Formation of a Magnetic Foam Catalyzed by Pd@FexO Heterodimers

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From Single Molecules to Nanostructured Functional Materials: Formation of a Magnetic Foam Catalyzed by Pd@FexO Heterodimers Muhammad Nawaz Tahir, Martin Klünker, Filipe Natalio, Bastian Barton, Karsten Korschelt, Sergii Shylin, Martin Panthöfer, Dr. Vadim Ksenofontov, Angela Möller, Ute Kolb, and Wolfgang Tremel ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00051 • Publication Date (Web): 07 Dec 2017 Downloaded from http://pubs.acs.org on December 10, 2017

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ACS Applied Nano Materials

From Single Molecules to Nanostructured Functional Materials: Formation of a Magnetic Foam Catalyzed by Pd@FexO Heterodimers Muhammad Nawaz Tahir1,2*‡, Martin Kluenker1‡, Filipe Natalio1,3, Bastian Barton1,4, Karsten Korschelt1, Sergii I. Shylin1, Martin Panthöfer1, Vadim Ksenofontov1, Angela Möller1, Ute Kolb1,5, and Wolfgang Tremel1* ‡

1

Equally Contributed Authorship

Institut für Anorganische Chemie und Analytische Chemie, Johannes Gutenberg-Universität,

Duesbergweg 10-14, 55128 Mainz, Germany 2

The Chemistry Department, King Fahd University of Petroleum and Minerals, Dhahran 31261, P.O. Box

5048, Saudi Arabia 3

Kimmel Center for Archaeological Science, Weizmann Institute of Science, Rehovot 76100, Israel

4

Fraunhofer-Institute for Structural Durability and System Reliability LBF, Division Plastics,

Schlossgartenstraße 6, 64289 Darmstadt, Germany 5

Institut für Angewandte Geowissenschaften, Technische Universität Darmstadt, Schnittspahnstraße 9,

64287 Darmstadt, Germany *

Correspondence should be addressed to: [email protected], [email protected]

Keywords: Pd; γ-Fe2O3; Fe3O4; nanochemistry; nanocatalysis; seed mediated growth, epitaxy, hybrid material, hydrosilylation

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Abstract Multicomponent nanostructures containing purely organic or inorganic as well as hybrid organic-inorganic components connected through a solid interface are, unlike conventional spherical particles, able to combine different or even incompatible properties within a single entity. They are multifunctional and resemble molecular amphiphiles, like surfactants or block copolymers, which makes them attractive for self-assembling complex structures, drug delivery, bioimaging or catalysis. We have synthesized Pd@FexO heterodimer nanoparticles to fabricate a macroporous, hydrophobic, magnetically active, three-dimensional (3D) and templatefree hybrid foam, capable to repeatedly separate oil contaminants from water. The Pd domains in the Pd@FexO heterodimers act as nanocatalysts for the hydrosilylation of polyhydrosiloxane and tetravinylsilane, while the FexO component confers magnetic properties to the final functional material. Pd@FexO heterodimers were synthesized by heterogeneous nucleation and growth of the iron oxide domain onto presynthesized Pd nanoparticles (NPs) at high temperatures in solution. The morphology, structure, and magnetic properties of the as-synthesized heterodimers were characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Mössbauer spectroscopy, and superconducting quantum interference device (SQUID). The epitaxial growth of FexO domain onto Pd was confirmed by high resolution transmission electron microscopy (HR-TEM). A potential application of the 3D hydrophobic magnetic foam was exploited by demonstrating its ability to soak oil beneath a water layer envisioning its use in oil sampling during oil prospection drilling or to remove oil films after oil spills.

Introduction The use of nanomaterials as catalysts in organic synthesis has attracted interest in research and technology because they combine exceptionally high surface area and reactive surfaces.1–3 Different types of nanoparticles have been used as catalysts in organic reactions: e.g. Cu NPs in ‘click’ cycloaddition reactions,4 twodimensional Au nanostructures for the selective oxidation of C-H bonds,5 Ru or Pt NPs for selective hydrogenation reactions,6 and Pd NPs for C-C and Si-C (hydrosilylation) tions.

1,8–10

7

coupling reactions under mild condi-

Nanocatalysts could be the best contenders for developing green and sustainable industrial process-

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es. The prime interest in this area was so far to mimic molecular catalysts used in conventional organic synthesis with nanomaterials containing the respective metal atoms. Recent developments in colloidal synthesis allow synthesizing NPs by joining two or more than two domains at the same platform. The option to graft different functions to a single particle in an individual fashion allows making materials with enhanced optical, magnetic and catalytic properties.12–16 In essence, the design and synthesis of multicomponent NPs with discrete domains that impart their functions in a synergistic manner may lead to new functional materials. Several multicomponent NPs with morphologies ranging from core-shell, dumbbell to epitaxially grown colloidal superparticles have been reported.17–21 Among these multicomponent NPs, heterodimers or Janus NPs have good prospects to perform complementary functions because their architecture offers a domain-specific surface chemistry.22,23 These heterodimers from metals and magnetic or semiconducting metal oxides and metal chalcogenides have a wide range of compositions to explore new chemical and physical properties.24–26 Nanomaterials containing Pd have received immense attention because they may catalyze organic reactions including Heck, Sonogashira, Suzuki, Stille, Negishi, Hiyama, Corriu-Kumada, Tsuji-Trost, and Ullmann reactions,3,8,9,27–30 while iron oxide NPs may serve as tool for magnetic separation.31–34 Here we report the syn-


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thesis of Pd@FexO heterodimer NPs, where the very small (≈ 5 nm) Pd domain catalyzes a hydrosilylation reaction. The surface availability of both components (metal and metal oxide) as well as their distinct properties offer a new route to a template-free synthesis of nanocomposite materials. With the proper choice of reactant structures we could synthesize a (poly)organosilicone-based three-dimensional (3D) hydrophobic, macroporous, magnetic composite with impregnated Pd@FexO NPs that act as “magnetic foam”. The potential application of the magnetic foam composite was demonstrated by removing oil contaminants from water. This magnetic foam could be manipulated with external magnetic field. The strategy could easily be extended for oil sampling during oil prospection drilling or oil film removal after an oil spill.35,36

Experimental Materials. Palladium(II) acetylacetonate (Pd(acac)2, Acros Organics), oleylamine (OAm, Acros Organics, 8090%,), 1-octadecene (ODE, Acros Organics, 90%), oleic acid (OAc, Fisher Scientific, Reagent Grade), tri-noctylphosphine (TOP, abcr, 97%), iron(III)chloride hexahydrate (FeCl3•6H2O, Chem Pur, 99+%), sodium oleate (C18H33O2Na, Sigma-Aldrich, >82% fatty acids (as oleic acid) basis, powder), nitrosonium tetrafluoroborate (NOBF4, Acros Organics, 97%), poly(dimethylsiloxane) (Sigma-Aldrich, vinyl terminated, Mw~25000), tetravinylsilane (abcr, 95%), poly(methylhydrosiloxane) (Sigma-Aldrich, trimethylsilyl terminated, Mn 17003200), cyclohexane (Fisher Scientific, Analytical Reagent Grade), ethanol (Sigma-Aldrich, p.a., >99.8%), hexane (Fisher Scientific, Analytical Reagent Grade), methanol (Fisher Scientific, Analytical Reagent Grade), dimethylformamide (DMF, Sigma-Aldrich, 99.8%), dichloromethane (CH2Cl2, Sigma-Aldrich, p.a., >99.9%), toluene (Sigma-Aldrich, p.a., >99.7%). Synthesis of Pd Seed Nanoparticles. The synthesis of the Pd seed NPs was adapted from Kim et al.37 In a 100 mL three-neck round bottom flask, 100 mg of Pd(acac)2 were dissolved in 1 mL of tri-n-octylphosphine under inert gas (Ar) conditions and stirred for 10 min. Subsequently, 10 mL of oleylamine were added. The mixture was heated to 250 °C with a rate of 2 °C/min and held at this temperature for 30 min. Afterwards the mixture was cooled slowly to room temperature. A black product was precipitated from the mixture by adding 15 mL of ethanol. The precipitate was separated by centrifugation (9000 rpm, 10 min, RT), dispersed in cyclohexane and washed twice by adding ethanol (cyclohexane:ethanol = 1:2), followed by centrifugation (9000 rpm, 10 min, RT). Finally, the product was dispersed in cyclohexane, flushed with Ar and stored at RT. Synthesis of Iron(III)oleate Complex. For the synthesis of the iron(III)oleate as precursor complex, 40 mmol of iron(III)chloride hexahydrate and 120 mmol of sodium oleate were dissolved in 80 mL of ethanol, 60 mL of Milli-Q water and 140 mL of hexane followed by heating at 70 °C for 4h as adapted from Park et al.38 Afterwards the organic phase was separated using separating funnel and washed once with a 1:1 Milli-Q water:methanol solution. The solvent was removed using a rotary evaporator, and the residue was dried under Schlenk line conditions at 10-3 mbar for 24h at RT and at 100 °C for 24h. Synthesis of Pd@FexO Nanoparticles. The Pd@FexO heterodimer NPs were synthesized by dissolving 900 mg (1 mmol) of the iron(III)oleate complex in 4 mL of 1-octadecene in a 100 mL three-neck round bottom flask under inert gas (Ar) conditions. Subsequently, 160 µL (0.5 mmol) of oleic acid and 160 µL (0.5 mmol) of oleylamine were added. After 10 min of stirring at RT, 10 mg of the prepared 5-7 nm Pd seed NPs dispersed in 1 mL of 1-octadecene were added to the reaction solution. The mixture was heated to 110 °C with a rate of 2 °C/min and held at this temperature for 20 min. The heating proceeded to 310 °C with 2 °C/min and kept for



30 min. Afterwards, the mixture was slowly cooled to room temperature. A black product was precipitated from the mixture by adding 15 mL of ethanol. The precipitate was separated by centrifugation (9000 rpm, 10 min, RT), dispersed in cyclohexane and washed twice by adding ethanol (cyclohexane:ethanol = 1:2) and centrifugation (9000 rpm, 10 min, RT). Finally, the product was dispersed in cyclohexane, flushed with Ar and stored at RT. The iron oxide NPs without Pd domain (FexO NPs) were synthesized accordingly to the heterodimer NPs without Pd seed particles. Surface Functionalization of Pd@FexO Nanoparticles. The hydrophilic surface functionalization of the assynthesized Pd@FexO heterodimer NPs was adapted from Dong et al.39 First, 5 mL of a 1 mg/mL dispersion of the Pd@FexO heterodimer NPs in hexane and a solution of NOBF4 (5.8 mg, 0.01 M) in 5 mL of dichloromethane was prepared. For better solubility of NOBF4 2 mL of DMF was added. Both solutions were combined, and the resulting mixture was slowly shaken for 10 min at RT. The mixture was then centrifuged (9000 rpm, 10 min, RT) to precipitate the NPs. The precipitate was re-dispersed in DMF and washed twice by adding toluene and cyclohexane (DMF:toluene:cyclohexane = 1:2:2) followed by one additional centrifugation (9000 rpm, 10 min, RT). Finally, the product was dispersed in DMF and stored at 8 °C. Preparation of Hydrophobic and Magnetic NP Foam Structure by Hydrosilylation. A 1 mg/mL dispersion of the hydrophilic functionalized Pd@FexO NPs in DMF was prepared and a second solution containing 250 mg of tetravinylsilane, 440 mg of poly(methylhydrosiloxane) and 100 mg of vinyl terminated poly(dimethylsiloxane) (equivalent to an 1:4:1 molar ratio with respect to the functional groups). 0.5 mL of the NP dispersion was added to the prepared silane/siloxane solution and the reaction mixture was placed in an oven at 80 °C for 96h. Afterwards the resulting NP foam residue was washed with cyclohexane and dried at 100°C for 30 min. Nanoparticle Characterization. Samples for TEM were prepared by placing a drop of dilute NP dispersion in cyclohexane on a carbon coated copper grid. TEM images for the characterization of size and morphology and surface rendering 3D tomography were obtained using a FEI Tecnai 12 equipped with LaB6 source at 120 kV and a twin-objective together with a Gatan US1000 CCD-camera (2kx2k pixels). High-resolution TEM, STEM and EDX data were obtained on a FEI Tecnai F30 S-TWIN TEM equipped with a field emission gun and operated at 300 kV. Atomic modelling and electron diffraction simulation were performed by Crystal Maker software. X-ray diffraction patterns were recorded on a Bruker AXS D8 Advance diffractometer equipped with a SolX energy dispersive detector in reflection mode using unfiltered MoKα radiation. Crystalline phases were identified according to the PDF–2 database using Bruker AXS EVA 10.0 software. Full profile fits (Le Bail / Pawley / Rietveld) were performed with TOPAS Academic 4.1 applying the fundamental parameter approach.40,41 57

Fe-Mössbauer spectra of powdered samples were recorded in transmission geometry with a 57Co source em-

bedded in a rhodium matrix using a conventional constant-acceleration Mössbauer spectrometer equipped with a helium cryostat at 5.5 K. Isomer shifts are given with respect to iron metal at ambient temperature. Simulations of the experimental data were performed with the Recoil software.42


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Figure 1. Synthesis of (a) Pd seed NPs, (b) the iron(III) oleate precursor complex and (c) Pd@FexO NPs. After functionalization with (d) NOBF4 the Pd@FexO NPs were used for a Pd catalyzed hydrosilylation reaction (e) of tetravinylsilane, poly(methylhydrosiloxane) and vinyl terminated poly(dimethylsiloxane) to form a hydrophobic and magnetic foam.

Magnetic susceptibility measurements were performed on a Quantum Design MPMS-XL SQUID magnetometer. FC and ZFC data were obtained in a temperature range between 5 and 300 K at 100 Oe. Hysteresis measurements were performed at 5 and 300 K. FT-IR spectroscopy was carried out on a Thermo Scientific Nicolet iS10 FT-IR Smart iTR infrared spectroscope equipped with Platinum-ATR (diamond crystal, one reflection) and OMNIC 8.1.210 software. Thermogravimetric analysis was performed on a Perkin Elmer Pyris 6 TGA instrument under nitrogen atmosphere (5 mL/min). The heat program was (i) 20 min at 30 °C and (ii) ramp of 10 °C/min from 30 °C to 600 °C.

Results and Discussion Figure 1 illustrates the synthesis of Pd@FexO heterodimers using Pd nanoparticles as seeds and an iron oleate complex as precursor for iron oxide. The Pd NPs used as seed particles were synthesized by decomposition/reduction of Pd(acac)2 using TOP and oleylamine at 250 °C (Figure 1a), whereas the FexO domain was formed by decomposition of iron oleate and heterogeneous nucleation/growth onto the pre-synthesized Pd seed NPs at 310 °C (Figure 1b and c). The subsequent surface functionalization (Figure 1d) of the assynthesized Pd@FexO heterodimer NPs was mandatory to transfer them into a polar organic solvent (DMF) to catalyze the hydrosilylation reaction between unsaturated silane and polysiloxane (Figure 1e). The Pd domain acted as nanocatalyst, and iron oxide imparted the magnetic character. By choosing the proper silane and siloxanes in an appropriate ratio, a 3-dimensionally organized network of polysiloxane with entrapped nanoparticles was obtained. The final composite material was macroporous and could be manipulated with an external magnetic field. Structure, Phase Composition and Morphology of Pd@FexO Heterodimers



Figure 2. TEM images of Pd@FexO NPs. (a) TEM image of Pd@FexO heterodimer NPS. (b) Surface rendering 3D tomography of octahedral and triangular shaped Pd@FexO NPs. (c) Holography phase image of the Pd[011] and Fe3O4[211] zone axis superimposed with a model of the atomic layers. (d) TEM image of Pd and FexO interface in red boxed region. (e) HR-TEM image of red boxed region from (d) with FFT and simulated ED pattern of Pd domain (1, cyan) and FexO domain (2, orange).

Figure 2 shows TEM and HR-TEM micrographs of Pd@FexO heterodimer NPs. The Pd domain in the Pd@FexO heterodimers is spherical with a diameter ≈ 5 nm (Figure 2a, Figure S1). The iron oxide domain is faceted (anisotropic) with octahedral morphology and a diameter ≈ 25 nm (along with a very small fraction of particles with triangular morphology) (Figure 2a). The faceted octahedral morphology of the iron oxide domains was confirmed by surface rendering 3D tomography (Figure 2b) and TEM tilting studies. For further examination of the domains and the interface, TEM holography from phase imaging was carried out (Figure 2c-e). Figure 2c shows an extended holography phase image of the Pd[011] and Fe3O4[211] zone axis superimposed with a model of the atomic layers to characterize the growth of the iron oxide on the Pd domain. The magnetite {11-3} layer with a d spacing of 0.48 nm grew on the Pd {-111} layer characterized by a d spacing of 0.23 nm. The lattice mismatch of approximately 7% caused a 6° tilted growth of the magnetite domain on Pd and enabled epitaxial growth of the FexO domain on the Pd seed particles. Holography of HR-TEM (Figure 2d) was performed on the Pd (Figure 2e, boxed region 1) and the iron oxide domains (Figure 2e, boxed region 2) extracting the FFT from the phase image. For the Pd domain, the simulated diffraction pattern of the Pd [011] zone axis matched well with the FFT. For the iron oxide domain, the simulated diffraction pattern of the magnetite [211] zone axis was also in agreement with the FFT. The chemical composition of both domains was confirmed by EDX, which show Pd signals for the Pd domain and Fe signals for the FexO domain of Pd@FexO NPs (Figure S2). Since the structure of iron oleate plays a major role in defining the morphology of iron oxide domain, the assynthesized iron oleate precursor was analyzed initially by FT-IR spectroscopy and thermogravimetric analysis (TGA).


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The FT-IR spectra of the surfactants, iron oleate and as-synthesized nanoparticles are presented in Figure S3. The FT-IR spectrum of the iron(III) oleate (Figure S3a) shows typical C-C and C-H vibrations in the region of 2800-3100 cm-1. The bands corresponding to the COO- symmetric and asymmetric stretching vibrations at 1400-1600 cm-1 were used to characterize the binding and coordination modes of the Fe3+ to the oleate moiety. The difference delta (∆) between the two modes indicates the type of coordination. For ∆ = 200-300 cm-1 a monodentate binding is likely. For ∆ = 110-200 cm-1 ionic or bridging coordination and for ∆ < 110 cm-1 chelating bidentate coordination can be assumed.43,44 The reduced difference results from the group symmetry of the chelating bidentate and bridging coordination which is the same as in the free ionic state. Accordingly, the frequency of the asymmetric vibration decreases and the frequency of the symmetric vibration increases relative to the monodentate binding mode.43 Here, ∆ = 128 cm-1 indicates a bridging coordination (Figure S3b).45 This also explains the faceted morphology of the iron oxide domain formed in the Pd@FexO heterodimer nanoparticles. It has been reported that iron oleate with bridging coordination leads to the formation of anisotropic iron oxide particles.45,46 The band at 1711 cm-1 can be assigned to the carbonyl vibration of free oleic acid.47 The IR spectrum in Figure S3c shows that the 5 nm Pd seed particles are mainly functionalized with TOP, whereas the Pd@FexO NPs display a surface functionalization of OAm and OAc (Figure S3d). The TGA data (Figure S4, Table S1) show the typical plot for iron oleate synthesized in ethanol. In the temperature range from 196 °C to 267 °C a dissociation of the symmetric oleate ligands of the quasi-octahedral Fe3+(oleate)3 complex takes place (lower binding energies).45 This step is attributed to the nucleation process.48 The second smaller weight loss between 267 °C and 331 °C is attributed to the dissociation of the third asymmetric ligand and characterizes the particle growth.49 These two weight losses are mainly due to the loss of CO2 due to a ketonic decarboxylation reaction.45 As this is not a redox reaction (supported XPS measurements reported by Bronstein et al.48) as-synthesized iron oleate shows only the presence of Fe3+species. The partial reduction to Fe2+, which leads to the formation of magnetite and maghemite in the iron oxide domain (vide infra), is likely to occur during the thermal decomposition of the remaining i.a. ketonic byproducts. Kwon et al.

50

demonstrated by thermogravimetric mass spectrometry that CO2 and H2 can be

detected during a second transition step at 320 °C leading to reduction of Fe3+ and the formation of a phase similar to magnetite. They assign this to the formation of thermal radicals resulting in the formation of CO and H2 after complete fragmentation.50 The DTA data (Figure S4a) and highest TGA mass loss of 24% (Figure S4b) reveal this decomposition step to take place between 331 °C and 374 °C with a maximum slope at 353 °C. Further smaller weight losses (11%, 374 °C - 424 °C and 13%, 424 °C - 468 °C) follow describing the complete desorption of the decomposed ligands as well as the vaporization of the organics leaving a 20% portion of iron oxide NPs.45

Magnetite vs. Maghemite in the FexO Domains Figure 3a displays the results of a Rietveld refinement for the as-synthesized Pd@FexO heterodimer NPs. According to the X-ray powder diffraction data, the sample contains three crystalline phases – a Pd side phase due to the presence of Pd domains and the reflections of two iron oxide main phases (Table S2). The Pd side phase with a weight fraction of 3.4% has a refined lattice parameter of 3.853(2) Å, which is close to the reported lattice parameter of Pd (3.879 Å), and a crystallite size of 3.5 nm which is comparable to the crystallite size of the Pd domain obtained from TEM imaging (5.2 nm ± 10%, Figure 2a, Figure S1).51 The distinction be-



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Figure 3. (a) Rietveld refinement to the powder XRD data of Pd@FexO NPs. Red dots mark the experimental data, the black line corresponds to the calculated pattern, the red line shows the difference between the experimental and calculated data. Black ticks mark reflections of Pd and iron oxide, respectively. Q = (4πsin(ϴ))/λ is the scattering vector. (b) 57Fe-Mössbauer spectrum of Pd@FexO NPs at 5.5 K. (c) Hysteresis measurements at 5 K and 300 K for Pd@FexO NPs.

tween γ-Fe2O3 and Fe3O4 based on X-ray diffraction data is difficult because of the structural and chemical similarity of both compounds. Maghemite is an iron deficient defect variant of magnetite and has a cubic structure as well, but with a slightly smaller lattice parameter.52 Additionally, reflection broadening for nanocrystalline materials occurs.53 Nevertheless, maghemite (60 wt%) co-occurs with magnetite (37 wt%). The crystallite size of the iron oxide was determined to be 16.6 nm, which is comparable to the iron oxide domain sizes extracted from the TEM images (Figure 2a). The distribution of Fe in the sample was clarified using Mössbauer spectroscopy (Figure 3b). The

57

Fe-

Mössbauer spectrum of the Pd@FexO NPs allows to correlate the data to those of the SQUID magnetic measurements (Figure 3c, Table S3). In Figure 3b the broad inner sextet (19% intensity) corresponds to Fe2+ in magnetite. The outer sextet (37% and 44% intensity, Table S4) has contributions from Fe3+ in magnetite and Fe3+ in maghemite.54,55 This is compatible with a total of 55.5% magnetite and 44.5% maghemite. Comparable FexO NPs of similar particle size showed a similar magnetite:maghemite distribution of 60:40 confirming that the iron oxide distribution does not depend on the heterodimeric nature of the NPs. In summary, an approximate 50:50 distribution of magnetite:maghemite can be derived from the Mössbauer data. According to HRTEM analysis (Figure 2) homogeneous FexO domains are present, giving no indication of separated magnetite and maghemite phase domains in the NPs. Therefore, each NP is like to contain a variable magnetite:maghemite two-phase distribution due to the spinel structure of both phases and octahedral site vacancies and fluctuating Fe2+ ion content in the octahedral sites. The Pd@FexO NPs show ferromagnetic behavior at 5K and superparamagnetic behavior at room temperature, which was extracted from temperature-dependent magnetization and hysteresis curves (Figure 3c). The saturation magnetization is a summation of the magnetic moments of all individual iron oxide domains; at 5 K and 300 K they are 72 emu/g and 65 emu/g, respectively for the Pd@FexO NPs (Figure 3c). This is lower than the saturation magnetization of both bulk magnetite (92 emu/g) and bulk maghemite (80 emu/g).56,57 These


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Figure 4. (a)Functionalized NPs dispersed in DMF compared to as synthesized NPs in cyclohexane. (b) Reaction mixture for hydrosilylation with and without NPs after reaction. (c) Magnetic property of NP foam. (d) Water drop on NP foam. Adsorption of hydrophobic liquid by NP foam (e-g) under water and (h-j) on water surface.

reduced values can be caused by the presence of both iron oxide phases, spin canting effects of the surface Feions or effects of the surfactants.58 (A detailed discussion of the Mössbauer spectra and magnetic measurements is given in the Supporting Information, Figure S5 – Figure S9). Catalytic Activity for Hydrosilylation The Pd@FexO NPs were tested for their activity to catalyze hydrosilylation reactions to synthesize porous silicones. Hydrosilylation, an addition of a silane to a double bond, is catalyzed typically by noble metals like Pd or Pt and undergoes a Heck cyclic reaction of oxidative addition and reductive elimination.59 We adopted this hydrosilylation for the reaction of a poly(methylhydrosiloxane) with double bond bearing vinyl terminated poly(dimethylsiloxane) and tetravinylsilane. This lead to the formation of a hydrophobic foam-like structure with Pd@FexO NPs entrapped inside the composite. The Si-C coupling reaction between alkene (C=C) and hydrosiloxane (Si-H) was confirmed by FT-IR spectroscopy (Figure S10). Tetravinylsilane showed a typical C-H stretching vibration associated with a double bond at 3049 cm-1 and a C=C stretching vibration at 1590 cm-1. The asymmetric C-H bending vibration at 1399 cm-1 is strong as well. This confirms the presence of the vinyl groups. The vinyl terminated poly(dimethylsiloxane) showed a strong band of the symmetric C-H bending vibration at 1258 cm-1 and a weak band of the asymmetric band. The asymmetric and symmetric Si-O-Si stretching vibration (1084 cm-1 and 1010 cm-1) merge into one broad band and the Si-C asymmetric stretching vibration appears as a strong band at 796 cm-1. Due to the methyl groups, the CH3 stretching vibration appears at 2967 cm-1. The third silyl component, poly(methylhydrosiloxane), additionally has a Si-H functional group



for hydrosilylation shown as stretching vibration at 2162 cm-1.60,61 In summary, the IR spectrum of the final sponge product displays the Si-C, Si-O-Si and C-H vibrations. It also shows the Si-H vibration which is probably due to the excess of poly(methylhydrosiloxane) used. This is supported by the missing bands associated with double bounds. Accordingly, all double bounds must have reacted in the hydrosilylation reaction. Surface Properties of “Magnetic” Silicone Foam Allow Collecting Oil The reactants designed for the hydrosilylation reaction were soluble in organic polar solvents like DMF. Therefore, the Pd@FexO NPs were surface functionalized to tune their solubility in DMF. The particles were treated with a NOBF4 solution to exchange the oleic acid and oleylamine surface ligands with BF4-. As shown in Figure 4a, this ligand exchange allowed to transfer the Pd@FexO NPs from cyclohexane to DMF. To start the reaction, the functionalized Pd@FexO heterodimer NPs were mixed with the reactants and heated at 80 °C for 4 days. The extended reaction time was necessary to achieve a stable structure due to the very small active Pd next to a much larger iron oxide domain. Figure 4b shows the reaction mixture after heat treatment with and without NPs, verifying that the reaction was catalyzed by the NPs and not triggered by heating. The resulting foam-like structure was magnetic (Figure 4c) and hydrophobic (Figure 4d). As demonstrated in Figure 4d the foam is superhydrophobic with a contact angle of 125.8° (average of three measurements). The foam had a hard texture and was slightly brittle upon cutting. TEM analysis verified that the Pd@FexO NPs were physically embedded in the hydrophobic foam, with the siloxane network acting as a “glue” (Figure S11). The hydrophobic magnetic foam floated on water and could be moved easily with a magnet (Video S1). When a hydrophobic liquid (with high density) was dropped beneath water (Figure 4e-g, Video S2) or made to float (with a low density such as oil) on the water surface (Figure 4h-j, Video S3) near the magnetic foam, the foam could be directed towards the oil patch and used to selectively adsorb the oil. The oil-loaded foam could be removed from the water with a magnet and the adsorbed oil could easily be expelled by rinsing the foam with a nonpolar solvent. Subsequently, the foam could be reused.

Conclusion In summary, we have developed a protocol to synthesize Pd@FexO heterodimer NPs with a spherical Pd (5 nm) component and faceted iron oxide domains (25 nm). Starting from an iron(III) oleate precursor we were able to obtain anisotropic iron oxide domains constituted of a ≈ 50:50 maghemite:magnetite with 65 emu/g saturation magnetization at 300 K. The Pd@FexO heterodimer NPs were surface functionalized to achieve a better dispersability in polar organic media to perform catalysis. With a proper choice of reactants and by exploring the individual properties of both domains of the heterodimer nanoparticles, we synthesized a hydrophobic, macroporous and magnetic foam that could be used for a reversible adsorption of nonpolar liquids. Such magnetically triggered hydrophobic foams could be a potential adsorbent for oil sampling in oil wells or for separating oil from water.

Associated Content TEM image of 5 nm Pd seed NPs (Figure S1), EDX spectrum of Pd@FexO NPs (Figure S2), IR spectra of iron(III) oleate, 5 nm Pd seed particles and Pd@FexO heterodimeric particles (Figure S3), thermogravimetric analysis of the iron(III) oleate precursor complex (Figure S4), 57Fe-Mössbauer spectrum of Pd@FexO NPs at 293 K (Figure S5), magnetic blocking temperature measurements for Pd@FexO NPs (Figure S6), TEM image of


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FexO NPs without Pd domain (Figure S7), magnetic blocking temperature and hysteresis measurements for FexO NPs (Figure S8), 57Fe-Mössbauer spectra of FexO NPs at 293 K and 5.5 K (Figure S9), IR spectra of magnetic foam (Figure S10), TEM image of Pd@FexO NPs embedded into hydrophobic foam structure (Figure S11), additional data to the thermogravimetric analysis, Rietveld refinement of powder XRD data, SQUID magnetic measurements and Mössbauer measurements (Table S1-S4), additional videos showing magnetically triggered movement of hydrophobic foam on water surface, adsorption of hydrophobic liquid under water and adsorption of hydrophobic liquid on water surface (Videos S1-S3). This material is available free of charge via the Internet at http://pubs.acs.org.

Author Information Corresponding Author * [email protected], [email protected]

Author Contributions All authors have given approval to the final version of the manuscript.

Acknowledgement This research was supported by the Max Planck Graduate Center (MPGC) through scholarships to M. K. and K. K. The facilities of the Electron Microscopy Center in Mainz (EZMZ) were supported by the State Excellence Cluster CINEMA. The authors acknowledge support through the SFB 1066 “Nanodimensionale polymere Therapeutika für die Tumortherapie”. A. M. acknowledges the Carl-Zeiss Stiftung for support. We acknowledge the collaboration of B.Sc. Nabil Boui.

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Formation of a Magnetic Foam Catalyzed by Pd@FexO Heterodimers

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