Understanding on the origin of formation and active sites for

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Understanding on the origin of formation and active sites for thiomolybdate [Mo3S13]2- clusters as hydrogen-evolution catalyst through the selective control of sulfur atoms Cheol-Ho Lee, Sungho Lee, Youn-Ki Lee, Yong Chae Jung, Yong-Il Ko, Doh C. Lee, and Han-Ik Joh ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.8b01034 • Publication Date (Web): 02 May 2018 Downloaded from http://pubs.acs.org on May 2, 2018

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Understanding on the origin of formation and active sites for thiomolybdate [Mo3S13]2- clusters as hydrogen-evolution catalyst through the selective control of sulfur atoms Cheol-Ho Leea,b,†, Sungho Leea,c,†, Youn-Ki Leea, Yong Chae Junga, Yong-Il Koa, Doh C. Leeb, and Han-Ik Johd,* a

Carbon Convergence Materials Research Center, Institute of Advanced Composite Materials, Korea Institute of Science and Technology (KIST), 92 chudong-ro, Bongdong-eup, Wanju, Jeollabukdo 55324, Republic of Korea.

b

Department of Chemical and Biomolecular Engineering (BK21+ Program), KAIST Institute for the Nanocentury, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 34141, Republic of Korea. c

Department of Nano Material Engineering, KIST School, University of Science and Technology, 217Gajeong-ro, Yuseong-gu, Daejeon 34113 Republic of Korea

d

Department of Energy Engineering, Konkuk University, 120 Neungdong-ro, Gwangjin-gu, Seoul 05029, Republic of Korea.

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ABSTRACT

[Mo3S13]2- clusters have become known as one of the most efficient catalysts for the hydrogen evolution reaction (HER) because most of the sulfur (S) atoms in the cluster are exposed, resulting in many active sites. However, the origin of the cluster formation and active S sites in the cluster is unknown, hindering the development of efficient catalysts. Herein, the mechanism of the transition from amorphous MoS3 to [Mo3S13]2- clusters is systematically investigated. In addition, the active S sites have been identified by the selective removal of S atoms via lowtemperature heat treatment. In summary, we believe that the clusters grow from amorphous MoS3 with apical S atoms, and bridging S atoms are the active HER sites in the [Mo3S13]2clusters. The clusters deposited on carbon nanotubes exhibited good electrochemical HER activity with a low onset potential of -96 mV, a Tafel slope of 40 mV/decade, and stability for 1000 cycles.

KEYWORDS: active site, carbon nanotube, hydrogen production, MoS3, Mo3S13, thermal treatment.

1. Introduction The hydrogen based energy cycle has received much attention as a future energy strategy. The production of molecular hydrogen (H2) using renewable energy and its utilization as an energy source in environmentally friendly fuel cells is desirable because of the current energy and environmental crisis, for example, exhaustion of fossil fuels, global warming, and air pollution. The electrolysis of water is the most representative clean technology for limitless H2 production, especially compared to the steam reforming of natural gas because no harmful by-products are


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produced. Even though the most efficient catalyst for the hydrogen evolution reaction (HER) is platinum (Pt), its high price and scarcity have driven extensive investigation into finding affordable replacement materials.1 Hence, the development of alternatives using low-cost and earth-abundant materials for electrocatalysts is a critical challenge. Among the various candidates, molybdenum disulfide (MoS2) shows excellent electrochemical HER activity and stability because of their low Gibbs free energy for hydrogen adsorption, nearly 0.08 eV, at the edge sites of MoS2, even though the basal plane is electrochemically inert.2-4 Crucial drawbacks of MoS2 as a catalyst are to reveal a lower electrochemical current density and higher overpotential than Pt, resulting from its relatively low electrical conductivity and small active sites.5 Therefore, extensive investigation into enlarging the active sites of MoS2 and enhancing the electrical conductivity of the catalysts by controlling the size and morphology of MoS2 on conducting carbon substrates6-10 or modifying the electronic structure of edge sites by exfoliation using metal cations or heteroatom doping.11-19 However, the small number of active sites in MoS2 is an intrinsic properties proved by density functional theory (DFT) calculations, and only 1/4 of edge atoms could be involved in hydrogen production compared to Pt.5 Controlling the atomic ratio between Mo and S has been studied significantly to overcome the intrinsic issue, leading to considerable changes in the crystallinity and number of active sites in MoSx. It is noted that crystalline MoS2 has only terminal S, but the electrochemical HER occurs at the edge sites, and bridge S of amorphous MoS3 unlike crystalline MoS2 participates in the HER.20 Recently, thiomolybdate [Mo3S13]2- clusters on graphite paper (GP) or highly oriented pyrolytic graphite (HOPG) have been reported as one of the most efficient HER catalysts because entire surface of the cluster with intrinsically exposed sulfurs would be active sites.21 Unfortunately, electrochemical cell electrodes, such as for fuel cells and water electrolysis, are


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usually prepared by wet chemistry and based on carbon powder, which has a larger surface area compared to GP. Considering practical and useful aspects, it is worthwhile to investigate the formation mechanism and structural and electrochemical properties of [Mo3S13]2- clusters deposited on commercial carbon materials. In particular, it is believed that clarifying the active S states among the three kinds of bonding between Mo and S in the cluster such as terminal S, bridge S, and apical S, is supremely important to improve HER catalysts. Herein, we report the transition process from amorphous MoS3 as a seed to [Mo3S13]2- clusters as functions of reaction time and additional supply of ammonium polysulfide. In addition, the cluster was incorporated on oxidized multi-walled carbon nanotube (O-CNT) as widely used electrochemical substrates to prepare practical HER catalysts and analyze activity and stability for HER. Compared to other non-precious metal based catalysts in strong acids, our nanocomposite materials exhibit unique onset potentials, Tafel slopes, and durability for the HER. To verify the active sites of the [Mo3S13]2- in the electrochemical reaction, the nanocomposites were heat-treated at various temperatures to modulate conformation selectively and their effects on HER performance were investigated. Understanding on the origin of the material formation and activity enables to propose strategies for further enhancing its properties.

2. Results and discussion The [Mo3S13]2- clusters were synthesized by the reaction of amorphous MoS3 as a seed and ammonium hydroxide solution. The amorphous MoS3 with HER activity is one of the most readily available materials prepared by facile wet chemistry unlike MoS2 that is synthesized under a high vacuum or at a temperature. In our previous study, we synthesized amorphous MoS3 on reduced graphene oxide, which shows excellent electrochemical HER activity because


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of improved electrical conductivity.7 However, the number of active sites in the amorphous MoS3 should significantly increase in order to improve the HER activity compared to Pt catalyst. In this regard, the transformation of amorphous MoS3 to [Mo3S13]2- cluster by facile and practical method is a convincing strategy to develop highly effective non-precious metal catalysts. Interestingly, there are two representative conflicting hypotheses for the formation of clusters derived from MoS3 with no experimental evidences. Muller et al. insisted that all of the MoS3 were ionized to sulfur and molybdenum ions in ammonium hydroxide solution followed by assembly of [Mo3S13]2- clusters from sulfur and molybdenum ions, thus, a shortage of S may induce a low transition yield.22 Meanwhile, Weber et al. claimed that some amounts of molybdenum sulfide clusters without apical S were selectively decomposed to S and Mo ions in ammonium hydroxide solution, and the remaining molybdenum sulfide clusters with apical S and decomposed S were rearranged to form crystalline (NH4)2Mo3S13 with irrespective of the amount of sulfur.23 Even though the transition mechanism is controversial, it is expected that the [Mo3S13]2- clusters would compensate relatively lower number of MoS3 active sites, leading to higher HER performance. To investigate the transition mechanism from amorphous MoS3 to crystalline (NH4)2Mo3S13, the chemical and structural properties were observed using X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD), respectively, as shown in Figure 1a-c. The Mo3d peaks of amorphous MoS3 were associated with two electronic states. A major doublet at lower binding energies of 229.3 and 232.5 eV indicates Mo4+ states, while a minor doublet at higher binding energies of 231.3 and 234.5 eV is attributed to the Mo5+ oxidation state. However, the Mo5+ doublet disappeared slowly in ammonium hydroxide solution, and only the Mo4+ peaks remained after 3 days. In the S2p spectra, the amount of apical/bridge S, which appear as a doublet at


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163.6 and 167.3 eV, decreased from 43.3 to 35.4 at% after 3 days. Interestingly, the content was slowly recovered up to 43.8 at% and the ratio of apical/bridge S to terminal S reached to approximately 7/6 after 7 days. Muller et al. reported that the sulfur in molybdenum sulfides could be easily dissolved by a nucleophilic reaction.24 Therefore, it could be speculated that amorphous MoS3 was separated to each defective MoSx clusters by nucleophilic attack of OH- in solvents for 3 day. After bi-reaction, ionized S might be used for increasing of the cluster crystallinity and size. In addition, the atomic ratio of S to Mo also increased from 2.99 to 4.27 as shown in Table S1. In the XRD diffractograms, the broad (002) peak of amorphous MoS3 disappeared, and peaks corresponding to crystalline (NH4)2Mo3S13 were observed with even a 1 day sample (Figure 1c). These results indicated that crystalline (NH4)2Mo3S13 was successfully transformed from amorphous MoS3. However, only 18.6 wt% of the (NH4)2Mo3S13 was obtained after 7 days (Table S2). To confirm the reason for the low transition yield, ammonium polysulfide of 10 mL was added to the ammonium hydroxide solution to increase the number of available S ions, which are expected to react with remained amorphous MoS3. However, there was no significant change in the content of (NH4)2Mo3S13 (Table S2), which is evidence that the low transition yield did not result from a shortage of sulfur ions, but the shortage of remained molybdenum sulfide clusters, as claimed by Weber et al.23 Based on the results of XPS spectra and XRD diffractograms, transition mechanism from amorphous MoS3 to (NH4)2Mo3S13 is proposed as shown in Figure 1d. Amorphous MoS3 is composed of a random arrangement of Mo3Sx (average value of x = 9) substructures as shown in Figure 1d. [20] According to Weber et al, barely ~35% of the Mo3Sx substructures contain apical S, and it is considered that only Mo3Sx units containing apical S are transformed to Mo3S13, whereas other Mo3Sx units are decomposed to Mo and S ions. A significant decrease in bridge S


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after 3 days indicates that shared disulfides between defective molybdenum sulfide clusters were degraded because of nucleophilic attack of OH-, leading to the separation of each molybdenum sulfide cluster.22,25 At the same time, some amounts of molybdenum sulfide clusters without apical S were completely dissociated to sulfur and molybdenum ions. Subsequently, sulfur ions were recombined with remained defective molybdenum sulfide clusters, and [Mo3S13]2- anions was formed ultimately. The [Mo3S13]2- anions were regularly arranged with ammonium cations and H2O, resulting in the (NH4)2Mo3S13 crystal structure (Figure S1). Therefore, the most of amorphous MoS3 with irregular shape was gradually converted to rod-shaped (NH4)2Mo3S13 crystals after 7 days as shown in Figure S2.26 The as-synthesized (NH4)2Mo3S13 was incorporated onto the surface of O-CNTs to prepare the nanocomposites, of which electrochemical HER activity was investigated. To synthesize the OCNT, multi-walled CNTs (MWCNTs) were oxidized using a mixture of H2SO4 and HNO3 (w/w = 7/3) at 50 oC for 6 h.[27] As a result, various kinds of oxygen functional groups such as hydroxyl, epoxide, and carboxyl group were introduced on the surface of MWCNTs as shown in Figure S3a, resulting in an increase of atomic content in oxygen from 3% to 18%. In addition, in the Raman spectrum, an intensity ratio of D to G peak increased from 0.29 to 0.61 after oxidation as shown in Figure S3b. The (NH4)2Mo3S13 powder (9 mg) was dissolved in dimethylformamide (DMF) of 1 mL and mixed with O-CNT powders of 6 mg followed by drying in a vacuum oven at 60 oC. As a result, [Mo3S13]2- clusters of 1~5 nm were sparsely dispersed on the surface of OCNT without significant aggregation as shown in Figure 2a, S4, and S5. Ammonium cations were evaporated as ammonia gas, and only remained [Mo3S13]2- anions are electrostatically attached on the surface of O-CNT during the drying of the mixture in an oven. Meanwhile, electron energy loss spectroscopy (EELS) mapping in Figures 2b-e confirms that molybdenum


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and sulfur atoms existed at the same positions, indicating the [Mo3S13]2- clusters, while carbon atom was observed in all the surface of the nanocomposites. Figure 3a shows electrochemical linear voltammograms of O-CNT, pristine [Mo3S13]2-, and [Mo3S13]2-/O-CNT nanocomposites (Mo3S13/O-CNT) in N2 gas and 0.5 M H2SO4 electrolyte. The overpotential of the nanocomposites at a current density of 10 mA/cm2 was significantly enhanced from -0.229 to -0.137 mV compared to pristine [Mo3S13]2- after correction for ohmic potential drop (iR) losses because the presence of CNTs increased the electrical conductivity of nanocomposites. In addition, the calculated Tafel slope of Mo3S13/O-CNT was approximately 40 mV/decade as shown in Figure 3b, while pristine [Mo3S13]2- had a Tafel slope of 51 mV/decade. It is well known that the rate-limiting step of catalysts for HER could be determined by Tafel slope.28 There are three steps for the HER in acidic media such as Volmer (H+ + e- → Hads, < 120 mV/decade), Heyrovsky (Hads + h+ + e- → H2, < 40 mV/decade), and Tafel reactions (Hads + Hads → H2, < 30 mV/decade), and these reactions indicate adsorption, desorption, and recombination, respectively. The Tafel slope value of our nanocomposites revealed that the HER followed the Volmer-Heyrovsky mechanism with the rate-limiting step of desorption of H2. In addition, when the catalytic durability of Mo3S13/O-CNT was measured for 1000 cycles in a potential range from -300 to 100 mV, only a negligible potential shift was observed without decline of current density because of rigorous H2 bubble formation on the surface of the electrode. The contents of three kinds of sulfurs in [Mo3S13]2- were controlled by a simple heat-treatment to verify an active site, on which HER takes place. Figure S6a represents a thermogravimetric analysis (TGA) thermogram of Mo3S13 in a N2 atmosphere, showing three distinct weight losses. The first decrease started from 80 oC due to the evaporation of H2O. It is evident that apical S was eliminated between 250 and 320 oC followed by removal of terminal S at a range of 380 to


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440 oC.26 Based on the thermograms, the Mo3S13/O-CNT were thermally treated at 280, 340, 400, and 460 oC for 30 min, respectively, to selectively control the content of sulfur structure. It was observed that the weight of [Mo3S13]2- decreased during the holding for 30 min in each temperature as shown in Figure S6b, indicating that sulfur in [Mo3S13]2- was reduced for all holdings, and it was more significant at 400 oC. Figure 4a shows the deconvoluted XPS S2p spectra of Mo3S13/O-CNT thermally treated at 280~460 oC. After thermal treatment at 280 oC, the ratio of apical/bridge S to terminal S and the atomic ratio of S/Mo decreased from 1.18 to 1.03 and from 4.27 to 3.67, respectively, as listed in Table S3. These results reveal that [Mo3S12]2- was formed by the elimination of apical S. Thermal treatment at 340 oC rendered significant decrease of a large amount of S, dominantly apical/bridge S, resulting in the low enough S/Mo ratio of 2.27 to disrupt the configuration of [Mo3S12]2-. Additional heat treatments at 400 and 460 oC induced a significant reduction in the content of bridge S, even though it is likely that only terminal S should be removed in this temperature range. Finally, only 6% of the bridge S in S2p spectra remained after heat treatment at 460 oC with decreasing an atomic ratio of S to Mo down to 2.09 (Table S3). Interestingly, there was a reduction in the amount of bridge S from 340 to 460 oC, which originated from that Mo atoms were covalently bonded with bridge S of neighboring clusters, and this aggregation of [Mo3S12]2- clusters rendered formation of poorly crystalline MoS2 structure as shown in Figure 4b.29 The XRD diffractograms of thermally treated Mo3S13/O-CNT in Figure S7 also revealed that a broad peak associated with (002) plane of MoS2 appeared and their intensity increased with increasing thermal treatment temperature. Therefore, this configurational change from bridge S to terminal S resulted in decease of bridge S, even though it was expected that only terminal S (not bridge S) could be eliminated after thermal treatment.


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The electrochemical HER activities of nanocomposites containing thermally treated Mo3S13/OCNT were investigated as shown in Figure 4c. The nanocomposites containing [Mo3S12]2- treated at 280 oC show comparable electrochemical activity to that of pristine Mo3S13/O-CNT. It is evident that the electrochemical HER activity was barely affected by the elimination of apical S, and apical S was not the active site for HER. However, the electrochemical HER activity was rapidly reduced in the nanocomposites containing [Mo3S12]2- treated at 340 and 400 oC, indicating that the deterioration of HER activity was highly related to the removal of bridge S. Furthermore, thermal treatment at 460 oC led to a more significant reduction in HER activity because of the conversion to MoS2. The quantitative HER activities of thermally treated Mo3S13/O-CNT were confirmed by calculation of the turnover frequency (TOF), as shown in Figure 4d. The TOF of pristine Mo3S13/O-CNT and Mo3S12/O-CNT at 250 mV were 2.50 and 2.38 s-1, respectively, even though TOF value rapidly decreased to 1.48, 1.07, and 0.96 s-1 with the thermal treatment of Mo3S13/O-CNT at 340, 400, and 460 oC, respectively, because of the removal of bridge S. These results confirmed that the bridge S act as active sites and play a crucial role in HER. In addition, the overpotential and Tafel slope values of Mo3S13/O-CNT deteriorated from -143 mV and 52 mV/dec to -173 mV and 93 mV/dec after thermal treatment to 460 oC, respectively (Figure S8). It is noted that the electrochemical HER activities of Mo3S13/OCNT at 400 and 460 oC did not increase but continuously decreased even though the most of bridge S in [Mo3S12]2- had been changed to terminal S at 400 and 460 oC. According to DFT calculations, the bridge S in molybdenum sulfide clusters such as [Mo3S13]2- and dimeric Mo2S12 have a relatively lower Gibbs free energy for hydrogen adsorption than terminal S.20,30 Consequently, the degeneration of the electrochemical HER activities of Mo3S13/O-CNT after heat treatment such as the TOF, overpotential and Tafel slope values originated from the


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deterioration in the hydrogen adsorption properties because of the elimination of bridge S. Therefore, it is believed that the bridge S in [Mo2S12]2- clusters are the major actives for electrochemical HER.

3. Conclusion In summary, we synthesized [Mo3S13]2- using amorphous MoS3, and investigated its structural transition. During the reaction, amorphous MoS3 was separated to molybdenum sulfide clusters, and some amounts of molybdenum sulfide clusters were completely degraded to S and Mo ions, simultaneously. Remained defective molybdenum sulfide clusters and S ions were reacted, and Mo3S13 crystals were formed. The as-synthesized Mo3S13/O-CNT showed good electrochemical HER performance and stability for 1000 cycles with a low overpotential of -137 mV at 10 mA/cm2 and Tafel slope of 40 mV/decade. The [Mo3S13]2- was reduced to [Mo3S12]2- and amorphous MoS2 during thermal treatment. The reduction in the electrochemical activity of Mo3S13/O-CNT was highly related to decrease of bridge S, thus, the electrochemical active site for HER in [Mo3S13]2- would be bridge S. Therefore, it is believed that apical S and bridge S play crucial role in remaining MoS3 structure as a seed in solvents for transformation process and become active sites in the cluster for HER, respectively.

ASSOCIATED CONTENT Supporting Information. Detailed experimental methods, schematic crystal structure of (NH4)2Mo3S13, TEM and SEM images, XRD diffractograms of molybdenum sulfides, TGA thermograms, and Raman and XPS spectra are available free of charge via the Internet at http://pubs.acs.org.


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AUTHOR INFORMATION Corresponding Author * Han-Ik Joh, Tel/Fax: +82-2-450-0545/+82-2-3437-8360, E-mail address: [email protected] Author Contributions †

These authors contributed equally to this study.

ACKNOWLEDGMENTS The authors acknowledge the financial supports for this work from the Korea Institute of Science and Technology (2Z05400, 2Z05360), Republic of Korea, “Human Resources Program in Energy Technology” of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), granted financial resource from the Ministry of Trade, Industry & Energy, Republic of Korea. (No. 20174010201540).

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[17] Li, D. J.; Maiti, U. N.; Lim, J.; Choi, D. S.; Lee, W. J.; Oh, Y.; Lee, G. Y.; Kim, S. O. Molybdenum Sulfide/N-Doped CNT Forest Hybrid Catalysts for High-Performance Hydrogen Evolution Reaction. Nano Lett. 2014, 14, 1228-1233. [18] Dai, X.; Du, K.; Li, Z.; Liu, M.; Ma, Y.; Sun, H.; Zhang, X.; Yang, Y. Co-Doped MoS2 Nanosheets with the Dominant CoMoS Phase Coated on Carbon as an Excellent Electrocatalyst for Hydrogen Evolution. ACS Appl. Mater. Interfaces 2015, 7, 27242-27253. [19] 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. [20] Ting, L. R. L.; Deng, Y.; Ma, L.; Zhang, Y.-J.; Peterson, A. A.; Yeo, B. S. Catalytic Activities of Sulfur Atoms in Amorphous Molybdenum Sulfide for the Electrochemical Hydrogen Evolution Reaction. ACS Catal. 2016, 6, 861-867. [21] Kibsgaard, J.; Jaramillo, T. F.; Besenbacher, F. Building an appropriate active-site motif into a hydrogen-evolution catalyst with thiomolybdate [Mo3S13]2− clusters. Nat. Chem. 2014, 6, 248-253. [22] Müller, A.; Diemann, E.; Krickemeyer, E.; Walberg, H.; Bögge, H.; Armatage, A. [Mo 3S(S2)6] 2-from amorphous MoS3 by the reaction with OH- and R=0.015 structure of (NH4) 2[Mo3S(S2)6]H2O. Eur. J. Solid State Inorg. Chem. 1993, 30, 565-572. [23] Weber, T.; Muijsers, J. C.; Niemantsverdriet, J. W. Structure of Amorphous MoS3. J. Phys. Chem. 1995, 99, 9194-9200.


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[24] Müller, A.; Reinsch, U. Activation and Sulfur-Atom Transfer Reaction of Cluster-Bonded S 22−-Bridge Ligands: Synthesis of the New Cluster [Mo3S4(CN)9]5− from [Mo3S(S2)6]2− and CN−. Angew. Chem. Int. Ed. 1980, 19, 72-73. [25] Tran, P. D.; Tran, T. V.; Orio, M.; Torelli, S.; Truong, Q. D.; Nayuki, K.; Sasaki, Y.; Chiam, S. Y.; Yi, R.; Honma, I.; Barber, J.; Artero, V. Coordination polymer structure and revisited hydrogen evolution catalytic mechanism for amorphous molybdenum sulfide. Nat. Mater. 2016, 15, 640-646. [26] Leist, A.; Stauf, S.; Loken, S.; Wolfgang Finckh, E.; Ludtke, S.; K. Unger, K.; Assenmacher, W.; Mader, W.; Tremel, W. Semiporous MoS2 obtained by the decomposition of thiomolybdate precursors. J. Mater. Chem. 1998, 8, 241-244. [27] Datsyuk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Chemical oxidation of multiwalled carbon nanotubes. Carbon 2008, 46, 833-840. [28] Conway, B. E.; Tilak, B. V. Interfacial processes involving electrocatalytic evolution and oxidation of H2, and the role of chemisorbed H. Electrochim. Acta 2002, 47, 3571-3594. [29] Hibble, S. J.; Feaviour, M. R. An in situ structural study of the thermal decomposition reactions of the ammonium thiomolybdates, (NH4)2Mo2S12 2H2O and (NH4)2Mo3S13 2H2O. J. Mater. Chem. 2001, 11, 2607-2614. [30] Huang, Z.; Luo, W.; Ma, L.; Yu, M.; Ren, X.; He, M.; Polen, S.; Click, K.; Garrett, B.; Lu, J.; Amine, K.; Hadad, C.; Chen, W.; Asthagiri, A.; Wu, Y. Dimeric [Mo2S12]2− Cluster: A Molecular Analogue of MoS2 Edges for Superior Hydrogen-Evolution Electrocatalysis. Angew. Chem. Int. Ed. 2015, 54, 15181-15185.


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Figure 1. Chemical and structural analyses during the transition from amorphous MoS3 to (NH4)2Mo3S13 using XPS spectroscopy and XRD diffraction. (a) Deconvoluted Mo3d spectra indicate that Mo5+ state (blue) disappeared gradually during reaction and only Mo4+ state (red) remained. (b) S2p can be deconvoluted into two doublets (S2p3/2 and S2p1/2): apical/bridge S (green) and terminal S (orange). (c) XRD diffractograms from 0 to 7 days indicate that crystalline (NH4)2Mo3S13 was successfully formed. (d) Schematic of transition mechanism from MoS3 to [Mo3S13]2- based on the XPS and XRD results.


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Figure 2. Morphology of Mo3S13/O-CNT nanocomposite. (a) HR-TEM microgram of Mo3S13/OCNT. (b) The corresponding high angle annular dark field (HAADF) microgram of Mo3S13/O-CNT and related EELS mapping images of (c) carbon, (d) sulfur, and (e) molybdenum. The position of molybdenum and sulfur indicated that [Mo3S13]2- clusters were deposited on the surface of the O-CNT.


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ACS Catalysis

Figure 3. Electrochemical HER properties of the Mo3S13/O-CNT nanocomposite. (a) Linear sweep curves indicate that HER activity of Mo3S13/O-CNT was highly enhanced compared to pristine [Mo3S13]2- and O-CNT. (b) Tafel plots of pristine [Mo3S13]2-, O-CNT, and Mo3S13/OCNT based on linear sweep measurements. (c) Stability tests of Mo3S13/O-CNT for 1,000 cycles from -0.3 to 0.1 V at a scan rate of 50 mV/s.


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Figure 4. Chemical and electrochemical properties of Mo3S13/O-CNT after thermal treatment at various temperatures. (a) Deconvoluted S2p spectra of thermally treated Mo3S13/O-CNT at 280, 340, 400, and 460 oC, and (b) suggested structural schemes during the thermal treatment. Pristine [Mo3S13]2- was changed to [Mo3S12]2- and MoS2 at a relatively high temperature. Orange and green colored S indicate terminal S and apical/bridge S, respectively. (c) Their electrochemical HER activity and (d) calculated TOF.


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Understanding on the origin of formation and active sites for

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