Single Crystal Oxide Surfaces Research Articles

Premodified Surface Method to Obtain Ultra-Highly Dispersed Metals and their 3D Structure Control on an Oxide Single-Crystal Surface.

Precise control of the three-dimensional (3D) structure of highly dispersed metal species such as metal complexes and clusters attached to an oxide surface has been important for the development of next-generation high-performance heterogeneous catalysts. However, this is not easily achieved for the following reasons. (1) Metal species are easily aggregated on an oxide surface, which makes it difficult to control their size and orientation definitely. (2) Determination of the 3D structure of the metal species on an oxide powder surface is hardly possible. To overcome these difficulties, we have developed the premodified surface method, where prior to metal deposition, the oxide surface is premodified with a functional organic molecule that can strongly coordinate to a metal atom. This method has successfully provided a single metal dispersion on an oxide single-crystal surface with the 3D structure precisely determined by polarization-dependent total reflection fluorescence X-ray absorption fine structure (PTRF-XAFS). Here we describe our recent results on ultra-high dispersions of various metal atoms on TiO2 (110) surfaces premodified with mercapto compounds, and show the possibility of fine tuning and orientation control of the surface metal 3D structures.

Ultra-high Dispersion of Metals on an Oxide Single-crystal Surface Premodified with a Functional Organic Molecule and Their 3D Structure Analysis by PTRF-XAFS Technique

Ultra-high Dispersion of Metals on an Oxide Single-crystal Surface Premodified with a Functional Organic Molecule and Their 3D Structure Analysis by PTRF-XAFS Technique

Formic Acid Dissociative Adsorption on NiO(111): Energetics and Structure of Adsorbed Formate

The dissociative adsorption of carboxylic acids on oxide surfaces is important for understanding adsorbed carboxylates, which are important as intermediates in catalytic reactions, for the organo-functionalization of oxide surfaces, and in many other aspects of oxide surface chemistry. We present here the first direct experimental measurement of the heat of dissociative adsorption of any carboxylic acid on any single-crystal oxide surface. The enthalpy of the dissociative adsorption of formic acid, the simplest carboxylic acid, to produce adsorbed formate and hydrogen (as a surface hydroxyl) on a (2 × 2)-NiO(111) surface is measured by single crystal adsorption calorimetry. The differential heat of adsorption decreases with formic acid coverage from 202 to 99 kJ/mol at saturation (0.25 ML). The structure of the adsorbed products is clarified by density functional theory (DFT) calculations, which provide energies in reasonable agreement with the calorimetry. These calculations show that formic acid readily...

Analysis of acid-base interactions at Al2O3 (11-20) interfaces by means of single molecule force spectroscopy

Single molecule force spectroscopy (SMFS) was employed to investigate the interaction forces between aliphatic amino, hydroxyl and ether groups and aluminum oxide single crystal surfaces in an aqueous electrolyte at pH=6. The force studies were based on the variation of the terminal group of polyethylene glycol which was bound via a SAu bond to the gold coated AFM tip. X-ray Photoelectron Spectroscopy (XPS) was performed to characterize the surface chemistry of the substrate. Force distance curves were measured between the PEG-NH2, OH and OCH3 functionalized atomic force microscope (AFM) tip and the non-polar single crystalline Al2O3(11-20) surface. The experimental results exhibit non-equilibrium desorption events which hint at acid-base interactions of the electron donating hydroxyl and amino groups with Al-ions in the surface of the oxide. The observed desorption forces for the NH2, OH/Al2O3(11-20) were in the range of 100–200pN.

Oxide Single-Crystal Surfaces: A Playground for Self-assembled Oxide Nanostructures

The different (structural and chemical) properties of oxide single-crystal surfaces that can be exploited for the growth of self-assembled oxide nanostructures are briefly reviewed. A large variety of nanostructures can be obtained, controlled by surface and interface structure and chemistry, which play a predominant role in their formation mechanisms at this nanometer scale. It is reminded that surface atomic order, surface steps, chemical terminations or heteroepitaxial strain can be used to generate various nanostructures such as nanodots, nanowires, nanostripes, with controlled size, morphology and spatial ordering.

Ordered Fe(II)Ti(IV)O3 Mixed Monolayer Oxide on Rutile TiO2(011).

Oxide monolayers supported or intermixed with an oxide support are potential nanocatalysts whose properties are determined by the interplay with the support. For fundamental studies of monolayer oxides on metal oxide supports, well-defined systems are needed, but so far, the synthesis of monolayer oxides with long-range order on single-crystal oxide surfaces is rare. Here, we show by a combination of scanning tunneling microscopy, photoemission spectroscopy, and density functional theory (DFT)-based computational analysis that the rutile TiO2(011) surface supports the formation of an ordered mixed FeTiO3 monolayer. Deposition of iron in a slightly oxidizing atmosphere (10(-8) Torr O2) and annealing to 300 °C results in a well-ordered surface structure with Fe in a 2+ charge state and Ti in a 4+ charge states. Low-energy ion scattering suggests that the cation surface composition is close to half Fe and half Ti. This surface is stable in ultrahigh vacuum to annealing temperatures of 300 °C before the iron is reduced. DFT simulations confirm that a surface structure with coverage of 50% FeO units is stable and forms an ordered structure. Although distinct from known bulk phases of the iron-titanium oxide systems, the FeTiO3 monolayer exhibits some resemblance to the ilmenite structure, which may suggest that a variety of different mixed oxide phases (of systems that exist in a bulk ilmenite phase) may be synthesized in this way on the rutile TiO2(011) substrate.

Exploring Zeolite Chemistry with the Tools of Surface Science: Challenges, Opportunities, and Limitations

The complexity of catalysts that the surface science community has been able to address has increased substantially in a systematic manner, starting with metal and oxide single crystal surfaces and evolving to an atomistic description of clusters and nanoparticles on well-defined, planar supports. The next step in adding complexity is now to address surfaces of porous oxide materials, in particular of zeolites, which are the most extensively used catalysts in the industry. The recently reported successful fabrication of well-ordered thin films, consisting of planar arrangement of aluminosilicate polygonal prisms on a metal substrate counting with highly acidic bridging hydroxyl groups on the surface, represents the limiting case of infinitely large pore and cages in zeolites. This model system allows one to study reactions catalyzed by zeolites using the toolkit of surface science. In this Perspective, we describe the zeolitic model system, with its virtues and limitations, as well as the challenges, opportunities and expectations for the future in modelling porous catalysts by a surface science approach. .

Influence of Adsorption Site and Wavelength on the Photodesorption of NO from the (Fe,Cr)3O4(111) Mixed Oxide Surface

The photochemical properties of nitric oxide on a mixed oxide single crystal surface was examined in ultrahigh vacuum (UHV) using temperature programmed desorption (TPD), photon stimulated desorption (PSD) and low energy electron diffraction (LEED). The mixed oxide was a 75% Fe and 25% Cr corundum (0001) oxide film prepared on an α-Al2O3(0001) crystal, however its surface became terminated with a magnetite-like (111) structure after sputter/anneal cleaning, leading to a surface designated of (Fe,Cr)3O4(111). TPD of NO from the (Fe,Cr)3O4(111) surface revealed three chemisorbed states at 220, ~315 and 370 K assigned to NO binding at Fe3+, Cr3+ and Fe2+ sites, respectively. No significant thermal chemistry of NO was detected. NO photodesorption, the primary photochemical pathway in UHV, was sensitive to the adsorption site, with rates at the three adsorption sites following the trend: Fe3+ > Fe2+ > Cr3+. Multiexponential rate behavior seen in the overall NO PSD spectra was linked directly to site heterogeneity being manifested as a convolution of the individual NO photodesorption rates at the three types of surface sites. The photodesorption rate with UV light (365 nm) was ~10 times greater than that in the visible, but the per-photon rates across the visible spectrum (from 460 tomore » 630 nm) were independent of the wavelength, which is suggestive of localized photon absorption at the adsorption site. Results in this study demonstrate that the adsorption site plays a critical role in determining photochemical rates on complex oxide surfaces. This work was supported by the US Department of Energy, Office of Basic Energy Sciences, Division of Chemical Sciences, Geosciences & Biosciences. Pacific Northwest National Laboratory (PNNL) is a multi-program national laboratory operated for DOE by Battelle. The research was performed using EMSL, a national scientific user facility sponsored by the Department of Energy's Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory.« less

Modifying Oxide Single Crystal Surfaces to Study Photoinduced Electron Transfer

Single crystal oxide semiconductor electrodes provide an ideal experimental system for the elucidation of the models and parameters that control light absorption, charge separation, and transport at semiconductor/liquid interfaces. Such fundamental investigations can prove very valuable in the understanding of dye sensitized nanocrystalline solar cells. The single crystal experimental system is much less complex than its nanocrystalline counterpart and the study of well-defined crystallographic surfaces allows for more definitive conclusions to be made concerning the relation between sensitizer attachment and surface structure and the sensitizer’s performance. I will report recent work on the sensitization of low index faces of both anatase and rutile forms of TiO2 and ZnO single crystals with dyes, quantum dots and polymers. We employ atomic force microscopy (AFM) to verify that atomically flat terraced oxide surfaces are prepared and to elucidate the structure of the sensitizers adsorbed on these surfaces. Photocurrent spectroscopy and attenuated reflection spectroscopy are used to correlate the photocurrent response with the optical absorption of the sensitizing species and the relative electron injection efficiencies of for instance dye monomers and aggregates. We have also recently begun to grow homoepitaxial layers on both anatase and rutile using atomic layer deposition (ALD). These layers can have superior electronic properties compared to the substrate crystal and allows us to control the doping density in the near surface region. The primary focus of this talk will be on CdSe and PbS quantum dots as sensitizers. PbS quantum dots adsorbed on anatase (001) surfaces irradiated with light of >2.5 times their band gap have been shown to produce more than one injected electron per incident photon (1), a process known as carrier multiplication or multiple exciton generation (MEG). Harnessing MEG may result in more efficient solar conversion devices.1. Justin B. Sambur, Thomas Novet and B. A. Parkinson, “Multiple Exciton Collection in a Sensitized Photovoltaic System”, Science, 330, 63-66, (2010)

Impact of a Mixed Oxide’s Surface Composition and Structure on Its Adsorptive Properties: Case of the (Fe,Cr)3O4(111) Termination of the α-(Fe,Cr)2O3(0001) Surface

Characterization of an α-(Fe0.75,Cr0.25)2O3(0001) mixed oxide single crystal surface was conducted using X-ray photoelectron spectroscopy (XPS), secondary ion mass spectrometry (SIMS), low-energy electron diffraction (LEED), and temperature-programmed desorption (TPD). After sputter/anneal cleaning in ultrahigh vacuum (UHV), the mixed oxide surface became terminated with a magnetite-like (111) structure based on the presence of (2 × 2) spots in LEED and Fe2+ in XPS. The composition of the surface was close to that of M3O4 based on XPS, with the metal (M) content ratio of Fe2+/3+ and Cr3+ being close to 1.4:1, despite the film’s bulk being prepared with a 3:1 ratio of these cations. Enrichment of the surface with Cr was not altered by high temperature oxidation in UHV but could be partially reversed by exposure to air. Adsorption of various probe molecules (NO, O2, CO2, and H2O) was used in an effort to identify the active cation sites present on the (Fe,Cr)3O4(111)-terminated surface. Although both XPS an...

Carbon dioxide adsorption on a ZnO(101̄0) substrate studied by infrared reflection absorption spectroscopy

The adsorption of carbon dioxide on the mixed-terminated ZnO(101[combining macron]0) surface of a bulk single crystal was studied by UHV Infrared Reflection Absorption Spectroscopy (IRRAS). In contrast to metals, the classic surface selection rule for IRRAS does not apply to bulk oxide crystals, and hence vibrational bands can also be observed for s-polarized light. Although this fact substantially complicates data interpretation, a careful analysis allows for a direct determination of the adsorbate geometry. Here, we demonstrate the huge potential of IR-spectroscopy for investigations on oxide single crystal surfaces by considering all three components of the incident polarized light separately. We find that the tridentate (surface) carbonate is aligned along the [0001] direction. A comparison to data reported previously for CO2 adsorbed on the surfaces of ZnO nanoparticles provides important insight into the role of defects in the surface chemistry of powder particles.

Polarization-Dependent Total-Reflection Fluorescence X-ray Absorption Fine Structure for 3D Structural Determination and Surface Fine Tuning

This short review reports the three dimensional structure analysis of oxide-supported metal species and the origin of the metal-support interaction, using single crystal oxide surfaces and polarization-dependent total-reflection X-ray absorption fine structure (PTRF-XAFS) method. The PTRF-XAFS revealed that metal species interact strongly with oxygen anions of oxide surfaces and that surface pre-modification is a promising way to stabilize the surface monoatomic species of easily-aggregating Cu and Au and to prepare finely-tuned surface metal–organic species with regulated molecular structures and orientations.

The Energetics of Supported Metal Nanoparticles: Relationships to Sintering Rates and Catalytic Activity

Transition metal nanoparticles on the surfaces of oxide and carbon support materials form the basis for most solid catalysts and electrocatalysts, and have important industrial applications such as fuel production, fuels, and pollution prevention. In this Account, I review my laboratory group's research toward the basic understanding of the effects of particle size and support material on catalytic properties. I focus on studies of well-defined model metal nanoparticle catalysts supported on single-crystalline oxide surfaces. My group structurally characterized such catalysts using a variety of ultrahigh vacuum surface science techniques. We then measured the energies of metal atoms in these supported nanoparticles, using adsorption calorimetry tools that we developed. These metal adsorption energies increase with increasing size of the nanoparticles, until their diameter exceeds about 6 nm. Below 6 nm, the nature of the oxide support surface reaches also greatly affects the metal adsorption energies. Using both adsorption calorimetry and temperature programmed desorption (TPD), we measured the energy of adsorbed catalytic intermediates on metal nanoparticles supported on single crystal oxide surfaces, as a function of particle size. The studies reveal correlations between a number of characteristics. These include the size- and support-dependent energies of metal surface atoms in supported metal nanoparticles, their rates of sintering, how strongly they bind small adsorbates, and their catalytic activity. The data are consistent with the following model: the more weakly the surface metal atom is attached to the nanomaterial, the more strongly it binds small adsorbates. Its strength of attachment to the nanomaterial is dominated by the number of metal-metal bonds which bind it there, but also by the strength of metal/oxide interfacial bonding. This same combination of bond strengths controls sintering rates as well: the less stable a surface metal atom is in the nanomaterial, the greater is the thermodynamic driving force for it to sinter, and the faster is its sintering rate. These correlations provide key insights into how and why specific structural properties of catalyst nanomaterials dictate their catalytic properties. For example, they explain why supported Au catalysts must contain Au nanoparticles smaller than about 6 nm to have high activity for combustion and selective oxidation reactions. Only below about 6 nm are the Au atoms so weakly attached to the catalyst that they bind oxygen sufficiently strongly to enable the activation of O₂. By characterizing this interplay between industrially important rates (of net catalytic reactions, of elementary steps in the catalytic mechanism, and of sintering) and their thermodynamic driving forces, we can achieve a deeper fundamental understanding of supported metal nanoparticle catalysts. This understanding may facilitate development of better catalytic nanomaterials for clean, sustainable energy technologies.

酸化物単結晶表面の平坦化におけるWetプロセスの化学

酸化物単結晶表面の平坦化におけるWetプロセスの化学

Angle resolved total reflection fluorescence XAFS and its application to Au clusters on TiO2(110) (1 * 1)

The bonding features and three dimensional structures of metal clusters on single crystal oxide surfaces can be determined by the polarization dependent total reflection fluorescence XAFS (PTRF-XAFS) method. K-edge XAFS has a cos2 θ polarization-dependence to give successfully three dimensional structure while L3 edge XAFS has a weaker polarization dependence though the L3 edge is suitable for Au and Pt samples. In order to overcome this issue, we have developed an angle-resolved total reflection fluorescence XAFS (ARTRF-XAFS) method to determine more precise three dimensional structures of Au clusters using the L3 edge by increase the number of data points. In this paper we described the methodologies of the ARTRF-XAFS and the results of Au clusters on the TiO2(110) surface.

A new dual-purpose ultrahigh vacuum infrared spectroscopy apparatus optimized for grazing-incidence reflection as well as for transmission geometries

A newly designed ultrahigh vacuum (UHV) infrared spectroscopy apparatus dedicated to the spectroscopic characterization of oxides, singles crystals as well as powders, is described. It combines a state-of-the-art vacuum Fourier transform infrared (FTIR) spectrometer (Bruker, VERTEX 80v) with a novel UHV system (PREVAC) consisting of load-lock, distribution, measurement, and magazine chambers. The innovative design allows carrying out both reflection-absorption IR spectroscopy experiments at grazing incidence on well-defined oxide single crystal surfaces and FTIR transmission measurements for powder particles. A further unique feature of the apparatus is the entirely evacuated optical path to avoid background signals from gas phase H(2)O, CO(2), and other species, thus creating the possibility to record high-quality IR data with high sensitivity and stability, an essential prerequisite for monitoring molecular species adsorbed on oxide single-crystal surfaces. The unique performance of this new apparatus with regard to the spectroscopic characterization of adsorbates on oxide single crystals as well as on powder particles is demonstrated by case studies for two different materials, TiO(2) and ZnO.

High Resolution Electron Energy Loss Spectroscopy on Perfect and Defective Oxide Surfaces

High resolution electron energy loss spectroscopy (HREELS) is a powerful method for the study of vibrational and electronic excitations at solid surfaces and has been extensively applied to metal single crystal surfaces. As a result of experimental difficulties, unfortunately, much less information is available on adsorbate vibrations at oxide surfaces. This review focuses on recent results showing the successful application of HREELS to study adsorption and reaction of molecules on metal oxide single crystal surfaces. The chemical reactivity of perfect surfaces is first investigated systematically using HREELS combined with thermal desorption spectroscopy (TDS) and low energy electron diffraction (LEED). Furthermore, it is demonstrated that the interaction of adsorbates with surface defects (in particular oxygen vacancies) can also be monitored by vibrational spectroscopy.

Vanadium oxide surfaces and supported vanadium oxide nanoparticles

The information obtained from the characterization of vanadium oxide single crystal surfaces is related to the study of vanadia nanoparticles supported on silica and alumina thin films, model systems for the so-called “supported monolayer vanadia catalysts”. It is found that these particles have properties similar to V2O3 surfaces, where the topmost V ions are involved in vanadyl groups and have a 5+ oxidation state. A vibrational spectroscopy investigation combined with DFT calculations show that the accepted interpretation of vibrational spectra from vanadia catalysts must be revised.

Charge transfers at metal/oxide interfaces: a DFT study of formation of Kδ+and Auδ−species on MgO/Ag(100) ultra-thin films from deposition of neutral atoms

Ultra-thin oxide films grown on a metal substrate and of thickness smaller than 1 nm may exhibit unusual properties with respect to thicker films or single crystal oxide surfaces. In a previous study [G. Pacchioni, L. Giordano and M. Baistrocchi, Phys. Rev. Lett., 2005, 94, 226104] we have suggested that a Au atom adsorbed on a MgO/Mo(100) thin film becomes negatively charged by direct electron tunneling from the Mo metal and that this is related to the low MgO/Mo(100) work function. Here we show, based on periodic DFT supercell calculations, that charge transfer can occur also in the opposite direction by adsorption of electropositive K atoms on MgO/Ag(100) films. We predict the occurrence of a charge transfer also for Au on MgO/Ag(100) films despite the fact that here the work function is 1 eV larger than in MgO/Mo(100). The formation of a layer of adsorbed negative (Au delta-/MgO/Ag) or positive (K delta+/MgO/Ag) adsorbates results in an increase or decrease, respectively, of the MgO/Ag(100) work function as predicted by the classical Gurney model for ionic adsorbates on metal surfaces.

偏光全反射蛍光EXAFSによる金属単原子の三次元吸着構造解析

We developed a UHV-Polarization dependent Total-Reflection Fluorescence Extended X-ray Absorption Fine Structure System (UHV-PTRF-EXAFS System), which enables us to prepare a clean and well-defined oxide single crystal surface. We have determined the adsorption sites and structures of highly dispersed Ni atoms on α-Al2O3(0001) and TiO2(110) using the system. In both cases, Ni atoms adsorbed at imaginary cation sites, where metal cations in next layer should be located. These results have indicated that Ni atoms selectively interact with oxygen dangling bonds.