Size-dependent exciton dynamics in TADF nanoparticles for efficient CO₂ photoreduction

Thermally activated delayed fluorescence (TADF) nanoparticles present a promising platform for solar CO₂ conversion owing to their long-lived excited states and favorable redox potentials. Here, we report the synthesis and characterization of TAPC : 3TPYMB nanoparticles with controlled diameters (40-211 nm) via a mini-emulsion method, and investigate the correlation between nanoscale morphology and exciton dynamics through steady-state and transient absorption spectroscopy. Medium-sized (∼133.7 nm) nanoparticles exhibited maximum CO production (17.06 ± 0.36 μmol) under visible light irradiation, representing 13-fold and 4-fold enhancements relative to ∼40.34 nm (1.29 ± 0.51 μmol) and ∼211.4 nm (4.45 ± 0.83 μmol) nanoparticles, respectively. In the medium-sized nanoparticles, surface-mediated non-radiative decay and bulk charge recombination are simultaneously suppressed, resulting in a maximum photocurrent density (6.43 μA cm^-2) and charge-transfer exciton dissociation. The size-dependent photocatalytic activity originates from the competition between surface and bulk energy loss mechanisms; small nanoparticles exhibit dominant surface quenching due to high surface-to-volume ratios, while large particles suffer from increased bulk recombination during extended charge migration. This work establishes a quantitative relationship of nanoparticle size, exciton dissociation, and catalytic activity in TADF-exciplex systems, providing a promising design principle for next-generation nanostructured photocatalysts in solar fuel production.