“…In addition, SO 2 formation was observed from metal/oxide interfaces with S at temperatures <640 K . In the current section, an attempt is made to correlate oxide dewetting and previously published findings on S-related degradation of protective oxide scales. − ,, For the present study, due to the inconvenient configuration of the thermocouple mounting as described in the Experimental Section, it is difficult to point out the temperature difference for oxide dewetting on S-free and S-covered Fe(111) surfaces.…”
Section: Resultsmentioning
confidence: 84%
“…The LEED pattern (not shown) shows a very diffuse (1 × 1) arrangement after oxidation, indicative of the growth of a disordered oxide on the Fe(111)(1 × 1)-S surface. There are no prominent line shape changes associated with the S signal (not shown), demonstrating that the (1 × 1)-S-covered Fe(111) under O 2 exposure at room temperature forms an oxide, rather than a surface S−O layer. 4 Fe(111)(1 × 1)-S after 1700 L exposure of O 2 at room temperature: (a) AES line shapes with increasing O 2 exposure; (b) a 50 nm × 50 nm STM image revealing island formation ( V = 0.5 V, I = 1 nA).…”
Section: Resultsmentioning
confidence: 98%
“…Previous studies 14 have consistently observed that oxide dewetting occurs at lower temperatures from S precovered surfaces than from clean polycrystalline iron samples. Oxide decomposition has been reported at 720 K on Fe(111)(1 × 1)-S when the oxide was formed at room temperature . In contrast, oxide layers formed at 300 K on S-free surfaces were stable upon heating to 720 K .…”
We report the first STM observations of the thermally induced dewetting of an iron oxide scale from
an Fe(111) surface. In addition, we report the influence of the presence of S at the metal/oxide interface
on substrate topography. Room-temperature oxidation of S-free and (1 × 1)-S-covered Fe(111) surfaces
at oxygen partial pressures of 1 × 10-7 to 5 × 10-7 Torr resulted in the growth of oxide islands, with the
only difference being the formation of larger islands in the latter case. Line shape analyses of the Fe(MVV)
peak indicated a similar growth mechanism in both cases, with the formation of Fe3O4 initially and Fe2O3
at higher exposures of O2. Flash annealing of the oxide formed on the S-free and (1 × 1)-S-covered surfaces
in ultrahigh vacuum (UHV) resulted in oxide dewetting, leaving larger oxide islands separated by S-free
or S-covered Fe regions, respectively. The presence of S at the metal/oxide interface causes oxide dewetting
to occur at lower temperatures than those on a S-free surface. In the case of the (1 × 1)-S-covered phase,
flash annealing to ∼720 K induces enhanced sulfur segregation apart from dewetting. When the S/Fe
Auger intensity ratio is sufficiently large prior to oxidation (>1.3), flash annealing causes dewetting of
the oxide scale, and the final S coverage induces the (2√3 × 1)R30° faceting transformation. However,
if the ratio is lower than ∼1.3, the faceting transition is not observed upon annealing, although oxide
dewetting and S segregation are noted. In other words, oxide dewetting is “decoupled” from faceting. The
additional sulfur segregation to the metal/oxide interface at comparatively lower temperatures is an
unexpected occurrence and occurs only if some interfacial sulfur is already present.
“…In addition, SO 2 formation was observed from metal/oxide interfaces with S at temperatures <640 K . In the current section, an attempt is made to correlate oxide dewetting and previously published findings on S-related degradation of protective oxide scales. − ,, For the present study, due to the inconvenient configuration of the thermocouple mounting as described in the Experimental Section, it is difficult to point out the temperature difference for oxide dewetting on S-free and S-covered Fe(111) surfaces.…”
Section: Resultsmentioning
confidence: 84%
“…The LEED pattern (not shown) shows a very diffuse (1 × 1) arrangement after oxidation, indicative of the growth of a disordered oxide on the Fe(111)(1 × 1)-S surface. There are no prominent line shape changes associated with the S signal (not shown), demonstrating that the (1 × 1)-S-covered Fe(111) under O 2 exposure at room temperature forms an oxide, rather than a surface S−O layer. 4 Fe(111)(1 × 1)-S after 1700 L exposure of O 2 at room temperature: (a) AES line shapes with increasing O 2 exposure; (b) a 50 nm × 50 nm STM image revealing island formation ( V = 0.5 V, I = 1 nA).…”
Section: Resultsmentioning
confidence: 98%
“…Previous studies 14 have consistently observed that oxide dewetting occurs at lower temperatures from S precovered surfaces than from clean polycrystalline iron samples. Oxide decomposition has been reported at 720 K on Fe(111)(1 × 1)-S when the oxide was formed at room temperature . In contrast, oxide layers formed at 300 K on S-free surfaces were stable upon heating to 720 K .…”
We report the first STM observations of the thermally induced dewetting of an iron oxide scale from
an Fe(111) surface. In addition, we report the influence of the presence of S at the metal/oxide interface
on substrate topography. Room-temperature oxidation of S-free and (1 × 1)-S-covered Fe(111) surfaces
at oxygen partial pressures of 1 × 10-7 to 5 × 10-7 Torr resulted in the growth of oxide islands, with the
only difference being the formation of larger islands in the latter case. Line shape analyses of the Fe(MVV)
peak indicated a similar growth mechanism in both cases, with the formation of Fe3O4 initially and Fe2O3
at higher exposures of O2. Flash annealing of the oxide formed on the S-free and (1 × 1)-S-covered surfaces
in ultrahigh vacuum (UHV) resulted in oxide dewetting, leaving larger oxide islands separated by S-free
or S-covered Fe regions, respectively. The presence of S at the metal/oxide interface causes oxide dewetting
to occur at lower temperatures than those on a S-free surface. In the case of the (1 × 1)-S-covered phase,
flash annealing to ∼720 K induces enhanced sulfur segregation apart from dewetting. When the S/Fe
Auger intensity ratio is sufficiently large prior to oxidation (>1.3), flash annealing causes dewetting of
the oxide scale, and the final S coverage induces the (2√3 × 1)R30° faceting transformation. However,
if the ratio is lower than ∼1.3, the faceting transition is not observed upon annealing, although oxide
dewetting and S segregation are noted. In other words, oxide dewetting is “decoupled” from faceting. The
additional sulfur segregation to the metal/oxide interface at comparatively lower temperatures is an
unexpected occurrence and occurs only if some interfacial sulfur is already present.
“…From our data, it is also not possible to determine whether sulfur and oxygen were randomly mixed, or segregated into separate domains when the (2 × 2)-S-covered Ni 3 Al(111) surface was exposed to submonolayer coverages of O 2 . The S (152) Auger peak did not exhibit any distinct line-shape changes 26 that would be indicative of variations in S-substrate bond properties upon oxidation. Therefore, without the aid of detailed I/V (38) Rosenhahn, A.; Schneider, J.; Kandler, J.; Becker, C.; Wandelt, K. Surf.…”
Section: Discussionmentioning
confidence: 93%
“…Similar results have been reported 7,8,[22][23][24][25] for the oxidation of various substrates precovered with sulfur. No prominent changes 26 were observed in the line shape of the sulfur transition (not shown) during O 2 exposure.…”
Section: A Oxidation and Oxide Morphologies At ∼300 Kmentioning
Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), and scanning tunneling microscopy (STM) have been used to investigate the growth, morphology, and thermal stability of the oxide prepared on a (2 × 2)-S-covered Ni3Al(111) surface. The results demonstrate that although sulfur significantly alters the oxidation rate and oxide morphology on the Ni3Al(111) substrate at room temperature, it does not inhibit the formation of the ordered γ′-Al2O3 at elevated temperatures. The oxide formed on the S-modified Ni3Al(111) surface at room temperature is stable up to at least 1100 K. Sulfur remains at the oxide-Ni3Al(111) interface during oxygen exposure at ∼300 K. During annealing, however, sulfur is removed from the oxide-alloy interface. Annealing from 300 to 1100 K results in the segregation of aluminum to the oxide-alloy interface, as evidenced by an enhancement in the Al(1396)/Ni(848) atomic ratio, and the appearance of a metallic aluminum peak in the Auger spectra.
Veranderringen der Zrisammensetzring sowie der geometrischert iind elektronisclteit S t r u k t u p von Ober-und Grenzflachen beeinflrissen in erheblichein MuJe die ntakroskopisclten Eigenschafien von Festkorpern. Mit Hiwe cler Pltotoelektronenspktroskopie konnenwesentliche Aussagen iiber diese Bereiclte erltulteri werclen .
SummaryPhotoelectron-Spectroscopy (XPS) is a n analytical method to get quantitative and qualitative informations about element distribution and chemical bonding on solid surfaces and in near surface layers. By means of recent piiblications about catalysis research, clectronics/electrotechnics as well as material development and refinement the performance of XPS will be outlined. 0 kristallographischen Eigenschaften 32 Vokirum in cler Prowis (1991) Nr. 1 S. 32-44 0 VCIl Verlagsge~ellscliaft mbH, D-6940 Weinheirn. 1991 0934-9758/91/0102-0032/53.50 + .25/0 -Vakirum in der Praxis (1Wl) Nr.1
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