Synthesis of Iron Oxide Nanorods Using a Template Mediated

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Synthesis of Iron Oxide Nanorods Using a Template Mediated Approach Hauke Kloust, Robert Zierold, Jan-Philip Merkl, Christian Schmidtke, Artur Feld, Elmar Pöselt, Andreas Kornowski, Kornelius Nielsch, and Horst Weller Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/acs.chemmater.5b00513 • Publication Date (Web): 23 Jun 2015 Downloaded from http://pubs.acs.org on July 1, 2015

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Chemistry of Materials

Synthesis of Iron Oxide Nanorods Using a Template Mediated Approach Hauke Kloust,a,b Robert Zierold,c Jan-Philip Merkl, a,b Christian Schmidtke, a,b Artur Feld, a,b Elmar Pöselt, a,b Andreas Kornowski,a,b Kornelius Nielsch, c, † Horst Weller a,b,d,e * a

b

Institute of Physical Chemistry, University of Hamburg, Grindelallee 117, 20146 Hamburg, Germany, The Hamburg c Center for Ultrafast Imaging, University of Hamburg, Luruper Chaussee 149, 22761 Hamburg, Germany, Institute of d Nanostructure and Solid State Applied Physics, Jungiusstrasse 11, 20355 Hamburg, Germany, Center for Applied e Nanotechnology (CAN) GmbH, Grindelallee 117, 20146 Hamburg, Germany, Department of Chemistry, Faculty of Science, King Abdulaziz University, P.O BOX 80203 Jeddah 21589, Saudi Arabia, †present address: IfW Leibniz Institute for Solid State and Material Research Dresden, Helmholtzstrasse 20, 01069 Dresden, Germany *[email protected] ABSTRACT: Herein, we present the formation of particularly small iron oxide nanorods following a one-step bottom up approach using iron oleate as precursor. By varying reaction temperature and/or time we achieve an excellent control over size. We assume a template mediated mechanism, whereby iron oxide nanodots string together in a row to form a rod. Magnetic characterization reveals superparamagnetic behavior as well as an easy axis of the magnetic anisotropy along the long axis of the formed nanorods caused by shape anisotropy.

Iron oxide nanocrystals which behave superparamagnetic are used in the biomedical field as well as in technical applica1-4 tions. For instance iron oxide-based contrast agents, such as Feridex™ and Resovist™ to name a few of them, are already applied in clinical use for magnetic resonance imaging 5 (MRI). The most scalable, chemical synthesis routes result 6, 7 in iron oxide nanocrystals with spherical shape. However, iron oxide nanocubes, which recently appeared, show an increased relaxivity compared to spherical crystals, and thus 8-10 constitute the second generation of MRI contrast agents. To further improve the functionality of iron oxide nanoparticles a pronounced magnetic anisotropy would be favorable. Since the magnetic crystalline anisotropy is material dependent, tuning the shape anisotropy of the nanocrystals might be a solution to increase the effective magnetic anisotropy. On the one hand, the shape of magnetic nanoparticles consisting of cobalt or nickel can be controlled by varying the 3, 11-13 organic ligands in the synthesis route. However, due to their toxic heavy metal content, these systems are not treated as biocompatible and thus cannot be used in clinic. On the other hand, the control of shape and aspect ratio of biocompatible iron oxide nanoparticles is quite challenging. Recently, a few strategies for tailor-made manipulation of the shape are reported in literature. For instance, nanorods with a length larger than 100 nm can be produced e.g. by hydro14, 15 16 thermal or by microwave-assisted synthesis. When

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shorter nanorods are required growth on seeds or on side 18 facets of FeO nanocubes can be consideres. Si et al. present a method for a solvothermal preparation of iron oxide nano19 rods from 58 to 250 nm. Other anisotropic nanoparticles such as nanowhiskers and tetrapods were produced by selective decomposition of an iron oleate complex or by a mix of 20, 21 three different surfactants, respectively. In this communication we present a versatile method that allows the synthesis of iron oxide nanorods directly from iron oleate in a one-step procedure. We propose a nonoriented attachment mechanism in the reaction sphere of a colloidal structure. This structure acts as the template for the attachment and may be destroyed by vacuum. We therefore assume that volatile compounds, such as water and ethanol, which occur during the iron oleate synthesis, form this soft template in combination with oleic acid and oleyl alcohol, which are present in the reaction mixture. The importance of the conservation of these colloidal structures is demonstrated by the transmission electron microscopy (TEM) images shown in Figure 1. If the iron oleate is only moderately dried after synthesis (10 mbar using a rotary evaporator), it consists of approximately 1 % water and ethanol (~6:1). By using this precursor as it is—without any drying of the reaction mixture—iron oxide nanorods are synthesized, as shown in Figure 1 A.

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Figure 1: Influence of the application of vacuum on the formation of iron oxide nanorods. A) Product of the nanorod synthesis, no vacuum was applied to the reaction solution. B) Vacuum was applied to the reaction solution and no rods were formed. C: Top histogram: nanorod length (blue), bottom: histogram of the nanorod width (red). -2

However, when lower pressure (~10 mbar) is applied to the reaction solution to remove the volatile compounds, no

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2 θ (°) Figure 2: A) TEM magnification of the iron nanorods. B) High resolution TEM image of iron oxide nanorods. C) XRD diffraction pattern and reference pattern of maghemite (00-039-1346) and magnetit (01-089-0691) and the XRD diffraction pattern of the nanorods.

formation of nanorods can be observed. The product of such a synthesis route is displayed in Figure 1 B and appears irregularly shaped, dot-like. Specially, no significant elongation of the nanocrystals can be found. The formed nanorods exhibit a mean length of approx. 24 nm and a mean diameter of 2.5 nm with an associated aspect ratio of 10. Compared to other template mediated synthesis of similar structures, the herein presented method allows the synthesis of particularly small and thin 22-24 The corresponding histograms of diameter and nanorods. length are displayed in Figure 1 C. Noticeably, the formation of the rod building template structure is not possible by purely adding the required amounts of water and ethanol to the reaction solution, after drying in high vacuum. Therefore, we conclude that the template formation needed for the nanorod formation can only take place during the iron oleate synthesis due to chemical reaction kinetics. A representative TEM micrograph of the iron oxide nanorods is shown in Figure 2A, where the alignment of singledot characteristic is apparent. The high resolution TEM image of iron oxide nanorods (Figure 2 B) confirms this characteristic and shows the twisted crystal-orientations of the single nanocrystals. To further confirm these twisted crystal-orientations the nanorods were investigated by Xdray diffraction (XRD). As reported by us, an alignment of single nanocrystals along preferred crystal facettes—referred as oriented attachment— leads to pronounced and sharp XRD-reflexes correlating with the respective crystallographic 25, 26 direction. In Figure 2C a XRD pattern of the nanorods, taken on a dried sample deposited on a silicon waver, is presented. This pattern shows typical broadened reflexes, taking the small crystallite domain sizes into account. As expected, does not show any sign of sharp reflexes. Therefore we conclude a non-oriented attachment mechanism during the ironrod synthesis. The position of the peaks corresponds to maghemite (00-039-1346) or magnetite (01-089-0691). Based on the measured X-ray diffraction pattern it is not possible to distinguish between this two crystalline phases of iron oxide.


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To determine the magnetic properties of the nanorods under concomitant analysis of their crystalline phase superconducting quantum interference device (SQUID) magnetometry was conducted. Therefore nanorods were dried on a glass/silicon substrate in the presence of a magnetic field: On the one hand, these measurements allow for insight in the micro magnetic properties, such as easy axis, coercive field, blocking temperature, to name a few of them. On the other hand, the saturation magnetization at high magnetic fields can be used to deduce the crystalline phase of the iron oxide. Temperature-dependent magnetization measurements (Figure 3 A) indicate superparamagnetic behavior of the nanorods by a vanishing remanence at around 27 ± 5 K and a sharp maximum in the zero-field-cooled (ZFC) measurement at 11 ± 1 K. Isothermal magnetization measurements were realized in two different mounting configurations, namely in-plane and out-of-plane as schematically drawn in the scheme in Figure 3 B. Low-temperature measurements exhibit the typical features of remanence and coercivity expected for magnetic nanorods in the blocked state (Figure 3 B). The saturation magnetization is estimated to be 370 ± 40 -3 emu cm by taking the dried amount of iron oxide nanorods into account; this value is close to the literature value of maghemite, thus supporting the presumption of mainly formed maghemite during the synthesis of iron oxide nanorods. Specifically, at 2 K the (inset in Figure 3 B), the squareness—remanent magnetization normalized to saturation magnetization—of the in-plane configuration amounts to 0.46 ± 0.01 compared to 0.31 ± 0.02 in the out-of-plane measurement. This observation indicates that the magnetic moment prefers to point along the long axis of the nanorods instead of a perpendicular alignment. Furthermore, the coercive field rises from 650 ±20 Oe to 1300± 20 Oe for the outof-plane and in-plane configuration, respectively. Since the nanorods can be treated as a connected row of spherical nanocrystals possessing various crystal orientations as proved by TEM, this difference between the two mounting configurations cannot only be explained by magnetocrystal27 line anisotropy. Applying the Jacobs-Bean model —chain of

Figure 3: A) Temperature-dependent SQUID measurements. Light-blue represents the data of a ZFC measurement with an obvious maximum at around 20 K. Remanence data is plotted in red. B) Magnetization isotherm measurement at 5 K. The scheme illustrates the two measurement directions. The inset shows a zoom-in of the 2 K-measurement revealing the large differences for the two mounting configurations.

Figure 4: Size control of the iron oxide nanorods. A) 150 °C, 1 min; B) 175 °C, 1 min; C) 200 °C, 30 min; D) 230 °C, 30 min; E) 245 °C, 30 min. F) Diagram showing the influence of the reaction temperature and time on the nanorod width.



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interacting spherical mono-domain particles—allows for the calculation of the switching field of a single nanorod with its long axis parallel to the external magnetic field. Assuming a -3 saturation magnetization of 400 emu cm for maghemite and ten particles forming a chain, the switching field can be calculated to 1310 Oe, being in good agreement with the experimental value. A deviation (of up to 60°) from the perfect parallel alignment would only result in a slight increase in the coercive field and rounding of the magnetization isotherms. In contrast, in the out-of-plane configuration—rod axis perpendicular to the field—a vanishing coercive field is theoretically expected. However, we attribute the apparent remanence and coercive field in the out-of-plane measurement to the magnetocrystalline anisotropy of the individual nanocrystals in the row. The coercive field of nanoparticles with a randomly oriented cubic anisotropy can be estimated 28 to be between 160 and 800 Oe assuming a magnetocrystal-5 -5 -3 line anisotropy constant of 1E -5 E erg cm as reported for 29-32 maghemite nanoparticles. The length as well as the diameter of the rods is tuned by reaction time and temperature as indicated by the transmission electron microscopy images displayed in Figure 4 A-E. The diameter of the nanorods increases with both with reaction time and temperature (see Figure 4 F). After 1 min at 150 °C iron oxide nanorods (A) with a width of 1.1 nm are synthesized. In the further course of the reaction, namely a slight increase of reaction temperature to 175 °C, the width increases to 1.4 nm (B). Subsequently the diameter reaches 2.5 nm after 30 min at 200 °C (C). Consecutive increasing of the reaction temperature results in a further increase of the diameter up to 6.0 nm (E). Remarkably, the length of the iron oxide nanorods is almost constant until 200 °C; thus, this synthetic approach allows to tune the aspect ratio by setting a specific reaction temperature. At temperatures above 200 °C (D and E) the rods shorten down to 16 nm under concomitant increase in width. After this transformation, the single nanodots are not distinguishable anymore. To conclude, we found a simple synthetic approach that directly allows the formation of iron oxide nanorods consisting of maghemite in a one-step synthesis. A templated effect is observed that is provided by a mixture of volatile compounds and can thus be degraded by vacuum. During the synthesis iron oxide nanodots are facilitated to attach to each other. In this non-oriented attachment nanorods, still containing the crystalline structure of the individual nanocrystals, are synthesized. Especially the width can precisely be tuned between 1 nm and 6 nm by changing the reaction temperature and reaction time. Temperature- and magnetic field-dependent magnetization measurements reveal superparamagnetic behavior of the prepared iron oxide nanorods. In the blocked state the nanorods exhibit a magnetic easy axis parallel to the long axis of the nanorods caused by the enhanced shape anisotropy.

ACKNOWLEDGMENT This work was supported by the EU within the FP 7 program (Vibrant, EU 228933), the State Excellence Initiative “Nanotechnology in Medicine” from the Free and Hanseatic City of 3 Hamburg and SFB 986 M (A.F. & H.W.) of the German Research Foundation (DFG). Financial support for RZ was provided by the DFG NI616/18-1. J.-P.M. and H.W. acknowledge the support of the Chemical Industry Fund, VCI:

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German Chemical Industry Association and the GermanAmerican Fulbright Program.

SUPPORTING INFORMATION Experimental procedure, Instrumentation and TEM histograms of iron oxide nanorods, this information is available free of charge via the Internet at http://pubs.acs.org

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Synthesis of Iron Oxide Nanorods Using a Template Mediated

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