Boosting visible-light hydrogen evolution of covalent-organic frameworks by introducing Ni-based noble metal-free co-catalyst

Journal Pre-Proof Boosting Visible-Light Hydrogen Evolution of Covalent-Organic Frameworks by Introducing Ni-based Noble Metal-Free Co-Catalyst Hong Dong, Xiang-Bin Meng, Xin Zhang, Hong-Liang Tang, Jun-Wang Liu, Jian-Hui Wang, Jin-Zhi Wei, Feng-Ming Zhang, Lin-Lu Bai, Xiao-Jun Sun PII: DOI: Reference:

S1385-8947(19)31745-0 https://doi.org/10.1016/j.cej.2019.122342 CEJ 122342

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Chemical Engineering Journal

Received Date: Revised Date: Accepted Date:

22 May 2019 17 July 2019 25 July 2019

Please cite this article as: H. Dong, X-B. Meng, X. Zhang, H-L. Tang, J-W. Liu, J-H. Wang, J-Z. Wei, F-M. Zhang, L-L. Bai, X-J. Sun, Boosting Visible-Light Hydrogen Evolution of Covalent-Organic Frameworks by Introducing Ni-based Noble Metal-Free Co-Catalyst, Chemical Engineering Journal (2019), doi: https://doi.org/10.1016/j.cej. 2019.122342

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Boosting Visible-Light Hydrogen Evolution of Covalent-Organic Frameworks by Introducing Ni-based Noble Metal-Free Co-Catalyst Hong Dong,‡ Xiang-Bin Meng,‡ Xin Zhang, Hong-Liang Tang, Jun-Wang Liu, Jian-Hui Wang, Jin-

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Zhi Wei, Feng-Ming Zhang,* Lin-Lu Bai*, and Xiao-Jun Sun*

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School of Materials Science and Engineering, College of Chemical and Environmental Engineering, Harbin University of Science and Technology. Harbin 150040, P.R. China.

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Corresponding author: *E-mail: [email protected] *E-mail: [email protected]

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‡ These authors contributed equally to this work.

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*E-mail: [email protected]

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Abstract

Covalent-organic frameworks (COFs) have been recognized as an emerging type of photocatalysts for H2 evolution and some of them have shown really outstanding photocatalytic activity with the

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help of Pt co-catalyst. To avoid the utilization of noble metal in COF-based photocatalysts, for the first time, we designed and constructed a series of nickel hydroxide-modified COF (TpPa-2) composite materials Ni(OH)2-X%/TpPa-2 (X: molar fraction of Ni(OH)2), which show apparently

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improved photocatalytic H2 evolution activity than that of the parent COF and the activity is comparable to that with Pt (0.3 wt%) co-catalyst. A series of Ni(OH)2-X%/TpPa-2 were prepared

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by in-situ adding TpPa-2 into the reaction system of Ni(OH)2, and the resulting Ni(OH)2-X%/TpPa-

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2 exhibit a novel sandwich-like morphology. The results of photocatalytic hydrogen evolution under visible light irradiation show that the optimized photocatalytic H2-evolution rate for Ni(OH)22.5%/TpPa-2 is up to 1895.99 μmol·h−1·g−1, which is about 26.3 times higher than that of the parent TpPa-2 and is one of the best performing 2D COF-based photocatalyst for H2 evolution. Further investigation confirm the improved activity should be attributed to the enhanced visible-light absorption of the composite materials contributed from Ni(OH)2 and the synergetic effect of Ni(OH)2 and metallic Ni derived from the in-situ reduction of Ni(OH)2, which promoted the separation of photogenerated electron–holes of the resulting materials. This work not only presents

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a series of new photocatalysts for efficient H2 evolution but also open an avenue for future design and synthesis of COF-based composite materials acting as a substitute of noble-metal-containing photocatalytic systems. Keywords: Covalent-organic frameworks • Ni(OH)2 • Hydrogen production • Co-catalyst

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1. Introduction

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Hydrogen is considered to be a substitute for fossil energy due to the high-energy capacity, environmental friendliness and recycling possibility.[1-4] Photocatalytic converting the solar energy

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into clean H2 energy has been regarded as one of the most ideal solution to resolve the global

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energy and environmental problems.[5-9] Therefore, enormous efforts have been devoted to develop efficient photocatalytic systems for hydrogen evolution via photoinduced water splitting. Conventional semiconductor photocatalysts, such as TiO2 and CdS, are still suffered from narrow

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light absorbance or photocorrosion, which severely restrict their practical applications.[10-14] In

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recent years, 2D layered photocatalysts, such as graphitic carbon nitride (also known as g-C3N4), has been proven to be prospective candidate for H2 production due to their facile synthesis, high physicochemical stability, and appealing electronic band structure.[15-18] However, the

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photocatalytic efficiency of pure g-C3N4 is still not ideal due to several obstacles, such as low surface area without forming textured pores, limited visible light absorbance of only blue light for

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the solar spectrum (460 nm), and high electron-hole recombination rate.[19-21] Especially the high photogenerated charge recombination rate in the polymer photocatalysts leads to noble metal cocatalyst is necessary in most case to get a remarkable catalytic activity.[22-26] Therefore, it is

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urgent to develop new photocatalysts that can efficiently split water into hydrogen under visible

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light irritation and without the utilization of noble metal at the same time. Covalent-organic frameworks, as a new class crystalline porous materials assembled with

organic molecules by covalent bonds, have been explored and applied in many areas,[27] such as gas separation and storage,[28, 29] chemical sensing,[30, 31] drug delivery,[32, 33] and catalysis.[34, 35] In recent years, COFs have been recognized as a new type of photoactive materials for light induced H2 evolution and some of them have exhibited really excellent photocatalytic activity.[36-41] In fact, COFs possess their inherent advantage as photocatalysts including the designable structures and band gap, long-range order structures, large surface areas

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and outstanding visible light absorbance.[20, 42-44] Moreover, most of COFs, especially these synthesized by Schiff-base reaction, usually show outstanding chemical stability in water solution.[45] Until now, most of reported COFs photocatalysts are main 2D structures with intense interaction between the adjacent layers. The π-π stacking in them mediates electronic interactions between the layers, thus providing another possible pathway for charge carrier transport besides

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transfer within the covalent sheets. Recently, our group firstly reported TpPa-COF, as a series of

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facile synthesized COFs, can act as promising H2 evolution photocatalysts and their activities are

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apparently better than that of the known g-C3N4 polymer photocatalyst.[46, 47] Nevertheless, like

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other organic polymer photocatalysts, the utilization of Pt co-catalyst is sill inevitable in almost all COF-based photocatalysts to promote the separation of photogenerated charges realizing the high catalytic activity.[38, 46, 48-50] The only exception is a recently attempt that dissolved a metallic

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Co2+ complex (chloro (pyridine) cobaloxime) into the reaction system to accept

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photogenerated eclectron from COF, and realized a H2 evolution rate of 782 μmol h−1 g−1.[37] In addition, previous investigations have indicated that Ni(OH)2 is an economical co-catalyst in photacatalytic H2 production. For example, Qiao group reported the enhanced visible-light

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photocatalytic H2 production by ZnxCd1-xS modified with earth-abundant Ni(OH)2 co-catalysts. The Ni(OH)2-loaded ZnxCd1-xS exhibits a very high photocatalytic H2-production rate of 7160 μmol hHowever, Ni(OH)2 has not been composited with COFs and if it is suitable for COF-based

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1g-1.[51]

photocatalysts as a co-catalyst to replace noble metals is also unknown until now. In this work, for the first time, we designed and constructed a series of nickel hydroxide-

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modified COF (TpPa-2) composite materials (Ni(OH)2-X%/TpPa-2), which show extremely

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improved photocatalytic H2 evolution activity than that of the parent COF. The effect of Ni(OH)2 loading on the photocatalytic H2-production rates of the resulting composite materials were also investigated and discussed. The results of photocatalytic hydrogen evolution under visible light irradiation (λ ≥ 420 nm) show that the optimized photocatalytic H2-evolution rate of Ni(OH)22.5%/TpPa-2 is up to 1895.99 μmol·h−1·g−1, which is about 26.3 times higher than that of the parent TpPa-2 under the same condition and is one of the best performing 2D polymer-based photocatalyst for H2 evolution. The photocatalytic activity of Ni(OH)2-2.5%/TpPa-2 is roughly equivalent to TpPa-2 with Pt loading of 0.3 wt%. Furthermore, The results of electrochemical impedance

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spectroscopy (EIS), photo-luminescence (PL), surface photovoltage spectroscopy (SPV) and transient photocurrent demonstrate that the improved activity should be attributed to the enhanced visible-light absorption of the composite materials contributed from Ni(OH)2 and the synergetic effect of Ni(OH)2 and metallic Ni derived from the reduction of Ni(OH)2, which promoted the

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separation of photogenerated electron–holes of the resulting materials.

materials. 2. Materials and methods

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Scheme 1. Schematic illustration for the synthesis a series of Ni(OH)2-X%/TpPa-2 composite

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2.1. Materials and Characterization.

All of the reagents and solvents were commercially available and used without further purification

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besides 1,3,5-triformylphloroglucinol (Tp) which was prepared from Phloroglucinol according to literature method.[52] The composition, structure and texture properties of the materials were

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investigated by Fourier transform infrared (FT-IR) spectra (Spectrum 100), X-ray powder diffraction (XRD) patterns (Bruker D8 X-ray diffractometer), scanning electron microscopy (SEM)

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micrographs (Hitachi S-4800), transmission electron microscopy (TEM) experiments (JEM-2100 electron microscope), thermogravimetric analyses (TGA) (SDTA851e). N2 adsorption-desorption isotherms were determined by Micromeritics ASAP 2020 analyzer at 77K. Before the measurement, the catalysts were degassed at 120 °C for 24 h. Optical properties were also studied by diffuse reflectance UV-vis spectroscopy (a Lambda 35 spectrometer) in the wavelength range of 200-800 nm. Electrochemical impedance spectroscopy (EIS) tests were conducted in the frequency range from 105 to 0.1 Hz. EIS, Mott-Schottky plot and photocurrent-time (I-T) profiles were recorded on the CHI660E electrochemical workstation with a standard three-electrode system with the

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photocatalyst-coated ITO as the working electrode, Pt plate as the counter electrode, and a saturated Ag/AgCl electrode as a reference electrode. During the measurement, a 300 W Xenon lamp with a 420 nm cut-off filter was used as the light source. A 0.25 M Na2SO4 solution was used as the electrolyte. The as-synthesized samples (2 mg) were added into 1 mL ethanol and 10 µL Nafion mixed solution, and the working electrodes were prepared by dropping the suspension (200 µL)

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onto an ITO glass substrate electrode surface and dried at room temperature. The

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photoluminescence (PL) spectra of the samples were measured with a SPEX Fluorolog-3

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spectrofluorometer at excitation wavelength of 380 nm. The surface photovoltage spectroscopy

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(SPV) measurements of the samples were carried out with a home-built apparatus equipped with a lock-in amplifier (SR830) synchronized with a light chopper (SR540). The powder sample was sandwiched between two ITO glass electrodes by which the outer electric field could be employed.

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The sandwiched electrodes were arranged in an atmosphere controlled container with a quartz window, and monochromatic light was obtained by passing light from a 500 W xenon lamp (CHF

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XQ500W, Global xenon lamp power) through a double prism monochromator (SBP300). The SPV signals are the potential barrier change of testing electrode surface between the presence of light and darkness.

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2.2. Synthesis of TpPa-1 and TpPa-2.

A vacuum valve measuring o.d × i.d. = 19 × 8 mm2 and length 150 mm was charged with Tp (63

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mg, 0.3 mmol), paraphenylenediamine (Pa-1, 48 mg, 0.45 mmol) for TpPa-1 and 2,5-dimethyl-pphenylenediamine (Pa-2, 61.3 mg, 0.45 mmol) for TpPa-2, 1.5 mL of 1,4-dioxane, 0.5 mL of 3 M

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aqueous acetic acid. This solution was then sonicated for 20 min to get a homogenous dispersion. The vacuum valve was then flash frozen at 77 K (liquid N2 bath) and degassed by three freeze-

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pump-thaw cycles. Subsequently, the vacuum valve was sealed under vacuum and then heated at 120 °C for 3 days. The precipitate was collected by centrifugation or filtration and washed with anhydrous THF (50 mL) thrice. The powder collected was soaked in anhydrous acetone for 48 h, replaced the solvent for 5~6 times and then dried at 180 °C under vacuum for 12 h. 2.3. Synthesis of Ni(OH)2. Typically, the Ni(NO3)2 (0.05 M) dropwise was added NaOH aqueous solution (30 mL, 0.25 M) under stirring. The resulting mixture was stirred for 6 h at room temperature. After that, the precipitate was washed three times with deionized water and ethanol, and dried at 60 °C for 8 h.

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2.4. Synthesis of Ni(OH)2/TpPa-2. TpPa-2 (0.05 g) was dispersed in NaOH aqueous solution (30 mL, 0.25 M), and Ni(NO3)2 (0.05 M) was added dropwise under stirring. The resulting mixture was stirred for 6 h at room temperature. After that, the precipitate was washed three times with deionized water and ethanol, and dried at 60 °C for 8 h. The molar ratio (R) of Ni(OH)2 to (TpPa-2 + Ni(OH)2 ) was 0.5, 1.0, 2.5, 4.0 and 5.0

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(mol %), and the resulting samples were labeled as Ni(OH)2-0.5%/TpPa-2, Ni(OH)2-1.0%/TpPa-2,

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Ni(OH)2-2.5%/TpPa-2, Ni(OH)2-4.0%/TpPa-2, and Ni(OH)2-5.0%/TpPa-2, respectively.

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2.5. Photocatalytic hydrogen evolution.

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All photocatalytic experiments were conducted in a 250 mL Pyrex reaction vessel via a photocatalytic H2 evolution activity evaluation system, where the photoreaction temperature was kept at a constant temperature (4 °C) with circulating water through a thermostat. In a typical

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reaction, 10 mg of the photocatalyst was suspended in PBS buffer solution (50 mL of 0.1 M solution at PH = 7) containing 100 mg of sodium ascorbate (SA) as sacrificial electron donor, and

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then the above mixed solution was dispersed in the ultrasound bath for 30 min. Then the solution was irradiated with a 300 W Xe lamp equipped with a 420 nm cut-off filter under vigorous stirring. The system was vacuumed at least 30 min to remove the dissolved oxygen with vacuum bump. The

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above reaction solution was stirred for 5 h and irradiated. The hydrogen evolved was determined by a GC112A gas chromatograph with TCD detector. The photocatalytic stability was performed for

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25 h in the same processing parameters except that the sacrificial regent was renewed for every 5 h in the aerobic condition.

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3. Results and discussion

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3.1. Preparation process and structure characterization of Ni(OH)2-X%/TpPa-2. TpPa-2 was prepared by solvothermal reaction of 1,3,5-triformylphloroglucinol (Tp) and 2,5dimethyl-p-phenylenediamine (Pa-2) at 120 °C for 3 days.[52] And a series of Ni(OH)2-X%/TpPa-2 (X: molar fraction) composite materials were prepared by in-situ adding exact amount of TpPa-2 into the synthetic reaction system of Ni(OH)2 containing different proportion NaOH and Ni(NO3)2 at room temperature (Scheme 1). The structure of TpPa-2 and Ni(OH)2-X%/TpPa-2 composites with different Ni(OH)2 content were initially confirmed by fourier transforminfrared (FT-IR) spectroscopy and powder X-ray diffraction (PXRD). The FT-IR spectra indicate the presence of

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characteristic bands corresponding to the stretching frequency of the C-N (1250 cm-1) and –CH3 (2882 cm-1) bonds for TpPa-2 compared with Tp and Pa-2 (Figure S1), which clearly revealed that the formation of the β-ketoenamine-linked framework structures. In addition, the Ni(OH)2X%/TpPa-2 composites show silimar FT-IR character with TpPa-2 (Figure S2), indicating that the structure of TpPa-2 has no obvious change after composited Ni(OH)2. It can be seen that the PXRD

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patterns of all composite materials and TpPa-2 exhibit intense peaks at 4.63°, corresponding to the

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reflection from the (100) plane of COF component (Figure 1a). In addition, no significant

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diffraction peaks of any Ni(OH)2 can be detected in composite materials, which could be attributed

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to the low content and weak crystallization of Ni(OH)2 in the samples. 3.2. Morphological and structure analysis.

Scanning electron microscopy (SEM) was used to observe the morphology of the as-prepared

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Ni(OH)2, TpPa-2 and Ni(OH)2-2.5%/TpPa-2 composite material. As shown in Figure 1b, pure

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TpPa-2 presents a typical flowerlike morphology similar to that reported previously.[52] The pure Ni(OH)2 displays an aggregational nano-sheet morphology with the diameter of single sheet around 100 nm (Figure 1c and S3). Interestingly, the SEM and TEM images of Ni(OH)2-2.5%/TpPa-2

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show a novel sandwich-like structure, in which Ni(OH)2 nanosheet were mainly embedded in TpPa2 nanosheets and with a part of agglomeration on the surface of TpPa-2 nanosheets (Figure 1d and

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S4). High resolution transmission electron microscopy (HRTEM) of Ni(OH)2-2.5%/TpPa-2 shows that small Ni(OH)2 nanosheets were embeded tightly with TpPa-2 nanosheets (Figure 1e). The observed distance between two adjacent lattice fringes of Ni(OH)2 is 0.214 nm, which is attributed

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to the (101) plane of hexagonal Ni(OH)2.[53, 54] Furthermore, the chemical composition of the

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Ni(OH)2-X%/TpPa-2 composites were determined by energy-dispersive X-ray spectroscopy (EDS) and inductively coupled plasma atomic emission spectroscopy (ICP-AES). The results of EDS and ICP-AES show the content of Ni is basically consistent with precursor added, respectively (Figure S5-S6, Table S1). In addition, TEM element mapping illustrates that the C, N and Ni elements are uniformly distributed in the Ni(OH)2/TpPa-2 composite material (Figure 1f), further demonstrating the existance of Ni(OH)2 in composite materials.

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Figure 1. (a) PXRD patterns of pure Ni(OH)2, TpPa-2 and a series of Ni(OH)2-X%/TpPa-2 composites. (b-d) SEM images for TpPa-2, Ni(OH)2 and Ni(OH)2-2.5%/TpPa-2, respectively. (e-f) HRTEM and EDS element mapping images of Ni(OH)2-2.5%/TpPa-2.

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Thermogravimetric analyses (TGA) under nitrogen atmosphere reveal that TpPa-2 and

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Ni(OH)2-2.5%/TpPa-2 composite material show thermal stability up to 400 °C (Figure S7). The permanent porosity of TpPa-2 and Ni(OH)2-2.5%/TpPa-2 were investigated by N2 absorption and desorption experiments at 77 K (Figure S8). All N2 absorption curves show reversible type I isotherm according to the IUPAC classification. The separate TpPa-2 possesses the largest Brunauer-Emmett-Teller (BET) surface area of 842.6 m2 g-1. However, the BET surface area of Ni(OH)2-2.5%/TpPa-2 decreased to 472.76 m2 g-1 for the presence of Ni(OH)2. The calculated pore size distributions of TpPa-2 is comparable to the reported value, while the obtained pore size of Ni(OH)2-2.5%/TpPa-2 is similar to that of the parent TpPa-2 (Figure S9).[52]

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3.3. XPS analysis. X-ray photoelectron spectroscopy (XPS) measurements were carried out to further analysis chemical composition and chemical status of elements of the as-prepared Ni(OH)2-2.5%/TpPa-2 sample. As shown in Figure 2a, the XPS survey spectrum of Ni(OH)2-2.5%/TpPa-2 obviously

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indicates the co-existence of C, N, O and Ni elements. The corresponding high resolution XPS

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spectra of C 1s, N 1s and Ni 2p of the Ni(OH)2-2.5%/TpPa-2 sample are displayed in Figure 2b–d, respectively. As shown in Figure 2b, three C 1s core-level peaks are obtained with binding energies

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(BEs) of 283.9, 285.2 and 288.3 eV, attributed to C–C, C-C=N and C-OH (C=O) bonds,

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respectively. The N 1s XPS spectrum displayed in Figure 2c can be deconvoluted into two distinct peak with BEs of 398.84 and 403.48 eV, which are assigned to C=N–C bonds and amino functional groups having a hydrogen atom (C-N-H), respectively. The Ni 2p XPS spectra is shown in Figure

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2d. Two major peaks with binding energies at 855.7 and 873.3 eV are observed corresponding to Ni

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2p3/2 and Ni 2p1/2, respectively. The spin-energy separation of 17.8 eV is also the characteristic of a Ni(OH)2 phase.[55-57] Thus, the XPS results can convincingly confirm the presence of Ni(OH)2 in

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the Ni(OH)2-2.5%/TpPa-2 sample.

Figure 2. XPS survey spectrum (a), and high-resolution XPS spectra of the C 1s region (b), N 1s region (c), and Ni 2p region (d) of Ni(OH)2-2.5%/TpPa-2. 3.4. Band structure analysis. The absorbance of the as-prepared TpPa-2 and Ni(OH)2-2.5%/TpPa-2 composite photocatalysts

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were measured by UV-vis diffuse reflectance spectra (DRS). As shown in Figure 3a, it could be observed that the Ni(OH)2-2.5%/TpPa-2 and TpPa-2 show stronger absorption below 620 nm region compared with pure Ni(OH)2. Differently, Ni(OH)2-2.5%/TpPa-2 exhibits stronger absorption in visible-light region for the presence of Ni(OH)2 in composite material. Furthermore, the corresponding band gaps for TpPa-2 was calculated to be 2.07 eV by Tauc plots derived from

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the corresponding DRS (Figure S10). The results of Mott-Schottky (MS) measurements indicate

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that the flat band position (Vfb) of Ni(OH)2-2.5%/TpPa-2 and TpPa-2 is approximately -0.78 and -

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0.67 eV vs. Ag/AgCl, respectively (Figure 3b and S11). Since it is generally believed that the

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bottom of the conduction band (CB) in many n-type semiconductors is more negative by about 0.10 eV than the Vfb,[58, 59] the CB of Ni(OH)2-2.5%/TpPa-2 and TpPa-2 was estimated to be -0.68 and -0.57 eV, respectively, satisfying thermodynamic requirements in the process of photocatalytic

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decomposition of water and hydrogen production as a result of being more negative than the redox potential of H+/H2 (-0.41 V vs. Normal Hydrogen Electrode (NHE), pH = 7). Combined with the

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band gap energy, the valence band (VB) position of TpPa-2 can be calculated, which is at 1.50 eV. The VB edge were more positive than the redox potential of water oxidation (0.82 V vs NHE, pH =

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7). Thus, TpPa-2 possess suitable band structure as photocatalysts for H2-production.

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Figure 3. (a) UV-vis diffuse reflection spectra of Ni(OH)2, TpPa-2 and Ni(OH)2-2.5%/TpPa-2. (b)

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Mott–Schottky plots of Ni(OH)2-2.5%/TpPa-2 and TpPa-2. (c-d) Comparison of photocatalytic H2 evolution activity of series of Ni(OH)2-X%/TpPa-2 and TpPa-2. (e) Comparison of the

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photocatalytic activity of Ni(OH)2-2.5%/TpPa-2, 0.30 wt%-Pt/TpPa-2 and 0.25 wt%-Pt/TpPa-2. (f) The photocatalytic stability of Ni(OH)2-2.5%/TpPa-2.

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3.5. Photocatalytic H2-production revolution. Encouraged by the above characterization results, photocatalytic H2-production activities for the

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series of Ni(OH)2-X%/TpPa-2 composites and TpPa-2 were evaluated under visible light irradiation ( ≥ 420 nm) using sodium ascorbate (SA) as a sacrificial agent. As shown in Figure 3c and 3d, TpPa-2 displays a photocatalytic H2 evolution rate of 72.09 μmol·h−1·g−1. After composited Ni(OH)2, H2 evolution rate of the series of Ni(OH)2-X%/TpPa-2 exhibit obvious increase tendency. Especially, the Ni(OH)2-2.5%/TpPa-2 (with 2.5 mol% Ni(OH)2) shows the optimal hydrogen production rate of about 1895.99 μmol·h−1·g−1, which is nearly 26.3 times higher than that of parent TpPa-2 photocatalyst under the same condition and superior to that of most of the reported COFs catalysts with noble metal co-catalysts (Table S2).[36, 38, 39, 46, 48, 49, 60-62] When the content

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of Ni(OH)2 increases from 0 to 2.5%, the photocatalytic activity also increases with the amount of Ni(OH)2. In addition, when the content of Ni(OH)2 is higher than 2.5 mol% in the resulting composite materials, the photocatalytic H2 evolution rate shows decease tendency, which maybe because excessive Ni(OH)2 maybe shield the surface of TpPa-2 and inhibit its visible-light absorption (Figure S12). Therefore, the Ni(OH)2-2.5%/TpPa-2

shows the optimal hydrogen

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production rate. Furthermore, Ni(OH)2-2.5%/TpPa-2 composite material also shows an apparently

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higher photocatalytic H2 production rate compared with the physical mixing sample of Ni(OH)2 and

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TpPa-2 through grinding under room temperature (Figure S13). To further confirm the advantage of

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composite, we also tested the photocatalytic activity of different platinum loadings on TpPa-2 as control and the results show that the catalytic activity of Ni(OH)2-2.5%/TpPa-2 is roughly

3.6. Photocatalytic results analysis.

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equivalent to that of TpPa-2 with 0.3 wt% Pt loading (Figure 3e and S14).

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For the purpose of evaluating the stable performance of Ni(OH)2-2.5%/TpPa-2 composite material, we conducted photocatalytic stability experiments. Notably, the Ni(OH)2-2.5%/TpPa-2 composite material shows excellent photocatalytic stability in the reused four cycles in 20 h (Figure 3f).

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Besides, the PXRD patterns and SEM morphology of Ni(OH)2-2.5%/TpPa-2 composite material are no obvious change before and after photocatalytic reaction (Figure S15-S16). These results further

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demonstrate the durable performance of the Ni(OH)2-2.5%/TpPa-2 composite material in the photocatalytic reaction. However, some weak NiO lattices appeared in HRTEM image of the Ni(OH)2-2.5%/TpPa-2 composite material after photocatalytic reaction (Figure S17). We also

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measured the XPS for the composite material after photocatalytic reaction, and the XPS spectrum of

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Ni 2p3/2 exhibits a weaker change after photocatalytic reaction (Figure 4), which is similar to other Ni(OH)2-contained

photocatalytic systems.[51, 63] It is observed that a new peak appeared at

854.4 eV, which indicates the formation of a small amount of NiO and is consistent with the observation of HRTEM. The reason of the formation of NiO should be attributed to some of Ni(OH)2 were in-situ reduced to Ni0 producing Ni nanoparticles. These Ni nanoparticles are easily oxidized into NiO when exposure to air after reaction.[63, 64] The metallic Ni formed in-situ could then capture the photogenerated electrons rapidly from the CB of TpPa-2, which significantly enhanced the charge separation and transfer efficiency in the synergetic effect between Ni(OH)2 and

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metallic Ni.

Figure 4. The high-resolution XPS spectra of the Ni 2p region of the Ni(OH)2-2.5%/TpPa-2 sample

W Xe lamp) using SA aqueous solution.

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3.7. Photocatalytic mechanism analysis.

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before and after photocatalytic H2 production for 5 h under visible light irradiation ( ≥ 420 nm, 300

To further deeply understand the photocatalytic H2 evolution mechanism, the separation of photogenerated electrons and holes in these materials were also investigated through

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electrochemical impedance spectroscopy (EIS), photo-luminescence (PL) emission spectroscopy, surface photovoltage spectroscopy (SPV) and transient photocurrent measurements. The EIS

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Nyquist plots of Ni(OH)2-2.5%/TpPa-2 and TpPa-2 were conducted in the frequency range from 105 to 0.1 Hz (Figure 5a). The semicircle in the Nyquist plot can be simulated well by an electrical equivalent-circuit model (Figure 5a inset). The results shown that Ni(OH)2-2.5%/TpPa-2 (Rt: 17085

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Ω) has a much smaller semicircle radius and lower Rt values than TpPa-2 (Rt: 20660 Ω), which

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indicates more efficient interfacial charge-carrier transfer in Ni(OH)2-2.5%/TpPa-2 composite. Linear sweep voltammetry polarization curves of TpPa-2 and Ni(OH)2-2.5%/TpPa-2 show that Ni(OH)2-2.5%/TpPa-2 exhibits much lower onset potential for electrocatalytic H2-evolution than TpPa-2 (Figure S18), indicating that Ni(OH)2 can act as co-catalyst to efficiently improve the photocatalytic H2 evolution activity.[25, 65] Moreover, the effective generation and instant separation of photoexicited charge carriers are prerequisites for photocatalytic reactions, which can be analyzed by photo-luminescence (PL) emission spectroscopy because photoluminescence stems from the recombination of free charge carriers. As shown in Figure 5b, the Ni(OH)2-2.5%/TpPa-2

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composite material shows a much weaker emission profile compared with TpPa-2, indicating a rapid charge transfer in Ni(OH)2-2.5%/TpPa-2. This is ascribed to the in-situ formation of metallic

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Ni, which facilitates the electrons migration.[51]

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Figure 5. (a) EIS Nyquist plots, (b) PL spectra with the excitation wavelength of 380 nm, (c) timeresolved PL spectra, (d) SPV spectra and (e) transient photo-current curves for TpPa-2 and Ni(OH)2-2.5%/TpPa-2, respectively. (f) Schematic illustration of the main mechanism of

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photocatalytic H2 evolution for Ni(OH)2-X%/TpPa-2.

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As shown in Figure 5c, time-resolved PL decay profiles of TpPa-2 and Ni(OH)2-2.5%/TpPa-2

at 480 nm emission show that the fluorescence lifetime of Ni(OH)2-2.5%/TpPa-2 (τ1=0.36 ns) is shorter compared with that of TpPa-2 (τ1=0.53 ns). This may be attributed to Ni(OH)2 can effectively accept the electrons transferred from TpPa-2 and add another decay channel to the excited states of the TpPa-2. To calculate the electron transfer rate (kET) of Ni(OH)2-2.5%/TpPa-2, we assume that the PL lifetimes of the TpPa-2 and Ni(OH)2-2.5%/TpPa-2 were given by τTpPa-2=1/k and τNi(OH)2-2.5%/TpPa-2 = 1/(k + kET).[66, 67] The kET can be calculated to be 8.9 ×108 s−1, which is larger than that of some reported semiconductor photocatalytic literatures,[68, 69] indicating that

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Ni(OH)2 as a charge collector can effectively transfer photogenerated electrons of Ni(OH)22.5%/TpPa-2 composite material. Besides, the SPV and transient photocurrent response of Ni(OH)2-2.5%/TpPa-2 composite material are significantly stronger than that of TpPa-2 (Figure 5d and 5e), suggesting that a higher separation efficiency of the photogenerated electron−hole pairs was achieved after composite. All of these results indicate that the improved activity of Ni(OH)2-

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2.5%/TpPa-2 should be attributed to the enhanced visible-light absorption of the composite material

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contributed from Ni(OH)2 and the synergetic effect of Ni(OH)2 and metallic Ni derived from the

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reduction of Ni(OH)2, which promoted the separation of photogenerated photo-generated electron–

3.8. Mechanism of photocatalytic H2 evolution.

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holes of the resulting materials.

On the basis of the above results, a possible mechanism for the separation and transport of the

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electron–hole pairs over the Ni(OH)2-X%/TpPa-2 is proposed in Figure 5f. Under visible light irradiation, TpPa-2 acts as a light harvester for its excellent light absorbance ability. Initially,

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Ni(OH)2-2.5%/TpPa-2 composite material shows moderate photocatalytic activity because pure Ni(OH)2 is an insulator that can hardly accept the photoinduced electrons from TpPa-2. However, because the CB potential of TpPa-2 (-0.57 V vs. NHE) is more negative than the CB of Ni2+/Ni (-

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0.23 V vs. NHE), photoinduced electrons from the CB of TpPa-2 could reduce partial Ni(OH)2 gradually to metallic Ni. The metallic Ni derived from the in-situ reduction of Ni(OH)2 could then

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extract the photogenerated electrons rapidly from the CB of TpPa-2, which greatly increases the charge separation and transfer efficiency in the Ni/Ni(OH)2-TpPa-2 system. This is in consist with

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the photocatalytic H2 evolution activity that a lower H2 evolution rate in the first reaction hour. After the initial hour period, Ni/Ni(OH)2-TpPa-2 shows accelerated photocatalytic activity. The

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reason is that the remaining Ni(OH)2 on the surface of TpPa-2 plays an important role to promote the dissociation of water molecules and the production of protons,[51, 70, 71] which are then adsorbed on the nearby metallic Ni formed in-situ and reduced by the photogenerated electrons trapped in metallic Ni to form H2 molecules. Therefore, the photocatalytic activity of Ni(OH)2X%/TpPa-2 composite materials have been significantly improved due to the synergetic effect between Ni(OH)2 and in-situ formed metallic Ni. In addition, in order to verify the effectiveness of Ni(OH)2 as co-catalyst to other COFs, we also chose another COF (TpPa-1) as precursor and synthesized Ni(OH)2-2.5%/TpPa-1 photocatalytic material (Figure S19). As expected, the Ni(OH)2-

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2.5%/TpPa-1 displays significantly enhanced photocatalytic hydrogen production activity for the existence of Ni(OH)2 as co-catalyst (Figure S20), which is nearly 8.6 times higher than that of the parent TpPa-1 photocatalyst under the same condition. The results indicated that Ni(OH)2 as cocatalyst may be effective to more COFs photocatalysts to enhance their photocatalytic hydrogen

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production activity.

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4. Conclusions

In summary, for the first time, visible-light active Ni(OH)2-X%/TpPa-2 photocatalysts were

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successfully prepared for efficient photocatalytic hydrogen production. The photocatalytic

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experiments results demonstrated that Ni(OH)2 is an effective co-catalyst to enhance the photocatalytic H2-production activity of TpPa-2 and its content exhibits an obvious influence on the resulting H2-production activity. The optimal Ni(OH)2 loading was determined to be 2.5 mol% in

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Ni(OH)2-X%/TpPa-2, and the corresponding H2-production rate was up to 1895.99 μmol h-1 g-1,

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which is nearly 26.3 times higher than that of the parent TpPa-2 under the same condition and is one of the best performing 2D polymer-based photocatalyst for H2 evolution. The photocatalytic activity of Ni(OH)2-2.5%/TpPa-2 is roughly equivalent to TpPa-2 with Pt loading of 0.3 wt%. The

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improved activity should be attributed to the enhanced visible-light absorption of the composite materials contributed from Ni(OH)2 and the synergetic effect of Ni(OH)2 and metallic Ni derived

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from the reduction of Ni(OH)2, which promoted the separation of photogenerated electron–holes of the resulting materials. This work not only presents a series of new photocatalysts for efficient H2 evolution but also open an avenue for further design and synthesis of COF-based composite

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materials acting as a substitute of noble-metal-containing photocatalytic systems.

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Acknowledgements This work was financially supported by the National Natural Science Foundation of China (No. 21676066), the Special Fund for Scientific and Technological Innovation Talents of Harbin Science and Technology Bureau (No. 2017RAQXJ057), Natural Science Foundation of Heilongjiang Province, China (No. B2017006, LH2019B026), and key Laboratory of Functional Inorganic Material Chemistry (Heilongjiang University), Ministry of Education Open Project Fund. Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version.

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Graphical Abstract

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Highlights: • For the first time, we designed and constructed a series of Ni(OH)2-modified COFs. • Ni(OH)2 acts as noble-metal free cocatalyst in catalyst system. • Ni(OH)2-X%/TpPa-2 photocatalyst is comparable to Pt-containing COF -based catalyts.

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• The synergistic effect of TpPa-2 and Ni(OH)2 has been systematic studied.

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• Ni(OH)2-X%/TpPa-2 photocatalysts master a high rate of electron transport.