Galvanic Replacement onto Complex Metal-Oxide Nanoparticles

Article pubs.acs.org/cm

Galvanic Replacement onto Complex Metal-Oxide Nanoparticles: Impact of Water or Other Oxidizers in the Formation of either Fully Dense Onion-like or Multicomponent Hollow MnOx/FeOx Structures Alberto López-Ortega,*,†,‡ Alejandro G. Roca,*,§ Pau Torruella,∥ Michele Petrecca,† Sònia Estradé,∥ Francesca Peiró,∥ Victor Puntes,§,⊥,∇ and Josep Nogués§,⊥ †

INSTM and Dipartimento di Chimica “U. Schiff”, Università degli Studi di Firenze, Sesto Fiorentino, I-50019 Firenze, Italy CIC nanoGUNE, E-20018 Donostia-San Sebastian, Spain § Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB, Bellaterra E-08193 Barcelona, Spain ∥ LENS-MIND-IN2UB, Departament d’Electrònica, Universitat de Barcelona, Martí i Franquès 1, E-08028 Barcelona, Spain ⊥ ICREA, Pg. Lluís Companys 23, E-08010 Barcelona, Spain ∇ Vall d’Hebron Institut de Recerca (VHIR), 08035 Barcelona, Spain ‡

S Supporting Information *

ABSTRACT: Multicomponent metal-oxide nanoparticles are appealing structures from applied and fundamental viewpoints. The control on the synthetic parameters in colloidal chemistry allows the growth of complex nanostructures with novel morphologies. In particular, the synthesis of biphase metaloxide hollow nanoparticles has been reported based on galvanic replacement using an organic-based seeded-growth approach, but with the presence of H2O. Here we report a novel route to synthesize either fully dense or hollow core/ shell metal-oxide nanoparticles (MnOx/FeOx) by simply adding or not oxidants in the reaction. We demonstrate that the presence of oxidants (e.g., O2 carried by the not properly degassed H2O or (CH3)3NO) allows the formation of hollow structures by a galvanic reaction between the MnOx and FeOx phases. In particular, the use of (CH3)3NO as oxidant allows for the first time a very reliable all-organic synthesis of hollow MnOx/FeOx nanoparticles without the need of water (with a somewhat unreliable oxidation role). Oxidants permit the formation of MnOx/FeOx hollow nanoparticles by an intermediate step where the MnO/Mn3O4 seeds are oxidized into Mn3O4, thus allowing the Mn3+ → Mn2+ reduction by the Fe2+ ions. The lack of proper oxidative conditions leads to full-dense onion-like core/shell MnO/Mn3O4/Fe3O4 particles. Thus, we show that the critical step for galvanic replacement is the proper seed oxidation states so that their chemical reduction by the shell ions is thermodynamically favored. replacement in aqueous media.21 This is an efficient way to build-up multicomponent systems that combine the distinct properties of the diverse counterparts in a hollow structure, leading to possible novel advanced applications in diverse fields22,23 such as catalysis,24 sensing,25,26 and lithium ion batteries.21,27 Here we present a facile synthetic approach to control the morphology of Mn−Fe oxide heterostructures, from hollow to full-dense core/shell, by controlling the amount of oxidizing agents (e.g., H2O or (CH3)3NO) during the synthesis. The role of these agents is promoting the oxidation of the MnO/Mn3O4 seeds to Mn3O4, thus allowing the galvanic process. It was

1. INTRODUCTION The design and synthesis of nanoparticles (NPs) by colloidal chemistry, with appealing physicochemical properties, is experiencing increasing interest,1,2 where the control of the growth processes generates a wide range of novel nanostructures with unique morphologies.3,4 Tailoring the composition, size, shape, and morphology of the NPs has permitted exquisite control of their functional properties, showing a huge capability in widespread technological applications.5−10 As an example, metal-oxide hollow NPs exhibit a great potential in energy storage,11 catalysis,12,13 electrorheological fluids,14 or drug delivery.15 Hollow NPs have been obtained by a variety of processes,16−18 where acid etching19 and Kirkendall processes20 are the most common for hollow metal-oxides. Moreover, recently, hollow iron oxide NPs have been obtained by galvanic © 2016 American Chemical Society

Received: September 6, 2016 Revised: October 13, 2016 Published: October 13, 2016 8025

DOI: 10.1021/acs.chemmater.6b03765 Chem. Mater. 2016, 28, 8025−8031

Article

Chemistry of Materials

(Experimental Section). TEM images and EELS analysis evidence that the presence of H2O in the reaction has a tremendous effect on the morphology of the heterostructures (Figures 1 and 2). NPs synthesized without H2O show the

found that H2O acts as an O2 carrier, the real oxidizing agent. Therefore, the oxidation capability of H2O strongly depends on the degree of degassing, where properly O2-free H2O does not form pure Mn3O4 NPs. On the other hand, the use of a pure organic oxidizing agent, (CH3)3NO, leads to a very reliable pathway to form hollow structures in an all-organic synthesis. In fact, this is the foremost evidence of the feasibility to synthesize hollow metal-oxide NPs by galvanic replacement without the presence of H2O.

2. EXPERIMENTAL SECTION Synthesis of Samples S1, CS1, and HW1. Manganese oxide nanoparticles were prepared by thermal decomposition of Mn(oleate)2 in 1-octadecene (sample S1).28 Subsequently, the S1 NPs were used as seeds to grow an iron oxide shell. In brief, a solution containing 40 mg of S1 NPs, 1.1 g of oleylamine, and 0.15 g of oleic acid in 50 mL of dibenzyl ether was magnetically stirred and degassed (10−2 Torr) at 90 °C. The mixture was heated up to 220 °C under N2 flow, and then a solution containing 0.05 g of Fe(acac)3 acetylacetonate, 0.5 g of oleylamine, and 0.1 g of oleic acid in 5 mL of dibenzyl ether was rapidly injected. The slurry was then kept at 220 °C for 1 h, and finally the flask was removed from the heating source and cooled down under nitrogen to room temperature (sample CS1). In a second approach, 0.5 mL of MiliQ H2O (>1% in weight) was added to the solution containing the seeds after the degassing process and before the injection of the iron precursor (sample HW1). Synthesis of Sample S2. A mixture of 1 g of manganese(II) acetylacetonate and 30 mL of oleylamine was degassed with several cycles of vacuum/nitrogen (10−2 Torr) at 80 °C and then heated up under magnetic stirring to 210 °C for 3 h.29 The slurry was removed from the heating source and cooled down to room temperature, opening the flask to air at 80 °C. All the synthesized nanoparticles were washed by several cycles of coagulation with ethanol, centrifugation at 5000 rpm, disposal of supernatant solution, and redispersion in hexane. Transmission Electron Microscopy (TEM). TEM images were obtained using a CM12 PHILIPS microscope with a LaB6 filament operated at 100 kV. The particle size and its standard deviation were obtained by calculating the number-average by manually measuring the diameters of more than 300 particles from the TEM images. High angle annular dark field (HAADF) images and electron energy loss spectroscopy (EELS) analysis were performed in a FEI Tecnai F20 equipped with a Quantum GIF EELS spectrometer. For the tomography reconstruction, a tilt series of 41 HAADF images from −71.8° to 48° was acquired every 3° in a JEOL 2010F TEM equipped with a field emission gun. The images were aligned and reconstructed with the Inspect3D software. The results were then processed for visualization and morphological analysis with Avizo6. X-ray Diffraction (XRD). The structure of the nanoparticles was investigated by XRD using a Bruker New D8 ADVANCE ECO diffractometer with Cu Kα radiation. Quantitative analysis of the XRD data was performed with a full pattern fitting procedure based on the fundamental parameter approach (Rietveld method) using the MAUD program.30 Fourier Transform Infrared Spectra (FTIR). FTIR were acquired in a 1600 PerkinElmer infrared spectrometer. Magnetometric Analysis. The magnetic properties of the nanoparticles were measured on tightly packed powdered samples using a vibrating sample magnetometer with a maximum field of 90 kOe (VSM, Quantum Design PPMS).

Figure 1. TEM images and particle size histograms for S1, CS1, and HW1 NPs. Scale bars correspond to 50 nm. The insets show schematic 3D images of the NPs (green, red, and gray colors refer to MnO, Mn3O4, and Fe3O4 phases, respectively).

expected spherical fully dense CS morphology with a 1−2 nm thick iron oxide shell (CS1).31,32 An EELS quantitative analysis33 (see Supporting Information, SI) reveals that the iron oxide phase in the shell can be assigned to magnetite (Fe3O4) and the core region is mainly composed by Mn2+ ions (MnO). In addition, the XRD patterns indicate the presence of Mn3O4 (Figure 3) although in a lower percentage than in the initial seeds (Table S1 and SI). In contrast, sample HW1, grown by adding H2O, exhibits a clear central void and a “crown” composed by small nanograins (Figure 1). EELS analyses reveal a Mn-rich inner ring surrounding the void (7−10 nm of diameter), with Fe ions in both inner and outer surfaces (Figure 2b and d). Tomography reconstruction of HW1 depicts a complex network of 5−10 nm holes that connect the hollow interior with the outside (Figure 4 and Supporting Video). The EELS analysis shows that while the Fe-phase is Fe3O4, similar to CS1, the Mn one is Mn3O4 (Figure S1 and SI).34,35 The XRD pattern supports the EELS results, since no traces of MnO (i.e., less than 5% in weight) have been observed (Figure 3 and Table S1). In concordance with the morpho-structural characterization, the magnetic properties of the three nanostructures (S1, CS1, and HW1) depend markedly on the sample, evidencing also the changes of the morphology. Figure S2a shows the 50 kOe field cool (FC) hysteresis loop for samples S1, CS1, and HW1 at 10 K. The CS character of the S1 sample is corroborated by the

3. RESULTS AND DISCUSSION In a first experiment, two different Fe−Mn oxide nanostructures were synthesized by seeded-growth31,32 starting from the MnO/Mn3O4 core/shell, CS, seeds (S1), and adding (HW1) or not (CS1) small traces of Milli-Q H2O (>1% w) into the reaction vessel prior to Fe-oxide shell precursor injection 8026


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Figure 2. (a, b) HAADF images and EELS mapping (red and green refer to the Fe and Mn L-edge signals). (c and d) Background and Fe elemental percentage profiles along two NPs for CS1 and HW1, respectively.

Figure 3. XRD pattern for S1 (top), CS1 (middle), and HW1 (bottom). The symbols correspond to the experimental data, the thick solid black lines correspond to the Rietveld refinement, and the thin lines show the contribution of each specific crystal structure: MnO (green), Mn3O4 (red), and Fe3O4 (blue).

presence of a large coercive field (HC) and exchange bias (HE), i.e., 5 and 3.4 kOe, respectively, characteristic of antiferromagnetic/ferrimagnetic (AFM/FiM) MnO/Mn3O4 CS NPs.36 On the other hand, both heterostructured NPs (samples CS1 and HW1) show smooth loops expected for FiM hard/soft exchange-coupled structures22 exhibiting some HE and a moderate HC. Although the presence of HE is expected for exchange coupled AFM/FiM systems, it can also be observed in coupled soft−hard FiM (or ferromagnetic) structures.37 Interestingly, even if both heterostructured NPs show similar HE (0.6 and 0.7 kOe for CS1 and HW1, respectively), their HC is rather different (i.e., 0.7 and 1.4 kOe for CS1 and HW1, respectively). By drawing an analogy with exchange biased systems,22,37 where HE ∝ 1/size(soft‑FM), it would indicate a similar iron oxide shell thickness in both systems. Conversely, the interpretation of the HC behavior is more complex, since due to the different morphology between CS1 and HW1, different induced demagnetization fields or possible variations in their magnetic switching behavior could arise.7,38,39 In fact, a closer inspection of the hysteresis loops of CS1 and HW1

Figure 4. 3D tomographic reconstruction of two NPs of the HW1 sample. Slices of the 3D tomographic reconstruction in the (a) XY, (b) front, (c) back, (d) XZ, and (e) YZ planes.

shows that while the loop for CS1 changes monotonically, for HW1 it appears to have some narrowing at low fields (see inset in Figure S2). This may indicate a change in the reversal mode, involving, for example, a vortex-like structure, as has been proposed for other hollow magnetic structures.7,38,39 In addition, zero field cool (ZFC) and FC magnetization curves of all three samples have a similar characteristic transition at ∼40 K, ascribed to the Curie temperature (TC) of the FiM Mn3O4 phase (Figure S2b).36 However, while for the initial MnO/Mn3O4 CS NPs (sample S1) the FC curve increases below TC, for both heterostructures (samples CS1 and HW1) a fast decrease of magnetization is observed. Recently this effect has been ascribed to the antiferromagnetic exchange coupling between Mn3O4 and Fe3O4 FiM phases in CS NPs.31 The existence of a clear AFM-coupling indicates a sharp interphase between Mn3O4 and Fe3O4 in both samples,31,32,40 as evidenced 8027


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Figure 5. TEM images for HW2 and HW3 NPs. Scale bars correspond to 20 nm.

that, even in the most oxidant conditions, the amount of oxygen available is negligible compared to that of refs 15 and 28. Moreover, the pH of water (slightly acid in the work of McDonagh et al.28 and Shin et al.15) probably also plays a key role in the dissolution of Mn2+ ions triggering the hollowing reaction, assisted by acid etching. In addition, in order to further elucidate the role played by the MnOx phase in the galvanic process, pure 11 (1) nm Mn3O4 NPs (sample S2; see Experimental Section and Figure S4) were used as seeds for the iron oxide growth in a similar approach as for the synthesis of sample CS1. Remarkably, in Figure 5a it can be observed that hollow Mn3O4−Fe3O4 heterostructures (sample HW2) were obtained without the use of small traces of H2O or any oxidation agent. Indeed, similarly as previously observed for sample HW1, as-synthesized HW2 nanoparticles depict 2−3 nm center voids and a 2 nm particle size increase due to the iron oxide deposition onto the manganese oxide seed surface (see Figures 5 and S4). However, in this case, the small size of the NPs results in the collapse of some of the particles, with more irregular shapes and the formation of small fragments.41 This probably indicates that, in smaller NPs, which should have better cationic diffusion, the galvanic reaction is more advanced, leading to the complete removal of the MnOx scaffold and partial collapse of the structures. Since galvanic replacement and transmetallization reactions require solid state diffusion, these results highlight how the morphology and size of the employed seeds are critical parameters in the final obtained nanostructures. These results corroborate the predominant role of the oxidation state of the manganese phase to activate the galvanic reaction. In addition, these results discard the possible secondary effects of H2O in the reaction mechanism, such as breaking down the dense structure of the Mn3O4 shell and permitting the formation of pores and, therefore, channels for iron precursor species to access the inner MnO core. Moreover, the formation of reactive species between the H2O and the organic solvent molecules can also be discarded to have any effect on the reaction mechanism. Thus, MnO oxidation to Mn3O4 could be understood through a classical diffusion mediated and growth mechanism. Similarly, the possible role of oleic acid/oleylamine in the hollowing process was also evaluated to discard any acid etching mediated process (see SI). The results reveal that the surfactants do not induce any hollowing to the S1 seed NPs. In addition, the use of oleic acid (with higher bonding strength to Mn) instead of oleylamine as a capping agent does not induce any modification of the reaction kinetics (see SI).41 Given the somewhat uncertain effect of water in the hollowing process and in order to design a simple and more reliable pathway to control the formation between MnO/ Mn3O4/Fe3O4 full-dense and hollow Mn3O4/Fe3O4 hetero-

from the EELS analysis. In order to corroborate the AFM coupling, FC curves at different fields have also been measured. For fields below a certain value (10 kOe and 5 kOe for samples CS1 and HW1, respectively) the magnetization decreases below TC, confirming the AFM coupling when the Mn3O4 becomes magnetically active. However, for sufficiently large fields, the Mn3O4 layer changes its magnetization direction, overcoming the AFM coupling with the Fe3O4 phase, resulting in an increase of the magnetization at TC (see Figure S2c). Nevertheless, for sample HW1, 50 kOe is not sufficient to induce a clear upturn of magnetization below TC. This indicates that while the change of morphology does not affect the existence of AFM-coupling, it does control the details of magnetic behavior.31 To gain a deeper understanding of the reaction mechanism in the hollowing process, the possible roles of the H2O and other synthetic parameters have been studied in detail. First, the solution containing the seeds S1 (MnO/Mn3O4 NPs) has been treated in different oxidative environments ranging from a high-oxidative atmosphere by bubbling compressed air at different temperatures (110 and 220 °C) to a nonoxidative medium (deoxygenated H2O injection) through an intermediate oxidative atmosphere (H2O2 injection). S1 seeds treated under a non-oxygenated atmosphere (samples DG110 and DG220; Figure S3a) show no change in their oxidation state, remaining mainly MnO. In contrast, when the heat treatment was carried out in a highly oxidative environment by bubbling compressed air (samples CA110 for the 110 °C aliquot and CA220 for the 220 °C aliquot; Figure S3b), a transformation to Mn3O4 is observed. Although Figure S3b shows that while at 110 °C the crystal structure remains unchanged, at 220 °C the XRD pattern turns to pure Mn3O4. A similar behavior has been observed after the injection of H2O2, i.e., highly concentrated O2-water (samples HO110 and HO220; Figure S3c), where the oxidative process occurs only at high temperature, i.e., 220 °C (Figure S3c). Interestingly, the TEM images corroborate that the injection of degassed H2O, compressed air bubbling, or H2O2 does not alter significantly the morphology of the NPs (Figure S3). Importantly, these experiments clearly demonstrate that pure water does not essentially affect the crystal structure of the seeds for the temperatures reached during the reaction, but if not properly degassed, it plays a key role acting as O2 carrier, leading to the oxidation of MnO into Mn3O4. Note that these results are different from the ones obtained by McDonagh et al.28 and Shin et al.,15 where hollow Mn3O4 structures are obtained by the Kirkendall effect when MnO/ Mn3O4 nanoparticles are exposed to a purely aqueous medium. The fact that for samples HW1 and HW3 the treatments are carried out in organic media, with only traces of oxygen, implies 8028


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replacement can be defined as the limiting step controlling the kinetics of the whole reaction mechanism.46 Conversely, in the case of fully dense core/shell structure (sample CS1), where no water was added, there was no hollowing effect because the process is thermodynamically not favored due to the presence of a Mn2+-rich structure in the core. As shown by XRD, the growth of Fe3O4 onto S1 leads to an increase of MnO in the core due to the release of Mn2+ ions from the Mn3O4. Hence, in this scenario, it seems that it is energetically more favorable that the released Mn2+ ions lead to the growth of the MnO core, than being dissolved and released outside the particle though channels or pinholes creating voids, which is the case when water or other oxidants are present. Notably, under these conditions (i.e., the preexistence of a MnO core in the seeds before galvanic reaction starts and an inert atmosphere) it is possible to form additional MnO phase from the Mn3+ release. On the other hand, when water or other oxidants are present, or in the case of using pure Mn3O4 as seeds (sample S2), only Mn3O4 is present before galvanic reaction starts; therefore, the reduction of Mn3+ does not lead to the formation of MnO (not favored due to the different metal-to-oxygen ratio of both structures without the preexistence of a MnO core) and is released to the media as free Mn2+ ions if water is present or forming a complex with free molecules of the surfactant. Thus, our results evidence that the unusual use of H2O in metal-oxide galvanic replacement reactions is not strictly necessary.21,24−26 In fact, the presence of H2O is revealed as playing the role of catalyzer to oxidize the MnO/Mn3O4 seeds due to the presence of oxygen. The stabilization of H2O in organic media is possible due to the formation of micelles21 in a water−organic emulsion (stabilized with oleic acid and oleylamine), favoring the complete oxidation of the MnO phase from MnO/Mn3O4 to Mn3O4. Indeed, the fact that we can obtain hollow structures starting from fully oxidized Mn3O4 seeds, without the presence H2O, demonstrates that what is important for the hollowing process is the oxidation state of the seed, i.e., the presence of Mn3+ throughout the seeds, to favor the Mn3+ → Mn2+ reduction of the inner parts by the Fe2+ ions. Interestingly, the possibility to develop a completely allorganic synthesis opens a new perspective in the synthesis of metal-oxide hollow structures by galvanic replacement reactions. It should be noted that while pure organic media permit the formation of a broad range of well-defined different types of NPs, the uncertain effect of water in the reaction system can modify and/or deteriorate the final properties of the assynthesized product. Certainly, taking into account the high reactivity of most of the transition metal oxides with water,47−49 the presence of small traces of the polar solvent can strongly affect and modify the physicochemical properties of the NPs by changing completely50 or partially (i.e., passivation)51 their structural and chemical composition.49 In this sense, given the appealing catalytic and gas sensing properties of the oxide hollow structures, the control and quality of their surfaces, such as their crystallinity and functionalization, is an essential parameter to develop reliable and scalable products. An additional advantage of an all-organic synthesis is the wellknown fact that the synthesis in organic media allows the possibility to have better control of the morphology7 and size36 of the NPs, which, in fact, is of vital importance to control their chemical activity. Finally, it is worth emphasizing that the possibility to use galvanic replacement reactions in oxide structures where two or

structures from MnO/Mn3O4 NPs, a pure organic oxidant, (CH3)3NO, was chosen to in situ oxidize sample S1 and posteriorly use it as seed to deposit the iron precursor (sample HW3; Figure 5). (CH3)3NO is a mild oxidant that is used, for example, in the formation of γ-Fe2O3 nanoparticles through oxidation of Fe NPs in organic media.42 This particular oxidant was chosen since it avoids the use of H2O to carry out the oxidation of MnO/Mn3O4 seeds so galvanic reaction can be carried out in organic media. Indeed, given the difficulty to control the concentration of O2 species in the H2O, the use of (CH3)3NO allows the complete control of the oxidation intermediate step in the reaction mechanism. Interestingly, similar to what has been previously observed in the case of H2O2 and compressed air, an excess of (CH3)3NO (weight ratio of 5:1 (CH3)3NO:S1) forces the complete oxidation of the MnO NPs at high temperatures (Figure S5a). As can be observed in Figure 5b, hollow structures (i.e., similar particle size increase and diameter of the voids than for sample HW1) can certainly be formed in a purely organic synthesis if a proper amount of an oxidizing agent (which favors the MnO → Mn3O4 transformation) is introduced in the reaction. Taking into account the previous results, it can be confirmed that the correct MnOx phase of the seeds and the iron oxide deposition onto their surface are the only driving parameters that trigger the hollowing process. Therefore, it can be assumed that the interaction between both parameters through a galvanic replacement reaction is the responsible mechanism for the formation of the carved structures. Moreover, it can be assumed that the galvanic process occurs as a solid−solid reaction at the interface of both materials. Namely, the apolar character of the solvent (i.e., benzyl ether, with a dielectric contestant ε = 4−5) avoids the presence of a proper electrolyte to carry out the conduction of electrons in the solution.43 In fact, even at an organic/aqueous interface of reverse vesicles, the only possible mechanism should occur at the interface between both oxide materials. Therefore, the chemical process can be defined as a galvanic reaction in an organic medium41 while the process can be extrapolated using the relative standard reduction potential values in water44 for Fe2+ and Mn3+ ions (i.e., EFe(III)/Fe(II) = 0.77 V and EMn(III)/Mn(II) = 1.51 V), where the initial Fe2+ ions (see SI) develop a spontaneous redox process (i.e., ΔEredox = 0.74 V, ΔG = −71 kJ), decreasing the amount of Mn3+ in the seed (being reduced to Mn2+) when Fe2+ is deposited on the outer and inner surfaces of the hollow structure. Moreover, the high reactivity of the Fe2+ ions and the metastability of the Fe1−xO structure at the nanoscale could trigger their oxidation to the more stable Fe3O4 structure (i.e., mixed Fe2+ and Fe3+ ions in an spinel phase).45 On the other hand Mn3O4 is a tetragonal spinel with a normal configuration where all trivalent cations are located in octahedral positions. Hence, the iron oxide growth based on the galvanic process could be interpreted by cationic replacement, where the partial Mn3+ → Mn2+ reduction permits the entrance of Fe3+ ions in the tetrahedral voids, thus keeping the spinel stoichiometry. That is, as the chemical reaction progresses, small pure Fe3O4 grains are formed on the surface and the electrons reduce the inner Mn3+ ions in the Mn3O4, forcing their dissolution through pinholes created in the nanoparticle, which allows the hollowing process. In fact, due to the strongly negative Gibbs free energy of the kinetic redox reaction, the thermodynamically favored Fe2+ nucleation (i.e., heterogeneous nucleation onto the Mn3O4 seeds) occurs instantly in comparison to the slower ionic displacement process. Thus, the diffusive 8029


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Chemistry of Materials Notes

more materials interact to generate a single heterostructure can open a new frontier in the synthesis of hollow nanoparticles. Namely, the formation of multiphase carved heterostructures, with their intrinsic large surface area, can be synergistically combined with the distinct physicochemical properties of a diverse type of materials, paving the way for novel multifunctional nanostructures.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the 2014-SGR-1015, MAT2010-20616C02, MAT2013-48628-R, MAT2013-41506, and CSD200900013 projects is acknowledged. ALO acknowledges the Juan de la Cierva Program (MINECO IJCI-2014-21530). ICN2 acknowledges support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295).

4. CONCLUSION Summarizing, the possibility to synthesize metal-oxide hollow structures by galvanic replacement using core/shell NPs seeds in either aqueous media or pure organic media is demonstrated. In addition, when Mn2+-rich structures act as seeds, small traces of oxidizing agents (H2O or (CH3)3NO) in the reaction can be used to tailor the morphology and the crystal structure of the synthesized NPs to obtain either fully dense MnO/Mn3O4/ Fe3O4 or biphase hollow Mn3O4/Fe3O4 heterostructures, evidencing that the Mn-oxide phase plays a critical role in the formation of metal-oxide hollow structures. We demonstrate that the role of H2O (or other oxidizers) is to act as carrier of O2, oxidizing the MnO core to Mn3O4 and allowing the concomitant Mn3+ → Mn2+ reduction to form hollow Mn3O4/ Fe3O4 structures when Fe2+ is deposited. The use of the organic oxidant (CH3)3NO, instead of the more commonly used H2O, has allowed us to design a more reliable synthetic pathway, for the first time in an all-organic approach, which permits a complete control of the oxidation intermediate step in the reaction mechanism to form biphase hollow Mn3O4/Fe3O4 nanostructures. Therefore, we propose the use of metal-oxide galvanic replacement reactions in a pure organic medium as a perspective for future work in more controllable hollow nanoparticles with better chemical properties for applications such as catalysis and sensing.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03765. EELS characterization. Structural data from XRD. Magnetization vs applied magnetic field hysteresis loops and ZFC magnetization vs temperature curves. Role of H2O and other synthetic parameters in the hollowing mechanism. Effect of oleic acid/oleylamine in the hollowing process. Capping ligand exchange. Fe precursor reduction (PDF) Video of the 3D tomographic reconstruction of two NPs of the Sample HW1 (AVI)

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REFERENCES

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Author Contributions

ALO, AGR, and JN conceived the idea. ALO and AGR synthesized the nanoparticles and performed the structural and magnetic measurements. MP performed IR measurements and analysis. PT, SE, and FP performed and analyzed the electron microscopy measurements. VP designed part of the experimental work. ALO, AGR, and JN wrote the manuscript. All authors contributed to revising the manuscript. 8030


Article

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Galvanic Replacement onto Complex Metal-Oxide Nanoparticles

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