Control of Superhydrophilic and Superhydrophobic Graphene Interface

Control of Superhydrophilic and Superhydrophobic Graphene Interface SUBJECT AREAS: MECHANICAL AND STRUCTURAL PROPERTIES AND DEVICES SURFACE CHEMISTRY Jing Dong1, Zhaohui Yao1, Tianzhong Yang2, Lili Jiang2 & Chengmin Shen2 1 School of Aerospace, Tsinghua University, Beijing 100084, PR China, 2Beijing National Laboratory of Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, PR China. MECHANICAL ENGINEERING APPLIED PHYSICS Received 22 January 2013 Accepted 3 April 2013 Published 25 April 2013 Correspondence and requests for materials should be addressed to Z.H.Y. (yaozh@ tsinghua.edu.cn) Superhydrophobic and superhydrophilic properties of chemically-modified graphene have been achieved in larger-area vertically aligned few-layer graphene nanosheets (FLGs), prepared on Si (111) substrate by microwave plasma chemical vapor deposition (MPCVD). Furthermore, in order to enhance wettability, silicon wafers with microstructures were fabricated, on which graphene nanosheets were grown and modified by a chemical method to form hydrophilic and hydrophobic structures. A superhydrophilic graphene surface (contact angle 06) and a superhydrophobic graphene surface (contact angle 152.06) were obtained. The results indicate that the microstructured silicon enhances the hydrophilic and hydrophobic wettabilities significantly. M any plants’ leaves have a special function of self-cleaning, especially the lotus, as has been noted for thousands of years. We call this kind of phenomenon the ‘Lotus effect.’ A leaf surface that has the ‘Lotus effect’ is superhydrophobic1, which indicates that the contact angle (CA) with a water drop is larger than 150u. One important feature of such a surface is anti-contamination, which means that contamination can be easily washed away by liquid. Barthlot and Neihuis2,3 studied the microstructure of lotus leaves and concluded that a lotus leaf’s mastoid structure in micrometer scale is the main reason for the lotus effect. Superhydrophobic and superhydrophilic surfaces are very useful in many applications, such as waterproof surfaces, anti-contamination surfaces, coatings and biomedical devices. Graphene is a two-dimensional crystal, which has been regarded as unstable4–6. In 2004, the discovery of graphene demonstrated that a single sheet of carbon atoms is stable at room temperature7,8. Graphene has attracted great interest because it has many extreme properties, such as mechanical stiffness9, high operating bandwidth10, and high thermal11 and electrical12 conductivity. In the past decade, different qualities of graphene were prepared through various routes, including mechanical exfoliation6, epitaxial growth on silicon carbide13, epitaxial growth on single crystal metal substrates14, chemical vapor deposition (CVD)15 and reduction of graphite oxide (GO)16. Pristine graphene is similar to a natural lotus leaf in that it exhibits hydrophobicity. But its wetting ability can be changed and enhanced by different modifications. First, multiple synthesis methods were used to get graphene with different microstructures, which enhanced the hydrophobic/hydrophilic properties; second, chemical modifications were presented to further improve the hydrophobic/hydrophilic properties. In previous studies, different wetting characteristics of graphene were obtained through chemical modification of Graphite Oxide (GO), aerogels of graphene and graphene-resin composite. GO has many hydrophilic functional groups such as carboxyl and hydroxyl, and further chemical modification can change these functional groups and get different interface properties. Moreover, GO has a flake-like structure in micron scale, which can be controlled to enhance the graphene’s particular wetting ability. Rafiee17 used a rapid thermal expansion route to expand and reduce the GO to prepare few-layer graphene. Then acetone or molecular water was attached to the surface of graphene sheets to change their wettability from superhydrophobic to superhydrophilic. Wong’s group reported that GO sheets were modified using octadecylamine (ODA) to form a superhydrophobic graphene surface18. Zhao et al., synthesized a superhydrophobic composite which has special structure in micron scale combining vinylidene fluoridehexafluoropropylene (PVDF-HFP) and graphene19. This material has a CA of 152u. Jin’s group20 also got a porous composite combining graphene and PVDF which is superhydrophobic. Liu’s group21 got a composite combining graphene and 2-methoxy-5-(20-ethyl-hexyloxy)-1,4-phenylene vinylene(MEH-PPV) particle, which has a CA of 152u. Graphene aerogel has a great specific surface area, which can enhance its wetting a lot. Lin22 got superhydrophobic graphene aerogel by rapid thermal expansion of GO, and then modified it by 1H,1H-2H,2H perfluorodecyl-trichlorosilane. The CA of graphene aerogels can be larger than 160u. SCIENTIFIC REPORTS | 3 : 1733 | DOI: 10.1038/srep01733 1 www.nature.com/scientificreports Although some research about superhydrophilic and superhydrophobic graphene was presented in recent years, there is little research about the wetting property of graphene synthesized by the MPCVD method. MPCVD is commonly used in industry, since it can prepare wafer-size homogeneous samples. MPCVD is used to synthesize fewlayer vertical graphene nanosheets on different kinds of bases, such as silicon23, nickel and titanium24. Maslesevic23 has synthesized vertical few-layer graphene by the MPCVD method, and his experiment showed that the graphene synthesized by MPCVD has a great field emission property. The microstructure of MPCVD graphene has vertical nanosheets, sharp edges and a large surface area, which may significantly improve the surface’s wetting ability. But no one has reported the wetting property of MPCVD graphene. Moreover, there is little research about the wettability of graphene on a patterned surface with microstructure. It has been reported that a silicon surface with micropillars has much better hydrophobicity than a smooth silicon surface25. Graphene synthesized on a silicon microstructure may have better wettability than graphene synthesized on a smooth silicon surface. Although MPCVD is a well developed technique and has been used by other groups, the recent investigations of vertical graphene prepared by MPCVD mainly focused on its field emission properties. The superhydrophilic and superhydrophobic properties of vertical graphene on the silicon surface have not been reported. In this paper, we report the control the superhydrophilic and superhydrophobic property of graphene on a wafer-size silicon surface. This may expand the application potential of graphene. Vertically aligned few-layer graphene nanosheets (FLGs) were prepared by microwave plasma chemical vapor deposition on a patterned silicon substrate. Then the surface of the FLGs was modified using a chemical method to get better superhydrophilic and superhydrophobic performance. (...truncated)