1wileyonlinelibrary.com www.particle-journal.com www. MaterialsViews.com Over years, layered transition metal dichalcogenides (TMDs) have shown vast potentials in catalyzing hydrogen evolution reaction (HER). While most current research works are exclusively focused on binary systems, very little attention has been paid to their ternary and quaternary alloys. In this work, a high-temperature solution method is employed to prepare 25 different Mo x W 1−x (S y Se 1−y ) 2 alloys with high yields. Resulting products are comprised of thin nanoflakes with cation and anion compositions independently tunable over the entire range. When evaluated as HER electrocatalysts in acids, these materials exhibit high activity and durability with small onset overpotentials and Tafel slopes. Furthermore, some general trend in their compositionactivity correlation is observed, and a few most active combinations successfully identified. This study may provide valuable insights in the future search of high-performance HER electrocatalysts based on TMDs.recently demonstrated a high-temperature solution method to prepare Mo(S x Se 1−x ) 2 alloy nanoflakes with monolayer or fewlayer thickness and fully tunable chemical composition. [18] It was evidenced that the incorporation of Se modified the d band electronic structure of Mo, and impacted its hydrogen adsorption free energy (ΔG H ). As a result, alloyed nanoflakes, especially with the chemical composition of MoSSe, were observed to have improved performance in comparison to either MoS 2 or MoSe 2 alone. [18] Encouraged by above progresses, we further propose that quaternary TMD alloys consisting of Mo, W, S, and Se may also hold unique promise for HER electrocatalysis. The compositional tunability on both cations and anions provides us two independent degrees of freedom to adjust the ΔG H , which, at the best scenario, may closely approach the thermoneutral, thus giving rise to optimal HER activity. Unfortunately, as far as we aware, there is no study available at present to explore quaternary TMD alloys for HER. To this end, in this work, we extend the high-temperature solution method developed by us earlier to the preparation of ternary or quaternary TMD alloys with a general formula of Mo x W 1−x (S y Se 1−y ) 2 . Their HER activities are systematically measured and compared in parallel. Through these experiments, we observe some general correlation between their electrocatalytic activity and chemical compositions, and identify a few most active combinations. Results and Discussion25 different alloy nanoflakes having a general formula of Mo x W 1−x (S y Se 1−y ) 2 (where x, y = 0, 0.33, 0.5, 0.67 or 1) were synthesized using an established high-temperature solution method by reacting the calculated amounts of MoCl 5 and/or WCl 6 with S and/or Se powders in a mixed solvent of oleylamine and 1-octadecene at 300 °C under N 2 atmosphere (see the Experimental for details). [16,18] Oleylamine was chosen as the solvent for its appreciable solubility toward both S and Se powders, forming...