A Solution-Based Anisotropic Template Route to Triangular Pyramid

May 18, 2009 - ABSTRACT: Solution-based fabrication of triangular pyramid polymer shells with one face open is demonstrated through deposition...
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CRYSTAL GROWTH & DESIGN

A Solution-Based Anisotropic Template Route to Triangular Pyramid Shells

2009 VOL. 9, NO. 7 3296–3300

Jian Lu, Dairong Chen,* Xiuling Jiao, and Wei Li Key Laboratory for Special Functional Aggregate Materials of Education Ministry, School of Chemistry and Chemical Engineering, Shandong UniVersity, Jinan 250100, P. R. China ReceiVed January 21, 2009; ReVised Manuscript ReceiVed April 29, 2009

ABSTRACT: Solution-based fabrication of triangular pyramid polymer shells with one face open is demonstrated through deposition onto a triangular-pyramid template with anisotropic surface properties. To our knowledge, such anisotropic coatings have not been achieved previously by solution-based methods. The polymer shells can be transformed to magnetic porous shells (a composite of iron nanoparticles embedded in a carbon matrix) which exhibit high saturation magnetization and which were found to adsorb dyes rapidly and in a reuseable fashion. The morphologies of the pyramid templates, the polymer coatings, and the magnetic shell were characterized in detail. It should be possible to make shells of many different compositions using polyhedral templates with anisotropic surface properties, such as the one we employ in this work. Introduction In the past decade, much attention has been paid to hollow nanostructures with various geometries for their mechanical, optical, electrical, chemical, and other properties.1 Therefore, various hollow nanostructures of inorganic materials with spherical or polyhedral geometries have been fabricated via hard template, soft template, or template-free methods.2 Despite a great deal of progress, the preparation of hollow polyhedral nanostructures remains a significant challenge to material scientists because the routes to the spherical structures do not generally apply for the polyhedrons. For example, soft templates, such as surfactant micelles, vesicles, and emulsion droplets, usually do not assume well-defined polyhedral shapes in order to minimize the interfacial energy.3 With the hard template method, besides a series of porous materials, nanotubes, nanorods, hollow spheres, etc., hollow polyhedrons have also been successfully prepared.1 With this approach, the coatings (or their precursors) deposit onto all exposed surfaces of the template. However, the hard template route to hollow polyhedrons introduces additional challenges, which range from the selective coating on the polyhedral templates’ surfaces to the difficulty in forming uniform shells with large variation in curvature.4 Hollow polyhedrons with open shells exhibit some interesting properties for their reduced symmetry or geometry and will be required in many cases for both fundamental research and practical applications.5 Thus, there are some investigations of their fabrication and properties.6,7 Notably, Odom et al. prepared hollow metallic pyramids with nanoscale tips using a combination of phase-shifting photolithography, wet-chemical etching, and electron beam deposition,7a and Whitesides’s group fabricated metallic pyramidal microstructures using pyramidal pits in the surface of an n-doped silicon substrate as templates, silicon serving as the cathode for the selective electro-deposition of metal in the pyramidal pits, and subsequent dissolution of the silicon substrate.7b However, to the best of our knowledge, there is no previous report of a solution-based hard template route for making open polyhedral shells due to the absence of polyhedral templates available for the synthesis. * Corresponding author. Fax: +86-531-88364281. Tel: +86-531-88364280. E-mail: [email protected].

Here, we present a novel anisotropic template-assisted solution route to triangular pyramid shells. In this approach, the Fe-Na hydroxysulfate templates with a shape of a triangular pyramid are first prepared. As the three side faces are equivalent and differ from the base in surface properties, including roughness, selective deposition of the Fe-ascorbic molecules only on the side faces is made possible, forming triangular pyramid polymer shells. The polymer shells are mesoporous and can be calcined into magnetic meso/microporous shells (composed of iron particles embedded in carbon) while maintaining the shell geometry. The magnetic shells are good adsorbents for dyes and can be recycled for use in this role. Experimental Section Synthesis. All reagents are analytical grade and were used without further purification. In a typical synthesis, into 12.0 mL of absolute ethanol 0.081 g (0.5 mmol) of FeCl3 and 0.519 g (1.5 mmol) of sodium dodecylbenzene-sulfonate (SDBS) are added under stirring to form a suspension after 0.5 h. Then, it is poured into a Teflon-lined autoclave and heated at 90 °C for 12.0 h. After that, it is cooled to room temperature, and 0.173 g (0.5 mmol) of SDBS and 50.0 µL diluted NH3 · H2O (50%v/v) are added, and then heated at 160 °C for 6.0 h. Cooling the sample to room temperature, the red precipitate (template) is collected by centrifugation. 0.040 g (0.25 mmol) of FeCl3 is dissolved in 12.0 mL of absolute ethanol to form a solution, and into it 0.060 g of template and 0.176 g (1.0 mmol) of ascorbic acid are added. The mixture is poured into a Teflon-lined autoclave and heated at 200 °C for 6.0 h and then cooled to room temperature. The black solid product is recovered by centrifugation, washed with water, and dried at 60 °C to obtain the triangular pyramidal shells. The triangular pyramidal shells are calcined at 600 °C for 2.0 h in a tubular furnace under N2 gas flow with a heating rate of 5.0 °C/min to form the magnetic porous carbon. Characterization. The morphology and microstructure of the products are characterized by a transmission electron microscope (TEM, JEM 100-CXII) with an accelerating voltage of 80 kV, high resolution transmission electron microscope (HR-TEM, GEOL-2010) with an accelerating voltage of 200 kV, and field emission-scanning electron microscope (FE-SEM JSM-6700F). X-ray diffraction (XRD) patterns are collected on a Rigaku D/Max 2200PC diffractometer with a graphite monochrometer and Cu KR radiation (λ ) 0.15418 nm). Thermogravimetric analysis (TGA) is carried out on a thermal analyzer (METTLER TOLEDO, TGA/SDTA851e) at a heating rate of 10 °C/min from room temperature to 1000 °C under air or N2 flow. Fourier transform infrared (FT-IR) spectra are recorded on a Nicolet 5DX FT-IR spectrometer using KBr pellet technique. Mo¨ssbauer studies are performed at room temperature using a 57Co:Rh source and a conventional constant acceleration drive Mo¨ssbauer spectrometer, and the isomer shift is given

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Figure 1. FE-SEM images (a) and FT-IR spectrum (b) of the templates (inset: magnified view). relative to that of R-Fe. N2 adsorption-desorption data are measured on a QuadraSorb SI apparatus at liquid N2 temperature (T ) -196 °C) after pretreatment at 100.0 °C for 24.0 h under vacuum, and surface area is determined by the Branauer-Emmet-Teller (BET) method. The magnetic properties are measured on a SQUID magnetometer (MPMSXL-7). Elemental analyses are conducted to identify the compositions of the polymer shells and triangular pyramidal templates. Dye Adsorption-Desorption Test. The adsorption-desorption measurements were conducted by mixing 10.0 mg of magnetic porous carbon and 20.0 mL dye’s solution (50 ppm, without pH adjustment) in a flask and putting it on a shaker table (220 rpm) at ambient temperature. The changes of the absorption at 554, 464, and 553 nm were applied to identify the concentrations of Rhodamine B (RB), Methyl Orange (MO) and Direct Blue (DB-78) as a function of the adsorption time using an UV-vis spectrophotometer (Lambda 35, Perkin-Elmer). Adsorption capacity was obtained after the equilibrium was achieved for 24.0 h. For the recycling experiment, the porous carbon was obtained on the flask’s bottom by a magnet. The supernatant was removed and 20.0 mL ethanol was added. After the sample was shaken for 5.0 min, the adsorbed dye was released into ethanol, which was removed by using a magnet to attract the porous carbon on the flask’s bottom. This procedure was repeated to examine the recycling adsorption capability of the magnetic porous carbon.

Results and Discussion The Triangular Pyramid Templates. In Figure 1, we show an SEM image of the anisotropic pyramid templates synthesized at 160 °C. A small amount of irregular nanoparticles is also present. The triangular pyramids have an edge length of 400-600 nm and are composed of stacked nanoplates of ∼10 nm thickness (Figure 1a, bottom right inset). Therefore, the pyramid’s base is smooth, while the other three side faces are relatively rough. The ED pattern of a single pyramid with slightly elongated diffraction spots shows its single-crystal nature and the oriented-aggregation behavior of the nanoplate units (Figure S1, Supporting Information), and the elongated diffraction spots might be due to the imperfect-orientation of the nanoplates.8 The pyramid template gradually shrinks to form amorphous structures under irradiation by electron beam, and can be dissolved in water leaving small irregular nanoparticles. 57 Fe Mo¨ssbauer spectra (Figure S2, Supporting Information) of the templates show a symmetric quadrupole doublet, and the IS and QS parameters indicate that all Fe3+ species are six coordinated in one geometry environment.9 The corresponding FT-IR spectrum (Figure 1b) shows a broad hydroxyl vibration adsorption at 3420 cm-1. Two strong absorptions at 1124 and 622 cm-1, and a weak one at 980 cm-1, are assigned to ν3, ν4, and ν1 vibrations of SO42- anions respectively.10 Weak C-H vibration absorptions ranging from 2800 to 3000 cm-1 and SdO symmetrical stretching adsorption in RSO3- group at 1148 cm-1 indicate the existence of a small amount of DBS molecules. The broad adsorption centered at 480 cm-1 is the characteristic Fe-O vibration.11 Combined with the further elemental analysis of the product (C-0.63%, H-0.77%, S-8.17%, Na-25.53%, Fe-

11.36%, Cl-28.33%, O-25.21% by weight percentage), it is deduced that the product mainly contains NaCl, Fe-Na hydroxysulfate (triangular pyramids) and a small amount of DBS residuals. We speculate that DBS species decompose at 160 °C to form the SO42- species with the hydrolysis of Fe3+ to form the Fe-Na hydroxysulfate during the solvothermal process.12 To investigate the formation process of the triangular pyramid templates, the detailed time-dependent solvothermal reaction at 160 °C is tracked. It is found that NaCl precipitated after FeCl3 and SDBS reacted at 90 °C for 12.0 h in ethanol. Into the above suspension, additional SDBS and diluted NH3 · H2O (50%v/v) are added, and heated at 160 °C for 2.0 h. TEM images show, at this stage, a large amount of nanoparticles as well as NaCl, but no triangular pyramids (Figure 2a). Further prolonging the reaction time to 3.0 h, triangular pyramids with side lengths of ca.250 nm begin to appear in TEM images while the amount of nanoparticles decreases (Figure 2b). The templates were washed out, and the HR-TEM image (Figure 2e) of the nanoparticles shows a lattice space of 1.1 nm, which is in accordance with the XRD reflection at 2θ ) 8.1° (d ) 1.09 nm, Figure 3 and S3). FT-IR spectrum (Figure 2f) shows both characteristic adsorptions of iron hydroxide and DBS groups. Strong adsorptions at 480 and 3422 cm-1 belong to the vibrations of Fe-O bonds and O-H groups. Absorptions of C-H, R-SdO, aromatic in DBS molecules respectively appear at 2800-3000, 1148, 1624, and 899 cm-1.10 It indicates that the nanoparticles should be partially hydrolyzed DBS salts of Fe3+ and Na+. As the reaction proceeds, the triangular pyramids gradually increase in number with the irregular nanoparticles decreasing, and the product consists mainly of triangular pyramids with side lengths of ca. 500 nm (Figure 2c,d). On the basis of the above experimental results, we can speculate on the likely process for the formation of the templates. When anhydrous FeCl3 is dissolved in ethanol, the Fe(OC2H5)xCl3-x might be formed, releasing Cl- ions. Then NaCl can form due to the reaction between Cl- and SDBS at 90 °C, which results in the dissolution of the DBS species. After the additional SDBS and dilute NH3 · H2O are added to the suspension and reacted at 160 °C, the Fe3+ partially hydrolyze to form the small irregular nanoparticles. The DBS groups in these nanoparticles are unstable at 160 °C, and decompose to form the Fe-Na hydroxysulfate nanoplates, which further self-aggregate to form the triangular pyramids due to their high surface energy. Under the solvothermal reaction conditions, continuous decomposition of DBS groups supplies the raw material for the growth of the triangular pyramids. The Polymer Shells. The preformed templates are added to the mixture of ascorbic acid and ethanol solution of FeCl3 to form a suspension, which is heated at 200 °C. The ascorbic acid gradually dissolves in ethanol and coats on the template’s

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Figure 4. FE-SEM (a and b) images of hollow polymer shells, and the corresponding FT-IR spectrum (c) and N2 adsorption-desorption isotherms (d) with the pore size distributions (the inset).

Figure 2. TEM images of the products as heating at 160 °C for (a) 2 h, (b) 3 h, (c) 4 h, and (d) 5 h, the inset in (b) is SAED pattern of a triangular pyramid. HR-TEM image (e) and FT-IR spectrum (f) of product in (b) after being washed by water.

Figure 3. XRD patterns of the products as the reactions at 160 °C for 2 h (a), 3 h (b), 4 h (c), 5 h (d), 7 h (e), and the template/polymer core-shell composite (f). The diffraction peaks at 2θ ) ca. 8.0° marked by “3” are assigned to the partially hydrolyzed DBS salts of Fe3+ and Na+ nanoparticles, and those marked by “ο” are attributed to the triangular pyramid templates.

surface for its superior coordination with metal ions.13 The ascorbic acid (AA) decomposes to form furfural molecules under the catalysis of Fe3+ (main product), which self-polymerize with the aldehyde groups, and chelate with Fe3+ species.14,15 The free Fe3+ species in solution synchronously incorporate into the polymer. Because of the different surface states of the base and side face of the template, the ascorbic acid molecules only coat the side faces, forming the core/half-shell structures (Figure S4,

Supporting Information). After removing the template, the FeAA polymer triangular pyramid shells are formed. The morphology of the Fe-AA polymer particles have been clearly observed by FE-SEM and TEM techniques. The FESEM image shows their uniform triangular pyramid shells with a side-edge length of 350-600 nm and shell thickness of 55-90 nm (Figure 4a,b). The selected area electronic diffraction (SAED) and XRD patterns indicate their amorphous nature (Figure S5, Supporting Information), and elemental analyses demonstrate the product mainly consists of C (41.95%), Fe (10.90%, calculated by the TG curve in Figure S6a, Supporting Information), O (42.30%), H (3.85%), and S (1.00%). A small quantity of S element might result from the incomplete removal of the template. 57Fe Mo¨ssbauer spectra recorded at room temperature (Figure S2, Supporting Information) are fitted with two symmetric quadrupole doublets, and the fitted parameters (Table S1, Supporting Information) indicate the character of an high spin state of Fe3+ located in octahedral sites.16 The isomer shift (IS) and quadrupole splitting (QS) values of two doublets rule out the existence of Fe0 and Fe2+, demonstrating that Fe3+ is not reduced by ascorbic acid.17 The FT-IR spectrum (Figure 4c) shows a broad absorption at 3421 cm-1 ascribed to the vibrations of intermolecular or intramolecular hydrogen bonds. The strong absorption at 1629 cm-1 is attributable to the CdC vibration within the furan ring of furfural molecules, and those at 1380 and 1150 cm-1 are assigned to the vibrations of the furan-rings’ skeleton and C-O-C bonds within the furfural molecules. The FT-IR spectrum indicates that the ascorbic acid is oxidized to form the furfural, the main oxidationdecomposition product of ascorbic acid in hypoxic condition,18,19 during the solvothermal process at 200 °C. Furfural molecules further polymerized to form a triangular pyramid shell with Fe3+ incorporated in it. N2 adsorption gives its BET surface area of 70.6 m2/g, and the mean size of the mesopores calculated by the BJH method is 3.9 nm (Figure 4d). Further experiments show that similar results would be obtained by replacing ascorbic acid with D-fructose or sucrose (Figure S7, Supporting Information). The anisotropic template might be applied to fabricate other pyramidal shells as ascorbic acid could coordinate with many other metal ions. The Magnetic Porous Carbon. The TG curves (Figure S6b, Supporting Information) of polymer shells under N2 atmosphere show a continuous weight loss from room temperature to 600

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Figure 5. FE-SEM (a) and TEM (b, c) images of magnetic porous carbon, and the corresponding TG/DSC curves in air atmosphere (d), N2 adsorption-desorption isotherms (e) with the pore size distributions (the inset), room-temperature magnetization curves (f) before (curve 1) and after (curve 2) repeated adsorption-desorption of RB molecules for 11 times.

°C ascribed to the carbonization of organic species. Then the porous carbon is obtained by calcining the pyramid polymer shells under suitable reaction conditions selected according to the TG curve. Energy-dispersive X-ray (EDX) spectroscopy shows the calcined product to be composed of C and Fe, and the XRD pattern (Figure S5b, Supporting Information) indicates only the crystalline phase present is R-Fe (JCPDS, No. 85-1410). The components of the carbonized product are mainly R-Fe and amorphous carbon, with the contents of 41.7 and 58.3% respectively calculated by the corresponding TG curve in air atmosphere (Figure 5d). SEM images (Figure 5a and its inset) show that the pyramid shells shrink with the side-edge length decreasing to 270-380 nm. At the same time, neighboring shells are often seen to have fused together. TEM and HR-TEM images (Figure 5b,c) further indicate that the calcined pyramid shells are constructed from R-Fe nanoparticles of ca. 5.0 nm embedded in amorphous carbon matrix with the mesopores of 3-5 nm. N2 adsorption-desorption isotherms also confirm the presence of mesopores with a mean size of ca. 3.8 nm, and micropores with an average size of 0.6 nm (Figure 5e). The BET surface area greatly increases to 399.1 m2/g, including mesopore surface area of 90.6 m2/g (by BJH method) and micropore surface area of 254 m2/g (by t-method). It indicates that the increasing of BET surface area results from the generated micropores during carbonization due to the decomposition of the polymer. The magnetic porous carbon shows obvious ferromagnetism, whose saturation magnetization value is ca. 39.3 emu/g (Figure 5f) with the remnant magnetization and coercivity respectively being 6.2 emu/g and 260 Oe. As superparamagnetism would be expected for 5 nm Fe nanoparticles,20 the enhanced remnant and coercivity might derived from irregular larger particles with the size of several tens of nanometers surrounding the shells (Figure S8, Supporting Information). The high saturation magnetization value due to the high iron content loading in the shell favors its use in magnetic separation applications. Magnetic porous carbon could be easily separated and collected using magnetic field, and would be greatly suitable for use as a magnetically separable catalyst, adsorbent, etc.21,22 However, only low loadings of the magnetic component can be obtained through conventional wet or ion exchange methods, necessitating procedures with multiple impregnation.23 Here, we

Figure 6. (a) Absorption kinetic curves of RB, MO, and DB-78 molecules on magnetic porous carbon, and (b) relative recycling adsorption capacities for RB after different cycles.

studied its adsorption kinetic and capacities of three kinds of dyes: Rhodamine B (RB, cationic dye), Methyl Orange (MO, anionic dye), and Direct Blue (DB-78, anionic dye). A higher adsorption rate on cationic dye than that on anionic dyes is observed. The final adsorption capabilities for contacting time of 24.0 h are 72.56 mg/g for RB, 69.79 mg/g for MO, and 81.43 mg/g for DB-78. Repeating adsorption test shows that >90% adsorption capability is maintained for the 11th adsorptiondesorption cycles (Figure 6). A slight decrease of the saturation magnetization value to 34.4 emu/g is observed after 11 times of adsorption-desorption cycles, potentially due to surface evolution of the R-Fe nanoparticles. Conclusions In conclusion, a novel anisotropic template method is developed for the fabrication of polymer triangular pyramid shells (containing Fe). The BET surface area of the polymer shells is 70.6 m2/g with a mean pore size of 3.9 nm. They can be transformed into magnetic carbon/Fe composites while keeping the shell geometry unchanged. Accompanying the change in composition, BET surface area increases to 399.1 m2/g. The magnetic porous carbon has a large saturation magnetization and exhibits a high adsorption rate and steady adsorption capacity for the dyes. The anisotropic triangular pyramid templates are composed of crystalline Fe-Na hydroxysulfate and enable the formation of the triangular pyramid shells. The formation of the pyramids is based on the generation of partially hydrolyzed DBS salts of Fe3+ and Na+, decomposi-

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tion of the DBS species to form Fe-Na hydroxysulfate particles, and their subsequent oriented-aggregation. This approach could be used to prepare other triangular pyramid shells due to the high avidity our template has toward many coordinating reagents. Acknowledgment. This work is supported by the National Natural Science Foundation of China (Grant No. 20671057), Program for New Century Excellent Talents in University (P. R. China). Supporting Information Available: TEM image of the triangular pyramids irradiated by electron beam for different times, Mo¨ssbauer spectra of the polymer shells and templates, XRD patterns of the washed products (by water) obtained at 160 °C for different times, FE-SEM and TEM images of the core/half-shell structures, XRD patterns of the polymer shells and magnetic porous carbon, TG/DSC curves of the polymer shell. This material is available free of charge via the Internet at http://pubs.acs.org.

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