Molecular adsorption states of O2 on W(111) at low temperatures (down to 5 K)

surface

science B

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Surface Science 377-379 (1997) 664-670

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Molecular adsorption states of O2 on W( 111) at low temperatures (down to 5 K) V.D. Osovskii a, Yu.G. Ptushinskii a, V.G. Sukretnyi ‘, B.A. Chuikov a, V.K. Medvedev a, Yu. Suchorski b** aInstitute of Physics of National Academy of Sciences of Ukraine, Prospect Nauki 46, Kyiv, UA-252022, Ukraine b Fritz-Haber-Institut der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Received 1 August 1996; accepted for publication 15 October 1996

Abstract

Adsorption of O2 on W( 111) at temperatures down to 5 K is studied using molecular beam (MB) and thermodesorption spectroscopy (TD) methods. The low-temperature TD spectra for the molecularly adsorbed 0, on W( 111) contain three peaks at 25, 27 and 60 K corresponding respectively to multilayer condensation, physisorption and weak molecular chemisorption. The role of the substrate structure for the 0, chemisorption is discussed on the basis of present results for W( 111) and earlier data for different metal surfaces. Field-effect on the O,-W( 111) interaction is studied using field ion appearance energy spectroscopy (FIAES) of 0: ions emitted from the apex plane of a [ 111l-oriented W field emitter tip. Applying a thermionic cycle, field-modified binding energies for O.JW( 111) are derived from the 02 appearance energies measured at 79 K. FIAES data reveals a field-adsorbed O2 layer as the origin of 0: field ions emitted from W( 111) for field ion imaging. Keywords: Adsorption kinetics; Chemisorption; desorption spectroscopy; Tungsten

Field ionization; Oxygen; Physical adsorption;

1. Introduction

Weakly bound molecular adsorption states play an important role in the description of dissociative chemisorption of gases on solid surfaces, being an essential intermediate state between the gas phase and the chemisorbed (dissociated) state [ 1,2]. Comprehensive examinations of dissociative chemisorption of oxygen on different metal surfaces allow one now to distinguish between two different types of intrinsic molecular precursors to dissociation: physisorbed and chemisorbed species [ 3-51. * Corresponding author.

Single crystal surfaces; Thermal

Recent studies of the molecular adsorption of oxygen on Pt( 111) have convincingly demonstrated a sequential character of these precursors: the physisorbed state of 0, on Pt( 111) is an intrinsic precursor to molecular chemisorption and the molecularly chemisorbed state acts as a precursor to dissociative chemisorption [4,5]. Recently, pioneering attempts to realize a nanostale visualization of catalytic surface reactions such as CO oxidation using the field ion microscope (FM) have succeeded for Pt [6] and Rh [7] surfaces. Such an experimental approach provides a direct molecular level view at the CO oxidation reaction, raising at the same time a

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number of questions concerning the field-modified kinetics of CO and oxygen adsorption. The detailed investigations [ 81 revealed a field-induced shift of the phase diagram of this reaction, which cannot be traced back to the field-moditled kinetic gas flux (field-compression effect [9]). Such a shift is supposed [ 10,111 to be mainly due to the differently field-modified weak molecular adsorption states for CO and 0,: the CO binding energy in such a state reaches 0.9 eV for different metal surfaces (Pt [ll], Rh [12], W [13]) in applied fields of 16 to 19 V nrr- l, in contrast to 0, (binding energies up to 0.22 eV were measured recently at 16Vnm-l for Pt(lll)-steps [ll]). To reveal the details of the field-induced contribution to the energetics of the O2 adsorption and, in this way, get more insight into field-modified adsorption kinetics for 0, on transition metal surfaces, it would be instructive to compare the data obtained in the field-free and high-field (FIM) conditions for the same metal surface. The present contribution describes such a comparative study performed for the O,/W(lll) system using two different experimental approaches: (i) molecular beam (MB) and thermodesorption (TD) spectroscopy for field-free (low-temperature) experiments; (ii) field ion appearance energy spectroscopy (FIAES) for high-field investigations.

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sample ([ 11 l]-oriented single W crystal of diameter 12 mm and thickness 0.5 mm). The temperature of the sample was measured using a tungsten-rhenium thermocouple calibrated by immersing into the different cryogenic liquids including helium. In the experimental procedure, an MB of 0, is directed towards the previously cleaned surface of a liquid helium-cooled (down to 5 K) sample and the time dependence of the intensity of the scattered beam (reverse flow) is measured. At the desired instant the incident oxygen flow is stopped by closing of the fast-acting (-6 ms) shutter. Simultaneously, the linear heating of the sample (at rates of 1 K s-l in the temperature range 5 to 30 K and 5 K s-l for higher temperatures, respectively) is started and a TD spectrum (Fig. 1) is recorded by the mass-spectrometric detector. The sticking coefficient S is determined from the time dependence of the intensity of reflected flux from the expression, AS(t)= 1-1(t)&

(1)

where I(t) and 1, are the detector ion currents for the scattered molecular beam at the instant t and at the onset of total reflection from the saturated surface of a sample, respectively. The surface density of adsorbed molecules

2. Experimental Field-free (low-temperature) experiments were carried out in a UHV set-up consisting of a 150-liter stainless steel chamber with a (liquid nitrogen cooled) jacket which is covered with a titanium layer. This allows for imitation of the “space’‘-vacuum at 10-lo-lO-ll Torr: the incident gas molecules get irreversibly attached to the nitrogen-cooled jacket walls of such a “black chamber”. This, in turn, makes it possible to form a molecular gas beam directed at the sample, and to ensure a single-tlight detection (by the mass-spectrometer detector) of molecules scattered (or desorbed) at the sample surface. The apparatus has been also equipped with an oxygen molecular beam source of an effusive type as well as with LEED- and Auger-systems for the control of the surface of a

‘20

30

40

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60

70

80

temperature(K) Fig. 1. Molecular desorption spectra for oxygen between 15 K and 80 K at different oxygen coverages (a-e) which are indicated in Fig. 2. Gas temperature, Z’s= 78 K.

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e-

b i

T t

-

1.0 1.5 2.0 2.5 coverage (monolayers)

L

L-

3.0

3.5

the specimen surface using Ne and 0, as imaging gases. After the W surface preparation (cleaning by visually controlled field evaporation and identification in FIM using Ne as imaging gas), the neon gas was replaced by oxygen, and the same surface was imaged with 0,’ ions. 0; field ions originating from selected ( 111) sites were analyzed with a five electrode electrostatic retarding potential analyzer [ 151 using a probe-hole technique. Field ion retardation curves (Fig. 3) were measured thus and values of appearance energy of 0: field ions, A(O:), were determined from the equation, A(O:)=&--ee6”“(0:),

Fig. 2. Sticking coefficient for oxygen as a function of oxygen coverage on W( 111) at different substrate temperatures: l-T, = 78 K, 2-T, = 35 K; 3-T, = 5 K. Oxygen coverages at which desorption spectra (shown in Fig. 1) were measured are indicated by dashed lines (a-e). Gas temperature, T,=78 K.

necessary for S(0) dependences (Fig. 2, 0 is oxygen coverage) is obtained from the expression,

(3)

where & is the work function of the retarder electrode and P(O,+ ) is the voltage applied between retarder and field emitter, just sufficient to collect the first 0,’ ions (onset value of the field ion retardation curve). The value of & Q, ret -

12

14

16

18

esW) 20

t n=vO

W>

s0

dt,

22 I

24 I

26 I

(2)

where v. is the known gas-kinetic molecular flux (MB intensity) which was set constant in the range 0.1 x 1014to 2.5 x 1014molecules cmP2 s-l with an accuracy of 2-3%. The molecular beam itself was oriented along the normal to the surface. The scattered (desorbed) molecules were detected in a solid angle of -0.2n, with the axis of detection being at 45” to the normal. The spatial distribution of the reverse flow molecules was found to be coverage- and temperature-independent within the accuracy of measurements [ 141. A detailed description of the apparatus itself and of the whole experimental procedure can be found in Ref. [ 141. For high-field experiments, the appearance energy spectroscopy of 0: ions field-emitted from a few surface sites of the apex plane of a [ 11 1]oriented W tip has been performed. We have used an atom-probe-like UHV set-up containing a FIM section which had been combined with an apparatus for a mass-to-charge-resolved retarding potential analysis. The FIM section allowed cleaning, shaping by field evaporation and visualization of

28 !! 4

J

160K

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-10

-12

-14

-16

-18

-20

-22

I

-24

6(V)

Fig. 3. Retarding potential analysis of 02 emitted at 12.9 and 14.4 V nm-l from a few, O,,-covered W( 111) sites at 79 and 240 K. At T. = 240 K, one finds A(O: ) = Z(0,) indicated by the dashed line. The Ne+ retardation curve measured at 160 K and 35.5 Vnm-’ for the same surface sites serves for the in situ determination of a CD,,value. This value is used for calibration of the upper energy scale.

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was obtained in situ from measurements of the onset voltage for field-ionized L(Ne% K, Ne at the emitter temperature of 160 K (Fig. 3). In the case of field ionization on non-accommodated Ne at elevated temperatures of the emitter (Tw = 160 K), the appearance energy of Ne+ is equal to I(Ne), the ionization energy of the free Ne atom [ 161. Thus the value of & can easily be obtained from the equation, I(Ne)=&-eeG”“(Ne+),,,

K.

(4)

Field penetration through the retarder meshes, which causes slight shifts of the onset values of retardation curves [ 161, is equal for different ions at the same ion beam energy (2 keV). Thus absolute values of the 02 appearance energy can be determined from data presented in Fig. 3. The calibration of the applied external field, FO, was based on the evaporation field strength of tungsten [ 171. A detailed description of the experimental set-up, the entire experimental procedure and a discussion of the physical significance of the appearance energy of field ions can be found elsewhere [ ll-13,16,18].

3. Results and discussion 3.1. Field-free (low temperature) experiments

Fig. 1 displays the field-free TD spectra for different initial molecular oxygen coverages on W( 111). The high-temperature parts of the spectra, corresponding to a desorption of the chemisorbed atomic oxygen, which are not related directly to the problems considered in the present study, are not shown in Fig. 1. The spectra display three weakly bound molecular adsorption states at 25, 27 and 60 K. Assuming the lifetime of an admolecule of 1 s at the desorption temperature, the (field-free) binding energies of O,/w( 111) can be estimated as 0.07,0.08 and 0.17 eV, respectively. The TD spectra shown in Fig. 1 are similar to the spectra measured earlier for OJW( 110) [ 191 as well as for O,/W( 100) and O,/W( 112) [ 14,201, and are practically identical with these in the temperature range 20 to 40 K. However, the peak observed in Ref. [ 141 at 45 K independently of

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surface orientation, appears in the present measurements at 60 K, indicating thus the sensitivity of the weakly chemisorbed 0, to the pronounced surface relief of the W( 111) surface, covered with atomic oxygen. The physical reason for such a sensitivity lies, in our opinion, in the more significant charge transfer from the metal into the n4 orbital of 0, in the case of W( 11l), as compared to the more close packed surfaces. Such a charge transfer distinguishes the chemisorbed O2 from the physisorbed one where no chemical bond to the surface via the rc* system exists [ 51. It is clear, that in the case of physisorption the sensitivity to the peculiarities of the electron density distribution near the surface is remarkably lower, leading to the same binding energy (0.08 eV) of 0, physisorption for the (loo), (llO), (112) [14,20] and (111) planes of W. Studies of the O2 adsorption on Pt confirm such a conclusion: the quite different electronic structure of Pt (111) in comparison with W( 111) leads to a significant difference in the binding energy of chemisorbed O2 (0.35 eV for Pt(ll1) [4] and 0.17eV for W(lll) in the present measurements), whereas the physisorption states are very similar (0.12 and 0.08 eV, respectively). The area under the thermal desorption peak is nearly proportional to the number of adsorbed molecules in the corresponding state. The analysis of the data shows that the filling of different O2 adsorption states (after or close to saturation of the chemisorbed atomic monolayer at nz 1 x 1015atoms cm-’ [21]) occurs successively: the more weakly bound state begins to be tilled after the preceding stronger bound state has been nearly filled. The states at 60 and 27 K have a limited capacity and get saturated with time. In contrary, the number of molecules in the 25 K state increases infinitely corresponding to the polylayer condensation of molecular oxygen. Thus the obtained data strongly suggest the growth of the oxygen cryocrystal at the W( 111) surface cooled down to 5 K. This growth is preceded by the filling of an atomic chemisorbed layer and of the molecularly chemisorbed and physisorbed oxygen layers. The field-free dependences of the sticking coefficient S on the O2 coverage on W( 111) measured at various substrate temperatures between 5 and 78 K are shown in Fig. 2. Within the

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monolayer oxygen coverage, measured values of S remain constant (close to unity) suggesting a precursor adsorption mechanism (at least at 78 K where the weakly bound molecular species are essentially depleted). Decrease of the substrate temperature (down to 5 K) does not lead to the changes in the sticking coefficient for 0 < 1 (compare curves 1, 2 and 3 in Fig. 2). This allows one to assume that the adsorption mechanism does not change in the temperature interval 5 to 78 K. It is to be noted that for submonolayer coverages the dissociative adsorption of oxygen on tungsten (even on the densely packed W( 110) surface) is reliably established down to 20 K [22,23]. Thus we suggest that in the present case the dissociative chemisorption takes place also at 5 K similarly as it occurs on W(lOO), where this was proven in measurements performed for different (to 200 K) gas temperatures [20]. The staggered form of the S(0) curves in Fig. 2 also indicates that an (extrinsic) precursor-mediated adsorption mechanism comes into play at T, 178 K similarly to that which was observed recently for W( 100) [20]. 3.2. High-Jeld (FIAES) experiments Recent appearance energy spectroscopy of CO+ emitted from Rh( 111) [24] detected the fielddependent terms in measured CO + appearance energies. Such terms supply information on the energetics of the molecule-surface interaction, provided the origin of field ions is clarified for particular species [25]. Detailed FIAES studies for several CO/metal systems [ 1l- 131, convincingly demonstrated that CO+ ions field emitted from all the studied surfaces originate from a weakly bound, highly mobile [26] CO layer via a field desorption process. New data for 0; /Pt( 111) [ 1l] also strongly suggest a weakly bound molecular oxygen layer as an origin of 0; ions emitted during FIM imaging of the Pt surface. To examine the field-dependence of the 0; appearance energy in the present case of W( 11 l), 0; retardation curves were recorded in the field range of 12 to 17 V nm-’ for a few surface sites of the apex plane of a [Ill]-oriented W tip at T, = 79 K. The measured curves (Fig. 3) show the same features as were recently observed for CO+

and 0; emitted from other metal surfaces: a high degree of spatial localization of the ionization process (reflected in small width of ion-energy distribution); values of 0: appearance energy obtained from these curves (Fig. 3) are higher than the ionization energy of a free O2 molecule; the 0; appearance energy increases with increasing field strengths, i.e. it contains a field-dependent term. Appearance energy of 0: emitted at elevated T, corresponds in turn to the ionization energy of a free O2 molecule (see curve for T,=240 K in Fig. 3), thus indicating ionization of non-accommodated O2 molecules. All these observed features agree well with the field desorption model of ion formation in the case of localized ionization of CO [24] and O2 [ 111 used as imaging gases in a FIM. In the general case of field desorption, the appearance energy of single-charged ions is given, following Forbes, by the equation, A=I+H(F)-Q(F),

(5)

derived from the theoretical consideration based on a thermionic cycle [25]. Here H(F) is the binding energy, in general field-dependent, and Q(fl is the activation energy for field desorption. As follows from the arguments of Refs. [27,28], the appearance energy of the field-desorbed ion should not depend on the temperature of the emitter in the range in which the contribution of thermally desorbed and post-ionized particles in the registered ion rate can be neglected. Indeed, the values of A for 0: /W( 111) were found in the present experiments to be independent of the emitter temperature in the range of 79 to 105 K. The applicability of Eq. (5) to the analysis of the energetics of field desorption was recently confirmed directly by the first measurements of absolute appearance energies of Li’ ions field-desorbed from W( 111) [29,30]. The values of the binding energy of Li in the first and second Li layer, which were determined using Eq. (5), are in perfect agreement with known thermodesorption data. Fig. 4 displays the experimental values of the term H(F) - Q(F) for O:/w( 111) as a nearly linearly increasing function of the applied electrostatic field strength Fo. The value of Q(F) characterizes the activation energy for 0: field

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Fo( VI nm)

Fig. 4. The term H-Q obtained from 0: appearance energies measured at 79 K for the oxygen-covered W( 111) surface as a function of the applied field strength, F,,.

desorption in the presence of a continuous supply of O2 molecules to the probed sites. Since the supply mechanism can affect the activation energies, as was shown in Ref. [29], the influence of varied supply mechanisms (surface diffusion, gas phase supply etc.) on the effective value of Q(F) should be taken into account. Fortunately in the present case this value, as estimated from our 0: rate measurements, does not exceed 0.1 eV, so the values along the vertical axis in Fig. 4. can be interpreted as the approximate binding energy of O2 molecules. Now we have to compare our field-free and high-field results in order to distinguish between two weakly bound molecular adsorption states (with a field-free binding energy of 0.08 and 0.17 eV respectively) as a possible origin of 0; ions registered in FIAES experiments. The fieldinduced increase of the 0: appearance energy measured in the present experiments is close to that measured for 0: ions emitted from Pt( 111) in a similar field range [ 111. For the O,/Pt( 111) system, a mechanism of 0,’ ion formation via field desorption from the weakly bound field-stabilized O2 layer (binding energy of which increases from 0.12 eV in a field-free case to 0.2 eV at fields of 10 to 14 V nn-l), was proposed in Ref. [ 111. In the present case of O,/W( 11 l), the O2 layer with the field-free binding energy of 0.08 eV plays a corresponding role in the origin of 0; ions. The nature of the field-induced bonds in the case of O,/W( 111) is qualitatively similar to that of fieldadsorbed noble gas atoms where a field-induced

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charge transfer [ 3 1] creates weak chemisorption bonds in addition to the physisorption (van der Waals) bonds. No significant participation of the weakly chemisorbed 0, (with the field-free binding energy of 0.17 eV) in 0: ion rate is expected because of peculiarities of the charge state (resembling negative 0; ions [5]) of such molecules. Surface geometric arrangement of such molecules (small adsorption length) also does not contribute to the likelihood of their ionization [32].

4. Conclusions Three weakly bound molecular adsorption states for oxygen on W( 11l), corresponding to multilayer condensation, physisorption and weak chemisorption have been detected using molecular beam and thermodesorption spectroscopy methods. Molecularly adsorbed oxygen serves as an extrinsic precursor to dissociative adsorption of oxygen on W(lll) in the temperature range 5 to 78 K. Comparison of molecular chemisorption states for different tungsten surfaces displays an influence of the substrate surface structure on the binding energy of weakly chemisorbed OZ. Applying of an external electrostatic field of the order of one volt per angstrom leads to the stabilization of the molecularly physisorbed oxygen due to the field-induced charge transfer. Field ion appearance energy spectroscopy data reveals the (field-stabilized) molecularly physisorbed oxygen layer as an origin of 0; ions emitted from W( 111) at the instance of the field ion imaging at 78 K.

Acknowledgements

One of the authors (Yu.S.) thanks Mr. M. Naschitzki for his excellent technical assistance. This work was supported in part by the European Community (INTAS Nos. 93-964 and 93-1525).

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