# Fileset

[Ball-milled MoS2 with graphene shows enhanced catalytic activity for hydrogen evolution reaction.pdf](https://mdr.nims.go.jp/filesets/9db69619-63df-4a67-a0d8-c99cbcac36a6/download)

## Creator

Linghui Li, Satish Laxman Shinde, Takeshi Fujita, Takahiro Kondo

## Rights

[Creative Commons BY Attribution 4.0 International](https://creativecommons.org/licenses/by/4.0/)

## Other metadata

[Ball-milled MoS2 with graphene shows enhanced catalytic activity for hydrogen evolution reaction](https://mdr.nims.go.jp/datasets/ad0879d4-6408-4fa0-ad11-a76b19f83764)

## Fulltext

Ball-milled MoS2 with graphene shows enhanced catalytic activity for hydrogen evolution reactionFull Terms & Conditions of access and use can be found athttps://www.tandfonline.com/action/journalInformation?journalCode=tsta20Science and Technology of Advanced MaterialsISSN: (Print) (Online) Journal homepage: www.tandfonline.com/journals/tsta20Ball-milled MoS2 with graphene shows enhancedcatalytic activity for hydrogen evolution reactionLinghui Li, Satish Laxman Shinde, Takeshi Fujita & Takahiro KondoTo cite this article: Linghui Li, Satish Laxman Shinde, Takeshi Fujita & Takahiro Kondo(29 May 2024): Ball-milled MoS2 with graphene shows enhanced catalytic activity forhydrogen evolution reaction, Science and Technology of Advanced Materials, DOI:10.1080/14686996.2024.2359360To link to this article:  https://doi.org/10.1080/14686996.2024.2359360© 2024 The Author(s). Published by NationalInstitute for Materials Science in partnershipwith Taylor & Francis Group.View supplementary material Accepted author version posted online: 29May 2024.Submit your article to this journal View related articles View Crossmark datahttps://www.tandfonline.com/action/journalInformation?journalCode=tsta20https://www.tandfonline.com/journals/tsta20?src=pdfhttps://www.tandfonline.com/action/showCitFormats?doi=10.1080/14686996.2024.2359360https://doi.org/10.1080/14686996.2024.2359360https://www.tandfonline.com/doi/suppl/10.1080/14686996.2024.2359360https://www.tandfonline.com/doi/suppl/10.1080/14686996.2024.2359360https://www.tandfonline.com/action/authorSubmission?journalCode=tsta20&show=instructions&src=pdfhttps://www.tandfonline.com/action/authorSubmission?journalCode=tsta20&show=instructions&src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/14686996.2024.2359360?src=pdfhttps://www.tandfonline.com/doi/mlt/10.1080/14686996.2024.2359360?src=pdfhttp://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2024.2359360&domain=pdf&date_stamp=29 May 2024http://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2024.2359360&domain=pdf&date_stamp=29 May 2024 Publisher: Taylor & Francis & The Author(s). Published by National Institute for Materials Science in partnership with Taylor & Francis Group. Journal: Science and Technology of Advanced Materials DOI: 10.1080/14686996.2024.2359360  Ball-milled MoS2 with graphene shows enhanced catalytic activity for hydrogen evolution reaction Linghui Li a, Satish Laxman Shinde b,c, Takeshi Fujita d, and Takahiro Kondo b,e,f* a Graduate School of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan b Department of Materials Science, Institute of Pure and Applied Sciences, University of Tsukuba, Tsukuba 305-8573, Japan cDepartment of Physics, Indian Institute of Technology Hyderabad, Kandi, Telangana, 502284, India d School of Engineering Science, Kochi University of Technology, Kochi 782-8502, Japan e Tsukuba Research Center for Energy Materials Science, Institute of Pure and Applied Sciences and R&D Center for Zero CO2 Emission with Functional Materials, University of Tsukuba, Tsukuba 305-8573, Japan f Advanced Institute for Materials Research, Tohoku University, Sendai 980-8577, Japan * Corresponding author. E-mail address: takahiro@ims.tsukuba.ac.jp (T. Kondo).  https://crossmark.crossref.org/dialog/?doi=10.1080/14686996.2024.2359360&domain=pdf ABSTRACT The hydrogen evolution reaction (HER) is an important phenomenon in water splitting. Consequently, the development of an active, earth-abundant, and inexpensive HER catalyst is highly desired. MoS2 has drawn considerable interest as an HER catalyst because it is composed of non-precious metal and exhibits high catalytic activity in the nanosheet form. In this study, size-controlled MoS2 particles were synthesized by ball milling. The as-prepared samples exhibited significantly enhanced electrochemical and catalytic properties compared to those of pristine bulk MoS2. Furthermore, the HER activity improved further upon the introduction of graphene into the as-prepared ball-milled samples. In particular, the MoS2 sample ball-milled for 12 h mixed with graphene exhibited optimal performance, showing an overpotential (160 mV at 10 mA cm−2) that was ~335 mV lower than that of pristine bulk MoS2. The superior catalytic activity was ascribed to the exposed edge sites, sulfur vacancies, and 1T phase of MoS2, as well as the noteworthy fortifying effect of the electronically conductive flexible material, graphene. The results provide a promising strategy for its application as an efficient and stable HER catalyst. KEYWORDS: molybdenum disulfide; hydrogen evolution reaction; electrocatalyst; edge site; 1T phase; sulfur vacancy 1. Introduction The adverse impact of fossil fuels on the environment and the need to sustainably advance human society have led to a recent focus on replacing fossil fuels with renewable energy [1]. To that end, hydrogen has been promoted as an ideal energy carrier since the early 1970s because of its high weight energy density and zero CO2 emissions [2]. Hydrogen can store the energy generated by renewable sources in chemical bonds and convert it back into electricity when needed through fuel cells or other devices. Hydrogen is mainly present in compounds such as  hydrocarbons and water on our planet and is primarily obtained through energy-intensive greenhouse-gas-emitting processes, such as the steam reforming of hydrocarbons [3]. However, achieving sustainable and economically feasible hydrogen production on an industrial scale is a major challenge [4]. One way to address this issue is to decompose water through electrolysis or photocatalysis, using renewable solar energy to separate water molecules directly or indirectly into their components [5,6]. The hydrogen evolution reaction (HER) is an extensively studied electrochemical process that involves the cathodic half-reaction of water decomposition [7]. It plays a crucial role in various energy-conversion devices, including hydroelectric and artificial photosynthetic cells. As its name suggests, the HER involves the reduction of protons or water molecules, leading to the release of gaseous hydrogen, as follows [8]:  2𝐻(𝑎𝑞)+ + 2𝑒− → 𝐻2(𝑔),   (1.1) where (aq) and (g) denote the form of H3O+ species in the aqueous solution and the gas phase, respectively. The standard reduction potential of the HER is defined as 𝐸𝐻+/𝐻20 = 0 𝑉 versus a normal hydrogen electrode (NHE) at pH 0. However, similar to many chemical reactions, the electrochemical processes occurring during the HER must overcome a certain activation energy barrier (that is, the overpotential) to proceed. Electrocatalysts are typically required to reduce the overpotential, thereby improving the reaction rate and efficiency. High-performance catalysts for the critical electrochemical HER should minimize the overpotential and thereby enhance its efficiency. Pt-group metals are considered remarkably  efficient HER electrocatalysts. However, the development of HER catalysts with exceptional activity using materials that are more readily available and cost-effective is challenging [9]. Molybdenum disulfide (MoS2), which has been widely studied as a hydrodesulfurization catalyst, has recently garnered attention as an HER electrocatalyst because of its high catalytic activity [10,11,12]. Both computational and experimental findings have confirmed that the HER activity originates from the edges of the MoS2 plates [13,14], whereas their basal planes are catalytically inactive. Additionally, recent studies have reported other catalytic active sites of MoS2 toward the HER. For example, MoS2 with the 1T structural phase exhibits high catalytic activity, even with substantial oxidation at the edge sites [12,15,16]. Furthermore, the catalytic activity of a MoS2 film increases with decreasing thickness and is potentially superior to that of edge-rich pyramidal MoS2 nanosheets [17]. In addition to the edge sites, the sulfur vacancies of MoS2 provide another set of major catalytically active sites for the HER [15,18]. Consequently, the exposure of more edges, converted to the octahedral (1T) structural phase, and sulfur vacancy engineering are expected to be viable strategies for synthesizing MoS2 with higher catalytic performance than that of bulk MoS2 [13,16,19,20]. One limitation of MoS2 catalysts is that their active sites are limited to edges [21]. To fully harness the potential of MoS2-based catalysts, there is a pressing need to increase the number of active sites, enhance the activity of these catalytic sites, and improve the electrical connections between the active sites and catalyst substrate [22]. One approach to optimizing charge transfer involves the use of a graphene support, which has been demonstrated to significantly enhance the HER kinetics [12].   MoS2 belongs to the category of quasi-two-dimensional transition metal dichalcogenides with a layered structure, and has recently gained significant attention owing to its unique electronic, optical, optoelectronic, and catalytic properties [23]. In its bulk form, MoS2 is an indirect-bandgap semiconductor with an energy gap of 1.29 eV, and it comprises multiple layers of S–Mo–S bonds, held together by weak van der Waals interactions. Each layer comprises a central plane of Mo atoms sandwiched between two S atoms, with strong covalent bonds and dipole formation between the positively charged Mo atoms and negatively charged S atoms. The Mo atoms in MoS2 adopt two different coordination modes: 1T phase and trigonal prismatic (2H and 3R phases), with the 2H phase being the most common in bulk MoS2. Notably, the crystal system can be exfoliated into individual layers owing to the weak van der Waals forces holding the S–Mo–S layers together [16,24]. These individual layers exhibit properties that differ significantly from those of the bulk material. However, although the conversion of 2H MoS2 into the 1T phase can lead to enhanced catalytic performance in the HER [25,26], the precise mechanisms underlying this improvement are not fully understood. Various methods have been developed to chemically exfoliate bulk MoS2 [16,24,25,27]. One approach involves the use of Li intercalation to construct single- or few-layered structures [23]. Alternatively, a bottom-up approach has been pursued to directly synthesize MoS2 nanostructures with a high density of edge sites [20,22]. Other methods include the chemical exfoliation of bulk MoS2 by sonication and solvent utilization [24,28]. Notably, all these methods involve wet-chemical processing schemes and may require potentially hazardous chemicals. Furthermore, chemical exfoliation tends to yield a minuscule amount of exfoliated material. Additionally, scaling up these methods can be challenging in  terms of economics, environmental concerns (for example, Li intercalation and solvent use), and technological feasibility (for example, sonication) [19]. In this context, ball milling, a simple and efficient method, shows promise for improving the catalytic properties of bulk MoS2 [10,19].  In this study, a straightforward and scalable method based on dry ball milling was adopted to synthesize size-controlled MoS2 with significantly enhanced electrochemical catalytic properties. The original material—pristine bulk MoS2 powder—was not subjected to any chemical reactions or agents. Comprehensive characterization was performed using techniques such as scanning electron microscopy (SEM), energy-dispersive X-ray (EDX) spectroscopy, and X-ray photoelectron spectroscopy (XPS) to track the morphological and chemical changes that occurred during ball milling. Additionally, the possible introduction of metallic impurities could affect the electrochemical and catalytic properties of the material was probed. Overall, this study was aimed at synthesizing size-controlled MoS2 with enhanced HER-catalyzing activity by increasing the number of edge sites, introducing the 1T phase, and introducing sulfur vacancies via the simple ball milling method, and examining the effects of each process step on the activity. 2. Materials and Methods 2.1 Materials MoS2 bulk powder, graphene nanoplates (GNPs; average number of layers: 5–7), Nafion (5 wt% in lower aliphatic alcohols and water), H2SO4 (97%), and commercial Pt/C (20 wt%  loading; matrix activated carbon support) were purchased from Sigma-Aldrich (Japan). The solvents were purchased from commercial sources. 2.2 Synthesis of ball-milled MoS2 A planetary ball mill (Fritsch PL-7, Flitch, Japan) whose container (capacity: 20 cm3) and balls (diameter: 1 cm) were made of ZrO2 was used for ball milling. Bulk MoS2 (100 mg) and six balls were placed in a container, and milling was performed at atmospheric pressure, a temperature of ~300 K, and a rotation speed of 400 rpm. Each cycle involved 5 min of rotation, followed by a rest period of 30 s (to prevent temperature increases during long-term operation). Ball milling was performed for 15 min, 30 min, 45 min, 1 h, 2 h, 4 h, 8 h, and 12 h. The ball-milled samples are denoted herein based on the ball-milling duration; for example, the MoS2 sample ball-milled for 30 min is named “MoS2-30 min.” 2.3 Preparation of “MoS2-ball milled + sonicated” ink  Ball-milled MoS2 powder (5 mg) was suspended in a mixture of ethanol (1 mL) and Nafion (50 µL), and the resulting sample was subjected to 2 h of bath sonication (500 W / 60 Hz, AS ONE Ltd., Japan), yielding a sample denoted as “MoS2-ball milled + sonicated” herein. 2.4 Preparation of “MoS2-ball milled + G” ink  Ball-milled MoS2 powder (5 mg) and GNPs (10 mg) were suspended in ethanol (1 mL) and then underwent 1 h of bath sonication (500 W / 60 Hz, AS ONE Ltd., Japan). A Nafion solution (50 µL) was added to the resulting mixture to prepare the ink for electrochemical measurements, denoted herein as “MoS2-ball milled + G.”  2.5 Characterization Powder X-ray diffractometry (XRD) was performed using a Rigaku MiniFlex instrument (Tokyo, Japan) with a Cu Kα X-ray source (λ = 1.540598 Å). Diffraction patterns were recorded using a D/teX Ultra silicon strip detector (Rigaku) at a speed of 0.05° s−1 up to 2θ = 80°, where θ is the angle of incidence. SEM and electron-probe microanalysis were performed using a JXA-8530F instrument (JEOL Ltd., Japan) at an operating voltage of 10 kV. Raman spectroscopy was conducted using a multichannel Raman imaging system (ST Japan Inc., Japan) at an incident wavelength of 532 nm. Transmission electron microscopy (TEM) and scanning TEM (STEM) images were captured using a JEM-2100F TEM/STEM (JEOL Ltd., Japan) equipped with an EDX spectrometer operating at 200 kV. Double-spherical aberration (Cs) correctors (CEOS GmbH, Heidelberg, Germany) were used to obtain high-contrast images with a point-to-point resolution of 1.4 Å. EDX profiles were collected using a JEOL JED-2300 T instrument. X-ray photoelectron spectroscopy (XPS) measurements were conducted using a JPS 9010 TR spectrometer (JEOL Ltd., Japan) equipped with a Mg Kα X-ray source (λ = 1253.6 eV). To that end, the pass energy was set to 10 eV, and each sample was placed on a piece of graphite tape. Because charge accumulation in the sample shifted the binding energy to higher values, the charge-up magnitude was calibrated using the C 1s peaks of graphene and graphite tape at 284.6 eV.    2.6 Calculation of crystal size The crystal size of the powder materials was calculated using the Scherrer equation (2.1). 𝐶𝑟𝑦𝑠𝑡𝑎𝑙 𝑆𝑖𝑧𝑒 = Ƙ𝜆𝛽 cos 𝜃,  (2.1) where Ƙ is a dimensionless shape factor, typically considered as ~0.9; λ is the X-ray wavelength (1.54060 Å (Cu Kα1) in this study); θ is the incidence angle of X-ray with respect to the sample holder; β is the full width at half maximum intensity of the peak. 2.7 Electrochemical measurements All electrochemical measurements were performed using a Corrtest CS2350H electrochemical workstation in aqueous 0.5 M H2SO4. The standard three-electrode setup comprised an active-material-loaded glassy carbon electrode (GCE) as the working electrode, a carbon rod as the counter electrode, and Ag/AgCl as the reference electrode. Except for the active material, all the electrodes were purchased from BAS Inc., Japan. For each measurement, the prepared ink (15 μL) was dropped thrice onto the polished surface of the GCE to achieve a final catalyst loading of 0.05 mg cm−2. Linear sweep voltammetry (LSV) curves were recorded at a scan rate of 5 mV s−1. All experiments were conducted at ambient temperature (298 ± 2 K), and the potentials were evaluated against the reversible hydrogen electrode (RHE) using equation (2.2). 𝐸𝑅𝐻𝐸 = 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.059 × 𝑝𝐻 + 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙0   = 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.059 × 0 + 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙0  = 𝐸𝐴𝑔/𝐴𝑔𝐶𝑙 + 0.199 𝑉 (at pH 0),          (2.2)  where EAg/AgCl is the measured potential, and E0Ag/AgCl is the standard potential of Ag/AgCl (saturated KCl) at 25 °C (0.199 V). The thermodynamic potential for the HER is 0 V; therefore, the absolute value of the potential obtained by LSV is the overpotential. To better elucidate the HER kinetics, Tafel slopes were derived from the LSV plots. To evaluate the electrocatalytic stability of the catalysts, the LSV curves before and after 6000 CV cycles (0.1 to −0.3 V vs. RHE) at a scan rate of 100 mV s−1 were compared.  3. Results and discussion 3.1 Characterization of ball-milled MoS2  Ball milling is a straightforward, effective method for exfoliating and decreasing the lateral dimensions of large quantities of layered materials. As shown in Figure 1, an industrial milling machine was used to produce small flakes of MoS2 with different thicknesses, starting from large grains of natural MoS2, followed by the addition of graphene to increase conductivity and realize superior electrocatalytic performance. SEM was performed to examine the milling-induced morphological changes. As shown in Figure 2, smaller MoS2 particles were produced via ball milling. Furthermore, XRD was conducted to evaluate the crystal structure and crystal size of each sample (Figure 3), with the Scherrer equation used for the calculations. The results (Table 1) indicated that there was no appearance of the other crystal structure, and the crystal size of MoS2 gradually reduced from 68 to 4 nm by ball milling.  High-resolution STEM was performed to further evaluate the structural properties of ball-milled MoS2. As shown in Figure 4a and b, the MoS2-30 min and MoS2-2 h samples were well crystallized and contained folded, bent layers. Notably, the 2H phase is the most common and stable configuration of MoS2 [26]. When MoS2 is multi-layered, the most stable phase is the 2H phase, and when it is single-layered, it is the 1H phase [29]. The difference in the structures of the 1T and 1H phases was identified by the contrast difference as reported previously [29]. The part captured in the photographs is single-layer MoS2; therefore, it is the 1H phase. According to the atomic structure model, the 1H and 1T phases existed simultaneously in the MoS2-12 h sample (Figure 4c and d), suggesting that ball milling caused the formation of the 1T phase, presumably through exfoliation. Further insights into the surface compositions of the materials were obtained by XPS. Specifically, the wide-scan compositional spectra of MoS2 (Figure S1) and the high-resolution Mo 3d and S 2p spectra (Figure S2) were acquired before and after milling for comparison. Notably, Si was detected in addition to Mo and S in the wide-scan spectra (Figure S1); Si originated from SiO2, which inevitably emerged during the XPS measurements as the samples were prepared by dripping the sample ink onto a glass slide. The high-resolution Mo 3d and S 2p XPS profiles (Figure S2) showed typical Mo 3d spectra for bulk MoS2, with sharp signals at approximately 229.5 and 232.6 eV, corresponding to Mo4+ 3d5/2 and Mo4+ 3d3/2, respectively. After ball milling, the MoS2-12 h sample exhibited a wider spectrum with peaks that shifted to higher energies. This can be ascribed to the presence of Mo atoms with higher oxidation states—that is, Mo5+ and Mo6+ [30,31]—which result from an oxidative process occurring during ball milling. Notably, a shift in the Mo 3d peaks toward higher energies has been  recorded in the presence of high surface-step densities [32]. Ball milling likely produced several surface step edges and defects while reducing the lateral size of the thin film. Similar conclusions were drawn from the S 2p spectra acquired before and after ball milling: the appearance of oxidized higher peak components in S 2p results in broader peaks possibly owing to the oxidation of surface sulfur because of ball milling. Elemental quantification was performed by acquiring narrow-scan XPS profiles, and the atomic percentage of each element was estimated (Table 2). The S/Mo ratio decreased slightly with increasing ball-milling time, and the longer the ball-milling time, the higher the sulfur defect; this implies that the milling did not induce a significant chemical transformation but created certain sulfur defects that potentially favored electrochemical reactions [33,34,35].  3.2 Electrochemical performance The electrocatalytic performances of the prepared HER catalysts were tested in a 0.5 M N2-saturated H2SO4 solution at a rotating speed of 1600 rpm. The electrochemical performance analysis of MoS2 samples ball-milled for different durations, with respect to commercial Pt/C as the reference (Figure S3), showed that the polarization curve gradually shifted to the right after ball milling. This implied that the overpotential decreased, signifying that ball milling undeniably improved the catalytic activity of MoS2. However, samples ball-milled for longer durations did not always exhibit superior activity. In particular, the 8 h and 12 h ball-milled samples performed worse than the other ball-milled samples. Based on the information obtained from SEM, the smaller MoS2 particles produced after prolonged ball-milling  agglomerated, which likely caused the exposed edges to be covered, thereby reducing the number of active sites and resulting in poor performance. To disperse the agglomerated particles and thereby obtain catalysts with superior performance, four of the samples were subjected to an additional two hours of bath sonication. As shown in Figure S4 and Table S1, the properties of each sample improved after sonication. Moreover, the longer the ball-milling duration—or alternatively, the smaller the particles—the greater the improvement after the additional sonication. Sonication caused the agglomerated particles to redisperse, exposing more edges and thereby improving the catalytic activity. Furthermore, the samples ball-milled for longer durations exposed more edges than those of the samples subjected to shorter ball-milling periods; additionally, the ball-milling process produced the 1T phase (Figure 4) and sulfur defects (Table 2). These results are consistent with previous discussions on the active sites of MoS2; thus, the presence of more exposed edges, the 1T phase, and sulfur vacancies is imperative to the HER catalyzing activity [14,15,18].  CV measurements were conducted in the presence of ferro/ferricyanide cyanide as an oxidation–reduction probe to evaluate the heterogeneous electron transfer between the material and the ferro/ferri ions in the solution. As shown in Figure S5, the CV profile of MoS2-12 h differed only slightly from that of bulk MoS2. Notably, the oxidation and reduction current intensities of the redox probe increased by ~25%, presumably owing to the ball-milling-induced increase and decrease in the electrochemical surface area and particle size of the material, respectively. Additionally, slightly faster electron transfer occurred after ball milling, resulting in less peak-to-peak separation between the oxidation and reduction profiles  of the reducing probe. This was also potentially due to the increased availability of the edge components, which have been proven to be remarkably potent active sites [36]. The as-prepared MoS2 samples were subsequently integrated with graphene—a typical conductive material—to improve their activity further [37,38]; this also helped maintain the morphology of the exfoliated sheets. As shown in Figure S6 and Table S2, the mixture of ball-milled MoS2 and graphene (MoS2 + G) samples exhibited superior performance. In particular, the MoS2-12 h + G sample showed an overpotential (160 mV) that was ~335 mV lower than that of pristine bulk MoS2. This enhanced activity can be ascribed to the improved electronic conductivity and suppressed agglomeration of MoS2 particles by mixing with flexible graphene: as shown in Figure S7, an SEM-based morphological analysis of MoS2-12 h + G shows that graphene plays an important role in connecting MoS2 and the electrode as well as the spacer to prevent agglomeration. Furthermore, the only-graphene sample exhibited low activity, and comparing the CV cycles with a large range of potential (Figure S8), there is a distinct difference in the current densities between the only-graphene sample and MoS2-12 h + G sample, thereby confirming the important role of graphene in providing conductivity. A performance comparison was performed between bulk MoS2 and the sonicated and graphene-incorporated variants of MoS2-12 h (Figure 5 and Table S3). The MoS2-12 h + G sample exhibited improved performance, with a lower overpotential and a smaller Tafel slope. The decrease in the Tafel slope of MoS2-12 h + G may be due to the introduction of graphene-supported conductivity in the materials (in other words, the larger Tafel slope is due to low conductivity), and the smaller Tafel slope also indicates its superior kinetics. Based on this finding, the graphene addition had a better effect on the catalyst performance than  sonication, presumably because graphene not only increased the electronic conductivity of the sample but also helped maintain the morphology and exposed edge state of MoS2 [39,40,41]. Furthermore, MoS2-12 h + G outperforms several other noteworthy MoS2-based electrocatalysts in the HER (Table S4 and Figure S9) [42,43,44,45,46,47,48]. Although certain catalysts exhibit better performance, they require additional and more complex procedures for operation. Therefore, the method used to prepare the graphene-integrated ball-milled MoS2 is more cost-effective than several other techniques[34,49,50,51]. Stability is another key factor in the evaluation of electrocatalysts. The electrochemical stability of MoS2-12 h + G was evaluated by comparing the LSV curves before and after 6000 CV cycles. As shown in Figure 5c, the MoS2-12 h + G sample cycled 6000 times continued to exhibit high catalytic activity, and the overpotential at 10 mA cm-−2 shifted by only 15 mV, suggesting its superior stability. Furthermore, the morphology of the MoS2-12 h + G sample did not change after the stability measurements (Figure S10). The surface elemental composition after 6000 CV cycles was evaluated by XPS (Figure S11), and the results revealed only a slight shift in the peaks after the long-term stability measurements, which were evidently due to slight oxidation on the surface. Collectively, these findings underscore the remarkable potential of the electrochemically stable graphene-incorporated ball-milled MoS2 for future applications. 4. Conclusion In this study, the size of MoS2 particles was controlled using a simple mechanical process—ball milling—and the as-prepared samples exhibited significantly enhanced electrochemical and catalytic properties compared to those of pristine bulk MoS2. Upon ball milling, a 25%  enhancement was observed in the current intensity, and peak-to-peak separation was achieved. The HER activity of the as-prepared ball-milled sample was further improved by introducing graphene. In particular, the MoS2-12 h + G sample exhibited superior performance, showing an overpotential (160 mV) that was ~335 mV lower than that of pristine bulk MoS2. These improved performance metrics were most likely due to the increased availability and density of edge planes in the material, which was achieved by the particle size reduction, the appearance of the 1T phase, and an increase in sulfur vacancies, in addition to the superior bolstering effect of graphene. Therefore, ball milling with graphene mixture shows promise as a new, scalable method for preparing size-controlled, highly active electrocatalysts. Acknowledgments The authors are grateful to Prof. Takeaki Sakurai and Dr. S. A. Pawar from the University of Tsukuba for providing experimental support during the early stages of this study. This study was supported by the Japan Science and Technology Agency (JST) through the Adaptable and Seamless Technology Transfer Program through Target-driven R&D (A-STEP) [grant number JPMJTR22T4]; JSPS KAKENHI [grant numbers JP19H02551, JP19H05790, JP19H05046:A01, JP21H00015:B01, JP21H05012, JP22K18964, JP23K26536, and JP23H01843]; and MHI Innovation Accelerator LLC.  References [1] Chow J, Kopp RJ, Portney PR. Energy resources and global development. Science. 2003;302(5650):1528–1531. [2] Crabtree GW, Dresselhaus MS, Buchanan MV. The hydrogen economy. Phys Today. 2004;57(12):39–44. [3] Turner JA. A realizable renewable energy future. Science. 1999;285(5428):687–689.  [4] Dawood F, Anda M, Shafiullah GM. Hydrogen production for energy: an overview. Int J Hydrogen Energy. 2020;45(7):3847–3869. [5] Tafel J. Über die Polarisation bei kathodischer Wasserstoffentwicklung [On the polarization during cathodic hydrogen evolution]. Z Phys Chem. 1905;50U:641–712. [6] Wang S, Lu A, Zhong CJ. Hydrogen production from water electrolysis: role of catalysts. Nano Convergence. 2021;8:4. [7] Dubouis N, Grimaud A. The hydrogen evolution reaction: from material to interfacial descriptors. Chem Sci. 2019;10(40):9165–9181. [8] Lasia A. Mechanism and kinetics of the hydrogen evolution reaction. Int J Hydrogen Energy. 2019;44(36):19484–19518. [9] Shi Y, Zhang B. Recent advances in transition metal phosphide nanomaterials: synthesis and applications in hydrogen evolution reaction. Chem Soc Rev. 2016;45(6):1529–1541. [10] Wang D, Wang Z, Wang C, et al. Distorted MoS2 nanostructures: an efficient catalyst for the electrochemical hydrogen evolution reaction. Electrochem Commun. 2013;34:219–222. [11] Yan K, Lu Y. Direct growth of MoS2 microspheres on Ni foam as a hybrid nanocomposite efficient for oxygen evolution reaction. Small. 2016;12(22):2975–2981. [12] Voiry D, Salehi M, Silva R, et al. Conducting MoS2 nanosheets as catalysts for hydrogen evolution reaction. Nano Lett. 2013;13(12):6222–6227. [13] Jaramillo TF, Jørgensen KP, Bonde J, et al. Identification of active edge sites for electrochemical H2 evolution from MoS2 nanocatalysts. Science. 2007;317(5834):100–102. [14] Zhang J, Wu J, Guo H, et al. Unveiling active sites for the hydrogen evolution reaction on monolayer MoS2. Adv Mater. 2017;29(42):1701955. [15] Yin Y, Han J, Zhang Y, et al. Contributions of phase, sulfur vacancies, and edges to the hydrogen evolution reaction catalytic activity of porous molybdenum disulfide nanosheets. J Am Chem Soc. 2016;138(25):7965–7972.  [16] Lukowski MA, Daniel AS, Meng F, et al. Enhanced hydrogen evolution catalysis from chemically exfoliated metallic MoS2 nanosheets. J Am Chem Soc. 2013;135(28):10274–10277. [17] Tan Y, Liu P, Chen L, et al. Monolayer MoS2 films supported by 3D nanoporous metals for high‐efficiency electrocatalytic hydrogen production. Adv Mater. 2014;26(47):8023–8028. [18] Li G, Zhang D, Qiao Q, et al. All the catalytic active sites of MoS2 for hydrogen evolution. J Am Chem Soc. 2016;138(51):16632–16638. [19] Ambrosi A, Chia X, Sofer Z, et al. Enhancement of electrochemical and catalytic properties of MoS2 through ball-milling. Electrochem Commun. 2015;54:36–40. [20] Maijenburg AW, Regis M, Hattori AN, et al. MoS2 nanocube structures as catalysts for electrochemical H2 evolution from acidic aqueous solutions. ACS Appl Mater Interfaces. 2014;6(3):2003–2010. [21] Chen Z, Forman AJ, Jaramillo TF. Bridging the gap between bulk and nanostructured photoelectrodes: the impact of surface states on the electrocatalytic and photoelectrochemical properties of MoS2. J Phys Chem C. 2013;117(19):9713–9722. [22] Chung DY, Park SK, Chung YH, et al. Edge-exposed MoS2 nano-assembled structures as efficient electrocatalysts for hydrogen evolution reaction. Nanoscale. 2014;6(4):2131–2136. [23] Joensen P, Frindt RF, Morrison SR. Single-layer MoS2. Mater Res Bull. 1986;21(4):457–461. [24] Coleman JN, Lotya M, O’Neill A, et al. Two-dimensional nanosheets produced by liquid exfoliation of layered materials. Science. 2011;331(6017):568–571. [25] Voiry D, Yamaguchi H, Li J, et al. Enhanced catalytic activity in strained chemically exfoliated WS2 nanosheets for hydrogen evolution. Nat Mater. 2013;12(9):850–855. [26] Wang D, Zhang X, Bao S, et al. Phase engineering of a multiphasic 1T/2H MoS2 catalyst for highly efficient hydrogen evolution. J Mater Chem A. 2017;5(6):2681–2688. [27] Eda G, Yamaguchi H, Voiry D, et al. Photoluminescence from chemically exfoliated MoS2. Nano Lett. 2011;11(12):5111–5116.  [28] Wang T, Liu L, Zhu Z, et al. Enhanced electrocatalytic activity for hydrogen evolution reaction from self-assembled monodispersed molybdenum sulfide nanoparticles on an Au electrode. Energy Environ Sci. 2013;6(2):625–633. [29] Eda G, Fujita T, Yamaguchi H, et al. Coherent atomic and electronic heterostructures of single-layer MoS2, ACS Nano. 2012;6(8):7311–7317. [30] Choi JG, Thompson LT. XPS study of as-prepared and reduced molybdenum oxides. Appl Surf Sci. 1996;93(2):143–149. [31] Al-Kandari H, Al-Kharafi F, Al-Awadi N, et al. The catalytic active sites in partially reduced MoO3 for the hydroisomerization of 1-pentene and n-pentane. Appl Catal A. 2005;295(1):1–10. [32] Mahatha SK, Menon KSR. Inhomogeneous band bending on MoS2 (0001) arising from surface steps and dislocations. J Phys: Condens Matter. 2012;24(30):305502. [33] Cheng Y, Song H, Wu H, et al. Defects enhance the electrocatalytic hydrogen evolution properties of MoS2‐based materials. Chem Asian J. 2020;15(20):3123–3134. [34] Joyner J, Oliveria EF, Yamaguchi H, et al. Graphene supported MoS2 structures with high defect density for an efficient HER electrocatalysts. ACS Appl Mater Interfaces. 2020;12(11):12629–12638. [35] Wang X, Zhang Y, Si H, et al. Single-atom vacancy defect to trigger high-efficiency hydrogen evolution of MoS2. J Am Chem Soc. 2020;142(9):4298–4308. [36] Ahmed SM, Gerischer H. Influence of crystal surface orientation on redox reactions at semiconducting MoS2. Electrochim Acta. 1979;24(6):705–711. [37] Li L, Hagiwara S, Jiang C, et al. Boron monosulfide as an electrocatalyst for the oxygen evolution reaction, Chem Eng J. 2023;471:144489. [38] Li L, Watanabe N, Jiang C, et al. Development of a highly stable nickel-foam-based boron monosulfide–graphene electrocatalyst with a high current density for the oxygen evolution reaction, Sci Technol Adv Mater. 2023;24(1):2277681. [39] Meng X, Yu L, Ma C, et al. Three-dimensionally hierarchical MoS2/graphene architecture for high-performance hydrogen evolution reaction. Nano Energy. 2019;61:611–616.  [40] Liao L, Zhu J, Bian X, et al. MoS2 formed on mesoporous graphene as a highly active catalyst for hydrogen evolution. Adv Funct Mater. 2013;23(42):5326–5333. [41] Behera SK, Deb P, Ghosh A. Mechanistic study on electrocatalytic hydrogen evolution by high efficiency graphene/MoS2 heterostructure. ChemistrySelect. 2017;2(13):3657–3667. [42] Liu Z, Gao Z, Liu Y, et al. Heterogeneous nanostructure based on 1T-phase MoS2 for enhanced electrocatalytic hydrogen evolution. ACS Appl Mater Interfaces. 2017;9(30):25291–25297. [43] Liu Z, Zhao L, Liu Y, et al. Vertical nanosheet array of 1T phase MoS2 for efficient and stable hydrogen evolution. Appl Catal B. 2019;246:296–302. [44] Yao Y, Ao K, Lv P, et al. MoS2 coexisting in 1T and 2H phases synthesized by common hydrothermal method for hydrogen evolution reaction. Nanomaterials. 2019;9(6):844. [45] Li Y, Yin K, Wang L, et al. Engineering MoS2 nanomesh with holes and lattice defects for highly active hydrogen evolution reaction. Appl Catal B. 2018;239:537–544. [46] Eftekhari A. Electrocatalysts for hydrogen evolution reaction. Int J Hydrogen Energy. 2017;42(16):11053–11077. [47] Murthy AP, Madhavan J, Murugan K. Recent advances in hydrogen evolution reaction catalysts on carbon/carbon-based supports in acid media. J Power Sources. 2018;398:9–26. [48] Chen B, Hu P, Yang F, et al. In situ porousized MoS2 nano islands enhance HER/OER bifunctional electrocatalysis. Small. 2023;19(14):2207177. [49] Mugheri AQ, Otho AA, Mugheri AA. Meritorious spatially on hierarchically Co3O4/MoS2 phase nanocomposite synergistically a high-efficient electrocatalyst for hydrogen evolution reaction performance: recent advances & future perspectives. Int J Hydrogen Energy. 2021;46(44):22707–22718. [50] Bolar S, Shit S, Murmu NC, et al. Activation strategy of MoS2 as HER electrocatalyst through doping-induced lattice strain, band gap engineering, and active crystal plane design. ACS Appl Mater Interfaces. 2021;13(1):765–780.  [51] Guo D, Wan Z, Fang G, et al. A tandem interfaced (Ni3S2‐MoS2)@TiO2 composite fabricated by atomic layer deposition as efficient HER electrocatalyst. Small. 2022;18(24):2201896.     Figures, tables, and captions   Figure 1. Synthesis of graphene-integrated ball-milled MoS2 electrocatalyst.       Figure 2. SEM images of (a) pristine bulk MoS2, (b) MoS2-30 min, (c) MoS2-2 h, (d) MoS2-4 h, (e) MoS2-8 h, and (f) MoS2-12 h.      Figure 3. XRD patterns of bulk MoS2 powder and MoS2 samples ball-milled for different durations.     Figure 4. STEM images of (a) MoS2-30 min and (b–d) MoS2-12 h.    Figure 5. (a) LSV curves for HER with the as-prepared MoS2 samples in aqueous 0.5 M H2SO4, and (b) corresponding Tafel plots. (c) LSV curves of the MoS2-12 h + G electrocatalyst recorded before and after 6000 cyclic voltammetry cycles (0.1 to −0.3 V vs. RHE) in aqueous 0.5 M H2SO4.     Table 1. Crystal sizes of MoS2 samples ball-milled for varying periods, calculated using the Scherrer equation.  Sample 2θ (degree) FWHM Crystallite size (nm) Bulk MoS2 13.63 0.12 68 MoS2-15 min 14.05 0.23 35 MoS2-30 min 14.03 0.30 26 MoS2-1 h 14.06 0.51 15 MoS2-2 h 14.05 0.90 8 MoS2-4 h 14.06 0.98 6 MoS2-12 h 14.05 1.20 4      Table 2. Atomic percentages of the main elements detected by XPS for pristine bulk MoS2 and MoS2 samples ball-milled for different durations. The values of Mo and S are derived from the areas of the Mo 3d peak and S 2p peaks with sensitivities (Mo 3d5/2:23.52 and S 2p3/2:7.18) determined using SpecSurf software (JEOL Ltd., Japan).  Sample (MoS2) Mo (at%) S (at%) S/Mo ratio MoS2 (S/Mo: 2/1) Bulk 44.6 55.4 1.24 2.00 Ball-milled 30 min 45.6 54.4 1.19 1.92 Ball-milled 1 h 46.5 53.5 1.15 1.85 Ball-milled 2 h 46.9 53.1 1.13 1.82 Ball-milled 4 h 47.5 52.5 1.10 1.77 Ball-milled 12 h 48.2 51.8 1.07 1.72       Li et al. reported MoS2-based electrocatalysts for the hydrogen evolution reaction. The superior catalytic activity was ascribed to exposed edge sites, sulfur vacancies, 1T phase, and the effect of graphene.     TOC    Supplementary information Ball-milled MoS2 with graphene shows enhanced catalytic activity for hydrogen evolution reaction  Table of contents Figures S1–S10 ……………………………………………… 1 Tables S1–S4 ………………………………………………… 8 Supplementary references …………………………..……..… 10    Supplementary figures  Figure S1. Wide-scan XPS profiles of bulk MoS2 before (black) and after (pink) ball milling for 12 h. (Si originates from SiO2, which is inevitably present during the XPS measurements, as each sample is prepared by dripping the sample ink onto a glass slide.)    Figure S2. High-resolution (a) Mo 3d and (b) S 2p XPS profiles of bulk MoS2 before (black) and after (pink) 12 h of ball milling.    Figure S3. LSV curves of as-prepared ball-milled samples, a commercial 20% Pt/C specimen, and bulk MoS2 in aqueous 0.5 M H2SO4.     Figure S4. LSV curves for the HER of as-prepared and sonicated samples in aqueous 0.5 M H2SO4.  Figure S5. Cyclic voltammograms of 5 mM [Fe(CN)6]4−/3− (vs. Ag/AgCl reference electrode) using bulk MoS2 and MoS2 ball-milled for 12 h, with 0.1 M KCl as the supporting electrolyte.      Figure S6. LSV curves for the HER of as-prepared and graphene-incorporated samples in aqueous 0.5 M H2SO4.  Figure S7. (a) SEM image and (b) electron-probe microanalysis mapping images of MoS2-12 h + G with Nafion.     Figure S8. Cyclic voltammograms of graphene and MoS2 -12 h + G sample in aqueous 0.5 M H2SO4.  Figure S9. Overpotentials of several MoS2-based catalysts at a current density of 10 mA cm−2 under acidic conditions [1–10].      Figure S10. (a) SEM image and (b) electron-probe microanalysis mapping images of MoS2-12 h + G with Nafion after stability measurements.     Figure S11. High-resolution (a) Mo 3d and (b) S 2p XPS profiles of MoS2 ball-milled for 12 h before (pink) and after (blue) 6000 CV cycles.       Supplementary tables Table S1. Comparison of as-prepared and sonicated samples  Sample Overpotential at 10 mA cm−2 (mV)  Sample  (with 2 h sonication) Overpotential at 10 mA cm−2 (mV) MoS2-30 min 368  MoS2-30 min + sonication 292 (76▼) MoS2-4 h 359  MoS2-4 h + sonication 262 (97▼) MoS2-8 h 408  MoS2-8 h + sonication 304 (104▼)  MoS2-12 h 380  MoS2-12 h + sonication 237 (143▼)  Table S2. Comparison of as-prepared and graphene-integrated samples.  Sample Overpotential at 10 mA cm−2  Sample (mixed with graphene) Overpotential at 10 mA cm−2 (mV)  (mV) MoS2-30 min 368  MoS2-30 min + G 228 (140▼) MoS2-4 h 359  MoS2-4 h + G 180 (179▼) MoS2-8 h 408  MoS2-8 h + G 169 (239▼)  MoS2-12 h 380  MoS2-12 h + G 160 (220▼)  Table S3. Comparison of certain as-prepared, and sonicated, and graphene-incorporated samples  Sample Overpotential  at 10 mA cm−2 (mV) Tafel slope (mV dec−1) Bulk MoS2 495 146 MoS2-12 h 380 200 MoS2-12 h + sonication 237 129 MoS2-12 h + G 160 86 20% Pt/C 44 25    Table S4. Comparison of the HER performances of previously reported MoS2-based electrocatalysts in acidic solutions.   Overpotential at 10 mA cm−2 (mV) Tafel slope (mV dec−1) Reference MoS2-12 h + G 160 86 This work MoS2/P-rGO 240 75 [1] E-MoS2/GCE 210 70 [2] GQDs-MoS2 200 43 [3] 1T-MoS2 220 61 [4] VS2/MoS2 177 55 [5] MoS2/g-C3N4 260 63 [6] MoS2 nanomesh 160 46 [7] MoS2/CNT/graphene 255 100 [8] 1T@2H-MoS2 250 88 [9] MoS2/BCTM 176 51 [10] 1rGO-MoS2 210 41 [11] MoS2 nano islands 248 84 [12]    References [1] Liu Y, Liu J, Li Z, et al. Exfoliated MoS2 with porous graphene nanosheets for enhanced electrochemical hydrogen evolution. Int J Hydrogen Energy. 2018;43(30):13946–13952. [2] Ji S, Yang Z, Zhang C, et al. Exfoliated MoS2 nanosheets as efficient catalysts for electrochemical hydrogen evolution. Electrochim Acta. 2013;109:269–275. [3] Guo J, Zhu H, Sun Y, et al. Doping MoS2 with graphene quantum dots: structural and electrical engineering towards enhanced electrochemical hydrogen evolution. Electrochim Acta. 2016;211:603–610. [4] Liu Z, Gao Z, Liu Y, et al. Heterogeneous nanostructure based on 1T-phase MoS2 for enhanced electrocatalytic hydrogen evolution. ACS Appl Mater Interfaces. 2017;9(30):25291–25297. [5] Chen X, Yu K, Shen Y, et al. Synergistic effect of MoS2 nanosheets and VS2 for the hydrogen evolution reaction with enhanced humidity-sensing performance. ACS Appl Mater Interfaces. 2017;9(48):42139–42148. [6] Fageria P, Sudharshan KY, Nazir R, et al. Decoration of MoS2 on g-C3N4 surface for efficient hydrogen evolution reaction. Electrochim Acta. 2017;258:1273–1283. [7] Li Y, Yin K, Wang L, et al. Engineering MoS2 nanomesh with holes and lattice defects for highly active hydrogen evolution reaction. Appl Catal B. 2018;239:537–544. [8] Murthy AP, Madhavan J, Murugan K. Recent advances in hydrogen evolution reaction catalysts on carbon/carbon-based supports in acid media. J Power Sources. 2018;398:9–26. [9] Yao Y, Ao K, Lv P, et al. MoS2 coexisting in 1T and 2H phases synthesized by common hydrothermal method for hydrogen evolution reaction. Nanomaterials. 2019;9(6):844.  [10] Qiao S, Zhao J, Zhang B, et al. Micrometer‒scale biomass carbon tube matrix auxiliary MoS2 heterojunction for electrocatalytic hydrogen evolution. Int J Hydrogen Energy. 2019;44(60):32019–32029. [11] Joyner J, Oliveira EF, Yamaguchi H, et al. Graphene supported MoS2 structures with high defect density for an efficient HER electrocatalysts. ACS Appl Mater Interfaces. 2020;12(11)12629–12638. [12] Chen B, Hu P, Yang F, et al. In situ porousized MoS2 nano islands enhance HER/OER bifunctional electrocatalysis. Small. 2023;19(14):2207177.