Self-Assembled Ferromagnetic Monodisperse Manganese Oxide

Mar 26, 2009 - Fax: +86-10-6275-4179. ... S. Lauzurica , R. G. Palgrave , I. P. Parkin , G. Sankar , M. Sikora , A. M. Stoneham , G. Thornton , and S...
0 downloads 0 Views 6MB Size
Download PDF
J. Phys. Chem. C 2009, 113, 6521–6528

6521

Self-Assembled Ferromagnetic Monodisperse Manganese Oxide Nanoplates Synthesized by a Modified Nonhydrolytic Approach Ya-Ping Du, Ya-Wen Zhang,* Ling-Dong Sun, and Chun-Hua Yan* Beijing National Laboratory for Molecular Sciences, State Key Laboratory of Rare Earth Materials Chemistry and Applications, PKU-HKU Joint Laboratory in Rare Earth Materials and Bioinorganic Chemistry, Peking UniVersity, Beijing 100871, China ReceiVed: December 30, 2008; ReVised Manuscript ReceiVed: March 3, 2009

Single-crystalline and monodisperse manganese oxide (MnO and Mn3O4) nanoplates have been synthesized via a modified nonhydrolytic approach, using manganous acetate as the precursor in oleylamine, and subsequent controlled oxidization with trimethylamine N-oxide. The formation of MnO nanoplates seems a highly kineticsdriven process, which proceeds under a quite high monomer concentration. Along with manipulating the post-treatment procedure and polarity of dispersants, the as-obtained nanoplates are aligned to form selfassembly nanoarrays either in “side-to-side” formation or “face-to-face” formation. Meanwhile, the MnO and Mn3O4 nanoplates display ferromagnetism at low temperatures. The as-obtained Mn3O4 nanoplates exhibit a coercive field of 8.9 kOe at 5 K, significantly higher than those of bulk Mn3O4 and thin films. Introduction Colloidal transition metal oxide nanocrystals with unique size and shape-dependent material properties and striking selfassembly ability are of increasing scientific and technological importance in board areas such as catalysis, sensors, information and energy storage, biomedicine, and so on.1 The efficient synthesis of diverse transition metal oxide nanocrystals with well-controlled size, morphology, phase purity, crystallinity, and chemical composition via facile and economical chemical approaches, and the designed assembly of these nanoscale building blocks into diverse advanced nanodevices via “bottom up” technology, have become the most stimulating subjects faced by many material chemists. So far, the nonhydrolytic route (thermolysis of metal-oxysalts precursors and its derivatives in high-boiling organic solvents) has been demonstrated as the versatile one for the synthesis of high-quality (single-crystalline, monodisperse, well-shaped, and phase-pure) colloidal transition metal oxide nanocrystals in many circumstances.2,3 Manganese oxides are important materials for applications in magnetics, electronics, catalysis, and biotechnology.4-7 In recent years, several groups have explored the size/shapecontrolled synthesis of 0D and 1D nanostructures of manganese oxides via some nonhydrolytic approaches, as well as their material properties. O’Brien et al. prepared size-controlled highquality MnO nanocrystals from dry manganous acetates in oleic acid/trioctylamine and observed their intriguing self-assembly ability in forming 3D superlattices.8 Park et al. synthesized sizecontrolled monodisperse Mn3O4 and MnO nanoparticles by thermal decomposition of [Mn(acac)2] (acac ) acetylacetonate) in oleylamine, and scrutinized their unique size-dependent magnetic properties.4a Also using Mn(acac)2 precursor in oleylamine but under different reaction conditions, Bruce et al. fabricated ordered three-dimensional (3D) arrays of monodisperse Mn3O4 core-shell nanoparticles.9 Hyeon et al. obtained uniform-sized MnO nanospheres and nanorods by using Mn-surfactant complex (prepared by reacting Mn2(CO)10 with * To whom correspondence should be addressed. Fax: +86-10-62754179. E-mail: [email protected] and [email protected].

oleylamine) as the precursors in trioctylphosphine (TOP).10 By thermolysis of manganous formates in oleic acid/oleylamine, our group demonstrated the simple synthesis of dispersible Mn3O4 nanocrystals, which showed interesting magnetic properties.11 In this paper, we report an efficient synthesis of monodisperse MnO nanoplates (2D nanostructure) by modifying a developed nonhydrolytic approach,4a,9 through thermal decomposition of manganous acetate precursor in oleylamine. Mn3O4 nanoplates are obtained by a controlled chemical oxidation from MnO nanoplates with trimethylamine N-oxide as an oxidant. The two kinds of nanoplates have been demonstrated to display interesting ferromagnetic properties. Experimental Section Materials. Mn(Ac)2(H2O)4 (A. R. grade), trimethylamine N-oxide (TMNO, 98% Alfa Aesar), oleic acid (OA; 90%, Alfa Aesar), oleylamine (OM; >80%, Acros), hexadecylamine (HDA; >92%, Merck-Schuchardt), dodecylamine (DDA; >98%, Acros), acetone (C3H6O, >99.8%), absolute ethanol (C2H6O, >99.7%), hexane (C6H6, >99.5%), toluene (C7H8, >99.1%), and commercial MnO2 powders were used as received. Synthesis of MnO Nanoplates. The synthesis was carried out with standard oxygen-free procedures. In a typical reaction, a slurry of Mn(Ac)2 in oleylamine (1:1 in molar ratio) was bubbled with Ar for about 20 min, and the stock solution was vacuumed at 120 °C for 30 min to remove water and oxygen. Then, the flask was heated to 200 °C for a reaction of 15 min under a continuous flow of argon. After the reaction, the flask was cooled to room temperature in open air. The addition of an excess amount of ethanol into the flask produced a brown suspension with some solid sediment. The as-formed precipitates were discarded after centrifugation at 3000 rpm for 10 min, and the residual suspension was sonicated for 10 min to form a clear solution. Removal of the insoluble material was done repeatedly (if any, by precipitation and centrifugation) by adding a mixture of hexane/acetone (v/v in 1:4, total volume ∼40 mL) to produce a brown powder. Further, if needed, the size-selection process was delicately conducted (see the Supporting Information for

10.1021/jp8114868 CCC: $40.75  2009 American Chemical Society Published on Web 03/26/2009

6522 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Figure 1. XRD pattern of MnO nanoplates.

experimental details). The as-precipitated nanocrystals were washed by ethanol several times and then dried in a vacuum desiccator at 60 °C overnight. The as-dried nanocrystals had a yield greater than 60%, and could be easily redispersed in various nonpolar organic solvents such as toluene. Synthesis of Mn3O4 Nanoplates. As-prepared MnO nanoplates, trimethylamine N-oxide (TMNO), and oleylamine (1:1 in molar ratio) were added into a three-necked flask at room

Du et al. temperature. The following synthetic procedure was the same as that used for the preparation of MnO nanoplates except that the reaction was performed at 210 °C for 5 h under an Ar atmosphere. Instrumentation. Powder X-ray diffraction (PXRD) patterns of the as-dried nanocrystals were recorded on a Rigaku D/MAX2000 diffractometer (Japan) with a slit of 0.5° at a scanning rate of 4 deg min-1, using Cu KR radiation (λ ) 1.5418 Å). Nanoparticle size and shape were examined by a Hitachi-H9000 (Japan) high-resolution TEM (HRTEM) operated at 300 kV. TEM and HRTEM samples were prepared by adding a drop of a dilute hexane/toluene dispersion of nanocrystals (with a small amount of oleylamine/oleic acid in it) onto a carbon-coated copper grid, and thereafter were dried in an ambient environment. The surface state of as-obtained nanocrystals was investigated by X-ray photoelectron spectroscopic (XPS) measurements, with the use of Al KR radiation (BE ) 1486.6 eV) as the X-ray excitation source, and an ion-pumped chamber (evacuated to 2 × 10-9 Torr) of an Escalad5 (U.K.) spectrometer. The thermogravimetry (TG) runs were performed with a Q600 SDT (U.S.A) instrument at a heating rate of 10 deg min-1 from room temperature to 600 °C, using R-Al2O3 as a reference. The magnetic properties of as-obtained nanocrystals were measured on a Quantum Design MPMS-XL-5 superconductive quantum interference device (SQUID) magnetometer (U.S.A.) from 5 to 300 K with a measuring field of 100 Oe. The

Figure 2. (a) TEM image of MnO nanoplates, the lower inset shows the schematic diagram of a nanoplate. (b) “Side-to-side” self-assembly nanoarrays and HRTEM (inset) images of MnO nanoplates. (c) “Face-to-face” self-assembly nanoarrays and HRTEM (inset) images of MnO nanoplates. (d) TEM image of highly ordered MnO nanoplate arrays with three-dimensional superlattice in some parts of TEM grids.

Synthesis of MnO and Mn3O4 Nanoplates

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6523

Figure 3. (a) XRD pattern of Mn3O4 nanoplates. (b) TEM image of Mn3O4 nanoplates, the lower inset shows the schematic diagram of a nanoplate. (c) Farfetched “side-to-side” self-assembly nanoarrays and HRTEM (inset) images of Mn3O4 nanoplates. (d) Farfetched “face-to-face” self-assembly nanoarrays and HRTEM (inset) images of Mn3O4 nanoplates.

temperature dependence of the susceptibility was investigated by cooling the dried sample at zero field and then stepping up the temperature (ZFC curve), or by cooling the sample in the presence of an external field (FC curve). Both curves were collected with an applied field of 100 Oe. The temperaturedependence magnetization of Mn3O4 nanoplates (FC curves) was measured at different applied magnetic fields of 500, 1 000, 10 000, 50 000, and 100 000 Oe. Results and Discussion Characterization of MnO and Mn3O4 Nanoplates. MnO Nanoplates. For as-prepared MnO nanoplates all the peaks on the PXRD pattern of Figure 1 match those of face-centered cubic (fcc) MnO (rock salt type, space group: Fm3jm), and the calculated lattice constant is a ) 4.451 Å (JCPDS: 72-1533). Figure 2a depicts the nanoarrays of near-monodisperse MnO nanoplates in the size of (10.2 ( 0.6) nm × (6.8 ( 0.4) nm. By tuning the polarity of the dispersant and further size-selection procedure, these 2D nanoplates can be partially aligned to form “side-to-side” and “face-to-face” self-assembled nanoarrays. For instance, as the dispersant is toluene/hexane (v/v 1:1), most of the MnO nanoplates lay on their bottom surfaces to give “sideto-side” pattern (Figure 2b). As a certain amount of ethanol is added into toluene/hexane (toluene/hexane/ethanol ) 1/1/0.5 in volume), nearly all the nanoplates stand on their side surfaces to form a “face-to-face” pattern (Figure 2c). If further increasing

the volume fraction of ethanol to 1/1/1, the solution becomes somewhat turbid with the formation of nanoplates precipitate. HRTEM analyses further confirm that as-obtained MnO nanoplates are single-crystalline, and are enclosed by {100} and {110} planes (see insets in panels b and c of Figure 2). The confined growth direction is along the 〈001〉 direction. Interestingly, a highly ordered 3D superlattice of MnO nanoplates is formed on some areas of the TEM grid (Figure 2d), as deposited from concentrated nanocrystal dispersion. Mn3O4 Nanoplates. Mn3O4 nanoplates are obtained by chemical oxidation of MnO nanoplates, with trimethylamine N-oxide (TMNO) as an oxidant. As shown in Figure 3a, the XRD pattern can be readily indexed to a tetragonal structure for the Mn3O4 nanoplates (hausmannite type, space group: I41/ amd). The calculated lattice constants of the nanoplates are a ) b ) 5.771 Å and c ) 9.756 Å (JCPDS: 24-0734). The TEM images shown in panels b-d of Figure 3 demonstrate the formation of nanoarrays composed of lying and/or standing Mn3O4 nanoplates in the size of (12.2 ( 0.6) nm × (5.6 ( 0.3) nm. The Mn3O4 nanoplates are also single crystallites, enclosed by {001} planes (see HRTEM insets in Figure 3c,d). XPS Characterization. XPS measurement is carried out to investigate the surface states of the MnO and Mn3O4 nanoplates. For a comparison, the XPS spectrum of bulk MnO2 is also measured. From Figure 4a, peaks attributable to the core levels of Mn 2p and O 1s are discernible for the three samples. The

6524 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Du et al.

Figure 4. XPS patterns of MnO and Mn3O4 nanoplates: (a) XPS survey spectra, (b) Mn 2p, (c) O 1s, and (d) Mn 3s. (e) TEM image of MnO nanocrystals synthesized from Mn(acac)2 in OM solvent.

appearance of the peaks ascribed to the core levels of N 1s and C 1s indicates the presence of OM ligands on the surfaces of the nanocrystals. The fitted Mn 2p3/2 and Mn 2p1/2 binding energies (BE) for the nanocrystals are shown in Figure 4b. It is evident that Mn atoms at the surface of the MnO nanocrystals have binding energies close to those of Mn3O4 rather than those of bulk MnO2. Accordingly, we propose that the surface of MnO nanoplates is partially oxidized to form a thin Mn3O4 layer, and is probably disordered.9 Figure 4c shows the fitted O1s spectra recorded for the nanocrystals, which are resolved into two components. The BE values of 531.2 and 529.6 eV are assigned to adsorbed oxygen species present on the nanoplate surfaces and lattice oxygen of manganese oxide, respectively.9,12 The average oxidation state of manganese in the nanoplates is further estimated from the multiplet splitting of the Mn 3s core level

spectrum.13 Figure 4d shows the fitted Mn 3s spectra recorded for the MnO and Mn3O4 nanoplates, which are resolved into two components. For the as-prepared MnO and Mn3O4 nanoplates, the energy splitting ∆E of Mn 3s is 5.69 and 5.45 eV, respectively, according to these data, the average oxidation state of manganese is calculated to be 2.12 for the MnO nanoplates and 2.63 for the Mn3O4 nanoplates,13b in good agreement with the expected values for bulk MnO (2.0) and Mn3O4 (2.67). Formation of MnO Nanoplates. A series of conditiondependent experiments were conducted to reveal the optimal conditions (including type of precursor, precursor concentration, solvent, reaction temperature, and reaction time) for the formation of phase-pure and monodisperse MnO nanoplates. Type of Precursor. Two kinds of readily available manganese compounds including acetylacetonate (Mn(acac)2) and acetate

Synthesis of MnO and Mn3O4 Nanoplates

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6525

Figure 5. TEM images of MnO nanocrystals obtained from the thermolysis of Mn(Ac)2 at 200 °C for 15 min under different conditions: (a) 2 mmol of Mn(Ac)2, 10 mmol of OM; (b) 10 mmol of Mn(Ac)2, 10 mmol of OA and OM, OA/OM ) 9/1; (c) 10 mmol of Mn(Ac)2, 10 mmol of HDA; and (d) 10 mmol of Mn(Ac)2, 10 mmol of DDA.

(Mn(Ac)2) were tested in the present synthesis. As seen from the TGA curves shown in Figure S1a,b in the Supporting Information, Mn(Ac)2 decomposed more rapidly than Mn(acac)2 at around 300 °C, and Mn(acac)2 could not completely decompose until the heating temperature reached 400 °C. Further synthetic experiments demonstrated that high-quality (10.2 ( 0.6) nm × (6.8 ( 0.4) nm MnO nanoplates could be synthesized from Mn(Ac)2 precursor (Figure 2a) in OM at 200 °C for 15 min. However, the use of Mn(acac)2 only produced nearmonodisperse (14.4 ( 2.1) nm polyhedral nanocrystals under the same conditions (Figure 4e). Therefore, we concluded that Mn(Ac)2 is a proper precursor for the preparation of high-quality MnO nanoplates. Precursor Concentration. The concentration of Mn(Ac)2 precursor was found to play a crucial role in obtaining MnO nanoplates. If the concentration of precursor was low, for example, when the molar ratio of Mn(Ac)2 to OM solvent was 0.5, only (6.5 ( 1.0) nm irregular MnO nanoparticles were obtained at 200 °C for 15 min (Figure 5a). As the ratio of Mn(Ac)2 to OM increased to 1:1, (10.2 ( 0.6) nm × (6.8 ( 0.4) nm MnO nanoplates were obtained (Figure 2a). Therefore, a rather high Mn(Ac)2 concentration facilitated the formation of manganese oxide nanoplates, indicating the nanoplate growth was a highly kinetics-driven process.14 SolWent. The presence of long-chain amine ligands (e.g., OM) in the solvent seemed a prerequisite to obtain phase-pure and monodisperse MnO nanoplates. The thermolysis of Mn(Ac)2 in OA yielded the matters containing no MnO. As a small amount of OA presented in the reaction solution, only ill-shaped

MnO nanocrystals were formed. For example, using 10 mmol of Mn(Ac)2 as the precursor, the reaction under OA/OM ) 9/1 (total: 10 mmol) at 200 °C for 15 min produced nonuniform MnO nanoplates ((12.5 ( 2.5) nm × (5.7 ( 1.3) nm, Figure 5b). Only in pure OM were high-quality MnO nanoplates ((10.2 ( 0.6) nm × (6.8 ( 0.4) nm) obtained at 200 °C for 15 min (Figure 2a). When employing other types of long-chain amine as solvents, such as hexadecylamine (HDA) or dodecylamine (DDA), the products were also nanoplates in the size of (9.6 ( 0.6) nm × (5.5 ( 0.8) nm and (7.1 ( 0.6) nm × (5.0 ( 0.4) nm, respectively (Figure 5c,d), indicating the existence of the same shaping effect on the MnO nanocrystals in these different long-chain amine solvents.4a,9,15 Reaction Temperature (T) and Time. Under a fixed ratio of Mn(Ac)2 to OM (1:1) and a reaction time of 15 min, high-quality MnO nanoplates ((10.2 ( 0.6) nm × (6.8 ( 0.4) nm) were obtained at 200 °C for 15 min (Figure 2a). As T was decreased from 200 to 180 °C, impure MnO species were formed, due to the unfinished thermolysis of the precursors at this low temperature. As T was increased to 220 °C, polydisperse MnO nanoplates ((10.3 ( 1.3) nm × (6.1 ( 0.8) nm)) were produced (Figure 6a). When T was further raised to 260 °C, polygonal MnO nanocrystals ((17.6 ( 1.8) nm) instead of nanoplates were formed (Figure 6b). At 200 °C, as the reaction time was shortened from 10 min to 5 and 0 min (Figure 6c,d), MnO nanoplates with broader size distributions (5 min: (9.6 ( 1.2) nm × (5.9 ( 0.8) nm; 0 min: (9.9 ( 1.1) nm × (6.1 ( 0.7) nm) were obtained, indicating a quick “size focusing” occurred during the reaction period from

6526 J. Phys. Chem. C, Vol. 113, No. 16, 2009

Figure 6. TEM images of MnO nanocrystals obtained from the thermolysis of Mn(Ac)2 in OM solvent (precursor/solvent ) 1) under different conditions: (a) 220 °C, 15 min; (b) 260 °C, 15 min; (c) 200 °C, 5 min; (d) 200 °C, 0 min; and (e) 200 °C, 30 min. (f) Photographs of colloid MnO and Mn3O4 nanoplates dispersed in toluene, which have been placed in an ambient environment for more than 6 months.

5 to 10 min. However, when the reaction time was prolonged to 30 min (Figure 6e), MnO nanoplates with a much broader size distribution ((18.2 ( 1.9) nm × (5.6 ( 1.7) nm) were formed, suggesting that a drastic “size defocusing” occurred. Therefore, these results demonstrated an Ostwald-ripening process existed in the formation of monodisperse MnO nanoplates at 200 °C.14b,c Plausible Pathway for the Formation of Manganese Oxide Nanoplates. In the synthesis of MnO nanoplates, as Mn(Ac)2 in OM solution was rapidly heated to its thermolysis temperature (around 200 °C) under an Ar atmosphere, some tiny gas bubbles were promptly emitted from the reaction system, which implied an instant decomposition of the precursor and a simultaneous formation of manganese oxide nuclei. As determined with a gas chromatograph-mass spectrometer, the major components of the gas bubbles were concluded to be CO2 and other small molecular species (Figure S2, Supporting Information). The CO2 species were supposed to act as the main oxidant in the formation of MnO nanocrystals.4a,8,11 With the as-formed MnO nuclei from the instant decomposition of Mn(Ac)2, the subsequent growth of MnO nanoplates at 200 °C under a considerably high Mn(Ac)2 concentration (Mn(Ac)2:OM ) 1:1) was proposed to proceed through a highly kinetics-driven process.14 At such a low temperature, the slow decomposition of the precursors produced fewer nuclei at the

Du et al. nucleation stage, and continuously released monomers to be consumed by the MnO nanocrystals in the growth stage. Under this condition, MnO nanocrystals grew more anisotropically, resulting in the formation of a kinetically metastable shape of nanoplates in the size of (10.2 ( 0.6) nm × (6.8 ( 0.4) nm) (Figure 2a). As the reaction went on at a high enough temperature of 260 °C, the rapid decomposition of the precursors yielded more nuclei at the nucleation stage, and simultaneously released more monomers for quick consumption by the MnO nanocrystals at the growth stage. Under this condition, thermodynamically stable MnO polyhedral nanocrystals were formed (Figure 6b). Through a chemical oxidation process with TMNO in OM at 210 °C for 5 h, MnO nanoplates were converted to Mn3O4 nanoplates. Figure 6f showed the photographs of colloidal monodisperse MnO and Mn3O4 nanoplates dispersed in toluene. There was no evident sedimentation even after having been stored for more than 6 months at room temperature. Generally, the crystal shape of the inorganic nanocrystals was dependent on the following factors: the crystalline phase of the nuclei, the selective adsorption of surfactant onto specific crystal planes, and the balance between the kinetic and the thermodynamic growth stages.14 For our MnO nanocrystals, their crystal phase was cubic, showing no structural anisotropy. We considered that the as-observed 2D growth mode for the MnO nanoplates probably originated from the selective adsorption of the capped oleylamine ligands onto the {001} crystal planes of the nanocrystals during the growth stage. The extremely high concentration of monomers tended to make the MnO nuclei undergo a fast 2D growth mode at a temperature around 200 °C.14a,b Under this condition, the shape transformation from dynamically stable nanoplates to thermodynamically stable nanopolyhedra would be restrained. As a result, the MnO nanoplates with the confined growth of {001} facets were formed. As the reaction temperature was raised to above 260 °C, the metastable MnO nanoplates were transformed into the thermodynamically stable nanoparticles in polyhedral shape (Figure 6b). Magnetic Properties of MnO and Mn3O4 Nanoplates. Early works have demonstrated that the small MnO nanoparticles (5-10 nm in diameter) show weak ferromagnetic behaviors at low temperature, although the bulk MnO has an antiferromagnetic transition at around 120 K.4a,16 Usually, this weak ferromagnetism has been attributed to the surface spin effects. The surface-to-volume ratio is very large for nanoscale antiferromagnets, and surface atoms possess reduced coordination, which may alter spin configurations throughout the nanocrystals. In this work, the observed weak ferromagnetism for the MnO nanoplates is ascribed to the presence of noncompensated surface spins on the antiferromagnetic core of the MnO nanoplates.9,10,17 The inset of Figure 7a shows the ZFC magnetization curve at an applied field (H) of 100 Oe, and the magnetic transition at the corresponding high temperature of TN ) 125 K is observed. As shown in Figure 7a, in a field of 100 Oe, ZFCFC measurements reveal obvious bifurcation below 40 K indicating a spontaneous magnetization. This magnetic phase transition may be responsible for tiny Mn3O4 impurities in the MnO nanoplates (as revealed by XPS analysis), since Mn3O4 nanoparticle samples show ferromagnetic behavior below the blocking temperature of about 40 K,18 and have larger magnetization than an antiferromagnet. In addition, the isothermal magnetization and the hysteresis loops are recorded at 5 K (Figure 7b), which displays a small but clear hysteresis with a coercive field of 1.6 kOe.

Synthesis of MnO and Mn3O4 Nanoplates

J. Phys. Chem. C, Vol. 113, No. 16, 2009 6527

Figure 7. (a) ZFC (filled symbols) and FC (open symbols) magnetization curves of as-prepared MnO nanoplates under an applied field of 100 Oe; the inset is the full-scan ZFC curve of MnO nanoplates measured in 100 Oe fields. (b) Hysteresis loops of MnO nanoplates at 5 K.

The temperature-dependent magnetization curve (FC-ZFC) of Mn3O4 nanoplates is shown in Figure 8a, which suggests ferromagnetic behaviors at low temperatures. Under zero-fieldcooling (ZFC) measurements at 100 Oe, the observed blocking temperature TB is 36 K for our Mn3O4 nanoplates. To determine the temperature of 36 K is a ferromagnetic order transition temperature rather than the spin reorientation and/or the spin-glasses transition temperatures,9,19a we have measured the temperature-dependence magnetization of the Mn3O4 nanoplates in different applied magnetic fields of 500, 1 000, 10 000, 50 000, and 100 000 Oe (Figure 8c). The transition temperature is found to be nearly a constant at 36 K for all curves, suggesting that the temperature of 36 K would be a blocking temperature.19b,c For typical superparamagnetic nanoparticles, if the interparticle magnetic interaction is weak, TB can be related via the following equation:20

KV ) 25kBTB where kB is the Boltzmann constant, K stands for the magnetic anisotropy constant, and V is the total volume of the particles. Here, the samples are supposed to be spherical. Compared to the isotropic nanopartilces (ideally: nanospheres) with the same diameters, the volume of nanoplates is obviously smaller. If we suppose the same value of K for Mn3O4 nanostructures, here, according to the equation above, TB of nanoplates will shift to a low temperature compared with that of nanospheres. This is consistent with the reported results, for instance, the observed blocking temperatures are around 40 K for 10 nm Mn3O4 nanoparticles.21 This trend follows the same size effect on the blocking temperature of Mn3O4 nanoparticles.

Figure 8. (a) ZFC (filled symbols) and FC (open symbols) magnetization curves of as-prepared Mn3O4 nanoplates under an applied field of 100 Oe. (b) Hysteresis loops of Mn3O4 nanoplates at 5 K. (c) FC magnetization curves of Mn3O4 nanoplates under different applied fields of 500, 1 000, 10 000, 50 000, and 100 000 Oe.

Different from the MnO nanoplates, the Mn3O4 nanoplates exhibit a much larger hysteresis loop with a coercive field of almost 8.9 kOe at 5 K (shown in Figure 8b), indicative of ferromagnetic components. This Hc value is much greater than that of Mn3O4 nanoparticles (≈7.6 kOe) determined at 5 K22b and is even greater than that of the bulk phase (≈2.8 kOe) or thin film (≈3.5 kOe).22a The coercivity is an extrinsic property of a magnet, depending not only on the spin carriers but also on the shape or size of the magnets.23 Conclusions We have demonstrated an efficient synthesis of singlecrystalline and monodisperse MnO nanoplates from thermal decomposition of high-concentration Mn(Ac)2 in oleylamine. By a controlled oxidation with trimethylamine N-oxide as oxidant, high-quality Mn3O4 nanoplates were obtained from the MnO nanoplates. The formation of kinetically metastable MnO nanoplates was concluded to be a highly kinetics-driven process,

6528 J. Phys. Chem. C, Vol. 113, No. 16, 2009 which occurred under a quite high monomer concentration. Through the manipulation of the post-treatment procedure and the polarity of dispersants, the as-obtained nanoplates were aligned to form self-assembly nanoarrays either in “side-to-side” or “face-to-face” formation. The MnO and Mn3O4 nanoplates displayed ferromagnetism at low temperatures. The as-obtained Mn3O4 nanoplates exhibited a coercive field of 8.9 kOe at 5 K, significantly higher than those of bulk Mn3O4 and thin films. Considering the magnetic properties and prominent self-assembly ability of the as-prepared manganese oxide nanoplates, they are expected to have the potential for constructing future self-assembled nanodevices. Acknowledgment. We gratefully acknowledge the financial aid from the MOST of China (Grant No. 2006CB601104) and the NSFC (Grant Nos. 20871006, 20821091, and 20671005). The authors also thank Prof. S. Gao and Mr. Xiuteng Wang for their kind help in the magnetic measurements. Supporting Information Available: Size-selection procedure, TG curves of Mn(Ac)2 and Mn(acac)2 (Figure S1), and GC-MS spectra of the gas components from the thermolysis of Mn(Ac)2 at 200 °C in oleylamine under a flow of Ar atmosphere (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) (a) Majetich, S. A.; Jin, Y. Science 1999, 284, 470. (b) Speliotis, D. E. J. Magn. Magn. Mater. 1999, 193, 29. (c) Zarur, A. J.; Ying, J. Y. Nature (London) 2000, 403, 65. (d) Murray, C. B.; Kagan, C. R.; Bawendi, M. G. Annu. ReV. Mater. Sci. 2000, 30, 545. (e) Cozzoli, P. D.; Kornowski, A.; Weller, H. J. Am. Chem. Soc. 2003, 125, 14539. (f) Hyeon, T. Chem. Commun. 2003, 927. (g) Epifani, M.; Arbiol, J.; Dı´az, R.; Pera´lvarez, M. J.; Siciliano, P.; Morante, J. R. Chem. Mater. 2005, 17, 6468. (h) Yavuz, C. T.; Mayo, J. T.; Yu, W. W.; Prakash, A.; Falkner, J. C.; Yean, S.; Cong, L. L.; Shipley, H. J.; Kan, A.; Tomson, M.; Natelson, D.; Colvin, V. L. Science 2006, 314, 964. (i) Xie, J.; Chen, K.; Lee, H. Y.; Xu, C. J.; Hsu, A. R.; Peng, S.; Chen, X. Y.; Sun, S. H. J. Am. Chem. Soc. 2008, 130, 7542. (2) (a) Trentler, T. J.; Denler, T. E.; Bertone, J. F.; Agrawal, A.; Colvin, V. L. J. Am. Chem. Soc. 1999, 121, 1613. (b) Hyeon, T.; Lee, S. S.; Park, J. N.; Chung, Y. H.; Na, H. B. J. Am. Chem. Soc. 2001, 123, 2798. (c) Jun, Y. W.; Casula, M. F.; Sim, J. H.; Kim, S. Y.; Cheon, J.; Alivisatos, A. P. J. Am. Chem. Soc. 2003, 125, 15981. (d) Park, J.; An, K.; Hwang, Y.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Hwang, N. M.; Hyeon, T. Nat. Mater. 2004, 3, 891. (e) Yin, M.; Wu, C. K.; Lou, Y. B.; Burda, C.; Koberstein, J. T.; Zhu, Y. M.; O’Brien, S. J. Am. Chem. Soc. 2005, 127, 9506. (f) Lu, A. H.; Salabas, E. L.; Schu¨th, F. Angew. Chem., Int. Ed. 2007, 46, 1222. (3) (a) Sun, S. H.; Zeng, H. J. Am. Chem. Soc. 2002, 124, 8204. (b) Jana, N. R.; Chen, Y. F.; Peng, X. G. Chem. Mater. 2004, 16, 3931. (c) Yin, M.; Gu, Y.; Kuskovsky, I. L.; Andelman, T.; Zhu, Y. M.; Neumark,

Du et al. G. F.; O’Brien, S. J. Am. Chem. Soc. 2004, 126, 6206. (d) Seo, W. S.; Shim, J. H.; Oh, S. J.; Lee, E. K.; Hur, N. H.; Park, J. T. J. Am. Chem. Soc. 2005, 127, 6188. (e) Shevchenko, E. V.; Talapin, D. V.; Kotov, N. A.; O’Brien, S.; Murray, C. B. Nature (London) 2006, 439, 55. (f) Jun, Y. W.; Choi, J. S.; Cheon, J. Angew. Chem., Int. Ed. 2006, 45, 3414. (4) (a) Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T. Angew. Chem., Int. Ed. 2004, 43, 1115. (b) Masala, O.; Seshadri, R. J. Am. Chem. Soc. 2005, 127, 9354. (c) Morales, M. A.; Skomski, R.; Fritz, S.; Shelburne, G.; Shield, J. E.; Yin, M.; O’Brien, S.; Leslie-Pelecky, D. L. Phys. ReV. B 2007, 75, 134423. (d) Latham, A. H.; Williams, M. E. Acc. Chem. Res. 2008, 41, 411. (5) (a) Tarascon, J. M.; Armand, M. Nature (London) 2001, 414, 359. (b) Larcher, D.; Sudant, G.; Leriche, J. B.; Chabre, Y.; Tarascon, J. M. J. Electrochem. Soc. 2003, 150, A133. (6) (a) Baldi, M.; Finocchio, E.; Milella, F.; Busca, G. Appl. Catal., B 1998, 16, 43. (b) Steven, L. S. Acc. Chem. Res. 2008, 41, 479. (7) (a) Na, H. B.; Lee, J. H.; An., K. J.; Park, Y. I.; Park, M.; Lee, I. S.; Nam, D. H.; Kim, S. T.; Kim, S. H.; Kim, S. W.; Lim, K. H.; Kim, K. S.; Kim, S. O.; Hyeon, T. Angew. Chem., Int. Ed. 2007, 46, 5397. (b) Jun, Y. W.; Seo, J. W.; Cheon, J. Acc. Chem. Res. 2008, 41, 179. (8) Yin, M.; O’Brien, S. J. Am. Chem. Soc. 2003, 125, 10180. (9) Jiao, F.; Harrison, A.; Bruce, P. G. Angew. Chem., Int. Ed. 2007, 46, 3946. (10) Park, J.; Kang, E.; Bae, C. J.; Park, J. G.; Noh, H. J.; Kim, J. Y.; Park, J. H.; Park, H. M.; Hyeon, T. J. Phys. Chem. B 2004, 108, 13594. (11) Sun, X.; Zhang, Y. W.; Si, R.; Yan, C. H. Small 2005, 1, 1081. (12) Apte, S. K.; Naik, S. D.; Sonawane, R. S.; Kale, B. B.; Pavaskar, N.; Mandale, A. B.; Das, B. K. Mater. Res. Bull. 2006, 41, 647. (13) (a) Chigane, M.; Ishikawa, M. J. Electrochem. Soc. 2000, 147, 2246. (b) Toupin, M.; Brousse, T.; Blanger, D. Chem. Mater. 2002, 14, 3946. (14) (a) Peng, X. G. AdV. Mater. 2003, 15, 459. (b) Si, R.; Zhang, Y. W.; Zhou, H. P.; Sun, L. D.; Yan, C. H. Chem. Mater. 2007, 19, 18. (c) Du, Y. P.; Zhang, Y. W.; Yan, Z. G.; Sun, L. D.; Gao, S.; Yan, C. H. Chem. Asian J. 2007, 2, 965. (d) Wang, D. S.; Zhen, W.; Hao, C. H.; Peng, Q.; Li., Y. D. Chem. Commun. 2008, 2556. (15) Rusakova, I.; Ould-Ely, T.; Hofmann, C.; Prieto-Centurio´n, D.; Levin, C. S.; Halas, N. J.; Lu¨ttge, A.; Whitmire, K. H. Chem. Mater. 2007, 19, 1369. (16) (a) Ahmad, T.; Ramanujachary, K. V.; Lofland, S. E.; Ganguli, A. K. J. Mater. Chem. 2004, 14, 3406. (b) Ghosh, M.; Biswas, K.; Sundaresana, A.; Rao, C. N. R. J. Mater. Chem. 2006, 16, 106. (17) (a) Lee, G. H.; Huh, S. H.; Jeong, J. W.; Choi, B. J.; Kim, S. H.; Ri, H. C. J. Am. Chem. Soc. 2002, 124, 12094. (b) Alvarez, G. S.; Sort, J.; Surin´ach, S.; Baru¨, M. D.; Nogue´s, J. J. Am. Chem. Soc. 2007, 129, 9102. (18) Dwight, K.; Menyuk, N. Phys. ReV. 1960, 119, 1470. (19) (a) Srinivasan, G.; Seehra, M. S. Phys. ReV. B 1983, 28, 1. (b) Zysler, R. D.; Winkler, E. Nanotechnology 2007, 18, 158001. (c) Du, J.; Hong, B. Nanotechnology 2007, 18, 158002. (20) Roth, W. L. Phys. ReV. 1958, 110, 1333. (21) (a) Shull, C. G.; Smart, J. S. Phys. ReV. 1949, 76, 1256. (b) Renninger, A.; Moss, S. C.; Averbach, B. L. Phys. ReV. 1966, 147, 418. (22) (a) Na, C. W.; Han, D. S.; Kim, D. S.; Parka, J.; Jeon, Y. T.; Lee, G.; Junga, M. H. Appl. Phys. Lett. 2005, 87, 142504. (b) Wang, N.; Guo, L.; He, L.; Cao, X.; Chen, C. P.; Wang, R. M.; Yang, S. H. Small 2007, 3, 606. (23) Kodama, R. H.; Makhlouf, S. A.; Berkowitz, A. E. Phys. ReV. Lett. 1997, 79, 1393.

JP8114868