Nanocatalysis MoS2/rGO: An Efficient Electrocatalyst for the Hydrogen Evolution Reaction

In this study, a systematic investigation of MoS2 nanostructure growth on a SiO2 substrate was conducted using a two-stage process. Initially, a thin layer of Mo was grown through sputtering, followed by a sulfurization process employing the CVD technique. This two-stage process enables the control of diverse nanostructure formations of both MoS2 and MoO3 on SiO2 substrates, as well as the formation of bulk-like grain structures. Subsequently, the addition of reduced graphene oxide (rGO) was examined, resulting in MoS2/rGO(n), where graphene is uniformly deposited on the surface, exposing a higher number of active sites at the edges and consequently enhancing electroactivity in the HER. The influence of the synthesis time on the treated MoS2 and also MoS2/rGO(n) samples is evident in their excellent electrocatalytic performance with a low overpotential.


Introduction
The present state of humanity is genuinely concerning due to the predominant use of current fuels, specifically fossil fuels.Not only are these fuels the primary contributors to climate change through the release of carbon dioxide [1,2], but they also pose a direct threat to human health.Additionally, relying solely on these fuels is unsustainable in meeting the growing energy demands, impacting both industries and modern civilization.This situation poses a significant risk to the global economy and the well-being of humanity as a whole [3,4].For these reasons, there is a growing interest in the search of cleaner fuels, with hydrogen emerging as the most promising alternative [5].Several processes for the production of hydrogen have been explored, including photovoltaic, wind and electrochemical water splitting [5][6][7][8][9][10][11], the electrochemical one being the most striking alternative, due to the evolution of hydrogen (HER) [12][13][14].
In this context, although the division of water by HER plays a fundamental role in the generation of hydrogen from hydrogen, it implies that the electrocatalyst to be used must possess outstanding physicochemical properties, such as fast kinetics in electron and proton transfer, a large number of active sites, a low energy barrier in water division, rapid adsorption/desorption of hydrogen ions, high electrocatalytic performance and low overpotential [15][16][17].However, the electrocatalysts with the aforementioned characteristics, such as Pt [18], Re [19], Pd [20], Ir [21], Rh [22] and Au [12], are noble metals that are both scarce and expensive, hindering their industrialization [23][24][25].Therefore, researchers aiming for the industrialization of hydrogen fuel have shifted their focus to exploring new electrocatalysts made from more abundant materials, especially transition metals [26].Among these, the most attractive options are transition-metal dichalcogenides (TMDs) such as WS 2 , ZrS 2 , MoSe 2 , ZrSe 2 , WSe 2 , ZrSSe and MoS 2 [27][28][29][30][31].The most striking of the dichalcogenides for HER applications is molybdenum disulfide (MoS 2 ), since it offers high electrocatalytic properties due to its high number of active sites, a highly stable crystalline structure, and a mode of highly efficient mass transport to the active center, favoring HER applications [32].These properties can be seen through theoretical calculations made by Nørskov et al., revealing that MoS 2 ranks at the top of the "hydrogen adsorption volcano".They reported a Gibbs free energy value in hydrogen adsorption (∆G H ) of ~0.08 eV, which is comparable to the previously mentioned precious-metal-based electrocatalysts [16].Due to its remarkable electrocatalytic properties, various MoS 2 synthesis strategies have been proposed such as mechanical stripping [33], colloidal methods [34], liquid phase stripping [35], chemical intercalation [36], etching [37], electrodeposition [38,39], the heterogeneous structure method [40,41], green synthesis [42,43] and chemical vapor deposition (CVD) [44].The heterogeneous structure synthesis method involves the growth of molybdenum disulfide on the substrate surface, with interactions with carbon-based nanomaterials [45].This method holds significant promise for potential use in large-scale industrial hydrogen production.In this context, several research groups have actively been searching for new materials with electrocatalysts properties [46][47][48][49][50][51].
Therefore, this work differs from the traditional methods of MoS 2 synthesis previously described, particularly from CVD methods that typically involve MoO 3 powders, which do not allow control over the homogeneity of MoS 2 deposition.In this study, a systematic investigation of MoS 2 nanostructure growth on a SiO 2 substrate was conducted using a two-stage process.Initially, a thin layer of Mo was grown through sputtering, followed by a sulfurization process employing the CVD technique.This two-stage process enables the control of diverse nanostructure formations of both MoS 2 and MoO 3 on SiO 2 substrates, as well as the formation of bulk-like grain structures.Subsequently, the addition of reduced graphene oxide (rGO) was examined, resulting in MoS 2 /rGO(n), where graphene is uniformly deposited on the surface, exposing a higher number of active sites at the edges and consequently enhancing electroactivity in the HER.The influence of the synthesis time on the treated MoS 2 and also on MoS 2 /rGO(n) samples is evident in their excellent electrocatalytic performance with a low overpotential.

Materials' Characterization by SEM, EDX, XPS and Raman
The FE-SEM images in Figure 1 show the formation of elongated microstructures of MoS 2 corresponding to the CVD-treated sample derived from a Mo thin film deposited for 25 min using sputtering.The color images correspond to EDS mappings of Mo, S and C in the micrographs.It is noticeable that the structures are primarily composed of S and Mo, represented by the highlighted colors in Figure 1.Additional information regarding the SEM images is provided in the Supplementary Material in Figures S1 and S2.From these figures, it can be observed that as the deposition time of the Mo thin film exposed to CVD thermal treatment increases, the type of formed structures changes.In Figure S1, for the thermal treatment carried out on the Mo thin film deposited for 15 min, only island-like structures are formed, whereas with an increase in the deposition time in sputtering, and consequently, the thickness of the Mo film, larger structures in the form of disks or bars are observed; the size of the MoS 2 nanoparticles can be seen in Table S1.As shown in Figure S2, EDS mappings are presented alongside the SEM images.In these mappings, sulfur is detected in the structures for each case; however, this could be attributed to false readings due to overlap with the molybdenum peak.Subsequent Raman and XPS analyses clarify this effect.XRD is an important characterization method for reflecting the composition and crystal structure of synthesized materials.Figure S3 shows the XRD of MoS2 heterostructures.The peaks at 14.4°, 33.00° and 61.7°, correspond to the (002), ( 101) and (110) 2H MoS2 phase planes (JCPDS card n°.37-1492).
The Raman spectra were obtained for MoS2 growth at several deposition times.Figure 2 show the characteristic peaks of MoS2 appearing at 387 and 412 cm −1 , observed in the sample at 25 min.The separation between those peaks (~25 cm −1 ) suggests a bulk-like structure [52].In the sample deposited for 30 min, a characteristic band of MoO3 is observed at 823 cm −1 , corresponding to the M=O stretch mode [53] for MoO3.In the case of the sample deposited for 15 min, the presence of MoO3 or MoS2 is not observed, indicating either a very thin deposition or the absence of material.XRD is an important characterization method for reflecting the composition and crystal structure of synthesized materials.Figure S3 shows the XRD of MoS 2 heterostructures.The peaks at 14.4 • , 33.00 • and 61.7 • , correspond to the (002), ( 101) and (110) 2H MoS 2 phase planes (JCPDS card n • .37-1492).
The Raman spectra were obtained for MoS 2 growth at several deposition times.Figure 2 show the characteristic peaks of MoS 2 appearing at 387 and 412 cm −1 , observed in the sample at 25 min.The separation between those peaks (~25 cm −1 ) suggests a bulk-like structure [52].In the sample deposited for 30 min, a characteristic band of MoO 3 is observed at 823 cm −1 , corresponding to the M=O stretch mode [53] for MoO 3 .In the case of the sample deposited for 15 min, the presence of MoO 3 or MoS 2 is not observed, indicating either a very thin deposition or the absence of material.XRD is an important characterization method for reflecting the composition and crystal structure of synthesized materials.Figure S3 shows the XRD of MoS2 heterostructures.The peaks at 14.4°, 33.00° and 61.7°, correspond to the (002), ( 101) and (110) 2H MoS2 phase planes (JCPDS card n°.37-1492).
The Raman spectra were obtained for MoS2 growth at several deposition times.Figure 2 show the characteristic peaks of MoS2 appearing at 387 and 412 cm −1 , observed in the sample at 25 min.The separation between those peaks (~25 cm −1 ) suggests a bulk-like structure [52].In the sample deposited for 30 min, a characteristic band of MoO3 is observed at 823 cm −1 , corresponding to the M=O stretch mode [53] for MoO3.In the case of the sample deposited for 15 min, the presence of MoO3 or MoS2 is not observed, indicating either a very thin deposition or the absence of material.In Supplementary Material Figure S4, a survey spectrum of the growth samples is presented.In these spectra, notable peaks include Mo 3d and Mo 3p, S 2p, corresponding to MoS 2 , along with Si 2p, Si 2s and O 1s peaks associated with the SiO 2 substrate.Finally, the C 1s peak attributed to adventitious carbon and rGO is observed.XPS high-resolution spectra of the Mo 3d region are show in Figure 3A.The spectra of sample growth at 25 min showed the Mo(IV) oxidation state (magenta) at 229.5 eV and its doublet at 232.7 eV of binding energy.The presence of the S 2s state at 227 eV (green), characteristic of MoS 2 , is also observed.The metal Mo 3d5/2-3/2 peak typically appears at 227.8 eV with a splitting of ~3.13 eV.For samples grown for 15 and 30 min, the Mo(VI) oxidation state (blue) was observed at a binding energy of 233.2 eV [54].In the XPS high-resolution spectra of the S 2p region in Figure 3B, a shift in the S 2p region was found at 163.9 eV [55] for the sample grown for 25 min.
In Supplementary Material Figure S4, a survey spectrum of the growth samples i presented.In these spectra, notable peaks include Mo 3d and Mo 3p, S 2p, corresponding to MoS2, along with Si 2p, Si 2s and O 1s peaks associated with the SiO2 substrate.Finally the C 1s peak attributed to adventitious carbon and rGO is observed.XPS high-resolution spectra of the Mo 3d region are show in Figure 3A.The spectra of sample growth at 25 min showed the Mo(IV) oxidation state (magenta) at 229.5 eV and its doublet at 232.7 eV of binding energy.The presence of the S 2s state at 227 eV (green), characteristic of MoS2 is also observed.The metal Mo 3d5/2-3/2 peak typically appears at 227.8 eV with a split ting of ~3.13 eV.For samples grown for 15 and 30 min, the Mo(VI) oxidation state (blue was observed at a binding energy of 233.2 eV [54].In the XPS high-resolution spectra o the S 2p region in Figure 3B, a shift in the S 2p region was found at 163.9 eV [55] for th sample grown for 25 min.In the 15 min sample, the observed shift corresponds to the characteristic sulfur tran sition in a non-stoichiometric molybdenum sulfide compound.In the 30 min sample, no sulfur presence was observed, corroborating the findings of the micro-Raman spectra which only detect the characteristic transition of Mo(VI) species at 233.1 eV.

Electrochemical Characterization
To determine the optimal growth time of MoS2 on the SiO2 surface with prominen electrocatalytic properties for HER applications, a study of the catalytic properties of th MoS2 synthesized for 0, 15, 20, 25 and 30 min was carried out by means of the LSV tech nique in 0.5 M H2SO4 with a scan rate of 2 mV s −1 (Figure S5).The study revealed overpo tentials (measured at 5 mA cm −2 ) of 0.761, 0.533, 0.423, 0.320 and 0.421 vs. RHE for 0, 15 20, 25 and 30 min, respectively.Therefore, based on the polarization curves and supported by the XPS in Figure 3, we can conclude that the optimal synthesis time for MoS2 was 25 min.At 15 and 20 min, there is a mixture of Mo(IV) and Mo(VI), and after 30 min, it begin to resemble MoO3 (see Figure S4).On the other hand, the influence of rGO to improve th catalytic properties of MoS2 was explored; On the other hand, the influence of rGO to In the 15 min sample, the observed shift corresponds to the characteristic sulfur transition in a non-stoichiometric molybdenum sulfide compound.In the 30 min sample, no sulfur presence was observed, corroborating the findings of the micro-Raman spectra, which only detect the characteristic transition of Mo(VI) species at 233.1 eV.

Electrochemical Characterization
To determine the optimal growth time of MoS 2 on the SiO 2 surface with prominent electrocatalytic properties for HER applications, a study of the catalytic properties of the MoS 2 synthesized for 0, 15, 20, 25 and 30 min was carried out by means of the LSV technique in 0.5 M H 2 SO 4 with a scan rate of 2 mV s −1 (Figure S5).The study revealed overpotentials (measured at 5 mA cm −2 ) of 0.761, 0.533, 0.423, 0.320 and 0.421 vs. RHE for 0, 15, 20, 25 and 30 min, respectively.Therefore, based on the polarization curves and supported by the XPS in Figure 3, we can conclude that the optimal synthesis time for MoS 2 was 25 min.At 15 and 20 min, there is a mixture of Mo(IV) and Mo(VI), and after 30 min, it begins to resemble MoO 3 (see Figure S4).On the other hand, the influence of rGO to improve the catalytic properties of MoS 2 was explored; On the other hand, the influence of rGO to improve the catalytic properties of MoS 2 was explored; as was recently reported, due to the strong van der Waals, the electrostatic interactions between rGO and MoS 2 are strong [56,57].In this sense, MoS 2 (synthesized for 25 min) was doped with different concentrations of rGO (1, 2 and 4 mg L −1 ), as seen in Figure S6.The correlation in the improvement of water reduction is evident in the LSV curves shown in Figure S5.The best performance for the HER was achieved with the MoS 2 /rGO(2) electrode instead of the MoS 2 /rGO(4).This could be attributed to the aggregation caused by the π-π interactions between the layers of rGO [58].This effect decreases the active sites of MoS 2 and causes MoS 2 /rGO(4) to require a higher overpotential than MoS 2 /rGO (2). Figure 4A depicts the LSV curve of the as-obtained Pt/C, bare SiO 2 , MoS 2 , SiO 2 /rGO and MoS 2 /rGO(2) at a scan rate of 2 mV s −1 .As it can be seen, the MoS 2 /rGO(2) electrode exhibited excellent electrocatalytic activity for HER compared to that of SiO 2 , MoS 2 and SiO 2 /rGO electrodes.Accordingly, a shift in the onset potential towards a less negative potential value and a benchmark current density of 10 mA cm −2 at a very low overpotential (−0.176V) can be noticed.An important aspect is to see by which mechanism it produces hydrogen, in which the adsorption of hydrogen is vital [59], and why the HER mechanism exists and whether it is Volmer-Tafel or Volmer-Heyrovsky, considering these Tafel plots were carried out.The catalytic activity of all the synthesized electrocatalysts was further investigated and studied via Tafel slope, as presented in Figure 4B.The Tafel slope was calculated using the observed LSV curves.The HER pathways and related kinetics were identified using the Tafel plots, generated with the equation overpotential (η) = a + b log |j|, where b is the Tafel slope, a is the intercept, η is the overpotential and j is the current density [60].The long-term durability of the MoS 2 /rGO(2) electrode was tested by recording LSV polarization curves before and after 1000, 2000 and 3000 consecutive potential cycles (scan rate 2 mV s −1 ), and by applying a constant current of 15 mA cm −2 for 27 h.The interaction between the nanostructures (interstitial S atoms) and rGO improves its stability and catalytic properties.Significant changes in the LSV were observed in MoS 2 ; however, this did not occur in MoS 2 /rGO(2) (Figure 4C), as it was also evident in the chronopotentiometric curve (Figure 4D).This suggests that the MoS 2 /rGO(2) presents efficient catalyst durability for HER applications, indicating that the interaction between MoS 2 and rGO improves its catalytic properties and provides greater stability.The overpotentials and Tafel slopes of the above-described materials were compared with those of other catalysts recently described in the literature (Table 1).The MoS 2 derivatives presented good electrochemical parameters, but MoS 2 /rGO(2) exhibited the best ones, and is expected to outperform all the others for HER.

Materials Synthesis
The growth of MoS 2 /rGO(n) nanostructures was conducted in a two-stage procedure.Initially, metallic molybdenum (Mo 99.95%) was deposited using sputtering on a 285 nm of thermal silicon dioxide for different synthesis times (15, 20, 25 and 30 min).Then, a treatment was performed using chemical vapor deposition (CVD) at 700 • C for 20 min.For this, 10 mg of sulfur powder was placed at the furnace entrance, and a mixture of Ar/H 2 gases in a 4:1 ratio was introduced.The distance between molybdenum and sulfur was kept at 10 cm.After the reaction, the oven was cooled to room temperature.To determine the optimal synthesis time for MoS 2 , 30 µL of aqueous reduced graphene oxide (rGO) was added via drop casting at different concentrations (1, 2 and 4 mg mL −1 ) to obtain MoS 2 /rGO(1), MoS 2 /rGO(2) and MoS 2 /rGO(4), in order to observe the influence of rGO on the catalytic properties of MoS 2 .

Materials Characterization
For the Raman spectroscopy characterizations, a Witech Alpha300 micro-Raman system (Oxford Instruments, Abingdon, UK) was used, employing a 514 nm laser with a 2 mm spot size and a power of 2 mW.XPS spectra were obtained using a Surface Analysis Station 1, model XPS RQ300/2 equipped with a DESA150 detector/2700 V (STAIB Instruments, Minneapolis, MN, USA), with aluminum X-ray radiation of 1486.6 eV.The shift due to charging of the XPS spectra was corrected using the characteristic peak of adventitious carbon (C 1s) at its position of 284.8 eV.The spectra were analyzed using Casa XPS software (version 2.3.25), and for fitting, a 70% Gaussian/30% Lorentzian symmetric line-shape was employed.High-resolution electron microscopy analyses were performed using the SU500 Hitachi equipment (Hitachi, Tokyo, Japan).The surface morphology of the MoS 2 (20, 25 and 30 min) and MoS 2 /nGO was investigated using a FE-SEM, Hitachi Brand, model SU5000, equipped with XFlash 6I30 detectors (Bruker brand, Billerica, MA, USA).The composition and distribution of the MoS 2 (20, 25 and 30 min) and MoS 2 /GO(n) were ascertained by performing elemental analysis using an EDX instrument.

Conclusions
A two-stage method was proposed for synthesizing MoS 2 nanostructures on the surface of a bare SiO 2 electrode, involving sputtering and chemical vapor deposition.The chemical composition of MoS 2 was controlled by maintaining a constant temperature of 700 • C during the thermal treatment, and the formation of nanostructures was explored using different thicknesses of molybdenum film.The two-stage method proposes an improvement in controlling the type of MoS 2 nanostructures and the formation of bulk-like grains, according to Raman spectroscopy analysis.Also, XPS spectra reveal the presence of oxidation states corresponding to Mo(IV), and the shift in the peaks of the S 2p orbital confirms the formation of MoS 2 .Subsequently, the synthesized MoS 2 was functionalized with rGO, and MoS 2 /rGO(2) demonstrated the best electrocatalytic performance.MoS 2 /rGO(2) exhibited enhanced electrocatalytic activity with a smaller Tafel slope of 63.7 mV dec −1 and a more positive onset overpotential of 176 mV, making it a promising electrocatalyst for HER in practical applications.The HER was observed to be strongly modulated by the heteroatom nature and content within the graphenic structure, as well as the interaction with MoS 2 .These results reveal the best performance for the HER in acidic media for MoS 2 /rGO(2).This is attributed to a precise amount of interstitial S atoms between graphene layers, which facilitates an increase in electron transfer.Additionally, the strong interaction between MoS 2 and rGO contributes to an expansion degree of the S-Mo-S bonds.The HER activity was improved by adjusting the ratio of MoS 2 and rGO.These results indicate that the MoS 2 /rGO electrocatalyst is a promising and cost-effective alternative for manufacturing electrodes, offering a potential solution for industrial-level hydrogen production.

Figure 3 .
Figure 3. XPS synthesis of MoS2/rGO(n) at 15, 20, 25 and 30 min (A,B).Magenta lines of group A correspond to Mo(IV) and the blue ones are Mo(VI) (A).High-resolution XPS peak-shape of Mo 3d and S 2p, respectively.

Figure 3 .
Figure 3. XPS synthesis of MoS 2 /rGO(n) at 15, 20, 25 and 30 min (A,B).Magenta lines of group A correspond to Mo(IV) and the blue ones are Mo(VI) (A).High-resolution XPS peak-shape of Mo 3d and S 2p, respectively.
: SEM images for MoS 2 samples grown through the thermal treatment of Mo thin films deposited via sputtering for 15 min (A), 20 min (E), 25 min (I) and 30 min (M).High magnification images are shown for MoS 2 surfaces prepared on Mo thin films deposited via sputtering for 15 min (A-D), 20 min (E-H), 25 min (I-L) and 30 min (M-P); Figure S2: SEM images for the MoS 2 samples grown via thermal treatment of Mo thin films deposited by sputtering for (A) 15, (D) 20, (G) 25, and (J) 30 minutes and EDS element mapping of Mo (B, E, H and K) and S (C, F, I and L) for each respective sample; Figure S3: X-ray diffraction patterns of MoS 2 ; Figure S4: Survey spectra for the samples after the CVD process.The highlighted regions correspond to the different spectral peaks of the elements involved in the CVD process, Mo,

Table 1 .
Comparison of HER performances non-noble metals.

Table 1 .
Comparison of HER performances non-noble metals.