Immobilized Single Molecular Molybdenum Disulfide on Carbonized

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Immobilized Single Molecular Molybdenum Disulfide on Carbonized Polyacrylonitrile for Hydrogen Evolution Reaction 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 ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.9b01266 • Publication Date (Web): 13 May 2019 Downloaded from http://pubs.acs.org on May 15, 2019

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Immobilized Single Molecular Molybdenum Disulfide on Carbonized Polyacrylonitrile for Hydrogen Evolution Reaction

Tamene Simachew Zeleke1, Meng-Che Tsai1, Misganaw Adigo Weret3, Chen-Jui Huang1, Mulatu Kassie Birhanu1, Tzu-Ching Liu1, Chiu-Ping Huang1,4, Yun-Liang Soo5, Yaw-Wen Yang6, Wei-Nien Su*2, and Bing-Joe Hwang*1,6,7 1NanoElectrochemistry

Laboratory, Department of Chemical Engineering, National Taiwan University

of Science and Technology, Taipei, 10607, Taiwan 2NanoElectrochemistry

Laboratory, Graduate Institute of Applied Science and Technology, National

Taiwan University of Science and Technology, Taipei, 10607, Taiwan 3Department

of Materials Science and Engineering, National Taiwan University of Science and

Technology, Taipei, 10607, Taiwan 4Material

and Chemical Research Laboratories, Industrial Technology Research Institute, Hsin-Chu

31040, Taiwan 5Department

of Physics, National Tsing Hua University, Hsin-Chu, 300, Taiwan

6National

Synchrotron Radiation Research Center, Hsin-Chu, 30076, Taiwan

7Applied

Research Center for Thin-Film Metallic Glass, National Taiwan University of Science and

Technology, Taipei 10607, Taiwan

Corresponding Authors *E-mail: [email protected] (Bing-Joe Hwang) *E-mail: [email protected] (Wei-Nien Su)

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ABSTRACT The designing of MoS2 catalyst having a large number of active sites and high site activity enables to improve catalytic activity toward hydrogen evolution reaction. Herein, we report a synthesis of low-cost and catalytically active immobilized single molecular molybdenum disulfide on carbonized polyacrylonitrile (MoS2-cPAN) electrocatalyst. From the Extended X-ray Absorption Fine Structure (EXAFS) spectra analysis, we found 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 (HAADF-STEM). Low coordination number and maximum utilization of single molecular MoS2 surface enable MoS2-cPAN to demonstrate significantly better electrochemical performance than the bulk MoS2 by two-order of exchange current density (jo) and turnover frequency (TOF (s-1)) to the hydrogen evolution. KEYWORDS: electrochemical reaction, single molecule, molybdenum disulfide, hydrogen evolution, renewable energy.

High fossil fuel consumption due to the world’s population growth results in a severe issue on global warming; this phenomenon accelerating and putting-forward the development of green and renewable energy usage.1,2 Hydrogen is one of the alternative renewable energy source with high energy density1,3,4 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 of a convenient and promising approach. Hydrogen production electrochemical from water at a standard condition needs free


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energy of ΔG = 237.1 kJ mol-1 and a voltage of 1.23 V versus RHE.6 In acidic electrolyte, hydrogen evolution reaction (HER) involves in three possible ways.7 Volmer step

* + H+ + e

Had

Heyrovsky step

H+ + e + Had

H2 + *

2Had

Tafel step

H2 + 2*

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 be 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 state-of-the-art catalysts for hydrogen evolution reaction.13 But, 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 and so on. Nowadays the transition metal dichalcogenide materials like MoS2 having 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 MoS2 catalyst with 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, nanoflower,4 and core-shell MoO3/ MoS224 and so on. (2) Chemical doping for the enhancement of the activity of edge sites.25-27 (3) Transformation 3 ACS Paragon Plus Environment

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of 2H MoS2 to metastable 1T phase (1T MoS2).28,29 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 (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 are 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 carbonaceous material due to non-intimate 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 attracts much attention due to its ultrahigh site activity and 100% catalysts’ utilization.35,36 Single molecule catalysts are usually found in homogeneous solution.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, and the electrocatalytic HER of MoS2-cPAN composite was performed and benchmarked. A single molecular MoS2 was prepared on carbonized PAN surface in stepwise. Initially, elemental sulfur is reacted with polyacrylonitrile, and the SPAN containing a sulfurcarbon bond has been formed. Secondly, sulfur in SPAN reacts with Li metal to form Li2S-cPAN electrochemically. The Li2S formed electrochemically reacts with Mo ions and give immobilized single molecular MoS2 on the surface of cPAN. The immobilized single molecular MoS2 synthesis scheme has been presented in Figure 1. The single molecular MoS2 on the surface of cPAN exhibits excellent electrochemical performance for hydrogen evolution reaction. Although a single molecular MoS2 surface has maximum catalytic performance, its conductivity could be questionable, since individual MoS2 is present independently with 4 ACS Paragon Plus Environment

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no interconnection among others. To improve its conductivity, a conductive support that stabilizes the super active molecule seems necessary. Hence, the support of conductive cPAN could stabilize and make single molecular MoS2 conductive through the electronic interaction (van der Waals forces) between the cPAN and molecular MoS2.

Figure 1. Schematic for the synthesis of single molecular MoS2 catalyst RESULTS AND DISCUSSION From the Scanning Electron Microscopy (SEM) image in Figure 2a, the morphology of highly magnified SPAN composite shows uniformly distributed spherical like nanoparticles, it confirms the formation of composite material after annealing 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 graphitic (002) plane, signifying successful carbonization of PAN.39 Through carbonization, the PAN crystal peak at 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 = 22o, 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°, 5 ACS Paragon Plus Environment

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implying the formation of crystalline LiF,41 possibly resulted from the decomposition of LiPF6 electrolyte or PVDF binder. The XRD 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 the MoS2 disperse homogeneously on carbonized PAN. The Energy Dispersive X-ray Spectrometry (EDS) mapping images in Figure 2d and 2e 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.


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Figure 2. (a) The 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. (d, e) EDS mapping (d) atomic Sulfur (e) atomic Molybdenum of MoS2-cPAN catalyst; shows 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 also observed on the map without the distribution of S and Mo atoms.

As shown in Figure 3a, the high magnification of Transmission Electron Microscopy (TEM) image of MoS2-cPAN doesn’t show the MoS2 molecule clearly. It supports that the MoS2 on the cPAN exist as very small in size. The HAADF-STEM images at high magnification in Figures 3b and 3c 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 7 ACS Paragon Plus Environment

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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 S atoms is estimated to be 0.78 nm. It is approximately comparable with the size of the experimentally found result. The slightly 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 G-to-D band ratio (IG/ ID) between MoS2-cPAN and SPAN changed (from 1 to 0.95). It shows the graphitization of PAN, while the reaction of SPAN with Mo atom to form a molecular MoS2 and MoS3 as mixture. The SS and CS bonds of SPAN were detected from the Raman spectra and these spectra appeared at 370 cm-1, 807 cm-1, and 932 cm-1.40,42,43 But these peaks were not observed on MoS2-cPAN. It is notifying 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 compared with the G-band 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 E12g 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 =14.32o, 33.01o, 39.09o, and 60.02o. It is indexed with MoS2 (JCPDS card No. 65-1915).


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b

c

10nm

10nm

5nm

e

d

12 10 8 6 4

f MoS2

MoS2 -cPAN

D

G

SPAN

MoS2

Intensity (a. u.)

14

a

Intensity (a. u. )

16

Counts (a.u. )

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

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MoS2-cPAN

MoS2-JCPDS

2 0 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

Particle size (nm)

400

800

1200

1600

Raman shift (cm-1)

2000

10

20

30

40

50

2  degree

60

70

80

Figure 3. (a) TEM image of MoS2-cPAN. (b) and (c) The HAADF-STEM images at different magnifications, the randomly dispersed bright spots on cPAN are individual MoS2 molecules. (d) The 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.

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 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 9 ACS Paragon Plus Environment

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rings. From XPS spectra of the C1s 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 oC has been graphitized and form 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 energies has some shift from the binding energy of the peaks of the bulk MoS2 (∆BE = 0.29 eV). The peak shift of Mo 3d for single molecular MoS2 has been observed, this may be due to high surface energy of the single atomic state tendency to form bonding.35 In the high resolution of C 1s spectra (Figure 4d), the SPAN’s C1s new peak at 287.9 eV corresponds to the formation of a new CS bond. Due to the formation of CS bond, the binding energies of CC/CC and CN slightly shift to higher values compared with other samples C 1s binding energies. For MoS2-cPAN, the C1s binding energies of CC/CC and CN peak appeared at 283.8 and 286.2 eV, respectively.50 It shows the shift of binding energies to lower values compared with the corresponding C1s 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 MoS2-cPAN can be also a possible cause for the shifting of C 1s to lower binding energy (Figure S7c and S8b). In addition, the newly formed MoS2 have some kinds of electronic interaction (electron transfer or van der


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Waals force) with the cPAN support and this interaction more likely causes the binding energy shifts to lower value.51 Mo 3d Mo

+4

b

3d5/2

S2s Mo

240

+6

S 2P

Intensity (a. u.)

Intensity (a. u )

a

S2P 3/2 S2P 1/2

3d5/2

236

232

228

Binding energy (eV)

c

224

172

168

164

160

156

Binding energy(eV)

d

Mo 3d

C1 s

Intensity (a. u. )

MoS2- cPAN

Intensity (a. u )

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

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MoS2-cPAN

MoS2 (Bulk)

240

235

230

Binding energy (eV)

C-S C-C/C=C C-N SPAN

cPAN

225

295

290

285

280

275

Binding energy (eV)

270

Figure 4. XPS spectra of Mo 3d, S 2p and C1s, high-resolution spectra 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 C1s for MoS2-cPAN, SPAN, and cPAN.

The local atomic structure of the MoS2-cPAN catalyst was characterized by the X-ray Absorption Near Edge Structure (XANES) and the Extended X-ray Absorption Fine Structure (EXAFS) of the Mo K-edge as shown in Figure 5. The pre-edge feature and the white line at the Mo K-edge of MoS2-cPAN exhibit some change compared to Mo foil and MoS2 (the reference). From Figure 5a, the absence of a pre-edge 11 ACS Paragon Plus Environment

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peak in MoS2 and MoS2-cPAN spectra (around 2001 eV), confirms that there is no an electron transition from 1s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 with relative to the Mo-foil and MoS2 ( the reference) (in the direction shown Figure 5I). It indicates that the presence of a molecular MoS3 species mixture with MoS2 in MoS2-cPAN, it is in agreement with our XPS results.53 The MoS2 spectrum shows a characteristic feature around 20011 eV. But, the spectrum of the MoS2cPAN 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 the inset (II) of Figure 5a, the closer inspection of the Mo XANES spectra of the white line (at 20030 eV) of both MoS2-cPAN and MoS2 (the reference) 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 Mo atom with another heteroatom. Both the 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 since there is a stronger interaction between sulfur and Mo atoms. But, the unfilled d-band of Mo in the single molecular MoS2 has been modified a little bit by the interaction with the support of cPAN. In order to make stable a single molecule, 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 results from (more available unoccupied d states) and /or more single molecular MoS2 interaction sites with cPAN. The sample of 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


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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 it revealed in Figure 5b. For the catalyst MoS2-cPAN (pink curve, Figure 5b), the main peak of MoS scattering clearly observed with lower intensity and bond length shifts to lower value. The presence of a single molecule MoS2 causes the shortness of 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 proportional to the coordination number and the number of vacancies. As presented in Figure S9 and Table 1, the coordination numbers of 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 4d orbital of Mo atoms are responsible for the excellent performance for the single molecular MoS2. The MoMo bond scattering is observed for MoS2 (the reference) and it has a coordination number of 6.0 1.6. But for MoS2-cPAN sample MoMo scattering is not observed. It indicates that the Mo atoms in the MoS2-cPAN sample have only bonding with S atoms (MoS bond distance = 1.95 0.05 Å) in the first coordination shell, but 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 results and with the support of TEM, HAADF-STEM, EELS map, 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 have exhibited great potential to achieve the atomic economy through maximum atomutilization efficiency57,58 and have been demonstrated excellent electrocatalytic performance.59,60 13 ACS Paragon Plus Environment

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Therefore, it is suggested that the MoS2 catalyst with a single molecule state would has 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 in Figure 5b of FT-EXAFS spectra, the MoMo bond is absence 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.

Table 1. FT-EXAFS analysis results for the reference sample (MoS2) and the single molecular MoS2. Sample

Shell

N

R (Å)

σ2 (Å2)

MoS

6.00.7

2.410.01

0.002

MoMo

6.01.6

3.160.01

0.002

MoS

1.10.03

1.950.05

0.009

MoS2 MoS2-cPAN

N: coordination number of each shell; R: bond length; σ2: Debye-Waller factor.


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a

3.5

Magnitude of F.T

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

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b

3.0

MoS2-cPAN Mo foil MoS2

2.5 Mo-Mo

2.0 1.5

Mo-S

1.0 0.5 0.0 0

1

2

3

4

5

6

R (Å)

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) FT-EXAFS spectra at Mo K edge of bulk MoS2, Mo foil and MoS2cPAN.

Electrochemical HER Performance According to the literature, the hydrogen evolution reaction’s state- of- art catalyst (Pt/C) exhibits high current density nearly at zero onset potential 48 and in recent times developed best MoS2-based catalysts show an onset potential of 0.090.25 V versus RHE.4,30-32 Here, our electrocatalyst of MoS2-cPAN shows the smallest onset potential of 0.025 V versus RHE.


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Figure 6a shows the polarization curve of cPAN, SPAN, MoS2, MoS2-cPAN, and Pt/C HER catalytic activity. All except MoS2-cPAN exhibit higher overpotential, too small exchange current density and a Tafel slope ≥ 120 mV per decade. But 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 two orders higher than the bulk MoS2. Proton adsorptions on the active site or hydrogen molecule evolution from the catalyst surface determine the rate of 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, while Tafel slope is 30 mV per decade (Heyrovsky) or 40 mV per decade the evolution of hydrogen molecule from the catalyst surface be 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 curve) on the surface of cPAN has 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, hydrogen evolution step (Volmer/ Heyrovsky reaction) becomes the rate-limiting step of the single molecular MoS2-cPAN catalyst. But for the MoS2 (black curve) the Volmer reaction; adsorption of H+ is a limiting of 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 hours of chronoamperometry measurement. It demonstrates the material of single molecular MoS2 on the surface of cPAN could be stable during the electrochemical experiments.


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Potential(V)

J(mA/cm-2)

-0.3

a

0 -5

cPAN

-10

SPAN MoS2 Pt/C MoS2-cPAN

-15 -20 -0.4 0

-0.2

-0.1

-1

140 mV dec -1 217 mV dec -1 120 mV dec

-0.2 MoS2 SPAN

cPAN

-1

-0.1

68 mV dec

MoS2-cPAN

0.0 -0.3

b

0.0

-1

32 mVdec

Pt/C

-2.0 -2.5 -3.0 -3.5 -4.0 -4.5 -5.0

log i (A/cm-2)

E/V vs RHE

c

0.8

d

-4

0.6

TOF(s-1)

J (mA/cm2)

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

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-8 MoS2- cPAN @ -0.18 V vs RHE

-12

0.4 0.2

-16

0.0 -20 0

4

8

12

Time (hr)

16

BM NP Flake QD

V

Catalysts

SV SM

Figure 6. Electrochemical analysis of MoS2-cPAN, MoS2, SPAN, cPAN and Pt/C. (a) Linear sweep voltammetry for MoS2-cPAN, MoS2, SPAN and PAN and Pt/C catalysts on 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).

The turns over frequency (TOF) of the catalysts are calculated using the following equation.


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𝑇𝑂𝐹(

)=

𝑠 -1

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𝑗𝑜(𝐴/𝑐𝑚2)

()

𝑠𝑖𝑡𝑒𝑠 𝑐 -19 # 𝑜𝑓 1.602 ∗ 10 * * 2(𝑒/𝐻2) 𝑒 𝑐𝑚2

The current densities (j) and exchange current density (jo) could be calculated from the polarization curve and Tafel plot, respectively. Whereas the density of active sites of the catalyst (sites cm-2) have 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 to calculate the number of active sites. The calculation of active sites have been done at which the reduction (Mo3+Mo0) taking place (0.3 to 0.0 V), and assuming that per MoS2 site 3 charge transfer occurs.24,63 The detail calculation is shown in Figure S12. The researchers reported that single pt atom catalysts improved catalytic activity ~ 10 times over the state-of-the-art Pt/C catalyst.55 Here our best immobilized single molecular MoS2 on cPAN electrocatalysts 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 mV, 100 mV and 200 mV with details shown in Figure S13. Overall, the TOF (s-1) values of MoS2-cPAN are two orders higher than that of the corresponding bulk MoS2. Table 2 shows the summarized onset potential, over potential at 10 mA cm-2, the current density at 300 mV versus RHE, exchange current density, and TOF(s-1) of the electrocatalysts that are presented from polarization curves of Figure 6a.


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Table 2. Electrochemical parameters of the catalysts in HER Onset potential (mV vs RHE)

η at 10 mA cm-2 (mV vs RHE)

Tafel plot (mV dec-1)

J (mA cm-2) (η =300mV)

jo (A cm-2)

TOF (s-1)

cPAN

-450

-

217

-0.14

3.3*10-6

-

SPAN

-230

-

140

-1.1

4.5*10-6

-

Bulk MoS2

-190

-460

120

-5.3

5.7*10-6

0.0046

MoS2-cPAN

-25

-185

68

-15

3.13*10-4

0.8

Catalyst

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 electrocatalyst. The single molecular nature of MoS2 on cPAN has been clarified by EXAFS and HAADF-STEM techniques. The as-prepared a single molecular MoS2 with unsaturated feature has grown homogenously on the cPAN template and its overpotential at 10 mA cm-2 is improved by ~3 times from the bulk MoS2. The exchange current density and TOF (s-1) in hydrogen evolution reaction of the single molecular MoS2 are approximately two orders higher than the bulk MoS2. The single molecular MoS2 boosts the activity of H2 evolution and this excellent activity’s attributed from the low-coordination and high vacancy of 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.


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METHODS Materials and Chemicals. Polyacrylonitrile (PAN, the average molecular weight of 150,000), sulfur powder (99.98%), MoS2 powder, anhydrous n-methylpyrrolidone (NMP), and polyvinylidene difluoride (PVDF) 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 mmole 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 (SEM) (EDX JSM 6500F, JEOL) together with EDX (Energy dispersive X-ray 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, German). The HAADF-STEM images and the EELS mapping images of the MoS2-cPAN sample were obtained by JEOL ARM 200F. The XRD spectra were characterized using a D2 Phaser XRD-300 W diffractometer with a Cu Kα radiation (l ¼ 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 beam line station in the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. XAS (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 beam line 20 ACS Paragon Plus Environment

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TPS BL-09A (Temporally Coherent X-ray Diffraction), at the National Synchrotron Radiation Research Center (NSRRC) in Hsinchu, Taiwan. Electrochemical Measurements. Preparation of the MoS2-cPAN composite on rotating disc electrode: 10 mg of catalyst was mixed with 1000 μL (v/v 1:3) mixture of water and ethanol, and 50 μL of 5 wt % Nafion solutions. The mixture was sonicated for 30 minutes 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 three electrode system. The catalyst on the electrode was used as the working electrode and also graphite rod and saturated Ag/AgCl was used as the counter and a reference electrode, respectively. Argon gas was bubbled into the 0.5 M H2SO4 electrolyte throughout the measurement. Linear sweep voltammetry (LSV) was performed in the range of linear sweeping 0 to -0.7 V vs RHE with a scan rate of 1 mV s-1 for the polarization curves.

ACKNOWLEDGMENTS This work was financially supported by the Ministry of Science and Technology (MOST) (106-2923-E 011-005, 105-3113-E-011-001, 105-ET-E-011-004-ET, 104-2923-M-011-002-MY3, 104-2911-1-011505-MY2, 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, Taiwan’s Deep Decarbonization Pathways toward a Sustainable Society Project (AS-KPQ-106- DDPP) from Academia Sinica as well as the facilities of support from National Taiwan University of Science and Technology (NTUST) and National Synchrotron Radiation Research Centre (NSRRC) are also acknowledged. ASSOCIATED CONTENT


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Page 22 of 31

Supporting Information Available: The document contains additional experimental details, including (i) Characterization (SEM, XPS and EDS) of SPAN. (ii) Characterization (SEM, in situ XRD) of Li2ScPAN. (iii) Characterization (TEM, HAADF-STEM, EELS map, and FT-EXAFS) of MoS2-cPAN. (iv) Characterization (XANSE) of MoO3 and compare with MoS2-cPAN. (v) Atomic ratio calculation of elements within MoS2-cPAN sample. (vi) Calculation of the Turnover Frequency (TOF) of MoS2 and MoS2-cPAN. (vii) Setup for the synthesis of SPAN, Li2S-cPAN, and MoS2-cPAN (PDF). This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Conflicts of Interests The authors declare no competing financial interest.

ORCID Bing-Joe Hwang:

0000-0002-3873-2149

Wei-Nien Su:

0000-0003-1494-267

Meng-Che Tsai:

0000-0002-1301-866X

Tamene Simachew Zeleke: 0000-0003-1605-9640


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Immobilized Single Molecular Molybdenum Disulfide on Carbonized

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