Fabrication of 3D biomimetic composite coating with broadband antireflection, superhydrophilicity, and double p-n heterojunctions

The traditional single material with two-dimensional (2D) biomimetic moth-eye structures is limited by its narrowband antireflection and single functional capability. To overcome these disadvantages, we exploited wet etching and hydrothermal synthesis coupled with chemical oxidation for fabricating a three-dimensional (3D) biomimetic moth-eye coating with ternary materials (polypyrrole nanoparticles, TiO2 nanorods, and Si micropyramids, i.e., PPy/TiO2/Si-p). This coating reduced the reflectivity to <4% at wavelengths ranging from 200 to 2,300 nm and exhibited remarkable superhydrophilicity with a low water contact angle of 1.8 degrees. Moreover, the composite coating had double p-n heterojunctions, allowing the high-efficiency separation of photogenerated carriers. The photo-current density of PPy/TiO2/Si-p was more than three times higher than that of TiO2/Si-p at a positive potential of 1.5 V. The proposed method provides a means to enhance solar energy conversion.