Water-Soluble Defect-Rich MoS2 Ultrathin Nanosheets for Enhanced

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Surfaces, Interfaces, and Catalysis; Physical Properties of Nanomaterials and Materials 2

Water-Soluble Defect-Rich MoS Ultrathin Nanosheets for Enhanced Hydrogen Evolution Jianfang Zhang, Yan Wang, Jiewu Cui, Jingjie Wu, Yang Li, Tianyu Zhu, Huirui Kang, Jingping Yang, Jian Sun, Yongqiang Qin, Yong Zhang, Pulickel M. Ajayan, and Yu-Cheng Wu J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.9b01121 • Publication Date (Web): 30 May 2019 Downloaded from http://pubs.acs.org on May 30, 2019

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The Journal of Physical Chemistry Letters

Water-Soluble Defect-Rich MoS2 Ultrathin Nanosheets for Enhanced Hydrogen Evolution

Jianfang Zhang1,2, Yan Wang1, *, Jiewu Cui1, Jingjie Wu2, *, Yang Li1, Tianyu Zhu1, Huirui Kang1, Jingping Yang1, Jian Sun1, Yongqiang Qin1, Yong Zhang1,4, Pulickel M Ajayan3,4,5, Yucheng Wu1,4,5 1

School of Materials Science and Engineering, Hefei University of Technology, Hefei 230009,

China 2

Department of Chemical and Environmental Engineering, University of Cincinnati,

Cincinnati, OH 45221, United States 3

Department of Material Science and NanoEngineering, Rice University, Houston, Texas

77005, United States 4

Base of Introducing Talents of Discipline to Universities for Advanced Clean Energy

Materials and Technology, Hefei 230009, China 5

China International S&T Cooperation Base for Advanced Energy and Environmental

Materials, Hefei 230009, China

*Corresponding authors: Dr. Yan Wang, [email protected]; Dr. Jingjie Wu, [email protected].

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Abstract In this work, we report a facile cryo-mediated liquid phase exfoliation method to synthesize water-soluble defect-rich MoS2 ultrathin nanosheets (d-MoS2 NS) with the assistance of NaBH4 in the solvent. The as-prepared d-MoS2 NS show enhanced electrocatalytic hydrogen evolution reaction (HER) performance in comparison to that of MoS2 NS due to surface hydrophilicity and abundant active edge sites. The formation process of the d-MoS2 NS with exposed edge sites is illustrated by investigating the influence of exfoliation time on their structural morphology. The optimal water-soluble d-MoS2 NS display excellent HER activities, including a low overpotential of 71.5 mV at a current density of -10 mA cm-2, a small Tafel slope of 58.3 mV dec-1, and good cycling stability.

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Hydrogen is regarded as one of the most attractive energy resources in the future to replace fossil fuels.1-2 Electrocatalytic hydrogen evolution reaction (HER) from water splitting is accepted as a sustainable method to generate hydrogen.3-5 Generally, Pt and other noble metal-based catalysts have been demonstrated to possess the highest electrocatalytic activity towards HER. However, the high cost and scarcity of noble metal-based catalysts restrict their widespread application.6-8 Therefore, there is a need to explore earth-abundant, low-cost alternative catalysts with high HER activity. Molybdenum disulfide (MoS2) has been extensively investigate for HER due to its natural abundance and low Gibbs free energy for hydrogen adsorption.9-16 Normally, bulk MoS2 exhibits low HER activities because its basal planes are catalytically inert.17-19 It has been shown both theoretically and experimentally that HER activities of MoS2 mainly arise from the edge sites of the two-dimensional MoS2 layers.20-28 As such, effort is being focused on increasing the density of edge sites in MoS2 in order to improve its HER activity.22,

29-37

However, developing a simple and large-scale

technique to directly synthesize MoS2 nanosheets with a high density of edge sties still remains a challenge. Recently, several methods have been reported to be able synthesize defect-rich MoS2 nanosheets with a high density of active edge sites.13,

26, 38-41

For example, Xie et al.

synthesized defect-rich MoS2 ultrathin nanosheets with exposed active edge sites by adjusting the amounts of thiourea during the solvothermal process.18 Yan et al. developed ultrathin MoS2 nanoplates with abundant active sites by the solvent-dependent method and achieved high HER activity with a small Tafel slope of 53 mV dec-1.42 Wu et al. demonstrated that MoS2 nanosheets with exposed edge sites prepared by a microdomain reaction method achieved a small Tafel slope of 68 mV dec-1.43 Li et al. utilized chemical vapor deposition (CVD) method to grow edge-enriched MoS2 nanoplatelet thin films.44

Wan et al. also

employed a CVD method to synthesize fractal-shaped single-layer MoS2.45 The assynthesized MoS2 with a large number of exposed edge sites showed superior HER activities 3 ACS Paragon Plus Environment


than the triangle-shaped MoS2. However, all the above methods suffer some drawbacks such as harsh conditions, tedious process and low yield.46 Therefore, an effective and scalable route to synthesize MoS2 nanosheets with a defect-rich structure to expose more active edge sites is urgently required. Liquid phase exfoliation is demonstrated to be a facile method to prepare MoS2 nanosheets.47-49 However, the synthesis of edge-abundant MoS2 nanosheets via liquid phase exfoliation method is rarely reported in the literature. Herein, we synthesize water-soluble defect-rich MoS2 ultrathin nanosheets (d-MoS2 NS) through a facile cryo-mediated liquid phase exfoliation method with the assistance of NaBH4. The NaBH4 is first used in liquid phase exfoliation. Pre-treatment in liquid nitrogen facilitates the efficiency of liquid phase exfoliation by weakening the interlayer van der Waals force between MoS2 layers. NaBH4 in the solvent continuously releases H2 gas that not only enters the MoS2 interlayer to cause abundant edge sites on the basal plane of MoS2 nanosheets but also alters the surface property from hydrophobic to hydrophilic in nature. The resulting dMoS2 NS has more exposed active site edges and is water-soluble. As an electrocatalyst for HER, the d-MoS2 NS exhibit a small overpotential of 71.5 mV at a current density of -10 mA cm-2 with a low Tafel slope of 58.3 mV dec-1.

Figure 1 (a) Schematic illustration of the synthesis process of cryo-mediated liquid phase exfoliation, (b) the formation mechanism of the defect-rich MoS2 nanosheets. 4 ACS Paragon Plus Environment

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The synthetisis process and formation mechanism of the d-MoS2 NS are illustrated in Figure 1. First, as shown in Figure 1a, bulk MoS2 powders are immersed in liquid nitrogen for 6 h. The cryo-pretreated MoS2 powders are immediately dispersed into the IPA/H2O mixture solvent after being removed from liquid nitrogen. For the formation of d-MoS2 NS, NaBH4 is added into the solvent during the liquid phase exfoliation process. As illustrated in Figure 1b, H2 gas from NaBH4 penetrates and enlarges the interlayer spacing between the MoS2 layers, causing them to be easily exfoliated into nanosheets with a large number of holes formed on the exposed Mo and S edges. As shown in Figure 2a and b, TEM images reveal the morphology of the d-MoS2 NS with numerous defects. The lateral size of nanosheets is found to be from tens to hundreds of nanometers. Obvious holes can be observed in d-MoS2 NS with exposed edge sites. The lattice spacing of 0.27 nm from the HRTEM in Figure 2d, corresponds to the interlayer spacing between (100) surfaces of MoS2. Selected area electron diffraction (SAED) pattern reveals clear diffraction spots with a hexagonal pattern, corresponding to the (100) and (110) faces of MoS2. Figure 2f shows the HRTEM image at the edge of a d-MoS2 NS. With the aid of Mo and S atoms superimposed over the micrograph, it can be clearly seen the presence of abundant Mo and S edges. The AFM image in Figure 2g clearly shows the presence of a large number of holes in the d-MoS2 NS. The thickness of dMoS2 NS is measured to be about 1.5 nm, as shown in Figure 2h, indicating single- or doublelayer of the d-MoS2 NS. Based on the TEM and AFM results, Figure 2i illustrates schematically the model of d-MoS2 NS. In contrast, MoS2 NS prepared by a similar process without the addition of NaBH4 show a different morphology. TEM image reveals a complete and unbroken nanosheet structure with a lateral size of about 2 m with the HRTEM image showing a lattice spacing of 0.27 nm, corresponding to the (100) faces of MoS2 (Figure S1a-c). The hexagonal pattern of diffraction spots in Figure S1d also reveal the (100) and (110) faces of MoS2. The AFM image (Figure S1e) shows a different morphology compared to that of dMoS2 NS. Although some holes are generated on the MoS2 nanosheets, they are more intact in 5 ACS Paragon Plus Environment


nature and larger in size compared to that of d-MoS2 NS which has more step edges. The thickness of MoS2 NS is measured to be 2.2 nm, which is slightly larger than that of d-MoS2 NS (Figure S1f). These results suggest that NaBH4 can reduce both the lateral size and layer thickness of d-MoS2 NS.

Figure 2 Characterization of the d-MoS2 NS: (a,b) TEM images, (c,d) HRTEM images, (e) SAED pattern, (f) identification of active sites/edges, (g) AFM image, (h) Height, (i) Schematic model structure. The morphologies of MoS2 NS and d-MoS2 NS shown by the SEM images of Figure 3a and b, clearly indicate they are very different. In the presence of NaBH4, the MoS2 nanosheets are broken into smaller pieces. The crystalline structures and surface chemical states of the MoS2 NS and d-MoS2 NS are analyzed by XRD, Raman and XPS. Figure 3c shows the XRD patterns of the bulk MoS2, MoS2 NS and d-MoS2 NS. Compared to bulk MoS2, the MoS2 NS and d-MoS2 NS show only one (002) peak at 2=14.4 in the XRD patterns, demonstrating the successful exfoliation of MoS2 nanosheets. It should be noted that the (002) peak of d6 ACS Paragon Plus Environment

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MoS2 NS is wider than that of MoS2 NS, as shown by the enlarged plot in the inset of Figure 3c. The Raman spectra in Figure 3d further confirm the existence of ultrathin nanosheet structure of the MoS2 NS and d-MoS2 NS, with the separation between the E2g and A1g peaks being smaller than that of bulk MoS2.47 Figure 3e displays the typical Mo 3d XPS spectra of MoS2 NS and d-MoS2 NS. The peaks at 232.6 and 229.5 eV correspond to the Mo 3d3/2 and Mo 3d5/2 of Mo4+, respectively, while the peak at 226.5 eV is due to S 2s.33 The small peak of Mo6+ at 235.8 eV is also observed in the two samples, which can be attributed to the slight oxidation of the surface region when exposed to air.50 The S 2p XPS spectra of MoS2 NS and d-MoS2 NS can be divided into two peaks at 162.3 and 163.5 eV (Figure 2f), corresponding to the S2- 2p3/2 and 2p1/2, respectively.33, 50

Figure 3 SEM images of (a) MoS2 NS, (b) d-MoS2 NS. (c) XRD patterns and (d) Raman spectra of bulk MoS2, MoS2 NS and d-MoS2 NS. XPS spectra of MoS2 NS and d-MoS2 NS: (e) Mo 3d, (f) S 2p. 7 ACS Paragon Plus Environment


It is worth pointing out that d-MoS2 NS become water-soluble with the addition of NaBH4. Previous reports have indicated that MoS2 powders are more likely to dissolve in mixed organic and water solvents. However, in our work, MoS2 powders can be completely dissolved in pure water in the presence of NaBH4. For comparison, we also prepared d-MoS2 NS by using IPA and H2O with a volume ratio of 1:1 as the solvent mixture. Interestingly, MoS2 powders can be well dispersed into IPA/H2O solvent during the liquid phase exfoliation process (Figure S2a), while a clear segregation of IPA and H2O appears after the synthesis process (Figure S2b). As shown in Figure S2c, after removal of IPA, d-MoS2 NS can be evenly dispersed in H2O. After centrifugation at 4000 rpm for 30 min, the d-MoS2 NS solution displays a brownish black color and remains stable for a long time (Figure S2d). The yield of d-MoS2 NS is estimated to be about 0.34 mg/ml, which is more than twice that of MoS2 NS (0.16 mg/ml). UV-vis spectrum of d-MoS2 NS shows higher absorption than that of MoS2 NS due to the darker color of the solution (Figure S3), which suggests a higher concentration of d-MoS2 NS in the solution.47, 51 In addition, to further confirm the watersoluble d-MoS2 NS, we adopt a series of IPA/H2O mixture solvents with the different volume ratio of 1:0, 4:1, 1:1, 1:4 and 0:1. As shown in Figure S4, after liquid phase exfoliation process, d-MoS2 NS become completely insoluble in pure IPA. When increasing the volume of H2O in the mixture, d-MoS2 NS are gradually dissolved into H2O with a clear dividing line in the solvent. The d-MoS2 NS can be completely dissolved when the volume ratio of IPA/H2O drops to 0:1. It can be seen from the top view of the solvent that the MoS2 powders are not dissolved in pure H2O (Figure S5). During the liquid phase exfoliation process by adding NaBH4, a hydrogen bubbles are generated and released from the solvent. Ultimately, the d-MoS2 NS are evenly dissolved in pure H2O. The contact angles of water droplets on the surface of MoS2 NS and d-MoS2 NS are also measured (Figure S6 and Video S1). MoS2 NS show a hydrophobic surface with a contant angle of about 109.8°, whereas d-MoS2 NS are


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hydrophilic with contact angle of almost 0°. These results suggest that NaBH4 contributes to making MoS2 NS water-soluble by changing their surface properties. We also investigate the influence of liquid phase exfoliation time on the morphology to get a further insight into the formation mechanism of d-MoS2 NS in the presence of NaBH4. From the TEM images of d-MoS2 NS obtained from different exfoliation times ranging from 1 to 7 h, it can be seen that the lateral size of d-MoS2 NS decreases with increasing exfoliation time (Figure 4a-d). The same results can also be observed by the AFM images in Figures 4e-h. After liquid phase exfoliation for 1 h, d-MoS2 NS exhibit a large area of nanosheets with some small holes. It is believed that the generated holes are attributed to the H2 bubbles released from NaBH4 during the liquid phase exfoliation process. With increasing exfoliation time, the holes become larger such that the nanosheets are broken into smaller pieces. At 7 h, most nanosheets are broken into nanodots. All the above results show that water-soluble dMoS2 NS with abundant exposed edge sites can be fabricated by cryo-mediated liquid phase exfoliation in the presence of NaBH4, which is expected to exhibit higher electrocatalytic activity towards HER.

Figure 4 TEM and AFM images of d-MoS2 NS prepared with different exfoliation times: (a,e) 1 h, (b,f) 3 h, (c,g) 5 h and (d,h) 7 h.



In order to confirm our expectation, electrochemical tests were carried out to study the HER activity of the water-soluble d-MoS2 NS prepared with an exfoliation time of 5 h. The dMoS2 NS catalysts were loading on a carbon cloth (CC) by a dip-coating method with a mass of 1.0 mg cm-2. For comparison, MoS2 NS catalysts with the same loading mass were prepared by the same dip-coating method. The SEM images and the corresponding EDS mapping show that the catalysts are uniformly dispersed on the CC (Figure S7). In addition, pure CC and 20% Pt/C electrodes were also evaluated for reference. Figure 5a shows the polarization curves of these electrodes. The 20% Pt/C electrode exhibits the best HER performance with a small overpotential of 45.6 mV at -10 mA cm-2. Pure CC electrode shows a very low current density, indicating negligible electrocatalytic performance. The MoS2 NS electrode displays a moderate electrocatalytic activity with an overpotential of 230.8 mV at 10 mA cm-2. As expected, the d-MoS2 NS electrode delivers an excellent electrocatalytic performance, comparable to that of the 20% Pt/C electrode, with a small overpotential of 71.5 mV at -10 mA cm-2. The d-MoS2 NS electrode also shows a competitive electrocatalytic activity towards HER compared to previously reported HER catalysts using other forms of MoS2 (Table S1). The corresponding Tafel plots are shown in Figure 5b. The 20% Pt/C electrode delivers the smallest Tafel slope of 30.9 mV dec-1. The d-MoS2 NS electrode shows a much lower Tafel slope of 58.3 mV dec-1 than that of MoS2 NS electrode (176.7 mV dec-1), revealing its improved electrocatalytic activity.17 It should be pointed out that the d-MoS2 NS electrode prepared with an exfoliation time of 5 h achieves the smallest Tafel slope compared to those prepared for 1, 3 and 7 h (Figure S8). The charge transport and kinetics of the electrode are investigated by electrochemical impedance spectroscopy (EIS) and the Nyquist plots are shown in Figure 5c. The charge transfer resistance value (Rct) fitted by the equivalent circuit is about 26.1  for the d-MoS2 NS electrode, which is smaller than that of the MoS2 NS electrode (37.6 ), demonstrating the fast charge transfer capability and superior kinetics process.45 The enhanced charge transport and kinetics can be attributed to 10 ACS Paragon Plus Environment

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the water-soluble property and more exposed active sites of the d-MoS2 NS electrode. The electrochemically active surface areas of d-MoS2 NS electrode can be evaluated from doublelayer capacitance (Cdl), where Cdl can be measured by CV at different scan rates (Figure S9). As shown in Figure 5d, the Cdl of d-MoS2 NS electrode (4.7 mF cm-2) is about twice that of MoS2 NS electrode (2.4 mF cm-2), indicating the larger surface areas of d-MoS2 NS electrode. Moreover, the stability of the catalysts was measured by continuous CV tests for 2000 cycles at a scan rate of 100 mV s-1 under a wide potential window ranging from -0.4 to 0.1 V vs. RHE. The current densities show no obvious degradation in the polarization curves of both dMoS2 NS and MoS2 NS electrodes after 2000 cycles (Figure 5e), confirming good stability of MoS2 catalysts. Besides, the durability of the catalysts was also checked by chronoamperometry test under an overpotential of 100 mV for 20 h. As can be seen in Figure 5f, the d-MoS2 NS and MoS2 NS electrodes exhibit relatively stable current density of -2.5 and -20 mA cm-2, respectively, further demonstrating long-term stability of MoS2 catalysts for HER. The Faradaic efficiency of the d-MoS2 NS electrode was measured by comparing the volume of H2 gas generated and quantity of charges passing the electrode by at an overpotential of 150 mV for 2 hours. The measured volume of H2 matches well with the calculated value, indicating the Faradaic efficency of H2 production is nearly 100% over this time period (Figure S10). All the above results indicate that the d-MoS2 NS electrode possesses a high electrocatalytic activity and good durability towards HER. The enhanced electrocatalytic performance of d-MoS2 NS is due to the defect-rich structure of and water soluble nature d-MoS2 NS, which accelerate the reaction kinetics between the electrode surface and electrolyte and providing more exposed active edge sites to improve the intrinsic electrocatalytic activity.



Figure 5 Electrochemical performance of the MoS2 NS and d-MoS2 NS electrodes: (a) polarization curves, (b) Tafel plots, (c) Nyquist plots, (d) Cdl, (e) polarization curves initially and after 2000 CV cycles at 100 mV s-1, and (f) time-dependent current density curves measured at an overpotential of 100 mV. In summary, water-soluble and defect-rich MoS2 nanosheets are synthesized by a facile cryo-mediated liquid phase exfoliation method with the assistance of NaBH4. The pretreatment in liquid nitrogen promotes the liquid phase exfoliation process. The presence of NaBH4 in the solvent not only creates abundant active edge sites on the basal plane of MoS2 nanosheets by releasing H2 gas but also alters the surface property from hydrophobic to hydrophilic in nature. As a result, the as-prepared water-soluble d-MoS2 NS electrode exhibits


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an excellent HER performance with a low overpotential of 71.5 mV at a current density of -10 mA cm-2 and a small Tafel slope of 58.3 mV dec-1, as well as good cycling stability. Acknowledgments This work is financially supported by the National Natural Science Foundation of China (51772072, 51701057, U1810204), the Natural Science Foundation of Anhui Province (1708085ME100), the Fundamental Research Funds for the Central Universities (PA2019GDQT0022, PA2019GDQT0015, S201910359043). We would also like to acknowledge the financial support from the 111 Project "New Materials and Technology for Clean Energy" (B18018). Dr. Y. Wang and Dr. J. W. Cui would like to thank the financial support from the China Scholarship Council during their visits to Prof. Pulickel M Ajayan’s group at Rice University for collaborative research. Supporting Information Experimental section, additional characterization data including TEM, AFM, SEM, UV-vis spectrum, photograph and electrochemical tests are presented. This material is available free of charge via the Internet at http://pubs.acs.org. References (1)

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