Spectral Engineering of Hybrid Biotemplated Photonic/Photocatalytic Nanoarchitectures

Abstract

Solar radiation is a cheap and abundant energy for water remediation, hydrogen generation by water splitting, and CO₂ reduction. Supported photocatalysts have to be tuned to the pollutants to be eliminated. Spectral engineering may be a handy tool to increase the efficiency or the selectivity of these. Photonic nanoarchitectures of biological origin with hierarchical organization from nanometers to centimeters are candidates for such applications. We used the blue wing surface of laboratory-reared male Polyommatus icarus butterflies in combination with atomic layer deposition (ALD) of conformal ZnO coating and octahedral Cu₂O nanoparticles (NP) to explore the possibilities of engineering the optical and catalytic properties of hybrid photonic nanoarchitectures. The samples were characterized by UV-Vis spectroscopy and optical and scanning electron microscopy. Their photocatalytic performance was benchmarked by comparing the initial decomposition rates of rhodamine B. Cu₂O NPs alone or on the butterfly wings, covered by a 5 nm thick layer of ZnO, showed poor performance. Butterfly wings, or ZnO coated butterfly wings with 15 nm ALD layer showed a 3 to 3.5 times enhancement as compared to bare glass. The best performance of almost 4.3 times increase was obtained for the wings conformally coated with 15 nm ZnO, deposited with Cu₂O NPs, followed by conformal coating with an additional 5 nm of ZnO by ALD. This enhanced efficiency is associated with slow light effects on the red edge of the reflectance maximum of the photonic nanoarchitectures and with enhanced carrier separation through the n-type ZnO and the p-type Cu₂O heterojunction. Properly chosen biologic photonic nanoarchitectures in combination with carefully selected photocatalyst(s) can significantly increase the photodegradation of pollutants in water under visible light illumination.

Keywords: ALD; Cu2O nanoparticles; UV-visible spectroscopy; ZnO; biotemplating; butterfly wing; hybrid photonic nanoarchitecture; p-n heterojunction; photocatalysis; spectral engineering.

Figure 1

Male Polyommatus icarus butterfly and typical sample structures used. (a) Dorsal wing surfaces of a male P. icarus specimen. Note the homogeneous blue coloration over the entire surface of the four wings; (b) Type-1 sample structure without 15 nm ZnO layer on the butterfly wing before the Cu₂O deposition; (c) Type-2 sample structure with 15 nm ZnO layer deposited by ALD on the butterfly wing before the Cu₂O deposition. Note that the various layer thicknesses are not drawn to scale.

Figure 2

Cu₂O nanoparticles stored in ethanol. (a) SEM micrograph of the nanoparticles drop dried on Si; (b) Extinction of the nanoparticles suspended in ethanol.

Figure 3

The effect of ethanol pretreatment and the reproducibility of the spectral measurements exemplified by the four wings of the same butterfly. (a) The reflectances of the four glass mounted wings measured before the ethanol pretreatment are shown, while in (b), the spectra of the same four wings, remeasured next day, are shown after two of the wings have undergone the ethanol pretreatment.

Figure 4

The modification of the optical properties of type-1 samples after the application of 120 µL of Cu₂O sol onto the wing. (a) Two wings after PMMA gluing onto glass, two wings after the ethanol pretreatment followed by mounting of the PTFE frame. (b) The ethanol-pretreated wings received the 3 × 40 µL Cu₂O sol.

Figure 5

Modifications of the optical properties of the type-1 and of the type-2 samples after the application of the increasing amounts of the Cu₂O sol onto the wing. (a) Peak wavelength and (b) peak intensity values are shown with their linear fits to guide the eye.

Figure 6

The effect of the deposited 15 nm thick ZnO layer onto the wings of P. icarus males.

Figure 7

Change in the reflectance of two P. icarus males’ wings conformally covered by 15 nm ZnO after the application of 120 µL of Cu₂O sol. (a) Reflectance of the 15 nm ZnO coated wings and (b) reflectance of the same wings after the application of 120 µL Cu₂O sol.

Figure 8

Optical and SEM micrographs of type-1 and type-2 samples after the application of 120 µL of Cu₂O sol. (a) Type-1 sample, optical microscope image recorded with crossed polarizers; (b) type-1 sample image recorded on a focus stacking optical microscope using partially crossed polarizers; (c) type-2 sample, optical microscope image recorded with crossed polarizers; (d) type-2 sample image recorded on a focus stacking optical microscope using partially crossed polarizers; (e) SEM micrograph of a type-1 sample; (f) SEM micrograph of a sample type-2; (g) detail of image in (e), note the clustering between the ridges; (h) detail of the image in (f), note the individual nanoparticles between the ridges marked by arrows.

Figure 9

Photographs (left) of the wing samples used in the photocatalytic experiments in as-prepared state and the corresponding reflectance spectra (right): (1) type-1 sample without ethanol pretreatment; (2) type-1 sample with ethanol pretreatment; (3) type-1 sample with 120 µL of Cu₂O sol drop dried; (4) type-2 sample with 120 µL of Cu₂O sol drop dried; (5) type-1 sample with 120 µL of Cu₂O sol drop dried, followed by the deposition of 5 nm of ZnO; (6) type-2 sample with 120 µL of Cu₂O sol drop dried, followed by the deposition of 5 nm of ZnO. The grey-shaded area marks the rhodamine B absorption band.

Figure 10

Absolute and relative reaction rates of the samples used to characterize the photocatalytic efficiency of the biotemplated photonic/photocatalytic nanoarchitectures. The value corresponding to bare glass was taken as unity for the calculation of the relative rates. The sample numbers on the bars are the same as those in Figure 9.