Subsurface Charge Repulsion of Adsorbed H-Adatoms on TiO₂(110)

Abstract

We have used noncontact atomic force microscopy (NC-AFM) and Kelvin probe force microscopy (KPFM) to directly visualize the presence of charged subsurface impurities on rutile TiO2(110). The subsurface charges add an additional electrostatic force between the sample and tip so that they appear as hillocks in the NC-AFM topography. Analysis of several subsurface defects in the same NC-AFM image reveals that the hillocks have discrete heights, which means that defects at different subsurface levels can be detected and distinguished. H adatoms, which are positively charged at the TiO2(110) surface, were found to be repelled by the buried positive charge, so that they form a ∼80–120 Å wide hydrogen-free zone around the charge. Thus, there is an opportunity to deliberately add dopants in order to exclude or perhaps even to confine certain adsorbates to a local region at the surface.

Atomic Force Microscopy for Imaging, Identification and Manipulation of Single Atoms

“…In the image of TiO 2 surface with adsorbed H atoms ( Fig. 1(c)), individual H atoms that are adsorbed on O atoms are imaged [8][9][10]. In the image of a single PTCDA molecule, chemical structure inside the molecule can be resolved by Pauli repulsive force even at room temperature [11].…”

Characterization of individual molecular adsorption geometries by atomic force microscopy: Cu-TCPP on rutile TiO2 (110)

“…31,32 In some regions, we could not detect defects at all, which is due to the influence of charged subsurface defects. 33 The ratio of the two aforementioned defects changed with time. At room temperature, residual water and hydrogen adsorbed continuously causing the accumulation of hydrogen adatoms on the surface.…”

Anticorrelation between Surface and Subsurface Point Defects and the Impact on the Redox Chemistry of TiO2(110)

“…[22][23][24] In particular, a previous STM study has demonstrated that adsorption of Cl 2 molecules on TiO 2 (110) is suppressed in the vicinity of positively charged subsurface impurities. [22][23][24] In particular, a previous STM study has demonstrated that adsorption of Cl 2 molecules on TiO 2 (110) is suppressed in the vicinity of positively charged subsurface impurities.…”

“…Currently, there are just a few reports available in the literature that concern the potential influence of extrinsic subsurface defects on the chemistry of well-characterized TiO 2 surfaces. [22][23][24] In particular, a previous STM study has demonstrated that adsorption of Cl 2 molecules on TiO 2 (110) is suppressed in the vicinity of positively charged subsurface impurities. [22] This observation has been attributed to a local increase in the effective electron affinity around the impurity (though no corresponding computational work has been carried out).…”

“…Specifically, the formation of O vacancies is found to be markedly inhibited around certain, positively charged subsurface defects. [21,23,24] It has been argued in the literature that an appearance of such features is caused by individual charged subsurface point defects, [21] in the same, well-established way as for conventional semiconductors (i.e., Si, GaAs). This is attributed not only to the dearth of V O sites around them, but also to the lack of other charge-donating defects in the nearest subsurface layers, which effectively constrains their direct effect on the redox chemistry at the reduced TiO 2 (110) surface.…”