Controlled Growth of Three Morphological Structures of Magnesium Hydroxide Nanoparticles by Wet Precipitation Method

ARTICLE IN PRESS

Journal of Crystal Growth 267 (2004) 676–684

Controlled growth of three morphological structures of magnesium hydroxide nanoparticles by wet precipitation method

Jianping Lv, Longzhen Qiu, Baojun Qu* State Key Laboratory of Fire Science, Department of Polymer Science and Engineering, University of Science and Technology of China, Hefei, Anhui 230026, PR China

Received 6 December 2003; accepted 15 April 2004

Communicated by J.M. Redwing

Abstract

Magnesium hydroxide nanoparticles with three structural morphologies, needle-, lamellar- and rod-like, were synthesized individually through solution precipitation in the presence of complexdispersants. The inﬂuence of synthesis parameters on the morphological characteristics and sizes of magnesium hydroxide nanoparticles precipitated in an aqueous medium was investigated, such as types of surfactants or water-soluble polymeric dispersants, reaction temperature and concentration, addition method and rate, and hydrothermal treatment of reactants. Special attention was given to obtain rod-shaped particles and stable colloidal dispersion. The obtained magnesium hydroxide nanoparticles were characterized in terms of morphology, particle size and crystal habits by transmission electron microscopy (TEM), ﬁeld emission scanning electron microscope (FESEM), and X-ray diffraction (XRD). r 2004 Elsevier B.V. All rights reserved.

PACS: 81.10.A; 61.10.N; 61.66.F

Keywords: A1. Crystal morphology; A1. X-ray diffraction; A2. Growth from solutions; A2. Hydrothermal crystal growth; B1. Complexdispersants; B1. Magnesium hydroxide

1. Introduction [2], the synthesis of nanoscale materials has been the focus of considerable interest because of their In recent years there has been growing interest potential applications in electronics, optics, cata- in materials with speciﬁc nanomorphologies be- lysis, ceramics, nanostructured composite materi- cause of the expectation of novel properties [1]. als, and nanodevices. However, large-scale Since the discovery of carbon nanotubes in 1991 synthesis of those materials is a challenge, because nanoparticles may adopt various shapes [3], which in turn play critical roles in determining their basic *Corresponding author. Tel.: +86-551-3607245; fax: +86- 551-3607245. properties. For example, magnesium hydroxide E-mail address: [email protected] (B. Qu). (MH) nanorods could be used as precursors for

J. Lv et al. / Journal of Crystal Growth 267 (2004) 676–684 677 the synthesis of MgO nanorods [4], which are sodium sulfate, polyvinypyrrolidone, polyglycol expected to have novel mechanical, catalytic and ether, polysorbate 80, polyvinyl alcohol, sodium electronic properties due to their extremely small polymethacrylate and polyacrylamide were sup- sizes, large anisotropy, and perfect crystallinity [5]. plied from Shanghai Chemical Reagent Co. Ltd, MH nanoneedles and nanolamellas could be good China, and used as received without further candidates for functional polymeric composites purification. and fiber hybrid materials as reinforcing agents or halogen-free flame-retardants [6,7]. In 1991, 2.2. Measurements Ultamapanya and coworkers prepared MH nanoparticles via a sol–gel technique followed by a Transmission electron microscope (TEM, HI- hypercritical drying procedure using metal-organic TACHI H-800) and field emission scanning precursors in the sol–gel procedure [8]. Recently, electron microanalyzer (FESEM, JSM-6700F, Li and Ding, and their coworkers proposed a route JEOL Lit, Japan) were used to observe the to synthesize MH nanorods [9,10], based on the morphology of the MH nanocrystals. The dried hydrothermal reaction of pure magnesium powder MH product was added into distilled water in an autoclave at higher temperature using long to form a suspension, which was stabilized by a reaction time and low reactant concentration. small amount (o0.2 wt%) of polyacrylamide Similarly, Yu and coworkers produced MH (MW=100,000) for preventing the nanoparticles nanoplates by the hydrothermal reaction of from agglomerating. commercial bulk magnesium oxide crystals [11]. The specific surface area was measured accord- All these chemical synthesis were, however, ing to the Brunauer–Emmett–Teller (BET) meth- characterized by problems such as relatively low od, employing nitrogen adsorption at liquid yields and were economically unacceptable. As a nitrogen temperature, by means of a Micromeritics result, it is still desired to develop a high-yield ASAP 2000 apparatus (Micromeritics Co., USA). method capable of generating nanosized MH in a MH samples (approximately 0.2 g) were first moderate quantity with a well-controlled dimen- outgassed for about 2 h at 150C under a residual sion and morphology. Although there are some pressure lower than 91.0 Pa, then for about 1.5 h at articles concern synthesis of nanosized MH 250C under a residual pressure lower than 2.6 Pa. particles with one or two morphologies [8–12], The X-ray diffraction (XRD) scans were recorded few concern the synthesis of three morphological at room temperature on a Philips X’ Pert MH using the wet precipitation method. PRO SUPER apparatus (Nicolet Instrument Co., In the present paper, we combine polymer- USA) using CuKa radiation with b-Ni filter assisted technology with the wet precipitation (l ¼ 1:541874 A)( at a scan rate of 0.0167/s. A method to prepare three morphological nanosized WQL-type instrument (made in China) was used MH, which are nanorod, nanoneedle and nanola- to measure the number-average size and weight- mellar. The effects of synthesis parameters such as average size of MH nanoparticles. reagent concentration, reaction temperature, addition speed, water-soluble polymer, and dispersant surfactant on the size and morphology were 2.3. Synthesis of MH nanoparticles investigated. MH nanocrystalline samples were synthesized by a wet chemical process, the so-called homo- 2. Experimental procedure geneous precipitation, in the presence of different dispersants. The reaction formula are given as 2.1. Materials follows: MgCl Á 6H O þ 2NH -MgðOHÞ Aqueous ammonia (25%), sodium hydroxide, 2 2 3 2 magnesium chloride hexahydrate, gelatin, lauryl þ2NH4Cl þ 6H2O; ARTICLE IN PRESS

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- for 24 h, yielding a white nanosized MH powder MgCl2 Á 6H2O þ 2NaOH MgðOHÞ2 (7.2 g, 98% yield, based on magnesium chloride þ2NaCl þ 6H O: 2 hexahydrate). In this method, MgCl2 Á 6H2O was used as a magnesium precursor. NH aqueous solution, 3 3. Results and discussion NaOH solution, or their mixture was used as precipitator. The different dispersants or surfac- 3.1. Size and morphology of MH crystals tants were used for preventing the products from aggregating. The pH value was monitored during MH nanocrystals synthesized by the wet pre- the precipitation reaction. The alkaline solution cipitation procedure were distinctly characterized was injected into the magnesium chloride solution according to their sizes and morphologies, which by a peristaltic pump at different speeds. Other- are strongly affected by water-soluble polymers, wise, the magnesium chloride solution was injected dispersant surfactants, alkaline concentration, into the alkaline solution. By controlling the reaction temperature, injecting rate, and hydro- injecting rate, the MH nanoparticles formed at thermal treatment. the initial stage of precipitation reaction could serve as seeds for the subsequent growth of MH 3.1.1. Surfactants and water-soluble polymeric crystallites. The use of a peristaltic pump provided dispersants a better way to control the nucleation step, and Surfactants or water-soluble polymeric disper- also allowed us to systematically investigate the sants had already been used as protecting agents in effects of reagent concentration, reaction tempera- the synthesis of nanoparticles with different ture, and drop-wise speed on the morphology and morphologies [13–16]. Surfactant molecules ab- size range of MH nanoparticles synthesized by sorbed on the nanoparticle’s surface can decrease solution precipitation approach. The reactant the surface energy and thus prevent the agglom- solution includes 2.5 mol/l magnesium chloride, eration of particles. Polymeric dispersants serve to 14.5 mol/l ammonia and 2 mol/l sodium hydro- stabilize the growing particles, and thus control xide. The chemical reaction was achieved in a the size of particles. In addition, strongly bound three-neck flask with a water circulation system dispersants would provide the protective poly- and a mechanical stirrer. The reaction temperature meric coatings of core-shell nanocomposites. The was varied in the range of 10–45C. The resulting nature of a polymeric dispersant is of importance, aqueous suspension was whitish and contained a which determines the properties of a nanocompo- solid (MH nanocrystals) content of 6.5 wt%. site material in a solution and in a solid state. In a typical procedure (for sample 6), 50 g of Polymers can either suppress or promote nuclea- 50 wt% magnesium chloride hexahydrate aqueous tion and growth, depending on the circumstances solution and 4 g of 10 wt% complexdispersant and the nature of materials [17]. The mean size aqueous solution of gelatin and lauryl sodium data and the ratios of weight-average sizes to sulfate were added into a 250 ml three-necked number-average sizes measured by WQL-type flask. Then, 20 g of 25 wt% ammonia water instrument in the presence of surfactants, disper- solution was injected into the above solution in sants or the complexof gelatin with lauryl sodium 1 h, and then 58 g of 8 wt% sodium hydroxide sulfate under same reaction conditions are listed in aqueous solution was injected in 2 h, under Table 1. It should be emphasized that the sizes vigorous stirring at 10C. The mixture was measured here were much larger than those continually stirred for 1 h, and then heated to obtained from TEM measurements described in 40C for 2 h before being cooled to room the following paragraph. However, the relative temperature. The resultant suspension was filtered comparison with each other is reasonable. and washed with water to remove the residual Some water-soluble polymers, such as polyvinyl impurities, such as sodium chloride and ammo- alcohol, sodium polymethacrylate and polyacryla- nium chloride. The final product was dried at 80C mide, were found to be not soluble in the reactant ARTICLE IN PRESS

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Table 1 Effect of surfactants/dispersants on MH size distribution

Dispersant/surfactant Mean size(mm) Size distribution weight- average/number-average Weight-average Number-average

None 4.76 0.77 6.18 Gelatin 1.67 0.69 2.42 Lauryl sodium sulfate 2.57 1.09 2.35 Polyvinylpyrrolidone 3.28 1.19 2.75 Polyglycol ether 3.75 1.21 3.10 Polysorbate 80 3.49 1.16 3.01 Gelatin+lauryl sodium sulfate 0.94 0.51 1.84

solution in our experiments even at very low obtained at different reaction conditions are given concentrations such as 0.5 wt%. From Table 1,it in Table 2. can be seen that all dispersants or surfactants play From Table 2, the addition of 5 wt% initial an important role in controlling particle size aqueous ammonia at different reaction tempera- distribution. Among them, gelatin and lauryl tures during the same injection time (sample 1– 3) sodium sulfate are more effective than others results in the formation of needle-like MH concerning the narrower size distribution of 2.42 nanoparticles, as shown as the TEM image in and 2.35, respectively. Moreover, when gelatin and Fig. 1(b). However, by increasing the initial lauryl sodium sulfate were used together in the aqueous ammonia concentration to 25 wt% (sam- ratio of 1:1 as a complexdispersant at the same ple 5–7), more regular lamellar nanocrystals total amount, the weight-average size, number- instead of needle-like ones were formed, as shown average size and their ratio decrease to the as the TEM image in Fig. 1(c). The morphology of minimum 0.94 mm, 0.51 mm, and 1.84, respectively. MH nanocrystal can be explained by the shape Without the addition of surfactants or water- factor (Fs), which is defined by the ratio of length soluble polymeric dispersants, the formed nano- to width of the particle. The Fs value was found to crystals tend to agglomerate into a large mass, decrease with an increase in the initial aqueous resulting into the widest size distribution of 6.18, ammonia concentration. as shown in Fig. 1(a). 3.1.3. Reaction temperature and injecting rate 3.1.2. Alkali and magnesium chloride hexahydrate It was found that raising reaction temperature concentrations had the same influence on MH morphology as The morphology and size distribution of the increasing the alkali concentration as discussed obtained MH particles were strongly affected by above. From Table 2, the temperature of 20C the initial reactant concentrations. It has been seems to be a critical one for forming different shown that the addition of a strong base into a morphologic structures of nanoparticles, because metal salt solution made it difficult to generate at this temperature one can observe that some monodispersed metal hydrous oxides [12].In lamellar crystals appeared (sample 3), as shown in this study, weak base aqueous ammonia was the TEM image in Fig. 1(d). This can be initially used to control the morphology and interpreted as due to the equal speed of crystal size of MH crystal, whereas the subsequent growth in various lattice planes at higher tempera- addition of 8 wt% sodium hydroxide as a strong tures (above 20C). When the reaction tempera- base aqueous solution was needed to raise the yield ture was raised over the threshold temperature, of MH nanoparticles from 75 to 98%. The such as 30C, MH crystals took lamellar shape morphologies and sizes of MH nanocrystallites completely, as shown in the image in Fig. 1(e) ARTICLE IN PRESS

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(sample 9). Furthermore, the mean diameter size increased with increased reaction temperature. It is strange that the thickness of the lamellar crystal was not sensitive to the change of temperature. It was found that the thickness kept constant at 30–40 nm at 20–50C, and increased to 70 nm at 60C. The XRD pattern for sample 1 with needle-like morphology is shown in Fig. 2. All diffraction peaks can be indexed as the hexagonal structure of MH (JCPDS file number 7-239). There is no additional phase peak in the XRD pattern. Therefore, the synthesized phase corresponds to pure MH, indicating that the synthesis reaction was completed. The significant peak broadening indicates that the MH particle has a very small grain size. The corresponding electron diffraction (ED) pattern, as shown as image in Fig. 1(f), presents several diffuse diffraction rings, indicating a fine crystallite size character. The ED pattern also confirms the hexagonal structure, clearly giving the diffraction rings of various planes. It should be pointed out that the additional aging of the final product at higher temperature had little influence on the crystallinity in our experiments. When the reaction temperature was as low as 2C (sample 1, 5 and 8), the resulted nanoparticles still present good crystallinity. After aging for 12 h at 80C for sample 1, there was no corresponding ED pattern change. Therefore, it can be concluded that the subsequent aging process did not improve the crystallinity. A possible explanation is that the slower injection speed of alkaline solution together with the complexof dispersants can avoid the rapid formation of severely aggregated MH nuclei that could result in poor crystallinity. High injecting rate such as first 5 wt% aqueous ammonia in 10 min, then 8 wt% sodium hydroxide in 20 min, surely caused the aggregation of MH Fig. 1. TEM micrographs of (a) without the addition of crystals as similar to that shown in Fig. 1(a). surfactant/dispersant, (b) 5 wt% initial aqueous ammonia at 2C in 3 h injecting time, (c) 25 wt% initial aqueous ammonia at It was very interesting to find that during a long 2C in 3 h injecting time, (d) at 20C with 5 wt% aqueous injection time of 7 h, adding 5 wt% aqueous ammonia in 3 h injecting time, (e) at 30C with 5 wt% aqueous ammonia first and then 8 wt% NaOH aqueous ammonia in 3 h injecting time, (f) ED pattern of needle-like solution to 40 wt% magnesium chloride hexahy- morphology for sample 1 synthesized at 2 C. drate (sample 4 in Table 1), in the presence of complexof gelatin and lauryl sodium sulfate with a slow heating process to 30C in 2 h and a subsequent cooling process to room temperature, ARTICLE IN PRESS

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Table 2 MH nanocrystallites obtained at different reaction conditions

Sample Reactant Initial ammonia Temp(C) Injection rate Injection rate Morphology Size (nm, TEM code injected concentration (wt%) (g/h)/ time (h)a (g/h)/ time (h)b or FESEM)

1 Alkali 5 2 100/1 29/2 Needle-like 5 100 2 Alkali 5 10 100/1 29/2 Needle-like 30 140 3 Alkali 5 20 100/1 29/2 Needle-like, 20–100 lamellar 4 Alkali 5 10 33/3 15/4 Rod-like 160 3000 5 Alkali 25 2 20/1 29/2 Lamellar 50 6 Alkali 25 10 20/1 29/2 Lamellar 150–200 7 Alkali 25 20 20/1 29/2 Lamellar 300–350 8 Saltc 2 25/2d Needle-like 10 60 9 Alkali 5 30 20/1 29/2 Lamellar 100–200

a 5 or 25 wt% ammonia solution. b 8 wt% aqueous sodium hydroxide. c 50 wt% aqueous magnesium chloride hexahydrate solution. d Injection rate (g/h)/time (h) of salt.

101 2000

1500 001

1000 110 102

intensity (a.u.) 500 100 111 103 201 0

10 20 30 40 50 60 70 80 2 theta Fig. 2. XRD pattern of needle-like morphology for sample 1 synthesized at 2 C, I001/I110=1.7.

resulted in the formation of rod-like nanoparticles, as shown as in Fig. 3. The rod-like nanoparticles have a maximum Fs of 112.5 calculated with 18 mm in length and 0.16 mm in diameter, which is in accordance with the XRD pattern, as shown in Fig. 4. The intensity ratio between reﬂections [0 0 1] and [1 1 0] gives a possible indication of a more pronounced orientation effect in the rod-like morphology due to the Fig. 3. (a) and (b) TEM micrographs of sample 4 with rod-like large aspect ratio of those particles. Comparing morphology, (c) synthesized without complexdispersant, (d) I001/I110 ratios in Figs. 2,4 and 6, it can be seen that ER for sample 4. ARTICLE IN PRESS

682 J. Lv et al. / Journal of Crystal Growth 267 (2004) 676–684 rod-like morphology exhibits a higher I001/I110 rod-like particle growth was controlled by restrict- ratio of 2.4 than that of 2.3 and 1.7 for lamellar- ing particle formation to conﬁned volumes of the and needle-like morphologies, which indicates a longer gelatin chain or by stabilizing the growing more pronounced orientation of the rodlets particle with surfactants or dispersants. towards the incident X-ray radiation. The prefer- It is admitted that preparing nanoparticles with ential orientations for rod-, lamellar- or needle-like rod structure is not so novel today, especially for morphological particles are not possible in the materials with layered structure. The appearance isotropic case of ‘‘sand rose’’ morphology whose of a nanorod structure in such a simple precipita- I001/I110 ratio is 0.51 reported by Henrist and tion reaction is still exciting. coworkers [12]. The ED pattern, as shown in Fig. 3(d), indicates 3.1.4. Hydrothermal treatment that MH nanorods have more perfect crystallinity. As MH crystal has the tendency to be in the It should be mentioned that the short MH nano- form of hexagonal platelets, hydrothermal treat- rods with low aspect ratio, as shown in Fig. 3(c), ment could have great effect on the morphology. were obtained in the absence of gelatin, which Fig. 5 shows the FESEM micrographs of the MH easily form the agglomeration. Therefore, complex particles before and after 12 h hydrothermal dispersant plays an important role in one-dimen- treatment at 180C. It can be clearly seen that sional growth of MH crystallites. Presumably, the the rough lamellar crystals have turned into the well-deﬁned hexagonal platelets with perfectly

2500 101 101 2000

1500 001 001 102 110 1000 102 (b) 100 111103 201

intensity (a.u.) 110

500 intensity (a.u.) 100 111 103 201 (a) 0 10 20 30 40 50 60 70 80 10 20 30 40 50 60 70 80 2 theta 2 theta Fig. 6. XRD patterns of sample 7. Curve a: untreated, I001/ Fig. 4. XRD pattern of sample 4 with rod-like morphology I110=2.3, Curve b: after 12 h hydrothermal treatment at 180 C, with I001/I110=2.4. I001/I110=1.9.

Fig. 5. FESTM micrographs of sample 7.(a) untreated, (b) after 12 h hydrothermal treatment at 180C. ARTICLE IN PRESS

J. Lv et al. / Journal of Crystal Growth 267 (2004) 676–684 683 smooth surface after hydrothermal treatment. The crystals were presumed to have monodispersed size range of about 350 nm in diameter and 40 nm in thickness. After hydrothermal treatment, there was no significant increase of their mean size as reported by Henrist [12]. This can be explained that the complexdispersant agents have encapsu- lated the particles and thus prevented the initial agglomeration of primary nuclei during the hydrothermal treatment process. Fig. 6 displays the XRD patterns of the sample before (a) and after 12 h hydrothermal treatment (b) at 180C. An overall enhancement of the crystallinity was clearly visible since the intensity of diffraction Fig. 7. TEM micrograph (a) and ER (b) for sample 8. peaks increases to a large extent in curve (b). Moreover, comparing the intensity ratio between reflections [0 0 1] and [1 1 0] gives a possible indication of a less orientation effect in the case and large specific surface area may have some of curve (b), due to particles exhibiting less of an practical applications as hydrated mineral aspect ratio. filler in water-soluble polymers, the synthesis of an emulsion-driven polymerization of hybrid 3.2. Formation of stable colloidal dispersion nanocomposite and the treatment of pollution water. In the precipitation process discussed, the alka- A precise mechanistic interpretation is beyond line aqueous solutions were injected to the the scope of this paper, but it can be postulated magnesium chloride solution in a normal way to that the pH value of the precipitation plays a form MH particles, which usually deposited to the crucial role during the crystal growth. Several bottom of flask in 1 h. However, when 50 wt% papers have determined the isoelectric point of magnesium chloride hexahydrate solution was MH particles in water, which is situated at ca. pH injected to the mixed alkaline solution of NaOH 12 [18,19]. In the beginning of the precipitation and ammonia (sample 8), much different phenom- process, the pH value is higher than 13. Therefore, enon happened in our experiments. MH particles the net residual electric charge on the surface of always floated in the upper part of the flask, a clear the particles is expected to be negative. The aqueous solution layer appeared in the bottom of repelling force of negative electric charges between flask, even existed as long as four months at nuclei can stop agglomeration. Furthermore, the ambient temperature. The BET information high concentration of hydroxyl ions results in an showed that the obtained MH particles had a extremely fast nucleation process that generates bigger surface area of 69 m2/g in the alternate tiny nuclei. Due to their high concentration injecting way than 41 m2/g obtained from the and small hydration sphere in solution, the normal injecting method. Moreover, the particles sodium ions or ammonium ions can probably be obtained from the alternate injecting way at 2Cin absorbed significantly onto all the facets of the 2 h injecting time took the needle shape in the nuclei, which hinder the incorporation of size of 10 60 nm, and still present good crystal- fresh magnesium ions and subsequent growth. linity (sample 8), as shown in Fig. 7. After washing Approaching the end of reaction, those very the suspension with distilled water to remove small particles tend to aggregate in order to water-soluble salts, a stable suspension with lower their surface energy, the agglomeration good flowability was then obtained. This stable step is, however, prevented by the complex suspension with a high solid/liquid ratio (20 wt%) dispersants. ARTICLE IN PRESS

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4. Conclusions Acknowledgements

MH nanoparticles with needle-, rod- and The authors gratefully acknowledge the support lamellar-like morphologies were prepared, respec- by the China National Key Basic Research Special tively, using wet-precipitation process under dif- Funds (NKBRSF item No. 2001cb409600). ferent synthesis conditions. Effects of several synthesis parameters on the three morphologies and size of MH particles were studied. References It has been shown that the alkali solution concentration is of prime importance besides the [1] Y. Xie, J. Huang, B. Li, B. Liu, Y.T. Qian, Adv. Mater. 12 complexdispersant: the use of lower concentration (2000) 1523. aqueous ammonia (5 wt%) promotes the forma- [2] S. Lijima, Nature 354 (1991) 56. tion of needle or rod morphology, while the [3] M.E. Spahr, P. Bitterli, R. Nesper, M. Muller,. F. synthesis driven with higher concentration aqu- Krumeich, H.U. Nissen, Angew. Chem. 110 (1998) 1339. eous ammonia (25 wt%) promotes the formation [4] M. Mckelvy, R. Sharma, A.G. Chizmeshya, R.W. of platelet-shaped particles. Those behaviors are Carpenter, K. Streib, Chem. Mater. 13 (2001) 921. attributed to the mechanism of nuclei growth. As [5] C.M. Lieber, A.M. Morales, P.E. Sheehan, E.W. Wong, 5 wt% aqueous ammonia injected, polymer dis- P. Yang, in: Chemistry on the Nanometer Scale, Proceed- persants might control the nuclei growth in some ings of the Robert A. Welch Foundation 40th Conference on Chemical Research, Houston TX, 1996. positions, resulting in the needle-like particles. In [6] R.C. Xie, B.J. Qu, Preparation of needle-shaped Mg(OH)2 the ultimate situation of very slow injecting rate, nanoparticles, Patent Application Number 00135436 of for example, 7 h for sample 4, rod-like particles P.R. China. appeared. With 25 wt% aqueous ammonia in- [7] M. Alexandre, G. Beye, C. Henrist, R. Cloots, ! # jected, nuclei could agglomerate and form lamel- A. Rulmont, R. Jerome, Ph. Dubois, Macromol. Rapid Commun. 22 (2001) 643. lar-like particles. In higher pH reaction [8] S. Ultamapanya, K.J. Klabunde, J.R. Schlup, Chem. circumstances, stable nanocrystal suspension was Mater. 3 (1991) 175. obtained. [9] Y. Li, M. Sui, Y. Ding, G. Zhang, F. Zhuang, C. Wang, The temperature also has a strong effect, mainly Adv. Mater. 12 (2000) 818. on the morphology and size of the particles. The [10] Y. Ding, G. Zhang, H. Wu, B. Hai, L. Wang, Y.T. Qian, Chem. Mater. 13 (2001) 435. particles show a tendency towards lamellar-like [11] J.C. Yu, A.W. Xu, L.Z. Zhang, R.Q. Song, L. Wu, J. Phys. form at 20 C with 350 nm in diameter, while at Chem. B. 108 (2004) 64. lower temperature, needle-like with 100 nm in [12] C. Henrist, J.P. Mathieu, C. Vogels, A. Rulmont, length is obtained. R. Cloots, J. Crystal Growth 249 (2003) 321. ! After submitting the platelet-shaped particles to [13] E. Matijevic, Chem. Mater. 13 (2001) 412. [14] J. Zhan, X. Yang, D. Wang, S. Li, Y. Xie, Y.T. Qian, Adv. a mild hydrothermal treatment, a pronounced Mater. 12 (2001) 1348. improvement of regularity of the particle mor- [15] N.A.D. Burke, H.D.H. Stover,& F.P. Dawson, Chem. phology was induced without an increase of their Mater, published on web 00/00/0000 PAGE EST: 9.6 mean size. [16] Y. Sun, Y. Xie, Adv. Mater. 14 (2002) 833. The described solution precipitation method for [17] J.D. Birchall, N.L. Thomas, J. Mater. Sci. 18 (1983) 2081. [18] V.A. Phillips, J.L. Kolbe, H. Opperhauser, J. Crystal preparing MH nanoparticles is promising for Growth 41 (1977) 228. industrial production in competition with other [19] T.E. Larson, A.M. Buswell, Ind. Eng. Chem. 32 (1940) approaches. 132.