Facilitated Unidirectional Electron Transmission by Ru Nano Particulars Distribution on MXene Mo2C@g-C3N4 Heterostructures for Enhanced Photocatalytic H2 Evolution

Precious metals exhibit promising potential for the hydrogen evolution reaction (HER), but their limited abundance restricts widespread utilization. Loading precious metal nanoparticles (NPs) on 2D/2D heterojunctions has garnered considerable interest since it saves precious metal consumption and facilitates unidirectional electron transmission from semiconductors to active sites. In this study, Ru NPs loaded on MXenes Mo2C by an in-site simple strategy and then formed 2D/2D heterojunctions with 2D g-C3N4 (CN) via electrostatic self-assembly were used to enhance photocatalytic H2 evolution. Evident from energy band structure analyses such as UV-vis and TRPL, trace amounts of Ru NPs as active sites significantly improve the efficiency of the hydrogen evolution reaction. More interestingly, MXene Mo2C, as substrates for supporting Ru NPs, enriches photoexcited electrons from CN, thereby enhancing the unidirectional electron transmission. As a result, the combination of Ru-Mo2C and CN constructs a composite heterojunction (Ru-Mo2C@CN) that shows an improved H2 production rate at 1776.4 μmol∙g−1∙h−1 (AQE 3.58% at 400 nm), which is facilitated by the unidirectional photogenerated electron transmission from the valence band on CN to the active sites on Ru (CN→Mo2C→Ru). The study offers fresh perspectives on accelerated unidirectional photogenerated electron transmission and saved precious metal usage in photocatalytic systems.


Introduction
The use of semiconductor photocatalysis for hydrogen generation is a hopeful solution for the issues of energy scarcity and pollution [1][2][3].However, the efficiency of electronic-hole pair separation and reduction reaction kinetics in surface water are crucial issues that impact photocatalytic hydrogen production [4][5][6].Various techniques, such as morphology modulation, valence band potential engineering, and heterogeneous structure building, have been utilized to address the aforementioned key problems [7][8][9][10].Precious metals, notably platinum (Pt), are recognized for their efficacy as co-catalysts to form heterojunctions with semiconductors by lowering the overpotentials required for surface water reduction reactions and enhancing charge transmission [11].Nevertheless, the limited availability and elevated costs hinder the widespread application of precious metals as co-catalysts [12,13].Hence, there has been a growing focus in recent years on minimizing the amount of precious metal co-catalysts used in photocatalytic hydrogen generation [14,15].Ruthenium (Ru), a relatively cost-effective metal from the platinum group, has Ru-H bond strengths comparable to those of Pt-H bonds, which suggests that it could be a viable substitute for Pt [16,17].To save on the use of precious metals, many studies have attempted to load Ru nanoparticles (NPs) onto other co-catalysts [18,19].Ru NPs can change the electronic structure of heterojunctions and introduce abundant active sites, resulting in several times the hydrogen production performance of the original catalyst [20,21].Incorporating Ru NPs onto two-dimensional (2D) structures, such as g-C 3 N 4 , is more achievable for the uniform distribution of active sites due to the extensive expanse of surface area [22,23].
2D g-C 3 N 4 is an ideal semiconductor photocatalyst because of its stable physicochemical properties and suitable band gap [24].Nevertheless, the extensive use of pure g-C 3 N 4 in photocatalysis is limited due to its slow charge transfer rate and rapid complexation of photoexcited electron-hole pairs [25].To address the mentioned issues, MXene Mo 2 C with superior conductivity can create a 2D/2D heterostructure with g-C 3 N 4 [26], with chemical bonds and van der Waals force accelerating electron migration, thus greatly facilitating the catalytic reaction [27].Furthermore, Mo 2 C exhibits excellent metal-like conductivity and a lower valence band position to form a Schottky junction with CN, thereby promoting the directional migration of photogenerated electrons via the junction [28].The metalloid's conductivity can be attributed to its Pt-like d-orbital structure, which is created by hybridizing Mo d-orbitals with C s/p-orbitals [29].However, the higher bond energy of the Mo-H bond (65-75 kcal mol −1 ) in Mo 2 C MXene results in robust adsorption of H + ions, hence exhibiting limited desorption of H 2 molecules from the Mo atoms [30,31].Therefore, it is essential to incorporate hydrogen evolution reaction (HER) activity into Mo 2 C to establish a balance in atomic hydrogen adsorption and desorption for improving photocatalytic hydrogen production [32].
This research combines a Ru-doped 2D MXene Mo 2 C co-catalyst with 2D g-C 3 N 4 layers to improve their hydrogen evolution efficiency.The conductive Mo 2 C enriches the photogenerated electrons generated in CN through the Schottky junction, as evidenced by time-resolved photoluminescence (TRPL) and electrochemical impedance spectroscopy (EIS).In addition, based on energy band structure analysis and the UV-Vis absorbance spectrum (UV-Vis), the active sites introduced by Ru NPs at the interface enhance the surface water reduction reaction, thereby guiding the enriched electrons to Ru.The Schottky barriers between Mo 2 C and CN inhibit the reverse flow and recombination of electron-hole pairs, thus facilitating unidirectional electron transmission from the semiconductor to the active sites (CN → Mo 2 C → Ru).As anticipated, the Ru-Mo 2 C@g-C 3 N 4 heterostructure demonstrated a notable hydrogen production rate of 1776.4 µmol•g −1 •h −1 and an apparent quantum efficiency (AQE) of 3.58% at 400 nm, surpassing the majority of reported Mo 2 C/g-C 3 N 4 photocatalytic systems.This study presents a strategy for unidirectional electron migration by loading trace precious metal NPs, which provides guidance for improving charge migration efficiency in photocatalytic hydrogen production.

Construction Strategy of Ru-Mo 2 C@CN
The production of Ru-Mo 2 C@CN photocatalysts followed the specific procedure outlined in Figure 1.Briefly, 2D MXene Mo 2 C was prepared by etching Mo 2 Ga 2 C with HCl-LiF in hydrothermal conditions, while Ru/Mo 2 C was fabricated by etching Mo 2 Ga 2 C with HCl-LiF-RuCl 3 .In the second step, Ru 3+ in the solution is transformed into Ru clusters attached to the Mo 2 C surface, with Ga being oxidized and subsequently eliminated.This redox reaction occurs spontaneously from a thermodynamic perspective.The Ru-Mo 2 C@CN was eventually formed via the electrostatic self-assembly of 2D g-C 3 N 4 nanosheets and MXene Ru-Mo 2 C layers.
redox reaction occurs spontaneously from a thermodynamic perspective.The Ru-Mo2C@CN was eventually formed via the electrostatic self-assembly of 2D g-C3N4 nanosheets and MXene Ru-Mo2C layers.

Synthesis and Structural Morphology of Ru-Mo2C@CN
The MXene Mo2C without Ru loading exhibits a typical layered structure, which will help prevent the aggregation of Ru nanoparticles (Figure S1).The scanning electron microscope (SEM) images (Figure 2a,b) displayed that the Ru-Mo2C nanosheets showed a flat and smooth 2D structure after etching, while the CN nanosheets showed a reticulated 2D structure, which created the possibility of electrostatic self-assembly between the two.High-resolution transmission electron microscopy (HRTEM) revealed the surface morphology, indicating the existence of Ru NPs in Figure 2c,d.As shown in Figure 2c, Ru supported on the Mo2C surface consists of nanoparticles smaller than 5 nm, which provide more abundant catalytic active sites for photocatalytic reactions compared with Ru clusters.The HRTEM image in Figure S2 also proves this conclusion.From the HRTEM image of Figure 2d, the Ru-Mo2C@CN is assembled from Mo2C nanosheets loaded with Ru NDs and 2D g-C3N4 nanosheets.Furthermore, as shown in Figure 2e-h, energy dispersive Xray spectroscopy (EDX) analysis indicated that Ru NPs are uniformly dispersed on the Mo2C surface.The above characterization indicates that MXene can serve as a carrier for the highly dispersed loading of precious metals.Even if the amount of precious metals added is small, they can uniformly adhere to the surface of MXene.
The absence of typical metallic Ru phase peaks in the XRD scan of Mxene could be attributed to the limited presence of Ru content on the surface (Figure 3a), falling below the detection threshold, or because the Ru grains are too small on the surface of Mxene to be diffracted during the X-ray diffraction (XRD) scanning.In addition, the (002) diffraction peak of Mo at 8.6° is somewhat shifted due to the introduction of Ru [33].The shifted peaks showed that Ru clusters disrupt the integrity of the lattice.X-ray photoelectron spectrometer (XPS) analysis provides additional details on the surface chemical properties of the Ru-Mo2C@CN photocatalyst.The N 1s spectra display three distinct peaks at 401.9, 400.2, and 398.7 eV, corresponding to terminal amino groups (C-N-Hx), tertiary nitrogen (N-C3), and sp2-hybridized nitrogen (C=N-C), as described in references [34,35].Ru-Mo2C@CN shows a positive shift in the typical peaks of the N spectra compared to CN, suggesting unidirectional electron transmission from CN to Ru-Mo2C.

Synthesis and Structural Morphology of Ru-Mo 2 C@CN
The MXene Mo 2 C without Ru loading exhibits a typical layered structure, which will help prevent the aggregation of Ru nanoparticles (Figure S1).The scanning electron microscope (SEM) images (Figure 2a,b) displayed that the Ru-Mo 2 C nanosheets showed a flat and smooth 2D structure after etching, while the CN nanosheets showed a reticulated 2D structure, which created the possibility of electrostatic self-assembly between the two.High-resolution transmission electron microscopy (HRTEM) revealed the surface morphology, indicating the existence of Ru NPs in Figure 2c,d.As shown in Figure 2c, Ru supported on the Mo 2 C surface consists of nanoparticles smaller than 5 nm, which provide more abundant catalytic active sites for photocatalytic reactions compared with Ru clusters.The HRTEM image in Figure S2 also proves this conclusion.From the HRTEM image of Figure 2d, the Ru-Mo 2 C@CN is assembled from Mo 2 C nanosheets loaded with Ru NDs and 2D g-C 3 N 4 nanosheets.Furthermore, as shown in Figure 2e-h, energy dispersive X-ray spectroscopy (EDX) analysis indicated that Ru NPs are uniformly dispersed on the Mo 2 C surface.The above characterization indicates that MXene can serve as a carrier for the highly dispersed loading of precious metals.Even if the amount of precious metals added is small, they can uniformly adhere to the surface of MXene.
The absence of typical metallic Ru phase peaks in the XRD scan of Mxene could be attributed to the limited presence of Ru content on the surface (Figure 3a), falling below the detection threshold, or because the Ru grains are too small on the surface of Mxene to be diffracted during the X-ray diffraction (XRD) scanning.In addition, the (002) diffraction peak of Mo at 8.6 • is somewhat shifted due to the introduction of Ru [33].The shifted peaks showed that Ru clusters disrupt the integrity of the lattice.X-ray photoelectron spectrometer (XPS) analysis provides additional details on the surface chemical properties of the Ru-Mo 2 C@CN photocatalyst.The N 1s spectra display three distinct peaks at 401.9, 400.2, and 398.7 eV, corresponding to terminal amino groups (C-N-H x ), tertiary nitrogen (N-C 3 ), and sp2-hybridized nitrogen (C=N-C), as described in references [34,35].Ru-Mo 2 C@CN shows a positive shift in the typical peaks of the N spectra compared to CN, suggesting unidirectional electron transmission from CN to Ru-Mo 2 C.This finding indicates that the close connections between Mo 2 C and CN could offer a successful route for electron transfer.Surprisingly, the XPS data (Table S1) showed that Ru NPs accounted for only 0.5% of the total surface elements in the Ru-Mo 2 C@CN, indicating that trace amounts of NPs can significantly reduce the dependence of HER on noble metals.The peaks at 484.3 and 462.1 eV in the Ru 3p spectra of Ru-Mo 2 C and Ru-Mo 2 C@CN are indicative of the Ru 3p 3/2 and 3p 1/2 associated with metallic Ru 0 [36,37], indicating the successful Ru NPs deposition on Mo 2 C (Figures 3d and S3).The Mo 3d spectrum of the Ru-Mo 2 C@CN sample (Figure 3e) can be deconvoluted into six peaks, which confirms the presence of Mo 4+ , Mo 5+ , and Mo 6+ species.The peaks at 229.7 (3d 5/2 ) and 232.9 eV (3d 3/2 ) are attributed to Mo 4+ ions, and the peaks observed at 233.0 (3d 5/2 ) and 236.2 (3d 3/2 ) belong to Mo 6+ ions [38,39], due to the partial oxidation of Mo 2 C. Additionally, the sub-oxide Mo 5+ peaks, which appear at 230.9 (3d 5/2 ) and 234.0 eV (3d 3/2 ), may contribute to the X-ray reduction of Mo 6+ in XPS tests [40,41].More importantly, the Mo 3D spectrum of Ru-Mo 2 C@CN exhibits a significant negative shift in comparison to the XPS spectra of the original Ru-Mo 2 C. The noticeable change is due to the successful interfacial interaction between the Mxene Mo 2 C and the CN nanosheets, resulting in a decrease in the bond strength of Mo-H in Ru-Mo 2 C@CN and boosting HER performance [42,43].This finding indicates that the close connections between Mo2C and CN could offer a successful route for electron transfer.Surprisingly, the XPS data (Table S1) showed that Ru NPs accounted for only 0.5% of the total surface elements in the Ru-Mo2C@CN, indicating that trace amounts of NPs can significantly reduce the dependence of HER on noble metals.The peaks at 484.3 and 462.1 eV in the Ru 3p spectra of Ru-Mo2C and Ru-Mo2C@CN are indicative of the Ru 3p3/2 and 3p1/2 associated with metallic Ru 0 [36,37], indicating the successful Ru NPs deposition on Mo2C (Figures 3d and S3).The Mo 3d spectrum of the Ru-Mo2C@CN sample (Figure 3e) can be deconvoluted into six peaks, which confirms the presence of Mo 4+ , Mo 5+ , and Mo 6+ species.The peaks at 229.7 (3d5/2) and 232.9 eV (3d3/2) are attributed to Mo 4+ ions, and the peaks observed at 233.0 (3d5/2) and 236.2 (3d3/2) belong to Mo 6+ ions [38,39], due to the partial oxidation of Mo2C.Additionally, the sub-oxide Mo 5+ peaks, which appear at 230.9 (3d5/2) and 234.0 eV (3d3/2), may contribute to the X-ray reduction of Mo 6+ in XPS tests [40,41].More importantly, the Mo 3D spectrum of Ru-Mo2C@CN exhibits a significant negative shift in comparison to the XPS spectra of the original Ru-Mo2C.The noticeable change is due to the successful interfacial interaction between the Mxene Mo2C and the CN nanosheets, resulting in a decrease in the bond strength of Mo-H in Ru-Mo2C@CN and boosting HER performance [42,43].The Fourier transform infrared spectroscopy (FTIR) of Figure 3f suggests that both Mo 2 C@CN and Ru-Mo 2 C@CN photocatalysts exhibit comparable characteristic peaks to CN.Two separate peaks around 806 and 884 cm −1 are likely caused by the vibrational modes related to the triazine units' breathing.Previous studies support the conclusion that triazine units are the primary molecular composition of CN materials, making this observation highly convincing [44].A broadband was observed in the 1233-1637 cm −1 region, which can be attributed to CN heterocyclic stretching vibration.In addition, the peaks at approximately 1243, 1330, 1412, 1464, 1572, and 1635 cm −1 are indicative of the presence of C-N bonds and C=N bond structures in the catalyst [44,45].A group of high points can be seen in the spectrum between 3068 and 3524 cm −1 , likely caused by the absorption of water molecules and subsequent stretching vibrations of N-H bonds [46].The appearance of spectral characteristic peaks of g-C 3 N 4 in the above-mentioned composite materials confirms that Mo 2 C doping does not damage the structure of CN.The Fourier transform infrared spectroscopy (FTIR) of Figure 3f suggests that both Mo2C@CN and Ru-Mo2C@CN photocatalysts exhibit comparable characteristic peaks to CN.Two separate peaks around 806 and 884 cm −1 are likely caused by the vibrational modes related to the triazine units' breathing.Previous studies support the conclusion that triazine units are the primary molecular composition of CN materials, making this observation highly convincing [44].A broadband was observed in the 1233-1637 cm −1 region, which can be attributed to CN heterocyclic stretching vibration.In addition, the peaks at approximately 1243, 1330, 1412, 1464, 1572, and 1635 cm −1 are indicative of the presence of C-N bonds and C=N bond structures in the catalyst [44,45].A group of high points can be seen in the spectrum between 3068 and 3524 cm −1 , likely caused by the absorption of water molecules and subsequent stretching vibrations of N-H bonds [46].The appearance of spectral characteristic peaks of g-C3N4 in the above-mentioned composite materials confirms that Mo2C doping does not damage the structure of CN.

Photocatalytic Performance
As shown in Figure 4a, the rapid rate of electron-hole pair complexation in CN leads to a low rate of H2 production through photocatalysis, with only minimal amounts of H2 detectable after 2 h of illumination.The combination of Mo2C and CN significantly

Photocatalytic Performance
As shown in Figure 4a, the rapid rate of electron-hole pair complexation in CN leads to a low rate of H 2 production through photocatalysis, with only minimal amounts of H 2 detectable after 2 h of illumination.The combination of Mo 2 C and CN significantly increases the H 2 production rate to 129.9 µmol•g −1 •h −1 by providing electron transfer pathways and reducing the recombination of photo-generated electron-hole pairs.The increase in hydrogen production rate indicates that Mo 2 C can serve as a co-catalyst to improve the efficiency of H 2 production in CN.Due to the large particle size of the bulk Mo 2 C, it could not be fully combined with CN.Therefore, a two-dimensional structure of Mxene Mo 2 C was chosen to bind with CN in this study.Substituting Mxene Mo 2 C for the original Mo 2 C in the composite material resulted in a renewed increase in the rate of photocatalytic H 2 production to 271.5 µmol•g −1 •h −1 , demonstrating that the 2D/2D pairing effectively enhanced the movement of photogenerated carriers, leading to a decrease in carrier recombination rate.It is common knowledge that Ru has emerged as a viable alternative to Pt for generating hydrogen through photocatalysis.The photocatalytic H 2 production performance of composite materials was further enhanced by doping 2D Mo 2 C with Ru.After introducing 0.5% Ru into Mo 2 C@CN, the photocatalysts reached a hydrogen production rate of 1776.4 µmol•g −1 •h −1 and exhibited an apparent quantum yield (AQE) of 3.58% at 400 nm (detailed calculations in attachment), surpassing the performance of most reported Mo 2 C/g-C 3 N 4 photocatalysts (Table S2).Importantly, to determine the successful construction of the heterojunction, we simply milled 10 mg of Ru-Mo 2 C with 90 mg of CN to obtain a physically mixed sample labeled Ru-Mo 2 C + CN.It was found that the hydrogen production rate of Ru-Mo 2 C + CN was much lower than that of Ru-Mo 2 C@CN, which can also be taken as evidence that the Ru-Mo2C@CN heterojunction was successfully constructed via electrostatic self-assembly (Figure S4).This excellent performance indicates that the 2D structure of Mo 2 C can serve as a good co-catalyst to improve the migration rate of photogenerated carriers, thereby further reducing the carrier complexation rate.In addition, the 2D structure of Mo 2 C can serve as a good intermediate carrier, providing more coverage areas for Ru as an active site.
original Mo2C in the composite material resulted in a renewed increase in the rate of photocatalytic H2 production to 271.5 µmol•g −1 •h −1 , demonstrating that the 2D/2D pairing effectively enhanced the movement of photogenerated carriers, leading to a decrease in carrier recombination rate.It is common knowledge that Ru has emerged as a viable alternative to Pt for generating hydrogen through photocatalysis.The photocatalytic H2 production performance of composite materials was further enhanced by doping 2D Mo2C with Ru.After introducing 0.5% Ru into Mo2C@CN, the photocatalysts reached a hydrogen production rate of 1776.4 µmol•g −1 •h −1 and exhibited an apparent quantum yield (AQE) of 3.58% at 400 nm (detailed calculations in attachment), surpassing the performance of most reported Mo2C/g-C3N4 photocatalysts (Table S2).Importantly, to determine the successful construction of the heterojunction, we simply milled 10 mg of Ru-Mo2C with 90 mg of CN to obtain a physically mixed sample labeled Ru-Mo2C + CN.It was found that the hydrogen production rate of Ru-Mo2C + CN was much lower than that of Ru-Mo2C@CN, which can also be taken as evidence that the Ru-Mo2C@CN heterojunction was successfully constructed via electrostatic self-assembly (Figure S4).This excellent performance indicates that the 2D structure of Mo2C can serve as a good co-catalyst to improve the migration rate of photogenerated carriers, thereby further reducing the carrier complexation rate.In addition, the 2D structure of Mo2C can serve as a good intermediate carrier, providing more coverage areas for Ru as an active site.Furthermore, it is crucial to consider the durability of the photocatalytic process, as demonstrated in Figure 4b, which illustrates the hydrogen generation rate by Ru-Mo2C@CN during a 10-h hydrogen production cycle.The photocatalyst showed good recyclability, as its hydrogen production rate decreased by less than 20% after five cycles.As shown in Figure S5, XRD images of the Ru-Mo2C@CN photocatalyst remain consistent before and after five cycles, indicating its good stability.

Photocatalytic Mechanism Analysis
To understand why the photocatalytic performance improved, the light absorption abilities of various photocatalysts were initially examined using UV-Vis.As shown in Figure 5a, the intrinsic absorption edges of Ru-Mo2C@CN redshift to 493 nm, compared to the original CN's inherent absorption edge at 468 nm.The redshift phenomenon is due to the strong visible light absorption ability of the Mo2C co-catalyst, indicating that Mo2C Furthermore, it is crucial to consider the durability of the photocatalytic process, as demonstrated in Figure 4b, which illustrates the hydrogen generation rate by Ru-Mo 2 C@CN during a 10-h hydrogen production cycle.The photocatalyst showed good recyclability, as its hydrogen production rate decreased by less than 20% after five cycles.As shown in Figure S5, XRD images of the Ru-Mo 2 C@CN photocatalyst remain consistent before and after five cycles, indicating its good stability.

Photocatalytic Mechanism Analysis
To understand why the photocatalytic performance improved, the light absorption abilities of various photocatalysts were initially examined using UV-Vis.As shown in Figure 5a, the intrinsic absorption edges of Ru-Mo 2 C@CN redshift to 493 nm, compared to the original CN's inherent absorption edge at 468 nm.The redshift phenomenon is due to the strong visible light absorption ability of the Mo 2 C co-catalyst, indicating that Mo 2 C can promote the unidirectional electronic transmission from CN to Ru-Mo 2 C. Compared with Mo 2 C@CN, the absorbance of the photocatalyst can be slightly improved in the case of Ru-Mo 2 C doping, which indicates that Ru NPs as a co-catalyst can enhance the light-harvesting ability of composite materials.In addition, the overall absorbance of Ru-Mo 2 C@CN showed a significant improvement, which may be related to the influence of Ru clusters on the Mo lattice.
As shown in Figure 5b, Kubelka-Munk functional plots of three photocatalysts were obtained based on the UV-visible photometric spectral transformation.The bandgap value of Ru-Mo 2 C@CN (2.37 eV) is slightly lower than that of Mo 2 C@CN (2.40 eV) and much lower than that of CN (2.52 eV), which demonstrates the formation of a Schottky junction between Mo 2 C and CN, leading to a decrease in the bandgap.CN has a higher bandgap value of 2.52 eV compared to Mo 2 C@CN with 2.40 eV, whereas Ru-Mo 2 C@CN has a lower bandgap value of 2.37 eV.The difference in bandgap can be attributed to the close contact between Mo 2 C and CN, which generates Schottky junctions and decreases the bandgap of Mo 2 C@CN and Ru-Mo 2 C@CN.Furthermore, the reduction in the band gap suggests that the presence of a Mo 2 C co-catalyst enhances the ability of CN to be stimulated by visible light, aligning with the enhanced photocatalytic efficiency of Ru-Mo 2 C@CN.
Molecules 2024, 29, 1684 7 of 13 can promote the unidirectional electronic transmission from CN to Ru-Mo2C.Compared with Mo2C@CN, the absorbance of the photocatalyst can be slightly improved in the case of Ru-Mo2C doping, which indicates that Ru NPs as a co-catalyst can enhance the lightharvesting ability of composite materials.In addition, the overall absorbance of Ru-Mo2C@CN showed a significant improvement, which may be related to the influence of Ru clusters on the Mo lattice.As shown in Figure 5b, Kubelka-Munk functional plots of three photocatalysts were obtained based on the UV-visible photometric spectral transformation.The bandgap value of Ru-Mo2C@CN (2.37 eV) is slightly lower than that of Mo2C@CN (2.40 eV) and much lower than that of CN (2.52 eV), which demonstrates the formation of a Schottky junction between Mo2C and CN, leading to a decrease in the bandgap.CN has a higher bandgap value of 2.52 eV compared to Mo2C@CN with 2.40 eV, whereas Ru-Mo2C@CN has a lower bandgap value of 2.37 eV.The difference in bandgap can be attributed to the close contact between Mo2C and CN, which generates Schottky junctions and decreases the bandgap of Mo2C@CN and Ru-Mo2C@CN.Furthermore, the reduction in the band gap suggests that the presence of a Mo2C co-catalyst enhances the ability of CN to be stimulated by visible light, aligning with the enhanced photocatalytic efficiency of Ru-Mo2C@CN.
To explore the effect of Mo2C on the semiconductor band gap, M-S measurements of the prepared catalyst were performed at a frequency of 200 Hz (Figure 5c).To find the flat band potential (EFB), extend the linear part of the M-S graph to the x-axis and locate the point where it intersects.In addition, the intercepts of the three catalysts in the M-S plot are all positive, indicating the characteristics of their n-type semiconductors [47].Generally speaking, the conduction band potential (ECB) of n-type semiconductors is 0.2 eV greater than the EFB [48].Therefore, the ECB for CN, Mo2C@CN, and Ru-Mo2C@CN concerning the Normal Hydrogen Electrode (NHE) are −1.33,−1.17, and −1.14 V, respectively.To explore the effect of Mo 2 C on the semiconductor band gap, M-S measurements of the prepared catalyst were performed at a frequency of 200 Hz (Figure 5c).To find the flat band potential (E FB ), extend the linear part of the M-S graph to the x-axis and locate the point where it intersects.In addition, the intercepts of the three catalysts in the M-S plot are all positive, indicating the characteristics of their n-type semiconductors [47].Generally speaking, the conduction band potential (E CB ) of n-type semiconductors is 0.2 eV greater than the E FB [48].Therefore, the E CB for CN, Mo 2 C@CN, and Ru-Mo 2 C@CN concerning the Normal Hydrogen Electrode (NHE) are −1.33,−1.17, and −1.14 V, respectively.Based on the preceding results, Figure 5d illustrates the band structure of Ru-Mo 2 C@CN.When exposed to light, a large quantity of photoexcited electrons are produced within CN and jump into the conduction band (CB).Photoexcited electrons from CN are moved to Mo 2 C with a lower CB via the Schottky junction, then quickly transferred to Ru NPs for the reaction with H + at the active site to produce hydrogen gas.The 0.16 V Schottky barriers between the CB of Mo 2 C and CN promote unidirectional electron migration.
To better understand the complexation process of photoexcited electron-hole pairs, photoluminescence (PL) spectra and TRPL spectra were used to analyze carriers' behavior.Figure 6a shows the PL spectra of CN, Mo 2 C@CN, and Ru-Mo 2 C@CN.Due to electronic transitions between bands, each photocatalyst exhibits a distinct emission peak at approximately 470-490 nm under excitation at 325 nm, which is consistent with previous research results.Mo 2 C@CN exhibits lower PL intensity than CN, indicating successful suppression of electron-hole recombination through rapid charge transfer from CN to Mo 2 C. The Ru-Mo 2 C@CN composite shows a slightly elevated PL intensity compared to Mo 2 C@CN, accompanied by a significant peak redshift.This observation suggests that the Ru NPs may act as catalytic sites for HER, thereby enhancing the production rate of electron-hole pairs within the system, which is consistent with the results of UV-vis testing.The more electron-hole pairs are generated, the more electron-hole pairs are recombined, ultimately leading to higher PL intensity.As shown in Figure 6b, the TRPL spectra of the prepared catalyst all conform to double exponential fitting (detailed data in Table S3).Among them, τ 1 denotes the duration of fluorescence decay for electrons transitioning from the conduction band to the valence band of CN, while τ 2 denotes the duration of fluorescence decay for the recombination of electron-hole pairs generated by light on the CN.The average fluorescence lifetime of the TRPL spectra was calculated by the following equation [49]: Molecules 2024, 29, 1684 9 of 13

Preparation of 2D g-C3N4 Nanosheets (CNs)
2D g-C3N4 nanosheets were prepared using urea as a precursor.According to the existing methods of our group [53], a ceramic crucible was filled with 25 g of urea, covered with a lid, then heated in a muffle furnace at a heating rate of 5 °C•min −1 until reaching 550 °C and maintained for 4 h.After cooling to room temperature, the resulting material was transferred to another ceramic crucible, heated again in the furnace at the same rate to 500 °C, and kept for 2 h.The final product, 2D g-C3N4 nanosheets, was obtained after grinding and labeling CN.Following the addition of Mo 2 C co-catalysts, both τ 2 and τ were significantly reduced, whereas the presence of Ru NPs had a minimal impact on τ 2 and τ.The decreased duration of τ 2 could be attributed to the quick transfer of electrons at the interface from the CN in an excited state to Mo 2 C. Further, Ru NPs barely affect the electron transfer rate.
To further investigate the effect of Ru-Mo 2 C on the interfacial electron migration rate in ternary catalysts, we used photoelectrochemical (PEC) detection to investigate the effect of Mo 2 C and Ru NPs on the migration rate of photogenerated electrons.The Nyquist plot of the EIS was conducted to analyze the characteristics of charge transfer resistance (Figure 6c), in which the resistance of all photocatalysts exhibited similar semicircle curves [50].The Ru-Mo 2 C@CN has the smallest arc radius, indicating the lowest resistance to charge transfer at the interface, which aligns with its superior photocatalytic activity.Moreover, compared with CN, Mo 2 C@CN and Ru-Mo 2 C@CN samples show higher photocurrent density (Figure 6d), indicating that the introduction of Mo 2 C improved the rate of electron generation and migration.Linear sweep voltammetry (LSV) can further demonstrate the functions of Ru-Mo 2 C as a co-catalyst in the photocatalytic process.As shown in Figure S6, the Ru-Mo 2 C@CN photocatalyst exhibits enhanced cathode current and a more advantageous potential in comparison to CN and Mo 2 C@CN.This indicates that Ru NPs can enhance the surface reaction [51].These results indicate that the Mo 2 C in Ru-Mo 2 C@CN can increase the rate of electron generation and migration, while the Ru on the surface can serve as an active site to significantly enhance the reduction effect of H + in hydrogen production reactions [52].2D g-C 3 N 4 nanosheets were prepared using urea as a precursor.According to the existing methods of our group [53], a ceramic crucible was filled with 25 g of urea, covered with a lid, then heated in a muffle furnace at a heating rate of 5 • C•min −1 until reaching 550 • C and maintained for 4 h.After cooling to room temperature, the resulting material was transferred to another ceramic crucible, heated again in the furnace at the same rate to 500 • C, and kept for 2 h.The final product, 2D g-C 3 N 4 nanosheets, was obtained after grinding and labeling CN. 1 g of LiF and 0.8 g of Mo 2 GaC were added to the inner tank of the reactor, then 30 mL of an 8 M HCl solution was added to the inner tank of the reactor and stirred for 12 h.Then, heat the reactor in a 180 • C oven for 24 h.After natural cooling to room temperature, let it stand for 8 h to obtain the black solid.Wash the black solid with water and ethanol again, and centrifuge three times to obtain the bottom sediment.Finally, the bottom sediment was placed in a vacuum drying oven at 50 • C and dried for 8 h to obtain MXene Mo 2 C. Ru-Mo 2 C was prepared in the same conditions as Mo 2 C. Differently, put 1 g of LiF, 0.8 g of Mo 2 Ga 2 C, and 40 mg of RuCl 3 •3H 2 O into the inner tank of the reactor before adding 30 mL of an 8 M hydrochloric acid solution.The remaining steps are identical to preparing Mo 2 C.

Preparation of Bulk Mo 2 C
A total of 1.18 g of (NH 4 ) 6 Mo 7 O 24 •4H 2 O was taken with 210 mg of C 2 H 4 N 4 and fully milled.Subsequently, the mixture was transferred into a tube furnace and subjected to heating at a rate of 5 • C•min −1 in the N 2 atmosphere condition until reaching a temperature of 750 • C, where it was maintained for 8 h.Following the natural cooling process to ambient temperature, thoroughly grind and calcine the product to obtain bulk Mo 2 C. 3.2.4.Preparation of MXene Mo 2 C@CN, Bulk Mo 2 C@CN, and Ru-Mo 2 C@CN Composites 10 mg of different Mo 2 C species and 90 mg of CN nanosheets were dispersed in 40 mL of an aqueous solution containing 50% ethanol and stirred overnight.The solid particles were separated through centrifugation, rinsed with deionized water, and subsequently desiccated in a vacuum oven to obtain MXene Mo 2 C@CN, bulk Mo 2 C@CN, and Ru-Mo 2 C@CN, respectively.

Figure 4 .
Figure 4. (a) The H 2 evolution efficiency of all photocatalysts; (b) stability test of Ru-Mo 2 C@CN.