Mixed Overlayer Research Articles

The role of surfactant in two−components structures formation on Si(111) surface

Surface structures and thermal stability of the ultrathin In films adsorbed on two different (α(√3 × √3) and β(√3 × √3)) Pb/Si(111) surfaces, varying in initial reconstructions, are investigated in situ with Low Energy Electron Diffraction (LEED) and X−ray Photoemission Spectroscopy (XPS). For the In/Si(111)−α(√3 × √3)−Pb system, In atoms invert their initial positions with the Pb surfactant at 470 K, resulting in formation of disordered mixed (In,Pb) overlayers. Moreover, Pb atoms located on the topmost layer desorb easily until the critical coverage is reached. Unreconstructed mixed (In,Pb) layers are also observed on the In/Si(111)−β(√3 × √3)−Pb system at room temperature due to In adsorption on Si surface in positions located between Pb atoms. Increasing the temperature of the samples yielded an arrangement of the adsorbate and surfactant atoms to 2D compounds with various periodic array. The new surface structures identified as (2 × 2) (4 × 1), and (√3 × √3) have been found to be formed in both systems. The detailed analyses of combining XPS and LEED data reveal that the phases can be built by either pure In or (In,Pb) compounds.

Interaction of HCl with a CeO2(111) Layer Supported on Ru(0001): A Theory and Experiment Combined Study

Ultrathin crystalline CeO2(111) films were grown on Ru(0001) in order to study the interaction of HCl with this surface as a first step of the Deacon reaction with experimental techniques, including low energy electron diffraction (LEED), scanning tunneling microscopy (STM), X-ray photoelectron spectroscopy (XPS), and thermal desorption spectroscopy (TDS) in combination with density functional theory calculations (DFT+U). HCl molecules adsorb dissociatively on CeO2(111), forming a well-ordered (3x3)R30° overlayer structure with one Cl and one H per surface unit cell. DFT calculations indicate that HCl adsorption is exothermic by 1.15 eV and proceeds via an acid–base reaction. The mixed overlayer structure is stabilized by Lewis acid–base pairing (∼0.4 eV). Stoichiometric CeO2(111) films are likely to be not very active in the Deacon process since at 800 K the recombination of adsorbed H* and Cl* to form HCl is far more preferred over Cl* + Cl* recombination to form the desired product Cl2.

Surface-Enhanced Raman Spectroscopy of Acetil-neuraminic Acid on Silver Nanoparticles: Role of the Passivating Agent on the Adsorption Efficiency and Amplification of the Raman Signal

We present a combined experimental and theoretical study dedicated to analyzing the surface-enhanced Raman spectra of solutions of citrate-covered silver nanoparticles (NPs) in the presence of acetil-neuraminic acid (Neu5Ac). The Raman signals from Neu5Ac (particularly the bands located at 1002 and 1237 cm–1) can easily be detected for concentrations as low as 1 mg/dl, providing outstanding molecular sensing properties for our synthesized Ag-NPs. When compared to its solid phase, Neu5Ac adsorption on citrate-covered Ag particles leads to enhanced Raman intensities; many vibrational frequencies are shifted; and relative intensities undergo significant changes. These variations in the spectra complicate molecular identification, especially in mixed overlayers as the ones considered in this work. Consequently, experimental results are discussed on the basis of extensive density functional theory (DFT) calculations on model citrate-covered silver clusters with coadsorbed Neu5Ac species. Several citrate–Neu5Ac...

Performance Study of Warm Mix Asphalt Fiber Mixture in the Thin Overlay

Ultrathin asphalt mixture used to overlay has better road performance. Asphalt mixture can be mixed and constructed at relatively low temperatures, basing on warm mix technologies, solving issues of the thin overlay fast cool and poor compaction. Warm mix overlays is a fine thin overlay, rutting resistance of concrete and thickness 2~3cm. It composed of high quality modified asphalt, aggregate gradations, warm mix agent and polyester fiber. Indoor experimental research analyzed the performance of warm mix of ultrathin asphalt mixture water stability, high temperature stability, rutting resistance, anti-slide and noise reduction. Tests show that 5% of asphalt, 0.3% mineral fiber is the optimum dosage of warm mixed agent.

Effects of a TiO 2 :CaO barrier layer on the back electron transfer in TiO 2 -based solar cells

Abstract A TiO 2 :CaO mixed overlayer was formed on TiO 2 photoelectrodes, and the resulting photoelectrodes were applied to dye-sensitized solar cells (DSSCs). DSSC with the mixed overlayer showed an increase in short-circuit current ( J sc ), open-circuit voltage ( V oc ), and fill factor ( FF ), resulting in a 17% enhancement in photovoltaic performance, compared to that of reference cell. It was found that, by incorporating the mixed overlayer, light harvesting and electron collection efficiency were enhanced, thereby increasing J sc . The enhancement in V oc resulted from a reduced back electron transfer, and the increased FF was attributed to a decreased series resistance and an increased shunt resistance.

Influences of Sr-Incorporated TiO2 Layer on the Photovoltaic Properties of Dye-Sensitized Solar Cells.

Effects of a mixed overlayer composed of TiO2 and TiSrO3 on the performance of dye-sensitized solar cells (DSSCs) were investigated. The surface of TiO2 photoelectrode formed on F-doped SnO2 (FTO) was modified by soaking it in a TiCl4:SrCl2 mixed aqueous solution with various molar ratios and then calcining to produce the TiCl4:SrCl2-treated TiO2 photoelectrode (Ti:Sr-TiO2/FTO). The highest power conversion efficiency (PCE) was obtained from DSSC with Ti:Sr(7:3)-TiO2/FTO, which was prepared from the mixed solution with the molar ratio of 7:3 (TiOl4:SrCl2). An enhancement in short-circuit photocurrent (J(sc)) and open-circuit voltage (V(oc)) of DSSC with Ti:Sr(7:3)-TiO2/FTO was achieved, compared to those of the reference device with Ti:Sr(10:0)-TiC2/FTO (i.e., TiO2-coated TiO2/FTO). The incorporation of the mixed overlayer on the nanoporous TiO2 photoelectorde led to an improvement in the electron collection efficiency by a prolonged electron lifetime, thereby increasing the J(sc) value. The increase in V(oc) value of the device with Ti:Sr(7:3)-TiO2/FTO was due to the suppression of the charge recombination between injected electrons and I3(-) ions.

Case study: performance effectiveness and cost-benefit analyses of open-graded friction course pavements in Tennessee

The effectiveness and cost-benefit of Open-Graded Friction Course (OGFC) pavements were analysed in this study by investigating the cost, field performance, short- and long-term effectiveness, and accident rate of OGFC projects in Tennessee. Paired sample t-tests were conducted to compare if there was significant difference between OGFC and non-OGFC sections. Investigation on the performance effectiveness showed that OGFC sections provided comparable performance level as traditional dense mix overlays and were effective in improving pavement performance and repairing deterioration except for rutting potential. However, no severe pavement deterioration was observed on the OGFC sections after more than four or five years of service, indicating sufficient durability of the OGFC used in Tennessee where mild freeze-thaw cycles are experienced annually. Although the unit cost ($/m3) of OGFC was about 42% higher than the cost of traditional dense mix overlays, it was observed that the accident rate was significantly reduced especially in rainy days after the OGFC treatment. The cost-benefit analyses based on the ratio of accident rate reduction over cost demonstrated that OGFC was significantly more cost-beneficial in improving driving safety and reducing accident rate in rainy days.

Two-dimensional Ostwald ripening on a patterned support and in a mixed overlayer

The author presents lattice Monte Carlo simulations of Ostwald ripening of islands formed of monomers with attractive nearest-neighbor interaction in two situations. The first one implies that monomers of one kind are located on a heterogeneous support which itself includes embedded frozen patterns of the Ostwald type. This feature is demonstrated to be able to terminate the island ripening. In the second case, the support is considered to be uniform while the overlayer is assumed to contain monomers of two kinds. This complicating factor is found to nearly not modify the ripening kinetics or make it somewhat slower despite appreciable correlations in the arrangement of different monomers.

Efficient dye-sensitized solar cells with surface-modified photoelectrodes

The influence of a mixed overlayer comprising titanium dioxide (TiO2) and niobium pentoxide (Nb2O5), which was coated on the surfaces of TiO2 nanoporous photoelectrodes, on the performance of dye-sensitized solar cells (DSSCs) was studied. The DSSCs with the TiO2:Nb2O5-coated photoelectrodes showed an increase in short-circuit current (Jsc) and open-circuit voltage (Voc), resulting in a power conversion efficiency of 10.37% compared to 7.90% for the reference cell with a TiO2-coated photoelectrode. The increase in Jsc was due to an improvement in the light harvesting and electron collection efficiency. The suppression of the charge recombination between the injected electrons and the I3− ions was found to increase the Voc value of the device with the TiO2:Nb2O5-coated photoelectrodes.

Enhancement of Electrocatalytic Activity of Novel Ruthenium Based Nanocatalyst Supported on Nitrogen Doped Carbons for Oxygen Reduction

Herein, ruthenium nanoparticles modified with mixed selenium and nitrogen overlayers (RuSexNy) were prepared via chelatation method onto the N-doped and undoped carbons (Norit and MWCNTs) under analogous experimental conditions. . The crystalographic data reveals the same type of crystallographic phases in RuSexNy crystallites, regardless of the doping of carbon support, nevertheless, as transmission electron microscopy measurements have shown the nanoparticles fabricated onto the N-doped carbons were found to be of smaller diameters. The studies of oxygen reduction reaction in acid (0.5 M H2SO4) medium using rotating ring disk voltammetry and impedance spectroscopy show that RuSexNy nanoparticles supported on the N-doped carbons exhibit higher ORR catalytic activity with generation of lower amounts of hydrogen peroxide. The spillover effect due to N- doping of carbon support was suggested to be responsible for improved decomposition of adsorbed ORR intermediates in carbon supported RuSexNy materials.

Thiol-based molecular overlayers adsorbed on C60: Role of the end-group and charge state on the stability of the complexes

We present pseudo-potential density functional theory calculations dedicated to analyze the stability and electronic properties of thiol-based molecular overlayers adsorbed on C60. We consider short molecules having a S atom as a headgroup, alkyl chains containing one to three C atoms, and a CH3 species as a terminal group. The thiol molecules are bonded to the carbon surface (through the S atom) with adsorption energies that vary in the range of ~1-2 eV and with S-C bond lengths of ~1.8 Å. For neutral C60(SCH3)n complexes, low energy atomic configurations are obtained when the thiol groups are distributed on the surface forming small molecular domains (e.g., pairs, trimers, or tetramer configurations of neighboring thiol molecules). In contrast, less stable random distributions are defined by orientationally disordered overlayers with highly distorted underlying carbon networks. The inclusion of London dispersion interaction slightly affects the structure of the molecular coating but increases the adsorption energies by values as large as 0.3 eV. Interestingly, the relative stability of the previous adsorbed phases differ from the one obtained when considering single sulfur adsorption on C60, a result that reveals the crucial role played by the terminal CH3 groups on the structure of the molecular coating. The positive (negative) charging of the [C60(SCH3)n](±q) complexes, with q as large as 8e, changes the geometrical structure and the chemical nature of the ligand shell inducing lateral molecular displacements, S-S bonding between neighboring thiols, as well as the partial degradation of the molecular coating. Finally, we consider the stability of two-component mixed overlayers formed by the coadsorption of CH3-, OH-, and NH2-terminated alkanethiols of the same length. In agreement with the results found on Au surfaces, we obtain lowest energy atomic configurations when molecular domains of a single component are stabilized on C60, a result that could be of fundamental importance in biomedical applications.

Molecular Oxygen Network as a Template for Adsorption of Ammonia on Pt(111).

Low-temperature scanning tunneling microscopy (STM) was used to observe a mixed NH3-O2 overlayer on Pt(111). At adsorption temperatures below 50 K, the chemisorbed O2 molecules form an ordered network at high coverages. The STM images reveal that this network features a distributed set of holes corresponding to on-top sites of the Pt lattice that are surrounded by two or three O2 molecules. Different hole-hole distances are observed with 0.73 nm most common. These holes in the O2 network act as preferential adsorption sites for the ammonia molecules leading to the formation of an NH3-O2 complex that serves as a precursor to ammonia oxydehydrogenation.

Electrocatalytic Activity toward Oxygen Reduction of RuSxNy Catalysts Supported on Different Nanostructured Carbon Carriers

Ruthenium admixed or nanoparticles modified with mixed selenium and nitrogen overlayers (RuSexNy) were fabricated onto different carbon nanostructured supports (that include multiwalled carbon nanotubes, Vulcan and Norit) under analogous experimental conditions. It is apparent from X-ray measurements that both Vulcan and Norit supported RuSexNy crystallites are characterized by the same type of crystallographic phases. Important parameters that influence electrocatalytic activity include the nature of functionalization of carbon support, in addition to its active surface area and morphology that affect distribution and size of active RuSexNy centers. Despite the same loading of ruthenium (40 μg cm−2), diameters of RuSexNy nanoparticles have somewhat differed and ranged from 2 to 6 nm, as it has been evident from transmission electron microscopy measurements. The systems have been evaluated with respect to oxygen reduction reaction in acid (0.5 M H2SO4) medium using rotating ring disk voltammetry and impedance spectroscopy. Having in mind such parameters as the half-wave reduction potential, the electroreduction current density, the heterogenous rate constant, the percent formation of the hydrogen peroxide undesirable intermediate as well as the overall charge transfer resistance appearing during oxygen reduction, the RuSexNy-based system utilizing functionalized Vulcan is characterized by the highest electrocatalytic activity.

Water and ammonia on Cu{110}: comparative structure and bonding

Water and ammonia are arguably the two most important inorganic molecular species in the modern world, and their interaction with metal surfaces is key to unlocking their further potential in a number of spheres. In this comparative study, conducted on the Cu{110} substrate, we present results from first-principles density functional theory that highlight the similarities and differences between these chemical cousins. We find that ammonia is less likely than water to undergo thermally induced partial dissociation, although we nevertheless identify the most likely product of electron-stimulated or defect-induced dissociation to be a surface amino species. We predict that ammonia, like water, will adopt a bilayer structure at high coverage, but that unlike water the net intermolecular interaction will be repulsive, despite the formation of a weak hydrogen-bonded network. Furthermore, we suggest that coadsorption of water and ammonia can give rise to an intimately mixed overlayer in which ammonia molecules are bound directly to the surface whilst water molecules are attached only via hydrogen bonds from below.

Computational Studies of Experimentally Observed Structures of Sulfur on Metal Surfaces

First-principles electronic structure calculations were carried out to examine the experimentally observed structures of sulfur on close packed surfaces of a number of important metals - Ag(111), Cu(111), Ni(111), Pt(111), Rh(111), Re(0001) and Ru(0001). At low coverages ({le} 1/3 ML), the prediction is consistent with the typical pattern of preferred sulfur occupancy of threefold hollow sites, notably the fcc site on the (111) surfaces and the hcp site on the (0001) surfaces. Theoretical confirmation for the existence of pure sulfur overlayer phases on Pt(111), Rh(111), Re(0001) and Ru(0001) at higher coverages (> 1/3 ML) was provided. For the ({radical}7 x {radical}7) phase seen on Ag(111), the most preferred structure identified for adsorbed S trimer consists of an S atom on the top site bonded to two S atoms situated on the nearest neighbor off-bridge site positions. Among the different densely packed mixed sulfur-metal overlayer models suggested for the ({radical}7 x {radical}7) phase on Cu(111), the structure which consists of metal and S atoms in a hexagonal-like arrangement on the top substrate was found to be the most energetically favorable. For the (5{radical}3 x 2) phase on Ni(111), the calculations confirm the existence of clock-reconstructed top layer metal atoms onto whichmore » sulfur atoms are adsorbed.« less

Proton transfer in water–hydroxyl mixed overlayers on Pt(1 1 1): Combined approach of laser desorption and spatially-resolved X-ray photoelectron spectroscopy

Proton transfer in water–hydroxyl mixed overlayers on a Pt(1 1 1) surface was studied by a combination of laser induced thermal desorption (LITD) method and spatially-resolved X-ray photoelectron spectroscopy (micro-XPS). The modulated pattern OH + H 2O/H 2O/OH + H 2O was initially prepared by the LITD method; vacant area with a 400 μm width was first formed in the mixed OH + H 2O overlayer by irradiation of focused laser pulses, and followed by refilling the vacant area with pure H 2O. Spatial distribution changes of OH and H 2O were measured as a function of time with the micro-XPS technique, which indicated that H 2O molecules in the central region flow into the OH + H 2O region. From quantitative analyses using a diffusion equation, we found that the proton transfer in the mixed overlayer consists of at least two pathways: direct proton transfer from H 2O to OH in the nearest site and the proton transfer to the next-nearest site via H 3O + formation. The time scale of first and second path was estimated to be 5.2 ± 0.9 ns and 48 ± 12 ns at 140 K, respectively. In the presence of water capping layer, however, the rate of proton transfer is reduced by an order of magnitude, which would be explained by peripatetic behavior of proton into H 2O capping layer.

First-principles studies of the [formula omitted] structure of sulfur on the Pd(1 1 1) surface

Density functional theory is used to investigate the ( √ 7 × √ 7 ) R 19.1 ° structure of sulfur on the Pd(1 1 1) surface. Among the different models that were considered, the densely packed mixed sulfur-metal overlayer structure proposed by Liu et al. [W. Liu, K.A.R. Mitchell, W. Berndt, Surf. Sci. 393 (1997) L119] was found to be the most energetically favorable. This model consists of arrangement of Pd triangles and pentagons on top of Pd(1 1 1) with the sulfur atoms at 3/7 monolayer coverage. The dominant mechanism for sulfur interaction with Pd in the overlayer region is the rehybridization of sulfur 3p and metal 4d bands. Simulated scanning tunneling microscopy image for this structure shows some similarity with that obtained from experiment. Our study confirms that surface sulfide with no structural resemblance to its bulk counterparts can form on Pd(1 1 1).

Intact and dissociative adsorption of water on Ru(0 0 0 1)

Water adsorption on Ru(0 0 0 1) has been examined to throw light on the controversy surrounding whether water adsorbs intact or as a partially dissociated overlayer. Below 140 K H 2O/D 2O adsorption produces overlayers which show vibrational spectra characteristic of an intact water bilayer. Above 150 K H 2O dissociates to form a mixed (OH/H 2O/H) overlayer, suppressing the molecular H 2O stretching and scissors modes seen in RAIRS of water bilayers to leave only out of plane bending modes. Dissociation of H 2O to form the mixed overlayer competes with H 2O desorption and this branching can be enhanced or suppressed by pre-adsorbing O or H.

Hydrogen bonding in mixed OH+H2O overlayers on Pt(111).

The stability of OH on Pt(111) has been investigated to determine the role of hydrogen bonding in stabilizing the overlayer. We find that the optimal structure is a mixed (OH+H2O) phase, confirming recent density-functional theory predictions. The reaction O+3H(2)O forms a hexagonal (sqrt[3]xsqrt[3])R30 degrees -(OH+H2O) lattice with a weak (3x3) superstructure, caused by ordering of the hydrogen bonds. The mixed overlayer can accommodate a range of H(2)O/OH compositions but becomes less stable as the H2O content is reduced, causing defects in the hydrogen-bonding network that lift the (3 x 3) superstructure and destabilize the overlayer.

Photoelectron spectroscopy studies of the Pd/Ru(0001) and (Cu + Pd)/Ru(0001)interfaces

The electronic structures of Pd and (Cu + Pd) overlayers onRu(0001) have been studied. Photoelectron emission spectroscopybased on synchrotron radiation and radiation from a Mg Kαx-ray source has been used to study the valence band of the surfaces and the Ru3d, Pd 3d and Cu 2p core levels. The interaction of Pd with Ru(0001) isstudied in the coverage range of submonolayer to several monolayers. Thecore level spectra of the Pd 3d5/2 level show a new component for Pdcoverages above 1 ML. In the submonolayer range the binding energy (BE)of the Pd 3d5/2 level is constant at 335.08 eV. A state characterizedas an interface state between the Pd adlayer and the Ru substrate isobserved at a BE of 1.22 eV. This state, which dominates the valenceband for submonolayer Pd coverages, has not been reported earlier andit is seen neither for clean Ru(0001) nor for multilayers of Pd. It is acharacteristic that the overlayer d bands are narrower in the low-coverage regime.Deposition of Cu, on a 0.4 ML Pd/Ru(0001) surface up to 1 ML totalcoverage of Pd and Cu, results in a mixed two-dimensional overlayer.