High-sensitivity Low-energy Ion Scattering Research Articles

Design of Cr-Free Promoted Copper-Iron Oxide-Based High-Temperature Water-Gas Shift Catalysts.

The effect of Ce addition to the Cr-free Al-promoted Cu-Fe oxide-based catalysts is investigated. Catalyst characterization (X-ray diffraction (XRD), in situ Raman spectroscopy, high-sensitivity low-energy ion scattering (HS-LEIS), Brunauer-Emmett-Teller (BET) analysis), CO-temperature-programmed reduction chemical probing, and steady-state WGS activity reveal that (i) in the absence of Al, Ce addition via coprecipitation has a detrimental effect on the catalytic activity related to the poor thermostability and formation of less active Ce-Cu-O NPs, (ii) the addition of Ce via coprecipitation also does not improve the performance of the CuAlFe catalyst because of the formation of a thick CeOx overlayer on the active Cu-FeOx interface, and (iii) impregnation of Ce onto the CuAlFe catalyst exhibits significant improvement in catalytic performance due to the formation of a highly active CeOx-FeOx-Cu interfacial area. In summary, Al does not surface-segregate and serves as a structural promoter, while Ce and Cu surface-segregate and act as functional promoters in Ce/CuAlFe mixed oxide catalysts.

Controlling the surface silanol density in capillary columns and planar silicon via the self-limiting, gas-phase deposition of tris(dimethylamino)methylsilane, and quantification of surface silanols after silanization by low energy ion scattering

Surface silanols (Si-OH) play a vital role on fused silica surfaces in chromatography. Here, we used an atmospheric-pressure, gas-phase reactor to modify the inner surface of a gas chromatography, fused silica capillary column (0.53 mm ID) with a small, reactive silane (tris(dimethylamino)methylsilane, TDMAMS). The deposition of TDMAMS on planar witness samples around the capillary was confirmed with X-ray photoelectron spectroscopy (XPS), ex situ spectroscopic ellipsometry (SE), and wetting. The number of surface silanols on unmodified and TDMAMS-modified native oxide-terminated silicon were quantified by tagging with dimethylzinc (DMZ) via atomic layer deposition (ALD) and counting the resulting zinc atoms with high sensitivity-low energy ion scattering (HS-LEIS). A bare, clean native oxide – terminated silicon wafer has 3.66 OH/nm2, which agrees with density functional theory (DFT) calculations from the literature. After TDMAMS modification of native oxide-terminated silicon, the number of surface silanols decreases by a factor of ca. 10 (to 0.31 OH/nm2). Intermediate surface testing (IST) was used to characterize the surface activities of functionalized capillaries. It suggested a significant deactivation/passivation of the capillary with some surface silanols remaining; the modified capillary shows significant deactivation compared to the native/unmodified fused silica tubing. We believe that this methodology for determining the number of residual silanols on silanized fused silica will be enabling for chromatography.

Area-Selective Atomic Layer Deposition of ZnO on Si\SiO2 Modified with Tris(dimethylamino)methylsilane.

Delayed atomic layer deposition (ALD) of ZnO, i.e., area selective (AS)-ALD, was successfully achieved on silicon wafers (Si\SiO2) terminated with tris(dimethylamino)methylsilane (TDMAMS). This resist molecule was deposited in a home-built, near-atmospheric pressure, flow-through, gas-phase reactor. TDMAMS had previously been shown to react with Si\SiO2 in a single cycle/reaction and to drastically reduce the number of silanols that remain at the surface. ZnO was deposited in a commercial ALD system using dimethylzinc (DMZ) as the zinc precursor and H2O as the coreactant. Deposition of TDMAMS was confirmed by spectroscopic ellipsometry (SE), X-ray photoelectron spectroscopy (XPS), and wetting. ALD of ZnO, including its selectivity on TDMAMS-terminated Si\SiO2 (Si\SiO2\TDMAMS), was confirmed by in situ multi-wavelength ellipsometry, ex situ SE, XPS, and/or high-sensitivity/low-energy ion scattering (HS-LEIS). The thermal stability of the TDMAMS resist layer, which is an important parameter for AS-ALD, was investigated by heating Si\SiO2\TDMAMS in air and nitrogen at 330 °C. ALD of ZnO takes place more readily on Si\SiO2\TDMAMS heated in the air than in N2, suggesting greater damage to the surface heated in the air. To better understand the in situ ALD of ZnO on Si\SiO2\TDMAMS and modified (thermally stressed) forms of it, the ellipsometry results were plotted as the normalized growth per cycle. Even one short pulse of TDMAMS effectively passivates Si\SiO2. TDMAMS can be a useful, small-molecule inhibitor of ALD of ZnO on Si\SiO2 surfaces.

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

Surface silanols (SiOH) are important moieties on glass surfaces. Here we present a tag-and-count approach for determining surface silanol densities, which consists of tagging surface silanols with Zn via atomic layer deposition (ALD) followed by detection of the zinc by high sensitivity-low energy ion scattering (HS-LEIS). Shards of fused silica were hydroxylated with aqueous hydrofluoric acid (HF) and then heated to 200, 500, 700, or 900 °C. These heat treatments increasingly condense and remove surface silanols. The samples then underwent one ALD cycle with dimethylzinc (DMZ) or diethylzinc (DEZ) followed by water. As expected, fused silica surfaces heated to higher temperatures showed lower Zn coverages. When fused silica surfaces treated at 200 °C were exposed to DMZ for two different dose times, the same sub-monolayer quantity of Zn was obtained by X-ray photoelectron spectroscopy (XPS). Surface cleaning/preparation immediately before HS-LEIS, including atomic oxygen treatment and annealing, played a critical role in these efforts. Surfaces treated with DMZ generally showed slightly higher Zn signals by LEIS. Using this methodology, a value of 4.59 OH/nm2 was found for fully hydroxylated fused silica. Both this result and those obtained at 500, 700, and 900 °C are in very good agreement with literature values.

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

Surface characterization of ultrathin atomic layer deposited molybdenum oxide films using high-sensitivity low-energy ion scattering

Few-layer, high quality, molybdenum oxide films were successfully grown using atomic layer deposition (ALD) and characterized using high-sensitivity low-energy ion scattering (HS-LEIS). The deposition quality, uniformity, and number of layers (thickness) of these films have a drastic effect on overall film properties and, therefore, on performance in electronic devices. In particular, achieving uniform and reproducible nucleation is important for creation of single-monolayer films. However, islanding often occurs during film growth in which film discontinuities or nonuniform thicknesses are formed, both of which are undesirable. We have investigated the uniformity and thickness control of molybdenum oxide films that are deposited via ALD and are precursors to MoTe2 transition metal dichalcogenides. HS-LEIS was used to assess surface coverage and islanding of thin MoOx films ranging in thickness from 0.2 nm to over 7 nm. The absence of a signal from the substrate indicated uniform nucleation and that complete surface coverage by MoOx occurred at a film thickness of approximately 0.6 nm (14 ALD cycles). Monte-Carlo-based simulations were used to predict LEIS spectra, which allowed for quantitative analysis of nucleation and film growth. These simulated spectra of few-layer films further confirmed that the grown films exhibited uniform nucleation.

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.

Calcium fluoride and gold reference by high sensitivity-low energy ion scattering

Information about the outermost atomic layer of a material is critically important for many processes and materials, including in catalysis, tribology, wetting, corrosion, and thin film growth. Low energy ion scattering (LEIS) is an extremely surface sensitive technique that can quantitatively analyze the outermost atomic layer of a material. Recent developments in LEIS have resulted in a particularly high sensitivity version of the technique known as high sensitivity LEIS (HS-LEIS). In LEIS and HS-LEIS, reference and/or standard materials allow straightforward quantitation of the spectra. In this submission, the authors show the HS-LEIS spectrum of a high purity calcium fluoride powder. CaF2 is a useful reference material for calcium in LEIS because it is relatively inexpensive, not hygroscopic, and available in high purity. Due to the applications/uses of calcium in different areas of science, including in catalysis, polymer light emitting diodes, organic photovoltaics, solid oxide fuel cells, and solar cells, these data may enable future work in these areas. A reference spectrum for gold is included with this submission.

Surface modification of SOFC cathodes by Co, Ni, and Pd oxides

The effect of adding transition-metal catalysts to the surfaces of Solid-Oxide-Fuel-Cell (SOFC) cathodes was examined. Monolayer amounts of Pd, Ni, and Co were deposited by atomic layer deposition (ALD) to cathodes prepared by impregnation of La0.8Sr0.2FeO3 (LSF), La0.8Sr0.2CoO3 (LSCo), and La0.6Sr0.4 Co0.2Fe0.8O3 (LSCF) into YSZ scaffolds. Incorporating the catalysts by ALD allowed modification of the surface composition without changing the cathode surface area or structure. X-ray Photoelectron Spectroscopy and High Sensitivity-Low Energy Ion Scattering were used to confirm ALD growth rates that were measured gravimetrically. Electrochemical impedance spectroscopy indicated that the addition of Pd had a small negative effect on all of the cathodes. Ni and Co exhibited only a small effect on LSCo and LSCF cathodes but both decreased the cathode impedance for LSF. It is suggested that an essential feature in cathode promotion by Co and Ni is that these cations are incorporated into the perovskite surface and thereby increase the concentration of oxygen vacancies, while Pd only blocked adsorption sites.

Tiny Ruthenium‐Cobalt‐Cobalt Hydroxide Nanoparticles Supported on Graphene for Efficiently Catalyzing Naphthalene Complete Hydrogenation

AbstractThe nanocatalyst of graphene‐supported ruthenium/cobalt/cobalt hydroxide nanoparticles (denoted as Ru/Co/Co(OH)2/G) was obtained at ambient temperature by using chemical reduction with hydrazine hydrate as reducing agent and galvanic replacement reaction methods. The nanostructure of Ru nanoclusters‐on‐cobalt‐on‐cobalt hydroxide nanoparticles (NPs) has been proved in Ru/Co/Co(OH)2/G by X‐ray diffraction (XRD), X‐ray photoelectron spectroscopy (XPS), transmission electron microscope (TEM), high resolution TEM (HRTEM), scanning transmission electron microscopy‐energy dispersive X‐ray spectroscopy (STEM‐EDS) elemental line scanning and mapping analysis, and high‐sensitivity low‐energy ion scattering (HS‐LEIS) measurement techniques. The Ru/Co/Co(OH)2/G catalyst showed excellent catalytic activity and 100% selectivity toward decalin for naphthalene hydrogenation reaction at the reaction temperature of 100 °C under initial 4.48 MPa hydrogen pressure probably owing to the synergetic effect of Ru, Co and Co(OH)2 active sites.

(Invited) Enhancing SOFC Anode Stability through Atomic Layer Deposition

Ni-YSZ cermet anodes are highly optimized to achieve good mechanical strength and electrochemical performance. However, they still suffer from sensitivity to impurities like sulfur, poor redox stability, and a tendency to form carbon fibers if the H2O:C ratio is too low or there are higher hydrocarbons in the feed to the cell. In this project we utilize atomic layer deposition (ALD) to grow thin oxide films onto the anode surface to enhance both the thermal stability and coking tolerance of the Ni-YSZ cermet. We have developed an ALD system that enables controlled, sub-monolayer conformal coating of mixed metal oxide films. The growth process, coverage, composition and operational stability of these films is carefully analyzed utilizing a unique High-Sensitivity Low Energy Ion Scattering (HS-LEIS) instrument that provides the composition of the outermost atomic surface layer of the material. This ex-situ characterization is closely correlated to the electrochemical performance of the anodes.

Functionalized Ionic Liquids Sputter Decorated with Pd Nanoparticles

The fabrication of surface clean palladium nanoparticles of 3–4nm was accomplished in imidazolium-based functionalized ionic liquids (ILs) having methoxy, cyano, and thio groups by magnetron sputtering deposition. The size of the NPs was strongly dependent on the surface composition and/or organisation of the ILs. The NP growth apparently occurred preferentially in the bulk of the fluids, whereas nucleation apparently occurred preferentially at the IL surface. Smaller NPs were detected close to the methoxy containing IL surface and were covered by at least one layer of IL ion pairs, as revealed by high-sensitivity low-energy ion scattering (HS-LEIS) measurements.

Ruthenium stabilized on transition metal-on-transition metal oxide nanoparticles for naphthalene hydrogenation

The multi-metallic nanocatalysts of ruthenium nanoclusters-on-transition metal/transition metal oxide nanoparticles (TM/TMO NPs) then supported on carbon (Ru/Ni/NiO/C or Ru/Co/Co3O4/C) were designed and synthesized. The Ni/NiO or Co/Co3O4 NPs strongly stabalized the ruthenium nanoclusters by the interfacial interaction among them. These catalysts exhibited high catalytic activity and 100% selectivity to decalin for naphthalene hydrogenation due to the synergy effect of multiple catalytic sites, where naphthalene was absorbed and activated at the TMO sites (NiO or Co3O4), H2 was activated at the Ru sites and it produced the activated H* species, H* was transferred to the surface of NiO or Co3O4 by the hydrogen spillover effect of TM (Ni or Co), reacting with the activated naphthalene and forming decalin. The nanostructures and synergetic effect of the Ru/Ni/NiO/C and Ru/Co/Co3O4/C catalysts were revealed by a series of techniques, such as high-resolution transmission electron microscope (HRTEM), temperature-programmed reduction (TPR), scanning transmission electron microscopy-energy dispersive X-ray spectroscopy (STEM-EDS) mapping, high-sensitivity low-energy ion scattering (HS-LEIS) and X-ray absorption spectroscopy (XAS). It is promising that the hydrogen storage can proceed at room temperature via catalyzing naphthalene hydrogenation over the Ru/Ni/NiO/C or Ru/Co/Co3O4/C catalyst.

Highly Dispersed Metal Carbide on ZIF-Derived Pyridinic-N-Doped Carbon for CO2 Enrichment and Selective Hydrogenation.

Catalytic conversion of CO2 into chemicals is a critical issue for energy and environmental research. Among such reactions, converting CO2 into CO has been regarded as a significant foundation to generate a liquid fuels and chemicals on a large scale. In this work, zeolitic imidazolate framework-derived N-doped carbon-supported metal carbide catalysts (M/ZIF-8-C; M=Ni, Fe, Co and Cu) with highly dispersed metal carbide were prepared for selective CO2 hydrogenation. Under the same metal loadings, catalytic activity for CO2 hydrogenation to CO follows the order: Ni/ZIF-8-C≈Fe/ZIF-8-C>Co/ZIF-8-C>Cu/ZIF-8-C. These catalysts are composed of carbide or metal supported on pyridinic N sites within the N-doped carbon structure. ZIF-8-derived pyridinic nitrogen and carbide effect CO2 adsorption, whereas dispersed Ni or Fe carbide and metal species serve as an active site for CO2 hydrogenation. The supported Ni catalyst exhibits extraordinary catalytic performance, which results from high dispersion of the metal and exposure of the carbide. Based on high-sensitivity low-energy ion scattering (HS-LEIS) and line scan results, density functional theory (DFT) was used to understand reaction mechanism of selective CO2 hydrogenation over Ni/ZIF-8-C. The product CO is derived mainly from the direct cleavage of C-O bonds in CO2 * rather than decomposition of COOH*. The CO* desorption energy on Ni/ZIF-8-C is lower than that for further hydrogenation and dissociation. Comparison of Ni/ZIF-8-C with ZIF-8-C indicates that the combined effects of the highly dispersed metal or carbide and weak CO adsorption result in high CO selectivity for CO2 hydrogenation.

Fabrication of naked silver nanoparticles in functionalized ionic liquids

Surface clean silver nanoparticles of 5-8 nm in size were prepared in functionalized imidazolium-based ionic liquids containing methoxy, cyano, and thio groups by magnetron sputtering deposition. The surface composition/organization of the ILs controls the size and location of NPs in these fluids. The simple model of metal–ligand interaction of surface-functionalized ILs can be used to explain and predict the structural organization of the sputtered NPs in these fluids. The sputtered NPs tend to penetrate deeper into the regions of the ILs containing lower coordinating groups (OMe), whereas those associated with strong sigma donating groups (CN and SH) tend to place the small nanoparticles close to the liquid surface. We estimate a single monolayer of IL surrounding the smaller Ag NPs located closer to the surface as determined by High-Sensitivity Low-Energy Ion Scattering (HS-LEIS).

Disclosure of the Surface Composition of TiO2-Supported Gold–Palladium Bimetallic Catalysts by High-Sensitivity Low-Energy Ion Scattering Spectroscopy

It is well-known that there is a critical relationship between the surface composition and activity for a bimetallic catalyst. However, the surface is normally reconstructed under different conditions, which makes the surface more complicated. In this work, TiO2-supported colloidal PdxAuy-NPs with different atomic ratios were prepared and treated under oxidation, reduction, and actual catalytic reaction conditions. The surface composition and structure were measured using X-ray photoelectron spectroscopy (XPS) and high-sensitivity low-energy ion scattering spectroscopy (HS-LEIS). Phase diagrams of the surface compositions under various conditions as a function of the bulk composition were established. Oxidation induces dealloying and enrichment of PdO on the surface, while H2 reduction results in realloying with just slight surface enrichment of Pd. Gold helps Pd to resist oxidation and enhances its activity for CO oxidation at low temperature.

Impact of Microstructure on MoS2 Oxidation and Friction.

This work demonstrates the role of microstructure in the friction and oxidation behavior of the lamellar solid lubricant molybdenum disulfide (MoS2). We report on systematic investigations of oxidation and friction for two MoS2 films with distinctively different microstructures-amorphous and planar/highly-ordered-before and after exposure to atomic oxygen (AO) and high-temperature (250 °C) molecular oxygen. A combination of experimental tribology, molecular dynamics simulations, X-ray photoelectron spectroscopy (XPS), and high-sensitivity low-energy ion scattering (HS-LEIS) was used to reveal new insights about the links between structure and properties of these widely utilized low-friction materials. Initially, ordered MoS2 films showed a surprising resistance to both atomic and molecular oxygens (even at elevated temperature), retaining characteristic low friction after exposure to extreme oxidative environments. XPS shows comparable oxidation of both coatings via AO; however, monolayer resolved compositional depth profiles from HS-LEIS reveal that the microstructure of the ordered coatings limits oxidation to the first atomic layer.

Combining Ru, Ni and Ni(OH)2 active sites for improving catalytic performance in benzene hydrogenation

In this study, the Ru0.04Ni0.96/C(T) catalysts were successfully prepared by the simple methods of hydrazine-reduction and galvanic replacement, where 0.04/0.96 and T represented the Ru/Ni atomic ratio and reducing temperature of the catalyst in N2+10%H2, respectively. The nanostructures of the Ru0.04Ni0.96 nanoparticles in the Ru0.04Ni0.96/C(T) catalysts were controlled by modulating their annealing temperature in N2+10%H2 and characterized by an array of techniques, including X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM), high resolution transmission electron microscopy (HRTEM), scanning transmission electron microscopy energy dispersive X-ray spectroscopy (STEM-EDS) mapping and high-sensitivity low-energy ion scattering (HS-LEIS). The Ru0.04Ni0.96/C(30) catalyst, which was composed of Ru clusters or single atoms supported on Ni/Ni(OH)2 nanoparticles, exhibited much better catalytic performance for benzene hydrogenation than the Ru0.04Ni0.96/C(T) catalysts reduced at above 30 °C, such as Ru0.04Ni0.96/C(160) with the nanostructure of partial Ru0.04Ni0.9 alloy and Ru0.04Ni0.96/C(280) with the nanostructure of complete Ru0.04Ni0.9 alloy. The reason was that the synergistic effect of multiple active sites – Ru, Ni and Ni(OH)2 sites was present in the Ru0.04Ni0.96/C(30) catalyst, where hydrogen was preferentially activated at Ru sites, benzene was probably activated at Ni(OH)2 surface and Ni acted as a “bridge” for transferring activated H∗ species to activated benzene by hydrogen spillover effect, hydrogenating and forming product – cyclohexane. This study also provided a typical example to illustrate that the synergy effect of multiple active sites can largely improve the catalytic hydrogenation performance.

Synthesis of Ru/CoNi crystals with different morphologies for catalytic hydrogenation

Cobalt–nickel alloy crystals with different morphologies, such as flower-like, column-like, mushroom-like and dendrite-like, were prepared by a facile hydrothermal or solvothermal reduction approach without the addition of any surfactant, using hydrazine hydrate as a reducing agent, ethanediamine as a capping agent and a mixture of nickel(II) chloride hexahydrate (NiCl2·6H2O) and cobalt(II) chloride hexahydrate (CoCl2·6H2O) as a precursor. The effect of hydrothermal temperature (120, 150 or 180 °C) and solvent (water or ethanol) on the shape of the CoNi crystals was investigated in this work. The corresponding Ru/CoNi catalysts (Ru-on-CoNi nanocrystals) were obtained via a galvanic replacement reaction. The sizes, element chemical states, morphologies and structures of the CoNi and Ru/CoNi samples were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), energy dispersive X-ray spectroscopy (EDS), X-ray photoelectron spectroscopy (XPS), and high-sensitivity low-energy ion scattering (HS-LEIS) techniques. The catalytic performance of the as-synthesized catalysts was evaluated by using the benzene hydrogenation reaction. The Ru/CoNi catalyst with a dendrite-like morphology exhibited the highest catalytic hydrogenation activity among the Ru/CoNi catalysts with different shapes. This was mainly due to its high Ru dispersion, many defect sites and positive synergistic effect compared with ruthenium, nickel and cobalt related species. Importantly, the cost of recycling the Ru/CoNi catalysts was relatively low because they could be recycled by magnetic separation.

In vacuo growth studies of Ru thin films on Si, SiN, and SiO2 by high-sensitivity low energy ion scattering

In vacuo high-sensitivity low energy ion scattering (HS-LEIS) has been used to investigate the initial growth stages of DC sputtered Ru on top of Si, SiN, and SiO2. The high surface sensitivity of this technique allowed an accurate determination of surface coverages and thicknesses required for closing the Ru layer on all three substrates. The Ru layer closes (100% Ru surface signal) at about 2.0, 3.2, and 4.7 nm on top of SiO2, SiN, and Si, respectively. In-depth Ru concentration profiles can be reconstructed from the Ru surface coverages when considering an error function like model. The large intermixing (4.7 nm) for the Ru-on-Si system is compared to the reverse system (Si-on-Ru), where only 0.9 nm intermixing occurs. The difference is predominantly explained by the strong Si surface segregation that is observed for Ru-on-Si. This surface segregation effect is also observed for Ru-on-SiN but is absent for Ru-on-SiO2. For this last system, in vacuo HS-LEIS analysis revealed surface oxygen directly after deposition, which suggests an oxygen surface segregation effect for Ru-on-SiO2. In vacuo XPS measurements confirmed this hypothesis based on the reaction of Ru with oxygen from the SiO2, followed by oxygen surface segregation.