PHYSICAL REVIEW B 71, 165437 共2005兲
Reduction of vanadium-oxide monolayer structures
J. Schoiswohl, S. Surnev,* M. Sock, S. Eck, M. G. Ramsey, and F. P. Netzer
Institut für Experimentalphysik, Karl-Franzens-Universität Graz, A-8010 Graz, Austria
G. Kresse
Institut für Materialphysik, Universität Wien, A-1090 Wien, Austria
共Received 17 June 2004; revised manuscript received 29 November 2004; published 29 April 2005兲
The reduction of vanadium oxide monolayer structures on Rh共111兲 has been investigated by variabletemperature scanning tunneling microscopy, low energy electron diffraction, photoelectron spectroscopy of the
core levels, and the valence band, and by probing the phonon spectra of the oxide structures in high-resolution
electron energy loss spectroscopy. A sequence of oxide phases has been observed following the reduction from
the highly oxidized 共冑7 ⫻ 冑7兲R19.1° V-oxide monolayer: 共5 ⫻ 5兲, 共5 ⫻ 3冑3兲rect, 共9 ⫻ 9兲, and “wagon-wheel”
oxide structures are formed with decreasing chemical potential of oxygen ␮O. The structures have been
simulated by ab initio density functional theory, and structure models are presented. The various V-oxide
structures are interrelated by common V u O coordination units, and the reduction progresses mainly via the
removal of V v O vanadyl groups. All oxide structures are stable at the appropriate ␮O only in the twodimensional V-oxide/Rh共111兲 phase diagram and are thus stabilized by the metal-oxide interface. The results
demonstrate that oxides in ultrathin layer form display modified physical and chemical properties as compared
to the bulk oxides.
DOI: 10.1103/PhysRevB.71.165437
PACS number共s兲: 68.47.Gh, 68.55.⫺a, 68.37.Ef, 71.15.Mb
I. INTRODUCTION
Ultrathin layers of metal oxides of thickness of one to a
few unit cells, often designated as oxide nanolayers, display
particular physical and chemical properties that are not
shared by their respective bulk counterparts. These properties
include new structural, stoichiometric, and bonding behavior
as well as a significantly modified chemical reactivity. Most
of these new properties are caused by the high surface-tovolume ratio of these systems and are determined by the
interaction at interfaces to the environment or to dissimilar
materials, which are necessary to support the nanostructures
in thin film form. In this study we have investigated the
chemical and structural properties of metal oxide nanolayers,
which are supported on a different metal surface, viz. vanadium oxides on Rh surfaces. Vanadium oxides in bulk phases
are interesting materials because of the possibility of different oxidation states of the V atoms and the resulting wealth
of structural and chemical behavior, which is associated with
different oxide phases due to the variable V cation valencies
and the different V u O coordination spheres.1 Vanadium oxides are components of important industrial catalysts for oxidation reactions,2 and in fact, vanadium is the most important
metal used in metal oxide catalysis for the manufacture of
high-value chemicals and in environmental pollution
control.3
Vanadium oxides in ultrathin film form have been investigated recently on Pd共111兲 and Rh共111兲 single crystal
surfaces.4–7 As shown recently, a peculiar growth behavior as
a function of oxide coverage and oxide layer thickness with
several interface stabilized structures has been found on
Rh共111兲. Here, this work was extended and we report a series
of vanadium-oxide monolayer structures on a Rh共111兲 surface, which develops in the presence of reducing environmental conditions. This structure sequence is significantly
different from that observed during growth.
1098-0121/2005/71共16兲/165437共8兲/$23.00
Vanadium-oxide nanostructures deposited by physical vapor deposition onto clean Rh共111兲 surfaces and subsequently
subjected to reducing conditions, i.e., to annealing in vacuum
or to the exposure of hydrogen, have been characterized by
variable-temperature scanning tunneling microscopy 共VTSTM兲, low energy electron diffraction 共LEED兲, photoelectron spectroscopy of core and valence states with use of synchrotron radiation 共XPS, UPS兲, and high resolution electron
energy loss spectroscopy 共HREELS兲. The oxide surface
structures and their development with the variation of the
chemical potential of oxygen ␮O have also been simulated
by ab initio density functional theory 共DFT兲 calculations. We
find that a sequence of new V-oxide phases evolves from the
highly oxidized as-deposited structures with decreasing ␮O,
which all are stable only in the quasi-2D limit of nanolayer
films. Although the present work has been conducted as a
model study on a well-characterized idealized system, we
believe that it has more general implications for the practical
purposes of oxide nanostructure fabrication.
II. EXPERIMENTAL AND COMPUTATIONAL
PROCEDURES
The experiments have been performed in three different
ultrahigh vacuum chambers, with typical base pressures
p = 1 ⫻ 10−10 mbar. The STM and HREELS measurements
have been carried out in our laboratories in Graz, in two
custom-designed systems. The STM system is equipped with
a variable-temperature STM 共Oxford Instruments兲, a LEED
optics, an CMA Auger electron spectrometer 共AES兲, and the
usual facilities for crystal cleaning and physical vapor
deposition.4 The STM images were recorded in a constant
current mode at room temperature or at elevated temperature,
with electrochemically etched W tips, which were cleaned in
situ by electron bombardment. The HREELS measurements
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©2005 The American Physical Society
PHYSICAL REVIEW B 71, 165437 共2005兲
J. SCHOISWOHL et al.
were performed with an ErEELS 31 spectrometer, as described in Ref. 8. The HREELS spectra were taken at room
temperature with a primary energy of 5.5 eV in specular reflection geometry, with a typical resolution of ⬃3.5 meV as
measured at the FWHM of the reflected primary peak.
High-resolution photoemission measurements with use of
synchrotron radiation were carried out at beamline I311 in
the Swedish synchrotron radiation laboratory MAX-lab in
Lund.9 The experimental end-station is equipped with a Scienta SES 200 hemispherical electron energy analyzer. Photon
energies of 620 eV and 110 eV have been employed for exciting V 2p core level and valence band spectra, respectively,
with corresponding energy resolutions of 250 meV and
100 meV. Clean Rh共111兲 surfaces were prepared by 1.5 keV
Ar+-ion sputtering, followed by annealing to 830 ° C for several minutes, and by heating cycles in O2 followed by a final
flash to 800 ° C. The cleanliness of the Rh共111兲 surfaces was
checked by AES, XPS or HREELS in combination with
LEED. LEED was also used to control the preparation of the
respective oxide surface structures in the different experimental systems. Vanadium-oxide films were prepared by reactive evaporation of V metal in 2 ⫻ 10−7 mbar O2 onto the
clean Rh共111兲 surface at 250 ° C or 400 ° C, and after deposition the sample was kept at these conditions for additional
5 minutes; the sample was subsequently cooled down to
T ⬍ 100 ° C in oxygen atmosphere to prevent the thermal reduction of the oxide. The vanadium deposition rate was
monitored by a quartz crystal microbalance and a rate of
0.2 monolayer/ min was typically employed. The V-oxide
coverage is given in monolayer equivalents 共MLE兲, where 1
MLE contains the same number of V atoms as one monolayer of Rh共111兲 atoms. The deposited V-oxide films were
subjected to reducing conditions by annealing in ultrahigh
vacuum or by exposure to a background pressure of H2 at
elevated temperature. In both cases, the structural evolution
of the V-oxide overlayers has been followed directly in the
VT-STM.
The DFT calculations were performed using the Vienna
ab initio simulation package 共VASP兲.10 The interaction between the valence electrons and the ionic cores was described by the projector augmented wave method in the
implementation of Kresse and Joubert,11 and the plane wave
cutoff was set to 250 eV. Generalized gradient corrections
were applied throughout this work.12 Generally four layer
thick slabs for the substrate and k-point grids corresponding
to 共8 ⫻ 8兲 points in the Brillouin zone of the primitive surface cell were used. The vibrational spectra of the considered
surface oxides were calculated using finite differences. To
this end, each atom in the oxide was displaced by 0.02 Å in
each direction, and the interatomic force constants were determined from the induced forces. The simulated STM images were calculated in the Tersoff Hamann approach.13 The
charge isosurfaces were evaluated at a value that the brightest spots are located 4 Å above the core of the topmost atom
in the surface oxide. For further calculational details we refer
to Ref. 7.
III. RESULTS
A. The highly oxidized „冑7 Ã 冑7…R19.1° vanadium-oxide layer
The reactive evaporation of vanadium on to the Rh共111兲
surface as described in the experimental section generates a
FIG. 1. 共Color online兲 共a兲 STM image of the 共冑7 ⫻ 冑7兲R19.1°
vanadium oxide on Rh共111兲 共800⫻ 800 Å2; tunneling conditions:
sample bias U = + 2 V; tunneling current I = 0.05 nA兲. 共b兲 Highresolution STM image of the 共冑7 ⫻ 冑7兲R19.1° V-oxide 共15
⫻ 15 Å2; U = + 0.75 V, I = 0.2 nA兲. A unit cell and the Rh共111兲 substrate direction is indicated. 共c兲 LEED pattern of the 共冑7
⫻ 冑7兲R19.1° V-oxide on Rh共111兲 共electron energy 50 eV兲. 共d兲 DFT
derived model of the 共冑7 ⫻ 冑7兲R19.1° vanadium oxide. Unit cell
and structural units are indicated.
highly oxidized vanadium-oxide overlayer at submonolayer
coverages. The STM image of Fig. 1共a兲 shows island structures of the 共冑7 ⫻ 冑7兲R19.1° V-oxide phase, which partially
cover the Rh共111兲 surface at a coverage of ⬃0.25 MLE.
Figure 1共b兲 displays a high resolution STM image of the
共冑7 ⫻ 冑7兲R19.1° structure, which shows a hexagonal
honeycomb-type network of bright protrusions; a 共冑7
⫻ 冑7兲R19.1° unit cell is indicated on the image. This unit
cell is also superimposed on the LEED pattern of this surface, shown in Fig. 1共c兲. The DFT calculations have established the model of the 共冑7 ⫻ 冑7兲R19.1° structure,7 that is
illustrated in Fig. 1共d兲. According to this model the 冑7 structure corresponds to a V3O9 oxide phase and contains identical pyramidal O4V v O building blocks 关squares in Fig.
1共d兲兴, which have the V atom in the center, four bridging O
atoms in the basal plane, and a vanadyl-type O atom at the
apex. The pyramids are linked together via the four bridging
O atoms in the basal plane as shown in the model, each 冑7
unit cell is formed by three of these formal VO3 units. Note
that these formal VO3 units are not in conflict with the maximal oxidation state of +5 of the V atoms, since the peripheral
four bridging O atoms are shared with the Rh substrate. Formal electron counting procedures are not applicable to a
metal surface, but a simple argument would be as follows:
each V atom donates two electrons to the double bonded
vanadyl O, and three electrons to the four surrounding O
atoms in the basal plane. To fill the 2p shell of these interfacial O atoms, 0.5 electron is missing, that must be provided
by the Rh surface.
The structure model of this highly oxidized V-oxide phase
is confirmed by the HREELS phonon spectrum 共see bottom
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PHYSICAL REVIEW B 71, 165437 共2005兲
FIG. 3. 共a兲 V 2p3/2 XPS core level spectra of various V-oxide
phases on Rh共111兲. 共b兲 Corresponding valence band spectra.
FIG. 2. 共Color online兲 STM images taken consecutively during
reduction of the 共冑7 ⫻ 冑7兲R19.1° vanadium-oxide phase on
Rh共111兲 at T = 400 ° C, pH2 = 1 ⫻ 10−9 mbar. The cumulative reduction time is indicated on each frame 共800⫻ 800 Å2 , U = + 2 V , I
= 0.1 nA兲.
curve in Fig. 4兲, where the characteristic stretching vibrations of the vanadyl V v O group at 128 meV along with the
vibrations of the bridging O atoms at 67 meV 共Ref. 7兲 are
clearly discerned. It is necessary to emphasize that the
共冑7 ⫻ 冑7兲R19.1° phase is a stable structure in the V-oxide/
Rh共111兲 phase diagram, but that it exists only in the single
layer limit, where it is stabilized by the Rh共111兲 interface.
B. Reduction sequence of the „冑7 Ã 冑7…R19.1°
vanadium-oxide structure
Exposing the as-deposited 共冑7 ⫻ 冑7兲R19.1° V-oxide layer
to H2 at elevated temperature leads to a transformation into a
sequence of reduced structures. Figure 2 shows a series of
STM images, starting from the 0.25 MLE 共冑7 ⫻ 冑7兲R19.1°
on Rh共111兲 surface 关Fig. 2共a兲兴, which have been recorded
consecutively at 400 ° C in 1 ⫻ 10−9 mbar H2; the time
elapsed during the reduction process is indicated on the images. The reduction begins with the introduction of disorder
into the 共冑7 ⫻ 冑7兲R19.1° structure—see Fig. 2共b兲. After
4530 s hydrogen exposure a new ordered structure appears,
which is characterized by a 共5 ⫻ 5兲 unit cell 关Fig. 2共c兲兴. This
structure is transformed into a 共5 ⫻ 3冑3兲-rectangular structure after 6250 s reduction time 关Fig. 2共d兲兴, which in turn
gives way to a 共9 ⫻ 9兲 structure after 17 350 s of reduction
关Fig. 2共e兲兴. The final, most reduced V-oxide phase in this
sequence is formed after 22 850 s hydrogen exposure 关Fig.
2共f兲兴: it is a complicated structure, which is designated as
“wagon-wheel” structure as explained below. Note that the
described sequence of structures has also been observed by
annealing in UHV from 250 ° C – 800 ° C. The various structures will be presented and discussed in more detail in Sec.
III C.
The V 2p XPS spectra confirm the reduction of the
V-oxide phases upon H2 exposure at elevated temperature. In
Fig. 3共a兲 V 2p3/2 XPS traces of the 共冑7 ⫻ 冑7兲R19.1°, the
共5 ⫻ 3冑3兲-rect, the 共9 ⫻ 9兲, and the “wagon-wheel” structures
are compared. The V 2p photoemission lines shift progressively from 515.4 eV for the 共冑7 ⫻ 冑7兲R19.1° structure to
lower binding energy, ending up at 513.2 eV for the “wagonwheel” structure. The V 2p photoemission lines are broad
and exhibit complicated shapes due to many-body and configuration interaction effects,14,15 we therefore refrain from a
peak decomposition analysis of the spectra. Moreover, it is
difficult to associate the absolute core level binding energies
of the cations with the oxidation state in ultrathin oxide layers by comparison with the respective core level binding energies measured on oxide bulk samples, because the influence of the interfacial bonding and the proximity of the
underlying metal surface modifies both the initial and final
state in the photoemission process. This can lead to erroneous assignments of the formal oxidation state and of the
stoichiometry as discussed in previous work.4,16 However,
the shifts of the V 2p core level peaks to lower binding
energy as apparent in Fig. 3 are clear indications of the progressive reduction of the V-oxide phases.
The valence band spectra of the various V-oxide structures are collected in Fig. 3共b兲. The intensity in the spectral
region between EF and 4 eV binding energy is due to photoemission from the overlapping V 3d and Rh 4d states.
However, since the photon energy of 110 eV is at the Cooper
minimum of the Rh 4d photoionization cross section,17 the V
3d contribution to the valence band should be dominant. The
region from 4 eV to 8 eV is due to the O 2p valence bands.
The V 3d intensity at the Fermi level increases in going from
the 共冑7 ⫻ 冑7兲R19.1° structure to the “wagon-wheel” phase
关from bottom to top in Fig. 3共b兲兴, with a sharp peak at the
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J. SCHOISWOHL et al.
FIG. 4. HREELS phonon spectra of various V-oxide phases on
Rh共111兲. Vertical bars indicate density functional theory predicted
dipole active modes.
Fermi level in the latter structure. This is due to the increasing V 3d density of states as a result of the progressive
reduction of the oxide phases. The spectral changes in the O
2p valence band region accompany the structural changes,
but have less diagnostic value apart from testifying the presence of oxygen at the surface. Thus, the prominent feature at
⬃6 eV in the valence band of the “wagon-wheel” structure
indicates that this most reduced structure still contains significant amounts of oxygen.
The HREELS phonon spectra provide very specific fingerprints of the different V-oxide structures 共Fig. 4兲. As mentioned above the 共冑7 ⫻ 冑7兲R19.1° structure is characterized
by the vanadyl V v O stretching frequency at 128 meV and
a vibrational loss peak at 67 meV due to vibrations involving
the bridging O atoms 共density functional theory predicts dipole active modes at 137 and 65 meV,7 in good agreement
with the experimental data兲. The 共5 ⫻ 3冑3兲-rect HREELS
spectrum still contains the vanadyl loss peak, albeit at reduced intensity and shifted to a somewhat lower loss energy
of 124 meV, and more complex loss structures at 52 meV
and 72 meV, the latter with a shoulder at ⬃80 meV. The
reduced intensity of the vanadyl loss suggests a reduction of
vanadyl groups in the 共5 ⫻ 3冑3兲-rect structure, and this is
important experimental input for the construction of structure
models in the DFT simulations. In contrast, the 共9 ⫻ 9兲 phase
displays no vanadyl loss peak, but the two broader loss structures at ⬃54 meV and ⬃72 meV. The “wagon-wheel” structure shows a simple vibrational loss spectrum with one single
peak at 62 meV and perhaps a weak lower-energy feature at
around 40 meV. The HREELS spectra thus indicate that the
reduction of the 共冑7 ⫻ 冑7兲R19.1° V-oxide progresses via the
loss of vanadyl V v O units, as discussed in the following
section.
FIG. 5. 共Color online兲 共a兲 STM image of the 共5 ⫻ 5兲 V-oxide on
Rh共111兲 共1000⫻ 1000兲 Å2, U = + 2 V, I = 0.02 nA兲. 共b兲 Highresolution STM image of the 共5 ⫻ 5兲 V-oxide 共40⫻ 40 Å2, U
= + 2 V, I = 0.02 nA兲. The inset shows a DFT simulated STM image. 共c兲 LEED pattern of the 共5 ⫻ 5兲 V-oxide on Rh共111兲 共electron
energy 50 eV兲. 共d兲 DFT model of the 共5 ⫻ 5兲 V-oxide. Unit cell and
structural units are indicated.
C. The reduced V-oxide structures: „5 Ã 5…, „5 Ã 3冑3…-rect, „9
à 9…, “wagon-wheel”
STM images of the 共5 ⫻ 5兲 V-oxide phase are displayed in
Figs. 5共a兲 and 5共b兲. Figure 5共a兲 shows well-ordered islands
of the 共5 ⫻ 5兲 structure at a nominal surface coverage of 0.15
MLE vanadium deposition. Figure 5共c兲 shows the LEED pattern of the corresponding surface. The high-resolution image
of Fig. 5共b兲 reveals that bright maxima are linked together in
a hexagonal honeycomb-type arrangement, so that a dark
depression is seen at the corners of the unit cell 共indicated on
the image兲. Note that the bright spots in between the 共5
⫻ 5兲 islands represent planar V6O12 oxide clusters which
have been addressed in detail in a previous publication.18
The DFT derived model of the 共5 ⫻ 5兲 structure in Fig.
5共d兲 discloses the structural details, hexagonal V6O12 units
共circle on the figure兲 are linked to O4V v O 共square on the
figure兲 and O3V v O 共triangle on the figure兲 units yielding a
V11O23 stoichiometry per 共5 ⫻ 5兲 unit cell. The overall oxide
stoichiometry is thus reduced from formally VO3 in the
共冑7 ⫻ 冑7兲R19.1° phase to VO2.09 in the 共5 ⫻ 5兲 structure. The
inset of Fig. 5共b兲 shows a simulated STM image as calculated with the model structure, the agreement with the experimental image is excellent and confirms the structure
model.
The 共5 ⫻ 3冑3兲-rect V-oxide structure is illustrated in Fig.
6. The STM images of Figs. 6共a兲 and 6共b兲 show nanometer
dimension 共5 ⫻ 3冑3兲-rect islands on the Rh共111兲 surface corresponding to a nominal coverage of 0.25 MLE and a highresolution view of this structure, respectively. This V-oxide
phase forms well-ordered islands of rectangular shape, which
are oriented along the 具11គ 0典 symmetry directions of the Rh
substrate. The sharp LEED pattern of this surface 关Fig. 6共c兲兴
confirms the high structural order. The 共5 ⫻ 3冑3兲-rect unit
cell, indicated in Fig. 6共b兲, contains a bright maximum in the
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FIG. 7. 共Color online兲 共a兲 STM image of the 共9 ⫻ 9兲 V-oxide on
Rh共111兲 共2000⫻ 2000 Å2, U = + 2 V, I = 0.05 nA兲. The inset shows
the LEED pattern of the 共9 ⫻ 9兲 phase 共electron energy 50 eV兲. 共b兲
High-resolution STM image of the 共9 ⫻ 9兲 V-oxide 共60⫻ 60 Å2,
U = + 2 V, I = 0.1 nA兲. The inset shows a DFT simulated STM image. 共c兲 DFT model of the 共9 ⫻ 9兲 V-oxide. Unit cell and structural
units are indicated.
FIG. 6. 共Color online兲 共a兲 STM image of the 共5 ⫻ 3冑3兲-rect
V-oxide on Rh共111兲 共1000⫻ 1000 Å2, U = + 2 V, I = 0.1 nA兲. 共b兲
High-resolution STM image of the 共5 ⫻ 3冑3兲-rect V-oxide 共33
⫻ 33 Å2, U = + 2 V, I = 0.1 nA兲. The inset shows a DFT simulated
STM image. 共c兲 LEED pattern of the 共5 ⫻ 3冑3兲-rect V-oxide phase
共electron energy 50 eV兲. 共d兲 DFT model of the 共5 ⫻ 3冑3兲-rect
V-oxide. Unit cell and structural units are indicated.
center and dark holes at the corners. According to the DFT
model 关Fig. 6共d兲兴 this structure contains hexagonal V6O12
共circle on the figure兲 and pyramidal O4V v O 共square on the
figure兲 with a unit cell content of V13O21, yielding a formal
VO1.6 stoichiometry. The STM simulation 关inset of Fig. 6共b兲兴
reveals that the bright maximum in the unit cell is due to the
vanadyl 共V v O兲 group of the tetragonal pyramidal structure
unit. The presence of vanadyl groups is also confirmed by
the vibration at 124 meV in the HREELS spectrum 共Fig. 4兲.
Similar to the 共5 ⫻ 5兲 structure, the 共5 ⫻ 3冑3兲-rect structure
constitutes a mixed-valent V-oxide with V atoms in different
oxygen coordination spheres. The DFT calculated dipole active vibrational frequencies are located at 130, 71, 55, and
33 meV, with an intensity distribution which resembles
closely the experimental ones. Additionally, a weak shoulder
is calculated at 84 meV 共which is also visible in the experimental data兲. These frequencies are in excellent agreement
with the experimental data, which is a further confirmation
for the suggested model 关as for the 共冑7 ⫻ 冑7兲R19.1° phase,
the frequency of the V v O stretch mode is somewhat too
high兴.
STM images of the 共9 ⫻ 9兲 V-oxide layer are presented in
Figs. 7共a兲 and 7共b兲. The 共9 ⫻ 9兲 islands display more rounded
boundaries and less well-defined geometrical shapes, however the sharp LEED pattern 关inset in Fig. 7共a兲兴 indicates a
high structural order within the islands. The high-resolution
STM image 关Fig. 7共b兲兴 shows circles of maxima with a depression in the center, which are arranged in a hexagonal
fashion. The centers of four hexagons define the 共9 ⫻ 9兲 unit
cell as indicated on the figure. The HREELS spectrum of the
共9 ⫻ 9兲 structure 共Fig. 4兲 reveals that vanadyl groups are absent in this structure. Based on the previous experience with
V-oxide structures on Rh共111兲 a structural model is shown in
Fig. 7共c兲. The model involves hexagonal V6O12 units 共circles
on the figure兲 and no pyramidal structures, and a V36O54
content of the 共9 ⫻ 9兲 unit cell is derived, which corresponds
to a formal V2O3 stoichiometry. The V 2p3/2 core level peak
at 514 eV binding energy is compatible with the formal V3+
oxidation state in V-oxide nanolayers.4 The model was relaxed using DFT calculations, and the final STM simulation
for tunnelling into empty states between 0 and 1 eV is shown
in the inset of Fig. 7共b兲. The agreement with experiment is
excellent, every single detail is reproduced leaving little
doubt about the correctness of the model.
The phase with the lowest oxidation state in the reduction
sequence of V-oxide nanolayers on Rh共111兲 is examined in
Fig. 8. The STM images 共a兲 and 共b兲 illustrate the morphology
and the atomic details of the structure, respectively, which
has been designated as “wagon-wheel” structure. The name
has been chosen because of the resemblance of the bright
array of maxima in the STM images 关Fig. 8共b兲兴 with the hub
and spokes of a wagon-wheel—a sketch of such a wagonwheel is drawn on the image. The hubs of the “wagonwheel” form a hexagonal lattice with a unit cell periodicity
of 18.9± 0.2 Å 关marked A on the image 8共b兲兴, which is ro-
FIG. 8. 共Color online兲 共a兲 STM image of the “wagon-wheel”
V-oxide on Rh共111兲 共1000⫻ 1000 Å2, U = + 2 V, I = 0.1 nA兲. 共b兲
High-resolution STM image of the “wagon-wheel” V-oxide 共80
⫻ 80 Å2, U = + 2 V, I = 0.1 nA兲. A “wagon-wheel” and two unit
cells A , B are drawn on the image. 共c兲 LEED pattern of the “wagonwheel” phase 共electron energy 60 eV兲. 共d兲 Autocorrelation diagram
of the STM image 共b兲. Unit cells A and B are indicated.
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tated by 21° ± 1° with respect to the 具11គ 0典 directions of the
Rh substrate. The periodicities of this structure are recognized best in an autocorrelation representation of the STM
image as displayed in Fig. 8共d兲. The measured unit cell dimensions are very close to a 共7 ⫻ 7兲R21.8° overlayer, suggesting a commensurate superlattice. This is compatible with
the observed LEED pattern 关see Fig. 8共c兲兴, which is consistent with the 共7 ⫻ 7兲R21.8° superstructure. The autocorrelation image reveals that around each corner maximum of the
unit cells A, a smaller hexagon of maxima appears, which
indicates a hexagonal basis lattice of the structure with unit
cell distances of 3.1± 0.2 Å labelled B in Figs. 8共b兲 and 8共d兲.
This sublattice has only moderate long range order as indicated by the wavy lines in the autocorrelation image. The
data suggest that the concept of a Moiré pattern might be at
the root of these experimental observations. Superimposing
the Rh substrate lattice with aRh = 2.69 Å and the V-oxide
lattice with aVOx = 3.1 Å and rotating the latter by 3.5° with
respect to the Rh 具11គ 0典 direction yields 18.8 Å and 21.8° for
the periodicity and rotation, respectively, of the Moiré pattern. This mirrors very closely the experimental observations
of the “wagon-wheel” lattice. The V 2p3/2 binding energy of
513.2 eV together with the significant amount of oxygen
present at the surface suggests an oxidation state of 2+ for
the V atoms. The HREELS spectrum 共Fig. 4兲 shows a single
loss peak at 62 meV and is similar to the loss spectrum
found for the so-called surface-V2O3 phase on Pd共111兲.4 The
latter structure has V atoms at the V u Pd interface and is O
terminated towards the vacuum side. The V density of the
“wagon-wheel” structure, with 0.73 MLE for a full monolayer, is however higher than that of the surface-V2O3 phase
共0.5 MLE兲. A further experiment provided additional hints
for the stoichiometry of the “wagon-wheel” phase, the structure has been obtained on the Rh共111兲 surface after deposition of 0.5 MLE of V atoms onto the O-precovered
Rh共111兲2 ⫻ 1-O structure 共O coverage 0.5 monolayer兲.
Taken together, these data suggest that the “wagon-wheel”
phase corresponds most likely to a VO stoichiometry, with V
atoms at the V u Rh interface and O atoms at the vacuum
side.
Overlayers with “wagon-wheel”-type structures seem to
be a more general phenomenon and have also been observed
in other systems, e.g., in the bimetallic Cr/ Pt共111兲 system19
or in the V-oxide/Pd共111兲 共Ref. 6兲 and Pd-TiO2共110兲 metal/
oxide systems.20 Following the model of Bennett et al. in
Ref. 20 for the so-called “pinwheel” superstructure of Pd on
TiO2共110兲 共which is very close to the “wagon-wheel” structure observed here兲 the geometric model of Fig. 9 has been
derived.兲
In Fig. 9共a兲 a superlattice with aOXIDE = 3.1 Å, rotated by
3.5° with respect to 具11គ 0典, has been superimposed on the
Rh共111兲 lattice. The Moiré pattern obtained by emphasizing
overlayer atoms, which are in or close to on-top and bridge
positions, with thicker lines reflects the brightness distribution in the experimental STM image of Fig. 8共b兲. This is also
illustrated in the billiard ball representation of Fig. 9共b兲,
where the adatoms in on-top and bridge positions on the
Rh共111兲 lattice are drawn with the stronger color tone,
whereas those in or close to threefold hollow positions are
FIG. 9. 共Color online兲 共a兲 and 共b兲 Billard ball models of the
“wagon-wheel” V-oxide structure. A “wagon-wheel” is indicated in
共b兲. 共c兲 DFT model of the “wagon-wheel” V-oxide. 共d兲 DFT simulated STM image.
colored less bright. The “wagon-wheel”-type pattern evolves
in remarkably good agreement with the experimental STM
contrast 关Fig. 8共b兲兴. Drawing on previous experimental and
theoretical evidence of V-oxides on Pd共111兲 and Rh共111兲
surfaces4,6,7 we presume that the maxima in the STM images
reflect the positions of the V atoms; these correspond to
spheres drawn in Fig. 9共b兲. The oxygen atoms required for
the VO model stoichiometry are located on top of the threefold hollow sites of the V layer, but they are not included in
Fig. 9共b兲 for the clarity of the presentation. Our model of the
VO “wagon-wheel” structure suggests thus a VO共111兲 overlayer; this oxide layer is V terminated at the interface to the
Rh and oxygen terminated at the surface to the vacuum. The
oxygen termination at the outer surface is stable for all
V-oxide layers on Pd共111兲 and Rh共111兲 surfaces, but the V
termination at the metal-oxide interface has only been found
for the more reduced oxide structures.4–6,21 The wagonwheel structure was also simulated using DFT. To estimate
the lattice constant of an ultrathin V u O overlayer on Rh,
four and three VO units 关共冑3 ⫻ 冑3兲 structure兴 were placed on
top of a Rh共2 ⫻ 2兲 slab. The three VO units, with a VO
in-plane lattice constant of 3.14 Å, are found to be energetically preferred over an epitaxial VO overlayer with a lattice
constant matching the Rh共111兲 surface. For the final modelling of the wagon-wheel structure, 37 VO units were placed
on top of a 共7 ⫻ 7兲R21.8° Rh substrate, and the model was
subjected to a gentle annealing between 800 and 500 K for
3 ps. The final structure and the STM simulation are shown
in Figs. 9共c兲 and 9共d兲 For this particular structure, the agreement with experiment is admittedly only modest, which we
tend to relate to a slight misplacement of the oxide layer with
respect to the substrate. In fact, the STM image is very sensitive to small variations of the surface oxide, STM
simulations of structural models obtained by relaxing snapshots of the finite temperature molecular dynamics showed
significant variations in the long range undulations of the
STM simulations, although the surface energies of all models
were within 5 meV. With the present computational re-
165437-6
REDUCTION OF VANADIUM-OXIDE MONOLAYER STRUCTURES
PHYSICAL REVIEW B 71, 165437 共2005兲
FIG. 11. Sequence of vanadium-oxide monolayer structures
with their unit cell stoichiometries forming on a Rh共111兲 surface as
a function of the chemical potential of oxygen ␮O.
peak has gained in intensity and sharpness and is
shifted somewhat towards lower binding energies. The onset
of the V states shifts progressively towards higher binding
energies in excellent agreement with the experimental observations. Overall, the DFT calculations resemble the measured spectra remarkably well, although the valence band
spectra alone would be insufficient to construct structural
models.
IV. CONCLUSIONS
FIG. 10. DFT derived atom resolved densities of states of various V-oxide phases on Rh共111兲. The shaded region refers to the
V v O related density of states. Solid lines, oxygen; dashed lines,
vanadium; dotted lines, rhodium in the topmost surface layer.
sources a more careful determination of the epitaxtial relationship between the surface oxide and Rh was however not
possible.
Finally, for the four phases considered in the DFT
calculations, the calculated densities of states are collected
in Fig. 10; the O, V, and Rh projected DOS are shown
as solid, dashed, and dotted lines, respectively. For the
共冑7 ⫻ 冑7兲R19.1° structure, the most pronounced feature is
the double peak at 3 and 3.5eV, which corresponds to the
experimental peak at 4.5 eV. Inspection of the ion decomposed DOS reveals that it is related to the vanadyl group.
A similar double peak was also observed in the valence
band of the vanadyl terminated V2O3 surface, both in
experiment22 and in DFT calculations.23 In DFT, the double
peak is slightly shifted towards lower binding energies,
which is a common problem related to the present density
functionals. Furthermore, a pronounced peak at higher
binding energies 共6 eV兲 is visible in the calculated DOS,
corresponding to the main peak in the experimental valence
band spectrum at 6.5 to 7 eV. It is noted that the vanadium
states hybridize significantly with the O2p levels, with the
onset of the antibonding V u O2p hybrid orbitals located
well above the Fermi level for the 共冑7 ⫻ 冑7兲R19.1° structure
共+1.5 eV兲.
For the 共5 ⫻ 3冑3兲-rect structure, the V v O double peak
is still present but only as two weak shoulders at 3 and
3.5 eV 共in the experimental spectrum this peak is not resolved兲. For the 共9 ⫻ 9兲 structure, the V v O related double
peak has vanished, and the main oxygen 2p related
In summary, a sequence of new vanadium-oxide
phases has been detected on Rh共111兲 upon reduction of
the highly oxidized 共冑7 ⫻ 冑7兲R19.1° V-oxide overlayer,
which is formed up to monolayer coverages on the
Rh共111兲 surface after reactive evaporation of V metal
in oxygen atmosphere at 250 ° C – 400 ° C substrate
temperature.7 The structures have been investigated experimentally by variable-temperature STM, LEED, photoelectron spectroscopy with use of synchrotron radiation,
and HREELS phonon spectroscopy, and have been analyzed
theoretically by DFT calculations. The sequence of structures
observed is 共冑7 ⫻ 冑7兲R19.1° → 共5 ⫻ 5兲 → 共5 ⫻ 3冑3兲-rect
→ 共9 ⫻ 9兲 → “ wagon-wheel” with decreasing chemical
potential of oxygen ␮O 共see Fig. 11兲.
The formal oxidation state of the V atoms decreases
from ⬃5+ in the 共冑7 ⫻ 冑7兲R19.1° structure to 2+ in the
“wagon-wheel” phase. The structures in this reduction
sequence are connected by the evolution of common
V u O coordination building units, with the reduction
proceeding mainly by a progressive loss of vanadyl V v O
groups. The here observed phases are stable at the appropriate ␮O only in the two-dimensional V-oxide/Rh共111兲
phase diagram, they are thus all stabilized by the oxideRh共111兲 interface.
Ultrathin layers of oxides on metal single crystal surfaces
constitute excellent model systems to study oxide materials
in nanometer dimensions at the atomic level, under controlled conditions. The results of this work indicate that
metal oxides in nanolayers exhibit new properties in terms of
structure and stoichiometry, with new bonding configurations
and particular metal-oxygen coordination spheres. In combination with noble metal surfaces as support materials oxide
nanolayers may show new chemical properties, with enhanced reactivity towards reducing or oxidizing conditions.
This is an important aspect for the promotion effect of metal
oxides in heterogeneous catalysis. Although the present
study has model character, the results may have more general
implications for the fabrication of nanostructured oxide systems.
165437-7
PHYSICAL REVIEW B 71, 165437 共2005兲
J. SCHOISWOHL et al.
ACKNOWLEDGMENTS
This work has been supported by the Austrian Science
Foundation, 关DOC兴 the PhD program of the Austrian Acad-
*Electronic address: [email protected]
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