Erik
Wetterskog
*ab,
Alice
Klapper
c,
Sabrina
Disch
d,
Elisabeth
Josten
ce,
Raphaël P.
Hermann
cf,
Ulrich
Rücker
c,
Thomas
Brückel
c,
Lennart
Bergström
a and
German
Salazar-Alvarez
*a
aDepartment of Materials and Environmental Chemistry, Arrhenius Laboratory, Stockholm University, Sweden. E-mail: erik.wetterskog@angstrom.uu.se; gersal@protonmail.com
bDepartment of Engineering Sciences, Ångström Laboratory, Uppsala University, Sweden
cJülich Centre for Neutron Science JCNS and Peter Grünberg Institut PGI, JARA-FIT, Forschungszentrum Jülich, 52425 Jülich, Germany
dDepartment of Chemistry, Universität zu Köln, 50939 Köln, Germany
eHelmholtz-Zentrum Dresden-Rossendorf, Institute of Ion Beam Physics and Materials Research, Bautzner Landstr. 400, 01328 Dresden, Germany
fMaterials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, 37831 Tennessee, USA
First published on 11th July 2016
A precise control over the meso- and microstructure of ordered and aligned nanoparticle assemblies, i.e., mesocrystals, is essential in the quest for exploiting the collective material properties for potential applications. In this work, we produced evaporation-induced self-assembled mesocrystals with different mesostructures and crystal habits based on iron oxide nanocubes by varying the nanocube size and shape and by applying magnetic fields. A full 3D characterization of the mesocrystals was performed using image analysis, high-resolution scanning electron microscopy and Grazing Incidence Small Angle X-ray Scattering (GISAXS). This enabled the structural determination of e.g. multi-domain mesocrystals with complex crystal habits and the quantification of interparticle distances with sub-nm precision. Mesocrystals of small nanocubes (l = 8.6–12.6 nm) are isostructural with a body centred tetragonal (bct) lattice whereas assemblies of the largest nanocubes in this study (l = 13.6 nm) additionally form a simple cubic (sc) lattice. The mesocrystal habit can be tuned from a square, hexagonal to star-like and pillar shapes depending on the particle size and shape and the strength of the applied magnetic field. Finally, we outline a qualitative phase diagram of the evaporation-induced self-assembled superparamagnetic iron oxide nanocube mesocrystals based on nanocube edge length and magnetic field strength.
In this work, we investigate the influence of particle size, shape and applied magnetic field on the formation of mesocrystals based on oleate-capped iron oxide nanocubes. We assembled nanocubes with four different edge lengths into highly ordered mesocrystals by controlled evaporation of the carrier solvent during drop-casting. We performed a full 3D characterization of the mesostructures using Grazing Incidence Small Angle X-ray Scattering (GISAXS) and determined the nanocube size and shape using Transmission Electron Microscopy (TEM). The interparticle distances in the mesocrystals were found to scale with the size and local curvature of the particles. The applied magnetic field strength influences the structure over several length scales and generates both single domain and complex multi-domain mesocrystals, and can even result in structures defined by ferrohydrodynamic instabilities. The micro- and mesostructure of the mesocrystals were analysed in detail by High Resolution Scanning Electron Microscopy (HRSEM) and image reconstruction techniques. Finally, we propose a qualitative micro- and mesostructural phase diagram of the nanocubes, based on edge length and applied magnetic field.
There is considerable difference in the translational order between the zero-field assemblies of the C086 and C096 nanocubes displayed in Fig. 1b. The C086 nanocubes form close packed arrays but lack long range order (cf. Debye–Scherrer ring in the inset of Fig. 1b) whereas the C096 nanocubes form large ordered mesocrystals (cf. spot pattern in the inset of Fig. 1b)—i.e. faceted, monodomain arrays of particles with a pronounced mesoscopic texture. The mesocrystals have flat top surfaces and uniform heights of 0.5–1 μm (see AFM images, Fig. S10†), suggesting that the vertical growth and thus the final thickness of the mesocrystals are limited by the thickness of the dispersion film.50 Drop-casting the C126 nanocubes also produces mesocrystals under zero-field conditions (Fig. 1b). The C126-based mesocrystals display several morphologies: cuboidal, hexagonal and truncated triangular platelets. The different mesocrystal morphologies are also associated with a distinct symmetry at the top surface layer characterized by 4-fold (cuboids) or 2-fold (hexagonal/triangular platelets) rotation axes. Evaporation-induced assembly of C136 nanocubes under zero-field conditions also generates mesocrystals, albeit with a considerably smaller domain size compared to mesocrystals produced from C096 and C126 (see Fig. 1b).
GISAXS patterns of nanocube mesocrystals assembled in zero-field are shown in Fig. 1c and corroborate the trend observed in the SEM images. The GISAXS pattern of a dispersion of C086 evaporated in zero-field displays broad reflections indicating a partially/short range ordered mesostructure of the dense packed array, as seen with SEM. In contrast, the C096 mesocrystals exhibit a highly ordered 3D mesostructure, clearly evidenced by the large number of sharp reflections in the scattering pattern. The structural analysis of the GISAXS patterns of C096-based mesocrystals reveals that they consist of a single mesostructure that has grown along two different orientations (see Fig. 2a and b). Analysis of the GISAXS data in Fig. 1c yields a body centered tetragonal (bct) lattice with a = b < c and the [001]MC-axis (the subscript denotes mesocrystal) parallel to the substrate normal. Additionally, we identify a second growth orientation (shown in Fig. 2b) corresponding to the [101]MC-orientation of the same lattice (indexing of this orientation is found in Fig. S5, ESI†). Qualitative analysis of the reflection intensities in the GISAXS patterns (see Fig. S7 and discussion, ESI†) was used to estimate the ratio of growth orientations in the C096- and C126-based mesocrystals. We found that C096 favours growth along the [001]MC-orientation (yielding square-shaped mesocrystals) whereas C126 favours growth in the [101]MC-orientation corresponding to a hexagonal or a truncated triangular mesocrystal habit. The different crystal habits can be linked to morphological differences of the nanocubes (nC096: 3.8 vs. nC126: 2.9). Particles closer to an ideal cube tend to deposit face-on on the substrate and form a square basal plane whereas more rounded cubes with a higher degree of blunting of the edges and corners can rotate and deposit edge-on on the substrate (see Fig. 2b), thereby forming pseudo-hexagonal (2-fold) layers. This is in line with our previous findings, where we observed a shift in the symmetry of the horizontal layers (from 4-fold to 6-fold) after morphological aging of the C086 nanocubes to a more rounded shape.46
Fig. 2 Representation of the two growth orientations of the body centered tetragonal (bct) mesocrystal lattice and the variation of the lattice parameters and volume fraction with the nanocube edge length. (a) Structural model of the [001]MC-oriented body centered tetragonal (bct) lattice. The face-to-face separation distance d is indicated. (b) Structural model of the [101]MC-oriented bct lattice. The unit cell of a rotated (and distorted) bct unit cell with lattice parameters ar, br and cr is indicated by dashed lines. The larger unit cell corresponds to an orthorhombic unit cell (see ESI† for more information). Nanocubes in the top layer of the [101]MC oriented bct lattice are shown in the inset. The flat substrate is indicated by the green slab. (c) Plot of the bct lattice parameters corresponding to the [001]MC unit cell and to the rotated [101]MC unit cell. Note that for cubes larger than C086 there are two growth orientations with overlapping symbols. (d) Plot of the nanocube volume fraction vs. nanocube edge length in the self-assembled mesocrystals. |
Remarkably, the GISAXS data in Fig. 1c (and Fig. S3, ESI†) reveal that the C086, C096, C126 systems and (in part) the C136 system are isostructural: each nanocube dispersion mesocrystallizes in a bct lattice. Similarly to the C096 system, the C126- and C136-based mesocrystals grow in two principal orientations: [001]MC and [101]MC with lattice parameters a, c, and ar, cr respectively (the subscript letter denotes rotation, see Fig. 2a and b). For the C136-based mesocrystals, there is a crossover from bct to sc (simple cubic) for the [001]MC-oriented lattice. The [101]MC-oriented structure remains isostructural with the other systems: a bct lattice with c-axis .
Orientational alignment of anisotropic nanocrystals has been reported to result from e.g. anisotropic van der Waals (vdW) interactions,51,52 whereas the role of the surfactant has been less clear. In this work, owing to the ability to determine particle positions in the mesocrystal together with the precise characterization of the size and shape of the monodisperse nanocubes, we can estimate the average separation distance of the nanocubes in the mesocrystals. The principal bct lattice parameters (a, ar), and (c, cr) vs. nanocube edge length (l) are plotted in Fig. 2c. For comparison, we have included the lattice parameters of the C086-based mesocrystals where a small magnetic field of μ0H ≈ 30 mT has been applied to promote the formation of an ordered bct lattice.30 For the C086–C126-based mesocrystals, we see that a depends linearly on the nanocube edge length l so that a/nm ≈ 7 + 0.7l. This implies a gradual decrease from ≈4.5(2) to 3.4(2) nm of the separation distance between the faces of the nanocubes that correspond to two squeezed oleate capping layers: d/nm = a − l = 7 − 0.3l (see Fig. S9†). This compression reflects the dynamic structure of the oleic acid double layer as a result of increasing vdW attraction between the nanocube faces. The oleic acid double layer appears to be incompressible beyond the minimum distance of ≈3.5 nm (cf. C126 and C136). This distance agrees reasonably well with twice the length of the L-form conformation of the oleic acid molecule (cf. β phase).53
Previous reports have suggested that a high (local) curvature increases the free volume for a grafted surfactant.54 This allows for a high degree of surfactant chain interdigitation, which was experimentally confirmed by comparing laureate-capped films (2D) with particles (3D).55 In the case of superellipsoids (n > 2), the Gaussian curvature approaches zero (are locally flat) at the centre of the face and increases rapidly towards its corners (shown in Fig. S2a, ESI†). In this logic, arrays of spherical particles (with Dsph ≈ lcub and n = 2) should have shorter interparticle distances than nanocubes interacting face-to-face due a higher degree of interdigitation. Indeed, arrays composed of nanospheres with Dsph = 9.2 ± 0.6 nm (afcc = 17.5(1) nm)46 display a significantly shorter interparticle distance of d = 3.2(2) nm compared to the interlayer face-to-face distances of the C086- and C096-based mesocrystals: d = 4.5(2)–4.1(2) nm.
Moreover, we notice that the two sets of nanocubes with more rounded morphologies, i.e. C086 and C126 (n = 2.7 and 2.9, respectively), form bct lattices with “short” c-axes ≈2l in each case implying a short intra-layer distance <1 nm in the tetragonal ABAB stacking (see Fig. 2a). The short intra-layer distance relates to interacting nanocube corners, i.e. surfaces with high local curvature that are expected to result in a high degree of interdigitation. The effective thickness of the “first” (A) nanocube layer (including the surfactant layers at the face of the cuboids corresponding to ≈4.5–3.5 nm) means that the nanocubes in the B-layer sit slightly recessed in the holes of the square lattice of the A-layer (see Fig. 2a). From a simple geometrical view, an increase of n results in a concomitant decrease of the hole size in the A-layer (see Fig. S2b, ESI†), thereby “pushing” out the particle in the B-layer. Indeed, the assembly of the relatively small, but less rounded C096 (n = 3.8) results in a bct lattice with c > 2l and a slightly longer intra-layer distance of ≈2 nm.
For the largest and relatively less rounded C136 nanocubes (n = 3.7) we observe two distinct mesostructures: a [101]MC-oriented bct lattice (see Fig. S5†) and a [001]MC-oriented sc lattice (with asc = abct, see Fig. 1) which is not observed for the other particle sizes. In a previous study,30 we attributed the preference for a bct lattice over a sc lattice to the exceptionally short interlayer corner-to-corner distance (≈0.6 nm) between the nanocubes in C086-based bct mesocrystals. As discussed above, the increase of n (which relates to a decrease in the degree of truncation) yields a longer c-axis, due to an associated decrease of the interstitial hole volume in the A-layer. This, in turn, will weaken the interlayer attraction and favour the transition to a sc structure. The experimental observations made in this work therefore confirm the previously suggested stability diagram30 derived for the C086 nanocubes. Compared to the C086 system, we observe an elongation of the c-axis with the decrease in truncation (cf. C096). For the largest cubes in this study (C136), the concomitant reduction of the interstitial hole volume relative to the cube volume leads to the formation of a simple cubic lattice. Interestingly, the simple cubic arrangement is actually less dense than the corresponding bct lattice: 40(2) vs. 46(3)% (see Fig. 2d).
The co-existence of two structures (bct, sc) in the C136 nanocube system suggests that their lattice energies are very close. We speculate that the structural divergence with respect to substrate orientation relates to the initial growth conditions. Assuming layer-by-layer growth, the hole volume of the 4-fold layers (see Fig. S2b, ESI†) is small compared to the nanocube, effectively rendering the surface flat. In contrast, the top layer of the [101]MC-oriented bct lattice (see Fig. 2b) is a surface with a much larger topographic roughness that can accommodate cubes in the next layer. The isostructurality of the nanocube mesocrystal system, together with the micro- and mesostructural diversity presented here, highlights its structural richness and sensitivity to small variations in particle size and shape.
Mesocrystallization with or without an applied magnetic field exhibits two notable differences. For the C096 nanocubes, the habits of the mesocrystals change drastically, from square to irregular branched shapes, in particular 4-pointed stars (see Fig. 3a and b and S11†). The GISAXS patterns reveal a change in the preferred growth orientation from [001]MC in zero field to [101]MC when the drop casting has been performed in a field of 65 mT (see Fig. S7 and discussion, ESI†). A similar field-assisted morphological crossover is observed for the C126 system (see Fig. 3c and d), although the in-field assembled mesocrystals are slightly smaller and less branched compared to the C096-based mesocrystals. For the C126 system the GISAXS analysis suggests a crossover from [101]MC under zero-field conditions to a slight preference for [001]MC for assembly at 65 mT. Analysis of HRSEM images also indicates a similar change in the growth direction (see Fig. S12†).
Two examples of irregularly shaped C096-based mesocrystals are shown in the tilted view in Fig. 4a and b. A central cross-section of a mesocrystal shown in Fig. 4c (here viewed parallel to the substrate) clearly demonstrates a high degree of order throughout the volume of the mesocrystal. The peculiar microstructures (a majority of them being 4-pointed stars) of the magnetic field-assembled mesocrystals originate from the intergrowth of several mesostructural domains that were analysed in detail using HRSEM.
Fig. 5a shows a mesocrystal where the different domains have been coloured according to the plane group symmetry of the top layer, with either a 2-fold (pmm) or 4-fold (p4mm) symmetry, cf. the green and blue models in Fig. 5b. The reciprocal lattice distances derived from FFT patterns show that the domains with 4-fold (p4mm) symmetry correspond to the (001)MC cleavage planes of the previously described bct lattice. The lattice parameter obtained from the analysis of the FFT of the SEM image, a = 13.5(3) nm, is in good agreement with that from GISAXS, a = 13.70(1) nm. Particles in domains with a 2-fold (pmm) symmetry have characteristic nearest neighbour distances of 13.5(3) nm and 15.2(3) nm. These distances match the (101)MC cleavage planes of the same bct lattice (see Fig. 5b), in perfect agreement with the GISAXS analysis. The co-existence of several structurally correlated domains within a single mesocrystal is further highlighted in Fig. 5c. The HR-SEM image shows the lattice of a 4-point mesocrystal star (Fig. 5d) featuring two epitaxial grain boundaries. Quite remarkably for the C096 mesocrystal system, the grain boundaries between the [001]MC and the [101]MC oriented domains are coherent (cf. side view, Fig. 5b), producing an interface that is almost free of strain. The coherent grain boundaries of the C096-based mesocrystals result from the particular dimensions of the bct unit cell: , see Fig. 5b and c. Moreover, intergrown [001]MC and [101]MC-domains of the C096- and C126-based mesocrystals are always found in the same relative (in-plane) orientation (shown in Fig. 5b). The observed preference for the (001)MC and (101)MC surface structures can be explained by noting that they are the two densest surface planes of a bct lattice with a = b < c.
At the particle level, a rotation of anisotropic nanocrystals can result from alignment of the magnetic easy axes of the nanocubes in the direction of the applied magnetic field. This gives rise to a global crystallographic texture.56 Electron diffraction from thin [001]MC-oriented C096 (bct) multilayers confirms the expected <100>-orientation of the spinel crystal axes with the substrate normal for mesocrystals with a 4-fold (p4mm) symmetry.43 For C096- (and C126)-based mesocrystals the particle volume fraction curves of the [001]MC and [101]MC-lattices overlap (see Fig. 2d), indicating that the nanocube orientation is maintained relative to the mesocrystal unit cell.46 Thus, rotation of a bct mesocrystal from a [001]MC- to a [101]MC-orientation will cause the <110>NC-crystal axes to lie approximately in the field direction, suggesting that the reorientation of C096-based mesocrystals is assisted by alignment of the nanocubes’ magnetic easy axes with the applied magnetic field.44,56,57 Indeed, in the recent study by Mehdizadeh Taheri et al.,28 a large applied magnetic field of μ0H = 1 T was found to directly determine the growth orientation for solution growth of nanocube mesocrystals.
For the C136, and to a lesser extent for the C126 nanocubes, assembly in an intermediate field of μ0Happ = 65 mT results in the formation of structures with a noticeable global anisotropy i.e. arrays of oriented mesocrystals (see Fig. 6a). In areas with a relatively high concentration (i.e. the substrate edges),60 arrays of mesoscopic pillars form (Fig. 6b). For the C136 nanocubes, the magnetic field guiding effect is so considerable that it can be observed directly in the scattering experiments.
Fig. 6c shows a GISAXS pattern of the zero-field assemblies of the C136 nanocubes, displaying a sc lattice. Assembly under μ0Happ = 65 mT (Fig. 6d) causes the GISAXS patterns of the in-field assembled C136 to smear out in broad rings. Below the dashed Yoneda line61 there is a SAXS pattern originating from the transmission of X-rays through the substrate. The transmission SAXS pattern displays a texture with Bragg spots smeared in the in-arc direction and is slightly tilted (≈5°, see Fig. 6e) with respect to the substrate reference frame/Yoneda line. The tilt angle represents the angle between the stray field and the substrate caused by a slight misalignment between the substrate and the centre of the magnet. The appearance of a tilted SAXS pattern and the complete loss of the spotted GISAXS pattern for the C136-based mesocrystals under an applied field represent a crossover where the field and its gradient, rather than the substrate orientation, define the mesocrystal growth orientations. In high fields (μ0Happ = 200 mT) we found that the smallest nanocubes (C086) form μm-sized pillars in a nearly hexagonal array and an interpillar spacing roughly equal to the average pillar diameter (see Fig. 6f and g). The pillars vary in height over the substrate surface, and can be found ranging from small protrusions to pillars with large aspect ratios, occasionally hollow with void interiors. HRSEM images reveal that the translational order of the nanocrystals in the base of these pillars is well-defined (see Fig. 6h). Further away from the substrate, the structural coherence is lost due to cracking and/or bending of the mesoscopic pillars. The loss of structural coherence results in GISAXS patterns with broad Debye–Scherrer rings (see Fig. S8, ESI†).
The field-response of magnetic nanoparticles can be understood in the framework of the Rosensweig (or normal-field) instabilities, which occur when a ferrofluid is subjected to a vertical magnetic field.22–24 Above a certain critical field strength, the fluid layer orders into a hexagonal array of pillars as a result of the competition between magnetic and surface forces. The characteristic spacing in the hexagonal pattern (instability wavenumber) follows an exponential decay with the thickness of the ferrofluid layer and a logarithmic increase with the applied field.62 Consequently, the large wavenumbers of the instability-generated patterns in Fig. 6 result from the limited critical film thickness in a typical drop-casting experiment, i.e. of the order of a few micrometres, and the relatively large applied field, i.e., μ0Happ = 65 mT. Similar field-induced patterns formed by spherical Co and γ-Fe2O3 and octahedral Fe3O4 nanoparticles have been investigated in some depth by the Pileni63,64 and Li et al.40 groups.
We suggest a qualitative phase diagram (shown in Fig. 7) that summarizes our observations for the iron oxide nanocube in this work. In the case of small nanocubes (l ≈ 8.5 nm) the application of a weak magnetic field during drop casting assists the formation of assemblies with long range order. Mehdizadeh Taheri et al. speculated on the existence of a lower-size limit for the assembly of iron oxide nanocubes,28 a limit which ultimately should depend on a number of other experimental parameters e.g. particle concentration, applied field, surfactant coverage, and chain length. Nonetheless, nanocubes with edge lengths between 9.6 and 13.6 nm form ordered single-domain mesocrystals in zero field. Upon the application of a moderately strong magnetic field (65 mT), nanocubes with edge lengths of 9.6 nm and 12.6 nm assemble into multidomain mesocrystals, composed of smaller mesocrystals in certain configurations. The small mesocrystals are fused over coherent grain boundaries and oriented primarily in two ways: with either [101]MC or [001]MC perpendicular to the substrate. We suggest that this reorientation follows the alignment of the magnetic easy axes of the iron oxide nanocubes with the applied magnetic field. When a strong magnetic field (200 mT) is applied to the smallest nanocubes (l = 8.6 nm) we observe the onset of Rosensweig instabilities that results in the formation of hexagonal patterns of nanocube pillars.
Footnote |
† Electronic supplementary information (ESI) available: Morphological characterization of the nanocubes, further GISAXS analysis and indexed patterns, AFM images, additional SEM images, mesocrystal surface reconstruction using SEM images, and magnetic characterization. See DOI: 10.1039/c6nr03776c |
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