Physical and optical properties of inverse opal CeO2 photonic crystals

Abstract

Inverse opal ceria (CeO2) films and powders, exhibiting three-dimensional ordered macroporous (3DOM) structures and a photonic band gap (PBG) in the visible region, were successfully fabricated using the colloidal crystal template approach. Colloidal crystals templates, comprising polymethylmethacrylate (PMMA) spheres of diameter ∼325 nm arranged on a face-centered cubic (fee) lattice, were prepared by self-assembly from aqueous colloidal suspensions of PMMA spheres. After drying, the interstitial spaces in the PMMA colloidal crystals were filled with a ceria sol-gel precursor, and then the resulting structure calcined at 400°C to remove the polymer template. The ceria inverse opals obtained were characterized by SEM, XRD, BET, porosity, and UV-vis transmittance measurements and showed fee ordering of macropores (diameter around 240 nm) within a CeO2 nanocrystal matrix. The CeO2 volume fraction in the inverse opals was 17-18 vol %, and its surface area was 51 m2 g-1. Both the PMMA colloidal crystals and CeO2 inverse opals behaved as 3-dimensional photonic crystals, with PBGs at 877 and 485 nm, respectively. Filling the macropores of the CeO2 inverse opal with solvent caused a redshift in the position of the PBG, with the magnitude of the shift being directly proportional to the refractive index of the solvent. Refractive index sensing with a sensitivity of n = 0.001 or better is achievable using inverse opal CeO2 thin films. Inverse opal CeO2 powders showed improved thermal stability at 800°C compared to non-networked ceria nanoparticles of similar initial crystallite size and surface area, suggesting that inverse opal architectures may be useful in applications where retention of large surface area during high temperature operation is important (e.g., heterogeneous catalysis). © 2008 American Chemical Society.