jpcc mn3o4

Easy Single-Step Route to Manganese Oxide Nanoparticles Embedded in Carbon and TheirMagnetic Properties

Sangaraju Shanmugam and Aharon Gedanken*Department of Chemistry and Kanbar Laboratory for Nanomaterials at the Bar-Ilan UniVersity Center forAdVanced 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 oxidenanoparticles embedded in an amorphous carbon. This was accomplished through a single-step directthermolysis of cetyltrimethylammonium permanganate. The as-synhtesized products were characterized bytransmission electron microscopy (TEM), scanning electron microscopy (SEM), high-resolution transmissionelectron microscopy (HRTEM), Raman microscope, X-ray diffraction (XRD), and X-ray photoelectronspectroscopy (XPS). The product consists mainly of Mn3O4 and of MnOOH coated with carbon. The carboncoating was observed when reaction was carried out at 400 °C and the average particle size is 9 nm. Theshape of the products can be controlled by varying parameters such as reaction temperature. As the temperatureincreases from 500 to 700 °C, larger spherical particles were observed without any carbon coating. The magneticproperties of as-synthesized products were evaluated using superconducting quantum interference devices(SQUID). The field-dependent magnetic measurements showed that nanoparticles embedded in carbon exhibiteda high coercivity value of 10.5 kOe at 2 K. The saturation magnetization values at 2 K are 42 and 46 emu/gfor the reaction temperatures of 500 and 600 °C, respectively.

1. Introduction

In recent years, the development of metal oxide nanoparticleshas attracted tremendous interest because of their potentialapplications in catalysis, energy storage, magnetic data storage,sensors, and biomedical application.1 Different oxides ofmanganese are possible due to the existence of Mn in variousoxidation states (II, III, IV, and VII). The magnetic, structural,and transport properties of these manganese oxides are ofconsiderable interest in understanding their unique propertiesfrom a fundamental point of view.2-4 Manganese oxide andoxyhydroxide one-dimensional nanostructured materials haveattracted a great deal of attention because of their low cost, highnatural abundance, and environmental compatibility.5 Manga-nese oxide materials find a wide range of applications, such asbatteries, catalysts, electrochromic, and magnetic materials.6Awide 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 ofmanganese, Mn3O4 is known to be an effective and inexpensivecatalyst for NOx and CO reduction, which provides a powerfulmethod of controlling air pollution.13 Mn3O4 is also used as acatalyst for the reduction of nitrobenzene or oxidation of methane.14

Another important application of Mn3O4 is being used as a rawmaterial for the production of soft magnetic materials such asmanganese zinc ferrite, which is useful for magnetic cores intransformers for power supply.15 Manganese oxides have been usedas electrochromic materials, and intensive research work wascarried out on these materials.16-18

Mn3O4 was usually prepared by the high-temperature calcina-tions of manganese oxides with a higher valence of manganese,hydroxides, and hydroxyoxides at 1000 °C in air.19 Variousmethods have been adapted to synthesize Mn3O4, viz., chemical

bath deposition, sol-gel technique, co-precipitation, and hy-drothermal 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., from48 to 72 h at different temperatures and pressures. The synthesesof carbon coating nanostructures of magnetic metal/carbonalways rely on very harsh conditions, such as arc techniques,22

catalytic chemical vapor deposition,23 magnetron and ion-beamcosputtering, and high-temperature annealing.24 The intrinsichigh-energy consumption and expensive hardware of thesetechniques are mainly responsible for the high cost of manu-facturing magnetic nanoparticles encapsulated in carbon and thuslimit their practical applications. Very recently, the magneticand microstructural properties of antiferromagnetic MnO nano-particles with ferrimagnetic Mn3O4 shells has been studied.25

Si et al. observed large coercivity for Mn3O4/MnO nanopar-ticles.26 Among them, MnO, Mn2O3, and Mn3O4 have a widerange of applications in catalysis and battery technologies.27

Here, we report the synthesis of manganese oxide nanopar-ticles embedded in carbon using a single-component precursor.The advantage of the present method is that the nanoparticleembedded in carbon is achieved in a single step. To the best ofour knowledge, manganese oxide nanoparticles embedded incarbon was not reported so far. The as-synthesized productswere characterized with various physicochemical techniques.The product consists of Mn3O4/MnOOH nanoparticles with anaverage size of 9 nm, and the particles are embedded in carbon,forming sheetlike structures in two-dimensional fashions. Byvarying the reaction temperature, we were able to synthesizemanganese oxide nanoparticle without any amorphous carbon.Another aspect of the paper is the as prepared nanoparticlesexhibit higher coercive fields when compared to the reportedvalues.

* To whom correspondence should be addressed. E-mail: [email protected].

J. Phys. Chem. C 2008, 112, 15752–1575815752

10.1021/jp806175y CCC: $40.75 2008 American Chemical SocietyPublished on Web 09/13/2008

2. Experimental Section

Preparation of the Precursor. The precursor cetyltrimethy-lammonium permanganate was prepared by using an aqueoussolution of potassium permanganate (KMnO4, 0.01 M) andcetyltrimethylammomium bromide (CTAB, 0.01 M). The ratiobetween the cation and anion is 1:1. An aqueous solution ofCTAB was added drop by drop to the KMnO4 solution withvigorous stirring.28 A purple gel was formed and was aged inair overnight, filtered, and washed with water several times. TheC, H, N analysis reveals that the ratio between the cation toanion is 1. The theoretical carbon content in the starting materialis 56.6 wt %the, and observed carbon content is 54.2 wt %.

The synthesis of Mn3O4 structures has been carried out in asingle-stage furnace with precisely controlled temperatures. Finedry powder of cetyltrimethylammonium permanganate was usedas the single-component precursor. For a typical synthesis, theprecursor was directly placed in a quartz boat and kept at thecenter of a quartz tube, which was placed inside a tubularfurnace. The temperature was raised 20 °C/min in the presenceof 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 as-synthesized product obtained in the quartz boat was used forcharacterization and magnetic studies. The yield of brownish-black product is 0.16 g which corresponds to 55% relative tothe starting material. Similar experiments were carried out atdifferent temperatures (500, and 600 °C) and different durationperiods. The carbon content in the products was determined byusing C, H, and N elemental analysis. A comparison of productmorphology, reaction parameters, and carbon content is pre-sented in Table 1.

Structural Characterization. The particle morphology wasstudied with transmission electron microscopy on a JEOL-JEM100 SX microscope, working at 80 kV accelerating voltage,and a JEOL-2010 high-resolution transmission electron micros-copy (HRTEM) instrument with an accelerating voltage of 200kV. Samples for TEM and HRTEM were prepared by ultrasoni-cally dispersing the products into absolute ethanol, then placinga drop of this suspension onto a copper grid coated with an

amorphous carbon film, and then drying under air. High-resolution scanning electron microscopy (HRSEM) of theobtained product was carried out on a JEOL-JSM 840 scanningelectron microscope operating at 10 kV. The X-ray diffractionmeasurements were carried out with a Bruker AXSD Advancepowder X-ray diffractometer with a Cu KR (λ ) 1.5418 Å)radiation source. The diffraction measurements were collectedfrom 20 to 80° at a speed of 1.2°/min. The elemental analysisof the sample was carried out by an Eager C, H, N, S analyzer.An Olympus BX41 (Jobin Yvon Horiba) Raman spectrometerwas employed, using the 514.5 nm line of an Ar laser as theexcitation source to analyze the nature of the carbon present inthe products. The X-ray photoelectron spectroscopy (XPS)measurements was carried out using JEOL JPS-900, in anultrahigh-vacuum (UHV), axis HS monochromatized Mg KRcathode source, at 75-150 W, using a low-energy electron floodgun for charge neutralization. Survey and high-resolutionindividual 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 powdersamples were analyzed with a Quantum Design MPMS-7.Detailed magnetic measurements, zero-field-cooled (ZFC) andfield-cooled (FC) magnetization vs temperature under field, andmagnetic hysteresis loops at several temperatures have beencarried out in order to study the magnetic properties of the as-synthesized manganese oxide nanoparticles. The saturation

Figure 1. (a, b) TEM images of product, showing the sheetlike morphology and consist of nanoparticles; (c) HRTEM image, showing latticefringes 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) C (wt %) product morphology size of crystals (nm) M (emu/g) coercivity (kOe)

1 400 °C, 3 h, argon 13.4 sheetlike 9 15 10.52 500 °C, 3 h, argon 0.05 chains with sperhical spheres 100-250 42 10.143 600 °C, 3 h, argon chains with spherical spheres 200-400 46 11.42

Figure 2. EDAX spectrum of the product obtained at 400 °C.

Manganese Oxide Particles Embedded in Carbon J. Phys. Chem. C, Vol. 112, No. 40, 2008 15753

magnetization and coercivity filed values were obtained fromthe hysteresis loops measured up to to a field of 6 T at 300, 10,

and 5 K. The temperature dependence of the magnetization wasmonitored by ZFC and FC experiments. The ZFC curve wasgenerated by first cooling the system in a zero field. Then thefield was applied (100 Oe), and magnetization was measuredwhile the temperature was increased to 300 K. The FC curvewas obtained in a similar way except that the sample was cooledin an applied field of 100 Oe.

3. Results and Discussion

The morphology of the product synthesized at 400 °C isshown in Figure 1. A typical TEM image shows that the productexhibits sheetlike shape. A closer look at the sheet indicatesthat the sheet consists of spherical-shaped nanoparticles. TheTEM image shows that the nanoparticles are embedded incarbon, which is highlighted with an arrow in Figure 1b. Theaverage size of the particles is 9 ( 1 nm. The highermagnification of the image shows lattice fringes, indicating theparticles are crystalline (Figure 1c). The EDAX analysis of thesheet shows the presence of Mn, O, and C without any otherimpurities (Figure 2). The carbon content in the product wasdetermined by using C, H, and N elemental analysis. Thecontents of C, H, and N were found to be 13.4, 0.6, and 0.6 wt%, respectively. The HRTEM image of the product indicatesthat the particles are arranged in such a way that the particlesare in close contact with other particles (Figure 3a). The well-resolved lattice fringes with a d-spacing value of 0.491 nmcorrespond to the (101) plane of cubic Mn3O4 (JCPDS 024-0734). The image also shows particles with a lattice distanceof 0.378 nm, which is corresponding to the γ-MnOOH (110)plane. From Figure 3b, it is clear that the disordered carboncoating on the Mn3O4 particle is evidenced (highlighted witharrows). The thickness of the carbon coating is around 4 nm.Figure 3b also reveals that the nature of carbon is disorderedlayers, corresponding to the nongraphitic, coallike lattice planesof the carbon, as the thermolysis reaction was carried out at400 °C.

The nature and type of carbon present in the product isanalyzed by Raman spectroscopy. The product exhibits twobroad peaks at 1328 and 1602 cm-1 (Figure 4). The band at1328 cm-1 corresponds to the D peak arising from the breathingmotion 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

Figure 3. (a) HRTEM image, showing lattice fringes of Mn3O4 andMnOOH. (b) HRTEM image, depicting amorphous carbon on thesurface of manganese oxide particles. The dotted arrows show thecarbon layers.

Figure 4. Raman spectrum of product obtained at 400 °C, showingthe presence of disordered graphitic carbon.

Figure 5. XRD pattern of the product obtained at 400 °C.

15754 J. Phys. Chem. C, Vol. 112, No. 40, 2008 Shanmugam and Gedanken

the nature of carbon.29 The measured ID/IG ratio is found to be1.1, suggesting that the carbon exists in a more disorderedgraphitic form (amorphous). A typical XRD pattern of theproduct is shown in Figure 5. The XRD peaks can be indexedto the tetragonal Hausmannite phase of Mn3O4 with a ) 0.576nm and c ) 0.946 nm in accordance with JCPDS No.24-0734.The XRD pattern also shows additional patterns, which cor-respond to the γ-MnOOH phase.

The composition and oxidation state of the product preparedat 400 °C was further analysized by the XPS spectroscopy. TheXPS survey spectrum shows the sheet consists of Mn, O, andC 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 splittingis the difference between BE values of Mn 2p3/2 and Mn 2p1/2

levels. The observed spin-orbit splitting is 11.7 eV, same asin manganese oxides. 30 The BE of the Mn 2p3/2 (641.0 eV)and spin-orbit splitting (11.7) is well-matched with the reportedvalue of Mn3O4. The discrimination between MnOOH andMn3O4 is difficult from the Mn 2p values, because, in both cases,manganese exists in the +3 oxidation state. Figure 6c shows abroad asymmetric peak, and the peak has been deconvoluted

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 underan applied field of 100 Oe.


into three symmetric peaks, C1, C2, and C3 (284.2, 286.4, and288.0 eV). The peak C1 (284.4 eV) originates from the fromC-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 carbonatoms bonded with one or two oxygen atoms, respectively.31

When the electronegative oxygen atoms are bonded to thecarbon, a positive charge is induced on the carbon atom. Hence,they can be assigned to alcoholic, ether (C-O) and ketone, oraldehyde (>CdO). These observations suggest that the carbonpresent in the product is amorphous carbon. The oxygen spectraalso give two peaks (O1 and O2) after a deconvulation (Figure6d). 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) at531.1 eV can be considered to pertain to oxygen in OH groupspresent in MnOOH. The Mn/O2- atomic ratio is in agreementwith the theoretical values calculated from the bulk composition.

The magnetic studies of Mn3O4 nanoparticles embedded inamorphous carbon were investigated using superconductingquantum interference devices (SQUID) at 300, 10, and 2 K.The hysteresis loops measured at different temperatures areshown in Figure 7a. The room-temperature magnetization studyshows the Mn3O4 is paramagnetic, and at low (42 K) temper-ature Mn3O4 exhibits ferromagnetic behavior. The saturatedmagnetization value is 15 emu/g, and remanence is 5.5 emu/gat 10 K. The remanence ratio (Mr/Ms) of Mn3O4 nanoparticlesis found to be 0.36. The interesting features of hysteresis loopsare as follows: (i) The sample does not saturate at a magneticfield of 5 T, which means the particles have a large anisotropyfield, and the anisotropy is directly related to the saturation;hence, the unsaturation of the hysteresis loop implies thepresence of ferromagnetic or antiferromagnetic fractions at lowtemp. (ii) The coercive field Hc (HR + HL)/2) of the product isvery large at 10 K (6.5 kOe), much higher than the reported

value for nanocrystalline Mn3O4 particles, and also at 2 K thecoercivity found to be 10.5 kOe. Buckelew et al. observed 8.8kOe coercivity at 2 K for Mn3O4 particles synthesized byhydrolysis of K2[Mn2(CN)6].33 We have thus obtained a largecoercivity, which is much larger than the values of 2.8 kOe forbulk samples,34 3.5 kOe for thin films,35 and 5.7 kOe fornanowires.36 To avoid dynamic coercivity, we have collectedthe hysteresis very slowly. Another important observation isthat the hysteresis loop shifts in both horizontal and verticaldirections even in the absence of cooling field. The horizontalhysteresis loop shift at 10 and 2 K are 1.0 and 4.8 kOe,respectively, in the negative direction in the field axis. The originof such high coercivity may be ascribed to the effect of shapeanisotropy of Mn3O4 nanoparticles and also due to the interfacialinteraction between the antiferromagnetic (γ-MnOOH) andferromagnetic (Mn3O4) phases, which results in the hysteresisloops shift. In our product, the nanoparticles are fixed inamorphous carbon, and therefore preferential orientation of themagnetic easy axis could also possible.

In our study, the presence of second-phase MnOOH antifer-romagnetic components could be responsible for such verticaland horizontal loop shifts. In fact, due to the large anisotropyof Mn3O4, KMn3O4 ) 1.4 × 106 erg/cm3 is expected to displaya small exchange bias that is only observable for very smallnanoparticles.34 Larger anisotropy results in greater exchangebias effect, since if a system has larger anisotropy, Mn3O4 willhave greater pinning effect over the other phase with lesseranisotropy, which creates difficulty in magnetization reversaland hence larger exchange bias.

The temperature-dependent magnetization studies measuredunder zero-field-cooled and field-cooled processes from 5 to300 K in a 100 Oe probe field are shown in Figure 7b. At roomtemperature Mn3O4 exhibits paramagnetic and ferromagnetic

Figure 8. SEM images of product obtained at (a) 500 and (b) 600 °C; (c,d) TEM images of product formed at 600 °C.


nature at low temperature (about 42 K). In our product, fromthe ZFC curve (filled symbols), an apparent transition fromparamagnetic to ferromagnetic behavior was observed at 41.5K, which is known to be the blocking temperature of Mn3O4.The observed blocking temperature is consistent with theliterature values.37

The thermolysis temperatures play an important role in themorphology of the product. We carried out the reactions at 500and 600 °C, and the XRD results indicate that as the reactiontemperature increases, the second-phase intensity, γ-MnOOH,decreased. The morphology of these products was measuredwith SEM and TEM, and the results are shown in Figure 8.The products obtained at 500 and 600 °C exhibit the particleswith spherical shape without any carbon on the surface. Whenthe reaction temperature increases, the particle size alsoincreases, giving rise to bigger particles sizes. It can be seenfrom the image that the product consists of spherical spheresforming chains. The size of the individual sphere is about 100nm. The length of the chains varies from 2 to 5 µm (Figure 8).The fusion of adjacent spheres is manifested as sharing thesubstructure. The linear alignment of several subunits wasevident inside some microspheres. The further association ofspheres led to higher levels of hierarchical structures. A typicalTEM image of microsphere chains is shown in Figure 8c. It isclear from the image that the joining of adjacent microspheres

is through the fusion process. The HRTEM image of the sphereis shows well-resolved fringes with a d-spacing value of 0.265nm, corresponding to the (303) plane of cubic Mn3O4 (Figure8d).

To compare the magnetic properties of product obtained at400 °C, we prepared manganese oxide nanoparticles withoutcarbon; this was accomplished by increasing the reactiontemperature. It was observed that when the reaction was carriedout at higher temperatures (500 and 600 °C), the resultingproducts 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 Table1. The saturation magnetization values at 2 K are 42 and 46emu/g for 500 and 600 °C, respectively (Figure 9a). The highmagnetization values observed for Mn3O4 particles obtained at500 and 600 °C could be attributed to the large particle sizeand the shape of the products when compared to Mn3O4-400,where the particle size is 9 nm, and also to high coercivities of10.1 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, inthe negative direction on the field axis at 2 K. The blockingtemperature of the product obtained at 500 °C is found to be42 K (Figure 9b).

4. Conclusions

In summary, by a one-step solid-state thermolysis of thecetyltrimethylammonium permanganate, we have successfullysynthesized manganese oxide nanoparticles embedded in amor-phous carbon. The formation of such Mn3O4-MnOOH nano-particles embeded in carbon was derived by the presence of anorganic structure-directing agent. The product mainly consistsof Mn3O4 along with γ-MnOOH nanoparticles embedded inamorphous carbon. The average size of the particle is 9 nm forthe sample prepared at 400 °C. The products obtained at 500and 600 °C exhibit particles with spherical shape without anycarbon around, and the sizes of the particles are in the range of200-400 nm. The magnetic properties of Mn3O4-MnOOHnanoparticles exhibit a coercivity value of 10.5 kOe, and theblocking temperature is 41.5 K. These nanoparticles showedloop shift (exchange bias) due to the coupling of the weaklyanisotropic MnOOH antiferromagnetic with highly anisotropicMn3O4 ferromagnetic particles. The magnetization values of theproducts obtained at 500 and 600 °C are in the range of 42-45emu/g.

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Figure 9. (a) Temperature-dependent hysteresis loops of manganeseoxide synthesized at 500 °C. The inset shows the variation of coercivity(Hc) vs temperature (T). (b) ZFC and FC magnetization curves underan applied field of 100 Oe.


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