pubs.acs.org/Langmuir © 2010 American Chemical Society
Nanometer to Submicrometer Magnesium Fluoride Particles with Controllable Morphology Asep Bayu Dani Nandiyanto,† Ferry Iskandar,‡ Takashi Ogi,§ and Kikuo Okuyama*,† †
Department of Chemical Engineering, Graduate School of Engineering, Hiroshima University, 1-4-1 Kagamiyama, Higashi Hiroshima 739-8527, Japan, ‡Department of Physics, Institut Teknologi Bandung, Jl. Ganesha no 10, Bandung 40132, Indonesia, and §Department of Chemical Engineering, Osaka Prefecture University, 1-1, Gakuen-cho, Naka-ku, Sakai, Osaka 599-8531, Japan Received March 25, 2010. Revised Manuscript Received June 8, 2010
Magnesium fluoride particles with controllable size (from several nanometers to submicrometers) and morphology (spherical and cubic forms) were successfully prepared via liquid-phase synthesis. The particles were synthesized from the reaction of MgCl2 and NH4F in an aqueous solution at 75 °C for 1 h under a nitrogen atmosphere. Control of particle size was accomplished mainly by changing the concentration of the reactants, which could be qualitatively explained by conventional nucleation theory. Flexibility of the process in controlling particle morphology, from a spherical to a cubical form, was predominantly achieved by varying the concentration of MgCl2. Since the same XRD pattern was detected in particles with varying morphologies, the shape transformation was due to changes in particle growth. With the ability to control particle size and morphology, the creation of other inorganic particles is possible and has potential for many field applications.
Introduction Magnesium fluoride (MgF2) is an attractive material due to its excellent properties: low refractive index, corrosion resistance, thermal stability, and significant hardness. Excellent performance makes this material useful for applications such as catalytic support, coating materials, and antireflective lenses.1 Several approaches to the synthesis of magnesium fluoride materials have been developed,2-4 which include physical (e.g., condensation from a suitable vapor phase, mechanical milling, laser dispersion, and molecular-beam epitaxy for coatings) and chemical methods (e.g., pyrolysis of suitable fluorinated precursor materials, direct interaction of hydrofluoric acid with a magnesium solution via liquid-phase synthesis, and chemical solution deposition followed by thermal calcination). Although the current preparation methods have shown potential for industrial application, several disadvantages have been noted: (i) most of the processes have the limitation of producing film material only; (ii) most require high-temperature processing, specific and complicated synthetic equipment, and rigid conditions; and (iii) most require the use of harmful and difficult-to-handle chemicals (e.g., hydrofluoric acid (HF),3 trifluoroacetate,5 etc.), creating conflicts with safety and environmental regulations in industrial applications. Alternative ways to circumvent the problems described above have been suggested by several research groups. Methods involving the direct interaction of magnesium and fluoride solutions (DIMFS) in an aqueous medium and changing the raw material into easy-to-handle chemicals have been proposed. Pietrowski et al. have reported the use of the DIMFS method and the use of NH4F as a fluoride source,1 which successfully produced monodispersed spherical particles of submicrometer size. However, this synthetic procedure has required the assistance of microwaves, *Corresponding author. E-mail:
[email protected]. (1) Pietrowski, M.; Wojciechowska, M. J. Fluorine Chem. 2007, 128(3), 219–223. (2) Rudiger, S.; Gross, U.; Kemnitz, E. J. Fluorine Chem. 2007, 128(4), 353–368. (3) Wojciechowska, M.; Nowinska, K.; Kania, W.; Nowacka, A. React. Kinet. Catal. Lett. 1975, 2(3), 229–236. (4) Grosso, D.; Boissiere, C.; Sanchez, C. Nature Mater. 2007, 6(8), 572–575. (5) Fujihara, S.; Tada, M.; Kimura, T. J. Sol-Gel Sci. Technol. 2000, 19(1-3), 311–314.
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which can be a problem for practical applications. Sekonkaev et al. have reported the use of NaF as the fluoride source using the DIMFS method.6 Particles about 50 nm in size with differing morphologies were effectively produced; unfortunately, the use of NaF affected the production of neighborite materials (materials with Na, Mg, and F components (NaMgF3)).7 Further, the Pietrowski et al. and the Sekonkaev et al. report included no mention of the feasibility for control of particle sizes. Chi et al. reported a technique for the control of particle sizes by adjusting the initial reactant concentrations.8,9 The size of metal fluoride could be controlled down to several nanometers, but the preparation of MgF2 particles was not reported in detail. Further, particle morphology also was not a focus of the Chi et al. study. The control of MgF2 particle size and morphology remains to be an unresolved issue.1 There has been no report on particle size controllability, especially reductions to several nanometers for this material type. In fact, nanometer-sized particles with controllable morphology are important because they exhibit unique properties (e.g., optical transparency).10,11 For this reason, the purpose of the present study was to reveal the details of the preparation of MgF2 particles with controllable size and shape. On the basis of our knowledge of gas-phase12-14 and liquidphase synthesis15 in controlling particle morphology, we report herein the synthesis of MgF2 particles from the reaction of Mg2þ (6) Sevonkaev, I.; Matijevic, E. Langmuir 2009, 25(18), 10534–10539. (7) Sevonkaev, I.; Goia, D. V.; Matijevic, E. J. Colloid Interface Sci. 2008, 317(1), 130–136. (8) Chi, C.; Green, D. A.; Mikeska, K. R.; Bekiarian, P. G. US2010/0012989, 2010. (9) Chi, C.; Green, D. A.; Mikeska, K. R.; Bekiarian, P. G. US2010/0019200, 2010. (10) Kwon, S. G.; Hyeon, T. Acc. Chem. Res. 2008, 41(12), 1696–1709. (11) Nandiyanto, A. B. D.; Hagura, N.; Iskandar, F.; Okuyama, K. Acta Mater. 2010, 58(1), 282–289. (12) Nandiyanto, A. B. D.; Iskandar, F.; Okuyama, K. Chem. Eng. J. 2009, 152(1), 293–296. (13) Nandiyanto, A. B. D.; Kaihatsu, Y.; Iskandar, F.; Okuyama, K. Mater. Lett. 2009, 63(21), 1847–1850. (14) Iskandar, F.; Nandiyanto, A. B. D.; Widiyastuti, W.; Young, L. S.; Okuyama, K.; Gradon, L. Acta Biomater. 2009, 5(4), 1027–1034. (15) Nandiyanto, A. B. D.; Kim, S. G.; Iskandar, F.; Okuyama, K. Microporous Mesoporous Mater. 2009, 120(3), 447–453.
Published on Web 06/17/2010
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Article Table 1. Summary of Magnesium Fluoride Particles Prepared under Different Conditions reactant concentration (mmol/L)
sample code
CNH4F
CMgCl2
NH4F/MgCl2
dp (nm)
Stdev (nm)
particle morphology
A B C D E F G H I J
5.0 8.0 2.0 8.0 14.0 2.0 2.0 2.0 5.5 7.5
150 250 60 60 60 150 600 2000 2000 2000
0.033 0.033 0.033 0.133 0.233 0.013 0.003 0.001 0.0028 0.0038
115 40 261 55 11 184 158 142 51 18
1.65 0.79 20.00 7.84 1.36 8.00 8.00 9.22 6.30 0.50
spheres spheres spheres spheres spheres spheres mixtures (spheres and cubes) cubes cubes spheres
and F- in an aqueous solution. The main difference in the synthesis of MgF2 particles, when using our method, is the ability to prepare particles with controllable size (from 6 to 300 nm) and morphology (from a spherical and a cubical form), which can easily be produced by means of an F/Mg ratio. Furthermore, the relationship of particle diameter and morphology with reactant concentration was investigated in detail, both experimentally and theoretically. We also developed an equation to predict particle size through reactant concentration adjustment. To the best of our knowledge, this is the first documented preparation of wellcontrolled particle size (from several nanometers) and morphology.
Experimental Section Synthesis of Magnesium Fluoride Particles. Magnesium fluoride particles were prepared using a liquid-phase synthesis method with a simple reaction between magnesium and fluoride sources (MgCl2 þ 2NH4F f MgF2 þ NH4Cl). Each reactant (MgCl2 and NH4F, purchased from Kanto Chemical Co., Inc., Japan, and used without further purification) was diluted in an aqueous solution. Both of the diluted reactants (each fixed at 40 mL) were added into a reactor system and used as a precursor. The reactor system itself was comprised of a batch glass reactor (300 mL of four-necked reactor), a magnetic stirrer, a mantle heater, a condenser, and a nitrogen gas inlet. The concentration of NH4F varied from 0.001 to 0.1 mol/L while MgCl2 ranged from 0.01 to 10 mol/L. Detailed variations of reactant concentrations are listed in Table 1. The precursors were vigorously mixed for several minutes to reach a homogeneous condition. The homogeneous mixtures were then heated to 75 °C and kept at this temperature for an hour under a nitrogen atmosphere. Next, the reacted solutions were cooled to room temperature. In order to collect the prepared particles and remove unreacted reactants and impurities, the cooled solutions were purified using a centrifugation process (15 000 rpm, 30 min, washing by ethanol). Characterizations. The prepared particles were characterized using a scanning electron microscope (SEM, Hitachi S-5000 operated at 20 kV) and a transmission electron microscope (TEM; JEM-3000F, JEOL, operated at 300 kV) to examine the size, morphology, and structure of the particles. The crystallinity of the samples was measured by X-ray diffraction (XRD; Rigaku Denki RINT2000, with Cu KR radiation, with angular domain between 20° and 80° (2θ)) and electron diffraction (SAED, coupled with the TEM equipment). Elemental mapping and chemical composition of the prepared particles were evaluated using a scanning transmission electron microscope (STEM) equipped with energydispersive X-ray spectroscopy (EDS).
Results and Discussion Results. Figure 1 shows SEM images of prepared particles produced with various NH4F/MgCl2 molar ratios. Relatively monodispersed particles with a spherical shape were examined in Langmuir 2010, 26(14), 12260–12266
all cases. A strong relationship between particle diameter and reactant ratio was revealed. Submicrometer particles were formed when the ratio of 0.033 was employed (Figure 1a). The increase in ratio allowed for the production of smaller particles (Figure 1b). Further increases in ratio resulted in the formation of nanoparticles (Figure 1c). The particles with NH4F/MgCl2 molar ratios of 0.033, 0.133, and 0.233 had sizes of 261, 55, and 11 nm, respectively. Figure 2 shows the effect of the reactant ratios on particle size. Final particle diameters as a function of the NH4F concentration, when performed at MgCl2 concentrations of 400, 500, 1000, and 2000 mmol/L, are shown. Spherical particles were observed for all cases (displayed in Supporting Information). A strong correlation between particle size and concentrations of both reactants (NH4F and MgCl2) was revealed. In order to confirm this relationship, simple data regressions were plotted and shown in Figure 2. Curve equations carried out at MgCl2 concentrations of 400, 500, 1000, and 2000 mmol/L could be written as eqs 1, 2, 3, and 4, respectively: ln dp ¼ - 0:25CNH4 F þ 5:90
ð1Þ
ln dp ¼ - 0:25CNH4 F þ 5:70
ð2Þ
ln dp ¼ - 0:25CNH4 F þ 5:52
ð3Þ
ln dp ¼ - 0:25CNH4 F þ 4:84
ð4Þ
where dp and CNH4F are the final mean particle diameter (measured from SEM images, in nm) and the concentration of NH4F (in mmol/L), respectively. The regressed data showed that for all variations the final size of the particles decreased with increasing fluoride concentration. Another correlation was also identified in Figure 2. As the concentration of MgCl2 increased, the particle size decreased. The detailed correlation figures between MgCl2 and particle size are displayed in the Supporting Information. The combination of “the effect of NH4F concentration on particle size” and “the interpolation data for the variation of MgCl2 concentration” resulted in the relationship of particle diameter with both reactants, as shown in the following equation: ln dp ¼ - 0:25CNH4 F - 0:0007CMgCl2 þ 6:13
ð5Þ
where CMgCl2 was the concentration of MgCl2 in the initial solution (in mmol/L). Figure 3 shows the effect of low NH4F/MgCl2 molar ratios on particle morphology. Spherical particles were synthesized when a high ratio was used (Figure 3a). Reduction of NH4F/MgCl2 molar ratios of a specific value caused the production of unique particles. As the ratio approached a specific value (NH4F/MgCl2 ratio of about 0.003), cubic particles started to appear (Figure 3b). Mixed particle shapes in spherical, cubic, and cubic-spherical DOI: 10.1021/la101194w
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Figure 1. SEM images of prepared particles with NH4F/MgCl2 ratios of 0.033 (sample C) (a), 0.133 (sample D) (b), and 0.233 (sample E) (c). All samples contained a MgCl2 concentration of 60 mmol/L.
Figure 2. Log particle size against detailed data on reactant concentration.
forms were observed. Further decreases in the ratio resulted in cubic particles (Figure 3c). In certain cases, smaller cubic particles were produced when increases in MgCl2 and NH4F concentration were combined (Figure 3d). Figure 4 shows TEM/SAED images of particles with various morphologies. Dense spherical and cubic particles were observed in parts a and b of Figure 4, respectively. The sizes and morphologies of particles, as assessed by TEM, were in good agreement with the SEM results (in Figures 1a and 3c). The high-resolution TEM and SAED results confirmed that the spherical particles had a polycrystalline structure, but the cubic particles had a relatively monocrystalline structure. 12262 DOI: 10.1021/la101194w
Figure 5 shows the elemental mapping of magnesium fluoride particles (11 nm in size). The STEM image of spherical-shaped nanoparticles is shown in Figure 5a. The mapping results showed that the particles had different components: one was magnesium (Figure 5b), and the other was fluoride (Figure 5c). Magnesium and fluoride were well-distributed inside the nanoparticles as prepared, confirming that the particles consisted of magnesiumfluoride compounds. The XRD patterns of prepared particles with different morphologies are presented in Figure 6. Different reactant ratios can lead to changes in material patterns and performances. Magnesium fluoride patterns were detected in all cases; the only difference appeared to be the intensity of the peaks that described dissimilar crystal sizes. The crystal sizes of nano- (sized 11 nm), spherical- (sized 261 nm), and cubic-shaped particles (sized 142 nm) were 3.3, 21.9, and 16.2 nm, respectively, as measured using the Scherer method. These results imply that a significant reactant ratio changed only the size of the crystalline particles (formation of very small crystalline) but did not substantially contribute to the material phase and pattern. Discussion. The present study was primarily directed toward investigation of the reactant concentration effect in the synthesis of monodispersed magnesium fluoride particles. This study used a single-step process, which was performed in the absence of additional components (e.g., chemicals, surfactants, etc.). The results demonstrate the possibility of producing particles with controllable morphology and size by suitable changes in reactant concentrations. The particle diameter could be controlled in sizes ranging from several to hundreds of nanometers, while the particle morphology could be created in several forms (spheres and cubes). The variation of concentration was the main focus of the present study, and further work on other parameters (e.g., temperature) will be reported in the future. Typically, the LaMer and Dinegar theory is used as a qualitative model to describe particle-formation phenomena. This theory Langmuir 2010, 26(14), 12260–12266
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Figure 3. SEM images of prepared particles with NH4F/MgCl2 ratios of 0.013 (sample F) (a), 0.003 (sample G) (b), and 0.001 (sample H) (c). Samples of (a) to (c) consisted of a NH4F concentration of 2 mmol/L. An additional figure (d) was the sample with the same amount of MgCl2 as (c), but with a NH4F concentration of 5.5 mmol/L (NH4F/MgCl2 ratio of 0.0028; sample I).
Figure 4. TEM with SAED analysis results of spheres (a) and cubes (b). Parts (a) and (b) were prepared using a NH4F/MgCl2 ratio of 0.033 (sample C) and 0.001 (sample H), respectively.
describes monomer formation, the nucleation step, and the growth process.16 Molar ratio was chosen as a comparison parameter because its discrepancies could be easily distinguished. In the present study, we focused of the NH4F/MgCl2 ratio as a molar ratio parameter. A high ratio meant there was a high amount of anions (i.e., fluoride ions) in the solution, which is identical to the trend toward a faster rate of monomer formation because these anions tend to actively attack and be adsorbed onto inorganic (16) Sugimoto, T.; Shiba, F.; Sekiguchi, T.; Itoh, H. Colloids Surf., A 2000, 164(2-3), 183–203. (17) Jovic, B. M.; Jovic, V. D.; Drazic, D. M. J. Electroanal. Chem. 1995, 399(1-2), 197–206. (18) Sugimoto, T. Monodispersed Particles; Elsevier: Amsterdam, 2001.
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compounds (i.e., magnesium ions).17,18 This faster formation rate allows rapid progress of particle evolution into further steps (nucleation and growth), causing the production of smaller particles. Conversely, a low ratio defines a low amount of fluoride ions in the initial solution, indicating retardation of the monomer formation rate. This circumstance causes an excess of unreacted monomer in the solution, resulting in a faster growth step and permitting the formation of particles with a larger size. A detailed explanation about LaMer and Dinegar Theory is represented in the Supporting Information. A nuclei number generated as an effect of different reactant concentrations is shown in Figure 7. To make an explicit calculation for the nuclei number, several conditions were assumed:16 DOI: 10.1021/la101194w
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Figure 5. Elemental mapping results of 11 nm prepared particles (sample E): (a) TEM analysis; (b) magnesium; (c) fluorine.
Figure 6. X-ray diffraction patterns of 11 nm spheres (sample E), 261 nm spheres (sample C), and 142 nm cubes (sample H).
(1) the monomer is supplied only from the chemical reaction; (2) critical supersaturation (Ccrit) is a minimal requirement for the nucleate to be discernible; (3) as the monomer concentration is lowered to below the Ccrit level, nucleus formation ceases, and the nucleus then starts to consume monomers to increase its weight (growth step process). The results in Figure 7 show that the number of nuclei (n) increased from 1012 to 1018 with increases in reactant concentration. The results were verified and agreed well with the above Lamer and Dinegar fundamental theory. The increase in reactant concentration caused a rapid conversion of reactants into monomers (increase in nuclei number), which would accelerate nuclei 12264 DOI: 10.1021/la101194w
Figure 7. Calculated number of nuclei as a function of MgCl2 and NH4F concentration.
formation but retard particle growth. Conversely, the decrease in reactant concentration resulted in fewer monomers. As a consequence, a short-nucleation stage occurred (which resulted in the production of fewer nuclei) and allowed the nuclei to catch more monomers and increase their weight during the particle growth stage. A detailed derivation of the nuclei number equation is shown in the Supporting Information. In some cases, when reactant ratios were relatively low, the transformation of particle shapes from spherical into cubic form could be observed (Figure 3). The spherical particles were obtained Langmuir 2010, 26(14), 12260–12266
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Figure 8. Effect of variation of MgCl2 and NH4F concentrations on magnesium fluoride particle size and morphology.
at relatively high molar ratios of NH4F/MgCl2, while the cubic particles were produced at low ratios. The XRD patterns for all shapes were the same (Figure 6), illustrating that the fundamental reason behind the shape transformation was not changes in material patterns or chemical structures. The XRD analysis showed that the peak among all samples was a characteristic of the cubic form of particles. On the basis of the results of the present study, we can hypothesize that particle growth was the reason for the shape transformations.19 When the ratio of NH4F/MgCl2 was very low, a limited amount of monomer was formed. This condition caused retardation of the particle growth rate, changes in the surface potential adsorption of monomers in the nuclei, and the growth of particles into their original form (cubic),16 which was confirmed to be a relatively monocrystalline structure by SAED analysis (Figure 4b). However, when the ratio is high, the number of anions also should be high. As the excess amount of anions increased, monomers were rapidly produced, allowing faster nuclei and crystal formation (very small crystalline generation). Enhanced particle growth from all particle surface faces progressed due to changes in specific surface energy.18 This phenomenon imparted a spherical shape and polycrystalline structure to the particles, which was verified by SAED analysis, as shown in Figure 4a. In addition, because the sample has a polycrystalline structure, a further confirmation of the SAED analysis will be conducted in a future study. Based on the experimental results, the reactant ratio can be used as a parameter to control particle size and morphology (Figure 8). Some combinations of this ratio resulted in the successful formation of particles with a spherical shape, while other combinations of reactant concentrations produced various morphologies (i.e., cubes). When NH4F/MgCl2 ratios above 0.003 were used in the process, spherical magnesium fluoride particles could be prepared (clear-patterned area). However, when the ratios were lower than 0.003, cubic shape particles were produced (blue-patterned area). For both shapes, particle size could be easily controlled by changing the reactant molar ratio. In (19) Sugimoto, T.; Zhou, X. P.; Muramatsu, A. J. Colloid Interface Sci. 2003, 259(1), 53–61.
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addition, increased reactant concentrations allowed the production of smaller particles. Recognition of Figure 8 came from the experimental results, in which the variation was within strictly specified concentrations (MgCl2 = 0.01-10 mol/L; NH4F = 0.001-0.1 mol/L). For this reason, the preparation of particles that are controllable for size and morphology underscores the need for further studies. We believe that insights gained from this type of research will contribute to more fabrication innovation.
Conclusions Magnesium fluoride particles with controllable size (from 6 to 300 nm) and morphology (spherical and cubic forms) were successfully prepared using a liquid-phase synthesis method. The effectiveness of this method in controlling particle diameter relied on changes in reactant concentration, which was qualitatively verified using conventional nucleation theory. The ability to create particles with various morphologies was due to changes in the NH4F/MgCl2 molar ratios. Spherical particles were prepared when the ratios were above 0.003. When the ratios decreased to below 0.003, the process led to the production of particles with a cubic shape. Because of the simple preparation procedures, use of relatively low-temperature processing, and employment of easy-to-handle chemicals, we believe that the present study provides important new information for the field of chemical and material science and engineering. Acknowledgment. A scholarship provided for A.B.D.N. by the Japanese Ministry of Education, Science, Sports, and Culture (MEXT) is gratefully acknowledged. We also thank the Hosokawa micron foundation for providing a research support grant for A.B.D.N. We thank Akihiro Oomura of the Dept. Chemical Engineering of Hiroshima University and Tuswadi of the Faculty of IDEC of Hiroshima University for assistance in this research. We are also grateful to Dr. Eishi Tanabe of the Hiroshima Prefectural Institute of Industrial Science and Technology for his help with the HR-TEM and for his consultation. DOI: 10.1021/la101194w
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Supporting Information Available: (1) LaMer and Dinegar theory, (2) nuclei number equation derivation, (3) LaMer and Dinegar diagram as a schematic explanation of particle formation at high (condition 1) and low (condition 2) molar ratios of NH4F/MgCl2, (4) log particle
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size against detailed data on reactant concentration (mean particle size vs MgCl 2 concentration), and (5) a brief of SEM results from Figure 2. This material is available free of charge via the Internet at http://pubs. acs.org.
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