Controllable Fabrication of Superhydrophobic Micro / nano Hierarchical Surface and Its Application for Energy Harvesting

Superhydrophobic surfaces have been widely used in antifouling, drag reduction, oil-water separation, and biomedicine applications. New triboelectric nanogenerator(TENG) applications have recently emerged for harvesting blue energy. However, large-area and controllable fabrication of superhydrophobic surfaces remains challenging. When a superhydrophobic surface is used in a TENG, the effects of its structure and morphology on the triboelectric output remain unknown. Here, a large-area, controllable, and scalable fabrication strategy is developed to manufacture superhydrophobic micro / nano hierarchical surface, and its application in a water-solid contact TENG is reported. A one-step photolithography and molding process is proposed to fabricate a micro / nano hierarchical surface composed of micropillars and nanopillars. The micropillars were formed using a lithographic mask equipped with a microhole array, and the nanopillars duplicated the structure of a V-shaped anodic aluminum oxide(AAO) mold. The structure of the micro / nano hierarchical surface can be controlled accurately and conveniently by regulating the size of the lithographic mask and AAO mold. Using scanning electron microscopy(SEM) observation, a series of micro / nano hierarchical structures were successfully fabricated. The micropillars and nanopillars showed high accuracy and good consistency. Subsequently, the hydrophobicity of the micro / nano hierarchical surface was analyzed using a contact-angle measurement system. The results indicate that, compared with the single-level nano surface and the single-level micro surface, the micro / nano hierarchical surface exhibits superior hydrophobic properties and achieves superhydrophobicity. This is because the surface exhibits a Cassie-Baxter wetting state, and the constructed micro / nano hierarchical structures can inhibit water droplets from immersing into the gap and obtain superhydrophobic properties. For the micro / nano hierarchical surface, as the center distance of the micropillars increased, the contact area between the surface and the water droplet decreased; thus, the hydrophobic performance was further enhanced. Specifically, when the center distance of the lithographic mask was 28 mu m, the fabricated micro / nano hierarchical surface showed a static contact angle of 158 degrees and a sliding angle of 2 degrees. Finally, the micro / nano hierarchical surface was applied for water energy harvesting. The effect of the surface structure on the output performance of the water-solid contact TENG was analyzed using a testing setup with water continuously dripping onto the surface. The results show that the constructed micro / nano hierarchical surface generates a more robust triboelectric output compared to the flat surface and the single-level microsurface. This outstanding output performance originated from the higher charge density caused by the larger friction surface and superhydrophobic properties. For the micro / nano hierarchical surface, as the center distance of the micropillars increased, the triboelectric output was further enhanced owing to the more hydrophobic structure. Specifically, at a flow rate of 8 mL / s, the output voltage and short-circuit current of the water-solid contact TENG based on the micro / nano hierarchical surface with a mask center distance of 28 mu m are approximately 46 V and 6.3 mu A, respectively. In addition, the effect of the flow rate on the output performance was studied. As the flow rate increased, the output peak value and output frequency increased, which was attributed to the sufficient friction and faster contact-separation process at high flow rates. Therefore, the proposed photolithography and molding process realizes the large-area and controllable superhydrophobic micro / nano hierarchical surface fabrication. Applying the micro / nano hierarchical surface in water-solid contact TENG demonstrates its suitable potential applications in water energy harvesting and self-powered sensor devices.