Synthesis of bipyridine-based covalent organic frameworks for visible-light-driven photocatalytic water oxidation

Journal Pre-proof Synthesis of bipyridine-based covalent organic frameworks for visible-light-driven photocatalytic water oxidation Jian Chen, Xiaoping Tao, Chunzhi Li, Yinhua Ma, Lin Tao, Daoyuan Zheng, Junfa Zhu, He Li, Rengui Li, Qihua Yang

PII:

S0926-3373(19)31017-3

DOI:

https://doi.org/10.1016/j.apcatb.2019.118271

Reference:

APCATB 118271

To appear in:

Applied Catalysis B: Environmental

Received Date:

19 August 2019

Revised Date:

26 September 2019

Accepted Date:

7 October 2019

Please cite this article as: Chen J, Tao X, Li C, Ma Y, Tao L, Zheng D, Zhu J, Li H, Li R, Yang Q, Synthesis of bipyridine-based covalent organic frameworks for visible-light-driven photocatalytic water oxidation, Applied Catalysis B: Environmental (2019), doi: https://doi.org/10.1016/j.apcatb.2019.118271

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Synthesis of bipyridine-based covalent organic frameworks for visible-lightdriven photocatalytic water oxidation Jian Chen,a,b,# Xiaoping Tao,a,c,# Chunzhi Li,a,b,# Yinhua Ma,b Lin Tao,a,b Daoyuan Zheng,a,b Junfa

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Zhu,d He Li,a,* Rengui Li,a,* and Qihua Yanga,* aState

authors contributed equally to this work.

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#These

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Key Laboratory of Catalysis, iChEM, Dalian Institute of Chemical Physics, Chinese Academy of Sciences, 457 Zhongshan Road, Dalian 116023, China. bUniversity of Chinese Academy of Sciences, Beijing 100049, China. cSchool of Chemistry and Materials Science, University of Science and Technology of China, Hefei 230026, China. dNational Synchrotron Radiation Laboratory and Department of Chemical Physics, University of Science and Technology of China, Hefei 230029, China.

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Corresponding authors:

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E-mail: [email protected]; [email protected]; [email protected].

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Graphical abstract

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Highlights 

It is the first example to realize the visible light driven water oxidation reaction with imine COF as semiconductor. Non-precious cobalt cation was used as co-catalyst for water oxidation reaction.



The role of cobalt cation as co-catalyst was elucidated.

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

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Abstract

Covalent organic frameworks (COFs) with band gap engineering characters are attractive organic

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semiconductors. Although several COFs are being utilized for photocatalytic H2 production and

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CO2 reduction, few of them can realize the challenging water oxidation under visible light. Herein, we present the visible-light-driven photocatalytic water oxidation on bipyridine-based COFs (Bp-

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COF), which was synthesized through the Schiff base condensation reaction. Bp-COF displays impressive visible-light-driven water oxidation activity with O2 evolution rate of 152 μmol g-1 h-1

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after coordinating with Co2+. Furthermore, the Bp-COF could also enable photocatalytic H2 production under visible light. The unique photocatalytic performance of BpCo-COF for both water oxidation and proton reduction could be attributed to its excellent light harvesting property, appropriate energy band structure, high porosity and wettability as well as coordinated cobalt for

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photocatalytic water splitting. This work demonstrates the potential of COFs as semiconductors for photocatalytic solar fuels conversion.

Key words: photocatalysis, covalent organic frameworks, visible light, water oxidation.

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1. Introduction With limited reserves of fossil fuels and sharply increased environmental concerns all over the world, the conversion of renewable solar energy into clean chemical fuels such as hydrogen via photocatalytic artificial photosynthesis has attracted much research attention.[1-3] The essential challenge in reaching such elusive goal is to develop novel semiconductor materials that enable

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wide-range light harvesting, efficient charge separation and charge utilization.[4,5] Inorganic semiconductors have been widely used for photocatalytic hydrogen production from water,

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however, it still lacks of inorganic semiconductors that possess visible light absorption properties and suitable band gap positions for water splitting.[6-10] Organic semiconductors are much less

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explored, but they are intriguing and emerging as promising candidates because of their adjustable

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electronic and structural properties.[11-15]

As an appealing class of crystalline porous materials, covalent organic frameworks (COFs)

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have gained significant attentions in various applications due to their modular synthetic versatility, highly accessible surface areas and, sometimes, good physicochemical stability.[16-22] Particularly,

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COFs have shown potential applications in photocatalytic solar energy conversion, e.g., photocatalytic H2 evolution and CO2 reduction.[23-32] For example, Lotsch et al. reported a series of tunable azine COFs in photocatalytic H2 production using triethanolamine (TEOA) as a

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sacrificial donor and Pt as the co-catalyst.[26] Cooper et al. synthesized sulfone-containing COFs which enabled photocatalytic hydrogen evolution from water in the presence of hole acceptors.[27] Recently, a two-dimensional COF incorporating with Re complex was used for photocatalytic CO2 reduction to CO in the presence of TEOA as a sacrificial donor.[28] Although successful applications of COFs in photocatalysis have been achieved, almost all these researches solely concern the utilization of photogenerated electrons for the reductive half reaction, e.g. H2

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production or CO2 reduction using sacrificial electron donors. Few of them can realize the much more challenging reaction, that is, photogenerated hole-involved water oxidation reaction for O2 evolution.[29] As the water oxidation reaction requires a four-electron-transfer process with O-H bond cleavage and O-O bond formation, in addition to the relative high overpotential with sluggish O-O bond formation kinetics, it has been widely recognized as the key and bottleneck in artificial

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photosynthesis.[33] Herein, we reported the synthesis of a two-dimensional bipyridine-based COFs (Bp-COF),

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which exhibits excellent activity in visible-light-driven photocatalytic water oxidation for O2

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evolution using cobalt-based co-catalyst. As far as we know, this is the first example of imine COFs that can realize photocatalytic water oxidation under visible light irradiation. Interestingly,

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this Bp-COF could also enable photocatalytic H2 production under visible light irradiation.

2.1 Synthesis of Bp-COF

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2. Experimental section

In a typical process of synthesis, 2,2′ -bipyridine-5,5′ -dicarboxaldehyde (Bp) (25.5 mg, 0.12

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mmol) was dissolved by sonication in a mixed solution of DMF (0.5 mL) and THF (3 mL) in a 10 mL glass tube without inert gas protection, followed by the addition of 1,3,5-tris(4aminophenyl)benzene (TAPB) (28.1 mg, 0.08 mmol). After that, CH3COOH aqueous solution (6

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M, 0.3 mL) was added dropwise to the reaction system. The tube was kept at 80 oC for 3 days. Then the resultant yellow color solid product was washed with THF followed by soxhlet extraction. At last, supercritical carbon dioxide drying process was used to dry the material. For screening the synthesis conditions, different types of solvents and temperature were used with the same acid concentration (details see Table S1). Considering the easy handle, high

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crystallinity and surface area for further investigation, DMF and THF were chosen as the solvents and the reaction was conducted under air at 80 oC (Figure S1 and S2). 2.2 Synthesis of BpCo-COF-x Bp-COF (20 mg) was dispersed in ethanol (10 mL), followed by the addition of a desired amount of ethanol solution containing Co(NO3)2 (1 mg/mL). After stirring the mixture at 60 oC for

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6 h, the solid product was filtered, washed several times with ethanol and followed with supercritical carbon dioxide drying process. The obtained solid product was denoted as BpCo-

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COF-x (x refers to the Co loading amount, Table S2).

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The coordination of Bp-COF with other metal ions (Mn2+, Fe3+, Ni2+, Cu2+) with 1 wt%

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content were conducted under same procedure as described above.

2.3 General procedures for photocatalytic oxygen and hydrogen evolution

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The photocatalytic oxygen evolution was operated in a closed gas circulation and evacuation system using a 300 W Xe lamp (Ushio-CERMAX LX300) and optical cutoff filter (kenko, L42; λ

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≥ 420 nm). The photocatalyst (10 mg) was dispersed in a Pyrex glass reaction cell containing 100 mL water and thoroughly degassed by evacuation. 5 mM AgNO3 was added as the electron acceptor. An on-line gas chromatograph (Shimadzu GC-8A, TCD, Ar carrier) equipped with a thermal conductive detector (TCD) was used to determine the amount of evolved O2. The rate of

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O2 evolution in the initial 1 hour was used for evaluating the photocatalytic activity of the photocatalysts. For the recycle experiment, the reaction system was fully degassed before the next run.

To confirm the origin of the detected O2, an isotope experiment was performed by using H218O as reagent (a 10 mL of H218O reaction scale). The gas after the photocatalytic oxygen

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evolution reaction was directly injected in to the mass spectrum analyzer (7000B GC-QqQ-MS equipped with a DB-5 capillary column). A peak at m/z = 36 was observed indicating that the produced 18O2 indeed originated from H218O specie rather than the decomposition of any oxygen species in the photocatalytic reaction. The peaks at m/z = 28 and 32 refer to the 14N2 and 16O2 in the air which were involved by the injection process.

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The AQE measurement was carried out using a Pyrex 5 top-irradiation-type reaction vessel and evacuation system using a 300 W Xe lamp (Ushio-CERMAX LX300). Band-pass filters

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(Asahi Spectra Co., FWHM: 10 nm) with λ at 420 nm were used. The number of photons reaching

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the reaction solution was measured using a calibrated Si photodiode (LS-100, EKO Instruments Co., LTD.), and the AQE () was calculated according to the following equation:

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 (%) = (AR / I )*100

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where A, R, and I represent a coefficient (8 for O2 evolution), the evolution rate of O2 in the initial one hour irradiation, and the absorption rate of incident photons, respectively. It was assumed that

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all incident photons were absorbed by the suspension. The total number of incident photons at the wavelength of 420 nm was measured to be 2.55 × 1020 photons h-1. The photodeposition of metal particles (Ru or Pt) was carried out under the irradiation of a

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300 W Xe lamp (Ushio-CERMAX LX300). Certain amount of Bp-COF or BpCo-COF-1 was dispersed in a solution of water (85 mL) and triethanolamine (TEOA, 15 mL). A certain amount of metal ion (a water solution of RuCl3 or H2PtCl6) was added into the system (For details, see Table S6). After three hours, the samples was washed with water and centrifuged for further characterization and evaluation. The photocatalytic hydrogen evolution reactions were carried out in a closed gas circulation and evacuation system using a 300 W Xe lamp (Ushio-CERMAX

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LX300) and optical cutoff filter (kenko, L42; λ ≥ 420 nm). The catalyst (10 mg) was dispersed in in a solution of water (85 mL) and triethanolamine (TEOA, 15 mL). The amount of evolved H2 was determined by an on-line gas chromatograph (Shimadzu GC-8A, TCD, Ar carrier) equipped with a thermal conductive detector (TCD). The rate of H2 evolution in the initial 1 hour was used for evaluating the photocatalytic activity of the photocatalysts.

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3. Results and discussion The Bp-COF was synthesized by heating 1,3,5-tris(4-aminophenyl)benzene (TAPB) and

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2,2′ -bipyridine-5,5′ -dicarboxaldehyde (Bp) in the presence of acetic acid without inert gas

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protection (Figure 1a). In the FT-IR spectrum of Bp-COF, the vibration peak of C=N could be clearly observed at 1624 cm-1, showing the successful condensation of aldehyde and amine group

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(Figure 1b). The weak vibration peak at 1700 cm-1 related to C=O stretching and two weak peaks at 3370 and 3450 cm-1 corresponding to the stretching vibrations of aromatic primary amine N-H

In the

13C

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indicate the existence of unreacted aldehydes and amine around the periphery of the Bp-COF.[34] CP TOSS NMR spectrum of Bp-COF, the peak at 157 ppm is assigned to C=N,

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indicating the formation of imine network (Figure 1c). Besides, all the chemical shifts of Bp-COF matched well with corresponding model compound (Figure S3). The combined results of FT-IR and 13C CP TOSS NMR confirmed the successful formation of imine-based framework. Thermal

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gravimetric analysis shows that Bp-COF is stable up to 400 °C in air (Figure S4). The SEM and TEM images show that Bp-COF was composed of aggregated irregular nanoparticles (Figure S5 and S6).

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(a)

TAPB 80oC, THF/DMF

+

air, 3 d

Bp-COF

Bp

g,h

-CHO

Bp-COF

TAPB

2000

1500

-1

i

1000

200

150

d l

k b

h c a

100

50

0

Chemical shift (ppm)

)

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Wavenumber (cm

e j

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Bp 3500 3000

g

i-k f

b-d a e,f

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(c)

-C=N -NH2

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(b)

Figure 1. (a) Schematic illustration for the synthesis of Bp-COF, (b) Fourier transform infrared (FT-IR) spectra of

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Bp-COF, TAPB and Bp, (c) 13C CP TOSS NMR spectrum of Bp-COF.

The Bp-COF exhibited strong XRD peaks at 2.42o, 4.10o, 4.72o, 6.24o, 8.24o, which can be

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assigned to the (100), (110), (200), (210), (220) facets of P3 space group, respectively (Figure 2a, red curve). Comparing to the staggered AB stacking models, the AA stacking mode can reproduce the peak position and intensity of the XRD pattern (Figure 2a and S7). The Pawley refinement yielded XRD patterns (black curve) are in good agreement with the experimentally observed ones,

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as evidenced by their negligible difference (blue curve). Based on the AA stacking mode of the 2D layers for Bp-COF (Figure 2b), lattice modeling and Pawley refinement figured out the optimized unit cell parameters (a = b = 44.17 Å, c = 3.72 Å, α= β= 90°, γ = 120°), which provided good agreement factors (Rp = 2.70% and Rwp = 3.80%) (Table S3). Based on the nitrogen sorption isotherms, the BET surface area and total pore volume of Bp-COF was ca. 1655 m2 g-1 and 1.2

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cm3 g-1 (Figure 2c). The pore size of Bp-COF is mainly distributed at 3.4 nm, similar to the theoretical result.

(b)

(a)

3.7 nm

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Intensity (a.u.)

Pawley refinement Experiment Simulated Difference

5

10

20

25

30

35

40

(d)

Potential / V vs. RHE

600

200 5.0

7.5

10.0

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2.5

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1.0

BpCo-COF-1

1

Pore Size (nm)

0

-0.86 V

LUMO H+/H2

Bp-COF

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3.4 nm

400

-0.95 V

-p

-1

3

Volume Uptake (cm /g STP)

15

2 Theta (degree)

(c) 800

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3.7 Å

1.46 V 2

1.23 V H2O/O2

1.54 V

HOMO

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Relative Pressure (P/P0)

Figure 2. (a) Bp-COF with the experimental profiles in red, Pawley-refined profiles in black, calculated eclipsed model profiles in dark cyan, and the differences between the experimental and refined PXRD patterns in blue, (b) views of space-filling models along the c-axis with the layer distances, (c) N2 sorption isotherm at 77 K (pore size

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distribution inset) of Bp-COF, (d) the scheme of calculated HOMO/LUMO band positions for Bp-COF and BpCoCOF-1.

Bp-COF shows an absorption edge extending to ca. 520 nm and the optical band gap of Bp-

COF was calculated to be 2.41 eV from Kubelka-Munk plots (Figure S8). The lowest unoccupied molecular orbital (LUMO) level of the Bp-COF is located at -0.95 V vs. RHE as determined by cyclic voltammetry measurement, which is in good agreement with the result obtained by Mott-

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Schottky plot (Figure S9 and S10). Considering the calculated band gap, the HOMO level of BpCOF appears at +1.46 V vs. RHE (Figure 2d). Also, we have calculated (using DFT) the molecular frontier orbitals of Bp-COF (Figure S11 to S14), both the theoretical and experimental results imply that the energy band structure of Bp-COF is thermodynamically feasible for both proton reduction and water oxidation reactions.

15N

enriched Bp-COF

BpCo-COF-1

-NO3 BpCo-COF-7.5

35003000 2000

* 1500

400

1000

300

N in imine and bipyridine

796.8

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amine N

Co2p1/2

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N in imine and bipyridine

Bp-COF

408 406 404 402 400 398 396 394 392 810

Binding energy (eV)

300

400

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Wavelength (nm)

781.4 eV

BpCo-COF-1

781.5 eV

CoO

780.9

Intensity (a.u.)

amine N

-100

Co2p3/2 (f)

(e)

Co-N

-NO3 N

0

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BpCo-COF-1

100

Chemical shift (ppm)

-1

Wavenumber (cm )

(d)

200

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15N enriched BpCo-COF-1

Bp-COF-Co-1 Bp-COF-Co-5 Bp-COF-Co-7.5 Bp-COF-Co-7.5*

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-C=N

805

800

795

790

Intensity (a.u.)

Bp-COF

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(c)

(b)

Absorbance (a.u.)

(a)

[Co(bpy)3]2+

781.1 eV

Co2O3

782.1 eV

BpCo-COF-1 after

782.1 eV

785

Binding energy (eV)

780

775 770

780

790

800

810

Photon energy (eV)

Figure 3. (a) FT-IR spectra, (b) 15N NMR spectra (star refers to side band), (c) UV-vis diffuse reflectance spectra of BpCo-COF-x, (d) N 1s XPS spectra of Bp-COF and BpCo-COF-1, (e) Co 2p XPS spectrum of BpCo-COF-1, (f) Co

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L-edge NEXAFS spectra.

However, without depositing of any cocatalysts, only trace amount of O2 can be detected

when using Bp-COF for photocatalytic water oxidation in the presence of AgNO3 as the electron acceptor. It is well-known that proper co-catalysts can lower the activation energy for surface redox reactions and significantly facilitate the photogenerated charge separation and transfer. Particularly, water oxidation reaction is usually faced with sluggish kinetics which requires

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overcoming large overpotential. Thus we introduced cobalt species as cocatalysts considering the wide-spread use of cobalt-based cocatalysts for water oxidation.[35-37] BpCo-COF-x (x refers to the Co loading amount, Table S2) samples were obtained by the coordination of Co ions with bipyridine units of Bp-COF. With the increase in Co content, the peak at 1385 cm-1 belonging to the anti-symmetric νa vibration band of -NO3 gradually increased in the FT-IR spectra of BpCoCOF-x (Figure 3a). No obvious change in the wavenumber of the vibration peak of -C=N was

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observed after the coordination of Co ions with Bp-COF, proving that Co did not coordinate with

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imine group.[38] 15N NMR spectrum of 15N-BpCo-COF-1 is almost identical to that of 15N-Bp-COF, which further confirmed that the Co is only coordinated with bipyridine motifs (Figure 3b). As the

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cobalt content increases from 0 to 7.5 wt%, the absorption edge of BpCo-COF-x red-shifts

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gradually from ~520 nm to more than 580 nm (Figure 3c). This is possibly due to the fact that the presence of Co(II) affects the electronic transition property of the COFs.[39] The LUMO and

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HOMO levels of BpCo-COF-1 were calculated to be -0.86 V and 1.54 V vs. RHE as determined with the assistant of the cyclic voltammetry and optical band gap measurement (Figure S8 and S9).

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As shown in Figure 2d, BpCo-COF-1 exhibited a more positive HOMO position than that of BpCOF, which is beneficial for photocatalytic water oxidation reactions. The successful introduction of Co(NO3)2 in BpCo-COF-1 was also proved by X-ray photoelectron spectroscopy (XPS) (Figure 3d and 3e). As shown in Figure 3d, there are two N species for Bp-COF with binding energy of

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398.7 and 399.5 eV respectively corresponding to pyridinic N (N in imine and bipyridine) and amine N. The existence of amine N is due to the unreacted amine on the periphery of the Bp-COF. The N 1s spectrum of BpCo-COF-1 clearly showed a new peak at 406.4 eV, corresponding to N of -NO3 group. The other broad peak in the range of 397~401 eV can be deconvoluted to three peaks. The peak at 398.7 eV decreased and a new peak at 399.8 eV could be clearly observed. The

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new peak is attributed to the N atom of pyridine unit coordination with Co(II). The binding energies of Co 2p1/2 and Co 2p3/2 respectively appeared at 796.8 and 780.9 eV and the satellite peak at 786.3 eV confirmed the presence of Co(II) (Figure 3e).[40] The cobalt L-edge near edge Xray absorption fine structure (NEXAFS) spectrum of BpCo-COF-1 showed two peaks at 781.4 eV (L3-edge) and 796.5 eV (L2-edge). The energy for L3-edge peak of BpCo-COF-1 was similar to CoO (781.5 eV) and cobalt(II) tris(2,2’-bipyridine) complex (781.1 eV), which further proved the

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Co(II) integrated in the COFs (Figure 3f).

(a)

(b)

(c)

-p

600

3

BpCo-COF-1 400

BpCo-COF-5

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200

BpCo-COF-1 BpCo-COF-5 BpCo-COF-7.5 0.0

0.2

0.4

0.6

0.8

Relative Pressure (P/P0)

1.0

2

4

6

8

10

Pore Size (nm)

(f)

(e)

(d)

100 nm

BpCo-COF-7.5

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0

Water uptake (cm3/g)

Volume Uptake (cm /g STP)

800

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Intensity

BpCo-COF-1

200

Bp-COF BpCo-COF-1 150

100

50

Bp-COF

25 nm

20

2 Theta (degree)

30

40

0 0.0

0.2

0.4 0.6 0.8 Relative Pressure (P/P0)

1.0

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Figure 4. (a) N2 sorption isotherms (measured at 77 K) and (f) NLDFT pore size distribution curves of BpCo-COFx, (c) HR-SEM and (d) TEM images of BpCo-COF-1, (e) PXRD patterns and (f) water adsorption isotherms of BpCOF and BpCo-COF-1.

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After the coordination with Co, BpCo-COF preserved high surface area, high porosity and large pore volume (Figure 4a, 4b and Table S4), particulate morphology (Figure 4c and 4d) and crystalline structure (Figure 4e). The wettability of photocatalysts has a big influence on the catalytic activity in H2O splitting because it affects the adsorption of H2O on the surface of the catalysts.[27] The water vapor uptake

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experiments were conducted for Bp-COF and BpCo-COF-1. As shown in Figure 4f, BpCo-COF1 showed much higher water uptake ability than that of Bp-COF. This is beneficial for

-p

(b)

(c) -2

300 200

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Intensity (a.u.)

400

m/z = 36

30 20

-1

10

-1

0

400

500

600

700

0

50

100

150

200

Time (s)

m/z

(f)

[email protected] [email protected]

500 400

120

300 200

60

0

300

35

Co

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Intensity (a.u.)

(e)

O2 evolution (umol g h )

40

0.5

0.0

30

180

1.0

-1

50

25

1.5

H2 Production (umol g )

4

-1

(d)

2 3 Reaction time (h)

-1

1

O2 Production (umol g h )

0

lP

100

0

Bp-COF BpCo-COF-1

@1.23 V VS RHE

2.0

Current Density (A cm )

Bp-COF BpCo-COF-1

-1

O2 Production (umol g )

(a) 500

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photocatalytic water splitting.

Ni No

Mn

Fe

800

Wavelength (nm)

Cocatalysts

100

Cu Blank

0 0

2

4 6 8 10 Reaction time (h)

12

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Figure 5. (a) Photocatalytic water oxidation performances under visible light irradiation (λ ≥ 420 nm), (b) mass spectra (m/z = 36) analysis of evolved O2 using H218O as water source, (c) photocurrent responses, (d) photocatalytic O2 evolution under different irradiation wavelength regions over BpCo-COF-1, (e) photocatalytic water oxidation performances of Bp-COF with different metal ions under visible light irradiation, (f) photocatalytic H2 evolution performances under visible light irradiation (λ ≥ 420 nm).

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To our delight, BpCo-COF-1 showed a remarkably enhanced photocatalytic activity compared with the bare Bp-COF in the presence of electron acceptor under visible light as shown in Figure 5a. To ascertain the origin of the as-produced O2, an isotope experiment was performed by using H218O as water source. A peak at m/z of 36 was observed, indicating that the produced 18O 2

indeed originated from H218O rather than the decomposition of any oxygen species in the

photocatalytic reaction (Figure 5b). The results of photoelectrochemical (PEC) performance

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showed that BpCo-COF-1 gave a much stronger photoanode current density than Bp-COF, which

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is in consistent with their photocatalytic activities (Figure 5c and S15). To verify the essential role of cobalt co-catalyst played in photocatalytic water oxidation, the photocatalytic activity of BpCo-

-p

COF with different amounts of Co was investigated (Figure S16). With the Co content increased

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from 0 to 1 wt%, the O2 evolution rate increased sharply to 152 μmol g-1 h-1. Further increasing the Co content resulted in a decline in photocatalytic activity. The apparent quantum efficiency (AQE)

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of photocatalytic water oxidation for BpCo-COF-1 under the wavelength of 420 nm monochromatic light was tested to be 0.46%. The O2 evolution ability of BpCo-COF-1 is superior

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to most of the reported COFs[29,41] or covalent triazine framework (CTFs) based catalysts[33,42,43], but lower than some metal-organic frameworks (MOFs) based catalysts[4,44] (Table S5). The HR-SEM and TEM images of BpCo-COF-1 after photocatalytic water oxidation showed the presence of Ag particles on its surface, further confirming that the photogenerated electrons

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indeed participate in the reduction of Ag+ to Ag (Figure S17). The existence of Ag NPs of recovered BpCo-COF-1 was also proved by PXRD and XPS analysis (Figure 6a and S18). This result validates the feasibility that the hole-involved oxygen evolution reaction takes place on the BpCo-COF. After reaction, high surface area was retained (Figure 6b and 6c). The almost unchanged FT-IR and 13C CP TOSS NMR spectra suggested the chemical stability of BpCo-COF-

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1 under light irradiation (Figure 6d and S19). However, the decrease in the crystallinity of BpCoCOF was observed after photocatalytic test (Figure 6a), showing the stability of crystalline structure of COFs under light irradiation should be further improved in the future. The energy for L3-edge peak of BpCo-COF-1 after O2 evolution shifted from 781.4 to 782.1 eV, indicating the presence of Co(III) (Figure 3f). In the FT-IR spectrum of reused BpCo-COF-1,

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no vibration peaks related with Co-O species could be observed (Figure 6d).[45] The oxygen content (analysed by XPS) for BpCo-COF-1 before and after photocatalytic water oxidation reaction was

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respectively of 13.2% and 11.0%, further showing that no CoO/Co3O4 was formed.

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Correspondingly, recycling and long-time scale photocatlaytic water oxidation reaction on BpCoCOF during 31 hours were also conducted (Figure 6e and 6f). The sustainable growth of the

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oxygen evolution also verified the stability of the catalyst. The decrease in water oxidation reaction activity after the first cycle might be related to the decreased crystallinity of COFs and the coverage

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of Ag NPs on the surface of COFs. The photocatalytic oxygen evolution of BpCo-COF under different wavelength ranges of incident light were also examined (Figure 5d). The result reveals

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that the trend of photocatalytic activity in different wavelengths is in good agreement with its light absorption spectrum, which implies that the reaction faithfully proceeded via photoabsorption by the photocatalyst. When other transition metal ions, e.g. Mn2+, Fe3+ and Ni2+ were used as cocatalysts, only Ni2+ could facilitate the photocatalytic water oxidation reaction, although it showed

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lower activity than that of Co2+ (Figure 5e).

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300

200

3

Intensity (a.u.)

(c)

(b)

Volume Uptake (cm /g STP)

(a)

100

JCPDS-4-783 Ag

0 40

60

80

0.0

0.2

2 Theta (degree) BpCo-COF-1 after OER Fresh BpCo-COF-1

0.6

0.8

350

7.5

10.0

300

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O2 Production (mol g )

1000

250

800

200

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400 200

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0

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0 4000 3500 3000 2500 2000 1500 1000 500

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1.0

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Produced O2 (umol g )

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Relative Pressure (P/P0)

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24

28

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Reaction time (h)

0

1

2

3

4

5

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7

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9

Reaction time (h)

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Wavenumber (cm )

Figure 6. (a) PXRD pattern, (b) N2 sorption isotherms (measured at 77 K) and (c) NLDFT pore size distribution curve

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of BpCo-COF-1 after the photocatalytic water oxidation reaction, (d) FT-IR spectra of fresh and used BpCo-COF-1, (e) long-time photocatalytic O2 evolution performance of BpCo-COF-2.5 under visible light irradiation (λ ≥ 420 nm), (f) stability test of photocatalytic O2 evolution performance for BpCo-COF-1 under visible light irradiation (λ ≥ 420

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nm).

Additionally, the photocatalytic hydrogen production on Bp-COF and BpCo-COF in the presence of TEOA were also evaluated using Pt and Ru NPs as co-catalysts. The HR-SEM and TEM images verified the existence of metal NPs on Bp-COF (Figure S20 and S21). As shown in

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Figure S22, [email protected] showed much higher activity than that of [email protected] (10.9 vs 2.3 μmol g-1 h-1). With the Pt loading amount increased from 0 to 0.4 wt% (determined by ICP, see Table S6), the H2 evolution rate of Bp-COF@Pt-x increased sharply to 24.6 μmol g-1 h-1 (Figure S23). As expected, the structure of Bp-COF was well maintained after the hydrogen evolution reaction (Figure S24). Encouragingly, BpCo-COF-1@Pt showed steady and higher

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photocatalytic H2 evolution activity (59.4 μmol g-1 h-1) than that of Bp-COF@Pt (Figure 4f). This proved that cobalt in BpCo-COF could not only increase the activity in photocatalytic oxygen evolution reaction but also the hydrogen evolution reaction. The H2 evolution activity of BpCoCOF-1@Pt is higher than that of some reported COFs (such as TaPa-2,[46] TP-EDDA[24] and N0COF[26]), but is still not high enough in comparison with COFs with better light-harvesting properties[29, 41, 46-49] (e.g. Sp2c-COFERDN,[29] g-C40N3-COF,[41] etc., see Table S7).

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To further investigate the role of cobalt played in the photocatalytic reaction, electrochemical

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impedance spectroscopy (EIS), photoluminescence (PL) emission spectra and transient absorption spectrum (TAS) were used to characterize both Bp-COF and BpCo-COF-1. As shown in Figure

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7a and 7b, the fitted values of Rsc and Rct for BpCo-COF-1 are much lower than those for Bp-

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COF, which implies that the presence of cobalt could greatly promote the charge transport inside the bulk as well as the charge transfer and injection across the semiconductor/electrolyte interface.

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The PL intensity of BpCo-COF-1 was quenched in comparison with that of Bp-COF, indicating the enhanced charge-separation efficiency (Figure S25). In addition, the transient absorption

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spectrum of the photogenerated electrons of Bp-COF gave a much faster decay of 0.28 μs compared to BpCo-COF-1 with a time constant of 1.00 μs, demonstrating the increase in the lifetime of photogenerated charges for BpCo-COF-1 with the addition of Co, which could hinder the charge recombination and thus promote the charge separation efficiency (Figure 7c). Combining with the

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above results, the activity enhancement of the Co coordinated in the Bp-COF towards photocatalytic water oxidation is proposed as shown in Figure 7d. The photogenerated charges are produced by light absorption of the photocatalysts, followed by separation and transferring to the interface of the material and water. The photogenerated electrons are captured by the Ag+ to produce Ag NPs, while the photogenerated holes tend to transfer to the Co2+ sites and oxidize the

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Co2+ to a higher chemical valence as detected in NEXAFS (Figure 3f). The high valence state cobalt would then react with the adsorbed H2O molecule at the interface to evolve oxygen, and

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returns back to the lower valence states, completing the whole reaction cycle.[50]

Figure 7. (a, b) Electrochemical impedance spectra (EIS) of Bp-COF and BpCo-COF-1, (c) kinetic profiles of

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transient absorption spectrum (TAS), (d) proposed mechanism of photocatalytic oxidation reaction on BpCo-COF.

4. Conclusion

In summary, we present the first example of Bp-COF with imine linkage that can perform

photocatalytic water oxidation under visible light irradiation so far. The Bp-COF with absorption edge expanding to ca. 520 nm exhibits visible light absorption properties and suitable band energy

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structure for both water oxidation and proton reduction. With Co2+ and Pt (Ru) NPs as co-catalysts, Bp-COF could enable the water oxidation half reaction for O2 evolution and proton reduction half reaction for H2 evolution under visible light, respectively. It should be noted that although the overall water splitting on the COFs is still challenging at the moment, both water oxidation and proton reduction reactions have been successfully achieved on this unique Bp-COF, manifesting

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its potential for photocatalytic solar energy conversion.

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Declaration of interests

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The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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The authors declare the following financial interests/personal relationships which may be considered as potential competing interests:

Acknowledgements

This work is dedicated to the 70th anniversary of Dalian Institute of Chemical Physics (DICP), CAS. This work was supported by the National Key R&D Program of China

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(2017YFB0702800), the NSFC (21733009, 21621063), Key Research Program of Frontier Sciences, CAS (QYZDY-SSW-JSC023) and the Strategic Priority Research Program of the Chinese Academy of Sciences (XDB17020200, XDA21010207).

Appendix A. Supplementary data Supplementary material related to this article can be found, in the online version, at doi: XXX.

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