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On the surface oxidation of a gold electrode in 1N H2S04 electrolyte
Surface Science 141 (1984) 515-532 North-Holland, Amsterdam
515
ON THE SURFACE OXIDATION H,SO, ELECTROLYTE M:PEUCKERT,
F.P. COENEN
OF A GOLD
ELECTRODE
IN 1N
and H.P. BONZEL
Institut ficr Grenzfliichenforschung und Vakuumphysik, Postfach 1913, D - 5170 Jiilich, Fed. Rep. of Germany
Recieved 11 January 1984; accepted for publication
Kernforschungsanlage
JiiIich GmbH,
8 March 1984
A single crystal Au(100) electrode in 1N H2S0, electrolyte has been potentiostatically polarized at potentials from 0.8 to 4.0 V versus standard hydrogen electrode (SHE). Cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) analysis of the surface gave evidence for water adsorption up to 1.5 V. An 0 1s signal with an electron binding energy of 532.4 eV was found. Beyond the region of transition from OH adsorption to bulk gold oxidation, i.e. 1.4 to 2 V, thick oxidic adlayers were grown. The Au 4f,,, and 4f,,, levels with binding energies of 86.1 and 89.8 eV, respectively, the 0 1s signal at 530.8 eV, as well as the O-to-Au stoichiometry of two, suggested an oxyhydroxide AuOOH for the chemical composition of these thick adlayers. This assignment is in accordance with two cathodic reduction peaks in the cyclic voltammograms, one being interpreted as hydrogenation of an oxidic species and the other as of a hydroxidic species. Thermal decomposition at 400-500 K led to a mixture of Au,O, and Au metal. The measured 0 1s binding energy was 530.0 eV. Above 600 K all oxygen was desorbed.
1. Intmduction By electrochemical polarization at highly positive potentials thick oxidic adlayers may be grown on the surface of a gold electrode. Though the phenomenon itself has been known for a long time, the chemical nature of the film is still controversial. In the beginning those films were thought to consist of some kind of peroxide AuO*, but it soon became evident that the adlayer is containing gold in the formal oxidation state of 3 + [1,2]. This finding could be confirmed by two independent X-ray photoelectron spectroscopy (XPS) studies by Kim et al. [3] and Dickinson et al. [4]. But those XPS spectra did not allow a further discrimination between alternative Au 3 + compounds. Thus, it still remains an open question whether the adlayer phase is a hydroxide Au(OH), [l-3,5], a hydrated oxide Au,O, * H,O [4,6-g], an oxyhydroxide, i.e. the so-called meta-gold acid AuOOH [7-91, an anhydrous oxide Au,O, [lo-141, or a non-stoichiometric mixture or an ordered layer structure of some of these substances [6,13-151. 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
516
M. Peuckert et al. / Surface oxidation of gold electrode
In the present study both electrochemical and XPS experiments were performed in order to elucidate this unresolved question. Other points that will be approached in this paper besides the chemical identification of the surface oxidation phase are the correlation between electrochemical and XPS data, the transition from mere adsorption to bulk oxidation, the thermal stability of the adlayer, and a comparison with the gas phase oxidation of gold in 0.1 MPa oxygen.
2. Experimental Electrochemical experiments were performed in 1N H,SO, electrolyte (Merck Titrisol 9981 in bidistilled water) at 298 K with a standard hydrogen electrode (SHE) as reference and a platinum wire as counter electrode. A gold single crystal cut for (100) surface orientation (6 X 7 X 1.5 mm3 size as to fit into the sample holder of the ultrahigh vacuum system) was used as working electrode. A second series of experiments using a polycrystalline gold electrode gave identical results to those obtained with the Au(100) electrode. The Au sample was suspended into the electrolyte by a gold wire. Initially it was cleaned by a number of successive cycles of electrochemical oxidation, thermal treatment in ultrahigh vacuum (UHV) and 4.5 keV argon sputtering. Carbon originating from the sample transferring was the only impurity found as checked by Auger electron spectroscopy (AES); there was no evidence for sulfur, as sulfate anions, that could stem from the electrolyte. Cyclic voltammograms were obtained at 0.1 V s-l scan rate on the polycrystalline Au sample. For X-ray photoelectron spectroscopy (XPS) analysis the gold electrode was taken out of the electrolyte with the potentiostat switched on, then rinsed with triply distilled water and immediately transferred into the roughing pump stage of the UHV system [16]. After the protective water film was pumped off, the sample was transferred into the main UHV analysis chamber (Leybold-Heraeus) where XPS data were collected at a set sample temperature (k 5 K). XPS spectra were taken at 300 W (Mg Ka X-ray source) in the constant pass energy mode at an analyzer pass energy of 100 eV corresponding to a resolution of 1.3 eV. Reduction of the sample due to X-ray radiation, as reported in ref. [4], has never been observed in this study. All binding energies are referred to the Fermi level (ELin = 1249.3 eV) that has been determined by measuring the valence band edge of a Pt sample. Since the Au 4f binding energies for the adlayer were well reproducible in the experiments with 5 min polarization time and the peak positions did not shift during the initial heat treatment, charging corrections did not seem to be necessary. Atomic sensitivity factors used for quantitative XPS analysis were 1.9 for the Au 4f,,, and 0.63 for the 0 1s level. The factors were calculated from ionization cross-sections, the mean free
M. Peuckert et al. / Surface oxidation of gold electrode
517
electron escape depths (- EEi6) and the transmission function of our spectrometer (- Ezs9) [17]. The peak intensity ratio Au4f,,,,-to-Auclf,,, was taken to be the theoretical 4 : 3 [17]. Whenever in this study reference is made to gold or oxygen XPS intensities or ratios thereof, always corrected intensities are meant. Those values represent a direct measure of the concentration of the respective element in the surface layer. Flash desorption spectra were taken in the same UHV system with a Leybold-Heraeus Quadruvak Q200 mass spectrometer with ion currents of 10-’ A, after 5 min pola~zation at 3 V versus SHE. Thermal oxidation in 0.1 MPa flowing 0, (Messer Griesheim 99.998% passed over a 4 A molecular sieve) was done for 18 h at 900 K in a quartz tube furnace. Flow rate was 1 cm3 s-l.
3.
Results
3.1. Electrochemical
measurements
Valuable information about the surface chemistry of a gold electrode in IN H,SO, electrolyte is contained in the cyclic volta~o~~s of such a system, fig. 1. By applying a sequence of triangular potential sweeps and recording the current response that flows through the gold working electrode, one obtains reproducible closed cycles of current I versus potential @. In fig. 1, a series of eight cyclic voltammograms is shown, that differ insofar as the maximum potential reached in one cycle is stepwise increased from @,, equal to 1.4 V (a), 1.5 V (b), 1.6 V (c), 1.7 V (d), 1.8 V (e), 2.0 V (f), 2.5 V (g) up to 3.0 V (h). The observed peaks can be related to adsorption and desorption reactions of surface species at distinct voltages, the potential being a measure of the affinity of the respective electrochemical reaction. The integrated areas under the voltammogram curves are proportional to the flow of charge corresponding to the electrosorptive process. Thus, for a surface coverage B = 1 with a monovalent adsorbate like H’ or OH- on an ideally smooth polycrystalline gold surface with 1.15 x 1015 atoms per cm2 (average value of (111) (100) and (110) plane) one can calculate the equivalent charge density as 184 PC cmd2. A realistic estimate for a practical electrode with corners and edges and a roughness factor of about 1.5 to 2 [4] would assign a charge density of about 300 &-50 PC cmp2 to a monolayer of univalent adsorbate species. Even for an annealed surface a certain roughness is introduced by the severe surface oxidation and reduction in the course of repetitive potential cycling. The adsorption of hydrogen in the low potential region, fig. 1, is almost negligible. Other workers [18,19] have reported values of surface coverage of 3 to 4% of a monolayer. Therefore, hydrogen adsorption cannot be used as a calibration standard for monolayer coverage, as it is
518
M. Peuckert et al. / Surface oxidation of gold electrode
OH
ADSORPTION
I OXIDATION
0.5
0
-0.5
2
@
f
0 -2 -L -6 -8 -10 -12 -?l
0
0.5 ELECTRODE
1.5
1.0 POTENTIAL
Cp vs SHE
I V
Fig. 1. cyclic voltartmograms of polycrystalline gold in N,-purged 1N H,SO, between 0 V and @ =1.4 V (a), 1.5 V (b), 1.6 V (c), 1.7 V (d), 1.8 V (e), 2.0 V (f), 2.5 V (g), and 3.0 V (h); sweep raTO.1 V s-l.
possible with platinum electrodes [16,30]. The integrated. anodic and cathodic areas of oxygen adsorption from fig. 1 can be plotted. against the maximum sweep potential am,. Such a plot, fig. 2, then quantitatively describes the degree of oxidation of the surface under the assumption of 300 PC cmv2 for one monolayer. Between 1.36 and 1.8 K the cathodic area linearly increases with @,_; beginning at 1.36 V (the onset’is seen in fig. l), at 1.5 V the first, at 1.6 V a second, and at 1.7 V a third monolayer .of adsorbed OH- species are completed. In other words, a monolayer of Au(OH), or half a monolayer of
M. Peuckert et al. / Surface oxidation of gold electrode
0 2.0
15 ELECTRODE
POTENTIAL
25 +,,,
519
3.0 vs
SHE
I V
Fig. 2. Integrated charge flows from voltammograms, fig. 1, of adsorption and desorption of oxygen species as a function of maximum scan potential a,,,,,; broken lines indicate charge densities for one monolayer of OH- adsorbate, and the first and second layer of oxidized Au3+ species.
Au,O, may be formed. Spectroscopic arguments that support the hydroxide formula against an oxide will be presented in section 3.2. The stepwise coordination of three OH- entities, or respectively the change from adsorption to bulk oxidation, also shows up in three anodic peaks that can be identified in fig. 1, peak (Y at 1.45, /? at 1.52 V and y at 1.64 V. A second Au(OH), monolayer, corresponding to about 2 mC cmm2, is completed at 2 V, fig. 2. An additional fourth anodic peak 6 at 1.78 V is found in the voltammograms, fig. 1. At higher voltages seemingly uninhibited bulk oxidation takes place, and the formation of dark brown adlayers is observed. Their thickness is mainly determined by the time of polarization and does not seem to reach a limiting value, since the oxidation product may be porous and tends to flake off the electrode. Around 2 V the electrocatalytic evolution of oxygen gas commences. Therefore the anodic section of the voltammograms cannot be used for a quantitative evaluation of the thick adlayer build up, fig. 2. The position of the cathodic reduction/desorption peak is also indicative of the reactions that occur at a certain potential [40,41]. As shown in fig. 3, from the lowest value of a,,, at which a cathodic signal can be discerned, i.e. - 1.5 V, the peak maximum is shifted to lower voltages as Gm,, is increased. If the related reaction were a mere adsorption/desorption process, one would not expect a peak shift, as in fact one can see in the case of a platinum electrode
520
M. Peuckert et al. / Surfoce oxidation
ELECTRODE
POTENTIAL
+,,x
of gold electrode
vs SHE I V
Fig. 3. Plot of position of cathodic oxygen desorption peak maxima from volt~mograms, versus ma~rn~rn electrode potential.
fig. 1,
[16]. But for bulk oxidation the reduction peak tends to shift with increasing layer thickness due to a potential drop across the adlayer. From this it seems as if there can hardly be made any distinction between bulk oxidation and a previous adsorption step. A second interesting feature of the cyclic voltammograms is the appearance of a shoulder on the reduction peak at maximum scanning potentials above 2 to 2.2 V, that is after completion of the second oxidized monolayer, see figs. 1 and 3. This can be interpreted by the presence of a second type of oxidation product. One possible identification of this compound would be to attribute the occurrence of peak B to the transition from hydroxide Au(OH), to an oxyhydroxide AuOOH species. But, such an assignment is rather speculative, because it is so far based only on the voltammograms which tell about currents that flow at a given potential, but not about the chemical species that transport the current or that are formed as reaction products. 3.2. XPS analysis X-ray photoelectron spectroscopy (XPS) offers a technique to determine the composition of an electrochemically oxidized electrode surface. Spectra that were obtained after 5 min of polarization in 1N H,SO, at potentials between
M. Peuckert et al. / Surface oxidation of gold electrode
521
0.8 and 4 V versus SHE are shown in fig. 4; quantitative data are summarized in table 1. At low voltages up to 1.5 V the gold 4f spectrum shows a sharp 7/2 and 5/2 doublet with electron binding energies of 84.0 f 0.1 eV and 87.6 f 0.1 eV that are typical of Au metal [17]. The Au 4f,,, signal full-width-at-halfmaximum (FWHM) is 1.7 eV. In the oxygen region there is a signal around 532.4 f 0.1 eV. Between 1.5 and 2.2 V versus SHE a second Au 4f doublet, 86.0 and 89.7 eV, emerges on the high binding energy side until this new signal remains the only one, indicating the gradual oxidation of the surface. Oxidation is also documented by a decrease in the density of occupied valence states (not shown, changes similar as in fig. 6 below). After 5 min of polarization at voltages above 2.2 V the layer is already thick enough (d > 5 nm) that the Au 4f XPS signal is no longer complicated by a superposition of the Au metal doublet. This has been a problem in earlier XPS studies [3,4] where the highest potential applied has been 2 V versus SHE. Thus, those spectra were not representing a uniform compound and therefore not suitable for a quantitative analysis of the adlayer. From the spectra between 2.2 and 4 V, presented in this paper, we can try to analyze the oxidation layer (table 1, fig. 4 spectra (g) and fig. 6 spectra (a)). The electron binding energies of 86.1 and 89.8 eV for the Au 4f,,, and 4f,,, level and the narrowness of the 7/2 signal (FWHM = 1.8 eV) show by comparison with
0 (lsl
XPS
N(E)
95
05
90 E,/eV
Fig. 4. XPS spectra SHE.
of Au(100) electrode
00
535
530 E,I
525
eV
after 5 min anodic polarization
at indicated
voltage versus
M. Peuckert et al. / Strrface oxidation of gold electrode
522
Table 1 Potential dependence of Au 4f and 0 1s binding energies (ev), signal full-width-at-half-maximum FWHM (ev) for Au 4f,,, and 0 Is, and corrected signal intensity ratio a) Spectra
@
Au 4f
(v)
0 Is
FWHM
10 /IA”
7/2
5/2 87.6 87.7 87.6 87.7 89.3 89.6 89.6 89.7
1.7 1.7 1.9 2.1
532.4 532.3 531.8 531.6 530.8
2.8 3.2 3.8 3.7
0.46 0.47 0.50 1.00 0.91
2.1 1.9
530.7 530.8 530.7
2.8 2.8
1.60 2.01 1.96
89.8 89.8
1.8 1.9
530.8 530.8
2.8 2.8
2.0 2.02
t c d e
0.8 1.2 1.5 1.9 2.0
f f
2.1 2.2 2.5
84.0 84.0 84.0 84.1 85.7 86.0 86.1 86.0
i j
3.0 4.0
86.1 86.1
a> Spectra binding
FWHM
taken after 5 min anodic polarization at potential @ versus SHE in IN HsSO, energies are reported relative to the Fermi level.
at 298 K;
literature data [3,4,17] that the adlayer contains gold in the formal oxidation state of 3 + , and that there is no mixture of different valence states and possibly only one single compound present (table 1, fig. 6 spectra (a)). More information is yielded by the 0 1s signal. This signal has a binding energy around 530.8 + 0.2 eV and is rather broad (FWHM = 2.8 V). E, has to be compared with 0 1s binding energies of other transition metal compounds. Typical values for a variety of oxides and also for adsorbed oxygen lie about
1
2 ELECTRODE
Fig. 5. Oxygen-to-gold
4
3 POTENTIAL
ratio as a function
vs SHE IV of polarization
potential.
hf. Peuckert et al. / Surface oxidation of gold electrode
Au
(LfI
523
XPS
N(E)
95
90
Es I
85
80
eV
Fig. 6. XPS spectra for thermal decomposition min polarization at 3 V versus SHE.
535
530
525
12LO
E,/eV
of electrochemical
12L5 E,,,
oxidation
1250
/ eV
layer on Au after 5
530 f 0.5eV [20-261. Most hydroxides give values of E, of 531.5 f 0.5 eV [16,20-23,271. Finally for the third alternative, that is adsorbed water or water of hydration, reported data are scattered around 533 f 1 eV [22-251. Therefore, the observed 0 1s signal at 530.8 eV appears to be composed of an oxidic and a hydroxidic peak, but no peak from water of hydration. Such an interpretation fits nicely with the observed average oxygen-to-gold stoichiometry, i.e., the corrected XPS Ols-to-Au4f intensity ratio of 2 f 0.2. All the features are best described by an oxyhydroxide AuOOH. Au(OH), and Au,O, may be excluded based on the observed broadness and binding energy of the 0 1s signal and the O/Au ratio; Au,O, - H,O, an alternate to AuOOH, should show an asymmetric 0 1s signal with a much larger FWHM. Whether the topmost layer also consists of AuOOH or rather Au(OH), or contains adsorbed water cannot definitely be decided. Having assigned the spectra of thick surface films grown at high potentials to a Au(III)oxyhydroxide, one can now turn to the interpretation of the XPS spectra, fig. 4, that are observed at intermediate voltages. The main species at 0.8 and 1.2 V seems to be strongly adsorbed water, because of the 0 1s signal at 532.4 eV (table 1). A value of 532.2 eV was found by Fisher and Gland [24] for water adsorbed on Pt(ll1) at 100 K. The corrected O-to-Au ratio of 0.47 is considerably high. A monolayer of adsorbed oxygen should only give a ratio of
M. Peuckert et al. / Surface oxidation
524
of gold
electrode
about 0.25 [16,17]. It seems as if the Helmholtz layer could be preserved during transfer into the UHV chamber as suggested by Hansen [28]. These findings are rather surprising and not yet understood, since one would expect that molecularly adsorbed water is easily desorbed at 300 K in UHV, as known for experiments on Pt(ll1) [23,24]. As seen from figs. 4 and 5, the transition from a metallic surface to a thick AuOOH adlayer occurs in a narrow potential range between 1.5 and 2.2 V. Oxidation around 2 V leads to broadening of the 0 1s signal and a shift to lower binding energies, meaning that the adsorbed water becomes split into hydroxyl groups and protons to form Au(OH), and eventually AuOOH. In fact, the 0 1s signals measured around 2 V represent a quite complex superposition from different species such as H,O, OH- and 02-. An interesting detail in fig. 5 is the reversal of the O/Au ratio between 1.9 and 2.0 V. Though initially thought to just be an experimental uncertainty, this effect proved to be reproducible. As a comparison of the respective 0 1s data shows, table 1, the central peak position concomitantly shifts from 531.6 to 530.8 eV binding energy when going from 1.9 to 2 V. This may be interpreted as a change from hydrated Au(OH), to AuOOH surface species, which would explain the observed break in the O-to-Au ratio plot, fig. 5. 3.3. Thermal treatment The thermal decomposition of an adlayer grown during 5 min potentiostatitally at 3 V versus SHE was followed by XPS, fig. 6 and table 2 [1,7]. The Table 2 Thermal decomposition of oxidation versus SHE in 1N H,SO, ‘) Spectra
T (K) 300 350 420
layer on gold electrode
Au 4f
after 5 min anodic polarization
FWHM
0 1s
FWHM
Io/I,,
7/2
512 89.8 89.6 89.2 81.6 89.2 87.7 87.6 87.6 87.6 87.6
1.8 3.0 -
530.8 530.6 530.1
2.8 2.7 2.6
2.0 1.40 0.79
-
530.0
2.5
0.66
2.4 1.8 1.8 1.8
530.0 530.1
2.5 _
0.40 0.1 0 0
87.5
1.7
530.1
2.5
0.2
d
410
e f f
520 570 670 110
86.1 86.0 85.6 84.2 85.5 84.2 84.0 83.9 83.9 83.9
i b,
300
83.9
at 3 V
a) Spectra measured at indicated temperature; electron binding energies (ev) are given relative to = corrected signal intensity Fermi level; full-width-at-half-maximum FWHM in eV; IO/I,, ratio of 0 Is and Au 4f,,,. b, After thermal oxidation for 18 h at 900 K in 0.1 MPa flowing 0,.
M. Peuckert et al. / Surface oxidation of gold electrode
525
spectra are almost the reverse series as in fig. 4 for the electrochemical build up. The Au 4f spectra show the transition from Au3+ to Au metal. The shift of the 0 1s peak is significant in two ways. Firstly, the decrease in binding energy from 530.8 to 530.0 eV suggests the dehydration to gold oxide Au,O,, characterized by the latter 0 1s binding energy. The dehydration seems to be a~ompanied by a decomposition reaction, so that not a pure film of Au,O,, but only a mixture of oxide and metal can be obtained. Secondly, the decrease of the FWHM value from 2.8 to 2.5 eV supports the notion that the 0 Is peak at 530.8 eV, as already discussed, may be composed of an oxide peak at 530.0 eV and a hydroxide peak at about 531.5 + 0.5 eV [16,20-271. The gradual filling of the 5d/6s valence band also shows the transition from an oxidized surface phase to the bulk metal [29]. Integration of the Au 4f and 0 1s signals gives a quantitative account of the thermal decomposition, fig. 7. Beginning slightly above room temperature, the AuOOH loses water and oxygen, and at 600 K all oxygen finally desorbs and leaves a clean gold surface. The thermal decomposition has also been followed by temperature programmed desorption spectroscopy (TPDS), fig. 8. Dehydration of the surface phase causes a broad water signal with a maximum at 400 K. Readsorption and subsequent desorption from the sample holder and transfer rod leads to a high background in the water TPD spectrum. OZ evolves from the gold oxide phase with a strong signal at 600 K (p peak). Two smaller 0, features at 400 K ( CX)and 890 K (y) may be attributed to partial reduction to Au metal parallel to dehydration at low temperatures (a), and final oxygen desorption (y). The sudden rise in 0, partial pressure at 600 K apparently causes release of H,O from the walls of the system such that a peak in H,O partial pressure occurs at 600 K; this peak is thus considered to be an artifact and not characteristic of
300
500 TEMPERATURE
900
700 I K
Fig. 7. Corrected XPS signal intensities for thermal decomposition of oxidation layer; after 5 min polarization at 3 V (*O), after 1 h poIarization at 3 V (X - - - x).
M. Peuckert et al. / Surface oxidation of gold electrode
526
the Au sample. The TPDS results agree with a stepwise decomposition from AuOOH to Au,O,/Au and Au. With very thick layers of several hundred A [6,7,13], grown during 1 h at 3 V, an interesting effect can be observed, fig. 7. First, up to about 400 K dehy~ation is found, then a further but less pronounced decrease of the O-to-Au ratio, and then an increase of the oxygen signal intensity is observed, until at temperatures as high as 900 K oxygen has disappeared. One possible explanation for this behaviour could be the dissolution of oxygen into the gold metal layer that is formed during the thermal treatment. This oxygen then finally is set free at considerably higher temperatures than those of the desorption of oxygen from the surface. The thick AuOOH surface phases were also found to be X-ray amorphous, since no diffraction patterns (Cu Ka: tube, 40 kV/40 mA) could be obtained. Treatment of a thick layer (grown during 1 h at 3 V) in 0.1 MPa 0, at 390 K for 1 h did not lead to a pure Au,O, phase, but the photoelectron spectra were similar to those of fig. 6, spectra (b), indicating the presence of Au metal. Oxidation of a clean Au(100) surface in 0.1 MPa of flowing 0, at 900 K for 18 h let to less than a monolayer of oxygen with an 0 1s binding energy of 530.1 eV (table 2, spectra (i)).
/ I
‘-‘,a
ml
Loo,
5Nl
600
h
1
700
800
TEMPERATURE
900
ICQO
I K
Fig. 8. Temperature programmed decomposition spectra, scan rate 5 K s-l; 0, x0.02 (- - -).
thinner adlayer,
IU. Peuckert et al. / Surface oxidation oj gold electrode
521
4. Discussion Based on the electrochemical and spectroscopic results presented here, one can try to draw a schematic picture of the surface chemistry on a gold electrode in contact with 1N H,SO,. In fig. 9 a tentative and simplified model of the surface composition is shown as a function of the applied potential versus SHE across the electrochemical double layer. In contrast to the behavior of a platinum electrode [16,30] there is almost no hydrogen adsorption observed in the low potential region. Reported coverages are 3 to 4% of a monolayer [18,19]. The electrocatalytic hydrogen evolution and hydrogen oxidation reactions take place on a largely H adsorbate free metal surface. At all potentials up to 1.3 V water is the primary adsorbate. Below the point of zero charge (PZC) of 0.18 V versus SHE [31] the preferential orientation will be with the hydrogen towards the metal surface, above the PZC bonding will most probably occur via the occupied non-bonding orbitals of the oxygen. In fact, it proved to be possible to transfer a thin water film into the UHV chamber [28]. This may not necessarily mean that the Helmholtz layer is preserved in the absence of electrolyte and applied potential, but rather that the sorption bond, that has been formed electrochemically is strong enough to survive the sample transfer. The adlayer may be stabilized by the ubiquitious carbon contaminant which was seen in the AES spectrum; sulfur has not been detected by either XPS or AES (see section 2) [42]. Between 1.3 and 1.5 V the water adlayer gets oxidized to an adsorbed layer of hydroxyl groups as seen from the cyclic voltammograms: Au. H,O,,
--, AuOH,,
+ H+ + e- .
0)
But there is no indication that oxidation stops at one monolayer of adsorbate, instead there is a continuous transition from OH adsorption to bulk hydroxide formation: AuOH,,
+ 2 H,O --, Au(OH),
+ 2 H+ + 2 e- .
(2)
The three anodic peaks, seen in fig. 1, at 1.45, 1.51 and 1.64 V may be attributed to those three oxidation steps described by reactions (1) and (2). Kirk et al. [32] found three similar anodic peaks on a gold electrode in alkali electrolyte and explained their observation by successive structure-sensitive adsorption of OH on (llO), (100) and (111) crystal faces of the polycrystalline sample. But, at least in our study with acid electrolyte, such an assignment to specific OH adsorption on different crystal planes can be excluded based on a number of arguments. The integrated charge density corresponds to a complete OH monolayer already at 1.5 V (fig. 2), therefore peaks fl and y fall into the range of incipient bulk oxidation and not of completion of a first OH monolayer. Also, one would expect three corresponding desorption peaks on the cathodic scan, which obviously is not the case, fig. 1. Finally, Ross’s [33]
I
1
Fig. 9. Model for surface composition
1.5
I t
of gold electrode
t
I
ELECTRODE
I
1
I
1
1
2.5
of applied
YS SHE/v
as a function
POTENTIAL
in acid electrolyte
2.0
potential.
I
!
I
1
1
3.0
hf. Peuckert et al. / Surface oxidation of gold electrode
529
work on single crystal platinum electrodes demonstrated that OH adsorption, at least on Pt, is not structure sensitive. It should rather be described as non-selective ionosorption. Around 1.7 V the first, around 2.0 V the second layer of Au(OH), is built up, fig. 2. A fourth anodic peak 6 at 1.78 V is observed, fig. 1. The XPS spectra taken in this range show a broad superposition of 0 1s signals of water, oxide and hydroxyl groups. Earlier XPS investigations were carried out in this potential range, therefore the 0 Is spectra were difficult to interpret and reported 0 1s binding energies differed by 1.5 eV between the two studies [3,4]. A new feature evolves at potentials above 2 V. As the adlayer thickness in creases beyond 2 monolayers of hydrated Au(OH),, a second cathodic reduction peak, B, figs. 1 and 3, appears as a shoulder, and the XPS 0 1s spectra give evidence for the presence of an oxyhydroxide, fig. 4. XPS spectra of thick uniform adlayers, by their binding energies, FWHM and corrected signal intensity, lead to the conclusion that in the high potential range the surface consists of AuOOH. Already above 2.2 V, film growth seems to proceed uninhibited up to several hundred A thickness, in accordance with refs. [6,7,13]. With this in mind, one may write the reaction for bulk oxidation at the hydroxide/metal interface [9] following reaction (1) like reaction (3): AuOH,,
+ H,O + AuOOH + 2 H+ + 2 e- .
(3)
Reactions (2) and (3) differ only in the degree of hydration of the product. The cathodic reduction may then formally be described by reactions (4) and (5): 3 AuOOH + 3 H+ + 3 e- + 2 AUK Au(OH),
+ 3 H+ + 3 e- + Au + 3 H,O.
+ Au,
(4) (5)
Though Au(OH), and AuOOH only differ in the degree of hydration, reduction of AuOOH according to reaction (4) is basically a hydrogenation reaction and will therefore most probably occur at a different potential than reaction (5). The appearance of a shoulder B in the cyclic voltammograms, figs. 1 and 3, concomitant with the formation of AuOOH as indicated by XPS, makes an assignment of peak B to reaction (4) and peak A to reaction (5) quite reasonable. Another explanation to account for two cathodic peaks, as given in refs. [6,10], proposes a layered oxide film composed of an inner oxide and a more hydrated outer oxide. An uniform oxyhydroxide, as shown in this study, can equally well explain the observed phenomenon. A thin surface film (in contact with electrolyte) of maybe one or two layers of Au(OH,) on a thick Au,O, adlayer (5 nm or more) could possibly lead to similar 0 1s signals as in fig. 4 spectra (g) and fig. 6 spectra (a), considering the mean 0 1s electron escape depth of 1.1 nm [16,17]; but a thin film of Au(OH),, on the other hand, cannot cause such a strong cathodic peak B as found in the voltammograms, fig. 1 scans (g) and (h). Also, broader Au 4f signals by superposition of two distinct oxidic and hydroxidic species should be anticipated.
M. Peuckert et al. / Surface oxidation of gold electrode
530
Electrocatalytic oxygen evolution commences around 2 V versus SHE, thus exhibiting a tremendous overpotential against the theoretical reversible oxygen potential of 1.23 V. The presence of an oxidized adlayer seems to be a requirement for water oxidation to occur [34]. The thermal degradation of electrochemically prepared Au(OH), or AuOOH does not lead to pure anhydrous Au,O,, figs. 6 and 7. Even in an atmosphere of 0.1 MPa of 0, decomposition takes place, approximately according to reaction (6): 6 AuOOH
+ Au,O,
+ 4 Au + 3 0, + 3 H,O.
(6)
The only product that is always found is a mixture of metal and oxide. From thin layers, all oxygen is desorbed at temperatures beyond 600 K, 2Au,O,+4Au+30,,
(7)
but with very thick oxidation layers, oxygen becomes trapped under the surface. Thus, in XPS the 0 1s signal intensity can reincrease above 600 K as the segregation of oxygen gets enhanced and oxygen diffuses to the surface and there desorbs. It is not yet clear how this “dissolved” oxygen should be described. Is it really dissolved in interstitial metal lattice positions, are there trapped Au,O, oxide clusters in a densely sintered metal matrix, or encapsulated oxygen gas in lattice imperfections of the gold? It is worthwhile to compare this seemingly unusual behaviour with the known chemistry of bulk gold oxides. Only hydrated Au,O, can be obtained as such. But also compounds with formal stoichiometries Au0 and Au,0 have been described in the early literature [9,35] and were eventually found to consist of mixtures of Au,O, with Au in the respective ratios. It may be suggested that the temperature ranges of restrained decomposition, fig. 7, with O-to-Au ratios of about 0.8 and 0.4 are related to the stability of these mixed phases, “AuO” and “Au *O”. All attempts to prepare oxide adlayers on the same sample in 0.1 MPa of oxygen gas at temperatures between room temperature and 900 K failed. Less than a complete monolayer of oxygen (0, < 1) was adsorbed after 18 h in 0.1 MPa 0, at 900 K (table 2, spectra (i)) [36,37]. An 0 1s binding energy of 530.1 eV (FWHM = 2.5 eV) supports the conclusions drawn in the foregoing sections. Extended surface oxidation as reported in other studies [38,39] may be attributed to contaminants like calcium.
5. Conclusions The surface electrochemistry on a gold electrode temperature has been investigated. Electrochemical XPS experiments were found to yield complementary
in 1N H,S04 at room cyclic voltammetry and data that are consistent
A4. Peuckert et al. / Surface oxidation of gold electrode
531
with each other. Occurrence of two cathodic peaks in the voltannnograms as well as the measured 0 1s and Au 4f binding energies and the atomic O-to-Au ratio of two are consistent with an oxyhydroxide AuOOH as the predominant surface phase, that grows under anodic polarization at voltages greater than 2.1 V versus SHE. The data allow the distinction of AuOOH from Au(OH),, Au,O, and Au,O, . xH,O, formulas that have previously been proposed for the oxidation adlayer. At lower voltages than 2.1 V the broad 0 1s signals suggest the presence of water and oxidic and hydroxidic species in adlayers only a few monolayers thick, while below 1.5 V adsorbed water is found as main surface species. Thermal treatment of a thick AuOOH adlayer does not lead to pure Au203, but to mixed phases of Au,O, and Au with O-to-Au ratios of 0.8 and 0.4, depending on temperature, and finally to a clean Au surface. At 900 K in 0.1 MPa of 0, gas only oxygen adsorption, but no bulk oxidation is observed.
Acknowledgement The authors thank G. Pirug for many helpful discussions.
References [l] F. Jima and 0. Buryanek, 2. Elektrochem. 29 (1923) 126. [2] F. Jirsa and H. Jelinek, Z. Electrochem. 30 (1924) 286. (31 KS. Kim, C.D. SeII and N. Winograd in: Proc. Electrocatalysis, Electrochem. Sot., Ed. M.W. Breiter (1974) p. 242. [4] T. Dickinson, A.F. Povey and P.M.A. Sherwood, J. Chem. Sot. Faraday Trans. I 71 (1975) 298. [5] (a) R. Cordova O., M.E. Martins and A.J. Arvia, J. Electrochem. Sot. 126 (1979) 1172; (b) R. Cordova O., M.E. Martins and A.J. Arvia, Electrochim. Acta 25 (1980) 453. [6] L.D. Burke and M. McRamr, J. Electroanal. Chem. 125 (1981) 387. [7] H.A. Laitinen and M.S. Chao, J. Electrochem. Sot. 108 (1961) 726. [8] S.B. Brummer and A.C. Makrides, J. EIectrochem. Sot. 111 (1964) 1122. [9] P. SchottBmder, Liebigs Ann. 217 (1883) 312, 340. [lo] J.-M. Cesbron et al., Compt. Rend. (Paris) C266 (1968) 1667. [ll] D.A.J. Rand and R. Woods, J. EIectroanaI. Chem. 31 (1971) 29. [12] J.W. Schultze and K.J. Vetter, Ber. Bunsenges. Physik. Chem. 75 (1971) 470. [13] M.M. Lohrengel and J.W. Schultze, Electrochim. Acta 21 (1976) 957. 1141 J.W. Schultze, Electrochim. Acta 17 (1972) 451. [15] E.J. Bard, Ed., Encyclopedia of Electrochemistry of the Elements, Vol. IV, No. 3, Gold (Dekker, New York, 1975). [16] M. Peuckert, F.P. Coenen and H.P. Bonzel, Electrochim. Acta, in press. [17] (a) Handbook of X-Ray Photoelectron Spectroscopy, Ed. G.E. Muilenberg (Perkin-Elmer, Physical Electronics Division, Eden Prairie, MN, 1979); (b) J.A. Scofield, J. Electron Spectrosc. Related Phenomena 8 (1976) 129; (c) C.D. Wagner, J. Electron Spectrosc. Related Phenomena 32 (1983) 99; (d) C.R. Bnmdle, J. Vacuum Sci. Technol. 11 (1974) 212.
532
M. Peuckert et al. / Surface oxidation of gold electrode
[18] M. Breiter, C.A. Knorr and W. Viilkl, Z. Elektrochem. 59 (1955) 681. [19] F.G. Will and C.A. Knorr, Z. Elektrochem. 64 (1960) 270. [20] N.S. McIntyre in: Applied Electron Spectroscopy for Chemical Analysis, Eds. H. Windawi and F.F.-L. Ho (Wiley, New York, 1982) p. 89. [21] CT. Au and M.W. Roberts, Chem. Phys. Letters 74 (1980) 472. [22] M.W. Roberts, Pure Appl. Chem. 53 (1981) 2269. [23] G.B. Fisher and B.A. Sexton, Phys. Rev. Letters 44 (1980) 683. [24] G.B. Fisher and J.L. Gland, Surface Sci. 94 (1980) 446. [25] P.R. Norton, J. Catalysis 36 (1975) 211. [26] G. BrodCn, G. Pirug and H.P. Bonzel, Chem. Phys. Letters 73 (1980) 506. [27] M. Peuckert and H. Ibach, Surface Sci. 136 (1984) 319. [28] W.N. Hansen, Surface Sci. 101 (1980) 109. [29] S. Hiifner, Topics Appl. Phys. 27 (1979) 173. [30] (a) P.N. Ross, J. Electrochem. Sot. 126 (1979) 67; (b) P.N. Ross, Surface Sci. 102 (1981) 463. [31] S. Trasatti, J. Electroanal. Chem. 33 (1971) 351. [32] D.W. Kirk, F.R. Foulkes and W.F. Graydon, J. Electrochem. Sot. 127 (1980) 1069. [33] P.N. Ross, Jr., J. Electrochem. Sot. 126 (1979) 78. [34] G. Grtineberg, Electrochim. Acta 10 (1965) 339. [35] G. K&s, Liebigs Ann. 237 (1887) 274. [36] P.C. Richardson and D.R. Rossington, J. Catalysis 20 (1971) 420. [37] D.D. Eley and P.B. Moore, Surface Sci. 76 (1978) L599. [38] M.A. Chester and G.A. Somorjai, Surface Sci. 52 (1975) 21. [39] P. Legare, L. Hilaire, M. Sotto and G. Maire, Surface Sci. 91 (1980) 175. [40] P. Stonehart, H.A. Kozlowska and B.E. Conway, Proc. Roy. Sot. (London) A310 (1969) 541. [41] H. Angerstein-Kozlowska, B.E. Conway and W.B.A. Sharp, J. Electroanal. Chem. 43 (1973) 9. [42] F.T. Wagner and P.N. Ross, Jr., J. Electrochem. Sot. 130 (1983) 1789.
515
ON THE SURFACE OXIDATION H,SO, ELECTROLYTE M:PEUCKERT,
F.P. COENEN
OF A GOLD
ELECTRODE
IN 1N
and H.P. BONZEL
Institut ficr Grenzfliichenforschung und Vakuumphysik, Postfach 1913, D - 5170 Jiilich, Fed. Rep. of Germany
Recieved 11 January 1984; accepted for publication
Kernforschungsanlage
JiiIich GmbH,
8 March 1984
A single crystal Au(100) electrode in 1N H2S0, electrolyte has been potentiostatically polarized at potentials from 0.8 to 4.0 V versus standard hydrogen electrode (SHE). Cyclic voltammetry and X-ray photoelectron spectroscopy (XPS) analysis of the surface gave evidence for water adsorption up to 1.5 V. An 0 1s signal with an electron binding energy of 532.4 eV was found. Beyond the region of transition from OH adsorption to bulk gold oxidation, i.e. 1.4 to 2 V, thick oxidic adlayers were grown. The Au 4f,,, and 4f,,, levels with binding energies of 86.1 and 89.8 eV, respectively, the 0 1s signal at 530.8 eV, as well as the O-to-Au stoichiometry of two, suggested an oxyhydroxide AuOOH for the chemical composition of these thick adlayers. This assignment is in accordance with two cathodic reduction peaks in the cyclic voltammograms, one being interpreted as hydrogenation of an oxidic species and the other as of a hydroxidic species. Thermal decomposition at 400-500 K led to a mixture of Au,O, and Au metal. The measured 0 1s binding energy was 530.0 eV. Above 600 K all oxygen was desorbed.
1. Intmduction By electrochemical polarization at highly positive potentials thick oxidic adlayers may be grown on the surface of a gold electrode. Though the phenomenon itself has been known for a long time, the chemical nature of the film is still controversial. In the beginning those films were thought to consist of some kind of peroxide AuO*, but it soon became evident that the adlayer is containing gold in the formal oxidation state of 3 + [1,2]. This finding could be confirmed by two independent X-ray photoelectron spectroscopy (XPS) studies by Kim et al. [3] and Dickinson et al. [4]. But those XPS spectra did not allow a further discrimination between alternative Au 3 + compounds. Thus, it still remains an open question whether the adlayer phase is a hydroxide Au(OH), [l-3,5], a hydrated oxide Au,O, * H,O [4,6-g], an oxyhydroxide, i.e. the so-called meta-gold acid AuOOH [7-91, an anhydrous oxide Au,O, [lo-141, or a non-stoichiometric mixture or an ordered layer structure of some of these substances [6,13-151. 0039-6028/84/$03.00 0 Elsevier Science Publishers (North-Holland Physics Publishing Division)
B.V.
516
M. Peuckert et al. / Surface oxidation of gold electrode
In the present study both electrochemical and XPS experiments were performed in order to elucidate this unresolved question. Other points that will be approached in this paper besides the chemical identification of the surface oxidation phase are the correlation between electrochemical and XPS data, the transition from mere adsorption to bulk oxidation, the thermal stability of the adlayer, and a comparison with the gas phase oxidation of gold in 0.1 MPa oxygen.
2. Experimental Electrochemical experiments were performed in 1N H,SO, electrolyte (Merck Titrisol 9981 in bidistilled water) at 298 K with a standard hydrogen electrode (SHE) as reference and a platinum wire as counter electrode. A gold single crystal cut for (100) surface orientation (6 X 7 X 1.5 mm3 size as to fit into the sample holder of the ultrahigh vacuum system) was used as working electrode. A second series of experiments using a polycrystalline gold electrode gave identical results to those obtained with the Au(100) electrode. The Au sample was suspended into the electrolyte by a gold wire. Initially it was cleaned by a number of successive cycles of electrochemical oxidation, thermal treatment in ultrahigh vacuum (UHV) and 4.5 keV argon sputtering. Carbon originating from the sample transferring was the only impurity found as checked by Auger electron spectroscopy (AES); there was no evidence for sulfur, as sulfate anions, that could stem from the electrolyte. Cyclic voltammograms were obtained at 0.1 V s-l scan rate on the polycrystalline Au sample. For X-ray photoelectron spectroscopy (XPS) analysis the gold electrode was taken out of the electrolyte with the potentiostat switched on, then rinsed with triply distilled water and immediately transferred into the roughing pump stage of the UHV system [16]. After the protective water film was pumped off, the sample was transferred into the main UHV analysis chamber (Leybold-Heraeus) where XPS data were collected at a set sample temperature (k 5 K). XPS spectra were taken at 300 W (Mg Ka X-ray source) in the constant pass energy mode at an analyzer pass energy of 100 eV corresponding to a resolution of 1.3 eV. Reduction of the sample due to X-ray radiation, as reported in ref. [4], has never been observed in this study. All binding energies are referred to the Fermi level (ELin = 1249.3 eV) that has been determined by measuring the valence band edge of a Pt sample. Since the Au 4f binding energies for the adlayer were well reproducible in the experiments with 5 min polarization time and the peak positions did not shift during the initial heat treatment, charging corrections did not seem to be necessary. Atomic sensitivity factors used for quantitative XPS analysis were 1.9 for the Au 4f,,, and 0.63 for the 0 1s level. The factors were calculated from ionization cross-sections, the mean free
M. Peuckert et al. / Surface oxidation of gold electrode
517
electron escape depths (- EEi6) and the transmission function of our spectrometer (- Ezs9) [17]. The peak intensity ratio Au4f,,,,-to-Auclf,,, was taken to be the theoretical 4 : 3 [17]. Whenever in this study reference is made to gold or oxygen XPS intensities or ratios thereof, always corrected intensities are meant. Those values represent a direct measure of the concentration of the respective element in the surface layer. Flash desorption spectra were taken in the same UHV system with a Leybold-Heraeus Quadruvak Q200 mass spectrometer with ion currents of 10-’ A, after 5 min pola~zation at 3 V versus SHE. Thermal oxidation in 0.1 MPa flowing 0, (Messer Griesheim 99.998% passed over a 4 A molecular sieve) was done for 18 h at 900 K in a quartz tube furnace. Flow rate was 1 cm3 s-l.
3.
Results
3.1. Electrochemical
measurements
Valuable information about the surface chemistry of a gold electrode in IN H,SO, electrolyte is contained in the cyclic volta~o~~s of such a system, fig. 1. By applying a sequence of triangular potential sweeps and recording the current response that flows through the gold working electrode, one obtains reproducible closed cycles of current I versus potential @. In fig. 1, a series of eight cyclic voltammograms is shown, that differ insofar as the maximum potential reached in one cycle is stepwise increased from @,, equal to 1.4 V (a), 1.5 V (b), 1.6 V (c), 1.7 V (d), 1.8 V (e), 2.0 V (f), 2.5 V (g) up to 3.0 V (h). The observed peaks can be related to adsorption and desorption reactions of surface species at distinct voltages, the potential being a measure of the affinity of the respective electrochemical reaction. The integrated areas under the voltammogram curves are proportional to the flow of charge corresponding to the electrosorptive process. Thus, for a surface coverage B = 1 with a monovalent adsorbate like H’ or OH- on an ideally smooth polycrystalline gold surface with 1.15 x 1015 atoms per cm2 (average value of (111) (100) and (110) plane) one can calculate the equivalent charge density as 184 PC cmd2. A realistic estimate for a practical electrode with corners and edges and a roughness factor of about 1.5 to 2 [4] would assign a charge density of about 300 &-50 PC cmp2 to a monolayer of univalent adsorbate species. Even for an annealed surface a certain roughness is introduced by the severe surface oxidation and reduction in the course of repetitive potential cycling. The adsorption of hydrogen in the low potential region, fig. 1, is almost negligible. Other workers [18,19] have reported values of surface coverage of 3 to 4% of a monolayer. Therefore, hydrogen adsorption cannot be used as a calibration standard for monolayer coverage, as it is
518
M. Peuckert et al. / Surface oxidation of gold electrode
OH
ADSORPTION
I OXIDATION
0.5
0
-0.5
2
@
f
0 -2 -L -6 -8 -10 -12 -?l
0
0.5 ELECTRODE
1.5
1.0 POTENTIAL
Cp vs SHE
I V
Fig. 1. cyclic voltartmograms of polycrystalline gold in N,-purged 1N H,SO, between 0 V and @ =1.4 V (a), 1.5 V (b), 1.6 V (c), 1.7 V (d), 1.8 V (e), 2.0 V (f), 2.5 V (g), and 3.0 V (h); sweep raTO.1 V s-l.
possible with platinum electrodes [16,30]. The integrated. anodic and cathodic areas of oxygen adsorption from fig. 1 can be plotted. against the maximum sweep potential am,. Such a plot, fig. 2, then quantitatively describes the degree of oxidation of the surface under the assumption of 300 PC cmv2 for one monolayer. Between 1.36 and 1.8 K the cathodic area linearly increases with @,_; beginning at 1.36 V (the onset’is seen in fig. l), at 1.5 V the first, at 1.6 V a second, and at 1.7 V a third monolayer .of adsorbed OH- species are completed. In other words, a monolayer of Au(OH), or half a monolayer of
M. Peuckert et al. / Surface oxidation of gold electrode
0 2.0
15 ELECTRODE
POTENTIAL
25 +,,,
519
3.0 vs
SHE
I V
Fig. 2. Integrated charge flows from voltammograms, fig. 1, of adsorption and desorption of oxygen species as a function of maximum scan potential a,,,,,; broken lines indicate charge densities for one monolayer of OH- adsorbate, and the first and second layer of oxidized Au3+ species.
Au,O, may be formed. Spectroscopic arguments that support the hydroxide formula against an oxide will be presented in section 3.2. The stepwise coordination of three OH- entities, or respectively the change from adsorption to bulk oxidation, also shows up in three anodic peaks that can be identified in fig. 1, peak (Y at 1.45, /? at 1.52 V and y at 1.64 V. A second Au(OH), monolayer, corresponding to about 2 mC cmm2, is completed at 2 V, fig. 2. An additional fourth anodic peak 6 at 1.78 V is found in the voltammograms, fig. 1. At higher voltages seemingly uninhibited bulk oxidation takes place, and the formation of dark brown adlayers is observed. Their thickness is mainly determined by the time of polarization and does not seem to reach a limiting value, since the oxidation product may be porous and tends to flake off the electrode. Around 2 V the electrocatalytic evolution of oxygen gas commences. Therefore the anodic section of the voltammograms cannot be used for a quantitative evaluation of the thick adlayer build up, fig. 2. The position of the cathodic reduction/desorption peak is also indicative of the reactions that occur at a certain potential [40,41]. As shown in fig. 3, from the lowest value of a,,, at which a cathodic signal can be discerned, i.e. - 1.5 V, the peak maximum is shifted to lower voltages as Gm,, is increased. If the related reaction were a mere adsorption/desorption process, one would not expect a peak shift, as in fact one can see in the case of a platinum electrode
520
M. Peuckert et al. / Surfoce oxidation
ELECTRODE
POTENTIAL
+,,x
of gold electrode
vs SHE I V
Fig. 3. Plot of position of cathodic oxygen desorption peak maxima from volt~mograms, versus ma~rn~rn electrode potential.
fig. 1,
[16]. But for bulk oxidation the reduction peak tends to shift with increasing layer thickness due to a potential drop across the adlayer. From this it seems as if there can hardly be made any distinction between bulk oxidation and a previous adsorption step. A second interesting feature of the cyclic voltammograms is the appearance of a shoulder on the reduction peak at maximum scanning potentials above 2 to 2.2 V, that is after completion of the second oxidized monolayer, see figs. 1 and 3. This can be interpreted by the presence of a second type of oxidation product. One possible identification of this compound would be to attribute the occurrence of peak B to the transition from hydroxide Au(OH), to an oxyhydroxide AuOOH species. But, such an assignment is rather speculative, because it is so far based only on the voltammograms which tell about currents that flow at a given potential, but not about the chemical species that transport the current or that are formed as reaction products. 3.2. XPS analysis X-ray photoelectron spectroscopy (XPS) offers a technique to determine the composition of an electrochemically oxidized electrode surface. Spectra that were obtained after 5 min of polarization in 1N H,SO, at potentials between
M. Peuckert et al. / Surface oxidation of gold electrode
521
0.8 and 4 V versus SHE are shown in fig. 4; quantitative data are summarized in table 1. At low voltages up to 1.5 V the gold 4f spectrum shows a sharp 7/2 and 5/2 doublet with electron binding energies of 84.0 f 0.1 eV and 87.6 f 0.1 eV that are typical of Au metal [17]. The Au 4f,,, signal full-width-at-halfmaximum (FWHM) is 1.7 eV. In the oxygen region there is a signal around 532.4 f 0.1 eV. Between 1.5 and 2.2 V versus SHE a second Au 4f doublet, 86.0 and 89.7 eV, emerges on the high binding energy side until this new signal remains the only one, indicating the gradual oxidation of the surface. Oxidation is also documented by a decrease in the density of occupied valence states (not shown, changes similar as in fig. 6 below). After 5 min of polarization at voltages above 2.2 V the layer is already thick enough (d > 5 nm) that the Au 4f XPS signal is no longer complicated by a superposition of the Au metal doublet. This has been a problem in earlier XPS studies [3,4] where the highest potential applied has been 2 V versus SHE. Thus, those spectra were not representing a uniform compound and therefore not suitable for a quantitative analysis of the adlayer. From the spectra between 2.2 and 4 V, presented in this paper, we can try to analyze the oxidation layer (table 1, fig. 4 spectra (g) and fig. 6 spectra (a)). The electron binding energies of 86.1 and 89.8 eV for the Au 4f,,, and 4f,,, level and the narrowness of the 7/2 signal (FWHM = 1.8 eV) show by comparison with
0 (lsl
XPS
N(E)
95
05
90 E,/eV
Fig. 4. XPS spectra SHE.
of Au(100) electrode
00
535
530 E,I
525
eV
after 5 min anodic polarization
at indicated
voltage versus
M. Peuckert et al. / Strrface oxidation of gold electrode
522
Table 1 Potential dependence of Au 4f and 0 1s binding energies (ev), signal full-width-at-half-maximum FWHM (ev) for Au 4f,,, and 0 Is, and corrected signal intensity ratio a) Spectra
@
Au 4f
(v)
0 Is
FWHM
10 /IA”
7/2
5/2 87.6 87.7 87.6 87.7 89.3 89.6 89.6 89.7
1.7 1.7 1.9 2.1
532.4 532.3 531.8 531.6 530.8
2.8 3.2 3.8 3.7
0.46 0.47 0.50 1.00 0.91
2.1 1.9
530.7 530.8 530.7
2.8 2.8
1.60 2.01 1.96
89.8 89.8
1.8 1.9
530.8 530.8
2.8 2.8
2.0 2.02
t c d e
0.8 1.2 1.5 1.9 2.0
f f
2.1 2.2 2.5
84.0 84.0 84.0 84.1 85.7 86.0 86.1 86.0
i j
3.0 4.0
86.1 86.1
a> Spectra binding
FWHM
taken after 5 min anodic polarization at potential @ versus SHE in IN HsSO, energies are reported relative to the Fermi level.
at 298 K;
literature data [3,4,17] that the adlayer contains gold in the formal oxidation state of 3 + , and that there is no mixture of different valence states and possibly only one single compound present (table 1, fig. 6 spectra (a)). More information is yielded by the 0 1s signal. This signal has a binding energy around 530.8 + 0.2 eV and is rather broad (FWHM = 2.8 V). E, has to be compared with 0 1s binding energies of other transition metal compounds. Typical values for a variety of oxides and also for adsorbed oxygen lie about
1
2 ELECTRODE
Fig. 5. Oxygen-to-gold
4
3 POTENTIAL
ratio as a function
vs SHE IV of polarization
potential.
hf. Peuckert et al. / Surface oxidation of gold electrode
Au
(LfI
523
XPS
N(E)
95
90
Es I
85
80
eV
Fig. 6. XPS spectra for thermal decomposition min polarization at 3 V versus SHE.
535
530
525
12LO
E,/eV
of electrochemical
12L5 E,,,
oxidation
1250
/ eV
layer on Au after 5
530 f 0.5eV [20-261. Most hydroxides give values of E, of 531.5 f 0.5 eV [16,20-23,271. Finally for the third alternative, that is adsorbed water or water of hydration, reported data are scattered around 533 f 1 eV [22-251. Therefore, the observed 0 1s signal at 530.8 eV appears to be composed of an oxidic and a hydroxidic peak, but no peak from water of hydration. Such an interpretation fits nicely with the observed average oxygen-to-gold stoichiometry, i.e., the corrected XPS Ols-to-Au4f intensity ratio of 2 f 0.2. All the features are best described by an oxyhydroxide AuOOH. Au(OH), and Au,O, may be excluded based on the observed broadness and binding energy of the 0 1s signal and the O/Au ratio; Au,O, - H,O, an alternate to AuOOH, should show an asymmetric 0 1s signal with a much larger FWHM. Whether the topmost layer also consists of AuOOH or rather Au(OH), or contains adsorbed water cannot definitely be decided. Having assigned the spectra of thick surface films grown at high potentials to a Au(III)oxyhydroxide, one can now turn to the interpretation of the XPS spectra, fig. 4, that are observed at intermediate voltages. The main species at 0.8 and 1.2 V seems to be strongly adsorbed water, because of the 0 1s signal at 532.4 eV (table 1). A value of 532.2 eV was found by Fisher and Gland [24] for water adsorbed on Pt(ll1) at 100 K. The corrected O-to-Au ratio of 0.47 is considerably high. A monolayer of adsorbed oxygen should only give a ratio of
M. Peuckert et al. / Surface oxidation
524
of gold
electrode
about 0.25 [16,17]. It seems as if the Helmholtz layer could be preserved during transfer into the UHV chamber as suggested by Hansen [28]. These findings are rather surprising and not yet understood, since one would expect that molecularly adsorbed water is easily desorbed at 300 K in UHV, as known for experiments on Pt(ll1) [23,24]. As seen from figs. 4 and 5, the transition from a metallic surface to a thick AuOOH adlayer occurs in a narrow potential range between 1.5 and 2.2 V. Oxidation around 2 V leads to broadening of the 0 1s signal and a shift to lower binding energies, meaning that the adsorbed water becomes split into hydroxyl groups and protons to form Au(OH), and eventually AuOOH. In fact, the 0 1s signals measured around 2 V represent a quite complex superposition from different species such as H,O, OH- and 02-. An interesting detail in fig. 5 is the reversal of the O/Au ratio between 1.9 and 2.0 V. Though initially thought to just be an experimental uncertainty, this effect proved to be reproducible. As a comparison of the respective 0 1s data shows, table 1, the central peak position concomitantly shifts from 531.6 to 530.8 eV binding energy when going from 1.9 to 2 V. This may be interpreted as a change from hydrated Au(OH), to AuOOH surface species, which would explain the observed break in the O-to-Au ratio plot, fig. 5. 3.3. Thermal treatment The thermal decomposition of an adlayer grown during 5 min potentiostatitally at 3 V versus SHE was followed by XPS, fig. 6 and table 2 [1,7]. The Table 2 Thermal decomposition of oxidation versus SHE in 1N H,SO, ‘) Spectra
T (K) 300 350 420
layer on gold electrode
Au 4f
after 5 min anodic polarization
FWHM
0 1s
FWHM
Io/I,,
7/2
512 89.8 89.6 89.2 81.6 89.2 87.7 87.6 87.6 87.6 87.6
1.8 3.0 -
530.8 530.6 530.1
2.8 2.7 2.6
2.0 1.40 0.79
-
530.0
2.5
0.66
2.4 1.8 1.8 1.8
530.0 530.1
2.5 _
0.40 0.1 0 0
87.5
1.7
530.1
2.5
0.2
d
410
e f f
520 570 670 110
86.1 86.0 85.6 84.2 85.5 84.2 84.0 83.9 83.9 83.9
i b,
300
83.9
at 3 V
a) Spectra measured at indicated temperature; electron binding energies (ev) are given relative to = corrected signal intensity Fermi level; full-width-at-half-maximum FWHM in eV; IO/I,, ratio of 0 Is and Au 4f,,,. b, After thermal oxidation for 18 h at 900 K in 0.1 MPa flowing 0,.
M. Peuckert et al. / Surface oxidation of gold electrode
525
spectra are almost the reverse series as in fig. 4 for the electrochemical build up. The Au 4f spectra show the transition from Au3+ to Au metal. The shift of the 0 1s peak is significant in two ways. Firstly, the decrease in binding energy from 530.8 to 530.0 eV suggests the dehydration to gold oxide Au,O,, characterized by the latter 0 1s binding energy. The dehydration seems to be a~ompanied by a decomposition reaction, so that not a pure film of Au,O,, but only a mixture of oxide and metal can be obtained. Secondly, the decrease of the FWHM value from 2.8 to 2.5 eV supports the notion that the 0 Is peak at 530.8 eV, as already discussed, may be composed of an oxide peak at 530.0 eV and a hydroxide peak at about 531.5 + 0.5 eV [16,20-271. The gradual filling of the 5d/6s valence band also shows the transition from an oxidized surface phase to the bulk metal [29]. Integration of the Au 4f and 0 1s signals gives a quantitative account of the thermal decomposition, fig. 7. Beginning slightly above room temperature, the AuOOH loses water and oxygen, and at 600 K all oxygen finally desorbs and leaves a clean gold surface. The thermal decomposition has also been followed by temperature programmed desorption spectroscopy (TPDS), fig. 8. Dehydration of the surface phase causes a broad water signal with a maximum at 400 K. Readsorption and subsequent desorption from the sample holder and transfer rod leads to a high background in the water TPD spectrum. OZ evolves from the gold oxide phase with a strong signal at 600 K (p peak). Two smaller 0, features at 400 K ( CX)and 890 K (y) may be attributed to partial reduction to Au metal parallel to dehydration at low temperatures (a), and final oxygen desorption (y). The sudden rise in 0, partial pressure at 600 K apparently causes release of H,O from the walls of the system such that a peak in H,O partial pressure occurs at 600 K; this peak is thus considered to be an artifact and not characteristic of
300
500 TEMPERATURE
900
700 I K
Fig. 7. Corrected XPS signal intensities for thermal decomposition of oxidation layer; after 5 min polarization at 3 V (*O), after 1 h poIarization at 3 V (X - - - x).
M. Peuckert et al. / Surface oxidation of gold electrode
526
the Au sample. The TPDS results agree with a stepwise decomposition from AuOOH to Au,O,/Au and Au. With very thick layers of several hundred A [6,7,13], grown during 1 h at 3 V, an interesting effect can be observed, fig. 7. First, up to about 400 K dehy~ation is found, then a further but less pronounced decrease of the O-to-Au ratio, and then an increase of the oxygen signal intensity is observed, until at temperatures as high as 900 K oxygen has disappeared. One possible explanation for this behaviour could be the dissolution of oxygen into the gold metal layer that is formed during the thermal treatment. This oxygen then finally is set free at considerably higher temperatures than those of the desorption of oxygen from the surface. The thick AuOOH surface phases were also found to be X-ray amorphous, since no diffraction patterns (Cu Ka: tube, 40 kV/40 mA) could be obtained. Treatment of a thick layer (grown during 1 h at 3 V) in 0.1 MPa 0, at 390 K for 1 h did not lead to a pure Au,O, phase, but the photoelectron spectra were similar to those of fig. 6, spectra (b), indicating the presence of Au metal. Oxidation of a clean Au(100) surface in 0.1 MPa of flowing 0, at 900 K for 18 h let to less than a monolayer of oxygen with an 0 1s binding energy of 530.1 eV (table 2, spectra (i)).
/ I
‘-‘,a
ml
Loo,
5Nl
600
h
1
700
800
TEMPERATURE
900
ICQO
I K
Fig. 8. Temperature programmed decomposition spectra, scan rate 5 K s-l; 0, x0.02 (- - -).
thinner adlayer,
IU. Peuckert et al. / Surface oxidation oj gold electrode
521
4. Discussion Based on the electrochemical and spectroscopic results presented here, one can try to draw a schematic picture of the surface chemistry on a gold electrode in contact with 1N H,SO,. In fig. 9 a tentative and simplified model of the surface composition is shown as a function of the applied potential versus SHE across the electrochemical double layer. In contrast to the behavior of a platinum electrode [16,30] there is almost no hydrogen adsorption observed in the low potential region. Reported coverages are 3 to 4% of a monolayer [18,19]. The electrocatalytic hydrogen evolution and hydrogen oxidation reactions take place on a largely H adsorbate free metal surface. At all potentials up to 1.3 V water is the primary adsorbate. Below the point of zero charge (PZC) of 0.18 V versus SHE [31] the preferential orientation will be with the hydrogen towards the metal surface, above the PZC bonding will most probably occur via the occupied non-bonding orbitals of the oxygen. In fact, it proved to be possible to transfer a thin water film into the UHV chamber [28]. This may not necessarily mean that the Helmholtz layer is preserved in the absence of electrolyte and applied potential, but rather that the sorption bond, that has been formed electrochemically is strong enough to survive the sample transfer. The adlayer may be stabilized by the ubiquitious carbon contaminant which was seen in the AES spectrum; sulfur has not been detected by either XPS or AES (see section 2) [42]. Between 1.3 and 1.5 V the water adlayer gets oxidized to an adsorbed layer of hydroxyl groups as seen from the cyclic voltammograms: Au. H,O,,
--, AuOH,,
+ H+ + e- .
0)
But there is no indication that oxidation stops at one monolayer of adsorbate, instead there is a continuous transition from OH adsorption to bulk hydroxide formation: AuOH,,
+ 2 H,O --, Au(OH),
+ 2 H+ + 2 e- .
(2)
The three anodic peaks, seen in fig. 1, at 1.45, 1.51 and 1.64 V may be attributed to those three oxidation steps described by reactions (1) and (2). Kirk et al. [32] found three similar anodic peaks on a gold electrode in alkali electrolyte and explained their observation by successive structure-sensitive adsorption of OH on (llO), (100) and (111) crystal faces of the polycrystalline sample. But, at least in our study with acid electrolyte, such an assignment to specific OH adsorption on different crystal planes can be excluded based on a number of arguments. The integrated charge density corresponds to a complete OH monolayer already at 1.5 V (fig. 2), therefore peaks fl and y fall into the range of incipient bulk oxidation and not of completion of a first OH monolayer. Also, one would expect three corresponding desorption peaks on the cathodic scan, which obviously is not the case, fig. 1. Finally, Ross’s [33]
I
1
Fig. 9. Model for surface composition
1.5
I t
of gold electrode
t
I
ELECTRODE
I
1
I
1
1
2.5
of applied
YS SHE/v
as a function
POTENTIAL
in acid electrolyte
2.0
potential.
I
!
I
1
1
3.0
hf. Peuckert et al. / Surface oxidation of gold electrode
529
work on single crystal platinum electrodes demonstrated that OH adsorption, at least on Pt, is not structure sensitive. It should rather be described as non-selective ionosorption. Around 1.7 V the first, around 2.0 V the second layer of Au(OH), is built up, fig. 2. A fourth anodic peak 6 at 1.78 V is observed, fig. 1. The XPS spectra taken in this range show a broad superposition of 0 1s signals of water, oxide and hydroxyl groups. Earlier XPS investigations were carried out in this potential range, therefore the 0 Is spectra were difficult to interpret and reported 0 1s binding energies differed by 1.5 eV between the two studies [3,4]. A new feature evolves at potentials above 2 V. As the adlayer thickness in creases beyond 2 monolayers of hydrated Au(OH),, a second cathodic reduction peak, B, figs. 1 and 3, appears as a shoulder, and the XPS 0 1s spectra give evidence for the presence of an oxyhydroxide, fig. 4. XPS spectra of thick uniform adlayers, by their binding energies, FWHM and corrected signal intensity, lead to the conclusion that in the high potential range the surface consists of AuOOH. Already above 2.2 V, film growth seems to proceed uninhibited up to several hundred A thickness, in accordance with refs. [6,7,13]. With this in mind, one may write the reaction for bulk oxidation at the hydroxide/metal interface [9] following reaction (1) like reaction (3): AuOH,,
+ H,O + AuOOH + 2 H+ + 2 e- .
(3)
Reactions (2) and (3) differ only in the degree of hydration of the product. The cathodic reduction may then formally be described by reactions (4) and (5): 3 AuOOH + 3 H+ + 3 e- + 2 AUK Au(OH),
+ 3 H+ + 3 e- + Au + 3 H,O.
+ Au,
(4) (5)
Though Au(OH), and AuOOH only differ in the degree of hydration, reduction of AuOOH according to reaction (4) is basically a hydrogenation reaction and will therefore most probably occur at a different potential than reaction (5). The appearance of a shoulder B in the cyclic voltammograms, figs. 1 and 3, concomitant with the formation of AuOOH as indicated by XPS, makes an assignment of peak B to reaction (4) and peak A to reaction (5) quite reasonable. Another explanation to account for two cathodic peaks, as given in refs. [6,10], proposes a layered oxide film composed of an inner oxide and a more hydrated outer oxide. An uniform oxyhydroxide, as shown in this study, can equally well explain the observed phenomenon. A thin surface film (in contact with electrolyte) of maybe one or two layers of Au(OH,) on a thick Au,O, adlayer (5 nm or more) could possibly lead to similar 0 1s signals as in fig. 4 spectra (g) and fig. 6 spectra (a), considering the mean 0 1s electron escape depth of 1.1 nm [16,17]; but a thin film of Au(OH),, on the other hand, cannot cause such a strong cathodic peak B as found in the voltammograms, fig. 1 scans (g) and (h). Also, broader Au 4f signals by superposition of two distinct oxidic and hydroxidic species should be anticipated.
M. Peuckert et al. / Surface oxidation of gold electrode
530
Electrocatalytic oxygen evolution commences around 2 V versus SHE, thus exhibiting a tremendous overpotential against the theoretical reversible oxygen potential of 1.23 V. The presence of an oxidized adlayer seems to be a requirement for water oxidation to occur [34]. The thermal degradation of electrochemically prepared Au(OH), or AuOOH does not lead to pure anhydrous Au,O,, figs. 6 and 7. Even in an atmosphere of 0.1 MPa of 0, decomposition takes place, approximately according to reaction (6): 6 AuOOH
+ Au,O,
+ 4 Au + 3 0, + 3 H,O.
(6)
The only product that is always found is a mixture of metal and oxide. From thin layers, all oxygen is desorbed at temperatures beyond 600 K, 2Au,O,+4Au+30,,
(7)
but with very thick oxidation layers, oxygen becomes trapped under the surface. Thus, in XPS the 0 1s signal intensity can reincrease above 600 K as the segregation of oxygen gets enhanced and oxygen diffuses to the surface and there desorbs. It is not yet clear how this “dissolved” oxygen should be described. Is it really dissolved in interstitial metal lattice positions, are there trapped Au,O, oxide clusters in a densely sintered metal matrix, or encapsulated oxygen gas in lattice imperfections of the gold? It is worthwhile to compare this seemingly unusual behaviour with the known chemistry of bulk gold oxides. Only hydrated Au,O, can be obtained as such. But also compounds with formal stoichiometries Au0 and Au,0 have been described in the early literature [9,35] and were eventually found to consist of mixtures of Au,O, with Au in the respective ratios. It may be suggested that the temperature ranges of restrained decomposition, fig. 7, with O-to-Au ratios of about 0.8 and 0.4 are related to the stability of these mixed phases, “AuO” and “Au *O”. All attempts to prepare oxide adlayers on the same sample in 0.1 MPa of oxygen gas at temperatures between room temperature and 900 K failed. Less than a complete monolayer of oxygen (0, < 1) was adsorbed after 18 h in 0.1 MPa 0, at 900 K (table 2, spectra (i)) [36,37]. An 0 1s binding energy of 530.1 eV (FWHM = 2.5 eV) supports the conclusions drawn in the foregoing sections. Extended surface oxidation as reported in other studies [38,39] may be attributed to contaminants like calcium.
5. Conclusions The surface electrochemistry on a gold electrode temperature has been investigated. Electrochemical XPS experiments were found to yield complementary
in 1N H,S04 at room cyclic voltammetry and data that are consistent
A4. Peuckert et al. / Surface oxidation of gold electrode
531
with each other. Occurrence of two cathodic peaks in the voltannnograms as well as the measured 0 1s and Au 4f binding energies and the atomic O-to-Au ratio of two are consistent with an oxyhydroxide AuOOH as the predominant surface phase, that grows under anodic polarization at voltages greater than 2.1 V versus SHE. The data allow the distinction of AuOOH from Au(OH),, Au,O, and Au,O, . xH,O, formulas that have previously been proposed for the oxidation adlayer. At lower voltages than 2.1 V the broad 0 1s signals suggest the presence of water and oxidic and hydroxidic species in adlayers only a few monolayers thick, while below 1.5 V adsorbed water is found as main surface species. Thermal treatment of a thick AuOOH adlayer does not lead to pure Au203, but to mixed phases of Au,O, and Au with O-to-Au ratios of 0.8 and 0.4, depending on temperature, and finally to a clean Au surface. At 900 K in 0.1 MPa of 0, gas only oxygen adsorption, but no bulk oxidation is observed.
Acknowledgement The authors thank G. Pirug for many helpful discussions.
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