Harnessing Slow Light in Optoelectronically Engineered Nanoporous Photonic Crystals for Visible Light-Enhanced Photocatalysis

Abstract

Spectrally tunable nanoporous anodic alumina distributed Bragg reflectors (NAA-DBRs) are modified with titanium dioxide (TiO2) coatings via atomic layer deposition and used as model optoelectronic platforms to harness slow light for photocatalysis under visible−NIR illumination. Photocatalytic breakdown of methylene blue (MB) with a visible absorbance band is used as a benchmark reaction to unveil the mechanism of slow light-enhanced photocatalysis in TiO2−NAA-DBRs with a tunable photonic stop band (PSB) and thickness of TiO2. Assessment of the optical arrangement between MB’s absorbance band and the PSB of TiO2−NAA-DBRs is used to identify and quantify slow light contributions in driving this model photocatalytic breakdown reaction. Our findings reveal that photodegradation rates rely on both the spectral position of PSB and thickness of the semiconductor. The performance of these photocatalysts is the maximum when the red edge of the PSB is spectrally close to the red or blue boundary of the MB’s absorbance band and to dramatically decrease within the absorbance maximum of MB due to light screening by dye molecules. It is also demonstrated that TiO2−NAA-DBRs featuring thicker photoactive TiO2 layers can harvest more efficiently incident slow light by generating extra pairs of charge carriers on the semiconductor coating’s surface. The crystallographic phase of TiO2 in the functional coatings is found to be critical in determining the performance of these model photocatalyst platforms, where the anatase phase provides ∼69% higher performance over its amorphous TiO2 form. This study provides opportunities toward the development of energy-efficient photocatalysts for environmental remediation and energy generation and other optoelectronic applications.