Green synthesis of heterolayered 2D nanohybrid catalytic hydrogel cell for environmentally-friendly water splitting

In this study, a straightforward, eco-friendly strategy for the phase modulation of MoS2 in 2D materials was developed for green-solvent-phase co-exfoliation and subsequent solvent-free drying/annealing processes, yielding binary-metal oxide nanoparticulate-decorated heterolayered 2D nanohybrids. As a green solvent, ethanol with poly(N-vinyl-2-pyrrolidone) provided an effective surface tension for liquid-phase exfoliation of the 2D materials (MoS2 and graphene). Electron injection of binary metal oxides (NiFe2O4) and graphene functional groups to the Mo 4d orbital played an important role in the modulating phase conversion of semiconductive MoS2 to metallic MoS2 in the nanohybrids. Moreover, the introduction of MoS2 and graphene tuned the diameter (∼6 nm) and effective crystalline domains of the NiFe2O4 nanoparticles in the resulting nanohybrid. The electrocatalytic activity of the nanohybrids exhibited outstanding performance in the hydrogen and oxygen evolution reactions due to the synergetic effects of the modified MoS2/graphene/NiFe2O4 components. Density functional theory calculations and the Tafel slope indicated that the combination of 1T-MoS2/graphene/NiFe2O4 nanohybrids achieved a lower activation barrier at the first hydrogen absorption step and the RDS for the interaction of adsorbed oxygen atoms with hydroxide in the hydrogen and oxygen evolution reactions. The nanohybrids were prepared as symmetric electrodes to assemble an environmentally-benign hydrogel cell for water-splitting systems, exhibiting excellent stability in deswelling–swelling cycles. We believe that the proposed process allows for phase-modulating 2D materials while alleviating the negative impact on the environment, creating desirable functionality for various catalytic applications.