Degradation of methylene blue by three-dimensionally ordered macroporous titania

Accelerated photodegradation of methylene blue (MB) over three-dimensionally ordered macroporous (3DOM) titania (pore sizes: 0.5 and 1 μm) is demonstrated. The catalysts were synthesized by colloidal crystal templating against polystyrene spheres using a metal alkoxide precursor. The 3DOM titania walls which are predominantly anatase (>98%) were decorated homogeneously with gold nanoparticles (5-7 nm) by pH-controlled precipitation of Au from HAuCl₄ using sodium hydroxide. A combination of powder X-ray diffraction (XRD), analytical transmission electron microscopy (ATEM) and X-ray photoelectron spectroscopy (XPS) confirmed the deposition of Au(O) on the 3DOM titania macroporous walls. Photocatalytic activity was monitored by following the degradation of MB with activity benchmarked against commercial P25 (Degussa) and powder titania prepared by hydrolysis of titanium ethoxide. Macroporous 3DOM titania with pore diameter 0.5 μm had the highest first-order rate constant of 0.042 min^-1 for decomposition of MB, compared to 0.025 min^-1 for P25 titania. Deposition of gold on the 3DOM titania surfaces decreased the reaction rate by covering the surface active sites.