Cicada wings are transparent to visible light due to unique cone-like “nano-nipples” that comprise their nanostructure. Such antireflective structures could be valuable in solar cells, where minimizing reflectance increases the amount of sunlight that is transmitted through the cell’s surface cell for conversion to electricity.

While scientists have succeeded in replicating the cicada wing’s nanostructure using various techniques—including nanoimprint lithography, molding with low surface-energy florin polymer, molding with thermoplastic polymers, and atomic layer deposition—none of these methods is easily scalable to industrial production quantities.

Now, researchers at the Shanghai Jiao Tong University (SJTU), China, used real cicada wings as templates to demonstrate a potentially scalable sol-gel method to replicate the wings’ nanostructure in TiO2 that provides antireflective properties in the visible wavelength from a wide range of incidence angles.

Previous polymeric versions of these structures have been made, but they suffered from the polymers’ susceptibility to photochemical deformation when exposed to sunlight and high temperatures, as is required in a solar cell. Furthermore, polymeric replicas were antireflective only when the incident light was normal to the surface.  In contrast, “Our biomorphic TiO2 has an optimally graded refractive index and that ultimately leads to angle-dependent antireflective properties in the visible light range,” says Wang Zhang, associate professor at the State Key Lab of Metal Matrix Composites at SJTU. “Furthermore, even after high calcination at 500°C, the antireflective structures retain their morphology and high-performance antireflection properties. These qualities should enable the coatings to withstand harsh environments and make them suitable for long-term applications.”

Because cicada wings are hydrophobic, Zhang and his colleagues had to use an ethanol/water solution in place of the normal aqueous sol-gel solution to get the TiO2 to coat the cicada wings. The wings of commercially obtained cicadas were pretreated with NaOH, dipped into the Ti-precursor solution and left immersed for 8 h at 60°C. Following removal from the solution, cleaning, and drying, the researchers calcined the wings in vacuum at 500°C for 60 min to burn out the organic template material, leaving behind TiO2 with the surface structure of the cicada wing (biomorphic TiO2). The researchers also prepared nontemplated TiO2 for comparison.

Scanning electron microscopy (SEM) images of cicada wings show highly ordered assemblies of nano-nipples on the surfaces arranged hexagonally, with the average basal diameter of 140 nm, top diameter of 60 nm, top spacing (center to center) of 160 nm, and estimated nano-nipple height of 200 nm.  For the TiO2 replicas, SEM revealed an average basal diameter of 175 nm, a top diameter of 75 nm, a top spacing (center to center) of 250 nm, and a height of 230 nm. The nano-nipple array structure of the cicada wing was thus retained in the biomorphic TiO2 after calcination, albeit with some distortion due to the high temperature calcination. X-ray diffraction showed some loss of crystallinity in the TiO2 because of the presence of amorphous carbon, but the carbon was completely removed by calcination, leaving behind only anatase phase biomorphic TiO2.

Angle-resolved spectrometry in the visible wavelength region (450–750 nm) showed a significant decrease in reflectivity in the biomorphic TiO2 compared with the nontemplated TiO2. A gradual change from 1.4% to 7.8% reflectance was observed in the reflectance spectra of the biomorphic TiO2 as the angle of incidence was changed from normal to 45°. Lower reflectivity means more sunlight reaches the circuitry of the solar cell to be converted into electricity, resulting in higher conversion efficiency. This angle-dependent antireflective property is attributed to an optimized gradient refractive index between air and TiO2 through antireflective structures on the surface.

“This study demonstrates the functionality, effectiveness and usefulness of the cicada wing structures,” Zhang concludes. “Therefore, combining this work with the nanoimprint method, we believe production of biomorphic TiO2 could be expended to commercial levels rapidly.”

Yiquan Wu, an associate professor in the School of Engineering at Alfred University who was not involved in this project, called the research “amazing” work, and added: “The technique is simple and low-cost, and therefore can be easily translated to industrial-scale manufacturing. This is a great example of how people can learn from nature to develop new materials and structures with novel functionalities.”

Read the abstract in Applied Physics Letters.