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Face specificity of the H2O adsorption and decomposition on Co surfaces — a LEED, UPS, sp and TPD study
590
Surface Science 117 (1982) 590-604 North-Holland Publishing Company
FACE SPECIFICITY OF THE H,O ADSORPTION AND DECOMPOSITION ON Co SURFACES - A LEED, UPS, sp AND TPD STUDY J.M. HERAS
*, II. PAPP ** and W. SPIESS
Institut fiir P&vsikuli.whe und Theoretische Chemie, Ciniversitijt Erlungen-Niirnherg
Erluqen.
Fed.
Rep. of Germany Received
14 September
1981; accepted
for publication
3 November
1981
The adsorption and decomposition of H,O has been investigated on three different Co surfaces (Co(OOOl), Co( I 120) and a “polycrystalline” Co(OOO1) surface) by LEED. UPS, sp and TPD studies, At about 100 K H,O is only molecularly adsorbed on all three surfaces with a maximum increase in surface potential of 1.2 V. The three valence orbitals of adsorbed H,O show a shift of binding energy dependent on the workfunction of the clean crystals. Multilayer adsorption is observed with UPS, No ordered overlayer can be detected at low temperatures. TPD experiments show that H,O is totally reversibly adsorbed on Co(Oo1) below 298 K. On Co{ 1120) and on “poiycrystalline” Co(OOO1) H,O starts already to decompose at 210K. On Co(OOO1) Hz0 decomposes above 350 K, but the rate of decomposition is much smaller than on Co(ll20).
There exist many different incentives to investigate the interaction of H,O with transition metal surfaces. The adsorption of H,O is a good example to study the transition from physisorption to multilayer adsorption at low temperatures and to study the transition from physisorption to decomposition at higher temperatures. Apart from these more theoretical considerations, the investigation of the interaction of H,O with metal surfaces is of course of great practical interest. The phenomena of corrosion and passivation of metal surfaces are naturally connected with water interaction. Knowledge about the interlayer water/metal surface is fundamental for electrochemistry. H,O is a common conta~nant in many catalytic reactions. These are the main reasons why the H,O interaction with metal surfaces was investigated with many different methods in recent years ]l- 121. The system Co/H,0 is of special interest since H,O is a reaction product during the Fischer-Tropsch synthesis on Co catalysts [13]. Heras et al. [3,5,6] * INIFTA, ** Lehrstuhl
Universidad Nacionai de La Plats, Argentina. fiir Technische Chemie, Ruhr-Universitat Bochum,
0039~6028/82/0000-0000/$02.75
Fed. Rep. of Germany.
0 1982 North-Holland
J. M. Hem et al. / Face specificity of H_,O adsorptim
591
have investigated the adsorption and decomposition of H,O on polycrystalline Co films with workfunction and resistivity measurements and thermal desorption mass spectrometry. They found that the surface topography of the Co films had a strong influence on the adsorption behaviour of H,O. The surface topography was altered by different annealing temperatures. On unsintered Co films, with low workfunction, H,O decomposes more readily than on sintered Co films with higher workfunction. X-ray diffraction studies on Co films [17] and LEED investigations of a Co foil [16] show that sintering of the polycrystalline samples at T -=c700 K produces only the hexagonal form of Co and an abundance of the basal plane in the surface. The above mentioned results were the basis of our investigations. In order to get a deeper insight into the dependence on surface topography of Co, we decided to investigate the interaction of H,O with a perfect Co(OOO1) surface, a Co(OOO1) surface roughened by Xe ion bombardment and a Co( 1120) surface representative for a surface with low workfunction. The Hz0 adsorption was followed by LEED, UPS, AES, TDS and sp measurements. 2. Experimental A detailed description of the experimental set up will be given elsewhere [14]. Only a short summary is given here. The main system consisted of a “Vacuum Generators” ion pumped LEED/AES apparatus with a four grid retarding field analyser. A base pressure of 5 X 10 -9 Pa was readily obtained. A quadrupole mass spectrometer allowed a residual gas analysis and a check on the purity of the gases used. A vibrating capacitor with a stainless steel gauze as reference electrode was used for surface potential measurements. For UPS studies a homebuilt microwave driven gas discharge UV lamp was added at an angle of 70” to the normal of the crystal. The Co(OOO1) and Co(l120) surfaces were cut from a single crystal bar received from “Materials Research Corporation”. The surface orientation was checked by Laue diffraction to be within f l/2’ of the desired orientation. Before mounting on a coolable crystal manipulator the crystals were mechanically polished. The Co single crystal surfaces were initially contaminated by Cl, S, N, and 0. They were usually cleaned by Xe ion bombardment at about 550 K and subsequently annealed at the same temperature. This temperature is well below the transition point of hcp Co into fee Co at about 700 K [ 151. A repeated transition through this point would lead to a polycrystalline Co sample [16]. The described cleaning cycle had to be repeated very often in order to get a reproducibly clean surface. The water was cleaned by a repeated distillation in vacuum before it was added via a leak valve. The mass spectrometric analysis showed that it was contaminated only by varying amounts of H,. This H, is most probably produced either by a decomposition of H,O on the metal parts of the system or by a displacement of H, by H,O from the pumps.
592
3. Results 3.1. Photoelectron
spectra
3.1.1. Adsorption temperature 100 to 120 K The first three figures show He I UP spectra of the adsorption of H,O at about 100 K on “perfect” Co(OOO1) (fig. l), on Co(OOO1) roughened by Xe ion bombardment at 298 K * (fig. 2, here the difference spectra are shown), and on Co( 1120) (fig. 3). Common to all three surfaces is a strong decrease in d-band emission and a growth of three additional emission peaks due to the increasing amount of adsorbed H,O. In fig. 2 a fourth maximum at about 16 eV is due to the shift of the onset of the secondary emission to a higher binding energy created by the increase in surface potential. Up to a H,O admission of 3 L the H,O peaks remain at constant binding energy on all three surfaces. Above a H,O exposure of 3 L a shift of the emission maxima to higher binding energy is observed. The energetic position, however, of the peaks due to water adsorption below 3 L is different on the three surfaces. In table 1 the binding energies of the H,O peaks at low coverages (G 3 L H,O) and higher coverages ( 16 to 8 L H,O) are reproduced together with the literature values for the valence orbitals of gaseous H,O. The assignment for gaseous H,O can also be applied to the adsorbed species. At low coverages a significant increase of the binding energy values is observed from “perfect” Co(OOO1) to Co(l120) (the accuracy is i-O.1 eV). The energetic position at about 6 to 8 L H,O is less dependent on the substrate. At high H,O doses (13.5 L HzO, cf. curve4 of fig. 3) a further shift is observed. At the same time the d-band of the Co substrate has almost vanished, indicative of a thick H,O layer. This high intensity of the photoemission peaks shows that the photon yield is much higher for a H,O layer than for the clean Co substrate. On all three surfaces the onset of the secondary electron emission is shifted by about 1.2 eV to higher binding energy. This indicates that the surface potential of the substrate increases by this amount. This shift is completed at about 2.6 L H,O admission. 3.1.2. Adsorption temperature 298 K No change in the UP spectrum of “perfect” Co(OOO1) is observed due to the admission of H,O at 298 K. Obviously no H,O is adsorbed at this temperature, which is also verified by sp measurements (see below). However, admission of H,O to Co( 1120) at 298 K changes the UP spectrum of the surface. This is shown in fig. 4. After a short exposure of H,O a single broad peak appears at about 6.1 eV (cf. fig. 4). 9.9 L water creates an increase of the 6.1 eV peak; a small peak at about 10 eV can also be observed. At a constant PH,O of about 6 X 10 -5 Pa these two peaks have grown considerably * This surface
will be called “polycqstalline”
Co(OOO1) in the following
593
15 Aev
10
5
%
-
Fig. I. He I UP spectra
of the Hz0
adsorption
on a “perfect”
Co(OOO1) surface
at 100 K.
in intensity. In the difference spectrum an additional small peak at about 13.5 eV can be detected. The peak at 6.1 eV is unsymmetric due to the overlap of a second peak at higher binding energy (cf. curve 4 of fig. 4). Curve 5 in fig. 4 represents the UP spectrum after a reduction in H,O pressure. The 10 eV peak and the shoulder at about 7 eV have almost vanished. On the “polycrystalline” Co(OOO1) surface the same effects as on Co(l120) can be observed. 3.1.3. Adsorption temperature 350 K Fig. 5 shows the He I UP spectra of clean “perfect” Co(OOO1) (curve l), of Co(OOO1) after admission of 617 L H,O (curve2) and after an admission of 1037 L Hz0 (curve3). The d-band emission decreases only slightly. At a binding energy of about 6 eV a peak is apparent due to the admission of H,O. The onset of the secondary electron cascade is shifted by about 0.5 eV to lower binding energy, indicating that the surface potential decreases by this amount. Similar results are obtained on Co( 1120) at 413 and 523 K.
J.M.
Heras et al. / Fctce specificity of h’,O udsorption
Fig, 2. He I UP difference spectra of the adsorption of Hz0 at 117 K on a "polyqstalhe" surface: (I) 0.9L HzO; (2) 1.5 L H,O; (3) 2.1 L H,O; (4) 2.7 L H,O: (5) 4.7 L Hz0
CO(OCOI)
(6)
6.7 L H,O.
3.2. Change
in surface potential
Fig. 6 shows the change in surface potential of the three single crystal surfaces and of a polycrystalline Co film (sintered at 478 K) due to the admission of water at 100 K (for the single crystals) and at 77 K (for the film).
5
tF
Fig. 3. He I UP spectra of the adsorption of H,O on a Co(Il?iO) surface at 100 K: surface; (2) 1.5 L H,O; (3) 7.5 L H,O; (4) 13.5 L H,O.
(1) clean
J. M. Hems et ul. / Face specificicv of H,O adsorption
595
Fig. 4. Influence of H,O admission on the He I UP spectrum of Co( 1120) at 298 K: (1) clean surface; (2) 2.4 L H,O; (3) 9.9 L H,O; (4) PH,O =6X IO-5 Pa; (5) PHZO=5X10-’ Pa.
15
5,
eV
Fig. 5. He I UP spectra surface: (2) 617 L H,O;
of H2Q admission (3) 1037 L H,O.
io a “perfect”
Co(0001)
surface
at 486 K: (I) clean
596
2
0
L
6
8
‘0 lb-
1
OJ 0
1
2
3
4
5
6 *
fig. 6. Change in surface potential due to the adsorption (O), on “polycrystalline” Co(OOO1) ( k ). on Co( 1120) film sintered at 478 K (~ ).
of H,O
at 100 K on “perfect”
Co(OOO1) Co
(A). and at 77 K on a polycrystalline
The preparation of the Co films is described in a paper by Heras et al. [3]. The maximum increase in surface potential is 1.20 k 0.02 V on all three surfaces. The differences in the initial slopes of the sp curves are caused by different measuring conditions: H,O was dose wise admitted to Co(l120), and the observed increase in sp as a function of H,O exposure compares well with that observed with UPS. On both Co(OOO1) surfaces the change in sp was investigated with a constant H,O pressure. This can simulate a low sticking coefficient because of a shadowing effect of the reference electrode and an increase in the H, contamination with increasing presence of H,O in the chamber (decomposition and evolution of Hz). The change in sp of both Co(OOO1) surfaces due to a dose wise admission of H,O measured with UPS is identical to that on Co( 1120); i.e. after 2.6 L H,O an increase of 1.2 t 0.1 V has been observed. No change in sp due to H,O admission is observed on “perfect” Co(OOO1) at room temperature. Fig. 7 shows the change in sp as a function of H ,O admission to Co( 1120) at 298 K. At PnO = 1.3 X 10 m6 Pa a fast increase in sp of about 30 mV is followed by a decrease in sp of about 40 mV. A reduction of PHzO creates a further decrease of 10 mV. A renewed H,O admission ( PHIO = 1.3 X 10 -’ Pa) followed by pressure reduction leads to a similar behaviour, but with a smaller amplitude. When PHzO increases, a further fast increase of sp with a subsequent slow decrease 1s observed (cf. fig. 7). A decrease in sp due to H,O exposure to Co(OOO1) and Co( 1120) is
J. M. Herus er al. / Face specificity of NJ0 udsorption
Fig. 7. Change time.
in surface
potential
due to H,O
admission
597
on Co( 1120) at 298 K as a function
of
observed at higher temperatures (cf. fig. 8). After about 130 L a saturation value of about -0.45 V is obtained, independent of temperature and surface. The slope of the initial change in sp of Co( 1120) increases with growing temperature. For Co(OOO1) the slope is much smaller than on Co(1 120).
-xw-
523K
df -4m
&‘8K -.L23K -
-
I -3&T-
Fig. 8. Change
-~-A~-.co(awl)
.
-*--
) co md
/------‘_
in sp of Co(WOl)
at 535 K and of Co( I 120) at 423 and 498 K due to H,O
exposure.
3.3. Thermal desorption studies Fig. 9 shows the decrease in sp due to the heating of H,O covered Co surfaces (heating rate approximately 3 to 5 K/min). For the Co(OOOl)/H,O system a steap decrease in sp is observed at about 155 K (within 10 K the surface potential decreases by 0.9 V). At about 250 K the sp value of the clean surface is obtained. No further change is observed. For the system “polycryslalline” Co(OOOl)/H,O the steep decrease in sp starts at 155 K and a change of -0.9 V is reached at 230 K, i.e. at clearly higher temperature (70K) than on “perfect” Co(OOO1). Then the sp stays constant up to 298 K. Above this temperature the change in sp is not drawn, since it is then influence by water interaction and the annealing of the “polycrystalline” Co(OOO1) sample. The low temperature part of the Co( 1 lqO)/H,O system is very similar to that of the ‘“polycrystalline” Co(OOO1) sample. The steep decrease is only shifted by about 5 K, a change of -0.9 V is reached at 245 K. After a plateau a further decrease is obtained at higher temperatures. At 305 K the surface potential of the clean surface is reached and at 390 K a sp value of 0.3 V below that of the clean surface is observed. The curve of the polycrystalline Co film exhibits a different behaviour. A decrease in sp of about 0.2 V between 100 and 130 K is followed by a plateau. Then the surface potential decreases continuously till it reaches at 400 K a value of 0.2 V below the sp of the clean film. The arrows of fig. 9 indicate the
-1.4 -
e-12.
!
-1.0. -0.8.
-0.6.
c
0’
-
Fig. 9. Change “polycrystalline” 478 K.
in sp due to heating of H,O Co(OOO1); (A) Co(lIZ0);
covered ( ------)
K
Co surfaces: (0) “perfect” Co(0001); (X) polycrystalline Co film annealed at
599
J. M. Hems et al. / Face specifici(v of H20 adsorption
15
r,eV --. Fig. 10. Effect of thermal desorption on the He I UP spectrum of a Hz0 covered “polycrystalline” Co(OOO1) surface: (1) clean surface at 116 K; (2) surface after 2.7 L Ha0 exposure at I16 K: (3) after warming to 208 K; (4) after warming to 297 K.
mass spectrometrically measured maximum rates of H,O desorption at 245 K and of H, desorption at 300 K from a H,O covered Co film. Fig. 10 shows the change in the He1 UP spectrum of a H,O covered “polycrystalline” Co(OOO1) surface during thermal desorption. Curve 1 represents the spectrum of the clean surface. Curve2 shows the spectrum after an admission of 2.7 L H,O at 116 K. The three H,O induced emission peaks have a binding energy of 7.0, 9.6 and 13.4 eV (cf. table 1). At a crystal temperature of 208 K the intensity of the additional emission maxima is drastically reduced and the low binding energy peak has been shifted to about 6 eV (cf. curve 3 of fig. 10). At 297 K only the 6 eV peak with reduced intensity remains visible (cf. curve4 of fig. 10). For the system Co(l lZO)/H,O similar results are obtained. For “perfect” Co(OOOl), however, only a reduction of the H,O induced peaks is observed
Table 1 Binding energy
in eV of the Hz0
Co(OOO1) perfect Co(OOO1) “polycrystalline” Co(ll~0) Gaseous Ha0 [ 181 Orbital assignment
induced
emission
G3 L Ha0 8 LH,O G3 L Ha0 6.7 L Ha0 <3LH,O 7.5 L Ha0
maxima 6.9 7.6 7.0 7.6 7.3 7.8 12.6
9.3 10.6 9.6 10.7 9.9 10.8 14.7
‘b,
3at
13.0 13.6 13.4 13.7 13.5 13.8 18.3 lb,
J. M. Herus et ul. / Fuce specifiat_v
600
A
A
A
0
D
0
A
A
A
0
H,O udsorption
a: A
A
A
A
0
0
q
0
0
0
A
A
A
A
A
A
0
of
q
Cl
0
0
0
OAAOAAO Fig.
I I. Schematic representation
during thermal about 250 K.
desorption
of the LEED pattern
and the spectrum
after H20
exposure
on Co( 1 IjO) at 4X0 K.
of the clean surface is obtained
at
3.4. LEED results No additional overlayer spots can be observed due to the adsorption of H,O on the three Co surfaces at low temperatures. Only a reduction in the intensity of the substrate pattern and an increase in the background intensity is obtained. At H,O exposures > 6 L the substrate spots have completely vanished. The LEED pattern of the substrate is restored, when the H,O covered crystals are warmed to room temperature. At temperatures above 400 K additional overlayer spots can be observed due to the interaction of H,O with Co(l120). At low exposures (up to 10 L H,O) a (4 X 1) overlayer structure appears and at higher exposures the LEED pattern of fig. 11 is obtained.
4. Discussion 4.1. Adsorption temperature 100 to 120 K First the region up to a H,O admission of about 3 L will be discussed. The three additional H,O peaks in the UP spectra (cf. figs. l-3) with binding energy values independent of coverage are created by the three valence orbitals of H,O (lb,, 3a, and lb,). This speaks for a molecular adsorption of H,O on the three Co surfaces at low temperatures. The distance in energy of the orbital emission in the adsorbed state is approximately the same as in the gaseous
J.M. Hews et (11./ Fuce specificit,: of H,O adsorption
601
=3.6 to 3.8 eV; AE(3a, - lb,),d =2.4 to 2.6 eV; -3a,),, = 3.6 eV [18]; AE(3a, - lb,),,, = 2.1 eV [IS]). This indicates AE(lb, -3a,),,, a weak interaction between the surface and the H,O molecules adsorbed. The three valence orbitals have different binding energy values on the three surfaces (cf. table 1). The binding energies measured in the UP spectra are referred to the Fermi level. Fig. 12 shows the UP spectra of the three clean Co surfaces. The secondary electron onset indicates that the “perfect” Co(OOO1) surface has the highest workfunction, that the “polycrystalline” Co(OOO1) surface has a workfunction which is about 0.7 eV lower, and that the Co( 1120) has the lowest workfunction. When it is assumed that the binding energy values of the H,O orbitals are referred to the vacuum level, then low E, values should be observed for the adsorption of H,O on a surface with high workfunction and vice versa. This is exactly what we find (cf. table 1). The obtained shift in binding energy is, however, smaller as the change in workfunction. This can be due to different initial state binding shifts because of different adsorption energies and due to different final state relaxation energy shifts on the three surfaces. A dependence of E, of orbitals of adsorbed species on the workfunction of the substrate has already been observed for the physisorption of Xe on transition metal surfaces [19,20], and this effects has been explained in a similar way. On all three single crystal surfaces and on a Co film the sp increases by 1.2 V due to the adsorption of H,O. From this increase can be concluded that H,O is bound via the oxygen to the surface. A similar increase in sp due to H,O adsorption on other transition metal surfaces has been reported in the literature [3]. Now the region of H,O admisssion above 3 L will be discussed. Only a small change in sp is found, but the intensities of the valence orbitals of H,O grow considerably. At the same time the d-band emission has almost vanished, which indicates multilayer-adsorption. The E, values of the valence orbitals state
(AE(lb,
Fig. 12. He I UP spectrum of clean “perfect” and of clean Co( 1120) (3):
Co(OOO1) (I), of clean “polycrystalline”
Co(OOO1) (2),
.I. M. Herus et al. / Frrce specifrci[v of H,O alsorption
602
are shifted to higher binding energy. The shift of the 3a, orbital is more pronounced than that of the others (cf. table 1). The observed shift can be explained in the following way: up to about 3 L H,O a monolayer is formed. Here the final state relaxation energy shift between adsorbed and free molecules is mainly caused by the screening of the hole created by photoemission by the metal electrons. Above 3 L the multilayer formation starts and the final state relaxation energy shift is more determined by electrons of the surrounding H,O molecules, which produces a higher binding energy of the valence orbitals. The more pronounced shift of the 3a, orbital can be tentatively explained by the assumption that the energy of the 3a, orbital is responsible for the angle between the two O-H bonds. This angle grows because of hydrogen bridge bonding within the multilayer. UPS investigations of the H,O adsorption on Co films [8] and on other transition metal surfaces [7.21] at low temperatures show also three emission peaks due to the valence orbitals of molecularly adsorbed H,O. No coverage dependence, however, has been investigated. From the LEED results can be concluded that Hz0 is adsorbed without long range order on all three single crystal surfaces of Co at low temperatures. This is in contrast to LEED results of the adsorption and condensation of H,O on single crystal surfaces of other metals where at low temperatures ordered ice structures were found (e.g. Ru(0001) [4], Ag( 111) [lo] and Pt( 111) [ 12.211). 4.2. Adsorption
temperature
298 K
Strong face specificity is found at this temperature. On Co(OOO1) no adsorption of H,O can be observed with UPS and sp measurements. On Co(1120) and “polycrystalline” Co(OOO1) Hz0 is adsorbed at room temperature. From fig. 4 can be concluded that H,O is mainly dissociatively adsorbed on the surface. The emission of about 6 eV is partly created by adsorbed oxygen. The peaks at about 6 and 10 eV are due to an adsorbed OH species. For this species two instead of three emission maxima are expected [24]. The small peak at 13.5 eV (only visible in the difference spectra) together with the shoulder at 7 eV and the peak at 10 eV indicate also the presence of molecularly adsorbed H,O on the surface. Similar conclusions can be drawn from the sp results at RT (cf. fig. 7). Additionally it can be seen that the decomposition of H,O is a kinetically controlled process. Due to the initial adsorption of molecular H,O the sp increases fast. This is followed by a decrease in sp caused by the decomposition of H,O on the surface. A reduction of H,O pressure in the gas phase creates a further decrease in sp because molecularly adsorbed H,O desorbs from the surface. From these observations a mechanism of H,O decomposition at RT can be evaluated: fast
* H,O,, H 2ogas
(increase
in sp)
SlOW - H,,
+ OH,,
(decrease
in sp),
J.M.
Heras ei al. / Fuce speciJici[v of H,O adsorption
603
or HZOgas ‘zt H,O,,
(increase
in sp)
slow -+ H, sas + O,, (decrease
in sp) .
The differences between “perfect” Co(OOO1) and the other two investigated surfaces are caused by a lower heat of adsorption on Co(OOO1). Atkinson et al. [22] investigated the H,O adsorption on MO films with XPS at room temperature. They found that H,O is molecularly and dissociatively adsorbed on MO films. Similar observations were made by Dwyer et al. [23] for the H,O interaction with Fe(OO1) at room temperature. 4.3. Adsorption
temperature
350 K
H,O is only dissociatively adsorbed at temperature above 350 K on Co( 1120) and Co(OOO1). This conclusion is drawn from the UP spectra, which show only a 6 eV peak after H,O admission. This peak is caused by 0 2p emission of adsorbed oxygen. The surface potential decreases on both surfaces and shows a saturation value of about 0.45 V at 120 L H,O adsorption (cf. fig. 8). The initial slopes of sp as a function of H,O exposure indicate that the decomposition of H,O is faster on Co(llz0) than on Co(OOO1). This is probably due to a lower activation energy of decomposition on Co( 1120). From the temperature dependence of the change in sp an activation energy of about 15 kJ/mol can be calculated for Co( 1120). A decrease in sp created by the decomposition of H,O at higher temperatures was also found on polycrystalline Fe [3], Co [3,5] and Ni films [ 11. On Co( 1120) ordered oxygen overlayer structures are formed above 400 K. An initial (4 X 1) overlayer is followed by a complicated structure (cf. fig. 1 l), which can be explained by a 1: ;>/‘I unit cell. 4.4. Thermal desorption studies The multilayers of H,O desorb completely below 140 K on all three single crystal Co surfaces. This can be concluded from UPS data. The desorption maximum of physisorbed H,O from “perfect” Co(OOO1) lies at 155 K. Only small amounts are still present on the surface above this temperature. At 250 K the clean surface is obtained, which means that H,O is only molecularly adsorbed on “perfect” Co(OOO1). From the change in sp during thermal desorption it can be assumed that different adsorption sites exist for H,O on Co(OOO1) (cf. fig. 9). From H,O covered Co( 1120) and “polycrystalline” Co(OO0 1) the main part of physisorbed H,O desorbs at higher temperatures (160-220 K) which is indicative of a higher heat of adsorption on the rougher planes. UP spectra (cf. fig. 10) tell us that already at 208 K H,O is partially dissociated on these
604
J.M.
Hews et cd. / Frtce specrficity of Hz0 adsorption
planes during thermal desorption. A photo electron peak at about 6 eV at 208 K indicates an oxygen or OH species on the surface (cf. fig. 10). The final sp value at 400 K of Co( 1120) lies about 0.3 V below that of the clean surface indicating the existence of adsorbed oxygen. which decreases the surface potential. This is also verified by UP data. The change in sp of the polycrystalline Co film during thermal desorption included in fig. 9 shows that H,O has an even higher heat of adsorption on this surface and that the maximum rate of H,O desorption is at 250 K and the maximum rate of H, desorption at 305 K. The final sp value lies as in the case of Co(ll20) below the sp of the clean surface because of the existence of dissociation products on the Co film.
Acknowledgement A grant of the Alexander by one of us (J.M.H.).
von Humboldt
Stiftung
is gratefully
acknowledged
References [l] J.M. Heras, L. Viscid0 and V. Amorebieta. Ber. Bunsenges. Physik. Chem. 82 ( 1978) 1035. [2] R. Ducros, M. Alnot. J.J. Ehrhardt, M. Housley. G. Piquard and A. Cassuto, Surface Sci. 94 (1980) 154. [3] J.M. Heras and E.V. Albano. in: Proc. 3rd European Conf. on Surface Science, Cannes, 1980. p. 291. [4] P.A. Thiel, F.A. Hoffmann and W.H. Weinberg, in: Proc. 3rd European Conf. on Surface Science. Cannes, 1980, p. 307. [S] J.M. Heras, L. Viscid0 and M. Amorebieta, Z. Physik. Chem. I I I (1978) 257. [6] J.M. Heras and L. Viscido. Appl. Surface Sci. 4 (1980) 238. [7] C.R. Brundle and F.A. Carley, Faraday Disc. Chem. Sot. 60 (1975) 51. [8] R.B. Moyes and M.W. Roberts, J. Catalysis 49 (1977) 216. [9] H. Ibach and S. Lehwald, Surface Sci. 91 (1980) 187. [lo] L.E. Firment and G.A. Somojai. Surface Sci. 84 (1979) 275. [ 1 l] B.A. Sexton, Surface Sci. 94 (1980) 435. [12] L.E. Firment and G.A. Somojai, J. Chem. Phys. 63 (1975) 1037. [13] H. Kblbel and E. Engelhardt, Erdol Kohle 3 (1950) 529. [I41 H. Papp, to be published. [lS] R.W. Lee, R. Alsenz. A. Ignatiev and M.A. Van Hove, Phys. Rev. B17 (1978) 1510. [I61 M.E. Bridge, CM. Comrie and R.M. Lambert. Surface Sci. 67 (1977) 393. (171 J.M. Heras, C. de Francesco and E. Toscano. Appl. Surface Sci. 3 (1979) 416. [I81 D.W. Turner, A.D. Baker, C. Baker and C.R. Brundle, Molecular Photoelectron Spectroscopy (Academic Press, London, 1970). [19] J. Ktippers, H. Michel, F. Nitschke. K. Wandelt and G. Ertl. Surface Sci. 89 (1979) 361. [20] J. Ktippers, F. Nitschke. K. Wandelt and G. Ertl, Surface Sci. 88 (1979) I. [21] G.B. Fisher and J.L. Gland, Surface Sci. 94 (1980) 446. [22] S.J. Atkinson, CR. Brundle and M.W. Roberts, Faraday Disc. Chem. Sot. 58 (1974) 62. [23] D.J. Dwyer, G.W. Simmons and R.P. Wei. Surface Sci. 64 (1977) 617. [24] C. Benndorf. C. Nobl. M. Rtisenberg and F. Thieme, Surface Sci. I I I (1981) 87.
Surface Science 117 (1982) 590-604 North-Holland Publishing Company
FACE SPECIFICITY OF THE H,O ADSORPTION AND DECOMPOSITION ON Co SURFACES - A LEED, UPS, sp AND TPD STUDY J.M. HERAS
*, II. PAPP ** and W. SPIESS
Institut fiir P&vsikuli.whe und Theoretische Chemie, Ciniversitijt Erlungen-Niirnherg
Erluqen.
Fed.
Rep. of Germany Received
14 September
1981; accepted
for publication
3 November
1981
The adsorption and decomposition of H,O has been investigated on three different Co surfaces (Co(OOOl), Co( I 120) and a “polycrystalline” Co(OOO1) surface) by LEED. UPS, sp and TPD studies, At about 100 K H,O is only molecularly adsorbed on all three surfaces with a maximum increase in surface potential of 1.2 V. The three valence orbitals of adsorbed H,O show a shift of binding energy dependent on the workfunction of the clean crystals. Multilayer adsorption is observed with UPS, No ordered overlayer can be detected at low temperatures. TPD experiments show that H,O is totally reversibly adsorbed on Co(Oo1) below 298 K. On Co{ 1120) and on “poiycrystalline” Co(OOO1) H,O starts already to decompose at 210K. On Co(OOO1) Hz0 decomposes above 350 K, but the rate of decomposition is much smaller than on Co(ll20).
There exist many different incentives to investigate the interaction of H,O with transition metal surfaces. The adsorption of H,O is a good example to study the transition from physisorption to multilayer adsorption at low temperatures and to study the transition from physisorption to decomposition at higher temperatures. Apart from these more theoretical considerations, the investigation of the interaction of H,O with metal surfaces is of course of great practical interest. The phenomena of corrosion and passivation of metal surfaces are naturally connected with water interaction. Knowledge about the interlayer water/metal surface is fundamental for electrochemistry. H,O is a common conta~nant in many catalytic reactions. These are the main reasons why the H,O interaction with metal surfaces was investigated with many different methods in recent years ]l- 121. The system Co/H,0 is of special interest since H,O is a reaction product during the Fischer-Tropsch synthesis on Co catalysts [13]. Heras et al. [3,5,6] * INIFTA, ** Lehrstuhl
Universidad Nacionai de La Plats, Argentina. fiir Technische Chemie, Ruhr-Universitat Bochum,
0039~6028/82/0000-0000/$02.75
Fed. Rep. of Germany.
0 1982 North-Holland
J. M. Hem et al. / Face specificity of H_,O adsorptim
591
have investigated the adsorption and decomposition of H,O on polycrystalline Co films with workfunction and resistivity measurements and thermal desorption mass spectrometry. They found that the surface topography of the Co films had a strong influence on the adsorption behaviour of H,O. The surface topography was altered by different annealing temperatures. On unsintered Co films, with low workfunction, H,O decomposes more readily than on sintered Co films with higher workfunction. X-ray diffraction studies on Co films [17] and LEED investigations of a Co foil [16] show that sintering of the polycrystalline samples at T -=c700 K produces only the hexagonal form of Co and an abundance of the basal plane in the surface. The above mentioned results were the basis of our investigations. In order to get a deeper insight into the dependence on surface topography of Co, we decided to investigate the interaction of H,O with a perfect Co(OOO1) surface, a Co(OOO1) surface roughened by Xe ion bombardment and a Co( 1120) surface representative for a surface with low workfunction. The Hz0 adsorption was followed by LEED, UPS, AES, TDS and sp measurements. 2. Experimental A detailed description of the experimental set up will be given elsewhere [14]. Only a short summary is given here. The main system consisted of a “Vacuum Generators” ion pumped LEED/AES apparatus with a four grid retarding field analyser. A base pressure of 5 X 10 -9 Pa was readily obtained. A quadrupole mass spectrometer allowed a residual gas analysis and a check on the purity of the gases used. A vibrating capacitor with a stainless steel gauze as reference electrode was used for surface potential measurements. For UPS studies a homebuilt microwave driven gas discharge UV lamp was added at an angle of 70” to the normal of the crystal. The Co(OOO1) and Co(l120) surfaces were cut from a single crystal bar received from “Materials Research Corporation”. The surface orientation was checked by Laue diffraction to be within f l/2’ of the desired orientation. Before mounting on a coolable crystal manipulator the crystals were mechanically polished. The Co single crystal surfaces were initially contaminated by Cl, S, N, and 0. They were usually cleaned by Xe ion bombardment at about 550 K and subsequently annealed at the same temperature. This temperature is well below the transition point of hcp Co into fee Co at about 700 K [ 151. A repeated transition through this point would lead to a polycrystalline Co sample [16]. The described cleaning cycle had to be repeated very often in order to get a reproducibly clean surface. The water was cleaned by a repeated distillation in vacuum before it was added via a leak valve. The mass spectrometric analysis showed that it was contaminated only by varying amounts of H,. This H, is most probably produced either by a decomposition of H,O on the metal parts of the system or by a displacement of H, by H,O from the pumps.
592
3. Results 3.1. Photoelectron
spectra
3.1.1. Adsorption temperature 100 to 120 K The first three figures show He I UP spectra of the adsorption of H,O at about 100 K on “perfect” Co(OOO1) (fig. l), on Co(OOO1) roughened by Xe ion bombardment at 298 K * (fig. 2, here the difference spectra are shown), and on Co( 1120) (fig. 3). Common to all three surfaces is a strong decrease in d-band emission and a growth of three additional emission peaks due to the increasing amount of adsorbed H,O. In fig. 2 a fourth maximum at about 16 eV is due to the shift of the onset of the secondary emission to a higher binding energy created by the increase in surface potential. Up to a H,O admission of 3 L the H,O peaks remain at constant binding energy on all three surfaces. Above a H,O exposure of 3 L a shift of the emission maxima to higher binding energy is observed. The energetic position, however, of the peaks due to water adsorption below 3 L is different on the three surfaces. In table 1 the binding energies of the H,O peaks at low coverages (G 3 L H,O) and higher coverages ( 16 to 8 L H,O) are reproduced together with the literature values for the valence orbitals of gaseous H,O. The assignment for gaseous H,O can also be applied to the adsorbed species. At low coverages a significant increase of the binding energy values is observed from “perfect” Co(OOO1) to Co(l120) (the accuracy is i-O.1 eV). The energetic position at about 6 to 8 L H,O is less dependent on the substrate. At high H,O doses (13.5 L HzO, cf. curve4 of fig. 3) a further shift is observed. At the same time the d-band of the Co substrate has almost vanished, indicative of a thick H,O layer. This high intensity of the photoemission peaks shows that the photon yield is much higher for a H,O layer than for the clean Co substrate. On all three surfaces the onset of the secondary electron emission is shifted by about 1.2 eV to higher binding energy. This indicates that the surface potential of the substrate increases by this amount. This shift is completed at about 2.6 L H,O admission. 3.1.2. Adsorption temperature 298 K No change in the UP spectrum of “perfect” Co(OOO1) is observed due to the admission of H,O at 298 K. Obviously no H,O is adsorbed at this temperature, which is also verified by sp measurements (see below). However, admission of H,O to Co( 1120) at 298 K changes the UP spectrum of the surface. This is shown in fig. 4. After a short exposure of H,O a single broad peak appears at about 6.1 eV (cf. fig. 4). 9.9 L water creates an increase of the 6.1 eV peak; a small peak at about 10 eV can also be observed. At a constant PH,O of about 6 X 10 -5 Pa these two peaks have grown considerably * This surface
will be called “polycqstalline”
Co(OOO1) in the following
593
15 Aev
10
5
%
-
Fig. I. He I UP spectra
of the Hz0
adsorption
on a “perfect”
Co(OOO1) surface
at 100 K.
in intensity. In the difference spectrum an additional small peak at about 13.5 eV can be detected. The peak at 6.1 eV is unsymmetric due to the overlap of a second peak at higher binding energy (cf. curve 4 of fig. 4). Curve 5 in fig. 4 represents the UP spectrum after a reduction in H,O pressure. The 10 eV peak and the shoulder at about 7 eV have almost vanished. On the “polycrystalline” Co(OOO1) surface the same effects as on Co(l120) can be observed. 3.1.3. Adsorption temperature 350 K Fig. 5 shows the He I UP spectra of clean “perfect” Co(OOO1) (curve l), of Co(OOO1) after admission of 617 L H,O (curve2) and after an admission of 1037 L Hz0 (curve3). The d-band emission decreases only slightly. At a binding energy of about 6 eV a peak is apparent due to the admission of H,O. The onset of the secondary electron cascade is shifted by about 0.5 eV to lower binding energy, indicating that the surface potential decreases by this amount. Similar results are obtained on Co( 1120) at 413 and 523 K.
J.M.
Heras et al. / Fctce specificity of h’,O udsorption
Fig, 2. He I UP difference spectra of the adsorption of Hz0 at 117 K on a "polyqstalhe" surface: (I) 0.9L HzO; (2) 1.5 L H,O; (3) 2.1 L H,O; (4) 2.7 L H,O: (5) 4.7 L Hz0
CO(OCOI)
(6)
6.7 L H,O.
3.2. Change
in surface potential
Fig. 6 shows the change in surface potential of the three single crystal surfaces and of a polycrystalline Co film (sintered at 478 K) due to the admission of water at 100 K (for the single crystals) and at 77 K (for the film).
5
tF
Fig. 3. He I UP spectra of the adsorption of H,O on a Co(Il?iO) surface at 100 K: surface; (2) 1.5 L H,O; (3) 7.5 L H,O; (4) 13.5 L H,O.
(1) clean
J. M. Hems et ul. / Face specificicv of H,O adsorption
595
Fig. 4. Influence of H,O admission on the He I UP spectrum of Co( 1120) at 298 K: (1) clean surface; (2) 2.4 L H,O; (3) 9.9 L H,O; (4) PH,O =6X IO-5 Pa; (5) PHZO=5X10-’ Pa.
15
5,
eV
Fig. 5. He I UP spectra surface: (2) 617 L H,O;
of H2Q admission (3) 1037 L H,O.
io a “perfect”
Co(0001)
surface
at 486 K: (I) clean
596
2
0
L
6
8
‘0 lb-
1
OJ 0
1
2
3
4
5
6 *
fig. 6. Change in surface potential due to the adsorption (O), on “polycrystalline” Co(OOO1) ( k ). on Co( 1120) film sintered at 478 K (~ ).
of H,O
at 100 K on “perfect”
Co(OOO1) Co
(A). and at 77 K on a polycrystalline
The preparation of the Co films is described in a paper by Heras et al. [3]. The maximum increase in surface potential is 1.20 k 0.02 V on all three surfaces. The differences in the initial slopes of the sp curves are caused by different measuring conditions: H,O was dose wise admitted to Co(l120), and the observed increase in sp as a function of H,O exposure compares well with that observed with UPS. On both Co(OOO1) surfaces the change in sp was investigated with a constant H,O pressure. This can simulate a low sticking coefficient because of a shadowing effect of the reference electrode and an increase in the H, contamination with increasing presence of H,O in the chamber (decomposition and evolution of Hz). The change in sp of both Co(OOO1) surfaces due to a dose wise admission of H,O measured with UPS is identical to that on Co( 1120); i.e. after 2.6 L H,O an increase of 1.2 t 0.1 V has been observed. No change in sp due to H,O admission is observed on “perfect” Co(OOO1) at room temperature. Fig. 7 shows the change in sp as a function of H ,O admission to Co( 1120) at 298 K. At PnO = 1.3 X 10 m6 Pa a fast increase in sp of about 30 mV is followed by a decrease in sp of about 40 mV. A reduction of PHzO creates a further decrease of 10 mV. A renewed H,O admission ( PHIO = 1.3 X 10 -’ Pa) followed by pressure reduction leads to a similar behaviour, but with a smaller amplitude. When PHzO increases, a further fast increase of sp with a subsequent slow decrease 1s observed (cf. fig. 7). A decrease in sp due to H,O exposure to Co(OOO1) and Co( 1120) is
J. M. Herus er al. / Face specificity of NJ0 udsorption
Fig. 7. Change time.
in surface
potential
due to H,O
admission
597
on Co( 1120) at 298 K as a function
of
observed at higher temperatures (cf. fig. 8). After about 130 L a saturation value of about -0.45 V is obtained, independent of temperature and surface. The slope of the initial change in sp of Co( 1120) increases with growing temperature. For Co(OOO1) the slope is much smaller than on Co(1 120).
-xw-
523K
df -4m
&‘8K -.L23K -
-
I -3&T-
Fig. 8. Change
-~-A~-.co(awl)
.
-*--
) co md
/------‘_
in sp of Co(WOl)
at 535 K and of Co( I 120) at 423 and 498 K due to H,O
exposure.
3.3. Thermal desorption studies Fig. 9 shows the decrease in sp due to the heating of H,O covered Co surfaces (heating rate approximately 3 to 5 K/min). For the Co(OOOl)/H,O system a steap decrease in sp is observed at about 155 K (within 10 K the surface potential decreases by 0.9 V). At about 250 K the sp value of the clean surface is obtained. No further change is observed. For the system “polycryslalline” Co(OOOl)/H,O the steep decrease in sp starts at 155 K and a change of -0.9 V is reached at 230 K, i.e. at clearly higher temperature (70K) than on “perfect” Co(OOO1). Then the sp stays constant up to 298 K. Above this temperature the change in sp is not drawn, since it is then influence by water interaction and the annealing of the “polycrystalline” Co(OOO1) sample. The low temperature part of the Co( 1 lqO)/H,O system is very similar to that of the ‘“polycrystalline” Co(OOO1) sample. The steep decrease is only shifted by about 5 K, a change of -0.9 V is reached at 245 K. After a plateau a further decrease is obtained at higher temperatures. At 305 K the surface potential of the clean surface is reached and at 390 K a sp value of 0.3 V below that of the clean surface is observed. The curve of the polycrystalline Co film exhibits a different behaviour. A decrease in sp of about 0.2 V between 100 and 130 K is followed by a plateau. Then the surface potential decreases continuously till it reaches at 400 K a value of 0.2 V below the sp of the clean film. The arrows of fig. 9 indicate the
-1.4 -
e-12.
!
-1.0. -0.8.
-0.6.
c
0’
-
Fig. 9. Change “polycrystalline” 478 K.
in sp due to heating of H,O Co(OOO1); (A) Co(lIZ0);
covered ( ------)
K
Co surfaces: (0) “perfect” Co(0001); (X) polycrystalline Co film annealed at
599
J. M. Hems et al. / Face specifici(v of H20 adsorption
15
r,eV --. Fig. 10. Effect of thermal desorption on the He I UP spectrum of a Hz0 covered “polycrystalline” Co(OOO1) surface: (1) clean surface at 116 K; (2) surface after 2.7 L Ha0 exposure at I16 K: (3) after warming to 208 K; (4) after warming to 297 K.
mass spectrometrically measured maximum rates of H,O desorption at 245 K and of H, desorption at 300 K from a H,O covered Co film. Fig. 10 shows the change in the He1 UP spectrum of a H,O covered “polycrystalline” Co(OOO1) surface during thermal desorption. Curve 1 represents the spectrum of the clean surface. Curve2 shows the spectrum after an admission of 2.7 L H,O at 116 K. The three H,O induced emission peaks have a binding energy of 7.0, 9.6 and 13.4 eV (cf. table 1). At a crystal temperature of 208 K the intensity of the additional emission maxima is drastically reduced and the low binding energy peak has been shifted to about 6 eV (cf. curve 3 of fig. 10). At 297 K only the 6 eV peak with reduced intensity remains visible (cf. curve4 of fig. 10). For the system Co(l lZO)/H,O similar results are obtained. For “perfect” Co(OOOl), however, only a reduction of the H,O induced peaks is observed
Table 1 Binding energy
in eV of the Hz0
Co(OOO1) perfect Co(OOO1) “polycrystalline” Co(ll~0) Gaseous Ha0 [ 181 Orbital assignment
induced
emission
G3 L Ha0 8 LH,O G3 L Ha0 6.7 L Ha0 <3LH,O 7.5 L Ha0
maxima 6.9 7.6 7.0 7.6 7.3 7.8 12.6
9.3 10.6 9.6 10.7 9.9 10.8 14.7
‘b,
3at
13.0 13.6 13.4 13.7 13.5 13.8 18.3 lb,
J. M. Herus et ul. / Fuce specifiat_v
600
A
A
A
0
D
0
A
A
A
0
H,O udsorption
a: A
A
A
A
0
0
q
0
0
0
A
A
A
A
A
A
0
of
q
Cl
0
0
0
OAAOAAO Fig.
I I. Schematic representation
during thermal about 250 K.
desorption
of the LEED pattern
and the spectrum
after H20
exposure
on Co( 1 IjO) at 4X0 K.
of the clean surface is obtained
at
3.4. LEED results No additional overlayer spots can be observed due to the adsorption of H,O on the three Co surfaces at low temperatures. Only a reduction in the intensity of the substrate pattern and an increase in the background intensity is obtained. At H,O exposures > 6 L the substrate spots have completely vanished. The LEED pattern of the substrate is restored, when the H,O covered crystals are warmed to room temperature. At temperatures above 400 K additional overlayer spots can be observed due to the interaction of H,O with Co(l120). At low exposures (up to 10 L H,O) a (4 X 1) overlayer structure appears and at higher exposures the LEED pattern of fig. 11 is obtained.
4. Discussion 4.1. Adsorption temperature 100 to 120 K First the region up to a H,O admission of about 3 L will be discussed. The three additional H,O peaks in the UP spectra (cf. figs. l-3) with binding energy values independent of coverage are created by the three valence orbitals of H,O (lb,, 3a, and lb,). This speaks for a molecular adsorption of H,O on the three Co surfaces at low temperatures. The distance in energy of the orbital emission in the adsorbed state is approximately the same as in the gaseous
J.M. Hews et (11./ Fuce specificit,: of H,O adsorption
601
=3.6 to 3.8 eV; AE(3a, - lb,),d =2.4 to 2.6 eV; -3a,),, = 3.6 eV [18]; AE(3a, - lb,),,, = 2.1 eV [IS]). This indicates AE(lb, -3a,),,, a weak interaction between the surface and the H,O molecules adsorbed. The three valence orbitals have different binding energy values on the three surfaces (cf. table 1). The binding energies measured in the UP spectra are referred to the Fermi level. Fig. 12 shows the UP spectra of the three clean Co surfaces. The secondary electron onset indicates that the “perfect” Co(OOO1) surface has the highest workfunction, that the “polycrystalline” Co(OOO1) surface has a workfunction which is about 0.7 eV lower, and that the Co( 1120) has the lowest workfunction. When it is assumed that the binding energy values of the H,O orbitals are referred to the vacuum level, then low E, values should be observed for the adsorption of H,O on a surface with high workfunction and vice versa. This is exactly what we find (cf. table 1). The obtained shift in binding energy is, however, smaller as the change in workfunction. This can be due to different initial state binding shifts because of different adsorption energies and due to different final state relaxation energy shifts on the three surfaces. A dependence of E, of orbitals of adsorbed species on the workfunction of the substrate has already been observed for the physisorption of Xe on transition metal surfaces [19,20], and this effects has been explained in a similar way. On all three single crystal surfaces and on a Co film the sp increases by 1.2 V due to the adsorption of H,O. From this increase can be concluded that H,O is bound via the oxygen to the surface. A similar increase in sp due to H,O adsorption on other transition metal surfaces has been reported in the literature [3]. Now the region of H,O admisssion above 3 L will be discussed. Only a small change in sp is found, but the intensities of the valence orbitals of H,O grow considerably. At the same time the d-band emission has almost vanished, which indicates multilayer-adsorption. The E, values of the valence orbitals state
(AE(lb,
Fig. 12. He I UP spectrum of clean “perfect” and of clean Co( 1120) (3):
Co(OOO1) (I), of clean “polycrystalline”
Co(OOO1) (2),
.I. M. Herus et al. / Frrce specifrci[v of H,O alsorption
602
are shifted to higher binding energy. The shift of the 3a, orbital is more pronounced than that of the others (cf. table 1). The observed shift can be explained in the following way: up to about 3 L H,O a monolayer is formed. Here the final state relaxation energy shift between adsorbed and free molecules is mainly caused by the screening of the hole created by photoemission by the metal electrons. Above 3 L the multilayer formation starts and the final state relaxation energy shift is more determined by electrons of the surrounding H,O molecules, which produces a higher binding energy of the valence orbitals. The more pronounced shift of the 3a, orbital can be tentatively explained by the assumption that the energy of the 3a, orbital is responsible for the angle between the two O-H bonds. This angle grows because of hydrogen bridge bonding within the multilayer. UPS investigations of the H,O adsorption on Co films [8] and on other transition metal surfaces [7.21] at low temperatures show also three emission peaks due to the valence orbitals of molecularly adsorbed H,O. No coverage dependence, however, has been investigated. From the LEED results can be concluded that Hz0 is adsorbed without long range order on all three single crystal surfaces of Co at low temperatures. This is in contrast to LEED results of the adsorption and condensation of H,O on single crystal surfaces of other metals where at low temperatures ordered ice structures were found (e.g. Ru(0001) [4], Ag( 111) [lo] and Pt( 111) [ 12.211). 4.2. Adsorption
temperature
298 K
Strong face specificity is found at this temperature. On Co(OOO1) no adsorption of H,O can be observed with UPS and sp measurements. On Co(1120) and “polycrystalline” Co(OOO1) Hz0 is adsorbed at room temperature. From fig. 4 can be concluded that H,O is mainly dissociatively adsorbed on the surface. The emission of about 6 eV is partly created by adsorbed oxygen. The peaks at about 6 and 10 eV are due to an adsorbed OH species. For this species two instead of three emission maxima are expected [24]. The small peak at 13.5 eV (only visible in the difference spectra) together with the shoulder at 7 eV and the peak at 10 eV indicate also the presence of molecularly adsorbed H,O on the surface. Similar conclusions can be drawn from the sp results at RT (cf. fig. 7). Additionally it can be seen that the decomposition of H,O is a kinetically controlled process. Due to the initial adsorption of molecular H,O the sp increases fast. This is followed by a decrease in sp caused by the decomposition of H,O on the surface. A reduction of H,O pressure in the gas phase creates a further decrease in sp because molecularly adsorbed H,O desorbs from the surface. From these observations a mechanism of H,O decomposition at RT can be evaluated: fast
* H,O,, H 2ogas
(increase
in sp)
SlOW - H,,
+ OH,,
(decrease
in sp),
J.M.
Heras ei al. / Fuce speciJici[v of H,O adsorption
603
or HZOgas ‘zt H,O,,
(increase
in sp)
slow -+ H, sas + O,, (decrease
in sp) .
The differences between “perfect” Co(OOO1) and the other two investigated surfaces are caused by a lower heat of adsorption on Co(OOO1). Atkinson et al. [22] investigated the H,O adsorption on MO films with XPS at room temperature. They found that H,O is molecularly and dissociatively adsorbed on MO films. Similar observations were made by Dwyer et al. [23] for the H,O interaction with Fe(OO1) at room temperature. 4.3. Adsorption
temperature
350 K
H,O is only dissociatively adsorbed at temperature above 350 K on Co( 1120) and Co(OOO1). This conclusion is drawn from the UP spectra, which show only a 6 eV peak after H,O admission. This peak is caused by 0 2p emission of adsorbed oxygen. The surface potential decreases on both surfaces and shows a saturation value of about 0.45 V at 120 L H,O adsorption (cf. fig. 8). The initial slopes of sp as a function of H,O exposure indicate that the decomposition of H,O is faster on Co(llz0) than on Co(OOO1). This is probably due to a lower activation energy of decomposition on Co( 1120). From the temperature dependence of the change in sp an activation energy of about 15 kJ/mol can be calculated for Co( 1120). A decrease in sp created by the decomposition of H,O at higher temperatures was also found on polycrystalline Fe [3], Co [3,5] and Ni films [ 11. On Co( 1120) ordered oxygen overlayer structures are formed above 400 K. An initial (4 X 1) overlayer is followed by a complicated structure (cf. fig. 1 l), which can be explained by a 1: ;>/‘I unit cell. 4.4. Thermal desorption studies The multilayers of H,O desorb completely below 140 K on all three single crystal Co surfaces. This can be concluded from UPS data. The desorption maximum of physisorbed H,O from “perfect” Co(OOO1) lies at 155 K. Only small amounts are still present on the surface above this temperature. At 250 K the clean surface is obtained, which means that H,O is only molecularly adsorbed on “perfect” Co(OOO1). From the change in sp during thermal desorption it can be assumed that different adsorption sites exist for H,O on Co(OOO1) (cf. fig. 9). From H,O covered Co( 1120) and “polycrystalline” Co(OO0 1) the main part of physisorbed H,O desorbs at higher temperatures (160-220 K) which is indicative of a higher heat of adsorption on the rougher planes. UP spectra (cf. fig. 10) tell us that already at 208 K H,O is partially dissociated on these
604
J.M.
Hews et cd. / Frtce specrficity of Hz0 adsorption
planes during thermal desorption. A photo electron peak at about 6 eV at 208 K indicates an oxygen or OH species on the surface (cf. fig. 10). The final sp value at 400 K of Co( 1120) lies about 0.3 V below that of the clean surface indicating the existence of adsorbed oxygen. which decreases the surface potential. This is also verified by UP data. The change in sp of the polycrystalline Co film during thermal desorption included in fig. 9 shows that H,O has an even higher heat of adsorption on this surface and that the maximum rate of H,O desorption is at 250 K and the maximum rate of H, desorption at 305 K. The final sp value lies as in the case of Co(ll20) below the sp of the clean surface because of the existence of dissociation products on the Co film.
Acknowledgement A grant of the Alexander by one of us (J.M.H.).
von Humboldt
Stiftung
is gratefully
acknowledged
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