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.
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.
Ionothermal synthesis of amorphous melon in a ternary salt melt (ZnCl 2 −LiCl−KCl) with lower melting points could promote the polycondensation process and improve the degree of crystallinity. As Zn 2+ ions tend to incorporate with edge nitrogen of carbon nitride via Lewis acid−base interactions, a series of crystalline hybrid poly heptazine imides (PHI), namely, Zn-PHI/PHI, integrated by van der Waals interactions, were generated as main products. Interestingly, the contents of doped Zn 2+ ions of the as-synthesized Zn-PHI/PHI gradually decrease from the bulk interior to surfaces, due to the evaporation of ZnCl 2 during thermal heating processes. Accordingly, a built-in electric field would be created in the catalyst, which largely accelerates a separation of the photogenerated charge carriers at the interfaces. Owing to the fast charge carrier separation and transfer, this hybrid polymer is active for photocatalytic water oxidation with an apparent quantum yield of up to 3.6% at 420 nm. This work offers a strategy for the rational design and synthesis of polymeric photocatalysts for water oxidation reaction.
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.
The
solar-driven reduction of carbon dioxide (CO2) using
nonmetal catalysts is an appealing solution to address the current
energy and environmental challenges. Herein, we synthesized nitrogen-containing
nanocarbons (NCNs) with a nitrogen content of 27.6 atom % by a photochemical
approach. We found that this NCNs structure can act as a catalyst
to drive the reduction of CO2 into carbon monoxide (CO).
The CO evolution activity over the NCNs reached 34.5 μmol/30
min (1 mg NCN) with a selectivity of 80.4%. The quantum efficiency
over the NCNs was 2.3% at 450 nm irradiation, which is comparable
to those of the metal-based catalysts for CO2 reduction
under visible light irradiation. The reaction processes of CO2 reduction were investigated by CO2 adsorption,
CO2 temperature-programmed desorption, and in situ diffuse
reflectance infrared Fourier transform spectroscopy. Results suggested
that the pyridinic nitrogen sites within NCNs were the main active
species for CO2 reduction catalysis. Besides, we observed
that nanocarbons with oxygen modification can also drive the CO2 reduction reaction. However, the promotion effect of the
nitrogen modification on CO2 reduction was better than
that of the oxygen modification on CO2 reduction. Additionally,
using this photochemical approach, we synthesized different nanocarbons
by changing carbon precursors, highlighting the versatility of this
approach. This work expands the application of nanocarbon for photosynthetic
CO2 conversion.
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