Polyhedron 28 (2009) 2119–2122
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Ovalbumin mediated synthesis of Mn3O4 Zehra Durmus a, Abdulhadi Baykal a,*, Hüseyin Kavas b, Mikail Direkçi c, Muhammet S. Toprak d a
Department of Chemistry, Fatih University, B.Çekmece, 34500 Istanbul, Turkey Department of Physics, Fatih University, B.Çekmece, 34500 Istanbul, Turkey Department of Physics, Bozok University, 66200 Yozgat, Turkey d Functional Materials Division, Royal Institute of Technology, SE16440 Stockholm, Sweden b c
a r t i c l e
i n f o
Article history: Received 18 February 2009 Accepted 24 March 2009 Available online 31 March 2009 Keywords: Sol–gel processes Nanocomposites Spinels Functional applications Ovalbumin
a b s t r a c t By using Mn2+ and Mn3+ salts, and freshly extracted ovalbumin, Mn3O4 nanocrystals have been synthesized successfully. The X-ray diffraction results indicated that the synthesized nanoparticles have only the spinel structure without the presence of any other phase impurities. As the ovalbumin–water mixture was highly basic, the process did not require any use of base to increase the pH where hydrolysis took place. A gel formed where water soluble ovalbumin proteins served as a perfect matrix for entrapment of metal ions (Mn2+ and Mn3+). Upon heat treatment, the dried gel precursor decomposed into nanocrystalline Mn3O4. The discrepancy between the crystallite size from XRD and particle size SEM analysis reveals polycrystalline nature of the synthesized particles with this route. EPR analysis of Mn3O4 shows a narrow and symmetric line indicating the absence of hyperfine splitting. Ó 2009 Elsevier Ltd. All rights reserved.
1. Introduction Manganese oxides have attracted considerable interest due to their wide range of applications in catalysis, ion-exchange materials, electrochemical materials, high-density magnetic storage media, solar energy transformation and molecular adsorption [1–6]. Hausmannite, Mn3O4 with the spinel structure, is an important oxide which can be used as an active catalyst for the oxidation of methane and carbon monoxide [7] and the selective reduction of nitrobenzene [8] as well as electrode materials for batteries; moreover, it is an effective material to prevent air pollution and limit the emission of NOx and volatile organic compounds from waste gases of different origins [9]. Various methods have been reported for the synthesis of Mn3O4 nanoparticles. Among these methods, high-temperature and hydrothermal methods are generally adopted in conventional processes for the synthesis of Mn3O4. Almost all oxides, hydroxides, carbonates, nitrates and sulfates of manganese can serve as precursors to produce Mn3O4 at about 1000 °C in air [10]. However, these processes are economically unviable. Recently, chemical bath deposition [11], sol–gel technique [12], oxidation–precipitation method [13], reflux [14] and the co precipitation method [15] have been employed to prepare Mn3O4. It is becoming very important to find simple and cost effective routes to synthesize Mn3O4 nanoparticles by application of cheap, non-toxic and environmentally friendly precursors. * Corresponding author. Tel.: +90 212 866 33 00x2061; fax: +90 212 866 34 02. E-mail address:
[email protected] (A. Baykal). 0277-5387/$ - see front matter Ó 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2009.03.026
Egg white proteins (ovalbumin) have various types of functional properties such as gelling, foaming, emulsification, heat setting and binding adhesion [16]. Besides the advantage of its solubility in water and its tendency to associate with metal ions in solution, egg white proteins have also been used as a binder cum gel to shape material, especially bulk and porous ceramics [17]. The use of egg white not only simplifies the process but also would provide another preference process to synthesize of nanocrystalline ceramic particles in an environmentally friendly and cheap way. Up to now, Dhara [17], Kumari et al. [18] and Maensiri et al. [19] used egg white as a water soluble polymeric matrix to produce nanocrystalline alumina, LiMn2O4 and NiFe2O4, respectively. Hereby, we report an innovative approach for the synthesis of Mn3O4 based on calcination of a dried gel precursor obtained by freshly extracted ovalbumin mediated synthesis. To the best of our knowledge, this study is the first report on the synthesis of Mn3O4 nanocrystals using egg white. 2. Experimental 2.1. Synthesis of Mn3O4 nanoparticles in the presence of egg white Mn (II) acetate dihydrate, (Mn(ac)2) 2H2O and Mn (III) acetylacetonate, Mn(acac)3, of reagent grade, were obtained from Merck. All chemicals were used without further purification. Firstly, water–egg white mixture was prepared (3:2 volume ratio). 60 ml of egg white was stirred with 40 ml deionized water with vigorously at room temperature until a homogeneous solution was obtained. Subsequently, stoichiometric amount of Mn(ac)2 and
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2 Theta (deg) Fig. 1. XRD pattern of Mn3O4 particles (obtained upon calcination of dried gel precursor 800 °C) and line profile fitting.
% Transmittance (a.u.)
X-ray powder diffraction (XRD) analysis was conducted on a Huber JSODEBYEFLEX 1001 diffractometer operated at 40 kV and 35 mA using Cu Ka radiation. Fourier transform infrared (FT-IR) spectra of the samples were recorded in transmission mode with a Perkin Elmer BX FT-IR infrared spectrometer using KBr diluted pellets in the range of 4000– 400 cm1. The thermal stability was determined by thermogravimetric analysis (TGA, Perkin Elmer Instruments model, STA 6000). The TGA thermogram was recorded for 5 mg of powder sample at a heating rate of 10 °C min in the temperature range of 30–800 °C under nitrogen atmosphere. Scanning electron microscopy (SEM) analysis was performed, in order to investigate the microstructure of the sample, using FEI XL40 Sirion FEG Digital Scanning Microscope. Samples were coated with gold at 10 mA for 2 min prior to SEM analysis. Transmission electron microscopy (TEM) analysis was performed using a FEI Tecnai G2 Sphera microscope. A drop of diluted sample in alcohol was dripped on the TEM grid. A conventional X-band (f = 9.8 GHz) Bruker EMX model EPR spectrometer employing an ac magnetic field (modulation frequency: 100 kHz) modulation technique was used for magnetic characterization. Operating conditions were 20.07 mW microwave power, 15 G modulation amplitude, center field 8000 G, time constant 40.96 ms and sweep time 41.94 s with multiple accumulations to enhance the signal-to-noise ratio.
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2.2. Characterization
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Mn(acac)3 (with the molar ratio of 1:2) were added slowly into egg white–water solution. During addition of Mn salts the solution was stirred magnetically at room temperature for 2 h to obtain a clear solution. Throughout the whole process, no pH adjustment was necessary as the pH of the solution was measured as 9. Then the yellow–orange mixed solution was evaporated by heating at 80 °C for several hours until a dried gel precursor was obtained. The dried mass was crushed into powder using a mortar and pestle and then calcined in a box-furnace at 800 °C 3 h in air.
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3. Results and discussion
Fig. 2. FT-IR spectrum of as-synthesized Mn3O4 upon calcination of the dried gel precursor at 800 °C.
3.1. XRD analysis A typical XRD pattern of the as-synthesized powder is shown in Fig. 1. All diffraction peaks were indexed to the tetragonal Mn3O4 (which are consistent bulk value ICDD Card no. 24-0734). The lattice parameters were calculated as a = 5.76(3) and b = 9.47(1) Å using the d-spacing values and the respective (h k l) parameters. The average size of the crystallites, DXRD, was estimated as 22 ± 10 nm from the diffraction pattern using the line profile fitting of Mn3O4 diffraction peaks as described in Wejrzanowski et al. [20] and Pielaszek [21]. The line profile, shown in Fig. 1, is fitted for 14 peaks with the following Miller indices: (1 0 1), (1 1 2), (2 0 0), (0 1 3), (2 1 1), (0 0 4), (2 2 0), (2 1 3), (2 0 4), (0 1 5), (3 1 2), (3 0 3), (3 2 1) and (2 2 4). 3.2. FT-IR analysis FT-IR analysis was performed for the sample prepared by the calcination of the drıed gel precursor and the spectrum is presented in Fig. 2. In the range of 1000–400 cm1, two main metal– oxygen bands at 606 and 478 cm1 were observed in the FT-IR spectrum which are associated with the coupling between Mn–O stretching modes of tetrahedral A- and octahedral B-sites [22,23].
3.3. Thermal analysis Thermal stability of the precursor powder and final powder has been analyzed using TGA. Thermograms for both samples are presented in Fig. 3. Calcined powder shows no weight loss, as all organic residues have been removed, that reveals the stability of obtained Mn3O4 nanoparticles. Precursor powder shows an overall weight loss of 75%. According to the differential thermogram, DTG, presented in the inset of Fig. 3 there is four-step weight loss (indicated with arrows) for the precursor powder, of mainly ovalbumin, matrix the first one at <200 °C, a shoulder at 240 °C, the most intense one at 300 °C and a secondary shoulder one at 450 °C. The observed thermogram perfectly matches that of pure ovalbumin reported by Kumari et al. [18] above 200 °C. Weight loss up to 200 °C is due to the adsorbed water and boiling of organic residues released from Mn salts. Weight loss around 240 °C suggests that water molecules in albumin are strongly bonded to the protein molecules and that it takes higher energies to remove them. The considerable weight loss at 300 °C and the shoulder at 450 °C is due to the decomposition of proteins in ovalbumin.
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Temperature ( C) Fig. 3. TGA thermograms of (a) as-made dried gel precursor; (b) final powder after calcination of dried gel precursor; and (c) differential thermogram, DTG, of the precursor in (a). Fig. 5. TEM micrograph of Mn3O4 particles obtained upon calcination of the dried gel precursor at 800 °C.
3.4. SEM analysis
Experimental Simulation EPR signal (a.u.)
SEM analysis was performed to investigate the morphology of synthesized Mn3O4 powder and micrographs taken at different magnifications are presented in Fig. 4. Particles were observed to have various morphologies and a wide range of sizes making it difficult to assess the average size from the micrographs. Non-spherical morphology of particles with smooth surfaces suggests the shape is controlled by the entrapment in the dried gel, mainly due to the ovalbumin proteins that confines and stabilizes the metal ions in the gel. Particles with well-defined smooth surfaces are in majority with sizes ranging from 150 nm up to 2 lm. The particle size is much larger than the crystallite size obtained from XRD indicating the polycrystalline nature of the observed particle morphologies. 3.5. TEM analysis Morphology of Mn3O4 particles were also investigated by TEM and a micrograph is presented in Fig. 5 which shows a strong agglomeration. Even though particles have lateral dimensions on the order of 1–2 lm levels, their high transparency under TEM reveals a sheet like morphology. 3.6. EPR analysis The magnetic properties of the as-synthesized Mn3O4 were measured using EPR technique in X band and the resultant spectrum is presented in Fig. 6. The EPR spectrum can be used to detect Mn2+ and Mn4+, whereas Mn3+ is usually not detected due to the
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Applied Field (Gauss) Fig. 6. First derivative of EPR signal (e) and simulation (–) for Mn3O4 particles obtained upon calcination of the dried gel precursor at 800 °C.
complete splitting of the energy levels (no ground state degeneracy) [24]. Fig. 6 shows room temperature EPR spectrum of Mn3O4 including a narrow and symmetric line with its Gaussian simulation signal. The hyperfine splitting is not observed. EPR lines have a peak to peak line-width of 195 G, resonance field of 3475 G and g value
Fig. 4. SEM micrographs of Mn3O4 particles, obtained upon the calcination of the dried gel precursor at 800 °C, at different magnifications.
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of 2.01. Srinivasan et al. [25] have reported on the temperature dependence of DH, Hr and g parameters of Mn3O4 in detail. Our results are in accordance with their room temperature observations, where DH, Hr and g have been reported as 405 Oe, 3170 Oe and 2.01 at 9 GHz. Huber et al. [26] suggested effective g value of the unlike spin in the exchanged–coupled system as follows:
g eff ¼
g A SA ðSA þ 1ÞNA þ g B SB ðSB þ 1ÞN B SA ðSA þ 1ÞN A þ SB ðSB þ 1ÞN B
where g A and g B are the g values, SA and SB are spin moments, N A and NB are the relative concentrations of A and B sites, respectively. At Mn+2[Mn+3]2 O4, by assuming that Mn+2 has g A ¼ 2 and using SA ¼ 5=2 and SB ¼ 2 are spin moments, N A ¼ 1 and NB ¼ 2 and experimental g value as g eff ¼ 2:01, the Mn+3 in B site has g value 2.02 that means unquenched orbital moment resulting unsplitted EPR line [25]. EPR spectrum and exchange coupled equation reveal the absence of hyperfine splitting in the final powder. 4. Conclusion A novel, environment friendly route utilizing freshly extracted ovalbumin, and Mn2+ and Mn3+ salts, has been successfully used for the synthesis of Mn3O4 nanocrystals. High pH value of ovalbumin–water mixture self-regulated the pH for hydrolysis of metal ions. Water soluble ovalbumin proteins formed a gel and served as a matrix for entrapment of Mn2+ and Mn3+ ions. The dried gel precursors decomposed into nanocrystalline Mn3O4 upon calcination. Crystallite size was estimated as 22 ± 10 nm by XRD line profile fitting and particle size observed from SEM is in the range of 150 nm–2 lm. This discrepancy indicated polycrystalline nature of synthesized nanoparticles. EPR analysis showed a narrow and symmetric line indicating the absence of hyperfine splitting. Presented synthetic route is simple, cost effective, cheap and environmentally friendly that can be applied for the synthesis of other oxide systems. Acknowledgements The authors are thankful to the Fatih University, Research Project Foundation (Contract No.: P50020803-2) and Turkish Ministry
of Industry and Trade (Contract No.: 00185.STZ.2007-2) for financial support of this study. The fellowship from Knut and Alice Wallenberg’s Foundation for Dr. M.S. Toprak is thankfully acknowledged (No.: UAW2004.0224).
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