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Journal ArticleDOI

Strategies to tailor the architecture of dual Ag/Fe-oxide nano-heterocrystals—interfacial and morphology effects on the magnetic behavior

29 Jun 2018-Journal of Physics D (IOP Publishing)-Vol. 51, Iss: 29, pp 295303

AbstractBifunctional nanostructured architectures have shown appealing properties, since a single entity can combine the diverse properties of its individual constituents. Particularly, by growing Fe-oxide domains over Ag nanoparticles, the plasmonic and superparamagnetic properties can be combined in a single particle. Beyond the multifunctionality of this system, there are several properties that emerge from intrinsic factors, such as: interface and/or morphology. In this study, we present the synthesis protocols to obtain two sets of heterocrystals, each one with different morphology: dimer and flower-like. In addition, the magnetization behavior of these hybrid nano-heterocrystals is investigated and discussed. These nanomaterials were built by a seed assisted heterogeneous nucleation process, carried out in organic solvents of high boiling point, using the same batch of silver nanoparticles with a mean size of 6 nm as seeds, and tuning the electron-donor capacity of the reaction environment at the thermal decomposition of the iron precursor. Ag/Fe3O4 heterocrystals with dimer and flower-like morphologies were obtained. The synthesis protocols for generating these types of nanomaterials are discussed step-by-step. Structural and morphological properties were determined by transmission electron microscopy, x-ray diffraction and x-ray absorption fine structure. DC magnetization results suggest that the silver/magnetite coupling generates an increase of the blocking temperature in comparison to those obtained from pure magnetite. This behavior could be linked to a possible increase in the magnetic anisotropy produced by an additional disorder at the Ag–Fe3O4 interface. The higher interface area of the Ag/Fe3O4 heterocrystals with flower-like architecture leads to a higher blocking temperature and a stronger magnetic anisotropy. These results are supported by AC susceptibility data.

Topics: Superparamagnetism (56%), Magnetic anisotropy (55%), Magnetization (53%), Nanomaterials (51%), Nucleation (51%)

Summary (2 min read)

Introduction

  • In the last years, a prominent part of the research in nanochemistry has focused on the preparation of multifunctional and heterostructured nanocrystals, capable of combining two or more inorganic materials into a single system through a direct atomic bond at the interface and without the presence of a molecular bridge [1-3].
  • In these peculiar nanostructures, the optical properties of metallic nanoparticles and the magnetic properties of Feoxide counterparts can be coupled.
  • The most common process to produce heterostructured nanocrystals is the so-called seed-assisted method, where a second component is heterogeneously nucleated over previously formed nanoparticles (“seeds”) of a first component.
  • Many efforts have been dedicated to the understanding of the synthesis conditions and mechanisms that lead to these different morphologies [2, 11-13], mainly because the macroscopic properties of the system can be controlled or tuned by the interface type and the interactions between the different constituents.
  • A discussion on the structural and optical properties is also presented to reinforce the results and the conclusions.

Experimental Details

  • All chemicals were used as received without further purification.
  • Finally, the system was cooled down to room temperature and the AgNPs were precipitated by the addition of ethanol and isolated by centrifugation at 3000 RPM for 15 minutes.
  • The isolated solid was washed 3 times before redispersed in hexane.
  • AgNPs were used as fixed seeds for Fe3O4 heterogeneous nucleation, also known as Ag/Fe3O4 heteroparticles formation.

Characterization

  • Phase composition and crystallographic characterization were carried out via X-ray diffraction (XRD) on a standard Rigaku diffractometer with Cu K-α radiation.
  • These measurements were performed at the Brazilian Synchrotron Light Laboratory (CNPEM/LNLS, Campinas, Brazil).
  • Absorption spectra, carried out on colloidal samples (nanoparticles dispersed in toluene), were recorded at room temperature by means of a UV-VisNIR Shimadzu UV-1603 using quartz cuvettes with 10 mm optical path length.
  • DC magnetization and AC magnetic susceptibility measurements were recorded with a MPMS-XL7 Quantum Design SQUID magnetometer and a PPMS Quantum Design magnetometer, respectively.
  • This mixture was stirred until the toluene was evaporated.

Results and Discussion

  • The two-step procedure employed for the Ag/Fe3O4 heteroparticles synthesis can be conveniently used to study the reaction mechanism and the conditions that lead to the formation of different heterostructure morphologies.
  • Some authors also suggest that Fe3O4 growth over the Ag seed does not always occur epitaxially, leading to nearly independent surface plasmonic properties of the metallic nanoparticles [19].
  • Furthermore, by comparing the obtained results from the Ag/Fe3O4 with those one from the Fe3O4 control sample, one can affirm that the environment of Fe-atoms located in the heterostructures are disposed to form an iron oxide with a stoichiometric Fe3O4 phase.

Conclusions

  • Controlled synthesis protocols were employed to obtain magnetic-plasmonic nanoheterostructures of Ag/Fe3O4 with two distinct morphologies.
  • In the flower-like sample, the dipolar interactions between the Fe-oxide nanoparticles are stronger than in dimer-like counterpart.
  • The maximum of the zero-field-cooling curve was shifted around 90 K and 170 K for dimer and flower-like samples, respectively.
  • Brazilian Nanotechnology National Laboratory (LNNano/CNPEM) is acknowledged for the use of TEM JEOL JEM 2100 (21789) facilities.

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Journal of Physics D: Applied Physics
ACCEPTED MANUSCRIPT
Strategies to tailor the architecture of dual Ag/Fe-oxide nano-
heterocrystals - Interfacial and morphology effects on the magnetic
behavior
To cite this article before publication: Pablo Tancredi et al 2018 J. Phys. D: Appl. Phys. in press https://doi.org/10.1088/1361-6463/aaccc3
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Strategies to tailor the architecture of dual Ag/Fe-oxide nano-heterocrystals - Interfacial
and morphology effects on the magnetic behavior
P. Tancredi
1
, O. Moscoso Londoño
2,3
*, P.C. Rivas Rojas
1
, U. Wolff
4
, L. M. Socolovsky
5
, M.
Knobel
2
, and D. Muraca
2
1
Laboratorio de Sólidos Amorfos, Instituto de Tecnologías y Ciencias de la Ingeniería “Hilario
Fernández Long”, Facultad de Ingeniería, Universidad de Buenos Aires CONICET, Buenos
Aires, Argentina
2
Laboratório de Materiais e Baixas Temperaturas, Instituto de Fïsica ‘Gleb Wataghin’,
Universidade Estadual de Campinas, Campinas, Brazil
3
Facultad de Ingeniería, Universidad Autónoma de Manizales, Antigua Estación del Ferrocarril,
Manizales, Colombia.
4
IFW Dresden, Leibniz Institute for Solid State and Materials Research, Dresden,
Helmholtzstrasse 20, 01069 Dresden, Germany
5
Facultad Regional Santa Cruz, Universidad Tecnológica Nacional - CIT Santa Cruz
(CONICET), Río Gallegos, Argentina
*Corresponding author: omoscoso@ifi.unicamp.br, oscar.moscosol@autonoma.edu.co
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Abstract
Bifunctional nanostructured architectures have shown appealing properties, since a single entity
can combine the diverse properties of its individual constituents. Particularly, by growing Fe-
oxide domains over Ag nanoparticles, the plasmonic and superparamagnetic properties can be
combined in a single particle. Beyond the multifunctionality of this system, there are several
properties that emerge from intrinsic factors, such as: interface and/or morphology. In this study,
we present the synthesis protocols to obtain two sets of heterocrystals, each one with different
morphology: dimer and flower-like. In addition, the magnetization behavior of these hybrid
nano-heterocrystals is investigated and discussed. These nanomaterials were built by a seed
assisted heterogeneous nucleation process, carried out in organic solvents of high boiling point,
using the same batch of silver nanoparticles with a mean size of 6 nm as seeds, and tuning the
electron-donor capacity of the reaction environment at the thermal decomposition of the iron
precursor. Ag/Fe
3
O
4
heterocrystals with dimer and flower-like morphologies were obtained. The
synthesis protocols for generating these types of nanomaterials are discussed step-by-step.
Structural and morphological properties were determined by transmission electron microscopy
(TEM), X-ray diffraction (XRD) and X-ray absorption fine structure (XAFS). DC magnetization
results suggest that the silver/magnetite coupling generates an increase of the blocking
temperature in comparison to those obtained from pure magnetite. This behavior could be linked
to a possible increase in the magnetic anisotropy produced by an additional disorder at the Ag-
Fe
3
O
4
interface. The higher interface area of the Ag/Fe
3
O
4
heterocrystals with flower-like
architecture leads to a higher blocking temperature and a stronger magnetic anisotropy. These
results are supported by AC susceptibility data.
Keywords: hybrid nanostructures, interface effects, magnetic-plasmonic properties,
superparamagnetism.
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Introduction
In the last years, a prominent part of the research in nanochemistry has focused on the
preparation of multifunctional and heterostructured nanocrystals, capable of combining two or
more inorganic materials into a single system through a direct atomic bond at the interface and
without the presence of a molecular bridge [1-3]. In these new heterocrystals, the properties of
the different components can be combined or can give rise to new features associated to
morphology or interface effects, factors that may partially govern the physicochemical behavior
of these new kind of systems.
Nanoparticles that combine a noble metal (Au, Ag) with ferrite type oxides (Fe
3
O
4
, CoFe
2
O
4
,
Co
3
O
4
, MnFe
2
O
4
) are among the systems that have gained major interest. In these peculiar
nanostructures, the optical properties of metallic nanoparticles and the magnetic properties of Fe-
oxide counterparts can be coupled. This dual behavior opens the door to new applications in
several fields, including catalysis [4, 5], antimicrobial agents [6, 7], magnetic and
photo/magnetic hyperthermia therapies [8, 9] and surface-enhanced Raman scattering substrates
(SERS) [10], among others.
The most common process to produce heterostructured nanocrystals is the so-called seed-assisted
method, where a second component is heterogeneously nucleated over previously formed
nanoparticles (“seeds”) of a first component. In this process, the heterogeneous nucleation of the
second material is favored over homogeneous nucleation, especially when there is some
compatibility between the cell parameters of the crystals [10]. Depending on the nucleation
events and the growth evolution, the formed heterostructures can show distinct architectures,
such as core-shell, dimer or flower-like. Many efforts have been dedicated to the understanding
of the synthesis conditions and mechanisms that lead to these different morphologies [2, 11-13],
mainly because the macroscopic properties of the system can be controlled or tuned by the
interface type and the interactions between the different constituents.
In this work, we study the synthesis and the magnetic response of two Ag/Fe
3
O
4
heterostructures
with different morphologies: dimer and flower-like type, prepared by a seed-assisted route in
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organic solvents of high boiling point. In particular, we investigate how the solvent nature,
associated to its electron-donor capacity, determines the final morphology. Correspondingly, we
performed a complete magnetic characterization by means of DC and AC magnetization
techniques to understand how the heterostructures architecture and the distribution of the Fe
3
O
4
domains define the macroscopic behavior of the samples. A discussion on the structural and
optical properties is also presented to reinforce the results and the conclusions.
Experimental Details
Materials. Silver nitrate (AgNO
3
, 99%, Sigma-Aldrich), sodium acetate (NaCH
3
COO, 99%,
Anedra), oleylamine (C
17
H
31
-CH
2
NH
2
, 99%, Sigma-Aldrich), oleic acid (C
17
H
31
-COOH, 90%,
Sigma-Aldrich), 1-octadecene (C
18
H
36
, 90%, Sigma-Aldrich), diphenyl ether ((C
6
H
5
)
2
O, 98%,
Sigma-Aldrich), iron(III) acetylacetonate (Fe(acac)
3
, 99%, Sigma-Aldrich) and 1,2-
hexadecanediol (C
16
H
34
O
2
, 90%, Sigma-Aldrich). All chemicals were used as received without
further purification.
Methods. Ag/Fe
3
O
4
nanoparticles were synthesized by a seed assisted heterogeneous nucleation
process in organic solvents of high boiling point, as described below.
Silver Seeds Nanoparticles (AgNPs) synthesis: AgNPs were prepared according to previous
reports [8, 12]. Briefly, 2 mmol silver acetate (AgCH
3
COO) were obtained by stochiometric
precipitation of AgNO
3
and NaCH
3
COO. The so-obtained white powder was dissolved in 2
mmol oleic acid, 2 mmol oleylamine and 5 mL 1-octadecene. The mixture was heated up to 175
°C under continuous stirring for 30 min to allow AgNPs formation. Finally, the system was
cooled down to room temperature and the AgNPs were precipitated by the addition of ethanol
and isolated by centrifugation at 3000 RPM for 15 minutes. The isolated solid was washed 3
times before redispersed in hexane.
Ag/Fe
3
O
4
heteroparticles formation: AgNPs were used as fixed seeds for Fe
3
O
4
heterogeneous
nucleation. 1 mmol Ag (as AgNPs), 0.3 mmol Fe(acac)
3
, 1.5 mmol 1,2-hexadecanediol, 0.75
mmol oleic acid, 1.5 mmol oleylamine and 5 mL solvent (1-octadecene, ODE or phenyl ether,
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Citations
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Abstract: Effect of Ag inclusions on magnetic properties and magnetic circular dichroism (MCD) of Fe3O4 nanoparticles (NPs) in the mixed system of Fe3O4 and Ag NPs in dependence on the relative concentration of the components is presented. The samples were synthesized by the thermal decomposition of the mixture of constant concentration of Fe(NO3)3·9H2O and varied concentration of AgNO3. The synthesized powdered samples consisted of Fe3O4 and Ag NPs located very close with each other, and in the most cases the Fe3O4 NPs were bordered with the Ag nanocrystals. The Ag introducing in the samples does not effect, practically, in the Fe3O4 NPs morphology and size distribution. At the same time, Ag NPs in the powdered samples cause a decrease in the Fe3O4 NPs magnetization and shift of the blocking temperature to lower temperatures, both approximately proportional to the Ag concentration. Most significant changes are revealed in the MCD spectra in the energy region of 1.2–2.2 eV. We have discussed the influence of the Ag NPs on the MCD spectra features in terms of the charge-transfer electron transitions.

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TL;DR: An oleate-based solvothermal approach to design silver-spinel ferrite flower-like NHs with magnetic and optical properties that are suitable for a wide range of applications is proposed.
Abstract: Artificial nano-heterostructures (NHs) with controlled morphology, obtained by combining two or more components in several possible architectures, make them suitable for a wide range of applications. Here, we propose an oleate-based solvothermal approach to design silver-spinel ferrite flower-like NHs. Small oleate-coated silver nanoparticles were used as seeds for the growth of magnetic spinel ferrite (cobalt ferrite and spinel iron oxide) nanodomains on their surface. With the aim of producing homogeneous flower-like heterostructures, a careful study of the effect of the concentration of precursors, the reaction temperature, the presence of water, and the chemical nature of the spinel ferrite was carried out. The magnetic and optical properties of the NHs were also investigated. A heterogeneous growth of the spinel ferrite phase on the silver nanoparticles, through a possible defect-assisted mechanism, was suggested in the light of the high concentration of stacking faults (intrinsic and twins) in the silver seeds, revealed by Rietveld refinement of powder X-ray diffraction patterns and High-Resolution electron microscopy.

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Abstract: Magnetoplasmonics nanoparticles encompass in a single nano-entity all the rich science and promising applications of the plasmonics and magnetic nanoworlds. The difficult liaison and a certain incompatibility between plasmonics and magnetic phenomena, due to the different chemical-physical origins and supporting materials, are overcome thanks to the design and synthesis of novel nanostructures. The variations of properties, interactions and synergies of both phenomena and materials demonstrate how rich and surprising the matter is at nanoscale and the promising applications. In fact, we show how not only light and magnetism can interplay but also other phenomena like forces, heat, electric field and chemical interactions, between others, can show synergism. Magnetoplasmonic systems are excellent benchmark materials to develop and investigate multi-responsive multifunctional nanosystems that now are required in an increasing number of technologies, such as biomedicine, pharmacy, catalysis, optoelectronics and data storage.

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Abstract: Multifunctional nano-heterostructures (NHSs) with controlled morphology are cardinal in many applications, but the understanding of the nanoscale colloidal chemistry is yet to be fulfilled. The stability of the involved crystalline phases in different solvents at mid- and high-temperatures and reaction kinetics considerably affect the nucleation and growth of the materials and their final architecture. The formation mechanism of manganese ferrite-based core–shell NHSs is herein investigated. The effects of the core size (8, 10, and 11 nm), the shell nature (cobalt ferrite and spinel iron oxide) and the polarity of the solvent (toluene and octanol) on the dissolution phenomena of manganese ferrite are also studied. Noteworthily, the combined use of bulk (powder X-ray diffraction, 57Fe Mossbauer spectroscopy, and DC magnetometry) and nanoscale techniques (HRTEM and STEM-EDX) provides new insights into the manganese ferrite dissolution phenomena, the colloidal stability in an organic environment, and the critical size below which dissolution is complete. Moreover, the dissolved manganese and iron ions react further, leading to an inverted core–shell in the mother liquor solution, paving the way to novel synthetic pathways in nanocrystal design. The MnFe2O4@CoFe2O4 core–shell heterostructures were also employed as heat mediators, exploiting the magnetic coupling between a hard (CoFe2O4) and a soft phase (MnFe2O4).

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Abstract: This paper reviews the recent research progress in syntheses and applications of dumbbell-like nanoparticles. It first describes the general synthesis of dumbbell-like nanoparticles containing noble metal and magnetic NPs/or quantum dots. It then outlines the interesting optical and magnetic properties found in these dumbbell nanoparticles. The review further highlights several exciting application potentials of these nanoparticles in catalysis and biomedicine. Keywords: Dumbbell nanoparticles, multifunctional nanoparticles, nanoparticle catalyst, nanomedicine 1. Introduction Dumbbell-like nanoparticles (DBNPs) described in this paper are referred to as those with two different functional NPs in intimate contact, as shown in Figure 1. The interfacial interactions originated from electron transfer across the nanometer contact at the interface of these two NPs can induce new property that is not present in the individual component NPs. For example, gold (Au) NPs are normally chemically inert but Au NPs deposited on a metal-oxide support, a structure similar to what is described in Figure 1, have shown high catalytic activity for CO oxidation.[1] In these Au-oxide composite catalyst systems, the choice of the oxide support plays an important role in the catalysis even thought the oxide is not in nanometer scale. This high activity of Au-oxide catalysts has been rationalized in terms of a junction effect, arising from transfer of electrons from the oxide support to the Fermi level of the adjacent Au NP.[2] Figure 1 Schematic illustration of a dumbbell-like nanoparticle sturcture described in this reveiw. With the example shown in the enhanced catalysis of the Au-oxide catalyst, the importance of studying the dumbbell structure is evident because each side of the NP in the structure is restricted to the nanometer scale and a small variation in electron transfer across the interface between these two limited electron “nanoreservoirs” may lead to drastic property change on each NP. Therefore, DBNPs offer an interesting platform for studying physical and chemical properties of the materials based not only on each NP dimension and shape but also on the communication between the two NPs. Furthermore, magnetically and optically active DBNPs containing two different chemical surfaces are particularly suitable for the selected NP functionalization with both targeting agent and drug molecules, facilitating their applications as multifunctional probes for target-specific imaging and delivery applications.[3] This report summarizes the recent research progress in preparing and studying DBNPs with interesting optical and magnetic properties. In particular the report focuses on the DBNPs containing noble metal NPs and magnetic NPs or quantum dots (QDs). The nanoscale junctions present in these structures allow electron transfer across the interface, changing the local electronic structure and therefore their physical and chemical properties. These DBNPs offer an ideal nano-system for catalytic and biomedical applications.

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Frequently Asked Questions (1)
Q1. What contributions have the authors mentioned in the paper "Strategies to tailor the architecture of dual ag/fe-oxide nano- heterocrystals - interfacial and morphology effects on the magnetic behavior" ?

Particularly, by growing Feoxide domains over Ag nanoparticles, the plasmonic and superparamagnetic properties can be combined in a single particle. In this study, the authors present the synthesis protocols to obtain two sets of heterocrystals, each one with different morphology: dimer and flower-like. These nanomaterials were built by a seed assisted heterogeneous nucleation process, carried out in organic solvents of high boiling point, using the same batch of silver nanoparticles with a mean size of 6 nm as seeds, and tuning the electron-donor capacity of the reaction environment at the thermal decomposition of the iron precursor. The synthesis protocols for generating these types of nanomaterials are discussed step-by-step. DC magnetization results suggest that the silver/magnetite coupling generates an increase of the blocking temperature in comparison to those obtained from pure magnetite.