Layered transition metal dichalcogenides are catalytically important compounds. Unlike the mounting interest in transition metal dichalcogenides such as MoS2 and WS2 for electrochemical applications, other metal chalcogenides with layered structure but different chemical composition have received little attention among the scientific community. One such example is represented by thallium(I) sulfide (Tl2S), a Group 13 chalcogenide, which adopts the peculiar anti-CdCl2 type structure where the chalcogen is sandwiched between the metal layers. This is the exact opposite of a number of transition metal dichalcogenides like 1T-MoS2 adopting the regular CdCl2 structure type. The electronic structure of Tl2S thus differs from MoS2. Such structure may provide a useful insight and understanding toward its electrochemical behavior in relation to the electrochemical properties of MoS2. We thus investigated the intrinsic electroactivity of Tl2S and its implications for sensing and energy generation, specifically the electrocatalytic properties toward the hydrogen evolution reaction (HER). We show that Tl2S exhibits four distinct redox signals at ca. 0.4 V, -0.5 V, -1.0 V and -1.5 V vs Ag/AgCl as a result of its inherent cathodic and anodic processes. We also demonstrate that Tl2S possesses slow electron transfer abilities with a rate (k(0)obs) as low as 6.3 × 10(-5) cm s(-1). Tl2S displays a competent performance as a HER electrocatalyst compared to a conventional glassy carbon electrode. However, the poor conductivity of Tl2S renders the HER electrocatalytic behavior second-rate compared to MoS2. Furthermore, we investigated the electronic properties of Tl2S and found that Tl2S exhibits an unusually narrow band dispersion around the Fermi level. We show here that anti-MoS2 structure of Tl2S is accompanied by highly unusual features.
Keywords: electrochemistry; hydrogen evolution; thallium(I) sulfide; transition-metal chalcogenides; two-dimensional materials.