Roadmap and Direction toward High-Performance MoS2 Hydrogen Evolution Catalysts

Enhanced Basal-Plane Catalytic Activity of MoS2 by Constructing an Electron Bridge for High-Performance Lithium-Sulfur Batteries

MoS2 is a promising sulfur host material for lithium-sulfur (Li-S) batteries, but its low conductivity and limited active edge sites largely inhibit the catalytic activity toward the conversion of lithium polysulfides (LiPSs). Herein, we propose an electron bridge strategy by combining interlayer structure modification and electronic modulation to activate the basal-plane catalytic activity of MoS2 for the highly efficient catalytic conversion of LiPSs. As validated by experimental characterizations and theoretical calculations, the proposed strategy not only creates a conductive network but also induces delocalized electron redistribution within the MoS2 basal planes, leading to facilitated interfacial charge transfer kinetics and accelerated LiPSs redox kinetics. Because of these advantages, the Li-S batteries assembled with regulated MoS2 demonstrate outstanding electrochemical performance even under practical conditions. This work demonstrates the effectiveness and potential of regulating the intrinsic basal-plane catalytic activity of transition-metal dichalcogenides for Li-S batteries and beyond.

Interfacing CuO, CuBi2O4, and protective metal oxide layers to boost solar-driven photoelectrochemical hydrogen evolution

This article reports the development of CuO|CuBi2O4 photocathodes stabilized by protective layers of TiO2, MgO, or NiO, with Pt or MoS2 nanoparticles serving as co-catalysts to facilitate H2 evolution. Most notably, this work demonstrates the first application of MgO as a protection/passivation layer for photocathodes in a water-splitting cell. All configurations of photocathodes were studied structurally, morphologically, and photoelectrochemically revealing that CuO|CuBi2O4|MgO|Pt photocathodes achieve the highest stable photocurrent densities of -200 μA cm-2 for over 3 hours with a Faradaic efficiency of ∼90%. Bias-free tandem water splitting was then performed by pairing this photocathode with a dye-sensitized TiO2 photoanode, producing H2 from neutral water without an external bias. This paper demonstrates key stability findings and proposes the use of spin-coated MgO, TiO2, and NiO as feasible earth-abundant protective materials to aid in the formation of a cheap and scalable tandem water splitting system. Charge transfer dynamics have also been probed by combining spectroelectrochemistry and in situ transient absorption spectroscopy.

Biphasic 1T/2H-MoS2 Nanosheets In Situ Vertically Anchored on Reduced Graphene Oxide via Covalent Coupling of the Mo-O-C Bond for Enhanced Electrocatalytic Hydrogen Evolution

Transition-metal dichalcogenides (TMDs) have recently emerged as promising electrocatalysts for the hydrogen evolution reaction owing to their tunable electronic properties. However, TMDs still encounter inherent limitations, including insufficient active sites, poor conductivity, and instability; thus, their performance breakthrough mainly depends on structural optimization in hybridization with a conductive matrix and phase modulation. Herein, a 1T/2H-MoS2/rGO hybrid was rationally fabricated, which is characterized by biphasic 1T/2H-MoS2 nanosheets in situ vertically anchored on reduced graphene oxide (rGO) with strong C-O-Mo covalent coupling. The rGO substrate improves the conductivity and ensures high-dispersed 1T/2H-MoS2 nanosheets to expose plentiful highly active edges. More importantly, the strong heterointerface electrical interaction by the C-O-Mo covalent bond can enhance the charge-transfer efficiency and reinforce structural stability. Furthermore, the integration with the appropriate 2H phase is in favor of stabilization of the metastable 1T phase; thus, the ratio of 1T and 2H was precisely regulated to balance activity and stability. With these advantages, the 1T/2H-MoS2/rGO catalyst presents a satisfactory activity and stability, as confirmed by the relatively low overpotential (268 and 140 mV at 10 mA cm-2) and the small Tafel slope (102 and 86 mV dec-1) in alkaline and acidic media, respectively. The theory calculations disclose that the electronic structure redistribution has been optimized via the strong coupled C-O-Mo heterointerface and phase interface, significantly reducing the adsorption free energy of hydrogen and improving intrinsic activity.

Phase-Engineered Dichalcogenides/Fluorine-Free V4C3Tx (T = OH, O) Heterostructures for pH-Universal Hydrogen Evolution Reaction

This research addresses the pH-dependency limitation in electrocatalytic hydrogen evolution reactions (HER) by creating heterostructures through the chemical bonding between 2D-dichalcogenides and V4C3Tx (T = OH, O) planes. The one-step solvothermal synthesis employed in this study constructs a synergistically interacted 1T phase of, e.g., MoS2 and V4C3Tx MXene, demonstrating an omnidirectional improvement on catalytic stability, active site exposure, surface area enlargement, electrical conductivity, and hence enhancement of water dissociation activities. Despite the notable progress in creating hydrogen production catalysts with ground breaking performances, a significant gap remains in the availability of catalysts capable of functioning effectively under high current densities. The catalyst 1T MoS2@V4C3Tx shows remarkable activities under the current density of 1000 mA cm-2, which require overpotentials of 16, 24, and 37 mV in 0.5 m H2SO4, 1 m KOH, and 0.1 m PBS electrolytes, respectively at 10 mA cm-2, and exhibits excellent HER performance with small overpotentials of 103.16 and 138 mV to achieve current densities of 500 and 1000 mA cm-2, respectively, with outstanding stability for 1000 cylic voltammetric cycle HER test without degradation in acidic media. Enhanced HER performance has also been observed in other 2D-dichalcogenides/V4C3Tx heterostructures, providing prospects for phase-engineered dichalcogenides/fluorine-free V4C3Tx composites for pH-universal HER.

Cu- and S-Doped Heteropolyacid Co2Mo10 as Electrocatalysts for Efficient Hydrogen Evolution

This study employed polyoxometalate Co2Mo10 as a precursor and a two-step method to prepare carbon cloth-supported CuxS-CoS2-MoS2 materials. The morphology and structure of the materials were characterized using XRD, XPS, SEM, TEM, and other techniques. Interestingly, changes in the reducing gas during the calcination process could adjust the product morphology, thereby altering catalytic activity. Electrochemical results indicated that the CuxS-CoS2-MoS2 nanomaterials prepared under an NH3 atmosphere exhibited unique morphology and structural advantages. They demonstrated significant electrocatalytic activity for the hydrogen evolution reaction (HER) in alkaline and acidic electrolytes (overpotentials of 108 and 196 mV at a current density of 10 mA cm-2, lower than the overpotentials of 143 and 226 mV obtained under a H2-Ar atmosphere during calcination) and excellent long-term durability. These findings provide insights and methods for synthesizing multicomponent electrocatalysts with enhanced catalytic performance.

Tuning Electronic and Functional Properties in Defected MoS2 Films by Surface Patterning of Sulphur Atomic Vacancies

Defects are inherent in transition metal dichalcogenides and significantly affect their chemical and physical properties. In this study, surface defect electrochemical nanopatterning is proposed as a promising method to tune in a controlled manner the electronic and functional properties of defective MoS₂ thin films. Using parallel electrochemical nanolithography, MoS₂ thin films are patterned, creating sulphur vacancy-rich active zones alternated with defect-free regions over a centimetre scale area, with sub-micrometre spatial resolution. The patterned films display tailored optical and electronic properties due to the formation of sulphur vacancy-rich areas. Moreover, the effectiveness of defect nanopatterning in tuning functional properties is demonstrated by studying the electrocatalytic activity for the hydrogen evolution reaction.

Tailored Growth of Transition Metal Dichalcogenides' Monolayers by Chemical Vapor Deposition

Here, results on the tailored growth of monolayers (MLs) of transition metal dichalcogenides (TMDs) are presented using chemical vapor deposition (CVD) techniques. To enable reproducible growth, the flow of chalcogen precursors is controlled by Knudsen cells providing an advantage in comparison to the commonly used open crucible techniques. It is demonstrated that TMD MLs can be grown by CVD on large scale with structural, and therefore electronic, photonic and optoelectronic properties similar to TMD MLs are obtained by exfoliating bulk crystals. It is shown that besides the growth of the "standard" TMD MLs also the growth of MLs that are not available by the exfoliation is possible including examples like lateral TMD1-TMD2 ML heterostructures and Janus TMDs. Moreover, the CVD technique enables the growth of TMD MLs on various 3D substrates on large scale and with high quality. The intrinsic properties of the grown MLs are analyzed by complementary microscopy and spectroscopy techniques down to the nanoscale with a particular focus on the influence of structural defects. Their functional properties are studied in devices including field-effect transistors, photodetectors, wave guides and excitonic diodes. Finally, an outlook of the developed methodology in both applied and fundamental research is given.

Trifunctional Graphene-Sandwiched Heterojunction-Embedded Layered Lattice Electrocatalyst for High Performance in Zn-Air Battery-Driven Water Splitting

Zn-air battery (ZAB)-driven water splitting holds great promise as a next-generation energy conversion technology, but its large overpotential, low activity, and poor stability for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution reaction (HER) remain obstacles. Here, a trifunctional graphene-sandwiched, heterojunction-embedded layered lattice (G-SHELL) electrocatalyst offering a solution to these challenges are reported. Its hollow core-layered shell morphology promotes ion transport to Co3S4 for OER and graphene-sandwiched MoS2 for ORR/HER, while its heterojunction-induced internal electric fields facilitate electron migration. The structural characteristics of G-SHELL are thoroughly investigated using X-ray absorption spectroscopy. Additionally, atomic-resolution transmission electron microscopy (TEM) images align well with the DFT-relaxed structures and simulated TEM images, further confirming its structure. It exhibits an approximately threefold smaller ORR charge transfer resistance than Pt/C, a lower OER overpotential and Tafel slope than RuO₂, and excellent HER overpotential and Tafel slope, while outlasting noble metals in terms of durability. Ex situ X-ray photoelectron spectroscopy analysis under varying potentials by examining the peak shifts and ratios (Co2+/Co3+ and Mo4+/Mo6+) elucidates electrocatalytic reaction mechanisms. Furthermore, the ZAB with G-SHELL outperforms Pt/C+RuO2 in terms of energy density (797 Wh kg-1) and peak power density (275.8 mW cm-2), realizing the ZAB-driven water splitting.

Enhancement of single-atom catalytic activity by the synergistic effect of interlayer charge transfer and magnetic coupling in an electride-based heterostructure

2D material-based single-atom catalysts have rapidly emerged and flourished in recent years due to their exceptional atomic utilization efficiency, adjustable catalytic activity, and remarkably high selectivity. The interface matching mechanism of 2D materials, influenced by van der Waals (vdW) interactions, presents a novel opportunity for constructing a heterostructure, further augmenting catalytic efficiency. In this work, the mechanism of performance regulation of magnetic transition-metal decorated MoS2 single-atom catalysis by importing a Gd2C electride substrate is investigated using first-principles calculations. The localization of d orbitals in transition-metals is weakened by adding a Gd2C substrate, thereby modulating the catalytic performance. Our findings demonstrate that the formation of an electron layer at the interface of the heterostructure by electride Gd2C induces a modification in the chemical environment of the MoS2 surface. The electron layer enhances the electron transfer during catalysis. Additionally, for the catalyst containing magnetic atoms, Gd2C can also achieve catalytic performance adjustment due to the magnetic coupling, similar to the effect of external magnetic fields. This study offers a novel concept and a pathway for enhancing the performance of single-atom catalysts through the construction of a heterostructure, capitalizing on the distinctive electron layer of an electride and its inherent high magnetic moments.

Synthesis of P-(NiCo)CO3/TiO2/Ti Self-Supported Electrode with High Catalytic Activity and Stability for Hydrogen Evolution

Hydrogen is considered an ideal clean energy due to its high mass-energy density, and only water is generated after combustion. Water electrolysis is a sustainable method of obtaining a usable amount of pure hydrogen among the various hydrogen production methods. However, its development is still limited by applying expensive noble metal catalysts. Here, the dissolution-recrystallization process of TiO2 nanotube arrays in water with the hydrothermal reaction of a typical nickel-cobalt hydroxide synthesis process followed by phosphating to prepare a self-supported electrode with (NiCo)CO3/TiO2 heterostructure named P-(NiCo)CO3/TiO2/Ti electrode is combined. The electrode exhibits an ultra-low overpotential of 31 mV at 10 mA  cm-2 with a Tafel slope of 46.2 mV dec-1 in 1 m KOH and maintained its stability after running for 500 h in 1 m KOH. The excellent catalytic activity can be attributed to the structure of nanotube arrays with high specific surface area, superhydrophilicity, and super aerophobicity on the electrode surface. In addition, the uniform (NiCo)CO3/TiO2 heterostructure also accelerates the electron transfer on the electrode surface. Finally, DFT calculations demonstrate that phosphating also improves the ΔGH* and ΔGH2O of the electrode. The synthesis strategy also promotes the exploration of catalysts for other necessary electrocatalytic fields.

Stabilizing and Activating Active Sites: 1T-MoS2 Supported Pd Single Atoms for Efficient Hydrogen Evolution Reaction

Metallic 1T-MoS2 with high intrinsic electronic conductivity performs Pt-like catalytic activity for hydrogen evolution reaction (HER). However, obtaining pure 1T-MoS2 is challenging due to its high formation energy and metastable properties. Herein, an in situ SO4 2--anchoring strategy is reported to synthesize a thin layer of 1T-MoS2 loaded on commercial carbon. Single Pd atoms, constituting a substantial loading of 7.2 wt%, are then immobilized on the 1T-phase MoS2 via Pd─S bonds to modulate the electronic structure and ensure a stable active phase. The resulting Pd1/1T-MoS2/C catalyst exhibits superior HER performance, featuring a low overpotential of 53 mV at the current density of 10 mA cm-2, a small Tafel slope of 37 mV dec-1, and minimal charge transfer resistance in alkaline electrolyte. Moreover, the catalyst also demonstrates efficacy in acid and neutral electrolytes. Atomic structural characterization and theoretical calculations reveal that the high activity of Pd1/1T-MoS2/C is attributed to the near-zero hydrogen adsorption energy of the activated sulfur sites on the two adjacent shells of atomic Pd.

Advancements in Rechargeable Zn-Air Batteries with Transition-Metal Dichalcogenides as Bifunctional Electrocatalyst

This review covers recent progress on transition metal dichalcogenides (TMDs) as bifunctional electrocatalysts for Zinc-air batteries (ZABs), emphasizing their suitable surface area, electrocatalytic active sites, stability in acidic/basic environments, and tunable electronic properties. It discusses strategies like defect engineering, doping, interface, and structural modifications of TMDs nanostructures for enhancing the performances of ZABs. Zinc-air batteries are promising energy storage devices owing to their high energy density, low cost, and environmental friendliness. However, the development of durable and efficient bifunctional electrocatalysts is a major concern for Zn-air batteries. In this review, we summarize the recent progress on transition metal dichalcogenides (TMDs) as bifunctional electrocatalysts for Zn-air batteries. We discuss the advantages of TMDs, such as high activity, good stability, and tunable electronic structure, as well as the challenges, such as low conductivity, poor durability, and limited active sites. We also highlight the strategies for fine-tuning the properties of TMDs, such as defect engineering, doping, hybridization, and structural engineering, to enhance their catalytic performance and stability. We provide a comprehensive and in-depth analysis of the applications of TMDs in Zn-air batteries, demonstrating their potential as low-cost, abundant, and environmentally friendly alternatives to noble metal catalysts. We also suggest future directions like exploring new TMDs materials and compositions, developing novel synthesis and modification techniques, investigating the interfacial interactions and charge transfer processes, and integrating TMDs with other functional materials. This review aims to illuminate the path forward for the development of efficient and durable Zn-air batteries, aligning with the broader objectives of sustainable energy solutions.

One Stone, Three-Birds Approach: Ultra-active Ru/N, S-MoO2/CNTs Electrocatalyst for Overall Water Splitting in Wide Electrode Applications (NF, GC, and CC)

The construction of exceptionally multifunctional electrocatalysts is essential for various applications, but it poses significant challenges. A novel electrocatalyst, denoted as Ru/N, S-MoO2/CNTs, was successfully synthesized using a combination of mechano-grinding and hydrothermal/calcination techniques. The Ru/N, S-MoO2/CNTs demonstrates ultrasmall overpotentials of 12 and 163 mV in NF, 51 and 167 mV in GCE, and 54 and 173 mV in CC for HER and OER, respectively, at a current density of 10 mA/cm2 in alkaline medium. To accomplish electrocatalytic OWS, a current density of 10 mA/cm2 can be obtained by using a cell voltage of 1.446 V. Theoretical studies demonstrated that the inclusion of Ru, N, and S triggers a change in the composition of MoO2; produces oxygen vacancies; and forms Ru, N, and S-oxygen-Mo catalytic centers. The combination of Ru, N, and S nanoclusters; Ru, N, and S-oxygen-Mo catalytic centers; and OVs-enriched MoO2 would position it among the top electrocatalysts.

Exfoliated 2D Nanosheet-Based Conjugated Polymer Composites with P-N Heterojunction Interfaces for Highly Efficient Electrocatalytic Hydrogen Evolution

We have achieved a significant breakthrough in the preparation and development of two-dimensional nanocomposites with P-N heterojunction interfaces as efficient cathode catalysts for electrochemical hydrogen evolution reaction (HER) and iodide oxidation reaction (IOR). P-type acid-doped polyaniline (PANI) and N-type exfoliated molybdenum disulfide (MoS2) nanosheets can form structurally stable composites due to formation of P-N heterojunction structures at their interfaces. These P-N heterojunctions facilitate charge transfer from PANI to MoS2 structures and thus significantly enhance the catalytic efficiency of MoS2 in the HER and IOR. Herein, by combining efficient sodium-functionalized chitosan-assisted MoS2 exfoliation, electropolymerization of PANI on nickel foam (NF) substrate, and electrochemical activation, controllable and scalable Na-Chitosan/MoS2/PANI/NF electrodes are successfully constructed as non-noble metal-based electrochemical catalysts. Compared to a commercial platinum/carbon (Pt/C) catalyst, the Na-Chitosan/MoS2/PANI/NF electrode exhibits significantly lower resistance and overpotential, a similar Tafel slope, and excellent catalytic stability at high current densities, demonstrating excellent catalytic performance in the HER under acidic conditions. More importantly, results obtained from proton exchange membrane fuel cell devices confirm the Na-Chitosan/MoS2/PANI/NF electrode exhibits a low turn-on voltage, high current density, and stable operation at 2 V. Thus, this system holds potential as a replacement for Pt/C with feasibility for applications in energy-related fields.

Defect-rich W1-xMoxS2 solutions for efficient H2 evolution in acidic electrolytes

An optimal W0.4Mo0.6S2 solid solution, equipped with rich intrinsic defects, exhibits excellent stability in both 0.5 M H2SO4 and 2.0 M NaCl, showing negligible activity degradation after continuous 50 hours of working, thereby showing outstanding practical prospects.

Developments in Nanostructured MoS2-Decorated Reduced Graphene Oxide Composite Aerogel as an Electrocatalyst for the Hydrogen Evolution Reaction

Developing lightweight, highly active surfaces with a high level of performance and great stability is crucial for ensuring the dependability of energy harvesting and conversion devices. Aerogel-based electrocatalysts are an efficient option for electrocatalytic hydrogen production because of their numerous benefits, such as their compatibility with interface engineering and their porous architecture. Herein, we report on the facile synthesis of a nanorod-like molybdenum sulfide-reduced graphene oxide (M-rG) aerogel as an electrocatalyst for the hydrogen evolution reaction (HER). The 3D architecture of the network-like structure of the M-rG hybrid aerogel was created via the hydrothermal technique, using a saturated NaCl solution-assisted process, where the MoS2 was homogeneously incorporated within the interconnected rGO aerogel. The optimized M-rG-300 aerogel electrocatalyst had a significantly decreased overpotential of 112 mV at 10 mA/cm2 for the HER in alkaline conditions. The M-rG-300 also showed a higher level of reliability. The remarkable efficiency of the HER involving the M-rG-300 is principally attributed to the excellent connectivity between the rGO and MoS2 in the aerogel structure. The efficient interconnection influenced the achievement of a larger electrochemically active surface area, increased electrical conductivity, and the exposure of more active sites for the HER. Furthermore, the creation of a synergistic effect in the M-rG-300 aerogel is the most probable mechanism to boost the electrocatalytic activity.

Engineered Two-Dimensional Transition Metal Dichalcogenides for Energy Conversion and Storage

Designing efficient and cost-effective materials is pivotal to solving the key scientific and technological challenges at the interface of energy, environment, and sustainability for achieving NetZero. Two-dimensional transition metal dichalcogenides (2D TMDs) represent a unique class of materials that have catered to a myriad of energy conversion and storage (ECS) applications. Their uniqueness arises from their ultra-thin nature, high fractions of atoms residing on surfaces, rich chemical compositions featuring diverse metals and chalcogens, and remarkable tunability across multiple length scales. Specifically, the rich electronic/electrical, optical, and thermal properties of 2D TMDs have been widely exploited for electrochemical energy conversion (e.g., electrocatalytic water splitting), and storage (e.g., anodes in alkali ion batteries and supercapacitors), photocatalysis, photovoltaic devices, and thermoelectric applications. Furthermore, their properties and performances can be greatly boosted by judicious structural and chemical tuning through phase, size, composition, defect, dopant, topological, and heterostructure engineering. The challenge, however, is to design and control such engineering levers, optimally and specifically, to maximize performance outcomes for targeted applications. In this review we discuss, highlight, and provide insights on the significant advancements and ongoing research directions in the design and engineering approaches of 2D TMDs for improving their performance and potential in ECS applications.

Constructing Dual-Phase Co9S8-CoMo2S4 Heterostructure as an Efficient Trifunctional Electrocatalyst for Oxygen Reduction, Oxygen Evolution and Hydrogen Evolution Reactions

Designing robust, efficient and inexpensive trifunctional electrocatalysts for the oxygen reduction reaction (ORR), oxygen evolution reaction (OER) and hydrogen evolution reaction (HER) is significant for rechargeable zinc-air batteries and water-splitting devices. To this end, constructing heterogenous structures based on transition metals stands out as an effective strategy. Herein, a dual-phase Co9S8-CoMo2S4 heterostructure grown on porous N, S-codoped carbon substrate (Co9S8-CoMo2S4/NSC) via a one-pot synthesis is investigated as the trifunctional ORR/OER/HER electrocatalyst. The optimized Co9S8-CoMo2S4/NSC2 exhibits that ORR has a half-wave potential of 0.86 V (vs. RHE) and the overpotentials at 10 mA cm-2 for OER and HER are 280 and 89 mV, respectively, superior to most transition-metal based trifunctional electrocatalysts reported to date. The Co9S8-CoMo2S4/NSC2-based zinc-air battery (ZAB) has a high open-circuit voltage (1.41 V), large capacity (804 mAh g-1) and highly stable cyclability (97 h at 10 mA cm-2). In addition, the prepared Co9S8-CoMo2S4/NSC2-based ZAB in series can self-drive the corresponding water-splitting device. The dual-phase Co9S8-CoMo2S4 heterostructure provides not only multi-type active sites to drive the ORR, OER and HER, but also high-speed charge transfer channels between two phases to improve the synergistic effect and reaction kinetics.

Molybdenum sulfide modified with nickel or platinum nanoparticles as an effective catalyst for hydrogen evolution reaction

In this study, we investigate the catalytic performance of molybdenum sulfide (MoS2) modified with either nickel (Ni) or platinum (Pt) nanoparticles as catalysts for the hydrogen evolution reaction (HER). The MoS2 was prepared on the TiO2 nanotube substrates via a facile hydrothermal method, followed by the deposition by magnetron sputtering of Ni or Pt nanoparticles on the MoS2 surface. Structural and morphological characterization confirmed the successful incorporation of Ni or Pt nanoparticles onto the MoS2 support. Electrochemical measurements revealed that Ni- and Pt-modified MoS2 catalysts exhibited enhanced HER activity compared to pristine MoS2. Obtained catalysts demonstrated a low onset potential, reduced overpotential, and increased current density, indicating efficient electrocatalytic performance. Furthermore, the Ni or Pt-modified MoS2 catalyst exhibited remarkable stability during prolonged HER operation. The improved catalytic activity can be attributed to the synergistic effect between metal nanoparticles and MoS2, facilitating charge transfer kinetics and promoting hydrogen adsorption and desorption. Incorporating Ni and Pt nanoparticles also provided additional active sites on the MoS2 surface, enhancing the catalytic activity.

Structural Aspects of MoS x Prepared by Atomic Layer Deposition for Hydrogen Evolution Reaction

Molybdenum sulfides (MoS x ) in both crystalline and amorphous forms are promising earth-abundant electrocatalysts for hydrogen evolution reaction (HER) in acid. Plasma-enhanced atomic layer deposition was used to prepare thin films of both amorphous MoS x with adjustable S/Mo ratio (2.8-4.7) and crystalline MoS2 with tailored crystallinity, morphology, and electrical properties. All the amorphous MoS x films transform into highly HER-active amorphous MoS2 (overpotential 210-250 mV at 10 mA/cm2 in 0.5 M H2SO4) after electrochemical activation at approximately -0.3 V vs reversible hydrogen electrode. However, the initial film stoichiometry affects the structure and consequently the HER activity and stability. The material changes occurring during activation are studied using ex situ and quasi in situ X-ray photoelectron spectroscopy. Possible structures of as-deposited and activated catalysts are proposed. In contrast to amorphous MoS x , no changes in the structure of crystalline MoS2 catalysts are observed. The overpotentials of the crystalline films range from 300 to 520 mV at 10 mA/cm2, being the lowest for the most defective catalysts. This work provides a practical method for deposition of tailored MoS x HER electrocatalysts as well as new insights into their activity and structure.

Construction of Co-Se-W at Interfaces of Phase-Mixed Cobalt Selenide via Spontaneous Phase Transition for Platinum-Like Hydrogen Evolution Activity and Long-Term Durability in Alkaline and Acidic Media

Cost-effective transition metal chalcogenides are highly promising electrocatalysts for both alkaline and acidic hydrogen evolution reactions (HER). However, unsatisfactory HER kinetics and stability have severely hindered their applications in industrial water electrolysis. Herein, a nanoflowers-shaped W-doped cubic/orthorhombic phase-mixed CoSe2 catalyst ((c/o)-CoSe2-W) is reported. The W doping induces spontaneous phase transition from stable phase cubic CoSe2 (c-CoSe2) to metastable phase orthorhombic CoSe2, which not only enables precise regulation of the ratio of two phases but also realizes W doping at the interfaces of two phases. The (c/o)-CoSe2-W catalyst exhibits a Pt-like HER activity in both alkaline and acidic media, with record-low HER overpotentials of 29.8 mV (alkaline) and 35.9 mV (acidic) at 10 mA cm-2, respectively, surpassing the vast majority of previously reported non-precious metal electrocatalysts for both alkaline and acidic HER. The Pt-like HER activities originate from the formation of Co-Se-W active species on the c-CoSe2 side at the phase interface, which effectively modulates electron structures of active sites, not only enhancing H2O adsorption and dissociation at Co sites but also optimizing H* adsorption to ΔGH* ≈ 0 at W sites. Benefiting from the abundant phase interfaces, the catalyst also displays outstanding long-term durability in both acidic and alkaline media.

On the origin of the improved hydrogen evolution reaction in Mn- and Co-doped MoS2

In the field of hydrogen production, MoS2 demonstrates good catalytic properties for the hydrogen evolution reaction (HER) which improve when doped with metal cations. However, while the role of sulfur atoms as active sites in the HER is largely reported, the role of metal atoms (i.e. molybdenum or the dopant cations) has yet to be studied in depth. To understand the role of the metal dopant, we study MoS2 thin films doped with Co and Mn ions. We identify the contribution of the electronic bands of the Mn and Co dopants to the integral valence band of the material using in situ resonant photoemission measurements. We demonstrate that Mn and Co dopants act differently: Mn doping favors the shift of the S-Mo hybridized band towards the Fermi level, while in the case of Co doping it is the less hybridized Co band that shifts closer to the Fermi level. Doping with Mn increases the effectiveness of S as the active site, thus improving the HER, while doping with Co introduces the metallic site of Co as the active site, which is less effective in improving HER properties. We therefore clarify the role of the dopant cation in the electronic structure determining the active site for hydrogen adsorption/desorption. Our results pave the way for the design of efficient materials for hydrogen production via the doping route, which can be extended to different catalytic reactions in the field of energy applications.

Impact of quantum size effects to the band gap of catalytic materials: a computational perspective

The evolution of nanotechnology has facilitated the development of catalytic materials with controllable composition and size, reaching the sub-nanometer limit. Nowadays, a viable strategy for tailoring and optimizing the catalytic activity involves controlling the size of the catalyst. This strategy is underpinned by the fact that the properties and reactivity of objects with dimensions on the order of nanometers can differ from those of the corresponding bulk material, due to the emergence of quantum size effects. Quantum size effects have a deep influence on the band gap of semiconducting catalytic materials. Computational studies are valuable for predicting and estimating the impact of quantum size effects. This perspective emphasizes the crucial role of modeling quantum size effects when simulating nanostructured catalytic materials. It provides a comprehensive overview of the fundamental principles governing the physics of quantum confinement in various experimentally observable nanostructures. Furthermore, this work may serve as a tutorial for modeling the electronic gap of simple nanostructures, highlighting that when working at the nanoscale, the finite dimensions of the material lead to an increase of the band gap because of the emergence of quantum confinement. This aspect is sometimes overlooked in computational chemistry studies focused on surfaces and nanostructures.

Conversion of Layered WS2 Crystals into Mixed-Domain Electrochemical Catalysts by Plasma-Assisted Surface Reconstruction

Electrocatalytic water splitting is crucial to generate clean hydrogen fuel, but implementation at an industrial scale remains limited due to dependence on expensive platinum (Pt)-based electrocatalysts. Here, an all-dry process to transform electrochemically inert bulk WS2 into a multidomain electrochemical catalyst that enables scalable and cost-effective implementation of the hydrogen evolution reaction (HER) in water electrolysis is reported. Direct dry transfer of WS2 flakes to a gold thin film deposited on a silicon substrate provides a general platform to produce the working electrodes for HER with tunable charge transfer resistance. By treating the mechanically exfoliated WS2 with sequential Ar-O2 plasma, mixed domains of WS2, WO3, and tungsten oxysulfide form on the surfaces of the flakes, which gives rise to a superior HER with much greater long-term stability and steady-state activity compared to Pt. Using density functional theory, ultraefficient atomic sites formed on the constituent nanodomains are identified, and the quantification of atomic-scale reactivities and resulting HER activities fully support the experimental observations.

Synthesis and Electrocatalytic Performance Study of Sulfur Quantum Dots Modified MoS2

The electrolysis of water for hydrogen production is currently receiving significant attention due to its advantageous features such as non-toxicity, safety, and environmental friendliness. This is especially crucial considering the urgent need for clean energy. However, the current method of electrolyzing water to produce hydrogen largely relies on expensive metal catalysts, significantly increasing the costs associated with its development. Molybdenum disulfide (MoS2) is considered the most promising alternative to platinum for electrocatalyzing the hydrogen evolution reaction (HER) due to its outstanding catalytic efficiency and robust stability. However, the practical application of this material is hindered by its low conductivity and limited exposure of active sites. MoS2/SQDs composite materials were synthesized using a hydrothermal technique to deposit SQDs onto MoS2. These composite materials were subsequently employed as catalysts for the HER. Research findings indicate that incorporating SQDs can enhance electron transfer rates and increase the active surface area of MoS2, which is crucial for achieving outstanding catalytic performance in the HER. The MoS2/SQDs electrocatalyst exhibits outstanding performance in the HER when tested in a 0.5 M H2SO4 solution. It achieves a remarkably low overpotential of 204 mV and a Tafel slope of 65.82 mV dec-1 at a current density of 10 mA cm-2. Moreover, during continuous operation for 24 h, the initial current density experiences only a 17% reduction, indicating high stability. This study aims to develop an efficient and cost-effective electrocatalyst for water electrolysis. Additionally, it proposes a novel design strategy that uses SQDs as co-catalysts to enhance charge transfer in nanocomposites.

Selective activation of MoS2 grain boundaries for enhanced electrochemical activity

Molybdenum disulfide (MoS2) has emerged as a promising material for catalysis and sustainable energy conversion. However, the inertness of its basal plane to electrochemical reactions poses challenges to the utilization of wafer-scale MoS2 in electrocatalysis. To overcome this limitation, we present a technique that enhances the catalytic activity of continuous MoS2 by preferentially activating its buried grain boundaries (GBs). Through mild UV irradiation, a significant enhancement in GB activity was observed that approaches the values for MoS2 edges, as confirmed by a site-selective photo-deposition technique and micro-electrochemical hydrogen evolution reaction (HER) measurements. Combined spectroscopic characterization and ab-initio simulation demonstrates substitutional oxygen functionalization at the grain boundaries to be the origin of this selective catalytic enhancement by an order of magnitude. Our approach not only improves the density of active sites in MoS2 catalytic processes but yields a new photocatalytic conversion process. By exploiting the difference in electronic structure between activated GBs and the basal plane, homo-compositional junctions were realized that improve the photocatalytic synthesis of hydrogen by 47% and achieve performances beyond the capabilities of other catalytic sites.

Precious Metal Free Hydrogen Evolution Catalyst Design and Application

The quest to identify precious metal free hydrogen evolution reaction catalysts has received unprecedented attention in the past decade. In this Review, we focus our attention to recent developments in precious metal free hydrogen evolution reactions in acidic and alkaline electrolyte owing to their relevance to commercial and near-commercial low-temperature electrolyzers. We provide a detailed review and critical analysis of catalyst activity and stability performance measurements and metrics commonly deployed in the literature, as well as review best practices for experimental measurements (both in half-cell three-electrode configurations and in two-electrode device testing). In particular, we discuss the transition from laboratory-scale hydrogen evolution reaction (HER) catalyst measurements to those in single cells, which is a critical aspect crucial for scaling up from laboratory to industrial settings but often overlooked. Furthermore, we review the numerous catalyst design strategies deployed across the precious metal free HER literature. Subsequently, we showcase some of the most commonly investigated families of precious metal free HER catalysts; molybdenum disulfide-based, transition metal phosphides, and transition metal carbides for acidic electrolyte; nickel molybdenum and transition metal phosphides for alkaline. This includes a comprehensive analysis comparing the HER activity between several families of materials highlighting the recent stagnation with regards to enhancing the intrinsic activity of precious metal free hydrogen evolution reaction catalysts. Finally, we summarize future directions and provide recommendations for the field in this area of electrocatalysis.

In Situ Probing the Structure Change and Interaction of Interfacial Water and Hydroxyl Intermediates on Ni(OH)2 Surface over Water Splitting

There is growing acknowledgment that the properties of the electrochemical interfaces play an increasingly pivotal role in improving the performance of the hydrogen evolution reaction (HER). Here, we present, for the first time, direct dynamic spectral evidence illustrating the impact of the interaction between interfacial water molecules and adsorbed hydroxyl species (OHad) on the HER properties of Ni(OH)2 using Au/core-Ni(OH)2/shell nanoparticle-enhanced Raman spectroscopy. Notably, our findings highlight that the interaction between OHad and interfacial water molecules promotes the formation of weakly hydrogen-bonded water, fostering an environment conducive to improving the HER performance. Furthermore, the participation of OHad in the reaction is substantiated by the observed deprotonation step of Au@2 nm Ni(OH)2 during the HER process. This phenomenon is corroborated by the phase transition of Ni(OH)2 to NiO, as verified through Raman and X-ray photoelectron spectroscopy. The significant redshift in the OH-stretching frequency of water molecules during the phase transition confirms that surface OHad disrupts the hydrogen-bond network of interfacial water molecules. Through manipulation of the shell thickness of Au@Ni(OH)2, we additionally validate the interaction between OHad and interfacial water molecules. In summary, our insights emphasize the potential of electrochemical interfacial engineering as a potent approach to enhance electrocatalytic performance.

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.

Boosting CO2 Hydrogenation to Methanol over Monolayer MoS2 Nanotubes by Creating More Strained Basal Planes

The controlled creation, selective exposure, and activation of more basal planes while simultaneously minimizing the generation and exposure of edge sites are crucial for accelerating methanol synthesis from CO2 hydrogenation over MoS2 catalysts but remain a bottleneck. Here, we report a facile method to fabricate heteronanotube catalysts with single-layer MoS2 coaxially encapsulating the carbon nanotubes (CNTs@MoS2) through host-guest chemistry. Inheriting the long tubular structure of CNTs, the grown MoS2 nanotubes exhibit significantly more basal planes than bulk MoS2 crystals. More importantly, the tubular curvature not only promotes strain and sulfur vacancy (Sv) generation but also preferentially exposes more in-plane Sv while limiting edge Sv exposure, which is conducive to methanol synthesis. Both the strain and layer number of MoS2 can be easily and finely adjusted by altering CNT diameter and quantity of precursors. Remarkably, CNTs@MoS2 with monolayer MoS2 and maximum strain displayed methanol selectivity of 78.1% and methanol space time yield of 1.6 g gMoS2-1 h-1 at 260 °C and GHSV of 24000 mL gcat.-1 h-1, representing the best results to date among Mo-based catalysts. This study provides prospects for novel catalyst design by synthesizing coaxial tubular heterostructure to create additional catalytic sites and ultimately enhance conversion and selectivity.

Precise Tuning of Bilayer Ultrasmall MoS2 Featuring Inhibition of Carrier Recombination and Fast Surface Chemistry for Green H2 Evolution

Achieving water splitting to produce green H2 , using the noble-metal-free MoS2 , has attracted huge interest from researchers. However, tuning the number of MoS2 layers precisely while obtaining small lateral sizes to surge the H2 -evolution rate is a tremendous challenge. Here, a bottom-up strategy is designed for the in situ growth of ultrasmall lateral-sized MoS2 with tunable layers on CdS nanorods (CN) by controlling the decomposition temperature and concentration of substrate seed (NH4 )2 MoS4 . Here, the bilayer MoS2 and CN coupling (2L-MoS2 /CN) exhibits the optimum photocatalytic activity. Compared to thicker MoS2 , the 2L-MoS2 has sufficient reduction capacity to drive photocatalytic H2 evolution and the ultrasmall lateral size provides more active sites. Meanwhile, the indirect bandgap, in contrast to the direct bandgap of the monolayer MoS2 , suppresses the carrier recombination transferred to 2L-MoS2 . Under the synergistic effect of both, 2L-MoS2 /CN has fast surface chemical reactions, resulting in the photocatalytic H2 -evolution rate of up to 41.86 mmol g-1 h-1 . A novel strategy is provided here for tuning the MoS2 layers to achieve efficient H2 evolution.

Water adsorption on MoS2 under realistic atmosphere conditions and impacts on tribology

Molybdenum disulfide (MoS2) is a 2D material widely used as a dry lubricant. However, exposure to water and oxygen is known to reduce its effectiveness, and therefore an understanding of the uptake of water is important information for mitigating these effects. Here we use grand canonical Monte Carlo simulations to rigorously study water adsorption on MoS2 surfaces and edges with different concentrations of defects under realistic atmospheric conditions (i.e. various temperatures and humidity levels). We find that the amount of water adsorbed depends strongly on the number of defects. Simulations indicate that defect sites are generally saturated with water even at low ppm levels of humidity. Water binds strongly to S vacancies on interlamellar surfaces, but generally only one water molecule can fit on each of these sites. Defects on surfaces or edges of lamellae also strongly attract water molecules that then nucleate small clusters of water bonded via hydrogen bonding. We demonstrate that water preferentially binds to surface defects, but once those are saturated at a critical humidity level of about 500-1000 ppm water, water binds to edge sites where it negatively impacts the tribological performance of MoS2.

Overall Spontaneous Water Splitting for Calcium Bismuthate Ca(BiO2)2: Flexible-Electronic-Controlled Band Edge Position and Adsorption-Site-Modulated Bond Strength

Eco-friendly photocatalysts for water splitting, highly efficient in oxygen/hydrogen evolution reactions, hold great promise for the storage of inexhaustible solar energy and address environmental challenges. However, current common photocatalysts rarely exhibit both H2 and O2 production performances unless some regulatory measures, such as strain engineering, are implemented. Additionally, the extensive utilization of flexible electronics remains constrained by their high Young's modulus. Herein, on the basis of density functional theory calculations, we identify a novel spontaneous oxygen-producing two-dimensional Ca(BiO2)2 material, which can efficiently regulate the electronic structures of the surface active sites, optimize the reaction pathways, reduce the reaction energy barriers, and boost the overall water-splitting activity through biaxial strain modulation. In detail, an unstrained Ca(BiO2)2 monolayer not only possesses a suitable band gap value (2.02 eV) to fulfill the photocatalytic water-splitting band edge relationships but also holds favorable transport properties, excellent optical absorption across the visible light spectrum, and spontaneous oxygen production under neutral conditions. More excitingly, under application of a 7% biaxial tensile strain modulation with an ideal biaxial strength of 32.35 GPa nm, the Ca(BiO2)2 monolayer not only maintains its structural integrity but also exhibits a completely spontaneous reaction for photocatalytic hydrogen precipitation with superior optical absorption. This can primarily be attributed to the substantial reduction of the potential barrier through strain engineering as well as the weakening of bond energy resulting from changes of the adsorption site as calculated by crystal orbital Hamiltonian population analysis. This flexible stretchable electronic modulated the photocatalyst behavior and bond energy of O-Bi and O-Ca interactions, offering outstanding potential for photocatalytic water spontaneous oxygen and hydrogen evolution among all of the reported metal oxides, and is more likely to become a promising candidate for future flexible electronic devices.

Recent Strategies for the Synthesis of Phase-Pure Ultrathin 1T/1T' Transition Metal Dichalcogenide Nanosheets

The past few decades have witnessed a notable increase in transition metal dichalcogenide (TMD) related research not only because of the large family of TMD candidates but also because of the various polytypes that arise from the monolayer configuration and layer stacking order. The peculiar physicochemical properties of TMD nanosheets enable an enormous range of applications from fundamental science to industrial technologies based on the preparation of high-quality TMDs. For polymorphic TMDs, the 1T/1T' phase is particularly intriguing because of the enriched density of states, and thus facilitates fruitful chemistry. Herein, we comprehensively discuss the most recent strategies for direct synthesis of phase-pure 1T/1T' TMD nanosheets such as mechanical exfoliation, chemical vapor deposition, wet chemical synthesis, atomic layer deposition, and more. We also review frequently adopted methods for phase engineering in TMD nanosheets ranging from chemical doping and alloying, to charge injection, and irradiation with optical or charged particle beams. Prior to the synthesis methods, we discuss the configuration of TMDs as well as the characterization tools mostly used in experiments. Finally, we discuss the current challenges and opportunities as well as emphasize the promising fields for the future development.

Nanocatalysis MoS2/rGO: An Efficient Electrocatalyst for the Hydrogen Evolution Reaction

In this study, a systematic investigation of MoS2 nanostructure growth on a SiO2 substrate was conducted using a two-stage process. Initially, a thin layer of Mo was grown through sputtering, followed by a sulfurization process employing the CVD technique. This two-stage process enables the control of diverse nanostructure formations of both MoS2 and MoO3 on SiO2 substrates, as well as the formation of bulk-like grain structures. Subsequently, the addition of reduced graphene oxide (rGO) was examined, resulting in MoS2/rGO(n), where graphene is uniformly deposited on the surface, exposing a higher number of active sites at the edges and consequently enhancing electroactivity in the HER. The influence of the synthesis time on the treated MoS2 and also MoS2/rGO(n) samples is evident in their excellent electrocatalytic performance with a low overpotential.

Dynamic coordination engineering of 2D PhenPtCl2 nanosheets for superior hydrogen evolution

Exploring the dynamic structural evolution of electrocatalysts during reactions represents a fundamental objective in the realm of electrocatalytic mechanism research. In pursuit of this objective, we synthesized PhenPtCl2 nanosheets, revealing a N2-Pt-Cl2 coordination structure through various characterization techniques. Remarkably, the electrocatalytic performance of these PhenPtCl2 nanosheets for hydrogen evolution reaction (HER) surpasses that of the commercial Pt/C catalyst across the entire pH range. Furthermore, our discovery of the dynamic coordination changes occurring in the N2-Pt-Cl2 active sites during the electrocatalytic process, as clarified through in situ Raman and X-ray photoelectron spectroscopy, is particularly noteworthy. These changes transition from Phen-Pt-Cl2 to Phen-Pt-Cl and ultimately to Phen-Pt. The Phen-Pt intermediate plays a pivotal role in the electrocatalytic HER, dynamically coordinating with Cl- ions in the electrolyte. Additionally, the unsaturated, two-coordinated Pt within Phen-Pt provides additional space and electrons to enhance both H+ adsorption and H2 evolution. This research illuminates the intricate dynamic coordination evolution and structural adaptability of PhenPtCl2 nanosheets, firmly establishing them as a promising candidate for efficient and tunable electrocatalysts.

2D-MoS2-supported copper peroxide nanodots with enhanced nanozyme activity: application in antibacterial activity

Peroxidase (POD)-like nanozymes are an upcoming class of new-generation antibiotics that are efficient for broad-spectrum antibacterial action. The POD-like activity employs the generation of reactive oxygen species (ROS), which have been utilized for bactericidal action. However, their intrinsic low catalytic activity and stability limit their bactericidal properties. In this study, we prepared a MoS2-based nanocomposite with copper peroxide nanodots (MoS2@CP) to achieve pH-dependent light-induced nanozyme-based antibacterial action. It has shown superior peroxidase and antibacterial activity at low pH. The mechanism behind the enhanced POD-like activity and high antibacterial activity was established. The mechanistic pathway involves estimating ROS generation, membrane depolarization, inner membrane permeabilization, metal ion release, and the effect of NIR on photothermal and photodynamic activities. Overall, our work highlighted the combinatorial approach for eradicating bacterial infections using enzyme-based antibacterial agents.

Enhanced electrocatalytic hydrogen evolution from nitrogen plasma-tailored MoS2 nanostructures

Two-dimensional (2D) layered transition metal dichalcogenides such as MoS2 have been viewed as the most favorable candidates for replacing noble metals in catalyzing the hydrogen evolution reaction in water splitting owing to their earth abundance, superb chemical stability, and appropriate Gibbs free energy. However, due to its low number of catalytic sites and basal catalytic inertia, the pristine MoS2 displayed intrinsically unsatisfactory HER catalytic activity. Here, the hydrogen evolution catalytic activities of nanostructured MoS2 powder before and after plasma modification with nitrogen doping were experimentally compared, and the influence of treatment parameters on the hydrogen evolution catalytic performance of MoS2 has been studied. The feasibility of regulating hydrogen evolution catalytic activity by nitrogen doping of MoS2 was verified based on density functional theory calculations. Our work demonstrates a more convenient and faster way to develop cheap and efficient MoS2-based catalysts for electrochemical hydrogen evolution reactions. Additionally, theoretical studies reveal that N-doped MoS2 exhibits strong hybridization between Mo-d and N-p states, causing magnetism to evolve, as confirmed by experiments.

Hydrogen-induced Sulfur Vacancies on the MoS2 Basal Plane Studied by Ambient Pressure XPS and DFT Calculations

Sulfur vacancy on an MoS2 basal plane plays a crucial role in device performance and catalytic activity; thus, an understanding of the electronic states of sulfur vacancies is still an important issue. We investigate the electronic states on an MoS2 basal plane by ambient-pressure X-ray photoelectron spectroscopy (AP-XPS) and density functional theory calculations while heating the system in hydrogen. The AP-XPS results show a decrease in the intensity ratio of S 2p to Mo 3d, indicating that sulfur vacancies are formed. Furthermore, low-energy components are observed in Mo 3d and S 2p spectra. To understand the changes in the electronic states induced by sulfur vacancy formation at the atomic scale, we calculate the core-level binding energies for the model vacancy surfaces. The calculated shifts for Mo 3d and S 2p with the formation of sulfur vacancy are consistent with the experimentally observed binding energy shifts. Mulliken charge analysis indicates that this is caused by an increase in the electronic density associated with the Mo and S atoms around the sulfur vacancy as compared to the pristine surface. The present investigation provides a guideline for sulfur vacancy engineering.

An advanced PdNPs@MoS2 nanocomposite for efficient oxygen evolution reaction in alkaline media

In response to the increasing availability of hydrogen energy and renewable energy sources, molybdenum disulfide (MoS2)-based electrocatalysts are becoming increasingly important for efficient electrochemical water splitting. This study involves the incorporation of palladium nanoparticles (PdNPs) into hydrothermally grown MoS2via a UV light assisted process to afford PdNPs@MoS2 as an alternative electrocatalyst for efficient energy storage and conversion. Various analytical techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and energy dispersive spectroscopy (EDS), were used to investigate the morphology, crystal quality, and chemical composition of the samples. Although PdNPs did not alter the MoS2 morphology, oxygen evolution reaction (OER) activity was driven at considerable overpotential. When electrochemical water splitting was performed in 1.0 M KOH aqueous solution with PdNPs@MoS2 (sample-2), an overpotential of 253 mV was observed. Furthermore, OER performance was highly favorable through rapid reaction kinetics and a low Tafel slope of 59 mV dec-1, as well as high durability and stability. In accordance with the electrochemical results, sample-2 showed also a lower charge transfer resistance, which again provided evidence of OER activity. The enhanced OER activity was attributed to a number of factors, including structural, surface chemical compositions, and synergistic effects between MoS2 and PdNPs.

Green Colloidal Synthesis of MoS2 Nanoflakes

Currently, two approaches dominate the large-scale production of MoS2: liquid-phase exfoliation, referred to as the top-down approach, and bottom-up colloidal synthesis from molecular precursors. Known colloidal synthesis approaches utilize toxic precursors. Here, an alternative green route for the bottom-up synthesis of MoS2 nanoflakes (NFs) is described. The NFs were synthesized by colloidal synthesis using [Mo(CH3COO)2]2 and a series of sulfur (S)-precursors including thioacetamide (TAA), 3-mercaptopropionic acid (3-MPA), l-cysteine (L-CYS), mercaptosuccinic acid (MSA), 11-mercaptoundecanoic acid (MUA), 1-dodecanethiol (DDTH), and di-tert-butyl disulfide (DTBD). While TAA, an S-precursor most commonly used for MoS2 NF preparation, is a known carcinogen, the other investigated S-precursors have low or no known toxicity. High-resolution scanning transmission electron microscopy (HR-STEM) and grazing incidence wide-angle X-ray scattering (GIWAXS) confirmed that in all cases, the syntheses yielded single-layer MoS2 NFs with lateral sizes smaller than 15 nm and a well-defined crystal structure. Electronic absorption and Raman spectra showed characteristic features associated with the MoS2 monolayers. The evolution of the absorption spectra of the growth solution during the syntheses reveals how the kinetics of the NF formation is affected by the S-precursor as well as the nature of the coordinating ligands.

Template-free scalable growth of vertically-aligned MoS2 nanowire array meta-structural films towards robust superlubricity

Two-dimensional (2D) molybdenum disulfide exhibits a variety of intriguing behaviors depending on its orientation layers. Therefore, developing a template-free atomic layer orientation controllable growth approach is of great importance. Here, we demonstrate scalable, template-free, well-ordered vertically-oriented MoS2 nanowire arrays (VO-MoS2 NWAs) embedded in an Ag-MoS2 matrix, directly grown on various substrates (Si, Al, and stainless steel) via one-step sputtering. In the meta-structured film, vertically-standing few-layered MoS2 NWAs of almost micron length (∼720 nm) throughout the entire film bulk. While near the surface, MoS2 lamellae are oriented in parallel, which are beneficial for caging the bonds dangling from the basal planes. Owing to the unique T-type topological characteristics, chemically inert Ag@MoS2 nano-scrolls (NSCs) and nano-crystalline Ag (nc-Ag) nanoparticles (NPs) are in situ formed under the sliding shear force. Thus, incommensurate contact between (002) basal planes and nc-Ag NPs is observed. As a result, robust superlubricity (friction coefficient μ = 0.0039) under humid ambient conditions is reached. This study offers an unprecedented strategy for controlling the basal plane orientation of 2D transition metal dichalcogenides (TMDCs) via substrate independence, using a one-step solution-free easily scalable process without the need for a template, which promotes the potential applications of 2D TMDCs in solid superlubricity.

Graphene Quantum Dot-Mediated Atom-Layer Semiconductor Electrocatalyst for Hydrogen Evolution

The hydrogen evolution reaction performance of semiconducting 2H-phase molybdenum disulfide (2H-MoS2) presents a significant hurdle in realizing its full potential applications. Here, we utilize theoretical calculations to predict possible functionalized graphene quantum dots (GQDs), which can enhance HER activity of bulk MoS2. Subsequently, we design a functionalized GQD-induced in-situ bottom-up strategy to fabricate near atom-layer 2H-MoS2 nanosheets mediated with GQDs (ALQD) by modulating the concentration of electron withdrawing/donating functional groups. Experimental results reveal that the introduction of a series of functionalized GQDs during the synthesis of ALQD plays a crucial role. Notably, the higher the concentration and strength of electron-withdrawing functional groups on GQDs, the thinner and more active the resulting ALQD are. Remarkably, the synthesized near atom-layer ALQD-SO3 demonstrate significantly improved HER performance. Our GQD-induced strategy provides a simple and efficient approach for expanding the catalytic application of MoS2. Furthermore, it holds substantial potential for developing nanosheets in other transition-metal dichalcogenide materials.

Coupling MoS2 nanosheets with CeO2 for efficient electrocatalytic hydrogen evolution at large current densities

We developed an efficient MoS2 nanosheet electrode coupled with CeO2via a hydrothermal process to facilitate water adsorption and dissociation, which displayed good HER activity and stability at a large current density of 500 mA cm-2. In situ Raman spectroscopy confirmed the formation of hydroxide ions based on the strengthening of the Ce-O bond during the HER.

Toolbox of Advanced Atomic Layer Deposition Processes for Tailoring Large-Area MoS2 Thin Films at 150 °C

Two-dimensional MoS₂ is a promising material for applications, including electronics and electrocatalysis. However, scalable methods capable of depositing MoS₂ at low temperatures are scarce. Herein, we present a toolbox of advanced plasma-enhanced atomic layer deposition (ALD) processes, producing wafer-scale polycrystalline MoS₂ films of accurately controlled thickness. Our ALD processes are based on two individually controlled plasma exposures, one optimized for deposition and the other for modification. In this way, film properties can be tailored toward different applications at a very low deposition temperature of 150 °C. For the modification step, either H₂ or Ar plasma can be used to combat excess sulfur incorporation and crystallize the films. Using H₂ plasma, a higher degree of crystallinity compared with other reported low-temperature processes is achieved. Applying H₂ plasma steps periodically instead of every ALD cycle allows for control of the morphology and enables deposition of smooth, polycrystalline MoS₂ films. Using an Ar plasma instead, more disordered MoS₂ films are deposited, which show promise for the electrochemical hydrogen evolution reaction. For electronics, our processes enable control of the carrier density from 6 × 10¹⁶ to 2 × 10²¹ cm^-3 with Hall mobilities up to 0.3 cm² V^-1 s^-1. The process toolbox forms a basis for rational design of low-temperature transition metal dichalcogenide deposition processes compatible with a range of substrates and applications.

Single transition metal-decorated C4N/MoS2 heterostructure for boosting oxygen reduction, oxygen evolution, and hydrogen evolution

Multifunctional electrocatalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and hydrogen evolution (HER) are required preconditions for the development of a highly promising new green energy conversion and storage technology. Herein, a comprehensive computation of the ORR, OER and HER catalytic performance for the pristine and metal-decorated C₄N/MoS₂ (TM-C₄N/MoS₂) is researched using density functional theory. Remarkably, Pd-C₄N/MoS₂ exhibits distinguished bifunctional catalytic performance with lower ORR/OER overpotentials of 0.34/0.40 V. Rh-C₄N/MoS₂ is the prospective trifunctional catalyst with the low ORR/OER/HER overpotentials of 0.48/0.55/-0.16 V, but its electrochemical stability needs to be further improved. Furthermore, the strong correlation between intrinsic descriptor (φ) and adsorption free energy of *OH verifies that the catalytic activity of TM-C₄N/MoS₂ is affected by active metal and surrounding coordination environment. The heap map has summarized the correlations of d-band center, adsorption free energy of reaction species, and φ as the vital parameter for ORR/OER overpotentials of designing catalysts. The electronic structure analysis uncovers the activity enhancement is due to the adjustable adsorption behavior of reaction intermediates on TM-C₄N/MoS₂. This finding paves the way to develop high-activity and multifunctional catalysts, making them suitable for multifunctional applications in the forthcoming critically needed green energy conversion and storage technologies.

Engineering the Local Atomic Configuration in 2H TMDs for Efficient Electrocatalytic Hydrogen Evolution

The introduction of heteroatoms is a widely employed strategy for electrocatalysis of transition metal dichalcogenides (TMDs). This approach activates the inactive basal plane, effectively boosting the intrinsic catalytic activity. However, the effect of atomic configurations incorporated within the TMDs' lattice on catalytic activity is not thoroughly understood owing to the lack of controllable synthetic approaches for highly doped TMDs. In this study, we demonstrate a facile approach to realizing heavily doped MoS₂ with a high doping concentration above 16% via intermediate-reaction-mediated chemical vapor deposition. As the V doping concentration increased, the incorporated V atoms coalesced in a manner that enabled both the basal plane activation and electrical conductivity enhancement of MoS₂. This accelerated the kinetics of the hydrogen evolution reaction (HER) through the reduced Gibbs free energy of hydrogen adsorption, as evidenced by experimental and theoretical analyses. Consequently, the coalesced V-doped MoS₂ exhibited superior HER performance, with an overpotential of 100 mV at 10 mA cm^-2, surpassing the pristine and single-atom-doped counterparts. This study provides an intriguing pathway for engineering the atomic doping configuration of TMDs to develop efficient 2D nanomaterial-based electrocatalysts.

Ultrafast Synthesis of Mo2C-Based Catalyst by Joule Heating towards Electrocatalytic Hydrogen Evolution Reaction

Developing earth-abundant electrocatalysts useful for hydrogen evolution reactions (HER) is critical for electrocatalytic water splitting driven by renewable energy. Molybdenum carbide (Mo2C) with the crystal structure of hexagonal symmetry has been identified to be an excellent HER catalyst due to its platinum-like electronic structure while the synthesis of Mo2C is generally time consuming and energy intensive. Herein, we demonstrated the ultrafast synthesis of a Mo2C-based electrocatalyst with Joule heating at 1473 K for only 6 s. Benefitting from several advantages including efficient catalytic kinetics, enhanced charge transport kinetics and high intrinsic activity, the as-prepared catalyst exhibited drastically enhanced HER performance compared with commercial Mo2C. It showed an overpotential of 288 mV for achieving a current density of −50 mA cm−2 and good stability, which highlighted the feasibility of the Joule heating method towards preparing efficient electrocatalysts.

1T-MoS2 Enriched Hierarchical MoS2 /MoO3 Produced by Phase Transformation for Efficient Hydrogen Evolution Reaction

In recent years, transition metal sulfides have been widely studied in the context of their use as electrocatalysts. The electrocatalytic propensity of the classical semiconductor MoS₂ , which exists in the 1T and 2H phase structures, has attracted extensive attention. Therefore, the synthesis of highly active and stable MoS₂ -based catalysts has become the goal of many research efforts. We recently developed a method that can be utilized to prepare the MoS₂ /MoO₃ heterojunction in a phase-controlled manner. 1T-MoS₂ phase enriched MoS₂ /MoO₃ heterojunction can be generated using a simple hydrothermal and acid treatment sequence and that the heterojunction has a unique three-dimensional structure, large active surface area, and therefore achieve a low overpotential and high catalytic current density, as well as long-term stability for the hydrogen evolution reaction.