Download Ru3(CO)12 Adsorption and Decomposition on TiO2

698
Langmuir 2002, 18, 698-705
Ru3(CO)12 Adsorption and Decomposition on TiO2
D. C. Meier,† G. A. Rizzi,‡ G. Granozzi,‡ X. Lai,† and D. W. Goodman*,†
Department of Chemistry, Texas A&M University, P.O. Box 30012,
College Station, Texas 77842-3012, and Dipartimento di Chimica Inorganica,
Metallorganica ed Analitica, University of Padova, Unità di Ricerca INFM, Via Loredan 4,
I-35131 Padova, Italy
Received July 9, 2001. In Final Form: October 19, 2001
Triruthenium dodecacarbonyl (Ru3(CO)12) adsorption and decomposition on titania (TiO2) were studied
using infrared reflection absorption spectroscopy, temperature-programmed desorption, Auger electron
spectroscopy, and scanning tunneling microscopy. It was found that the vapor-deposited cluster adsorbs
with the Ru3 plane oriented normal to the surface. The clusters form an amorphous layer at 90 K, order
at 195 K, and begin to decompose in a vacuum at ca. 250 K. The clusters decompose in two stages, with
complete decomposition occurring above 600 K. The clusters are highly dispersed on the surface and
decompose cleanly to form small ruthenium clusters up to 3 nm across and single atom rows up to 20 nm
long.
Introduction
Metal carbonyls are important as precursors in catalysis
and thin film technologies; thus, their surface chemistry
is of fundamental and practical interest. The adsorption
and decomposition in ultrahigh vacuum (UHV) of metal
carbonyls on various surfaces have been the subject of
several recent investigations (refs 1-4 and references
therein). While their decomposition to metal clusters for
heterogeneous catalysts (both mono- and bimetallic) has
been the subject of numerous studies (see for example ref
5), it is only recently that metal carbonyls have been used
as sources for the deposition of ultrathin oxide and/or metal
films on semiconductors and metals.6 Some metal carbonyls are sufficiently volatile7 such that introduction into
a UHV chamber is practical via an effusion cell. The
pressure during the deposition (10-8-10-9 Torr) favors
decarbonylation for certain systems, allowing the deposition of virtually carbon-free, highly dispersed metal
clusters. This approach has recently been used to produce
epitaxial films of RuO2 by dosing Ru3(CO)12 onto a heated
TiO2(110) substrate under oxygen ambient.8
Ruthenium and ruthenia/titania composites are of
interest for a variety of fundamental and practical reasons.
Ruthenium is known to be active for low-temperature
methane coupling9 and is active for CO and CO2 methanation, particularly when partially oxidized on titania
supports.10 Ruthenia/titania mixed metal oxides are used
in dimensionally stable anodes (DSA).11 Mixed oxides are
* To whom correspondence should be addressed.
† Texas A&M University.
‡ University of Padova.
(1) Clowes, S. K.; Seddon, E. A.; McCash, E. M. Surf. Sci. 2000, 464,
L667.
(2) Fachini, E. R.; Cabrera, C. R. Langmuir 1999, 15, 717.
(3) Sun, L.; McCash, E. M. Surf. Sci. 1999, 422, 77.
(4) Huang, W.; Zybill, C. E.; Luo, L.; Hieringer, W.; Huang, H. H.
Organometallics 1998, 17, 5825.
(5) Psaro, R.; Recchia, S. Catal. Today 1998, 41, 139.
(6) Rizzi, G. A.; Zanoni, R.; Di Siro, S.; Perriello, L.; Granozzi, G.
Surf. Sci. 2000, 462, 187.
(7) Chandra, D.; Garner, M. L.; Lau, K. H. J. Phase Equilib. 1999,
20, 565.
(8) Rizzi, G. A.; Magrin, A.; Granozzi, G. Surf. Sci. 1999, 443, 277.
(9) Lenz-Solomun, P.; Wu, M. C.; Goodman, D. W. Catal. Lett. 1994,
25, 75.
(10) Thampi, K. R.; Kiwi, J.; Gratzel, M. Nature 1987, 327, 506.
(11) Beer, H. B. J. Electrochem. Soc. 1980, 127, C303.
also used to chemically fine-tune catalysts. For example,
by varying the composition ratio of silica/alumina, the
acidity of the composite oxide can be adjusted.12,13 In this
regard, ruthenia/titania has shown promise as a binary
metal oxide support, particularly since ruthenia and
titania have similar crystalline structures. It has been
shown that ruthenium can be incorporated into the titania
crystal lattice via decomposition of Ru3(CO)12.14 Furthermore, while the two oxides are similar in structure, their
electronic properties are quite different as a result of
varying degrees of d-shell filling. Titania is an electronpoor semiconductor; ruthenia is relatively electron-rich.
Since the activity of supported metal catalysts can depend
profoundly on the precise nature of the oxide support, a
selection of electronically tunable substrates should allow
these effects to be quantified and optimized.
At temperatures as low as 573 K in an oxygen
atmosphere (10-6 Torr), metallic Ru particles, formed after
the loss of the CO ligands from the parent carbonyl, are
easily oxidized to obtain a carbon-free RuO2 epitaxial film.
Such behavior has been demonstrated by in situ angleresolved photoelectron diffraction (XPD) studies.15 At
higher substrate temperatures, the formation of a
Ti(1-x)RuxO2 binary surface oxide is favored.14-16 X-ray
photoelectron spectroscopy (XPS) results indicate that Ru3(CO)12 is adsorbed on TiO2(110) at room temperature,
resulting in a saturated surface of a partially decarbonylated species. Annealing this surface at 473 K leads to
the evolution of CO, suggesting some further decomposition of the precursor. Eventually at 573 K, metallic Ru
nanoparticles are obtained.
In a previous study14 in which polycrystalline TiO2 was
impregnated at room temperature with Ru3(CO)12, the
formation of subcarbonyl species bonded via Ru-O-Ti
bridges was reported. Temperature-programmed desorption (TPD) indicated that the parent cluster is physisorbed
(12) Busca, G. Phys. Chem. Chem. Phys. 1999, 1, 723.
(13) Daniell, W.; Schubert, U.; Glockler, R.; Meyer, A.; Noweck, K.;
Knozinger, H. Appl. Catal., A 2000, 196, 247.
(14) Rizzi, G. A.; Magrin, A.; Granozzi, G. Phys. Chem. Chem. Phys.
1999, 1, 709.
(15) Rizzi, G. A.; Sambi, M.; Magrin, A.; Granozzi, G. Surf. Sci. 2000,
454, 30.
(16) Kim, Y. J.; Gao, Y.; Chambers, S. A. Appl. Surf. Sci. 1997, 120,
250.
10.1021/la011041r CCC: $22.00 © 2002 American Chemical Society
Published on Web 01/10/2002
Ru3(CO)12 Adsorption and Decomposition on TiO2
Figure 1. Selected IRAS spectra of 3 langmuir Ru3(CO)12/
TiO2/Mo(110) at a series of temperatures in UHV. After heating
to 295 K, no IRAS features are visible. Recooling the sample
to 90 K and exposing to CO yields the bottom spectrum, also
devoid of IRAS features (see text).
at room temperature and begins to decompose via CO
evolution at 363 K. Extended X-ray absorption fine
structure (EXAFS) and infrared data suggest the formation of [Ru3(CO)6(µ-OTi)3] following an anneal to 473 K.17
This system serves as the benchmark for comparison of
the present study with previous work.
Here, we report the use of infrared reflection absorption
spectroscopy (IRAS) and TPD to study the adsorption and
decomposition of Ru3(CO)12 on TiO2 and the potential of
this carbonyl precursor as a metal source for synthesizing
thin films. Scanning tunneling microscopy (STM) of a TiO2(110) single crystal exposed to Ru3(CO)12 vapor was used
to acquire images before and after heat treatment at 573
K in an effort to develop a more complete picture of the
decomposition process.
Experimental Section
The spectroscopic portion of this study was performed in a
UHV chamber (base pressure less than 2 × 10-10 Torr) equipped
with an Auger electron gun and cylindrical mirror analyzer
(Perkin-Elmer), a quadrupole mass analyzer (QMS, UTI Instruments), a low-energy electron diffraction (LEED, Perkin-Elmer)
spectrometer, and an infrared cell separated from the main
chamber by a two-stage sliding seal. This apparatus has been
described in detail elsewhere.18,19 The main chamber is also fitted
with an effusion cell doser for the introduction of metal carbonyls
or other solids with suitable vapor pressures. The doser is
constructed of a perforated tantalum foil (H. Cross Co.) pocket
mounted inside a UHV stainless steel tube and held in place
(17) Asakura, K.; Bando, K. K.; Iwasawa, Y. J. Chem. Soc., Faraday
Trans. 1990, 86, 2645.
(18) Campbell, R. A.; Goodman, D. W. Rev. Sci. Instrum. 1992, 63,
172.
(19) Leung, L. W. H.; He, J. W.; Goodman, D. W. J. Chem. Phys.
1990, 93, 8328.
Langmuir, Vol. 18, No. 3, 2002 699
Figure 2. Expanded IRAS spectrum from Figure 1 at 195 K.
The line spectrum overlay is taken from Battiston et al. (ref
24); dashed lines denote equatorial stretching modes, and the
solid line is the pure axial mode.
with threaded rods. The doser is isolated from the main analysis
chamber by a gate valve and can be evacuated independently of
the main chamber and the infrared cell through an attached
needle valve. This construction allows for the evacuation of
ambient atmosphere without significant contamination of the
UHV system. The doser’s position on the main chamber also
allows for the sample to be protected from contamination by
insertion into the IR cell when the gate valve is initally opened.
Using this configuration, outgassing of the doser could be
accomplished independent of the sample.
The IRAS cell is fitted with flange-mounted CaF2 windows in
the infrared beam path. The optics and windows are aligned
such that the angle of incidence is 85° from surface normal, an
ideal angle for IRAS.20 The infrared spectrometer used for data
collection is a Mattson Cygnus 100 in single-beam mode using
an MCT detector. All spectra collected in this study are averages
of 512 scans at 4 cm-1 resolution.
The sample consists of a Mo(110) single crystal (Single Crystal
Limited) that is mounted on the sample probe with a tantalum
wire loop spot-welded around the edge of the sample and a type
C thermocouple (wire supplied by H. Cross Co.) spot-welded to
the back. This mount allows for resistive heating to 1500 K and
liquid nitrogen cooling to 90 K. The sample was cleaned by
alternating annealing at 800 K in 5 × 10-6 Torr O2 and electron
beam heating to 2000 K until no carbon or oxygen was observed
using Auger electron spectroscopy (AES). The Ti doser was
constructed by wrapping titanium wire (99.99+%, Goodfellow)
around a tungsten wire loop (H. Cross Co.) that could be heated
resistively. The doser was outgassed thoroughly by resistive
heating prior to use. Oxygen (99.99%, Botco, Bryan, TX) and
carbon monoxide (99.99%, Matheson Gas Products) were further
purified by fractional condensation and transferred to glass bulbs.
The CO bulb was left immersed in liquid nitrogen during
experiments in order to condense volatile metal carbonyls that
are typically formed when CO comes in contact with the steel
(20) Hoffmann, F. M. Surf. Sci. Rep. 1983, 3, 107.
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Langmuir, Vol. 18, No. 3, 2002
Figure 3. An illustration of the two possible orientations of
the Ru3(CO)12 molecule with respect to the surface. Since only
modes perpendicular to the surface plane are visible in IRAS,
it can be used to determine molecular orientation.
gas manifold. Ru3(CO)12 (99%, Strem Chemicals, Inc.) was
introduced into the effusion doser as received.
The titanium oxide thin films were produced by evaporation
of titanium metal onto the Mo(110) crystal at 650 K in a
background of 5 × 10-6 Torr O2. The films were annealed to 800
K in this oxygen background in order to ensure full oxidation
and to eliminate adventitious carbon that was occasionally found
in the films prior to annealing. Details of the film synthesis can
be found elsewhere;21 essentially, these films are the rutile phase
with a preponderance of (110) facets. Ru3(CO)12 was then dosed
on the films at 90 K at a number of exposures; the exposure
pressure for the carbonyl species was typically 1.8 × 10-9 Torr.
IRAS and TPD were performed on the samples prior to decarbonylation, and AES was carried out afterward. TPD data were
collected from 100 to 800 K at 28 atomic mass units and 5 K s-1.
AES was collected from 20 to 600 eV at 3 kV beam voltage.
Because Ru remained on the surface after the decomposition of
Ru3(CO)12, the resulting composite films were evaporated from
the sample by an e-beam flash to 2000 K. This procedure produces
a clean Mo(110) substrate upon which a new titania film could
be synthesized.
The imaging portion of this study was performed in a combined
high-pressure cell/multitechnique UHV analysis chamber (base
pressure less than 2 × 10-10 Torr) that is equipped with a doublepass cylindrical mirror analyzer for AES and XPS, LEED optics
(Perkin-Elmer), a QMS (UTI Instruments), and a UHV scanning
tunneling microscope (Omicron). The apparatus consists of four
main sections: a high-pressure cell, a surface analysis chamber,
a sample preparation chamber, and a STM chamber; details are
available elsewhere.22
A Type C thermocouple (wire provided by H. Cross Co.) was
attached to the edge of a TiO2(110) crystal (Commercial Crystal
Laboratories) using high-temperature ceramic adhesive (AREMCO 571). The thermocouple was used to measure the surface
temperature and to calibrate a pyrometer (OMEGA OS3700)
prior to its disconnection for the STM measurements. The
(21) Lai, X.; Guo, Q.; Min, B. K.; Goodman, D. W. Surf. Sci. 2001,
487, 1.
(22) Xu, C.; Oh, W. S.; Liu, G.; Kim, D. Y.; Goodman, D. W. J. Vac.
Sci. Technol., A 1997, 15, 1261.
Meier et al.
Figure 4. Expanded IRAS spectrum from Figure 1 at 195 K.
The line spectrum overlay is taken from Asakura et al. (ref 17).
Note the substantial difference in position and relative intensity
of the low-energy peaks.
pyrometer was then used to measure the temperature in
subsequent annealing experiments. The crystal was prepared
for experiments by alternating argon ion sputtering with hightemperature (1000 K and higher) annealing. This cleaning
procedure has been described in detail elsewhere.23 Following
cleaning, the sample was introduced into the high-pressure cell
to which the Ru3(CO)12 effusion cell was attached. After exposure
to the cluster vapor, the sample was removed to the vacuum
chamber in order to conduct the annealing and STM experiments.
Results and Discussion
Figure 1 shows the carbonyl stretching region of the
IRAS spectrum of Ru3(CO)12 (exposure equal to 3 langmuir,
where 1 langmuir ) 1 × 10-6 Torr s) adsorbed on TiO2 at
90 K, as well as spectra acquired as the sample was heated.
At 90 K, two broad features are apparent at 2084 and
2047 cm-1. Both features exhibit a tail sloping to the lowenergy side. As the temperature is increased, the feature
at 2047 cm-1 decreases slightly in intensity while the 2084
cm-1 feature increases slightly in intensity, with the lowenergy tail diminishing. Between 180 and 195 K, the
spectrum transforms sharply in that a symmetric feature
appears at 2029 cm-1 with a small shoulder at 2041 cm-1.
The strong feature at 2084 cm-1 also becomes more
symmetric at this temperature. As the temperature is
increased further, the entire spectrum red-shifts slightly
and becomes broader and more asymmetric until no
features are apparent at 295 K. This sample was then
cooled to 90 K, and 1 × 10-5 Torr CO was introduced; no
absorption features were apparent at this temperature
and pressure.
(23) Xu, C.; Lai, X.; Zajac, G. W.; Goodman, D. W. Phys. Rev. B 1997,
56, 13464.
Ru3(CO)12 Adsorption and Decomposition on TiO2
Figure 5. CO TPD of Ru3(CO)12/TiO2/Mo(110) at a series of
exposures. The temperature at which IRAS features are no
longer visible (295 K) is marked for reference. Note the
substantial CO desorption above 295 K.
Figure 2 is an expanded view of the spectra taken at
195 K, where a transmission infrared overlay collected
from Ru3(CO)12 dissolved in hexane24 is also shown for
comparison. The peaks are presented as-acquired with
their relative peak intensities; each is labeled with its
assigned symmetry, either A2′′ (for the vibrational mode
fully axial with respect to the Ru3 plane) or E′ (for the
equatorial vibrational modes). The spectral features for
the adsorbed species are blue-shifted from those of the
dissolved species; however, they are quite similar in
amplitude and relative position to those lines assigned to
E′ modes. This phenomenon can be explained in light of
the IRAS selection rule, which forbids absorption by any
vibrational mode that has no surface normal dynamic
dipole moment.20 The consequences of this selection rule
are illustrated in Figure 3, which shows a simplified
diagram of the Ru3(CO)12 molecule adsorbed on titania
with the Ru3 plane either parallel or perpendicular to the
surface plane. For the parallel case, all axial carbonyls
are aligned normal to the surface, allowing IRAS excitation
of only the axial vibrational modes; for the perpendicular
case, only equatorial dynamic dipole moments are normal
to the surface, resulting in IRAS excitation of only the
equatorial modes.
To explain these experimental findings, a comparison
with infrared experimental data on similar systems is
useful. It is unlikely that the observed peaks can be
attributed to species arising from the complete decomposition of the original trinuclear cluster since these
features do not correspond to any of those measured for
CO adsorbed on an analogous Ru/TiO2 powdered system.
(24) Battiston, G. A.; Sbrignadello, G.; Bor, G. Inorg. Chem. 1980,
19, 1973.
Langmuir, Vol. 18, No. 3, 2002 701
These powdered samples show transmission IR features
at 2140 cm-1 (w) and 2085 cm-1 (w) and a broad
heterogeneous band with its maximum located around
2060 cm-1 (s).25 There is also no agreement between the
spectra of this study and that of CO adsorbed on metallic
single crystals, such as CO/Ru(001), which have infrared
absorption features at 2080 cm-1 (w) and 2048 cm-1 (s).26
Furthermore, a significant breakdown in symmetry due
to strong adsorbate-support interactions is unlikely, as
the increased spectral complexity expected to result from
such interactions does not appear. See, for example, the
infrared spectra of Ru2Os(CO)12, RuOs2(CO)12,27 FeRu2(CO)12, and Fe2Ru(CO)12,28 systems in which the molecular
symmetry has been reduced from D3h to C2v. A similar
symmetry reduction would be consistent for Ru3(CO)12
oriented with the Ru3 plane normal to the substrate,
assuming there is a sufficiently strong adsorbatesubstrate interaction.
Considering these points, the IRAS spectra are most
consistent with random initial adsorption (indicated by
the broad heterogeneous features at low temperature) with
subsequent ordering between 180 and 195 K into a
geometry where the Ru3 plane is perpendicular to the
surface. As previously discussed, only equatorial vibrational modes should be observed when the Ru3 plane is
perpendicular to the surface. Comparison of the surface
spectra to the solution spectra yields similar peak
amplitude and relative position, with the notable exception
that the axial mode observed in the solution phase is
invisible in IRAS. IRAS data cannot be used to determine
how individual ruthenium atoms are oriented with respect
to the surface, nor can IRAS show whether there is a
preferred molecular orientation with respect to the
substrate atomic rows; however, the orientation of the
Ru3 plane with respect to the surface plane is clear.
These results contrast with those in which Ru3(CO)12
was adsorbed onto a titania powder by wet impregnation.17
The bands that Asakura et al. assign to physisorbed Ru3(CO)12 nearly match the two most intense bands of the
sample dissolved in hexane (2065 cm-1 (s) and 2030 cm-1
(s)), while those features observed following a heat
treatment at 473 K (2139 cm-1 (w), 2081 cm-1 (s), and
2009 cm-1 (s), as shown in the line spectrum overlay in
Figure 4) are somewhat different than those observed in
the thin film sample of this study (2085 cm-1 (s), 2041
cm-1 (w), and 2029 cm-1 (w)). It is evident from the IRAS
data alone that the thin film sample and the powdered
sample are not entirely analogous. It is unclear whether
these differences are due to the phase of the substrate
(the powder is expected to contain substantially more
anatase than the thin film), the nature of the deposition
method, or both. In any case, since the infrared spectra
of the sample formed by wet impregnation is isotropic, no
direct molecular orientation comparison can be made; the
variations in peak positions and relative intensities
between the two methods are the primary evidence that
the sample formed by vapor deposition differs from that
formed by wet impregnation. Thus, the evidence of this
work, which indicates a perpendicular molecular orientation, need not be considered contradictory to that of
Asakura et al., which suggests a parallel orientation.
Finally, following elimination of the IRAS signal at 295
K, no IRAS features are seen upon introduction of CO at
(25) Robbins, J. L. J. Catal. 1989, 115, 120.
(26) Peden, C. H. F.; Goodman, D. W.; Weisel, M. D.; Hoffmann, F.
M. Surf. Sci. 1991, 253, 44.
(27) Battiston, G. A.; Bor, G.; Dietler, U. K.; Kettle, S. F. A.; Rossetti,
R.; Sbrignadello, G.; Stanghellini, P. L. Inorg. Chem. 1980, 19, 1961.
(28) Yawney, D. B. W.; Stone, F. G. A. J. Chem. Soc. A 1969, 502.
702
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Meier et al.
Figure 6. AES spectra of the TiO2/Mo(110) sample prior to (top) and following (bottom) Ru3(CO)12 exposure (5 langmuir) and TPD
to 800 K.
90 K. Two possible reasons for this result are either
complete cluster desorption or decomposition of the cluster
in such a way that few ruthenium sites are available for
CO adsorption (due to insertion and oxidation of ruthenium into the titania lattice). The TPD and AES results
reported here together with the results of Rizzi et al.14
suggest that the latter is the case.
Figure 5 shows CO TPD data for several Ru3(CO)12
exposures. Most notably, CO evolution occurs in two stages
at all coverages. For the 0.5 langmuir exposure, the first
decomposition feature appears at 300 K, followed by a
second at 500 K. At this low coverage, the peak areas are
approximately identical. As coverage is increased, a
number of things occur. First, the second desorption peak
increases substantially with respect to the first while
shifting to lower temperature (360 K at 5 langmuir). Also,
at the higher coverages two new features emerge: the
first, centered at about 150 K, is attributed to multilayer
desorption, while the second appears around 610 K.
The assignment of the TPD decomposition features is
aided substantially by the temperature-dependent IRAS
spectra discussed above. Note that at 295 K there are no
features in the IRAS spectrum that correspond to the TPD
desorption feature at 300 K (as marked in Figure 5).
However, the TPD data show that the CO does not fully
desorb from the surface until 500 K or higher; a substantial
amount of CO remains on the surface after IRAS no longer
shows any evidence of it. Since the IRAS features have
been assigned to the equatorial carbonyl vibrational modes
of the cluster with the Ru3 plane oriented perpendicular
to the surface, it follows that these modes disappear with
the initial carbonyl elimination from the cluster. This does
not necessarily indicate that exclusively equatorial carbonyl groups are desorbing; the equatorial vibrational
modes would also disappear upon dissociation of Ru-Ru
bonds, resulting in the demolition of the original cluster.
Furthermore, since no other bands are visible after the
first stage of the decomposition, it follows that the
remaining surface CO is bound tangential to the surface
plane, much like the axial CO molecules in the adsorbed
cluster. It has been suggested previously by Asakura et
al.17 that the initial desorption leaves a ruthenium
polynuclear subcarbonyl species bound to the surface, that
is, Ru3(CO)6(µ-OTi)3. While this could be a plausible
interpretation of the current data, other possibilities must
be considered in light of the previously discussed discrepancies between the vapor-deposited and wet-impregnated samples. One such possibility could be the formation
Ru3(CO)12 Adsorption and Decomposition on TiO2
Langmuir, Vol. 18, No. 3, 2002 703
Figure 7. STM of Ru3(CO)12/TiO2(110) after annealing in a vacuum at 600 K for 60 s. Note the highly dispersed clusters and the
numerous stripes of adatoms along the [001] surface vector.
of single-metal subcarbonyl species, such as Ru(CO)2 and
Ru(CO)3. These species have been reported on TiO2
previously25 and could account for the increased intensity
of the high-temperature decomposition peak with respect
to the low-temperature peak as the cluster coverage is
increased. In effect, initial decomposition of the adsorbed
cluster at low coverage would produce 3Ru(CO)2 + 6CO,
resulting in the nearly equal CO TPD peak areas during
cluster decomposition. As coverage increases, greater
interaction between Ru(CO)2 and desorbing carbonyls
could result in the formation of some Ru(CO)3 with a
stoichiometry of XRu(CO)3 + (3 - X)Ru(CO)2 + (6 - X)CO, where X ranges from 0 to 3 and increases with
coverage. This explanation allows for up to a 3:1 peak
area ratio between the second and first decomposition
features at high coverage, which is consistent with the
TPD results. As both of these single-metal ruthenium
subcarbonyl species could be planar, their formation would
be consistent with the IRAS data assuming that their
molecular planes are mostly parallel to the surface.
AES verifies that the cluster decomposition is complete
and clean at 800 K. Figure 6 shows the Auger of the TiO2/
Mo(110) system before and after 5 langmuir Ru3(CO)12
exposure and TPD to 800 K. The adsorbed ruthenium is
plainly seen (259 eV), but an unfortunate coincidence of
the ruthenium and carbon features requires comparison
to the published peak shapes.29 The ratio of the positive
amplitude of the derivative feature to the total amplitude
is approximated as a linear combination of the ratios found
(29) Davis, L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.;
Weber, R. E. In Handbook of Auger Electron Spectroscopy, 2nd ed.;
Physical Electronics Division, Perkin-Elmer Corp.: Eden Prairie, MN,
1976.
for the pure elements. The relative concentration of
ruthenium can then be found by using the usual elemental
composition equation, thereby factoring in the Auger
sensitivities for the respective features. This computation
indicates that approximately 90% clean ruthenium remains on the surface following Ru3(CO)12 decomposition,
assuming no corrections are necessary to compensate for
slight variations in the electronic structures of the
adsorbed species.
Regardless of the differences between the decomposition
characteristics of the vapor-deposited sample and those
of the wet-impregnated sample, the vapor deposition
method clearly results in highly dispersed ruthenium
clusters on the rutile surface. Figure 7 shows a large field
STM image of the clean TiO2(110) after exposure to Ru3(CO)12 and vacuum annealing at 600 K for 60 s. The surface
is covered with clusters of dimensions up to 3 nm in
diameter. While there is some agglomeration at step edges,
it is not noticeably more abundant than clustering on
terraces. Also visible in the image are a number of bright
“stripes” of atoms parallel to the [001] surface vector with
lengths approaching 20 nm. Finally, there are single bright
atomic spots, a phenomenon previously assigned to
interstitial titanium ions moving from the bulk to the
surface upon annealing, albeit at somewhat higher
temperatures than here.23 Due to the chemical insensitivity of STM, it cannot be conclusively determined whether
the stripes are due entirely to interstitial transport,
ruthenium atoms, or some combination thereof.
An expanded image of the same surface is shown in
Figure 8. In the center of the image, a cluster of 1.3 nm
diameter and 0.2 nm thickness is outlined (A), a size
corresponding to a ruthenium cluster numbering on the
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Langmuir, Vol. 18, No. 3, 2002
Meier et al.
Figure 8. Expanded view of Figure 7. Two features (A and B) discussed in the text are circled. Feature A is accompanied by a
cross section showing a cluster thickness of 0.2 nm, which is on the order of a single atomic layer.
order of 14-20 atoms. Just below that cluster, there is a
structure (B) which appears as three bright spots that are
collinear with the TiO2 [001] vector directly atop features
previously assigned to exposed rows of titanium ions in
the rutile structure. These spots are flanked on each side
by slightly dimmer spots that are atop the dark rows
previously assigned to the TiO2 bridging oxygen.30 While
it is tempting to assign this structure to three adjacent
Ru(CO)2 units arising from the decomposition of a single
trinuclear cluster, it must be stressed that structures such
as these are not common; longer stripes or larger multicluster features make up the preponderance of the
surface features (as shown in Figure 7). Further study is
required to determine whether these are in fact the result
(30) Onishi, H.; Fukui, K.; Iwasawa, Y. Bull. Chem. Soc. Jpn. 1995,
68, 2447.
of decomposed single trinuclear clusters or simply a
fortuitous collection of interstitials; however, such features
are consistent with the decomposition scheme in which
perpendicularly adsorbed molecular Ru3(CO)12 decomposes into IRAS-invisible Ru(CO)2 collinear with the [001]
surface vector, which at higher temperatures further
decomposes to form three ruthenium atoms on the surface
(Figure 9).
The STM evidence provides another indirect means to
compare vapor-deposited samples to wet-impregnated
ones. While the clusters of this study are at most a few
tens of atoms in size, EXAFS studies performed on wetimpregnated samples indicate high Ru-Ru coordination
and therefore larger three-dimensional particles than
those shown in the present work.31
Ru3(CO)12 Adsorption and Decomposition on TiO2
Langmuir, Vol. 18, No. 3, 2002 705
which vapor-deposited Ru3(CO)12 decomposes on titania
surfaces. It is found by IRAS that deposition at low
temperature gives rise to a disordered film that orders as
the temperature increases into an adsorbed system in
which largely unperturbed clusters are oriented with the
Ru3 plane perpendicular to the surface plane. These
clusters begin to decompose at 250 K and are invisible in
IRAS starting from 295 K. TPD results confirm that
decomposition occurs in two stages and is complete at 600
K. This result is consistent with the formation of planar
single-metal ruthenium subcarbonyls adsorbed parallel
to the surface which then further decompose to form
ruthenium aggregates. STM results show clusters up to
3 nm in diameter located on both terraces and step edges
as well as rows of atoms oriented atop the [001] rows of
titanium ions.
Figure 9. Proposed mechanism for Ru3(CO)12 decomposition
including an adsorbed Ru(CO)2 subcarbonyl species.
Conclusions
We have developed, through a combination of techniques, a comprehensive proposal for the mechanism by
Acknowledgment. We acknowledge with pleasure the
support of this work by the Department of Energy, Office
of Basic Energy Sciences, Division of Chemical Sciences.
G.G. and G.A.R. gratefully acknowledge funding to this
project coming from “Progetto Finalizzato Materiali Speciali per Tecnologie Avanzate II” of the CNR, Rome, and
from Ministero della Ricerca Scientifica e Tecnologica
(MURST) through the National Program “Strati ultrasottili di ossidi e solfuri inorganici: crescita, caratterizzazione e reattività superficiale”. D.C.M. gratefully acknowledges Dr. Timothy R. Hughbanks for providing
helpful insights into the symmetry elements and vibrational characteristics of these clusters.
LA011041R
(31) Mizushima, T.; Tohji, K.; Udagawa, Y.; Ueno, A. J. Am. Chem.
Soc. 1990, 112, 7887.