Optical Control of Selectivity of High Rate CO2 Photoreduction Via Interband- or Hot Electron Z-Scheme Reaction Pathways in Au-TiO2 Plasmonic Photonic Crystal Photocatalyst

Abstract Photonic crystals consisting of TiO2 nanotube arrays (PMTiNTs) with periodically modulated diameters were fabricated using a precise charge-controlled pulsed anodization technique. The PMTiNTs were decorated with gold nanoparticles (Au NPs) to form plasmonic photonic crystal photocatalysts (Au-PMTiNTs). A systematic study of CO2 photoreduction performance on as-prepared samples was conducted using different wavelengths and illumination sequences. A remarkable selectivity of the mechanism of CO2 photoreduction could be engineered by merely varying the spectral composition of the illumination sequence. Under AM1.5 G simulated sunlight (pathway#1), the Au-PMTiNTs produced methane (302 μmol g c a t . - 1  h−1) from CO2 with high selectivity (89.3 %). When also illuminated by a UV-poor white lamp (pathway#2), the Au-PMTiNTs produced formaldehyde (420 μmol g c a t . - 1  h−1) and carbon monoxide (323 μmol g c a t . - 1  h−1) with almost no methane evolved. We confirmed the photoreduction results by 13C isotope labeling experiments using GC MS. These results point to optical control of the selectivity of high-rate CO2 photoreduction through selection of one of two different mechanistic pathways. Pathway#1 implicates electron-hole pairs generated through interband transitions in TiO2 and Au as the primary active species responsible for reducing CO2 to methane. Pathway#2 involves excitation of both TiO2 and surface plasmons in Au. Hot electrons produced by plasmon damping and photogenerated holes in TiO2 proceed to reduce CO2 to HCHO and CO through a plasmonic Z-scheme.