Ammonium Ions Intercalated 1T-MoS2 for Enhanced Electrochemical Hydrogen Production

Abstract

Molybdenum disulphide (MoS2), a 2D layered transition metal dichalcogenide (TMDC) is being studied as an effective catalyst for Hydrogen Evolution Reaction (HER) in electrochemical water splitting. Optoelectronic properties of nanostructured MoS2 make them a potential materials for energy conversion and energy storage.[1-2] MoS2 exhibits 2 major stacking polytypes, 2H and 1T based on their coordination symmetry, where 2H is trigonal prismatic (D3h), 1T exists in octahedral coordination (Oh). 2H phase is a semiconductor where sulfur edges are only active, but 1T phase is metallic with both edge and basal planes active for HER. However, 1T is metastable in nature and easily converted into 2H [3]. To enhance the HER rate, alkali metals (Na, K, Li) are intercalated into 1T MoS2. The intercalation of alkali metals (Li, Na, Mg) can increase the d-spacing and electronic conductivity between interlayer and reduce the barrier to the diffusion of large-sized ions.[4] Here we have reported a facile one-pot hydrothermal synthesis of 1T-MoS2 at various temperatures (120-200oC) to perform as effective catalysts for HER. The d-spacing of inter-layer expanded 1T-MoS2at various temperatures are 0.92nm. 1T MoS2 was prepared by hydrothermal method at various temperatures (120-200oC) using ammonium molybdate tetrahydrate and thiourea in aqueous solution as precursors. The calculated molar ratio of Mo:S in the solution was 1:4. Ammonium ions released from thiourea and ammonium molybdate intercalated into the layers of MoS2 which eventually increases the d-spacing. The crystallinity and phases of the MoS2 sample were analyzed using X–ray diffraction. In contrast to bulk 2H phase, the ammonium ion intercalated 1T phase shows two new peaks at 9.5o (002) and 18.95o (004) with a d-spacing of ~9.27Å and 4.67Å, respectively. The ammonium ion intercalation increases the d-spacing and thereby shifts in the (002) plane from 14.3o to 9.5o is observed. The change in the d spacing is 3.3 Å and matches with the size of hydrogen bonding diameter in ammonium ions (∼ 3.5 Å).[5] The crystallinity of the MoS2sample increased with increasing temperature. The sulfur composition is being further examined using X-ray photoelectron spectroscopy. The onset potential for HER decreases with increase in the crystallinity. The hydrogen evolution current also shows significant increase with increase in crystallinity. The enhanced interlayer spacing increases the diffusion of larger Hydronium ions into the layer which directly enhances the hydrogen evolution rate. Hydrogen evolution reaction mechanism is being studied using electrochemical studies such as Tafel polarization.