Optical and Photocatalytic Properties of Three-Dimensionally Ordered Macroporous Ta2O5 and Ta3N5 Inverse Opals

Abstract

Colloidal crystal templating is a simple yet remarkably versatile synthetic strategy toward inverse opal (IO) photonic crystals for optical sensing and catalytic applications. Herein, we report the successful fabrication of tantalum (V) oxide, Ta2O5, inverse opal thin films and powders using the colloidal crystal templating method, utilizing poly(methyl methacrylate) (PMMA) colloidal crystals as sacrificial templates and TaCl5 as the tantalum source. The Ta2O5 IO thin films and powders showed structural color at ultraviolet (UV) and visible wavelengths, with the photonic band gap (PBG) position along the [111] direction increasing linearly with the diameter of macropores (D) in the inverse opals and also the refractive index of the medium filling the macropores, in excellent accord with a modified Bragg’s law expression. Thermal ammonolysis of the Ta2O5 inverse opals at 700 °C yielded well-ordered Ta3N5 IO films and powders possessing high specific surface areas (37 m2 g-1) and a semiconductor band gap of 2.0-2.1 eV. A Pt/Ta3N5 IO photocatalyst delivered a H2 production rate of ∼300 μmol g-1 h-1 in aqueous methanol (10 vol % MeOH) under visible-light irradiation (300 W Xe lamp, λ ≥ 420 nm), approximately twice that achieved using conventional Pt/Ta3N5 powder photocatalysts (161 μmol g-1 h-1, 8.4 m2 g-1). Results demonstrate that inverse opal engineering is an effective approach for realizing Ta2O5 IO thin films for sensing applications and Ta3N5 IOs with enhanced photocatalyst performance.