Visible Light-Promoted Plasmon Resonance to Induce "Hot" Hole Transfer and Photothermal Conversion for Catalytic Oxidation

Titanium dioxide (TiO2) semiconductor photocatalysts were photosensitized to the visible spectrum with gold nanospheres (AuNSs) and gold nanorods (AuNRs) to study the ethanol photo-oxidation cycle, with an emphasis toward driving carbon-carbon (C-C) bond cleavage at low temperatures. The photocatalysts exhibited a localized surface plasmon resonance (SPR) that was harnessed to drive the complete photo-oxidation of formic acid (FA) and ethanol (EtOH) via augmented carrier generation/separation and photothermal conversion. Contributions of transverse and longitudinal localized SPR modes were decoupled by irradiating AuNSs-TiO2 and AuNRs-TiO2 with targeted wavelength ranges to probe their effects on plasmonically assisted photocatalytic oxidation of FA and EtOH. Photocatalytic performance was assessed by monitoring the yield of gaseous products during photo-oxidation experiments using a gas chromatography-mass spectrometry multiple headspace extraction (GC-MS-MHE) analysis method. The complete oxidation of EtOH to CO2 under visible-light irradiation was confirmed by GC-MS-MHE for both AuNSs and AuNRs on TiO2 at room temperature. Photothermal and local field enhancements were found to aid in selectively cleaving the C-C bond in EtOH to form FA, while FA was further oxidized to CO2 by plasmon-induced electron transfer mechanisms. Under visible-light (>420 nm) irradiation, carrier generation/separation, and photothermal conversion was achieved, resulting in the photogenerated "hot" holes driving the photo-oxidation primarily on the gold nanoparticles. Specifically, plasmonic enhancement by AuNR-TiO2 enhances EtOH oxidation, providing a method to selectively cleave C-C bonds.