Formation of 3D interconnectively macro/mesoporous TiO₂ sponges through gelation of lotus root starch toward CO₂ photoreduction into hydrocarbon fuels

A particular TiO₂ sponge, consisting of macroporous framework with interconnected mesoporous channels, was fabricated through a co-gelation of lotus root starch (LRS) with TiO₂ precursor, followed by lyophilization and subsequent calcination. This strategy advantageously inherits both the traditional hard-templating technique for well-defined 3D predesigned macroporous architecture and soft-templating techniques for interpore connectivity. The resulting TiO₂ sponge exhibits about a 2.60 fold improvement in CO₂ photoconversion rate (CH₄: 5.13 ppm h^-1) compared to the referred TiO₂ (1.97 ppm h^-1) formed in the absence of the LRS. The generation rate of CH₄ over macro/mesoporous TiO₂ sponge could be further significantly enhanced to 11.95 ppm h^-1 by co-loading Pt (0.9 wt%) and Cu (1.7 wt%) as co-catalysts by improvement of the separations of the photogenerated electron-hole pairs. The higher photocatalytic activity of the macro/mesoporous TiO₂ sponge can be attributed to the following three reasons: (1) macroporous architecture favors gas diffusion of the reactants and the products; (2) macroporous architecture also promotes the multiple-reflection effect occurring inside the interior macrocavities, which enables trapping (or harvest) the incident light in the photocatalyst for a longer duration and bring forth more opportunities for light absorption; and (3) the mesoporous structure enhances gas capture/adsorption of the reactants and provides more reaction sites.