Enhanced Catalytic Activities of Surfactant-Assisted Exfoliated WS2

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Enhanced Catalytic Activities of Surfactant-Assisted Exfoliated WS2 Nanodots for Hydrogen Evolution Xue Zhao, Xiao Ma, Jian Sun, Dehui Li, and Xiurong Yang ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.5b06653 • Publication Date (Web): 19 Jan 2016 Downloaded from http://pubs.acs.org on January 20, 2016

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Enhanced Catalytic Activities of Surfactant-Assisted Exfoliated WS2 Nanodots for Hydrogen Evolution Xue Zhao†,‡, Xiao Ma†,‡, Jian Sun†, Dehui Li†,‡ and Xiurong Yang*,† †

State Key Laboratory of Electroanalytical Chemistry, Changchun Institute of Applied

Chemistry, Chinese Academy of Sciences, Changchun, Jilin 130022, China. E-mail: [email protected] ‡

University of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS WS2 nanodots, octahedral (1T), hexagonal (2H), hydrogen evolution ABSTRACT WS2 nanodots were prepared by liquid-phase exfoliation of bulk WS2 crystals in surfactant aqueous solution with the aid of ultrasonication. Their behaviors on catalyzing hydrogen evolution reaction (HER) were investigated after drop-casting them onto a GCE. Based on the optical and electron characterizations, the nanodots were identified with a high concentration of octahedral phase of WS2 that showed better catalysis properties than the hexagonal WS2. From the polarization curve, the Tafel slope was estimated to be 51 mV per decade and the onset potential was 90 mV, indicating good catalytic performance of such nanodots. Our results

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suggest that surfactant-mediated exfoliation is an environmentally benign method to synthesize WS2 nanodots for improved catalyzing HER. Electrocatalytic hydrogen evolution reaction (HER) is considered a potentially “clean” method by consuming continuous supplies of electric energy and generates hydrogen, which is regarded as an excellent energy carrier in the future.1 Although platinum-based catalysts usually exhibit good HER performances, the resource scarcity and high cost of platinum metal have hindered their large-scale applications.2 Low efficiencies and high onset potentials of non-noble-metal compounds are the major problems encountered in search of materials to substitute Pt-based catalysts.3 The identification of the transition-metal dichalcogenides (TMDs) as potential efficient catalysts for HER has opened up an exciting new path for HER.4 Both computational and experimental results have confirmed that the electrocatalytic HER activities mainly stem from the edge sites rather than the basal planes.5 As a result, much effort has been focused on trying to acquire TMDs nanomaterials with a plethora of edge site. Among them, the most common pathway to achieve this was through increasing the edges of these layered materials by introducing nanosize dimensions.6-8 TMDs in the form of MS2 consist of strong planar covalent bonding among the atoms S-M-S on the latitude dimension (M means transition metal and S equals to chalcogenide).8 Each layer binds to each other loosely via van der Waals interactions.9 This weak connection makes it possible for the formation of two-dimensional nanosheets which exhibit thickness-dependent physical properties.10 TMDs can be exfoliated through “top-down” micromechanical into their individual nanosheets by electrochemical, mechanical and/or chemical means.11,12 For example, chemically exfoliated metallic MoS2 nanosheets by lithium intercalation exhibit enhanced HER performance.13 Engineering the size and dimension of layered materials is one of the most


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fascinating ways to endow them with novel properties. Therefore, it is believed that, when the lateral size of MS2 ulteriorly decreases and forms MS2 nanodots, the HER performance increases due to the quantum confinement14 and edge effects.8 Specifically, MoS2 ultra small nanoparticles prepared by ultrasonication in DMF have enrichment of S edges, making them high efficient HER catalysts.8 Recently, the approaches by using a combination of grinding and sonication in N-methyl-2-pyrrolidone (NMP)15 and isopropanol (IPA)4 have been proposed, which exhibited high yields. Exfoliation in aqueous solution instead of organic solvent would be a better option if feasible, since it is environmentally friendly. The method of surfactant delamination of TMDs in aqueous solution has been reported, however, such method for the synthesis of ultra-small TMDs suffered from low yields during synthesis, or formed as by-products when aiming to prepare nanosheets.16,17 Herein, we demonstrate a facile and general protocol to prepare WS2 nanodots, denoted as WS2NDs, by ultrasonication in surfactant aqueous solution. The MS2 crystals mainly have three crystal phases, ie. octahedral (1T), hexagonal (2H and 3R), which provide foundation for tailoring the atomic structures as well as electronic properties. In detail, the two hexagonal phases, 2H (anti-parallel) and 3R (parallel), are semiconducting, of which the former is more stable. The octahedrally coordinated 1T phase also exists and is predicted to be metallic. It is not stable in the bulk form and is associated solely with TMDs monolayers.18 It is worth mentioning that the 2H-MS2 is able to transform to 1T phase by electron doping (electron irradiation or introducing Li ions).19 Notably, distorted 1T-MS2 exhibited high HER performance. It was reported that increasing 1T content of the catalyst has an effect on its performance in HER, mainly on decreasing the onset potential, because that charge transfer kinetic in metallic 1T-MS2 is faster than in 2H phase.20 In recent years, many efforts have been made to control over the crystal phases of TMDs by modification of the


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synthetic method, such as utilizing vacancy-induced two-step hydrothermal method21 or controlling the reactivity of precursor during colloidal synthesis22. However, the crystal phase of TMDs exfoliated in surfactant aqueous solution was not thoroughly investigated. Herein, we selected WS2NDs as the prototype to research the crystal phases of exfoliated TMDs in great details by both optical and electron characterization. In this work, we have prepared WS2NDs with high quality and uniformity dispersed in aqueous solution. Further, the obtained WS2NDs were drop-casted onto glassy carbon electrode. An HER electrode with low onset potential (90mV) and small Tafel slope (51mV dec-1) was obtained. Remarkably, a high concentration of 1T-WS2 incorporation in WS2NDs was responsible for the enhanced catalytic activities. To the best of our knowledge, this is the first report of researching the crystal phases of exfoliated WS2NDs and utilizing WS2NDs for catalyzing HER.

RESULTS AND DISCUSSION

Characterization of WS2NDs. With sonication, bulk crystals of WS2 were exfoliated in 0.3 mmol L-1 PEG-PPG-PEG Pluronic® (a non-ionic surfactant, hereafter called F108) aqueous solution, leading to the formation of nanodots, as schematically depicted in Scheme 1. The concentration of as-fabricated WS2 nanodots suspension, denoted as WS2NDs, was determined to be approximately 0.16 mg mL-1. The overall yield was rather high after collecting the sediment. Although dispersed concentrations of the nanodots increased with longer ultrasonication time, the concentrations were high enough by ultrasonication for 4 h (Figure S1). Therefore, taking into account the energy consumption, 4 h sonication time was used in all the


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following research. The WS2NDs were uniform in size without aggregation as shown in the TEM image (Figure 1a). As shown in Figure 1b, the lattice spacing of about 0.21 nm can be indexed to (104) planes of WS2 crystal. HRTEM and fast Fourier transform pattern indexed unequivocally to 1T-WS2 were shown in Figure 1c. The existence of 1T-WS2 polymorph indicated that we successfully incorporated 1T polymorph into the matrix of 2H-WS2 crystal.23 In addition, the HRTEM of 2H-WS2 was provided in Figure S2 for comparison. The particle size distributions showed that the lateral sizes of WS2NDs were of 2.7 ± 0.8 nm (Figure 1d). The atomic force microscopy (AFM) characterization (Figure 1e) and the height profile (Figure 1f) confirmed that the height of the WS2NDs was 0.7 nm, which was about the size of single layer.24 The WS2NDs dispersion was negatively charged, with a zeta potential of -7.3 ± 0.6 mV. The surface charge induced electrostatic repulsion between WS2NDs, which kept them stable in the aqueous solution. Furthermore, the TEM and SEM graphs of the products from different stages of the exfoliation and purification process were shown (Figure S3 and S4). It was indicative that the surfactant was almost completely removed by comparing Figure S3a and S3b. The centrifugation rate during the purification process had an effect on the sizes of the nanodots (Figure S3c). In addation, Figure 1g presented a typical Tyndall effect for collidal dispersion and that indicated the existence of very small particles in the solution. Also the UV-vis spectrum of WS2NDs was shown in Figure 1g. The peaks at 665 nm were the characteristic absorption bands of exfoliated WS2 in solution. Besides, the threshold at 395 nm and 450 nm were attributed to the direct transition from the deep valence band to the conduction band, and the characteristic emission peaks at 665 nm came from the direct excitonic transitions at the K point of the Brillouin zone, indicating the existence of 2H-WS2 in WS2NDs.25 It is worth mentioning that the


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1T-WS2 showed no obvious absorption peaks in UV-vis spectrum.22 Other kinds of surfactant also work for preparing such WS2NDs except that the yields are much lower. (Figure S5 and S6). WS2 is 2H phase in nature, while a metallic 1T phase also exists but is not stable in the bulk form.26,27 It is challenging to use X-ray diffractograms (XRD) to characterize the 1T-WS2 and 2H-WS2 nanostructures (Figure 2a). The WS2NDs samples were sufficiently crystalline to exhibit well-resolved diffraction peaks.28 The peaks that were marked with asterisks were attributed to the known 2H-WS2 pattern (JCPDs No. 08-0237). The diffraction peaks of pristine bulk WS2 which all corresponded to the 2H-WS2 pattern were also shown for comparison (Figure S7b). For the analysis of the crystal structures, the difficulty we faced was that an indexed structure for the thermodynamically unstable 1T-WS2 was absent. To overcome this problem, we simulated a XRD pattern using the reported ReS2 structures with the lattice parameters extracted from the HRTEM images and obtained the patterns shown in Figure 2a.22,29 In order to further identify the phase compositions, annealing experiment was conducted. The obtained samples were denoted as WS2NDs-XXX for convenience, where XXX denotes the annealed temperature in °C. As shown in Figure 2a, all the diffraction peaks of WS2NDs-300 corresponded well to the 2H-WS2 (Figure S8). All diffraction peaks of WS2NDs can be indexed from either 2H-WS2 or 1T-WS2. Hence, we can draw a conclusion that the two phases coexisted in the polymorphic WS2NDs. Raman spectroscopy measurements were also performed to further confirm the phase classification (Figure 2b). The characteristic Raman shifts at 366 and 431cm-1 expected for the E12g and A1g were clearly observed, respectively. In addition, the additional weak peaks in the lower frequency regions were previously referred as J1, J2, and J3, corresponding to modes that were only active in 1T-type WS2 and not allowed in 2H-WS2.13,30 Besides, the Raman spectra


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were monitored as a function of the annealing temperature. The spectra revealed that the intensities of the Raman active modes from the 1T phase decreased with annealing temperature, while the peaks corresponding to the 2H phase became shaper. Spectrum of bulk 2H WS2 was also provided for comparison. Moreover, since the positions of E12g and A1g peaks were reported to change with the number of monolayers, a weak down shift observed for bulk WS2 in comparison with the WS2NDs indicated that the WS2NDs contained fewer layers. A third way to distinguish 1T-WS2 and 2H-WS2 is the utilization of X-ray photoelectron spectroscopic (XPS). Considering the fact that the tungsten signal is sensitive to its oxidation state and coordination geometry, the deconvolution of the W4f 7/2 and W4f 5/2 core level peaks would benefit us distinguish the discrepancy between the 1T and 2H structures. Extracted phase compositions showed that the as-deposited WS2NDs consisted of major amounts (>80 %) of the 1T phases (Table S1 and Table S2). It is known that the 1T phase relaxed to the stable 2H structures on annealing (>100 °C). In order to further identify the phase compositions of WS2NDs, the XPS of bulk WS2 and WS2NDs as a function of annealing temperatures were investigated (Figure 3). The 1T peaks were gradually quenched after annealing above 100 °C. The intensity of the 2H-WS2 peaks increased slightly as the temperature increased. Such trend was in good agreement with the transformation of 1T-WS2 to 2H-WS2. After annealing at temperatures above 200 °C, the material was predominantly in 2H phase, which was similar to the bulk WS2. These interpretations together with the aforementioned characterization results solidly confirm the crystal phase co-existence of both 2H-WS2 and 1T-WS2. The appearance of 2H is thought to be the intact crystal phase from the bulk form, while the 1T phase was introduced during the ultrasonication in surfactant aqueous solution. This is different from the reports on WS2 exfoliated NMP and IPA with hundreds of nanometers in lateral sizes which


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were all 2H-WS2, as well as WS2 exfoliated by lithium intercalation which resulted in distorted 1T-WS2. Similarly, in the S2p region of the spectra, the peaks were shifted to lower binding (Figure 3c). No peaks were observed around 168 eV, which indicated that sulfur atoms remained non-oxidized. We demonstrated that it was possible to achieve a very high concentration (>80%) of the metallic phase in WS2NDs, which may lead a dramatic enhancement in the catalytic activities.20 Such phase conformation was desired in electrocatalytic hydrogen evolution, as the exfoliation at room temperature may lead to better catalytic performance. Meanwhile, the surface elemental compositions of WS2NDs were confirmed with XPS. WS2NDs showed W: S = 28.6: 71.4, which was smaller than the stoichiometric ratio of WS2 that is 1:2, being attributed to more sulfur edge after ultrasonication. The peaks at 32.4 and 34.6 eV can be attributed to W4f 7/2 and W4f 5/2 orbitals, respectively. They were in good agreement with the binding energies of W (IV) in WS2. It is worth noting that there was a slight oxidation peak appeared for tungsten oxide. It was reported that slight oxidation of TMDs can improve the density of the active sites, which can enhance the catalytic activities of WS2NDs. Nonetheless, exhaustive oxidation should be avoided.30 The compositional feature of the WS2NDs was also characterized by EDX measurement along with TEM (Figure S9). We can confirm that the nanodots were comprised of WS2. Analogous experiments with exfoliated WS2 in F108 aqueous solution yielded results similar to MoS2, which was also HER-active TMDs. In this work, the surfactant aqueous solution was successfully used to split WS2, MoS2 and graphite into ultrasmall nanodots (Figure S4, S10, S11, S12 and S13). It is believed that the same method can be used to prepared black phosphorus nanodots, which has aroused tremendous research interest recently.31


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Evaluation of electrocatalytic activity. To evaluate the electrochemical performance of the WS2NDs for the HER, the LSV measurement was performed as described in the Experimental Section. The polarization curves of all samples were shown in Figure 4a. They displayed the normalized current density versus voltage (j versus V) for the WS2NDs along with WS2NDs-300, bulk WS2, bare glass carbon electrode and Pt. Exfoliated WS2NDs exhibited good electrocatalytic activities with a low onset potential of ~90 mV, where the hydrogen evolution was significantly observed. As the control groups, polarization curves of WS2NDs-300, bulk WS2 and bare glass carbon electrode were measured, which exhibited high onset overpotential (180mV, 290mV, 350mV) as well as low cathodic current density. In comparison, the Pt/C exhibited excellent HER activities. The catalytic activity of WS2NDs was comparable to that of platinum and has lower energy consumption in catalytic process as well. These benefits suggest the good catalytic activities of WS2NDs, and the size reduction has led to an improvement in the HER activity. Further insight into the catalytic activity of the WS2NDs was obtained by extracting the slopes from the Tafel plots shown in Figure 4b. Tafel equation of each samples display Tafel slopes of 30, 51, 59 and 119 mV decade-1 for Pt, WS2NDs, WS2NDs-300, bulk WS2 and bare GCE, respectively. A smaller Tafel slope indicated a faster increment of HER velocity with the increasing of potential. The WS2NDs displayed enhanced activities with small slope values, which may be caused by the incorporation of 1T phase. It has been demonstrated that Tafel slope values can be used to determine which HER reaction path predominates for a specific catalyst.32 The WS2NDs has a Tafel slope of 51 mV decade-1, and the Volmer-Heyrovsky mechanism of HER was applicable to the WS2NDs. Such mechanism involved a fast discharge reaction and then a rate determining ion and atom reaction (Equation S1 and S2).33 However, Platinum-


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catalyzed HER was found to proceed through the Volmer-Tafel mechanism with fast reaction rate and a Tafel slope of 30 mV per decade (Equation S1 and S3).34 Compared to recently reported Pt-free catalysts for HER, Tafel slope of WS2NDs was not outperformed some of these novel catalysts, such as CoP/CC35, MoO2@PC-RGO36, MoS2/CoSe237, MoS2/RGO32. Nonetheless, the prepared process of WS2NDs was simple than most of the reported nanocomposites (Table S3). The HER activity of WS2NDs outperformed most of the pure TMDbased catalysts8,38 as well as WS2 hybrid catalysts39,40 in terms of both onset potential and Tafel slope (Table S4). The most inherent measurement of catalytic activities for HER is the exchange current density (j0).41 It can be obtained by fitting the logi-E data to the Tafel equation and using extrapolation method. In detail, the 20 % Pt/C revealed a j0 of 0.59 mA·cm-2, the WS2NDs yielded a j0 at 0.11 mA·cm-2 and the WS2NDs-300 yielded a j0 at 1.23*10-3 mA·cm-2 .The data for the WS2NDs was high and was proportional to the quantity of evolved hydrogen. The value was higher than that of most previous reports for pure TMD-based catalysts2,8 and was also comparable to most Pt-free catalysts42-46 (Table S3). The large exchange current density could be attributed to the improved numbers of edges, which benefited from the edge exposed as the size decreased. Stability is another important requirement for catalyst. As shown in Figure 5, a long-term cycling test of the WS2NDs was performed. Only a slight activity loss was observed after 1000 cycles, indicating that the good durability of WS2NDs in a long-term electrochemical process.

Discussion for the preparation process of WS2NDs. Computational studies have estimated the exfoliation (surface) energy of WS2 as greater than 250 mJ m-2.47 It has been reported that when the surface energy of the solvent matches that of exfoliated nanodots, maximum dispersion of


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exfoliated WS2 can be achieved.48 Unfortunately, the most useful solvent of all, water, has a surface tension of 72 mJ m-2 and cannot be used alone to exfoliate WS2.49 Surfactants can lower the liquid-vapor interfacial energy of water to an optimum range that corresponds to the energy required to separate the sheets beyond the range of the van der Waals forces. On the other hands, the surfactant acts as a stabilizer to prevent rapid re-aggregation driven by the large surface energy. Modelling has shown that if the surface energy of the solvent is similar to that of the layered material, the energy difference between the exfoliated and re-aggregated states will be very small, eliminating the driving force for re-aggregation. Based on the research, when the amount of F108 added was approximately 0.098% w/w, the surface tension of the suspension would reduce to the optimum value of ~41 mJ m−2.50 To this end, it can not only lower the liquid-vapor interfacial energy of the solution to an optimum range, but also create an extra repulsive term, preventing the re-aggregation of the exfoliated nanodots. In addition, these nanodots can be stabilized by electrostatic repulsion as characterized by zeta potential. After centrifugation, zeta potential of the nanodots changed from -7.3 to -6.6 mV, indicating that charges were suppressed at the surfaces of the nanodots. This was consistent with the removal of most surfactant from the nanodots surface. The dispersion was highly stable that no precipitation was observed after being stored for weeks under ambient conditions. When the layered materials were exposed to ultrasonic waves in a solvent, these waves generate cavitation bubbles that were collapsed into high-energy jets, breaking up the layered crystallites, producing exfoliated nanodots, and transforming the original trigonal-prismatic 2H-WS2 structure to octahedrally coordinated W atoms.51 The crystal structure of 1T-WS2 is very similar to that of 2H-WS2.52 Therefore, the 1T phase is a good candidate for incorporating another phase of WS2 into the 2H-WS2 matrix, which is a concept called “phase incorporation”. More importantly, the


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W atoms in 1T-WS2 have identical atomic positions as in 2H-WS2; therefore, the incorporated 1T-WS2 does not change the distributions of W ions in the 2H-WS2.21 It is believed that ultrasonication has intentionally introduced atom vacancies to the 2H-WS2 matrix, followed by crystal structure rearrangement. The role of sulfur vacancy is reported to be similar to that of a lithium ion occupying the interlayer S-S tetrahedron site in lithiated WS2 which undergoes a 2H1T phase transition.53 Since the probe sonication leads to the exclusive formation of few layered nanosheets of MS2,16 bath sonication was chosen and utilized. During the bath sonication, different positions of the vessel may experience different forces, leading to the formation of nanodots with different sizes. By centrifugal separation, the nanodots can be obtained with relatively uniform sizes. Exfoliation of WS2 by F108 is particularly important because this technique (1) does not need high temperature, (2) creates more catalytic edge sites on WS2 and (3) can be achieved in aqueous solutions instead of organic solvents. Findings on exfoliation of WS2NDs and enhanced catalytic activity highlight not only the potential applications of F108 as effective exfoliation agent but also its possible applications on other layered materials.

CONCLUSION Herein we reported a simple and universal approach to prepare WS2NDs by “top-down” synthesis from bulk WS2. Different from the ion exfoliation method, this process is quick, easy, and insensitive to ambient conditions. The WS2NDs presented an outstanding HER performance with an onset potential as low as 90 mV, a Tafel slope of 51 mV decade-1, an exchange current density of 0.11 mA cm-2 as well as excellent stability. The versatility of the solvent-exfoliation method enables straightforward creation of hybrid dispersions and films by simply adding


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another material to the dispersion. Combined with the scalability of this method, the asexfoliated WS2NDs could be potentially interesting catalysts for photocatalysis, which remain to be explored.

METHODS Preparation of monodispersed WS2NDs: A suspension of WS2NDs interspersed in aqueous solution was prepared using a one-pot liquid exfoliation technique involving bath sonication of WS2 flakes (Sigma-Aldrich) in surfactant aqueous solution. The main surfactant used was nonionic pluronic surfactant F108 with a molecular weight of ~14.6 kDa (F108, HO(C2H4O)141(C3H6O)44(C2H4O)141H). In short, WS2 powder (5 mg, Aldrich) was added to F108 solution (50 mL, 0.3 mmol L-1). This mixture was ultrasonicated by a KQ-200 KDE sonicator (Shanghai, China) at 200W for 4 h. Ice-water bath was applied to the reaction mixture. The suspension was then centrifuged at 6000 rpm for 10 min to remove unexfoliated powder. The supernatant was collected by pipette, then centrifuged at 12000 rpm for 30 min. The precipitate was collected and washed with de-ionized water twice. WS2NDs dispersion with a calculated concentration as high as 0.16 mg mL−1 was prepared by re-dispersing the precipitation into MilliQ water with half of the volume before centrifugation (25 mL). The color of colloidal suspension changed with the volume of the water increased. Some other surfactants were also used to substitute F108 in the study. Annealing was conducted in a tube furnace under the protection of argon at 100 °C, 200 °C, 300 °C for 1 h after drop-casting on silicon wafer and air drying. Electrochemical measurements: Electrochemical measurements of various samples were performed in H2SO4 (0.5 M) solution using the standard three-electrode configuration and with a carbon rod as counter electrode. Generally, Pt/C (1 mg, 20 %) and Nafion (30 µL, 5 wt%) were dispersed in water-ethanol solution (1 mL, v/v = 4:1) followed by sonication for 30 min to form


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a homogeneous ink. The as-prepared WS2NDs dispersion (1 mL) was mixed with Nafion (30 µL, 5 wt%) followed by sonication for 5 min. The obtained catalyst ink was loaded onto a glassy carbon electrode (GCE, 5 mm in diameter), respectively. The surface area of the prepared electrodes with approximately one continuous layer of WS2NDs showed a calculated loading of 16.3 µg cm-2. While other material modified electrodes for control groups were prepared by the same method and with the same loading as WS2NDs. The HER activity was evaluated by linear sweep voltammetry on rotating disk electrode with a rotation rate of 1600 rpm and a scan rate of 100 mV·s-1. In all measurements, Ag/AgCl electrode was used as the reference and the potential values after iR-corrected were normalized to reverse hydrogen electrode (RHE) (Figure S14). In 0.5 M H2SO4, E (RHE) = E (Ag/AgCl) + 0.202 V. Stable polarization performance curves were recorded after 10 cycles in the electrolyte de-aerated with N2 for 1h.

FIGURES

Figure 1. TEM image (a), high-resolution TEM images (b and c-left), fast Fourier Transform of the images (c-right) and the particle size distributions (d) are shown. A typical atomic force microscopy (AFM) image of WS2NDs (e) and the corresponding height profile (f) are shown.


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Separation of the supernatant after centrifugation at 6000 rpm yielded dark liquid as shown along with the UV-vis spectra of WS2NDs (g).

Figure 2. (a) XRD patterns of WS2NDs (blue) and calculated diffraction peaks for a hypothetical 1T-WS2 with the ReS2 structure (top). The XRD patterns of WS2NDs-300 (gray) and JCPDs No. 08-0237 (bottom) for 2H-WS2 were also shown for comparison. (b) Raman spectra of bulk WS2 and WS2NDs as a function of annealing temperature.


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Figure 3. (a) XPS spectra showing W4f core level peak regions for bulk WS2, WS2NDs and WS2NDs annealed at 100 °C, 200 °C, 300 °C; (b) Extracted relative fraction of 2H and 1T components of WS2NDs; (c) The binding energy of S 2p for the five samples.


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Figure 4. a) Polarization performance curves on a rotating glassy carbon disk electrode recorded in 0.5M H2SO4 for various electrocatalysts. b) The Tafel plots in which only the linear portions were shown to give a clear vision.


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Figure 5. Durability test for the WS2NDs. Negligible HER current was lost after 1000 cycles of cyclic voltammetry from -0.4 to 0.1 V vs. RHE.

SCHEMES

Scheme 1. Illustration of the preparation procedure.


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ASSOCIATED CONTENT Supporting Information. Additional information includes UV-vis spectra of WS2NDs in aqueous solution prepared with different sonication time; HRTEM image of the WS2 nanoparticle whose crystal phase is mostly 2H-WS2; TEM images of the products from different stages of the exfoliation process; SEM images of the WS2, MoS2 starting powders and the sediment of WS2, MoS2 collected after centrifugation; Photographs of WS2 dispersed in solvents with different kinds of surfactants; The TEM image, HRTEM image, UV-vis spectrum and the corresponding height profile of WS2NDs prepared in 0.6 mmol L-1 CTAB solution; XRD patterns of WS2NDs annealed at 250 oC and bulk WS2; XPS peak positions obtained after deconvolution; Relative percentages of 1T-WS2, 2HWS2 and atom percentage of element tungsten for the 4 samples described in the main text; EDX spectrum of WS2NDs; XRD of MoS2NDs and bulk MoS2; XPS spectra for bulk MoS2, MoS2NDs samples; Extracted relative fractions of 2H and 1T components of MoS2NDs; The XPS patterns of S 2p for the five samples; TEM image, height profile, and UV-vis spectrum of exfoliated MoS2NDs prepared in both 0.3 mmol L-1 F108 and 0.6 mmol L-1 CTAB solution; The TEM image and UV-vis sperum of exfoliated graphite nanodots. This material is available free of charge via the Internet at http://pubs.acs.org.

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; Fax: +86 431 85689278


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. 21175124, 21435005).

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ToC figure


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Enhanced Catalytic Activities of Surfactant-Assisted Exfoliated WS2

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