Molybdenum DisulfideâBlack Phosphorus Hybrid Nanosheets as a

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Molybdenum Disulfide-Black Phosphorus Hybrid Nanosheets as a Superior Catalyst for Electrochemical Hydrogen Evolution Rong He, Jian Hua, Anqi Zhang, Chuanhao Wang, Jiayu Peng, Weijia Chen, and Jie Zeng Nano Lett., Just Accepted Manuscript • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 13, 2017

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Revised ms# nl-2017-01334j, 06/2017

Molybdenum Disulfide-Black Phosphorus Hybrid Nanosheets as a Superior Catalyst for Electrochemical Hydrogen Evolution Rong He,†,|| Jian Hua,†,|| Anqi Zhang,† Chuanhao Wang,† Jiayu Peng,† Weijia Chen,† Jie Zeng*,† †

Hefei National Laboratory for Physical Sciences at the Microscale, Key Laboratory of

Strongly-Coupled Quantum Matter Physics of Chinese Academy of Sciences, Department of Chemical Physics University of Science and Technology of China, Hefei, Anhui 230026, P. R. China

*To whom correspondence should be addressed. E-mail: [email protected] ||

These authors contributed equally.

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Abstract Engineering electronic properties is a promising way to design nonprecious-metal or earth-abundant catalysts towards hydrogen evolution reaction (HER). Herein, we deposited catalytically active MoS2 flakes onto black phosphorus (BP) nanosheets to construct the MoS2-BP interfaces. In this case, electrons flew from BP to MoS2 in MoS2-BP nanosheets because of the higher Fermi level of BP than that of MoS2. MoS2-BP nanosheets exhibited remarkable HER performance with an overpotential of 85 mV at 10 mA cm-2. Due to the electron donation from BP to MoS2, the exchange current density of MoS2-BP reached 0.66 mA cm-2, which was 22 times higher than that of MoS2. In addition, both the consecutive cyclic voltammetry and potentiostatic tests revealed the outstanding electrocatalytic stability of MoS2-BP nanosheets. Our finding not only provides a superior HER catalyst, but also presents a straightforward strategy to design hybrid electrocatalysts.

Keywords: hydrogen evolution reaction, black phosphorus, molybdenum disulfide, electron transfer

Table of Content

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Hydrogen serves as a future energy resource to reduce the dependence on fossil fuels by virtue of the highest mass energy density and renewability.1,2 Electrochemical water splitting represents one of the most convenient and promising approaches to hydrogen generation.3,4 Currently, commercial electrocatalysts towards hydrogen evolution reactions (HER) are typically composed of noble metals, especially Pt and Pd.5-7 The scarcity and high cost of noble-metal catalysts have hampered the large-scale deployment of HER technologies. As such, increasing attention has been paid to applying nonprecious metal catalysts for HER. Thanks to the tremendous efforts from many research groups, the past decade has witnessed spectacular success in developing a myriad of catalysts, including carbon materials, alloys, transition metal compounds as well as their combinations.8-20 Recently, molybdenum disulfide (MoS2) has drawn significant attention among the HER catalysts, because density function theory (DFT) shows that the free energy of H adsorption (∆GH*) on MoS2 edge sites was close to thermo-neutral.21 The electrocatalytic activity for HER is closely associated with the number of the active sites. In the case of pristine MoS2, only the edge sites serve as active sites.22 For this reason, great efforts have been made to increase the number of active sites for MoS2 catalysts.23-30 For example, a facile method of oxygen plasma exposure and hydrogen treatment on pristine monolayer MoS2 was reported to introduce more edge sites via the formation of defects.31 As another example, the vertical aligned MoOx/MoS2 nanotubes were developed to confine MoS2 nanosheets in a low angle, zig-zag configuration for a high density of active sites.32 Moreover, the introduction of S-vacancies and strain can convert the inert basal planes of monolayer 2H-MoS2 to be active for HER.33 Besides the number of active sites, the intrinsic exchange current density (j0) also serves as a crucial factor for HER activity of MoS2 catalysts according to Butler-Volmer equation.34 Unfortunately, j0 of pristine MoS2 is two orders of magnitude lower than that of Pt, which significantly limits the HER activity of MoS2.35 Based on the analysis above, increasing j0 of MoS2 catalysts becomes critical to optimize catalysis towards HER. According to the volcano-type relationship between j0 and ∆GH*, pristine MoS2 exhibits weak adsorption of H atoms, corresponding to a relative low j0.36 During the adsorption process, proton (H+) obtains an electron to form a H adatom that binds to the surface of MoS2. As such, ∆GH* can be reduced by injecting electrons on MoS2 catalysts. Therefore, it is highly desired to accumulate electrons on MoS2 for improved j0 through the construction of MoS2-based hybrid catalysts.


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Herein, we deposit MoS2 flakes on highly conductive nanosheets to modify their electronic properties for enhanced HER activity. Black phosphorous (BP), a metal-free layered semiconductor, serves as a promising candidiate owing to its high carrier mobility.37,38 In this case, electrons transferred from BP to MoS2 due to the higher Fermi level of BP than that of MoS2. The as-synthesized MoS2-BP nanosheets exhibited remarkable HER performance with an overpotential as low as 85 mV (at 10 mA cm-2). Due to the electron accumulation on MoS2, the j0 of MoS2-BP reached 0.66 mA cm-2, which was 22 times higher than that of MoS2. In addition, both the consecutive cyclic voltammetry (CV) and potentiostatic tests revealed the outstanding electrocatalytic stability of MoS2-BP nanosheets. To begin with, BP nanosheets were mechanically exfoliated from bulk BP crystals, and MoS2 flakes were synthesized according to the previous work.39,40 Typically, MoS2-BP nanosheets were prepared by sonicating the as-synthesized BP and MoS2 in ethanol, followed by solvothermal treatment. As shown in Figure S1, pristine MoS2 flakes and BP nanosheets were free-standing with lateral sizes of about 100 nm and 10 µm, respectively. Figure 1a shows a transmission electron microscopy (TEM) image of a typical MoS2-BP nanosheet. Two MoS2 flakes were located at the corners of the BP nanosheet. The high resolution TEM (HRTEM) image of the MoS2-BP nanosheets was presented in Figure 1b. The distinct lattice fringes with interplanar spacing of 0.34 and 0.26 nm were assigned to the (021) plane of BP and the (100) plane of hexagonal 2H-MoS2, respectively. Figure S2 shows the Moire patterns of a MoS2-BP nanosheet, indicating that MoS2 and BP nanosheets were stacked closely to form good interfaces. Furthermore, the atomic force microscope (AFM) was used to measure the thickness of MoS2-BP nanosheets (Fig. S3). The maximum thickness of the as-synthesized MoS2-BP nanosheets was 9 nm, corresponding to the sum of the thicknesses of an individual MoS2 flake (4 nm) and a BP nanosheet (5 nm). This result demonstrates that MoS2 flakes were supported onto the BP nanosheet, instead of forming lateral heterostructures. The mass content of MoS2 in MoS2-BP nanosheets was determined to be 3.0 % by using inductively coupled plasma atomic emission spectroscopy (ICP-AES). In order to characterize the composition and elemental distribution of MoS2-BP nanosheets, scanning transmission electron microscopy-energy dispersive X-ray (STEM-EDX) analysis was conducted. The existence of P, Mo, and S elements in the obtained nanocrystals was revealed by the EDX spectrum in Figure S4. The STEM and its corresponding elemental mapping images of a typical MoS2-BP nanosheet revealed that P was


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homogeneously distributed over the entire nanosheet, whereas Mo and S mainly gathered at two corners of the nanosheet (Fig. 1c-f). In addition, EDX line-scanning across the entire nanosheet in Figure 1g-h further demonstrated the spatial distribution of MoS2 on the BP nanosheets. The structure of the as-synthesized MoS2-BP nanosheets was determined using Raman and X-ray diffraction (XRD) measurements. Figure S5 shows the Raman spectra of BP, MoS2, and MoS2-BP nanosheets. The peaks located at 363 cm-1, 440 cm-1, and 467 cm-1 were assigned to A1g , B2g, and A2g modes of BP, respectively.41 As for MoS2-BP nanosheets, the intensity of peaks at 380 cm-1 and 405 cm-1, corresponding to the E12g and A1g modes of MoS2, significantly weakened. Moreover, compared with the profile of pristine BP, no obvious shift was observed for the peaks in MoS2-BP nanosheets, which could be attributed to the low mass loading of MoS2. To verify this point, we provided the Raman spectrum of MoS2-BP nanosheets with 20% mass content of MoS2. Due to the electron transfer in the MoS2/BP interface, the A2g peak of the newly prepared MoS2-BP nanosheets shifted to higher wavenumber compared with that of BP, whereas the A1g peak shifted to lower wavenumber compared with that of MoS2. The preserved crystal structure of BP in MoS2-BP nanosheets was also verified by XRD patterns, where three sets of characteristic diffraction peaks for BP were observed (Fig. S6). To investigate the variation in surface electron states of MoS2-BP nanosheets, we conducted X-ray photoelectron spectroscopy (XPS) and Mott-Schottky measurements. In the XPS survey spectra, weak signals of Mo and S were observed in MoS2-BP nanosheets (Fig. S7). Figure S8 shows the P 2p XPS spectra of BP and MoS2-BP nanosheets. The peaks located at 129.6, 130.5, and 133.2 eV were assigned to P 2p1/2, P 2p3/2, and PxOy, respectively. As shown in the magnified P 2p XPS spectra, the P peaks of MoS2-BP showed a shift of 0.1 eV to higher binding energy compared with those of BP nanosheets (Fig. 2a). The S 2p XPS spectra of MoS2 and MoS2-BP nanosheets were shown in Figure 2b. Notably, the peaks for both S 2p1/2 and 2p3/2 shifted to lower binding energies by 0.8 eV in MoS2-BP nanosheets compared with those of MoS2 nanosheets. Meanwhile, binding energies of Mo 3d3/2 and 3d5/2 in MoS2-BP nanosheets were also negatively shifted by 0.7 eV compared with those of MoS2 nanosheets (Fig. 2c). These results indicated the electron transfer from BP to MoS2 in MoS2-BP nanosheets. Moreover, in P 2p and Mo 3d XPS spectra, the peak corresponding to Mo-P bonds was not observed, which demonstrated that the Van der Waals force was the driving force to enable the formation of MoS2/BP interfaces. The Mott-Schottky plots were further obtained to qualitatively determine


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the positions of Fermi level (EF) (Fig. 2d). The flat-band potential (UF) of BP nanosheets was about -0.29 V, which was more negative than that (-0.21 V) of MoS2 flakes. Given that UF denotes the EF position of a semiconductor with respect to the electrochemical potential of redox couple in the electrolyte, the EF of BP was higher than that of MoS2 flakes. As a result of EF equilibrium, the electrons were transferred from BP to MoS2 in BP-MoS2 nanosheets. We further analyzed the band gap of MoS2 in MoS2-BP nanosheets by Tauc/Davis-Mott plots in the near-infrared region. As shown in Figure S9, the band gap of MoS2 in MoS2-BP nanosheets was narrower than that of MoS2 flakes, demonstrating the electron accumulation on MoS2 in MoS2-BP nanosheets. The catalytic activity of MoS2-BP nanosheets for HER was evaluated in comparison with BP, MoS2, and commercial Pt/C. The electrochemical measurements of various samples were carried out in 0.5 M H2SO4 solution using a typical three-electrode configuration with the loading of 0.102 mg cm-1 on glassy carbon electrodes. MoS2 flakes were loaded on active carbon (MoS2/C) at the same mass loading as that of MoS2-BP nanosheets. Figure 3a shows the linear scan voltammetry (LSV) curves of BP, MoS2/C, MoS2-BP, and commercial Pt/C. Both pristine BP and MoS2/C displayed rather poor HER performance. Instead, MoS2-BP nanosheets exhibited a substantial enhancement in hydrogen evolution activity. To exclude the possibility that the remarkable HER performance originated from the hydrothermal treatment during the preparation of MoS2-BP, we also conducted the HER tests for BP and MoS2 after the same hydrothermal treatment as that for MoS2-BP (Figure S10a). At -0.3 V vs RHE, the current density of BP remained unchanged after the hydrothermal treatment, whereas that of MoS2 flakes showed a slight increase (Figure S10b). The minor enhancement of HER activity was likely attributed to the expanding interlayer spacing of MoS2 flakes42. Figure 3b shows the overpotentials of the three catalysts at current density of 10 mA cm-2. Compared with BP and MoS2, MoS2-BP nanosheets exhibited a much lower overpotential of 85 mV. Such value was lower than recently reported metallic MoS2 nanosheets or edge-oriented MoS2 nanosheets43,44. The dramatic enhancement was ascribed to the electron transfer from BP to MoS2 through heterojunction contact. To gain insight into the HER kinetics of the as-synthesized catalysts, Tafel plots and electrochemical impedance spectroscopy (EIS) curves were analyzed. Tafel slopes quantitatively show the rate-determining step of the HER process. Generally, under acidic conditions, hydrogen


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evolution undergoes a multistep reaction process for converting H+ to H2, which can be described by two different mechanisms with three principle steps, commonly known as the Volmer, Heyrovsky and Tafel reactions: H+ (aq) + e-→Hads (Step 1, Volmer reaction) Hads + H+ (aq) + e-→ H2 (g) (Step 2, Heyrovsky reaction) Hads + Hads→ H2 (g) (Step 2’ , Tafel reaction) Theoretically, the Tafel slopes of Volmer, Heyrovsky, and Tafel reactions are calculated to be approximately 118, 39 and 29 mV decade-1, respectively.45 As shown in Figure 3c, BP and MoS2/C exhibited Tafel slopes of 161 and 117 mV decade-1, respectively. This result indicates that Volmer reaction was the rate-limiting step of HER in the two catalysts. Contrastingly, MoS2-BP nanosheets showed a Tafel slope of 68 mV decade-1. This transition reveals that either transfer of electrons to MoS2 or the formation of H adatoms at MoS2/electrolyte interface is no longer the major step that limits the HER activity of MoS2-BP nanosheets. The variation in rate-limiting steps indicates that abundant electrons were accumulated on MoS2 nanosheets. Figure 3d shows the Nyquist plots and the corresponding equivalent circuit of the three catalysts. Based on the radius of the semicircular Nyquist plots, the charge-transfer resistances (RCT) of MoS2/C and MoS2-BP nanosheets were calculated to be 35 Ω and 5 Ω, respectively. This result demonstrated that the Faradaic process of MoS2-BP nanosheets was much faster than that of MoS2 flakes. In MoS2-BP nanosheets, the accumulated electrons on MoS2 facilitated the coupling of electrons with protons at the catalysts-electrolyte interface, resulting in the superior HER kinetics. The inherent mechanism of enhanced HER kinetics was further investigated. The underpotential deposition of Cu (Cu-UPD) is a well-established method to calculate the number of active sites46. Figure 3e shows the stripping peaks of Cu adatoms on glassy carbon, MoS2/C, and MoS2-BP catalysts. Compared to glassy carbon, Cu adatoms were electrochemically oxidized on catalysts at higher potentials, indicating that the adsorbed Cu atoms on catalysts derived from the UPD process. According to the integral area of stripping peaks, the MoS2-BP catalysts possessed the same amounts of active sites as MoS2/C catalysts. This result was consistent with the double-layer capacitance (Cdl) measurements (Fig. S11). The Cdl value of


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MoS2-BP was quite similar to that of MoS2/C, suggesting that the involvement of BP did not increase the electrochemical active surface area. Figure 3f shows the geometrical j0 at the thermodynamic redox potential (η=0). Determined by the intercept of the linear region of the Tafel plots, j0 of MoS2-BP nanosheets was calculated to be 0.66 mA cm-2. Such value was 22 times higher than that of MoS2/C, demonstrating that ∆GH* of MoS2 was significantly reduced. We compared the HER activities and exchange current densities of MoS2-BP with that of recently reported MoS2-based catalysts47. The MoS2-BP nanosheets displayed the largest exchange current density among the listed catalysts (Table S1). The decreased ∆GH* was also confirmed by DFT calculations. As shown in Figure S12, the ∆GH* of MoS2-BP was 0.1 eV lower than that of MoS2. As such, the injected electrons from BP enhanced the activity of original active sites in MoS2, rather than increased active sites. We also evaluated the long-term stability of the MoS2-BP catalysts by applying CV ranging from 0.05 to -0.35 V versus RHE at a scan rate of 50 mV s-1 at room temperature. From the LSV curves shown in Figure 4a, the catalytic activity of MoS2-BP nanosheets was retained in the current after 10,000 cycles. This result confirmed the excellent stability of MoS2-BP during the consecutive CV tests. Moreover, Figure 4b shows the durability of MoS2-BP nanosheets by applying scanning at a constant overpotential of 80 mV. Even after 104 seconds, MoS2-BP nanosheets exhibited excellent stability with less than 3% degradation of current density. As evidenced by the TEM image and XRD spectrum after the potentiostatic test, MoS2-BP nanosheets showed negligible change, suggesting the potential use of the MoS2-BP catalysts over a long time in an electrochemical process (Fig. S13). In summary, we constructed MoS2-BP nanosheets through the deposition of MoS2 on BP nanosheets. In MoS2-BP nanosheets, effective electron transfer from BP to MoS2 was achieved because of the higher Fermi level of BP than that of MoS2. The as-synthesized MoS2-BP nanosheets exhibited remarkable HER activity with an overpotential of 85 mV at 10 mA cm-2. Due to the electron donation of BP, the j0 of MoS2-BP reached 0.66 mA cm-2, which was 22 times than that of MoS2. Moreover, MoS2-BP nanosheets exhibited outstanding electrocatalytic stability. The success of engineering electronic properties by forming MoS2-BP heterostructures presents a straightforward strategy to design hybrid electrocatalysts.


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ASSOCIATED CONTENT Supporting Information. Experimental details, TEM images of BP and MoS2 nanosheets, Moire patterns of MoS2-BP nanosheets, AFM image and height profile of BP, MoS2, and MoS2-BP nanosheets, EDX spectrum of MoS2-BP nanosheets, Raman spectra of BP, MoS2, and MoS2-BP nanosheets, XRD patterns of BP, MoS2, and MoS2-BP nanosheets, XPS survey spectra of BP, MoS2, and MoS2-BP nanosheets, P 2p XPS spectra of BP and MoS2-BP nanosheets, Tauc/Davis-Mott plots of MoS2 and MoS2-BP nanosheets, HER activity of BP and MoS2/C after the hydrothermal treatment, double-layer capacitance measurements of MoS2 and MoS2-BP nanosheets, HER activities and j0 on recently reported MoS2-based catalysts, DFT calculations for ∆GH* on MoS2 and MoS2-BP, TEM image and XRD pattern of MoS2-BP nanosheets after the potentiostatic test. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *[email protected] Author Contributions R.H. and J.H. contributed equally. The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS This work was supported by Collaborative Innovation Center of Suzhou Nano Science and Technology, MOST of China (2014CB932700), NSFC (21573206 and 51371164), Key Research Program of Frontier Sciences of the CAS (QYZDB-SSW-SLH017), Strategic Priority Research Program B of the CAS (XDB01020000), and Fundamental Research Funds for the Central Universities.

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(38) Batmunkh, M., Bat-Erdene, M.; Shapter, J. G. Adv. Mater. 2016, 28, 8586-8617. (39) Luo, Z.; Maassen, J.; Deng, Y.; Du, Y.; Garrelts, R. P.; Lundstrom, M. S.; Ye, P. D.; Xu, X. Nat. Commun. 2015, 6, 8572. (40) Xie, J.; Zhang, H.; Li, S.; Wang, R.; Sun, X.; Zhou, M.; Zhou, J.; Lou, X. W.; Xie, Y. Adv. Mater. 2013, 25, 5807-5813. (41) Wu, J.; Mao, N.; Xie, L.; Xu, H.; Zhang, J. Angew. Chem. Int. Ed. 2015, 54, 2396-2399. (42) Gao, M.-R.; Chan, M. K.Y.; Sun, Y. Nat. Commun. 2015, 6, 7493. (43) Geng, X.; Sun, W.; Wu, W.; Chen, B.; Al-Hilo, A.; Benamara, M.; Zhu, H.; Watanabe, F.; Cui, J.; Chen, T. Nat. Commun. 2016, 7, 10672. (44) Sun, Y.; Alimohammadi, F.; Zhang, D.; Guo, G. Nano Lett. 2017, 17, 1963-1969. (45) Pentland, N.; Bockris, J. O. M.; Sheldon, E. J. Electrochem. Soc. 1957, 104, 182-194. (46) Voiry, D.; Yamaguchi, H.; Li, J.-W.; Silva, R.; Alves, D. C. B.; Fujita, T.; Chen, M.-W.; Asefa, T.; Shenoy, V. B.; Eda, G.; Chhowalla, M. Nat. Mater. 2013, 12, 850-855. (47) Morales-Guio, C. G.; Stern, L.-A.; Hu, X. Chem. Soc. Rev. 2014, 43, 6555-6569.


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Nano Letters

Figure 1. (a) TEM image of a typical MoS2-BP nanosheet. (b) HRTEM images of the region marked in (a). (c-f) STEM and STEM-EDX elemental mapping images of the MoS2-BP nanosheet. (g, h) Compositional line profiles of P, S and Mo from the MoS2-BP nanosheet recorded along two different directions marked by orange lines in panel (c).


Nano Letters

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Figure 2. (a) P 2p XPS spectra of BP and MoS2-BP nanosheets. (b) S 2p XPS spectra of MoS2 and MoS2-BP nanosheets. (c) Mo 3d XPS spectra of MoS2 and MoS2-BP nanosheets. (d) Mott-Schottky plots of BP and MoS2. The capacitance C was obtained from impedance measurements. The dash line shows the baseline of the curve. The intersection values show the flat band potentials.


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Nano Letters

Figure 3. (a) LSV curves of BP, MoS2/C, and MoS2-BP catalysts for hydrogen evolution in 0.5 M H2SO4. The Pt/C catalyst is presented as a reference. (b) Enlarged LSV plots derived from a. The dash line shows the current density of 10 mA cm-2. (c) Tafel plots of BP, MoS2/C, MoS2-BP, and Pt/C catalysts. (d) Nyquist plots of BP, MoS2/C, MoS2-BP, and Pt/C catalysts. (e) Cu stripping curves on glassy carbon, MoS2/C and MoS2-BP. (f) The exchange current densities (j0, geometrical) of BP, MoS2/C, MoS2-BP, and Pt/C catalysts.


Nano Letters

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Figure 4. (a) LSV curves of MoS2-BP nanosheets after 10,000 cycles. (b) Plot of current density (j) versus time for MoS2-BP nanosheets at a constant overpotential of 80 mV.


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