Magneto-Optical Modulation on Colloid Cu–Ni Nanocomposite

Article pubs.acs.org/JPCC

Magneto-Optical Modulation on Colloid Cu−Ni Nanocomposite Alexandr V. Vinogradov,*,†,‡ A. A. Levshanov,‡ M. A. Kashirin,§ A. V. Agafonov,‡ and Vladimir V. Vinogradov†,‡ †

ITMO University, St. Petersburg 197101, Russian Federation G.A. Krestov Institute of Solution Chemistry, Russian Academy of Sciences, Ivanovo 153045, Russian Federation § Voronezh State Technical University, Voronezh 394026, Russian Federation ‡

S Supporting Information *

ABSTRACT: In the present work, we report, for the first time, the synthesis of a Cu−Ni-nanoalloy-based composite demonstrating remarkable magnetic activity as compared to that of the Ni nanoparticles. Apart from its possessing 3 times as much magnetic permeability as Ni powder, we have discovered unique properties exhibited by the product nanoparticles in the colloidal state in aqueous solutions. By applying an external magnetic field, a three-dimensional optical modulation with a transmission coefficient of 20−90% was observed.

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INTRODUCTION Using colloidal chemistry for adjusting the growth of crystalline particles from solutions is one of the top areas of developing materials science predicted in the near future.1−7 This is due to the fact that employing traditional physical techniques for the formation of nanosized particles usually requires “rigid” setting the synthesis procedure without providing the ability to control and change the morphology of the produced particles in a wide range.8−10 Meanwhile, it is well-known that for nanoscale objects the role of structure can sometimes be superior to that of chemical composition, thus determining the final properties of the obtained product.11 Materials that exhibit several unique properties within a single system are of special importance.12 For instance, the combination of plasmonic, fluorescence, and magnetic properties within a single system has become a whole research area being motivated mainly by biomedical multitasking applications, such as sensing and manipulation,13,14 but also motivated from a fundamental point of view for investigating fortification or weakening effects. As a result, researchers come across exhibition of combination effects serving as an example of synergy by applying several external parameters. The most relevant example is a nanoalloy based on Ni−Cu compounds with different stoichiometry.8,9,15 In general, there are three main alloy classes of this type: (1) segregated alloy of core−shell type, (2) subcluster alloy, and (3) mixed Cu−Ni alloy. The group of D. Avnir16 has pioneered the studies on chirality of these compounds, first systematizing the general principles of constructing the structure of alloys of this type. The main problem in the interpretation of the properties of such materials is the determination of quantitative phase composition, which, depending on the applied methods of analysis, may be significantly different from reality. A common feature of all of the Cu−Ni systems is increased © XXXX American Chemical Society

mechanical durability as compared to the initial pure substances, as well as remarkable plasmonic properties of colloidal particles adjusted by an external magnetic field.8 In general, magneto-optic effects studied for materials of this type were usually attributed to a direct dependence on the content of the original components and their stoichiometric ratio. Thus, it is assumed that the Cu content of more than 60 wt % results in loss of ferromagnetic properties in a Cu−Ni nanoalloy, acquiring diamagnetic17 or paramagnetic17 state. Taking into account the temperature dependence, the critical content for the ferromagnetic properties should be provided by the alloys Cu0,7Ni0,3.18 The sole factors responsible for nonlinear changes in the properties of Cu−Ni alloys, providing a huge impact on the total magnetic response of the system, are the structural parameters, such as the size and shape of the formed particles. In particular, in ref 11, it was shown that a change in magnetic properties of Ni nanoparticles may vary several times upon transition from nano to micrometer conditions. At the same time, the presence of the atomic shell covering the Cu−Ni nanoalloy core contributed to exhibiting opposite properties according to the scheme diamagnet−antiferromagnet−ferromagnet. This structural feature is essential for forming peapodlike structures of ferromagnetic particles in aqueous solutions. For instance, in their explanation of the mechanism of interaction of spherical particles in solution from a position of dipole−dipole interaction, the authors23 have shown that the key point of this self-organization is the presence of a nonmagnetic layer on the surface of ferroparticles determining the “flexibility” of chains in solution. Similar magneto-optic Received: November 24, 2014 Revised: December 23, 2014

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composites. Visualization of the formation of the composite with Cu and Ni segregation areas is shown in Figure 1A. It is well-known that the aggregation of metal nanoparticles (with a diameter of up to 30 nm) is inevitably associated with the presence of interplay factors, according to the DLOV theory. The absence of stabilizers and strong protonating agents in our case promoted the dominance of van der Waals interactions, resulting in self-assembly of the nanoparticles into large spherical micrometer formations, Figure 2A,B. Assembly

effects were also found in other systems, in particular, the Fe3O4@SiO2 composite, which implied the use of sequential accumulation and stabilization technique to produce elongated particles.19 However, the presence of a core−shell structure is not the only example that illustrates such behavior of the particles. Thus, we assume that the supramolecular selfassembly of magnetic particles into thread-like formations can occur in structures with nonmagnetic areas connected to the ferromagnetic phase. Moreover, the ways to self-assemble the magnetic nanoparticles in colloidal solutions promoting exhibition of synergistic activity particularly useful for highspeed optical modulation of light by an external magnetic field have not been determined yet.

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RESULTS AND DISCUSSION In this work, we have used a new single-stage method of producing Cu−Ni alloy nanocomposite, Figure 1A, which self-

Figure 1. Visualization of (A) facile formation of nanoscale Cu−Ni particles and (B) self-assembly in water media.

Figure 2. (A,B) TEM and (C,D) HRTEM images of as-prepared Cu− Ni nanocomposite. Light-blue lines stand for copper-rich particle areas, while yellow lines stand for nickel-rich areas.

assembles into elongated thread-like structures, Figure 1B, exhibiting more than 3 times as much magnetic permeability as nickel nanoparticles, Supporting Information Table S1. Formation of Cu−Ni alloy in solutions by reduction of metal ions is due to similar values of the lattice parameters for Cu and Ni (3.62 and 3.54 Å, respectively), as well as small positive enthalpies of solution for Cu in Ni and Ni in Cu.8 The mechanism of polyol formation of copper and nickel nanoparticles was excellently described in ref 26, providing the possibility of obtaining monodisperse spherical microagglomerates using concentration factors. Producing such structural arrangement is possible at low concentrations of NaOH by stabilizing metal centers with glycolate structures possessing bidentate nature of complexes with acetylacetone. Decreasing the growth of crystal nuclei of metals results in an increase in the chain length of the polyol,26 which may lead to stabilization of the particles in microrange. Surprisingly, the formation of microsized clusters with narrow size distribution will only occur for nanosize conditions of the individual particles. However, in most cases, the formation of Cu−Ni alloys using solution methods does not prevent segregation of phases due to bulk diffusion of the components and their intermediates. The amount of formed intermediate products, such as glycols, oxides, glycolates, and their complexes, slows the diffusion of nickel atoms into the copper atoms and vice versa, producing areas with varying composition,8 Figure 1. This leads to a change in functional properties of the synthesized

of particles with narrow size distribution in the range of about 100 nm apparently occurs as a result of residual magnetization, leading to directional growth of elongated thread-like structures, Figures 1B and 2A,B. This is mainly due to a huge difference in the magnitudes of magnetic forces of particle interaction and intermolecular bonds.24 Controlling the formation of Cu−Ni alloy particles was provided by temperature conditions and slow reduction in ethylene glycol in the absence of modifiers. This approach was selected with the aim to demonstrate the effect of the segregation of the ferromagnetic phase on the formation of thread-like clusters in aqueous solutions, Figure 1A, with a high magnetic response. According to TEM and HRTEM, Figure 2, microsized (a diameter of about 100 nm) particles with narrow size distribution self-assembled into chain-like structures were produced. A detailed analysis, Figure 2C,D, clearly reveals the nanoscale nature of crystalline formations, with a size of about 5 nm, which the Cu−Ni nanoalloy composite consists of. A high-resolution TEM (HRTEM) reveals an fcc structure imaged along the [111] zone-axis, corresponding to alloy formation. The observed reflections correspond to (110) lattice planes in the fcc structure. The d values for (110) are calculated to be 2.51, 2.58, and 2.58 Å from three different orientations, which correspond to d (111) of 2.05, 2.09, and 2.09 Å, respectively. The d value for Cu(111) is 2.087 Å (ICSD file 41508), and the d value for B

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Figure 3. EDS mapping of as-synthesized Ni−Cu composite illustrating the segregation of a nickel-rich area.

Figure 4. (A) Optical microscope images and light modulations caused by vertical rotation of magnetic field. (B) Transmittance change kinetics for a colloidal solution of the Cu−Ni nanoparticles under the influence of horizontal rotation of magnetic field.

Ni(111) is 2.034 Å (ICSD file 52265). The precision with which the (110) d values can be measured allows one to determine the presence of individual areas of Cu, Ni, or the alloy. Moreover, the EDS image mapping data clearly reveal the segregation of nickel areas, indicating polyphase composition of the material at the nanolevel, Figure 3. At the same time, using powder X-ray diffraction data, Supporting Information Figure S1, we have determined the composition of the synthesized powder to be the alloy Cu3Ni,20,21 which is consistent with the stoichiometric composition calculated using energy-dispersive analysis spectra. However, given the dimensional nature of the synthesized particles and surface segregation of copper−nickel areas, we classify the composition of the synthesized materials as a Cu−Ni nanoalloy composite with the above composition. The EDS mapping of as-prepared sample detected only Ni and Cu atoms, Figure 3. The compositions of the Cu−Ni nanoparticles were coincident with the molar ratios [Cu2+]: [Ni2+] used for the synthesis. This stoichiometric ratio was selected to demonstrate the extraordinary properties of the magneto-optical composite colloidal particles, Figure 4, because in the classical interpretation of the Cu−Ni alloy magnetism for such stoichiometry of a material only paramagnetic properties25 can be detected using diffraction data. The detection of ferromagnetic properties at a higher copper content is presumably due to the presence of nickel clusters segregation

areas,9 which can be detected using HRTEM, Figure 2D, and EDS analysis, Figure 3. Because self-assembled clusters are nanocrystalline formations exhibiting high ferromagnetic activity, in the absence of an external magnetic field thread-like self-assembled particles can partially disperse, Figure 1B-3. This behavior of the peapod-like structures can be more clearly observed through dark-field optical microscopy by examining a thin film of peapod solution in water cover glass, Figure 4A (in contrast to Ni particles, Supporting Information Figure S3). As seen in Figure 4, a conversion of the light beam in several directions is observed. In the absence of an external magnetic field, peapod-like structures are well dispersed in water, showing various chain lengths due to their random orientations. When a magnetic field is horizontally applied, transmittance of the colloidal solution from 23% to 87% is observed, Figure 4B, and vertical rotation provides polarization of the monochrome beam, Figures 4A and 5B, with the light polarization degree of up to 80%. In this Article, we have demonstrated, for the first time, the use of colloidal solution for a 3D manipulation of light. Not only is this effect new to this kind of well-studied systems, but also in many ways is a key approach to a threedimensional manipulation of the light beam. Transmittance change kinetics shown for three time ranges reveal a high C

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this is high magnetic permeability of the synthesized nanoalloy particles as compared to that of Ni nanoparticles, Figure 6. Magnetic permeability (derivative graph) at room temperature of both samples (Cu−Ni alloy composite and Ni powders, Supporting Information Figure S1) has a maximum at 200 Oe, whereas in the alloy the value of μ is 2−3 times higher, Figure 6b. An increase in copper content in the alloy leads to an increase in the atomic disorder, so we have an increase in coercive force. At the same time, copper atoms reduce magnetocrystalline anisotropy, and we observe an increase in residual magnetization. Given that specific magnetization of saturating composite with respect to Ni powder increases, the susceptibility of the alloy appears to be higher. Increasing magnetic field strength decreases permeability, and upon reaching 600 Oe permeability values are identical. The as-synthesized Cu−Ni composite reaches saturation in magnetic fields of ∼1000 Oe, whereas the Ni powder sample gets saturated when an external magnetic field strength is over 2000 Oe. These data are consistent with the structure of the produced associates, because high arrangement of magnetic areas promotes an increase in permeability under the influence of an external magnetic field. With narrow size distribution of nanocrystalline formations, microsized particles assembled into thread-like structures interact with an external magnetic field step by step, providing a synergistic effect. Thus, the first stage apparently involves superparamagnetic orientation of nanoparticles by singledomain arrangement, resulting in a spatial assembly of microparticles, which in turn leads to their spontaneous magnetization at the second stage, providing ferromagnetic properties for the composite, Figure 6a. Collective response of the triple composite to an external magnetic field in the solution is evidence of a sufficiently strong magnetostatic interaction. Its estimation may be based on the model of dipole−dipole interaction in the absence of an external magnetic field,22,23 Figure 6c. Mathematical calculation yields a maximum interaction energy of 20 kT for the composite and one-half of this value for nickel powder, given the formation of clusters of up to 20 nm. In the latter case, arranged chains were not observed in the solution in the absence of a magnetic field. In fact, once a particle is in the vicinity of the other particles and their moments can interact, they participate in the formation of a chain. Thus, in contrast to paramagnetic particles, one requires very low field strength to start the process of forming a chain. In this study, a relatively weak magnetic field of 10 G was

Figure 5. Polarization of a monochrome light beam in a colloidal solution of Cu−Ni alloy composite: (A) without an external magnetic field, and (B) under the influence of an external magnetic field.

degree of particle relaxation while shifting the magnet to extreme angular positions. The presence of an external elliptical polarized light field along each particle organizing elongated ensembles results in an intensification of this effect by imposing an external polarized cloud along the distribution vector. Because the formation of these chains is determined by the energy of the dipole−dipole interaction, we can definitely say that the degree of polarization of the light beam is determined by two factors: (1) the magnitude of the structural organization that corresponds to the scattering direction, and (2) the magnetic interactions responsible for the degree of polarization of the light wave. Given that the synthesized composite corresponds to the maximum value of each factor, we may declare a synergistic increase in the magneto-optical anisotropy of light for the synthesized composite. Thus, by comparing the obtained data with the applied magnetic field, Figure 5B, and without it, Figure 5A, the degree of polarization with respect to the diameter ratio d1/d2 in the horizontal plane amounted to 80%. In particular, the results obtained significantly outperform those of magnetic-field optical modulation for such systems as Fe3O4@SiO2, where the transmission/absorption maxima ratio was 3.5 times lower than in the present case. The reason for

Figure 6. (a) Magnetization and (b) magnetic susceptibility versus applied field for Cu−Ni nanocomposite in contrast to Ni powder. (c) Dependence of magnetic component of interaction energy on the relative position of magnetic dipoles of particles. D

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applied to initiate formation of chains. Even weaker (2.1 G) field was used for the manipulation of preformed chains. With altering the direction of the field, it is possible to manipulate elongated structures, performing their rotation (aligning the magnetic moment along the field), Figure 4. The effect of scattered light polarization by transmitting photons through a colloidal solution of spherical composite particles is described by the Mi model, suggesting the microsized state. Moving on to organized clusters elongated along one axis and possessing a magnetic moment, as in the present case, we can expect an intensification of these effects along the planes of distribution, Figure 4A, by increasing the polarization of the light wave in a magnetic field. Explaining this phenomenon, which was also observed in other systems,19,23 is possible by assuming the combined impact of scattering photons and direct polarization of the light beam. Obviously, the main light scattering occurs on the surface of the solid-phase particles. The shape of the light beam transmitted through the sample is thus determined by the morphology of a particle or cluster on the surface of which scattering occurs. This effect is now used for dynamic light scattering.

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CONCLUSIONS We have applied a single-stage synthesis for the first time to produce thread-like associates of Cu−Ni nanoalloy composite. It is shown that microsized particles with a diameter of about 100 nm and narrow size distribution consist of nanocrystallites with a size of about 5 nm, which correspond to the composition Cu−Ni alloy with metallic embedments of copper and nickel. Employing methods of spectroscopy and detection of light beam position, a three-dimensional manipulation of light using a colloidal solution of the synthesized sample and an external magnetic field was studied for the first time. It is shown that attaining discovered effects is due to threadlike self-assembly of the particles, with narrow size distribution in nano- and micro-ranges. In contrast to Ni nanoparticles, high arrangement of the particles promoted a synergistic increase in magnetic permeability with identical level of magnetic response.

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ASSOCIATED CONTENT

* Supporting Information S

Experimental details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: vinogradoff[email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Russian Government, Ministry of Education. (Research was made possible due to financing provided to the Customer from the federal budget aimed at maximizing Customer’s competitive advantage among world’s leading educational centers.) We are grateful to the Center for Nanoscience and Nanotechnology at Hebrew University for assistance in performing HRTEM experiments.

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REFERENCES

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