Improvement of Thermal Stability of Maghemite Nanoparticles Coated

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Improvement of Thermal Stability of Maghemite Nanoparticles Coated With Oleic Acid and Oleylamine Molecules: Investigations Under Laser Irradiation Yassine El Mendili, Fabien Grasset, Nirina Randrianantoandro, Nicolas Nerambourg, Jean-Marc Greneche, and Jean-François Bardeau J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b00819 • Publication Date (Web): 21 Apr 2015 Downloaded from http://pubs.acs.org on April 26, 2015

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The Journal of Physical Chemistry

Improvement of Thermal Stability of Maghemite Nanoparticles Coated With Oleic Acid and Oleylamine Molecules: Investigations Under Laser Irradiation Yassine El Mendili1* a), Fabien Grasset2 b), Nirina Randrianantoandro1, Nicolas Nerambourg2, Jean.-Marc Greneche1, Jean-François Bardeau1* 1

LUNAM Université, Institut des Molécules et Matériaux du Mans, UMR CNRS 6283, Université du Maine Avenue Olivier Messiaen, 72085 Le Mans Cedex 9, France

2

Université de Rennes 1, Institut des Sciences Chimiques de Rennes, UMR UR1-CNRS 6226, Campus de Beaulieu, 35042 Rennes Cedex, France a)

Present address: SUBATECH, CNRS-IN2P3, Ecole des Mines de Nantes, Université de Nantes, 4 rue Alfred Kastler, BP 20722, 44307 Nantes Cedex 03, France.

b)

Present address: CNRS – SAINT_GOBAIN, UMI 3629, Laboratory for Innovative Key Materials and Structures–LINK, National Institute of Material Science (NIMS), GREEN/MANA Room 512, 1-1 Namiki, 3050044 Tsukuba, Japan *Corresponding authors: Y. El Mendili: [email protected], J.-F. Bardeau: [email protected]

Abstract We investigated the influence of the coating of maghemite nanoparticles (NPs) with oleic acid and oleylamine molecules on the thermal stability of maghemite and on the γ → α-Fe2O3 phase transformation. The uncoated maghemite NPs were synthetized by co-precipitation and the coated NPs by thermal decomposition of organometallic precursors. The morphology and size of the coated NPs were characterized by transmission electron microscopy, and magnetic and structural properties by 57Fe Mössbauer and Raman spectroscopies. The phase stability of coated maghemite NPs was examined under in situ laser irradiation by Raman spectroscopy. The results indicate that coated γ-Fe2O3 NPs are thermally more stable than the uncoated NPs: the phase transformation of maghemite into hematite was observed at 15 mW for uncoated NPs of 4 nm, whereas it occurs at 120 mW for the coated NPs of similar size. The analysis of the Raman baseline profile reveals clearly that the surface coating of maghemite NPs results both in reducing the number of surface defects of nanoparticles and in delaying this phase transition. Keywords : Maghemite, nanoparticles, laser irradiation, Raman spectroscopy, phase transition

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1. Introduction Magnetic iron oxide nanoparticles (NPs) are of great interest for researchers from a broad range of disciplines, including magnetic fluids, data storage, magnetoresistors, gas sensors, catalysis, and bio-applications. [1–9] They can also be used as heat sources for hyperthermia [12, 13]: indeed, this heat treatment which is rather riskless to the living organisms aims to kill cancer cells by raising the cell temperature, using the heat dissipated from magnetic NPs exposed in an alternating current magnetic field [14,15]. This treatment modality complements currently available treatments including chemotherapy, radiation therapy, surgery, gene therapy and immunotherapy for cancer. In general, magnetic particles generate heat in an external AC magnetic field from several physical mechanisms. They include relaxation losses or hysteresis losses, which strongly depend on the frequency of the external field, as well as the nature of the NPs, such as their size and their surface modifications [16, 17]. In conclusion this form of therapy requires a magnetic material with the controlled size and morphology and with the optimum heat evolution.

Thus, there is a technological challenge to control the thermal and thus the structural stability of NPs. Due to their high surface-to-volume ratio and therefore high surface energies, nanoparticles dispersed into a liquid tend to aggregate so as to minimize the surface energies. Ferrofluids are most commonly dispersions of maghemite (γ-Fe2O3) NPs: indeed, maghemite behaves as a ferrimagnet and plays a significant role among iron oxides. In addition, maghemite NPs have numerous applications like recording, memory devices, magnetic resonance imaging, drug delivery or cell targeting [10, 11].

Under heat treatment and laser irradiation, cubic ferrimagnetic maghemite transforms irreversibly into the rhombohedral antiferromagnetic hematite (α-Fe2O3) [18, 19]. This structural transition is being investigated with the aim of increasing the temperature limit of stability of the maghemite phase and to maintain its magnetic properties, allowing thus to extend the range of applications of γ-Fe2O3. Therefore, it becomes crucial to develop protection strategies to chemically stabilize the magnetic NPs and prevent against aggregation, degradation or phase transition. These


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strategies comprise grafting or coating with organic species, such as surfactants or polymers, or coating with an inorganic layer, such as silica. [20-27] It is noteworthy that in many cases the surface coating not only stabilizes the NPs, but can also be used for further functionalization and thus becomes one of the potential methods for challenging the toxicity of NPs and improving their biocompatibility [28-30]. The use of surfactant molecules, such as oleic acid and oleylamine can easily functionalize iron oxide NPs to be hydrophobic surfaces [31-34]. Indeed, recent studies showed that oleic acid is one of the best candidates for the stabilization of magnetite (Fe3O4). [35, 36] In the present study, maghemite NPs coated with oleic acid and oleylamine surfactants have been synthetized with two different sizes (6 and 15 nm) and characterized by transmission electron microscopy (TEM),

57

Fe Mössbauer and micro-Raman spectroscopies.

The effects of laser irradiation on structural stability of the coated NPs have been studied by using in situ Raman spectroscopy and compared to uncoated maghemite nanoparticles.

2. Experimental 2-1 Synthesis of uncoated γ -Fe2O3 nanoparticles γ-Fe2O3 nanoparticles (NPs) were prepared according to the Massart’s method [37] with the cationic precursors used in the form of metallic salts soluble in the water. The experimental procedure was detailed in our recent studies [18, 19]. Co-precipitation method is a simple route of synthesizing maghemite and other ferrite NPs from ferric and ferrous salts. With the appropriate ratios of the precursor salts, it is possible to obtain narrow size distribution of spherical NPs (Figure S1) [38, 39].

2-2 Elaboration of coated γ-Fe2O3 A stabilized colloidal dispersion of maghemite NPs in cyclohexane was prepared according to a procedure described by Sun et al. [33]. The main idea was to start with Fe(acac)3 compound as a precursor for high-temperature decomposition reaction in presence of phenyl ether, alcohol, oleic acid and oleylamine.



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The synthesis was carried out using standard airless procedures and commercially available reagents without purification. In this procedure, maghemite NPs are synthesized in 20 mL phenyl ether by reduction of iron(III) acetylacetonate using 1,2-hexadecanediol. Prior to the reaction, a surfactant mixture (ratio=1) of pure oleic acid (cis-9-octadecenoic acid) and pure oleylamine (cis-1-amino-9-octadecene) is added to stabilize the NPs against flocculation by adsorbing onto the particle surfaces. The reactant mixture is heated to 265 °C to reflux for 30 min. After cooling down to room temperature, the particles are precipitated to the bottom of the flask both by adding ethanol (20 mL) to the solution and by using a permanent magnet (0.1 T). Subsequently, the supernatant is removed and the particles are again dispersed in 20 mL phenyl ether to repeat the above described synthesis procedure twice. The average particle size is then increased by the seeded growth of the particles prepared in the first step. After the last growth step, the supernatant is removed and the precipitated iron oxide is dissolved into 5 mL cyclohexane. Finally, ferrofluids are washed three times with 5 mL ethanol. In each purification step, the precipitates are dried using a N2 flow and redispersed in 5 mL pure cyclohexane. An idealized scheme of the oleic acid and oleylamine-capped γ-Fe2O3 NPs is displayed in Fig. 1. The length of the full extended molecules in the trans configuration was estimated to be 1.7 nm by geometric modeling.

Fig. 1. Idealized models of maghemite nanoparticles coated with oleic acid and oleylamine.

2-3 Structural analysis


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The size distribution of NPs was inspected by a Transmission Electron Microscope (MET, JEOL 2100) operating at 200 kV. Every sample was prepared according to the following protocol: a very small amount of sample was crushed in an agate mortar containing absolute ethanol. A drop of the suspension is deposited on a copper grid covered with amorphous carbon membrane holes. The grid was dried and then inserted into the TEM.

2-3 Magnetic properties The Mössbauer spectra were recorded at 10, 77K and 300 K, using a standard transmission geometry equipped with a conventional constant acceleration spectrometer and a 57

Co source diffused into a Rh matrix. In-field Mössbauer spectra were obtained by means of

a commercial cryomagnetic device where the 8T external magnetic field is oriented parallel to the γ-radiation on the sample, the source remaining unpolarized. The hyperfine parameters were refined by using MOSFIT, a Lorentzian line-fitting program. [40]

2-4 Raman analysis The Raman spectra were recorded at room temperature in the backscattering configuration on a T64000 Jobin-Yvon (Horiba) spectrometer under a microscope with a 100x objective focusing the 514 nm line from an Argon–Krypton ion laser (coherent, Innova). The spot size of the laser was estimated to 0.8 µm. Measurements using different laser output powers between 2 mW and 200 mW were carried out. Raman spectra were systematically recorded twice with an integration time of 600s.

3. Results and discussion 3.1. TEM Analysis The synthesized particles were well crystallized with a size of approximately 6 and 15 nm. The nanoparticles present well-defined facets and flat surfaces as seen from TEM Micrographs (Figure 2). It can also be noticed that the size distribution in each size group of the maghemite NPs is very narrow (Figure S2, S3). TEM images confirm that the oleic acid/oleylamine coating was efficient to avoid nanoparticle aggregation and the intervals of



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each neighboring NPs is uniform and estimated to 3.3 nm which is in good agreement with the presence of a compact monolayer of extended molecules in the trans configuration on nanoparticle surfaces [31]. .

50 nm

50 nm

Fig. 2. Transmission electron microscope (TEM) micrographs (200 X 200 nm) of faceted maghemite nanoparticles of a) 15 nm and b) 6 nm of size, both coated by oleic acid/oleylamine molecules.

3.2. Mössbauer Analysis Mössbauer spectrometry is a powerful tool to discriminate the different iron species in oxide phases, including magnetite and maghemite. Indeed, although the crystalline structures of these two compounds are close, the magnetite is a mixed valence compound consisting of Fe (II) and Fe (III) having distinct hyperfine parameters. In this crystal structure, Fe (II) ions and half of the Fe (III) ions occupy octahedral sites and the other half of the Fe(III) occupies tetrahedral sites. In the case of maghemite, the iron atoms are in the oxidation state (III) and are distributed in the tetrahedral and octahedral sites. The maghemite structure can be obtained by creating 8/3 vacancies out of the 24 Fe sites in the cubic unit cell of magnetite. These vacancies are known to be located in the octahedral sites [41]. The first objective is therefore to identify the presence of maghemite and/or magnetite in our samples and to quantify their relative proportions. At 300K and 77K, the hyperfine structures (Figure S4 and S5) are fairly consistent with the presence of maghemite only with superparamagnetic relaxation phenomena, in agreement with those usually observed in literature.


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Consequently, we performed in-field low temperature Mössbauer measurements to resolve the hyperfine structure and to estimate accurately the Fe populations located in both sites [42]. Indeed, the application of an external magnetic field parallel to the γ-beam, allows to split the hyperfine structure into two different magnetic contributions attributed to tetrahedral and octahedral Fe species and to estimate accurately their respective proportions. Figure 3 compares the Mössbauer spectra recorded at 10K under an 8T external magnetic field applied parallel to the γ-beam while the refined values of hyperfine data are listed in Table 1. In addition to the splitting into two resolved magnetic sextet originating from the ferrimagnetic structure of maghemite, one observes that the intermediate 2-5 lines does not completely disappear only in the case of the smallest NPs. One should conclude that they exhibit a canted magnetic surface layer particularly the octahedral Fe species while the other two sets of NPs behave as collinear ferrimagnets. Assuming a core-shell magnetic model, the thickness of the canted layer is roughly estimated at 0.4 nm for spherical NPs of about 4 nm [43], i.e. about a single atomic layer, that is slightly smaller than the two atomic layers usually observed in most of as-prepared NPs. From Fig.6, the absence of the intermediate lines in the in-field spectrum corresponding to the second set of 6nm NPS suggests that neither the surface magnetic layer nor the magnetic core is canted. Such a feature differs from expected one as the canting surface effect is usually observed on NPS the size of which is less than about 10 nm, because the surface/volume ratio exceeds about 5% [42, 45]. At this stage, it seems pertinent to take also into account the elaboration route and the morphology of NPs. Indeed, spherical NPs originate some topological disorder at the surface favoring magnetic frustration, surface anisotropy [45] and surface pinning resulting from to the lack of coordinating oxygen atoms, i.e. canted magnetic structure; on the opposite, facetted NPs do possess regular and flat surfaces preventing from a canted surface magnetic structure. This infield Mössbauer study allows different magnetic superficial behaviors for the present sets of NPS to be clearly distinguished: one does conclude that the number of surface defects of coated maghemite NPs is significantly reduced compared to that of as-prepared NPs: this is consistent with the different morphologies with facetted NPS as illustrated by TEM (see previous section), in contrary to native NPS which are usually spherical.



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Sample

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δ (mm/s)

2ε (mm/s)

Beff (T)

θ(°)

Bhyp (T)

%

±0.01

±0.01

±0.5

±5

±0.5

±2

d = 4 nm

Site T

0.34

0.36

60.6

Improvement of Thermal Stability of Maghemite Nanoparticles Coated

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