adsorption of oxygen on gold

240 Surface Science 141 (1984) 240-254 North-Holland, Amsterdam

THE ADSOR~ION OF OXYGEN ON GOLD

N.D.S. CANNING, D. OUTKA and R.J. MADIX

Department of Chemical Engineering, Stanford University, Stanford, CaIijotxia 94305, USA

Received 25 October 1983; accepted for publicatidn 21 February 1984

The oxidation of gold has been studied under UHV conditions by AES, XPS, and TDS. The

previously reported adsorbed oxygen state, which formed by heating the sample above 600 K in

10W5 Torr of oxygen and which remained after subsequent heating to 1100 K in vacua, has been

shown to result from the reaction of oxygen with silicon diffusing from the bulk. No oxygen

adsorption was detected on a clean sample for oxygen pressures up to 10m4 Torr and sample

temperatures between 300-600 K. Chemisorption of oxygen atoms could be induced by placing a

hot platinum filament close to the sample during exposure to oxygen. The activation energy for

desorption of this oxygen state was estimated from the thermal desorption spectra to be about 163

kJ mol-‘. The chemisorbed oxygen atoms and the oxygen associated with silicon were dis-

tinguished by different O(ls) binding energies (529.2 and 532.3 eV respectively) and by different

O(KVV) Auger fine structure.

1. Introduction

In the last decade several investigators have published conflicting results concerning the interaction of oxygen with gold surfaces [l-4]. While all of the studies agree that no oxygen is adsorbed on the clean gold surface at room temperature, there is marked disagreement over the behavior at higher temperatures. Schrader [l] studied the oxidation of a (111) gold surface in 5 X lob7

Torr of oxygen at several temperatures between 373 and 873 K and concluded that activated adsorption of oxygen occurred to form a disordered chemisorbed layer. Chesters and Somorjai [2] in their study of the oxidation of Au(ll1) and Au(S)-(6(111) X (100)) reached similar conclusions. They also observed new diffraction features which they attributed to the. adsorbed oxygen in a square surface oxide mesh. The stepped surface behaved identically to the (111) surface; no enhancement of the rate of adsorption by the steps was observed. In both of these studies the authors found that the surface oxide was remarkably stable, disappearing only upon heating the sample above 1073 K for several hours. It was noted [1,2] that this behavior was very different from that observed for bulk gold oxide, Au,O,, which cannot be formed by direct reaction of oxygen and gold and which decomposes near 410 K. The greater

0039-6028/84/$03.00 0 Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)

N. D. S. Canning et al. / Adrorption of oxygen on gold 241

stability of the surface oxide compared to the bulk oxide was attributed to the coordinative unsaturation of the surface metal atoms which causes the heat of formation of the surface oxide to be much larger than the heat of formation pf the bulk oxide [l]. This very stable state of adsorbed oxygen was also reported by Bazhutin et al. [5] after heating a gold foil to 900 K in 5 X lo-’ Torr of oxygen. However, a different state of oxygen was detected by these authors after exposing the foil to atomic oxygen; this state desorbed between 500-600 K without conversion to the strongly bound state.

In sharp contrast to the above results Eley and Moore [3] found that a clean polycrystalline gold sample exposed to 3 X lop5 Torr of oxygen for an hour at temperatures between 300-1000 K did not adsorb oxygen.

The presence of impurities, which may alter the activity of the surface towards oxygen adsorption, may explain some of the differences in the above results. In this context several studies have shown that calcium at the surface of gold can lead to greatly increased rates of adsorption of oxygen [4,6]. These studies disagree on the maximum oxygen coverage attained in the presence of calcium. One study indicated that all the oxygen was associated with calcium, forming CaO at the surface [4] while another suggested that spillover of oxygen from calcium to gold could occur leading to a much higher O/Ca ratio than expected for CaO formation [6]. Spillover implies that oxygen bound to calcium should have a similar stability to oxygen bound to gold. This seems

unlikely in view of the heats of formation of bulk CaO and Au,O, which are - 151.8 kcal mol-’ [7] and -0.9 kcal mol-’ [8] respectively. In the absence of spillover it is unlikely that surface calcium was responsible, in general, for the contradictory results for the oxidation of gold, since in each study mentioned above, calcium was below the limit of detection by AES (the intensity of the Ca(KW) transition in CaO is larger than the O(KW) transition by a factor of 1.3 [9], so Ca should be more easily detected than oxygen). The results presented below suggest that a higher O/Ca ratio than that expected for CaO was probably due to the undetected presence of silicon which also reacts with oxygen, rather than due to oxygen spillover from calcium.

The irreproducibility in results for the formation of the strongly bound oxide on gold is analogous to the confusion in the literature over the existence of very stable bulk oxides of other noble metals such as platinum or palladium. Several reports have indicated that silicon must be present for these oxygen states to form [lO,ll] although a recent study of the oxidation of platinum disputes this [12].

Reported activation energies for desorption of chemisorbed oxygen from surfaces of group VIII and Ib metals are collected in table 1. The role of lateral interactions among adsorbed oxygen atoms has not been quantitatively de- termined in all of these studies, which is unfortunate since ideally the zero coverge limiting value of the activation energy, E,, should be compared when looking for periodic trends in the adsorbate-substrate binding energy. How-

242 N. D.S. Canning et al. / Adsorption of oxygen on gold

Table 1

Collected activation energies for oxygen atom recombination and desorption and the temperatures

of the peaks in the desorption spectra; references are indicated by the numbers in parentheses; the

data for gold are from the present work

co Ni cu

Stable

oxide

Stable

oxide

Stable

oxide

Rh 1341 Pd [35] Ag [361

T, W E (kJ mol-‘)

looo-1100 830-900 SOL-600

234 230 173

Ir [37] Pt [38] AU

Tp W 900-1100 690-800 654 E (kJ mol-‘) 230-270 200 163

ever the reported activation energies do show periodic trends similar to those observed for the stabilities of the bulk oxides shown in table 2. The

metal-oxygen interaction energy increases from right to left in each period and decreases moving down each group. According to periodic trends then, surface oxides of gold which are stable above 1000 K in UHV certainly would not be expected to form.

The results given below show that the stable high temperature oxide state on

gold is attributable to the undetected presence of silicon, which diffuses out of the bulk and reacts with oxygen to form a very stable SiO, layer at temperatures above 300 K. In addition the state of chemisorbed oxygen atoms bonded to gold, formed only by prior dissociation of O,, has been characterized by AES, XPS and TDS.

2. Experimental

The experiments were performed in a vacuum system consisting of a stainless steel bell jar, evacuated by an ion pump to an ultimate pressure of

Table 2 Decomposition temperatures for bulk metal oxides decomposing to metal and oxygen in air

Rh

(F&O) 1300 K [8]

Pd

(PdO) 1148 K [40]

Ag

(Ag,O) 450 K VI (Rho,) 1320 K [39]

Ir

(IrO,) 1373 K [40]

Au

;tO,) 923 K [40] (Au,O,) 423 K [40]

N.D.S. Canning et al. / Adsorption of oxygen on gold 243

8 x lo-” Torr after bake out. A double pass CMA (PHI) was used to record the Auger and photoelectron spectra. The Auger spectra were excited by an electron beam with a primary energy of 2 keV, incident along the surface normal. A peak-to-peak modulation of 2 V was used routinely, but for high

resolution spectra 0.5 V was employed. The energy scale of the Auger spectra was referenced to the Fermi level of the sample and adjustment for the work function of the spectrometer was made by setting the energy of the gold N,N,O,, transition to 239 eV [13]. The X-ray photoelectron spectra were recorded using the CMA in the constant pass energy mode (P = 50 eV). The spectra were collected digitally in a Tracer Northern signal analyser which enabled co-adding of scans to improve the signal-to-noise ratio of the spectra. Mg Ka radiation (400 W at grazing incidence) was employed to excite the photoelectrons. The binding energy scale of each spectrum was referenced to the sample Fermi level; the binding energy of the Au(4f,,,) peak was then 83.8 eV [14].

The gold sample was cut from a gold rod which was found to be a twinned crystal. After several cycles of ion bombardment and annealing the LEED pattern showed that the surface had faceted, though high background intensity and poorly defined diffraction spots indicated a large amount of surface disorder remained. A precise characterization of the surface atomic structure has thus not been attempted. The sample, with dimensions 10 mm diameter x 2 mm thick, was mounted in a tantallum holder with a chromel-alumel thermo- couple held in physical contact with the edge of the crystal. The sample was heated radiatively by a tungsten filament mounted behind it. A clean sample was obtained by cycles of oxidation (PO, = 10m6 torr, T= 900 K for 1 h) and argon ion bombardment with the sample at room temperature. In this way the

calcium and silicon impurities in the near surface region were reduced to below the limits of detection by AES after 70-80 h treatment. However, prolonged annealing of the sample at 900 K for 2 h would again lead to the detection of

silicon at the surface. Continued treatment over the course of several weeks sufficiently depleted the sample of silicon to allow thermal desorption spectra to be obtained without segregation of impurities to the surface.

In order to induce adsorption of atomic oxygen on the clean gold surface, a platinum filament was placed at 1 cm from the surface during the oxygen dose. For filament temperatures above 1450 K platinum metal was deposited on the gold and was detected by the growth of new Auger features at 1967 and 2044 eV. At 1400 K no deposition of platinum was detected. No measurable increase in the sample tempertature was produced by the hot filament.

Thermal desorption spectra were recorded using a UT1 mass spectrometer in line of sight to the crystal to monitor the oxygen flux from the surface. The temperature ramp was provided by a current jump through the heating filament. The heating rate was found to be constant over the temperature range for which 0, desorption occurred.


3. Results and discussion

3.1. Thermal oxidation of gold

Exposing a clean sample to oxygen partial pressures between 10-6-10-’ Torr for 1 h at room temperature did not lead to any detectable adsorption of oxygen. However, initially if the sample was heated to 600 K at 10W6 Torr oxygen pressure, adsorption did occur. The adsorbed oxygen was retained even after heating the sample to 1000 K for 1 h in vacuum. However the amount of oxygen adsorbed in this state depended on the previous history of the sample,

suggesting the role of impurities. After several weeks of further cleaning no oxygen was adsorbed by this procedure.

XI

dN(E)

dE

x4

f

(b)

i

(Cl

ji

30 130 230 330 430 5301

1550 1650

KE/eV

Fig. 1. Auger spectra recorded after exposing the sample to oxgyen at 600 K: (a) clean surface: (b)

33 L 0,; (c) 100 L 0,; (d) 900 L 0,.

N. D.S. Canning et al. / Adsorption of oxygen on gold 245

Although calcium was not detectable following the prolonged heat treatment that led to the formation of the adsorbed oxygen state, a careful examination of the Auger spectra showed that silicon was present whenever the oxygen was detected (fig. 1). The ratio of the intensities of the Si(KLL) transition at 1605 eV to the O(KW) transition at about 508 eV was identical to that in SiO, [13]. Further spectroscopic evidence, to be given below, indicates that all of the adsorbed oxygen was associated with the silicon and further supports the SiO, stoichiometry.

Detection of silicon was particularly difficult since the most intense silicon Auger feature (at 94 eV for pure silicon) shifts to 78 eV in SiO, [15] and so is

obscured by the gold transition at 69 eV. This is clearly illustrated in fig. 2, which shows the effect of exposing a surface with some segregated silicon present to oxygen at room temperature. Spectrum 2a was recorded after ion bombardment of the surface and no silicon could be detected. Spectrum 2b

followed annealing of the sample to 600 K for 20 min. The two transitions at 88 and 94 eV have been observed previously for silicon atoms in a metallic envionment [16]. The silicon KLL transition was detected at 1618 eV. After exposing this surface to 1000 L of oxygen at room temperature spectrum 2c was recorded. Spectrum 2d was taken after an additional 1000 L exposure. The

AU

(a 1

(b 1

30 c

50 70 so 110 130 1550 1650

KE/cV

Fig. 2. The effect of room temperature oxidation on the silicon Auger spectra: (a) clean gold surface; (b) sample annealed to 600 K for 20 min; (c) 0, exposure 1000 L; (d) 0, exposure 2000 L.

246

x2

x2 " I

i)

N. D.S. Canning et al. / Adsorprion of oxygen on gold

Au(4f-I 1 ‘, 12

j-l A?.(4 f5,2’ i.!,.

‘1. ‘* ‘I. I, I. ,P. I !’ I I.

‘I’ 1: + 1. .1”!,:

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.I .I :, I ( ‘: , :, : r , :

4358c/s .;: ; I. ‘, ‘.

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. I. 1:

!, 1’ ‘\ :: r: I ” ., .. ,. ‘, I’

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1 X’ .._,,__ “__.’ .“. .: r: ”

,,..-‘.. ., ,: I: ‘_ . :\ ._._. ,J .._’ .\ ,..I’ P_____,--. ,I

\ . e-. ‘. _.___ . . (b )

, --; . . . . . c

,F. ,,A:-~‘,: . . .._ _ _ .i __.,. _... k.....’

.._... Y. (a)

__..,.”

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O(lS)

52 9.2 +.,. ;,’

980~1s ::: ..:’ :‘.‘., :.: ..,

:f j, ,c+ >: .:: ‘:.: (C )

:: :c .:’ :... :. ‘;

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i, . . <:.% ;: ‘?.

: : ‘..

$f’ Y:,

. . . . . . +c\” .G’ ?: . ..., ‘,s .,.... ;..i. +.:si. . .

n,.<.wi’- .i’ “I b) Xl ..‘. . . . . .

:; .:’ : . . . .

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:’ _.C. . . :

,I c. ..: .:.. (a 1

;: ‘t : .‘.’ ;c’

.; ‘k. ..,-.. -.;+ ,,,;,, , ‘i :‘.

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82 .PG I,..,,.> . . . . :.:::“-% ._ ,.... “...$j@’ ”

(,,, ..\i . . . . . . ..‘...iC.

Xl ‘“...~w:,,,.sM.,Y+

524 529 534 539 544 549

8Wev

Fig. 3. (i) Au(4f) and (ii) O(k) XPS spectra: (a) clean gold surface; (b) after heating the sample in oxygen (1000 L, T- 600 K); (c) after exposing the clean gold sample to oxygen with a hot Pt filament close to the surface.

N.D.S. Canning et al. / Aakorption of oxygen on gold 247

Si(KLL) transition was unaltered in intensity following the oxygen treatment but was shifted to 1605 eV. Oxygen adsorption caused the growth of an O(KW) feature and large shifts of the Si(KW) features, leading to the apparent disappearance of the transitions at 88 and 94 eV indicating that the oxygen was bonding to the silicon. The shift in the Si(KLL) transition from 1618 to 1605 eV is consistent with the formation of SiO, [13]. These results agree with a recent report that gold monolayers and multilayers can catalyse the oxidation of silicon surfaces at room temperature leading to complete oxidation of silicon atoms alloyed with the gold to SiO, [17].

Further evidence that the strongly bound oxygen state formed upon heating a gold crystal in oxygen is due to the formation of SiO, is given by the XPS results shown in fig. 3 and by the oxygen Auger spectra in fig. 4. These spectra were recorded following an oxygen dose of 1000 L with the sample at 600 K. The XPS spectrum 3(ii)b shows a single O(ls) peak with a FWHM of 2.0 eV

dN(E) dE

Au-O (b)

, 440 460 480 500 520 540

KE/eV

Fig. 4. High resolution O(KW) Auger spectra for the oxygen state produced by: (a) heating the gold sample in oxygen (30 L, T= 600 K); (b) exposing the room temperature sample to oxygen

atoms. Both spectra were recorded under identical conditions, the resolution was 2 eV. Spectrum (c) was recorded following the oxidation of a Cu(ll1) surface (300 L, T= 300 K) and is identical to the spectrum of Cu,O.


which is close to the instrumental resolution of 1.8 eV. The O(ls) binding energy of this state was 532.3 k 0.2 eV. This binding energy is close to that expected for SiO, on gold (532:3 k 0.2) estimated from the known Si(2p)-O(ls) separation in pure SiO, (429.6 eV [IS]) and the previously reported Si(2p) binding energy of oxidized silicon in the presence of gold multilayers (103.0 eV

[17]). The silicon XPS was too weak to be detected in our experiments. The width of the oxygen peak suggests that only one chemical form of oxygen is present. The Au(4f) features were attenuated following oxygen exposure and the formation of this strongly bound oxygen state, but no chemical shift due to oxide formation was observed (fig. 3(i)b). This contrasts with the noticeable

chemical shift of the Au (4f) levels produced by a similar coverage of oxygen following adsorption of oxygen atoms on the clean surface (see below).

The Auger O(KVV) lineshape of the thermally induced oxygen state shown in fig. 4a is identical to that of pure SiO, [19]. It was noticed that for a high coverage of this oxygen state prolonged exposure to the electron beam when recording the oxygen Auger spectrum led to a shift of the main negative feature from 504 to 509 eV. Such beam-induced effects for SiO, have previously been investigated [19] and account for the difference in the Auger spectra

for the strongly bound oxygen state reported in this study and that reported by

the Russian workers mentioned earlier [5]. The absence of a chemical shift of the Au(4f) features following the

formation of the thermally induced adsorbed oxygen state indicates that no gold oxides are present. The Si(KW) and Si(KLL) Auger spectra from this “oxidized” surface indicate that silicon is present and is chemically associated

with oxygen. The single O(ls) XPS peak suggests all of the oxygen is associated with the surface silicon. The binding energy of the O(ls) electrons and the fine structure of the O(KVV) Auger transitions are consistent with the presence of

SiO, .

3.2. Adsorption of atomic oxygen

When the clean gold surface at room temperature was exposed to oxygen in the presence of a platinum filament at 1400 K a new adsorbed oxygen state was detected with very different physical characteristics from the thermally produced oxygen state. At 1400 K no platinum deposition on the gold could be detected so that the new oxygen state was not associated with surface platinum. The XPS and AES spectra of this new oxygen state are compared with those of the strongly bound oxygen state in figs. 3 and 4. The O(ls) binding energy of 529.2 eV is typical for atomic oxygen bound to metal surfaces and compares well with that reported by Evans et al. [20] for the adsorbed oxygen state on gold produced by exposing the sample to oxygen in a microwave discharge. The Au(4f) transitions were affected by the presence of adsorbed atomic oxygen, showing small shoulders about 1.4 eV higher in binding energy than

N.D.S. Canning et al. / Adwrption of oxygen on gold 249

the main peaks following oxidation. A comparison of this binding energy shift

with those observed for mono-nuclear and polynuclear gold complexes [21,22] (summarized in table 3) suggests a formal oxidation state of + 1 for the surface

gold atoms. Several factors may limit the validity of this comparison, however. First,

relaxation effects for the gold atoms at the surface may differ from those in inorganic complexes, causing variations in the binding energy shifts for the two environments for a given oxidation state. Second, an increase in the covalent character of the metal-oxygen bond in the oxides compared to the metal-ligand bond in the inorganic complexes may well lead to a reduction in the binding energy shifts for a given formal oxidation state of the metal. This seems to be true for Au,O, which exhibits a binding energy shift of 1.9 eV for the Au

(4f,,,) feature compared to the clean metal [23]. A much larger shift would be predicted from the data for inorganic complexes. Consequently the formal oxidation state of the surface gold atoms associated with formation of the surface oxide is not clear from the observed binding energy shift alone, making determination of the stoichiometry of the surface oxide difficult.

The overall shape and position of the fine structure in the O(KW) Auger spectrum of this oxygen species on gold is typical of atomic oxygen adsorbed on metal surfaces and has been interpreted in terms of the closed shell 02- ion [24]. The spectrum is compared with that for chemisorbed oxygen on copper [25] in fig. 4, where it is seen that all the features are identical. On copper the feature at 520 eV has been assigned to an interatomic Auger transition involving the metal valence electrons [26] and its width has been shown to be sensitive to the oxidation state of the metal: Cu,O gives a well defined feature as in fig. 4, while for CuO it is not fully resolved from the major intra-atomic feature at 514 eV owing to the broader valence band of CuO compared to Cu,O. It is tempting to interpret the 520 eV feature in the case of gold in a similar manner, attributing it to the presence of Au+ ions. However, in the absence of UPS data for different oxidation states of gold and without O(KW) spectra for bulk Au203, the sensitivity of this feature to the oxidation state of gold must remain a matter of speculation.

The maximum coverage of chemisorbed atomic oxygen was’estimated by comparison of the Auger peak-to-peak height ratio O(515 eV)/Au(239 eV), which was 2.0 at saturation, with that reported for the (2 x 2) oxygen adlayer on Pt(ll1) (corresponding to a coverage of 3.8 X lOi atoms cm-* [27]). After

Table 3 The dependence of the binding energy of the Au(4f,,,) level on the formal oxidation state (OS) of gold from data in refs. [21,22]

BE (eV) OS

83.9 84.7 85.5 87.5 0.0 +os +1.0 +3.0

250 N.D.S. Canning et al. / Adwption of oxygen on gold

correction for the relative sensitivity of the Au(239 eV) and the Pt(237 eV) transitions of the clean metals [13], a coverage of 2.9 x 1Or5 atoms cm-* was obtained, which corresponds to a relative coverage of at least two monolayers, assuming that the sample surface was atomically smooth. A comparison of the XPS intensity ratio of the O(ls) signal at saturation and the Au(4p,,,) signal of the clean surface recorded under the same conditions with the

O(ls): Pt(4p,,z) intensity ratio of the (2 X 2)0 adlayer [27] gives a similar coverage estimate of 2.5 X 1015 atoms cm-2, after allowance was made for the dependence of the ionization cross sections of 0, Au and Pt on photon energy. The ionization cross sections of the elements calculated by Scofield [28] were

used to make these corrections. It is difficult to determine unambiguously if all the oxygen is present just as

chemisorbed atoms at the metal surface or if a true oxide layer has begun to form on the metal. The high oxygen concentration and the observation of a chemical shift of the Au(4f) levels are consistent with formation of an oxide at the surface since oxygen chemisorption usually ceases at much lower coverages and does not produce noticeable chemical shifts of the metal core levels. However, the oxide layer which is formed can be rapidly and completely removed by reaction with formic acid at room temperature [29], suggesting that the oxygen has not penetrated deep into the gold lattice. The XPS spectra support this contention. A single O(ls) peak with a binding energy characteris- tic of adsorbed atomic oxygen was observed at 529.2 eV. This is considerably different from the O(ls) binding energy previously reported for bulk Au,O, at 531.8 eV [23]. These results can be compared with those for oxygen adsorption

on Ag(ll0) where two O(ls) signals were observed at 529.2 and 531.2 eV,

assigned respectively to reactive, chemisorbed oxygen atoms and to unreactive oxygen atoms which had penetrated the surface of the crystal lattice [30].

A stoichiometry for the surface oxide layer may be estimated from a

comparison of the intensity of the chemically shifted Au(4f,,,) peak and the O(ls) peak. The shifted Au(4f,,2) peak intensity was obtained by subtraction of a clean surface Au(4f,,,) spectrum normalized to the unshifted Au(4f,,,) feature in the spectrum of the oxidized surface. It was assumed that the features in the difference spectrum were only due to the chemical shift of the 4f levels of gold atoms associated with oxygen. It was further assumed that the

oxygen forms a homogeneous layer with all the gold atoms in the layer characterized by this shifted peak. The oxygen-to-gold intensity ratio together with published atomic sensitivity factors [14] gave the ratio of oxygen to gold in this layer as 2.0 k 0.5, suggesting that a thin layer of gold oxide has begun to form. This surface oxide must be somewhat different from the bulk oxide since the Au(4f,,,) and O(ls) photoelectron binding energies are somewhat different. The observation that the oxide layer stops growing at an oxygen coverage of about 2.0 indicates that it is a superficial oxide stabilized by the surface. This implies there is an energy barrier to the formation of the bulk oxide from the surface oxide.

N. D. S. Canning et al. / Adsorption of oxygen on gold 251

I

495 544 592 640 688 735

T/K

Fig. 5. Thermal desorption spectra following the adsorption of atomic oxygen on the gold surface for varying initial coverages up to saturation. The heating rate was 21.5 K s-l. The mass spectrometer was tuned to m/e 32.

XPS and AES showed that the oxygen completely desorbed upon heating the crystal above 670 K. Thermal desorption spectra are shown in fig. 5 for varying initial coverges of adsorbed atomic oxygen. At intermediate coverages

252 N.D.S. Canning et al. / Adsorption of oqgen on gold

two desorption rate maxima were observed. As the initial coverage was increased the lower temperature maxima grew without a shift in position and the higher temperature peak was removed entirely. Since the rate of recombination of atomic oxygen is expected to be second order in the atomic oxygen concentration, the apparent first order behavior of the desorption peak at 654 K must result from the presence of attractive lateral interactions between oxygen atoms in a manner similar to that observed for oxygen on Ag(ll0) [31]. It is not clear why the high temperature state was lost with increasing coverage. Further desorption studies form structurally well defined surfaces are in progress to clarify this point. An estimate of the activation energy for oxygen atom recombination and desorption was extracted from the data by applying Redhead’s formula for the activation energy of a first order desorption process

and assuming a pre-exponential factor of lOI s-’ [32]. This yield a value Ed = 162.5 kJ mol-‘, which is just a little lower than the activation energy for

desorption of oxygen from silver [31]. If the dissociative adsorption of oxygen on gold were non-activated this desorption activation energy would be equal to

the heat of adsorption. The desorption activation energy is much larger than the heat of formation of the bulk gold oxide, and this may be traced to two causes. First, heats of adsorption of oxygen on noble metals are often larger than the heat of formation of the bulk oxide due to the unique properties of the surface environment [32]. Second, if adsorption is activated (with activation energy E,) the desorption activation energy is larger than the heat of adsorption by Ea. The observation that oxygen will not adsorb at room temperature on gold indicates that adsorption is activated. Comparison of Ed with the heat of formation of bulk gold oxide indicates that 0 < E, -c 38 kcal mall’.

4. Summary

In conclusion it has been shown that the strongly bound oxide on gold, previously reported by several authors, is probably attributable to the formation of SiO, from Si which diffuses out of the crystal on heating and is not due

to an activated adsorption of oxygen. For a clean crystal no adsorption was detected following oxygen exposures in excess of 5 x lo5 L and for crystal temperatures up to 600 K where rates of desorption of adsorbed atomic oxygen become significant. This indicates that the sticking probability for oxygen on this gold sample must be below 10p5, a much smaller value than that observed

for Ag(ll0) where S,, = 3 x 1O-3 [31]. It was shown that gold will adsorb

oxygen atoms at room temperature and that the activation energy for recombination and desorption of these atoms was about 163 kJ mol-‘. Both adsorbed atomic oxygen and oxygen associated with silicon on the gold surface have been characerized by XPS and high resolution Auger spectroscopy.

The inability of gold to adsorb oxygen under our experimental conditions is attributable to an activation barrier to adsorption. This activation energy for

N. D.S. Canning et al. / Ahorption of oxygen on gold 253

adsorption might be measured by studying the adsorption at higher oxygen

pressures than those accessible in our UHV apparatus or by employing molecular beam techniques. However, the results presented here indicate that great care must be taken in cleaning the samples for any attempt to measure this activation energy to be successful and to avoid interference form SiO, formation. These results also raise questions on the nature of the adsorbed oxygen states which have been observed following the high pressure oxidation of gold samples. Hopefully careful XPS and AES studies will resolve some of these questions.

Acknowledgements

The authors gratefully acknowledge the support of the National Science Foundation through grant NSF CPE 80-23815Al. Acknowledgement is also made to the Donors of the Petroleum Research Fund, administered by the

American Chemical Society for partial support of this research.

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adsorption of oxygen on gold

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