Chemical Synthesis and Characterization of Highly Oil Dispersed MgO Nanoparticles

Transcription

1 J. Ind. Eng. Chem., Vol. 12, No., (200) Chemical Synthesis and Characterization of Highly Oil Dispersed MgO Nanoparticles Jo-Yong Park, Yun-Jo Lee, Ki-Won Jun, Jin-Ook Baeg, and Dae Jae Yim* Korea Research Institute of Chemical Technology (KRICT), P.O. Box 7, Yuseong, Daejeon , Korea *EMAX Solutions CO., LTD, 81-5 Gyesan-dong, Gyeyang-gu, Incheon , Korea Received March 17, 200; Accepted July 20, 200 Abstract: Preparation of highly oil dispersed MgO nanoparticles (NPs) was carried out at room temperature via precipitation of Mg(OH) 2 from aqueous media using Mg(NO 3 ) 2 as a starting precursor and NaOH as a precipitant. The Mg(OH) 2 precursor was converted into MgO particles having face-centered cubic structures through calcination treatment in air at 500 o C for 4 h. The freshly calcined material had a high specific surface area (91.4 m 2 /g). An SEM study revealed the presence of nanoparticles of spherical shape (20~50 nm). The NPs of MgO remained dispersed in oil through the capping action of oleic acid after heating them under reflux at 10 o C in organic solvent. An IR study indicated the presence of hydroxyl (OH - ) groups at the surface of the MgO particles; these units interact with the carboxylate groups of oleic acid molecules to cap the MgO particles and, thus, to help disperse them in a given solvent. The results are quite reproducible and the dispersion stability was maintained for long durations. Keywords: MgO, nanoparticles, capping, dispersion Introduction 1) Oxide nanomaterials are finding a wide range of applications as catalysts and starting materials for preparing advanced structural ceramics because they possess a unique property, i.e., high reactivity, resulting from their high specific surface area, controlled size, and distribution. Magnesium oxides are also used in variety of industrial applications, e.g., as heat-resistant and high-temperature insulating materials and as fuel-oil additives [1-3]. In addition, MgO is also used as a heat-resistant glass composite in liquid crystal display panels, electroluminescence display panels, plasma display panels, and fluorescent display tubes. Recently, it was reported [4,5] that MgO possesses good additive properties in heavy fuel-oil because of its high oil dispersion ability and large specific surface area. Furthermore, MgO has exhibited high activity for the cleaning of fuel-oil pipelines, avoidance of sludge formation in tanks, and protection of boilers against corrosion, thereby increas- To whom all correspondence should be addressed. ( yjlee@krict.re.kr, kwjun@krict.re.kr) ing these system s lifetimes. Conventionally, MgO is synthesized by the decomposition of various magnesium salts in air [,7]. However, the MgO particles produced by this method are usually large and of irregular size, with low specific surface area. MgO NPs with narrow size distributions can be prepared by using different chemical routes [8]; the oxide morphology and particle sizes are dependent on such preparation conditions as the ph, temperature of precipitation, concentration or ionic strength, and calcination temperature/time [11]. Therefore, research continues into achieving better control over the sizes of MgO particles through smart chemical processing of materials. A novel method that produces MgO NPs with controlled spherical shapes, uniform narrow size distributions, and high specific surface areas is desirable [4]. These properties ultimately govern the dispersion behavior in mineral oils [5,]. In this paper, we report a novel method for the preparation of MgO NPs and their further dispersion in oil by using commercially available magnesium nitrate as raw material. The method involves the following three steps: i) Precipitation of magnesium cations in alkaline media

2 Chemical Synthesis and Characterization of Highly Oil Dispersed MgO Nanoparticles 883 Figure 1. Schematic illustration of the formation of capped MgO. as Mg(OH) 2 by using NaOH, NH 4 OH, or KOH as precipitant. ii) Magnesium hydroxide Mg(OH) 2 precipitate thus obtained is further calcined at a slightly higher temperature (shown by DTA/TGA as 450 o C) for 4 h to generate nano-sized MgO particles having high specific surface areas. iii) MgO NPs are capped with oleic acid under reflux to obtain a highly dispersed phase of MgO in a given solvent. The methodology described above is useful for the preparation of nano-sized MgO particles having high specific surface areas and improved oil-dispersion properties. Experimental Synthesis of Nano-sized MgO Mg(NO 3 ) 2 H 2 O (99 %, Samchun Co.) and NaOH (98 %, Samchun Co.) were used as starting materials. In a typical procedure for the preparation of MgO, 0.1 M Mg(NO 3 ) 2 was dissolved in 0 ml of deionized water; 0.2 M aqueous NaOH solution was added under constant stirring [1-3]. The molar ratio of metal ions to hydroxide ions was maintained at 1:2. The mixture was then stirred for 2 h at room temperature to form magnesium hydroxide. Then, two samples of magnesium hydroxide were prepared using different washing procedures. Sample 1 was washed with deionized water only, while sample 2 was washed first with deionized water and subsequently with ethyl alcohol. The samples of washed magnesium hydroxide were dried at 1 o C for 12 h and calcined at 500 o C for 4 h in air to obtain MgO NPs. Figure 2. TGA/DTA thermograms of Mg(OH) 2. Capping of MgO The MgO ( g, 0. mol in Mg) NPs were dispersed in kerosene ( ml) solvent and then oleic acid (0~7.1 g, 0~0.0 mol) as the capping agent was added to the mixture. The mixed solution was heated under reflux at various temperatures between 0 and 200 o C for 1 h. The capped solution was left for 5 h in kerosene at room temperature and the dispersion (D b ) of MgO NPs was determined by measuring the quantity of settled MgO. A schematic representation of the formation of MgO capped with oleic acid is illustrated in Figure 1. Characterization of MgO NPs The surface morphological features (shape and particle size) of the MgO NPs were measured using a Philips XL-30 scanning electron microscope (SEM). The EDAX technique was used for the stoichiometric analysis of the elements of magnesium and oxygen in MgO. The specific surface areas and particle sizes of samples 1 and 2 of MgO NPs were determined from their nitrogen adsorptions through the BET method using a Micromeritics TriStar 3000 apparatus. Powder X-ray diffractograms were recorded using a Model Rigaku-D/Max-2200 V X-ray diffractometer with CuK α radiation (λ= A ) and a Ni filter. The surface morphologies and particle sizes of the prepared MgO were determined using a Field Emission Scanning Electron Microscope (FE-SEM; Model JEOL-JSM 700 F). The capped powder was characterized using Fourier Transform Infrared Spectroscopy (Model: JASCO FTIR) and TGA/DTA thermal studies (Model: SETARAM, TGDTA92) at a heating rate of 5 o C/min under a static air atmosphere. The dispersion (D b ) of the highly oil-dispersed MgO colloid

3 884 Jo-Yong Park, Yun-Jo Lee, Ki-Won Jun, Jin-Ook Baeg, and Dae Jae Yim (a) Figure 3. XRD patterns of Mg(OH) 2 and MgO. solution was calculated based on the quantity of settled MgO in a specified quantity of kerosene. Results and Discussion Thermal Analysis of Mg(OH) 2 Figure 2 presents the TGA/DTA curves of the asprepared magnesium hydroxide. The TGA curve of the Mg(OH) 2 shows a weight loss occurring in two steps. The first small weight loss is due to the removal of adsorbed water/alcohol below 0 o C and the second exothermic loss of 30.7 % is attributable to decomposition of Mg(OH) 2 and crystallization of MgO particles. The theoretical weight loss (30.87 %) calculated by conversion of the hydroxide precursor to MgO is in good agreement with the measured value. The process accounts for ca. one third of the total weight loss in the whole decomposition process from 0 to 800 o C according to the TGA result, with the major loss occurring between 300 to 400 o C [11]. Thus, DTA/TGA results indicated that the formation temperature of MgO was 450~500 o C in air. X-ray Diffraction (XRD) Patterns of Mg(OH) 2 and MgO NPs Figure 3 presents the XRD patterns of as-dried Mg (OH) 2 and MgO NPs. The as-dried precipitate showed all of the reflections corresponding to the Mg(OH) 2 phase. (b) Figure 4. SEM images of MgO calcined at 500 o C for 4 h. Mg(OH) 2 was washed with (a) water and (b) water and alcohol. XRD confirmed that on calcination at 500 o C, Mg(OH) 2 was completely converted to MgO having a face-centered cubic phase [12]. The particle size calculated using the Scherrer formula was 20 nm in the case of the sample washed with alcohol. These nanoparticles may tend to form aggregates, and may form large soft agglomerates through inter-particle interaction resulting from electrostatic and van der Waals forces. Thus, the XRD study confirmed the formation of NPs of MgO after calcination in air. Specific Surface Area (BET) Measurement The particle size was calculated according to the formula D p = 3 / S ρ, where D p is the size of the particles, S is the specific surface area, and ρ is the Table 1. Specific Surface Area Values and Estimated MgO Particle Sizes Sample no. 1 2 S (m 2 /g) Particle size (nm, BET) Particle size (nm, SEM) 50~0 30~500 Crystallite size (nm, XRD) 33 20

4 Chemical Synthesis and Characterization of Highly Oil Dispersed MgO Nanoparticles 885 (a) (b) Figure 5. EDAX patterns of (a) uncapped and (b) capped MgO at 10 o C. density of MgO (3.58 g/cm 3 ) [13]. It is evident from Table 1 that the MgO sample 2 had a smaller particle size and, thus, a larger specific surface area than sample 1. Sample 1 had a poor microstructure and its particles Figure. IR spectra of the uncapped and capped MgO. appear to be agglomerated as shown in the SEM study. Scanning Electron Microscopy (SEM) SEM images of the MgO samples calcined at 500 o C for 4 h in air are shown in Figure 4. The average particle size of the MgO NPs fell in the range from 20 to 50 nm, depending on the procedure used to wash Mg(OH) 2. The as-prepared, ethyl alcohol-washed MgO sample (sample 2) exhibited a smaller and more spherical morphology than did the water-only-washed sample (sample 1) [Figure 4(b)]. The MgO NP sample 1 had an irregular shape and poor microstructure resulting from agglomeration of MgO particles [Figure 4(a)]. Therefore, the use of a low-surface-tension solvent, such as ethyl alcohol, during washing treatment was a critical parameter in protecting agglomeration of the nanoparticles and producing mondisperse MgO NPs. EDAX Spectra of Uncapped and Capped MgO NPs In Figure 5(a), we observe the peaks corresponding to Mg and O elements with an average atomic percentage ratio of 58:42. The EDAX spectrum in Figure 5(b) shows that not only do the Mg and O signals exist but so does a C signal arising from the capping material. The presence of the C atom signal and the higher percentage of oxygen Table 2. EDAX Elemental Analysis of the Uncapped and Capped MgO Uncapped MgO Capped MgO Element Wt% At% Wt% At% CK OK MgK Total

5 88 Jo-Yong Park, Yun-Jo Lee, Ki-Won Jun, Jin-Ook Baeg, and Dae Jae Yim Table 3. Dispersion (D b) of MgO Colloid Solution Under Different Conditions Sample no. MgO (g) Oleic acid (g) Kerosene (g) Refluxing Temp. ( o C) D b (%) * * MgO was prepared from water-only-washed Mg(OH) in the sample of MgO capped with oleic acid reflects the presence of the oleic acid units. IR Studies of Uncapped and Capped MgO NPs In Figure, the weak peak that appears at 17 cm -1 is attributable to the presence of C-O groups; the main peaks at 1454 and 155 cm -1 are due to the O-C-O groups from oleic acid units used as the capping material. The band at 285~2927 cm -1 in the IR spectrum corresponds to C-H vibrations arising from the alkyl chains of the oleic acid units. The IR spectrum of the as-prepared MgO NPs shows a peak at 3434 cm -1 that is typical of O-H vibrations for adsorbed species; this signal was modified and became broad in the capped MgO particles, which is indicative of the interaction of OH species with the carboxylate groups from oleic acid leading to capping action. Thus, this IR spectroscopic study confirmed the interaction between the OH species from the oxide surface and the carboxylate groups of the oleic acid units, leading to capping action and, hence, dispersion of the MgO particles in a given solvent. Evaluation for Dispersion (D b ) Test Table 3 shows the dispersions (D b ) of the MgO colloid solutions capped with varying amounts of oleic acid and subjected to various reflux temperatures in kerosene. All of the samples were washed with water and then alcohol, except for MgO NP sample 9, which was washed with water alone. We define the dispersion (D b ) as follows (Figure 7): D b = Total MgO - Settled MgO Total MgO 0 = A A+B 0 (1) where D b is the dispersion of the MgO colloidal solution in kerosene. Samples of 1 g of MgO were added in 20 Figure 7. Dependency of MgO oil-dispersion on reflux temperature for capping. ml of kerosene and then the dispersion of the MgO in kerosene was measured based on the amount of unsettled MgO after 5 h. The dispersion was highest when the oleic acid content was g (0.021 mole) per g (0. mole) of MgO at a reflux temperature of 10 o C. Figure 7 shows the variation in dispersion of the MgO NPs as a function of the reflux temperature. An increase in the reflux temperature increased the dispersion of MgO NPs up to 10 o C, but thereafter we observed a slight decrease in the dispersion. This result indicates that maximum capping was achieved at that temperature, and that a further increase in temperature is not helpful for increasing the capping action. Conclusion MgO NPs were prepared using a simple precipitation

6 Chemical Synthesis and Characterization of Highly Oil Dispersed MgO Nanoparticles 887 process in alkaline media. The washing procedure was an important parameter controlling the agglomeration tendency of the particles. Washing Mg(OH) 2 in an organic solvent (ethyl alcohol) having a low critical surface tension lowered the degree of agglomeration and, subsequently, led to smaller particle sizes. Stable colloid solutions of MgO in oil were successfully prepared by capping the MgO NPs with oleic acid through the reaction of the hydroxyl groups on the MgO surface with the carboxyl groups of the oleic acid units. Passivation of the Mg NPs surface with long-chain aliphatic groups (oleic acid) prevented the MgO NPs from agglomerating and settling in the oil solvent. Acknowledgments We are grateful for the financial support of the Korea Energy Management Corporation under the Clean Energy R&D Program of the Ministry of Commerce, Industry, and Energy, Korea. References 1. A. Poullikkas, Energy Conversion Manage., 45, 17 (2004). 2. X. L. Guo, Z. G. Liu, X. Y. Chen, S. N. Zhu, S. B. Xiong, W. S. Hu, and C. Y. Lin, J. Appl. Phys., 29, 132 (199). 3. Y. R. Li, Z. Liang, Y. Zhang, J. Zhu, S. W. Jiang, and X. H. Wei, Thin Solid Films, 489, 245 (2005). 4. J. Davies, J. W. Laxton, and M. J. Owers, J. Inst. Energy, 14, 21 (1981). 5. J. C. Brien, J. Inst. Energy, 15, 115 (1982).. H. R. Oswald and R. Asper, Physics and Chemistry of Mineral with Layered Structures, R.M.A. Kieth Ed., Vol. 1, Reidel, Dordrecht (1977). 7. M. Laska, J. Valtyni, and P. Fellner, Cryst. Res. Tech., 28, 931 (1993). 8. S. Makhluf, R. Dror, Y. Nitzan, Y. Abramovich, R. Jelinek, and A. S. Makhluf, Adv. Funct. Mater and references therein, 15, 1708 (2005). 9. H. S. Choi and S. T. Hwang, J. Mater. Res., 15, 842 (2000).. C. Henrist, J. Crystal Growth, 249, 321 (2003). 11. (a) J. M. Hur, I. S. Kim, W. K. Lee, S. H. Cho, C. S. Seo, and S. W. Park, J. Ind. Eng. Chem.,, 442 (2004). (b) K. D. Kim, S. S. Kim, and H. T. Kim, J. Ind. Eng. Chem., 11, 584 (2005). 12. Joint Committee of Powder Diffraction Standards (JCPDS) File No C. W. Robert, J. A. Melvin, and H. B. Willian, The CRC Handbook of Chemisty and Physics, th Edn., CRC Press, Boca Raton, FL (198).