prepared the negative replica of Au directly from cicada template and then used it as a mold to fabricate the positive replica of PMMA (polymethyl methacrylate) 30. Many researchers have made different approaches to replicate the negative replica of cicada wings nano-structure such as metal deposition 30, nanoimprint lithography 31, 32, and low-surface-energy resin 33. Therefore, on large scale existing of these two dimensional nano-nipple arrays structure of cicada wing was found to be a most useful natural biotemplate during the replication process. Chitin, a crystalline polymer with a high Young’s modulus of 7–9 GPa and high molecular weight, is the most important component of the cicada wing, and has a key role in preserving the original nanostructure of the wing surface 29. Among these biological prototypes, cicada wing is one of the most promising template due to its highly ordered hexagonal nano-nipple arrays structure, which plays an important role in reducing light reflection over a broad range of visible wavelength 24. In fact, nature provides us an astonishing variety of structures from biological system such as periodic structure of butterfly wings 21, 22, cicada wings 23, 24, green leaves 25, 26, the sea-mouse spines 27 and the insects compound eyes 28. However, it’s very challenging for new technology to design and produce such effective nano-architectures with high symmetry, while one can easily find these architectures in nature. To improve light absorption, these structures allow the incident light through cavities and reduces the optical loss due to the multiple light scattering and reflection. Moreover, it has been also reported that different micro-nano structures significantly enhances the light harvesting and photocatalytic property of TiO 2 such as nanotube arrays 17, ordered mesoporous structure 18, micro-hole arrays 19, inverse opal structures 20. Even though the above-mentioned methods partially enhance the photocatalytic property of TiO 2, yet still it limits their efficiency due to the thermal instability, lower redox potential of the photo-generated electrons and reduced electron-hole separation 11. These techniques involved doping of TiO 2 with metallic or nonmetallic elements 11, sensitizing TiO 2 with a second photoactive component such as ruthenium complex 13, quantum dots 14, organic dyes 15, and narrow bandgap semiconductors 16. Therefore, several techniques have been applied to exploit most of the energy spectrum (visible region) which is about 40% of the total energy 12. However, owing to their wide band gap (i.e., 3.2 eV for anatase and 3.0 eV for rutile), TiO 2 absorb mainly UV light which is only 4% of the entire solar spectrum on earth 11. It has potential applications in decomposition of organic pollutants 7, 8 water splitting 9 and photo energy conversion 10, etc. TiO 2 has been extensively used as a photocatalytic material due to its significant photostability, environmentally friendliness, high chemical stability, non-toxicity and low-cost 6. Photolysis of organic containments has paid a great attention in environmental cleaning and water purification 1, 2, 3, 4, 5. This work provides a new insight to design such a structure which may lead to a range of novel applications. Moreover, the biomorphic Ag-TiO 2 showed more absorption capability in the visible wavelength range. This high-performance photocatalytic activity of the biomorphic Ag-TiO 2 may be attributed to the nano-holes structure, localized surface plasmon resonance (LSPR) property of the Ag nanoparticles, and enhanced electron-hole separation. The biomorphic Ag-TiO 2 with nano-holes structure showed superior photocatalytic activity compared to the biomorphic TiO 2 and commercial Degussa P25. It was observed that the biomorphic Ag-TiO 2 showed remarkable photocatalytic activity by degradation of methyl blue (MB) under UV-vis light irradiation. The Ag nanoparticles (10 nm–25 nm) were homogeneously decorated on the surface and to the side wall of nano-holes structure. The nano-holes array structure was well maintained after calcination in air at 500 ☌. The negative replica of biomorphic TiO 2 with nano-holes structure has been effectively fabricated directly from nano-nipple arrays structure of cicada wings by using a simple, low-cost and highly effective sol-gel ultrasonic method.
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