Langmuir 1987, 3, 423-428 In concluding we would like to comment on the usefulness of EHT for chemisorption studies. Due to its simplicity it is a useful tool in handling large numbers of atoms, and its qualitative nature suits it to comparative studies of relative differences between molecules. Its major shortcoming is the incorrect spacing of T-T* energy levels, which leads to unrealistic charge transfers. In future studies more sophisticated methods which place orbitals closer to their correct position will be employed.
423
Acknowledgment. Support from the Air Force Office of Scientific Research (Grant AFOSR-86-0050) is acknowledged with thanks. Thanks are also expressed to Dr. Gregory R. Schoofs and Dr. Leiland C. Allen for helpful discussions. A. K. Myers is appreciative of a graduate fellowship from the National Science Foundation. Registry No. Ni, 7440-02-0;benzene, 71-43-2;toluene, 108-88-3; o-xylene, 95-47-6; m-xylene, 108-38-3;p-xylene, 106-42-3;mesitylene, 108-67-8.
Tunnel Processes in Photoelectrochemical Reactions of Passive Films Ulrich Stimming Electrochemistry Laboratory, Department of Chemical Engineering and Applied Chemistry, Columbia University, New York, New York 10027 Received September 4, 1986. I n Final Form: January 14, 1987 Previous experimental results for photocurrent measurements on passive films on iron, tantalum, and zirconium and on Xe-implanted passive films on hafnium indicate a behavior different from that of bulk crystalline materials. A discussion is presented that tries to explain cathodic photocurrents that are observed at potentials above the flatband potential and wavelength-dependent potentials where the photocurrent changes sign. The model emphasizes tunneling processes of photoexcited carriers from the passive film directly into the electrolyte. Depending on conditions such as photon energy and field, a direct competition of tunneling with and against the field which correspondsto anodic and cathodic photocurrents, respectively, is possible. Model calculations are used to support the proposed mechanism. Introduction Although well-known for a long time,l photoelectrochemistry with passive films has only recently developed into a technique for the characterization of passive films under in situ conditions. In principle, such measurements, i.e., the photocurrent as a function of wavelength and electrode potential, allow one to determine important solid-state properties of the passive film such as bandgap energy and flatband potential. The basis for such an analysis is usually the photoelectrochemical behavior of bulk crystalline semiconductors. The photoexcitation process is considered to be a band-to-band excitation with a subsequent transport of the photoexcited carrier in the bands. It has recently been recognized, however, that the description of passive films in terms of bulk semiconductors is only a crude approximation. In thin films, the band structure normal to the surface is not fully developed or films are amorphous. In either case, it can be expected that a relatively high number of localized states exist in what is the bandgap in a crystalline material. These localized states are believed to be important for the photoelectrochemical behavior of passive films.2 Recent model calculations based on the Poole-Frenkel effect3 showed that in fact a subbandgap response can be considerable if the electric field in the passive film is high enough. In this paper a model will be presented which is based on the presence and involvement of localized states of the passive f i i in the photoelectrochemical process. On (1) Becquerel, E. C. R . Hebd. Seances Acad. Sci. 1839, 9,561. (2) Stimming, U.Electrochim. Acta 1986, 31, 415. (3) Stimming, U.;Newmark, A. R. Electrochim. Acta, in press. Newmark, A. R.; Stimming, U. J.Electroanal. Chem. 1986, 204, 197.
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this basis, experimental results will be explained which are incompatible with the model of bulk crystalline materials. Photoelectrochemical Processes Involving Localized States A photocurrent can be thought to be composed of three processes: the absorption of a photon, the motion of the photoexcited carriers (e- and h+) in the electrode to the surface and to the backside contact, respectively, and the charge-transfer reaction at the electrode-electrolyte interface. The situation is illustrated for a bulk crystalline n-type semiconductor in Figure 1. The first step is a band-to-band excitation. The second one is a transport of the charge carrier in the respective band, which happens, usually, at the bottom of the conduction band and the top of the valence band, respectively, if photoemission processes are excluded. The third step is usually not considered to be rate determining provided electronic states are available in the electrolyte at the energy of the charge transfer. In the presence of localized states the following additional processes are possible: (i) absorption can occur from extended to localized states, from localized to extended states, and from localized to localized states; (ii) electron transport can happen by various means which will be discussed later; and (iii) interfacial charge transfer can occur at any energy in between the band edges depending on the distribution function of the localized states. This situation is illustrated in Figure 2. In Figure 2a the various possibilities of a photon absorption involving localized states are shown. Excitation at a given photon energy involving extended states is also possible to energies higher than E, and from energies lower than E,. Such 0 1987 American Chemical Society
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424 Langmuir, Vol. 3, No. 3, 1987
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semiconductor electrolyte Figure 1. Photoinduced process in an n-type semiconductor under depletion conditions.
1
I
I X
a) Figure 2. Absorption processes in an amorphous semiconductor showing excitation from extended to localized, localized to extended, and localized to localized states (a);part b gives a schematic representation of a possible density of states distribution in the bandgap.
I
X Figure 3. Charge-transfer paths at an amorphous passive filmelectrolyte interface for photoexcited electron and holes in localized and extended states. contributions, however, are less probable if a distribution function as shown schematically in Figure 2b is assumed. This may be different for other kinds of distribution functions. The possibilities of interfacial charge transfer are illustrated in Figure 3. Under depletion conditions in the film, either with a uniform potential distribution as shown in Figure 3 or a nonuniform potential distribution in the film, photoexcited carriers in the film can create anodic and cathodic currents. For an insulating film or an n-type film under the conditions shown in Figure 3, anodic photocurrents will be expected, but a transfer of electrons against the field is, in principle, possible as well. The latter seems to be quite important for passive films
I
X Figure 4. Escape paths of a photoexcited electron from a localized state: (1)hopping or tunneling process in the direction of the field into the conduction band; (2) multiple hopping or tunneling with the field to the metal; (3) tunneling against the field into the electrolyte.
as w i l l be shown later. The corresponding situation would hold for p-type conducting films under depletion conditions. An important factor contributing to the probability of charge transfer is the distribution of occupied and unoccupied electronic states in the electrolyte which is governed by the kind and concentration of reduced and oxidized species, re~pectively.4.~ Some of the possibilities of charge transport in the film involving localized states are shown in Figure 4. It illustrates the case for the photoexcited electrons and for straight band edges that, i.e., the thickness of the depletion layer, d,, is larger than the film thickness, d,, ddl > d,. For the situation of a space charge layer, the processes are comparable. In principle, the same situation can also be discussed for the holes as minority as well as majority carriers. (1)After photoexcitation of an electron onto an unoccupied localized state, it has the possibility of being transferred into the conduction band. This can be an overbarrier or a tunneling process. Details will be discussed later. (2) If the photoexcited electron ends up at an electronic state of lower energy, multiple hopping is possible. This situation has been discussed by Hills for the dark conductivity of amorphous materials. For high fields and if states are close together so that the barrier becomes thin enough, tunneling becomes more favorable. The formalism in terms of the field dependence is similar to the single trap situation. Another formalism to calculate the probability of the electron transport in the film is the percolation theory.' (3) The electron also has a certain probability of being transferred directly to the electrolyte. This represents a cathodic current while the other processes are anodic ones. Processes 1and 3 will be discussed later in more detail.
Experimental Results from Literature In the following, some experimental results of photoelectrochemical behavior with passive films are reviewed that show characteristics which are different from those of bulk crystalline materials. All but one system belong to the group of the so-called valve metals whose passive films usually have bandgap energies above 3 eV, but the discussed effects are not restricted to that group. Typical features are (i) photocurrent transienh that change from (4) Gerischer, H. 2.Phys. Chem., N.F. 1960,26,223,325; 1961,27,48. (5) Gerischer, H. In Physical Chemistry;Eyring, H., Henderson,D., Jost, W., Eds.; Academic: New York, 1970; Vol. IXA. (6) Hill, R. M. Philos. Mag. 1971, 23, 59. (7) Mott, N. F.; Davis, E. A. Electronic Processes in Noncrystalline
Solids; Clarendon: Oxford, 1979.
Langmuir, Vol. 3, No. 3, 1987 425
Tunneling in Reactions of Passive Films
t
-3
Zr in 1 M Na,SO,
Uox= 3V vs nhe
hv /eV 4.0
a
t
t
On
Off
t t
t t
On O f f
On O f f
t t
On O f f
Figure 5. Photocurrent transients observed with passive iron at potentials above the flatband potential shown for increasing potential from left to right, taken from ref 8, 9.
0.4 Ta /Ta 2O w
5
-
-1d
5 0.2-
0
>
> -
2
3
F / I O ~ (V/cm)
0.035
1
4.0
45
50
Figure 7. Photocurrent potential curves for a passive film on zirconium (formationpotential 3 V vs. SHE in 1 M Na2S04)at various photon energies, taken from ref 11, 12.
hv/eV
4
HfO, (10% Xe) in 0.5M H,SO,
Figure 6. Potentials U,on passive tantalum as a function of photon energy, taken from ref 10.
U/V vs nhe
I
1.6
I
CI
* 0.8 anodic to cathodic a t potentials above the flatband po0.2 tential; (ii) potentials Uowhere the photocurrent changes sign, which are not identical with the flatband potential ( Uo > Ufb);(iii) potentials Uowhich are a function of the photon energy; and (iv), related to the latter, photocurrent spectra with anodic and cathodic currents where the X photon energy, hvo, at which the photocurrent changes sign +k is potential dependent. The systems are passive i r ~ n , ~ , ~ passive tantalum,'O passive zirconium," and heavily xenon-implanted passive films on hafnium.I2 Passive Iron. Passive films on iron show generally ancdic photocurrents as reviewed in ref 2. After prolonged polarization of the electrode (>24 h) the anodic photocurrent transients change to cathodic steady-state currents at potentials above the flatband p ~ t e n t i a l . ~Figure ,~ 5 shows that effect. While the transient anodic photocurrent 3 4 5 has a strong potential dependence, the cathodic photocurrent is potential independent. hv /eV Passive Tantalum. Passive films on tantalum have a bandgap energy around 3.9 eV with a fairly pronounced Figure 8. Photocurrent spectra of passive films on hafnium (d, tailing to lower photon energies.1° Upon cathodic pre= 50 nm) which were heavily ion implanted with xenon (lo%), treatment, a large subband-gap response is observed.2 It taken from ref 12. has been found that the potential dependence of the 6. Uochanges from approximately 0.4 V at h v = 3.6 eV photocurrent obtained from cyclic voltammetry exhibits to below 0.1 V at 5.0 eV. a pronounced hysteresis. In the scan to lower potentials, Passive Zirconium. The bandgap energy of zirconium the potential Uo where the photocurrent changes sign passive films has been determined to be about 4.8 eV; shows a signifcant dependence on the photon energy. This further results of photoelectrochemical investigations of should not be the case if Uois identical with the flatband passive films on zirconium are discussed elsewhere.'l For potential, U,. The dependence U,(hv) is shown in Figure thin films, d < 10 nm, U,, values are found to depend on the photon energy. Photocurrent potential curves for (8)Stimming, U. In Passiuity of Metals and Semiconductors; Frovarious photon energies are shown in Figure 7. Recording ment, M., Ed.; Elsevier: Amsterdam, 1983;p 477. photocurrent spectra at low potentials results in anodic (9)Searson, P.;Stimming, U.; Latanision, R. M. In Surfaces, Inhiband cathodic photocurrents (see Figure 2 in ref 11). The ition and Passivation; McCafferty, E., Brodd, R. J., Eds.; The Electrochemical Society: Princeton, NJ, 1986; p 175. photon energy hvoat which the photocurrent changes sign (10)White, T. M.S. Thesis, Columbia University, 1985. White, T.; is potential dependent. It increases with decreasing poStimming, U., manuscript in preparation. tential, i.e., the lower the potential the higher is the photon (11) Newmark, A. R.; Stimming, U. Langmuir, in press. energy at which a cathodic photocurrent is still observed. (12) Newmark, A. R.; Stimming, U., unpublished results.
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photocurrent considerably as compared to classical transitions. This enhancement is more pronounced at lower photon energies. The combined probability of the electron leaving the trap in the direction of the field, which includes classical transitions, direct tunneling, and phonon assisted tunneling, is given by3 = E, - OF'/*- 2kT hE=O
eXP( -4 (2m*)'i2
P A S S I V E FILM
(Ei - AE)3/2X
ELECTROLYTE
Figure 9. More detailed energetic situation of a photoexcited electron on a localized state (see Figure 4) experiencing a Coulombic force that is modifed by a superimposedelectricfield: (1) classical overbarrier process; (2) phonon-assisted tunneling; (3) direct tunneling, all in the direction of the field as treated in ref 3; (4) classical overbarrier process; (5) direct tunneling process against the field. x denotes the distance of the localized state from the passive film-electrolyte interface.
Xe-Implanted Passive Films on Hafnium. Previous investigations of xenon-implanted passive films on hafnium revealed a shift of the photocurrent onset in the photocurrent spectra to lower photon energies by approximately 2 eV as compared to unimplanted films.13 This can be explained by a disordering induced by the ion implantation process.14 Figure 8 shows photocurrent spectra with anodic and cathodic photocurrents for heavily xenon-implanted Hf02 films (10 atom % Xe).12 The cathodic portions increase with decreasing electrode potential. The results are similar to those obtained for thin passive films on zirconium. Discussion In the discussion we want to focus on one specific mechanism: the escape of a photoexcited charge carrier from a localized state as shown in Figure 4. This process seems to be important for the explanation of the experimental results described above. Let us assume a photoexcitation of an electron to a localized state; the photoinduced creation of a hole on a previously occupied localized state is, in principle, equivalent. In order to observe a photocurrent, the charge carrier has to be removed from the trap. Assuming that the electron is surrounded by a Coulombic well, the ionization energy, Ei, is given by the energy difference between the trap and the conduction band. Under superposition of an electric field, the barrier is reduced by an amount /3F1l2,which is the so-called Poole-Frenkel effect, and the escape probability, PpF, is given by PpF
s)
= exp[-(Ei - /3F'/2)/kT]
(1)
where /3 = (e3/~eo?r)1/2. Figure 9 describes the energetic situation of the electron in a schematic representation. The classical process described by the Poole-Frenkel effect is indicated by path 1. For high fields the barrier can become thin enough so that tunneling processes through the barrier become possible as well. This is shown in Figure 9 by path 2 for phonon-assisted tunneling and path 3 for direct tunneling. This situation has been discussed previously on the basis of model calculations for a number of parameter^.^ The results show that at fields F > lo5 V/cm tunneling can become significant, enhancing the (13) Danzfuea, B.; Schultze, J. W.; Stimming, U.; Meyer, 0. Mater. Sci. Eng. 1985, 69,273. (14) Stimming, U. Nucl. Instrum. Meth. B,in press.
(2)
Equation 2 describes process 1in Figure 4. If the trap is close enough to the surface, a direct escape of the electron into the electrolyte is possible as a competing process. This can be, in principle, a classical as well as a tunneling process, which is indicated by paths 4 and 5, respectively. The barrier for the process against the field which is given by h E ( x ) = e2/4attox + eFx (3) is so much higher than the one in the field direction that the probability for path 4 is always much lower than for path 1at U > U,. This is not the case for the tunneling processes. The probability for path 5 strongly depends on the distance, Ax, between the position of the trap and the surface. From the simple Gamov equation for a rectangular barrier, the probability, P,, is given by15
P, = exp[-2A~/h(2m*hE)'/~]
(4)
Inserting the expression for the field-dependent barrier height
AE = Ei - e2/4?rt~oAx+ eFAx/2
(5)
we obtain the probability in its field dependence. Calculations using eq 4 and 5 show that for reasonable assumptions for Ax, such as 1 or 2 nm, a considerable probability for path 5 exists. Thus, path 5 represents a competing reaction to paths 1-3. (It is assumed, of course, that for path 5 the hole moves easily toward the backside contact.) Any influence should be detectable since the sign of the current associated with path 5 is negative. In order to quantify the influence of the cathodic tunneling path, the overall probability, P, for an escape from the trap has been calculated. It is obtained by adding the probabilities of eq 2 and eq 4 and 5, giving P, a negative sign to account for the cathodic direction: P = Pa - P, (6) In order to study the influence of various parameters, calculations have been performed for the probability P as a function of the field for various ionization energies, Ei, of the trap. Ei relates to the photon energy, assuming photoexcitation from an extended to a localized state (see the discussion in ref 3 for such an assumption). The smaller Ei is, the larger is the corresponding photon energy; hv of the subbandgap response is given by the difference between the bandgap energy and the ionization energy of the trap h v = E, - Ei (7) (15)Duke, C. B. Tunneling in Solids;Academic: New York, 1969.
Tunneling in Reactions of Passive Films
-id
0
,
I
1
3
2
Langmuir, Vol. 3, No. 3, 1987 427
I
I
4
5
6
o (v/cm) Figure 10. Model calculations according to eq 6 for various ionization energies, Ei, for a distance Ax = 1 nm; m* = 0.5mE. F/I
0
1
2
3
4
5
6
o (V/cm) Figure 11. Model calculations according to eq 6 for various ionization energies, Ei, for a distance Ax = 2 nm; m* = 0.5mE. F/I
Results of such calculations are shown in Figure 10. The trap distance from the surface is assumed to Ax = 1 nm and curves are shown for Ei = 0.25-1.0 eV. The higher Ei is, Le., the smaller hv is, the higher is the field at which the probability switches sign. The latter can be seen from the points where the curves dip down to low probabilities: at these points the probability changes sign corresponding to anodic photocurrents at higher fields and cathodic photocurrents at lower fields. These points depend on Ei since for the cathodic process only the barrier height changes with Ei while in the anodic direction for paths 2 and 3, which are the more probable paths at high Ei?both barrier height and barrier thickness change with Ei. The slope for the anodic process is considerably higher than the one for the cathodic process. This results from the different field dependence of Paand P, as expressed in eq 2 and eq 4 and 5, respectively. A less favorable condition for the cathodic process exists assuming a greater distance of the trap level from the surface, e.g., Ax = 2 nm. Calculations for this condition are shown in Figure 11 for Ei = 0.25-1.0 eV. Even under these conditions the change of sign occurs at fields considerably above the flatband potential and a t quite moderate photon energies of subbandgap illumination. In order to compare the calculations with photocurrent measurements the following assumptions have to be made. The field has to be related to the electrode potential which is given by where Ap,, = U - U,. In a passive film with a linear potential drop where the thickness d = do,, the thickness of the passive film, the field is proportional to the electrode
potential. For a semiconducting film, d can be approximated by the thickness, ddl,of the depletion layer which depends on the square root of the electrode potential. Then the field also depends on the square root of the electrode potentid2 The observation of a cathodic current at the energy of the trap level requires the presence of unoccupied electronic states at the same energy, as represented by a chemical species of suitable redox potential in the electrolyte. It may also be discussed that surface states exist at the energy of the trap which would be able to mediate the charge transfer. A mediation of charge transfer in photoinduced reactions has been discussed previously.16 If the results of the calculations are compared with the experimental results, for instance for passive zirconium as shown in Figure 7 , the similarity is obvious. The results of Figure 7 are obtained for thin films; for thicker films cathodic photocurrents cannot be observed directly. A wavelength-dependent potential, Uo,however, indicates the same situation. The lower the photon energy the higher is the relative contribution of the cathodic process which shifts Uo in positive directions with decreasing photon energy. Such a dependence has been observed for passive zirconium and tantalum and xenon-implanted passive films on hafnium. In the latter case it has been found that the anodic photocurrent increases more steeply than the cathodic, which is predicted by the model calculations shown in Figures 10 and 11. A wavelength-dependent Uohas also been discussed for bulk crystalline and even single-crystalline TiOz e1ectrodes.l' That interpretation, in contrast to the one given here, makes surface states created by the illumination process responsible for the effect. It should be noted, however, that the wavelength dependence of Uodescribed in ref 17 is opposite to ours. This indicates that the process discussed in ref 17 is different from the one discussed here. The same model as outlined in the calculations which were performed as a function of the field can also explain the change in sign of the photocurrent in the spectral dependence. The latter has been observed for thin films on zirconium and xenon implanted hafnium passive films (Figure 8). At lower potentials, Le., at lower fields, the cathodic process is more favorable which results in photon energies, hvo,at which the sign change occurs, which are at higher hv. With increasing potential, hvo is shifted to lower values and eventually cannot be observed anymore. The influence of the cathodic photocurrent can also have an impact on the evaluation of the bandgap energy of the passive film. Since the overall (anodic) photocurrent is diminished by the cathodic process which is stronger at lower rather than at higher photon energies, at low potentials the apparent bandgap energy is shifted to higher values. This, in fact, has been observed for zirconium where a potential-dependent apparent bandgap energy has been found; this is discussed e1sewhere.l' The experimental results for passive iron are somewhat different: In photocurrent transients at potentials above the flatband potential, a steady-state cathodic current is observed after an anodic transient. This can also be explained by a tunneling process from a localized state to the electrolyte. The fact that it is restricted to a certain potential range may be due to a mediation of the charge transfer by surface states which only exist at a certain (16) Finlayson, M.F.;Wheeler, B. L.; Kakuta, N.; Park, K.-H.; Bard, A. J.; Campion, A.; Fox, M. A.; Webber, S. E.; White, J. M. J . Phys. Chem. 1985,89, 5676. (17) Sprunken, H.R.;Schumacher, R.; Schindler, R. N. Faraday Discuss. Chem. SOC.1980, 70, 55.
Langmuir 1987, 3, 428-433
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energy. In capacity measurements, a strong frequencydependent capacity has been observed at higher potentials which was discussed as a contribution of surface states.8 In photocurrent spectra, a potential-dependent onset energy similar to the one described for thin films on zirconium is observed a t low potentials. This is another indication that cathodic photocurrents are important at low potentials. The experimental results for passive iron, however, are not yet as detailed as for the other systems, allowing for no quantitative interpretation.
Conclusions Detailed results of photocurrent measurements on passive films revealed the presence of cathodic photocurrents at potentials above the flatband potential. The potentials, Uo,at which the photocurrent changes sign are found to be wavelength dependent which indicates that Uois not the flatband potential. It is also found that U,,
is usually more positive than U, values obtained from capacity measurements. Passive films are generally highly disordered or amorphous. This means that a large number of localized states are present in what is the bandgap of the corresponding crystalline material. For a photoexcitation process that involves localized states, the removal of the photoexcited charge carrier from the localized state can be rate determining? The model presented here takes into account reverse tunneling, i.e., tunneling processes against the field, which can result in cathodic photocurrents. The model calculations generate results that are very similar to experimental results and which are able to explain experimental results described above.
Acknowledgment. The help of Andrea Newmark in preparing the manuscript and performing the model calculations is greatly appreciated. Financial support from the National Science Foundation and Columbia University is gratefully acknowledged.
EXAFS Study of the Nickel Oxide Electrode? J. McBreen and W. E. O'Grady* Department of Applied Science, Brookhaven National Laboratory, Upton, New York 11973
K. I. Pandya and R. W. Hoffman Department of Physics, Case Western Reserve University, Cleveland, Ohio 44106
D. E. Sayers Department of Physics, North Carolina State University, Raleigh, North Carolina 27650 Received September 25, 1986 Transmission EXAFS has been used, in situ, to follow the changes in the structure of Ni(OH)2as it is being electrochemically oxidized in concentrated alkali electrolyte. Changes occurring during repeated oxidation and reduction of the electrode were also studied. EXAFS results reveal changes occurring in the xy plane of the crystal lattice. Lack of interplanar interactions indicates a loose disordered stacking of planes in Ni(OH)2. The interatomic distances and coordination numbers for Ni(OH)2are similar to those determined from X-ray diffraction. Oxidation of Ni(OH)*to the trivalent state results in a contraction of the Ni-0 and Ni-Ni distances in the xy plane. Reduction of the trivalent material yields similar a m considerable disorder interatomic distances to unoxidized Ni(OH)2. A single oxidation-reduction cycle c in the xy plane. This may facilitate subsequent electrochemical oxidation. The cell and techniques reported here are generally applicable to the study of oxidation-reduction processes in other hydrous oxides. Two advantages of the EXAFS technique are the ability to do in situ structure determinations and facility with which short-range order can be probed, even in highly disordered materials.
Introduction Oxidation-reduction processes in transition-metal hydrous oxides are important in charge storage,' electrocatalysis,2 corro~ion,~ and electrochromic d e v i ~ e s . ~These reactions often occur via topochemical reactions that yield amorphous products. The X-ray diffraction patterns for these products are characterized by broad lines of reduced intensity. In some cases there is even total absence of a diffraction pattern. The lack of structural information has caused much confusion in the field. The oxidation of Ni(OHI2is a case in point. It has been the subject of most studies and several excellent When Ni(OH)2is precipitated from a nickel salt solution by hot concentrated alkali, the hydroxide formed is re+This research was performed under the auspices of the US. Department of Energy under Contract DE-AC02-76CH00016.
ferred to 88 j3-Ni(OH)2.8 This material has a brucite C6 type structure. The hexagonal unit cell has the formula Ni(OH)2and dimensions a = 3.126 A and c = 4.605 A. The fractional coordinates of the atoms are, for nickel, 0, 0,O (1) Steele, B. C. H. In Power Sources for Electric Vehicles; McNicol, B. D., Rand, D. A. J., Eds.; Elsevier: Amsterdam, 1984; pp 697-719. (2) Trasatti, S.; Mi,G. In Electrodes of Conductiue Metallic Oxides; Trasatti, S., Ed.;Elsevier:. Amsterdam, 1981; pp 521-626. (3) OGrady, W. E. J. Electrochem. SOC.1980,127,555. (4) Gotteefeld, S.; Mchtyre, J. D. E.; Bani, G.; S h y , J. L. Appl. Phys. Lett. 1978, 33, 208. (5) Milaer, P.C.; Thomas,U. B. In Advances in Electrochemistry and Electrochemical Engineering; Delahay, P., Tobias, C. W., Eds.; Wiley: New York, 1967; Vol. 5, p 42. (6) Briggs, G. W. D. In Chemical Society Specialist Periodical Reports, Electrochemistry; The Chemical Society: London, 1974; Vol. 4, pp 33-54. (7) McEwen, R. S. J. Phys. Chem. 1971, 75, 1782. (8)Cairns, R. W.; Ott, E. J . Am. Chem. SOC.1933, 55, 527.
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