Adsorption structures of NO/Pt„111… investigated by scanning tunnelingmicroscopy
M. Matsumotoa)
Institute for Molecular Science, Myodaiji, Okazaki 444-8585, Japan, and Institute of Industrial Science,
University of Tokyo, Minato-ku, Tokyo 106-8558, Japan

N. Tatsumi, K. Fukutani, and T. Okanob)
Institute of Industrial Science, University of Tokyo, Minato-ku, Tokyo 106-8558, JapanT. Yamada, K. Miyake, K. Hate, and H. Shigekawac)
Institute of Applied Physics, Center for TARA, and CREST, Japan Science and Technology Corporation(JST), University of Tsukuba, Tsukuba 305-8573, Japan~Received 23 October 1998; accepted 3 May 1999!
The adsorption structure of nitric oxide ~NO! on Pt~111! was studied at 10 and 70 K by scanningtunneling microscopy ~STM!. The island growth modes at both temperatures are similar except forthe domain size of the 232 structure. In these low temperature region, two phases can coexist atmedium coverages. These phases are assigned to the two NO species occurring at different
stretching-vibrational frequencies observed in the previous vibrational spectroscopic studies. The
relative location of two different species observed by STM and its stretching-vibrational frequencies
suggests that the adsorption sites of NO on the Pt~111! surface at low and high coverages
correspond to the hollow and the on-top sites, respectively. © 1999 American Vacuum Society.
@S0734-2101~99!22404-9#
I. INTRODUCTION
The adsorption and reaction of nitric oxide ~NO! on transition metal surfaces have been the objects of considerable
interest due to their importance in the catalytic conversion of
automotive exhaust.1–3 In addition, the unpaired electron in
the antibonding 2p* orbital of NO increases the variety ofreactions, which has attracted much attention of chemists formany years. Many efforts have been made to determine theadsorption structures of NO on the platinum surfaces,4–11 but
there still remains a controversy.
It is known that NO adsorbs molecularly on the Pt~111!
surface. In the temperature range between 80 and 160 K, a
1490 cm21 peak at low coverages and a 1710 cm21 peak athigh coverages are detected by electron energy-loss spectroscopy ~EELS!4,5 and infrared absorption spectroscopy~IRAS!.6–8 These peaks were first assigned to the N–Ostretching vibrations of monomer and dimer species,
respectively.4 Later, Gland and Sexton adopted a ‘‘site
model’’ based on a comparison of the stretching-vibrationalfrequency of the adsorbed NO with that of NO in nitrosyl
complexes.5 They suggested that NO occupies a bridge site
at low coverages and transits to an on-top site as the coverage increases. This site model has been supported by manygroups.6,8,9 On the contrary, from a dynamical low-energy
electron diffraction ~LEED! analysis, Materer et al., concluded that the three-fold face-centered-cubic ~fcc! hollow
site is the most probable adsorption site for the 232 orderedstructure of NO on Pt~111! at the saturation coverage ofu NO50.25 ML.10 Their report reopened a controversy over
the adsorption structure of NO on Pt~111!. Ge and King cala!

Electronic mail: masuakiiis.u-tokyo.ac.jp
Electronic mail: okano@iis.u-tokyo.ac.jpc!
Electronic mail: hidemi@ims.tsukuba.ac.jp
b!

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culated the adsorption energies of NO on Pt~111! using
density-functional theory ~DFT! with the generalized gradient approximation ~GGA!.11 They also found that the fcc
hollow site is most stable, supporting the LEED result.
Recently, Loffreda et al. evaluated the stretchingvibrational frequencies of NO on Pd~111! and Rh~111! by
DFT calculations.12 They found that the vibrational frequency of NO on the most stable fcc hollow site is in better
agreement with the experimental value than that on thebridge site which has been regarded as the most probableadsorption site of NO so far. Their results posed a generalquestion on the adsorption site determination from the comparison of the vibrational frequency to that of nitrosyl complexes. Thus, the structure determination of NO is not yet
solved not only on the Pt~111! but also on other transition
metal surfaces.
Scanning tunneling microscopy ~STM! is one of the mostpowerful techniques for the determination of surface structures. However, the number of reported STM studies of adsorbed gas molecules on metal surfaces is still limited. For
NO on Pt, only one report for NO/Pt~001! by Song et al. has
been published before.13 To our knowledge, NO/Pt~111! has
not yet been investigated by means of STM.
In this article, we report the STM observation of the NO
Molecules on Pt~111! at 10 and 70 K. Measurement at low
temperatures enables us not only to observe the mobile gas
molecule fixed in the measurement time scale but also toobserve several different metastable species at the same time.
This is necessary for the identification of the different species which has the same periodicity, because a simultaneous
atomic resolution of the substrate and the adsorbate is notfrequently possible in STM. On the basis of the coveragedependence of the STM images and the data reported in pre-0734-2101/99/17„4…/1577/4/$15.00©1999 American Vacuum Society

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Matsumoto et al.: Adsorption structures of NO/Pt„111… investigated by STM

FIG. 1. STM images of NO/Pt~111! at 10 K: ~a! u NO'0.03 ML, ~b! u NO
'0.06 ML, ~c! medium, and ~d! high coverages. ~a!,~b!,~d!: V tip50.5 V,
I t 50.5 nA, ~c!: V tip50.1 V, I t 55 nA. ~a!,~b!: current image, ~c!,~d!: topographic image.

vious LEED and vibrational studies, the adsorption structureof NO molecules on the Pt~111! surface is discussed.
II. EXPERIMENTThe STM measurements were carried out in an ultrahighvacuum chamber equipped with a low-temperature STM unit
~Unisoku, USM-601! and LEED.14 The base pressure waslower than 131028 Pa. The Pt~111! sample was cleaned byrepeated cycles of Ar1 ion sputtering and annealing at 1400
K. To reduce carbon contaminations, repeated cycles of annealing under 231025 Pa of oxygen at 1100 K and flashing
to 1400 K were performed. The sample was heated with a Ta
heater and the temperature was measured with an opticalpyrometer throughout the cleaning process. The sample
cleanliness was checked by LEED and STM. The STM unit
is attached to the bottom of a liquid helium reservoir directly,
allowing measurements at 9 K with liquid helium. Using
liquid nitrogen as a cryogen, the lowest measurement temperature of about 70 K was achieved by evacuating the cryostat to reduce the vibration caused by bubbling of liquid
nitrogen. Measurements in the temperature ranges of 20–70
K and 90–250 K were difficult because of the large thermal
drift. The sample temperature was measured with a Au–Fe
0.07%/chromel thermocouple attached to the STM unit. NO
gas was introduced into the vacuum chamber through a variable leak valve and a 1/8 in. capillary tube directed to thesample surface. The NO coverage was estimated from the
STM image. The scale was calibrated by measuring STM
images of the Si~111!-737 structure at several temperatures.
III. RESULTS AND DISCUSSION
STM images of NO/Pt~111! after exposing the sample to
NO at 10 K are shown in Fig. 1. The tip bias (V tip) and thetunnel current (I t ) of our STM measurements were 0.1–0.5
V and 0.5–1 nA, respectively. The occupied states are obJ. Vac. Sci. Technol. A, Vol. 17, No. 4, Jul/Aug 1999

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served. The parameters for all images are given in the figure
captions. Under these conditions, NO molecules are observed as bright spots. Any dips, which were assigned tooxygen or nitrogen atoms in the STM images of
NO/Ru~0001!,15 could not be observed in our STM imagesof NO/Pt~111! at 10 and 70 K. This shows that the NO does
not dissociate on Pt~111! in this temperature region. Whenthe tunnel resistance was reduced (V tip50.1 V, I t 510 nA!,
individual platinum atoms could be resolved. However, in
this case, the images of NO particles were distorted by theinteraction of the tip and the molecules, and we could not
determine the adsorption site from the STM images directly.
As revealed by repeated scans of the same surface area, theNO molecules were not displaced or desorbed by the interaction with the tip under the conditions presented in thisreport.
At very low coverages @Fig. 1~a!, 0.03 ML#, most of theNO molecules adsorb on the surface randomly. Some ofthem already form dimers or trimers. The distance between
the molecules in these dimers and trimers is about 0.55 nm,
which is about twice the nearest neighbor Pt–Pt distance of
the Pt~111! surface. Therefore, these dimers are different
from the gas phase dimers, in which the distance between themolecules is 0.224 nm. At medium coverages, these dimers
and trimers grow into 232 islands @Fig. 1~b!, 0.06 ML#.
These islands are small and they consist of small domains
whose phases do not match with each other. The number ofthe molecules in one domain is less than ten. This is considered to be caused by the very low mobility of NO at 10 K.
As the coverage increases, a second phase of adsorbed molecules appears on the surface. In Fig. 1~c!, the bright spots
construct 232 structures in the region from the left-bottom
to the right-top side of the image. In addition, less brightspots can be seen in the space between the 232 islands of thebright spots and also construct 232 structures. These twokinds of bright spots show the presence of two different molecular species, as will be discussed later. At very high coverages, NO condenses on the surface in multilayers @Fig.
1~d!#. In this case only a rough surface can be seen by STM
without any regular structures on the surface. The estimationof the coverage and the number of layers from the multilayer image is difficult because the second layer molecules
cannot be observed by STM.
STM images after exposing the sample to NO at 70 K areshown in Fig. 2. At very low coverages @Fig. 2~a!, 0.04 ML#,
many isolated molecules and some 232 islands exist on thesurface. As the coverage increases @Fig. 2~b!, 0.14 ML#, 232
islands grow in size and number. The islands consist of morethan 20 molecules, which means they are larger than at 10 K.
This reflects the higher mobility of NO at 70 K. Several
bright spots indicated by arrows exist in Fig. 2~b! and, at
higher coverages, also construct 232 structures as indicatedin Fig. 2~b!. At very high coverages, NO also condenses on
the surface @Fig. 2~d!#. These images indicate that the adsorption structure of NO at 70 K is qualitatively similar tothat at 10 K. The main difference between them is the do-

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FIG. 2. STM images of NO/Pt~111! at 70 K: ~a! u NO'0.03 ML, ~b! u NO
'0.14 ML, ~c! medium, and ~d! high coverages. ~a!,~b!,~d!: V tip50.5 V,
I t 50.5 nA,~c!: V tip50.1 V, I t 51 nA. ~a!: current image, ~b!,~c!,~d!: topographic image.

main size which is closely related to the mobility of the NO
molecules.
We focus on the two different molecular species observedin Fig. 1~c!. A large scale image where both species coexist
is shown in Fig. 3~a!. The characteristic features can be summarized as follows. ~i! Two kinds of molecular species arepresent on the surface. ~ii! Both species construct 232 structures. ~iii! The heights of the bright and the less bright species are 0.087 and 0.04 nm, respectively, as indicated in the
profile plot of Fig. 3~b!. ~iv! The bright ~‘‘higher’’! species islocated on the threefold hollow site of the less bright~‘‘lower’’! one.
According to the previous vibrational studies at a sample
temperature of about 100 K, the stretching-vibrational frequencies of N–O on Pt~111! at low and high coverages are'1490 and '1710 cm21,4–6 respectively. The 232 LEED
pattern was observed only at high coverages, previously.4,6
The LEED pattern was observed at a coverage where only
the vibrational frequency of 1490 cm21 is measured by IRAS
at 90 K and it was confirmed that the 232 LEED pattern also
appears under this condition.16 Furthermore, it was shownthat the LEED intensity versus energy ~I–V! curves change
at higher coverages. This suggests the presence of two different 232 phases of NO/Pt~111!.
The measurement temperature of our STM is lower than
JVST A - Vacuum, Surfaces, and Films

FIG. 3. ~a! STM topographic image of NO adsorbed on Pt~111! at 10 K.
u NO'0.15 ML. V tip50.1 V, I t 51 nA. ~b! Profile plot along A2A 8 line in
Fig. 2~a!. The heights of higher and lower species are 0.087 and 0.04 nm,
respectively. ~c! A model for NO/Pt~111! at low temperature. The closed
and open circles correspond to the lower and the higher species, respectively. The thick line indicates the domain boundary of NO adsorbates. The
thin line indicating the platinum lattice is drawn on the assumption that thelower species adsorbs on the threefold hollow site of Pt~111!.

that in those studies. However, in IR spectra measured at 25
K, no additional peaks appear before condensation occurs.17
We also confirmed that the two kinds of species exist after
annealing the sample to 100 K. Therefore, the structure of
NO/Pt~111! at the temperatures of our measurements islikely to be similar to the one at about 100 K. In our coverage dependent STM measurements, the lower species first
appears, while the higher species appears at higher coverages. Therefore, we suppose that the lower and higher species in the STM can be assigned to the two species observedin the vibrational studies at the frequencies of 1490 and 1710
cm21, respectively. Due to the higher mobility at higher temperatures, molecules will adsorb to yield a dense layer oflower species before the higher species grows, but in the

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low-temperature region, both species can coexist even at low
coverages.
From the height analysis, we found that the higher species
is twice as high as the lower one. But both the heights ofthese species as well as the difference in heights between
them are considerably shorter than the N–O distance ~0.117
nm! in gas phase. The heights in the STM images do notindicate the real corrugation. The heights rather reflect theelectronic structure of the surface near the Fermi level. Similar two types of STM images have been measured for the
carbon monoxide ~CO! molecules on Pt~111! previously.18
The heights were determined to be 0.044 and 0.007 nm inthis case. These images were calculated by electron scattering quantum chemical technique and assigned to the two
distinct binding sites ~the on-top and the bridge sites,
respectively!.19 The STM image of CO on the hollow site
was also calculated and it was suggested that the height ofthe species on this site is lower than that on the bridge site.
No calculations of adsorbed NO molecules have been performed yet, but the difference of the apparent height can also
be related to the difference of the adsorption site because of
the similarity of the molecular orbital and bonding mechanism between NO and CO. From the relative locations of thetwo species in the STM, two combinations of the site occupation can be expected. One possibility is the occupation ofhollow @hexagonal-close-packed ~hcp! or fcc# and the on-top
sites, and the other is the occupation of the fcc and the hcp
hollow sites. The occupation of the bridge and the on-top
sites which has been adopted previously is not preferable
because a molecule on the on-top site does not sit on thecenter of the three molecules on the bridge site. The large
difference in the stretching-vibrational frequencies of N–O isinconsistent with the occupation of both hollow sites. Therefore, we suggest that the lower and the higher species adsorb
on the hollow and the on-top sites of the Pt~111! surface,respectively. We cannot determine which hollow site, fcc or
hcp, is preferable from our STM results only. The dynamical
LEED analysis concluded that the fcc hollow is the mostprobable adsorption site of NO,10 and it was suggested to be
the structure at low coverages.16 The DFT calculation
showed that the fcc hollow is the most stable adsorption
site.11 This also suggests that the fcc hollow is the first occupied site at low coverages. From this discussion, we suppose that the lower and higher species adsorb on the fcc
hollow and the on-top sites, respectively. The model for the
STM image of Fig. 3~a! is shown in Fig. 3~c!.

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IV. CONCLUSIONS
STM images of NO/Pt~111! were measured at 10 and 70
K. The adsorption structure at 70 K is similar to that at 10 K
except for the 232 domain size. At low coverages, the NO
molecules adsorb on the surface randomly. As the coverage
is increased, they construct two different phases of 232
structures. And at high coverages, they condense in multilayers. The higher and the lower 232 species observed at medium coverages are assigned to the stretching-vibrational frequencies of 1490 and 1710 cm21 measured in the previous
vibrational studies, respectively. From the relative locationsof these two different species in the STM images, the adsorption sites of the lower and the higher species are supposed tobe the fcc hollow and the on-top sites, respectively.
ACKNOWLEDGMENTS
The authors greatly appreciate the fruitful discussions
with Dr. H. Aizawa of the University of Tokyo and Dr. M.
Wilde of the Institute of Industrial Science, University ofTokyo. This work was supported by the Shigekawa Projectof TARA, University of Tsukuba. The support of a Grant-inAid for the Scientific Research from the Ministry of Education, Science, Sports and Culture of Japan is also acknowledged.
M. Shelef and G. W. Graham, Catal. Rev. Sci. Eng. 36, 433 ~1994!.
K. C. Taylor, Catal. Rev. Sci. Eng. 35, 457 ~1993!.
V. Schmatloch and N. Kruse, Surf. Sci. 269/270, 488 ~1992!.
4
H. Ibach and S. Lehwald, Surf. Sci. 76, 1 ~1978!.
5
J. L. Gland and B. A. Sexton, Surf. Sci. 94, 355 ~1980!.
6
B. E. Hayden, Surf. Sci. 131, 419 ~1983!.
7
D. S. Dunn, M. W. Severson, J. L. Hylden, and J. Overend, J. Catal. 78,
225 ~1982!.
8
M.-B. Song, M. Suguri, K. Fukutani, F. Komori, and Y. Murata, Appl.
Surf. Sci. 79/80, 25 ~1994!.
9
W. Ranke, Surf. Sci. 209, 57 ~1989!.
10
N. Materer, A. Barbieri, D. Gardin, U. Starke, J. D. Batteas, M. A. VanHove, and G. A. Somorjai, Surf. Sci. 303, 319 ~1994!.
11
Q. Ge and D. A. King, Chem. Phys. Lett. 285, 15 ~1998!.
12
D. Loffreda, D. Simon, and P. Sautet, Chem. Phys. Lett. 291, 15 ~1998!.
13
M.-B. Song, K. Momoi, and M. Ito, Jpn. J. Appl. Phys., Part 2 36, L1528~1997!.
14
H. Shigekawa, K. Miyake, M. Ishida, K. Hata, H. Oigawa, Y. Nannichi,
R. Yoshizaki, A. Kawazu, T. Abe, T. Ozawa, and T. Nagamura, Jpn. J.
Appl. Phys., Part 2 35, L1081 ~1996!; H. Shigekawa, K. Hata, K. Miyake,
M. Ishida, and S. Ozawa, Phys. Rev. B 55, 15448 ~1997!.
15
T. Zambelli, J. Wintterlin, J. Trost, and G. Ertl, Science 273, 1688 ~1996!.
16
N. Tatsumi, M. Matsumoto, H. Aizawa, S. Tsuneyuki, K. Fukutani, andT. Okano, J. Vac. Soc. Jpn. ~submitted!.
17
J. Yoshinobu and M. Kawai, Chem. Lett. 1995, 605 ~1995!.
18
J. A. Stroscio and D. M. Eigler, Science 254, 1319 ~1991!.
19
M.-L. Bocquet and P. Sautet, Surf. Sci. 360, 128 ~1996!.
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