Structural, Optical, and Catalytic Support Properties of γ‑Al2O3 Inverse Opals
journal contributionposted on 26.03.2015, 00:00 by Geoffrey I. N. Waterhouse, Wan-Ting Chen, Andrew Chan, Haishun Jin, Dongxiao Sun-Waterhouse, Bruce C. C. Cowie
Colloidal crystal templating is a versatile and inexpensive method for the fabrication of 3-dimensional photonic crystals. Here we describe the successful use of the method to fabricate γ-alumina (γ-Al2O3) inverse opal thin films and powders, possessing pseudo photonic bandgaps (PBGs) along the  direction at visible wavelengths. The optical properties of γ-Al2O3 inverse opal films were investigated in detail for the first time and closely obeyed a modified Bragg’s law expression. The PBGs red-shifted on immersion in organic solvents, with the magnitude of the shift being directly proportional to the solvent refractive index. Calcination of the γ-Al2O3 inverse opals (BET area 250–275 m2 g–1) at temperatures from 550 to 1200 °C resulted in the stepwise transformation γ-Al2O3 → δ-Al2O3 → θ-Al2O3 → α-Al2O3. The onset temperatures for the latter polymorphic transitions were ca. 50–100 °C higher for Al2O3 inverse opals compared to a sol–gel alumina nanopowder, suggesting that the inverse opal architecture imparts sintering resistance. Au/γ-Al2O3 catalysts synthesized using γ-Al2O3 inverse opal supports demonstrated excellent activity for CO oxidation, with 69% CO conversion being achieved at 20 °C and near-complete conversion at 150 °C. The hierarchical porosity and high specific surface areas of γ-Al2O3 inverse opal powders make them near ideal supports for catalytic applications that traditionally utilize γ-alumina.