Design of MoS2/Graphene van der Waals Heterostructure as Highly Efficient and Stable Electrocatalyst for Hydrogen Evolution in Acidic and Alkaline Media

Synthesis, Modulation, and Application of Two-Dimensional TMD Heterostructures

Two-dimensional (2D) transition metal dichalcogenide (TMD) heterostructures have attracted a lot of attention due to their rich material diversity and stack geometry, precise controllability of structure and properties, and potential practical applications. These heterostructures not only overcome the inherent limitations of individual materials but also enable the realization of new properties through appropriate combinations, establishing a platform to explore new physical and chemical properties at micro-nano-pico scales. In this review, we systematically summarize the latest research progress in the synthesis, modulation, and application of 2D TMD heterostructures. We first introduce the latest techniques for fabricating 2D TMD heterostructures, examining the rationale, mechanisms, advantages, and disadvantages of each strategy. Furthermore, we emphasize the importance of characteristic modulation in 2D TMD heterostructures and discuss some approaches to achieve novel functionalities. Then, we summarize the representative applications of 2D TMD heterostructures. Finally, we highlight the challenges and future perspectives in the synthesis and device fabrication of 2D TMD heterostructures and provide some feasible solutions.

Encapsulating CoNi nanoparticles into nitrogen-doped carbon nanotube arrays as bifunctional oxygen electrocatalyst for rechargeable zinc-air batteries

The high theoretical specific energy and environmental friendliness of zinc-air batteries (ZABs) have garnered significant attention. However, the practical application of ZABs requires overcoming the sluggish kinetics associated with oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Herein, 3D self-supported nitrogen-doped carbon nanotubes (N-CNTs) arrays encapsulated by CoNi nanoparticles on carbon fiber cloth (CoNi@N-CNTs/CFC) are synthesized as bifunctional catalysts for OER and ORR. The 3D interconnected N-CNTs arrays not only improve the electrical conductivity, the permeation and gas escape capabilities of the electrode, but also enhance the corrosion resistance of CoNi metals. DFT calculations reveal that the co-existence of Co and Ni synergistically reduces the energy barrier for OOH conversion to OH, thereby optimizing the Gibbs free energy of the catalysts. Additionally, analysis of the change in energy barrier during the rate-determining step suggests that the primary catalytic active center is Ni site for OER. As a result, CoNi@N-CNTs/CFC exhibits superior catalytic activity with an overpotential of 240 mV at 10 mA cm-2 toward OER, and the onset potential of 0.92 V for ORR. Moreover, utilization of CoNi@N-CNTs/CFC in liquid and solid-state ZABs exhibited exceptional stability, manifesting a consistent cycling operation lasting for 100 and 15 h, respectively.

Vertical heterostructure of graphite-MoS2 for gas sensing

2D materials, given their form-factor, high surface-to-volume ratio, and chemical functionality have immense use in sensor design. Engineering 2D heterostructures can result in robust combinations of desirable properties but sensor design methodologies require careful considerations about material properties and orientation to maximize sensor response. This study introduces a sensor approach that combines the excellent electrical transport and transduction properties of graphite film with chemical reactivity derived from the edge sites of semiconducting molybdenum disulfide (MoS2) through a two-step chemical vapour deposition method. The resulting vertical heterostructure shows potential for high-performance hybrid chemiresistors for gas sensing. This architecture offers active sensing edge sites across the MoS2 flakes. We detail the growth of vertically oriented MoS2 over a nanoscale graphite film (NGF) cross-section, enhancing the adsorption of analytes such as NO2, NH3, and water vapor. Raman spectroscopy, density functional theory calculations and scanning probe methods elucidate the influence of chemical doping by distinguishing the role of MoS2 edge sites relative to the basal plane. High-resolution imaging techniques confirm the controlled growth of highly crystalline hybrid structures. The MoS2/NGF hybrid structure exhibits exceptional chemiresistive responses at both room and elevated temperatures compared to bare graphitic layers. Quantitative analysis reveals that the sensitivity of this hybrid sensor surpasses other 2D material hybrids, particularly in parts per billion concentrations.

Nickel Nanoparticles Protruding from Molybdenum Carbide Micropillars with Carbon Layer-Protected Biphasic 0D/1D Heterostructures for Efficient Water Splitting

It remains a tremendous challenge to achieve high-efficiency bifunctional electrocatalysts for both the hydrogen evolution reaction (HER) and the oxygen evolution reaction (OER) for hydrogen production by water splitting. Herein, a novel hybrid of 0D nickel nanoparticles dispersed on the one-dimensional (1D) molybdenum carbide micropillars embedded in the carbon layers (Ni/Mo2C@C) was successfully prepared on nickel foam by a facile pyrolysis strategy. During the synthesis process, the nickel nanoparticles and molybdenum carbide were simultaneously generated under H2 and C2H2 mixed atmospheres and conformally encapsulated in the carbon layers. Benefiting from the distinctive 0D/1D heterostructure and the synergistic effect of the biphasic Mo2C and Ni together with the protective effect of the carbon layer, the reduced activation energy barriers and fast catalytic reaction kinetics can be achieved, resulting in a small overpotential of 96 mV for the HER and 266 mV for the OER at the current density of 10 mA cm-2 together with excellent durability in 1.0 M KOH electrolyte. In addition, using the developed Ni/Mo2C@C as both the cathode and anode, the constructed electrolyzer exhibits a small voltage of 1.55 V for the overall water splitting. The novel designed Ni/Mo2C@C may give inspiration for the development of efficient bifunctional catalysts with low-cost transition metal elements for water splitting.

A quadruplet 3-D laser scribed graphene/MoS2, functionalised N2-doped graphene quantum dots and lignin-based Ag-nanoparticles for biosensing

Troponin I is a protein released into the human blood circulation and a commonly used biomarker due to its sensitivity and specificity in diagnosing myocardial injury. When heart injury occurs, elevated troponin Troponin I levels are released into the bloodstream. The biomarker is a strong and reliable indicator of myocardial injury in a person, with immediate treatment required. For electrochemical sensing of Troponin I, a quadruplet 3D laser-scribed graphene/molybdenum disulphide functionalised N2-doped graphene quantum dots hybrid with lignin-based Ag-nanoparticles (3D LSG/MoS2/N-GQDs/L-Ag NPs) was fabricated using a hydrothermal process as an enhanced quadruplet substrate. Hybrid MoS2 nanoflower (H3 NF) and nanosphere (H3 NS) were formed independently by varying MoS2 precursors and were grown on 3D LSG uniformly without severe stacking and restacking issues, and characterized by morphological, physical, and structural analyses with the N-GQDs and Ag NPs evenly distributed on 3D LSG/MoS2 surface by covalent bonding. The selective capture of and specific interaction with Troponin I by the biotinylated aptamer probe on the bio-electrode, resulted in an increment in the charge transfer resistance. The limit of detection, based on impedance spectroscopy, is 100 aM for both H3 NF and H3 NS hybrids, with the H3 NF hybrid biosensor having better analytical performance in terms of linearity, selectivity, repeatability, and stability.

Two-Dimensional Nanoarchitectonics for Two-Dimensional Materials: Interfacial Engineering of Transition-Metal Dichalcogenides

Transition-metal dichalcogenides (TMDs) have attracted increasing attention in fundamental studies and technological applications owing to their atomically thin thickness, expanded interlayer distance, motif band gap, and phase-transition ability. Even though TMDs have a wide variety of material assets from semiconductor to semimetallic to metallic, the materials with fixed features may not show excellence for precise application. As a result of exclusive crystalline polymorphs, physical and chemical assets of TMDs can be efficiently modified via various approaches of interface nanoarchitectonics, including heteroatom doping, heterostructure, phase engineering, reducing size, alloying, and hybridization. With modified properties, TMDs become interesting materials in diverse fields, including catalysis, energy, electronics, transistors, and optoelectronics.

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.

Highly efficient MoS2/WS2 heterojunctions for the CO2 reduction reaction: strong electronic transmission

Transition metal dichalcogenides (TMDs) possess several advantages, such as high conductivity, stable structure, and low cost, making them promising catalysts for carbon dioxide electroreduction. However, the high overpotential and the desorption characteristics of the reaction products during the reduction of carbon dioxide present significant challenges in the field of catalysis. In this study, we have further enhanced the catalytic activity of the original WS2 structure by constructing a heterojunction. We systematically investigate the catalytic activity of MoS2/WS2 heterojunctions supported by transition metals using density functional theory (DFT) calculations. The findings of this study are as follows: (1) the unique multiphase structure enhances the catalytic performance for CO2 reduction. (2) After constructing the MoS2/WS2 heterojunction, the electronic properties and conductivity of the heterojunction can be significantly enhanced, thereby facilitating the catalytic reduction of carbon dioxide. The Cu loading on the Cu@MoS2/WS2 heterojunction significantly reduces the overpotential, with a very low limit potential of -0.58 V. The adsorption behavior of CO on the Cu@MoS2/WS2 heterojunction was evaluated using adsorption energy, desorption energy, and density of states (DOS). The appropriate interaction between CO and Cu@ MoS2/WS2 promotes the reduction of CO2 to CO and facilitates smooth desorption of CO, demonstrating a strong catalytic effect on the CO2 reduction reaction (CO2RR). Therefore, it can be seen that Cu@MoS2/WS2 may be considered as potential single-atom catalysts (SACs) for CO2 reduction electrocatalysts. Finally, it is hoped that our results will provide theoretical support for the development of efficient CO2 reduction catalysts.

Alkylamine-Grafted Molybdenum Disulfide Nanosheets for Enhanced Dispersion Stability and Lubricity Properties

The unique structure and ultralow interlayer shear strength give molybdenum disulfide (MoS2) materials a broad prospect for energy savings, economic benefits, and extended operating life of lubrication systems. Herein, we prepared an effective integration strategy to prepare novel small-sized and chemically grafted MoS2 to solve the problems of poor dispersibility and easy agglomeration of MoS2. The MoS2 powder was stripped and oxidized to generate active centers using acid oxidation and high-speed ultrasonic crushing to obtain two different types of alkylamine chemically, covalently grafted, oxidized MoS2 nanosheets as lubricant additives to achieve friction reduction and antiwear. The chemical changes and structural characteristics of different types of alkylamine molecules upon covalent interaction with oxidized MoS2 were investigated in detail by FTIR, XPS, TGA, XRD, and TEM analyses. The results showed that the alkylamine-grafted MoS2 oxide nanosheets had good dispersion in 15# industrial white oil, and friction experiments confirmed that the alkylamine-grafted MoS2 oxide (MoS2-O-OLA) nanosheets exhibited better friction and wear resistance such that, compared with pure 15# industrial white oil, the 0.02 wt % MoS2-O-OLA nanosheets could significantly reduce friction (36.2%) and wear (22.4%). The field-emission scanning electron microscopy (FESEM) and EDS analyses of the wear surface showed that MoS2-O-OLA nanosheets play an important role in improving tribological properties by generating interlayer slippage at the steel ball contact interface, thereby forming surface protection and a uniform oil film.

In-plane gate graphene transistor with epitaxially grown molybdenum disulfide passivation layers

We demonstrate in-plane gate transistors based on the molybdenum disulfide (MoS₂)/graphene hetero-structure. The graphene works as channels while MoS₂ functions as passivation layers. The weak hysteresis of the device suggests that the MoS₂ layer can effectively passivate the graphene channel. The characteristics of devices with and without removal of MoS₂ between electrodes and graphene are also compared. The device with direct electrode/graphene contact shows a reduced contact resistance, increased drain current, and enhanced field-effect mobility. The higher field-effect mobility than that obtained through Hall measurement indicates that more carriers are present in the channel, rendering it more conductive.

Efficient Hydrogen Evolution via 1T-MoS2 /Chlorophyll-a Heterostructure: Way Toward Metal Free Green Catalyst

Electrocatalytic hydrogen evolution reaction (HER) is regarded as a sustainable and green way for H₂ generation, which faces a great challenge in designing highly active, stable electrocatalysts to replace the state-of-art noble metal-platinum catalysts. 1T MoS₂ is highly promising in this regard, but the synthesis and stability of this is a particularly pressing task. Here, a phase engineering strategy has been proposed to achieve a stable, high-percentage (88%) 1T MoS₂ /chlorophyll-a hetero-nanostructure, through a photo-induced donation of anti-bonding electrons from chlorophyll-a (CHL-a) highest occupied molecular orbital to 2H MoS₂ lowest unoccupied molecular orbital. The resultant catalyst has abundant binding sites provided by the coordination of magnesium atom in the CHL-a macro-cycle, featuring higher binding strength and low Gibbs-free energy. This metal-free heterostructure exhibits excellent stability via band renormalization of Mo 4d orbital which creates the pseudogap-like structure by lifting the degeneracy of projected density of state with 4S in 1T MoS₂ . It shows extremely low overpotential, toward the acidic HER (68 mV at the current density of 10 mA cm^-2 ), very close to the Pt/C catalyst (53 mV). The high electrochemical-surface-area and electrochemical turnover frequency support enhanced active sites along with near zero Gibbs free energy. Such a surface-reconstruction strategy provides a new avenue toward the production of efficient non-noble-metal-catalysts for the HER with the aim of green-hydrogen production.

Role of introduced Se element and induced anion vacancies in Mo(SSe)2-x/G van der Waals heterostructure for enhanced hydrogen evolution reaction

The Gibbs free energy of hydrogen adsorption at the edge of molybdenum disulfide (MoS₂) is close to that of Pt, meaning that MoS₂ is the best candidate to replace Pt-based materials. However, easy agglomeration between layers to mask active sites, lack of catalytic activity in the basal planes, and poor electronic conductivity make MoS₂ exhibit dissatisfactory hydrogen evolution reaction (HER) catalytic performance. Here, we successfully construct a van der Waals heterostructure stacked alternately with Mo(SSe)_2-x and graphene (Mo(SSe)_2-x/G) to enhance its catalytic ability. The introduction of Se into MoS₂ and the thermal treatment induce the sample to generate more anion vacancies. Theoretical and experimental results demonstrate the constructed van der Waals heterostructure, the introduced Se element, and the increased anion vacancies are in favor of promoting the number of active sites and improving the electronic conductivity of the catalyst. Therefore, Mo(SSe)_2-x/G exhibits superior HER catalytic performance (the overpotentials of 137 mV and 136 mV at a current of 10 mA cm^-2) and long-term stabilities (>90 h and 140 h at a current density of 20 mA cm^-2) in both acidic and alkaline media.

First-principles-driven catalyst design protocol of 2D/2D heterostructures for electro- and photocatalytic nitrogen reduction reaction

Ammonia synthesis from nitrogen is a vital process and a necessity in a variety of applications including energy, pharmaceutical, agricultural, and chemical applications. The electro- and photocatalytic nitrogen reduction reactions (NRRs) are promising sustainable processes operated under milder conditions than the conventional Haber-Bosch process. However, the main pain points of these catalytic processes are their low selectivity and low efficiency. This perspective presents the recent status and the design protocols for developing promising 2D/2D heterojunction catalysts for the NRR, using the first-principles approach. The current theoretical studies are briefly discussed, and available methods are suggested for the development and design of new potential 2D/2D heterojunctions as efficient electro- and photo-NRR catalysts.

Facet synergy dominant Z-scheme transition in BiOCl with enhanced 1O2 generation

BiOCl powders with different morphology were obtained through self-assembling. Their photocatalytic performance was tested through degradation of organic dye and mechanism of photocatalytic for obtained samples were investigated. Relevant characterization demonstrated that facet synergy was a main reason of photocatalytic performance promotion due to changed facet exposure and proportion under self-assembling. Theory and experimental analysis manifested that synergistic facet stimulated Z scheme transition in samples with lower (001) facet proportion, which provided favorable condition of ¹O₂ generation and simultaneously generated prominent charge separation. This work unveiled the facet synergy dominant photocatalytic performance improvement in self-assembling system of BiOCl and verified decisive role of facet proportion in constructing Z-scheme facet junction, which also prompted possibility of improving ¹O₂ generation through facet engineering under self-assembling.

Platinum-like HER onset in a GNR/MoS2 quantum dot heterostructure through curvature-dependent electron density reconfiguration

Tailoring the curvature-directed lattice strain in GNRs along with optimal surface anchoring of molybdenum disulfide (MoS₂) quantum dots (QDs) can lead to a unique heterostructure with Pt-like HER activity (onset potential -60 mV). The curvature-induced electronic charge redistribution at the curved region in the graphene nanoribbons allows a facile GNR-MoS₂ interfacial charge transfer in the heterostructure, making the interfacial sulfur (S) more active towards the HER. The density functional theory (DFT) calculations confirmed electronically activated interfacial S-based catalytic centers in the curved GNR-based heterostructure leading to Pt-like HER activity.

Janus MoPC Monolayer with Superior Electrocatalytic Performance for the Hydrogen Evolution Reaction

Designing the earth's abundant and high-performance electrocatalysts, which possess high stability, excellent electrical conductivity, inherent active sites, and catalytic activity identical with Pt, is challenging but crucial for the hydrogen evolution reaction (HER). By first-principles structure search simulations, we identify a new two-dimensional (2D) MoPC material with the Janus structure as a promising catalyst. This novel 2D monolayer has superior stability and metallic conductivity. Especially, it exhibits a remarkable HER catalytic activity, where all of the constituent atoms, including Mo, P, and C, can uniformly act as active sites in view of the near-zero ΔG_H* value. Its active site density counts up to 1.46 × 10¹⁵ site/cm², larger than that of many reported materials and even comparable to Pt. The excellent HER catalytic activity can also be maintained at a very high H coverage with or without external strain. The MoPC monolayer can produce H₂ spontaneously through the favorable Volmer-Heyrovsky pathway. The detailed catalytic mechanism behind the high HER activity has been also analyzed. Our work provides a feasible action for the experimental synthesis of excellent HER catalysts.

Three-dimensional cross-linked Co-MoS2 catalyst on carbon cloth for efficient hydrogen evolution reaction

MoS₂-Based materials are promising hydrogen evolution reaction (HER) electrocatalysts. However, their HER activities are restrained by the poor population of HER activated edge centers, the large area exposed HER inert basal planes, and low conductivity. Fixing these problems on one system is an effective strategy, but it remains a challenge due to the harsh synthetic conditions. Herein, cobalt carbonate hydroxide (CoCH) nanosheets were used as the substrate for preparing a three-dimensional self-supported cross-linked (3DSC) Co-MoS₂ nanostructured HER catalyst, which possesses abundant active centers and fast electronic transfer ability. In addition, Co activates the basal-plane sulfur atom in MoS₂ to be the HER reactive center effectively. Benefiting from these advantages, 3DSC Co-MoS₂ electrode integrated on carbon cloth (CC) shows that it can drive the current density of 10 and 100 mA cm^-2 with only 40 and 119 mV overpotentials, respectively, which is superior to other MoS₂-based HER catalysts reported recently. This research provides a facile strategy for the improvement of efficient HER electrocatalysts.

Edge-Rich Reduced Graphene Oxide Embedded in Silica-Based Laminated Ceramic Composites for Efficient and Robust Electrocatalytic Hydrogen Evolution

To mitigate the energy crisis and environmental pollution, efficient and earth-abundant hydrogen evolution reaction (HER) electrocatalysts are essential for hydrogen production through electrochemical water splitting. Graphene-based materials as metal-free catalysts have attracted significant attention but suffer from insufficient activity and stability. Therefore, a novel and economical approach is developed to prepare highly active, robust, and self-supported reduced graphene oxide (rGO)/SiO₂ ceramic composites as electrocatalysts in HER. Through intercalation and pressure sintering, the rGO sheets are parallelly aligned and embedded into a dense and chemically inert SiO₂ matrix, ensuring the electrical conductivity and stability of the prepared composites. After directional cutting, the edges of the oriented rGO sheets become fully exposed on the composite surface, acting as highly electrocatalytic active sites in HER, as confirmed by density functional theory calculations. The 4 vol% rGO/SiO₂ composite displays superior electrocatalytic performance, featuring a low overpotential (134 mV) at a current density of 10 mA cm^-2 , a small Tafel slope (103 mV dec^-1 ), and excellent catalytic durability in 0.5 m H₂ SO₄ . This study provides a new yet cost-effective strategy to prepare metal-free, robust, and edge-rich rGO/ceramic composites as a highly electrocatalytic active catalyst for HER applications.

Van der Waals Heterostructures-Recent Progress in Electrode Materials for Clean Energy Applications

The unique layered morphology of van der Waals (vdW) heterostructures give rise to a blended set of electrochemical properties from the 2D sheet components. Herein an overview of their potential in energy storage systems in place of precious metals is conducted. The most recent progress on vdW electrocatalysis covering the last three years of research is evaluated, with an emphasis on their catalytic activity towards the oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER). This analysis is conducted in pair with the most active Pt-based commercial catalyst currently utilized in energy systems that rely on the above-listed electrochemistry (metal-air battery, fuel cells, and water electrolyzers). Based on current progress in HER catalysis that employs vdW materials, several recommendations can be stated. First, stacking of the two types vdW materials, with one being graphene or its doped derivatives, results in significantly improved HER activity. The second important recommendation is to take advantage of an electronic coupling when stacking 2D materials with the metallic surface. This significantly reduces the face-to-face contact resistance and thus improves the electron transfer from the metallic surface to the vdW catalytic plane. A dual advantage can be achieved from combining the vdW heterostructure with metals containing an excess of d electrons (e.g., gold). Despite these recent and promising discoveries, more studies are needed to solve the complexity of the mechanism of HER reaction, in particular with respect to the electron coupling effects (metal/vdW combinations). In addition, more affordable synthetic pathways allowing for a well-controlled confined HER catalysis are emerging areas.

A review on vertical and lateral heterostructures of semiconducting 2D-MoS2 with other 2D materials: a feasible perspective for energy conversion

Fossil fuels as a double-edged sword are essential to daily life. However, the depletion of fossil fuel reservoirs has increased the search for alternative renewable energy sources to procure a more sustainable society. Accordingly, energy production through water splitting, CO₂ reduction and N₂ reduction via photocatalytic and electrocatalytic pathways is being contemplated as a greener methodology with zero environmental pollution. Owing to their atomic-level thickness, two-dimensional (2D) semiconductor catalysts have triggered the reawakening of interest in the field of energy and environmental applications. Among them, following the unconventional properties of graphene, 2D MoS₂ has been widely investigated due to its outstanding optical and electronic properties. However, the photo/electrocatalytic performance of 2D-MoS₂ is still unsatisfactory due to its low charge carrier density. Recently, the development of 2D/2D heterojunctions has evoked interdisciplinary research fascination in the scientific community, which can mitigate the shortcomings associated with 2D-MoS₂. Following the recent research trends, the present review covers the recent findings and key aspects on the synthetic methods, fundamental properties and practical applications of semiconducting 2D-MoS₂ and its heterostructures with other 2D materials such as g-C₃N₄, graphene, CdS, TiO₂, MXene, black phosphorous, and boron nitride. Besides, this review details the viable application of these materials in the area of hydrogen energy production via the H₂O splitting reaction, N₂ fixation to NH₃ formation and CO₂ reduction to different value-added hydrocarbons and alcohol products through both photocatalysis and electrocatalysis. The crucial role of the interface together with the charge separation principle between two individual 2D structures towards achieving satisfactory activity for various applications is presented. Overall, the current studies provide a snapshot of the recent breakthroughs in the development of various 2D/2D-based catalysts in the field of energy production, delivering opportunities for future research.

A palladium doped 1T-phase molybdenum disulfide-black phosphorene two-dimensional van der Waals heterostructure for visible-light enhanced electrocatalytic hydrogen evolution

The electrocatalytic hydrogen evolution reaction (HER) is a green chemistry route for sustainable energy production. Compared to 2H-phase molybdenum disulfide (MoS₂), the 1T-phase MoS₂ (1T-MoS₂) has higher theoretical activity and faster charge transfer kinetics, but the HER performance of 1T-MoS₂ is commonly hindered by limited active edge/defect as well as poor structural stability. Herein, we synthesize a well-defined 2D vdW heterostructure composed of Pd doped 1T-MoS₂ and black phosphorus (BP) nanosheets via electrostatic self-assembly. The spontaneous Pd doping under mild reaction conditions could introduce catalytically active sulfur vacancies in MoS₂ without triggering a wide range of 1T to 2H phase transformation. The hetero-interfacial charge transfer from BP to Pd-1T-MoS₂ can effectively improve the intrinsic activity of Pd-1T-MoS₂ with a relatively low S vacancy concentration and simultaneously stabilize the 1T-phase structure. Due to the wide-range light absorption of BP nanosheets and the high carrier mobilities of 2D materials, the HER activity of the obtained Pd-1T-MoS₂/BP could be further enhanced under ≥420 nm visible light illumination.

Modulation of electronic structures in two-dimensional electrocatalysts for the hydrogen evolution reaction

The electrocatalytic hydrogen evolution reaction (HER) has attracted substantial attention owing to its important role in realizing economic and sustainable hydrogen production via water electrolysis. Designing two-dimensional (2D) materials with large surface area, highly exposed surface sites and facile charge transport pathways is highly attractive for promoting the HER activity of the earth-abundant catalysts, and conducting rational modulations in the electronic structures is considered to be promising in further optimizing the intrinsic HER activity and thus realizing promoted HER performance. In this Feature Article, we systematically summarize recent progress in the modulation of the electronic structures of 2D HER electrocatalysts via multiple strategies including elemental doping, formation of alloyed structures, defect engineering, facet engineering, phase regulation, interface engineering and hybridization of the nanocatalysts with 2D substrates, and discuss the role of electronic structures in optimizing the intrinsic HER activity of 2D HER catalysts. We anticipate that this Feature Article will offer helpful guidance for oriented design and optimization of efficient electrocatalysts for scalable and economic hydrogen production.

A Universal Process: Self-Templated and Orientated Fabrication of XMoO4 (X: Ni, Co, or Fe) Nanosheets on MoO2 Nanoplates as Electrocatalysts for Efficient Water Splitting

Fabrication of superior nonprecious electrocatalysts is essential for water electrolysis. Herein, the epitaxial growth of the XMoO₄ (X = Ni, Co, Fe) nanosheets on the hexagonal MoO₂ nanoplates are carried out. The preoxidation of MoO₂ nanoplate is fatal to the epitaxial growth of a nanosheets array on MoO₂ nanoplates. The hierarchical heterostructure of the vertically aligned NiMo nanosheets on MoO₂ nanoplate (NiMo/MoO₂) is well-maintained in the process of in situ topotactic reduction transformation from NiMoO₄·xH₂O/MoO₂. Attributing it to the rich electroactive sites from nanosheets array, together with the intrinsic electrocatalytic performance of NiMo alloy, the as-engineered NiMo/MoO₂ as electrocatalyst exhibits admirable hydrogen evolution reaction (HER) activity with a small onset potential of -12 mV vs RHE (1 mA cm^-2) and a tafel value of 43.6 mV dec^-1 at alkaline media. Furthermore, the obtained CoMoO₄/MoO₂ possesses excellent oxygen evolution performance, which is verified by an ultralow overpotential of 230 mV@10 mA cm^-2, small Tafel slope (51 mV dec^-1), and robust durability. The developed NiMo/MoO₂ and CoMoO₄/MoO₂ electrocatalysts are assembled into an alkaline electrolyzer, which affords a cell potential of 1.51 V at 10 mA cm^-2, as well as outstanding operational durability, which is superior to the typically constructed 20 wt % Pt/C-RuO₂ system (1.59 V at 10 mA cm^-2). Hence, the universal strategy using MoO₂ nanoplates as Mo source and epitaxial substrate may be extended to explore and construct economical and superior Mo-based electrocatalysts for water electrolysis.

NiFe2O4 hollow nanoparticles of small sizes on carbon nanotubes for oxygen evolution

CNT-supported Ni–Fe bimetallic oxide hollow nanoparticles with an ultra-small size based on Kirkendall effect are fabricated and this catalyst exhibits excellent OER performances and robust stability, superior to the benchmark IrO₂ catalyst.