Anal. Chem. 1980, C. J. "Analyzing Syncrude of Utah Coal", (DOE, ERDA BERC, Bartlesville, OK) BERC/RI 75:7, 1975. (5) Altgelt, K. H., Gouw, T. H., Ed. "Chromatography in Petroleum Analysis", Vol. 11; Marcel Dekker: New York, 1979; Chapters 9 and 16. (6) Jackson, L. P.; Allbright, C. S.; Jensen. H. B. "Characteristics of Synthetic Crude Oil Produced by In-Situ Combustion Retorting", Am. Chem. Soc., Div. Fuel Chem., P r e p . 1974, 79(2), 175. (7) McKay, J. F.; Weber, J. H.; Latham, D. R . Anal. Chem. 1976, 48, 891-898. (8) Uden, P. C.; Carpenter, A. P.; Hackett, H. M.; Henderson, D. E.; Siggia, S. Anal. Chem. 1979, 51. 38-43. (9) Popl, M,.; Stejskal, M.; Mostecky, S. Anal. Chem. 1975, 47, 1947-1950. ( I O ) Suatoni, J. C.; Swab, R. E. J . Chromatogr. Sci. 1975, 73, 361-366. (11) Swansiger, J. T.; Dickson, F. E.; Best, H. T. Anal. Chem. 1974, 46, 730-734. (12) Dark, W. A.; McFadden, W. H. J . Chromatogr. Sci. 1978, 76, 289-293. (13) Clark, B. R.; Ho, C-H.; Jones, A. R. "Approaches to Chemical Class Analyses of Fossil Derived Materials", ACS Division of Analytical Chemistry paper #190, ACS National Meeting, March 1977. (14) Schabron, J. F.; Hurtubise, R. J.; Silver, H. F. Anal. Chem. 1977, 49, 2253-2260. (15) Hurtubise, R. J.; Skar, G. T.; Poulson, R. E. Anal. Chim. Acta 1978, 97, 13-19. (16) Goeckner, N. A.; Griest, W. H. Sci. Total Environ. 1977, 8 , 187-193. (17) Jones, A. R.; Guerin, M. R.; Clark, 6. R. Anal. Chem. 1977, 49, 1766- 177 1. (18) Schmeltz, I.Phytochemistry 1967, 6, 33-38. (19) Bjorseth, A. Anal. Chim. Acta 1977, 94, 21-27. (20) Henneberg, D.; Henrichs, V.; Schomburg, G. Chromatographia1975, 8, 449-45 1. (21) Fenimore, D. C.; Whitfwd, J. H.; Davis, C. M.; Zlatkis, A. J . Chromatogr. 1977. 140, 9-15. (22) Crawford, K. W.; Prien, C. H.; Baboolal, L. B.; Shih, C. C.; Lee, A. A. A Preliminary Assessment of the Environmental Impacts from Oil Shale Developments", EPA Report 600/7-77-069, 1977, p 92 (Available
52, 1657-1662
(23)
(24) (25)
(26)
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through the National Technical Information Service, Springfield, Va. 22 161). Guerin, M. R.; Griest, W. H.; Ho, C-H.; Shults. W. D. "Chemical Characterization of Coal Conversion Pilot Plant Material" I n Proceedings of the 3rd Environmental Protection Conference, Chicago, Ill., ERDA-92, 1975, p 670. Guerin, M. R.; Epler, J. L.; Griest, W. H.; Clark, B. R.; Rao, T. K. "Carcinogenesis, Polynuclear Aromatic Hydrocarbons", Jones, P. W., Eds. Raven Press, New York, 1978; Vol. 3, pp 21-23. Freudenthal, R. I., Peterson, M. R.; Fruchter. J. S. "Studies of Materials Found in Products and Wastes from Coal-Conversion Processes", in Pacific Northwest Laboratory Annual Report for 1978 to the DOE Assistant Secretary for Environment, PNL-2850 PT3 UC-11, Richland, Wash., 1979, p 1.43. Wise, S. A.; Chesler, S.N.; Hertz, H. S.:Hilpert. L. R.: May, W. E. Anal. Chem. 1977. 49, 2306-2310.
RECEIVED for review February 25,1980. Accepted May 5,1980. Partial financial support from the Office of Health and Environmental Research of the Department of Energy and from the Office of Energy, Minerals, and Industry within the Office of Research and Development of the U.S. Environmental Protection Agency under the Interagency Energy/Environment Research and Development Program, is gratefully acknowledged. In order to specify procedures adequately, it has been necessary to identify some commercial materials in this report. In no case does such identification imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the material identified is necessarily the best available for the purpose.
Electron Spectroscopy for Chemical Analysis of Thin Film CopperXuprous Oxide Standards Prepared in Situ by High Temperature Solid-state Electrolysis Noriyuki Watanabe and Theodore Kuwana" Department of Chemistty, The Ohio State University, Columbus, Ohio 432 10
The in-situ preparation of clean cuprous oxide in the high vacuum chamber of the ESCA/Auger spectrometer was accomplished by high temperature solid-sfafeelectrochemistry. The separator membrane which served as the ionic conductor of the electrochemical cell was zirconia which transported oxygen ion. The elemental atomic ratios of copper to oxygen were adjusted by coulometric titration of a thin Cuo/Cu,O film. ESCA and Auger spectra of the Cuo/Cu20films were obtained and, assuming the correctness of the copper to oxygen stoichiometry as determined by coulometry, ESCA quantltation parameters were evaluated.
Electron spectroscopy for chemical analysis (ESCA or XPS) as a method for quantitation of surface elemental analysis has progressed rapidly during the past few years. In addition to the experimental studies (1-IO), much effort has been directed to solving various parameters theoretically, such as photoionization cross section (11, 12),electron mean free path in various matrices (13),asymmetry factor due to the different angular dependence of photoelectron (14),and electron kinetic energy dependence of the instrument (15). All of these factors are important considerations in the use of ESCA for elemental quantitation. The validity of these parameters has been tested 0003-2700/80/0352-1657$01 .OO/O
and verified for few cases of simple materials such as pure metals (16). The verification for compounds is more difficult (17) and one problem of quantitation is that of preparing standards with known Stoichiometry and composition. Contamination and adsorption of foreign gases such as 02,N2, CO, C02, H20, organics, etc., can also occur, particularly when samples are prepared outside of the vacuum chamber. Also, surface sputtering with various gases, e.g., 02+ or Ar+ has been known to change the surface composition. Thus, there is always an element of risk involved when sputtering is used prior to acquisition of a spectrum. In this study, we describe the use of a high temperature solid-state electrolysis for the in-situ ultra-high vacuum (UHV) sample preparation and the subsequent quantitation of elemental ratios by ESCA. Metal or metal oxide samples can be coulometrically oxidized or reduced with the quantity of charge, q , in coulombs deteryining the metal to oxygen stoichiometry. Such coulometric titrations have been previously used in the determination of phase boundaries and of thermodynamic parameters in the iron-oxygen system by the use of a solid electrolyte of stabilized zirconia, which has been known as a good oxygen anion conductor at high temperatures (18, 19). Stout et al. (20) used this technique previously to make thin iron oxide films in the AES-ESCA chamber. The advantage of the coulometric method is the CZ 1980 American Chemical Society
1658
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
21n
____(
Figure 1. Structure of cell for high temperature electrochemistry. 1: metal thin film; 2: zirconia crucible; 3: reference electrode, Cu/Cu,O; 4: platinum ribbon; 5: thermocouple,Pt-Pt 13%Rh;6: ceramic tube
with groove; 7: electrical leads for heating; 8: thermal shields; 9: supporting plate, Macor machinable glass; 10: copper plug
ability t o perform repeated oxidation-reductions of the metal/metal oxide films reversibly without exposing the sample to the atmosphere outside of the chamber. The copper/copper oxide system was chosen in the present work as a model system for study because it appeared to fulfill the requirements of (a) relatively rapid rate of metallic copper oxidation or copper oxide reduction, (b) ease of preparing uniform thin films by vapor deposition, (c) compatibility of the Cuo/Cu oxide with the oxygen vapor pressure in the spectrometer sample chamber, and (d) the lack of satellite structure in the ESCA spectrum for copper and cuprous oxide. T h e one concern was the relatively high vapor pressure of metallic copper at the temperatures employed. However, results indicated t h a t the copper loss rate was rather insignificant during the duration of the experiment.
EXPERIMENTAL ESCA and Auger spectra were obtained using a Physical Electronics Inc., PHI Model 548 spectrometer equipped with a magnesium X-ray source. The ESCA spectra were taken using a 50-eV pass energy at which the resolution of the analyzer was estimated to be ca. 1.2 eV. Electrolyses at high temperature were performed at less than 6 X lo4 Torr using the cell shown in Figure 1. A thin film of copper was vapor deposited onto the bottom surface of the zirconia crucible used as the oxygen anion conductor a t high temperature. The thickness of the metal film was evaluated as ca. 5300 8, from integrated electric charge, assuming the formation of Cu20. The zirconia crucible used was Degussit ZR23 purchased from Degussa. Outer diameter, length, and thickness of the crucible were 10, 15, and 1mm, respectively. The surface of the zirconia crucible onto which copper was vapordeposited was first polished with emery paper and then 0.3-pm alumina powder until a mirror-like finish was obtained. It was then washed with methanol and distilled water in an ultrasonic washer. A thin foil of copper (99.9% purity) was used as the source for the vapor deposition. The foils were etched in dilute acid and then washed with distilled water prior t o use. The crucible was heated by a tantalum wire (0.25-mmdiameter) which was wrapped along the outer grooves of a lava stone ceramic tube. The crucible and the heating coil were surrounded triply by steel sheets separated from each other for thermal shielding under vacuum. A mixture of copper and cuprous oxide (1:2) served simultaneously as the oxygen buffer and the reference electrode. A platinum ribbon and one of the thermocouple wires were used as the electrical leads for the electrolysis. A temperature up t o 850 "C can be maintained effectively on the crucible by the tantalum coil heater at a power consumption of less than 60 W (e.g., 2.3 A, 25 V). Operational conditions of less than 6 X lo-' Torr can be attained in the sample chamber after initial degassing which requires one or two cycles of heating the crucible. The cell potential between the metallic or the oxidized copper film and the reference electrode was maintained by a conventional potentiostat (the reference electrode lead was common with the auxiliary electrode). The electrolysis currents were measured by means of an operational amplifier in the current-follower mode. The copper film working electrode was maintained at virtually ground during electrolyses.
XlOO
Current-time curves for oxidation and reductions of Cu and Cu,O. a: Oxidation at 770 O C and an applied potential of +350 mV. b: Reduction at 770 O C and an applied potential of -400 mV Figure 2.
Table I. Total Electrical Charge for Successive Titrations Q, Oxidationa Q, Reductiona 1st experiment
average 0
0.313 0.324 0.319 0.3 24 0.311
0.322 0.322 0.319 0.316 0.308
0.318 0.005
0.317 0.005
2nd experiment 0.350 0.349 0.358 average 0
a
Unit, coulomb.
b
0.351 0.346
0.354
0.352 0.350 0.004 0.003 u = standard deviation.
RESULTS A N D DISCUSSION Oxidation or reduction of the Cuo/Cu oxide film could be completed within 15 min at 770 OC. Figure 2 shows typical current-time (i-t) curves for the oxidation of the metallic copper and the reduction of copper oxide at this temperature. The background current was less than 1 pA after completion of the electrolysis. Hence, the electronic conduction of the zirconia was negligibly small. There was a slight change in the shape of the i-t curves with repeated redox cycling but the general features remained the same. At the cell potentials applied (+350 mV for oxidation and -400 mV for reduction) the current rose much more rapidly for the oxidation (Figure 2 , trace a) than reduction but slowly decayed during the course of reaction. On the other hand, the reductive current (Figure 2, trace b) rose less rapidly and remained nearly constant for a period of several minutes at ca. 0.8 mA and then decayed relatively rapidly when the reduction was nearly completed. The integrated charge for the oxidation was equal to that for the reduction. Table I shows the total electrical charges for the complete, successive oxidation followed by reduction for two sets of experiments using two individual films. The reproducibility and the reversibility in the oxidation and the subsequent reduction of the Cuo/CuzOfilm are clearly evident from the data in Table I. After 12 cycles of the redox titration, the total charge per oxidation or reduction decreased by ca. 5 % . This decrease was attributed to loss of Cuo due t o high temperature vaporization. Figure 3 shows the calculated equilibrium oxygen vapor pressure of Cuo/Cu oxides (21). Considering the equilibrium oxygen vapor pressure shown in Figure 3 and the oxygen partial pressure in the chamber of ESCA (estimated
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
l
1659
10-1'
1d9 t c
-4+r Xl
I
Flgure 3. Equilibrium oxygen vapor pressure vs. inverse of temperature. a: 2Cu 4- '/, 0,= Cu,O. b: Cu,O '/, 0, = 2CuO (broken line indicates pressure of chamber under electrolysis)
+
9
Blinding
Energy
'
( x l ~ d e ~ 1)
Flgure 4. ESCA spectra of electrolyzed film. a: Reduced film. b: oxidized film
value), oxidation should be performed a t the lowest possible temperature consistent with the kinetics of the process. On the other hand, it is advantageous for reductions to be done at higher temperature. Thus, most oxidations were performed at 650 or 700 O C , although the time required for complete oxidation was increased to that required when the temperature was 770 "C. The reductions were done at 750 "C. It is unlikely for cupric oxide to form by electrolysis under these conditions. Changes in the Cuo/Cu oxide composition could also be seen as viewed by the color changes of film. Figures 4 and 5 show the low resolution ESCA and AES spectra for the oxidized and reduced films after high temperature electrolysis. T h e ESCA spectra of reduced copper did not exhibit any other peaks than those assignable to elemental copper. AES spectra similarly showed copper structures although a small oxygen peak was noted. Carbon peaks were absent. Occassionally a small AES peak at 250 (fl) eV appeared, most often on the oxidized copper surface. This peak may be due to the presence of trace potassium which may occur as a contaminant of the zirconia. Considering the relatively high sensitivity of potassium in AES (22) and its negligible effect to the Cu/CupO quantitation, it was not considered to be significant to the present objectives of the work reported herein. Additional experiments are in progress to confirm the identity and source of this 250-eV AES peak. While the intensities of the copper peaks in the ESCA spectra were decreased by oxidation, the oxygen peaks appeared and increased. These oxygen peaks disappeared completely after reduction. There were no satellite structures
1
I 3
I
5 Kinetic
1
7
9
Energy ( x 1 0 0 ~ ~ )
Figure 5. AES spectra of electrolyzed film. a: Reduced film. b: Oxidized film
in the spectrum of either reduced or oxidized copper films. A charge shift of about 10 eV was observed for the fully oxidized copper films. If the binding energy of the copper 2p3/, peak for the fully oxidized film was assumed to be equal to that reported in the literature for cuprous oxide, the binding energy assignments for all the other copper peaks and the oxygen 1s were consistent with those of cuprous oxide (23-26). The oxidation product under the experimental conditions was cuprous oxide and there was no evidence of cupric oxide formation. The shape of the copper L3M4,5M4,5 Auger peaks in the ESCA spectra, as will be discussed later, also provides additional support for the formation of cuprous oxide. Quantitation in ESCA. Since copper and cuprous oxide are diamagnetic, the complications in the measurement of peak areas in ESCA spectra caused by multiple splitting or shake up processes are minimized (24-26). The intensity of the photoelectron detected for a homogeneous solid of infinite thickness compared with the mean free path of the electron is given by ( 4 ) :
I = AFTna4BX
(1)
where A is area from which photoelectrons are detected, F is the incident X-ray flux, T i s the transmission function of the analyzer, rz is the number of atoms concerned per unit volume, cr is the photoionization total cross-section, 4 is the production efficiency of electrons emitted with normal photoelectron energy, 0 is the angular distribution factor which depends on the instrumental arrangement, and X is the mean free path of the photoelectron in the sample. Carbon contamination, which must be usually considered, was negligibly small for both oxidized and reduced copper films since carbon peaks were absent from both low and high resolution ESCA and AES spectra. Also, vapor deposition of the Cu onto the highly polished mirror-like finish of the zirconia substrate minimized surface roughness. Thus, no attempt was made to correct for these two factors. For a constant resolution CMA (cylindrical mirror analyzer) with retarding grid as used in this study, the area from which photo-emitted electrons are analyzed is proportional to the inverse of kinetic energy of electron ( 1 5 ) . Consequently, A in equation 1might be involved in T. Penn has tabulated the electron mean free paths for inelastic scattering as a function of kinetic energy for many elemental solids and has derived formulas for calculating the mean free paths in compounds (13). The mean free the paths for copper and cuprous oxide were calculated from these formulas. Although it is difficult to evaluate @, @ or more so its ratio can be approximately assumed as unity for the lines which apparently do not have satellite structure, as in our case. Intensities of each subshell depend on the relative angle between incident X-ray and
1660
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980 A
I\
b-2
/1
5
10
nin.
x100-
995
990
75
Figure 6. High resolution ESCA spectra of electrolyzed film. a: Reduced film. b: Oxidized film. 1: copper 2pSl2 ( X 1); 2: copper 3 p ( X 10); 3: oxygen 1s ( X 10)
Figure 7. Electrolysis curves for half oxidation and corresponding reduction. a: Oxidation at 690 OC and at an applied potential of +350 mV; integrated electrical charge was 0.178 coulomb. b: Reduction at 730 'C and at an applied potential of -500 mV; integrated charge was 0.176 coulomb
acceptance of the analyzer ( 1 4 ) . The ratios of the intensities of the p subshell to that of the 0 1s shell decrease by 3 to 4% depending on 3p or 2p subshell for the CMA which has an acceptance angle of 42.3'. For two elements in a single homogeneous solid, 11 - nlC718ltZhl _ -
12
(2)
nzC7282t1X2
where ZIis the intensity of one element and Z2 is the intensity of the second element; is the kinetic energy of the photoelectron, and the parameters of analyzer A and F can be cancelled out. The X-ray flux is assumed to be constant during the experiment. Figure 6 shows the high resolution ESCA spectra of copper 2p3/,/3p and oxygen Is for the reduced and fully oxidized form. The oxygen peak a t 530 eV was completely absent for the reduced film, leaving a background due to the copper Auger. Intensities were measured by integration of the curves, using a base line drawn to the base a t both sides of the peak. Table I1 shows the relative intensities predicted by Equation 2 compared to the experimental values. The values of photoionization cross section given by Scofield were used in the calculation (12). The mean free paths of photoelectrons in copper metal and cuprous oxide were calculated according to Penn's method (13). Relative intensities in the second and third rows of Table I1 are given as values corrected by the mean free paths of the photoelectron in copper or cuprous oxide, the kinetic energy dependence of the analyzer, and the angular dependence of the subshell electron. Those in the first row are given as ratios of the photoionization cross section. T h e theoretical relative intensity of 2 ~ ~ / ~ was / 3 paffected considerably by the corrections because of the large difference of kinetic energy between these subshells. Measured values are shown for three redox cycles. Although there were some dispersion and certain deviation from the prediction, measured relative intensities among copper subshells for both oxidized and reduced films agreed well with the values predicted, particularly with the 2p3/,/3p ratio. This internal consistency allows the comparison of intensity of oxygen to copper and the derivation of atomic ratio of oxygen to copper in oxidized film. Oxygen 1s intensity was compared with those of copper 3p and 2p3p Both of them resulted in almost the same atomic ratio as seen in Table 11. The experimental value of 0.44 was in fairly good agreement with the theoretical value of 0.50. This suggests the validity of the photoionization cross section used, the corrections involved, and the method of sample preparation. This was confirmed further by the results discussed in the following section.
Titratad
a m o u n t o f oxygen
Figure 8. a: AES peak to peak ratio of oxygen to copper vs. titrated amount of oxygen. 0: By peak to peak ratio of oxygen to copper in AES. b: Atomic ratio of oxygen to copper determined by ESCA quantitation vs. titrated amount of oxygen. 0: By relative intensity of oxygen 1s to copper 3p. X: By relative intensity of oxygen 1s to copper
2P3/2
Preparation of Nonstoichiometric Cuprous Oxide. Nonstoichiometric cuprous oxides were prepared by partial coulometric titration of the sample. The sample partially titrated was maintained at high temperature for about 30 min to attain homogeneity in the composition before cooling and taking ESCA and AES spectra. The back-reaction during this equilibration period was negligibly small as monitored by integrating the electrical charge for the back-titration (reduction), which was almost the same as that of the corresponding oxidation. Typical i-t curves for a partial oxidation and its corresponding reaction are shown in Figure 7 . In contrast to the i-t curve shown for the reduction in Figure 2b, the i-t in this case for an incompletely oxidized film increased almost immediately to its maximum value after the potential was applied. This could be attributed to the faster diffusion of ions in the nonstoichiometric cuprous oxide compared to the stoichiometric oxide. The titrated amount of oxygen was defined as the ratio of the integrated electrical charge for a partial oxidation to that of a complete oxidation. Figure 8a provides the plot of the ratio of AES peak-to-peak intensity of oxygen (510 eV, KLL) to that of copper (919 eV, L3M4,,M4,,)against the titrated amount of oxygen. The slope of the linear plot was consistent with that derived from the relative Auger sensitivities of oxygen and copper determined by same type of analyzer (22). Atomic ratios of oxygen to copper determined by ESCA, quantitated as described in the previous section, was plotted vs. the titrated amount of oxygen in Figure 8b. The agreement
ANALYTICAL CHEMISTRY, VOL. 52, NO. 11, SEPTEMBER 1980
1661
Table 11. Quantitation in ESCA relative intensities 3d 0 Is 2Pwz 2P,/, 3P - __ 2P,,, 3P 3s 3P 3P
2P,,,
2P,,2
321
301
copper 3P 3s 1179 1132
15.87
8.1
2.28
0.805
0.658
2.85
1.94
5.31
5.11
12.9
12.5
13.5
9.09
1.89
5.98 observed relative intensities, three redox cycles
5.72
14.8
14.4
15.5
10.4
E ,
eV
relative cross sections (12 ) calculated mean Cu free pathsaSb ( 1 3 )h
cU,o
oxygen 3d 1s 1251 724
reduced
atomic ratio of O/Cuc
2.83
0.289
10.3
2.71
0.824
1.89
10.1
2.71
0.284
3
2.00 1.97 2.05
11.0 11.4 11.8
2.90 2.98 3.06
0.403 0.418 0.430
1' 2'
1.99 2.09
3'
2.11
10.3 10.5 9.20
2.90 3.26 3.04
0.457 0.404 0.352
1 2
oxidized
0 1s
~~
6.96
1.25
2P,/, 0.180
0.73
0.072
-
0.622 0.652 0.631
0.0606 0.0624 0.0686
1' 2'
0.43
3'
0.43
0.42 0.43 0.48
0.45
a hcu = ~ / ( 2 3 . 6 ( ln 3.21)):hcuz0= ~ / ( 1 9 . 7 ( l n 3.01)); A . Predicted relative intensities corrected with the mean free paths of electron in copper or cuprous oxide, kinetic energy dependence of analyzer, and angular dependence of subshell electron. Calculated from the relative intensities predicted and measured for fully oxidized copper _ . film. -
a t 914 and 919 eV decreased while a new peak with kinetic energy of 917-918 eV appeared. An uncertainty still remains in the identification of the satellite lines of L3M4,5M4,5 in the Auger spectra of copper metal (27-32). Careful spectral deconvolution studies of AES and ESCA spectra of quantitatively prepared in-situ samples will provide a means toward further assignment and characterization.
CONCLUSIONS
I
I
910
I
I
920 K i n e t i c Energy ( e v )
Figure 9. Copper Auger L,M,,M,, spectra in high resolution ESCA spectra. a: Reduced film, b: '/, oxidized film, c: '/* oxidized film, d:
3/4
oxidized film, e:
full-oxidized film
between the AES and ESCA determined oxygen to copper ratios with those of the coulometrically determined stoichiometry supports the validity of this method for the preparation of standards. The agreement between the calculated and experimental ESCA results is also gratifying. T h e binding energies of the copper subshells and oxygen Is in the ESCA spectra remain essentially independent of the extent of oxidation. However, significant changes were observed for the copper Auger peaks in the ESCA spectra with lines oxidation. Figure 9 shows the copper Auger L3M4,5M4,5 as a function of the oxidation. In the fully reduced Cuo spectrum (Figure 9, curve a) the main line at kinetic energy of 919 eV (B.E. at 335 eV) with shoulders at ca. 921 and 914 eV appear. As the Cuo is oxidized (curves b to e), the peaks
Known nonstoichiometric compositions of cuprous oxide could be made readily and conveniently in the UHV spectrometer chamber by coulometry. Another method of attaining nonstoichiometric composition may be by controlling the potential between the metal and the reference electrode. However, the time required will be considerably lengthened. There are several problems which must be considered in the choice of the metal/metal oxide system. First of all, the required equilibrium pressure of oxygen for the metal oxide at high temperature must be within the range available in the spectrometer chamber. Second, the kinetics of the oxidation/reduction of the metal and the extent to which the reaction proceeds should be favorable. Also, the melting point and vapor pressure of the metal should be consistent with the temperature required. These requirements will limit the applicability of the method to only a few metal oxide systems. Perhaps, half of the first row transition metals can be studied without any major change in the present cell design. Metals such as tungsten and ruthenium appear to be promising candidates. With the C u / C u 2 0 system, the high-temperature redox characteristics will be investigated when small amounts of other metals and metal oxides are introduced as additional components. The stoichiometry and the redox states of the metals and their oxides will be monitored by ESCA and Auger during electrolysis. It will be particularly interesting in those cases where metallic alloys or mixed compound formation occurs. Of course, a variety of other ionic salts may be amenable to such in-situ preparation by the judicious choice of the composition of the solid-state, ionic conductor. Further studies along these lines a r e u n d e r investigation.
ACKNOWLEDGMENT We thank Roy Tucker of the Department of Physics at The Ohio State University for his kind help in preparing the vapor
1662
Anal. Chem. 1980, 52, 1662-1667
deposited films. T h e assistance of Lorraine Siperko in this work is hereby gratefully acknowledged.
LITERATURE CITED Wagner, C. D. Anal. Chem. 44, 1050 (1972). Swingle, R. S.Anal. Chem. 47, 21 (1975). Berthou, H.; Jorgensen, C. K. Anal. Chem. 47, 482 (1975). Wagner, C. D. Anal. Chem. 49, 1282 (1977). Carter, W. J.; Schweitzer. G. K.; Carlson. T. A. J . Electron Spectrosc. Relat. Phenom. 5 , 827 (1974). (6) Evans, S.; PrRchard, R. G.; Thomas, J. M. J . Electron. Spectrosc. Rebt. Phenom. 14. 341 11978). (7) Powell, C. J..Appl.‘Surf: Sci. 1, 186 (1978). (8) Powell, C. J . ASTM STP643, 5 (1978). (9) Wagner, C. D.; ASTM STP643, 31 (1978) (IO) Salvati, L.; Carter. W. J.; Hercules, D. M. ASTM STP 643 47 1978). (11) Nefedov, V. I.; Sergushin, N. P.; Band, J. M.: Trzhaskovskaya, M. B. J . Electron Spectrosc. Relat. Phenom. 2 , 383 (1973). (12) Scofield, J. H. J. .Electron. Spectrosc. Relat. Phenom. 8 , 129 1976). (13) Penn, D. R. J . .Electron. Spectrosc. Relat. Phenom. 9. 29 1976). (14) Reilman, R. F.; Msezane, A,; Manson. S. T. J . Electron. Spc trosc . Relat. Phenom. 8 , 389 (1976). (15) Palmberg, P. W. J . Vac. Sci. Techno/. 12, 379 (1975). (16) Brillson, L. J.; Ceasar, G. P. Surf. Sci. 58, 457 (1976). (17) Dreiling. M. J. Surf. Sci. 71, 231 (1978). (18) Rlzzo. F. E.: Smith. J. V. J . Phvs. Chem. 72. 485 11968). (19) Rizzo; F. E.;’Gordon, R. S.; Cutler, I.B. J . Electrochem. S i c . 116, 266 (1969). (1) (2) (3) (4) (5)
(20) Stout, D.A.; Gavelli, G.; Lumsden, J. B.; Staehle, R. W. Surf. Sci. 69, 741 (1977). (2 1) Barin, I.; Knacke, 0. “Thermochemical Properties of Inorganic Substances”, Springer-Verlag: Berlin, 1973. (22) Davis., L. E.; MacDonald, N. C.; Palmberg, P. W.; Riach, G. E.; Weber, R. E. “Handbook of Auger Electron Spectroscopy”; Physical Electronics Industries, Inc.: Eden Prairie, Minn., 1972. (23) Rosencwaig, A.; Wertheim, G. K. J . Electron Spectrosc. Rebt. Phenom. 1, 493 (1972-1973). (24) Larson, P. E. J . Electron Spectrosc. Relat. Phenorn. 4, 213 (1974). (25) Schon, G. Surf. Sci. 35, 96 (1973). (26) McIntyre, N. S.; Cook, M. G. Anal. Chem. 47, 2208 (1975). (27) Schon. G. J . Ekctron Spectrosc. Rebt. Phenom. 1, 377 (1972-1973). (28) Yin, L.; Tsang, T.; Adler, I. J . Electron Spectrosc. Relat. Phenom. 9, A _ 7. I(1976)
(29) Andrews, P. T.; Weightman. P. J . Electron Spectrosc. Relat. Phenorn. 15, 133 (1979). (30) Roberts, E. D.; Weightman, P.; Johnson, C. E. J . Phys. C., Solidstate Phvs. 6. L301 (1975). (31) Anionides, E.; Janse.’E. C.; Sawatzky, G. A. Phys. Rev. 8 . 15, 1689, 4596 (1977). (32) McGuire, E. J. Phys. Rev. 6.17, 182 (1978).
RECEIVED for review March 7 , 1980. Accepted May 27, 1980. The authors gratefully acknowledge support of this work by the National Science Foundation.
Determination of Trace Levels of Nitric Oxide in Aqueous Solution Oliver C. Zafiriou” Department of Chemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543
Mack McFarland NOAAIERL Aeronomy Laboratory, Boulder, Colorado 80303
Nitric oxide at trace levels M) in aqueous solution is determlned by using a flow system to equilibrate the solution with a gas stream and measurlng NO, with a chemiiuminescence detector. A standard pNoprepared by dynamic dilution of gases is used to calibrate the detector and to test the system for adsorptlve and artifact effects. NO signals are differentiated from interferents by criteria based on the gas/solution partition coefficient of NO, its low boiling point, or reverslble formatlon of the Fe(N0)” complex. The precision of the technique at the M [NO], level is --f3% and the accuracy is estimated to be f20%; a determination requires about 2 min. The versatility of the method and its appllcability to environmental measurements are illustrated by relevant examples.
available, few data exist concerning its occurrence and behavior in the environment. Nitric oxide reacts with oxygen both in the gas-phase (7) and in solution (8) with rates proportional to [N0I2,so that concentration steps in the presence of oxygen prior to analysis lead to losses due to NO2 formation. In this paper, we describe a rapid, sensitive, and precise approach to determining traces (to M) of NO in aqueous media, including oxygenated solutions. Specificity criteria for confirming that NO causes the signal are presented. The basic approach involves equilibrating the highly insoluble NO,, in solution with a flowing gas stream (“stripping”) and measuring NO, with a sensitive, stable chemiluminescence detector. Preliminary applications to studies of trace NO, under conditions of interest in marine and natural water chemistry illustrate the versatility of the approach.
Nitric oxide (NO) is a chemically unusual and environmentally significant radical, stable to self-reaction but an excellent free radical trap (1). In the atmosphere, it is a well-known pollutant (2)and an important constituent of clean air (3, 4). Very recently it has become clear that aqueous solutions of NO may also be environmentally significant. Photochemically generated NO has been detected in seawater (5) and presumably forms in other natural waters also. Biogeochemical processes also produce NO, which is an intermediate in some denitrifications (6). However, since suitable methods for determining traces of NO,, have not been
EXPERIMENTAL Apparatus. The basic equipment consists of (A) a distribution system to provide and route gas mixtures of various compositions, including known trace NO mixtures for calibrating the detector and preparing aqueous sofutions of known NO vapor pressure, pN0,by equilibration, (B) a chamber to contain the sample and permit removal of NO,, by intimately contacting the gas and sample phases (“stripping”),and (C) a chemiluminescence NO detector. Figure 1 shows a versatile configuration for the principA components. A 300-W Xe arc UV light source and a calibrated broadband UV radiometer have been added for photochemical studies. The gas distribution system provides constant flow to the detector with mass flow controllers, yielding constant sensitivity
0003-2700/80/0352-1662$01 .OO/O
62 1980 American Chemical Society
