Table IV. Pure Compounds Eluted from Activity I Neutral Alumina Using the Solvent Sequence Indicated Solvent Hexane Toluene
Chloroform (I) Chloroform (11)
Te trahydrofuran/ethanol
Compound eluted Octacosane Octah y drophenanthrene Naphthalene Dibenzofuran Phenyl ether Phenanthrene Dibenzo thiophene Fluoranthene Pyrene Chrysene Phenyl ether Dimethyl quinolene Diphenylamine Carbazole Benzoquinoline o,rn,-Cresol l-Hvdroxvfluorene
Table 111lists elemental analysis data for some fractions isolated from two CQ-Steam products. The toluene eluate is essentially free of nitrogen compounds, while the second chloroform fraction includes about 70% of the nitrogen compounds in the original oil. This separation of nitrogen compounds from ethers and hydroxyl compounds allows accurate compound assignments by mass spectrometry of major hydrocarbon and heteroatom compounds. Elution data for pure compounds spiked into an SRC sample are shown in Table IV. The fraction where a given
compound appeared was consistent with the HRMS data (Table 11)and with the solvent polarity and adsorbent activity for the system used. The order of elution was the same as that shown in Table I. Some polymeric material of varying polarity may be eluted in these fractions along with the compound classes indicated. Because of their high molecular weight, these polymeric materials do not volatilize into the MS or GC-MS and, therefore, they do not interfere with characterization of the lower molecular weight material by these methods.
ACKNOWLEDGMENT The authors thank Dean H. Neal for performing experimental work in this study.
LITERATURE CITED (1) H. R. Appel, and I. Wender, Div. Fuel Chem. Prep., Am. Chem. Soc., 12 (3), 220 (1968). (2) J. E. Schiller, Hydrocarbon Process, 56, 147 (1977). (3) L. R. Snyder and B. E . Buell, Anal. Chem., 40, 1295 (1968). (4) H. J. Coleman, J. E. Dooley, D. E. Hirsch, and C. J. Thompson, Anal. Chem., 45, 1724 (1973). (5) D. A. Lane, H. K. Moe, and M. Katy, Anal. Chem., 45, 1776 (1973). (6) D. M. Jewell, E. W. Albaugh, B. E. Davis, and R. G. Ruberto, Ind. Eng. Chem. Fundam., 1 3 , 278 (1974). (7) M. Farcasiu, fuel, 56 (1) 9 (1977). (8) W. R. Middleton, Anal. Chem., 40, 1839 (1968).
RECEIVED for review March 8, 1977. Accepted May 5, 1977. Work by D.R.M. done in part under an ERDA-University of North Dakota fellowship. Reference to specific brands and models is for identification purposes only, and does not represent endorsement by ERDA.
X-ray Photoelectron/Auger Electron Spectroscopic Studies of Tin and Indium Metal Foils and Oxides Albert W. C. Lin, Neal R. Armstrong,’ and Theodore Kuwana” Department of Chemistry, Ohio State University, Columbus, Ohio 432 10
X-ray photoelectron spectroscopy (XPS or ESCA) and Auger electron spectroscopy (AES) have been applled to the surface analysis of “standards” of metal folk and oxides of two elements, tin and Indium. For the metal foils, the surface was Initially in an oxidized state whlch could be removed by argon Ion sputterlng to reveal the pure metal. On the surfaces of the tin and lndlum foils, the oxygen to metal atomic ratios were close to the expected values for SnO, and In2OB. These ratios changed wlth depth proflllng untll the pure metal was exposed. The energetlcs and the atomlc ratlos were also obtalned for several powder samples of SnO,, SnO, and 1n2O3standards. Although some changes in the ratios occurred with depth proflling of these standards, beam damage wlth decomposition was noted only for SnO. The problems and approaches to the identificatlonof elemental composition and to quantitation of oxygen to metal atomlc ratios are discussed.
The application of the various surface electron spectroscopic methods such as Auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS or ESCA) to surface analyses Present address, Department of Chemistry, Michigan State University, East Lansing, Mich. 1228
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
of metals and metal oxides is now well recognized and documented (1-18). In a recent paper (19),we reported the results of our initial efforts at applying ESCA/Auger methods to the evaluation of the elemental composition and the valence of electrode surfaces, namely to those of antimony doped tin oxide and tin doped indium oxide on transparent substrates. These oxide surfaces have been of interest to us for several years as part of our development and application of optically transparent electrodes (20). The objectives of the ESCA/ Auger analysis are twofold: 1)to determine the elemental stoichiometryon the surface of these oxides, and 2) to correlate the surface composition and valence with the electrochemically derived evidence of surface changes as a function of the electrode potential or electrode pretreatments. Of particular concern was the quantitation of intermediate oxidation states which could be present between the fully oxidized and reduced states (19). However, a prerequisite to a detailed quantitative analysis of these electrodes was the need for ESCA/Auger information on tin, tin oxide, indium, and indium oxide standards whose composition and valences were known. Although several Auger kinetic energies and ESCA binding energies have appeared in the literature (18,21-27) for Sn and In metal and oxides, there has been no attempt for a quantitative examination which included evaluation of the composition in terms of stoichiometry (e.g., oxygen to metal
atomic ratios). The analytical determination of the absolute quantities of surface composition is complicated by factors which are quite difficult to evaluate. These factors included for AES, the ionization cross sections, electron escape depths, roughness factors, matrix effects, various electron and ion beam induced artifacts, and other instrumental variables (28-33). For ESCA, similar factors must be considered (17, 18). In view of these complicating factors, it is important to accurately determine the binding energy (BE) and to evaluate methods of quantitation of surface concentration when two or more oxidation states of the same element are present. The objectives of this paper are therefore to examine some of these complications and to attempt the unambiguous identification and quantitation of surface species using standard materials such as high purity metal oxides and pure metals of tin and indium. For the metals, AES and ESCA analyses were performed before and after successive argon ion etching which revealed compositional profiles of the subsurface regions. The understanding gained from the analysis of these standards has been extended to an in-depth ESCA/Auger study of the doped tin oxide and indium oxide electrodes. The results for these electrodes will be reported in a separate paper.
EXPERIMENTAL Stannic oxide (Baker Analyzed Reagent, 98.0% pure and Spex Industry, Inc., 99.9% pure), stannous oxide (Pfaltz and Bauer, Inc., and Baker Analyzed Reagent) and indium oxide (Spex Industry, Inc., 98.5% pure) were used without any further purification. Stannous oxide was subjected to a high temperature oxidation to stannic oxide by thermogravimetric methods and was found to be 94.0% pure, assuming that the weight change was primarily due to the conversion of SnO to SnOz. Samples of these oxides in powder form were pressed onto In, Pb, or Sn foils (34) for ESCA/Auger surface analyses. The stoichiometry of these compounds was checked by elemental analysis and was compared to the results of the surface studies. Tin foil (Fisher Scientific Co., 99.99% pure) and indium foil (Pfaltz and Bauer, Inc., 99.99% pure) were used without any pretreatment. Carbon as a contaminant, e.g., hydrocarbons, COz,etc., was observed under some experimental conditions. Most of the ESCA data were obtained using a Physical Electronics, Inc., (PHI) Model 548 ESCA/Auger Spectrometer which was equipped with a Mg source (Mg Ka1,J. ESCA data of the metal foils were also obtained using the high resolution, Hewlett-Packard Model 5950A (Al Kal,J instrument with a quartz crystal monochromator (courtesy of N. Winograd, Purdue University). The pressure in the analyzer chambers was maintained at less than lo-’ Torr during analyses. For the PHI instrument, the Mg x-ray beam was operated at a power of 400 W. The resolution (AEIE)of the instrument was checked with Au, Ag, Cu, Sn, and In foils and an average value of resolution equal to 0.018 was obtained. The full width at half maximum (FWHM) of each species as a function of pass energy was studied (see later discussions). A pass energy of 25 eV which was a compromise between the resolution and the S/Nratio, was selected for most of the high resolution runs. Auger analyses were carried,out on a Varian CMA-1 analyzer and a PHI 548 ESCA/AU 1‘ spectrometer operated at approximately lo-’ Torr. A p g a r y electron beam energy of 1 keV and current of 30 pA for P k I and 2 keV, 50 pA for the Varian were used, respectively. The modulation voltage was 2 V peak-to-peak at 16 kHz. The Auger signal, dN(E)/dE, was measured with a lock-in amplifier using a time constant of 0.3 s. All spectra were taken with the same experimental conditions. An argon ion gun (PHI) with a voltage of 1 kV and an emission Torr was employed for depth profiling current of 10 mA at 5 X studies. Using similar conditions, a sputtering rate of 4-5 &min was observed on a vapor deposited film of Ti on quartz. The Ti film thickness of 200 i 10 was measured by optical interferometry. In the Varian instrument, depth profiling was accomplished using a Varian scanning Ar ion gun with a voltage at 600 V and an emission current of 40 mA at a pressure of ca. 5 X Torr.
Binding energies of the ESCA transitions were corrected for charging effect by referencing to the C(ls) peak which was assumed to have a BE of 284.4 eV (35). For the Sn and In metal foils, the BE’Swere cross-checked with results obtained from the Hewlett-Packard instrument which employed an electron flood gun for charge neutralization of the surface. Values of the ESCA FWHM reported in the text have been corrected for the K a z contribution whereas the spectra shown in the figures are the actual ones reproduced without change. The possible damages caused by electron and x-ray beams were carefully examined by checking the variation of the oxygen to metal atomic ratios as a function of the analysis time. Data acquisition, storage, and processing, particularly for obtaining signal averaged ESCA spectra, were accomplished using a NOVA 800 minicomputer (Data General Corporation) which was equipped with 32K core of memory, 2 Diablo disks (1.2 M bits) and r-y plotting facilities. Procedure. Computer Deconvolution of Spectra. ESCA spectra were “deconvoluted” into separate spectral components in a fashion similar to published methods (2,4-11). They were computer-simulated by inputting the following parameters: 1) the slope of the linear spectral background; 2) the binding energy of each component; 3) the full width at half maximum (FWHM) of each component; 4) peak height of each component; and 5) the percentage of Gaussian contribution to the shape of the spectral band. Parameter 5 was evaluated for the 3d transition of Sn and In standards or a Au (40 line changing the percentage of Gaussian contribution to fit the high and low sections of the simulated Lorentzian curves. This Gaussian contribution to the ESCA spectra varied considerably with experimental conditions. In the PHI instrument, parameters 3 and 5 were dependent on the pass energy of the analyzer; e.g., at pass energy of 25 eV, FWHM and peak shape were 1.5 eV and 50% Gaussian for tin(1V) oxide powder. If these FWHMs were plotted as a function of the pass energies, linear plots resulted at high energies (36). The slopes of these plots were similar from element to element. The value of the slope was determined by the resolution ( a E / E ) of the instrument. At low pass energies, the limits of the FWHMs were either the value of the FWHM of the Mg x-ray source or the FWHM of the element in solid phase. In a few of the elements the value of FWHM becomes constant at low pass energies (for example, ca. 1.0 eV for Sn 3d6pin tin foil). FWHM of elements can be estimated therefore in some cases, by the extrapolation of these FWHM vs. pass energy plots to the limiting, constant FWHM value. A problem in evaluating FWHM at low pass energies is the loss of signal intensity which then gives a lower signal to noise ratio. Of course, signal averaging methods can be used to improve the S/N. Undoubtedly, some variations in spectral band shape and width will occur depending on individual instruments. It was assumed that the spectral band shape (parameter 5) was constant for all the transitions at a given pass energy (25 eV setting on PHI). Parameters 2 and 3 were evaluated from spectra of pure standards of known composition. Parameters 1, 2, and 3 were then held constant for each simulation. Parameter 4 was varied for each component until a “best fit” was attained for the entire spectrum. Each simulated spectrum was also corrected for the satellite peaks which are present in the PHI instrument (without a monochromatic x-ray source). The Mg KaZsatellite contributed on the high binding energy side (0.33 eV from KaI) of each spectral band and were not normally resolved. By measuring the intensity of the Ka3,., satellites that could be resolved, the contribution of the Ka2 to the Kal band was then calculated and deconvoluted by the computer simulation. The relative positions and intensities of the satellites with respect to the Kal used were -0.33, +8.4, and +10.15 eV, and 50, 12.8, and 6.970, respectively (37). Calculation of Atomic Ratios. The atomic ratio of oxygen to metal No/NM, has proved to be the most informative parameter that could be determined by AES or ESCA to indicate surface stoichiometry of the metal oxides. Several methods exist for the computation of these ratios for both AES and ESCA. Method 1, used in the ESCA studies, involves the computation of N ~ / N M by correction of the ratio of the O(1s) and metal (3d6por 3d3p) intensities, Zo/ZM, for differences in photoelectric cross section,
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8 , JULY 1977
1229
Table I. Comparison of Calculations of Oxygen/Metal Ratioa ESCA -
AES
0bserved
Observed peakarea ( N o / (No/ Carter ( N ~ / N M ) , (NOINM)2 g ratio NM), NM)~ g et aleb Species intensity ratio SnO, powder 0.75 t 0.02 1.95 t 0.05 2.0d 1.03 0.47 1.66 2.0d 1.20 1.73 (AIC 2.3 i 0.1 _0.46 1.62 1.95 -1.69 0.88 c 0.02 2.29 * 0.05 (B) Sn foil oxidized surface 0.8 i 0.1 2.0 * 0.3 2.1 * 0.3 0.46" 1.63 1.96 1.70 SnO powder 1.60 1.39 0.38 1.33 0.60 i 0.02 1.57 i 0.05 1.61 t 0.09 (A) 0.65 i 0.02 1.70 * 0.05 1.7 * 0.1 0.37 1.31 1.58 1.37 (B) 2.37 i 0.09 1.5d 0.63 0.40e 1.27 1.5d 1.18 1.35 In,O, powder 0.79 * 0.03 In foil oxidized surface 0.7 t 0.3 2.1 i 0.9 1.3 t 0.3 0.31" 0.98 1.16 1.04 The results reported here are averages of at least five AES measurements and two ESCA measurements on each sample. For AES, metal M,N,,,N,,, or MsN4,,N49,, and oxygen KL, ,L,,,were used for the calculations. For ESCA, peak areas of Equation 6 of kef. 17. Different commercial sources. The standard metal ( 3 d J 1 2and ) O(1s) were employed. powders were assumed stoichiometric. " More than one kind of oxygen species were observed. Only the peak area of the metal oxide oxygen species was counted.
__ __ __
__ __
__
__
These photoelectric cross sections ( U M ~and u o X ) have been previously calculated by Scofield (38). Method 2 involves the computation of N o / N by ~ correction of lollM with the intensity ratio of oxygen to metal in a standard material of known composition, e.g, Sn02,or InpOs,
A variation to Method 1 has been reported by Carter et al. (17) where the differences in escape depths (inelastic mean free path, IMFP) of the oxygen and metal electrons due to differences in their kinetic energies were corrected. For the Sn, In, and 0 transitions, these corrections are small. A comparison of the three methods for tin and indium oxides will be discussed. Atomic ratios were calculated from Auger spectra using similar considerations. Method 1 for AES calculations uses intensity ratios for oxygen and metal corrected by ratios of electron impact ionization cross section, a~~ and uoe. These cross sections were obtained from literature values of the gas phase elements (33). This type of correction assumes that the Auger transition probability for each element is approximately 1.0 (30) and that the IMFP for each element is nearly equal because of the small differences in kinetic energies and of similarity in matrix. Method 2 for AES calculations proceeds exactly as for Method 2 ESCA calculations with intensity ratios in the unknown corrected by intensity ratios computed for a standard material. A comparison between Method 1 and Method 2 calculations for the SnOz and InlOs standard materials was carried out by ratioing the N o / N values ~ computed by each method,
(3) where g can also be written,
(4) The cross sections, u~~ and uoi, represent either the photoelectric cross section or the electron impact ionization cross section depending upon whether ESCA or AES calculations are being considered. A g factor of near 1.0 can indicate agreement between the predicted and actual cross sections. The g factor includes consideration for the ratios of IMFP's, backscattering coefficients, etc., for each element and is similar to the modification factor recently proposed by Palmberg (28). RESULTS AND DISCUSSION Comparison between Methods 1and 2 for Computation of Atomic Ratios. The values of N o / N Mcomputed from AES and ESCA data for Method 1, 2, and Carter et al. are listed in Table I. The M4N4,jN4,j,M5N4,5N4,jtransition 1230
ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
100 200
300
400
500
600
KINETIC ENERGY (eV)
Figure 1. Auger spectra of metal oxide powders. ( a ) SnO,, ( b ) In,OB
intensities of Sn and In, and the KL2,3L2,3 transition intensity of oxygen were used for the AES computations. The O(ls) and Sn or In (3d3j2)peak areas were used for the ESCA computations. Metal (3d5p) peaks were also used for computations. The No/NM ratios were within *0.09 of those calculated using the (3d3I2)peaks. Values of g reasonably near 1were observed for all computations involving the S n 0 2 and InsOBstandards except for the AES data for In203. A g value of near 0.6 could result from decomposition of the In203 surface during analysis. This should have resulted in a time dependent atomic ratio, which was not observed if the beam voltage was kept a t 1 keV. The low AES g value for In203 is likely due to an error in the value of qneused. This value (uhe = 3.0 X lO-''/lO e-) was determined by extrapolation from other MNN literature values (33). An error of 40% in the cross-section value would explain the error in the g factor. ESCA g factors were slightly higher than unity for reasons which are discussed below. Tin(1V) Oxide. Auger and ESCA results of each standard material will be discussed in the order of Sn02, Sn foil, SnO, In203,and In foil. A typical Auger spectrum of SnOz is shown in Figure 1, trace a. The tin doublet a t 425 and 432 eV corresponds to the core, core-valence M N N transitions (M4N4,5N4,5, M5N4,5N4,5) and the oxygen KL2,3L2,3 transition appears a t 510 eV. In Figure 1,trace b, the In20:{spectrum
Table 11. Binding Energiesa and FWHM of Standard Materials at Pass Energy of 25 eV Species Sn foile
M(3d5/21 484.5 t 0.2
O(1s)
(1.0)f Sn foil
oxidized surface SnO, powder In foile
486.2.t 0.2 (1.5) 486.2 +. 0.2 (1.45) 443.6 f 0.2
815.7 i. 0.2
M(3d3,,)o(1~)- M(M,N, gN4 5 ) M ( 3 d ~ ) ~ M(3d5/ Z 1“ M(3dI/z )A 8.4 t 0.1 (45.6 f 0.2) 331.2 t 0.2
820.6 * 0.2
8.4
t
0.1
43.9 + 0.1
334.4
i
0.2
820.6 i 0.2
8.4
f
0.1
43.9
334.4
f
0.2
842.7
7.6
i
0.1
M(n44N4,5N4,5)
530.1 t 0.2g (1.65) 530.1 t 0.2
0.1
i
(1.65)
__
f
0.2
399.1 * 0.2
(86.4 i 0.2)
(1.08)
530.0 t 0.zg 846.9 f 0.2 7.6 i 0.1 85.3 t 0.2 402.4 + 0.2 (1.7) 444.5 f 0.2 846.9 * 0.2 7.6 0.1 85.5 f 0.2 402.4 i 0.2 530.1 t 0.2g (1.65) (1.7) All binding energies were corrected for charge shift. Difference in binding energies of M13d,/,) and M(3d5/,). Difference in binding energies of metal oxide O(1s) and M(3d,,,). For the metal foils, the differences between O(1s) of the corresponding metal oxide and M(3d512.)of the metal foil, are shown in parentheses. Difference in M(M,N,,,N,,,) Auger “binding energy” and M(3d, ? ) binding energy. e Both metal foils have been sputtered clean by an Ar‘ ion gun. All FWHM’s have been corrected /or contributions from Koc,. Two forms of oxygen species were observed. Only the data of metal oxide oxvgen are reDorted here. In foil oxidized surface In,O, powder
444.7
i
0.2
(1.7)
S n 0 ~ powder
(bl
300
h
i
1
495
1
1
,
1
1
490
,
/
,
,
,
,
,
,
,
485 B i n d i n g Energy
,
,
,
480’
/
,
’
,
,
,
1
1
1
,
530
525
lev1
Figure 3. ESCA high resolution spectra of SnO, powder. ( a ) As received, ( b ) after 30 min of Ar’ sputtering at 1 kV, 10 mA, and 5 X Torr Figure 2. Schematic diagram of Auger MNN transitions
is similar in overall appearance to that of SnOz except for the shift in the energy for the metal peaks. The two metal transitions involve the ejection of an Auger electron from the 4d orbital. A schematic of the metal transitions is shown in Figure 2. T h e Auger values of the oxygen to metal atomic ratio, N o / N M ,for the tin and indium standards are tabulated in ) ~ a value of 1.95 f 0.05 which Table I. For SnOz, ( N o / N Mhas is in satisfactory agreement with the reference stoichiometric value of 2.00 (Method 2). A high resolution ESCA spectrum of an SnOz standard sample is shown in Figure 3, trace a. The Sn metal 3d line (BE of 3d5p = 486.2 eV) has a FWHM of 1.45 eV and the O(ls) band showed only one symmetric component which was centered a t 530.1 eV (FWHM = 1.65 eV). Data are summarized in Table 11. This O(1s) spectrum is similar to those reported for other metal oxides (2, 4, 12, 18, 22). Also tabulated in Table I1 are the values of the differences between the BE’S of O(1s) and Sn(3dsjz), and the x-ray induced, Auger at 820.6 eV and the Sn(3de/z). These differences are the most reliable indicator of the chemical state of the surface since they are free from charging complications (39)* Atomic ratios were computed from the ESCA peak areas of each component and are summarized in Table I. Normalization of these peak areas by the published x-ray cross sections produced (No/Np,& values which were less than that of the stoichiometric value for the oxide. Reduction of the oxide was not evident during analysis. Thus, the low (No/NM)
could arise because the literature cross-section values were in error. Method 2 thus assumes a stoichiometry of 2.00 for the standard sample of SnOz and normalizes all other ESCA intensities or peak areas to that of this standard. I t should be noted that the atomic ratios obtained using Methods 1and 2 are quite different for Auger and ESCA. As shown below, the high resolution ESCA spectrum allows discrimination between adsorbed oxygen and metal oxide oxygen, whereas the AES spectrum does not make such distinction. Tin Foil. The Auger spectra of tin foil before and after various times of sputtering are shown in Figure 4. It can be seen that these spectra change dramatically with sputtering. Initially, the spectrum was that of the oxide, SnOz, on the surface of the foil. The positions of the Sn and 0 peaks as well as the No/NMratio were close to that of SnOz standard (see Table I and Table 11). With sputtering, the No/NMratio decreased until oxygen was completely removed (Figure 4, trace d). The resulting spectrum was that of tin metal with the tin MNN doublets a t 430 and 437 eV. The plasmon transitions at 416 and 423 eV are distinctive for the Sn metal (arrows, traces c and d of Figure 4) and are absent in the metal oxides. Also it should be noticed that the Sn MNN doublets are progressively shifted in kinetic energy (ca. 5 eV total) and are less well defined during the intermediate stages of sputtering. Although ESCA data were obtained with both the PHI and Hewlett-Packard instruments, only the P H I data will be presented here. For tin foil, initially one Sn component was observed with a 3d5p BE of 486.2 eV and FWHM of 1.5 eV (Figure 5, trace a). After sputtering (Figure 5, traces b and c) two components were seen with B E of 486.1 eV (FWHM ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
1231
,
~
Sn F o i l
1
f 300
4?
b)
T
IK
(1)
I /
I
,
400 600 K i n e t i c Energy ( e v )
Figure 4. Auger spectra of Sn foil at various Ar' sputtering times. The sputtering condition is the same as in Figure 3, ( a ) 0 min, ( b )7 min, (c)8.5 min, ( d ) 31 min Sn F o i l Sn ad
1
I
I
I
,
i\ 3fK
I
I
,
I
825
425 485
Binding Energy (eV)
FIqure 5. ESCA Sn(3d)high resolution spectra of Sn foil at various Ar sputtering times. The sputtering condition and times are the same as in Figure 4 = 1.5 eV) and 484.5 eV (FWHM = 1.0 eV). After removal of
ca. 150 A (31 min of sputtering, Figure 5 , trace d ) , only the peak due to Sn metal was seen with BE = 484.5 and FWHM = 1.0 eV. The corresponding spectra for O(1s) are shown in Figure 6. Initially, a broad peak with maximum at 530.1 eV with FWHM of 2.2 eV was present. This peak can be deconvoluted into two components; the minor one being at a BE of 531.7 eV. With sputtering (traces b and c , Figure 6) the broad peak gave way to a narrower peak at 530.1 eV (FWHM = 1.6 eV) which finally disappeared (trace d , Figure 6) as the base Sn metal was exposed. The BE of the minor component on the surface was consistent with an adsorbed oxygen species, possibly a water of hydration of the metal oxide. Similar ANALYTICAL CHEMISTRY, VOL. 49, NO. 8, JULY 1977
I
I
I
I
525
f
Energy (eV) 8pO
Binding
1232
I
Sn F o i l
I
490
I
Figure 6. ESCA O(1s) high resolution spectra of Sn foil at various Ar' sputtering times. The sputtering condition and times are the same as in Figure 4
830
495
,
535 530 Binding Energy ( e V )
I
)O
1
430
I
435
I
44(
K i n e t i c Energy ( e V )
Flgure 7. X-ray induced Auger spectra of Sn foil at various Ar' sputtering times. The sputtering condition and times are the same as in Figure 4 oxygen species have been suggested for oxide samples ( I I , 1 2 ) . The 530.1 eV peak was undoubtedly due to the oxygen of SnOz. The x-ray induced Auger transitions were also observed and followed as a function of surface sputtering of the Sn foils. As seen in Figure 7, a new Auger MNN doublet appeared corresponding to energies approximately 5 eV lower than the surface oxide transitions. These Auger levels, M4N4,5N4,5 and M5N4,5N4,5, lie at or near the valence band of the solid and are extremely sensitive to changes in the chemical environment. These new transitions were reponsible for the variations in the shape of the Auger dN(E)/dE signals as a function of sputtering depth (i.e., changes in the chemical composition) which were previously observed in Figure 4,trace b. They were not resolvable or identifiable because the energy reso-
S r 0 powder
Figure 8. ESCA high resolution spectra of SnO powder. (a) As received, ( b )after 30 min of Ar’ sputtering at 1 kV, 10 mA, and 5 X Torr
lution was inadequate for their observation in the usual AES experiments. Sn(I1) Oxide Powder. As indicated by other workers (26, 40), there is very little shift in the BE between the oxidation states of Sn. Thus, the Auger and ESCA spectra (Figure 8) of SnO appeared nearly identical to that of SnOg with the exception of the FWHM’s and the O/M ratios. Two different sources of SnO samples were studied. At the surface prior to any sputtering, the BE’s of the Sn(3d5,J and O(1s) are within experimental error of that for SnOg. More particularly, the ABE’s of SnO are essentially identical to those of SnOz. New, very small x-ray induced Auger transitions were apparent for the SnO powder a t higher KE’s than those reported for the SnOg powders. The major x-ray induced transitions, however, remained unchanged. The ESCA bands for the Sn(3d5p) transition showed a FWHM of 1.85 to 2.3 eV which was 0.4-0.8 eV greater in width than those observed for SnOz. The variation in FWHM depended on the source and the extent of the charge broadening. The similarity in the ESCA binding energies between SnO and SnOz is perplexing in view of the chemical difference associated with these two oxidation states. There is little difference in the energies of core-level electrons. Nevertheless, some differences exist in the ESCA and AES spectra of these oxides which make possible their identification. The AES (No/NM)ratios for the SnO powder were 1.6-1.7 compared to 1.95 for SnOz. The ESCA ratios were 1.2-1.3 for SnO compared to 1.66 for SnOz. Sputtering of the surface of these two oxides also produced differences. For the SnOg samples, the AES and ESCA No/NM ratio decreased slightly with sputtering (e.g., AES ratio decreased from 1.95 to steady state value of ca. 1.5) and no new Sn species were observed in the ESCA spectrum (Figure 3, trace b ) . For SnO, not only did the No/NMratio change but also a new Sn band was observed in ESCA at lower binding energies (Figure 8, trace b). This band was identified as metallic tin, Sn’. This decomposition occurred even when the ion beam accelerating voltage and current were kept below 1 kV and 10 mA. Results suggest that sputtering of the SnOg surface removed all adsorbed oxygen (e.g., water) and possibly produced a suboxide with oxygen to tin atomic ratio less than 2/1 whereas the sputtering of the SnO surface caused decomposition to form metallic tin. Since tin(I1) oxide may disproportionate in air, it is possible that there is a tin(1V) oxide layer with metallic tin on the top of the tin(I1) oxide. This hypothesis could explain the greater FWHM of SnO than that of SnOz and the higher No/NM ratio than the expected stoichiometry. However, no metallic tin was observed on the SnO powders before Ar’ sputterings. The ABE’s of the main Sn(3d5I2) of SnO powders after Ar’ sputterings which caused the appearance of metallic tin, are close to those of SnOz within experimental error. The results
verify earlier conclusions (26, 40) that the chemical shift of the Sn(3d) band between SnO and SnOg is small. Powder Mixture of Tin(1V) a n d Tin(I1) Oxides. Known mixtures of tin(1V) and tin(I1) oxide powders were used to further check the NO/NM ratio and BE’S of the tin element at various oxidation states. Several mixtures with various weight percentages of SnO and SnOz were prepared. Initially, each of the Sn(3d) and O(1s) lines exhibited only one symmetric component with a FWHM of 1.6 and 1.45 eV, respectively. The BE’s and ABE’s for the mixtures were within experimental error of those for the tin(1V) oxide powder. The No/NM ratios for the mixtures were close to those values expected by computing the No/ NMratios of the pure powders from their weight percentages. For instance, a mixture of 76.71% tin(I1) oxide (source B) and 23.29% ~ tin(1V) oxide (source B) had experimental (NO/N M )ratios of 1.35 f 0.07 and 1.64 f 0.04 for ESCA and AES, respectively, while the expected values were 1.37 (for ESCA) and 1.84 (for AES). The possible presence of a SnOz layer on the top of SnO powders does not affect the validity of the result, because the expected values were calculated from the observed No/NM ratios for SnOz and “SnO” powders. The homogenity of the mixtures was checked by comparing (No/NM)I ratios at different sampling areas. The average values are reported above. After 30 min of Ar sputtering (1 kV, 10 mA), the No/NM ratios decreased to values which depended on the composition of the mixtures. A new low binding energy component of the Sn(3d) lines was observed as expected from the decomposition of tin(I1) oxide caused by the sputtering. The new component was identified as metallic tin from ABE’s and was consistent with results for the sputtering of tin(I1) oxide discussed previously. Auger a n d ESCA Results for In203a n d I n Foils. Referring to trace b of Figure 1,it can be seen that the overall Auger spectrum for h203 powder is very similar in shape to that for SnOz except for the shift in the energy for the metal. The M4N4,5N4,5 and M5N4,5N4,6 transitions appeared at 399 and 405 eV, respectively, while the oxygen KL2,3L2,3 transition was the same as that for oxygen in SnOz at 510 eV. The ESCA spectrum of IngOBwas also similar to that of SnOz except that the O(1s) band showed two components, i.e., metal oxide and adsorbed oxygen species. The BE’s and ABE’s are tabulated in Table 11. These BE’S for In in Inz03were in good agreement with those previously reported (18,25). Since the value of the BE for oxygen is the same in both Ins03 and SnOz, the ABE’s, particularly those for the difference between O(1s) and In or Sn(3d5,J, are good indicators for qualitatively examining and comparing In and Sn in various oxides. The FWHM value for In& (1.65 eV at a pass energy of 25 eV) was also similar to SnOz (1.45 eV) (Table 11). The average value of the (No/NM)lratio for In203was high (2.37) for Auger and low for ESCA (1.27) compared to the predicted value of 1.5. The ESCA value was lower than the Auger value because only the peak area of the oxygen(1s) band which could be attributed to Inz03was used for quantitation. The subtraction of any oxygen due to adsorbed water or other sources of oxygen (oxygen peak deconvoluted into two components) lowered the (No/NM)I ratio (from 1.75 to 1.27). For Auger analysis, the (NO/NM)lratio was higher than 1.5 because the peak intensity including the total oxygen content, irrespective of sources, and because the electron impact ionization cross section may have been in error. Similar to Sn foil surface, the Auger spectrum of In foil surface was very similar to that of h203 powder. After 39 min of argon ion sputtering, a spectrum of clean In metal with two plasmon peaks at 381 and 392 eV appeared. The shift of the In MNN doublets (from 399,405 eV to 404,410 eV) and the less defined doublet were observed during the sputtering. ANALYTICAL CHEMISTRY, VOL. 49, NO.
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I n Foil I n 3d
Foil
Clean I n
ir
83eV-10 I e V -
-10 l e v 4 7 eV+
+II
n
f
2u3d5 3d34
1
46 2
Figure 11. of I n foil I
l
l
1
I
I
455
l
l
I
I
I
I
I
I
I
I
450 445 B i n d i n g Energy ( e V )
I
440
Figure 9. ESCA In(3d) high resolution spectra of In foil at various Art sputtering times. The sputtering condition is the same as in Figure 3. (a) 0 min, ( b ) 1 min, (c)9 min, ( d ) 39 min I n Foil 0 IS
T 300
(d)
1
535
1
,
1
1
1
1
,
1
1
1
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1
530 525 Binding Energy (eV)
Flgure 10. ESCA O(1s) high resolution spectra of In foil at various Ar' sputtering times. The sputtering condition and times are the same as in Figure 9 Figures 9 and 10 show the ESCA high resolution In(3d) and O(1s) spectra for In foil before and after different sputtering times with an argon ion beam. The In 3d512 peak was symmetric at the foil surface with a BE of 444.7 eV and FWHM of 1.7 eV. When the surface was sputtered, the In peaks became broader and could be deconvoluted into two components; one with a BE similar to Inz03and the other to In metal (see trace c, Figure 9). With further sputtering, the peaks narrowed and the 3d5p peak had a BE of 443.6 eV with a FWHM of 1.1eV. Assuming that the surface of the In foil was initially fully oxidized, the BE difference between the fully oxidized and the reduced In 3d5p peak was 1.1eV. The value is within the range of reported values of 0.4 to 1.35 eV (18, 21-25). The BE and ABE values for In foil are summarized in Table 11. 1234
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I
I
452 442 B i n d i n g Energy (eV)
ESCA In(3d) spectrum and
I
43 2
its characteristic loss structure
The O(1s) spectrum (Figure 10) of the In foil surface could be deconvoluted into two components, one with a BE of 532.0 eV (FWHM = 1.7 eV) and the other a t 530.2 eV (FWHM = 1.7 eV). These values are in agreement with those assigned to adsorbed oxygen and metal oxide oxygen species previously seen at the Sn foil surface. As expected, the O(1s) component at 532.0 eV disappeared completely prior to the disappearance of the oxide oxygen when the surface was argon ion sputtered. An ESCA NOINM ratio of the In foil surface was computed from the In(3daI2)peak area and the O(1s) peak area of the 530.2 eV component. The (No/NM)lratio was found to be 2.1 for AES and 0.98 for ESCA. The reasons for this difference are similar to the case of Inz03. The x-ray induced Auger peaks were present at 846.9 and 842.7 eV for In203and In metal, respectively. The charge independent ABE's for the In (Auger) and In(3dsIz)bands were 402.4 and 402.2 eV for I n z 0 3powder and the oxidized In foil surface respectively, while a value of 399.1 eV was determined for Ino metal (Table 11). Two new peaks appeared for the ESCA spectrum of the clean In foil a t an energy of 11.7 eV higher than the two In(3d) peaks (see Figure 11). Similar peaks were observed for clean Sn foil with an energy of 14.1 eV higher than the Sn(3d) peaks. It seemed reasonable to assign these peaks to plasmon or other energy loss transitions (41). The correlation between the appearance of these peaks and the plasmon loss peaks from Auger spectra of the metals (both Sn and In) confirmed this assignment.
CONCLUSION Previous studies of metal oxide electrodes and other metal oxide surfaces have relied on several assumptions for quantitation of the surface composition. We have explored several analytical computational methods for the S n 0 2 and Inz03 systems and must conclude that several difficulties remain for computation of atomic ratios using calculated photoelectric or electron impact ionization cross sections. Calculation of No/NM ratios from AES or ESCA intensities can be expected to give lower than the true ratios if these intensities are corrected only for cross-section differences as in Method 1 or if an additional correction for inelastic mean free path differences is included (17). It is clear that calculations of No/NM ratios are best made by comparison of the spectral intensities of the unknown metal oxide with a standard of known stoichiometry and similar matrix such as SnOz or h203 powders. Since the AES atomic ratio calculations do not discriminate between various forms of oxygen on the surface, these ratios can be expected to be higher in some cases than those computed by ESCA.
Interpretation of ESCA binding energy data for these metal oxides should be made mainly from the binding energy differencesof the various oxygen and metal transitions, since these ABE’s are the ones free from charging complications (39). It is clear that it is very difficult to distinguish the intermediate oxidation states of Sn and In from the fully reduced and oxidized forms. Linear correlations of binding energy and oxidation number are not observed for the SnOz, SnO, Sn system. The N o / N Mratios, however, serve to distinguish the partially reduced metal oxide, since they are consistently lower than those computed for SnOz. Decomposition of SnO to Sno by ion etching also serves for identification purposes. The quantitations reported herein are important prerequisites for a detailed understanding of the surface composition of SnOz and Inz03electrodes, which are reported in a subsequent paper.
ACKNOWLEDGMENT We greatly appreciate the assistance of J. Lumsden (OSU) for use of the Varian Auger instrument and Paul Uglum and Nick Winograd (Purdue University) for obtaining ESCA spectra on the Hewlett-Packard ESCA instrument.
(13) (14) (15) (16) (17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35)
LITERATURE CITED (1) C. R. Brundle, J . Eiectron Spectrosc. Relat. Phenom., 5, 291 (1974). . (2) 0. C. Allen, P. M. Tucker, A. Capon, and R. Parsons, J , E l e c h o a ~ lChem. Interfacial Electrochem., 50, 335 (1974). (3) A. Cimino and B. A. DeAngelis, J. Catal , 36, 11 (1975). (4) N. S.McIntyre and M. G. Cook, Anal. Chem., 47, 2208 (1975). (5) J. S.Hammond, S . W. Gaarenstroom, and N. Winograd, Anal. Chem., 47, 2193 (1975). K. S. Kim and N. Winograd, J. Catal., 35, 66 (1974). K. S. Kim and N. Winograd, Surf. Sci., 43, 625 (1974). K. S.Kim, A. F. Gossmann, and N. Winograd, Anal. Chem., 46, 197 (1974). K. S.Kim, T. J. O’Leary, and N. Winograd, Anal. Chem., 45, 2214 (1973). K. S. Kim and R. E. Davis, J . Electron Specfrosc. Relat. Phenom., 1, 251 (1972/3). K. S. Kim, N. Winograd, and R. E. Davis, J. Am. Chem. Soc., 93, 6296 (197 1). T. Robert, M. Bartel, and G. Offergeld, Surf. Sci., 33 123 (1972).
(36) (37) (38) (39) (40) (41)
G. Schon, Surf. Sci., 35, 96 (1973). M. Seo, J. B. Lumsden, and R. W. Staehle, Surf. Sci., 42, 337 (1974). M. P. Seah, Surf. Sci., 40, 595 (1973). M. P. Seah, Surf. Sci., 32, 703 (1972). W. J. Carter and G. K. Schweitzer, J. Electron Spectrosc. Rebt. &nom., 5, 827 (1974). R. S.Swingle and W. M. Riggs, Crit. Rev. Anal. Chem., 5, 267 (1975). N. R. Armstrong, A. W. C. Lin, M. Fujlhira, and T. Kuwana, Anal. Chem., 48, 741 (1976). T. Kuwana and N. Winograd, “Spectroelectrochemistry at Optically Transparent Electrodes,” in “Electroanalytical Chemistry, ’ Vol. 7, A. J. Bard, Ed., Marcel Dekker, New York, N.Y., 1974, and references therein. R . Holm and S.Storp, Appl. Phys., 9, 217 (1976). J. B. Bosnell and R. Wagnorne, Thin Solid Films, 15, 141 (1973). C. D. Wagner and P. Bioen, Surf. Sci., 35, 82 (1973). G. E. McGuire, G. K. Schweitzer, and T. A. Carlson, Inorg. Chem., 12, 2450 (1973). V. I. Nefedov, B. F. Druzsinszklj, N. P. Szergusin, J. V. Szalin, and Gyula 6s GBti, Magy. Kem. Foly., 81, 495 (1975). P. A. Grutsch, M. V. Zeller, and T. P. Fehlner, Inorg. Chem., 12, 1431 (1973). W. E. Morgan and J. R. VanWazer, J. Phys. Chem., 77, 964 (1973). P. W. Palmberg, J . Vac. Sci. Techno/., 13, 214 (1976). D. J. Pocker, R. W. Springer, F. E. Ruttenberg, and T. W. Haas, J . Vac. sci. Techno/., 13, 507 (1976). H. E. Bishop and J. C. Riviere, J. Appl. Phys., 40, 1740 (1969). F. Meyer and J. J. Vrakking, Surf. Sci., 33, 271 (1972). J. T. Grant, T. W. Haas, and J. E. Houston, Surf. Sci., 42, 1 (1974). J. J. Vrakklng Bnd F. Meyer, Phys. Rev. A , 9, 1932 (1974). G. E. Theriault, T. L. Barry, and M. J. B. Thomas, Anal. Chem., 47, 1492 (1975). K. Siegbahn, C. Vordling, A. Fanlman, R. Nordberg, K. Hamrin, J. Hedman, G. Johansson, T. Bergmark, S. E. Karlsson, I. Lindgren, and B. Lindberg, “ESCA-Atomic, Molecular, and Solid State Structure Studied by Means of Electron Spectroscopy,” Almquist and Wiksells, Uppsala, Sweden, 1967. P. W. Palmberg, J . Nectron Spectrosc. Re/. Phenom., 5, 691 (1974). N. Beatham and A. F. Orchard, J. Electron Spectrosc. Re/. Phenom., 9, 129 (1976). J. H. Scofield, J . Necfron Spectrosc. Rei. Phenom., 8, 129 (1976). C. D. Wagner, Faraday Discuss. Chem. Soc., 60, 291 (1975). G. W. Grynkewich, B. Y. K. Ho, T. J. Marks, D. L. Tomaja, and J. J. Zuckerman, Inorg. Chem., 12, 2522 (1973). R. A. Pollak, L. Ley, F. R. McFeely, S. P. Kowalczyk, and D. A. Shirley, J . Nectron Spectrosc. Re/. Phenom., 3, 381 (1974).
RECEIVED for review February 16, 1977. Accepted April 25, 1977. The financial support provided by NSF grants C H E 73-04882 and C H E 76-04911 is gratefully acknowledged.
Determination of Organic Carbon By Thermal Volatilization-Plasma Emission Spectrometry D. G. Mitchell,” K. M. Aldous, and E. Canelli Division of Laboratories a n d Research, N e w York State Department of Health, Albany, N e w York
Organic carbon is determined by differential volatilization and plasma emission spectrometry. Water or suspended particulate samples are dispensed into platinum boats acd dried at 85 ‘C for 10 min. Oxidant is added, then the boats are injected Into a furnace at 850 ‘C, at which temperature organic carbon volatilizes several seconds before inorganic carbon. The resulting vapor is fed to a microwave-excited plasma, and carbon emission at 193.0 nm is measured. For a 400-pL allquot, callbration curves are linear up to 16 pg C, the detection limit is 0.4 kugl dissolved organic carbon, and the relative standard deviation is typically 3% at the 5 pg C level. The instrument has a number of advantages for routine organic carbon analysis: (i) Analysis is rapid, typically with an elapsed tlm@of 10 min and an analysis rate of 20 samples per hour. (ii) Samples usually do not require pretreatment other than drying. (iii) Moderate amounts of inorganlc carbon do not interfere. (iv) Refractory organic compounds can be determined with excellent recoveries.
12201
Determination of organic carbon in natural waters, waste waters, suspended particulates, sludges, and sediments is one of the most important environmental analyses. Organic carbon levels indicate the potential oxygen demand. Monitoring of sewage treatment plant effluent and receiving waters is a common application and necessitates many routine determinations. Both particulate organic carbon (POC) and total organic carbon (TOC) are measured. In general, samples are acidified, and dissolved COPand COz from bicarbonate and carbonate are removed. Organic carbon is then (i) oxidized by combustion or wet oxidation to COP and detected using an infrared or a thermal conductivity detector or (ii) oxidized, then reduced to CH4 and determined using a flame ionization detector (1-6). These procedures are precise but they have several important practical disadvantages, particularly for routine analysis. The elimination of inorganic carbonates by treatment with acid is time-consuming, may introduce contamiANALYTICAL CHEMISTRY, VOL. 49, NO. 8 , JULY 1977
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