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Atomic Force Microscopy Studies for Investigating the Smectic Structures of Colloidal Crystals of β-FeOOH Hideatsu Maeda* National Institute for Advanced Interdisciplinary Research, National Institute of Bioscience and Human-technology, and Joint Research Center for Atom Technology, 1-1-4 Higashi, Tsukuba, Ibaraki 305, Japan
Yoshiko Maeda† University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305, Japan Received June 21, 1995. In Final Form: November 15, 1995X Smectic, or multilayer, structures in dry sol sediments of β-FeOOH (ferric oxyhydroxide) have been investigated using an atomic force microscope (AFM). The AFM technique has provided three-dimensional information of the smectic structures in a nondestructive manner. The shading in the AFM images of the dry sol surfaces evidently shows that in the iridescent (red, orange, yellow-green, green, and blue) regions, the β-FeOOH crystals are standing upright at a tilt (with respect to the plane of the smectic layer) and form approximately a square lattice. In contrast, in the noniridescent (gray and brown) regions, the crystals lie parallel or randomly oriented to each other. By determining the tilt angle of the crystals in the layer and the refractive index of the layer, we have found the thickness of the layer to be close to the wavelength of visible light in the layer. This supports an earlier periodic-spacing model that attempts to explain the origin of the iridescent colors. The AFM images also show that the individual crystal surface is homogeneous crystallographically, thus disputing an earlier bundle-structure model of individual β-FeOOH crystals. Furthermore, the AFM images show that the crystals have stepped surfaces, tapered ends, and either convex or concave ends, thus indicating nonuniform crystal growth. The dry sol surfaces of β-FeOOH have been further investigated with a scanning electron microscope (SEM) on a larger scale than is used with AFM. In the SEM images, we have found the following: (1) The in-layer structure of the smectics is, at least locally, square. Although defects destroy the two-dimensional crystalline order, a longerranged bond-orientational order still exists. (2) The smectic structures exhibit bending of the directors (which is usually observed in nematics) and overall layer undulation. We have also observed the coexistence of several smectic domains, each of which has a distinct orientation, and mono- and several-folded layers confined in the sols. The smectic structures of dry β-FeOOH sols have been found to be obtained not only from suspensions with attractive-force interparticle interactions but also from those with repulsive-force interactions.
Introduction Zocher and Heller found a stable, ordered structure in the sol sediments of β-FeOOH, called Schiller layers, which exhibit brilliant interference colors.1-3 However, they had no detailed information on the structure of the layers. By using an electron microscope technique in which ultrathin sectioning is applied, Watson et al.4 investigated the internal structure and assembly of β-FeOOH particles. They found that the β-FeOOH particle is a rodlike single crystal with a length of about 350 nm and with square sides of about 55 nm. They also found that these crystals are arranged in an orthogonal array, with long axes of the crystals oriented approximately parallel to each other. However, they had no details concerning the crystal configurations in the Schiller layers. On the basis of their observations, they speculated that the origin of the interference colors is the lateral periodicity between the parallel-aligned crystals. Maeda et al. found (using a scanning electron microscope (SEM) and an optical microscope) that an order-disorder phase separation occurs in β-FeOOH sols (suspensions) and that the ordered phase has a smectic structure composed of parallel layers in which β-FeOOH crystals are oriented approximately perpendicular to the layer.5,6 † Present address: Tsukuba Uni Network Co., Ltd., 4-1-15 Kasuga, Tsukuba, Ibaraki 305, Japan. X Abstract published in Advance ACS Abstracts, February 15, 1996.
(1) Zocher, H. Z. Anorg. Allg. Chem. 1925, 147, 91. (2) Zocher, H.; Heller, W. Z. Anorg. Allg. Chem. 1930, 186, 75. (3) Heller, W. Comptes Rendus, 1935, 201, 831. (4) Watson, J. H. L.; Cardell, R. R., Jr.; Heller, W. J. Phys. Chem. 1962, 66, 1757. (5) Maeda, Y.; Hachisu, S. Colloids Surf. 1983, 6, 1.
On the basis of the optical microscope observation, they showed that another possible origin of the iridescent colors is the visible light interference due to the spatial periodicity perpendicular to the layer. However, they did not determine a refractive index of the smectic layer, which is needed to examine interference conditions necessary for iridescent colors. Watson et al. also carried out electron microscopic studies of β-FeOOH crystals (using ultrathin sectioning).4 From the results, they concluded that the β-FeOOH crystal is a hollow rod with an internal diameter of 3 nm and an external diameter of 6 nm. By using a high-resolution electron microscope, Galbraith et al. showed no evidence for such a structure.7 Thus, to clarify the crystal structure, we devised a direct, high-resolution technique. Measurements using an atomic force microscope (AFM) can provide quantitative information on atomic structures and three-dimensional (3D) shapes of individual colloidal crystals, as well as their assemblies, without damaging the sample. This nondestructive characteristics is a great advantage over electron microscopic measurements, because the required ultrathin sectioning needs troublesome pretreating to cut the sample, possibly altering or damaging intact structures of individual crystals and/or their assembly. Furthermore, although SEM measurements can provide useful information of two-dimensional (2D) smectic structures, the required metal coating on the sol surfaces degrades the information on the iridescent colors. This degradation is a serious disadvantage when trying to determine the smectic structure of β-FeOOH crystals and the origin of the iridescent colors. (6) Maeda, Y.; Hachisu, S. Colloids Surf. 1983, 7, 357. (7) Galbraith, S. T.; Braid, T.; Fryer, J. R. Acta Crystallogr. 1979, A35, 197.
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Figure 1. (a) A transmittance electron microscope image of β-FeOOH crystals. (b) Distributions of length and width of the crystals.
We used an AFM that had an optical microscope with a CCD camera, allowing us to precisely position the cantilever on desired regions. By using the AFM, we obtained images for the 3D structures of iridescent and noniridescent regions in dry β-FeOOH sol surfaces and for the 3D shape and surface structure of individual β-FeOOH crystals. Experimental Section Following a procedure by Zocher et al.,1,2,4 we prepared samples of β-FeOOH sols by first producing 7 L dilute suspensions of FeCl3 (30-40 mmol-1) at room temperature. The crystallization of β-FeOOH proceed slowly (i.e., over a 6-month period) in flasks; eventually, a dense sediment deposited at the bottom of each flask. We then purified the sediments using dialysis and obtained the sols with about pH 4. By varying the pH of the sols (or by adding HCl), we were able to change the surface charge of the crystals. For the sols prepared at pH 1.8, the interparticle interactions are attractive, which we called attractive sols; for sols at pH 4, the interactions are repulsive, which we call repulsive sols. Using a transmission electron microscope (TEM; JEOL, JEM100B), we determined the average length and width of the β-FeOOH crystal to be 349 and 64 nm, respectively. Figure 1a is a TEM image of the β-FeOOH crystals, showing rodlike particles with rounded ends. From this image, we determined the distributions of the crystal length (in the range between 300 and 400 nm) and the width (between 40 and 90 nm), which are shown in Figure 1b. The small standard deviations indicate that the crystal size was highly monodisperse. Dry sols were prepared by slow drying iridescent sediments in the purified β-FeOOH sols. In this experiment, we used dry sols preserved at 25 °C for more than 10 years. Thus, the smectic structures formed in the sols are considered to be completely in the equilibrium state.
Figure 2. (a) Coexistence of three smectic domains I, II, and III of β-FeOOH crystals. In domain I, only the crystal ends are visible, as they are standing upright (horizontal smectic layer). In contrast, in domains II and III, only the crystal sides are visible, as they are laying down (vertical smectic layer). (b) Smectic layers corresponding to domain I. (c) A schematic figure of the three domains I, II, and III. We examined the 2D surface structure of the dry sols of β-FeOOH using an SEM with a maximum magnification of 200 000 (Hitachi, S-500A) and determined the 3D structure of the β-FeOOH crystals and their assembly using an AFM equipped with an optical microscope and a CCD camera for precise cantilever positioning (Olympus, OMPM). The AFM was operated at a scanning rate of 2 s/line in a repulsive force region (∼10-9). Silicon nitride cantilevers were supplied by Olympus. To examine the interference condition of visible light in the β-FeOOH layer, we determined the refractive index of the thin layer (deposited on a quartz plate) using an autoellipsometer with a He-Ne laser (Mizoziri, DHA-XA).
Results and Discussion (1) SEM Observations of Dry Attractive-Sol Surfaces. (i) Morphology of Smectic Layer Surfaces. The dry sols denoted in this section are those obtained from the attractive sols (suspensions). Figure 2b is a top view of the dry attractive-sol surfaces showing the coexistence of three smectic domains (I, II,
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and III), and Figure 2a is a smectic structure corresponding to domain II. Figure 2c is a schematic figure of Figure 2b showing well-ordered lattice and layer structures (i.e., an ideal crystalline structure) that does not account for fluctuations in position and orientation that occur in real smectic structures of the sols. Domain I is a top view of a smectic layer in which only the square ends of the crystals are visible, indicating that the crystals were standing upright in the layer. We call this a horizontal smectic layer. Domains II and III are side views of the smectic structures in which the crystals are laying down, which we call a vertical smectic layer, and the layer orientations of both domains are distinct. From the figures, we found that the boundaries of the three domains coincided well with each other without any noticeable gaps, suggesting that the crystals can easily go in and out from the layer edges. Furthermore, we found that although the crystal ends were arranged approximately in a square lattice when looked at under high magnification, under a lower magnification lattice deformations and dislocations were visible (refer to Figure 3a). This indicates that in-plane translational order is short ranged, while tetratic bondorientational order still remain long ranged. The locally square in-plane order is due to a packing effect, because the β-FeOOH crystals have a cross section but no specific interparticle interactions such as ferromagnetic dipoledipole interactions. Figure 3b shows layer undulations and dislocations of the smectic structure. In addition, a curved smectic structure is visible in Figure 3c, where the gradual layerby-layer change in the average crystal orientation is formed in an arc. This type of director pattern is similar to the so-called bending structure seen in nematic liquid crystals but is usually not observed in smectic liquid crystals. Thus, these findings (the overall layer undulation and the bending of the director of the smectics) indicate that the coupling between director and layer normal is anomalously weak in these lyotropic smectics. (ii) Formation of Multilayer Structures. It is of great interest how the multilayer structures of β-FeOOH are formed in the sol (suspension) states. Using an optical microscope, Maeda et al. suggested the following formation mechanism of the multilayer structures: First, in attractive-interaction suspensions, the crystals form bundlelike reversible aggregates (corresponding to the monolayers denoted in this paper), and then, the bundles grow and finally form smectic structures.5 Our SEM observations of the dry sol surfaces confirm this mechanism by showing (1) monolayers with a diameter of approximately 1.5 µm corresponding to a cluster of 100-300 crystals (Figure 4a), (2) single-, and double-, and seven-folded layers confined in the horizontal smectic layers (Figure 4b-d), and (3) the coexistence of smectic domains, each with a distinct layer orientation (Figure 2b). Each domain frequently consisted of more than 100 layers, and the crystal number in each layer frequently exceeded the order of 104. These can be considered as “freeze-frames”, showing scenes at different times in the multilayer formation process. From these results, we have inferred the following mechanism: first, β-FeOOH crystals aggregate laterally at first to form monolayers; then the monolayers grow and deposit vertical or horizontal to the bottom plane of the glass vessel; finally, the layers continue to grow, eventually assembling and forming the domain structures. (2) SEM Observations of Dry Repulsive-Sol Surfaces. Maeda et al. found iridescent sediments not only in the attractive sols but also in the repulsive sols.5 Similarly, we have found smectic structures not only in the dry attractive-sols (Figures 2, 3, and 4) but also in dry repulsive-sols (Figure 5), indicating that interparticle
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Figure 3. (a) An arrangement of the crystal ends of β-FeOOH in the horizontal smectic layer surface. (b) Deformations and dislocations in the vertical smectic layers of β-FeOOH crystals. (c) Curved smectic layers of β-FeOOH crystals.
attractive forces are not necessary for the formation of the smectic structures. Using computer simulations, Frenkel et al. showed that spherocylindrical particles that have hard core repulsions undergo four phase-transitions as the number density of the particle increases: isotropicnematic, nematic-smectic, nematic-columnar, and smectic-solid.8,9 They concluded that hard core repulsions (or excluded volume effects) are essential for the formation of such stable, ordered structures in hard spherocylindrical particle systems. Thus, the occurrence of the smectic phases in the dry repulsive sols (that we have found) and (8) Frenkel, D.; Mulder, D. M. Mol. Phys. 1985, 55, 689. (9) Veerman, J. C. A.; Frenkel, D. Phys. Rev. A, 1991, 43, 4334.
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Figure 4. (a) Deposited monolayers of β-FeOOH crystals. (b) A vertical monolayer of β-FeOOH crystals confined in the horizontal smectic layers. (c) Vertical multilayers of β-FeOOH crystals confined in the horizontal smectic layers, in which a slightly bent double layer is visible. (d) Vertical seven-folded layers of β-FeOOH crystals confined in the horizontal smectic layers.
the repulsive suspensions (that Maeda et al. found) are consistent with the result predicted by Frenkel et al. using computer simulations. Furthermore, investigations of colloidal suspensions of tobacco mosaic virus (TMV) also support predictions by Frenkel et al. In fact, TMV suspensions consist of rigid rodlike particles with electrostatic repulsive interactions of hard rods where various phases including smectic and colloidal crystalline structures were observed.10-14 An iridescent phase was found in TMV suspensions by Oster.10 This is considered to arise from either smectic-A or colloidal crystalline phases.11,14 Using a combination of optical and X-ray experiments, Wen et al.12 determined the smectic-A (SA) phase in TMV suspensions by observations of a periodic layer structure and a liquid-like order within layers. They also found the characteristic SA-type focal conic texture and in-layer undulational fluctuations. It is (10) Oster, G. J.Gen. Physiol. 1950, 33, 445. (11) Kreibig, U.; Wetter, C. Z. Naturforsch., Teil. 1980, 35, 750. (12) Wen, X.; Meyer, R. B.; Casper, D. L. D. Phys. Rev. Lett. 1989, 63, 2760. (13) Meyer, R. B. in Dynamics and Patterns in Complex Fluids; Onuki, A., Kawasaki, K., Eds.; Springer Proceedings in Physics; SpringerVerlag: Berlin, 1990; Vol. 52, p 62. (14) Fraden, S.; Casper, D. L. D.; Philips, W. Biophy. J. 1982, 37, 97a.
of great interest and significance to compare these findings with our results (observed in the β-FeOOH colloidal systems). (3) AFM Observations of Dry Attractive-Sol Surfaces. Parts a to c of Figure 6 show optical microscope images of red, green-blue, and gray-brown regions in a dry sol surface of β-FeOOH crystals, respectively. To explain the origin of iridescent colors in β-FeOOH sol sediments, two models have been proposed, one by Watson et al.4 and one by Maeda et al.5 Watson et al. speculated that the iridescent colors are caused by visible light interference due to a lateral periodicity between parallel-oriented crystals. This model requires that the interparticle distance be similar to the wavelength of visible light. Maeda et al. proposed that the interference colors can be induced by a spatial periodicity perpendicular to the plane of the smectic layers. To clarify the mechanism for the iridescent colors, we obtained 3D AFM images of the iridescent and noniridescent regions of β-FeOOH crystals. (i) Observations of Smectic Structures. Figure 7a is an AFM image for the red region in the dry sol surface of β-FeOOH, in which crystal ends form deformed square lattices. Parts b and c of Figure 7 are enlargements of the shading images in Figure 7a showing that the crystals
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Figure 5. (a) A SEM image for the smectic layers of the dry repulsive sol surface of β-FeOOH crystals. The layer dislocations are seen around the center of this picture. (b) A SEM image of the dry sol surface of β-FeOOH crystals. Randomoriented and ordered regions of the crystals are visible.
were square rods standing upright at a tilt and had tapered ends and stepped sides. Figure 8a shows an AFM image for the green region in the dry sol surface of β-FeOOH crystals. Figure 8b is a shading image of Figure 8a, in which the crystals were standing in an ordered lattice, similar to that seen in the red region. In this figure, not only the ends but also the sides (the dark, shaded parts) of the crystal are visible, indicating that the crystals were tilted in the layer. Again, as in the red region, stepped sides are visible. Furthermore, Figure 8c shows either convex or concave ends of the crystals in the green region, indicating nonuniform crystal growth of β-FeOOH. Figure 9a is an AFM image for the boundary region between the gray and green regions of the dry sol surface, showing that the crystals were laying down (gray region) and standing upright (green region) in the layer. Furthermore, parts b and c of Figure 9 are the top views of an enlarged AFM image for a gray (noniridescent) region of the dry sol surface and a further enlarged shading AFM image of this same region, respectively. These images indicate that the iridescent colors in the dry sol surfaces are not due to the periodicity of the lateral interparticle distance in each layer. We estimated the interparticle
Figure 6. Optical microscopic images for the iridescent regions of the dry β-FeOOH sol surfaces (a magnification of 650): (a) red, (b) green-blue, and (c) gray regions.
distance to be 63-64 nm (Figure 10c), which is too short for visible light interference. Figure 10a shows a cross section of a sol surface, which we obtained by cutting the red-region surface shown in Figure 7c along the direction indicated by the arrow. The curve indicates that the crystals were tilted parallel to each other. From this curve, we estimated the average tilt angle and width of the crystals to be 26° and 63 nm, respectively. Figure 10b shows a cross section of a sol surface in the gray region (obtained by cutting the surface shown in Figure 9b along the direction indicated by the arrow). In this figure, three rods are visible, from which we then estimated the average length (L) of the rod (or crystal) to be 340 nm. In Figure 10c, the length and width of the β-FeOOH crystal obtained by AFM are compared with those obtained by TEM. They are in good agreement with each other.
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Figure 8. (a) An AFM image for the dry sol surface of β-FeOOH crystals in the green region. (b) A shading image of (a). The dark parts are the crystal sides, and the bright parts are the other sides or the ends. The stepped structures are seen, particularly on the dark sides. (c) An enlarged AFM image of the several crystal ends, which are seen to be convex or concave. Figure 7. (a) An AFM image for the dry sol surface of β-FeOOH crystals in the red region. (b) An enlarged shading image of the dry sol surface of β-FeOOH crystals. The crystals are standing upright at a tilt. (c) An enlarged AFM image of (b), showing stepped and tapered structures on the most of the crystal surfaces.
(ii) Interference of Visible Light Due to Spatial Periodicity of Smectic Layers. As mentioned above, we found that in the iridescent regions of the dry sol surfaces, the crystals were standing upright at a tilt, whereas in the noniridescent regions they were laying. Thus, we expected that the origin of the iridescent colors is due to the visible light interference in the layers composed of standing crystals. To verify this, we determined the wavelength of visible light in the β-FeOOH layer and a periodic spacing perpendicular to the layer plane. We calculated the wavelength λ of light in the layer as
λ ) λ′/n
(1)
where λ′ is the wavelength of light in a vacuum and n is the refractive index of the layer. We determined the value of n to be 1.667 by using an autoellipsometer with a HeNe laser (more details will be published elsewhere). Thus, λ′ in the range of visible light from 430 (purplish blue) to 650 nm (red) is transformed into λ from 258 to 390 nm. According to the model for iridescent colors proposed by Maeda et al.,5 the periodic spacing in the multilayered structures corresponds approximately to the layer thickness. The layer thickness can be calculated as L cos θ, where L is the particle length and θ is the tilting angle of the crystals. In fact, L values are approximately 300400 nm (as shown in Figure 1), and θ values are 0 to 30° from the AFM sectionings of the surfaces (e.g., 26° as seen from Figure 10a). Thus, the layer periodicities can be easily calculated to be 260-400 nm, corresponding to approximately the wavelength of the purplish blue and
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Figure 10. (a) A cross section of Figure 7c along the direction indicated by the arrow, showing that the crystals are standing at a tilt. (b) A cross section of Figure 9b along the direction indicated by the arrow, showing that the crystals are laying down. (c) Comparison of width (W) and length (L) of the β-FeOOH crystals determined from the AFM images of (a) and (b) with those determined from the TEM images of Figure 1.
evidence to the bundle-structure model proposed by Watson et al. Summary
Figure 9. (a) An AFM image for the boundary between the gray and green, in which the β-FeOOH crystals are laying down (gray region) and standing upright (green region), respectively. (b) An enlarged AFM image for the dry sol surface of β-FeOOH crystals in the gray region. (c) A further enlarged, shading AFM image of the sol surface of (b). The laying square rods (β-FeOOH crystals) are visible, showing uniform surfaces crystallographically.
red colors respectively. This close agreement between the values of λ (the wavelength of light) in the layer and the values of L cos θ (the layer periodicities) shows that the origin of the iridescent colors of the dry sol surface can be explained by the model proposed by Maeda et al. rather than by the model by Watson et al. (iii) Observations of Individual Particles. Watson et al. proposed that the individual β-FeOOH crystals could be described as a bundle of rods, each of which is hollow, with an internal diameter of 3 nm and an external diameter of 6 nm.4 On the contrary, Galbraith et al. concluded that the hollow-rod structure is just an artifact coming from the irradiation damage in the electron micrograph measurement and that the crystals are actually homogeneous crystallographically.7 Thus, we used AFM to clarify this structure. Figure 9c shows that the crystal surfaces are homogeneous, giving no supporting
AFM has been shown to be the useful tool for a nondestructive investigation of 3D structures of colloidal crystals of β-FeOOH and their assemblies. In fact, the following determinations can be made: (1) The size of the β-FeOOH crystal was exactly determined without any sample pretreating as required in TEM. (2) Each β-FeOOH crystal is shown to have stepped surfaces, and tapered and convex (or concave) ends, suggesting nonuniform crystal growth. Furthermore, the individual crystals are shown to have no bundle structure as proposed by Watson et al. but a crystallographically homogeneous structure. (3) The dry sol surfaces of β-FeOOH can be microscopically observed without any loss of information about the colors. Actually, the crystals are found to be standing parallel to one another in each iridescent region and found to be laying to form a layered or randomly oriented structure in the noniridescent regions. (4) Each layer thickness obtained by AFM is found to be in good agreement with the wavelength of visible light in the layer (calculated by determining the refractive index of the thin layers of β-FeOOH), supporting one of the periodic-spacing models for the origin of the iridescent colors proposed in earlier works. Accordingly, SEM can be considered to be a suitable tool for observing assemblies of colloidal crystals on a larger scale than is possible with AFM. Using SEM, in fact, the coexistence of the smectic domains of β-FeOOH has been observed in the dry sol surfaces. The domains frequently consist of more than 100 layers, where layer undulation and layer dislocation are observed. In each layer (of the smectics), the number of crystals frequently exceeds the order of 105, where the translational order of the crystals is short ranged but the bond-orientational order is long ranged. LA950495J
