1992
The Journal of Physical Chemlstfy, Vol. 82, No. 18,
D. S. Zingg and D. M. Hercules
1978
Electron Spectroscopy for Chemical Analysis Studies of Lead Sulfide Oxidation D. S. Zlngg and Davld M. Hercules* Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania 15260 (Received May 3, 1978) Publication costs assisted by the University of Pittsburgh
ESCA has been used to study the oxidation of lead sulfide. Initially sulfur loss, as a function of oxidation time, was followed and found to be significant. Using peak ratios it was discovered an additional lead species was present which did not contain sulfur. Spectral data indicate that the species is probably PbO. Additionally, PbO was found to react with the products of PbS oxidation to form PbS04. This indicates that PbO is an intermediatein the oxidation of PbS. The intermediatewas produced from PbS and the effect of varying S O z / O ~ ratios was studied. Two products were formed. With high SOz/Oz ratio PbS03was formed, with low SOz/Oz ratios PbS04.
Introduction Since the early 1940's, a great deal of work has centered around understanding the surface oxidation of lead sulfide. The surface oxidation is important for two reasons. First, naturally occurring lead sulfide is separated from other sulfide minerals by a floatation process which uses xanthate collectors. Oxygen appears to affect this separation process. Second, and more important, is the effect which oxygen has on both the photoconductivity and the photoelectric response of PbS. Thus, work aimed at understanding both the final oxidation products and the mechanism of PbS oxidation has been carried out and much of this work is in disagreement. Table I summarizes some of the oxidation products proposed. Because ESCA is both surface sensitive (sampling depth -20 A) and has the ability to distinguish between chemical species, it is ideally suited for studying PbS oxidation. ESCA has been used previously to determine the final oxidation products of PbS oxidation,l but no systematic study devoted to the elucidation of the reaction mechanism was attempted. Our study re-examines the final oxidation products and, in addition, presents a systematic study of the reaction mechanism. Experimental Section An AEI ES2OOA electron spectrometer with an AEI DSlOO data system was used to record all ESCA spectra. This spectrometer used an aluminum anode (A1 K a = 1486.6 eV) which is operated at 1 2 kV and 20 mA. The instrument operates at a vacuum of 5 X Torr. The digital data obtained were processed using an H P 2114A computer. Overlapping peaks were deconvoluted using a Dupont 310 curve resolver. The probe system used allowed transport of the sample from the spectrometer to the reaction chamber without exposure to air. The probe and reaction chamber design have been described p r e v i o u ~ l y .The ~ ~ ~probe was used without modification. The reaction chamber system was modified by placing a glass U-tube in the flow stream leading to the reaction chamber. This is shown in Figure 1.
Lead sulfide (99.9991%, Apache Chemical, Seward, Ill.) samples were prepared by vacuum deposition onto aluminum supports in a vacuum chamber operated in the 5 X lo4 Torr range. The sample was mounted on the probe and never again exposed to air. Standard spectra were obtained for PbO, PbS04 (both 99.999% Apache Chemical), PbS03 (Pfaltz & Bauer, Stanford, Conn.), and PbS203 (Ventron, Beverly, Mass.). Each spectrum was run at least three times. All binding energies are reported as the mean 0022-3654/78/2082-1992$0 1.OO/O
TABLE I: ProDosed Oxidation Products products conditions 170-280 C in air PbSO, PbSO, t PbO t SO, 250 " C in 0, PbS,O, 0, PbO t S air PbO. PbSO, 0.1 Torr 0,; 500 " C 4PbO-PbSO, t PbSO, 230-265 "C in air
ref 2 3 4 5 6 7
TABLE 11: ESCA Binding Energies of Pb 4f, S 2s, and 0 1s Lines binding energies, eV compd Pb 4f,,, s 2s 0 Is PbS 138.0 i 0.1 225.5 * 0 . 1
a
PbO
137.9 -I: 0.1
PbSO, PbSO, PbS,O,
139.7 * 0.1 138.8 * 0.1 138.6 * 0.1
232.7 * 230.9 * 237.7 c 226.1 *
Rhombic form of PbO.
0.1 0.1 0.1 0.1
531.1 P 529.1 * 531.7 * 531.0 P 531.3 i
O.la O.lb 0.1 0.1 0.1
Tetragonal form of PbO.
f standard deviation and are referenced to the C 1s line at 285.0 eV. Referencing was checked by using Au 4f peaks of vapor deposited gold.
Results and Discussion In order to identify the chemical nature of the oxidation products of PbS, the ESCA spectra of PbO, PbS04PbS03, and PbSZO3were obtained. The peak positions of the Pb 4f, S 2s and the 0 Is lines are given in Table 11. The S 2s line was used instead of the stronger S 2p line because the S 2p peak position co-incided with the energy loss peak of the P b 4f lines, making area calculations difficult and obscuring the first appearence of initially small sulfate peaks. The wide scan in Figure 2 shows the positions where the various lead and sulfur species occur. The high background under the S 2p is apparent. Because the sulfur line exhibits such a large chemical shift per unit change in oxidation state, the position of the S 2s peak is a good indication of the oxidation state of sulfur. The Pb 4f lines also shift slightly as a function of the sulfur oxidation state, but this shift is not as large as that of the sulfur and is therefore only used as a check on conclusions drawn from the sulfur shift. As is evident from Table 11, all lines of the proposed products should be distinguishable from one another except for the lead 4f peaks of PbO in the presence of PbS. In this case the lead peaks of PbO occur at approximately the same energy as those of PbS. In order to get an idea 0 1978 American Chemical Society
The Journal of Physical Chemistry, Vol. 82, No. 18, 1978 1993
ESCA Studies of Lead Sulfide Oxidation
Flgure 1. Modified reaction chamber: (A) sample probe, (6)reaction chamber, (C) furnace, (D) gas inlet, (E) glass U-tube, (F) gas outlet.
'\
j/
%: - ;
--I__
146
' _ _ ,,\
140
134 234 228 Binding E n e r g y ( e V )
222
Figure 3. Pb 4f and S 2s lines of lead sulfide: (A) freshly vacuum deposited, (6) after oxidation for 5 min, (C) after oxidation for 36 h. Oxidation with dry O2at 200 OC, flowrate 60 mL/min.
I
1000
I
BOO
1
600
I
I
I
I
400
200
I
I
I
I
SPs/Pb 4 f ratio x PbO intensity
I
.-
0
Binding Energy ( e V )
Flgure 2. Wide scan of vacuum deposited lead sulfide with AI Kcu radiation.
of PbO surface concentrations peak ratios were used. Major Products. Upon reacting a freshly sublimed PbS film with dry O2 at 200 "C, a new sulfur line appeared at 232.7 eV as shown in Figure 3. This line continued to grow at the expense of the S 2s line due to lead sulfide at 225.5 eV. In addition the appearence of the new sulfur line, the P b 4f line, showed a high binding energy shoulder. Deconvoluting the Pb 4f envelope showed the presence of two peaks. The high binding energy P b 4f peak at 139.4 eV appears to be associated with the S 2s at 232.7 eV because their ratio remains essentially constant. In addition, the low binding energy P b 4f peak at 138.0 eV is not solely due to PbS since the ratio of the S 2s at 225.5 eV to the P b 4f at 138.0 eV is not constant. Also, when the peak due to the lead sulfide at 225.5 eV has disappeared, a low energy shoulder at approximately 138.0 eV is still present as can be seen in Figure 3C. Since this remaining low binding energy lead peak cannot be due to any sulfurcontaining lead species, it is concluded that it is due to PbO. This PbO species is not a final product, instead it is felt to be an intermediate, because extensive reaction with O2 leads to the complete disappearence of this peak. Reaction Mechanism. The presence of PbO indicates that the reaction is possibly proceeding by the following mechanism: PbS + 3/z02 PbO + SOz (1) PbO
+ '/202
-+
SO2
+
PbS04
(2)
This mechanism has been proposed previously by Hillenbrand.1° In order to verify the mechanism, ESCA was used to follow the loss of sulfur as a function of oxidation time. This loss is shown in Figure 4, curve A. The graph shows
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20 IXJ
od'
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I
I
I
I
I
I
5
IO
15
20
25
30
35
Oxidation Time (min)
Flgurb 4. Rehtive intensity vs. oxidation time: (0)sulfur loss (calculated as the area under all S 2s lines); (X) increase of nonsulfur lead species (PbO). Oxldation conditions are the same as for Figure 3.
an initial loss of sulfur, which appears to reach a steady state after about 15 min. The fact that a loss in sulfur intensity occurs is especially significant, because the peak area ratio of the S 2s to Pb 4f in PbS04 is greater than that of PbS (0.0432 vs. 0.0362). Therefore if no loss of sulfur was occurring the curve in Figure 4A should show an increase. The sulfur is probably lost as SOz, which agrees with previous workers who reported evolution of SO2from the reaction v e s ~ e l The . ~ ~ system ~ reaches a steady state when the PbO surface concentration is increased, by oxidation and loss of SOz, to the point where SOz is no longer desorbed. Instead the SO2reacts with a PbO species as soon as it is formed. This is confirmed by comparing Figures 4A and 4B. Figure 4B is a plot of the lead oxide surface concentration vs. oxidation time. The lead oxide P b 4f line intensity was calculated as follows: (Pb 4f)pbo = (Pb 4f)tota1- [(Pb 4f)pbs + (Pb 4f)PbSOdI (3) where (Pb 4f) corresponds to the area of the P b 4f signal due to the species indicated. Since the area ratio of S 2s
I994
The Journal of Physical Chemistry, Vol. 82, No. 18, 1978
D. S. Zingg and D. M. Hercules
0
IS
1:
B
'I 146
140
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222
538 532 526 Binding Energy ( e V )
Binding E n e r g y i e V )
Flgure 5. PbS oxidation by 12% O2 in He: (A) freshly deposited PbS, (8) after 1 h of oxidation; (C) after 3 h of oxidation. Oxidation at 200 "C, flowrate 60 mL/min.
1'-
Figure 6. 0 I s change upon oxidation of PbS in 12% O2 in He: (A) after 1 h of oxidation, (8)after 3 h of oxidation. Conditions are the same as those for Figure 5.
to P b 4f is known for both species (0.0432 for PbS04 and 0.0362 for PbS), eq 3 can be rearranged to
(4) where (S 2s) is the area under the S 2s line centered at the indicated binding energy. The nonsulfur lead species is considered to be PbO since it occurs at the same binding energy as PbS. Since curve 4A reaches a steady state at the same time as curve 4B, the indication is that PbO and SOz are the two species responsible for the steady state. This would be expected based on the proposed mechanism given in eq 1 and 2. According to the mechanism, if PbS is subjected to oxidation in an atmosphere with a low partial pressure of oxygen, the only product obtained would be PbO. A comparison of Figure 5A with 5B shows that reacting PbS with 12% Oz/He, the S 2s line at 225.5 eV decreased as was seen in Figure 3 but with no aparent increase in the S 2s line at 232.7 eV. This is carried to completion in Figure 5C, in which no sulfur lines appear. This indicates that an excess of oxygen, over and above that need for initial oxidation of PbS, is needed to form PbS04. In addition to seeing no new sulfur species no shift in the Pb 4f lines is observed, only a slight broadening. This also indicates the formation of PbO since the P b 4f lines for PbO and PbS are at approximately the same binding energy. Further evidence for the formation of PbO is shown in Figure 6. This figure shows the 0 1s spectra which were taken at the same time as the spectra in Figures 5B and 5C. By comparing Figures 6A and 6B with Figures 5B and 5C, it is seen that as sulfur is lost (as PbO is produced) a low binding energy shoulder at 529.1 eV appears in the 0 1s spectrum. This shoulder corresponds to the tetragonal form of PbO. Its presence gives conclusive evidence for the presence of PbO, since it is the lowest binding energy oxygen peak seen in any of the spectra. Although its presence is indicative of PbO, its absence does not necessarily indicate the absence of PbO. PbOtetregond is rapidly transformed to PbOrhombicin the occurs presence of O2.l1 The 0 1s peak due to PbOrhombic
s 2s A
I
146
140
134
235
I
229
223
Binding Energy ( e V )
Figure 7. Change in Pb 4f and S 2s spectra of PbO upon reaction wRh the oxidation products of PbS: (A) before reaction, (B) after reaction with the oxidation products of PbS for 1 h. Reaction with dry 02, temperature 200 OC (U-tube and furnace), flowrate 60 mllmin.
at 531.1 eV approximately the same binding energy as all the other 0 1s lines and therefore cannot be used to draw any conclusions. Although the evidence supports the presence of PbO, it was necessary to determine if the species evolved during the oxidation of PbS would react with PbO. To conduct this experiment the modified reaction chamber flow stream was used. In the glass U-tube was placed a sample of PbS; a lead oxide sample was placed on the probe and both the probe and the U-tube heated to the same temperature, in a flow of gas. Figure 7A shows the initial PbO spectra, The spectra obtained after flowing He at 200 "C were not different than the spectra shown in Figure 7A. When oxygen was used in the flow stream, the spectra changed drastically as shown in Figure 7B. The most dramatic
The Journal of Physical Chemistry, Vol. 82, No. 18, 1978
ESCA Studies of Lead Sulfide Oxidation
0
1995
IS
A
J B
J 146
u
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536 Binding Energy ( e V )
Binding E n e r g y ( e V 1
Figure 8. 0 1s spectra before and after reaction of PbO with the oxidation products of PbS: (A) before reaction, (6)after reaction with the oxidation products of PbS for 1 h. Conditions same as for Figure 7.
Figure 9. Effect of S02/02ratio on Pb 4f and S 2s spectra. SOz/02 is allowed to react with the interme’diate (PbO), which is produced by the reaction of PbS with 12% O,/He: (A) after reaction of the intermediate with 100% SO2,(6) after reaction of the intermediate with 12% S02/02.Temperature 200 OC, flowrate 60 mL/min.
change was the appearance of an S 2s peak at 232.7 eV indicating the formation of PbS04. In addition to the S 2s line indicating the formation of PbS04, the P b 4f line shifted approximately 1eV which confirms the presence of PbS04. The 0 1s spectra before and after the reaction with oxygen are shown in Figure 8. Initially, the 0 1s peak due to PbOtetragond is present but on oxidation only one peak appears. This peak could correspond to either to PbOrhoInbic or to the sulfate oxygen in PbSO4 On the basis of the P b 4f and the S 2s binding energy it is safe to say that the 0 1s peak also corresponds to oxygen in PbS04. Another parameter investigated was the effect of the two extremes of SO2 concentration on the final products. Initially, PbO was produced on the surface by reacting PbS with 12% 02/He. This PbO formed was then allowed to react with pure SO2and 12% SO2in Oz. The final spectra are shown in Figure 9. The reaction of this sample with pure SO2is shown in Figure 9A. The Pb 4f line indicates the presence of two different lead species. The low binding energy tail is due to some unreacted PbO. The new sulfur peak present at 230.5 eV is too low for the S 2s line due to PbS04, therefore is concluded to be due to PbSOB. The P b 4f binding energy of 138.7 eV confirms this. When this sample was oxidized in pure O2 the peak maximum shifted to 232.7 eV m expected for PbS04. The reactions occurring there are probably the following: PbO + SOz PbS03
-
PbS03 + ‘ / 2 0 2
+
PbS04
In the subsequent experiment, a new sample of PbS was oxidized to form PbO, and then exposed to 12% SOz in 02.The spectra are shown in Figure 9B. When compared with Figure 9A the most dramatic change is the shift in the S 2s line. When pure SOz was used in the flow stream, as in Figure 9A, the S 2s peak appeared at 230.5 eV, corresponding to PbSO3. With O2 present in the flow
stream, as in Figure 9B, this peak is shifted to 232.7 eV, a binding energy characteristic of PbS04. This is furthur evidence supporting the mechanism proposed in eq 1and 2.
Conclusions 1. As found by Manocha and Park,l the final product of PbS oxidation is PbS04, in presence of excess 02. 2. In addition, it was found that another nonsulfur lead species was present in the final oxidation product. This is probably the reason for the low binding energy value for the Pb 4f lines obtained by Manocha and Park for their oxidized PbS samples. 3. By making use of low partial pressures of O2 in a flow system the intermediate was isolated and identified as PbO. 4. The PbO intermediate was found to produce either PbS03 or PbS04 depending on the SOz/02 ratio. This may explain some reports of PbS03 being the final oxidation product. 5. The reports of a mixed PbO-PbS04 crystal may actually be the final product (PbSO,) plus some remaining intermediate (PbO) resulting from the loss of sulfur as SO2. 6. The formation of PbS203would be easily identifiable in ESCA due to the two sulfur peaks. This species was never seen.
Acknowledgment. This work was supported by the National Science Foundation under Grant CHE76-19452. References and Notes (1) (2) (3) (4) (5) (6) (7)
(8) (9) (10) (11)
A. S. Manocha and R. L. Park, Appl. Surf. Sci., 1, 129 (1977). H. Hagihara, J. Phys. Chem., 56, 610 (1952). H. T. Minden, J . Phys. Chem., 23, 1948 (1955). G. W. Poling and J. Leja, J. Phys. Chem., 67, 2121 (1963). B. Reuter and R. Stein, 2. Electrochem., 61, 440 (1957). H. Wilman, Proc. Phys. Soc., 60, 117 (1948). D. H. Klrkwood, Trans AIM€, 233, 708 (1965). K. T. Ng and D. M. Hercules, J . Phys. Chem., 80; 2094 (1976). T. A. Patterson, J. C. Carver, D. E. Leyden, and D. M. Hercules, J . Phys. Chem., 80, 1700 (1976). L. J. Hillenbrand, J. Phys. Chem., 73, 2902 (1969). K. S. Kim and N. Winograd, Chem. Phys. Lett., 18, 209 (1973).
