Combining the lotus leaf effect with artificial photosynthesis: regeneration of underwater superhydrophobicity of hierarchical ZnO/Si surfaces by solar water splitting

Abstract

Fabrication of stable superhydrophobic surfaces in dynamic circumstances is a key issue for practical uses of non-wetting surfaces. However, superhydrophobic surfaces have finite lifetime in underwater conditions due to the diffusion of gas pockets into the water. To overcome this limited lifetime of underwater superhydrophobicity, this study introduces a novel method for regenerating a continuous air interlayer on superhydrophobic ZnO nanorod/Si micropost hierarchical structures (HRs) via the combination of two biomimetic properties of natural leaf: superhydrophobicity from the lotus leaf effect and solar water splitting from photosynthesis. The designed n/p junction in the ZnO/Si HRs allowed for highly stable gas interlayer in water and regeneration of the underwater superhydrophobicity due to the unique ability of the surface to capture and retain a stable gas layer. Furthermore, we developed a model to determine the optimum structural factors of hierarchical ZnO/Si surfaces that aid the formation of an air interlayer to completely regenerate the superhydrophobicity. We also verified that this model satisfactorily predicted the regeneration of underwater superhydrophobicity under various experimental conditions. The regenerative method developed in this work is expected to broaden the range of potential applications involving superhydrophobic surfaces and to create new opportunities for related technologies. NPG Asia Materials (2015) 7, e201; doi:10.1038/am.2015.74; published online 17 July 2015 INTRODUCTIONSuperhydrophobic surfaces that mimic lotus leaves have been extensively studied over the past several decades, and various micro/nanostructures have been developed to create bio-inspired superhydrophobic surfaces. [1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] In recent years, the non-wetting property of superhydrophobic surfaces submerged in water has attracted much attention because it has potential applications in drag reduction, anti-fouling, anti-corrosion, waterproof devices, microchannels, anti-icing and other non-wetting related applications. [16][17][18][19][20][21][22][23][24][25] Practical applications of non-wetting surfaces, however, have been impeded by the limited stability of their underwater superhydrophobicity. [26][27][28] According to the Cassie-Baxter model, the presence of an air interlayer on the submerged surface causes the non-wetting behavior, and thus, the stability of underwater superhydrophobicity is determined by the lifetime of the air (or gas) interlayer captured by the superhydrophobic surface. 29 However, the gas interlayer is unstable due to diffusion of the gas into the water. 30 According to previous studies, the gas diffusion rates are strongly affected by physical (hydrostatic pressure and micro/nanostructures of the surface) and chemical (surface energy) factors. 26,31

Bio-inspired dewetted surfaces based on SiC/Si interlocked structures for enhanced-underwater stability and regenerative-drag reduction capability

“…In this study we have developed new SiC/Si interlocked hierarchical structures using a carbothermal reduction based synthesis method 55 . Compared to previously reported structures, our surface drastically improved the lifetime of the air interlayer and showed the highest stability of underwater superhydrophobicity due to its unique networking structure 47 56 . According to our drag reduction measurements, our superhydrophobic SiC/Si hierarchical surface showed 56% drag reduction effect compared to a flat Si surface.…”

Superhydrophobic drag reduction in laminar flows: a critical review

“…This semi-active method was proven effective under near-realistic conditions, such as a very high liquid pressure (up to 7 atm) and a defective surface. Lee and Yong (2015) also adopted a similar hierarchically structured surface to regenerate a gas layer on SHPo surface. But, they used solar energy instead of electrical energy to initiate the electrochemical gas generation.…”

“…The design approach to help keeping the plastron on a SHPo surface includes making the surface resistant to a wetting transition (Carlborg et al 2008;Lee and Kim 2009;Barthlott et al 2010;Carlborg and van der Wijngaart 2011) and adding a built-in mechanism that replenishes the gas on the surface on demand Kim 2011b, 2012;Lee and Yong 2015). The approach can be categorized depending on whether or not external energy is used, i.e., passive versus active approach, as summarized in Fig.…”

Deciphering buried air phases on natural and bioinspired superhydrophobic surfaces using synchrotron radiation-based X-ray phase-contrast imaging

“…We expect that this study will deepen the understanding of the physical mechanisms of superhydrophobicity and open up new possibilities for the design of functional surfaces and bioinspired materials. [27][28][29][30][31] …”