Boosting Photocatalytic Hydrogen Evolution Achieved by NiSx

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C: Energy Conversion and Storage; Energy and Charge Transport

Boosting Photocatalytic Hydrogen Evolution Achieved by NiSx Coupled with G-CN@ZIF-67 Heterojunction 3

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Yongke Zhang, and Zhiliang Jin J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b04695 • Publication Date (Web): 12 Jul 2019 Downloaded from pubs.acs.org on July 17, 2019

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The Journal of Physical Chemistry

Boosting Photocatalytic Hydrogen Evolution Achieved by NiSx Coupled with g-C3N4@ZIF-67 Heterojunction

Yongke Zhang1,2,3, Zhiliang Jin1,2,3* 1. School of Chemistry and Chemical Engineering, North Minzu University, Yinchuan 750021, P.R.China 2. Ningxia Key Laboratory of Solar Chemical Conversion Technology, North Minzu University, Yinchuan 750021, P.R.China 3. Key Laboratory for Chemical Engineering and Technology, State Ethnic Affairs Commission, North Minzu University, Yinchuan 750021, P.R.China

Corresponding author: [email protected]

Abstract: g-C3N4@ZIF-67 composite photocatalysts supported NiSx are successfully prepared by hydrothermal method using g-C3N4 and ZIF-67 as carriers. The synthesized composite catalyst has an efficient photocatalytic H2 production effect. Under visible light irradiation, the maximum H2 production within 5 h over the [g-C3N4@ZIF-67/(10%wt)NiSx] photocatalyst is 208 μmol, which is 8.32 times higher than that of pure g-C3N4. Based on the analysis of SEM and TEM, it is not only known that g-C3N4 and ZIF-67 provide space for the loading of NiSx nanoparticles, but also that ZIF-67 as a carrier framework can effectively reduce the particle size of NiSx nanoparticles and increase the dispersion of NiSx. Further studies of PL, TRPL, i-t, LSV, EIS and Mott-Schottky curves, we can not only know that the modification of NiSx nanoparticles do enhance the electron transfer ability, but also that the matched CB position between the g-C3N4 and ZIF-67 provide a feasible thermodynamic path for the transmission of electrons. Based on the DFT calculations, both NiS2 and Ni3S4 show the metallic characteristics, which means the outstanding electrical conductivity of them and implies the excellent capability to transport electrons. The work function of NiSx with metallic properties is more negative than that of

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semiconductor, which indicates that the loading of NiSx can further promotes the separation of electrons, thereby improving the H2 production efficiency.


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1. Introduction With the worldwide energy crisis and environmental pollution, people pay great attention to the new pollution-free clean production.1-3 Photocatalytic technology is considered as one of the most potential technologies to solve energy and environmental problems, and its core is photocatalyst. The landmark research work in the field of photocatalysis is the photolysis of water to hydrogen on single crystal titanium dioxide electrode discovered by Fujiyama and Honda in 1972.4 Subsequently, a variety of photocatalysts have been developed and utilized, including metal oxides such as Fe2O3,5 ZnO,6 TiO2,7,8 WO3,9 metal sulfides such as CdS,10 CuInS2,11 metal halides such as BiOI,12 BiOBr,13 BiOCl,14 metal salt compounds such as BiVO4,15 Ag3PO4,16 and so on. In recent years, metal-free photocatalysts have been explored by many researchers because of their abundant storage, low cost, easy availability, and no secondary pollution to the environment. As the metal-free photocatalyst, graphite phase C3N4 (g-C3N4) is first reported in 2009.17,18 Its structure and properties are easy to control, and its physical and chemical stability is good. In recent years, it has successfully attracted the widespread attention of many international scientific researchers and stimulated their research interest.In order to enhance the photocatalytic activity of g-C3N4, many methods of modification of g-C3N4 have emerged. For example, a few template method and non-template method are used to change the microstructure, element doping is used to control the band gap, the surface modification of photocatalyst, and compounding with other semiconductor materials. (1) In terms of microstructure: Yang G. D. and Lin B. et al. prepared the novel SiO2/g-C3N4 core-shell nanosphere synthesized by annealing the mixture of silica nanospheres.19,20 Wang Y. and Gong Y. T. reported the polymeric mesoporous carbon graphitic nitrides (mpg-C3N4) and ordered mesoporous graphitic carbon nitrides (ompg-C3N4) with different surface area and morphology.21 The adjustment of the microstructure can not only increase the surface area of the catalyst to promote the mass transfer process, but also increase the amount of the active site on the catalyst surface, thereby optimizing the energy band structure and light absorption performance, and significantly improving the photocatalytic activity. (2) In terms of the polymerization process: g-C3N4 is easily synthesized by thermal polymerization of some precursors such as melamine, dicyandiamide, cyanamide, urea, thiourea. (3) In terms of element doping: Zhang L. Z. and Dong G. H. et al. theoretically demonstrate that carbon self-doping can



induce intrinsic electronic and band structure change of g-C3N4 to increase the electrical conductivity and photoreactivity.22 Qiao S. Z. and Ma T. Y. et al. reported a flexible non-metal phosphorus-doped graphitic carbon nitride nano-flowers, which accelerated charge transfer and increased high active surface area.23 In brief, doping a small amount of metal or non-metal element in g-C3N4 can change its energy band structure, optimize its light absorption capacity to a certain extent, and promote the separation of photogenerated electron-hole pairs, thereby improving its photocatalytic performance. (4) In terms of construction of heterojunctions: Yu J. G.and Fu J. W. et al. synthesized a ultrathin 2D/2D WO3/g-C3N4 S-scheme heterojunction photocatalysts, which inhibits the recombination of useful electrons and holes.24 Zou Z. G. and Hao X. Q. et al. prepared a novel ZnS/g-C3N4 photocatalyst, which possess a low over-potential and extended absorption in the visible light region.25 In conclusion, the heterojunction26 formed by coupling g-C3N4 with other semiconductor materials can accelerate the separation and transfer of photogenerated electrons and holes, thereby greatly improving the photocatalytic efficiency. Metal-organic framework materials (MOFs), composed of central metal ions and organic ligands, have many properties such as large specific surface area, regular porous structure, adjustable organic ligands and polymetallic sites.27-29 In addition, the MOFs’ intrinsic porous structure and hybrid nature afford special opportunities for further tuning and functionalization.30 In terms of medical imaging,31 drug delivery,32 gas storage/separation33 and photocatalysis,34 etc., the MOFs materials have good practicability. Many researchers make use of MOFs to synthesize some metal oxides such as Fe2O3,35 Co3O4,36 nano-carbon materials37 and bimetal oxides such as CuO@NiO,38 which can be used in fields such as photocatalysis, electrocatalysis and photoelectrocatalysis and so on. In recent years, due to their good conductivity, special metalloid properties and excellent catalytic properties, transition metal sulfides such as MoS2,39 WS2,40, NiSx,41 and so on have received the attention of many researchers. They can provide reactive sites to accelerate the transfer of electrons, thereby increasing catalytic activity. In this paper, we construct an EosionY (EY)-sensitized efficient photocatalytic H2 production system of g-C3N4@ZIF-67/NiSx. Under visible light irradiation, the EY dye acts as a photosensitizer, which broadens the visible light absorption range and absorption intensity of the semiconductor and forms an effective separation


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of the photogenerated charge. Furthermore, it is the the synergy between the g-C3N4, ZIF-67 and NiSx that improves the efficiency and activity of photocatalytic H2 production. Through a series of characterization results, we put forward a possible mechanism of photocatalytic H2 production.

2. Experimental section 2.1 Photocatalyst preparation All the reagents are of analytical grade and are used without further purification. All experiments use the deionized water. Co(NO3)2·6H2O (99%), Polyvinylpyrrolidone (PVP-K29-32, 99%), Ni(CH3COO)2·4H2O (99%), Thioacetamide (TAA, 98%) and Methanol (99.5%) are purchased from Shanghai Aladdin Biochemical Technology Co., Ltd. Melamine (99%) and 2-methylimidazole (98%) are purchased from Shanghai Macklin Biochemical Co., Ltd. 2.1.1 Preparation of g-C3N4 We use the method reported in the previous literature to prepare g-C3N4.42 First of all, a crucible containing 25 grams of melamine was put into a muffle furnace. Then, the melamine is heated from room temperature to 550 °C at a heating rate of 2.3 °C/min by a program temperature control method and maintained at this temperature for 4 hours. Subsequently, when the temperature of the muffle furnace drops to room temperature, the sample in the crucible is poured into an agate mortar and ground uniformly, thus the bright yellow g-C3N4 is obtained. 2.1.2 Preparation of ZIF-67(Co) The ZIF-67(Co) material is synthesized by referring to the previously reported literature.36,43 The specific preparation process is as follows: First, prepare the solution A: a certain amount of Co(NO3)2·6H2O and PVP is poured into a 50 ml beaker, and 50 ml of methanol is added and stirred to make it uniform. Then, prepare the solution B: Pour a certain amount of 2-methylimidazole into a beaker containing 50 ml methanol and stir to make it even. Finally, we pour the solution B into solution A slowly while stirring solution A. When the mixed solution is allowed to stand for 48 h, the purple ZIF-67(Co) precipitate is formed. Then, the ZIF-67(Co) precipitate is washed several times with methanol and dried at 60 ℃.



2.1.3 Preparation of g-C3N4@ZIF-67/NiSx The composite catalysts of [g-C3N4@ZIF-67/(x%wt)NiSx] with different contents of NiSx [m(NiSx)/m(g-C3N4@ZIF-67)×100%= 2.5,5,10,20,40] are prepared by the following synthetic method. Take the preparation of [g-C3N4@ZIF-67/(10%wt)NiSx] sample as an example. Firstly, 60 ml DI water is poured into a 80 ml beaker. 100 mg g-C3N4 is poured into the beaker and treated with ultrasound for 15 minutes. Then, 100 mg ZIF-67 is poured into the beaker and treated with ultrasound for 15 minutes. Soon afterwards, 42 mg of Ni(CH3COO)2 · 4H2O and 62 mg of thioacetamide are poured into the beaker and treated with ultrasound for 15 minutes. Finally, the mixed solution is transferred to an 80 mL autoclave and kept at 160 ℃ for 12 h. When the reaction system is cooled to room temperature, the sample is obtained by centrifugation, washing and drying. g-C3N4/NiSx, ZIF-67/NiSx and NiSx samples are prepared by the same method. Scheme 1 shows the schematic illustration for the fabrication of g-C3N4, ZIF-67(Co), NiSx, g-C3N4@ZIF-67 and [g-C3N4@ZIF-67/NiSx].


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Scheme 1 Synthetic scheme for the preparation of g-C3N4, ZIF-67(Co), NiSx, g-C3N4@ZIF-67 and [g-C3N4@ZIF-67/NiSx].

2.2 Photocatalytic H2 measurements Firstly, 15 mg of the catalyst sample, 20 mg of EY dye and a magnetic stir bar are added to the 62 ml reaction bottle (Perfectlight). Secondly, 30 ml of 15% triethanolamine (TEOA) aqueous solution is added to the reaction flask and sonicated for 10 minutes. Subsequently, the system is replaced with N2 about 30 min to remove O2. Finally, we place the reactor on the multi-channel reaction system (PCX50A Discover) and extract 0.5 ml of gas every half an hour to analyze through gas chromatography (Tianmei GC7900, TCD, 13X column, N2 as the carrier). We use the external standard method to calculate the amount of H2 production. Next, we test the apparent quantum efficiency (AQE) of the [g-C3N4@ZIF-67/(10%wt)NiSx] composite catalyst under the same conditions as the above H2 production process. The M300 xenon lamp light source (Microsolar, Perfect Light) with 420, 450, 475, 500, 520 and 500 nm band pass filters is used as light source. The PL-MW200 photoradiometer (Perfect Light) is used to measure the photon flux of the incident light. The reaction solution is irradiated for 1 h to detect the H2 production. At the same time, the AQE at different wavelengths is calculated according to the equation (1).

AQE 

2  the number of evolved hydrogen molecules  100% the number of inident photos

(1)

2.3 Photoelectrochemical measurements All photoelectrochemistry experiments are tested on the electrochemical workstation (VersaSTAT4-400, AMETEK) in a homemade standard three-electrode cell. The reference electrode, counter electrode and working electrode are respectively saturated calomel electrode (SCE), Pt electrode and FTO (1*2 cm2) drop-coating homogeneous catalyst. The electrolyte is Na2SO4 aqueous solution (0.2 mol L−1 ). The incident light source is a 300 W xenon lamp with a 420 nm filter. The photocurrent current density-time curve has a test bias of 0 V, and the opening



and closing cycles are 4 times. The linear scanning voltammetry test has a voltage sweep range of -0.6 to 0.2 V and a scan rate of 50 mV/s. The AC impedance spectrum has a test frequency range of 10 kHz to 0.1 Hz. 2.4 Characterization The crystalline structure, surface element composition and the UV-vis diffuse reflectance spectra are respectively tested by X-ray diffraction analysis (XRD, Rigaku RINT-2000), X-ray photoelectron spectroscope (XPS, ESCALAB 250Xi) and UV-2550 (Shimadzu) spectrometer (BaSO4 as the reference). The content of Ni and S elements of the composite photocatalyst is measured by Agilent ICP-OES 730. The images of samples’ morphology are characterized by a field-emission scanning electron microscope (FESEM, JSM-6701F) and a transmission electron microscopy (TEM, Tecnai G2-TF30). The nitrogen adsorption-desorption isotherms are measured at 77 K with an ASAP 2020 M instrument and analyzed by the Brunauer-Emmett-Teller (BET) equation. The pore size distribution plots are obtained by the Barret-Joyner-Halenda (BJH) model. We use the the FluoroMAX-4 spectrometer and the Single Photon Counting Controller: FluoroHub (HORIBA) to test the photoluminescence (PL) experiment and ime-resolved PL (TRPL) experiment, respectively. We use Material Studio software to construct the crystal structure model. At the same time, we use the density functional theory as implemented in CASTEP module in Material Studio software package for DFT calculations, which mainly include the structure optimizations, electronic band structure and density of states. The ultrasoft pseudopotential with a kinetic energy cutoff of 550 eV for the wave function expansion is adopted to describe core and valence electrons. The generalized gradient approximation (GGA) with Perdew-Burke-Ernzerhof (PBE) functional employed to deal with the exchange-correlation energy of electrons. The K-point mesh based on the Monkhorst-Pack scheme is set to be 2×2×1 for NiS2 and Ni3S4, respectively.

3. Results and discussion 3.1 XRD, SEM, and TEM analysis


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3.1.1 XRD analysis Fig.1 shows the XRD pattern of g-C3N4, ZIF-67(Co), NiSx, g-C3N4/NiSx, ZIF-67/NiSx and [g-C3N4@ZIF-67/(40wt%)NiSx] photocatalysts. For g-C3N4, it can be clearly seen that there are two characteristic diffraction peaks at 13.1° and 27.4°, which correspond to the (100) crystal plane and the (002) crystal plane of g-C3N4, respectively.42,44 The (100) crystal plane is a structure formed by stacking in-plane repeating structural units, whose interplanar spacing is 0.681 nm.45 The (002) crystal plane is a typical layered conjugated aromatic structure, whose interplanar spacing is 0.325 nm.42 The diffraction angle of the characteristic peak of ZIF-67(Co) is mainly concentrated between 5° and 40°,46 which is consistent with the results reported in previous literature. There are five obvious characteristic peaks at 7.5°, 10.4°, 12.7°, 18.1° and 26.7°, which are matched well with the (011), (002), (112), (222) and (134) crystal plane, respectively.47 In the pattern of XRD characteristic diffraction peak of NiSx, there are four obvious characteristic peaks located at 31.38° (200), 35.23° (210), 38.76° (211) and 48.05° (220) can be indexed, which is in agreement with the cubic NiS2 (JCPDS#88-1709) with a space group of Pa-3 (no. 205), while other diffraction peaks located at 16.21° (111), 26.21° (220), 31.26° (311), 37.91° (400), 49.95° (511) and 54.72° (440) can be indexed respectively, which is consistent with the cubic Ni3S4 (JCPDS#47-1739) with a space group of Fd-3m (no. 227). For g-C3N4/NiSx, we can see the characteristic diffraction peaks of g-C3N4 and NiSx. Similarly, for ZIF-67/NiSx, the characteristic diffraction peaks of ZIF-67 and NiSx can be seen. For [g-C3N4@ZIF-67/(40wt%)NiSx], we can not only see the characteristic diffraction peaks of g-C3N4 and ZIF-67, but also the characteristic diffraction peaks of NiSx, which indicates that the complexes mainly consist of g-C3N4, ZIF-67 and NiSx.



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Fig. 1 The XRD patterns of g-C3N4, ZIF-67, NiSx, g-C3N4/NiSx, ZIF-67/NiSx, [g-C3N4@ZIF-67/(40wt%)NiSx] and the JCPDS of NiS2 and Ni3S4.

Taking the [g-C3N4@ZIF-67/(20%wt)NiSx] sample as an example, in order to obtain the exact contents of NiSx in the composites, we further test the content of Ni and S elements in the composite photocatalyst by ICP-OES. The results are shown in Table 1. It can be seen that the sum of the mass of the Ni and S elements is 128911 mg. Through calculation, we can get that the NiSx accounts for 14.8% of the composite catalyst g-C3N4@ZIF-67. It is further speculated that we can know that the exact contents of NiSx in the composite catalyst g-C3N4@ZIF-67 is reduced, but as the Ni ions increase, the percentage of NiSx as a whole also shows an increasing trend.

Table 1 The analysis of element content in [g-C3N4@ZIF-67/(20%wt)NiSx] sample by ICP-OES.

Sample quality/g

Measured element

Element content/mg•kg-1

0.0417

Ni

47610.0

0.0417

S

81301.0

3.1.2 SEM analysis In order to acquire and observe the patial morphology, the particle size, the sample


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distribution, these samples are characterized and analyzed by SEM. Fig. 2(A-F) are the SEM images of g-C3N4, ZIF-67(Co), NiSx nano-particles and [g-C3N4@ZIF-67/(10%wt)NiSx]. In Fig. 2(A), the structure of the g-C3N4 exhibits a layered sheet structure and also presents a porous structure, which is consistent with previous reports in the literature.42 In Fig. 2(B,C), we can not only see that the ZIF-67(Co) presents a three-dimensional structure of rhombic dodecahedron, but also see that their distribution is relatively uniform and their surface is relatively smooth. In Fig. 2(D), it can be seen that the NiSx powders are agglomerated and easy to form micro-spheres with diameters

of

approximately

200-300

nm.

Fig.

2(E,F)

are

the

SEM

images

of

[g-C3N4@ZIF-67/(10%wt)NiSx] at low magnification and high magnification, respectively. By observing Fig. 2(E,F), on the one hand, some NiSx nanoparticles are loaded onto the surface of g-C3N4 nanosheets, on the other hand, the other NiSx nanoparticles are loaded onto the surface of ZIF-67(Co). Further observation Fig. 2(F), the spatial structure of the rhombic dodecahedron of ZIF-67 has not been destroyed and still exists after loading. In addition, we can see that the diameter of NiSx nanoparticle is reduced, which indicates that the introduction of ZIF-67 as a carrier framework can effectively reduce the particle size NiSx nanoparticles and effectively slow down the agglomeration of NiSx.



Fig. 2 SEM images: (A) g-C3N4; (B,C) ZIF-67(Co); (D) NiSx; (E,F) [g-C3N4@ZIF-67/(10%wt)NiSx].

3.1.3 TEM, HRTEM, EDX and EDS mapping analysis In order to observe the internal structure, morphology and contact form between g-C3N4, ZIF-67 and NiSx in the composite catalyst, the [g-C3N4@ZIF-67/(10%wt)NiSx] sample are characterized by TEM, HRTEM and EDX via field emission electron microscopy. Fig. 3(A,B) display the TEM images of [g-C3N4@ZIF-67/(10%wt)NiSx] at different magnifications from low to high. It can be seen that the g-C3N4 sample exhibits a two-dimensional nanosheet layer


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structure, which not only contains many wrinkles but also irregular shapes. These thin nano-sheets are beneficial to the loading of other catalysts and the adsorption of dye molecules. The spatial structure of rhombic dodecahedron of ZIF-67 remains substantially unchanged. Part NiSx nanoparticles are loaded onto the surface of the g-C3N4, another portion adhering to the surface of ZIF-67, the particle size has been reduced to a certain extent. The results demonstrate the presence of C, N, Co, Ni and S elements, which can be further demonstrated by the subsequent EDS and elemental surface scanning imaging. From the HRTEM image of [g-C3N4@ZIF-67/(10%wt)NiSx] of Fig. 3(C), the close contact interfaces among the g-C3N4, ZIF-67 and NiSx can be clearly seen. Further observation, it’s can be found that there are two distinct and clear interfaces in the composite catalyst. There are two different lattice spacings of 0.28 nm and 0.34 nm, which are correspond to the (200) crystal plane of NiS248 and the (220) crystal plane of Ni3S4,49 respectively, which is consistent with the results of XRD. The above results further prove that the close interface between the g-C3N4, ZIF-67 and NiSx does exist, and the close contact of the interface will facilitate the transfer of photogenerated carriers at the contact interface.

Fig. 3 (A,B) TEM images of [g-C3N4@ZIF-67/(10%wt)NiSx]; (C) HRTEM image of [g-C3N4@ZIF-67/(10%wt)NiSx] and (D) the EDX spectrum of [g-C3N4@ZIF-67/(10%wt)NiSx].



Fig. 3(D) is the energy dispersive X ray spectroscopy (EDX) spectrum of the samples of [g-C3N4@ZIF-67/(10%wt)NiSx] and the results demonstrate the presence of C, N, Co, Ni and S elements. Elemental surface scanning imaging was performed on a typical and selected area of the [g-C3N4@ZIF-67/(10%wt)NiSx] sample, and the results are shown in Fig. 4. The sample contains C, N, Co, Ni and S elements, all of which are evenly distributed. It’s further confirmed that C, N, Co, Ni and S elements coexist in [g-C3N4@ZIF-67/(10%wt)NiSx] sample, which is consistent with the EDX and XRD test results.

Fig. 4 The EDS mapping pattern of [g-C3N4@ZIF-67/(10%wt)NiSx] sample.

3.2 XPS, BET and UV–vis analysis 3.2.1 XPS analysis In order to obtain the surface chemical composition and valence states of the elements in the composite catalyst, XPS characterization of sample [g-C3N4@ZIF-67/(10%wt)NiSx] is carried out. As can be seen from the survey spectrum of Fig. 5(A), the composite contains C, N, Co, S and


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Ni elements. Fig. 5(B) is the fine spectrum of C1s, and two peaks with binding energies of 284.8 eV and 288.3 eV can be observed. The peaks at 284.8 eV belong to the sp2 hybrid orbital C atom and the exogenous carbon pollution from XPS instrument itself, while the peaks at 288.3 eV belong to the N-C=N coordination bond.45 Fig. 5(C) is the fine spectrum of N1s. Three peaks with binding energy of 401.2 eV, 399.7 eV and 398.6 eV can be observed. The peak at 401.2 eV belongs to N atom in C-N-H functional group, the peak at 399.7 eV belongs to N atom in N-(C)3 functional group, and the peak at 398.6 eV is assigned to the C=N-C coordination bond.50 From the Fig. 5(D), it can be seen that the the Co 2p fine spectrum can be deconvoluted into two spin-orbit doublets including six different peaks. The peaks at 778.2 eV(Co 2p3/2) and 793.4 eV(Co 2p1/2) belong to the first doublet.51,52 The peaks at 780.3 eV(Co 2p3/2) and 797.8 eV(Co 2p1/2) belong to the second doublet.51,52 At the same time, the other peaks are attributed to the satellites. From the Fig. 5(E), we can see that the the Ni 2p fine spectrum can be deconvoluted into six different peaks. The binding energies at 855.5 eV(Ni 2p3/2) and 873.3 eV(Ni 2p1/2) are corresponded to Ni2+2p3/2 and Ni2+ 2p1/2.53,54 The binding energies at 852.9 eV(Ni 2p3/2) and 870.1 eV(Ni 2p1/2) show that the higher valence state of Ni species other than Ni2+.53,54 At the same time, the other peaks at 859.8 eV and 878.6 eV are attributed to the satellites. In Fig. 5(F), S 2p fine spectrum has two peaks at 162.5 eV and 161.3 eV, which are assigned to S 2p1/2 and S 2p3/2, respectively.47



Fig. 5 XPS patterns of [g-C3N4@ZIF-67/(10%wt)NiSx] sample. (A) Survey, (B) C 1s, (C) N 1s, (D) Ni 2p, (E) Co 2p and (F) S 2p scan spectra.

3.2.2 BET analysis The pore size and morphological structure of the catalyst determine the performance of the catalyst, which is closely related to its application. In order to obtain the specific parameters such as specific surface area, pore volume, and pore size distribution of the sample, we tested them using the ASAP 2020 instrument. The method adopted is the BET nitrogen adsorption capacity method, and the BET equation is shown in formula (2):


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Va c( p / p* )  Vma (1  p / p* )1  ( c  1) p / p*  V a  the absorption amount under pressure p; Vma  the saturated adsorption capacity of monolayer;

(2)

p*  the saturated vapor pressure; c  Adsorption constant; The N2 adsorption-desorption isotherms and pore size distribution curves (insert) of g-C3N4, ZIF-67, NiSx, g-C3N4/NiSx, ZIF-67/NiSx and [g-C3N4@ZIF-67/(10%wt)NiSx] are shown in Fig. 6(A-F). The physical adsorption desorption performance parameters of the above samples are shown in Table 2. In Fig. 6(B), because when the relative pressure is 0.1, the isotherm of ZIF-67(Co) reaching equilibrium, the adsorption and desorption curve of ZIF-67 belongs to the type I isotherm. It is the type I isotherm that makes ZIF-67 possess a strong adsorption capacity.55 Further analyzing the pore size distribution curve and the average pore size, we can draw a conclusion that the ZIF-67 material belongs to the mesoporous material and its pore size is very close to that of the micropore. Except for ZIF-67, the adsorption and desorption isotherms of other samples belong to type IV isotherms, according to BDDT classification, they belong to mesoporous materials.56 Due to their adsorption process and desorption process cannot completely coincide, according to IUPAC classification, their hysteresis loops is H3 type, which indicate the existence of a cracked pore structure.56



Page 18 of 44

Fig. 6 The N2 adsorption-desorption isotherms and pore size distribution curves (insert): (A) g-C3N4; (B) ZIF-67; (C) NiSx; (D) g-C3N4/NiSx; (E) ZIF-67/NiSx and (F) [g-C3N4@ZIF-67/(10%wt)NiSx].

Further analysis of Table 2, it can be seen that the specific surface area of pure g-C3N4, ZIF-67, NiSx, g-C3N4/NiSx, ZIF-67/NiSx and [g-C3N4@ZIF-67/(10%wt)NiSx] are 5, 879, 8, 53, 498 and 359 m2g-1, respectively. The specific surface area and pore volume of g-C3N4 and NiSx are relatively small. Pure ZIF-67 material shows the largest specific surface area, which fully shows the excellent performance of MOF material and provides a lot of space for the loading of other materials and the adsorption of dye molecules. In addition, the higher specific surface area and larger pore volume of the composite sample are beneficial to increase the number of active sites, thereby enhancing photocatalytic activity. Moreover, the mesoporous structure enables light to be transmitted to the pore walls of the catalyst and enables higher mobility of the carriers, which contributes to an improvement in photocatalytic activity.57

Table 2 The Physical adsorption performance parameters.

SBET

Pore volume

Average pore

(m2g-1)a

(cm3g-1)b

size (nm)b

g-C3N4

5

0.03

21.28

ZIF-67(Co)

879

0.058

8.99

NiSx

8

0.028

19.32

g-C3N4/NiSx

53

0.16

12.45

ZIF-67/NiSx

498

0.029

10.69

g-C3N4@ZIF-67/(10wt%)NiSx

359

0.046

11.77

Samples

a Obtained from BET method; b Relative pressure (P/P0) was 0.99.


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3.2.3 UV–vis analysis The optical absorption characteristics of the samples are investigated by UV-vis. Fig. 7(A) shows that the absorption boundaries of all the samples are in the visible range, which can be attributed to the band gap transition of g-C3N4. The absorption edge of pure g-C3N4 is at 450 nm, which is consistent with previous reports in the literature.42 The NiSx nanoparticles sample exhibits a strong absorption intensity from 300 to 800 nm. As depicted in Fig. 7(C), the UV-vis spectra of ZIF-67 displays two absorption features, which include a a broad absorption in UV region (

Boosting Photocatalytic Hydrogen Evolution Achieved by NiSx

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