Influence of intrinsic parameters on the particle size of magnetic spinel nanoparticles synthesized by wet chemical methods

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Review

Influence of intrinsic parameters on the particle size of magnetic spinel nanoparticles synthesized by wet chemical methods H. Shokrollahi a,∗ , L. Avazpour b a b

Electroceramics Group, Department of Materials Science and Engineering, Shiraz University of Technology, Shiraz 13876-71557, Iran Department of Materials Engineering, Isfahan University of Technology, Isfahan 84156-83111, Iran

a r t i c l e

i n f o

Article history: Received 28 July 2015 Received in revised form 9 October 2015 Accepted 18 October 2015 Available online xxx Keywords: Magnetic spinel ferrite Nanoparticle Internal parameter Wet chemical methods Particle size

a b s t r a c t The synthesis of magnetic spinel ferrites at the nanoscale is a field of intense study, because the mesoscopic properties enable their novel applications. Spinel nanoparticles have a promising role because of their extraordinary properties compared with those of micro and macro scale particles. Several colloidal chemical synthetic procedures have been developed to produce monodisperse nanoparticles of spinel ferrites and other materials using sol–gel, co-precipitation, hydrothermal, and microemulsion techniques. To improve the synthesis method and conditions, quality and productivity of these nanoparticles, understanding the effect of extrinsic (pH, temperature, and molecular concentration) and intrinsic parameters (site preferences, latent heat, lattice parameters, electronic configuration, and bonding energy) on the particle size during synthesis is crucial. In this review, we discuss the effect of the intrinsic parameters on particle size of spinel ferrites to provide an insight to control their particle size more precisely. © 2016 Chinese Society of Particuology and Institute of Process Engineering, Chinese Academy of Sciences. Published by Elsevier B.V. All rights reserved.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Intrinsic parameters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Enthalpy of formation (latent heat) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Site preferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Electronic configuration and bonding energy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Lattice parameter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 00

Introduction Magnetic materials have played a number of crucial roles in the daily life (Kurmude, Barkule, Raut, Shengule, & Jadhav, 2014; Gul, Ahmed, & Maqsood, 2008) and in some cases, there are no alternative materials (Sharifi, Shokrollahi, Doroodmand, & Safi, 2012). Among magnetic materials, magnetic oxides or ferrites have received special attention because of chemical stability as well as high electrical resistivity (Rodriguez & Fernández-García, 2007; Shokrollahi & Janghorban, 2007; Shokrollahi, 2008; Faraji,

∗ Corresponding author. Tel.: +98 37270047; fax: +98 37270047. E-mail address: [email protected] (H. Shokrollahi).

Yamini, & Rezaee, 2010). The extrinsic properties of these materials are highly dependent on their microstructure and control of domains. The efficient use of ferrimagnetic compound materials (ferrites) is dependent on improving their design and controlling their microstructure. The effect of particle size on microstructure is evident. Therefore, understanding the effect of extrinsic (pH, temperature, and molecular concentration) and intrinsic parameters (site preferences, latent heat, lattice parameters, electronic configuration, and bonding energy) on the particle size during synthesis is important. Spinel type magnetic oxides are exemplified by the spinel structure ferrite (MFe2 O4 ) where M is a divalent metal ion and Fe is a trivalent (ferric) iron ion. The spinel structure can be described by the formula AB2 O4 where A and B refer to tetrahedral (8(a)), and octahedral (16(d))

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Please cite this article in press as: Shokrollahi, H., & Avazpour, L. Influence of intrinsic parameters on the particle size of magnetic spinel nanoparticles synthesized by wet chemical methods. Particuology (2016), http://dx.doi.org/10.1016/j.partic.2015.10.004

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Fig. 1. Cubic spinel structure of CoFe2 O4 . The white spheres represent the oxygen atoms, and the tetrahedral and octahedral sites are indicated with blue and orange polyhedrons, respectively (Andersen & Christensen, 2015). Reproduced by permission of The Royal Society of Chemistry. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

cation sites, respectively (see Fig. 1 from Andersen & Christensen, 2015). The oxygen anions are arranged in a face-centered cubic lattice. Each unit cell contains eight formula units with O2− anions at the 32(e) sites, and M3+ and M2+ cations occupying the 16(d) ¯ and 8(a) sites. The space group of ferrites is Fd3m, and the lattice ˚ The occupation of the tetrahedral parameters are typically ∼8.5 A. site entirely with a divalent transition metal (such as Zn) produces a normal spinel structure, whereas occupation of the octahedral site with the divalent transition metal ions yields an inverse spinel structure. If divalent transition metal ions are present on both A and B sub-lattices, the structure is mixed or disordered (Willard, Nakamura, Laughlin, & McHenry, 1999; Harrison & Purnis, 1996). Spinel ferrite nanoparticles with a high surface to volume ratio have received great attention because of their useful, nonlinear optical behavior, increased mechanical strength, enhanced diffusivity and suitable specific heat, high electrical and high magnetic properties (Gubin, Koksharov, Khomutov, & Yurkov, 2005; Chauhan et al., 2013; Kruis, Fissan, & Peled, 1998; Gleiter, 2000). These properties make them good candidates for application in magnetic fluids, magnetic data storage devices, magnetic information storage, xerography, electronics (recording media), catalysis, magnetic diagnosis, as well as therapeutics and environmental remediation (Solyman, 2006; McCarthy & Weissleder, 2008; Jordan, Scholz, Wust, Fähling, & Felix, 1999; Ozatay, Mather, Thiele, Hauet, & Braganca, 2009; Bystrzejewski, Lange, Huczko, Elim, & Ji, 2007; Yu, Oduro, Tam, & Tsang, 2008; Hong et al., 2008; Carta et al., 2010). Furthermore, during the last two decades, due to the rapid development of nanotechnology, wet chemical synthesis methods have been widely used to synthesize nanostructures (Veiseh, Gunn, & Zhang, 2010; Ahmed, Okasha, & El-Dek, 2008; Lakshman, Rao, & Mendiratta, 2002). Numerous publications about magnetic spinel materials have described efficient routes to attain shape-controlled, highly stable, and narrow sized distributed magnetic nanoparticles (Leslie-Pelecky et al., 1998; Iida, Takayanagi, Nakanishi, & Osaka, 2007; Wei et al., 2012). Several chemical methods including co-precipitation (Sugimoto, 2000; Pillai & Shah, 1996; Zhang, Zhong, Yu, Liu, & Zeng, 2009; Qu et al., 2006), microemulsion (Liu, Zou, Rondinone, & Zhang, 2000; Bellusci et al., 2007), sol–gel (Lee & Kim, 2005), and hydrothermal routes (Xu & Teja, 2008; Hao

Fig. 2. Parameters influencing particle size.

& Teja, 2003) have been proposed by different researchers to synthesize magnetic nanoparticles. As mentioned before, the particle size strongly affects the properties and applications of spinel materials, and the synthesis method directly influences the particle size (Kasapoglu, Birsöz, Baykal, Köseoglu, & Toprak, 2007; Ahmed & El-Khawlani, 2009). The particle size can be changed by the extrinsic synthesis variables such as pH, temperature, and molecular concentration (Kim, Mikhaylova, Zhang, & Muhammed, 2003; Meng et al., 2012; Long et al., 2008), as well as the intrinsic parameters, including site preference, latent heat, electronic configuration, bonding energy, and lattice parameters (Qu et al., 2006; Sun, 2007; Atif, Hasanain, & Nadeem, 2006) (see Fig. 2). Numerous investigations have focused on the extrinsic variables (Nishimura, Abe, & Inoue, 2002; Tada, Hatanaka, Sanbonsugi, Matsushita, & Abe, 2003). For example, Joseyphus, Narayanasamy, Shinoda, Jeyadevan, and Tohji (2006) have shown that for Mn0.67 Zn0.33 ferrite, the decrease in particle size for higher concentrations of the oxidant (KNO3 ) is because of the faster oxidation of the ferrous ion to the ferric state, and consequently an increase in the nucleation rate of ferrite nanoparticles. However, lower molar concentrations of the oxidant have resulted in a wide distribution of particles with large diameters because of the varying time intervals for nucleation (Joseyphus et al., 2006; LaMer & Dinegar, 1950). Another extrinsic key point is the quantity of the chelating agent. Sajjia, Oubaha, Prescott, and Olabi (2010) indicated that, in the synthesis of cobalt ferrite, the use of larger quantities of the chelating agent tends to produce larger particles (at the same heat treatment temperature). The pH is another effective extrinsic parameter, as demonstrated by Hosono et al. (2009). The particle size of magnetite decreases from 14 to 12 nm as the reaction pH increases from 9.5 to 11.2. This finding indicates the variation in nucleation rate, which is a function of reaction pH. The nature of the base is the next extrinsic parameter to influence the particle size. Three bases have been used as co-precipitating agents: NaOH, CH3 NH2 , and NH3 (Shokrollahi, 2008). Auzans, Zins, Blums, and Massart (1999) have shown that for Mn–Zn ferrite in the case of NaOH, with a ∼0.4 M base concentration, there is a pH = 12.5–13.0 after coprecipitation. For the two other bases, CH3 NH2 and NH3 , the base concentration of 0.8–1.0 M results in the pH after co-precipitation of 10.5–11.0 and 9.5, respectively. To explain the effect of pH, we should mention that most of the physicochemical properties

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of oxide particles in aqueous medium can be controlled by the proton adsorption–desorption equilibrium at the solid–solution interface. The net surface charge depends on the medium’s pH. The net surface charge is zero when the pH equals the point of zero charge (PZC), negative when pH > PZC and positive when pH < PZC. At a fixed ionic strength, the magnitude of the charge increases with the increase in ıpH = |pH–PZC| up to a maximum value. The charge magnitude depends on the structure and composition of the surface (Auzans et al., 1999; Jolivet, Chaneac, Prene, Vayssieres, & Tronc, 1997). In addition, at a fixed pH, the magnitude of the charge varies with the ionic strength through a screening effect between charged sites, which depends on the size, charge, and concentration of the counterions balancing the surface charge of the particle. The surface charge promotes strong interactions with solvent molecules. Through the screening effect, the counterions affect the extension and structure of the solvation layer (Jolivet et al., 1997). For the spinel iron oxides, the maximum value of the charge magnitude is of the order of 1 C/m2 and the PZC is of the order of seven (Jolivet, Massart, & Fruchart, 1983). The magnetite cannot acquire a notable positive charge in acidic medium without transforming into maghemite, but maghemite particles are stable far from the PZC in both acidic and alkaline media. Tourinho, Franck, and Massart (1990) have shown that for Mn ferrite, three bases resulted in a significant decrease in the particle size as follows: DNaOH > DCH3NH2 > DNH3 . For Mn–Zn ferrite, the same trend has been reported. For Co ferrite, Sajjia et al. (2010) reported the effect of increasing the percentage of citric acid, which resulted in a wide range of particle sizes (20–250 nm). Avazpour, Toroghinejad, and Shokrollahi (2015) used ethylene glycol and ethylenediaminetetraacetic acid precursors in a sol–gel synthesis for Co ferrite. They showed that with an increase in precursor concentration, the crystallite size and particle size (20–70 nm) decreased. The type of base and the rate of mixing can affect the particle size. Maaz, Mumtaz, Hasanain, and Ceylan (2007) have reported that for Co ferrite nanoparticles, the slow rates of mixing resulted in larger particle size as the growth rate begins to exceed the nucleation rate. As compared with the literature, despite investigations on the effect of extrinsic parameters, the intrinsic parameters have less been considered by researchers during the last two decades. For this reason, this paper provides a comprehensive study on the effect of intrinsic parameters on the particle size of magnetic spinel ferrites synthesized by wet chemical methods. It is important to note that in this review, we have used both particle size and crystal size terminology, their trends are similar to each other.

Intrinsic parameters Enthalpy of formation (latent heat) The crystal growth in the solution depends on various parameters. The most important one is the molecular concentration of the material approaching the surface of the tiny crystal during the growth process. If there is the liberation of latent heat at the surface, the local temperature is normally higher than the solution temperature. As the surface temperature increases, the molecular concentration at the crystal surface decreases, preventing crystal growth (Maaz et al., 2007). Therefore, if the formation of a spinel is more exothermic compared with those of other spinels, it will decrease the molecular concentration at the crystal surface and obstruct the grain growth (Maaz et al., 2007; Upadhyay, Verma, & Anand, 2004). The free energies of formation will be comparable with the enthalpy of formation for normal spinels, and somewhat more negative for spinels with intermediate or inverse cation distributions (Gul et al., 2008). The largely inverse ferrites

3

Table 1 Change in electronic entropy (J/(kmol)) with disorder for selected transition metal ions. Cation 3+

V Cr3+ Mn3+ Mn2+ Fe2+ Co3+ Co2+ Ni2+ * ** ***

Electronic configuration 2

d d3 d4 d5 d6 d6 d7 d8

Sel tet *

Sel oct **

Sel ***

0 R ln 3 R ln 3 0 R ln 2 – 0 R ln 3

R ln 3 0 R ln 2 0 R ln 3 R ln 3 R ln 3 0

−9.13 −9.13 3.37 0 −3.37 −3.37 −9.13 9.13

Electronic configurational entropy for tetrahedral sites. Electronic configurational entropy for octahedral sites Vibrational entropy; R is the gas constant.

including Mg ferrite and Co ferrite have enthalpies of formation 12.5 kJ/mol less negative than the corresponding aluminates and gallates (Domide, Walter, Behrens, Kaifer, & Himmel, 2011). There are several published research articles in this area. For example, Navrotsky and Kleppa (1968) have shown that the formation of Zn ferrite is more exothermic compared with that of Ni ferrite. Zn ferrite has a normal cation distribution “forced” by the strong tetrahedral site preference of the Zn2+ ions, and has an enthalpy of the formation of −11.3 kJ/mol. Thus, if zinc is introduced into the system, more heat will be liberated, decreasing the molecular concentration at the crystal surface and obstructing the grain growth (Maaz et al., 2007; Upadhyay et al., 2004). Sharifi and Shokrollahi (2012) have confirmed that the introduction of Zn in Co1−x Znx ferrite (x = 0.50–0.75) causes crystallite size to decrease from 10 to 6 nm. Velmurugan, Venkatachalapathy, and Sendhilnathan (2010) have shown the average crystallite size (dXRD ) decreases from 9 to 7 nm when the partial substitution of zinc increases (x = 0.0–1.0) in Ni–Zn ferrite. Site preferences Theoretical calculations have indicated that the difference in lattice energy between normal and inverse 2–3 spinels is low. Therefore, the cation distribution is determined largely by the site preference energies of the individual cation. Configurational electronic entropy or electronic entropy is the entropy of a system attributable to electron’s probabilistic occupation of states. In other words, the electronic entropy can be attributed to the site preferences. The electronic entropy will reflect any configurational disorder present in the spinel, and at high temperature, a considerable “entropy stabilization” may result from this disorder (Domide et al., 2011). The grain growth may be obstructed when the cationic preferences are not fully satisfied. When the cationic preferences are not fully satisfied, a main part of energy is consumed in the cations internal displacement or configurational entropy (De Boer, Van Santen, & Verwey, 1950; Gorter, 1954; Rath et al., 2002). Table 1 shows the change in electronic entropy with disorder for some transition metal ions. The dependence of grain size on zinc concentration in Zn-substituted-cobalt ferrite can be related to the site preferences of Co, Zn, and Fe in ferrite systems. Zn2+ ions in the spinel structure have a very strong preference for tetrahedral sites, and Ni2+ ions have a similar strong preference for octahedral sites. Moreover, Fe3+ ions have a stronger preference for the tetrahedral sites compared with the octahedral sites. Thus, the formation of Ni ferrite is the most favorable because both Fe3+ and Ni2+ occupy their preferred sites (Gul et al., 2008). As Zn is introduced in the system, it forces Fe3+ to occupy octahedral sites, and the situation becomes less favorable. In this area, in addition to Gul et al. (2008), other investigators such as Sharifi and Shokrollahi (2012), Upadhyay et al. (2004), and Navrotsky and Kleppa (1968) have obtained similar results. These findings are also reflected in the formation of the pure

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Fig. 3. Variation in particle size with Zn concentration (Arulmurugan et al., 2005). Reproduced by permission of Elsevier publishing.

phase ferrite where the conditions are more stringent to make Znrich ferrites compared with those of Ni-rich ferrites. Arulmurugan, Jeyadevan, Vaidyanathan, and Sendhilnathan (2005) have shown that the particle size decreases by an increase in the Zn concentration for Co–Zn ferrite as well as for Mn–Zn ferrites as shown in Fig. 3. Sharifi and Shokrollahi (2013), and Mohseni, Shokrollahi, Sharifi, and Gheisari (2012) have shown that for Mnx Mg0.5−x Zn0.5 Fe2 O4 (where x = 0.0, 0.1, 0.2, 0.3, 0.4, and 0.5) the particle size is significantly dependent on the Mn substitution in such a way that as the Mn content increases, the particle size increases. The dependence of the particle size on Mn concentration may also be related to the site preferences of Mn, Zn, Mg, and Fe in the ferrite structure. As mentioned before, the Zn2+ ions in the spinel structure have a very strong preference for tetrahedral sites. The Mg2+ ions suggest a preference for octahedral site, and the Mn2+ ions prefer both octahedral and tetrahedral sites. Mn2+ is uniformly distributed among the different sites and has a higher probability of being absorbed by a nucleus. Zn2+ forces Fe3+ to occupy the octahedral sites and leads to the cationic site preferences, which are not fully satisfied. As compared with the value of crystal field stabilization energy (CFSE), the electronic entropy (Sel ) effect is small. However, according to Dunitz and Orgel (1957), Sel for V3+ reinforces the relatively small excess octahedral CFSE for this ion. For clearance, let us remind one more quantity: octahedral site preference energy (OSPE), which is defined as the octahedral CFSE less the tetrahedral CFSE. For V3+ , octahedral CFSE equals to 160 kJ/mol and Tetrahedral CFSE 106 kJ/mol, as a result octahedral Site Preference becomes 54 kJ that is small compared to Sel for this ion which is 211 kJ/mol (White, 2013), keeping all V3+ spinels exclusively normal despite this ion’s large ionic radius. However, Sel for Ni2+ favors the tetrahedral

Fig. 4. Variation of crystallite size with Al concentration (wt%) (redrawn using data extracted from Deraz, 2012).

Fig. 5. Variation of particle size with increase in Zn concentration (x = 0.1–0.5) (Arulmurugan et al., 2006). Reproduced by permission of Elsevier publishing.

site, contributing toward the randomization of NiAl2 O4 (O’Neill & Navrotsky, 1984). Deraz (2012) studied the effects of Al doping on the values of crystallite size of cobalt ferrite. The results showed an increase in crystallite size by increasing doping level because of the preferential occupation of Al ions on tetrahedral sites, resulting decreasing occupancy of tetrahedral sites by Fe3+ ions, which leads to formation of a partially inversed structure. Variation in particle size with Al concentration is plotted in Fig. 4. Electronic configuration and bonding energy Because the electron orbitals tend to be completed such as those with Zn2+ (3d10 ) and Cd2+ (4d10 ) or to be half-completed such as Mn2+ (3d5 ), the interaction tendency with environmental anions decreases, and the particle size decreases (Arulmurugan,

Fig. 6. Transmission electron micrographs of the prepared samples: (a) Mn0.9 Zn0.1 Fe2 O4 , (b) Mn0.7 Zn0.3 Fe2 O4 , and (c) Mn0.5 Zn0.5 Fe2 O4 (Arulmurugan et al., 2006). Reproduced by permission from Springer publishing.

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Vaidyanathan, Sendhilnathan, & Jeyadevan, 2006). They have exhibited that as the Zn content in the Co1−x Znx ferrite increases, the particle size decreases (Fig. 5). This finding may be explained by the unbalanced electronic configuration of Co2+ (3d7 ), and its tendency to interact with ligands and oxygen anions compared with Zn2+ (3d10 ), which has a complete electronic configuration. The complete electronic structure of Zn2+ leads to very little covalent interaction and tendency toward the extension between Zn2+ and its ligand. Furthermore, it has been reported (Domide et al., 2011; Navrotsky & Kleppa, 1968) that the smaller particle size of the samples doped with Zn ions are due to the lower bonding energy of Zn2+ –O2− (159 kJ/mol) compared with that of Co2+ –O2− (384 kJ/mol). Arulmurugan et al. (2006) have revealed that the average particle size decreases from 11.3 to 8.5 nm when the partial substitution of zinc (x = 0.1–0.5) with manganese increases in Mn1−x Znx ferrite. The transmission electron micrographs showed that as the Zn content increases, the grain size decreases (Fig. 6). Kaur, Rana, and Tarsikka (2012) prepared Mg1−x Cdx Fe2 O4 (x = 0.0, 0.2, 0.4, and 0.6) nanoparticles. As Cd2+ (4d10 ) (with completed electron orbital) is introduced to the lattice with increasing Cd content, the particle size of samples decreased. Singhal, Jauhar, Chandra, and Bansal (2013) confirmed these results. A similar trend has been reported in studies of zinc-substituted nickel ferrite with different zinc content in bulk and nanoparticle samples (Kurmude et al., 2014).

5

Lattice parameter Many researchers (Feng, Guo, Xu, Qi, & Zhang, 2007; Wang, Zeng, Peng, & Chen, 2004; Guo, Yan, Cui, Wang, & Bai, 2004) have investigated the dependency of the particle size on the lattice parameters in ferrites. They have shown that as the lattice parameter increases, the microstrain, surface energy, and the particle size increase. Feng et al. (2007) have synthesized Mn1−x Znx ferrite nanoparticles by the hydrothermal method, and showed that as the Zn content increases, the particles size decreases. This finding can be explained by the following reasons: the radius ˚ is larger than that of Zn2+ ions (0.74 A). ˚ of Mn2+ ions (0.93 A) 2+ When the Mn ions are replaced by Zn2+ ions at the A site, the lattice parameter becomes smaller by increasing the Zn content. This action leads to a decrease in the particle size, and an increase in the surface energy. Arulmurugan et al. (2006) have shown that for Mn–Zn ferrite and Co–Zn ferrite the same trend occurs and the lattice parameter decreases by introducing Zn2+ in the system (Table 2). In another investigation, Dahotre and Singh (2011) have synthesized Mn1−x Znx ferrite (x = 0–1) by the sol–gel technique, and observed that as the zinc content decreases, the lattice constant ‘a’, crystal density, and crystallite size increase. Ahmed and El-Khawlani (2009) have focused on the variation in crystal size with the lattice parameter in the Co–Mg ferrite (Co1−x Mgx Fe2 O4 ). They stated

Fig. 7. SEM micrographs of: (a) CoFe2 O4 , (b) Ni0.2 Co0.8 Fe2 O4 , (c) Ni0.4 Co0.6 Fe2 O4 , (d) Ni0.6 Co0.4 Fe2 O4 , (e) Ni0.8 Co0.2 Fe2 O4 , and (f) NiFe2 O4 (Kasapoglu, Birsöz, Baykal, Köseoglu, & Toprak, 2007). Reproduced by permission from Springer publishing.

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Table 2 Variation in particle size with lattice parameter in selected ferrites. Lattice parameter (Å)

References

– – – 7.0

8.359 8.396 8.403 8.438

Gul et al. (2008)

Mn0.875 Zn0.099 Fe1.94 O4 Mn0.789 Zn0.21 Fe1.96 O4 Mn0.699 Zn0.295 Fe1.98 O4 Mn0.591 Zn0.399 Fe1.96 O4 Mn0.499 Zn0.495 Fe1.99 O4

11.3 10.8 10.1 9.2 8.5

8.442 8.440 8.439 8.436 8.409

Arulmurugan et al. (2005)

Co0.901 Zn0.102 Fe1.96 O4 Co0.81 Zn0.204 Fe1.98 O4 Co0.70 Zn0.31 Fe1.96 O4 Co0.61 Zn0.41 Fe1.96 O4 Co0.51 Zn0.5 Fe1.95 O4

11.1 10.9 9.8 9.0 8.8

8.390 8.387 8.385 8.383 8.356

Arulmurugan et al. (2005)

Sample Ni0.75 Zn0.25 Fe2 O4 Ni0.50 Zn0.50 Fe2 O4 Ni0.25 Zn0.75 Fe2 O4 ZnFe2 O4

Particle size (nm)

that the increase of the crystal size with the lattice parameter is a normal trend because of the induced microstrain resulting from the replacement of Mg2+ by Co2+ up to x = 0.8. After x = 0.8, a surface relaxation occurs. Zn2+ ions commonly substitute for Co2+ at the tetrahedral sites, resulting in an increase in the lattice parameter ˚ compared with because of the larger ionic radius of Zn2+ (0.74 A) ˚ However, Gul et al. (2008) have shown that that of Co2+ (0.58 A). the addition of Zn2+ in Ni ferrite causes the Fe3+ ions to migrate from the A site to the B site because of the larger ionic radius of ˚ compared with that of Fe3+ ions (0.67 A). ˚ Because of Zn2+ (0.74 A) the lattice expansion, the lattice parameter increases (Table 2). Sharifi and Shokrollahi (2013) measured the lattice parameter as a function of the Mn concentration in a Co–Mn–Zn ferrite. The results revealed that the unit cell parameters increase from 0.841 to 0.844 A˚ with the Mn concentration in the lattice until the substitution of 0.4 of “A” sites with manganese and 0.1 with cobalt. ˚ is smaller than that This is because the radius of Co ions (0.58 A) ˚ (Arulmurugan et al., 2005). The increase in the of Mn ions (0.66 A) lattice constant obeys Vegard’s law (Whinfrey, Eckart, & Tauber, 1960). When all the cobalt ions are replaced with manganese, the lattice parameter decreases. This decrease may be related to the declining of the configurational entropy in the spinel structure by reducing the ions (O’Neill & Navrotsky, 1983). Substituting Zn for Fe increases the value of lattice parameter for CoFe2−x Znx O4 (x = 0.1, 0.2, and 0.3) as compared with that for CoFe2 O4 . The radius of Zn2+ ions in tetrahedral and octahedral sites is larger than that of Fe3+ ions; thus, each of the sites replacing Fe3+ with Zn2+ is expected to increase ‘a’ (Somaiah, Jayaraman, Joy, & Das, 2012). Increase in lattice constant with zinc content in Ni1−x Znx Fe2 O4 (x = 0.0–1.0) is reported by Kurmude et al. (2014) that is in good agreement with previously mentioned results (Kurmude et al., 2014). A similar trend was shown for Ni-substituted cobalt ferrite by Lu et al. (2007). Because the ionic radius of Ni2+ is smaller than that of Co2+ , the replacement of Co by Ni ions leads to a decrease of the lattice parameter. Fig. 7 shows the SEM micrograph of Ni-substituted cobalt ferrite. A summary of lattice parameter changes because of ion substitution is shown in Table 2. Conclusions From this review, we conclude that for the wet-chemically synthesized spinel ferrite, the intrinsic parameters have a strong influence on the particle size, as do the extrinsic ones. • If the formation of ceramic is more exothermic, as compared with those of other ceramics, the particle size will decrease. Znsubstituted ferrite is an example of obstructing crystal growth.

• The grain growth may be promoted when the cationic preferences are fully satisfied. • The grain growth may be obstructed when the cationic preferences are not fully satisfied. • If the electron orbital is complete or half-complete, the interaction tendency with environmental anions will decrease, and consequently the particle size will decrease. • Depending on ionic radii, the crystal expands or shrinks, and the lattice parameter will change. As the ferrite nanoparticles can be considered for potential applications in data storage and transmission, spintronics, microwave devices, and sensors, these conclusions will help researchers and R&D staff to adjust the particle size of wetchemically synthesized spinel ferrites more accurately and minimize its deviation. There are many novel applications for Co-ferrite nanoparticles including drug delivery, hyperthermia, memory devices instead of magnetite (Fe3 O4 ); In addition, Co-ferrite materials with lesser iron concentration compared to magnetite, current commercial drug carrier and contrast agent, are good candidate for these purposes because iron ions are poorly distinguishable from those of hemoglobin contents. As a result, adjusting nanoparticles’ size in paramagnetic region is crucial.

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