English

• In recent years, due to global energy problems and environmental degradations caused by excessive carbon dioxide emissions from the large-scale use of fossil energy, the development and utilization of green energy has aroused great attentions [1-4]. Photocatalytic hydrogen production was considered to be one of the most promising ways to produce renewable green energies [5-8]. Therefore, it is crucial to design and prepare high efficiency photocatalysts. Many fancy semiconductor materials, such as metal sulfides, metal oxides, graphite carbon nitride (g-C₃N₄), metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) have been developed as effective photocatalysts to promote the process of photocatalytic hydrogen production [9,10]. However, there are still a few challenges hinder the effective photocatalysis, such as insufficient utilization of active sites, low utilization of photoelectrons, high cost and so on [11,12]. Therefore, it is necessary to develop new photocatalysts to solve these problems.

  COFs is a new class of crystalline materials, which were constructed by covalent bonding of organic monomers. Their open and infinite structures with various topologies endow COFs with permanent porosity, long-range order, and rigid backbones, and render them potential value in wide applications including drug delivery, energy storage and conversion devices, gas storage and separations, and heterogeneous catalysis.

  With large surface area and excellent visible light activity, COFs show great application prospects in the field of photocatalysis for hydrogen production [13-16]. As a kind of COFs, covalent triazine frameworks (CTFs) was a two-dimensional ordered crystalline organic semiconductor material composed of monomers connected with triazine rings. The layered structure with large conjugated systems and strong interlayer interactions makes the material owing excellent visible light response ability. In 2008, Thomas et al. synthesized CTF-1 for the first time [17]. However, due to the high reaction temperature, the CTF-1 skeleton has been partially carbonized to form a deep black powder CTF-1, whose photocatalytic hydrogen evolution reaction (HER) activity is very low. To avoid skeleton carbonization, Wang et al. used NaCl-KCl-ZnCl₂ ternary eutectic salt (ES) as catalyst to synthesize CTF-ES₂₀₀ with good crystallinity at a lower temperature [18], which significantly improved its photocatalytic HER activity. However, CTFs used as one component photocatalyst also has the problem of fast recombination of electrons and holes (e-h) pairs and weak photoelectron mobilities. One of effective strategies to solve this problem is to construct heterogeneous structures. The N content of CTFs is very high due to the triazine linking group in the structure. This high N content contributes to the generation of heteroatomic effects, resulting in an abundance of catalytic reaction centers and functional centers [19,20]. Because of the presence of C═N in the triazine structure, CTF has high thermal and chemical stability. These features make CTFs excellent carriers for the construction of heterojunction photocatalysts [21]. In 2020, Bi et al. reported that loading CdS quantum dots onto CTF to form heterojunctions significantly improves its HER activity [22]. Subsequently, in 2022, Zou et al. modified CTF with CoP to form a heterojunction structure, significantly improving the charge separation efficiency of the material, and its photocatalytic HER ability was even better than that of precious metal based photocatalysts [23]. Therefore, construction of heterojunction is a very effective strategy to improve the photoelectron utilization of photocatalysts.

  In heterojunction engineering, the S-scheme heterojunction in the type Ⅱ heterojunction has a very high photoelectron utilization, which is mainly due to the interface electric field (IEF) between the two semiconductor materials. The presence of IEF realizes effective carrier charge separation and significantly improves the catalytic efficiency of the photocatalysts. The powerful role of IEF in facilitating carrier charge separation in type Ⅱ heterojunctions has been demonstrated in plenty literatures [24,25]. In contrast, the role of IEF in type Ⅰ heterojunctions has usually been ignored, and the e-h pairs separation efficiency of type Ⅰ p-n heterojunctions has often been seriously underestimated. Therefore, it is a significant topic to construct effective IEF in p-n heterojunctions to enhance the photogenerated carrier separation ability in type Ⅰ heterojunctions.

  NiS is a typical p-type semiconductor, usually having a smaller band gap structure and higher conductivities [26,27]. These characteristics make NiS a broad application prospect in the field of photocatalysis. However, the size of NiS synthesized by routine methods is relatively small, difficult to recover after testing, and easy to agglomerate, becoming a formidable obstacle to large scale application of NiS in the field of photocatalytic HER. Loading these tiny NiS particles onto other semiconductors materials to form heterostructures can solve the agglomeration problem and meanwhile the uniform loading can effectively maintain the high exposure of active sites [28]. For example, in 2021, Zhang and Chen et al. loaded NiS nanoparticles onto CdS to form heterostructures and got an increased photocatalytic hydrogen production rate of 18.1 mmol g^-1 h^-1 [29]. Subsequently, Yang et al. loaded NiS on polymerized carbon-oxygen semiconductors for photocatalytic total hydrolysis, and its performance was even better than that of precious metal-based photocatalysts [30]. As a two-dimensional layered COFs, CTF-ES₂₀₀ is a stable n-type semiconductor photocatalytic material with good visible light response. Therefore, we propose to construct a p-n heterostructure by loading tiny NiS nanoparticles onto CTF-ES₂₀₀ to achieve efficient photocatalytic HER.

  Herein, the p-n heterojunction photocatalyst (NiS/CTF-ES₂₀₀) has been successfully prepared through loading tiny NiS nanoparticles onto CTF-ES₂₀₀ as substrate in a green, simple and rapid photodeposition method. The optimal HER rate of 9NiS/CTF-ES₂₀₀ is 22.98 mmol g^-1 h^-1, and the HER rate can maintain 85% of the initial rate after four cycles, indicating that this photocatalyst has good stabilities. Experimental results shows that an IEF has been successful constructed in this type Ⅰ p-n heterojunction. Under the electric field force of IEF, electrons in the conduction band of CTF-ES₂₀₀ have been accumulated, making itself a high reducibility in HER, and effectively improving the separation efficiency of the e-h pairs. Furthermore, XPS and EXAFS data demonstrate that an electron transport channel has been fabricated through the formation of Ni-N bond, further accelerating the transfer efficiency of the interface carriers. The successful construction of this kind of type Ⅰ p-n heterojunction provides a new idea for the design of high efficiency photocatalysts.

  First, CTF-ES₂₀₀ with good crystallinity and high N content was synthesized by ternary eutectic salt system. Subsequently, we used a green, simple and rapid photodeposition method to load tiny NiS nanoparticles onto CTF-ES₂₀₀ to form NiS/CTF-ES₂₀₀ composites to construct p-n heterojunction for photocatalytic HER. The synthesis process of the material is shown in Fig. 1a. To verify the integrity of the synthesized material, the photocatalysts were tested by Fourier transform infrared spectroscopy (FT-IR) and powder X-ray diffraction (PXRD), respectively. As shown in Figs. S2a and b (Supporting information), the sample has two peaks at 1350 cm^-1 and 1510 cm^-1, which are C—N breathing vibration and tensile vibration of the triazine ring, respectively. The peak at 2230 cm^-1 may be related to oligomers left behind during the synthesis of CTFs [31,32]. CTF-ES₂₀₀ has a strong PXRD peak at 7.8°, which is related to its {101¯0} crystal face [18]. The FT-IR and PXRD data of CTF-ES₂₀₀ are consistent with the literature, indicating that CTF-ES₂₀₀ has been successfully synthesized. It can be seen from the figure that after loading NiS by light deposition method, the diffraction peak of the sample does not change significantly, indicating that the structure of CTF-ES₂₀₀ is well preserved after modification. In addition, no significant NiS diffraction peak is observed in the figure, indicating the low loading content of NiS.

  Figure 1

  

  Figure 1.  (a) Synthesis diagram of NiS/CTF-ES₂₀₀ photocatalyst. TAA is a thioacetamide. (b, c) TEM images of NiS/CTF-ES₂₀₀. (d, e) HRTEM images of NiS/CTF-ES₂₀₀.

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  The morphology, microstructure and element distribution of the prepared photocatalysts were observed by scanning electron microscope (SEM), transmission electron microscope (TEM) and energy dispersive spectrometer (EDS). As shown in Fig. S3a (Supporting information), CTF-ES₂₀₀ presented an irregular blocky shape, which was caused by the stacking of layered structures. The NiS/CTF-ES₂₀₀ samples showed a morphology similar to that of CTF-ES₂₀₀ (Fig. S3b in Supporting information), indicating that our method did not alter the structure of CTF-ES₂₀₀. However, no NiS was observed in the NiS/CTF-ES₂₀₀ samples, which might be due to their small particle size [26,33]. TEM images (Figs. 1b and c) of NiS/CTF-ES₂₀₀ showed that CTF-ES₂₀₀ presents an obvious layered structure, and the surface of CTF-ES₂₀₀ is loaded with NiS nanoparticles with a size of about 5–10 nm. It can be seen from Figs. 1d and e that the lattice spacing of the nanoparticles is 0.20 nm and 0.26 nm, corresponding to the (102) and (101) crystal faces of hexagonal NiS (α-NiS), respectively [34,35]. In addition, the EDS spectrum (Figs. S2c and d in Supporting information) displayed that the four elements C, N, Ni and S were evenly distributed, which also indicated that the NiS nanoparticles were uniformly loaded onto CTF-ES₂₀₀, further indicating the successful synthesis of the NiS/CTF-ES₂₀₀ composite material [26]. Furthermore, the inductively coupled plasma emission spectrometer (ICP-OES) test results show that the actual concentration of NiS in 9NiS/CTF-ES₂₀₀ is about 3.78 wt%.

  The surface chemical states of the NiS/CTF-ES₂₀₀ photocatalysts were analyzed by X-ray photoelectron spectroscopy. It was observed that the CTF-ES₂₀₀ samples consisted mainly of the elements C and N, and the 9NiS/CTF-ES₂₀₀ consisted mainly of the elements Ni, S, C and N (Fig. S4 in Supporting information). High-resolution XPS spectra of C 1s, N 1s, Ni 2p, and S 2p further provided the states of the elements in 9NiS/CTF-ES₂₀₀. (Figs. 2a and b) The C 1s XPS spectra of each sample were decomposed into two peaks. The strong peak at 284.8 eV was attributed to the C in the benzene ring, while the other peak at 286.72 eV was attributed to the C in the triazine ring, and it could be seen that there is no shift of the binding energy peaks of C after photodeposition of NiS [36]. There was a strong binding energy peak at 399.33 eV for N 1s in the 9NiS/CTF-ES₂₀₀ sample, which undergoes a positive shift of 0.19 eV compared to the binding energy in the pristine CTF-ES₂₀₀. This positive shift in binding energy indicates that there is a strong electron interaction between NiS and CTF-ES₂₀₀, the surface electron density changes and the electrons on CTF-ES₂₀₀ were transferred to NiS. The Ni 2p spectrum had two main peaks at 856.13 and 873.80 eV (Fig. 2c), which are in agreement with Ni 2p_3/2 and Ni 2p_1/2 for Ni²⁺. The other two peaks at 861.88 eV and 880.00 eV belong to the satellite peaks of Ni 2p_3/2 and Ni 2p_1/2 of Ni²⁺ [26,37]. The above results indicate that the nickel element in 9NiS/CTF-ES₂₀₀ exists in the form of Ni²⁺. In addition, two weaker NiS/CTF-ES₂₀₀ peaks, Ni⁰ 2p_3/2 and Ni⁰ 2p_1/2, were observed at 853.48 eV and 870.69 eV, respectively, which might be due to the partial reduction of Ni²⁺ with the presence of metallic Ni. In addition, the S 2p spectra showed two peaks at 168.84 eV and 163.28 eV (Fig. 2d), which corresponded to S 2p_3/2 and S 2p_1/2 of S^2- [38,39]. The above XPS analysis shows that the electrons on CTF-ES₂₀₀ were transferred to NiS after NiS guest material loading.

  Figure 2

  

  Figure 2.  (a) C 1s XPS spectrum. (b) N 1s XPS spectrum. (c) Ni 2p XPS spectrum. (d) S 2p XPS spectrum. (e) Ni K-edge XANES. (f) Fourier-transformed EXAFS spectra for the Ni K-edge of the sample.

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  In order to further explore the chemical valence and coordination environment of Ni elements in NiS/CTF-ES₂₀₀ catalyst, X-ray absorption near edge structure (XANES) spectra and extended X-ray absorption fine structure (EXAFS) spectra of NiS/CTF-ES₂₀₀ catalyst were collected and compared with standard nickel foil and NiO. As shown in Fig. 2e, the edge shape of the XANES curve of NiS/CTF-ES₂₀₀ is similar to that of Ni foil and NiO reference material, indicating that Ni oxidation states existing in NiS/CTF-ES₂₀₀ is mainly in the form of bivalent Ni, which is consistent with the above XPS results [40]. It could be seen from the Fig. 2f that in the EXAFS spectrum of NiS/CTF-ES₂₀₀ sample, there was only one large peak at 1.60 Å, matching the first coordination shell of Ni-O, Ni-C or Ni-N. According to the wavelet transform map (Fig. S5 in Supporting information), the maximum strength of NiS/CTF-ES₂₀₀ is 4.7 Å^-1, which is mainly the contribution of the Ni-S bond in NiS, while the maximum strength of NiO is about 7.1 Å^-1, which rules out the possibility of Ni-O bond [41]. Combining with the above results of XPS which showed that, the binding energy of C element did not change before and after loading NiS, while the binding energy of N element changes, we can draw a conclusion that the 1.60 Å peak corresponds to the first coordination shell of Ni-N [42,43]. The formation of the chemical bond between nickel and nitrogen effectively helps to construct an efficient interfacial electron transport channel between NiS and CTF-ES₂₀₀, which will effectively increasing the charge transfer in photocatalytic HER [44]. In contrast to Ni foil, the 2.16 Å peak corresponding to the Ni-Ni bond in the spectrum is absent in the NiS/CTF-ES₂₀₀ spectrum [42].

  Hydrogen production experiments were performed under visible light (λ > 420 nm) (Fig. 3a). The hydrogen production rate of 9NiS/CTF-ES₂₀₀ can reach up to 22.98 mmol g^-1 h^-1 (AQE = 3.46%, λ = 475 nm), which is higher than all NiS contained non-metallic semiconductor composites. After four hydrogen production tests, the hydrogen production efficiency of NiS/CTF-ES₂₀₀ catalyst can still reach more than 85% of the first cycle (Fig. 3b). Also there is no significant change in the PXRD pattern of the sample after soaking in 10% TEOA solution and after 1 cycle hydrogen evolution test. Above two results both shows that this composite material have a very good stability (Figs. S6a and b in Supporting information).

  Figure 3

  

  Figure 3.  (a) Photocatalytic H₂ evolution rates of the samples. (b) Photocatalytic H₂ evolution of 9NiS/CTF-ES₂₀₀ in four consecutive cycles. (c) Steady-state PL spectra with EY. (d) The Mott-Schottky test for 9NiS/CTF-ES₂₀₀. (e) Transient photocurrent responses (under the Xenon lamp, λ > 420 nm) and (f) EIS curves of the electrodes.

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  The photoluminescence (PL) spectra of the photocatalyst were measured at the excitation wavelength of 380 nm. The results were shown in Fig. 3c. All samples have a distinct luminescence peak at 535 nm. The lower the peak value, the higher the separation efficiency of e-h pairs and the better the photocatalytic activity of the catalyst. After photodeposition of NiS, the intensity of the PL peak of the photocatalyst decreased, indicating that the strong interaction between CTF-ES₂₀₀ and NiS can effectively separate the photogenerated e-h pairs. Among all the composite catalysts, 9NiS/CTF-ES₂₀₀ showed the lowest PL peak, which was consistent with the best hydrogen production performance of 9NiS/CTF-ES₂₀₀. It is further shown that the construction of NiS/CTF heterojunction could reduce the recombination rate of e-h pairs, which can greatly improve the H₂ generation activity.

  The band structure of NiS/CTF-ES₂₀₀ photocatalyst was studied by UV–vis diffuse reflectance spectroscopy (DRS). The visible light absorption of composites was significantly increased after NiS loading. In addition, the band gap can be deduced from the UV–vis DRS spectra of the semiconductors by the Kubelka-Munk method (Figs. S7a and b in Supporting information) [45,46]. The calculated bandgaps of CTF-ES₂₀₀ and 9NiS/CTF-ES₂₀₀ are 3.18 eV and 2.56 eV, respectively. The band gap of the composite decreases with the increase of the NiS load, and the band gap reaches the smallest when the NiS load is 9 wt%. As the NiS loading amounts increases further, the bandgap value increases slightly, possibly due to agglomeration of NiS particles.

  We also performed a set of Mott-Schottky (M-S) tests. The flat-band potential (V_FB) of CTF-ES₂₀₀ and NiS were calculated. The values are −0.89 V vs. Ag/AgCl (= −0.69 V vs. NHE), 0.80 V vs. Ag/AgCl (=1.00 V vs. NHE) (Figs. S8a and S9 in Supporting information), which indicates that CTF-ES₂₀₀ is an n-type semiconductor and NiS is a p-type semiconductor. Because the conduction band potential (V_CB) of n-type semiconductor is 0.1–0.2 V lower than that of plain band, and the valence band potential (V_VB) of p-type semiconductor is 0.1–0.2 V higher than that of plain band, So the V_CB and NiS V_VB of CTF-ES₂₀₀ are −0.89 V and 1.20 V (relative to NHE), respectively [47-49]. After loading NiS, the V_FB of 9NiS/CTF-ES₂₀₀ is approximately reduced to −1.02 V relative to Ag/AgCl (relative to NHE = −0.82 V) (Fig. 3d). In addition, with the increase of NiS loading, the conduction band value of the composite gradually decreases (Figs. S8b and c in Supporting information). The more negative the V_CB value of the composite, the stronger the reduction ability to promote the hydrogen precipitation reaction. When the load reaches 12 wt%, the conduction band value increases and the reduction capacity decreases, which is consistent with the results of the above hydrogen production test. The above test results show that CTF-ES₂₀₀ has a positive slope in Mott-Schottky diagram, while NiS has a negative slope. A p-n heterojunction has been successfully constructed by these two materials, which has a suitable band structure to realize efficient photocatalytic HER.

  We monitor the change in photocurrent density over time (Fig. 3e). It could be observed that the 9NiS/CTF-ES₂₀₀ material exhibited the highest photocurrent density. This greater response could be ascribed to the close interaction between NiS and CTF-ES₂₀₀, which facilitates effective charge transfer and reduces e-h pairs recombination. Electrochemical impedance spectroscopy (EIS) can display charge transfer resistance (Fig. 3f). The Nyquist plot of 9NiS/CTF-ES₂₀₀ shows the smallest semicircle, indicating the fastest charge transfer capability and the lowest resistance. In comparison to pure CTF-ES₂₀₀, NiS deposited CTF-ES₂₀₀ exhibits smaller semicircles, indicating that loading a reasonable amount of NiS onto CTF-ES₂₀₀ can effectively reduce the impedance of the material.

  In addition, the photothermal effect of semiconductor materials with relatively narrow band gaps in heterojunctions is also beneficial to photocatalytic reactions [44]. Infrared thermal images of NiS, CTF-ES₂₀₀ and NiS/CTF-ES₂₀₀ under visible light were collected (Figs. 4a-c). Under simulated visible light irradiation, the temperature of NiS rises and stabilizes to 103 ℃ within 80 s, the temperature of CTF-ES₂₀₀ rises slowly to 69.8 ℃ within 160 s, and the temperature of NiS/CTF-ES₂₀₀ can rise to a higher temperature 86.9 ℃ within 160 s. This indicates that NiS has a strong photothermal effect. The increase of catalyst temperature accelerates photoexcited charge migration and surface reaction kinetics, thus promoting the HER [50,51]. These results indicate that the photocatalytic HER activity of CTF-ES₂₀₀ can be greatly enhanced by loading NiS.

  Figure 4

  

  Figure 4.  (a-c) Timeline photothermal mapping images of samples. (d-f) Band structure and electron transport mechanism of samples.

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  The mechanism of this photocatalyst can be summarized as follows: NiS is a p-type semiconductor and CTF-ES₂₀₀ is an n-type semiconductor, there is an obvious difference between the two Fermi levels. After the formation of NiS/CTF-ES₂₀₀ composites, the electrons in the CTF-ES₂₀₀ are spontaneously transferred to NiS driven by this Fermi energy difference. This is consistent with the XPS analysis results. Finally, their Fermi levels beginning to converge (Fig. 4e). The energy bands of CTF-ES₂₀₀ and NiS are bent due to electron accumulation and electron loss, respectively. At the same time, IEF is constructed at the interface of composite materials. Under visible light, the photocatalyst NiS/CTF-ES₂₀₀ is excited to produce e-h pairs. Based on the synergistic effect of band bending, IEF and coulomb repulsion, some of the photogenerated holes in CTF V_VB recombine with electrons in NiS V_CB and some transfer to NiS V_VB under the effect of IEF, resulting in the accumulation of photoelectrons on the V_CB of CTF-ES₂₀₀ [44]. The photocatalytic mechanism is shown in Figs. 4d-f. Therefore, the electrons retained in CTF-ES₂₀₀ have a high reducing power to catalyze water decomposition to produce hydrogen [25,29]. More holes were accumulated in the V_VB of NiS, which significantly improved the separation efficiency of e-h pairs. This electron transfer principle is similar to S-scheme heterojunction. At the same time, the ground state EY is excited by light and transforms into the excited state EY^1*. Through the intersystem transition, EY^1* transforms into a more stable triplet excited state EY^3* [52]. The sacrificial agent TEOA occupies the holes on NiS, and TEOA is oxidized to TEOA⁺, which helps EY^3* to convert to EY^- [53,54]. The electrons produced by EY^- are transferred to the V_CB of CTF-ES₂₀₀, which further accumulates electrons on the V_CB of CTF-ES₂₀₀.

  In conclusion, the 9NiS/CTF-ES₂₀₀ photocatalyst has the highest photocatalytic HER performance compared to other NiS loaded non-metallic semiconductor materials. This outstanding performance can be attributed to the following points: (1) IEF in this type Ⅰ p-n heterojunction prevents the electrons flowing from CTF to NiS, causing electron accumulation on the V_CB of CTF-ES₂₀₀; meanwhile it accelerates holes transfer to the V_VB of NiS, resulting in an effective separation of e-h pairs; (2) The formation of Ni-N bond at the interface builds an efficient electron transport channel, accelerating the transfer of interface carriers, and further improving the utilization rate of photoelectrons.

  In summary, we synthesized a high efficiency photocatalyst through depositing p-type semiconductor NiS nanoparticles onto n-type semiconductor CTF-ES₂₀₀ by a simple green photodeposition method, forming a type Ⅰ p-n heterojunction for photocatalytic HER. 9NiS/CTF-ES₂₀₀ has the best photocatalytic HER performance, and the hydrogen production rate is 22.98 mmol g^-1 h^-1, which is significantly higher than that of pure CTF-ES₂₀₀. Moreover, it still has high HER activities after four cycles. The IEF in p-n heterojunction leads to the accumulation of photoelectrons on the V_CB of CTF-ES₂₀₀, making CTF-ES₂₀₀ maintain a high reductiveness in HER, and significantly improves the utilization of photoelectrons. Furthermore, the XPS and EXAFS data show that an efficient electron transport channel is constructed through the formation of Ni-N bond, which further accelerates the interface carrier transport efficiency. This study provides a theoretical basis for the mechanism analysis of IEF in type Ⅰ heterojunction and also pave a new avenue for heterojunction design in high activity photocatalysts synthesis.

  Declaration of competing interest

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

  CRediT authorship contribution statement

  Tao Zhou: Writing – original draft, Software, Formal analysis, Data curation, Conceptualization. Xu Han: Formal analysis, Investigation. Wangwang Shen: Formal analysis. Fang Ji: Software. Menglong Liu: Software. Yingyu Song: Software. Wen-Wen He: Project administration, Resources, Supervision, Writing – review & editing.

  Acknowledgments

  This work was financially supported by the National Natural Science Foundation of China (No. 22271022), and the Science and Technology Development Planning of Jilin Province (No. YDZJ202201ZYTS342). This work was also supported by the China Scholarship Council (CSC, No. 201802335014). The authors acknowledge Specreation Instruments Co., Ltd. for the provision of instrument (Table XAFS-500A).

  Supplementary materials

  Supplementary material associated with this article can be found, in the online version, at doi:10.1016/j.cclet.2024.110415.

  
  
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  Figure 1  (a) Synthesis diagram of NiS/CTF-ES₂₀₀ photocatalyst. TAA is a thioacetamide. (b, c) TEM images of NiS/CTF-ES₂₀₀. (d, e) HRTEM images of NiS/CTF-ES₂₀₀.

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  Figure 2  (a) C 1s XPS spectrum. (b) N 1s XPS spectrum. (c) Ni 2p XPS spectrum. (d) S 2p XPS spectrum. (e) Ni K-edge XANES. (f) Fourier-transformed EXAFS spectra for the Ni K-edge of the sample.

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  Figure 3  (a) Photocatalytic H₂ evolution rates of the samples. (b) Photocatalytic H₂ evolution of 9NiS/CTF-ES₂₀₀ in four consecutive cycles. (c) Steady-state PL spectra with EY. (d) The Mott-Schottky test for 9NiS/CTF-ES₂₀₀. (e) Transient photocurrent responses (under the Xenon lamp, λ > 420 nm) and (f) EIS curves of the electrodes.

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  Figure 4  (a-c) Timeline photothermal mapping images of samples. (d-f) Band structure and electron transport mechanism of samples.

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