in the Presence of Polymers and Surfactants: Nanocrystals - American

Morphological Control of Hydrothermal Ni(OH)2 in the Presence of Polymers and Surfactants: Nanocrystals, Mesocrystals, and Superstructures Maria Teresa Buscaglia,† Vincenzo Buscaglia,*,†,‡ Carlo Bottino,† Massimo Viviani,†,‡ Roxane Fournier,§ Mohamed Sennour,| Sabrina Presto,⊥ Rinaldo Marazza,‡,⊥ and Paolo Nanni†,‡,#

CRYSTAL GROWTH & DESIGN 2008 VOL. 8, NO. 10 3847–3855

Institute for Energetics and Interphases, National Research Council, Via De Marini 6, I-16149 Genoa, Italy, Italian InteruniVersity Consortium on Materials Science and TechnologysINSTM, Via G. Giusti 9, I-50121 Florence, Italy, ESIREM, UniVersite´ de Bourgogne, AVenue A. SaVary 9, 21078 Dijon Cedex, France, Centre des Mate´riaux, Ecole des Mines de Paris, BP 87, 91000 EVry, France, Department of Chemistry and Industrial Chemistry, UniVersity of Genoa, Via Dodecaneso 31, I-16146 Genoa, Italy, and Department of Process and Chemical Engineering, UniVersity of Genoa, Fiera del Mare, P.le Kennedy, I-16149 Genoa, Italy ReceiVed May 27, 2008

ABSTRACT: Polymers with different hydrophilic groups [polyvinylpyrrolidone (PVP), ammonium polyacrylate (APA), and hydroxypropylmethyl cellulose (HPMC)] and surfactants [cetyltrimethylammonium bromide (CTAB) and sodium dodecylbenzensulfonate (SDBS)] were used as additives to modify the crystallization of β-Ni(OH)2 in hydrothermal conditions. Marked morphological changes in the β-Ni(OH)2 particles were observed depending on the additive concentration and on the duration of the hydrothermal treatment. The final morphology is the result of a complex, time-dependent self-assembly and growth process. Well-defined particles with sizes from submicrometer range to a few micrometers corresponding to hexagonal lamellae, hexagonal tabular mesocrystals, rosette- and flowerlike aggregates of lamellae, hexagonal prismatic mesocrystals, and acicular nanocrystals were easily obtained after a short time (2-24 h) aging at 150-200 °C. With PVP and CTAB, there is evidence of a growth process dominated by self-assembly of nanocrystals to produce mesocrystals. The formation of spherical superstructures (with SDBS, up to 70 µm in diameter) and hollow spheres (with PVP) is observed at long times (>24 h) as a result of solvent-mediated recrystallization processes, like Ostwald ripening. The overall results show that hydrothermal synthesis of β-Ni(OH)2 in the presence of polymers with hydrophilic groups and surfactants is a versatile tool for crystal morphogenesis. Introduction Nickel hydroxide is a material with important applications in energy conversion and energy storage devices. It is the active component of the positive electrode in alkaline rechargeable batteries (Ni-Cd, Ni-Zn, and Ni-Fe)1 and is also used as a precursor for the preparation of porous and composite nickel electrodes for solid-oxide fuel cells and molten carbonate fuel cells1,2 by sintering in a reducing atmosphere. In general, the performances of the devices are strongly dependent on size, morphology, and crystal structure of Ni(OH)2. Two polymorphic forms of nickel hydroxide, R and β, exist. The hexagonal β phase has a brucitelike hexagonal structure with a ) 3.12 Å and c ) 4.60 Å consisting of the ordered stacking of Ni(OH)2 layers with a Ni-Ni distance of 4.60 Å. The β modification is usually obtained by controlled addition of a base (NH4OH, NaOH, KOH, tetramethylammonium hydroxide, and methylamine) to a solution containing a soluble Ni salt (nitrate, acetate, chloride, etc.) and consists of very thin hexagonal platelets with a diameter of 50-200 nm.3-11 Treatment in hydrothermal conditions results in well-crystallized platelets. Ammonia evaporation from aqueous solutions of the Ni(NH3)62+ complex results in larger platelets (≈1 µm) whose thickness depends on * To whom correspondence should be addressed. Tel: +39-010-6475708. Fax: +39-010-6475700. E-mail: [email protected]. † National Research Council. ‡ Italian Interuniversity Consortium on Materials Science and TechnologysINSTM. § Universite´ de Bourgogne. | Ecole des Mines de Paris. ⊥ Department of Chemistry and Industrial Chemistry. # Department of Process and Chemical Engineering.

the nickel concentration.12 Use of ethylenediamine and hexamethylenetetramine as precipitating agent produces spherical aggregates of thin lamellae.13,14 Flowerlike aggregates of hexagonal platelets were obtained by hydrothermal treatment of a Ni-dimethylglyoxime precursor.15 Synthesis of β-Ni(OH)2 particles with different morphologies is not trivial and was mainly performed using templated growth. Nanorods were fabricated by the hydrothermal process inside the cavity of carbon nanotubes.16 Nanotubes can be prepared by impregnation and precipitation using a porous alumina membrane as a template.17 Bidimensional arrays of Ni(OH)2 hollow spheres on indium tin oxide substrate were fabricated using polystyrene microspheres as templates.18 Inorganic structures with welldefined size and morphology, often originated by self-assembly of nano building blocks (mesocrystals), can be obtained in the presence of suitable organic molecules, such as polymers with hydrophilic groups, double-hydrophilic block copolymers, and surfactants.19 However, the use of organic additives to direct the morphogenesis of Ni(OH)2 has received little attention. Coudun and Hochepied20 have systematically investigated the formation of elongated nickel hydroxide particles originated by the stacking of thin rounded platelets (pancake morphology) induced by sodium dodecylsulfate. Hydrothermal synthesis of β-Ni(OH)2 in the presence of glycerol produced spherical aggregates of nanosheets.21 Spherical aggregates of nanoplatelets were also prepared by reverse microemulsions using cetyltrimethylammonium bromide (CTAB) as a surfactant and urea as a base.22 In the present study, we report the preliminary results of the effect of some surfactants and polymers with different functional

10.1021/cg800555x CCC: $40.75  2008 American Chemical Society Published on Web 09/09/2008

3848 Crystal Growth & Design, Vol. 8, No. 10, 2008

groups on the morphology of β-Ni(OH)2 particles prepared by hydrothermal synthesis using aqueous ammonia as base. The effect of additive concentration and duration of hydrothermal treatment was investigated in detail. Experimental Section Hydrothermal Synthesis. Nichel hydroxide, β-Ni(OH)2, particles were obtained by means of hydrothermal synthesis with and without the addition of crystal modifiers. In a typical synthesis, Ni(NO3)2 · 6H2O (Aldrich) is dissolved in an aqueous ammonia solution at room temperature obtaining a clear blue liquid with [Ni] ) 0.2 M ([Ni] indicates the overall nickel concentration in solution). The ratio R ) [OH-]/[Ni] was varied in the range 4-10 for syntheses performed without additives, whereas it was 8 for all experiments carried out with organic additives. The organic compound was added to this solution under vigorous stirring. Five different organic molecules, three polymers and two surfactants, were investigated as follows: polyvinylpyrrolidone (PVP, grade K30, Fluka, MW 40000), ammonium polyacrylate (APA), hydroxypropylmethyl cellulose (HPMC, Aldrich, MW 86000), cetyltrimethylammonium bromide (CTAB, Aldrich), and sodium dodecylbenzensulfonate (SDBS, Aldrich). The APA solution was prepared by titrating an aqueous solution of poly(acrylic acid) (63% in water, Acros, MW 2000) with a NH4OH solution (28% in water, Aldrich) up to pH 10. Different values, 0.1-1, of the ratio S ) [Ni]/[additive] were employed. In the case of CTAB and SDBS, the additive concentration corresponded to the effective concentration of the organic molecules according to their formula unit. In the case of polymeric additives (PVP, APA, and HPMC), the additive concentration was defined as the concentration of monomeric units calculated using the average molecular weight reported by the producer. Forty milliliters of the final nickel solution was transferred in a 125 mL stainless steel PTFE-lined autoclave (model PA4748, Parr Instrument Co.), heated in an oven at 3 °C min-1 and kept at the reaction temperature for 2-96 h. Syntheses were carried out at 90, 150, and 200 °C. The resulting precipitate was collected, washed several times with distilled water, and finally ovendried. Characterization. The phase composition of the precipitate was investigated by X-ray powder diffraction (Philips PW1710, Co KR radiation, 20-100° 2θ scan, step 0.025° 2θ, sampling time 10 s). The crystallite size was calculated from the breadth of the XRD peaks by means of the Scherrer equation after correction for instrumental broadening using a silicon standard. The morphology of the particles was investigated by scanning electron microscopy (SEM) using a LEO 1450 VP instrument. High-resolution transmission electron microscopy (HRTEM) observations and electron diffraction (ED) were conducted using a FEI Tecnai F 20 microscope operated at 200 kV. In both cases, a small amount of precipitate was dispersed in acetone, and a drop of the resulting suspension was placed on an aluminum stub (SEM) or onto a carbon-coated copper grid (TEM) and dried. The TEM was equipped with a energy-dispersive electron microprobe (EDS). The specific surface area, SBET, was determined by nitrogen physisorption (BET method, model ASAP 2010, Micromeritics). The equivalent BET diameter, dBET, was calculated by the formula dBET ) 6/FSBET, where F is the density.

Results Hydrothermal Synthesis of Ni(OH)2 without Additives (R ) 4-10). When the synthesis is carried out for 2 h at 90-200 °C with R ) 4 or at 90-150 °C with R ) 8-10, the final product always consists of β-Ni(OH)2 thin hexagonal platelets with a diameter of the order of 100-200 nm (see Figure S1a of the Supporting Information). The larger hexagonal faces correspond to the (001) crystallographic planes. This is the most common morphology reported in the literature. Particles obtained at a lower temperature are more rounded without sharp edges. Hydrothermal treatment at 200 °C with R in the range 8-10 results in the formation of irregular aggregates of thick hexagonal platelets (see Figure S1b of the Supporting Information). In this case, morphology evolution is clearly driven by Ostwald ripening.

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Hydrothermal Synthesis of Ni(OH)2 with PVP (R ) 8). The formation of dense spherical aggregates of hexagonal tabular crystals is observed at 150-200 °C with S ) 0.1 (Figure 1a). The aggregates have a rather broad diameter distribution of 3-10 µm. At 150 °C and S ) 1, the dominant morphology observed after 2 h hydrothermal treatment corresponds to tabular hexagonal crystals with uniform diameter of 4-5 µm (Figure 1b). Many of these crystals show secondary lamellae, which have started to grow on the hexagonal face with a slightly different orientation; that is, the basal plane of the secondary lamellae form an angle with the basal plane of the main structure. Careful observation of the lateral morphology reveals that the tabular crystals are generated by the stacking of many thin platelets (thickness ≈ 100 nm). Therefore, the particles should be referred to as mesocrystals. A mesocrystal is a superstructure composed of individual nanocrystals aligned along a common crystallographic direction and with external crystal faces.19 However, there is a certain degree of disorder in the alignement of the lamellae (Figure 1b,c), and the misalignment is higher for the lamelle that are closer to the surface of the structure. The growth of new secondary lamellae continues with increasing temperature and time, leading to the formation of flowerlike aggregates as a result of the progressive increase of the inclination of the lamellae (Figure 1c,d). The diameter of these structures remains practically unchanged for times of 2-24 h. In contrast, the lamellae have the tendency to cement together with increasing time and their thickness increases up to ≈300nm (Figure 1d). Hydrothermal treatment for 48 h determines a complete recrystallization of the flowerlike structures with the formation of either single or aggregated spherical particles (Figure 1e). The surface shows a fine lamellar structure (thickness of the lamellae, 50-70 nm). These particles were embedded in resin and sectioned by polishing with SiC emery paper and diamond paste. Observation of the polished section by SEM (Figure 1f) shows that the particles are hollow spheres with lamellar microstructure. Some insight into the internal structure of the Ni(OH)2 particles is provided by the analysis of the broadening effects observed in the diffraction patterns (Figure 2). Anisotropic broadening is indicated by the sharper peaks corresponding to (100) and (110) reflections in comparison to the neighboring peaks. Analysis of broadening effects by means of the Gaussian quadratic plot23 after deconvolution for instrumental broadening indicates that (i) peaks with l ) 0 are less broadened and mainly affected by size broadening and (ii) peaks with k ) 0 and l > 0 are broader and affected by both size and strain broadening. The size of the coherently diffracting domains was calculated by considering the size broadening contribution only. The particles obtained after 2 h of treatment at 150 and 200 °C consist of domains with diameter of ≈40 nm ([100] direction) and thickness of ≈30 nm ([001] direction). The HRTEM images (see Figure 3 as an example) of the particles show the presence of many defects, such as dislocations and misalignements. Contrast variations might hint at the presence of small pores. These nanometer-length scale features strongly suggest that the particles are originated by oriented aggregation of primary nanocrystals. Dislocations and small misalignments are the consequence of imperfect oriented attachment of the primary building units24 and provide an explanation for the observed strain broadening of the XRD peaks. Aging for 48 h determines an increase of the domain diameter to ≈70 nm, whereas the thickness is practically unchanged. Observation of Figure 2 also indicates that the (001) peak of the powders obtained after 2 h has a much higher relative intensity if compared to the theoretical pattern (PDF

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Figure 1. Morphology of β-Ni(OH)2 particles grown in hydrothermal conditions in the presence of PVP. Conditions: (a) 150 °C, 2 h, S ) 0.1. (b) 150 °C, 2 h, S ) 1. The inset shows the lateral growth morphology of the hexagonal particles. (c) 200 °C, 2 h, S ) 1. (d) 200 °C, 24 h, S ) 1. Inset: detail of the surface structure. (e) 200 °C, 48 h, S ) 1. Inset: detail of the surface structure. (f) 200 °C, 48 h, S ) 1, cross-section of the particles.

14-117). Because the powder is slightly pressed in a plastic sample holder for collection of the XRD pattern, this determines the preferential orientation of the hexagonal tabular mesocrystals and indicates that the hexagonal face is a (001) face. In contrast, the relative intensities of the pattern corresponding to the precipitate obtained after 48 h are in good agreement with the expected intensities because spherical particles can not give preferred orientation effects. Overall, the above results indicate that the Ni(OH)2 particles obtained in the presence of PVP have a hierarchical superstructure corresponding to nanocrystals with a thickness of about 30 nm arranged in larger lamellae with thickness of the order of 100 nm, which in turn form hexagonal mesocrystals at short aging times (2 h) or spherical hollow particles after long time (g48 h) treatment.

Hydrothermal Synthesis of Ni(OH)2 with APA (R ) 8). Synthesis at 90 °C with S ) 1 produces a colloidal suspension. The precipitate obtained by centrifugation of the suspension is predominantly amorphous. The EDS spectra indicate that the precipitate contains Ni and O and might be composed of amorphous Ni(OH)2. SEM observation has also revealed the presence of fibers and filaments with a length of 10-50 µm and diameter of 0.5-3 µm (Figure 4a). TEM observation has shown neither a substructure nor lattice fringes inside these structures, and accordingly, the ED pattern was that of an amorphous material. EDS spectra collected through the TEM again showed the presence of Ni and O. Two hours of hydrothermal treatment at 150 °C leads to the formation, beside the amorphous phase, of rosettelike aggregates of β-Ni(OH)2


Figure 2. XRD patterns (Co KR radiation) of β-Ni(OH)2 precipitates obtained after 2 and 48 h of hydrothermal treatment at 150-200 °C in the presence of PVP, S ) 1.

Figure 3. HRTEM image of a β-Ni(OH)2 particle grown in hydrothermal conditions in the presence of PVP, S ) 1, 150 °C, 2 h. Lattice fringes correspond to the 101 planes. Defects such as dislocations and misalignments are apparent when the micrograph is viewed along a low angle.

thin platelets with a diameter of 1-2 µm (Figure 4b). Prolonged treatment (24 h) at 150 °C results in the further growth of these aggregates up to a diameter of 3-4 µm and the crystallization of the amorphous phase to β-Ni(OH)2, which forms a compact layer with columnar microstructure at the bottom of the hydrothermal vessel. Thus, the rosettelike structures appear as laying on a carpet with a fine lamellar pattern (Figure 4c). Hydrothermal Synthesis of Ni(OH)2 with HPMC (R ) 8). Observation of the precipitate resulting from 2 h treatment at 150 °C (S ) 0.1) shows thin but relatively large platelets (diameter ≈ 1 µm) and aggregates of intergrown platelets (see Figure S2a of the Supporting Information), both composed of β-Ni(OH)2. This morphology is quite stable, and only a thickening of the platelets is observed after 48 h of aging (Figure S2b of the Supporting Information). A similar situation was observed at 200 °C. Hydrothermal Synthesis of Ni(OH)2 with CTAB (R ) 8). At 150 °C and S ) 0.1 after 2 h of hydrothermal treatment, the predominant morphology of β-Ni(OH)2 corresponds to prismatic crystals with hexagonal base (diameter, 2-3 µm). Both

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single crystals and groups of two or more intergrown crystals can be observed (Figure 5a). Prolonged hydrothermal treatment (24 h) results in the coarsening of these crystals, up to ≈8 µm in diameter (Figure 5b). A very similar morphology is observed in powders produced at 200 °C. When the precipitate is subjected to moderate sonication, the crystals are partially eroded at the surface and show a substructure (Figure 5c,d) corresponding to small building units (length, 0.4-1 µm; thickness, 0.1-0.2 µm) with their longer dimension preferentially oriented parallel to the basal plane of the hexagonal prism. Prolonged sonication results in the complete disintegration of the particles. The existence of a substructure is supported by a uniform broadening of the XRD peaks (not shown), which gives a crystal size of ≈70 nm. These observations indicate that the observed prismatic particles probably correspond to mesocrystals. Hydrothermal Synthesis of Ni(OH)2 with SDBS (R ) 8). The behavior of the system after hydrothermal treatment at 150-200 °C is strongly dependent on the value of S. When S ) 1, a blue precipitate is observed after the reaction, but it dissolves during the washing with water. When S ) 0.1, a mixture of acicular crystals and small equiaxed particles (200-300nm) is obtained. Only when S ) 0.5, the main reaction product corresponds to acicular crystals with length of 0.5-2 µm and diameter of ≈100-300 nm (Figure 6a), which form big shapeless agglomerates. The XRD pattern (Figure 7) corresponds to β-Ni(OH)2. HRTEM observation shows single crystal whiskers (Figure 8a). The lattice fringes (Figure 8b) correspond to (001) crystallographic planes (d ) 0.46 nm) arranged orthogonally to the major axis of the whisker. This indicates that the whiskers have preferentially grown in the [001] direction. With increasing time (24 h), beside large shapeless agglomerates of acicular nanocrystals (0.5-1 µm in length), relatively big spherules (10-30 µm) with different surface structures appear (Figure 6b-d). Some of the spheres have a porous and rough surface lacking of specific features. When broken, these particles show an internal structure corresponding to a starlike radial aggregate of acicular crystals (Figure 6b). Other spheres have a well-defined lamellar surface structure (Figure 6c). Spheres in which both surface morphologies coexist are also observed (Figure 6d). Quite likely, the different surface morphologies correspond to different stages of evolution, suggesting a gradual transition from radial aggregates of whiskers (Figure 6b) to ordered superstructures of lamellar crystals (Figure 6c). After 48 h of hydrothermal treatment, the large shapeless aggregates of acicular nanocrystals have largely disappeared, leaving spheres with diameters up to 50 µm and well-defined lamellar surface structures (Figure 6e). Nearly all of the spheres show a large crack along the equatorial circumference (Figure 6c,e). These particles were embedded in resin and sectioned by polishing with SiC emery paper and diamond paste. Observation of the particle section by SEM shows an evident radial structure (Figure 6f) with internal cracks and channels. Only limited further growth was observed for much longer times (96 h) when the precipitate is almost completely composed of big spheres up to 60-70 µm in diameter. The surface lamellar structure becomes even more evident. For all precipitates obtained after 2-96 h of treatment, the XRD patterns (Figure 7) only show the peaks of β-Ni(OH)2. The precipitate collected after 96 h of treatment at 150 °C was converted into NiO (Figure 7) by 2 h of calcination in air at 400 °C. The specific surface area of the calcined particles, which preserve the original spherical morphology, was 100 m2 g-1, which is a rather high surface area in comparison to other values reported in the literature.7,8,11 The NiO crystallite size, calculated



Figure 4. Morphology of β-Ni(OH)2 particles grown in hydrothermal conditions in the presence of APA, S ) 1. Conditions: (a) 90 °C, 2 h; (b) 150 °C, 2 h; and (c) 150 °C, 24 h.

from the breadth of the (200) peak by means of Scherrer equation after correction of the instrumental broadening, was 7 nm. Discussion The stability of amino complexes Ni(NH3)n2+ decreases with increasing temperature, and consequently, formation of Ni(OH)2 occurs in hydrothermal conditions although the starting Ni2+ammonia solution with a total nickel concentration of 0.2 mol L-1 is stable at room temperature. As indicated by the above results, surfactants and polymers with hydrophilic groups exert a remarkable level of control on the growth of the β-Ni(OH)2 particles and are responsible for the wide range of morphologies and structures observed. The organic additives can affect the precipitation process in different and complex ways. First, some of the organic molecules used in this study can chemically interact with the nickel aqueous species [Ni2+ and Ni(NH3)n2+] originated after dissolution of the Ni salt in the ammonia solution. It is well-known that organic molecules with donor nitrogen atoms, such as ethylenediamine, propylenediamine, pyridine, etc., as well as bidentate carboxylate anions form stable complexes with Ni2+.25 Polyacrilic acid strongly interacts with divalent cations,26 and according to our results, also, SDBS can bind to Ni2+. Consequently, supersaturation can be lowered by the presence of some of the additives in comparison to that of the Ni2+-ammonia solution at the same temperature. This can have a significant impact on the precipitation process, because nucleation and molecular growth mechanisms change with supersaturation. Besides these direct chemical effects, polymers and surfactants can preferentially adsorb on some specific crystal surfaces,

producing a variation of the solid-liquid interface energy and, in turn, a different crystal habit. This seems the origin of the whiskers observed in the presence of SDBS (Figures 6a and 8) after 2 h of treatment at 150 °C. Even more important, macromolecules and surfactants can bind to nanocrystal surfaces promoting nonclassical crystal growth by oriented aggregation and self-assembly in mesocrystals with iso-oriented mosaic texture.19 The adsorption of organic molecules stabilizes the nanocrystal against further molecular growth and can promote interaction between specific crystal faces, either because some surfaces do not have adsorbed molecules or by formation of interparticle organic bilayers.19,27 Consequently, these stabilized nanocrystals can act as building blocks in aggregation-based processes of crystal growth although growth by oriented attachment has also been observed in the absence of organic molecules.24 Selective adsorption of ionic polymers and surfactants on some crystal surfaces can also determine the formation of an electrostatic multipole field, which can direct the aggregation process.19,27 Building blocks with anisotropic shape can also produce complex, highly organized superstructures by oriented aggregation at different length scales.19,27,28 The morphology evolution observed for Ni(OH)2 in the presence of a high PVP concentration (S ) 1) is a clear example of the formation of superstructures by polymer-induced ordered aggregation (Figure 1b-f). The tabular hexagonal mesocrystals observed after 2 h at 150 and 200 °C (Figure 1b,c) as well as the spherical hollow particles obtained after 48 h at 200 °C (Figure 1e,f) consist of lamellae with a thickness of ≈100 nm, and as indicated by XRD, the lamellae are composed of nanocrystals with a thickness of ≈30 nm. Similar superstructures were observed for hematite particles obtained by hydrolysis of


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Figure 5. Morphology of β-Ni(OH)2 particles grown in hydrothermal conditions in the presence of CTAB, S ) 0.1. Conditions: (a) 150 °C, 2 h; (b) 150 °C, 24 h, after moderate sonication; and (c and d) 200 °C, 24 h, after moderate sonication.

iron-polyolate complex followed by hydrothermal treatment,29 for ZnO synthesized in the presence of PVP30 and for hydrophilic copolymer-mediated crystallization of CaCO3.31 The stacking of the lamellae is not completely ordered (Figure 1b,c), and the lamellae originated at a later time are progressively more tilted leading to the formation of spherical, flowerlike aggregates for long aging times (Figure 1d). In addition, secondary nucleation and growth of new lamellae with very different orientation are also observed. Overall, this growth mechanism, schematically represented in Figure 9, resembles the generation of spherical superstructures by a fractal growth process, as described by Busch et al. for fluoroapatite32 and by Co¨lfen et al. for CaCO3, BaCO3, and other carbonates.33 On increasing the duration of the hydrothermal treatment to 48 h, the flowerlike aggregates transform in hollow spherical particles (Figures 1e,f and 9) with a radial lamellar structure. Quite likely, this shell originates from recrystallization of the outer part of the flowerlike aggregates at the expenses of the inner part, which acts as a sacrificial material, by means of a dissolution-repricipation process similar to Ostwald ripening. The inner part of the aggregates is probably more disordered and/or composed of smaller primary nanocrystals, and this provides the driving force for the recrystallization process. A similar mechanism was proposed for the formation of spherical CaCO3 hollow particles34 and probably holds for other examples of hollow structures recently reported in the literature.35 An oriented aggregation mechanism can be also invoked to explain the formation of the hexagonal prismatic mesocrystals in the presence of CTAB (Figure 5). The easy disintegration of these structures by moderate sonication indicates that surfactant molecules as well as water molecules adsorbed at the surface

of the building blocks remain occluded inside the mesocrystals. Likewise, the formation of the fiberlike Ni(OH)2 amorphous structures observed at 90 °C (Figure 4a) might occur by polymer-mediated directional aggregation of amorphous nanoparticles, as described for the growth of fiberlike crystals of BaSO4 and BaCrO4 in the presence of sodium polyacrylate and other hydrophilic polymers.26 The formation of a colloidal amorphous phase at short times indicates a strong interaction between Ni2+ and APA, a behavior similar to that observed with Ca2+ and Ba2+ ions. The fiberlike morphology might result from heterogeneous nucleation of aggregates of amorphous particles on the walls of the vessel followed by further growth by polymer-controlled assembly of amorphous nanoparticles. Hydrothermal treatment with APA at higher temperature determines the formation of rosettelike aggregates of hexagonal or rounded lamellae as also happens using HPMC. This morphology seems rather favored for β-Ni(OH)2 and has been observed in several previous studies,12-15 even in the absence of organic additives. These aggregates do not evolve toward the formation of hollow spheres as happens in the presence of PVP probably because of the more open microstructure. Morphology evolution from isolated whiskers (Figure 6a) to starlike radial aggregates of whiskers (Figure 6b) observed in the case of SDBS addition looks to be driven by a mechanism different from ordered aggregation and polymer-directed selfassembly. A mechanism based on coarsening and recrystallization of pre-existing spherical particles could be excluded after careful SEM observation of the precipitate obtained after 2 h of treatment at 200 °C. In contrast, many spherical cavities with diameter from a few micrometers to few tens of micrometers were revealed inside the shapeless powder agglomerates forming



Figure 6. Morphology of β-Ni(OH)2 particles grown in hydrothermal conditions in the presence of SDBS, S ) 0.5. Conditions: (a) 150 °C, 2 h. (b-d) 150 °C, 24 h. The inset of part c shows the surface structure. (e) 150 °C, 48 h. (f) Cross-section of a particle obtained after 48 h at 150 °C.

the precipitate. These cavities could have originated from particles of an intermediate SDBS-Ni2+ compound, which were redissolved during the hydrothermal treatment or from SDBS vesicles that acted as templates. Quite likely, the starlike aggregates have developed at the interior of some of the spherical cavities existing inside the powder agglomerates by a dissolution-repricipitation process, with the acicular crystals growing from the cavity surface toward the center. Prolonged hydrothermal treatment (up to 96 h) determines a significant coarsening of the spheres (from 10-20 µm after 24 h to 60-70 µm after 96 h) by Ostwald ripening at the expense of the powder agglomerates, which are gradually dissolved acting as a source of material. Recrystallization of the surface of the spheres (Figure 6c,d) with formation of a lamellar structure probably occurs when the particles are no longer embedded in the powder agglomerates and come in contact with the solvent. The above results provide a suggestion for new approach to the synthesis

of spherical structures. Intentional or accidental use of cavities associated with gas bubbles dispersed in liquids or at the liquid/ air interface as templates for fabrication of hollow particles and other superstructure has been already reported.36 Hollow particles can also be created using micelles and vesicles generated in situ by surfactants inside aqueous solutions.37 According to the present results, cavities inside powder agglomerates or powder beds in contact with a solvent can be used to prepare spherical superstructures and other morphologies. The cavities can be created using suitable sacrificial templates, which can be dissolved by increasing temperature or changing the solvent composition. Conclusions In summary, surfactants and polymers with hydrophilic functional groups have been successfully used as crystal


Figure 7. XRD patterns (Co KR radiation) of β-Ni(OH)2 precipitates obtained after 2-96 h of hydrothermal treatment at 150 °C in the presence of SDBS, S ) 0.5. The top pattern corresponds to the precipitate collected after 96 h and transformed to NiO by 2 h of calcination at 400 °C.

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Figure 9. Suggested mechanisms for the growth of different morphologies of β-Ni(OH)2 in the presence of PVP. (a) Primary nanocrystals. (b) Self-assembly of nanocrystals in platelets. (c) Tabular mesocrystals via lamellae intermediates. (d and e) Formation of a spherical flowerlike aggregate by a branching process. (f) Creation of a hollow sphere by Ostwald ripening.

modifiers during hydrothermal synthesis of β-Ni(OH)2 to obtain particles with different morphologies, including hexagonal platelets, hexagonal tabular mesocrystals, rosette- and flowerlike aggregates of lamellae, hexagonal prismatic mesocrystals, whiskers, spherical aggregates, spherical superstructures, and hollow spheres. We have used organic molecules in a barely systematic way, just by trial and error, as a strategy to generate different and, sometimes, complex material architectures. The exact role of each molecule in the growth process is not exactly known, and it was not possible to predict the morphology produced by a specific additive. Nevertheless, the results of this preliminary investigation show the potential of organic molecules with specific chemical groups to control particle shape and size during the hydrothermal synthesis of nickel hydroxide. The additive concentration and the duration of the hydrothermal treatment have a significant influence on particle morphology. In the case of PVP and CTAB for short aging times (2 h), there is evidence of a nonclassical growth process by ordered aggregation of nanocrystals mediated by the organic molecules adsorbed on the solid surfaces. For long hydrothermal treatments (>24 h) in the presence of PVP and SDBS, morphology evolution is dominated by solvent-mediated recrystallization processes, like Ostwald ripening. The ability to tailor the morphology of β-Ni(OH)2 may provide a useful tool for the controlled synthesis of particles of this compound and, in turn, the fabrication of electrode materials with well-defined and/or complex microstructure to improve the performance of electrochemical devices such as alkaline batteries and fuel cells. Acknowledgment. Financial support from the Ministero dell’Universita` e della Ricerca under the FISR project “Nanosistemi Inorganici e Ibridi per lo Sviluppo e l’Innovazione di Celle a Combustibile” is gratefully acknowledged. Supporting Information Available: SEM pictures of β-Ni(OH)2 particles obtained in the absence of organic additives (Figure S1) and SEM pictures of β-Ni(OH)2 particles obtained with addition of HPMC (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

References Figure 8. Morphology of a β-Ni(OH)2 acicular nanocrystal grown in hydrothermal conditions in the presence of SDBS, S ) 0.5, 150 °C, 2 h. (a) TEM, low magnification. (b) HRTEM image.

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in the Presence of Polymers and Surfactants: Nanocrystals - American

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