Monolayer-Precision Synthesis of Molybdenum Sulfide Nanoparticles and Their Nanoscale Size Effects in Hydrogen Evolution Reaction

Metal sulfide-based nanostructured materials have emerged as promising catalysts for hydrogen evolution reaction (HER), and significant progress has been achieved in enhancing their activity and durability for the HER. The understanding of nanoscale size-dependent catalytic activities can suggest critical information regarding catalytic reactivity, providing the scientific basis for the design of advanced catalysts. However, nanoscale size effects in metal sulfide-based HER catalysts have not yet been established fully, due to the synthetic difficulty in precisely size-controlled metal sulfide nanoparticles. In this work, we prepared molybdenum sulfide (MoS₂) nanoparticles with monolayer precision from one to four layers with the nearly constant basal plane size of 5 nm, and explored their size-dependent catalytic activity in the HER. Using density functional theory (DFT) calculations, we identified the most favorable single-, double-, and triple-layer MoS₂ model structures for the HER, and calculated elementary step energetics of the HER over these three model structures. Combining HER activity measurements and the DFT calculation results, we establish that the turnover frequency of MoS₂ nanoparticles in the HER increases in a quasi-linear manner with decreased layer numbers. Cobalt-promoted MoS₂ nanoparticles also exhibited similar HER activity trend. We attribute the higher HER activity of smaller metal sulfide nanoparticles to the higher degree of oxidation, higher sulfur coordination number, formation of the 1T phase, and lower activation energy at the rate-determining step. This insight into the nanoscale size-dependent HER activity trend will facilitate the design of advanced HER catalysts as well as other hydrotreating catalysts.