Slow photon photocatalytic enhancement of H2 production in TaON inverse opal photonic crystals

Abstract

In this study, tantalum oxynitride inverse opal (TaON IO) photonic crystals with different macropore diameters (D) were synthesized in a two-step process involving colloidal crystal templating and thermal nitridation, then applied as visible light-driven photocatalysts for hydrogen production. The TaON inverse opals showed photonic band gaps (PBGs) at visible wavelengths, with the PBG position redshifting as the diameter of the macropores increased in accordance with a modified Bragg's law expression. By aligning the electronic absorption edge of TaON (Eg ∼ 2.4 eV) with the blue edge (short-wavelength side) or red edge (long-wavelength side) of the PBGs, slow photon enhancement of photocatalytic H2 generation was realized. H2 production tests conducted in 10 vol% methanol containing H₂PtCl6 under Xe lamp irradiation (150 W) showed that the hydrogen production rate was enhanced by ∼1.3–1.4 times when the TaON absorption edge (∼510 nm) aligned with blue edge of the PBG (0.3421 mmol g−1 h−1) or the red edge of the PBG (0.3254 mmol g−1 h−1) in the inverse opals. The red edge enhancement was due to increased light absorption and charge carrier generation in TaON, whereas the blue edge effect was likely due to suppression of electron-hole pair recombination. Results demonstrate that photonic crystal engineering to exploit slow photon effects is a viable approach for boosting photocatalytic hydrogen production rates.