migration of hydrogen atom 23 , the stochastic motion of hydrogen atom induced by inelastic tunneling electrons 3 , manipulation of oxygen adatom 27 and lateral tip induced excitation of the single molecule 35 have been clarified. However, the lateral manipulation of the single hydrogen atom to the desired position on rutile TiO 2 surface has been poorly reported experimentally up to now 10 , although it could provide an efficient means of the arrangement of the on-surface water splitting reactions, dissociation dynamics, which critically control the efficiency of heterogeneous catalytic reactions on this surface 1 – 23 .…”

“…Moreover, on the rutile TiO 2 surface, the vertical desorption of hydrogen atom 9 , 10 , 16 , 18 , 21 , reversible migration of hydrogen atom 23 , the stochastic motion of hydrogen atom induced by inelastic tunneling electrons 3 , manipulation of oxygen adatom 27 and lateral tip induced excitation of the single molecule 35 have been clarified. However, the lateral manipulation of the single hydrogen atom to the desired position on rutile TiO 2 surface has been poorly reported experimentally up to now 10 , although it could provide an efficient means of the arrangement of the on-surface water splitting reactions, dissociation dynamics, which critically control the efficiency of heterogeneous catalytic reactions on this surface 1 – 23 . Especially, elucidating the interaction force between tip and water configuration on the TiO 2 surface is important for answering the current controversial question of whether a proton atom should remain as one water molecule or two distinct hydroxyls from the mechanical point of view 1 – 8 .…”