Immobilized Single Molecular Molybdenum Disulfide on Carbonized

Immobilized Single Molecular Molybdenum Disulfide on Carbonized Polyacrylonitrile for Hydrogen Evolution Reaction Downloaded via BUFFALO STATE on July 23, 2019 at 15:24:38 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

Tamene Simachew Zeleke,† Meng-Che Tsai,† Misganaw Adigo Weret,§ Chen-Jui Huang,† Mulatu Kassie Birhanu,† Tzu-Ching Liu,† Chiu-Ping Huang,†,∥ Yun-Liang Soo,⊥ Yaw-Wen Yang,# Wei-Nien Su,*,‡ and Bing-Joe Hwang*,†,#,¶ †

NanoElectrochemistry Laboratory, Department of Chemical Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan ‡ NanoElectrochemistry Laboratory, Graduate Institute of Applied Science and Technology, National Taiwan University of Science and Technology, Taipei 10607, Taiwan § Department of Materials Science and Engineering, National Taiwan University of Science and Technology, Taipei 10607, Taiwan ∥ Material and Chemical Research Laboratories, Industrial Technology Research Institute, Hsin-Chu 31040, Taiwan ⊥ Department of Physics, National Tsing Hua University, Hsin-Chu 300, Taiwan # National Synchrotron Radiation Research Center, Hsin-Chu 30076, Taiwan ¶ Applied Research Center for Thin-Film Metallic Glass, National Taiwan University of Science and Technology, Taipei 10607, Taiwan S Supporting Information *

ABSTRACT: Designing a MoS2 catalyst having a large number of active sites and high site activity enables the catalytic activity toward the hydrogen evolution reaction to be improved. Herein, we report the synthesis of a low-cost and catalytically active immobilized single molecular molybdenum disulfide on carbonized polyacrylonitrile (MoS2-cPAN) electrocatalyst. From the extended X-ray absorption fine structure spectra analysis, we found that the as-prepared material has no metal−metal scattering and it resembles MoS2 with a molecular state. Meanwhile, the size of the molecular MoS2 has been estimated to be about 1.31 nm by high-angle annular dark-field scanning transmission electron microscopy. A low coordination number and maximum utilization of the single molecular MoS2 surface enable MoS2-cPAN to demonstrate electrochemical performance significantly better than that of bulk MoS2 by two orders of exchange current density (jo) and turnover frequency to the hydrogen evolution. KEYWORDS: electrochemical reaction, single molecule, molybdenum disulfide, hydrogen evolution, renewable energy

H

electrolyte, the hydrogen evolution reaction (HER) involves three possible pathways.7

igh fossil fuel consumption due to the world’s population growth results in a severe issue of global warming, with this phenomenon accelerating and putting forward the development of green and renewable energy usage.1,2 Hydrogen is one of the alternative renewable energy sources with high energy density,1,3,4 and utilization of hydrogen energy leads to substantial reductions in fossil fuel consumption and CO2 emissions.5 Hydrogen production from water splitting electrochemically is one convenient and promising approach. Electrochemical hydrogen production from water at a standard condition needs a free energy of ΔG = 237.1 kJ mol−1 and a voltage of 1.23 V versus RHE.6 In acidic © 2019 American Chemical Society

Volmer step

* + H+ + e− F Had

Heyrovsky step

Tafel step

H+ + e− + Had F H 2 + *

2Had F H 2 + 2*

Received: February 14, 2019 Accepted: May 13, 2019 Published: May 13, 2019 6720

DOI: 10.1021/acsnano.9b01266 ACS Nano 2019, 13, 6720−6729

Article

www.acsnano.org

Cite This: ACS Nano 2019, 13, 6720−6729

Article

ACS Nano

Figure 1. Schematic for the synthesis of a single molecular MoS2 catalyst.

Figure 2. (a) SEM image of the SPAN powder. (b) XRD for sulfur, PAN, SPAN, commercial Li2S, and in situ XRD of Li2S-cPAN. (c) SEM image of molecular MoS2 deposited on carbonized PAN. EDS mapping of (d) atomic sulfur and (e) atomic molybdenum of the MoS2-cPAN catalyst, showing the homogeneous distribution of atomic S and Mo mapping on the surface of cPAN, respectively. Both atoms have the same pattern and holes (cPAN) on the SEM are observed on the map without the distribution of S and Mo atoms.

with a large number of active sites and high site activity (turnover frequency, TOF).20 Common strategies to increase the surface area and the number of active sites are (1) fabrication of efficient MoS2 in different morphologies to increase the active edge sites like nanowires,21 nanoparticles,1 defect-rich films,22 mesoporous films,23 porous nanosheets,12 vertical nanoflakes, nanoflowers,4 and core−shell MoO3/ MoS2;24 (2) chemical doping for the enhancement of the activity of edge sites;25−27 (3) transformation of 2H MoS2 to the metastable 1T phase (1T MoS2),28,29 in which the 1T phase MoS2 has highly active edge sites and basal planes, but its application is limited due to unsteady nature of the phase;30 and (4) activating the basal plane through introducing sulfur vacancies,31,32 strain,31 and/or grain boundaries.32 In general, to improve the turnover frequency, overpotential, and the current density of the HER, different structural engineerings were performed during the synthesis of MoS2.6 Supporting MoS2 with conductive and high surface area material is also one of the ways to enhance the catalytic activity, too. Carbonaceous supporting materials facilitate electron transfer and enhance catalytic performance. Even though it is challenging to attach active MoS2 with carbona-

The overall mechanism of HER occurs either in the Volmer/ Heyrovsky (reaction rate limited by the desorption step) or the Volmer/Tafel (reaction rate limited by the adsorption step).8,9 The most excellent HER catalysts have near zero hydrogen binding energy (HBE); that is, the binding energy between the surface and intermediates is neither too weak nor too strong.6,10,11 Extensive studies are going on to develop advanced hydrogen evolution electrocatalysts, which help to enhance the current density and to reduce the overpotential.12 Platinum (Pt) and its alloys are the most effective and stateof-the-art catalysts for the hydrogen evolution reaction.13 However, its shortage and high cost mainly limit its applications.14 Investigation of low-cost, abundant, and highly active HER catalysts is still challenging.15 Recently, scholars are striving to replace the precious Pt electrocatalysts16 with the earth-abundant and inexpensive transition metal of sulfides,6,15 selenides,17,18 nitrides,19 phosphides,16 etc. Nowadays, transition metal dichalcogenide materials, like MoS2, have distinctive thermal, electrical, optical, and mechanical properties, and these properties enable the material to be a potential and promising candidate for HER. The HER activity of MoS2 can be improved by designing a MoS2 catalyst 6721


Article

ACS Nano

Figure 3. (a) TEM image of MoS2-cPAN. (b,c) HAADF-STEM images at different magnifications; the randomly dispersed bright spots on cPAN are individual MoS2 molecules. (d) Size distribution histogram of single molecular MoS2 for the sample MoS2-cPAN. The average particle size is about 1.31 nm. (e) Raman spectra of the MoS2, MoS2-cPAN, and SPAN by a laser with an excitation wavelength of 532 nm. (f) XRD patterns of MoS2 and MoS2-cPAN.

conductivity, a conductive support that stabilizes the highly active molecule seems necessary. Hence, the support of conductive cPAN could stabilize and make single molecular MoS2 conductive through electronic interaction (van der Waals forces) between the cPAN and molecular MoS2.

ceous material due to the nonintimate relation between both materials,33 their interactions (van der Waals forces) could increase or weaken the hydrogen binding energy by sticking Mo edges strongly to the support.34 Recently, a single atom catalyst has attracted much attention due to its ultrahigh site activity and 100% catalyst utilization.35,36 Single molecule catalysts are usually found in homogeneous solutions.37 To the best of our knowledge, there is no report on immobilized single-molecule catalysts. Herein, we report a synthetic approach to fabricate immobilized single molecular molybdenum disulfide on the surface of carbonized polyacrylonitrile (PAN), and the electrocatalytic HER of the MoS2-cPAN composite was performed and benchmarked. A single molecular MoS2 was prepared on the carbonized PAN (cPAN) surface in a stepwise fashion. Initially, elemental sulfur was reacted with polyacrylonitrile, and the sulfur−polyacrylonitrile (SPAN) containing a sulfur−carbon bond was formed. Second, sulfur in SPAN reacted with Li metal to form Li2S-cPAN electrochemically. The Li2S formed electrochemically reacted with Mo ions and gave immobilized single molecular MoS2 on the surface of cPAN. The immobilized single molecular MoS2 synthesis scheme is presented in Figure 1. The single molecular MoS2 on the surface of cPAN exhibits excellent electrochemical performance for the hydrogen evolution reaction. Although a single molecular MoS2 surface has a maximum catalytic performance, its conductivity could be questionable as individual MoS2 is present independently with no interconnection among others. To improve its

RESULTS AND DISCUSSION From the scanning electron microscopy (SEM) image in Figure 2a, the morphology of a highly magnified SPAN composite shows uniformly distributed spherical-like nanoparticles, confirming the formation of a composite material after being annealed with sulfur.38 The SEM image of the electrode in Figure S2a clearly shows the SPAN composite material cast with carbon. In Figure S2b, the SEM image of Li2S-cPAN shows the formation of dense film on the carbonized PAN electrode surface. From the X-ray diffraction (XRD) analysis, crystalline sulfur peaks are not seen in the as-prepared SPAN in Figure 2b. This indicates that sulfur already reacted with PAN to form SPAN. A broad diffraction peak at 2θ = 25.6° on SPAN material is the graphitic (002) plane, signifying successful carbonization of PAN.39 Through carbonization, the PAN crystal peak at the 2θ = 17° (110) plane completely disappeared.40 The in situ XRD pattern of the Li2S-cPAN in Figure S3 shows a broad peak at 2θ = 22°, and it is from the Kapton tape which kept the samples from moisture and air. The sharp peaks at 2θ = 38, 44.2, 64.67, and 77.81° indicate the formation of crystalline LiF, 41 possibly resulting from decomposition of LiPF 6 electrolyte or polyvinylidene difluoride binder. The XRD 6722


Article

ACS Nano

Figure 4. XPS high-resolution spectra of Mo 3d, S 2p, and C 1s in the energy regions of (a) Mo 3d of MoS2-cPAN, (b) S 2p of MoS2-cPAN, (c) Mo 3d of single molecular and bulk MoS2, and (d) XPS spectra of C 1s for MoS2-cPAN, SPAN, and cPAN.

S atoms is estimated to be 0.78 nm. It is approximately comparable with the size of the experimentally found result. The slight deviation from the experimental data might result from the aggregation of molecules. In general, the SEM, TEM, HAADF-STEM images, and EELS map of the MoS2-cPAN sample reveal that the MoS2 molecules anchored on the surface of carbonized PAN. As shown in Figure 3e, the Raman spectroscopic data of the G- to D-band ratio (IG/ID) between MoS2-cPAN and SPAN changed (from 1 to 0.95). It shows the graphitization of PAN, whereas the reaction of SPAN with a Mo atom form molecular MoS2 and MoS3 as a mixture. The S−S and C−S bonds of SPAN were detected from the Raman spectra, and these spectra appeared at 370, 807, and 932 cm−1.40,42,43 However, these peaks were not observed on MoS2-cPAN. It is informative that the bond between C−S and S−S has been cleaved, after the reaction of SPAN with Mo4+. The blue shift of the G-band is observed after MoS2 is formed on the surface of carbonized PAN when it is compared with that of the Gband of SPAN. It confirms that there is the formation of a new product on the carbonized PAN surface. Raman spectra of the crystalline MoS2, which is synthesized from the commercial Li2S and molybdenum(V) chloride salt, revealed feature peaks at 375 cm−1 (in-plane 1E2g mode) and 401 cm−1 (out-of-plane A1g mode).44,45 However, the characteristics of Raman feature peaks associated with crystalline MoS2 are not observed in the MoS2-cPAN, which confirms that the products became either molecular or amorphorous.5,46 The lack of the peaks on the XRD spectra of MoS2-cPAN material in Figure 3f further confirms the material’s amorphous or molecule nature,5,47 but the crystalline MoS2 reveals the XRD peaks at 2θ = 14.32, 33.01, 39.09, and 60.02°. It is indexed with MoS2 (JCPDS card No. 65-1915).

pattern of commercial Li2S shows the peaks at 2θ = 26.1, 30.21, 44, and 50.4°, but no Li2S peaks can be observed in the in situ XRD pattern of Li2S-cPAN. It implies that the formed Li2S on the carbonized PAN surface is either amorphous or in a molecular state. From the SEM image (shown in Figure 2c), the morphology reveals that the MoS2 disperses homogeneously on carbonized PAN. The energy-dispersive X-ray spectrometry (EDS) mapping images in Figure 2d,e show that both molybdenum and sulfur have the same pattern of atomic distribution, and also the nanometer scale holes (cPAN without molecular MoS2 layer) have been observed on the map in the absence of Mo and S atoms. The strongly correlated pattern of Mo and S implies that the formed MoS2 molecules are uniformly distributed on the support, and there are no isolated Mo or S atoms. As shown in Figure 3a, the high-magnification transmission electron microscopy (TEM) image of MoS2-cPAN does not show the MoS2 molecule clearly. It supports that the MoS2 on the cPAN exists as very small in size. The high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) images at high magnification in Figure 3b,c show that bright spots on the dark background are distributed on the surface of cPAN. These bright spots in the HAADF-STEM image are supposed to be individual immobilized single MoS2 molecules randomly anchored on the surface of cPAN. The electron energy loss spectroscopy (EELS) map of Mo and C atoms in Figure S6 also clearly show that the Mo atoms are anchored on the surface of carbon atoms. The size of the molecular MoS2 has been estimated experimentally in Figure 3d, and the average size is about 1.31 nm. The rough calculation of the size of MoS2 without considering the bond length and bond angle between Mo and 6723


Article

ACS Nano

Figure 5. (a) Mo K-edge XANES spectra of bulk MoS2, Mo foil, and MoS2-cPAN; (I) and (II) are enlarged XANES spectra of (a). (b) FTEXAFS spectra at the Mo K-edge of bulk MoS2, Mo foil, and MoS2-cPAN.

energies have some shift from the binding energy of the peaks of the bulk MoS2 (ΔBE = 0.29 eV). The peak shift of Mo 3d for single molecular MoS2 has been observed, and this may be due to high surface energy of the single atomic state tendency to form bonds.35 In the high-resolution C 1s spectra (Figure 4d), the SPAN’s new C 1s peak at 287.9 eV corresponds to the formation of a new C−S bond. Due to the formation of the C−S bond, the binding energies of C−C/CC and C−N slightly shift to higher values compared with other sample C 1s binding energies. For MoS2-cPAN, the C 1s binding energies of C−C/ CC and C−N peaks appeared at 283.8 and 286.2 eV, respectively.50 It shows the shift of binding energies to lower values compared with that of the corresponding C 1s in SPAN. It implies that the sulfur molecules have already detached from the SPAN backbone atoms and made a new bond with Mo4+ to form MoS2. The less amount of quaternary nitrogen in MoS2cPAN can also be a possible cause for the shifting of C 1s to lower binding energy (Figures S7c and S8b). In addition, the newly formed MoS2 have some kinds of electronic interactions (electron transfer or van der Waals force) with the cPAN support, and this interaction more likely causes the binding energy to shift to a lower value.51 The local atomic structure of the MoS2-cPAN catalyst was characterized by X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) of the Mo K-edge, as shown in Figure 5. The preedge feature and the white line at the Mo K-edge of MoS2cPAN exhibit some change compared to Mo foil and MoS2

At high temperature, the reaction of PAN and sulfur is reported as it produces covalently cross-linked Sx with the carbon rings.43 Here, in order to know the cross-linking between C, S, and N within the SPAN composite, X-ray photoelectron spectroscopy (XPS) analyses were conducted and are depicted in Figure S7. As shown in Figure S7a, the SPAN sulfur’s 2p spectra resolution peak of 2p3/2 around 161.3 eV and the 2p1/2 around 163.18 eV are the characteristic peaks of the C−S bond. The S 2p3/2 peak around 166.7 eV and the S 2p1/2 peak at 165.4 are ascribed to the S−S bond of sulfur short-chain bonded on carbon rings. From the XPS spectra of the C 1s in Figure S7b, the peaks at 288.6, 287.9, and 285.2 eV represent the binding energy of C− N, C−S, and C−C/CC, respectively.5 Thereby the SPAN synthesized at 350 °C has been graphitized and forms the bonded sulfur. The chemical states of MoS2-cPAN composites were further examined by XPS. In Mo 3d spectra convolution (Figure 4a), the peaks at 232.9 and 229.5 eV are attributed to the doublet of Mo 3d3/2 and 3d5/2, respectively, and confirm the existence of Mo4+. The shoulder peak at 227.1 eV reveals the peak of S 2s. In S 2p spectra (Figure 4b), the peaks at 162.2 and 161.1 eV correspond to the energy of S 2p1/2 and 2p3/2, respectively. These peak positions belong to Mo4+ and S2− oxidation states in MoS2.48,49 The peaks at 236.5 and 233.6 eV are the binding energy of Mo 3d3/2 and 3d5/2 of Mo6+, respectively. It implies that the molecular MoS2 is mixed with molecular MoS3. From the XPS (Figure 4c) spectra of Mo 3d, the single molecular MoS2 doublet peak splitting binding 6724


Article

ACS Nano (the reference). From Figure 5a, the absence of a pre-edge peak in MoS2 and MoS2-cPAN spectra (around 2001 eV) confirms that there is no electron transition from 1s to 4d,52 but from the closer inspection of the pre-edge peak region of Mo K-edge spectra, the MoS2-cPAN spectrum is slightly shifted to higher energy relative to the Mo foil and MoS2 (the reference) (in the direction shown in Figure 5a,I). It indicates the presence of a molecular MoS3 species mixture with MoS2 in MoS2-cPAN and is in agreement with our XPS results.53 The MoS2 spectrum shows a characteristic feature around 20011 eV, but the spectrum of the MoS2-cPAN differs from the MoS2 for a partial erosion of the characteristic feature at 20011 eV. The increasing intensity in the edge before the white line (20021 eV) is due to the presence of a MoS3 mixture within MoS2-cPAN.54 As shown in inset (II) of Figure 5a, closer inspection of the Mo XANES spectra of the white line (at 20030 eV) of both MoS2-cPAN and MoS2 (the reference) shows they are very similar. Therefore, we could be sure that the synthesized material has Mo−S coupling as the MoS2 (the reference), but the Mo foil has no white line intensity (20030 eV) due to the absence of coupling of the Mo atom with another heteroatom. Both MoS2-cPAN and MoS2 (the reference) are characterized by a slight difference in the white line intensity. An increase in the white line intensity indicates a decrease in the number of electrons in the d orbital as there is a stronger interaction between sulfur and Mo atoms. However, the unfilled d-band of Mo in the single molecular MoS2 has been modified slightly by the interaction with the support of cPAN. In order to make a single molecule stable, there could be an interaction more likely with the support. Furthermore, the average resulting in a broader white line intensity of single molecular MoS2 on cPAN is from more available unoccupied d states and/or more single molecular MoS2 interaction sites with cPAN. The sample of the MoS2 (reference) white line is sharper than the single molecular MoS2. It implies that the single molecular MoS2 crystalline order is too poor (not crystalline).51,52,55 Therefore, the MoS2-cPAN could improve the catalytic activity for reactions involving donating and/or accepting of electrons. For the single molecular MoS2, the Mo atoms have a different coordination environment and less coordination number, which makes the molecule higher in site activity. The Fourier transformed EXAFS spectrum of bulk MoS2 (black curve) shows two distinct peaks of Mo−S and Mo−Mo scattering, as revealed in Figure 5b. For the catalyst MoS2cPAN (pink curve, Figure 5b), the main peak of Mo−S scattering is clearly observed with lower intensity, and bond length shifts to a lower value. The presence of a single molecule MoS2 causes the shortness of the Mo−S bond of the MoS2-cPAN catalyst, and it could affect its local structure.53 The M−S peak intensity of the MoS2-cPAN sample is less intense than the bulk MoS2 sample. The peak intensity is related to the coordination number and the number of vacancies. As presented in Figure S9 and Table 1, the coordination numbers of the Mo−S shell for a single molecular MoS2 and bulk MoS2 are about 1.1 ± 0.03 and 6.0 ± 0.7, respectively. The unsaturated coordination environments of the active centers in single-atom catalysts have been proven to significantly improve their catalytic activity.36,56 Hence, it is suggested that the low coordination and high vacancy of the 4d orbital of Mo atoms are responsible for the excellent performance for the single molecular MoS2.

Table 1. FT-EXAFS Analysis Results for the Reference Sample (MoS2) and Single Molecular MoS2a sample MoS2 MoS2-cPAN

shell

N

R (Å)

σ2 (Å2)

Mo−S Mo−Mo Mo−S

6.0 ± 0.7 6.0 ± 1.6 1.1 ± 0.03

2.41 ± 0.01 3.16 ± 0.01 1.95 ± 0.05

0.002 0.002 0.009

N is the coordination number of each shell; R is the bond length; σ2 is the Debye−Waller factor. a

The Mo−Mo bond scattering is observed for MoS2 (the reference), and it has a coordination number of 6.0 ± 1.6. For MoS2-cPAN sample, Mo−Mo scattering is not observed. It indicates that the Mo atoms in the MoS2-cPAN sample only bond with S atoms (Mo−S bond distance = 1.95 ± 0.05 Å) in the first coordination shell, with no bonding with the neighbor Mo atom (no Mo−Mo bond) in the second coordination shell for MoS2-cPAN. So based on this Fourier transformed EXAFS result and with the support of TEM, HAADF-STEM, EELS mapping, XRD, XPS, EDS mapping, and Raman analyses, it is concluded that MoS2 on the surface of carbonized PAN exists in the molecular state. The single-atom catalysts exhibit great potential to achieve atomic economy through maximum atom utilization efficiency57,58 and have demonstrated excellent electrocatalytic performance.59,60 Therefore, it is suggested that the MoS2 catalyst with a single molecule state would have maximum catalytic performance for electrochemical reactions due to its higher site activity and site populations per mass. In general, an amorphous substance’s internal structure has interconnected structural blocks or low connectivity between the building blocks.61 As shown by the FT-EXAFS spectra in Figure 5b, the Mo−Mo bond is absent in MoS2-cPAN, so the MoS2 could exist as molecular nature. This molecular nature could attribute the material to be amorphous, and it is also confirmed from XRD analysis. Electrochemical HER Performance. According to the literature, the hydrogen evolution reaction’s state-of-the-art catalyst (Pt/C) exhibits high current density nearly at zero onset potential,48 and recently, the best MoS2-based catalysts with an onset potential of 0.09−0.25 V versus RHE have been developed.4,30−32 Here, our electrocatalyst MoS2-cPAN shows the smallest onset potential of 0.025 V versus RHE. Figure 6a shows the polarization curves of cPAN, SPAN, MoS2, MoS2-cPAN, and Pt/C HER catalytic activity. All except MoS2-cPAN and Pt/C exhibit higher overpotentials, too small exchange current density, and a Tafel slope ≥120 mV per decade. However, the single molecular MoS2 on the surface of cPAN highly improved both the current density and overpotential for HER. The MoS2-cPAN catalyst overpotential at 10 mA cm−2 is improved by ∼3 times from the bulk MoS2. Both the exchange current density and the turnover frequency of MoS2-cPAN are approximately 2 orders higher than that of the bulk MoS2. Proton adsorptions on the active site or hydrogen molecule evolution from the catalyst surface determine the rate of the hydrogen evolution reaction.24 If the reactions’ Tafel slope is 120 mV per decade, proton adsorptions on the active site (Volmer step) would be the rate-limiting step, whereas the Tafel slope is 30 mV per decade (Heyrovsky) or 40 mV per decade, indicating the evolution of the hydrogen molecule from the catalyst surface is the rate-limiting step.8,24,62,63 As shown in Figure 6b, the Tafel slope of bulk MoS2 (black curve) is 120 mV per decade, and the single molecular MoS2 (pink 6725


Article

ACS Nano

Figure 6. Electrochemical analysis of MoS2-cPAN, MoS2, SPAN, cPAN, and Pt/C. (a) Linear sweep voltammetry for MoS2-cPAN, MoS2, SPAN, PAN, and Pt/C catalysts on a glassy carbon electrode. (b) Tafel slopes of MoS2-cPAN, MoS2, SPAN, cPAN, and Pt/C. (c) Stability at constant potential (−0.18 V versus RHE) electrolysis of MoS2-cPAN catalysts on carbon paper with loading of 0.34 mg cm−2. (d) Comparison of the turnover frequency of H2 per MoS2 molecules of best reported MoS2-based electrocatalysts: MoS2 nanoparticle (NP),64 MoS2 flakes,65 quantum dot (QD) MoS2,66 strain and sulfur vacancy (SV) MoS2, sulfur vacancy (V) MoS2,31 single molecular MoS2 (SM, this work), and BM (bulk MoS2, this work).

Table 2. Electrochemical Parameters of the Catalysts in HER catalyst

onset potential (mV vs RHE)

cPAN SPAN bulk MoS2 MoS2-cPAN

−450 −230 −190 −25

η at 10 mA cm−2 (mV vs RHE)

Tafel plot (mV dec−1)

J (mA cm−2) (η = 300 mV)

jo (A cm−2)

TOF (s−1)

−460 −185

217 140 120 68

−0.14 −1.1 −5.3 −15

3.3 × 10−6 4.5 × 10−6 5.7 × 10−6 3.13 × 10−4

0.0046 0.8

respectively. The density of active sites of the catalyst (sites cm−2) has been calculated from cyclic voltammograms.24,64,66 Cyclic voltammograms were measured by applying a potential at higher oxidation potentials to convert MoS2 particles to Mo6+,63 which enable calculation of the number of active sites. The calculation of active sites has been done in which the reduction (Mo3+ − Mo0) takes place (0.3−0.0 V) and it is assumed that per MoS2 site a 3 charge transfer occurs.24,63 The detail calculation is shown in Figure S12. The researchers reported that a single Pt atom catalyst improved the catalytic activity ∼10 times over the state-of-theart Pt/C catalyst.55 Here, our best immobilized single molecular MoS2 on cPAN electrocatalyst TOF (at 0 V versus RHE) as a function of MoS2 molecule is 0.8 s−1, and it is higher than those of the best characterized MoS2 edge sites (shown in Figure 6d). The TOF (s−1) values for both MoS2 and MoS2-cPAN have been calculated at the overpotential (η) of 0, 100, and 200 mV, with details shown in Figure S13. Overall, the TOF (s−1) values of MoS2-cPAN are 2 orders higher than that of the corresponding bulk MoS2. Table 2 shows the summarized onset potential, overpotential at 10 mA cm−2, current density at 300 mV versus RHE, exchange current

curve) on the surface of cPAN is 68 mV per decade. Here, the decrease of the Tafel slope of the single molecular MoS2 on cPAN catalyst is an indication for enhancement of proton absorption on the surface of the catalyst. Therefore, the hydrogen evolution step (Volmer/Heyrovsky reaction) becomes the rate-limiting step of the single molecular MoS2cPAN catalyst. For the Volmer reaction of MoS2 (black curve), adsorption of H+ limits the reaction due to the inefficient active edge of the material. As depicted in Figure 6c, the graph of current density versus time shows the current density value changes only about 9% over the 18 h of chronoamperometry measurement. It demonstrates that the material of single molecular MoS2 on the surface of cPAN could be stable during the electrochemical experiments. The turnover frequency of the catalysts are calculated using the following equation. jo (A/cm 2)

TOF (s−1) = # of

sites cm 2

× 1.602 × 10−19

( ce )*2(e/H2)

The current densities (j) and exchange current density (jo) could be calculated from the polarization curve and Tafel plot, 6726


Article

ACS Nano density, and TOF (s−1) of the electrocatalysts that are presented from polarization curves of Figure 6a.

working electrode, and also a graphite rod and saturated Ag/AgCl were used as the counter and reference electrodes, respectively. Argon gas was bubbled into 0.5 M H2SO4 electrolyte throughout the measurement. Linear sweep voltammetry was performed in the 0 to −0.7 V versus RHE range of linear sweep with a scan rate of 1 mV s−1 for the polarization curves.

CONCLUSION In conclusion, we report a synthesis approach for the preparation of low-cost and highly active immobilized single molecular MoS2 on the surface of cPAN electrocatalysts. The single molecular nature of MoS2 on cPAN has been clarified by EXAFS and HAADF-STEM techniques. The as-prepared single molecular MoS2 with unsaturated features was grown homogeneously on the cPAN template, and its overpotential at 10 mA cm−2 was improved by ∼3 times from the bulk MoS2. The exchange current density and TOF (s−1) in the hydrogen evolution reaction of the single molecular MoS 2 are approximately 2 orders higher than that of the bulk MoS2. The single molecular MoS2 boosts the activity of H2 evolution, and this excellent activity is attributed to the low coordination and high vacancy of the 4d orbital of Mo atoms. This work helps to develop highly active single molecular catalysts and explore the fundamental reaction mechanism on the surface of immobilized single molecular catalysts.

ASSOCIATED CONTENT S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsnano.9b01266. Characterization (SEM, XPS, and EDS) of SPAN; characterization (SEM, in situ XRD) of Li2S-cPAN; characterization (TEM, HAADF-STEM, EELS map, and FT-EXAFS) of MoS2-cPAN; characterization (XANES) of MoO3 and comparison with MoS2-cPAN; atomic ratio calculation of elements within the MoS2-cPAN sample; calculation of the turnover frequency of MoS2 and MoS2-cPAN; and setup for the synthesis of SPAN, Li2S-cPAN, and MoS2-cPAN (PDF)

AUTHOR INFORMATION

METHODS

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

Materials and Chemicals. Polyacrylonitrile (average molecular weight of 150 000), sulfur powder (99.98%), MoS2 powder, anhydrous N-methylpyrrolidone, and polyvinylidene difluoride were purchased from Sigma-Aldrich. All chemicals were used without further purification. Synthesis of SPAN. SPAN was synthesized through carbonization of polyacrylonitrile with sulfur powder.38 The brief procedure is described in Figure S14. Synthesis of Li2S-cPAN and MoS2-cPAN. The Li2S-cPAN was prepared via electrochemical synthesis through reduction of Li+ on SPAN cathode material,42 and the MoS2-cPAN was prepared by the reaction of 0.125 mmol MoCl5 with the Li2S-cPAN solution. The brief procedures are sketched in Figure S15. Characterizations. The morphology of the catalyst was identified by field-emission scanning electron microscopy (EDX JSM 6500F, JEOL) together with EDX analysis with a beam voltage of 15 kV. The elemental composition of the composite material was analyzed by elemental analysis (elementar Vario EL cube, for NCSH, Germany). The HAADF-STEM images and the EELS mapping images of the MoS2‑cPAN sample were obtained by a JEOL ARM 200F instrument. The XRD spectra were characterized using a D2 Phaser XRD-300 W diffractometer with a Cu Kα radiation (λ = 1.5418 Å) with a beam source operated at 30 kV and 10 mA. The Raman spectra were collected using on a ProMaker confocal microscope system about 10 s exposure and 15 accumulations for the solid sample operating at 532 nm and 20 mW as the excitation source and laser power, respectively. The XPS (PHI, 1600S) was performed at the 24A beamline station in the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. X-ray absorption spectroscopy was conducted at the beamline station (07A1) in the National Synchrotron Radiation Research Center (NSRRC), Hsinchu, Taiwan. In situ XRD measurements were conducted at beamline TPS BL-09A (temporally coherent X-ray diffraction) at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. Electrochemical Measurements. Preparation of the MoS2cPAN composite on rotating disc electrode followed this process: 10 mg of catalyst was mixed with 1000 μL (v/v 1:3) of a mixture of water and ethanol and 50 μL of a 5 wt % Nafion solution. The mixture was sonicated for 30 min to form a homogeneous ink mixture. Finally, 15 μL of ink suspension was cast onto a 0.196 cm2 glassy carbon electrode. The MoS2-cPAN electrochemical performance measurement was done using galvanostatic Autolab (PGSTAT302N) of a threeelectrode system. The catalyst on the electrode was used as the

ORCID

Tamene Simachew Zeleke: 0000-0003-1605-9640 Yun-Liang Soo: 0000-0002-1683-3141 Wei-Nien Su: 0000-0003-1494-2675 Bing-Joe Hwang: 0000-0002-3873-2149 Notes

The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology (MOST) (108-3116-F-011-001-CC1, 1062923-E 011-005, 105-3113-E-011-001, 105-ET-E-011-004-ET, 104-2923-M-011-002-MY3, 104-2911-1-011-505-MY2, and 103-2221-E-011-156-MY3), the Applied Research Center for Thin-Film Metallic Glass from the Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education of Taiwan, and Taiwan’s Deep Decarbonization Pathways toward a Sustainable Society Project (AS-KPQ-106- DDPP) from Academia Sinica. Facilities support from National Taiwan University of Science and Technology (NTUST) and National Synchrotron Radiation Research Centre (NSRRC) is also acknowledged. REFERENCES (1) 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. (2) Birhanu, M. K.; Tsai, M.-C.; Kahsay, A. W.; Chen, C.-T.; Zeleke, T. S.; Ibrahim, K. B.; Huang, C.-J.; Su, W.-N.; Hwang, B.-J. Copper and Copper-Based Bimetallic Catalysts for Carbon Dioxide Electroreduction. Adv. Mater. Interfaces 2018, 5, 1800919. (3) Li, Y.; Cain, J. D.; Hanson, E. D.; Murthy, A. A.; Hao, S.; Shi, F.; Li, Q.; Wolverton, C.; Chen, X.; Dravid, V. P. Au@MoS2 Core-Shell Heterostructures with Strong Light-Matter Interactions. Nano Lett. 2016, 16, 7696−7702. (4) Lu, X.; Lin, Y.; Dong, H.; Dai, W.; Chen, X.; Qu, X.; Zhang, X. One-Step Hydrothermal Fabrication of Three-Dimensional MoS2 6727


Article

ACS Nano

Reaction: Ultralow Kinetic Barriers with High Active Site Density. ACS Catal. 2017, 7, 5097−5102. (22) 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. (23) Kibsgaard, J.; Chen, Z.; Reinecke, B. N.; Jaramillo, T. F. Engineering the Surface Structure of MoS2 to Preferentially Expose Active Edge Sites for Electrocatalysis. Nat. Mater. 2012, 11, 963−969. (24) Cummins, D. R.; Martinez, U.; Sherehiy, A.; Kappera, R.; Martinez-Garcia, A.; Schulze, R. K.; Jasinski, J.; Zhang, J.; Gupta, R. K.; Lou, J.; Chhowalla, M.; Sumanasekera, G.; Mohite, A. D.; Sunkara, M. K.; Gupta, G. Efficient Hydrogen Evolution in Transition Metal Dichalcogenides via a Simple One-Step Hydrazine Reaction. Nat. Commun. 2016, 7, 11857. (25) Wang, H.; Tsai, C.; Kong, D.; Chan, K.; Abild-Pedersen, F.; Nørskov, J. K.; Cui, Y. Transition-Metal Doped Edge Sites in Vertically Aligned MoS2 Catalysts for Enhanced Hydrogen Evolution. Nano Res. 2015, 8, 566−575. (26) Pető , J.; Ollár, T.; Vancsó, P.; Popov, Z. I.; Magda, G. Z.; Dobrik, G.; Hwang, C.; Sorokin, P. B.; Tapasztó, L. Spontaneous Doping of the Basal Plane of MoS2 Single Layers through Oxygen Substitution under Ambient Conditions. Nat. Chem. 2018, 10, 1246− 1251. (27) Luo, Y.; Li, X.; Cai, X.; Zou, X.; Kang, F.; Cheng, H.-M.; Liu, B. Two-Dimensional MoS2 Confined Co(OH)2 Electrocatalysts for Hydrogen Evolution in Alkaline Electrolytes. ACS Nano 2018, 12, 4565−4573. (28) Lukowski, M. A.; Daniel, A. S.; Meng, F.; Forticaux, A.; Li, L.; Jin, S. Enhanced Hydrogen Evolution Catalysis from Chemically Exfoliated Metallic MoS2 Nanosheets. J. Am. Chem. Soc. 2013, 135, 10274−10277. (29) Yin, Y.; Han, J.; Zhang, Y.; Zhang, X.; Xu, P.; Yuan, Q.; Samad, L.; Wang, X.; Wang, Y.; Zhang, Z.; Zhang, P.; Cao, X.; Song, B.; Jin, S. 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, 7965−7972. (30) 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. (31) Li, H.; Tsai, C.; Koh, A. L.; Cai, L.; Contryman, A. W.; Fragapane, A. H.; Zhao, J.; Han, H. S.; Manoharan, H. C.; AbildPedersen, F.; Norskov, J. K.; Zheng, X. Corrigendum: Activating and Optimizing MoS2 Basal Planes for Hydrogen Evolution through the Formation of Strained Sulphur Vacancies. Nat. Mater. 2016, 15, 364. (32) Li, G.; Zhang, D.; Qiao, Q.; Yu, Y.; Peterson, D.; Zafar, A.; Kumar, R.; Curtarolo, S.; Hunte, F.; Shannon, S.; Zhu, Y.; Yang, W.; Cao, L. All the Catalytic Active Sites of MoS2 for Hydrogen Evolution. J. Am. Chem. Soc. 2016, 138, 16632−16638. (33) Yoo, H.; Tiwari, A. P.; Lee, J.; Kim, D.; Park, J. H.; Lee, H. Cylindrical Nanostructured MoS2 Directly Grown on CNT Composites for Lithium-Ion Batteries. Nanoscale 2015, 7, 3404− 3409. (34) Tsai, C.; Abild-Pedersen, F.; Norskov, J. K. Tuning the MoS2 Edge-Site Activity for Hydrogen Evolution via Support Interactions. Nano Lett. 2014, 14, 1381−1387. (35) Wan, J.; Chen, W.; Jia, C.; Zheng, L.; Dong, J.; Zheng, X.; Wang, Y.; Yan, W.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. Defect Effects on TiO2 Nanosheets: Stabilizing Single Atomic Site Au and Promoting Catalytic Properties. Adv. Mater. 2018, 30, 1705369. (36) Chen, Y.; Ji, S.; Chen, C.; Peng, Q.; Wang, D.; Li, Y. SingleAtom Catalysts: Synthetic Strategies and Electrochemical Applications. Joule 2018, 2, 1242−1264. (37) Pegis, M. L.; Wise, C. F.; Martin, D. J.; Mayer, J. M. Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts. Chem. Rev. 2018, 118, 2340−2391.

Nanoflower Using Polypyrrole as Template for Efficient Hydrogen Evolution Reaction. Sci. Rep. 2017, 7, 42309. (5) Ye, J.; He, F.; Nie, J.; Cao, Y.; Yang, H.; Ai, X. Sulfur/Carbon Nanocomposite-Filled Polyacrylonitrile Nanofibers as a Long Life and High Capacity Cathode for Lithium-Sulfur Batteries. J. Mater. Chem. A 2015, 3, 7406−7412. (6) Benck, J. D.; Hellstern, T. R.; Kibsgaard, J.; Chakthranont, P.; Jaramillo, T. F. Catalyzing the Hydrogen Evolution Reaction (HER) with Molybdenum Sulfide Nanomaterials. ACS Catal. 2014, 4, 3957− 3971. (7) Zheng, J.; Sheng, W.; Zhuang, Z.; Xu, B.; Yan, Y. Universal Dependence of Hydrogen Oxidation and Evolution Reaction Activity of Platinum-Group Metals on pH and Hydrogen Binding Energy. Sci. Adv. 2016, 2, No. e1501602. (8) Benson, J.; Li, M.; Wang, S.; Wang, P.; Papakonstantinou, P. Electrocatalytic Hydrogen Evolution Reaction on Edges of a Few Layer Molybdenum Disulfide Nanodots. ACS Appl. Mater. Interfaces 2015, 7, 14113−14122. (9) Kucernak, A. R.; Zalitis, C. General Models for the Electrochemical Hydrogen Oxidation and Hydrogen Evolution Reactions: Theoretical Derivation and Experimental Results under Near MassTransport Free Conditions. J. Phys. Chem. C 2016, 120, 10721− 10745. (10) Ding, Q.; Song, B.; Xu, P.; Jin, S. Efficient Electrocatalytic and Photoelectrochemical Hydrogen Generation Using MoS2 and Related Compounds. Chem. 2016, 1, 699−726. (11) Zhuang, H.; Tkalych, A. J.; Carter, E. A. Surface Energy as a Descriptor of Catalytic Activity. J. Phys. Chem. C 2016, 120, 23698− 23706. (12) 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. (13) Mahmood, J.; Li, F.; Jung, S.-M.; Okyay, M. S.; Ahmad, I.; Kim, S.-J.; Park, N.; Jeong, H. Y.; Baek, J.-B. An Efficient and pH-Universal Ruthenium-Based Catalyst for the Hydrogen Evolution Reaction. Nat. Nanotechnol. 2017, 12, 441−446. (14) Gong, M.; Zhou, W.; Tsai, M. C.; Zhou, J.; Guan, M.; Lin, M. C.; Zhang, B.; Hu, Y.; Wang, D. Y.; Yang, J.; Pennycook, S. J.; Hwang, B. J.; Dai, H. Nanoscale Nickel Oxide/Nickel Heterostructures for Active Hydrogen Evolution Electrocatalysis. Nat. Commun. 2014, 5, 4695. (15) Wang, D. Y.; Gong, M.; Chou, H. L.; Pan, C. J.; Chen, H. A.; Wu, Y.; Lin, M. C.; Guan, M.; Yang, J.; Chen, C. W.; Wang, Y. L.; Hwang, B. J.; Chen, C. C.; Dai, H. Highly Active and Stable Hybrid Catalyst of Cobalt-Doped FeS2 Nanosheets-Carbon Nanotubes for Hydrogen Evolution Reaction. J. Am. Chem. Soc. 2015, 137, 1587−92. (16) Liu, W.; Hu, E.; Jiang, H.; Xiang, Y.; Weng, Z.; Li, M.; Fan, Q.; Yu, X.; Altman, E. I.; Wang, H. A Highly Active and Stable Hydrogen Evolution Catalyst Based on Pyrite-Structured Cobalt Phosphosulfide. Nat. Commun. 2016, 7, 10771. (17) Rowley-Neale, S. J.; Foster, C. W.; Smith, G. C.; Brownson, D. A. C.; Banks, C. E. Mass-Producible 2D-MoSe2 Bulk Modified ScreenPrinted Electrodes Provide Significant Electrocatalytic Performances Towards the Hydrogen Evolution Reaction. Sustainable Energy Fuels 2017, 1, 74−83. (18) Wang, H.; Kong, D.; Johanes, P.; Cha, J. J.; Zheng, G.; Yan, K.; Liu, N.; Cui, Y. MoSe2 and WSe2 Nanofilms with Vertically Aligned Molecular Layers on Curved and Rough Surfaces. Nano Lett. 2013, 13, 3426−3433. (19) Xie, J.; Li, S.; Zhang, X.; Zhang, J.; Wang, R.; Zhang, H.; Pan, B.; Xie, Y. Atomically-Thin Molybdenum Nitride Nanosheets with Exposed Active Surface Sites for Efficient Hydrogen Evolution. Chem. Sci. 2014, 5, 4615−4620. (20) Zhang, G.; Liu, H.; Qu, J.; Li, J. Two-Dimensional Layered MoS2: Rational Design, Properties and Electrochemical Applications. Energy Environ. Sci. 2016, 9, 1190−1209. (21) Ling, C.; Ouyang, Y.; Shi, L.; Yuan, S.; Chen, Q.; Wang, J. Template-Grown MoS2 Nanowires Catalyze the Hydrogen Evolution 6728


Article

ACS Nano

(55) Sun, S.; Zhang, G.; Gauquelin, N.; Chen, N.; Zhou, J.; Yang, S.; Chen, W.; Meng, X.; Geng, D.; Banis, M. N.; Li, R.; Ye, S.; Knights, S.; Botton, G. A.; Sham, T.-K.; Sun, X. Single-Atom Catalysis Using Pt/Graphene Achieved through Atomic Layer Deposition. Sci. Rep. 2013, 3, 1775. (56) Yang, X.-F.; Wang, A.; Qiao, B.; Li, J.; Liu, J.; Zhang, T. SingleAtom Catalysts: A New Frontier in Heterogeneous Catalysis. Acc. Chem. Res. 2013, 46, 1740−1748. (57) Chen, Y.; Huang, Z.; Ma, Z.; Chen, J.; Tang, X. Fabrication, Characterization, and Stability of Supported Single-Atom Catalysts. Catal. Sci. Technol. 2017, 7, 4250−4258. (58) Zhang, Y.; Guo, L.; Tao, L.; Lu, Y.; Wang, S. Defect-Based Single-Atom Electrocatalysts. Small Methods 2018, 1800406. (59) Zhang, L. Z.; Jia, Y.; Gao, G. P.; Yan, X. C.; Chen, N.; Chen, J.; Soo, M. T.; Wood, B.; Yang, D. J.; Du, A. J.; Yao, X. D. Graphene Defects Trap Atomic Ni Species for Hydrogen and Oxygen Evolution Reactions. Chem. 2018, 4, 285−297. (60) Liu, P. X.; Zheng, N. F. Coordination Chemistry of Atomically Dispersed Catalysts. Natl. Sci. Rev. 2018, 5, 636−638. (61) Ojovan, M. I.; Lee, W. E. Connectivity and Glass Transition in Disordered Oxide Systems. J. Non-Cryst. Solids 2010, 356, 2534− 2540. (62) Sheng, W.; Gasteiger, H. A.; Shao-Horn, Y. Hydrogen Oxidation and Evolution Reaction Kinetics on Platinum: Acid vs Alkaline Electrolytes. J. Electrochem. Soc. 2010, 157, B1529. (63) Chen, Z.; Cummins, D.; Reinecke, B. N.; Clark, E.; Sunkara, M. K.; Jaramillo, T. F. Core-Shell MoO3-MoS2 Nanowires for Hydrogen Evolution: A Functional Design for Electrocatalytic Materials. Nano Lett. 2011, 11, 4168−4175. (64) 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. (65) Kong, D.; Wang, H.; Cha, J. J.; Pasta, M.; Koski, K. J.; Yao, J.; Cui, Y. Synthesis of MoS2 and MoSe2 Films with Vertically Aligned Layers. Nano Lett. 2013, 13, 1341−1347. (66) Qiao, W.; Yan, S.; Song, X.; Zhang, X.; Sun, Y.; Chen, X.; Zhong, W.; Du, Y. Monolayer MoS2 Quantum Dots as Catalysts for Efficient Hydrogen Evolution. RSC Adv. 2015, 5, 97696−97701.

(38) Wang, J.; Yang, J.; Xie, J.; Xu, N. A Novel Conductive Polymer−Sulfur Composite Cathode Material for Rechargeable Lithium Batteries. Adv. Mater. 2002, 14, 963−965. (39) Kuo, C. F. J.; Weret, M. A.; Hung, H. Y.; Tsai, M. C.; Huang, C. J.; Su, W. N.; Hwang, B. J. Sulfurized-Poly(acrylonitrile) Wrapped Carbon-Sulfur Composite Cathode Material for High Performance Rechargeable Lithium-Sulfur Batteries. J. Power Sources 2019, 412, 670−676. (40) Kim, J.-S.; Hwang, T. H.; Kim, B. G.; Min, J.; Choi, J. W. A Lithium-Sulfur Battery with a High Areal Energy Density. Adv. Funct. Mater. 2014, 24, 5359−5367. (41) Mäntymäki, M.; Hämäläinen, J.; Puukilainen, E.; Sajavaara, T.; Ritala, M.; Leskelä, M. Atomic Layer Deposition of LiF Thin Films from Li(thd), Mg(thd)2, and TiF4 Precursors. Chem. Mater. 2013, 25, 1656−1663. (42) Wei, S.; Ma, L.; Hendrickson, K. E.; Tu, Z.; Archer, L. A. MetalSulfur Battery Cathodes Based on PAN-Sulfur Composites. J. Am. Chem. Soc. 2015, 137, 12143−12152. (43) Zhang, S. Understanding of Sulfurized Polyacrylonitrile for Superior Performance Lithium/Sulfur Battery. Energies 2014, 7, 4588−4600. (44) Shi, Y.; Zhou, W.; Lu, A. Y.; Fang, W.; Lee, Y. H.; Hsu, A. L.; Kim, S. M.; Kim, K. K.; Yang, H. Y.; Li, L. J.; Idrobo, J. C.; Kong, J. Van Der Waals Epitaxy of MoS2 Layers Using Graphene as Growth Templates. Nano Lett. 2012, 12, 2784−2791. (45) Cai, L.; He, J.; Liu, Q.; Yao, T.; Chen, L.; Yan, W.; Hu, F.; Jiang, Y.; Zhao, Y.; Hu, T.; Sun, Z.; Wei, S. Vacancy-Induced Ferromagnetism of MoS2 Nanosheets. J. Am. Chem. Soc. 2015, 137, 2622−2627. (46) Zhang, X.; Meng, F.; Mao, S.; Ding, Q.; Shearer, M. J.; Faber, M. S.; Chen, J.; Hamers, R. J.; Jin, S. Amorphous MoSxCly Electrocatalyst Supported by Vertical Graphene for Efficient Electrochemical and Photoelectrochemical Hydrogen Generation. Energy Environ. Sci. 2015, 8, 862−868. (47) Saha, N.; Sarkar, A.; Ghosh, A. B.; Dutta, A. K.; Bhadu, G. R.; Paul, P.; Adhikary, B. Highly Active Spherical Amorphous MoS2: Facile Synthesis and Application in Photocatalytic Degradation of Rose Bengal Dye and Hydrogenation of Nitroarenes. RSC Adv. 2015, 5, 88848−88856. (48) Youn, D. H.; Jang, J. W.; Kim, J. Y.; Jang, J. S.; Choi, S. H.; Lee, J. S. Fabrication of Graphene-Based Electrode in Less Than a Minute through Hybrid Microwave Annealing. Sci. Rep. 2015, 4, 5492. (49) Turner, N. H.; Single, A. M. Determination of Peak Positions and Areas from Wide-Scan XPS Spectra. Surf. Interface Anal. 1990, 15, 215−222. (50) Frey, M.; Zenn, R. K.; Warneke, S.; Müller, K.; Hintennach, A.; Dinnebier, R. E.; Buchmeiser, M. R. Easily Accessible, Textile FiberBased Sulfurized Poly(acrylonitrile) as Li/S Cathode Material: Correlating Electrochemical Performance with Morphology and Structure. ACS Energy Lett. 2017, 2, 595−604. (51) Kim, Y.; Jackson, D. H.; Lee, D.; Choi, M.; Kim, T. W.; Jeong, S. Y.; Chae, H. J.; Kim, H. W.; Park, N.; Chang, H.; et al. In Situ Electrochemical Activation of Atomic Layer Deposition Coated MoS2 Basal Planes for Efficient Hydrogen Evolution Reaction. Adv. Funct. Mater. 2017, 27, 1701825. (52) Chen, W. F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.; Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Hydrogen-Evolution Catalysts Based on Non-Noble Metal Nickel-Molybdenum Nitride Nanosheets. Angew. Chem., Int. Ed. 2012, 51, 6131−6135. (53) Lassalle-Kaiser, B.; Merki, D.; Vrubel, H.; Gul, S.; Yachandra, V. K.; Hu, X.; Yano, J. Evidence from In Situ X-Ray Absorption Spectroscopy for the Involvement of Terminal Disulfide in the Reduction of Protons by an Amorphous Molybdenum Sulfide Electrocatalyst. J. Am. Chem. Soc. 2015, 137, 314−321. (54) Cesano, F.; Bertarione, S.; Piovano, A.; Agostini, G.; Rahman, M. M.; Groppo, E.; Bonino, F.; Scarano, D.; Lamberti, C.; Bordiga, S.; Montanari, L.; Bonoldi, L.; Millini, R.; Zecchina, A. A. Model Oxide Supported MoS2 HDS Catalysts: Structure and Surface Properties. Catal. Sci. Technol. 2011, 1, 123−136. 6729


Immobilized Single Molecular Molybdenum Disulfide on Carbonized

Recommend Documents