V2O3(0001)/Au(111) and /W(110): Growth, Electronic Structure and Adsorption Properties
Résumé
In this work, we firstly showed that it is possible to grow thin
V2O3(0001) films on Au(111) and W(110). The preparation process
consists of an evaporation of metallic vanadium in an oxygen
atmosphere, followed by an annealing at 700 K in 5.10-8 mbar
of oxygen. The low energy electron diffraction (LEED) patterns
obtained for both substrates exhibit sharp spots, indicating a
well-defined surface structure. The stoichiometry of the film has
been characterized by X-ray photoelectron spectroscopy (XPS) and
near edge X-ray absorption fine structure spectroscopy (NEXAFS).
The XP spectrum in the binding energy range 500-540 eV shows three
features corresponding to the V 2p3/2, V 2p1/2 and O 1s
lines, respectively. Relevant parameters for the determination of
the stoichiometry of the oxide are the distance between the O 1s
and V 2p signals, the Full Width at Half Maximum (FWHM) and the
shape of the spectra. Our spectra show good agreement with those
found in the literature for V2O3 single crystals. V L-edge NEXAFS
spectra present noticeable chemical shifts characteristic of the
different vanadium valencies and their shape depends implicitly on
the local symmetry of the vanadium cation. Each vanadium oxide
type therefore displays a typical spectrum. A comparison of our
spectrum to reference spectra permits the identification of our
vanadium oxide thin film to V2O3.
We proved with infrared absorption spectroscopy (IRAS) the
existence of two possible terminations of the V2O3 (0001) surface.
These two terminations differ only by the presence or not of
oxygen atoms on the top of the surface, forming vanadyl groups
with the surface vanadium atoms. The first termination, called
-V=O termination, is obtained after the preparation process. The
second termination - the -V termination - is obtained by heating
the -V=O surface up to 600 K with electron bombardment.
We studied with UV photoelectron spectroscopies (UPS), XPS and
NEXAFS the electronic structure of our V2O3 (0001) thin films. The
UP spectra of the -V=O terminated surface clearly show a gap for
the -V=O terminated surface. These data therefore evidence a metal
to insulator transition induced by the formation of the vanadyl
groups on the surface. This result is confirmed by our NEXAFS O K
edge and XPS results. The NEXAF O K edge spectra consist of two
features. The first one is attributed to the tansition to the
unoccupied V 3d egΠ and a1g (t2g) states with O 2p
character and the second one to the unoccupied V 3d egΣ states.
For the -V=O termination, both features of the spectrum exhibit a
shift towards higher energy relative to the spectrum for the -V
termination. This shift can be explained by the changes in the
electronic structure due to the metal to insulator transition. The
XP spectra exhibit enhanced satellite features in the case of the
-V=O termination, which can be attributed to poorly screened final
states. We also observed a shift of the O 2p band towards lower
binding energies for the -V=O terminated surface relative to the
-V terminated surface. We tried to explain this phenomenon with a
band bending model. Finally, we proposed two models for the
surface geometry of the -V=O terminated surface. In the first one,
the oxygen atoms sit on top of the vanadium atoms. In the second
one, the oxygen atoms sit on quasi regular bulk positions.
We performed high resolution electron energy loss spectroscopy
(HREELS) measurements and presented a phonon spectrum for each
termination. Differences in phonon intensities observed between
both surface terminations can be interpreted as a screening effect
of electronic carriers. We compared our spectra with a spectrum of
the isomorphic Cr2O3(0001) and found out that the
metal-oxygen bond is not so strong in V2O3 as in Cr2O3.
We studied the water adsorption properties of both surface
terminations. The experiment consists of the adsorption of water
at 90 K, yielding the formation of ice on the sample surface. The
sample then is heated up to 190 K. The species present on the
surface at this temperature are analyzed with UPS, XPS and HREELS.
The adsorption path seems to depend on both the termination and
the exposure. We observed molecularly adsorbed water on both
surface terminations for low exposures. The adsorbed water shows
only weak interaction with the substrate. For large exposures,
water dissociates and OH- groups were detected. When the OH-
desorb of the primary -V=O terminated surface, the surface left is
-V terminated. In the case of the -V=O terminated surface, the
water molecule is assumed to adsorb on the surface vanadium atom
through its oxygen atom. The oxygen double bonded to the vanadium
can interact with the hydrogen of the water molecule to form a OH
radical, breaking its double bond to the vanadium. This
dissociation mechanism may imply charge redistribution, explaining
why the V 3d emission in UPS increases upon water adsorption. This
model explains why the vanadyl oxygen atoms desorb with the OH
groups. For the -V terminated surface, we observed a charge
transfer from the V 3d substrate to the adsorbate, producing
OH- groups. Therefore, we proposed a model in which the
vanadium a1g or egΠ orbital forms a Σ bond with oxygen
lone-pair orbitals of OH-.
We performed CO2 adsorption experiments with UPS, XPS, HREELS and
IRAS. The UP results for the -V=O surface exhibit small features
which we assigned to physisorbed CO2. The CO2 adsorption on the
-V terminated surface is more complex. The analyze of the IRAS
results leads us to the conclusion that CO2 adsorbs in a bent
configuration. With UPS and XPS, we could evidence the formation
of carbonates upon heating up to 200 K.
The CO adsorption properties follow a similar trend as for CO2 :
only small quantities adsorb on the -V=O surface while the -V
surface seems to be much more reactive. On the -V=O surface, CO
adsorbs molecularly and we concluded from the angle resolved UPS
data that the CO molecule is strongly tilted on the surface. With
NEXAFS and IRAS, we showed the formation of CO2 on the -V
surface.
To our knowledge, we are the first to report a surface effect
resulting in a metal to insulator transition. This very complex
phenomenon is very exciting for the surface scientist. Further
work on V2O3 (0001) should therefore involve theorists in order to
explain properly why the formation of vanadyl groups on the
surface induces a metal to insulator transition. A simulation of
the angle resolved UPS data could determine which model for the
surface geometry is correct. Further experimental work could be
thermal desorption spectroscopy (TDS) and IRAS with isotopes in
order to identify the formation path of CO2 by CO adsorption on
the -V terminated surface.
V2O3(0001) films on Au(111) and W(110). The preparation process
consists of an evaporation of metallic vanadium in an oxygen
atmosphere, followed by an annealing at 700 K in 5.10-8 mbar
of oxygen. The low energy electron diffraction (LEED) patterns
obtained for both substrates exhibit sharp spots, indicating a
well-defined surface structure. The stoichiometry of the film has
been characterized by X-ray photoelectron spectroscopy (XPS) and
near edge X-ray absorption fine structure spectroscopy (NEXAFS).
The XP spectrum in the binding energy range 500-540 eV shows three
features corresponding to the V 2p3/2, V 2p1/2 and O 1s
lines, respectively. Relevant parameters for the determination of
the stoichiometry of the oxide are the distance between the O 1s
and V 2p signals, the Full Width at Half Maximum (FWHM) and the
shape of the spectra. Our spectra show good agreement with those
found in the literature for V2O3 single crystals. V L-edge NEXAFS
spectra present noticeable chemical shifts characteristic of the
different vanadium valencies and their shape depends implicitly on
the local symmetry of the vanadium cation. Each vanadium oxide
type therefore displays a typical spectrum. A comparison of our
spectrum to reference spectra permits the identification of our
vanadium oxide thin film to V2O3.
We proved with infrared absorption spectroscopy (IRAS) the
existence of two possible terminations of the V2O3 (0001) surface.
These two terminations differ only by the presence or not of
oxygen atoms on the top of the surface, forming vanadyl groups
with the surface vanadium atoms. The first termination, called
-V=O termination, is obtained after the preparation process. The
second termination - the -V termination - is obtained by heating
the -V=O surface up to 600 K with electron bombardment.
We studied with UV photoelectron spectroscopies (UPS), XPS and
NEXAFS the electronic structure of our V2O3 (0001) thin films. The
UP spectra of the -V=O terminated surface clearly show a gap for
the -V=O terminated surface. These data therefore evidence a metal
to insulator transition induced by the formation of the vanadyl
groups on the surface. This result is confirmed by our NEXAFS O K
edge and XPS results. The NEXAF O K edge spectra consist of two
features. The first one is attributed to the tansition to the
unoccupied V 3d egΠ and a1g (t2g) states with O 2p
character and the second one to the unoccupied V 3d egΣ states.
For the -V=O termination, both features of the spectrum exhibit a
shift towards higher energy relative to the spectrum for the -V
termination. This shift can be explained by the changes in the
electronic structure due to the metal to insulator transition. The
XP spectra exhibit enhanced satellite features in the case of the
-V=O termination, which can be attributed to poorly screened final
states. We also observed a shift of the O 2p band towards lower
binding energies for the -V=O terminated surface relative to the
-V terminated surface. We tried to explain this phenomenon with a
band bending model. Finally, we proposed two models for the
surface geometry of the -V=O terminated surface. In the first one,
the oxygen atoms sit on top of the vanadium atoms. In the second
one, the oxygen atoms sit on quasi regular bulk positions.
We performed high resolution electron energy loss spectroscopy
(HREELS) measurements and presented a phonon spectrum for each
termination. Differences in phonon intensities observed between
both surface terminations can be interpreted as a screening effect
of electronic carriers. We compared our spectra with a spectrum of
the isomorphic Cr2O3(0001) and found out that the
metal-oxygen bond is not so strong in V2O3 as in Cr2O3.
We studied the water adsorption properties of both surface
terminations. The experiment consists of the adsorption of water
at 90 K, yielding the formation of ice on the sample surface. The
sample then is heated up to 190 K. The species present on the
surface at this temperature are analyzed with UPS, XPS and HREELS.
The adsorption path seems to depend on both the termination and
the exposure. We observed molecularly adsorbed water on both
surface terminations for low exposures. The adsorbed water shows
only weak interaction with the substrate. For large exposures,
water dissociates and OH- groups were detected. When the OH-
desorb of the primary -V=O terminated surface, the surface left is
-V terminated. In the case of the -V=O terminated surface, the
water molecule is assumed to adsorb on the surface vanadium atom
through its oxygen atom. The oxygen double bonded to the vanadium
can interact with the hydrogen of the water molecule to form a OH
radical, breaking its double bond to the vanadium. This
dissociation mechanism may imply charge redistribution, explaining
why the V 3d emission in UPS increases upon water adsorption. This
model explains why the vanadyl oxygen atoms desorb with the OH
groups. For the -V terminated surface, we observed a charge
transfer from the V 3d substrate to the adsorbate, producing
OH- groups. Therefore, we proposed a model in which the
vanadium a1g or egΠ orbital forms a Σ bond with oxygen
lone-pair orbitals of OH-.
We performed CO2 adsorption experiments with UPS, XPS, HREELS and
IRAS. The UP results for the -V=O surface exhibit small features
which we assigned to physisorbed CO2. The CO2 adsorption on the
-V terminated surface is more complex. The analyze of the IRAS
results leads us to the conclusion that CO2 adsorbs in a bent
configuration. With UPS and XPS, we could evidence the formation
of carbonates upon heating up to 200 K.
The CO adsorption properties follow a similar trend as for CO2 :
only small quantities adsorb on the -V=O surface while the -V
surface seems to be much more reactive. On the -V=O surface, CO
adsorbs molecularly and we concluded from the angle resolved UPS
data that the CO molecule is strongly tilted on the surface. With
NEXAFS and IRAS, we showed the formation of CO2 on the -V
surface.
To our knowledge, we are the first to report a surface effect
resulting in a metal to insulator transition. This very complex
phenomenon is very exciting for the surface scientist. Further
work on V2O3 (0001) should therefore involve theorists in order to
explain properly why the formation of vanadyl groups on the
surface induces a metal to insulator transition. A simulation of
the angle resolved UPS data could determine which model for the
surface geometry is correct. Further experimental work could be
thermal desorption spectroscopy (TDS) and IRAS with isotopes in
order to identify the formation path of CO2 by CO adsorption on
the -V terminated surface.
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