Epitaxial growth of Pd nanoparticles on molybdenum disulfide by sonochemistry and its effects on electrocatalysis

In this work, we have synthesized Pd/MoS₂ samples in which about 7 nm sized Pd NPs are formed on a MoS₂ surface with the Pd content varied from 14.4 wt% to 33.6 wt% and studied their electrocatalytic activity for oxygen reduction reaction (ORR) in an alkaline medium. The syntheses were achieved by single-step sonochemical reactions between palladium acetylacetonate and 2H-MoS₂ in ethylene glycol. Based on the structural characterization data, the Pd NPs are firstly grown epitaxially on the MoS₂ surface in the [1-10]_Pd[100]_MoS2 and (111)_Pd(001)_MoS2 relationship. As the Pd-content increases, Pd NPs are formed on top of the first layer of Pd NPs as well as on the available MoS₂ surface. The epitaxially grown Pd NPs experience a tensile strain and charge-transfer to MoS₂, which raises the d-band center of Pd, lowering the on-set potential in ORR. On the other hand, the enhanced adsorption of O₂ on MoS₂ can facilitate the ORR kinetics of Pd NPs. The observed ORR data on Pd/MoS₂ can be explained as the net result of these two opposing effects of the MoS₂ support.