Characterization of Supermagnetic Cobalt Ferrite Submicrometer Particles Fabricated Under γ–Irradiation

: Magnetic cobalt ferrite has wide spread applications, especially as catalyst for the conversion of alkenes to related aldehydes. Despite several studies found in the literature concerning the fabrication of cobalt ferrite, none has reported on gamma irradiation as a tool for the synthesis of submicrometer sized inverse spinel CoFe 2 O 4 . The actual investigation reports on the application of γ-irradiation method for the synthesis of superparamagnetic CoFe 2 O 4 , using metal salts precursors and organic reagents. The material fabrication occurs in two main steps as sofar described. The obtained powder was isolated after irradiation and was characterized using X-ray diffraction method, transmission electron microscopy, FT-IR spectroscopy, Raman spectroscopy, UV-visible measurements and vibrating sample magnetometer. Furthermore, the X-ray diffraction data revealed the presence of a reverse spinel structure. The magnetic properties of the fabricated powder exhibited the measured lower coercivity and remanence, demonstrating that the spinel powders are made of superparamagnetic particles and finally, to gain information about the photocatalytic properties of the synthetized material, the room temperature recorded optical measurements for different samples proved that these powder materials may probably exhibit new opportunities which could improve their high photocatalytic efficiency under visible light. The prepared materials could be used as potetentiel candididates for the oxidation of organics compounds.


Introduction
Recently nanoscience and technology is exponentially improving owing to their potential applications. Nano-sized materials made of cobalt ferrite (CoFe 2 O 4 ) could be used both from fundamental research and for various physical, chemical and biological applications. They are used for their thermal, mechanical and chemical stability [1], magnetism and electronics properties [2], sensor design [3], catalysis [4 -7], and medicine [8][9].
The ferrites spinel has a cubic close-packed structure consisting of oxygen arrangement, in which cations with small radius value less than 1Å (M 2+ ) occupies tetrahedral and octahedral sites. This structure is represented by M 2+ O.
The potential applications of cobalt ferrite needs to gain a fine control of the morphology and the size of the fabricated material. There is a need to find preparative methods, which allow the sythesis of microstructural materials with specific composition and morphology by controlling physicochemical parameters.
As a piece of information, the irradiation of pure water under γ -ray leads to the formation of different species such as: HO 2  -) are respectively the oxidizing and reducing species, which can readily interact with dissolved transition metal ions (M n+ ) to change their oxidation states. The condensation and the formation of oxide particles are due to the conversion of dissolved metal species to oxidation states with low solubility. However, to avoid the particles agglomeration the use of an appropriated surfactants is necessary [17,18].
The fabrication at room temperatures in an aqueous environment under gamma irradiation of nanoparticles made of metal oxides offers some benefits over the conventional methods. The gamma irradiation does not required toxic or hard reagents, and does not necessitating complex purification steps and is able of controlling the particle size and structure. The size of nanoparticles depends of the choice of solvents and stabilizer, the pH during synthesis, and absorbed dose.
The objective of the present investigation is to report on γirradiation as a facile route to prepare the submicrometer sized particles made of cobalt ferrite, which have been prepared at room temperature and ambient pressure. The fabricated material was characterized to find out about its chemical composition, structure, optical and magnetic properties.

Sample Preparation
The reagents used in the present investigation for the preparation of the spinel made of CoFe 2 O 4 were of highpurity analytical grade and were used as received: cobalt (II) The starting solutions made of inorganic precursor containing both iron (III) and cobalt (II) were prepared by mixing an appropriated amount distilled water and the all were stirred till to obtain a lucid solution.
The procedure for preparing cobalt ferrite composite includes two steps (refer to Table 1 The particles were precipitated and isolated by centrifugation after irradiation. To remove the by-products, these particles were washed several times with distilled water, absolute alcohol, and acetone, and finally they were dried in electric oven at 60°C for six hours. The black product constituted of pure cobalt ferrite was obtained and characterized. The preparation of the solutions of 2 4 Fe CoO is detailed in Table 1.

Techniques of Characterization
The prepared powders products (before and after γ-irradiation) were identified by X-ray powder diffraction (XRD) patterns, employing a D/MAX-2550 X-ray diffractometer with Cu-Kα radiation (λ=1.54056 Å) with a nickel filter (Rigaku Co., Japan). The identification of the characteristic functional groups in the synthesized material was made using a FT-IR spectrophotometer (SHIMADZU model). A field emission scanning electron microscopy (FE-SEM; JEOL JSM-6700F) well equipped with an energy dispersive X-ray (EDAX) spectrophotometer was used to find out about the surface morphology, the size of particles and the elemental compositions of prepared powder. The morphology and the particles size were determined by transmission electron microscopy with an accelerating voltage of 200 kV. Magnetic measurements were made at room temperature using a vibrating sample magnetometer. The sample material was performed through Raman measurements, which were carried out at room temperature to find out about the compositional and the structure of the fabricated material.

XRD Studies
The X-ray diffraction technique (diffractograms recorded in the 2θ range from 15 to 80°) was used to identify the phase composition of the fabricated power and the patterns from samples 1 and 5 were similar ( refer respectively to Figure  1(a) and 1(b)). The crystalline nature of the prepared products is proven via the sharpness of diffractogram peaks. For sample 3 (The XRD pattern not shown here), a little amount of CoO could be identified as a secondary phase and at low irradiation dose (about 11 kGy), the Xray spectrum (not shown) mainly indicated the presence of cobalt ferrite and certain unidentified phases peaks. All observed peaks were indexed to the pure face centered cubic CoFe 2 O 4 with inverse spinel structure (JCPDS N° 22-1086) [26][27][28]. No other phases (or impurities) were observed, indicating the purity of the fabricated product. The location of these peaks matched well with the structure of an inverse spinel (TR 2 O 4 ), where T is the tetrahedral site and R the octahedral site. In the spinel, the full Co 2+ ions and one half of Fe 3+ ions are located in the octahedral site, while the other half of Fe 3+ ions are located in the octahedral sites.

TEM Images
The TEM images of the fabricated spinel made of CoFe 2 O 4 are exhibited in Figure 2. One can observe that the irradiation dose plays an important role in the morphology of the prepared spinels. From Figure 2 (b, c), one can observed that the sample is mainly consisted of snowball-spindle micrometer particles morphology. Some mesoporous structures were observed and could enhance the photocatalytic performance of the fabricated material. The particle get bigger when the absorbed dose increases.

Energy Dispersive Studies (EDS)
The EDS analysis of the prepared sample 1 CoFe 2 O 4 sample as presented in the spectrum of Figure 3. Based on the energies appeared in the presented spectrum, it can be concluded that the mainly chemical composition of the prepared powder was very close to the planned one. The spectrum shows the presence of Co, Fe, and O elements. The experimental atomic percentages of Co, O and Fe are found to be 15.61%, 56.67%, and 27.72% respectively. The EDS spectrum showed strong signals of cobalt and iron in fabricated sample. No additional peaks were observed, which confirmed that the cobalt ferrite synthesized by γ -ray irradiation was pure. The EDS spectrum supports others characterization techniques. The calculated atomic ratio of Co: Fe: O is consistent with the real composition of cobalt ferrite.

Analysis by FT-IR Spectroscopy
In order to ascertain the purity and nature of metal the prepared spinels, the FT-IR spectroscopy investigation was carried out in the region from 4000 to 400 cm -1 . The recorded information is shown in Figure 4. The band located at 3412cm -1 is attributed to OH stretching of the capping agent, polyvinyl alcohol used for the preparation of CoFe 2 O 4 spinel. The positioned band at 1387cm -1 can be assigned to the binding vibrations of water molecules absorbed on the synthesized CoFe 2 O 4 surface. The peaks positioned at 608 and 432cm -1 are due to stretching vibration of Metal-Oxygen bond in octahedral and tetrahedral sites [29,30]. The observed absorption band located at 432 cm −1 could be associated to the tetrahedral cation (Fe 3+ -O 2−, in α-Fe 2 O 3 ) vibration mode of prepared submicrometer spinel particles [31,32].

Raman Phonon Modes Analysis
To gain information about the vibrational energy states within the synthetized sample, we used Raman spectroscopy. Measurements were made at room temperature ( Figure 5). From, this figure, one can clearly see that all the peaks characteristic of cobalt spinel ferrites appear in the spectra, which confirms the formation of cubic cobalt ferrite [33][34][35][36].

UV-Visible Measurements
To gain information about the photocatalytic properties of the fabricated material, the optical measurements for different samples were recorded at room temperature using a UV-Visible absorption spectrophotometer as shown in Figure  6. The spectrum exhibits two absorption bands at 335 and 520 nm, where the first one could be assigned to the O 2− to Co 2+ charge transfer process and the second band could be related to the O 2− to Fe 3+ charge transfer [39]. These two characteristic bands are expected for the spinel made of CoFe 2 O 4 . From the spectrum, one can see that the fabricated samples exhibited an absorption from UV to visible light region, which demonstrates the possibility of these materials as catalysts under visible light.

Magnetic Properties
The synthetized spinel magnetic properties (sample of 30kGy gamma irradiated dose) at room temperature (300 K) is shown in figure 7, where the magnetization (M) as a function of the applied field (H) is presented. The remanence of the fabricated spinel could be extrapolated to 150 emu g -1 at 10kOe. The saturation magnetization is decreased, which is due mostly to the existence of surfactants on the surface of the spinel constituted of CoFe 2 O 4 . The measured lower coercivity and remanence are demonstrated that the spinel powders are made of superparamagnetic particles [40].
The superparamagnetic properties observed in the present investigation could be due to the special morphology of the prepared CoFe 2 O 4 sample [41]. The gamma irradiation could be expected as a new route for preparing other metal oxides semiconductor.

Radiolytic Formation of CoFe 2 O 4 Mechanism
To illustrate the production of CoFe 2 O 4 under γirradiation process, the following mechanisms could be suggested. The radiolysis of water leads to the production of free radicals such as: In alkaline media, the reducing species H· are converted to eaq , which mainly play the major role in the course of reaction. The 2-propanol (CH 3 ) 2 CHOH) scavenges both reducing and oxidizing species (H· and OH·) but under the given condtion, this alcohol is inert in reacting with the hydrated electron eaq [45]. The hydrated electron eaq reacts rapidly with the hydroxyl radical OH·, leading to the deprotonation of OH· radical, which is converted to Ovia the following reactions [46,47] Then the reactions of Ospecies with the reducing species such H· and eaq could produce of OHaccording to the reactions: O -+ H· 2OH - Under alkaline condition, the formation of the spinel cobalt ferrite, CoFe 2 O 4 could be due to ferric oxyhydroxide, FeO (OH) reduction by hydrated electron (produced upon gamma irradiation), in the presence of stable cobalt ion, Co 2+ species.

Conclusion
In the present investigation, gamma irradiation technology has been applied at room temperature, ambient pressure to prepare submicrometer 2 4 CoFe O particles. This spinel was fabricated without any kind of catalysts in water system. All characterization methods employed to get information about the structure, the composition, the purity and the nature of the prepared powder showed that, the fabricated sample was made of highly pure cobalt ferrite, which is exhibiting photo absorption from ultraviolet domain to visible light region, which implies the possibility for the prepared material to be efficiently used as photocatalytist under visible light.
The gamma irradiation could be expected as a new route for preparing other metal oxides semiconductor.
The degradation of textile dyes and the oxidation of alkenes (using the prepared material as catalyst) are under investigation and the results will be submitted soon.