Langmuir 1998, 14, 2535-2540
2535
X-ray Photoelectron Spectroscopy and Time-of-Flight Secondary Ion Mass Spectrometry Study of Hg(I) and Sulfate Adsorption Processes Accompanying the Coulostatic Underpotential Deposition of Mercury on Gold Xiangqun Zeng, Shiva Prasad,† and Stanley Bruckenstein* Department of Chemistry, University at Buffalo, SUNY, Buffalo, New York 14260-3000 Received September 26, 1997. In Final Form: December 29, 1997 Time-of-flight secondary ion mass spectrometry and X-ray photoelectron spectroscopy successfully clarified the processes occurring on gold electrode surface that accompany coulostatic underpotential deposition (UPD) of mercury. They showed that a sulfur-containing species was adsorbed. It corresponded to the adsorption of 28% Hg(I) bisulfate on the gold during the UPD process. This work confirmed Shay and Bruckenstein’s deductions, based on electrochemical quartz crystal microbalance studies, that 34% Hg(I) bisulfate was adsorbed.
1. Introduction Application of modern surface techniques, for example, x-ray photoelectron spectroscopy (XPS), to study the films or adsorbed layers formed on the electrode surface has been shown to have considerable value. For example, Hammond and Winograd1,2 studied the underpotential deposition (UPD) of silver and copper on platinum electrodes by XPS and found that the binding energy of UPD Cu and Ag shifted in a more negative direction compared with the bulk metals. The Ag 3d5/2 UPD binding energy shift, compared with the clean bulk metal value of 368.2 eV, averaged -0.65 ( 0.1 eV. The UPD Cu 2p3/2 binding energy shift was -0.95 eV compared with bulk Cu. Significantly, these shifts are insensitive to the parameters responsible for the observed multiple UPD anodic stripping peaks and the binding energy shifts did not change with the UPD coverage. In another XPS study, Adzic et al.3 found that for a partial monolayer of lead on gold, UPD lead had binding energy peaks at 138.5 eV (4f7/2) and 143.6 eV (4f5/2). These binding energies are much higher than those they found for pure lead, 137 eV (4f7/2 ). They rationalized this positive shift of binding energy for the lead UPD monolayer as caused by the oxidation of UPD lead during handling as it was transferred from the electrochemical cell to the XPS vacuum chamber. Hanson and Yeager4 studied UPD lead films on Ag(110) using XPS and observed two different chemical states of the UPD lead. In this investigation, and the previous ones, only the binding energy shifts were characterized. No attempt was made to use the XPS data to find surface coverages of the UPD metal layer or of other species that might have also been adsorbed on the surface during the UPD process. † Permanent address: Departmento de Engenharia Quimica, Universidade Federal da Paraiba, Campos II, Cidade Unersitaraia, Brazil.
(1) Hammond, J. S.; Winograd, N. J. Electroanal. Chem. 1977, 80, 123. (2) Hammond, J. S.; Winograd, N. J. Electroanal. Chem. 1977, 124, 826. (3) Adzic, R.; Yeager, E.; Cahan, B. D. J. Electrochem. Soc. 1974, 121, 474. (4) Hanson, M. E.; Yeager, E. Electrochemical Surface Science Molecular Phenomena at Electrode Surface; Soriaga, Ed.; American Chemical Society: Washington, DC, 1988; Chapter 10, p 141.
The present work shows the advantages of using XPS to quantitate the surface coverage of species that form at a gold electrode during the coulostatic underpotential deposition of mercury. Time-of-flight, secondary ion mass spectrometry (TOF-SIMS) has high sensitivity for detecting surface species and was used to show the presence of surface species that the XPS technique could not find. The XPS and TOF-SIMS results, combined with earlier electrochemical5 and electrochemical quartz crystal microbalance (EQCM) mass data,6 lead to a picture of the gold surface that is in reasonable agreement with the models deduced from the latter techniques. 2. Experimental Section 2.1. Reagents. Solutions were prepared with deionized water provided by a Milli-Q reagent grade water system (Millipore Corp., Bedford, MA). The stock mercury(I) perchlorate solution was prepared from reagent grade chemicals using Pugh’s method as described by Hassan.7 The supporting electrolyte used was 0.2 M H2SO4. 2.2. Instruments. XPS experiments were performed using a Physical Electronics Laboratories (PHI) spectrometer (model 5100) equipped with a Mg/Ti dual anode source and an Al/Be window. An achromatic Mg KR X-ray (1253.6 eV) source was operated at 300 W (15 kV and 20 mA). Spectra were obtained at a takeoff angle 45°. Data analysis was performed using a Perkin-Elmer 7500 Professional computer running PHI XPS software (version 2.0). The TOF-SIMS instrument we used was a PHI 7200 TOFSIMS model 07-850 equipped with a Cs+ ion gun. The primary ion energy was 8 keV and the primary ion beam current was between 0.2 and 0.3 nA. The sputtering area in the dc mode was 1000 × 1000 µm. For the dynamic TOF-SIMS experiments, the ion dosage during the entire sputtering process was typically 1.0 × 1012 ions/cm2. Spectra were obtained from 200 × 200 µm regions. The resolution was 128 pixels × 128 pixels. Data were conducted using TOF-PAC software. 2.3. Electrodes. (A) A 10 MHz AT cut quartz crystal (International Crystal Co., Oklahoma City, OK) with vacuumdeposited gold electrodes on both sides of the quartz wafer was used for the TOF-SIMS experiments. One gold electrode served as the working electrode. Its area was 0.25 cm2. These kinds (5) Lindstrom, T. R.; Johnson, D. C. Anal. Chem. 1981, 53, 1855. (6) Shay, M.; Bruckenstein, S. Langmuir 1989, 5, 280. (7) Hassan, M. Z. Ph.D. Thesis, State University of New York at Buffalo, 1977, Chapter 2.
S0743-7463(97)01067-6 CCC: $15.00 © 1998 American Chemical Society Published on Web 04/09/1998
2536 Langmuir, Vol. 14, No. 9, 1998 of crystals were also used in previous EQCM experiments by Shay and Bruckenstein.6 (B) A rectangular gold plate working electrode (1.0 cm × 1.2 cm) was used for the XPS and TOF-SIMS experiments. The gold plate electrode had a small flag (area 0.12 cm2) used to connect it to the potentiostatic circuit. The roughness factor of the working electrode was ∼1.63 as determined by the method of Brummer and Makrides.8 2.4. Electrode Pretreatment. Electrochemical treatment of the electrode was done in a 0.2 M H2SO4 supporting electrolyte using a standard three-electrode potentiostatic arrangement. Electrode potentials were recorded with respect to the saturated calomel reference electrode (SCE). The electrode’s potential was cycled between +1.4 and -0.2 V for approximately 15 min at 100 mV/s. Then it was reduced at 0.0 V for 2 min. The working electrode was emersed from the electrolytic cell under potential control (at 0.0 V) and transferred to the XPS instruments within 10 min. No special precautions were taken during sample transfer. 2.5. Experimental Protocol. 2.5.1. XPS Experiments on Gold Plate Electrodes. Four series of XPS experiments were done with the same gold plate electrode after the following different treatments. Sample 1: Heat treatment. The gold electrode was heated at 900 °C in an air oven for 4 h, allowed to cool to room temperature, and, without any electrochemical treatment, was rinsed with deionized water and dried with nitrogen. Then the XPS spectrum was obtained. Sample 2: Emersion from supporting electrolytes without rinsing. The electrode was electrochemically pretreated as described in section 2.4, emersed from the electrolyte at 0.0 V without rinsing, and dried with nitrogen. Then the XPS spectrum was obtained. Sample 3: Emersion from supporting electrolytes with rinsing. The electrode was electrochemically pretreated as described in section 2.4, emersed from the electrolyte at 0.0 V, rinsed with deionized water, and dried with nitrogen. Then the XPS spectrum was obtained. Sample 4: Coulostatic UPD Hg in 0.2 M H2SO4. The electrode was electrochemically pretreated as described in section 2.4. Then it was immersed in 2 × 10-5 M Hg22+ solution free of oxygen at an open circuit for five minutes to form Hg(UPD). It was then emersed, rinsed with deionized water and dried with nitrogen. Then the XPS spectrum was obtained immediately. 2.5.2. TOF-SIMS Experiment Using Evaporated Gold Electrodes. (A) Sample 5. Emersion of a reduced gold electrode from supporting electrolyte. The gold working electrode of the 10 MHz AT quartz crystal was pretreated the same way as sample 3. Then it was emersed, rinsed with deionized water and dried with a nitrogen stream, and immediately transferred to the TOFSIMS apparatus. An experiment was immediately done to obtain the blank TOF-SIMS spectrum. (B) Sample 6. Emersion of a gold electrode with UPD mercury from the supporting electrolyte. After the sample 5 TOF-SIMS spectrum was taken, the same electrode was pretreated again the same way as before. Then it was immersed in a deoxygenated solution of 2 × 10-5 M Hg(I) and 0.2 M H2SO4 through which nitrogen was bubbling. No connection to the gold electrode and the potentiostat was established. UPD of mercury occurred under open-circuit conditions in the oxygen-free solution for 15 min.6 The electrode was emersed, rinsed with deionized water, and dried with a nitrogen stream. Then the crystal was transferred to the TOF-SIMS apparatus, and a TOF-SIMS spectrum of the UPD coated electrode was obtained immediately.
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Figure 1. XPS multiplex spectra at the gold plate electrode in Hg 4f binding energy region: sample 1, pure gold after heat treatment; sample 3, reduced gold electrode emersed at 0.0 V with rinsing; sample 4, reduced gold electrode open circuited to form Hg(UPD), emersed with rinsing.
Hg22+ + Au f Au-Hgsurface + Hg2+
(1)
Reaction 1 occurs instead of the usual comproportionation reaction, reaction 2
Hg2+ + Hg(liq.) f Hg22+
(2)
because reaction 3 produces a monolayer of very stable surface species, Au-Hgsurface.
Hg(liq.) + Au f Au-Hgsurface
(3)
3.1. Interpretations of XPS Binding Energy Data. Shay and Bruckenstein6 interpreted their EQCM studies as showing that the following reaction spontaneously produces UPD mercury at a reduced, open-circuited gold electrode:
The species Au-Hgsurface corresponds to UPD mercury. Once a full monolayer forms, the disproportionation reaction ceases on the time scale of our experiment. Untereker et al.9 pointed out that coulostatic UPD formation shifts the electrode potential to more positive potentials and that the disproportionation reaction (Hg22+ + Au f Au-Hgsurface + Hg2+) at open circuit shifts the potential to a more negative value. These opposing potential shifts lead to the formation of a monolayer UPD Hg that is completed at the Nernst potential for the liquid mercury couple in the supporting electrolyte. So UPD Hg behaves like pure metal, liquid mercury. The latter result generally is not true for other UPD processes where more than one monolayer of UPD species is required to reach the potential characteristic of the bulk metal. Figure 1 gives the XPS multiplex spectra for sample 1, 3, and 4 experiments. Additional peaks in the binding energy region 85-110 eV exist in the sample 4 spectrum. The two peaks in sample 4 are identified as Hg 4f5/2 and 4f7/2. The binding energies of Hg 4f5/2 and 4f7/2 are the same as pure Hg metal as shown in Table 1. However, Hammond and Winograd’s studies mentioned above show that UPD formation usually shifts the UPD deposit’s binding energy to a value more negative than the bulk value for the UPD species. They explain this as caused by an increase of cohesive energy between the UPD species and the substrate. The unexpected binding energy of UPD mercury behavior is probably associated with the fact that the potential of the spontaneously formed UPD mercury layer in the mercurous sulfate solution is indistinguishable from that of a liquid mercury electrode. Ordinarily, several UPD layers are required before the potential of a bulk metal electrode is reached. Changes in oxygen peak shapes and binding energy in the XPS spectra (Figure 2) for samples 1-4 were
(8) Brummer, S. B.; Makrides, A. C. J. Electrochem. Soc. 1964, 111, 1122.
(9) Untereker, D. F.; Sherwood, W. G.; Bruckenstein, S. J. Electrochem. Soc. 1978, 125, 380.
3. Results and Discussion
UPD of Hg on Au
Langmuir, Vol. 14, No. 9, 1998 2537 Table 1. Binding Energies for Gold Plate Electrodes (eV) sample
C(1s)
O(1s)
Au 4f5/2
Au 4f7/2
sample 1, gold after heat treatment sample 2, emersion without rinsing sample 3, emersion with rinsing sample 4, UPD Hg at open circuit with rinsing
284.8 284.8 284.8 284.8
532.5 532.5 532.8 532.0
87.4 87.6 87.4 87.6
83.6 84.0 83.5 84.0
Hg4f5/2
Hg4f7/2
S(2p) 168.4
103.8
99.8
Figure 2. XPS multiplex spectra for the gold plate electrode in O 1s binding energy region: sample 1, pure gold after heat treatment; sample 2, reduced gold electrode emersed at 0.0 V without rinsing; sample 3, reduced gold electrode emersed at 0.0 V with rinsing; sample 4, reduced gold electrode open circuited to form Hg(UPD), emersed with rinsing. All these samples were pretreated differently, so that the spectral differences indicate the oxygen atomic concentration characteristic of each pretreatment and impurities introduced in the transfer from the electrochemical cell to the XPS experiment.
observed, indicating changes in the chemical environment of oxygen. Contamination by atmosphere oxygen existed for all samples. However, in sample 1, nonatmospheric oxygen comes from a surface gold oxide produced during heat pretreatment. In sample 2, no surface gold oxide contribution can exist because the gold had been reduced electrochemically, consequently all nonatmospheric oxygen comes from adsorbed SO42- and HSO4- in the emersed double layer. A lesser contribution of oxygen from SO42and HSO4- is seen for sample 3 due to removal of the emersed double layer by rinsing with water. In sample 4, additional nonatmospheric oxygen probably comes from specifically adsorbed Hg2(HSO4)2 and causes a broadening of the oxygen peak. The sulfur binding energy data provide valuable evidence about sample history as seen in Figure 3, the high-resolution multiplex XPS spectra in the S2p binding energy region for the four samples. An obvious S2p binding energy peak exists in sample 2 that is absent in samples 1 and 3. We identify it as S2p from SO42- by its binding energy shift. The results confirm that the emersed double layer ions remains on the surface despite being placed in ultrahigh vacuum under X-ray bombardment and exposure to laboratory air. In sample 3, the disappearance of the S2p peak shows that rinsing the electrode after emersion can partially or completely remove the double layer. A small S2p peak seen in sample 4 suggests that this sulfur differs from double layer sulfate. We attribute this peak to specifically adsorbed Hg2(HSO4)2 that is hard to remove by rinsing with water. 3.2. Quantitative Interpretation of XPS Results. For all four samples, carbon, oxygen, sulfur, gold, and mercury were analyzed at high resolution and their percent atomic concentration, %AC, values were calculated. These results are shown in Table 2. The sampling depths of the various atomic species are similar (Table 3). Consequently, we have assumed that escape depth of the
Figure 3. XPS multiplex spectra in the S 2p binding energy region at a gold plate electrode: sample 1, pure gold after heat treatment; sample 2, reduced gold electrode emersed at 0.0 V without rinsing; sample 3, reduced gold electrode emersed at 0.0 V with rinsing; sample 4, reduced gold electrode open circuited to form Hg(UPD), emersed with rinsing. Table 2. Atomic Concentrations Found at the Gold Plate Electrode sample
%O
%C
% Au
sample 1 sample 2 sample 3 sample 4 sensitivity factor
18.78 22.09 10.34 14.24 0.66
55.76 56.41 56.26 52.97 0.25
25.46 18.54 32.91 25.94 4.95
% Hg 4f 0 0 5.21 5.5
%S 0 2.96 0.49 1.64 0.54
emitted photons is the same for all the five elements, and we use a sampling depth of five atomic layers in the following discussion. The limits of detection for the XPS experiment are typically ∼0.1% AC.10 As shown in Table 2, carbon, oxygen, and gold were detected in all samples. Carbon and oxygen mainly come from the contamination that occurs during the transfer through air to the spectrometer, and gold comes from the substrate. The atomic concentration of carbon hardly changes. (10) Weitzsacker, C. L.; Gardella, J. A. Energy Fuels 1996, 10, 141.
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Table 3. Sampling Depths at Gold,a Calculated from Reference 12 inelastic mean free path λ (Å) average thickness of a monolayer sampling depth (Å) number of sampling layers
Au
Hg
C
O
18.9 2.57 13.36 5.2
22.7 2.90 16.05 5.5
13.66 2.07 9.66 6.6
19.8 2.65 14.0 7.47
a Calculated from: Seah, M. P.; Dench, W. A. Surf. Interface Anal. 1979, 1, 2.
The gold atomic concentration decreases as that of the oxygen increases. The gold atomic concentration detected in sample 4 is smaller than that detected in sample 3. We attribute this to the surface monolayer of UPD Hg in sample 4 that reduces the sampling depth in gold by one layer, from five to four. This assumption predicts that sample 4 will have a gold atomic concentration 4/5 of the atomic concentration in sample 3, i.e., 4/5 × 32.91% ) 26.3%. We found it to be 25.9%. Analogous reasoning can be applied to interpret the gold atomic concentration in sample 1. It has a surface oxide layer, so that the sampling depth is the same as for sample 4 that has a monolayer of a mercury UPD species. This predicts the observed result that the same atomic concentration of gold should be found for samples 1 and 4. Sample 3 has a much smaller oxygen atomic concentration than sample 2. Both were identical reduced samples emersed from the sulfuric acid solution, but only sample 3 was rinsed with water. This difference is caused by the oxygen in the emersed double layer originating from SO42- and HSO4-. Sample 4, a reduced gold electrode that was open circuited to form UPD mercury and then rinsed with water after emersion, had a larger atomic oxygen concentration than sample 3, which had not been open circuited. This increase in oxygen concentration is consistent with a strongly adsorbed sulfate-containing species, such as the previously postulated adsorbate, Hg2(HSO4)2. The oxygen atomic concentration in sample 1 (the gold electrode that was only heat treated) is higher than those in sample 3 and sample 4. This result can be rationalized by a surface oxide formed during heat treatment. Atmospheric contamination is another oxygen source for all four samples. The diagrams in Figure 4 summarize our picture of the surface history for the four samples prepared using the gold plate electrode. 3.3. Correlation of XPS and EQCM Results. The calculation of quantitative compositions of the surface region sampled in the XPS experiment is based on the diagrams given in Figure 4. We start by assuming that atmospheric oxygen contamination is the same for sample 3 and sample 4. Then, the total percent atomic oxygen concentration in sample 4 is the sum of atmospheric oxygen contamination and the oxygen derived from SO42and/or HSO4- that remains on the electrode after emersion. The S2p binding energy peak signal in Figure 3 for sample 3, the reduced gold electrode rinsed with water after emersion, is very small compared to the signal for sample 4. This shows that the emersed double layer was almost completely removed by rinsing with water. Consequently, we assume the oxygen in sample 3 comes mainly from the atmosphere contamination with a minor contribution from oxygen associated with sulfate species in the emersed double layer that did not wash off. The latter oxygen contribution was determined by using the sulfur atomic percent concentration for sample 3, given as 0.49% in Table 2 for the S2p binding energy peak signal. We assumed that one sulfur atom is associated with four
Figure 4. Schematic surface history of experiments on gold plate electrode: sample 1, pure gold under heat treatment; sample 2, reduced gold electrode emersed at 0.0 V without rinsing; sample 3, reduced gold electrode emersed at 0.0 V with rinsing; sample 4, reduced gold electrode open circuited to form Hg(UPD), emersed with rinsing. Two possible surface arrangements of Hg(I) and bisulfate.
oxygen atoms. Thus, 1.96% of atomic concentration of oxygen in sample 3 is ascribed to remaining emersed sulfate species, and we calculate that oxygen contamination from the atmosphere in sample 3 is 8.38% using the data in Table 2 (10.34% - 1.96% ) 8.38%).
UPD of Hg on Au
Langmuir, Vol. 14, No. 9, 1998 2539
Next, we corrected the total sulfur AC% (1.64%) for the sulfur background (0.49%) obtained in sample 3 to obtain the sulfur on the UPD surface to be 1.15%. From the formula Hg2(HSO4)2, we see that sulfur to Hg22+ ratio is 1 to 1. So the Hg AC% obtained experimentally (5.21%) should be the sum of Hg(UPD) and Hg22+. Therefore the ratio of adsorbed Hg2(HSO4)2 to Hg(UPD) should be given by %AC Hg22+/(total %AC Hg - %AC Hg22+), i.e.,
1.15%/(5.21% - 1.15%) ) 28.3% This value is very close to the one obtained in Shay and Bruckenstein’s6 study. They found the UPD layer to consist of 1.7 × 10-9 mol/cm2 of Hg(0) and 0.29 × 10-9 mol/cm2 of Hg2(HSO4)2. Thus their ratio of Hg2(HSO4)2 to Hg(0) is about 34% (0.29 × 10-9 mol/cm2 × 2/1.7 × 10-9 mol/cm2 ) 34%). These values are lower than Lindstrom and Johnson’s5 reported ratio of Hg(0)/Hg(I) of 1/2. We do not consider this difference significant because their experiments were done at a controlled potential while ours were done at open circuit. These two studies are in general agreement with the early EQCM analysis of Shay and Bruckenstein.6 3.4. Stripping Hg(0) after XPS Experiment. After the XPS experiment for sample 4, the electrode was immersed in the oxygen-free 0.2 M H2SO4 solution with the potential set to 0.0 V. The potential was then stepped to 1.20 V, and the oxidation charge was recorded as a function of time. This charge was corrected for the blank response in 0.2 M H2SO4. The charge required to oxidize the adsorbate from the gold substrate is 80% of that required for a monolayer of Hg(0) adatoms, which was calculated from the charge required to strip a monolayer of oxygen atoms from our electrodes, 1.655 mC. The XPS and TOF-SIMS data below confirm the presence of a surface adsorbate containing sulfur, and the EQCM data of Shay and Bruckenstein imply that this species is bisulfate. Consequently, we suggest that the models shown in the diagrams of Figure 4 as possibilities for the relative positions of Hg(0), Hg22+, and HSO4- with respect to the gold surface. The one in which Hg(0) and Hg22+ reside side by side is consistent with the more detailed model suggested by Lindstrom and Johnson. The charge for Hg(0) oxidation was 1318 µC (1.6 × 10-9 mol/cm2), which is UPD Hg oxidization charge. Adsorbed Hg(I) remains on the gold surface. This number is smaller than the Shay and Bruckenstein’s6 results (1.7 × 10-9 mol/cm2 for one monolayer of Hg(0)). Most likely some Hg evaporated in the ultrahigh vacuum of the XPS system. 3.5. TOF-SIMS Experimental Results. Figure 5 gives the TOF-SIMS negative survey spectra for samples 5 and 6 obtained at the same evaporated gold electrode. Both samples were first treated the same way as sample 3, that is, they were reduced at 0.0 V. Sample 5 was emersed from the sulfuric acid supporting electrolyte at 0.0 V (no mercury (I) was present) and rinsed with water. Then sample 6 was exposed to a Hg(I) solution at opencircuit to form UPD Hg before emersion and rinsing with water. Sample 6 had larger peaks at m/z ) 96 and 97 than sample 5. These peaks can be assigned to SO4- and HSO4-. No detectable negative Hg ion peak was evident for sample 6. Also, no mass peaks assignable to Hg could be detected in a TOF-SIMS positive survey spectrum. This is not surprising as Hg has zero electron affinity and consequently gives a zero negative ion yield. Also, Hg has very poor positive ion yields because of its large ionization potential (10.44 eV). Consequently, the TOFSIMS data are inconclusive on the issue of the presence of surface mercury.
Figure 5. TOF-SIMS negative survey spectra on an evaporated gold electrode: sample 5, reduced gold electrode emersed at 0.0 V with rinsing; sample 6, reduced gold electrode open circuited to form Hg(UPD), emersed with rinsing.
Figure 6. TOF-SIMS depth profile of a Hg (UPD) treated evaporated gold electrode.
The principal anion in the sulfuric acid supporting electrolyte is HSO4-. We previously assumed6 that Hg22+ adsorbed on the UPD layer of Hg(0) existed as Hg22+(HSO4-)2, that is, the counterion for adsorbed mercurous ion in the compact double layer would be a bisulfate ion. The TOF-SIMS results in Figure 5 are consistent with the prior EQCM work. These spectra confirm that the sulfate species on the Hg(UPD)-treated sample is more strongly bound (harder to rinse off) than those on a reduced gold electrode that does not have UPD mercury (more readily rinsed off). The result of a TOF-SIMS depth profile experiment is given in Figure 6 for an evaporated gold electrode with a UPD Hg deposit. The decreasing intensity of SO4and HSO4- with sputtering time establishes the existence of SO42- and HSO4- at the gold surface. We ascribe the presence of the sulfate species to adsorbed Hg2(HSO4)2 on the UPD mercury-covered gold surface. The SO4- and HSO4- depth profiles show the two decay regimes typical of ion beam mixing. The initial steep decay corresponds to the direct removal of the overlayer species. The subsequent slower exponential decay of the overlayer species signal is caused by more-or-less homogeneous mixing of the overlayer species with the substrate that occurs over the depth of primary ion penetration. Only the small fraction of the overlayer material in the outermost surface layer of the mixed layer is accessible to sputtering. The remaining material is continuously mixed deeper into the sample as sputtering proceeds.
2540 Langmuir, Vol. 14, No. 9, 1998
The gold intensity increases with the removal of surface Hg layers by sputtering and the gold intensity becomes constant when all the Hg has been removed. The complete removal of the Hg layer can be inferred from the SO4- and HSO4- profiles. At longer sputtering time (>100 s), possibly one monolayer, the SO4- and HSO4- signals decrease and reach a constant level. At short sputtering times, the surface overlayer is not removed and the “depth profile” experiment only shows the time dependence of some negative secondary ions emitted from the original uppermost monolayer.11 However, the exponential time dependence of the secondary ion current is not seen for Au and O components although SO4- and HSO4- show approximately the exponential decay at very short times. The behavior of SO4- and HSO4- at longer times is probably caused by the “knock in” effect. In summary, we have direct evidence that SO4and HSO4- only exist on the surface. The oxygen intensity at the surface is higher than those for HSO4- and SO4-. The presence of native oxide contamination from the atmosphere and from the adsorbed bisulfate contribute to the surface oxygen concentration. The XPS experiments described in the previous section also show that oxygen is present in every sample. The oxygen depth profile also showed that O is present in the deeper layers. Because the relationship between concentration and intensity for sputtered ions varies over a large range in a TOF-SIMS experiment, it is impossible to make any quantitative statements about the relative concentrations of oxygen and sulfate on the electrode surface. (11) Benninghoven, A. Surf. Sci. 1973, 33, 27.
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4. Conclusion XPS results on a gold electrode with a deposit of UPD mercury are consistent with the view that (a) the mercury species are mostly Hg(0) and (b) 28% of a monolayer of Hg2(HSO4)2 is adsorbed. TOF-SIMS confirmed sulfate species adsorption. These two studies are in agreement with the early EQCM analysis of Shay and Bruckenstein6 but differs somewhat from the earlier work by Lindstrom and Johnson,5 which involved the direct reduction of Hg(II) to UPD mercury at controlled potential. Their coulometric data were interpreted as indicating a close packed layer of mercury species with an atom ratio of Hg(I)/Hg(0) of 1/2. We believe these differences arise because our experiments were done at open circuit. TOF-SIMS and XPS are valuable complements to electrochemical techniques for the study of anion and cation adsorption during UPD processes because they provide information not easily obtained by other means. The emersed double layer can be detected on the electrode surface even after exposure to air during transfer from the electrochemical cell to the ultrahigh vacuum system. Washing the electrode after emersion can partially or completely remove the emersed double layer. The extent of removal depends on the presence of other surface adsorbed species and the electrode’s rational potential when the gold is emersed. Acknowledgment. We thank the University at Buffalo/SUNY and the National Science Foundation (Grant CHE-9616641) for financial support. We also thank Dr. Rich Nowak and Dr. Ewa Pater for the assistance. LA971067B
