Covalent organic frameworks with high quantum efficiency in photocatalytic hydrogen evolution: mediating charge separation


 Compared with inorganic semiconductors, the difficulty of exciton dissociation is one of the main reasons for the lower photocatalytic activity of organic semiconductors. In this work, we report that the charge carrier lifetime is dramatically prolonged by incorporating a suitable donor-acceptor (β-ketene-CN) pair to a covalent organic framework nanosheet (CN-CON). CN-CON showed remarkably high apparent quantum efficiency up to 82.6% at 450 nm in photocatalytic H2 evolution, superior to all the COFs reported so far. The charge carrier kinetic analysis and femtosecond transient absorption spectroscopy characterizations verified that CN-CON had intrinsically lower exciton binding energies and hence longer-lived charge carriers than the corresponding CON without CN unit. This work provides an excellent model for gaining insight into the nature of ultrashort-lived active species in polymeric organic photocatalysts.

including energy minimization. The AB stacking structure was built with the similar process as described above, with the exception that a supercell with double c value was selected as the initial cell of staggered structure. The cell optimized from the Universal force fields was subsequently refined using the Pawley refinement method in Reflex tools.

Computational method of charge distribution
We have employed the VASP [1,2] to perform all the spin-polarized density functional theory (DFT) calculations within the generalized gradient approximation (GGA) using the Perdew-Burke-Ernzerhof (PBE) [3] formulation. We have chosen the projected augmented wave (PAW) potentials [4] to describe the ionic cores. Take valence electrons into account using a plane wave basis set with a kinetic energy cutoff of 450 eV. Partial occupancies of the Kohn−Sham orbitals were allowed using the Gaussian smearing method and a width of 0.05 eV. The electronic energy was considered self-consistent when the energy change was smaller than 10−5 eV. A geometry optimization was considered convergent when the energy change was smaller than 0.03 eV/Å. The brillouin zone is sampled with 1 × 1 × 1 Gamma mesh [5] .

AQE measurement
The AQE was measured using a 300 W Xe lamp (PLS-FX300, Perfectlight) with different band-pass filters of 400, 450, 500, 550, 600 and 650 nm. The number of incident photons reaching the solution was measured using a calibrated Si photodiode (LS-100, EKO Instruments Co., LTD). The numbers of photons were counted according to a litrature method [6] using photon-to-current conversion with a Si photodiode and a multimeter in the device shown in Supplementary Figure 20. The AQE was calculated using the following equation: The numbers of photons were measured at various positions by sliding the position of a Si photodiode with an interval of 3 mm, the total incident photons can be then calculated by numerically integrating the obtained data over the entire light acceptance area (7.5 cm ×7.5 cm) of the reactor. [6] The total numbers of incident photons at the wavelength of 400, 450, 500, 550 600 and 650 nm were summarized in Supplementary Figure 20.

Photocatalytic hydrogen evolution with thin CN-CON film
The thin CN-CON film deposited on glass support was prepared as follows. minutes. The reaction system was irradiated with a 300 W Xe lamp (PLS-FX300, Perfectlight) for the time specified using cut-on filters (λ > 420 nm) without stirring.
Gas samples were taken with a gas-tight syringe (Hamilton 1700) and the HER rate was analyzed by GC.

Photocatalytic oxygen evolution
A flask with quartz filter was charged with CN-CON (20 mg), a certain amount of Co(NO3)2 as a co-catalyst, 100 mL water containing 5 mmol AgNO3, and 100 mg of La2O3 as pH buffer agent. The resulting mixture was sonicated for 10 minutes before degassing by Ar bubbling for 30 minutes. The reaction system was irradiated with a 300 W Xe lamp (PLS-FX300, Perfectlight) for the time specified using cut-on filters (λ > 420 nm). Gas samples were taken with a gas-tight syringe (Hamilton 1700) and run on a gas chromatograph (Agilent 8860) equipped with Molecular Sieve 5A column connected to thermal conductivity detector.