Eu3+

Apr 9, 2009 - cruller-like, and flower-like, were hydrothermally synthesized by simply adjusting the ratios of surfactant polyethylene glycol-6000 (PE...
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Bisurfactant-Controlled Synthesis of Three-Dimensional YBO3/Eu3+ Architectures with Tunable Wettability Ying-Feng Xu, De-Kun Ma,* Xi-An Chen, Dong-Peng Yang, and Shao-Ming Huang* Nanomaterials and Chemistry Key Laboratory, Wenzhou University, Wenzhou, Zhejiang 325027, People’s Republic of China Received January 18, 2009. Revised Manuscript Received March 2, 2009 Three-dimensional (3D) architectures of YBO3/Eu3+ with different morphologies such as nest-like, rose-like, cruller-like, and flower-like, were hydrothermally synthesized by simply adjusting the ratios of surfactant polyethylene glycol-6000 (PEG-6000) to octadecylamine (ODA). These 3D architectures were all self-assembled by nanoflakes. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field emission scanning electron microscopy (FE-SEM), and photoluminescence (PL) spectra were used to characterize the morphology and structures of the samples. PEG-6000, ODA, and the ODA/PEG ratio played important roles in the formation process of various architectures. Rose-like architecture was chosen as a candidate, and the formation mechanism of the architecture was proposed on the basis of XRD analysis and SEM observation of the products at different reaction periods of time. As-synthesized samples displayed strong emission located at 591, 610, and 615 nm. Water contact angle measurements indicated that the films fabricated by the samples obtained under the different ratios of PEG-6000/ODA could exhibit tunable wettability ranging from superhydrophilicity to superhydrophobicity. This kind of one-pot bisurfactantcontrolled hydrothermal synthesis method reported here provides a new strategy to realize the surfaces of functional materials with tunable wettability.

Introduction In recent years, the surfaces of functional materials with tunable wettability have attracted much attention because of their usefulness in fundamental research and industrial applications.1 For example, the superhydrophilic surface generated by UV irradiation has been successfully used as a transparent coating with antifogging and self-cleaning properties.2 On the other hand, superhydrophobic surfaces have many important practical applications including the prevention of the adhesion of snow to antennas, corrosion protection, the fabrication of microfluidic devices, and the self-cleaning of traffic signals.3-7 There have been some successful methods for tuning the wettability of materials from superhydrophilicity to superhydrophobicity, such as light irradiation,8-11 electrical potential,12 temperature change,13 and *To whom correspondence should be addressed. Telephone: +86-57788373031. Fax: +86-577-88373064. E-mail: [email protected] (D.-K.M.); [email protected] (S.-M.H.). (1) (a) Sun, T.; Feng, L.; Gao, X.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (b) Lahann, J.; Mitragotri, S.; Tran, T.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (c) Lim, H. S.; Han, J. T.; Kwak, D.; Jin, M.; Cho, K. J. Am. Chem. Soc. 2006, 128, 14458. (2) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431. (3) Cheng, Y.; Rodak, D. E. Appl. Phys. Lett. 2005, 86, 144101. (4) Kousik, G.; Pitchumani, S.; Renganathan, N. G. Prog. Org. Coat. 2001, 43, 286. :: (5) Zhai, L.; Berg, M. C.; Cebeci, F. Cu.; Kim, Y.; Milwid, J. M.; Rubner, M. F.; Cohen, R. E. Nano Lett. 2006, 6, 1213. (6) Blossey, R. Nat. Mater. 2003, 2, 301. (7) Ferrari, M.; Ravera, F.; Liggieri, L. Appl. Phys. Lett. 2006, 88, 203125. (8) Ichimura, K.; Oh, S. K.; Nakagawa, M. Science 2000, 288, 1624. (9) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62. (10) Feng, X. J.; Zhai, J.; Jiang, L. Angew. Chem., Int. Ed. 2005, 44, 5115. (11) Lim, H. S.; Kawk, D. H.; Lee, D. Y.; Lee, S. G.; Cho, K. J. Am. Chem. Soc. 2007, 129, 4128. (12) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371. (13) Crevoisier, G. D.; Fabre, P.; Corpart, J. M.; Leibler, L. Science 1999, 285, 1246.

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treatment of selective solvents.14 Among these approaches, a photo-induced surface wettability transition usually occurs in transition-metal oxides, such as ZnO,9 TiO2,10 and V2O5.11 Applying electrical potential, temperature, and solvent treatment to change wetting behavior is only suitable for special organic molecules.12-16 Therefore, it is still desirable to exploit new method to control the wettability of materials easily switching between superhydrophilicity and superhydrophobicity. The studies on biological surfaces have demonstrated that special wettability on functional surfaces can be achieved through the cooperation between the chemical composition and the surface micro- and nanostructures.17-19 On the basis of this theory, a number of strategies have been developed to fabricate an artificial superhydrophobic surface by a two-step process.20-24 The two-step process usually includes the generation of geometric microstructures by various means and then the modification of a low-surface-energy organic substance, such as fluoroalkylsilane, n-dodecanethiol, poly(dimethylsiloxane) vinyl terminated, and oligo(p-phenylenevinylene)s. Herein, we design a facile one-step hydrothermal route to obtain a superhydrophobic surface by (14) Minko, S.; Muller, M.; Motornov, M.; Nitschke, M.; Grundke, K.; Stamm, M. J. Am. Chem. Soc. 2003, 125, 3896. (15) Xu, L. B.; Chen, W.; Mulchandani, A.; Yan, Y. S. Angew. Chem., Int. Ed. 2005, 44, 6009. (16) Sun, T.; Wang, G.; Feng, L.; Liu, B.; Ma, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2004, 43, 357. (17) Feng, L.; Li, S.; Li, Y.; Li, H.; Zhang, L.; Zhai, J.; Song, Y.; Liu, B.; Jiang, L.; Zhu, D. Adv. Mater. 2002, 14, 1857. (18) Gao, X.; Jiang, L. Nature 2004, 432, 36. (19) Lee, W.; Jin, M. K.; Yoo, W. C.; Lee, J. K. Langmuir 2004, 20, 7665. (20) Qu, M. N.; Zhang, B. W.; Song, S. Y.; Chen, Li.; Zhang, J. Y.; Cao, X. P. Adv. Funct. Mater. 2007, 17, 593. (21) Zhang, X.; Shi, F.; Yu, X.; Liu, H.; Fu, Y.; Wang, Z. Q.; Jiang, L.; Li, X. Y. J. Am. Chem. Soc. 2004, 126, 3064. (22) Guo, Z. G.; Zhou, F.; Hao, J. C.; Liu, W. M. J. Am. Chem. Soc. 2005, 127, 15670. (23) Guo, Z. G.; Liu, W. M.; Su, B. L. Appl. Phys. Lett. 2008, 92, 063104. (24) Sampath, S.; Vakayil, K. P.; Robert, P.; Ayyappanpillai, A. Angew. Chem., Int. Ed. 2008, 47, 5750.

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modulating YBO3/Eu3+ geometric microstructures and surface energy at the same time. This novel one-pot approach involves the use of two kinds of different surfactants polyethylene glycol-6000 (PEG-6000) and octadecylamine (ODA) to control the geometric microstructures and surface energy of the samples by adjusting the ratios of PEG-6000/ODA. YBO3/Eu3+, one of the best red phosphors to be widely applied to PDPs, has attracted much attention because of its low toxicity, strong luminescence intensity, high chemical stability, vacuum ultraviolet transparency, and exceptional optical damage threshold.25,26 Various techniques have been developed to synthesize YBO3/Eu3+ phosphors by far, such as solid-state reactions,26a co-precipitation and sol-gel methods,27 microwave heating,28 spray pyrolysis,29 hydrothermal and solvothermal routes,30 and an electrospinning method.31 Controlled synthesis of well-defined crystals with uniform size and morphology is of great importance and interest for their theoretical study and potential practical applications in the future. Although YBO3/Eu3+ nanoparticles,30c nanotubes, and nanowires,31 drum-like microcrystals,32 and donut-like assemblies have been successfully synthesized,30b few reports concerned the controlled synthesis of YBO3/Eu3+ with diversiform morphologies. In this work, we present a bisurfactant-controlled hydrothermal route to synthesize YBO3/ Eu3+ architectures with various morphologies, including nest-like, rose-like, cruller-like, and flower-like architectures. Furthermore, the wettability of films obtained by drop-casting an ethanolic suspension of as-synthesized YBO3/Eu3+ architectures onto Si wafers was also investigated.

Experimental Section Synthesis of YBO3/Eu3+ Architectures. All of the chemicals are of analytical-grade reagents purchased from Shanghai Chemical Corporation and used without further purification. In a typical procedure for the synthesis of YBO3/Eu3+ crullerlike architecture (sample I), 2.7 mmol of Y(NO3)3 3 6H2O was added into 30 mL of aqueous solution containing 0.3 mmol of Eu(NO3)3 3 6H2O. The solution was stirred for 5 min, and then 4 mmol of H3BO3, 2 mmol of PEG-6000, and 1 mmol of ODA were introduced into the above solution, respectively. After that, the pH value of the stock solution was adjusted to 8 by NH3 3 H2O solution under vigorous agitation. Then, the solution was poured into a stainless-steel autoclave with a Teflon linear of 40 mL capability and heated at 220 °C for 24 h. After the autoclave was cooled to room temperature, the products were separated centrifugally and washed with ultrapure water and absolute ethanol for 3 times. Then, the products were dried under vacuum at 60 °C for 4 h. Other YBO3/Eu3+ architectures were synthesized in a manner similar to that for the sample I, (25) (a) Lin, Z. S.; Wang, Z. Z.; Chen, C. T. Chem. Phys. Lett. 2004, 399, 125. (b) Lin, Z. S.; Lin, J.; Wang, Z. Z.; Wu, Y. C.; Ye, N.; Chen, C. T.; Li, R. K. J. Phys.: Condens. Matter 2001, 13, 369. (26) (a) Ren, M.; Lin, J. H.; Dong, Y.; Yang, L. Q.; Su, M. Z.; You, L. P. Chem. Mater. 1999, 11, 1576. (b) Wu, X. Y.; Hong, G. Y.; Zeng, X. Q.; You, H. P.; Kim, C. H.; Pyun, C. H.; Bal, H. S.; Yu, B. Y.; Kwon, I. E.; Park, C. H. Chem. J. Chin. Univ. 2000, 21, 1658. (27) Boyer, D.; Bertrand, C. G.; Mahiou, R.; Caperaa, C.; Cousseins, J. C. J. Mater. Chem. 1999, 9, 211. (28) Li, Y. Y.; Peng, M. L.; Feng, S. H. Chin. Chem. Lett. 1996, 7, 387. (29) Kim, K. N.; Jung, H. K.; Park, H. D.; Kim, D. J. Mater. Res. 2002, 17, 907. (30) (a) Jiang, X. C.; Yan, C. H.; Sun, L. D.; Wei, Z. G.; Liao, C. S. J. Solid State Chem. 2003, 175, 245. (b) Jiang, X. C.; Sun, L. D.; Yan, C. H. J. Phys. Chem. B 2004, 108, 3387. (c) Li, Z. H.; Zeng, J. H.; Li, Y. D. Small 2007, 3, 3438. (d) Zhang, J.; Lin, J. J. Cryst. Growth 2004, 271, 207. (e) Yang, J.; Zhang, C. M.; Li, C. X.; Yu, Y. N.; Lin, J. Inorg. Chem. 2008, 47, 7262. (f) Yang, J.; Li, C. X.; Zhang, X. M.; Quan, Z. W.; Zhang, C. M.; Li, H. Y.; Lin, J. Chem. Eur. J. Chem. 2008, 14, 4336. (31) Song, H. W.; Yu, H. Q.; Pan, G. H.; Bai, X.; Dong, B.; Zhang, X. T.; Hark, S. K. Chem. Mater. 2008, 20, 4762. (32) Jiang, X. C.; Sun, L. D.; Feng, W.; Yan, C. H. Cryst. Growth Des. 2004, 4, 517.

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Xu et al. except that the molar ratios of ODA/PEG-6000 were 1:1 (sample II), 1:4 (sample III), 1:8 (sample IV), and 1:12 (sample V), respectively. Additionally, control experiments were performed in the absence of ODA (sample VI), PEG-6000 (sample VII), and PEG-6000 and ODA (sample VIII), respectively. Different hydrothermal treatment time (0.5, 1, 2, 3, 4, 8, and 16 h) was selected to investigate the evolutional process of YBO3/Eu3+ rose-like architecture. The detailed experimental conditions for the synthesis of the samples are summarized in Table S1 (see the Supporting Information). Fabrication of YBO3/Eu3+ Films. YBO3/Eu3+ films were obtained by drop-casting an ethanolic suspension of as-synthesized YBO3/Eu3+ samples onto Si wafers. Characterizations. Powder X-ray diffraction (XRD) was carried out with a Bruker D8 Advance X-ray diffractometer using Cu KR radiation (λ = 0.154 18 nm) at a scanning rate of 8°/ min in the 2θ range from 10° to 70°. Field emission scanning electron microscopy (FE-SEM) images were taken on a Nova NanoSEM 200 scanning electron microscope (FEI, Inc.). IR spectra were obtained on a Bruker EQUINOX 55 Fourier transform infrared (FTIR) spectrometer ranged from 4000 to 500 cm-1. The samples and KBr crystal were ground together, and then, the mixture was pressed into a flake for IR spectroscopy. Photoluminescence spectra were recorded on a Fluoromax-4 spectrofluorometer (HORIBA Jobin Yvon, Inc.) equipped with a 150 W xenon lamp as the excitation source. The wettability of the as-prepared YBO3/Eu3+ films was analyzed by measurement of the water contact::angles (CAs) using an Easydrop contact angle system (KRUSSGmbH, Germany). All of the measurements were performed at room temperature.

Results and Discussion The crystallinity and phase purity of the products were examined by powder XRD. Figure 1 shows typical XRD patterns of the as-synthesized products in the absence of surfactants (Figure 1a) and at the assistance of PEG-6000 with ODA (Figure 1b), respectively. All of reflections can be indexed to pure hexagonal phase, which is in agreement with the reported data (JCPDS 160277). As can be seen from the XRD patterns, the products have high crystallinity. This will be beneficial to luminescence of the phosphor because high crystallinity generally means less traps.30c Through the comparison of parts a and b of Figure 1, it can draw a conclusion that the addition of surfactants will not affect the crystallinity of the products in the present reaction system. Figure 2 shows the FTIR spectra of YBO3/Eu3+ samples I and VIII. The peaks between 800 and 1200 cm-1 are typical IR 26a According to absorption of the polyborate group B3O99 . 33 a previous report, IR absorption peaks in the region of 800950 cm-1 are assigned to ring-stretching vibration modes and the peaks in the region of 950-1200 cm-1 are assigned to terminalstretching vibration modes. IR absorption peaks at 2918, 2850, and 1635 cm-1 in Figure 2a result from characteristic vibration of ODA. A sharp peak at 1380 cm-1 is ascribed to absorption vibration of NO3 impure ions from reaction solution. No characteristic absorption peaks of PEG-6000 were observed, indicating easy removal of PEG-6000 molecules by washing with ultra-pure water and absolute ethanol. It was found that part absorption peaks of sample I were red-shifted in comparison to those of sample VIII. It is accepted that force constants become lower and the IR absorption wavelength will be longer when the bond distances become longer. Therefore, a red shift of IR absorption indicates that surfactant molecules act on the surfaces of assynthesized sample I and possibly lengthen B-O bond distances. (33) Wei, Z. G.; Sun, L. D.; Liao, C. S.; Yin, J. L.; Jiang, X. C.; Yan, C. H. J. Phys. Chem. B 2002, 106, 10610.

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Figure 1. XRD patterns of the as-synthesized products (a) in the absence of surfactants (sample VIII) and (b) in the presence of PEG-6000 and ODA (sample I). Figure 3. Panoramic FE-SEM images of the samples synthesized (a) without any surfactants (sample VIII), (b) in the presence of 1 mmol ODA (sample VII), (c) in the presence of 2 mmol of PEG6000 (sample VI), and (d) in the presence of 1 mmol of ODA and 2 mmol of PEG-6000 (sample I).

Figure 2. FTIR spectra of the as-synthesized products (a) in the presence of PEG-6000 and ODA (sample I) and (b) in the absence of surfactants (sample VIII).

Figure 3 describes the typical panoramic FE-SEM images of as-synthesized samples. As can be seen from Figure 3a, the products directly obtained without use of any surfactants consist of a large quantity of microspheres (sample VIII). Careful observation on an individual microsphere shows that the microsphere was assembled from interlaced nanoflakes. When 1 mmol of ODA was introduced into the hydrothermal system, the morphology of the products became nest-like architecture (sample VII in Figure 3b). Rose-like architectures were obtained with 2 mmol of PEG-6000 in place of 1 mmol of ODA (sample VI in Figure 3c). To add in 2 mmol of PEG-6000 and 1 mmol of ODA at the same time, the morphology of the products took on crullerlike architectures (sample I in Figure 3d). Nest-like, rose-like, and cruller-like architectures are also fabricated by nanoflakes. However, as far as these architectures are concerned, assembly modes and extent of nanoflakes are distinctly different from each other. In the process of solution-phase synthesis, ODA can be used as soft template,34 an activation reagent and ligand,35 and stabilizer (34) Choudhury, S.; Bagkar, N.; Dey, G. K.; Subramanian, H.; Yakhmi, J. V. Langmuir 2002, 18, 7409. (35) Lee, S. H.; Kim, Y. J.; Park, J. Chem. Mater. 2007, 19, 4670.

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to control the morphologies of the products.36 In our reaction system, ODA plays multiform roles. First, it is an effective stabilizer that limits the crystal growth at high temperature. The N atom of the ODA molecule has strong coordination ability, which can easily stick to the surface of many inorganic crystal nucleuses, similar to oxygen and sulfur ligands. Second, ODA molecules adsorb on the surfaces of primary particles and act as a structure-directing reagent. It can direct primary particles to assemble into definite morphologies through molecular interaction. By comparison of parts a and b of Figure 3, it was found that the size of the products became small (The size of samples VIII and I are in the range of 10-20 and 5-8 μm, respectively) and appeared as new nest-like architectures in the presence of ODA. This result confirmed that ODA molecules behaved as a stabilizer and structure-directing reagent in the present reaction system. However, when the molar quantum of ODA was increased to 2 mmol, the morphology of the resultant products remains nest-like architecture (see Figure S1 in the Supporting Information). Therefore, we decided to introduce another organic addition PEG-6000 into the reaction system to fabricate the YBO3/Eu3+ crystal with various morphologies. PEG with different molecular weights has been successfully used to prepare various morphologies, such as nanorods and nanowires,37 nanocubes and nanoboxes,38 three-dimensional architectures,39 etc. PEG has hydrophilic -O- and hydrophobic CH2-CH2 on the long chains. PEG-6000 could play a similar role to ODA. It could bond on the surface of firstborn crystal nucleuses because of the coordination action of oxygen atoms, limit the crystal growth, and act as structure-directing reagent as well. As shown in Figure 3c, the size of the products became small (in the range of 5-8 μm) and appeared as new rose-like architectures in (36) Xu, J.; Ge, J. P.; Li, Y. D. J. Phys. Chem. B 2006, 110, 2497. (37) (a) Wang, W. Z.; Zhan, Y. J.; Wang, G. H. Chem. Commun. 2001, 7, 27. (b) Xiong, Y. J.; Xie, Y.; Wu, C. Z.; Yang, J.; Li, Z. Q.; Xu, F. Adv. Mater. 2003, 15, 405. (38) (a) Gou, L. F.; Murphy, C. J. J. Mater. Chem. 2004, 14, 735. (b) Wang, W. Z.; Poudel, B.; Wang, D. Z.; Ren, Z. F. Adv. Mater. 2005, 17, 2110. (39) Xu, F.; Xie, Y.; Zhang, X.; Wu, C. Z.; Wang, X.; Hong, J.; Tian, X. B. New J. Chem. 2003, 27, 1331.

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Figure 4. Panoramic FE-SEM images of the samples synthesized at the different ratios of PEG-6000/ODA: (a) 1:1, (b) 4:1, (c) 8:1, and (d) 12:1.

Figure 6. FE-SEM images of the products at the different reaction time: (a) 0.5, (b) 2, (c) 3, (d) 4, (e) 8, and (f) 16 h.

Figure 5. XRD patterns of the products synthesized at (a) 0.5, (b) 1, (c) 2, and (d) 4 h.

the presence of PEG-6000. This result confirms that PEG-6000 molecules can act as the same function as ODA. Distinctively, the products exhibited flower-like morphology when the quantity of PEG-6000 was increased to 4 mmol (see Figure S2 in the Supporting Information). Therefore, we design to set the molar quantity of ODA as 1 mmol and simultaneously vary that of PEG6000 to adjust the morphologies of the products. As can be seen from Figure 4, as-synthesized products take on a different appearance in the presence of different ratios of PEG-6000/ODA. The morphologies of the products were clearly changed after ODA and PEG-6000 were joined together, compared to only the use of ODA or PEG-6000 (parts b and c of Figure 3). Therefore, it can be deduced that morphological diversification can be attributed to the cooperative action of ODA and PEG-6000. To understand the formation mechanism of YBO3/Eu3+ architectures, rose-like architecture was chosen as a candidate for detailed study. XRD analysis and SEM observation at the different reaction periods of time were carried out. Figure 5 shows 7106 DOI: 10.1021/la9002109

the XRD pattern of the samples hydrothermally synthesized at 0.5, 1, 2, and 4 h, respectively. As can be seen from Figure 5a, the products consist of a large quantity of amorphous precursors and a spot of crystal nucleuses at the initial 0.5 h. When reaction time was prolonged to 1 h, large numbers of YBO3/Eu3+ crystal nucleuses appear in solution. As the reaction time extended to a longer time, the crystallinity of the products is further improved, judging from the relative intensity of corresponding diffraction peaks in parts b-d of Figure 5. To intuitively understand the morphology evolution process of rose-like architecture, SEM observation on the samples at the different periods of time was performed. Parts a-f of Figure 6 describe SEM images of the products hydrothermally synthesized at 0.5, 2, 3, 4, 8, and 16 h, respectively. As can be seen from Figure 6a, the products are irregular small particles within initial 0.5 h. With the reaction time extended to 2 h, these particles were transformed into nanoflakes. Immediately, these firstborn nanoplates began to assemble into multilayered hierarchical structures (see Figure 6c). After 4 h of growth, these multilayered structures were twisted into loose conglomeration. A further increase in the reaction time led to the formation of 3D rose-like compact microspheres (as the white arrowheads show in Figure 6e). Upon aging for a longer period up to 16 h, as-obtained products basically grew into rose-like architectures. According to the above-mentioned analysis, the formation mechanism of rose-like architecture was deduced as follows. First, Y3+ and H3BO3 form amorphous precursors under the alkaline condition. Then, these amorphous precursors will be converted into crystal nucleuses under hydrothermal conditions quickly. Because of the intrinsical hexagonal structure and selective adsorption of surfactant molecules on crystal faces of crystal nucleuses, YBO3 crystal nucleuses will Langmuir 2009, 25(12), 7103–7108

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Figure 7. Schematic illustration for the formation process of rose-like architectures.

anisotropically grow into nanoflakes. This kind of phenomena has been observed in the process of hydrothermal growth of YBO3/Eu3+ crystals.30d Through molecular interaction between surfactant molecules adsorbed on the nanoflakes, these primary building blocks will assemble into multilayered hierarchical structures next. Because sphericity has a minimal surface energy, driven by the minimization of the total energy of the system, multilayered hierarchical structures will transform into spherical conglomeration shortly. After further hydrothermal treatment, these loose conglomerations will gradually form more compact rose-like microspheres through adjusting spatial orientation of surfactant molecules adsorbed on them. The final products become uniform rose-like microspheres through a long hydrothermal growth process. A simple schematic illustration for the formation of the process was given in Figure 7. The formation mechanisms of other YBO3/Eu3+ architectures are similar to that of the rose-like architecture. The morphological difference lies in the use of different ratios of PEG-6000/ODA that lead to different assembly modes and extent of primary nanoflakes. The photoluminescence (PL) emission spectra of the samples obtained in the absence/presence of surfactants are shown in Figure 8. All of spectra consist of sharp peaks located at the same wavelengths: 580 (5D0 f 7F0), 591 (5D0 f 7F1), 610 and 625 (5D0 f 7F2), and 650 and 657 nm (5D0 f 7F3) of Eu3+. As can be seen, for sample VIII, the ratio of the red emission at 610 nm to the orange one at 591 nm (R/O value) was much higher than those of sample I. The red emission at 610 nm from the 5D0 f 7F2 transition is a typical electric dipole transition, while the orange emission at 591 nm from the 5D0 f 7F1 transition is a typical magnetic dipole transition. The relative intensity of them strongly depends upon the local symmetry of Eu3+ ions, and a higher symmetry of crystal field around Eu3+ ions will result in a lower R/O value.40 From PL spectra, it can be concluded that the addition of surfactants will improve the degree of order of YBO3/ Eu3+ crystal near the surface and thus influence its chromaticity. Figure 9 show optical images of a water droplet on the films prepared by different YBO3/Eu3+ architectures. As can be seen from Figure 9a, a drop of water spreads extensively on the surface of the film prepared by sample VIII with a CA of less than 5°, indicating that the surfaces of the sample are superhydrophilicity. The reason is due to the capillary effect. When the sample was prepared only in the presence of PEG-6000 (sample VI), the asobtained film also shows superhydrophilicity (Figure 9b). (40) Judd, B. R. Phys. Rev. 1962, 127, 750.

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Figure 8. PL spectra of (a) the sample I synthesized in the presence of PEG-6000 and ODA and (b) the sample VIII synthesized without use of any surfactants.

However, when sample VII was prepared only in the presence of ODA, the as-obtained film has a 126° CA (Figure 9c). The obvious difference of CAs can be chiefly ascribed to different surface energy between PEG-6000 and ODA. PEG-6000 is amphiphilic, and ODA is hydrophobic. Therefore, the sample prepared by ODA exhibits high CA. In general, the wettability of solid surfaces is governed by both the chemical composition and the geometrical microstructure.16-19 When the different molar ratios of ODA/ PEG-6000 were used, the surfaces of corresponding films would possess different microstructures and chemical composition and thus display different wettability. As can be seen in parts d-h of Figure 9, the CAs of samples II, I, III, IV, and V are 140°, 160°, 150°, 145°, and 125°, respectively. For a liquid droplet on a flat film, the wettability is determined by the surface free energy of a solid substrate, which is commonly given by the Young’s equation (eq 1) cos θ ¼ ðγsv  γsl Þ=γlv

ð1Þ

where θ is the contact angle in the Young’s mode and γlv, γsv, and γsl are the different surface tensions (liquid/vapor, solid/vapor, and solid/liquid) involved in the system. Because the molar quantities of ODA are the same and PEG-6000 molecules are easily washed off in present system, the surface energy should have a lesser influence on CA. In actuality, CA is usually modified for complex surfaces with hierarchical architectures associated with large DOI: 10.1021/la9002109

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Figure 9. Optical images of a water droplet on the films prepared by (a) sample VIII, (b) sample VI, (c) sample VII, (d) sample II, (e) sample I, (f) sample III, (g) sample IV, and (h) sample V.

surface roughness r.17,41 Here, r is defined as the ratio of the actual over the apparent surface area. According to the Wenzel model,42 the apparent CA θr for a rough surface is given by (eq 2) cos θr ¼ r cos θ

ð2Þ

From eq 2, it can be found that, if the CA of a liquid on a smooth surface is large than 90°, the apparent angle on a rough surface will be larger. When θ > 90°, the larger the roughness r, the larger the CA. The transition from Figure 4a (sample II) to Figure 4d (sample V) is accompanied by a change of the surface roughness r (sample III > sample IV > sample II > sample V), which is mainly attributed to the difference of CA (sample III > sample IV > sample II > sample V). Sample I has the largest CA (160°), displaying superhydrophobicity, which was probably attributed to its largest roughness r. The detailed mechanisms need to be clarified.

Conclusions In conclusion, three-dimensional YBO3/Eu3+ architectures with various morphologies have been successfully synthesized (41) Zhu, Y.; Zhang, J. C.; Zheng, Y. M.; Huang, Z. B.; Feng, L.; Jiang, L. Adv. Funct. Mater. 2006, 16, 568. (42) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988.

7108 DOI: 10.1021/la9002109

by a bisurfactant-controlled hydrothermal route. PEG-6000 and ODA could behave as an effective stabilizer and structure-directing reagent to control the morphology of the products, respectively. PEG-6000 and ODA adsorbed on the surfaces of primary nanoflakes could cooperate to direct their self-assembly and adjust morphologies of the resultant crystals through molecular interaction when they were used at the same time. The addition of surfactants would not affect the crystallinity of the products in the present hydrothermal system. However, the R/O values of the samples would be lowered, which suggested that the addition of surfactants would improve the degree of order of YBO3/Eu3+ crystal near the surface. On the basis of XRD analysis and SEM observation of the products at the different reaction periods of time, the formation of rose-like architecture went through nucleation, anisotropic growth, assembly, further assembly, and a further growth process. CA measurement indicated that cruller-like architectures had the largest CA (160°), which was probably attributed to the largest roughness r. The tunable wettability of the three-dimensional YBO3/Eu3+ architecture surface together with strong luminescence make them promising potential applications in many fields. Acknowledgment. The authors appreciate partial financial support from the National Natural Science Foundation of China (Grant 50772076) and the Bureau of Science and Technology of Wenzhou, China (G20070092). Supporting Information Available: Experimental conditions for synthesis of the samples (Table S1), FE-SEM image of the sample synthesized with the assistance of 2 mmol of ODA (Figure S1), and FE-SEM image of the sample synthesized with the assistance of 4 mmol of PEG-6000 (Figure S2). This material is available free of charge via the Internet at http://pubs.acs.org.

Langmuir 2009, 25(12), 7103–7108