Path-Dependent Self-Assembly of Magnetic Anisotropic Colloidal Peanuts

Here we present the field induced self-assembly of anisotropic colloidal particles whose shape resembles peanuts. Being made up of hematite core and silica shell, these particles align in a direction perpendicular to the applied external magnetic field. Using small-angle X-ray scattering with microradian resolution (μrad-SAXS) in sedimented samples, we have found that one can tune the self-assembled structures by changing the time of application of the external field. If the field is applied after the sedimentation, the self-assembled structure is a nematic one, while dipolar chains are formed if the field is applied during the sedimentation process. Interestingly, within each chain particles form a smectic phase with defects. Further, these aforementioned nematic and smectic phases are of oblate type in spite of the prolate shape of the individual particles. For dipolar chains, an unusual diffraction peak shape has been observed with highly anisotropic tails in the transverse direction (perpendicular to the external field). The peak shape can be rationalized by considering the fact that the dipolar chains can act as a building block aligned along the field direction to form a para-nematic phase.


■ INTRODUCTION
The ability to manipulate the self-assembly in colloidal systems has opened up an alternative route toward the development of smart materials with designable properties. 1,2 As a result, a considerable amount of research has been carried out over the last couple of decades to understand as well as to manipulate the self-assembled structures formed by colloidal particles possessing basic shapes like spheres, 3,4 rods, 5−7 disks, 8 and ellipsoids. 9,10 Recent progress in the synthetic techniques of colloidal particles has resulted in the fabrication of particles with complex shapes resembling peanuts, 11−13 dumbbells, 14−16 polyhedras, 17,18 and octapods, 19 to name but a few. Controlling the self-assembly of these particles requires a thorough control over the interparticle interactions. Additionally, there are some other major challenges that also need to be overcomecontrol over particle orientation, their localization, and their registry into self-assembled structures. Therefore, the possibility to explore the different self-assembled structures formed from diverse complex building blocks has contributed significantly toward reinvigorating colloidal science in recent times.
One of the simplest ways to regulate the self-assembly process is through an external stimulus. Of all the different colloidal particles that can in principle be manipulated using external stimuli, magnetic colloids have attracted much attention. The primary reason behind this is the fast and reversible nature of the dipole−dipole interactions together with their wide-ranging applicability in diverse fields including photonics, 20,21 drug delivery, 22,23 patterning, 24,25 and magnetic levitation, 26,27 to name but a few. In order to fully exploit the potential of stimuli responsive self-assembled structures, one needs to understand not only the final structure but also the kinetics of the self-assembly processes. Thus, acquired knowledge would be particularly helpful in avoiding the undesirable metastable phases that might restrict the possibility of obtaining the desired smart synthetic materials. Understanding the path dependent self-assembly process will have wider implications on different areas of science and engineering, e.g., condensed matter physics and metallurgy, with impact on applications that range from obtaining tailor-made metallic alloys and ceramics to modern shape-memory materials. 28,29 In this article, we present our investigations on the stimuli responsive self-assembly of micron-sized anisotropic peanutshaped colloids in sedimented samples using μrad-SAXS. Sedimentation has been used with an intention to efficiently sample the phase behavior of these particles as a function of packing fraction. Being made up of hematite core and silica shell, they orient with their long axes in planes perpendicular to the field direction. We demonstrate that the self-assembled structures strongly depend on when the external field is switched onduring or after the sedimentation process. When the field is applied after sedimentation, an oblate nematic phase results with the nematic director oriented along the short axes of the particles. However, the scenario changes drastically once the field is applied during the sedimentation process. In the latter case, dipolar chains form due to the dipole−dipole interaction between the particles. In addition, we also observed a peculiar stretching of the diffraction peaks perpendicular to the direction of the dipolar chains. We believe that in this case the dipolar chains can act as elongated building blocks which align along the field direction to form a para-nematic phase. Owing to the large particle size and high density, their gravitational length is quite small; as a result, we did not observe any variation in the self-assembled structures as a function of sedimentation height.

■ SYNTHESIS AND CHARACTERIZATION METHODS
Synthesis. Micron-sized hematite peanuts were synthesized following the sol−gel method proposed by Sugimoto et al. 30 In a typical synthesis, 90 mL of 6.0 M NaOH (Merck) was slowly added in approximately 5 min to 100 mL of a magnetically stirred 2.0 M FeCl 3 (Sigma-Aldrich, puriss. p.a.) solution followed by the addition of 10 mL of a 0.25 M Na 2 SO 4 (Acros, 99%) solution. Subsequently, the obtained condensed iron hydroxide gel was aged at 100°C for 8 days. The resulting precipitated particles were then purified by repeated centrifugation and redispersion cycles in water.
In the next step, the hematite particles were coated with an additional silica layer by following Graf et al. 31 As an initial step, the particles were coated with a sterically stabilizing layer by adding excess of PVP to an aqueous hematite particle dispersion while stirring. The dispersion was stirred overnight and finally was washed with ethanol. The silica coating was performed in a 3 L round-bottom flask under mechanical stirring and ultrasonication. A mixture of 1 L ethanol, 100 mL water, and 10 mL of a 1 wt % TMAH solution was prepared and 10 g of 10 wt % PVP-stabilized hematite dispersion in ethanol was added. Subsequently, TEOS solutions were added using a peristaltic pump operating at 16 mL/h. The TEOS solution was prepared by mixing 10 mL of TEOS and 10 mL of ethanol. To prevent aggregation, a solution of 20 g PVP in 100 mL water was added to the dispersion, which was then sonicated for 2 more hours and stirred overnight. The silicacoated particle dispersion was washed several times with ethanol and then with water.
Characterization Methods. The size and shape of the peanut-shaped colloidal particles were characterized using transmission electron microscopy (TEM) (Philips TECNAI 10) (Figure 1a). The length of the particles and the diameter of their lobes are found to be 1723 ± 50 nm and 740 ± 50 nm, respectively, by analyzing more than 100 particles in the TEM images.
For SAXS measurements, dispersions of peanuts ∼5 wt % were placed in capillaries with internal dimensions of 100 × 4 × 0.2 mm 3 (W3520 Vitrocom) that were flame-sealed and stored vertically and left undisturbed for 24 h for particle sedimentation. In order to investigate the self-assembled structures, the sedimentation profiles were scanned over the full length of the capillary along the vertical directions while Z = 0 was set at the bottom of the capillaries. SAXS measurements were performed at BM26B beamline at ESRF, Grenoble. We have employed a μrad-SAXS setup which involves compound refractive lenses. 32 The 13 kev X-ray beam was focused on a CCD X-ray detector which was placed at a distance of 7.45 m from the samples. The data have been recorded with a Photonic science detector with pixel size of 24 × 24 μm 2 . The detector was protected from the direct X-ray beam using a wedge-shaped beam-stop that shades the detector. The capillaries were oriented vertically with their length (100 mm) parallel to the gravitational field. All the measurements were carried out in the presence of external magnetic fields which were applied using a permanent magnet. The direction of the magnetic field (B), X-ray beam, and gravity were perpendicular to each other as shown in Figure  1b.

■ RESULTS AND DISCUSSION
The self-assembly of peanut shaped colloidal particles was studied in the presence of external magnetic fields under two different conditions. In the first case, the magnetic field was applied after the sedimentation process was over, whereas in the second case, the field was applied while the particles were still sedimenting. Interestingly, the subsequent self-assembled structures exhibited a strong dependence upon when the field was applied as discussed in the following subsections.
Route 1: Application of the Magnetic Field after Sedimentation. In this case, the filled-in capillaries were stored vertically unperturbed for 24 h. Once the particles had formed thick sedimented layers, magnetic fields of different magnitudes were applied and the measurements were done. For all the magnetic fields measured (12.3 mT ≤ B ≤ 380 mT), we found nicely ordered nematic phases, the representative diffraction pattern of which is shown in Figure  2a. However, the aforementioned nematic phase is quite different from that normally observed for rod-like objects. 5−7 For colloidal rods, a nematic phase is expected when L/D > 3.4. 33 In addition, the nematic order for this phase is expected to be along the long axes of the particles. In the present case, although the L/D ∼ 2.3, we still observe a nematic phase in the presence of an external field. However, in the absence of external field, we observe an isotropic phase as expected, which does not show any variation in the nearest neighbor distance as a function of height from the bottom of the capillary (Supporting Information). Further, the nematic order of the field induced self-assembled phase is along the short axes of the particles. From that point of view, this nematic phase is similar to the one that is observed for colloidal platelets where the nematic director is along the short axes of the particles. 8 In a nutshell, one can say that although the particles are prolate in shape, the nematic phase formed by them has the characteristics of the one formed by oblate particles. Having obtained similar results for colloidal ellipsoids (unpublished results) leads us to a conclusion that the aforementioned feature seems The Journal of Physical Chemistry B pubs.acs.org/JPCB Article to be generic for rod-like anisotropic colloidal particles with hematite core. These observations can be rationalized by considering the fact that in both these cases the dipole moments lie in a plane perpendicular to the hematite c-axis which is parallel to the long axes of the particles. 11 As a result, in the presence of an external magnetic field the particles tend to align with their short axes parallel to the field direction. However, their long axes can still rotate around the magnetic field as shown in Figure 2b. This very peculiar arrangement of the dipole moments makes hematite ellipsoids and peanuts form oblate nematic phases in the presence of an external field in spite of the prolate shape of the particles. The nearest neighbor distance, d, did not show any significant change when the magnitude of the magnetic field varied. Figure 2c shows the intensity profiles for different magnetic fields along the direction of the field shown by the white line in Figure 2a. Only at the maximum field (380 mT) does one find a slight shift in the peak position toward a higher q or lower d value. This indicates that at very high field value the resulting dipole−dipole attraction is significant, resulting in the formation of a more close-packed structure.
Due to the large size of the particles, their gravitational length, L g = k B T/M b g (k B T being the thermal energy, M b being the buoyant mass, and g being the gravitational acceleration), which is a measure of the balance between the thermal energy and the gravitational force, came out to be very small, ∼175 nm. As a result, dense sediments of the particles were formed, where gravitational compression occurred due to the mass of the particles lying above. Due to this very high gravitational compression, we did not observe any variation of the nearest neighbor distance as a function of height (Z) from the bottom of the capillaries (Figure 2d).
We have characterized the nematic phase by the orientational order parameter, S 2 . Following Purdy et al., 34,35 the azimuthal intensity distribution (between the red dotted lines in Figure 2a) can be fitted with where I 0 is the normalizing factor, ω is the azimuthal angle, and f(ω) is the orientational distribution function where the parameter A determines the width of the distribution function and P(ω) is the Legendre polynomial  Figure 3a shows the fit of the experimental data for different magnetic fields using the Purdy model, while Figure 3b represents the variation of S 2 as a function of magnetic field B at a particular height (Z = 2.0 mm) from the bottom of the capillaries. The analysis revealed that the orientational order develops as a function of the field strength, starting from a low value (<0.5), S 2 reaches a value of 0.85 at 380 mT. Similar analysis for the data at different heights of the capillaries indicate that there is not much variation of S 2 as a function of Z (Figure 3c,d).
Route 2: Application of the Magnetic Field during Sedimentation. In this case, the magnetic fields were applied soon after filling the capillaries with colloidal dispersion and continued during the sedimentation process. Figure 4a shows a representative diffraction pattern under this condition. An interesting feature that is obvious from the diffraction pattern is the anisotropy in the peaks perpendicular to the field direction. We have tried to come up with a plausible explanation in order to rationalize this observation by considering the fact that these particles form dipolar chains.
These chains can in principle be considered as prolate objects that align along the field direction to form a para-nematic phase. They show a quasi-periodic structure along their axes, which is also the external field direction. However, in the perpendicular direction, they scatter up to q ⊥ ∼ 2π/d eff , with d eff being the effective diameter of the chains. Using SAXS data d eff is found to be 1957 nm, which is in good agreement with the length of the particles. Further, the intensity profile along the direction of the field shows the appearance of a secondorder peak (Figure 4b), the q value of which is double that of the first-order peak indicating the formation of a smectic phase. The nearest neighbor distance along this direction is found to be 1150 nm, which corresponds to the width of a single layer. We have used double Lorentzian function (I = I 0 + (2A 1 /π)· (w 1 /(4 × (q − q c1 ) 2 + w 1 2 )) + (2A 2 /π)·(w 2 /(4 × (q − q c2 ) 2 + w 2 2 ))) to fit our experimental data and found the width of the first peak to be 0.0018 nm −1 indicating that around three layers are correlated. Surprisingly, just like the nematic phase (formed in the first case when the magnetic fields were applied after sedimentation), this smectic phase also has an oblate nature though the particles are prolate in shape.
To further probe the structure, we obtained the images of these self-assembled structures in real space using bright field microscopy. Representative bright field micrograph shown in Figure 4c proves that indeed under this particular condition these particles self-assembled into dipolar chains. In addition,  Figure 4c (which is characteristics of smectic phase) with lots of defects. As already mentioned, as a signature of this smectic phase a second-order peak can be seen in the diffraction pattern (Figure 4a,b) as well as in the FFT (Figure 4d).

■ CONCLUSION
In conclusion, we have shown that one can tune the selfassembled structures formed by anisotropic magnetic colloidal peanuts in the presence of external magnetic field by changing the time of application of the external field. In the present case, depending on the time of application of the external field, two different self-assembled phases results. A nematic phase was obtained when the magnetic field was applied after the sedimentation process, while a phase made up of local dipolar chains resulted if the field was applied during the sedimentation process. Further, within each chain particles form smectic phase with lots of defects. Another interesting observation was that, although the particles are prolate in shape, the nematic and smectic phase formed by them has the characteristics of the one formed by oblate shaped particles. We believe this to be a generic feature for prolate-shaped, hematite anisotropic magnetic colloids with high dipolar moment. Our results will have direct implications in situations where manipulation of colloidal self-assembly using a external field will be employed as an alternative route toward the design of new materials. Our results indicate that it is not only important to study the final self-assembled structures but also the kinetic pathway leading to the final structures. Thus, acquired knowledge can be used to avoid metastable states while developing application oriented smart materials.
■ ASSOCIATED CONTENT
Representative diffraction pattern as well as intensity profiles corresponding to the isotropic phase formed in absence of an external field (PDF)