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Aligned Silicon Carbide Nanowire Crossed Nets with High Superhydrophobicity Jun Jie Niu,* Jian Nong Wang, and Qian Feng Xu School of Materials Science and Engineering, Shanghai Jiao Tong UniVersity, Shanghai, 200240, People’s Republic of China ReceiVed February 15, 2008. ReVised Manuscript ReceiVed April 17, 2008 Aligned silicon carbide nanowire crossed nets (a-SiCNWNs) were directly synthesized by using a vapor-solid reaction at 1100 °C. Zinc sulfide was used as catalyst to assist the growth of a-SiCNWNs with small size and crystal structure. After functionalization with perfluoroalkysilane, a-SiCNWNs showed excellent superhydrophobic property with a high water contact angle more than 156 ( 2°, compared to random nanowires (147 ( 2°) and pure silicon wafers (101 ( 2°). The topographic roughness and chemical modification with CF2/CF3 groups contributed the better superhydrophobicity. Furthermore, the as-grown SiCNWNs can be scraped off and coated on other substrates such as pure silicon wafers. The novel nanowire coating with good superhydrophobicity displays extensive applications in silicon-related fields such as solar cells, radar, etc.
Introduction The water-repelling lotus leaf with self-cleaning ability inspires scientists to develop superhydrophobic artificial surface with a water contact angle (CA) greater than 150°.1 Superhydrophobic materials show wide applications in fields of various coatings, textiles, microfluidic systems, etc.2 Particularly in some harsh environments, such as the outside wall of a skyscraper, a superhydrophobic surface would display a self-cleaning function, which means dirt and debris can be easily removed by rain droplets, saving a great deal of maintenance time and cost. Similar with the lotus effect, wettability of a solid surface can be controlled by modulating topographical microstructure and chemical composition. High surface roughness increases the amount of air-trapping pores, while chemical coating lowers the free surface energy (Cassie Baxter model).3 A large geometric surface area, compared to the projected area on a rough surface, requires a high energy barrier to produce a solid-liquid interface. Therefore, when the surface energy is lower than that of water, the surface displays a water-repelling superhydrophobicity.4 One-dimensional nanomaterials with high surface area make them particularly appealing for sensors and other precise applications. However, sensitivity to environmental humidity should be addressed with the effective devices running. As a result, superhydrophobic one-dimensional nanomaterials are being studied widely.4–9 It has been reported that the nanostructures with aligned carbon nanotubes (CNTs),10 polymer
* Corresponding author: tel, +86-21-54743182; e-mail,
[email protected]. (1) Lee, H. J.; Michielse, S. J. Polym. Sci. 2007, 45, 253. (2) Cyranoskl, D. Nature 2001, 414, 240. (3) Verplanck, N.; Galopin, E.; Camart, J. C.; Thomy, V. Nano Lett. 2007, 7, 813. (4) Lau, K. K. S.; Bico, J.; Teo, K. B. K.; Chhowalla, M.; Amaratunga, G. A. J.; Milne, W. I.; Mckinley, G. H.; Gleason, K. K. Nano Lett. 2003, 3, 1701. (5) Ma, M. L.; Mao, Y.; Gupta, M.; Gleason, K. K.; Rutledge, G. C. Macromolecules 2005, 38, 9742. (6) Teare, D. O. H.; Spanos, C. G.; Ridley, P.; Kinmond, E. J.; Roucoules, V.; Badyal, J. P. S. Chem. Mater. 2002, 14, 4566. (7) Woodward, L.; Schofield, W. C. E.; Roucoules, V.; Badyal, J. P. S. Langmuir 2003, 19, 3432. (8) Feng, L.; Song, Y.; Zhai, J.; Liu, B.; Xu, J.; Jiang, L.; Zhu, D. Angew. Chem. 2003, 115, 824. (9) Feng, X.; Feng, L.; Jin, M.; Zhai, J.; Jiang, L.; Zhu, D. J. Am. Chem. Soc. 2003, 126, 62. (10) Li, S.; Li, H.; Wang, X.; Song, Y.; Liu, Y.; Jiang, L.; Zhu, D. J. Phys. Chem. B 2002, 106, 9274.
nanofibers,11 ZnO nanorods,9 and SiO2 nanowires12 all exhibited superhydrophobicity. Among them, Jiang’s group has made some progress in CNTs, polymers, and inorganic oxide nanotubes.8,13–16 Huang et al. have reported a superhydrophobic CNT surface with a CA of 159°.17 Furthermore, the reversible electrowetting of liquid droplets in air and oil environments on superhydrophobic silicon nanowires (SiNWs) has been studied.3 Superhydrophobic silicon oxide nanowires with a CA of 150° were also reported recently.12,18 Silicon carbide nanowire (SiCNW), as a wide band gap semiconductor, displays extensive applications in many harsh conditions due to the high mechanical strength, high chemical stability, low induced activity, etc. To date, several techniques have been used to synthesize SiCNWs, including CNTs confined reaction,19 carbothermal reduction,20 metal-assisted vapor–liquid– solid (VLS) growth,21 etc. The solid-vapor reaction with CNTs and volatile oxide is often used.19 Lee and co-workers reported the fabrication of β-SiCNWs through hot filament chemical vapor deposition (HFCVD).22 Deng et al. synthesized single-crystal SiCNWs on a SiC ceramic substrate by catalyst-assisted thermal heating.23 Although various methods are applied, the synthesis of orderly SiCNW arrays via a simple process still remains a challenge.24,25 It is desirable to explore a feasible technique to synthesize single-crystal aligned silicon carbide nanowire crossed (11) Feng, L.; Li, S.; Li, H.; Zhai, J.; Song, Y.; Jiang, L.; Zhu, D. Angew. Chem., Int. Ed. 2002, 41, 1221. (12) Coffinier, Y.; Janel, S.; Addad, A.; Blossey, R.; Gengembre, L.; Payen, E.; Boukherroub, R. Langmuir 2007, 23, 1608. (13) Wang, S. T.; Liu, H. J.; Liu, D. S.; Ma, X. Y.; Fang, X. H.; Jiang, L. Angew. Chem., Int. Ed. 2007, 46, 1. (14) Hong, X.; Gao, X. F.; Jiang, L. J. Am. Chem. Soc. 2007, 129, 1478. (15) Jiang, L.; Zhao, Y.; Zhai, J. Angew. Chem., Int. Ed. 2004, 43, 4338. (16) Jin, M. H.; Feng, X. J.; Feng, L.; Sun, T. L.; Zhai, J.; Li, T. J.; Jiang, L. AdV. Mater. 2005, 17, 1977. (17) Huang, L.; Lau, S. P.; Yang, H. Y.; Leong, E. S. P.; Yu, S. F.; Prawer, S. J. Phys. Chem. B 2005, 109, 7747. (18) Georg, R. J. A.; Jung, S.; Ximmermann, J.; Gautschi, H. P.; Marquardt, K.; Seeger, S. AdV. Mater. 2006, 18, 2758. (19) Dai, H. J.; Wong, E. W.; Lu, Y. Z.; Fan, S. S.; Lieber, C. M. Nature 1995, 375, 769. (20) Ye, H.; Titchenal, N.; Gogotsi, Y.; Ko, F. AdV. Mater. 2005, 17, 1531. (21) Zhang, H. F.; Wang, C. M.; Wang, L. S. Nano Lett. 2002, 2, 941. (22) Zhou, X. T.; Wang, N.; Lai, H. L.; Peng, H. Y.; Bello, I.; Wong, N.; Lee, C. S.; Lee, S. T. Appl. Phys. Lett. 1999, 74, 3942. (23) Deng, S. Z.; Li, Z. B.; Wang, W. L.; Xu, N. S.; Zhou, J.; Zheng, X. G.; Xu, H. T.; Chen, J.; Lee, J. C. Appl. Phys. Lett. 2006, 89, 023118-1. (24) Pan, Z.; Lai, H. L.; Au, F. C. K.; Duan, Z.; Zhou, W.; Shi, W.; Wang, N.; Lee, C. S.; Wong, N. B.; Lee, S. T.; Xie, S. AdV. Mater. 2000, 12, 1186. (25) Li, Z.; Zhang, J.; Meng, A.; Guo, J. J. Phys. Chem. B 2006, 110, 22382.
10.1021/la800494h CCC: $40.75 2008 American Chemical Society Published on Web 06/04/2008
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Figure 1. AFM images of a-SiCNWNs with low magnification (a), high magnification (b), and 3-D graphics (c and d).
nets (a-SiCNWNs). For potential applications, much effort on SiCNWs has been processed, especially in field emission and mechanical property with reinforcing the strength of a ceramicmatrix composite.26,27 With high surface area and high roughness, the promising superhydrophobicity with SiCNWs has stimulated our research interest. To date, as we know, the novel superhydrophobicity of a-SiCNWNs is not even reported. In this paper, aligned SiCNW crossed nets were directly synthesized on a silicon substrate by a ZnS-assisted vapor-solid reaction. As a comparison, random SiCNWs with big size and poor crystallization were also obtained without ZnS. After modification with organic perfluoroalkysilane, the a-SiCNWNs exhibited excellent superhydrophobicity with a contact angle greater than 156 ( 2°, compared to random nanowires (147 ( 2°) and pure silicon wafer (101 ( 2°). The in situ superhydrophobic nanowires on a silicon wafer display useful applications in many water-repelling semiconductor devices such as the wings of solar cells, surface of radar, etc. Furthermore, SiCNWNs can be conveniently scraped off and a superhydrophobicity with high (26) Wong, E. W.; Sheehan, P. E.; Lieber, C. M. Science 1997, 277, 1971. (27) Han, X. D.; Zhang, Y. F.; Zhang, X. N.; Zhang, Z.; Hao, Y. J.; Guo, X. Y.; Yuan, J.; Wang, Z. L. Nano Lett. 2007, 7, 452.
CA was simultaneously obtained after coated on a new silicon wafer. This shows that the novel material can indeed be processed in wide self-cleaning fields. Finally, a superhydrophobic mechanism related to the surface microstructure and chemical modification is discussed in detail.
Experimental Details SiCNWs were directly synthesized by the vapor-solid reaction of carbon oxide and silicon wafer without any metal catalyst in a simple alumina-ceramic tube furnace.28 In a typical experiment, carbon powders were deposited on the tube inside by sintering at a high temperature. Pieces of silicon wafers with or without ZnS powders on ceramic boats were sent to the middle of the furnace. Then the furnace was heated to 1100 °C. An argon gas flow was initiated at a rate of ∼16 L/h. After reaction for 2-3 h, the as-grown samples with yellow-black color were taken out for further investigation. The obtained samples (with and without ZnS) were chemically modified by vaporized perfluoroalkysilane gas at a temperature less than 200 °C. The period was controlled so that the thickness of organic film was not larger than 5 nm. Furthermore, the a-SiCNWN film was scraped off the substrate and then coated on a new silicon wafer by using an immersing method. Subsequently, (28) Niu, J. J.; Wang, J. N. J. Phys. Chem. B 2007, 111, 4368.
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Figure 2. FESEM images of the as-prepared SiCNWs: a-SiCNWNs (a) (inset is the enlarged image) and oriented fan-out structures (b); (c) random SiCNWs grown without ZnS; (d) size distribution of the SiCNWs with and without ZnS.
the coated film was modified with perfluoroalkysilane under the same process. As a comparison, a pure silicon wafer was also modified under the exact same procedure. The aligned structure and surface roughness were determined by atomic force microscopy (AFM, Nanoscope IIIa, Digital Instruments, USA) with contact mode. Field emission scanning electric microscopy (FESEM, FEI Sirion) and high-resolution field emission transmission electron microscopy (HR-FETEM, JEM 2010F) at an acceleration voltage of 200 kV were carried out to observe the morphology and crystal lattice, respectively. The crystal structure was analyzed by X-ray diffraction using Cu KR radiation (XRD, Rigaku, D/MAX 2550). The chemical composition of the sample was analyzed by X-ray photoelectron spectroscopy (XPS, Kratos Axis Ultra DLD XPS, UK) with a power of 150 W. A monochromatic Al KR X-ray source (1486.6 eV) was operated in constant analyzer energy (CAE) mode (CAE was 160 eV for survey spectra and 80 eV for highresolution spectra) by applying the electromagnetic lens mode. The water contact angle was measured with 3 µL of deionized water by a contact angle system (OCA 20, Dataphysics, Germany). All the CA measurements were checked in ambient atmosphere at room temperature. Because of the slight deviation during the measurement, an average value of CA was used after measuring several spots on one surface.
Results and Discussion Figure 1 shows AFM images of the highly oriented SiCNW arrays. As can be seen from the low-magnification image (Figure 1a), large scales of a-SiCNWNs with high density are uniformly dispersed. The magnified image indicates that each SiCNW possesses a uniform and smooth surface (Figure 1b). The threedimensional (3-D) AFM images more clearly present the highly aligned array by varying the visual angle (Figure 1, panels c and d). The nanowire surface is clean and free of nanoparticles and impurities. The presence of the interface induces an accented
surface, as shown in Figure 1, panels a and b. Thus, a high roughness can be obtained by scanning different regions. A root mean square roughness of ∼4.0 nm was calculated within a region of 3 × 3 µm (Figure 1b). This value was received with a low dimensionality of small area. However, the altitude is not uniform even some regions remain many porous morphologies. As a result, a high root mean square roughness with several hundreds of nanometers (up to ∼613 nm) was usually obtained within the larger scanning regions. Surface morphology can also be observed from FESEM images. As shown in Figure 2a, quantities of oriented SiCNWNs formed a film of tens of micrometers on the substrate. The crimped surface with high roughness can be clearly seen from the figure. The enlarged image clearly shows the interspaces between compact nanowire crossed nets (Figure 2a inset). The amplified FESEM image in Figure 2b indicates that the a-SiCNWNs formed several fan-out structures with a radius of 3-7 µm. Different from a-SiCNWNs, plenty of disordered SiCNWs with coiled morphology were obtained by the same process but without ZnS (Figure 2c). The figure shows that scales of SiCNWs with relatively thick size were randomly distributed. The diameter distribution by calculating hundreds of SiCNWs is presented in Figure 2d. The calculated data show that size distribution of the sample with ZnS is narrow with 7-9 nm while that with free of ZnS displays a wide range at ∼70 nm. Obviously, the size is enlarged nearly 10 times if ZnS was not used. A homogeneous morphology with a small diameter of ∼8 nm is observed from the TEM image (Figure 3a). The symmetrical electronic diffraction (ED) pattern (inset) shows a good crystal structure with main direction of SiC (111) face. The clear diffraction spots indicate a standard diamond-like face-centered cubic structure.
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Figure 3. (a) TEM image of a-SiCNWNs with ZnS. (b) HR-FETEM image of the SiCNW with ZnS. (c) TEM image of random SiCNWs without ZnS. The insets are the corresponding ED patterns, respectively. (d) XRD patterns of the SiCNWs with and without ZnS. The Si signal comes from silicon substrate.
The unambiguous lattice fringes in the HR-FETEM image (Figure 3b) depict a single-crystal nature of a SiCNW with a d-spacing of ∼0.25 nm corresponding to the (111) plane spacing. It indicates that the nanowire grew along the orientation of (111) or with a small angle (see the arrows in Figure 3b). However, the random SiCNWs display a poor quality. The magnified TEM image shows the nanowire surface is uniform but with a large diameter of tens of nanometers (Figure 3c). The corresponding crystal nature of both samples was characterized by XRD patterns, as expressed in Figure 3d. The significantly strong peak of (111) face indicates that the sample is mainly crystal with (111) direction, which is consistent to the analysis in HR-FETEM image (Figure 3b). It is evident that the intensity of (111) peak with ZnS is absolutely stronger than the one without ZnS. The stronger diffraction spots are simultaneously observed from ED patterns (Figure 3a and c insets). Furthermore, the peak width is relatively broader than that of ZnS-free (Figure 3d). The analysis confirms that ZnS can not only assist the formation of aligned arrays and improve the crystal degree but also decrease the size. More details with structures and growth mechanism can be referred from ref.28 Wettability of the samples was checked by a water contact angle instrument. Because of the existing polar OH groups, original SiCNWs were almost hydrophilic with a very low CA. After modification with perfluoroalkysilane, the CA was sharply increased to a high value, as shown in Figure 4. A CA of 156 ( 2° (average value for several spots) with a-SiCNWNs was observed, indicating a superhydrophobic surface (Figure 4a). A slightly lower CA of 147 ( 2° with the disordered SiCNWs was also obtained (Figure 4b). As a reference, the chemically functionalized pure silicon wafer only showed a low CA of 101
( 2°, as presented in Figure 4d. The chemical stability was also investigated. A very weak CA hysteresis of less than 5° was observed even after exploring in air for 1 week (Figure 4c). All these data confirm an excellent superhydrophobicity for the modified a-SiCNWNs. As a good candidate for superhydrophobic material, aSiCNWNs can be scaled up and scraped off the substrate. Thus, the current material may be used as a coating on many surfaces such as glass, metal, and ceramic. On the basis of this assumption, a tentative sample was scraped off and covered on a new silicon wafer. The modified new surface also displayed superhydrophobic property. Figure 5A shows that the CA is higher than 140 ( 2° with the scraped sample. Compared with a pure silicon wafer, the CA has been strongly enhanced nearly 40°. The stability and lifetime with a reinforcing polymer such as epoxy resin become more important in future industrial applications. This will be further investigated in our next work. It is known that hydrophobicity of a solid surface depends on both the roughness and surface free energy.16 A high roughness can be feasibly achieved by the special topographical structure of present a-SiCNWNs. As one of the most effective chemical compositions, fluorine-contained perfluoroalkysilane was employed to decrease the surface free energy. Therefore, the modified a-SiCNWNs displayed a good superhydrophobicity (Figure 4). Provided the error of CA measurement can be ignored, the roughness of the samples with and without ZnS is basically similar but a slightly higher value was found with the aligned sample. As analyzed from AFM images, the a-SiCNWNs possess a slightly stronger root mean square roughness (average value is higher by tens of nanometers within a region of 20 × 20 µm) because of
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Figure 4. Water contact angles of modified a-SiCNWNs (a), random SiCNWs (b), the sample of (a) after exploring in air for 1 week (c), and a pure silicon wafer (d).
the bigger gurgitation induced by thick film and debris. Thus, a slightly higher CA with a-SiCNWNs was observed (Figure 4). As a counterpart, the surface of pure silicon wafer is very smooth with a low roughness and thus contributes a low CA of 101° even after modification. Herein the mechanism of chemical modification is suggested by analyzing the chemical reaction and surface composition before and after treatment. The perfluoroalkysilane is favorably hydrolyzed to produce OH groups along the following equation:In
general, the wall of as-prepared SiCNW will be slightly oxidized and thusformaverythinoxideshellunderthepresentfree-vacuumsystem.28 The generated silicon oxides easily carry more OH groups and react with the hydrolyzed perfluoroalkysilane,29 as illustrated below: The
final product remains CF2 and CF3 groups, which will play a key role in lowering the surface free energy.12,30 Furthermore, the
reverse-hydrolyzed reaction between solid surface and perfluoroalkysilane induces a good superhydrophobic stability with time (Figure 4c). Chemical composition variation of the samples before and after modification was analyzed by XPS (Figure 5B). Figure 5B shows the XPS surveys of original a-SiCNWNs (in order to generate more OH groups, the as-prepared sample was directly taken out from the reaction region at high temperature), the modified sample, and the modified sample eroded by the 400 keV ion gun with ∼3 min. It is clear that four binding energy peaks of 101.0, 151.7, 283.05, and 532.2 eV are found among the three samples. The values of 101.0 and 151.7 eV correspond well to the Si 2p and Si 2s in SiC. The peak of 283.05 eV is confirmed to come from the C 1s in SiC. The peak at 532.2 eV is due to O 1s, mainly from the silicon oxide. The peak of O 1s is strong in the original sample, which means more oxides on surface, as was desired. The peak of C 1s with high-resolution spectra can be more clearly observed in Figure 6a. Thus, the SiC composition is confirmed before and after modification. As can be seen from Figure 5B, a strong peak at 688.8 eV due to the F 1s appears after modification. It is believed to originate from CF2 and CF3 groups. In the enlarged XPS spectrum of C 1s (Figure 6a), the peaks at 293.4, 291.3, and 284.8 eV are attributed to the CF3, CF2, and CH2 functional groups, respectively. These data indicate that the perfluoroalkysilane successfully reacted with SiCNWs and generated low-energy fluorine-contained groups. After erosion by an ion gun for 3 min, the perfluoroalkysilane can be partly destroyed, as displayed in Figure 5. It can be seen that the peak intensities of 688.8, 291.3, and 284.8 eV decrease; even the peak at 293.4 eV has disappeared. This line variation indicates that (29) Tsoi, S. F.; Fok, E.; Sit, J. C.; Veinot, J. G. C. Langmuir 2004, 20, 10771.
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Figure 5. (A) Water contact angle of scraped SiCNWNs coated on a new silicon wafer. (B) XPS surveys of a-SiCNWNs before (c) and after (a) modification with perfluoroalkysilane. (b) The sample of (a) after eroding for 3 min with a 400 keV ion gun.
the perfluoroalkysilane film on SiCNWs is very thin (about 1-5 nm). On the contrary, the peaks at 101.0 and 283.05 eV related to the Si 2p and C 1s of SiC are strengthened (Figures 5B and 6). It demonstrates that the perfluoroalkysilane has nearly been removed and SiCNWs emerged after a short time of ion eroding. The XPS data show a good agreement with the above mechanism analysis. Finally, Cassie and Baxter’s equation of cos θtrue ) f1 cos θ f2 is used to estimate the ratio of air-trapping action in current modified a-SiCNWNs.31 Here f1 and f2 are the area fractions of solid and vapor on the surface, respectively, and f1 + f2 ) 1. θtrue and θ are the water CAs of modified a-SiCNWNs and pure silicon wafer. In the present experiments, the θ true is 156° and the θ is supposed to be 101°. After calculation, the values of f1 ) 0.107 and f2 ) 0.893 are obtained. It proves that the air-trapping action plays a main role on the surface with high roughness and low free energy.
Conclusion In summary, self-organized a-SiCNWNs were directly synthesized by a simple vapor-solid reaction with carbon oxide (30) Kim, S. H.; Kim, J. H.; Kang, B. K.; Uhm, H. S. Langmuir 2005, 21, 12213. (31) Cassie, A. B. D.; Baxter, S. Trans. Faraday Soc. 1944, 40, 546.
Figure 6. High-resolution XPS spectra of C 1s (a) and F 1s (b) of modified and eroded a-SiCNWNs.
and silicon wafers at 1100 °C. Oriented SiCNW arrays with small size and good crystal structure were obtained under the assistance of ZnS while a random sample was received without ZnS. After modification with perfluoroalkysilane, a-SiCNWNs showed a better self-cleaning superhydrophobicity with a high CA of 156°. The high roughness induced by a special geometric microstructure and low surface energy caused by fluorineincluded groups (CF2 and CF3) contribute to the superhydrophobicity. The nanowires can be mixed with other compounds and conveniently coated on desired surfaces. Therefore, the current a-SiCNWNs with excellent superhydrophobicity presents potential applications on various surfaces of glass, metal, and other silicon-related substrates. Acknowledgment. This work was sponsored by the Shanghai Educational Development Foundation (No. 2007CG14) and the Shanghai-Applied Materials Research and Development Fund (No. 06SA06). We thank the Instrumental Analysis Center of Shanghai Jiao Tong University for their great help with measurements. LA800494H
