Cellulose Acetate-Directed Growth of Bamboo-Raft-like Single

(26) (a) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. N. AdV. Funct. Mater. 2002, .... copper grid upon which the selenium nanorafts were deposited. The...
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Langmuir 2007, 23, 7321-7327

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Cellulose Acetate-Directed Growth of Bamboo-Raft-like Single-Crystalline Selenium Superstructures: High-Yield Synthesis, Characterization, and Formation Mechanism Ji-Ming Song, Yong-Jie Zhan, An-Wu Xu, and Shu-Hong Yu* DiVision of Nanomaterials and Chemistry, Hefei National Laboratory for Physical Sciences at Microscale, Department of Chemistry, UniVersity of Science and Technology of China, Hefei, Anhui 230026, The People’s Republic of China ReceiVed January 29, 2007. In Final Form: April 14, 2007 High-yield synthesis of bamboo-raft-like single-crystalline selenium superstructures has been realized for the first time via a facile solvothermal approach by reducing SeO2 with ethylene alcohol in the presence of cellulose acetate. The formation of a raftlike superstructure with various forms is strongly dependent on the temperature, amount of cellulose acetate, reaction time, and even preheating treatment. The suitable amount of cellulose acetate is essential for the formation of elegant and uniform raft Se. The morphology, microstructure, optical properties, and chemical compositions of bamboo-raft-like selenium were characterized using various techniques (X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy (TEM), high-resolution (HR) TEM, X-ray photoelectron spectroscopy, UV-vis spectroscopy, FTIR spectroscopy, and Raman spectroscopy). A possible growth mechanism has been proposed. Such special superstructures could provide a useful precursor for potential applications.

1. Introduction The properties of nanomaterials are determined significantly by their structure, i.e., by their dimensions, crystallinity, and geometry.1 A variety of nanostructures have been synthesized, including cages,2 cylindrical wires,3 rods,4 cables,5 diskettes,6 springs,7 tubular structures,8 and other forms. Among them, twodimensional (2D) nanomaterials (belts or flake) are of extraordinary importance because of their special properties and potential applications in construction of nanoscale electronic and optoelectronic devices, which has initiated intense interest in the synthesis of such beltlike nanostructures.9 Self-assembly of 2D arrays represents a first step toward the construction of designed superstructure materials. The technological importance of 2D nanoscale assembly has been widely recognized as evidenced by the development of 2D colloidal arrays10 for use as coatings,11 chemical sensors,12 and photonic crystals.13 In current literature, 2D array self-assemblies have mainly been obtained through the Langmuir-Blodgett (LB) technique and oriented aggregation * To whom correspondence should be addressed. E-mail: shyu@ ustc.edu.cn. Fax: +86 551 3603040. (1) (a) Peng, X. G.; Manna, L.; Yang, W.; Wickham, D. J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (b) Sun, X. M.; Li, Y. D. AdV. Mater. 2005, 17, 2626. (2) (a) Saito, Y.; Matsumoto, T. Nature 1998, 392, 237. (b) Tenne, R.; Homyonfer, M.; Feldman, Y. Chem. Mater. 1998, 10, 3225. (3) (a) Trentler, T. J.; Hickman, K. M.; Goel, S. C.; Viano, A. M.; Gibbons, P. C.; Buhro, W. E. Science 1995, 270, 1791. (b) Mozetic, M.; Cvelbar, U.; Sunkara, M. K.; Vaddiraju, S. Adv. Mater. 2005, 17, 2138. (c) Lee, S. T.; Wang, N.; Zhang, Y. F.; Tang, Y. H. MRS Bull. 1999, 24, 36. (4) (a) Dai, H. J.; Wong, E. W.; Lu, Y. Z.; Fan, S. S.; Lieber, C. M. Nature 1995, 375, 769. (b) Jana, N. R. Small 2005, 1, 875. (c) Link, S.; El-Sayed, M. A. J. Appl. Phys. 2002, 92, 6799. (5) Luo, L. B.; Yu, S. H.; Qian, H. S.; Zhou, T. J. Am. Chem. Soc. 2005, 127, 2822. (6) Dai, Z. R.; Pan, Z. W.; Wang, Z. L. J. Am. Chem. Soc. 2002, 124, 8673. (7) Kong, X. Y.; Wang, Z. L. Nano Lett. 2003, 3, 1625. (8) Ajayanp, M.; Iijima, S. Nature 1992, 358, 23. (9) Ding, Y.; Wang, Z. L. J. Phys. Chem. B 2004, 108, 12280. (10) (a) Fo¨rster, S.; Antonietti, M. AdV. Mater. 1998, 10, 195. (b) Kiely, C. J.; Fink, J.; Brust, M.; Bethell, D.; Schiffrin, D. J. Nature 1998, 396, 444. (c) Shenton, W.; Pum, D.; Sleytr, U. B.; Mann, S. Nature 1997, 389, 585. (11) Smits, C.; Van Duijneveldt, J. S.; Dhont, J. K. G.; Lekkerkerker, H. N. W.; Briels, W. J. Phase Transitions 1990, 21, 157. (12) Lee, K.; Asher, S. A. J. Am. Chem. Soc. 2000, 122, 9534.

Scheme 1. Conformations of Substituted Monosaccharide Units in CDA

of precursors such as zero-dimensional (0D) and one-dimensional (1D) nanoparticles on a solid substrate (such as a silicon wafer, mica, a quartz plate, and other metal lamellae).14 In addition, quasi-two-dimensional (2D) beltlike nanostructured oxides (ZnO, SnO2, In2O3, and CdO) were originally synthesized by Wang et al. using a thermal evaporation of oxide powders at high temperature (1000-1400 °C).15 Afterward, many kinds of nanobelts have been fabricated, with typical examples including Si,16 Zn,17 TiO2,18 PbO,19 Fe2O3,20 ZnS,21 ZnSe,22 and Au.23 Trigonal selenium (t-Se), a p-type, extrinsic semiconductor with an indirect band gap of about 1.6 eV,24 has been widely used in the production of photocells, photographic exposure (13) (a) Rogach, A.; Susha, A.; Caruso, F.; Sukhorukov, G.; Kornowski, A.; Kershaw, S.; Mohwald, H.; Eychmuller, A.; Weller, H. AdV. Mater. 2000, 12, 333. (b) Mayers, B. T.; Gates, B.; Xia, Y. AdV. Mater. 2000, 12, 1629. (c) Reese, C. E.; Guerrero, C. D.; Weissman, J. M.; Lee, K.; Asher, S. A. J. Colloid Interface Sci. 2000, 232, 76. (14) (a) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407. (b) Clark, T. D.; Ferrigno, R.; Tien, J.; Paul, K. E.; Whitesides, G. M. J. Am. Chem. Soc. 2002, 124, 5419. (c) Vidoni, O.; Reuter, T.; Torma, V.; Meyer-Zaika, W.; Schmid, G. J. J. Mater. Chem. 2001, 11, 3188. (15) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Science 2001, 291, 1947. (16) Shi, W. S.; Peng, H. Y.; Wang, N.; Li, C. P.; Xu, L.; Lee, C. S.; Kalish, R.; Lee, S. T. J. Am. Chem. Soc. 2001, 123, 11095. (17) Wang, Y. W.; Zhang, L. D.; Meng, G. W.; Liang, C. H.; Wang, G. Z.; Sun, S. H. Chem. Commun. 2001, 2632. (18) Jung, J. H.; Kobayashi, H.; Bommel, K. J. C.; Shinkai, S.; Shimizu, T. Chem. Mater. 2002, 14, 1445. (19) Pan, Z. W.; Dai, Z. R.; Wang, Z. L. Appl. Phys. Lett. 2002, 80, 309. (20) Wen, X. M.; Wang, S. H.; Ding, Y.; Wang, Z. L.; Yang, S. H. J. Phys. Chem. B 2005, 109, 215. (21) Gong, J. F.; Yang, S. G.; Duan, J. H.; Zhang, R.; Du, Y. W. Chem. Commun. 2005, 351. (22) Yao, W. T.; Yu, S. H.; Huang, X. Y.; Jiang, J.; Zhao, L. Q.; Pan, L.; Li, J. AdV. Mater. 2005, 17, 2799-2802. (23) Zhang, J. L.; Du, J. M.; Han, B. X.; Liu, Z.; Jiang, T.; Zhang, Z. F. Angew. Chem., Int. Ed. 2006, 45, 1116.

10.1021/la700230d CCC: $37.00 © 2007 American Chemical Society Published on Web 05/25/2007

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Figure 1. FESEM images (a-c) and TEM image (d) of raftlike t-Se: (a) at a low magnification; (b) a typical flake raftlike t-Se particle at high magnification; (c) end of a flake raftlike t-Se particle; (d) detailed surface structure of the raftlike t-Se particle.

meters, pressure sensors, and electrical rectifiers, due to its high photoconductivity, excellent spectral sensitivity, and large piezoelectric, thermoelectric, and nonlinear optical responses.25 There are several approaches to prepare selenium nanomaterials. a-Se spherical colloids (0D) with a size distribution of less than 5% have been achieved by adopting a more viscous solvent (ethylene glycol) as the reaction medium in place of water.26 Recently, several approaches have been employed to fabricate selenium nanowires or nanorods (1D),27 including the use of cytochrome c3,28 refluxing process,29 interface growth,30 sonochemical synthesis,31and chemical vapor deposition.32 Zhang et al.33 reported the synthesis of selenium nanotubes in the presence of cetyltrimethylammonium bromide via the influence of an ultrasonic and a water bath. At present, selenium nanobelts (quasi-2D) have been reported by a thermal evaporation route at high temperature (600 °C)34 and hydrothermal synthesis.35 Se nanobelts were synthesized with cellulose as both a reducing (24) Stuke, J. In Selenium; Zingaro, R. A., Cooper, W. C., Eds.; Van Nostrand Reinhold: New York, 1974; pp 177-178. (25) (a) Li, H. T.; Rerensburger, P. J. J. Appl. Phys. 1963, 34, 1730. (b) Greenwood, N. N.; Eamshaw, A. Chemistry of the Elements, 2nd ed.; Pergamon Press: Oxford, 1997; Chapter 16. (c) Handbook of Chemistry and Physics, 83rd ed.; CRC Press: Cleveland, OH, 2002; Chapter 12. (26) (a) Gates, B.; Mayers, B.; Cattle, B.; Xia, Y. N. AdV. Funct. Mater. 2002, 12, 219. (b) Jeong, U.; Xia, Y. N. AdV. Mater. 2005, 17, 102. (27) (a) Gao, X.; Gao, T.; Zhang, L. J. Mater. Chem. 2003, 13, 6. (b) Jiang, Z. Y.; Xie, Z. X.; Xie, S. Y.; Zhang, X. H.; Huang, R. B.; Zheng, L. S. Chem. Phys. Lett. 2003, 368, 425. (c) Cheng, B.; Samulski, E. T. Chem. Commun. 2003, 2024. (d) Zhang, H.; Yang, D.; Ji, Y.; Ma, X.; Xu, J.; Que, D. J. Phys. Chem. B 2004, 108, 1179. (e) Tang, Z. Y.; Wang, Y.; Sun, K.; Kotov, N. A. AdV. Mater. 2005, 17, 358. (28) Abdelouas, A.; Goug, W. L.; Lutze, W.; Shelnutt, J. A.; Franco, R.; Moura, I. Chem. Mater. 2000, 12, 1510. (29) Gates, B.; Yin, Y.; Xia, Y. J. Am. Chem. Soc. 2000, 122, 12582. (30) Song, J. M.; Zhu, J. H.; Yu, S. H. J. Phys. Chem. B 2006, 110, 23790. (31) Mayers, B. T.; Liu, K.; Sunderland, D.; Xia, Y. N. Chem. Mater. 2003, 15, 3852. (32) Cao, X. B.; Xie, Y.; Li, L. AdV. Mater. 2003, 15, 1914. (33) Zhang, S. Y.; Liu, Y.; Ma, X.; Chen, H. Y. J. Phys. Chem. B 2006, 110, 9041. (34) Cao, X. B.; Xie, Y.; Zhang, S. Y.; Li, F. Q. AdV. Mater. 2004, 16, 649. (35) (a) Lu, Q. Y.; Gao, F.; Komarneni, S. Chem. Mater. 2006, 18, 159. (b) Xie, Q.; Dai, Z.; Huang, W. W.; Ma, D. K.; Hu, X. K.; Qian, Y. T. Cryst. Growth Des. 2006, 6, 1514.

Figure 2. (a) XRD pattern of the as-prepared nanorafts. (b) Raman scattering spectrum of the nanorafts. The resonance peak at 233.8 cm-1 is a characteristic stretching mode of t-Se.

and a morphology-directing agent in solution under hydrothermal conditions.35a Se nanobelts with diameters of ∼80 nm and lengths

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Figure 3. (a) A typical TEM image of the end of one nanoraft. The inset shows the enlarged area of (a). (b) SAED pattern taken from the middle portion of an individual raft shown in (a). (c, d) HRTEM images of different areas of an individual raft as marked in (a).

up to a few hundred micrometers were prepared by reducing SeO2 with glucose under the same hydrothermal conditions.35b So far, no method has been developed for the synthesis of twodimensional bamboo-raft-like selenium superstructures. To the best of our knowledge, such Se structures have not been reported hitherto and it is a novel morphology for selenium and other elements. Cellulose acetate (CA), first prepared in 1865, is the acetate ester of cellulose. Acetate is derived from cellulose by deconstructing wood pulp into purified fluffy white cellulose. This material is obtained by replacing some of the -OH groups shown in Scheme 1 by -COOCH3 groups. The anhydroglucose unit is the fundamental repeating structure of cellulose and has three hydroxyl groups which can react to form acetate esters. The most common form of cellulose acetate fiber has an acetate group on approximately two of every three hydroxyls. This cellulose diacetate (CDA) is known as a secondary acetate or simply as “acetate”. Cellulose acetate is usually used as a film base in photography and as a component in some adhesives; it is also used as a synthetic fiber. In this paper, we report the high-yield synthesis of novel 2D Se hierarchical superstructures constructed by parallel nanowires, via a facile solvothermal approach, in which precursor SeO2 was reduced by ethylene alcohol in the presence of cellulose acetate. Bamboo-raft-like single-crystalline selenium superstructures were obtained by the solvothermal approach. 2. Experimental Section 2.1. Materials and Synthetic Procedure. The cellulose acetate used in the experiment, which being directly purchased from the Shanghai Chemical Reagent Co. is chemically pure, while other chemicals are all analytical grade, and all reagents are used as received

without further purification. In a typical procedure, 0.075 g of cellulose acetate was added to 20 mL of ethylene glycol, and the mixture was then transferred into a 25 mL Teflon-lined autoclave (with a filling ratio of ca. 80%). The autoclave was closed and kept at 160 °C for 3 h until a homogeneous and clear solution was achieved. After the autoclave cooled, 0.070 g of SeO2 was added to the clear solution under stirring. The autoclave was closed and kept at 160 °C for 36 h again. After the solution was cooled to room temperature naturally, the obtained light red solid products were collected by centrifuging the reaction mixture; they were then washed with acetone and absolute ethanol several times each and dried in a vacuum at 60 °C for 6 h for further characterization. 2.2. Characterization. The phase purity of the as-prepared products was determined by X-ray diffraction (XRD) using a Philips X’Pert PRO SUPER X-ray diffractometer equipped with graphitemonochromatized Cu KR radiation (λ ) 1.54178 Å). To obtain further evidence for the purities and compositions of the as-prepared products, the X-ray photoelectron spectra were used, which were recorded on an ESCALab MKII X-ray photoelectron spectrometer, using Mg KR radiation as the exciting source. The Raman spectra were performed with 488 nm laser excitation with a micro-Raman system, which was modified by coupling an Olympus microscope to a Spex 1740 spectrometer with a CCD detector. UV-vis spectra were recorded on a UV-2501PC/2550 at room temperature (Shimadzu Corp., Japan). The composition of the product was measured by a fully functional environmental scanning electron microscope (XL30ESEM) at an acceleration voltage of 20 kV. Scanning electron microscopy (SEM) and field emission scanning electron microscopy (FESEM) were applied to investigate the size and morphology and were carried out with a Hitachi X-650 scanning electron microanalyzer and a field emission scanning electron microanalyzer (JEOL6700F), respectively. Transmission electron microscopy (TEM) photos were taken with a Hitachi H-800 transmission electron microscope at an acceleration voltage of 200 kV. High-resolution

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transmission electron microscopy (HRTEM) photos and selected area electron diffraction (SAED) patterns were obtained on a JEOL2010 transmission electron microscope.

3. Results and Discussion 3.1. Synthesis of Bamboo-Raft-like Single-Crystal Se Superstructures. High-yield synthesis of bamboo-raft-like single-crystalline selenium superstructures has been realized for the first time via a facile solvothermal approach by reducing SeO2 with ethylene alcohol in the presence of cellulose acetate. Figure 1 shows representative FESEM images of the collected product. The low-magnification image shown in Figure 1a reveals that the as-prepared product consists of bamboo-raft-like superstructures, with a yield higher than 95%. The lengths and widths of the rafts both reach several tens of micrometers; the widths are even longer than the lengths for some of them. The thicknesses of these rafts are about tens of nanometers. Each end of the raft is sawtooth-like. A high-magnification SEM image indicated that the raft has been constructed by a row of bamboolike rods which are parallel with each other and are aligned side by side (Figure 1b-d). A single rod can be seen clearly at the end of a raftlike structure, and even the individual rod can peel off from the raft body as shown in Figure 1b (indicated by arrowheads). The surface of the raft is rough, and even the landscape of the packed rods can be seen clearly even though they were packed rather close (Figure 1d). The powder XRD patterns (Figure 2a) show that the sample is trigonal selenium without other impurities. All the sharp and strong reflection peaks could be indexed as a trigonal phase of selenium, indicating that the as-prepared specimens are highly crystalline and very pure. The lattice parameters calculated from the diffraction pattern are a ) 4.367 Å and c ) 4.950 Å, which agree well with the standard literature values (Joint Committee on Powder Diffraction Standards, JCPDS No. 06-0362). The diffraction peak of (100) is more intensified and usually stronger than the (101) peak reported in literature.27d,34 The morphology and structure of the nanorafts were further characterized by TEM, SAED, and HRTEM. The tip of a single rod comprises many crystal whiskers with a diameter of less than 20 nm (Figure 3a). HRTEM images in Figure 3c,d show lattice spacings of ca. 3.0, 3.8, and 5.0 Å, respectively, corresponding to the lattice spacings of the (100) planes, (101) planes, and (001) planes for trigonal selenium, respectively. The corresponding SAED pattern taken along the [110] direction on the nanoraft indicated that it is single crystalline (Figure 3b). The combination of the results by HRTEM and the SAED pattern confirmed that the axis of the nanoraft is along the [001] direction, the nanoraft is structurally uniform, and no dislocation is observed in the examined area. The energy-dispersion X-ray fluorescence analysis (EDAX) spectrum confirmed that the nanorafts are composed of pure selenium (see Supporting Information Figure S1). In the EDAX spectrum, the detected copper arises from the carbon-coated copper grid upon which the selenium nanorafts were deposited. The Raman scattering spectrum (Figure 2b) provided further evidence that the selenium nanorafts are in the trigonal phase, due to the presence of its characteristic resonance peak of t-Se located at ∼235 cm-1, whereas those of amorphous selenium and monoclinic selenium are centered at ∼264 and ∼256 cm-1, respectively. The peak at 436.4 cm-1 can be attributed to the second-order spectra of trigonal selenium.36 The peak at 141.6 cm-1 is the transverse optical phonon mode (E bond bending (36) Lucovsky, G.; Mooradian, A.; Taylor, W.; Wright, G.; Keezer, B. R. C. Solid State Commum. 1967, 5, 113.

Figure 4. XPS spectra of the Se rafts: (a) XPS survey of the Se 3d region; (b) XPS survey pattern of the bamboo-raft-like Se flakes obtained at 160 °C for 36 h using Al KR radiation as the excitation source. There are no labeling peaks; all are the Auger peaks of the Se element.

mode).37 The results of XPS have been analyzed to give the surface composition of the sample obtained (Figure 4). The strong peak at 54.9 eV corresponds to the Se 3d binding energy of Se(0) as shown in Figure 4a. The electron binding energies of Se 3d for Se(-2), Se(+4), and Se(+6) are ca. 53, 59, and 61 eV, respectively (Figure 4b). Thus, no peaks for selenium oxide or other impurity peaks are observed, indicating a high purity of the product. The FTIR spectrum and TGA show that there is no residual cellulose acetate in the sample (see Supporting Information Figures S2 and S3). UV-vis absorbance spectroscopy was also used to characterize the samples (see Supporting Information Figure S4). The Se rafts prepared with CA show the broadening of the absorption peak from 555 to 615 nm. It might be associated with the variation of the surfaces and assembly behaviors of 2D nanostructures. 3.2. Formation Mechanism of Bamboo-Raft-like Se Superstructures. To understand the formation mechanism as discussed above, a series of experiments were performed. Cellulose acetate cannot be dissolved in ethylene glycol at room temperature; thus, a preheating treatment was performed to make cellulose acetate swell and dissolve in ethylene glycol before SeO2 was added to the reaction vessel. Directly mixing cellulose acetate and SeO2 in ethylene glycol can also result in the formation of similar raftlike structures (Figure 5a,b); however, most of the rafts have a higher aspect ratio than those obtained by a preheating treatment reaction. In addition, the two edges of the rafts along the lateral direction become more uniform and regular, and the (37) (a) Rajalakshmi, M.; Arora, A. K. Nanostruct. Mater. 1999, 11, 399. (b) Li, X. M.; Li, Y.; Li, S. Q.; Zhou, W. W.; Chu, H. B.; Chen, W.; Li, I. L.; Tang, Z. K. Cryst. Growth Des. 2005, 5, 911.

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Figure 5. FESEM images of selenium samples without preheating treatment: (a) at a low magnification; (b) enlarged images; (c) a single raftlike t-Se in (a); (d) end of a flake raftlike t-Se particle.

length of individual nanowires is longer, compared with that observed in the case of the preheating process (Figure 5a,b). A well-defined raft is shown in Figure 5c,d. The results suggested that the initial solubility of cellulose acetate in ethylene glycol has a remarkable influence on the growth process, shape, and sizes of the Se rafts. To obtain a uniform morphology, we put the mixture of ethylene alcohol and cellulose acetate into the oven at 160 °C for 3 h to get a clear solution at first, and then the other reactant, selenium dioxide (SeO2), was introduced into the reaction system. The presence of cellulose acetate is essential for the formation of Se nanorafts. No nanorafts can be obtained, and instead aggregated particles are obtained in the absence of cellulose acetate, which could be due to a too fast reaction speed in ethylene alcohol at 160 °C. At the same time, some familiar surfactants, such as poly(vinylpyrrolidone) (PVP), poly(vinyl alcohol) (PVA), cetyltrimethylammonium bromide (CTAB), sodium dodecyl sulfate (SDS), were tested in this reaction system in place of cellulose acetate; no raft structures were obtained. The reduction reaction cannot happen in pure water; however, ethylene glycol serves as both a reducing agent and a solvent in the present reaction system.26 In addition, the formation of raft superstructures is also dependent on temperature and the amount of cellulose acetate. The reaction cannot take place when the temperature is below 150 °C; on the contrary, a conglomeration of Se is obtained when the temperature is above 180 °C. The most appropriate dosage range of cellulose acetate is 0.05-0.1 g (ca. 0.2-0.4 wt %) in this reaction system. The formation process of the raft superstructures has also been investigated. Figure 6 shows the FESEM images of intermediate products from samples. Centipede-shaped hierarchical structures formed after reaction for 12 h, and then “the feet” of the centipede became close-packed, while its surface remained crude in the

Figure 6. FESEM images of selenium intermediate products in typical synthesis conditions [(a) 12 h; (b) 24 h] demonstrating the formation mechanism for the centipede-shaped structures.

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Figure 7. HRTEM images of the obtained selenium samples prepared in 12 h (a, b) and 24 h (c, d).

next 12 h reaction (Figure 6b). Eventually, the raftlike body formed after continuing reaction for another 12 h. The shape and structures of the intermediate products are similar to the MgO fishbone fractal nanostructures and ZnO hierarchical heteronanostructures in the thermal evaporation/reaction process at 1600 and 900 °C, respectively.38,39 This time-dependent shape evolution process of the rafts suggested that the particles formed at initial stages tend to grow further preferentially along certain crystallographic directions. The centipede-shaped samples and their crystallinity have been examined by HRTEM. The HRTEM images taken on selected areas of the edges and the tips of an individual foot of the centipede-shaped samples indicated lattice spacings of 5.0 and 2.2 Å, corresponding to those for the (001) and (110) planes, respectively (Figure 7a). The “foot” grew preferentially along the [001] direction, which is perpendicular to the [110] and [100] directions (Figure 7b-d). The raft structures almost totally disappeared (Figure 8), while bundlelike selenium particles formed if 0.468 g (8 mmol) of NaCl was added to this reaction system under other identical experimental conditions, implying that the addition of NaCl increases the ionic strength in the reaction media and has a significant influence on the interaction between growing Se nanostructures and cellulose acetate molecules and also on the acting forces among cellulose acetate molecular chains. The above raftlike Se superstructures can be classified into a special example of a “mesocrystal” which was recently proposed (38) Zhu, Y. Q.; Hsu, W. K.; Zhou, W. Z.; Terrones, M.; Kroto, H. W.; Walton, D. R. M. Chem. Phys. Lett. 2001, 347, 337. (39) Lao, J. Y.; Wen, J. G.; Ren, Z. F. Nano Lett. 2002, 2, 1287.

Figure 8. FESEM images of the selenium sample with addition of 0.8 mmol of NaCl to the typical procedure.

as a new concept for the preparation of highly ordered structures with the simultaneous formation and assembly of modular crystals through a nonclassical crystallization process which does not proceed through ion-by-ion attachment, but by a modular nanobuilding-block route.40 The intermediate stage of the raftlike

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become straight due to the rigidity of Se atom anisotropic crystal growth along the c axis. In the vapor-solid mechanism,38,39 the trunk stem forms first, and then the sub-branches of the fishbone grow from its surface blisters. Unlike the vapor-solid mechanism, herein one-dimensional Se rods arrange from their lateral sides to form raftlike structures without the undergoing trunk stem process (see the perfect edges along the direction which is perpendicular to the nanofiber axis). The intermolecular and intramolecular hydrogen bonds in cellulose allow adjacent molecular chains (broken lines in Figure 9) to be formed. The balance and/or their synergistic effects of both intermolecular hydrogen bonds and van der Waals forces together could make the nanorods hold together and connect each other from their lateral sides and thus form bamboo rafts.

4. Conclusion

Figure 9. Top: Schematic representation of the cellulose conformation. Bottom: Schematic illustration of the proposed formation mechanism of bamboo raft selenium superstructures: (I) possible conformation of cellulose acetate in ethylene glycol; (II) selenium atoms deoxidized and selectively adsorbing on the surfaces of the fibers; (III) directed anisotropic growth of Se structures under the control of cellulose acetate; (IV) further crystallization, fusion, oriented attachment, and Ostwald ripening process for the formation of final raftlike superstructures.

superstructures can be clearly observed by comparison with the middle part of the raft and the edge area. The interspaces between adjacent rods can be filled in by a Se source from subsequent reaction, the fusion of the rodlike building blocks, oriented attachment, and the Ostwald ripening process; thus, raftlike superstructures are finally obtained. On the basis of all the above results and analysis, the formation mechanism of the raftlike Se superstructures has been proposed. Cellulose acetate, as a cellulose derivate, has a conformation similar to that of cellulose. The cellulose conformation41 is illustrated in Figure 9 (upper part). Cellulose fibers enwind each other into a bundle before they can totally dissolve. These fibers will stretch with increasing temperature. Selenium atoms reduced from SeO2 will be adsorbed at the surfaces of these fibers with the functional sites. Owing to its intrinsic anisotropic structure, the crystallization of Se proceeds along the c axis to form onedimensional structures, which leads to the formation of nanorods.33,42 The fibers with flexibility and flexual character will (40) (a) Co¨lfen, H.; Antonietti, M. Angew. Chem., Int. Ed. 2005, 44, 5576. (b) Co¨lfen, H. Top. Curr. Chem., in press. (41) Kennedy, J. F.; Phillips, G. O.; Wedlock, D. J.; Williams, P. A. Cellulose and its DeriVatiVes:Chemistry, Biochemistry and Applications; Halsted Press: Chichester, U.K., 1985; Chapter 5. (42) Mayers, B.; Xia, Y. J. Mater. Chem. 2002, 12, 1875.

In summary, the cellulose acetate-directed bamboo-raft-like single-crystalline selenium superstructure, a novel morphology for the selenium element, has been realized for the first time via a facile solvothermal approach by reducing SeO2 with ethylene alcohol in the presence of cellulose acetate with a yield higher than 95%. The lengths and widths of the raft both reach several tens of micrometers, and the thicknesses of these rafts are about tens of nanometers. The formation of a raftlike superstructure with various forms is strongly dependent on the temperature, reaction time, and even preheating treatment. A suitable amount of cellulose acetate is essential for the formation of such raft superstructures. The formation mechanism of the raftlike superstructures has been proposed. The raftlike superstructures made of very crystalline Se nanowires could provide a chance to further investigate the difference in the conductibility along the transverse direction and longitudinal direction of the raftlike Se, and their other application, as mesoscale building blocks, which may be used as thin membranes and light guide materials with an anisotropic property. Acknowledgment. This work is supported by the National Science Foundation of China (NSFC) (Grant Nos. 20325104, 20621061, 20671085, and 50372065), the 973 project (Grant 2005CB623601), special funding from the Centurial Program of the Chinese Academy of Sciences, and the Scientific Research Foundation for the Returned Overseas Chinese Scholars supported by the State Education Ministry, the Specialized Research Fund for the Doctoral Program (SRFDP) of Higher Education State Education Ministry, and the Partner-Group of the Chinese Academy of Sciences-the Max Planck Society. Supporting Information Available: EDX pattern and FTIR and UV-vis spectra. This material is available free of charge via the Internet at http://pubs.acs.org. LA700230D