Adsorption of CO on the unreconstructed and reconstructed Ir(100) surface

Adsorption of CO on the unreconstructed and reconstructed Ir(100) surface

Surface Science 245 (1991) 65-71 North-IIolland 65 Adsorption of CO on the unreconstructed Ir( 100) surface and reconstructed G. Kisters, J.G. Che...

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Surface Science 245 (1991) 65-71 North-IIolland

65

Adsorption of CO on the unreconstructed Ir( 100) surface

and reconstructed

G. Kisters, J.G. Chen ‘, S. Lehwald and H. Ibach Institut ftir Grenzfi’iichenforschung und Vakuumphysik, Received

19 September

1990; accepted

for publication

Forschungszentrum 5 November

Jiilich, Postfach 1913, W-51 70 Jiilich, Germany

1990

The adsorption of CO on both the (1 x 1) and the (5 X 1) reconstructed Ir(lOO) surface at room temperature has been investigated using electron energy loss spectroscopy (EELS) and LEED. CO is adsorbed on both surfaces at all coverages in on-top sites. All four vibrational modes of the adsorbate have been detected. Upon adsorption of CO on the (5 X 1) surface the reconstruction is locally lifted giving rise to a (1 x 1) LEED pattern. Adsorption of CO on the (1 X 1) surface leads to a c(2 X 2) structure. The vibrational frequencies of the CO-molecules on both surfaces differ only slightly. At saturation the iridium-CO and the C-O stretching frequencies are 485 and 2075 cm-’ on the (5 X 1) and 497 and 2068 cm-r on the (1 X 1) surface, respectively. The frequency of the rotational mode of the CO molecule is found to be 425 cm-’ and the frustrated translation at 53 cm-‘, both showing no dispersion along PM direction. The C-O stretching vibration shows dispersion due to dipole-dipole interaction, also when the overlayer is not ordered.

From previous investigations it is well known that the Ir(100) surface undergoes an (1 X 1) --f (5 x 1) surface reconstruction and that just like on the Pt and Au (100) surface the top Ir atom layer is nearly hexagonally close packed [l-3]. The hexagonal layer fits to the substrate in “bridge registry” with a buckling of 0.5 A [2]. Stable and atomically clean surfaces of both Ir(lOO)-(1 x 1) and Ir(lOO)-(5 x 1) can be obtained by controlling experimental conditions [2-51. An investigation of the surface phonon dispersion on both surfaces showed that a large amount of surface stress is involved on the (1 x 1) surface and that the surface stress is the driving force for the reconstruction [51. Similar to the Pt(lOO) surface, where a different activity with respect to hydrogen and oxygen chemisorption is reported for the reconstructed ’ Present address: Exxon Research 08801, USA.

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and unreconstructed surface, respectively [t;], also for the Ir(lO0) surface, different adsorption characteristics have been found for the two structures concerning adsorption of oxygen and other gases [3,4]. For CO adsorption on Ir(lOO), UPS spectra were reported after a - 7 L CO dose on both surface structures which indicate molecular adsorption and look quite similar for both surfaces [4]. From this it was concluded that there is essentially no difference in the reactivity of CO on Ir(lOO)-(1 x 1) and on (5 X 1) [4], although adsorption of CO on the (5 X 1) surface gives rise to a structural change from (5 X I) to a (1 X 1) LEED pattern [7]. Vibrational analysis of CO adsorption on Iridium was so far only reported for the fr(ll0) and (111) surfaces [S]. On both faces CO is molecularly adsorbed for all coverages in on-top site. At saturation the iridium-CO and the C-O stretching frequencies were found to be 490 and 2075 cm-l on the (110) and 490 and 2050 cm- ’ on the Ir( 111) surface, respectively [ 81. In this paper we report an EELS investigation of the CO vibrations on both the Ir(lOO)-(1 X 1) and the (5 X 1) surface. The results confirm earlier

B.V. (North-Holland)

66

G. Kisters et al. / CO adwrpiion

on the Ir(IOO)-(I x I) and the (5 x I) surface

findings and reveal that also on the Ir(lOO) surfaces CO is adsorbed in on-top sites only. The disper-sion of the CO modes is measured along TM direction.

2. Experimental

details

The experiments were carried out in an UHV chamber equipped with a double-pass electron energy loss spectrometer (EELS), a 3-grid LEED optics, an Auger electron spectrometer, and a mass spectrometer. The base pressure of the chamber was in the 2 X lo-” mbar range. The EELS spectrometer contained space charge corrected monochromators [9] and a computer-controled voltage supply which provides high stability and a low noise level ( < 0.2 meV). This enabled us to obtain EELS spectra with a resolution (FWHM) as good as 7.9 cm-’ (0.98 mev) in the elastic beam. A sample spectrum for this high resolution, recorded on an Ir(lOO)-(1 X 1) surface exposed to 16 L (1 langmuir = 1 X 10e6 Torr . s) CO at 300 K, is shown in fig. 1. The spectrum is recorded in steps of 2cm-’ with a sampling time of 3 s per channel. The count rate is - 7 x lo4 counts/s in the elastic Energy 188

50

-58

channel and - 150 counts/s in the loss peaks. In the other EELS spectra reported here the resolution was normally set within the range of 15-25 cm-’ to gain intensity especially in the off-specular spectra. The electron impact energies were in the range of 5-50 eV. The scattering plane was -aligned parallel to the [OOl] (I?M) azimuth of the sample. The Ir(lOO) surface was cut and polished to within 0.5” of the desired orientation. The crystal was initially cleaned by cycles of Ne ion sputtering at 300 K for 5 min followed by annealing at 1400 K for 5 min. The final stages of cleaning involved exposure of the surface to - 10 L of oxygen at 1200-1300 K followed by annealing for 1 min at 1400 K. After these cleaning procedures the surface was free of oxygen and the impurity level of carbon was less than 1% near the surface region according to AES. The cleanliness of the surface was also assured by EELS. The Ir(100) surface obtained after the above procedures showed a sharp (5 x l)-LEED pattern indicating the surface reconstruction. The preparation of a clean Ir(lOO)-(1 x 1) unreconstructed surface has been described previously [2-51. The procedure used was the following: a Loss CmeV) 158 280

258

388

2

u-lh)

l

-590

9

IElm

50e Energy

Loss

ISa3

2868

2588

(l/cm)

Fig. 1. High resolution EELS spectrum of an Ir(lOO)-(1 X 1) surface exposed to 16 L CO at 300 K. The electron eV, the specular scattering angle 62”. The energy resolution (FWHM) is 0.98 meV = 7.9 cm-‘.

impact

energy

is 5

61

G. Kisters et al. / CO arlsorpiion on the Ir(loO)-(I x I) and the (5 x I) surface

clean (5 x 1) reconstructed surface was exposed to 45 L of 0, at - 450 K followed by a slow heating of the crystal to 740 K. This resulted in an oxygen-covered surface with an Ir(lOO)-(1 x 1)-O LEED pattern. The oxygen was then removed by exposing to 5 L of H, at - 525 K followed by a slow heating to 700 K. After the above chemical treatment a clean, unreconstructed Ir(lOO) surface characterized by a sharp (1 X l)-LEED pattern was routinely obtained. The surface was found to be free of oxygen and the impurity level of carbon was less than 1% near the surface region according to AES. The cleanliness again was assured by EELS. Only spectra with a very high magnification of about lo4 compared to the elastic beam indicated traces of oxygen as identified by a small vibrational loss at - 540 cm-‘. Occasionally small dipole active losses were observed around 125 and 195 cm-‘, respectively, on both clean surfaces, which were probably caused by steps or defects. The unreconstructed (1 X 1) surface is stable up to 700 K. Heating the (1 X 1) surface to 1400 K gave

rise to the thermally irreversible (5 X 1) reconstruction of the surface. The dosing of the sample with CO and other gases was performed with a calibrated system. The amount of the gas to which the surface was exposed was controlled by measuring the gas pressure in a small calibrated volume using a spinning rotor gauge. The surface was exposed to the gas by opening a valve to a tube directed towards the sample with the surface positioned a few millimeters in front of the open end of the tube. The amount of gas in the volume was converted to a dose in langmuir by assuming that all gas molecules hit the surface. The valve towards the tube was opened stepwise so that the pressure in the UHV-chamber did not exceed 1.5 x 10-‘” mbar.

3. Results and discussion

Electron energy loss spectra have been recorded (in specular direction) after exposing both the (1 X 1) and the (5 X 1) surface to increasing

a

Ir(lOO)-(1x1) + CO

,a0

E,=5ev 9,. Bt~69'

b

Ir(lOO)-(5x1)+ co E,=SeV 8;+69" 485

T .300K

2o176

I

I

16 L

20,27

0

1000

2027

2000

Energy Loss km-')

Energy Loss (cm-')

Fig. 2. EELS spectra recorded in specular direction after exposing (a) the Ir(lOO)-(1 X 1), (b) the Ir(lOO)-(5 X 1) surface to 0.5 and 16 L of CO, respectively, at room temperature. The energy resolution is set to 20 cm-‘. The spectra are recorded in 5 cm-’ intervals with 2 s sampling time per channel.

68

G. Kisters et al. / CO adsorption on the Ir(IOO)-(1 x I) and the (5 x I) surface

amounts of CO at room temperature. Two sample spectra at very low and at saturation coverage on each surface are shown in fig. 2. They exhibit two loss-peaks caused by the dipole active Ir-CO and C-O stretching vibration, respectively. Thus CO is molecularly adsorbed, not tilted, and the C-O stretching frequency well above 2000 cm-’ indicates adsorption in on-top sites. Also in the coverage range between 0.5 and 16 L no additional losses are observed. In the spectrum of 16 L CO on the (1 X 1) surface a loss around 125 cm-’ appears. It is not an eigenmode of the CO-molecule, as will also be supported by the off-specular measurements. Exposing the (1 X 1) surface to 16 L of CO led to a c(2 x 2) LEED pattern as described later. For a c(2 x 2) structure on a fee (100) surface the M-point of the surface Brillouin-zone is folded back to the r-point, and therefore the Rayleigh mode S,(M) would be dipole allowed for CO adsorbed in on-top sites, as observed for Cu(lOO)-c(2 x 2) CO [lo]. The Rayleigh mode S, on the clean Ir(lOO)-(1 X 1) surface has been found to have a frequency of 132 cm-’ at M [5]. Taking into account the mass loading effect of the surface by the CO, the frequency would be expected to be around 125 cm-‘. A loss around 125 cm-’ has been observed, however, often also at coverages where no c(2 x 2) exists, also on the clean surfaces and also with adsorbed oxygen. Therefore the main contribution to this loss may be due to an iridium vibration caused by steps or defects which is intensity enhanced when decorated with an adsorbate. In the spectra of figs. 1 and 2 the intensity of the C-O stretching loss is equal or smaller than that of the Ir-CO loss. This is caused by the high resolution and the fact that the sharp focusing conditions of the lenses are not completely maintained with scanning. When the impact energy is raised (less sensitive focusing conditions) or the lens potentials are reoptimized on the C-O loss line, then its intensity is about 3 times the intensity of the Ir-CO loss. In fig. 3 the frequencies of the Ir-CO and the C-O stretching vibration on both surfaces are collected as a function of CO exposure. The frequency of the C-O stretching vibration is within the limit of experimental error nearly the same on

_ -L u

IdlOO)-(Sxl)+CO : q Ir(1001-(1x1)+ Co : x T=300K 0 900 x

2080-

2MO-

B 8

E L!J z

p

x

x

9 : x

x0 : 2040 e” q

2020 -

0

4

8

12

16

CO exposure (L) Fig. 3. Frequencies of the IF-CO and C-O stretching vibration on the Ir(lOO)-(1 Xl) and (5X1) surface as a function of CO exposure at room temperature.

both surfaces. It increases from 2027 to about 2070 cm-’ with coverage. The frequency of the Ir-CO stretch is about 480 cm-’ on the (5 X 1) surface and about 490 cm-’ when CO is adsorbed on the (1 x 1) surface. The frequencies are in the same range as those reported for CO adsorption on the Ir(ll0) and (111) surfaces [8]. When the reconstructed (5 X 1) surface was exposed to increasing amounts of CO the (5 X 1) LEED pattern changed gradually to a (1 X 1) LEED pattern, often with faint streaks in (01) and (10) direction. The (1 x 1) LEED pattern was obtained after an exposure of - 15 L. Upon heating the sample to 700 K the CO desorbed around 650 K and the (5 x 1) LEED pattern reappeared, as also has been observed previously [7]. This and the identical CO frequencies observed on both surfaces indicate that the reconstruction is locally liftet upon CO adsorpotion. Exposing the clean unreconstructed (1 X 1) surface up to 16 L or more of CO in front of the open-end tube mostly led to a c(2 x 2) LEED pattern. Frequently, however, depending on how rapidly the valve to the tube had been opened, which determines the “local pres-

G. Kisters et al. / CO adsorption on the Ir(IOO)-(I x 1) and the (5 x I) surface

sure” in front of the surface, a (1 X 1) LEED pattern was obtained. The (1 x 1) LEED pattern, however, was not stable. After l-2 h in vacuum or within 1-2 min under the electron beam of the LEED gun (100-200 eV) a c(2 x 2) LEED pattern appeared. Heating the sample to 700 K desorbed the CO and the (1 x 1) LEED pattern reappeared. The c(2 x 2) LEED pattern obtained with CO on the unreconstructed (1 X 1) surface and the (1 X 1) LEED pattern obtained with CO on the reconstructed (5 x 1) surface were stable with time. Prolonged electron bombardment caused in both cases desorption and also dissociation of CO as indicated by carbon left on the surface after heating to 700 K. The CO coverage on the surface in case of the c(2 x 2) LEED pattern on the unreconstructed surface and in case of the (1 x 1) LEED pattern on the reconstructed surface seems to be 8 = 0.5. This is estimated by comparing the Auger peak to peak ratios of the carbon 272 eV and the Ir 229 eV line to the same ratio on a c(2 X 2) structure obtained by diffusing carbon from the bulk to the surface. This is a somewhat rough estimate only: When an Auger spectrum is recorded over the range of the carbon line directly after the electron beam has been directed on the sample, the spectrum exhibits the well known 272 eV carbon line and in addition a more intense line at 264 eV, which has also been reported on Ir(ll1) [ll]. Within l-2 min the 264 eV line disappears, however, and only the 272 eV line is left and stays stable. This suggests that in the spot area of the electron beam (2.5 kV) the CO is partially dissociated and partially desorbed as indicated by the intensity decrease of the oxygen Auger-line. Both amounts have been included in the estimation of the CO coverage being about t9 = 0.5. Exposing the (5 x 1) surface to 2 16 L of CO only occasionally led to very faint c(2 x 2) extra spots in addition to the (1 X 1) spots. This indicates that long range order is hindered on the surface with locally lifted reconstruction. This is supported by the observation that the diffuse elastic intensity as measured off-specular by EELS was about 5 times higher on the (5 x 1) + CO than on the (1 x 1) + CO surface.

69

EELS spectra recorded in specular direction on the unreconstructed surface having been exposed to 16 L CO exhibited the same Ir-CO and C-O stretching frequencies, respectively, independent whether the LEED pattern was c(2 X 2) or (1 X 1). We did not investigate whether at higher ambient CO pressures more CO could be adsorbed on the Ir(lOO) surface giving rise to more dense layers and further LEED structures as has been observed for Pt(lOO)-(1 x 1) [12] and other fee (100) surfaces

v31.

The frequencies of the two dipole active CO stretching vibrations suggest adsorption in on-top sites on Ir(100). To ensure this we recorded offspecular spectra to reveal the other CO normal modes, which are not totally symmetric in this adsorption site and therefore observable only offspecular. These are the CO rotational mode, the frequency of which is expected to be slightly below the Ir-CO stretching vibration and the frustrated translation of the CO molecule, the frequency of which is expected to be below 100 cm-’ [13-151. The off-specular spectra were recorded with the scattering plane aligned-- along [OOl] direction, i.e. for wavevectors along rM direction. Sets of many spectra at different impact energies and scattering angles were recorded to tune into high cross sections for the different modes. Spectra were taken on the CO saturated unreconstructed (1 x 1) surface exhibiting both a c(2 X 2) or a (1 X 1) LEED pattern and also on the CO saturated reconstructed (5 x 1) surface. A sample spectrum recorded on the unreconstructed surface, which reveals all carbon monoxide modes at the same scattering conditions is shown in fig. 4. The loss at 54 cm-’ is assigned to the frustrated translational mode, 425 cm-’ to the rotational mode, 487 cm-’ is the Ir-CO stretch and 2005 cm-’ the C-O stretching vibration, in complete agreement with an on-top adsorption site [14,15]. The frequencies obtained at different wave vectors along TM are collected in the dispersion curves of fig. 5. The Ir-CO stretching mode shows only little dispersion from 495 to about 480 cm-’ and the rotational CO mode around 425 cm-’ shows nearly no dispersion as has also been observed for CO on Ni(ll0) [15]. Also the translational CO mode

G. Kisters et al. / CO adsorption on the Ir(IOO)-(1 x I) and the (5 x I) surface

70

4

IrllOO)-llxll +16L CO

-500

0

I

1000

2000

Energy Loss km-'1 Fig. 4. EELS FM on the T= 300 K; sampled

spectrum recorded off-specular at 3 = 0.6 along Ir(lOO)-(1 X 1) surface dosed with 16 L CO at E, =15 eV, AE = 25 cm-‘. The spectrum is at 5 cm ’ intervals with 5 s sampling time.

Iri1001-l5xl~+l6LCO: q Ir(lOO)-(lx1)+16LCO: x 2100 I “xi 2050

‘i

2000

t

T

; kl

.n E z F

500

x

450

%

x

I

400

T

60

x

20 I

0

I

0.2

0.4

I

0.6

F

I

0.8

1 1.0 fi

<= 0,,11.64 A-' Fig. 5. Disperion curves along TM for the saturated Ir(lOO)-CO surface. CO is adsorbed in on-top site, the mode around 52 cm-’ is the CO-translational mode, that around 425 cm-’ is the rotational mode.

around cm shows no dispersion on Ir(100) whereas in the compressed (2 x l)-CO structure on Ni(ll0) this mode showed strong dispersion due to direct CO-CO interaction [15]. For on-top CO of the non-compressed c(4 x 2) overlayer on Pt(ll1) a translational CO mode around 56 cm-’ and showing also no dispersion is reported from He-atom scattering [16] in agreement with our findings on Ir(lOO). The C-O stretching vibration shows a rather large dispersion from about 2070 cm-’ down to about 2000 cm-’ around l= 0.6 within the Brillouin-zone, because this vibration has a large dipole moment and its dispersion has been shown to be caused by dipole-dipole interaction [15,17]. For the C-O stretching vibration we have collected dispersion data in fig. 5 which were obtained on saturated CO overlayers on the reconstructed (5 X 1) surface showing a (1 X 1) LEED pattern (squares) and which were obtained on CO overlayers on the unreconstructed Ir(lOO)-(1 x 1) surface exhibiting a (1 x 1) or a c(2 X 2) LEED pattern (crosses). The data (0) show that because of the long range dipole-dipole interaction the C-O stretch shows dispersion also when the overlayer is not ordered but sufficiently densely packed. In a dilute disordered layer the C-O stretch vibration showed no dispersion [17]. For a c(2 x 2) overlayer the M point is folded back to the r point and hence the C-O stretching frequency should be identical at M and r. Fig. 5 shows that on a well ordered c(2 x 2) overlayer we really measured a frequency of about 2065 cm-’ at u. For apparently not so well ordered c(2 X 2) structures or (1 X 1) overlayers which changed during the time of recording the off-specular spectra into a c(2 x 2) layer, fig. 5 shows that the observed frequencies go upwards within the second half of the Brillouin zone but only until - 2045 cm-’ at a. The basic theory of dipole interactions in adsorbed molecular layers has been reviewed by Willis et al. [18]. The dispersion relation w(q,,) due to dipole interaction has been evaluated for an ordered monolayer. For a partially filled monolayer or taking disorder into account the frequency shift with coverage or isotopic mixing has been calculated only for q,, = 0 [19-221. The influence

G. Kisters et al. / CO acisorption on the Ir(IOO)-(I x 1) and the (5 x I) surface

of dipole interaction on the disperion in case of disorder has not yet been explicitly treated. Our measurements show that there is dispersion and on that the ~c_~ loss peak observed off-specular nonordered CO overlayers was always broadened with a high-frequency shoulder.

4. Conclusions The vibrational analysis of CO adsorption on the unreconstructed (1 X 1) and the reconstructed (5 x 1) Ir(100) surface using EELS revealed, that CO is adsorbed on both surfaces at all coverages in on-top sites only. On the (5 x 1) surface the reconstruction is locally lifted upon CO adsorption, on the (1 X 1) surface a c(2 X 2) overlayer is observed. On both surfaces the CO coverage is about 8 = 0.5. -- By in- and off-specular measurements (along I?M) all four normal modes of the adsorbed CO could be detected. The frequencies differ only slightly on both surfaces. The Ir-CO and the C-O stretching vibrations are found at 485 and 2075 cm-’ on the (5 x 1) and at 497 and 2068 cm-’ on the (1 X 1) surface, respectively. The rotational mode is found to be 425 cm-’ and the frustrated translational mode at 53 cm on both surfaces. Both latter modes are practically dispersionless in the noncompressed overlayers. The C-O stretching vibration, the dispersion of which is caused by dipole-dipole interaction, shows a dispersion downwards by about 70 cm-‘. This is observed both in ordered and in non-ordered overlayers.

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