Spectral tuning of biotemplated ZnO photonic nanoarchitectures for photocatalytic applications

Abstract

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, and moreover, to optimize charge carrier transfer. Further enhancement can be achieved by using photonic nanostructures, which exhibit photonic band gap (PBG). Structurally coloured 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 of ZnO on the wings of various butterfly species (Arhopala asopia, Hypochrysops polycletus, Morpho sulkowskyi, Polyommatus icarus) possessing structural colour 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.

Keywords: atomic layer deposition; biotemplating; butterfly wing; photocatalysis; photonic nanoarchitecture; structural colour.

Figure 1.

(a) Absorbance of ZnO [35] and Rh B [36], and the transmittance of the glass cuvette used for photodegradation measurements; (b) reflectance of butterfly wings conformally covered by ZnO thin films. The gray band marks the Rh B absorption range, while the dashed line indicates absorption maximum.

Figure 2.

Optical microscopy and spectral characterization of (a–e) Morpho sulkowskyi and (f–j) Hypochrysops polycletus wings in pristine state and after the conformal deposition of ZnO by ALD. (b), (g) 10 nm, (c), (h) 15 nm and (d), (i) 20 nm of ZnO layer thicknesses are shown. The corresponding reflectance spectra are shown in (e) and (j), respectively.

Figure 3.

Peak shift of the reflectance of different butterfly wings with the increasing thickness of the deposited ZnO layer. Hypochrysops polycletus, Morpho sulkowskyi and Polyommatus icarus are shown in pristine states and when 10–15–20 nm of ZnO coatings were deposited.

Figure 4.

Cross-sectional TEM and SEM images of the cover scales of Hypochrysops polycletus (a) before and (b) after 20 nm ZnO deposition. One may observe that neither the micrometer scale, nor the nanometer-scale structure was affected. (c) The conformal ZnO coverage of 20 nm is shown after the chitinous template was removed by oxidation in air for 3 h at 500°C.

Figure 5.

(a) Reaction rate versus time for bare glass, ZnO covered glass, pristine Morpho sulkowskyi wing and 15 nm ZnO covered M. sulkowskyi wing. (b) Absorption spectra of Rh B measured during the 120-minute-long photodecomposition measurement on the 15 nm ZnO-covered M. sulkowskyi wing.

Figure 6.

Reaction rate as a function of the overlap between the wing reflectance spectra of the biotemplated samples and the absorption spectrum of Rh B (figure 1b). Linear relationship was found between the overlap integral and the photocatalytic activity except for Arhopala asopia, where higher-than-expected reaction rate was measured, and thus it was left out from the fit.

Figure 7.

UV-visible absorption spectra recorded in the course of hydroquinone (H₂Q, 60 µM) photolysis in the presence of 15 nm ZnO-coated Morpho sulkowskyi wing. Conditions are identical to the photocatalytic conditions applied for Rh B.