3614
J. Phys. Chem. C 2007, 111, 3614-3623
Nonaqueous Synthesis of Manganese Oxide Nanoparticles, Structural Characterization, and Magnetic Properties Igor Djerdj,*,†,§ Denis Arcˇ on,‡ Zvonko Jaglicˇ ic´ ,⊥ and Markus Niederberger*,†,# Max Planck Institute of Colloids and Interfaces, Colloid Chemistry, Research Campus Golm, 14424 Potsdam, Germany, Institute “Jozˇef Stefan”, JamoVa 39, 1000 Ljubljana, SloVenia, Department of Physics, Faculty of Science, Bijenicˇ ka 32, 10000 Zagreb, Croatia, Institute of Mathematics, Physics and Mechanics, Jadranska 19, 1000 Ljubljana, SloVenia, and ETH Zu¨rich, Department of Materials, Wolfgang-Pauli-Strasse 10, 8093 Zu¨rich, Switzerland ReceiVed: NoVember 6, 2006; In Final Form: January 4, 2007
The synthesis, structural characterization, and magnetic properties of crystalline manganese oxide nanoparticles are presented. The procedure is based on the reaction of benzyl alcohol with the two precursors: potassium permanganate KMnO4 and manganese(II) acetylacetonate Mn(acac)2. Depending on the precursor used, the composition of the final product can be varied in such a way that in the case of KMnO4 mainly Mn3O4 is formed, whereas Mn(acac)2 leads predominantly to MnO. Rietveld refinement of the XRD powder patterns, high-resolution transmission electron microscopy (HRTEM), selected area electron diffraction (SAED), and energy-dispersive X-ray (EDX) analysis, as well as electron energy loss spectroscopy (EELS) were employed for the structural characterization of the as-synthesized compounds. Especially the MnO manganosite nanocrystals exhibit some interesting features. HRTEM investigations point to the formation of a superstructure, which can be described as an ordered Mn vacancy cubic superstructure with the general formula of Mn0.875Ox and a lattice parameter of 8.888 Å. The SQUID measurement proves a superparamagnetic behavior of the MnO nanoparticles.
Introduction Manganese oxides are important materials in many applications such as catalysis, electrodes, high-density magnetic storage media, ion exchangers, sensors, molecular adsorption, and electronics.1-4 Particularly Mn3O4 (hausmannite) is known to be an efficient catalyst in the decomposition of waste gas NOx, reduction of nitrobenzene, and oxidation of methane.5 It has been widely used for the preparation of Li-Mn-O electrodes for rechargeable lithium batteries and for soft magnetic materials such as manganese zinc ferrite, which is applicable as magnetic cores in transformers for power supplies.6 Magnetic properties drastically change when the particle size becomes comparable with the characteristic length of the magnetic interaction or the length of spin diffusion. This means that nanoscale materials can have magnetic properties that are considerably different from those of the bulk form. Bulk Mn3O4 undergoes the ferrimagnetic transition at a Curie temperature of TC ) 42 K, but the nanoparticles exhibit a size-dependent TC. It was found, for instance, that Mn3O4 nanoparticles with diameters of 6, 10, and 15 nm show a TC ) 36, 40, and 41 K, respectively.2 Nanocrystalline MnO, on the other hand, is ferromagnetic, although its bulk counterpart is antiferromagnetic with a Nee´l temperature of TN ) 122 K or TN ) 118 K.8,9 At temperatures above TN the MnO lattice has the cubic NaCltype structure. The antiferromagnetic transition at TN is ac* Authors to whom correspondence should be addressed. E-mail:
[email protected] (I.D.);
[email protected] (M.N.). † Max Planck Institute of Colloids and Interfaces. ‡ Institute “Joz ˇ ef Stefan”. § Department of Physics, Faculty of Science. ⊥ Institute of Mathematics, Physics and Mechanics. # ETH Zu ¨ rich.
companied by a cubic-to-rhombohedral lattice distortion and is driven by nearest-neighbor and next-nearest-neighbor antiferromagnetic interactions.9 The magnetic moments on the Mn atoms align within the (111) planes, forming ferromagnetic (111)-type sheets that stack antiferromagnetically along the [111] axis. Nanocrystalline manganese oxides MnO and Mn3O4 were synthesized with various methods from different precursors. Several synthesis protocols have been reported so far. Thermal evaporation of MnCl2 powders resulted in MnO and Mn3O4 nanowires.7 The thermal decomposition route was followed in the synthesis of uniform-sized MnO nanospheres (5-40 nm in diameter) and nanorods (7-10 nm in diameter and 30-140 nm in length) from Mn-surfactant complexes.10 Yang et al. reported the synthesis of Mn3O4 polyhedral nanocrystals employing the microwave-assisted solution-based method starting from Mn(CH3COO)2 and (CH2)6N4 at 80 °C.5 A two-step route to Mn3O4 nanocrystallites has been reported, starting from the hydrothermal reaction between KMnO4 and toluene in water, which yielded γ-MnOOH nanowires. In the subsequent step the solvothermal treatment of γ-MnOOH in ethylenediamine and ethylene glycol at 150 °C was performed, leading to the formation of Mn3O4 nanocrystallites.6 To conclude this short literature overview about different synthesis approaches to MnO and/or Mn3O4 nanoparticles, recently the preparation of MnO nanohexapods was reported.11-13 In the past few years we demonstrated the versatility of surfactant-free synthesis routes to nanocrystalline metal oxides in organic solvents.14-17 We found that in general the nonaqueous synthesis of nanoparticles seems to provide better control over particle size, shape, crystallinity, and surface properties in comparison to aqueous media.17 In this paper we extend the
10.1021/jp067302t CCC: $37.00 © 2007 American Chemical Society Published on Web 02/10/2007
Nanocrystalline Manganese Oxides applicability of the benzyl alcohol route to another important class of nanomaterials, namely, MnO and Mn3O4. In addition to the synthesis approach, we present a thorough structural characterization together with a detailed study of the magnetic properties. Clear experimental evidence has been found for a superstructure in MnO nanoparticles, which was modeled as Mn vacancy ordering. Experimental Details Materials. Manganese(II) acetylacetonate (99.99+%), potassium permanganate (99.99+%), and anhydrous benzyl alcohol (99.8%) were obtained from Aldrich and used as received. The solvothermal treatment was performed in Parr acid digestion bombs with 45 mL Teflon cups. Synthesis. All synthesis procedures were carried out in a glovebox (O2 and H2O < 0.1 ppm). The sample, which contains Mn3O4 as the dominant phase, was synthesized in the way that 158 mg of KMnO4 (1 mmol) was added to 20 mL of benzyl alcohol. The other sample, which contains MnO as the dominant phase, was prepared by adding 253 mg of Mn(acac)2 (1 mmol) to benzyl alcohol. In both cases, the reaction mixture was transferred into a Teflon cup of 45 mL inner volume, slid into a steel autoclave, and carefully sealed. The autoclave was taken out of the glovebox and heated in a furnace at 200 °C for 2 days. The resulting brown suspensions were centrifuged in order to separate the precipitate from the mother liquid. Excess organic impurities were removed by repeated washing steps in 10 mL of high-purity ethanol and subsequently dried in air at 60 °C. Characterization. X-ray powder diffraction (XRD) patterns were measured in reflection mode with Cu KR radiation on an X’Pert PRO diffractometer (PANalytical manufacturer) equipped with a X’Celerator detector. The patterns were measured in the 2θ range from 10° to 110° with a scanning step of 0.016° and a fixed counting time of 1000 s. The instrumental contribution to the peak broadening caused by instrumental aberrations was removed by the deconvolution method with highly crystalline NIST SRM 660a LaB6 powdered sample as a standard. Transmission electron microscopy (TEM) measurement was performed on a Zeiss EM 912Ω instrument at an acceleration voltage of 120 kV, while high-resolution transmission electron microscopy (HRTEM) characterization was done using a Philips CM200-FEG microscope (200 kV, Cs ) 1.35 mm) equipped with a Gatan energy filter for electron energy loss spectroscopy (EELS) measurements. The HRTEM image simulation was done using the JEMS software.18 EELS spectra were recorded in an image mode with a dispersion of 0.2 eV per channel from sufficiently thin regions of the sample in order to minimize the multiple loss contributions. The energy resolution, estimated from the full width at half-maximum of the zero-loss peak, was 1 eV. The EELS spectra of the samples were recorded at a collection angle of 8 mrad. The recorded core loss spectra have been subjected to deconvolution using the Fourier ratio method with low-loss spectra recorded in equivalent conditions in order to reduce multiple loss events. Finally, the backgrounds under the edges were subtracted using the standard AE-r background model. The samples for TEM characterization were prepared in a way that one drop of the dispersion of as-synthesized powder in ethanol was deposited onto a copper grid covered by an amorphous carbon film. To prevent agglomeration of nanoparticles the copper grid was put on a filter paper at the bottom of a Petri dish. The recorded XRD powder patterns were subsequently processed with the Rietveld method19 using the program FULLPROF.20 On the basis of the choice of the structural model
J. Phys. Chem. C, Vol. 111, No. 9, 2007 3615 as well as the type of profile function, the program simulates the XRD powder patterns and compares them with experimental ones in the least-squares comparison mode. Thus, a certain number of least-squares structural and microstructural parameters were refined. In this refinement, the scale factor, the background coefficients, the zero point of the detector, and the unit cell parameters were simultaneously refined, followed by the refinement of the Gaussian half-width parameters, U, V, W, and the Lorentzian half-width parameters, X, Y. These parameters define the diffraction profile function, which was chosen to be the modified Thompson-Cox-Hastings pseudo-Voigt (T-C-H pV),21 making the size-strain analysis straightforward. In our approach we assumed that the line broadening of the deconvoluted profile occurred due to the small crystallite size and the presence of lattice microstrain, and thereby the values of half-width parameters V, W, and X were kept constant at instrumental values determined by the NIST SRM 660a LaB6 standard. The background was taken to be the polynomial function of 2θ of the fourth order, because only in this case was the best background modeling obtained. The quality of the Rietveld refinement was evaluated in terms of the discrepancy factor (profile-weighted residual error), Rwp, Bragg discrepancy factor RB, and the goodness-of-fit indicator, GOF. The magnetization at applied field was measured with a commercial Quantum Design SQUID magnetometer, equipped with a 5 T superconducting magnet. Zero-field and field-cooled runs were performed between room temperature and 2 K in static magnetic fields of 0.01, 0.1, and 1 T. EPR experiments were performed with a Bruker E580 spectrometer operating in X-band (9.63 GHz). Small changes in the Q-factor of the cavity were corrected with a reference sample in the second cavity. The Oxford Instruments continuous flow cryostat EPR900 combined with a temperature controller ITC 503 was used, and temperature stability was better than ( 0.1 K over the entire temperature range. Results and Discussion The crystallinity and phase composition of the final products were investigated by X-ray powder diffraction (XRD). The structure of the manganese oxides MnO and Mn3O4 was refined by the Rietveld method applied on the XRD powder patterns, enabling the calculation of unit cell parameters, fractional atomic coordinates, average crystallite size, and microstrain. Figure 1 shows the typical Rietveld refinement output plot of the sample synthesized from Mn(acac)2 as the precursor (a) and from KMnO4 (b). Concerning Figure 1a all strong peaks are undoubtedly indexed to a face-centered cubic MnO phase (mineralogical name manganosite), which is isostructural with NaCl. However, careful inspection of the asymmetric necks of the 111 and 220 peaks at 2θ ) 34.91° and 58.72° of manganosite indicates the presence of another phase, which can be assigned to tetragonal Mn3O4, mineralogical name hausmannite. Accordingly, the sample synthesized from Mn(acac)2 as the precursor is not phase pure but a mixture of MnO (manganosite) as the dominant phase and Mn3O4 (hausmannite) as the minor component. This result is rather surprising, as benzyl alcohol is well-known to be a reductive medium in the synthesis of metal oxide nanoparticles. However, in this case Mn2+ is partially oxidized to Mn3+ yielding (Mn2+1/3Mn3+2/3)3O4 with a mixed valence. In contrast to that, the reductive influence of benzyl alcohol is obvious in the case of KMnO4 as the precursor. According to the XRD pattern in Figure 1b, both oxides are again present, but this time with Mn3O4 as dominant phase. Already a simple comparison of the intensity of the strongest lines, the 211 reflection at 2θ
3616 J. Phys. Chem. C, Vol. 111, No. 9, 2007
Djerdj et al. ( 0.8) wt % of Mn3O4. No peaks from other Mn-O phases or other byproducts originating from the synthesis procedure were detected, indicating a high purity of the manganese oxides obtained by this benzyl alcohol route. The results of Rietveld refinement of the XRD powder patterns are summarized in Table 1. The lattice parameter values of MnO-manganosite and Mn3O4-hausmannite agree well with the reported data for the bulk crystals: a0 ) 4.445 Å (ICDD PDF no. 7-230) and a0 ) 5.7621 Å, c0 ) 9.4696 Å (ICDD PDF no. 24-734), respectively. Besides instrumental broadening of the diffraction profiles, which was taken into account in the deconvolution procedure, we assumed that the main sources of line broadening are a small crystallite size and lattice microstrain. With the use of the same procedure as described elsewhere, the average crystallite size and microstrain of MnO and Mn3O4 were calculated and are given in Table 1.24 Usually, the average crystallite sizes are expressed in terms of different directions in reciprocal space. However, in the present case the line broadening is hkl independent (isotropic), making the labeling of the average crystallite size with hkl superfluous. It is also important to stress that the crystallite size calculated in this way is the volumeweighted average crystallite size. The isotropic broadening of the diffraction lines points to a spherical crystallites shape. The average crystallite size of MnO equals 21.5(1) nm, which is considerably larger than that of Mn3O4 (3.4(3) nm) in the sample synthesized from Mn(acac)2. The opposite holds for the sample prepared from KMnO4, where the average crystallite size of Mn3O4 is larger than the one of MnO, but the difference in sizes is not so pronounced. Regarding lattice microstrain, their appreciable values point to the presence of lattice defects in the nanoparticles. In both samples the dominate components have the same microstrain value of 0.12%, while for the minor components the microstrain values are strongly related to their crystallite size, i.e., the smaller the cystallites, the larger the microstrain. This finding is in accordance with the general rule that the concentration of defects accommodated in nanoparticles is related to their large surface-to-volume ratio, which is scaled with the particle size. The quality of fit is estimated in terms of R-values, Rwp and RB, goodness-of-fit indicator GOF (Table 1), and in the visual appearance of the difference curve. According to all of these judgment criteria Rietveld refinement was successfully applied. Representative TEM images of the manganese oxide nanoparticles are displayed in Figure 2a (MnO as dominant phase) and Figure 5a (Mn3O4 as dominant phase), clearly proving that
Figure 1. XRD data and Rietveld refinement plots of the two manganese oxide samples. (a) MnO (dominant phase) + Mn3O4 (minor phase) synthesized from Mn(acac)2. (b) Mn3O4 (dominant phase) + MnO (minor phase) synthesized from KMnO4. The experimental data is shown in red, the calculated patterns in black, and the difference curves in blue. The short vertical bars in green represent the positions of the Bragg reflections of MnO and Mn3O4.
) 36.08° of Mn3O4 and the 200 reflection at 2θ ) 40.54° of MnO, clearly supports this finding. The intensity of the Mn3O4 peak is much higher than the one of MnO, implying a considerably higher content of Mn3O4 in the product. According to the adopted procedure of Hill and Howard, the phase composition (in wt %) was calculated.22,23 The sample synthesized from Mn(acac)2 consists of (79.2 ( 1.2) wt % of MnO and (20.8 ( 0.6) wt % of Mn3O4, while the sample originated from KMnO4 comprises (13.4 ( 0.4) wt % of MnO and (86.6
TABLE 1: Crystallographic Data and Refined Values of the Structural Parameters for the Mn-O System Calculated by Rietveld Refinement of the XRD Powder Patterns MnO(d) + Mn3O4(m)
sample phase space group lattice parameter (Å) cell volume (Å3) Mn site x y z O site x y z isotropic ave crystallite size (nm) average maximum microstrain e (×104) phase composition (wt %) RB (%) Rwp (%) GOF
MnO manganosite Fm-3m (225) a ) 4.444(1)
Mn3O4(d) + MnO(m) MnO manganosite Fm-3m (225) a ) 4.430(1)
0 0 0
Mn3O4 hausmannite I41/amd (141) a ) 5.788(1) c ) 9.393(3) 314.7(1) Mn1 Mn2 0 0 0.5 0.25 0.5 0.875
0 0 0
Mn3O4 hausmannite I41/amd (141) a ) 5.764(4) c ) 9.462(1) 314.36(4) Mn1 Mn2 0 0 0.5 0.25 0.5 0.875
0.5 0.5 0.5 21.5(1) 12.2(3) 79.2 ( 1.2 5.1
0 0.474(3) 0.254(1) 3.4(3) 85.9(2) 20.8 ( 0.6 11.3
0.5 0.5 0.5 20.1(3) 52.7(1) 13.4 ( 0.4 17.4
0 0.473(1) 0.257(1) 25.4(5) 12.2(4) 86.6 ( 0.8 18.0
87.765(4)
8.5 1.2
86.94(2)
3.5 1.0
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Figure 2. (a) TEM overview of MnO nanoparticles; (b) corresponding SAED pattern with two well-resolved and indexed reflections of Mn3O4; (c) HRTEM image of a part of a single-crystalline MnO nanoparticle; (d) corresponding power spectrum of part c; (e) fully indexed power spectrum; (f) corresponding SAED pattern with the two principal reflections (111, 200) and the superlattice reflection (0.5 0.5 0.5); (g) Fourier-filtered image of part c taken with reflections from its power spectrum (arrows denote the lattice fringes corresponding to the superstructure); (h) simulated image based on the model shown in Figure 3; (i) EDX spectrum of MnO nanoparticles.
the obtained samples are composed of well-defined nanoparticles with a low degree of agglomeration. This finding is remarkable taking into account that no surfactant was used during the synthesis. However, in the present case obviously benzyl alcohol or its condensation products act as stabilizing agent. Concerning the shape of the Mn3O4 nanoparticles (Figure 5a) various morphologies were observed including platelike forms, spherules, ellipsoids, cubes, and rhombohedra. On the other hand, the shapes in the case of MnO are more uniform with nanoplatelets as the main species. The particle diameters are in the range from 12 to 88 nm with the average value of 37 nm for the MnO-dominated compound, and from 15 to 69 nm (average value of 32 nm) for the Mn3O4-dominated product. The average particle diameter was estimated by measuring 100 particles in magnified TEM images. Comparison of the average (volume-weighted) crystallite size calculated from XRD with the average diameters extracted from TEM images reveals that the nanoparticles are as a matter of fact composed of single crystals in the nanometric size region. The small discrepancy between the average values, for instance, in the case of Mn3O4 the average crystallite size is 25.4 nm and the average particle diameter equals 32 nm, can be attributed to the limited statistical assembly for TEM (100 particles) introducing higher uncertainties in the calculation of the average values.
Figure 3. Atomic model of the Mn0.875Ox superstructure viewed along [010] (manganese atoms, purple; oxygen atoms, red; Mn vacancies, yellow). The projection of the superstructural unit cell is denoted by the white square.
The phase composition deduced from XRD analysis is further confirmed by SAED. For the Mn3O4-dominated sample the situation is quite clear, because the reflections of both compounds are well resolved in the XRD pattern. However, in the MnO-dominated sample the 101 reflection of Mn3O4 (2θ )
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Djerdj et al.
Figure 4. (a) [1-12] HRTEM image of a single-crystalline MnO nanoparticle; (b) corresponding power spectrum of part a; (c) Fourierfiltered image of part a taken with reflections from its power spectrum; (d) simulated image based on the model shown in Figure 3.
17.98°) is hardly visible. Therefore, SAED is additionally applied to prove the presence of Mn3O4. As a matter of fact, in the SAED pattern displayed in Figure 2b, the 101 and 112 reflections of Mn3O4 are observed. These two reflections do not coincide with any reflection of MnO, corroborating the twophase character of the MnO sample. Further insight into the microstructure of MnO and Mn3O4 nanocrystallites was achieved by HRTEM measurements. Figure 2c shows the HRTEM image of a part of one nanoparticle of the MnO-dominated sample. According to the corresponding power spectrum (Figure 2d) and the selected area electron diffraction (SAED) pattern (Figure 2f), the isolated nanoparticle is a single crystal. The observed lattice spacing of 2.581 Å, which belongs to the spot marked with A in Figure 2d, corresponds to the (111) planes of fcc MnO. However, this HRTEM image together with its power spectrum and corresponding SAED pattern shows a peculiar feature associated with the spot B in the power spectrum. The corresponding d-value equals 5.078 Å, which is exactly the double of spot A. This reflection can be indexed only with rational Miller indices 0.5 0.5 0.5. Moreover, the whole power spectrum as well as the corresponding SAED pattern is indexed in terms of rational indices (Figure 2e). Such a configuration of spots in the power spectrum is characteristic of the [0-11] zone axis orientation of the MnO nanocrystal displayed. The indexing of the power spectrum and the SAED pattern with rational numbers points to a superstructure. Besides the fundamental fcc reflections indexed with integers, the spots close to the transmitted beam circle in the SAED pattern are then superlattice spots. A careful look at the Fourier-filtered HRTEM image in Figure 2g clearly shows arrays of spots. One bright line of spots (denoted by arrowheads) alternates with a less bright one. The distance between two lines of spots with the same brightness is exactly the double (5.078 Å) of the distance between arrays of brightless bright spots (2.581 Å). Such a regular arrangement of double arrays of spots found in the HRTEM image is characteristic for the presence of a superstructure. The appearance of a superstructure in the MnO nanocrystals is thus confirmed in direct
Figure 5. (a) TEM overview of Mn3O4 nanoparticles; (b) HRTEM image of a part of a single-crystalline nanoparticle; (c) corresponding power spectrum of part b with the two principal reflections (011, 200); (d) corresponding SAED pattern; (e) EDX spectrum of Mn3O4 nanoparticles.
(HRTEM) as well as in reciprocal (SAED) space. According to Figure 2e the assigned rational Miller indices can be turned into integers by simple multiplication by a factor of 2, implying that the new integer Miller indices, assigned to the spots in the power spectrum and in the SAED pattern of the MnO nanocrystal oriented in the [0-11] zone axis, describe the structure of MnO with a doubled value of the unit cell parameter, i.e., the lattice parameter of such a MnO supercell (superlattice, superstructure) equals 8.888 Å. The superstructures are more common for ordered alloys, intermetallics, and ternary oxides. In these cases below some temperature the constituent metallic elements tend to occupy specific regular positions, thus forming an ordered crystal superlattice. It is also well-documented that the modulated charge ordering and vacancy ordering result in superstructure formation.25,26 The unit cell of such a superlattice is composed of several (integer number) unit cells of the original parental structure. However, in this case there are no foreign atoms in the MnO structure as evidenced by EDX analysis. Characteristic EDX spectra are displayed in Figures 2i and 5e for the MnO- and Mn3O4-dominated samples, respectively. The elemental composition extracted from EDX (displayed in atomic fraction) of the MnO compound was found to be 46.1% Mn and 53.9% O, and 48.5% Mn and 51.5% O for Mn3O4. Although there is no proof for foreign atoms in the MnO system, some kind of ordering had to occur in order to detect the superstructure itself. One possibility constitutes the regular distribution of
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TABLE 2: Crystallographic Data for the Mn0.875Ox Superstructurea
a
formula
Mn0.875Ox
space group lattice parameter (Å) cell volume (Å3) Z
Fm-3m (225) a ) 8.888 702.121 32
label
site
x
y
z
0 Mn1 Mn2 O1 O2
4a 4b 24d 8c 24e
0 0.5 0 0.25 0.25
0 0 0.25 0.25 0
0 0 0.25 0.25 0
The vacant Mn Site is denoted by 0.
manganese or oxygen vacancies or interstitials in the MnO crystal matrix. It is unlikely that the oxygen vacancies are responsible for the superstructural ordering, because HRTEM is less sensitive to light atoms. Therefore, ordered Mn vacancies are responsible for the superstructure. With the use of an image simulation technique, HRTEM images supported this conclusion. According to SAED and FFT analysis of the HRTEM image 2c the superstructure is a cubic fcc structure with a lattice parameter that is the doubled value of the lattice parameter for bulk MnO. The superstructural model is visually displayed in Figure 3 in the projection along the b-axis. The manganese atoms are removed from the corners and centers of the faces in a simple doubled unit cell of MnO (yellow color), leaving behind an ordered system of Mn vacancies. A simple calculation shows that the Mn deficiency in such a model is 12.5%, i.e., the formula of the defective MnO structure is Mn0.875Ox. The crystallographic data for this model of the superstructure are summarized in Table 2. On the basis of the proposed vacancyordered superstructural model, the HRTEM image simulation was performed using the multislice method, and considering the experimental conditions of HRTEM imaging and specific zone axis. Taking the experimental parameters of CM200-FEG (Cs ) 1.35 mm, Cc ) 2 mm, ∆E ) 1 eV, ∆f ) 4 nm, objective aperture radius ) 10 nm-1) into account, the image is simulated at Scherzer defocus (-68 nm) and at the sample thickness of 2.5 nm (Figure 2h). The bright contrast in the simulated image corresponds to the Mn vacancy column, because in HRTEM imaging the atom deficiency columns leave the signature as a bright contrast. The visual inspection of the simulated image shows that it nicely matches with the Fourier-filtered HRTEM image of a MnO nanocrystal oriented in the [0-11] zone (Figure 2g). In this way, the HRTEM measurement corroborates the validity of the proposed superstructure model based on Mn vacancies ordering. To prove that this example is not an exceptional case, another HRTEM image with [1-12] orientation of the MnO crystal with respect to the electron beam is displayed in Figure 4a. The corresponding power spectrum (Figure 4b) also shows reflections that can be indexed only in terms of rational numbers, pointing again to the formation of a superstructure. The Fourier-filtered image (Figure 4c) clearly exhibits the interchange of two lattice fringes in the [-111] direction that are mutually shifted in the [110] direction by d220 (1.57 Å). The simulated image (sample thickness t ) 4.9 nm, defocus value δz ) -68 nm) unequivocally confirms the superstructural model as it also fits nicely with the experimental one. It is important to highlight that the MnO superstructure is not detected by powder XRD but only with local probing (HRTEM and SAED) at the level of the single nanocrystal. To further support the hypothesis that the ordered Mn2+ vacancies are responsible for the superstructure formation, the site
occupancy factors of the Mn2+ site in MnO is also calculated within the Rietveld refinement of the XRD pattern. However, one has to keep in mind that XRD is not the best probe for the extraction of the chemical occupancy factor in this system due to the high correlation of occupancy and isotropic temperature factors (which were not refined in this work). Therefore, the values extracted should be considered with certain precaution in comparison on an absolute scale. Thus, 90.1(3)% of the Mn2+ sites in the MnO nanoparticles are occupied by Mn2+ cations, and the rest, 9.9(3)%, of the Mn2+ sites are vacant or, in other words, occupied by Mn2+ vacancies. This result agrees well with HRTEM data that proposed a vacancy-ordered superstructure with 87.5% Mn2+ sites occupied by Mn2+ cations and 12.5% vacant Mn2+ sites. Of course, such a superstructural model raises the question about charge balance. In the absence of other impurities the charge compensation can generally be achieved in two ways: (i) creation of interstitials of the same (in this case Mn) cation and (ii) formation of compensative oxygen vacancies. On the basis of the experimental XRD pattern, Rietveld refinement of the oxygen occupancy proposed that 4.2(3)% of oxygen sites are vacant. Radler et al. reported that bulk MnO manganosite exhibits a large range of nonstoichiometry up to 15%. They reported that a deviation of 9% from stoichiometry resulted in an interstitials occupation of 2% for Mn.27 Accordingly, we assume that oxygen vacancies (as calculated from experimental XRD patterns) as well as Mn interstitials compensate the extra charge produced by the Mn vacancies ordered superstructure. We have shown that very small crystals of Mn3O4 are also detected with XRD in this MnO-dominated sample. In general, benzyl alcohol is known to be a reducing solvent, which rarely oxidizes the metals of the precursors into higher oxidation states. However, in this MnO-dominated system oxidation of Mn2+ to Mn3O4 occurred. A possible explanation for this unusual behavior lies in the fact that in the reaction system of metal acetylacetonate-benzyl alcohol often organic species such as acetone, benzaldehyde, or benzoic acid are formed that can act as oxidation agents.17 Figure 5b shows a typical HRTEM image of a part of a singlecrystalline Mn3O4 nanoparticle in the Mn3O4-dominated sample. This atomic-resolved image reveals negligible defects in the lattice planes of the tetragonal Mn3O4 hausmannite crystal. The lattice fringes and zone axis identification were performed with the help of the corresponding power spectrum (Figure 5c). Two basis vectors of the projection of reciprocal space are identified in terms of Miller indices as 200 and 011. Consequently, the crystal orientation with respect to the incident electron beam was determined as the [0-11] zone axis. The SAED pattern (Figure 5d) further confirms the single crystallinity and the zone axis orientation. Figure 6 shows the manganese L2,3-edge spectra of the Mn3O4 [Mn3O4(d)] and MnO [MnO(d)] dominated samples after multiple loss deconvolution and the background subtraction. They consist of two white lines, L3 and L2, owing to the transitions from the 2p3/2 and 2p1/2 core states to the 3d unoccupied ones localized on the excited manganese ions. The shape and position of the L2,3 white lines in manganese compounds are sensitive to the oxidation state of the manganese ion. For Mn3O4(d) the Mn L3-edge exhibits a rather asymmetric peak consisting of two peaks at 640.7 and 641.7 eV. The L3 peak position tends to shift to higher energy with increasing formal valence of the manganese ion, which agrees with our results. The EELS spectrum of MnO(d) shows a single
3620 J. Phys. Chem. C, Vol. 111, No. 9, 2007
Figure 6. Manganese L2,3-edge spectra from Mn3O4(d) and MnO(d) samples after deconvolution of multiple loss events and background subtraction (d ) dominant phase).
L3 peak at an energy of 640.4 eV, which corresponds to Mn2+ion, while in Mn3O4(d) the higher energy peak corresponds to Mn3+. The peak splitting observed in Mn3O4 hausmannite is an evidence for the presence of the mixed manganese valence of 2+ and 3+ (ideally containing Mn3+ and Mn2+ in the ratio of 2:1). However, also in the MnO(d) case the L3 peak shows a weak asymmetry toward higher energy, which corroborates the results from the XRD powder pattern that proved the presence of Mn3+ ions in the form of Mn3O4 within MnO as the main phase. Accordingly, the EELS spectrum of this phase mixture shows the presence of Mn3+ ions, evidenced as a small asymmetry of the L3 peak. According to literature the height of the white line and the ratios of the area are sensitive to the oxidation state of 3d transition metals.28,29 Area and height ratios were calculated after background subtraction and subsequent extrapolation of the step levels back from the tails of each of the L3 and L2 peaks. Finally, the areas above the step function and below the peaks were measured. This procedure gave L3-to-L2 white line height ratios of 2.57 (Mn3O4) and 3.74 (MnO) and 2.83 (Mn3O4) and 3.39 (MnO) for the area ratios. The L3 peak height for Mn3O4 was determined from the more intense 2+ edge. On the basis of the 2j + 1 degeneracy of the initial core state for manganese, the L3-to-L2 white line ratio should be equal to the statistical values of 2.30 However, in real spectra this is not completely true. Sparrow et al. found that the L3-to-L2 white line ratio is related to the occupancy of the 3d orbitals on the metal ions, i.e., the ratio is maximum for the d5 electron configuration and decreases toward both the d0 and d10 configuration.31 Our results are consistent with this prediction, because for MnO (d3 configuration) the L3-to-L2 white line ratios (height, area) are higher in comparison to those of Mn3O4 (mixture of d3 and d2 configurations). Our L3-to-L2 white line ratios are greater than the nominal value of 2, but the deviation is higher in the area white line ratios. If one compares our results for L3-to-L2 white line ratios for MnO manganosite with the corresponding results of Garvie and Craven, one can notice that the latter values are higher (6.89 in Garvie and Craven’s work, 3.74 in the present work).32 This is another indirect indication for the presence of Mn3+ valence, which leads to a decrease of the L3-to-L2 white line ratio. The magnetic properties of both samples MnO(d) and Mn3O4(d) were investigated. Figure 7 shows the temperature dependences of the magnetic susceptibility. For MnO(d) the magnetic
Djerdj et al.
Figure 7. Temperature dependence of the magnetic susceptibility for (a) the MnO(d) sample and (b) the Mn3O4(d) sample. Insets: Enlarged region around 120 K. Please note the differences in the scales for the magnetic susceptibility. The applied magnetic field was 100 Oe in both cases.
susceptibility between room temperature and ∼130 K follows a Curie-Weiss type of behavior
χ)
C + χdia T-θ
C is the Curie constant, θ is the Curie-Weiss temperature, and χdia is the diamagnetic contribution. A least-squares fit of the data between 300 and 150 K leads to C ) 3.82(20) emu K/mol and θ ) -462(17) K. Although the Curie-Weiss temperature is fairly large, we note that it is still smaller than the one measured in bulk MnO,33 where it amounts to -548 K. The reduction of the Curie-Weiss temperature (and thus the effective exchange interaction between the Mn moments) is likely to be a consequence of the presence of disorder and defects in the nanoparticles. Another interesting observation is that the effective moment calculated from the Curie constant equals µeff ) pµB ) 5.5(2)µB. This moment is slightly smaller than the one expected for Mn2+ moments (µeff(Mn2+) ) 5.92µB). A possible reason for this discrepancy can be also the deficiency of Mn atoms on Mn2+ sites in the structure as inferred from the superstructural investigation described above. The anomaly (bump) in the magnetic susceptibility at ∼130 K (inset in Figure 7a) seems to coincide well with the Nee´l transition temperature for bulk MnO (TN ) 122 K).33 However, for a MnO antiferromagnetic powder, one expects the susceptibility to decrease below TN.34 This has not been observed in our casesthe magnetic susceptibility monotonically increases with decreasing temperatures. One may thus conclude that the anomaly at ∼130 K either (i) signals the tendency for the development of a long-range magnetic order, which in nanoparticles is not possible to truly develop or (ii) due to some magnetic anisotropy interactions the antiferromagnetic ordering is affected and changed to a weak ferromagnetic state. In such a case the magnetic anisotropy should arise from the surface effects which are strongly enhanced at the nanoscale.35 In any case, it turns out that the magnetic properties are strongly affected by the size of the MnO nanoparticles and their magnetic ground state may be altered. On further cooling of the MnO(d) sample another magnetic transition at TN2 ) 41.7(5) K was found. According to the literature this temperature is the transition temperature to a ferrimagnetically ordered state of Mn3O4.36 This result provides another proof that the sample contains Mn3O4 as impurity,
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Figure 9. Temperature dependence of the inverse susceptibility of the Mn3O4(d) sample. Vertical dotted lines mark the two transition temperatures of the MnO and Mn3O4 phases.
Figure 8. (a) Hysteresis curve measured in the MnO(d) sample at T ) 60 K (black curve) and at T ) 5 K (red curve). Inset: Enlarged view of the T ) 5 K magnetic hysteresis curve. (b) Low-temperature ZFC/FC curves measured in different magnetic fields ranging between 100 and 10 000 Oe. Please note the shift of the ZFC maxima toward lower temperatures in higher magnetic fields.
however, without telling whether the nanocrystals are a mixture of MnO and a few Mn3O4 nanoparticles or Mn3O4 impurities incorporated in the MnO nanoparticles. In an attempt to answer this question Figure 8a displays the magnetic hysteresis curve measured at low temperatures. If the Mn3O4 and MnO phases were mixed within the same nanoparticles, then a very narrow hysteresis is expected (Mn3O4 defects embedded in MnO show a hysteresis with a width of 5 Oe), displaced by about 25 Oe to negative fields.34 In our sample, the width of the hysteresis curve is 450 Oe and its displacement is negligible. We can thus conclude that the two competing phases, MnO and Mn3O4, are not present in the same particle. Finally, the observation of the thermal hysteresis effects and maxima in the ZFC curves below 42 K is discussed (Figure 8b). We stress that the maxima are strongly field dependent. In measurements performed at 100 Oe the ZFC maximum appears at 29(1) K, while in the 10 000 Oe experiment, the ZFC maximum shifted to 19(1) K. Such behavior is typical for superparamagnetic (monodomain) particles nicely corroborating with the electron microscopy studies. A ZFC maximum recently measured by Park et al. for 5 nm Mn3O4 nanospheres was found to be around 15 K.10 Because our samples have a much broader size distribution, the maximum measured in the ZFC curve only provides an effective blocking temperature. The study of the magnetic susceptibility of the Mn3O4(d) sample shows that the magnetic structure of Mn3O4 is very
complex. In the spinel lattice Mn2+ (S ) 5/2) ions occupy tetrahedral sites, while Mn3+ (S ) 2) are located on the octahedral sites.36 The ferrimagnetic transition temperature in bulk Mn3O4 is at TN ) 42 K, but the canted-spin order of the Yafet-Kittel type is realized below T ) 33 K, and a spinspiral structure along the [010] axis has been observed in the temperature range of 33 K < T < 39 K. The temperature dependence of the magnetic susceptibility of Mn3O4(d) sample is shown in Figure 7b. The magnetic susceptibility is strongly enhanced compared to that of the MnO(d) sample due to a larger effective moment of the Mn3O4 phase (theoretically one would expect µeff ) 9.15µB). According to Figure 9 the temperature dependence of the reciprocal susceptibility has a typical “hyperbolic” dependence usually encountered in ferrimagnets. Intriguingly, the effective magnetic moment, which can be calculated from the room-temperature value of the magnetic susceptibility, is only µeff ) 6.6(2)µB, i.e., much smaller than the expected value. A plausible explanation for such an observation would be either the presence of short-range order effects that exists even far above TN or a sizable fraction of MnO impurities. To get deeper insight into the magnetic properties of MnO(d) and Mn3O4(d) nanopowders we also performed a temperature-dependent X-band EPR study. A comparison of roomtemperature X-band EPR spectra measured in bulk MnO powder, MnO(d) nanopowder, and Mn3O4(d) nanopowder is shown in Figure 10. In comparison to bulk MnO powder, the MnO(d) nanopowder exhibits a dramatic broadening of the EPR spectra. The line width increases from the peak-to-peak EPR value 383(2) G in bulk to 791(5) G in the nanopowder. However, the effective g-factor is nearly the same in both samples. A likely explanation for such a pronounced broadening is the presence of defects in the nanopowder, leading to strains and thus a broad distribution of the magnetic anisotropy interactions. On the other hand the room-temperature spectrum of Mn3O4(d) nanopowder is strongly shifted toward lower fields. Its effective g-factor value is 2.84(1). The line width of the signal is in this case already 1180(8) G. These values are rather unusual for Mn3O4 particles, i.e., a very narrow resonance at g ) 2.005 with a line width of ∼100 G has been measured in Mn3O4 particles embedded in MnO.37 Since the g-factor for Mn2+ is expected to be fairly close to 2, we attribute the shift in the g-factor to Mn3+ ions. We stress once again, that Mn3+ ions sit on the octahedral sites. Such a deviation of the effective g-factor
3622 J. Phys. Chem. C, Vol. 111, No. 9, 2007
Djerdj et al. both transition temperatures. The tendency of decreasing the line width with decreasing temperature just proves once again that the Mn3+ ions in the Mn3O4 nanopowder are in states with unquenched orbital moment. The temperature dependence of the EPR line width could thus be due to strong spin-phonon coupling as proposed in ref 39. Conclusion
Figure 10. Comparison of the room-temperature X-band EPR spectra measured in bulk MnO powder, MnO(d) nanopowder, and Mn3O4(d) nanopowder.
Figure 11. Comparison of the line width of the EPR spectra measured in bulk MnO powder, MnO(d) nanopowder, and Mn3O4(d) nanopowder.
thus implies a significant distortion of the octahedral environment in the Mn3O4 nanopowder, resulting in a crystal field effect, which shifts the Mn3+ g-factor. In Figure 11 a comparison of the temperature dependence of the line width of the EPR spectra in all three samples is given. In bulk MnO the line width, as expected, diverges when approaching TN from above. Such a behavior is a clear manifestation of the development of spin correlations at higher temperatures and critical fluctuations just above TN. The EPR signal completely disappears below TN since it is very difficult to detect antiferromagnetic resonance in a powder sample. On the other hand, in MnO(d) nanopowder a maximum in the line width 20 K above the bulk TN temperature is found. The divergence of the EPR line width is strongly suppressed. Below the maximum, the EPR line width rapidly drops from 2200(10) G at 155 K to 820(5) G at 125 K. However, the EPR signal does not disappear at lower temperatures in contrast to powdered MnO. Upon cooling, the line width gradually increases with decreasing temperature, showing another anomaly at 42 K. The behavior is somehow reminiscent of the behavior found in MnO phases grown in different nanoporous materials.38 It corroborates with the susceptibility data, suggesting that the true long-range magnetic order cannot establish in our MnO nanoparticles. On the other hand, in the Mn3O4 nanopowder the line width decreases with decreasing temperature with two anomalies at
The nonaqueous sol-gel reaction of manganese(II) acetylacetonate with benzyl alcohol yielded a phase mixture of manganese(II) oxide (manganosite) as the main and manganese(II, III) oxide (hausmannite) as the minor component. The analogous reaction of potassium permanganate with benzyl alcohol resulted in the inverse situation, where manganese(II, III) oxide represents the main and manganese(II) oxide the minor component. The manganese oxides obtained are in the form of nanoparticles with various shapes, including nanoplatelets, spherules, ellipsoids, cubes, and rhombohedra for Mn3O4 and mostly nanoplatelets for MnO. The nanoparticles are single crystalline with random orientation with respect to the electron beam. In the crystal system of MnO manganosite a superstructure is observed, which can be well modeled as an ordered Mn vacancies superstructure with the chemical formula of Mn0.875Ox and lattice parameter of 8.888 Å. This interesting and also surprising result corroborates with the refined site occupancies of Mn (90.1(3)%) and O (95.8(3)%). The presence of oxygen vacancies partially compensates the extra charge originated from the Mn vacancies ordering. Analysis of the position and shape of the white lines on the manganese L2,3-edge spectra revealed the presence of manganese with the two valencies, 2+ and 3+, in both samples. The magnetic characterization of the MnO nanoparticles clearly revealed their superparamagnetic (monodomain) behavior, which is different from previous reports that predicted their weak ferromagnetic character. We also found that the enhanced magnetic anisotropy and defects may alter the antiferromagnetic ordering in MnO nanoparticles, resulting in a finite magnetic moment below 120 K. The decrease of the effective magnetic moment for Mn2+ in the MnO nanoparticles compared to the expected value can be interpreted in terms of the ordered Mn vacancies superstructure in the MnO. Acknowledgment. We thank the Max Planck Society for financial support. Furthermore, we express our gratitude to Professor Robert Schlo¨gl and Dr. Dang Sheng Su from the FritzHaber-Institute (FHI) for the use of the electron microscope. We also thank Dr. Marc Willinger (FHI) for EELS acquisition and Edi Kranjc (Institute of Chemistry, Ljubljana) for XRD recording. References and Notes (1) Armstrong, A. R.; Bruce, P. G. Nature 1996, 381, 499-500. (2) Seo, W. S.; Jo, H. H.; Lee, K.; Kim, B.; Oh, S. J.; Park, J. T. Angew. Chem., Int. Ed. 2004, 43, 1115-1117. (3) Anilkumar, M.; Ravi, V. Mater. Res. Bull. 2005, 40, 605-609. (4) Shen, Y. F.; Zerger, R. P.; DeGuzman, R. N.; Suib, S. L.; McCurdy, L.; Potter, D. I.; O’Young, C. L. Science 1993, 260, 511-515. (5) Yang, L.-X.; Zhu, Y.-J.; Tong, H.; Wang, W.-W.; Cheng, G.-F. J. Solid State Chem. 2006, 179, 1225-1229. (6) Zhang, Y. C.; Qiao, T.; Hu, X. Y. J. Solid State Chem. 2004, 177, 4093-4097. (7) Na, C. W.; Han, D. S.; Kim, D. S.; Park, J.; Tae Jeon, Y.; Lee, G.; Jeon, Y. T.; Lee, G.; Jung, M.-H. Appl. Phys. Lett. 2005, 87, 142504-1142504-3. (8) Ghosh, M.; Biswas, K.; Sundaresan, A.; Rao, C. N. R. J. Mater. Chem. 2006, 16, 106-111.
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