Surface Science 454–456 (2000) 101–105
www.elsevier.nl/locate/susc

Study of the adsorption structure of NO on Pt(111) byscanning tunneling microscopy and high-resolution
electron energy-loss spectroscopy
Masuaki Matsumoto a, *, Katsuyuki Fukutani a, Tatsuo Okano a, Kouji Miyake b,
Hidemi Shigekawa b, Hiroyuki Kato c, Hiroshi Okuyama c, Maki Kawai ca Institute of Industrial Science, University of Tokyo, Megro-ku, Tokyo 153-8505, Japanb Institute of Applied Physics, CREST, Japan Science and Technology Corporation (JST), University of Tsukuba,Tsukuba 305-8573, Japanc RIKEN (The Institute of Physical and Chemical Research), 2-1 Hirosawa, Wako, Saitama 351-0198, JapanAbstract
The structure of nitric oxide (NO) on a Pt(111) surface was studied by scanning tunneling microscopy (STM )and high-resolution electron energy-loss spectroscopy (HREELS ). The coexistence of two species is observed by STM
after saturating the surface with NO at 70 K and annealing to 215 K. Two pairs of NMO and PtMN stretchingvibrational frequencies are observed in HREELS spectra after annealing, which are assigned to the two speciesobserved by STM. Comparison between the spectra of HREELS and infrared absorption spectroscopy (IRAS)
indicates that IRAS is less sensitive to the lower-frequency mode. A new model for the adsorption structure of NO
on the Pt(111) surface is proposed and discussed. © 2000 Published by Elsevier Science B.V. All rights reserved.
Keywords: Chemisorption; Electron energy loss spectroscopy ( EELS ); Electron stimulated desorption (ESD); Nitrogen oxides;
Platinum; Scanning tunneling microscopy; Surface structure, morphology, roughness, and topography; Vibrations of adsorbedmolecules

1. Introduction
The vibrational modes of an adsorbate areclosely related to its adsorption structure on thesurface. Therefore, vibrational spectra measured
by high-resolution electron energy-loss spectroscopy (HREELS) or infrared absorption spectroscopy (IRAS ) are utilized for structure
determination of adsorbates. However, structure
determination on the basis of vibrational data issometimes misleading. Additional information* Corresponding author. Fax: +81-3-5452-6132.
E-mail address: masuaki@iis.u-tokyo.ac.jp (M. Matsumoto)obtained by other techniques such as thermal
desorption spectroscopy ( TDS ), low-energy
electron diffraction (LEED) or scanning tunnelingmicroscopy (STM ) are indispensable to determine
the structure correctly. The structure of NO on
Pt(111) is one of the cases that has not yet beenclarified successfully on the basis of vibrational
studies [1–11].
NO adsorbs molecularly on the Pt(111) surface
at low temperatures (80–160 K ). In the vibrational
spectra [1–4], an NMO stretching vibrationalmode is observed at 1490 cm−1 at low coverages.
As the coverage increases, a new vibrational mode
appears at 1710 cm−1. On the basis of this meas-0039-6028/00/$ - see front matter © 2000 Published by Elsevier Science B.V. All rights reserved.
PII: S0 0 39 - 6 0 28 ( 00 ) 0 02 6 6 -1

102

M. Matsumoto et al. / Surface Science 454–456 (2000) 101–105

urement, Gland and Sexton proposed a model inwhich NO first adsorbs on the bridge site andtransits to the on-top site with increasing coverage[2]. This model has been supported to date [4–7,12] in spite of several inconsistencies. The modeat 1490 cm−1 that was assigned to NO on thebridge site disappears only in IRAS spectra [4]but remains with reasonable intensity in HREELS
spectra up to saturation coverage [1,2]. A large
desorption peak of molecular NO is observed atabout 200 K by TDS. However, the origin of thispeak is not well understood because no significant
change has been observed in the vibrational spectra
after annealing to 260 K [2,4]. The saturation
coverage was determined to be 0.54 monolayers(1 ML=1.5×1015 cm−2) by X-ray photoelectronspectroscopy ( XPS) [8]. This value is much larger
than the value of 0.25 ML in Gland and Sexton’smodel. In our previous report, two different species
were observed and assigned to the two modes
observed in the vibrational spectra [9]. However,as the surface temperature in the measurements inRef. [9] was lower than the condensation temperature (80 K ) of NO, three-dimensional growthoccurred before completion of the 2×2 layer. In
addition, these results could not be compareddirectly with the results of the vibrational studies[1,2,4], which were performed at higher temperatures (90–160 K ). Hence, in this work, theadsorption structure after annealing above 200 K
was studied by using STM and HREELS.
Annealing is useful to construct a well-ordered
2×2 structure and reveals the coexistence structureof two NO species at high coverages. The effect ofannealing on the HREELS spectra and the comparison with IRAS spectra are discussed
qualitatively.

2. Experimental
The STM and HREELS experiments were carried out in two separate ultrahigh vacuum ( UHV )chambers. The STM chamber [9] and theHREELS chamber [13] have been described indetail elsewhere. The measurements and exposure
to NO were carried out at about 70 K (STM ) and90 K (HREELS). Each Pt(111) sample wascleaned by repeated cycles of Ar+-ion sputteringand annealing at 1400 K, and by repeated cyclesof annealing under 2×10−5 Pa of oxygen at
1100 K and flashing to 1400 K. The cleanliness ofthe sample was checked by STM or HREELS.
After cleaning, each sample was exposed to NOup to saturation. The surface was then annealed
to 200–250 K and cooled again to the respective
measurement temperature. A molecular desorptionof NO is observed at 190 K, and no other molecular desorption is observed in the temperature
region between 200 and 270 K in TDS spectra[2,4]. The species that desorb at 190 K can beremoved by keeping the sample at 200 K forseveral seconds. The reproducibility of the construction of a well-ordered 2×2 structure byannealing was confirmed by STM. Therefore, theadsorption structure of NO in the STM andHREELS experiments is identical after annealing
to 200–250 K.

3. Results and discussionA topographic STM image of NO/Pt(111) measured at 70 K after annealing to 215 K is showninFig. 1a(V=−0.1 V, I=0.5 nA,
sampletunnel
7.6 nm×7.6 nm). A well-ordered 2×2 structure isobserved on the surface. It is noticed that theupper triangle in the unit cell is darker than thelower one, suggesting that the two different NO
species denoted by a and b are present on thesurface. The b species appears to be higher thanthe a species. This is shown more clearly in Fig. 1b(V=−0.1 V, I=0.5 nA, 5.7 nm×5.7 nm),sampletunnelwhich was measured after electron irradiation:
after the measurement of Fig. 1a, the feedback
loop of the STM was briefly turned off and a
voltage pulse (−2 V, 100 ms) was applied to theSTM tip. This caused partial desorption of NO
molecules. The mechanism of this electron-stimulated desorption will be discussed elsewhere [14].
Since the electron irradiation was conducted in thearea on the right side of Fig. 1b, only the moleculesin the right half (‘A’) were affected. In this area,
most of the b species are desorbed but the a species
remain on the surface. In contrast, both species (aand b) can be observed in region ‘B’. The two

M. Matsumoto et al. / Surface Science 454–456 (2000) 101–105

103Fig. 1. (a) Topographic STM image of NO on Pt(111) after annealing to 215 K. A well-ordered 2×2 structure is constructed on thesurface. Two species (a and b) appearing at different heights are present on the surface. (b) Magnified image of the same area as (a)after electron irradiation from the STM tip. Most of the b species in the region ‘A’ are desorbed by the electron irradiation. Several
unit cells indicated by c are bright in the whole area.

species both construct 2×2 structures, with the bspecies being located on the three-fold hollow siteof the a species. The heights of the a and b species
are 0.05 and 0.09 nm, respectively, which are thesame as the values of the two species described in
our previous STM report [9]. As the geometricstructures are also identical in both measurements,
the a and b species in Fig. 1a are identical to the‘lower’ and ‘higher’ species in Ref. [9]. While thepresence of the coexistence structure was attributed
to the low measurement temperature in Ref. [9],Figs. 1a and b clearly show that the coexistence
structure still remains at temperatures above
200 K. The NO coverage estimated from Fig. 1ais 0.5 ML.
HREELS spectra of NO on Pt(111) measured
at 90 K are shown in Fig. 2. All spectra weremeasured at an incident angle of 60° in a specularreflection. The azimuthal direction and the energyof the incident electrons were [2: 11] and 7.1 eV,
respectively. At low coverages ( Fig. 2a), two loss
peaks are observed at 330 (n M ) and 1484
Pt N(a)(n) cm−1, which can be assigned to the stretchNO(a)ing vibrations of PtMN (n M ) and NMO (n ),NO
Pt N
respectively. At saturation coverage (Fig. 2b), twon M modes are observed at 290 (n∞ M ) andPt N(a)Pt N

Fig. 2. HREELS spectra of NO on Pt(111): (a) at low coverages
at 90 K, (b) at saturation coverage at 90 K and (c) after annealing to 200 K.

450 (n M ) cm−1 and three n modes appear atPt N(b)
NO1444 (n∞), 1508 (n) and 1710 (n) cm−1.
NO(a)
NO(c)
NO(b)
The features agree well with previous EELS spectra

104

M. Matsumoto et al. / Surface Science 454–456 (2000) 101–105[1,2] but n∞and n
are well resolved in ourNO(a)NO(c)
spectrum by virtue of higher energy resolution andsensitivity. After annealing to 200 K (Fig. 2c),ndisappears and small shifts of the otherNO(c)modes are observed. The presence of two pairs ofn M and n is consistent with the finding of two
Pt NNOspecies in the STM images of Fig. 1. According to
the coverage dependence of the STM image,nand nwere assigned to the ‘lower’ andNO(a)NO(b)‘higher’ species in Fig. 3 of Ref. [9], because the‘lower’ species was noticed at low coverages beforethe ‘higher’ species appeared. As both species arepresent after annealing, n∞and n
can beNO(a)NO(b)
assigned to the a and the b species in Fig. 1b,
respectively. It is suggested that the n mode ofNO
the a species shifts from 1484 cm−1 (n) toNO(a)1444 cm−1 (n∞) by co-adsorption of the bNO(a)species at high coverages.
The relative integrated intensity of I/In∞nin Fig. 2c is 0.25. This is much smallerNO(a)than NO(b)thevalue of 1 expected from the site occupancy
deduced by STM. The attenuation of the lowerfrequency band was also observed in IRAS andexplained by an ‘intensity transfer’ induced bydynamic dipole–dipole coupling in the case of
densely packed adlayers of CO on Pt(111) [15,16 ].
A similar attenuation mechanism was suggested
for the IRAS spectra of NO on Pt(111) [17]. It istherefore suggested that the growth of natNO(b)high coverages causes the attenuation of n∞,NO(a)and the growth of ncauses further attenuation
NO(c)
of n∞
below the detection limit of IRAS. ThisNO(a)is consistent with the observation that annealing
causes desorption of the nspecies and increases
NO(c)the intensity of the n∞mode. Because there areNO(a)scattering processes other than dipole scattering inHREELS, e.g., impact scattering, the effect of the‘intensity transfer’ is considered to be smaller thanin IRAS and thus the lower-frequency moderemains observable in HREELS. This result showsthat the loss of peaks in IRAS does not always
indicate the loss of the vibrational mode, and greatcare has to be taken in the analysis of IRASspectra.
Concerning the adsorption structure, the coverage estimated above and the azimuthal direction
confirmed by the Laue method restrict the possiblecombinations of site occupations to the two candi-Fig. 3. Models for the adsorption structure of NO on Pt(111)showing different combinations of site occupations: (a) facecentered cubic (fcc) hollow+on-top model (h=0.5 ML); (b)hexagonal close-packed (hcp) hollow+fcc hollow model (h=0.5 ML); and (c) three-site model (fcc hollow+on-top+hcphollow) for saturation coverage (h=0.75 ML).

dates shown in Figs. 3a and b. One combination
is face-centered-cubic (fcc) hollow and on-top(Model a, Fig. 3a), and another one is hexagonalclose-packed (hcp) hollow and fcc hollow (Modelb, Fig. 3b) for the a and b species, respectively.
Ge and King showed that the fcc hollow site is
the most stable adsorption site on the bare Pt(111)
surface [10]. This suggests that the adsorption site
populated first is the fcc hollow site. Since thecoordination number is closely related to n , theNOlarge difference between nand n∞
is inconNO(b)NO(a)
sistent with Model b. Furthermore, previous
LEED analysis showed that NO adsorbs at the fcc
hollow site at a coverage of 0.25 ML [11]. Amongthe two possible combinations, therefore, Model ais the most probable structure at 0.5 ML.
Assuming that the NO coverage is 0.5 ML for
the sample after annealing, the saturation coverage
at 90 K is estimated to be 0.74 ML by comparing
the intensity of corresponding TDS spectra. Thisvalue is larger than the value of 0.54 ML obtainedby Kiskinova et al. [8], which is based on theassumption that the saturation coverage ofCO/Pt(111) is 0.6. However, their value agrees
better with our result if one considers that thesaturation coverage of CO is actually 0.67–0.71[18–20].
At saturation coverage, an additional nNO(c)mode appears at 1508 cm−1 in the HREELS

M. Matsumoto et al. / Surface Science 454–456 (2000) 101–105

spectrum in Fig. 2b. The small frequency difference
between n∞
and n, as well as the saturation
NO(a)
NO(c)
coverage of 0.74 ML, suggest that an additional
species adsorbs probably on the hcp hollow site,the model of which (Model c) is shown in Fig. 3c.
The nfor this species is not observed byPt–N
HREELS probably because the mode is very close
to the n∞mode and cannot be resolved well.
Pt–N(a)The bright image in the whole unit cell at thedefects indicated by c in Fig. 1b also suggests thepresence of a species on the hcp hollow site. Asimilar three-site occupation model has recently
been reported for the adsorption of NO on
Rh(111) [21,22], Pd(111) [22] and Ru(001) [23].
Therefore, this kind of structure will be a common
feature of NO adsorption on transition metal
surfaces. The disappearance of the nmodeNO(c)after annealing shows that desorption of the molecules on the hcp hollow site is the origin of thepeak at 190 K in TDS spectra [2,4].
This three-site model offers an unambiguous
assignment for the site selectivity observed in thephotoinduced processes of NO on Pt(111). The
dissociation of NO was observed at low coverages,
while molecular desorption was observed at highcoverages by laser irradiation at 193 nm [7].
Furthermore, the rotational distribution of thedesorbed NO was dependent on the sample temperature: the rotational distribution wasBoltzmann with a rotational temperature of 500 K
at saturation coverage at 90 K, and nonBoltzmann with a spin–orbit state inversion after
annealing to 220 K [12,24]. Our three-site model
indicates that NO adsorbed on the fcc hollow site
is decomposed into atomic nitrogen and oxygen,and NO on the on-top and the hcp hollow sites ismolecularly desorbed by laser irradiation with
non-Boltzmann and Boltzmann distribution in therotational motion, respectively.

4. Conclusions
STM images and HREELS spectra of NO on
Pt(111) were measured after annealing theNO-saturated surface to 200–250 K. The coexistenceof two NO species is observed by STM and eachcorresponds to two pairs of n and n M modes
NOPt Ndetectable by HREELS. Based on the STM images

105

and the vibrational spectra, a new model for theadsorption structure of NO/Pt(111) is proposed.

Acknowledgements
This work was supported by the Shigekawa
Project of TARA, University of Tsukuba. The
support of a Grant-in-Aid for Scientific Researchand Creative Basic Research from the Ministry ofEducation, Science, Sports and Culture of Japanis also acknowledged.

References[1] H. Ibach, S. Lehwald, Surf. Sci. 76 (1978) 1.
[2] J.L. Gland, B.A. Sexton, Surf. Sci. 94 (1980) 355.
[3] D.S. Dunn, M.W. Severson, J.L. Hylden, J. Overend,J. Catal. 78 (1982) 225.
[4] B.E. Hayden, Surf. Sci. 131 (1983) 419.
[5] V.K. Agrawal, M. Trenary, Surf. Sci. 259 (1991) 116.
[6 ] J. Yoshinobu, M. Kawai, Chem. Lett. 1995 (1995) 605.
[7] M.-B. Song, M. Suguri, K. Fukutani, F. Komori, Y.
Murata, Appl. Surf. Sci. 79/80 (1994) 25.
[8] M. Kiskinova, G. Pirug, H.P. Bonzel, Surf. Sci. 136(1984) 285.
[9] M. Matsumoto, N. Tatsumi, K. Fukutani, T. Okano, T.
Yamada, K. Miyake, K. Hata, H. Shigekawa, J. Vac. Sci.
Technol. A 17 (1999) 1577.
[10] Q. Ge, D.A. King, Chem. Phys. Lett. 285 (1998) 15.
[11] N. Materer, A. Barbieri, D. Gardin, U. Starke, J.D. Batteas, M.A. VanHove, G.A. Somorjai, Surf. Sci. 303(1994) 319.
[12] K. Fukutani, Y. Murata, R. Schwarzwald, T.J. Chuang,Surf. Sci. 311 (1994) 247.
[13] H. Kato, J. Yoshinobu, M. Kawai, Surf. Sci. 427–428(1999) 69.
[14] M. Matsumoto, K. Fukutani, T. Okano, T. Yamada, M.
Miyake, H. Shigekawa, in preparation.
[15] M.W. Severson, C. Stuhlmann, I. Villegas, M.J. Weaver,J. Chem. Phys. 103 (1995) 9832.
[16 ] I. Villegas, M.J. Weaver, J. Chem. Phys. 101 (1994) 1648.
[17] I. Villegas, R. Gómez, M.J. Weaver, J. Phys. Chem. 99(1995) 14 832.
[18] G. Ertl, M. Neumann, K.M. Streit, Surf. Sci. 64 (1977) 393.
[19] N.R. Avery, J. Chem. Phys. 74 (1981) 4202.
[20] B.N.J. Persson, M. Tüshaus, A.M. Bradshaw, J. Chem.
Phys. 92 (1990) 5034.
[21] I. Zasada, M.A. VanHove, G.A. Somorjai, Surf. Sci. Lett.
418 (1998) L89.
[22] D. Loffreda, D. Simon, P. Sautet, Chem. Phys. Lett. 291(1998) 15.
[23] M. Stichler, D. Menzel, Surf. Sci. 391 (1997) 47.
[24] S.A. Buntin, L.J. Richter, D.S. King, R.R. Cavanagh,J. Chem. Phys. 91 (1989) 6429.