The interaction of molecular water with the GaP(110) surface has been studied under ultrahigh vacuum conditions with a combination of x-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD) methods. An adlayer of water at 1 ML coverage (referenced to the number of undercoordinated Ga surface atoms) desorbs from the GaP(110) surface over the temperature range of 250-370 K in TPD experiments. This water monolayer exhibited a c(2 × 2) surface structure, observed with both LEED and STM, consisting of alternating OH and H2O surface-bound species, as determined by XPS data. However, at water coverages below 1 ML, the surface concentrations of OH species were higher than that of molecular H2O, and in the 0-0.75 ML surface coverage range, the (OH:H2O) ratio was constant at 2:1. We also observed another stable surface adlayer at 4/3 ML water coverage that exhibited quasi-ordering with a c(3 × 2) LEED pattern. TPD spectra showed that this extra 1/3 ML desorbed near 210 K before the onset of the desorption of the 1 ML coverage.

Acetic acid adsorption and reactions at multiple surface coverage values on Ni(110) were studied with temperature-programmed desorption (TPD) and infrared reflection absorption spectroscopy (IRAS) at 90-500 K. The experimental measurements were interpreted with density functional theory (DFT) calculations that provided information on adsorbate geometries, energies, and vibrational modes. Below the monolayer saturation coverage of 0.36 ML at 90 K, acetic acid adsorbs mostly molecularly. Above this coverage, a physisorbed layer is formed with dimers and catemers, without detectable monomers. Dimers and catemers desorb as molecular acetic acid at 157 and 172 K, respectively. Between 90 and 200 K, the O-H bond in acetic acid breaks to form bridge-bonded bidentate acetate that becomes the dominant surface species. Desorption-limited hydrogen evolution is observed at 265 K. However, even after the acetate formation, acetic acid desorbs molecularly at 200-300 K due to recombination. Minor surface species observed at 200 K, acetyls or acetates with a carbonyl group, decompose below 350 K and generate adsorbed carbon monoxide. At 350 K, the surface likely undergoes restructuring, the extent of which increases with acetic acid coverage. The initial dominant bridge-bonded bidentate acetate species formed below 200 K remain on the surface, but they now mostly adsorb on the restructured sites. The acetates and all other remaining hydrocarbon species decompose simultaneously at 425 K in a narrow temperature range with concurrent evolution of hydrogen, carbon monoxide, and carbon dioxide. Above 425 K, only carbon remains on the surface.

Scanning electrochemical microscopy (SECM) has previously been employed in probing photoelectrochemical processes at semiconductor surfaces. However, the spatial resolution of these studies has not yet matched the nanoscale SECM resolution attained without substrate illumination. Herein, we introduce nanoscale photo-SECM with a glass-sealed, polished tip simultaneously serving as a nanoelectrode and a light guide to produce a microscopic light spot on the substrate surface. The advantages of this approach are demonstrated by comparing current transients obtained using through-tip and global illumination of the sample. The spot of light on the substrate surface facing the nanotip was sufficiently bright to measure the diffusion-controlled positive feedback current in good agreement with the theory. We employed this approach for high-resolution photoelectrochemical mapping of ferrocenemethanol oxidation and oxygen evolution reactions at the Nb:TiO2 rutile (110) single crystal surface. The images obtained using 40-50 nm radius tips showed only minor and random variations in photoelectrochemical reactivity for both processes, pointing to essentially uniform distribution of the Nb dopant over the TiO2 surface and no measurable segregation on the ∼50 nm scale.

The diffusion of noble gas argon in the near-surface region of rutile TiO2 has been explored with the Temperature Programmed Out-Diffusion (TPOD) method. The Ar atoms were deposited in a several-nanometer-deep layer of the single-crystal TiO2(110) surface by bombardment with 1-5 keV energy Ar ions. Subsequently, in the TPOD experiments this sample was heated at a linear rate and the out-diffusion of argon was monitored with a mass-spectrometer. Surface conditions were probed with Auger Electron Spectroscopy (AES) and Low-Energy Electron Diffraction (LEED). The experimental results were analyzed with the aid of numerical simulations. The measurements showed a dependence of the Ar diffusion rates on the concentration of buried argon and composition of the surface layer. The kinetic parameters of Ar diffusion in pristine rutile TiO2 were estimated as E-a = 104 kJ/mol and D-0 = 6 . 10(-9) m(2)/s. A distinctive diffusion regime, related to the rock salt TiO phase formation, were identified.

Guaiacol (2-methoxyphenol, C6H4(OH)-(OCH3)) adsorption and reactions on a Pt(100) surface were studied with infrared reflection-absorption spectroscopy (IRAS) and temperature programmed desorption (TPD) measurements at different surface coverage values from 100 to 800 K. In addition, density functional theory (DFT) calculations were used to determine geometries, adsorption energies, and vibrational frequencies for adsorption structures. Depending on surface coverage, guaiacol formed one or two physisorbed states. At low coverage, a single state with a desorption peak at 225 K was observed. At high coverage, two physisorbed states were observed with desorption peaks at 195 and 225 K. At temperatures above 225 K, after the desorption of physisorbed layers, a dissociatively adsorbed structure, C6H4O(OCH3) + H, was observed. Recombinative molecular guaiacol desorption was detected at 320 K. The dissociatively adsorbed structure was stable up to 337 K when C-O bonds began to break. Molecularly adsorbed guaiacol in horizontal (flat-lying) configurations bound through its benzene ring was not observed under all tested conditions. Similarities of vibrational spectra and desorption measurements for a Pt(100) surface in this study and a Pt(111) surface reported previously demonstrate that the obtained results are generally valid for low index Pt crystal planes and, more importantly, for catalytic Pt nanoparticles.

Scanning tunneling microscopy (STM) has been used to obtain the aerial distribution of bridge-bonded hydroxyl groups (HOb) on a ruffle TiO2(110) surface, modified with a well-defined nanoscale strain field. Our study makes use of earlier findings that 5-30 nm wide locally strained areas of the surface can be formed via low-energy Arion bombardment combined with a thermal treatment. These strained areas appear as protrusions in the STM images, resulting from subsurface argon-filled cavities. our STM images show that the local surface concentration of OHb groups is lower on the protrusions. This lowering of concentration has been interpreted as a reduction in the local H absorption. energy, Delta E, a result similar to that observed on metals. In this paper, analysis of the reduction in this O-H bond energy-across the surface shows a strong correlation between Delta(EOH) and the characteristic surface strain Value, S. The Delta E-OH values have been calculated through a subtraction of the contribution of the repulsive dipole dipole interaction between OHb groups. This interaction has been estimated from an analysis of the radial distribution of OHb pairs in the STM images. The measured linear relation between the reduction in O-H bond energy and the surface strain has been estimated to be Delta E-OH (meV)approximate to 11.S (%).

An atomically resolved study of an adsorbate photoreaction on the surface of an isolated TiO2 nanocrystal is reported. The crystal is grown in situ on Au(111) in an ultrahighvacuum chamber. The experiments use scanning tunneling microscopy (STM) backed by temperature-programmed desorption (TPD) to determine the surface coverage of trimethylacetic acid (TMAA) on the nano crystal surfaces before and after irradiation with monochromated 305 nm UV light. A detailed determination of the surface structure of the 1-3 nm thick and 10-30 nm wide nanocrystals is presented and the importance of moire effects in controlling the reaction sites shown. The normalized TMAA photodesorption quantum efficiency from Au-supported TiO2 nanocrystals was found to be 4 times lower compared to the same reaction on rutile TiO2(110) surface, a result consistent with the lower fraction of light adsorbed in the nanocrystals.

The application of elastic lattice strain is a promising approach for tuning material properties, but the attainment of a systematic approach for introducing a high level of strain in materials so as to study its effects has been a major challenge. Here we create an array of intense locally varying strain fields on a TiO2 (110) surface by introducing highly pressurized argon nanoclusters at 6-20 monolayers under the surface. By combining scanning tunneling microscopy imaging and the continuum mechanics model, we show that strain causes the surface bridge-bonded oxygen vacancies (BBOv), which are typically present on this surface, to be absent from the strained area and generates defect-free regions. In addition, we find that the adsorption energy of hydrogen binding to oxygen (BBO) is significantly altered by local lattice strain. In particular, the adsorption energy of hydrogen on BBO rows is reduced by similar to 35 meV when the local crystal lattice is compressed by similar to 1.3%. Our results provide direct evidence of the influence of strain on atomic-scale surface chemical properties, and such effects may help guide future research in catalysis materials design.

A surface moire pattern is used to examine the site-dependent variation in the surface reactivity of an organic molecule on an epitaxial-layered system. Our experiment examines chemisorption of trimethyl acetic acid (TMAA) molecules on epitaxial monolayers of TiO on Au(111). This layered system is shown to exhibit a 6-fold pinwheel moire pattern, such that each pinwheel consists of six interlocking triangles (with both a clockwise and counterclockwise orientation) around a central hub. The unit cell for this pinwheel moire pattern is measured to be 28 angstrom x 28 angstrom. The evolution of the TMAA chemisorption geometry on these patterned layers as a function of coverage is investigated by atomic-resolution scanning-tunneling-microscopy (STM) imaging. TMAA is preferentially chemisorbed, most probably to form trimethyl acetate (TMA), at the center of the fixed pinwheels at low coverage (similar to 0.02 ML). This chemisorption pattern evolves into a 3-fold symmetric triad at the center of each pinwheel as the coverage of TMA increases to similar to 0.06 ML. Finally at saturation coverage, the chemisorbed TMA groups leave an empty region in the central region of each triangle even, indicating a significant difference in adsorption energy between adsorption at the hub of the pinwheel and at the center of each triangle. These results show the utility of moire patterns in understanding the role of substrate-overlayer atomic registry to probe surface reactions.

We report an experimental and theoretical investigation of the decomposition (partial oxidation) of deuterated methanol (CD3OD) on a single-crystal Fe3O4(111) surface. The crystal surface contains majority areas of a Fe-terminated Fe3O4(111) surface as well as smaller regions of O-terminated FeO(111) or biphase surface reconstruction. Our investigation uses a combination of scanning tunneling microscopy, temperature-programmed desorption, and density functional theory calculations to examine the surface reactions and adsorbates as a function of coverage. Our studies show that the reaction of methanol on this iron–oxide surface is highly sensitive to atomic-level surface reconstructions.

Arrays of highly strained 5-25 nm-wide regions have been prepared on rutile TiO2(110) surface through a low energy Ar ion bombardment technique. Using scanning tunneling microscopy (STM) and an innovative STM tip-triggered nanoexplosion approach we show experimentally that the protrusions arise from subsurface Ar-filled pockets. Continuum mechanics modeling gives good estimates of the corresponding elastic deformation. Surface strain values of up to 4% have been deduced.

We have studied the adsorption and tip-induced chemistry of 2-chloroanthracene on TiO2(110). STM images show that at 135 K and low coverage, i.e., similar to 0.1 ML, these molecules are physisorbed along the five-coordinated titanium rows on the rutile(110) surface as a result of electrostatic interaction. Applying electric pulses >2.5 V from the STM tip to individual molecules causes either desorption or dissociation of the molecules, as indicated by the changes in the STM images. We have observed dissociative electron capture of a single 2-chloroanthracene molecule, which leaves behind a surface chlorine atom adsorbed in the on-top configuration on a surface Ti atom. The threshold energy required for the dissociation was found to be similar to 2.7 eV.

The reactivity of 2-propanol with TiO2 nanocrystals supported on Au(111) was studied by temperature-programmed desorption (TPD) and scanning tunneling microscopy (STM). The nanocrystals, which are grown through oxidation of a TiAu surface alloy, had an average height of 1nm and width of 15nm with a dominantly hexagonal morphology. The desorption of propanol and propanol-derived products from the TiO2 nanocrystal surfaces was observed in the 270–570K temperature range and could be distinguished from desorption of propanol from the Au(111) surface below 270K. With increasing propanol coverage, the TiO2-related TPD peaks were occupied before the appearance of any Au(111)-related peaks. Our calculations showed that the TiO2 nanocrystals were saturated at 0.4ML of local propanol surface concentration, where 1ML≡5.2×1014cm−2 refers to surface density of five-coordinated Ti atoms on rutile(110). Our TPD measurements showed that 61% of this adsorbed propanol desorbed molecularly at 310K, while 23% dehydrated into propene and 6% dehydrogenated to form acetone, both products desorbing in the 370–570K temperature range. The desorption temperatures of products from supported TiO2 nanocrystals were shown to depend strongly on the morphology of the nanocrystals.

Adsorption of anthracene and 4-bromobiphenyl on a TiO2 rutile(110) surface has been studied under ultrahigh vacuum conditions with scanning tunneling microscopy (STM) and thermal programmed desorption (TPD). Multilayer and single-layer desorption peaks were observed at 270 and 360 K for anthracene and at 260 and 360 K for bromobiphenyl, respectively. STM images taken at room temperature show that both molecules form periodic patterns on the TiO2 surface with individual molecules aligned along the 5-coordinated Ti atom row of the substrate. Individual molecules of the organic adsorbates are mobile on the surface in the [001] direction parallel to the atomic rows. The observed lateral alignment of the molecules suggests attractive interactions in the case of anthracene and repulsive in the case of bromobiphenyl.

We report on a new surface phase of the Co-vicinal-Cu(111) system which exhibits self-assembled uniform Co quantum wires that are stable at 300K. STM-imaging measurements show that wires will self-assemble within a narrow range of Co coverage and, within this range, the wires increase in length as coverage is increased. The STM images show that the wires form along the leading edge of the step rise, differentiating it from previously theoretically predicted atomic-wire phases. The formation of relatively long laterally un-encapsulated one- and two-atom wires also differentiates it from past experimentally observed step-island formation. Furthermore, our experiments also show directly that the Co wires coexist with another Co phase that had been previously predicted for growth on Cu(111). Our observations allow us to comment on the formation kinetics of the atomic-wire phase and on the fit of our data to a recently developed lattice-gas model.