Biomimetic graphene surfaces with superhydrophobicity and iridescence.

Triggered by the fantastic functions and bright appearance of biological systems in nature, enormous efforts have been devoted to biomimetic fabrication. For instance, butterfly wings and red rose petals have attracted increasing attention due to their excellent water repellency and splendid structural color. Consequently, colorful superhydrophobic surfaces have become a hot topic with significance in both fundamental research and practical applications. Previous studies have shown that hierarchical micro-/nanostructures on biosurfaces play a critical role in the multifunctional acquisition. On the one hand, the highly rough textures trap a wealth of air bubbles at the interface preventing a water droplet from spreading; thus, the surfaces exhibit a high water contact angle (CA>1508). Along this line, a variety of water-repellent surfaces with multiscale structures have been achieved by classical “top-down” and “bottomup” approaches. On the other hand, as inspired by many natural species that use structural color as a warning or protection, the surface microstructures are not randomly distributed but rigidly arranged in periodic micro-patterns, and therefore triggered light diffraction and scattering contribute to the brilliant appearance. However, due to technical challenges in the fabrication of uniform and well-defined nanostructures in the long-range order, most of the superhydrophobic surfaces hardly show any structural color. To date, only a few attempts to such multifunctional surfaces have been realized. For instance, Gu et al. fabricated colloidal photonic crystal films with both structural color and superhydrophobicity at the cost of long time and high temperature. Jiang et al. reported multicolor superhydrophobic coatings that depend on metal ions for the appearance of color; in their study, individual samples exhibited a single color. Wu et al. fabricated superhydrophobic surfaces with iridescence by employing multiple procedures including interference, surface modification, and chemical plating. Consequently, a facile and convenient approach for surfaces with superhydrophobicity and iridescent structural color is in urgent need. Unlike the classical slippery superhydrophobic surface represented by the famous self-cleaning lotus leaf, rose petals possess a sticky superhydrophobicity, exhibiting both a high water contact angle (CA>1508) and strong adhesion. Because of the adhesive force, flowers are able to maintain a fresh appearance, as a water droplet cannot roll off effortlessly but stays stably on the surface without any movement. To date, by tailoring the chemical composition, the geometrical structure, or interfacial capillary and van der Waals forces, superhydrophobic surfaces with high adhesion were successfully prepared. Due to their representativeness in wetting behavior and their research value in various realms, such as liquid transportation in microfluidic systems and biomedical applications, high-adhesion surperhydrophobic surfaces become increasingly important. Recently, graphene has attracted much attention due to its unique single atom-layer structure, which contributes to its wide applications in nanoelectronics, sensing devices, energy storage, and even tissue engineering. For example, graphene has proven to be a promising biocompatible scaffold that could accelerate the specific differentiation of human mesenchymal stem cells (hMSCs) into bone cells. In this case, the cell adhesion on the graphene surface plays a critical role in the long-term differentiation; therefore, a precise control of the surface wettability of graphene becomes a significant issue. Generally, superhydrophobic graphene substrates could be fabricated by using an irregular stack of graphene oxides (GO) prepared by chemical oxidation of graphite. In this procedure, to reduce the surface energy, the hydrophilic oxygen-containing groups on the GO surface have to be removed beforehand. To the best of our knowledge, modulation of the wetting property of graphene by micro-/nanostructuring and simultaneous control of chemical composition have not been realized. Moreover, despite the pioneering biomimetic fabrications of periodic micro-/nanostructures based on a wide range of materials, a bioinspired graphene surface with properties comparable to natural surfaces has not been reported yet. [a] J.-N. Wang, R. -Q. Shao, Dr. Y. -L. Zhang, L. Guo, Prof. H. -B. Sun State Key Laboratory on Integrated Optoelectronics College of Electronic Science and Engineering Jilin University 2699 Qianjin Street, Changchun 130012 (P. R. China) Fax: (+86)431-85168281 E-mail : yonglaizhang@jlu.edu. cn hbsun@jlu.edu. cn [b] H. -B. Jiang, D. -X. Lu, Prof. H. -B. Sun College of Physics Jilin University 2699 Qianjin Street, Changchun 130012 (P. R. China) Supporting information for this article is available on the WWW under http://dx.doi. org/10.1002/asia.201100882.