Physicochemical Studies of Complex SilverâMagnetite

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PhysicoChemical Studies of Complex Silver-Magnetite Nano Heterodimers with Controlled Morphology Oscar Moscoso, Diego Muraca, Pablo Tancredi, Carlos Cosio Castañeda, Kleber R. Pirota, and Leandro M Socolovsky J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/jp501453m • Publication Date (Web): 27 May 2014 Downloaded from http://pubs.acs.org on May 29, 2014

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The Journal of Physical Chemistry

Physicochemical Studies of Complex SilverMagnetite Nano Heterodimers with Controlled Morphology Oscar Moscoso-Londoño†, Diego Muraca‡,*, Pablo Tancredi†, Carlos Cosio-CastañedaΔ, Kleber R. Pirota‡, and Leandro M. Socolovsky† †

Laboratorio de Sólidos Amorfos (LSA), INTECIN, Facultad de Ingeniería, Universidad de

Buenos Aires – CONICET, C1063ACV Buenos Aires, Argentina ‡

Laboratorio de Materiais e Baixas Temperaturas (LMBT), Instituto de Física “GlebWataghin”,

Universidade Estadual de Campinas, CEP 13083-859 Campinas-SP, Brazil Δ

Departamento de Física y Química Teórica, Facultad de Química, Universidad Nacional

Autónoma de México, Ciudad Universitaria, D.F. 04510, México Keywords: Heterodimer structures; magnetic nanoparticles; magnetic properties; interacting superparamagnetic model; thermal decomposition; self-assembly. Abstract: This work discusses the influence of synthesis conditions on self-assembly capability and morphology of obtained Ag–Fe3O4 nano-heterostructures. Samples were synthesized in two steps, first silver nanoparticles were synthesized then, used as seeds for the growth of iron oxide nanoparticles in a second step. The silver nanoparticles size was tuned

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changing the oleylamine (OAm) and oleic acid (OA) ratio, which enable us to study the influence of chemical agents and seed size on the final magnetic nanoparticles morphology. The mechanism during the formation of these heterostructures has been discussed by several authors; however it remains an open issue. In this paper we extend the discussion and advance on the understanding of synthesis condition, related to silver sizes, chemical agents and physical properties on the obtained nanoparticles. In our Ag-Fe3O4 system, two types of heterostructures were obtained; dimer, flower or combination of the two. We have found that the final shape depends on silver seed size, as well the polarity of the chemical agents used during the synthesis. We made an exhaustive study of the relationship between magnetic properties and structural features. The morphology and size distributions of the heterostructures were analyzed with transmission electron microscopy (TEM). Introduction Heterostructures are composed of noble metals – iron ferrites nanoheterostructures are interesting bifunctional materials, in which nanoparticles of magnetic iron oxide and noble metal are stuck together. In this way, it is possible to combine the optical properties of a noble metal, like silver or gold, and the high magnetization and superparamagnetic properties associated with the iron ferrite nanoparticles. These heterodimer nanoparticles, sometimes called Janus nanoparticles, have enormous potential for biomedical application such as hyperthermia treatment, magnetic resonance imaging contrast agents, magnetic separation, among others

1,2

They can also be used in technological applications3, taking advantage of the incorporation of the metal ions into iron oxides nanoparticles, for example in switch contacts, welding, catalysis,4 amongst others.


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Diverse routes for synthesizing Ag (or Au) – MFe2O4 (M = Fe, Co or Mn) have been reported. One pioneer work reported by L. Zhang et al. 5 suggests that the heterodimer structure is determined through epitaxial growth of Ag on the Fe3O4 seeds in non-polar solution. With a similar synthesis, a more contemporary work shows that changing reaction times and noble metal (Au instead of Ag) make it possible to obtain Core-Shell Fe3O4 @Au structures.6,7 Since then, several articles have been published, varying the route of synthesis, concentration of seed nanoparticles, as well as the type and size of the iron oxide used, 8-18 aiming to improve and explain observed magnetic, optical and electrical properties. Nevertheless, formation mechanism was not sufficiently studied, thus leaving unfinished questions. In this paper, we report the synthesis of Ag-Fe3 O4 heterodimer structures, first varying the polarity of the chemical agents, and second using different Ag seed sizes. The main objective is to go further into the mechanism of the formation of these structures. Also we present magnetic properties analysis of these systems related to the final structure obtained. Experimental Section General synthesis of Ag–Fe3O4 heterostructures. Ag-Fe2MO4 (M = Fe2+) heterodimer nanoparticles were synthesized by thermal decomposition of organometallic precursors. The first reaction involves the synthesis of Ag nanoparticles that, in a second step, will be used as seeds for growing magnetite nanoparticles by an in-situ procedure, forming a nano sized heterostructure. Synthesis of Ag seeds. Ag nanoparticles, namely Ag seeds throughout this manuscript, were obtained by thermal decomposition of Ag(PPh3)2NO3 in benzyl ether solution. The silver complex was prepared from stoichiometric amounts of silver nitrate and triphenylphospine


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(PPh3) in acetonitrile. A white solid is obtained after heating the solution at 80°C. Subsequently, the white solid is dispersed in benzyl ether, oleylamine (OAm) and oleic acid (OA). In order to obtain different samples, different quantities of OA were added to produce specific Ag particles size and surface charge. The mixture is loaded into a three-neck round bottom flask with a reflux condenser, a thermometer and a magnetic stir bar in an argon atmosphere. The reaction mixture was heated at a rate of 3 °C/min up to 100 °C and a then at a rate of 2 °C/min up to 300 °C. In both thermal treatments, after the temperature was stabilized, reaction conditions were maintained for 30 min. Ag nanoparticles were centrifuged several times with toluene-ethanol solution, and dried in vacuum. Ag seeds are labeled as OA6-Ag, OA10-Ag and OA14-Ag, for samples prepared with OAm:OA (molar ratios) of 6:6, 6:10, and 6:14, respectively Synthesis of Ag-Fe3O4 (samples MOA:6, MOA:10 y MOA:14) using as seeds Ag nanoparticles previously synthesized. These Ag-Fe3O4 heterostructures were obtained by mixing the aforementioned Ag seeds, Fe(acac)3, OAm, OA, 1,2-hexadecanediol and benzyl ether in a molar relation of 3:2:1:6:6:10:263, respectively. Reaction mixture was kept at 80 °C in an argon atmosphere for 30 min. Then, temperature was slowly raised up to 300 °C with a heating rate of 2 °C/min. After 40 min of reaction, the heater was turned off and the solution was brought to room temperature. The samples were dispersed, then precipitated and after that centrifuged with toluene-ethanol several times and finally dried in vacuum. Throughout this paper the following nomenclature for samples is used: MOA:6 , MOA:10 and MOA:14 for samples prepared with OAm:OA ratios of 6:6, 6:10 and 6:14, respectively. Synthesis of Ag-Fe3O4 (samples SPHE and SOTD) keeping Ag seeds size and using two solvents with different polarity. Some changes were introduced with respect to the previous synthesis. The main difference involves the use of two different solvents; octadecene and phenyl


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ether (see supplementary information for synthesis details). SOTD y SPHE correspond to samples prepared with octadecene and phenyl ether, respectively. Characterization. Structural characterization was carried out via X-ray Diffraction (XRD), in a commercial diffractometer; and with Small Angle X-ray Scattering (SAXS), measured at the SAXS line (National Synchrotron Light Laboratory, LNLS, CNPEM, Campinas, Brazil). Morphology and size distribution of the samples have been obtained by Transmission Electron Microscopy (TEM). Magnetic properties were studied by SQUID magnetometry between 5 and 300 K. Results and Discussion Ag seeds size distributions. Ag seeds were studied by XRD (see supplementary information. Figure S.1) and SAXS in order confirm the phase of the Ag seed and to obtain a detailed analysis of particle size distributions and aggregation state. Figure 1. a and c show the particle size distributions of OA10-Ag and OA14-Ag samples (obtained from SAXS measurements, red lines), respectively. SAXS size distribution was calculated using GNOM software.19 Two major peaks were observed for each size distribution profile. OA10-Ag sample shows a narrow peak at D = 23 nm, and a small peak is observed at D = 77 nm. For OA14-Ag sample, a peak is observed at D = 32 nm, and a second peak, broader and less defined, has a maximum at D = 120 nm.


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Figures 1. a and c show the particle size distribution of the OA10-Ag and OA14-Ag samples obtained from SAXS measures (red lines) and size distribution histograms with their corresponding lognormal fits, obtained by analyzing TEM images. In figures b and d the size distribution and its lognormal fit for Fe3O4 nanoparticles in heterostructures are shown.. TEM size distribution histograms were obtained from several TEM images of Ag-Fe3O4 samples. Their fit to a lognormal curve (black lines) is also displayed in figure 1 a and c. It is worth noting that in Ag seeds, size distribution profiles obtained by SAXS (red line) and TEM


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are similar for the first peak. As expected, this agreement confirms that the size of Ag NPs is preserved in the synthesis final step. Also from TEM images, only individual particles of mean value 23 nm and 32 nm were observed, for OA10-Ag and OA14-Ag respectively. This confirms that the second correlation length population could be caused by nanoparticle agglomerations, though electrostatic interaction of the first population during the colloidal suspension in the SAXS acquisition. This is the first evidence of possible charge compensation mediated by stabilization agent, from where on the OA14-Ag the charge compensation would be greater than the OA10-Ag. Fe3O4 size distributions obtained from analysis of TEM images. Size distributions profiles show that the Fe3O4 nanoparticles in the heterostructure have a narrow size distribution with a mean size of 7 nm and 8 nm for MOA:10 and MOA:14 samples, respectively (Figure 1. b and d). As will be shown later, TEM images also reveal the presence of alone magnetite nanoparticles. However, the size distribution histogram takes into account both populations, joined to Ag seeds and alone Fe3O4 nanoparticles, necessary to magnetic properties analysis. TEM information of the Ag-Fe3O4 system (sample MOA:10 and MOA:14). Images of the Ag-Fe3O4 systems (Figure. 2) shows nanoparticles with two different contrasts associated with two different materials (silver and magnetite). Darkness profile in TEM images of a particle with homogeneous density is proportional to atomic number, Z.2,20 Hence, Ag particles, which have higher Z compared with Fe3O4 particles, exhibit darker contrast.


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Figure 2. TEM images of Ag-Fe3O4 heterostructures. MOA:10 (a and b) and M OA:14 (c and d). TEM images reveal that Fe3O4 nanoparticles nucleate, grow and self-assemble (during the synthesis final step) using Ag nanoparticles as fixed seeds. This evolution leads to the formation of two types of heterostructures, dimer or “flower” (Figure 2). These two types of structures were obtained in the studied samples. In MOA:10 sample predominate the dimer or dumbbell-like structure, while in MOA:14 one the flower-like is the structure that prevail. These possible structures, dimer or “flower”, are derived from the atom-by-atom growth of the magnetite nanoparticles on the Ag seeds.


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Our interpretation is that when Fe3O4 nanoparticles starts to nucleate on the Ag surface a charge in the polarized plane at the interface is induced. This new deficiency must be compensated, in this case by free electrons from the Ag nanoparticles. The way in which compensation is carried out can lead to the formation of distinct heterostructures.21 In previous work, Heng Yu et al21 demonstrated that for Au-Fe3O4 systems, the solvent polarity plays an important role in the final conformation structure. They argue that the increase in solvent polarity could compensate the charge induced by the polarized plane growth of the Fe3O4 nanoparticles, allowing the nucleation on multiple facets. Under this perspective, the mechanism of charge compensation induced by a specific compound in the reaction mixture (solvent or a stabilization agent) is a crucial issue to define the heterostructure conformation. Also it will determine the final structure and shape, as well as the magnetic and electronic properties. In the samples under study (MOA:10 and MOA:14), the amount of solvent (benzyl ether) is kept constant, therefore the differences in the charge compensation process are associated to the amount of stabilizing agent (OA) used in the synthesis. In Ag nanoparticles synthesized by the employed method, the interaction with the metal surface is mediated by the electron-dense carboxylic group of the OA. A denser presence of these groups over the surface will produce the charge compensation needed to allow nucleation on multiple facets. This type of charge compensation causes the formation of flower-like heterostructures in the sample MOA:14, the one with the biggest amount of OA. On the other hand, in the MOA:10 sample, the charge compensation induced by stabilization agent (OA) is not enough to let multiple facets growth, thus producing dumbbell-like heterostructures. Also, as will be discussed later, the difference in


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the Ag seeds size between the two samples can contribute to the formation of these two types of heterostructures. To verify that the formation of different heterostructures occurs when the amount of stabilizing agent (OA) is constant (OA concentrations in the same order of OA14-Ag sample) and the compensating charge is modified by varying the solvent, others two samples were synthesized (SPHE and SOTD). For this two solvents with different polarity were employed. First Ag-Fe3O4 (SPHE) heterostructure was synthesized using phenyl ether as solvent. It is known that this organic compound, with increased polarity, contains an ether group which act as charge compensator. On the other hand, the second Ag-Fe3O4 heterostructure (SOTD) was synthesized using octadecene as solvent, this compound is known for being non-polar. Figure 3 shows TEM images of the as-prepared Ag-Fe3O4 heterostructures (SPHE and SOTD). As in previous samples, the darker regions correspond to Ag NPs and low contrast structures correspond to Fe3O4 NPs. From the images it can be observed the formation of dimerlike heterostructures when octadecene is used as solvent; while, when phenyl ether is used instead, “flower”-like heterostructures are obtained.


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Figure 3. TEM images of Ag-Fe3O4 NPs prepared with different solvents: (a) dimer-like heterostructures prepared with octadecene (SOTD) and (b) “flower”-like heterostructures prepared with phenyl ether (SPHE). Red dotted circles indicate the attached particles of iron oxide and silver. Then, our results prove that synthesis performed with different polarity organic solvents lead to different heterostructure conformations. Unlike the samples MOA:10 and MOA:14, on SPHE and SOTD samples, charge compensation is influenced by the polarity of the solvent. In that way, if a solvent with increased polarity is used, their charges act to allow nucleation of iron oxide NPs on multiples facets on the surface of Ag NPs. When a non-polar solvent is used, charge compensation is carried out by the stabilizing agent (OA). These results correlate with results shown by Yu et. al for Au-Fe3O4 heterostructures.21 Moreover, several studies have analyzed other conditions that lead to the different final conformations of M-Fe3O4 heterostructures. For example, Ag seeds of same size but varying its concentration could change the final structure of this complex nanoparticles.22 In this sense, high


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concentration of Ag seeds produce dimeric structures, while low concentrations produce flowerlike heterostructures. Also, the size of the Ag seed has great influence on the final conformation. This assumption is based on the fact that the electron delocalization influences differently when Ag seeds are large or small. i. e. if the seeds are small, the delocalization could affect different facets on the surface, leaving them unsuitable for multi-nucleation; while if the seeds are larger, the delocalization affects the Ag nanoparticle core. Considering that Ag seeds size would influence the charge compensation, we synthesized silver nanoparticles of greater size. These seeds are labeled as OA6-Ag. Figure 4. a shows the particle size distribution from SAXS (red line) and TEM (black line and histogram) of OA6-Ag sample. From SAXS size distributions two peaks are observed; the first corresponds to Ag nanoparticles with mean diameter of 73 nm. Other peak, observed at D = 266 nm are electrostatic agglomerates of particles of the first populations. From TEM, only one peak is revealed (78 nm), which is in good agreement with the first peak showed by SAXS size distributions. Also in TEM images, no Ag nanopartocles with D > 250 nm are observed. Figure 4. b displays the size distribution of the Fe3O4 NPs on the MOA:6 sample. The size distribution was obtained by measuring more than 300 nanoparticles (joined to Ag seeds and alone). From the analysis of these images, these nanoparticles have a size distribution slightly broader compared to the distributions reported for samples MOA:10 and MOA:14. Figure 4. c and d show the TEM images of the Ag-Fe3O4 heterostructures (MOA:6) prepared using OA6-Ag as seeds. In the as-synthesized heterostructure a mix of dumbbell and “flower”-like structures (multiple facets) is observed.


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Figure 4. (a) show particle size distribution by SAXS and TEM of OAg-Ag. (b) particle size distribution of Fe3O4 NPS in MOA:6 sample. (c and d) TEM images of Ag-Fe3O4 heterostructures prepared using OAg-Ag as seeds. To expand the discussion regarding the influence of Ag size on the formation of the achieved structures, the results obtained above were compared with the results presented for samples MOA:10 and MOA:14. For this purpose, and considering the low charge compensation by stabilization agent, only the size of Ag seeds would influence the charge compensation and hence the formation of different heterostructures. Subsequently, we can see that under approximated condition 2 ≤ size

Ag seed

/ size

Fe3O4

≤ 4 favors the formation of a dimer or

dumbbell-like structure, also noted in others works.18,21 On the other hand, when the size of the Ag seeds is slightly greater than the previous condition, “flower” (or core-satellite) nanoparticles


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are the predominant heterostructure. This type of structure has been previously reported for similar systems14,16,22,23 in which the reported condition was 3 ≤ size Ag seed / size Fe3O4 ≤ 5. In our results we note that the conditions previously described are conserved in almost all cases. As described above, there may be different forms of charge compensation in MOA:6, MOA:10 and MOA:14 samples: by size and/or by stabilization agent. Table 1 shows the influence of the charge compensation mechanisms and the final heterostructure obtained

Sample (OAm:OA ratio) Ag-Fe3O4 (6:6) Ag-Fe3O4 (6:10) Ag-Fe3O4 (6:14)

Heterostructure conformation

Influence Labeled as

charge compensation by size

charge compensation by stabilization agent

MOA:6

High

Low

Dimer and Flower

MOA:10

Low

Medium

Dimer

MOA:14

Low-Medium

High

Flower

Table 1. Influence of the charge compensation mechanisms. This influence is labeled as High, Medium or Low according to the impact of each mechanism in the sample preparation. Notice that for the M OA:14 sample charge compensation by size was considered as “Low-Medium”, but could be little bigger than the M OA:10 sample because a small difference on sizes In table 2 shows the influence of the charge compensation mechanism on SOTD and SPHE samples. In these two samples, the influence by size and by stabilization agent is not taken in account, since parameters such as the Ag seeds size and amount of stabilizing agent were kept constant.


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Sample (Solvent) Ag-Fe3O4 (octadecene)

Influence Labeled as

charge compensation

Heterostructure conformation

by solvent SOTD

Low

Dimer

SPHE

High

Flower

Ag-Fe3O4 (phenyl ether)

Table 2. Influence of the charge compensation mechanisms on SOTD and SPHE samples. Note that the influence by size and by stabilization agent is not taken in account, since parameters such as the Ag seeds size and amount of stabilizing agent were kept constant. Magnetic measurements of samples MOA:6, MOA:10 and MOA:14. Experimental fielddependent magnetization curves (symbols) at T = 300 K and T = 5 K are shown in figure 5 a, b and c. From these measurements Ag-Fe3O4 at 300 K the superparamagnetic behavior is confirmed, with low coercive fields of 4 Oe, 20 Oe and 19 Oe for MOA:6, MOA:10 and MOA:14 samples respectively. Temperature dependence of the coercive fields Hc(T) for the Ag-Fe3O4 samples are shown at the top left inset of the figure 5 a, b and c. In all samples, and at temperatures below 50 K the coercive fields has T0.5 dependence, typical for a system with random distributions of anisotropies. 24 At low temperatures, hysteresis loops show increased coercive fields, e.g. at T = 5 K are obtained Hc values of 370 Oe for sample MOA:6, 322 Oe for sample MOA:10 and 394 Oe for sample MOA:14. Above 50 K, Hc(T) behavior deviates from the T1/2 curve, suggesting that the process is influenced by other effects, like dipolar interaction25. Hc


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values at low temperature (5 K), reported in table S.1 (see supplementary information), are similar for all Ag-Fe3O4 samples. The obtained magnetization saturation values (Ms) for Ag-Fe3O4 samples were 3.2 emu/g, 6.5 emu/g and 5.8 emu/g for the 6:6, 6:10 and 6:14 OAm:OA ratios respectively. The significant small value of Ms, in comparison with Fe3O4 in bulk (84 emu/g), is due to the relative low mass percentage of Fe3O4 component in these heterostructures and the small sizes of the magnetic nanoparticles. Similar superparamagnetic Fe3O4 nanoparticles with an average size of 7 nm, have a saturation magnetization (Ms) of about 26 emu/g at T = 300 K16,26 Considering that the magnetite nanoparticles in the heterostructure have a narrow size distribution with a maximum of around of 7 nm (Figure. 1 b, d and figure 4. b), we can consider 26 emu/g as a reference value to calculate, in a first approximation, the relative amount of magnetite nanoparticles present in Ag-Fe3O4 samples. Based on this assumption the obtained values were 13 wt%, 25 wt% and 21 wt% for MOA:6, MOA:10 and MOA:14 samples respectively. Reduction in saturation magnetization is usually attributed to the effect of the nanometer scale of the particle size on the surface/volume ratio, resulting in spin canting due to the spin frustration 27-29 or poor crystallization of the Fe3O4 phase which reduces magnetic moment of the iron oxide nanoparticles.30,31 Furthermore, TEM images show some isolated Fe3O4 nanoparticles with size below 3 nm, which gives a superparamagnetic contribution to the measures and lower magnetic moment per particle than for the 7 nm ones.


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Figure 5. M vs. H curves at T = 5 K and 300 K for MOA:6 (a), MOA:10 (b) and MOA:14 (c). The top left inset shows is a zoom-in of the low field region showing the coercive field and remanence at T = 5 K and 300 K. In the bottom right insets are displayed the temperature dependence of the coercive field (solid line represents the tendency). (d), (e) and (f) show the true and apparent mean magnetic moment values as function of temperature for Ag-Fe3O4 samples. Symbols represent the values obtained from the fit and the straight lines represent the tendencies. According to D. Kechrakos et. al,32 dipolar interactions acts to decrease magnetic response of the system, suggesting that magnetization approximates to saturation slower than corresponding non-interacting superparamagnetic systems. Hence, in the case of the sample


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MOA:10, which is already saturated at H = 16 kOe, should present a smaller influence of dipolar interactions. It is important to notice that the majority of the heterostructures found in this sample present a dumbbell-like structure. In contrast, and due to the possible existence of stronger-dipolar interactions among Fe3O4 nanoparticles, the samples MOA:6 and MOA:14 do not reach saturation even at H = 20 kOe and T = 5 K. Due the shape of these samples (containing flower-like or core-satellite heterostructures), we can infer that the existence of this type of heterostructures promotes the increase in the strength of dipolar interactions. Then, based on the theory proposed in the cited reference32 and comparing M vs H curves at T = 300 K of these two samples, the MOA:14 sample should has greater influence of dipolar interactions between the magnetic nanoparticles. To better understand the role of magnetic interactions in our systems, more quantitative analysis were performed. Langevin equation was used to fit the M vs H curves, from which size distribution histograms where obtained (Figure 1. b, d and figure 2. b). These indicated that the samples have size distributions which follow a lognormal distribution. Therefore, the magnetization contribution of the unblocked (superparamagnetic) Fe 3O4 nanoparticles, M (H,T) is described by: 𝑀 𝐻, 𝑇 =

where 𝐿

𝜇 𝑎𝑝𝑝 𝐻 𝐾𝐵 𝑇

𝜇 𝐻 ∞ 𝜇 𝐿 𝑘𝑎𝑝𝑝𝑇 0 𝑎𝑝𝑝 𝐵

𝑓 𝜇𝑎𝑝𝑝 𝑑𝜇𝑎𝑝𝑝 ,

(1)

is the Langevin function, kB is the Boltzman constant, T the temperature, µapp is

the apparent magnetic moment (called apparent because the fit does not take into account interactions among nanoparticles) and f(µapp) represents the lognormal distribution function of the apparent moments which is given by:


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𝑁

𝑓 𝜇𝑎𝑝𝑝 =

2𝜋 𝜇 𝑎𝑝𝑝

exp − 𝜍

ln 2 𝜇 𝑎𝑝𝑝 𝑥 0 2𝜍 2

,

(2)

where x0 is the median of the distribution and σ is the standard deviation. The mean of the apparent magnetic moments is given by ‹µapp› = µapp exp(σ2/2).33 In this way, the parameters µapp and σ were obtained and their corresponding fitting curves (black continuous lines) are displayed in figure 5. a, b and c. The apparent mean magnetic moment (‹µapp›) obtained at T = 300 K are in good agreement with values reported for superparamagnetic systems. 34 The figure 5 c, d and f (gray symbols) shows the ‹µapp› values obtained by equation 1. The obtained values increase with T until a certain value is reached. However, in correspondence with the behavior of Ms as function of T, the particle magnetic moments are expected to remain almost constant for low temperatures and decrease as the temperature approaches the Curie temperature. This unrealistic behavior was reported and explained by P. Allia et. al35 for magnetic systems with dipolar interactions between particles. To obtain the thermal dependence of a real magnetic moment, they introduce a phenomenological approach modifying the temperature on the standard Langevin function, in a model known as T* or the Allia model. They suggested the existence of dipolar field which acts by altering in a random way the magnetic moments, their directions, sign and magnitude. This effect is analogous to the effect produced by the temperature, such that the temperature in the Langevin function can be written as: Ta = T + T* when T* is related to the dipolar energy, εD = KBT* = αMs2/kBT, where α is a proportionality constant. Based on previously mentioned results, it is possible to assume that in our systems the magnetization is described by a modified Langevin function: 𝑀 𝐻, 𝑇 =

∞ 𝜇 𝑡𝑟 𝐻 𝜇 𝐿 0 𝑡𝑟 𝑘 𝐵 (𝑇+𝑇 ∗)

𝑓 𝜇𝑡𝑟 𝑑𝜇𝑡𝑟

(3)


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The above equation represents the magnetic regimen in which the dipolar interaction effect is taken into account and where µtr represent the true magnetic moment. T* determination is carried out starting from the low – field susceptibility equation for interacting superparamagnetic systems: 𝜒 = 3𝑘

𝑁𝜇 2 𝐵 (𝑇+𝑇

∗)

, using the conditions Nd3 = 1 (d is average

distance between particles) and Ms = Nμ it is simple to obtain: 𝜌 𝜒

= 3𝑘𝐵 𝑁

𝑇 𝑀𝑠2

+ 3𝛼

(4)

where ρ is related to the distribution moments defined as: 𝜌 = 𝜇2 / 𝜇 2 . In all samples a linear dependence of ρ/χ as function of T/Ms2 was observed, with the exception of the data obtained at T = 5 K. This deviation could be ascribed to a model failure when the system is completely blocked. However, the linear trend of the other data allowed us to determine α and therefore T*. Based on the above mentioned T* model and using the fitting procedure of reference.34 ‹µtr› as function of T was calculated (Figure 5. d, e and f. Black symbols). In all cases, at low temperature ‹µtr› values are larger than ‹µapp› ones, while at T = 200 K and 300 K the values are closer. It is well-known that the true mean magnetic moment value at T = 5 K present a deviation from expected behavior because at this temperature the majority of Fe3O4 nanoparticles are in the blocked regime. In table S.1 (see supplementary information) the parameters obtained through the fit with equation 1 and 3 are reported. The ‹µtr› tendencies obtained for MOA:10 and MOA:14 is observed a monotonic decrease, except for the data obtained at T = 5 K. This behavior is related with shape of heteroestructures, where in these two samples a dominant heterostructure persists, dimer or flower for MOA:10 and MOA:14 respectively. On the other hand, the mean true magnetic moment values ‹µtr› of the


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sample MOA:10 are greater in comparison with those of MOA:14 sample (Figure 5 e and f) at all temperatures. This is in corresponds with our previous hypothesis, where a smaller effect of dipolar interactions (dimer configuration) gives rise to higher magnetic response. Also, it reinforces the fact that in the MOA:14 sample (flower configuration) the contact between surfaces of the Ag - Fe3O4 and Fe3O4 - Fe3O4 nanoparticles generates an increase in exchange coupling and in dipole – dipole interactions. This assumption is also in agreement with M vs H measurements in which the MOA:14 sample reaches saturation at a higher magnetic field, and with lower values of the mean true magnetic moment (‹µtr›) obtained for this sample. The decrease of the true magnetic moment in sample MOA:6 is steeper because of the presence of the two types of heterostructures (dimer and flower). This would give rise to a more complex magnetization process, such that the information obtained for MOA:6 sample from the standard and modified Langevin equation cannot be corrected. Moreover, it should be noted that the dipolar interactions would have positive implication on hyperthermia treatment. Recent studies show that such interactions increases the specific absorption rate (SAR) due a crossover between Neel to Brown dynamic.36,37 Figure 6. a, b and c show the T dependence dc magnetization for MOA:6, MOA:10 and MOA:14 samples. Measurements were performed in a zero field cooling (ZFC) and field cooling (FC) modes with H = 50 Oe from 5 K to 300 K. ZFC-FC curves for the three samples are typical of superparamagnetic systems. However, some differences were observed, which are associated with the shape of the structure achieved. All MZFC curves exhibits a well-defined maximum, ascribed to the blocking temperature (TB). The system can be defined as blocked for values of T < TB and superparamagnetic if T > TB. The obtained values for TB are: 35 K, 63 K and 22 K for


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samples MOA:6, MOA:10 and MOA:14 respectively. In sample MOA:10 (TB = 63 K), the curve is distributed over a broad range of temperatures in comparison to the other two Ag-Fe3O4 samples. At a first glance, one might believe that these differences are due to an increased interaction strength (of dipolar type) or a greater size distribution of Fe 3O4 nanoparticles in sample MOA:10. However, these assumptions can be discarded because, (1) the histograms obtained from the TEM images show a narrow size distribution of the Fe 3O4 nanoparticles, even slightly narrower in comparison to the other two Ag-Fe3O4 samples (MOA:6 and MOA:14). (2) The results obtained from M vs. H measurements demonstrate an increase in the dipolar interactions when flower-like structures are the predominant form. In this sense, these differences can be explained from the different electronic configurations due influence of Ag nanoparticles like major spin transfer (from the magnetite to the silver) or changes on the magnetic orbital13. However, further studies are required to confirm these hypotheses. We have observed that in MOA:10 and MOA:14 the irreversibility temperature was near the blocking one, and no Morin (typically of hematute) were detected38. For MOA:6 the Verwey transition TV is evident (TV = 120 K). TV is typically observed in well crystallized Fe 3O4 nanoparticulated systems, at TV = 120 K.


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Figure 6. ZFC-FC curves of the samples MOA:6, MOA:10 and MOA:14 respectively, the insets show the inverse of the magnetization as function of temperature and the linear fitting of the CurieWeiss law (solid line). Temperature dependence of the inverse magnetization is also shown on insets of the figure 6. At high temperatures, it follows the Curie-Weiss law: 𝜒 =

𝐶sp 𝑇−Θsp

, where Csp is the Curie

molar constant, T is the absolute temperature and Θsp is the Curie–Weiss temperature. The measures show that the inverse susceptibility (directly related to the magnetization with χ = M H1

) is not linear in the whole temperature range as a consequence of the magnetic interparticle

interactions (with greater strength in samples MOA:6 and MOA:14). Another source of deviation can be related to the spin frustration which is manifested in the surface of the nanoparticles. Even in the most diluted superparamagnetic system the ideal Curie-law is never observed.35 Deviations from linearity at very low temperatures can be ascribed to some type of particle blocking. So that one can infer that in MOA:10 sample, which has a higher value of Θsp (~133 K) this blocking would be related to the dimer-like heterostructure. To compare magnetic moments obtained from the M vs. H curves, the Curie – Weiss law was used to obtain the effective magnetic moment, such that 𝜇𝑒𝑓𝑓 = 3𝑘𝐵 /𝑁𝜇0 (1/𝐶)

1/2

, where

C is the Curie constant. The results obtained were 7.3 x 10 3 μB, 15.3 x 103 μB and 3.0 x 103 μB for MOA:6, MOA:10 and MOA:14 samples respectively. It is remarkable that these results have the same order of magnitude than those obtained from the M vs. H curves at T = 300 K. Conclusions


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We have presented a detailed and extensive study of synthesis conditions, structural, magnetic properties and mechanism formation of magnetic heterostructures composed by AgFe3O4. From these studies we conclude that in a two-step synthesis, Fe3O4 use Ag nanoparticles as fixed seeds, to grow as shell layer. This growth is restricted to a charge distribution on the silver nanoparticle, limiting the possible facets where the oxide could nucleate and grow. In this sense, the charge compensation in the Ag seed associated with the amount of stabilizing agents in the surface that will determine the final magnetic nanostructure. Also, the Ag sized nanoparticles used as seeds could affect the final structure in physical properties. For example, high values of oleic acid (OA) and silver nanoparticles of about 32 nm creates a high charge compensation and leads the formation of “flower” type structure, whereas low amounts of oleic acid (OA) and silver particles of 23 nm decrease this charge compensation, inhibits the Fe 3O4 nucleation in multiple facets and favors the dumbbell-like heterostructures. The final shapes of the obtained nanoparticles were correlated with magnetic properties. From these studies all the samples present near superparamagnetic behavior, at room temperature. It is interesting to notice that in all cases the irreversibility temperature was near the blocking one, an observation not reported on this type of systems before. To better understand the magnetic behavior of these samples a superparamagnetic-based Langevin model was considered. From this model, we observed that the dipolar interaction strongly affect the magnetic moment of the nanoparticles. Hence, it was necessary to consider a dipolar interaction contribution on the Langevin model to obtain a more accurate value of magnetic moment as function of temperature. Based on this, we show that the sample with flower structure presents the higher dipolar interaction than the heterodimers samples, possible due to the proximity of the magnetic particles on the edges of the silver particles. These conformations results in a reduction


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of the magnetic moment per magnetic nanoparticle when the dipolar interaction increases. The effect on dipolar interaction could have positive implication on hyperthermia treatment because the specific heat absorption is enhanced under an alternating magnetic field. Acknowledgements The authors wish to acknowledge Argentine agencies CONICET and ANPCyT, Mexican CONACyT, and Brazilian FAPESP for financial support. CNPEM (Campinas, Brazil) is acknowledged for the use of synchrotron light (SAXS line, LNLS), and TEM (C2Nano). O. Moscoso-Londoño would like to thank COLCIENCIAS, Colombia. We thank P. Mendoza Zélis and D. F. Coral for their help with M vs. H fitting.

Corresponding Author Diego Muraca [email protected] UNICAMP/IFGW-Instituto de Fisica Gleb Wataghin LMBT-Laboratorio de Materiais de Baxas Temperaturas Rua Sergio Buarque de Holanda, 777 Cidade Universitaria, Zeferino Vaz, Barao Geraldo, CEP 13083-859 Campinas-SP Brazil Ph. No. 55-19-3521-5504, 55-19-3521-5504 (LMBT). Tel. (55) 19 98668481 - (55) 19 98668481


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Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Abbreviations C OA, Oleic Acid; OAm: Oleylamine; TEM, Transmission Electron Microscopy; SAXS, Angle X-ray Scattering; XRD, X-ray Diffraction; Hc, Coercive fields; Ms, Magnetization Saturation; TB, blocking temperature; ZFC, zero field cooling; FC, field cooling.

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Table of Contents Graphic

Schematic representation of concentration of oleic acid during synthesis and respectively shapes of nano-heteroestructures. AO:10) low amount of oleic acid and obtained heterodimer structures. AO) hight amount of oleic acid and a satellite like structure obtained.

Brief Summary The influence of synthesis conditions on the capability of self-assembly and the obtained final morphology of Ag–Fe3O4 nano-heterostructures have been studied. The mechanism formation of these structures were discussed and related magnetic properties were analyzed.


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