Theoretical design of novel Janus transitional metal dichalcogenides for catalytic water splitting

Abstract

In recent years, two-dimensional (2D) nanomaterials have attracted immense research interest from the international scientific community. Transition metal dichalcogenides (TMDs), a unique class of 2D materials in which one layer of transition metal atoms is sandwiched between two layers of chalcogenide atoms, boast fantastic mechanical, optical, and electronic properties. While conventional 2D TMDs are typically nanosheets composed of one transition metal species and one dichalcogenide species with a stoichiometric ratio of 1:2, e.g., MoS2 monolayers, Janus TMD monolayers comprise two layers of different chalcogenide species and exhibit an asymmetric out-of-plane structural configuration, and they are expected to possess a relatively wide variety of desirable optical and electronic properties.

First, the thermodynamic stability of different possible configurations of Pd-based Janus TMD monolayers, namely, the 1T, 2H, and pentagonal phases, was analysed through comprehensive density functional theory (DFT) calculations to determine their most thermodynamically favourable ground-state configurations. With regard to previous experimental studies that established the pentagonal phase as the true ground-state configuration of bulk PdS2 and PdSe2, the Janus Pd-based TMD monolayers were hypothesised to exhibit such a unique configuration in lieu of the more common 1T and 2H phases in many 2D TMDs. The structural, electronic, and optical properties of three Janus Pd-based TMD monolayers were thus investigated. Based on the previous study of a PdSeO3 monolayer that fulfils the thermodynamic requirements for facilitating one-step water splitting as a photocatalyst, we assessed the applicability of the three Janus Pd-based TMD monolayers as catalysts for one-step photocatalytic water splitting, particularly through evaluating their band structure and optical absorbance properties. We have established that the monolayers exhibit decent thermal stability and are semiconductors with moderately wide band gaps, which can be modulated via strain engineering. As water-splitting photocatalysts, the valence and conduction bands of the monolayers flawlessly engulf the reduction and oxidation potentials of water, and their photogenerated charge carriers are capable of facilitating the key half reactions of water splitting, namely, the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER), simultaneously on different active sites. Furthermore, the monolayers possess remarkable anisotropic optical absorbance properties in the visible and ultraviolet regions of the electromagnetic spectrum, indicating that they are capable of effectively harvesting photons from direct sunlight as photocatalysts.

Secondly, the effects of self-intercalation on the HER catalytic performance of six Mo- and W- based Janus TMD bilayer systems were explored. The systems were constructed through the insertion of transition metal atoms between two identical Janus TMD monolayers while preserving their individual structures. Based on comprehensive DFT calculations, the most thermodynamically favourable ground-state configurations of the pristine Mo- and W- based Janus TMD monolayers were determined be the common distorted 1T' configuration, and their basal site activity with respect to the HER was evaluated by predicting the Gibbs free energy of hydrogen adsorption of their potential active sites. While most of the pristine Janus TMD monolayers were considered mediocre HER electrocatalysts, the pristine SMoTe monolayer was identified as a relatively promising candidate for facilitating the HER, despite its catalytic performance being marginally inferior to that of (111) Pt. We proposed to enhance the HER catalytic performance of the Mo- and W- based Janus TMD monolayers through the design of self-intercalated Janus TMD bilayers, e.g., by inserting Mo atoms between two identical SMoTe monolayers to form a self-intercalated SMoTe bilayer system, which would potentially create active basal sites on the surfaces of the bilayer frameworks via the redistribution of charge facilitated by the intercalated metal ions. Our results indicate that the introduction of intercalated metal ions have dramatically boosted the basal site activity of all the modified Janus TMD systems with respect to the HER. Notably, the HER catalytic performance of the self-intercalated SMoTe bilayer system was found to be significantly superior to that of (111) Pt, rendering it an exceptional HER electrocatalyst that outclasses its monolayer predecessor. Additionally, in this study, the effects of intercalated-atom concentration on the HER catalytic performance of the self-intercalated Janus TMD bilayers have also been investigated, revealing that their HER catalytic performance can be further optimised through modulation of the concentration of the intercalated transition metal atoms.

Thirdly, the incorporation of single-atom metals (SAMs) into Janus TMD monolayers was conducted with the objective of improving their performance as water-splitting electrocatalysts. Specifically, the basal site activity of SAM-incorporated Mo-, W-, and Cr- based Janus TMD monolayers with respect to the HER, the OER, and the oxygen reduction reaction (ORR) was systematically investigated through simulations employing comprehensive DFT calculations. The SAMs explored in this study are Pd, Pt, and Ni. Certain pristine TMD monolayers, such as the SMoTe and SCrSe monolayers, have been established as excellent HER electrocatalysts. Additionally, in the vast majority of cases, the incorporation of the SAMs, in particularly Pt, into the pristine Janus TMD monolayers corresponds to a massive improvement in their HER catalytic performance. Unfortunately, all the pristine TMD monolayers were determined to display extremely substandard catalytic performance with respect to both the OER and the ORR. Through the introduction of SAMs, the OER/ORR catalytic performance of all the Janus TMD monolayers have undergone a significant boost, resulting in the identification of multiple exceptional OER, ORR, and bifunctional OER/ORR electrocatalysts. The boost in catalytic performance of the SAM-incorporated Janus TMD monolayers can be attributed to the direct participation of their d-electrons in the reactions and the modulation of their d-states, which is a consequence of strong electronic metal–support interactions between the SAMs and the underlying Janus TMD framework facilitating the formation of thermodynamically favourable metal–support bonds and the redistribution of charge.

Ultimately, a plethora of concerns must be addressed before a theoretical material can be successfully fabricated and adopted as an effective water-splitting photo/electrocatalyst, which include, but are not limited to, the compatibility between its proposed reaction mechanisms and the individual potentials of charger carriers, material cost, and the chemical stability of the catalyst in an aquatic environment. In this Ph.D. study, the water-splitting capability of Janus TMD monolayers and their derivatives are evaluated through the identification of potential active sites on their surfaces and the investigation of their respective free energy changes with respect to the adsorption/desorption of HER and OER intermediates. The findings from this study provide insight on the intrinsic properties of 2D Janus TMDs, and they are instrumental in revealing the underlying mechanisms of such materials in terms of facilitating the HER and the OER, thereby elucidating the rational design of water-splitting photocatalysts and electrocatalysts.