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In situ modification of Fe and Ni magnetic nanopowders produced by the electrical explosion of wire
Journal of Alloys and Compounds 586 (2014) S483–S488
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In situ modification of Fe and Ni magnetic nanopowders produced by the electrical explosion of wire I.V. Beketov a,b,⇑, A.P. Safronov a,b, A.V. Bagazeev a, A. Larrañaga c, G.V. Kurlyandskaya b,c, A.I. Medvedev a,b a
Institute of Electrophysics, RAS, Urals Branch, Ekaterinburg, Russia Ural Federal University, Ekaterinburg, Russia c University of the Basque Country UPV-EHU, Bilbao, Spain b
a r t i c l e
i n f o
Article history: Available online 13 February 2013 Keywords: Electric explosion of wire Nanoparticles Magnetic measurements Surface modification
a b s t r a c t Iron and nickel spherical magnetic nanoparticles (MNPs) with mean diameter 50–80 nm were prepared by the electric explosion of wire. The advantage of this method is that it provides a very high production rate up to 200 g/h and requires low energy consumption of 25 kW h/kg. The as-prepared MNPs were modified in situ in hexane, toluene, chloroform, and the solutions of polystyrene in toluene, in an inert atmosphere at room temperature in order to provide both the stability at ambient conditions and tunable surface properties. Hexane was an inert liquid for Fe and Ni MNPs. In the case of toluene the surface of asprepared Fe or Ni MNPs acted as a catalyst for the condensation reaction of aromatic organic compounds forming irregular polycyclic structures. The treatment with toluene led to the deposition of carbon and formation of ‘‘hair-like’’ ordered structures on the surface of the MNPs. Chloroform chemically interacted with Fe MNPs, forming FeCl2. The treatment of MNPs with the solution of polystyrene in toluene resulted in the adsorption of polymer and the formation of 3–6 nm polymeric coatings on the surface. The obtained magnetic properties (like the magnetization value for Ni MNPs up to 52 emu/g and Fe MNPs up to 179 emu/g) at room temperature make these MNPs competitive candidates for nanocomposites for microwave and other technological applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Highly dispersed metallic nanoparticles (MNPs) are intensely studied for the prospective usage in technological and biomedical applications, due to the variety of their unique properties [1,2]. In composites containing MNPs their high specific surface area is important, as it provides enhanced interaction at the phase boundaries. Therefore the major importance of chemical interactions on the surface of MNPs could not be overestimated both for fundamental studies and practical applications. At the same time, due to the high reactivity of the MNPs, the chemical composition on their surface is difficult to maintain. In general, chemical surface properties of MNPs strongly depend on the fabrication technology. In this respect, the methods based on high energy dispersion and metal condensation in the inert gas provide intact metal surface of the ascondensed MNPs. However, their oxidation upon exposure to ambient atmosphere is inevitable. Moreover, typically, rapid oxidation of
⇑ Corresponding author at: Institute of Electrophysics, RAS, Urals Branch, Ekaterinburg, Russia. E-mail addresses: [email protected] (I.V. Beketov), [email protected] (A.P. Safronov), [email protected] (A.V. Bagazeev), [email protected] (A. Larrañaga), [email protected] (G.V. Kurlyandskaya), medtom@iep. uran.ru (A.I. Medvedev). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.152
the active metallic surface leads to the combustion of MNPs making them highly pyrophoric. The pyrophoricity is a serious obstacle for their application which can be reduced by the MNPs surface passivation, i.e. formation of a barrier layer on the surface preventing rapid oxidation. The nature of the barrier layer may be different: controlled slow oxidation of MNPs [3], formation of a carbon coating on the surface [4], coating by some other modifiers [5]. The passivation of the MNPs’ surface causes the changes in its chemical nature. It should definitely be taken into account considering the research studies and prospective applications. For example, the phase boundaries in MNP composites are affected by the barrier layer rather than by metal itself. As the barrier layer is created specifically for the prevention of pyrophoricity, it is hard to expect its optimum performance at the interfaces in the composite. Therefore, instead of the general all-purpose surface passivation of the MNPs, it is desirable to maintain their tunable specific modification for the application in particular composites. The optimal way to carry it out is to provide surface modification in situ directly in the course of MNPs’ production taking an advantage of the high chemical activity of non-passivated metal surface. Initially pure ferromagnets like iron and nickel are useful model materials for surface modification studies because their key parameters are well known in a bulk state. At the same time, it was shown earlier that fundamental magnetic characteristics
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become susceptible to variations of size and shape at nanoscale [6,7]. Lowering magnetization of strongly bound Fe MNPs for different systems was previously discussed in literature [7,8]: a strongly attached surfactant molecule is expected to perturb the electronic structure lowering the spin. For example, the reduction in magnetization can be increased with the increase of polymer polarity [9]. The above mentioned effect is not limited to organic surfactants: coating by magnetic iron oxides of iron MNPs may affect the coercivity [10]. Metallic nanoparticles (iron, nickel, cobalt, etc.) hold a particular interest for researches as ideal candidates for heterogeneous catalysis, environmental applications and possible absorbers in microwave devices [11]. One of the goals therefore is to obtain magnetic MNPs with reasonably high macroscopic magnetization values. In this work we have prepared Fe and Ni MNPs by the high energy dispersion method of the electrical explosion of wire (EEW) and performed the in situ surface modification by several organic liquids and monomeric, oligomeric, and polymeric organic compounds. The structure, morphological features and magnetic properties were studied by different techniques. We confirm that spherical MNPs with tunable magnetic properties can be fabricated by EEW in large quantities, with low energy consumption and low environmental cost that make them attractive for a variety of technological applications. 2. Experimental Fe and Ni MNPs were produced by the EEW [12]. The method is based on the evaporation of a portion of metal wire by electric high power pulse in the inert atmosphere. Further condensation of the expanding metal vapours results in formation of spherical MNPs. EEW is an ecologically safe method, providing production rates up to 200 g/h. It is characterized by low energy consumption of about 25 kW h/kg. Moreover, it ensures the fabrication of spherical MNPs at properly selected conditions for many materials. The details of the elaborated EEW procedure are given elsewhere [13]. The high-temperature gas-phase deposition of carbon on the surface of the Ni MNPs was made by the additional feeding of butane – C4H10 (the flow rate of 0.5 cm3/s) into the working gas before entering the explosion chamber. The lowtemperature liquid-phase modification of MNPs was provided by immersing asprepared MNPs in hexane, toluene, chloroform or toluene solutions of polystyrene (PS) with molecular mass 2.7 105 in the special hopper attached to the EEW facility. The specific surface of the produced MNPs was determined by low temperature nitrogen adsorption using Micromeritics TriStar 3000. Phase composition was characterized by X-ray diffraction, XRD (Bruker D8 DISCOVER). Transmission electron microscopy including high resolution (TEM and HTEM) was performed using JEOL JEM2100 microscope operating at 200 kV. Combined thermal analyses with simultaneous quadruple mass spectrometry (QMS) were carried out using NETZSCH STA409. Magnetic measurements were performed at room and cryogenic temperatures by a vibrating sample magnetometer (Cryogenics Ltd. VSM), a superconducting quantum device (Quantum Design MPMS-7) and Physical Property Measurement System Quantum Design (PPMS). Primary magnetization curves of all samples were recorded prior to the application of a high field in order to compare the demagnetizing fields of fabricated MNPs to demagnetizing fields estimated for ideally spherical MNPs with the same saturation magnetization [11]. Zero-field cooled (ZFC) and field cooled (FC) thermomagnetic curves were measured under the applied magnetic field H = 50 Oe. The magnetization values in a field of 65 kOe were designated as the saturation magnetization, Ms. In all the cases under consideration, magnetization values, M, in the field of 18 kOe were very close to the magnetization values measured in the maximum applied field of 65 kOe: 100 (Ms M(H = 18 kOe))/Ms 1.34%.
3. Results and discussion We have elaborated two different approaches for the surface modification of MNPs prepared by the EEW. The first one is the deposition of carbon onto the surface of as-condensed metal particles dispersed in the inert gas phase at high temperature. If a small amount of any volatile hydrocarbon is added to nitrogen circulating in the explosion chamber, the hydrocarbon splits to the elements due to the high temperature in a plasma filament formed by the explosion of a wire, and during the process of MNPs’ condensation carbon
deposits onto their surface. This highly productive way lacks in flexibility – (i) not all of the EEW MNPs can be modified in this way; (ii) high temperature gas-phase modification is restricted almost solely to carbon shells. In this respect the second approach – lowtemperature liquid-phase modification of as-prepared active surface provides a variety of opportunities for tunable modification of MNPs. The nature of the liquid modifier is very important: in the present work we have tested hexane, toluene and chloroform for this purpose. The proper choice of liquid opens the path to further modification of as-prepared MNPs by substances which are both soluble in the liquid and have high absorption capacity on the active surface. This is the only way to modify the MNPs with polymers. It the present work Fe and Ni MNPs were studied focusing on the possibility of formation of polystyrene shells on their surfaces. 3.1. High-temperature gas-phase deposition of carbon onto Ni Ni MNPs can be surface modified by carbon in situ during hightemperature gas-phase deposition. Carbon is highly soluble in liquid Fe and it therefore dissolves in the liquid droplets of dispersed Fe in the explosion chamber rather than deposits on the MNPs’ surface. If the electric explosion of iron wire is performed in the presence of volatile hydrocarbon the majority of obtained products are various Fe carbide phases. Under the same conditions the deposition of carbon onto Ni particles goes smoothly and XRD reveals 100% of metal Ni phase. Selected properties of Ni/C MNPs are given in Table 1. The total carbon content in the samples was evaluated by thermal analysis with simultaneous QMS. While heated up to 1000 °C the carbon either in free form or as a part of organic compounds is oxidized to carbon dioxide, (this quantity is measured by QMS). The total carbon content in Ni/C sample was 2.0% (Table 1). The TEM image of Ni/C (Fig. 1) shows a highly ordered carbon structure on the surface of Ni/C particle. Up to 10 parallel layers can be clearly seen in the carbon shells of the MNPs (Fig. 1a). This number of layers is probably the highest that can be formed as a result of carbon condensation. The number of carbon layers varies from one particle to another (Fig. 1b). No expansion of carbon structure apart from the particles was observed: in each case the structure was tightly attached to the surface. The estimation of interlayer distance value was 0.32 nm, which is typical for graphite. Fig. 1b shows the monolayer of carbon upon the surface of a small particle which might be attributed to graphene. 3.2. Interaction of Fe and Ni EEW nanopowders with hexane, toluene and chloroform Hexane is an inert liquid for Fe and Ni MNPs, even if they are immersed in it immediately after EEW. The dominant crystalline phases are a-Fe and Ni (Table 1). QMS analysis does not reveal any carbon in Ni-I MNPs being heated up to 1000 °C in an oxidizing atmosphere. A small amount of carbon was found in Fe-I sample. Such content corresponds to the initial composition of iron wire used in EEW. Most likely this carbon forms part of a-Fe ferrite phase. Both the crystalline structure and the carbon content are essentially the same for Fe-I and Ni-I samples immersed in hexane and Fe and Ni samples passivated by the controlled surface oxidation. However, the measurable amount of oxide phase is detected for Fe sample in the latter case. Samples Fe-II and Ni-II immersed in toluene differed from Fe-I and Ni-I in one aspect: the total carbon content determined by QMS analysis was much higher. While there was no carbon in Ni, Ni-I samples, and a very small amount of carbon, originated from the composition of wire in Fe, Fe-I samples, the total carbon content in Fe-II and Ni-II was substantially increased. TEM image presented in Fig. 2 reveals the location of extra carbon on the surface of Fe MNPs.
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I.V. Beketov et al. / Journal of Alloys and Compounds 586 (2014) S483–S488 Table 1 Selected properties of Fe and Ni MNPs treated by liquids. Sample
Liquid
S (m2/g)
dBET (nm)a
Crystalline phases (%)
Total C content (%, weight)b
Ni-I Ni-II Ni/C Fe
Hexane Toluene – –
13.2 12.9 16.5
51 52 47
– 1.3 2.0 0.08
Fe-I Fe-II
Hexane Toluene
9.5 8.3
81 94
Fe-III
Chloroform
24.2
82
Ni – 100% Ni – 100% Ni – 100% a-Fe – 95.0; c-Fe – 1.9 Fe oxides – 3.1 a-Fe – 97.7; c-Fe + C – 2.3 a-Fe – 88.6; c-Fe + C – 6.7 Fe oxides – 4.7 a-Fe – 47.2; c-Fe + C – 1.5 FeCl2 – 51.2
0.08 0.96 0.74
a Calculated based on the specific surface area and the mean average density of the phases using the equation for the spherical particles: dBET = 6/(S q); BET – Brunauer– Emmett–Teller physical adsorption formalism. b Measured by thermal analysis with simultaneous QMS.
Fig. 1. TEM image of Ni/C MNPs with carbon shells formed by high-temperature gas-phase deposition.
One can clearly see the crystalline lattice of the particle surrounded by the ‘‘hair’’ of loose carbon structures. Similar structures were observed at Ni-II surface. Although they are irregular, one can notice the multiple fragments of jammed double layers. The carbon structures do not stick to the surface of the particle, but expand further forming curls, which make the whole structure ‘‘hair-like’’.
Fig. 2. TEM image of Fe-II nanoparticle with carbon structures formed by toluene treatment at ambient temperature.
To the best of our knowledge the formation of such irregular ‘‘hairlike’’ carbon structures at ambient temperature under the simple immersing of Fe and Ni MNPs into toluene has not been reported yet. We suppose that the surface of as-prepared metal particles acts as a catalyst for the condensation reaction of aromatic organic compounds forming irregular polycyclic structures. Initially these structures are located on the surface but then they are expelled from it upon the further growth and form curly structures apart. The detailed research on this novel observation requesting studies of the reaction mechanism, kinetics, and chemical analysis of the structures, falls beyond the scope of the present work. Perhaps, the difference between carbon structures on the surfaces of either Fe or Ni MNPs (Figs. 1 and 2) with those mentioned in the literature [14–16] stems from the distinct mechanisms of carbon condensation at high temperature and ambient conditions. The conventional mechanism of high-temperature formation of graphene sheets, carbon nanotubes or other ordered carbon structures includes the thermal decomposition of a carbon-containing precursor to the elements and further reassembling of carbon in the ordered structure. This mechanism includes the dissolution of amorphous carbon in the metal lattice at high temperature with further migration to the surface and the formation of an ordered carbon structure on the lattice as a template. At ambient conditions there is no room for elemental carbon in the liquid phase and apparently it cannot be dissolved in metal. Therefore, it is reasonable to assume that the mechanism of carbon deposition in the liquid phase should be different and should necessarily include condensation of molecular blocks, which might be aromatic solvent molecules or some products of their pre-condensation. The principal possibility of such polycyclo condensation is reported in
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the literature [17,18]. As the metal surface acts as a catalyst for this reaction, the products can migrate from it, forming expanded irregular structures. Sample Fe-III (Table 1), which was immersed in chloroform, is substantially different from Fe, Fe-I, and Fe-II. The crystalline structure of Fe-III is dominated by rokuhnite (FeCl2 phase). It means that in this case Fe MNPs are the major reagents in the chemical interaction with the liquid. Such chemical transformation of Fe in chloroform takes place only at the active surface of as-prepared Fe MNPs. The passivated Fe sample, which was immersed in chloroform for several weeks showed no chemical interaction with the liquid. The possible chemical reaction of Fe-III with chloroform might be written as follows:
2CHCl3 þ Fe ¼ FeCl2 þ CHCl2 CHCl2
ð1Þ
In principle, Eq. (1) describes the first step of the possible polycondensation of chlorinated alkanes. Thermal analysis with simultaneous QMS for Fe-III sample reveals the evolution of H2O, CO2, 35 Cl, 37Cl, H35Cl, H37Cl, which are the products of thermal destruction both of FeCl2 and polymeric chlorinated alkanes. Thus, the obtained experimental results show that the as-prepared Fe and Ni EEW MNPs can interact with several organic liquids. Hexane is an inert liquid to the active surface of Fe and Ni. However, for the same reason it is also a poor solvent for the majority of polymeric modifiers. In the case of chloroform metal MNPs act as a reagent in chemical interaction, and their chemical and phase structure changes irreversibly. Therefore, chloroform should not be used as a liquid for surface modification of Fe and Ni MNPs. In case of toluene MNPs act as a catalyst in the process of carbon condensation on the surface, and the phase structure of the particles remains essentially intact. Therefore toluene was chosen as a carrier liquid for the modification of Fe and Ni nanoparticles by polystyrene (PS). 3.3. Modification of Fe and Ni EEW nanoparticles by polysterene in toluene solution Fe and Ni nanopacticles are subjected to the irreversible adsorption of the polymer when immersed into the toluene solution of PS. The adsorbed quantity of the polymer, which depended on its concentration in liquid, was measured by thermal analysis with simultaneous QMS. The isotherm of PS adsorption on Fe is given in Fig. 3. It is a typical saturation curve fitted well by the Langmuir equation [19]. The phase structure of the Fe/PS and Ni/PS samples modified by polystyrene in toluene is essentially the same as for other Fe and Ni EEW MNPs: nickel is represented as a single metal phase; in the Fe/
PS case the iron phase structure is similar to the iron phase structure of Fe-II sample. The same is true for the total carbon content. Fig. 4 shows TEM image of Fe/PS nanoparticles. Contrary to the carbon structures, which are formed on the surface of Fe-II particles in toluene (Fig. 2), PS shells on the particles are amorphous, relatively uniform in thickness and they are attached to the surface with no jammed expanded structures. We suppose that the adsorption of polymer on the surface of MNPs takes place more rapidly than chemical condensation of toluene molecules and while the active surface is blocked by the adsorbed polymer, the formation of ‘‘hair-like’’ carbon structures is prevented. 3.4. Magnetic properties of modified Fe and Ni EEW nanoparticles Fig. 5 shows hysteresis loops M(H) with primary magnetization curves measured at room temperature for all samples under consideration (see also Table 2). Insets of Fig. 4 shows typical selected examples of the ZFC–FC temperature dependence of the magnetization in the range 5–310 K. To obtain zero-field-cooling (ZFC) data, the samples were cooled at zero-field conditions from room temperature down to 5 K, and then the magnetization was recorded with an increasing temperature under the applied constant magnetic field of 50 Oe. To obtain FC data, the process was repeated under the same magnetic field applied both while cooling and consequent heating of the sample. In all cases, the low temperature value for the magnetization measured after zero-fieldcooling is non-zero and rather high: for example, MZFC(T = 5 K) 0.6MFC(T = 5 K) for Fe/PS and MZFC(T = 5 K) 0.7MFC(T = 5 K) for Ni/PS MNPs respectively. An increase of the MZFC magnetization was observed as the temperature increased for all samples showing no clear maximum on the ZFC curve. MFC(T) magnetization showed very small change, being almost constant for all samples. Similar ZFC–FC curves’ shape for nickel MNPs surrounded by uniform-size NiO shell in carbon matrix was previously reported by Fernández-García et al. [8] where the observed very wide MZFC(T) maximum was explained by the wide distribution of the sizes of MNPs and effects of collective frozen state related to the influence of the magnetic ‘‘dead’’ layer. In the present work the origin of the lack of clear maximum on the ZFC curve is different. The average sizes of the EEW Fe and Ni MNPs are at least one order of magnitude higher (Table 1) and therefore they are well above the critical sizes of the transition into superparamagnetic state for all studied samples [20]. It is important to mention the existence of two critical sizes for iron and nickel spherical MNPs: the first critical diameter is related to the transition into superparamagnetic state (Dspm) and the second one is the so-called limit of the single domain state (Dsd). Despite the variation in these numbers reported for ambient conditions by different authors, especially for the Dspm limit, it is widely accepted [20] that Dspm < 20 nm in the case of Ni and Dspm 10 nm for Fe; Dsd 14 nm for Fe and Dsd 50–60 nm in the case of Ni. Analysis of the sizes of all studied MNPs (Table 2) indicates that: (a) All their sizes are well above the superparamagnetism (SPM) limit. (b) The iron nanoparticles are certainly in multidomain states. (c) The nickel nanoparticles’ sizes are close to single domain transition limit.
Fig. 3. Isotherm of PS adsorption on Fe/PS sample.
Further insight for the understanding of magnetic behaviour comes from the analysis of magnetization vs. the applied field curves (Fig. 5). Rather substantial coercivity around 100 Oe (see also Table 2) and non-zero remanence (Mr – remanent magnetization) were observed for all MNPs. As an example Fig. 6 shows M(H) curves of Ni-II MNPs measured at low and room temperatures:
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Fig. 4. TEM image of Fe/PS (a) and Ni/PS, and (b) MNPs.
Fig. 6. Hysteresis loops for Ni-II MNPs.
Fig. 5. Hysteresis loops measured at 300 K for MNPs listed in Table 2: Fe-based MNPs (a); Ni-based MNPs (b). The insets show a zoom-in of the hysteresis behaviour at low field (left) and examples of ZFC–FC magnetization curves (right) of the selected samples under an applied magnetic field H = 50 Oe.
Table 2 Selected properties of the EEW Fe and Ni. Sample
dXRD (nm)
Liquid
Hc (Oe)
Ms (emu/g)
M s (emu/g)
Ni-I Ni-II Ni/C Ni/PS 0.5% PS Fe-I Fe-II Fe/PS 0.5% PS
40 40 30/10 30 60 40 40
Hexane Toluene Hexane Toluene Hexane Toluene Toluene
260 230 100 280 290 250 260
52 49 39 43 175 151 155
52 50 40 43 175 162 156
MNPs: dXRD – mean diameter defined by XRD. Ms – net magnetization measured at a temperature of 300 K; Hc – coercivity. Ms – magnetization value recalculated for pure metallic phase of the MNP.
Mr/Ms 0.26 at room and Mr/Ms 0.36 T = 5 K temperature. The absence of saturation at low temperatures is an interesting feature of some composites and MNPs, which in special cases can be explained by the surface spin canting and large surface magnetic anisotropy [21]. The observed tendency to saturation even in the field of about 20 kOe seems discarding that possibility for MNPs studied in the present work.
It is also interesting to mention that even at room temperature the magnetization curves are close to saturation at reasonably low magnetic fields: for all MNPs M(H = 18 kOe) 0.987Ms. One can use the saturation magnetization approach of the high field M(H) in order to insure the saturation magnetization value [22]. Thus, for Ni-II MNPs we obtained Ms 55.5 emu for T = 5 K, which is only about 5% lower than the one expected for bulk fcc Ni [21] and very close to the one obtained directly from the experimental M(H) dependence. Therefore, eventually, we concluded that there was no need to apply established formalism of the fitting of ZFC–FC curves by the non-interacting superparamagnetic system model because the MNPs appeared to be in ferromagnetic state: the magnetization curves showed clear hysteresis and magnetization saturation, even at room temperature (Fig. 5). Analysis of M(H) curves shows a clear similarity in each MNPs group with close values of coercivity (Hc), except for Ni/C MNPs with carbon shell. Again, all of the observed Ms values are lower than the corresponding Ms value of the bulk. Apart from the size effects it is a consequence of complex surface modification and changes in the composition of the samples (Table 2). Therefore we have recalculated magnetization value for pure metallic phase of the MNPs (M s ) taking into account the real composition of the MNPs, percentage of pure metallic core, densities and magnetization values of the additional phases using the corresponding number for bulk materials [20,22]. For example, M s is quite reasonably close to Ms of the pure bulk iron being only 23% lower in the case of Fe-I sample. Gangopadhyay et al. [10] discussed the fact that coatings by magnetic iron oxides of iron MNPs affected the coercivity strongly. We did not observe the last effect in present studies, perhaps due to a bigger size of EEW MNPs. Encapsulated metallic
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magnetic MNPs with such high saturation magnetization values are ideal candidates for being used as tunable microwave absorbers in a wide range of frequencies [11]. Significant reduction of coercivity was observed in the case of Ni/C composite MNPs. According to XRD and TEM studies, they consist of 30 nm diameter Ni core and 5 nm wide graphite shells. One, therefore, can explain the change in coercivity as a consequence of the reduction of the interparticle interactions due to the increase of the distances between the magnetic cores and/or transition into single domain state for a majority of the MNPs of the ensemble. Although additional studies with MNPs of smaller size are necessary, Ni/C nanoparticles are good candidates for the development of non-agregated magnetic nanofluids. Finally we would like to attract attention to the shape of fabricated EEW MNPs which is a critically important parameter for many technological and especially biological applications. It is well known that, despite many efforts and the variety of different techniques tested, the shape of the MNPs is still one of the most difficult parameters to control [13]. Figs. 1, 2 and 4 show clearly that EEW MNPs are essentially spherical. As an additional and complementary way to evaluate the degree of sphericity one can use well established methods of analysis of the shape of the primary magnetization curves: the initial slope of the 4pM vs. H curve is close to 3 for spherical MNPs [11,23,24]. Let us give just one example for comparison. It was found as a result of the analysis (Fig. 5) that for Fe-I sample the cut point of the slope of the primary magnetization curve was very close to the field corresponding to spherical MNPs: 4pMs 3H with an accuracy of about 5%. 4. Conclusions Iron and nickel spherical magnetic MNPs were prepared by the EEW technique which provides a very high production rate up to 200 g/h and requires low energy consumption of 25 kW h/kg. The as-prepared MNPs were modified in situ at room temperature in hexane, toluene, chloroform, and the solutions of polystyrene in toluene in an inert atmosphere in order to provide both the stability at ambient conditions and tunable surface properties. Hexane was an inert liquid for Fe and Ni MNPs. In the case of toluene the surface of as-prepared Fe or Ni MNPs acted as a catalyst for the condensation reaction of aromatic organic compounds. The treatment with toluene led to the deposition of carbon on the surface of the MNPs. Chloroform chemically interacted with Fe MNPs, forming FeCl2. The treatment of MNPs with the solution of polystyrene in toluene resulted in the adsorption of polymer and the formation of 3–6 nm polymeric coatings. The obtained properties of magnetic nanoparticles with mean diameter 50–80 nm (like the magnetization value for Ni MNPs up to 52 emu/g and up to 179 emu/g for Fe MNPs) at room temperature make them competitive candidates for nanocomposites focused on technological applications. Acknowledgments This work was supported by RFBR 10-02-96015, UrFU 215 and MAT2011-27573-C04 project of the Spanish Government grants.
Selected measurements were performed at SGIker service of UPV-EHU. We thank S.M. Bhagat and I. Orue for special support. References [1] J. Tang, L. Feng, J. Wiemann, Negative magnetoresistance of g-Fe2O3 observed in g-Fe2O3/Ag granular nanocomposites, Appl. Phys. Lett. 74 (1999) 2522– 2524. [2] J. Llandro, J.J. Palfreyman, A. Ionescu, C.H.W. Barnes, Magnetic biosensor technologies for biomedical applications: a review, Med. Biol. Eng. Comput. 48 (2010) 977–998. [3] Y.-S. Kwon, A.A. Gromov, J.I. Strokova, Passivation of the surface of aluminum nanopowders by protective coatings of the different chemical origin, Appl. Surf. Sci. 253 (2007) 5558–5564. [4] E. Athanassiou, R. Grass, W. Stark, Large-scale production of carbon-coated copper nanoparticles for sensor applications, Nanotechnology 17 (2006) 1668– 1673. [5] X.P. Xia, S.Z. Cai, C.S. Xie, Preparation, structure and thermal stability of Cu/ LDPE nanocomposites, Mater. Chem. Phys. 95 (2006) 122–129. [6] G.V. Kurlyandskaya, J. Cunanan, S.M. Bhagat, J.C. Aphesteguy, S.E. Jacobo, Fieldinduced microwave absorption in Fe3O4 nanoparticles and Fe3O4/polyaniline composites synthesized by different methods, J. Phys. Chem. Solids 68 (2007) 1527–1532. [7] Y.-W. Jun, J.-W. Seo, J. Cheon, Nanoscaling laws of magnetic nanoparticles and their applicabilities in biomedical sciences, Acc. Chem. Res. 41 (2008) 179–189. [8] M.P. Fernandez-Garcia, P. Gorria, M. Sevilla, M.P. Proença, R. Boada, J. Chaboy, A.B. Fuentes, J. Blanco, Enhanced protection of carbon-encapsulated magnetic nickel nanoparticles through a sucrose-based synthetic strategy, J. Phys. Chem. 115 (2011) 5294–5300. [9] D.L. Huber, Synthesis, properties, and applications of iron nanoparticles, Small 5 (2005) 482–501. [10] S. Gangopadhyay, G.H. Hadjipanayis, B. Dale, S.M. Sorensen, K.J. Klabunde, V. Papaefthymiou, A. Kostikas, Magnetic properties of ultrafine iron particles, Phys. Rev. B 45 (1992) 9778–9787. [11] G.V. Kurlyandskaya, S.M. Bhagat, C. Luna, M. Vazquez, Microwave absorption of nanoscale CoNi powders, J. Appl. Phys. 99 (104308) (2006) 1–5. [12] Yu.A. Kotov, Electric explosion of wires as a method for preparation of nanopowders, J. Nanoparticle Res. 5 (2003) 539–550. [13] G.V. Kurlyandskaya, S.M. Bhagat, A.P. Safronov, I.V. Beketov, A. Larrañaga, Spherical magnetic nanoparticles fabricated by electric explosion of wire, AIP Adv. 1 (2011) 042122. [14] Y. Yao, Zh. Li, Z. Lin, K.-S. Moon, J. Agar, C. Wong, Controlled growth of multilayer, few-layer, and single-layer graphene on metal substrates, J. Phys. Chem. C 115 (2011) 5232–5238. [15] J.A. Rodrıguez-Manzo, C. Pham-Huu, F. Banhart, Graphene growth by a metalcatalyzed solid-state transformation of amorphous carbon, ACS Nano 5 (2011) 1529–1534. [16] Zh. He, J.-L. Maurice, A. Gohier, C.S. Lee, D. Pribat, C.S. Cojocaru, Iron catalysts for the growth of carbon nanofibers: Fe, Fe3C or both?, Chem Mater. 23 (2011) 5379–5387. [17] M. Muller, C. Kubel, K. Mullen, Giant polycyclic aromatic hydrocarbons, Chem. Eur. J. 4 (1998) 2099–2109. [18] J. Wu, Z. Tomovic, V. Enkelmann, K. Mullen, From branched hydrocarbon propellers to c3-symmetric graphite disks, J. Org. Chem. 69 (2004) 5179–5186. [19] P.C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, Marcel Dekker, New York, 1997. pp. 333–335. ´ Handley, Modern Magnetic Materials, John Wiley & Sons, New York, [20] R.C. O 1972. p. 740. [21] J. Lee, T. Isobe, M. Senna, Preparation of ultrafine Fe3O4 particles by precipitation in the presence of PVA at high pH, J. Colloid Interface Sci. 177 (1996) 490–494. [22] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley Publishing Company, 1972. pp. 233–236. [23] J.S. Ramachandran, S.M. Bhagat, J.L. Peng, M. Rubinstein, FMR of powder La0.7Ca0.3MnO3, Solid State Commun. 96 (1995) 127–131. [24] I.V. Beketov, A.P. Safronov, A.I. Medvedev, J. Alonso, G.V. Kurlyandskaya, S.M. Bhagat, Iron oxide nanoparticles fabricated by electric explosion of wire: focus on magnetic nanofluids, AIP Adv. 2 (2012) 022154.
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
In situ modification of Fe and Ni magnetic nanopowders produced by the electrical explosion of wire I.V. Beketov a,b,⇑, A.P. Safronov a,b, A.V. Bagazeev a, A. Larrañaga c, G.V. Kurlyandskaya b,c, A.I. Medvedev a,b a
Institute of Electrophysics, RAS, Urals Branch, Ekaterinburg, Russia Ural Federal University, Ekaterinburg, Russia c University of the Basque Country UPV-EHU, Bilbao, Spain b
a r t i c l e
i n f o
Article history: Available online 13 February 2013 Keywords: Electric explosion of wire Nanoparticles Magnetic measurements Surface modification
a b s t r a c t Iron and nickel spherical magnetic nanoparticles (MNPs) with mean diameter 50–80 nm were prepared by the electric explosion of wire. The advantage of this method is that it provides a very high production rate up to 200 g/h and requires low energy consumption of 25 kW h/kg. The as-prepared MNPs were modified in situ in hexane, toluene, chloroform, and the solutions of polystyrene in toluene, in an inert atmosphere at room temperature in order to provide both the stability at ambient conditions and tunable surface properties. Hexane was an inert liquid for Fe and Ni MNPs. In the case of toluene the surface of asprepared Fe or Ni MNPs acted as a catalyst for the condensation reaction of aromatic organic compounds forming irregular polycyclic structures. The treatment with toluene led to the deposition of carbon and formation of ‘‘hair-like’’ ordered structures on the surface of the MNPs. Chloroform chemically interacted with Fe MNPs, forming FeCl2. The treatment of MNPs with the solution of polystyrene in toluene resulted in the adsorption of polymer and the formation of 3–6 nm polymeric coatings on the surface. The obtained magnetic properties (like the magnetization value for Ni MNPs up to 52 emu/g and Fe MNPs up to 179 emu/g) at room temperature make these MNPs competitive candidates for nanocomposites for microwave and other technological applications. Ó 2013 Elsevier B.V. All rights reserved.
1. Introduction Highly dispersed metallic nanoparticles (MNPs) are intensely studied for the prospective usage in technological and biomedical applications, due to the variety of their unique properties [1,2]. In composites containing MNPs their high specific surface area is important, as it provides enhanced interaction at the phase boundaries. Therefore the major importance of chemical interactions on the surface of MNPs could not be overestimated both for fundamental studies and practical applications. At the same time, due to the high reactivity of the MNPs, the chemical composition on their surface is difficult to maintain. In general, chemical surface properties of MNPs strongly depend on the fabrication technology. In this respect, the methods based on high energy dispersion and metal condensation in the inert gas provide intact metal surface of the ascondensed MNPs. However, their oxidation upon exposure to ambient atmosphere is inevitable. Moreover, typically, rapid oxidation of
⇑ Corresponding author at: Institute of Electrophysics, RAS, Urals Branch, Ekaterinburg, Russia. E-mail addresses: [email protected] (I.V. Beketov), [email protected] (A.P. Safronov), [email protected] (A.V. Bagazeev), [email protected] (A. Larrañaga), [email protected] (G.V. Kurlyandskaya), medtom@iep. uran.ru (A.I. Medvedev). 0925-8388/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.jallcom.2013.01.152
the active metallic surface leads to the combustion of MNPs making them highly pyrophoric. The pyrophoricity is a serious obstacle for their application which can be reduced by the MNPs surface passivation, i.e. formation of a barrier layer on the surface preventing rapid oxidation. The nature of the barrier layer may be different: controlled slow oxidation of MNPs [3], formation of a carbon coating on the surface [4], coating by some other modifiers [5]. The passivation of the MNPs’ surface causes the changes in its chemical nature. It should definitely be taken into account considering the research studies and prospective applications. For example, the phase boundaries in MNP composites are affected by the barrier layer rather than by metal itself. As the barrier layer is created specifically for the prevention of pyrophoricity, it is hard to expect its optimum performance at the interfaces in the composite. Therefore, instead of the general all-purpose surface passivation of the MNPs, it is desirable to maintain their tunable specific modification for the application in particular composites. The optimal way to carry it out is to provide surface modification in situ directly in the course of MNPs’ production taking an advantage of the high chemical activity of non-passivated metal surface. Initially pure ferromagnets like iron and nickel are useful model materials for surface modification studies because their key parameters are well known in a bulk state. At the same time, it was shown earlier that fundamental magnetic characteristics
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become susceptible to variations of size and shape at nanoscale [6,7]. Lowering magnetization of strongly bound Fe MNPs for different systems was previously discussed in literature [7,8]: a strongly attached surfactant molecule is expected to perturb the electronic structure lowering the spin. For example, the reduction in magnetization can be increased with the increase of polymer polarity [9]. The above mentioned effect is not limited to organic surfactants: coating by magnetic iron oxides of iron MNPs may affect the coercivity [10]. Metallic nanoparticles (iron, nickel, cobalt, etc.) hold a particular interest for researches as ideal candidates for heterogeneous catalysis, environmental applications and possible absorbers in microwave devices [11]. One of the goals therefore is to obtain magnetic MNPs with reasonably high macroscopic magnetization values. In this work we have prepared Fe and Ni MNPs by the high energy dispersion method of the electrical explosion of wire (EEW) and performed the in situ surface modification by several organic liquids and monomeric, oligomeric, and polymeric organic compounds. The structure, morphological features and magnetic properties were studied by different techniques. We confirm that spherical MNPs with tunable magnetic properties can be fabricated by EEW in large quantities, with low energy consumption and low environmental cost that make them attractive for a variety of technological applications. 2. Experimental Fe and Ni MNPs were produced by the EEW [12]. The method is based on the evaporation of a portion of metal wire by electric high power pulse in the inert atmosphere. Further condensation of the expanding metal vapours results in formation of spherical MNPs. EEW is an ecologically safe method, providing production rates up to 200 g/h. It is characterized by low energy consumption of about 25 kW h/kg. Moreover, it ensures the fabrication of spherical MNPs at properly selected conditions for many materials. The details of the elaborated EEW procedure are given elsewhere [13]. The high-temperature gas-phase deposition of carbon on the surface of the Ni MNPs was made by the additional feeding of butane – C4H10 (the flow rate of 0.5 cm3/s) into the working gas before entering the explosion chamber. The lowtemperature liquid-phase modification of MNPs was provided by immersing asprepared MNPs in hexane, toluene, chloroform or toluene solutions of polystyrene (PS) with molecular mass 2.7 105 in the special hopper attached to the EEW facility. The specific surface of the produced MNPs was determined by low temperature nitrogen adsorption using Micromeritics TriStar 3000. Phase composition was characterized by X-ray diffraction, XRD (Bruker D8 DISCOVER). Transmission electron microscopy including high resolution (TEM and HTEM) was performed using JEOL JEM2100 microscope operating at 200 kV. Combined thermal analyses with simultaneous quadruple mass spectrometry (QMS) were carried out using NETZSCH STA409. Magnetic measurements were performed at room and cryogenic temperatures by a vibrating sample magnetometer (Cryogenics Ltd. VSM), a superconducting quantum device (Quantum Design MPMS-7) and Physical Property Measurement System Quantum Design (PPMS). Primary magnetization curves of all samples were recorded prior to the application of a high field in order to compare the demagnetizing fields of fabricated MNPs to demagnetizing fields estimated for ideally spherical MNPs with the same saturation magnetization [11]. Zero-field cooled (ZFC) and field cooled (FC) thermomagnetic curves were measured under the applied magnetic field H = 50 Oe. The magnetization values in a field of 65 kOe were designated as the saturation magnetization, Ms. In all the cases under consideration, magnetization values, M, in the field of 18 kOe were very close to the magnetization values measured in the maximum applied field of 65 kOe: 100 (Ms M(H = 18 kOe))/Ms 1.34%.
3. Results and discussion We have elaborated two different approaches for the surface modification of MNPs prepared by the EEW. The first one is the deposition of carbon onto the surface of as-condensed metal particles dispersed in the inert gas phase at high temperature. If a small amount of any volatile hydrocarbon is added to nitrogen circulating in the explosion chamber, the hydrocarbon splits to the elements due to the high temperature in a plasma filament formed by the explosion of a wire, and during the process of MNPs’ condensation carbon
deposits onto their surface. This highly productive way lacks in flexibility – (i) not all of the EEW MNPs can be modified in this way; (ii) high temperature gas-phase modification is restricted almost solely to carbon shells. In this respect the second approach – lowtemperature liquid-phase modification of as-prepared active surface provides a variety of opportunities for tunable modification of MNPs. The nature of the liquid modifier is very important: in the present work we have tested hexane, toluene and chloroform for this purpose. The proper choice of liquid opens the path to further modification of as-prepared MNPs by substances which are both soluble in the liquid and have high absorption capacity on the active surface. This is the only way to modify the MNPs with polymers. It the present work Fe and Ni MNPs were studied focusing on the possibility of formation of polystyrene shells on their surfaces. 3.1. High-temperature gas-phase deposition of carbon onto Ni Ni MNPs can be surface modified by carbon in situ during hightemperature gas-phase deposition. Carbon is highly soluble in liquid Fe and it therefore dissolves in the liquid droplets of dispersed Fe in the explosion chamber rather than deposits on the MNPs’ surface. If the electric explosion of iron wire is performed in the presence of volatile hydrocarbon the majority of obtained products are various Fe carbide phases. Under the same conditions the deposition of carbon onto Ni particles goes smoothly and XRD reveals 100% of metal Ni phase. Selected properties of Ni/C MNPs are given in Table 1. The total carbon content in the samples was evaluated by thermal analysis with simultaneous QMS. While heated up to 1000 °C the carbon either in free form or as a part of organic compounds is oxidized to carbon dioxide, (this quantity is measured by QMS). The total carbon content in Ni/C sample was 2.0% (Table 1). The TEM image of Ni/C (Fig. 1) shows a highly ordered carbon structure on the surface of Ni/C particle. Up to 10 parallel layers can be clearly seen in the carbon shells of the MNPs (Fig. 1a). This number of layers is probably the highest that can be formed as a result of carbon condensation. The number of carbon layers varies from one particle to another (Fig. 1b). No expansion of carbon structure apart from the particles was observed: in each case the structure was tightly attached to the surface. The estimation of interlayer distance value was 0.32 nm, which is typical for graphite. Fig. 1b shows the monolayer of carbon upon the surface of a small particle which might be attributed to graphene. 3.2. Interaction of Fe and Ni EEW nanopowders with hexane, toluene and chloroform Hexane is an inert liquid for Fe and Ni MNPs, even if they are immersed in it immediately after EEW. The dominant crystalline phases are a-Fe and Ni (Table 1). QMS analysis does not reveal any carbon in Ni-I MNPs being heated up to 1000 °C in an oxidizing atmosphere. A small amount of carbon was found in Fe-I sample. Such content corresponds to the initial composition of iron wire used in EEW. Most likely this carbon forms part of a-Fe ferrite phase. Both the crystalline structure and the carbon content are essentially the same for Fe-I and Ni-I samples immersed in hexane and Fe and Ni samples passivated by the controlled surface oxidation. However, the measurable amount of oxide phase is detected for Fe sample in the latter case. Samples Fe-II and Ni-II immersed in toluene differed from Fe-I and Ni-I in one aspect: the total carbon content determined by QMS analysis was much higher. While there was no carbon in Ni, Ni-I samples, and a very small amount of carbon, originated from the composition of wire in Fe, Fe-I samples, the total carbon content in Fe-II and Ni-II was substantially increased. TEM image presented in Fig. 2 reveals the location of extra carbon on the surface of Fe MNPs.
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Liquid
S (m2/g)
dBET (nm)a
Crystalline phases (%)
Total C content (%, weight)b
Ni-I Ni-II Ni/C Fe
Hexane Toluene – –
13.2 12.9 16.5
51 52 47
– 1.3 2.0 0.08
Fe-I Fe-II
Hexane Toluene
9.5 8.3
81 94
Fe-III
Chloroform
24.2
82
Ni – 100% Ni – 100% Ni – 100% a-Fe – 95.0; c-Fe – 1.9 Fe oxides – 3.1 a-Fe – 97.7; c-Fe + C – 2.3 a-Fe – 88.6; c-Fe + C – 6.7 Fe oxides – 4.7 a-Fe – 47.2; c-Fe + C – 1.5 FeCl2 – 51.2
0.08 0.96 0.74
a Calculated based on the specific surface area and the mean average density of the phases using the equation for the spherical particles: dBET = 6/(S q); BET – Brunauer– Emmett–Teller physical adsorption formalism. b Measured by thermal analysis with simultaneous QMS.
Fig. 1. TEM image of Ni/C MNPs with carbon shells formed by high-temperature gas-phase deposition.
One can clearly see the crystalline lattice of the particle surrounded by the ‘‘hair’’ of loose carbon structures. Similar structures were observed at Ni-II surface. Although they are irregular, one can notice the multiple fragments of jammed double layers. The carbon structures do not stick to the surface of the particle, but expand further forming curls, which make the whole structure ‘‘hair-like’’.
Fig. 2. TEM image of Fe-II nanoparticle with carbon structures formed by toluene treatment at ambient temperature.
To the best of our knowledge the formation of such irregular ‘‘hairlike’’ carbon structures at ambient temperature under the simple immersing of Fe and Ni MNPs into toluene has not been reported yet. We suppose that the surface of as-prepared metal particles acts as a catalyst for the condensation reaction of aromatic organic compounds forming irregular polycyclic structures. Initially these structures are located on the surface but then they are expelled from it upon the further growth and form curly structures apart. The detailed research on this novel observation requesting studies of the reaction mechanism, kinetics, and chemical analysis of the structures, falls beyond the scope of the present work. Perhaps, the difference between carbon structures on the surfaces of either Fe or Ni MNPs (Figs. 1 and 2) with those mentioned in the literature [14–16] stems from the distinct mechanisms of carbon condensation at high temperature and ambient conditions. The conventional mechanism of high-temperature formation of graphene sheets, carbon nanotubes or other ordered carbon structures includes the thermal decomposition of a carbon-containing precursor to the elements and further reassembling of carbon in the ordered structure. This mechanism includes the dissolution of amorphous carbon in the metal lattice at high temperature with further migration to the surface and the formation of an ordered carbon structure on the lattice as a template. At ambient conditions there is no room for elemental carbon in the liquid phase and apparently it cannot be dissolved in metal. Therefore, it is reasonable to assume that the mechanism of carbon deposition in the liquid phase should be different and should necessarily include condensation of molecular blocks, which might be aromatic solvent molecules or some products of their pre-condensation. The principal possibility of such polycyclo condensation is reported in
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the literature [17,18]. As the metal surface acts as a catalyst for this reaction, the products can migrate from it, forming expanded irregular structures. Sample Fe-III (Table 1), which was immersed in chloroform, is substantially different from Fe, Fe-I, and Fe-II. The crystalline structure of Fe-III is dominated by rokuhnite (FeCl2 phase). It means that in this case Fe MNPs are the major reagents in the chemical interaction with the liquid. Such chemical transformation of Fe in chloroform takes place only at the active surface of as-prepared Fe MNPs. The passivated Fe sample, which was immersed in chloroform for several weeks showed no chemical interaction with the liquid. The possible chemical reaction of Fe-III with chloroform might be written as follows:
2CHCl3 þ Fe ¼ FeCl2 þ CHCl2 CHCl2
ð1Þ
In principle, Eq. (1) describes the first step of the possible polycondensation of chlorinated alkanes. Thermal analysis with simultaneous QMS for Fe-III sample reveals the evolution of H2O, CO2, 35 Cl, 37Cl, H35Cl, H37Cl, which are the products of thermal destruction both of FeCl2 and polymeric chlorinated alkanes. Thus, the obtained experimental results show that the as-prepared Fe and Ni EEW MNPs can interact with several organic liquids. Hexane is an inert liquid to the active surface of Fe and Ni. However, for the same reason it is also a poor solvent for the majority of polymeric modifiers. In the case of chloroform metal MNPs act as a reagent in chemical interaction, and their chemical and phase structure changes irreversibly. Therefore, chloroform should not be used as a liquid for surface modification of Fe and Ni MNPs. In case of toluene MNPs act as a catalyst in the process of carbon condensation on the surface, and the phase structure of the particles remains essentially intact. Therefore toluene was chosen as a carrier liquid for the modification of Fe and Ni nanoparticles by polystyrene (PS). 3.3. Modification of Fe and Ni EEW nanoparticles by polysterene in toluene solution Fe and Ni nanopacticles are subjected to the irreversible adsorption of the polymer when immersed into the toluene solution of PS. The adsorbed quantity of the polymer, which depended on its concentration in liquid, was measured by thermal analysis with simultaneous QMS. The isotherm of PS adsorption on Fe is given in Fig. 3. It is a typical saturation curve fitted well by the Langmuir equation [19]. The phase structure of the Fe/PS and Ni/PS samples modified by polystyrene in toluene is essentially the same as for other Fe and Ni EEW MNPs: nickel is represented as a single metal phase; in the Fe/
PS case the iron phase structure is similar to the iron phase structure of Fe-II sample. The same is true for the total carbon content. Fig. 4 shows TEM image of Fe/PS nanoparticles. Contrary to the carbon structures, which are formed on the surface of Fe-II particles in toluene (Fig. 2), PS shells on the particles are amorphous, relatively uniform in thickness and they are attached to the surface with no jammed expanded structures. We suppose that the adsorption of polymer on the surface of MNPs takes place more rapidly than chemical condensation of toluene molecules and while the active surface is blocked by the adsorbed polymer, the formation of ‘‘hair-like’’ carbon structures is prevented. 3.4. Magnetic properties of modified Fe and Ni EEW nanoparticles Fig. 5 shows hysteresis loops M(H) with primary magnetization curves measured at room temperature for all samples under consideration (see also Table 2). Insets of Fig. 4 shows typical selected examples of the ZFC–FC temperature dependence of the magnetization in the range 5–310 K. To obtain zero-field-cooling (ZFC) data, the samples were cooled at zero-field conditions from room temperature down to 5 K, and then the magnetization was recorded with an increasing temperature under the applied constant magnetic field of 50 Oe. To obtain FC data, the process was repeated under the same magnetic field applied both while cooling and consequent heating of the sample. In all cases, the low temperature value for the magnetization measured after zero-fieldcooling is non-zero and rather high: for example, MZFC(T = 5 K) 0.6MFC(T = 5 K) for Fe/PS and MZFC(T = 5 K) 0.7MFC(T = 5 K) for Ni/PS MNPs respectively. An increase of the MZFC magnetization was observed as the temperature increased for all samples showing no clear maximum on the ZFC curve. MFC(T) magnetization showed very small change, being almost constant for all samples. Similar ZFC–FC curves’ shape for nickel MNPs surrounded by uniform-size NiO shell in carbon matrix was previously reported by Fernández-García et al. [8] where the observed very wide MZFC(T) maximum was explained by the wide distribution of the sizes of MNPs and effects of collective frozen state related to the influence of the magnetic ‘‘dead’’ layer. In the present work the origin of the lack of clear maximum on the ZFC curve is different. The average sizes of the EEW Fe and Ni MNPs are at least one order of magnitude higher (Table 1) and therefore they are well above the critical sizes of the transition into superparamagnetic state for all studied samples [20]. It is important to mention the existence of two critical sizes for iron and nickel spherical MNPs: the first critical diameter is related to the transition into superparamagnetic state (Dspm) and the second one is the so-called limit of the single domain state (Dsd). Despite the variation in these numbers reported for ambient conditions by different authors, especially for the Dspm limit, it is widely accepted [20] that Dspm < 20 nm in the case of Ni and Dspm 10 nm for Fe; Dsd 14 nm for Fe and Dsd 50–60 nm in the case of Ni. Analysis of the sizes of all studied MNPs (Table 2) indicates that: (a) All their sizes are well above the superparamagnetism (SPM) limit. (b) The iron nanoparticles are certainly in multidomain states. (c) The nickel nanoparticles’ sizes are close to single domain transition limit.
Fig. 3. Isotherm of PS adsorption on Fe/PS sample.
Further insight for the understanding of magnetic behaviour comes from the analysis of magnetization vs. the applied field curves (Fig. 5). Rather substantial coercivity around 100 Oe (see also Table 2) and non-zero remanence (Mr – remanent magnetization) were observed for all MNPs. As an example Fig. 6 shows M(H) curves of Ni-II MNPs measured at low and room temperatures:
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Fig. 4. TEM image of Fe/PS (a) and Ni/PS, and (b) MNPs.
Fig. 6. Hysteresis loops for Ni-II MNPs.
Fig. 5. Hysteresis loops measured at 300 K for MNPs listed in Table 2: Fe-based MNPs (a); Ni-based MNPs (b). The insets show a zoom-in of the hysteresis behaviour at low field (left) and examples of ZFC–FC magnetization curves (right) of the selected samples under an applied magnetic field H = 50 Oe.
Table 2 Selected properties of the EEW Fe and Ni. Sample
dXRD (nm)
Liquid
Hc (Oe)
Ms (emu/g)
M s (emu/g)
Ni-I Ni-II Ni/C Ni/PS 0.5% PS Fe-I Fe-II Fe/PS 0.5% PS
40 40 30/10 30 60 40 40
Hexane Toluene Hexane Toluene Hexane Toluene Toluene
260 230 100 280 290 250 260
52 49 39 43 175 151 155
52 50 40 43 175 162 156
MNPs: dXRD – mean diameter defined by XRD. Ms – net magnetization measured at a temperature of 300 K; Hc – coercivity. Ms – magnetization value recalculated for pure metallic phase of the MNP.
Mr/Ms 0.26 at room and Mr/Ms 0.36 T = 5 K temperature. The absence of saturation at low temperatures is an interesting feature of some composites and MNPs, which in special cases can be explained by the surface spin canting and large surface magnetic anisotropy [21]. The observed tendency to saturation even in the field of about 20 kOe seems discarding that possibility for MNPs studied in the present work.
It is also interesting to mention that even at room temperature the magnetization curves are close to saturation at reasonably low magnetic fields: for all MNPs M(H = 18 kOe) 0.987Ms. One can use the saturation magnetization approach of the high field M(H) in order to insure the saturation magnetization value [22]. Thus, for Ni-II MNPs we obtained Ms 55.5 emu for T = 5 K, which is only about 5% lower than the one expected for bulk fcc Ni [21] and very close to the one obtained directly from the experimental M(H) dependence. Therefore, eventually, we concluded that there was no need to apply established formalism of the fitting of ZFC–FC curves by the non-interacting superparamagnetic system model because the MNPs appeared to be in ferromagnetic state: the magnetization curves showed clear hysteresis and magnetization saturation, even at room temperature (Fig. 5). Analysis of M(H) curves shows a clear similarity in each MNPs group with close values of coercivity (Hc), except for Ni/C MNPs with carbon shell. Again, all of the observed Ms values are lower than the corresponding Ms value of the bulk. Apart from the size effects it is a consequence of complex surface modification and changes in the composition of the samples (Table 2). Therefore we have recalculated magnetization value for pure metallic phase of the MNPs (M s ) taking into account the real composition of the MNPs, percentage of pure metallic core, densities and magnetization values of the additional phases using the corresponding number for bulk materials [20,22]. For example, M s is quite reasonably close to Ms of the pure bulk iron being only 23% lower in the case of Fe-I sample. Gangopadhyay et al. [10] discussed the fact that coatings by magnetic iron oxides of iron MNPs affected the coercivity strongly. We did not observe the last effect in present studies, perhaps due to a bigger size of EEW MNPs. Encapsulated metallic
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magnetic MNPs with such high saturation magnetization values are ideal candidates for being used as tunable microwave absorbers in a wide range of frequencies [11]. Significant reduction of coercivity was observed in the case of Ni/C composite MNPs. According to XRD and TEM studies, they consist of 30 nm diameter Ni core and 5 nm wide graphite shells. One, therefore, can explain the change in coercivity as a consequence of the reduction of the interparticle interactions due to the increase of the distances between the magnetic cores and/or transition into single domain state for a majority of the MNPs of the ensemble. Although additional studies with MNPs of smaller size are necessary, Ni/C nanoparticles are good candidates for the development of non-agregated magnetic nanofluids. Finally we would like to attract attention to the shape of fabricated EEW MNPs which is a critically important parameter for many technological and especially biological applications. It is well known that, despite many efforts and the variety of different techniques tested, the shape of the MNPs is still one of the most difficult parameters to control [13]. Figs. 1, 2 and 4 show clearly that EEW MNPs are essentially spherical. As an additional and complementary way to evaluate the degree of sphericity one can use well established methods of analysis of the shape of the primary magnetization curves: the initial slope of the 4pM vs. H curve is close to 3 for spherical MNPs [11,23,24]. Let us give just one example for comparison. It was found as a result of the analysis (Fig. 5) that for Fe-I sample the cut point of the slope of the primary magnetization curve was very close to the field corresponding to spherical MNPs: 4pMs 3H with an accuracy of about 5%. 4. Conclusions Iron and nickel spherical magnetic MNPs were prepared by the EEW technique which provides a very high production rate up to 200 g/h and requires low energy consumption of 25 kW h/kg. The as-prepared MNPs were modified in situ at room temperature in hexane, toluene, chloroform, and the solutions of polystyrene in toluene in an inert atmosphere in order to provide both the stability at ambient conditions and tunable surface properties. Hexane was an inert liquid for Fe and Ni MNPs. In the case of toluene the surface of as-prepared Fe or Ni MNPs acted as a catalyst for the condensation reaction of aromatic organic compounds. The treatment with toluene led to the deposition of carbon on the surface of the MNPs. Chloroform chemically interacted with Fe MNPs, forming FeCl2. The treatment of MNPs with the solution of polystyrene in toluene resulted in the adsorption of polymer and the formation of 3–6 nm polymeric coatings. The obtained properties of magnetic nanoparticles with mean diameter 50–80 nm (like the magnetization value for Ni MNPs up to 52 emu/g and up to 179 emu/g for Fe MNPs) at room temperature make them competitive candidates for nanocomposites focused on technological applications. Acknowledgments This work was supported by RFBR 10-02-96015, UrFU 215 and MAT2011-27573-C04 project of the Spanish Government grants.
Selected measurements were performed at SGIker service of UPV-EHU. We thank S.M. Bhagat and I. Orue for special support. References [1] J. Tang, L. Feng, J. Wiemann, Negative magnetoresistance of g-Fe2O3 observed in g-Fe2O3/Ag granular nanocomposites, Appl. Phys. Lett. 74 (1999) 2522– 2524. [2] J. Llandro, J.J. Palfreyman, A. Ionescu, C.H.W. Barnes, Magnetic biosensor technologies for biomedical applications: a review, Med. Biol. Eng. Comput. 48 (2010) 977–998. [3] Y.-S. Kwon, A.A. Gromov, J.I. Strokova, Passivation of the surface of aluminum nanopowders by protective coatings of the different chemical origin, Appl. Surf. Sci. 253 (2007) 5558–5564. [4] E. Athanassiou, R. Grass, W. Stark, Large-scale production of carbon-coated copper nanoparticles for sensor applications, Nanotechnology 17 (2006) 1668– 1673. [5] X.P. Xia, S.Z. Cai, C.S. Xie, Preparation, structure and thermal stability of Cu/ LDPE nanocomposites, Mater. Chem. Phys. 95 (2006) 122–129. [6] G.V. Kurlyandskaya, J. Cunanan, S.M. Bhagat, J.C. Aphesteguy, S.E. Jacobo, Fieldinduced microwave absorption in Fe3O4 nanoparticles and Fe3O4/polyaniline composites synthesized by different methods, J. Phys. Chem. Solids 68 (2007) 1527–1532. [7] Y.-W. Jun, J.-W. Seo, J. Cheon, Nanoscaling laws of magnetic nanoparticles and their applicabilities in biomedical sciences, Acc. Chem. Res. 41 (2008) 179–189. [8] M.P. Fernandez-Garcia, P. Gorria, M. Sevilla, M.P. Proença, R. Boada, J. Chaboy, A.B. Fuentes, J. Blanco, Enhanced protection of carbon-encapsulated magnetic nickel nanoparticles through a sucrose-based synthetic strategy, J. Phys. Chem. 115 (2011) 5294–5300. [9] D.L. Huber, Synthesis, properties, and applications of iron nanoparticles, Small 5 (2005) 482–501. [10] S. Gangopadhyay, G.H. Hadjipanayis, B. Dale, S.M. Sorensen, K.J. Klabunde, V. Papaefthymiou, A. Kostikas, Magnetic properties of ultrafine iron particles, Phys. Rev. B 45 (1992) 9778–9787. [11] G.V. Kurlyandskaya, S.M. Bhagat, C. Luna, M. Vazquez, Microwave absorption of nanoscale CoNi powders, J. Appl. Phys. 99 (104308) (2006) 1–5. [12] Yu.A. Kotov, Electric explosion of wires as a method for preparation of nanopowders, J. Nanoparticle Res. 5 (2003) 539–550. [13] G.V. Kurlyandskaya, S.M. Bhagat, A.P. Safronov, I.V. Beketov, A. Larrañaga, Spherical magnetic nanoparticles fabricated by electric explosion of wire, AIP Adv. 1 (2011) 042122. [14] Y. Yao, Zh. Li, Z. Lin, K.-S. Moon, J. Agar, C. Wong, Controlled growth of multilayer, few-layer, and single-layer graphene on metal substrates, J. Phys. Chem. C 115 (2011) 5232–5238. [15] J.A. Rodrıguez-Manzo, C. Pham-Huu, F. Banhart, Graphene growth by a metalcatalyzed solid-state transformation of amorphous carbon, ACS Nano 5 (2011) 1529–1534. [16] Zh. He, J.-L. Maurice, A. Gohier, C.S. Lee, D. Pribat, C.S. Cojocaru, Iron catalysts for the growth of carbon nanofibers: Fe, Fe3C or both?, Chem Mater. 23 (2011) 5379–5387. [17] M. Muller, C. Kubel, K. Mullen, Giant polycyclic aromatic hydrocarbons, Chem. Eur. J. 4 (1998) 2099–2109. [18] J. Wu, Z. Tomovic, V. Enkelmann, K. Mullen, From branched hydrocarbon propellers to c3-symmetric graphite disks, J. Org. Chem. 69 (2004) 5179–5186. [19] P.C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, Marcel Dekker, New York, 1997. pp. 333–335. ´ Handley, Modern Magnetic Materials, John Wiley & Sons, New York, [20] R.C. O 1972. p. 740. [21] J. Lee, T. Isobe, M. Senna, Preparation of ultrafine Fe3O4 particles by precipitation in the presence of PVA at high pH, J. Colloid Interface Sci. 177 (1996) 490–494. [22] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley Publishing Company, 1972. pp. 233–236. [23] J.S. Ramachandran, S.M. Bhagat, J.L. Peng, M. Rubinstein, FMR of powder La0.7Ca0.3MnO3, Solid State Commun. 96 (1995) 127–131. [24] I.V. Beketov, A.P. Safronov, A.I. Medvedev, J. Alonso, G.V. Kurlyandskaya, S.M. Bhagat, Iron oxide nanoparticles fabricated by electric explosion of wire: focus on magnetic nanofluids, AIP Adv. 2 (2012) 022154.