Langmuir 2003, 19, 7569-7572
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Synthesis and Characteristics of Sm2O3 and Nd2O3 Nanoparticles Tong Liu,* Yaohua Zhang, Huaiyu Shao, and Xingguo Li The State Key Laboratory of Rare Earth Materials Chemistry and Applications, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China Received February 27, 2003. In Final Form: June 21, 2003 A novel method has been used to synthesize rare earth oxide nanoparticles. Sm and Nd hydride nanoparticles were first produced by a hydrogen plasma-metal reaction. The evaporation rate of Sm is about three times that of Nd. Nd2O3 nanoparticles were then directly synthesized from the passivation of Nd hydride nanoparticles and Sm2O3 nanoparticles with a pure cubic structure from the heat treatment of Sm3H7 nanoparticles at 673 K in air. The morphology and crystal structure of the prepared powders were investigated by transmission electron microscopy, X-ray diffraction, and Fourier transform infrared spectroscopy. Both the Sm2O3 and the Nd2O3 nanoparticles were spherical in shape, with a mean particle diameter of about 40 and 17 nm, respectively. The particle size of the Sm2O3 nanoparticles changed very little after the sintering of the Sm hydride nanoparticles at 473 and 673 K.
1. Introduction Nanoparticles are ultrafine powders with particle sizes in the range of 1-100 nm; therefore, a considerable fraction of atoms locate in the disordered interfacial structure, giving rise to novel physical and chemical properties. Owing to their rich valence states, vast surface areas, and varying electronic structures, metal oxide nanoparticles, especially the transition metal oxides, have wide applications, from catalysis, electronics, and sensors to optics and magnetics. The production of oxide nanoparticles has been achieved by methods such as hydrolysis, pyrolysis, and sol-gel and gas-phase condensation techniques.1-6 The oxides of rare earths such as samarium, cerium, and neodymium have many important applications, including high-efficiency phosphors and catalysts. They show good catalytic properties in several reactions, including the synthesis of ammonia and oxidative coupling of methane.7-9 The large ratio of the surface area to the volume of the nanoparticles is very promising to improve their properties. In recent years, the rare earth doped luminescent nanoparticles prepared by precipitation have been extensively studied and showed strong photoluminescence.10-12 However, the production and properties of pure rare earth oxide nanoparticles are still lacking in study. Reverchon and co-workers have used the thermal * Author to whom correspondence should be addressed. (1) Bellon, K.; Chaumont, D.; Stuerga, D. J. Mater. Res. 2001, 16, 2619. (2) Jiang, Y. D.; Guo, C. J. Powder Technol. 1992, 72, 101. (3) Depero, L. E.; Bonzi, P.; Musici, M.; Casale, C. J. Solid State Chem. 1994, 111, 247. (4) Martı´nez, B.; Roig, A.; Molins, E.; Gonza´lez-Carren˜o, T.; Serna, C. J. J. Appl. Phys. 1998, 83, 3256. (5) Oh, I. H.; Hong, S. A.; Sun, Y. K. J. Mater. Sci. 1997, 32, 3177. (6) Balachandran, U.; Siegel, R. W.; Liao, Y. X.; Askew, T. R. Nanostruct. Mater. 1995, 5, 505. (7) Kadowakim, Y.; Aika, K. J. Catal. 1996, 161, 178. (8) Murata, S.; Aika, K. J. Catal. 1992, 136, 118. (9) Livage, J.; Henry, M.; Sanchez, C. Prog. Solid State Chem. 1988, 18, 259. (10) Lee, M. H.; Oh, S. G.; Yi, S. C. J. Colloid Interface Sci. 2000, 226, 65. (11) Huang, H.; Xu, G. Q.; Chin, W. S.; Gan, L. M.; Chew, C. H. Nanotechnology 2002, 13, 318. (12) Jiang, Y. D.; Wang, Z. L.; Zhang, F. L.; Paris, H. G.; Summers, C. J. J. Mater. Res. 1998, 13, 2950
decomposition of samarium acetate prepared by supercritical antisolvent precipitation to produce samarium oxide without pores, and a catalytic test of ethane oxidative dehydrogenation showed higher selectivity to ethylene in comparison with conventional samples.13 On the other hand, the mean particle size of samarium oxide produced by this method was still large, about 100 nm in average. A forced hydrolysis technique was also used for preparing Sm2O3 nanoscale powders with the average size of about 40 nm at low processing temperatures and good catalytic properties.14 Fourier transform infrared (FTIR) spectra indicated the formation of Sm2O3, but the result of XRD showed that the produced nanoparticles were not pure and a certain amount of impurity was found. Nanoparticles of cerium oxide have been prepared by mixing a cerium nitrate solution with an ammonium reagent; however, the production rate was lower.15 The hydrogen plasma-metal reaction (HPMR) method is suitable to prepare better metallic ultrafine particles (UFPs) industrially at low cost.16-17 However, it has never been used to synthesize rare earth oxide nanoparticles. In this paper, we try to synthesize pure Sm and Nd oxide nanoparticles through the HPMR method followed by oxidation treatment. The first step is to fabricate hydride nanoparticles; the second is to oxidize them at room or elevated temperatures. The crystal structure and characteristics of nanoparticles are investigated. 2. Experimental Section The experimental equipment for producing hydride nanoparticles, primarily containing an arc melting chamber and a collecting system, was described previously.16 The purities of the bulk Sm and Nd ingots were 99.9%. UFPs were produced by arc-melting the Sm and Nd bulk ingots weighing 30 g in a 90% (13) Reverchon, E.; Della, P. G.; Sannino, D.; Lisi, L.; Ciambelli, P. Stud. Surf. Sci. Catal. 1998, 118, 349. (14) Gao, J. Z.; Zhao, Y. C.; Yang, W.; Tian, J. N.; Guan, F.; Ma, Y. J.; Hou, J. G.; Kang, J. W.; Wang, Y. C. Mater. Chem. Phys. 2003, 77, 65. (15) Zhang, F.; Chan, S. W.; Spanier, J. E.; et al. Appl. Phys. Lett. 2002, 80 (1), 127. (16) Li, X. G.; Chiba, A.; Takahashi, S. J. Magn. Magn. Mater. 1997, 170, 339. (17) Liu, T.; Leng, Y. H.; Li, X. G. Solid State Commun. 2003, 125, 391.
10.1021/la034350l CCC: $25.00 © 2003 American Chemical Society Published on Web 08/01/2003
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Table 1. Characteristics of Sm and Nd in HPMR Processing element
Rp
Sm Nd
0.453 0.285
evaporation phase produced specific surface speed (mol/h) after passivation area (m2/g) 0.239 0.076
Sm3H7 Nd2O3
23.8 48.7
Ar and 10% H2 mixture of 0.1 MPa. The flow rate of the circulation gas for the collection of nanoparticles was 100 L/min. The arc current and voltage were selected as 200 A and 28 V, respectively. After passivated in a 99% Ar + 1% O2 atmosphere for 24 h, the nanoparticles were taken out of the arc melting chamber. For comparison, the other experiment for the samarium hydride nanoparticles without passivation was also done by refilling the chamber with air at 1 atm after HPMR processing. The crystal structures of the powder samples were determined by X-ray diffraction (XRD) using monochromatic Cu KR radiation. FTIR was used to determine the chemical band of Sm-O. The morphology, size distribution, and shape of particles were observed by transmission electron microscopy (TEM) using a 200 kV JEOL EX microscope and Bruanuer-Emmett-Teller (BET) gas adsorption.
Figure 1. XRD patterns of nanoparticles: (a) samarium hydride with passivation and (b) samarium hydride without passivation.
3. Results and Discussion Synthesis of Sm Hydride and Nd Oxide Nanoparticles. Table 1 summarizes characteristics of Sm and Nd in HPMR processing. The evaporation speed of Sm is 0.239 mol/h, about three times that of Nd. It is noted that the evaporation rate of metal during HPMR is proximately proportional to the reaction parameter Rp of the elements in the following equation:18
Rp ) (-∆Hr/Ls)[NH2(T)/NH2(273)]
(1)
where ∆Hr is the reaction enthalpy between hydrogen and the metal, Ls is the vaporization heat of the metal at temperature T, NH2(T) and NH2(273) are the densities of molecular hydrogen in metal at temperature T and 273 K, respectively. According to this model, the Rp values of Sm and Nd are calculated to be 0.453 and 0.285, respectively. This ratio of Sm to Nd is a little bit lower than the evaporation speed ratio in this work. It is known that all of the rare earth metals react directly with hydrogen to form dihydrides or trihydrides, which are normally nonstoichiometric, usually exhibiting very wide existence ranges. The rate at which the reaction proceeds depends on the temperature, hydrogen pressure, and condition of the metal surface. In the case of the HPMR process, Sm and Nd vapors are formed under hydrogen plasma and, thermodynamically, are unable to react with hydrogen under such high-temperatures on the basis of the following reaction:
2Sm/Nd + H2 f 2Sm/NdHx + Q
(2)
where Q means the exothermic heat of the reaction. According to the Nd/Sm-H phase diagram and pressurecomposition isotherm diagram,19 rare earth nanoparticles start to adsorb hydrogen and form hydride while they are cooled to about 1000 K. Because the rare earth metal particles in this experiment are in the nanoscale range, the extremely large surface area and high activity make hydrogen liable to be absorbed by rare earth nanoparticles during the cooling process and even at room temperature. In addition, that no oxide film is formed on the nanoparticles in this reducing gas also accelerates the hydro(18) Ohno, S.; Uda, M. Trans. Jpn. Inst. Met. 1984, 48, 640. (19) Thaddeus, B. M.; Joanne, L. M.; Lawrence, H. B.; Hugh, B.; Linda, K. Binary Alloy Phase Diagrams; American Society for Metals: Materials Park, Ohio, 1986.
Figure 2. XRD pattern of the Nd hydride nanoparticles after passivation.
genation. In this case, Nd/Sm nanoparticles adsorb a large amount of hydrogen even at such a high cooling rate. Figure 1 shows XRD patterns of the Sm hydride sample after passivation and without passivation. It is found that very pure Sm hydride UFPs (Sm3H7) with a tetragonal structure are formed after passivation. The weak peak at 32° is noticeable and exhibits the presence of a small amount of samarium oxide after passivation, as was observed in other metallic UFPs.17 With respect to the Sm hydride sample without passivation, the nanosized powders start to fire violently after the refilling of air. The XRD pattern in Figure 1b shows that a single phase of Sm2O3 with monoclinic crystal structure is produced. It seems that the Sm hydride nanoparticles are very active just like active metals, and it is very necessary to prevent the fine Sm hydride particles from burning and enlarging the particle size. From the XRD pattern, as is shown in Figure 2, it can be seen that the Nd hydride nanoparticles are oxidized totally even during the passivation process and very pure Nd2O3 with a cubic structure is synthesized instead of hydride, such as in the case of Sm. It is indicated that Nd hydride nanoparticles are more active than Sm hydride nanoparticles, resulting in the full oxidation of Nd hydride during the passivation. Figure 3a,b shows TEM images of Sm3H7 nanoparticles after passivation and Sm2O3 UFPs produced from the hydride nanoparticles without passivation, and Figure 4 shows the TEM bright field image of Nd2O3 nanoparticles produced from the passivation of hydride nanoparticles.
Sm2O3 and Nd2O3 Nanoparticles
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Figure 3. TEM bright field image of UFP samples: (a) Sm3H7 nanoparticles after passivation and (b) Sm2O3 powder prepared from hydride nanoparticles without passivation.
Figure 4. TEM bright field image of Nd2O3 nanoparticles prepared from hydride nanoparticles with passivation.
It is revealed from the TEM observations that the passivated nanoparticles, Sm3H7 and Nd2O3, are nearly spherical in shape. The particle size of Sm hydride ranges from 10 to 100 nm with a mean diameter of about 40 nm, and the Nd2O3 particle size is much smaller than that of the Sm hydride, varying from 5 to 50 nm with the average diameter of about 20 nm. The BET gas adsorption method shows that the mean particle sizes of the Sm3H7 and Nd2O3 UFP samples predicted from the specific surface area are 36 and 17 nm, respectively, in good agreement with TEM observation. The difference of particle size between Sm3H7 and Nd2O3 contributes to the fact that the evaporation rate of Sm is so high that small particles collide and grow by coalescence during the cooling process. The smaller the particle size of Nd hydride nanoparticles, the more active the Nd hydride nanoparticles than the Sm hydride nanoparticles, leading to the total oxidation of Nd hydride. It is also implied that the oxidation of Nd hydride during the passivation does not enlarge the particle size obviously. The Sm2O3 powders produced from hydride nanoparticles without passivation are larger than 100 nm as the result of coalescence and agglomeration during burning (Figure 3b). Some researchers have once used the gas-phase condensation technique to produce oxide nanoparticles, such as Cr2O3 and ZnO, with the mean particle sizes of about several tens of nanometers, by introducing oxygen into the chamber right after the preparation of the metal nanoparticles.6,20 Nevertheless, the violent oxidation leads to the increasing of the particle size in our work because of the extremely high activity of rare earth hydride nanoparticles. Thus, the direct oxidation method without passivation is not suitable to fabricate rare earth oxide nanoparticles. Synthesis of Samarium Oxide Nanoparticles. Metallic oxide nanoparticles can be produced by the oxidation of metallic nanoparticles at certain conditions. (20) Mayo, M. J.; Siegel, R. W.; Liao, Y. X.; Nix, W. D. J. Mater. Res. 1992, 7, 973.
Figure 5. XRD patterns of the samarium hydride nanoparticles heated at different temperatures: (a) as prepared, (b) 473 K, (c) 573 K, and (d) 673 K.
Figure 6. FTIR patterns of the samarium hydride nanoparticles heated at different temperatures: (a) as prepared, (b) 473 K, and (c) 673 K.
Because the samarium hydride nanoparticles are not as active as the neodymium hydride nanoparticles in this work, samarium oxide nanoparticles cannot be synthesized directly by passivation. The subsequent heat treatment in air is, thereby, used to form nanosized samarium oxides on the basis of the following reaction:
4Sm3H7 + 9O2 f 6Sm2O3 + 14H2
(3)
After the samarium hydride nanoparticles are heated at 473 K for 1 h, they are oxidized to Sm2O3 with both monoclinic and cubic crystal structures (Figure 5b). Then, most of the nanoparticles with monoclinic structures change into cubic structures at 573 K, as is seen in
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Figure 7. TEM bright field images of the samarium hydride nanoparticles heated at different temperatures: (a) 473 K and (b) 673K.
Figure 5c. The particles possess single cubic crystal structures after sintering at 673 K (Figure 5d). It is interesting to find that the samarium oxide in the cubic crystal structure is more stable than that in the monoclinic structure. The produced Sm oxide powders with monoclinic structure from the hydride without passivation are probably attributed to the quick heating and cooling during the burning of the hydride nanoparticles. Figure 6 shows FTIR spectra of nanoparticles heated at different temperatures. For the as-prepared Sm hydride nanoparticles after passivation, only one characteristic absorption peak of the Sm2O3 crystal is noticeable at 860 cm-1 in Figure 6a due to the stretching vibration of Sm(III)-O groups. This also indicates a small amount of hydride oxidation after passivation, as was proved by XRD observation. After the heat treatment at 473 and 673 K, the intensity of the peak at 860 cm-1 increases; moreover, another characteristic peak is observed at 730 cm-1 both in part b and in part c of Figure 6, as a result of the severe oxidation of the hydride nanoparticles. It is also revealed from Figure 6 that the absorption peaks are broadened as a result of the finer particle size and many more surface atoms in the Sm2O3 nanoparticles than in the conventional sample. Figure 7 shows the TEM images of nanoparticles heated at different temperatures. The samarium oxide powders sintered at 473 and 673 K are still in the nanoscale region with a mean particle size of about 40 nm, and they change quite little compared with the as-prepared Sm hydride nanoparticles in Figure 3a. The catalytic properties of these rare earth oxide nanoparticles are being further studied.
4. Conclusion The HPMR method is first used to prepare rare earth oxide nanoparticles with a high production rate. The evaporation rate of Sm is about three times that of Nd, resulting in a larger particle size of Sm hydride than that of Nd hydride. Sm3H7 and Nd2O3 nanoparticles with mean particle sizes of 36 and 17 nm are formed after the passivation of Sm and Nd hydride nanoparticles, respectively. Sm2O3 particles with monoclinic structure prepared from hydride without passivation are larger than 100 nm as the result of coalescence and agglomeration during burning. The direct oxidation method is not suitable to fabricate rare earth oxide nanoparticles. Sm2O3 nanoparticles with monoclinic and cubic crystal structures are synthesized by the oxidation of Sm hydride nanoparticles at 473 K with a mean particle size of about 40 nm. Sm2O3 nanoparticles with a single cubic structure are formed at 673 K, and the particle size of the Sm hydride nanoparticles change very little during sintering at 473 and 673 K, compared with samarium hydride nanoparticles. Acknowledgment. This work was supported by NSFC of China (Nos. 20025103, 50274002, and 20221101). The authors acknowledge Associate Professor Gai and Liao Fuhui for their kind technical help with the TEM and XRD measurements. LA034350L
