Hollow Superparamagnetic Microballoons from Lifelike, Self-Directed

Hollow Superparamagnetic Microballoons from Lifelike, Self-Directed Pickering Emulsions Based on Patchy Nanoparticles Tim Granath,†,‡ Angela Sanchez-Sanchez,§ Aleksey Shmeliov,⊥ Valeria Nicolosi,⊥ Vanessa Fierro,§ Alain Celzard,§ and Karl Mandel*,†,‡ †

Fraunhofer Institute for Silicate Research ISC, Neunerplatz 2, 97082 Wuerzburg, Germany Department of Chemical Technology of Materials Synthesis, University of Wuerzburg, Roentgenring 11, 97070 Wuerzburg, Germany § Institut Jean Lamour, UMR CNRS-Université de Lorraine n°7198, ENSTIB, 27 Rue Philippe Séguin, CS 60036, Epinal 88026 Cedex, France ⊥ Trinity College Dublin, School of Chemistry, School of Physics & CRANN, Dublin 2, Ireland ‡

S Supporting Information *

ABSTRACT: Herein, the formation of hollow microballoons derived from superparamagnetic iron oxide nanoparticles with silica patches is reported. Depending on the experimental conditions, single- or multishelled superparamagnetic microballoons as well as multivesicular structures were obtained. We show how such structural changes follow a lifelike process that is based on selfdirecting Pickering emulsions. We further demonstrate that the key toward the formation of such complex architectures is the patchy nature of the nanoparticles. Interestingly, no well-defined ordering of patches on the particles surface is required, unlike what theorists formerly predicted. The resultant hollow microballoons may be turned into hollow carbonaceous magnetic microspheres by simple pyrolysis. This opens the way to additional potential applications for such ultralightweight (density: 0.16 g·cm−3) materials. KEYWORDS: patchy nanoparticles, microballoons, self-directed Pickering emulsions, hollow particles, lifelike structures

T

be theoretically obtained through the suitable design of the surface pattern of patches.5 In a later work, Jankowski and Glotzer published a conceptual approach for screening and designing patchy particles for optimized self-assembly propensity.6 Dias et al. numerically investigated the irreversible aggregation of patchy spherical colloids on a flat substrate depending on their number of patches.9 Rezvantalab and Shojaei-Zadeh studied, also numerically, the adsorption of spherical patchy particles onto a flat oil−water interface depending on both amphiphilicity and size of the patches. They showed that amphiphilic particles are more effective for interface stabilization than their homogeneous counterparts.11 A promoted effectivity toward oil−water interface stabilization by amphiphilic/Janus/patchy particles has also been described for particle-based emulsion stabilization, i.e., for the so-called Pickering emulsion formation.12,13

remendous progress has been made in recent years regarding the synthesis possibilities of nanoparticles of any kind, designed for use as nano building blocks.1−4 The next step forward in nanoscience and technology is to combine and to assemble such “artificial supergiant atoms” into complex artificial architectures. By doing so, secondary structures with distinct properties can be achieved. The key toward such complex architectures is the nanoparticles themselves. An atom is able to form only certain bonds with other atoms, ultimately yielding a specific molecule. Transferring such a concept to nanoparticles requires that the latter do not have isotropic or homogeneous but more differentiated surfaces. In other words, the nanoparticles need to have Janus properties or be equipped with patches of different chemical functionalities at their surface. Theorists such as those from Glotzer’s group5,6 and others7−11 already predicted that complex structures can be assembled in a controlled way by applying precisely engineered patchy particles. For instance, Zhang and Glotzer performed molecular simulations to study the self-assembly of patchy particles. They demonstrated that some specific structures can © 2016 American Chemical Society

Received: September 8, 2016 Accepted: October 26, 2016 Published: October 26, 2016 10347

DOI: 10.1021/acsnano.6b06063 ACS Nano 2016, 10, 10347−10356

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ACS Nano Recently, the successful practical synthesis of Janus and patchy particles has been reported more and more frequently.13−18 Extensive research focused on showing that it is possible to control the subnano arrangement of patches on the nanoparticles’ surfaces. For instance, Perro et al. were able to control the dimension of the modified patch area through wax-in-water emulsions by varying the surfactant concentration.19 Bao et al. investigated the yield of silver deposition on silica spheres and the resultant morphology, depending on the reaction temperature.20 The group of Kretzschmar controlled the coverage area of silver deposition by changing the precuring time of a PDMS mask to produce surface-anisotropic polystyrene spheres by electroless deposition.21 The same group also published a method to determine the patch geometry of patchy particles produced via glancing angle deposition by varying the angle of incidence of the vapor rays.22 In the present work, we experimentally achieved the next level, i.e., the building of complex architectures based on patchy nanoparticles. Magnetic nanoparticles with patchy silica were indeed shown to form hollow superparamagnetic microballoons with a lifelike shell behavior by self-directing Pickering emulsions. We justified the use of the term “lifelike” and demonstrated that the key toward the microballoon-shaped structural entity lies in the patchy nature of the nano building blocks. Theorists most of the time relate the formation of complex structures to the existence of very well-defined ordering of patches on the particles surface. In contrast, we showed here that structure formation only depends on the patchy nature of nanoparticles without the need of a highly ordered nature of these patches. A detailed characterization of patchy nanoparticles by transmission electron microscopy is presented in the Supporting Information. Herein, nanoparticles with superparamagnetic properties were used as building blocks for complex structures as this class of nanoparticles may provide materials with a whole set of functionalities from which, for instance, magnetically manipulatable/stirrable,23,24 heatable,25,26 or imageable27,28 materials can be created. The formation of more complex architectures with these multifunctional building blocks is expected to yield still unpredicted properties, potentially valuable for future applications. Besides the patchy particles, an organosilane/methacryloxypropyl trimethoxysilane (MEMO) was employed in our work to support the formation of complex secondary nanostructures. Organosilanes such as MEMO are indeed intrinsically hydrophobic. However, upon hydrolysis of the siloxane groups, organosilanes can gradually become hydrophilic at one end of the molecule as pictured in Figure 1. The change of hydrophilicity can be exploited for nanoparticle modification at liquid interfaces which in our works serves for the formation of complex nanoparticle architectures.

Figure 1. Organosilanes such as MEMO can gradually change their hydrophobic/hydrophilic character depending on the extent of hydrolysis.

well-known result in the case of bare iron oxide nanoparticles originally dispersed in an aqueous phase at acidic pH.29

Figure 2. Reaction of magnetic nanoparticles at an oil−water interface upon stirring with MEMO, leading to the expected phase transfer from water to oil for standard magnetite nanoparticles (a) but to a very different outcome for iron oxide nanoparticles that possess silica patches at their surface (b).

However, something remarkable happens with nanoparticles bearing patches at their surface. For the present superparamagnetic iron oxide nanoparticles with a patchy silica surface, whose synthesis was recently published,30 no phase transfer was indeed observed, but a rather creamy product resulted instead, as can be seen in Figure 2b. Detailed investigations on the iron oxide nanoparticles with their patchy silica surface can be found in Figures S1−S3 of the Supporting Information. The product obtained after drying at 45 °C overnight from the system based on patchy nanoparticle building blocks turned out to be made of hollow microballoons; see Figure 3. From the literature, it is known that modifying particles with MEMO can justify their hydrophilicity for better stabilization of Pickering emulsions.31,32 It is also known that MEMO, as an oil dispersed in water, attracts water-dispersed particles to selfassemble at the droplet interface, thus forming a thermodynamically stable Pickering emulsion.33,34 To the best of our knowledge, MEMO has never been used so far in the role of a connecting cosurfactant through its hydrolyzed form. Beyond that, we found that the role of the nature of the nanoparticles’ surface is indeed crucial to form the microballoons reported herein. This becomes obvious when comparing Figure 3 with the structure obtained from bare iron oxide nanoparticles. In the latter case, only undefined lumps resulted from drying in the same conditions; see Figure S4 of the Supporting Information. It is worth mentioning that undefined lumps were also obtained when completely isotropically silica-coated iron oxide nanoparticles were used; see

RESULTS AND DISCUSSION Reaction of Iron Oxide Nanoparticles Bearing Silica Patches with Organosilanes in an Emulsion Process. Consider a system based on chemically uniform hydrophilic nanoparticles dispersed in water coming into contact with an oil phase which contains dissolved organosilane. In our case, the oil phase is cyclohexane and the silane is MEMO. At the interface, the organosilane gradually hydrolyzes and therefore may react with the nanoparticles. Upon silanization at the interface, these nanoparticles are rendered hydrophobic. This eventually results in a phase transfer of the nanoparticles from water to the oil phase. Figure 2a shows such an expected and 10348


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Figure 3. Laser scanning microscopy (a−d) and scanning electron microscopy (e−h) images at different magnifications of hollow microballoons built from patchy iron oxide nanoparticle building blocks. Images of partly fractured balloons were chosen to visualize the hollow character of the particles.

Figure 4. Suggested routes for the formation of hollow microballoons by silanization of patchy silica−iron oxide nanoparticles and subsequent clustering (h-MEMO = hydrolyzed MEMO).

Figure S5 of the Supporting Information. High-resolution transmission electron microscopy (HRTEM) images showing the difference in micrograph appearance for (a) uncoated, bare iron oxide nanoparticles, (b) iron oxide nanoparticles with silica patches, and (c) iron oxide nanoparticles covered with an excess of silica can be found in the Supporting Information (Figure S6). To rule out the possibility of different nanoparticle interactions in dispersion during reaction with MEMO in the three following cases, dynamic light scattering (DLS) measurements on all three nanoparticle dispersions were carried out (Figure S7 of the Supporting Information): (1) bare iron oxide nanoparticles, (2) iron oxide nanoparticles with silica patches, and (3) fully silica coated iron oxide nanoparticles. From DLS, it was found that the nanoparticle size in dispersion only slightly increases from bare iron oxide nanoparticles in dispersion to the fully silica modified ones. This can be explained by a slightly larger size of the individual particles with more silica being present on their surface. However, it can be clearly stated that the dispersion state is the same in all cases, and therefore, any different nanoparticle−nanoparticle interaction which might change the synthesis result when trying to form the microballoons can be excluded. Thus, taking into account these findings and TEM studies, the microballoon formation can be truly attributed to the patchy character of the nanoparticles.

As expected from the images of Figure 3, such hollow microballoons are obtained in the form of an extremely lightweight fluffy powder, which almost hovers in air (see the Supporting Information for a video showing the particles in motion). The skeletal material density of the microballoons, i.e., the density of the shell, was found to be about 2.2 g cm−3 as measured by helium pycnometry. Thus, the calculated density of exemplarily selected hollow microballoons with a diameter of 20 μm and a shell thickness about 250 nm is only about 0.16 g· cm−3. This value is remarkably low for a magnetically easily manipulatable micro-object (see the video in the Supporting Information and Figure S8a for the magnetic properties) compared to the original density of iron oxides such as magnetite, which is about 5.2 g·cm−3. Even if the microballoons were still completely filled with residual oil (cyclohexane), a density of only about 0.88 g·cm−3 would result. The theoretical calculations yielding the values for the density of the microballoons are presented in detail in the last section of the Supporting Information. As such lightweight hollow magnetic microballoons may have great potential for various applications, their reproducible manufacturing is of key importance and so is the understanding of their formation mechanism. Therefore, the formation principles were investigated much more closely as described below. 10349


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Figure 5. Evolution of hollow microballoons: silanized, patchy nanoparticles form lifelike clusters together with oil. With increasing reaction/ stirring time from 1 h (a) to 3 h (b), 5 h (c), 9 h (d), and then 48 h (e), the oil migrates to the center, forming a soft core. Because of their lifelike behavior, the nanoparticle-based shell supports this migration (from a to d) until the hollow structure collapses (e). The bottom row in the figure shows the powder obtained after drying, whose volume increases until collapse.

Mechanisms of Hollow Superparamagnetic Microballoon Formation. First of all, it is observed that intense stirring of the biphasic system, i.e., water containing the patchy particles and oil containing the silane, leads to the formation of an emulsion. The reaction of the silane with the nanoparticles, whenever these two species encounter each other, is of particular importance. The particle surface met by the silane can indeed be either iron oxide (core) or silica (patches). Under the chosen conditions at pH 1−2, corresponding to the region of the isoelectric point (IEP) of silica,30 the silica patches are the regions of lowest reactivity of the nanoparticles’ surface. In contrast, the iron oxide surfaces are positively charged and acidic and are therefore the preferred reaction spots for the silane. Thus, the silanol groups prefer to react with, and attach to, the iron oxide surface. Ultimately, the patchy particles therefore bear both hydrophobic and hydrophilic regions. The former ones correspond to silanized iron oxide, whereas the latter are the silica patches. Such hydrophobic−hydrophilic patchy particles are expected to be ideal for preparing Pickering emulsions.13 Considering the whole emulsion system, there are two conceivable reaction routes that might occur after silane hydrolysis and that might lead to the hollow microballoons shown in Figure 3: (1) The amphiphilic silanes behave as typical surfactants and form micelles. A reaction may take place at the micellar interface between the nanoparticles and the silane groups, forming a thin nanoparticle shell (reaction path 1 in Figure 4). (2) The amphiphilic silanes react with the nanoparticle surface, followed by nanoparticle clustering, which results in compact structures and thicker shells (reaction path 2 in Figure 4). The reaction mechanism that actually prevails in the system can be identified by looking at the obtained product volume

after different reaction times and by studying the appearance of the nanostructures at the microscopic level. In fact, it is remarkable that the volume of the powder obtained after drying increased drastically with stirring time before it suddenly reduced to only a tiny fraction after the emulsion was stirred for too long a time; see Figure 5. The observation of the evolution of one microballoon over time at the microscopic level reveals that the initially solid spheres gradually develop a hollow core. This cavity grows with time, yielding a thinner and thinner shell until collapse (Figure 5). The aforementioned observations are not typical for particlestabilized emulsions and cannot be explained with reaction path 1 as depicted in Figure 4. However, it can be explained by assuming aggregation and clustering, i.e., by reaction path 2 as depicted in Figure 4, which is therefore assumed to account for the particle evolution. The nanoparticle clustering and/or aggregation in emulsion as described and observed here is known for other particle-based emulsion reactions, as those, for instance, reviewed by Binks12 and Walther and Müller.13 The formation of hollow cores such as those observed here can be explained in detail as follows: (i) Initially, each hydrophobic nanoparticle patch region adsorbs oil. (ii) Thereafter, the amphiphilic nanoparticles form clusters between which oil is trapped. (iii) The oil coalesces into droplets and more efficiently retains nanoparticles as larger clusters at their surface. Simultaneously, additional amphiphilic particles add to the clusters at the water−oil interface and shield the oil (Pickering effect). This ultimately results in compact structures behaving as nanosized oil reservoirs. As the reaction medium is acidic, the condensation of the silane is slow, which prevents cross-linking. The latter point is important as the solidification of the hollow spherical structure is now hindered. The remaining nanoparticle mobility within the yet flexible cluster 10350


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Figure 6. Structures formed from the patchy nanoparticles resemble those known from living systems: unilamellar vesicles (a), multilamellar vesicles (b), as well as multivesicular vesicles (c). Furthermore, structures known from geoscience are observed (d). (a, c) Laser-scanning micrographs; (b, d) scanning electron micrographs.

Figure 7. C, O, Fe, and Si line-scanned EDX profiles across the thickness of microballoon shells reveal a slight gradient of increased iron oxide nanoparticle concentration toward the water interface. This principle holds for both types of emulsions: (a) with excess of water compared to oil or (b)with excess of oil compared to water. The yellow circle in the inset marks the region of the shell where the line-scanned EDX analysis was performed. Note: at% denotes the relative atomic fractions calculated from the sum of the elements C, O, Fe, and Si.

known from geoscience, was found when a more viscous oil such as xylene was used; see Figure 6d. In geology, it is caused by trapped gas bubbles in the cooling of lava. Here, the higher viscosity and the slowly progressive condensation prevent further vesicle release. ULV may preferably result from nanoparticle-stabilized emulsion droplets where a particle mono- or multilayer forms a “breathable skin” caused by the interstices between the grains (note that the term “breathable skin” was introduced in the context of Janus grains by de Gennes35). MLV and MVV may also build up based on the Pickering effect, as they might be a product of initially formed clusters, within which oil coalesced in different places. The structure depicted in Figure 6d suggests how new particles are born: As the soft core grows, some material is released, which is a well-known process in living, biological systems, so that new particles form from formerly built ones. While the mother generation of particles increases the diameter of its hollow core, a daughter generation of new particles is formed from the released material. Smaller vesicles are then released from lysis- and cytosis-like processes, which ultimately explain the observed volume expansion for the

structure enables the oil to gradually migrate to the center within the cluster. In other words, it has to be assumed that the whole cluster system is in motion during the evolution of the microballoons; i.e., it behaves like it living. As described below and shown in Figure 6, the phenomena described above may lead to all kinds of evolving vesicular structures that can be unilamellar or multilamellar or even multivesicular. Finally, when the structure has evolved in a way that the oil core is too large with respect to the shell thickness, the microballoons collapse; see the far right column of Figure 5. The assumption of clustering-based particle evolution provides us with an explanation for the growth of the hollow core in a self-consistent manner. However, with progressing reaction time, it has also been observed that the number of microballoons increased. To explain this, further investigations were carried out regarding the formation mechanisms within the emulsion. Therefore, samples were quenched at different stages of their synthesis and investigated by laser scanning microscopy. All kinds of vesicular structures such as unilamellar vesicles (ULV), multilamellar vesicles (MLV), as well as multivesicular vesicles (MVV) were observed; see Figure 6a−c. In addition to these structures, a sort of vesicular structure, 10351


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Figure 8. Results of SEM (a−c), EDX (d), Raman (e), and XRD (f) studies on carbonized microballoons. Raman spectra were obtained for three zones of the hollow microballoons carbonized at 900 °C; their intensity was shifted by 1000 counts for clarity. Zones 1 and 2 correspond to different areas of the spheres, whereas zone 3 corresponds to a through-thickness area of the shell.

patchy particles. The use of amphiphilic particles indeed helps to reduce the liquid−liquid interface energy and yields a higher shell stability.13 To further support that the patchy nature of the nanoparticles is the key toward the formation of hollow balloon architectures by lifelike, self-directing Pickering emulsions, different parameters were varied. As described in the beginning of the Results and Discussion, additional experiments were carried out with pure iron oxide nanoparticles, i.e., without silica patches (see Figure S4 of the Supporting Information) and with iron oxide nanoparticles homogeneously coated with silica (see Figure S5 of the Supporting Information). In none of these cases was it possible to create hollow microballoon structures: only lumps were obtained. Any dependence on the degree of dispersion was ruled out as well by DLS investigations, as explained previously (see also Figure S7 of the Supporting Information). To finally rule out any major influence coming from the nature of the silane, we repeated the process again but (i) only with the silane and without any nanoparticle and (ii) with the patchy nanoparticles but with different silanes such as cyclohexyltrimethoxysilane and propyltrimethoxysilane. For case (i), no microparticles could be obtained, whereas case (ii) showed that most of the silanes yielded the same kind of hollow microballoons. All of these additional experiments led to the conclusion that the patchy nature of the nanoparticles is definitely the key toward the complex particle architecture reported herein. Materials Modification and Potential Applications. Technological applications of the present magnetic microballoons are currently considered in fields wherein either their ultralow density or their hollow character, or both, might be important features. As expected, due to their extreme lightness, the microballoons almost hover in air and float on liquids while at the same time are magnetically steerable. Thus, for instance, magnetically controlled fluidized bed reactors based on catalytically activated microballoons can be imagined. Moreover, the particles might be interesting candidates for collecting oil spills. Figure S9 thus presents a series of images showing magnetic microballoons that were added to a water surface covered with oil and, subsequently, magnetically collected together with the oil. Potential electromagnetic applications making use of the hollow nature of the microballoons are also predicted with

product recovered after the reaction is stopped at different stirring times. In nature, systems and evolution conditions comparable to those reported here can be found. The most prominent example is the cell membrane,36 also a highly flexible shell. The artificial membrane herein is the shell of the microballoons. As demonstrated above, the patches play a decisive role; thus the patchy particles can be seen as a key toward the complexity of the lifelike matter. Eventually, a last level of complexity can be found in the system, again made possible by the patchy nature of the nano building blocks. It was indeed observed that the emulsion formation is self-directing. The microballoon formation occurs for both oil-rich or water-rich systems. Empirically, it was found that balloons, harvested after synthesis in a semidried state and subsequently mechanically fractured, lost either water or oil from their interior, depending on whether their synthesis had been conducted in excess of oil or water, respectively. Scanning electron microscopy energy dispersive X-ray analyses (SEM-EDX), although only of a semiquantitative nature, can give an additional hint supporting the observation of self-directing assembly. Figure 7 shows two high-resolution SEM images with EDX analysis of two shells of particles for systems with an excess of water on one hand (water to oil volume ratio 5:3, Figure 7a) and with an excess of oil (water to oil volume ratio 3:5, Figure 7b) on the other hand. It can be seen that the shell is always composed of iron oxide and silica. The silica originates from the patches of the particles and from the organosilane. EDX reveals that there is a slight gradient of magnetic nanoparticle content as seen by the Fe profile. Figure 7 shows that such a gradient is diametrically opposite for the two systems (a) and (b). Therefore, it can be concluded that the oil to water ratio also directs the assembly of the particles. This happens in a way that the nanoparticle concentration during shell formation increases toward the shell−water interface. The opposite behavior of the Si gradient is most probably due to the Si content coming from unreacted silane, which has a higher affinity toward the oil-rich side of the balloons. In the case of multishelled microballoons such as those shown in Figure 6b, alternating gradients of nanoparticle concentrations in the shells can be observed within one MLV. This finding strongly supports that a Pickering emulsion was initially formed and stabilized the droplet system. Theorists predict that Pickering emulsions form quite easily from Janus or 10352


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larger magnetic domains were formed. The magnetic properties of the particles are shown in Figure S8b. The assumption that the domains increased is consistent with the observation that the crystallite size of the particles for the samples was found to be about 32 nm (determined by application of Scherrer’s equation), i.e., the iron oxide nanoparticles which had a size of 10 nm grew in the furnace process to >30 nm and got reduced into pure Fe. It can be concluded that a highly interesting material can be obtained that is magnetic and composed of carbon combined with an ultralightweight hollow structure. Investigations of the electromagnetic properties of such magnetic hollow carbon microspheres are presently underway.

regard to their expected interaction with electromagnetic waves. For instance, such kinds of particles could be used as antireflective coatings in a range of frequencies depending on the microballoons’ diameters and shell thicknesses.37 In this context, it is worth noting that the particles can furthermore be easily turned into hollow magnetic carbon spheres, giving them electrical conductivity. Such behavior was expected as this kind of material was indeed already reported from the pyrolysis of various iron-containing organic38−40 or organometallic41−45 precursors. With this goal in mind, the microballoons were simply pyrolyzed at 900 °C under inert atmosphere. The silane (MEMO) was the source of carbon, and the latter was integrated into the hollow structure after pyrolysis. Figure S10 (Supporting Information) shows the TGA of the microballoons undergoing pyrolysis. A substantial weight loss of about 37 wt % occurred between 250 and 555 °C due to the thermal decomposition of MEMO, giving rise to released gases (H2O, CO, and CO2, among others) and to a partial conversion into carbon. Before carbonization, samples had BET surface areas (SBET) of 3 and 191 m2 g−1 for collapsed and noncollapsed hollow microballoons, respectively. After heating to only 300 °C, the hollow microballoons had an intermediate SBET of 60 m2 g−1. Application of the nonlinear density functionality theory (DFT) to the nitrogen adsorption isotherms led to surface areas, SDFT, ranging from 14 to 276 m2 g−1. Finding higher values than those of SBET indicates the presence of a fraction of narrow microporosity. It is indeed well-known that the BET measurement method underestimates the surface area as soon as narrow pores exist.46 After carbonization at 900 °C, the surface areas decreased for all materials, and SDFT were found to be lower than 50 m2 g−1. Such a decrease is related to that of the porosity in the whole range of investigated pore sizes; see Table S1. Figure 8 shows SEM images (a−c), energy dispersive X-ray spectroscopy (EDX) (d), Raman spectra (e), and an X-ray diffraction pattern (f) of the magnetic hollow carbon microballoons. From the SEM images, it can be seen that some balloons partly collapsed or just broke into hemispheres, but most of them retained their shape. The carbon that was found besides Si and Fe in the EDX spectra could be more closely classified by Raman spectroscopy. Raman spectra exhibited broad D and G bands at 1330 ± 5 and 1600 ± 5 cm−1, respectively, as well as a shallow valley in between and a shoulder at 1190 cm−1. These features are typical of highly disordered carbons.47 Spectra obtained at different places on the microballoon surface (zones 1 and 2 in Figure 8e) did not show significant differences in terms of D/G intensity ratio (1.00 and 1.01, respectively), whereas the spectrum obtained in the thickness of the shell (zone 3) had a lower D/G ratio (0.966). As such a ratio is often considered as a graphitization indicator,48 these results suggest that the carbon present at the microballoon surface is a little less organized than the one incorporated inside the shell. This finding is rather logical if one considers that the carbon located within the shell was submitted to a confinement effect as well as a close contact with silica, both known to be in favor of a more organized carbon texture. From XRD it can be seen that the nano iron oxide turned into pure iron, which is related to the reducing power of the carbonization process. The particles are still magnetic, but superparamagnetic (anhysteretic) iron oxide turned into ferromagnetic (hysteretic) iron, thereby suggesting that much

CONCLUSION AND OUTLOOK In the present work, the theoretical prediction that patchy particles yield remarkable results in terms of Pickering emulsion stabilization effects was experimentally demonstrated. Furthermore, the patchy character of the nanoparticles was found to enable the formation of complex secondary nanostructure entities such as hollow superparamagnetic microballoons. Those hollow magnetic microballoons showed, during their evolution, a lifelike behavior which was accounted for by the patchy nature of the nano building blocks. The resultant structures are remarkably lightweight and almost hovering in air. Due to their extremely low density and to their hollow structure, such magnetic microparticles may have various potential applications. Particularly interesting is the possibility of turning such spheres into hollow magnetic carbon microballoons by simple pyrolysis, with expected electromagnetic applications. METHODS Materials. Iron(III) chloride hexahydrate (FeCl3·6H2O, 99%), iron(II)chloride tetrahydrate (FeCl2·4H2O, 99%), ammonia solution (aqueous NH3, 28−30 wt %), p-xylene (99%), octadec-1-ene (99%), octan-1-ol (99%), and n-heptane (99%) were obtained from Sigma-Aldrich (Germany). 3Methacryloxypropyltrimethoxysilane (MEMO, 98%) and cyclohexane (100%) were obtained from VWR International (Germany). Tetraethyl orthosilicate (TEOS, 99%), cyclohexyltrimethoxysilane (97%), and propyltrimethoxysilane (97%) were obtained from abcr GmbH (Germany). Nitric acid (HNO3, 1 M, diluted from a 53 wt % solution) was obtained from Otto Fischar GmbH & Co. KG (Germany). Ethanol (EtOH, 99%, denatured with 1% methyl ethyl ketone) was obtained from CSC JÄ KLECHEMIE GmbH & Co. KG (Germany). Sodium silicate (water glass) solution (36 wt %, molar ratio of SiO2/Na2O = 3:1, Na2Si3O7) was obtained from Fischer Chemicals (Germany). All reagents were used without further purification. Synthesis of Superparamagnetic Iron Oxide Nanoparticles. Superparamagnetic iron oxide nanoparticles were synthesized by the co-precipitation method,49 and stabilization was performed as described in a previous publication.50 In brief, 2.16 g (8 mmol) of FeCl3·6H2O and 795 mg (4 mmol) of FeCl2·4H2O were dissolved in 100 mL of deionized water. Then 5 mL of an aqueous ammonia solution (NH3 (aq), 28− 30%) was quickly added under stirring. The black precipitate was magnetically separated and washed until neutral pH. Then 10 mL of 1 M nitric acid (HNO3) was added and subsequently diluted with 20 mL of water to yield a pH of below 1. 10353


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Optical micrographs. Optical images were obtained using a Keyence 3D laser scanning confocal microscope VK-X 200 series (controller, VK-X 200; measuring unit, VK-X 210). SEM. SEM was carried out with a Zeiss Supra 25 SEM at 2 keV. EDX were performed at 10 keV at a working distance of 9 mm. TEM and ADF-STEM. Patchy iron oxide nanoparticles were deposited onto ultrathin carbon film on holey carbon. Imaging and EDX characterization were carried out using a TEM FEI Titan with Si(Li) EDX detector. The FEI Titan was operating at 300 keV. DLS. To quantify the particle size in aqueous dispersions, the hydrodynamic size of the particles was measured by DLS using Malvern Zetasizer NanoZS. Magnetic Measurements. Magnetic properties of the hollow microballoons were studied with a vibrating sample magnetometer (VSM, VersaLab 3 T Cryogen-free vibrating sample magnetometer) by cycling the applied field from −30 to +30 kOe for two times with a step rate of 100 Oe s−1. The temperature was set to 20 °C. Helium Pycnometry. Skeletal densities were determined by helium pycnometry (Accupyc II1340, Micromeritics). The samples were previously degassed at 60 °C under vacuum. The analysis was performed at 25 °C in order to avoid He adsorption. The measurement cell was filled with as much material as possible so that the typical uncertainty was less than 2%. Nitrogen and Carbon Dioxide Adsorption. Pore texture parameters were obtained by nitrogen and carbon dioxide adsorption at −196 and 0 °C, respectively, using a Micromeritics ASAP 2020 automatic apparatus. Samples were degassed for 48 h under vacuum at 60 or 125 °C depending on whether the materials were characterized by gas adsorption before or after carbonization, respectively. The measurement cell was filled with enough material so that the absolute surface area that was measured was always higher than 15 m2. As a result, the typical uncertainty on the specific surface area, i.e., with respect to the sample weight, was always lower than 5%. The pore size distribution (PSD), the micropore volume (Vμ‑DFT), and the surface area (SDFT) were determined by application of the nonlinear density functional theory to the CO2 and N2 adsorption isotherms.58 The SAIEUS software, provided by Micromeritics, allowed us to obtain one single PSD from the two adsorption isotherms, nitrogen and carbon dioxide. Nitrogen adsorption data were treated for obtaining the BET surface area, SBET,59 the micropore volume by applying the Dubinin−Radushkevich (DR) model (Vμ‑DR, N2),60 the total pore volume, V0.97,61 and the mesopore volume, Vm, calculated as the difference V0.97 − Vμ‑DFT. Additional assessment of microporosity was obtained by application of the DR model to the CO2 adsorption isotherm (Vμ‑DR, CO2). Raman Spectrometry. Raman spectra were obtained with a Horiba XploRa Raman spectrometer without sample preparation. The spectra were collected under a microscope using a 100× objective. The Raman-scattered light was dispersed by a holographic grating with 1200 lines per mm and detected by a CCD camera. A laser of wavelength 532 nm filtered at 10% of its nominal power was used. The corresponding incident power, around 1.8 mW, was low enough to avoid any heating or damage of the samples. Each spectrum was obtained by accumulation of two spectra recorded from 50 to 5000 cm−1 over 10 to 120s, depending

Synthesis of Patchy Nanoparticles. Following previous work,30 a dispersion of iron oxide nanoparticles with silica patches was prepared by adding 5 mL of TEOS to the suspension of superparamagnetic iron oxide nanoparticles described above after dilution to 100 mL with stirring (which yielded a pH of about 1−2). After 1 h reaction time, the product was aged for at least 1 day. Synthesis of Densely Silica-Coated Magnetite. Two routes were used to obtain densely silica-coated iron oxide nanoparticles. The first method (route A) was performed by adding 5 mL of TEOS to the as-prepared patchy nanoparticle suspension after 24 h of aging to fully cover the remaining free iron oxide surfaces. After 1 h reaction time, the product was again aged for at least 1 day. For the secondary route (route B), 40 mL of the magnetic nanoparticle fluid was further diluted with 40 mL of H2O and subsequently added to 200 mL of ethanol. Subsequently, the procedure followed the protocol, published by Sun et al., which is based on an initial seed coating of the iron oxide nanoparticles with a water glass source prior to TEOS modification to ultimately yield a dense silica shell around the nanoparticles.51 Concentration and pH of the final product were adjusted by dilution with H2O and dropwise addition of 1 M nitric acid to pH 2 prior to the synthesis attempt to form microballoons. Synthesis of Microballoons. The microballoon synthesis is based on a concept that simultaneously combines the surface modification of nanoparticles by a coupling agent like MEMO52−54 with phase transfer from an aqueous phase to an organic phase as already known for surfactant-assisted paths.55−57 Typically, 210 mL of the acidic aqueous nanoparticle dispersion was carefully covered with 200 mL of oil such as cyclohexane. Then 1240 μL (5.2 mmol) of MEMO was slowly dropped into the oil phase. After that, vigorous stirring was performed for several hours. Thus, a stable emulsion that contained thin-walled emulsion droplets was obtained. The vigorous stirring period ranging from 1 to 9 h determined the type of resultant microballoons as shown in Figure 5. Longer stirring times (at least 48 h) led to collapsed microballoons. After the mixing was stopped, typical emulsion processes such as coalescence, creaming, and sedimentation occurred. Immediate washing with ethanol was observed to destroy the still living and yet evolving shells around the droplets and therefore the complex spheres. Thus, allowing the reaction product to age without stirring for at least a couple of hours is a crucial step since further condensation of the silanol groups appears to be dramatically slowed, thus resulting in the formation of fixed shells. To obtain the microballoons in powder form, the product was dried at 45 °C after washing overnight (18 h) in air. To test the influence of the nature of the silane on the formation of microballoons, other silanes such as cyclohexyltrimethoxysilane and propyltrimethoxysilane were used. The synthesis was the same as described above except that MEMO was replaced by an equimolar amount of the newly selected silane. Carbonization of Microballoons. A microballoon sample (∼0.25 g) was carbonized at a low heating rate (1 °C min−1) under nitrogen flow (80 mL min−1) up to the target temperature (900 °C, 1 h) and was subsequently cooled to room temperature in order to recover the carbonized material. The carbonization yield was 46.65%. 10354


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ACS Nano

(9) Dias, C. S.; Araújo, N. A. M.; Telo da Gama, M. M. Effect of the Number of Patches on the Growth of Networks of Patchy Colloids on Substrates. Mol. Phys. 2015, 113, 1069−1075. (10) Bianchi, E.; Blaak, R.; Likos, C. N. Patchy Colloids: State of the Art and Perspectives. Phys. Chem. Chem. Phys. 2011, 13, 6397−6410. (11) Rezvantalab, H.; Shojaei-Zadeh, S. Designing Patchy Particles for Optimum Interfacial Activity. Phys. Chem. Chem. Phys. 2014, 16, 8283−8293. (12) Binks, B. P. Particles as Surfactantssimilarities and Differences. Curr. Opin. Colloid Interface Sci. 2002, 7, 21−41. (13) Walther, A.; Müller, A. H. E. Janus Particles: Synthesis, SelfAssembly, Physical Properties, and Applications. Chem. Rev. (Washington, DC, U. S.) 2013, 113, 5194−5261. (14) Shah, A. A.; Schultz, B.; Kohlstedt, K. L.; Glotzer, S. C.; Solomon, M. J. Synthesis, Assembly, and Image Analysis of Spheroidal Patchy Particles. Langmuir 2013, 29, 4688−4696. (15) Pawar, A. B.; Kretzschmar, I. Fabrication, Assembly, and Application of Patchy Particles. Macromol. Rapid Commun. 2010, 31, 150−168. (16) Rodríguez-Fernández, D.; Liz-Marzán, L. M. Metallic Janus and Patchy Particles. Part. Part. Syst. Charact. 2013, 30, 46−60. (17) Loget, G.; Kuhn, A. Bulk Synthesis of Janus Objects and Asymmetric Patchy Particles. J. Mater. Chem. 2012, 22, 15457−15474. (18) Kaewsaneha, C.; Tangboriboonrat, P.; Polpanich, D.; Eissa, M.; Elaissari, A. Preparation of Janus Colloidal Particles via Pickering Emulsion: An Overview. Colloids Surf., A 2013, 439, 35−42. (19) Perro, A.; Meunier, F.; Schmitt, V.; Ravaine, S. Production of Large Quantities of “Janus” Nanoparticles Using Wax-in-Water Emulsions. Colloids Surf., A 2009, 332, 57−62. (20) Bao, H.; Peukert, W.; Taylor, R. K. One-Pot Colloidal Synthesis of Plasmonic Patchy Particles. Adv. Mater. 2011, 23, 2644−2649. (21) Cui, J.-Q.; Kretzschmar, I. Surface-Anisotropic Polystyrene Spheres by Electroless Deposition. Langmuir 2006, 22, 8281−8284. (22) Pawar, A. B.; Kretzschmar, I. Patchy Particles by Glancing Angle Deposition. Langmuir 2008, 24, 355−358. (23) Lyubchanskii, I. L.; Dadoenkova, N. N.; Lyubchanskii, M. I.; Shapovalov, E. A.; Rasing, T. Magnetic Photonic Crystals. J. Phys. D: Appl. Phys. 2003, 36, R277−R287. (24) Chong, W. H.; Chin, L. K.; Tan, R. L. S.; Wang, H.; Liu, A. Q.; Chen, H. Stirring in Suspension: Nanometer-Sized Magnetic Stir Bars. Angew. Chem., Int. Ed. 2013, 52, 8570−8573. (25) Laurent, S.; Dutz, S.; Häfeli, U. O.; Mahmoudi, M. Magnetic Fluid Hyperthermia: Focus on Superparamagnetic Iron Oxide Nanoparticles. Adv. Colloid Interface Sci. 2011, 166, 8−23. (26) Gonzales-Weimuller, M.; Zeisberger, M.; Krishnan, K. M. SizeDependant Heating Rates of Iron Oxide Nanoparticles for Magnetic Fluid Hyperthermia. J. Magn. Magn. Mater. 2009, 321, 1947−1950. (27) Veiseh, O.; Gunn, J. W.; Zhang, M. Design and Fabrication of Magnetic Nanoparticles for Targeted Drug Delivery and Imaging. Adv. Drug Delivery Rev. 2010, 62, 284−304. (28) Lam, T.; Pouliot, P.; Avti, P. K.; Lesage, F.; Kakkar, A. K. Superparamagnetic Iron Oxide Based Nanoprobes for Imaging and Theranostics. Adv. Colloid Interface Sci. 2013, 199−200, 95−113. (29) Mandel, K.; Szczerba, W.; Thünemann, A. F.; Riesemeier, H.; Girod, M.; Sextl, G. Nitric Acid-Stabilized Superparamagnetic Iron Oxide Nanoparticles Studied with X-Rays. J. Nanopart. Res. 2012, 14, 1066. (30) Mandel, K.; Straßer, M.; Granath, T.; Dembski, S.; Sextl, G. Surfactant Free Superparamagnetic Iron Oxide Nanoparticles for Stable Ferrofluids in Physiological Solutions. Chem. Commun. (Cambridge, U. K.) 2015, 51, 2863−2866. (31) Yin, D.; Zhang, Q.; Yin, C.; Zhao, X.; Zhang, H. Hollow Microspheres with Covalent-Bonded Colloidal and Polymeric Shell by Pickering Emulsion Polymerization. Polym. Adv. Technol. 2012, 23, 273−277. (32) Yin, D.; Zhang, Q.; Zhang, H.; Yin, C. Fabrication of Covalently-Bonded Polystyrene/SiO2 Composites by Pickering Emulsion Polymerization. J. Polym. Res. 2010, 17, 689−696.

on the sample. Several zones of each sample were investigated for checking the homogeneity. TGA. Thermogravimetric analysis was carried out with a STA 449F3 Jupiter (NETZSCH) thermobalance by heating ∼20 mg sample under argon flow up to the target temperature (900 °C, 2 °C min−1). The released gases (H2O, CO, and CO2) were analyzed in continuous mode with a QMS 403D Aëolos (NETZSCH) mass spectrometer. XRD. X-ray diffraction (XRD) was performed on dried powder samples. A PANalytical Empyrean Series 2 X-ray diffractometer employing Cu Kα radiation (λ = 0.15406 nm) was used (step size 0.013 2θ, typical count time 58.4 s). Indication of reflections was carried out with reference to the International Centre for Diffraction Data PDF-4. The crystallite size of the iron nanoparticles was determined on the basis of Scherrer’s equation using the Highscore Plus PANalaytics software to measure the fwhm of the most prominent Fe XRD peaks (referenced to the LaB6 standard).

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.6b06063. Figures S1−S10 and Table S1 (PDF) Magnetic microballoons in motion (AVI)

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Tel: +49 931 4100 402. Notes

The authors declare no competing financial interest.

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Hollow Superparamagnetic Microballoons from Lifelike, Self-Directed

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