Preparation of MgO Nanoparticles

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example, in earlier studies, if synthesis from dilute ... magnesium oxide and analytical-grade nitric acid. Magnesium oxide nanoparticles were synthesized in.

ISSN 0020-1685, Inorganic Materials, 2007, Vol. 43, No. 5, pp. 502–504. © Pleiades Publishing, Inc., 2007. Original Russian Text © P.P. Fedorov, E.A. Tkachenko, S.V. Kuznetsov, V.V. Voronov, S.V. Lavrishchev, 2007, published in Neorganicheskie Materialy, 2007, Vol. 43, No. 5, pp. 574–576.

Preparation of MgO Nanoparticles P. P. Fedorov, E. A. Tkachenko, S. V. Kuznetsov, V. V. Voronov, and S. V. Lavrishchev Laser Materials and Technologies Research Center, Prokhorov General Physics Institute, Russian Academy of Sciences, ul. Vavilova 38, Moscow, 119333 Russia e-mail: [email protected] Received September 22, 2006

Abstract—MgO nanoparticles have been prepared via hydroxide precipitation from aqueous solutions, followed by the thermal decomposition of the hydroxide. The nanoparticles inherit the platelike shape from the hydroxide and break into isometric particles upon significant superheating. The particle size of the synthesized magnesium oxide powders varies from 30 to 75 nm, depending on the annealing temperature. DOI: 10.1134/S0020168507050111

INTRODUCTION Advances in nanotechnology, a relatively young area of materials research, have led to revision and new interpretation of earlier known scientific facts. For example, in earlier studies, if synthesis from dilute solutions yielded a colloidal solution of a precipitate with low to zero filterability, this was considered a negative effect. Conversely, there is now increasing interest in such substances because the effect in question is connected with the preparation of nanometer- and submicron-sized particles, which possess special properties, differing from the properties of bulk materials. An interesting effect was found in studies of the formation of yttria nanoparticles through thermal decomposition of a basic nitrate hydrate [1–4]: adjusting precipitation conditions, in particular, using some surfactants, one can obtain precursor particles in the form of nanometer-thick platelets. The Y2O3 particles resulting from subsequent thermal decomposition of the precursor inherit the platelike shape from the precursor particles. Considerable superheating causes the platelets to break into spherical nanoparticles, which is accompanied by a reduction in lattice microstrain to almost zero. The purpose of this work was to ascertain whether this mechanism of nanoparticle formation is universal. The model systems used were magnesium hydroxide and magnesium oxide. Magnesium hydroxide has a layered, brushite-type structure [5] and typically consists of platelike particles. Magnesium oxide (periclase) crystallizes in cubic symmetry (NaCl structure); platelike particles are uncommon for MgO. Rao and Pitzer [6] reported that the DTA curve of Mg(OH)2 showed, in addition to the endotherm due to hydroxide decomposition, an exothermic peak near 560°C, which

was obviously caused by recrystallization of the MgO formed. EXPERIMENTAL The starting chemicals used were reagent-grade magnesium oxide and analytical-grade nitric acid. Magnesium oxide nanoparticles were synthesized in two steps: precipitation of magnesium hydroxide, followed by calcination at different temperatures until MgO formation. Magnesium hydroxide was obtained by adding increments of aqueous ammonia to a magnesium nitrate solution until precipitation, with constant stirring using a magnetic stirrer. The magnesium nitrate solution (0.207 M) was prepared by dissolving magnesium oxide in a threefold excess of nitric acid. The precipitate was washed with distilled water and dried on filter paper using a lamp (at a temperature of about 40– 50°C). The samples thus prepared were characterized by x-ray diffraction (XRD). In particular, we evaluated the size of coherent scattering domains (CSDs) and lattice microstrain ε as described by Jiang et al. [7]. XRD patterns were collected on a DRON-4 powder diffractometer (CuKα radiation, pyrolytic graphite monochromator). Polycrystalline silicon (220 reflection, 2θ = 47.34°) was used as a peak-shape standard. The CSD size and ε in magnesium oxide were determined by analyzing the 200 diffraction line profile (2θ = 42.98°). The morphology of powder specimens was examined by scanning electron microscopy (SEM) on a JEOL 5910. To prevent surface charging, a gold layer was evaporated on the surface of the specimens. Thermal analysis (DTA + TG + DTG) was performed at a heating rate of

502

PREPARATION OF MgO NANOPARTICLES

503

itate consists of irregularly shaped quasi-two-dimensional micron-sized particles (flakes). The DTA curve of the precipitate (Fig. 2) shows two endothermic peaks. The lower temperature endotherm seems to correspond to the removal of adsorbed water (weight loss of 7.1%). The endotherm between 312 and 393°C is due to magnesium hydroxide decomposition by the reaction Mg(OH)2 = MgO + H2O. 1 µm Fig. 1. SEM micrograph of the magnesium hydroxide precipitate.

10°C/min (MOM Q-1500 D thermoanalytical system, alundum crucibles). RESULTS AND DISCUSSION The lattice parameters of the magnesium hydroxide precipitate were a = 3.14 Å and c = 4.77 Å, in good agreement with JCPDS PDF data (no. 76-0667). The SEM micrograph in Fig. 1 demonstrates that the precip-

The weight loss calculated for this reaction is 31%; the measured weight loss was 31.5%. The total weight loss was 38.6%, which is attributable to the fact that the sample was not dried before thermal analysis. The conversion of magnesium hydroxide to magnesium oxide is characterized by a large endotherm. Based on thermal analysis data, we selected temperatures to which the precipitate was heated in order to obtain magnesium oxide: 480, 730, and 1000°C. XRD examination of the resultant powders showed that MgO was obtained at the three temperatures (table). The SEM micrograph in Fig. 3a demonstrates that, during magnesium hydroxide calcination at 730°C and oxide formation, the platelike shape of the particles persists, even though it is a nonequilibrium shape for MgO. At high temperature, the platelets break into isometric particles (Fig. 3b).

DTG

TG DTA

312°C 393°C

T

Fig. 2. Thermal analysis results for the magnesium hydroxide precipitate (368-mg sample). INORGANIC MATERIALS

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CSD size and lattice microstrain in MgO powders after heating to different temperatures Sample

Reflection

2θ, deg

2ω, deg

β, deg

0.101

0.127

DCSD , nm

ε × 103

a, Å

Si

220

MgO, 480°C

200

42.753

0.443

0.587

30

5.1

4.226

MgO, 730°C

200

42.768

0.426

0.524

52

5.9

4.225

MgO, 1000°C

200

42.848

0.358

0.436

75

5.0

4.218

Thus, during magnesium hydroxide decomposition, we observe the same effect as in the case of basic yttrium nitrate: the oxide particles inherit the platelike shape from the precursor particles, which is a nonequilibrium shape for the reaction product, and then break into isometric nanoparticles. In contrast to yttria, the process in the system under consideration is only gradual. No exothermic peak was detected in the DTA curve of the precipitate. It seems likely that the mechanism responsible for the change in particle shape depends on the heating conditions. Note that the lattice parameter of the nanoparticles is slightly larger than that of bulk samples (a = 4.209 Å, JCPDS PDF, no. 77-2364). It is reasonable to expect

that platelike MgO nanoparticles, containing a high density of structural defects, exhibit increased reactivity. CONCLUSIONS A procedure was developed for the preparation of magnesium oxide nanoparticles via hydroxide precipitation from aqueous solutions. The particle shape was shown to persist during magnesium hydroxide conversion to magnesium oxide. Subsequent heating leads to the formation of isometric nanoparticles from nonequilibrium platelets. The large heat effect of magnesium hydroxide decomposition leads to the formation of MgO nanoparticles. REFERENCES

(a)

(b)

1 µm

0.5 µm

Fig. 3. SEM micrographs of magnesium oxide after heating to (a) 730 and (b) 1000°C.

1. Baranchikov, A.E., Ivanov, V.K., Dmitriev, A.V., et al., Chemical Transformations of Basic Yttrium Nitrates during Ultrasonic–Hydrothermal Treatment, Zh. Neorg. Khim., 2006, vol. 51, no. 11, pp. 1797–1803. 2. Basiev, T.T., Voronov, V.V., Kuznetsov, S.V., et al., Preparation of Yttria Nanoparticles, XI Natsional’naya konferentsiya po rostu kristallov, NKRK-11 (XI Natl. Conf. on Crystal Growth, NCCG-11), Moscow, 2004, p. 77. 3. Basiev, T.T., Fedorov, P.P., Konyushkin, V.A., et al., Preparation of Yttrium Oxide Nanoparticles, Int. Conf. Crystal Materials 2005, Kharkov, 2005, p. 60. 4. Fedorov, P.P., Basiev, T.T., Voronov, V.V., et al., Preparation of Nanocrystalline Materials through Decomposition of High-Energy States, V Natsional’naya konferentsiya po primeneniyu rentgenovskogo, sinkhrotronnogo izluchenii, neitronov i elektronov dlya issledovaniya nanomaterialov i nanosistem, RSNE NANO-2005 (V Natl. Conf. on the Application of X-rays, Synchrotron Radiation, Neutrons, and Electrons in Characterization of Nanomaterials and Nanosystems, XSNE-2005), Moscow: Shubnikov Inst. of Crystallography, Russ. Acad. Sci., 2005, p. 172. 5. Chalyi, V.P., Gidrookisi metallov (Metal Hydroxides), Kiev: Naukova Dumka, 1972. 6. Rao, C.N.R. and Pitzer, K.S., Thermal Effects in Magnesium and Calcium Oxides, J. Phys. Chem., 1960, vol. 64, no. 2, pp. 282–283. 7. Jiang, H.G., Ruhle, M., and Lavernia, E.J., On the Applicability of X-ray Diffraction Line Profile Analysis in Extracting Grain Size and Microstrain in Nanocrystalline Materials, J. Mater. Res., 1999, vol. 14, no. 2, pp. 459–559. INORGANIC MATERIALS

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