Bandgap-dependent Pt size on covalent organic frameworks: the COF structure outweighs the Pt cocatalyst size in...

Photocatalytic hydrogen evolution from water splitting using covalent organic frameworks (COFs) heavily relies on platinum (Pt) as a benchmark cocatalyst. However, it remains unclear whether the Pt cocatalyst size or the intrinsic electronic structure of the COF host plays a more decisive role in determining the photocatalytic activity. In this work, we investigated this question using three ketoenamine-linked COFs (TPPA, TPBD, and TPDT). We discovered a bandgap-dependent effect: the narrower-bandgap COF (TPPA) deposited larger Pt nanoparticles (3-5 nm), whereas the wider-bandgap COFs (TPBD and TPDT) deposited smaller ones (2-3 nm). Mechanistically, this effect is attributed to the differences in exciton binding energy across the COFs, which govern the charge separation kinetics during photodeposition. Surprisingly, despite the successful tuning of the Pt size, the photocatalytic hydrogen evolution rates showed no correlation with the Pt particle size. Instead, the intrinsic exciton binding energy and charge dissociation efficiency of the COF scaffolds played the dominant role. The photocatalytic activity followed the order TPPA > TPBD > TPDT, which aligns with the trend in exciton binding energy (TPPA: 44.72 meV and TPDT: 147.6 meV) rather than the Pt size. These findings challenge the conventional assumption that minimizing the Pt cocatalyst size universally enhances activity and highlight that optimizing the exciton binding energy of the COF host is more critical than cocatalyst size engineering.