Photosynthetic conversion of CO2 into fuel and chemicals is a promising but challenging technology. The bottleneck of this reaction lies in the activation of CO2, owing to the chemical inertness of linear CO2. Herein, we present a defect‐engineering methodology to construct CO2 activation sites by implanting carbon vacancies (CVs) in the melon polymer (MP) matrix. Positron annihilation spectroscopy confirmed the location and density of the CVs in the MP skeleton. In situ diffuse reflectance infrared Fourier transform spectroscopy and a DFT study revealed that the CVs can function as active sites for CO2 activation while stabilizing COOH* intermediates, thereby boosting the reaction kinetics. As a result, the modified MP‐TAP‐CVs displayed a 45‐fold improvement in CO2‐to‐CO activity over the pristine MP. The apparent quantum efficiency of the MP‐TAP‐CVs was 4.8 % at 420 nm. This study sheds new light on the design of high‐efficiency polymer semiconductors for CO2 conversion.

Conventional polymerization for the synthesis of carbon nitride usually generates amorphous heptazine‐based melon with an abundance of undesired structural defects, which function as charge carrier recombination centers to decrease the photocatalytic efficiency. Herein, a fully condensed poly (triazine imide) crystal with extended π‐conjugation and deficient structure defects was obtained by conducting the polycondensation in a mild molten salt of LiCl/NaCl. The melting point of the binary LiCl/NaCl system is around 550 °C, which substantially restrain the depolymerization of triazine units and extend the π‐conjugation. The optimized polymeric carbon nitride crystal exhibits a high apparent quantum efficiency of 12 % (λ=365 nm) for hydrogen production by one‐step excitation overall water splitting, owing to the efficient exciton dissociation and the subsequent fast transfer of charge carriers.

The Frenkel exciton dissociation efficiency and the subsequent free charge carrier migration rate mainly determine the photocatalytic property of polymeric carbon nitride (PCN). Herein, by postcalcination of heptazine-based PCN in molten salts at mild temperature (673 K), we are able to design a series of crystalline PCNs with a promising triazine–heptazine hybrid structure. The molecular triazine–heptazine junctions promote the exciton splitting, while the ordered in-plane packing structure facilitates the hot charge carrier migration. Accordingly, the optimal triazine–heptazine-based PCN exhibits a dramatically enhanced activity toward one-step excitation photocatalytic overall water splitting to generate H2 and O2 with visible light over heptazine-based pristine PCN, by a factor of ∼34 times.

No abstract

Conventional polymerization for the synthesis of carbon nitride usually generates amorphous heptazine‐based melon with an abundance of undesired structural defects, which function as charge carrier recombination centers to decrease the photocatalytic efficiency. Herein, a fully condensed poly (triazine imide) crystal with extended π‐conjugation and deficient structure defects was obtained by conducting the polycondensation in a mild molten salt of LiCl/NaCl. The melting point of the binary LiCl/NaCl system is around 550 °C, which substantially restrain the depolymerization of triazine units and extend the π‐conjugation. The optimized polymeric carbon nitride crystal exhibits a high apparent quantum efficiency of 12 % (λ=365 nm) for hydrogen production by one‐step excitation overall water splitting, owing to the efficient exciton dissociation and the subsequent fast transfer of charge carriers.