Transition metal doping activated basal-plane catalytic activity of two-dimensional 1T'-ReS2 for hydrogen evolution reaction: a first-principles calculation study

Oxygen-Vacancy-Induced Enhancement of BiVO4 Bifunctional Photoelectrochemical Activity for Overall Water Splitting

Hydrogen generation via photoelectrochemical (PEC) overall water splitting is an attractive means of renewable energy production so developing and designing the cost-effective and high-activity bifunctional PEC catalysts both for the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) has been focused on. Based on first-principles calculations, we propose a feasible strategy to enhance either HER or OER performance in the monoclinic exposed BiVO4 (110) facet by the introduction of oxygen vacancies (Ovacs). Our results show that oxygen vacancies induce charge rearrangements, which enhances charge transfer between active sites and adatoms. Furthermore, the incorporation of oxygen vacancies reduces the work function of the system, which makes charge transfer from the inner to the surface more easily; thus, the charges possess stronger redox capacity. As a result, the Ovac reduces both the hydrogen adsorption-free energy (ΔGH*) for the HER and the overpotential for the OER, facilitating the PEC activity of overall water splitting. The findings provide not only a method to develop bifunctional PEC catalysts based on BiVO4 but also insight into the mechanism of enhanced catalytic performance.

Single Transition-Metal Atom Anchored on a Rhenium Disulfide Monolayer: An Efficient Bifunctional Electrocatalyst for the Oxygen Evolution and Oxygen Reduction Reactions

Developing efficient oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) bifunctional electrocatalysts is attractive for rechargeable metal-air batteries. Meanwhile, single metal atoms embedded in 2D layered transition metal chalcogenides (TMDs) have become a very promising catalyst. Recently, many attentions have been paid to the 2D ReS2 electrocatalyst due to its unique distorted octahedral 1T' crystal structure and thickness-independent electronic properties. Here, the catalytic activity of different transition metal (TM) atoms embedded in ReS2 using the density functional theory is investigated. The results indicate that TM@ReS2 exhibits outstanding thermal stability, good electrical conductivity, and electron transfer for electrochemical reactions. And the Ir@ReS2 and Pd@ReS2 can be used as OER/ORR bifunctional electrocatalysts with a lower overpotential for OER (ηOER) of 0.44 V and overpotentials for ORR (ηORR) of 0.26 V and 0.27 V, respectively. The excellent catalytic activity is attributed to the optimal adsorption strength for oxygen intermediates coming from the effective modulation of the electronic structure of ReS2 after Ir/Pd doping. The results can help to deeply understand the catalytic activity of TM@ReS2 and develop novel and highly efficient OER/ORR electrocatalysts.

Universal Platform for Robust Dual-Atom Doped 2D Catalysts with Superior Hydrogen Evolution in Wide pH Media

Layered 2D transition metal dichalcogenides (TMDs) have been suggested as efficient substitutes for Pt-group metal electrocatalysts in the hydrogen evolution reaction (HER). However, poor catalytic activities in neutral and alkaline electrolytes considerably hinder their practical applications. Furthermore, the weak adhesion between TMDs and electrodes often impedes long-term durability and thus requires a binder. Here, a universal platform is reported for robust dual-atom doped 2D electrocatalysts with superior HER performance over a wide pH range media. V:Co-ReS2 on a wafer scale is directly grown on oxidized Ti foil by a liquid-phase precursor-assisted approach and subsequently used as highly efficient electrocatalysts. The catalytic performance surpasses that of Pt group metals in a high current regime (≥ 100 mA cm-2) at pH ≥ 7, with a high durability of more than 70 h in all media at 200 mA cm-2. First-principles calculations reveal that V:Co dual doping in ReS2 significantly reduces the water dissociation barrier and simultaneously enables the material to achieve the thermoneutral Gibbs free energy for hydrogen adsorption.

Two-dimensional materials as catalysts, interfaces, and electrodes for an efficient hydrogen evolution reaction

Two-dimensional (2D) materials have been significantly investigated as electrocatalysts for the hydrogen evolution reaction (HER) over the past few decades due to their excellent electrocatalytic properties and their structural uniqueness including the atomically thin structure and abundant active sites. Recently, 2D materials with various electronic properties have not only been used as active catalytic materials, but also employed in other components of the HER electrodes including a conductive electrode layer and an interfacial layer to maximize the HER efficiency or utilized as templates for catalytic nanostructure growth. This review provides the recent progress and future perspectives of 2D materials as key components in electrocatalytic systems with an emphasis on the HER applications. We categorized the use of 2D materials into three types: a catalytic layer, an electrode for catalyst support, and an interlayer for enhancing charge transfer between the catalytic layer and the electrode. We first introduce various scalable synthesis methods of electrocatalytic-grade 2D materials, and we discuss the role of 2D materials as HER catalysts, an interface for efficient charge transfer, and an electrode and/or a growth template of nanostructured noble metals.

2D Metal-Free BSi5 with an Intrinsic Metallicity and Remarkable HER Activity

One of the most urgent and attractive topics in electrocatalytic water splitting is the exploration of high-performance and low-cost catalysts. Herein, we have proposed three fresh two-dimensional nanostructures (BSi5, BSi4, and BSi3) with inherent metallicity contributed by delocalized π electrons based on first-principles calculations. Their planar atoms arrangement, akin to graphene, is in favor of the availability of active atoms and H adsorption/deadsorption. Among them, the BSi5 monolayer shows the best HER activity, even superior to a commercial Pt catalyst. Moreover, its extraordinary HER activity can be maintained under high H coverage and large biaxial strain, mainly originating from the fact that B 2pz orbital electrons are responsible for the B-H interaction. Further analysis reveals that there appears to be a linear correlation between the magnitude of B 2pz DOS at the Fermi level and Gibbs free energy in both three proposed nanostructures and five hypothetical B-Si nanostructures. Our work represents a significant step forward toward the design of metal-free HER catalysts.

Strain engineering of hyperbolic plasmons in monolayer carbon phosphide: a first-principles study

Natural and tunable in-plane hyperbolic plasmons have so far been elusive, and hence few two-dimensional hyperbolic materials have been theoretically and experimentally discovered. Here, comprehensive first-principles calculations were conducted to study the electronic and plasmonic properties of biaxially strained monolayer carbon phosphide (β-CP). We found that (i) a compressed β-CP hosts strong anisotropic Dirac-shaped fermions with robust modulated Fermi velocity, (ii) for biaxial strain of -3% an unprecedented ultra-wide hyperbolic window is extended continuously from terahertz (9 THz) to mid-visible (blue light, 693 THz), (iii) the tunable optical Van Hove singularity as the origin of hyperbolic plasmons in deformed β-CP is disclosed, (iv) an elliptic to hyperbolic transition in the σ-near-zero regime is demonstrated in terahertz frequencies (9 THz), (v) the propagation angle of the concave wavefront can be actively tuned using biaxial strains, and (vi) hyperbolic dispersion reorientation from one principal axis to another orthogonal one under compressive strains larger than 8% is observed. This study sheds new light on the unique properties of hyperbolic two-dimensional (2D) materials having exotic optoelectronic characteristics which are promising candidates for anisotropic light control with ultimate dexterity in the flat optics.

Orbital Modulation with P Doping Improves Acid and Alkaline Hydrogen Evolution Reaction of MoS2

There has been great interest in developing and designing economical, stable and highly active electrocatalysts for the hydrogen evolution reaction (HER) via water splitting in an aqueous solution at different pH values. Transition-metal dichalcogenides (TMDCs), e.g., MoS₂, are identified to be promising catalysts for the HER due to the limited active sites at their edges, while the large basal plane of MoS₂ is inert and shows poor performance in electrocatalytic hydrogen production. We theoretically propose orbital modulation to improve the HER performance of the basal plane of MoS₂ through non-metal P doping. The substitutional doping of P provides empty 3p_z orbitals, perpendicular to the basal plane, can enhance the hydrogen adsorption for acid HER and can promote water dissociation for alkaline HER, which creates significant active sites and enhances the electronic conductivity as well. In addition, 3P-doped MoS₂ exhibits excellent HER catalytic activity with ideal free energy at acid media and low reaction-barrier energy in alkaline media. Thus, the doping of P could significantly boost the HER activity of MoS₂ in such conditions. Our study suggests an effective strategy to tune HER catalytic activity of MoS₂ through orbital engineering, which should also be feasible for other TMDC-based electrocatalysts.

Recent Progress in Research on Ferromagnetic Rhenium Disulfide

Since long-range magnetic ordering was observed in pristine Cr₂Ge₂Te₆ and monolayer CrCl₃, two-dimensional (2D) magnetic materials have gradually become an emerging field of interest. However, it is challenging to induce and modulate magnetism in non-magnetic (NM) materials such as rhenium disulfide (ReS₂). Theoretical research shows that defects, doping, strain, particular phase, and domain engineering may facilitate the creation of magnetic ordering in the ReS₂ system. These predictions have, to a large extent, stimulated experimental efforts in the field. Herein, we summarize the recent progress on ferromagnetism (FM) in ReS₂. We compare the proposed methods to introduce and modulate magnetism in ReS₂, some of which have made great experimental breakthroughs. Experimentally, only a few ReS₂ materials exhibit room-temperature long-range ferromagnetic order. In addition, the superexchange interaction may cause weak ferromagnetic coupling between neighboring trimers. We also present a few potential research directions for the future, and we finally conclude that a deep and thorough understanding of the origin of FM with and without strain is very important for the development of basic research and practical applications.

Electrocatalytic activity of a β-Sb two-dimensional surface for the hydrogen evolution reaction

Hydrogen energy is considered to be one of the most promising clean energy sources. The development of highly active, low-cost catalysts, and good stability is essential for hydrogen production. Herein, the catalytic activity of a two-dimensional β-Sb surface doped with main-group elements (N, P, As, O, S, Se, and Te) for the hydrogen evolution reaction (HER) was investigated by density functional theory, and the catalytic activity of the β-Sb monolayer can be improved by doping group VIA atoms. The catalytic activity of Se@Sb and O@Sb structures at the doping concentration of 2.78% and the S@Sb structure at the doping concentration of 5.56% may be as good as the Pt(111) surface, while keeping energetically stable. In addition, the catalytic performance could be optimized under biaxial strain. Further analysis suggests that the activity is caused by hole states in the lone pair electrons, which are created by the group VIA atom dopants. And our work also reveals that the density of states at the Fermi level could be an appropriate descriptor of the hydrogenation Gibbs free energy. This work not only proposes a novel non-platinum HER catalyst but also provides physical foundations for further application on antimonene-based catalysts.

A density functional theory study on the strain modulated electronic and photocatalytic properties of a GaSe monolayer for photocatalytic water splitting and artificial photosynthesis

The strain modulated electronic and photocatalytic properties of GaSe monolayer for photocatalytic water splitting and artificial photosynthesis using DFT study.

Concerted effect of Ni-in and S-out on ReS2 nanostructures towards high-efficiency oxygen evolution reaction

Herein, a one-step hydrothermal reaction is developed to synthesize Ni-doped ReS2 nanostructure with the sulphur defect. The material exhibited excellent OER activity with a current density of 10 mAcm-2 at...

Electrochemical Capacitor Performance of Nanotextured Carbon/Transition Metal Dichalcogenides Composites

Transition metal dichalcogenides (TMDs) are emerging low-dimensional materials with potential applications for electrochemical capacitors (EC). Here, physicochemical and electrochemical characterizations of carbon composites with two sulfides ReS₂ and FeS₂ are reported. To enhance conductivity, multiwalled carbon nanotubes (NTs) serve as a support for ReS₂ while 3D graphene-like network (3DG) is utilized for FeS₂ deposition. Unique structure of carbon/TMDs composites allows a faradaic contribution of sulfides to be exploited. Capacitance values, charge/discharge efficiency, capacitance retention, charge propagation, cyclabilty, and voltage limits of EC with carbon/sulfide composites in aqueous neutral solutions (Li₂ SO₄ , Na₂ SO₄ ) are analyzed. Special attention is devoted to energetic efficiency of capacitive charge/discharge processes. Structure-to-capacitance correlation for the composites with various TMDs loading is thoroughly emphasized. The more defected structure of layered NTs/ReS₂ composite is responsible for the lower capacitor voltage (0.8 V) owing to quicker electrolyte decomposition. Additionally, the catalytic effect of Re for hydrogen evolution reaction plays a crucial role in EC voltage restriction. Contrary, the operating voltage of capacitor based on 3DG/FeS₂ is able to be extended until 1.5 V in sodium sulfate electrolytic solution.

Multilevel Theoretical Screening of Novel Two-Dimensional MA2Z4 Family for Hydrogen Evolution

The synthesis of 2D MoSi₂N₄ marked the birth of a new family of 2D MA₂Z₄ materials, whose potential applications are expected to sweep the field of nanoscale devices and catalysis in the future. In this work, we propose a multilevel screening workflow to systematically explore the mechanic stabilities, electronic properties, and hydrogen evolution performances of the 2D MA₂Z₄ family, among which seven stable, metallic, and highly active 2D MA₂Z₄ monolayers (2H-α-VGe₂N₄, 2H-α-NbGe₂N₄, 2H-α-TaGe₂N₄, 2H-α-NbSi₂N₄, 2H-β-VGe₂N₄, 2H-β-NbGe₂N₄, and 2H-β-TiGe₂P₄) are predicted as promising hydrogen evolution reaction (HER) catalysts with near-zero hydrogen adsorption free energy (ΔG_H). The lowest unoccupied state energy (E_LUS) of the MA₂Z₄ basal plane is identified as a concise descriptor to influence the electron filling and eventually determine its ΔG_H. The criteria of -6.0 < E_LUS < -5.6 eV is confirmed as the HER active window to explore novel HER catalysts. In particular, group-VI-terminated MA₂Z₄ basal planes have a higher E_LUS (> -5.6 eV), which leads to weak H adsorption and poor HER activities accordingly.

Phase Transitions and Water Splitting Applications of 2D Transition Metal Dichalcogenides and Metal Phosphorous Trichalcogenides

2D layered materials turn out to be the most attractive hotspot in materials for their unique physical and chemical properties. A special class of 2D layered material refers to materials exhibiting phase transition based on environment variables. Among these materials, transition metal dichalcogenides (TMDs) act as a promising alternative for their unique combination of atomic-scale thickness, direct bandgap, significant spin-orbit coupling and prominent electronic and mechanical properties, enabling them to be applied for fundamental studies as catalyst materials. Metal phosphorous trichalcogenides (MPTs), as another potential catalytic 2D phase transition material, have been employed for their unusual intercalation behavior and electrochemical properties, which act as a secondary electrode in lithium batteries. The preparation of 2D TMD and MPT materials has been extensively conducted by engineering their intrinsic structures at the atomic scale. In this study, advanced synthesis methods of preparing 2D TMD and MPT materials are tested, and their properties are investigated, with stress placed on their phase transition. The surge of this type of report is associated with water-splitting catalysis and other catalytic purposes. This study aims to be a guideline to explore the mentioned 2D TMD and MPT materials for their catalytic applications.

MX Anti-MXenes from Non-van der Waals Bulks for Electrochemical Applications: The Merit of Metallicity and Active Basal Plane

Two-dimensional transition-metal compounds (2DTMCs) are promising materials for electrochemical applications, but 2DTMCs with metallicity and active basal planes are rare. In this work, we proposed a simple and effective strategy to extract 2DTMCs from non-van der Waals bulk materials and established a material library of 79 2DTMCs, which we named as anti-MXenes since they are composed of one M atomic layer sandwiched by two X atomic layers. By means of density functional theory computations, 24 anti-MXenes were confirmed to be thermodynamically, dynamically, mechanically, and thermally stable. The metallicity and active basal plane endow these anti-MXenes with potential as excellent electrode materials, for example, as electrocatalysts for hydrogen evolution reactions (HER). Among the noble-metal free anti-MXenes with favorable H-binding, CuS can boost HER at the whole range of H coverages, while CoSi, FeB, CoB, and CoP show promise for HER at some specific H coverages. The active sites are the tetra-coordinating nonmetal atoms at the basal planes, thus rendering a very high density of active sites for these materials. CoB is also a promising anode material for lithium-ion batteries, showing low Li diffusion energy barriers, a very high capacity, and a suitable open circuit voltage. This work promotes the "computational exfoliation" of 2D materials from non-van der Waals bulks and exemplifies the applications of anti-MXenes in various electrochemical processes.

Construction of heterojunctions between ReS2 and twin crystal ZnxCd1−xS for boosting solar hydrogen evolution

Nanoflower-like ReS₂ anchoring on nanotwins Zn_xCd_1−xS greatly boosts photocatalytic hydrogen evolution rate with 31-times higher than pure phase P-ZCS.

2D Re-Based Transition Metal Chalcogenides: Progress, Challenges, and Opportunities

The rise of 2D transition-metal dichalcogenides (TMDs) materials has enormous implications for the scientific community and beyond. Among TMDs, ReX2 (X = S, Se) has attracted significant interest regarding its unusual 1T' structure and extraordinary properties in various fields during the past 7 years. For instance, ReX2 possesses large bandgaps (ReSe2: 1.3 eV, ReS2: 1.6 eV), distinctive interlayer decoupling, and strong anisotropic properties, which endow more degree of freedom for constructing novel optoelectronic, logic circuit, and sensor devices. Moreover, facile ion intercalation, abundant active sites, together with stable 1T' structure enable them great perspective to fabricate high-performance catalysts and advanced energy storage devices. In this review, the structural features, fundamental physicochemical properties, as well as all existing applications of Re-based TMDs materials are comprehensively introduced. Especially, the emerging synthesis strategies are critically analyzed and pay particular attention is paid to its growth mechanism with probing the assembly process of domain architectures. Finally, current challenges and future opportunities regarding the controlled preparation methods, property, and application exploration of Re-based TMDs are discussed.

Phase Evolution of Re1-xMoxSe2 Alloy Nanosheets and Their Enhanced Catalytic Activity toward Hydrogen Evolution Reaction

Two-dimensional ReSe₂ has emerged as a promising electrocatalyst for the hydrogen evolution reaction (HER), but its catalytic activity needs to be further improved. Herein, we synthesized Re_1-xMo_xSe₂ alloy nanosheets with the whole range of x (0-100%) using a hydrothermal reaction. The phase evolved in the order of 1T″ (triclinic) → 1T' (monoclinic) → 2H (hexagonal) upon increasing x. In the nanosheets with x = 10%, the substitutional Mo atoms tended to aggregate in the 1T″ ReSe₂ phase with Se vacancies. The incorporation of the 1T' phase makes the alloy nanosheets more metallic than the end compositions. The 10% Mo substitution significantly enhanced the electrocatalytic performance toward HER (in 0.5 M H₂SO₄), with a current of 10 mA cm^-2 at an overpotential of 77 mV (vs RHE) and a Tafel slope of 42 mV dec^-1. First-principles calculations of the three phases (1T″, 2H, and 1T') predicted a phase transition of 1T″-2H at x ≈ 65% as well as the production of a 1T' phase along the composition tuning, which are consistent with the experiments. At x = 12.5%, two Mo atoms prefer to form a pair along the Re₄ chains. Gibbs free energy along the reaction path indicates that the best HER performance of nanosheets with 10% Mo originates from the Mo atoms that form Mo-H when there are adjacent Se vacancies.

Adatom Doping of Transition Metals in ReSe2 Nanosheets for Enhanced Electrocatalytic Hydrogen Evolution Reaction

Two-dimensional Re dichalcogenide nanostructures are promising electrocatalysts for the hydrogen evolution reaction (HER). Herein, we report the adatom doping of various transition metals (TM = Mn, Fe, Co, Ni, and Cu) in ReSe₂ nanosheets synthesized using a solvothermal reaction. As the atomic number of TM increases from Mn to Cu, the adatoms on Re sites become more favored over the substitution. In the case of Ni, the fraction of adatoms reaches 90%. Ni doping resulted in the most effective enhancement in the HER catalytic performance, which was characterized by overpotentials of 82 and 109 mV at 10 mA cm^-2 in 0.5 M H₂SO₄ and 1 M KOH, respectively, and the Tafel slopes of 54 and 81 mV dec^-1. First-principles calculations predicted that the adatom doping structures (TMs on Re sites) have higher catalytic activity compared with the substitution ones. The adsorbed H atoms formed a midgap hybridized state via direct bonding with the orbitals of TM adatom. The present work provides a deeper understanding into how TM doping can provide the catalytically active sites in these ReSe₂ nanosheets.

Nanoscale engineering and Mo-doping of 2D ultrathin ReS2 nanosheets for remarkable electrocatalytic hydrogen generation

Two-dimensional (2D) lamellar ReS2 nanosheets are considered a promising electrocatalyst for the hydrogen evolution reaction (HER) but suffer from poor intrinsic conductivity and catalytically inert basal planes. In this work, sub 50 nm hierarchical Mo-doped ReS2 nanospheres consisting of numerous few-layered and defect-rich nanosheets are designed and synthesized as robust and efficient HER catalysts. On the one hand, the small size of the hierarchical structure, the disordered basal planes and the expanded interlayer endow the nanosheets with plentiful defects, thereby resulting in abundant exposed active sites. On the other hand, Mo-doping offers the nanosheets with some electronic benefits of unsaturated electrons, improved intrinsic conductivity, and optimized hydrogen adsorption free energy (ΔGH) of the basal planes. Owing to the synergistic effects, the 10%Mo-ReS2 catalyst exhibits an optimized catalytic activity with striking kinetic metrics of a small Tafel slope of 62 mV dec-1, a low overpotential of 81 mV at 10 mA cm-2, and a long operation stability of 50 h, and its performance is among the best of ReS2-based catalysts. This work provides a new approach for gaining the structural and electronic benefits of ReS2 catalysts by combinational nanoscale engineering and heteroatom doping.

Functionalization of two-dimensional 1T'-ReS2 with surface ligands for use as a photocatalyst in the hydrogen evolution reaction: a first-principles calculation study

Density functional theory calculations were performed to tune the band edge positions of two-dimensional 1T'-ReS2 by functionalization with surface ligands. A shift in the band edge was caused by the intrinsic dipole of the ligand and the induced dipole at the ligand/ReS2 interface. The upward shift in the band edge was tuned over a large range by choosing suitable polar ligands, controlling the surface coverage by the ligand, functionalizing the ligand and building heterostructures. The C6H5CN/ReS2/MoS2 and C6H5CH2CN/ReS2/MoS2 van der Waals heterostructures are ideal candidates for use as photocatalysts in the splitting of water as a result of their strong absorption in the visible region, suitable band edge positions, reduced electron-hole recombination and good stability. Our findings show the potential for creating novel photocatalysts based on van der Waals heterostructures of ReS2.

Robust ferromagnetism in Cr-doped ReS2 nanosheets demonstrated by experiments and density functional theory calculations

As one of the transition metal dichalcogenides (TMDs), ReS₂ displays several outstanding properties, while the intrinsically nonmagnetic property limits its applications in spin-related devices. In this study, we selected Cr as the dopant to realize the robust room-temperature ferromagnetism in Cr-doped ReS₂ (Cr-ReS₂) nanosheets. The saturation magnetization (M _s ) of the samples can be tuned by changing the Cr concentration. Density functional theory calculation results reveal that Cr dopant can provide the magnetic moments and stable ferromagnetic coupling in Cr-doped ReS₂ system. This finding provides an effective approach for designing the new magnetic TMDs in spintronics devices.