Tunable geometry and wettability of organosilane nanostructured surfaces by water content

Abstract

A facile method to fabricate nano-architecture organosilane surfaces with tunable geometry is presented. By changing the water content of the reaction system, surface geometries with controlled shape, size and quantity are obtained. The corresponding wettability variation can vary from hydrophobic to superhydrophobic with the surface topography evolution.

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Surfaces with extreme wetting properties are currently attracting a great deal of interest for both scientific research and practical applications.^1–5 The special wettabilities result from the simultaneous control of surface compositions and microstructures.^6–10 Many studies have shown that a superhydrophobic surface can be fabricated by modification of the rough surface with low surface energy material, or enhancing the roughness of hydrophobic surfaces.^11–15 Organosilanes have been widely known because of their potential advantages of low surface free energy, chemical stability and biocompatibility.^16,17 They has been used in many contexts to alter and control surface physiochemical properties such as adhesion, adsorption and surface activity.^18,19 Organosilanes (R_nSiX_4−n, n = 0–3) owning the hydrolysable group (usually X = Cl, OR, NMe₂) in its molecule, can easily graft to the surface by covalent attachment with chemical bond (Si–O–Si_s).²⁰

So far, most studies about organosilane covered the preparation and performance of silane monolayers.^21,22 In recent years, researchers have also used organosilane to prepare 3D structured surfaces with various nanostructures through phase separation or vapor phase plasma induced polymerization.^23–26 Gao et al. have prepared a perfectly hydrophobic surface via solution phase reaction.²³ Seeger et al. have grafted silicone nanofilaments to different material surfaces, which are rendered superhydrophobic via vapor phase reaction route.²⁴ Researchers also have studied the impact of the monomer concentration, humidity, reaction time on the surface topography and corresponding wettability. It is well known that water is the requisite material in the polycondensation of organosilane monomers, so that the water content is an important parameter to affect the surface structure, but seldom studies was focus on the relationship between water content and surface morphology as well as wettability. In this communication, by changing the water content of the reaction system, we have carried out a detailed investigation of the influence of water content on surface structure and on wettability. The surface morphology could be tuned from nanoprotuberances to nanofibres and finally to nanospheres. And the corresponding wettability of the surfaces also varies from hydrophobic to superhydrophobic with the morphology evolution.

The general steps for the hydrolysis and polycondensation of methyltrichlorosilane (MTS) to form polyorganosiloxane surfaces are exhibited in Scheme 1. MTS molecules graft onto the hydroxylated surface. At the same time, with the presence of water molecules, monomers hydrolyze and polycondense to form a cross-linked network. Finally, the rough surface is obtained by extraction in ethanol. Obviously, water content directly affects the extent of crosslinking. Here, our study mainly focused on the influence on the MTS-derived surface structure and the further impact on its wettability when the water content is changed.

Scheme 1 Schematic illustration of polyorganosiloxane surface formation. MTS monomers graft onto the hydroxylated substrate to form Si–O bonds. Meanwhile, in the presence of a trace of water, silanes react with adjacent silane molecules to form a network structure of Si–O–Si. Finally, the rough surface is becoming to form with the rapid extraction of toluene from ethanol.

Because only trace of water is needed in this reaction, directly adding water into the reaction system requires very sophisticated equipment. Here we used a simple method, namely, mixing water-saturated toluene (WT) with anhydrous toluene (AT) in different ratios to adjust water content. In other words, WT content is the volume ratio of WT and the sum of WT and AT. For example, water-saturated toluene content 30% represents the WT : AT = 3 : 7.

To evaluate the effect of water content on the geometry and wettability of MTS nano-architecture surfaces, five different WT contents (0%, 30%, 60%, 80%, and 100%) were selected to carry silylation reaction with 0.0106 M monomer concentration for 1 h. To characterize the varieties of nano-architectured surfaces, the surface topography was investigated by scanning electron microscopy (SEM), as shown in Fig. 1. In AT solution, only a sporadic distribution of some nanoscale protuberances formed on the substrate (Fig. 1a). When the water-saturated toluene content reached 30%, as shown in Fig. 1b, a layer of nano-protuberances with the size of 82.7 ± 16.3 nm appeared on the surface evenly. With the water-saturated toluene content increasing to 60% (Fig. 1c), nano-protuberances on the surface decrease except for a handful of discrete fibres appearing on the surface. The discrete fibres are irregularly bent and hooked. When the WT content was further increased to 80%, a kind of complicated 3D nanofibre network could be clearly seen on the surface (Fig. 1d). Fig. 1e showed the surface morphology when WT (100%) was used, the fibres become sparser, while a number of nanospheres emerged (see the cross-section SEM images in the ESI, Fig. S1†). The results showed that water content plays an important role in controlling the MTS's polymerization on glass surfaces, and then affecting the shape, size and quantity of the surface structure.

Fig. 1 SEM images of MTS-derived surface generated under various system water contents at 0.0106 M and 1 h: (a) 0%; (b) 30%; (c) 60%; (d) 80%; (e) 100%. (f) A hand-drawn diagram of the clear description of nano-protuberances, discrete fibres, fibre network, and nanospheres.

Water is an indispensible reagent in the silanization reaction, so the water content directly affects the formation of cross-linked networks. As MTS is a kind of multi-functional monomer, each monomer mostly reacts with three water molecule to hydrolyze. Due to the dependence on water content, the quantity of Si–O–Si bond formed by hydrolysis and polycondensation is different. Accordingly the progress and the form of polycondensation can be controlled. The reaction solution is close to anhydrous when the concentration is 0%, besides some water molecules which adsorbed on the surface, so that most of the monomers can't participate in the hydrolysis reaction. With the increase of water content, the monomer reaction of covalent adsorption on surfaces and polymerization to form polysiloxanes are existent simultaneously. Both 1D and 3D polymerization of monomer reactions may occur on the surface. 1D growth leads to the growth of nanofibres, while 3D growth contributes to the increase of nanospheres. Different water content will promote polymerization in different ways, subsequently, different surface structures occurred (see the possible mechanism of the reaction in the ESI, Fig. S2†).

We have noted that there is a strong correlation between the water content and the surface geometry of the MTS-derived nanoarchitectures. Correspondingly, we have also studied the wetting properties of the MTS-derived surfaces by contact angle (CA) measurement at different water-saturated toluene content. The relationship between wettability and water-saturated toluene content is as shown in Fig. 2. At 0%, the surface exhibits a CA of 94 ± 1.1°, which shows that the surface is hydrophobic. With the increase of WT content, the CA is gradually increased. When the water-saturated toluene content is 80%, the CA reaches a maximum value, as high as 170.1 ± 1.5° (superhydrophobicity), at the same time, the surface exhibits extremely low hysteresis as Fig. 2b showed. But in the largest water-saturated toluene content of 100%, the water CA becomes small slightly and decreases to 151.5 ± 2.3°.

Fig. 2 a) Dynamic water CA measurements on the MTS-derived surfaces as a function of water content at 0.0106 M and 1 h. The insets are the water droplet profiles of the relative water CAs. b) Shapes of water droplets, taken at different stages during the CA measurement process on the MTS-derived surface at a water content of 80%. The image 1 was taken before the water drop contacted the surface; image 2 was taken when the water drop contacted the surface; image 3 obtained as the water drop was leaving the surface; and image 4 was taken after the water droplet left the surface.

To thoroughly understand the variation of the wettability of the MTS-derived surfaces with the change in water-saturated toluene content, the surface chemical composition and the surface structure, which are two main factors dominating the surface wettability, are considered. MTS is a relatively low free-energy compound. And the as-prepared MTS grafted surface is rough, as seen in Fig. 1. When the water droplets contact with the surfaces, the air can be trapped in the rough grooves. The hydrophobicity of a rough surface can be enhanced by increasing the proportion of air/water interface, and ultimately achieve superhydrophobicity (see ESI, Table S1 for the details†).

Concentration-dependent experiments show that a concentration of around 0.0106 M is appropriate for the formation of ideal superhydrophobic surfaces. At about 0.00212 M, some discrete submicron spheres are formed on the glass substrate (Fig. 3a) and the CA is only about 135°. At about 0.106 M, the submicron spheres on the substrate become significantly larger (Fig. 3b) and the CA is about 102°. When the concentration is above 0.5 M, the polymerization prefers to occur in the solution rather than on the glass substrate. Some spheres fall onto the substrate but are easily rinsed off. Therefore, a suitable concentration is important for a stable superhydrophobic surface.

Fig. 3 SEM images of the MTS-derived surfaces obtained by immersing the substrate in a) 0.00212 M and b) 0.106 M solutions of methyltrichlorosilane at the water-saturated toluene content of 80%.

In conclusion, organosilane nano-architectured surfaces with tunable surface geometry and corresponding wettability have been fabricated by controlling the water content. With the increase of water content, the MTS-derived surface morphology evolved from nano-protuberances to discrete nanofibres and finally developed to nanospheres. Further influence on wettability is conducted, and the corresponding CA variation is obvious. The progress of hydrolysis and polymerization cause the growth of the surface structure to form different surface morphologies. And the change in the surface roughness makes the surface wettability varied. We believe this study can provide a guideline for the design of a relevant surface structures to achieve desired surface-wetting properties, which is certainly significant for future industrial applications.

Acknowledgements

The authors acknowledge the National Natural Science Foundation of China (20703007, 20801057) and the Fundamental Research Funds for the Central Universities (2009QN049) for financial support.

Notes and references

1. H. Y. Erbil, A. L. Demirel, Y. Avcı and O. Mert, Science, 2003, 299, 1377.
2. R. Wang, K. Hashimoto, A. Fujishima, M. Chikuni, E. Kojima, A. Kitamura, M. Shimohigoshi and T. Watanabe, Nature, 1997, 388, 431.
3. X. F. Gao and L. Jiang, Nature, 2004, 432, 36.
4. W. C. Wu, X. L. Wang, D. A. Wang, M. Chen, F. Zhou, W. M. Liu and Q. J. Xue, Chem. Commun., 2009, 1043.
5. T. L. Sun, G. J. Wang, L. Feng, B. Q. Liu, Y. M. Ma, L. Jiang and D. B. Zhu, Angew. Chem., Int. Ed., 2004, 43, 357.
6. R. N. Wenzel, Ind. Eng. Chem., 1936, 28, 988; A. B. D. Cassie and S. Baxter, Trans. Faraday Soc., 1944, 40, 546.
7. T. Nishino, M. Meguro, K. Nakamae, M. Matsushita and Y. Ueda, Langmuir, 1999, 15, 4321.
8. A. Lafuma, D. Quéré, Nat. Mater; Tsujii, Angew. Chem., Int. Ed., 2005, 44, 3453.
9. L. Feng, S. H. Li, Y. S. Li, H. J. Li, L. J. Zhang, J. Zhai, Y. L. Song, B. Q. Liu, L. Jiang and D. B. Zhu, Adv. Mater., 2002, 14, 1857.
10. G. Qing and T. L. Sun, Adv. Mater., 2011, 23, 1615; T. L. Sun and G. Qing, Adv. Mater., 2011, 23, H57.
11. T. Onda, S. Shibuichi, N. Satoh and K. Tsujii, Langmuir, 1996, 12, 2125.
12. H. Yan, K. Kurogi, H. Mayama and K. Tsujii, Angew. Chem., Int. Ed., 2005, 44, 3453.
13. S. T. Wang, L. Feng, H. Liu, T. L. Sun, X. Zhang, L. Jiang and D. B. Zhu, ChemPhysChem, 2005, 6, 1475.
14. X. Zhang, F. Shi, X. Yu, H. Liu, Y. Fu, Z. Q. Wang, L. Jiang and X. Y. Li, J. Am. Chem. Soc., 2004, 126, 3064.
15. M. H. Jin, X. J. Feng, J. M. Xi, J. Zhai, K. W. Cho, L. Feng and L. Jiang, Macromol. Rapid Commun., 2005, 26, 1805.
16. J. Zimmermann, F. A. Reifler, G. Fortunato, L.-C. Gerhardt and S. Seeger, Adv. Funct. Mater., 2008, 18, 3662.
17. D.-A. E. Rollings, S. Tsoi, J. C. Sit and J. G. C. Veinot, Langmuir, 2007, 23, 5275.
18. Y. Arima and H. Iwata, J. Mater. Chem., 2007, 17, 4079.
19. C. Zhao, I. Witte and G. Wittstock, Angew. Chem., Int. Ed., 2006, 45, 5469.
20. E. G. Rochow, J. Am. Chem. Soc., 1945, 67, 963.
21. A. Y. Fadeev, R. Helmy and S. Marcinko, Langmuir, 2002, 18, 7521.
22. B. Y. Chow, D. W. Mosley and J. M. Jacobson, Langmuir, 2005, 21, 4782.
23. L. Gao and T. J. McCarthy, J. Am. Chem. Soc., 2006, 128, 9052.
24. G. R. J. Artus, S. Jung, J. Zimmermann, H.-P. Gautschi, K. Marquardt and S. Seeger, Adv. Mater., 2006, 18, 2758.
25. H. S. Khoo1 and F.-G. Tseng, Nanotechnology, 2008, 19, 345603.
26. M. H. Jin, J. Wang, X. Yao, M. Y. Liao, Y. Zhao and L. Jiang, Adv. Mater., 2011, 23, 2861.

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Footnote

† Electronic supplementary information (ESI) available: Experimental section and cross-section view. See DOI: 10.1039/c1py00246e

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