In Situ Polycondensation Synthesis of NiS-g-C3N4 Nanocomposites for Catalytic Hydrogen Generation from NaBH4

The nanocomposites of S@g-C3N4 and NiS-g-C3N4 were synthesized for catalytic hydrogen production from the methanolysis of sodium borohydride (NaBH4). Several experimental methods were applied to characterize these nanocomposites such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and environmental scanning electron microscopy (ESEM). The calculation of NiS crystallites revealed an average size of 8.0 nm. The ESEM and TEM images of S@g-C3N4 showed a 2D sheet structure and NiS-g-C3N4 nanocomposites showed the sheet materials that were broken up during the growth process, revealing more edge sites. The surface areas were 40, 50, 62, and 90 m2/g for S@g-C3N4, 0.5 wt.% NiS, 1.0 wt.% NiS, and 1.5 wt.% NiS, respectively. The pore volume of S@g-C3N4 was 0.18 cm3, which was reduced to 0.11 cm3 in 1.5 wt.% NiS owing to the incorporation of NiS particles into the nanosheet. We found that the in situ polycondensation preparation of S@g-C3N4 and NiS-g-C3N4 nanocomposites increased the porosity of the composites. The average values of the optical energy gap for S@g-C3N4 were 2.60 eV and decreased to 2.50, 2.40, and 2.30 eV as the NiS concentration increased from 0.5 to 1.5 wt.%. All NiS-g-C3N4 nanocomposite catalysts had an emission band that was visible in the 410–540 nm range and the intensity of this peak decreased as the NiS concentration increased from 0.5 to 1.5 wt.%. The hydrogen generation rates increased with increasing content of NiS nanosheet. Moreover, the sample 1.5 wt.% NiS showed the highest production rate of 8654 mL/g·min due to the homogeneous surface organization.


Introduction
Research on graphitic carbon nitride (g-C 3 N 4 ) has received a lot of attention due to its structure and substantial chemical and physical characteristics, such as superior electrical conductivity and high mechanical strength [1][2][3]. Moreover, the use of g-C 3 N 4 in photocatalysis, electrocatalysis, photovoltaic devices, and bioimaging applications has a lot of potential [4][5][6]. It is composed of several abundant elements and is the most stable allotrope of carbon nitrides under ambient conditions. Graphite and g-C 3 N 4 have a similar structure, however one by one, nitrogen and carbon atoms make up the hexatomic ring. A significant planer network structure is developed by the covalent connections that are formed by each carbon atom with three nitrogen atoms. The sp 2 hybridized C and N atoms are organized in a six-member stalked ring; providing the semiconductor properties of g-C 3 N 4 . In the optical range of 260-320 nm, g-C 3 N 4 has a significant UV-Vis absorption peak. The π-π* electron transfer for g-C 3 N 4 with s-triazine rings is responsible for the absorption peak, which is located at about 250 nm. Moreover, the n-π* electron transfer involving a lone pair of electrons on nitrogen atoms in the g-C 3 N 4 produced the absorption peak at 320 nm [7,8]. The electrical features and surface chemical properties of g-C 3 N 4 doped with heteroatoms (such as oxygen, sulfur, phosphorous, and boron) can be tuned, which is advantageous for expanding their applications in bioimaging and biosensing. For prove conductivity and surface area by elemental doping, converting into nanosheets, and combining with metal nanoparticles and other carbon nanomaterials. This in turn will suppress the electron-hole recombination. The in situ polycondensation process can produce metal/g-C 3 N 4 nanocomposites. Meanwhile, this synthesis procedure enables the direct formation of nanocomposites as well as microstructure control. Moreover, this technique has excellent aggregate elimination/reduction along with ideal and reproducible properties.
The current study aims to find an appropriate precursor, morphology, exfoliation condition, and fabrication processes for g-C 3 N 4 in order to improve the catalytic activity. Furthermore, NiS nanostructures can be integrated in situ into the g-C 3 N 4 nanosheet to increase the electroactive surface area and electrical conductivity. The in situ polycondensation process using different ratios of nickel chloride and thiourea at 550 • C for 120 min synthesized the NiS-g-C 3 N 4 nanocomposite samples. The structural measurements for these nanocomposites was conducted using XRD, FTIR, and ESEM techniques. Moreover, an extensive study of the methanol hydrolysis of sodium borohydride was completed. Finally, the hydrogen catalytic efficiency of prepared materials was examined at methanolysis of NaBH 4 . The hydrogen generation rates increased with increasing NiS nanosheet content. Moreover, the sample 1.5 wt.% NiS showed the highest production rate of 8654 mL/g·min.

Experimental
Loba Chemi, Mumbai, India supplied the chemicals (nickel chloride hexahydrate, thiourea, and sodium borohydride). The absolute methanol was provided by Sigma-Aldrich, Darmstadt, Germany. All provided chemicals were directly used without extra purification.
The bulk S@g-C 3 N 4 was synthesized by heating 10.0 g of thiourea powder in a porcelain crucible covered with a lid in the muffle furnace at 550 • C (ramping rate = 3.0 • /min) and maintaining the temperature for 2 h. The yellow solid mass in a crucible was then allowed to cool to room temperature. Using an agate mortar, the resultant bulk S@g-C 3 N 4 was crushed into a fine powder. The in situ polycondensation process using different ratios of nickel chloride and thiourea at 550 • C for 120 min synthesized NiS-g-C 3 N 4 nanocomposite samples. In a typical synthesis, 10 g of thiourea powder and 0.5, 1.0 and 1.5 wt.% of nickel chloride were ground in an agate mortar for 30 min. After that, the powder was transferred to porcelain crucibles covered with a lid inserted inside a muffle furnace. The furnace operated at 550 • C for 120 min at a heating rate of 3.0 • C/min. The obtained nanocomposite samples were allowed to cool and then ground.
X-ray diffraction studies can analyze structural factors such as crystallinity, grain size, strain, phase composition, and structural defects. The XRD spectra were recorded from a Shimadzu XRD 700 instrument utilizing a Cu kα wavelength of 1.54056 Å. Cu kα radiation was created by using a Cu source as an X-ray source. The data scans were collected at the 2Theta range 5.0-80 • with a count rate of 0.2 • /min. The crystal structure was identified by comparing the diffraction pattern of the synthesized nanocomposite with the JCPDS files in the database. FTIR data were collected using a Shimadzu FTIR spectrometer-100 Tracer. The frequency ranges that can be examined are generally in the 4,000,399 cm −1 range. The sensitive characterization instrument ESEM exposes surface morphology and when coupled with an EDX (energy dispersive X-ray analysis) accessory, determines the elemental composition of materials. High-resolution 3D micrographs of the morphology were provided utilizing the ESEM technique. ESEM images and EDX data were acquired using a Thermo Fisher Quattro environmental scanning electron microscope (ESEM). Transmission electron microscopy (TEM) has become an essential tool in medical, biological, and materials' research because of its high magnifications. An investigation of TEM microscopy was carried out using a Thermo Fisher Scientific Talos F200i TEM/STEM electron microscope. The most generally used technique for calculating the specific surface area of produced material is the Brunauer-Emmett-Teller (BET) method. The BET technique involves the multi-layer adsorption of chemically inert N 2 gas with relative pressure and gas volume adsorbed in cm 3 /gm. The samples were degassed at 100 • C overnight to remove trapped moisture molecules. The samples were subjected to N 2 gas at 77 K to record the adsorption-desorption isotherm on NOVA 4200e surface area analyzer. As a significant characterization tool in the field of photocatalysis, UV-Vis spectroscopy is a non-destructive method for analyzing optical characteristics such as absorbance, reflectance, transmittance, and bandgap energy that are related to the chemical composition of the material. In order to move electrons from their ground state to their excited state for electronic spectroscopy, it is necessary to absorb photons in the UV-visible region of the spectrum. The UV-Vis spectra were recorded on Agilent Cary 60 Spectrophotometer. The photoluminescence (PL) approach has been widely applied in the field of photocatalysis to study surface processes. Photoluminescence (PL) spectra at room temperature were obtained using a Cary Eclipse fluorescence spectrometer with a 350 nm excitation wavelength.
The hydrogen catalytic efficiency of prepared materials was evaluated as follows. Typically, 20 mg of the nanocomposite sample was mixed with 0.25 g of NaBH 4 and 10 mL of methanol was added without stirring. The hydrogen gas volume was recorded via the water displacement method. The measurements were completed at a temperature of 30 • C.

Results and Discussion
XRD is a non-destructive analytical method that provides details of the physical characteristics and crystalline structure of materials. The results of XRD data for the synthesized S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposite catalysts are presented in Figure 1. The catalyst included a variety of compounds which were discovered using XRD analysis. The presence of two major peaks at 13.08 • and 27.20 • were observed in the spectrum of S@g-C 3 N 4 . The small diffraction peak at 13.08 • matched a (100) plane with a distance of 0.676 nm [26,27]. The highest diffraction peak with reflection (002) was a typical interlayer stacking peak for graphitic C 3 N 4 material and located at 2θ = 27.20 • , which occurs because the atomic radius of sulfur is larger than carbon and nitrogen. Further, small diffraction peaks of sulfur were observed in the XRD spectrum of S@g-C 3 N 4 [28]. For NiS-g-C 3 N 4 at 0.5, 1.0, and 1.5 wt.%, the peak (002) moved to 27.55 • , 27.66 • and 27.64 • , respectively. The positions of these reflections shifted to higher angles and thus lowered d-values with increase in NiS content. This indicates a structural contraction along the layer-stacking direction, presumably because of a more extended condensation of the carbon nitride chains [29]. Moreover, this shift occurs because of the reduced size (layer thickness) in carbon nitride sheets [30].
The  . The synthesis of the NiS-g-C 3 N 4 nanocomposites is confirmed by the existence of the two phases. Meanwhile, the crystallite size (D) is inversely proportional to the diffraction peak broadening (β) as proposed by the Scherer equation [31,32]: Accordingly, the calculations of b at the diffraction peak with reflection (002) for S@g-C 3 N 4 decreased as the NiS concentration increased from 0.5 to 1.5 wt.%. This reveals the small crystallite domains of S@g-C 3 N 4 . The calculation of NiS crystallites reveals an average size of 8.0 nm for 0.5 wt.% NiS, 1.0 wt.% NiS, and 1.5 wt.% NiS. Moreover, the shift of peak position for the (002) plane after the growth of NiS at different content (0.0-1.5 wt.%) confirms the successful formation of nanocomposites. Figure 2 shows the FTIR spectra of S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposite samples. The vibrations of C-O stretching and C-OH stretching for the S@g-C 3 N 4 sample located at 1010 and 1132 cm −1 [33]. This result indicates the existence of both hydroxyl (C-OH), carbonyl (C=O), and carboxylic (COOH). The vibrations of N-H stretching or the H 2 O adsorption showed two peaks at 3100-3300 cm −1 [34]. The absorption bands between 1229 and 1628 cm −1 observed in the spectrum of S@g-C 3 N 4 correspond to the typical stretching modes of CN heterocycles [35]. It was determined that the physically adsorptive CO 2 from the environment was responsible for a weak band at 2336 cm −1 [30]. Moreover, another weak peak connected to the C=N bond was seen at 2170 cm −1 [36]. (1) Accordingly, the calculations of b at the diffraction peak with reflection (002) for S@g-C3N4 decreased as the NiS concentration increased from 0.5 to 1.5 wt.%. This reveals the small crystallite domains of S@g-C3N4. The calculation of NiS crystallites reveals an average size of 8.0 nm for 0.5 wt.% NiS, 1.0 wt.% NiS, and 1.5 wt.% NiS. Moreover, the shift of peak position for the (002) plane after the growth of NiS at different content (0.0-1.5 wt.%) confirms the successful formation of nanocomposites. Figure 2 shows the FTIR spectra of S@g-C3N4 and NiS-g-C3N4 nanocomposite samples. The vibrations of C-O stretching and C-OH stretching for the S@g-C3N4 sample located at 1010 and 1132 cm −1 [33]. This result indicates the existence of both hydroxyl (C-OH), carbonyl (C=O), and carboxylic (COOH). The vibrations of N-H stretching or the H2O adsorption showed two peaks at 3100-3300 cm −1 [34]. The absorption bands between 1229 and 1628 cm −1 observed in the spectrum of S@g-C3N4 correspond to the typical stretching modes of CN heterocycles [35]. It was determined that the physically adsorptive CO2 from the environment was responsible for a weak band at 2336 cm −1 [30]. Moreover, another weak peak connected to the C=N bond was seen at 2170 cm −1 [36]. The medium intense sharp band at around 804 cm −1 suggests the samples consist of triazine or heptazine building blocks [37]. The bands at 1205 and 1311 cm −1 indicate the presence of C-N (sp 3 ) and C-N(-C)-C bonds [38]. Moreover, the positions of these peaks slightly shift to a higher wavenumber after the growth of NiS at 1.5 wt.%. These outcomes provide evidence of the successful preparation of NiS-g-C3N4 nanocomposites and agree with the XRD investigations. The medium intense sharp band at around 804 cm −1 suggests the samples consist of triazine or heptazine building blocks [37]. The bands at 1205 and 1311 cm −1 indicate the presence of C-N (sp 3 ) and C-N(-C)-C bonds [38]. Moreover, the positions of these peaks slightly shift to a higher wavenumber after the growth of NiS at 1.5 wt.%. These outcomes provide evidence of the successful preparation of NiS-g-C 3 N 4 nanocomposites and agree with the XRD investigations.
The morphology and structure of the synthesized S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposite samples were investigated by ESEM images. In Figure 3, S@g-C 3 N 4 shows that a 2D sheet structure was achieved for g-C 3 N 4 materials. NiS-g-C 3 N 4 nanocomposites showed the sheet materials were broken up during the growth process in Figure 3, revealing more edge sites. This aligns with the XRD result showing that NiS-g-C 3 N 4 is less ordered and crystalline by virtue of the broader spectral peaks.
The EDX spectra displayed in Figure S1 (Supplementary Materials) of the NiS-g-C 3 N 4 nanocomposite samples confirm the presence of all the elements supposed. Table 1 also displays the weight percent of the elements found on the samples' surfaces. Moreover, Figure S1 shows no identifiable peaks for any other elements besides Ni, S, C, and N, demonstrating that the NiS-g-C 3 N 4 nanocomposite samples are of high purity.    Figure 4. The lamellar structure with sheet morphology is seen in the images. The image of S@g-C3N4 reveals stack layers that are connected with XRD and ESEM measurements. Moreover, the sheet materials in 1.5 wt.% NiS nanocomposite sample were broken up throughout the growing process.  TEM images of S@g-C 3 N 4 and 1.5 wt.% NiS nanocomposite samples are shown in Figure 4. The lamellar structure with sheet morphology is seen in the images. The image of S@g-C 3 N 4 reveals stack layers that are connected with XRD and ESEM measurements. Moreover, the sheet materials in 1.5 wt.% NiS nanocomposite sample were broken up throughout the growing process. An adsorbent surface area, which is directly related to the number of active sites for adsorption, has a significant impact on its catalytic activity. We measured surface area using the N2 adsorption-desorption isotherm at 77 K for the S@g-C3N4 and NiS-g-C3N4 nanocomposite samples ( Figure 5). All the samples exhibited type IV isotherm with no saturation implying mesoporous nature. BET plots give specific surface areas of 40, 65, 66, and 83 m 2 /g for S@g-C3N4, 0.5 wt.% NiS, 1.0 wt.% NiS, and 1.5 wt.% NiS, respectively. The surface area represents the effect of the in situ polycondensation preparation of S@g-C3N4 and NiS-g-C3N4 nanocomposites. The BJH pore volume of S@g-C3N4 is 0.18 cm 3 , which increases to 0.20 cm 3 in 1.5 wt.% NiS owing to the incorporation of NiS into the nanosheet. We found that the in situ polycondensation preparation of S@g-C3N4 and NiS-g-C3N4 nanocomposites increased the porosity of the composites, which allowed for more interaction with ions and faster electron transport for catalytic activity [39]. An adsorbent surface area, which is directly related to the number of active sites for adsorption, has a significant impact on its catalytic activity. We measured surface area using the N 2 adsorption-desorption isotherm at 77 K for the S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposite samples ( Figure 5). All the samples exhibited type IV isotherm with no saturation implying mesoporous nature. BET plots give specific surface areas of 40, 65, 66, and 83 m 2 /g for S@g-C 3 N 4 , 0.5 wt.% NiS, 1.0 wt.% NiS, and 1.5 wt.% NiS, respectively. The surface area represents the effect of the in situ polycondensation preparation of S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites. The BJH pore volume of S@g-C 3 N 4 is 0.18 cm 3 , which increases to 0.20 cm 3 in 1.5 wt.% NiS owing to the incorporation of NiS into the nanosheet. We found that the in situ polycondensation preparation of S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites increased the porosity of the composites, which allowed for more interaction with ions and faster electron transport for catalytic activity [39].
The absorbance properties of S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposite samples were measured with the help of UV-visible spectroscopy as shown in Figure 6a. The absorbance curve displays a significant absorption band centered around 322 nm that corresponds to n→π* electronic transitions. Heterocyclic aromatics showed the band gap absorption around 400 nm that corresponds to the characteristic π-π* transitions [8]. It is also expected that the disorder in nanocomposites will result in separated electron and hole states with energies in the band gap, causing the Urbach tail in the optical absorption spectrum, which broadens the absorption even more. Moreover, a shoulder appeared at 400 nm that showed a red shift when NiS content varies from 0.0-1.5 wt.%. The red shift in adsorption revealed the ease of production of photo-induced electrons and holes. The absorbance properties of S@g-C3N4 and NiS-g-C3N4 nanocomposite samples were measured with the help of UV-visible spectroscopy as shown in Figure 6a. The absorbance curve displays a significant absorption band centered around 322 nm that corresponds to n→π* electronic transitions. Heterocyclic aromatics showed the band gap absorption around 400 nm that corresponds to the characteristic π-π* transitions [8]. It is also expected that the disorder in nanocomposites will result in separated electron and hole states with energies in the band gap, causing the Urbach tail in the optical absorption spectrum, which broadens the absorption even more. Moreover, a shoulder appeared at 400 nm that showed a red shift when NiS content varies from 0.0-1.5 wt.%. The red shift in adsorption revealed the ease of production of photo-induced electrons and holes. The optical energy gap (Eopt) is an important parameter to estimate the electronic structure of the S@g-C3N4 and NiS-g-C3N4 nanocomposites. Eopt is calculated by evaluation of the straight lines intercept at zero photon absorption from the plots of (αhν) 2 vs. photon energy (hν) shown in Figure 6b as follows [40][41][42]: The average values of the optical energy gap for S@g-C3N4 were 2.60 eV that decreased to 2.50, 2.40, and 2.30 eV as the NiS concentration increased from 0.5 to 1.5 wt.%. This decrease in the energy gap is explained by the development of additional energy levels or changes in the g-C3N4 electronic structure [43,44].
Physical and chemical characteristics of materials are measured in photoluminescence by employing photons to produce excited electronic states in the material system The optical energy gap (E opt ) is an important parameter to estimate the electronic structure of the S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites. E opt is calculated by evaluation of the straight lines intercept at zero photon absorption from the plots of (αhν) 2 vs. photon energy (hν) shown in Figure 6b as follows [40][41][42]: The average values of the optical energy gap for S@g-C 3 N 4 were 2.60 eV that decreased to 2.50, 2.40, and 2.30 eV as the NiS concentration increased from 0.5 to 1.5 wt.%. This decrease in the energy gap is explained by the development of additional energy levels or changes in the g-C 3 N 4 electronic structure [43,44].
Physical and chemical characteristics of materials are measured in photoluminescence by employing photons to produce excited electronic states in the material system and evaluating the optical emission when these states relax. This in turn induces electronhole pairs that recombine after a lifetime in excited states. The key factors influencing a catalyst's capacity to catalyze a reaction are the electrical and structural defects as well as the recombination of electron-hole pairs [45]. Figure 7 shows the emission spectra of S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites. All NiS-g-C 3 N 4 nanocomposite catalysts have an emission band that is visible in the 410-540 nm range and is composed of n-π* transitions [46]. The intensity of this peak decreased as the NiS concentration increased from 0.5 to 1.5 wt.%. This could be because electron-hole pairs are produced quickly while the pair recombination process is delayed. Further, the photo-induced electron-hole pair can transfer easily at the interface of NiS/g-C 3 N 4 nanocomposite. As a result, we expect that the nanocomposite sample 1.5 wt.% NiS will show high catalytic performance concerning the other samples.  In self-hydrolysis, sodium borohydride solutions become chemically stable and do not produce substantial quantities of H2 at ambient conditions. In pure water, sodium borohydride undergoes self-hydrolysis, consuming H3O + ions, and causing a pH rise that lowers the rate at which hydrogen is produced [47]. Methanol is one of the highest reactivity reagents toward sodium borohydride and is the lightest alcohol, which makes it a suitable alternative for water in the reaction that produces hydrogen. Another benefit of employing methanol is that it lowers the reactant mixture's freezing point, making it feasible to generate hydrogen at temperatures lower than 273 K with rapid hydrogen synthesis and quick reaction initiation-impossible when using pure water. Moreover, methanol regeneration may be employed as a possible high gravimetric density hydrogen storage device, and sodium borohydride methanolysis has been presented as a practical process for hydrogen production at low temperatures [48]. In self-hydrolysis, sodium borohydride solutions become chemically stable and do not produce substantial quantities of H 2 at ambient conditions. In pure water, sodium borohydride undergoes self-hydrolysis, consuming H 3 O + ions, and causing a pH rise that lowers the rate at which hydrogen is produced [47]. Methanol is one of the highest reactivity reagents toward sodium borohydride and is the lightest alcohol, which makes it a suitable alternative for water in the reaction that produces hydrogen. Another benefit of employing methanol is that it lowers the reactant mixture's freezing point, making it feasible to generate hydrogen at temperatures lower than 273 K with rapid hydrogen synthesis and quick reaction initiation-impossible when using pure water. Moreover, methanol regeneration may be employed as a possible high gravimetric density hydrogen storage device, and sodium borohydride methanolysis has been presented as a practical process for hydrogen production at low temperatures [48].
According to the Langmuir-Hinshelwood mechanism, catalysts whose surfaces are linked with the amino group effectively contribute to the hydrolysis or methanolysis of NaBH 4 and for hydrogen generation. Meanwhile, methanol and NaBH 4 molecules adsorb on the catalyst's surface [49,50]. On the other hand, Michaelis-Menten stated that the active sites of the catalyst adsorb NaBH 4 without methanol [51,52]. The aforementioned information leads to the conclusion that catalyst surface characteristics are crucial for the evolution of hydrogen gas.
Nanocomposite catalysts are employed to accelerate the kinetics of sodium borohydride hydrolysis in stable solutions, resulting in a significant increase in hydrogen production. The particle size and degree of dispersion are other factors that affect a catalyst's activity. In order to speed up the reaction and lower the catalyst loading, small particle size and excellent dispersion encourage extensive catalyst interaction with the NaBH 4 solution. A methanolysis experiment was performed in order to check the role of S@g-C 3 N 4 and NiSg-C 3 N 4 nanocomposites in the hydrogen evolution from NaBH 4 . The data were recorded at 30 • C and plotted in Figure 8. S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites were added and led to an increase in the maximum quantity of hydrogen. The sample 1.5 wt.% NiS showed the fastest hydrogen generation performance. In methanol, the NaBH 4 material broke down into Na + and BH − 4 ions. Moreover, the surface of S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites adsorbed BH − 4 ions. The efficient catalyst adsorbs more BH − 4 ions in a short time and thus produces more hydrogen.
Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of and NiS-g-C3N4 nanocomposites in the hydrogen evolution from NaBH4. The data we recorded at 30 °C and plotted in Figure 8. S@g-C3N4 and NiS-g-C3N4 nanocomposites we added and led to an increase in the maximum quantity of hydrogen. The sample 1.5 wt. NiS showed the fastest hydrogen generation performance. In methanol, the NaBH4 mat rial broke down into Na + and BH ions. Moreover, the surface of S@g-C3N4 and NiS-C3N4 nanocomposites adsorbed BH ions. The efficient catalyst adsorbs more BH ion in a short time and thus produces more hydrogen. The rate of hydrogen production greatly determines the efficiency of the catalyst speed up the reaction. Hydrogen evolution rates (r) of the S@g-C3N4 and NiS-g-C3N4 nan composites are calculated with help of the following equations using the H2 volume (V the mass of catalyst (mcat), and time of reaction (t) [20,53]: The rate of hydrogen production greatly determines the efficiency of the catalyst to speed up the reaction. Hydrogen evolution rates (r) of the S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites are calculated with help of the following equations using the H 2 volume (V), the mass of catalyst (m cat ), and time of reaction (t) [20,53]: The hydrogen evolution curves displayed in Figure 8 were used to calculate the hydrogen generation rate. The calculated production rates are plotted in Figure 9 for the S@g-C 3 N 4 and NiS-g-C 3 N 4 samples. The hydrogen generation rates increased with the increase in the NiS nanosheet content. Moreover, the sample 1.5 wt.% NiS showed the highest production rate of 8654 mL/g·min. The nanocomposite 1.5 wt.% NiS had the highest generation rate among others due to the promising surface design [54]. In this context, NaBH 4 decomposes in methanol into Na + and BH − 4 ions. The large surface area of the nanocomposite sample 1.5 wt.% NiS helps the adsorption of more BH − 4 ions. As a result, the production of hydrogen from methanolysis of NaBH 4 will be accelerated. Additionally, this rate of hydrogen evolution (8654 mL/g·min) exceeds the rates for R-TiO 2 -NH 2 (3525 mL/g·min) [50], SiO 2 @PAA (5120 mL/g·min) [55], ZIF-67@GO (3200 mL/g·min) [56], and Ru/NiO-Ni foam (6000 mL/g·min) [57]. the production of hydrogen from methanolysis of NaBH4 will be accelerated. Additionally, this rate of hydrogen evolution (8654 mL/g.min) exceeds the rates for R-TiO2-NH2 (3525 mL/g.min) [50], SiO2@PAA (5120 mL/g.min) [55], ZIF-67@GO (3200 mL/g.min) [56], and Ru/NiO-Ni foam (6000 mL/g.min) [57].

Conclusions
The in situ polycondensation method was implemented for the preparation of S@g-C3N4 and NiS-g-C3N4 and was employed for catalytic hydrogen production from the methanolysis of sodium borohydride. The incorporation of NiS during the growth process played a major role in the enhancement of the surface area and porosity of the S@g-C3N4 composites. The 1.5 wt.% NiS sample had the highest surface area of 90 m 2 /g compared with the 0.5 wt.% NiS and 1.0 wt.% NiS samples. The pore volume of S@g-C3N4 was 0.18 cm 3 , which was reduced to 0.11 cm 3 in 1.5 wt.% NiS owing to the incorporation of NiS particles into the nanosheet. We found that during the in situ polycondensation prepara- Figure 9. H 2 production rates for S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites.

Conclusions
The in situ polycondensation method was implemented for the preparation of S@g-C 3 N 4 and NiS-g-C 3 N 4 and was employed for catalytic hydrogen production from the methanolysis of sodium borohydride. The incorporation of NiS during the growth process played a major role in the enhancement of the surface area and porosity of the S@g-C 3 N 4 composites. The 1.5 wt.% NiS sample had the highest surface area of 90 m 2 /g compared with the 0.5 wt.% NiS and 1.0 wt.% NiS samples. The pore volume of S@g-C 3 N 4 was 0.18 cm 3 , which was reduced to 0.11 cm 3 in 1.5 wt.% NiS owing to the incorporation of NiS particles into the nanosheet. We found that during the in situ polycondensation preparation of S@g-C 3 N 4 and NiS-g-C 3 N 4 nanocomposites increased the porosity of the composites. The average value of the optical energy gap for S@g-C 3 N 4 was 2.60 eV and decreased to 2.30 eV because of the 1.5 wt.% NiS incorporation. The NiS-g-C 3 N 4 catalysts showed an emission band in the 410-540 nm range and the intensity of this peak decreased as the NiS concentration increased from 0.5 to 1.5 wt.%. The hydrogen generation rates increased with the increase in the NiS nanosheet content. The sample 1.5 wt.% NiS showed the highest production rate of 8654 mL/g·min compared with others due to the promising surface design. The large surface area of the nanocomposite sample 1.5 wt.% NiS helps for adsorption of more BH − 4 ions. As a result, the production of hydrogen from methanolysis of NaBH 4 will be accelerated. All of these results enhance the possibility of using 1.5 wt.% NiS as a promising catalyst for the production of hydrogen from NaBH 4 methanolysis.

Data Availability Statement:
The corresponding author will make the data available on request.