Large-Scale Synthesis of Graphene-Like MoSe2 Nanosheets for

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Large Scale Synthesis of Graphene-Like MoSe2 Nanosheets for Efficient Hydrogen Evolution Reaction Chu Dai, Zhaoxin Zhou, Chen Tian, Yong Li, Chao Yang, Xueyun Gao, and Xike Tian J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b11423 • Publication Date (Web): 02 Jan 2017 Downloaded from http://pubs.acs.org on January 2, 2017

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The Journal of Physical Chemistry

Large Scale Synthesis of Graphene-Like MoSe2 Nanosheets for Efficient Hydrogen Evolution Reaction Chu Dai, Zhaoxin Zhou *, Chen Tian , Yong Li, Chao Yang, Xueyun Gao and Xike Tian* Faculty of Materials Science and Chemistry, China University of Geosciences, Wuhan 430074, P. R. China. *Corresponding author: Prof. Zhaoxin Zhou and Prof. Xike Tian. E-mail: [email protected], Tel.: +86 27 6788 4574, Fax: +86 27 6788 4574; [email protected], Tel.: +86 27 6788 4574, Fax: +86 27 6788 4574.

ABSTRACT

Two-dimensional (2D) materials have attracted great attention by researchers due to their fascinating properties and promising applications. However, the synthesis methods for few layers are usually difficult to expand to large area applications because of their low yield. In this paper, graphene-like MoSe2 nanosheets are successfully and scaleable synthesized by a facile and lowcost hydro-thermal method under the synergy of PVP and graphene. The ultrathin MoSe2 nanosheets are typically 1-3 layers, which are confirmed by HRTEM. This unique structure makes this MoSe2 electrode material show superior activity towards the electrocatalytic hydrogen production with a low Tafel slope about 70 mV·dec-1. Furthermore, the synthesized graphene-like MoSe2 nanosheets had a high stability during the electrocatalytic process and we nearly cannot find the degradation after 1000 cyclic voltammetric sweeps.

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INTRODUCTION Two-dimensional (2D) materials have attracted great attention of researchers due to their fascinating properties and promising applications.1−5As known to all, graphene has extraordinary properties ,such as optical, electronic, thermal and mechanical properties together with a superior surface area, which offers great benefits when they are used in transistors and as electrochemical electrodes.6However, their switch current ability in transistors are seriously reduced because of the zero band gap of pristine graphene.7 Even though some works have been made based on the graphene surface functionalization or put them under external electric or strain fields, the band gaps can be changed little .8 On the other hand, considering their photoresponse, the band-gap energies is an important factor for 2D semiconductors when they are applied in optoelectronic devices. And monolayers of these 2D semiconductors, such as MoS2, MoSe2, WS2, and WSe2, could result in substantial changes in their electronic structures and bring new optoelectronic properties and applications in devices.9-12 MoSe2 a typical layered semiconducting material, was proven to be an interesting narrowband-gap semiconductor material and belongs to the family of Transition metal chalcogenides (TMDs).13-14 Its layered structure is similar to graphite, both the individual layers are interacted with weak van deer Waals. The general forms of MoSe2 are widely used for different purposes, such as intercalation,

15

lubricants,

16

catalysts,

17

electrodes18 and so on. When decreasing

thickness of the MoSe2 layers to few-layers or a monolayer, the band structure can be changed from the bulk indirect structure to direct structure. And the band gap changed correspondingly from 1.1eV to 1.5 eV.19,20 Therefore, monolayer or few-layers MoSe2 can be a promising candidate for applications in the optical devices, especially in the electrical fields because of its tunable properties.


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Besides, hydrogen is considered to be a promising alternative for fossil fuels, due to the superior advantages, such as high energy density and environment benign et al. 21,22 Being one of the potential earth-abundant materials, MoSe2 has aroused wide attention as the Pt substitute towards the electrocatalytic hydrogen production due to their tunable band structure and rich electronic properties. Previous studies showed the active sites in MoSe2 nanosheets are distributed over the unsaturated Se-edges, which are beneficial to the HER process as the same with MoS2.23 Though majority of papers reported on the synthesis of MoS2 for HER, the more metallic nature compared with MoS2 and the lower Gibbs free energy for hydrogen adsorption onto MoSe2 edges than MoS2 with higher coverage of hydrogen adsorption, makes MoSe2 captured more attention recently.24,25And in order to improve the HER activity, we can either expose more edges of MoSe2, namely increase more active sites or enhance the electrical conductivity of the material by constructing hybrids with other conductive substrates. Majority of papers have reported on the synthesis of MoS2. 26-35While it is difficult to prepare Se-based TMDs (MSe2) than MS2 with large scale because of the lower chemical reactivity of Se.36,37 Though several previous approaches used for material manufacturing have been mostly based on mechanical exfoliation20,28(the “Scotch tape method”), liquid exfoliation, chemical vapor deposition (CVD)29,30, colloidal synthesis31, chemical exfoliation32,33(lithium intercalation– exfoliation), and electrochemical exfoliation.34 The problem is that they have limitations in size, reproducibSility, and low throughput. For example, Gi Woong Shim 35et al reported synthesis of MoSe2 monolayers in large area. However, the process is complicated and needs to be carried out under high temperature. What’s more, it is not safe when using H2 in the selenization process and the extra H2Se gas generate in this process needs further treatment. Thus, the preparation of MoSe2 few layers in large area has still been a great challenge.



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In this work , we develop a novel, general ,facile ,and scaleable method to synthesize graphene-like ultrathin MoSe2 nanosheets through a hydrothermal route under mild conditions from the reaction of NCSeCH2COONa, (NH4)6Mo7O24·4H2O , graphene oxide (GO) and a certain amount of polyvinyl pyrrolidone (PVP) as the surfactant. And both the graphene and PVP play important roles in this reaction. By introducing PVP into this reaction, it could change the morphology of MoSe2 and effectively prevent the agglomeration of carbon atoms in the reduced graphene (RGO) sheets. As an excellent dispersant with layer structure, RGO could offer a platform that makes MoSe2, expose more active sites while twined with RGO. Besides, due to the excellent electrical conductivity, RGO can accelerate electron conduction in the prepared samples, which makes graphene-like MoSe2 nanosheets suitable for HER catalysts. Hence their HER activity were also studied in our work.

METHODS

Synthesis of the graphene-like MoSe2 nanosheets To explore the effect of PVP in this reaction, we do a control experiment without adding graphene in the reaction. Briefly, 0.5mmol (NH4)6Mo7O24·4H2O (i.e. 3.5 mmol Mo) and 7 mmol NCSeCH2COONa powder is dissolved in 60mL H2O-ethylene glycol mixture solution with a volume ratio of 1:1. Then add 0.1g PVP and appropriate amount of graphene oxide to the mixture solution. After well mixed, the solution is transferred into the Teflon-lined autoclave for 24 h at 210℃. The collected black precipitates were washed with DI water and absolute ethyl alcohol for at least 3 times, respectively. Then the products are dry-vacated overnight. To make comparisons, we also do the comparable experiment without adding the surfactant. The effects of different surfactants on the morphology and electrochemical properties


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of

the

prepared

samples

were

also

investigated.

Other

surfactants,

such

as

Cetyl Trimethyl Ammonium Chloride (CTAC) and Sodium Dodecyl Benzene Sulfonate (SDBS) were added to each solution to make comparisons of the difference of different surfactant. The graphene-like MoSe2 nanosheets is synthesized by adding appropriate amount of graphene oxide to the above PVP assisted homogeneous solution dropwise. GO is prepared based on the previously modified Hummers’ method38 and its concentration of GO is calculated to be 5.525g /L. According to the content of GO in the preparation process, we named the prepared samples as PVP-MoSe2@G1 and PVP-MoSe2@G2 with a GO amount of 2% and 4%, respectively. The obtained solution is stirred for 12 h and it is transferred into the Teflon-lined autoclave (24 h at 210℃). The subsequent operations are the same as the above process.

Characterization

X-ray diffraction (XRD, Bruker AXS D8-Focus) is used to characterize the crystalline structure of the prepared samples. The surface morphologies of the samples are investigated with the field emission scanning electron microscopy (FESEM, Hitachi SU-8010) equipped with an attached Oxford Link ISIS energy-dispersive X-ray spectroscopy (EDS) apparatus and transmission electron microscopy (TEM, Philips CM 12). The chemical compositions are further analyzed by X-ray photoelectron spectroscopy (XPS, VG Multilab 2000). The differences of different samples structure are obtained by Fourier transform infrared spectrometer (FTIR, Bruker, VERTEX 70). The structures of the samples are further invested by High-resolution Transmission Electron Microscopy (HRTEM, JEM 2010).



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Electrochemical measurements

Prior to all electrochemical measurements, 4 mg of catalyst and 30 µL of Nafion solution are added to 1 mL water-ethanol solution (volume ratio: 3:1) respectively, which was sonicated for 60 min to form a homogeneous solution. 5 µL of the above dispersion were loaded onto the glassy carbon electrode (GCE) with 3 mm diameter. All the electrochemical experiments are conducted using a CHI660C electrochemical workstation in a standard three-electrode system, where Ag/AgCl (saturated KCl), graphite rod and the modified GCE are used as the reference electrode, counter electrode and the working electrode, respectively. The potential is calibrated to the reversible hydrogen electrode (RHE). Line sweep voltammetry is performed at a scan rate of 5 mV·s−1. The Nyquist plots are obtained with frequencies ranging from100 kHz to 0.01 Hz under the over potential of 200 mV.

RESULTS AND DISCUSSON

The crystal structure of these as-prepared MoSe2 microspheres and MoSe2@G nanocomposites were systematically investigated by using XRD patterns. Figure 1a shows the XRD patterns of graphene-like ultrathin MoSe2 nanosheets (PVP-MoSe2@G). The peaks at 32°,38°and 56° as shown in the XRD spectrum are in agreement with hexagonal MoSe2 (JCPDS No. 17-0887). It indicates the high purity of the prepared samples. While the XRD diffraction peak at 13.7° shifts to small angular direction. It is likely to be the oxygen incorporate enlarges the interlayer spacing, thus decreases the angle. And the XRD patterns of other products at different reactions were shown in Figure S1. The entire diffraction peaks were well matched with the JCPDS Card, No.17-0887. Figure 1b shows the SEM image of PVP-MoSe2@G1 nanohybrids, the MoSe2 nanosheets intermingle with the graphene nanosheets. This can be easily observed from Figure


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1c and Figure 1d, which are the TEM image of PVP-MoSe2@G1 nanohybrids. The EDS element mapping analysis in Figure S2 also indicates the homogeneous distribution of C, O, Mo and Se. It confirms the graphene nanosheets are uniform adhere to the MoSe2 layers. Besides, the atomic ratio of Se to Mo with about 2:1 can further indicate the sample is MoSe2. The morphology of without adding surfactant or adding different surfactants were shown in Figure S3. We can clearly see that different surfactants have different effects on the morphology of the same material. And both the surfactant and graphene play synergistic effect in the form of the graphene-like ultrathin MoSe2 nanosheets. The structure of the PVP-MoSe2@G1 nanohybrids is further confirmed using HRTEM (Figure 2). Figure 2a shows the top-view HRTEM images of the PVP-MoSe2@G1. From Figure 2a, there are numbers of MoSe2 crystal fringes are dispersed onto the amorphous graphene nanosheets .The green and orange part of the Figure 2a can be amplified as shown in Figure 2b and Figure 2c, respectively. From the Figure 2b and Figure 2c, we can clearly see the inter-planar spacing of 0.24 nm and 0.28 nm, and they are correspond to the d spacing of the (103) and (100) planes of the MoSe2 (JCPDS Card, No.17-0887), respectively. Besides, we can observe the rich defects (marked in red) from the HRTEM images of the PVP-MoSe2@G1 nanohybrids, which can increase the electrocatalytic active sites and the catalytic ability. The side-view HRTEM image (Figure 2d) shows the interlayer spacing of 0.71 nm, which is greater than that of the ordinary MoSe2 of 0.65 nm from the d spacing of the (100) plane of the MoSe2. The thickness of the curled edges is measured to be 2.13 nm, which is three times of the interlayer spacing of 0.71 nm. It implies the curled edge of the MoSe2 nanosheets consists of 3 layers. Figure 2e shows the crystal structure of MoSe2 with the interlayer spacing to be 0.65nm. The crystal structure of



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MoSe2 with the interlayer spacing of 0.71nm is shown in Figure 2f, which may attribute to the oxygen incorporate. In order to study the reason for the expansion of the interlayer spacing, FTIR and XPS were carried out. FTIR spectrums of different samples are shown in Figure 3. And the peaks on the FTIR spectrum belonging to the corresponding vibrational modes for prepared samples are shown in Table S1. Figure 3a shows the FTIR spectrum of pure MoSe2 and it no obvious peaks. From the FTIR spectrum of the pure PVP (Figure 3b), it shows a broad speak centered at 3434 cm−1 corresponding to -OH stretching and a peak at 2955 cm−1 is corresponding to C-H stretching. The peak at 1661 cm−1 was the characteristic peak of C=O stretching vibration while the peaks at 1291 cm−1 and 1440 cm−1 was attributed to the stretching vibration of C-N and the attachment of -CH2 in pyrrole ring of PVP. 39,40 From Figure 3, we can clearly see that after adding the PVP in this reaction process, the samples of PVP-MoSe2 (Figure 3c) and PVPMoSe2@G1 (Figure 3d) present the almost the same peaks with the pure PVP (Figure 3b). What’s more, a red-shift from 1661 cm−1 to 1644 cm−1 was observed for C=O due to the weak bond of the partial donation of oxygen with the molybdenum.41 And the C-O stretching vibration of the graphene can also be seen at the 944 cm−1.42 Thus it is clear that the prepared MoSe2 contain PVP. While when not adding the PVP, the MoSe2 presents weak peaks. All spectra contain two peaks located at 2329 cm−1 and 2368 cm-1, corresponding to the C=O stretching mode of intercalated CO2. These peaks indicate the CO2 molecules physical adsorption on the surface of the prepared samples.42 Besides, the XPS results of the as-prepared samples are shown in Figure 4. For the C 1s spectra (Figure 4a), the binding energies at 284.5eV, 285.5eV 286.7eV and 288.3eV are related to C-C, C-N, C-O and C=O43, which indicated that PVP and MoSe2 are well dispersed onto the


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graphene surface. For the O 1s spectra (Figure 4b), the peaks at 531.41, and 532.56 indicate the existence of C-C, C=O. Besides, the binding energy at 530.50 eV in O 1s spectra was due to Mo (IV)-O, thus verifying the successful oxygen incorporation in the MoSe2 nanosheets.44,45 The characteristic peaks at 228.50 eV (Mo 3d5/2) and 231.58 eV ( Mo 3d3/2) in Figure 4c indicated the oxidation state of Mo was IV.31,46 The Se 3d region of the PVP-assisted MoSe2@G1 nanohybrids (Figure 4d) exhibits 3d5/2 and 3d3/2 peaks at 53.8 and 54.9eV, which are consistent with a divalent sulfide ion (Se2−).46 Thus, it may be due to the oxygen incorporate that enlarges the interlayer spacing , and the experimental result is consistent with the result of XRD. The possible process for forming the oxygen incorporate may contribute to the oxygen in PVP that combine with the metal ions in the reaction.39,47 It can be shown in Figure 5. First, the PVP would attach to the surface of graphene oxide nanosheets due to the strong adsorption. When the reaction started, the Mo6+ and graphene oxide could be reduced to Mo4+ and graphene by ethanediol, respectively. During this process, the Mo4+ could be well dispersed due to the large space steric hindrance of PVP and the hydrogen-bond interaction of the metal ions and oxygen of the graphene. Besides, the organic selenium can decompose to H2Se, CO2 and NH3 in this acid solution system. Finally the Mo4+ would combine with Se2- which forms MoSe2. And the oxygen incorporate may form in this process. Due to such oxygen incorporate in the PVP-MoSe2@G1, we can observe the rich defects (marked in red) from the HRTEM images of the PVP-MoSe2@G1 nanohybrids, which can increase the electrocatalytic active sites and the catalytic ability. We believe that the large number of defects and the few layers of MoSe2 nanosheets observed in our experiment results would play important roles in the HER.



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The HER activity of different surfactant synthesized MoSe2 were studied at first. As mentioned above, different surfactants have different impacts on the samples’ morphology, which would present different electric-catalytic activity. As shown in Figure S4, the polarization curves of the different surfactant synthesized MoSe2 are carried out. Obviously, we can find that when adding the surfactant to the reaction solution, the products’ HER activities get improved greatly. While when comparing different surfactants, the current density of PVP-MoSe2 is higher than that of the other surfactants assisted MoSe2 at the onset overpotential of −0.4 V (vs. RHE). By adding the surfactant , the size of the product gets smaller and the morphology of the product emerges more nanosheets on the edge , which would increase the number of active sites for the HER, thus improve the HER activity. Besides, the nanostructure engineering of the conjugation of MoSe2 with RGO could improve charge transfer behavior and bring new properties. So the HER activities of PVP-MoSe2@G deposited on a glassy carbon electrode in a 0.5 M H2SO4 solution are investigated. The commercial Pt electrode was used as a reference under the same condition and exhibits superior catalytic activity towards hydrogen production and the onset over potential is nearly zero. Figure 6a shows the polarization curves of MoSe2, PVP-MoSe2 and the PVP-MoSe2@G1. Compared to MoSe2, the PVP-MoSe2 and the PVP-MoSe2@G1 possess much lower onset overpotential of 0.18V and 0.15 V, respectively. Lower onset overpotential of PVP-MoSe2@G1 should come from two aspects: (1) RGO as an excellent dispersant with layer structure offer a platform that makes MoSe2, exposed more active sites, (2) RGO can accelerate electron conduction due to the excellent electrical conductivity, thus resulting in a great enhancement towards the electrocatalytic hydrogen production.48 It is not always the higher the mass fraction of graphene, the better the electrocatalytic activities of the samples. As seen in Figure S5, with the increased


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loading of RGO, the electrocatalytic activity decreased .The current density of PVP-MoSe2@G2 is even lower than that of the PVP-MoSe2 at the onset overpotential of −0.4 V (vs. RHE), which directly indicates the worse electrocatalytic activity. Too much graphene supporting on MoSe2 would decrease the electrocatalytic ability perhaps due to the blockage of active sites that only existing on the edges of MoSe2. So when increasing the content of graphene, it is actually reducing the active sites of the MoSe2 and the electrocatalytic ability would be reduced correspondingly. Moreover, Tafel slope could reflect the inherent property of an electrode (determined by the rate-limiting step in the HER process)49, which is hence an important factor of an electrocatalyst. The HER is the half reaction of water splitting and contains three basic steps for converting H+ to H2 in an acidic medium.50 As shown in Equation 1, the initial reaction is an initial discharge step named as Volmer reaction. The second and third reactions were electrochemical desorption step (Heyrovsky reaction, Equation 2) and a recombination step (Tafel reaction, Equation 3), respectively. Supposed that the Volmer, Heyrovsky or Tafel reaction was the rate-determining step separately, the Tafel slopes should be about 120, 40 or 30 mV·dec-1 according to the previous reports31 on kinetic models for the HER. In a typical HER process, the rate-determining often comes from the combination of the above steps such as Volmer–Heyrovsky mechanism or Volmer-Tafel mechanism, that lead to the production of H2 molecules.

H3O+ + e- → Hads + H2O

(Equation 1)

Hads + H3O+ + e- → H2 + H2O (Equation 2) Hads + Hads → H2

(Equation 3)



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Figure 6b shows the corresponding Tafel plots of MoSe2, PVP-MoSe2, PVP-MoSe2@G1 and Pt. Their Tafel slopes are 177, 84, 70 and 30 mV ·dec-1, respectively. This result indicates that the PVP-MoSe2 presents excellent electrochemical performance among the MoS2 or MoSe2based HER catalysts. A comparison of the HER activity between different catalysts is provided in Table S2. It also indicates that the Volmer reaction is the rate-determining step in the HER process.23,51 Through this discharge step ,the protons in the H2SO4 solution can be converted into active hydrogen on the catalyst surface. In addition, we can find that Tafel slope decreased greatly when adding the surfactant PVP which could be the effects of PVP that changed the size and morphology of MoSe2 and increased more active cites thus enhance the HER activities. While when adding the surfactant PVP and decorating RGO, we can find the Tafel slope decreased further. This can be the synergistic effect of PVP and RGO in the prepared samples. On one hand, PVP and RGO can efficiently prevent the aggregation of the MoSe2 layers to expose more active sites of the electrode material because they are excellent dispersants. On the other hand, the excellent electrical conductivity of RGO can accelerate electron fast transport in the electrode reactions as they can offer a platform and form an interconnected conducting network. In order to prove the interface reactions, intrinsic conductivity and get a better understanding of the kinetics of the different catalysts under the HER process, electrochemical impedance spectroscopy (EIS) is carried out under frequencies ranging from 100 kHz to 0.01 Hz .The insert of Figure 6c shows the equivalent circuit model of these electrode materials, which is used to illustrate the performances of the electrodes. And the Rct, Rs and CPE in the Figure 6c are corresponding to the charge transfer resistance, the series resistance and the constant phase element, respectively. From the Figure 6c, the semicircle in the Nyquist plot can be assigned to


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the Rct, which is caused due to the H+ reduction at the electrode-electrolyte interface. And Rs is attributed to the x-intercept of Nyquist plots. Obviously, we can see a significantly decreased Rct for MoSe2 (1306 Ω), PVP-MoSe2 (160 Ω) and PVP-MoSe2@G1 (50 Ω). The EIS results show that adding surfactant and graphene indeed optimized the electrical properties and enhanced the intrinsic conductivity. As a splendid conductive substrate, graphene improves the interdomain conductivity of the PVP-MoSe2@G electrode material, which enhances the electrochemical reaction. At last, durability is another important element, which is used to evaluate whether the electrode material is an advanced electrocatalyst. The durability of the PVP-MoSe2@G1 electrocatalysts has been cycled continuously for 1000 cycles. As shown in Figure 6d, a negligible decay can be observed which indicates the excellent stability of the PVP-MoSe2@G electrocatalyst in a longterm electrochemical process. Above all, the PVP-MoSe2@G nanohybrids have excellent catalyst activity and high stability in the HER.

CONCLUSIONS

In conclusion, we have successfully developed a method for scaleable synthesized graphene-like MoSe2 ultrathin nanosheets through adding the surfactant in the reaction process and adding the graphene as the dispersant. These ultrathin nanosheets make this electrode material exhibit excellent catalytic property with a lower tafel slope of 70 mV·dec-1. And the mechanism of HER is according to the Volmer-Heyrovsky mechanism. In addition, the graphene-like MoSe2 ultrathin nanosheets can be cycled continuously for 1000 cycles without any loss in electrocatalytic ability, exhibiting high durability. This work successfully indicates that the PVP



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assisted and graphene supported defects-rich and ultrathin MoSe2 nanosheets are feasible by a facile hydrothermal method and exhibit high electrocatalytic ability for the HER.

ASSOCIATE CONTENT

Supporting Information Available: Supplementary information provides additional Figures and Tables described in this paper, which is available free of charge via the Internet at http://pubs.acs.org.

NOTES

The author declares no competing financial interests.

ACKNOWLEDGEMENTS

This work was supported by the Foundation for Innovative Research Groups of The National Natural Science Foundation of China (No. 41521001), the National Natural Science Foundation of China (Grant No. 51371162) and the Fundamental Research Funds for the Central Universities, China University of Geosciences (Wuhan).

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Figures:

Figure 1 (a) XRD patterns of the PVP- MoSe2@G1; (b) SEM images of PVP-MoSe2 @G1; (c,d) TEM images of PVP-MoSe2 @G1.



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Figure 2 (a) Top-view HRTEM image; (b,c) enlarge images of green and orange part of the Figure 2a ;(d) side-view HRTEM image of the curled edge of the ultrathin PVP-MoSe2@G1 nanosheets;(e) Crystal structure of MoSe2; (f)Crystal structure of oxygen incorporate MoSe2;


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Figure 3 FTIR spectrums of different samples: (a) MoSe2; (b) pure PVP; (c) PVP-MoSe2; (d)PVP-MoSe2@G1.



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Figure 4 XPS of the as-prepared PVP-MoSe2@G1 ultrathin nanosheets: (a) C 1s ; (b) O 1s ; (c) Mo 3d ; (d) Se 3d .


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Figure 5 The reaction process of forming the final PVP-MoSe2@G1 ultrathin nanosheets.



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Figure 6 (a) Polarization curves and (b) corresponding Tafel plots of MoSe2, PVP-MoSe2, PVPMoSe2@G1 and Pt; (c) Nyquist plots of MoSe2, PVP-MoSe2, PVP-MoSe2@G1; (d) The initial polarization curves of the PVP-MoSe2@G1nanohybrids and after 1000 cycles.


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Table of Contents Graphic Large scale synthesis of graphene-like MoSe2 nanosheets for efficient hydrogen evolution reaction


Large-Scale Synthesis of Graphene-Like MoSe2 Nanosheets for

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