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Ultrastable and efficient visible-light-driven hydrogen production based on donor-acceptor copolymerized covalent organic polymer Yaoyao Liu, Zhijian Liao, Xiangliang Ma, and Zhonghua Xiang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b10022 • Publication Date (Web): 20 Aug 2018 Downloaded from http://pubs.acs.org on August 21, 2018

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ACS Applied Materials & Interfaces

Ultrastable and Efficient Visible-Light-Driven Hydrogen Production Based On Donor-Acceptor Copolymerized Covalent Organic Polymer

Yaoyao Liu, Zhijian Liao, Xiangliang Ma and Zhonghua Xiang*

State Key Laboratory of Organic-Inorganic Composites, College of Chemical Engineering, College of Energy, Beijing University of Chemical Technology, Beijing 100029, P. R. China

*E-mail address: [email protected]

Keywords Conjugated microporous polymer; visible-light-driven hydrogen evolution; organic photocatalyst; water splitting; stable photocatalytic activity

Abstract Developing stable and efficient photocatalysts for H2 production under visible light is still a big challenge. In this work, a novel covalent organic polymer (COP)-based photocatalyst with trace ending groups was prepared by the efficent irreversible kinetic coupling reaction, i.e., nickel(0)-catalyzed Yamamoto-type Ullmann cross-coupling, using pyrene as electron donor and countpart, e.g., phenanthrolene, benzene, pyrazine, as electron acceptor. The newly developed optimal photocatalyst 1 ACS Paragon Plus Environment

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(termed as COP-TP3:1) has a 14-fold improvement in the H2 evolution rate from 3 µmol h-1 to 42 µmol h-1 under visible light compared with the sample without donor-acceptor structure. Moreover, COP-TP3:1 also performs excellent photocatalytic activity under different water quality (deionized water, municipal water, commercial mineral water and simulated seawater (NaCl 3wt %)). Significantly, ignored decrease of H2 evolution can be observed after twenty hours cycling H2 production, and the performance is only reduced by about 7% even after discontinuous cycles of photocatalysis and storage for a month. The donor-acceptor units with trace ending groups contribute to suppress electron-holes recombination kinetics and the N coordination sites in electron-acceptors conduce to anchor Pt (as the cocatalyst) onto the surface of photocatalyst, both of which are conducive to the outstanding photocatalytic activity and stability. Accordingly, this work can provide guidance to design stable and efficient photocatalyst by copolymerization for visible-light-driven H2 production.

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1. INTRODUCTION The increasing shortage of fossil fuel and environmental pollution have received tremendous attention in the search for clean and sustainable energy sources.1-2 Hydrogen energy is regarded as one of the most ideal alternative candidates for the replacement of fossil fuels.3-5 Photocatalytic H2 evolution under solar illumination has been considered to be a promising technology for green energy resource.6-7 So far, various inorganic photocatalysts for hydrogen evolution, such as, metal sulfides,8 nitrides9-10 and phosphides11-12 have been explored, however, most of them mainly respond to UV-light that occupies less than 5% of the solar spectrum.3 Therefore, it remains a major challenge to develop efficient and stable visible light photocatalysts for H2 production.13 In the past few years, organic semiconductors with numerous merits,14-15 such as, well-defined structure,16-17 controllable surface area,18 π-conjugated system,19-20 versatile functional groups,21-22 have been designed as promising photocatalysts for H2 production under visible light. However, due to the high exciton binding energy of organic polymer semiconductors and rapid recombination kinetics of photoexcited electron-hole pairs,13 these photocatalysts show moderate activity during water splitting. Therefore, it is highly desired to overcome these disadvantages. By molecular design,23 the optical gap of microporous polymers has been fine tuned over a broad range (1.94−2.95 eV), the absorption spectrum of which can be shifted from 400 nm to 600 nm for improving visible light photocatalytic hydrogen evolution.18 Exfoliating stacked conjugated microporous polymers into ultrathin nanosheets can 3 ACS Paragon Plus Environment


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accelerate photoexcited charge carriers to rapidly reach the surface of polymers to drive redox reactions which contributes to suppress undesirable electron–hole recombination.24 In addition to above strategies, constructing electron-donor-acceptor organic polymers is also considered as an efficient route,25 not only to strengthen visible light havesting, but also to boost the separation and transport of photogenerated electron-hole pairs.26-27 In view of the excellent property of donor-acceptor structure, when introducing electron-withdrawing groups into electron-donor-based frameworks to synthesize donor-acceptor polymers, the electrons will transfer from donor to acceptor molecules, where the Highest Occupied Molecular Orbital is offered by the donor units, and the Lowest Unoccupied Molecular Orbital is located on the acceptor molecules.28 This feature contributes to accelerate the intramolecular charge transfer transition and efficient separation of photo-generated

electron-holes

under

long-wavelength-light

irradiation,

then

improving photocatalytic hydrogen evolution.29 Recently, benzothiadiazole and tricyanomesitylene as electron-withdrawing unit were incorporated into the phenyl-based polymer backbones to form donor-acceptor polymers for improving photocatalytic activity.26-27 Via significant efforts, big strides towards improving visible-light-driven photocatalytic efficiency have been made, however, the rate of hydrogen evolution begins to decrease from ten hours or twenty hours during the photocatalytic cyclic tests, which is still far from meeting industrial requirements of stability.30-31 Nevertheless, the long-term durability and regenerability of organic photocatalysts is also another critical issue for practical application. 4 ACS Paragon Plus Environment

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Nickel(0)-catalyzed Yamamototype Ullmann cross-coupling has been widely developed as an efficient condensed polymerization to prepare highly porous polymers with ultrahigh stability and trace terminal groups, e.g., porous aromatic frameworks (PAFs),32-34 covalent organic polymers (COPs).35 The covalent organic polymers (COPs) have many intriguing properties, such as, well-defined chemical composition, large conjugated system, remarkable hydrothermal stability and versatility in molecular design, which have been widely applied in carbon dioxide capture,36 oxygen reduction reactions,37-38 bandgap engineering39 and water splitting.40-41 Herein, we synthesized a series of COPs using 1, 3, 6, 8-tetrabromopyrene (TBP) as electron donor and 3, 8-Dibromo-phenanthroline (DBP) as electron acceptor to build a donor-acceptor structure, which enhances visible light absorption and boosts photo-induced

charge

carriers.

For

the

control

experiment,

two

weaker

electron-withdrawing units (benzene and pyrazine) were incorporated into pyrene-based covalent organic polymers. The as-synthesized materials were termed as COP-TPx:y, x:y represents the ratio of TBP and DBP. Among them, COP-TP3:1 exhibits a remarkable H2 evolution rate of 42 µmol h-1 along with great stability under visible light (λ ≥ 400 nm) and an apparent quantum yield (AQY) of 1.5% at 400 nm. After discontinuous cycles of photocatalysis and storage for a month, no obvious decrease of H2 evolution can be observed. Moreover, COP-TP3:1 also performs excellent photocatalytic activity under different water quality (deionized water, municipal water, commercial mineral water and simulated seawater (NaCl 3wt %)). 5 ACS Paragon Plus Environment


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2. EXPERIMENTAL SECTION 2.1. Materials. All reagents were obtained from commercial sources. 1, 3 ,6 ,8-tetrabromopyrene (TBP), 3, 8-Dibromophenanthroline (DBP), Triethanolamine (TEOA),

1,

4-Dibromobenzene

(DB),

2,5-Dibromopyrazine

(DPZ),

1,

5-Cyclooctadiene (cod) and N, N-dimethylformamide (DMF), trichloromethane (CHCl3), tetrahydrofuran (THF), bis (1, 5-cyclooctadiene) nickel (0) (Ni(cod)2) and 2, 2’-bipyridyl were purchased from J&K; Hydrochloric acid (HCl) was obtained from Beijing Chemical Works. 2.2. Preparation of Photocatalysts. The COP-TPx:y and other polymers were synthesized in the light of our previous literature method.36, COP-TP3:1,

42

Typically, for

0.5 g Ni(cod)2 and 0.2839 g 2, 2’-bipyridyl were dissolved in 65 ml dry

DMF, then, 0.222 mL cod was added to the above solution. Afterwards, TBP (0.1335 g) and DBP (0.029 g) were added to the resulting solution. The reaction temperature was maintained at 85°C overnight under nitrogen atmosphere. After the resulting solution was cooled down to the room temperature, the concentrated HCl (2.5 ml) was added to the above suspension, then stirring for one hour. After filtration, the filter cake was washed by CHCl3, THF and H2O, respectively. The synthesized sample was dried at 80 ℃ in vacuum oven for overnight. 2.3. Photocatalytic Activity Evaluation. The measurement was carried out according to our previous literature method.40 Photocatalytic hydrogen evolution was meansured in a 150 mL closed quartz vessel containing 50 ml water. Then, 10 mg COP-TPx:y was dispersed in the above water, 10 mL Triethanolamine and a certain 6 ACS Paragon Plus Environment

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amount of H2PtCl6 (Pt, 3 wt %) were added into the above solution. Ultrasonic treatment was carried out for 30 min, then the air in vase was removed via nitrogen bubbling. The reactant suspension was illuminated under a 300 W Xe-lamp. The produced hydrogen was detected via gas chromatography. 2.4. The Apparent Quantum Yield (AQY) measurement. The apparent quantum yield (AQY) was carried out under different monochromatic light which was obtained by 300 W Xe light irradiation with different optical filters (400, 450, 500, and 550 nm). The apparent quantum yield was carried out by the equation below:43-44 Apparent Quantum Yield = (The reacted electrons) / (The incident photons)×100% ଶ×௠×ே×ா

=

ௌ×௉×்

×100%

E is the photon energy, m is the produced hydrogen molecules, N is Avogadro constant, S is the illumination area, T is the irradiation time.44 2.5. Electrochemical measurements. The fluorine doped tin oxide (FTO) was used as working electrode. Then 4 mg COP-TPx:y was dispersed into 1 ml ethanol containing 20 µl Nafion (0.5 wt%) and ultrasonic treatment was carried out for 30 min. After that, 40 µL of the as-prepared suspension was dropped on FTO glass with an active area 1 cm2. Electrochemical measurement was carried out using a three electrode system with 0.1 M Na2SO4 electrolyte, among which, the FTO was a working electrode, the Pt slice was a counter electrode and the saturated Hg/HgCl2 was a reference electrode. The Mott−Schottky plot was tested with a bias potential that ranged from -1.0 to 1.0 V (vs Hg/HgCl2) at 1 KHz frequency. The photocurrent curve of COP-TPx:y was implemented under visible light without an external bias 7 ACS Paragon Plus Environment


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potential. (ERHE = EHg/HgCl2 + 0.059 PH + 0.2438) 2.6. Characterization. The spin-trapped paramagnetic oxygen species were detected via electron spin resonance (ESR) spectra (Bruker 500, Germany). The valence band spectra and work function (ϕ) of these COPs were obtained from ultraviolet photoelectron spectroscopy (UPS) (VG Scienta R4000) with He I light (21.2 eV) and −5 V bias. The UV–Vis diffuse reflectance spectra was performed via integrating sphere method using UV-visible spectrophotometer (TU-1901). The Brunauer-Emmett-Teller (BET) surface area was carried out by N2 adsorption / desorption isotherms (P/P0 = 0.05 ~ 0.30, Micromeritics ASAP 2020). 13C solid-state nuclear magnetic resonance spectra (NMR) was performed at 75.5 MHz (Bruker AV300). The morphologies of COP-TPx:y were analyzed via Scanning electron microscopic (SEM, Hitachi S-4700, Japan) and Transmission electron microscopic (TEM, Hitachi-7700, Japan). The copolymerizing condition was characterized by Fourier transform spectrophotometry (FT-IR, Bruker Vertex 70V). The electron density around different elements of these COP materials was measured via X-ray photoelectron

spectroscopy

(XPS,

ESCALAB

250).

The

steady-state

photoluminescence (PL) spectra and Powder X-ray diffraction (PXRD) of the COP samples were carried out via Fluorescence Spectrophotometer (Hitachi F-7000, Japan) and D/MAX 2000 instrument, respectively. Theoretical calculation was carried out according to density functional theory (DFT), which used the DMOL package with the form of Perdewe-Beckee-Ernzerhof (PBE) and the generalized gradient approximation (GGA). 8 ACS Paragon Plus Environment

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2.7. Preparation of samples for Ultraviolet photoelectron spectroscopy (UPS). The silicon wafer was cleaned via ultra-sonication for 30 min using ethanol. Then 1 mg synthesized COP samples were uniformly dispersed into 20 mL ethanol and ultrasonically treated for 30 min. 10 µl of the as-prepared homogeneous suspension was dropped on silicon wafer, after dried, another 10 µl suspension was dropped and repeated this stretch 10 times. Finally, a thin film would be formed on the silicon wafer. 3. RESULTS AND DISCUSSION 3.1 Morphology, Structure and Physicochemical properties. As shown in the Scheme 1, the copolymerization of COP-TPx:y was carried out by Ni-catalyzed Yamamoto type Ullmann cross-coupling reaction.36,

42

COP-TP1:0 was synthesized

according to the reported Suzuki−Miyaura polycondensation.18 The successful synthesis of the COP-TPx:y is further confirmed by solid-state shown in Figure S1. The characteristic

13

13

C NMR spectra

C NMR peak of the central pyrene units in

the COP-TPx:y at 127 ppm can easily be found. Carbon signal of bonding two different molecules can be found at 134 ppm for COP-TP1:0 to COP-TP1:4 and 131 ppm

for

COP-TP0:1.



Scheme. 1 Schematic illustration of synthetic route for COP-TPx:y using 1, 3, 6, 8-tetrabromopyrene (TBP) with 3, 8-Dibromophenanthroline (DBP) by the nickel(0)-catalyze Yamamoto-type Ullmann cross-coupling reaction .

These results manifest the conversion of the precursors into the respective COP-TPx:y. Moreover, FT-IR spectra (Figure 1a and S2) was adopted to characterize the

Figure 1. (a) Transmission FT-IR spectra of COP-TPx:y as KBr pellets, (b) Nitrogen adsorption and desorption isotherm curves of COP-TPx:y, (c) UV−vis diffuse reflectance spectra of COP-TPx:y.

copolymerization condition of COP-TPx:y. The typical signal of FT-IR spectra at approximately 1590 cm-1 is attributed to the skeleton vibration of the aromatic nucleus 10 ACS Paragon Plus Environment

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in COP-TPx:y and the signals at 1430 cm-1 can be ascribed to the C=N stretching modes in COP-TPx:y.26 When increasing the ratio of DBP, the signals at 1430 cm-1 has been strengthened. Raman spectra was further employed in Figure S4, the two prominent peaks around ≈1350 and ≈1600 cm−1 are attributed to the D and G bands of aromatic rings, respectively.24 The Brunauer-Emmett-Teller (BET) surface areas and porous stuctures of COP-TPx:y were investigated via nitrogen gas absorption measurements (Figure 1b). The hysteresis loop of all COP-TPx:y is presented on the basis of N2 adsorption/desorption curves at relatively low pressure due to the existence of mesoporous in the COP networks. The average pore sizes are concentrate upon 3-6 nm approximately in the light of the figure of pore size distribution (Figure S5), which is decided via BJH technology. The BET surface area of COP-TPx:y is summarized in Table S2 and COP-TP1:0 shows the largest area 640 m2 g-1. The BET surface area of COP-TPx:y decreases with the increasing number of electron-acceptor DBP, probably due to the larger conjugated system which leads to severe π-π stacking and tier upon tier in the bulk.26 When without TBP, COP-TP0:1 performs the smallest BET surface area 35 m2 g-1. The scanning electron microscopy (SEM) was employed to study the morphologies of these COP materials (Figure S6). The degree of aggregation increases with the augmentation of electron-withdrawing units from COP-TP1:0 to COP-TP0:1. The agglomerates gradually grow larger with the layers stacking together severely which is in agreement with nitrogen gas absorption analysis. Owing to the 11 ACS Paragon Plus Environment


disordered property of copolymerized structures, the COPs showed amorphous texture, which is determined via the powder X-ray diffraction (PXRD) (Figure S8). The N 1s XPS peaks of the COP-TPx:y shift to lower binding energy with the DBP increases as shown in Figure S10. Such lower-energy shifts should be ascribed to the increase of electron density around the N atom in DBP on account of the intermolecular π–π interaction.45 3.2 Optical property, Band structure and Separation of electron-holes. The UV-vis diffuse reflectance spectra (DRS) of COP-TPx:y is shown in Figure 1c. All samples

Figure 2. (a) The valance band (VB) spectra were measured with UPS, (b) Band structure diagram for COP-TPx:y, (c) Mott-Schottky plots of COPs, (d) Photocurrent response for COP-TPx:y under visible light irradiation. 12 ACS Paragon Plus Environment

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clearly show the major absorption peak located at nearby 450 nm, which can be attributed to π → π* electron transition in the conjugated system.46 These optical absorption band edge can even reach to 700 nm, from which the calculated bandgap fall into 1.76 to 1.84 eV from COP-TP1:0 and COP-TP0:1, respectively, based on the Kubelka-Munk function (Figure S11).47 Not only is the proper bandgap helpful for enhancing the visible light absorption, but also the appropriate conduction band level of COP-TPx:y is extremely important for hydrogen evolution. We used ultraviolet photoelectron spectroscopy (UPS) to determine the valance band energy (Ev), which was measured via VG Scienta R4000 instrument with He I light (21.2 eV) and −5 V bias as exhibited in Figure 2a, S13 and S14. The Kubelka–Munk plot shows that the band gap of COP-TPx:y and Figure 2b further shows the position of conduction band (Ec) and valance band energy (Ev). Owing to the effect of donor-acceptor groups, COP-TP3:1 exhibits the lowest conduction band energy -0.39 eV which decreases 0.34 eV and 0.32 eV, respectively, compared with COP-TP1:0 and COP-TP0:1. Such result suggests the enhanced thermodynamic driving force for improving electron mobility and facilitate photo-induced electrons to move to the reactive sites.21 The Mott−Schottky curves (Figure 2c) was employed to reveal the relative conduction band (CB) of COPs. From the Mott−Schottky curves, the positive slop can be observed which manifest the character of n-type semiconductors.48 The conduction band (CB) of COP-TP3:1 was -0.39 eV and other CB of samples were shown in Figure 2c, which was consistent with the UPS results. The steady-state photoluminescence (PL) spectra with an excitation wavelength of 13 ACS Paragon Plus Environment


450 nm for COP-TPx:y is shown in Figure 3a. COP-TP3:1 exhibited the strongest intensity which manifest that the number of photo-induced charge carriers was the most and the nonradiative transition channel was suppressed.49 When the content of pyrene increased, the COPs show a gradual redshift owing to the ring-strain of pyrene.18 Meanwhile, the photoexcited holes and electrons of COP-TP3:1 could be tested via DMPO-assisted electron spin resonance (ESR) spectra (Figure 3c and 3d), which can monitor the spin-trapped paramagnetic oxygen species •O2− and •OH.50 COP-TP3:1 shows no signal under dark demonstrating no existence of electron-holes. However, under visible light illumination, the peak in Figure 3c is assigned to DMPO-•O2−

Figure 3. (a) Photoluminescence spectra (PL) of the COP-TPx:y measured in the solid state (λexcitation = 450 nm), (b) Time-resolved transient photoluminescence (PL) decay 14 ACS Paragon Plus Environment

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of COP-TPx:y (The inset shows average lifetime τa), (c) ESR spectra for DMPO-•O2− over COP-TP3:1, (d) ESR spectra for DMPO-•OH over COP-TP3:1. product, which indicates visible-light photocatalytic activity of COP-TP3:1.51-52 Because the potential for generating H2O/•OH or OH-/•OH radicals should be 1.99 eV and 2.38 eV vs. RHE respectively and the valance band (VB) of COP-TP3:1 is 1.45 eV

vs. RHE, the DMPO-•OH signal was not detected even under visible light. The above result confirms that the valance band (VB) of COP-TP3:1 is less than 1.99 eV from another point of view, which is consistent with the UPS results. To better understand the role of donor-acceptor molecules, photocurrent response was carried out (Figure 2d). The photocurrent of COP-TP3:1 outdistances COP-TP1:0 and COP-TP0:1 which demonstrates that the combination of donor-acceptor molecules can accelerate to generate more electron-holes and the ratio of donor-acceptor molecules is also crucially important. To elucidate the recombination dynamics of photogenerated electron-holes, Time-resolved transient photoluminescence spectra (Figure 3b) at the corresponding emission peaks was carried out. The average lifetimes of COP-TPx:y were from 1.01 to 1.66 ns, among them, COP-TP3:1 shows the longest lifetime. The longer average lifetime implies the recombination of electron-holes was restrained.53-54 3.3 Photocatalytic Hydrogen Evolution. The photocatalysis experiments of the COP-TPx:y were performed using 50 mL deionized water containing 10 ml triethanolamine (TEOA) which was used as sacrificial electron donor under visible light (λ ≥ 400 nm). Hexachloroplatinic acid was used to form platinum (Pt2+) as 15 ACS Paragon Plus Environment


cocatalyst to reduce the overpotential. For further characterizing whether the Pt2+ is coordinated with N sites of COP-TP3:1 during photocatalytic reaction, XPS (X-ray photoelectron spectroscopic) spectra (Figure 4a and 4b) was carried out. Pt element in COP-TP3:1 is Pt2+ according to Pt 4f7/2 peak at 72.4 eV (Figure 4b). The peaks of COP-TP3:1 at 399.2 eV (Figure 4a) belong to the sp2-bond nitrogen group confirming the formed coordinate bonds of Pt2+ with N atoms because of their lone-pair electrons.55 Moreover, the N 1s peak of COP-TP3:1 after photocatalysis is shifted to higher energy being ascribed to the reduced electron density of the N atoms, which is a typical feature for coordination of Pt2+ with N atoms from phenanthroline monomers(Figure 4b).55-57 The performance of hydrogen production is illustrated in Figure 5a. Among them, COP-TP3:1 exhibits the highest hydrogen evolution of 42 µmol h-1 (equal to 4200 µmol

Figure 4. (a) XPS spectra of COP-TP3:1 before reaction, (b) XPS spectra of COP-TP3:1 after reaction. g-1 h-1) under visible light irradiation (λ ≥ 400 nm), which is much better than other


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organic polymers (see Table S3 for more comparison information). On the one hand, the high photocatalytic activity of COP-TP3:1 can be attributed to the large BET surface area and porosity which enhanced mass transport in the porous structures.58-59 The exist of donor-acceptor molecules can help to facilitate the separation of electron-holes (Figure 3b). On the other hand, The formed coordination of Pt2+-N from phenanthroline donors60 can also shorten the distance of charge transportation and boost to transmit the charge carriers to reactive sites, hence leading to high hydrogen evolution

61, 55

Such

results can be also confirmed by theoretical calculations on the basis of density functional theory (DFT) (Figure S15). When the electron-withdrawing property of electron-acceptor molecule strengthens, the Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital can be well separated where HOMO is offered from the donor units and LUMO is located on the acceptor moiety,62 which contributes to promote electron-holes separation and enable charge carriers to be easily collected. The calculation results are consistent with the time-resolved transient



Figure 5. (a) Photocatalytic hydrogen evolution of COP-TPx:y, (b) Photocatalytic hydrogen evolution of COP-TP3:1 in different water quality, (c) Apparent quantum yield (AQY) of COP-TP3:1 at 400 nm, 450 nm, 500 nm, 550 nm. (d) Stability test of COP-TP3:1 during photocatalytic hydrogen production.

photoluminescence spectra (Figure 3b). Though the lifetime of COP-TP1:0 is shorter than COP-TP1:2 or COP-TP1:4 and the BET surface area is larger than COP-TP1:2 or COP-TP1:4, the photocatalytic efficiency of COP-TP1:0 is the lowest. Such performance should be ascribed to following results: i) the highest conduction band energy which decreases the thermodynamic driving force for hydrogen evolution; ii) having no electron-acceptor units leading to generate too small amount of photoinduced charge carriers; iii) short of reactive sites due to the


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disappearance of N atoms. As the number of electron-acceptor molecules increases, the BET surface areas (Table S2) decrease and lifetimes become longer, resulting in decreased photocatalytic hydrogen production for COP-TP1:2 and COP-TP1:4. Hence, copolymerizing

electron-donor

molecules

with

appropriate

number

of

electron-withdrawing molecules is of crucial importance to donor-acceptor system. The apparent quantum yields (AQY) of COP-TP3:1 were also measured at different monochromatic light, as exhibited in Figure 5c. The highest AQY of 1.5% was achieved at 400 nm. With regard to the remarkable photocatalytic activity, COP-TP3:1 exhibits a great stability after discontinuous photocatalytic test and shelved for a month as shown in Figure 5d, only 7% decrease of H2 evolution can be observed. What’s more, COP-TP3:1 also performs excellent photocatalytic activity under different water quality (Figure 5b) and the photocatalytic efficiency still maintains 85% even in simulated seawater. 3.4 The Effect of Electron-Acceptor on Donor-Acceptor System. For better understanding the effect of electron-acceptor molecule on donor-acceptor system, we choose two weaker electron-withdrawing units, pyrazine and benzene. The copolymerization was carried out via the same type Ullmann cross-coupling reaction at 85℃ for overnight, using TBP with 1,4-Dibromobenzene and 2,5-Dibromopyrazine respectively, termed as CMP-DBPy and COP-TZ (Figure S17). The synthesis of the CMP-DBPy is according to the published method

39

. The morphology of these two

COPs was investigated via SEM (Figure S7), the FT-IR spectra and XRD was shown in Figure S3 and S9. The UV-vis spectra of COP- TPx:y is shown in Figure S18. The 19 ACS Paragon Plus Environment


narrower bandgap and shorter average lifetime of CMP-DBPy and COP-TZ than COP-TP3:1 indicate that the easier recombination of electron-holes for CMP-DBPy and COP-TZ (Figure S12 and S19). The theoretical calculations (Figure S16) also illustrate that the HOMO and LUMO is not separated clearly in contrast to COP-TP3:1 which demonstrates the easier recombination of electron-holes for CMP-DBPy and COP-TZ. As shown in Figure S20, the photocurrent of CMP-DBPy and COP-TZ is too small compared with COP-TP3:1 manifesting that the number of photoinduced charge carriers is very low. The CMP-DBPy and COP-TZ exhibit poor photocatalytic efficiency (Figure S21) by introducing the weaker electron-withdrawing units into donor-acceptor system, due to the small quantity of photo-induced charge carriers and easier recombination of electron-holes. Meantime, maximizing H2 production of the co-polymers might be a balance between the intensity of visible light absorbed and the thermodynamic driving force for proton reduction.63 4. CONCLUSION In summary, the COPs can be rationally designed for suitable energy band structure based on donor-acceptor strategy, which contribute to accelerate efficient charge carriers dissociation and enhance the photocatalytic hydrogen evolution. The results of experiment and theoretical calculation confirm the above donor-acceptor strategy for suppressing the recombination of electron-holes. Using Pt as cocatalyst, COP-TP3:1 exhibits an efficient hydrogen evolution 42 µmol h-1 and apparent quantum yields (AQY) 1.5% at 400 nm. Furthermore, the photocatalytic efficiency of COP-TP3:1 still maintains 85% even in simulated seawater. The superior 20 ACS Paragon Plus Environment

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photocatalytic efficiency of COP-TP3:1 is ascribed to the high production of photo-induced charge carriers, efficient electron-holes separation, fast charge carrier transportation. Meantime, the COP-TP3:1 exhibits ultrahigh stability and regenerability, after discontinuous cycles of photocatalysis and storage for a month, no obvious decrease of H2 evolution can be observed. Accordingly, we expect that this work can pave the way to design stable and efficient photocatalyst by copolymerization for overall water splitting under broad-spectrum irradiation. Supporting Information Supporting Information is available free of charge on the ACS Publications website at DOI:10.1002/ (please add manuscript number). Element analysis, Solid state 13C CP/MAS NMR spectra, FT-IR, Raman spectra, Pore size distribution, SEM images, PXRD, XPS, Bandgap energy, UV spectroscopy, Theoretical calculations using DFT method, Schematic illustration of synthetic route, UV−vis diffuse reflectance spectra, Time-resolved transient photoluminescence decay, Periodic on/off photocurrent response, Time-Course photocatalytic hydrogen evolution, Comparison of photocatalytic activity.

AUTHOR INFORMATION Corresponding Author *E-mail address: [email protected] Notes The authors declare no conflict of interest. ACKNOWLEDGEMENTS



This work was supported by National Natural Science Foundation of China (51502012; 21676020, 21620102007); Beijing Natural Science Foundation (2162032, 17L20060); Young Elite Scientists Sponsorship Program by China Association for Science and Technology (2017QNRC001); the Fundamental Research Funds for the Central Universities by Ministry of Education of the People's Republic of China (buctrc201420; buctrc201714; ZD1502), the “111” project of China by State Administration of Foreign Experts Affairs (B14004) and Distinguished scientist program at BUCT (buctylkxj02). We are thankful to Prof. Yufei Zhao from Beijing University of Chemical Technology for helpful discussion. REFERENCES (1) Chen, S.; Takata, T.;Domen, K. Particulate Photocatalysts for Overall Water Splitting. Nat. Rev. Mater. 2017, 2, 17050-17067. (2) Bai, S.; Wang, L.; Li, Z.;Xiong, Y. Facet-Engineered Surface and Interface Design of Photocatalytic Materials. Adv. Sci. 2017, 4, 1600216-1600223. (3) Wang, W.; An, T.; Li, G.; Xia, D.; Zhao, H.; Yu, J. C.; Wong, P. K. Earth-abundant Ni2P/g-C3N4 Lamellar Nanohydrids for Enhanced Photocatalytic Hydrogen Evolution and Bacterial Inactivation Under Visible Light Irradiation. Appl.

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