Abstract
Recent years have seen a surge in the use of low-dimensional transition metal dichacolgenides, such as MoS2, as catalysts for the electrochemical hydrogen evolution reaction. In particular, sulfur vacancies in MoS2 can activate the inert basal plane, but that requires an unrealistically high defect concentration (~9%) to achieve optimal activity. In this work, we demonstrate by first-principles calculations that assembling van der Waals heterostructures can enhance the catalytic activity of MoS2 with low concentrations of sulfur vacancies. We integrate MoS2 with various two-dimensional nanostructures, including graphene, h-BN, phosphorene, transition metal dichacolgenides, MXenes, and their derivatives, aiming to fine-tune the free energy of atomic hydrogen adsorption. Remarkably, an optimal free energy can be achieved for a low sulfur vacancy concentration of ~2.5% in the MoS2/MXene-OH heterostructure, as well as high porosity and tunability. These results demonstrate the potential of combining two-dimensional van der Waals assembly with defect engineering for efficient hydrogen production.
Highlights
It was demonstrated that edges of MoS2 are active sites for hydrogen evolution reaction (HER),[4,5,6] and plenty of strategies have been developed to enhance the catalytic activity either by maximizing the exposed edge sites through synthesis of nanostructures,[7,8,9,10,11] or by increasing the intrinsic activity of MoS2 through electronic structure modifications via chemical doping or using strain.[12,13,14,15]
In this work, aiming to optimize the interfacial interaction and enhance the catalytic capability of MoS2, by means of firstprinciples calculations based on density functional theory (DFT), we investigated a variety of van der Waals heterostructures constructed by assembling MoS2 with other common 2D structures, including graphene, h-BN, black phosphorene, transition metal dichacolgenides (TMDs) (MoS2, WS2 and MoSe2) and transition metal carbides/ nitrides (Ti2C, Ti3C2, V2C and Ti2N) that are termed as MXenes
Before studying 2D heterostructures, we first investigated the relationship between concentration of VS on MoS2 and the catalytic activity towards HER
Summary
Owing to their low cost, earth abundance and high activity, transition metal dichacolgenides (TMDs), such as MoS2, have become a promising alternative to platinum (Pt) for catalyzing hydrogen (H2) production from water in the last decade.[1,2,3] It was demonstrated that edges of MoS2 are active sites for hydrogen evolution reaction (HER),[4,5,6] and plenty of strategies have been developed to enhance the catalytic activity either by maximizing the exposed edge sites through synthesis of nanostructures,[7,8,9,10,11] or by increasing the intrinsic activity of MoS2 through electronic structure modifications via chemical doping or using strain.[12,13,14,15] To increase the overall catalytic performance, it is highly desirable to utilize the basal plane of MoS2 because it provides the most possible active sites towards HER. Phase transition from semiconducting 2H to metallic 1T/1T′ phase dramatically enhances the catalytic activity of MoS2.16–18 the 1T/1T′ phase of MoS2 is metastable and would be transformed to the more stable 2H phase under irradiation or mild heating conditions,[19,20] severely limiting its practical applications Both theoretical and experimental studies showed that the inert basal plane of MoS2 can be activated by creating sulfur vacancy (VS).[21,22,23,24] In particular, Hong et al presented a detailed study on point defects in 2D MoS2 and found that VS is the most energy-favorable defect on the basal plane.[23] Li et al explored the catalytic properties of various active sites in MoS2, and found that VS on the basal plane provides one major active site for HER in addition to edges.[24] Interestingly, it was experimentally shown that the catalytic performance strongly depends on the concentration of VS (VS%), for which an optimal HER activity can only be achieved when VS% reaches ~9%. Recent advances in van der Waals heterostructures have invoked substantial interest in tailoring the electronic properties of 2D structures through interfacial coupling.[31,32] It was shown that by combination of different materials to form heterostructures, novel physical phenomena and electronic behaviors arise that cannot be derived from their constituent layers.[33,34] In particular, as catalyst for HER, Liu et al synthesized 1T phase of MoS2 on flexible single-walled carbon nanotube (SWNT) and found that MoS2/SWNT composite exhibits increased activity.[35]
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