Synergistic Effect of MoS2 Nanosheets and VS2 for the Hydrogen

Subscriber access provided by READING UNIV

Article

Synergistic Effect of MoS2 nanosheets and VS2 for Hydrogen Evolution Reaction with Enhanced Humidity Sensing Performance Xiaofan Chen, Ke Yu, Yuhao Shen, Yu Feng, and Zi-Qiang Zhu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b14957 • Publication Date (Web): 09 Nov 2017 Downloaded from http://pubs.acs.org on November 10, 2017

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Synergistic Effect of MoS2 Nanosheets and VS2 for Hydrogen Evolution Reaction with Enhanced Humidity Sensing Performance Xiaofan Chen, † Ke Yu, †, ‡,* Yuhao Shen, † Yu Feng, † and Ziqiang Zhu† †

Key Laboratory of Polar Materials and Devices (Ministry of Education of China), Department

of Electronic Engineering, East China Normal University, Shanghai 200241, China ‡

Collaborative Innovation Center of Extreme Optics, Shanxi University, Taiyuan, Shanxi

030006, China

ABSTRACT: As a typical transition metal dichalcogenides (TMDs), MoS2 has been a hotspot of research in many fields. In this work, the MoS2 nanosheets were compounded on 1T-VS2 nanoflowers (VS2@MoS2) successfully by two-step hydrothermal method for the first time and their hydrogen evolution properties were studied mainly. The higher charge transfer efficiency benefiting from the metallicity of VS2 and the greater activity due to more exposed active edge sites of MoS2 improve the hydrogen evolution reaction (HER) performance of the nanocomposite electrocatalyst. Adsorption and transport of intermediate hydrogen atom by VS2 also enhances the hydrogen evolution efficiency. The catalyst shows a low onset potential of 97 mV, a Tafel slopt as low as 54.9 mV dec-1 and good stability. Combined the electric conductivity of VS2 with the physicochemical stability of MoS2, VS2@MoS2 also exhibits excellent humidity properties.

KEYWORDS: MoS2, VS2, VS2@MoS2, Nanomaterials, Hydrogen evolution reaction, Humidity sensor

ACS Paragon Plus Environment

1


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 31

INTRODUCTION Electrocatalytic splitting of water through hydrogen evolution reaction (HER) is a promising method for its high-efficiency and non-pollution to solve the energy crisis.1 The catalyst is the key factor influencing the speed or efficiency for HER, which requires low overpotential and high stability. Due to the high cost of precious metals with high catalytic activity, the replacement of them has been investigated all over the world. 2D TMDs are inexpensive and abundant candidates for HER. Owing to the alternating sheets structure with transition-metal atom layer sandwiched between two chalcogen atom layers, all of the catalytically active edge sites are exposed, which is the critical advantage to improve HER properties.2 As a typical transition metal sulfide, MoS2 with a band gap range from 1.2 to 1.8 eV as the thickness decreased to monolayer has been developed into ultrasensitive and broadband photodetector3 and floating gate memory-based monolayer MoS2 transistor4. More than that, MoS2 also has excellent chemical stability and potential hydrogen evolution activity, and the theoretical calculations show that the binding energy of hydrogen atoms on the edge of layered MoS2 is close to that of platinum.5 MoS2 can be devided into three types of atom arrangements (1T-MoS2, 2H-MoS2, and 3R-MoS2) based on different crystal structures.6 2H and 3R phase have trigonal prismatic coordination and perform as semiconductor. While 1T phase with trigonal antiprismatic (or octahedral) coordination is metallic.7 1T-MoS2 is demonstrated to exhibit excellent HER catalytic performances due to active sites both on the edges and basal plane.8 However, 1T-MoS2 and 3RMoS2 are both metastable.6 So most attention has been concentrated on 2H-MoS2 because it is stable and easy to obtained.


2

Page 3 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Due to the van der Waals force between the two layers, MoS2 nanosheets are easy to reunite in the process of synthesis and their conductivity is poor, which hinders the further improvement of hydrogen evolution performance. In the past decade, the main research directions of the catalysts for HER of MoS2 are to improve catalytic activity by thinning layers9, 10 and defecting11-13, or to enhance electron transport by doping14, 15 and designing synergetic composites16-18. And the last method is more common and easy to achieve. Chen et al. grew MoS2 on porous MoO2 via hydrothermal method to enhanced conductivity, which achieved a large current density (10 mA cm-2 at -0.24 V) and a small Tafel slope of 76.1 mV dec-1.19 Liu et al. combined MoS2 nanoparticles with mesoporous graphene, exhibiting excellent electrocatalytic activity and fast charge transfer kinetics.20 Both theoretical and experimental studies proved that the choice of substrates could influence the energy of hydrogen adsorption.21,

22

Different from semiconducting MoS2, VS2, as another

typical TMDs without bandgap, has shown excellent metallicity in the application of supercapacitors23 and moisture sensors24. Hydrogen evolution properties of VS2 have already been studied as well.25 Because of good conductivity and active sites, layered VS2 have great potential in HER. In this work, in order to compensate for the poor conductivity and expose more active sites on the edges of 2H-MoS2, we developed an active catalyst for HER of MoS2 nanosheets via a modified two-step hydrothermal method on 1T-VS2 nanoflowers for the first time (Scheme 1). On one hand, with MoS2 nanosheets growing vertically on the surface, VS2 can prevent the cluster of MoS2 effectively resulting in the formation of thinner nanosheets, so that more active sites can be exposed. The S of VS2 also can be as active sites to absorb and transport intermediate hydrogen atom and their basal-plane exhibits unusual self-optimizing performance


3


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 4 of 31

as they catalyse hydrogen evolution.26. On the other hand, the highly conductive VS2 nanoplates contribute to the high electrical conductivity of VS2@MoS2 and facilitate a fast electron-transfer process by enhancing the electrical contact between the active sites and the electrodes.27 Besides, VS2 nanoflowers with the stable structure can provide stable and shape-controlled skeleton inhibiting the decomposition of MoS2 on the surface which will contribute to the structure stability of active electrocatalysts.28, 29 Based on the above designed experiment, improved HER properties of VS2@MoS2 catalyst have been obtained with a lower onset potential and Tafel slope, and the stability is optimized as well which are much better than that of pure VS2 and MoS2. First-principle calculations were also implemented to analyze the HER activity of the VS2@MoS2. The results of the calculated electronic structure prove that near the Fermi level, electrons can be transferred from VS2 substrates to MoS2-edge due to the interband charge transport which is related to the quantum tunneling towards the interlayer barriers. In addition, we also reported the humidity sensing properties of VS2@MoS2. The humidity sensor conducting the current in the form of hydronium ion, consequently, moisture stimuli is converted into electric signals. As mentioned above, the conductivity of the composite and the ability to absorb hydrogen ions have been improved. Due to the high electropositivity of V4+, VS2 ultrathin nanosheets was verified to be a good humidity sensitive material.24 VS2 can be oxidized easily with poor physicochemical stability, especially in the humid environment. Whereas MoS2 with good stability could isolate VS2 from oxygen to keep the metallicity of VS2. Moreover，MoS2 owns outstanding virtues, such as a great surface-area-to-volume ratio, high carrier mobility, and low noise level. MoS2 nanosheets act as an anchor in the VS2@MoS2 hybrid and plays a dominant role in eliciting the sensor response.30 Experiment has shown that the humidity sensing properties and stability VS2@MoS2 composite are much better than pure MoS2


4

Page 5 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


and VS2. With the application of 2D materials in flexible electronics, we use PET film as the substrate instead of traditional ceramic substrate, which will have good prospect in wearable domain.

Scheme 1. Scheme for the fabrication of VS2@MoS2. EXPERIMENTAL SECTION Synthesis of VS2 nanoflowers. Typically，0.7 g ammonium vanadate (NH4VO3) was added into 54 mL solution containing with 45 mL deionized water and 9 mL ammonia (NH3·H2O). After magnetic stirring for 15 minutes, 2.4 g thioacetamide was add to the above solution as the S sources followed by another 15-minite stirring until all the reactant dissolved. The homogeneous solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave and heated at 160 °C for 20 h. The pure black VS2 precipitates were gathered by ultrasonication (in ice water) and centrifugation, washed with distilled water or ethanol, and dried at 60 °C at 6 h in a vacuum drying oven. Preparation of VS2@MoS2 nanocomposites. Take 0.2 g as-prepared VS2 black powder into beaker A with 40 mL distilled water for 30-minite ultrasonication in ice water to prevent oxidation. At the same time, 0.5 g sodium molybdate dihydrate (Na2MoO4·2H2O) and 0.8 g


5


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 6 of 31

thiourea (CH4N2S) were solved into beaker B with 30 mL distilled water using 0.4 g oxalic acid to regulate PH value followed by magnetic stirring for 20 minutes. Then the suspension in beaker A was poured into beaker B to be stirred for intensive mixing. The mixed liquid was transferred into the 100 mL autoclave heated at 200 °C for 24 h. By the same cleaning step, the VS2@MoS2 nanocomposite was eventually gathered after being dried at 60 °C at 6 h in a vacuum drying oven and annealed at 550 °C for 1 h with protecting gas to increase the crystallinity before further characterization. Characterization. X-ray diffraction (XRD) was measured by Bruker D8 Adbance diffractometer using monochromatized Cu-K radiation with λ of 1.5418 Å. Raman spectrum were acquired on a Jobin-Yvon LabRAM HR 800 micro-Raman spectrometer. X-ray photoelectron spectra (XPS) were performed on a Kratos Axis ULTRA X-ray photoelectron spectrometer with monochromatic Al Kα radiation. Field emission scanning electron microscopy (FE-SEM) measurements were carried out on a JEOL JSM-6700F SEM and a JEOL 2010 field emission electron microscope at an acceleration voltage of 200 kV was employed to get the transmission electron microscopy (TEM) images. UPS and NEXAFS experiments were performed at the Catalysis and Surface Science Endstation at the BL11U beamline in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. A sample bias of −5 V was applied to observe the secondary electron cutoff (SEC) with a photon energy of 40 eV. The work function (ϕ) can be determined by the difference between the photon energy and the binding energy of the secondary cutoff edge. The S L-edge NEXAFS were measured in total electron yield (TEY) mode. Calculations. The first-principles calculations were performed by Vienna Ab-initio Simulation Package (VASP) with the projector augmented wave (PAW) The generalized gradient


6

Page 7 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


approximation (GGA) in the scheme of the Perdewe Burkee Ernzerhof (PBE) was used for the exchange correlation functional. The energy cutoff was set to 500 eV, and a Monkhorste Pack kpoint mesh of 5 × 5 × 1 was used during all the supercell calculations. The residual forces for each ion converged less than 0.02 eV/A after structure optimization . Band structure calculations were performed along the paths, connecting the high-symmetry points: G (0, 0, 0), K (-1/3, 2/3, 0), and M (0, 0.5, 0) in the k-space. Electrochemical Measurements. 2 mg VS2@MoS2 black powder and 20 µL of Nafion solution (5 wt %, DuPont) were dispersed in 400 µL of an isopropanol/water mixed solvent with the volume ratio of 1:3 by sonication for 30 minutes to form a homogeneous ink. Then 10 µL of the dispersion was loaded onto a glassy carbon electrode (3 mm diameter) and dried at 40 °C in vacuum. The electrochemical measurements were performed in a three-electrode system at temperature by CHI660D electrochemical workstation with Pt wire as the counter electrode, Ag/AgCl as the reference electrode and the as-prepared glassy carbon electrode as working electrode. All three electrodes worked in an N2-saturated 0.5 M H2SO4 electrolyte. Fabrication and measurement of the humidity sensor. Take a small amount of sample as moisture sensitive active material into the centrifuge tube (1.5mL) mixed with a few drops of ethanol. After ultrasonic treatment until the sample evenly dispersed, the obtained slurry was spincoated onto polyethylene terephthalate (PET) flexible substrate with Ag interdigitated electrodes (inset of Figure 7a) and letting it air-dry. The humidity sensor worked in closed glass vessel where humidity environments were controlled by saturated salt aqueous solutions (LiCl, MgCl2, Mg(NO3)2, NaCl, KCl and KNO3, which yielded 11, 33, 54, 75, 85 and 95% RH, respectively.). The measurements were carried out on CHS-1 intelligent humidity sensitive system.


7


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 8 of 31

RESULTS AND DISCUSSION As the XRD of as-prepared VS2, MoS2 and VS2@MoS2 shown in Figure 1a, pure VS2 and MoS2 are assigned to hexagonal VS2 (JCPDS-36-1139) and hexagonal phase of MoS2 (JCPDS 371492) respectively. The feature peaks of VS2 locate at 15.45 degrees, 35.77 degrees and 45.26 degrees, respectively, corresponding to the crystal face (001), (101), (102). The main peaks of MoS2 are observed at 14.38 degrees, 33.5 degrees and 59.06 degrees, matching the crystal face (002), (101), (110). (002) diffraction peak is high and sharp, indicating that MoS2 has good crystallization and good layered structure. For the composites, there’s no high-indexed diffraction peaks indicating that the short-range structure distribution of MoS2 nanosheets contributing to more active sites. Strong (002) peak of VS2@MoS2 represents good stacked structures, however, compared with the pure MoS2, the degradation of (002) peaks is likely due to the decrease of MoS2 layers. The Raman spectra (Figure 1b) reveals that the nanocomposites displays four main peaks containing two peaks of VS2 at 280 and 404 cm-1 corresponding to the vibration Eg mode and Ag mode (out of plane) and two peaks of MoS2 at 381 and 407 cm-1 corresponding to the in-layer E vibration mode and out-of-plane A1g vibration mode. XPS was

performed to explore the elemental composition and chemical state of VS2@MoS2. Figure 1c shows that two characteristic peaks locate at 523.7 and 516.3 eV arose from V4+ 2p1/2 and V4+ 2p3/2 orbitals, indicating the oxidation state of V. As shown in the Figure 1d,e, the peaks fitting analysis of Mo4+ 3d3/2 (232.5 eV) and Mo4+ 3d5/2 (229.3 eV) orbitals confirms the presence of Mo4+ and the signal peaks at 163.9 and 162.7 eV are assigned to S2+ 2p1/2 and S2+ 2p3/2 levels. In addition, S L-edge NAXAFS spectra of VS2@MoS2 hybrid in Figure S1 indicates that MoS2 and VS2 are not stacked on the macroscopic scale, but form a heterojunction with high lattice match resulting in bonding effect between two atomic layers of two materials. It can be interpreted as


8

Page 9 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


that 4d orbit of Mo atom will be out of plane interacting with S-2p orbit of VS2. At the same time, 3d orbit of V atom will be out of plane interacting with S-2p orbit of MoS2. Figure S1 also shows that some unsaturated S2- exist, which is beneficial for HER. And we also calculate the work function of MoS2 and VS2 through UPS test to support the electron transfer. The results are shown in Figure S2. To confirm the direction of electron transfer, XPS spectra of Mo 3d in pure MoS2 is compared with that of Mo 3d in VS2@MoS2, which is added to Figure S3 in Supporting Information. It is known that electrons transferred to the d orbit of Mo atom will result in the shift of Mo 3d3/2 and Mo 3d5/2 peaks to lower binding energy. Therefore, the observed peak shift of VS2@MoS2 in Figure S3 indicates that the electrons are transferred from VS2 to MoS2.

Figure 1. (a) XRD patterns of VS2 and VS2@MoS2. (b) Raman spectra of MoS2, VS2 and VS2@MoS2. High-resolution XPS spectra: (c) V 2p. (d) Mo 3d. (e) S 2p.


9


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 10 of 31

FE-SEM and TEM images of VS2 nanoflowers are shown in Figure 2. Figure 2a displays that flower-like VS2 is stacked by a large number of VS2 nanoplates in different directions. The radius of single VS2 nanoflower and the average thickness of the nanoplate are approximately 8 µm and 100 nm (seen from the inset of Figure 2a), respectively. HRTEM were further performed to confirm the crystallinity of VS2. Figure 2b show VS2 nanoplates with a d-spacing of ~ 5.76 Å corresponding to the (001) plane. In Figure 2c and d, each facets of (102) and (101) can be well indexed and the crystallinity of (101) facet is better than (102) facet, which is consistent with the result of XRD. In order to reduce error, the atomic spacing of VS2 was obtained from the average values of several intensity peaks (the inset of Figure 2d). According to Ostwald ripening process known for the growth of flowerlike metal sulfide structure, the formation of flower-like VS2 probably involves two steps: initial nucleating and crystal growth.31 Firstly, the functional groups -NH2, -SH react with V ions dissociating from NH4VO3 in the reaction vessel to form V-S complexes followed by decomposing to shape VS2 nuclei for further growth. Then the flowerlike structure formed by VS2 nanoplates weakly stacking together and self-assembly. To unstack the layers, ammonia was added into the reactants forming NH3-intercalated VS2, which could thin the VS2 nanoplates and no other ions residual in the assembled structures after the evaporation of NH3.


10

Page 11 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 2. (a) Medium-magnification and high-magnification. (Inset) SEM image of VS2 nanoflowers. (b and c) HRTEM image of the VS2. (d) HRTEM image of VS2. (Inset) intensity signals along the red dotted lines. After hybrid, the morphology structure and composition distribution of the nanocomposite was observed in Figure 3. As Figure 3a-c shown, the structure of VS2 nanoflowers remain the same, which providing a stable skeleton for MoS2. Aggregation occur on the pure MoS2 nanosheets leading to countless nanospheres, since the pure MoS2 2D nanopetals will curl freely to a closed structure, and eventually form surface petals to reduce dangling bonds and reduce surface energy. While as for the composite, in the process of sodium molybdate reduced to MoS2, VS2 acts as a templet so that MoS2 nanosheets grew on the surface of VS2 uniformly and vertically. The morphology of the VS2@MoS2 is further displayed via TEM detection. A low-magnification TEM image (Figure 3d) shows the full view of composite structure. The selected area electron diffraction (SAED) (inset of Figure 3d) shows a reciprocal lattice of hexagonal crystal projected along (001) direction, which demonstrates that the VS2 thin films stack along the c axis. Moreover, a large number of MoS2 edge sites can be seen, which are beneficial for the HER


11


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 12 of 31

electrochemical catalysis. The high-magnification TEM image (Figure 3e) shows that the lattice spacing d = 0.62 nm, which means it coincides well with the crystal face in MoS2 (002) plane. To further demonstrate the interfacial microstructure features of composites, we provide HRTEM for composites in Figure S4. Energy dispersive X-ray (EDX) elemental mapping images (Figure 3f-i) demonstrate that all the elements of V, S, Mo are distributed uniformly on the nanocomposites.


12

Page 13 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. (a-c) SEM image of VS2, MoS2 and VS2@MoS2. (d)TEM image of VS2@MoS2. (Inset) SAED pattern of VS2. (e)HRTEM image of VS2@MoS2. (Inset) HRTEM image of VS2. (f) TEM image of VS2@MoS2. (g-i) Elemental mapping of V, S and Mo. Electrochemical measurements were carried out on a typical three-electrode system (Figure S5). H2 bubbles are easily to produce on the surface of glassy carbon work electrode with active materials, and the speed of hydrogen production is also considerable here. Comparative studies were performed on VS2, MoS2 and VS2@MoS2 nanocomposites. As shown in Figure 4a, both of the pure VS2 and MoS2 show a poor HER activity with high onset potential of 602 mV and 327 mV, respectively. In contrast, VS2@MoS2 exhibits a better HER property. More HER active sites are exposed due to the unique growth pattern and high temperature annealing results in less impurities and higher crystallinity so that the onset potential of VS2@MoS2 down to 97 mV. Moreover, as the conductive substrate, VS2 provides abundant electrons for the H+ combined with -S in MoS2 via interlayer electron transfer and with the increase of the overpotential, VS2 gradually has a contribution to HER itself for which the nanocomposites still keep a low overpotential of 177 mV when the current density reaches to 10 mA cm-2. While for the two pure samples, a high overpotential need to be attained at 979 mV and 611 mV, respectively. To set a control experiment, we also prepared the 40% commercial Pt/C electrocatalysts for the referential measurements with almost no onset potential. For the further analysis, Tafel slops of all the samples were derived from linear sweep voltammetry (LSV) on the Tafel equation η = b log (j/j0) (η is overpotential, b is the Tafel slope, j is the current density, and j0 is the exchange current density). As can be seen from Figure 4b, the Tafel slope of VS2@MoS2 (54.9 mV dec-1) is nearest to that of 40% commercial Pt/C electrocatalysts (35.4 mV dec-1) indicating the highest efficiency of the catalytic reaction. By contrast, high values of 133.7 mV dec-1 and 105.6 mV


13


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 14 of 31

dec-1 are obtained for the pure VS2 and MoS2 catalysts, respectively. The HER properties of the MoS2 based hydrogen evolution cathode materials in previous reported were listed for comparison (see Table 1). Table 1. Comparison of HER activity data among various catalysts.

Catalyst

Loading amount (mg cm-2)

Onset potential (mV)

Tafel slope (mV dec-1)

Overpotential / Current density (mV) / (mA cm-2)

Reference

One layer MoS2

-

119

140

-

9

t -Bu-Li exfoliated MoS2

-

160

94

~400 / 10

10

Defect-rich MoS2

0.285

120

50

200 / 13

11

V0.09Mo0.91S2

0.285

130

69

~240 / 10

14

MoS2@Fe3O4

0.285

110

52

~235 / 10

32

MoS2/MoO2

0.22

104

76.1

240 / 10

19

0.285

126

90

244 / 10

33

0.285

97

54.9

177 / 10

This work

Cu-MoS2 /RGO VS2@MoS2

In acidic electrolytes, the HER is mainly influenced by two processes.18 First is a discharge process that hydrogen proton integrates with electron to form an intermediate-state adsorbed hydrogen atom on the surface of the catalyst called Volmer reaction:


14

Page 15 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


H3O+ + e- → Hads + H2O

=

.

≈ 120 mV

(1)

(2)

This process occurs on VS2 and MoS2 at the same time, while the adsorbed hydrogen atoms are instability and most of them are transferred on MoS2. The results of linear sweep voltammetry and Tafel slope of VS2 were conformed. Next is the desorption process of hydrogen including either an electrochemical desorption step called Heyrovsky reaction : H3O+ + Hads + e- →H2 + H2O .

= () ≈ 40 mV

(3)

(4)

or a recombination step called Tafel reaction : Hads + Hads →H2

=

.

≈ 30 mV

(5)

(6)

The obtained Tafel slope of the composite catalyst in our work is 54.9 mV dec-1 suggesting a Volmer-Heyrovsky HER processes. There may be two reasons for the experimental value higher than the theoretical value, one is that when the surface adsorbed hydrogen atom reaches a certain coverage rate, the discharge process will be limited, the other is the hydrogen produced during the reaction hindered the contact between the catalyst and the electrolyte, which could be proved by the inset of Figure S5. In addition, some external experimental error such as ambient temperature and electrolyte concentration also influence the results. Electrical impedance spectroscopy (EIS) was employed to learn the electrode kinetics of MoS2 and VS2@MoS2 catalysts for HER. As shown in Figure 4c, the semicircles reflect the charge-


15


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 16 of 31

transfer resistance (Rct) between electrode and electrolyte. The R-C equivalent circuit (inset of Figure 4c) was used to research the kinetic differences of different catalysts. The electrical resistance of VS2@MoS2 decreases a lot compared with MoS2 catalyst suggesting higher conductivity and more active edge sites. In addition, cyclic voltammetry measurements with different scanning rates were introduced to characterize the charge storage capacity and effective reaction area of the catalysts, as shown in Figure 4d and e. Through the calibration of differential output current at 0.25 V, Figure 4f was gained according to the current voltage relationship. Calculation shows that Cdl of VS2@MoS2 and pure MoS2 are 15.02 and 7.13 mF cm-2 respectively which demonstrates that the double-layer capacitance of the composite materials increases obviously (more than one time), by contrast, pure MoS2 shows a low double-layer capacitance. By compounding, MoS2 can achieve greater surface activity, as a result, electrons can be stored more and transferred faster, conducing to the improvement of HER performance.


16

Page 17 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 4. (a) HER polarization curves of various samples as indicated. (b) Corresponding Tafel plots. (c) Nyquist plots of MoS2 and VS2@MoS2. (Inset) A common R-C equivalent circuit. (d and e) Cyclic voltammetry curves of MoS2 and VS2@MoS2 at various scan rates in the region of 0.15-0.35 V versus RHE. (f) Linear fitting for the capacitive currents of MoS2 and VS2@MoS2 versus scan rates.


17


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 18 of 31

Besides the HER activity, stability is also a significant evaluation for the catalysts. The long-time cycling was adopted to test the stability of the catalysts performed in Figure 5. As can be seen from Figure 5a and b, after 1000 CV cycles, both of the onset potential and the current density at the same overpotential of MoS2 decrease apparently, but the HER activity of VS2@MoS2 depresses few. Furthermore, a 20-hour galvanostatic measurement at a current density of 20 mA cm-2 was exhibited in Figure 5c. The overpotential required to reach to the certain current density for VS2@MoS2 is more stable than that of VS2 and MoS2 which benefits from the synergistic effect of VS2 and MoS2 in the composite material. MoS2 is compounded with VS2 formating the metal semiconductor junction. Because of the interaction between the two kinds of materials, the Schottky barrier of metal semiconductor junction decreases. So that near the Fermi level, metal d electrons can be transfer from V to Mo continuously, which improve the efficiency of charge transmission and persistence, and the adsorption of hydrogen atom on S-VS2 can be passed to SMoS2 to improve HER efficiency. At the same time, MoS2 with good physical and chemical stability has blocked the contact between vanadium sulfide and oxygen so that it can be in a stable state for a long time.

Figure 5. (a and b) Stability test of VS2@MoS2 and MoS2 through potential cycling, polarization curves before and after 1000 potential cycles, (c) Potential versus time at a constant current density of 20 mA cm-2 for 20 h.


18

Page 19 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 6 is a simple moisture sensitivity test for the three samples, which proves our inference about the improvement of VS2@MoS2. As can be seen from impedance versus RH curves of VS2@MoS2 at various frequencies (inset of Figure 6a), VS2@MoS2 humidity sensor shows different properties under different frequencies. The complex impedances are greatly affected by frequency at low RH, and tend to be consistent at high RH. Corresponding to the same relative humidity, the impedance decreases as the frequency increases. This is because the water molecules cannot be polarized at high frequency, resulting in lower device resistance.34 Besides, the impedances show the best linearity at 100Hz which is the most suitable frequency for measuring the properties of the VS2@MoS2 humidity sensor. Dehumidification characteristics usually lag behind the absorption process on account of more demands of energy, so we investigated the humidity hysteresis loop of the VS2@MoS2 composites sensor (Figure 6a) with a small amount of hysteresis (the humidity difference at the same impedance value before and after the cycle). Figure 6b represents the response and recovery curves of the three humidity sensors measured at 100 Hz and RH range from 11 to 95%. VS2@MoS2 humidity sensor performs a good response/recovery property with response/recovery time as 23 s / 13 s, better than VS2. In order to detect the stability of the device, the change of impedance with time was studied under different humidity every 5 days. As can be seen from the Figure 6c, pure MoS2 is stable but not sensitive to humidity with the sensibility (S = Im (11 %) / Im (95 %)) of 35.6. The pure VS2 humidity sensor has a high sensibility of 3909, while after 30 days, the sensibility reduced to 1160.8 which shows a poor stability. For VS2@MoS2, slight fluctuations occur and the sensibility is kept around 5798.5, indicating that the nanocomposite sensor has good long-term stability. And the sensing response of VS2@MoS2 is better than that of VS2 for humidity detection, indicating that MoS2 is superior in capturing water molecules. The humidity sensing


19


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 20 of 31

mechanism can be inferred from the complex impedance spectra at different humidity (Figure 6d). At low RH, water molecules are initially adsorbed in the form of chemisorption on the active sites of VS2@MoS2 and dissociated into two hydroxyl groups.35 Then the water molecules are adsorbed on the surface of a physical layer through the double hydrogen bonds, which cannot move freely, leading to a high impedance. With the increase of humidity, a large number of water molecules are adsorbed on the surface of the material physically by single hydrogen bond, and gradually form an approximate liquid water layer. At this point, protons can be passed through neighboring water molecules forming a Grotthuss chain reaction Cycle Mechanism, which greatly reduces the impedance of the sensor.36

Figure 6. (a) Humidity hysteresis characteristics of VS2@MoS2. (Inset) Humidity sensor with flexible substrate and impedance versus RH curves of VS2@MoS2 at various frequencies. (b) Response and recovery properties of sensors with MoS2, VS2 and VS2@MoS2. These curves


20

Page 21 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


were measured from 11% to 95% RH. (c) Long-time stability test of three samples under various humidity (d) Complex impedance property of VS2@MoS2 at the different RH. Both the improvements of VS2@MoS2 nanocomposites for the HER property and humidity characteristics can be contributed to the higher electrical conductivity and faster electron-transfer process so that the HER rate of the VS2@MoS2 catalyst and humidity sensitivity of VS2@MoS2 sensor are increased, respectively. And for HER, not only electrons but also the adsorbed hydrogen atoms are transfered from VS2 to MoS2, which further improves the hydrogen evolution efficiency. In addition, MoS2 grows vertically on the surface of VS2 in form of thinner nanosheets with more exposed active sites. To analyze the electronic structure of MoS2-edge nanosheet composited with VS2, we consider two types of stacked bilayer structural model as presented in Figure 7a. Without hydrogen adsorbed, we have compared the band structure of this two cases. As shown in Figure 7b, because of the different band dispersion near Fermi level Ef of different stacking cases we will obtain different electron transport from valance to conduction band, which will further influence the interband charge transfer related to tunneling towards different interlayer barrier. In fact, this metal-semiconductor Schottky barrier is tunable because the different metal-induced gap states (different band dispersion) will influence the effect of Fermi level pinning (FLP) which acts as a role of further tuning the Schottky barrier.37, 38 To further investigate the hydrogen evolution ability, we calculated the Gibbs free energies of different adsorption sites. It is demonstrated that as for A-B stacking case, when a H atom adsorbed in the inner sites and Mo and S edge sites (shown in Figure 7a), the Gibbs free energies were calculated to be -0.73, 0.14, and 0.31 eV, respectively. And as for A-A stacking case, the corresponding Gibbs free energies to the H atom adsorbed in the inner sites and Mo and S edge sites were calculated to be -0.70, 0.08, 0.30 eV,


21


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 22 of 31

respectively. The difference of hydrogen adsorption free energy in the two stacking cases can be mainly attributed to the different interband charge transfer which stems from interlayer barrier of metal d electron from VS2 to MoS2. The value of free energies in isolated MoS2 is close to the previous calculation in our group (when a H atom adsorbed in the basal plane and Mo and S edges, the Gibbs free energies were calculated to be -1.17, 0.12, and 0.63 eV, respectively)39, both of that of two stacking cases are closer to 0. The electron transport process may be that the electron transfer between metal d electrons attribute to the tunneling effect, then the electrons from metal d to sulfur electrons followed by transferring to H atoms belongs to the charge transfer of neighboring atoms. Moreover, there exists that the unstable H* may actually transfer from VS2 to MoS2 since the calculated hydrogen adsorption free energy of VS2 monolayer is 0.21 eV,40 larger than that of MoS2-edge nanosheet (inner sites and Mo edge sites) of our cases. It is a general recognition that the good HER performance for the catalyst is considered as the Gibbs free energy of adsorbed H is close to zero .Thermodynamically, if the hydrogen adsorption is endothermic, the generation of surface H* would be hindered, whereas it is too exothermic, the desorption of H* to form H2 would be difficult.41 Considering that the optimal value of free energy should be around 0 eV, which is the ideal state to bind H atom neither too weakly nor too strongly, we show that the Mo edge sites in A-A stacking case possess highest HER activity compared with other adsorbed cases. The calculated value 0.08 eV is comparable to that of Ptfree catalyst (≈0.09 eV). Figure 7c shows the calculated 3D charge density difference surface of this case. It could be clearly seen that the charge transfer mainly occurs between the adjacent S atoms and the H atom. Therefore, the enhanced charge density of the edge sites mainly contribute to the fast charge transfer in HER activity.


22

Page 23 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) Two types of stacked bilayer structural model and their adsorption configurations. (b) Band structures of A-B stacking case (left) and A-A stacking case (right) of VS2@MoS2-edge nanosheet.(c) 3D charge density difference surface of A-A stacking case. The cyan and yellow regions represent the charge depletion and accumulation space, respectively.


23


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 24 of 31

CONCLUSIONS In summary, we have reported a VS2@MoS2 nanomaterial with MoS2 nanosheets growing vertically on VS2 nanoflowers via a modified two-step hydrothermal method for the first time. Owing to the high conductivity and rich exposed edge active sites, the VS2@MoS2 nanocatalyst exhibited an excellent HER performance with a low onset potential, large cathodic currents and a small Tafel slope. Meanwhile, good structural and physicochemical stability of the nanocomposite, together with the high conductivity, resulting in improved humidity characteristics. We believe that VS2@MoS2 nanomaterial reported in this work may also be applied to other fields and this thought of choosing different materials to synthetize can be contributed to other material systems. ASSOCIATED CONTENT Supporting Information S L-edge NEXAFS spectra of VS2@MoS2 hybrid and pure MoS2, UPS of (a) MoS2 and (b) VS2, XPS spectra of Mo 3d for pure MoS2 and VS2@MoS2 composite, HRTEM of VS2@MoS2 hybrid, Three-electrode system equipment for HER test with VS2@MoS2 as the working electrode. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author * Tel.: +86 21 54345198; E-mail address: [email protected] (Ke Yu) Notes


24

Page 25 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The authors declare no competing financial interest. ACKNOWLEDGMENT The authors acknowledge financial support from the NSF of China (Grant Nos. 61574055, 61474043), the Open Project Program of Key Laboratory of Polar Materials and Devices, MOE (Grant No. KFKT20140003), East China Normal University, and the Catalysis and Surface Science Endstation in the National Synchrotron Radiation Laboratory (NSRL) in Hefei, China. REFERENCES (1) Norskov, J. K.; Christensen, C. H. Toward Efficient Hydrogen Production at Surfaces. Science 2006, 312, 1322-1323. (2) Voiry, D.; Yang, J.; Chhowalla, M. Recent Strategies for Improving the Catalytic Activity of 2D TMD Nanosheets Toward the Hydrogen Evolution Reaction. Adv.Mater. 2016, 28, 61976206. (3) Wang, X.; Wang, P.; Wang, J.; Hu, W.; Zhou, X.; Guo, N.; Huang, H.; Sun, S.; Shen, H.; Lin, T.; Tang, M.; Liao, L.; Jiang, A.; Sun, J.; Meng, X.; Chen, X.; Lu, W.; Chu, J. Ultrasensitive and Broadband MoS2 Photodetector Driven by Ferroelectrics. Adv. Mater. 2015, 27, 65756581. (4) Wang, J.; Zou, X.; Xiao, X.; Xu, L.; Wang, C.; Jiang, C.; Ho, J. C.; Wang, T.; Li, J.; Liao, L. Floating Gate Memory-based Monolayer MoS2 Transistor with Metal Nanocrystals Embedded in the Gate Dielectrics. Small 2015, 11, 208-213. (5) Jaramillo, T. F.; Jorgensen, K. P.; Bonde, J.; Nielsen, J. H.; Horch, S.; Chorkendorff, I. Identification of Active Edge Sites for Electrochemical H2 Evolution from MoS2 Nanocatalysts. Science 2007, 317, 100-102.


25


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 31

(6) Wang, T.; Chen, S.; Pang, H.; Xue, H.; Yu, Y. MoS2-Based Nanocomposites for Electrochemical Energy Storage. Adv Sci 2017, 4, 1600289. (7) Ambrosi, A.; Sofer, Z.; Pumera, M. 2H → 1T phase transition and hydrogen evolution activity of MoS2, MoSe2, WS2 and WSe2 strongly depends on the MX2 composition. Chemi. Commun. 2015, 51, 8450-8453. (8) Voiry, D.; Salehi, M.; Silva, R.; Fujita, T.; Chen, M.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Conducting MoS2 nanosheets as catalysts for hydrogen evolution reaction. Nano Lett. 2013, 13, 6222-6227. (9) Yu, Y.; Huang, S. Y.; Li, Y.; Steinmann, S. N.; Yang, W.; Cao, L. Layer-dependent Electrocatalysis of MoS2 for Hydrogen Evolution. Nano Lett. 2014, 14, 553-558. (10) Ambrosi, A.; Sofer, Z.; Pumera, M. Lithium Intercalation Compound Dramatically Influences the Electrochemical Properties of Exfoliated MoS2. Small 2015, 11, 605-612. (11) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Defect-rich MoS2 Ultrathin Nanosheets with Additional Active Edge Sites for Enhanced Electrocatalytic Hydrogen Evolution. Adv.Mater. 2013, 25, 5807-5813. (12) Tao, L.; Duan, X.; Wang, C.; Duan, X.; Wang, S. Plasma-engineered MoS2 Thin-film as an Efficient Electrocatalyst for Hydrogen Evolution Reaction. Chemi. Commun. 2015, 51, 7470-7473. (13) Ye, G.; Gong, Y.; Lin, J.; Li, B.; He, Y.; Pantelides, S. T.; Zhou, W.; Vajtai, R.; Ajayan, P. M. Defects Engineered Monolayer MoS2 for Improved Hydrogen Evolution Reaction. Nano Lett. 2016, 16, 1097-1103.


26

Page 27 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(14) Sun, X.; Dai, J.; Guo, Y.; Wu, C.; Hu, F.; Zhao, J.; Zeng, X.; Xie, Y. Semimetallic Molybdenum Disulfide Ultrathin Nanosheets as an Efficient Electrocatalyst for Hydrogen Evolution. Nanoscale 2014, 6, 8359-8367. (15) Yang, L.; Fu, Q.; Wang, W.; Huang, J.; Huang, J.; Zhang, J.; Xiang, B. Large-area Synthesis of Monolayered MoS(2(1-x))Se(2x) with a Tunable Band Gap and Its Enhanced Electrochemical Catalytic Activity. Nanoscale 2015, 7, 10490-10497. (16) Mcateer, D.; Gholamvand, Z.; Mcevoy, N.; Harvey, A.; O’Malley, E.; Duesberg, G. S.; Coleman, J. N. Thickness Dependence and Percolation Scaling of Hydrogen Production Rate in MoS2 Nanosheet and Nanosheet-Carbon Nanotube Composite Catalytic Electrodes. Acs Nano 2016, 10, 672-683. (17) Zhang, B.; Liu, J.; Wang, J.; Ruan, Y.; Ji, X.; Xu, K.; Chen, C.; Wan, H.; Miao, L.; Jiang, J. Interface Engineering: the Ni(OH)2/MoS2 Heterostructure for Highly Efficient Alkaline Hydrogen Evolution. Nano Energy 2017, 37, 74-80. (18) Li, Y.; Wang, H.; Xie, L.; Liang, Y.; Hong, G.; Dai, H. MoS2 Nanoparticles Grown on Graphene: An Advanced Catalyst for the Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2011, 133, 7296-7299. (19) Yang, L.; Zhou, W.; Hou, D.; Zhou, K.; Li, G.; Tang, Z.; Li, L.; Chen, S. Porous Metallic MoO2-supported MoS2 Nanosheets for Enhanced Electrocatalytic Activity in the Hydrogen Evolution reaction. Nanoscale 2015, 7, 5203-5208. (20) Liao, L.; Zhu, J.; Bian, X.; Zhu, L.; Scanlon, M. D.; Girault, H. H.; Liu, B. MoS2 Formed on Mesoporous Graphene as a Highly Active Catalyst for Hydrogen Evolution. Adv. Funct. Mater. 2013, 23, 5326-5333.


27


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 31

(21) Chen, W.; Santos, E.; Zhu, W.; Kaxiras, E.; Zhang, Z. Tuning the Electronic and Chemical Properties of Monolayer MoS2 Adsorbed on Transition Metal Substrates. Nano Lett. 2013, 13, 509-514. (22) Tsai, C.; Abild-Pedersen, F.; Nørskov, J. K. Tuning the MoS2 Edge-site Activity for Hydrogen Evolution via Support Interactions. Nano Lett. 2014, 14, 1381-1387. (23) Feng, J.; Sun, X.; Wu, C.; Peng, L.; Lin, C.; Hu, S.; Yang, J.; Xie, Y. Metallic Few-layered VS2

Ultrathin

Nanosheets:

High

Two-dimensional

Conductivity

for

In-plane

Supercapacitors. J. Am. Chem. Soc. 2011, 133, 17832-17838. (24) Feng, J.; Peng, L.; Wu, C.; Sun, X.; Hu, S.; Lin, C.; Dai, J.; Yang, J.; Xie, Y. Giant Moisture Responsiveness of VS2 Ultrathin Nanosheets for Novel Touchless Positioning Interface. Adv.Mater. 2012, 24, 1969-1974. (25) Chia, X.; Ambrosi, A.; Lazar, P.; Sofer, Z.; Pumera, M. Electrocatalysis of Layered Group 5 Metallic Transition Metal Dichalcogenides (MX2, M = V, Nb, and Ta; X = S, Se, and Te). J. Mater. Chem. A 2016, 4, 14241-14253. (26) Liu, Y.; Wu, J.; Hackenberg, K. P.; Zhang, J.; Wang, Y. M.; Yang, Y.; Keyshar, K.; Gu, J.; Ogitsu, T.; Vajtai, R.; Lou, J.; Ajayan, P. M.; Wood, B. C.; Yakobson, B.I. Self-optimizing, Highly Surface-active Layered Metal Dichalcogenide Catalysts for Hydrogen Evolution. Nat. Energy 2017, 6, 17127. (27) Liang, H.; Shi, H.; Zhang, D.; Ming, F.; Wang, R.; Zhuo, J.; Wang, Z. Solution Growth of Vertical VS2 Nanoplate Arrays for Electrocatalytic Hydrogen Evolution. Chem. Mater. 2016, 28, 5587-5591.


28

Page 29 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(28) Ding, S.; Zhang, D.; Chen, J. S.; Lou, X. W. Facile Synthesis of Hierarchical MoS2 Microspheres Composed of Few-layered Nanosheets and Their Lithium Storage Properties. Nanoscale 2012, 4, 95-98. (29) Wan, Z.; Shao, J.; Yun, J.; Zheng, H.; Gao, T.; Shen, M.; Qu, Q.; Zheng, H. Core-shell Structure of Hierarchical Quasi-hollow MoS2 Microspheres Encapsulated Porous Carbon as Stable Anode for Li-ion Batteries. Small 2014, 10, 4975-4981. (30) Zhang, D.; Sun, Y.; Li, P.; Zhang, Y. Facile Fabrication of MoS2-Modified SnO2 Hybrid Nanocomposite for Ultrasensitive Humidity Sensing. ACS Appl. Mater. Interfaces 2016, 8, 14142-14149. (31) Cao, F.; Hu, W.; Zhou, L.; Shi, W.; Song, S.; Lei, Y.; Wang, S.; Zhang, H. 3D Fe3S4 Flower-like Microspheres: High-yield Synthesis via a Biomolecule-assisted Solution Approach, Their Electrical, Magnetic and Electrochemical Hydrogen Storage Properties. Dalton Trans. 2009, 38, 9246-9252. (32) Zhang, X.; Ding, P.; Sun, Y.; Wang, Y.; Wu, Y.; Guo, J. Shell-core MoS2 Nanosheets@Fe3O4 Sphere Heterostructure with Exposed Active Edges for Efficient Electrocatalytic Hydrogen Production. J. Alloys Compd. 2017, 715, 53-59. (33) Li, F.; Zhang, L.; Li, J.; Lin, X.; Li, X.; Fang, Y.; Huang, J.; Li, W.; Tian, M.; Jin, J.; Li, R. Synthesis of Cu-MoS2/rGO Hybrid as Non-noble Metal Electrocatalysts for the Hydrogen Evolution Reaction. J. Power Sources 2015, 292, 15-22. (34) Wang, J.; Wan, H.; Lin, Q. Properties of a Nanocrystalline Barium Titanate on Silicon Humidity Sensor. Meas. Sci. Technol. 2003, 14, 172-175.


29


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 30 of 31

(35) He, Y.; Zhang, T.; Zheng, W.; Wang, R.; Liu, X.; Xia, Y.; Zhao, J. Humidity Sensing Properties of BaTiO3 Nanofiber Prepared via Electrospinning. Sens. Actuators, B 2010, 146, 98-102. (36) Lu, Y.; Wang, Z.; Yuan, S.; Shi, L.; Zhao, Y.; Deng, W. Microwave-hydrothermal Synthesis and Humidity Sensing Behavior of ZrO2 Nanorods. RSC Adv. 2013, 3, 1170711714. (37) Liu, Y.; Stradins, P.; Wei, S. H. Van der Waals Metal-semiconductor Junction: Weak Fermi Level Pinning Enables Effective Tuning of Schottky Barrier. Sci. Adv. 2016, 2, e1600069. (38) Liu, Y.; Xiao, H.; Goddard, W. A. Schottky-Barrier-Free Contacts with Two-Dimensional Semiconductors by Surface-Engineered MXenes. J. Am. Chem. Soc. 2016, 138, 1585315856. (39) Guo, B.; Yu, K.; Li, H.; Song, H.; Zhang, Y.; Lei, X.; Fu, H.; Tan, Y.; Zhu, Z. Hollow Structured Micro/Nano MoS2 Spheres for High Electrocatalytic Activity Hydrogen Evolution Reaction. ACS Appl. Mater. Interfaces 2016, 8, 5517-5525. (40) Tsai, C.; Chan, K.; Nørskov, J. K.; Abild-Pedersen, F. Theoretical Insights into the Hydrogen Evolution Activity of Layered Transition Metal Dichalcogenides. Surf. Sci. 2015, 640, 133-140. (41) Xing, J.; Jiang, H. B.; Chen, J. F.; Li, Y. H.; Wu, L.; Yang, S.; Zheng, L. R.; Wang, H. F.; Hu, P.; Zhao, H. J.; Yang, H. G. Active Sites on Hydrogen Evolution Photocatalyst. J. Mater. Chem. A 2013, 1, 15258-15264.


30

Page 31 of 31

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TOC


31

Synergistic Effect of MoS2 Nanosheets and VS2 for the Hydrogen

Recommend Documents