J. Phys. Chem. 1995,99, 11967-11973
11967
EXAFS Investigations of Zn(I1) in Concentrated Aqueous Hydroxide Solutions Kaumudi I. Pandya? Andrea E. Russell: J. McBreen? and W. E. O'Grady*J' Physics Department, North Carolina State University, Raleigh, North Carolina 27695, Department of Chemistry, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, U.K., Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973, and Code 6170, Naval Research Laboratory, Washington, D.C. 20375-5000 Received: March 28, 1995@
The structure of the Zn2+ species in concentrated aqueous solutions of NaOH, KOH, RbOH, and CsOH was investigated with extended X-ray absorption fine structure (EXAFS) spectroscopy. The results show that the Zn2+ species exist in a tetrahedral configuration with a Zn-0 bond distance of 1.96 f 0.01 A, independent of the cation of the hydroxide electrolyte. No evidence for higher Zn-HzO coordination shells, aggregation of the Zn(OH)42- tetrahedra, or Zn-Zn interactions were found. The necessity of including the multiplescattering contributions to obtain a complete and correct EXAFS analysis is clearly shown.
Introduction
The coordination of the Zn2+ ion in concentrated aqueous hydroxide solutions has been the focus of a long debate. Since 1903 numerous attempts have been made to unambiguously identify the zinc-containing ions using various electrochemical and spectroscopic techniques. The earliest determinationswere made by electrochemical methods, including measurements of the potential of cells containing zinc in aqueous sodium hydroxide solutions'$2and potentiometric titration^.^ These measurements indicated the presence of two species, Zn0z2and HZnOz-. However, such electrochemical methods were complicated by the presence of junction potentials. Further, potentiometric titrations by Dirkse? using cells in which the junction potentials were negligible, indicated that the zinc existed as the zincate ion, Zn(OH)42-. Subsequent R a m a ~ ~infrared ,~.~ reflection: and proton NMR7,8 experiments provided further evidence of the tetrahedral Zn(OH)42- species. However, there continues to be a significant discussion about the structure of the zincate ion because the solubility of ZnO in concentrated hydroxide solutions formed by the anodic oxidation of zinc electrodes is higher than that obtained from the equilibration of solid ZnO with the concentrated hydroxide solutions at the same temperature. A number of authors have claimed that Zn(OH)42- by itself will not account for the reported solubility. In attempts to explain the solubility observed, several other species have been suggested, such as [Zn(OH)3(H20)]- and [Zn(OH)2(H20)2].8-11Polymers formed by linking the Zn(OH)4 tetrahedra via bridges between two oxygen atoms have also been proposed. In a Raman and 67ZnNMR study of unsaturated and nearly saturated solutions, Cain et al. found that the structure of Zn species was consistent with that of tetrahedral Zn(OH)42with some possibility of aggregation of the Zn(OH)42- tetrahedra via hydrogen bonding or bridging oxygens at high zinc concentration^.^ A preliminary 67ZnNMR, neutron diffraction, and EXAFS study proposed a general formula for zincate ion of [Zn(OH)42H20I2-, with a structure consisting of a planar Zn(OH)42- with two H20 molecules in the axial positions.I2 In a subsequent decomposition kinetics study these same authors proposed that the supersaturated species were bridged zincate North Carolina State University. University of Newcastle upon Tyne. 5 Brookhaven National Laboratory. II Naval Research Laboratory. Abstract published in Advance ACS Abstracts, July 1, 1995. +
@
dimers.I3 In a recent EXAFS study McBreen supported the [Zn(OH)42H20I2- structure proposed by Debiemme-Chouvy et al. McBreen suggested that the Zn is in a tetragonally distorted octahedral coordination consisting of planar Zn(OH)4 species with a Zn-0 bond length of 1.96 A and two axial waters with a Zn-H2O bond length of 3.34 A.14 Both McBreen and Debiemme-Chouvy found that the EXAFS data for the saturated and supersaturated solutions were i d e n t i ~ a l . ' ~ . ' ~ Nevertheless, even in the saturated solutions the structure of the zincate ion has not been definitively resolved, and discrepancy still exists whether the Zn2+ ion is in a tetrahedral or octahedral configuration. In this paper, we report EXAFS data obtained from a series of concentrated aqueous hydroxide solutions (NaOH, KOH, RbOH, CsOH) saturated with Zn2+. The purpose of this study is to establish the structure/configuration of the zincate ion in saturated aqueous hydroxide solutions and to demonstrate the necessity of utilizing multiple scattering in EXAFS data analysis. This is necessary to avoid attributing a peak at a higher bond distance, due to multiple scattering, to a higher coordination shell, leading to the interpretation of additional coordinating ligands or to zincate aggregation/ bridging, as done in several recent publication^.'^.'^ Experimental Section Sample Preparation. Zinc oxide (New Jersey Zinc USP19) and potassium hydroxide (J.T. Baker Co., low-chloride pellets) were used as received to prepare the electrolyte (0.74 M ZnO 8.4 M KOH). Electrolyte samples were also prepared containing 0.74 M ZnO in 8.4 M NaOH (Mallinckrodt), CsOH (Aldrich), and RbOH (Aldrich, 50% wt aqueous solution). In addition to the electrolyte samples a solid ZnO standard was prepared as a BN pellet. X-ray Absorption Experiments. EXAFS measurements were performed at beamline X-l1A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. Thin layers of the electrolyte sample (-0.25 mm thick) were held in a polyacrylic cell for liquid samples with 0.75 mm thick polyethylene windows.15 The EXAFS data for the electrolyte samples and the ZnO solid standard were collected at the K-edge of zinc (9659 eV) at room temperature in the transmission mode. The NSLS storage ring operated at 2.52 GeV beam energy with ring currents between 40 and 200 mA. The monochromator was operated in the two-crystal mode using Si( 111) crystals. The monochromator was detuned by 20% to reject the higher
+
0022-3654/95/2099-11967$09.00/0 0 1995 American Chemical Society
11968 J. Phys. Chem., Vol. 99,No. 31, 1995
Pandya et al.
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14
t
16
Figure 1. EXAFS spectra for the (a) ZnONaOH, (b) ZnOKOH, (c) ZnORbOH, and (d) ZnO/CsOH samples.
harmonics present in the beam, and the energy calibration was achieved by measuring the EXAFS of a 7 p m thick Cu foil (K-edge 8979 eV). The intensities of the incident (lo) and transmitted (I) X-rays were measured with gas-filled ion chambers. The gas compositions of the ion chambers were adjusted such that the l o and I chambers absorbed 15% and 80% of the incoming X-rays, respectively. Each absorption spectrum required approximately 20 min to measure. Two scans were measured on each sample. These scans were compared for consistency, and data analysis was carried out on one of the sets of data. Data Analysis and Results
EXAFS has proven to be a very useful tool for probing local atomic environments in a variety of systems. The materials studied can be in the form of a solid, liquid, or gas and do not need to possess any long-range order. Analysis of the EXAFS spectra yields the interatomic distances (R), the numbers of neighbors in coordination shells (N), and a measure of the disorder of the system, the Debye-Waller factors (a2).Recently, EXAFS has been applied to the study of several electrolyte solution systems.16-20 The technique can even make it possible to sort out ion-ion and ion-solvent interactions. However, the analysis of the EXAFS spectra is not straightforward and relies on comparison with the backscattering phase shift and amplitude functions from reference compounds or theoretical calculations for each absorber-backscatterer pair. The EXAFS analysis is generally performed using a singlescattering approximation. In this paper, it is shown that the multiple-scattering processes become important in the analysis of higher shells, which if not included in the analysis, may result in an incorrect structure determination.
Figure 2. RSFs, A&= 2.5-13.4 A-': (a) k3 weighted; (b) k' weighted, (- NaOH), (- - - KOH), and (.*. CsOH).
The details of the basic data analysis procedures have been described previously.*' Briefly, an EXAFS function X(k) was separated from the measured absorption data by a second-order polynomial removal from the pre-edge data, a cubic spline background removal from the post-edge data, and normalization to a per atom basis by dividing through by the edge-jump. The normalized raw (unfiltered) EXAFS spectra for all the solutions are shown in Figure 1. The signal-to-noise ratio is excellent for all the samples with the exception of the ZnO/CsOH sample, for which the sample thickness was not optimal due to the large Cs absorption. The corresponding radial structure functions (RSFs) are shown in Figure 2. The ZnO/CsOH sample is not included in this figure because of its lower signal-to-noise ratio. The RSF as well as the imaginary component of the Fourier transformation (1") are almost identical for all the samples, indicating that the local structure of Zn is not affected by the cations of the hydroxides (NaOH, KOH, RbOH, or CsOH). Fi ure 2a,b show a major peak centered at approximately 1.6 followed by a broad peak between 2.4 and 3.6 A. Preliminary data analysis showed that the first peak is due to oxygen neighbors. The second peak gives information about the long-range structure of the Zn ions. Although its intensity is substantially lower than that of the first peak, the excellent S/N ratio allows a detailed analysis to be made. Reference Compounds. Experimental backscattering phase shifts and amplitude functions were obtained from the EXAFS of crystalline ZnO and Cu metal. The normalized EXAFS data x(k) measured at room temperature for the ZnO standard and the corresponding RSF are shown in Figure 3. The coordination shells in the RSF are well separated, allowing the Zn-0 shell EXAFS function to be isolated by carrying out an inverse Fourier transformation. The crystallographic data22 and the Fourier transformation parameters used for isolating the various interactions are listed in Table 1. The first Zn-Zn shell in ZnO
1
Zn(I1) in Concentrated Aqueous Hydroxide Solutions
J. Phys. Chem., Vol. 99, No. 31, I995 11969 TABLE 2: Fourier Transformation Parameters Used for Isolating the First-Shell Zn-0 Interaction sample k weighting Ak (kl)Ar (A) Ak fitting ZnO/NaOH 3 2.5-14.0 0-2.10 2.5-14.0 ZnO/KOH 3 2.5-13.4 0-2.10 2.5-13.0 ZnOmbOH 3 2.5-13.4 0-2.10 2.5-13.0 ZnOICsOH 3 2.5-12.0 0-2.10 2.5-10.0 ~~
- 15 0
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R(N Figure 3. (a) EXAFS spectrum for the ZnO model compound and (b) corresponding RSF (k3weighted, Ak = 2.5-14.0 A-1). TABLE 1: Crystallographic Data and the Fourier Transformation Parameters Used To Prepare the Reference Zn-0 and Cu-Cu Interactions
shell Zn-0
cu-cu
Ak (.k')
Ar (A)
N
R(A)
2.5- 14.0 2.5-14
0-2.10 1.5-2.7
4 12
1.97 2.552
contains two subshells, Zn metal is hexagonal close packed, and its first Zn-Zn shell also contains two subshells, making it impossible to use either of these materials for a Zn-Zn reference standard. It has been demonstrated that it is possible to use the phase and amplitude of the element adjacent to the X-ray-absorbing element in the periodic table as a reference standard.23 Hence, the f i s t Cu-Cu interaction in Cu metal was 'used as the reference to analyze the Zn-Zn interaction in the solutions. Analysis of the First Shell. The fust-shell Zn-0 interaction was analyzed using the reference phase and amplitude functions derived from the EXAFS of crystalline ZnO. The Fourier transformation parameters used for isolating the interaction are listed in Table 2. An isolated EXAFS function was fitted in k space using a nonlinear least squares fitting routine. The quality of the fit was monitored in both k space and r space using the k l - p technique.24 The experimental and the fitted spectra for the ZnO/NaOH sample are shown in Figure 4, and the structural parameters are listed in Table 3. Similar high-quality fits (not shown) were obtained for the ZnOKOH, ZnORbOH, and ZnO/ CsOH samples. The EXAFS analysis was also carried out using 0.5 M ZnS04 aqueous solution containing Zn(H~0)6~complexes as the reference compound (Nref = 6, Rref = 2.05 A), and similar results are obtained. These results show that Zn has four nearest oxygen neighbors and the Zn-0 bond length is 1.96 8, for all the solutions. Analysis of the Second Shell. The analysis of the second shell is complicated because of its lower intensity. Its nature was further investigated by carrying out various Fourier transformations. Figure 5 compares RSFs for the ZnO/NaOH sample, for 2.6 < k -= 13.4 8,-' (solid line) and 3.5 k 13.4 (dotted line). Surprisingly,the intensity of the second shell decreases significantly if the low-k region ( k -= 3.5 is
excluded from the Fourier transformation range, indicating the presence of low-2 neighbors such as oxygen. To further investigate this possibility, a Zn-0 phase corrected RSF was calculated (Figure 6). For the first Zn-0 shell, the magnitude and the imaginary component of the FT coincide.25 However, for the second shell, the magnitude and the imaginary component are out-of-phase by n radians. Hence the possibility of distant oxygen neighbors is ruled out. Several attempts were made to analyze this interaction using single-scattering (SS) and multiple-scattering (MS) contributions as follows. Single-Scattering Model. The EXAFS contribution of the second shell was examined in detail to allow a comparison with the previous studies which have suggested a Zn-H2O hell.'^,'^ For all the samples, the EXAFS contributions were isolated using Fourier filtering (k3 weighted, Ak = 2.5-13.5 Ar = 2.2-3.6 8,). An isolated EXAFS function was analyzed using nonlinear least square fitting and log-ratio methods in the k space interval 2.5 k 8.0 8,-' using Zn-0 as the reference interaction. An unconstrained fit in k space resulted in negative coordination numbers. Physically meaningful structural parameters were obtained only if a large value for EO (inner potential) was used. It was possible to obtain an acceptable fit only by constraining EO. In that case the structural parameters were N = 2.5 f 0.5, R = 3.37 f 0.05 A, Aa2 = 0.02 & 0.01 A2, and EO = -10 f 30 eV. Further analysis using the logratio method did not agree with these results. Hence, the possibility of a Zn-H2O shell is rejected because it was not possible to obtain agreement between the log-ratio analysis method and the phase-corrected RSF method. The possibility of dimeri~ation'~ or aggregation/bridging9of zincate ions was investigated by analyzing the second peak assuming a Zn-Zn interaction. The isolated EXAFS functions were fitted using the Cu-Cu interaction as the reference phase and amplitude functions. Although the quality of fit (not shown) was excellent, the structural parameters were meaningless (e.g. N > 6 ) . Hence, the possibility of a Zn-Zn interaction is also rejected. Multiple-Scattering Model. The analysis of the first shell showed that Zn is surrounded by four oxygen neighbors at 1.96 8, (Table 3). This is in agreement with the previous IR and Raman result^^,^.^ which have suggested that Zn is in tetrahedral configuration. The analysis of the second shell assuming Zn-0 or Zn-Zn interactions did not result in satisfactory results, as discussed above. The fact that the major contribution of the second shell is below 3.5 8,-' suggests that it is not due to a high-2 scatterer like Zn. The only remaining explanation is that the second shell is due to the multiple-scattering processes occurring within the f i s t coordination shell. Recently, it has been shown that the contribution of the noncollinear multiplescattering processes is significant, and it should be included in the EXAFS analysis of the higher ~ h e l l s . ~ ~ - ~ O Assuming a tetrahedral configuration, the FEFF (version 5.03) code26 was used to model the EXAFS contributions of the single-scattering and multiple-scattering processes occumng within a Zn04 tetrahedron. The Zn(OH)4 tetrahedron is represented as ZnO4 because the scattering amplitude of hydrogen is negligible. A schematic of the various singlescattering and multiple-scattering paths is shown in Figure 7. The details of the multiple-scattering paths are given in Table
Pandya et al.
11970 J. Phys. Chem., Vol. 99, No. 31, 1995 0.4 0.2 h
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0
2
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Figure 4. Experimental (-) and fitted (- - -) spectra for the Zn-0 first-shell EXAFS of the ZnO/NaOH sample: (a) k3 weighted; (b) k' weighted. Corresponding RSF (Ak = 2.5-13.4 kl): (c) k3-weighted RSF; (d) kl-weighted RSF.
a
TABLE 3: Structural Parameters for the First-Shell Zn-0 Interactiona sample
interaction
N
ZnO/NaOH ZnO/KOH ZnO/RbOH ZnO/CsOH
Zn-0 Zn-0 Zn-0 Zn-0
4.02 4.02 4.10 4.18
R
(A)
1.96 1.96 1.96 1.96
Aa2 (A2)
Eo (eV)
0.0002 0.0003 0.0002 0.0006
-2.1 -1.0 -2.0 -2.0
Error in coordination numbers 15%; error in bond distances 0.01 error in Debye-Waller factors 4~10%.
a
A;
4. The single-scattering (SS) path represents scattering of the photoelectron only once by an oxygen neighbor before returning to the central Zn atom. The triangular path MS(1) represents noncollinear double scattering of a photoelectron by two oxygen atoms before retuming to the central atom. The path MS(2) represents scattering of the photoelectron by an oxygen atom twice. Thus, the photoelectron travels to an oxygen atom twice. The path MS(3) is the same as MS(2) except that the second scattering is from a different oxygen neighbor. It is important to note that the central (excited) Zn atom is also a scatterer in the triple-scattering processes MS(2) and MS(3). The total path length of the photoelectron is 3.92 8, for S S , 7.12 8, for MS(l), and 7.84 8, for MS(2) and MS(3). The resulting peaks in the RSF are expected to be around 1.96 8, for S S , 3.56 8, for MS(l), and 3.92 A for MS(2) and MS(3). For the initial calculations, the values of So2 (amplitude reduction factor) and u2 were chosen to be 1 and 0, respectively, for all the paths ( S S and MS), and the path length in r space was limited to 4 8, (contributions of total path lengths longer than 8 8, were negligible).
Figure 8a,b shows the calculated EXAFS spectrum and the corresponding RSF, respectively, for a ZnO4 tetrahedron. For comparison, the contributions of the single-scattering and multiple-scatteringprocesses are also shown. In agreement with the experimental data (Figure 2), the calculated spectrum shows a broad peak between 2 and 4 8, in the RSF (Figure 8b), and there is a phase difference of approximately 7t radians between the single-scattering and multiple-scattering contributions (Figure 8a). Figure 9 shows the RSFs of the calculated EXAFS function for 2.5 < k < 14.0 and 3.5 < k < 14.0 A-l, respectively. As seen in the experimental data (Figure 5), the intensity of the second peak (MS contribution) strongly depends upon the choice of lower limit of Fourier transformation. Thus,
a 6
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4
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2
0
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Figure 5. RSFs for the ZnO/NaOH sample. Ak = 2.5-13.4 A-l (solid line) and Ak = 3.5-13.4 h-' (dotted line): (a) k3 weighted; (b) k' weighted.
it is confirmed that the Zn is in a tetrahedral configuration and the second broad peak in the RSF is due to the multiplescattering processes. Finally, the FEFF-calculated phase and amplitude functions were used to analyze the first-shell Zn-0 interaction for the ZnO/NaOH sample to determine the values of So2 (amplitude reduction factor), a2,and EO(inner potential). The isolated first shell EXAFS was fitted in 2.5 < k < 13.5 8,-' keeping N = 4 fixed. The results are listed in Table 5. Using So2 = 0.85, the values of u2and EOfor the multiple-scattering paths were varied to obtain a good fit for 2.5 < k < 13.5 8,-' and 0 < r 4 8,.
Zn(I1) in Concentrated Aqueous Hydroxide Solutions
J. Phys. Chem., Vol. 99, No. 31, 1995 11971
t
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A-').
d 10
TABLE 4: Details of the Scattering Paths for a Zn04 Tetrahedron scattering tvDe
2
15
Figure 7. Schematics of the scattering paths: (a) single scattering S S ; (b) noncollinear double scattering MS(1); (c) collinear triple scattering MS(2); (d) noncollinear triple scattering MS(3).
path
1
8
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SS Z n - 0 MS(1)Z n - 0 - 0 MS(2) Zn-0-Zn-0 MS(3) Zn-0-Zn-0
single scattering noncollinear double scattering collinear triple scattering noncollinear triple scattering
4 12 4 12
1.96 3.56 3.92 3.92
The results are listed in Table 5. The experimental and calculated data are compared in Figure 10. The overall agreement between the FEFF calculations and the experimental data is satisfactory. For the first Zn-0 shell, an excellent agreement in phase as well as amplitudes is obtained. For the MS paths, the phase of the theoretically calculated and the experimental spectra are in good agreement, but the amplitude of the calculated spectrum is slightly lower than that of the experimental spectrum. However, if the value of So2 is increased from 0.85 to 1.0, an excellent agreement in the MS amplitudes is obtained. These differences can be attributed to several factors which are known to affect the low-k region such as the amplitude reduction factor So2 for the theoretical calculations3' and the background removal technique for the experimental data.32 The theoretical calculations presented here do not take into account the ionic charges of Zn and 0. The contribution of the MS paths was calculated using the same value of So2,
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Figure 9. RSFs for the theoretically calculated EXAFS spectra, Ak = 2.5-13.4 A-' (-)and Ak = 3.5-13.4 (- - -): (a) k3 weighted; (b) k' weighted.
that for the SS path (first shell). Normally, the So2 is treated as a constant parameter, and its value typically ranges from 0.6 to
11972 J. Phys. Chem., Vol. 99, No. 31, 1995 0.4
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k(L-') Figure 10. Experimental (-) and calculated (- - -) EXAFS spectra for the ZnO/NaOH sample and corresponding RSFs (Ak = 2.5-14.0 and c) k' weighted; (b and d) k3 weighted. TABLE 5: Structural Parameters for the ZnO/NaOH Sample Obtained Using FEFF Calculations path N R (A) so2 u2(A2) (eV)
ss MS(1) MS(2)
MS(3)
4 12 4 12
1.96 3.56 3.92 3.92
0.85 0.85 0.85 0.85
0.0045 0.005 0.006 0.006
-3 -3 -3
-3
0.9. However, So2shows significant k-dependence in the low-k region, and in some cases, its value can reach unity.33 As the contribution of the MS paths is primarily in the low-k region, a higher value of So2 is required to model the amplitude of the MS paths as found here. Discussion EXAFS analysis is generally carried out in the singlescattering approximation, and the contribution of the multiplescattering processes is ignored except in special collinear configurations (focusing effect). As a result, information about the site symmetry is not obtained. In the present study, we have included the multiple-scattering processes in the analysis of the higher shells. The EXAFS results unambiguously show that the Zn2+ ions are in a tetrahedral configuration and the Zn-0 bond distance is 1.96 A. This structure is consistent with most of the previously reported spectroscopic data mentioned above, in particular the Raman evidence that the Zn ions are tetrahedrally c~ordinated.~,~ The presence of the second shell in the RSF is explained on the basis of multiple-scattering processes. This is not in agreement with the neutron diffractionf2 and EXAFSI4 studies, which suggested a distorted octahedral configuration. The fact that this peak remains unchanged in the supersaturated solutionsf4 in 12 M KOH further rules out the possibility of a Zn-HzO shell. No evidence of aggregation or bridging between the Zn-tetrahedra was found. This work shows that the accurate determination of the coordination of complex ions beyond the first coordination shell must include the contribution of the multiple-scattering process. The simple example presented in this paper, ZnO in concentrated alkaline solutions, clearly demonstrates the errors in interpretation which may occur if the multiple-scattering contributions are neglected. Without these contributions it is possible to force a fit of the data, which suggested a second Zn-HzO shell, in agreement with the previous EXAFS s t ~ d i e s . ' ~However, .~~
.&I):
(a
when the multiple-scattering pathways were taken into account, the data strongly supported the presence of the tetrahedral coordinated Zn(OH)42- with a Zn-0 coordination distance of 1.96 A. The results of this study do not support the presence of other Zn species, such as [Zn(OH)3(H20)]-, [Zn(OH)2(H~0)21,or polymeric [Zn(OHk], which have previously been proposed to account for the supersaturation during the oxidation of zinc electrodes in concentrated potassium hydroxide. An explanation of the supersaturation consistent with only Zn(OH)d2- as the Zn(I1) species is possible by considering the reactions (in KOH solution).
2K+
+ Zn(0H);-
s K,Zn(OH),
The oxidation product of the zinc electrode is initially Zn(0H)d2-, which eventually precipitates to form KzZn(OH)4. This species is metastable with respect to ZnO and decomposes to produce ZnO according to this equation.34 K,Zn(OH), s KOH
+ ZnO + H20
This decomposition reaction releases hydroxide and potassium ions, which then allow further discharge of the zinc electrode. Acknowledgment. The authors gratefully acknowledge the support of the U.S. Department of Energy, Division of Material Sciences, under Contract No. DE-FG05-89ER45384, for its role in development and operation of Beam Line X l l A at the National Synchrotron Light Source (NSLS). The NSLS is supported by the Department of Energy, Division of Material Sciences, under Contract No. DE-AC02-76CH00016. This work was supported by the U.S.Office of Naval Research. References and Notes (1) Bodlander, G. Ber. Dtsch. Chem. Ges. 1903, 36, 3933. (2) Kunschert, F. Z. Anorg. Chem. 1904, 41, 337. (3) Hildebrand, J. H.; Bowers, W. G. J . Am. Chem. Soc. 1916, 16, 795. (4)Dirkse, T. P. J . Electrochem. SOC. 1954, 101, 328. (5) Lipincott, E. R.; Psellos, J. A,; Tobin, M. C. J. Chem. Phys. 1952, 20, 536.
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