EK-CER researchers have investigated the photocatalytic properties of hybrid nanostructures produced using butterfly wings coated with zinc oxide

Certain semiconductors like zinc oxide (ZnO) and titanium dioxide (TiO₂) can induce such chemical modifications: they absorb light, and the excitations produced can induce chemical transformations in the undesired substance dissolved in water (for example colorant residues, drugs traces, microplastic, etc.), this way reducing their harmful effects. Unfortunately, the physical properties of both of the above-mentioned materials mean that they can only be used for photocatalysis if illuminated with ultraviolet (UV) light. This makes their application for water treatment complicated and expensive. It could be very advantageous if their photocatalytic activity could be enhanced in the visible spectral range, making possible in this way the use of glass, or of transparent plastics.

One possible way to achieve this is the use of special nanostructures that can affect light propagation. These nanoarchitectures are knows as photonic crystals. Photonic crystals are special nanocomposites of two transparent materials with different optical properties (Fig. 1) in such a way that the arrangement of constituting materials is alternating regularly on a scale comparable with the wavelength of the light. This means that the light of certain wavelength cannot propagate in the photonic nanoarchitecture and is completely reflected from its surface.

This kind of nanocomposite can be produced in laboratory conditions, but the procedure is time consuming, highly trained operators are required and often harmful substances must be used. However, biologic evolution also ‘discovered’ this type of nanoarchitectures many million years ago. Photonic crystal type nanoarchitectures – built from chitin and air – occur on the wings of numerous butterfly species: these nanoarchitectures play a decisive role in the sexual communication of butterflies. The larvae of the butterflies are herbivorous, and frequently feed on the leaves of agricultural plants. These include the caterpillars of the Common Blue (Polyommatus icarus) (Fig. 2) bred by us, which feed on the leaves of common bird’s-foot trefoil (Lotus corniculatus). This means that by breeding butterflies it is possible to produce photonic nanoarchitectures in a cheap and environmentally friendly way. When breeding in the lab, one single breeding pair may have as many as 600 or 700 descendants.

Because one component of the nanostructure of butterflies is air, the effective surface of photonic nanoarchitecture is very large. This offers further advantages as the area of liquid-solid interaction is much larger when compared with a flat surface like glass.

With modern materials science methods such as atomic layer deposition (ALD), it is possible to coat the chitin based photonic nanoarchitectures with a conformal ZnO layer with the thickness of a few nanometers. With the thickness of the ZnO layer, it is possible to tune the color of the reflected light (Fig. 3).

Using butterfly wings covered by a few nanometers of ZnO, researchers of the ELKH Centre for Energy Research (EK-CER) in collaboration with the Lepidoptera Collection of the Hungarian Natural History Museum, have shown that the enhancement of the photocatalytic effects increases as the overlap between the reflectance of the butterfly wing and the absorption of the test substance (rhodamine B) dissolved in water increases. Researchers have demonstrated that the properly nanostructures ZnO may possess catalytic activity in the visible spectral range, too. The magnitude of this effect can be tuned by the well-chosen parameters of the biologic photonic nanoarchitecture can be used as a template (Fig. 4.).

This work was funded by the Priority Project 139/2021 of the Centre for Energy Research and Project no. TKP2021-NKTA-05 implemented with the support provided by the Ministry of Innovation and Technology of Hungary from the National Research, Development and Innovation Fund, financed under the TKP2021 funding scheme.

Piszter, G., Kertész, K., Nagy, G., Baji, Z., Horváth, Zs. E., Bálint, Zs., Pap, J. S., & Biró, L. P. (2022). Spectral tuning of biotemplated ZnO photonic nanoarchitectures for photocatalytic applications. Royal Society Open Science, 9(7). https://doi.org/10.1098/rsos.220090