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Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method
Materials Chemistry and Physics xxx (2015) 1e5
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Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method A. Bouremana a, A. Guittoum b, *, M. Hemmous b, D. Martínez-Blanco c, Pedro Gorria d, J.A. Blanco e, N. Benrekaa a a
LPM, Faculty of Sciences, USTHB, BP 32, El-Alia, Bab Ezzouar, Algiers, Algeria Nuclear Research Centre of Algiers, 02 Bd Frantz Fanon, BP 399, Alger-Gare, Algiers, Algeria SCTs, University of Oviedo, EPM, 33600 Mieres, Spain d n, Spain Department of Physics & IUTA, EPI, University of Oviedo, 33203 Gijo e Department of Physics, University of Oviedo, Calvo Sotelo St., 33007 Oviedo, Spain b c
h i g h l i g h t s Pure Nickel nanoparticles have been synthesized by a chemical reaction process. Different morphologies were observed with the change of NaOH concentration. The coercive field increases with increasing the NaOH concentration and depends on the shape of nanoparticles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2014 Received in revised form 1 February 2015 Accepted 5 May 2015 Available online xxx
Powder samples containing high purity nickel nanoparticles (NPs) were prepared by hydrothermal method from Ni(II) chloride hexahydrate (NiCl2$6H2O) under the presence of sodium hydroxide (NaOH) with different concentrations between 5 and 25 mol/L. The synthesis of the NPs occurs through chemical reduction at relatively low temperature (140 C). The Ni NPs have a face-centred cubic (fcc) crystal structure with a lattice parameter value close to that of pure Ni (a ¼ 3.52 Å). The average crystallite size determined from x-ray diffraction is around 20 nm, except for the sample synthesized under the highest NaOH concentration (25 mol/L), which has the largest average size (>30 nm). The powder morphology at the sub-micrometre length scale looks like agglomerates of Ni-NPs that drastically changes their shape depending on the NaOH concentration, from flower (5 mol/L) to a dendritic-like (25 mol/L). All the samples are ferromagnetic at room temperature with saturation magnetization values between 50 and 52emu/g, and a coercive field that increases with the NaOH concentration from around 135 (5 mol/L) up to 180Oe (25 mol/L). © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic materials Nanostructures Powder diffraction SEM Hysteresis
1. Introduction Nowadays, the synthesis of new nanomaterials with controllable chemical composition, crystalline structure, morphology, size or shape that could enable to predict the physicalechemical properties and their subsequent modification for application purposes is a major challenge for researchers in nanotechnology [1e3]. In particular, magnetic nanomaterials have received much attention because of their excellent suitability for a great variety of
* Corresponding author. E-mail address: [email protected] (A. Guittoum).
technological applications such as high-density data storage, micro-electronic devices, medical diagnosis, magnetic separation or catalysis among others [4e6]. The magnetic properties of nanoparticles (NPs) are often dominated by their size and shape, but also by the matrix and the agglomeration state, hence, it is important to have the capability for controlling the size as well as the morphology during the synthesis process [1]. A major interest has been devoted to ferromagnetic NPs, particularly Nickel, due to the broad field of potential technological applications such as magnetic fluids, biotechnologies, magnetic resonance imaging, magnetic separation, or data storage [7e10]. In parallel, several methods have been developed to elaborate Ni NPs with diverse morphologies and size distributions in
http://dx.doi.org/10.1016/j.matchemphys.2015.05.015 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015
2
A. Bouremana et al. / Materials Chemistry and Physics xxx (2015) 1e5
different matrices; modified polyol-process, solvothermal method, g-ray irradiation technique, Sol-Gel or pyrolysis among others [11e16]. However, the hydrothermal route is distinguished from others by its low cost and simplicity, and also because a number of parameters can be fine-tuned in order to control the NP shape and size [8,17]. Until now, several works devoted to the study of the physical properties of Ni NPs elaborated by hydrothermal method with different morphologies including dendritic, spherical and flowerlike have been published [8,18,19]. From these reported data, it follows that the adequate choice of synthesis parameters (precursor concentration, annealing temperature, annealing time, type of ligand or PH), which will influence the size and shape of Ni NPs and consequently their magnetic properties, is still a challenging topic of research. In this sense, we believe that sodium hydroxide, NaOH, could be a key parameter for the hydrothermal synthesis of Ni NPs. Indeed, we have previously reported [17] that the NaOH concentration has a clear influence on the shape and consequently, on the magnetic behaviour of Ni nanoparticles prepared from a sulphate (NiSO4.6H2O) precursor. In the present work, we have used a chloride precursor (NiCl2.6H2O) as a stating material with various NaOH concentrations for the preparation of Ni NPs. The present investigation is motivated by the lack, as far as we know, of a detailed study dealing with the effect of NaOH on the physical properties of Ni nanoparticles prepared from a chloride (NiCl2.6H2O) precursor. In this perspective, we have focused on the effect of NaOH concentration on the crystal structure, morphology, microstructure and room temperature magnetic properties of Ni NPs prepared by hydrothermal route. 2. Experimental procedure 2.1. Starting materials and NP synthesis The chemical reagents used in this work were manufactured by sigma Aldrich without additional purification. Nickel (II) chloride hexahydrate (NiCl2$6H2O), sodium hydroxate (NaOH), ethylenediamine (EDA) and hydrazine hydrate (N2H4$H2O) were used as starting materials. A typical synthesis procedure consists in dissolving 4 g (5 mol/L) of NaOHin20 mL of deionized water, then 1 mL of NiCl2$6H2O (0.5 mol/L) and 0.35 mL of ethylenediamine (EDA) were introduced under vigorous stirring for 60min. After that, 0.25 mL hydrazine hydrate (N2H4$H2O 30%) were added and followed by 30min of continuous stirring. The mixed solution was loaded into a Teflon cup, sealed and maintained at 140 C for 4 h and then cooled down to room temperature. The black fluffy product floating on the solution was collected using a magnetic bar, and then rinsed with distilled water and absolute ethanol. The process was repeated several times in order to remove any alkali salt and impurities. The final product was dried in air at 40 C for 4 h. A series of four samples with different concentration of NaOH (5, 10, 15 and 25 mol/L) was prepared following the above mentioned procedure. The samples will be named here after as Ni5, Ni-10, Ni-15 and Ni-25.
squares procedure was used in order to minimize the difference between the experimental and the simulated patterns. In this sense, Rwp (weighted residual factor), RB (Bragg factor) and Rexp (expected residual factor) were considered as the reliability parameters for the fit. The morphology and chemical composition of the Ni powder samples were investigated by means of a JEOL JSM6610LV scanning electron microscope (SEM) equipped with Energy Dispersive X-ray analyzer (EDX). The room temperature magnetization vs. applied magnetic field loops, M(H), were measured in a vibrating sample magnetometer VSM model EV9 between ± 22 kOe. Up to 450 experimental points were collected for each M(H) loop. 3. Results and discussion 3.1. Crystal structure and microstructure The room temperature X-ray diffraction patterns of the samples Ni-5, Ni-10, Ni-15 and Ni-25 are depicted in Fig. 1. The five intensity peaks located at 2q angular positions around 44.5 , 51.8 , 76.3 , 92.9 and 98.4 , correspond to the (111), (200), (220), (311) and (222) Bragg reflections of a face centred cubic (fcc) crystal structure with a lattice parameter, a z 3.52 Å (see Table 1) that can be ascribed to Ni. There are not any additional peaks coming from impurity phases (such as Nickel hydroxide, Ni(OH)2, or nickel oxide, NiO), except for the Ni-5 sample (around 1%, see bottom panel in Fig. 2), thus evidencing the effectiveness of the synthesis route to obtain pure metallic Ni NPs. In Fig. 2, the full-profile fit of the XRD patterns corresponding to samples Ni-5 and Ni-10 are shown as an example of the fit reliability. Let us concentrate now on the results obtained from the fullprofile fit of the XRD patterns. The value of the lattice parameter remains almost unaffected by the NaOH concentration (see Table 1). In turns, the average crystallite size varies with the NaOH concentration. A decrease from 24 nm (Ni-5) to 18 nm (Ni-10 and Ni-15) is observed, while this value is above 30 nm for the Ni-25 sample. These findings suggest that an excess of NaOH provokes the nucleation of larger Ni NPs. This behaviour of the average crystallite size is different from that found in the case of Ni NPs
2.2. Sample characterization Room temperature x-ray diffraction (XRD) patterns of Ni NPs powders were recorded with a PANalytical X-Pert Pro diffractometer equipped with a Cu Ka radiation (l ¼ 1.5418 Å). The scanning angular range in 2q was from 15 to 120 with steps of 0.04 . The values for the structural parameters (lattice parameter, a and average crystallite size,) were obtained from the full-profile refinement of the XRD patterns by means of the MAUD software [20] based on the Rietveld method [21,22]. The Marquardt least-
Fig. 1. Room temperature x-ray diffraction patterns of Ni-NPs samples.
Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015
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Table 1 Estimated values of the lattice parameters, a(Ǻ), and average crystallite size,(nm), of Ni NPs samples obtained from the Rietveld refinement of the XRD patterns. Sample
NaOH (mol/L)
Ni wt (%)
Lattice parameter, a (Å)
Average crystallite size, (nm)
Rwp (%)
GOF (%)
Ni-5 Ni-10 Ni-15 Ni-25
5 10 15 25
99 100 100 100
3.526 3.524 3.524 3.524
24 18 19 32
2.3 1.8 1.8 2.0
1.9 1.4 1.5 1.6
(2) (9) (8) (4)
(1) (1) (1) (1)
units look like dandelion-like flowers, the Ni-15 sample (Fig. 3e and f) exhibits aggregates formed by almost spherical and/or polyhedral cores resembling a bunch of grapes. Interestingly, when the NaOH concentration increases up to 25 mol/L (Ni-25 sample, see Fig. 3g and h), a dendritic-like morphology appears. Hence, the micrographs show a fractal dendritic structure with a trunk formed of several branches distributed along different directions. These findings tell us that the NaOH concentration has a large role to play in modifying the final morphology of the nanostructured Ni powder samples. It is worth noting that the powder morphology observed in the samples elaborated following the same synthesis route but using a chloride precursor [17] instead of a sulphate one, is markedly different. A good example of that is the case for a concentration of 15 mol/L of NaOH. We observed quasi-spherical aggregates with the chloride precursor, but flower-like micronic entities are present when sulphate is utilized [17]. Along with the clear effect on the powder morphology, the nature of the precursor seems to have a significant influence in the growth mechanism of Ni nanoparticles. Therefore, the NaOH concentration and the precursor are important factors, which have to be carefully selected, if the NP shape, size and morphology of the powder samples have to be tuned. The chemical composition of all samples has been determined by EDX analysis. Only peaks associated with nickel are observed. 3.3. Magnetic behaviour
Fig. 2. Observed (dots) and calculated (solid line) XRD patterns of samples Ni-10 (upper panel) and Ni-5 (bottom panel). The vertical bars indicate the position of the Bragg reflections. The observed-calculated difference is depicted at the bottom of each figure. Small reflections below 2q ¼ 50 in the pattern corresponding to Ni-5 sample come from Ni(OH)2hydroxide impurity phase (1%).
obtained from sulphate precursor (NiSO4$6H2O) and using the same values for the NaOH concentration. In that case, the average size was larger (z40 nm) and almost constant for all the samples [17]. Hence, this comparison hints that the use of different precursors (chloride or sulphate) plays an important role in the average size of the nanoparticles.
3.2. Morphology evolution Fig. 3 shows some selected SEM images collected under different magnifications of the samples Ni-5, Ni-10, Ni-15 and Ni25. In these images we can observe that the powders are formed by individual entities with sizes ranging between 1 and 5 mm. Moreover, each of those entities is composed itself by an agglomeration of smaller particles within the nanometre length scale, and changes its morphology depending on the NaOH concentration. Indeed, for the Ni-5 sample (Fig. 3a and b) a flower-like shape units constituted by many joined sword-like filaments attached to spherical cores having different diameters is clearly distinguished. However, while for the Ni-10 sample (Fig. 3c and d), the individual
The magnetization curves, M(H), for the five different Ni samples are shown in Fig. 4 a. All the samples exhibit the typical hysteresis loop characteristic of a ferromagnetic system. The value of the saturation magnetization, MS, was estimated from the fit of the data in the high magnetic field region (H > 5 kOe) to an approachto-saturation law [23]:
a b M ¼ MS 1 2 þ cH H H
(1)
where a is a coefficient related to inclusions and/or microstrain, bis due to crystal anisotropy and c is ascribed to an independent field susceptibility. In our case, the term a/H is very small and can be discarded [24]. The estimated values for MS from the fit to Equation (1) are in between 50 and 52emu/g (see Table 2) similar to the values found for Ni-NP synthesized using sulphide precursor [17], and very close to that for pure Ni bulk (54.39emu/g) [23]. In addition, we have also obtained the magnitude of the coercive field, HC, from the hysteresis loops (in Fig. 4b) a zoom of the central region of the loops is depicted) as the semi sum of the magnetic field values corresponding to zero magnetization. The HC values for all the NiNP samples are around 135Oe (see Table 2), except that corresponding to Ni-25 sample, which is somewhat larger (ca. 180Oe, see Table 2). The dendritic morphology together with larger average NP size may be regarded as the main factors responsible for that due to an increase in the shape magnetic anisotropy respect to the other samples exhibiting more isotropic morphologies (spherical or flower-like shaped). Lastly, it is interesting to note that the coercive field values
Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015
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A. Bouremana et al. / Materials Chemistry and Physics xxx (2015) 1e5
Fig. 3. SEM images of Ni-NPs samples collected at different magnifications. Images (a)e(b), (c)e(d), (e)e(f) and (g)e(h) correspond to samples Ni-5, Ni-10, Ni-15 and Ni-25, respectively.
obtained in this work are lower than those already reported for Ni NPs prepared by hydrothermal method from chloride precursor; XM. Liu et al. found a value of 215Oe for dendritic Ni NPs [18]; C. Jiang et al. reported 232Oe for flower-like Ni hierarchical microstructures [25] and A. Yimamu et al. obtained 140 and 95Oe for flower-like and polyhedron-like Ni NPs [26], respectively. Taking into account that the synthesized nickel nanoparticles in all of these referred works (including this work) have average sizes below the upper limit for a magnetic single-domain, we suggest that macroscopic effects (shape, size and morphology of the micronic agglomerates of NPs) are the responsible for the different value of the coercive field, due
to the major role that magnetic shape anisotropy plays in these systems. 4. Summary and conclusions Pure Nickel nanoparticles have been synthesized by a chemical reaction process at low temperature (140 C) with varying the concentration of sodium hydroxide between 5 and 25 mol/L. The Ni NPs exhibit fcc crystal structure with a lattice parameter almost identical than that of pure Ni. The average crystallite size is around 20 nm for all the samples excluding that with the highest NaOH
Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015
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Table 2 Estimated values of the room temperature saturation magnetization, MS, and coercive field, HC, of Ni-NPs samples. Sample
NaOH (mol/L)
Coercive field, HC (Oe)
Saturation magnetization MS (emu/g)
Ni-5 Ni-10 Ni-15 Ni-25
5 10 15 25
135 137 129 179
52 52 50 51
(3) (3) (3) (3)
(1) (1) (1) (1)
agglomerates that modify the magnetic shape anisotropy and consequently both the value of the coercive field and the low-field permeability. Acknowledgements We thank SCT's of the University of Oviedo for the assistance in the XRD and magnetometry measurements. The authors from Spain acknowledge financial support from Spanish MINECO (research project MAT2011-27573-C04-02). References
Fig. 4. (a) Room temperature hysteresis loops for Ni-NPs samples. (b) Zoom of the low applied magnetic field region.
concentration (25 mol/L), which exceeds 30 nm. The morphology of the Ni powders at the micrometre length scale evolves form flowerlike to spherical aggregates and then to a dendritic microstructure. These findings indicate that the NaOH concentration affects not only the shape and morphology of the micronic entities, but also plays an important role in the growth of the Ni nanoparticles that are the fundamental blocks forming the micronic entities by agglomeration. Therefore, the NaOH concentration as well as the type of the precursor has to be carefully chosen in order to control the shape and morphology of the powders together with the NP average size. Lastly, the samples exhibit a ferromagnetic behaviour, with saturation magnetization values close to that of pure Ni and a coercive field that is related with the morphology of the NP
[1] D. Vollath, Nanomaterials: an Introduction to Synthesis, Properties and Applications, Wiley-VCH, Weinheim, 2008. [2] Guozhong Cao, Nanostructures and Nanomaterials: synthesis, Properties & Applications, Imperial College Press, London, 2004. [3] N. Rinaldi-Montes, et al., Nanoscale 6 (2014) 457. [4] A. Datta, M. Pal, D. Chakravorty, D. Das, S.N. Chintalapudi, J. Magn. Magn. Mater. 205 (1999) 301. ndez-García, P. Gorria, J.A. Blanco, A.B. Fuertes, M. Sevilla, R. Boada, [5] M.P. Ferna che, Phys. Rev. B81 (2010) 094418. J. Chaboy, D.S. Schmool, J.-M. Grene [6] A. Biffis, N. Orlandi, B. Corain, Adv. Mater. 15 (2003) 1551. [7] A.H. Lu, E.L. Salabas, F. Schüth, Angew. Chem. Int. Ed. 46 (2007) 1222. [8] Z. Wang, J. Zou, Z. Ding, J. Wu, P. Wang, S. Jin, H. Bi, Mat. Chem. Phys. 142 (2013) 119. ndez-García, P. Gorria, M. Sevilla, M.P. Proença, R. Boada, J. Chaboy, [9] M.P. Ferna A.B. Fuertes, J.A. Blanco, J. Phys. Chem. C 115 (2011) 5294. [10] M.C. Daniel, D. Astruc, Chem. Rev. 104 (2004) 293. [11] K. Yu, D.J. Kim, H.S. Chung, H.Z. Liang, Mater. Lett. 57 (2003) 3992. [12] P. Gorria, M. Sevilla, J.A. Blanco, A.B. Fuertes, Carbon 44 (2006) 1954. [13] M. Tang, G. Huang, S. Zhang, Y. Liu, X. Li, X. Wang, X. Pang, H. Qiu, Mat. Chem. Phys. 145 (2014) 418. ndez-García, M. Sevilla, J.A. Blanco, A.B. Fuertes, Phys. Stat. [14] P. Gorria, M.P. Ferna Solidi (RRL) 3 (2009) 4. [15] H. Zhao, X. Han, L. Zhang, G. Wang, C. Wang, X. Li, P. Xu, Rad. Phys. Chem. 80 (2011) 390. [16] F. Jia, L. Zhang, X. Shang, Y. Yang, Adv. Mater. 20 (2008) 1050. [17] A. Bouremana, A. Guittoum, M. Hemmous, B. Rahal, J.J. Sunol, D. MartínezBlanco, J.A. Blanco, P. Gorria, N. Benrekaa, J. Magn. Magn. Mater. 358e359 (2014) 11. [18] X.M. Liu, S.Y. Fu, J. Crys. Growth 306 (2007) 428. [19] H. Chen, C. Xu, C. Chen, G. Zhao, Y. Liu, Mater. Res. Bull. 47 (2012) 1839. [20] L. Lutterotti, S. Matthies, H.-R. Wenk, IUCr: Newsletter of the CPD, vol. 21, 1999, p. 14. http://www.ing.unitn.it/~maud/. [21] H.M. Rietveld, J. Appl. Cryst. 22 (1969) 65. [22] R.A. Young, The Rietveld Method, Oxford University Press/IUCr, Oxford, 1993. [23] B.D. Cullity, Introduction to Magnetic Materials, Addison-Wesley, Reading MA, 2009. el, J. Phys. Radium 9 (1948) 184. [24] L. Ne [25] C. Jiang, G. Zou, W. Zhang, W. Yiu, Y. Qian, Mater. Lett. 60 (2006) 2319. [26] A. Yimamu, S. Beysen, D. Peng, Y. Aierken, Particuology 10 (2012) 392.
Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015
Contents lists available at ScienceDirect
Materials Chemistry and Physics journal homepage: www.elsevier.com/locate/matchemphys
Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method A. Bouremana a, A. Guittoum b, *, M. Hemmous b, D. Martínez-Blanco c, Pedro Gorria d, J.A. Blanco e, N. Benrekaa a a
LPM, Faculty of Sciences, USTHB, BP 32, El-Alia, Bab Ezzouar, Algiers, Algeria Nuclear Research Centre of Algiers, 02 Bd Frantz Fanon, BP 399, Alger-Gare, Algiers, Algeria SCTs, University of Oviedo, EPM, 33600 Mieres, Spain d n, Spain Department of Physics & IUTA, EPI, University of Oviedo, 33203 Gijo e Department of Physics, University of Oviedo, Calvo Sotelo St., 33007 Oviedo, Spain b c
h i g h l i g h t s Pure Nickel nanoparticles have been synthesized by a chemical reaction process. Different morphologies were observed with the change of NaOH concentration. The coercive field increases with increasing the NaOH concentration and depends on the shape of nanoparticles.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 25 July 2014 Received in revised form 1 February 2015 Accepted 5 May 2015 Available online xxx
Powder samples containing high purity nickel nanoparticles (NPs) were prepared by hydrothermal method from Ni(II) chloride hexahydrate (NiCl2$6H2O) under the presence of sodium hydroxide (NaOH) with different concentrations between 5 and 25 mol/L. The synthesis of the NPs occurs through chemical reduction at relatively low temperature (140 C). The Ni NPs have a face-centred cubic (fcc) crystal structure with a lattice parameter value close to that of pure Ni (a ¼ 3.52 Å). The average crystallite size determined from x-ray diffraction is around 20 nm, except for the sample synthesized under the highest NaOH concentration (25 mol/L), which has the largest average size (>30 nm). The powder morphology at the sub-micrometre length scale looks like agglomerates of Ni-NPs that drastically changes their shape depending on the NaOH concentration, from flower (5 mol/L) to a dendritic-like (25 mol/L). All the samples are ferromagnetic at room temperature with saturation magnetization values between 50 and 52emu/g, and a coercive field that increases with the NaOH concentration from around 135 (5 mol/L) up to 180Oe (25 mol/L). © 2015 Elsevier B.V. All rights reserved.
Keywords: Magnetic materials Nanostructures Powder diffraction SEM Hysteresis
1. Introduction Nowadays, the synthesis of new nanomaterials with controllable chemical composition, crystalline structure, morphology, size or shape that could enable to predict the physicalechemical properties and their subsequent modification for application purposes is a major challenge for researchers in nanotechnology [1e3]. In particular, magnetic nanomaterials have received much attention because of their excellent suitability for a great variety of
* Corresponding author. E-mail address: [email protected] (A. Guittoum).
technological applications such as high-density data storage, micro-electronic devices, medical diagnosis, magnetic separation or catalysis among others [4e6]. The magnetic properties of nanoparticles (NPs) are often dominated by their size and shape, but also by the matrix and the agglomeration state, hence, it is important to have the capability for controlling the size as well as the morphology during the synthesis process [1]. A major interest has been devoted to ferromagnetic NPs, particularly Nickel, due to the broad field of potential technological applications such as magnetic fluids, biotechnologies, magnetic resonance imaging, magnetic separation, or data storage [7e10]. In parallel, several methods have been developed to elaborate Ni NPs with diverse morphologies and size distributions in
http://dx.doi.org/10.1016/j.matchemphys.2015.05.015 0254-0584/© 2015 Elsevier B.V. All rights reserved.
Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015
2
A. Bouremana et al. / Materials Chemistry and Physics xxx (2015) 1e5
different matrices; modified polyol-process, solvothermal method, g-ray irradiation technique, Sol-Gel or pyrolysis among others [11e16]. However, the hydrothermal route is distinguished from others by its low cost and simplicity, and also because a number of parameters can be fine-tuned in order to control the NP shape and size [8,17]. Until now, several works devoted to the study of the physical properties of Ni NPs elaborated by hydrothermal method with different morphologies including dendritic, spherical and flowerlike have been published [8,18,19]. From these reported data, it follows that the adequate choice of synthesis parameters (precursor concentration, annealing temperature, annealing time, type of ligand or PH), which will influence the size and shape of Ni NPs and consequently their magnetic properties, is still a challenging topic of research. In this sense, we believe that sodium hydroxide, NaOH, could be a key parameter for the hydrothermal synthesis of Ni NPs. Indeed, we have previously reported [17] that the NaOH concentration has a clear influence on the shape and consequently, on the magnetic behaviour of Ni nanoparticles prepared from a sulphate (NiSO4.6H2O) precursor. In the present work, we have used a chloride precursor (NiCl2.6H2O) as a stating material with various NaOH concentrations for the preparation of Ni NPs. The present investigation is motivated by the lack, as far as we know, of a detailed study dealing with the effect of NaOH on the physical properties of Ni nanoparticles prepared from a chloride (NiCl2.6H2O) precursor. In this perspective, we have focused on the effect of NaOH concentration on the crystal structure, morphology, microstructure and room temperature magnetic properties of Ni NPs prepared by hydrothermal route. 2. Experimental procedure 2.1. Starting materials and NP synthesis The chemical reagents used in this work were manufactured by sigma Aldrich without additional purification. Nickel (II) chloride hexahydrate (NiCl2$6H2O), sodium hydroxate (NaOH), ethylenediamine (EDA) and hydrazine hydrate (N2H4$H2O) were used as starting materials. A typical synthesis procedure consists in dissolving 4 g (5 mol/L) of NaOHin20 mL of deionized water, then 1 mL of NiCl2$6H2O (0.5 mol/L) and 0.35 mL of ethylenediamine (EDA) were introduced under vigorous stirring for 60min. After that, 0.25 mL hydrazine hydrate (N2H4$H2O 30%) were added and followed by 30min of continuous stirring. The mixed solution was loaded into a Teflon cup, sealed and maintained at 140 C for 4 h and then cooled down to room temperature. The black fluffy product floating on the solution was collected using a magnetic bar, and then rinsed with distilled water and absolute ethanol. The process was repeated several times in order to remove any alkali salt and impurities. The final product was dried in air at 40 C for 4 h. A series of four samples with different concentration of NaOH (5, 10, 15 and 25 mol/L) was prepared following the above mentioned procedure. The samples will be named here after as Ni5, Ni-10, Ni-15 and Ni-25.
squares procedure was used in order to minimize the difference between the experimental and the simulated patterns. In this sense, Rwp (weighted residual factor), RB (Bragg factor) and Rexp (expected residual factor) were considered as the reliability parameters for the fit. The morphology and chemical composition of the Ni powder samples were investigated by means of a JEOL JSM6610LV scanning electron microscope (SEM) equipped with Energy Dispersive X-ray analyzer (EDX). The room temperature magnetization vs. applied magnetic field loops, M(H), were measured in a vibrating sample magnetometer VSM model EV9 between ± 22 kOe. Up to 450 experimental points were collected for each M(H) loop. 3. Results and discussion 3.1. Crystal structure and microstructure The room temperature X-ray diffraction patterns of the samples Ni-5, Ni-10, Ni-15 and Ni-25 are depicted in Fig. 1. The five intensity peaks located at 2q angular positions around 44.5 , 51.8 , 76.3 , 92.9 and 98.4 , correspond to the (111), (200), (220), (311) and (222) Bragg reflections of a face centred cubic (fcc) crystal structure with a lattice parameter, a z 3.52 Å (see Table 1) that can be ascribed to Ni. There are not any additional peaks coming from impurity phases (such as Nickel hydroxide, Ni(OH)2, or nickel oxide, NiO), except for the Ni-5 sample (around 1%, see bottom panel in Fig. 2), thus evidencing the effectiveness of the synthesis route to obtain pure metallic Ni NPs. In Fig. 2, the full-profile fit of the XRD patterns corresponding to samples Ni-5 and Ni-10 are shown as an example of the fit reliability. Let us concentrate now on the results obtained from the fullprofile fit of the XRD patterns. The value of the lattice parameter remains almost unaffected by the NaOH concentration (see Table 1). In turns, the average crystallite size varies with the NaOH concentration. A decrease from 24 nm (Ni-5) to 18 nm (Ni-10 and Ni-15) is observed, while this value is above 30 nm for the Ni-25 sample. These findings suggest that an excess of NaOH provokes the nucleation of larger Ni NPs. This behaviour of the average crystallite size is different from that found in the case of Ni NPs
2.2. Sample characterization Room temperature x-ray diffraction (XRD) patterns of Ni NPs powders were recorded with a PANalytical X-Pert Pro diffractometer equipped with a Cu Ka radiation (l ¼ 1.5418 Å). The scanning angular range in 2q was from 15 to 120 with steps of 0.04 . The values for the structural parameters (lattice parameter, a and average crystallite size,
Fig. 1. Room temperature x-ray diffraction patterns of Ni-NPs samples.
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Table 1 Estimated values of the lattice parameters, a(Ǻ), and average crystallite size,
NaOH (mol/L)
Ni wt (%)
Lattice parameter, a (Å)
Average crystallite size,
Rwp (%)
GOF (%)
Ni-5 Ni-10 Ni-15 Ni-25
5 10 15 25
99 100 100 100
3.526 3.524 3.524 3.524
24 18 19 32
2.3 1.8 1.8 2.0
1.9 1.4 1.5 1.6
(2) (9) (8) (4)
(1) (1) (1) (1)
units look like dandelion-like flowers, the Ni-15 sample (Fig. 3e and f) exhibits aggregates formed by almost spherical and/or polyhedral cores resembling a bunch of grapes. Interestingly, when the NaOH concentration increases up to 25 mol/L (Ni-25 sample, see Fig. 3g and h), a dendritic-like morphology appears. Hence, the micrographs show a fractal dendritic structure with a trunk formed of several branches distributed along different directions. These findings tell us that the NaOH concentration has a large role to play in modifying the final morphology of the nanostructured Ni powder samples. It is worth noting that the powder morphology observed in the samples elaborated following the same synthesis route but using a chloride precursor [17] instead of a sulphate one, is markedly different. A good example of that is the case for a concentration of 15 mol/L of NaOH. We observed quasi-spherical aggregates with the chloride precursor, but flower-like micronic entities are present when sulphate is utilized [17]. Along with the clear effect on the powder morphology, the nature of the precursor seems to have a significant influence in the growth mechanism of Ni nanoparticles. Therefore, the NaOH concentration and the precursor are important factors, which have to be carefully selected, if the NP shape, size and morphology of the powder samples have to be tuned. The chemical composition of all samples has been determined by EDX analysis. Only peaks associated with nickel are observed. 3.3. Magnetic behaviour
Fig. 2. Observed (dots) and calculated (solid line) XRD patterns of samples Ni-10 (upper panel) and Ni-5 (bottom panel). The vertical bars indicate the position of the Bragg reflections. The observed-calculated difference is depicted at the bottom of each figure. Small reflections below 2q ¼ 50 in the pattern corresponding to Ni-5 sample come from Ni(OH)2hydroxide impurity phase (1%).
obtained from sulphate precursor (NiSO4$6H2O) and using the same values for the NaOH concentration. In that case, the average size was larger (z40 nm) and almost constant for all the samples [17]. Hence, this comparison hints that the use of different precursors (chloride or sulphate) plays an important role in the average size of the nanoparticles.
3.2. Morphology evolution Fig. 3 shows some selected SEM images collected under different magnifications of the samples Ni-5, Ni-10, Ni-15 and Ni25. In these images we can observe that the powders are formed by individual entities with sizes ranging between 1 and 5 mm. Moreover, each of those entities is composed itself by an agglomeration of smaller particles within the nanometre length scale, and changes its morphology depending on the NaOH concentration. Indeed, for the Ni-5 sample (Fig. 3a and b) a flower-like shape units constituted by many joined sword-like filaments attached to spherical cores having different diameters is clearly distinguished. However, while for the Ni-10 sample (Fig. 3c and d), the individual
The magnetization curves, M(H), for the five different Ni samples are shown in Fig. 4 a. All the samples exhibit the typical hysteresis loop characteristic of a ferromagnetic system. The value of the saturation magnetization, MS, was estimated from the fit of the data in the high magnetic field region (H > 5 kOe) to an approachto-saturation law [23]:
a b M ¼ MS 1 2 þ cH H H
(1)
where a is a coefficient related to inclusions and/or microstrain, bis due to crystal anisotropy and c is ascribed to an independent field susceptibility. In our case, the term a/H is very small and can be discarded [24]. The estimated values for MS from the fit to Equation (1) are in between 50 and 52emu/g (see Table 2) similar to the values found for Ni-NP synthesized using sulphide precursor [17], and very close to that for pure Ni bulk (54.39emu/g) [23]. In addition, we have also obtained the magnitude of the coercive field, HC, from the hysteresis loops (in Fig. 4b) a zoom of the central region of the loops is depicted) as the semi sum of the magnetic field values corresponding to zero magnetization. The HC values for all the NiNP samples are around 135Oe (see Table 2), except that corresponding to Ni-25 sample, which is somewhat larger (ca. 180Oe, see Table 2). The dendritic morphology together with larger average NP size may be regarded as the main factors responsible for that due to an increase in the shape magnetic anisotropy respect to the other samples exhibiting more isotropic morphologies (spherical or flower-like shaped). Lastly, it is interesting to note that the coercive field values
Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015
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A. Bouremana et al. / Materials Chemistry and Physics xxx (2015) 1e5
Fig. 3. SEM images of Ni-NPs samples collected at different magnifications. Images (a)e(b), (c)e(d), (e)e(f) and (g)e(h) correspond to samples Ni-5, Ni-10, Ni-15 and Ni-25, respectively.
obtained in this work are lower than those already reported for Ni NPs prepared by hydrothermal method from chloride precursor; XM. Liu et al. found a value of 215Oe for dendritic Ni NPs [18]; C. Jiang et al. reported 232Oe for flower-like Ni hierarchical microstructures [25] and A. Yimamu et al. obtained 140 and 95Oe for flower-like and polyhedron-like Ni NPs [26], respectively. Taking into account that the synthesized nickel nanoparticles in all of these referred works (including this work) have average sizes below the upper limit for a magnetic single-domain, we suggest that macroscopic effects (shape, size and morphology of the micronic agglomerates of NPs) are the responsible for the different value of the coercive field, due
to the major role that magnetic shape anisotropy plays in these systems. 4. Summary and conclusions Pure Nickel nanoparticles have been synthesized by a chemical reaction process at low temperature (140 C) with varying the concentration of sodium hydroxide between 5 and 25 mol/L. The Ni NPs exhibit fcc crystal structure with a lattice parameter almost identical than that of pure Ni. The average crystallite size is around 20 nm for all the samples excluding that with the highest NaOH
Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015
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Table 2 Estimated values of the room temperature saturation magnetization, MS, and coercive field, HC, of Ni-NPs samples. Sample
NaOH (mol/L)
Coercive field, HC (Oe)
Saturation magnetization MS (emu/g)
Ni-5 Ni-10 Ni-15 Ni-25
5 10 15 25
135 137 129 179
52 52 50 51
(3) (3) (3) (3)
(1) (1) (1) (1)
agglomerates that modify the magnetic shape anisotropy and consequently both the value of the coercive field and the low-field permeability. Acknowledgements We thank SCT's of the University of Oviedo for the assistance in the XRD and magnetometry measurements. The authors from Spain acknowledge financial support from Spanish MINECO (research project MAT2011-27573-C04-02). References
Fig. 4. (a) Room temperature hysteresis loops for Ni-NPs samples. (b) Zoom of the low applied magnetic field region.
concentration (25 mol/L), which exceeds 30 nm. The morphology of the Ni powders at the micrometre length scale evolves form flowerlike to spherical aggregates and then to a dendritic microstructure. These findings indicate that the NaOH concentration affects not only the shape and morphology of the micronic entities, but also plays an important role in the growth of the Ni nanoparticles that are the fundamental blocks forming the micronic entities by agglomeration. Therefore, the NaOH concentration as well as the type of the precursor has to be carefully chosen in order to control the shape and morphology of the powders together with the NP average size. Lastly, the samples exhibit a ferromagnetic behaviour, with saturation magnetization values close to that of pure Ni and a coercive field that is related with the morphology of the NP
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Please cite this article in press as: A. Bouremana, et al., Microstructure, morphology and magnetic properties of Ni nanoparticles synthesized by hydrothermal method, Materials Chemistry and Physics (2015), http://dx.doi.org/10.1016/j.matchemphys.2015.05.015