* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
Download Characterization of the citrate precursor, used for
Spectrum analyzer wikipedia , lookup
Ellipsometry wikipedia , lookup
Electron paramagnetic resonance wikipedia , lookup
Reflection high-energy electron diffraction wikipedia , lookup
Ionic compound wikipedia , lookup
Scanning tunneling spectroscopy wikipedia , lookup
Magnetic circular dichroism wikipedia , lookup
Atomic absorption spectroscopy wikipedia , lookup
Astronomical spectroscopy wikipedia , lookup
Two-dimensional nuclear magnetic resonance spectroscopy wikipedia , lookup
Particle-size distribution wikipedia , lookup
X-ray photoelectron spectroscopy wikipedia , lookup
Vibrational analysis with scanning probe microscopy wikipedia , lookup
Chemical imaging wikipedia , lookup
Stability constants of complexes wikipedia , lookup
Mössbauer spectroscopy wikipedia , lookup
Rutherford backscattering spectrometry wikipedia , lookup
Cent. Eur. J. Chem. • 7(3) • 2009 • 415-422 DOI: 10.2478/s11532-009-0054-7 Central European Journal of Chemistry Characterization of the citrate precursor, used for synthesis of nanosized Mg-Zn ferrites Research Article Violeta D.Kassabova-Zhetcheva Department Technology of Silicates, University of Chemical Technology and Metallurgy, Sofia 1756, Bulgaria Received 22 September 2008; Accepted 07 January 2009 Abstract: The citrate precursor has been used to synthesize nanocrystalline Mg-Zn-ferrites. The nature of the prepared precursor is characterized and compared with those of the precursors studied earlier, prepared by the same process. The study has been performed by inorganic and organic elemental analyses, Fourier Transformed Infrared Spectroscopy (FTIR), Mössbauer spectroscopy, X-ray Photoelectron Spectroscopy (XPS), Electron Paramagnetic Resonance (EPR), Electronic absorption spectrometry in the UV-VIS region, Differential Thermal analysis/ Thermogravimetry (DTA-TG) analyses, and X-ray diffraction (XRD) analysis. The collected results determined the precursor as a coordination polymer with monomer unit (NH4)4{M [Fe(C6H5O7)2]2}, where M=Zn or Mg. Keywords: Iron(III) citrate complexes • Mg-ferrite • Zn- ferrite • Citrate method © Versita Warsaw and Springer-Verlag Berlin Heidelberg. 1. Introduction The citrate precursor technique has been used to synthesize nanocrystalline Mg-Zn-ferrites with respect to their application in thermal cancer therapy [1]. The citrate precursors are widely used to produce a number of multi-component, phase-homogeneous nanosized ferrites [2-8]. According to its creators, the technique comprises in the obtaining of amorphous mixture retaining all the necessary metallic ions and citric acid in pure homogeneity [2]. From previous research studies, the citrate precursors are presented by the formulas: Zn3Fe6(C6H5O7)8•(12+n)H2O [3]; Ni3Fe6O4(C6H6O7)8•6H2O [4]; Co3Fe6(C6H6O7)8•6H2O [5] which defined them as mixtures of metal ions and citric acid. The authors in the following articles determined the precursors for ferrites of Mg(II), Ca(II) and Zn(II) as citrate complexes: Mg3[Fe(C6H5O7)2]2•10H2O [6]; Ca3[Fe(C6H5O7)2]2•8H2O [6] and Zn3[Fe(C6H5O7)2]2•12H2O [7]. The researchers in the next study described the citrate precursor for Ni2Fe2O4 as a linear-type polymer [(C6H6O7)4NiFe2]n [7]. The complexes described in [6,7] consist of citrate anions C6H5O73-, without information about the usage of a base. In [8], ammonium hydroxide was used, but the possibility for participation of NH4+ ions in the formed complex was not discussed. The possibility for a complex formation in the studied system: citric acid-Fe(III)-Mg(II)-Zn(II)-NH4+, was adopted due to the well known chelating properties of citric acid. The complex compounds that form the ions of magnesium and zinc, depend on the pH rate and the amount of citric acid. According to the dissociation scheme of citric acid [9,10], at a pH rate of about 6, theions of Zn(II) and Mg(II) may form the following complexes: [MC6H6O7]0, and [MC6H5O7]-, where M=Zn(II) and Mg(II). The coordination chemistry of citric acid - Fe(III) system is studied taking into account the biological importance only [10-12]. The established results in contributed to the better elucidation of the complex formation between Fe(III) and citric acid. It is important to note that two iron(III) citrate complexes, crystallized from solution at pH 6 and pH 7, consisted of ammonium ions as outer coordination sphere [12]. The authors have established that the complexes corresponded to the following formulas: (NH 4 ) 4 [Fe(C 6 H 5 O 7 )(C 6 H 6 O 7 )]• 3H 2 O, and (NH4)5[Fe(C6H5O7)2]•2H2O. The question of whether mixtures of initial compounds, complexes of citric acid or coordination * E-mail: [email protected] 415 Unauthenticated Download Date | 6/15/17 4:11 PM Characterization of the citrate precursor, used for synthesis of nanosized Mg-Zn ferrites polymers are formed needs further clarification. Moreover, the question regarding the type of citrate complexes formed and whether the ammonium ions take part in the citrate precursor for mixed Mg-Zn ferrites has not yet been answered. Thus, the aim of this paper is to study the citrate precursor for Mg-Zn ferrites which will contribute to the better understanding and controlling of the process of synthesis. 2. Experimental Procedures The citrate precursor for Mg-Zn-ferrites is prepared by the same mode described in [1]. In brief, it includes obtaining a mixed solution of stoichiometric quantities of Mg(NO3)2•6H2O, Zn(NO3)2•6H2O, and ferric citrate FeC6H5O7•H2O. The ratio of metal ions to anhydrous citric acid (C6H8O7) used is 1:3. Ammonia hydroxide is used for correcting the pH rate up to 6. The obtained solution is heated at 80°C. Solely, in this study, before the solution becomes viscous, the evaporation was stopped and crystalline compound was formed in open air for a month. The obtained compound was stored in a desiccator over P2O5. The examined sample was named “Precursor” and was prepared according to the formula Mg1-xZnxFe2O4, where: x=0.3. The sample “Precursor” was heat treated in the dryer at 100°C for 4 hours, after that the burned material was calcined at 250°C, 350°C, 450°C, and 600°C. The concentration of Fe(III) is determined by chemical titration. For determination of Zn and Mg, a method of Atomic Absorption Spectrometry (AAS) was applied by using a spectrophotometer Perkin-Elmer 5000. The content of C, H, and N in the obtained precursor was determined by the common organic analysis method using a Karlo Erba analyzer. The DTA-TG of the precursor was carried out in atmospheric air using a Paulik-PaulikErdey (MOM,Hungary) derivatograph. The heating rate was 10°C min-1 and amount of the sample was 100 mg. The X-ray diffraction patterns of studied sample were taken at room temperature (RT) in the Bragg-Brentano geometry (2θ from 5° to 80°), with Philips APD 15. The diffractogram was obtained using Cu Kα radiation (λ = 1.54178 Å). The FTIR-spectra were collected for disk specimens mixed with KBr using a Bruker Equinox 55 spectrometer in the range 4000-400 cm-1. Mössbauer spectroscopy gave additional information about the valence state of iron ions and their coordination. The transmission type spectrum was recorded with an electromechanical spectrometer Wissenschaftliche Elektronik GMBH, using a 57Co/Cr source in constant acceleration mode at RT. The velocity was calibrated by α-Fe standard at room temperature. The experimentally obtained spectrum was evaluated by a program, which assumes a Lorentzian shape of the Mössbauer spectral lines. The following parameters were determined isomer shift (IS), quadrupole splitting (QS), as well as the line width (FWHM), and the relative weight (G) of each component. The EPR spectrum was registered as the first derivative of the absorption signal with an X-band ERS-220/Q spectrometer at RT. Electronic absorption spectra were taken at RT with a UV-VIS spectrometer Cary 100 within the range of 200 to 800 nm. The UV-VIS spectra of the sample “Precursor” were recorded after dissolving in distilled water in ratio: 1:1 and 1:2, and for ratio 1:1 after a week under sunlight. The X-ray Photoelectron spectra were obtained using un-monochromatized Al Kα (1486.6 eV) radiation in a VG ESCALAB MK II electron spectrometer under base pressure of 1×10-8 Pa and a total instrumental resolution of 1 eV. The charging effects were corrected by using the C1s peak as reference at binding energy (BE) of 284.6 eV. The photoelectron spectra of C1s, O1s, N1s, Fe2p, Mg1s, and Zn2p were recorded and corrected by subtracting a Shirley-type background and then were quantified using the peak area and Scofield’s photoionization cross-sections. 3. Results and Discussion 3.1 FTIR- study The FTIR-spectrum of the studied precursor is presented in Fig. 1. The small shoulder at 3435 cm-1 confirms the presence of lattice water [13]. The free ammonium ion has four modes of vibration: a non-degenerate (۷1), a doubly degenerate (۷2), and two triply degenerate vibrations (۷3 and ۷4). All four vibrations are Raman active, whereas only ۷3 and ۷4 are IR-active. The fundamental frequencies ۷1, ۷2, ۷3, and ۷4 for the free ammonium ion are 3040, 1680, 3145, and 1400 cm-1 [14]. The infrared spectrum of NH4+ in a symmetrical environment contains the bands at 3145 and 1400 cm-1 only [14]. Ammonium absorption bands of the sample “Precursor” correspond to the NH4-bending vibration (۷4) at 1437 cm-1 and a series of overlapping bands from 2797 to 3213 cm-1. The overlapping bands arise from the NH4+ stretching vibration ۷3, combination mode ۷2 + ۷4, and overtones 2۷2 and 2۷4 [14]. The appearance of IR-inactive bands and overtones corresponds to the distorted Td symmetry of ammonium ion [13], probably as a result of interaction with the crystal field of citrate ligands. Such NH4+ vibrational spectrum is well known because it corresponds to those in many ammonium salts 416 Unauthenticated Download Date | 6/15/17 4:11 PM Violeta D.Kassabova-Zhetcheva [15]. Thus, a formation of ammonium salt in the sample “Precursor” can be expected. The frequency shift of the IR-active bands, with respect to the theoretical positions from 3150 to 3213 cm-1 and from1400 to 1437 cm-1 were assigned to formation of hydrogen bonds in the sample “Precursor” with NH4+ participation [15]. The formation of citrate complexes is proven by the doublets located at 1617 and 1578 cm-1 due to antisymmetric stretching vibration, and at 1415 and 1399 cm-1 due to symmetrical stretching vibration of ionized carboxylate groups [5,6,10,13]. The band positions are close to the ones cited in the analogous citrate precursors for ferrites, which are listed in Table 1. The IR-spectrum of the sample “Precursor” differs with a presence of two pronounced doublets for antisymmetric and symmetric stretching vibrations which reveals the occurrence of two non-equivalent carboxylate anions (Fig. 1). This fact could be explained by the presence of two types of coordination, which are deduced by the difference (∆) of antisymmetric and symmetric wave numbers (Table 1) [13]. As a result, a bidentate coordination could be assigned to Fe(III) ions. A sharp and distinct band at 566 cm-1 ascertains the bonding of Fe(III) to the citrate ligands in octahedral geometry [5,6]. Probably, in the unidentate type of coordination, the ions of Zn(II) and Mg(II) are involved. The two bands at 524 and 480 cm-1 were assigned to Zn-O and Mg-O bonds [16,17]. The most common coordination for d10 cations of Zn is tetrahedral and they mainly form covalent bonds with ligands [18]. For the d° cations of Mg, interactions with ligands are predominantly columbic or ionic, however, the tetrahedral geometry is also characteristic of their complexes [18]. The studied sample does not show any absorption bands above 1700 cm-1 indicating the absence of an undissociated -COOH group [18]. The deformation vibrations at 2601, 2527, 2050, 1288, 1262, 1200, 1137, and 1077 cm-1 are characteristic of the undissociated –OH group [16,19]. For the sample “Precursor”, the sharp and intense band observed at 1079 cm-1 is characteristic for the metal hydroxyl deformation modes and may be related to Fe-OH hydrogen bonding [16,19]. Normally, the metalhydroxyl deformation vibration is located at 1030 cm-1 and shifting the band to higher frequencies can be obtained by increased strength of hydrogen bonds and high degree of depolarization of O-H bond due to strong Lewis acid nature of Fe(III) ion [11]. The FTIR-spectra of the heat treated sample “Precursor” are shown in Figs. 2a-e. As can be seen, the bands of carboxylate anions are still present in the recorded spectra up to 250°C (Figs. 2a, b). Thus, the citrate complexes were thermally stable up to temperatures of the ignition of the auto-combustion process. The thermal decomposition of the citrate precursor began at 350°C and led to the initial formation of metal oxycarbonate complex. The spectral evidence Table 1. Infrared data for citrate precursors, citrate ionic compound, and citrate complex compound ۷as(COO-) ,cm-1 ۷ s(COO-) , cm-1 Δ = ۷as -۷s,cm-1 Citrate ionic compound, Na3 Citrate [19] 1609 1402 207, free of coordination “Precursor” for Mg0.7 Zn0.3Fe2O4 1617 1578 1399 1415 218, unidentate 163, bidentate Sample Citrate precursor for MgFe2O4, Mg3[Fe(C6H5O7)2]2•10H2O[6] 1680 1410 270, unidentate Citrate precursor for ZnFe2O4, Zn3[Fe(C6H5O7)2]2•12H2O [7] 1622 1385 237, unidentate Citrate precursor for NiFe2O4, [(C6H6O7)4NiFe2]n [8] Citrate complex compound, (NH4)5 Fe(C6H4O7)2•2H2O [11] Figure 1. FTIR-spectrum of sample “Precursor”. 1595 1389 206, unidentate from1626 to1586 from1436 to1372 ~200, unidentate Figure 2. FTIR-spectra of sample “Precursor” heat treated at: a) 100°C, b) 250°C, c) 350°C, d) 450°C and e) 600°C. 417 Unauthenticated Download Date | 6/15/17 4:11 PM Characterization of the citrate precursor, used for synthesis of nanosized Mg-Zn ferrites Figure 3. Mössbauer spectrum of sample “Precursor”. Figure 4. . EPR X-band spectrum of sample “Precursor”. (Fig. 2c) is revealed as splitting of the doubly degenerate vibration ν3 into two bands located at 1596 and 1409 cm-1 and appearing of infrared inactive ν1 at 1119 cm-1 [13,20]. With the augmentation of temperature from 450°C to 600°C, the bands assigned to oxycarbonate formation varied from 1506 to 1517 cm-1 and from 1415 to 1427 cm-1 along with bands between 1106 and 1119 cm-1. The splitting of the degenerate vibration ν3 was lowered with the rising of temperature and it can be suggested that bidentate oxycarbonate complex became unidentate [13]. 3.3 EPR study 3.2 Mössbauer spectroscopy study The experimentally obtained Mössbauer spectrum (Fig. 3) consists of line shoulders and central doublet (Db) part. The Mössbauer data shown in Table 2 indicate that all iron ions are in high spin (S = 5/2), in oxidation state +3, and in octahedral coordination. The coordination number of six for iron(III) satisfied by two citrate ligands which bind to iron through the oxygen atoms of the carboxylate groups [5,6]. The values of Mössbauer parameters for sample “Precursor” are close to those cited for complexes of magnesium bis(citrate) ferrate(III) decahydrateand zinc bis(citrate) ferrate(III) [6]. Table 2. In Fig. 4 is shown the EPR X-band spectrum of sample “Precursor”, which was registered as the first derivative of the absorption signal. It consists of one wide asymmetric line with width at about 149G (149.10-4 T) (Fig. 4). This broad single line between g = 1.89 and g = 1.96 in the spectrum is probably due to spin-spin interaction of Fe(III)-Fe(III) pairs [21]. It can be inferred that Fe centers are part of a polymer structure. The octahedral coordinated Fe(III) ions show an EPR signal of g-value of about 2 and an octahedral coordination can be assigned to Fe(III) ions of the sample “Precursor”. For comparison, a g-factor of citrate complex (NH4)5[Fe(C6H4O7)2]•H2O is 4.3 and consistent with the presence of high-spin rhombic Fe(III) species [10]. The result obtained for sample “Precursor” is in good agreement with the previously established data from Mössbauer spectroscopy. The spikes in the low field part can not be interpreted. 3.4 UV-VIS study The electronic absorption spectrum (Fig. 5, green line) of the dissolved sample “Precursor” in ratio 1:1 to distilled water, consisted of a broad, weak maximum located at 455 and strong, well-defined maximum at 394 nm. After the scanning of the diluted solution (ratio 1:2), the maxima appeared with a very small displacement at 453 nm and 380 nm (Fig. 5, brown line). Mössbauer parameters of different citrate precursors and citrate complex compound. Sample Component / Spectral view IS mm s-1 QS mm s-1 FWHM mm s-1 G, % “Precursor” Fe octa/br sym db 0.41 0.62 0.50 100 Citrate precursor for MgFe2O4 [6] Fe3+ octa/br sym db 0.38 0.59 - - Citrate precursor for ZnFe2O4 [7] Fe3+ octa/br sym db 0.42 0.58 - - Citrate complex compound (NH4)5[Fe(C6H4O7)2] •H2O Fe3+ octa/br sym db 0.38 1.80 - - 3+ 418 Unauthenticated Download Date | 6/15/17 4:11 PM Violeta D.Kassabova-Zhetcheva Figure 5. UV-VIS scans of sample “Precursor diluted in ratio: 1) 1:1, green line; 2) 1:2, brown line; and 3) 1:1, aged under sunlight, red line. The maximum at about 450 nm can be assigned to the 6A1 + 6A1 → 4T1(4G) + 4T1(4G) excitation of an Fe(III) – Fe(III) pair, which is as result of the magnetic coupling of electronic spins of next-nearest neighbor Fe(III) ions and referred to as a pair excitation or double excitation process [23]. A feature near 390 nm correspond to the 6A1 → E4 (4D) ligand field transitions of Fe(III) [25]. Judging by the very pale-yellow color of studied sample, a weak magnetic coupling can be suggested [21]. These observations gave rise to the assumption for the existence of a polymeric structure in the studied sample “Precursor”, which coordinated well with EPR results. From the UV-VIS spectrum of the mononuclear iron citrate complex, the only one ill-defined peak at 450 nm was established without assignment [10]. The last scan (Fig. 5, red line) was recorded after the aging of the dissolved sample “Precursor” (ratio 1:1) for a week under sunlight. During aging, the color turned green which can be explained with the formation of the Fe(II) citrate complex. The spectral view is a featureless scan that is characteristic for citrate complexes of Fe(II) [24]. In this way, the oxidation state of iron ions in the sample “Precursor” has been ascertained as “3+”. Figure 6. C1s XPS spectrum of sample “Precursor”. Figure 7. O1s XPS spectrum of sample “Precursor”. 3.5 XPS study To further examine the chemical structure of the sample “Precursor”, the XPS spectra of C1s, O1s, N1s, and Fe2p core levels were obtained (Figs. 6-9). The C1s spectrum was fitted with three peaks (Fig. 6). The C1s peak situated at 284.6 eV is ascribed to the carbon atoms in the aliphatic chain (C-C) [25]. The peak at 286.6 eV can be assigned to a C-OH group [26]. The peak situated at 288.0 eV was assigned to the carboxylate (-COO-) moiety [27]. No C1s corresponding to carboxylic carbon (-COOH) appeared in the spectrum, indicating the absence of free acid in the studied sample [27]. The fitting of O1s spectrum revealed the existence of three peaks situated at 529.9 eV, 531.4 eV, and 532.9 eV (Fig. 7). The peak at 529.9 eV is consistent with the data previously obtained for Fe-O bonding in hematite Figure 8. N1s XPS spectrum of sample “Precursor”. 419 Unauthenticated Download Date | 6/15/17 4:11 PM Characterization of the citrate precursor, used for synthesis of nanosized Mg-Zn ferrites [28]. This assigning of the photoelectron peak suggests that six oxygen ions were octahedrally coordinated to Fe(III) in the sample “Precursor”. The single and symmetric peak at 531.4 eV revealed the presence of two symmetric oxygen atoms in the carboxylate (-COO-) moiety and absence of C=O bonds in the sample [29]. The peak at 532.9 eV was assigned to C-OH group [26]. The O1s peaks for Mg-O and Zn-O bindings reported at about 530.5 eV and 531.2 eV, respectively [30,31]. Due to overlapping by the neighboring O1s regions, these peaks are not pronounced. The resolved N1s spectrum is due to two peaks (Fig. 8). The first peak situated at 399.7 eV was attributed to existence of NH3 and the second one situated at 401.5 eV was assigned to ammonium ion NH4+ [26]. The Fe2p peak with the specific spin-orbit split in two components (2p3/2,1/2) is shown in Fig. 9. To conclude from the Mössbauer, EPR, UV-VIS spectral data and assignment of the O1s core region, the Fe2p line-shape of sample “Precursor” was analyzed with respect to the fitting of peak positions for high spin Fe(III) compound. A large amount of XPS studies on the iron oxidation state in the iron oxides and reported binding energies value of Fe 2p3/2 peak, spread over broad superposed ranges: between 709.1 and 710.65 eV for FeO, between 707.9 and 710.7 eV for Fe2O4, and between 710.6 and 711.5 eV for Fe2O3 [32]. To infer the oxidation state of iron from the Fe 2p XPS core-level spectrum of sample “Precursor” turned out to be a difficult task because the cited data correspond to environment of oxygen ligands. The previous XPS data about iron citrate complexes are not found. The BE of Fe2p3/2 and Fe2p1/2 peaks in the analyzed sample “Precursor” are 709.74 eV and 723 eV respectively, along with satellites appeared as peak shoulders at 713.4 eV and 727 eV. It can be seen that BE value of Fe2p3/2 peak is lower than that cited for “3+” oxidation state in the Fe2O3 [32]. Since, the Fe(III) ion has a configuration of d5 in its ground state, probably the lowering of BE value of Fe2p3/2 peak can be explained by a process, which occurs in the final state via charge transfer from ligand to Fe(III) so as to form Fe3+ 2p1/21□2p3/243d6 or Fe3+ 2p1/22 2p3/23□d6 (□ – hole) and, since such transfer costs relatively little energy, such process will lower the binding energy [33]. Moreover, as the electronegativity of the ligand decreases, the electron density surrounding the Fe cation increases, meaning that the nucleus is more shielded, as a result the binding energy associated with ferric 2p3/2 photoelectron peak also decreased [35]. Identically, in the case of sample “Precursor”, the citrate ligand is a much weaker crystal field splitting ligand than that of O2- ligand, therefore the Fe(III) ions in the sample “Precursor” do not resemble the Fe(III) ions found in the iron oxides [34]. The sample “Precursor” present a single photoelectron Mg 1s peak at 1304.2 eV which can be related to the Mg–O binding and a single photoelectron Zn 2p3/2 peak at 1021.9 eV which can also be assigned to the Zn-O binding [31]. Figure 9. XPS spectrum from the Fe2p level of sample “Precursor”. Figure 10. XRD patterns of the sample “Precursor”. 3.6 XRD Study The formed precursor is crystalline, contrary to that previously reported as X-ray amorphous [1,5-7]. The XRD-patterns of the studied precursor are shown in Fig. 10. The sample “Precursor” consists of a lot of initial compounds, which extremely bothers the phase-identification. Moreover, there is not reference data concerning similar complicated compositions. Definitely, the comparison with PDF (Powder Diffraction Files) base leads to the conclusion that the sample “Precursor” is not a re-crystallized mixture of initial salts. Also, the XRD-patterns of the studied sample cannot be interpreted as phase-formation of the complex 420 Unauthenticated Download Date | 6/15/17 4:11 PM Violeta D.Kassabova-Zhetcheva Figure 11. DTA-TG traces of the sample “Precursor”. compounds (NH 4 ) 4 [Fe(C 6 H 5 O 7 )(C 6 H 6 O 7 )]•3H 2 O or (NH4)5[Fe(C6H5O7)2]•2H2O [11]. The presence of NH4NO3 was identified with JCPDS 47-0867. The X-ray diffraction patterns show that the interplanar distances of one of the crystal phases excising in the studied sample “Precursor” are very close to ammonium citrate (NH4)3C6H5O7 (JCPDS 45-1540). This fact could be interpreted as an indication of ammonium citrate being present in the system [35]. 3.7 Elemental analysis The results of inorganic and organic elemental analyses are shown in Table 3. The theoretically calculated gross formula of the studied sample Mg2.35ZnFe6.7C82H156O116N28 and such derived from elemental analyses Mg2.39ZnFe6.9C83H161O119N26 are in good agreement. The gross formula of the sample “Precursor” ascertained the assumption that the compound with high molecular weight like polymer is formed. Table 3. The elemental content of the sample “Precursor”. Elemental content, % Mg Zn Fe C H N Calculated 1.68 1.47 9.61 25.44 4.00 9.96 Observed 1.66 1.48 9.83 25.19 4.10 9.34 3.8 DTA-TG analysis The DTA and TG traces of sample “Precursor” (Fig. 11) revealed that the multi-step decomposition process took place. Such decomposition path is described in [5-7] and differs from one-step decomposition process, which was established in [36]. The DTA trace was started with a small endothermic effect at 120°C with corresponding weight loss of 9%. This event was attributed to the dehydratation process, which ended at about 180°C. The amount of hydrated water was determined at about 0.5 mol. The second weight loss of 50% and an exothermic event at 260°C indicate the beginning of the rapid decomposition process, which steeply passed trough the next exothermic event at about 400°C. The accompanied weight loss reached 73%. Based on the thermo-gravimetric calculations, it was believed that at this stage the formation of the intermediate compound, probably oxycarbonate Mg2.39ZnFe6.9(CO3)O5, took place. The formation of oxycarbonate compound may be explained with polymer structure of citrate precursor, which provides closeness between initial elements and prevents the fast cleavage leading to the formation of related metal oxides as it stands in [5,6]. The intermediate remains stable up to 440°C. At 490°C a decomposition of oxycarbonate took place, followed by fast exo-process which was accompanied by a mass loss of 80% and was attributed to the crystallization of mixed Mg-Zn ferrite phase at 540°C. The further decomposition of residual oxycarbonate with weight loss of 4% was characterized by an endothermic event at 580°C. The weight of the final residue is 16% of the initial weight, coinciding with the calculated value of Mg0.7Zn0.3Fe2O4, 16.7%. 4. Conclusions The collected data indicated that citrate precursor used for synthesis of Mg-Zn ferrites represents a coordination polymer. It was believed that monomer unit is (NH4)4{M [Fe(C6H5O7)2]2}, where M is Zn(II) or Mg(II). Both citrate ligands are three-ionized, and as such they utilize one terminal as well and central carboxylate to bind in a bidentate fashion to Fe(III). In order to satisfy the coordination requirements of the octahedral Fe(III), the hydroxyl moiety is bound by hydrogen bond to Fe(III). The third, terminal, ionized carboxylate group of both citrate ligands are coordinated to Mg(II) or Zn(II) ions. In this way, the ions of Mg and Zn bridged neighbor citrate ligands and form chains. Probably, the NH4+ ions linked polymeric chains by hydrogen bonds and as a result a 3D-polymeric network is formed. Additionally, the NH4+ promotes crystal packing. The elemental and DTA-TG analyses have ascertained the theoretically calculated gross formula of the citrate precursor as Mg2.35ZnFe6.7C82H156O116N28. The polymer structure of the citrate precursor allowed converting the gel into corresponding Mg-Zn ferrite powders, avoiding the stage of a solid state synthesis from corresponding oxides. This is possible due to structure-based ligand design. Since the polymer chains are sequences of the octahedral citrate complexes of Fe(III) and tetrahedral 421 Unauthenticated Download Date | 6/15/17 4:11 PM Characterization of the citrate precursor, used for synthesis of nanosized Mg-Zn ferrites citrate complexes of Zn(II) or Mg(II), they served as a template for the spinel lattice in the precursor stage. During the calcinations, the cations rested on the previously set positions as a result of the thermal stability of the citrate complexes and the formed oxycarbonate intermediate complex. Thus, by using citrate ligands it could be possible to tailor the cation distributions which are crucial for the magnetic properties of the ferrites Acknowledgements I am grateful to Prof. Isabelle Gautier-Luneau and Prof. Athanasios Salifoglou for kindly provided articles. References [1] V.D. Kassabova-Zhetcheva, L.P. Pavlova, B.I. Samuneva, Z.P. Cherkezova-Zheleva, I.G. Mitov, M.T. Mikhov, Cent. Eur. J. Chem. 5, 107 (2007) [2] C.Marcilly, P. Courty, B. Delmon, J. Am. Cer. Soc. 53, 56 (1970) [3] N.S.Gajbhiye, U. Bhattacharya, V.S. Darshane, Thermochim. Acta 264, 219 (1995) [4] N.S. Gajbhiye, S. Prasad, Thermochim. Acta 285, 325 (1996) [5] S. Prasad, A. Vijayalakshmi, N.S. Gajbhiye, J. Therm. Anal. Calorim. 52, 595 (1998) [6] B.S. Randhawa, M. Kaur, J. Radioanal. Nucl. Chem. 261, 569 (2004) [7] B.S. Randhawa, M. Kaur, J. Radioanal. Nucl.Chem. 256, 509 (2003) [8] C.-Y.Zhang, X.-Q. Shen, J.-X. Zhou, M.-X, Jing, K. Cao, J. Sol-Gel Sci. Tech. 42, 95 (2007) [9] J.-H. Choy, Y.-S. Han, J. Mater. Chem. 7, 1815 (1997) [10] J.L. Pierre, I. Gautier-Luneau, BioMet. 13, 91 (2000) [11] M. Matzapetakis, C.P. Raptopoulou, A. Tsonos, V. Papaefthymiou, N. Moon, A. Salifoglou, J. Am. Chem. Soc. 120, 13266 (1998) [12]I. Gautier-Luneau, C. Merle, D. Phanon, C. Lebrun, F. Biaso, G. Serratrice, J.-L. Pierre, Chem. Eur. J. 11, 2207 (2005) [13]K. Nakamoto, Infrared and Raman spectra of Inorganic and Coordination Compounds, 3rd edition (Wiley Interscience Publication, New York, 1978) [14]V. Busigny, P. Cartigny, P. Philippot, M.Javoy, Am. Mineralog. 89, 1625 (2004) [15]S. Petit, D. Righi, J. Madejova, A. Decarreau, Clay Mineral. 33, 579 (1998) [16]V.I. Sumin De Portilla, Am. Mineralog. 61, 95 (1976) [17]A. Moses Ezhil Raj, L.C. Nehru, M. Jayachandran, C. Sanjeeviraja, Cryst. Res. Techol. 42, 867 (2007) [18]D. Crerar, S. Wood, S. Brantley, Can. Mineralog. 23, 333 (1985) [19]J. Aikaite, O. Gyliene, O. Nivinskiene, Chemija (Vilnius) 14, 135 (2003) [20]J. Perez-Ramirez, G. Mul, F.F. Kapteijn, J.A. Muolijn, J. Mater. Chem. 11, 2529 (2001) [21]I. Petrov, F. Yude, L.V. Bershow, S.S. Hafner, H. Kroll, Am. Mineralog. 74, 604 (1989) [22]T.R.N. Kutty, M. Nayak, Mater. Res. Bull. 34, 249 (1999) [23]D.M. Sherman, T. Davit Whaite, Am. Mineralog. 70, 1262 (1985) [24]A.J. Francis, C.J. Dodge, App. Env. Microbiol. 59, 109 (1993) [25]M. Rjeb, A. Labzour, A. Rjeb, S. Sayouri, M. Chafil El Idrissi, S. Massey, A. Adnot, D. Roy, M. J. Cond. Mater. 5, 168 (2004) [26]S.J. Kerber, J.J. Bruckner, K. Wozniak, S. Seal, S. Hardcastle, T.L. Barr, J. Vac. Sci. Technol. A 14, 1314 (1996) [27]A. Dmitriev, H. Spillmann, S. Stepanow, T. Strunskus, C. Woll, A.P. Seitsonen, M. Lingenfelder, N. Lin, J.V. Barth, K. Kern, ChemPhysChem. 7, 2197 (2006) [28]J. Lutzenkirchen, Surface Complexation Modeling (Elsevier Academic Press, New York, 2006) 54 [29]N. Wu, L. Fu, M. Su, M. Aslam, K.C. Wong, V.P. Dravid, Nano Lett. 4, 383 (2004) [30]S. Altieri, S.F. Contri, S. Agnoli, S.Valeri, Surf. Sci. 566–568, 1071 (2004) [31]B.Y. Zhu, H.I. Elim, Y.-L. Foo, T. Yu, Y. Liu, W. Ji, J.-Y. Lee, Z. Shen, A. Thye-Shen Wee, J. ThiamLeong Thong, C.-H. Sow, Adv. Mater. 18, 587 (2006) [32] R. Turcu, D. Bika, L. Vekas, N. Aldea, D. Makovei, A. Nan, O. Pana, O. Marinica, R. Grecu, C.V.L. Pop, Rom. Rep. Phys. 58, 359 (2006) [33]G. Van der Laan, C. Westra, C. Hass, G.A. Sawatzky, Phys. Rev. B 23, 4369 (1981) [34]A.P. Grosvenor, B.A. Kobe, M.C. Biensinger, N.C. McIntyre, Surf. Interface Anal. 36, 1564 (2004) [35]M. Getsova, D. Todorovsky, V. Enchev, I. Wawer, Monatshef. Chem. (Chem. Mon.) 138, 389 (2007) [36]C. Cannas, A. Falqui, A. Musinu, D. Peddis, G. Piccaluga, J. Nanopart. Res. 8, 255 (2006) 422 Unauthenticated Download Date | 6/15/17 4:11 PM