Photon-induced chemical vapor deposition of molybdenum on Pt(1 1 1)

Article abstract of DOI:10.1016/j.susc.2004.06.207

Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been employed to study the adsorption and photon-induced decomposition of Mo(CO)₆. Mo(CO)₆ adsorbs molecularly on a Pt(111) surface with weak interaction at 100 K and desorbs intact at 210 K without undergoing thermal decomposition. Adsorbed Mo(CO)₆ undergoes decarbonylation to form surface Mo(CO)_x (x ≤ 5) under irradiation of ultraviolet light. The Mo(CO)_x species can release further CO ligands to form Mo adatoms with CO desorption at 285 K. In addition, a fraction of the released CO ligands transfers onto the Pt surface and subsequently desorbs at 350-550 K. The resulting Mo layer deposited on the Pt surface is nearly free of contamination by C and O. The deposited Mo adatoms can diffuse into the bulk Pt at temperatures above 1070 K.

Full text of DOI:10.1016/j.susc.2004.06.207

Surface Science 565 (2004) 81–88
www.elsevier.com/locate/susc
Photon-induced chemical vapor deposition of molybdenum
on Pt(111)
a
Chun-Chuan Yeh , Ying-Huang Lai , Wen-Yi Chu ,
a
a
b
Chuin-Tih Yeh , Wei-Hsiu Hung
a,c,
*
a
National Synchrotron Radiation Research Center, R&D Road VI, Hsin-chu 300, Taiwan
b
Department ofChemistry, Tsing-Hua University, Hsinchu 300, Taiwan
c
Department ofChemistry, National Taiwan Normal University, Taipei 116, Taiwan
Received 1 December 2003; accepted for publication 9 June 2004
Available online 21 July 2004
Abstract
Temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS) have been employed to
study the adsorption and photon-induced decomposition of Mo(CO)6. Mo(CO)6 adsorbs molecularly on a Pt(111) sur-
face with weak interaction at 100 K and desorbs intact at 210 K without undergoing thermal decomposition. Adsorbed
Mo(CO)6 undergoes decarbonylation to form surface Mo(CO)x (x 6 5) under irradiation of ultraviolet light. The
Mo(CO)x species can release further CO ligands to form Mo adatoms with CO desorption at 285 K. In addition, a frac-
tion of the released CO ligands transfers onto the Pt surface and subsequently desorbs at 350–550 K. The resulting Mo
layer deposited on the Pt surface is nearly free of contamination by C and O. The deposited Mo adatoms can diffuse
into the bulk Pt at temperatures above 1070 K.
ꢀ 2004 Elsevier B.V. All rights reserved.
Keywords: Molybdenum; Platinum; Chemical vapor deposition; X-ray photoelectron spectroscopy; Thermal desorption
1. Introduction
Platinum is the most widely used metallic cata-
lyst in a direct methanol fuel cell (DMFC). The
addition of a second metal to a Pt cathode can sig-
nificantly modify catalytic properties of the surface
[1]. Much effort has been focused on preparing
bimetallic catalysts and examining their chemical
*
Corresponding author. Address: National Synchrotron
Radiation Research Center, R&D Road VI, Hsin-chu 300,
Taiwan. Tel.: +886 3 5780281; fax: +886 3 5783813.
E-mail addresses: whung@cc.ntnu.edu.tw, whung@srrc.
gov.tw (W.-H. Hung).
0039-6028/$ - see front matter ꢀ 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.susc.2004.06.207

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C.-C. Yeh et al. / Surface Science 565 (2004) 81–88
and physical properties. Bimetallic Mo/Pt alloys
have emerged as an exceptionally active electrode
material for oxidation of methanol in aqueous
acidic electrolytes [2].
2. Experiments
Experiments were performed in a UHV cham-
ber (base pressure ꢀ2 · 10ꢁ10 Torr), which was
equipped with a quadrupole mass spectrometer
and an electron-energy analyzer. A Pt(111) sample
was spot-welded on two Ta wires in turn mounted
on a copper block. The Pt sample could be cooled
to 100 K with liquid nitrogen via conduction
through the copper block and heated resistively.
The Pt surface was cleaned through Ar+-sputter-
ing, followed by thermal annealing to 1000 K in
an oxygen atmosphere (ꢀ10ꢁ7 Torr) to remove
residual carbon from the surface. When the sur-
face was further annealed to 1200 K, it exhibited
XPS features characteristic of clean Pt, with no
evidence for either carbon or oxygen being
present.
Before use, the Mo(CO)6 compound (98%,
Strem) was purified through sublimation and
pumping, and was then introduced onto the Pt sur-
face via a stainless-steel doser that was previously
passivated under prolonged exposure of Mo(CO)6.
During dosing, the partial pressure of Mo(CO)6
was controlled at 5 · 10ꢁ9 Torr; the Pt surface
was placed ꢀ2 cm before the doser to minimize
contamination of the UHV system with Mo com-
plexes. A Hg–Xe arc lamp (PTI Inc., 200 W)
served as a source of UV light for photon-induced
decomposition. A water filter was mounted before
the sample to diminish infrared irradiation that
might produce heating of the sample. The filtered
illumination on the sample was used at a flux
ꢀ0.1 W/cm2. The sample was kept at a tempera-
ture below 140 K during exposure to UV light.
XPS measurements were conducted at the wide-
range beamline of National Synchrotron Radia-
tion Research Center in Taiwan; the incident angle
of the photon beam was 45ꢁ from the surface nor-
mal. Emitted photoelectrons were collected with
an electron analyzer oriented 10ꢁ from the surface
normal in an angle-integrated mode. For measure-
ments at varied temperatures, the sample was
heated to a desired temperature at a rate of 1 K/
s and cooled immediately to 100 K, at which time
XPS spectra were recorded. Mo(CO)6 is highly
sensitive to X-ray irradiation and can undergo
photo-dissociation. To suppress interference
Several procedures to produce such bimetallic
surface have been reported, using bulk alloys, or
adsorption of transition-metal atoms onto bulk
Pt by physical, chemical or electrochemical means
[1,3]. Molybdenum is a highly refractory metal,
having a high boiling point; this property imposes
a difficulty in physical vapor deposition, for in-
stance, contamination caused by outgassing at ele-
vated temperature [4]. This metal deposited on a Pt
surface through spontaneous electrochemical de-
position invariably showed contamination due to
carbon that was difficult to remove [5]. In perform-
ing UHV experiments, chemical vapor deposition
(CVD) was therefore applied to decorate a metal
surface with other metals on heating a sample in
the presence of their carbonyl complexes. An
advantage of a carbonyl complex as a CVD pre-
cursor is that the metal has oxidation state zero
in such a complex, which hence involves neither
reduction nor oxidation, and requires no addi-
tional reagent, in the deposition of metal.
In the preparation of a catalyst, the Mo crys-
tal was formed via thermal decarbonylation of
Mo(CO)6 on surfaces of partially dehydroxylated
alumina [6,7]. Nanoscale Mo clusters were ob-
tained after thermal vapor deposition of Mo(CO)6
on Au(111); these clusters grew preferentially at
elbow sites [8]. Ni-Mo alloy was formed upon ther-
mal liberation of CO from Mo(CO)6 on a Ni sur-
face [9]. We found that Mo(CO)6 adsorbed
molecularly on a Pt surface at 100 K and desorbed
at 210 K without thermal decomposition. Photoly-
sis of Mo(CO)6 adsorbed on several metallic sur-
faces (i.e. Ag, Cu, Rh, and Mo) has been
reported [10–14]. In this article, we report that
Mo adatoms are deposited on a Pt(111) surface
via photon-induced chemical deposition of
Mo(CO)6. We utilized X-ray photoelectron spec-
troscopy (XPS) and temperature-programmed
desorption (TPD) measurements to investigate
the variation of surface composition during ther-
mal reaction and photo-induced dissociation of
Mo(CO)6, and accordingly elucidated the reaction
mechanism.

C.-C. Yeh et al. / Surface Science 565 (2004) 81–88
83
resulting from photo-dissociation, we completed
recording of each XPS spectrum within 2 min,
and the probed area of the surface was changed
after each XPS measurement.
durations. The signal at m/e = 181 indicates the
most abundant fragment in the mass spectrum of
Mo(CO)6; its intensity was then recorded for
molecular desorption of Mo(CO)6. Only a desorp-
tion feature of Mo(CO)6 is observed at 210 K.
Desorption of CO (m/e = 28) was also observed
with a sharp maximum at 210 K and a broad peak
in a temperature range between 350 and 500 K.
The former feature characteristic of CO desorp-
tion is identical to that of molecular Mo(CO)6
desorption and is thus attributed to fragmentation
of desorbed Mo(CO)6 molecules upon electron
bombardment in the ionization region of the mass
spectrometer. The high-temperature CO desorp-
tion feature resembles that of CO chemisorbed
on a clean Pt surface [15]. The desorption, thus
attributed to originate from CO molecules ad-
sorbed on the Pt surface, is discussed later.
The quadrupole mass spectrometer, that ren-
dered analysis of desorption products in the TPD
measurement, was enclosed in a differentially
pumped cylinder, at the end of which is a skimmer
with an entrance aperture (diameter 2 mm). For
TPD measurement, the sample surface was placed
about 2 mm before the aperture and in line of sight
of the ionizer of the mass spectrometer; TPD scans
were recorded on ramping the sample at a linear
rate of ꢀ2 K/s.
3. Results and discussion
Fig. 1 shows TPD spectra taken from a clean Pt
surface exposed to Mo(CO)6 at 100 K for various
The Pt 4f core-level spectrum is not significantly
altered upon exposure of the Pt surface to
Mo(CO)6, except its decreased intensity. Thus,
Fig. 2 shows only Mo 3d, C 1s and O 1s spectra
that are recorded from a Pt surface exposed to
Mo(CO)6 for 60 s at 100 K and subsequently an-
nealed to various temperatures. At 100 K, only a
Mo 3d spin–orbit doublet is observed for all dura-
tions of exposure, implying that Mo(CO)6 adsorbs
molecularly on the surface without fragmentation.
A shake-up satellite feature is observed ꢀ5.5 eV
greater than the Mo 3d doublet, which is also seen
in C and O 1s spectra. This shake-up process is
proposed to involve a transition of metal-to-ligand
charge transfer (4d ! 2p*) as observed in the gas-
eous phase [16].
On the sample being heated to 220 K, the
Mo 3d peak disappeared because all adsorbed
Mo(CO)6 molecules desorbed intact from the sur-
face as shown from TPD data. This behavior indi-
cates that Mo(CO)6 molecules adsorbed weakly on
the surface and underwent no thermal reaction. A
small proportion of carbon and oxygen that per-
sisted on the surface after desorption of Mo(CO)6
is attributed to adsorbed CO molecules. Adsorbed
CO might originate from CO as an impurity in the
Mo(CO)6 sample, rather than decarbonylation of
Mo(CO)6, as XPS data show no Mo-containing
species left on the surface after a sample had been
heated to 220 K. CO that is inevitably present in
Fig. 1. TPD scans of Mo(CO)6 (m/e = 181) and CO (m/e = 28)
for a Pt(111) surface exposed to Mo(CO)6 at 100 K for various
durations.

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C.-C. Yeh et al. / Surface Science 565 (2004) 81–88
peared at 570 K. Their disappearance corresponds
to desorption of CO in a range 350–500 K (as
shown in Fig. 1).
UV radiation is known to induce the photo-
dissociation of an adsorbate, and the resulting
fragments can undergo thermal decomposition
upon subsequent heating, rather than desorption
of intact adsorbate [10]. Fig. 3 shows XPS spec-
tra of Mo 3d, C 1s and O 1s for a Pt surface
after exposure to Mo(CO)6 for 60 s at 100 K
and subsequent irradiation with UV light for var-
ious durations. The intensities of Mo 3d, C 1s
and O 1s due to absorbed molecular Mo(CO)6
decreased with increased duration of UV irradia-
tion. New Mo-containing species that formed,
having Mo 3d electrons at large binding energies,
are attributed to Mo subcarbonyl species (Mo
(CO)x, x 6 5), which were generated via photon-
induced liberation of CO from Mo(CO)6. In
previous work involving measurement of IR and
UV–vis spectra, Mo subcarbonyl species were pro-
posed to form upon adsorption and thermal
decomposition on dehydroxylated alumina at
298 K [19].
Upon exposing Mo(CO)6 to UV light, the
chemical shift of Mo 3d to larger binding energy
resulted from the decreased coordination number
and altered conformation of Mo and its CO lig-
ands. A dative bond is known to form in metal-
carbonyl complexes, which purportedly involves
electron donation from 5r of CO into an unfilled
metal orbital and a back p-bonding donation from
a filled metal orbital into an empty CO 2p* orbital
[20]. The extent of this bonding that affects signif-
icantly the electronic density on the center metal is
consequently related to the binding energy of its
core electron. UV irradiation causes liberation of
CO ligands from Mo(CO)6, resulting in a de-
creased coordination number of Mo to CO lig-
ands; their bonding configuration also alters. As
a result, the electronic density of the Mo atom is
diminished and the binding energy of Mo 3d in-
creases. In contrast to the chemical shift of Mo
3d, the C and O 1s core-level electrons of Mo-
(CO)x shift to smaller binding energy as shown
in Fig. 3(b). The integrated intensities of C and
O 1s decreased because adsorbed Mo(CO)6 under-
went decarbonylation upon UV exposure.
Fig. 2. XPS spectra of: (a) Mo 3d and (b) C and O 1s for a Pt
surface exposed to Mo(CO)6 at 100 K for 60 s and then
annealed to various temperatures.
equilibrium with a Mo(CO)6 solid sample was
introduced onto the surface while dosing
Mo(CO)6, even though the Mo(CO)6 solid sample
was subjected to prolonged evacuation before use.
The CO molecule can adsorb at two adsorption
sites (on-top and bridge) on Pt(111) with binding
energies 534.1 and 532.4 eV of O 1s and 286.9
and 286.1 eV of C 1s, respectively [17,18], as
shown in Fig. 2(b). The XPS intensities of C and
O 1s became gradually attenuated on annealing
the sample above 350 K and completely disap-

C.-C. Yeh et al. / Surface Science 565 (2004) 81–88
85
Fig. 4. TPD scans of CO (m/e = 28) recorded from a Pt surface
exposed to Mo(CO)6 for 60 s at 100 K, followed by exposure to
UV irradiation for various durations.
tion, decreased with increased UV exposure. This
behavior is consistent with the argument that UV
irradiation can induce decarbonylation of ad-
sorbed Mo(CO)6 molecules. Furthermore, two
CO desorption features appear with a sharp fea-
ture at 285 K and a broad feature over a tempera-
ture range 350–550 K. The intensity of the former
desorption feature initially increased with in-
creased duration of UV exposure but then de-
creased on prolonged duration. As described
previously, an intermediate Mo(CO)x formed via
photon-induced decarbonylation of Mo(CO)6.
We propose that the desorption feature at 285 K
originates from thermal decarbonylation of Mo-
(CO)x. The total amount of CO in Mo(CO)3 in-
creased initially with increased UV exposure but
eventually decreased on prolonged UV exposure
because Mo(CO)x was also subject to further pho-
ton-induced decarbonylation to form lower sub-
carbonyl Mo complexes.
Fig. 3. XPS spectra of: (a) Mo 3d and (b) C and O 1s for a Pt
surface exposed to Mo(CO)6 for 60 s at 100 K, followed by
exposure to UV irradiation for various durations.
Fig. 4 shows TPD scans of CO (m/e = 28) taken
from a Pt surface that had been exposed to
Mo(CO)6 for 60 s at 100 K and subsequently irra-
diated with UV light for various durations. The
intensity of the CO feature at 210 K, due to an ion-
ization fragment of molecular Mo(CO)6 desorp-
The broad feature of CO desorption at 350–550
K, which resembles that of CO absorbed on a
clean Pt surface, is attributed to desorption of
CO impurity in Mo(CO)6 as described previously.
However, the integrated area of CO desorption

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C.-C. Yeh et al. / Surface Science 565 (2004) 81–88
obtained from the Mo(CO)6-covered Pt surface
after UV irradiation was significantly greater than
that without UV irradiation. This additional CO
desorption may indicate that a fraction of the
CO ligands in Mo(CO)x intermediate transfers
onto the Pt surface during thermal decarbonyla-
tion and subsequently desorbs at the same temper-
ature as observed on the CO desorption from a
clean Pt surface.
On the other hand, for the decarbonylation of
Mo(CO)6, formation of Mo(CO)3 as a relatively
stable species is supported by quantitative meas-
urement of the amounts of CO evolved during
activation of a catalyst at 373 K [21]. In addition,
removal of all CO ligands from Mo(CO)6 was
completed at temperatures above 573 K [20,22].
Thus, an alternative mechanism could be pro-
posed, according to which the CO desorption at
285 K originates from decarbonylation of Mo-
(CO)x into Mo(CO)3 whereas the broad CO
desorption at 350–550 K is mainly due to further
thermal decarbonylation of Mo(CO)3 into Mo
(Mo(CO)3 ! Mo + 3CO).
Fig. 5. TPD scans of CO (m/e = 28) recorded from a Pt surface
exposed to Mo(CO)6 at 100 K for various durations, followed
by exposure to UV irradiation for 60 s. Dashed lines serve to
highlight desorption features.
Fig. 5 shows TPD scans in which a Pt surface
was exposed to Mo(CO)6 for varied durations
and then irradiated with UV light for a constant
duration 60 s. At small coverage, only a CO
desorption band was observed between 350–550
K, whereas the additional CO desorption feature
appears at 285 K for large exposures and increases
with increased exposure of Mo(CO)6. For sub-
monolayer coverages, the fraction of photons ad-
sorbed in Mo(CO)6 decomposition is insignificant
relative to the total number of incident photons
during UV exposure. Thus, the extent of photo-
decomposition of Mo(CO)6 is independent of the
Mo(CO)6 coverage at a fixed duration of UV
exposure. If the thermal decarbonylation of
Mo(CO)x proceeds through an intermediate of
Mo(CO)3, the ratio of desorption of CO generated
from decarbonylation of Mo(CO)x at 285 and
350–550 K, respectively, is expected not to vary
with the coverage for a constant UV exposure.
The amount of the latter CO desorption was ob-
tained by integrating the desorption peak and sub-
tracting it from that obtained without UV
irradiation. The resulting ratio increased with in-
creased duration of exposure (coverage) (as shown
in Fig. 5). In addition, the ratio obtained at a large
duration of Mo(CO)6 exposure (200 s) is ꢀ1.3,
which is greater than the maximum stoichiometric
proportion (0.67) expected from the decomposi-
tion Mo(CO)x via a Mo(CO)3 intermediate. Thus,
a better explanation for CO desorption is the first
mechanism, in which Mo(CO)x decomposes to
form an Mo adatom and releases all CO ligands
in one step, instead of proceeding through a
metastable intermediate. At small coverages, the
released CO ligand transfers onto the Pt surface
and subsequently desorbs at 350–550 K. At greater
coverage, fewer empty sites on the Pt surface are
available for CO adsorption and a smaller fraction
of released CO transfers onto the surface. The CO
ligands released from Mo(CO)x decomposition
can either directly evolve or transfer onto the Pt
surface.
Fig. 6 shows XPS spectra of Mo 3d, C and O 1s
for a Pt surface exposed to Mo(CO)6 for 60 s at
100 K, irradiated with UV light for 100 s, and then
heated to various temperatures. On annealing the
UV-irradiated surface to 270 K, the intensity of
C and O 1s assigned to Mo(CO)6 decreased due

C.-C. Yeh et al. / Surface Science 565 (2004) 81–88
87
decarbonylated to form Mo adatoms, based on
the preceding argument. At higher temperature,
the surface CO gradually desorbs and the peak
width of Mo 3d doublet becomes smaller. Upon
the sample being annealed to 670 K, surface CO
are completely desorbed and C and O 1s XPS data
show that the resulting Mo layer was nearly free of
carbon and oxygen. In addition, the Mo 3d feature
became an intense and well resolved doublet with
d5/2 binding energy 228.1 eV and an asymmetric
tail at the high-energy side, which is characteristic
of metallic Mo. However, we also found that a Mo
film was obtained from photon-induced decompo-
sition of a thick physisorbed Mo(CO)6 multilayer
and contained C and O impurities.
Upon the resulting Mo-deposited sample being
annealed to ꢀ1000 K, the XPS spectrum of depos-
ited Mo 3d was unaltered, indicating no formation
of Mo nanoparticles, as has been proposed for the
thermal vapor decomposition of Mo(CO)6 on Au
[23,24]. The total intensity of the Mo 3d signal de-
creased and a new Mo 3d component was ob-
served with d5/2 binding energy 229.2 eV, on
annealing the sample beyond 1070 K, as shown
in Fig. 7. Diffusion of Mo into bulk Pt has been
Fig. 6. XPS spectra of: (a) Mo 3d and (b) C and O 1s for a Pt
surface exposed to Mo(CO)6 for 60 s at 100 K, followed by
exposure to UV irradiation for 100 s and annealed to various
temperatures.
to desorption of intact Mo(CO)6. Between 270 and
370 K, the intensity of C and O 1s signals gradu-
ally decreased, and these signals shifted toward
smaller binding energy, resulting from decarbony-
lation of Mo(CO)x. Furthermore, Mo 3d became
a partially resolved doublet because Mo(CO)x
Fig. 7. Thermal evolution of Mo 3d for a Pt surface exposed to
Mo(CO)6 for 60 s at 100 K, followed by exposure to UV
irradiation for 100 s and annealing at 670, 1070 and 1120 K.

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C.-C. Yeh et al. / Surface Science 565 (2004) 81–88
proposed to attain a solubility limit (saturation),
resulting in formation of a PtMo2 alloy [4]. This
new feature is thus attributed to subsurface Mo
formed on diffusion of deposited Mo into bulk
Pt. This Mo-deposited Pt surface serves as a model
surface in our continuing studies of Mo/Pt electro-
catalysts of a fuel cell, in which we compare ther-
mal desorption and decomposition of CO and
methanol on Mo-covered and Mo/Pt alloy-cov-
ered Pt surfaces.
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