Core and valence band photoelectron spectroscopic studies of nickel

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Anal. Chem. 1993, 65, 2276-2281

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Core and Valence Band Photoelectron Spectroscopic Studies of Nickel Oxidation in an Anaerobic Liquid Cell Yuanling LiangJ Dilip K. Paul? Yaoming Xie? and Peter M. A. Sherwood'J Department of Chemistry, Willard Hall, Kansas State University, Manhattan, Kansas 66506,and Department of Chemistry, St. Mary College, Leavenworth, Kansas 66048

An apparatus is described that allows studies to be made of the solid-liquid interface. The apparatus is linked to an X-ray photoelectron spectrometer,allowing an investigation of the changes in surface chemistry at this interface. The corrosion of nickel under various conditions is examined using this apparatus. This study focuses on the surface chemical changes associated with the exposure of nickel to deoxygenated and oxygenated solutions of water and sodium chloride. Deoxygenated water causes negligible oxidation, but oxygenated water causes a slight increase in oxidation. The presence of chloride ions causes a dramatic increase in nickel corrosion. Angleresolved studies show that the surface is enriched in adsorbed water and nickel hydroxide.

investigation of films or adsorbed layers on metal and other surfaces. Electrochemistry is a valuable method for understanding the electrochemical processes themselves, and XPS can identify the chemical nature and thickness of films or adsorbed layers formed by the electrochemical process. Our group has been one of many to develop special approaches to reduce or eliminateatmospheric oxidation or contamination in transfer of the electrode from the electrochemical cell to a spectrometer.4-21 The idea of using an anaerobic cell, in other words, an electrochemicalcellthat can be fitted into a subsidiary vacuum chamber attached to a surface science instrument, has been explored by several workers (a small selection of appropriate references being indicated above4-21 and further examples being found in a review by P.M.A.S. in 19852). In our group we have developed three designs, the first being used in 1975,'s the second in the early 1 9 8 0 ~and , ~ the third the cell reported here. Such an apparatus is not trivial to operate, though we have found the design reported here to be the most reliable and easy to operate of the three cells. In this work we add the collection and analysis of the valence band region to the core studies. We have found this region to be a powerful addition to the core region, often giving spectra that are more sensitive to subtle differences in the surface chemistry.21-2s In this paper we report the application of this cell to the study of the oxidation of nickel. Nickel is an important component of many alloys, due to ita relative noncorrodibility. There is also considerable interest in nickel due to ita use in

INTRODUCTION An analytical technique that is usable in solution is the best approach for examination of the chemistry that occurs at the solid-liquid interface. Important advances have been achieved using methods such as infrared and Raman spectroscopy for examining this interface in situ. However, these approaches often use experimental probes that are not inherently surface sensitive, which often cannot provide all the necessary chemical informationabout the system. Other, inherently surface sensitive approaches thus have an important contribution to make, although they require the surface to be removed from ita liquid environment and placed into an ultrahigh vacuum (UHV) system. This approach needs to be carried out in a way that minimizes the possibility of further chemical change and surface contamination during the transfer process and in the UHV system. Surface hydrocarboncontamination can substantially inhibit the use of some surface-sensitive spectroscopies. Core and valence band X-ray photoelectron spectroscopy (XPS or ESCA) has considerable potential for the investigation of practical surfaces and, in particular, is a valuable tool for the investigation of electrochemical and corrosion processses.1-21 The combination of XPS with electrochemical studies provides a powerful tool for the formation and ~~

Kansas State University. St. Mary College. (1)Sherwood, P. M. A. In Contemporary Topics in Analytical and Clinical Chemiutry;Herculea, D.M.,Hieftje,G. M.,Snyder, L. R.,Ereneon, M. A.,Eds.;Plenum Preas: New York and London, 1982;Vol. 4,Chapter 7,pp 205-293. (2) Sherwood, P. M. A. Chem. SOC.Rev. 1988,14(I),1-44. (3)Hoppe, H.W.; Strehblow, H. H. Surf. Interface Anal. 1989,14, 121-131. (4)Kim,K. S.;Winograd, N.; Davis, R. E. J.Am. Chem. SOC.1971,93, 6296-6297. (5)Hubbard, A. T. CRC Crit. Rev. Anal. Chem. 1973,3,201-243. (6)Hubbard, A. T. Acc. Chem. Res. 1980,13,177-184. (7)Hubbard, A. T.; Ishikawa, R. M.; Katekkaru, J. J. Electroanal. Chem. Interfacial Electrochem. 1978,86,271-288. t

t

0003-2700/93/0365-2278504.0010

(8)Wieckowski, A.; Roeasco, S. D.; Schardt, B. C.; Stickney, J. L.; Hubbard, A. T. Inorg. Chem. 1984,23,565-569. (9)Wagner, F. T.; Roes, P. N., Jr. J. Electrochem. SOC.1983, 130, 1789-1790. (lO)Yeager, E. B.; O'Grady, W. E.; Woo, M. Y. C.; Hagans, P. J. Electrochem. SOC.1978,125,348-349. (11) O'Grady, W. E.; Woo, M. Y. C.; Hagans, P. L.; Yeager, E. B. J. Vac. Sci. Technol. 1977,14,365-368. (12)Sukhotin, A. M.; Shlepakov, M. N.; Kostikov, Yu, P.; Strykanov, V. S. Electrokhimiya 1982,18,285-287. (13)Pou, T. E.;Murphy, 0. J.; Yong, V.; Bockris, J. OM.; Tongson, L. L. J. Electrochem. SOC.1984,131,1243-1251. (14)Hammond, J. S.;Winograd, N. J.Electroanal. Chem. Interfacial Electrochem. 1977,78, 55-69. (15)McIntyre, N. S.;Sunder, S.; Shoesmith, D. W.; Stanchell, F. W. J. Vac. Sci. Technol. 1981,18,714-721. (16)Neff, H.; Foditach, W.; Kotz, R. J. Electron Spectrosc. Relat. Phenom. 1984,33,171-174. (17)Fleiech,T.;Shepard,A.T.;Ridley,T.Y.;Vaughn, W.E.;Winograd, N.; Baittinger, W. E.; Ott, G. L.; Delgaes, W. N. J. Vacuum Sci. Technol. 1978,15,1756-1760. (18)Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Chem. Soc., Faraday Trans. 1 1977,73,327-343. (19)Ansell, R. 0.; Dickinson, T.; Povey, A. F.; Sherwood, P. M. A,;J. Electroanul. Chem. Interfacial Electrochem. 1979,98,69-77. (20)Sherwood,P. M. A.;Welsh, I. D.; Hercules, D. M. Appl. Spectrosc. 1988,42,658-666. (21)Sherwood, P. M. A.; Thomas, S. J. Chem. SOC.,Faraday Trans. 1993,89,263-266. (22)Welsh, I. D.; Sherwood,P. M. A. Chem. Mater. 1992,14,133-140. (23)Thomas, S.;Sherwood,P. M. A. Anal. Chem. 1992,64,2488-2495. (24)Welsh, 1. D.;Sherwood, P. M. A. In Advances in corrosion protection by organic coatings; Scantleburg, D., and Kendig, M., Eds.; R o c . Electrochem. SOC.1989,8*13,417-429. (25)Xie, Y.; Sherwood, P. M. A. Chem. Mater. 1991,3,164-168. @J

1893 Amerlcan Chemlcal Society

ANALYTICAL CHEMISTRY, VOL. 65, NO. 17, SEPTEMBER 1, 1993

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alkaline batteries and as anode material in fuel cells. The surface chemistry associated with nickel corrosion is clearly very important indeveloping these applications. The currentvoltage curve of the metal shows several regions10 including active, passive, and transpassive regions. Many workers have studied the surface chemistry associated with electrochemical teratment.lJQ121*21Studies of the effect of gaseous water and oxygen on nickel single crystal3245and polycrystallinesurfaces have been reported. However, there has been little investigation of the effect of aqueous solutions on nickel oxidation.m+41 The effects of both oxygenated and deoxygenated solutions of water and chloride on the metal are reported in this paper. The anaerobic cell approach allows us to start with a clean and clearly characterized surface as well as to examine each step of the process. EXPERIMENTAL SECTION General Information. XPS measurements were made with a VSW HA100 spectrometerhaving a base pressure of 1W0Torr using achromic Mg Ka X-radiation (line width approximately 0.7 eV) with a power of 300 W. Spectra were recorded in fixed analyzer transmission (FAT) mode with a pass energy of 25 eV to achieve maximum instrument resolution (substantially better than the X-ray line width). Data were usually collected with at least 17 points/eV to be sure to identify any subtle features that might be lost at lower resolution and a larger step size. The spectrometer energy scale was calibrated using copper.*2 While the anaerobic cell approach minimizes carbon contamination, calibration could be performed by using the small amount of residual carbon as a reference, with the C 1s line set to a binding energy of 284.6 eV. Argon ion sputtering was carried out with the use of a B21 saddle-field ion source sputter ion gun at 2.6 kV and 1mA with an argon pressure of 1O-g Torr. Ultra high-purity argon was used for etching and as an inert atmosphere in the experiments described below. Nickel foil was obtained from Alfa (with 99.994% purity), polished mechanicallywith AlzOa (44pm), degreased with acetone, and cleaned with quadruply distilled water before insertion into the anaerobic chamber. No aluminum was found in XPS spectra recorded after polishing. The samplewas argon ion etched until no oxide could be detected. This generally required 1.5-2 h of etching using the mild etching conditions described above. The sample was rotated during the argon ion bombardment to ensure uniform etching. After this preparation, the sample was immersed into solution in an argon atmosphere. The sample was removed from the electrolyte and rinsed with deoxygenated quadruply distilled water three or four times. The chamber was then evacuated and the sample pumped to an ultrahigh vacuum before analysis by XPS. (26) Zumin, M.; Ives, M. B. J. Electrochem. SOC.1979,126,470-474. (27) Armstrong,R. D.; Henderson, M. J. Electroanal. Chem.Interface Electrochem. 1972,39, 222-224. (28) Marcue, P.; Olefjord, I.; Oudar, J. Corros. Sei. 1984,24,269-278. (29)Sato, N.; Okanmto, G. J. Electrochem. SOC.1963,110,605-614. (30) Herbelin, J. M.; Marcue, P. Proceedings of the Symposium of the Application of Surface Analysis Methods to EnvironmentallMaterial Interactions; Symposium on the application of SurfacaAnalyeis Methods to EnvironmentaVMaterial Interaction, Seattle, WA, 1990; pp 222-234. (31) Marcue, P.; Oudar,J.; Olefjord, I.J. Microsc. Spectrosc. Electron. 1979,4,63-72. (32) Hopster, H.; Brundle, C. R. J. Vac. Sci. Technol. 1979,16,548565. (33) Benndorf, C.; NBbl, C.; Thieme, F. Surf. Sci. 1982,121,249-259. (34) Benndorf,C.;NBbl,C.;Rllaenburg,M.;Thieme,F.Surf.Sci. 1981, 111,87-101. (35) Madey, T. E.; Netzer, F. P. Surf. Sci. 1982,117,549-560. (36) Benndorf, C.; N6b1, C.; Rllaenberg, M.; Thieme, F. Appl. Surf. Sci. 1982,11~12,803-811. (37) Page, P. J.; Trimm, D. L.; Williams,P. M. J. Chem. SOC.,Faraday Trans 1 1974, 7,1769-1792. (38) Brundle, C. R.; Carley, A. F. J. Chem. SOC.,Faraday Trans. 1 1976,6,51-70. (39) Arfelli, M.; Ingo, G. M.; Mattogno, G.; Beccaria, A. M. Surf. Interface Anal. 1990,16,299-303. (40)Beccaria, A. M.; Poggi, G. Corrosion 1986,42,470-475. (41) Kishi,K.;Miyoe.hi,H. J. ElectronSpectrosc.Relat. Phenom. 1991. 53, 237-249. (42) Surf. Interface Anal. 1991, 17,889-891 (ASTM, E902-88).

P t electrode

K'

Solution Outlet

Figure 1. Schematicdiagram of the spectrometer chamber and the anaerobiccell chamber: A, spectrometer UHV chamber: B, anaerobic cell main UHV chamber; C and D, subsldlary vacuum chambers: HI

and HP,O-ring seals: K1 and K2 O-ring seals;gl, gate valve to sample Preparation chamber and additional sample lntroductlon system: 92, 93,and g4, 6-In. gate valves; E, electrochemlcal cells; R, reference electrode chamber; F, Ion etchlng gun; J, x, y, z manlpulator.

Nonlinear backgrounds were removed from the spectra using the previously described Nonlinear least squares curve fitting was performed using a Gaussian/Lorentzian peak shape,-which included X-radiation satellites in the fit. Nickel metal features included an exponential tail to account for conductionband interaction,- with satellite features associated with the fitted components also included. Anaerobic Cell. Figure 1 shows a schematic diagram of the anaerobic cell chamber with ita associated electrochemical cell. The anaerobic cell chamber is connected to the main XPS chamber through a 6-in. UHV gate valve (g2). This valve allows the two chambers to be pumped to UHV separately, or together. The anaerobic cell chamber can be separated into three regions (B-D)that are separated by two additional6-in.UHV gate valves (g3, g4). Region B is the location where the sample surface is sputtered, exposed to solution, or electrochemically treated. The sample can be moved in three dimensions, and rotated a full 360' around the sample probe axis, by a precision x , y, z manipulator. The glass electrochemcialcell and the upper Luggin capillary tube normally reside in regions C and D and can be completely removed for cleaning. Glass parta are only inserted into region B for the liquid exposure and electrochemical experiments when region B is raised to a positive pressure of an inert gas (argon or nitrogen). The electrochemical cell is a single piece of glassware consisting of a modified beaker assembly with a lower drain tube. It can be moved up and down on an O-ring seal (Hl) fitted to the bottom of region D. The glass drain tube is sealed by another O-ring seal (Kl) at its bottom end. The O-ring seal (Hl) is thus a substantial distance (35 cm) from the gate valve 84. The cell is normally stored below gate valve g4 (as shown in Figure 1). It can be moved into the UHV region B by (43) Sherwood, P. M. A. In Practical Surface Analysia by Auger and PhotoelectronSpectroscopy;Brigga,D., Seah,M. P., Eda.;Wdey London, __ 1983; pp 445-475. (44) Sherwood,P.M.A.InData AnalysisinXPSandAESinPractical Electron Spectroscopy; Brigge, D., Seah, M. P., Eds.; Wiley: New York, 1990; Appendix 3, pp 555-586. (46) Ansell, R. 0.;Dickinson, T.; Povey, A. F.; Sherwood, P. M. A. J. Electroanal. Chem. Interfacial Electrochem. 1979, 98, 79-89. (46) Li, C. P.; David, A. P.; Hercules, D. M. Appl. Spectrosc. 1984,38,

88&886. (47) Proctor, A.; Sherwood, P. M. A. J. Electron Spectrosc. Relat. Phenom 1982,27, 39-56. (48) Proctor, A.; Hercules, D. M. Appl. Spectrosc. 1984,38,505-618.

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Electrolvte

Quadruply distilled rater

Reference electrode

.7

Metal tube

Luggin Capillary

,

Main XPS chamber

Fritted glass Metal tube Electrochem.

1 Solution Outlet Figure 2. Expanded view of the electrochemical cell assembly.

pushing the cell vertically up on the O-ring seal H1, once g4 is opened and the chamber B raised to a positive pressure of an inert gas. Figure 2 shows an expanded view of the cell region in a configurationcorrespondingto a typical experimental situation. Figure 2 shows the sample at the end of the x, y, z mainuplator immersed in the cell, and the Luggin capillary moved from chamber C so that is located near the sample surface. A counter electrode in a “subbeaker” (separated from the rest of the cell by a glass frit) is provided to allow electrochemical experiments to be performed. An electricalcontact from the platinum counter electrodeis attached to an electricalfeed-throughlocated in region D. The upper region C contains a double O-ring seal (H2) separated from the gate valve g3 by a considerable (25 cm) distance. The Luggin capillary is O-ring sealed to fit concentrically into a larger diameter metal tube so that both tubes can be moved independently up and down. These tubes are normally stored just above the gate valve g3 as shown in Figure 1. The Luggin capillary tube can be lowered into the cell when the chamber B is raised to a positive pressure of an inert gas. The metal tube is notched so that it can be used to rotate the sample through 90° and place it in the cell as shown in Figure 2. Solution entry is achieved through the Luggin capillary tube. A reference electrode is located in the compartment R (Figure 1) for electrochemicalexperiments. Introduction of liquid into the cell, and solution draining of the cell are gravity driven. The x, y, z manipulator is driven by an electric motor in the z direction with stops to prevent any accidental collision of the sample with closed gate valves. The use of a positive pressure of an inert gas during movement of the glass tubes in chambers C and D ensures that no atmospheric leaks occur. The location of the O-ring seals far from the UHV gate valves g3 and g4 minimize contamination of the UHV chamber B. The cell has a capacity of about 50 mL. All washing operations use deoxygenated quadruply distilled water that is contained in one of two liquid reservoirs. The second reservoir is used to contain the solution for the liquid exposure experiments. These reservoirs are attached to the top of the Luggin capillary so that no air contamination of the solution is possible. The solution in the reservoirs is routinely deoxygenated with an inert gas prior to use. The sample can be of any geometry or size that will fit in the chamber. After the experiment the sample is alwayswashed with deoxygenated quadruply distilled water. Pressures in the UHV chamber B are routinely obtainable in the 10-lO-Torrrange.

RESULTS AND DISCUSSION Nickel-Water System. The effects of immersion of a sputtered-clean nickel metal surface in deoxygenated and oxygenated water were investigated. The quadruply distilled water samples were prepared by passing either ultrahighpurity argon (or nitrogen) or oxygen through the water for several hours, respectively. The initial metal samples were prepared by argon ion etching and verified by XPS studies as being metal immediately before the water-exposure experiments. In the case of exposure to deoxygenated water, two different exposure times of 30 min and 2.5 h were used. The latter time corresponded to a further 2-h exposure of the sample that had been exposed for 30 min. A clean nickel metal surface was immersed in oxygenated water for 2 h. The results of these experiments are shown in Table I and Figures 3 and 4. Figure 3 shows the XPS spectra of the Ni 2~312and 0 1s regions fitted to a number of component peaks. In all cases the 0 ls/Ni Auger spectra (Figure 3) were curve fitted into four peaks, at 529.9,531.7,533.3, and 538.6 eV, corresponding to 0%species,l9~39,49-52OH-species/adsorbed ~ a t e r , 1 9 ~ 3 9 * ~ ~ ~ ~ 5 ~ adsorbed water, 1 9 3 & ~ - 5 3and the LzMzsM23 nickel Auger peak,” respectively. The relative intensity of the nickel Auger peak around 539 eV compared to the 0 1s peak shows the relative amount of nickel and oxygen species. Curve fitting of Ni 2p312spectra (Figure 3a) gives three different nickel species, corresponding to metal and its associated satellite features, nickel oxide, and nickel hydroxide in agreement with literature values and the reference compounds. Each nickel metal type used in the fit is chosen to have an initially chosen peak shape and width and associated satellite or satellites that take the values that we have determined for the individual compounds. The fitting process allows these values to vary as the fit converges; however, final fitted values for these parameters were close to those found for the pure compounds. The 95% confidence limits on the peak position and percentage of the total area are shown in the tables. The metal peak is found at 852.4 eV19,39.m3with a satellite at 858.9,M~wand the NiO peak is found at 854.2 eV19p391m3with satellites at 855.7 eV46953 and 860.7.*,53 In addition, the Ni(OH)z peak is found at 856.1 eV1939,m3with a satellite at 862.9 eV.w.53 Figure 4 shows the valence band region for these experiments. The etched metal shows the characteristic metal spectrum that contains the expected satellitew@at a binding energy of about 6 eV. The broad hump around 20 eV, may be due to the presence of a small amount of residual carbon. Exposure to deoxygenated and oxygenated water leads to the appearance of a broad principally 0 2s feature at about 27 eV together with some weak features at about 19 and 14 eV due to principally C 2s and C 2p features, respectively. Clearly the metal has an affinity for residual hydrocarbons, since the amount of hydrocarbon is substantially greater than that found for the metal exposed to either deoxygenated or oxygenated sodium chloride solution (see below) in the same apparatus. One also notes that the hydrocarbon intensity grows with time. Thus the spectrum shown in Figure 4d (49) Marcus, P.; Olefjord, I. Surf. Interface Anal. 1982,4, 29-33. (50) Fontaine, R.; Feve, L.; Buvat, J. P.; Schoeller, C.; Caillat, R. J. Microsc. Spectrosc. Electron. 1989, 14, 453-470. (51) Langell, M. A.; Furstenal, R. P. Appl. Surf.Sci. 1980,26,445-460. (52) Moroney, L. M.; Smart, P. S. C.; Roberta, M. W. J. Chern. SOC., Faraday tram^. 1, 1983, 79, 1769-1778. (53) Kim, K. S.; Winograd, N. Surf. Sci. 1974, 43, 625-643. (54) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moudler, J. F.; Mullenberg, G. E. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer: Eden Prairie, MN, 1979. (55) Kemeny, P. C.; Schevchik, N. J. Solid State Cornrnun. 1976,17, 255-258. (56) Hiifner, S.; Wertheim, G. K. Phys. Lett. 1975,51A, 299-300.

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Table I. Core Binding Energies (eV) and Related Details for Nickel Samples in W a t e r %

NiO etched

fhwm DeOdwater, 30 min fhwm DeOdwater,b 2 h fhwm oxygenated water 2h fhwm

NiO

Ni 2 ~ ~ 1 2 %

Ni(OH)2

%

%

0%

0 1s OH%

0 Is/ HzO

%

NiAwr

852.4(0.0) lOO(O.00) 1.1 852.5(0.1) 77.64(0.43) 854.1(2.9) 17.91(1.37) 856.2(13.2) 4.45(0.91) 529.8 2.77 531.8 44.34 533.2 52.89 1.79 1.1 2.0 2.4 1.3 1.9 1.9 852.5(0.1) 69.56(0.50) 854.1(2.3) 22.89(1.62) 856.2(10.6) 7.55(1.06) 529.8 2.85 531.9 44.35 533.3 52.79 3.13 1.1 2.0 2.4 1.4 2.1 2.1 852.6(0.2) 67.37(0.64) 854.2(2.4) 27.03(2.20) 856.3(19.2) 5.60(1.54) 530.0 4.03 531.9 53.85 533.5 42.13 3.18

2.0

1.1

2.4

1.3

2.1

2.1

a DeOdwater, deoxygenated quadruply distilled water. fwhm, full width a t half-maximum. Figures in parentheses represent 95% confidence limita (*2 standard deviations). The 0 1s peak positions were fixed and so do not have confidence levels shown. Ni 2p spectrum for IOo takeoff angle. 0 1s spectrum for 45O takeoff angle.

*

Original

O l s / N i Auger

Smoothed

n u)

4

4

-4

G

3 h

I

- 1 bU

A

1

38 866

858

850

542 536

8

I 38 8

Binding Energy (eV) 530

B i n d i n g Energy ( e V ) Flgure 3. Core photoelectron spectra of nickel metal exposed to oxygenated and deoxygenated water: (a)argon Ion etched nickel metal sample, (b) etched metal after exposure to deoxygenated quadruply distilled water for 30 mlns, (c) sample (b) after a further 2 4 water exposure, and (d) etched metal after exposure to oxygenated water for 2 h. initially appears similar to that of Figure 4c, but the amount of hydrocarbon grows together with the background slope to higher binding energy (indicative of the effect of a surface overlayer). We will discuss the affinity of the metal for hydrocarbon in another paper,a7where we discuss hydrocarbon polymerization due to the breakdown of a corrosion inhibitor on the metal. All the valence band spectra are indicative of a principally metal spectrum, with features indicative of a surface film involving adsorbed water and hydrocarbon. No metal oxide features are seen in any of these spectra. This is consistent with the core results discussed above. One should note that the valence band region gives information from a greater depth than the core region (57) Liang,Y.; Paul,D.; Sherwood,P. M. A. Chem.Mater., submitted.

Flgure 4. Valence band photoelectron spectra of nlckel metal exposed to oxygenated and deoxygenated water: (a) argon Ion etched nlckel metal sample, (b) etched metal after exposure to deoxygenated quadruply dlstllled water for 30 mlns, (c)sample (b) after a further 2 4 water exposure, and (d) etched metal after exposrue to oxygenated water for 2 h. due to the significantly greater kinetic energies (and thus escape depths) of the photoelectrons involved. These spectra show that there is a considerableamount of adsorbed water for both types of experiment, with the peak at 533.3 eV being more intense than the 531.7-eV peak in the deaerated water experiments. We believe that most of the surface oxygen comes from adsorbed water in the deoxygenated water experiment, since the Ni 2p region shows only a small amount of oxide/hydroxide formation. The overall increase of adsorbed water on the sample with increasing exposure time is clearly seen by comparing the relative Ni Auger and 0 1s intensity in Figure 3a-c and Table I. Table I indicates that uncertainty in peak position increases as the amount of the component decreases in the fitted envelope. Thus Ni(OH)Z,the lowest percentage component present in all the spectra, is found to have the greatest uncertainty in ita peak position in the fit. It is significant to note that the

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ANALYTICAL CHEMISTRY, VOL. 65, NO. 17, SEPTEMBER 1, 1993

Table 11. Core Binding Energies (eV) and Related Details for Nickel Samples in Sodium Chloride (1 M) Solution. Ni 2 ~ 3 1 2 0 Is NiO % NiO % Ni(0H)z % 0% % OH% H2O % etched

852.4(0.0) lOO(O.00)

1.1 fhwm 1M NaCl/DeOz, 852.5(0.1) 37.38(0.23) 854.1(0.8) 47.39(0.9) 1 h, 40° fwhm 1M NaCl/ 0 2 , l h, 10’ b fwhm 40° fwhm 75’ b fwhm

0 Is/ NiAwer

856.3(0.9)

15.23

529.8 25.7

531.6 60.54 533.4 13.76 4.32

1.1 2.1 2.4 1.4 1.7 1.7 852.7(0.1) 31.51(0.37) 854.3(1.0) 52.24(0.76) 856.4(15.5) 16.70(0.38) 530.1 21.43 531.8 58.03 533.5 20.54 10.1 1.1 2.1 2.4 852.5(0.1) 36.59(0.35) 854.2(0.9) 50.85(0.50) 856.3(7.2) 1.1 2.0 2.4 852.5(0.1) 41.10(0.42) 854.1(0.9) 49.00(1.38) 856.2(6.7) 1.1 2.0 2.4

12.56 g.gO(0.94)

1.4 1.8 1.8 529.9 22.39 531.6 60.19 533.3 17.08 6.20 1.4 1.8 1.8 530.0 25.28 531.7 60.67 533.5 14.05 4.48 1.4 1.8 1.8

NaCl/DeOz, 1M sodium chloride solution in deoxygenated quadruply distilled water. NaC1/02,1 M sodium chloride solution in oxygenated quadruply distilled water. fwhm, full width at half-maximum. Figures in parentheses represent 95% confidence limits (+2 standard deviations). The 0 Is peak positions were fixed and so do not have confidence lvels shown. Takeoff angle, the angle between sample surface and the X-ray gun.

uncertainty in the percentage of each component in the fit is much less than the uncertainty in peak position. Fitting a complex nickel envelope with a number of component compounds with associated satellite peaks will always have some error; however, the fits that we present here show consistent trends in the variation of the amounts of the component compounds. Clearly the immersion of nickel in oxygenated water causes additional oxidation as might be expected. The overall amount of oxygen is much the same as in the deoxygenated exposure case (note the 0 ls/Ni Auger intensity ratio), but the amount of oxidation (as opposed to water adsorption) is substantially greater (as indicated by the amount of NiO and Ni(OH)2) by about 50%. One also notes that there is about twice as much NiO as there is Ni(OH)2in all cases independent of exposure time or the presence of absorbed oxygen in the water. The slow oxidation of nickel is consistent with its expected reactivity and is consistent with studies using gaseousexposure to water or oxygen.32~36.58 Nickel-Sodium Chloride Solution System. The effect of exposure of a clean nickel metal surface to deoxygenated and oxygenated sodium chloride solutions was investigated. The procedure followed was the same as that for the water experiments described above, except that a 1 M solution of sodium chloride was used. Nickel samples were washed with deoxygenated quadruply distilled water at least three times after the exposure. The results of these experiments are shown in Table I1 and Figures 5-7, showing the 0 Is, Ni 2p3/2, and valence band regions,respectively. The Ni 2p and 0 1sregions of the oxygenated solution were also measured a t three different takeoff angles, giving information that has a variable amount of information from the surface region. The results of the curve fitting, which uses the same parameters as those in Table 1, are shown in Table 11. Figure 5d indicates the nickel LzMzsM23 region as “d”, the adsorbed water region as “c”, the OH- species/adsorbed water as “b”, and 0%as “a”. Figure 6d indicates the nickel metal region as “a” with its satellite at ”e”, the NiO region as “b” with its satellites at “d“ and “f“, the Ni(OH)2as “c” with its satellite at “g”. It should be noted that the uncertainties in peak position and the percentage of each component present are much less in Table I1than in Table I due to the larger amount of oxidized species present for the data presented in Table 11. Comparison of the XPS spectra for both the water and sodium chloride solution experiments indicates that extensive (58) Norton, P. R.; Tapping, R. C.; Goodale, J. W. Surf. Sci. 1977,65, 13-36.

544

538

528

544

538

528

Binding Energy ( e V ) Flgure 5. 0 1s core photoelectron spectra of nickel metal exposed to 1 M sodlum chiorlde solution: (a) argon Ion etched nlckel metal sample, (b) etched metal after exposure to a deoxygenated solution of 1 M sodium chloride in quadruply distilled water with a takeoff angle of 40°, (c) as wlth (b) but for an etched metal sample exposed to an oxygenatedsolution of sodium chloride. (d-f) show the effect of takeoff angle on the spectrum shown In (c) for angles of lo’, 40°, and 75’.

oxidation of nickel occurs in the presence of chloride ions. This substantial oxidation is seen for both deoxygenated and oxygenated solutions. The relative Ni Auger to 0 1sintensity increases when oxygenated sodium chloride solution is used (Table 11, last column), though there is almost the same amount of NiO and Ni(OH)2 for both oxygenated and deoxygenated solutions as indicated in Table 11. Angleresolved studies in the Ni 2p3/2 and 0 ls/Ni Auger regions show that the amount of metal increases with increase in takeoff angle, corresponding to spectra that become increasingly less surface sensitive. One also finds a substantial reduction, by a factor of 2, of the amount of Ni(OH)2 as the takeoff angle changes from 10’ to 75’. This clearly shows that Ni(OH)2 is located in the surface region. The amount of NiO in the Ni 2p region shows a smaller amount at low takeoff angle, but less substantial changes than found for the hydroxide. Comparison of the ratio of the 0 ls/NiA,, intensity indicates a general reduction in the overall amount

ANALYTICAL CHEMISTRY, VOL. 65, NO. 17, SEPTEMBER 1, 1993

.3

,

I

I

852

864

n

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852

Binding Energy ( e V ) Flgure 6. Ni 2 ~ 3 core ~ 2 photoelectron spectra of nickel metal exposed to 1 M sodium chlorkle solution: (a) argon ion etched nickel metal

sample, (b) etched metal after exposure to a deoxygenated solution of 1 M sodium chloride in quadruply distilled water with a takeoff angle of 40°, (c) as with (b) for an etched metal sample exposed to an oxygenated solution of sodium chloride. (d-1) show the effect of takeoff angle on the spectrum shown in (c)for angles of loo, 40°, and 75'.

Original

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2281

surface region, as indicated by the decrease in the water contribution to the overall 0 1s intensity with increasing takeoff angle. This probably corresponds to a falloff in adsorbed water with depth, as expected. Adsorbed water probably contributes to both the 533.3- and 531.7-eV peaks, the latter also corresponding to hydroxide. The valence band region (Figure 7) shows the substantial oxidation reflected in the core region. No C13s peak is seen (no C12s or C12p core peaks are seen either), indicating that chloride ions were effectively removed from the film by washing the sample with deoxygenated quadruply distilled water. The spectrum shows the appearance of oxide by the presence of three clear features in the near-band edge region. One of these features is the metal feature together with the metal satellite around 6 eV. The metal feature near the band edge is broadened by the presence of oxide, whose principally nickel 3d region is at -2.5 eV higher binding energy than the metal. In addition, the intense satellite of the oxidelhydroxide is seen at -10 eV binding energy (its presence marked by an arrow in Figure 6). The principally 0 2s region is broad indicative of the presence of oxide, hydroxide, and water. In our previous studies of the oxides, hydroxides, and oxyhydroxidessg we found that the hydroxide has an 0 2s feature at -3 eV higher binding energy than that found in the oxide. The relative amount of oxide to metal is less in the valence band region than the core region, as expected due to the greater depth probed by this region. It is important to note that there is practically no hydrocarbon visible in Figure 7. Since the experimental conditionswere similar to those in the water case (in terms of solution exposure, use of the same cell for solution exposure, etc.), we consider that this difference is caused by the much greater affinity for hydrocarbon of the metal as opposed to the oxide. Our results do not support the suggestion of Marcussothat chloride ions can be trapped during the growth of the passive film. Certainly it is likely that chloride ions can initiate pitting.39 The removal of chloride ions by washing may show that the films formed on the metal are porous, and some loss of metal may occur in the formation of soluble (possiblynickelcontaining) species. The nickel oxidelhydroxide film is probably not very protective against further attack.

CONCLUSIONS

38

8

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B i n d i n g E n e r g y (eV) Flgure 7. Valence band photoelectron spectra of nickel metal exposed to 1 M sodium chlorlde solution: (a) argon ion etched nickel metal sample, (b) etched metal after exposure to a deoxygenated solution of 1 M sodium chloride In quadruply distilled water, (c)as with (b) for an etched metal sample exposed to an oxygenated solution of sodium chloride. The arrow points to the feature that arises from the intense satellite of nickel oxide/hydroxide.

of all forms of oxygen as the takeoff angle becomes less surface sensitive, confirming the presence of oxygen-containing species in the surface region. The 0 1s region shows relatively less nickel oxide in the topmost surface region, as indicated by the increase in the oxide contribution to the overall 0 1s intensity with increasing take off angle. In contrast, the 0 1sregion shows relatively more adsorbed water in the topmost (59) Sherwood, P. M. A.; et al., to be published

This study illustrates the use of an anaerobic cell to investigate the corrosion and oxidation of nickel. The experimental process is well controlled, and the surface chemistry is monitored during the study. The study shows that nickel can be transferred without any significant oxidation. The studies of nickel in quadruply distilled water and 1 M sodium chloride solution show that oxidation of nickel by water is very slow, but substantial in sodium chloride solution. The film is composed of a mixture of NiO and Ni(OH)z, which is probably a porous film that may provide limited protection to further attack. This suggests that oxide formationwill increase with exposuretime. Dissolved oxygen leads to a significant, but not substantial, increase in the amount of oxidation in both water and sodium chloride solutions.

ACKNOWLEDGMENT This material is based upon work supported by the National Science Foundation under Grant CHE-8922538. Ths US. Government has certain rights in this material. We are grateful to the U.S.Department of Defense for funding the X-ray diffraction equipment.

RECEIVED for review February 8, 1993. Accepted May 14, 1993.