J. CHEM. SOC. FARADAY TRANS., 1994, 90(9), 1271-1278 Valence and Core Photoemission of the Films formed Electrochemically on Nickel in Sulfuric Acid Yuanling Liang and Peter M. A. Sherwood" Department of Chemistry, Willard Hall, Kansas State University, Manhattan, KS 66506,USA Dilip K. Paul Department of Chemistry, St. Mary College, Leaven worth, KS 66048,USA The nature of the anodic films formed on nickel in 0.5 mol dm-3 sulfuric acid after polarization in the passive and transpassiveo state has been investigated by X-ray photoelectron spectroscopy. It is found that the film thickness (10-25 A) grows linearly with potential in the passive region. Film thickness drops at the beginning of the transpassive region, due to the further oxidation of NiO to Ni0,H (which may be soluble in the electrolyte).The results from anaerobic cell and ex situ experiments were compared in order to study the effect of atmo- spheric conditions on the electrochemical treatment. Experiments performed in the anaerobic cell show strong sulfate adsorption and the trapping of sulfate ions in the passive film. No significant sulfate adsorption is found in the ex situ experiments, presumably because the air-formed oxide layer prevents these ions from being trapped in the film. The passivation of nickel in sulfuric acid solution has been studied in detail for many years with different techniques.'-' ' Most of the published data show that the electrochemical behaviour of nickel depends upon the time of polarization and the pretreatment of the electrode.The passive layer formed is a mixture of, or a bilayer structure of, NiO and Ni(OH),. With an increase in potential the composition of the film changes with the NiO content in the film increasing with a surface analysis with XPS, the correlation of the electro- chemical behaviour with the nature of the species on the surface is achieved. To avoid artifacts such as oxide growth during exposure to the oxygen of the laboratory atmosphere, sample transfer from the electrolyte to the UHV system was performed in a specially designed anaerobic chamber. The use of ultra-high purity (UHP) argon as a protective gas pre- vents any further oxidation. On the other hand, a number of decline in the amount of Ni(OH), comp~nent.~*~-~~,~~-~ reactions occur under conditions where the electrode may The thickness of the layer is in the range of 1.1-1.7 nm deter- mined by coulometry,' electrochemical methods,' nuclear microanalysis and X-ray photoelectron spectroscopy (XPS).*-''*' Ho wever, there are still many unsolved ques- tions. Some controversies still exist among various investigators concerning the formation, composition, structure and proper- ties of the passive layer, despite several careful investiga- tions.In particular: (i) Nature and thickness of the passive film; some authors have reported a linear increase in film thickness in the passive region with p~tential.'*'~ Other authors suggest that the thickness of the passive layer was potential-independent.'*' (ii) The presence of sulfate ions in the passive film; we found sulfate ions on the electrode at almost all potentials, and a very small amount of sulfide present in the active and prepassive regions.Some previous workers have indicated that nickel sulfate exists as a constitu- ent of the passive Droste and Feller3 and Marcus and Grimalg have suggested that the sulfate was a kind of contamination on the passive film surface, the HS0,-orSob2-ions being adsorbed by the oxide film. In order to understand this system, surface analysis is needed in addition to electrochemical methods. Surface analysis methods in ultra-high vacuum (UHV), like XPS, compliment several in situ experiments in providing detailed chemical In XPS studies the core level spectra may become ambiguous due to the overlapping of a large amount of multiplet splitting and other satellite peaks.In such cases the valence band spectrum may become a powerful tool for the determination of the chemical nature of the surface layer. The aim of this work was to examine the passive layer in detail after a variety of electrochemical treatments, including treatment in the preactive, passive and transpassive regions. By combining the controlled electrochemical treatment and have either an adsorbed layer of oxygen or a very thin oxide film. The film may play a role in the electrochemical reactions either directly by modifying the energetics of adsorption of reactants or intermediates, or indirectly by changing the potential distribution at the surface.Thus a set of parallel experiments was also performed on the bench (the ex situ experiment) to investigate the effect of air exposure on the electrode surface chemistry. Experimental Instrumentation XP spectra were collected on a VSW HA10 spectrometer with a second UHV system used for electrochemical experi- ments. The design of this chamber and the operation of its \ I. 1.5 1.o 0.5 0.0 -0.2 potential/Vvs. SCE Fig. 1 Cyclic voltammogram of nickel metal in 0.5 mol dm-3 sul- furic acid solution with a scanning rate of 40 mV s-' 1272 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 1 Curve-fitting parameters for Ni 2p3,, and 0 1s core XPS regions ~ ~~ Ni 2p reference in addition to this main sat. 1" sat.2" sat. 3" 0 1s work Ni metal 852.4 858.8 19, 34, 37-39 1.3' 5.2' NiO 854.1 2.2' 855.6 2.5' 860.9 5.P 864.4 5.P 529.9 1.5' 19, 35-39 Ni(OH), 856.4 2.6' 858.0 2.7b 862.5 5.P 531.8 1.9' 19, 34, 36-38 H,O -- - 533.6 1.9' 14,38 NiSO, 856.3 2.4' 858.2 2.7' 861.3 4.9' 532.3 1.9' 37, 38 " Sat. = satellite peak. ' FWHM. electrochemical cell has been described elsewhere.2 1,22 Both UHV systems could achieve a base pressure of lo-'' Torr. All XPS data were recorded in the fixed analyser transmis- sion (FAT) mode with a pass energy of 25 eV for core regions and 50 eV for the valence band region. Achromic Al-Ka radi- ation (1486 eV with a linewidth of ca. 0.85 eV) was used at a power of 250 W (10 kV, 25 mA).Data were collected with at least 17 points per eV to be sure to identify any subtle fea- tures that might be lost at lower resolution and larger step size. The spectrometer energy scale was calibrated using copper.32 Peak positions were referenced to the C 1s peak (284.6 eV), due to residual hydrocarbon on the sample surface. Argon-ion sputtering was carried out using of a B21 saddle-field ion source sputter ion gun operated at 2.8 kV and 1 mA. Ultra-high-purity argon was used for etching and as an inert atmosphere in the anaerobic cell experiments described below. Two three-electrode cells were used for anaerobic cell experiments and ex situ experiments, respec- tively. Potentiostatic polarization and cyclic voltammetry A B C D 175 167 537 529 880 856 40 8 binding energy/eV Fig.2 XP spectra for nickel metal polarized to different potentials in sulfuric acid (0.5 mol dm-3) solution within the anaerobic cell. Spectra are shown for the S 2p, 0 1s and Ni 2p core regions and the valence band region. The curve-fitting results are shown for the 0 1s region. (a) Shows an etched nickel metal sample. (b)-(f)shows results of polarizing a nickel electrode to different potentials for 15 min. All potentials other than -0.6 V were prepolarized to -0.6 V for 5 min; (b) -0.6, (c) +0.7, (6)+ l.l,(e) +2.0, (f) +2.5 V. were performed using a Thompson Ministat Research Poten- tiostat (model 402R). Sample Preparation Nickel foil from Alfa (of 99.994% purity) was used as the source of the nickel electrode. 0.5 rnol dm-' sulfuric acid solution was prepared from the concentrated acid (with 95.0%-98.0% concentration) obtained from Fisher (of an ACS specified purity) and quadruply distilled water.All potentials were measured and quoted with respect to the saturated calomel reference electrode (SCE), which is 0.245 V with respect to standard reversible hydrogen electrode, in 0.5 rnol dm-3 sulfuric acid solution. All of the sulfuric acid solu- tions and washing water were deaerated with UHP nitrogen for at least 8 h and then exposed to UHP argon for 10-15 min. A B C D ' -., ., . . i.* '.<..<;;<*.:, ,,..,. . :' v 172 164 534 526 880 856 40 8 binding energy/eV Fig.3 XP spectra for nickel metal polarized to different potentials in sulfuric acid (0.5 mol dm-3) solution for ex situ experiments. Spectra are shown for the S 2p, 0 1s and Ni 2p core regions and the valence band region. The curve-fitting results are shown for the 0 1s region. (a) Shows an etched nickel metal sample. (b)-(f)show results of polarizing a nickel electrode to different potentials for 15 min. All potentials, other than -0.6 V, were polarized to -0.6 V for 5 min; (b) -0.6,(c) +0.7,(d) +l.l,(e) +2.0,(f) +2.5 V. J. CHEM.SOC. FARADAY TRANS., 1994, VOL. 90 A B h v)Y.-C 862 852 862 852 binding energy/eV Fig. 4 Curve-fitting results for the Ni 2p,,, core XPS region for nickel polarized to different potentials in sulfuric acid (0.5 mol dmP3) solution for 15 min in both anaerobic cell and ex situ experiments.All potentials, other than -0.6 V, were prepolarized to -0.6 V for 5 min. (a)-0.6,(b) +0.7,(c) + 1.1, (6)+2.0, (e) +2.5 V. The nickel metal samples were polished mechanically with alumina (44pm). No trace of aluminium was found on the polished metals following XPS examination. They were also degreased with acetone and cleaned with quadruply distilled water. The metal thus had only an air-formed film on the surface. Two types of experiments were conducted. The anaerobic cell experiments were performed in the special designed anaerobic chamber discussed previously.2 'st2 These experi- ments involved an additional cleaning procedure for the samples by argon-ion sputtering for 1.5-2 h on each side with the surface purity monitored by XPS.The sample was rotated during the argon-ion bombardment to allow even treatment of the sample. This further cleaning procedure enabled us to remove all of the contamination and oxide resi- dues formed during exposure to air, resulting in reliable and reproducible starting conditions. The clean sample was then exposed to a positive pressure of an UHP argon atmosphere in the anaerobic cell before the electrochemical treatment. For the ex situ experiments, the sample, cleaned as described above except for the argon-ion sputtering, was immediately immersed in the deaerated 0.5 mol dm-, sulfuric acid and polarized. In all of the electrochemical treatments, the samples were prepolarized at -0.6 V for 5 min to remove oxide film, then pulsed to a chosen potential: -0.6, +0.7, +1.1, +2.0 and +2.5 V,respectively, and maintained there for 15 min.All the experiments were carried out at room temperature. After fin- ishing the electrochemical treatment, the electrode was removed from the solution with the potentiostat still switched on, and then washed eight times with deaerated quadruply distilled water. Finally, the sample was dried by evacuation (anaerobic cell experiments), or by gently absorbing the water with adsorbent paper (ex situ experiments). Afterwards the sample was transferred to the XP spectrometer chamber. For ex situ experiments, the transfer procedure took <5 min. The anaerobic cell experiments eliminated exposure of the samples to any oxygen in either the gaseous or solution state, except for the oxygen produced from the electrochemical reaction itself.1273 Curve Fitting Table 1 gives all of the parameters used for curve fitting of the 0 1s and Ni 2p3,, spectra. The fitting of these spectra uses the approach that we have used previou~ly,~~.~~ and is consistent with the fitting of these regions by other workers cited in Table 1. The 0 1s regon was fitted to three or four peaks (vide infra) and the detailed curve fits of this region are shown in Fig. 2 and 3 (later). The Ni 2p3,, region had a non- linear background removed using the Tougaard method4' and the detailed curve fits of this region are given in Fig.4 (later). A non-linear least-squares curve-fitting program was used with a mixed Gaussian/Lorentzian (G/L) peak-The peak profile was kept the same in all cases. Exponential asymmetric tailing, to account for conduction band interaction (CBI) effects, was added to the basic G/L lineshape for nickel metal. All the other peak profiles remained symmetric. Ka,, X-ray satellites were also included in the curve-fitting routine. All of the fitting param- eters, such as peak position, peak width and peak separation between the main peak and the satellites, were chosen according to previous work and curve fitting results of stan- dard compounds (as indicated in Table 1). Results Electrochemistry Nickel exhibits a typical active-passive behaviour.The potentiodynamic polarization curves of nickel in 0.5 mol dm-3 sulfuric acid solution can be found el~ewhere.~*~-'~ Fig. 1 shows the cyclic voltammogram of nickel recorded in 0.5 mol dm-3 sulfuric acid solution with a scan rate of 40mV s-'. The polarization potentials used to treat the samples were chosen so that there was one in the cathodic region (-0.6 V), two in passive region ( +0.7 and + 1.1 V), and two in the transpassive region ( +2.0 and +2.5 V), respectively. Surface Analysis Anaerobic Cell Experiments Fig. 2 shows the typical XP spectra of a nickel electrode in 0.5 mol dm-, sulfuric acid solution as a function of electrode potential for treatment in the anaerobic cell. 0 1s spectra were curve-fitted with four peaks, which correspond to 02-, OH-, H20 and respectively.The OH- region also contains some water intensity.22 The fitting parameters are listed in Table 1, the fitting results are given in Table 2, and the approximate film composition is given in Table 3. Table 3 has been presented so that the reader can obtain a rough comparison between the information provided by the 0 1s and the N 2p regions. All the atomic ratio data assumes the very crude model of a uniform homogeneous surface layer, and are adjusted for differences in cross-section and analyser transmission function. The table explains how the percent- ages of the oxides are obtained from the percentage peak areas of Table 2. It should be noted that the ratio shown in column A is obtained assuming that NiO contributes one oxygen atom and Ni(OH), two oxygen atoms.The percent- ages of oxidized nickel species would not expected to be the same for the 0 1s region as the Ni 2p region since the Ni 2p electrons have a lower kinetic energy (635 eV) than the 0 1s electrons (956 eV) and are thus more surface sensitive. Since we know that the oxide lies in the outer region of the film we would expect the percentages of oxidized nickel to be greater J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 Table 2 Curve-fitting results for the 0 1s and Ni 2p,,, core XPS regions 2” Ni metal NiO Ni(OH), 0,-OH-‘ H2O so,,-potential peak % peak % peak % peak % peak % peak % peak % ex situ -0.6 852.3 70.8 854.0 856.2 10.3 530.1 8.9 531.8 64.2 533.6 (0.0) (0.2) (0.5) (0.6) (0.3) (0.2) (0.02) (0.1) (0.0) (0.1) 1.3’ 2.2’ 2.5’ 1.5‘ 2.P 2.P +0.7 852.4 27.1 854.1 856.5 24.1 529.9 23.0 531.8 66.0 533.6 (0.0) (0.3) (0.2) (1.2) (0.3) (0.0) (0.0) (0.0) (0.0) (0.1) 1.3’ 2.2’ 2.6’ 1.5‘ 2.P 2.P +1.1 852.3 18.6 854.1 856.2 26.7 529.8 25.0 531.9 65.4 533.6 --(0.1) (0.3) (0.2) (1.3) (0.43) (0.0) (0.0) (0.0) (0.0) (0.1) 1.3‘ 2.2’ 2.6’ lSb 2.1‘ 2.1‘ +2.0 852.4 48.7 854.1 856.4 19.0 530.0 19.5 53 1.8 61.8 533.6 (0.0) (0.3) (0.2) (1.2) (0.3) (0.0) (0.0) (0.0) (0.0) (0.1) 1.3’ 2.2’ 2.6’ 1.5‘ 2.P 2.P +2.5 852.3 21.1 854.0 856.3 23.1 529.8 26.7 531.8 58.3 533.6 (0.0) (0.3) (0.2) (1*4) (0.4) (0.0) (0.0) (0.0) (0.0) (0.1) 1.3‘ 2.2’ 2.6’ 1.5’ 2.P 2.P anaerobic cell -0.6 852.5 83.1 854.0 856.3 --531.5 43.0 533.5 532.2 35.3 (0.0) (0.3) (2-4) (2.6) (3 (0.0) (-) (-1 (0.0)1.3’ 2.2’ 2.5’ 1.9’ 1.9‘ 1.9’ +0.7 854.4 45.5 854.0 856.4 530.0 3.0 531.7 34.7 533.6 532.3 48.2 (0.0) (0.2) (0.5) (1.4) (-1 0.0) (-1 0.0) (-) (-3 (0.2)1.3’ 2.2’ 2.6’ 1.5’ 1.9’ 1.9‘ 1.9’ + 1.1 852.4 27.4 854.1 856.5 530.0 10.6 531.7 37.9 533.7 532.3 37.9 (0.1) (0.6) (0.6) (1-7) (0.0) (0.0) (-1 (0.0) (-1 (-9 (0.0)1.3’ 2.2’ 2.6’ 1.5’ 1.9’ 1.9’ 1.9’ +2.0 852.4 59.6 854.1 856.3 529.9 2.1 531.6 39.2 533.6 532.3 43.4 (0.0) (0.3) (0.7) (0.8) (-3 (0.0) (-4 0.0) (-1 (-3 (0.0)1.3’ 2.2’ 2.5’ 1.5‘ 1.9‘ 1.9’ 1 .9’ +2.5 852.5 19.1 854.1 856.5 529.8 11.0 531.7 31.4 533.7 532.4 44.3 (0.0) (0.05) (0.4) (1.3) (-3 (0.0) (-4 (0.0) (-) (3 (0.0)1.3‘ 2.2’ 2.5’ 1.5‘ 1.9* 1 .9’ 1.9‘ a Peak = peak centre.% = % area of the Ni 2p or 0 1s core XPS region. The potential is in V us. SCE. ’FWHM. ‘This peak includes some H20intensity. in this region. However, the trends are the same for both also contains some intensity due to adsorbed water. Columns regions. The accuracy of the percentage of Ni(OH), is limited A and C should be roughly the same (as observed), and differ by the fact that its Ni 2p and 0 1s features are similar to by the amount of water in the film. The S : 0 atomic ratio is NiS04 and by the fact that the 0 1s region for hydroxide only accurate to about +5% due to the low intensity of the S Table 3 Approximate composition information from core XPS regions“ ~~ ~ ~~ 0 :Ni atomic S : 0 atomic 0 :Ni atomic 0 : Ni atomic % 0 1s as NiO (%) Ni(OH), (%) ratio from ratio from ratio ratio from sulfate Ni Q,,, S 2p/O 1s adjusted for 0 Is :Ni 2p,,, from S :0 Nimetal(%) Ni2p 0 1s Ni2p 0 1s curve peak ratio sulfate peak ratio atomic ratio potential 1 2 3 4 5 A B C D E ex situ -0.6 70.8 18.9 7.6 10.3 22.4 0.40 0.03 0.81 0.86 12+0.7 27.1 48.8 24.8 24.1 22.7 0.97 0.07 0.97 1.08 24 + 1.1 18.6 54.5 30.7 26.7 25.4 1.08 0.06 1.08 1.23 24 +2.0 48.7 32.3 16.6 19.0 21.2 0.70 0.03 0.79 0.85 12 +2.5 21.1 55.8 49.1 23.1 31.6 1.02 0.06 1.62 1.84 24 anaerobic cell -0.6 83.1 7.6 0 9.4 14.2 0.26 0.13 0.49 0.66 52 +0.7 45.5 24.3 4.9 30.3 28.1 0.85 0.14 1.17 1.62 56 + 1.1 27.4 42.6 18.0 30.0 32.2 1.03 0.12 1.29 1.7 48 +2.0 59.6 14.6 2.5 25.8 23.3 0.66 0.17 0.79 1.19 68+2.5 19.1 44.8 21.3 36.2 30.5 1.17 0.17 1.60 1.94 68 ~~~~ ~ “ Columns 1,2 and 4 are taken from the curve fit of the Ni 2p,,, region in Table 2. Column 3 is calculated by multiplying the percentage of the 0,-components in the 0 1s region of Table 2 by the 0 :Ni atomic ratio shown in column D.Column 5 is calculated by multiplying the percentage of the OH- component in the 0 1s region of Table 2 by the half the 0 :Ni atomic ratio shown in column D [since Ni(OH), has two oxygen atoms for each nickel atom]. Column 5 is further modified for the ex situ experiment by reducing the percentage of the OH- component in the 0 1s region of Table 2 by the percentage shown in column E since the ex situ data has no sulfate component fitted in the 0 1s region.Column C is obtained by adjusting the data in column D for sulfate not included in column A using column B. J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2p region. Changing the method from background subtrac- tion in the Ni 2p region will also change the percentages, the iterative non-linear meth~d~'.~~giving lower percentages than the Tougaard method4' used here, though the trends are the same. Cathodic Region. After argon-ion sputtering, the intensity of the 0 1s signal was extremely weak. No 0 2s signal is found in the valence band region (which is less surface sensi- tive due to the higher electron kinetic energy). This indicated that the surface is almost oxide free when the sample is exposed to the electrolyte. Cathodic polarization of the sample at -0.6 V gave some oxidized nickel.We believe that the oxidized nickel came from reaction with the deoxy-genated water, since the amount of oxidation was consistent with that found for clean metal samples exposed to deoxy- genated water in our previous studies2, in the anaerobic cell. Since the initial metal sample had no oxide present, and the electrochemical conditions would be expected to reduce any oxidized nickel species on the metal surface to metal, reaction with water would be expected to account for all the observed oxidation. Substantial amounts of adsorbed water and sulfate were indicated by the significant 0 1s peak and a weak 0 2s feature [due the lesser surface sensitivity of the valence band region (oide supra)].The S 2p region shows two peaks. The peak at higher binding energy (169.1 eV) corresponds to sulfate, and the peak at lower binding energy (163.5 eV) to sulfide. Sulfide results from the reduction of the adsorbed sulfate at the nickel surface. Passioe Region. In the passive region [Fig. 2(c), (41, the signal due to the oxidized nickel increases with increasing potential. For example, there is more NiO after polarization to 1.1 than to 0.7 V, which can be seen from the curve fitting results of the 0 1s and Ni 2p regions (Fig. 2B and Table 2). The percentage of Ni(OH), remains approximately constant as the polarizing potential increases. Ni(OH), could be slightly dissolved by sulfuric acid, and a significant amount of the Ni(OH), content may be formed after the sample was removed from the acid solution.After leaving the acid solu-tion all of the samples have the same environment, so it is reasonable to assume that they form almost the same amount of Ni(OH), . The increase in the intensity of the sulfate signal indicates that the amount of sulfate on the nickel surface increased with increasing potential (see Fig. 2A). At the same time the sulfide signal at lower binding energy is extremely weak and finally disappears after anodic polarization. Since the main peak of the Ni 2p signal from NiSO, and the first satellite peak (see Table 1) is at about the same position as that of Ni(OH), ,it is difficult to distinguish these compounds in the Ni 2p region. However, the NiSO, signal can be seen from the S 2p region and in the valence band spectra.The two small peaks at around 12.8 eV and 15.5 eV (Fig. 2D) are caused by sulfate. According to our standard spectra4, the separation between these peaks is about 2.7 eV for sulfate and around 3.9 eV for hydrogensulfate, and the relative inten- sities of the two peaks are different. For sulfate the peak at higher binding energy is more intense than the one at the lower binding energy, but the situation is reversed for hydro- gensulfate. The shoulder of the Ni 3d signal makes the rela- tive intensity of these two peaks less easy to determine, but the separation between the two peaks is clear and this allows us to identify the sulfur as sulfate and not hydrogensulfate.The 0 2s region is dominated by oxygen originating from sulfate and hydroxide. Transpassioe Region. In the transpassive region, the signal due to oxidized nickel decreases significantly at 2.0 V and then increases at 2.5 V. From both the 0 1s and Ni 2p regions (Table 2) we can see that the most significant change occurs for the NiO component, the change of Ni(OH), com-ponent being less than 7%. This result suggests the further oxidation of NiO and/or Ni(OH), to form NoO,H, which can be dissolved by sulfuric acid and would result in the thin- ning of the passive layer. The oxidation and reduction of the Ni(OH),/NiO,H pair has been discussed elsewhere as a main process for the nickel The small change in the relative intensity of the Ni(OH), component further supports the suggestion that most of the Ni(OH), was formed after the nickel electrode was removed from the acid solution.At 2.5 V oxygen was vigorously released and the formation of NiO becomes the dominant process again, resulting in the second- ary passive region. In this set of experiments, the sulfate signal can be seen in both the core level and valence band spectra. The washing procedure was the same as that used in our earlier work,,, where it was found that three washings removed all of the soluble sodium and chloride species after immersion of nickel into sodium chloride solution. In this work we washed each sample eight times. However, there was still a significant amount of sulfate left on the nickel surface as described above.It has been suggested that sulfate is a kind of contami- nation layer adsorbed on the oxidized nickel surface. Others have suggested that the sulfate is incorporated within the oxide layer, since it cannot be removed by washing the sample. Since NiSO, is a soluble compound, and our pre- vious work suggests that the cleaning procedure can remove soluble species on the nickel surface, it is unlikely that the sulfate exists as a physically adsorbed layer on the oxidized nickel surface. Ex situ Experiments Fig. 3 shows the XPS results for the ex situ experiments, with the curve-fitting data being shown in Table 2, and the approximate films composition in Table 3. The results show almost the same oxidation pattern as that of the anaerobic cell experiments (note the Ni 2p and 0 1s regions in both experiments).Cathodic Region. Cathodic reduction at -0.6 V shows some oxidation [NiO and Ni(OH),] on the electrode surface. As in the anaerobic cell case we suppose that this is caused by reaction with water, but one notes that the oxidation level is about twice as high, presimably because not all the air formed oxidation was removed by electrochemical reduction in this region. The fact that the NiO concentration is nearly twice as great as the Ni(OH), concentration supports this contention. Passive Region. In the passive region an increase in anodic polarization potential causes the intensity of the oxidized nickel signals to increase and the intensity of the metal signals to decrease.NiO is accumulated in the passive layer. No significant change in the amount of Ni(OH), was observed. Transpassioe Region. In the transpassive region, the inten- sity of the oxidized nickel signals decreased at 2.0 V and increased again at 2.5 V. The difference in these results from those of the anaerobic cell experiments is that only a very weak sulfur signal due to sulfate (S 2p) was observed. No significant sulfur contribution to the valence band spectra was seen. The two-peak feature at 12.8 eV and 15.5 eV was not observed. It is reasonable to assume that the washing procedure for these two sets of experiments should not cause this significant difference in sulfate concentration if the sulfate was simply adsorbed by the oxidized nickel film.Nevertheless, in order to check the effect of the washing procedure, an additional experiment was done. In this experiment, the nickel sample was cleaned in the normal manner (oide supra) and then argon-ion sputtered to remove all of the oxide layer. After this cleaning procedure the nickel sample was removed from the UHV chamber and electrochemically polarized imme- diately. This procedure was kept exactly the same as that of the anaerobic cell experiments, except that the nickel sample was exposed to air before electrochemical treatment. The sample was dried in air after treatment, and transferred into the UHV chamber within 5 min. This experiment gave the same result as that of the normal ex situ experiments, namely an extremely weak sulfate (S 2p) signal, and no sulfate signal in the valence band and other core regions. Details of the Curve Fitting of the Ni 2p,,, Region Fig.4 gives the curve-fitting results for the Ni 2p,,, region. All fitting parameters are given in Table 1, which uses peak positions and peak widths obtained from the spectra of stan-dard samples and the literature. The peak intensities were variable values determined by the fitting program. Three components, nickel metal, NiO, and Ni(OH),-NiS04 were used to fit all of the spectra. NiSO, cannot be distinguished from Ni(OH), ,since both of them have a Ni 2p,,, main peak and first satellite peak at almost the same binding energy. There is, however, a difference in position of the second satel- lite peak for these two compounds (Table 1).The fitting results are given in Table 2. We can see that the ex situ experiments always exhibit more oxidation than that of anaerobic cell experiments at the same potential, except at 2.5 V. We believe this to be caused by the additional oxida- tion of nickel by air in the ex situ case. Column B of Table 3 gives the S : 0 atomic ratio. This ratio is very small and has an almost constant value. This ratio is about three times greater in anaerobic cell experi- ments than the in ex situ experiments. Discussion Differences between the Anaerobic Cell and ex situ Experiments It is important to reconcile the differences between the ex situ and anaerobic cell experiments in the amount and ease of removal of surface sulfate.The experiments above suggest that the difference arises from the fact that the ex situ experi-ments have an air-formed oxide/hydroxide layer whereas the anaerobic cell experiments have an argon-ion etched metal surface. The argon-ion etching would be expected from previous studies of the oxidized nickel system48*49 to lead mainly to sputtering (for example 400 eV argon ions lead to about 70% sputtering and 30% reduction). Our saddle field gun provides uniform etching, though it is possible that some surface roughening occurs. Certainly our XPS studies show that the etched surface is almost completely metallic. We believe that in the anaerobic cell experiments sulfate ions were chemisorbed by the clean nickel surface.Sub- sequent electrochemical oxidation leads to the formation of an oxidized layer over the adsorbed sulfate layer. The sulfate ions were thus trapped between the nickel metal and the passive layer. This would explain why the sulfate ions cannot be washed away and thus they contribute significantly to the valence band and core level spectra. In the ex situ experi-ments, the sulfate ions are adsorbed onto an already air- oxidized metal. In this situation these ions are easily removed by washing. A second difference between the ex situ and the anaerobic cell experiments is that the former experiments show more adsorbed water and hydroxide ions in the valence band J.CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 region. This is evident from the more intense 0 2s signal over most of the potential range for the ex situ case (compare Fig. 2 and 3). Our work indicates that we cannot obtain a clean nickel metal surface by simple electrochemical reduction treatment, even at -0.6 V which is far below the redox potential of nickel metal. This reductive treatment still gives a significant amount of NiO and Ni(OH), in the case of the ex situ experi-ments, and a small amount of nickel hydroxide and/or sulfate in the anaerobic cell experiments. As discussed above this oxidation is almost entirely caused by reaction with water in the anaerobic cell case, combined with air oxidation during transfer in the ex situ experiment.Some residual strongly che- misorbed water may result in both cases. Estimating the Thickness of the Oxidation Layer It is possible to obtain an estimate of the oxide layer thick- ness using a simple intensity model, though one should make clear that such an approach assumes a uniform surface and a homogeneous mixture of NiO and Ni(OH), ,which we point out below is a rather crude approximation.50i51 This simple model uses an expression derived from the familiar intensity expressions where the intensity ratio of metal to oxidized nickel is given by where N, and N, are the number fraction of nickel atoms of NiO and Ni(OH), ,respectively. In this case we can assume a,,, = a,, (a,,, and a,, are the photoelectron cross-sections of the metal and the oxide, respectively), K the spectrometer factor, K, is obtained from the spectra using the previously described method48 as 1.36 for Ni(metal)/Ni(OH), and 0.98 for Ni(metal)/NiO. r~ is the reciprocal of the mean escape depth (A), D is the density of the atom in the material under investigation and x is the thickness of the film. Table 4 shows the values of G, D and molecular weight for this calcu- lation.8.1O Results of this evaluation for nickel samples polarized at different potentials are given in Fig.5. In the passive region the oxide thickness increases linearly with the electrode potential. The thickness fell around 2.0 V and increased again at 2.5 V. The oxide layer for the ex situ experiments was always thicker than the anaerobic cell thicknesses, except at 2.5 V.The minimum thickness amounts to 4 A arising from the film formed by adsorbed water and/or sulfate species. Previous angle-dependent XPS studies,, show (as expected) Table 4 Calculation parameters for the passive layer thickness &' Dlg cm -molecular weight /g mol-' Ni 0.18 8.90 58.71 NiO 0.10 6.67 74.71 Ni(0H) 0.073 4.15 92.7 1 NiOOH -4.68 91.71 J. CHEM. SOC. FARADAY TRANS., 1994, VOL. 90 2.5 -2.0 -E5. 1.5-v) tY .-u 1.0-5 0.5 -0.0 4 I 1 I I I I i -1.0 -0.5 0.0 0.5 1.0 1.5 2.0 2.5 potentialp vs. SCE Fig. 5 Change in the total thickness of the oxidized nickel layer, formed in 0.5 mol dm-3 sulfuric acid solution, with the polarization potential.The polarization time is 15 min. that it is more likely that NiO is found in the inner part of the film, with an accumulation of hydroxide and water in the outer parts of the layer. The sulfate ions may exist either in the inner part (anaerobic cell experiments) or outer part (ex situ experiments) as discussed above, though this is dificult to determine by angle-dependent XPS. Clearly the film is much more complex than the simple homogeneous model that we have used to obtain these thickness values, and so such values should be regarded as a crude estimate of the film composition. Conclusions Comparison of the data from both anaerobic cell and ex situ experiments for nickel oxidation illustrates the importance of conducting such experiments under anaerobic conditions.Air-formed oxide films on the metal cannot be removed by electrochemical reduction, and this film can substantially affect the amount and ease of removal of sulfate ions. The use of this approach has allowed us to support and extend the conclusions of our previous study.8 The main chemical changes resulting from electrochemical oxidation are: (i) the film present on the electrode surface in the passive region is composed of NiO and Ni(OH),, the former being the passive The average thickness of this layer is in the region over the potential region where passivation occurs. On increasing the potential, the thickness of the passive layer increases linearly. (iii) At the beginning of the transpassive region, NiO and/or Ni(OH), could be further oxidized to form NiO,H, which can be dis- solved by acid solution and result in the breakdown of the passive layer.At higher potential (about 2.5 V) vigorous evolution of oxygen occurs, and a second passive region occurs. (iv) The adsorption of sulfate ions depends upon the pretreatment of the nickel electrode. Sulfate ions be chemi- cally adsorbed on a clean nickel metal surface, and then be subsequently trapped within the passive layer. Electrochemi- cal treatment of metal in the ex situ experiment where the metal has an air-formed oxide layer probably leads to adsorption of sulfate by the oxide layer, from which sulfate is easily removed by washing. This material is based upon work supported by the National Science Foundation under Grant No.CHE-8922538. The US Government has certain rights in this material. 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