A facile preparation of WS2 nanosheets as a highly effective HER catalyst

Tungsten Tungsten (2019) 1:101–109 https://doi.org/10.1007/s42864-019-00008-7 www.springer.com/42864 ORIGINAL PAPER A facile preparation of WS2 nanosheets as a highly effective HER catalyst Xiangyong Zhang1 · Hao Fei1 · Zhuangzhi Wu1,2 · Dezhi Wang1,2 Received: 1 February 2019 / Revised: 7 March 2019 / Accepted: 11 March 2019 / Published online: 24 April 2019 © The Nonferrous Metals Society of China 2019 Abstract Tungsten disulfide (WS2) has been considered as a promising hydrogen evolution reaction (HER) candidate due to its high activity, robust chemical stability, and earth-abundant resources. However, the inert basal planes and low electrical conductivity greatly hinder its development in HER. Increasingly, the density of active sites through the structural designing is one of the most effective strategies to enhance the HER performance. Herein, we report a facile one-step hydrothermal method for synthesizing flower-like WS2 nanosheets for highly efficient HER. Besides, the effect of preparation temperature is also been discussed. The optimized WS2 nanosheets exhibit the enhanced HER activity in strong acidic solutions with a low Tafel slope and a good stability. The improvement of the HER performance can be attributed to sheet-like nanostructures, which greatly increase the edge sites and defects, resulting in a high density of exposed active sites. Besides, these sheet-like nanostructures also can make the acidic electrolyte easily accessible to the surface of WS2 and accelerate the electron transfer rate. Keywords Tungsten disulfide · Nanosheet · Electrocatalyst · Hydrogen evolution reaction · Hydrothermal method 1 Introduction Exploring renewable carbon-free energy alternatives is one of the most promising pathways for alleviating the energy and environmental crisis [1, 2]. Hydrogen has been considered as one of the most promising clean energy carriers because of its high energy density and no pollutant product [3]. Moreover, electrochemical water splitting is a highly efficient sustainable hydrogen production route [4]. However, the corresponding hydrogen production process always requires excellent catalysts to achieve fast kinetics and lower the overpotential for HER. Up to now, although platinum (Pt) and other Pt-group metals exhibit the best catalytic activity for HER, the scarcity and high cost impede their widespread applications [5]. Thus, there is still an urgent * Zhuangzhi Wu * Dezhi Wang 1 School of Materials Science and Engineering, Central South University, Changsha 410083, China 2 Key Laboratory of Ministry of Education for Non-ferrous Materials Science and Engineering, Changsha 410083, China demand for developing earth-abundant catalysts to replace these noble metal catalysts for effective HER. Recently, a large variety of non-precious metal candidates, including transition metal carbides [6–9], phosphides [10–13], chalcogenides [14–20] and so on, have been investigated and shown striking HER performances. Among these alternatives, tungsten disulfide (WS2) has received persistent interest for its high activity, robust chemical stability, and earth-abundant resources [21]. However, both theoretical and experimental studies revealed that the edge state possesses a lower hydrogen adsorption Gibbs free energy, which means that HER performance mainly arises from the edge site, while the large-area basal planes are catalytically inert and useless for HER [22–24]. Moreover, as a semiconductor material, the activity of WS2 is primarily limited by its low electrical conductivity, which restricts charge transfer kinetics for HER [25]. Considering the above two factors, there are generally two routes to enhance the catalytic activity of WS2. One is to increase the density of the exposed active sites. Among large amounts of methods, structural designing is one of the most effective strategies for increasing the number of active sites [26, 27]. In our previous work [28], the WS2 nanosheets (NSs) with loosely stacked layers were successfully obtained by a mechanical activation strategy. This special nanostructure provides highly exposed rims and 13 Vol.:(0123456789) 102 edges for HER. Cheng et al. [29] synthesized the ultrathin WS2 nanoflakes by the high-temperature solution-phase method. The obtained catalyst possessed abundant edges and the ultrathin thickness. By directly vulcanizing the WO3 nanosheets, Shang et al. [30] fabricated a WS2/WO3 heterostructure which could expose abundant active sites for HER. The other route is to improve the electrical conductivity. Sun et al. [31] synthesized the N-doped WS2 nanosheets by onestep sol-gel process and found that N doping in W S2 might be an effective way to improve the intrinsic conductivity of WS2. Duan et al. [32] fabricated 3D W S2 nanolayers@ heteroatom-doped graphene films via a vacuum-filtration process, and the conductive network of graphene sheets greatly accelerates the charge transfer kinetics. Despite these developments, the HER activity of WS2 is still much lower than that of Pt. Thus, it is still a big challenge to further improve the HER performance of WS2. Herein, the flower-like WS2 nanosheets were synthesized via a facile one-pot hydrothermal method. This unique nanostructure not only can greatly increase the edge sites and defects, resulting in a high density of exposed active sites, but also can make the acidic electrolyte easily accessible to the surface of W S2 and accelerate the electron transfer rate. Moreover, the HER performance of W S2 is improved by controlling the reaction temperature. As a result, the optimized WS2 NSs exhibit a good HER activity in an acidic solution with a small Tafel slope of 70 mV dec−1 and a good stability. 2 Experimental 2.1 Materials Sodium tungstate dihydrate (Na 2WO 4·2H 2O), sodium hy p o p h o s p h i t e ( Na H 2 P O 2 · H 2 O ) , t h i o a c et a m i d e (CH3CSNH2) and bulk W S2 were purchased from Aladdin. The Pt/C catalyst (20 wt.%) was purchased from the Johnson Matthey and Nafion solution (5 wt.%) was purchased from the DuPont. 2.2 Synthesis of WS2 Typically, 0.99 g Na2WO4·2H2O, 0.32 g NaH2PO2·H2O and 1.13 g CH3CSNH2 were dissolved into 50 mL distilled water. After being stirred to form a transparent solution, 0.75 mL concentrated HCl was dropped into the solution and continuously stirred for 30 min. Then, the solution was transferred into a 100 mL Teflon-lined stainless-steel autoclave, and heated in an oven maintained at 180–220 °C for 24 h. After being naturally cooled to room temperature, the asprepared samples were obtained by centrifugation, washed 13 X. Zhang et al. by absolute ethanol and deionized water for several times, and dried at 80 °C overnight. 2.3 Preparation of working electrodes In a typical procedure, 3 mg of the catalyst was added into the solution containing 80 μL Nafion solution (5 wt.%), 0.2 mL absolute ethanol and 0.8 mL deionized water. After being sonicated for 30 min, 5 mL of the formed (...truncated)