Aggregation induced edge sites actuation of 3D MoSe2/rGO electrocatalyst for high‐performing water splitting system

2D materials are regarded as promising electrocatalysts for water splitting because of their advances in providing ample active sites and improving electrochemical reaction kinetics. 2D MoSe2 has a greater intrinsic electrical conductivity and lower Gibbs free energy for reactant adsorption. However, there is still room for improvement in the electrocatalytic performance of MoSe2 for high‐performance electrochemical water splitting devices. Herein, the in situ preparation of heterostructure made of covalently bonded MoSe2 and rGO is reported. The obtained electrocatalyst contains the aggregated 3D structured MoSe2 over rGO, which is covalently bonded together with more edge sites. The active edge sites of MoSe2/rGO are dynamically involved in the electrocatalytic activity while facilitating electron transfer. Hence, the MoSe2/rGO heterostructure requires a low cell voltage of 1.64 V to reach 100 mA cm−2 in water splitting with high reaction kinetics. The aggregated MoSe2 over rGO with more edge sites exposed by the 3D structure of MoSe2 and the interfacial covalent bond in between them provides a favorable electronic structure for the HER and OER with low overpotentials and high current densities and enhances the stability of the electrocatalyst. This work presents an attractive and cost‐effective electrocatalyst suitable for industrial‐scale hydrogen fuel production.

massive disparity between energy demand and supply. [2]The limited efficiency and energy transportation difficulties advocate hydrogen as an effective energy carrier for a sustainable and trustworthy future. [3]Electrochemical water splitting devices using water and renewable electricity are considered clean and eco-friendly methods to produce hydrogen. [4]he simultaneous occurrence of the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode in an electrochemical cell plays a crucial role in facilitating efficient hydrogen production.The heterogeneous electrocatalyst facilitates the cathodic HER and anodic OER, which includes active sites for analyte adsorption and enhances the reaction kinetics. [5]ompared to homogeneous catalysis, heterogeneous catalysis offers many benefits, including easy recycling, operating under extreme heat and pressure, and significant industrial value.Since they catalyze more than 85% of all commercial electrocatalytic processes, heterogeneous electrocatalysts are the choice of chemists worldwide. [6,7]10][11] Compared to other heavy metals in the transition metal family, molybdenum (Mo) is considered reasonably safe for living things.Mo has been discovered to be a crucial trace element for the typical growth of plants and animals; no cases of poisoning have yet been reported. [12,13]nvironment-friendly Mo-based electrocatalysts have been employed as efficient electrocatalysts for several processes, including hydrogen evolution, oxygen evolution, nitrogen reduction, and hydrogenolysis.Moreover, Mo-based electrocatalysts are extensively studied for water splitting applications due to their similar Fermi level and 3d-orbital structure to noble metal Pt, low toxicity, high abundance, and easier H bond formation. [14]However, the highly empty orbitals of Mo unfavorably promote strong adsorption of H, requiring higher energy to desorb H 2 from the surface.[25][26] While 2D MoSe 2 is expected to provide a large surface area with more active sites for the adsorption/desorption of reactants/products.2D MoSe 2 is reported to be excellent at reducing hydrogen in an acidic environment; its HER is slow in an alkaline environment due to the high hydrolysis energy and activation energy. [27,28]lso, 2D MoSe 2 faces shortcomings, such as less active sites, less activation of active sites, low electrical conductivity, and instability, which can be addressed in two ways: (1) By using structural engineering, more vibrant places can be created for the electrocatalytic reactions, [2,11,16] (2) electronic modification to increase intrinsic activity. [29,30]heoretical and practical results show that electrocatalyst performance correlates with the number and efficiency of active sites and their presence for a longer duration.[33][34] The basal surfaces are made up of catalytically inert sheets, and the edges contain several unsaturated bonds and, thus, unpaired electrons.[37][38][39] The general property of 2D MoSe 2 is the crumpling of their sheets due to the presence of van der Waals force between the Mo and Se layers.Compositing the electrically conductive materials with redox-active 2D MoSe 2 can achieve remarkably enhanced electrocatalytic activity and stability. [31,32,40,41]In addition, it is important to consider the formation of MoSe 2 on the conductive substrate and the bonding between the single components in a heterostructure, as it can significantly impact the active sites and the catalytic properties of the materials.44] Herein considering the crucial aspects of designing an electrocatalyst for OER, HER, and water splitting, we have synthesized MoSe 2 loaded on rGO as a 3D/2D heterostructure.
This study utilizes an in situ design approach to synthesize the electronically regulated MoSe 2 /rGO heterostructure with exposed edge morphology.The 3D architecture consists of MoSe 2 flower-like nanosheets on rGO, formed through the aggregation of 2D MoSe 2 nanosheets.This aggregation of 2D nanosheets on rGO vertically results in flower-like MoSe 2 structures over rGO, enhancing the utilization of electrocatalytic active edges within the MoSe 2 nanosheets.Hence, through the structural engineering of the MoSe 2 /rGO heterostructure, we create dynamic sites that are conducive to electrocatalytic reactions.Additionally, the electronic structure modification stemming from covalent bonding at heterointerfaces substantially reduces the energy barrier for interfacial charge transfer. [45]The hetero-interfaced structure of MoSe 2 nanosheets with rGO enhances the electron density change within their components, leading to the activation of active edge sites in MoSe 2 /rGO.This activation facilitates accelerated reaction kinetics for both OER and HER.MoSe 2 /rGO exhibits more significant OER activity with an overpotential of 265 mV at 10 mA cm −2 due to the simultaneous increase in active edge sites and electronic structure modification.Due to the lower kinetic energy barrier in an alkaline medium, HER activity of MoSe 2 /rGO was also enhanced and displayed an overpotential of 136 mV at 10 mA cm −2 .Subsequently, the MoSe 2 /rGO exhibits a relatively lower cell voltage of about (1.64 V at 100 mA cm −2 in) than the pristine MoSe 2 during the overall water splitting.The percentage of evolved hydrogen and oxygen through the experiment is almost similar to the theoretical results.This study highlights the importance of effectively designing 3D/2D-derived electrocatalysts for water splitting application.

RESULTS AND DISCUSSION
The synthesis process of pristine MoSe 2 and MoSe 2 /rGO heterostructure is schematically shown in Figure 1A.By using the facile process of solvothermal, MoSe 2 /rGO heterostructure was prepared.Graphene oxide (GO) was mixed with the precursors of molybdenum and selenide in the stochiometric amount to obtain MoSe 2 /rGO.MoSe 2 /rGO heterostructure without any impurities was produced after carbonization and proper washing of the as-prepared electrocatalysts.A similar experimental procedure (excluding rGO) was followed to prepare pristine MoSe   and the desired MoSe 2 /rGO heterostructure was produced with advantageous aggregation in MoSe 2 nanosheets. [46]-ray diffraction patterns of the prepared electrocatalysts were examined to study their heterostructure formation and composition (Figure 1B).The obtained electrocatalysts were identified as MoSe 2 and MoSe 2 /rGO heterostructure, according to the following X-ray diffractions.
Diffraction peaks for MoSe 2 can be attributed to the (002), (100), and (110) planes, respectively, at 2θ = 12.9 • , 31.6 • , and 55.9 • . [47]The JCPDS card no of the MoSe 2 2Htype crystal structure and the space group is 29-0914 and P63/mmc, respectively. [46]The broader nature of the diffraction peaks in the XRD pattern indicates the presence of multilayer MoSe 2 with a lamellar structure. [48]Yet, the same diffraction peaks as that of pristine MoSe 2 are visible in the MoSe 2 /rGO heterostructure except for the presence of a peak at 41 • of the plane (103). [49]The absence of a peak at 41 • in MoSe 2 is due to its poor crystallinity. [50]After the heterostructure formation, the crystallinity was enhanced with the proper construction of the lamellar structure of MoSe 2 on rGO.In addition to these peaks, the presence of rGO is confirmed by the distinctive diffraction peak caused by its presence at about 2θ = 25 • .The Raman spectral lines of MoSe 2 /rGO and MoSe 2 /rGO heterostructure are presented in the range of 1000-4000 cm −1 (Figure 1C).The Raman peaks in the MoSe 2 /rGO heterostructure can be found at about 1344 and 1584 cm −1 , and they are caused by the D-band and Gband of rGO, respectively.The D-band and G-band indicate the presence of rGO in the MoSe 2 /rGO heterostructure.The Raman spectra of the pristine MoSe 2 did not contain the Dband or G-band.Thus, Raman spectroscopy also showed that the MoSe 2 /rGO heterostructure was successfully fabricated.
XPS measurements were conducted to investigate the electronic interaction and bonding environment of the designed heterostructure.The results are displayed in Figure 1D-F.The Mo 3d spectrum is shown in Figure 1D; two binding energies of Mo 3d 3/2 and Mo 3d 5/2 can be distinguished.[53] In MoSe 2 /rGO heterostructure, Mo 3d deconvoluted spectrum has peaks at 228.5 and 231.5, and 232.5 eV due to the presence of Mo─Se, Mo 3+ , and Mo 6+ , respectively. [51,52]The negative shift was observed in the peaks of the MoSe 2 /rGO heterostructure due to the formation of the heterostructure, demonstrating the formation of interfacial Mo─Se bond between MoSe 2 and rGO.This negative shift is attributed to the strong electronic interactions between MoSe 2 and rGO.Such strong electronic interactions can improve the electrocatalytic activity of the MoSe 2 /rGO. [54,55]Besides, the peak at the lower binding energy is due to Mo-edge sites. [53,56]The Se 2p spectrum in Figure 1E presents the binding energy peak at 54.7 eV and 55.5 eV of MoSe 2 and 54.5 and 55.2 eV of MoSe 2 /rGO heterostructure, referring to 3d 5/2 and 3d 3/2 , respectively. [57]imilar to Mo 3d spectrum, the Se 3d spectra have a lower binding energy peak at 54.5 eV due to Se-edge sites. [58]The electron clouds will be drawn towards the Mo and Se because it is more electronegative than rGO, and this will result in a transfer of electrons between their distinct components, as shown by the negative shift of the peaks in Mo 3d and Se 3d.[37][38][39] The C 1s spectra of pristine MoSe 2 and MoSe 2 /rGO heterostructure are shown in Figure 1F.The binding energy peaks of the deconvoluted C 1s spectra can be seen at 284.6 and 286.6 eV, which correspond to the C─C and C═O bonds in MoSe 2 /rGO heterostructure, respectively. [59,60]C═O resulting from the existence of surface oxidized carbon. [61]It is important to note that the peak at 282.5 eV represents the formation of the Mo─O─C covalent bond between 3D and 2D structures. [44]The presence of carbon peaks in pristine MoSe 2 is due to the background carbon from the analysis grid.
MoSe 2 possesses a graphene-like structure, high electrical properties, and a substantial specific surface area, which enhances electrochemical activity and electronic conductivity, making it favorable for achieving higher rates of HER and OER.However, MoSe 2 alone is not a commercially viable electrocatalyst due to its poor intrinsic conductivity, and limited active sites.Besides, the lack of edge active sites in pristine MoSe 2 is a drawback when considering MoSe 2 as an electrocatalyst.By combining MoSe 2 with rGO, the shortcomings of MoSe 2 are addressed.
The rGO acts as a foundation for producing MoSe 2 nanosheets with exposed edges and provides a support structure for their growth.MoSe 2 /rGO heterostructure was produced by assembling an ensemble of MoSe 2 2D nanosheets on rGO.This assembly resulted in the formation of a 3D MoSe 2 structure, with flower-like grown nanosheets covalently bonded to rGO, ensuring the exposure of a greater number of activated edge sites.The field emission scanning electron microscopy (FESEM) analysis clearly shows the presence of MoSe 2 on the thin layer of rGO in the MoSe 2 /rGO heterostructure, as depicted in Figure 2A-C.SEM images of the MoSe 2 /rGO heterostructure (Figure 2B,C) exhibit aggregated 3D structured and high-quality MoSe 2 nanosheets decorating the surface of rGO.This compositing of MoSe 2 and rGO creates numerous heterointerfaces, facilitating faster electron flow during electrochemical reactions through abundant heterojunctions.
Transmission electron microscopy (TEM) analysis of the MoSe 2 /rGO heterostructure, shown in Figure 2D-J, further confirms the strong adhesion of MoSe 2 nanosheets to the rGO layer.The presence of a thin rGO layer promotes the anchoring of MoSe 2 nanosheets, leading to formation of a 3D structure, resembling flower-like arrangements of MoSe 2 over rGO.This unique architecture enables the efficient uti-lization of electrocatalytic active edges within the MoSe 2 nanosheets, overcoming the limitation of pristine MoSe 2 .High-resolution TEM image (Figure 2E) reveals the crystal structure of MoSe 2 , where the (100) plane is identifiable.The selected area electron diffraction (SAED) image (Figure 2F) verifies the presence of the ( 110

Evaluation of the electrocatalytic HER performances
Even though MoSe 2 is a good choice of low-cost nonnoble electrocatalyst, the electrical conductivity is only along the two dimensions. [62]Constructing 3D/2D heterostructures with conductive carbonaceous material like rGO will enhance the conductivity and improve the charge transfer.Moreover, the strong interfacial interaction of the MoSe 2 aggregated in the 3D structure with 2D rGO is resulted in heterostructure with more active edge sites, further facilitating the electrocatalytic activity. [63]The computational analyses suggest that the MoSe 2 with thermoneutral hydrogen adsorption energy holds a higher H coverage and can perform HER electrocatalysis with a lower Tafel slope and overpotential. [62,63]urther, the formation of 3D/2D heterostructure with rGO can form a specific interfacial coupling state between the two nanosheets.The MoSe 2 sheet is activated by the metallic state of molybdenum diselenide introduced by the electron transfer associated reduction to form a strong contact with rGO. [63]nd hence the MoSe 2 /rGO 3D/2D heterostructure is prepared by merging two layers of reactive surfaces to be used as HER electrocatalyst.The HER electrocatalytic qualities of prepared electrocatalysts and Pt/C can be assessed using linear sweep polarization measurements to concentrate hydrogen generation during electrolysis.The HER activities were evaluated utilizing a three-electrode setup using LSV profiles at a scan rate of 5 mV s −1 in 1 M KOH electrolyte.The results of MoSe 2 /rGO heterostructure, pristine MoSe 2 , and bare CC were compared with a high electrocatalytic material of Pt/C loaded carbon cloth (CC) as an electrode for the evaluation of HER.
The overpotential of bare CC, pristine MoSe 2 and MoSe 2 /rGO heterostructure, and Pt/C was determined using LSV curves.Hg/HgO was used as the reference electrode, while a graphite rod served as the counter electrode.The LSV profiles of the electrocatalysts Pt/C, MoSe 2 /rGO heterostructure, pristine MoSe 2 , and bare CC are shown in Figure 3A.A low overpotential of 54 mV is displayed by the commercialized Pt/C electrode for electrocatalysis to drive the current density of 10 mA cm −2 .At 10 mA cm −2 , the overpotential of the bare CC, pristine MoSe 2 , and MoSe 2 /rGO heterostructure is 587, 180, and 136 mV, respectively.The bare CC has high overpotential.The excellent electrocatalytic capabilities of edge-exposed MoSe 2 /rGO heterostructure are further observed at the high current density of 1000 mA cm −2 (Figure 3B).The HER overpotential (394 mV @1000 mA cm −2 ) generated by MoSe 2 /rGO heterostructure is comparable to those of several MoSe 2 -based electrocatalysts that have been reported, and commercial Pt/C in 1 M KOH electrolyte, as shown in Table S1.The improved electrocatalytic activity of the MoSe 2 /rGO heterostructure, in which reactant adsorption and product desorption occur on the edge sites, is primarily due to the morphology of the aggregated MoSe 2 nanosheets on rGO.Further, the rapid flow of electrons from the substrate to the active edges of the electrocatalyst is made possible by the strong contact in the heterostructure between the MoSe 2 and rGO. [49]Hence, it is evident that the exposure of more active sites by the precisely defined, flower like heterostructure of MoSe 2 aggregates with rGO is crucial to its HER electrocatalytic behavior in water splitting.
The Tafel slope is an important consideration when evaluating the electrocatalytic activity of HER. Figure 3C shows the Tafel plots of Pt/C, pristine MoSe 2 , bare CC, and MoSe 2 /rGO heterostructure.The large Tafel slope value at 0.1 V for the high electrocatalytic Pt/C electrode was 88 mV dec −1 .The MoSe 2 /rGO heterostructure yielded a Tafel slope value of 86 mV dec −1 at 0.2 V, which is less than that of pristine MoSe 2 (145 mV dec −1 ) and bare CC (273 mV dec −1 ).The Tafel plots were fitted, and their results offer a valid link between intrinsic electrocatalyst behavior to their rate-limiting steps. [64]These findings suggest that adding MoSe 2 to rGO may enhance its kinetics during HER activity.Furthermore, the critical kinetic parameters for HER, such as mass activity (MA) and turnover frequency (TOF) of the prepared electrocatalysts, were calculated to study their intrinsic characteristics. [65,66]When calculating the mass activity (MA) of the HER electrocatalysts, the current density is normalized by the loading of the electrocatalyst.Compared to Pt/C and MoSe 2 /rGO heterostructure, pristine MoSe 2 has a lower MA at all overpotentials.At a lower overpotential of 200 mV, Pt/C has a more considerable hydrogen evolution mass activity (139 A g −1 ) than MoSe 2 /rGO heterostructure (36 A g −1 ).Surprisingly, at an overpotential value of 300 mV, the MA of MoSe 2 /rGO heterostructure (257 A g −1 ) is higher than Pt/C (242 A g −1 ) and rapidly increases at high current densities with overpotential values above 300 mV (Figure S2).The turnover frequency (TOF), which measures how many electrocatalytic reaction cycles occur per site and per unit of time, can be used to quantify the efficiency of an active electrocatalytic site at the molecular level. [66]TOF was analyzed for Pt/C, pristine MoSe 2 , and MoSe 2 /rGO heterostructure, and it was found that the MoSe 2 has less TOF relatively, similar to MA (Figure 3D).TOF of MoSe 2 /rGO heterostructure was inferior to Pt/C till reaching the potential of 300 mV.The kinetics of MoSe 2 /rGO heterostructure became better than Pt/C from 300 mV and achieved a greater value of 0.000425 s −1 at 380 mV.There are two possible mechanisms in HER, such as Volmer-Heyrovsky pathways or Volmer-Tafel pathways.The Tafel slope of MoSe 2 /rGO heterostructure (86 mV dec −1 ) proposes a rapid HER mechanism resulting in the Heyrovsky pathway decided by the Volmer-Heyrovsky mechanism (Figure 3E). [67,68]Therefore, MoSe 2 /rGO heterostructure follows the Volmer-Heyrovsky mechanism as shown below, To condense the mass transportation effect, we have reduced the catalyst loading and analyzed the hydrogen evolution reaction of MoSe 2 /rGO heterostructure and pristine MoSe 2 . [69,70]As depicted in Figure S3a, employing a catalyst loading of 0.5 mg cm −2 , the MoSe 2 /rGO heterostructure consistently demonstrates higher current density than pristine MoSe 2 across all potentials.For example, at an overpotential of 10 mV, the MoSe 2 /rGO heterostructure achieves a current density of 145 mA cm −2 , surpassing the current density of pristine MoSe 2 (229 mA cm −2 ).This pattern echoes the trend observed with catalysts having 1.5 mg cm −2 loadings.In line with the same conditions, the MoSe 2 /rGO heterostructure showcases superior MA and TOF, estimated to be greater compared to pristine MoSe 2 (Figure S3b,c).Overall, these findings confirm the augmented intrinsic activity of the MoSe 2 /rGO heterostructure in comparison to pristine MoSe 2 , both in terms of TOF and MA.Concurrently, Tafel analysis was executed at lower catalyst loadings to delve into the intrinsic kinetics of alkaline HER for the synthesized catalysts.It is noteworthy that as the catalyst loading decreases, there is no change in the Tafel slope of pristine MoSe 2 (145 mV dec −1 ) (Figure S3d).This suggests that hydrogen mass transport becomes less dominant, signifying that the rate of HER is influenced by a combination of the Volmer and Heyrovsky steps.Conversely, the Tafel slope of the MoSe 2 /rGO heterostructure (79 mV dec −1 ) is reduced at lower catalyst loadings.This implies that the HER process in the MoSe 2 /rGO heterostructure is primarily limited by the Heyrovsky step. [69,70]obustness is a crucial component of a valuable HER electrocatalyst.Long-term cycling of chronoamperometric performance was carried out for MoSe 2 /rGO heterostructure at a static overpotential of −0.3 V versus RHE to attain a current density of 370 mA cm −2 for 24 h in order to examine solidity in an alkaline solution (Figure 3F).The strong interaction between MoSe 2 and rGO in heterostructure can be credited for the electrocatalytic capabilities of HER for 20 h of operation without deterioration.After the HER activity, a postmortem analysis of MoSe 2 /rGO was done with the aid of XRD, SEM and XPS (Figure S4).The XRD spectrum of the MoSe 2 /rGO after the HER activity of 24 h determined that the crystal phase is not revealing the significantly noted change.The high-resolution XPS results (Figure S5) indicated the compositional changes on the surface of electrocatalyst after the HER electrocatalytic activity.A slight variation was observed in the Mo 3d and Se 3d spectra, the fitted spectrum showed an obvious shift in the binding energy peak.After HER, MoSe 2 /rGO XPS characteristic peaks of Mo 3d and Se 3d shifts to the higher binding energy by about 0.5 eV compared to the XPS characteristic peak before HER activity (Figures S5a,b).The partial surface oxidation of MoSe 2 /rGO during HER activity can be attributed to the adsorbed intermediate on the surface. [71,72][74] Meanwhile, the morphology of the MoSe 2 /rGO has not changed significantly after the stability test.As depicted in Figure S6, this underscores the structural stability of MoSe 2 /rGO following HER activity, further confirming the robust interaction between MoSe 2 and rGO particles.The overall morphology of MoSe 2 /rGO remained nearly unaffected after electrocatalytic activity, with exposed edge sites retaining their well-defined structure.

Evaluation of the electrocatalytic OER performances
The dynamic behavior of transition metal chalcogenides under high OER potentials causes notable modifications on their surface and subsurface during the reaction conditions.Throughout the OER process, these materials undergo substantial surface reconstructions involving changes in composition, phase or morphology under oxidizing potentials. [75]Ultimately, this leads to the development of self-assembled layers of amorphous metal oxides/(oxy)hydroxides on the surface of the precatalyst in alkaline electrocatalytic environment. [76,77]These newly formed metal oxides/(oxy)hydroxides act as the active layers driving the OER process.It is worth noting that the structure and morphology of the precatalyst can significantly influence the extent and progression of this reconstruction.Greater electrochemically active surface areas tend to lead to swifter and more profound surface reconstruction in catalyst species. [78]n the present study, MoSe 2 is strategically designed with rGO to enhance the presence of active edges in MoSe 2 .This innovative approach is complemented by the MoSe 2 /rGO heterostructure, where distinct electronegativities synergistically facilitated efficient electron mobility between the two components.This intricate electron transfer mechanism was pivotal in influencing the electron density at the edge sites, characterized by their presence of unsaturated bonds.Consequently, the MoSe 2 /rGO heterostructure with active edge sites exhibited a pronounced constructive influence on its surface reconstruction, bolstered by its intricate structural and morphological attributes.Using the conventional three-electrode cell arrangement, we assessed the electrocatalytic OER activity of RuO 2 , bare CC, pristine MoSe 2 , and MoSe 2 /rGO heterostructure in an alkaline electrolyte.As can be seen in Figure 4A, the current density rapidly increased as the positive potential rose, illustrative of the high OER activity of produced electrocatalysts.The overpotential of MoSe 2 /rGO heterostructure was 265 mV at 10 mA cm −2 anodic current density, exceeding bare CC (425 mV) and pure MoSe 2 (307 mV) while being on par with RuO 2 (218 mV).
The overpotentials of electrocatalysts at 100, 200, 300, and 400 mA cm −2 were also evaluated (Figure 4B).The OER activity of MoSe 2 /rGO heterostructure is comparable to com-mercial RuO 2 .Even though the onset of RuO 2 is before MoSe 2 /rGO, the reaction kinetics of MoSe 2 /rGO heterostructure is dramatically increasing with the potential increment.In contrast, the kinetics of RuO 2 is reducing at high current density, indicating the better kinetics of MoSe 2 /rGO heterostructure than RuO 2 .This result shows the withstanding ability of the MoSe 2 /rGO heterostructure, which can reach high current density with low applied potential.Besides, MoSe 2 /rGO heterostructure attained a significantly smaller Tafel slope at 0.3 V in comparison with the RuO 2 and pristine MoSe 2 , respectively, further indicating the more significant favorable OER reaction kinetics of the former (Figure 4C).The Tafel values of RuO 2 , bare CC, pristine MoSe 2 , and MoSe 2 /rGO heterostructure are 97, 428, 109, and 77 mV dec −1 at the respective potentials.Comparisons were made between the overpotential values of the prepared electrocatalysts, RuO 2 , and previously published MoSe 2 -based electrocatalysts (Table S1).
The two most critical kinetic parameters for OER, MA, and TOF reveal the fundamental properties of electrocatalytic activity. [65,66]The current density was calculated and normalized by the loaded amount of electrocatalyst, and the results of the MA of the OER electrocatalysts are shown in Figure S7.The MA of RuO 2 is better at the low potential range and mediocre at the high potential range.MoSe 2 /rGO heterostructure can reach the MA of 630 A g −1 at 510 mV, reaching 1000 mA cm −2 .A further crucial kinetic measure for OER is TOF, which reveals the fundamental properties of electrocatalytic activity (Figure 4D).Supporting the calculated MA values, the TOF of MoSe 2 /rGO heterostructure also displays substantially more significant values than pristine MoSe 2 and the bare CC.TOF of MoSe 2 /rGO heterostructure at 510 mV to reach 1000 mA cm −2 is 0.000223 s −1 .The significantly improved MA and TOF confirmed the considerable intrinsic activity improvement of MoSe 2 /rGO heterostructure.The present work advances a comprehensive perspective, proposing the intricacies of the absorbate evolution process and unveiling the underlying OER mechanism intrinsic to the MoSe 2 /rGO heterostructure precatalyst.Through a series of four meticulously coordinated protonelectron transfer steps at the active site (as illustrated in Figure 4E and detailed in Equations ( 1)-( 5)), the electrocatalytic OER mechanism of the prepared heterostructure is discussed.Another hydroxyl radical then couples with a proton and an electron from hydroperoxide to form a water molecule, which facilitates the restoration of the active edge site and the evolution of oxygen. [79]The stability of MoSe 2 /rGO heterostructure was tested over 24 h at a constant potential of 1.65 V versus RHE, reaching 380 mA cm −2 , as shown in Figure 4F.The 380 mA cm −2 current density is kept constant over a lengthy period of time without degrading.These findings demonstrate the extraordinary electrocatalytic stability of MoSe 2 /rGO heterostructure during OER.
To investigate both surface reconstruction and the adsorption of intermediates after OER activity, we conducted thorough analyses utilizing XRD, XPS, and SEM techniques on the MoSe 2 /rGO heterostructure.Notably, the XRD spectrum of the MoSe 2 /rGO following 24 h of OER activity exhibited no substantial changes in its crystal phase (Figure S8).In particular, slight changes were discernible in the Mo 3d and Se 3d spectra, with fitted peaks displaying a clear shift in binding energy.Post-OER, the characteristic XPS peaks of Mo 3d and Se 3d in MoSe 2 /rGO underwent an upward shift of approximately 1 eV in binding energy, in contrast to the peaks observed before OER activity (Figure S9).This shift is attributed to surface oxidation during the OER process.[74] Meanwhile, the morphology of MoSe 2 /rGO exhibited minimal change after the stability test.As depicted in Figure S10, this underscores the structural stability of MoSe 2 /rGO following OER activity, further confirming the robust interaction between MoSe 2 and rGO particles.The overall morphology of MoSe 2 /rGO remained nearly unaffected after OER, with exposed edge sites retaining their well-defined structure.This preservation of morphological integrity played a pivotal role in upholding the stable performance of MoSe 2 /rGO during OER.

Water splitting activity of MoSe 2 /rGO electrocatalyst
To boost overall water splitting activity in an alkaline electrolyte, a bifunctional electrocatalyst with strong HER and OER electrocatalytic activities can be utilized.After the individual OER and HER electrocatalytic study of pristine MoSe 2 and MoSe 2 /rGO heterostructure, both were used as bifunctional electrocatalysts for both the anode and the cathode in a two-electrode system, as shown in Figure 5A.Surprisingly, MoSe 2 /rGO heterostructure in 3D/2D form with aggregated MoSe 2 nanosheets produces 10 mA cm −2 at a cell voltage of just 1.64 V at 100 mA cm −2 in 1.0 M KOH which is higher than that of pristine MoSe 2 (1.76 V) (Figure 5B).To confirm the hydrogen (H 2 ) and oxygen (O 2 ) producing ability of MoSe 2 /rGO heterostructure, the amount of H 2 and O 2 evolved during electrocatalytic reaction in water splitting system for 30 mins were quantified.Continuous H 2 and O 2 production were observed for MoSe 2 /rGO heterostructure, and produced gases were measured by the experimental arrangement as shown in the Figure 5C. Figure 5D displays the typical time courses of H 2 and O 2 evolution over the MoSe 2 /rGO heterostructure with the applied respective potentials.The theoretical value of gases evolved was calculated and compared with the experimental values.The theoretical and practical results of the evolved gases were almost equal, presenting a Faradaic efficiency of ≈98%.The chronoamperometric analysis revealed that the MoSe 2 /rGO heterostructure was driven steadily at a voltage of 1.9 V for more than 70 h to achieve the current density of 340 mA cm −2 during water splitting (Figure 5E).A steady current density of about 340 mA cm −2 is maintained, which is likely due to the durability of the electrocatalyst.Furthermore, at applied potentials of 1.83, 1.99, and 2 V, the MoSe 2 /rGO total water splitting system attained current densities of 200, 600, and 1000 mA cm −2 .The electrocatalyst appears to be stabilized by rGO based on the stability of the MoSe 2 /rGO heterostructure that has been observed.The total water splitting activity of MoSe 2 /rGO heterostructure is comparable to a number of previously reported alkaline electrolyzers using Mo-based electrocatalysts (Table S1).Its high activity and more stable operation in water splitting were related to the morphology of MoSe 2 /rGO with improved exposed edges in the heterostructure.The electron transport channel is enhanced by the interface interaction of MoSe 2 and rGO, which also contributes to its most excellent electrocatalytic activity toward OER and HER in the water splitting system.

Evaluation of electrochemical active surface area and electrochemical impedance
To further prove the electrocatalytic activity of the prepared electrocatalysts, electrochemical active surface area (ECSA) and electrochemical impedance spectroscopy (EIS) were investigated.Consideration of electrochemical doublelayer capacitance (C dl ) is an acceptable method for estimating ECSA.The ECSA for MoSe 2 and MoSe 2 /rGO heterostructures, respectively, was calculated using the published methodology to be 470 cm 2 and 1227.5 cm 2 , respectively. [80]o estimate the C dl parameters for MoSe 2 and MoSe 2 /rGO electrocatalysts in the non-Faradaic region, CV was measured at scan rates of 10 to 100 mVs −1 in 1 M KOH as shown in Figure S11.The C dl values were calibrated by measuring the slope of the current difference (j) at 0.025 V versus RHE.The C dl values for the MoSe 2 and MoSe 2 /rGO electrocatalysts were 18.8 and 49.1 mF, respectively (Figure 6A).Because of its morphology featuring the vertically aggregated nanosheets, MoSe 2 /rGO heterostructure has a higher C dl than MoSe 2 .The observed results show that the electrocatalytic properties of MoSe 2 /rGO heterostructure are significantly influenced by greater electrochemically active surface area with active edges and vertically aggregated heterostructure with more interface between MoSe 2 and rGO.
The electrochemical impedance spectroscopy (EIS) plots were utilized to illustrate the electrocatalytic kinetics and interface interactions.In Figure 6B, the Nyquist Impedance graphs of both the MoSe 2 and MoSe 2 /rGO electrocatalysts are presented, which offers insights into electrochemical processes occurring at the electrode-electrolyte interface.The high, mid, and low-frequency curves in the Nyquist plot of EIS provide information about the solution resistance and charge transfer resistance (kinetics) at high frequencies and the diffusion processes (mass transport) at low frequencies, respectively. [54]To fit the impedance data of the prepared electrocatalysts, an equivalent circuit model (as depicted in the inset) was employed.Here, R s symbolizes the electrolyte resistance, and R ct represents the charge transfer resistance at the electrode and electrolyte interface.The respective fitted values can be found in Table S2.
Comparatively, the R s is lower for the MoSe 2 /rGO heterostructure than for pristine MoSe 2 .Likewise, the observed R ct values for MoSe 2 /rGO are lower than those for pristine MoSe 2 .The assessment of ion-diffusion resistance within the electrode was deduced from the linear tail evident in the lowfrequency region. [81]Notably, the angle of the tail, surpassing 45 • in relation to the real axis, substantiates the superficial movement of the electrolyte onto the MoSe 2 /rGO surface, manifesting capacitive behavior.
Figure 6C shows Nyquist admittance graphs for MoSe 2 and MoSe 2 /rGO heterostructure that were produced in the knee-frequency-dominated 150 kHz to 10 mHz frequency range.Pristine MoSe 2 and MoSe 2 /rGO heterostructure have been shown to have knee frequencies of 1963.14 and 3216.03Hz, respectively.The knee frequency identifies the highest frequency at which capacitive behavior takes control, and resistive behavior becomes insignificant.The MoSe 2 /rGO heterostructure exhibits a higher knee frequency than the others, which supports the lower charge transfer resistance. [82]Figure 6D shows bode plots of pristine MoSe 2 and MoSe 2 /rGO heterostructure.The Bode plot is used to calculate the time constant at a low frequency using the formula τ = 1/ω p , where τ is the time constant, and ω p is the characteristic frequency.The corresponding time constants of pristine MoSe 2 and MoSe 2 /rGO heterostructure at a 60 • phase angle show that the latter electrocatalyst takes a substantially shorter amount of time to adsorb and desorb the reactants, intermediates, and products.The Nyquist plot of pristine MoSe 2 and MoSe 2 /rGO heterostructure was analyzed as a function of potential from 1.2 to 1.8 V versus RHE (Figure 6E).Due to R s and R ct , the produced electrocatalysts display small and large semicircles at high-mid and midlow frequencies.In every applied potential, MoSe 2 /rGO has a lower R s than pristine MoSe 2 .The Nyquist plot semicircle at low frequency (high Z) demonstrates that the R ct value for MoSe 2 /rGO is considerably lower.The various semicircle diameters at the mid-low area indicate their decreasing R ct to speed up the slow reaction kinetics.The OH − ions were absorbed on the surface of the electrocatalyst as the voltage increased from 1.2 to 1.8 V versus RHE, lowering the C dl .
The decrease in C dl and the reduction in charge transfer resistance show that the reaction kinetics of MoSe 2 /rGO increased when the potential was raised, making it easier for the rate-determining steps to pass the potential barrier. [83]urther, the Bode plots were drawn employing |Z| versus ω and ω versus V on the primary and secondary vertical axes, respectively (Figure 6F).These frequency-dependent analyses showed a decline in impedance with rising potential, corroborating the aforementioned occurrence.Consequently, the MoSe 2 /rGO heterostructure, featuring prominently exposed edges, engendered an augmentation in the number of electroactive sites.This distinctive architectural configuration yielded remarkable electrocatalytic proficiency for pivotal electrochemical processes encompassing the HER, OER, and water splitting.Notably, these achievements were accompanied by a pronounced reduction in overpotential, signifying enhanced overall performance in water splitting.This favorable outcome is underpinned by a confluence of contributing factors.The abundant active edges on the heterostructure provide ample sites for catalytic reactions to transpire.Additionally, the advantageous morphology of the MoSe 2 component in relation to the reduced graphene oxide (rGO) component further enhances the catalytic attributes of the MoSe 2 /rGO heterostructure.Moreover, the synergistic interplay between MoSe 2 and rGO within the MoSe 2 /rGO heterostructure proves instrumental.This collaboration brings about variations in electron density within the components of heterostructure, an aspect that significantly augments the efficiency of electron transport and catalytic kinetics at the critical electrode-electrolyte interface.

CONCLUSION
In conclusion, the overall electrocatalytic performance of the prepared MoSe 2 /rGO is highly attributed to the 3D/2D heterostructure with exposed active sites, interfacial coupling induced electron density redistribution of the heterostructure, formation of strong interfacial coupling state, surface reconstruction during OER, and rapid interlayer mass and charge transport.The effective manipulation of the MoSe 2 with more active edge sites and heterointerface offers perspectives on how to create high-performance electrocatalysts that can efficiently split water.One-pot solvothermal processing was used to create the MoSe 2 /rGO heterostructure.The aggregated MoSe 2 composited with rGO demonstrated exceptional electrocatalytic activity and long-term stability for HER, OER, and water splitting.When the MoSe 2 /rGO heterostructure is used as a bifunctional electrocatalyst to drive the electrocatalytic full water splitting, just a 1.64 V overpotential is required at 100 mA cm −2 .The catalyst showed good endurance and an almost unchanged overpotential during the durability test, which lasted more than 70 h due to the covalent bond between 3D and 2D structures.This research is intended to point the way toward the surface regulation of 3D materials to be used as an effective bifunctional electrocatalyst in water splitting.C for further characterization.The graphene oxide (GO) of 140 mg was added with the starting precursors and followed the same procedure to attain the heterostructured electrocatalyst, MoSe 2 /rGO, with the edge active sites for the hydrogen and oxygen evolution reactions.

Material characterization
Powder X-ray diffraction (XRD) was carried out using Cu K radiation (λ = 1.5418 ), the Rint 1000, Rigaku, Japan.NRS-5100 was used to characterize Raman spectra.On a Multilab 2000 (UK), X-ray photoelectron spectroscopy (XPS) was carried out.The Hitachi S-4700 (Japan) and the JEM-2000 EX-II, JEOL (Japan) were used to produce the field emission scanning electron microscopy (FESEM) and high-resolution transmission electron microscopy (HRTEM) images, respectively.

Electrochemical measurements
All electrochemical measurements were performed using a standard three-electrode system on a BioLogic VMP3 multichannel electrochemical workstation.Linear sweep voltammetry (LSV) was carried out during the electrocatalytic studies of OER and HER with a carbon cloth brush coated with electrocatalysts serving as the working electrode in a 1 M KOH electrolyte.The counter and reference electrodes were made of a graphite rod, and Hg/HgO and the reference electrode were precisely calibrated both before and after the tests to guarantee accuracy.Electrocatalyst powder, carbon black, and polyvinyl pyrrolidone were combined and coated on the carbon cloth substrate to create the working electrodes (anode and cathode).In a two-electrode system using the electrocatalyst-coated substrate as the anode and cathode in the 1.0 M KOH aqueous electrolyte, the water splitting performance of the produced electrodes was assessed.
) and (100) planes of MoSe 2 .Detailed images in Figure 2G-J and Figure S1 vividly depict the MoSe 2 nanosheets adhering to the surface of rGO in the MoSe 2 /rGO heterostructure.EDAX analysis confirms the presence of all constituent elements: Mo, Se, and C (Figure 2G-J).

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I G U R E 3 (A) HER LSV curves of pristine MoSe 2 , Pt/C, MoSe 2 /rGO heterostructure and bare CC with a scan rate of 5 mV s −1 .(B) Overpotentials obtained from LSV. (C) Corresponding HER Tafel plots.(D) Corresponding turnover frequency at various overpotentials.(E) HER mechanism on MoSe 2 /rGO heterostructure and (F) Chronoamperometry curves of MoSe 2 /rGO heterostructure at −0.3 V versus RHE.

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I G U R E 5 (A) Schematic representation of using MoSe 2 /rGO heterostructure as bifunctional electrocatalyst.(B) LSV curves of the fabricated MoSe 2 /rGO and pristine MoSe 2 based water-splitting system.(C) Digital image of water-splitting system, (D) theoretically calculated and experimentally measured H 2 and O 2 volume, and (E) chronoamperometry curve of the MoSe 2 /rGO-based water-splitting system.

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I G U R E 6 (A) Electrochemical active surface area of pristine MoSe 2 and MoSe 2 /rGO heterostructure.Electrochemical impedance spectroscopy of pristine MoSe 2 and MoSe 2 /rGO heterostructure.(B) Nyquist impedance plot.(C) Nyquist admittance plot.(D) Bode plot.(E) Nyquist plots as a function of potential and (F) Bode plot (|Z| vs ω) as a function of applied potential. 2

Preparation of MoSe 2 and MoSe 2 /rGO
MoSe 2 was synthesized using a facile solvothermal synthesis process using starting precursors of sodium molybdate hexahydrate (Na 2 MoO 4 ⋅6H 2 O), Se metal, hydrazine hydrate (NH 2 NH 2 ⋅H 2 O) and ethanol (C 2 H 6 O) as a solvent.Initially, the 3 mmol of Na 2 MoO 4 ⋅6H 2 O was dissolved in the mixed solvent of water (H 2 O) (25 mL) and ethanol (25 mL).Se metal was dissolved in 5 mL of NH 2 NH 2 .H 2 O solution, and then it was added dropwise to the above solution.The above mixture was transferred to the stainless-steel Teflon lined container, and heat treatment was carried out at 200 • C for 16 h.After this process, the sample was washed with H 2 O and C 2 H 6 O to remove the impurities, and then the sample was dried at 80