DOI: 10.5061/dryad.w9ghx3fr8
title: Spectral tuning of biotemplated ZnO photonic nanoarchitectures for photocatalytic applications

description: 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.
description: 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).