Operando spectroscopy for functional materials
Solar Fuels
Solar hydrogen generation from water represents a compelling component of a sustainable future energy portfolio. Recently, chemically-robust heptazine-based polymers known as graphitic carbon nitrides (g-C3N4) have emerged as promising photocatalysts to drive hydrogen evolution with visible light, while withstanding harsh catalytic environments.
The Schlenker group has undergone various experimental studies of these heptazine-based catalysts, including kinetic studies of carbon nitride and mechanistic studies of model molecular units.
Photochemistry of metal-free solar water splitting
Using visible and NIR ultrafast transient absorption, we observed the first direct spectroscopic characterization of the photoinduced electron transfer cascade between graphitic and exfoliated carbon nitride that results in a near doubling of the hydrogen evolution rate. This correlation between electron transfer and photocatalytic activity provides unique new insight into structural modifications aimed at controlling charge separation dynamics to improve the activity of carbon-based photocatalysts.
Mechanistic photocatalytic studies using model compounds
To gain mechanistic understanding of heptazine-based photochemistry, we synthesized and studied TAHz, a model molecular photocatalyst chemically related to carbon nitride. On the basis of time-resolved photoluminescence (TR-PL) spectroscopy, we kinetically reveal a new feature that emerges in aqueous dispersions of TAHz. Using global target analysis, we spectrally and kinetically resolve the new emission feature to be blue shifted from the steady-state luminescence, and observe a fast decay component exhibiting a kinetic isotope effect (KIE) of 2.9 in H2O versus D2O, not observed in the steady-state PL. From ab initio electronic-structure calculations, we attribute this new PL peak to the fluorescence of an upper excited state of mixed nπ*/ππ* character. In water, the KIE suggests the excited state is quenched by proton-coupled electron transfer, liberating hydroxyl radicals that we detect using terephthalic acid. Our findings are consistent with recent theoretical predictions that heptazine-based photocatalysts can participate in proton-coupled electron transfer with H2O.
