The photocatalytic activity of a flat surface can be increased by micro- and nanostructuring the interface to increase the area of the contact surface between the photocatalyst and the solute, moreover, optimize charge carrier transfer. Further enhancement can be achieved by using photonic nanostructures, which exhibit photonic band gap (PBG). Structurally colored butterfly wings offer a rich “library” of PBGs in the visible spectral range which can be used as naturally tuned sample sets for biotemplating. We used conformal atomic layer deposition (ALD) of ZnO on the wings of various butterfly species (Arhopala asopia, Hypochrysops polycletus, Morpho sulkowskyi, Polyommatus icarus) possessing structural color extending from the near UV to the blue wavelength range, to test the effects arising from the nanostructured surfaces and from the presence of different types of PBGs. Aqueous solutions of rhodamine B were used to test the enhancement of photocatalytic activity that was found for all ZnO coated butterfly wings. The best reaction rate of decomposing rhodamine B when illuminated with visible light was found in 15 nm ZnO coated M. sulkowskyi wing the reflectance of which had the highest overlap with the absorption band of the dye and had the highest reflectance intensity.

1. Microscopy

Optical microscope images were taken with the ×20 objective of a Nikon Eclipse LV150N device using extended depth of focus (EDF) mode which resulted high depth of field images of the otherwise significantly textured butterfly wing surfaces. The butterfly wing samples were prepared for electron microscopy using standard techniques. The samples were examined via scanning electron microscopy (SEM) and cross-sectional transmission electron microscopy (TEM) imaging using Thermo Fisher Scientific Scios 2 DualBeam (Waltham, MA, USA) and Philips CM20 (Eindhoven, The Netherlands) systems, respectively.

2. Reflectance spectroscopy

Optical reflectance measurements were carried out using a fiber optic Avantes (Apeldoorn, The Netherlands) system consisting of an AvaSpec-HERO spectrophotometer, an AvaLight-DH-SBAL stabilized UV-Vis light source, an integrating sphere (AvaSphere-30-REFL) for light collection, and a WS-2 diffuse tile as a reference. Reflectance of pristine and ZnO coated wing pieces were measured in 200-950 nm wavelength range. Data analysis was performed using OriginPro 2021 (OriginLab Corporation, Northampton, MA, USA) software.

3. Photocatalytic degradation of rhodamine B (Rh B)

The photocatalytic activity was evaluated based on the removal of Rh B (15 µM) from its unbuffered solution upon illumination using MilliQ ultrapure water as solvent. The ZnO samples were placed vertically in 20 mL of the Rh B solution in a glass cuvette with magnetic stirring at room temperature. A heat free 300 W xenon lamp (ASAHI MAX-301), with fiber optics was applied as light source supplying an adjustable square-shaped beam (1.5×1.5 cm square-shaped illumination). The distance between the lamp and the catalytic surface immersed in the cuvette was 6 cm (corresponding to ~100 mW/cm2 light power). The rate of degradation was followed by an Agilent Cary 60 UV–Vis spectrophotometer equipped with an immersion probe (l = 1 cm) that was placed inside the solution out of the illumination area. Spectra were collected every fifth minute during the 2 hours of reaction time. Degradation of Rh B was followed at the wavelength of its absorption maximum at 554 nm. Conversion was calculated as (A0-Ai)/A0, where A0 is the initial absorbance of Rh B at 554 nm, and Ai is the absorbance at a given point in the reaction. Soaking tests of the surfaces in Rh B solution before their use ruled out any detectable role of initial adsorption of the dye in the change of absorbance (note the high reaction volume over catalytic surface area ratio).