A g-c3n4 / nis composite photocatalyst, a preparation method thereof and application thereof in photoelectrocatalytic overall water splitting

Abstract

This invention relates to a g-C3N4@NiS composite photoelectrocatalyst and its preparation and application. The catalyst has a flower-like hierarchical porous structure, composed of g-C3N4 nanospheres and NiS nanoparticles supported thereon. The g-C3N4 nanospheres are assembled from stacked nanosheet layers. The NiS nanoparticles and g-C3N4 nanospheres form an in-situ covalently bonded heterojunction through Ni-S and Ni-N bonds, constructing a p-n heterostructure. The preparation method involves mixing melamine, glucose, CTAC, nickel acetate, and thiourea, and simultaneously achieving the condensation polymerization of g-C3N4 and the growth of NiS using a low-temperature one-pot hydrothermal method. Under alkaline conditions, the HER overpotential of this catalyst is only -2 mV at 10 mA cm⁻¹. ‑2 The overpotential of OER is 270 mV at 10 mA cm⁻¹. ‑2 In the dual-electrode total water splitting system, only 2.4 V is required to achieve 20 mA cm⁻¹. ‑2 It achieves a high current density and operates stably for over 24 hours, exhibiting excellent catalytic activity and stability in seawater. This invention boasts advantages such as a simple synthesis process, strong interfacial coupling, superior catalytic performance, and resistance to seawater corrosion.

Description

Technical Field

[0001] This invention relates to the fields of photoelectrocatalytic materials and new energy technologies, and in particular to a g-C3N4 / NiS composite photoelectrocatalyst, its preparation method, and its application in photoelectrocatalytic water splitting. Background Technology

[0002] With the increasing severity of the energy crisis and environmental problems, the large-scale production of green hydrogen energy has become a research focus. Photoelectrochemical (PEC) water splitting can directly convert solar energy into chemical energy and is considered an ideal route for hydrogen production. Currently, noble metal-based catalysts (Pt / C, RuO2, IrO2) are expensive and have poor stability, making them difficult to apply on a large scale. Graphite-like carbon nitride (gC3N4) has a suitable band structure, good visible light response, and chemical stability, making it a highly promising photoelectrode material. However, it suffers from problems such as small specific surface area, severe recombination of photogenerated carriers, and limited single catalytic function. Nickel sulfide (NiS) has excellent conductivity and abundant active sites, and can form heterojunctions with gC3N4 to achieve efficient charge separation and bifunctional catalysis. Existing gC3N4 / NiS composite materials are mostly synthesized stepwise, resulting in complex processes, weak interfacial bonding, and poor structural controllability, making large-scale preparation difficult.

[0003] For example, patent CN107892284 A discloses a NiS / C3N4 binary composite, and patent CN117299176 A discloses a V-NiS2 microsphere-attached porous gC3N4 composite photocatalyst. These technical solutions have the following essential differences and disadvantages: The preparation methods differ: all the aforementioned patents employ a stepwise synthesis method, which involves first preparing porous gC3N4 through complex steps such as high-temperature calcination, and then growing NiS or NiS2 on it using a hydrothermal method. This stepwise method is lengthy, energy-intensive (requiring calcination at >550℃), and has poor batch stability, making it unsuitable for large-scale production.

[0004] Different interface bonding methods: In composite materials synthesized by stepwise method, the two components are mostly physically mixed or have weak interfacial contact, resulting in low charge transport efficiency.

[0005] The material structure and performance are different: the above patents mainly focus on photocatalytic degradation or hydrogen production, and do not demonstrate their dual-function (simultaneously driving HER and OER) water splitting performance.

[0006] Therefore, there is an urgent need to develop a low-temperature, one-pot, in-situ bonding, and structurally uniform preparation technology to obtain a highly efficient and stable photoelectrocatalyst with both HER and OER functions. This is of great significance for promoting the practical application of photoelectrocatalytic water splitting. Summary of the Invention

[0007] The purpose of this invention is to provide a g-C3N4 / NiS composite photoelectrocatalyst, its preparation method, and its application in photoelectrocatalytic water splitting. The catalyst has a flower-like hierarchical porous structure, is prepared by a one-pot method, and achieves in-situ covalent coupling between g-C3N4 and NiS, thereby forming a stable pn heterojunction. This catalyst has strong light absorption performance, efficient charge separation performance, and excellent bifunctional photoelectrocatalytic performance, and can be directly used for photoelectrocatalytic water splitting in alkaline and natural seawater systems.

[0008] The objective of this invention can be achieved through the following technical solutions: On the one hand, the present invention provides a g-C3N4 / NiS composite photocatalyst, the catalyst having a flower-like hierarchical porous structure, composed of g-C3N4 nanospheres and NiS nanoparticles in situ supported on its surface, wherein the g-C3N4 nanospheres are assembled by stacking nanosheet layered structures; The NiS nanoparticles and the g-C3N4 nanospheres form an in-situ covalently bonded heterostructure interface through Ni-S bonds and Ni-N bonds, constructing a pn heterostructure; in the heterostructure interface, Ni atoms coordinate with both S atoms and N atoms in g-C3N4 to form an N-Ni-S coordination configuration.

[0009] Preferably, the g-C3N4 / NiS composite photocatalyst possesses dual-functional photocatalytic activity of HER and OER.

[0010] More preferably, the NiS nanoparticles are uniformly loaded on g-C3N4 nanospheres.

[0011] Preferably, the g-C3N4 / NiS composite photocatalyst has a diameter of 0.5~3 μm and a specific surface area of ​​2~5 m². 2 / g, pore volume 0.018~0.042 cm³ 3 / g, with an average pore size of 3.0~17.1 nm.

[0012] In this invention, the specific surface area of ​​the g-C3N4 / NiS composite photoelectrocatalyst is nearly twice that of pure g-C3N4.

[0013] More preferably, the NiS nanoparticles have a particle size of 20~50 nm, and the g-C3N4 nanospheres have a particle size of 0.5~3 μm.

[0014] More preferably, the loading of the NiS nanoparticles is (1.7~80.7):0.25 based on the mass ratio of nickel to melamine.

[0015] Preferably, the g-C3N4 / NiS composite photocatalyst is prepared by low-temperature one-pot hydrothermal method, which is simultaneously polycondensation-sulfidation of g-C3N4 precursor and NiS precursor, and achieves in-situ covalent coupling between NiS and g-C3N4.

[0016] Secondly, the present invention provides a method for preparing the g-C3N4 / NiS composite photocatalyst, comprising the following steps: S1: Melamine, glucose, hexadecyltrimethylammonium chloride, nickel acetate, and thiourea are added sequentially to deionized water and magnetically stirred at room temperature to form a homogeneous precursor solution; S2: Transfer the precursor solution from step S1 to the reactor and carry out a hydrothermal reaction. After the reaction is complete, allow it to cool naturally. S3: The product obtained after the reaction in step S2 is centrifuged, washed, and vacuum dried to obtain the g-C3N4 / NiS composite photocatalyst.

[0017] Preferably, in step S1, the molar ratio of nickel acetate to thiourea in the precursor solution is 1:2~4, the mass ratio of melamine to glucose is 0.25:0.30~0.40, and the mass ratio of melamine to nickel acetate is 0.25:1.7~80.7.

[0018] More preferably, in step S1, the molar ratio of nickel acetate to thiourea in the precursor solution is 1:3.

[0019] More preferably, in step S1, the mass ratio of melamine to glucose in the precursor solution is 0.25:0.35.

[0020] Preferably, in step S1, the concentration of nickel acetate is 0.25~10 mol / L.

[0021] Preferably, in step S1, the concentration of hexadecyltrimethylammonium chloride is 3~3.5 g / L, more preferably 3.25 g / L.

[0022] Preferably, in step S1, the magnetic stirring speed is 400~600 rpm and the time is 20~80 h.

[0023] More preferably, in step S1, the magnetic stirring speed is 500 rpm and the time is 24 h.

[0024] Preferably, in step S2, the parameters of the hydrothermal reaction include: heating to 140~180 ℃ at a heating rate of 4.5~5.5 ℃ / min and holding at that temperature for 8~32 h.

[0025] More preferably, in step S2, the parameters of the hydrothermal reaction include: heating to 160 ℃ at a heating rate of 5 ℃ / min and holding at that temperature for 24 h.

[0026] Preferably, in step S3, the centrifugal washing refers to washing with deionized water 2-4 times and washing with ethanol 2-4 times in sequence.

[0027] More preferably, in step S3, the centrifugal washing refers to washing three times with deionized water and three times with ethanol in sequence.

[0028] Preferably, in step S3, the vacuum drying process involves a vacuum degree ≤ -0.09 MPa, a temperature of 60~80 ℃, and a time of 10~14 h.

[0029] More preferably, in step S3, the vacuum drying temperature is 70 °C and the time is 12 h.

[0030] Further preferably, the preparation method of the g-C3N4 / NiS composite photocatalyst includes the following steps: dissolving 0.25 g melamine, 0.35 g glucose, and 0.13 g CTAC (hexadecyltrimethylammonium chloride) in 40 mL deionized water and stirring until clear, as solution A; dissolving 0.16 mol nickel acetate and 0.48 mol thiourea in the same system and continuing stirring for 24 h to form a homogeneous suspension; then transferring the suspension to an 80 mL polytetrafluoroethylene-lined stainless steel autoclave and heating it to 160 ℃ at 5 ℃ / min for 24 h; after the reaction, allowing it to cool naturally, washing the product three times each with deionized water and ethanol by centrifugation, and vacuum drying at 70 ℃ for 12 h to obtain the g-C3N4 / NiS flower-shaped composite material; wherein, the mass ratio of melamine to glucose is 1:1.4; the concentration of CTAC is 3.25 g / L; the molar ratio of nickel acetate to thiourea is 1:3; and the stirring speed is 500 rpm. rpm; drying vacuum degree ≤ -0.09 MPa; the reaction temperature is 160 ℃, and the reaction time is 24 h.

[0031] Thirdly, the present invention provides an application of the g-C3N4 / NiS composite photocatalyst, wherein the g-C3N4 / NiS composite photocatalyst possesses dual photocatalytic activity of HER and OER, and can be used for photocatalytic total water splitting.

[0032] More preferably, the g-C3N4 / NiS composite photocatalyst, under alkaline conditions, has a 10 mA cm⁻¹ [response value missing]. -2 At current density, the HER overpotential is -2 mV, and the OER overpotential is 270 mV.

[0033] Preferably, a dual-electrode system is used, with a xenon lamp as the light source, to perform photoelectrochemical catalytic water splitting on the electrolyte at 23~27 °C. The electrolyte includes any one of KOH alkaline solution, simulated seawater, natural seawater, and freshwater.

[0034] More preferably, both the positive and negative electrodes of the dual-electrode system use carbon paper as a conductive substrate, and the g-C3N4 / NiS composite photocatalyst is loaded onto the carbon paper with a loading amount of 4.5~5.5 mg cm⁻¹. -2 The carbon paper has a thickness of 0.15~0.20 mm and a resistivity ≤5 mΩ cm. 2 .

[0035] More preferably, the power of the xenon lamp is 300 W (300 W cm⁻¹). -2 ).

[0036] More preferably, the concentration of the KOH alkaline solution is 1 M.

[0037] Preferably, the NiS in the g-C3N4 / NiS composite photocatalyst can be reconstructed in situ to form the γ-NiOOH active phase during the photocatalytic process, which significantly improves the OER kinetics and long-term stability.

[0038] More preferably, the molar ratio of H2 to O2 generated during the total water splitting process is close to 2:1, resulting in high Faraday efficiency and stable operation for more than 24 hours.

[0039] More preferably, when the g-C3N4 / NiS composite photoelectrocatalyst is applied to photoelectrochemical water splitting, it can achieve 20 mA cm⁻¹ at a potential of 2.4 V in a 1 MKOH alkaline electrolyte. -2 The stable total hydrolysis current.

[0040] More preferably, when the g-C3N4 / NiS composite photoelectrocatalyst is applied to photoelectrochemical water splitting, a higher potential of 2.8 V is required in the natural seawater system to achieve 20 mA cm⁻¹. -2 The current density.

[0041] More preferably, the seawater system has a pH of 7.29, [Cl... - =34200 ppm.

[0042] This invention provides a low-temperature one-pot preparation method for g-C3N4 / NiS composite photoelectrocatalysts and their application in photoelectrocatalytic water splitting. The method involves mixing melamine, glucose, CTAC, nickel acetate, and thiourea, followed by a hydrothermal reaction at 160°C for 24 hours to synthesize a hierarchical pn-type hybrid material with a flower-like microstructure (i.e., a multi-level structure formed by the coiled assembly of nanosheets). The resulting g-C3N4 / NiS heterojunction achieves multifunctional enhancements in light absorption performance, active site density, and space charge separation efficiency due to its unique electronic coupling effect and synergistic catalytic interface. Notably, the optimal junction with Ni-SCN synergistic coordination exhibits excellent performance under alkaline conditions: it can stably achieve 10 mA cm⁻¹ at low overpotentials of 270 mV (OER) and 2 mV (HER). -2 The water splitting current density was significantly improved. Notably, the bifunctional photoelectrode coupling system exhibited a low tank voltage of 2.4 V and long-lasting durability. Through in-situ Raman spectroscopy combined with theoretical calculations, it was discovered that the structural reconstruction of the coordination sites promoted the transformation of the γ-NiOOH active phase during water splitting, thereby significantly reducing the hydrogen adsorption energy and the adsorption energy barrier of oxygen-containing intermediates. This invention provides a new approach for designing highly efficient and multifunctional photoelectrodes, and has significant application value in the field of solar energy conversion and storage devices.

[0043] Compared to existing g-C3N4 / NiS composite materials, this invention employs a low-temperature (140-180℃) one-pot hydrothermal method to simultaneously react the g-C3N4 precursor (melamine) and the NiS precursor (nickel acetate, thiourea), obtaining the composite material in one step. This process is extremely simple, energy-saving, and environmentally friendly.

[0044] Secondly, this invention utilizes a one-pot method to achieve in-situ covalent bonding between NiS and g-C3N4, forming a Ni-SCN coordination interface. EXAFS characterization confirmed the simultaneous existence of Ni-N bonds (coordination number 0.53) and Ni-S bonds (coordination number 5.78), a strongly coupled interface at the atomic level that is difficult to achieve using stepwise methods.

[0045] Finally, the catalyst prepared by this invention has a unique flower-like hierarchical porous structure, and its specific surface area is significantly increased (up to 5.007 m²). 2 It exhibits excellent bifunctional photoelectrocatalytic water splitting performance (HER overpotential -2 mV, OER overpotential 270 mV) and stability in long-term operation in seawater.

[0046] Compared with the prior art, the present invention has the following beneficial effects: (1) The synthesis process is extremely simple, and the low-temperature one-pot method can be used for large-scale production: Traditional g-C3N4-based heterojunction materials mostly adopt high-temperature calcination above 550℃ or a two-step process of "polymerization-composite", which not only has high energy consumption and large equipment investment, but also easily causes grain coarsening and pore collapse. This invention adopts a low-temperature one-step hydrothermal strategy to achieve in-situ polycondensation of g-C3N4 and simultaneous growth of NiS under mild conditions of 140~180℃. It does not require high-temperature calcination, multi-step reaction, or post-processing modification, which significantly simplifies the preparation process and reduces energy consumption and equipment requirements. The reaction system is uniform, the conditions are controllable, and the repeatability is good, which is suitable for laboratory scale-up preparation and industrial mass production, and solves the technical problems of stepwise synthesis, interface contamination, and large batch differences of traditional heterojunction materials.

[0047] (2) In-situ covalent bonding to construct a strongly coupled heterojunction interface: During the preparation process, Ni atoms simultaneously form Ni–S–CN coordination bonds with S atoms and N atoms in g-C3N4 in situ, achieving atomic-level close contact and constructing a stable pn heterojunction with an internal electric field. This strongly coupled interface significantly reduces charge transport impedance, prolongs carrier lifetime, and significantly suppresses photogenerated electron-hole recombination, thereby improving photoelectrocatalytic efficiency from the structural source and solving the bottlenecks of physical mixing, poor interfacial contact, slow charge transfer, and easy detachment of conventional composite catalysts.

[0048] (3) Hierarchical porous structure, exposing high-density active sites: The obtained g-C3N4 / NiS composite material exhibits a flower-like hierarchical porous morphology, with a specific surface area nearly twice that of pure g-C3N4. The abundant pores and good connectivity can significantly improve electrolyte wettability, ion transport rate, and the number of exposed active sites. The abundant edge sites, defect sites, and coordinated unsaturated Ni sites work synergistically to provide sufficient reaction sites for HER and OER, achieving high-efficiency catalysis with high current density and low overpotential.

[0049] (4) Outstanding bifunctional catalytic performance, simultaneously driving HER and OER efficiently: The prepared composite material is a bifunctional photoelectrocatalyst, exhibiting excellent hydrogen evolution (HER) and oxygen evolution (OER) activities in an alkaline system. At 10 mA / cm², -2 At current density, the HER overpotential is as low as -2mV, the OER overpotential is only 270mV, and the Tafel slope is significantly lower than that of pure g-C3N4 and pure NiS, resulting in a significant improvement in reaction kinetics, which can simultaneously meet the requirements for efficient cathodic reduction and anodic oxidation.

[0050] (5) Excellent water splitting performance and stable and durable two-electrode system: Using g-C3N4 / NiS as both photocathode and photoanode, a two-electrode photoelectrocatalytic water splitting system was assembled. In alkaline electrolyte, only 2.4 V is required to achieve 20 mAcm. -2It has a stable current density and can operate continuously and stably for more than 24 hours without significant decay. The molar ratio of H2 to O2 generated is close to 2:1, and the Faraday efficiency is high. It solves the problems of most catalysts that can only perform single functions, have large pressure drop in total water splitting, and have poor stability.

[0051] (6) Resistant to seawater corrosion, it can be directly used for hydrogen production from natural seawater: The sulfate species generated in situ on the material surface and the reconstructed γ-NiOOH active phase can effectively resist Cl. - Despite corrosion, it can still work stably in natural seawater and simulated seawater. Although the impedance of the seawater system is slightly higher, it still maintains good photoelectrocatalytic response and long-term durability, expanding the practical application potential of photoelectrocatalytic hydrogen production in seawater resource utilization, island hydrogen production, and near-shore new energy scenarios.

[0052] (7) In-situ self-reconstruction to generate an active phase, resulting in a superior catalytic mechanism: During the OER process, NiS sites can spontaneously reconstruct in-situ into the γ-NiOOH active phase, optimizing the adsorption energy barrier of oxygen-containing intermediates and significantly improving the intrinsic activity and stability of OER. Combined with DFT calculations, it is shown that interfacial coordination regulation improves the hydrogen adsorption free energy (ΔG). H* Approaching the ideal value, the energy barrier for the conversion of oxygen-containing intermediates is reduced, achieving synergistic enhancement of photocatalysis and electrocatalysis.

[0053] (8) The raw materials are cheap and readily available, and there are no precious metals, resulting in a significant cost advantage: The melamine, glucose, thiourea, nickel acetate and other raw materials used in this invention are all bulk chemical raw materials, which are inexpensive and widely available. No precious metals such as Pt, Ru and Ir are used in the whole process, which greatly reduces the cost of catalysts and is suitable for large-scale industrial applications, providing a feasible solution for low-cost green hydrogen energy preparation. Attached Figure Description

[0054] Figure 1 A schematic diagram of the one-pot preparation of CN / NiS composite photocatalysts; Figure 2 XRD spectra of CN, NiS, and CN / NiS and their corresponding standard cards; Figure 3 XRD spectra of CN / NiS without the addition of thiourea at (a) different hydrothermal temperatures, (b) hydrothermal times, (c) and (d) composite ratios; Figure 4 Infrared spectra of CN, NiS, and CN / NiS; Figure 5 Raman spectra of CN, NiS, and CN / NiS; Figure 6 Morphological characterization images of different samples: (a) CN, (b) NiS, (c) SEM images of CN / NiS; (d) High-magnification image of selected area; Figure 7Elemental analysis images for CN / NiS: (a) Elemental distribution mapping image and corresponding elemental distribution map of selected regions; (b) EDS energy spectrum and corresponding atomic weight percentage (table). Figure 8 Comparison of transient photocurrent response (OPCOD) for CN, NiS, and CN / NiS; Figure 9 High-resolution XPS spectra of CN, NiS and CN / NiS samples: (a) C 1s, (b) N 1s, (c) Ni 2p, (d) S 2p; Figure 10 Normalized (a) Ni K-edge XANES spectrum and (b) Ni K-edge FT-EXAFS spectrum for CN / NiS and control samples (Ni foil, NiO, NiS); Figure 11 FT-EXAFS fitting curve analysis for NiS and CN / NiS; Figure 12 The comparative absorption spectra of CN, NiS, and CN / NiS are shown. Figure 13 A comparative study of the PEC and EC properties of CN / NiS materials in HER and OER; Figure 14 Comparison of LSV curves for HER and OER of CN, NiS, and CN / NiS materials with different NiS loadings; Figure 15 The effects of hydrothermal temperature (a) and hydrothermal time (b) on the PEC properties of 0.16CN / NiS material; Figure 16 For (a) different current densities obtained through global optimization (η=10, 20, 50 mA cm⁻¹) -2 (b) Comparison of Tafel curves for HER and OER of CN / NiS materials; Figure 17 XPS spectra of (a) Ni 2p and (b) S 2p of the photoanode after the OER reaction of CN / NiS material; Figure 18 (a) Data plot of gas evolution behavior of CN / NiS photoelectrode under constant potential; (b) Faraday efficiency plot of CN / NiS photoelectrode under applied anode and cathode bias. Figure 19 (a) Schematic diagram of a dual-electrode electrolytic cell using a CN / NiS bifunctional photoelectrode; (b) Water decomposition polarization curves of the CN / NiS||CN / NiS system in KOH solution (black line) and seawater (red line); Figure 20(a) Data graph of gas evolution behavior of bifunctional CN / NiS photoelectrode under multiple potentials (inset: gas release phenomenon on electrode surface); (b) Chronoamperometry curves of photoelectrolysis of CN / NiS in KOH solution (black line) and seawater (red line) under a bias voltage of 2.4V; (c) Comparison of actual gas production during long-term operation in alkaline aqueous solution under constant voltage of 2.4V; (d) Comparison of actual gas production during long-term operation in alkaline aqueous solution under constant voltage of 2.4V. Figure 21 The results show (a) a comparison of polarization curves and Tafel slopes (inset) in different electrolyte systems (KOH-black line, seawater-red line, KOH+NaCl-blue line) and (b) the results of long-term stability tests. Figure 22 (a) Normalized Ni K-edge XANES spectra of CN / NiS, CN / NiS-OER, and reference samples (Ni foil, NiO, NiS); (b) Ni K-edge Fourier transform EXAFS spectra of CN / NiS, CN / NiS-OER, and reference samples; (c) In-situ Raman spectra of the initial CN, NiS, and CN / NiS photoelectrodes in the potential range of 1.0–2.0 V (vs. RHE) as a function of potential. Figure 23 The theoretical simulation structures for the optimal adsorption configurations of (a) CN / NiS, (b) H*, (c) *OH, (d) *O, and (e) *OOH are shown in the top view and bottom view, where * indicates an intermediate state. Brown, blue, red, yellow, gray, and light pink spheres represent C, N, O, S, Ni, and H atoms, respectively. Figure 24 The optimized crystal structures of the original (a) CN, (b) NiS, and (c) composite CN / NiS are shown (top: top view; bottom: side view). The brown, blue, yellow, and gray spheres in the figures correspond to C, N, S, and Ni atoms, respectively. Figure 25 The optimized adsorption configurations of the active sites on the surface of the original CN material for the intermediates *H, *OH, *O and *OOH are shown in the top view and the bottom view. * indicates the intermediate adsorption state, and the brown, blue, red, yellow, gray and light pink spheres represent C, N, O, S, Ni and H atoms, respectively. Figure 26 Optimized adsorption configurations of *H, *OH, *O, and *OOH intermediates on a NiS substrate surface (top: top view; bottom: side view); where * represents intermediate adsorption states, and red, yellow, gray, and light pink spheres represent O, S, Ni, and H atoms, respectively; Figure 27The graphs show the free energy changes of the HER reaction steps on the surfaces of CN, NiS, and CN / NiS materials under voltage conditions of U=0 V; and the free energy changes of the OER reaction steps on the surfaces of CN, NiS, and CN / NiS materials under voltage conditions of U=0 V (top) and U=1.23 V (bottom). Figure 28 The optimized adsorption configurations of *H, *OH, *O, and *OOH intermediates at the surface active sites of the CN / NiS heterojunction are shown in the top view and the bottom view. * indicates the intermediate adsorption state, and the brown, blue, red, yellow, gray, and light pink spheres represent C, N, O, S, Ni, and H atoms, respectively. Figure 29 This is a schematic diagram of the reaction mechanism of the bifunctional CN / NiS catalyst; Figure 30 Comparison of (a) Tauc curves and (b) valence band X-ray photoelectron spectra of different catalysts based on UV-Vis spectral data; Figure 31 IPCE for different photoelectrodes.

[0055] Figure 32 This is a diagram showing the pore structure distribution of CN / NiS material.

[0056] Figure 33 The HER polarization curves of CN / NiS material prepared by the one-pot method of this invention are compared with those of CN / NiS material prepared by the three-step method of the comparative example. Detailed Implementation

[0057] The present invention will now be described in detail with reference to the accompanying drawings and specific embodiments. These embodiments are based on the technical solution of the present invention and provide detailed implementation methods and specific operating procedures. However, the scope of protection of the present invention is not limited to the following embodiments.

[0058] Unless otherwise specified, the reagents, methods, instruments, and equipment used in this invention are conventional in the art. Unless otherwise specified, the reagents and materials used in the following examples are all commercially available.

[0059] Example 1 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Add 0.25 g melamine, 0.35 g glucose, 0.13 g CTAC and 0.2 ml acetic acid to 40 mL deionized water in sequence, and stir magnetically at 500 rpm for 30 min to obtain clear solution A; (2) Add 0.01 mol nickel acetate and 0.03 mol thiourea (Ni:S=1:3) to solution A, and continue stirring for 24 h to form a light green suspension; (3) Transfer the suspension to an 80 mL polytetrafluoroethylene-lined stainless steel autoclave, heat it to 160 °C at 5 °C / min, and hold it at that temperature for 24 h; then allow it to cool naturally. (4) The product was centrifuged and washed (3 times each with deionized water and ethanol), and vacuum dried at 70 °C for 12 h to obtain flower-shaped 0.01 CN / NiS powder, which was recorded as "0.01 CN / NiS-160 °C-24h".

[0060] Example 2 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Solution A is the same as in Example 1; (2) Add 0.04 mol nickel acetate and 0.12 mol thiourea (Ni:S=1:3), and perform the remaining operations as in Example 1; (3) Recorded as “0.04 CN / NiS-160℃-24h”.

[0061] Example 3 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Solution A is the same as in Example 1; (2) Add 0.16 mol nickel acetate and 0.48 mol thiourea, and perform the remaining operations as in Example 1; (3) Recorded as “0.16 CN / NiS-160℃-24h”.

[0062] It should be noted that subsequent Examples 4-8 all use the sample prepared in this Example (i.e., 0.16 CN / NiS-160℃-24h) as the object for performance testing and comparison.

[0063] Example 4 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Solution A is the same as in Example 1; (2) Add 0.40 mol nickel acetate and 1.20 mol thiourea, and perform the remaining operations as in Example 1; (3) is recorded as “0.40 CN / NiS-160℃-24h”.

[0064] Example 5 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Keep the amount of nickel acetate and thiourea constant at 0.16 mol, and only change the hydrothermal temperature: set it to 140 ℃ for 24 h; (2) Recorded as “0.16 CN / NiS-140℃-24h”.

[0065] Example 6 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Keep the amount of nickel acetate and thiourea constant at 0.16 mol, and only change the hydrothermal temperature: set it to 180 ℃ for 24 h; (2) Recorded as “0.16 CN / NiS-180℃-24h”.

[0066] Example 7 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Keep the amount of nickel acetate and thiourea constant, and only change the hydrothermal time: set the temperature to 160 ℃ for 8 h; (2) Recorded as “0.16 CN / NiS-140℃-8h”.

[0067] Example 8 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Keep the amount of nickel acetate and thiourea constant, and only change the hydrothermal time: set the temperature to 160 ℃ for 16 h; (2) Recorded as “0.16 CN / NiS-140℃-16h”.

[0068] Example 8 The low-temperature one-pot preparation method of the g-C3N4 / NiS composite photocatalyst of the present invention specifically includes the following steps: (1) Keep the amount of nickel acetate and thiourea constant, and only change the hydrothermal time: set the temperature to 160 ℃ for 32 h; (2) Recorded as “0.16 CN / NiS-140℃-32h”.

[0069] Comparative Example 1 Preparation of single g-C3N4: Remove nickel acetate and thiourea according to Example 3, with the other conditions being the same; denoted as "CN".

[0070] Comparative Example 2 Preparation of single NiS: Melamine, glucose, CTAC and acetic acid were removed according to Example 3, and the other conditions were the same; it is referred to as "NiS".

[0071] Comparative Example 3 Stepwise preparation of gC3N4 / NiS composite material: First, remove nickel acetate and thiourea as in Example 3, with the remaining conditions the same; denoted as "CN". Then add 0.16 mol nickel acetate and 0.48 mol thiourea, with the remaining operations the same as in Example 1; denoted as "0.16 CN / NiS-Compair".

[0072] The hybrid material of Example 3 with a hierarchical structure was prepared in one step by simultaneously performing direct hydrothermal aging and pyrolysis of melamine and in-situ growth of NiS. Figure 1 XRD patterns ( Figure 2 The XRD pattern shows that the characteristic peaks at 13.2° and 27.4° correspond to the (100) and (002) crystal planes of gC3N4, respectively, confirming that the CN (i.e., the gC3N4 / C composite material) obtained in this invention maintains typical in-plane conjugated structure and interlayer stacking characteristics. Specifically, the XRD pattern of the single CN prepared in Comparative Example 1 shows that the carbonaceous components generated by glucose pyrolysis under hydrothermal conditions are combined with gC3N4 formed by melamine condensation to form a gC3N4 / C structure. The diffraction peak of graphene oxide (GO) generated by glucose pyrolysis at 10.6° shows that its π-π stacking distance with CN has decreased from the reported 0.86 nm to 0.83 nm, indicating that the introduction of glucose enhances the π-π stacking interaction.

[0073] In Example 3, the CN / NiS material retained the characteristic CN peaks while exhibiting a significant decrease in peak intensity, indicating that it maintained a good structure even after co-existence with NiS. In the preparation system of this invention, glucose plays multiple structure-directing roles. On one hand, the carbonaceous intermediates formed by the hydrothermal pyrolysis of glucose can act as structure-directing agents; their surface oxygen-containing functional groups (such as hydroxyl and carboxyl groups) can interact with NiS. 2+Coordination interactions occur, thereby regulating the nucleation and growth environment of NiS and promoting the transformation of NiS crystals from the pure phase R3m(160) space group to the P63 / mmc(194) space group. On the other hand, the carbonaceous components formed by glucose carbonization are combined with gC3N4 to construct a g-C3N4 / C composite structure. At the same time, the graphene oxide (GO) generated by glucose cleavage forms a π-π stacking interaction with g-C3N4 (interlayer spacing of about 0.83 nm), which helps to facilitate the rapid transfer of interfacial charges. Under the above synergistic effect, there is no longer any free GO in the final product, which is reflected by the disappearance of the GO diffraction peak in the XRD pattern. Therefore, the addition of glucose is crucial for forming the flower-like hierarchical porous structure of the present invention, realizing the in-situ covalent bonding of NiS and g-C3N4, and obtaining excellent photoelectrocatalytic performance. Control group experiment (without thiourea, Figure 3 (a) The XRD pattern of the product is completely consistent with that of the pure comparative example 1, confirming that the formation of Ni-SCN bonds and the symmetry transformation of NiS crystals require precise coordination of the sulfur source. By comparing different aging temperatures ( Figure 3 (b) and time ( Figure 3 (c) The sample synthesized under optimized conditions was found to have a higher CN diffraction peak ( Figure 3 (d) The effect gradually weakened with the increase of NiS loading, which once again confirmed that NiS nanoparticles were successfully grown in situ on the CN framework.

[0074] Fourier transform infrared spectroscopy ( Figure 4 Analysis showed that Example 3 completely retained the CN skeleton structure, and the characteristic absorption peaks included the triazine ring (810 cm⁻¹). -1 CN heterocycles (1750-1300 cm⁻¹) -1 ) and the stretching vibrations of adsorbed water molecules and NH bonds (2900-3500 cm⁻¹) -1 Only 614 cm -1 The newly emerging Ni-S characteristic vibrational peak confirms the successful recombination of NiS. Simultaneously, the C=S bond stretching vibration peak increased from 740 cm⁻¹. -1 Redshifted to 716 cm -1 This confirms the formation of metal-sulfur coordination bonds rather than sulfur doping or nickel integration, providing direct evidence for the bridging of CN and NiS through the Ni-SCN interface. Raman spectroscopy (200-3300 cm⁻¹) -1 , Figure 5 Further, it is shown that Example 3 also has a defect band of CN (1328 cm). -1 ), graphite band (1627 cm) -1 ) and NiS lattice vibrations (501 cm⁻¹) -1 Characteristic peaks of ).

[0075] Scanning electron microscopy (SEM) morphological observation showed that pure comparative example 1 was composed of smooth, regular microspheres (0.5-3 μm in diameter) stacked together. Figure 6 (a) shows a different morphology from that formed by the traditional hydrothermal polymerization of glucose-melamine. Compared to pure comparative example 1 and agglomerated comparative example 2 nanoparticles ( Figure 6 (b)), the composite material in Example 3 exhibits a hierarchical flower-like structure ( Figure 6 (c) High-magnification SEM shows that its petal-like structure is formed by the interlacing of rolled nanosheets to form a porous network. Figure 6 (d)), the specific surface area is twice that of pure comparative example 1 (Table 1). Element distribution map ( Figure 7 The results show that Ni and S elements are uniformly distributed within the CN framework, combined with broad-spectral absorption ( Figure 8 ) and active site exposure data (Table 1), pore structure distribution map ( Figure 32 This confirms that the heterojunction structure is beneficial for enhancing light absorption and catalytic activity. For example... Figure 32 As shown, the pure CN sample exhibits a clear pore size distribution in the ranges of approximately 2–10 nm and 10–80 nm, indicating that it contains both mesopores and macropores. The pore size distribution curve of the 0.16CN / NiS composite material shows a bimodal characteristic, with a clear pore volume distribution in the ranges of 2–5 nm and 10–100 nm, confirming the simultaneous presence of mesopores and macropores in the material, forming a typical hierarchical porous structure.

[0076] Table 1. Specific surface area and average pore size of photocatalysts To further characterize the surface chemical state of the samples prepared in Comparative Example 1, Comparative Example 2, and Example 3, X-ray photoelectron spectroscopy (XPS) was used for analysis. High-resolution C1s spectra ( Figure 9 (a) shows that there are three peaks at 288.90, 285.48 and 284.80 eV, corresponding to sp in the NC=N bond, respectively. 3 / sp 2 Hybrid carbon, C-NH2 groups, and surface-adsorbed carbon species (CC / C=C). N 1s spectrum ( Figure 9 (b) The characteristic peaks are located at 402.73, 400.44, and 399.14 eV, respectively, belonging to the C-NH2, N-(C)3, and CN=C structures, and a π-electron excitation peak (eV) was also observed. The C1s and N1s peak positions of the composite material in Example 3 are basically consistent with those of the pure comparative example 1, indicating that the original structure of graphitic carbon nitride is preserved. Ni 2p fine spectral analysis ( Figure 9(c) shows that the characteristic peaks in the ranges of 853-858 eV (Ni 2p3 / 2) and 870-876 eV (Ni 2p1 / 2) originate from Ni. 2+ (853.12 and 875.31 eV), Ni 0 (857.34 and 870.37 eV) and corresponding satellite peaks (861.76 and 879.97 eV). Compared with the original Comparative Example 2, Ni in Example 3 2+ The signal enhancement is significant, confirming the successful formation of divalent NiS on the CN surface. (S 2p spectrum) Figure 9 (d) The characteristic peaks at 161.17, 162.28, and 163.29 eV can be identified as sulfides (S). 2- S 2p1 / 2 and S 2p3 / 2, polysulfides (Sx 2- ), and sulfate (SO4) at 168.80 eV. 2- The latter may originate from surface oxidation during the hydrothermal synthesis process. The presence of sulfate helps improve the chlorine resistance and stability of the material in seawater environments. Notably, compared to the 164.7 eV lattice sulfur characteristic in pure Comparative Example 2, Example 3 exhibits significantly higher chlorine resistance in S... 2- The enhanced signal in the region indicates the formation of CS bonds, rather than NS bonds. This result confirms the heterojunction interface bridging structure constructed via Ni-SCN bonds. Furthermore, the strong interactions between the heterojunction interfaces lead to negative shifts in the C 1s binding energies and positive shifts in the S 2p binding energies. This effect is thought to promote interfacial charge separation, thereby enhancing the photocatalytic performance of the material.

[0077] To elucidate the atomic coordination environment of the composite material in Example 3, further investigation was conducted using X-ray near-edge absorption structure (XANES) and extended X-ray absorption fine structure (EXAFS) techniques. Figure 10 As shown in (a), the NiK-edge XANES spectrum edge position of Example 3 is between that of the Ni foil and the NiO standard sample, similar to the characteristics of Comparative Example 2. This result indicates the presence of Ni in the material. 0 and Ni 2+ The mixed valence state is consistent with the aforementioned XPS (Ni 2p) analysis results. Except for the Ni at 8334 eV, which is the same as in Comparative Example 2. 2+ Besides the 1s-3d transition peak, Example 3 exhibits a 1s-4pz transition characteristic peak at 8339 eV, leading to a difference in spectral shape. This phenomenon reveals a special electronic structure with D4h symmetry. The R-space Fourier transform curve of Comparative Example 2 (…) Figure 10(b) shows a significant Ni-S coordination peak at 1.82 Å, while the typical coordination characteristics of Ni foil (2.05 Å) and NiO (1.53 Å Ni-O, 2.45 Å Ni-O-Ni) are not observed. In contrast, the EXAFS spectrum of Example 3 shows a Ni-N bond with a bond length of 1.41 Å and a Ni-S bond with a bond length of 1.89 Å. The presence of the Ni-N bond directly confirms that the Ni atom forms a direct chemical coordination bond with the N atom in the CN matrix (i.e., it is in the first coordination shell of the Ni atom), while the presence of the Ni-S bond confirms the formation of the NiS phase. The coexistence of these two bonds proves that the Ni atom is simultaneously coordinated with the S atom and the N atom in CN, forming the N-Ni-S coordination structure described in this invention. To accurately quantify the coordination parameters of the Ni K-edge in Example 3, Fourier transform EXAFS fitting was performed ( Figure 11 The average Ni-S coordination number of the original Comparative Example 2 was 6.3 (Table 2), indicating that the Ni atoms exhibited an octahedral NiS6 configuration; while in Example 3, the Ni-S coordination number decreased to 5.78, and a Ni-N coordination of 0.53 was detected. This result confirms that heterointerface bridging leads to the formation of an unsaturated coordination structure, which is consistent with the hexagonal crystal system transformation observed by XRD. Notably, the specific interaction between Ni and the adjacent triazine unit may modulate the intrinsic electronic structure of CN and extend its charge lifetime by reducing structural crystallinity (XRD) and N signal intensity (XPS) while maintaining the Ni-S coordination environment. Figure 8 Based on the above analysis, it can be inferred that a novel heterojunction structure between CN and NiS was successfully constructed via a one-step hydrothermal method. This interface structure with synergistic coordination properties enhances optical absorption (…). Figure 12 With its superior charge separation efficiency compared to single components, it is expected to exhibit excellent catalytic activity.

[0078] Table 2. Fitting parameters of extended X-ray absorption fine structure (EXAFS) for various samples at the Ni K-edge. (a) CN: coordination number; (b) R: distance between absorbing and backscattering atoms; (c) σ 2 : Debye-Waller factor, used to account for thermal and structural disorder; (d) ΔE(0): internal potential correction; R factor represents the goodness of fit. Based on the experimental EXAFS fitting results of copper foil, CN is fixed to the known crystallographic value, S0 2 It was set to 0.78. A reasonable range for EXAFS fitting parameters is: 0.700 < S0. 2 < 1.000; CN > 0; σ 2 (Å) 2)>0; |ΔE0| < 30 eV; R factor < 0.02.

[0079] The HER performance of the photocathode in Example 3 was systematically evaluated using a three-electrode system in a 1.0 M KOH electrolyte. Figure 13 As shown, the significant improvement in photoelectrochemical (PEC) polarization curves compared to the pure electrochemical (EC) system is mainly attributed to the intrinsic property of the material to convert light energy into electron energy. Compared to the pure material of Comparative Example 1, the composite material of Example 3 exhibits superior catalytic activity under the same conditions as the NiS content gradually increases (0.01~0.4). Figure 14 Among them, Example 3, with an optimal NiS loading of 0.16, exhibited superior performance compared to intrinsic NiS in LSV testing, confirming that the heterostructure can effectively reduce overpotential and improve reaction kinetics. To investigate the regulatory effect of different hydrothermal temperatures (140–180 °C) and reaction times (8–32 h) on the heterointerface barrier of Example 3, the structure-property relationship of the Example 3 samples was systematically studied. Figure 15 The results showed that Example 3, prepared by reacting at 160 °C for 24 hours, exhibited the best HER performance. Figure 16 (a) Only -2, -116, and -261 mV overpotentials are required to drive 10, 20, and 50 mA cm⁻¹, respectively. -2 The current density. Based on its superior performance, this optimized sample, Example 3 (160 ℃-24h), was uniformly named CN / NiS in this study. Compared with other metal chalcogenides / nitrides / phosphides, this naming emphasizes the key role of the heterointerface in reducing the reaction barrier. Tafel slope analysis obtained by fitting the HER polarization curve shows that ( Figure 16 (b) Example 3 (72.01 mV dec -1 The Tafel slope of the first example is significantly smaller than that of the second example (324.97 mV dec). -1 ) and Comparative Example 2 (89.8 mV dec -1 This result confirms that, under alkaline conditions, when the formation of H* is the rate-determining step, the composite material of Example 3 exhibits the fastest reaction kinetics following the Volmer-Heyrovsky mechanism.

[0080] The OER performance of the CN / NiS photoanode was systematically tested under the same alkaline conditions. Compared with Comparative Example 1 and Comparative Example 2, the increase in the content of Comparative Example 2 also significantly improved the OER activity. Figure 14 When the current density reaches 10, 20, and 50 mA cm⁻¹ -2At that time, the optimized preparation of Example 3 (160 °C-24 h) required only 270, 310 and 464 mV overpotentials, respectively. Figure 16 (a) shows superior performance compared to most reported transition metal chalcogenides / nitrides / phosphides. Notably, during anodic scanning, Example 3 observed a significantly enhanced Ni oxidation peak (Ni) before the OER onset potential. 0 / Ni 2+ →Ni 3+ orNi 4+ This characteristic peak was confirmed as a potential active site for OER. Fine spectral analysis of the photoanode Ni 2p after the OER reaction showed that the peaks at 856.08 eV (Ni 2p3 / 2) and 861.98 eV (Ni 2p1 / 2) correspond to Ni... 2+ and Ni 3+ Species indicate the formation of the γ-NiOOH phase ( Figure 17 (a)). Meanwhile, Bode plot analysis under alkaline conditions indicates that, under oxidation potential, the sulfur component in the initial catalyst NiS (which can be considered a pre-catalyst) dissolves as water-soluble oxygen-containing anions, leading to a decrease in its binding energy. Figure 17 (b) This phenomenon significantly promotes the formation of the nickel active phase on the CN framework. Based on the above results, the Tafel slopes of Comparative Example 1, Comparative Example 2, and Example 3 were calculated to be 372.95, 233.35, and 198.75 mV dec, respectively. -1 This data confirms that Example 3 exhibits the best reaction kinetics in alkaline OER.

[0081] In addition to the current curve, the amount of gas evolution at different potentials in Example 3 was quantitatively monitored using an online trace gas chromatography system with an automatic sampling interval of 60 minutes. The results showed that the amount of gas evolution on the photoelectrode of Example 3 increased proportionally with the increase of the cathode (0~-0.6 V vs / RHE) and anode (1.4~2.0 V vs / RHE) potentials. Figure 18 (a) Due to the slow kinetics of water oxidation, the amount of H2 and O2 gas produced simultaneously under strong cathode bias is approximately twice that under anodic bias for the same reaction time. Faraday efficiency calculated based on H2 production shows that under low negative bias, the initial Faraday efficiency (44%) increases to a peak (163%) with increasing potential and then slowly decreases (77%). Figure 18(b) Although the Faraday efficiency evolution shows the same trend as the anode bias gradually increases, its value, which is significantly higher than that under cathode conditions, clearly reflects the superior kinetic characteristics dominated by alkaline OER. In addition, the actual H2 to O2 ratio released under moderate bias is consistent with the theoretical value of 2:1, and the Faraday efficiency is higher than 100%, indicating that this material has potential advantages in water splitting applications.

[0082] Based on the excellent activity exhibited by Example 3 in the OER and HER reactions, a dual-electrode electrolysis system using Example 3 as both the anode and cathode was further constructed to evaluate its total water splitting performance. Figure 19 (a) In a 1.0 MKOH electrolyte, the CN / NiS||CN / NiS system requires a cell voltage of 2.4 V to achieve 20 mA cm⁻¹. -2 current density ( Figure 19 (b) This performance is competitive with previously reported bifunctional catalytic systems. The gas released by high-density bubbles on the electrode surface was monitored in situ. Figure 20 (a) Illustration) It was found that when the operating voltage increased from 1.2 V to 2.4 V, the production of H2 and O2 increased significantly in a stoichiometric ratio (2:1) and reached a maximum. Figure 20 (a)). In a 24-hour durability test conducted at a constant voltage of 2.4 V ( Figure 20 (b) Voltage fluctuations caused by bubble desorption were observed in the initial stage, but the system still exhibited better stability than MoS2 / Ni3S2 (10 hours) and Cu2S@NF (20 hours). Notably, during the continuous alkaline water splitting process, the production of H2 and O2 increased linearly over 12 hours until the system saturated, and the gas production capacity was fully restored within the following 12 hours after bubble desorption. Figure 20 (c) Given the excellent performance of this bifunctional photoelectrode under alkaline conditions, the study further evaluated its application potential in natural seawater systems. The results showed that a higher potential (2.8 V) was required in the seawater system to achieve 20 mA cm⁻¹ compared to the alkaline medium (2.4 V). -2 current density ( Figure 19 (b)). After 24 hours of continuous testing, the cumulative H2 production in seawater was only 1 / 7 of that under alkaline conditions. Figure 20 (d) Based on the stability test data, it can be inferred that although exogenous components in seawater can promote the HER process, they have a significant inhibitory effect on the OER reaction.

[0083] By observing the competing reactions during the photoelectrolysis of seawater, the catalytic activity of Example 3 in different electrolytes was systematically compared, revealing the relevant resistance characteristics of the OER process. In the simulated seawater system (KOH + NaCl), the OER reaction was carried out at 50 mA cm⁻¹. -2 Only 460 mV overpotential is required at current density ( Figure 21 (a) shows superior performance compared to pure KOH electrolyte (550 mV), a result consistent with 27 mV dec -1 The Tafel slope reflects the excellent reaction kinetics (illustration). However, in natural seawater systems, the photoelectrolysis system requires a higher overpotential (510 mV) to reach 10 mA cm⁻¹. -2 The low current density is mainly attributed to the blockage of surface active sites caused by the large adsorption of chloride ions, ultimately reducing the reaction efficiency. Stability tests further confirm the advantages of this bifunctional catalyst in practical applications. In the pure KOH system, the current density exhibits a tower-like increase in the first 4 hours followed by a gradual decline, and then remains stable over the next 24 hours. Figure 21 (b) It is worth noting that, in the simulated seawater system (KOH+NaCl), apart from the initial current abrupt change caused by the dynamic reconstruction of nickel sulfide, the tailing phenomenon observed in the subsequent timeline indicates that an active phase with chloride ion corrosion protection has formed on the material surface, thereby enhancing the OER activity. Long-term testing of the natural seawater system shows a flat current curve, and its performance degradation is mainly due to the increased impedance caused by side reactions such as chloride precipitation, as well as the negative impact of complex components such as sulfate, magnesium, and calcium ions. The core objective of this invention is to develop photoelectrocatalysts with intrinsic bifunctional properties, rather than optimizing for low activity under extreme conditions (this will be a future research direction). Nevertheless, the significant catalytic activity and long-term stability exhibited in Example 3 indicate that this material has potential application value in alkaline water electrolysis and whole seawater splitting.

[0084] Previous studies have shown that transition metal chalcogenides (especially nickel sulfide) are readily oxidized under alkaline OER conditions, transforming into catalytically active (hydroxy) hydroxide phases. This is consistent with the Ni 2p spectrum observed in this study. 3+ The characteristic peaks and sulfur leaching results are consistent. Figure 17 XPS analysis confirmed the formation of the γ-NiOOH active phase, which plays a decisive role in enhancing OER activity. To clarify the structural evolution of NiS after the OER reaction, XANES spectroscopy was used for ex-situ analysis. Comparison of the Ni K-edge absorption spectra of Ni foil, NiO, and NiS standard samples revealed that the absorption edge of the Example 3-OER sample after the OER reaction exhibited a significantly higher energy shift, and its spectral characteristics differed significantly from those of the original Example 3. Figure 22 (a) confirmed that it has Ni3+ The formation of the high-valence NiOOH phase. FT-EXAFS quantitative analysis showed that, compared to precursor Example 3, the bond lengths of Ni-N (1.36 Å), Ni-S (1.92 Å), and Ni-Ni-O (2.28 Å) in the OER-reacted sample were significantly elongated. This result stems from the reconstruction of the NiS6 octahedral structure and the formation of high-valence NiOOH. Theoretically, sulfur-bridged Ni... 2+ NiS originally has t2g 6 eg2 electronic configuration. According to the Yang-Shao-Horn principle, higher valence Ni in this system can optimize the number of electrons filling the eg orbitals (<2), thus significantly improving OER activity. This active phase with a special electronic structure (especially strongly Lewis acidic Ni) 3+ The synthesized photocatalyst (with specific adsorption sites) effectively promotes the initial adsorption and activation of OH-, thereby optimizing the subsequent conversion process of oxygen-containing intermediates and ultimately enhancing the overall catalytic activity. The optical properties of the synthesized photocatalyst were further investigated using UV-Vis spectroscopy. Figure 12 As shown in (a), the UV-Vis absorption spectrum of the nanoparticles in Comparative Example 2 exhibits two significant absorption peaks at wavelengths of 250 nm and 403 nm. These absorption peaks indicate that Comparative Example 2 has a high absorption capacity in the visible light region. With the loading of nanoparticles from Comparative Example 2, the absorption capacity of the composite photocatalyst in Example 3 in the visible light region was significantly improved.

[0085] To gain a deeper understanding of structural differences and local coordination environments, this study systematically verified the phase transition dynamics of the OER process using in-situ Raman spectroscopy at different potentials. Unlike Raman spectroscopy records excited by near-infrared light, the original Comparative Example 1 sample, at an excitation wavelength of 532 nm, showed phase transitions at 467, 1220, and 1620 cm⁻¹. -1 Several poorly resolved characteristic peaks were observed at the point, which correspond to the layer deformation vibration, C(sp2) bending vibration, and C=N stretching vibration modes, respectively. Figure 22 (c) However, no significant changes in the Raman signal were observed within the applied voltage range of 1.0–2.0 V. Similarly, the original Comparative Example 2 sample, which had a broad peak (the superposition peak of the Eg mode and the SS pair) and a CO shoulder peak (the interaction between sulfur dissolution and the soluble CO2 surface), remained stable after treatment within the same bias voltage range, with only a 550 cm⁻¹ peak. -1 The out-of-plane stretching vibration (νNi-O) from the NiOOH phase shows a weak evolution (orange columnar region). For the original Example 3 composite, its Raman spectrum retains the same two main peaks as Comparative Example 2 (gray columnar region), without showing the disappearance of the characteristic peaks of Comparative Example 1. Furthermore, at 1068 cm⁻¹... -1The newly appearing S=O characteristic peak originates from SO4, a surface oxidation product of sulfur dissolved under alkaline conditions. 2- 1.2 At the initial stage of the V potential, a broad peak with a blue shift (900~1050 cm⁻¹) gradually appeared in the sample of Example 1. -1 ), corresponding to NiOO - The characteristic vibrations (orange columnar region) are observed, while the S=O peak below 1.0 V and the CO / C=N mixed peak (gray columnar region) disappear. As the potential continues to increase (from gray to orange columnar region), the ν Ni–O vibrational band of γ-NiOOH continues to strengthen, indicating that SO4 in Example 3 initially... 2- The rapid breaking of the S=O bond facilitates the OO coupling process, thereby promoting the continuous formation of the NiOOH phase. This is consistent with the catalytic performance test results, further verifying the positive role of this active phase. During the anodic oxidation process, the disappearance of the CO / C=N mixed peak (gray to orange columnar region) after 1.2 V was also observed, accompanied by the strengthening of the Ni-N bond (see...). Figure 22 (b) FT-EXAFS spectrum). Due to CO3 2- The formation of the intercalated NiOOH phase consumes CO groups. The results further confirm that the formation of the NiOOH active phase may weaken the C=N interaction in the polymer CN and enhance the Ni-N bond, thereby effectively strengthening the interface engineering of the heterojunction in Example 3 and ultimately achieving higher OER activity and long-term stability.

[0086] To investigate the excellent bifunctional activity of Example 3 in overall water splitting, the structural model of Example 3 was constructed based on density functional theory (DFT). Figure 23 (a) A theoretical simulation was performed and compared with Comparative Example 1 and Comparative Example 2 alone. Figure 24 In the HER kinetics, the adsorption free energy of *H (ΔG) H* ΔG is widely considered a key theoretical parameter for evaluating catalyst performance. Specifically, the ΔG of the catalysts in Comparative Example 1 and Comparative Example 2 is... H* The values ​​are -1.83 eV and -0.24 eV, respectively. Figure 27 (a) indicates that both surfaces have a strong binding capacity for the H intermediate. Figure 25 and Figure 26 HER polarization curves were plotted on the stepwise synthesized sample (denoted as 0.16 CN / NiS-Compair) prepared in Comparative Example 3. Figure 33The test results show that the onset potential of the 0.16CN / NiS sample is approximately -0.05 V (vs. RHE), significantly higher than the -0.15 V (vs. RHE) of the control sample. This indicates that the one-pot in-situ composite sample of this invention has a lower HER reaction energy barrier, making the reaction easier to initiate. At a current density of 10 mAcm⁻¹... -2 At this potential, the 0.16CN / NiS sample required only -20 mV, while the control sample required an overpotential as high as -170 mV. Furthermore, at a potential of -0.6 V (vs. RHE), the 0.16CN / NiS sample achieved a current density of -0.18 A cm⁻¹. -2 It is approximately 0.03 A cm⁻¹, compared to the control sample. -2 The results indicate that the CN / NiS composite material prepared by in-situ covalent bonding in a one-pot method exhibits significantly better HER catalytic activity than materials prepared by stepwise synthesis. This performance advantage is attributed to the in-situ covalent coupling interface (Ni-SCN coordination bond and N-Ni-S coordination structure) described in this invention, which achieves atomic-level close contact and efficient charge transfer, thereby significantly reducing the HER overpotential and improving reaction kinetics.

[0087] In contrast, ΔG in Example 3 H* (0.13 eV) is close to zero potential, indicating the coordination configuration of its interface S site ( Figure 23 (b) Top and side views) facilitate the cascade process of hydrogen adsorption and desorption following the Volmer-Heyrovsky pathway under alkaline conditions. On the other hand, by calculating the four elementary steps of the OER ( Figure 23 (c), (d), and (e) Top view and magnified side view of the structural model) showing the Gibbs free energy change (ΔGn, n=1-4) to explain the experimentally observed OER activity. Figure 27 (a)). Under zero electrode potential conditions (U = 0 V vs. SHE), the overpotential (η) was confirmed to be equal to the difference between the actual potential (maximum ΔGn divided by charge e) and the standard Nernst potential (1.23 V vs. SHE). Calculations were made based on the adsorption model of *OH, *O, and *OOH intermediates in the OER reaction pathway. Figure 25 and Figure 26 Intrinsic CN itself is a good OER photocatalyst, with a theoretical η of 0.50 V and a rate-determining step (RDS) of O intermediate formation (ΔG2 = 1.73 eV); while Comparative Example 2 has a higher η (1.35 V), with RDS being the conversion of *O to *OOH (ΔG3 = 2.58 eV). Figure 27(b) The figure above). X-ray absorption spectroscopy and in-situ Raman spectroscopy data show that orbital hybridization of the Ni site with the oxygen-containing species resulted in Example 1 achieving the lowest η (0.47 V) in the *OOH formation step (ΔG3 = 1.70 eV), which is highly consistent with the results of a smaller Tafel slope. Compared to Comparative Example 1 and Comparative Example 2 ( Figure 28 After applying a voltage of 1.23 V, the free energy of each reaction step in Example 3 decreased significantly. Figure 27 (b) The figure below confirms that its OER process is thermodynamically superior and conforms to the recognized adsorption evolution mechanism (AEM).

[0088] Based on the experimentally observed lower overpotential and smaller Tafel slope, the improved reaction kinetics at the coordination sites can be attributed to the optimized charge transfer efficiency resulting from the interface engineering strategy. To further elucidate the catalytic mechanism, the reaction process mechanism of the system in Example 3 was analyzed and proposed. Figure 29 Based on the Tauc curve and price band XPS ( Figure 30 Before contact, the CB and VB positions of the materials were: n-type CN (CB: -0.36 eV, VB: 2.31 eV) and p-type NiS (CB: -0.56 eV, VB: 1.48 eV), respectively. After contact, the band gap of Example 3 (2.54 eV) was narrower than that of the original Comparative Example 1 (2.67 eV), confirming that the heterojunction formed an intermediate band structure at Fermi level equilibrium. This pn heterojunction construction resulted in an interleaved band arrangement, inducing a built-in electric field at the contact interface. Under the combined action of light irradiation and bias voltage, the conduction band electrons and valence band holes of CN were driven to migrate to the corresponding bands of the adjacent Comparative Example 2. This redistribution of charge (CB: -0.36 eV, VB: 2.01 eV) with strong redox potential will continue to participate in the subsequent water reduction / oxidation reaction. Detailed observations of sulfur leaching behavior revealed that low-energy unsaturated sulfur sites can capture CN conduction band electrons through strong electron interactions via -CS- bonds (charge transfer bridges), thereby modulating the electronic state of sulfur atoms. In this case, the sulfur sites on both sides of the metal comparative example 2 continuously consume accumulated electrons, enhancing the affinity for H* coupling to form H2, thus promoting the water reduction half-reaction with a lower energy barrier. Consistent with in-situ XAS and Raman results, the NiS valence band in Example 3 can be optimized for Ni. 2+ The electronic configuration enhances Ni 3+ -OOH orbital hybridization synergistically lowers the *OOH formation energy barrier, thereby improving oxygen evolution performance. In addition to the charge transfer bridge formed by NCS-Ni coordination, the strengthened N-Ni bonds can create a higher density of Ni on the CN polymer backbone. 2+Active sites are identified to achieve highly efficient full-reaction activity. Therefore, in the bifunctional photoelectrochemical water splitting process, the heterojunction designed in Example 3 can increase the exposure of active sites, accelerate charge / mass transfer kinetics, and optimize the binding energy of reaction intermediates through energy level synergy, ultimately achieving excellent HER / OER activity and higher IPCE efficiency at 420 nm. Figure 31 ).

[0089] The above description of the embodiments is provided to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A g-C3N4 / NiS composite photocatalyst, characterized in that, The catalyst has a flower-like hierarchical porous structure, consisting of g-C3N4 nanospheres and NiS nanoparticles supported on its surface in situ. The g-C3N4 nanospheres are assembled by stacking nanosheet layered structures. The NiS nanoparticles and the g-C3N4 nanospheres form an in-situ covalently bonded heterostructure interface through Ni-S and Ni-N bonds, thus constructing a pn heterostructure.

2. The g-C3N4 / NiS composite photocatalyst according to claim 1, characterized in that, The g-C3N4 / NiS composite photocatalyst has a diameter of 0.5~3 μm and a specific surface area of ​​2~5 m². 2 / g, pore volume 0.018~0.042 cm³ 3 / g, with an average pore size of 3.0~17.1 nm, the NiS nanoparticles have a particle size of 20~50 nm, and the g-C3N4 nanospheres have a particle size of 0.5~3 μm.

3. The g-C3N4 / NiS composite photocatalyst according to claim 1, characterized in that, The product was prepared by simultaneous polycondensation-sulfidation of g-C3N4 precursor and NiS precursor using a low-temperature one-pot hydrothermal method, achieving in-situ covalent bonding between NiS and g-C3N4.

4. A method for preparing a g-C3N4 / NiS composite photocatalyst as described in any one of claims 1-3, characterized in that, Includes the following steps: S1: Melamine, glucose, hexadecyltrimethylammonium chloride, nickel acetate, and thiourea are added sequentially to deionized water and magnetically stirred at room temperature to form a homogeneous precursor solution; S2: Transfer the precursor solution from step S1 to the reactor and carry out a hydrothermal reaction. After the reaction is complete, allow it to cool naturally. S3: The product obtained after the reaction in step S2 is centrifuged, washed, and vacuum dried to obtain the g-C3N4 / NiS composite photocatalyst.

5. The preparation method of the g-C3N4 / NiS composite photocatalyst according to claim 4, characterized in that, In step S1, the precursor solution contains a nickel acetate to thiourea molar ratio of 1:2-4, a melamine to glucose mass ratio of 0.25:0.30-0.40, and a melamine to nickel acetate mass ratio of 0.25:1.7-80.7; the nickel acetate concentration is 0.25-10 mol / L, and the hexadecyltrimethylammonium chloride concentration is 3-3.5 g / L; the magnetic stirring speed is 400-600 rpm, and the stirring time is 20-80 h.

6. The preparation method of the g-C3N4 / NiS composite photocatalyst according to claim 4, characterized in that, In step S2, the parameters of the hydrothermal reaction include: heating to 140-180 ℃ at a heating rate of 4.5-5.5 ℃ / min and holding at that temperature for 8-32 h.

7. The preparation method of the g-C3N4 / NiS composite photocatalyst according to claim 4, characterized in that, In step S3, the centrifugal washing refers to washing with deionized water 2-4 times and washing with ethanol 2-4 times in sequence. The vacuum drying has a vacuum degree ≤-0.09 MPa, a temperature of 60-80 ℃, and a time of 10-14 h.

8. The application of the g-C3N4 / NiS composite photocatalyst as described in any one of claims 1-3, characterized in that, The g-C3N4 / NiS composite photocatalyst possesses dual photocatalytic activity of HER and OER, and can be used for photocatalytic total water splitting.

9. The application according to claim 8, characterized in that, A dual-electrode system is used with a xenon lamp as the light source to perform photoelectrochemical catalytic water splitting on an electrolyte at 23~27℃. The electrolyte includes any one of KOH alkaline solution, simulated seawater, natural seawater, and freshwater. In this dual-electrode system, both the positive and negative electrodes use carbon paper as a conductive substrate, and the g-C3N4 / NiS composite photoelectrocatalyst is loaded onto the carbon paper at a loading of 4.5~5.5 mg cm⁻¹. -2 The carbon paper has a thickness of 0.15~0.20 mm and a resistivity ≤5 mΩ cm. 2 .

10. The application according to claim 8, characterized in that, In the g-C3N4 / NiS composite photocatalyst, NiS can be reconstructed in situ to form the γ-NiOOH active phase during the photocatalytic process.

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