Vertically Aligned Ultrathin 1T-WS2 Nanosheets Enhanced the Electrocatalytic Hydrogen Evolution

Vertically Aligned Ultrathin 1T-WS2 Nanosheets Enhanced the Electrocatalytic Hydrogen Evolution Efficient evolution of hydrogen through electrocatalysis holds tremendous promise for clean energy. The catalytic efficiency for hydrogen evolution reaction (HER) strongly depends on the number and activity of active sites. To this end, making vertically aligned, ultrathin, and along with rich metallic phase WS nanosheets is effective to maximally unearth the catalytic performance of WS nanosheets. Metallic 1T polymorph combined with vertically aligned ultrathin WS nanosheets on flat substrate is successfully prepared via one-step simple hydrothermal reaction. The nearly vertical orientation of WS nanosheets enables the active sites of surface edge and basal planes to be maximally exposed. Here, we report vertical 1T-WS nanosheets as efficient catalysts for hydrogen evolution with low overpotential of 118 mV at −2 −1 10 mA cm and a Tafel slope of 43 mV dec . In addition, the prepared WS nanosheets exhibit extremely high stability in acidic solution as the HER catalytic activity and show no degradation after 5000 continuous potential cycles. Our results indicate that vertical 1T-WS nanosheets are attractive alternative to the precious platinum benchmark catalyst and rival MoS materials that have recently been heavily scrutinized for hydrogen evolution. Keywords: Electrocatalysis, Hydrogen evolution reaction, WS nanosheets, Metallic 1T phase Background due to their high abundance and cost-efficiency [22–27]. Hydrogen, as a clean fuel, has been considered as a However, bulk WS is a poor HER catalyst. At present, promising alternative for traditional fossil fuels in the fu- the effective routs for the synthesis of monolayer or few ture [1, 2]. A tremendous amount of effort thus has been layers TMDCs nanosheets are chemical exfoliation and made to pursue sustainable and efficient hydrogen pro- chemical vapor deposition (CVD). Normally, the chem- duction. The electrocatalytic hydrogen evolution reac- ical exfoliation needs n-butyllithium, which is a danger- tion (HER) is considered one of the most important ous solvent resulting from the highly pyrophoric pathways to produce hydrogen efficiently [3–5]. The property in air [28–31]. CVD method incurs expensive most effective HER electrocatalysts up to now are based apparatus, high temperature, and vacuum [32–34]. noble metals (e.g., platinum and palladium) [6, 7]. How- Therefore, an effective and environment-friendly strategy ever, the high cost and scarcity of noble metals largely for large-scale preparation of ultrathin WS nanosheets impede their practical utilization. Therefore, developing is highly desirable. effective HER electrocatalysts with cheap and earth Both experimental and computational studies confirm abundance still remains urgent. that the HER activity of TMDCs was mainly resulting In the search for nonprecious metal catalysts for the from the rare edge surfaces, rather than basal planes HER, transition metal dichalcogenides (TMDCs) have [35, 36]. Stimulated by this understanding, intense in- been proposed as promising candidates [8–21]. WS - vestigations have been concentrated on developing based electrocatalysts have been extensively investigated highly nanostructured TMDCs to maximize the num- ber of exposed edge sites, including crystalline and amorphous materials [37–41], metallic 1T polymorph * Correspondence: wanglonglu@hnu.edu.cn; yinkai@hnu.edu.cn [42, 43], vertically aligned structures [44, 45], and molecu- State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, People’s Republic of China lar mimics [46]. Although outstanding accomplishment, Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. He et al. Nanoscale Research Letters (2018) 13:167 Page 2 of 9 many actual challenges yet need to enhance the activity Characterization and stability of WS -based catalysts. The morphologies and microstructures of WS nano- 2 2 Herein, we highlight a pathway to fulfill the assign- sheets were characterized via field emission scanning ment. Ultrathin WS nanosheets with perpendicular electron microscope (FESEM, Hitachi, Japan) and trans- orientation and 1T metallic phase feature exhibit high mission electron microscopy (TEM, Tecnai F20). The activity and stability towards HER in acidic water. Its fast energy-dispersive X-ray spectroscopy (EDS) mapping −1 kinetic metrics (e.g., the Tafel slope of 43 mV dec ) images were captured on a Tecnai G2 F20 S-TWIN indicate superior electrocatalytic activity. This study atomic resolution analytic microscope. The binding en- hints at the promise of cheap and efficient HER ergies of W and S were determined by X-ray photoelec- electrocatalysts by one-step hydrothermal process. tron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific, England) using an Al-Kα X-ray source. Experimental Section Electrochemical Measurements Synthesis of the Vertical 1T-WS Nanosheets All electrochemical measurements were performed at Vertical 1T-WS nanosheets were manufactured by a room temperature on a standard three-electrode electro- simple hydrothermal method on titanium substrate. In a lytic system. The saturated calomel electrode (SCE), car- typical procedure, thiourea (CS(NH ) , i.e., 0.4104 g) and bon stick electrode and titanium substrate growth 2 2 hexaammonium heptatungstate ((NH ) W O , i.e., 0. directly with WS nanosheets were served as reference 4 6 7 24 267 g) were dissolved in 32 mL deionized water under electrode, counter and working electrode, respectively. vigorous stirring to form a homogeneous solution. Ti- As for reference, titanium substrate with deposited Pt/C −2 tanium substrate (1 × 4 cm) was carefully cleaned with and WS nanosheets (approximately 100 μgcm ) also concentrated hydrochloric solution, deionized water, and was regarded as working electrode. The HER activities absolute ethanol in an ultrasound bath each for 10 min. were conducted by linear sweep voltammetry (LSV) −1 The titanium substrate (against the wall) and the aque- solution with a scan rate of 5 mV s . The stability was ous solution were transferred to a 40 mL Teflon-lined tested by taking continuous cyclic voltammograms at a −1 stainless steel autoclave. The autoclave was sealed and scan rate of 50 mV s from − 0.4 to 0.1 V with maintained at 200 °C for 7 h and then enabled to cool 5000 cycles. The striking stability was further down to room temperature within 15 min using cooling demonstrated by using chronoamperometry (j~t) at water. A dark thin film was extracted from the autoclave 160 mV. All the measurements were performed in 0.5 M and subsequently rinsed with deionized water and abso- H SO without iR compensated. The electrolyte solution 2 4 lute ethanol, and dried at 60 °C under vacuum. The was purged with high purity nitrogen (N ) for half an loading mass of WS nanosheets was determined by hour to remove the dissolved oxygen before testing. weighing the titanium substrate before and after hydro- Under without special emphasis, all the potentials were thermal process; a surface density of approximately here referenced to the reversible hydrogen electrode −2 100 μgcm was obtained. (RHE) using the following equation: EðÞ RHE ¼ EðÞ SCE þ 0:24 V þ 0:059  pH Synthesis of the Flat 1T-WS Nanosheets For the synthesis of flat 1T-WS nanosheets, 0.267 g (NH ) W O and 0.4104 g CS(NH ) were dissolved in Results and Discussion 4 6 7 24 2 2 32 mL deionized water under vigorous stirring to form a Characterization Supports of Catalysts clear solution. Then, the solution was transferred into a Figure 1a shows the scanning electron microscopy 40 mL Teflon-lined stainless steel autoclave, maintained (SEM) image of the prepared vertical 1T-WS nano- at 200 °C for 7 h, and allowed to cool to room sheets with dimensions of ca. 2 μm, which indicated that temperature naturally. The final product was washed nanosheets were exceedingly large. As shown in Fig. 1b, with deionized water and absolute ethanol for several the nanosheets are nearly perpendicular to the electrode times and dried at 60 °C under vacuum. Specifically, Ti substrate, which facilitates the exposure of WS edge the obtained WS catalyst was dispersed in an ethanol sites as edge-oriented grapheme on carbon nanofiber −1 solution with a concentration of 0.8 mg ml .Then, [47]. The cross profile of vertical 1T-WS nanosheets is we loaded the WS catalyst or Pt/C on titanium shown in Additional file 1: Figure S1. Meanwhile, criss- substrate by a drop-casting method with a mass load- cross rather than stack occurred between nanosheets. −2 ing of approximately 100 μgcm as well. All the Such an open structure is supposed to allow the fast materials were purchased from SinoPharm and used transportation of proton throughout the catalyst and without further purification. utilize the basal planet sites for HER as well. Vertical He et al. Nanoscale Research Letters (2018) 13:167 Page 3 of 9 1.5 1.8 2.1 2.4 2.7 Size (µ m) c d 200 nm 100 nm Fig. 1 a–b Top-down SEM image of the prepared vertical 1T-WS nanosheets on Ti substrate. c–d HAADF-STEM of 1T-WS nanosheets 2 2 1T-WS nanosheets in Fig. 1c are extremely transparent, The HAADF-SEM image (Fig. 2a) and homogeneously implying that formed nanosheets were ultrathin. The no- distributed W and S component elements from the ticeable distortion of nanosheets (Fig. 1d) helps to de- corresponding energy-dispersive X-ray (EDX) mapping crease their high surface energy to make the WS stable (Fig. 2b, c) further reveal the successful synthesis of WS 2 2 as independent ultrathin nanosheet units. Meanwhile, nanosheets. In addition, the elemental mapping overlap- the luminous line in Fig. 1c, d indicated that prepared ping of S and W (Fig. 2d) was dovetailing well and WS nanosheets hold excellent conductivity, which is evidenced convincingly the WS nanosheets formed. 2 2 vital for electrocatalytic HER. Meanwhile, elemental analysis using EDS shows the Fig. 2 HAADF-STEM image (a) and corresponding elemental mapping (b for S, c for W, d for S and W) for the 1T-WS nanosheets Percentage (%) He et al. Nanoscale Research Letters (2018) 13:167 Page 4 of 9 homogeneous distribution of W and S in WS nano- nanosheets, which are susceptible to oxidation [28]. It is sheets (Additional file 1: Figure S2). worth noting that a slight oxidation of TMDs can im- The precise microscopic knowledge of nanostructure prove the density of the active sites, which can enhance materials is of fundamental importance. In Fig. 3a, the the catalytic activities of nanosheets. Nonetheless, ex- high-resolution TEM image (HRTEM) shows the disor- haustive oxidation should be avoided [10]. The relative dered structure of WS nanosheets. Moreover, these percentages of 1T-WS and 2H-WS obtained by inte- 2 2 2 WS nanosheets with a thickness of about four layers gration of the W4f peak were 70 and 30%, respect- 2 7/2 are dominated by well-defined crystalline edges, thus in- ively. Such high concentration of the metallic phase in creasing the density of active sites. To better understand WS nanosheets may lead to a dramatic enhancement in the atomic structure, we have further utilized the Z- the catalytic activities [30]. Such phase conformation contrast. As shown in Fig. 3b, c, the crystal structure of was desired in electrocatalytic hydrogen evolution. Sim- the sheets is not the hexagonal packing usually observed ultaneously, S 2p region of the spectra (Fig. 4b), the for 2H-WS but rather corresponding to 1T-WS struc- peaks located at 161.6 and 162.7 eV, are assigned to 2 2 ture. It is obvious that S atoms are evenly distributed be- S2p and S2p , respectively [49]. Moreover, the atom 3/2 1/2 tween the W and W sites to form a 1T phase, as shown ratio of W and S in the vertical 1T-WS nanosheets by in Fig. 3d. Meanwhile, metallic 1T phase could be XPS and ICP (in Additional file 1: Table S1) was 1:1.96 converted into semiconducting 2H phase after 300 °C and 1:1.94, respectively. annealing treatment, as shown in Additional file 1: Raman spectroscopy measurements were also per- Figure S3. formed to further confirm the phase classification. X-ray photoelectron spectroscopy (XPS) was able to Figure 5a presents Raman spectra collected from vertical confirm the chemical state and composition. All XPS 1T-WS nanosheets grown on Ti substrate. Due to spectra were calibrated using the C 1s peak at 284.8 eV. the polarization dependence, out-of-plane A is Meanwhile, XPS could distinguish 1T- and 2H-WS as preferentially excited for edge-terminated nanosheets, well. As shown in Fig. 4a, the 2H-WS features two whereas the in-plane E is preferentially excited for 2 2g characteristic peaks at around 34.49 and 31.94 eV, corre- terrace-terminated nanosheets, as illustrated in Fig. 5b. −1 sponding to W4f and W4f of 2H-WS components, The characteristic Raman shifts at 343 and 411 cm 5/2 7/2 2 1 1 respectively, while the 1T-WS displays the presence of expected for the E and A were clearly observed, 2 2g g new chemical species clearly shifted toward lower respectively [50]. In addition, the additional peaks in the binging energies (33.54 and 31.29 eV, corresponding to lower frequency regions were previously referred as W4f and W4f of 1T-WS components) [48]. The J1, J2, and J3, corresponding to modes that were only 5/2 7/2 2 result suggests nanosheets were the mixture of 1T- and in 1T-type WS and not allowed in 2H-WS [22]. In 2 2 2H-WS . The nanosheets also contain a small amount of the Additional file 1:FigureS4,theJ1, J2,andJ3 tungstate, as evidenced by the signal at 35.14 eV, which peaks after annealing were quenched, which also verify corresponds to a W4f species. These results are con- the transformation from 1T phase to 2H phase. These 7/2 sistent with the known metallic nature of 1T-WS interpretations together with the aforementioned ab c Fig. 3 HRTEM image of a vertical 1T-WS nanosheets and b, c false-color images responding to the amplification of a. Intensity profiles along the light-blue line indicated in image b is shown in image d He et al. Nanoscale Research Letters (2018) 13:167 Page 5 of 9 Fig. 4 XPS spectra of W 4f (a) and S 2p (b) binding energy of vertical 1T-WS nanosheets characterization results solidly confirm the formation of TMDCs HER catalysts to minimizing ohmic loss, as vertical 1T-WS nanosheets. the interlayer conductivity is 2 order of magnitude lower than intralayer conductivity [8, 51]. Electrons Evaluation of Electrocatalytic Activity are required to traverse the van der Waals gaps to To assess electrocatalytic performance of vertical 1T- move between the individual layers; therefore, vertical WS nanosheets in HER, measurements are performed nanostructure does favor for electrons shuttle [44]. in a 0.5 M H SO solution using a typical three- Besides, the vertical 1T-WS nanosheets after anneal- 2 4 2 electrode cell setup. For reference purposes, Ti substrate ing at 300 °C were investigated as well (in with a drop-cast commercial Pt benchmark (Pt/C) and Additional file 1:FigureS5),and the hydrogen evolu- WS nanosheets catalysts has also been used as the tion performance significantly decrease. working electrode. Tafel plot in Fig. 6b is used to determine the Tafel The polarization curves of all samples are shown in slope, which is an important parameter describing HER Fig. 6a. The vertical 1T-WS nanosheets exhibit a activity of catalysts. The linear part of vertical 1T-WS 2 2 low overpotential of 118 mV (V vs RHE), compared nanosheets Tafel plot under small overpotential is fitted −1 to the overpotential of 230 mV for WS nanosheets to give a Tafel slope of 43 mV dec , which is smaller −2 at 10 mA cm . It indicated that rich metallic than those of previously reported values (in Table 1 and polymorph (~ 70%) in basal planes and exposed edge Additional file 1: Table S2, including WS /MoS -based 2 2 sites of vertical 1T-WS nanosheets can significantly catalysts). Tafel slope is associated with the elementary increase the electrochemical HER activity. In steps in HER. The first step of HER is a discharge step addition, the structure of vertical 1T-WS nanosheets (Volmer reaction, Eq. 1) in which protons are adsorbed guarantees efficient charge flow from the conductive to active sites on the surface of the catalysts and support to active surface site along individual layers. combined with electrons to form adsorbed hydrogen It is in fact a general consideration in designing atoms. It is followed by a desorption step (Heyrovsky Fig. 5 a Raman spectrum of vertical 1T-WS nanosheets. b Schematics of preferentially excited A Raman mode for edge-terminated nanosheets 2 g (top) and E mode for terrace-terminated nanosheets (bottom) 2g He et al. Nanoscale Research Letters (2018) 13:167 Page 6 of 9 a b 0 0.5 -10 Vertical 1T-WS nanosheets -20 0.4 Pt/C -30 WS nanosheets -40 0.3 -1 b=52 mV dec -50 -60 0.2 -70 Veritical 1T-WS nanosheets -1 b=43 mV dec -80 Pt/C 0.1 WS nanosheets -90 -1 b=30 mV dec -100 0.0 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.0 0.4 0.8 1.2 1.6 Potential (V versus RHE) Decade (log|j|) cd -10 -20 -20 -40 -30 -60 -40 Initial -80 After 5000 cycles -50 -100 -60 -0.3 -0.2 -0.1 0.0 0.1 0.2 0 5 10 15 20 25 30 Potential (V versus RHE) Time (h) Fig. 6 a Polarization curves and b Tafel plots of Pt/C, WS nanosheets, and vertical 1T-WS nanosheets in 0.5 M H SO at a scan rate of 2 2 2 4 5mV/s. c Durability test showing negligible current loss even after 5000 CV cycles and d time dependence of the current density curve at an overpotential of 160 mV versus RHE for vertical 1T-WS nanosheets (no iR compensation) Table 1 Summary of literature catalytic parameters of various WS or WS -based catalysts, recently 2 2 −1 −2 Catalysts Onset overpotential [mV] Tafel slopes [mV decade ] η@j = 10mA cm [mV] Ref. 1T-WS nanosheets ~ 100 60 250 [22] WS nanoribbons – 109 > 420 [24] WS NRs-CH OH – 86 260 2 3 WS NRs-H O – 68 225 2 2 WS NRs-250 °C – 97 313 2H-WS nanoflake 100 48 – [25] WS NDs 90 51 [26] WS NDs − 300 °C 180 59 Bulk-WS 270 119 2H-WS nanosheets 60 72 ~ 160 [54] 2H-WS 282 110 – [55] Au/2H-WS 233 57.5 – Annealed WS 140 – [56] WS /rGO 150–200 58 – WS nanotubes – 113 – [57] VA WS nanosheets 30 61 136 [58] WS nanosheets – 97 236 [59] rGO/WS nanosheets – 73 229 1T-WS nanosheets 100 43 118 This work -2 -2 j (mA cm ) j(mA cm ) geo geo -2 j(m mA c ) Overpotential(V verse RHE) geo He et al. Nanoscale Research Letters (2018) 13:167 Page 7 of 9 reaction, Eq. 2) or a combination step (Tafel reaction, effectively. Hence, such nanostructure catalysts com- Eq. 3)[52, 53]. bined with the scalability of the hydrothermal synthe- sis can be readily applied in diverse water electrolysis þ − H O þ e →H þ H O ð1Þ 3 ads 2 as low-cost, high-performance, and stable HER catalyst. þ − H þ H O þ e →H þ H O ð2Þ ads 3 2 2 H þ H →H ð3Þ ads ads 2 Additional file Additional file 1: Fig S1. The cross profile SEM image of the prepared Under a special set of conditions, when the Volmer re- vertical 1T-WS nanosheets on Ti substrate. Fig S2. Whole-energy spectra action is the rate-determining step of HER, a slop of ca. of vertical 1T-WS nanosheets. Fig S3. (a) and (b) are false-color images −1 responding to vertical 1T-WS nanosheets transform into 2H-WS 2 2 120 mV dec should result, while a rate-determining nanosheets after 300 °C annealing treatment, respectively. Fig S4. Raman Heyrovsky of Tafel reaction should produce slope of ca. spectrum of vertical 1T-WS nanosheets (bottom) transform into 2H-WS 2 2 −1 30 and 40 mV dec , respectively [52, 53]. In this work, nanosheets (up) after 300 °C annealing treatment. Fig S5. Polarization curves of vertical 1T-WS nanosheets after annealing at 300 °C in 0.5 M it seems that free energy barrier of discharge step is H SO at a scan rate of 5 mV/s. Fig S6. Variation of current density versus 2 4 reduced to be comparable with that of the following the potential as a function of the pH for the vertical 1T-WS nanosheets. The desorption or combination step, resulting in the slope of highest current density is obtained for the lowest pH, consistent with the −1 solution having the highest proton concentration. Table S1. Element 43 mV dec for vertical 1T-WS nanosheets. Mean- analyses of the vertical 1T-WS nanosheets. Table S2. Summary of literature while, the key step in HER is the adsorption of the pro- catalytic parameters of various MoS or MoS -based catalysts, recently. 2 2 ton on the active site. To asses this, we have varied the (DOCX 1570 kb). pH, as shown in Additional file 1: Figure S6. We found that the vertical 1T-WS nanosheets are active over a Abbreviations wide range of pH although the activity decreases when CVD: Chemical vapor deposition; EDS: Energy-dispersive X-ray spectroscopy; increasing the pH from 0 to 7, which results from the HER: Hydrogen evolution reaction; LSV: Linear sweep voltammetry; strong diminution of the quantity of protons available. RHE: Eversible hydrogen electrode; SCE: Saturated calomel electrode; TEM: Transmission electron microscopy; TMDCs: Transition metal Stability is another important criterion for electrocata- dichalcogenides; XPS: X-ray photoelectron spectroscopy lysts. To assess the long-term durability of vertical 1T- WS nanosheets in an acid environment, continuous Funding HER by CV in the cathodic potential window at an ac- This work was supported by the National Natural Science Foundation of celerated scanning rate of 5 mV/s were conducted. The China (51478171 and 51778218). polarization curves before and after cycling are recorded under quasi-equilibrium conditions. Polarization curves Availability of Data and Materials after the 5000 cycles almost overlay the curve of the ini- We declared that materials described in the manuscript, including all tial cycle with negligible loss of cathodic current, as relevant raw data, will be freely available to any scientist wishing to use them for non-commercial purposes, without breaching participant shown in Fig. 6c. It confirms that vertical 1T-WS nano- confidentiality. sheets are stable in acidic electrolyte and remain intact through repeated cycling. Meanwhile, vertical 1T-WS Authors’ Contributions nanosheets associated ability to continuously catalyze The work presented here was carried out in collaboration between all the generation of H was examined using chronoam- the authors. QH, KY, and LW synthesized and characterized the prepared perometry (j-t). This quasi-electrolysis process was catalysts, analyzed the data, performed the statistical analysis, and wrote the manuscript. LW and SL conceived the idea of the study and carefully conducted at a constant of 160 mV in 0.5 M H SO 2 4 checked the manuscript. All authors discussed the results and commented on (Fig. 6d). Remarkably, the H evolution can proceed the manuscript. All authors read and approved the final manuscript. −2 at a sustained current density of − 21 mA cm even over 30 h of continuous operation, indicating the Competing Interests ultrahigh stability of vertical 1T-WS nanosheets. The authors declare that they have no competing interests. Conclusions In summary, we have developed a simple, eco-friendly, Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in and effective hydrothermal method for the synthesis published maps and institutional affiliations. of vertical 1T-WS nanosheets. The vertical 1T-WS 2 2 nanosheets, with metallic polymorph and exposed Author details State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan edge sites, represent a novel structure of layered ma- University, Changsha 410082, People’s Republic of China. School of Physics terials. The unique structure paves the ways to utilize and Electronics, Hunan University, Changsha 410082, People’s Republic of the edges and planes of layered materials more China. He et al. Nanoscale Research Letters (2018) 13:167 Page 8 of 9 Received: 21 March 2018 Accepted: 8 May 2018 22. 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Vertically Aligned Ultrathin 1T-WS2 Nanosheets Enhanced the Electrocatalytic Hydrogen Evolution

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Abstract

Efficient evolution of hydrogen through electrocatalysis holds tremendous promise for clean energy. The catalytic efficiency for hydrogen evolution reaction (HER) strongly depends on the number and activity of active sites. To this end, making vertically aligned, ultrathin, and along with rich metallic phase WS nanosheets is effective to maximally unearth the catalytic performance of WS nanosheets. Metallic 1T polymorph combined with vertically aligned ultrathin WS nanosheets on flat substrate is successfully prepared via one-step simple hydrothermal reaction. The nearly vertical orientation of WS nanosheets enables the active sites of surface edge and basal planes to be maximally exposed. Here, we report vertical 1T-WS nanosheets as efficient catalysts for hydrogen evolution with low overpotential of 118 mV at −2 −1 10 mA cm and a Tafel slope of 43 mV dec . In addition, the prepared WS nanosheets exhibit extremely high stability in acidic solution as the HER catalytic activity and show no degradation after 5000 continuous potential cycles. Our results indicate that vertical 1T-WS nanosheets are attractive alternative to the precious platinum benchmark catalyst and rival MoS materials that have recently been heavily scrutinized for hydrogen evolution. Keywords: Electrocatalysis, Hydrogen evolution reaction, WS nanosheets, Metallic 1T phase Background due to their high abundance and cost-efficiency [22–27]. Hydrogen, as a clean fuel, has been considered as a However, bulk WS is a poor HER catalyst. At present, promising alternative for traditional fossil fuels in the fu- the effective routs for the synthesis of monolayer or few ture [1, 2]. A tremendous amount of effort thus has been layers TMDCs nanosheets are chemical exfoliation and made to pursue sustainable and efficient hydrogen pro- chemical vapor deposition (CVD). Normally, the chem- duction. The electrocatalytic hydrogen evolution reac- ical exfoliation needs n-butyllithium, which is a danger- tion (HER) is considered one of the most important ous solvent resulting from the highly pyrophoric pathways to produce hydrogen efficiently [3–5]. The property in air [28–31]. CVD method incurs expensive most effective HER electrocatalysts up to now are based apparatus, high temperature, and vacuum [32–34]. noble metals (e.g., platinum and palladium) [6, 7]. How- Therefore, an effective and environment-friendly strategy ever, the high cost and scarcity of noble metals largely for large-scale preparation of ultrathin WS nanosheets impede their practical utilization. Therefore, developing is highly desirable. effective HER electrocatalysts with cheap and earth Both experimental and computational studies confirm abundance still remains urgent. that the HER activity of TMDCs was mainly resulting In the search for nonprecious metal catalysts for the from the rare edge surfaces, rather than basal planes HER, transition metal dichalcogenides (TMDCs) have [35, 36]. Stimulated by this understanding, intense in- been proposed as promising candidates [8–21]. WS - vestigations have been concentrated on developing based electrocatalysts have been extensively investigated highly nanostructured TMDCs to maximize the num- ber of exposed edge sites, including crystalline and amorphous materials [37–41], metallic 1T polymorph * Correspondence: wanglonglu@hnu.edu.cn; yinkai@hnu.edu.cn [42, 43], vertically aligned structures [44, 45], and molecu- State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan University, Changsha 410082, People’s Republic of China lar mimics [46]. Although outstanding accomplishment, Full list of author information is available at the end of the article © The Author(s). 2018 Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. He et al. Nanoscale Research Letters (2018) 13:167 Page 2 of 9 many actual challenges yet need to enhance the activity Characterization and stability of WS -based catalysts. The morphologies and microstructures of WS nano- 2 2 Herein, we highlight a pathway to fulfill the assign- sheets were characterized via field emission scanning ment. Ultrathin WS nanosheets with perpendicular electron microscope (FESEM, Hitachi, Japan) and trans- orientation and 1T metallic phase feature exhibit high mission electron microscopy (TEM, Tecnai F20). The activity and stability towards HER in acidic water. Its fast energy-dispersive X-ray spectroscopy (EDS) mapping −1 kinetic metrics (e.g., the Tafel slope of 43 mV dec ) images were captured on a Tecnai G2 F20 S-TWIN indicate superior electrocatalytic activity. This study atomic resolution analytic microscope. The binding en- hints at the promise of cheap and efficient HER ergies of W and S were determined by X-ray photoelec- electrocatalysts by one-step hydrothermal process. tron spectroscopy (XPS, K-Alpha 1063, Thermo Fisher Scientific, England) using an Al-Kα X-ray source. Experimental Section Electrochemical Measurements Synthesis of the Vertical 1T-WS Nanosheets All electrochemical measurements were performed at Vertical 1T-WS nanosheets were manufactured by a room temperature on a standard three-electrode electro- simple hydrothermal method on titanium substrate. In a lytic system. The saturated calomel electrode (SCE), car- typical procedure, thiourea (CS(NH ) , i.e., 0.4104 g) and bon stick electrode and titanium substrate growth 2 2 hexaammonium heptatungstate ((NH ) W O , i.e., 0. directly with WS nanosheets were served as reference 4 6 7 24 267 g) were dissolved in 32 mL deionized water under electrode, counter and working electrode, respectively. vigorous stirring to form a homogeneous solution. Ti- As for reference, titanium substrate with deposited Pt/C −2 tanium substrate (1 × 4 cm) was carefully cleaned with and WS nanosheets (approximately 100 μgcm ) also concentrated hydrochloric solution, deionized water, and was regarded as working electrode. The HER activities absolute ethanol in an ultrasound bath each for 10 min. were conducted by linear sweep voltammetry (LSV) −1 The titanium substrate (against the wall) and the aque- solution with a scan rate of 5 mV s . The stability was ous solution were transferred to a 40 mL Teflon-lined tested by taking continuous cyclic voltammograms at a −1 stainless steel autoclave. The autoclave was sealed and scan rate of 50 mV s from − 0.4 to 0.1 V with maintained at 200 °C for 7 h and then enabled to cool 5000 cycles. The striking stability was further down to room temperature within 15 min using cooling demonstrated by using chronoamperometry (j~t) at water. A dark thin film was extracted from the autoclave 160 mV. All the measurements were performed in 0.5 M and subsequently rinsed with deionized water and abso- H SO without iR compensated. The electrolyte solution 2 4 lute ethanol, and dried at 60 °C under vacuum. The was purged with high purity nitrogen (N ) for half an loading mass of WS nanosheets was determined by hour to remove the dissolved oxygen before testing. weighing the titanium substrate before and after hydro- Under without special emphasis, all the potentials were thermal process; a surface density of approximately here referenced to the reversible hydrogen electrode −2 100 μgcm was obtained. (RHE) using the following equation: EðÞ RHE ¼ EðÞ SCE þ 0:24 V þ 0:059  pH Synthesis of the Flat 1T-WS Nanosheets For the synthesis of flat 1T-WS nanosheets, 0.267 g (NH ) W O and 0.4104 g CS(NH ) were dissolved in Results and Discussion 4 6 7 24 2 2 32 mL deionized water under vigorous stirring to form a Characterization Supports of Catalysts clear solution. Then, the solution was transferred into a Figure 1a shows the scanning electron microscopy 40 mL Teflon-lined stainless steel autoclave, maintained (SEM) image of the prepared vertical 1T-WS nano- at 200 °C for 7 h, and allowed to cool to room sheets with dimensions of ca. 2 μm, which indicated that temperature naturally. The final product was washed nanosheets were exceedingly large. As shown in Fig. 1b, with deionized water and absolute ethanol for several the nanosheets are nearly perpendicular to the electrode times and dried at 60 °C under vacuum. Specifically, Ti substrate, which facilitates the exposure of WS edge the obtained WS catalyst was dispersed in an ethanol sites as edge-oriented grapheme on carbon nanofiber −1 solution with a concentration of 0.8 mg ml .Then, [47]. The cross profile of vertical 1T-WS nanosheets is we loaded the WS catalyst or Pt/C on titanium shown in Additional file 1: Figure S1. Meanwhile, criss- substrate by a drop-casting method with a mass load- cross rather than stack occurred between nanosheets. −2 ing of approximately 100 μgcm as well. All the Such an open structure is supposed to allow the fast materials were purchased from SinoPharm and used transportation of proton throughout the catalyst and without further purification. utilize the basal planet sites for HER as well. Vertical He et al. Nanoscale Research Letters (2018) 13:167 Page 3 of 9 1.5 1.8 2.1 2.4 2.7 Size (µ m) c d 200 nm 100 nm Fig. 1 a–b Top-down SEM image of the prepared vertical 1T-WS nanosheets on Ti substrate. c–d HAADF-STEM of 1T-WS nanosheets 2 2 1T-WS nanosheets in Fig. 1c are extremely transparent, The HAADF-SEM image (Fig. 2a) and homogeneously implying that formed nanosheets were ultrathin. The no- distributed W and S component elements from the ticeable distortion of nanosheets (Fig. 1d) helps to de- corresponding energy-dispersive X-ray (EDX) mapping crease their high surface energy to make the WS stable (Fig. 2b, c) further reveal the successful synthesis of WS 2 2 as independent ultrathin nanosheet units. Meanwhile, nanosheets. In addition, the elemental mapping overlap- the luminous line in Fig. 1c, d indicated that prepared ping of S and W (Fig. 2d) was dovetailing well and WS nanosheets hold excellent conductivity, which is evidenced convincingly the WS nanosheets formed. 2 2 vital for electrocatalytic HER. Meanwhile, elemental analysis using EDS shows the Fig. 2 HAADF-STEM image (a) and corresponding elemental mapping (b for S, c for W, d for S and W) for the 1T-WS nanosheets Percentage (%) He et al. Nanoscale Research Letters (2018) 13:167 Page 4 of 9 homogeneous distribution of W and S in WS nano- nanosheets, which are susceptible to oxidation [28]. It is sheets (Additional file 1: Figure S2). worth noting that a slight oxidation of TMDs can im- The precise microscopic knowledge of nanostructure prove the density of the active sites, which can enhance materials is of fundamental importance. In Fig. 3a, the the catalytic activities of nanosheets. Nonetheless, ex- high-resolution TEM image (HRTEM) shows the disor- haustive oxidation should be avoided [10]. The relative dered structure of WS nanosheets. Moreover, these percentages of 1T-WS and 2H-WS obtained by inte- 2 2 2 WS nanosheets with a thickness of about four layers gration of the W4f peak were 70 and 30%, respect- 2 7/2 are dominated by well-defined crystalline edges, thus in- ively. Such high concentration of the metallic phase in creasing the density of active sites. To better understand WS nanosheets may lead to a dramatic enhancement in the atomic structure, we have further utilized the Z- the catalytic activities [30]. Such phase conformation contrast. As shown in Fig. 3b, c, the crystal structure of was desired in electrocatalytic hydrogen evolution. Sim- the sheets is not the hexagonal packing usually observed ultaneously, S 2p region of the spectra (Fig. 4b), the for 2H-WS but rather corresponding to 1T-WS struc- peaks located at 161.6 and 162.7 eV, are assigned to 2 2 ture. It is obvious that S atoms are evenly distributed be- S2p and S2p , respectively [49]. Moreover, the atom 3/2 1/2 tween the W and W sites to form a 1T phase, as shown ratio of W and S in the vertical 1T-WS nanosheets by in Fig. 3d. Meanwhile, metallic 1T phase could be XPS and ICP (in Additional file 1: Table S1) was 1:1.96 converted into semiconducting 2H phase after 300 °C and 1:1.94, respectively. annealing treatment, as shown in Additional file 1: Raman spectroscopy measurements were also per- Figure S3. formed to further confirm the phase classification. X-ray photoelectron spectroscopy (XPS) was able to Figure 5a presents Raman spectra collected from vertical confirm the chemical state and composition. All XPS 1T-WS nanosheets grown on Ti substrate. Due to spectra were calibrated using the C 1s peak at 284.8 eV. the polarization dependence, out-of-plane A is Meanwhile, XPS could distinguish 1T- and 2H-WS as preferentially excited for edge-terminated nanosheets, well. As shown in Fig. 4a, the 2H-WS features two whereas the in-plane E is preferentially excited for 2 2g characteristic peaks at around 34.49 and 31.94 eV, corre- terrace-terminated nanosheets, as illustrated in Fig. 5b. −1 sponding to W4f and W4f of 2H-WS components, The characteristic Raman shifts at 343 and 411 cm 5/2 7/2 2 1 1 respectively, while the 1T-WS displays the presence of expected for the E and A were clearly observed, 2 2g g new chemical species clearly shifted toward lower respectively [50]. In addition, the additional peaks in the binging energies (33.54 and 31.29 eV, corresponding to lower frequency regions were previously referred as W4f and W4f of 1T-WS components) [48]. The J1, J2, and J3, corresponding to modes that were only 5/2 7/2 2 result suggests nanosheets were the mixture of 1T- and in 1T-type WS and not allowed in 2H-WS [22]. In 2 2 2H-WS . The nanosheets also contain a small amount of the Additional file 1:FigureS4,theJ1, J2,andJ3 tungstate, as evidenced by the signal at 35.14 eV, which peaks after annealing were quenched, which also verify corresponds to a W4f species. These results are con- the transformation from 1T phase to 2H phase. These 7/2 sistent with the known metallic nature of 1T-WS interpretations together with the aforementioned ab c Fig. 3 HRTEM image of a vertical 1T-WS nanosheets and b, c false-color images responding to the amplification of a. Intensity profiles along the light-blue line indicated in image b is shown in image d He et al. Nanoscale Research Letters (2018) 13:167 Page 5 of 9 Fig. 4 XPS spectra of W 4f (a) and S 2p (b) binding energy of vertical 1T-WS nanosheets characterization results solidly confirm the formation of TMDCs HER catalysts to minimizing ohmic loss, as vertical 1T-WS nanosheets. the interlayer conductivity is 2 order of magnitude lower than intralayer conductivity [8, 51]. Electrons Evaluation of Electrocatalytic Activity are required to traverse the van der Waals gaps to To assess electrocatalytic performance of vertical 1T- move between the individual layers; therefore, vertical WS nanosheets in HER, measurements are performed nanostructure does favor for electrons shuttle [44]. in a 0.5 M H SO solution using a typical three- Besides, the vertical 1T-WS nanosheets after anneal- 2 4 2 electrode cell setup. For reference purposes, Ti substrate ing at 300 °C were investigated as well (in with a drop-cast commercial Pt benchmark (Pt/C) and Additional file 1:FigureS5),and the hydrogen evolu- WS nanosheets catalysts has also been used as the tion performance significantly decrease. working electrode. Tafel plot in Fig. 6b is used to determine the Tafel The polarization curves of all samples are shown in slope, which is an important parameter describing HER Fig. 6a. The vertical 1T-WS nanosheets exhibit a activity of catalysts. The linear part of vertical 1T-WS 2 2 low overpotential of 118 mV (V vs RHE), compared nanosheets Tafel plot under small overpotential is fitted −1 to the overpotential of 230 mV for WS nanosheets to give a Tafel slope of 43 mV dec , which is smaller −2 at 10 mA cm . It indicated that rich metallic than those of previously reported values (in Table 1 and polymorph (~ 70%) in basal planes and exposed edge Additional file 1: Table S2, including WS /MoS -based 2 2 sites of vertical 1T-WS nanosheets can significantly catalysts). Tafel slope is associated with the elementary increase the electrochemical HER activity. In steps in HER. The first step of HER is a discharge step addition, the structure of vertical 1T-WS nanosheets (Volmer reaction, Eq. 1) in which protons are adsorbed guarantees efficient charge flow from the conductive to active sites on the surface of the catalysts and support to active surface site along individual layers. combined with electrons to form adsorbed hydrogen It is in fact a general consideration in designing atoms. It is followed by a desorption step (Heyrovsky Fig. 5 a Raman spectrum of vertical 1T-WS nanosheets. b Schematics of preferentially excited A Raman mode for edge-terminated nanosheets 2 g (top) and E mode for terrace-terminated nanosheets (bottom) 2g He et al. Nanoscale Research Letters (2018) 13:167 Page 6 of 9 a b 0 0.5 -10 Vertical 1T-WS nanosheets -20 0.4 Pt/C -30 WS nanosheets -40 0.3 -1 b=52 mV dec -50 -60 0.2 -70 Veritical 1T-WS nanosheets -1 b=43 mV dec -80 Pt/C 0.1 WS nanosheets -90 -1 b=30 mV dec -100 0.0 -0.3 -0.2 -0.1 0.0 0.1 0.2 0.0 0.4 0.8 1.2 1.6 Potential (V versus RHE) Decade (log|j|) cd -10 -20 -20 -40 -30 -60 -40 Initial -80 After 5000 cycles -50 -100 -60 -0.3 -0.2 -0.1 0.0 0.1 0.2 0 5 10 15 20 25 30 Potential (V versus RHE) Time (h) Fig. 6 a Polarization curves and b Tafel plots of Pt/C, WS nanosheets, and vertical 1T-WS nanosheets in 0.5 M H SO at a scan rate of 2 2 2 4 5mV/s. c Durability test showing negligible current loss even after 5000 CV cycles and d time dependence of the current density curve at an overpotential of 160 mV versus RHE for vertical 1T-WS nanosheets (no iR compensation) Table 1 Summary of literature catalytic parameters of various WS or WS -based catalysts, recently 2 2 −1 −2 Catalysts Onset overpotential [mV] Tafel slopes [mV decade ] η@j = 10mA cm [mV] Ref. 1T-WS nanosheets ~ 100 60 250 [22] WS nanoribbons – 109 > 420 [24] WS NRs-CH OH – 86 260 2 3 WS NRs-H O – 68 225 2 2 WS NRs-250 °C – 97 313 2H-WS nanoflake 100 48 – [25] WS NDs 90 51 [26] WS NDs − 300 °C 180 59 Bulk-WS 270 119 2H-WS nanosheets 60 72 ~ 160 [54] 2H-WS 282 110 – [55] Au/2H-WS 233 57.5 – Annealed WS 140 – [56] WS /rGO 150–200 58 – WS nanotubes – 113 – [57] VA WS nanosheets 30 61 136 [58] WS nanosheets – 97 236 [59] rGO/WS nanosheets – 73 229 1T-WS nanosheets 100 43 118 This work -2 -2 j (mA cm ) j(mA cm ) geo geo -2 j(m mA c ) Overpotential(V verse RHE) geo He et al. Nanoscale Research Letters (2018) 13:167 Page 7 of 9 reaction, Eq. 2) or a combination step (Tafel reaction, effectively. Hence, such nanostructure catalysts com- Eq. 3)[52, 53]. bined with the scalability of the hydrothermal synthe- sis can be readily applied in diverse water electrolysis þ − H O þ e →H þ H O ð1Þ 3 ads 2 as low-cost, high-performance, and stable HER catalyst. þ − H þ H O þ e →H þ H O ð2Þ ads 3 2 2 H þ H →H ð3Þ ads ads 2 Additional file Additional file 1: Fig S1. The cross profile SEM image of the prepared Under a special set of conditions, when the Volmer re- vertical 1T-WS nanosheets on Ti substrate. Fig S2. Whole-energy spectra action is the rate-determining step of HER, a slop of ca. of vertical 1T-WS nanosheets. Fig S3. (a) and (b) are false-color images −1 responding to vertical 1T-WS nanosheets transform into 2H-WS 2 2 120 mV dec should result, while a rate-determining nanosheets after 300 °C annealing treatment, respectively. Fig S4. Raman Heyrovsky of Tafel reaction should produce slope of ca. spectrum of vertical 1T-WS nanosheets (bottom) transform into 2H-WS 2 2 −1 30 and 40 mV dec , respectively [52, 53]. In this work, nanosheets (up) after 300 °C annealing treatment. Fig S5. Polarization curves of vertical 1T-WS nanosheets after annealing at 300 °C in 0.5 M it seems that free energy barrier of discharge step is H SO at a scan rate of 5 mV/s. Fig S6. Variation of current density versus 2 4 reduced to be comparable with that of the following the potential as a function of the pH for the vertical 1T-WS nanosheets. The desorption or combination step, resulting in the slope of highest current density is obtained for the lowest pH, consistent with the −1 solution having the highest proton concentration. Table S1. Element 43 mV dec for vertical 1T-WS nanosheets. Mean- analyses of the vertical 1T-WS nanosheets. Table S2. Summary of literature while, the key step in HER is the adsorption of the pro- catalytic parameters of various MoS or MoS -based catalysts, recently. 2 2 ton on the active site. To asses this, we have varied the (DOCX 1570 kb). pH, as shown in Additional file 1: Figure S6. We found that the vertical 1T-WS nanosheets are active over a Abbreviations wide range of pH although the activity decreases when CVD: Chemical vapor deposition; EDS: Energy-dispersive X-ray spectroscopy; increasing the pH from 0 to 7, which results from the HER: Hydrogen evolution reaction; LSV: Linear sweep voltammetry; strong diminution of the quantity of protons available. RHE: Eversible hydrogen electrode; SCE: Saturated calomel electrode; TEM: Transmission electron microscopy; TMDCs: Transition metal Stability is another important criterion for electrocata- dichalcogenides; XPS: X-ray photoelectron spectroscopy lysts. To assess the long-term durability of vertical 1T- WS nanosheets in an acid environment, continuous Funding HER by CV in the cathodic potential window at an ac- This work was supported by the National Natural Science Foundation of celerated scanning rate of 5 mV/s were conducted. The China (51478171 and 51778218). polarization curves before and after cycling are recorded under quasi-equilibrium conditions. Polarization curves Availability of Data and Materials after the 5000 cycles almost overlay the curve of the ini- We declared that materials described in the manuscript, including all tial cycle with negligible loss of cathodic current, as relevant raw data, will be freely available to any scientist wishing to use them for non-commercial purposes, without breaching participant shown in Fig. 6c. It confirms that vertical 1T-WS nano- confidentiality. sheets are stable in acidic electrolyte and remain intact through repeated cycling. Meanwhile, vertical 1T-WS Authors’ Contributions nanosheets associated ability to continuously catalyze The work presented here was carried out in collaboration between all the generation of H was examined using chronoam- the authors. QH, KY, and LW synthesized and characterized the prepared perometry (j-t). This quasi-electrolysis process was catalysts, analyzed the data, performed the statistical analysis, and wrote the manuscript. LW and SL conceived the idea of the study and carefully conducted at a constant of 160 mV in 0.5 M H SO 2 4 checked the manuscript. All authors discussed the results and commented on (Fig. 6d). Remarkably, the H evolution can proceed the manuscript. All authors read and approved the final manuscript. −2 at a sustained current density of − 21 mA cm even over 30 h of continuous operation, indicating the Competing Interests ultrahigh stability of vertical 1T-WS nanosheets. The authors declare that they have no competing interests. Conclusions In summary, we have developed a simple, eco-friendly, Publisher’sNote Springer Nature remains neutral with regard to jurisdictional claims in and effective hydrothermal method for the synthesis published maps and institutional affiliations. of vertical 1T-WS nanosheets. The vertical 1T-WS 2 2 nanosheets, with metallic polymorph and exposed Author details State Key Laboratory of Chemo/Biosensing and Chemometrics, Hunan edge sites, represent a novel structure of layered ma- University, Changsha 410082, People’s Republic of China. School of Physics terials. The unique structure paves the ways to utilize and Electronics, Hunan University, Changsha 410082, People’s Republic of the edges and planes of layered materials more China. He et al. Nanoscale Research Letters (2018) 13:167 Page 8 of 9 Received: 21 March 2018 Accepted: 8 May 2018 22. 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Nanoscale Research LettersSpringer Journals

Published: May 31, 2018

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