Grazing Incidence X-ray Photoelectron Spectroscopy Research Articles

Effect of Underlying Cobalt Content on Oxygen Reduction Reaction Activity at Pt-Skin/Pt100-XCox (111) Single Crystal Electrodes

The R&D of highly active and durable cathode catalysts for the oxygen reduction reaction (ORR) is very important for polymer electrolyte fuel cells (PEFCs). Bimetallic alloys of Pt such as Pt-Co exhibit higher ORR activities than that of pure Pt.1 For polycrystalline Pt alloy electrodes, the kinetically-controlled area-specific current densities (jk ) for the ORR strongly depend on the bulk Co composition.2 It is essential to analyze the mechanism of the increased ORR activity at well-defined Pt-alloys by multilateral techniques. Recently, we have demonstrated the composition dependence of jk at a series of (111), (110), and (100) faces of Pt100−xCox single crystal electrodes, on which well-defined Pt-skin layers were formed by annealing in H2.3 The Pt-skin/Pt73Co27(111) electrode exhibited the highest jk value, which was about 27 times higher than that on pure Pt(111). In the present research, we have examined the effect of layer-by-layer composition, measured under electrochemical conditions, of three different Pt100−xCox(111) electrodes with varying Co bulk content (x = 17, 25, 30 at.%) on the ORR activity. The Pt100−xCox(111) single crystal electrodes (ca. 3.5 mm in diameter and 1.5 mm in thickness) were prepared in the same manner as described in our previous work.3 The Co composition in bulk was evaluated by use of X-ray photoelectron spectroscopy (XPS). Prior to the all measurements, the Pt100−xCox(111) electrodes were heat-treated at 1170 K in 100% H2 to prepare the well-defined surface. In-situ surface X-ray scattering (SXS) measurements were carried out at synchrotron radiation facilities (SPring-8 and the Photon Factory) in the same manner as described previously.4 After filling the electrochemical cell with N2-purged 0.1 M HClO4, the working electrode was polarized at 0.4 V vs. RHE. Structure parameters from the topmost to fourth layers were determined from the least-squares fitting to X-ray reflectivity data [(00) crystal truncation rod (CTR)] with a kinematical calculation based on a specific interfacial model. Figure 1 shows the dependence of jk at 0.9 V vs. RHE on x, together with layer-by-layer composition of Pt and Co analyzed by in situ SXS. The composition of the topmost layer was found to be Pt 98 at.% for all three alloys, regardless of Co content, i.e., a Pt-skin layer was formed on the surface of the Pt-Co(111) single crystal electrodes. In contrast, the Co content in the second layer changed significantly with the Co content in bulk. It is striking that the second layer of Pt75Co25(111) (Pt3Co(111)) was nearly pure Co, 98 at.%, where the jk value was close to the maximum value, ca. 25 times higher than that of pure Pt(111). At Co 30 at.% in bulk, jk decreased, and the Co content in the second layer (32 at.%) was very similar to that in bulk. The Co content in the third layer ranged from ca. 20 at.% to ca. 30 at.% in the three kind electrodes. Thus, the ORR activities of Pt-skin/Pt100−xCox(111) electrodes were strongly correlated with the enrichment of Co in the second layer. It was found by ex-situ angle-resolved, grazing-incidence XPS (ARGI-XPS) for Pt-skin/ Pt73Co27(111) that Co in the subsurface layers was positively charged, which was completely different from the electronic state of Co(II)O.4 Such an electronically positive state of Co is reasonably explained by an electron transfer from Co into the Pt-skin layer, which has been linked to a d-band center downshift. This work was supported by funds for the SPer-FC project from NEDO and a Grant-in-Aid (No. 25410007) for Scientific Research MEXT of Japan. We wish to thank Dr. K. Tamura of the Japan Atomic Energy Agency and Drs. I. Hirosawa and T. Watanabe of SPring-8 for their help with the CTR fitting program and SXS measurements, respectively. The synchrotron radiation experiments were performed as projects approved by the PAC (No. 2016G059) and by JASRI (2016A1184 and 2016B1551). References V. R. Stamenkovic, B. S. Mun, K. J. J. Mayrhofer, P. N. Ross, N. M. Markovic, J. Rossmeisl, J. Greeley, J. K. Nørskov, Angew. Chem. Int. Ed., 45, 2897 (2006).T. Toda, H. Igarashi, H. Uchida, M. Watanabe, J. Electrochem. Soc. 146, 3750 (1999).S. Kobayashi, M. Wakisaka, D. A. Tryk, A. Iiyama, H. Uchida, J. Phys. Chem. C, 121,11234 (2017).S. Kobayashi, M. Aoki, M. Wakisaka, T. Kawamoto, R. Shirasaka, K. Suda, D. A. Tryk, J. Inukai, T. Kondo, H. Uchida, ACS Omega, 3, 154 (2018). Figure 1

EC-XPS Analysis of Oxygen Species Adsorbed on Pt-Nanoparticle Model Electrodes

The research and development of highly active cathode catalysts for the oxygen reduction reaction (ORR) is one of the most important subjects to achieve high efficiency in polymer electrolyte fuel cells (PEFCs). In order to understand the ORR mechanism and find clues to the design of high-performance cathode catalysts, it is essential to analyze surface the oxidation states of catalysts and the adsorbed oxygen species in the ORR. Recently, we have clarified the surface oxidation states of Pt polycrystalline and low-index single-crystal electrodes as a function of the electrode potential by using X-ray photoelectron spectroscopy combined with an electrochemical cell (EC–XPS).1 In practical PEFCs, however, Pt nanoparticles supported on carbon black have been employed as the anode and cathode catalysts. In the present research, we have prepared a model electrode of Pt nanoparticles supported on a carbon substrate and have examined the electronic structure of the Pt nanoparticles and the oxygen species adsorbed on the surface by using EC–XPS. Pt nanoparticles were prepared by a colloidal method,2 and dispersed on polished glassy carbon (GC). It was clarified by scanning electron microscopy (SEM) that Pt nanoparticles of ca. 3 nm diameter were well dispersed on the carbon substrate. We employed angle-resolved, grazing incidence X-ray photoelectron spectroscopy (AR-GIXPS), which is more surface-sensitive than conventional XPS, in order to analyze the small amount of Pt nanoparticles supported on the flat GC substrate. Figure 1 shows Pt 4f XP spectra of the Pt/GC model electrode after emersion from the 0.1 M HF solution at various electrode potentials. It was found that the peaks assigned to Pt 4f7/2 and 4f5/2 shifted to higher binding energies with increasing electrode potential. This peak shift can be ascribed to the surface core level shift induced by the surface oxidation of the Pt nanoparticles. At E > 1.1 V, peaks for bulk PtO, with binding energies of 73 and 77 eV, commenced to increase due to further oxidation of the Pt nanoparticles. Figure 2a shows O 1s XP spectra at the Pt/GC model electrode and a GC electrode (without any Pt) at 0.9 V, indicated by red and black lines, respectively. The O1s spectrum of the Pt/GC model electrode was found to include photoelectron signals attributed to oxygen species formed on the GC substrate, such as quinone and carboxyl groups. Then, we extracted the photoelectron signals originating from oxygen species adsorbed on the Pt nanoparticles by subtracting the O1s spectrum of the GC from that of the Pt/GC model electrode. Figure 2b shows the O1s difference spectrum for oxygen species adsorbed on the Pt nanoparticles. Thus, we succeeded in deconvoluting the difference spectrum into three components, H2Oad, OHad and Oad formed on the Pt nanoparticles. On the basis of the deconvoluted photoelectron intensities, it was found that H2Oad decreased with increasing electrode potential, while OHad increased. The Oad species was found to appear at E > 0.8 V. The onset potential of the OHad formation at the Pt/GC electrode was less positive than that for polycrystalline Pt, suggesting that the surface oxidation proceeded more easily at the Pt nanoparticle surfaces than at the polycrystalline surface. This work was supported by funds for the SPer-FC Project of NEDO, Japan References 1 M. Wakisaka, H. Suzuki, S. Mitsui, H. Uchida, and M. Watanabe, Langmuir, 25, 1897 (2009). 2 M. Watanabe, M. Uchida, and S. Motoo, J. Electroanal. Chem., 199, 311 (1986). Figure 1

Enhanced Corrosion Resistance of 316L Stainless Steel Interstitially Hardened with Nitrogen or Carbon

Case hardening by traditional carburization or nitridation methods has long been recognized to produce wear resistant surfaces in steels. When these methods are applied to stainless steels, corrosion performance is degraded because of carbide and nitride formation. However, low temperature gas-phase and plasma interstitial hardening (IH) processes relying on paraequilibrium kinetics have been developed (1-5) that allow substantial amounts of carbon or nitrogen to be introduced into stainless steels without formation of carbides or nitrides. Paraequilibrium refers to the concept that diffusion kinetics of substitutional solutes, such as Cr and Ni, diffuse slower than interstitial solutes, such as carbon or nitrogen. Substitutional solutes are effectively immobile, whereas interstitial solutes can diffuse into the alloy to depths of 10-30 µm. Surface carbon or nitrogen concentrations on the order of 12 at% or greater can be obtained, resulting in a hardened surface. This modified case-hardened region has been referred to as S-phase or expanded austenite. The industrial implementation of the IH treatment with carbon (IH-C) or nitrogen (IH-N) is straightforward, is a non-line of sight process, maintains sample dimensions, and is relatively inexpensive. A growing body of work on IH-C treated stainless steel alloys shows that gas phase IH-C treatment improved localized corrosion, fatigue, and wear resistance of the treated materials compared to the untreated alloys (5-9).The origin of the hardening and the improved corrosion resistance for IH-C treated 316L stainless steels is the “colossal” supersaturation of interstitial carbon. The corrosion resistance of stainless steel involves a Cr2O3-rich passive film. In previous work, grazing incidence X-ray photoelectron spectroscopy (GI-XPS) was used to determine the composition and measure the thickness of these Cr-rich passive films developed during anodic polarization at selected potentials for both IH-C and non-treated 316L stainless steel samples. Since no Cl-was observed in the passive oxide film of both the IH-C and non-treated samples at any of the potentials examined, and the chemical composition was the same for IH-C and non-treated samples at each potential, we suggest that passive film breakdown is of chemo-mechanical origin [8,9].In this work, exploration into the nature of passive oxide film breakdown was extended to gas phase IH-N. The experimental work conducted follows that of IH-C in References 7 and 9. This includes determining the polarization behavior in 0.6 M NaCl solutions, performing potentiostatic experiments at selected potentials below the pitting potential with subsequent XPS, and impedance measurements on the electronic response of the oxide film. XPS will be used to determine the oxide film thickness and composition of the potentiostatically polarized samples. These results will then be compared to the results reported in References 7 and 9. Acknowledgements The authors gratefully acknowledge the Office of Naval Research and the Naval Research Laboratory for financial support of this work. References (1) Z.L. Zhang and T. Bell, Surf. Eng., 1,131 (1985).(2) P.C. Williams and S.C. Marx: U.S. Patent 6,093,303, July 25, 2000.(3) Y. Cao, F. Ernst, and G.M. Michal, Acta Mater., 51, 4171 (2003).(4) T. Christiansen and M.A.J. Somers, Surf. Eng., 21,445 (2005).(5) G.M. Michal, F. Ernst, H. Kahn, Y. Cao, F. Oba, N. Agarwal, and A.H. Heuer, Acta Mater., 54, 1597 (2006).(6) A.H. Heuer, H. Kahn, J. O'Donnell, G.M. Michal, R.J. Rayne, F.J. Martin, and P.M. Natishan, Electrochem. and Solid State Lett., 13 (12) C37-C39 (2010).(7) F.J. Martin, E.J. Lemieux, T.M. Newbauer, R.A. Bayles, P.M. Natishan, H. Kahn, G.M. Michal, F. Ernst, and A.H. Heuer, Electrochem. and Solid State Lett., 10 (12) C76-C78 (2007).(8) A.H. Heuer, H. Kahn, P.M. Natishan, F.J. Martin, and L.E. Cross, Electrochimica Acta, 58,157 (2011). (9) A.H. Heuer, H. Kahn, F. Ernst, G.M. Michal, D. Hovis, R.J. Rayne, F.J. Martin, and P.M. Natishan, Acta Materialia, 60, 716 (2012).

Enhanced corrosion resistance of interstitially hardened stainless steel: Implications of a critical passive layer thickness for breakdown

Immense interstitial hardening of 316L austenitic stainless steel via low-temperature paraequilibrium carburization also leads to greatly improved corrosion resistance. Both the hardening and the improved corrosion resistance owe their origin to a “colossal” supersaturation of interstitial carbon. The corrosion resistance of stainless steel involves a Cr 2O 3-rich passive film, and the composition and thickness of the passive film developed during anodic polarization at various potentials were determined for both carburized and non-treated steels using grazing incidence X-ray photoelectron spectroscopy. Passive oxide film breakdown is a necessary step in pitting corrosion, and appears to occur in these steels at a critical film thickness of ≈3 nm. We suggest that this breakdown is of chemomechanical origin. Long wavelength thickness perturbations occur during film growth to reduce the strain energy density in the passive film arising from intrinsic and electric field-induced stresses. At the critical thickness, the localized thinning is sufficient to lead to dielectric breakdown and nucleation of pitting corrosion. The improved corrosion resistance for the carburized material results from thinner passive films at a given potential and hence a delay in the detrimental effect of the thickness perturbations.

Grazing Incidence X-Ray Photoelectron Spectroscopy:

Grazing Incidence X-ray Photoelectron Spectroscopy (GIXPS) is a method that offers promise as a nondestructive technique to measure the thickness and chemistry of ultrathin gate dielectric films. It combines aspects of x-ray reflectivity and conventional x-ray photoelectron spectroscopy (XPS). Collimated x-rays are incident on the sample at a grazing angle. A series of photoemission spectra are obtained over a range of incidence angles in the vicinity of the critical angle for total external reflection. The advantage of this method is the incorporation of the optical constants of the layers, as well as the photoemission cross sections of the elements and the inelastic attenuation lengths of the escaping photoelectrons, to fit the nonlinear variation of the photoemission spectra as a function of angle. The x-ray field variation with angle within individual layers provides additional constraints beyond standard angle-resolved XPS for interpreting the densities and thicknesses of multiple layers. The application of this method to the analysis of oxide on Si is described.

Grazing‐incidence x‐ray photoemission spectroscopy and the accuracy of thickness measurements of CMOS gate dielectrics

Grazing-incidence x-ray photoelectron spectroscopy (GIXPS) is a method that offers promise as a non-destructive technique to measure the thickness and chemical state of ultrathin gate dielectric films. This method utilizes the non-linear dependence of the photoelectron yield on the radiation incidence angle to characterize a given layer structure. The accuracy of the results also depends on the input parameters, such as photoemission cross-sections, indices of refraction and electron attenuation lengths within the incident x-ray energy range of application (1–2 keV). Uncertainties in these parameters have consequences not only for GIXPS but also for other techniques that rely on them for materials analysis. We present a study of the impact of uncertainty in the photoionization cross-sections and atomic scattering factors on the GIXPS measurement of film thickness and density. A test of the accuracy of the index of refraction is demonstrated by measurements of a native oxide layer on Si at incident energies below and above the Si K edge. We also discuss the effects of surface and interface roughness. Published in 2001 by John Wiley & Sons, Ltd.

Silicon Oxynitride Film Thickness Measurements Using HRTEM and Grazing Incidence X-Ray Photoelectron Spectroscopy (GIXPS)

The ability to accurately and precisely measure the thickness of ultrathin (∼ 3 nm) dielectric films, used as gate dielectrics in integrated circuits, is critical to the continued success of the semiconductor manufacturing industry. Many metrology tools have been applied to this problem in the past, but recent research has focussed on ellipsometry, x-ray reflectivity, secondary-ion mass spectrometry, capacitance-voltage curves, medium energy ion scattering, high resolution transmission electron microscopy (HRTEM), and grazing incidence x-ray photoelectron spectroscopy (GIXPS). Unfortunately, these techniques disagree about a given film's thickness by amounts larger than their individual precisions. To support the statistical process control methodologies used in production wafer fabrication, these disagreements need to be investigated and the true accuracy and precision of the tools need to be determined. This work compares the ability of two techniques, HRTEM and GIXPS, to measure the thickness of silicon oxynitride films on silicon substrates.

Development of a Titanium Nitride Thin Film Standard

ABSTRACTTitanium nitride films are used in a variety of applications in the semiconductor industry, but the stoichiometry and homogeneity of the films can be difficult to control and characterize. The National Institute of Standards and Technology (NIST) is developing a TiN thin film on silicon to be used as a physical and chemical standard for the semiconductor industry. A prototype 100 nm thick film has been manufactured by dual ion beam deposition. NIST is characterizing the film by x-ray interferometry for thickness and density, neutron depth profiling for nitrogen content and homogeneity as a function of depth, neutron activation analysis for titanium content, and x-ray fluorescence spectromety, electron probe microanalysis, grazing incidence x-ray photoelectron spectroscopy, and transmission electron microscopy for homogeneity and composition. The intent of this research is to create a TiN film of known thickness, density, stoichiometry, and homogeneity that can be used by the semiconductor industry as a primary, traceable standard for characterization of industry thin films. The intent of the standard is to promote more accurate characterization and improved TiN manufacturing capabilities as well as to aid manufactures in more easily employing ISO 9000 related traceability and quality assurance practices.

Grazing-incidence x-ray photoelectron spectroscopy from multilayer media: Oxidized GaAs(100) as a case study.

At the energies of interest in x-ray photoelectron spectroscopy (XPS), total external reflection of the x-ray beam occurs from a smooth surface at small incidence angles. The penetration of the x rays into the material is strongly attenuated at these angles and surface sensitivity is enhanced in the XPS yields. As the incidence angle is increased, the x rays penetrate more deeply into the material and the XPS signal contains a larger contribution from the bulk. By exploiting this angle-dependent x-ray penetration depth, it is possible to obtain depth-dependent XPS spectra from which the concentration profiles of the photoelectron-emitting atoms can be inferred. In this paper we develop a general formalism for calculating grazing-incidence XPS (GIXPS) yields from multilayer media. A quantitative analysis of GIXPS spectra acquired from an oxidized GaAs(100) surface that was annealed to remove oxidized As will be discussed. The results show that this annealed oxide is composed of ${\mathrm{Ga}}_{2}$${\mathrm{O}}_{3}$ and that the oxide-GaAs substrate interface is rough.

Grazing-incidence x-ray photoemission spectroscopy investigation of oxidized GaAs(100): A novel approach to nondestructive depth profiling

When a beam of x rays (∼1 keV) impinges on a flat surface at grazing angles (≲3°) the x rays undergo total external reflection. Under these conditions, the penetration depth of the x rays can become comparable to photoelectron escape depths and the photoelectron yields from the surface are enhanced. As the incidence angle of the x rays is increased, the x-ray penetration depth increases and the photoelectron yields contain a larger contribution from deeper layers within the sample. By exploiting this depth-dependence of photoelectron yields as a function of incident x-ray beam angle, it is possible to obtain information about the depth distribution of the photoelectron emitting atoms. We have used this novel application of grazing-incidence x-ray photoelectron spectroscopy (GIXPS) to study the ultraviolet oxidized GaAs(100) surface. This oxide/GaAs surface is particularly well suited for study with GIXPS because a variety of oxide phases are formed during oxidation and questions concerning stratification of these phases can be addressed. Much insight into the composition and depth-dependence of these oxide phases can be obtained by directly comparing the spectra collected at different x-ray incidence angles.