Synthesis of NiGa Layered Double Hydroxides. A Combined Extended X-ray Absorption Fine Structure, Small-Angle X-ray Scattering, and Transmission Electron Microscopy Study. 1. Hydrolysis in the Pure Ni 2+ System Guillaume Defontaine,* ,² Laurent J. Michot, ² Isabelle Bihannic, ² Jaafar Ghanbaja, ‡ and Vale´rie Briois § Laboratoire Environnement et Mine´ ralurgie (LEM), CNRS INPL ENSG UMR 7569, BP 40, 54501 Vandoeuvre le` s Nancy France, Service Commun de Microscopie Electronique a` Transmission, Faculte´ des Sciences, Universite´ Henri Poincare´ , BP 239, 54500 Vandoeuvre le` s Nancy France, and Laboratoire pour l’Utilisation du Rayonnement Electromagne´ tique (LURE), CNRS UMR 130, BP 34, 91898 Orsay Cedex France Received July 15, 2003. In Final Form: September 26, 2003 Takovites are nickel-based layered double hydroxides (LDHs) with a general formula that can be written as Ni1-xAlx(OH)2,A z- x/z, yH2O, where A is a compensating interlayer anion. As in some other LDH samples, the positive charge of the layer can be adjusted upon synthesis and various anions can be exchanged in the interlayer region. It is then important to understand the synthesis pathway of these materials. We then undertook a study on the hydrolytic behavior of pure Ni salts and mixtures of Ni and Ga salts. This paper focuses only on the hydrolysis of Ni 2+ ions carried out by base addition. The samples will be defined by their hydrolysis ratio R ) [OH - ]/[Ni 2+ ]. For all R values, colloids are observed in the final suspensions. Each hydrolyzed sample was studied by Ni K-edge extended X-ray absorption fine structure (EXAFS) to obtain information on the local structure of the colloids. Small-angle X-ray scattering (SAXS) and transmission electron microscopy (TEM) are also used to obtain concomitant information at larger scale. EXAFS results reveal that upon Ni 2+ hydrolysis by NaOH, the number of nickel neighbors increases with R, atomic distances corresponding only to edge-sharing nickel octahedra. These results could then be interpreted as revealing the precipitation of increasing amounts of Ni(OH)2, with increasing R values. However, SAXS curves display a significant evolution with R. Modeling shows the presence of rod-shaped colloids for low R values. These rodlike particles progressively disappear in favor of disklike platelets, that are the only species present in suspension for R ) 2.0. Such an evolution is confirmed by TEM analysis. Introduction The speciation of most transition metals in aqueous solutions is largely controlled by hydrolysis and conden- sation reactions. 1 In some cases, the pathway leading from a solution of a metal salt to a colloidal suspension of metallic oxide or hydroxide involves numerous steps where monomers condense to form various oligomers and/or polymers constituted by metallic cations under different conformations. These steps depend on both the chemical properties of metallic cations and characteristics of the aqueous media (pH, Eh, ionic strength, etc.). The iden- tification of these various steps is of crucial importance in many fields (water treatment, materials science, environmental science, etc.) but is often a complicated task requiring the combined use of various analytical techniques. In that context the combination of small-angle X-ray scattering (SAXS), extended X-ray absorption fine structure (EXAFS), and nuclear magnetic resonance (NMR) has been applied in recent years to unravel the hydrolytic behavior of various trivalent and tetravalent metal salts, e.g., lanthanum, 2 titanium, 3 chromium, 4-6 ferric iron, 7-9 aluminum, 10-16 and gallium. 17,18 Less at- tention has been dedicated to divalent metallic cations such as Cu 2+ , Ni 2+ , Mg 2+ , etc., despite their ability to give * To whom correspondence may be addressed. E-mail: guillaume.defontaine@ensg.inpl-nancy.fr. ² Laboratoire Environnement et Mine´ralurgie (LEM), CNRS INPL ENSG UMR 7569. ‡ Service Commun de Microscopie Electronique a` Transmission, Faculte´ des Sciences, Universite´ Henri Poincare´. § Laboratoire pour l’Utilisation du Rayonnement Electromag- ne´tique (LURE), CNRS UMR 130. (1) Baes, C. F., Jr.; Mesmer, R. E. The hydrolysis of cations; Wiley: New York, 1976. (2) Ali, F.; Chadwick, A. V.; Smith, M. E. J. Mater. Chem. 1997, 7, 285-291. (3) Chemseddine, A.; Moritz, T. Eur. J. Inorg. Chem. 1999, 2, 235- 245. (4) Stu¨ nzi, H.; Rotzinger, F. P.; Marty, W. Inorg. Chem. 1984, 23, 2160-2164. (5) Jones, D. J.; Rozie` re, J.; Mairelles-Torres, P.; Jimenez-Lopez, A.; Olivera-Pastor, P.; Rodriguez-Castellon, E.; Tomlinson, A. A. G. Inorg. Chem. 1995, 34, 4611-4617. (6) Roussel, H.; Briois, V.; Elkaim, E.; de Roy, A.; Besse, J.-P. J. Phys. Chem. 2000, 104, 5915-5923. (7) Tchoubar, D.; Bottero, J. Y.; Quienne, P.; Arnaud, M. Langmuir 1991, 7, 398-402. (8) Bottero, J. Y.; Tchoubar, D.; Arnaud, M.; Quienne, P. Langmuir 1991, 7, 1365-1369. (9) Bottero, J. Y.; Manceau, A.; Villieras, F.; Tchoubar, D. Langmuir 1994, 10, 316-319. (10) Akitt, J. W.; Farthing, A. J. Magn. Reson. 1978, 32, 345-352. (11) Akitt, J. W.; Farthing, A. Chem. Soc., Dalton. Trans. 1981, 1617- 1623. (12) Akitt, J. W.; Elders J. M. Chem. Soc., Dalton. Trans. 1988, 1347- 1355. (13) Akitt, J. W.; Kettle, D. Magn. Reson. Chem. 1989, 27, 377-379. (14) Bottero, J. Y.; Cases, J. M.; Fiessinger, F.; Poirier, J. E. J. Phys. Chem. 1980, 84, 2933-2939. (15) Bottero, J. Y.; Tchoubar, D.; Cases, J. M.; Fiessinger, F. J. Phys. Chem. 1982, 86, 3667-3673. (16) Bottero, J. Y.; Axelos, M.; Tchoubar, D.; Cases, J. M.; Fripiat, J. J.; Fiessinger, F. J. Colloid Interface Sci. 1987, 117, 47-57. (17) Michot, L. J.; Montarge` s-Pelletier, E.; Lartiges, B. S.; d’Espinose de la Caillerie, J.-B.; Briois, V. J. Am. Chem. Soc. 2000, 122, 6048- 6056. (18) Pokrovski, G. S.; Schott, J.; Hazemann, J. L.; Farges, F.; Pokrovski, O. S. Geochim. Cosmochim. Acta 2002, 66, 4203-4222. 10588 Langmuir 2003, 19, 10588-10600 10.1021/la035279m CCC: $25.00 © 2003 American Chemical Society Published on Web 11/08/2003