MoO3/S@g-C3N4 Nanocomposite Structures: Synthesis, Characterization, and Hydrogen Catalytic Performance

Hydrogen production as a source of clean energy is high in demand nowadays to avoid environmental issues originating from the use of conventional energy sources i.e., fossil fuels. In this work and for the first time, MoO3/S@g-C3N4 nanocomposite is functionalized for hydrogen production. Sulfur@graphitic carbon nitride (S@g-C3N4)-based catalysis is prepared via thermal condensation of thiourea. The MoO3, S@g-C3N4, and MoO3/S@g-C3N4 nanocomposites were characterized using X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), Field Emission Scanning Electron Microscope (FESEM), STEM, and spectrophotometer. The lattice constant (a = 3.96, b = 13.92 Å) and the volume (203.4 Å3) of MoO3/10%S@g-C3N4 were found to be the highest compared with MoO3, MoO3/20-%S@g-C3N4, and MoO3/30%S@g-C3N4, and that led to highest band gap energy of 4.14 eV. The nanocomposite sample MoO3/10%S@g-C3N4 showed a higher surface area (22 m2/g) and large pore volume (0.11 cm3/g). The average nanocrystal size and microstrain for MoO3/10%S@g-C3N4 were found to be 23 nm and −0.042, respectively. The highest hydrogen production from NaBH4 hydrolysis ~22,340 mL/g·min was obtained from MoO3/10%S@g-C3N4 nanocomposites, while 18,421 mL/g·min was obtained from pure MoO3. Hydrogen production was increased when increasing the masses of MoO3/10%S@g-C3N4.


Introduction
The use of conventional energy sources, i.e., fossil fuels, has led to the global climate change crisis, which has become a serious environmental issue [1,2]. The negative impact of using such energy sources resides in their emissions of toxic gases such as NO X , CO X , and SO X . Exposure to these gases at certain concentrations may cause serious health issues in humans and animals [3][4][5]. Therefore, people have been stimulated to use renewable energy sources like solar cells and hydrogen production, aiming to maintain the stability of the environment [1,2,6]. Solar cells have been used for the conversion of sunlight to power. The efficiency of solar cells is limited to the condition of the weather, hence, the solar cell produces more power on a sunny day compared with a cloudy day. In addition, the power generated by the solar cells must be stored in batteries to be used whenever needed. Economically, generating power from solar cells is not favorable due to their low efficiency and high cost of energy storage. Therefore, hydrogen production has superior proprieties to solar cells, and so it becomes a hotspot topic nowadays. Hydrogen exists in nature as an abundant element i.e., water [7].
Nanomaterials have a significantly large surface area to volume ratio due to their small dimensions. The surface properties of nanomaterials will have an impact on the entire material, particularly when their sizes are comparable in terms of length [2,6]. Thus, the properties of the bulk materials can be improved upon or modified. Since a pioneer study that was published in 1972 about the use of TiO 2 electrodes for water splitting into hydrogen [8], many photocatalytic studies have been conducted for hydrogen production using nanomaterials i.e., metal oxides, carbon nitride, nanosheets doped with sulfur,

Nanocomposite Preparation
Sulfur@carbon nitride nanosheet was prepared via thermal condensation. 15 g of thiourea are inserted into a porcelain crucible and heated at 550 • C for 2 h at a heating rate of 3.0 • C/min. The crucible was taken from the furnace and the yellow powder was washed with distilled water.
MoO 3 /S@g-C 3 N 4 catalyst nanocomposites were prepared with different proportions of carbon nitride. The powders of MoO 3 (90, 80, 70 wt%) and S@g-C 3 N 4 (10, 20 and 30 wt%) were mixed in ethanol for 1 h on a magnetic stirrer at 300 K. Then, the solution was subjected to an ultrasonic bath for 1 h. After that, the solution was dried at 100 • C overnight. The obtained powder was ground very well for 30 min.

Characterization of Nanocomposite
An effective method for determining the crystalline structure based on the interaction of materials and electromagnetic radiation is the XRD analysis. The data of XRD spectra were conducted using a Shimadzu diffractometer (XRD 7000, Kyoto, Japan). Software can be employed to index the crystal structures of samples by comparing the obtained XRD patterns to the crystalline database. Identification of the functional groups in the material can be completed with the use of FTIR spectroscopy. The ATR spectra were obtained using a Shimadzu spectrometer (FTIR-Tracer 100, Kyoto, Japan). The scanning electron microscope is a tool for the morphological analysis of materials that scans the surface with a focused electron beam. FESEM micrographs were recorded on the Quattro ESEM's environmental scanning electron microscope (Thermo Fisher Scientific, Waltham, MA, United States). The samples were placed on carbon tape and their surface was coated with gold. STEM microscopy analysis was completed on a Talos F200i TEM/STEM electron microscope (Thermo Fisher Scientific, Waltham, MA, USA). In order to investigate the physical characteristics of porous materials, such as specific surface area, pore size distribution, and pore volume, we have completed N 2 sorption/desorption analysis. NOVA 4200e chemo-physisorption surface area analyzer was used to get the conventional N 2 adsorption isotherms point-by-point by measuring the quantity of nitrogen adsorbed and the equilibrium pressure at 77 K. The samples were initially outgassed for a continuous 24 h at 150 • C and 1 millitorr vacuum. The sample is cleaned and ready for the collection of adsorption data after this outgassing procedure. The UV-Vis analysis is an important technique for determining a semiconductor band gap and for analyzing a photocatalyst capacity for absorption. The optical absorption spectroscopy data were conducted on a Thermo Scientific Evolution 200 UV-Vis spectrophotometer (Waltham, MA, USA). The samples were dissolved in distilled water via ultrasonic waves to get a suspension.

Hydrogen Catalytic Performance
Catalytic hydrogen evolution at room temperature was used to assess the catalytic activity of the prepared composites. The hydrogen catalytic tests were completed in a 250 mL conical flask with 0.25 g of NaBH 4 solution that was hydrolyzed with 10.0 mL methanol. The reaction temperature for the system was constant as the glass flask was placed in a water bath. No stirring was employed in the reaction flask. The temperature of the water bath was kept constant at 25 • C. The masses of catalyst and NaBH 4 were mixed very well and then inserted into the conical flask. After that, 10 mL methanol was added to the glass flask and the stopwatch started simultaneously. The volume of generated hydrogen gas was measured by measuring the displacement of the water level in the burette.
The S@g-C 3 N 4 diffraction peaks at 27.5 • and 13.02 • correspond to the (002) and (100) planes, which is consistent with the interplanar staking peaks characteristic of aromatic systems and the inter-layer structure packing, respectively [35]. In the case of MoO 3 /S@g-C 3 N 4 composites, however, the S@g-C 3 N 4 diffraction peaks are not clearly recognized. This outcome indicates that S@g-C 3 N 4 nanosheets coat the surface of the MoO 3 nanocrystal. All of the diffraction peaks of MoO 3 /S@g-C 3 N 4 composites are observed to shift from 27.2 • to 27.4 • with different S@g-C 3 N 4 concentrations, and this is a result of the two lines overlapping. Consequently, the different concentration of S@g-C 3 N 4 in the expanded XRD diffraction pattern confirms the coexistence of MoO 3 and g-C 3 N 4 in the MoO 3 /S@g-C 3 N 4 composites. Moreover, the less-ordered crystalline structure of S@g-C 3 N 4 is expected to show an intense thermal etching process with many defects, which is intended to improve catalytic activity.
The plots obtained by the Rietveld refinement of MoO 3 , MoO 3 /10%S@g-C 3 N 4 , MoO 3 / 20S@g-C 3 N 4 , and MoO 3 /30S@g-C 3 N 4 are shown in Figure 1b-d. Crystallography open database (COD) was used to find matching reference patterns, which were used to set the initial values for space group, cell parameters, and atom coordinates. The background was improved by applying the cosine Fourier series with six different coefficients that could be modified, and the Bragg reflection profile was characterized by the Thompson-Cox-Hastings pseudo-Voigt function. Several factors were refined, including unit cell parameters, scale factor, structure factor, occupancy, position parameters, etc. The observed and calculated diffractograms are in good agreement for both types of synthesis processes, as shown in Figure 1c,d. Also, Table 1 provides an overview of the comparison of lattice parameters and quality of fit. It is crucial to note that there is a change in lattice parameters for the MoO 3 /10%S@g-C 3 N 4 sample when compared to other concentrations of wt% S@g-C 3 N 4 ; an increase in lattice constants that can be ascribed to the S@g-C 3 N 4 doping on MoO 3 structure. In particular, an increase in the value of the lattice constant "b" was found at 10 wt% MoO 3 /S@g-C 3 N 4 concentration and a decrease in the lattice parameter "b" was observed for samples containing 20 wt% and 30 wt% of S@g-C 3 N 4 . The decrease in the lattice parameter "b" suggests that the MoO 3 crystal structure is under compression [36]. Previous work has shown that the lattice parameter can be increased due to the oxygen vacancies, and that might be decreased by raising the annealing temperature, which decreases oxygen vacancies and enhances crystallinity [37][38][39].

FTIR Structure Analysis
The FT-IR spectra of MoO3, S@g-C3N4, and different concentrations of MoO3/S@g-C3N4 composites are shown in Figure 2. The peak at 1643 cm −1 for pure S@g-C3N4 is attributable to C=N stretching vibration modes, while the peaks at 1242, 1322, 1405 cm −1 , and 1563 cm −1 are correlated with aromatic C-N stretching [40,41]. The band at 809 cm −1 corresponds to the C-N heterocycles with out-of-plane bending modes [42]. The vibrations in pure MoO3 appeared at approximately 561, 866, and 990 cm −1 , which are formed by the

FTIR Structure Analysis
The FT-IR spectra of MoO 3 , S@g-C 3 N 4 , and different concentrations of MoO 3 /S@g-C 3 N 4 composites are shown in Figure 2. The peak at 1643 cm −1 for pure S@g-C 3 N 4 is attributable to C=N stretching vibration modes, while the peaks at 1242, 1322, 1405 cm −1 , and 1563 cm −1 are correlated with aromatic C-N stretching [40,41]. The band at 809 cm −1 corresponds to the C-N heterocycles with out-of-plane bending modes [42]. The vibrations in pure MoO 3 appeared at approximately 561, 866, and 990 cm −1 , which are formed by the oxygen stretching mode associated with three metal atoms, the oxygen stretching mode in the Mo-O-Mo units, and the Mo=O stretching mode, respectively [43,44]. The characteristic vibrations for MoO 3 and S@g-C 3 N 4 persist in MoO 3 /S@g-C 3 N 4 composites and the absorption bands in MoO 3 /S@g-C 3 N 4 slightly enhanced as the S@g-C 3 N 4 concentration increases. These outcomes agree with the XRD structural data.
oxygen stretching mode associated with three metal atoms, the oxygen stretchin in the Mo-O-Mo units, and the Mo=O stretching mode, respectively [43,44]. The teristic vibrations for MoO3 and S@g-C3N4 persist in MoO3/S@g-C3N4 composites absorption bands in MoO3/S@g-C3N4 slightly enhanced as the S@g-C3N4 concentra creases. These outcomes agree with the XRD structural data.

ESEM and STEM Microscopy
The morphology of materials composites enhances the understanding of thei structure and contributes to the identification of suitable applications. Therefore, faces of the MoO3/S@g-C3N4 nanocomposites were scanned using the FESEM mic and the images were collected in Figure 3a-e. The S@g-C3N4 shown in Figure 3a t form of scattered flakes because of the sticky layers [45]. While the micrograph fo nanocrystals showed orthorhombic shapes (Figure 3b). In the images of MoO3/S@ nanocomposites (Figure 3c-e), MoO3 crystals appear wrapped with thin random f carbon nitride. Moreover, the thickness of the S@g-C3N4 layers increases with inc content from 10 to 30%. These observations are evidence of the strong interaction b MoO3 and S@g-C3N4.

ESEM and STEM Microscopy
The morphology of materials composites enhances the understanding of their microstructure and contributes to the identification of suitable applications. Therefore, the surfaces of the MoO 3 /S@g-C 3 N 4 nanocomposites were scanned using the FESEM microscope and the images were collected in Figure 3a-e. The S@g-C 3 N 4 shown in Figure 3a takes the form of scattered flakes because of the sticky layers [45]. While the micrograph for MoO 3 nanocrystals showed orthorhombic shapes (Figure 3b). In the images of MoO 3 /S@g-C 3 N 4 nanocomposites (Figure 3c-e), MoO 3 crystals appear wrapped with thin random flakes of carbon nitride. Moreover, the thickness of the S@g-C 3 N 4 layers increases with increasing content from 10 to 30%. These observations are evidence of the strong interaction between MoO 3 and S@g-C 3 N 4 .
In Figure 3f, a STEM image of the MoO 3 /10%S@g-C 3 N 4 was shown and the formation of the sticky flakes coating the surface of MoO 3 was confirmed. It also showed the mapping of Mo, oxygen, sulfur, carbon, and nitrogen atoms. The distribution of elements is homogeneous and thus confirms the formation of the nanocomposite. Moreover, the STEM image confirms the scans of SEM.  In Figure 3f, a STEM image of the MoO3/10%S@g-C3N4 was shown and the formation of the sticky flakes coating the surface of MoO3 was confirmed. It also showed the mapping of Mo, oxygen, sulfur, carbon, and nitrogen atoms. The distribution of elements is homogeneous and thus confirms the formation of the nanocomposite. Moreover, the STEM image confirms the scans of SEM.

Surface Area and Pore Size
The adsorption−desorption isotherms for the MoO3/S@g-C3N4 nanocomposites are displayed in Figure 4. These isotherms belong to type IV mesoporous materials. The surface area of samples is often determined using a traditional Brunauer-Emmet-Teller (BET) model. Moreover, the surface area BET for these nanocomposites was determined. The surface area values are 40.0, 19.0, 22, 9.0, and 2.5 m 2 /g for the samples S@g-C3N4, MoO3, MoO3/10%S@g-C3N4, MoO3/20%S@g-C3N4, and MoO3/30%S@g-C3N4, respectively.

Surface Area and Pore Size
The adsorption−desorption isotherms for the MoO 3 /S@g-C 3 N 4 nanocomposites are displayed in Figure 4. These isotherms belong to type IV mesoporous materials. The surface area of samples is often determined using a traditional Brunauer-Emmet-Teller (BET) model. Moreover, the surface area BET for these nanocomposites was determined.  The nanocomposite sample MoO3/10%S@g-C3N4 revealed a higher surface area. The increased surface area promotes the reaction rate simply by introducing more active sites to the reactants. The BJH pore volume data were 0.18, 0.1, 0.11, 0.04, and 0.01 cm 3 /g for the samples S@g-C3N4, MoO3, MoO3/10%S@g-C3N4, MoO3/20%S@g-C3N4, and MoO3/30%S@g- The nanocomposite sample MoO 3 /10%S@g-C 3 N 4 revealed a higher surface area. The increased surface area promotes the reaction rate simply by introducing more active sites to the reactants. The BJH pore volume data were 0.18, 0.1, 0.11, 0.04, and 0.01 cm 3 /g for the samples S@g-C 3 N 4 , MoO 3 , MoO 3 /10%S@g-C 3 N 4 , MoO 3 /20%S@g-C 3 N 4 , and MoO 3 /30%S@g-C 3 N 4 , respectively. Accordingly, the nanocomposite sample MoO 3 /10%S@g-C 3 N 4 showed a higher surface area and large pore volume. Large pore volumes work in a different way, in that larger pore volumes can offer more interior voids as nano-reactors to physically confine the reactants in specific areas, enhance active species concentrations, and therefore dynamically promote mass transfer. Therefore, this sample contains more active sites and has a high adsorption capacity.

UV-Vis Spectroscopy
The optical measurements aim to calculate the optical band gap of S@g-C 3 N 4 , MoO 3 / 10%S@g-C 3 N 4 , MoO 3 /20%S@g-C 3 N 4 , MoO 3 /30%S@g-C 3 N 4 , and MoO 3 nanocomposites. We demonstrated optical absorption spectra to elucidate the optical properties using a spectrophotometer. Due to n→π* electronic transitions, the absorbance spectra exhibit a high absorption peak centered at 322 nm. We use Tauc's plot and the ASF formula to determine the optical band gap. For crystalline materials, the following equations are used to study the absorption coefficient and incident photon energy [46,47]: where α (v) is the absorption coefficient, E gab is the optical gap, hv represents incident photon energy, k is a constant, r is the optical charge carrier direct transition index which is equal to 1/2. λ gab , E gab represents the optical gap (E gab (eV) = 1239.83 λ gab ), and h and c are Plank's constant and the light velocity, respectively. In order to calculate the optical band gap, we use Beer-Lambert's law. This law is determined by A(λ) = α 1 2.303 α 2 where α 1 and α 2 are the solution concentration and absorbance, respectively. Thus, Equation (2) becomes [48]: where D is represented as D = k (hc) m−1 α 1 2.303 . Figure 5 shows the plot of A λ λ 2 against λ −1 , where we extrapolate the straight-line portion of this plot at A λ λ 2 = 0. Then, we determined the E ASF gap value, which was 2.4 eV for S@g-C 3 N 4 nanosheet. Moreover, the energy gaps of MoO 3 , MoO 3 /10%S@g-C 3 N 4 , MoO 3 /20%S@g-C 3 N 4 , and MoO 3 /30%S@g-C 3 N 4 nanostructures were 3.86, 4.14, 4.0, and 4.12 eV, respectively. The energy gap for MoO 3 /10%S@g-C 3 N 4 is that of MoO 3 , MoO 3 /20%S@g-C 3 N 4 , and MoO 3 /30%S@g-C 3 N 4 . The lattice constant and the volume of 10%S@g-C 3 N 4 , MoO 3 are increased leading to higher band gap energy [49]. The efficiency of the photocatalyst is enhanced by increasing the band gap energy. The increased band gaps are attributed to the strong quantum effect produced by the ultra-thin atomic-thick S@g-C 3 N 4 nanosheets. For example, doping CdS, Fe 2 O 3 , and WO 3 with TiO 2 improve the optical absorption and charge carrier separations. The larger the band gaps, the greater the reductive capacity, and hence a more favorable thermodynamic driving force for H 2 generation. The sample (MoO 3 /10%S@g-C 3 N 4 ) is expected to improve hydrogen production [50]. band gap energy [49]. The efficiency of the photocatalyst is enhanced by increasing the band gap energy. The increased band gaps are attributed to the strong quantum effect produced by the ultra-thin atomic-thick S@g-C3N4 nanosheets. For example, doping CdS, Fe2O3, and WO3 with TiO2 improve the optical absorption and charge carrier separations. The larger the band gaps, the greater the reductive capacity, and hence a more favorable thermodynamic driving force for H2 generation. The sample (MoO3/10%S@g-C3N4) is expected to improve hydrogen production [50]. Mulliken electronegativity theory was used to predict the location of the CB and VB edges of MoO3/S@g-C3N4. Accordingly, the following formula can be used to estimate the location of the valence band (EVB) [51]: where χ is the MoO3 electronegativity (6.4 eV), E is electron free energy (4.5 eV), and Eg is the determined band gap [52]. Further, the position of conduction band (ECB) is estimated according to valence band and the band gaps [53]: The calculated values of valence band positions are 3.83, 3.97, 3.90, and 3.96 eV for MoO3, MoO3/10%S@g-C3N4, MoO3/20%S@g-C3N4, and MoO3/30%S@g-C3N4 nanocomposites. The conduction band energies for the same nanocomposite samples are 0.03, −0.11, Figure 5. Plots of (a) optical absorption and (b) ASF graphs for nanostructures.
Mulliken electronegativity theory was used to predict the location of the CB and VB edges of MoO 3 /S@g-C 3 N 4 . Accordingly, the following formula can be used to estimate the location of the valence band (E VB ) [51]: where χ is the MoO 3 electronegativity (6.4 eV), E is electron free energy (4.5 eV), and E g is the determined band gap [52]. Further, the position of conduction band (E CB ) is estimated according to valence band and the band gaps [53]: The calculated values of valence band positions are 3.83, 3.97, 3.90, and 3.96 eV for MoO 3 , MoO 3 /10%S@g-C 3 N 4 , MoO 3 /20%S@g-C 3 N 4 , and MoO 3 /30%S@g-C 3 N 4 nanocomposites. The conduction band energies for the same nanocomposite samples are 0.03, −0.11, −0.04, and −0.1 eV. The activity of a catalyst for each reaction is influenced by changes in the conduction and valence bands caused by coating with S@g-C 3 N 4 .

Hydrogen Catalytic Performance
Self-hydrolysis of NaBH 4 at room temperature generates a very low volume of hydrogen due to an increase in pH during the hydrolysis reaction. The main reason for the pH raise is the induced by-product of strongly basic sodium metaborate (NaBO 2 ) ion [54]. Therefore, to implement an H 2 economy, the development of a catalyst with the ability to cause a high generation rate at room temperature is necessary and essential. High hydrogen generation rates with great control are accomplished via a catalyst. Primary alcohols are used as reactants in the place of water or as a partial substitute for water in a different method of producing hydrogen from sodium borohydride. Methanol is the lightest alcohol and has the highest reactivity toward sodium borohydride, making it an effective alternative to water as a reactant for the generation of hydrogen [55]. The measurements of hydrogen evolution from the reaction of NaBH 4 -methanolysis are shown in Figure 6. 20 mg of catalysts (MoO 3 , S@g-C 3 N 4 , and MoO 3 /S@g-C 3 N 4 ) were used to test their efficiency in producing hydrogen. The catalysts connected with the amino group catalyze the NaBH 4 hydrolysis and methanolysis for the production of hydrogen according to the mechanism of Langmuir-Hinshelwood; as the molecules of methanol and NaBH 4 adsorbed on the surface of the catalyst [56]. Otherwise, NaBH 4 can be adsorbed without methanol on the catalyst surface as described by the mechanism of Michaelis-Menten [57]. It can be concluded from the foregoing that the surface properties of the catalyst are of great importance in the production of hydrogen. gen generation rates with great control are accomplished via a catalyst. Primary alcohols are used as reactants in the place of water or as a partial substitute for water in a different method of producing hydrogen from sodium borohydride. Methanol is the lightest alcohol and has the highest reactivity toward sodium borohydride, making it an effective alternative to water as a reactant for the generation of hydrogen [55]. The measurements of hydrogen evolution from the reaction of NaBH4-methanolysis are shown in Figure 6. 20 mg of catalysts (MoO3, S@g-C3N4, and MoO3/S@g-C3N4) were used to test their efficiency in producing hydrogen. The catalysts connected with the amino group catalyze the NaBH4 hydrolysis and methanolysis for the production of hydrogen according to the mechanism of Langmuir-Hinshelwood; as the molecules of methanol and NaBH4 adsorbed on the surface of the catalyst [56]. Otherwise, NaBH4 can be adsorbed without methanol on the catalyst surface as described by the mechanism of Michaelis-Menten [57]. It can be concluded from the foregoing that the surface properties of the catalyst are of great importance in the production of hydrogen. In methanol, NaBH4 is initially decomposed into Na + and BH4 ions. A second stage might be the adsorption of the produced BH4 ions onto the charged surface of the MoO3/S@g-C3N4 catalyst. Thus, the large positive area located on the surface of the catalyst will increase the adsorption of BH4 ions. 1.0 mole of H2 is produced by the interaction of the hydrogen atom with a negative charge in the structure of the catalyst complex-H that is induced because of electron transfer and the hydrogen atom with positive charge of methanol. BH3 and the methoxy ion of 3.0 mole of methanol combine simultaneously to generate B(CH3O). Finally, 4H2 is generated and NaB(OCH3)4 was produced [56,57]. In methanol, NaBH 4 is initially decomposed into Na + and BH 4 ions. A second stage might be the adsorption of the produced BH 4 ions onto the charged surface of the MoO 3 /S@g-C 3 N 4 catalyst. Thus, the large positive area located on the surface of the catalyst will increase the adsorption of BH 4 ions. 1.0 mole of H 2 is produced by the interaction of the hydrogen atom with a negative charge in the structure of the catalyst complex-H that is induced because of electron transfer and the hydrogen atom with positive charge of methanol. BH 3 and the methoxy ion of 3.0 mole of methanol combine simultaneously to generate B(CH 3 O). Finally, 4H 2 is generated and NaB(OCH 3 ) 4 was produced [56,57].
From Figure 6, it can be seen that the maximum amount of hydrogen produced is accelerated with the addition of nanocomposites. In addition, the fastest hydrogen evolution was revealed with the sample MoO 3 /10%S@g-C 3 N 4 . The methanolysis of NaBH 4 material includes two products of Na + and BH 4 − ions. Thereafter, the developed BH 4 − ions are adsorbed on the charged surface of the MoO 3 /10%S@g-C 3 N 4 . Thus, a catalyst with a high positively-charged surface adsorb BH 4 − ions in short times [58]. Moreover, the fitting of hydrogen volume versus time for S@g-C 3 N 4 , MoO 3 , MoO 3 /10%S@g-C 3 N 4 , MoO 3 /20%S@g-C 3 N 4 , and MoO 3 /30%S@g-C 3 N 4 nanocomposites give slopes of 1. 60, 8.0, 8.71, 6.22, and 4.74, respectively. The nanocomposite sample MoO 3 /10%S@g-C 3 N 4 showed the highest slope (8.71) and thus possesses the highest hydrogen generation rate. Figure 7 shows a fast hydrogen generation as the MoO 3 /10%S@g-C 3 N 4 loading is increased from 0.0 to 30 mg. This may be due to the wide energy gap of the MoO 3 /10%S@g-C 3 N 4 catalyst, which possesses a high separation of charge carriers [59]. Moreover, the amino group from S@g-C 3 N 4 that is coated on the surface of MoO 3 motivates the hydrogen production from NaBH 4 .
(8.71) and thus possesses the highest hydrogen generation rate. Figure 7 shows a fast hydrogen generation as the MoO3/10%S@g-C3N4 load creased from 0.0 to 30 mg. This may be due to the wide energy gap of the MoO3/ C3N4 catalyst, which possesses a high separation of charge carriers [59]. Moreo amino group from S@g-C3N4 that is coated on the surface of MoO3 motivates the h production from NaBH4. One of the most crucial factors that must be taken into account while de engineering solutions for hydrogen energy applications, is hydrogen generating H2 generation rate (r) for the MoO3/%S@g-C3N4 catalyst is estimated from the f relation [60,61]: where V denotes the H2 volume, mcat is the mass of the catalyst, and t is the time tion. The H2 generation rates that were obtained based on the data in Figure 7, n less, have decreased. As the values of r were 28,767, 22,340, and 15,905 mL/g.min and 30 mg of MoO3/10%S@g-C3N4 catalyst, respectively. A reduction in the cataly ity of the methanolysis process is the end outcome, which is the blockage of the active sites as a result of the catalyst active sites becoming saturated [58]. Figure 8 shows the influence of nanocomposite catalysts on the rate of hydro erated. The nanocatalyst MoO3/10%S@g-C3N4 achieved a higher generation rate One of the most crucial factors that must be taken into account while developing engineering solutions for hydrogen energy applications, is hydrogen generating rate. The H 2 generation rate (r) for the MoO 3 /%S@g-C 3 N 4 catalyst is estimated from the following relation [60,61]: where V denotes the H 2 volume, m cat is the mass of the catalyst, and t is the time of reaction.
The H 2 generation rates that were obtained based on the data in Figure 7, nevertheless, have decreased. As the values of r were 28,767, 22,340, and 15,905 mL/g·min at 10, 20, and 30 mg of MoO 3 /10%S@g-C 3 N 4 catalyst, respectively. A reduction in the catalytic activity of the methanolysis process is the end outcome, which is the blockage of the catalyst active sites as a result of the catalyst active sites becoming saturated [58]. Figure 8 shows the influence of nanocomposite catalysts on the rate of hydrogen generated. The nanocatalyst MoO 3 /10%S@g-C 3 N 4 achieved a higher generation rate of 22,340 mL/g·min. The higher generation rate of composite comes due to the separation of charge carriers and positively charged areas on the surface of the catalyst.
The comparison of hydrogen evolution rate for different catalyst materials is recorded in Table 2. Moreover, this value of hydrogen evolution rate (22,340 mL/g·min) is higher than the rates achieved in the literature [58,[61][62][63][64][65]. This remarkable development in the performance of the MoO 3 /S@g-C 3 N 4 catalyst indicates its priority in the production of hydrogen from sodium borohydride. Nanomaterials 2023, 13, x FOR PEER REVIEW 12 of 15 mL/g.min. The higher generation rate of composite comes due to the separation of charge carriers and positively charged areas on the surface of the catalyst. The comparison of hydrogen evolution rate for different catalyst materials is recorded in Table 2. Moreover, this value of hydrogen evolution rate (22,340 mL/g.min) is higher than the rates achieved in the literature [58,[61][62][63][64][65]. This remarkable development in the performance of the MoO3/S@g-C3N4 catalyst indicates its priority in the production of hydrogen from sodium borohydride.

Conclusions
The MoO3/g-C3N4 nanocomposites were functionalized for hydrogen production. Graphitic carbon nitride (g-C3N4)-based catalysis is prepared via thermal condensation of thiourea. The MoO3, g-C3N4, and MoO3/g-C3N4 nanocomposite catalysts were characterized by using XRD, FTIR, FESEM, STEM and spectrophotometer. When g-C3N4 and MoO3 were combined, the g-C3N4 was coated on the surface of MoO3 and turned to be positively charged. The lattice constant and the volume of MoO3/10-C3N4 was found to be the highest compared with MoO3, MoO3/20-C3N4, and MoO3/30C3N4. The nanocomposite sample

Conclusions
The MoO 3 /g-C 3 N 4 nanocomposites were functionalized for hydrogen production. Graphitic carbon nitride (g-C 3 N 4 )-based catalysis is prepared via thermal condensation of thiourea. The MoO 3 , g-C 3 N 4 , and MoO 3 /g-C 3 N 4 nanocomposite catalysts were characterized by using XRD, FTIR, FESEM, STEM and spectrophotometer. When g-C 3 N 4 and MoO 3 were combined, the g-C 3 N 4 was coated on the surface of MoO 3 and turned to be positively charged. The lattice constant and the volume of MoO 3 /10-C 3 N 4 was found to be the highest compared with MoO 3 , MoO 3 /20-C 3 N 4 , and MoO 3 /30C 3 N 4 . The nanocomposite sample MoO 3 /10%S@g-C 3 N 4 showed a higher surface area and large pore volume. In addition, the combination of MoO 3 /10-C 3 N 4 has a wide band gap energy of~4.14 eV, whereas MoO 3 has an energy band gap of 3.86 eV. The average nanocrystal size and microstrain for MoO 3 /10-C 3 N 4 were found to be 23 nm and −0.042, respectively. The highest hydrogen production from NaBH 4 hydrolysis~22,340 mL/g·min was obtained from MoO 3 /10-C 3 N 4 nanocomposites, while 18,421 mL/g·min was obtained from pure MoO 3 . Hydrogen production was increased when increasing the masses of MoO 3 /10-C 3 N 4 .