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Enhanced electrocatalytic hydrogen evolution from large scale, facile prepared, highly crystalline WTe2 nanoribbon with Weyl semimetallic phase Jie Li, Meiling Hong, Leijie Sun, Wenfeng Zhang, Haibo Shu, and Haixin Chang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13387 • Publication Date (Web): 13 Dec 2017 Downloaded from http://pubs.acs.org on December 15, 2017

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ACS Applied Materials & Interfaces

Enhanced electrocatalytic hydrogen evolution from large scale, facile prepared, highly crystalline WTe2 nanoribbon with Weyl semimetallic phase Jie Li,1 Meiling Hong,1,2 Leijie Sun,1 Wenfeng Zhang,1 Haibo Shu,3 Haixin Chang1,*

1

Center for Joining and Electronic Packaging, State Key Laboratory of Material

Processing and Die & Mould Technology, School of Materials Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, China 2College of Chemistry and Environmental Engineering, Wuhan Institute of Technology, Wuhan 430073, China 3College of Optical and Electronic Technology, China Jiliang Univeristy, Hangzhou 310018, China

* Email: [email protected]

Abstract: WTe2 is one of most important layered transition metal dichalcogenides (TMDs) and exhibits various prominent physical properties. All the present methods for WTe2 preparation need strict conditions such as high temperature or cannot be applied in large scale which limit the practical applications. In addition, most studies on WTe2 focus on its physical properties while the electrochemical properties are still illusive with little investigation. Here we develop a facile and scalable two-step method to synthesize high quality WTe2 nanoribbon crystal with 1T’Weyl semimetal phase for the first time. We can get highly crystalline 1T’-WTe2 nanoribbon on a large scale through this two-step method. In addition, the electrochemical tests show thatWTe2nanoribbon exhibits smaller overpotential and much better hydrogen evolution reaction (HER) catalytic performance than other tungsten-based sulfide and selenide (WS2, WSe2) nanoribbons under same morphology and preparation condition. 1

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WTe2 nanoribbons show a Tafel slope of 57 mV/dec, which is one of best values for TMDs catalysts and about 2 and 4 times smaller than that 135 mV/dec for 2H-WS2nanoribbonand 213 mV/dec for 2H-WSe2nanoribbon, respectively. 1T’-WTe2nanoribbon also show ultrahigh stability in 5000 cycles and 20 h at 10 mA/cm2. The better performance is attributed to high conductivity of semimetallic 1T’-phase-stable WTe2nanoribbonwith one or two order higher charge transfer rate than normally semiconducting 2H-stable WS2 and WSe2 nanoribbons. These results open the door of electrochemical applications of Weyl semimetallic TMDs.

Keywords: transition metal dichalcogenides, two-dimensional layered materials, WTe2, nanoribbon, Weyl semimetal phase, hydrogen evolution reaction, water splitting, electrocatalysis

INTRODUCTION Layered transition metal dichalcogenides (TMDs) exhibit comprehensive electronic properties with a wide range of application in catalysis,1-7 energy storage,7-11 sensors12-13 and electronics.14-19 Tungstenditellurium (WTe2), one of most important layered TMDs, is now drawing lots of attention.20-22 In contrast with other counterparts such as MoS2,MoSe2, WS2 and WSe2 with stable semiconducting 2H phase under ambient condition, monoclinic metallic phase is the most stable polymorph for WTe2 (1T’ phase). Tungsten atomic plane is sandwiched between two tellurium atomic layers, and tungsten atom is surrounded by six tellurium atoms, forming an octahedral coordination. However tungsten atom slightly shifts from the center of the octahedron due to the strong W–W bond, resulting in the semimetallic state of WTe2.21, 23-24In the view of unique electronic structure, WTe2 exhibits various prominent physical properties including the giant and nonsaturated positive magnetoresistance at low temperature,23, 25-26 superconductivity under high pressure,27-29quantum spin Hall (QSH) effect and the candidate of type-ⅡWeyl semimetal,20, 30-32 which enable the promising application in magnetic storage, spintronics and quantum computations. These make WTe2one of most important 2


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layered TMDs.

However, all the preparation methods either need strict condition such as high temperature or cannot be synthesized in a large scale, which severely limit the practical applications of WTe2. The first kind of synthetic method for WTe2 is based on the direct reaction of tellurium with tungsten for few days under high temperature (about 1000℃) and low pressure. This method can get crystallized bulk WTe2whichalways be used to explore its intrinsic physical properties.21, 23, 25, 33-35

Recent reports reveal that chemical vapor deposition method (CVD) can be

applied to grow WTe2single crystal or WTe2film.36-38 The thickness of resulted WTe2 can be tuned by modulating growth condition. Additionally, molecular beam epitaxy (MBE) is also applied to synthesize high-quality W(Mo)Te2 so that they can directly characterize the intrinsic atomic structure and electronic structure in WTe2.39All the mentioned methods need strict condition or cannot be adapted to large scale for practical applications. Besides, most studies on WTe2 focus on physical properties while the electrochemical properties are still illusive with little investigation. Transition metal dichalcogenides, such as MoS(Se)2, WS(Se)2, have also been proved to be promising candidate for replacing the noble Pt electrode in the field of hydrogen evolution reaction (HER).3, 40-41Nevertheless, one of the assignable questions for MoS(Se)2 and WS(Se)2is the low electrical conductivity of them because the ambient-stable hexagonal 2H phase is semiconducting. Developing nanostructured TMDs with metallic phases have been regarded as the most useful methods to prove the electrical conductivity of TMDs.42-46

Herein, we develop a facile and scalable method to synthesize high qualityWTe2nanoribboncrystal with 1T’Weyl semimetal phase for the first time. First, WO3-xnanoribbon was prepared through a simple hydrothermal method and the prepared WO3-xnanoribbon was tellurized in a CVD system. Through this two-step method we can get highly crystalline 1T’-WTe2nanoribbon on a large scale and the resultant morphology of WTe2 crystal can be tuned by regulating the first step. In 3



addition, WTe2nanoribbon exhibits smaller overpotential and much better catalytic performance than other tungsten-based sulfide and selenide TMDs nanoribbons under same morphology and reacting condition.WTe2nanoribbon show a Tafel slope of 57 mV/dec, which is one of the best values for TMDs catalysts and about 2 and 4 times smaller than that 135 mV/dec for 2H-WS2nanoribbon and 213 mV/dec for 2H-WSe2nanoribbon. 1T’-WTe2 nanoribbon also show high stability in 5000 cycles and 20 h at 10 mA/cm2. The better performance is attributed to high conductivity of 1T’-phase-stable WTe2nanoribbon with one or two order higher charge transfer rate than normally semiconducting 2H-stable WS2 and WSe2nanoribbons.These results open the door of electrochemical application of Weyl semimetals.

EXPERIMENTAL SECTION

Preparation ofWO3-x. Synthesis of WO3 nanoribbons The WO3-x stripes were synsthesized by a facile solvothermal method. Typically, Ammonium metatungstate hydrate (AMT) of0.3g was dissolved in 60mL deionized water. Then thiourea (0.2g) was added into the aqueous solution. After sonication for 10min, the mixture solution was transferred into a 100mL Taflon-lined stainless autoclave and heated to 180℃ for 12 h. After the hydrothermal reaction, the autoclave was naturally cooled down to room temperature. Blue precipitates were collected and washed with deionized water and absolute ethanol for several times. Finally, the resulting precipitates were freeze-dried, grind and get blue WO3-x powder. Synthesis of microspheres WO3 Spherical WO3 was prepared with a modified previous reported method. 0.3g of WCl6 used as precursors for W was dissolved in 60 mL acetic acid with intensive stirring. And then the solution was transferred to a 100mL Taflon-lined stainless autoclave and heated to 180℃ for 12 h. The product was separated by centrifugation and washed with DI water and ethanol for few times. Finally, the resulting precipitates 4


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were freeze-dried.

Preparation of WTe(Se or S)2. A two-zone furnace was used to synthesize WTe2, WSe2 and WS2under similar condition. As for WTe2, 0.4g tellurium powder and 60mg prepared WO3-x powder were put into a quartz boat separately. Subsequently, the quartz boat was loaded into the center of the individual zone. After vaccum the furnace, the heating zone was heated to 650℃ with 20 min and kept this temperature for 1h under a N2/H2 flowing gas (N2=15sccm, H2=20sccm). After the tellurization, the furnace was cooled to room temperature by opening the furnace.

WSe2 and WS2 were prepared through a modified method. According to the low melting point of selenium and sulphur, the selenium or sulphur was placed in a separate quartz boat and loaded at the upper heating zone. The quartz boat with WO3-x was put into the second heating zone maintained at 650℃ during the reaction. The temperature for the upper heating zone was set 200℃ for sulphur, 300℃ for selenium. The two heating zone were heated to the setting temperature at the same time and maintained for 2h. Finally, the furnace was cooled down to room temperature. Characterizations. X-ray diffraction (XRD) patterns was measured by X-ray diffraction meter (PANalytical B.V., Empyrean ) to indentify all of the prepared materials. Raman spectrometer (LabRAM HR800) with a 100× objective and a power of 5mW was used to obtain the corresponding Raman spectra. The morphology of the samples were characterized by field-emission scanning electron microscopy (FE-SEM, FEI, Nova NanoSEM 450).X-ray photoelectron spectroscopy (XPS)was conducted to analyze the binding energy with a XPS spectrometer (AXIS-ULTRA DLD-600W). The scanning transmission electron microscopy (STEM) measurements were made using a Tecnai G2 F30 instrument. Electrochemical Measurements. 5



All the electrochemical measurements were conducted in a typical three-electrode system in 0.5M H2SO4. Glassy-carbon loaded with prepared materials as working electrode. A Ag/AgCl electrode and a graphite rod served as reference and counter electrode, respectively. A 5mg of sample was dispersed in 135 μL DI water and 45 μL ethanol, and then 20 μL of Nafion was added. The mixed solution was sonicated for 30min to get a homogenous catalyst ink. The catalyst ink of 4μLwas dropped onto the surface of a glassy-carbon electrode with 3 mm diameter and dried. All the potentials mentioned in the article were converted to reversible hydrogen electrode (RHE), E(RHE)=E(Ag/AgCl)+0.059*pH+0.197 V. Linear sweep voltammetry (LSV) measurements were conducted between 0 V and −0.8 V versus Ag/AgCl with a scan rate of 5 mV s−1.Impedance measurements were conducted at a selected overpotential in a frequency range of100k−0.01 Hz. Cyclic voltammetry (CV) tests were performed with different scan rates. Electrochemical pre-treatment was performed by applying a potential of +1.4 V (oxidation) and -1.2 V (reduction) for a period of 600 s.

RESULTS AND DISCUSSION

We developed a two-step fabrication process to prepare the tungsten ditelluride nanoribbons as shown in Figure 1 (more details in experimental section). Briefly, we first synthesized the precursor of WO3-x by a solvothermal method (Figure 1a).AMT and thiourea were dissolved into a mixture solution and reacted under 180℃ for 12h. We got light blue precipitates after the hydrothermal reaction. The as-prepared WO3-xwas then tellurized into WTe2at 650℃ under a reducing atmosphere. In Figure 1c, we can see that the blue powder turned into black powder after tellurization, and the product was characterized by XRD and Raman tests.WS2 and WSe2 were also prepared by replacing the tellurium source with sulfur and selenium source, respectively.

Figure 2c shows the XRD pattern of product of the first step before tellurization. The 6


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diffraction peaks at 13.8°, 23.1°, 24.2°, 27.0°, 27.9°, 33.7°, 36.6°, 37.3°, and 39.2° can be assigned to the diffraction from (100), (002), (110), (102), (200), (112), (202), (210) and (211) planes of hexagonal WO3 according the standard diffraction patterns of JCPDS No. 33-1387. We note that the color of the hexagonal WO3 is light blue, which indicate the existence of oxygen vacancy.47X-ray photoelectron spectroscopy (XPS) was conducted to analyze more details about the stoichiometric ratio between tungsten and oxygen. Figures S1a and S1b show the spectrum of the W 4fand O 1s, respectively. It can be seen that the spectra of the W 4f can be deconvoluted into four peaks. The peaks locating at 36.1 and 38.3 eV are attributed to core level of W6+ 4f7/2 and W6+ 4f5/2 and the another two peaks locating at relatively lower binding energy should be identified as W 4f5/2 (35.8 eV) andW4f5/2 (37.9 eV) of W5+,4+confirming the presence of oxygen vacancy.47-48The three deconvoluted peaks at 530.2, 530.7 and 531.1 eV of the spectra of O 1s can be attributed to the binding energy of W-O, W=O and W-OH, respectively. So we should identify the product from the first step as WO3-x.47Figure 1a,b are the SEM images of WO3-x and reveal the nanoribbon morphology of prepared WO3-x.

In order to confirm whether tungsten ditelluride is formed, XRD pattern and Raman spectra of products from the second step was carried out (Figure 3). In Figure 3a, all the peaks located at 12.5°, 14.6°, 29.1°, 32.0°, 34.2° and 38.6°match well with the diffraction of planes of WTe2 for (002), (011), (111), (103), (113), (024), respectively (JCPDS No.24-1352). The XRD result shows that the prepared WTe2 have a orthorhombic structure (Pmn21, with a cell unit of a=3.48, b=6.28 and c=14.05Å), in agreement with the fact that the WTe2is stabilized with orthorhombic structure in contrast to the WS(Se)2 which have a stable 2H phase in nature.49-50Wedon’t find additional peaks from WO3-x in the XRD result, implying that WO3-x can be completely tellurized into WTe2by the second step. Raman spectra of the prepared WTe2 show four prominent peaks at the frequencies of120.9, 140.1, 159.3 and 206.4 cm-1(Figure 3b). Those peaks can be attributed to A31 , A41 , A71 and A91 vibration modes of 1T’semimetallic Weyl phase WTe2which agree well with previous 7



reports.36-37, 51XPS tests were also conducted to analyze the surface states and chemical compositions of the prepared WTe2. XPS survey spectra shows the presence of W and Te in the obtained WTe2 nanoribbon (Figure S2) and Figures3c-dshow the XPS spectra of W 4d and Te 3dwith both of them deconvoluted into four peaks. The Te 3d core level peaks locate at binding energy of 583.6 eV and 573.3 eV can be attributed to Te 3d3/2 and Te 3d5/2 of WTe2 nanoribbon. W 4d3/2, and W 4d5/2peaks of WTe2nanoribbon are detected at 256.6 eV and 243.9 eV, respectively. These results are comparable to previous reports.21, 37-38 Those peaks located at relatively higher binding energy in XPS spectra of W 4d and Te 3d may be due to the oxidation during the ambient exposure. In addition, the molar ratio of W/Te is calculated to be about 1:1.98, very close to the stoichiometric ratio of WTe2.By the way, commercial WO3was also used to prepare WTe2 with the same method.WTe2@defect-rich and WTe2@defect-free were name for synthesized WO3-x with rich defects and commercial WO3, respectively. We find that the product is also WTe2from commercial WO3but crystallinity is not as well as that from synthesized defect-rich WO3 (Figure S3).We believe that the defects in synthesizedWO3-x can promote the tellurization which makes the reaction easier. Considering the phase transition of MoTe2by changing the temperature, higher temperature of tellurization (700℃ and 750℃) were also adopted for preparing WTe2nanoribbon. Figure S4shows the XRD and Raman spectra of as-prepared WTe2nanoribbonwith different tellurization temperature. The XRD results are almost same for different temperature and the additional vibration modes in the Raman spectra to characterize phase change for TMDs are not detected. The results are consistent with previous experimental and theoretical studies that 1T’ phase is the thermodynamic stable polymorph for WTe2.49-50

The microstructure of as-prepared WTe2 nanoribbon was further characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). As shown in Figure 4a, b, the WTe2has a nanoribbon morphology with a length of 3-5μm and width of 100-200 nm and thickness of 20-80 nm, similar with the 8


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precursor of WO3-x without any aggregation after tellurization at 650℃.Moreover, elemental mapping of a single nanoribbon (Figure 4c-e) was further carried out to illustrate the distribution of W and Te species distribute homogenously in the whole nanoribbon. Low-resolution TEM image in Figure 4f also verify the nanoribbon morphology of WTe2. The corresponding selected area electron diffraction (SAED) pattern illustrates that the as prepared WTe2 possesses good crystalline and rectangular SAED pattern further confirms the orthorhombic, 1T’ semimetallic Weyl phase of WTe2 consistent with previous reports.35, 38, 52-53High-resolution TEM images reveal the as-prepared WTe2 possesses a very good crystalline quality. Figure 4h shows the inverse Fourier transform image from the region marked with white square in Figure 4g.The lattice spacing of 0.7 nm can be indexed to the (002) plane of WTe2.EDX spectrum of the resulted WTe2, as shown in Figure 4i, illustrating the both existence of tungsten and telluride element and confirming the ratio of W to Te is 1:2, which is consistent with the previous XPS results.

Note that, to some extent, we can tune the morphology of WTe2 by modulating the morphology of precursor of WO3-x in the first step. For an example, WO3 microspheres are synthesized by tuning the reaction condition (more details in experimental section), as shown in Figure S5. And then, 1T’-WTe2 microspheres are prepared through second step (Figure S6). Like WTe2 nanoribbons, the morphology of WTe2 microspheres is the same as the WO3. Moreover, WS(Se)2 nanoribbon were also synthesized with similar method and the corresponding XRD and Raman tests were performed (supporting information Figure S7). The diffraction peaks in Figure S7a, b match well with the planes of 2H WS2 (JCPDS No.08-0237) and 2H WSe2 (JCPDS No. 38-1388). As shown in Figure S7c, d, the characteristic out-of-plane (A1g ) and in-plane (E12g) Raman-active modes of WS2 and WSe2 exist. We noticed that different phase structures were obtained even with same method. Actually, experimental and theoretical study have shown that W(Mo)S2, W(Mo)Se2 and MoTe2 are 2H phase under ambient conditions. In contrast with other counterparts, 1T’ is the thermodynamic stable polymorph for WTe2.49-50 9



Considering the good conductivity of WTe2, the as-synthesized samples were used to measure HER performance. Figure 5a shows the polarization curves of as-prepared tungsten-based samples and glassy carbon (GC).It is clear that the HER performance of WTe2 is much better than that of other tungsten-based 2H phases WS2 and WSe2samples and has much larger catalytic current. The current density increase abruptly after 0.35 V with an overpotential of 0.43 V at 10 mA/cm2. The Tafel slope plots in Figure 5bwere obtained by converting the polarization curves(η= b log j + a). Compared with WSe2 (213 mV/dec) and WS2 (135 mV/dec), WTe2 has the smallest Tafel slope of 57 mV/dec which is one of the best values for TMDs catalysts. The value of 57 mV/dec is larger than Heyrovsky reaction (40 mV/dec) and smaller than Volmer reaction (120 mV/dec), which demonstrates Volmer−Heyrovsky reaction mechanism is operative during the electrochemical reactions.54-56The electrochemical impedances spectroscopy(EIS) tests of tungesten-based samples were conducted to analyze the charge transfer process. As shown in Figure 5c, three samples display huge difference in charge transfer resistance (Rct), in which the Rct for WSe2, WS2 are two and one order magnitude larger than that of WTe2 (85 Ω).The relatively small Rct indicates the good conductivity of WTe2 benefits the charge transfer during the electrochemical reactions. As discussed in a previous report,57 capacitance values of group 6B chalcogenides are directly related to the electrochemical active surface area. In order to evaluate electrochemical active surface area of prepared samples, the double layer capacitance (Cdl) was measured by the cyclicvoltammetry (CV) method (As shown in Figure 6). The Cdl of WTe2 electrode can be calculated to be0.080μ F/cm2 which is much larger than that of WS2 (0.028μF/cm2)and that of WSe2 (0.012 μF/cm2), indicating the larger effective active surface area of WTe2.As shown in Figure S8, the HER catalytic performance of WTe2sphere was also conducted and the sample displays similar HER behavior with that of nanoribbons. We note that WTe2 nanoribbon show comparable even better Tafel slope but worse overpotential than the 1T’ MoS2/WS2few layers or mixtures of TMDs and conductive materials as claimed in previous reports (Table S1).10, 58-60It may be related with the fact that, compared 10


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with few layer 1T’ MoS2/WS2 nanosheets, WTe2 nanoribbon are much thicker with high crystalline quality and few defects and therefore may lack of active sites and large exposed surfaces of hydrogen evolution. Additionally, the charge transfer ability and catalytic properties can be tuned by reductive or oxidative electrochemical treatment.61-62 As shown in Figure S9, catalytic properties of WTe2 become worse with smaller catalytic current after oxidative electrochemical treatment while smaller overpotential are observed for reductive treatments, which are similar with the result in previous report62. We believe WTe2 would be a potential candidate for HER by inducing more defects in ultrathin few layer WTe2 nanosheets with more exposed surfaces and defects.

In order to elucidate the WTe2 improvement of HER performance, electronic band structures and adsorption free energy of H (ΔGH)are calculated for bilayer2H-WS2, 2H-WSe2 and 1T’-WTe2(Figure 7). As shown in Figure 7a,7b, 7c,2H-WS2 and WSe2 exhibit indirect band gaps with a band gap of 1.74 eV and 1.46 eV for WS2 and WSe2, respectively. Meanwhile, the electronic structure calculation show 1T’-WTe2 is a gapless semimetal meaning that electron transfer rate can be enhanced during the electrochemical catalytic reaction as shown in other reports.58 The calculations are consistent with the above EIS results that 1T’-WTe2 has the smallest charge transfer resistance. To get further insight into the different HER performance among W dichalcogenides, the calculations of adsorption free energy of H (ΔGH) were also conducted. As is well-known, the ΔGH value for a good catalyst should be close to zero.55-56, 63 In Figure7d, the values of ΔGH for WSe2 and WS2 are 2.2 and 2.1 eV, respectively. However, ΔGH for 1T’-WTe2 is much smaller than that of WSe2/WS2 indicating the HER activity is improved. In the view of fact that edge sites of TMDs are active for HER, the distorted octahedral structure of 1T’-WTe2 has two different sites for hydrogen adsorption and the corresponding ΔGH were calculated (as shown in Figure 7). We can see that the value of ΔGH for X site (~0.93 eV) is much smaller than the value of Y site (~1.8 eV). Therefore, we assume that the Te-sites close to W atom should dominated the HER activity for 1T’-WTe2. 11



Furthermore, as one of most important parameters of practical application, the electrochemical stability was also tested by continuous cyclic voltammetry and chronoamperometry (Figure 8). Compared with the initial polarization curve, the polarization curve show no distinct change in the current density even after 5000 cycles, indicating the catalyst is very stable during electrochemical hydrogen evolution. The SEM image of 1T’-WTe2after 5000 cycles in Figure S10a, b show no obvious morphology change. In Figure S9c, we can see that the characteristic vibration modes (A71 and A91 ) can be detected indicating the stability 1T’-WTe2of after the long-term HER test. The chronoamperometry test also demonstrates the excellent stability of WTe2 electrode in 0.5 M H2SO4 solution. Even though there is some fluctuation at the beginning few hours, which may be due to the appearance of bubbles on the electrode surface, the electrode scan keep efficiently working at ~ 10 mA/cm2 for 20 hours with no degradations.

CONCLUSION In this study, a facile and scalable two-step method was developed to synthesize 1T’-WTe2nanoribbon with large scale and high quality for the first time. We can tune the morphology of WTe2 with unique electrochemical HER properties. The HER catalytic performance of 1T’-WTe2 nanoribbon is very stable under long time, continuous HER tests and much better than that of 2H-WS2 and 2H-WSe2nanoribbon prepared from similar method due to the high conductivity from semimetallic phase of WTe2. The results show the great potentials of TMDs semimetals.

Acknowledgements. The authors would like to acknowledge support from the National Natural Science Foundation of China (grant nos.51402118,61775201,51502101, 61674063), National Key Research and Development Program of China (No. 2016YFB070070-2), National Basic Research Program of China (No. 2015CB258400), and the Fundamental Research Funds for the Central Universities in Huazhong University of Science and Technology (grant no. 2015QN006). AFM, XRD, Raman, SEM, TEM and XPS tests from Analytical Center of Huazhong University are acknowledged. 12


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Figures

Figure 1 Schematic diagram of the experimental setup.(a) First step, fabrication of WO3-xnanoribbon by hydrothermal method and (b) Second step, tellurization of WO3-xnanoribbon to synthesize 1T’-WTe2nanoribbon. (c) The obtained WO3-x (light blue powder) and 1T’-WTe2nanoribbons (black powder) from (a) and (b) step, respectively.

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Figure 2 (a, b) SEM images from prepared WO3-xnanoribbon. (c) XRD Pattern for WO3-xnanoribbon.

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Figure 3 (a), (b) The XRD and Raman characterization of the as synthesized WTe2nanoribbon. (c), (d) XPS spectra of the Te 3d (c) and W 4d (d) of WTe2nanoribbon.

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Figure 4 Morphology and composition analysis of WTe2nanoribbon.(a-c) SEM images of as prepared WTe2nanoribbon, and the corresponding Te, W elemental EDX mappings (d,e). (f) Low-resolution TEM images of WTe2nanoribbon. Inset: the corresponding electron diffraction pattern. (g) High-resolution TEM image of (f). (h) The inverse Fourier transform image from the region marked with white square in (g). (i) EDX spectrum of the resulted WTe2 nanoribbon. 21



Figure 5 (a) Polarization curves of glass carbon (GC) and WS2, WSe2, WTe2 nanoribbon catalysts. (b) Tafel plots obtained from the polarization curves. (c) Nyquist plots of different samples. (d) Linear fitting of the capacitive currents of the catalysts vs. scan rates.

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Figure 6 (a-c) Cyclic voltammetry curves of WS2(a), WSe2(b) and WTe2(c) in the region of 0-0.1 V vs Ag/AgCl at different scan ratesof20,40,60,80,100,120,140,160,180,200 mV/s

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Figure 7 (a-c) The atomic structure and corresponding electronic band structures for bilayer 1T’-WTe2 (a), 2H-WSe2 (b) and 2H-WS2 (c), respectively. (d) X, Y denote two different adsorption sites of 1T’-WTe2 structure (left); Calculated adsorption free energy of H (ΔGH) for 2H-WSe2, 2H-WS2, 1T’-WTe2-Y and 1T’-WTe2-X (right).

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Figure 8 Cycling stability of 1T’-WTe2nanoribbon catalysts. (a) Cycling test for WTe2nanoribbon catalyst before and after 1000 and 5000 polarization scanning cycles. (b) Longtime HER stability in 20 h with a current density of about 10mA/cm2.

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Enhanced Electrocatalytic Hydrogen Evolution ... - ACS Publications

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