Simultaneous Synthesis of Dendritic Superstructural and Fractal

Crystal Growth & Design , 2006, 6 (3), pp 769–773 ... synthesized in the same reaction system via facile vegetal bi-templates, mung bean sprouts (MB...
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Simultaneous Synthesis of Dendritic Superstructural and Fractal Crystals of BaCrO4 by Vegetal Bi-templates Yun

Yan,†

Qing-Sheng

Wu,*,†

Li

Li,†

and Ya-Ping

Ding‡

Department of Chemistry, Tongji UniVersity, Shanghai 200092, People’s Republic of China, and Department of Chemistry, Shanghai UniVersity, Shanghai 200444, People’s Republic of China

CRYSTAL GROWTH & DESIGN 2006 VOL. 6, NO. 3 769-773

ReceiVed August 7, 2005; ReVised Manuscript ReceiVed NoVember 18, 2005

ABSTRACT: Two novel dendritic superstructural and fractal BaCrO4 crystals were simultaneously synthesized in the same reaction system via facile vegetal bi-templates, mung bean sprouts (MBS). The two dendritic crystals in different shapes were grown on the outer surface and the inner stem wall of MBS. The two BaCrO4 dendrites are characterized by scanning electron microscopy (SEM), X-ray diffraction (XRD), and photoluminescence (PL). The luminescent property indicates that the products have a broad emission band peak may have applications in an electronic light device. A presumable mechanism is also given. 1. Introduction The fabrication and self-organization of inorganic materials on different scales with special size and well-defined shape are attracting increasing interest, for they are key challenges in material chemistry research, as well as practical applications. Different morphologies of inorganic materials, including rods,1 bundles,2 plates,3 funnel-like shapes,4 dendrites,5 and cubes,6 have been synthesized by various methods. Among these shapes, dendritic formations have been focused on due to their importance connected to some fractal growth phenomena7 and crystallography research and because they have wide applications in microdevices. Diverse strategies involving hydrothermal synthesis,8 vapor deposition method,9 evaporation-induced selfassembly (EISA),10 and ultraviolet irradiation photoreduction technique11 have been adopted to fabricate them. The growth of dendritic crystals is a profound example among a wide range of pattern-forming phenomena in nature and biology; it is thus obvious to search for biological templates to direct dendritic growth. There has been a trend of employing the information processing and sensing capabilities of biological templates to obtain ordered, hierarchical structures or nanostructures in specific shapes.12 Among them, templates of organism structures are attracting more and more interest due to their inherent hierarchical ordered superstructures, such as tobacco mosaic virus,13 silica algae,14 Equisetum arVense,15 eggshell membrane,16 etc. Most methods using biotemplates to synthesize well-defined nanostructures or superstructures, however, are performed under harsh conditions within relatively high temperature or extreme pH values or followed by complicated procedures or require other modifiers. Few ordered, hierarchical structures are produced in a heterogeneous environment at ambient temperature and pressure and pH values close to neutral with no additive. Barium chromate, an oxidizing agent and photocatalyst,17,18 has been frequently investigated because of its wide application and the crystal habit of shape-sensitivity,19 which enlightens us to induce the crystal to well-defined shape and structure through precise template/condition controls. Till now, a conjoint dendritic fractal pattern has still not been synthesized, even though barium chromate is relatively apt to undergo fractal patterning. S. H. * Corresponding author. E-mail: [email protected]. † Tongji University. ‡ Shanghai University.

Yu et al. have prepared complex, similar fiber bundles controlled by PEG-b-PMAA-PO3H2,19 as well as high-ordered funnellike BaCrO4 superstructures with a complex form and a remarkably self-similar growth pattern under the control of polyacrylate.4 L. M. Qi et al. reported the synthesis of hierarchical superstructures consisting of nanobelts by adjusting the mixing ratio, r, in catanionic reverse micelles.20 In previous work, most effort was made on the command of reverse micelles and microemulsions or polymer crystal growth modifiers, but the control function of vegetal templates on fractal patterns was not referred. In this paper, mung bean sprout (MBS), having a uniaxial pore structure with a biological surface pattern, was adopted to induce hierarchical and fractal structures as a dualfunction template with both nucleation position control and crystal growth/assembly control in the whole formation process. Mung bean sprout (MBS), the sprout form of mung bean, is an ideal biological template.21 It is a traditional vegetable in China and Southeast Asia and largely cultivated in many developing countries. MBS is available in supermarkets with no seasonal limitations and is extremely inexpensive, remaining stable at temperatures between 0 and 35 °C and at pH values from 4 to 10. Besides, MBS consists of large amounts of celluloses, proteins, and more than 17 kinds of amino acids, such as aspartic acid, glutamic acid, valine, etc. Statistically speaking, there are 27.4 g of amino acids contained in every 100 g of dry MBS. In principle, these various functionalities such as hydroxyl, primary amide, carboxylate, and amine groups should offer a wide variety of nucleation sites for surfacecontrolled inorganic deposition,13a which could be exploited in the biomimic synthesis of inorganic materials such as hierarchical superstructures and fractals in specific shapes. The epidermal surface and the inner stem of MBS have different structures. Figure 1 shows the outside surface (Figure 1a) and the inside stem (Figure 1b) patterns of it. It can be seen from Figure 1a that the epidermis is not totally smooth but has well-distributed grooves whose widths are 80-120 µm and diameters are 1020 µm. Figure 1b is the cross section of the inner stem of MBS. Stems were composed of many canaliculi of uniform size, whose apertures were about 100 µm. In addition, there are many pleats on the inner stem wall. Inspired by these different but specific structures between the outside and the inside of MBS, in association with the functionalities contained in the sprout, we employed mung bean sprout as a facile vegetal bi-template for two dendritic patterns of BaCrO4 superstructural and fractal crystals at the same time.

10.1021/cg050386v CCC: $33.50 © 2006 American Chemical Society Published on Web 01/26/2006

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2. Experimental Section

Figure 1. SEM images of (a) the epidermal surface of mung bean sprout (outside) and (b) the cross section of the interior stem of mung bean sprout (inside).

These different barium chromates were concurrently grown on the surface and the inner stem wall of MBS through a biomineralization process. Compared with other biomimetic systems or biotemplates, our approach requires neither harsh conditions nor extra modifiers. The luminescent properties of barium chromate at microscale were characterized,22 which exhibit good light performance and may be applied to make tuned laser materials, photocatalytic devices, and electrochromic devices.

2.1. Synthesis Method. Mung bean sprouts were purchased from a supermarket. Prior to the experiment, the plants were washed with distilled water several times to ensure that no impurities were attached on them. (i) The MBS (except laminae) were immersed in a solution of 0.1 mol L-1 BaCl2 for 8 h. (ii) The MBS were taken out of the BaCl2 solution and washed with distilled water; then the MBS were immersed in a solution of 0.1 mol L-1 K2CrO4, and five minutes later on the surface of the mung bean sprouts light yellow precipitates began to appear. After 1 h, precipitates began to appear on the inner stem of the MBS, and the preferential deposition location was toward the ends of the stem, consistent with a capillary-driven mechanism. The MBS were kept immersed in the solution for 8 h, until the reaction was complete. (iii) After being washed with distilled water and absolute ethanol repeatedly, the different products, accreted inside and outside of the MBS, were successively collected into two beakers. (iv) Before the precipitates were characterized, all precipitates were carefully washed repeatedly with distilled water and absolute ethanol, followed by centrifugation, to ensure that no residual MBS was attached on the products. 2.2. Characterization Method. Powder X-ray diffraction (XRD) patterns were recorded on a Philips PW1700 X-ray diffractometer (XRD) with Cu KR radiation (λ ) 0.154 18 nm) (Holand). Scanning electron microscopy (SEM) measurements were performed with a fieldemission environmental scanning electron microscope FEI/ Phillps XL30 ESEM-FEG (SEM, Holand). IR spectra were measured with a NEXUS870 FT-IR spectrometer (Nicolet). Photoluminescence (PL) spectra were obtained with a Perkin-Elmer luminescence spectrophotometer (LS-55). All measurements were carried out at room temperature.

3. Results and Discussion 3.1. Morphologies and Structures. The morphology image of the as-grown BaCrO4 (outside and inside of the mung bean sprouts) was obtained using SEM, as shown in Figure 2. The outside products exhibit quite a number of dendritic hierarchical architectures with a length of 200-400 µm along the trunk, and its side branches are about 50-150 µm long (Figure 2a). Further observation (Figure 2b,c) demonstrates that all the dendrites were assembled from rectangular blocks.

Figure 2. SEM images of (a-c) as-prepared BaCrO4 (outside) sample and (d,e) as-prepared BaCrO4 (inside) sample. Scale bars: (a) 200 µm; (b,c) 20 µm; (d) 200 µm; (e) 60 µm.

Dendritic Superstructural and Fractal Crystals of BaCrO4

Figure 3. XRD patterns of (a) as-prepared BaCrO4 (outside) sample and (b) as-prepared BaCrO4 (inside) sample.

Figure 4. FT-IR spectra of BaCrO4 products prepared in different experimental procedures: (A) whole spectras(a) pure mung bean sprout, (b) mung bean sprout in Ba2+ solution, and (c) mung bean sprout in CrO42- solution; (B) magnification spectra of characteristic peaks.

The inside products exhibit dendritic fractals showing selfsimilarity (Figure 2d). Further observation (Figure 2e) reveals that there is a single integrity, which greatly differs from the outer products. The branches extended themselves like a balm, much bigger than that of the outside samples, about 1000 µm long, and the length of the side branch is about 500-700 µm (Figure 2d,e).

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Figure 3 shows XRD patterns of the as-prepared BaCrO4 from the outer surface of the MBS (a) and from the inner surface of the MBS (b). Figure 3a,b shows that the two samples are in the same space group, Pnma. The peaks correspond to 〈hkl〉 values of (210), (211), (203), and (312), which are in good agreement with reference to the unit cell of the orthorhombic structure (JCPDS File No. 35-0642), a ) 9.105 Å, b ) 5.541 Å, and c ) 7.343 Å. 3.2. Optical Properties. 3.2.1. FT-IR Spectroscopy. To reveal the formation process of barium chromate, pure MBS, MBS immersed in Ba2+ after the first synthesis step, and MBS immersed in CrO42- after the second step were taken out to be examined by Fourier transform infrared (FT-IR) spectroscopy. Before examination, each MBS was washed with distilled water to ensure that no ion solution was left in the MBS. Then the MBS selected from different steps were pressed to become thin membranes and dried for examination. Figure 4a-c) displays the FT-IR spectra of the pure MBS, the MBS immersed in Ba2+ after the first synthesis step, and the MBS immersed in CrO42after the second step. It can be seen from Figure 4c that after immersion in CrO42-, a new absorption band was observed on the MBS compared with those in Figure 4a,b. The reference bands were observed around 850-950 and around 400 cm-1, which were ascribed to the Cr-O stretching mode (ν1, ν3) and O-Cr-O symmetric bending mode (ν2, ν4) in the CrO42tetrahedron, respectively.23 In addition, the new absorption band is not as sharp or strong, which implies binding between the MBS and the CrO42- ion are not very compact. It may be ascribed to the electrostatic interactions24 that combine the CrO42- ion with the MBS during the biomineralization process. Through FT-IR analysis, it was revealed that during the deposition, nucleation, and growth process of BaCrO4, the MBS biosystem not only acts as a spatial confinement but also plays a role in molecular recognition. 3.2.2. PL Spectroscopy. Till now, few reports of BaCrO4 preparation referred to the luminescence properties, largely because the isoelectronic system CrO42- is nonluminescent because of rapid radiationless deactivation.25 Here, we examined the PL spectra of the as-prepared barium chromate at room temperature, and our result shows it to have a potential application in optical devices. The luminescence spectra (Figure 5a,b,c) show that BaCrO4 has a broad emission band peak when excited at 276, 448, and 488 nm, respectively. When they were excited at 276 nm, the external BaCrO4 samples have emission band peaks at 327 and 421 nm (Figure 5a1), and the internal ones have emission band peaks at 325 and 421 nm (Figure 5a2). When they were excited at 454 and 488 nm, the broad emission band peaks appeared at 607 nm (both the external and the internal) and 640 nm (both the external and the internal),

Figure 5. PL spectrum of (1) as-prepared BaCrO4 (outside) sample and (2) as-prepared BaCrO4 (inside) sample: (a) λex ) 276 nm; (b) λex ) 454 nm; (c) λex ) 488 nm.

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Figure 6. SEM images of BaCrO4 prepared at different times: (a) outside products obtained after 5 min; (b) outside products obtained after 1 h; (c) outside products obtained after 6 h; (d) inside products obtained after 5 min; (e) inside products obtained after 1 h; (f) inside products obtained after 6 h. Scale bars: (a) 5 µm; (b,c) 10 µm; (d) 50 µm; (e) 200 µm; (f) 1 µm.

respectively (Figure 5b,c). This phenomenon may be ascribed to emission from a metastable triplet state of the chromate ion.26 Based on these diverse luminescent properties, the dendriticshaped BaCrO4 have potential application in electronic device such as tuned laser materials and photocatalytic and electrochromic devices. 3.3. Effect of Time. To observe the growth process, timedependent scanning electronic microscopy experiments were conducted. Dispersed particles were initially formed outside and inside of MBS (Figure 6a,d). As time goes by, those particles gradually aggregated and grew into a network pattern both outside and inside of the MBS (Figure 6b,e). After 6 h, the crystal pattern began to exhibit some typical branches, which tend toward dendritic patterns (Figure 6c,f). 3.4. Mechanism Exploration. A probable mechanism for the development of the dendritic superstructural and fractal crystals is proposed. Biologically, the MBS template is different between its external and internal portions, from its spatial structures to various organic group arrangements. The main groups include hydroxyl, primary amide, carboxylate, and some phospholipid groups. According to the theory of soft-hard acid-base, Ba2+ ion belongs to hard acid, which will preferentially combine with hard alkaline groups, such as oxygen in the hydroxyl and carboxylate groups. Therefore, Ba2+ ion was first combined on the wall of the MBS containing many hydroxyl and carboxylate groups mainly by electrostatic attraction. After the CrO42- ion entered the MBS, BaCrO4 molecules were formed, and then amorphous nuclei were continuously produced on the interface; later nanoparticles came into being (Figure 6a,d). This step was supported by earlier findings that the superstructures do not really grow from a supersaturated ion solution but by aggregation/transformation of the primary clusters formed.19,27 Then, induced by the biotemplate, nanoparticles were assembled to net framework (Figure 6b,e), followed by the aggregation/growth into dendritic fractal and superstructal crystals (Figure 2). Living biotemplates, which possess dual-function on each with both nucleation position control and crystal growth control, played a decisive role in the whole formation process. During nucleation, templates are dominant in the nucleation positions through special confinement. During the further aggregation and growth period, the external and internal surface templating groups preferentially adsorbed on some crystal faces and inhibit these faces from further growth. Meanwhile, the special function

of organic groups induced nanocrystals to directionally assemble and orientationally grow. In the end, the outside and inside dendritic patterns were formed. The detailed mechanism needs to be further investigated in future work, since there is still no method to explore the detailed arrangement of chemical groups on the MBS. 4. Conclusion In summary, two different superstructural and fractal dendritic crystals of barium chromate were successfully synthesized through biological functions of vegetal bitemplates. This method is mild, convenient, and “green” because of the absence of surfactant and organic solvent28 and provides a new route to fabrication of other inorganic salt superstructures or fractals. The detailed formation mechanism may be very complicated, and further investigation is to be continued. The investigation of their luminescence properties indicates that microscale BaCrO4 has a broad emission band peak and may be applied in tuned light devices. Acknowledgment. The authors thank the National Natural Science Foundation of China (Grant No. 20471042), the Doctoral Program Foundation of the Ministry of Education of China (Grant No. 20040247045), and the Nano-Foundation of Shanghai in China (Grant No 0452nm075) for financial support. References (1) (a) Puntes, V. F.; Krishnan, K. M.; Alivisatos, A. P. Science 2001, 291, 2115. (b) Yang, P. D.; Lieber, C. M. Science 1996, 273, 1836. (c) Wang, J. W.; Li, Y. D. Mater. Chem. Phys. 2004, 87, 420. (2) (a) Moore, D. F.; Ding, Y.; Wang, Z. L. J. Am. Chem. Soc. 2004, 126, 14372. (b) Jiang, C.; Zhang, W.; Zou, G.; Yu, W.; Qian, Y. J. Phys. Chem. B 2005, 109, 1361. (3) (a) Yu, J. C.; Xu, A. W.; Zhang, L. Z.; Song, K. Q.; Wu, L. J. Phys. Chem. B 2004, 108, 64. (b) Chen, S. H.; Carroll, D. L. Nano Lett. 2002, 2, 1003. (4) Yu, S. H.; Antonietti, M.; Colfen, H.; Hartmann, J. Nano Lett 2003, 3, 379. (5) (a) Kuang, D. B.; Xu, A. W.; Fang, Y. P.; Liu, H. Q.; Frommen, C.; Fenske, D. AdV. Mater. 2003, 20, 1747. (b) Ma, Y. R.; Qi, L. M.; Ma, J. M.; Chang, H. M. Cryst. Growth Des. 2004, 4, 351. (c) Zhang, J.; Sun, L. D.; Jiang, X. C.; Liao, C. S.; Yan, C. H. Cryst. Growth Des. 2004, 4, 309. (6) (a) Wang, D. B.; Mo, M. S.; Yu, D. B.; Xu, L. Q.; Li, F. Q.; Qian, Y. T. Cryst. Growth Des. 2003, 3, 717. (b) Sun, Y. G.; Xia, Y. N. Science 2002, 298, 2176. (7) Vicsek, T. Fractal Growth Phenomena, 2nd ed.; World Scientific: Singapore, 1992.

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