ON-OFF Switching of Photocatalytic Hydrogen Evolution by Built-in Pt-Nitrogen-Carbon Reticular Heterojunctions

COF engineering with a built-in, high concentration of defined N-doped sites overcomes the "black-box" drawback of conventional trial-and-error N-doping methods (used in polymeric carbon nitride and graphene), that hamper a directed evolution of functional carbon interfaces based on structure-reactivity guidelines. The cutting-edge challenge is to dissect the many complex and interdependent functions that originate from reticular N-doping, including modification of the material optoelectronics, band alignments, interfacial contacts and co-localization of active-sites, producing a multiple-set of effectors that can all play a role to regulate photocatalysis. Herein, an ON-OFF gated photocatalytic H₂ evolution (PHE) is dictated by the Pt-N_Pyridine-carbon active sites and probed with a dual COF platform, based on stable β-ketoenamine connectivities made of triformylphloroglucinol (Tp) as the acceptor knots and 1,4-diaminonaphtalene (Naph) or 5,8-diaminoisoquinoline (IsoQ) as donors. Our results showcase two novel COF-Naph-Tp and COF-IsoQ-Tp frameworks featuring quasi-identical slip-stacked microporous structure, and similar surface area, band gap, light harvesting envelope up to 700 nm, fluorescence emission profile/lifetime, and PEIS response at the surface/water interface (R_ct=16-10±4 KΩ). A divergent behaviour is indeed observed for COF-IsoQ-Tp with record photoelectrochemical outputs (J=-16 μA cm^-2, R_t=3 KΩ at 0.40 V vs RHE) and two orders of magnitude higher rate of PHE (11.3 mmol g^-1 h^-1, λ>400 nm, pH 5) compared to the inactive COF-Naph-Tp analogue. It turns out that PHE is regulated by the isoquinoline residues at the COF pores where emergent Pt-N_Pyridine-carbon functional heterojunctions are formed upon photo-deposition of Pt nanoparticles as co-catalysts, as probed by combined XPS and DFT calculations evidence. This work sets a key guideline to direct the design of carbon-based materials encoding the installation of metal-nitrogen-carbon active sites within tailored coordination environments enabling the catalytic performance.