Synchrotron Radiation X-ray Photoemission Spectroscopy Research Articles

Rationally-designed sandwiched nanostructures boosting Fe-N based catalysts toward efficient oxygen reduction electrocatalysis in acidic medium

The development of acidic-available noble-metal-free oxygen reduction reaction (ORR) catalysts with high activity and good long-term durability is of significant importance for efficient proton exchange membrane fuel cells, but is still very challenging. Herein, we develop originally a facile wet-chemical-adsorption, pyrolysis and post-etching strategy to effectively intercalate Fe clusters among two nitrogen-doped carbon (NC) layers, forming unique hollow spherical nanostructures with sandwiched NC/Fe/NC shells as an active ORR catalyst. Thanks to the sandwiched nanostructure and the active Fe-N species, this as-prepared hollow sandwiched NC/Fe/NC catalyst could present superior ORR activity in an acidic medium, with a nice onset potential of 0.92 V and decent diffusion-limited current density of ~5.1 mA/cm2. The NC/Fe/NC catalyst manifests strong methanol tolerance and outstanding durability during a long-term acidic ORR operation, being a promising alternative to Pt-based catalysts toward efficient proton exchange membrane fuel cells. Synchrotron radiation characterizations and X-ray photoemission spectroscopy reveal at the atomic-level that the abundant robust Fe–N bonds presented in the sandwiched shells of hollow NC/Fe/NC spherical catalyst contribute substantially to high electrochemical activity and superior corrosion-resistance for efficient 4e¯ ORR in acidic electrolyte.

Interface atom mobility and charge transfer effects on CuO and Cu2O formation on Cu3Pd(111) and Cu3Pt(111)

We bombarded mbox{text{Cu}_{3}text{Pd}(111)} and mbox{text{Cu}_{3}text{Pt}(111)} with a 2.3 eV hyperthermal oxygen molecular beam (HOMB) source, and characterized the corresponding (oxide) surfaces with synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS). At 300,text{K}, CuO forms on both mbox{text{Cu}_{3}text{Pd}(111)} and mbox{text{Cu}_{3}text{Pt}(111)}. When we increase the surface temperature to 500,text{K}, mbox{text{Cu}_{2}text{O}} also forms on mbox{text{Cu}_{3}text{Pd}(111)}, but not on mbox{text{Cu}_{3}text{Pt}(111)}. For comparison, mbox{text{Cu}_{2}text{O}} forms even at 300,text{K} on Cu(111). On mbox{text{Cu}_{3}text{Au}(111)}, mbox{text{Cu}_{2}text{O}} forms only after 500,text{K}, and no oxides can be found at 300,text{K}. We ascribe this difference in Cu oxide formation to the mobility of the interfacial species (Cu/Pd/Pt) and charge transfer between the surface Cu oxides and subsurface species (Cu/Pd/Pt).

Observation of nitrogen species at Al2O3/NO2/H-diamond interfaces by synchrotron radiation x-ray photoemission spectroscopy

Two nitrogen-related peaks were successfully detected by synchrotron radiation x-ray photoemission spectroscopy (XPS) at the interface of the Al2O3/NO2/H-diamond structure, which is used for metal–oxide–semiconductor field-effect transistors. The 399-eV peak was attributed to the C‒NH2 or C‒N bond, which was formed by electron transfer from the NO2 molecule to the H-diamond surface and subsequent decomposition of NO2 molecules at the H-diamond surface. The 407-eV peak was attributed to NO3− bond, which was formed by the decomposition of N2O4 molecules. We confirmed that N species are localized at the interface by changing the photoemission angle in XPS. The N interface density at the (111) interface was about twice that at the (001) interface. This difference is caused by C‒H bond density on different surface orientations. The band alignments were determined to be type II (staggered type), and the valence band offset (ΔEV) was determined to be 3.9 ± 0.1 eV for (001) and 4.3 ± 0.1 eV for (111) surface orientations. These results agree well with the higher hole sheet concentration on the (111) interface than on the (001) interface.

Redox Properties of Cu2O(100) and (111) Surfaces

Intense research efforts are directed toward Cu and Cu2O based catalysts as they are viewed as potential replacements for noble metal catalysts. However, applications are hampered by deactivation, e.g., through facile complete oxidation to CuO. Despite the importance of the redox processes for Cu2O catalysts, a molecular level understanding of the deactivation process is still lacking. Here we study the initial stages of oxidization of well-defined Cu2O bulk single crystals of (100) and (111) termination by means of synchrotron radiation X-ray photoemission spectroscopy (XPS) and scanning tunneling microscopy (STM). Exposure of the (100) surface to 1 mbar O2 at 25 °C results in the formation of a 1.0 monolayer (ML) CuO surface oxide. The surface is covered by 0.7 ML OH groups from trace moisture in the reaction gas. In contrast, neither hydroxylation nor oxidation was observed on the (111) surface under similar mild exposure conditions. On Cu2O(111) the initial formation of CuO requires annealing to ∼400 ...

Surface temperature dependence of oxidation of Cu3Au(111) by an energetic oxygen molecule

We report a study on the surface-temperature (Ts) dependence of oxidation process at Cu3Au(111) by using a hyperthermal oxygen molecular beam and synchrotron-radiation x-ray photoemission spectroscopy. The O-1s spectra and the corresponding O-uptake curves demonstrate that Cu2O domains grow effectively at high Ts of 400 and 500 K. The simple analysis of the O distribution suggests that the temperature-induced atomic diffusion causes the Cu2O domains growing thicker at 500 K. The oxidation of Cu3Au(111) is less efficient at Ts = 300–500 K than that of Cu(111), demonstrating that the protective nature of Cu3Au against oxidation, in comparison to Cu, remains even at high Ts.

Surface-Confined Polymerization of Halogenated Polyacenes: The Case of Dibromotetracene on Ag(110)

On-surface synthesis of thin organic and organometallic films in a bottom-up fashion has become a promising approach for the development of new nanotechnologies. In this work we studied 5,11-dibromotetracene (C18H10Br2) as a prototypical case of rodlike polyaromatic molecules functionalized with two bromine atoms on the sides. The adsorption and temperature-stimulated transformations of dibromotetracene assemblies on Ag(110) have been investigated by a combination of synchrotron radiation X-ray photoemission spectroscopy (XPS), near-edge X-ray absorption spectroscopy (NEXAFS), scanning tunneling microscopy (STM), and density functional theory (DFT) calculations. Upon the contact with the Ag substrate, the Br–C bonds are promptly cleaved at room temperature, and Ag-coordinated protopolymers are formed along the [001] substrate direction. The organometallic dimers and trimers remain on the surface up to 523 K. The stabilization of the protopolymers is driven by the substrate anisotropy and weak interactions...

In Situ SR-XPS Observation of Ni-Assisted Low-Temperature Formation of Epitaxial Graphene on 3C-SiC/Si.

Low-temperature (~1073 K) formation of graphene was performed on Si substrates by using an ultrathin (2 nm) Ni layer deposited on a 3C-SiC thin film heteroepitaxially grown on a Si substrate. Angle-resolved, synchrotron-radiation X-ray photoemission spectroscopy (SR-XPS) results show that the stacking order is, from the surface to the bulk, Ni carbides(Ni3C/NiCx)/graphene/Ni/Ni silicides (Ni2Si/NiSi)/3C-SiC/Si. In situ SR-XPS during the graphitization annealing clarified that graphene is formed during the cooling stage. We conclude that Ni silicide and Ni carbide formation play an essential role in the formation of graphene.

Effect of water vapor and hydrogen treatments on the surface structure of Ni 3 Al foil

Abstract We have developed a water vapor treatment followed by hydrogen reduction to modify the surface structure of Ni 3 Al foils in order to obtain high catalytic activity. The Ni 3 Al foils were heat treated in water vapor at 873 K for 1 h followed by H 2 reduction at 873 K for 1 h. The effects of the water vapor treatment and the H 2 reduction on the surface structure of the Ni 3 Al foils were investigated by means of scanning electron microscopy and synchrotron radiation X-ray photoemission spectroscopy. Both Ni and Al in the surface layer of the Ni 3 Al foil were oxidized during the water vapor treatment; fine NiO particles with a high density were formed on the outermost surface, accompanied by the formation of oxide layers of Al(OH) 3 and NiAl 2 O 4 /Al 2 O 3 beneath the NiO particles. The NiO particles were reduced to metallic Ni and the Al(OH) 3 was decomposed to Al 2 O 3 , whereas the NiAl 2 O 4 and Al 2 O 3 remained unchanged during the subsequent H 2 reduction, forming a Ni-enriched porous structure on the surface layer of NiAl 2 O 4 /Al 2 O 3 .

Simple, green, and clean removal of a poly(methyl methacrylate) film on chemical vapor deposited graphene

The clean removal of a poly(methyl methacrylate) (PMMA) film on graphene has been an essential part of the process of transferring chemical vapor deposited graphene to a specific substrate, influencing the quality of the transferred graphene. Here we demonstrate that the clean removal of PMMA can be achieved by a single heat-treatment process without the chemical treatment that was adopted in other methods of PMMA removal. The cleanness of the transferred graphene was confirmed by four-point probe measurements, synchrotron radiation x-ray photoemission spectroscopy, optical images, and Raman spectroscopy.

Hydrosilylation of Styrene on Water-Saturated Si(001)-2×1 at Room Temperature

The Si(001)-2×1 surface saturated by water is characterized by the passivation of nearly all of the dimerized atoms by H/OH terminations, except isolated dangling bonds, whose areal density is in the range of 1.5 ± 0.2 × 10–2 defects per Si atom. Therefore the water-saturated Si(001)-2×1 surface presents similarities with the defective H-terminated Si(001)-2×1 surface (presence of monohydrides and isolated dangling bonds), on which alkene molecules are known to be grafted via a radical-based hydrosilylation mechanism initiated at silicon dangling bonds. These common features stimulated the present study devoted to the reactivity of the water-saturated surface (n+-doped substrate) with styrene (H2Cα═CβH–C6H5) at room temperature. Using synchrotron radiation X-ray photoemission spectroscopy (XPS), our aim was to estimate the extent of styrene growth and to characterize the chemistry of the adsorbed molecule. XPS showed that styrene does react with the surface: after an exposure of 6.7 L (900 s × 0.75 × 10–8 Torr), we estimate that about 0.2 molecules per Si dimer (∼0.1 molecule per Si atom) are grafted on the surface. The C 1s XPS spectrum is consistent with a hydrosilylation product, Si–CαH2–CβH2–C6H5. Indeed we found that the C 1s spectral shape of styrene mono-σ bonded to the water-covered surface is markedly different from that of styrene di-σ bonded to the clean Si(001)-2×1 surface, confirming the specificity of the reaction product formed on the former surface. Mechanistically, a radical-based hydrosilylation reaction is the most plausible, as theoretical works indicate that the activation barrier of the latter mechanism is much lower than that of a direct, concerted mechanism. The C 1s spectral shape also excludes a reaction of the molecule with surface hydroxyls, leading to the formation of monohydroxyls CβOH(H) (radical-based reaction) or of Si–O–C linkages (Markovnikov or anti-Markovnikov addition).

Enhancement of Intragrain Critical Current Density in Bi-Based Superconductor by Substitutional Structural Defects

We synthesized Ca-rich Bi-based superconducting whiskers by an Al <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> -seeded glassy quenched platelet method. The grown whiskers were precisely characterized by synchrotron radiation X-ray photoemission spectroscopy and high-resolution transmission electron microscopy. The Ca-rich Bi-based superconducting whiskers show a high critical current density of 2 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sup> A/cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> at 40 K in self-field. We found that excess Ca <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> ions substitute for the Sr <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2+</sup> sites and cause nano domains with shorter-period modulation within the conventional 2212 crystal structure. The embedded nano crystalline domains can result in structural distorted defects which work as strong pinning center.

A Synchrotron Radiation X-ray Photoemission Spectroscopy Study of n-Propyltriethoxysilane Adsorption on Si(001)-2 × 1 at Room Temperature

The reaction of n-propyltriethoxysilane (PTES) with clean Si(001)-2 × 1 at room temperature is studied by synchrotron radiation X-ray photoemission spectroscopy (XPS). It is shown that PTES dissociatively adsorbs on the surface via the scission of at least two Si−O bonds. Adsorbate geometries are proposed accounting for the XPS data, and possible reaction paths are discussed, considering the zwitterionic nature of the surface silicon dimers. The understanding of the reactivity of the triethoxysilane termination on clean silicon paves the way to its possible use as an anchoring unit enabling the grafting of complex multifunctional molecules.

Synchrotron radiation x-ray photoemission spectroscopy and high-resolution transmission electron microscopy analysis of Bi2Sr2Can−1CunOy superconducting whiskers with high critical current density

Using synchrotron radiation x-ray photoemission spectroscopy and high-resolution transmission electron microscopy, we have studied Ca-rich Bi-based superconducting whiskers grown by an Al2O3-seeded glassy quenched platelet method. The Ca-rich Bi-based superconducting whiskers show a high critical current density of 2×105 A/cm2 at 40 K in self-field. We found that excess Ca2+ ions substitute for the Sr2+ sites and cause pillar-shaped nanocrystalline domains with shorter period modulation embedded in the base crystalline. The embedded nanocrystalline domains can result in structural distorted defects which work as strong pinning center.

Electronic structure and chemical reaction of Ca deposition on regioregular poly(3-hexylthiophene) surfaces

Conjugated polymer, regioregular poly(3-hexylthiophene) (rr-P3HT), films were prepared by spin-coating the rr-P3HT chloroform solution onto clean silicon wafer surfaces. The chemical reaction and electronic structure of Ca deposition on rr-P3HT surfaces were in situ investigated by synchrotron radiation photoemission spectroscopy (SRPES) and X-ray photoemission spectroscopy (XPS). Upon Ca deposition, Ca-induced band bending of rr-P3HT is observed. In addition, Ca atoms preferentially react with S atoms of rr-P3HT. No obvious reaction between Ca and C atoms can be found. Through the investigation of the evolution of valence band spectra and secondary electron cut-off of rr-P3HT during the process of Ca deposition, an energy level alignment diagram at the Ca/rr-P3HT interface is derived.

Following the oxidation of yttrium silicide epitaxially grown on Si(1 1 1) by core level photoemission spectroscopy

We have identified, by means of synchrotron radiation X-ray photoemission spectroscopy, several core-level shifted components in the Si-2p photoemission core level peak from a thin yttrium silicide layer epitaxially grown on a Si(1 1 1) surface. We have unequivocally assigned these components to different environments of the Si atoms in the silicide structure. This information has been used to monitor a surface oxidation process promoted by room temperature oxygen adsorption, identifying the final product of this reaction as a silicate-type ternary compound.

Adsorption of benzonitrile onSi(001)−2×1at 300 K

We have studied the adsorption geometries of benzonitrile (C{sub 6}H{sub 5}-C{identical_to}N) deposited on Si(001)-2x1 at 300 K, using synchrotron radiation x-ray photoemission spectroscopy and x-ray absorption spectroscopy. The experimental data are interpreted through density functional theory calculations of ground and excited states. It is shown that the molecule chemisorbs under multiple forms. While the literature indicated that adsorption is highly selective at 110 K (bonding via the cyano), at 300 K, cyano bonding is observed concomitantly with other geometries for which the molecule interacts with the surface by the phenyl ring. Among the phenyl-bonded species, the cyano group can remain unreacted or can make a dativelike bond with the substrate via its nitrogen end (the latter species is a minority species at surface saturation). Real-time kinetic studies show that all species are present on the surface with the same distribution from the very beginning of the adsorption process.

XPS Study of C－I Covalent Bond on Single-walled Carbon Nanotubes (SWNTs)

A simple method was used to synthesize the iodinated multi hydroxylated SWNTs(SWNTols) by the reac-tion of SWNTols with NaI in the presence of strong oxidant N-chloro-p-toluenesulfonamide. The formation of the C – I covalent bond was verified by comparing the electron binding energies of iodine of I-SWNTols and the reference materi-als, which were measured with the synchrotron radiation X-ray photoemission spectroscopy (SRXPS) as well as the conven-tional X-ray photoelectron spectroscopy (XPS). This result will be useful in applications of SWNTs in the biomedical and pharmaceutical sciences.

Interfacial reconstruction in the system Pb/Ag(1 1 0)

We have examined the Pb/Ag(1 1 0) adsorption system using low energy electron diffraction (LEED) and synchrotron radiation X-ray photoemission spectroscopy (SRXPS). At a temperature of 120 K, layer-by-layer growth occurs and no superstructure is observed by LEED. On heating a two monolayer film to 230 K, or on deposition of two monolayers at 300 K temperature, a c(2 × 4) superstructure is observed. We interpret this as a reconstruction involving a (1 × 2) Ag substrate, and a buckled hexagonal overlayer of Pb. The reconstruction is unusual as it is induced at a solid–solid interface, rather than by an adsorbate at the solid–vacuum interface. Core level shifts of the Pb 5d and 4f and Ag 3d levels are also reported.

NO adsorption on the Rh(110) surface: kinetics and composition of the adlayer studied by fast XPS

The adsorption and dissociation of NO on the Rh(110) surface were studied by synchrotron radiation X-ray photoemission spectroscopy at temperatures in the range 210–370 K. The O 1s or N 1s spectra were collected every 14 s while the surface was continuously exposed to a steady NO gas pressure. The difference in the binding energies for the atomic oxygen (O 1s ≤530.2 eV), atomic nitrogen (N 1s ∼397.2 eV) and molecular upright bonded NO molecules (O 1s ≥531.0 eV and N 1s ∼400 eV) allowed us to distinguish these surface species and to follow the evolution of the adsorbate layer. In addition to these dominating surface species a new species, characterized by O 1s binding energy of 530.7 eV and N 1s binding energy similar to that of the atomic nitrogen, was detected within a narrow coverage range. This state is tentatively assigned to a “lying down” NO bonding configuration, detectable at the timescale of the measurements. The uptake plots, constructed using the integrated intensity of the deconvoluted O 1s and N 1s spectra, are used to elucidate the effect of the reaction temperature and surface coverage and composition on the kinetics of dissociative and molecular NO adsorption of Rh(110).

Acetylene gas as a carbon source: An x-ray photoemission spectroscopy and near-edge x-ray absorption fine structure spectroscopy study of its stability on Si(111)-7×7

The electronic structure and bonding geometry of acetylene adsorbed at room temperature on Si(111)-7×7 is studied by a combination of synchrotron radiation x-ray photoemission spectroscopy and of near-edge x-ray absorption fine structure spectroscopy. Then the stability of the molecule, submitted to thermal annealings and to synchrotron white beam irradiation is examined. The possibility of using acetylene gas as a carbon source for the fabrication of silicon-carbon compounds (or for the formation of abrupt carbon/silicon interfaces) is discussed.