J . Phys. Chem. 1990, 94, 21-26
21
ARTICLES Extended X-ray Absorption Fine Structure Investigations of Nickel Hydroxkies K. I. Pandya,t W. E. O'Grady,f D. A. Corrigan,s J. McBreen,I and R. W. Hoffman**t Department of Physics, Case Western Reserve University, Cleveland, Ohio 441 06, Code 61 70, Surface Chemistry Branch, Naval Research Laboratory, Washington, DC 20375-5000, Department of Applied Science, Brookhaven National Laboratory, Upton, New York 1 1 973, and Physical Chemistry Department, General Motors Research Laboratories, Warren, Michigan 48090 (Received: June 23, 1988; In Final Form: April 21, 1989)
The extended X-ray absorption fine structure (EXAFS) technique was used to investigate the structures of a-Ni(OH), and P-Ni(OH), samples at room temperature and at liquid nitrogen temperature. The EXAFS data for P-Ni(OH), were analyzed by using the reference phase and amplitude functions derived from the NiO and Ni foil EXAFS spectra. The EXAFS spectra of a-Ni(OH), were analyzed by using NiO as well as P-Ni(OH), as the reference compounds. A contraction in the Ni-Ni distance of 0.05 A in the lattice of a-Ni(OH), was observed in comparison to P-Ni(OH),. Analysis of in situ EXAFS spectra of an a-Ni(OH), film indicated subtle changes upon hydration including an increase in the Ni-0 coordination number.
Introduction Nickel hydroxides are important materials with applications in nickel batteries,' fuel cell electrodes,2 electr~lyzers,~ electrosynthesis: and electrochromic device^.^ The electrochemistry of these materials involves several phases and the oxidation-reduction reactions often occur via topochemical reactions that lead to amorphous products.6 Previously, X-ray diffraction (XRD) was used to characterize nickel hydroxides and their oxidation products. However, due to the disordered nature of these compounds, X-ray diffraction has provided limited structural information. The structures of nickel hydroxides depend upon the method of preparati~n.~The two main classes of structure are a wellcrystallized o-Ni(OH), phase and a poorly crystallized, hydrated a-Ni(OH),! phase. The electrochemistry of these phases differs ~ignificantly.~-~ Thus, the structure of the nickel hydroxide materials has considerable impact on the performance of nickel batteries and other devices, and much effort has been devoted to studying the various nickel hydroxide phases and their interconThe @-Ni(OH), phase has been studied by X-ray diffraction6J0 and neutron diffraction" measurements. From these measurements, it is now well established that this material has a C6brucite-type structure with a hexagonal unit cell having dimensions a = 3.126 A and c = 4.605 A. The fractional coordinates of the atoms are for nickel O,O,O and for oxygen 113,213,~and 2/3,1/3,z, where z lies between 0.22 and 0.25. The structure consists of stacked layers with each layer having a hexagonal arrangement of nickel atoms with octahedral coordination of oxygen atoms, three lying above the plane and three lying below the plane. Figure 1 shows the structure of P-Ni(OH),, including the arrangement within the basal plane and the stacking along the c axis. The OH bonds are parallel to the c axis. The proposed structures of these phases are based on X-ray diffraction patterns with few sharp lines8 This has prompted the application of other techniques such as infrared and Raman spectro~copy.'~-'~ However, a clear picture of the structure of these phases is still lacking. EXAFS is a new technique which can provide valuable structural information about materials lacking 'Case Western Reserve University. Naval Research Laboratory. (General Motors Research Laboratories. Brookhaven National Laboratory. f
0022-3654/90/2094-0021$02.50/0
long-range order.I5 Recently, EXAFS was applied to the investigation of the oxidation of P-Ni(OH), in plastic-bonded nickel hydroxide electrodes.I6 In that study, a sizable contraction of the Ni-Ni and Ni-0 bond distances accompanying the oxidation of 0-NiOOH was measured. In the present work, EXAFS is used to reveal the subtle differences between a-Ni(OH), and P-Ni(OH),. Using X-ray diffraction results, Bode proposed a layered structure for ~ u - N i ( 0 H similar )~ to that for /3-Ni(OH)2.6 This proposed structure of hexagonal layers of nickel with octahedrally coordinated oxygen is essentially identical with that shown for P-Ni(OH), in Figure 1, except that between the (001) planes there are disordered water layers resulting in an increase in the c-axis spacing to about 8 A. Vibrational spectroscopy studies indicate hydrogen bonding between the lamellar hydroxyl groups and the interlamellar water. In addition to the increase in c-axis spacing, Bode reported a small contraction in the lattice parameters within the nickel hydroxide layers for a-Ni(OH),. Subsequently, McEwen claimed this small contraction was actually due to experimental artifacts of the X-ray diffraction experiment.IO However, examination of the diffraction patterns for CY-N~(OH)~ reveals very broad lines which make the determination of lattice spacings difficult at best.E In the present work, EXAFS is used ~~~
(1) Falk, S. U.; Salkind, A. J. Alkaline Storage Batteries; Wiley: New York, 1969. (2) Bacon, F. T. J. Electrochem. SOC.1979, 126, 7C. (3) Hall, D. E. J. Electrochem. SOC.1983, 130, 317. (4) Manandar, K.; Pletcher, D. J. Appl. Electrochem. 1979, 9, 707. (5) Carpenter, M. K.; Conell, R. S.; Corrigan, D. A. Solar Energy Mat.
1987, 16, 333. (6) Bode, H.; Dehmelt, K.; Witte, J. Electrochim. Acta 1966, 1 1 , 1079. (7) Briggs, G.W. D. In Specialist Periodical Reports-Electrochemistry; The Chemical Society: London, 1974; Vol. 4.
( 8 ) Oliva, P.; Leonard, J.; Laurent, J. F.; Delmas, C.; Braconnier, J. J.; Figlarz, M.; Fievet, F.; deGuibert, A. J. Power Sources 1982, 8, 229. (9) Visscher, W.; Barendrecht, E. J. Electroanal. Chem. 1983, 154, 69. (IO) McEwen, R. S . J. Phys. Chem. 1971, 75, 1782. (11) Szytula, A.; Murasik, A.; Balanda, M. Phys. Stat. Sol B 1971, 43, 125. (12) Jackovitz, J. F. In Proceedings of the Symposium on the Nickel Electrode; Gunther, R. G . , Gross, S . Eds.; The Electrochemical Society: Pennington, NJ, 1982; p 48. (13) Figlarz, M.; LeBihan, M. C.R. Acad. Sci. Paris 1971, 272, 580. (14) Desilvestro, J.; Corrigan, D. A.; Weaver, M. J. J. Electrochem. Soc. 1988, 135, 885. (15) Koningsberger, D. C. Trends Anal. Chem. 1981, I, 16. (16) McBreen, J.; OGrady, W. E.; Pandya, K. 1.; Hoffman, R. W.; Sayers, D. E. Langmuir 1987, 3, 428.
0 1990 American Chemical Society
22
Pandya et al.
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990
sample and the analysis was carried out on the averaged data.
ONi @ O H
a
b
Figure 1. Crystal structure of @-Ni(OH),. (a) Within the basal plane, (b) stacking along the c axis.
to obtain very accurate coordination distances which unambiguously demonstrate a small contraction in lattice parameters within the nickel hydroxide layers in ~ y - N i ( 0 H ) ~ . Experimental Section Bulk Sample Preparation. NiO and P-Ni(OH), were obtained as reagent grade powders from Alpha Products. a-Ni(OH)’ was prepared by electroprecipitation from 0.1 M Ni(N03)2solution onto nickel foils by cathodic polarization at 8 mA/cmZ for 15 min. The a-Ni(OH)2 films deposited were rinsed for 30 min in distilled water and dried under vacuum. The films were then scraped off the nickel foil. The identity of the NiO, P-Ni(OH)2, and cyNi(OH)2 samples were confirmed by X-ray diffraction measurements. For EXAFS measurements, self-supporting wafers were fabricated from the nickel hydroxide and nickel oxide powders mixed with boron nitride. About 10 mg of the sample was thoroughly mixed with about 100 mg of boron nitride and pressed into uniform pellets of 1.3 cm in diameter. This yielded about 5 mg/cm2 of nickel in the sample. In Situ a-Ni(OW2Film Sample. Thin films of a a-Ni(OH), were deposited onto a 1000-AAu/Melinex substrate.” Deposition was achieved from 0.05 M Ni(N03)’ by cathodic polarization at 1.6 mA/cm2 for 1000 s which produced 5 mg of a-Ni(OH)’ on a 6.25-cm2 area. After they were briefly rinsed with distilled water, two thin film samples were immersed in 1 M KOH electrolyte in a thin-layer cell. (The use of two adjacent thin film samples increased the sample absorbance and minimized pin hole effects.) The thin-layer cell avoided excessive attenuation of the X-ray beam as it passed through the 0.5” acrylic plastic windows and 0.5 mm thickness of electrolyte. EXAFS measurements were performed within 30 min after preparation of the thin film samples. X-ray Absorption Experiments. EXAFS measurements were performed at beamline X-1 1A of the National Synchrotron Light Source (NSLS) at Brookhaven National Laboratory. The nickel K edge (8333 eV) X-ray absorption spectra for a 7-pm thick nickel foil, NiO, cy-Ni(OH),, and @-Ni(OH),samples were measured at 297 and 77 K with the storage ring operating at 2.52 GeV beam energy and beam currents 40-200 mA. The monochromator was operated in the two-crystal mode using Si( 11 1) crystals. The incident and transmitted X-ray intensities were measured with gas-filled ion chambers. The gas compositions were adjusted such that the first and second ion chambers absorbed 15% and 80% of the incoming X-rays, respectively. The in situ X-ray absorption measurements on the thin film of a-Ni(OH)’ were taken at room temperature. Nickel foil was used as a reference for calibrating the energy scale. Each absorption spectrum required approximately 20 min to measure. Two scans were measured on each ( 1 7) Kordesch, M. E.; Hoffman, R. W. Thin Solid Films 1983, 107, 365.
Data Analysis The EXAFS function x ( k ) is defined as x ( k ) = ( p - po)/po, where 1.1 and po are the X-ray absorption coefficients of the absorbing atom in the material of interest and in the free state, respectively. The difference ( p - po) depends upon the local structure of the absorbing atom and represents the EXAFS. The division by po normalizes the EXAFS oscillations to a per atom basis. The wave vector k of the ejected photoelectron is given by k = [2m/h2(hu - Eb - Eo)]’/’, where m is the mass of the electron, Y is the frequency of the X-ray photons, Eb is the binding energy of the electron, and Eo is the correction to the binding energy caused by the atomic potentials. The EXAFS oscillations were separated from the absorption background by using a cubic spline background removal technique and were subsequently normalized to a per atom basis by dividing by the step height of the absorption edge. Since the reference compounds were normalized in the same way, a step normalization can be applied. The theoretical expression which relates the measured EXAFS parameters in the single scattering approximation is given by18-2’
x(k) = Z A j ( k ) sin (2kRj + 4 j ( k ) ) I
(1)
The EXAFS equation is a superposition of contributions from different coordination shells. Here j refers to the jth coordination shell, and Rj is the average interatomic distance from the absorber atom to the neighbor atoms in thejth shell. 4Ak) is the total phase shift suffered by the electron in the scattering process. Aj(k) is the amplitude function of the jth shell and is given by A j ( k ) = -S02(k) Nj k R;
Fj(k) e-2(Rt-A)/k2c?k2
(2)
where N j is the average coordination number and Fj(k) is the backscattering amplitude of the atoms in the jth shell. ut is a Debye-Waller term which accounts for the thermal vibrations (assuming harmonic vibrations) and static disorder (assuming Gaussian pair distribution) present in the material. SO2(k)is an amplitude reduction factor which accounts for the relaxation of the absorbing atom and multielectron excitation (shake up/off‘) processes a t the absorbing atom. X(k) is the mean free path of the photoelectron and A is a correction factor ( A N R , ) to the mean free path since So2(/?)and Fj(k) already account for most of the photoelectron energy losses in the first coordination shell. The EXAFS function of a particular shell was isolated by applying a Fourier transform followed by an inverse Fourier transform. The limits of the forward transform range were chosen to coincide with the nodes of the x ( k ) function in order to reduce the termination errors which arise from the finite range of the Fourier transformation. Similarly, the limits for the inverse transform were chosen to coincide with the nodes in the imaginary part of the complex Fourier transform. The EXAFS function of a particular shell was analyzed by using the phase and amplitude functions derived from suitable reference compounds. A fitting in k space was then performed using an iterative least-squares technique. The parameters obtained from the fitting were checked by comparing the Fourier transforms of the experimental EXAFS with the fitted EXAFS function. When a kn-weighted Fourier transformation is applied to an EXAFS signal containing high Z scatterers, side lobes are introduced in R space on both sides of the peak.2z*23When peaks due to other shells are close to the main peak, these side lobes may interfere with the signals of the (18) Sayers, D. E.;Bunker, B. A. In X-ray Absorption; Koningsberger, D. C., Prins, R., Eds.; W h y : New York, 1987; pp 211-253. (19) Sayers, D. E.; Stern, E. A.; Lytle, F. W. Phys. Rev. Letr. 1971, 27, 1204. .-.
(20) Stern, E. A. Phys. Reo. B. 1974, 10, 3027. (21) Lee, P. A.; Pendry, J. B. Phys. Rev. B. 1975, I I , 2795. (22) Van Zon, J. B. A. D.; Koningsberger, D. C.; Van’t Blik, H. F. J.; Sayers, D.E. J Chem. Phys. 1985,82, 5742. (23) Marques, E. C.; Sandstrom, D. R.; Lytle, F. W.; Greegor, R. B. J . Chem. Phys. 1982, 77, 1027.
The Journal of Physical Chemistry, Vol. 94, No. 1, 1990 23
EXAFS Investigations of Nickel Hydroxides I "
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nearby shells making the isolation of these peaks difficult. The intensity of the side lobes may be reduced to a considerable extent by increasing the weighting factor of the Fourier transformation. However, it also amplifies the noise at high k values. The most important disadvantage in using higher weighting factors is that the Fourier transform becomes less sensitive to the contributions of low Z scatters (like oxygen) which have their scattering amplitude primarily at low k values. In such cases, a phase- and amplitude-corrected Fourier transformation is an appropriate choice because it reduces the intensity of the side lobes to a great extent. As a result the ambiguity during inverse Fourier transformation is minimized and a single shell can be more readily isolated.
Results The analysis of an EXAFS spectrum requires knowledge of the phase and amplitude functions for a specific absorber-scatterer pair. These functions can be calculated theoretically or can be extracted experimentally from the EXAFS spectrum of a reference compound of known structure. We used NiO and Ni foil as the reference compounds. The phase and amplitude functions derived from the first Ni-O and Ni-Ni shells of NiO were used to analyze the first Ni-0 and Ni-Ni shells of nickel hydroxide samples, respectively. The phase and amplitude functions derived from the fourth shell of Ni foil were used in analyzing the third interplanar Ni-Ni shell of the nickel hydroxides. The EXAFS spectra of a-Ni(OH), were also analyzed by using P-Ni(OH), as the reference compound. Reference Compounds. The EXAFS data x(k) measured at 77 K for the Ni foil and NiO samples are shown in Figure 2a,b. The corresponding radial structure functions for Ni foil and NiO are displayed in Figure 2, c and d, respectively. The coordination shells in the radial structure function of Ni foil are well separated, so the first Ni-Ni shell EXAFS function can easily be filtered out. The parameters used for isolating this first shell Ni-Ni EXAFS function are listed in Table I. For the Ni-O EXAFS, a k'-weighted Fourier transform is an appropriate choice because it emphasizes the contribution of the oxygen atoms. However, a kl-weighted Fourier transform shows significant overlap of the Ni-0 and Ni-Ni shells. Even in the k3-weighted Fourier transform, some overlap of these shells exists, which is seen as a side lobe (shoulder) on the low-R side of the Ni-Ni shell (Figure 2d). The Ni-O and Ni-Ni EXAFS contributions were separated by applying a phase-corrected Fourier transform and applying the difference file technique. The following sequence of steps describes the procedure that was employed to isolate the Ni-0 and the Ni-Ni shell contributions from the NiO EXAFS spectrum.
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TABLE I: Fourier Transform Parameter and Crystallographic Data for the Reference Compounds ref compd temp, K shell k" Ak. A-I AR. A-I N R . A
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1.35-2.70 4.30-5.00 2.48-3.38 0.00-2.18 2.60-3.62 0.00-2.04 5.42-6.48
12 12 12 6 6 6 6
2.49 4.97 2.94 2.08 3.13 2.07 6.26
Using the phase function obtained from the first shell of Ni foil, a phase-corrected Fourier transform was carried out on the NiO EXAFS spectrum. In order to obtain a perfectly symmetric peak in the imaginary part of the Fourier transform which peaks at the same R value as the absolute magnitude (IFTI) of the Fourier transform, a small Eo (inner potential) correction was necessary. This can be achieved by varying Eoof the NiO EXAFS spectrum as suggested by Lee and Ber~i.,~Instead, Eo for the Ni foil EXAFS spectrum was varied with the stipulation that the phase function obtained from this treatment of the Ni spectrum be used to carry out a phase-corrected Fourier transformation on NiO resulting in the desired symmetric Ni-Ni peak. An Eo correction of 4.5 eV was necessary. The symmetric Ni-Ni contribution from the radial structure function was filtered by performing an inverse Fourier transform (AR= 2.45-3.38 A) to give the Ni-Ni reference spectrum. This isolated Ni-Ni EXAFS contribution was subtracted from the NiO EXAFS spectrum in k space. The resulting difference spectrum was Fourier transformed (k', Ak = 2.5-12 A-') and subsequently back transformed (AR = 0-2.18 A) to give the Ni-0 reference spectrum. P-Ni( OH),.The normalized EXAFS spectrum measured at 77 K for crystalline P-Ni(OH), is presented in Figure 3a. The corresponding radial structure function is shown in Figure 3b and it exhibits three well-resolved peaks. The first peak is attributed to the first Ni-0 shell, the second peak is attributed to the first Ni-Ni shell (cf. Figure 1). The third peak at 6 A is relatively strong and corresponds to the third interplanar Ni-Ni shell. The first Ni-Ni shell contribution was isolated by performing a k3weighted Ni-Ni phase-corrected Fourier transform followed by an inverse transform (see Table 11). This isolated single-shell Ni-Ni EXAFS function was fitted in k space over the interval During the initial fit, the four variables N , R, Ak = 4-14 A d , and AEo were allowed to vary. From the results of this initial fit, an EXAFS function was calculated, Fourier transformed, and inverse transformed in exactly the same way as the P-Ni(OH), EXAFS spectrum to produce a new reference spectrum. This new (24) Lee, P. A.; Beni, G. Phys. Reu. B. 1977, 15, 2862.
24
The Journal of Physical Chemistry, Vol. 94, No. 1 , 1990
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TABLE 11: Fourier Transform Parameter Used for Isolating Ni-Ni and Ni-O Interactions from the Experimental Data Using NiO (77 K) as the Referencea sample temp, K shell kn Ak, A R , 8, P-Ni(OH), 77 Ni-Ni k' 3.16-15.01 2.60-3.62 Ni-0 k' 3.16-12.00 0.00-1.98 k' 3.16-15.01 5.42-6.48 Ni-Ni 297 Vi-Ni k' 3.16-15.12 2.60-3.64 k' 3.16-12.00 0.00-2.00 Ni-0 k3 3.16-15.12 5.44-6.26 Ni-Ni a-Ni(OH), 77 Ni-Ni k' 3.20-14.73 2.56-3.62 k' 3.20-12.00 0.00-1.98 Ni-0 k3 3.20-14.73 5.26-6.34 Ni-Ni 297 Ni-Ni k' 3.19-14.05 2.54-3.62 k' 3.19-12.00 0.00-2.00 Ni-0 k' 3.19-1 4.05 5.02-6.18 Ni-Ni a-Ni(OH), 297 Ni-Ni k' 3.15-14.06 2.58-3.60 in situ Ni-0 k' 3.15-12.00 0.00-2.00 Ni-Ni k3 3.15-14.06 5.30-6.30 "In each spectrum the first Ni-Ni shell is isolated after a Ni-Ni phase-corrected Fourier transform is applied and the Ni-0 shell is isolated after the Ni-Ni shell contribution is subtracted.
reference spectrum was used in the successive fitting procedures because it contained the same transformation artifacts as the measured EXAFS function and provided a more rapid covergence in the fitting. The measured spectrum was fitted in the following ways: Initially all four parameters ( N , R, ha2,aEo)were allowed to vary and a good fit in k space and in R space was obtained. Then the values of R and AE, were kept constant allowing only N and Po2 to vary during the fitting. This method provided a good way of checking the correlation between the two sets of parameters R, AEo and N, Au2. After the values of N and Au2 were optimized, the values of R and AEo were checked keeping the values of N and Au2 fixed during the fitting. As a final check
TABLE III: Fourier Transform Ranges Used for Isolating Ni-Ni and Ni-O Contributions from the Experimental Data Using B-Ni(OH), (77 K ) as the Referencen sample temp, K shell k" Ak, 8,-' A R , 8, P-Ni(OH), 297 Ni-Ni k' 3.30-14.60 2.56-3.66 Ni-0 k' 3.30-12.00 0.00-2.00 Ni-Ni k' 3.30-14.60 5.30-6.30 k' 3.28-14.73 a-Ni(OH), 77 Ni-Ni 2.56-3.62 Ni-0 k' 3.28-12.00 0.00-2.00 Ni-Ni k' 3.28-14.73 5.28-6.30 297 Ni-Ni k3 3.30-14.07 2.52-3.60 Ni-0 kl 3.30-12.00 0.00-1.96 k' 3.30-14.07 5.30-6.20 Ni-Ni a-Ni(OH), 297 Ni-Ni k' 3.22-14.07 2.56-3.66 kl 3.22-12.00 0.00-2.04 in situ Ni-0 Ni-Ni k' 3.22-14.07 5.3 8-6.34 " I n each spectrum the first Ni-Ni shell is isolated after a Ni-Ni phase-corrected Fourier transform is applied and the Ni-0 shell is isolated after the Ni-Ni shell contribution is substracted.
N a n d R were kept fixed and Au2 and AE, were allowed to vary and a good fit was obtained. The best fit results are listed in Table IV. An EXAFS function was calculated from these fit results. Figure 4a compares the isolated experimental and calculated Ni-Ni EXAFS functions. Parts b and c of Figure 4 compare the corresponding imaginary parts and magnitude of the radial structure functions. The calculated Ni-Ni EXAFS was subtracted from the measured EXAFS function of @-Ni(OH), in k space. From the resulting difference file, the Ni-0 EXAFS contribution was isolated by applying a k'-weighted Fourier transform (Table I). Following the same procedure that was used for fitting the Ni-Ni EXAFS function, the Ni-0 EXAFS function was fitted in the k-space interval Ak = 3.6-10 A-' with a kl-weighting factor and
EXAFS Investigations of Nickel Hydroxides
The Journal of Physical Chemistry, Vol. 94, No. 1. 1990 25
TABLE I V Structural Parameter@Obtained from the EXAFS Analysis Using NiO (77 K) as the Reference sample temp, K shell N R , 8, Ao2, A2 A&, eV O-Ni(OHL 77 Ni-0 6.0 2.07 -0.0012 -1.89 , Ni-Ni 6.2 3.13 0.0002 -1.85 Ni-Ni 6.8 6.26 0.0018 -0.13 297 Ni-0 5.9 2.07 0.0004 -3.07 Ni-Ni 6.0 3.12 0.0026 -1.31 Ni-Ni 6.0 6.26 0.0054 -1.92 5.6 2.05 0.0009 -1.14 a-Ni(OH), 77 Ni-0 Ni-Ni 5.7 3.08 0.0024 -2.15 0.94 297 Ni-0 6.2 2.05 0.002 Ni-Ni 5.3 3.08 0.0049 -2.16 a-Ni(OH), 297 Ni-0 7.2 2.06 0.0019 -1.88 in situ Ni-Ni 6.2 3.09 0.004 -0.65 I-
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"Accuracies: N f 10%. Aa2 f 15%, R f 0.01 8,. using the phase and amplitude functions derived from the Ni-0 EXAFS. The best fit results are given in Table IV. Figure 4d compares the isolated experimental and the calculated Ni-0 EXAFS functions. Parts e and f of Figure 4 compare the corresponding imaginary parts and magnitudes of the Fourier transforms of the experimental and calculated EXAFS. To check the quality of the Ni-Ni fit results, the calculated Ni-0 EXAFS function was subtracted from the measured @Ni(OH)2 EXAFS spectrum. A Ni-Ni phase- and amplitudecorrected Fourier transformation was carried out on this difference spectrum. If the Ni-Ni peak in the imaginary part of the Fourier transform was symmetric, the fit parameters were considered to be satisfactory. When it was not symmetric, the inverse transform on the difference spectrum was reanalyzed until symmetry was achieved . The third peak in the radial structure function of @-Ni(OH), is unusually strong and requires special treatment. As seen in Figure 1, the third interplanar Ni-Ni shell is shadowed by the first Ni atom shell. This intervening first shell of atoms acts as a lens for the electron wave and enhances the contribution of the third-shell atoms. This is known as the "focusing effectnZ5and is also present for the fourth shell in the FCC structure. It was found that the fourth shell of nickel foil was a suitable reference and it gave meaningful fit results. Figure 4g compares the isolated experimental and calculated third-shell Ni-Ni EXAFS functions. Parts h and i of Figure 4 compare the corresponding imaginary parts and magnitudes of the radial structure functions. The parameters used for isolating this third Ni shell EXAFS function are given in Table I1 and the best fit results are listed in Table IV. A set of reference spectra for the Ni-0 and Ni-Ni shell interactions were prepared from the P-Ni(OH), (77 K) EXAFS spectrum by using the best fit results (Table I). These reference spectra were used to analyze the P-Ni(OH), (297 K) and aNi(OH)2 EXAFS spectra. The Fourier transform parameters used for separating the various contributions from the @-Ni(OH), (297 K) EXAFS spectrum are listed in Table I1 and Table 111. Tables IV and V list the fit results obtained. a-Ni( OH)2. The normalized EXAFS spectrum measured at 77 K for a-Ni(OH)2is presented in Fi ure 3c. The corresponding radial structure (k3, Ak = 3.2-14.7 is shown in Figure 3d. The normalized EXAFS spectrum measured at 297 K for the CX-N~(OH film ) ~ prepared on Au/Melinex and measured in situ is shown in Figure Sa. The corresponding radial structure function is shown in Figure 5b. The EXAFS spectra of in situ and powder ~ x - N i ( 0 Hwere )~ analyzed by using the experimental phase and amplitude functions derived from NiO as well as P-Ni(OH)2. The parameters used for isolating the Ni-0 and the Ni-Ni contributions are listed in Tables 11 and 111. The fit results obtained are listed in Tables IV and V. The third interplanar shell was fitted in the k-space
1-I)
( 2 5 ) Teo, B. K. In EXAFS Spectroscopy: Technique and Applications; Tea, B. K.. Joy, D. C., Eds.; Plenum: New York, 1981; p 13.
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8
R8,
Figure 5. (a) Normalized EXAFS spectrum of the a-Ni(OH), film in 1 M KOH. (b) Radial structure function (k3,Ak = 3.22-14.06A-') for the a-Ni(OH), film. TABLE V Structural ParametersaObtained from the EXAFS Analysis Using B-Ni(OH), (77 K) as the Reference sample temp, K shell N R, 8, nu2, A2 A& eV P-Ni(OH), 297 Ni-0 6.0 2.06 0.0017 -2.81 Ni-Ni 6.0 3.12 0.0027 -2.63 Ni-Ni 5.5 6.21 0.0037 -0.94 ~ t - N i ( 0 H ) ~ 77 Ni-0 5.8 2.05 0.0016 -2.26 Ni-Ni 5.7 3.07 0.0029 0.02 Ni-Nib 4.0 6.15 0.0017 1.12 2.0 6.10 0.0047 -9.98 a-Ni(OH), 297 Ni-0 6.0 2.04 0.0025 -2.81 Ni-Ni 5.5 3.07 0.0055 -1.98 Ni-Nib 4.0 6.14 0.0068 -1.05 2.0 6.10 0.0119 -12.91 a-Ni(OH), 297 Ni-0 7.3 2.05 0.0036 -2.34 in situ Ni-Ni 6.5 3.09 0.0044 -1.32 film Ni-Nib 4.0 6.18 0.0037 0.50 2.0 6.12 0.0115 -9.97
*
OAccuracies: N f lo%, Aa2 15%, R f 0.01 A. bResults of twoshell fit; accuracies N f 20%, R f 0.1 8,. interval Ak = 5-14 A-l with ko and k' weighting factors. Since this peak was broader than the corresponding peak in 8-Ni(OH)2, a two-shell fit was utilized. The constraint imposed on the fit was that the total coordination number be 6. The results were optimized by using the previously discussed procedures. The best fit results are listed in Table V.
Discussion @-Ni(OH),. The radial structure functions for /3-Ni(OH)2 are shown in Figure 3b. The EXAFS results for @-Ni(OH), (77 K) indicate coordination shells with 6.2 nickel atoms at 3.13 8, and 6.0 oxygen atoms at 2.07 A. Similar results were obtained at room temperature. Both sets of results are in good agreement with the shells of 6 nickel atoms at 3.13 A and 6 oxygen atoms at 2.14 A calculated from the X-ray diffraction results. For the Ni-0 shell, the EXAFS analysis give the interatomic bond distance to be 2.07 f 0.01 A, which is slightly lower than the value 2.14 8, found by X-ray diffraction and reported by McEwen.l0 However, X-ray diffraction gives the coordinates of oxygen atoms (1/3, 2/3, Z ) and (2/3, 1/3, Z),where Z varies between 0.22 and 0.25. Taking Z = 0.25, the resulting Ni-0 bond distance is 2.14 A, while taking Z = 0.22, the resulting Ni-0 bond distance is 2.07 8, in very close agreement with the EXAFS result. The EXAFS results can also be compared with the ionic radii of Ni2+and 0'-. The ionic radius of Ni2+with 6 neighbors is 0.69 f 0.01 8, and
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Pandya et al.
The Journal of Physical Chemistry, Vol. 94, No. 1, I990
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a Ni(OH),
i
material arises from thermal sources. These values are relative to well crystallized NiO at 77 K. The Aa2 values of P-Ni(OH), show very little difference from that of NiO at 77 K, indicating that there is very little structural or static disorder in the sample. a-Ni(OH),. The radial structure functions of a-Ni(OH), measured at 77 and 297 K are shown in Figure 3d. The EXAFS' results for a-Ni(OH), measured at liquid nitrogen temperature indicate coordination shells of 5.7 nickel atoms at 3.08 8, and 5.6 oxygen atoms at 2.05 A surroundin each nickel atom in the lattice. There is a contraction of 0.05 in the nickel coordination shell of a-Ni(OH), compared to that of P-Ni(OH),. Similar results were obtained at room temperature. As with P-Ni(OH),, no contribution from other coordination shells was evident except the coplanar nickel shell which is enhanced due to the focusing effect. We did not observe any contribution from the interlamellar water layer as proposed by Bode et aL6 since the EXAFS amplitude from such a layer would be very small. The results for a-Ni(OH), did indicate a higher degree of disorder for all coordination shells as evidenced by the increased Debye-Waller factors as seen in Table IV. The third coordination peak in the radial structure function was of lower amplitude than the corresponding peak in P-Ni(OH),; consequently, it was more difficult to fit. The difficulties in fitting this peak arise from structural disorder effects coupled with multiple-scattering process. A reasonable fit could only be obtained from a two-shell model with the total coordination number fixed at 6. The EXAFS results for the a-Ni(OH), film in 1 M KOH electrolyte indicated a coordination shell of 6.2 nickel atoms at 3.09 8, and 7.2 oxygen atoms at 2.06 A surrounding each nickel atom in the lattice. The results differ from the ex situ a-Ni(OH), results in that there is a 20% increase in the coordination number of oxygen. The significance of this change is not clear but it may signal the start of the aging process which converts a-Ni(OH), to P-Ni(OH), in aqueous solution.* a-Ni(OH), Lattice Contraction. The radial structure functions for a-Ni(OH), and P-Ni(OH), are compared in Figure 6. Despite similarities, EXAFS results show significant differences between a-Ni(OH), and P-Ni(OH)? Compared to P-Ni(OH), the Ni-Ni distance is contracted by 0.05 A. These results were obtained at 77 K where the disorder due to thermal vibrations is small. Essentially the same results were obtained from 297 K EXAFS measurements. This contraction was observed when both NiO and P-Ni(OH), were used as the reference compound. Thus, we are confident that the 0.05-8, contraction is real and not an apparent contraction due to the structural disorder.,' This small yet significant contraction may result from hydrogen bonding between the hydroxide groups and the interlamellar water as proposed by Bode.6 Regardless, it is indicative of significant chemical differences between a-Ni(OH), and @-Ni(OH)2which can be expected to have an effect on the electrochemistry of these materials.
1
R Angstroms Figure 6. Radial structure functions for a-Ni(OH)2and @-Ni(OH),.For cr-Ni(OH)2,Ni-Ni peaks are shifted toward lower R values, indicating a contraction in bond distances. the ionic radius of oxygen is 1.35-1.40 A.26 Taking the ionic radius of 0,- to be 1.37 A, the sum of the two radii is 2.07 A. Thus, the EXAFS results are in agreement with the sum of ionic radii. As in previously reported measurements16 the possible contribution from the two nickel atoms located in the adjacent hexagonal planes at a distance of 4.60 8, was not observed. Similarly, the contributions from the 6 coplanar nickel atoms at 5.42 8, and the 12 nickel atoms at 5.57 8, in adjacent planes were not observed. These missing coordination shells in adjacent planes may indicate platelet-shaped crystals of P-Ni(OH), with only a few hexagonal layers stacked together. Alternatively, a high amplitude is not expected from these shells due to the 1/R2 fall off of the magnitude and the damping terms which reduces the amplitude greatly. The third peak in the radial structure function of P-Ni(OH), is unusually strong and coincides with the third interplanar shell of 6 nickel atoms at 6.26 8,. By comparing the amplitude and phase functions of the fourth shell of nickel foil, the fourth nickel shell of NiO and the third coplanar nickel shell of P-Ni(OH), it was observed that the fourth shell of nickel foil would provide a suitable reference spectrum. A good fit of this shell was obtained using the phase and amplitude functions derived from the fourth nickel shell of the nickel foil sample. Meaningful fit results were not obtained with the fourth nickel shell of NiO perhaps due to the interference of coplanar oxygen atoms (not present in /3Ni(OH),). As seen in Table IV, relatively higher values of AEo are obtained for this shell. This is not unexpected since the multiple scattering process in nickel metal involving Nio-Nio-NiO interactions is used to model the multiple scattering process in Ni(OH), involving Ni2+-Ni2+-Ni2+ interactions. Comparing the values of Aa2 for B-Ni(OH), measured at 77 and 297 K, we can obtain the contributions of the static and thermal disorder. Comparing the values of Aa2 = 0.0002 A2 for the first nickel shell of P-Ni(OH), (measured at 77 K) and Au2 = 0.0026 A2 for the first nickel shell of P-Ni(OH), (measured at 297 K) suggests that the primary source of disorder in this
Acknowledgment. We are thankful to Dr. D. C. Koningsberger (Eindhoven University, The Netherlands) for providing the data analysis programs and for giving guidance on their use. We acknowledge support of the U S . Department of Energy, Division of Material Sciences, under contract DE-AS05-80-ER10742 for its role in development and operation of Beam Line X-11 at NSLS. The NSLS is supported by Department of Energy, Division of Materials Science and Division of Chemical Science, under contract DE-AC02-76CH00016. K.I.P. and R.W.H. were supported in part by NASA under Grant NAG-3-694. Registry No. Ni(OH),, 12054-48-7.
( 2 6 ) Wells, A. F. Srrucrural Inorganic Chemistry; Oxford University Press: London, 1975; pp 257-259.
(27) Eisenberger, P.; Brown, G.S . Solid Srare Commun. 1979, 29, 481.
