Straight and Branched Goethite Topology by Oriented Attachment at

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DOI: 10.1021/cg900392m

Straight and Branched Goethite Topology by Oriented Attachment at High pH

2010, Vol. 10 504–509

Xiaobo Mao,† Hongchao Yang,‡ Xuemei Zhou,† Chenxuan Wang,† Yinshu Wang,‡ Yanlian Yang,† Chen Wang,† and Gang Liu*,† †

National Center for Nanoscience and Technology, Beijing 100190, P. R. China and ‡Department of Physics, Beijing Normal University, Beijing 100875, P. R. China Received April 8, 2009; Revised Manuscript Received December 22, 2009

ABSTRACT: In this study, we report novel anisotropic nanostructures of goethite (alpha-FeOOH) nanocrystallites induced by oriented attachment at high pH in the absence of organic additives. Combined transmission electron microscopy, highresolution transmission electron microscopy, and atomic force microscopy analyses shed new light on different stages of the oriented attachment process. It is found that a straight prerod intermediate was assembled by crystallographically aligned nanoparticles with a rectangular shape; a V-shaped structure with an angle of (118.5 ( 3.5)° was connected by apexes of two needle-shaped rods sharing three comparable dimensions, while a Y shape was constructed by a V-shaped rod and a straight one through planar stacking. This is the first example of such a straight and branched topology achieved at high pH without the assistance of organic additives. Our findings will be potentially applicable to a range of naturally abundant colloidal nanocrystallites (e.g., metal (oxyhydr)oxide and sulfide minerals) occurring in the evolution of hydro-, bio-, and geochemical systems. The results from this study could provide broad implications in self- and directed-engineering complex nanostructures through the oriented attachment mechanism.

1. Introduction In recent years, using self- and directed-assembly as a “bottom-up” route to build complex nanostructures is receiving great attention.1 Of particular interest is to tailor topologically complex shapes with novel optical, magnetic, electrical, and catalytic properties using solution-based methodologies mediated by mechanisms such as oriented attachment (i.e., aggregation). Penn and Banfield first reported the mechanism of oriented attachment.2-4 In terms of coarsening or recrystallization processes after nucleation, oriented attachment is quite distinct from the classical coarsening mechanism - Ostwald ripening. Driven by the reduction of surface energy, Ostwald ripening, which has been described in terms of growth of large particles at the expense of smaller ones, is an atom-mediated growth process. In contrast, oriented attachment, which occurs with coherent structural accord at the interface among primary nanoparticles, is a particle-mediated growth process. Oriented attachment has proved to be one of the promising approaches which enable the formation of anisotropic nanocrystals and shape control in inorganic nanomaterials,5-12 such as CdTe,13 PbSe,14 CdSe,15 ZnS,16 TiO2,17 ZnO,18 MnO,19 CdO,20 SnO2,21 WO3,22 CaCO3,23 Ag,24 and Pt.25 Various anisotropic assemblies, such as onedimensional (1D) wires and rods, two-dimensional (2D) disks and prisms, and branched shapes (e.g., tripods, tetrapods, and dendritic structures), have been addressed as potential “bottom-up” nanobuilding blocks. Understanding the details of an oriented attachment process has both practical and academic interest. Nevertheless, most studies rely on additives (e.g., stabilizer ligands, organic surfactants, and templates) to induce directed-assembly rather than self-assembly in the oriented attachment process. On the other hand, the perception *To whom all correspondence should be addressed. E-mail: liug@ nanoctr.cn. Fax: (þ86) 10-62656765. pubs.acs.org/crystal

Published on Web 01/12/2010

of different stages of the oriented attachment process and its driving force still remains elusive. Insights into oriented attachment not only can help to unravel the mechanisms of growth and crystallization at the nanoscale, but allow extending the general applicability of oriented attachment to construct unique and useful nanomaterials and nanodevices using anisotropic nanobuilding blocks. In this study, we present the first example of straight and branched topology assembled by goethite nanocrystallites at high pH without the assistance of organic additives. As a stable iron oxyhydroxide, goethite is extremely common in soils and sediments at and near the Earth’s surface,26,27 and has found many technological applications such as an industrial pigment,28 magnetic recording media precursor,29 mineral liquid crystal colloid,30 and environmental contaminant (metal, anionic, and organic) scavenger for wastewater treatment.31-33 Herein, using a combination of transmission electron microscopy (TEM), high-resolution TEM, and atomic force microscopy (AFM), we gain insights into different stages of the oriented attachment process which governs intermediate formation as well as complex shape evolution. 2. Experimental Section As shown in our previous work,34-36 six-line ferrihydrite (Fe5HO8 3 4H2O) nanoparticles of 3 and 6 nm in size are denoted as Fhyd3 and Fhyd6, respectively. The synthesis methods for Fhyd3 nanoparticles and goethite nanorods were reported previously.31,34-37 Briefly, at a rate of 4.58 mL/min, 1.0 L of 0.48 M NaHCO3 (analytical grade, Beijing Chemical Reagents Company) was added dropwise using a peristaltic pump to a continuously stirred 1.0 L solution of 0.40 M Fe(NO3)3 3 9H2O (analytical grade, Beijing Chemical Reagents Company) to form a homogeneous dark brownish suspension. The resulting suspension was placed into 200 mL Nalgene bottles and microwaved until boiling occurred. During heating, the bottles were shaken every 40 s for an even heating over r 2010 American Chemical Society

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the suspension. Immediately after heating, the suspension was cooled rapidly inside an icy water bath to room temperature, and dialyzed using a membrane (MWCO = 2000, SpectraPor) in Milli-Q water (18 MΩ 3 cm in resistivity, Millipore Corporation) for three days, with the water changing three times each day. Finally, the suspension was placed in a hood to dry. For the synthesis of goethite nanorods, dialyzed suspensions of Fhyd3 nanoparticles were quickly adjusted to a pH of 12 using a 5 M sodium hydroxide (analytical grade, Beijing Chemical Reagents Company), and the resulting suspensions were aged at 90 °C for 30 and 90 h, respectively. Then, the aged suspensions were dialyzed using a membrane (MWCO = 2000, SpectraPor) in Milli-Q water (18 MΩ 3 cm in resistivity, Millipore Corporation) for three days, with the water changing three times each day. Finally, the suspension was placed in a hood to dry. Powder X-ray diffraction (XRD) data were collected using a Shimadzu X-ray diffractometer (XRD-6000) with CuKR radiation (λ = 0.154178 nm). Measurement was in the 2θ range of 15-85° with a scanning step of 0.68 (deg/min). The diffraction patterns were compared to the reference powder diffraction files (PDF) for six-line ferrihydrite (29-0712) and goethite (81-0463). TEM images were obtained using a Tecnai G2 20 S-TWIN microscope operating at 200 keV. Ferrihydrite and goethite as dry powders were dissolved into Milli-Q water and sonicated in a cool water bath until a homogeneous suspension was formed. Second, a drop of sample suspension was spreaded onto an amorphous holeycarbon film supported by a standard TEM grid (Beijing XinXingBaiRui Technology Co., Ltd.). The excess suspension was removed with filter papers. Finally, all TEM samples were allowed to dry under ambient conditions. No sample damage was observed during highresolution TEM studies. Tapping-mode AFM measurements were carried out under ambient conditions using a Nanoscope IIIa (Veeco). The probes were MPP-11100-10 (Veeco) with a nominal spring constant of 40 N/m and a resonant frequency of 300 kHz. A silicon wafer was used as a substrate for the deposited samples. Prior to each use, the silicon wafer was sequentially cleansed with acetone and ethanol, and thoroughly rinsed with Milli-Q water. A drop of sample suspension was spreaded onto the silicon substrates and dried in air.

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Figure 1. Powder XRD patterns of 6 nm six-line ferrihydrite (Fhyd6), 3 nm six-line ferrihydrite (Fhyd3), and goethite prepared by aging Fhyd3 suspensions at pH 12 and 90 °C for 30 (Gt30) and 90 h (Gt90), respectively.

3. Results and Discussion Figure 1 shows experimental powder XRD patterns of sixline ferrihydrite nanoparticles and reaction products. Products derived by aging Fhyd3 suspensions at pH 12 and 90 °C for 30 and 90 h are denoted as Gt30 and Gt90, respectively. The XRD pattern for Fhyd6 (with an average particle size of (5.8 ( 0.5) nm shown in Figure S1 in the Supporting Information) displays a typical structure for six-line ferrihydrite. The pattern for Fhyd3 shows that the predominant phase is six-line ferrihydrite which is mixed with minor goethite phase. The XRD peaks for aged samples are indexed to a predominant orthorhombic phase of goethite (space group Pbnm, No. 62) with lattice constants of a = 0.4596, b = 0.9957, and c = 0.3021 nm. Figure 1 indicates that after aging for 30 h, ferrihydrite was completely transformed to goethite. Aging from 30 to 90 h apparently increases the product crystallinity, manifested by (110) peak narrowing. Figure 2a shows the TEM image of Fhyd3 nanoparticles with a minor portion of coexisting goethite nanorods. The nanoparticles are quasispherical in shape. A close inspection of the images, collected from different parts of the sample surface, shows an average particle size of (2.8 ( 0.6) nm. As to Gt30, the TEM image (Figure 2b) displays elongated nanorods and a significant fraction of quasi-spherical nanoparticles. Figure 2c is a corresponding tapping-mode AFM topographical image, showing that the nanorods lie flat on the substrate with the crystallographic c-axis approximately parallel to the surface. According to XRD data in Figure 1, these nanorods and nanoparticles are ascribed to goethite nanocrystallites. The majority of nanorods exhibit faceted ends with a needle shape typical for goethite. The preferential growth along the c-axis

Figure 2. (a) TEM image of quasi-spherical Fhyd3 nanoparticles. (b) TEM image of Gt30 nanoparticles and nanorods. (c) Tappingmode AFM topographical image (292 nm  292 nm) of goethite nanorods deposited on a silicon wafer. The cross-sectional view corresponds to the line drawn in the image.

with high energy facets is in accord with a kinetic growth mode.5 For needle-shaped nanorods with one end nearly perpendicular to the surface, we observed a flat apex end (Figure S2 in the Supporting Information). Statistical analysis of nanorod dimensional distributions based on TEM and AFM images indicates that the length, width, and height is (76 ( 27), (10 ( 3), and (5.9 ( 1.3) nm, respectively. The average size of goethite nanoparticles is (4.5 ( 0.8) nm based on TEM measurements. In addition to elongated goethite nanorods, anisotropic aggregates with a high degree of linearity were observed. Figure 3a shows a representative high-resolution TEM image of an individual straight aggregate. The aggregate apparently

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Figure 3. (a) Representative high-resolution TEM image of a straight goethite prerod intermediate derived from Gt30. The rectangle-shaped nanoparticles marked by yellow dotted lines are the individual segments which are crystallographically aligned with respect to each other. Insets (1-3) show corresponding FFT analyses for the white, blue, and red square regions, respectively. (b) Tappingmode AFM 3D topographical image of straight and quasi-straight aggregates of goethite nanoparticles on a silicon substrate.

comprises a number of nanoparticles with a rectangular shape, yet not attached. The dimpled boundaries between adjacent nanoparticles are clearly seen, implying that the aggregate is an intermediate in the early growth stage. The aggregate at this stage can be described as “prerod aggregate”, analogous to “prewire aggregate” reported in the literature.15 The lattice fringe spacings are measured to be 0.274 nm, exclusively consistent with the interplane distance of goethite (130) planes. Fast Fourier transform (FFT) (insets 1-3 in Figure 3a) analysis indicates that all segments display the same crystallinity. Moreover, these rectangle-shaped segments share the same width of 2.8 nm and are crystallographically aligned with respect to each other to present a tendency of epitaxial assembly. The alignment probably results from the interplay of interparticle forces, such as dipolar attraction.38 Unlike previous studies13-15,18,37,39 showing the oriented attachment by partially or completely fused nanoparticles with spherical shapes, Figure 3a provides the first direct evidence that even prior to attachment, a

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Figure 4. (a) Representative TEM image of a V-shaped nanorod with an apparent boundary between two straight rods connected to each other, highlighted by white dashed circles. The white arrow indicates the boundary; (b) representative TEM image of a Yshaped nanorod with apparent boundary formed by attachment of a V-shaped rod with a straight one, highlighted by white dashed circles. The white arrows indicates the boundary.

straight prerod intermediate with a high-aspect ratio (15) is composed of rectangle-shaped nanoparticles with low-aspect ratios (1.3-3.4) which share the same crystallographical structure and orientation. Prerod intermediates could serve as an “embryo” for the growth of straight nanorods. Further evidence for nanoparticle oriented alignment and fusion is provided by a tapping-mode AFM three-dimensional (3D) topographical image (Figure 3b). Apparently, goethite nanoparticles are aligned to form straight and quasi-straight aggregates. With increasing aging time, goethite nanocrystallites show significant changes on structures and morphologies. Aging Fhyd3 suspensions at pH 12 and 90 °C for 90 h led to 78% straight nanorods, 15% branched nanorods with a V shape, and 7% branched nanorods with a Y shape (for example, Figure S3 in the Supporting Information). More complex branched shapes (data not shown) were also observed with very low appearance frequency. The vast majority of straight nanorods (Figure S3a in the Supporting Information) appear needle-shaped with the ends well faceted. On the basis of TEM and AFM measurements, the length, width, and height for the straight nanorods are (86 ( 30), (13 ( 5), and (7.0 ( 1.5) nm, respectively. Analogous to “prerod aggregate”, Figure 4,

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Figure 5. Representative morphologies and structures of straight, V-shaped, and Y-shaped goethite nanorods derived from Gt90. (a) Tappingmode AFM 3D topographical image of three types of rods deposited on a silicon wafer. (b) High-resolution TEM image of a straight needleshaped rod. Inset: FFT analysis. (c) TEM image of a V-shaped rod. (d) High-resolution TEM image shows lattice continuity across a V junction of the square area in (c). Inset: FFT analysis. (e) TEM image of a Y-shaped rod. (f) High-resolution TEM image depicts lattice continuity across a Y junction of the square area in (d). Inset: FFT analysis.

panels a and b display the intermediate stage for the formation of V- and Y-shaped rods, respectively. Prior to complete smoothing by Ostwald ripening, apparent boundaries between the connecting parts are shown. A representative tapping-mode AFM 3D topographical image of straight, V, and Y shapes is delineated in Figure 5a. High-resolution TEM image (Figure 5b) and FFT analysis (inset of Figure 5b) of a straight needle-shaped rod indicate the lattice fringes are 0.496, 0.255, and 0.250 nm, which are well ascribed to the goethite (020), (021), and (101) planes, respectively. The (021) planes often are apex facets of needle-shaped goethite nanorods.32 The interplanar angle between (020) and (021) planes is measured to be ca. 120°, which is very close to the theoretical value of 121.2° (calculations based on eq 1 in the Supporting

Information). For V-shaped rods (Figure 5c and S3b, Supporting Information), the dimensions for two branches are quite uniform, with (56 ( 14) nm in length, (15 ( 4) nm in width, and (6.4 ( 0.8) nm in height for all branches, respectively. The (021) facets (shown in Figure 5b) are one of preferential contact planes used by two rods to form a V shape, given the fact that branching angle R is (118.5 ( 3.5)° based on statistical analysis. Hence, oriented attachment rather than random attachment can rationalize the formation of V-shaped rods. The FFT patterns (inset of Figure 5d) display lattice fringes of 0.252 and 0.250 nm, which are consistent with the (021) and (101) planes of goethite. A close look into the V shape (Figure 5d) reveals that lattice fringes (e.g., d = 0.252 nm) continuously span the entire V-shaped

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structure. Further, no apparent grain boundaries and other defect microstructures2-4,40,41 were observed in the junction region. The present results suggest that oriented attachment between two straight needle-shaped rods occurs preferentially on the (021) high-energy apex surfaces via an end-to-end epitaxial contact mode, and eliminates two high-energy surfaces to form one V-shaped rod. The Y-shaped nanorod (Figures 5e and S3c, Supporting Information) apparently comprises three segments like one stem with two branches. Figure 5f shows that the widths for the stem and branches are different, with 18, 14, and 10 nm for the stem and two branches, respectively. The stem and the rightmost branch are almost parallel relative to each other. The angle R between the stem and the leftmost branch is 116°, and the angle β between two branches is 64°. Statistical analysis indicates that for Y-shaped rods, R is (117.5 ( 4.0)°, which is very close to the R value of (118.5 ( 3.5)° as seen in V-shaped nanorods. The FFT patterns (inset in Figure 5f ) indicate that the Y-shaped nanorod displays crystalline lattice spacings of 0.500, 0.254, and 0.251 nm, in agreement with the goethite (020), (021), and (101) planes, respectively. High-resolution TEM micrograph (Figure 5f) with well-resolved lattice fringes (e.g., d = 0.254 nm) reconfirms that the Y-shaped rod is a nanocrystal. Hence, it is reasonable for a Y-shaped rod to be a combination of a V-shaped rod with a straight one via an epitaxial attachment mode to enhance the packing density.42 This behavior is probably driven by minimizing the interface mismatch energy via a side-by-side assembling route (i.e., planar stacking along specific crystal planes through inherent van der Waals attraction).43 In the present work, not every V and Y shape observed exhibits single-domain nanocrystallinity, in part owing to the requirement of proper crystal alignment with respect to the incident electron beam, and in part owing to the imperfect oriented attachment. On the basis of the results from the current study, in Figure 6 we propose a scheme of topological evolution mediated by aging Fhyd3 suspensions at pH 12 and 90 °C between 30 and 90 h. In particular, oriented attachment can induce the formation of straight prerod intermediates and advanced shapes. A V-shaped rod was formed by connecting high-energy apex surfaces of two rods which share three comparable dimensions in terms of length, width, and height. Planar stacking between a V-shaped rod and a straight one subsequently generates a Y-shaped structure. The formation of V and Y topology is essentially a facet-mediated aggregation and sequential elimination of surfaces from high- to low-energy, owing to significant contribution of surface energy to the total free energy in a nanoscaled system.5 Different from previous work addressing the oriented attachment using zero-dimensional (0D) isotropic nanoparticles,2,4,18 our results show compelling evidence that using 1D anisotropic building blocks oriented attachment not only enables the formation of straight prerod intermediates but 2D assembly into branched nanocrystals, keeping in mind that the aging experiment in this work was performed at a relatively low temperature of 90 °C in the absence of organic additives. Nevertheless, a high pH value (i.e., a high OH- ion concentration) in aqueous solution implies a high chemical potential which favors anisotropic growth, such as the formation of elongated nanowires.44,45 On the basis of our direct observations, we suggest that high pH could be a driving force for the oriented attachment process enabling the growth of anisotropically shaped goethite. Similarly, a recent study on titania by Finnegan et al.46 showed that the oriented attachment can be controlled by pH value and

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Figure 6. Schematic diagram illustrating topological evolution from ferrihydrite nanoparticles to goethite advanced shapes, obtained by aging Fhyd3 suspensions at pH 12 and 90 °C between 30 and 90 h. (a) Precursor ferrihydrite nanoparticles. (b) Goethite nanorods formed kinetically, and prerod aggregates by oriented attachment. (c) Spatially separated needle-shaped rods grown by intraparticle ripening. (d) Formation of branched shapes by oriented attachment. End-to-end adhesion by high-energy apex facets between two straight needle-shaped rods sharing comparable three dimensions leads to a V-shaped rod; side-by-side planar stacking between a V-shaped rod and a straight one generates a Y-shaped structure. (e) Ostwald ripening contributes to smoothening the V- and Y-shaped junctions.

temperature. In addition, thermally driven Brownian motion could regulate nanorod 3D rotation and collision during the oriented attachment process.4,47 As a result, maximization of 2D coherent crystallographic contact and subsequent fusion between the nanorods to form a nanocrystal is achieved. Complementary with the oriented attachment, classical Ostwald ripening which has been described in terms of growth of large particles at the expense of smaller ones contributes to smoothening of the branched junctions. In order to control the contents of different shaped nanostructures, we have conducted additional experiments at different pH values (2, 4, 6, 8, 10), reaction temperature (75 °C), and aging times (up to 360 h). We found that the formation of anisotropic shapes by oriented attachment is enhanced at higher pH and higher temperature. For instance, aging 90 h at pH 10 and 90 °C produces only dominant straight nanorods (Figure S4a, Supporting Information). At pH 12 and 75 °C, aging for a longer time (up to 360 h) could induce some unconventional shapes (Figure S4b, Supporting Information) but to a lesser degree compared with those at pH 12 and 90 °C. 4. Conclusions In summary, in the absence of organic additives, aging ferrihydrite suspensions at pH 12 and 90 °C between 30 and 90 h led to goethite with various types of anisotropies. Studies using high-resolution TEM and AFM shed new light on different stages of oriented attachment which accounts for topological evolution. In detail, a straight prerod intermediate was assembled by crystallographically aligned nanoparticles with a rectangular shape; a V-shaped structure with an angle of (118.5 ( 3.5)° was connected by apexes of two needle-shaped

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rods sharing three comparable dimensions, while a Y shape was constructed by a V-shaped rod and a straight one through planar stacking. To our knowledge, no study dealing with such straight and branched topology achieved at high pH without the use of organic additives has been reported. The results of this study could provide implications in self- and directed-engineering complex nanostructures through the oriented attachment mechanism. In addition, our findings will be potentially applicable to a range of naturally abundant colloidal nanocrystallites (e.g., metal (oxyhydr)oxide and sulfide minerals) occurring in the evolution of hydro-, bio-, and geochemical systems. Acknowledgment. G.L. gratefully acknowledges the financial support of this work from National Center for Nanoscience and Technology, Beijing, China. Note Added after ASAP Publication. This paper was published on the Web on January 12, 2010, with an error to Figure 5. The corrected version was reposted on the Web January 14, 2010. Supporting Information Available: TEM image of Fhyd6; AFM image of apex end of a needle-shaped goethite nanorod; TEM images of straight and branched goethite nanostructures at different pH values, temperatures, and aging times; theoretical calculations determining an angle between two planes for an orthorhombic symmetry. This material is available free of charge via the Internet at http://pubs.acs.org.

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