Calcium and fluorine signals in HS-LEIS for CaF2(111) and powder-Quantification of atomic surface concentrations using LiF(001), Ca, and Cu references

The powder of CaF2 has been proposed as a practical reference for the quantitation of Ca and F in low energy ion scattering (LEIS) analysis. It is chemically inert, easy to clean, and inexpensive. LEIS is extremely surface sensitive. Thus, in contrast to surface analytic techniques such as x-ray photoelectron spectroscopy and secondary ion mass spectroscopy, the surface termination of a sample is clearly reflected in the LEIS results. It is, thus, unlikely that in LEIS, the F/Ca ratio for CaF2 is 2.0. This paper supports the reliability of the CaF2 powder reference by evaluating the calcium and fluorine atomic surface concentrations, roughness factor and shows that the surface termination of the powder is the same as that of CaF2(111). The CaF2 samples are treated by annealing at 725 K and measured at 625 K. The presented spectra are practically free of contamination. Ion scattering by LiF (001), an evaporated Ca layer, and a Cu foil are used as basic references for Ca and F. The atomic sensitivity factors and the relative sensitivity factors have been determined for F, Ca, and Cu (3 keV He+, 145 degrees). The F/Ca atomic ratio is found to be the same (2.3 +/- 0.1) for CaF2(111) and its powder. For the powder, the Ca and F signals are reduced by a factor of 0.77 +/- 0.03 in comparison with those for the single crystal

Application of low-energy ion scattering to studies of growth

The possibilities of low-energy ion scattering spectroscopy for surface analysis are discussed. By choosing the proper experimental conditions the structure of single crystal surfaces can be determined very accurately. Furthermore, low-energy ion scattering (LEIS) can be used to determine the atomic composition of the outermost atomic layer of any (crystalline or not) surface. This extreme surface sensitivity makes LEIS an ideal tool to investigate growth mechanisms. The results for calcined powders of oxidic spinels suggest that only one or two principal planes occur at the surface. This greatly facilitates the interpretation and application of LEIS to growth on powders. Some applications to study the growth mechanism of supported catalysts are presented

Quantitative analysis of calcium and fluorine by high-sensitivity low-energy ion scattering: Calcium fluoride

Low-energy ion scattering (LEIS) probes the atomic composition of the outer surface. Well-defined reference samples are used for the quantitation. For elements like fluorine and calcium, it is not easy to find suitable, clean, and homogeneous references, since fluorine is a gas and calcium is a very reactive metal. In contrast to surface analytic techniques such as XPS, the extreme surface sensitivity of LEIS makes it difficult to use stable compounds like CaF2 as reference, since these compounds are not homogeneous at the atomic scale. With LEIS, CaF2 is not expected to show an atomic ratio F/Ca = 2.0. Thus, before CaF2 can be used as reference, its atomic surface concentrations have to be determined. Here, 3-keV He+ scattering by a LiF(001) single crystal, an evaporated layer of Ca, and a Cu foil are used as basic references. High-purity CaF2 is available in two forms: a single crystal and a powder. For a practical reference, powders are preferred, since if bulk impurities segregate to the surface, they will be dispersed over a large surface area. It is found that both CaF2 (111) and powder have similar F/Ca atomic ratios. This confirms the F termination for both samples. For the powder, the F and Ca signals are reduced by 0.77 ± 0.03 in comparison with those for the single crystal. The atomic sensitivity factors and relative sensitivity factors have been determined for F, Ca, and Cu

Surface active sites present in the orthorhombic M1 phases: Low energy ion scattering study of methanol and allyl alcohol chemisorption over Mo-V-Te-Nb-O and Mo-V-O catalysts

The outermost surface compositions and chemical nature of active surface sites present on the orthorhombic (M1) Mo-V-O and Mo-V-Te-Nb-O phases were determined employing methanol and allyl alcohol chemisorption and surface reaction in combination with low energy ion scattering (LEIS). These orthorhombic phases exhibited vastly different behavior in propane (amm)oxidation reactions and, therefore, represented highly promising model systems for the study of the surface active sites. The LEIS data for the Mo-V-Te-Nb-O catalyst indicated surface depletion for V ()23%) and Mo ()27%), and enrichments for Nb (+55%) and Te (+165%) with respect to its bulk composition. Only minor changes in the topmost surface composition were observed for this catalyst under the conditions of the LEIS experiments at 400 °C, which is a typical temperature employed in these propane transformation reactions. These findings strongly suggested that the bulk orthorhombic Mo-V-Te-Nb-O structure is terminated by a unique active and selective surface layer in propane (amm)oxidation. Moreover, direct evidence was obtained that the topmost surface VOx sites in the orthorhombic Mo-V-Te-Nb-O catalyst were preferentially covered by chemisorbed allyloxy species, whereas methanol was a significantly less discriminating probe molecule. The surface TeOx and NbOx sites on the Mo-V-Te-Nb-O catalyst were unable to chemisorb these probe molecules to the same extent as the VOx and MoOx sites. These findings suggested that vastly different catalytic behavior exhibited by the Mo-V-O and Mo-V-Te-Nb-O phases is related to different surface locations of V5+ ions in the orthorhombic Mo-V-O and Mo-V-Te-Nb-O catalysts. Although the proposed isolated V5+ pentagonal bipyramidal sites in the orthorhombic Mo-V-O phase may be capable of converting propane to propylene with modest selectivity, the selective 8-electron transformation of propane to acrylic acid and acrylonitrile may require the presence of several surface VOx redox sites lining the entrances to the hexagonal and heptagonal channels of the orthorhombic Mo-V-Te-Nb-O phase. Finally, the present study strongly indicated that chemical probe chemisorption combined with low energy ion scattering (LEIS) is a novel and highly promising surface characterization technique for the investigation of the active surface sites present in the bulk mixed metal oxides

A study of the surface region of the Mo-V-Te-O catalysts for propane oxidation to acrylic acid

The bulk mixed Mo - V - Te oxides possess high activity and selectivity in propane oxidation to acrylic acid and represent well-defined model catalysts for studies of the surface molecular structure-activity/selectivity relationships in this selective oxidation reaction. The elemental compositions, metal oxidation states, and catalytic functions of V, Mo, and Te in the surface region of the model Mo - V - Te - O system were examined employing low energy ion scattering (LEIS) and X-ray photoelectron spectroscopy (XPS). This study indicated that the surfaces of these catalysts are terminated with a monolayer, which possesses a different elemental composition from that of the bulk. The rates of propane consumption and formation of propylene and acrylic acid depended on the topmost surface V concentration, whereas no dependence of these reaction rates on either the surface Mo or Te concentrations was observed. These findings suggested that the bulk Mo - V - Te - O structure may function as a support for the unique active and selective surface monolayer in propane oxidation to acrylic acid. The results of this study have important practical consequences for the development of improved selective oxidation catalysts by introducing surface metal oxide components to form new surface active V - O - M sites for propane oxidation to acrylic acid

The surface of catalytically active spinels

By substitution of the Mn and Co cations in different sites in the spinel structure of Mn3O4 and Co3O4, by other cations which are not active in the selective reduction of nitrobenzene to nitrosobenzene, the role of these sites in the catalytic reaction can be studied. By combining activity measurements with quantitative surface analysis by low-energy ion scattering, which only probes the outermost atomic layer, the catalytic performance can be related to the surface composition. The results suggest that octahedral sites are exposed almost exclusively at the surface of the spinel oxide powders and that only these sites participate in the reaction. The only two low-index planes of the spinel structure which can satisfy this condition are identified as B(111) and D(110). Although the present results pertain to Mn3O4 and Co3O4, it is believed that the absence of occupied tetrahedral sites at the surface is a more general property of spinels

The surface composition/organization of ionic liquids with Au nanoparticles revealed by high sensitivity Low Energy Ion Scattering.

International audienceHigh-sensitivity low-energy ion scattering (HS-LEIS) analysis was used to elucidate the outermost layer of both functionalized and non-functionalized imidazolium ionic liquids (ILs). The IL outermost layer is composed of all atoms of both cations and anions. The HS-LEIS analyses also allow for quantitative measurement of the thickness of IL overlayers on Au nanoparticles prepared by sputter deposition, which was shown to be a monolayer of ions, as predicted by density functional theory calculations

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

Analysis of the outer surface of platinum-gold catalysts by low-energy ion scattering

Low-energy ion scattering (LEIS) can be used to selectively analyse the atomic composition of the outer atomic layer of a catalyst, i.e., precisely the atoms that largely determine its activity and selectivity. It is shown how a new development in LEIS significantly improves its mass resolution. Using this advanced separation and quantification of signals from platinum and gold, the atomic composition of the outer surface of a realistic supported platinum-gold bimetallic system can be determined for the first time