Chalcogen Vacancy Engineering in 1T-TiS₂, 1T-TiSe₂ and 1T-TiTe₂ Monolayers for Enhanced HER Activity: A DFT Study

Electrocatalytic water splitting offers a sustainable pathway for green hydrogen production, yet its widespread adoption requires low-cost and earth-abundant alternatives to platinum-group metal catalysts. Herein, we present a systematic density functional theory (DFT) study of chalcogen vacancy engineering on the catalytic performance of 1T-TiX₂ (X = S, Se, Te) monolayers in hydrogen evolution reaction (HER). Our calculations reveal that pristine 1T-TiX₂ surfaces exhibit poor hydrogen adsorption, with 1T-TiS₂ lying on the strong-binding side, while 1T-TiSe₂ and 1T-TiTe₂ reside on the weak-binding side of the volcano curve, explaining their unfavourable catalytic activity. The introduction of single chalcogen vacancies dramatically shifts all systems toward the volcano apex, with defective 1T-TiS₂ achieving a near-thermoneutral Gibbs free energy of hydrogen adsorption (ΔGH*) of -0.08 eV, thermodynamically comparable to the benchmark Pt (111) value (-0.09 eV). This promising computational result requires experimental validation. Defect formation energies are positive for all systems (3.53 eV, 2.73 eV, and 2.12 eV for S, Se, and Te vacancies, respectively), indicating thermodynamic stability of the vacancy configurations under computational chemical-potential conditions. Electronic structure analysis further demonstrates that vacancy-induced metallization generates prominent states at the Fermi level, thereby enhancing charge-transfer kinetics. Notably, 1T-TiS₂ undergoes a semimetallic to metallic transition upon S-vacancy creation, whereas 1T-TiSe₂ and 1T-TiTe₂ show moderate electronic enhancement. This work establishes chalcogen-vacancy engineering as a universal strategy for activating the 1T-TiX₂ basal planes. It identifies defective 1T-TiS₂ as the most promising, cost-effective, and non-precious HER catalyst within the 1T-TiX₂ family, providing design principles for next-generation sustainable hydrogen production technologies.