Sulfur-doped graphene/transition metal dichalcogenide heterostructured hybrids with electrocatalytic activity toward the hydrogen evolution reaction

facile route for the preparation of molybdenum disulfide ( MoS2) and tungsten disulfide ( WS2), uniformly deposited onto sulfur- doped graphene ( SG), is reported. The realization of the SG/ MoS2 and SG/ WS2 heterostructured hybrids was accomplished by employing microwave irradiation for the thermal decomposition of ammonium tetrathiomolybdate and tetrathiotungstate, respectively, in the presence of SG. Two different weight ratios between SG and the inorganic species were used, namely 3 : 1 and 1 : 1, yielding SG/ MoS2 ( 3 : 1), SG/ MoS2 ( 1 : 1), SG/ WS2 ( 3 : 1) and SG/ WS2 ( 1 : 1). SG and all newly developed hybrid materials were characterized by ATR- IR and Raman spectroscopy, TGA, HR- TEM and EELS. The electrocatalytic activity of the SG/ MoS2 and SG/ WS2 heterostructured hybrids was examined against the hydrogen evolution reaction ( HER) and it was found that the presence of SG not only significantly improved the catalytic activity of MoS2 and WS2 but also made it comparable to that of commercial Pt/ C. Specifically, hybrids containing higher amounts of SG, namely SG/ MoS2 ( 3 : 1) and SG/ WS2 ( 3 : 1), exhibited extremely low onset overpotentials of 26 and 140 mV vs. RHE, respectively. The latter results highlighted the beneficial role of SG as a substrate for immobilizing MoS2 and WS2 and stressed its significance for achieving optimum electrocatalytic performance toward the HER. Finally, examination of the Tafel slopes as extracted from the electrocatalytic polarization curves, manifested the adsorption of hydrogen as the rate- limiting step for SG/ MoS2 ( 3 : 1), while for SG/ WS2 ( 3 : 1) the electrochemical desorption of adsorbed hydrogen atoms to generate hydrogen was revealed to be the rate- limiting step.