Development of highly active hydrogen evolution reaction (HER) catalysts composed of reduced graphene oxide and amorphous molybdenum sulfides derived from (NH4)2MoOmS4-m (m=0, 1, and 2)

Photochemically synthesized catalysts using (NH4)(2)MoOmS4-m (m = 0, 1, and 2) as raw materials were studied for the purpose of hydrogen evolution reaction (HER) from H2O. Each starting material is found to generate a composite of MoSx/RGO covered by MoO3 layer, where RGO is reduced graphene oxide. XRD, TEM, XPS, and elemental analysis of the resulting MoSx/RGO reveals that MoSx exists in an amorphous phase and is composed of Mo(IV)S-x (x = 3). The HER activity of each prepared composite is estimated by eta(10) (the overpotential at 10 mA cm(-2) current density), Ts (the Tafel slope) from linear sweep voltammetry (LSV), and turnover frequency (TOF). The composites from m = 0, 1, and 2 show eta(10) : 198, 206, and 297 mV vs. RHE, Ts: 49.1, 46.9, and 89.1 mV dec(-1), and TOF: 0.16, 0.20, and 0.10 s(-1), respectively. From the viewpoint of Ts and TOF, the composites from m = 0 and 1 display a remarkably efficient activity, but they show the decline of HER performance with the electrochemical reaction time and cycle. The composite from m = 2, however, shows less activity than the others. The origins of the prominent efficiency of the composites from m = 0 and 1 are predicted as follows: (1) Photoirradiation of the starting materials on RGO leads to a selective and confined formation of amorphous Mo(IV)S-x, which is constructed, intriguingly, by abundant reactive bridging S-2(2-) ligands. (2) The HER efficiency is related to the presence of the Mo(IV) centers and the bridging S-2(2-) ligands. The Ts data suggest that the catalytic HER proceeds by a Volmer-Heyrovsky pathway.