New surfaces stabilized by adsorbate-induced faceting.

Faceting is a form of self-assembly of single-crystal surfaces at the nanometer-scale in which an initially planar surface converts to a 'hill-and-valley' structure, exposing new faces of low-index surfaces. Our recent studies revealed that, upon annealing in O(2), three-sided nanoscale pyramids form on Ir(210) exposing smooth {311} and partially restructured (110) faces. Through a combination of scanning tunneling microscopy and density functional theory, we identify this structure to be a stepped double-missing-row reconstruction, which is only stable on nanopyramidal facets, not on a planar Ir(110) surface. This faceting-enabled stabilization of a hitherto unstable surface points to a new approach to prepare nanoscale model catalysts for structure-sensitivity studies in heterogeneous (electro-)catalysis with high selectivity and reactivity.

Nanoscale surface chemistry over faceted substrates: structure, reactivity and nanotemplates.

Faceting is a form of self-assembly at the nanometre-scale on adsorbate-covered single-crystal surfaces, occurring when an initially planar surface converts to a "hill and valley" structure, exposing new crystal faces of nanometre-scale dimensions. Planar metal surfaces that are rough on the atomic scale, such as bcc W(111), fcc Ir(210) and hcp Re(1231), are morphologically unstable when covered by monolayer films of oxygen, or by certain other gases or metals, becoming "nanotextured" when heated to temperatures above approximately 700 K. Faceting is driven by surface thermodynamics (anisotropy of surface free energy) but controlled by kinetics (diffusion, nucleation). Surfaces can spontaneously rearrange to minimize their total surface energy (by developing facets), even if this involves an increase in surface area. In this critical review, we discuss the structural and electronic properties of such surfaces, and first principles calculations are compared with experimental observations. The utility of faceted surfaces in studies of structure sensitive reactions (e.g., CO oxidation, ammonia decomposition) and as templates for growth of metallic nanostructures is explored (122 references).

First-principles studies on oxygen-induced faceting of Ir(210).

Density functional theory calculations were performed to obtain an atomistic understanding of facet formation on Ir(210). We determined geometries and energetics of clean and oxygen-covered surfaces of planar Ir(210) as well as Ir(311) and two types of Ir(110) surfaces, which are involved in faceting by forming three-sided nanopyramids. Using the energies together with the ab initio atomistic thermodynamics approach, we studied the stability of substrate and facets in the presence of an oxygen environment. Our results show that facets are stable over the entire temperature range at which oxygen is adsorbed on the surface at coverages >/=0.45 physical ML, supporting the picture of a thermodynamic driving force. We also investigated the dependence of the phase diagram on the choice of the exchange-correlation functional and obtained qualitatively the same behavior. Finally, this work helps to better understand reactivity and selectivity of O-covered planar and faceted Ir surfaces in catalysis.

Electron-induced oxygen desorption from the TiO2(011)-2x1 surface leads to self-organized vacancies.

When low-energy electrons strike a titanium dioxide surface, they may cause the desorption of surface oxygen. Oxygen vacancies that result from irradiating a TiO2(011)-2x1 surface with electrons with an energy of 300 electron volts were analyzed by scanning tunneling microscopy. The cross section for desorbing oxygen from the pristine surface was found to be 9 (+/-6) x 10(-17) square centimeters, which means that the initial electronic excitation was converted into atomic motion with a probability near unity. Once an O vacancy had formed, the desorption cross sections for its nearest and next-nearest oxygen neighbors were reduced by factors of 100 and 10, respectively. This site-specific desorption probability resulted in one-dimensional arrays of oxygen vacancies.

Facet stability in oxygen-induced nanofaceting of Re(1231).

The stability of the various facets in oxygen-induced faceting of Re(1231) has been studied by low-energy electron diffraction, scanning tunneling microcopy, and synchrotron-based high-resolution X-ray photoemission spectroscopy. When Re(1231) is annealed at 800-1200 K in oxygen (10(-7) Torr), the surface becomes completely covered with nanometer-scale facets, and its morphology depends on the substrate temperature and oxygen exposure. Especially, the (1121) facet competes with the (1011) facet in determining the surface morphology, and the stability of each facet relies on oxygen coverage. Using density functional theory, the O-Re binding energies on the facets for various oxygen concentrations are calculated to explain how the oxygen coverage affects the anisotropy of surface free energy, which in turn determines the morphology of the faceted surface.

Structure sensitivity in the oxidation of CO on Ir surfaces.

We report results on the catalytic oxidation of carbon monoxide (CO) over clean Ir surfaces that are prepared reversibly from the same crystal in situ with different surface morphologies, from planar to nanometer-scale facets of specific crystal orientations and various sizes. Our temperature-programmed desorption (TPD) data show that both planar Ir(210) and faceted Ir(210) are very active for CO oxidation to form CO2. Preadsorbed oxygen promotes the oxidation of CO, whereas high coverages of preadsorbed CO poison the reaction by blocking the surface sites for oxygen adsorption. At low coverages of preadsorbed oxygen (< or = 0.3 ML of O), the temperature Ti for the onset of CO2 desorption decreases with increasing CO coverage. At high coverages of preadsorbed oxygen (> 0.5 ML of O), T(i) is < 330 K and is independent of CO coverage. Moreover, we find clear evidence for structure sensitivity in CO oxidation over clean planar Ir(210) versus that over clean faceted Ir(210): the CO2 desorption rate is sensitive to the surface morphological differences. However, no evidence has been found for size effects in CO oxidation over faceted Ir(210) for average facet size ranging from 5 to 14 nm. Energetically favorable binding sites for O/Ir(210) are characterized using density functional theory (DFT) calculations.

Coupling of carbon monoxide molecules over oxygen-defected UO2(111) single crystal and thin film surfaces.

While coupling reactions of carbon-containing compounds are numerous in organometallic chemistry, they are very rare on well-defined solid surfaces. In this work we show that the reductive coupling of two molecules of carbon monoxide to C2 compounds (acetylene and ethylene) could be achieved on oxygen-defected UO2(111) single crystal and thin film surfaces. This result allows in situ electron spectroscopic investigation of a typical organometallic reaction such as carbon coupling and extends it to heterogeneous catalysis and solids. By using high-resolution photoelectron spectroscopy (HRXPS) it was possible to track the changes in surface states of the U and O atoms as well as identify the intermediate of the reaction. Upon CO adsorption U cations in low oxidation states are oxidized to U4+ ions; this was accompanied by an increase of the O-to-U surface ratios. The HRXPS C 1s lines show the presence of adsorbed species assigned to diolate species (-OCH=CHO-) that are most likely the reaction intermediate in the coupling of two CO molecules to acetylene and ethylene.

Decomposition of ammonia and hydrogen on Ir surfaces: structure sensitivity and nanometer-scale size effects.

The adsorption and decomposition of ammonia and hydrogen have been studied on surfaces of clean planar Ir(210) and clean nanoscale-faceted Ir(210), which are prepared from the same crystal in situ. We find evidence for structure sensitivity in recombination and desorption of H2 and in thermal decomposition of NH3 on clean planar Ir(210) versus clean faceted Ir(210). Moreover, the decomposition kinetics of NH3 on faceted Ir(210) exhibit size effects on the nanometer scale, which is the first observation of size effects in surface chemistry on an unsupported monometallic catalyst with controlled and well-defined structure and size.

Nanoscale surface chemistry.

We report evidence in several experiments for nanometer-size effects in surface chemistry. The evidence concerns bimetallic systems, monolayer films of Pt or Pd on W(111) surfaces. Pyramidal facets with [211] faces are formed on annealing on physical monolayer of Pt, Pd on a W(111) substrate, and facet sizes increase with annealing temperature. We used synchrotron radiation-based soft x-ray photoemission to show that monolayer films of Pt, Pd, on W "float" on the outer surface, whereas multilayer films form alloys on annealing. Acetylene reactions over bimetallic planar and faceted Pd/W surfaces exhibit size effects on the nanometer scale, that is, thermal desorption spectra of reactively formed benzene and ethylene (after acetylene adsorption) change systematically with facet size. In the second case, the decomposition of C(2)H(2) over planar and faceted Ir(210) surfaces also exhibits structure sensitivity; temperature programmed desorption of H(2) from C(2)H(2) dissociation depends on the nanoscale surface structure. Finally, we have characterized interactions of Cu with the highly ordered S(4 x 4)/W(111) surface. The substrate is a sulfur-induced nanoscale reconstruction of W(111) with (4 x 4) periodicity, having broad planar terraces (approximately 30 nm in width). Fractional monolayers of vapor-deposited Cu grow as three dimensional clusters on the S(4 x 4) surface over a wide coverage range. At low Cu coverage (< or = 0.1 ML), Cu nanoclusters nucleate preferentially at characteristic 3-fold hollow sites; we find a clear energetic preference for one type of site over others, and evidence for self-limiting growth of nanoclusters.