Microemulsion Synthesis of Nd0.5Ca0.5MnO3 and Nd0.5Sr0.5MnO3 Nanoparticles
Z. Q. Wang1, K. B. Yin1, F. Gao1, K. F. Wang1, Z. F. Ren2, and J. -M. Liu1,3
1Department of Physics, Nanjing University, Nanjing, 210093, China, People's Republic of
2Department of Physics, Boston College, Boston, MA, 02467
3International Center for Materials Physics, Chinese Academy of Sciences, Shenyang, 110016, China, People's Republic of
ABSTRACT
Nd0.5Ca0.5MnO3 (NCMO) and Nd0.5Sr0.5MnO3 (NSMO) nanoparticles have been synthesized using microemulsion synthesis method, with hexamethylene alkyl, a mixture of OP and 1- hexanol, NaOH as oil phase, surfactant, and precipitating agent, respectively. The phase formation of NCMO and NSMO nanoparticles was examined. The final NCMO and NSMO nanoparticles have average particle size of 24 and 50 nm, respectively, and present high-quality crystallinity. Measurements of the magnetic properties suggest that the charge-order state favored for bulk NCMO phase collapses in NCMO nanoparticles. The spin freezing behavior for both NCMO and NSMO nanoparticles was identified.
INTRODUCTION
Hole doped perovskite manganites with general formula R1-xAxMnO3 (R=Rare earth ion, A=Alkali earth ion) have attracted considerable attention because of their unusual magnetic and electronic properties (e.g. colossal magnetoresistance and charge ordering) and potential technological applications (e.g. high density magnetic storage and field sensors) [1-5]. Several techniques for the preparation of these manganites, including conventional solid-state sintering [3,5], sol-gel [6,7], and hydrothermal reaction [8-10], have been examined. The conventional solid-state sintering needs high reaction temperature (usually as high as 1400oC), complex operating procedures. Furthermore, the products obtained have large particle size of several micrometers. In comparison with solid-state sintering, sol-gel and hydrothermal methods possess advantages such as low reaction temperature, simple operating procedures etc. In addition, nanostructured manganites can be produced using these two methods. However, the sol-gel method has low yield of products because of the need of template providing nanochannels for the growth of manganites. Though hydrothermal synthesis is performed under a temperature below 240oC and have been successfully applied to the preparation of many ternary or binary oxides, only a very limited number of doped manganites can be synthesized by this method and most of the products have particle sizes as large as several micrometers [9,10].
Microemulsion synthesis has been used to prepare nanosized binary and ternary oxides such as TiO2, LaMnO3, ZnAl2O4, PbZrO3 [11-13]. In a typical microemulsion synthesis, an oil phase, a surfactant phase and an aqueous phase are mixed, resulting in a microemulsion system consisting of numerous nanosized aqueous droplets due to the uniform dispersion of the aqueous phase in the continuous oil phase. Precipitating reactions occur when the aqueous droplets containing desirable reactants collide with each other, leading to the formation of nanosized precursor particles. Nanoparticles are obtained after the calcination of precursors at different temperatures.
Mater. Res. Soc. Symp. Proc. Vol. 962 © 2007 Materials Research Society 0962-P10-17
Recently, La0.8Sr0.2M1−xRhxO3 (where M=Mn, Co, Fe and x=0, 0.1) used for catalysts was synthesized using microemulsion method, with n-heptane, C11E2 and ammonium oxalate as oil phase, surfactant and precipitating agent, respectively[14]. In the microemulsion process, types of oil phases, surfactants, and volume ratios between the three phases are key factors for controlling the final particle sizes. In this work, microemulsion method was applied to synthesize Nd1-xSrxMnO3 and Nd1-xCaxMnO3 nanoparticles, selecting hexamethylene alkyl, the mixture of OP and 1-hexanol, and NaOH as oil phase, surfactant, and precipitating agent, respectively, and their microstructures and magnetic behaviors were characterized subsequently.
EXPERIMENT DETAILS
The synthesis process proceeds as the following steps: (1) Identical weight amount of OP(C34H62O11) and 1-hexanol(C6H14O) were mixed as surfactant phase, and was mixed under stirring with hexamethylene alkyl(C6H12) according to the volume ratio of 1:2 to obtain the organic phase. (2) Some amount of Nd2O3 was dissolved in HNO3 aqueous solution, then Sr(NO3)2/Ca(NO3)2 and MnCl2•4H2O were added according to a molar ratio of Nd:Sr/Ca:Mn=
1:1:2. Afterwards the aqueous solution was mixed into the organic phase according to the volume ratio of 2:15 to obtain microemulsion I. All the above precursor salts have purity of
>99.5%. (3) An aqueous solution containing NaOH with molar concentration 10 times of MnCl2
was mixed with the organic phase at the volume ratio of 2:15 to produce microemulsion II. (4) The microemulsion I and II were mixed with the 1:1 volume ratio and vigorously stirred for an hour, then aged for 12h. (5) The final microemulsion was centrifugated and the separated precipitates were washed in turn with ethanol, deionized water and acetone. This step was repeated for three times and the powder finally obtained was dried in air at 80oC for 8h. (6) The as-dried powder was subsequently annealed in air at 600oC and 800oC, respectively, for 3h.
The as-dried powder and annealed powder were subjected to X-ray diffraction (XRD) and high-resolution transmission electron microscopy (HRTEM) for microstructure examinations.
The atomic ratios between the cation elements in the annealed powder were determined by inductively coupled plasma atomic emission spectroscopy (ICP-AES). The magnetic properties were measured using a superconducting quantum interference device (SQUID) magnetometer.
RESULTS and DISCUSSION Nd0.5Ca0.5MnO3 nanoparticles
Fig.1 shows the XRD and TEM results of the powders dried at 80oC and annealed at 600oC and 800oC. In the XRD spectra of the as-dried powders, large amounts of Nd(OH)3 and CaCO3
along with a small quantity of Ca(OH)2, are revealed. No compound containing manganese element is present, possibly because it is included in amorphous state. TEM image of this powder clearly suggests that compounds take the fibre morphology, 2-25 nm in width and several tens of nanometers in length. In the XRD spectra of the sample annealed at 600oC, a new phase is identified which can be indexed to Nd0.5Ca0.5MnO3 (NCMO) but there still exists some unknown impurity. Annealing at 800oC for 3h eliminates the impurity and produces pure NCMO phase (Fig.1 left panel (c)). This phase has the orthorhombic structure with lattice constants a=0.5392, b=0.7589, c=0.5376nm. Further determination of the Nd:Ca atomic ratio for the powder annealed at 800oC gives a result of 0.50:0.51. The morphology of this pure phase is
shown in Fig.1 right panel (b). It is seen that the particles are agglomerated together, but their boundaries are distinguishable, and an average particle diameter of ~24nm can be measured. The selected area electron diffraction (SAED) pattern along [2 0 -2] (Fig.1 right panel (c)) clearly indicates that the NCMO particles are single crystals. Fig.1 also shows the HRTEM image of one nanoparticle and the lattice separation of ~0.269nm corresponds exactly to the (121) planes of NCMO. Both SAED and HRTEM images suggest that the NCMO nanoparticles are of good crystalinity quality. It is clear that the broadening of the XRD pattern of the pure NCMO particles results from the fine grain size.
From the above results, it is seen that NCMO nanoparticles have been successfully synthesized through the microemulsion method. After reactions between the cations of Nd3+, Ca2+, and Mn2+ with OH- in the microemulsions, precursor compounds with nanoscale sizes are formed, which is crucial for controlling the final NCMO particle sizes during the subsequent annealing at high temperature.
Figure 1. Left panel: XRD spectra of the powder dried at 80oC (a), annealed at 600oC for 3h (b), and annealed at 800oC for 3h (c) during preparation of NCMO nanoparticles.
Right panel: TEM images of as-dried powder at 80oC(a), NCMO powder annealed at 800oC for 3h(b), SAED patterns of a NCMO nanoparticle along [2 0 -2] (c), HRTEM image of a nanoparticle, the lattice separation is 0.269nm, corresponding to the spacing between (121) planes of NCMO(d).
Results of field-cooled (FC) and zero-field-cooled (ZFC) magnetization measurements for pure NCMO nanoparticles are reported in Fig.2. For the bulk Nd0.5Ca0.5MnO3, charge ordering (CO) and anti-ferromagnetic (AFM) phase transitions occur with decreasing temperature, which are characterized on the magnetization curve as a relatively large peak at TCO=240-250K and a relatively small peak at TN=140-160 K (sometimes the small peak cannot be revealed), respectively [15-17]. The FC and ZFC magnetization curves of the present NCMO nanoparticles coincide as a straight line in the temperature range of 300-120K. This suggests that the CO phase in bulk NCMO are completely melted due to the reduction of particle size to nanometer scale.
The existence or disappearance of AFM phase in the NCMO nanoparticle cannot be concluded since the AFM transition peak in bulk one is very small and sometimes not shown at all [16-18].
Melting (collapse) of CO phase in manganites was reported to be caused by high magnetic field, electric field, electron irradiation, chemical doping, or external pressure [2,4,5,15,16,18-20].
Recently, partial suppression of CO phase due to the reduction of particle size to nanometer scale in Pr0.5Sr0.5MnO3 nanoparticles (30 and 40 nm) and Pr0.5Ca0.5MnO3 nanowires (50nm in diameter and several micrometers in length) were also found [20,21].
At 110K, both the FC and ZFC magnetizations rise abruptly, indicating a paramagnetic to ferromagnetic transition. The FC magnetization begins to deviate from the ZFC one and the difference between them increases with decreasing temperature. The ZFC magnetization curve peaks at about 30 K, below which the magnetization decreases with the decrease of the temperature, characterizing a spin freezing behavior [22].
Figure 2. ZFC and FC magnetization of NCMO nanoparticles as function of
temperature.
Nd0.5Sr0.5MnO3 nanoparticles
During preparation of NSMO, phases detected by XRD in the precipitates obtained after reaction are Nd(OH)3 and SrCO3 which take fibre morphology (Fig.3), similar cases as found in the preparation of NCMO. After annealing at 600oC for 3h, the nanosized precursor compounds (perhaps the manganese appears in the amorphous state since no traces both in XRD and TEM analysis are available) react with each other to form NSMO phase, but the reaction is incomplete and Nd(OH)3 and SrCO3 as impurities can be detected through XRD(Fig.3). For the sample annealed at 800oC for 3h, all the XRD peaks can be well indexed to perovskite orthorhombic structure of NSMO with lattice constants of a=0.5431, b=0.7625, c=0.5477nm. Measurements of the chemical compositions of this sample by ICP-AES gives the Nd:Sr molar ratio of 0.52:0.48, in accord with the XRD result. Fig.3 (b)right panel presents typical TEM morphology of NSMO particles annealed at 800oC. The average particle size is ~50 nm. The SAED pattern along [0 0 - 4] suggests that some nanoparticles are single crystals while the others are polycrystalline, as indicated by the SAED diffraction rings. HRTEM images with lattice separations corresponding to spacings between (101), (200)/(121), and (224) are observed (only that between (200)/(121) is shown in Fig.3). Both SAED patterns and HRTEM image suggest the high-quality crystalline structure of the NSMO nanoparticles.
Fig.4 presents the ZFC and FC magnetization of the NSMO nanoparticles. When cooling from room temperature, a transition from paramagnetic to ferromagnetic state occurs at about 250K as indicated by the abrupt rise of both ZFC and FC magnetization. The ZFC and FC curves diverge from each other at this temperature point, with the FC one continuing to increase when
decreasing the temperature while the ZFC one forming a broad peak around 120K, suggesting occurrence of the spin freezing behavior in the NSMO nanoparticles.
CONCLUSIONS
To summarize, microemulsion synthesis method has been successfully applied to prepare NCMO and NSMO nanoparticles, with hexamethylene alkyl, a mixture of OP and 1-hexanol, and NaOH as oil phase, surfactant, and precipitating agent, respectively. XRD and HRTEM analysis show that NCMO and NSMO have the average particle sizes of 24 and 50 nm, respectively, and have high quality crystalline structures. Charge order state favored in bulk NCMO particles was found to have collapsed in the NCMO nanoparticles. Both NCMO and NSMO nanoparticles exhibit spin freezing behavior.
Figure 3. Left panel: XRD results of the powders dried at 80oC(a), annealed at 700oC for 3h(b), and annealed at 800oC for 3h(c) during preparation of the NSMO nanoparticles.
Right panel: TEM images of the as-dried powder at 80oC(a), NSMO nanoparticles annealed at 800℃ for 3h(b), SAED patterns of NSMO nanoparticle along [0 0 -4] (c), SAED diffraction rings(d), HRTEM image of a nanoparticle, the lattice separation is 0.271nm, corresponding to the spacing between (200) planes of NSMO (e).
Figure 4. ZFC and FC magnetization of NSMO nanoparticles as function of temperature.
ACKNOWLEDGMENTS
The authors acknowledge the support from the Natural Science Foundation of China (50528203, 50332020, 10021001), National Key Projects for Basic Research of China (2002CB613303, 2006CB0L1002), China Postdoctoral Science Foundation (20060390275) and Jiangsu Postdoctoral Research Foundation (0601001A).
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