Surface Science 437 (1999) 38–48
www.elsevier.nl/locate/susc
Vanadium oxides thin films grown on rutile TiO (110)-(1×1)
2
and (1×2) surfaces
Q. Guo, S. Lee, D.W. Goodman *
Department of Chemistry, Texas A&M University, PO Box 30012, College Station, TX 77842-3012, USA
Received 14 December 1998; accepted for publication 10 May 1999
Abstract
Vanadium oxides on rutile TiO (110)-(1×1) and (1×2) surfaces have been prepared under ultrahigh vacuum
2
conditions by evaporation of vanadium in background O and characterized by various vacuum surface analytical
2
techniques. Auger electron spectroscopy (AES), low-energy electron diffraction (LEED), high-resolution electron
energy loss spectroscopy (HREELS ) and X-ray photoelectron spectroscopy ( XPS ) including core-level and X-rayinduced valence band, indicate the formation of V O on both substrates at vanadia coverages greater than one
2 3
monolayer equivalent (MLE ). The dispersion of vanadia is higher on the (1×2) surface than on the (1×1) surface.
At lower coverages <1 MLE, the interface shows two chemical states of vanadium, V3+ and V4+. © 1999 Elsevier
Science B.V. All rights reserved.
Keywords: Auger electron spectroscopy; Electron energy loss spectroscopy (EELS ); Low energy electron diffraction (LEED); X-ray
photoelectron spectroscopy; Vanadium oxide; Titanium oxide; Metal–insulator surfaces; Single crystal epitaxy
1. Introduction
Vanadium oxide catalysts are an industrial
mainstay for a variety of chemical processes.
Vanadium oxide supported on titanium oxide, for
instance, is the most common commercial catalyst
for o-xylene oxidation to phthalic anhydride [1].
In practice, vanadium pentoxide ( V O ) is the
2 5
most commonly used vanadium oxide catalyst.
The surface of this catalyst, however, may contain
a lower oxidation state of vanadium oxide, which
could play an important role in catalytic reactions
since the oxidation state of vanadium is known to
* Corresponding author. Tel.: +1-409-845-0214;
fax: +1-409-845-6822.
E-mail address: [email protected]
(D.W. Goodman)
govern reactivity. Typically, monolayer or submonolayer quantities of oxovanadium deposited on a
support yield the most active catalyst [2]. It has
been found that some reactions can be controlled
by the in-plane metalMoxygen bond strength
( VMOMV ) rather than the strength of the terminal bond ( VNO) [3]. It has been proposed that
the number of participating surface vanadium
oxide sites is related to the reducibility of the
VMOMsupport bond [3].
There have been studies on the interaction of
vanadia with titania, including powders and single
crystals [4–11]. It has been reported that the
vanadia surface structure is determined by its
coverage and temperature. At low coverages,
vanadia interacts with the titania support and is
stabilized as V4+ at high temperatures [4]. Zhang
and Henrich [5] studied the interaction of vanadia
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Q. Guo et al. / Surface Science 437 (1999) 38–48
39
with TiO (110) by photoemission including X-ray
2
photoelectron spectroscopy ( XPS), ultraviolet
photoelectron spectroscopy ( UPS), other electron
spectroscopies, and work-function measurements.
The results indicate the formation of lower oxides
of vanadium that interact weakly with the support.
Angle-resolved XPS studies on V O –TiO (ana2 5
2
tase) suggest the formation of a VMTiMO bond
at the vanadia–titania interface [9]. Sambi et al.
studied the growth and structure of epitaxial VO
2
on TiO (110) by deposition of vanadium followed
2
by annealing in oxygen [11].
In this paper, we report the results of depositing
vanadia onto TiO (rutile) (110)-(1×1) and
2
-(1×2) surfaces by evaporating vanadium in O
2
(6×10−7 Torr) at 570 K. The experiments utilized
Auger electron spectroscopy (AES ), low-energy
electron diffraction (LEED), XPS including corelevel and X-ray-induced valence band and highresolution electron energy loss spectroscopy
(HREELS).
2. Experiments
The experiments were carried out in two ultrahigh vacuum ( UHV ) chambers, one with a base
pressure of 2×10−10 Torr, equipped with reverse
view optics for LEED, electron optics with a
single-pass cylindrical mirror analyzer (CMA) for
AES, and a LK-2000 model HREELS spectrometer. The second chamber has a base pressure of
5×12−10 Torr and is equipped with XPS, LEED
and AES. The TiO (110) sample (10 mm×
2
5 mm×1 mm) was mounted on an Mo plate
(1 mm thickness) as shown in Fig. 1. To increase
the contract area and ensure good thermal contract, the edges of the TiO (110) sample were
2
attached to Ta foil which was spot-welded to the
Mo plate (Fig. 1A). A W–5%Re/W–26%Re thermocouple was attached to the sample with Ta foil
and a W clip to ensure good thermal contact. Two
Ta wires were then spot-welded to the backside of
the Mo plate for heating. The final sample assembly is shown in Fig. 1B. The sample could be
resistively heated to 1500 K and to 2500 K by an
electron beam heater mounted at the back of the
sample; it could be cooled to 95 K with liquid
Fig. 1. Sample mounting. (A) The sample is mounted on an
Mo plate by Ta foils, spot-welded for good thermal contact.
(B) Final assembly.
N . The TiO (110) sample was cleaned with dilute
2
2
nitric acid and then washed in water and acetone
solutions prior to insertion into the UHV chamber.
After several annealing cycles at ~1100 K for 5–
10 min, LEED showed an excellent (1×1) pattern.
Impurity levels in all cases measured by AES were
limited to carbon (<1%). The sample was annealed
at 1100 K in ~10−6 Torr oxygen to remove any
carbon, followed by a second anneal for 5–10 min
at ~1100 K in vacuo. The (1×2) surface was
prepared by several cycles of annealing to
~1300 K for 5–10 min.
The V doser consisted of a vanadium wire
wrapped tightly around a W filament; before evaporation, the source was thoroughly degassed. The
vanadia was grown on the TiO (110) substrate by
2
40
Q. Guo et al. / Surface Science 437 (1999) 38–48
evaporation of V in ~10−7 Torr O at 570 K
2
followed by a 580 K anneal in 10−6 Torr O , then
2
further annealed at 580 K in vacuo. The doser
current was calibrated to control the amount of
vanadium deposited. The deposition rate of vanadium is about 0.5 monolayer per minute.
A primary electron energy of 3 keV was used
in the AES measurements; in XPS, the Mg Ka
line was used. The binding energy (BE ) was calibrated using the Au(4f ) (BE=84.0 eV ) and
Cu(2p) (BE=932.7 eV ) features of metallic Au
and Cu respectively; a pass energy of 11.75 eV was
used for data acquisition. In HREELS, incident
energies E of 4.5 to 6.0 eV with a typical resolution
p
of 9-12 meV (73–97 cm−1), measured as full-width
at half maximum (FWHM ) of the elastic peak,
were selected for the surface vibrational studies.
For electronic excitations, an E of 25 eV was used
p
with a resolution of 25–30 meV at FWHM. The
HREELS data were recorded at 60° with respect
to the surface normal at the indicated
temperatures.
3. Results and discussion
3.1. Vanadia on TiO (110)-(1×1)
2
The most thermodynamically stable surface of
TiO is TiO (110), which has two surface struc2
2
tures, depending on preparation, characterized by
(1×1) and (1×2) LEED patterns. In our experiments, after repeated annealing cycles to ~1100 K,
AES indicated a clean TiO (110) substrate with a
2
sharp (1×1) LEED pattern. With increasing coverages of V in O at 570 K, the AES intensity of
2
the Ti feature gradually decreased while that of
the V
feature increased, as shown in Fig. 2. At
LMM
approximately ten monolayer equivalents (MLE )
coverage of V (the growth of V is not necessarily
layer-by-layer and so the coverage of vanadia film
is given as MLE) the AES showed only V and O
AES transitions. In a previous study [12], the
relative intensities of the oxygen to vanadium
Auger transitions were used to determine the surface composition of vanadium oxides; however,
quantitative analysis is difficult owing to the overlapping Auger peaks of oxygen and vanadium.
Alternatively, the ratio of the intensity of the
vanadium transitions at 470 and 437 eV has been
used to distinguish different phases of vanadia
[13]. The ratio of V(470)/V(437) varies smoothly
with the oxide state of vanadium oxide, with the
oxide species containing vanadium in higher oxidation states yielding smaller V(470)/V(437) ratios
[13]. For example, using a hemispherical analyzer
the V(470)/V(437) ratio is 0.67 for VO and 1.13
2
for VO [13]. From the AES spectrum of V O
2 3
films grown on Au(111), a V(470)/V(437) peakto-peak ratio of 0.9 was found using a double pass
CMA [14]. An ordered V O film grown on
2 3
Al O –Mo(110) yields a V(470)/V(437) ratio of
2 3
approximately 1.0 using a single-pass CMA [15].
In Fig. 2, at coverage of 6 MLE, AES still shows
a Ti signal from the substrate. At 10 MLE coverage, however, the surface is covered completely by
vanadia, with a V(470)/V(437) AES peak-to-peak
ratio of ~1.0, consistent with the formation of
VO.
2 3
The various vanadia samples were characterized
by XPS. On clean TiO (110)-(1×1), the binding
2
energies of Ti(2p ) and O(1s) are 459.5 eV and
3/2
530.8 eV respectively, in good agreement with previously reported data [16–19]. Fig. 3 shows the
XPS spectra of V(2p) and O(1s) core levels as a
function of the vanadium coverage in O . On the
2
clean surface, oxygen satellite peaks at 522.3 and
520.6 eV are observed as a result of Mg Ka and
3
Ka radiation. The interference of the V(2p )
4
1/2
feature and the satellite peaks makes qualitative
analysis difficult; therefore, only the V(2p ) data
3/2
are discussed. At coverages ≤1 MLE, the
V(2p ) binding energy is 516.2 eV. At vanadium
3/2
coverages >1 MLE this value shifts to 515.8 eV,
whereas the O(1s) binding energy is 530.3 eV.
Previous studies have indicated that the binding
energies of V(2p ) emission are 516.7 eV,
3/2
516.0 eV and 515.5 eV for V O , VO and V O
2 5
2
2 3
respectively [20–24]. Among these, the line-shape
of V(2p ) and V(2p ) from V O ( V3+) is much
3/2
1/2
2 3
broader. At coverages >1 MLE, the vanadium
binding energies and peak shapes are very similar
to those obtained from single crystals of V O [25]
2 3
and ordered V O films, consistent with the forma2 3
tion of V O . Furthermore, the binding energy of
2 3
the O(1s) feature, 530.3 eV at 20 MLE in Fig. 3,
Q. Guo et al. / Surface Science 437 (1999) 38–48
41
Fig. 2. AES of the vanadia–TiO (110)-(1×1) surface as a function of vanadium coverage in O .
2
2
Fig. 3. XPS of vanadia–TiO (110)-(1×1) as a function of coverage. The spectra were acquired at 300 K with an Mg Ka source
2
(1253.6 eV ).
is very close to the value reported for V O [20,23].
2 3
Also, our results are consistent with recent work
by Zhang and Henrich [5], who found that V O
2 3
was formed from the interaction of vanadium with
rutile TiO in 1×10−7 Torr O . However, in our
2
2
experiments at lower coverages (≤1.0 MLE ), the
516.2 eV binding energy suggests vanadium with a
higher oxidation state at the interface. Binding
energies of 516.3 to 516.5 eV have been reported
for VO [11]. The binding energy of the V(2p )
2
3/2
feature could be altered by surface charging, since
the binding energy of the O(1s) features is also
42
Q. Guo et al. / Surface Science 437 (1999) 38–48
shifted in the same direction with increasing coverage. This possibility can be excluded from the
Ti(2p ) XPS measurements. The binding energy
3/2
of the Ti(2p ) feature is shifted from 459.5 eV
3/2
for the clean substrate to 459.3 eV for a surface
with 10 MLE vanadia. This 0.2 eV shift is lower
than the 0.4 eV shift of the V(2p ) feature at
3/2
similar coverage.
Since valence-band spectra can provide additional information regarding the chemical state of
the vanadia films, experiments were carried out
using X-ray-induced valence-band spectroscopy.
Various vanadium oxide surfaces show clear
differences in the valence band. In VO , for exam2
ple, the highest valence band varies between 0.3
and 0.7 eV, depending upon the particular phase
[21]. The O(2p) band exhibits a broad structure
consisting of two features at 4.8 and 7.0 eV. For
V O , the 3d band is centered at ~1.3 eV with a
2 3
bandwidth of ~3 eV. Two features at 0.8 and
1.2 eV below the Fermi level are observed for a
cleaved V O single crystal surface corresponding
2 3
to the V(3d ) a and ep levels respectively [25]. A
1g
g
small dispersion is observed in the 3d band in
V O [26 ]. The O(2p) band consists of two features
2 3
at 4.9 and 6.5 eV. The X-ray-induced valence-band
spectrum for 10 MLE vanadia coverage is shown
in Fig. 4. The location of the Fermi level is in
good agreement with previous studies, where it
was argued that the Fermi level lies within the
overlapping a and ep levels of the V(3d) band
1g
g
[21,25]. Fig. 4 shows a broad feature at 1.2 eV in
the 3d band, consistent with V O at the higher
2 3
coverages. These results are in very good
agreement with the results of Zhang and Henrich
[5]. Unfortunately, data were not obtained at
lower coverages owing to the low sensitivity of the
X-ray-induced valence-band measurements. In
addition, overlap of the O(2p) bands of the substrate and vanadia makes detailed data analysis
difficult.
The various vanadia surfaces were also characterized using vibrational spectroscopy. Fig. 5
shows an HREEL spectrum of a clean
TiO (110)-(1×1) substrate at a resolution of
2
6.7 meV at FWHM. The spectra were obtained at
room temperature at an incidence angle of 60°
from the surface normal. Three fundamental
losses, at 47 meV (n ), 55 meV (n ) and 95.6 meV
1
2
(n ), as well as overtone losses and combinations
3
are observed (Fig. 5). These results agree well with
previous studies [27,28]. Fig. 6 shows the change
Fig. 4. X-ray-induced valence-band spectroscopy for 10 MLE vanadia on TiO (110)-(1×1). The spectrum was acquired with a Mg
2
Ka source (1253.6 eV ).
Q. Guo et al. / Surface Science 437 (1999) 38–48
43
Fig. 5. HREEL spectrum on TiO (110)-(1×1) with a primary energy of 6.0 eV and resolution of 6.7 meV at FWHM. The spectrum
2
was collected at 60° with respect to the surface normal at 300 K.
Fig. 6. HREEL spectra of vanadia–TiO (110)-(1×1) as a function of vanadia coverage.
2
in the loss features with vanadia coverage; the loss
peaks have been normalized to the elastic peak.
Initial deposition of V, 0.3–0.6 MLE, causes a
shift of the 95.6 meV loss feature to 94.7 meV
(Fig. 6). At 1.2 MLE coverage, this loss feature
shifts to 93.7 meV, and at 3.0 MLE, to 93.0 meV.
At higher coverages, 6.0 MLE and 10.0 MLE, the
primary loss positions are at 88 meV and 87 meV
respectively. At 10.0 MLE coverage an additional
loss feature is apparent at 48 meV; this compares
44
Q. Guo et al. / Surface Science 437 (1999) 38–48
to the loss features at 47 and 55 meV from the
clean substrate. The changes in the surface phonon
structure can be seen as well by monitoring the
combination feature at 191 meV. The fact that the
loss at 191 meV on the clean substrate changes to
187 meV at 1.2 MLE coverage suggests restructuring of the surface. An ordered V O film on
2 3
Al O –Mo (110) studied by HREELS showed a
2 3
primary loss feature between 78.0 and 82.0 meV
depending upon the surface temperature [15]. The
slight shift of the surface phonon frequencies with
temperature is due to the change in lattice constant
as a function of temperature (see below for details).
At higher vanadia coverages, the loss feature at
87 meV is broader and less intense ( Fig. 6). Since
LEED shows a faceted vanadia structure with a
very weak (1×1) pattern from the substrate at
this coverage, a rougher surface leading to a
broader and weaker phonon peak is expected.
Note that the broad loss feature at 87 meV may
include some signal from the substrate, suggesting
that the loss at 87 meV results from a combination
of disordered V O and TiO .
2 3
2
The electronic structure of vanadia–TiO
2
(110)-(1×1) was measured as a function of coverage using electron energy loss spectroscopy
( EELS) with a primary energy of 25 eV. For an
idealized TiO (110)-(1×1) surface the loss region
2
between 0.5 and 3.5 eV should be featureless,
except for a defect feature at approximately 0.8 eV
[29]. It is noteworthy that a loss is observed at
~1.9 eV at coverages between 0.3 and 0.6 MLE
on the (1×1) surface (Fig. 7). This loss shifts to
lower energy with increasing coverage. At 3.0 MLE
coverage, this loss is at 1.3 eV. With increasing
coverage, this feature disappears and an exponentially increasing feature is observed (see Fig. 7).
Unfortunately, no EELS data have been published for V O ; however, the electronic structure
2 3
of ordered V O films on alumina supported on
2 3
an Mo(110) surface has been studied [15]. The
V O has a V3+ (d2) electronic configuration and
2 3
undergoes a metal–insulator phase transition as a
function of temperature [30]. V O is metallic at
2 3
room temperature and becomes a magnetic insulator at temperatures below ~170 K [21,30–37].
Electron energy loss of a thick V O (0001) film
2 3
shows a difference within the low energy loss
Fig. 7. Electron energy loss spectra of vanadia on
TiO (110)-(1×1) as a function of coverage. The spectra were
2
acquired at 300 K with a primary energy of 25 eV at an incident
angle of 60°.
region as a function of temperature for the specular
scattering direction. At room temperature the
electron energy losses corresponding to the d–d
transitions of the metallic phase of V O are at 0.9
2 3
and 1.6 eV, whereas the 0.9 eV energy loss shifts
to 1.2 eV at 100 K in the insulator phase.
The electron energy loss line-shapes are very
characteristic of the various phases of V O . In the
2 3
metallic phase, the loss feature decays exponentially within the 0.5 to 3.5 eV region with two very
weak losses at 0.9 and 1.6 eV. In the insulator
phase, the exponential decay disappears and the
loss at 1.2 eV is much more pronounced due to an
energy increase in the a level within the 3d band
1g
at lower temperatures. A dispersion of the loss
energy as a function of the incident electron angle
h and scattering angle h has been observed. The
i
s
experimental results indicate that the losses at
0.9 eV and 1.6 eV are not altered when h =h .
i s
However, in the case of h ≠h , a 0.3 eV shift of
i s
the maximum is observed. At room temperature,
for instance, the loss of 1.6 eV with respect to the
Q. Guo et al. / Surface Science 437 (1999) 38–48
specular direction shifts to ~1.9 eV upon changing
the scattering angle to 20–30° off specular.
Therefore, the loss at 1.9 eV at lower V O cover2 3
ages in Fig. 7 is most likely caused by V O par2 3
ticles with different orientations. At this coverage,
very small particles, which do not yield an LEED
pattern, are present. However, at higher coverages
of V O , a faceted LEED pattern with high back2 3
ground [very weak (1×1) substrate pattern] is
observed, consistent with a disordered film. This
rough, disordered film yields broad EELS features,
as shown in Fig. 7.
Regarding the possibility of the formation of
VO , this phase of vanadia undergoes a metal–
2
insulator phase transition as a function of temperature. It is metallic above 340 K with a rutile
structure; below 340 K it becomes insulating with
a monoclinic structure. Using transmission EELS,
Abe et al. studied the insulator phase of VO at
2
room temperature [38]. Loss features at 0.9 and
1.2 eV were observed and assigned to d–d transitions [38]. These features are very similar to those
found for a V O film at room temperature as
2 3
discussed above; however, considerable differences
45
in the width of the d–d transition are observed
for V O compared with VO . In V O this width
2 3
2
2 3
is ~2.8 eV [15], and in VO it is ~1.4 eV [38].
2
The width of the d–d transition in the present
studies is ~3 eV, suggesting the formation of
V O . Since XPS indicates a higher oxidation state
2 3
of vanadium upon initial deposition of vanadia,
some V4+ species are assumed to be present at the
interface. Various studies have shown that vanadium oxide is modified by the titanium oxide
substrate and this oxide–oxide interaction leads to
a spreading of vanadium oxide to form a thin
‘monolayer’ film ([1] and references cited therein).
This vanadia film can interact with the substrate
to form an interface layer with a higher oxidation
state of vanadia, for instance, a VMOMTi complex. This VMOMTi interface oxide layer may
give rise to the loss feature at 1.9 eV. TiO interacts
2
with V O as well. In the V O –TiO system, a
2 5
2 5
2
V4+ species, which interacts strongly with the
support, is observed at the oxide interface [4] The
V4+ species so formed can diffuse into the titania
(rutile) since the TiMO and VMO bonds have
similar polarities [4,6 ].
Fig. 8. Electron energy loss spectra from various TiO (110) structures. Lower: (1×1) LEED structure; middle: (1×1) LEED structure,
2
streaked along the [010] direction; upper: (1×2) LEED structure. The loss at 0.82 eV is a typical surface defect state, as discussed
in the text.
46
Q. Guo et al. / Surface Science 437 (1999) 38–48
3.2. Vanadia on TiO (110)-(1×2)
2
After annealing to ~1300 K, LEED data indicate that the TiO (110)-(1×1) surface reconstructs
2
to a (1×2) structure in agreement with previous
studies [27,39]. It has been proposed that the
(1×2) surface arises either by a missing-row of
oxygen or by the formation of Ti O [40,41].
2 3
HEELS data are unique for the particular surface
structure of titania. Fig. 8 shows the EELS data
for the surfaces corresponding to the (1×1), the
(1×1)-streaked along the [010] direction, and the
(1×2) LEED structures. A loss at 0.82 eV is
apparent for the (1×1)-streaked surface and
is even more pronounced for the (1×2) surface.
This loss indicates the presence of surface defects
arising from Ti3+ sites at oxygen vacancies
[18,29,42].
Apparently, the density of defects is greater on
the (1×2) surface than on the (1×1)-streaked
surface, consistent with the XPS results. Fig. 9
shows XPS spectra from TiO (110)-(1×1) and
2
(1×2) reconstructed surfaces. On the (1×2) surface, a shoulder at 457.4 eV, due to Ti3+ surface
Fig. 9. XPS of the TiO (110)-(1×1) and (1×2) surfaces. For
2
the (1×2) surface (solid circle) a very weak peak at 457.4 eV
is assigned to Ti3+.
Fig. 10. XPS of vanadia–TiO (110)-(1×2) as a function of
2
vanadia coverage. The spectra were acquired at 300 K with an
Mg Ka source (1253.6 eV ).
defects [18], is evident. AES, HREELS and XPS
data as a function of vanadium coverage on the
(1×2) surface were measured as well. The results
are in close agreement to the corresponding data
found for the (1×1) surface.
Fig. 10 shows XPS spectra of the (1×2) surface
as a function of vanadia coverage. At ≤1 MLE,
the V(2p ) emission at 516.1 eV indicates a rela3/2
tive high oxidation state of vanadium. At coverages
>1 MLE, the data suggest a fractional component
of V O , similar to the (1×1) surface. It must be
2 3
pointed out that after annealing the sample in
10−7 Torr oxygen at 600 K for 5–10 min (without
vanadium source on) surface defects were still
detected by HREELS, and the (1×2) LEED
pattern was retained. Furthermore, a shoulder at
low binding energy in the Ti 2p spectrum after
XPS is also evidence of Ti3+. However, a deposition of vanadium in 10−7 Torr oxygen at 570 K
for 30 s, equal to 0.3 MLE, results in the disappearance of surface Ti3+ states measured by XPS.
Q. Guo et al. / Surface Science 437 (1999) 38–48
Therefore, the surface defect is suggested to be
healed by vanadia. Moreover, differences are evident in the electronic structure at the lower vanadia
coverages. Fig. 11 shows EELS results of vanadia
deposited on the (1×2) surface as a function of
coverage. As discussed above, the loss at ~0.8 eV
from the clean (1×2) surface is indicative of
surface defects. At 0.1 MLE coverage a loss feature
at 1.0 eV is observed, and the intensity of the loss
at 0.8 eV due to surface defects is attenuated. From
0.3 to 0.6 MLE, the energy loss maximizes at
~1.3 eV (see Fig. 11). This contrasts with the
(1×1) surface, where the loss feature maximizes
at a higher energy at this coverage (see Fig. 7). At
vanadia coverages ≥10 MLE the loss maximum
is typical of V O , as seen for the (1×1) surface.
2 3
Note that, for the initial coverage, the loss at
1.3 eV on the (1×2) surface is not observed on
the (1×1) surface. Lack of a loss feature at 1.9 eV
at coverages of 0.3 and 0.6 MLE on the (1×2)
Fig. 11. Electron energy loss spectra of vanadia on
TiO (110)-(1×2) as a function of coverage. The spectra were
2
acquired with a primary energy of 25 eV and an incident angle
of 60°.
47
surface suggests a structural modification of the
V O . In Fig. 11 the surface defect state for the
2 3
clean (1×2) surface at ~0.8 eV is absent and a
new loss feature at 1.0 eV appears upon the initial
deposition of vanadia. The vanadia, therefore, is
assumed to occupy the defect sites. It is likely that
at the vanadia–titania interface a two-dimensional
VMOMTi structure is formed and that this leads
to a reduction in intensity of the d–d transition in
the EELS experiments.
Two interesting results have been obtained: (i)
the defect sites on the clean (1×2) surface are
occupied by vanadia species upon initial deposition; and (ii) a V4+ species possibly forms at the
vanadia–titania interface.
XPS measurements further reveal that the
vanadia species are more dispersed on the (1×2)
surface than on the (1×1) surface. Fig. 12 shows
the change of the XPS V/Ti ratio as a function of
coverage for the (1×1) and the (1×2) surfaces.
On the (1×2) surface, at coverages ≥5 MLE, the
V/Ti ratio is higher than for the (1×1) surface,
indicating that vanadia is more thoroughly dis-
Fig. 12. The V/Ti XPS ratio as a function of vanadia coverage
on Ti(110)-(1×1) and (1×2).
48
Q. Guo et al. / Surface Science 437 (1999) 38–48
persed on the (1×2) surface than on the (1×1)
surface. Surface defects likely play an important
role regarding the relative abilities of these surfaces
to disperse vanadia.
4. Conclusion
Vanadia on rutile titanium oxide (110)-(1×1)
and (1×2) surfaces has been prepared in UHV
conditions and characterized by various vacuum
surface analytical techniques, including AES,
LEED, HREELS and XPS. The results indicate
that V O is formed on both substrates at coverages
2 3
>1 MLE. However, at coverages <1 MLE, two
chemical states, V3+ and V4+, are present at the
vanadia–titania interface; V4+ preferentially occupies the defect sites. The dispersion of vanadia is
greater on the (1×2) surface compared with the
(1×1) surface.
Acknowledgements
We acknowledge with pleasure the support of
this work by the Department of Energy, Office of
Energy Sciences, Division of Chemical Sciences.
Helpful discussions with Dr Kent Davis are also
acknowledged.
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