Phase-Transition of Mo2 C Induced by Tungsten Doping as Heterointerface-Rich Electrocatalyst for Optimizing Hydrogen Evolution Activity

Electrochemical hydrogen evolution reaction (HER) from water splitting driven by renewable energy is considered a promising method for large-scale hydrogen production, and as an alternative to noble-metal electrocatalysts, molybdenum carbide (Mo2 C) has exhibited effective HER performance. However, the strong bonding strength of intermediate adsorbed H (Hads ) with Mo active site slows down the HER kinetics of Mo2 C. Herein, using phase-transition strategy, hexagonal β-Mo2 C could be easily transferred to cubic δ-Mo2 C through electron injection triggered by tungsten (W) doping, and heterointerface-rich Mo2 C-based composites, including β-Mo2 C, δ-Mo2 C, and MoO2 , are presented. Experimental results and density functional theory calculations reveal that W doping mainly contributes to the phase-transition process, and the generated heterointerfaces are the dominant factor in inducing remarkable electron accumulation around Mo active sites, thus weakening the Mo─H coupling. Wherein, the β-Mo2 C/MoO2 interface plays an important role in optimizing the electronic structure of Mo 3d orbital and hydrogen adsorption Gibbs free energy (ΔGH* ), enabling these Mo2 C-based composites to have excellent intrinsic catalytic activity like low overpotential (η10 = 99.8 mV), small Tafel slope (60.16 dec-1 ), and good stability in 1 m KOH. This work sheds light on phase-transition engineering and offers a convenient route to construct heterointerfaces for large-scale HER production.

A Highly Active Porous Mo2C-Mo2N Heterostructure on Carbon Nanowalls/Diamond for a High-Current Hydrogen Evolution Reaction

Developing non-precious metal-based electrocatalysts operating in high-current densities is highly demanded for the industry-level electrochemical hydrogen evolution reaction (HER). Here, we report the facile preparation of binder-free Mo2C-Mo2N heterostructures on carbon nanowalls/diamond (CNWs/D) via ultrasonic soaking followed by an annealing treatment. The experimental investigations and density functional theory calculations reveal the downshift of the d-band center caused by the heterojunction between Mo2C/Mo2N triggering highly active interfacial sites with a nearly zero ∆GH* value. Furthermore, the 3D-networked CNWs/D, as the current collector, features high electrical conductivity and large surface area, greatly boosting the electron transfer rate of HER occurring on the interfacial sites of Mo2C-Mo2N. Consequently, the self-supporting Mo2C-Mo2N@CNWs/D exhibits significantly low overpotentials of 137.8 and 194.4 mV at high current densities of 500 and 1000 mA/cm2, respectively, in an alkaline solution, which far surpass the benchmark Pt/C (228.5 and 359.3 mV) and are superior to most transition-metal-based materials. This work presents a cost-effective and high-efficiency non-precious metal-based electrocatalyst candidate for the electrochemical hydrogen production industry.

Current Progress of Mo-Based Metal Organic Frameworks Derived Electrocatalysts for Hydrogen Evolution Reaction

As an important half-reaction for electrochemical water splitting, electrocatalytic hydrogen evolution reaction suffers from sluggish kinetics, and it is still urgent to search high efficiency non-platinum-based electrocatalysts. Mo-based catalysts such as Mo2 C, MoO2 , MoP, MoS2 , and MoNx have emerged as promising alternatives to Pt/C owing to their similar electronic structure with Pt and abundant reserve of Mo. On the other hand, due to the adjustable topology, porosity, and nanostructure of metal organic frameworks (MOFs), MOFs are extensively used as precursors to prepare nano-electrocatalysts. In this review, for the first time, the progress of Mo-MOFs-derived electrocatalysts for hydrogen evolution reaction is summarized. The preparation method, structures, and catalytic performance of the catalysts are illustrated based on the types of the derived electrocatalysts including Mo2 C, MoO2 , MoP, MoS2 , and MoNx . Especially, the commonly used strategies to improve catalytic performance such as heteroatoms doping, constructing heterogeneous structure, and composited with noble metal are discussed. Moreover, the opportunities and challenges in this area are proposed to guide the designment and development of Mo-based MOF derived electrocatalysts.

TiO2 Nanotubes Decorated with Mo2C for Enhanced Photoelectrochemical Water-Splitting Properties

The presence of Ti3+ in the structure of TiO2 nanotube arrays (NTs) has been shown to enhance the photoelectrochemical (PEC) water-splitting performance of these NTs, leading to improved results compared to pristine anatase TiO2 NTs. To further improve the properties related to PEC performance, we successfully produced TiO2 NTs using a two-step electrochemical anodization technique, followed by annealing at a temperature of 450 °C. Subsequently, Mo2C was decorated onto the NTs by dip coating them with precursors at varying concentrations and times. The presence of anatase TiO2 and Ti3O5 phases within the TiO2 NTs was confirmed through X-ray diffraction (XRD) analysis. The TiO2 NTs that were decorated with Mo2C demonstrated a photocurrent density of approximately 1.4 mA cm-2, a value that is approximately five times greater than the photocurrent density exhibited by the bare TiO2 NTs, which was approximately 0.21 mA cm-2. The observed increase in photocurrent density can be ascribed to the incorporation of Mo2C as a cocatalyst, which significantly enhances the photocatalytic characteristics of the TiO2 NTs. The successful deposition of Mo2C onto the TiO2 NTs was further corroborated by the characterization techniques utilized. The utilization of field emission scanning electron microscopy (FESEM) allowed for the observation of Mo2C particles on the surface of TiO2 NTs. To validate the composition and optical characteristics of the decorated NTs, X-ray photoelectron spectroscopy (XPS) and UV absorbance analysis were performed. This study introduces a potentially effective method for developing efficient photoelectrodes based on TiO2 for environmentally sustainable hydrogen production through the use of photoelectrochemical water-splitting devices. The utilization of Mo2C as a cocatalyst on TiO2 NTs presents opportunities for the advancement of effective and environmentally friendly photoelectrochemical (PEC) systems.

Effect of Mo2C Addition on the Tribological Behavior of Ti(C,N)-Based Cermets

Due to the excellent properties of Ti (C,N)-based ceramics, such as high hardness, excellent wear resistance, exceptional thermal deformation resistance, and sound chemical stability, they have been widely used in cutting tools or molds. Thus, revealing their tribological behavior against hard materials is of great significance. Some studies have reported the tribological behavior of Ti(C,N)-based cermets and hard cermets, but so far, the effects of Mo2C additions on the frictional properties of Ti(C,N)-based cermets are still unclear. In this study, Ti(C,N)-10WC-1Cr3C2-5Co-10Ni-x Mo2C cermets (x = 4, 6, 8, 10 and 12 wt.%) were sintered using a vacuum hot-pressing furnace. Furthermore, the core-rim morphologies of the sintered samples were observed in SEM images. Then, the wear resistance of the cermets was studied against a Si3N4 ball at a 50 N load using the fretting wear test. Finally, the wear mechanism was characterized using a combination of SEM, EDS and XPS. The experimental results indicated that the wear mechanisms of the cermets were mainly abrasive wear, adhesive wear, and the formation of an oxide film. As the content of Mo2C increased from 4 wt.% to 12 wt.%, the friction coefficient and wear volume had a variation law of first decreasing and then decreasing, and reached minimum values at 6 wt.% and 12 wt.%, and the lowest friction coefficient and wear rate were 0.49 and 0.9 × 10-6 mm3/Nm, respectively. The 6 wt.% Mo2C greatly improved the hardness and fracture toughness of the cermet, while the 12 wt.% Mo2C promoted the formation of an oxide film and protected the friction surface. The cermet with 6 wt.% Mo2C is recommended because it has comprehensive advantages in terms of its mechanical properties, tribological properties, and cost.

Ni-Doped Mo2C Anchored on Graphitized Porous Carbon for Boosting Electrocatalytic N2 Reduction

Facilitating the efficient activation of N₂ molecules and inhibiting the competing hydrogen evolution reaction remain a challenge in the nitrogen reduction reaction (NRR). A heteroatom doping strategy is an effective way to optimize the energy barrier during the NRR process to improve the catalytic efficiency. Herein, we report Ni-doped Mo₂C anchored on graphitized porous conductive carbon for regulating the electronic structure and catalytic properties of electrocatalysts toward NRR. Benefiting from the porous structure and graphitization features of the carbon matrix, more active sites and high electronic conductivity were achieved. Meanwhile, with the doping of Ni atoms, the electronic configuration near the Ni-Mo active sites was optimized and the adsorption of N₂ on them was also promoted due to the increased electron transfer. Moreover, the lowered energy barrier of the NRR process and the suppressed hydrogen adsorption on the active site all resulted in the high catalytic activity and selectivity of the catalyst. Therefore, a high NH₃ yield rate of 46.49 μg h^-1 mg^-1 and a faradic efficiency of 29.05% were achieved. This work not only validates the important role of heteroatom doping on the regulation of NRR catalytic activity but also provides a promising avenue for the green synthesis of NH₃.

Preparation of Three-Dimensional Mo2C/NC@MXene and Its Efficient Electromagnetic Absorption Properties

The positively charged MoO₃/PDA microspheres are obtained by stacking and assembly of the sheet structure, and the negatively charged MXene nanosheets are wrapped on the surface through the principle of electrostatic self-assembly. After annealing, a nitrogen-doped carbon composite and a MXene-coated Mo₂C wave absorber are obtained. The formation of the wrinkled surface provides a complex pore structure, and the multiple interface reflections between the nanosheets enhance the absorption performance. The existence of heterogeneous interfaces and the uneven distribution of space charges accumulated between the interfaces effectively reduce the minimum reflection loss (RL_min). This work explores the effects of the ratio between MoO₃/PDA and MXene nanosheets and loading amount on the microwave absorption properties. Mo₂C/NC@MXene-2 obtained when the ratio of the two is 3:1 has the best absorption performance under 25% loading. The RL_min is -59.36 dB, and the corresponding effective absorption bandwidth (EAB) is 4.6 GHz at 2.5 mm. This work expands the new applications of MXene-based and Mo₂C-based materials and has a guiding significance for the design of electrostatic self-assembly materials.

Hierarchical Mo2C@CNT Hybrid Structure Formation for the Improved Lithium-Ion Battery Storage Performance

2-D transition metal carbides (TMCs)-based anode materials offer competitive performance in lithium-ion batteries (LIBs) owing to its excellent conductivity; cheaper, flexible uses; and superior mechanical stability. However, the electrochemical energy storage of TMCs is still the major obstacle due to their modest capacity and the trends of restacking/aggregation. In this report, the Mo₂C nanosheets were attached on conductive CNT network to form a hierarchical 2D hybrid structure, which not only alleviated the aggregation of the Mo₂C nanoparticle and facilitated the rapid transference of ion/electron, but also adapted effectually to the hefty volume expansion of Mo₂C nanosheets and prevented restacking/collapse of Mo₂C structure. Benefitting from the layered Mo₂@CNT hybrid structure, the charge/discharge profile produced a 200 mAh g⁻¹ discharge-specific capacity (second cycle) and 132 mAh g⁻¹ reversible-discharge discharge-specific capacity (after 100 cycles) at 50 mA g⁻¹ current density, with high-speed competency and superior cycle stability. The improved storage kinetics for Mo₂@CNT hybrid structure are credited to the creation of numerous active catalytic facets and association reaction between the CNT and Mo₂C, promoting the efficient electron transfer and enhancing the cycling stability.

Mo2C/C Hierarchical Double-Shelled Hollow Spheres as Sulfur Host for Advanced Li-S Batteries

One of the major challenges in the sulfur cathode of the Li-S batteries is to achieve high sulfur loading, fast Li ions transfer, and lithium polysulfides (LiPSs) shuttling suppressing simultaneously. This issue can be well solved by the development of molybdenum carbide decorated N-doped carbon hierarchical double-shelled hollow spheres (Mo2C/C HDS-HSs). The mesoporous thick inner shell and the central void of the HDS-HSs achieve the high sulfur loading, facilitate the ion/electrolyte penetration, and accelerate the charge transfer. The microporous thin outer shell suppresses the LiPSs shuttling and reduces the charge/mass diffusion distance. The double-shelled hollow structure accommodates the volume expansion during lithiation. Furthermore, Mo2C/C composition renders the HDS-HSs cathode with improved conductivity, enhanced affinity to LiPSs, and accelerated kinetics of LiPSs conversion. The structural and compositional advantages render the Mo2C/C/S HDS-HSs electrode with the high specific capacity, excellent rate capability, and ultra-long cycling stability in the composed Li-S batteries.

Tremella-like Mo and N Codoped Graphitic Nanosheets by In Situ Carbonization of Phthalocyanine for Potassium-Ion Battery

A tremella-like Mo and N codoped graphitic nanosheet array supported on activated carbon (Mo₂C-MoC/AC-N) is prepared via in situ carbonization of nitrogen-rich cobalt phthalocyanine nanoparticulates anchored on activated carbon as a high-performance anode for potassium-ion batteries. The nanosheets about 5 nm thick are uniformly distributed on the surface of activated carbon for fast K-ion intercalation, and the abundant micropores in activated carbon provide additional adsorption sites of potassium ions, forming a three-dimensional architecture for potassium storage. The 3.9 atom % Mo in Mo₂C-MoC/AC-N is in the form of Mo₂C and MoC flakes (around 1:1) attached to the graphitic nanosheets. X-ray diffraction (XRD) analysis revealed that the reaction with Mo₂C (forming K₂C) happens mainly at 0.8-0.4 V, while the reaction with MoC (forming K₂C) occurs primarily at 0.4-0.01 V. The N doping (9.6 atom %) causes an interlayer spacing expansion of 0.3 Å in the graphitic nanosheets, beneficial to the potassium-ion insertion reaction to form KC₈ at 0.4-0.01 V. The Mo₂C-MoC/AC-N anode exhibits a capacity of 457.5 mA h g^-1 at a current density of 0.05 A g^-1 and an excellent capacity of 144.4 mA h g^-1 at a high current of 5 A g^-1 with a capacity loss rate of 0.49‰ per cycle.

Synthesis of Ultrathin and Grid-Structural Carbon Nanosheets Coupled with Mo2 C for Electrocatalytic Hydrogen Production

Molybdenum carbide possessing a Pt-like d-band electronic structure is considered as one of potential candidates of electrocatalysts and it shows intrinsic catalytic property. However, a high carbonizing temperature easily leads to the coalescence of nanoparticles (NPs). Here, we propose a simple sol-gel route to achieve high dispersity of carbide NPs by designing a Mo-involved xerogel. The results show that molybdenum carbide NPs are dispersed and anchored on the nitrogen-doped carbon nanosheets (Mo₂ C@NC). Ultrathin carbon layers resemble graphene and the network structures act as a support of carbide NPs, which can hinder NPs' coalescence effectively. Nanpoparticles cross-coupled on network-structure nanosheets display the grid shapes. Electrochemical studies indicate that Mo₂ C@NC material exhibits outstanding hydrogen evolution performance in alkaline electrolyte.

Bimetallic MOFs-derived coral-like Ag-Mo2C/C interwoven nanorods for amperometric detection of hydrogen peroxide

Coral-like Ag-Mo₂C/C-I and blocky Ag-Mo₂C/C-II composites were obtained from one-step in situ calcination of [Ag(HL)₃(Mo₈O₂₆)]_n·nH₂O [L: N-(pyridin-3-ylmethyl) pyridine-2-amine] under N₂/H₂ and N₂ atmospheres, respectively. The coral-like morphology of Ag-Mo₂C/C-I is composed of interwoven nanorods embedded with small particles, and the nano-aggregate of Ag-Mo₂C/C-II is formed by cross-linkage of irregular nanoparticles. The above composites are decorated on glassy carbon electrode (GCE) drop by drop to generate two enzyme-free electrochemical sensors (Ag-Mo₂C/C/GCE) for amperometric detection of H₂O₂. In particular, the coral-like Ag-Mo₂C/C-I/GCE sensor possesses rapid response (1.2 s), high sensitivity (466.2 μA·mM^-1·cm^-2), and low detection limit (25 nM) towards trace H₂O₂ and has wide linear range (0.08 μM~4.67 mM) and good stability. All these sensing performances are superior to Ag-Mo₂C/C-II/GCE, indicating that the calcining atmosphere has an important influence on microstructure and electrochemical properties. The excellent electrochemical H₂O₂ sensing performance of Ag-Mo₂C/C-I/GCE sensor is mainly attributed to the synergism of unique microstructure, platinum-like electron structure of Mo₂C, strong interaction between Mo and Ag, as well as the increased active sites and conductivity caused by co-doped Ag and carbon. Furthermore, this sensor has been successfully applied to the detection of H₂O₂ in human serum sample, contact lens solution, and commercial disinfector, demonstrating the potential in related fields of environment and biology. Graphical abstract.

Structural Evolution and Underlying Mechanism of Single-Atom Centers on Mo2C(100) Support during Oxygen Reduction Reaction

The single-metal atoms coordinating with the surface atoms of the support constitute the active centers of as-prepared single-atom catalysts (SACs). However, under hash electrochemical conditions, (1) supports' surfaces may experience structural change, which turn to be distinct from those at ambient conditions; (2) during catalysis, the dynamic responses of a single atom to the attack of reaction intermediates likely change the coordination environment of a single atom. These factors could alter the performance of SACs. Herein, we investigate these issues using Mo₂C(100)-supported single transition-metal (TM) atoms as model SACs toward catalyzing the oxygen reduction reaction (ORR). It is found that the Mo₂C(100) surface is oxidized under ORR turnover conditions, resulting in significantly weakened bonding between single TM atoms and the Mo₂C(100) surface (TM@Mo₂C(100)_O* term for SAC). While the intermediate in 2 e^- ORR does not change the local structures of the active centers in these SACs, the O* intermediate emerging in 4 e^- ORR can damage Rh@ and Cu@Mo₂C(100)_O*. Furthermore, on the basis of these findings, we propose Pt@Mo₂C(100)_O* as a qualified ORR catalyst, which exhibits extraordinary 4 e^- ORR activity with an overpotential of only 0.33 V, surpassing the state-of-the-art Pt(111), and thus being identified as a promising alternative to the commercial Pt/C catalyst.

Synergizing Mo Single Atoms and Mo2 C Nanoparticles on CNTs Synchronizes Selectivity and Activity of Electrocatalytic N2 Reduction to Ammonia

Previous research of molybdenum-based electrocatalysts for nitrogen reduction reaction (NRR) has been largely considered on either isolated Mo single atoms (MoSAs) or Mo carbide particles (e.g., Mo₂ C) separately, while an integrated synergy (MoSAs-Mo₂ C) of the two has never been considered. The theoretical calculations show that the Mo single atoms and Mo₂ C nanoparticles exhibit, respectively, different catalytic hydrogen evolution reaction and NRR selectivity. Therefore, a new role-playing synergistic mechanism can be well enabled for the multistep NRR, when the two are combined on the same N-doped carbon nanotubes (NCNTs). This hypothesis is confirmed experimentally, where the MoSAs-Mo₂ C assembled on NCNTs (MoSAs-Mo₂ C/NCNTs) yields an ammonia formation rate of 16.1 µg h^-1 cm_cat ^-2 at -0.25 V versus reversible hydrogen electrode, which is about four times that by the Mo₂ C alone (Mo₂ C/NCNTs) and 4.5 times that by the MoSAs alone (MoSAs/NCNTs). Moreover, the Faradic efficiency of the MoSAs-Mo₂ C/NCNTs is raised up to twofold and sevenfold of the Mo₂ C/NCNTs and MoSAs/NCNTs, respectively. The MoSAs-Mo₂ C/NCNTs also demonstrate outstanding stability by the almost unchanged catalytic performance over 10 h of the chronoamperometric test. The present study provides a promising new prototype of synchronizing the selectivity and activity for the multistep catalytic reactions.

Molybdenum Carbide-Embedded Multichannel Hollow Carbon Nanofibers as Bifunctional Catalysts for Water Splitting

With the environmental pollution and non-renewable fossil fuels, it is imperative to develop eco-friendly, renewable, and highly efficient electrocatalysts for sustainable energy. Herein, a simple electrospinning process used to synthesis Mo₂ C-embedded multichannel hollow carbon nanofibers (Mo₂ C-MCNFs) and followed by the pyrolysis process. As prepared lotus root-like nanoarchitecture could offer rich porosity and facilitate the electrolyte infiltration, the Mo₂ C-MCNFs delivered favourable catalytic activity for HER and OER. The resultant catalysts exhibit low overpotentials of 114 mV and 320 mV at a current density of 10 mA cm^-2 for HER and OER, respectively. Furthermore, using the Mo₂ C-MCNFs catalysts as a bifunctional electrode toward overall water splitting, which only needs a small cell voltage of 1.68 V to afford a current density of 10 mA cm^-2 in the home-made alkaline electrolyzer. This interesting work presents a simple and effective strategy to further fabricating tunable nanostructures for energy-related applications.

Fabrication of MoOx/Mo2C-Layered Hybrid Structures by Direct Thermal Oxidation of Mo2C

Two-dimensional (2D) Mo₂C, as a new member of transition metal carbides, has many intriguing properties and potential applications in superconductors and electronic devices. The thermal stability of 2D materials is essential for the performance of the related devices, especially the ones with a vertical heterostructure. However, rare reports have demonstrated the thermal stability of Mo₂C and the effects of thermal stability on its performance. Here, we propose a facile and controllable method to directly oxidize Mo₂C to MoO_x, forming a MoO_x/Mo₂C heterostructure. During the oxidization process, an in situ technique is employed to uncover the transformation and thermal stability of the Mo₂C. The chemical vapor deposition Mo₂C shows high structural stability below 550 °C in Ar or below 350 °C in O₂, which demonstrates the high thermal stability and antioxidation of the Mo₂C film. The metallic Mo₂C is gradually oxidized to semiconducting MoO_x as the temperature increases above 350 °C. The oxidization rate can be easily controlled by adjusting the oxidation temperature and time. Further, the obtained MoO_x/Mo₂C vertical hybrid structure shows obvious Schottky junction behaviors, strongly indicating the perfect interfacial contact between the component layers. This work offers a new strategy for the controllable fabrication of high-quality 2D heterostructures.

Synthesis of Mo2C and W2C Nanoparticle Electrocatalysts for the Efficient Hydrogen Evolution Reaction in Alkali and Acid Electrolytes

The synthesis of low cost, high efficacy, and durable hydrogen evolution electrocatalysts from the non-noble metal group is a major challenge. Herein, we establish a simple and inexpensive chemical reduction method for producing molybdenum carbide (Mo2C) and tungsten carbide (W2C) nanoparticles that are efficient electrocatalysts in alkali and acid electrolytes for hydrogen evolution reactions (HER). Mo2C exhibits outstanding electrocatalytic behavior with an overpotential of -134 mV in acid medium and of -116 mV in alkaline medium, while W2C nanoparticles require an overpotential of -173 mV in acidic medium and -130 mV in alkaline medium to attain a current density of 10 mA cm-2. The observed results prove the capability of high- and low-pH active electrocatalysts of Mo2C and W2C nanoparticles to be efficient systems for hydrogen production through HER water electrolysis.

Electric Field-Driven Interfacial Alloying for in Situ Fabrication of Nano-Mo2C on Carbon Fabric as Cathode toward Efficient Hydrogen Generation

A binderless composite cathode for efficient electrocatalytic hydrogen evolution reaction (HER), Mo₂C-decorated carbon cloth (denoted as CC/MC), is simply fabricated via a novel and unique strategy which involves a solid-solid phase interfacial electrochemical reaction between carbon fiber and bulk-MoS₂ in molten NaCl-KCl (700 °C). MoS₂, evenly coated on carbon cloth (CC), is electrochemically reduced in situ and readily reacts with the carbon fibers of CC current collector to form a Mo₂C nanoparticle layer. The experiment and calculation results show that the applied electric field results in a declining migration barrier of Mo vacancies in Mo₂C lattice, which promotes the diffusion of Mo atoms into carbon across the interfacial Mo₂C layer, thereby impelling the combination of Mo with C in depth. The electrochemical tests indicate that the optimized cathode (CC/MC-2) exhibits a small overpotential of 134.4 mV at 10 mA cm^-2 and stays stable for HER in acidic media. The catalytic capacity for N₂ reduction of CC/MC-2 is analyzed. In addition, a Ni-doped Mo₂C-modified carbon fabric electrode with enhanced HER activity (η₁₀ = 96.6 mV) can be prepared through a similar process.

MoS2-Based Photodetectors Powered by Asymmetric Contact Structure with Large Work Function Difference

Self-powered devices are widely used in the detection and sensing fields. Asymmetric metal contacts provide an effective way to obtain self-powered devices. Finding two stable metallic electrode materials with large work function differences is the key to obtain highly efficient asymmetric metal contacts structures. However, common metal electrode materials have similar and high work functions, making it difficult to form an asymmetric contacts structure with a large work function difference. Herein, Mo₂C crystals with low work function (3.8 eV) was obtained by chemical vapor deposition (CVD) method. The large work function difference between Mo₂C and Au allowed us to synthesize an efficient Mo₂C/MoS₂/Au photodetector with asymmetric metal contact structure, which enables light detection without external electric power. We believe that this novel device provides a new direction for the design of miniature self-powered photodetectors. These results also highlight the great potential of ultrathin Mo₂C prepared by CVD in heterojunction device applications.

Rational Design of Atomic Layers of Pt Anchored on Mo2 C Nanorods for Efficient Hydrogen Evolution over a Wide pH Range

Transition metal carbide compound has been extensively investigated as a catalyst for hydrogenation, for example, due to its noble metal-like properties. Herein a facile synthetic strategy is applied to control the thickness of atomic-layer Pt clusters strongly anchored on N-doped Mo₂ C nanorods (Pt/N-Mo₂ C) and it is found that the Pt atomic layers modify Mo₂ C function as a high-performance and robust catalyst for hydrogen evolution. The optimized 1.08 wt% Pt/N-Mo₂ C exhibits 25-fold, 10-fold, and 15-fold better mass activity than the benchmark 20 wt% Pt/C in neutral, acidic, and alkaline media, respectively. This catalyst also represents an extremely low overpotential of -8.3 mV at current density of 10 mA cm^-2 , much better than the majority of reported electrocatalysts and even the commercial reference catalyst (20 wt%) Pt/C. Furthermore, it exhibits an outstanding long-term operational durability of 120 h. Theoretical calculation predicts that the ultrathin layer of Pt clusters on Mo-Mo₂ C yields the lowest absolute value of ΔG_H* . Experimental results demonstrate that the atomic layer of Pt clusters anchored on Mo₂ C substrate greatly enhances electron and mass transportation efficiency and structural stability. These findings could provide the foundation for developing highly effective and scalable hydrogen evolution catalysts.

Controlled Growth of an Mo₂C-Graphene Hybrid Film as an Electrode in Self-Powered Two-Sided Mo₂C-Graphene/Sb₂S0.42Se2.58/TiO₂ Photodetectors

The monotonic work function of graphene makes it difficult to meet the electrode requirements of every device with different band structures. Two-dimensional (2D) transition metal carbides (TMCs), such as carbides in MXene, are considered good candidates for electrodes as a complement to graphene. Carbides in MXene have been used to make electrodes for use in devices such as lithium batteries. However, the small lateral size and thermal instability of carbides in MXene, synthesized by the chemically etching method, limit its application in optoelectronic devices. The chemical vapor deposition (CVD) method provides a new way to obtain high-quality ultrathin TMCs without functional groups. However, the TMCs film prepared by the CVD method tends to grow vertically during the growth process, which is disadvantageous for its application in the transparent electrode. Herein, we prepared an ultrathin Mo₂C—graphene (Mo₂C—Gr) hybrid film by CVD to solve the above problem. The work function of Mo₂C—Gr is between that of graphene and a pure Mo₂C film. The Mo₂C—Gr hybrid film was selected as a transparent hole-transporting layer to fabricate novel Mo₂C—Gr/Sb₂S_0.42Se_2.58/TiO₂ two-sided photodetectors. The Mo₂C—Gr/Sb₂S_0.42Se_2.58/TiO₂/fluorine-doped tin oxide (FTO) device could detect light from both the FTO side and the Mo₂C—Gr side. The device could realize a short response time (0.084 ms) and recovery time (0.100 ms). This work is believed to provide a powerful method for preparing Mo₂C—graphene hybrid films and reveals its potential applications in optoelectronic devices.

Molybdenum Carbide Nanodots Enable Efficient Electrocatalytic Nitrogen Fixation under Ambient Conditions

Electrocatalytic nitrogen fixation is considered a promising approach to achieve NH₃ production. However, due to the chemical inertness of nitrogen, it is necessary to develop efficient catalysts to facilitate the process of nitrogen reduction. Here, molybdenum carbide nanodots embedded in ultrathin carbon nanosheets (Mo₂ C/C) are developed to serve as a catalyst candidate for highly efficient and robust N₂ fixation through an electrocatalytic nitrogen reduction reaction (NRR). The as-synthesized Mo₂ C/C nanosheets show excellent catalytic performance with a high NH₃ yield rate (11.3 µg h^-1 mg^-1 _Mo2C ) and Faradic efficiency (7.8%) for NRR under ambient conditions. More importantly, the isotopic experiments using ¹⁵ N₂ as a nitrogen source confirm that the synthesized ammonia is derived from the direct supply of nitrogen. This result also demonstrates the possibility of high-efficiency nitrogen reduction even though accompanied with vigorous hydrogen evolution.

Molybdenum Carbide Nanodots Enable Efficient Electrocatalytic Nitrogen Fixation under Ambient Conditions

Electrocatalytic nitrogen fixation is considered a promising approach to achieve NH₃ production. However, due to the chemical inertness of nitrogen, it is necessary to develop efficient catalysts to facilitate the process of nitrogen reduction. Here, molybdenum carbide nanodots embedded in ultrathin carbon nanosheets (Mo₂ C/C) are developed to serve as a catalyst candidate for highly efficient and robust N₂ fixation through an electrocatalytic nitrogen reduction reaction (NRR). The as-synthesized Mo₂ C/C nanosheets show excellent catalytic performance with a high NH₃ yield rate (11.3 µg h^-1 mg^-1 _Mo2C ) and Faradic efficiency (7.8%) for NRR under ambient conditions. More importantly, the isotopic experiments using ¹⁵ N₂ as a nitrogen source confirm that the synthesized ammonia is derived from the direct supply of nitrogen. This result also demonstrates the possibility of high-efficiency nitrogen reduction even though accompanied with vigorous hydrogen evolution.

Scalable Synthesis of a Ruthenium-Based Electrocatalyst as a Promising Alternative to Pt for Hydrogen Evolution Reaction

Designing highly active, stable, and cost-efficient electrocatalysts as alternatives to replace Pt is extremely desirable for hydrogen evolution reaction (HER). Despite much progress that has been made based on complete nonprecious metals (NPMs), very few NPM catalysts have shown comparable performance to Pt-based catalysts. Herein, a cost-efficient, environmentally friendly, and scalable method to synthesize a novel ruthenium(Ru)-doped transition-metal carbide (Mo₂C) hybrid catalyst was proposed. The hybrid nanoparticles were uniformly distributed and strongly embedded in a biomass-derived highly porous N-doped carbon framework. In particular, Mo₂C@Ru exhibited a Pt-like remarkable electrocatalytic performance for HER, and it only required an extremely low overpotential of 24.6 mV to reach the current density of 10 mA cm^-2. Furthermore, our density functional theory calculations indicated that the nanocomposite exhibits improved metal-hydrogen binding and favorable hydrogen adsorption energy, which is comparable to that of Pt. The facile and scalable synthesis methodology, the relatively low cost, and the excellent electrochemical HER performance comparable to that of commercial Pt/C suggest that the Mo₂C@Ru electrocatalyst is a promising alternative to Pt for large-scale hydrogen production.

Atomic H-Induced Mo2C Hybrid as an Active and Stable Bifunctional Electrocatalyst

Mo₂C nanocrystals (NCs) anchored on vertically aligned graphene nanoribbons (VA-GNR) as hybrid nanoelectrocatalysts (Mo₂C-GNR) are synthesized through the direct carbonization of metallic Mo with atomic H treatment. The growth mechanism of Mo₂C NCs with atomic H treatment is discussed. The Mo₂C-GNR hybrid exhibits highly active and durable electrocatalytic performance for the hydrogen-evolution reaction (HER) and oxygen-reduction reaction (ORR). For HER, in an acidic solution the Mo₂C-GNR has an onset potential of 39 mV and a Tafel slope of 65 mV dec^-1, and in a basic solution Mo₂C-GNR has an onset potential of 53 mV, and Tafel slope of 54 mV dec^-1. It is stable in both acidic and basic media. Mo₂C-GNR is a high-activity ORR catalyst with a high peak current density of 2.01 mA cm^-2, an onset potential of 0.93 V that is more positive vs reversible hydrogen electrode (RHE), a high electron transfer number n (∼3.90), and long-term stability.