Sonochemical Synthesis of Cobalt Ferrite Nanoparticles

Cobalt ferrite being a hardmagneticmaterial with high coercivity andmoderatemagnetization has foundwide-spread applications. In this paper, we have reported the sonochemical synthesis of cobalt ferrite nanoparticles usingmetal acetate precursors.The ferrite synthesis occurs in three steps (hydrolysis of acetates, oxidation of hydroxides, and in situmicrocalcination ofmetal oxides) that are facilitated by physical and chemical effects of cavitation bubbles. The physical and magnetic properties of the ferrite nano-particles thus synthesized have been found to be comparable with those reported in the literature using other synthesis techniques.


Introduction
The spinel ferrite nanoparticles have exceptional electronic and magnetic properties, which are quite different from the bulk materials [1].As a result, the magnetic ferrite nanoparticles have found wide applications in information storage systems, ferrofluids, and medical applications like magnetic drug delivery and hyperthermia for cancer treatment [2].Among metal ferrites, zinc, nickel, and cobalt ferrites have been mostly applied.Cobalt ferrite (CoFe 2 O 4 ) is a hard magnetic material that is known to have high coercivity and moderate magnetization [3].On nanoscale, the cobalt ferrite achieves properties of high saturation magnetization, high coercivity, strong anisotropy, high mechanical hardness, and high chemical stability [4].Conventional techniques for synthesis of metal nanoparticles (including cobalt ferrite particles) are sol-gel method [5,6], microemulsions [7,8], reverse micelles [9], autocombustion, [10] and coprecipitation [11].A relatively new technique for synthesis of ferrite nanoparticles is the sonochemical route [12][13][14][15][16], in which the reaction mixture is exposed to ultrasound irradiation.Spectacular physical and chemical effects induced by ultrasound bring about the synthesis of metal ferrites from the metal salt precursors, usually acetates.The hydrolysis of acetates, followed by oxidation of the hydroxides to oxides and the reaction between oxides to yield ferrites is brought about by ultrasound and its secondary effect, cavitation.Cavitation is essentially nucleation, growth and transient implosive collapse of gas bubbles driven by ultrasound wave [17].This technique has been well demonstrated for zinc ferrites.In our previous papers [15,16], we have tried to illuminate the links between physics of ultrasound and cavitation and the chemistry of zinc ferrite nanoparticles.It was revealed in these studies that chemical species produced during transient cavitation induce the hydrolysis and oxidation of hydroxides, while shock waves generated by transient cavitation bring about the calcination of metal oxides to form ferrites [18].
In this study, we have extended the same theme for synthesis of cobalt ferrite.We would like to specifically mention that sonochemical preparation of cobalt ferrite nanoparticles has already been reported by Shafi et al. [19].However, in their study the precursors were the metal carbonyls, which were sonochemically decomposed in decalin solvent to yield amorphous oxides that further reacted to form ferrites (with external calcination at 450 and 700 ∘ C) that also had amorphous character.The major problem with this route is the use of toxic and hazardous metal carbonyl precursors.Our approach is more environmentally friendly in that the precursors used by us are cobalt acetate and iron (II) acetate.
The characterization of the particles has been made using XRD, FE-SEM, and VSM.

Materials and Methods
The experimental parameters as well as analysis in this paper are based on results of these previous papers, as well as some preliminary studies (not reported in this paper).
The main hypothesis on which this paper is based is that physical and chemical effects of cavitation bubbles induce the chemistry of ferrite synthesis.The metal acetates first undergo hydrolysis, thermal decomposition, and oxidation of hydroxides thus formed gives metal oxides, which are insoluble and precipitate in reaction mixture.The metal oxide particles get drifted in the shock waves generated by cavitation bubbles and collide at high velocities.The energy generated from these collisions can induce reaction leading to formation of ferrites.All of these steps have been well proven by previous researchers.Sonochemical hydrolysis and thermal decomposition are well-known reactions (for example Kotronarou et al. [20]).High energy collisions between particles induced by shock waves have also been proven by Doktycz and Suslick in a landmark paper [18].

Materials.
The following chemicals have been used for the synthesis: cobalt acetate tetrahydrate (AR Grade, Merck), iron (II) acetate (AR Grade, Alfa Aesar), and NaOH pellets (Merck).All chemicals were used as received without any further purification.All the reaction mixtures were prepared using deionized water from Milli-Q Synthesis unit (Model: Elix 3, Millipore Ⓡ , USA).

Experimental Setup.
A schematic diagram of the experimental setup is shown in Figure 1.All experiments were carried out with 20 mL of Millipore water in a borosilicate glass beaker.A desired amount of cobalt acetate was taken with iron (II) acetate and was placed on a hot plate (Remi Equipments Ltd., Model: 1-MLH).The ultrasound was applied for 30 min using a high grade titanium alloy ultrasonic probe (dia 13 mm) with a frequency of 20 kHz (Model: VCX-500, 20 kHz, 500 W).The probe was operated at 20% amplitude with theoretical power dissipation of 100 W (500 W × 0.20 = 100 W).The microprocessor based control unit of the processor had facilities of automatic frequency tuning and amplitude compensation, which delivers constant power to the reaction mixture during the sonication period, irrespective of the changes occurring in the reaction medium.
The acoustic pressure amplitude generated by the ultrasound probe was measured using calorimetric method as mentioned by Sivasankar et al. [21].For 20% of the theoretical maximum power, the pressure amplitude produced by the probe was determined as 1.5 bar.

Synthesis
Procedure.An amount of 0.708 g of cobalt acetate (0.2 M) was taken in a 20 mL of Millipore water with 1.39 g of iron (II) acetate (0.4 M) making the molar ratio of cobalt acetate to iron (II) acetate 1 : 2. The solution was subjected to sonication for 30 min in a pulse mode (20 s on −5 s off) with actual total sonication time of 24 min.The temperature of the solution was maintained at 70 ∘ C (±4 ∘ C) during the experiment.The initial pH of the reaction mixture in all protocols was 4.5.The pH of the solution was adjusted to the desire value by dropwise addition of either 0.1 N NaOH or 0.1 N HCl.No external H 2 O 2 was added to the reaction mixture during the synthesis of cobalt ferrite nanoparticles, as required for the oxidation of ferrous hydroxide.The exact chemistry of the cobalt ferrite synthesis is given by following chemical reactions: Transient cavitation in the reaction mixture leads to in-situ generation of H 2 O 2 as follows.During expansion of the cavitation bubble, large amount of water vapor evaporates into the bubble and diffuses to the core of the bubble.During the ensuing compression phase, the bubble contracts and the vapor starts diffusing back to the bubble wall (or gas-liquid interface).However, during final moments of the collapse, the bubble wall velocity becomes extremely fast and all of the vapor that has entered the bubble cannot escape (or diffuse back to the bubble wall and condense).This water vapor gets entrapped in the bubble and is subjected to extreme conditions (∼5000 K temperature and ∼500 bar pressure) generated in the bubble at the moment of transient collapse and undergoes dissociation to generate radicals [22]: The hydroxyl radicals generated by the bubble can recombine to yield hydrogen peroxide that leads to oxidation of hydroxide to oxides: Alternatively, direct thermal decomposition of hydroxide particles can also occur in the thin liquid shell surrounding the cavitation bubble in which the temperature reaches moderately high values (∼400-600 K) at the moment of transient collapse of the cavitation bubbles [20,23].In our previous papers [24][25][26][27][28][29][30][31][32], we have reported extensive simulations of cavitation bubble dynamics using the diffusion limited model of Toegel et al. [33] under variety of conditions that give quantitative confirmation of above physical and chemical effects of cavitation bubble dynamics.We refer the interested readers to these papers for greater details on this subject.A blackish solid precipitate was obtained as a product after 30 min of sonication.The precipitate was separated by centrifugation for 20 min at 6000 rpm (make: Hermile, Model: Z300).The solid product was then dried in a hot air oven for overnight at 100 ∘ C and characterized using powder X-ray diffraction (make: Bruker, Model: Advanced D8) to ascertain formation of ferrite phase.
In an interesting paper, Dang et al. [38] have reported the sonochemical synthesis of BaTiO 3 particles through sol suspension route using BaCl 2 and TiCl 4 precursors.In a subsequent paper, Yasui and Kato [39] have presented a numerical simulation of oriented aggregation of sonochemically synthesized BaTiO 3 nanocrystals.In the study of Dang et al. [38], the Ti-based sol suspension was obtained by first preparing a solution of BaCl 2 and later adding TiCl 4 to it.Later the pH of the sol suspension was adjusted to 14, with temperature being 80 ∘ C.This suspension was sonicated at different power levels to yield BaTiO 3 due to reaction induced between colloidal particles by collisions between them driven by transient cavitation events.A similar route such as this may also occur in the present case leading to ferrite formation.However, we are unable to distinguish between this route and the mechanism that we have proposed above.

Characterization of the Solid Product.
It should be specifically mentioned that external calcination was not applied to the solid product obtained after sonication.Characterization of the dried solid powder resulting from experiments was done using X-ray powder diffractometer (make: Bruker, Model: Advanced D8) with monochromatic CuK  ( = 1.5406Å) radiation operated at 40 kV and 40 mA in the range from 20 to 70 ∘ .The mean crystallite particle size ( xrd ) of the product was calculated from the most intense peak observed at (311) using Debye-Scherer equation [40,41]:  xrd = 0.9 × /( × cos ), where  is the X-ray wavelength,  is the half width of the relevant diffraction peak, and  is the Bragg's angle.The morphology of the cobalt ferrite nano-particles was determined using field emission scanning electron microscope (FE-SEM, Model: SIGMA VP, make: Carl Zeiss Microscopy GmbH, Germany).The magnetic characteristics of the ferrite particles were determined using vibrating sample magnetometer (VSM, make: Lakeshor, Model: 7410).

Results and Discussion
The results of the experiments have been presented in Figures 2, 3, and 4. The X-ray diffractogram of the solid product was obtained after sonication.The diffraction peaks corresponding to characteristic crystallographic planes at spinel structure of ferrites [(220), (311), (400), (511), and (440)] are evident in Figure 2. The ferrite phase is relatively pure in that no presence of other phases like hematite [(-Fe 2 O 3 ) between ( 220) and (311) or Fe 3 O 4 at (333)] or cobalt oxide is seen.The average crystal size of particles, as calculated from Debye-Scherer formula, is 24 nm.It is noteworthy that the cobalt ferrite was formed during sonication itself, without external calcination of the material, confirms our earlier hypothesis of in-situ micro-calcination of the oxide precursors induced due to the energetic collisions between the particles, as they get drifted in high pressure amplitude shock wave generated by the cavitation bubble.The FE-SEM micrographs of the as-synthesized ferrite particles are shown in Figure 3.These micrographs show agglomerates of nearly spherical particles.Determination of exact particle size from these pictures is thus difficult.Nonetheless the crystalline structure of the particles can be deduced from the uniformity of the particle sizes.The room temperature magnetization curve of the as-synthesized cobalt ferrite particles is given in Figure 4.The saturation magnetization of the sample is 25.7 emu/g, with magnetic remanence of 8.78 emu/g.These values are much smaller than the magnetic saturation reported for bulk CoFe 2 O 4 material, that is, 72 emu/g [19,42].Significantly lower values of saturation magnetization observed for our particles confirm its nanocrystalline nature.Shafi et al. [19] have discussed possible causes that lead to this effect.The size of the magnetic particle determines its energy in an external field.This relation is essentially through the number of magnetic molecules in an individual magnetic domain.
With reduction in particle size, the energy of the particle decreases.As the energy becomes comparable to  (: Boltzmann constant, : Temperature): thermal fluctuations will dominate causing reduction in the total magnetic moment.The reduction in saturation magnetization could be consequence of noncollinear spin arrangement.Cobalt ferrite is essentially an inverse spinel with collinear ferrimagnetic spin structure in the bulk form.However, for particle size in nanorange, there could be non-collinear spin arrangement at or near the surface of the particle that causes reduction in saturation magnetization.Another property which is of relevance to saturation magnetization is the surface-tovolume ratio.The larger this ratio (as for the particles in nanometer range) the magnetic molecules on surface lack proper coordination due to which their spins are disturbed, and total magnetization is reduced.Goodarz Naseri et al. [34] have reported that the relationship between coercivity field and remanence ratio and the saturation magnetization is not simple in that no definite trends are observed.Goodarz Naseri et al. [34] have concluded that the value of remanence ratio (), which varies between 0 and 1, is representative of existence or absence of different types of intergrain group exchanges.For  < 0.5, the particles interact by magnetostatic interaction, while  = 0.5 indicates randomly oriented noninteracting particles, which undergo coherent rotations.For 0.5 <  < 1 indicates existence of exchange coupling particles.According to this criterion, the particles interaction in the as-synthesized cobalt ferrite is through magnetostatic interaction.
Table 1 gives a comparative account of the results of present study with published literature.It could be seen that the particle size as well as magnetization characteristics of the as-synthesized particles is at par with the particles synthesized using conventional techniques.

Conclusion
In this study, we have explored the sonochemical route of synthesis of cobalt ferrite nano-particles.The precursors are the metal acetates that are hydrolyzed and oxidized under ultrasound irradiation to form metal oxides.In-situ microcalcination of the oxide particles induced due to high energy collisions between the particles due to the shock waves generated by cavitation bubble causes synthesis of ferrite.Most notably, no external calcination is needed.Characterization of as-synthesized particles with XRD, FE-SEM, and VSM reveals that ferrite particles have inverse spinel crystalline structure.The physical and magnetic properties of the as-synthesized particles are comparable with those synthesized using conventional techniques.

Table 1 :
Characterization of cobalt ferrite nanoparticles and comparison with the literature.