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Layer-by-Layer Polymeric Supramolecular Structures Containing Nickel Hydroxide Nanoparticles and Microcrystallites Aurora Marie Fojas,† Erin Murphy, and Pieter Stroeve* Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA), Department of Chemical Engineering and Materials Science, University of California at Davis, Davis, California 95616
Supramolecular polymer structures were assembled using the layer-by-layer, polyionic deposition technique. Depositing alternating layers of a polycation, poly(diallyldimethylammonium chloride), and a polyanion, poly(styrenesulfonate), formed the thin films. The nucleation of nickel hydroxide nanoparticles was achieved using a cyclical absorption-oxidation process in which nickel ions were absorbed into the polymer matrix (which bind to the sulfonate groups) and then oxidized in an alkali solution. Ultraviolet-visible spectroscopy was used to monitor the deposition of the thin film, as well as the subsequent nucleation and growth of nanoparticles within the supramolecular structure. Characterization of the nanoparticles using Fourier transform infrared spectroscopy showed the presence of the R form of nickel hydroxide, and X-ray diffraction analysis showed a mixture of both R and β forms of nickel hydroxide. Transmission electron microscopy images revealed fibrous, needlelike microcrystalline structures, increasing in size with absorption-oxidation cycles, surrounded by nanoparticles of approximately 5-50 nm in size. Introduction Nickel hydroxide has been shown to exist in a variety of forms.1-3 The two most common forms are the R and β forms of Ni(OH)2. R-Ni(OH)2 consists of stacked Ni(OH)2-x layers intercalated with various anions or water molecules and is categorized as being isostructural with hydrotalcite-like compounds. The anhydrous β-Ni(OH)2 does not have intercalated species and is structurally brucite-like.4 Nickel hydroxide is an important material because of the relative ease with which it is transformed into other materials. Nickel oxides, which can be obtained through heating of nickel hydroxide, are industrially used in catalysis.5-7 Nickel hydroxide is also used as an additive in lubricants and has been shown to provide good anticorrosion properties to surfaces.8 One of the most prominent applications of nickel hydroxide is in rechargeable battery systems, such as the nickel-cadmium battery. Overall, the simplified chemistry at a charged cathode of a battery9 is NiOOH + e- + H+ T Ni(OH)2. During recharging, nickel hydroxide can be oxidized to nickel oxyhydroxide, which is the starting material at the positive electrode. However, nickel systems are at a disadvantage when compared to other more expensive and more hazardous batteries, such as lithium systems, because they do not yield as high an energy density,10 and over repeated charging-discharging cycles, they lose their storage capacity. Reducing the size of materials changes their electrochemical, catalytic, and optical properties, a characteristic apparent in current quantum dot research.11 Likewise, nickel hydroxide is a promising candidate for crystal size reduction, yielding better electrochemical * To whom correspondence should be addressed. E-mail:
[email protected]. Telephone: (530) 752-8778. Fax: (530) 752-8778. † Present address: School of Engineering, Tufts University, Medford, MA 02155.
properties, according to research done by Watanabe et al.12 If electrodes in nickel systems could be coated with nickel hydroxide nanoparticles easily and reliably, then lower-cost battery systems could be created which could yield increased energy density outputs and larger capacities. Furthermore, coatings of nanoparticles of nickel hydroxide would be of interest in catalytic applications.5-7 To encourage small particulate growth, a medium with constrained growth sites would provide a useful environment for nucleation. Polymers lend themselves to this characteristic. Polymers can be formed into membranes or coatings of various shapes and thicknesses. It has been shown by Decher et al.13-16 that ultrathin, multilayer, ionic polymer assemblies can be coated reproducibly on supports and that these layerby-layer structures are stable. Furthermore, it has been shown that these ultrathin films can be used to grow nanoparticles of metal hydroxides, oxides, sulfides, and sulfates.17-20 In this research, monolayers of polyions were deposited on a substrate using the layer-by-layer technique.13-16 A polycation, poly(diallyldimethylammonium chloride) (PDDA), and a polyanion, poly(styrenesulfonate) sodium salt (PSS), were adsorbed onto substrates in an alternating manner to build up ultrathin films. Nickel hydroxide particles were nucleated within the polymeric, supramolecular structure through cyclic exposure to a solution of nickel metal ions and an alkali solution under an inert environment. It has previously been shown that the sulfonate groups in PSS are strong binding sites for metal ions and that the bound ions are available for chemical reaction.17-20 The present work is focused on producing the ultrathin films and characterizing the nickel hydroxide particles. Experimental Section Sodium hydroxide and hydrochloric acid (reagent grade) were purchased from Fisher Scientific Chemical Co. PDDA and PSS were obtained from Polysciences Inc.
10.1021/ie010689w CCC: $22.00 © 2002 American Chemical Society Published on Web 04/02/2002
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Nickel(II) chloride hexahydrate was supplied by Aldrich Chemical Co. All reagents were used as received. Solutions were prepared using purified, deionized water (18.1 MΩ).17-20 Quartz slides, which were used as substrates for ultraviolet-visible (UV-vis) analysis and X-ray diffraction (XRD) films, were thoroughly cleaned by soaking in a piranha solution bath [H2O2:H2SO4 ) 3:7 (v/v)] for 1 h, followed by a rinse and sonication in deionized water.20 Extreme caution must be used when handling piranha solutions because the mixture is extremely corrosive and is known to cause serious damage to skin and tissues. The ZnSe substrates for Fourier transform infrared (FTIR) analysis were cleaned by soaking in methylene chloride for 30 min and then rinsed with purified water. The polyionic solutions of PDDA and PSS were both prepared in 20 mM concentrations; the former was dissolved in deionized water, while the latter was dissolved in a 0.1 M NaOH solution and then adjusted to a pH of 4.5 by dropwise addition of 0.1 M HCl. All concentrations were calculated based on the monomer molecular weights. Polymer films were assembled on substrates using the layer-by-layer technique similar to procedures described elsewhere.17-19 A PDDA/PSS deposition cycle is termed the deposition of one layer pair. Deposition always ended with the first half of a layer pair in order to terminate the film with a PDDA layer.17-20 Nucleation of Ni(OH)2 nanoparticles within the polymer matrix occurred by cycling the polymer-coated substrates in Ni2+ and NaOH solutions. The Ni2+ solutions were prepared in concentrations of 4-40 mM by dissolving nickel(II) chloride hexahydrate in purified water. Solutions of 0.1-1.0 M NaOH were made using purified water and 10 M NaOH. Before cycling, all solutions were vigorously sparged for 45 min with nitrogen gas to remove any dissolved oxygen in solution. In Schlenken tubes, under nitrogen gas, the polymercoated substrates were exposed to the Ni2+ solution for 2 min. They were rinsed with degassed and purified water to remove excess ions absorbed in the film and then exposed to the NaOH solution for 7 min. Substrates were rinsed with degassed and purified water and dried completely before the process was repeated. UV-vis absorption was carried out on a Cary 3 absorption spectrophotometer. Quartz slides with 0-12.5 layer pairs were used, and nucleation was performed with 6.5 layer pairs through 0-12 cycles of 20-40 mM Ni2+ and 1.0 M NaOH solutions. FTIR spectroscopy of the samples was carried out on a Nicolet Prote´ge´ 460 spectrophotometer with a KBr beam splitter and an MCT/A detector. Clean ZnSe substrates were used, and nucleation was carried out with 4.5 layer pairs through 12 cycles of 40 mM Ni2+ and 0.1 M NaOH. The low concentration of NaOH for the FTIR experiments was necessary because strong bases can attack ZnSe. XRD data were taken as described before.17-20 To obtain sufficient material, the samples for XRD were formed on quartz slides by depositing 10.5 layer pairs and cycling 20 times with 30 mM Ni2+ and 1.0 M NaOH solutions. A powder sample was obtained by scraping off the prepared films from six quartz slides. Samples for transmission electron microscopy (TEM) studies were prepared on Formvar-coated grids (300 mesh) with 2.5 layer pairs with cycles of 4 mM Ni2+ and 1.0 M NaOH solutions. The lower Ni2+ solution concentration
Figure 1. (a) UV-vis absorption for a number of layer-pair (LP) depositions (without absorption-oxidation cycles). (b) Absorbance at 225 nm with respect to the number of layer-pairs deposited.
was required to obtain a good contrast and to have fewer particles in an image. Images of the films were taken on a Hitachi 6010 scanning TEM. Results UV-vis spectroscopy was used to monitor the layerpair deposition. Figure 1a shows that the polymer films deposited exhibit an absorption band at 225 nm wavelength, which is an indicator for the aromatic rings in PSS.17-20 The absorption of the film at 225 nm increases linearly with the number of layer pairs deposited, as seen in Figure 1b. Nucleation and growth of nanoparticles were also monitored by UV-vis spectroscopy. Figure 2a shows the absorption of a 6.5 layer-pair film after it was subjected to a number of absorptionoxidation cycles. The bases of all of the UV-vis spectra rise as the number of cycles increases. Figure 2b shows that the absorption of the film at 300 nm increases linearly with the number of cycles. Identification of the nanoparticles grown was determined via FTIR, TEM, and XRD analysis. In FTIR spectra published in the literature3 (not shown), β-Ni(OH)2 spectra show a peak at about 3650 cm-1 due to a non-hydrogen-bonded OH- stretch and absorption at 540 and 470 cm-1, while R-Ni(OH)2 spectra show a broad band from 3600 to 3000 cm-1 due to hydrogenbonded OH- stretching, as well as vibrations in the 1600-1000 cm-1 region and at 640 and 470 cm-1. The FTIR results from our work, shown in Figure 3, show a very strong broad band centered at 3300 cm-1, two small
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Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002 Table 1. Model d Spacings for r- and β-Ni(OH)2, as Well as Ni(OH)2 Made from Nickel Chloride Precursorsa Model dobs (Å) hkl
stabilized R
hkl
stabilized β
003 006 101 105 107 110
7.89 3.89 2.542 2.321 2.222 1.501
001 100 101 102 110
4.62 2.706 2.332 1.755 1.563
dobs (Å) for Ni(OH)2 from Rajamathi et al. 5.4 2.691 1.755 4.41 2.326 1.558 a
Both sets of data are taken from Rajamathi et al.3
Figure 2. (a) UV-vis absorption spectra for 6.5 layer pairs cycled in a 40 mM NiCl2‚6H2O solution and in a 1.0 M NaOH solution. (b) Absorption at 300 nm with respect to the number of cycles.
Figure 3. FTIR transmission spectrum for 4.5 layer pairs with 12 cycles in 40 mM NiCl2‚6H2O and 0.1 M NaOH on a ZnSe substrate.
peaks around 2900 cm-1, and various broad peaks in the 1600-1000 cm-1 region. The small sharp peaks around 2900 cm-1 are indicative of -CH3 modes and stem from polymer contributions to the spectra.21 The broad band at 3300 cm-1 and various bands between 1600 and 1000 cm-1 are both characteristic of the R form of nickel hydroxide. Model d spacings from XRD results for stabilized R and β forms of Ni(OH)2, as well as published XRD data for Ni(OH)2 prepared from nickel chloride precursors, are taken from Rajamathi et al.3 and are shown in Table 1. Figure 4a shows XRD results from our work (10.5
Figure 4. XRD data where (a) dotted and solid bars denote expected peaks of models R- and β-Ni(OH)2, respectively, and dashed-line bars denote expected peaks of Ni(OH)2 synthesized from a NiCl2 precursor and (b) fitted Gaussian curves (dotted lines) sum to a curve (dashed line) similar to that of the actual XRD data (solid line).
layer pairs, 20 cycles). Peaks are observed at 2θ ) 16.19, 21.80, 34.41, and 60.40°. These peaks were converted to d spacings using Bragg’s law, nλ ) 2d sin θ, where λ is the wavelength of the X-ray (Cu KR line ) 1.540 562 Å), n is an integer, and θ is the angle of diffraction in radians. Converting 2θ into d spacings shows results with peaks at d ) 5.47, 4.07, 2.60, and 1.53 Å, respectively. Figure 4b shows Gaussian curves fitted to the data at 2θ ) 30-45°. The sum of these Gaussian curves closely matches the actual XRD data. In parts a and b of Figure 5, TEM images (2.5 layer pairs, 8 cycles) show fibrous, needlelike microcrystalline particles surrounded by a network of fibrous, needlelike nanoparticles. Magnifications of 25K and 50K show the
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Figure 5. TEM images of 2.5 layer pairs with 8 cycles of 4 mM NiCl2‚6H2O at (a) 25K and (b) 50K magnifications.
fibrous nanoparticles ranging from 5 to 50 nm in length, while the larger particles are about 140-420 nm in length and 40 nm wide. Discussion Procedures for layer pair deposition initially corresponded with those described by Dante et al.20 and followed PDDA adsorption with a purified water rinse before exposure to PSS. However, upon comparison with films produced by the method described in the present work and elsewhere,17-19 it was determined that fol-
lowing PDDA adsorption with a dilute HCl rinse increased the adsorption of the PSS layer deposited directly afterward. In this work the latter procedure was used. In parts a and b of Figure 1, UV-vis results indicate that polymer layers were deposited evenly using the procedures used previously.17-19 The absorption of light at 225 nm increased linearly with the number of layer pairs. Because PSS absorbs light at 225 nm, the linear increase shows a uniform deposition of PSS with each layer. The absorption of light by the film increases as particles are nucleated and grown within the polymer matrix.17-20 The uniform increase of the spectra seen in Figure 2a is clearly visible. Particles formed within the film impart a visible color to the film after repeated cycling. The spectra show that there is a linear relationship between the amount of nickel hydroxide formed within the polymer structure and exposure to the cycling solutions. To avoid any distortion that might be seen from the PSS aromatic ring absorption, the 300 nm data were chosen to be representative of the change in absorption for the 6.5 layer-pair film in the visible range. The linearity of the results given in Figure 2b indicates that controlling the amount of nanoparticles deposited within the film can be done directly by exposure to the number of cycling solutions. The FTIR spectrum in Figure 3 shows a strong, broad band centered at 3300 cm-1. This broad band is indicative of hydrogen-bonded OH- stretches and is a major characteristic of the R-Ni(OH)2 FTIR spectrum.21 The multiple bands in the 1600-1000 cm-1 region are due to strong vibrations from intercalated anions and also are indicative of the R form of nickel hydroxide. The major characteristics of the β form, such as a strong spike at about 3650 cm-1, are not readily observed, but it is possible that trace amounts of the β form are present within the sample; its contributions can be masked by the strong broad peak of the R form. The FTIR spectra below 670 cm-1 were not taken, so correlation with absorption bands under 1000 cm-1 could not be made. The FTIR results show a dominance of R-Ni(OH)2 within the film, with possibly trace amounts of the β form present. In Figure 4a, the XRD data, namely, d spacings of 5.47, 2.60, and 1.53 Å, are indicative of β-Ni(OH)2. The large intensity of the peak at 2.60 Å is almost 4 times larger than the other characteristics of the XRD curve. Its asymmetry and range indicate that it is a broad peak representative of β-Ni(OH)2, but its size seems out of place with respect to the peak, 2θ (d) ) 21.80° (4.07 Å), which indicates the presence of the R form. All other peaks indicate the presence of the β form. It seems likely that contributions from both R and β forms cause the shape of the spectrum. In Figure 4b, the sum of two broad β bands at 2θ (d) ) 34.41° (2.60 Å) and 37.80° (2.38 Å), created by fitting two Gaussian curves to the actual data, produces an asymmetric curve which matches the data well. With XRD, broad bands are expected when a range of crystal sizes is present and the bands can run into each other and effectively combine to produce a broad peak. If this range of crystal sizes holds true, then one can speculate that R-form contributions over the d ) 2.54-2.22 Å range, together with the broad β bands at d ) 2.706 and 2.338 Å, are causing the large peak present over 2θ ) 30-45°. The spectrum supports the presence of β form and a trace of R form within the film.
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TEM images, e.g., Figure 5 a,b, show a network of fibrous, needlelike nanoparticles surrounding fibrous, needlelike microcrystalline structures. In a single absorption-oxidation cycle (not shown), only a few nanoparticles are seen. With more cycles, additional nanoparticles are nucleated in the films, while existing particles grow in size. The R form of Ni(OH)2 has been described in the literature as ribbonlike8 and fibrous in nature.4 The microcrystals seen in the images in our work resemble single strands of fibrous conglomerations. The needlelike particles in Figure 5a,b look identical to TEM images of R-Ni(OH)2 particles.4,8 Crystal growth would stem from the areas where initial particle nucleation occurred, presumably the nanoparticles. However, the microcrystalline size indicates that the particles are not restricted to the void spaces within the PSS layer. The sizes of the microcrystals (140-420 nm in length and 40 nm in width) shown in the TEM images indicate that, as the Ni(OH)2 particles grow, they expand into, and presumably grow out of, the film with increased cycles of absorption-oxidation. Growing the Ni(OH)2 within the polymeric assembly helps prevent conglomeration of the particles but does not hinder their size. The β form, which has been described as having a hexagonal, platelet-like structure,3 could not be identified in our TEM images, even though its presence is indicated in the samples prepared for XRD analysis. It is possible that the fibrous microcrystals and perhaps the fibrous nanoparticles contain trace amounts of the β phase. Results derived in this work can be explained by the R form being preferentially created within the film for relatively few cycles and low NaOH concentrations. In samples prepared for XRD, the R form could have been partially converted to the β form because more cycles were used and the NaOH concentration was higher. It has been shown that the R form converts to the more stable and anhydrous β form over time, at high temperatures, and in alkali solutions.1,2,22-24 Because nucleation of nickel hydroxide within the film requires exposure to an NaOH solution, it is likely that R-Ni(OH)2 is formed within the film initially but that it is converted to the β form after repeated cycling. The TEM images in Figure 5a,b were only exposed to 8 cycles, so prolonged exposure to NaOH did not occur. Samples for FTIR, which were exposed to 12 cycles, were prepared using a weaker alkali solution (0.1 M) and indicated the presence of mainly R form. The XRD data indicate a strong presence of β-Ni(OH)2 because of the method of preparation, namely, 20 cycles in a 1.0 M NaOH solution. The alkali exposure over a relatively long period of time can cause conversion from R to β forms in the 10.5 layer-pair film. Images with TEM could not be taken for 20 cycles and 1.0 M NaOH because the density of crystals was so large that mainly a black image was obtained. We see that Ni(OH)2 is nucleated and grown within the polyion films. The results show that the R form is present for a relatively low number of cycles and a low concentration of NaOH, while the β form is present when the number of cycles is relatively large (20) and the NaOH concentration is high (1.0 M). The method as described in this work can be used to coat substrates with ultrathin polymeric films containing particles of nickel hydroxide. These films may be useful in applications in batteries and in catalysis.
Conclusion The present work confirms the uniform coating of ultrathin supramolecular structures on several substrates using the layer-by-layer deposition technique. The nucleation of nickel hydroxide nanoparticles was achieved using a cyclical absorption-oxidation process in which nickel ions were absorbed into the ultrathin polymer film and then oxidized in an alkali solution. UV-vis spectroscopy was used to monitor the deposition of the thin film, as well as the subsequent nucleation and growth of particles within the supramolecular structure. Based on FTIR measurements, the particles in the films were shown to be mainly R-Ni(OH)2. XRD data indicated the presence of a mixture of R- and β-Ni(OH)2 probably because of repeated exposure over many cycles to a 1.0 M NaOH solution. The structures seen in TEM imaging show fibrous microcrystals within a network of fibrous nanoparticles consistent with R-Ni(OH)2. Acknowledgment This work was supported by grants from the National Science Foundation through NSF-MRSEC (DMR9808677) and NSF-REU (DMR-9820149) through the Center on Polymer Interfaces and Macromolecular Assemblies (CPIMA). A.M.F. was a SURE student at CPIMA. Literature Cited (1) Oliva, P.; Leonardi, J.; Laurent, J. F.; Delmas, C.; Braconnier, J. J.; Figlarz, M.; Fievet, F. Review of the Structure and the Electrochemistry of Nickel Hydroxides and Oxyhydroxides. J. Power Sources 1982, 8, 229. (2) Bernard, M. C.; Bernard, P.; Keddam, M.; Senyarich, S.; Takenouti, H. Characterisation of New Nickel Hydroxides During the Transformation of R Ni(OH)2 to β Ni(OH)2 by Ageing. Electrochem. Acta 1996, 41, 91. (3) Rajamathi, M.; Subbanna, G. N.; Kamath, P. V. On the Existence of a Nickel Hydroxide Phase Which is Neither R nor β. J. Mater. Chem. 1997, 7, 2293. (4) Jeevanandam, P.; Koltypin, Y.; Gedanken, A. Synthesis of Nanosized R-Nickel Hydroxide by a Sonochemical Method. Nanoletters 2001, 1, 263. (5) Mango, F. D. Transition Metal Catalysis in the Generation of Natural Gas. Org. Geochem. 1996, 24, 977. (6) Roginskaya, Y. E.; Morozova, O. V.; Lubnin, E. N.; Ulitina, Y. E.; Lopukhova, G. V.; Trasatti, S. Characterization of Bulk and Surface Composition of CoxNi1-xOy Mixed Oxides for Electrocatalysis. Langmuir 1997, 13, 4621. (7) Andreev, A.; Khristov, P.; Losev, A. Catalytic Oxidations of Sulfide Ions over Nickel Hydroxides. Appl. Catal. B 1996, 7, 225. (8) Zhang, J.; Yang, S.; Xue, Q. Preparation and Characterization of Ni(OH)2 Nanoparticles Coated with Dialkyldithiophosphate. J. Mater. Res. 2000, 15, 541. (9) Watanabe, K.; Kumagai, N. Thermodynamic Studies of Cobalt and Cadmium Additions to Nickel Hydroxide as Material for Positive Electrodes. J. Power Sources 1998, 76, 167. (10) Reisner, D. E.; Salkind, A. J.; Strutt, P. R.; Xiao, T. D. Nickel Hydroxide and Other Nanophase Cathode Materials for Rechargeable Batteries. J. Power Sources 1997, 65, 231. (11) Wang, Y.; Herron, N. Nanometer-sized Semiconductor Clusters: Materials Synthesis, Quantum Size Effects, and Photophysical Properties. J. Phys. Chem. 1991, 95, 525. (12) Watanabe, K.; Kikuoka, T.; Kumagai, N. Physical and Electrochemical Characteristics of Nickel Hydroxide as a Positive Material for Rechargeable Alkaline Batteries. J. Appl. Electrochem. 1995, 25, 219. (13) Decher, G. Layered Nanoarchitectures via Directed Assembly of Anionic and Cationic Molecules. Compr. Supramol. Chem. 1996, 9, 507. (14) Decher, G.; Hong, J. Fine-tuning of the Film Thickness of Ultrathin Multilayer Films Composed of Consecutively Alternating
Ind. Eng. Chem. Res., Vol. 41, No. 11, 2002 2667 Layers of Anionic and Cationic Polyelectrolytes. Prog. Colloid Polym. Sci. 1992, 89, 160. (15) Decher, G. Fuzzy Nanoassemblies: Toward Layered Polymeric Multicomposites. Science 1997, 277, 1232. (16) Decher, G.; Lvov, Y.; Schmitt, J. Proof of Multilayer Structural Organization in Self-assembled Polyanion-polycation Molecular Films. Thin Film Solids 1994, 244, 772. (17) Zhang, L.; Dutta, A.; Jarero, G.; Stroeve, P. Nucleation and Growth of Cobalt Hydroxide Crystallites in Organized Polymeric Multilayers. Langmuir 2000, 16, 7095. (18) Dutta, A.; Jarero, G.; Zhang, L.; Stroeve, P. In-situ Nucleation and Growth of γ-FeOOH Nanocrystallites in Polymeric Supramolecular Assemblies. Chem. Mater. 2000, 12, 176. (19) Dutta, A.; Ho, T.; Zhang, L.; Stroeve, P. Nucleation and Growth of Lead Sulphide Nano- and Micro-Crystallites in Supramolecular Polymer Assemblies. Chem. Mater. 2000, 12, 1042. (20) Dante, S.; Hou, Z.; Risbud, S.; Stroeve, P. Nucleation of Iron Oxy-Hydroxide Nanoparticles in Layer-By-Layer Polyionic Assemblies. Langmuir 1999, 15, 2176.
(21) Surca, A.; Orel, B.; Pihlar, B. Characterisation of Redox States of Ni(La)-Hydroxide Films Prepared Via the Sol-gel Route by Ex Situ IR Spectroscopy. J. Solid State Electrochem. 1998, 2, 38. (22) Fievet, F.; Figlarz, M. Preparation and Study by Electron Microscopy of the Development of Texture with Temperature of a Porous Exhydroxide Nickel Oxide. J. Catal. 1975, 39, 350. (23) McEwen, R. S. Crystallographic Studies on Nickel Hydroxide and the Higher Nickel Oxides. J. Phys. Chem. 1971, 75, 1782. (24) Freitas, M. B. J. G. Nickel Hydroxide Powder for NiOOH/Ni(OH)2 Electrodes of the Alkaline Batteries. J. Power Sources 2001, 93, 163.
Received for review August 20, 2001 Revised manuscript received January 22, 2002 Accepted January 23, 2002 IE010689W
