Two-dimensional hybrid nanosheets of tungsten disulfide and reduced graphene oxide as catalysts for enhanced hydrogen evolution.

Monolayers of transition-metal dichlacogenides (TMDs) have been recently receiving interest for both fundamental and technological investigations. One key to the realization of the potential of TMDs is the synthesis of high-quality materials. One interesting TMD compound is WS2 in which the electrical properties can be varied from metallic and semiconducting by tuning the crystal structure and the number of layers. Conventionally monoor few-layered WS2 can be obtained by mechanical exfoliation or grown by chemical vapor deposition (CVD), using precursors such as WOCl4 and HS(CH2)2SH. [1a,2] In addition, chemical methods for large-scale synthesis of layered WS2 have been reported. For example, WS2 nanosheets with lateral dimensions of 100 nm have been synthesized from one-dimensional (1D) W18O49 with assistance of surfactants through a rolling-out method. Recently, a powder of WS2 nanosheets was obtained by a two-step process that involves mixing WO3 and S by ball milling and heating the mixture and S powder to 600 8C in Ar. A solid-state reaction with tungstic acid and thiourea in N2 atmosphere at 773 K has also produced layered WS2. [5] The hydrothermal reaction is known to be a facile method for large scale manufacturing of TMD nanosheets at relatively low temperature. The synthesis of MoS2 sheets has been well studied using precursors such as sodium molybdate and thioacetamide or thiourea as the S source. Although inorganic fullerene-like (IF) WS2 nanoparticles, 1D nanotubes, or 1D rods have been obtained by the hydrothermal method, synthesis of WS2 sheets by the hydrothermal reaction has yet to be realized. The primary reason for this is due to the fact that the WOx precursor required for the formation of WS2 nanosheets does not occur in 2D form. Instead WOx prefers to form 1D or zero-dimensional (0D) nanostructures. Thus sulfurization of the WOx favors the formation of 0D fullerene-like or 1D nanotube/nanorod like WS2 nanostructures. This is in contrast to MoS2 nanosheets in which the MoO3 precursor is a layered compound. [9,10] The absence of a facile WS2 nanosheet synthesis method has also prevented the study of WS2/graphene hybrid structures, despite their potentially useful applications. We therefore develop a hydrothermal method for synthesis of WS2 nanosheets and then we integrate rGO nanosheets into the reactor to fabricate novel WS2/rGO hybrids. We report detailed structural analyses of the synthesized products and investigate their potential catalysts for the hydrogen evolution reaction (HER). The primary uniqueness of our work is the synthesis of WS2 and rGO/WS2 nanosheets using a scalable hydrothermal method and their implementation as efficient catalysts for HER. MoS2 nanostructures are promising electrocatalysts for H2 production. The overpotential of MoS2 catalysts is 200– 150 mV and Tafel slopes are in a range of 55–40 mVdec 1 (millivolts per decade). Recent results have suggested that WS2 nanosheets could be interesting as HER electrocatalysts. An overpotential of 60 mV and a Tafel slope of about 70 mVdec 1 have been measured for WS2 sheets synthesized by the ball-milling method while values of 150 mVand about 70 mVdec 1 have been obtained for WS2 particles on carbon cloth. Voiry et al. showed that both the overpotential ( 100 mV) and Tafel slopes (60 mVdec ) can be lowered by used WS2 nanosheet catalysts that contain a high concentration of the metallic 1T phase. Herein, we report the synthesis of WS2 and WS2/rGO nanosheets using an one-pot hydrothermal reaction process at low temperature. We show that WS2 nanosheets are selectively fabricated using tungsten chloride and thioacetamide precursors. We also show that WS2 nanosheets readily hybridize with rGO nanosheets when GO is added in the reaction vessel. This is one of the first reports on selective synthesis of WS2 and WS2/rGO hybrid nanosheets by the hydrothermal reaction. We further demonstrate that WS2/ rGO nanosheets exhibit good catalytic activity for hydrogen evolution. Based on impedance measurements, the better catalytic performance is attributed to enhanced charge trans[*] J. Yang, S. J. Ahn, D. Kang, A. Y. Kim, Prof. H. S. Shin Interdisciplinary School of Green Energy and Low Dimensional Carbon Materials Center Ulsan National Institute of Science and Technology(UNIST) UNIST-gil 50, Ulsan 689-798 (Republic of Korea) E-mail: shin@unist.ac.kr D. Voiry, Prof. M. Chhowalla Materials Science and Engineering Department Rutgers University 607 Taylor Road, Piscataway, NJ (USA) E-mail: manish1@rci.rutgers.edu [] These authors contributed equally to this work.