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ISRN Spectroscopy

Volume 2012 (2012), Article ID 706398, 8 pages

http://dx.doi.org/10.5402/2012/706398

## Nanoparticles: Synthesis, Characterization, and Dielectric Properties

^{1}Laboratoire des Matériaux Utiles, Institut National de Recherche et d’Analyse Physico-Chimique, Pôle Technologique de Sidi-Thabet, 2020 Tunis, Tunisia^{2}Unité de Radiotraitement, Centre National des Sciences et Technologies Nucléaires (CNSTN), Pôle Technologique de Sidi Thabet, 2020 Tunis, Tunisia

Received 11 December 2011; Accepted 2 January 2012

Academic Editors: J. M. Saniger and M. Soylak

Copyright © 2012 Hassouna Dhaouadi et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

#### Abstract

nanoparticles were prepared by a simple chemical route using cetyltetramethylammonium bromide (CTAB) as a template agent. nanocrystals present an octahedral shape, and their crystallite size varies between 20 and 80 nm. They were characterized by XRD, SEM, DTA/TG, and IR spectroscopy. XRD studies confirm the presence of a highly crystalline phase. The Rietveld refinement of the X-ray diffraction data confirms that nanoparticles crystallize in the tetragonal system with space group I41/amd. DTA/TG and XRD measurements demonstrate the phase transition toward a spinel structure between 25 and . The electrical conductivity increases between 80 and , suggesting a semiconducting behaviour of . Both dielectric dispersion (*ε*′) and dielectric loss (*ε*′′) were investigated from 80 and in the frequency range of 10 Hz–13 MHz. The dielectric properties showed typical dielectric dispersion based on the Maxwell-Wagner model.

#### 1. Introduction

Metal oxide nanocrystals are widely applied in catalysis, energy storage, magnetic data storage, sensors, and ferrofluids [1–4]. Tetramanganese oxides (Mn_{3}O_{4}) are particularly used as main sources of ferrite materials [5] and applied in numerous industrial fields such as magnetic [6], electrochemical [7], and catalysis [8, 9]. The particle size and morphology may be controlled by using various methods including solvothermal/hydrothermal [10, 11], vapor phase growth [12], vacuum calcining precursors [13], thermal decomposition [14], ultrasonic, gamma, and microwave irradiation [15–17], and chemical liquid homogeneous precipitation [18]. Therefore, Mn_{3}O_{4} compounds show distinct shapes including nanoparticles [19], nanorods [20], nanowires [21], and tetragonal particles [22].

The electrical and magnetic properties of numerous nanomaterials are completely different from those of their bulk counterparts. Changes in dielectric properties were attributed to changes in particle size, shape, and boundaries [23, 24]. The modified dielectric properties were used as capacitors, electronic memories, and optical filters. Materials exhibiting a giant dielectric constant were already reported elsewhere [25–27]. The high dielectric permittivity and the low loss factors over a wide frequency range are always of a great interest [28]. In a previous work, we synthesized Mn_{3}O_{4} nanomaterials with lozenge morphology under hydrothermal treatment [29]. The temperature dependence of the conductivity between 25 and 220°C obeys the Arrhenius law with activation energy of 0.48 eV. However, the present work describes the preparation of Mn_{3}O_{4} nanomaterials with octahedral shape at room temperature by using a precipitation method. The magnetic properties of the Mn_{3}O_{4} nanocrystals are also studied by the electron paramagnetic resonance (EPR) technique. To the best of our knowledge, only a limited number of studies of the dielectric properties of nanostructured Mn_{3}O_{4} were reported [30–32]. The scope of the present work is to study the dielectric properties of nanostructured Mn_{3}O_{4} nanoparticles as a function of the frequency and the temperature.

#### 2. Experimental Procedure

##### 2.1. Sample Preparation

The synthesis of Mn_{3}O_{4} nanoparticles was performed at room temperature, using the precipitation route [18]. 0.22 mol of MnCl_{2}·6H_{2}O (Aldrich) was added to 0.44 mol of NaOH (Fluka) aqueous solution. Then 1.37 mmol cetyltetramethylammonium bromide (CTAB) (Sigma) was dispersed and homogenized under slowly magnetic stirring during 24 hours. The precipitate was filtered and carefully washed with distilled water for several times. The resulting product was dried in the oven at 80°C for 24 hours.

The formation of Mn_{3}O_{4} compound in the alkaline aqueous solution may be formulated by the following reactions:

Manganese ions are firstly reduced in alkaline media to Mn(OH)_{2} and then decomposed into MnO (see (2) and (3)). Finally, MnO is oxidized to Mn_{3}O_{4} by the atmospheric oxygen.

##### 2.2. Characterisation Techniques

X-ray powder diffraction (XRD) measurements were performed using a Panalytical XPERT PRO MPD diffractometer operating with Cu K*α* radiation (). Crystallographic data (lattice parameters, space group) were determined by using the Highscore software. The XRD pattern was analyzed by the Fullprof program [33] based on the Rietveld method [34] in order to calculate the cell parameters.

Infrared (IR) spectra were recorded in the wavenumber range of 4000–400 cm^{−1} with a “Nicolet 380 Spectrometer.” The samples were dispersed in pure KBr pellets. Thermal analyses were performed by using the “SetaramSetsys 1750” operating from 25 to 800°C at a heating rate of 5°C·min^{−1} under air atmosphere. Scanning electron microscopy (SEM) images were obtained by a Zeiss Stereoscan 440 instrument. Conductivity measurements were carried out from 80 to 240°C with 5–20°C steps by checking the complex impedance spectroscopy with a Hewlett Packard 4129A impedance analyzer. The signal frequency ranged from 10 to 13 MHz.

#### 3. Results and Discussion

##### 3.1. Crystal Structure, Size, and Morphology

Figure 1 shows XRD patterns of as-prepared Mn_{3}O_{4} nanoparticles. The diffraction peaks correspond to the tetragonal Mn_{3}O_{4} single phase (JCPDS Card 01-080-0382). The Rietveld refinement procedure was performed for the investigation of the crystalline structure of as-prepared nanocrystallites. Figure 1 presents the experimental and calculated patterns together with the differential one, deduced from the Rietveld refinement. Mn_{3}O_{4} X-ray data were refined in the tetragonal system (space group I41/amd). An excellent agreement between experimental and calculated patterns is obtained (). The refined cell parameters of Mn_{3}O_{4} structure are Å, Å, and Å^{3}. The structure was refined according to these parameters: , , , and .

The crystallite sizes were calculated from the major (211) diffraction peak using the Debye Scherrer approximation [35] and are about 50 nm. The morphology of Mn_{3}O_{4} nanoparticles is shown in Figure 2. The formation of Mn_{3}O_{4} nanoparticles is confirmed. Aggregated nanoparticles with octahedral shapes are observed, and their diameter varies between 20 and 80 nm. The SEM results of the Mn_{3}O_{4} nanoparticles agree with the XRD data.

##### 3.2. IR Absorption Spectroscopy

The IR absorption spectra of the pure CTAB and Mn_{3}O_{4} nanopowders calcinated at different temperatures for 2 h are shown in Figure 3. The IR spectrum of as-prepared Mn_{3}O_{4} nanoparticles without any thermal treatment displays three significant absorption bands in the range of 400–650 cm^{−1} (Figure 3(b)). The vibration frequency located at 629 cm^{−1} is characteristic of Mn–O stretching modes in tetrahedral sites, whereas the vibration frequency located at 510 cm^{−1} corresponds to the distortion vibration of Mn–O in an octahedral environment. The third vibration band, located at 410 cm^{−1}, can be attributed to the vibration of manganese species (Mn^{3+}–O) in the octahedral site of Mn_{3}O_{4} [36–39]. Moreover, the broad band at 3436 cm^{−1} and the narrow one at 1646 cm^{−1} correspond to the O–H vibrating mode of the adsorbed water. The two intense adsorption bands located at 2920 and 2840 cm^{−1} are assigned to the symmetric and asymmetric stretching modes of the –CH_{2} group of the CTAB surfactant bounded to the surface of Mn_{3}O_{4}, respectively. A comparison of the IR spectrum for pure CTAB (Figure 3(a)) and the spectra relative to the Mn_{3}O_{4} nanoparticles (Figures 3(b), 3(c), and 3(d)) shows invariant frequency positions at 2920, 2840 cm^{−1}, suggesting the absence of intermolecular interactions (C–H*⋯*O) between oxygen atoms of Mn_{3}O_{4} nanoparticles and the –CH_{2} groups belonging to the CTAB surfactant. The difference between symmetric and asymmetric scissoring vibrations of (CH_{3})_{3}–N^{+} group of the pure CTAB (1482 cm^{−1}) and CTAB capped on the Mn_{3}O_{4} nanoparticles surface (1429 cm^{−1}) reveals that CTAB adsorbed on the nanoparticle surface interacts strongly with oxygen atoms of Mn_{3}O_{4} (Figure 3(b)). The single band located at 960 cm^{−1} on CTAB spectrum (Figure 3(a)) and the narrow one at 955 cm^{−1} (Figure 3(b)) for bounded CTAB may be assigned to the (C–N^{+}) stretching mode. The frequency shift may be caused by the interaction between (CH_{3})_{3}–N^{+} group and the oxidized metal surface [40]. This type of interaction was also observed between the polymer hydroxypropyl cellulose (HPC) used as capping agent and the silver metal surface [41].

During the heat treatment of Mn_{3}O_{4}, the progressive removal of the organic surfactant is well demonstrated by the IR spectra (Figure 3(b)–(f)) recorded at different temperatures. After two hours of thermal treatment at 500°C, vibration bands corresponding to the surfactant molecules were not observed on IR spectra, indicating that all organic groups were removed. During the heating process, a clear difference is observed in the IR spectra, in the range 700–400 cm^{−1}, corresponding of the stretching bands of Mn–O. This change in the position bands between 300 and 700°C can be related to the phase transformation of Mn_{3}O_{4}. This result is in a good agreement with X-ray diffraction data.

##### 3.3. Thermal Analyses

In order to examine the thermal stability of Mn_{3}O_{4} nanoparticles, thermal gravimetric (TG) and differential thermal analyses (DTA) were carried out between 25 and 800°C (Figure 4). In the temperature range of 25–250°C, the weight loss of 9.15% can be related to the release of weakly adsorbed water molecules and to the removal of the organic entities of CTAB surfactant. The broad exothermic peak at about 350°C, associated with the mass gain, is about 0.4% (Figure 4). As confirmed by earlier studies [42–44], this thermal event can be corroborated by the fact that Mn_{3}0_{4} (, ) can be transformed into Mn_{5}O_{8} (, ) in the temperature range of 300–450°C [45]. The new (Mn_{5}O_{8}) phase is formed according to the following reaction:

Mn_{5}O_{8} phase is isostructural with Cd_{2}Mn_{3}O_{8} and crystallizes in the monoclinic system (C2/m) [46]. Mn_{5}O_{8} was observed during the decomposition of manganese oxalate [47] as well as in the transformation of hydrohausmannite into anhydrous hausmannite [46, 48]. The oxidation of Mn^{3+} to Mn^{4+} in the temperature range of 300–450°C leads to the crystallization of the Mn_{5}O_{8} phase. After the exothermic peak at 350°C the mass remains relatively constant.

The endothermic event at 575°C, associated with the weight loss of 2.02% (calculated weight loss 1.98%), are related to the consequent release of oxygen due to the reduction of Mn_{5}O_{8} to Mn_{2}O_{3}. The reaction scheme can be written as follows:

The weight loss is still constant at temperature higher than 700°C, indicating the completion of the thermal decomposition of the intermediate phase (Mn_{5}O_{8}) and the beginning of the crystallization of Mn_{2}O_{3} as a new phase.

In order to give a plausible interpretation of the thermal phenomena, we also performed an investigation by X-ray diffraction analysis at different calcination temperatures for 2 h. Several samples of Mn_{3}O_{4} nanoparticles were heated in electric oven (under air atmosphere) at different temperatures: 25, 300, 500, and 700°C, and they are immediately submitted to X-ray diffraction measurements (Figure 5). The X-ray diffraction of the product treated at 300°C is similar to that of the bare sample. This means that Mn_{3}O_{4} nanoparticles are stable up to 300°C. In the temperature range of 25–300°C, XRD peaks gradually become sharper and higher, indicating an improved crystallinity. The X-ray diffraction spectrum at 500°C shows some additional peaks (Figure 5(c)). These diffraction peaks cannot be attributed to Mn_{2}O_{3}, and they are distinguishable from those of Mn_{3}O_{4}. They are observed at 21.741, 31.916, 35.400, 39.250, 47.801, and 66.190° and indexed to (−201), (020), (−401), (002), (−421), and (−223) Bragg peaks, respectively. The corresponding phase is identified as Mn_{5}O_{8} (JCPDS card no. 00-039-1218) [44]. It was already established that Mn_{5}O_{8} is stable up to 550°C and transforms to Mn_{2}O_{3} and Mn_{3}O_{4} phases [49]. In the present case, after calcination at 700°C, the XRD spectrum (Figure 5(d)) shows that all diffraction peaks of Mn_{5}O_{8} and Mn_{3}O_{4} phases disappear. In the same way, a new phase Mn_{2}O_{3} appears (JCPDS card no. 00-41-1442) with typical peaks at 19.02, 23.28, 33.07, 35.87, 38.49, 55.44, and 65.92°. These peaks are indexed to (200), (211), (222), (321), (400), (440), and (622), respectively.

Paramagnetic properties of Mn_{3}O_{4} nanoparticles calcined at different temperatures (25°C, 300°C, and 500°C) and cooled at room temperature were studied by the EPR technique (Figure 6). The paramagnetic properties were used in order to study the valence state changes of manganese element as a function of the temperature. Figures 6(a) and 6(b) indicate that the spectra mainly consist in one resonance line centered at that characterizes the unique paramagnetic center (Mn^{2+}) at each resonance feature. The broad EPR line seems to be originating from the electrostatic and dipolar interactions between the manganese ions with mixed valence states. Figure 6(c) shows two resonance peaks corresponding to the paramagnetic centers Mn^{2+} and Mn^{4+}, respectively. This indicates the presence of two Mn_{5}O_{8} (, ) and Mn_{3}O_{4} (, ) phases around 500°C. Such a result is in good agreement with the XRD and thermal data and confirms the presence of two phases Mn_{3}O_{4} and Mn_{5}O_{8} around 500°C.

##### 3.4. Conductivity Study

The temperature variation of the electrical conductivity between 80 and 300°C is represented in Figure 7 in the form of Ln (* σ*T) versus 10

^{4}/T. The electrical conductivity increases with increasing temperature. In the temperature range 80–140°C, the electrical conductivity approximately presents an Arrhenius-type behavior with an activation energy of . Around 140°C, a break in the curve is observed, accompanied by an increase of the activation energy (). This behaviour may be related to the removal of water molecules, residual ions, and organic surfactant capped on the surface of Mn

_{3}O

_{4}particles. This probably affects the conductivity mechanism. It was reported elsewhere that the change in the slope does not occur for Mn

_{3}O

_{4}nanoplatelet preliminarily heated at 300°C before being used in the conductivity study [29]. Larson and Arnott have measured the electrical conductivity of Mn

_{3}O

_{4}bulk materials in the range 0–200°C. The obtained activation energy is 0.75 eV [50].

##### 3.5. Dielectric Analysis

The complex dielectric permittivity is represented by *ε** = *ε*′− i*ε*′′, where * ε*′ is the real part of the dielectric permittivity that describes the stored energy, while

*ε*′′ is the imaginary part of dielectric permittivity related to the dissipation energy (or loss of energy) within the medium [51]. Figure 8(a) shows that the dielectric constant (

*ε*′) decreases continuously at lower frequencies and remains constant at higher frequencies giving rise to a plateau. The monotonous decrease in the value of dielectric constant on increasing frequency may be attributed to a combined contribution due to electronic, ionic, and interfacial polarization at low frequencies [52]. The observed dielectric dispersion at low frequencies can be explained on the basis of the Maxwell-Wagner theory of interfacial polarization [53]. According to the Maxwell-Wagner model, the dielectric structure of ferrites consists in two layers. The first layer represents a large number of grains that act as conducting layer at higher frequencies and the other layer consists in grain boundaries that act as highly resistive medium at lower frequencies. Similar results were observed for ferrite nanoparticles (Mn

_{0.2}Ni

_{0.8}Fe

_{2}O

_{4}) [54] as well as in bulk SrNb

_{2}O

_{6}material [55]. At low frequencies, the polarization process in Mn

_{3}O

_{4}nanoparticles is explained as a local displacement of electrons by hopping mechanism between Mn

^{2+}and Mn

^{3+}and an orientation of electric dipole in the direction of the applied field. With increasing the frequency, electric dipoles, and the Mn

^{2+}/Mn

^{3+}electron transfer, the net displacement of charge in one direction decreases and then the dielectric constant decreases. The high dielectric values of the Mn

_{3}O

_{4}nanoparticles are observed for other compounds such as ZnO nanotubes [56] and Mn

_{0.2}Ni

_{0.8}Fe

_{2}O

_{4}nanoparticles [54] and for the bulk Mn-Mg spinel ferrite [57]. The large value of dielectric constant is due to the fact that Mn

_{3}O

_{4}acts as a nanodipole under electric fields. The small-size particles involve large number of particles per unit volume resulting in an increase of the dipole moment per unit volume and the high dielectric constant.

The temperature dependence of dielectric permittivity (*ε*′) at selected frequencies is represented in Figure 8(b). The dielectric permittivity increases gradually with the temperature until reaching a maximum value of . On the other hand, relaxor ferroelectrics show a broad peak for the permittivity variation with the temperature at large frequency dispersion [58]. In the present case, a broad peak was observed in the temperature range of 90–120°C at low frequencies (Figure 8(b)). Other compounds show relaxor dielectric response such as CrTiNbO_{6} [59] and FeTiNbO_{6} [59]. The relatively insignificant variation of dielectric permittivity with the temperature observed at higher frequencies is attributed to atomic and electronic polarizations.

The imaginary part of the dielectric permittivity *ε*′′ shows higher values that decrease rapidly with the frequency and increase as the temperature rises (Figure 8(c)). For nanometer-sized particles, the contribution of the interfacial loss and the loss from electrical conductivity is dominant at lower frequencies, but at high frequencies these losses are negligible. This may explain the decrease in dielectric constant (*ε*′′) observed at high frequencies. The large value of the dielectric constant *ε*′′ at lower frequency is due to the predominance of Mn^{2+} ions, oxygen vacancies, and grain boundary defects [60, 61]. Figure 8(d) indicates that the imaginary part *ε*′′ is fluctuating and increasing as a function of the temperature at higher frequencies. This suggests that the temperature dependency of the imaginary part is strongly dependent on the frequency range.

#### 4. Conclusion

Mn_{3}O_{4} nanoparticles were successfully prepared by using the precipitation method at room temperature in the presence of CTAB, as a template agent. X-ray structural analyses confirmed that Mn_{3}O_{4} nanoparticles crystallize in the tetragonal system. From XRD and SEM results, Mn_{3}O_{4} nanoparticles exhibit a crystallite size between 20 and 80 nm. XRD and DTA/TG analyses show the apparition of the metastable Mn_{5}O_{8} phase at about 350°C. The paramagnetic properties of Mn_{3}O_{4} samples were studied by the EPR technique in order to study the valence state changes with the temperature and to confirm the presence of the metastable Mn_{5}O_{8} phase around 350°C. Dielectric measurement results show that *ε*′ and *ε*′′ decrease as the frequency increases. The dielectric dispersion *ε*′ and the dielectric absorption *ε*′′ of Mn_{3}O_{4} nanoparticles are found to be slightly temperature dependent at higher frequency. These results indicate that the dielectric properties showed a usual dielectric dispersion explained on the basis of the Maxwell-Wagner model.

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