Rational design of defect-engineered TMDs: Unlocking active sites for selective capture and catalysis in MoS2, MoSe2, and MoTe2

Molybdenum-based transition-metal dichalcogenides (TMDs) are promising catalysts for key electro- and photochemical reactions, including CO₂ reduction (CRR), N₂ reduction (NRR), and hydrogen evolution (HER). However, their catalytic performance is inherently limited by the low reactivity of their basal planes, necessitating structural modifications to expose chemically active transition-metal sites. Here, we provide fundamental insights into chalcogen-vacancy engineering in Mo-based TMDs. Using large-scale density functional theory (DFT) computations, including NVT ab initio molecular dynamics (AIMD) and density functional perturbation theory (DFPT), we examine 400 adsorption cases across three TMD monolayers — MoS₂, MoSe₂, and MoTe₂ — considering both pristine and defective structures with three chalcogen-vacancy sizes, as well as six molecular species (N₂, O₂, NO, CO, CO₂, and NO₂). Our findings reveal that vacancy effects are highly selective, with adsorption enhancements varying significantly by molecular species. While larger vacancies generally strengthen adsorption across all TMDs, they also amplify intrinsic physicochemical differences. MoTe₂ exhibits the highest binding energies and molecular deformation, followed by MoSe₂ and MoS₂. Notably, vacancy-engineered TMDs demonstrate promising adsorption for N₂ and CO₂, with activation-to-binding ratios surpassing many conventional catalysts. By strategically selecting TMD compositions and tailoring vacancy sizes, adsorption strength and molecular activation can be finely optimized, leading to distinct thermodynamic favorability. Our results show defective MoS₂ favors CO₂ capture and activation for CRR but suppresses NRR and modestly limits HER, whereas MoTe₂ suppresses HER while promoting both NRR and CRR. These insights establish chalcogen selection as critical parameter in defect engineering, paving the way for rational design of advanced catalytic materials.