Probing depth of soft x‐ray absorption spectroscopy measured in total‐electron‐yield mode

Abbate, M.; Goedkoop, J.B.; Groot, Frank M. F. de; Grioni, M.; Fuggle, J. C.; Hofmann, S.; Petersen, H.; Sacchi, M.

doi:10.1002/sia.740180111

Cited by 192 publications

(141 citation statements)

References 19 publications

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“…similar to the largest probing depth in our experiments. [37] It is also noteworthy that in the case of CH 3 OH:O 2 =3:1 the O:Cu stoichiometry close to the surface is similar to that of Cu 2 O. The valence band spectra in Fig.…”

Section: Resultsmentioning

confidence: 67%

Methanol Oxidation on a Copper Catalyst Investigated Using in Situ X-ray Photoelectron Spectroscopy

Bluhm¹,

Hävecker²,

et al. 2004

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The surface and near-surface region of an active catalyst and the adjacent gas-phase reactants were investigated simultaneously under reaction conditions using in situ X-ray photoelectron spectroscopy (XPS). This investigation of methanol oxidation on a copper catalyst showed that there was a linear correlation between the catalytic activity of the sample and the presence of a sub-surface oxygen species that can only be observed in situ. The concentration profile of the sub-surface oxygen species within the first few nanometers below the surface was determined using photon energy-dependent depth-profiling. The chemical composition of the surface and the near-surface region varied strongly with the oxygen-tomethanol ratio in the reactant stream. The experiments show that the pure metal is not an active catalyst for the methanol oxidation reaction, but that a certain amount of oxygen has to be present in the sub-surface region to activate the catalytic reaction. Oxide formation was found to be detrimental to formaldehyde production. Our results demonstrate also that for an understanding of heterogeneous catalysts a characterization of the surface alone may not be sufficient, and that sub-surface characterization is essential.

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Section: Resultsmentioning

confidence: 67%

Methanol Oxidation on a Copper Catalyst Investigated Using in Situ X-ray Photoelectron Spectroscopy

Bluhm¹,

Hävecker²,

et al. 2004

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“…19 One detector measures the gas phase, while a second detector (close to the sample surface) detects both the gas phase and surface signal, where "surface" is defined here as the top 4 nm of the sample. The surface signal can be revealed through subtraction of the gas-phase signal.…”

Section: Methodsmentioning

confidence: 99%

In-Situ Soft X-ray Absorption of Over-exchanged Fe/ZSM5

et al. 2003

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In-situ soft X-ray absorption spectroscopy (XAS) has been applied to study the iron redox behavior in overexchanged Fe/ZSM5. The Fe L 2,3 XAS and O K spectral shapes of the Fe/ZSM5 surface have been measured during heat treatments and reduction/oxidation cycles. Charge-transfer multiplet calculations provide a detailed understanding of the L 2,3 spectra of iron in Fe/ZSM5. The oxidized form of Fe/ZSM5 contains Fe III ions in an octahedral surrounding, with a total crystal field splitting of ∼1.0 eV. This value is significantly smaller than that for Fe 2 O 3 , which is indicative of a much weaker Fe-O bonding. The reduced form of Fe/ZSM5 has Fe II ions in a tetrahedral oxygen surrounding. The Fe L 2,3 spectra show that iron in calcined Fe/ZSM5 is reduced in 15 min to an average valence state of 2.65, under 10 mbar of pure helium at room temperature. This value has a relative uncertainty on the order of 0.01. Heating in helium up to 350°C under the same pressure further reduces the iron valence to 2.15. The oxygen spectra show that the autoreduction is accompanied by a loss of molecular oxygen and water. Reoxidation with 5% O 2 in helium yields a valence of >2.90 after 10 min.

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“…As a consequence, except for the few materials for which the sampling depth has been experimentally determined, the depth sensitivity of this technique is still under discussion. Previous studies on transition metals and rare earths showed a strong dependence of d on the investigated material, in the range 1.5 -2.5 nm for metallic Fe, Ni, Co, Dy and Tb for photon energies in the range 700 -900 eV [4,5,6,7,8,9]. Because of the different electronic structure between metals and insulators, in particular the gap in the density of states near the Fermi level in the latter that will influence the secondary electron cascade, insulators are expected to have a larger electron sampling depth.…”

Section: Introductionmentioning

confidence: 99%

“…Because of the different electronic structure between metals and insulators, in particular the gap in the density of states near the Fermi level in the latter that will influence the secondary electron cascade, insulators are expected to have a larger electron sampling depth. Little literature is available on this topic [9,10,11,12]. Gota et al [11] found an unexpectedly high value for the sampling depth of the iron magnetic oxide Fe 3 O 4 of 4.5 nm, in comparison to the one known for metallic Fe in the range 1.7 -2.2 nm, while d ≈ 2nm was found both in Ta 2 O 5 at the O K-3 edge [9] and in LaFeO 3 at the Fe L 2,3 edge [12].…”

Section: Introductionmentioning

confidence: 99%

“…Little literature is available on this topic [9,10,11,12]. Gota et al [11] found an unexpectedly high value for the sampling depth of the iron magnetic oxide Fe 3 O 4 of 4.5 nm, in comparison to the one known for metallic Fe in the range 1.7 -2.2 nm, while d ≈ 2nm was found both in Ta 2 O 5 at the O K-3 edge [9] and in LaFeO 3 at the Fe L 2,3 edge [12]. Recently, transition-metal oxides with the perovskite structure and their interfaces have been intensely investigated, especially using XAS techniques to probe their interfaces [13,14] because of their potential application in spintronic or superconducting devices.…”

Section: Introductionmentioning

confidence: 99%

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Electron sampling depth and saturation effects in perovskite films investigated by soft x-ray absorption spectroscopy

et al. 2014

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Knowledge of the electron sampling depth and related saturation effects is important for quantitative analysis of X-ray absorption spectroscopy data, yet for oxides with the perovskite structure no quantitative values are so far available. Here we study absorption saturation in films of two of the moststudied perovskites, La 0.7 Ca 0.3 MnO 3 (LCMO) and YBa 2 Cu 3 O 7 (YBCO), at the L 2,3 edge of Mn and Cu, respectively. By measuring the electron-yield intensity as a function of photon incidence angle and film thickness, the sampling depth d, photon attenuation length λ and the ratio λ/d have been independently determined between 50 and 300 K. The extracted sampling depth d LCMO ≈ 3 nm for LCMO at high temperatures in its polaronic insulator state (150 -300 K) is not much larger than values reported for pure transition metals (d Co or Ni ≈ 2 -2.5 nm) at room temperature, but is smaller than d YBCO ≈ 3.9 nm for metallic YBCO that is in turn smaller than the value reported for Fe 3 O 4 (d Fe3O4 ≈ 4.5 nm). The measured d LCMO increases to 4.5 nm when LCMO is in the metallic state at low temperatures. These results indicate that a universal rule of thumb for the sampling depth in oxides cannot be assumed, and that it can be measurably influenced by electronic phase transitions that derive from strong correlations.

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Probing depth of soft x‐ray absorption spectroscopy measured in total‐electron‐yield mode

Cited by 192 publications

References 19 publications

Methanol Oxidation on a Copper Catalyst Investigated Using in Situ X-ray Photoelectron Spectroscopy

Methanol Oxidation on a Copper Catalyst Investigated Using in Situ X-ray Photoelectron Spectroscopy

In-Situ Soft X-ray Absorption of Over-exchanged Fe/ZSM5

Electron sampling depth and saturation effects in perovskite films investigated by soft x-ray absorption spectroscopy

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