As the rate-determining half reaction (OER) in the overall CO ₂ photoreduction, the four-electron-involving OER on S-vacancy sites was favored, therefore facilitating the hole-elimination/proton release and unleashing the CO ₂ reduction half reaction.

Graphitic carbon nitride has long been considered incapable of splitting water molecules into hydrogen and oxygen without adding small molecule organics despite the fact that the visible-light response and proper band structure fulfills the proper energy requirements to evolve oxygen. Herein, through in-situ observations of a collective C = O bonding, we identify the long-hidden bottleneck of photocatalytic overall water splitting on a single-phased g-C₃N₄ catalyst via fluorination. As carbon sites are occupied with surface fluorine atoms, intermediate C=O bonding is vastly minimized on the surface and an order-of-magnitude improved H₂ evolution rate compared to the pristine g-C₃N₄ catalyst and continuous O₂ evolution is achieved. Density functional theory calculations suggest an optimized oxygen evolution reaction pathway on neighboring N atoms by C–F interaction, which effectively avoids the excessively strong C-O interaction or weak N-O interaction on the pristine g-C₃N₄.

The surge in anthropogenic CO₂ emissions from fossil fuel dependence demands innovative solutions, such as artificial photosynthesis, to convert CO₂ into value-added products. Unraveling the CO₂ photoreduction mechanism at the molecular level is vital for developing high-performance photocatalysts. Here we show kinetic isotope effect evidence for the contested protonation pathway for CO₂ photoreduction on TiO₂ nanoparticles, which challenges the long-held assumption of electron-initiated activation. Employing isotopically labeled H₂O/D₂O and in-situ diffuse reflectance infrared Fourier transform spectroscopy, we observe H⁺/D⁺-protonated intermediates on TiO₂ nanoparticles and capture their inverse decay kinetic isotope effect. Our findings significantly broaden our understanding of the CO₂ uptake mechanism in semiconductor photocatalysts.

Direct solar-to-hydrogen conversion from pure water using all-organic heterogeneous catalysts remains elusive. The challenges are twofold: (i) full-band low-frequent photons in the solar spectrum cannot be harnessed into a unified S₁ excited state for water-splitting based on the common Kasha-allowed S₀ → S₁ excitation; (ii) the H⁺ → H₂ evolution suffers the high overpotential on pristine organic surfaces. Here, we report an organic molecular crystal nanobelt through the self-assembly of spin-one open-shell perylene diimide diradical anions (:PDI^2-) and their tautomeric spin-zero closed-shell quinoid isomers (PDI^2-). The self-assembled :PDI^2-/PDI^2- crystal nanobelt alters the spin-dependent excitation evolution, leading to spin-allowed S₀S₁ → ¹(TT) → T₁ + T₁ singlet fission under visible-light (420 nm~700 nm) and a spin-forbidden S₀ → T₁ transition under near-infrared (700 nm~1100 nm) within spin-hybrid chromophores. With a triplet-triplet annihilation upconversion, a newly formed S₁ excited state on the diradical-quinoid hybrid induces the H⁺ reduction through a favorable hydrophilic diradical-mediated electron transfer, which enables simultaneous H₂ and O₂ production from pure water with an average apparent quantum yield over 1.5% under the visible to near-infrared solar spectrum.