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Reduction under hydrogen of ferrite MFe2O4 (M: Fe, Co, Ni) nanoparticles obtained by hydrolysis in polyol medium: A novel route to elaborate CoFe2, Fe and Ni3Fe nanoparticles
Journal of Alloys and Compounds 536S (2012) S381–S385
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Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Reduction under hydrogen of ferrite MFe2 O4 (M: Fe, Co, Ni) nanoparticles obtained by hydrolysis in polyol medium: A novel route to elaborate CoFe2 , Fe and Ni3 Fe nanoparticles N. Ballot, F. Schoenstein, S. Mercone, T. Chauveau, O. Brinza, N. Jouini ∗ Laboratoire des Sciences des Procédés et des Matériaux, CNRS, LSPM – UPR 3407, Université Paris 13, PRES Sorbonne-Paris-Cité, 99 Avenue J.-B. Clément, 93430 Villetaneuse, France
a r t i c l e
i n f o
Article history: Received 9 July 2011 Received in revised form 29 December 2011 Accepted 3 January 2012 Available online 11 January 2012 Keywords: Nanoparticles Oxide reduction Spinel Metal Alloy Magnetism
a b s t r a c t A novel method to process metal and various alloy particles of nanometric size is described. The first step consists in the elaboration of MFe2 O4 (M: Fe, Ni or Co) spinel nanoparticles in polyol medium via hydrolysis and the second one in gently reducing these latter under hydrogen at 300 ◦ C. X-ray diffraction analysis shows that pure Fe and CoFe2 alloy are well obtained by reducing Fe3 O4 and CoFe2 O4 , respectively. This is not the case when we try to reduce NiFe2 O4 . A mixture of Fe and Ni3 Fe is observed. TEM analysis reveals that the size of metal particles stays within the range of a few tenths of nm up to 150 nm, while the precursors (MFe2 O4 ) never exceed 5 nm. Our results show that the formation of metal particles occurs via two main steps: (i) reduction of the spinel oxide nanoparticles into metal ones and (ii) aggregation of the latter, leading to larger metal nanoparticles. Magnetic measurements indicate that the as-obtained metallic materials have good magnetic properties mainly affected by the sizes of the nanoparticles and the purity of the reduced phases. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the last three decades, several works have focused on the synthesis of metal and oxide nanoparticles in polyol medium [1,2]. For a given system, the synthesis of metal or the corresponding oxide depends on the competition between the two main reactions occurring in such medium: reduction and hydrolysis [3]. To the best of our knowledge, this mechanism is well optimized for easily reducible element such as Ni and Co. In the case of iron, reduction has been hardly achieved in polyol medium. Indeed, the reducing power of polyol is not sufficient to obtain metal iron starting from iron (II) and/or iron (III) salts. Starting from iron (II) salt, a dismutation reaction is observed leading to a mixture of metal and iron (III) ion [4]. Nowadays nanoparticles of iron, iron–cobalt and iron–nickel alloys are of great interest for applications due to their magnetic properties [5]. The aim of this work is to describe a novel strategy allowing an easy elaboration of such nanomaterials. This novel method is mainly based on the high reactivity of oxide nanoparticles toward reducing agents due to the very high surface/volume ratio [6,7]. Such reactivity allowed to elaborate several metal nanoparticles from corresponding oxides such as Ni from
∗ Corresponding author. E-mail address: [email protected] (N. Jouini). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.01.019
NiO [8], Ni–Fe from NiFe2 O4 [9]. The novelty of our work consists on combing soft chemistry (forced hydrolysis) with gentle heat treatment to elaborate easily metal and alloys. In the first step, forced hydrolysis in polyol medium allowed to process oxide spinel nanoparticles of controlled size not exceeding 5 nm. In the second step, the as-obtained oxide nanoparticles are reduced into metal by a gentle heat treatment under hydrogen gas. Such novel strategy presents two main advantages in the elaboration of nanoparticles (i) it offers an alternative route friendly environmental in comparison with chemical routes involving complex mechanism and several chemicals [8], (ii) starting from oxide nanoparticles, the solid–gas reaction (reduction) will likely lead to metal particles with size remaining in the nanometer range. 2. Experimental 2.1. Synthesis The first step of the synthesis of metal nanoparticles is to process spinels MFe2 O4 nanoparticles (M: Fe, Co or Ni) via forced hydrolysis in a polyol medium [3]. The second step is to reduce them by hydrogen gas at low temperature. CoFe2 O4 and Fe3 O4 were synthesized, starting from an amount of precursors salts of 50 mmol of FeCl3 ·6H2 O and 25 mmol of Co(CH3 COO)2 ·4H2 O for CoFe2 O4 and for Fe3 O4 , 37.5 mmol of FeCl3 ·6H2 O. The acetate ratio defined as [OAc]/[M] (M stands for the total amount of metallic elements), was fixed to 3. These salts along with the aforementioned amount of sodium acetate were added into 125 ml of in 1,2-propanediol. The mixture was then heated at 160 ◦ C for 6 h with at a rate of 6 ◦ C/min.
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NiFe2 O4 nanoparticles were obtained by mixing 63.75 mmol of Ni(CH3 COO)2 ·4H2 O and 85 mmol of FeCl3 anhydrous with an acetate ratio of 2.6 in 125 ml of 2-hydroxyethyl ether (DEG). The solution was then heated up to the boiling temperature of the liquid (200 ◦ C) with a heating rate of 6 ◦ C/min for 6 h [10]. The mixtures were then cooled down to room temperature. Spinel solids were separated from liquid by centrifugation and washed several times with ethanol, acetone and water. They were then dried in an oven at 50 ◦ C. 2.2. Spinels reduction Spinels cobalt, nickel ferrites and ferrite nanoparticles were gently reduced in an oven under continuous hydrogen flow fixed at 150 ml/min. Hydrogen was generated by a Hydrogen gas generator 20H, made by Domnick Hunter. 200 mg of each of the powders were introduced in the oven in a ceramic boat. Temperature range was of 250–500 ◦ C and time range of 3–6 h. Powders were then collected in a glove compartment under argon. 2.3. Characterization techniques The crystalline structure of the samples was identified from X-ray diffraction (XRD) patterns recorded in 2 range 35–155◦ with an incident beam of 30◦ , using a ˚ and a four circle diffracBrukerTM SRA18 cobalt rotating anode ((K␣1 ) = 1.7889 A) tometer equipped with a curved linear position-sensitive detector (2 = 0.015◦ ). XRD were background-extracted using Fityk as the interfacing program. Crystalline phases were identified using Match software [11]. Cell parameters were manually determined and phase quantification were determined from XRD data by using the MAUD program [12]. Crystallite sizes were, finally, calculated with the Williamson–Hall peak broadening analysis [13]. Transmission electron microscopy samples were prepared by dispersing nanoparticles in ethanol and placing one drop of the suspension on a copper grid with a carbon membrane film. Images were taken using JEOL-2011 operating at 200 kV. The program ImageJ was used to measure the diameter distribution of about 100 nanoparticles for each sample. Zero field-cooling and field cooling magnetization (ZFC/FC) and hysteresis loop measurements were carried out by using a superconducting quantum interference device (SQUID) at CRISMAT Laboratory at ENISCAEN. ZFC and FC magnetization were measured in the temperature range of 4–400 K at a magnetic field of 400 Oe and hysteresis loop measurements were collected in the magnetic field range of −5 T/5 T at 4 and 380 K. For magnetic measurements, in order to avoid mechanical rotation of powders during the changing sign of the magnetic field, nanoparticles were blocked into an epoxy matrices (with vitreous temperature around 380 K) inside gelatin standard pills.
3. Results and discussion The main goal of this work was to completely reduce the spinel nanoparticles into pure metal nanoparticles of sizes as close as possible to the ones of the precursors spinel (i.e. less than 10 nm). With regards to the reduction-step, two factors, namely time and temperature, were particularly studied for this purpose. X-ray diffraction and TEM showed that the general optimal conditions are as follows: (i) temperature of 300 ◦ C and (ii) time: 6 h. At lower temperatures, no reduction occurs and at higher temperature a significant growth of particles is observed. Results showed that 6 h is the minimum amount of time necessary to obtain completely or almost completely reduced nanoparticles (percent of oxide inferior to 5% in mass). 3.1. X-ray diffraction characterization X-ray diffraction patterns of reduced samples along with the corresponding spinels are indicated in Fig. 1. As shown, the metal patterns present narrow peaks whereas the corresponding spinels show very wide ones. This indicates a significant crystallite growth during reducing process despite the low temperature applied. Cobalt ferrite (Fig. 1(a) – top graph) is reduced into the bcc CoFe2 ˚ phase (Fig. 1(b) – top graph) with a cell parameter of 2.862(2) A, very close to that previously reported [14,15]. It should however be noted that XRD analysis shows the presence of traces of CoFe2 O4 not exceeding 5 wt%. This may be due to incomplete reduction of the starting spinel or to oxidation of the as-obtained metal particle by air atmosphere. The crystallite size of the resulting alloy remains
Fig. 1. XRD patterns showing (a) oxide a) CoFe2 O4 , b) Fe3 O4 , and c) NiFe2 O4 and (b) their respectively reduced particles a) CoFe2 , b) Fe, and c) Ni3 Fe/Fe.
in the nanometer range but it is significantly higher (55 nm) than that of the initial CoFe2 O4 (3.5 nm). For Fe3 O4 , the reduction led to the formation of bcc Fe. The inferred cell parameter 2.867(3) A˚ is very close to that reported in the JCPDS No. 96-900-8537. As for CoFe2 , the crystallite size of Fe nanoparticles (140 nm) is very high in comparison to that of the Fe3 O4 nanoparticles (4.5 nm). Again, light traces of precursor Fe3 O4 (<5 wt%) are still observed by XRD analysis. Nickel ferrite, however, behaves differently. Indeed, while the cobalt and iron spinels led to nearly pure phases (CoFe2 and Fe), nickel ferrite reduction yielded to a mixture of Ni3 Fe and Fe with 57.4 wt% and 42.6 wt% respectively. The cell parameter observed for Ni3 Fe is 3.59(4) A˚ (space group Pm–3m), which corresponds at the one reported in the JCPDS No. 96-901-3942. The sizes of crystallite calculated are 10.5 nm and 48 nm for the nickel-iron and iron phases, respectively. It is well-known that metal nanoparticles are subject to easy oxidation when exposed to air. X-ray diffraction analysis shows no significant changes are observed when our samples are kept in normal conditions (room temperature and atmospheric pressure) for five months. This may indicate that the oxidation layer thickness is below the limit of XRD detection (5%).
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Fig. 2. TEM micrographs with histograms of NP sizes of (a) CoFe2 O4 and (b) reduced CoFe2 , (c) Fe3 O4 and (d) reduced Fe and (e) NiFe2 O4 and (f) reduced Ni3 Fe.
3.2. TEM characterization TEM micrographs (Fig. 2) allow the determination of particle morphology and diameter. As previously observed, TEM images also show that nickel spinel behaves differently. Reduction to Ni3 Fe nanoparticles of sizes remaining in the nanometer range (12 nm) occurs without the presence of spurious oxide precursor, but Fe nanoparticles slightly larger (48 nm) are also found at the end of the process. On the other hand, reduction of cobalt and iron spinel MFe2 O4 (M = Co or Fe) led to CoFe2 particles of 55 nm in diameter and bcc Fe of significantly larger size (145 nm), respectively. These sizes are very close to the crystallite sizes determined by XRD analysis and discussed above, which indicate that the particles are monocrystalline. The slight difference between the crystallite size obtained after calculations based on X-ray patterns and those based on TEM image analysis, suggests that not all the particles are exactly spherical. Moreover, volumes in TEM measurements are not as high as those in XRD data.
In conclusion, it is interesting to note that starting from spinel spherical nanoparticles smaller than 5 nm, a gentle reduction by hydrogen gas leads to monocrytalline nanoparticles of significantly larger size. The mechanism likely responsible for this enhanced growth may pass through two mains steps: (i) during the first part of the reduction process, spinel nanoparticles are again reduced into very small nanoparticles of either metal or alloy (ii) immediately after, a second process occurs and the in situ metal nanoparticles diffuse to each other and fuse together due to their high reactivity, leading to larger monocrystalline nanoparticles. 3.3. Magnetic characterization Magnetization as function of the applied magnetic field measured at 4 K and 380 K for the reduced spinels are shown in Fig. 3(a) and (b) respectively. In Table 1 we report the main important features of these hysteresis loops, i.e. the saturated magnetization (Ms) measured at 4 K and the estimation of coercive behaviors at
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Fig. 4. Susceptibility at 400 Oe as function of temperature of (from the top) cobalt ferrite nanoparticles (open circles), iron nano-particles (black circles) and nickel ferrite nano-particles (gray rhombus). The bound at 380 K is due to the approaching vitreous temperature of epoxy matrices in which samples were prepared.
Fig. 3. Hysteresis magnetic loop of cobalt ferrite nano-particles (open circles), iron nano-particles (black circles) and nickel ferrite nano-particles (gray rhombus) at (a) 4 K and (b) 380 K.
both 4 K and 380 K. The MS measured for all samples at low temperature and high magnetic field (5 T) lies in between the MS of pure Ni (57 emu/g) and that of pure Fe (221 emu/g) in agreement with the chemical composition inferred from X-ray analysis to our nanoparticles. Despite of the small number of experimental points we measured at low field, it is reasonable to discuss the observable estimations of the coercive fields. The reduced cobalt spinel (CoFe2 ) confirms the highest coercivity (see Table 1) due to the high magnetocrystalline anisotropy of cobalt as reported in literature. The coercive field of this sample observed in the hysteresis loops is coherent with the value reported for Fe–Co alloys (500–1000 Oe) processed by hydrogen plasma–metal reaction and having almost similar particles sizes [16]. All samples present an open hysteresis loop up to high temperature showing the lack up to 380 K of a superparamagnetic phase. Table 1 Saturated magnetization and coercivity field for the three reduced samples. Ms4K (emu/g) CoFe2 Fe Ni3 Fe/Fe
178 186 103
Hc4K (Oe) 1125 470 530
Hc380K (Oe) 800 270 150
In order to study this latter, we show in Fig. 4 the temperature dependence of the susceptibility measured in between 4 K and 400 K. The data was collected in the standard zero-field-cooling (ZFC) and field-cooling (FC) conditions under the application of a 400 Oe magnetic field. All the ZFC/FC curves show a relaxing behavior around 380 K which is due to the approaching vitreous temperature of epoxy pills used for preparation of SQUID samples. All systems studied present a magnetic irreversible behavior. This irreversibility can belong to superparamagnetism of very small nanoparticles below the so-called blocking temperature (i.e. temperature below which the magnetization appears to be blocked) or to the depinning of domains in ferromagnetic material [17]. The superparamagnetic behavior occurs when the particles become a single magnetic domain system and this is possible when their size is below a critical value (DC ) depending on the composition. For reduced iron spinel, no distinct maximum in the ZFC/FC magnetization curves was observed below 400 K, thus revealing pure ferromagnetic behavior. Furthermore, the ZFC susceptibility increases very slightly and FC susceptibility remains almost constant. For this sample, the particle dimension (145 nm) is above the critical size (DC ≈ 14 nm) for which Fe particles become a single domain magnet [18]. Such behavior was previously observed for Fe particles with 65 nm in diameter [19]. Dakhlaoui Omrani et al. also observed this phenomenon for Ni particles with a diameter of 170 nm [17]. In these cases, this irreversibility between ZFC/FC susceptibilities can mainly originate from the domain wall depinning [20].
N. Ballot et al. / Journal of Alloys and Compounds 536S (2012) S381–S385
In the case of reduced cobalt and nickel spinel, the irreversibility is more pronounced than for the reduced iron sample. For reduced nickel spinel, we obtained two phases: 57% of Ni3 Fe particles with a size of 12 nm (i.e. less than the critical sizes of Ni and Fe [16]) and 43% of bcc Fe nanoparticles with size of 48 nm (i.e. significantly higher than the DC size of Fe (14 nm)). Thus, the magnetism of this sample can be a resultant of both superparamagnetism arising from Ni3 Fe mono-domain particles and ferromagnetic poly-domain Fe particles. In the case of CoFe2 , TEM showed that the nanoparticles have size around 55 nm, which is lower than the critical size of a cobalt single domain (70 nm) but significantly higher than that of iron (14 nm) [18]. However, the pronounced irreversibility observed between ZFC/FC is in favor of superparamagnetic behavior rather than of wall depinning in multi-domain particles. The blocking temperature is probably higher than 380 K since the hysteresis loop remains open at this temperature, as already reported. This means that the magnetic behavior of this alloy may be governed by the size of the cobalt single domain. It should however be noted, that ZFC/FC curves of this sample, present a particular behavior around 30 K (decreasing in FC and a maximum in ZFC). This can be due to very small nanoparticles existing along with those having 55 nm in size. Thus 30 K can be considered as the blocking temperature of such very small nanoparticles. The existence of two blocking temperatures was previously reported for Fe [21], for Co fibers [22] and for Fe nanoparticles embedded in NiO matrix [23]. Further characterizations (ZFC/FC measurements at high temperature, AC susceptibility, Mossbauer spectroscopy. . .) studies are in progress for better understanding of the magnetic behaviors of the as-obtained alloys. 4. Conclusion Spinel nanoparticles of MFe2 O4 (M = Co, Ni or Fe) were processed by forced hydrolysis in a polyol medium. Taking advantage of their high surface reactivity, these spinel nanoparticles were gently reduced under hydrogen flow at 300 ◦ C. This led to nanoparticles of CoFe2 , Fe and Ni3 Fe with sizes not exceeding 150 nm. TEM and XRD analysis revealed the monocrystalline character of these nanoparticles. Magnetic measurements indicate that the resulting materials demonstrate a magnetic behavior depending on two main factors (i) chemical composition of the reduced nanoparticles and (ii)
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particle size. bcc Fe nanoparticles with 140 nm in diameter present a ferromagnetic character similar to that of the corresponding bulk iron while the CoFe2 alloy nanoparticles display a magnetic behavior approaching superparamagnetism. In the case of nickel spinel reduced nanoparticles, the two phases obtained present a mix of their magnetic behavior and thus a more complex feature. Current studies are in progress to gain more insight into these behaviors. Acknowledgements We thank very much S. Hebert and A. Pautrat at the CRISMAT Laboratory-ENSICAEN for their precious help with the magnetic measurements. The IFR Paris Nord Plaine de France (PPF) has supported this work. References [1] M. Figlarz, F. Fievet, E.U. Patent 0113281 (1985). [2] C. Feldmann, J. Merikhi, Colloid Interface Sci. 223 (2000) 229. [3] L. Poul, S. Ammar, N. Jouini, F. Fiévet, F. Villain, J. Sol–Gel Sci. Technol. 26 (2003) 26. [4] G. Viau, F. Fiévet-Vincent, F. Fiévet, J. Mater. Chem. 6 (1996) 1047. [5] Y. Soumare, C. Garcia, T. Maurer, G. Chaboussant, F. Ott, F. Fievet, J.-Y. Piquemal, G. Viau, Adv. Funct. Mater. 19 (2009) 1. [6] S.-C. Yang, W.-N. Su, S.D. Lin, J. Rick, J.-H. Cheng, J.-Y. Liu, C.-J. Pan, D.-G. Liu, J.-F. Lee, T.-S. Chan, H.-S. Sheu, B.-J. Hwang, Appl. Catal. B: Environ. 106 (2011) 650. [7] S.S.A. Syed-Hassan, C.-Z. Li, Int. J. Chem. Kinet. (2011) 667. [8] M. Bhattacharya, Metall. Mater. Trans. B 42B (2011) 380. [9] J.S. Lee, T.H. Kim, J.H. Yu, S.W. Chung, Nanostruct. Mater. 9 (1997) 153. [10] S. Chkoundali, S. Ammar, N. Jouini, F. Fievet, P. Molinié, M. Danot, F. Villain, J-M. Grenèche, J. Phys.: Condens. Matter 16 (2004) 4357. [11] http://www.crystalimpact.com/match/default.htm, June 2011. [12] L. Lutterotti, Department of Materials Science and Industrial Technologies, University of Trento, Italy, http://www.ing.unitn.it/∼maud/, June 2011. [13] M. Andasmas, P. Langlois, N. Fagon, Th. Chauveau, A. Hendaoui, D. Vrel, Powder Technol. 207 (2011) 305. [14] I.S. Jurca, N. Viart, C. Mény, C. Ulhaq-Bouillet, P. Panissod, G. Pourroy, Surf. Sci. 529 (2003) 215. [15] W.C. Ellis, E.S. Greiner, Trans. Am. Soc. Met. 29 (1941) 415. [16] X.G. Li, T. Murai, T. Saito, S. Takahashi, J. Magn. Magn. Mater. 190 (1998) 277. [17] A. Dakhlaoui Omrani, M.A. Bousnina, L.S. Smiri, M. Taibi, P. Leone, F. Schoenstein, N. Jouini, Mater. Chem. Phys. 123 (2010) 821. [18] D.L. Leslie-Pelecky, Chem. Mater. 8 (1996) 1770. [19] Y.L. Hou, S. Gao, J. Alloys Compd. 365 (2004) 112. [20] C. Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia, R. Vijayaraghavan, Appl. Phys. Lett. 77 (2000) 895. [21] M.P. Fernandez, D.S. Schmool, A.S. Silva, M. Sevilla, A.B. Fuertes, P. Gorria, J.A. Blanco, J. Magn. Magn. Mater. 322 (2010) 1300. [22] A. Dakhaloui, L.S. Smiri, G. Babadjian, F. Shoenstein, P. Molinié, N. Jouini, J. Phys. Chem. C 112 (2008) 14348. [23] S.P. Pati, B. Bhushan, A. Basumallick, S. Kumar, D. Das, Mater. Sci. Eng. B 176 (2011) 1015.
Contents lists available at SciVerse ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
Reduction under hydrogen of ferrite MFe2 O4 (M: Fe, Co, Ni) nanoparticles obtained by hydrolysis in polyol medium: A novel route to elaborate CoFe2 , Fe and Ni3 Fe nanoparticles N. Ballot, F. Schoenstein, S. Mercone, T. Chauveau, O. Brinza, N. Jouini ∗ Laboratoire des Sciences des Procédés et des Matériaux, CNRS, LSPM – UPR 3407, Université Paris 13, PRES Sorbonne-Paris-Cité, 99 Avenue J.-B. Clément, 93430 Villetaneuse, France
a r t i c l e
i n f o
Article history: Received 9 July 2011 Received in revised form 29 December 2011 Accepted 3 January 2012 Available online 11 January 2012 Keywords: Nanoparticles Oxide reduction Spinel Metal Alloy Magnetism
a b s t r a c t A novel method to process metal and various alloy particles of nanometric size is described. The first step consists in the elaboration of MFe2 O4 (M: Fe, Ni or Co) spinel nanoparticles in polyol medium via hydrolysis and the second one in gently reducing these latter under hydrogen at 300 ◦ C. X-ray diffraction analysis shows that pure Fe and CoFe2 alloy are well obtained by reducing Fe3 O4 and CoFe2 O4 , respectively. This is not the case when we try to reduce NiFe2 O4 . A mixture of Fe and Ni3 Fe is observed. TEM analysis reveals that the size of metal particles stays within the range of a few tenths of nm up to 150 nm, while the precursors (MFe2 O4 ) never exceed 5 nm. Our results show that the formation of metal particles occurs via two main steps: (i) reduction of the spinel oxide nanoparticles into metal ones and (ii) aggregation of the latter, leading to larger metal nanoparticles. Magnetic measurements indicate that the as-obtained metallic materials have good magnetic properties mainly affected by the sizes of the nanoparticles and the purity of the reduced phases. © 2012 Elsevier B.V. All rights reserved.
1. Introduction In the last three decades, several works have focused on the synthesis of metal and oxide nanoparticles in polyol medium [1,2]. For a given system, the synthesis of metal or the corresponding oxide depends on the competition between the two main reactions occurring in such medium: reduction and hydrolysis [3]. To the best of our knowledge, this mechanism is well optimized for easily reducible element such as Ni and Co. In the case of iron, reduction has been hardly achieved in polyol medium. Indeed, the reducing power of polyol is not sufficient to obtain metal iron starting from iron (II) and/or iron (III) salts. Starting from iron (II) salt, a dismutation reaction is observed leading to a mixture of metal and iron (III) ion [4]. Nowadays nanoparticles of iron, iron–cobalt and iron–nickel alloys are of great interest for applications due to their magnetic properties [5]. The aim of this work is to describe a novel strategy allowing an easy elaboration of such nanomaterials. This novel method is mainly based on the high reactivity of oxide nanoparticles toward reducing agents due to the very high surface/volume ratio [6,7]. Such reactivity allowed to elaborate several metal nanoparticles from corresponding oxides such as Ni from
∗ Corresponding author. E-mail address: [email protected] (N. Jouini). 0925-8388/$ – see front matter © 2012 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2012.01.019
NiO [8], Ni–Fe from NiFe2 O4 [9]. The novelty of our work consists on combing soft chemistry (forced hydrolysis) with gentle heat treatment to elaborate easily metal and alloys. In the first step, forced hydrolysis in polyol medium allowed to process oxide spinel nanoparticles of controlled size not exceeding 5 nm. In the second step, the as-obtained oxide nanoparticles are reduced into metal by a gentle heat treatment under hydrogen gas. Such novel strategy presents two main advantages in the elaboration of nanoparticles (i) it offers an alternative route friendly environmental in comparison with chemical routes involving complex mechanism and several chemicals [8], (ii) starting from oxide nanoparticles, the solid–gas reaction (reduction) will likely lead to metal particles with size remaining in the nanometer range. 2. Experimental 2.1. Synthesis The first step of the synthesis of metal nanoparticles is to process spinels MFe2 O4 nanoparticles (M: Fe, Co or Ni) via forced hydrolysis in a polyol medium [3]. The second step is to reduce them by hydrogen gas at low temperature. CoFe2 O4 and Fe3 O4 were synthesized, starting from an amount of precursors salts of 50 mmol of FeCl3 ·6H2 O and 25 mmol of Co(CH3 COO)2 ·4H2 O for CoFe2 O4 and for Fe3 O4 , 37.5 mmol of FeCl3 ·6H2 O. The acetate ratio defined as [OAc]/[M] (M stands for the total amount of metallic elements), was fixed to 3. These salts along with the aforementioned amount of sodium acetate were added into 125 ml of in 1,2-propanediol. The mixture was then heated at 160 ◦ C for 6 h with at a rate of 6 ◦ C/min.
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NiFe2 O4 nanoparticles were obtained by mixing 63.75 mmol of Ni(CH3 COO)2 ·4H2 O and 85 mmol of FeCl3 anhydrous with an acetate ratio of 2.6 in 125 ml of 2-hydroxyethyl ether (DEG). The solution was then heated up to the boiling temperature of the liquid (200 ◦ C) with a heating rate of 6 ◦ C/min for 6 h [10]. The mixtures were then cooled down to room temperature. Spinel solids were separated from liquid by centrifugation and washed several times with ethanol, acetone and water. They were then dried in an oven at 50 ◦ C. 2.2. Spinels reduction Spinels cobalt, nickel ferrites and ferrite nanoparticles were gently reduced in an oven under continuous hydrogen flow fixed at 150 ml/min. Hydrogen was generated by a Hydrogen gas generator 20H, made by Domnick Hunter. 200 mg of each of the powders were introduced in the oven in a ceramic boat. Temperature range was of 250–500 ◦ C and time range of 3–6 h. Powders were then collected in a glove compartment under argon. 2.3. Characterization techniques The crystalline structure of the samples was identified from X-ray diffraction (XRD) patterns recorded in 2 range 35–155◦ with an incident beam of 30◦ , using a ˚ and a four circle diffracBrukerTM SRA18 cobalt rotating anode ((K␣1 ) = 1.7889 A) tometer equipped with a curved linear position-sensitive detector (2 = 0.015◦ ). XRD were background-extracted using Fityk as the interfacing program. Crystalline phases were identified using Match software [11]. Cell parameters were manually determined and phase quantification were determined from XRD data by using the MAUD program [12]. Crystallite sizes were, finally, calculated with the Williamson–Hall peak broadening analysis [13]. Transmission electron microscopy samples were prepared by dispersing nanoparticles in ethanol and placing one drop of the suspension on a copper grid with a carbon membrane film. Images were taken using JEOL-2011 operating at 200 kV. The program ImageJ was used to measure the diameter distribution of about 100 nanoparticles for each sample. Zero field-cooling and field cooling magnetization (ZFC/FC) and hysteresis loop measurements were carried out by using a superconducting quantum interference device (SQUID) at CRISMAT Laboratory at ENISCAEN. ZFC and FC magnetization were measured in the temperature range of 4–400 K at a magnetic field of 400 Oe and hysteresis loop measurements were collected in the magnetic field range of −5 T/5 T at 4 and 380 K. For magnetic measurements, in order to avoid mechanical rotation of powders during the changing sign of the magnetic field, nanoparticles were blocked into an epoxy matrices (with vitreous temperature around 380 K) inside gelatin standard pills.
3. Results and discussion The main goal of this work was to completely reduce the spinel nanoparticles into pure metal nanoparticles of sizes as close as possible to the ones of the precursors spinel (i.e. less than 10 nm). With regards to the reduction-step, two factors, namely time and temperature, were particularly studied for this purpose. X-ray diffraction and TEM showed that the general optimal conditions are as follows: (i) temperature of 300 ◦ C and (ii) time: 6 h. At lower temperatures, no reduction occurs and at higher temperature a significant growth of particles is observed. Results showed that 6 h is the minimum amount of time necessary to obtain completely or almost completely reduced nanoparticles (percent of oxide inferior to 5% in mass). 3.1. X-ray diffraction characterization X-ray diffraction patterns of reduced samples along with the corresponding spinels are indicated in Fig. 1. As shown, the metal patterns present narrow peaks whereas the corresponding spinels show very wide ones. This indicates a significant crystallite growth during reducing process despite the low temperature applied. Cobalt ferrite (Fig. 1(a) – top graph) is reduced into the bcc CoFe2 ˚ phase (Fig. 1(b) – top graph) with a cell parameter of 2.862(2) A, very close to that previously reported [14,15]. It should however be noted that XRD analysis shows the presence of traces of CoFe2 O4 not exceeding 5 wt%. This may be due to incomplete reduction of the starting spinel or to oxidation of the as-obtained metal particle by air atmosphere. The crystallite size of the resulting alloy remains
Fig. 1. XRD patterns showing (a) oxide a) CoFe2 O4 , b) Fe3 O4 , and c) NiFe2 O4 and (b) their respectively reduced particles a) CoFe2 , b) Fe, and c) Ni3 Fe/Fe.
in the nanometer range but it is significantly higher (55 nm) than that of the initial CoFe2 O4 (3.5 nm). For Fe3 O4 , the reduction led to the formation of bcc Fe. The inferred cell parameter 2.867(3) A˚ is very close to that reported in the JCPDS No. 96-900-8537. As for CoFe2 , the crystallite size of Fe nanoparticles (140 nm) is very high in comparison to that of the Fe3 O4 nanoparticles (4.5 nm). Again, light traces of precursor Fe3 O4 (<5 wt%) are still observed by XRD analysis. Nickel ferrite, however, behaves differently. Indeed, while the cobalt and iron spinels led to nearly pure phases (CoFe2 and Fe), nickel ferrite reduction yielded to a mixture of Ni3 Fe and Fe with 57.4 wt% and 42.6 wt% respectively. The cell parameter observed for Ni3 Fe is 3.59(4) A˚ (space group Pm–3m), which corresponds at the one reported in the JCPDS No. 96-901-3942. The sizes of crystallite calculated are 10.5 nm and 48 nm for the nickel-iron and iron phases, respectively. It is well-known that metal nanoparticles are subject to easy oxidation when exposed to air. X-ray diffraction analysis shows no significant changes are observed when our samples are kept in normal conditions (room temperature and atmospheric pressure) for five months. This may indicate that the oxidation layer thickness is below the limit of XRD detection (5%).
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Fig. 2. TEM micrographs with histograms of NP sizes of (a) CoFe2 O4 and (b) reduced CoFe2 , (c) Fe3 O4 and (d) reduced Fe and (e) NiFe2 O4 and (f) reduced Ni3 Fe.
3.2. TEM characterization TEM micrographs (Fig. 2) allow the determination of particle morphology and diameter. As previously observed, TEM images also show that nickel spinel behaves differently. Reduction to Ni3 Fe nanoparticles of sizes remaining in the nanometer range (12 nm) occurs without the presence of spurious oxide precursor, but Fe nanoparticles slightly larger (48 nm) are also found at the end of the process. On the other hand, reduction of cobalt and iron spinel MFe2 O4 (M = Co or Fe) led to CoFe2 particles of 55 nm in diameter and bcc Fe of significantly larger size (145 nm), respectively. These sizes are very close to the crystallite sizes determined by XRD analysis and discussed above, which indicate that the particles are monocrystalline. The slight difference between the crystallite size obtained after calculations based on X-ray patterns and those based on TEM image analysis, suggests that not all the particles are exactly spherical. Moreover, volumes in TEM measurements are not as high as those in XRD data.
In conclusion, it is interesting to note that starting from spinel spherical nanoparticles smaller than 5 nm, a gentle reduction by hydrogen gas leads to monocrytalline nanoparticles of significantly larger size. The mechanism likely responsible for this enhanced growth may pass through two mains steps: (i) during the first part of the reduction process, spinel nanoparticles are again reduced into very small nanoparticles of either metal or alloy (ii) immediately after, a second process occurs and the in situ metal nanoparticles diffuse to each other and fuse together due to their high reactivity, leading to larger monocrystalline nanoparticles. 3.3. Magnetic characterization Magnetization as function of the applied magnetic field measured at 4 K and 380 K for the reduced spinels are shown in Fig. 3(a) and (b) respectively. In Table 1 we report the main important features of these hysteresis loops, i.e. the saturated magnetization (Ms) measured at 4 K and the estimation of coercive behaviors at
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Fig. 4. Susceptibility at 400 Oe as function of temperature of (from the top) cobalt ferrite nanoparticles (open circles), iron nano-particles (black circles) and nickel ferrite nano-particles (gray rhombus). The bound at 380 K is due to the approaching vitreous temperature of epoxy matrices in which samples were prepared.
Fig. 3. Hysteresis magnetic loop of cobalt ferrite nano-particles (open circles), iron nano-particles (black circles) and nickel ferrite nano-particles (gray rhombus) at (a) 4 K and (b) 380 K.
both 4 K and 380 K. The MS measured for all samples at low temperature and high magnetic field (5 T) lies in between the MS of pure Ni (57 emu/g) and that of pure Fe (221 emu/g) in agreement with the chemical composition inferred from X-ray analysis to our nanoparticles. Despite of the small number of experimental points we measured at low field, it is reasonable to discuss the observable estimations of the coercive fields. The reduced cobalt spinel (CoFe2 ) confirms the highest coercivity (see Table 1) due to the high magnetocrystalline anisotropy of cobalt as reported in literature. The coercive field of this sample observed in the hysteresis loops is coherent with the value reported for Fe–Co alloys (500–1000 Oe) processed by hydrogen plasma–metal reaction and having almost similar particles sizes [16]. All samples present an open hysteresis loop up to high temperature showing the lack up to 380 K of a superparamagnetic phase. Table 1 Saturated magnetization and coercivity field for the three reduced samples. Ms4K (emu/g) CoFe2 Fe Ni3 Fe/Fe
178 186 103
Hc4K (Oe) 1125 470 530
Hc380K (Oe) 800 270 150
In order to study this latter, we show in Fig. 4 the temperature dependence of the susceptibility measured in between 4 K and 400 K. The data was collected in the standard zero-field-cooling (ZFC) and field-cooling (FC) conditions under the application of a 400 Oe magnetic field. All the ZFC/FC curves show a relaxing behavior around 380 K which is due to the approaching vitreous temperature of epoxy pills used for preparation of SQUID samples. All systems studied present a magnetic irreversible behavior. This irreversibility can belong to superparamagnetism of very small nanoparticles below the so-called blocking temperature (i.e. temperature below which the magnetization appears to be blocked) or to the depinning of domains in ferromagnetic material [17]. The superparamagnetic behavior occurs when the particles become a single magnetic domain system and this is possible when their size is below a critical value (DC ) depending on the composition. For reduced iron spinel, no distinct maximum in the ZFC/FC magnetization curves was observed below 400 K, thus revealing pure ferromagnetic behavior. Furthermore, the ZFC susceptibility increases very slightly and FC susceptibility remains almost constant. For this sample, the particle dimension (145 nm) is above the critical size (DC ≈ 14 nm) for which Fe particles become a single domain magnet [18]. Such behavior was previously observed for Fe particles with 65 nm in diameter [19]. Dakhlaoui Omrani et al. also observed this phenomenon for Ni particles with a diameter of 170 nm [17]. In these cases, this irreversibility between ZFC/FC susceptibilities can mainly originate from the domain wall depinning [20].
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In the case of reduced cobalt and nickel spinel, the irreversibility is more pronounced than for the reduced iron sample. For reduced nickel spinel, we obtained two phases: 57% of Ni3 Fe particles with a size of 12 nm (i.e. less than the critical sizes of Ni and Fe [16]) and 43% of bcc Fe nanoparticles with size of 48 nm (i.e. significantly higher than the DC size of Fe (14 nm)). Thus, the magnetism of this sample can be a resultant of both superparamagnetism arising from Ni3 Fe mono-domain particles and ferromagnetic poly-domain Fe particles. In the case of CoFe2 , TEM showed that the nanoparticles have size around 55 nm, which is lower than the critical size of a cobalt single domain (70 nm) but significantly higher than that of iron (14 nm) [18]. However, the pronounced irreversibility observed between ZFC/FC is in favor of superparamagnetic behavior rather than of wall depinning in multi-domain particles. The blocking temperature is probably higher than 380 K since the hysteresis loop remains open at this temperature, as already reported. This means that the magnetic behavior of this alloy may be governed by the size of the cobalt single domain. It should however be noted, that ZFC/FC curves of this sample, present a particular behavior around 30 K (decreasing in FC and a maximum in ZFC). This can be due to very small nanoparticles existing along with those having 55 nm in size. Thus 30 K can be considered as the blocking temperature of such very small nanoparticles. The existence of two blocking temperatures was previously reported for Fe [21], for Co fibers [22] and for Fe nanoparticles embedded in NiO matrix [23]. Further characterizations (ZFC/FC measurements at high temperature, AC susceptibility, Mossbauer spectroscopy. . .) studies are in progress for better understanding of the magnetic behaviors of the as-obtained alloys. 4. Conclusion Spinel nanoparticles of MFe2 O4 (M = Co, Ni or Fe) were processed by forced hydrolysis in a polyol medium. Taking advantage of their high surface reactivity, these spinel nanoparticles were gently reduced under hydrogen flow at 300 ◦ C. This led to nanoparticles of CoFe2 , Fe and Ni3 Fe with sizes not exceeding 150 nm. TEM and XRD analysis revealed the monocrystalline character of these nanoparticles. Magnetic measurements indicate that the resulting materials demonstrate a magnetic behavior depending on two main factors (i) chemical composition of the reduced nanoparticles and (ii)
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particle size. bcc Fe nanoparticles with 140 nm in diameter present a ferromagnetic character similar to that of the corresponding bulk iron while the CoFe2 alloy nanoparticles display a magnetic behavior approaching superparamagnetism. In the case of nickel spinel reduced nanoparticles, the two phases obtained present a mix of their magnetic behavior and thus a more complex feature. Current studies are in progress to gain more insight into these behaviors. Acknowledgements We thank very much S. Hebert and A. Pautrat at the CRISMAT Laboratory-ENSICAEN for their precious help with the magnetic measurements. The IFR Paris Nord Plaine de France (PPF) has supported this work. References [1] M. Figlarz, F. Fievet, E.U. Patent 0113281 (1985). [2] C. Feldmann, J. Merikhi, Colloid Interface Sci. 223 (2000) 229. [3] L. Poul, S. Ammar, N. Jouini, F. Fiévet, F. Villain, J. Sol–Gel Sci. Technol. 26 (2003) 26. [4] G. Viau, F. Fiévet-Vincent, F. Fiévet, J. Mater. Chem. 6 (1996) 1047. [5] Y. Soumare, C. Garcia, T. Maurer, G. Chaboussant, F. Ott, F. Fievet, J.-Y. Piquemal, G. Viau, Adv. Funct. Mater. 19 (2009) 1. [6] S.-C. Yang, W.-N. Su, S.D. Lin, J. Rick, J.-H. Cheng, J.-Y. Liu, C.-J. Pan, D.-G. Liu, J.-F. Lee, T.-S. Chan, H.-S. Sheu, B.-J. Hwang, Appl. Catal. B: Environ. 106 (2011) 650. [7] S.S.A. Syed-Hassan, C.-Z. Li, Int. J. Chem. Kinet. (2011) 667. [8] M. Bhattacharya, Metall. Mater. Trans. B 42B (2011) 380. [9] J.S. Lee, T.H. Kim, J.H. Yu, S.W. Chung, Nanostruct. Mater. 9 (1997) 153. [10] S. Chkoundali, S. Ammar, N. Jouini, F. Fievet, P. Molinié, M. Danot, F. Villain, J-M. Grenèche, J. Phys.: Condens. Matter 16 (2004) 4357. [11] http://www.crystalimpact.com/match/default.htm, June 2011. [12] L. Lutterotti, Department of Materials Science and Industrial Technologies, University of Trento, Italy, http://www.ing.unitn.it/∼maud/, June 2011. [13] M. Andasmas, P. Langlois, N. Fagon, Th. Chauveau, A. Hendaoui, D. Vrel, Powder Technol. 207 (2011) 305. [14] I.S. Jurca, N. Viart, C. Mény, C. Ulhaq-Bouillet, P. Panissod, G. Pourroy, Surf. Sci. 529 (2003) 215. [15] W.C. Ellis, E.S. Greiner, Trans. Am. Soc. Met. 29 (1941) 415. [16] X.G. Li, T. Murai, T. Saito, S. Takahashi, J. Magn. Magn. Mater. 190 (1998) 277. [17] A. Dakhlaoui Omrani, M.A. Bousnina, L.S. Smiri, M. Taibi, P. Leone, F. Schoenstein, N. Jouini, Mater. Chem. Phys. 123 (2010) 821. [18] D.L. Leslie-Pelecky, Chem. Mater. 8 (1996) 1770. [19] Y.L. Hou, S. Gao, J. Alloys Compd. 365 (2004) 112. [20] C. Mazumdar, R. Nagarajan, L.C. Gupta, B.D. Padalia, R. Vijayaraghavan, Appl. Phys. Lett. 77 (2000) 895. [21] M.P. Fernandez, D.S. Schmool, A.S. Silva, M. Sevilla, A.B. Fuertes, P. Gorria, J.A. Blanco, J. Magn. Magn. Mater. 322 (2010) 1300. [22] A. Dakhaloui, L.S. Smiri, G. Babadjian, F. Shoenstein, P. Molinié, N. Jouini, J. Phys. Chem. C 112 (2008) 14348. [23] S.P. Pati, B. Bhushan, A. Basumallick, S. Kumar, D. Das, Mater. Sci. Eng. B 176 (2011) 1015.