Ion Scattering Spectroscopy Research Articles

Size-Dependent Energy and Adhesion of Pd Nanoparticles on Graphene on Ni(111) by Pd Vapor Adsorption Calorimetry

Carbon-supported late transition-metal nanoparticles are promising catalysts and electrocatalysts for wide-ranging applications. However, experimental investigations of the bonding energetics of metal nanoparticles on carbon supports are very limited. Here, we report heats of adsorption of Pd vapor deposited onto single-layer graphene(0001) supported on Ni(111) at 100 and 300 K as Pd grows particles of well-defined size in the range from three atom clusters to 6 nm diameter. Sizes were determined from He+ low-energy ion scattering (LEIS). In this size range, the differential heat of Pd adsorption increases from 228 kJ/mol to within 10 kJ/mol of the heat of sublimation of bulk Pd (377 kJ/mol). The chemical potential of metal atoms in these nanoparticles as a function of average particle size was determined from these results. The adhesion energy at the Pd/graphene(0001)/Ni(111) interface was extracted from these data and found to be 3.5 J/m2 for the largest Pd particles. For the three metal elements that have now been studied (Pd, Ni, and Ag), we found an increase in metal/graphene(0001)/Ni(111) adhesion energy with metal carbophilicity, which we defined here as the heat of C atom adsorption on that metal’s (111) surface estimated from published density functional theory calculations.

Enhancing the Catalytic Activity of Mo(110) Surface via Its Alloying with Submonolayer to Multilayer Boron Films and Oxidation of the Alloy: A Case of (CO + O2) to CO2 Conversion

In-situ formation of boron thin films on the Mo(110) surface, as well as the formation of the molybdenum boride and its oxide and the trends of carbon monoxide catalytic oxidation on the substrates formed, have been studied in an ultra-high vacuum (UHV) by a set of surface-sensitive characterization techniques: Auger and X-ray photoelectron spectroscopy (AES, XPS), low-energy ion scattering (LEIS), reflection-absorption infrared spectroscopy (RAIRS), temperature-programmed desorption (TPD), electron energy loss spectroscopy (EELS) and work function measurements using the Anderson method. The boron deposited at Mo(110) via electron-beam deposition at a substrate temperature of 300 K grows as a 2D layer, at least in submonolayer coverage. Such a film is bound to the Mo(110) via polarized chemisorption bonds, dramatically changing the charge density at the substrate surface manifested by the Mo(110) surface plasmon damping. Upon annealing of the B-Mo(110) system, the boron diffuses into the Mo(110) bulk following a two-mode regime: (1) quite easy dissolution, starting at a temperature of about 450 K with an activation energy of 0.4 eV; and (2) formation of molybdenum boride at a temperature higher than 700 K with M-B interatomic bonding energy of 3.8 eV. The feature of the formed molybdenum boride is that there is quite notable carbon monoxide oxidation activity on its surface. A further dramatic increase of such an activity is achieved when the molybdenum boride is oxidized. The latter is attributed to more activated states of molecular orbitals of coadsorbed carbon monoxide and oxygen due to their enhanced interaction with both boron and oxygen species for MoxByOz ternary compound, compared to only boron for the Mox'By' double alloy.

Modeling experimental low energy ion scattering multi-angle maps with molecular dynamics FAN

Multi-angle maps obtained with low energy ion scattering (LEIS) are not only a convenient way to visualize the atomic-level surface structure of single crystals, but also provide a comprehensive data set that is useful for quantitative fitting with models. However, modeling these maps is often difficult due to the complexity of the scattering process, as well as the number of simulations needed to accurately reproduce a single experimental map. For example, traditional molecular dynamics (MD) and binary collision approximation (BCA) simulations typically require on the order of one billion simulated ion trajectories to obtain sufficient statistics. This problem is exacerbated when modeling impact-collision ion scattering spectroscopy (ICISS) maps, as backscattering cross sections are much smaller than forward-scattering cross sections. A workaround to this problem was given by Neihus and Spitzl in the form of FAN (Niehus and Spitzl, 1991), a simulation that approximates the angular dependence of the backscattering signal by only simulating ion trajectories that reach and emanate from lattice atoms, using a BCA code. This greatly reduces the number of trajectories that must be calculated, though it can oversimplify some of the more complex collision sequences. In this work, we develop a fully three-dimensional FAN code that uses MD to simulate trajectories rather than the BCA. Our approach exploits the improved speed of FAN, while retaining the more accurate ion trajectory calculations of MD. We apply the MD-FAN simulation to model LEIS measurements on two surfaces. First, we benchmark our MD-FAN simulation in a comparison to a BCA simulation for 3 keV He+ incident on W(111) that has been previously reported in Wong et al. (2020). The MD-FAN map had stronger agreement with the experimental ICISS measurements than did the BCA map, despite containing nearly 100-fold fewer ion trajectories. Second, to obtain experimental data sets for a more complex surface, we perform experimental ICISS measurements using a 3 keV He+ beam on a magnetite single crystal, Fe3O4(001) to generate a multi-angle map. The simulated MD-FAN map for the Fe3O4(001) surface once again agreed well with the experimental map, allowing for quantitative comparisons. The MD-FAN simulation also provided insight into the scattering processes, revealing that some of the observed pattern arises from 3 keV He+ that have backscattered into the detector from Fe atoms more than 2 nm below the surface.

Nanoarchitectonics of CuNi bimetallic nanoparticles in ionic liquids for LED-assisted synergistic CO2 photoreduction

An efficient and robust photocatalysts have been developed by combining bimetallic CuNi nanoparticles and TiO2. Simple co-reduction of CuCl2.2H2O and NiCl2.6H2O in 1-n-butyl-methylimidazolium tetrafluoroborate (BMIm.BF4) ionic liquid affords small sized (4–5 nm) bimetallic CuNi NPs. Low-Energy Ion Scattering (LEIS) and magnetic measurements analyses reveals that the NPs are alloyed nanostructure-having Ni enriched surface. CuNi NPs decorated on P25-TiO2 remarkably promote the generation of CH3OH and syngas as compared to their monometallic counter-parts using 1200 mW LEDs of 450 nm (visible) light irradiations in an optofluidic microreactor. The system continuously produces CH3OH with a rate of 102.1 μm.g−1.h−1 (AQY 0.90%) and syngas (H2 118.8 and CO 24.6 μmol.g−1.h−1), represents a high yield and quantum efficiency. Where as, its counter-part, Ni/TiO2, generate CH3OH with a rate of 37.0 μm.g−1.h−1, H2 200.0 μmol.g−1.h−1 and CO 15.1 μmol.g−1.h−1 having 0.33, 0.59 and 0.1% AQY, respectively. The presence of Cu in CuNi NPs significantly boosts the catalytic performance via metal dilution and synergistic effect, causes the red shift and increases the conductivity, while Ni increases the band gap, thereby achieving a record high CH3OH rate generation.

Structure and Properties of Ultrathin SiOx Films on Cu(111).

The metal-oxide interface plays a crucial role in catalysis, and it has attracted increasing interest in recent years. Cu/SiO2, as a common copper-based catalyst, has been widely used in industrial catalysis. However, it is still a challenge to clarify the structures of the interface of Cu-SiOx and the effect on catalytic performance. Herein, we prepared ultrathin SiOx films by evaporating Si onto a Cu(111) surface followed by annealing in an O2 atmosphere, which were characterized by various surface science techniques. The results showed that a SiOx film could grow nearly layer-by-layer on the Cu(111) surface in the present condition. Both X-ray photoelectron spectroscopy (XPS) and high-resolution electron energy loss spectroscopy (HREELS) results confirmed the presence of Cu-O-Si and Si-O-Si species. Thermal stability experiments illustrated that the well-ordered silica films were stable under annealing in vacuum. The feature of CO adsorption suggested a CO-Cuδ+ species induced from the Cuδ+-O-Si. Low-energy ion scattering spectroscopy (LEIS) and XPS results demonstrated that some Cu2O appeared on the surface when the 1 ML SiOx/Cu(111) was exposed in O2 at 353 K.

Characterization of the Interfacial Structures of Core/Shell CdSe/ZnS QDs.

Core/shell quantum dots (QDs) have been extensively studied, yet their optical properties widely vary among studies. Such variation may arise from the variation in interfacial structures induced by the subtle difference in each synthetic procedure. Here, we studied the interfacial structures of CdSe/ZnS QDs using the time-of-flight medium energy ion-scattering spectroscopy (TOF-MEIS), which offers the radial elemental distributions as well as the overall elemental compositions of QDs. The TOF-MEIS spectra provided strong evidence for the existence of an alloyed layer at the interface between CdSe and ZnS in typical CdSe/ZnS QDs. On the basis of the emission and absorption spectra of QDs sampled during the synthesis, we conclude that such interfacial alloying is caused by the dissolution of CdSe seeds during the synthesis steps. Such a dissolution mechanism is further corroborated by the observation that the ligand environment of solvent (X or L type) leads to different shapes of interfaces.

A Tag-and-Count Approach for Quantifying Surface Silanol Densities on Fused Silica Based on Atomic Layer Deposition and High-Sensitivity Low-Energy Ion Scattering

A Tag-and-Count Approach for Quantifying Surface Silanol Densities on Fused Silica Based on Atomic Layer Deposition and High-Sensitivity Low-Energy Ion Scattering

Elucidating the underlying surface chemistry of Sn/Al2O3 catalysts during the propane dehydrogenation in the presence of H2S co-feed

Incipient wetness impregnation was used to deposit dispersed tin (Sn) atoms on γ-Al2O3. The morphology and structure of the catalysts were characterized using Raman, X-ray Photoelectron Spectroscopy (XPS), and low energy ion scattering (LEIS) combined with N2-physisorption and in situ infrared diffuse reflectance spectroscopy (DRIFTS) using NH3 and CO2 as probe molecules, H2- temperature-programmed reduction (H­2-TPR), and steady-state reaction kinetics measurements. The effects of H2S on the surface chemical composition and structure of the resulting catalysts were systematically studied. The results showed isolated and well-dispersed Sn oxide sites of the deposited catalyst with nanoparticles observed only at relatively high coverages of 10% Sn at a coverage ranging from 0.48 to 2.8 Sn atoms nm−2. DRIFT spectroscopy evidenced a decrease in the number of Lewis acid sites on H2S treated Sn/Al2O3 suggesting surface tin species re-dispersing and blocking the active sites of γ-Al2O3. Overall, the surface analysis results presented here suggest that the active catalytic sites in the presence of H2S involve Al2O3 rather than bulk sulfide and Sn atoms dispersed over alumina towards propane dehydrogenation.

Elemental determination of a solid surface by the Low Energy Ion Scattering technique

Elemental determination of a solid surface by the Low Energy Ion Scattering technique

How oxygen passivates polycrystalline nickel surfaces.

The passivation of polycrystalline nickel surfaces against hydrogen uptake by oxygen is investigated experimentally with low energy ion scattering (LEIS), direct recoil spectroscopy (DRS), and thermal desorption spectroscopy (TDS). These techniques are highly sensitive to surface hydrogen, allowing the change in hydrogen adsorption in response to varying amounts of oxygen exposure to be measured. The chemical composition of a nickel surface during a mixed oxygen and hydrogen exposure was characterized with LEIS and DRS, while the uptake and activation energies of hydrogen on a nickel surface with preadsorbed oxygen were quantified with TDS. By and large, these measurements of how the oxygen and hydrogen surface coverage varied in response to oxygen exposure were found to be consistent with predictions of a simple site-blocking model. This finding suggests that, despite the complexities that arise due to polycrystallinity, the oxygen-induced passivation of a polycrystalline nickel surface against hydrogen uptake can be approximated by a simple site-blocking model.

Oxidation of Sn at the Cluster–Support Interface: Sn and Pt–Sn Clusters on TiO2(110)

The growth of Sn and Pt–Sn clusters on TiO2(110) has been studied by scanning tunneling microscopy, X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS), and density functional theory (DFT). At low Sn coverages (0.02 ML), single-layer high clusters of SnOx are formed with a narrow size distribution and uniform spatial distribution. XPS experiments indicate that these clusters consist of oxidized Sn, and the corresponding reduction in the TiO2 substrate is observed. At higher Sn coverages, the surface is still dominated by two-dimensional clusters of SnOx, but larger three-dimensional clusters of metallic Sn also appear. As the Sn coverage is increased, the number of three-dimensional clusters increases, and the ratio of Sn/SnOx increases, suggesting that SnOx and reduced TiOx form at the cluster–support interface. When Pt is deposited on top of the Sn/SnOx clusters, the relatively mobile Pt atoms diffuse across the TiO2 surface and become incorporated into existing Sn/SnOx clusters. Furthermore, the addition of Pt to the Sn/SnOx clusters causes the reduction of SnOx to metallic Sn and the oxidation of Ti3+ to Ti4+; this behavior is attributed to the formation of Pt–Sn alloy clusters, which results in the diffusion of Sn away from the interface with the TiO2 support. In contrast, when Sn is deposited on an equal coverage of Pt clusters, new Sn/SnOx clusters are formed that coexist with Pt–Sn clusters. However, the surfaces of both Pt on Sn and Sn on Pt clusters are Sn-rich due to the lower surface free energy of Sn compared to Pt. DFT calculations demonstrate that M–TiO2 bonding is favored over M–M bonding for M = Sn, unlike for transition metals such as M = Pt, Au, Ni, and Co. Furthermore, the substantial charge transfer from Sn to TiO2 leads to dipole–dipole repulsion of Sn atoms that prevents agglomeration into the larger clusters that are observed for the mid-late transition metals. DFT studies also confirm that the addition of Pt to a Sn cluster results in strong Pt–Sn bond formation and diminished Sn–O interactions.

LiOx-modification of Ni and Co3O4 surfaces: An XPS, LEIS and LEED study

LiOx was deposited at room temperature by physical vapor deposition (PVD) on polycrystalline Ni foil and Co3O4(111) thin film, creating uniform model systems well-suited for surface-sensitive characterization by X-ray photoelectron spectroscopy (XPS), low energy ion scattering (LEIS) or low energy electron diffraction (LEED). In the case of Ni, about 15 layers of LiOx film were grown under the current conditions either stepwise or continuously, with XPS analysis indicating a deposition rate of 0.16 and 0.24 ML/min, respectively. Li 1s and O 1s spectra revealed that Li2O and to a lesser extent LiOH were preferentially formed. The stability of the LiOx films was examined in UHV, upon annealing at 573 K and upon hydrogen reduction at 723 K. On the more reactive Co3O4(111) film grown on Ir(100), the Li accommodation rate was about twice as high, at least within the first minutes of deposition. Post-deposition LEED showed an obscured cobalt oxide diffraction pattern, not unexpected in light of the LiOx deposited. On both substrates, LEIS characterization of Li (≈ 103 eV) was prevented by the high background in this kinetic energy region, due to surface roughness and unspecific scattering. Still, LiOx deposition was evident from the vanished LEIS signals of Ni or Co. The prepared LiOx-modified surfaces may serve as starting point for the future growth of epitaxial LixCoO2 model systems.

Surface Analysis of Perovskite Oxynitride Thin Films as Photoelectrodes for Solar Water Splitting.

Perovskite oxynitride semiconductors have attracted huge interest recently as promising photoelectrode materials for photoelectrochemical (PEC) water splitting. Depicted by, the extensive studies of the PEC activity of oxynitride powder-based photoelectrodes and/or deposited thin-film electrodes. High-crystalline-quality, oxynitride thin films grown by physical vapor deposition are ideal model systems to study the fundamental physical and chemical properties of the surface of these materials, including their evolution. In this work, using a combination of high-sensitivity low-energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS), we monitor surface evolution of LaTiOxNy (LTON) and CaNbOxNy (CNON) thin films before and after the PEC characterizations. The as-prepared epitaxial LTON films show a preferential LaO termination at the surface layers, followed by a Ti-enriched subsurface. Whereas, the polycrystalline CNON thin films exhibit a non-uniform surface, with a mixed surface termination and a significant Ca-segregated subsurface. After the PEC characterizations, additional precipitated LaO species are found on the outer surface of the LTON epitaxial films. However, no significant surface change is observed on the polycrystalline CNON films by LEIS. The XPS analysis shows, an increase of the oxidized Ti and Nb cations (Ti4+ and Nb5+) after the PEC reaction in the LTON and CNON films, respectively. The initial drops in photocurrent for the LTON and CNON films are attributed to the changes in the surface chemical status. This work provides insight into the surface characteristics and evolution of LTON and CNON oxynitride thin films as photoelectrodes for PEC applications.

Nb Texture Evolution and Interdiffusion in Nb/Si-Layered Systems.

In this paper, we present a detailed study on the microstructure evolution and interdiffusion in Nb/Si-layered systems. Interlayer formation during the early stages of growth in sputter-deposited Nb-on-Si and Si-on-Nb bilayer systems is studied in vacuo using a high-sensitivity low-energy ion-scattering technique. An asymmetric intermixing behavior is observed, where the Si-on-Nb interface is ∼2× thinner than the Nb-on-Si interface, and it is explained by the surface-energy difference between Nb and Si. During Nb-on-Si growth, the crystallization of the Nb layer occurs around 2.1 nm as-deposited Nb thickness with a strong Nb(110)-preferred orientation, which is maintained up to 3.3 nm as-deposited Nb thickness. A further increase in the Nb layer thickness above 3.3 nm results in a polycrystalline microstructure with a reduced degree of texture. High-resolution cross-sectional transmission electron microscopy imaging is performed on Nb/Si multilayers to study the effect of the Nb layer texture on interdiffusion during low-temperature annealing. Nb/Si multilayers with amorphous 2 nm Nb layers and strongly textured 3 nm thick Nb layers, with limited grain-boundary pathways for diffusion, show no observable interdiffusion during annealing at 200 °C for 8 h, whereas in a Nb/Si multilayer with polycrystalline 4 nm thick Nb layers, a ∼1 nm amorphous Nb/Si interlayer is formed at the Si-on-Nb interface during annealing.

Zinc and copper, by high sensitivity-low energy ion scattering

Low energy ion scattering (LEIS) is an extremely surface sensitive technique that can quantitatively analyze the outermost atomic layer of a material. In LEIS and high sensitivity-low energy ion scattering (HS-LEIS), straightforward quantitation is available using reference and/or standard materials. Here, we present the HS-LEIS spectra of zinc obtained with 3 keV 4He+ and 4 keV 20Ne+ projectile ions. Zinc is an important material with a wide range of applications. Thus, these spectra should be useful standards/references for future applications. A high purity zinc foil was used for these measurements after the removal of the oxide layer. As a reference for the instrumental sensitivity, the spectra for Cu from a high purity foil are also included with this submission. Atomic sensitivity and relative sensitivity factors for Zn and Cu are reported.

Ni-modified Fe3O4(001) surface as a simple model system for understanding the oxygen evolution reaction

Electrochemical water splitting is an environmentally friendly technology to store renewable energy in the form of chemical fuels. Among the earth-abundant first-row transition metal-based catalysts, mixed Ni-Fe oxides have shown promising performance for effective and low-cost catalysis of the oxygen evolution reaction (OER) in alkaline media, but the synergistic roles of Fe and Ni cations in the OER mechanism remain unclear. In this work, we report how addition of Ni changes the reactivity of a model iron oxide catalyst, based on Ni deposited on and incorporated in a magnetite Fe3O4(001) single crystal, using a combination of surface science techniques in ultra-high vacuum such as low energy electron diffraction (LEED), x-ray photoelectron spectroscopy (XPS), low-energy ion scattering (LEIS), and scanning tunneling microscopy (STM), as well as atomic force microscopy (AFM) in air, and electrochemical methods such as cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) in alkaline media. A significant improvement in the OER activity is observed when the top surface presents an iron fraction among the cations in the range of 20-40%, which is in good agreement with what has been observed for powder catalysts. Furthermore, a decrease in the OER overpotential is observed following surface aging in electrolyte for three days. At higher Ni load, AFM shows the growth of a new phase attributed to an (oxy)-hydroxide phase which, according to CV measurements, does not seem to correlate with the surface activity towards OER. EIS suggests that the OER precursor species observed on the clean and Ni-modified surfaces are similar and Fe-centered, but form at lower overpotentials when the surface Fe:Ni ratio is optimized. We propose that the well-defined Fe3O4(001) surface can serve as a model system for understanding the OER mechanism and establishing the structure-reactivity relation on mixed Fe-Ni oxides.

Analysis of InGaP(001) surface by the low energy ion scattering spectroscopy

Ion scattering spectroscopy, which is a variation of low energy ion scattering (LEIS) that employs glancing scattering angles, is performed on InGaP(001) surfaces. LEIS energy distribution are simulated by computer simulation along the <110> and <ī10> direction, and the match of the positions of the flux peaks shows that the top three atomic layers are bulk-terminated. A newly observed feature are identified as a minimum in the multiple scattering when the ion beam incidence is along a low index direction. Calculated trajectories of scattered ions. This new method for analysis of large-angle LEIS data was shown to be useful for accurately investigating complex surface structures.

Activity for the ORR on Pt-Pd-Co ternary alloy electrodes is markedly affected by surface structure and composition

The oxygen reduction reaction (ORR) has been studied on single crystals of the ternary alloys Pt45Pd45Co10 and Pt77Pd8Co15 in O2-saturated 0.1 M HClO4 using a hanging meniscus rotating disk electrode. Specific activity for the ORR depends markedly on the composition and surface structure of the single crystal electrodes. The activity on Pt45Pd45Co10(hkl) increases in the order Pt45Pd45Co10(100) ≈ Pt45Pd45Co10(111) < Pt45Pd45Co10(110), whereas the activity on Pt77Pd8Co15(hkl) is Pt77Pd8Co15(100) < Pt77Pd8Co15(110) < Pt77Pd8Co15(111). The ORR activity of Pt45Pd45Co10(110) is 2.6 times higher than that of Pt3Co(110), which is known to have high ORR activity. The activity of Pt77Pd8Co15(111) is 2.4 times higher than that of Pt3Co(111). Low energy ion scattering spectroscopy (LE-ISS) shows that Pt and Co coexist at the topmost layer of Pt77Pd8Co15(111). Pt, Pd and Co atoms are dissolved uniformly in the inner layers according to elemental mapping by scanning transmission electron microscopy with energy-dispersive X-ray spectroscopy (STEM-EDS).

On the Origin of Reduced Cytotoxicity of Germanium-Doped Diamond-Like Carbon: Role of Top Surface Composition and Bonding.

This work attempts to understand the behaviour of Ge-induced cytotoxicity of germanium-doped hydrogen-free diamond-like carbon (DLC) films recently thoroughly studied and published by Jelinek et al. At a low doping level, the films showed no cytotoxicity, while at a higher doping level, the films were found to exhibit medium to high cytotoxicity. We demonstrate, using surface-sensitive methods—two-angle X-ray-induced core-level photoelectron spectroscopy (ARXPS) and Low Energy Ion Scattering (LEIS) spectroscopy, that at a low doping level, the layers are capped by a carbon film which impedes the contact of Ge species with tissue. For higher Ge content in the DLC films, oxidized Ge species are located at the top surface of the layers, provoking cytotoxicity. The present results indicate no threshold for Ge concentration in cell culture substrate to avoid a severe toxic reaction.

Biocompatibility study of nanocomposite titanium boron nitride (TiBN) thin films for orthopedic implant applications

Abstract Thin films of Titanium Boron Nitride (TiBN) were sputtered onto SS 316 L substrates and were characterized for their microstructure and biocompatibility. The X-ray diffraction (XRD) showed reflections of TiN and an absence of boron containing phases. High resolution transmission electron microscopy (HRTEM) mapping, ion scattering spectroscopy (ISS), and X-ray photoelectron spectroscopy (XPS) determined the presence of boron. The mechanical properties of the thin films were characterized by both nanoindentation and block on ring wear tester which indicated a hardness of 13 GPa and a friction coefficient of 0.27, respectively. The biocompatibility screening of the thin films was performed via hemocompatibility studies, cytotoxicity assay, mineralization assay and monitoring its bioactivity in SBF. The results revealed non-cytotoxic and non-thrombogenic nature of the sputtered TiBN thin films. In addition, mineralization studies demonstrated the ability of thin films to facilitate calcium mineralization by MC3T3-E1 cells. The corrosion behavior of the TiBN coated specimen was monitored electrochemically by impedance spectroscopy and potentiodynamic polarization techniques. A positive shift in Ecorr value and a higher Rct value compared to uncoated control indicated the protection of the underlying metal by the sputtered films.