Engineering of Broadband Nanoporous Semiconductor Photonic Crystals for Visible-Light-Driven Photocatalysis

A new class of semiconductor photonic crystals composed of titanium dioxide (TiO2)-functionalized nanoporous anodic alumina (NAA) broadband-distributed Bragg reflectors (BDBRs) for visible-light-driven photocatalysis is presented. NAA-BDBRs produced by double exponential pulse anodization (DEPA) show well-resolved, spectrally tunable, broad photonic stop bands (PSBs), the width of which can be precisely tuned from 70 +/- 6 to 153 +/- 9 nm (in air) by progressive modification of the anodization period in the input DEPA profile. Photocatalytic efficiency of TiO2-NAA-BDBRs with tunable PSB width upon visible-NIR illumination is studied using three model photodegradation reactions of organics with absorbance bands across the visible spectral regions. Analysis of these reactions allows us to elucidate the interplay of spectral distance between red edge of TiO2-NAA-BDBRs' PSB, electronic bandgap, and absorbance band of model organics in harnessing visible photons for photocatalysis. Photodegradation reaction efficiency is optimal when the PSB's red edge is spectrally close to the electronic bandgap of the functional semiconductor coating. Photocatalytic performance decreases dramatically when the red edge of the PSB is shifted toward visible wavelengths. However, a photocatalytic recovery is observed when the PSB's red edge is judiciously positioned within the proximity of the absorption band of model organics, indicating that TiO2-NAA-BDBRs can harness visible electromagnetic waves to speed up photocatalytic reactions by drastically slowing the group velocity of incident photons at specific spectral regions. Our advances provide new opportunities to better understand and engineer light-matter interactions for photocatalysis, using TiO2-NAA-BDBRs as model nanoporous semiconductor platforms. These high-performing photocatalysts could find broad applicability in visible-NIR light harvesting for environmental remediation, green energy generation, and chemical synthesis.