Spatial Regulation of Acceptor Units in Olefin‐Linked COFs toward Highly Efficient Photocatalytic H2 Evolution

Abstract Covalent organic frameworks (COFs)‐based photocatalysts have received growing attention for photocatalytic hydrogen (H2) production. One of the big challenges in the field is to find ways to promote energy/electron transfer and exciton dissociation. Addressing this challenge, herein, a series of olefin‐linked 2D COFs is fabricated with high crystallinity, porosity, and robustness using a melt polymerization method without adding volatile organic solvents. It is found that regulation of the spatial distances between the acceptor units (triazine and 2, 2'‐bipyridine) of COFs to match the charge carrier diffusion length can dramatically promote the exciton dissociation, hence leading to outstanding photocatalytic H2 evolution performance. The COF with the appropriate acceptor distance achieves exceptional photocatalytic H2 evolution with an apparent quantum yield of 56.2% at 475 nm, the second highest value among all COF photocatalysts and 70 times higher than the well‐studied polymer carbon nitride. Various experimental and computation studies are then conducted to in‐depth unveil the mechanism behind the enhanced performance. This study will provide important guidance for the design of highly efficient organic semiconductor photocatalysts.

A flask was charged with a given mass of the COFs powder, 10 mL Triethanolamine (TEOA), 90 mL water solution, and a certain proportion hexachloroplatinic acid (50mM aqueous solution) as a Pt precursor. Then the resulting suspension was transferred into a Pyrex top-irradiation reaction vessel connected to a closed gas system. The resulting suspension was ultrasonicated for 20 min before degassing. The reaction mixture was evacuated several times to ensure complete removal of air and then was illuminated with a 300 W Xe light source (λ ＞ 420 nm) with appropriate filters for the period specified. The temperature of the reaction solution was maintained at room temperature by the flow of cooling water. The evolved gases were analyzed by gas chromatography with argon as the carrier gas. Hydrogen was detected with a thermal conductivity detector (TCD) referencing standard gas with a known concentration of hydrogen. After the photocatalysis experiment, the COFs were recovered by washing with methanol before drying at 120 °C.

Photocatalytic oxygen evolution
50 mg of Pt@COFs was well dispersed by ultrasonication in an aqueous solution (100 mL) containing, Ce(NH 4 ) 2 (NO 3 ) 6 (0.1 mol L −1 ) as electron acceptor and La 2 O 3 (0.2 g) as pH buffer. The suspension was poured into a Pyrex top-irradiation reaction vessel connected to a closed gas system. Then it was evacuated several times to completely remove air prior to irradiation under a 300 W Xe lamp (λ ＞ 420 nm). The temperature of the reaction solution was maintained at room temperature by the flow of cooling water. The evolved gases were analyzed by gas chromatography with argon as the carrier gas. Oxygen was detected with a thermal conductivity detector (TCD) referencing against standard gas with a known concentration of oxygen.
Photocatalytic overall water splitting 50 mg of Pt@COFs was well dispersed by ultrasonication in 100 mL of deionized water.
The suspension was poured into the Pyrex top-irradiation reaction vessel connected to a closed gas system. Then it was evacuated several times to completely remove air prior to irradiation under a 300 W Xe lamp (λ ＞ 420 nm). The temperature of the reaction solution was maintained at room temperature by the flow of cooling water. The evolved gases were analyzed by gas chromatography with argon as the carrier gas.
The irradiation area was controlled as 3.14 × 2.6 2 cm 2 . The AQY was calculated according to the follow Eq.: ; Where, N e is the amount of generated electrons for H 2 , N p is the amount of incident photons, M is the amount of H 2 molecules (mol) during 0.5 hour, N A is Avogadro constant (6.022 × 10 23 mol -1 ), h is the Planck constant (6.626 × 10 -34 J·s), c is the speed of light (3 × 10 8 m/s), S is the irradiation area (m 2 ), P is the intensity of irradiation light (W /m 2 ), t is the photoreaction time (t=1800 s), λ is the wavelength of the monochromatic light (m).

The Solar-to-hydrogen (STH) measurements
The STH energy conversion efficiency (η) was calculated according to the following Eq.: ; where R H , ΔG 0 , P, and S denote the rate of H 2 evolution (mol s −1 ) in photocatalytic water splitting, standard Gibbs energy of water (237.13 × 10 3 J mol −1 ), intensity of simulated sunlight (0.1 W cm −2 ), and irradiation area (21.2 cm 2 ), respectively.

Structure simulations.
Structural modeling of all the COFs was generated using the Accelrys Materials Studio software package. The lattice model was geometry optimized using the Forcite module. Pawley refinement was applied to define the lattice parameters.

Computation methods
The calculations were carried out using the Vienna Ab Initio Simulation Package (VASP) [1] with the frozen-core projector-augmented wave (PAW) method [2] . The generalized gradient approximation in the Perdew-Burke-Ernzerhof (GGA-PBE) [3] function was employed for the exchange-correlation energy. a cutoff energy of 400 eV was selected for the plane-wave expansion. The convergence criteria for the force and electronic self-consistent iteration were set to 0.03 eV/Å and 10 -4 eV, respectively. The Gamma k-point was used to sample the Brillion zone.
Adsorption energies were calculated according to Ex/slab is the total energy of the slab with adsorbents in its equilibrium geometry, Eslab is the total energy of the bare slab, and Ex is the total energy of the free adsorbents in the gas phase.
Therefore, the more negative the Eads, the stronger the adsorption.

COFs Activation:
The as-synthesized olefin-linked COFs were Soxhlet extracted with tetrahydrofuran and methanol for 48 h. And then these materials were activated by supercritical CO 2 for 3 h.

Electrochemical measurements
Cyclic voltammetry measurements were performed in a typical three electrode cell system with a scan rate of 0.05 V/s. The cleaned ITO glass was coated with NKCOFs suspension solutions (1 mg/mL in EtOH and H 2 O (V : V=1 : 1) with a few droplets of 5 wt% Nafion) and then dried in air as the working electrode. The Pt flake and Ag/AgCl electrodes were used as counter electrode and reference electrode, respectively. For photocurrent intensity response and flat band potential (E fb ) measurements, the electrolyte was changed to 0.1 M Na 2 SO 4 aqueous solution, the reference electrode was altered to Ag/AgCl electrode and the light source was provided by a 300 W Xe-lamp with an ultraviolet cut-off filter (λ > 420 nm).