1650
Anal. Chem. 1985, 57, 1650-1657
Experimental and Theoretical Studies of Copper Ion Exchange on Compacted Lead Ion Selective Membrane Powders Vaneica Young'
Department of Chemistry, Texas A&M University, College Station, Texas 77843
Angular distribution XPS analysis and scannlng electron microscopy have been used to study copper Ion exchange on compacted lead ion selective electrode membrane powders. Untreated pellets, ammoniacal EDTA treated pellets, and perchloric acid treated pellets were studied. It Is shown that reductlve Ion exchange occurs. Because of this process, the Ag2S component of the pellet is not immune to interaction with Cu( 11) as expected from simple solubility consideratlons. I t Is demonstrated that these results can be predicted a priori using simple band structure caiculatlons.
Ion interference in solid-state ion-selective electrodes has classically been modeled as a surface ion exchange process followed by diffusion into the bulk (1). In order to determine the effect on the electrode potential, selectivity coefficients are often calculated; these coefficients are the product of two terms: an exchange equilibrium constant and an ion mobility ratio. More recently, it has been shown that bromide ions are rapidly incorporated into the bulk of silver chloride electrodes and a disruptive metathetic displacement mechanism has been proposed to explain the experimental results (2). Nevertheless, it was shown that the equilibrium composition of the membrane is in good agreement with the predictions of the exchange equilibrium constant. It appears, then, that this model is valid when the membrane in question is an electronic insulator. A model considering the effects of bulk concentrations, time, temperature, and stirring rate has recently been published ( 3 ) . Many ion-selective solid-state membranes are fabricated from sparingly soluble salts which are electronic semiconductors. One such example is the lead ion selective electrode membrane which is fabricated from a mixture of lead sulfide and silver sulfide. Both substances are semiconductors, E , = 0.37 f 0.06 eV and 1.00 f 0.08 eV for lead and silver suKdes, respectively (4). In such a case, the previously described model is not completely valid because it neglects a very real possibility: the occurrence of charge transfer processes between the membrane and the interfering ion. The occurrence of such processes can enhance corrosion processes on the membrane and even enhance minor corrosion pathways. Such is the case for Cu(I1) exchange on lead ion selective electrode membranes. In a previous fixed angle XPS study (5),we assumed a simple ion exchange process although it was noted that there was an increase in the membrane sulfate contaminant. In this paper, we have used angular distribution XPS to investigate the actual change in surface composition as a function of depth (6-8) for compact lead ion selective membrane powders which are initially untreated, treated with ammoniacal EDTA, and treated with 0.1 F HC104. The changes in the surface speciation which occurs for the initial states have recently been reported (9). The interaction of Cu(I1) with the pellet is explored theoretically using three-dimensional LCAO band Present address: Department of Chemistry, University of Florida, Gainesville, FL 32611.
structure calculations. Mechanisms are proposed for the oxygen corrosion and the Cu(I1) enhanced corrosion of these membrane powders. EXPERIMENTAL SECTION
The powders were prepared by coprecipitation of PbS and AgzS (1:l mol ratio) from a solution containing AgN03 and Pb(N03)z by addition of a solution of NazS. The precipitate was filtered, washed, and dried at 110 "C overnight. The powders are compacted into 7 mm diameter pellets using a Quick Press. Pellets were either untreated or exposed to 1 mL of 0.01 F ammoniacal EDTA or 0.1 F HCIOl for 15 min, washed in a deionized water jet, and blotted dry on a Chemwipe. All pellets were then immersed for 15 min in 1 mL of 0.1 F copper(I1) nitrate prepared from the reagent grade salt. The pellets were then washed in a deionized water jet and dried in nitrogen at room temperature. XPS spectra were recorded at takeoff angles of la0, 38",and 58" on a Hewlett-Packard 5950A ESCA spectrometer equipped with an angular distribution probe (Surface Science Model 259) and a dedicated memory upgraded HP 2825A computer system. The instrumentation and the computer software have been previously described (9). All binding energies have been referenced to the ubiquitous hydrocarbon contamination peak at 285 eV. The cross sections used for atom ratio calculations are the asymmetry corrected Scofield photoionization cross sections for the HP 5950A ESCA spectrometer (10). Micrographs for the pellets were recorded on a JEOL 25 SI1 scanning electron microscope at magnifications of 450X and 4500X. RESULTS AND DISCUSSION
SEM analysis has previously shown that the surfaces of untreated P b ISE membrane pellets consist of almost undistorted packed spheres with very small interparticle boundaries. By contrast, EDTA and HC104 treated pellets showed large regions on coalesced particles with large interparticle boundaries. The latter pellets also showed clusters of particles scattered on the surfaces and craters and crevices which undoubtedly reflect preferential attack of the reagents on certain regions of the pellet (9). In Figure 1,we show SEM micrographs of these same pellets after treatment with 0.1 F Cu(I1) solution. As before, the surface layer of almost undistorted spheres is lost from the untreated pellet. More significantly, a number of rodlike structures appear on the surfaces. They occur with high frequency on the pellet initially treated with EDTA, with low frequency on the pellet initially untreated, and are not apparent on the pellet initially treated with HC104. The dimensions of the structures are 5 2 Fm X 10 Fm, too small for us to characterize by Auger analysis with the instruments available to us at the present time. Relevant results have also been obtained from the angular distribution XPS studies. All of the pellets exhibit copper peaks and 50-eV scans of the Cu 2p level reveal the absence of satellites (Figure 2). This observation is consistent with the presence of "CuS" ( 1 1 , l . Z )the presence of CuSz (13), or the presence of Cu(1) salt. Candidates for the latter include CuzS, CuzO, and CuC03. Copper(1) sulfate decomposes in water. We may conclude that CuO, Cu(OH),, and the basic Cu(I1) carbonates are absent as these compounds exhibit satellite peaks on the Cu 2p levels. When the Cu 2p3p level is examined at high resolution, we find that the level can be
0003-2700/85/0357-1650$01.50/00 1985 American Chemical Society
ANALYTICAL CMMISTRY. VOL. 57. NO. 8. J U Y 1985 .
..
F @ n 1. SEM rnlaogaphs of (a) untreated Pb ISE pellets. (b) EDTA treated pb ISE pellets. and (c) %IO, treated Pb ISE pellets a with Cu(1l) solutbn (1. 450X: 2. 4500X).
resolved into two Gaussian peaks a t 932.9 f 0.2 eV and 933.9 0.2 eV. These peaks have an intensity ratio of (2.0 f 0.3):l ( F i i 3). They may represent chemically inequivalent forms of copper or they may reflect the inadequacy of fitting routines
*
10.51
h treabnent
which use only symmetric fitting functions. That is, a single chemical form of copper may be present With asymmetric core level lines. The Pb 4f levels exhibit a triplet of doublets (Figure 4) of which the Pb 4f,,, components occur at 137.4
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1975
Cu Exchanged Pellets Initially
untreated
Initially EDTA treated
A
i
e=='
I 936
033 8 E(cV1
930
936
833
930
936
Flgure 3. Example spectra of the Cu 2p,,,
I
9M
933 6 E(eV)
6 E(eV)
level In high resolution.
Cu Exchanged Initially CDTA t r e 3 t e a
1
145
142
139
136
B E(eV)
I 145
'842
139
136
B E(eV)
Figure 4. High resolution spectra for the Pb 4f level (see ref 9 for spectra of Cu(I1) exposed untreated pellets).
f 0.2 eV, 138.4 & 0.2 eV, and 139.3 & 0.2 eV, respectively. These same peaks have been observed on untreated, EDTA treated, and HCIOl treated pellets (9, 14) and have been assigned to PbS, PbO containing chemisorbed COZand H20, and PbS04, respectively. The Ag 3d level exhibits a single
spin-orbit split doublet; the Ag 3dsjzcomponent has a binding energy of 368.1 & 0.2 eV. In agreement with previous results, these results suggest that silver is present only as AgzS. In Figure 5, we compare the sulfur 2p levels with the corresponding levels for the pellets before copper ion exposure. The
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
174
168
162
Sulfur
2p
Spectra
I
U
%e
156 174
8 E(CV)
-
162
1
”
156 174
”
168
’
”
162
1053
’
0 E BY)
0 E levi
Flgure 5. Comparison of initial and final SlP levels.
differences in the spectra are readily apparent. Notice that in the sulfide region, a much broader band is observed for the copper treated pellets and that the structure observed suggests at last a doublet of doublets. Because the spin-orbit splitting for the S 2paI2and S 2 ~ , peaks , ~ is less than 1.0 eV, it is not easy to resolve the level into fours peaks with the correct intensity ratio. Since we were unable to obtain low values of x2 (goodness of fit parameter), we decided to resolve the level into two Gaussian peaks to similate two unresolved doublets. The higher binding energy Gaussian has a maximum at 164.0 f 0.3 eV. Again we recognize the possibility that Cu(I1) interaction may simply give a sulfide level subject to asymmetry broadening on the high binding energy side. Comparing the sulfate regions of the spectra, we notice that the Cu(I1) treated membranes have additional intensity on the low binding energy side. The binding energy of this shoulder is 167.9 eV and is characteristic of sulfur in the +4 oxidation state (15). We now utilize the time normalized intensity data in order to analyze the resulte. A good starting point is to assume that simple ion exchange occurs so that the lower binding energy bands in the S 2p spectra are due to PbS, Ag2S, and “CuS”, i.e., covellite. Previously we have shown that the ratio [Pb(137.6 eV) + Ag/2]/sulfide is equal to unity within experimental error (the overall value is 1.0 f 0.1 (9)), as one expects when this peak is due entirely to sulfide. Thus we calculate values for the ratio [Pb(137.6 eV) + Ag/2 + Cu]/sulfide assuming a single chemical form of copper. The results are shown in Table I as case I. It is obvious that the values are much lower than unity (the overall value is 0.6 f 0.1). The results imply an enhancement of sulfur in the membrane surface. Another possiblity is that the excess intensity in the sulfide region is due to elemental sulfur. Let us assume that the higher binding energy Gaussian is due to elemental sulfur. Thus, for case 11,we assume that the intensity in the “sulfide” region is due to PbS + Ag2S + CuS + So. Calculated values for the ratio [Pb(137.6 eV) + Ag/2 + Cu + S]/”sulfide” are shown in Table I. These values are much closer to unity (overall value is 1.0 f 0.2). A third case assumes that the intensity in the sulfide region is due to PbS + AgzS Cu2S. Calculated values for the ratio [Pb(137.6) + Ag/2 + 2Cu]/ “sulfide” are shown in Table I as case 111. These values are also close to unity (overall value is 0.9 f 0.2). For case IV, we assume that the intensity in the sulfide region is due to PbS + AgzS + C U ~ + S So. Calculated values for the ratio [Pb(137.6) + Ag/2 + Cu/2 + s]/“sulfide” are also shown in
+
Table I. Metal to S Ratio for Five Possible Speciations
e treatment none case I case I1 case I11 case IV case V
18O
38”
58’
0.6 0.9
0.6 0.9 1.1 0.7 0.5
0.8 0.8 0.8
0.7
0.6
0.9
0.8 0.8
0.8 0.8
0.7
with ammoniacal EDTA case I case I1 case I11 case IV case V with HCIOl case I case I1 case I11 case IV case V
0.6 1.4 0.9 0.9 0.7 0.6 1.1 0.9 0.9 0.8
1.1
0.7
0.7 0.6
0.5
0.7 0.6
0.7
0.5
1.0
0.8 0.7 0.8 0.7
1.1 0.8 0.6
Table I. There is greater deviation from unity (overall value is 0.8 f 0.1). As a fifth case, we may assume that there is no copper sulfide species in the membrane, i.e., copper exists as CuzO or Cu2C03or some combination of the two. The intensity in the sulfide region must then be due to PbS + Ag2S + So. Deviation from unity for the calculated values of the ratio [Pb(137.6) + Ag/2 s]/“sulfide” (Table I; overall value = 0.6 f 0.1) is identical with that for case I. There is a sixth case-the intensity in the sulfide region can be due to PbS + Ag2S + Cu2S + CuS + So. Unfortunately, there are not enough degrees of freedom in the intensity data to treat this case. We conclude that cases I and V are not consistent with the data. We recognize case VI as being intermediate between cases 11and IN,and as such it would be expected to be a better candidate than case IV itself. Thus, on the basis of the above results, cases 11, 111, and VI are viable candidates. At this point, we can make a number of significant observations independent of which case is the correct one. CuS2 is metallic; it has the structure Cu+S2-(13). Covellite is actually a mixed valence compound with the structure [CU~~+S~-CU+S~-] (16).Thus the result indicate that copper occurs as Cu(1) in the membrane. It is obvious that charge transfer occurred between Cu(I1) and the membrane and that
+
1654
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985 Untreated pellet Ini t la I ly Treated with C u ( E )
EDTA Treated pellet Initially Treated wlth C u ( T [ )
HC104 Treated pellet Initra I ly Treated wlth Cu ( E )
- t
6
-2
Q
t
w-
0.50
-
t
0
Figure 6.
40
20
80
60
-
0
20
60
40
80
20
0
-
40
60
-
0 (Degrees )
0 ( Degrees )
Depth
0 (Degrees)
Depth
Depth
80
Initial and final state sulfur distributions: 0, sulfide: 0 , S 2p at 164 eV; 0,sulfate. Untreated Pellet _ _ In _ - I. it ial y_ - Treated with Cu(E)
EDTA Treated Pellet
100
b" .
1
--___ Initially _
_.__--
Treated with Cu(II)
-
100
w-
-Treated with CU(E)
+-----+-----+
. . d
-
0 50 -
050-
b '
Q
20
40
60
-
0 (Degrees) Depth
80
20
40
60
80
20
-
40
60
80
-
e ( Degrees)
8 ( Degrees 1 Depth
Depth
Figure 7. Initial and final state silver and copper distributions: 0, silver: 0 , copper.
the process causes the oxidation of some sulfide to either Szor ST So. Oxygen corrosion of PbS has been shown to give PbO and PbS04 as the final products (17); hence interaction of the membrane with Cu(I1) enhances an alternant oxidation pathway. It is also clear that it is invalid to fit the Cu 2p3I2 level with two symmetric peaks. Because both covellite and CuSz are conductors, the deep hole produced by core ionization causes electron-hole excitation at the Fermi level; this process causes asymmetry on the high binding energy side of core levels and a proper fit can only be made with Doniach-Sunjic functions (18). We now turn to the literature for help in further defining the possibilities. It has been shown that at 25 "C, CuSz is stable only above 8 kbar, but it is metastable for periods of several months (19). Since these reactions were conducted at atmospheric pressure, it seems that 111is not a viable candidate even though it fits the experimental data. The remaining cases indicate that elemental sulfur is formed on the membrane surfaces. Coetzee et al. (20) reported detecting sulfur on copper ion selective electrodes by carbon disulfide extraction. Consequently, we tried extracting an untreated lead ion membrane pellet and untreated, EDTA treated, and HC104treated membrane pellets after treatment with Cu(I1) with spectrometric grade carbon disulfide. The results were inconclusive. Sulfur gave a band with A,, = 375 nm, but a spectrum of CS2 vs. CS2 gave a small peak at the same X with absorbance equal 0.09. Evidently the spectrophotometric grade carbon disulfide contains a small sulfur contamination and the tolerances of the matched cells do not allow complete cancellation. We then went to the solvent extraction literature where we found that sulfur at a concentration of 5-40 pg/mL can be detected by spectrophotometric analysis a t 275 nm if cyclohexane is used (21). How-
+
ever, even with monolayer sulfur coverage on both surfaces of the pellet, the total amount of sulfur present would be only -0.09 pg. It is not possible for us to detect this amount of sulfur spectrophotometrically even with a microcuvette (400 pL volume). Comparisons of the distribution of the various species with the initial distributions are shown in Figures 6-8. Notice in particular the results obtained for pellets initially treated with perchloric acid. Perchloric acid treatment virtually eliminates lead from the surface of the pellet, but after Cu(I1) treatment the amount of silver in the pellet surface is reduced by a factor of 5. The same kind of changes are observed for the ammoniacal EDTA pellets, but to a lesser extent. Simple ion exchange of Cu(I1) with Pb(I1) can be described by the equation Cu2+(aq)
+ PbS(s)
CuS(s)
=i
+ Pb2+(aq)
This equation implies that there is no change in the oxidation state of Cu(I1). We have demonstrated above that the experimental results are not consistent with such a process. It is worthwhile to consider whether or not simple three-dimensional LCAO band structure calculations can predict this result. The band structure for PbS has been calculated (22, 23). Qualitative molecular orbital concepts have been used to study the relative binding energies of metal d and ligand p orbitals in Cu2S and Ag,S (24). No theoretical band structure for covellite has been calculated. It is not possible to compare the literature band structures for PbS and AgzS directly, because different methods were used to do the calculations. PbS has the rock salt structure, and we may calculate a theoretical band structure for covellite by completely replacing Pb by Cu in the PbS structure. This exercise allows
ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985 Untreated pellet Initially ___ Treated w i t h C u ( l I )
EDTA Treated pellet Initially Treated with Cu
--:
______
3 2 3 [
______
e
31
_ _ - - --
OZ:l
.6 .-.
021:
. 6
*
-
---+
d
--2--_
w-
O’z:
a
i
HClQ
-----
_-----
40
20
60
80
0
o’2:
with C u ( I I )
_ _ _ _ _ *---- --
-
E
----_
r
r
0
t-
:O
+
Treated pellet
- - - - - - - Initially -Treated
(E)
-4
1655
20
-
60
40
80
20
-
40
60
80
-
0 ( Degrees 1
0 ( Degrees)
0 ( Degrees)
Depth
Depth
Depth
Flgure 8. Initial and final state lead distributions: 0, lead sulfide; 0 , oxygen containing lead species: 0,lead sulfate. a ) Lead Sulfide
D)
c ) Silver Sulfide
Adsorbate Levels
L r X K r L r XK Figure 9. Energy dispersion curves for (a) PbS, (b) “CuS”, and (c) Ag,S.
us to obtain an “adsorbate” band for copper. The room temperature stable form of Ag@, acanthite, has a complex structure; however, the high temperature form has &(I) partly in tetrahedral and partly in octahedral coordination (4).For the latter form, we may obtain a qualitative picture of the valence band structure as follows. First we assume that half of the silver ions and all of the sulfide ions form a fcc lattice with the cell dimensions determined by the sulfide ions. Then we assume that the remaining silver ions plus the lattice sulfide ions would give the zinc blende structure; i.e., we place the extra silver ions in tetrahedral sites. We then approximate the band structure of Ag2S as a composite of the fcc and zinc blende calculations. The positions of the band due to the sulfide ions are not changed much for the fcc VI. the zinc blende calculations, hence we postulate that the latter calculation allows us to deduce the band for tetrahedral silver. The matrix elements for the fcc calculations were generated by using the equations of Slater and Koster (25) while those for the zinc blende calculation were generated by using the equations of Harrison (26). Initially, we used the HermanSkillman orbital energies for the various atomic levels (26); however both the Cu 3d and Ag 4d values are too large and the calculated valence band density of states is not in qualitative agreement with experiment (16,27, 28). Therefore, we have used the atomic data of Moore (29)to calculate orbital energies as shown in Table 11. Since no atomic data exist for the 6s level of lead, we have used the Herman-Skillman values for both the P b 6s and P b 6p levels. We feel this is justified, because the agreement between the Herman-Skillman orbital energy for P b 6p and the orbital energy for P b 6p calculated from atomic data is very good. The complex Hermitian matrices which are obtained are solved by using
r
L
r
X K
r
Table 11. Orbital Energies for the Valence Levels of Sulfur, Copper, Silver, and Lead orbital energy Herman-Skillman atomic data
atom
level
sulfur
3P 3s
10.27 20.80
10.36 19.29
copper
4P 4s 3d
1.83 6.92 20.14
3.91 7.72 9.21
silver
5P 5s 4d
2.05 6.41 19.21
3.83 7.59 11.54
lead
6P 6s
5.77 12.07
5.65
oxygen
2P
14.14
12.61
a program, written by us, which incorporates the EISPACK subroutines (30). The calculated energy dispersion curves are shown in Figure 9. The simple LCAO calculation on lead sulfide is in qualitative agreement with previous results; however, a band gap of -0.13 eV (the Pb 6s and Pb 6p bands overlap slightly) is predicted. Because the calculated overlap is small, a “pseudogap” is predicted. In this case, one finds localized states between the valence and conduction bands (31). Hence the calculations predict a narrow band gap semiconductor with E, = 0.13 eV. This value is a factor of 3 too small. The calculations on Ag2S are consistent with the expectation that silver is present as Ag+ (the Ag 4d bands lie beneath the top
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ANALYTICAL CHEMISTRY, VOL. 57, NO. 8, JULY 1985
Table 111. Postulated Mechanism for Cu(I1) Exchange at “Lead”Sites in the Metal Ion Sublattice
equation
comment
1. Cu2”(aq)F? Cu2+(ad) 2. Cu2+(ad)F? Cut(ad) H* 3. Cut(ad) + V‘M t CU-M 4. PbM t v ‘ ~PbZt(ad) 5 . Pb2+(ad)F? Pb2+(aq) 6. h* Ss F? Sts 7. h* Scs z S2+S 8. S+s + Sts ii V‘s (SJS 9. h* + (S2)s t (Szt)s 10.s+s + s2+s rt VIS (S2+)s overall for Cu(I1) interaction: Cuzt(aq) + PbM li Pb2+(aq)+ CU-M+ h*
+
contact absorption charge transfer
+
+ +
Table IV. Postulated Mechanism for Cu(I1) Exchange at “Silver” Sites in the Metal Ion Sublattice
+ +
elemental sulfur
s2S2-
of the sulfide 3p band); however a band gap of 3.04 eV is calculated. This value is a factor of 3 too large. Now let us see what these results predict about Cu(I1) interaction with pellets of P b ISE membrane powders. Notice that the Cu 3d levels lie lower than the P b 6s band of PbS. Thus, the calculations predict that electron transfer will occur from the P b 6s band to Cu(I1) to form Cu(1). However, as shown by the SEM results, these membranes are particulate in nature. I t is fair to ask if band structure calculations have any relevance to such systems. We have shown previously that the particles have an average diameter of -0.25 pm (9); this is considerably larger than the particle size at which a transition to bulk electronic properties occur (32). Thus, the band structure calculations reveal the electron energies within an individual particle. At the boundary between any two particles, we have a discontinuity. At discontinuities, band bending occurs. This is a fundamental process which will occur at solid-solid, solid-liquid, or solid-gas boundaries. For compacted ZnO particles, it has been shown that the bands bend up at the interparticle boundaries (33). Thus, we expect the calculated bands to bend up at the membrane particle boundaries. This means oxidative attack by Cu(I1) will be even more favorable at the interparticle boundaries. We conclude that when a band structure calculation indicates oxidative attack, the probability of occurrence is certain. When a band structure calculation indicates that oxidative attack should not occur, there is still a probability that it can occur at interparticle boundaries. On the basis of the experimental results and the theoretical calculations, a sequence of events involving the formation or consumption of point defects, occurring when a lead ISE membrane pellet is exposed to Cu(I1) solution, may be postulated as shown in Table 111. Although several notation schemes are in common use, we employ here the notation of Kroger and Vink (34) as frequently modified in the catalysis literature (35). Consider a pure ionic compound M2+AZ-.There are two sublattices-a metal ion sublattice and an anion sublattice. A resident occupied lattice site is represented as MM or AAand is considered to have a charge of zero. An unoccupied lattice site, vacancy, is symbolized by V with a symbol subscript to indicate the sublattice in which it occurs (VMor V,) and a superscript to indicate the charge. A vacancy has a charge equal and opposite to that of the ion which belongs in that location. Unit negative charge is symbolized by the superscript “prime” and unit positive charge by the superscript (but “asterisk” is now commonly used in the catalysis literature). Thus a vacancy in the metal ion sublattice of M2+A2-is VIM. For OCcupied sites, plus and minus are often used in the catalysis literature to indicate the site charge. (The reason this is not done for vacancies is because vanadium has the symbol V, so that ambiguities are present.) Thus one may find an N+ ion in the metal ion sublattice of M2+A2-represented as NM- or NIMand an X- ion in the anion sublattice as XA+,XA’or XA*. “s’’
equation
comment
1. CuZt(aq)s Cu2+(ad) contact absorption 2. Cu2+(ad)t Cu’(ad) + h* charge transfer 3. Cut(ad) + V‘M e CU-, 4. Ag-, s V‘, + Agt(ad) 5. Agt(ad) s Agt(aq) 6. 10 as in Table I11 11. Cut(ad) + Agt, s Cuti Agt(ad) overall for Cu(I1) interaction: Cu2’(aq) + Ag-, s Agt(aq) + Cu-, + h* Cu2+(aq)+ Agt, s Agt(aq) + Cut, h*
-
+
+
An interstitial is indicated by the symbol of the occupant with the subscript ‘5’’ and a superscript to indicate the charge. Since interstitial sites are normally empty, an occupied interstitial carries a charge equal in sign and magnitude to that of the occupant. For a mixed salt, naming point defects can become more difficult. We can take the present case as an example. Should we assume that there are two metal ion sublattices (one for lead and one for silver) or should we assume a single metal ion sublattice which can be occupied by two different kinds of ions? Because we have assumed the latter in the band structure calculation, we indicate the mixed salt as MS. When a silver ion is present in the metal ion sublattice, Ag-M, another silver ion must be present in an interstitial site. Thus the pair Ag-MAgf,(note: this does not imply a localized pair, Ag+, could be surrounded by PbM)has the same charge as PbM. Initially, the Cu(I1) ions are present at the outer Helmholtz plane (OHP) of the pellet and the concentration of Cu(I1) at this plant is expected to be equal to the bulk solution phase concentration of Cu(I1). Contact adsorption (eq 1) of Cu(I1) may be defined as a process by which Cu(I1) moves from the OHP to the inner Helmholtz plane (IHP) (36). However, electron transfer will occur from the pellet to adsorbed Cu(I1) (eq 2 ) . Here h* indicates that a hole is formed in the valence band of the pellet. What is the fate of Cu+ (ad)? It can enter a metal ion vacancy, V’lM (eq 3); but the effective charge of the vacancy, 2-, is reduced by only I since Cu(I1) carries a charge of only If (Cu-M). Since there can be no exchange current, lead in a lattice site must be displaced to the IHP and must subsequently be desorbed (eq 4 and 5). The overall equation for the reductive ion exchange of Cu(I1) is shown a t the bottom of the table. The holes produced must, of course, be consumed. Steps 6 and 7 show that elemental sulfur can be produced by the stepwise absorption of two holes by a sulfide ion. Another possible product is Sc which can be produced by two pathways: the combination of two S- ions to give S z - followed by absorption of a hole (steps 8 and 9) or combination of S- with neutral sulfur (step 10). These latter processes each require the adsorption of three holes. It is important to realize that if Cu(1) can migrate on the membrane surface, it can also exchange with silver. The postulated sequence of events is shown in Table IV. The only new wrinkle is that Cu(1) can replace silver in interstitial sites. Notice that exchange of Cu(1) and Ag(1) is predictable from soubility considerations since the Ksp values are similar. Exchange of Cu(I1) with Ag(1) is not predicted from K,, considerations. Thus, theoretical predictions are in accord with experimental observations. The experimental results show an enhancement of sulfate and the presence of intensity as a shoulder on the low binding energy side of the Szpsulfate peak. This latter peak is most likely due to adsorbed sulfur-oxygen species. Thus it is obvious that oxidative corrosion has been enhanced on the pellets. In order to explain these results, we first consider the oxygen corrosion of PbS. The postulated sequence of events
ANALYTICAL CHEMISTRY, VOL.
Table V. Postulated Mechanism for Oxygen Corrosion of Lead Sulfide equation
1. Odg)
comment
* Odad)
2. 02(ad) s 20(ad) 3. O(ad) s V'Pb + h* + 0's 4. Ss + h* e S'S 5. S'S + h* s S2+s 6.0's ~r h* + 0 s 7. 20(ad) StS + SO^)'^ 8. SO^)'^ O+s s (SO& V', 9. O(ad) + (SO& (SO& 10. (S02)+s + V', + e- + (SOz)(ad) 11. ( S O d a d ) s S O k d
+
+
+
contact absorption dissociation charge transfer formation o f neutral sulfur formation o f oxide
57, NO. 8, JULY 1985
1057
enhance the most probable corrosion pathways and activate alternate pathways. We have also demonstrated the value of band structure calculations as a predictive tool in elucidating the effect of interfering ions on such membranes. As an aside, we note that conflicting reports for various corrosion products on semiconducting sulfides may be due to the presence of ions in solution which can enhance alternant pathways. Registry No. Cu, 7440-50-8; Pb, 7439-92-1; PbS, 1314-87-0; Ag,S, 21548-73-2. LITERATURE CITED
formation o f sulfite formation o f sulfate
is shown in Table V. Numerous corrosion products for oxidation of PbS have been proposed (37);this mechanism can account for So, PbO, PbSO,, and PbS04. In step 3, 0- is formed at what would be sulfide sites in the extended lattice. Sites which would be occupied by lead in the extended lattice are vacant, hence V I P b . The ion 0- has been shown to be the ultimate form of adsorbed oxygen on silver metal (38)and has been shown to be the species which migrates into bulk strontium metal during its oxidation (39). Such migration can explain the presence of oxide below the surface. The formation of sulfite and sulfate has been shown as localized at the surface (i.e., O(ad) is involved), although equations similar to 7 and 9 could be written using 0- instead of O(ad) (implying that surface and subsurface formation of sulfite and sulfate can occur). Experimentally, sulfate is preferentially segregated at the surface, so that subsurface sulfur-oxygen containing anions must migrate to the surface to give qualitative agreement with experiment. Because of the mass of the ions, such migrations should be much less probable than 0- migration. Therefore, these processes have been omitted from Table V, although there is no evidence to rigorously exclude the migration of sulfur-oxygen containing anions. As expected in any multiple product system, many of the steps compete for intermediates. Zingg and Hercules (17) have shown that the final products of PbS corrosion are PbO and PbS04. This means that of the competing processes (eq 5 and 7), eq 5 is slow (i-e., sulfur-oxygen containing species are formed preferential to neutral sulfur). This can be understood because the concentration of holes is limited. The holes produced in eq 3 are consumed in eq 4 to produce the intermediate S-needed by both steps. Notice that eq 6 and 8 compete. If there is a limitation in the amount of SOz- (caused by limited O(ad) or due to desorption of SOz), the oxide will be preferentially formed. If O(ad) is plentiful, then sulfate will be formed. These predictions are in accord with the results of Zingg and Hercules (17). Now, let us consider what happens when Cu(I1) is present in solution. Since the processes shown in Tables I11 and IV generate holes, it is evident that the formation of SS+ (eq 4) and Ss2+ (eq 5) are both enhanced. Hence, we get increased sulfate and the formation of Sz-and/or neutral sulfur. CONCLUSIONS
It has been shown that the simple ion exchange model is n o t generally valid for solid-state ion-selective electrode membranes when the membrane components are electronic semiconductors. It has been shown that interfering ions can
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RECEIVED for review January 24,1984. Resubmitted December 26, 1984. Accepted March 28, 1985. These results were reported in part at the 184th National Meeting of the American Chemical Society, Kansas City, MO, Sept 12-17,1982. Partial support of the Robert A. Welch Foundation, Grant No. A-771, is gratefully acknowledged.
