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J. Phys. Chem. C 2008, 112, 15752–15758
Easy Single-Step Route to Manganese Oxide Nanoparticles Embedded in Carbon and Their Magnetic Properties Sangaraju Shanmugam and Aharon Gedanken* Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center for AdVanced Materials and Nanotechnology, Bar-Ilan UniVersity, Ramat-Gan, 52900, Israel ReceiVed: July 13, 2008; ReVised Manuscript ReceiVed: July 29, 2008
We present a facile and simple solid-state thermolysis approach for the formation of manganese oxide nanoparticles embedded in an amorphous carbon. This was accomplished through a single-step direct thermolysis of cetyltrimethylammonium permanganate. The as-synhtesized products were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), Raman microscope, X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The product consists mainly of Mn3O4 and of MnOOH coated with carbon. The carbon coating was observed when reaction was carried out at 400 °C and the average particle size is 9 nm. The shape of the products can be controlled by varying parameters such as reaction temperature. As the temperature increases from 500 to 700 °C, larger spherical particles were observed without any carbon coating. The magnetic properties of as-synthesized products were evaluated using superconducting quantum interference devices (SQUID). The field-dependent magnetic measurements showed that nanoparticles embedded in carbon exhibited a high coercivity value of 10.5 kOe at 2 K. The saturation magnetization values at 2 K are 42 and 46 emu/g for the reaction temperatures of 500 and 600 °C, respectively. 1. Introduction In recent years, the development of metal oxide nanoparticles has attracted tremendous interest because of their potential applications in catalysis, energy storage, magnetic data storage, sensors, and biomedical application.1 Different oxides of manganese are possible due to the existence of Mn in various oxidation states (II, III, IV, and VII). The magnetic, structural, and transport properties of these manganese oxides are of considerable interest in understanding their unique properties from a fundamental point of view.2-4 Manganese oxide and oxyhydroxide one-dimensional nanostructured materials have attracted a great deal of attention because of their low cost, high natural abundance, and environmental compatibility.5 Manganese oxide materials find a wide range of applications, such as batteries, catalysts, electrochromic, and magnetic materials.6A wide variety of morphological structures of manganese oxides, ranging from single crystals and thin films to nanowires, nanosheets, and nanoparticles, has been reported.7-12 Among the oxides of manganese, Mn3O4 is known to be an effective and inexpensive catalyst for NOx and CO reduction, which provides a powerful method of controlling air pollution.13 Mn3O4 is also used as a catalyst for the reduction of nitrobenzene or oxidation of methane.14 Another important application of Mn3O4 is being used as a raw material for the production of soft magnetic materials such as manganese zinc ferrite, which is useful for magnetic cores in transformers for power supply.15 Manganese oxides have been used as electrochromic materials, and intensive research work was carried out on these materials.16-18 Mn3O4 was usually prepared by the high-temperature calcinations of manganese oxides with a higher valence of manganese, hydroxides, and hydroxyoxides at 1000 °C in air.19 Various methods have been adapted to synthesize Mn3O4, viz., chemical * To whom correspondence should be addressed. E-mail: gedanken@ mail.biu.ac.il.
bath deposition, sol-gel technique, co-precipitation, and hydrothermal and thermal decomposition in organic solvents.20 When the hydrothermal method is used, it leads to Mn3O4 formation through hydroxide followed by partial oxidation.21 The hydrothermal method requires long reaction time, i.e., from 48 to 72 h at different temperatures and pressures. The syntheses of carbon coating nanostructures of magnetic metal/carbon always rely on very harsh conditions, such as arc techniques,22 catalytic chemical vapor deposition,23 magnetron and ion-beam cosputtering, and high-temperature annealing.24 The intrinsic high-energy consumption and expensive hardware of these techniques are mainly responsible for the high cost of manufacturing magnetic nanoparticles encapsulated in carbon and thus limit their practical applications. Very recently, the magnetic and microstructural properties of antiferromagnetic MnO nanoparticles with ferrimagnetic Mn3O4 shells has been studied.25 Si et al. observed large coercivity for Mn3O4/MnO nanoparticles.26 Among them, MnO, Mn2O3, and Mn3O4 have a wide range of applications in catalysis and battery technologies.27 Here, we report the synthesis of manganese oxide nanoparticles embedded in carbon using a single-component precursor. The advantage of the present method is that the nanoparticle embedded in carbon is achieved in a single step. To the best of our knowledge, manganese oxide nanoparticles embedded in carbon was not reported so far. The as-synthesized products were characterized with various physicochemical techniques. The product consists of Mn3O4/MnOOH nanoparticles with an average size of 9 nm, and the particles are embedded in carbon, forming sheetlike structures in two-dimensional fashions. By varying the reaction temperature, we were able to synthesize manganese oxide nanoparticle without any amorphous carbon. Another aspect of the paper is the as prepared nanoparticles exhibit higher coercive fields when compared to the reported values.
10.1021/jp806175y CCC: $40.75 2008 American Chemical Society Published on Web 09/13/2008
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Figure 1. (a, b) TEM images of product, showing the sheetlike morphology and consist of nanoparticles; (c) HRTEM image, showing lattice fringes of manganese oxide particles. An arrow shows the carbon.
TABLE 1: Reaction Parameters, Product Morphology and Magnetic Properties of Manganese Oxide Nanoparticles expt no.
reacn params (temp, time, atmosphere)
1 2 3
400 °C, 3 h, argon 500 °C, 3 h, argon 600 °C, 3 h, argon
C (wt %)
product morphology
size of crystals (nm)
M (emu/g)
coercivity (kOe)
13.4 0.05
sheetlike chains with sperhical spheres chains with spherical spheres
9 100-250 200-400
15 42 46
10.5 10.14 11.42
2. Experimental Section Preparation of the Precursor. The precursor cetyltrimethylammonium permanganate was prepared by using an aqueous solution of potassium permanganate (KMnO4, 0.01 M) and cetyltrimethylammomium bromide (CTAB, 0.01 M). The ratio between the cation and anion is 1:1. An aqueous solution of CTAB was added drop by drop to the KMnO4 solution with vigorous stirring.28 A purple gel was formed and was aged in air overnight, filtered, and washed with water several times. The C, H, N analysis reveals that the ratio between the cation to anion is 1. The theoretical carbon content in the starting material is 56.6 wt %the, and observed carbon content is 54.2 wt %. The synthesis of Mn3O4 structures has been carried out in a single-stage furnace with precisely controlled temperatures. Fine dry powder of cetyltrimethylammonium permanganate was used as the single-component precursor. For a typical synthesis, the precursor was directly placed in a quartz boat and kept at the center of a quartz tube, which was placed inside a tubular furnace. The temperature was raised 20 °C/min in the presence of argon gas. Thermolysis was carried out at 400 °C for 3 h, and thereafter the furnace was cooled to room temperature. Argon flow was maintained throughout the experiment. The assynthesized product obtained in the quartz boat was used for characterization and magnetic studies. The yield of brownishblack product is 0.16 g which corresponds to 55% relative to the starting material. Similar experiments were carried out at different temperatures (500, and 600 °C) and different duration periods. The carbon content in the products was determined by using C, H, and N elemental analysis. A comparison of product morphology, reaction parameters, and carbon content is presented in Table 1. Structural Characterization. The particle morphology was studied with transmission electron microscopy on a JEOL-JEM 100 SX microscope, working at 80 kV accelerating voltage, and a JEOL-2010 high-resolution transmission electron microscopy (HRTEM) instrument with an accelerating voltage of 200 kV. Samples for TEM and HRTEM were prepared by ultrasonically dispersing the products into absolute ethanol, then placing a drop of this suspension onto a copper grid coated with an
amorphous carbon film, and then drying under air. Highresolution scanning electron microscopy (HRSEM) of the obtained product was carried out on a JEOL-JSM 840 scanning electron microscope operating at 10 kV. The X-ray diffraction measurements were carried out with a Bruker AXSD Advance powder X-ray diffractometer with a Cu KR (λ ) 1.5418 Å) radiation source. The diffraction measurements were collected from 20 to 80° at a speed of 1.2°/min. The elemental analysis of the sample was carried out by an Eager C, H, N, S analyzer. An Olympus BX41 (Jobin Yvon Horiba) Raman spectrometer was employed, using the 514.5 nm line of an Ar laser as the excitation source to analyze the nature of the carbon present in the products. The X-ray photoelectron spectroscopy (XPS) measurements was carried out using JEOL JPS-900, in an ultrahigh-vacuum (UHV), axis HS monochromatized Mg KR cathode source, at 75-150 W, using a low-energy electron flood gun for charge neutralization. Survey and high-resolution individual metal emissions were taken at a medium resolution, with a pass energy of 50 eV and a step of 1 eV. Magnetic Measurements. Magnetic properties of powder samples were analyzed with a Quantum Design MPMS-7. Detailed magnetic measurements, zero-field-cooled (ZFC) and field-cooled (FC) magnetization vs temperature under field, and magnetic hysteresis loops at several temperatures have been carried out in order to study the magnetic properties of the assynthesized manganese oxide nanoparticles. The saturation
Figure 2. EDAX spectrum of the product obtained at 400 °C.
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Figure 5. XRD pattern of the product obtained at 400 °C.
and 5 K. The temperature dependence of the magnetization was monitored by ZFC and FC experiments. The ZFC curve was generated by first cooling the system in a zero field. Then the field was applied (100 Oe), and magnetization was measured while the temperature was increased to 300 K. The FC curve was obtained in a similar way except that the sample was cooled in an applied field of 100 Oe. 3. Results and Discussion
Figure 3. (a) HRTEM image, showing lattice fringes of Mn3O4 and MnOOH. (b) HRTEM image, depicting amorphous carbon on the surface of manganese oxide particles. The dotted arrows show the carbon layers.
Figure 4. Raman spectrum of product obtained at 400 °C, showing the presence of disordered graphitic carbon.
magnetization and coercivity filed values were obtained from the hysteresis loops measured up to to a field of 6 T at 300, 10,
The morphology of the product synthesized at 400 °C is shown in Figure 1. A typical TEM image shows that the product exhibits sheetlike shape. A closer look at the sheet indicates that the sheet consists of spherical-shaped nanoparticles. The TEM image shows that the nanoparticles are embedded in carbon, which is highlighted with an arrow in Figure 1b. The average size of the particles is 9 ( 1 nm. The higher magnification of the image shows lattice fringes, indicating the particles are crystalline (Figure 1c). The EDAX analysis of the sheet shows the presence of Mn, O, and C without any other impurities (Figure 2). The carbon content in the product was determined by using C, H, and N elemental analysis. The contents of C, H, and N were found to be 13.4, 0.6, and 0.6 wt %, respectively. The HRTEM image of the product indicates that the particles are arranged in such a way that the particles are in close contact with other particles (Figure 3a). The wellresolved lattice fringes with a d-spacing value of 0.491 nm correspond to the (101) plane of cubic Mn3O4 (JCPDS 0240734). The image also shows particles with a lattice distance of 0.378 nm, which is corresponding to the γ-MnOOH (110) plane. From Figure 3b, it is clear that the disordered carbon coating on the Mn3O4 particle is evidenced (highlighted with arrows). The thickness of the carbon coating is around 4 nm. Figure 3b also reveals that the nature of carbon is disordered layers, corresponding to the nongraphitic, coallike lattice planes of the carbon, as the thermolysis reaction was carried out at 400 °C. The nature and type of carbon present in the product is analyzed by Raman spectroscopy. The product exhibits two broad peaks at 1328 and 1602 cm-1 (Figure 4). The band at 1328 cm-1 corresponds to the D peak arising from the breathing motion of sp2 rings, and the band at 1602 cm-1 is a G band. The ratio between the D and G bands is found to correlate to
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J. Phys. Chem. C, Vol. 112, No. 40, 2008 15755
Figure 6. XPS spectra of the product synthesized at 400 °C; (a) survey scan; (b) Mn 2p core level; (c) C 1s and (d) O1s spectra.
Figure 7. (a) Temperature-dependent hysteresis loops of manganese oxide synthesized at 400 °C and (b) ZFC and FC magnetization curves under an applied field of 100 Oe.
the nature of carbon.29 The measured ID/IG ratio is found to be 1.1, suggesting that the carbon exists in a more disordered graphitic form (amorphous). A typical XRD pattern of the product is shown in Figure 5. The XRD peaks can be indexed to the tetragonal Hausmannite phase of Mn3O4 with a ) 0.576 nm and c ) 0.946 nm in accordance with JCPDS No.24-0734. The XRD pattern also shows additional patterns, which correspond to the γ-MnOOH phase. The composition and oxidation state of the product prepared at 400 °C was further analysized by the XPS spectroscopy. The XPS survey spectrum shows the sheet consists of Mn, O, and C elements (Figure 6a). The concentration of C is found to be
15%, which is in good agreement with elemental analysis results. The high-resolution spectrum of Mn 2p is given in Figure 6b. The obtained binding energy (BE) values of Mn 2p3/2 and 2p1/2 are 641.0 and 652.7 eV, respectively. The spin-orbit splitting is the difference between BE values of Mn 2p3/2 and Mn 2p1/2 levels. The observed spin-orbit splitting is 11.7 eV, same as in manganese oxides. 30 The BE of the Mn 2p3/2 (641.0 eV) and spin-orbit splitting (11.7) is well-matched with the reported value of Mn3O4. The discrimination between MnOOH and Mn3O4 is difficult from the Mn 2p values, because, in both cases, manganese exists in the +3 oxidation state. Figure 6c shows a broad asymmetric peak, and the peak has been deconvoluted
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Figure 8. SEM images of product obtained at (a) 500 and (b) 600 °C; (c,d) TEM images of product formed at 600 °C.
into three symmetric peaks, C1, C2, and C3 (284.2, 286.4, and 288.0 eV). The peak C1 (284.4 eV) originates from the from C-C and C-H forms of sp2 carbon, while the peaks at 286.4 (major), and 288.0 eV can be assigned to sp3-hybridized carbon atoms bonded with one or two oxygen atoms, respectively.31 When the electronegative oxygen atoms are bonded to the carbon, a positive charge is induced on the carbon atom. Hence, they can be assigned to alcoholic, ether (C-O) and ketone, or aldehyde (>CdO). These observations suggest that the carbon present in the product is amorphous carbon. The oxygen spectra also give two peaks (O1 and O2) after a deconvulation (Figure 6d). The peak (O1) at 529.6 eV can be attributed to oxygen (O2-) in the lattice of Mn-O-Mn,32 whereas the peak (O2) at 531.1 eV can be considered to pertain to oxygen in OH groups present in MnOOH. The Mn/O2- atomic ratio is in agreement with the theoretical values calculated from the bulk composition. The magnetic studies of Mn3O4 nanoparticles embedded in amorphous carbon were investigated using superconducting quantum interference devices (SQUID) at 300, 10, and 2 K. The hysteresis loops measured at different temperatures are shown in Figure 7a. The room-temperature magnetization study shows the Mn3O4 is paramagnetic, and at low (42 K) temperature Mn3O4 exhibits ferromagnetic behavior. The saturated magnetization value is 15 emu/g, and remanence is 5.5 emu/g at 10 K. The remanence ratio (Mr/Ms) of Mn3O4 nanoparticles is found to be 0.36. The interesting features of hysteresis loops are as follows: (i) The sample does not saturate at a magnetic field of 5 T, which means the particles have a large anisotropy field, and the anisotropy is directly related to the saturation; hence, the unsaturation of the hysteresis loop implies the presence of ferromagnetic or antiferromagnetic fractions at low temp. (ii) The coercive field Hc (HR + HL)/2) of the product is very large at 10 K (6.5 kOe), much higher than the reported
value for nanocrystalline Mn3O4 particles, and also at 2 K the coercivity found to be 10.5 kOe. Buckelew et al. observed 8.8 kOe coercivity at 2 K for Mn3O4 particles synthesized by hydrolysis of K2[Mn2(CN)6].33 We have thus obtained a large coercivity, which is much larger than the values of 2.8 kOe for bulk samples,34 3.5 kOe for thin films,35 and 5.7 kOe for nanowires.36 To avoid dynamic coercivity, we have collected the hysteresis very slowly. Another important observation is that the hysteresis loop shifts in both horizontal and vertical directions even in the absence of cooling field. The horizontal hysteresis loop shift at 10 and 2 K are 1.0 and 4.8 kOe, respectively, in the negative direction in the field axis. The origin of such high coercivity may be ascribed to the effect of shape anisotropy of Mn3O4 nanoparticles and also due to the interfacial interaction between the antiferromagnetic (γ-MnOOH) and ferromagnetic (Mn3O4) phases, which results in the hysteresis loops shift. In our product, the nanoparticles are fixed in amorphous carbon, and therefore preferential orientation of the magnetic easy axis could also possible. In our study, the presence of second-phase MnOOH antiferromagnetic components could be responsible for such vertical and horizontal loop shifts. In fact, due to the large anisotropy of Mn3O4, KMn3O4 ) 1.4 × 106 erg/cm3 is expected to display a small exchange bias that is only observable for very small nanoparticles.34 Larger anisotropy results in greater exchange bias effect, since if a system has larger anisotropy, Mn3O4 will have greater pinning effect over the other phase with lesser anisotropy, which creates difficulty in magnetization reversal and hence larger exchange bias. The temperature-dependent magnetization studies measured under zero-field-cooled and field-cooled processes from 5 to 300 K in a 100 Oe probe field are shown in Figure 7b. At room temperature Mn3O4 exhibits paramagnetic and ferromagnetic
Manganese Oxide Particles Embedded in Carbon
J. Phys. Chem. C, Vol. 112, No. 40, 2008 15757 is through the fusion process. The HRTEM image of the sphere is shows well-resolved fringes with a d-spacing value of 0.265 nm, corresponding to the (303) plane of cubic Mn3O4 (Figure 8d). To compare the magnetic properties of product obtained at 400 °C, we prepared manganese oxide nanoparticles without carbon; this was accomplished by increasing the reaction temperature. It was observed that when the reaction was carried out at higher temperatures (500 and 600 °C), the resulting products consist of Mn3O4 without any carbon (see Table 1). The magnetic studies of products synthesized at 500 and 600 °C were measured, and the obtained results were given in Table 1. The saturation magnetization values at 2 K are 42 and 46 emu/g for 500 and 600 °C, respectively (Figure 9a). The high magnetization values observed for Mn3O4 particles obtained at 500 and 600 °C could be attributed to the large particle size and the shape of the products when compared to Mn3O4-400, where the particle size is 9 nm, and also to high coercivities of 10.1 and, 11.4 kOe for samples prepared at 500 and 600 °C, respectively. For samples prepared at 500 and 600 °C systems, the hysteresis loop shifts are 0.96 and 1.1 kOe, respectively, in the negative direction on the field axis at 2 K. The blocking temperature of the product obtained at 500 °C is found to be 42 K (Figure 9b). 4. Conclusions
Figure 9. (a) Temperature-dependent hysteresis loops of manganese oxide synthesized at 500 °C. The inset shows the variation of coercivity (Hc) vs temperature (T). (b) ZFC and FC magnetization curves under an applied field of 100 Oe.
nature at low temperature (about 42 K). In our product, from the ZFC curve (filled symbols), an apparent transition from paramagnetic to ferromagnetic behavior was observed at 41.5 K, which is known to be the blocking temperature of Mn3O4. The observed blocking temperature is consistent with the literature values.37 The thermolysis temperatures play an important role in the morphology of the product. We carried out the reactions at 500 and 600 °C, and the XRD results indicate that as the reaction temperature increases, the second-phase intensity, γ-MnOOH, decreased. The morphology of these products was measured with SEM and TEM, and the results are shown in Figure 8. The products obtained at 500 and 600 °C exhibit the particles with spherical shape without any carbon on the surface. When the reaction temperature increases, the particle size also increases, giving rise to bigger particles sizes. It can be seen from the image that the product consists of spherical spheres forming chains. The size of the individual sphere is about 100 nm. The length of the chains varies from 2 to 5 µm (Figure 8). The fusion of adjacent spheres is manifested as sharing the substructure. The linear alignment of several subunits was evident inside some microspheres. The further association of spheres led to higher levels of hierarchical structures. A typical TEM image of microsphere chains is shown in Figure 8c. It is clear from the image that the joining of adjacent microspheres
In summary, by a one-step solid-state thermolysis of the cetyltrimethylammonium permanganate, we have successfully synthesized manganese oxide nanoparticles embedded in amorphous carbon. The formation of such Mn3O4-MnOOH nanoparticles embeded in carbon was derived by the presence of an organic structure-directing agent. The product mainly consists of Mn3O4 along with γ-MnOOH nanoparticles embedded in amorphous carbon. The average size of the particle is 9 nm for the sample prepared at 400 °C. The products obtained at 500 and 600 °C exhibit particles with spherical shape without any carbon around, and the sizes of the particles are in the range of 200-400 nm. The magnetic properties of Mn3O4-MnOOH nanoparticles exhibit a coercivity value of 10.5 kOe, and the blocking temperature is 41.5 K. These nanoparticles showed loop shift (exchange bias) due to the coupling of the weakly anisotropic MnOOH antiferromagnetic with highly anisotropic Mn3O4 ferromagnetic particles. The magnetization values of the products obtained at 500 and 600 °C are in the range of 42-45 emu/g. References and Notes (1) (a) Schmid, G. Chem. ReV. 1992, 92, 1709. (b) Andres, R. P.; Bielefeld, J.; D.; Henderson, J. I.; Janes, D. B.; Kolagunta; V., R.; Kubiak, C. P.; Mahoney, W. J.; Osifchin, R. G. Science 1996, 273, 1690. (2) Nayak, S. K.; Jena, P. Phys. ReV. Lett. 1998, 81, 2970. (3) Lidstrom, E.; Hartmann, O. J. Phys.: Condens. Matter 2000, 12, 4969. (4) Pask, J. E.; Singh, J.; Mazin, I. I.; Hellberg, C. S.; Kortus, J. J. Phys. ReV. B 2001, 64, 24403. (5) (a) Wu, M.-S. Appl. Phys. Lett. 2005, 87, 153102. (b) Chang, J.K.; Tsai, W.-T. J. Electrochem. Soc. 2003, 150, A1333. (6) (a) Shen, Y. F.; Zerger, R.P.; Deguzman, R. N.; Suib, S. L.; Mccurdy, L.; Potter, D. I.; O’Young, C. L. Science 1993, 260, 511. (b) Feng, Q.; Karoh, H.; Ooi, K.; Tani, M.; Nakacho, Y. J. Electrochem. Soc. 1994, 141, L135. (c) Armstrong, A. R.; Bruce, P. G. Nature 1996, 381, 499. (d) Lidstrom, E.; Hartmann, O. J. Phys.: Condes. Mater. 2000, 12, 161. (e) Torresi, S. C. D.; Gorenstein, A. Electrochim. Acta 1992, 37, 2015. (7) (a) Brock, S. L.; Duan, N.-G.; Tian, Z.-R.; Giraldo, O.; Zhou, H.; Suib, S. L. Chem. Mater. 1998, 10, 2619. (b) Feng, Q.; Kanoh, H.; Ooi, K. J. Mater. Chem. 1999, 9, 319. (8) Yang, X.-J.; Tang, W.-P.; Feng, Q.; Ooi, K. Cryst. Growth Des. 2003, 3, 409.
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JP806175Y
