Effect of starting solution acidity on the characteristics of CoFe2O4 powders prepared by solution combustion synthesis

Author’s Accepted Manuscript Effect of starting solution acidity on the characteristics of CoFe2O4 powders prepared by solution combustion synthesis B. Pourgolmohammad, S.M. Masoudpanah, M.R. Aboutalebi www.elsevier.com/locate/jmmm

PII: DOI: Reference:

S0304-8853(16)32168-0 http://dx.doi.org/10.1016/j.jmmm.2016.10.073 MAGMA61986

To appear in: Journal of Magnetism and Magnetic Materials Received date: 11 September 2016 Revised date: 8 October 2016 Accepted date: 15 October 2016 Cite this article as: B. Pourgolmohammad, S.M. Masoudpanah and M.R. Aboutalebi, Effect of starting solution acidity on the characteristics of CoFe2O powders prepared by solution combustion synthesis, Journal of Magnetism and Magnetic Materials, http://dx.doi.org/10.1016/j.jmmm.2016.10.073 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Effect of starting solution acidity on the characteristics of CoFe2O4 powders prepared by solution combustion synthesis

B. Pourgolmohammad, S. M. Masoudpanah*, M. R. Aboutalebi School of Metallurgy & Materials Engineering, Iran University of Science and Technology (IUST), Narmak, Tehran, Iran * Corresponding author. Tel.: +98 21 77240540, Fax:+98 21 77240480. [email protected]

Abstract Cobalt ferrite (CoFe2O4) nanoparticles were synthesized at the different pH values of starting solution, adjusted by NH4OH, using solution combustion method. Theoretical calculations and Fourier transform infrared spectroscopy analysis were carried out for determination and controlling the chelated species in solution. The phase evolution, surface area, morphology and magnetic properties of the combusted CoFe2O4 powders have been investigated by thermal analysis, X-ray diffractometry, electron microscopy, adsorption–desorption and vibrating sample magnetometer. The combustion rate mainly depends on pH which affects the phase and crystallite size. Furthermore, the specific surface area of the porous CoFe2O4 powders decreases from 66.25 to 27.09 m2/g by the increase of pH from 2 to 10. The combusted CoFe2O4 powders exhibit ferromagnetic properties which the highest saturation magnetization of ~63.7 emu/g was achieved at pH of 2. Furthermore, the coercivity increases from 1112 to 1225 Oe by the increase of pH due to the decreasing of crystallite size.

Keywords Cobalt ferrite; Solution combustion synthesis; Acidity; Magnetic property;

1. Introduction Combustion synthesis which also known as self-propagating high-temperature synthesis (SHS) is an effective energy saving and low-cost method for production of various advanced materials [13]. The initial heterogeneous mixture in combustion synthesis is ignited by an external thermal source which leads to propagation of a rapid high-temperature (1000–3000 °C) reaction wave in

1

a self-sustained manner, causing the formation of the solid material without involving any additional energy [4, 5]. However, the coarse initial powders (10–100 m) along with the high reaction temperatures result in the well-crystalline products with the low surface area and coarse sizes. Therefore, the other treatments such as mechanical activation, intensive milling and chemical dispersion are applied for reducing the particle size after combustion, which increase the preparation steps and cost [6-8].

Solution combustion synthesis (SCS) which begins from an aqueous reactive solution is a versatile, simple and rapid process and suitable for rapid and direct synthesis of mass nanomaterials [9, 10]. Solution combustion synthesis employs an exothermic reaction between an oxidizer, typically metal salts, like nitrates, sulfates and carbonates and an organic fuel such as urea, glycine, citric acid and sucrose [11]. The combustion reaction releases enough thermal energy for the formation of oxides, although the required heat input to trigger the combustion reaction is not high [12]. Among various metal salts, the metal nitrates are generally utilized due to the lower decomposition temperature, good solubility in water and the efficient oxidizing NO3− groups [13]. Moreover, the urea (CO(NH2)2) and glycine (NH2CH2COOH) are usually used as fuels due to their low cost, good availability, high exothermicity as well as their coordination ability toward cations [14]. The complex formation helps to achieve a more homogeneous mixing and hence avoids cations segregation which is beneficial for producing multicomponent oxides. However, many efforts have been performed to modify the SCS parameters such as fuel type [15], fuel to oxidizer ratio [16] and pH [17] for adjusting the physicochemical properties of the synthesized nanoparticles, like the phase, morphology, particle size and surface area. For example, the pH value of the starting solution can be effective on preventing from selective precipitation and/or phase separation during the evaporation of solvents [18]. Wu et al. also showed that the combustion reaction of the dried gels are more completed with the increase of pH which results in the pure ferrite nanoparticles [19].

Spinel magnetic ferrites, especially cobalt ferrite (CoFe2O4), have been considered as wellknown materials for electronic devices, magneto-optical recording and purification of contaminated water [20-22]. Owing to the great application potentials of cobalt ferrite in energy storage [23], catalysis [24], sensing [25] and adsorption and separation [26], magnetic

2

nanoparticles with the high surface area have been of keen interest recently. Among various synthesis methods such as coprecipitation [27], hydrothermal [28] and thermal decomposition [29], SCS has attracted considerable attention due to its ability for production of the high surface area CoFe2O4 powders for the biomedical applications such as magnetic resonance imaging, cell separation and hyperthermia tumor treatment [30-33].

In the present study, the effects of the pH values of the starting solution on the phase, morphology, particle size, specific surface area and magnetic properties are analyzed to achieve reliable conditions for solution combustion synthesis of cobalt ferrite nanoparticles. Our results show that the combustion reaction rate of the solution depends on pH which affect the physicochemical properties, especially specific surface area, of the combusted CoFe2O4 powders. 2. Experimental procedure The required amounts of glycine (C2H5NO2), ferric nitrate (Fe(NO3)3.9H2O) and cobalt nitrate (Co(NO3)2.6H2O) obtained from Merck Co. were dissolved in the distilled water in which the fuel to oxidant molar ratio was considered as unity. After homogenization, the pH value of the solution was adjusted to 2, 7 and 10 using 25 wt.% ammonia (NH4OH) solution. The dark brown homogeneous solution was poured into a dish and heated till to transform into a gel while by further heating up to a certain temperature, ignition reaction started from a point which was the most ready for this ignition and then the combustion front propagated spontaneously towards the walls of the dish due to the exothermic reaction. The combusted powders were hand-crushed with a pestle for the following characterization. IR spectra in the range of 400–4000 cm−1 were measured by Fourier transform infrared (FTIR) spectrometer (8500S SHIMADZU). Thermal decomposition of the dried gel at 80 °C was examined by simultaneous differential thermal and thermogravity analysis (DTA/TGA) in air with the heating rate of 5 °C/min on the STA BäHR 503 instrument. The phase evolution was analyzed by Philips X’pert X-ray diffractometer (XRD) using monochromatic CuKα radiation. The average crystallite size was also calculated from the width (311) peak using Scherrer’s equation. The XRD patterns were also submitted to a crystal

3

structure analysis by the Rietveld method using Highscore plus software. The cation distribution was obtained from the analysis of XRD patterns by Bertaut method in which the observed intensity ratios were compared with the calculated intensity ratios [34].

The morphology and microstructure were examined by TESCAN Vega II field emission scanning electron microscopy (FESEM)

The Brunauer–Emmett–Teller (BET) surface area was measured by N2 gas adsorption technique using a PHS-1020 instrument, after degassing at 250 °C for 5 h. The Barrett–Joyner–Halenda (BJH) cumulative volume of pores was calculated from the adsorption branch of the isotherms. The equivalent particle size was calculated based on the BET surface area as follows (1): (1) where DBET is the equivalent particle size (nm),  is the theoretical density of CoFe2O4 (5.3 g/cm3) and SBET stands for the BET surface area (m2/g). A vibrating sample magnetometer (Meghnatis Daghigh Kavir Co., Iran) was also employed to measure the magnetic properties of the samples at room temperature.

3. Results and discussion Glycine as a simple amino acid can effectively complex various metal ions because it contains a carboxylic acid group at one end and an amino group at the other end [14]. Therefore, the glycine molecules prevent the selective precipitation of cations and maintain the compositional homogeneity. However, the ionization and complexing ability of glycine with cations mainly depends on pH and the fuel to oxidant molar ratio, as shown in Fig. 1. In Fig. 1(a), it is demonstrated that the Fe3+ cations easily complex with the glycine in the form of Fe(NH2CH2COO) as pH approaches to 8 for the fuel to oxidant molar ratio of 0.75. As the fuel/oxidant molar ratio increases, the relative concentration of Fe(NH2CH2COO) complex increases near to 100% as pH>6. In Fig. 1(b), it is shown that the various forms of the Co2+– glycine complexes varies with the pH values: the relative concentration of Co(NH2CH2COO) initially increases from pH of 6 and approaches to the local maximum concentration of about

4

60% at pH of 8, then gradually goes down near to 0 at pH≈10 for the fuel/oxidant molar ratios of 1 and 1.25, whereas the Co(NH2CH2COO) complex remains at 20% as pH>10 for the fuel/oxidant molar ratio of 0.75. However, the relative concentration of Co(NH2CH2COO)2 increases at pH=7 and approaches to ~ 75% at pH>10 for the fuel/oxidant molar ratio of 0.75 which it increases to ~97% with the increase of the fuel to oxidant molar ratio. Consequently, more stable complexes of Fe3+ and Co2+ cations exist in the solutions with the higher pH and fuel/oxidant molar ratios. The existence of these complexes within the solution inhibits the selective precipitation and facilitates the formation of single phase cobalt ferrite nanoparticles.

Fig. 2 shows FTIR spectra of the dried gels at 80 °C prepared with the different pH values. The broad band in the region of 3700–3000 cm-1 can be ascribed to the symmetric and antisymmetric stretching vibration modes of the hydroxyl group originating from the organic phases and the residual water [35]. The absorption bands at 1380, 827 and 900 cm-1 correspond to the NO3−

ions which demonstrate that the NO3−

exists as a group in the structure of gel during

the gelation of solution formed from nitrates and glycine [36]. The rocking vibration of glycine NH2 group is the cause of absorption bands at about 1112 and 1038 cm−1. Furthermore, the NO stretching vibration leads to the absorption bands at 1460 and 688 cm−1. The band at 595 cm-1 belongs to Fe(OH)2+ species [37, 38]. The absorption bands at about 1640 and 1758 cm-1 in IR spectra of the dried gel prepared at pH of 2 correspond to the deformation vibration of NH2 and stretching vibration of C=O in COOH group, respectively [38]. Therefore, the glycine coordinates to metal cation as monodentate ligand due to the simultaneous presence of NH2 and COOH, as shown in Fig. 3a. With the increasing of pH, the absorption bands appear at 1620 and 1357 cm-1 corresponding to the asymmetric and symmetric stretching vibrations of dissociated COO−

group. Consequently, the

bidentate ligand of glycine (Fig. 3b) can be formed with the increase of pH which can be established by appearing the absorption band at 518 cm-1 for the M-O bonds [39]. The more chelated species leads to the more homogeneity of cations which helps in the formation of cobalt ferrite nanoparticles.

5

TGA/DTA curves of the dried gels prepared at the different pH values are shown in Fig. 4. The vaporization of absorbed water and dehydration reaction of gel result in the weight loss of ~8% in the temperature range of 25-180 °C [40]. A sharp weight loss of ~79% on the TGA curve for pH of 2 which is accompanied with a sharp exothermic peak at 180 °C can be related to the exothermic combustion reaction between glycine and nitrate ions in the gels with the liberation of H2O, CO2 and N2. Above 190 °C, almost no weight loss take place indicating the completion of thermal decomposition of the gel. However, the lower weight loss of about ~38% along with the exothermic peak at 175 °C can be observed for pH of 7, may be due to the existence of carboxylate (COO−) ions which retard the combustion reaction. Moreover, the gradual weight loss of ~15% from 190 to 700 °C is related to the slow oxidation of solid organic residues. Consequently, the need more energy for decomposition of bidentate ligands at pH of 7 leads to the lower overall weight loss of ~57%. However, the sharp weight loss of ~72% at about 180 °C with the following gradual weight loss of ~19% is observed for pH of 10, in spite of the existence of bidentate ligands. The higher overall weight loss of ~98% at pH of 10 can be attributed to the existence of more NH4NO3 which can be decomposed to NOx and O2 oxidants [18].

Fig. 5 shows XRD patterns of the combusted powders at the different pH values. The patterns show that the combusted powders at pH of 2 and 10 are almost single phase cobalt ferrite (JCPDS card no. 22-1086). However, the powders synthesized at pH of 7 contain some impurity phases of -Fe2O3 and CoO, due to the slow decomposition of its gel which prevents simultaneous decomposition of the ferric and cobalt complexes and results in the formation of the metal oxides separately. Consequently, the fast combustion reaction at pH of 2 leads to the formation of single phase cobalt ferrite nanoparticles, despite the existence of free cations. The chelated species and liberation of enough energy during the combustion reaction at pH of 10 make easily formation of single phase CoFe2O4 nanoparticles. The Rietveld refinement results of the powders synthesized at pH of 2 are shown in Fig. 6. The fitting qualities of the pattern, Rexp, Rwp, and 2 are 22%, 25%, and 1.07, respectively, showing a fairly good refinement. Table 1 presents the cation distribution, lattice parameter, crystallite size and magnetic properties of the CoFe2O4 powders. Cation distribution shows that the

6

CoFe2O4 powders have a partially inverse structure in which the divalent cations (Co2+) are distributed in tetrahedral (A-sites) and octahedral (B-sites) sites. It can be seen the inversion coefficient defined as the fraction of Fe3+ cations in tetrahedral sites (A-sites) depends on the synthesis conditions. The insufficient distribution of cations in the combusted powders at pH of 7 results in the lower inversion coefficient [41]. However, the higher inversion coefficient for pH of 10 is due to the more homogeneous distribution of cations, originating of the coordinated species. The higher inversion can also be confirmed by the closer lattice parameter with the theoretical lattice parameter (a = 0.8392 nm) for the complete inverse cobalt ferrite [42]. Table 1 also illustrates that the crystallite size decreases from 41 to 32 nm by increasing of pH on account of the slower combustion reaction at the high pH values.

The adsorption–desorption isotherms and the SEM and TEM micrographs of the combusted CoFe2O4 powders at the different pH values are shown in Fig. 7. Table 2 also presents the specific surface area (SBET), equivalent particle size (DBET) and pore volume. The combusted CoFe2O4 powders show type IV isotherms with H3 hysteresis according to the International Union of Pure and Applied Chemistry (IUPAC) classification, which corresponds to the presence of fragile porous particle agglomerations. The specific surface area decreases from 66.25 to 27.09 m2/g by the increase of pH which may be due to the broadening thermal decomposition for the higher pH values, leading to the hard agglomeration among active primary crystallites [18]. However, the rapid thermal decomposition of the gels at pH of 2 results in the generation of large amount of gases (such as, CO, CO2, NO, NO2, NH3, water vapor, etc.). It not only helps in preventing the formation of a dense structure and results in a higher surface area, but also disintegrates the large particles to yield nanoparticles, as confirmed by the higher pore volume at pH of 2 (Table 2). Consequently, the slow rate of decomposition along with the relatively weak exothermic peak at pH of 7 results in the agglomeration of the primary crystallites and the lowest pore volume (Table 2). Furthermore, the equivalent particle size, DBET, calculated from the BET surface area is larger than the crystallite size, DXRD, calculated from the XRD patterns which shows the combusted CoFe2O4 powders at pH values of 7 and 10 are not single crystals, but they contain several crystallites, are polycrystalline as confirmed by the TEM images of the samples.

7

The evolution of specific surface area and cumulative pore volume as a function of pH values of the starting solution is also confirmed by the SEM and TEM images (Fig. 7). The particles are associated in agglomerations smaller than 30 m, which is in accordance to the type H3 hysteresis shown by the adsorption–desorption isotherms. The SEM images revealed the combusted powders at low pH values consist of porous and loosely bounded particles which are turned to bulk with the increasing of pH. The foamy structure and plenty of tiny pores are induced by the reactions between fuels and oxidizers which are resulted in the large amount of escaping gases and short combustion duration. These reaction features inhibit crystal growth and favor the synthesis of porous particles constructed by crystallites [43]. This particular kind of morphology is consistent with the large cumulative pore volumes (Table 2). However, the bulky morphology of the combusted powders at the higher pH values can also be attributed to the establishment of polymeric network between the chelated species which postpones thermal decomposition and results in the low specific surface areas [44].

The TEM images of the combusted CoFe2O4 powders at the different pH values are also shown as inset in Fig. 7. The powders contain many spherical like nanoparticles with the average size of about 50, 39 and 37 nm for pH values of 2, 7 and 9, respectively, which are fairly in agreement with the XRD crystallite sizes.

The magnetization curves of the CoFe2O4 powders prepared at the different pH values are shown in Fig. 8. The saturation magnetization (Ms) and coercivity (Hc) values are also presented in Table 1. The magnetic properties of spinel ferrite can be mainly modified by the phase, particle size, and cation distribution and so on. The coercivity (Hc) of the CoFe2O4 powders increases from 1112 to 1225 Oe with the increasing of pH from 2 to 7 due to the decrease of the crystallite size. The crystallite size of 40 nm is a single-domain size limit for cobalt ferrite which the decrease of Hc for D<40 nm is due to the prevailing of the thermal energy on the anisotropy energy, leading to thermally assisted jumps over the anisotropy barriers. However, the increase of Hc with decreasing size for D<40 nm can be attributed to the enhanced role of the surface and its strong anisotropy [45].

8

The low saturation magnetization of the combusted CoFe2O4 powders at pH of 7 can be attributed to the presence of antiferromagnetic -Fe2O3 and impurity CoO phases, as evidenced from the XRD results (Fig. 5). The highest saturation magnetization of 63.7 emu/g for pH of 2 is due to the nearly pure of spinel phase and large crystallite size [46]. In spite of purity of the combusted powders at pH of 9, their saturation magnetization (61.1 emu/g) is lower than that of the powders obtained at pH of 2 due to their higher inversity. The spinel ferrites with a cubic crystal structure have two different crystallographic sublattices for magnetic ions, i.e., tetrahedral (A) and octahedral [B] sublattices. The magnetic order in magnetic spinel ferrite mainly results from superexchange interaction between the magnetic ions in the A and B sublattices mediated by oxygen ions. According to the Neel model, A–B superexchange interactions are predominant over intrasublattice A–A and B–B interactions and the saturation magnetization is given by the vector sum of the net magnetic moments of the individual A and B sublattices [44, 47]. The more Fe3+ cations with the ionic magnetic moments of 5B in octahedral (A) sites in the higher inversity decreases the magnetic moment of the B-site and then the total magnetic moments.

4. Conclusions CoFe2O4 powders have been prepared by solution combustion synthesis using glycine as fuel and metal nitrates as oxidizer at the different pH (2, 7 and 10) of the starting solution. The pH values take effect on the cations distribution in the solution and the combustion rates through the formation of the different chelated species and then have influences on the structure, surface area, morphology and magnetic properties. The slow combustion rate for pH of 7 results in the formation of the impure -Fe2O3 and CoO phases together with the cobalt ferrite phase, causing the lowest saturation magnetization (58.8 emu/g). The morphology of the combusted powders changes from sponge to bulk accompanied by the decreasing of surface area with the increase of pH. The highest saturation magnetization of ~63.7 emu/g was achieved for pH of 2 while its lowest coercivity (1112 Oe) was attributed to the large crystallite size (~41 nm).

References 1. A. S. Mukasyan, P. Epstein, P. Dinka, “Solution combustion synthesis of nanomaterials”, Proceedings of the Combustion Institute, 31(2007)1789–1795

9

2. H. C. Yi and J. J. Moore, “Self-propagating-high-temperature-combustion-synthesis-SHS-ofpowder-compacted-materials”, Journal of Materials Science, 25(1990)1159-1168 3. S. T. Aruna, A. S. Mukasyan, “Combustion synthesis and nanomaterials”, Current Opinion in Solid State and Materials Science, 12(2008)44–50 4. A. G. Merzhanov, “The chemistry of self-propagating high-temperature synthesis”, J. Mater. Chem., 14(2004)1779–1786 5. A. Varma, V. Diakov, E. Shafirovich, “Heterogeneous combustion: recent developments and new opportunities for chemical engineers”, J AIChE, 51(2005)2876–2884 6. F. Bernard and E. Gaffet, “Mechanical alloying in SHS research”, Int. J. Self-Propag. HighTemp. Synth., 10(2001)109-132. 7. L. Stobierski, Z. Wegrzyn, J. Lis and M. Buck, “SHS synthesis of nanocomposite AlN–SiC powders”, Int. J. Self Propag. High-Temp. Synth., 10(2001)217-228. 8. I. P. Borovinskaya, T. I. Ignat'eva, V. I. Vershinnikov, G. G. Khurtina and N. V. Sachkova, “Preparation of ultrafine boron nitride powders by self-propagating high-temperature synthesis”, Inorg. Mater., 39(2003)588-593. 9. W. Wen and J. M. Wu, “Nanomaterials via solution combustion synthesis: a step nearer to controllability”, RSC Adv., 4(2014)58090-58100 10. F. Li, J. Ran, M. Jaroniec and S. Z. Qiao, “Solution combustion synthesis of metal oxide nanomaterials for energy storage and conversion”, Nanoscale, 7(2015)17590–17610 11. J. J. Kingsley and K. C. Patil, “A novel combustion process for the synthesis of fine particle α-alumina and related oxide materials”, Mater. Lett., 6(1988)427-432. 12. K. Rajeshwar and N. R. de Tacconi, “Solution combustion synthesis of oxide semiconductors for solar energy conversion and environmental remediation”, Chem. Soc. Rev., 38(2009)19841998. 13. J. S. Zhang, Q. J. Guo, Y. Z. Liu and Y. Cheng, “Preparation and Characterization of Fe2O3/Al2O3 Using the Solution Combustion Approach for Chemical Looping Combustion’,Ind. Eng. Chem. Res., 51(2012)12773-12781. 14. L.A. Chick, L.R. Pederson, G.D. Maupin, J.L. Bates, L.E. Thomas and G. J. Exarhos, “Glycine-nitrate combustion synthesis of oxide ceramic powders”, Mater. Lett., 10(1990)6-12 15. A. Naveenkumar, P. Kuruva, C. Shivakumara, and C. Srilakshmi, “Mixture of fuels approach for the synthesis of SrFeO3−δ nanocatalyst and its impact on the catalytic reduction of nitrobenzene”, Inorg. Chem., 53(2014)12178−12185 16. K. V. Manukyan, A. Cross, et al., “Solution Combustion Synthesis of Nano-Crystalline Metallic Materials: Mechanistic Studies”, J. Phys. Chem. C, 117(2013)24417−24427 17. S. Salem, “Rapid combustion synthesis of pure nanocrystalline gahnite: Effect of solution pH on powder characteristics”, Thermochimica Acta, 609(2015)75–81 18. T. Peng, X. Liu, K. Dai, J. Xiao, H. Song, “Effect of acidity on the glycine–nitrate combustion synthesis of nanocrystalline alumina powder”, Materials Research Bulletin, 41(2006)1638–1645 19. K.H. Wu, C.H. Yu, Y.C. Chang, D.N. Horng, “Effect of pH on the formation and combustion process of sol–gel auto-combustion derived NiZn ferrite/SiO2 composites”, Journal of Solid State Chemistry 177 (2004) 4119–4125

10

20. Y.H. Hou, Y.L. Huang, S.J. Hou, S.C. Ma, Z.W. Liu, Y.F. Ouyang, “Structural, electronic and magnetic properties of RE3+-doping in CoFe2O4: A first-principles study”, Journal of Magnetism and Magnetic Materials, 421(2017)300-305. 21. S. Agrawal, A. Parveen,, A. Azam, “Structural, electrical, and optomagnetic tweaking of Zn doped CoFe2-xZnxO4-δ nanoparticles”, Journal of Magnetism and Magnetic Materials, 414(2016)114-152 22. D. H. K. Reddy and Y. S. Yun, “Spinel ferrite magnetic adsorbents: Alternative future materials for water purification?”, Coordination Chemistry Reviews 315 (2016) 90–111 23. A. K. Rai, J. Gim, T. V. Thi, D. Ahn, S. J. Cho and J. Kim, “High Rate Capability and Long Cycle Stability of Co3O4/CoFe2O4 Nanocomposite as an Anode Material for High-Performance Secondary Lithium Ion Batteries”, J. Phys. Chem. C, 118(2014)11234−11243 24. D. Zhang, X. Pu, Y. Gao, C. Su, H. Li, W. Hang, “One-step combustion synthesis of CoFe2O4–graphene hybrid materials for photodegradation of methylene blue”, Materials Letters, 113(2013)179–181. 25. E. Ranjith Kumar, A.S. Kamzin, K. Janani, “Effect of annealing on particle size, microstructure and gas sensing properties of Mn substituted CoFe2O4 nanoparticles”, Journal of Magnetism and Magnetic Materials, 417(2016)122–129. 26. C. Charanab and V. K. Shahi, “Cobalt ferrite (CoFe2O4) nanoparticles (size: 10 nm) with high surface area for selective nonenzymatic detection of uric acid with excellent sensitivity and stability”, RSC Adv., 6(2016)59457-59467 27. N. Millot, S.L. Gallet, D. Aymes, F. Bernard, Y. Grin, “Spark plasma sintering of cobalt ferrite nanopowders prepared by coprecipitation and hydrothermal synthesis”, J. Eur. Ceram. Soc., 27(2007)921–926. 28. M. Penchal Reddy, A.M.A. Mohamed, X.B. Zhou, S. Du, Q. Huang, “A facile hydrothermal synthesis, characterization and magnetic properties of mesoporous CoFe2O4 nanospheres”, Journal of Magnetism and Magnetic Materials, 388(2015)40–44 29. M. Houshiar, F. Zebhi, Z. Jafari Razi, A. Alidoust, Z. Askari, “Synthesis of cobalt ferrite (CoFe2O4) nanoparticles using combustion, coprecipitation, and precipitation methods: A comparison study of size, structural, and magnetic properties”, Journal of Magnetism and Magnetic Materials 371 (2014) 43–48 30. T. Prabhakaran, J. Hemalatha, “Combustion synthesis and characterization of cobalt ferrite nanoparticles”, Ceramics International, 42(2016)14113–14120 31. G. Yang, F. He, et al., “A cheap and facile route to synthesize monodisperse magnetic nanocrystals and their application as MRI agents”, Dalton Transition, 44(2015)247–253 32. J. Li, M. Chen, Z. Gao, J. Du, W. Yang, M. Yin, “Effective approach towards Si-bilayer-IDA modified CoFe2O4 magnetic nanoparticles for high efficient protein separation”, Colloids and Surfaces B: Biointerfaces, 146(2016)468-474. 33. S. Liébana Viñas, K. Simeonidis, et al., “Tuning the magnetism of ferrite nanoparticles”, Journal of Magnetism and Magnetic Materials, 415(2016)20–23 34. L. P. Skolnick, S. Kondo, L. R. Lavine, “An improved X-ray method for determining cation distribution in ferrites”, Journal of Applied Physics, 29(1958)198–203 35. G. Socrates, Infrared and Raman Characteristic Group Frequencies, Wiley, New York NY, 2001. 11

36. P. Ma, Q. Geng, X. Gao, S. Yang, G. Liu, “Solution combustion of spinel CuMn2O4 ceramic pigments for thickness sensitive spectrally selective (TSSS) paint coatings”, Ceramics International 42 (2016) 11966–11973 37. A. Shanmugavani, R. K. Selvan, S. Layek, L. Vasylechko, C. Sanjeeviraja, “Influence of pH and fuels on the combustion synthesis, structural, morphological, electrical and magnetic properties of CoFe2O4 nanoparticles”, Materials Research Bulletin, 71(2015)122–132 38. K. Nakamoto, Infrared Spectra of Inorganic and Coordination Compounds, 2nd ed., John Wiley & Sons, 1970, pp. 232–244 39. L.B. Tahar, H. Basti, et al., “Co1−xZnxFe2O4 (0 ≤ x ≤ 1) nanocrystalline solid solution prepared by the polyol method: Characterization and magnetic properties”, Materials Research Bulletin, 47(2012)2590–2598. 40. R. Liu, H. Fu, H. Yin, P. Wang, L. Lu, Y. Tao, “A facile sol combustion and calcination process for the preparation of magnetic Ni0.5Zn0.5Fe2O4 nanopowders and their adsorption behaviors of Congo red”, Powder Technology 274 (2015) 418–425 41. S. Javadi, S. M. Masoudpanah, A. Zakeri, “Conventional versus microwave combustion synthesis of CoFe2O4 nanoparticles”, Journal of Sol-Gel Science and Technology, 79(2016)176–183 42. A. Manikandan, R. Sridhar, S. Arul Antony, S. Ramakrishna, “A simple aloe vera plantextracted microwave and conventional combustion synthesis: Morphological, optical, magnetic and catalytic properties of CoFe2O4 nanostructures”, Journal of Molecular Structure, 1076(2014)188–200 43. Z. Cao, M. Qin, et al., “One pot solution combustion synthesis of highly mesoporous hematite for photocatalysis”, Ceramics International, 41(2015)2806–2812 44. S. Salem, “Relationship between gel rheology and specific surface area of nano-sized CoAl2O4 powder manufactured by autoignition technique”, Materials Letters, 139(2015) 498– 500 45. K. Maaz, A. Mumtaz,, S.K. Hasanain, A. Ceylan, “Synthesis and magnetic properties of cobalt ferrite (CoFe2O4) nanoparticles prepared by wet chemical route”, Journal of Magnetism and Magnetic Materials 308 (2007) 289–295 46. E. Ranjith Kumar, R. Jayaprakash, T. Prakash, “The effect of annealing on phase evolution, microstructure and magnetic properties of Mn substituted CoFe2O4 nanoparticles”, Journal of Magnetism and Magnetic Materials, 358-359 (2014) 123–127. 47. S. Chikazumi, Physics of ferromagnetism, Oxford University Press, Oxford, 1997, p. 502.

12

Fig. 1. The calculated concentration of ferric and cobalt species in the solution, as a function of pH and the fuel/oxidant molar ratio (L: symbols for NH2CH2COO- ligand). Fig. 2. FTIR spectra of the dried gels synthesized at the different pH values. Fig. 3. (a) Monodentate and (b) bidentate coordination of glycine to metal atom. Fig. 4. TGA/DTA curves for the dried gels prepared at pH of (a) 2, (b) 7 and (c) 10. Fig. 5. XRD patterns of the combusted powders at the different pH values. ( :CoO and Fe2O3)

: -

Fig. 6. XRD pattern and Rietveld refinement results of the combusted powder at pH of 2. (Iobs.: observed intensity, Icalc.: calculated intensity, Iobs.-Icalc.: the residual) Fig. 7. Adsorption-desorption isotherms and the corresponding SEM and TEM (as inset) micrographs of the combusted CoFe2O4 powders at pH of (a, b) 2, (c, d) 7 and (e, f) 10. Fig. 8. Magnetization curves of the CoFe2O4 powders synthesized at the different pH values.

Table 1. Cation distribution, lattice parameter, crystallite size and magnetic properties as a function of pH pH Cation distribution a (nm) DXRD (nm) Ms (emu/g) Hc (Oe) 2 (Co0.05Fe0.95)A[Co0.95Fe1.05]BO4 0.8383 41 63.7 1112 7 (Co0.07Fe0.93)A[Co0.93Fe1.07]BO4 0.8381 32 58.8 1218 10 (Co0.03Fe0.97)A[Co0.97Fe1.03]BO4 0.8389 33 61.1 1225

Table 2. Dependence of specific surface area, SBET, equivalent particle size, DBET, and pore volume on pH values. pH SBET (m2/g) DBET (nm) Pore volume (cm3/g) 2 66.25 17.1 0.25 7 33.04 34.3 0.11 10 27.09 41.8 0.18

13

Highlights  -Formation of the different chelated species between glycine and cations as a function of pH..  -The phase and crystallite size of the CoFe2O4 powders depends on pH through combustion rate.  -The BET surface areas decrease from 66.25 to 27.09 m2/g with the increasing of pH.  -The highest Ms of ~63.7 emu/g along with the lowest Hc of 1112 Oe were achieved for pH=2.

14

(a) Co2+

FeL

Fe 3+

80 % Species

80 % Species

(b)

100

100

60 40 0.75 1 1.25

20

CoL2

60 40 0.75 1 1.25

20

CoL

0

0 2

4

6

pH

8

10

12

2

4

6

pH

8

10

Fig. 1. The calculated concentration of ferric and cobalt species in the solution, as a function of pH and the fuel/oxidant molar ratio (L: symbols for NH2CH2COO- ligand).

12

1380

1460

pH=2

1112 1038 900 827 688 595

1380 1357

1620

590 518

pH=7

1758 1640

Transmition (%)

588 520

pH=10

4000

3600

3200

2800

2400 2000 1600 Wavenumber (cm-1)

1200

800

Fig. 2. FTIR spectra of the dried gels synthesized at the different pH values.

NH2 M

NH2CH2COOH

M

CH2 O

C O

(a)

(b)

Fig. 3. (a) Monodentate and (b) bidentate coordination of glycine to metal atom.

400

180 °C

100

14 (a)

Weight (%)

6

60 40

2

20

-2

0

-6 0

DT (mV)

10

80

100 200 300 400 500 600 700 Temperature (°C) 14 (b)

100

Weight (%)

6

175 °C

60 40

2

20

-2

0

DT (mV)

10

80

-6 0

100 200 300 400 500 600 700 Temperature (°C) 14 (c) 10

80 180 °C

60

6

40

2

20

-2

0

-6 0

DT (mV)

Weight (%)

100

100 200 300 400 500 600 700 Temperature (°C)

Fig. 4. TGA/DTA curves for the dried gels prepared at pH of (a) 2, (b) 7 and (c) 10.

(620)

(533)

(440)

(511)

(422)

(400)

(311) (222)

(220)

Intensity (arb. units)

pH=10

pH=7

pH=2

20

30

40

50 2 theta (°)

60

70

Fig. 5. XRD patterns of the combusted powders at the different pH values. ( :CoO and Fe2O3)

80 : -

Intensity (arb. u.)

Iobs. Iobs.-Icalc. Icalc.

20

30

40

50 2 theta (°)

60

70

80

Fig. 6. XRD pattern and Rietveld refinement results of the combusted powder at pH of 2. (Iobs.: observed intensity, Icalc.: calculated intensity, Iobs.-Icalc.: the residual)

Table 1. Cation distribution, lattice parameter, crystallite size and magnetic properties as a function of pH pH 2 7 10

Cation distribution (Co0.05Fe0.95)A[Co0.95Fe1.05]BO4 (Co0.07Fe0.93)A[Co0.93Fe1.07]BO4 (Co0.03Fe0.97)A[Co0.97Fe1.03]BO4

a (nm) 0.8383 0.8381 0.8389

DXRD (nm) 41 32 33

Ms (emu/g) 63.7 58.8 61.1

Hc (Oe) 1112 1218 1225

140

(a)

(b)

Va /cm3 (STP) g-1

120 100 80 60

40 20

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 p/p0

Va /cm3 (STP) g-1

60

(c)

(d)

40

20

0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 p/p0

Va /cm3 (STP) g-1

100

(e)

(f)

80 60

40 20 0 0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1 p/p0 Fig. 7. Adsorption-desorption isotherms and the corresponding SEM and TEM (as inset) micrographs of the combusted CoFe2O4 powders at pH of (a, b) 2, (c, d) 7 and (e, f) 10.

Table 2. Dependence of specific surface area, SBET, equivalent particle size, DBET, and pore volume on pH values. SBET (m2/g) 66.25 33.04 27.09

pH 2 7 10

DBET (nm) 17.1 34.3 41.8

Pore volume (cm3/g) 0.25 0.11 0.18

75 60 45

M (emu/g)

30 15 0

-15 -30 pH=2 pH=7 pH=10

-45

-60 -75 -10 -8

-6

-4

-2 0 2 H (kOe)

4

6

8

10

Fig. 8. Magnetization curves of the CoFe2O4 powders synthesized at the different pH values.