TiS2 Monolayer as an Emerging Ultrathin Bifunctional Catalyst: Influence of Defects and Functionalization

2D Transition Metal Chalcogenides (TMDs) for Electrocatalytic Hydrogen Evolution Reaction: A Review

Since the MoS2 synthesis, the family of two-dimensional transition metal chalcogenides (TMDs) have been intensively explored theoretically and experimentally. TMDs endowed with adjustable electronic, physical and chemical properties lead to increasing interest in the application of energy storage, molecule detection and catalysis. In the mini review, we present a forward-looking summary of 2D TMDs in hydrogen evolution electrocatalysis, including synthesis methods, hydrogen evolution performance, and optimization strategies. This review will deepen the fundamental understanding of the physical-chemical properties of TMDs with different phases and contribute unveil the universal principle among electronic configuration, atomic arrangement, physical and chemical property for the material design.

ALD-grown two-dimensional TiSx metal contacts for MoS2 field-effect transistors

Metal contacts to MoS2 field-effect transistors (FETs) play a determinant role in the device electrical characteristics and need to be chosen carefully. Because of the Schottky barrier (SB) and the Fermi level pinning (FLP) effects that occur at the contact/MoS2 interface, MoS2 FETs often suffer from high contact resistance (Rc). One way to overcome this issue is to replace the conventional 3D bulk metal contacts with 2D counterparts. Herein, we investigate 2D metallic TiSx (x ∼ 1.8) as top contacts for MoS2 FETs. We employ atomic layer deposition (ALD) for the synthesis of both the MoS2 channels as well as the TiSx contacts and assess the electrical performance of the fabricated devices. Various thicknesses of TiSx are grown on MoS2, and the resultant devices are electrically compared to the ones with the conventional Ti metal contacts. Our findings show that the replacement of 5 nm Ti bulk contacts with only ∼1.2 nm of 2D TiSx is beneficial in improving the overall device metrics. With such ultrathin TiSx contacts, the ON-state current (ION) triples and increases to ∼35 μA μm-1. Rc also reduces by a factor of four and reaches ∼5 MΩ μm. Such performance enhancements were observed despite the SB formed at the TiSx/MoS2 interface is believed to be higher than the SB formed at the Ti/MoS2 interface. These device metric improvements could therefore be mainly associated with an increased level of electrostatic doping in MoS2, as a result of using 2D TiSx for contacting the 2D MoS2. Our findings are also well supported by TCAD device simulations.

Substrate effect on hydrogen evolution reaction in two-dimensional Mo2C monolayers

The physical and chemical properties of atomically thin two-dimensional (2D) materials can be modified by the substrates. In this study, the substrate effect on the electrocatalytic hydrogen evolution reaction (HER) in 2D Mo₂C monolayers was investigated using first principles calculations. The isolated Mo₂C monolayer shows large variation in HER activity depending on hydrogen coverage: it has relatively low activity at low hydrogen coverage but high activity at high hydrogen coverage. Among Ag, Au, Cu, and graphene substrates, the HER activity is improved on the Ag and Cu substrates especially at low hydrogen coverage, while the effects of the Au and graphene substrates on the HER activity are insignificant. The improvement is caused by the charge redistribution in the Mo₂C layer on the substrate, and therefore the HER activity becomes high for any hydrogen coverage on the Ag and Cu substrates. Our results suggest that, in two-dimensional electrocatalysis, the substrate has a degree of freedom to tune the catalytic activity.

Anisotropic Thermoelectric Materials: Pentagonal PtM2 (M = S, Se, Te)

We here report a new pentagonal network structure of the PtM₂ (M = S, Se, Te) monolayers with the P2₁/c (no. 14) space group. The electronic structure and thermoelectric properties of the pentagonal PtM₂ monolayers are calculated through the VASP and BoltzTraP codes. We verify their dynamic and thermodynamic stabilities by calculating their phonon spectra and simulating ab initio molecular dynamics. It is found that the new material belongs to the medium-wide indirect band gap semiconductors from the PBE and HSE06 methods. At 300 K, the lattice thermal conductivities (K_l) of the pentagonal PtTe₂ in the x and y directions are the smallest among these three materials, being 1.77 and 5.17 W/m K, respectively. The anisotropic zT values (2.60/1.14) in the x/y direction of the pentagonal PtTe₂ at 300 K are much greater than those of the pentagonal PtSe₂ (1.75/0.82) and the pentagonal PtS₂ (0.58/0.16) at 300 K. Importantly, the p-type pentagonal PtTe₂ also has excellent thermoelectric properties at 600 K, with a zT value of 5.03 in the x direction, indicating that the p-type pentagonal PtTe₂ has a good application potential in the thermoelectric field.

First-Principles Investigation of β-FeOOH for Hydrogen Evolution: Identifying Reactive Sites and Boosting Surface Reactions

Akaganeite (β-FeOOH) is a widely investigated candidate for photo(electro)catalysis, such as water splitting. Nevertheless, insights into understanding the surface reaction between water and β-FeOOH, in particular, the hydrogen evolution reaction (HER), are still insufficient. Herein, a set of first-principles calculations on pristine β-FeOOH and halogen-substituted β-FeOOH are applied to evaluate the HER performance through the computational hydrogen electrode model. The results show that the HER on β-FeOOH tends to occur at Fe sites on the (010) surface, and palladium and nickel are found to serve as excellent co-catalysts to boost the HER process, due to the remarkably reduced free energy change of hydrogen adsorption upon loading on the surface of β-FeOOH, demonstrating great potential for efficient water splitting.

Combinatorial Design and Computational Screening of Two-Dimensional Transition Metal Trichalcogenide Monolayers: Toward Efficient Catalysts for Hydrogen Evolution Reaction

Recent experiments showed that some layered ternary transition metal trichalcogenide compounds are efficient catalysts for the hydrogen evolution reaction (HER). Motivated by these, we have combinatorially designed and computationally screened, through an efficient, automated approach based on density functional theory, single layers of such compounds, including those not reported in widely used crystal structure database like the International Crystal Structure Database (ICSD), for their efficiency as HER catalysts. On the basis of our theoretical prediction of overpotentials determined from the reaction coordinate mapping corresponding to the HER mechanism, 13 of these compounds are found to be promising catalysts, out of which three are suggested to be as efficient as platinum, the best known HER catalyst to date.

HfS2 and TiS2 Monolayers with Adsorbed C, N, P Atoms: A First Principles Study

First principles density functional theory was used to study the energetic, structural, and electronic properties of HfS 2 and TiS 2 materials in their bulk, pristine monolayer, as well as in the monolayer structure with the adsorbed C, N, and P atoms. It is shown that the HfS 2 monolayer remains a semiconductor while TiS 2 changes from semiconductor to metallic behavior after the atomic adsorption. The interaction with the external atoms introduces localized levels inside the band gap of the pristine monolayers, significantly altering their electronic properties, with important consequences on the practical use of these materials in real devices. These results emphasize the importance of considering the interaction of these 2D materials with common external atomic or molecular species.

Achieving high hydrogen evolution reaction activity of a Mo2C monolayer

Two-dimensional Mo2C materials (1T and 2H phases) have emerged as promising electrocatalysts for the hydrogen evolution reaction (HER) due to their low cost, inherent metallicity, and high stability. Unfortunately, the catalytic activity of Mo2C is lower than that of Pt, and it needs to be substantially improved for practical applications. It is necessary and urgent to consider the effect of synergetic interactions among defects, functions, and strain on the HER activity. In this study, the geometric structures, electronic properties, and the HER activity of the Mo2C monolayer, with vacancy defects (i.e. Mo and C), oxygen functionalization, and strain, are studied by using first-principles calculations. According to our results, the combination of Mo vacancies, which can be obtained under C-rich conditions, and oxygen functionalization is the most effective way to improve the HER activity of 1T- and 2H-Mo2C. Considering the abundant active sites and optimal Gibbs free energy of hydrogen adsorption, the 1T phase we obtained shows excellent HER activity even at high H coverage and improves the utilization of active sites, for which the HER activity is comparable to that of Pt. This can be attributed to the fact that oxygen atoms gain more electrons from Mo2C, which weakens the strength of the O-H bond. Our work provides not only an opportunity to better understand the catalytic mechanism, but also a guide to achieving high HER activity of a Mo2C monolayer.