Anal. Chem. 1986, 58,2637-2640
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Quantitative Oxidation-State Determination of Titanium and Manganese in Binary Oxide Mixtures by Photoelectron Spectroscopy Ronald E. Negri,’ J a m e s W. Taylor,* Charles E. Beall, a n d Thomas M. Rossee12
Department of Chemistry, University of Wisconsin, Madison, Wisconsin 53706
The TiO,, TIO, MnO,, and MnO transition-metal 3p subsheii photoelectron bindlng energies were measured by using synchrotron radiation. With the synchrotron radiation tuned to maximize the sensitivity of the measurement and by use of curve fitting to decompose the overlapping spectral lines, the chemical shtft between the 3p subshells of the two oxides of a given transitlon metal is shown to be large enough to measure one chemical state quantitatively in the presence of the other. The problem of sample charging was minimized by pressing the powders into metal screens, by using an electron flood gun, and by referencing the binding energies to the oxygen 2s line. The relative standard deviatlon of the relative concentrations, using the calibration-curve method, is 14.0% in the mole ratio range of 0.33-3.0. These results demonstrate that a quantitatlve oxidation-state determination of Ti and Mn in binary oxide mixtures can be accomplished. I t Is also shown that the reiatlve-sensitivity-factor method can be used to determine the relative surface concentrations of Ti and Mn valence states (separated by at least two oxidation-state units) In single-phase oxide matrkes with a relative standard deviation of 1 1 2 % , H the 3p transition Is used and photon energles that maximize sensitivity are employed.
One of the goals in these laboratories is to develop a method, using photoelectron spectroscopy quantitatively to determine the oxidation state of first-row transition metals in insulating oxide matrices. This measurement is important to many diverse areas such as heterogeneous catalysis (active site (1) and support characterization (2)) and lunar geochemistry (accurate prediction of the chemical composition of lunar rocks based on limited analyses ( 3 ) ) .As a first step, we measured the energy dependence of the Ti, Cr, and Mn 3p subshell photoionization cross sections from 90 to 200 eV by using synchrotron radiation to find the photon energy (and other experimental conditions) that would maximize the sensitivity ( 4 ) . From the measured 3p cross sections and an evaluation of the light flux available from the monochromator at a given resolution, it was determined that photon energies of 90, 100, ,and 105 eV for Ti, Cr, and Mn, respectively, would maximize signal levels. A second conclusion of the study was that the relative 3p photoionization cross sections for several different oxides of the same transition metal (for Ti, Cr, and Mn) are the same within the estimated experimental error of 115%. The next step in the development of this analytical method would be to demonstrate a quantitative oxidation-state determination for one of these first-row transition metals by using the experimental conditions that maximize sensitivity. Previous XPS analyses of mixed chemical states of Ti, Cr, or Present address: Physical Electronics Division, Perkin-Elmer
Corp., Eden Prairie, MN 55344. Present address: Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, T N 37831.
Mn either have been semiquantitative or have differentiated only between the metal and one of its oxides. Several investigators (5-7), for example, have determined semiquantitatively the relative amounts of TiO, Ti203, and Ti02 in thin films by spectral decomposition of the chemically shifted 2p lines. McIntyre and Zetaruk (8) quantitatively analyzed Cr-Cr203 mixtures by using both the 2p and the 3p lines. Currently, however, there are no reports of quantitative chemical-state determinations for Ti, Cr, or Mn in mixed oxide samples by photoelectron spectroscopy. The purpose of this study is to investigate the possibility for a quantitative oxidation-state determination of T i or Mn in binary oxide mixtures. The 3p line was chosen for the analysis because it is accessible by the synchrotron radiation from the Wisconsin Tantalus I electron-storage ring. Unlike conventional sources, synchrotron radiation can be tuned to a photon energy that maximizes the 3p photoionization cross section and hence the sensitivity of the measurement ( 4 ) . Unfortunately, because the 3p line width in first-row transition metals is so broad, oxidation-state shifts are difficult to resolve. By use of this particular transition, it becomes necessary, therefore, to determine valence states that are separated by a t least two oxidation-state units and to use curve fitting to separate the overlapping chemically shifted lines. Both the relative-sensitivity-factor method and the calibration-curve method of quantitative analysis will be examined for applicability to this problem. EXPERIMENTAL SECTION Apparatus. Photoelectron spectra were obtained by using synchrotron radiation from the 240-MeV Tantalus I electronstorage ring operated by the Synchrotron Radiation Center of the University of Wisconsin. The light was dispersed by a 2-m “grasshopper”grazing-incidence monochromator (9)with a grating of 600 lines/mm, Au overcoat, and 8.3 A/mm dispersion (at the exit slit). Photon energies of 90 and 105 eV were used for the Ti and Mn spectra, respectively. A one-half scale version of the Helmer analyzer, modified so that it has a symmetric nearly Gaussian line shape ( l o ) ,was set to a pass energy of 48 eV and run in the preretarding mode. The overall instrumental energy resolution was 1.8 eV as determined by the fwhm of the Au 4f, line. The photoelectron takeoff angle relative to the sampie surface was approximately 65”. The operating pressure of the ion-pumped sample chamber was between 1 X and 3 x torr for all measurements. Thermal electrons from a heated filament were used to reduce sample charging. Data were collected and stored on a Commodore 64 microcomputer, which also controlled the spectrometer. Data analysis and plotting were carried out on a Harrisl7 minicomputer with a CALCOMP plotting facility. Reagents. The transition-metal oxides were obtained commercially (Alfa Products, Thiokol/Ventron Division, Danvers, MA) as powders with a purity of 99+% or better. Procedure. The powders were weighed to k0.1mg and then mixed in a Wig-L-Bug blender for 2 min. Because the powders are different colors, any gross inhomogeneities could be detected. In addition to mixing, the blender also grinds the powders so that fresh particle surfaces and a more even particle-size distribution are obtained. The powders were pressed into pellets, 12.7 mm
0003-2700/86/0358-2637$01.50/00 1986 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
Table I. Transition-Metal 3p Subshell Binding Energies (eW
Ti0 TiOz MnO MnOz
lit."
measd
36.0
36.1 f 0.2 37.5 f 0.2
37.5 47.7 49.5
47.9 f 0.2 49.2 f 0.2
"From ref 11. Uncertainty not reported. in diameter and approximately 1mm thick, with a hydraulic press. Simultaneously,an 88% transmitting 28 lines/cm OFHC-copper mesh (BMC Industries, Interconics Division, Buckbee-Mears Operation, St. Paul, MN) was pressed into the front surface of the pellet to reduce sample charging. Except when loaded into the vacuum chamber, the samples were handled in a glovebag under dry-nitrogen positive pressure and stored in a desiccator. To determine the surface morphology and particle size distribution of the samples, scanning electron micrographs of the pressed pellets were recorded at magnifications of lOOOX and 1OOOOx. XPS spectra were obtained by averaging repetitive scans in alternating directions across the desired kinetic energy interval. The scan direction was alternated to eliminate the bias caused by the constantly decreasing signal due to storage-ringbeam decay and adventitious sample contamination. Scanning was continued until peak heights of 25000-50000 counts were obtained;the signal was approximately 5OOO counts higher than the background level. Because the synchrotron radiation was focused to a 2 mm vertical by 5 mm horizontal spot on the sample, zero-order light could cause problems due to localized sample heating. To avoid these problems, photoemission experiments were performed only on portions of samples that were not exposed to zero-order light. Because the binding energy of the oxygen 2s subshell is nearly constant among the oxides of Ti and for those of Mn (7,II), the position of the oxygen 2s line can be used as an internal binding-energy standard. Hence, an oxygen 2s spectrum was obtained before and after each of the transition-metal 3p spectra so that the transition-metal binding energies could be referenced to the oxygen 2s position at 23.0 eV. The only sample for which the oxygen 2s position was found to be charge-shiftedwas that of pure TiOz. The spectrometer binding-energy scale was calibrated at 83.8 eV for the Au 4f7/2line. All of the samples, except those containing Ti02, were ionsputter cleaned with a modified PHI Model 4-131F sputter gun for 5 min (3 X 10" torr Ar, 10-pA beam current, 30-mA emission current, 700-V beam voltage, 600-V focus voltage) before the first spectrum was obtained. For TiO, MnO, and MnOz, the transition-metal 3p line exhibited no change in width, in shape, or in binding energy (relative to the oxygen 2s line) after cleaning; i.e., there was no evidence for transition-metal ion reduction. This observation is supported by Kim et al. (12) who did not observe transition-metal ion reduction after sputter cleaning for many compact oxides. Moreover, Kelly (13,14) did not observe any preferential sputtering of oxygen from thick Ti0 or MnO samples. Storp and Holm (15) noted that many thick or compact oxides are stable; i.e., no reduction can be detected by photoelectron spectroscopy after sputter cleaning. By contrast, the Ti 3p line of Ti02 broadened markedly after ion sputter cleaning, in agreement with the severe changes that Thomas (16) observed in the Auger spectrum of TiOz after sputter cleaning. Thomas ascribed these changes to reduction of the titanium ion from the cleaning operation. Because of this behavior, samples containing Ti02 were not sputter cleaned.
RESULTS AND DISCUSSION Transition-Metal Binding Energies. The measured transition-metal 3p subshell binding energies of the four oxides along with the values obtained by Rao et al. (11) are listed in Table I. The estimated uncertainty of our measured values is 10.2 eV. Although b o et al. did not report an experimental uncertainty, a reasonable value of 10.2 eV can be assumed. Thus, within the combined experimental errors, the binding-energy values reported here are the same as those reported by Rao et al. The binding-energy shift between the 3p sub-
J
l
51.0
.
.
l
.
4
,
.
48.5
,
.
.
,
.
.
.
,
46.0
,
.
43.5
Binding Energy (eV) Figure 1. Steps in the background determinationfor a typical 3p peak
(a) fitted with a straight-line background, (b) after subtraction of the linear background, and (c) after subsequent removal of the inelastic tail.
shells is 1.4 eV for the two titanium oxides and 1.3 eV for the two manganese oxides. Background Determination and Area Measurement. Figure 1 displays the result of each step in the background determination method used in this work. Figure l a shows a typical transition-metal 3p peak and a straight line fit to the sloping background. Satisfactory results were obtained by assuming that the slope of the background is nearly constant in the region of a peak. Figure I b shows the same peak after subtraction of the linear background. At kinetic energies lower than that of the peak, the base line level is constant and higher than it is at kinetic energies higher than that of the peak. The difference is assumed to arise entirely from 3p photoelectrons, which were inelastically scattered before leaving the sample. The inelastic contribution to the peak was removed by using the empirical method of Shirley (17). In this procedure a quantity proportional to the integrated intensity at higher kinetic energy is subtracted from the intensity at each point of the peak. Because the oxides of a given transition metal are similar to one another in structure and electronic properties (relative to other types of materials), the degree of inelastic scattering should also be similar ( 4 ) . The experimentally measured inelastic contributions for the two oxides of T i and for those of Mn were the same within 1 5 % . In agreement with this observation, Sayers and Armstrong (7) found that in XPS analyses of Ti surfaces, using the 2p line, the variation in background signal intensity with oxide stoichiometry is small. Figure ICshows the observed peak after subtraction of the linear background and removal of the inelastic tail. Figure 2 shows how the areas under the two chemically shifted lines were obtained; the spectral components shown are corrected for the background signal. A curve-fitting procedure (developed in these laboratories) was used with the assumption that there are two lines under the peak at binding energies corresponding to that of the two pure oxides. The line shapes were assumed to be Gaussian and, because the experimentally observed line widths for the two pure oxides of each transition metal were the same, the two line widths were assumed to be equal. The curve-fitting procedure then consisted of adjusting the line width and the two areas until
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
2639
Table 11. M 0 2 / M 0 Mole and Area Ratios M
mole ratio
area ratio"
Ti Ti Ti
0.334 1.004 2.98 0.332 1.003 3.00
0.248, 0.390 0.872, 1.061 2.83, 3.57 0.315 0.991, 1.316 2.96
Mn Mn Mn a
Corrected for the analvzer detection efficiencv.
If the relative concentration of different valence states of a
I
51.0
.
.
.
48.0
.
,
,
.
.
.
45.0
Binding Energy (eV) Figure 2. Typical area measurement results for a series of Mn mixtures. The solid line is the 3p peak after background correction and the dashed line represents the fitted curves for (a) MnO,, (b) 75% Mn0,-25% MnO, (c) 50% Mn0,-50% MnO, (d) 25% Mn0,-75% MnO, and (e) MnO.
the sum of squared deviations between the fitted and experimental curve was minimized. A simple Gaussian function was found to be satisfactory for simulating the line shape because multiplet splitting broadens the natural Lorentzian core line (18)and because the instrument line shape (Le., the combined line shape of the dispersed synchrotron radiation and the analyzer transmission) is nearly Gaussian (10). Relative-Sensitivity-Factor Method of Quantitative Analysis. The most common method of quantitative analysis in photoelectron spectroscopy uses relative sensitivity factors. In this method the intensity of a photoelectron peak, I , for a thick homogeneous sample with a smooth surface, is given by (19) I = fAnaXyC4T (1) where f is the incident photon flux in photons/(cm2 s), A is the sample area in cm2 from which photoelectrons are detected, n is the analyte concentration in atoms per cm3, u is the total cross section in cm2 per atom for photoionization of the particular level, and X is the mean free path in centimeters for the photoelectron in the sample matrix. The remaining factors are efficiency factors: y is the fraction of photoelectric transitions from the given level that result in an ion in the ground state and a photoelectron of the appropriate kinetic energy (i.e., the fraction of transitions that are actually measured), C is the fractional efficiency of emergence through a contaminating layer, 4 is the angular efficiency of the instrument arrangement, and T is the inherent efficiency of detection by the spectrometer. Relative sensitivity factors, S, are used to convert relative peak areas to relative numbers of atoms in the detected volume. These factors are normally (19) used in the manner ni = I i / S i (2) where (except for intensity and concentration) Siincludes all of the terms described in eq 1. Hence, atomic ratios are given by %/nz = Ul/~Z)(S2/SJ (3) and relative atomic concentrations are given by ci = n i / C n i
(4)
given element is desired, the areas under different chemically shifted peaks of the same line are used. The factor f$ is the same for all chemical states, and the factor XC is nearly the same because the kinetic-energy separation between the chemical states is very small. Furthermore, because the factor AT varies as the inverse of the electron kinetic energy (for spectrometers that scan with a retarding field), the variation in AT can be calculated with high precision. Thus, variability in sensitivity factors among chemical states of the same element arises only from the factor uy. Therefore, to determine the precision of an atomic ratio calculated by using eq 3, the indeterminate errors in the area ratio and the sensitivity-factor ratio (or cry ratio) are combined through a propagation-of-error computation. A similar method can be used to compute the precision of a relative atomic concentration calculated with eq 4. It is invalid to apply the sensitivity-factor method to the quantitative surface analysis of two-phase samples such as the transition-metal oxide mixtures because the assumption of sample homogeneity is violated (19). By contrast to singlephase samples where XC is nearly identical for different chemical states, the photoelectron mean free path and the amount of adventitious contamination may differ for the two powders. Thus, for heterogeneous samples, the factor XC is not the same for different chemical states. In addition, this quantitative method cannot be used to determine the bulk concentration of two-phase mixtures because the relationship between the surface and the bulk concentration is unknown. One powder may shadow the other resulting in a surface concentration that differs from that of the bulk (19, 20). It is valid, however, to combine the results obtained in this study with those obtained from our prior study in order to determine the precision of this quantitative method when it is applied to single-phase samples. The mole and area ratios of the binary oxide mixtures are listed in Table 11. The area ratios are corrected for the analyzer detection efficiency (i.e., for the factor AT). This correction introduces negligible error because its magnitude is less than 3% and because the kinetic energies are measured with 0.1 '70 precision. The effect of microscopic surface roughness of packed powders on XPS intensity ratios was recently assessed by Young and McCaslin (21). Morphology was described in the XPS intensity equation by a functional description of profilometry data. For their untreated pressed powder samples, it was found that for takeoff angles greater than 18" the surface appears to be smooth. Roughness, therefore, had a negligible effect on their intensity ratios. Scanning electron micrographs of our pellets showed that the particle size ranged from 0.1 to 3.0 wm. Because the particle size and morphology of our samples are similar to the untreated samples of Young and McCaslin and because of the relatively large and constant photoelectron takeoff angle used in our experiment (65O), our area ratios should be unaffected by the microscopic roughness of the surfaces. From our previous study on single-phase oxides ( 4 ) ,it was shown that for photon energies between 100 and 150 eV, the
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ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
variation in uy among the chemical states of a given transition metal for several oxides of Ti, Cr, and Mn is 515% for the 3p subshell. Therefore, under these measurement conditions, the factor S2/S1 in eq 3 is equal to unity with a relative precision of 515%. From the present study, in which the same measurement conditions were used, the variation in the area ratio for replicate measurement on a giuen sample can be used as a valid measure of the uncertainty in the intensity ratio. The average relative standard deviation of the replicate area ratios in Table I1 is 20%. By combination of the uncertainty in the area ratio with that of the sensitivity-factor ratio using a propagation-of-error computation, it can be shown that the atomic ratios computed from eq 3 have an a relative standard deviation of 525%, and the relative concentrations computed from eq 4 have a relative standard deviation of 512%. Thus, the sensitivity-factor method can be used to determine the relative surface concentrations of Ti and Mn valence states (separated by a t least two oxidation-state units) in single-phase oxide matrices with a relative standard deviation of 112% if the 3p line is chosen and photon energies that maximize sensitivity are used. The range of mole ratios is limited to approximately 0.33-3.0 due to the difficulty in detecting one oxide (at low concentration) in the presence of the other (at high concentration). Calibration-Curve Method of Quantitative Analysis. Calibration curves can be used validly for the quantitative bulk analysis of two-phase mixtures by using photoelectron spectroscopy (20, 22). Most commonly, area ratio is plotted as a function of mole ratio, and a straight line is fit to the data. Once again, variation in the factor AT can be calculated with high precision, and the factor f$ is the same for all chemical states. Unlike sensitivity factors, however, standard curves are valid for the analysis of two-phase materials because the factors uy and XC, as well as the shadowing effect, are taken into account by the slope of the calibration curve. Although these factors are not the same for the two chemical states, their ratio is constant and characteristic of any particular pair of compounds which have been prepared in the same manner. In other words, as long as the slope of the standard curve is constant, it does not matter what value it has. This is the basis for the calibration-curve method. Standards can provide a high degree of accuracy for a quantitative XPS analysis, but their use presents problems in the preparation and retention of purity for the standard curve. The standards and samples must be handled and cleaned uniformly to minimize surface effects. It is necessary, therefore, to prepare local calibration standards as nearly like the samples of interest as possible and to handle the samples in exactly the same manner as the calibration mixtures. Differences in any step of the sample preparation will adversely affect the accuracy of the results. The slope of the resulting calibration curve depends strongly upon the mixing, grinding, and cleaning procedures used for sample preparation. The area ratios were plotted as a function of the mole ratios for the two transition-metal mixtures. The data are listed in Table 11. Assuming y intercepts of zero, weighted least-squares straight-line fits to the data yield a slope of 0.98 with a standard deviation of 0.07 for T i and a slope of 1.07 with a standard deviation of 0.09 for Mn. Furthermore, the relative standard deviation of the mole ratios determined by the analysis was 6.8% for T i and 8.0% for Mn, and the relative standard deviation of the relative concentrations was 3.4% for Ti and 4.0% for Mn. Because of the difficulty in separating the overlapping lines, the mole ratio range was limited to 0.33-3.0. Outside this range, the signal due to one oxide (at low concentration) cannot be detected in the presence of the signal arising from the other (at high concentration).
Thus, standard curves can be used quantitatively to determine the bulk concentration of T i and Mn valence states (separated by a minimum of two oxidation-state units) with a relative standard deviation of 54% in the mole ratio range of 0.33-3.0. These results demonstrate that a quantitative oxidation-state determination of T i and Mn in binary oxide mixtures is feasible if curve fitting is used to resolve the overlapping chemically shifted 3p lines.
CONCLUSION This work has demonstrated that it is feasible to use synchrotron radiation as a source for quantitative XPS measurements. Among its unique advantages are high intensity and energy tunability. Furthermore, this work has demonstrated that a quantitative oxidation-state determination of Ti and Mn in binary oxide mixtures can be accomplished for valence states that are separated by at least two oxidation-state units. In order to separate adjacent chemical states, a transition more narrow than the 3p must be used. The higher energy and flux available from the new 1-GeV ring a t the Wisconsin Synchrotron Radiation Center will provide the opportunity to study the first-row transition-metal 2p subshell as a replacement for the 3p subshell. The use of the 2p line should improve the resolution of contiguous oxidation states because the spin-orbit splitting of the 2p transition is resolved and, thus, its natural line width is narrower than that of the 3p transition. Additionally, the 1-GeV ring will provide the opportunity to examine the energy dependence of the 2p transition to determine the energy of maximum sensitivity in a manner analogous to our previous study ( 4 ) . Registry No. Ti02, 13463-67-7; TiO, 12137-20-1; Mn02, 1313-13-9;MnO, 1344-43-0;Ti, 7440-32-6;Mn, 7439-96-5. LITERATURE CITE9 Andersson. S. L. T. J . Chem. SOC.,Faraday Trans. I 1979, 7 5 , 1356-1370. Tauster, S. J.; Fung, S. C.; Baker, R. T. K.; Horsley, J. A. Science 1981, 211, 1121-1125. Criswell, D.. Ed. Proc. Lunar Sci. Conf., 7th 1978. Rosseel, T. M.; Taylor, J. W.; Negri, R. E. Anal. Chem. 1985, 5 7 , 2665-2690. Armstrong, N. R.; Quinn, R. K. Surf. Sci. 1977, 6 7 , 451-468. Quinn, R. K.: Armstrong, N. R. J . Hectrochem. SOC. 1978. 125, 1790- 1796. Sayers, C. N.; Armstrong, N. R. Surf. Sci. 1978, 7 7 , 301-320. McIntyre, N. S.;Zetaruk, D. G. J. Vac. Sci. Technoi. 1977, 14, 181-185. Brown, F. C.; Bachrach, R. 2.; Hagstrom, S. B. M.;Lien, N.; Pruett, C. H. I n IVth International Conference on VUV Radiation Physics; Koch, E., Haensel, R., Kunz, C. Eds.; Pergamon-Vieweg: Braunschweig, 1974. Negri, R. E.; Taylor, J. W. Rev. Sci. Instrum., in press. Rao, C. N. R.; Sarma, D. D.; Vasudevan S.: Hegde. M. S. R o c . R . SOC.London, A 1979, 367, 239-252. Kim, K. S.;Baltlnger, W. E.; Amy, J. W.; Winograd, N. J. Necrron Spectrosc. Relat. Phenom. 1974, 5 , 351-367. Kelly, R. Nucl. Instrum. Methods 1978, 149. 553-558. Kelly, R. Surf. Sci. 1980, 100, 85-107. Storp, S.;Holm, R. J. Nectron Spectrosc. Relat. fhenom. 1979, 16, 183-193. Thomas, S. Surf. Sci. 1976, 5 5 , 754-758. Shirley, D. A. Phys. Rev. 8 1972, 5 , 4709-4714. Carbon, T. A. Faraday Discuss. Chem , SOC, 1975, 6 0 , 30-36. Wagner, C. D.Anal. Chem. 1977, 4 9 , 1282-1290. Wyatt, D. M.;Carver J. C.; Hercules D. M. Anal. Chern. 1975, 4 7 , 1297-1301. Young. V.; McCaslin, P. C. Anal. Chem. 1985, 5 7 , 880-886. Swartz, W. E.; Hercules D. M. Anal. Chem. 1971, 4 3 , 1774-1779.
RECEIVEDfor review December 30, 1985. Resubmitted June 19, 1986. Accepted June 19, 1986. This research was supported by NSF Grant CHE-8121205, by a grant from the Chevron Corp., by the Wisconsin Alumni Research Foundation, and partially supported by a grant from General Motors Corp. The Procter and Gamble Corp. provided a nine-month fellowship to R.E.N. and the SRC is operated under NSF Grant DMR-8421292.
