Surface Chemical Reactivity Research Articles

Alkaline-Earth Oxide Nanoparticles Obtained by Aerogel Methods. Characterization and Rational for Unexpectedly High Surface Chemical Reactivities

Aerogel preparation of MgO and CaO nanoparticles involves a sol−gel approach where methoxides are converted to hydroxide gels followed by hypercritical drying and vacuum dehydration. This produces, for these particular hydroxides/oxides, ultrafine particulates rather than monoliths. These particulate materials have exhibited unexpectedly high surface chemical reactivities that have allowed their successful use as high-capacity destructive adsorbents for toxic chemicals, including chlorocarbons, organophosphorus compounds, and acid gases. Detailed characterization involving X-ray diffraction (XRD), gas adsorption/pore distribution analysis, infrared, TEM, AFM, XPS, probe molecule adsorption, and elemental analysis has allowed a rationale to be developed that helps explain the high chemical reactivities observed, especially for CaO. Pore volume and size distribution, unusual surface morphologies with a high ratio of edge ion/surface ions, and trace residual (persistent) surface −OH and −OCH3 seem to be the main factors allowing these nanoparticulates to be isolable and stable and yet highly reactive, and results for CaO are emphasized herein.

Metal pattern fabrication using the local electric field of a conducting atomic force microscope probe

Two approaches were developed using an ambient atomic force microscope (AFM) to pattern metal in either positive or negative tone. The difference in surface chemical reactivity between exposed and unexposed regions was used to selectively attach colloidal Pd(II) nanoparticles capable of initiating the deposition of electroless (EL) Ni. In one case, EL Ni was selectively deposited in the unexposed regions of an organosilane film. In the second case, EL Ni was selectively deposited directly on an AFM oxide pattern. Based on the different binding selectivities of the Pd(II) catalysts, selective deposition on AFM patterns was attributed to the presence of a carbon residue in AFM oxide patterns formed under ambient conditions. The patterned Ni films were used as a mask for pattern transfer into the underlying substrate by reactive ion etching.

Catalytic properties of nanostructured metal oxides synthesized by inert gas condensation

The correlation between defects in metal oxides and activity in heterogeneous catalytic oxidation was investigated. Processing of nanocrystalline metal oxides by inert gas condensation was employed to synthesize oxygen-deficient nonstoichiometric cerium oxide. Comparison with stoichiometric nanocrystalline ceria revealed the effect of nonstoichiometry on surface chemical reactivity. Stoichiometric nanocrystalline cerium dioxide was investigated for catalytic activity in two reactions; oxidation of CO by (i) SO 2 and (ii) O 2. Catalytic activity was found at significantly different temperatures of 560°C in the former and 360°C in the latter reaction. Complementary temperature programmed reduction (TPR) studies were performed and two distinct signals were found, which correlated with the light-off temperatures for each of the catalytic reactions. This demonstrated that the oxygen intermediates, participating in the redox mechanisms were different for both reactions. Nonstoichiometric nanocrystalline CeO 2−x exhibited catalytic activity in CO oxidation by O 2 and a corresponding TPR-signal at 200°C, which suggested the presence of an even more reactive oxygen species. Fourier-transform infrared spectroscopy (FT-IR) and electrical conductivity were used to study the interaction of chemisorbed CO and O 2 with the metal oxides. Surface defects and chemisorbed oxygen were discussed as possible origins of enhanced catalytic activity due to their effects on the nature of surface oxygen species. The combination of these studies yielded detailed insight into the different reaction mechanisms and the chemical interaction between solid surface and gaseous species that are critical towards the engineering of catalytic and gas sensor materials.

Atomic-level spatial distributions of dopants on silicon surfaces: toward a microscopic understanding of surface chemical reactivity

We have investigated the interaction of phosphine (PH 3) and diborane (B 2H 6) with the Si(001) surface using scanning tunneling microscopy, infrared spectroscopy, and ab initio molecular orbital calculations. Experiment and theory show that the formation of PSi heterodimers is energetically favorable compared with formation of PP dimers. The stability of the heterodimers arises from a large strain energy associated with formation of PP dimers. At moderate P coverages, the formation of PSi heterodimers leaves the surface with few locations where there are two adjacent reactive sites. This in turn modifies the chemical reactivity toward species such as PH 3, which require only one site to adsorb but require two adjacent sites to dissociate. Boron on Si(001) strongly segregates into localized regions of high boron concentration, separated by large regions of clean Si. This leads to a spatially-modulated chemical reactivity which during subsequent growth by chemical vapor deposition (CVD) leads to formation of a rough surface. The implications of the atomic-level spatial distribution of dopants on the rates and mechanisms of CVD growth processes are discussed.

Continuous wave electron paramagnetic resonance spectroscopy in the investigation of the surface properties and chemical reactivity of an ionic oxide (MgO)

The surface reactivity of metal oxides has been studied by continuous wave electron paramagnetic resonance spectroscopy and is discussed by means of selected examples concerning magnesium oxide. In particular, the influence of external physical or chemical treatments experienced by the solid on its surface structure and reactivity is illustrated. Examples are given of the effects of progressive dehydration of the solid, doping with transition metal ions and, particularly, addition of alkali metal vapours.

Adlayer core-level shifts of admetal monolayers on transition-metal substrates and their relation to the surface chemical reactivity.

Using density-functional theory, we study the electronic and structural properties of a monolayer of Cu on the fcc(100) and (111) surfaces of the late 4d transition metals, as well as a monolayer of Pd on Mo bcc(110). We calculate the ground states of these systems, as well as the difference of the ionization energies of an adlayer core electron and a core electron of the clean surface of the adlayer metal. The theoretical results are compared to available experimental data and discussed in a simple physical picture; it is shown why and how adlayer core-level binding energy shifts can be used to deduce information on the adlayer's chemical reactivity. \textcopyright{} 1996 The American Physical Society.

Chemical Approach to Nanofabrication: Modifications of Silicon Surfaces Patterned by Scanning Probe Anodization

Scanning probe microscope-induced local oxidation of a material surface with adsorbed water is a recent nanolithographic technology. We applied this technique to the nanoscale patterning of hydrogen-terminated silicon (Si-H) surfaces. Using the silicon oxide (SiOx) patterns as masking, examples of two types of pattern transfer method through area-selective chemical modification were demonstrated. Nanostructures of substrate Si or deposited gold were fabricated by wet chemical etching or electroless plating, respectively. These area selectivities arose from the difference in surface chemical reactivities between anodic SiOx and the surrounding Si-H. The oxidation chemistry is discussed in terms of anodization.

The influence of the surface nature on scattering, fragmentation and deposition processes in C60+ collisions with solid surfaces

Abstract Surface collision experiments with C 60 + ions under UHV conditions were carried out to study the influence of the surface atomic mass, structure, and chemical reactivity of a variety of crystalline surfaces on different collision processes. The impact energy was varied from about 100 to 700 eV. Collisions with graphite (0001), diamond (111), and tungsten diselenide (0001) yield scattered C 60 + ions which were analyzed with respect to their final kinetic energy and rate of metastable fragmentation by means of velocity selective time-of-flight mass spectroscopy. With increasing mass of the surface atoms, the final kinetic energy decreases along with a rise of the final internal energy as deduced from the rate of metastable fragmentation. In strong contrast, C 60 + collisions with Ni(100) and Cu(100) did not result in a detectable yield of scattered ions. Auger electron spectroscopy (AES) showed carbon deposition on these two surfaces. However, the surface carbon concentration as a function of the C 60 + exposure dose depicts remarkable differences. On Ni(100), the carbon concentration rapidly rises at low exposure doses until saturation is reached. By means of low energy electron diffraction (LEED), a c(2×2) carbon superstructure was found. On Cu(100), the carbon concentration steadily rises with increasing slope. The blurring of the Cu(100) electron diffraction spots gives evidence that the surface becomes damaged under C 60 + bombardment resulting in carbon-binding defects. Finally we show that C 60 + collisions with fullerite surfaces are nearly elastic for impact energies up to 250 eV. Rainbow scattering was observed for this surface. For higher impact energier sputtering becomes dominant.

Review: Surface tension and its relations with adsorption, vapourization and surface reactivity of liquid metals

The interactions of a gaseous phase with a liquid metallic surface are analysed in terms of surface tension, adsorption and chemical reactivity. It is shown that thermodynamic equilibrium considerations are not sufficient to interpret surface tension measurements especially at high temperatures, and that diffusion and interactions in the vapour phase must also be considered. Surface tension isotherms are discussed and models allowing calculations of maximum adsorption and surface tension values are presented. These are of particular interest when dealing with systems forming very stable oxides. The influence of surface thermodynamic quantities on chemical reactivity is also shown by means of classical thermodynamic reasoning. One of the main results presented here is that reactions favoured in the bulk phase may not take place at the surface, where the presence of the excess free energy term may favour another reaction, thermodynamically impossible in the bulk phase.

Dry etching of GaAs with Cl 2: correlation between the surface Cl coverage and the etching rate at steady state

While dry etching of GaAs with chlorine is technologically important for manufacturing semiconductor devices, little is known conclusively about the surface chemical reactivity responsible for this etching process. In this work, modulated molecular beam scattering (MMBS) has been combined with temperature-programmed reaction (TPR) and Auger electron spectroscopy (AES) to study the reaction of molecular chlorine with GaAs. The MMBS and AES results indicate that the surface coverage of chlorine during steady state etching over the temperature range of 350–650 K is in the monolayer regime. Above 700 K the surface is chlorine free. A direct correlation is observed between the number of vacant surface sites and the Cl 2 reaction probability. This result suggests a Langmuir adsorption model for the surface reaction, and it is shown that such a model combined with the product evolution kinetics determined from TPR studies successfully simulates the temperature dependence of the chlorine evolved from the surface during modulated molecular beam scattering.

Molecular chemistry at well-defined surfaces

Surface chemistry studies are described in which molecular layers chemisorbed at well-defined Pt or Ag electrode surfaces from aqueous solutions were characterized by means of ex–situ UHV surface analysis techniques such as Auger electron spectroscopy (AES), electron energy–loss spectroscopy (EELS), and low-energy electron diffraction (LEED), and in–situ electrode–solution interfacial characterization methods such as cyclic voltammetry (CV) and coulometry. Types of compounds studied include alcohols, ethers, aldehydes, ketones, carboxylic acids, amines, amino–acids, amides, cyanides, thiocyanides, halides, mercaptans, thiols, sulfones, sulfonic acids, benzenes, five– and six– membered heterocyclic aromatics, multinitrogen heteroaromatics, fused–ring aromatics, and deuterated molecules. Molecules chemisorbed at electrode surfaces are often not removed by rinsing, facilitating investigation by electrochemical methods, and arc stable also in vacuum, permitting identification and characterization of the adsorbed species by AES, EELS and LEED and related methods. Molecular stoichiometry (elemental composition) and packing density (surface concentration, molecules per unit area) are measured by means of AES. Surface molecular orientation can be deduced from molecular packing densities, supplemented with adsorbate vibrational spectra, electrochemical reactivity, LEED structure and other related observations. Studies of series of related adsorbates offer the advantage that additional insight can be gained from intercomparison of molecular surface behavior as a function of systematic structural and chemical differences. Electrochemical reactivity is found to be a function of molecular orientation: horizontally oriented adsorbates at Pt surfaces in most cases undergo complete electrochemical oxidation to CO2 and other oxides, while vertically oriented adsorbates undergo less extensive oxidation; reversible redox centers such as the hydroquinone and catechol moieties retain their electrochemical reactivity and reversibility at electrode surfaces only when the redox center is pendant, and direct attachment of a redox center at surfaces results in loss of redox electro- activity.Close similarity between adsorbed layer EELS spectra and the IR spectra of the unadsorbed liquid or vapor is usually found for vertically oriented adsorbates for which adsorption docs not substantially perturb the main–frame molecular structure. Surface chemical phenomena involving molecular adsorbates at electrode surfaces, including molecular Packing density, mode of attachment, surface orientation, long–range surface order, molecular structure, spectroscopic behavior, and chemical and electrochemical reactivity, are greatly influenced by factors such as adsorbate molecular structure, electrode material, crystallographic structure of electrode surface, electrode potential, solution pH, adsorbate concentration, and temperature.

Bimetallic Surface Chemistry

Bimetallic surface chemistry plays a crucial role in a number of tech­ nologically important areas, including catalysis, magneto-optical films, microelectronics fabrication, electrochemistry, corrosion passivation, and structural materials. With the advent of ultrahigh vacuum surface ana­ lytical techniques and the development of efficient methods for surface preparation, it has become possible since approximately 1970 to prepare clean single-crystal bimetallic surfaces that are well characterized with respect to the geometric locations of the two different metal elements and that are very homogeneous in this surface structure (i.e. having an ideal surface unit cell with few defects in repeating this unit across the surface). These well-defined bimetallic surfaces have offered for the first time an ability to correlate surface chemical reactivity with atomic-level surface structure. Such structure/function relationships must ultimately form the basis for any predictive ability in tailor-making bimetallic surfaces to have desirable reactive properties. This recognition has motivated a great deal of stud y over the past two decades concerning the structure of single-crystal bimetallic surfaces and their surface reactivity with respect to adsorption, desorption, decomposition reactions, and catalytic processes. Although this field is still in its infancy, a great deal has already been learned and trends arc beginning to emerge that give some predictive ability with respect to the surface structures assumed by bimetallic surfaces and their corresponding reactivity toward simple molecules. The aim of this chapter

Formation of In/GaP(1̄ 1̄ 1̄) interface studied by energy loss spectroscopy, x-ray photoelectron spectroscopy, and ultraviolet photoelectron spectroscopy

Submonolayer to several monolayers of In were evaporated onto the GaP(1 1 1) (1×1) clean surface at room temperature to investigate the formation of In/GaP(1 1 1) interface. From the electron energy loss spectra, it has been shown that the surface electronic states related with the Ga dangling bonds are significantly affected by the deposited In atoms even at initial stage. The growth mode of In overlayers on GaP(1 1 1) is island like rather than layer‐by‐layer form. By increasing the amount of deposited In atoms, the In islands become larger and thicker, as evaluated from the relative peak intensities of the bulk and plasmon losses. The core‐level photoelectron specrtra show there is no indication of the occurrence of In–Ga interfacial exchange reaction. The valence band ultraviolet photoelecton spectroscopy reveals a new peak structure at the energy of 5.2 eV below Fermi level, which is believed to be attributed by metallic In. In the case of Ga on InP, it has already been verified that the Ga–In interchange reaction does exist and is quite remarkable. It does not seem sufficient to explain the irreversibility of surface interchange reaction between Ga/InP and In/GaP simply by means of thermodynamical consideration. The surface migration coefficient and the stability of metallic clusters may play a complementary role in determining the surface chemical reactivity.

The surface structure and chemical reactivity of Rh(111)-(2 × 2)-3NO by HREELS and dynamical LEED analysis

The surface structure and chemical reactivity of nitric oxide on Rh(111) has been studied over the temperature range of 120–480 K by high-resolution electron energy loss spectroscopy (HREELS) and dynamical low-energy electron diffraction (LEED) analysis. HREELS results indicate that the adsorption sites of NO depend strongly on the surface coverage and adsorption temperature. At one-half monolayer coverage a c(4 × 2) LEED pattern is observed, with only bridge sites occupied (NO stretching frequency of 1590 cm −1). At saturation coverage and 120 K, only disordered bridge-site adsorption is observed (NO stretching frequency of 1635 cm −1). Saturation adsorption at a surface temperature between 250 and 350 K leads to NO occupation of top sites ( v(NO) = 1830 cm −1) in addition to bridge sites ( v(NO) = 1515 cm −1), in an ordered (2 × 2) lattice. A dynamical LEED intensity analysis of the (2 × 2) structure shows that each unit cell contains two molecules in near-top-sites and one in a bridge site. All the NO molecules are perpendicular to the surface with bond lengths of 1.15 Å. The RhN bond length is 2.07 Å at the near-top site and 2.05 Å at the bridge site. The NO surface structure is similar to a previously analyzed (2 × 2) structure at high CO coverage. This is the first LEED structural analysis for a pure molecular NO overlayer on a metal single crystal surface.

The surface structure and chemical reactivity of Rh(111)-(2 × 2)-3NO by HREELS and dynamical LEED analysis

The surface structure and chemical reactivity of Rh(111)-(2 × 2)-3NO by HREELS and dynamical LEED analysis

Pt 50Ni 50(110) alloy: Chemical surface reactivity with respect to the pure components

Pt 50Ni 50(110) alloy: Chemical surface reactivity with respect to the pure components

The influence of incorporated and adsorbed fluoride on the dissolution of powdered and pelletized hydroxyapatite in fluoridated and non-fluoridated acid buffers.

The effects of fluoride (F) incorporated into hydroxyapatite (HA), adsorbed onto the HA surface, and fluoride in solution were studied during HA dissolution. Dissolution rates at pH 5.0 and 25 degrees C were determined in fluoridated (0.1, 5.0 micrograms/mL) and non-fluoridated buffers for the following powdered and pelletized synthesized HAs: non-fluoridated HA, partially-fluoridated (100, 1000, 37,000 micrograms/g) HA, and surface-adsorbed fluoridated (100, 1000 micrograms/g) HA. The dissolution rate curves were used to derive two components we have called 'diffusion' and 'surface chemical reactivity'. With powders, the rate-determining factor was surface chemical reactivity during the first 60 min. Diffusion was reduced by increasing the F levels in the apatite or buffer. In non-fluoridated buffers, diffusion was reduced 29% for apatite with 100 micrograms/g incorporated F and 99% with 37,000 micrograms/g. Diffusion was reduced by 37% and 81% by 0.1 and 5.0 micrograms/mL F, respectively, in the buffer. With pelletized HA, the rate-determining factor during the first 10 min was surface chemical reactivity. Diffusion then became relatively more important, but the diffusion rate was independent of F content. During the dissolution of powdered apatites, F was taken up by the remaining apatite solid, as evidenced by the lower F concentration in solution than that calculated to be released during dissolution. There was also a decrease in the F concentration of the 0.1 microgram/mL buffer as reaction progressed. Pelletized apatites did not release enough F to influence dissolution significantly. Surface-adsorbed F was more effective than incorporated F in reducing HA dissolution.(ABSTRACT TRUNCATED AT 250 WORDS)

Catalytic oxidation of methanol on LnCoO 3 ( Ln = LaEu) perovskite oxides

The catalytic activity of LnCoO 3 ( Ln = LaEu) in the reaction of methanol oxidation has been studied. After the injection of methanol into O 2N 2 passing over the catalyst, the conductivity change of the thin oxide film was observed. These conductivity changes have been closely related to the surface chemical catalytic reactivity. The activity was related to the binding energy of oxygen, ΔH(metal-O), which coordinates to the metal ions in LnCoO 3 perovskite. The activity of LnCoO 3 was in the following order: Sm ⋍ Eu > Nd > La, where the activity was measured at a temperature of which a response ratio attains 10%. Further, from the results of the reduction of SmCoO 3 and the measurement of reaction kinetics, the mechanism of the methanol oxidation reaction on SmCoO 3 was discussed.

Surface coordination chemistry of platinum studied by thin-layer electrodes. Surface chemical reactivity of aromatic and quinonoid compounds adsorbed in specific orientational states

Donnees sur les reactivites chimiques de composes diphenoliques et quinonoides particuliers. Etude de 5 reactions compte tenu du mode de fixation, de la temperature et du potentiel d'electrode: 1) de la reactivite electrochimique reversible a l'etat adsorbe comparee a celle a l'etat libre; 2) de l'oxydation electrocatalytique de composes presentant des etats d'orientation multiples; 3) de l'hydrogenation catalytique de l'hydroquinone et de derives thiophenoliques fixes sur S; 4) des reactions d'echange entre des adsorbats aromatiques non identiques et; 5) des reactions de substitution entre des composes aromatiques et des halogenures

Bitumen–silica model composites: The role of the filler

AbstractThe storage moduli of bitumen–silica model composites obey a simple relationship: E′ = E′0 + Aϕψ, where ϕ is the filler volume ratio and ψ the bitumen–silica interfacial area for a unit volume of composite. This equation is experimentally varified taking various silicas of different surface areas or chemical surface reactivity. The influence of the interface properties on the mechanical glass transition temperature is also examined.