Intrinsic Diffusion of Hydrogen on Rutile TiO₂(110)

Abstract

The combined experimental and theoretical study of intrinsic hydrogen diffusion on bridge-bonded oxygen (BBO) rows of TiO 2(110) is presented. Sequences of isothermal scanning tunneling microscopy images demonstrate a complex behavior of hydrogen formed by water dissociation on BBO vacancies. Different diffusion rates are observed for the two hydrogens in the original geminate OH pair suggesting the presence of a long-lived polaronic state. For the case of separated hydroxyls, both theory and experiment yield comparable temperature-dependent diffusion rates. Density functional theory calculations show that there are two comparable low energy diffusion pathways for hydrogen motion along the BBO from one BBO to its neighbor, one by a direct hop and the other by an intermediate minimum at a terrace O. The values of kinetic parameters (prefactors and diffusion barriers) determined experimentally and theoretically are significantly different and indicate the presence of a more complex diffusion mechanism. We speculate that the hydrogen diffusion proceeds via a two-step mechanism: the initial diffusion of localized charge, followed by the diffusion of hydrogen. Both experiment and theory show the presence of repulsive OH-OH interactions.

An experimental setup combining a highly sensitive detector for reaction products with a mass-selected cluster source and a low-temperature STM for advanced nanocatalysis measurements

“…The O vac density estimated from Figure 4a is 5.4% monolayer (ML), where 1 ML is defined as the density of unit cells of the TiO 2 (110)-(1×1) surface, i.e., 5.2 × 10 14 cm −2 . This O vac concentration is comparable to those found in former studies for reduced crystals [41][42][43][44][45][46][47]. Pt + 1 has been deposited with a kinetic energy of 7 eV by means of the mass-selected cluster source on the clean TiO 2 (110)-(1×1) surface kept at RT.…”

“…The observed atomic contrast is well known to be dominated by electronic effects and the bright and dark lines correspond to the [001]-oriented rows of fivefold coordinated Ti and bridging O atoms, respectively [41]. A number of point defects are also seen, which are mostly O vac and some hydroxyls (OH), the latter resulting from the dissociation of residual water molecules at O vac sites [44][45][46]. Both defects are discerned by the apparent height of 0.3 ± 0.1Å (O vac ) and 0.6 ± 0.1Å (OH) with respect to the O rows.…”

Anomalous energy pathway of vacancy migration and self-diffusion in hcp Ti

“…The energy barrier or migration pathway is commonly thought to follow a bellshaped profile with a "transition state" structure at its peak: this structure possesses a single imaginary frequency in its vibrational spectrum. Energy profiles that deviate from a bell-shaped profile have been noted commonly for diffusion involving interstitial sites, 7,9,10 and hence the bell-shaped profile is not universal. However, such behavior has not been noted for diffusion in most of the pure elemental solids such as fcc Al, 1 bcc Mo, 4 and hcp Mg. 6 The only observation of the flat shape or even the slightly double-peaked shape profile for vacancy migration was for hcp Zr by Verite et al 11 from first-principles calculations.…”

“…[1][2][3][4][5][6][7][8][9][10] During diffusion, a migrating atom passes through an energy barrier in moving from a local energy minimum site to another equivalent nearest neighbor site. The energy barrier or migration pathway is commonly thought to follow a bellshaped profile with a "transition state" structure at its peak: this structure possesses a single imaginary frequency in its vibrational spectrum.…”

Fundamental Reactions on Rutile TiO2(110) Model Photocatalysts Studied by High-Resolution Scanning Tunneling Microscopy

“…[21]). Without the co-adsorbed water, the reaction may stop after the formation of the OH_O and OH_OH species, since thermally activated diffusion of the H adatoms is not feasible at $190 K [21,90]. In Figure 4.11, water species in the form of water dimers are denoted using the labels A n , B n , C n and D, respectively, where n indicates the estimated number of containing H atoms, that is, A n with n ¼ 4 denotes a stoichiometric water dimer.…”