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Superhydrophobic Thermoplastic Polyurethane Films with Transparent/Fluorescent Performance Shengyang Yang, Lifang Wang, Cai-Feng Wang, Li Chen, and Su Chen* State Key Laboratory of Material-Oriented Chemical Engineering and College of Chemistry and Chemical Engineering, Nanjing University of Technology, Nanjing 210009, P. R. China Received September 1, 2010. Revised Manuscript Received October 6, 2010 In this paper, we report a simple and versatile route for the fabrication of superhydrophobic thermoplastic polyurethane (TPU) films. The approach is based on octadecanamide (ODAA)-directed assembly of nanosilica/TPU/ ODAA hybrid with a well-defined sheetlike microstructure. The superhydrophobic hybrid film shows a transparent property, and its water contact angle reaches as high as 163.5° without any further low surface energy treatment. In addition, the superhydrophobic TPU hybrid film with fluorescent properties is achieved by smartly introducing CdTe quantum dots, which will extend potential application of the film to optoelectronic areas. The resulting fluorescent surface produced in this system is stable and has a water contact angle of 172.3°. This assembly method to control surface structures represents an intriguing and valuable route to tune the surface properties of organic-inorganic hybrid films.
Introduction Wettability of solid surfaces is the focus of much attention not only because it plays a critical role in interface chemistry and understanding the surface microstructures, but also because it is an important character that can be assimilated into numerous fundamental and industrial applications.1 It is well-known that surfaces with a water contact angle (CA) greater than 150° are called superhydrophobic surfaces, which can be widely applied in self-cleaning surfaces, microfluidic devices, biological fields, and so forth.2 In general, superhydrophobic surfaces can be achieved by tailoring the chemical composition and hierarchical micro/ nanostructures of solid surfaces.3 Up to now, a variety of elegant routes for the fabrication of superhydrophobic surfaces have been *To whom correspondence should be addressed. E-mail: chensu@ njut.edu.cn. (1) (a) Wenzel, R. N. Ind. Eng. Chem. 1936, 28, 988. (b) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546. (c) Barthlott, W.; Neinhuis, C. Planta 1997, 202, 1. (d) Neinhuis, C.; Barthlott, W. Ann. Bot. 1997, 79, 667. (e) Shibuichi, S.; Yamamoto, T.; Onda, T.; Tsujii, K. J. Phys. Chem. 1996, 100, 19512. (f) Onda, T.; Shibuichi, S.; Satoh, N.; Tsujii, K. Langmuir 1996, 12, 2125. (g) Lafuma, A.; Quere, D. Nat. Mater. 2003, 2, 457. (h) 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. (2) (a) Blossey, R. Nat. Mater. 2003, 2, 301. (b) Gao, X.; Jiang, L. Nature 2004, 432, 36. (c) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644. (d) Ma, M. L.; Hill, R. M. Curr. Opin. Colloid Interface Sci. 2006, 11, 193. (e) Wang, S.; Feng, L.; Jiang, L. Adv. Mater. 2006, 18, 767. (f) Chen, S.; Hu, C.; Chen, L.; Xu, N. Chem. Commun. 2007, 1919. (3) (a) Quere, D. Rep. Prog. Phys. 2005, 68, 2495. (b) Li, X.; Reinhoudt, D.; Crego-Calama, M. Chem. Soc. Rev. 2007, 36, 1350. (c) Roach, P.; Shirtcliffe, N. J.; Newton, M. I. Soft Matter 2008, 4, 224. (4) (a) Wu, Y.; Sugimura, H.; Inoue, Y.; Takai, O. Chem. Vap. Deposition 2002, 8, 47. (b) Liu, H.; Feng, L.; Zhai, J.; Jiang, L.; Zhu, D. B. Langmuir 2004, 20, 5659. (c) Zhu, L. B.; Xiu, Y. H.; Xu, J. W.; Tamirisa, P. A.; Hess, D. W.; Wong, C. P. Langmuir 2005, 21, 11208. (5) (a) Li, J.; Fu, J.; Cong, Y.; Wu, Y.; Xue, L.; Han, Y. Appl. Surf. Sci. 2006, 252, 2229. (b) Li, Y.; Jian, W.; Song, Y.; Xia, X. Chem. Mater. 2007, 19, 5758. (c) Yuan, Z.; Chen, H.; Tang, J.; Gong, H.; Liu, Y.; Wang, Z.; Shi, P.; Zhang, J.; Chen, X. J. Phys. D: Appl. Phys. 2007, 40, 3485. (d) Li, Y.; Cai, W.; Duan, G.; Cao, B.; Sun, F.; Lu, F. J. Colloid Interface Sci. 2005, 287, 634. (6) (a) Shiu, J.; Kuo, C.; Chen, P.; Mou, C. Chem. Mater. 2004, 16, 561. (b) Lai, Y.; Lin, C.; Wang, H.; Huang, H.; Zhuang, H.; Sun, L. Electrochem. Commun. 2008, 10, 387. (c) Yao, T.; Wang, C.; Lin, Q.; Li, X.; Chen, X.; Wu, J.; Zhang, J.; Yu, K.; Yang, B. Nanotechnology 2009, 20, 065304. (d) Pozzato, A.; Dal Zilio, S.; Fois, G.; Vendramin, D.; Mistura, G.; Belotti, M.; Chen, Y.; Natali, M. Microelectron. Eng. 2006, 83, 884. (e) Kim, T.; Baek, C.; Suh, K.; Seo, S.; Lee, H. Small 2008, 4, 182.
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developed, including chemical/physical vapor deposition (CVD/ PVD),4 template-based techniques,5 lithography,6 sol-gel processing,7 electrospinning,8 various assembly means, and others.9 Moreover, to construct hierarchical structure and tailor surface topography, a great many materials have been applied based on advanced micro- and nanofabrication techniques, ranging from inorganic nanoparticles to bulk polymeric materials.10 However, most of these cases are limited to lab investigation, since the involved materials or end product films are costly, unstable, and fragile.11 Recently, growing interest has been paid to assembly of organic-inorganic nanocomposites for the preparation of superhydrophobic films with tailored structure, promising strength and desired steadiness.12 Jiang et al. reported the fabrication of a (7) (a) Tadanaga, K.; Kitamuro, K.; Matsuda, A.; Minami, T. J. Sol-Gel Sci. Technol. 2003, 26, 705. (b) Shirtcliffe, N. J.; McHale, G.; Newton, M. I.; Perry, C. C. Langmuir 2003, 19, 5626. (c) Yang, S.; Chen, S.; Tian, Y.; Feng, C.; Chen, L. Chem. Mater. 2008, 20, 1233. (d) Manca, M.; Cannavale, A.; De Marco, L.; Arico, A. S.; Cingolani, R.; Gigli, G. Langmuir 2009, 25, 6357. (8) (a) Acatay, K.; Simsek, E.; Ow-Yang, C.; Menceloglu, Y. Z. Angew. Chem., Int. Ed. 2004, 43, 5210. (b) Ma, M. L.; Hill, R. M.; Lowery, J. L.; Fridrikh, S. V.; Rutledge, G. C. Langmuir 2005, 21, 5549. (c) Ma, M. L.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742. (d) Tang, H.; Wang, H.; He, J. J. Phys. Chem. C 2009, 113, 14220. (e) Wu, W.; Zhu, Q.; Qing, F.; Han, C. C. Langmuir 2009, 25, 17. (f) Lu, X.; Zhou, J.; Zhao, Y.; Qiu, Y.; Li, J. Chem. Mater. 2008, 20, 3420. (9) (a) Genzer, J.; Efimenko, K. Science 2000, 290, 2130. (b) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (c) Lee, E. J.; Kim, J. J.; Cho, S. O. Langmuir 2010, 26, 3024. (d) Peng, J.; Yu, P.; Zeng, S.; Liu, X.; Chen, J.; Xu, W. J. Phys. Chem. C 2010, 114, 5926. (e) Guo, Z. G.; Zhou, F.; Hao, J. C.; Liu, W. M. J. Am. Chem. Soc. 2005, 127, 15670. (f) Zhao, N.; Shi, F.; Wang, Z.; Zhang, X. Langmuir 2005, 21, 4713. (g) Niemietz, A.; Wandelt, K.; Barthlott, W.; Hoch, K. Prog. Org. Coat. 2009, 66, 221. (10) (a) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62. (b) Huang, L.; Lau, S. P.; Yang, H. Y.; Leong, E. S. P.; Yu, S. F. J. Phys. Chem. B 2005, 109, 7746. (c) Piech, M.; Sounart, T. L.; Liu, J. J. Phys. Chem. C 2008, 112, 20398. (d) Daoud, W. A.; Xin, J. H.; Tao, X. M. J. Am. Ceram. Soc. 2004, 87, 1782. (e) Erbil, H. Y.; Demirel, A. L.; Avci, Y.; Mert, O. Science 2003, 299, 1377. (11) Zhang, X.; Shi, F.; Niu, J.; Jiang, Y.; Wang, Z. J. Mater. Chem. 2008, 18, 621. (12) (a) Nishimoto, S.; Sekine, H.; Zhang, X.; Liu, Z.; Nakata, K.; Murakami, T.; Koide, Y.; Fujishima, A. Langmuir 2009, 25, 7226. (b) Srinivasan, S.; Praveen, V. K.; Philip, R.; Ajayaghosh, A. Angew. Chem., Int. Ed. 2008, 47, 5750. (c) Li, X.; Ding, B.; Lin, J.; Yu, J.; Sun, G. J. Phys. Chem. C 2009, 113, 20452. (d) Song, X.; Zhai, J.; Wang, Y.; Jiang, L. J. Phys. Chem. B 2005, 109, 4048. (e) Jiang, Y.; Wang, Z.; Yu, X.; Shi, F.; Xu, H.; Zhang, X. Langmuir 2005, 21, 1986.
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superhydrophobic surface via combination of self-assembly of alkylsilane and its fluorinated analogue on a laser-etching roughening silicon surface.12d An electrostatic self-assembly route was also employed to prepare superhydrophobic organic-inorganic hybrid films by Cho’s group.13 Very recently, great effort has been devoted to fabricate superhydrophobic films with transparent, responsive, and conductive properties.14 In addition, there are sparse examples on the fabrication of the superhydrophobic films with special optical properties in these cases.15 Char and co-workers prepared smart superhydrophobic films exhibiting multicolor, utilizing hydrophobic fluorophores, such as nanocrystals (NCs) and organic dye, to incorporate into the hydrophobic polystyrene (PS) cores of charged block copolymer micelles.16 Although little progress has been achieved in these systems, there is still need to explore an alternative and reliable strategy to produce superhydrophobic films with sufficient stability, along with the ability to easily scale up in practice. In our previous work, we introduced a convenient and inexpensive approach for the fabrication of durable superhydrophobic organic-inorganic (PS/SiO2) films via a sol-gel process.7c Herein, we report a simple route for the fabrication of superhydrophobic thermoplastic polyurethane (TPU) films exhibiting transparent or fluorescent properties based on an octadecanamide (ODAA)-directed assembly process. TPU is one of the most versatile engineering thermoplastics with hard and soft segments, exhibiting the best performance of rubber and plastic.17 The cadmium telluride (CdTe) nanocrystal is one of the important promising semiconductor materials for applications in optoelectronics.18 As a consequence, if TPU films with superhydrophobicity and fluorescence were further developed, it would confer them more widely practical applications. In particular, these triggers are promisingly transmitted to photovoltaic devices because of their considerable efficiency, low-cost and because they are maintenance-free. Moreover, in this case, the formation mechanism, surface morphology, and optical properties of hybrid superhydrophobic films are deeply discussed. Significantly, the assembly of inorganic semiconductor nanoparticles into an engineering thermoplastic polymer matrix, involving a simple process, inexpensive materials, stability, multifunction, and easy scaling up, is a powerful alternative for the achievement of functional hybrid surfaces.
Experimental Section Materials. Thermoplastic polyurethane (TPU 95A) was purchased from Bayer and used without any treatment. Octadecanamide (ODAA) and γ-glycidoxypropyltrimethoxysilane (GPS) were from Hebao Shanghai Chemical Co., Ltd. and Jiangsu Chenguang Coincident Dose Co., Ltd., respectively. Nanosilicas (A150, round-shaped, average particle size 14 nm, specific surface area 150 m2/g) were supplied by Shenyang Chemical Co., Ltd. (13) Han, J. T.; Zheng, Y.; Cho, J. H.; Xu, X.; Cho, K. J. J. Phys. Chem. B 2005, 109, 20773. (14) (a) Li, Y.; Liu, F.; Sun, J. Chem. Commun. 2009, 2730. (b) Yabu, H.; Shimomura, M. Chem. Mater. 2005, 17, 5231. (c) Xia, F.; Zhu, Y.; Feng, L.; Jiang, L. Soft Matter 2009, 5, 275. (d) Zhu, W.; Feng, X.; Feng, L.; Jiang, L. Chem. Commun. 2006, 2753. (e) Han, J. T.; Kim, S. Y.; Woo, J. S.; Lee, G. W. Adv. Mater. 2008, 20, 3724. (f) Luo, C.; Zuo, X.; Wang, L.; Wang, E.; Song, S.; Wang, J.; Wang, J.; Fan, C.; Cao, Y. Nano Lett. 2008, 8, 4454. (15) (a) Li, Y.; Li, C.; Cho, S. O.; Duan, G.; Cai, W. Langmuir 2007, 23, 9802. (b) Shen, P.; Uesawa, N.; Inasawa, S.; Yamaguchi, Y. Langmuir 2010, 26, 13522. (c) Gu, Z.; Uetsuka, H.; Takahashi, K.; Nakajima, R.; Onishi, H.; Fujishima, A.; Sato, O. Angew. Chem., Int. Ed. 2003, 42, 894. (16) Hong, J.; Bae, W. K.; Lee, H.; Oh, S.; Char, K.; Caruso, F.; Cho, J. Adv. Mater. 2007, 19, 4364. (17) Liff, S. M.; Kumar, N.; McKinley, G. H. Nat. Mater. 2007, 6, 76. (18) Enzenroth, R. A.; Barth, K. L.; Sampath, W. S.; Manivannan, V.; Kirkpatrick, A. T.; Noronha, P. J. Sol. Energy Eng. 2009, 131, 021012.
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Cadmium chloride (CdCl2), tellurium powder, and NaBH4 were purchased from Aldrich and used as received. Toluene, N,Ndimethylformamide (DMF), dodecanethiol (DDT), and acetone were supplied by Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China) and used as purchased without further purification. High-purity water with a resistivity of 18 MΩ 3 cm-1 was used in the experiments. Modification of Nanosilicas. The nanosilicas (5 g, specific surface area ∼150 m2/g) were dispersed and refluxed in a mixture of GPS (2 g) and toluene (100 mL) to introduce epoxy groups onto the surface of nanosilicas under N2 atmosphere at 115 °C for 24 h. The sample was separated from the solution by centrifugation, washed with absolute ethanol for five times, and then dried under vacuum at ambient temperature. Synthesis of CdTe NCs. CdTe precursor solution was prepared by adding freshly oxygen-free NaHTe solution to a N2-deaerated CdCl2 solution at pH of about 11 in the presence of thioglycolic acid (TGA) as stabilizer ([Cd] = 0.02 mol/L, [TGA] = 0.036 mol/L, [Te] = 0.01 mol/L). Under microwave irradiation (900 W), the reaction mixture was heated to 98 °C to yield yellow (18 h) and red-emitting (36 h) QDs. Transfer of CdTe Quantum Dots (QDs) into DDT. The process was carried out by a similar procedure found in previous work.19 In a typical process, 20 mL of the above CdTe QDs in water was injected into a 150 mL flask, and 50 mL of acetone was added, followed by the addition of 20 mL of DDT. The whole procedure was maintained at 60 °C with slight stirring. And then the transferred CdTe QDs were carefully decanted for subsequent use.
Preparation of Transparent Superhydrophobic TPU Film. First, 0.2 g of modified nanosilicas was thoroughly dispersed in 8 g of toluene for at least 30 min in an ultrasonic bath, and then a solution of ODAA (0.3 g) in toluene/DMF (4 g/4 g) was added dropwise slowly with continuous stirring. Subsequently, a solution of TPU (0.8 g) in DMF (8 g) was added to the above mixture and laid in ultrasonic bath for an additional 2 h. Finally, the resulting solution was spin-casted onto a clean glass slide at room temperature for 5 h to obtain a film, which was then retained at 70 °C for 48 h in an oven. The control sample for the superhydrophobic nanosilica/ODAA composite film was similarly obtained without adding TPU solution.
Preparation of Fluorescent Superhydrophobic TPU Film. To achieve bifunctional photoluminescent-superhydrophobic films, 4 g of the above transferred CdTe nanocrystals was added into the pre-prepared nanosilica/TPU/ODAA hybrid solution. Moreover, the process of film formation for the attainment of photoluminescent and superhydrophobic film was carried out as described above. Characterization. The morphology of the resulting films was determined by scanning electron microscopy (SEM, QUANTA 200) and transmission electron microscopy (TEM,:: JEOL JEM2010).:: Contact angles were measured on a KRUSS DSA100 (KRUSS, Germany) contact-angle system using a 5 μL water droplet at ambient temperature. Fourier transform infrared (FTIR) spectra were recorded on a Nicolet 6700 FT-IR spectrometer. Photoluminescence (PL) spectra were measured on a Cary Eclipse fluorescence/photoluminescence spectrophotometer at room temperature operating with a 350 nm laser beam as a light source and Xe lamp as excited source; the excitation and emission slits were 5 nm. The thermal decomposition analyses of the sample were carried out by simultaneous thermogravimetry-differential scanning calorimetry (TG-DSC), performed with a NETZSCH STA 409 PG/PC thermal analysis system at 10 K 3 min-1 under a flow of nitrogen. Transmittance of visible light was evaluated by a UV/vis/NIR scanning spectrophotometer (Lambda 950, PerkinElmer) using air as reference. (19) Gaponik, N.; Talapin, D. V.; Rogach, A. L.; Eychmuller, A.; Weller, H. Nano Lett. 2002, 2, 803.
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Figure 1. (a, b) Top-view SEM images of the surface of nanosilica/ODAA hybrid film. Inset image indicates the CA of a 5 μL water drop on the corresponding surface. (c) SEM image of the nanosilica/ODAA hybrid transferred from the nanosilica/ODAA mixture.
Figure 2. (a, b) Top-view SEM images of the surface of nanosilica/TPU/ODAA hybrid film. (c) Profile of a water droplet (5 μL) on the nanosilica/TPU/ODAA hybrid film. (d) Photograph of a glass substrate covered with the superhydrophobic film. (e) Optical microscope photograph of the nanosilica/TPU/ODAA hybrid objects in the mixture solution of DMF and toluene. (f, g) SEM images of the nanosilica/ TPU/ODAA hybrid objects transferred from the solution.
Results and Discussion A surface with flowerlike hierarchical microstructures was prepared by assembling nanosilicas with ODAA. First, in order to gain favorable miscibility of nanosilicas in the organic matrix, we employed GPS to functionalize the surface of nanosilicas (Supporting Information Scheme S1). The FT-IR spectra of bare and GPS-modified nanosilicas are presented in Figure S1 in the Supporting Information. New peaks in the spectra of modified nanosilicas occurred at 915 cm-1 (epoxy group), 1736 cm-1 (ester group), 2940 cm-1 (νCH2), and 2870 cm-1 (νC-H), and they confirm the existence of GPS on the surface of modified nanosilicas. Then, the functionalized nanosilicas were incorporated into ODAA via the reaction between epoxy groups on the surface of nanosilicas and amine groups in the ODAA (Supporting Information Scheme S2). It is worth noting that the excessive ODAA is not necessary to be rinsed with solvents since, residual ODAA can still serve to construct microstructures for the formation of a superhydrophobic surface. The FT-IR spectra of the nanosilica/ ODAA composites, along with those of modified nanosilicas and ODAA, are shown in Figure S2 in the Supporting Information. The chemical bonding action between epoxy and amine groups is validated by the disappearance of the characteristic absorption of the epoxy group at about 915 cm-1, which suggests that nanosilicas have been well-incorporated with ODAA. A SEM image presents the flowerlike microstructures of the hybrid film derived from the nanosilica/ODAA mixture (Figure 1a). Via scrutiny of the microstructures, it has been found that the film presents petallike sub-microstructures (Figure 1b). Importantly, the film with hierarchical structures exhibits distinct superhydrophobic properties, showing the CA of about 156.7° 18456 DOI: 10.1021/la103496t
(Figure 1a inset). In this case, this nanosilica/ODAA hybrid in the mixture can be further evolved to sheetlike nanostructure by leaving the mixture for about 1 day at room temperature (Figure 1c). Thermogravimetric analysis (TGA) was conducted to analyze the thermal behavior and composition of the hybrid shown in Figure 1c (Supporting Information Figure S3). The TGA curve in Figure S3b in the Supporting Information reveals that the weight loss of the hybrid is ∼60%, which is favorably consistent with the content of ODAA. This evidence further confirms the nanosilicas are well embedded into the hybrids. Although the above nanosilica/ODAA hybrid film reveals a superhydrophobic property, this film is fragile and opaque, limiting its further application. Generally, the superhydrophobic film with desirable stability, transparency, strength, and flexibility is highly needed for practical purposes. To this end, herein, we introduce TPU into the nanosilica/ODAA hybrid solution in virtue of its moderate stiffness, high extensibility, and easy processability.17 The microstructure of the nanosilica/TPU/ODAA hybrid film observed by top-view SEM is shown in Figure 2a, b. As shown in Figure 2a, the uniform pattern of the film is constructed from the regular nanosheets. These ordered nanosheets with width of about 1.5 μm, thickness of about 200 nm, and length of several micrometers are organized nearly vertically on the glass substrate (Figure 2b). Their density is about 600 000 sheets per square millimeter, and the top (Figure 2a, b) or side view (Supporting Information Figure S4) also validates the nanosilicas have been well-dispersed and incorporated into hybrid film. The apparent water CA of the nanosilica/TPU/ODAA hybrid film is as high as 163.5° (Figure 2c), and the sliding angle (SA) is less than 3° (Supporting Information Figure S5). A 10 μL water droplet could Langmuir 2010, 26(23), 18454–18458
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Figure 3. Schematic representation for the formation process of nanosilica/TPU/ODAA hybrid film and corresponding objects in the solution.
easily roll off the horizontal film (movie S1 in the Supporting Information), showing ultralow surface adhesion and its potential for industrial applications in self-cleaning and anticorrosion. Significantly, unlike previous flowerlike microstructures, these nanosheets in the TPU hybrid film are perpendicularly aligned linear structures over the whole substrate. This drives the formation of an optically transparent nanosilica/TPU/ODAA hybrid film (Figure 2d). And the optical transmittance spectra of the film is shown in Figure S6 in the Supporting Information, in which the optical transmittance of the film is higher than 70% within the wavelengths ranging from 420 to 760 nm. Typically, the introduction of amorphous nanosilicas into the polymer matrix to produce superhydrophobic films has been extensively applied. However, in many previous reports, nanosilicas tend to aggregate owing to their high surface energy and the strength of obtained films is also fragile.20 In our case, the surface of nanosilicas is functionalized with epoxy groups, which can further react with ODAA, thus resulting in a homogeneous and stable dispersion of nanosilicas in polymer hosts. As a consequence, the morphology of the hybrid film is remarkably different from those of the films that involve the layer-by-layer (LBL) deposition technique.21 Thus, we believe that the method herein provides a simple alternative way to achieve superhydrophobic films with high performance. To gain better understanding of the variation of the morphologies for the nanosilica/TPU/ODAA hybrid, we employed the optical microscope to characterize them in real-time in solution. As seen in Figure 2e, the nanosilica/TPU/ODAA hybrids showed dominant hierarchical dendriform structures in solution which had been placed at room temperature for 1 day. The formation of dendritic structures in the solution could be ascribed to the selfassembly of substructural nanosheets. After carefully transferring the nanosheets from the solution onto a clean glass slide, the structures of hybrid objects were characterized by SEM. Figure 2f and g clearly presents a parallel structure of nanosilica/TPU/ ODAA hybrids to the nanosilica/ODAA hybrids shown in Figure 1c. On the basis of these results, we suppose that the sheetlike microstructure is the consequence of the ODAA-induced assembly. An idealized sketch of the microstructure formation of nanosilica/ TPU/ODAA hybrid is pursued in Figure 3. The previous nanosilica/TPU/ODAA hybrids expressly demonstrate an aligned and uniform sheetlike nanostructure when assembled on a glass slide. It could be a result of the effect of ODAA molecules associated with
the well film-formation property of TPU. In the nanosilica/ODAA system, the hybrids tend to gather for the achievement of flowerlike microstructures without the TPU matrix, along with the volatilization of solvents. However, this behavior is strongly restricted in the presence of TPU, which offers a spatial confined medium and limits the movement of nanosilicas, allowing for beamed growth of hybrids on the substrate. The pure ODAA film formed from the mixture solution of ODAA and DMF/toluene reveals a prickly flowerlike hierarchical microstructure (Supporting Information Figure S7), which also validates the action of ODAA in assembly. The structures of the nanosilica/TPU/ODAA hybrids gained from the solution take on a dendrite morphology because little spatial confinement effect induces easy assembly process. Moreover, the effect may also be attributed to nonequilibrium nucleation and disorder-to-order microscale transformations that are affected by the surrounding environment and the intrinsic traits of hybrids.22 The results described above have successfully displayed the assembly structures of aligned nanosheets for the superhydrophobic film with transparency, but the application spectrum can be further extended via the functionalization of the film. To diversify the applications of our superhydrophobic film, especially potentials in solar cells, flexible electronics, and lightemitting displays,23 we naturally introduced CdTe quantum dots (QDs) into the above nanosilica/TPU/ODAA hybrid film. First, CdTe with yellow emission (PL emission peak at about 595 nm), synthesized in aqueous solution for 18 h by microwave, was transferred into dodecanethiol (DDT) with the use of acetone. Figure 4a shows the schematic photographs of CdTe QDs before (1, 2) and after (3, 4) transfer into DDT. The photos of 1 and 3 were taken under daylight, while the photos of 2 and 4 were taken under ultraviolet lamp. It is noteworthy that the blue-shift emission and slightly lower PL intensity are observed when the CdTe QDs are transferred into DDT (Figure 4b). In virtue of the interactions between the exterior cadmium ions of CdTe QDs and DDT molecules, the reduced molar ratio of Cd/Te results in the little decreases of PL intensity, as well as subsequent blue-shift of PL due to the decreased diameter of the CdTe QDs.2f,24 Subsequently, the transferred CdTe QDs in DDT were added into the nanosilica/TPU/ODAA hybrid solution and subjected to ultrasonication for an additional 30 min. The TEM image of the mixing solution shows that the silica particles have an average diameter of ∼25 nm and are well dispersed (Figure 5a). The silica nanoparticles could be homogeneously dispersed into the TPU
(20) (a) Shang, H. M.; Wang, Y.; Limmer, S. J.; Chou, T. P.; Takahashi, K.; Cao, G. Z. Thin Solid Films 2005, 472, 37. (b) Rao, A. V.; Kulkarni, M. M.; Amalnerkar, D. P.; Seth, T. J. Non-Cryst. Solids 2003, 330, 187. (21) Zhang, L.; Chen, H.; Sun, J.; Shen, J. Chem. Mater. 2007, 19, 948.
(22) Mann, S. Nat. Mater. 2009, 8, 781. (23) (a) Dharmadasa, I. M. Curr. Appl. Phys. 2009, 9, E2. (b) Shvydka, D.; Parsai, E.; Kang, J.; Pearson, D. Med. Phys. 2007, 34, 2629. (24) Bao, H.; Gong, Y.; Li, Z.; Gao, M. Chem. Mater. 2004, 16, 3853.
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Figure 4. (a) Photographs of the phase transfer of CdTe QDs from water into DDT. Pictures of (1, 2) and (3, 4) were taken before and after adding the solution of DDT in acetone, respectively; pictures (1, 3) were taken under daylight, while pictures (2, 4) were taken under a UV lamp. (b) Fluorescence emission spectra of CdTe QDs before (black line) and after (red line) transfer into DDT solution.
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Figure 6. (a, b) SEM images of the film formed after adding DDT solution of CdTe into the nanosilica/TPU/ODAA hybrid mixture. (c) Photographs (taken under UV lamp) of fluorescent superhydrophobic films on the glass substrates. (d) Shape of a water droplet (5 μL) on the fluorescent (yellow) superhydrophobic film.
hydrophobic films with other color emissions can be easily tuned by using CdTe QDs with different sizes. For example, when the synthesized CdTe nanocrystals with red emission (36 h) were transferred into DDT, the homogeneous superhydrophobic film with red fluorescence could be easily obtained on a clean glass substrate (Figure 6c). We believe that this reliable strategy to facilely produce multifunctional films may have tremendous promise for applications in high-performance functional devices.25
Conclusions Figure 5. TEM images of nanosilica/TPU/ODAA/CdTe QDs hybrid with (a) low and (b) high magnification. Inset image shows a HRTEM image of CdTe QDs.
matrix without evident aggregation. The magnified TEM image indicates that a layer of TPU has wrapped on the nanosilica (Figure 5b), which can be further confirmed from the existence of CdTe QDs around the silica particle. The inset high-resolution TEM (HRTEM) picture given in Figure 5b shows the wellresolved lattice fringes of CdTe QDs. Figure 6a and b shows the SEM images of the as-prepared film from the solution of nanosilica/TPU/ODAA/CdTe QDs. The film exhibits combined petal- and sheetlike nano/microstructures, which is similar to the morphology of the above nanosilica/ ODAA and nanosilica/TPU/ODAA hybrid films. Without any further modification, the binary film revealed surprising superhydrophobic and fluorescent properties (Figure 6c). The fluorescence spectrum is displayed in Figure S8 in the Supporting Information, in which the peak of the spectrum is in good agreement with the PL results shown in Figure 4b. The water CA of the film reaches 172.3° (Figure 6d). The measurement of static CA takes on impossibility because of extremely low affinity for water (movie 2 in the Supporting Information). A water droplet can easily roll off the film even when the film is placed flat. This suggests that the film with drastic nonstickiness behaves with perfect superhydrophobicity for self-cleaning application. Super18458 DOI: 10.1021/la103496t
In summary, a simple and faithful procedure for superhydrophobic films has been demonstrated based on ODAA-directed assembly. The transparent superhydrophobic film is developed by direct assembly of nanosilica/TPU/ODAA hybrids under certain circumstances without any further treatment. After introducing CdTe QDs, the superhydrophobic film shows a favorable fluorescent property, which will be expected for applications in optoelectronic domains. This is the first example concerning the fabrication of superhydrophobic surfaces with tunable optical properties that could be varied from transparency to diverse fluorescence. Moreover, the development of the process described in this paper, as well as the extension of our methodology, will no doubt facilitate the achievement of other multifunctional superhydrophobic hybrid films. Acknowledgment. This work was supported by the National Natural Science Foundation of China-NSAF (Grant 10976012), the National Natural Science Foundation of China (Grants 21076103 and 21006046), and the Natural Science Foundation for Jiangsu Higher Education Institutions of China (Grants 07KJA53009 and 09KJB530005). Supporting Information Available: Additional SEM images; CA, SA, TGA analyses; and video files. This material is available free of charge via the Internet at http://pubs.acs.org. (25) (a) Hou, L.; Wang, C.; Chen, L.; Chen, S. J. Mater. Chem. 2010, 20, 3863. (b) Borras, A.; Groning, P. Langmuir 2010, 26, 1487.
Langmuir 2010, 26(23), 18454–18458
