Igor Lyubinetsky

Intrinsic Diffusion of Hydrogen on Rutile TiO2(110)

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.

Formation of O adatom pairs and charge transfer upon O2 dissociation on reduced TiO2(110)

Scanning tunneling microscopy and density functional theory have been used to investigate the details of O(2) dissociation leading to the formation of oxygen adatom (O(a)) pairs at terminal Ti sites. An intermediate, metastable O(a)-O(a) configuration with two nearest-neighbor O atoms is observed after O(2) dissociation at 300 K. The nearest-neighbor O(a) pairs are destabilized by Coulomb repulsion of charged O(a)s and separate further along the Ti row into energetically more favorable second-nearest neighbor configuration. The potential energy profile calculated for O(2) dissociation on Ti rows and following O(a)s separation strongly supports the experimental observations. Furthermore, our results suggest that the itinerant electrons associated with the O vacancies (V(O)) are being utilized in the O(2) dissociation process at the Ti row. Experimentally this is supported by the observation that not all V(O)s can be healed by O(2) exposure at 300 K, as some V(O)s becoming less reactive due to supplying certain charge to O(a)s. Further, theoretical results show that at least two oxygen vacancies per O(2) molecule are required in order for the O(2) dissociation at the Ti row to become viable.

FORMATION OF SINGLE-PHASE OXIDE NANOCLUSTERS: CU2O ON SRTIO3(100)

Selective formation of the single-phase nanoclusters of Cu2O on SrTiO3(100) substrates in the size range of 10–50 nm is found to occur only in a very narrow oxygen plasma-assisted molecular-beam epitaxy growth parameter window, in comparison with the bulk phase diagram (for oxygen pressure versus temperature). There are distinctive parameter regions, where multiple phaselike forms coexist (CuO/Cu2O and Cu2O/Cu), in agreement with theoretical prediction for small systems, and as opposite to the sharp phase boundaries for the bulk. Observed changes in the nanocluster composition are found to correlate with differences in cluster morphologies.

Epitaxial growth and microstructure of Cu2O nanoparticle/thin films on SrTiO3(100)

Cuprous oxide (Cu2O) was grown on SrTiO3 (STO)(100) by oxygen plasma-assisted molecular-beam epitaxy. The microstructure of the grown layer and the Cu valence state were analysed using x-ray diffraction (XRD), x-ray photo-electron spectroscopy (XPS), atomic force microscopy (AFM) and cross-sectional transmission electron microscopy (TEM) as well as electron diffraction. The grown layer was dominated by the Cu2O phase, possessing an epitaxial orientation of and with respect to the substrate. The morphology of the Cu2O film shows a dependence on the growth rate. Typically, fast growth will lead to the formation of a thin film with a relatively smooth surface. Slow growth will lead to the development of nanoparticles, featuring the formation of Cu2O pyramids. The pyramids are invariantly defined by the Cu2O{111} planes. Given the fact that the {111} planes correspond to the lowest surface energy of Cu2O, slow growth will give the system enough time to allow it to adopt the pyramid configuration by which the overall energy of the system is minimized.

Reproducible tip fabrication and cleaning for UHV STM

Several technical modifications related to the fabrication and ultra-high vacuum (UHV) treatments of the scanning tunneling microscope (STM) tips have been implemented to improve a reliability of the tip preparation for high-resolution STM. Widely used electrochemical etching drop-off technique has been further refined to enable a reproducible fabrication of the tips with a radius <or= 3 nm. For tip cleaning by a controllable UHV annealing, simple and flexible setup has been developed. Proper W tip preparation has been demonstrated via an imaging of the TiO2 (110) surface atomic structure.

Focused-ion-beam directed self-assembly of Cu2O islands on SrTiO3(100)

Nanoscale islands of Cu2O have been synthesized on single-crystal SrTiO3 (100) substrates using oxygen plasma-assisted molecular-beam epitaxy (MBE). Island growth location has been controlled by using an ex situ Ga+ focused ion beam (FIB) to modify the growth surface in discrete locations prior to island synthesis. The FIB modifications have generated surface topography with lateral dimensions of 150–200nm. Ex situ atomic force microscopy study after island growth reveals that certain FIB substrate modification and MBE growth condition combinations lead to directed self-assembly of metal oxide islands at the edges of the FIB modified zones.

Experimental Persistence Probability for Fluctuating Steps

The persistence behavior for fluctuating steps on the Si(111)-(sqrt[3]xsqrt[3])R30 degrees -Al surface was determined by analyzing time-dependent STM images for temperatures between 770 and 970 K. Using the standard persistence definition, the measured persistence probability displays power-law decay with an exponent of theta=0.77+/-0.03. This is consistent with the value of theta=3/4 predicted for attachment-detachment limited step kinetics. If the persistence analysis is carried out in terms of return to a fixed-reference position, the measured probability decays exponentially. Numerical studies of the Langevin equation used to model step motion corroborate the experimental observations.

Dimerization Induced Deprotonation of Water on RuO2(110).

RuO2 has proven to be indispensable as a co-catalyst in numerous systems designed for photocatalytic water splitting. In this study, we have carried out a detailed mechanistic study of water behavior on the most stable RuO2 face, RuO2(110), by employing variable-temperature scanning tunneling microscopy and density functional theory calculations. We show that water monomers adsorb molecularly on Ru sites, become mobile above 238 K, diffuse along the Ru rows, and form water dimers. The onset for dimer diffusion is observed at ∼277 K, indicating a significantly higher diffusion barrier than that for monomers. More importantly, we find that water dimers deprotonate readily to form Ru-bound H3O2 and bridging OH species. The observed behavior is compared and contrasted with that observed for water on isostructural rutile TiO2(110).

Adsorption states and mobility of trimethylacetic acid molecules on reduced TiO2(110) surface

Combined scanning tunneling microscopy (STM), X-rays photoelectron spectroscopy (XPS) and density functional theory (DFT) studies have probed the bonding configurations and mobility of trimethylacetic acid (TMAA) molecules on the TiO(2)(110) surface at RT. Upon TMAA dissociation through deprotonation, two distinctly different types of stable chemisorption configurations of the carboxylate group (TMA) have been identified according to their position and appearance in STM images. In configuration A, two carboxylate O atoms bond to two Ti(4+) cations, while in configuration B one O atom fills the bridging oxygen vacancy (V(O)) with the other O bounded at an adjacent regular Ti(4+) site. Calculated adsorption energies for the configurations A and B are comparable at 1.28 and 1.36 eV, respectively. DFT results also show that TMA may rotate at RT about its O atom that filled the V(O) (in configuration B), with a rotation barrier of approximately 0.65 eV. Both the observation of the constant initial sticking coefficient and preference for TMAA molecules to dissociate at selective sites indicate that TMAA adsorption is mediated by a mobile precursor state. Several possible molecular (physisorbed) states of TMAA have indeed been identified by DFT, all being highly mobile at RT. In contrast, the TMA diffusion in the chemisorbed (dissociative) state is a very slow with a calculated barrier of 1.09 eV for diffusion along the Ti row.

Probing equilibrium of molecular and deprotonated water on TiO2(110)

Significance Understanding how water binds and dissociates on surfaces has broad implications in a vast range of physical and chemical processes. The relative stability of molecularly and dissociatively bound water has been debated for decades on many oxide surfaces, but it has never been successfully measured. Our study describes unique instrumentation, direct measurements, and a state-of-the-art computation and theory approach that yield a detailed kinetic and dynamic description of water deprotonation equilibrium on TiO2(110), a prototypical surface commonly used in mechanistic studies of photocatalytic water splitting. This unique study demonstrates that the molecularly bound water on TiO2(110) is preferred over the surface-bound hydroxyls by only 0.035 eV. Understanding adsorbed water and its dissociation to surface hydroxyls on oxide surfaces is key to unraveling many physical and chemical processes, yet the barrier for its deprotonation has never been measured. In this study, we present direct evidence for water dissociation equilibrium on rutile-TiO2(110) by combining supersonic molecular beam, scanning tunneling microscopy (STM), and ab initio molecular dynamics. We measure the deprotonation/protonation barriers of 0.36 eV and find that molecularly bound water is preferred over the surface-bound hydroxyls by only 0.035 eV. We demonstrate that long-range electrostatic fields emanating from the oxide lead to steering and reorientation of the molecules approaching the surface, activating the O–H bonds and inducing deprotonation. The developed methodology for studying metastable reaction intermediates prepared with a high-energy molecular beam in the STM can be readily extended to other systems to clarify a wide range of important bond activation processes.