Carbonaceous Material Modified MoO2 Nanospheres with Oxygen Vacancies for Enhanced Visible-Light Photocatalytic Oxidative Coupling of Benzylamine

Surface oxygen vacancy (OV) plays a pivotal role in the activation of molecular oxygen and separation of electrons and holes in photocatalysis. Herein, carbonaceous materials-modified MoO₂ nanospheres with abundant surface OVs (MoO₂/C-OV) were successfully synthesized via glucose hydrothermal processes. In situ introduction of carbonaceous materials triggered a reconstruction of the MoO₂ surface, which introduced abundant surface OVs on the MoO₂/C composites. The surface oxygen vacancies on the obtained MoO₂/C-OV were confirmed via electron spin resonance spectroscopy (ESR) and X-ray photoelectron spectroscopy (XPS). The surface OVs and carbonaceous materials boosted the activation of molecular oxygen to singlet oxygen (¹O₂) and superoxide anion radical (•O₂^-) in selectively photocatalytic oxidation of benzylamine to imine. The conversion of benzylamine was 10 times that of pristine MoO₂ nanospheres with a high selectivity under visible light irradiation at 1 atm air pressure. These results open an avenue to modify Mo-based materials for visible light-driven photocatalysis.

NIR-II Responsive Molybdenum Dioxide Nanosystem Manipulating Cellular Immunogenicity for Enhanced Tumor Photoimmunotherapy

Photothermal therapy (PTT) in the second near-infrared (NIR-II) window has emerged as a better candidate for deep-tissue tumor elimination. More interestingly, the photothermal ablated tumor cells also manifest somewhat immunostimulation potency to elicit antitumor immunity, although most dying cells are undergoing apoptosis that is commonly considered as immunologically silent. Here, a NIR-II responsive nanosystem is established for tumor photoimmunotherapy using molybdenum dioxide (MoO₂) nanodumbbells as the nanoconverter. Meanwhile, an apoptosis-blocking strategy is proposed to regulate the cell death pattern under NIR-II laser irradiation in order to improve the immunogenic cell death. The nanoformulation can efficiently block caspase 8-dependent apoptotic pathway in photothermal ablated tumor cells and transform into more immunogenic death patterns, thereby activating systemic immunity to inhibit tumor growth and metastasis. In addition, this strategy also helps enhance the body's responses to α-PD-1 immune checkpoint inhibitor, which implies a potential optimal combination for cancer immunotherapy.

Encapsulating MoO2 Nanocrystals into Flexible Carbon Nanofibers via Electrospinning for High-Performance Lithium Storage

Design and synthesis of flexible and self-supporting electrode materials in high-performance lithium storage is significant for applications in the field of smart wearable devices. Herein, flexible carbon nanofiber membranes with uniformly distributed molybdenum dioxide (MoO₂) nanocrystals are fabricated by a needlefree electrospinning method combined with the subsequent carbonization process, which exhibits outstanding structural stability under abrasion and deformation. The as-fabricated lithium-ion batteries (LIBs) exhibit a high discharge of 450 mAh g⁻¹ after 500 cycles at 2000 mA g⁻¹ by using the MoO₂/C nanofiber membrane as the self-supporting anode. Further, the nanofibers structure remains intact after 500 cycles, which reflects the excellent stability of the materials. This study provides a simple and effective method for the preparation of MoO₂/C nanofiber materials, which can not only maintain its excellent electrochemical and physical properties, but also easily realize large-scale production. It is undoubtedly beneficial for the development of flexible LIBs and smart wearable devices.

Confining Ultrafine MoO2 in a Carbon Matrix Enables Hybrid Li Ion and Li Metal Storage

Poor cycle and rate performance caused by volume effects and sluggish kinetics is the main bottleneck for most lithium-ion battery (LIB) anode materials run on the conversion reaction. Although nanostructure engineering has shown to be an effective method to reduce the undesirable volume effects, cycling instability usually remains in nanostructured electrodes owning to particle aggregation in discharge and loss of active materials in charge. Here, to make these kinds of materials practical, we have developed a structure of ultrafine MoO₂ nanoparticles (<3 nm) confined by a conductive carbon nanosheet matrix (MoO₂/C). Instead of running on the conversion mechanism, the Li storage in the MoO₂/C composite is through a two-step mechanism in discharge: intercalation followed by the formation of metallic Li, acting as a hybrid host for both Li ion intercalation and metallic Li plating. The Li-storage mechanism has been revealed by in situ X-ray diffraction analysis and in situ scanning transmission electron microscopy with corresponding electron energy loss spectrum analysis, which explains the natural origin of such high capacity along with good cyclability. This unique MoO₂/C structure exhibits an excellent discharge capacity (810 mAh g^-1 at 200 mA g^-1) and cyclability (75% capacity retention over 1000 cycles). The carbon sheet plays a vital role in both a conductive network and a structure supporter with a robust confining effect that keeps the size of MoO₂ uniformly under 3 nm even after high-temperature calcination. Our finding provides insights for the design of next-generation LIB anode materials with high capacity and longevity.

Insight of Enhanced Redox Chemistry for Porous MoO2 Carbon-Derived Framework as Polysulfide Reservoir in Lithium-Sulfur Batteries

As a promising energy-storage system, lithium-sulfur batteries (LSBs) with a high energy density suffer from the polysulfide shuttle effect and sluggish reaction kinetics, which have been studied for a few decades. Incorporation of polar metal oxides is an efficient addition for LSBs to suppress the dissolution of soluble polysulfides, increase the utilization of sulfur, and improve cycling stability. Herein, a model (MoO₂/C-NCs) based on a porous octahedral carbon framework decorated with MoO₂ nanoparticles (MoO₂ NPs) as a sulfur host is proposed. Adsorption experiments of lithium polysulfides (LiPSs) to MoO₂/C-NCs and cyclic voltammetry analysis showed that the MoO₂ NPs facilitate interfacial charge transfer and provide numerous active sites for the electrochemical redox reactions of LiPSs. Density functional theory calculations further reveal that LiPSs are diffused and strongly adsorbed on the surface of MoO₂ NPs because of the powerful van der Waals forces via Mo-S and Li-O bonds, which helps achieve a stable long-term cycling performance. As a result, the fabricated LSBs display a high initial specific capacity of 1317 mA h g^-1 at 0.2C and a promising capacity of 602 mA h g^-1 and a capacity retention of 65.6% at 1C when proceeding to 500 cycles.

Ultrathin Molybdenum Dioxide Nanosheets as Uniform and Reusable Surface-Enhanced Raman Spectroscopy Substrates with High Sensitivity

Metal oxides have advantages over the traditional noble metals to be used as substrate materials for surface-enhanced Raman spectroscopy (SERS) with low cost, versatility, and biocompatibility, but their enhancement factors are generally quite low with a poor limit of detection. Here, ultrathin molybdenum dioxide (MoO₂ ) nanosheets synthesized by chemical vapor deposition demonstrated in large area are used as SERS substrates with superior signal uniformity in the whole area with a limit of detectable concentration down to 4 × 10^-8 m and enhancement factor up to 2.1 × 10⁵ , exceeding that of 2D materials and comparable to that of noble metal films. More practically important, the planar MoO₂ substrate is more robust than noble metals and shows excellent reusability and uniformity, which is usually prohibited for nanostructured or nanoparticle-based metal oxide substrates. The enhancement is mainly attributed to the surface plasmon resonance effect as evidenced by the first principle calculations and UV-vis absorption spectroscopy characterization, which can be further increased by decreasing the thickness of the MoO₂ nanosheets. The overall superior performance makes the MoO₂ nanosheets an ideal substrate for practical SERS applications.

Engineering High-Performance MoO2 -Based Nanomaterials with Supercapacity and Superhydrophobicity by Tuning the Raw Materials Source

Herein, a simple self-assembly method is proposed for the fabrication of MoO₂ -based superhydrophobic material with record high contact angles (contact angle up to about 173°) for conductive metal oxides on hard/soft substrates. The spin-coated surface demonstrates excellent oil-water separation efficiency (>98%) after 50 cycles and robust corrosion resistance after immersion into different pH solutions for 20 d. These water-resistant coatings retain excellent superhydrophobicity after oil immersion, knife-scratch, and long-cycle sandpaper abrasion, which is not observed on most artificial surfaces. Meanwhile, the functionality switching from superhydrophobicity to supercapacity, which have an inverse relationship in aqueous solutions because of poor electrode wettability, is achieved simply by editing the raw materials source. Tuning of the raw materials leads to the same product MoO₂ /graphitic carbon with different morphologies and functionalities. Different from superhydrophobic MoO₂ /carbon ball flowers, MoO₂ nanotubes with carbon exhibit excellent supercapacity with a large gravimetric capacitance and great cycling stability.

Edge-Riched MoSe2 /MoO2 Hybrid Electrocatalyst for Efficient Hydrogen Evolution Reaction

Molybdenum diselenide (MoSe₂ ) is widely considered as one of the most promising catalysts for the hydrogen evolution reaction (HER). However, the absence of active sites and poor conductivity of MoSe₂ severely restrict its HER performance. By introducing a layer of MoO₂ on Mo foil, MoSe₂ /MoO₂ hybrid nanosheets with an abundant edge and high electrical conductivity can be synthesized on the surface of Mo foil. Metallic MoO₂ can improve the charge transport efficiency of MoSe₂ /MoO₂ , thereby enhancing the overall HER performance. MoSe₂ /MoO₂ exhibits fast hydrogen evolution kinetics with a small overpotential of 142 mV versus RHE at a current density of 10 mA cm^-2 and Tafel slope of 48.9 mV dec^-1 .

MnCo2 O4 /MoO2 Nanosheets Grown on Ni foam as Carbon- and Binder-Free Cathode for Lithium-Oxygen Batteries

Carbon is usually used as cathode material for Li-O₂ batteries. However, the discharge product, such as Li₂ O₂ and LiO₂ , could react with carbon to form an insulating lithium carbonate layer, resulting in cathode passivation and capacity fading. To solve this problem, the development of non-carbon cathodes is highly desirable. Herein, we successfully synthesized MnCo₂ O₄ (MCO) nanoparticles anchored on porous MoO₂ nanosheets that are grown on Ni foam (current collector) (MCO/MoO₂ @Ni), acting as a carbon- and binder-free cathode for Li-O₂ batteries, in an attempt to improve the electrical conductivity, electrocatalytic activity, and durability. This MCO/MoO₂ @Ni electrode delivers excellent cyclability (more than 400 cycles) and rate performance (voltage gap of 0.75 V at 5000 mA g^-1 ). Notably, the battery with this electrode exhibits a high energy efficiency (higher than 85 %). The advanced electrochemical performance of MCO/MoO₂ @Ni can be attributed to its high electrical conductivity, excellent stability, and outstanding electrocatalytic activity. This work offers a new strategy to fabricate high-performance Li-O₂ batteries with non-carbon cathode materials.

Constructing MoO2 Porous Architectures Using Graphene Oxide Flexible Supports for Lithium Ion Battery Anodes

Graphene oxide flexibly supported MoO2 porous architectures (MoO2/GO) by decomposition of the prepared ammonium molybdate/GO preforms is fabricated. Focused ion beam microscope analysis shows that the inside structures of the architectures strongly depend on the percentages of the GO used as flexible supports: micrometer scale MoO2 particulates growing on the GO (micrometer MoO2/GO), 3D honeycomb-like nanoarchitectures (MoO2/GO nanohoneycomb), and layered MoO2/GO architectures are achieved at the percentage of GO at 4.3, 15.2, and 20.8 wt%, respectively. The lithium storage performance of the MoO2/GO architectures strongly depends on their inside structures. At the current density of 100 mA g-1, the capacities of the micrometer MoO2/GO, MoO2/GO nanohoneycomb, and layered MoO2/GO remain at 901, 1127, and 967 mAh g-1 after 100 cycles. The average coulombic efficiencies of micrometer MoO2/GO, MoO2/GO nanohoneycomb, and layered MoO2/GO electrodes are 97.6%, 99.3%, and 99.0%. Moreover, the rate performance shows even cycled at a high current density of 5000 mA g-1, the MoO2/GO nanohoneycomb can deliver the capacity as high as 461 mAh g-1. The MoO2/GO nanohoneycomb exhibits best performance attributed to its unique nanohoneycomb structure constructed with ultrafine MoO2 fixed on the GO flexible supports.

Formation of Mo-Polydopamine Hollow Spheres and Their Conversions to MoO2 /C and Mo2 C/C for Efficient Electrochemical Energy Storage and Catalyst

Highly uniform hierarchical Mo-polydopamine hollow spheres are synthesized for the first time through a liquid-phase reaction under ambient temperature. A self-assembly mechanism of the hollow structure of Mo-polydopamine precursor is discussed in detail, and a determined theory is proposed in a water-in-oil system. Via different annealing process, these precursors can be converted into hierarchical hollow MoO₂ /C and Mo₂ C/C composites without any distortion in shape. Owing to the well-organized structure and nanosize particle embedding, the as-prepared hollow spheres exhibit appealing performance both as the anode material for lithium-ion batteries and as the catalyst for hydrogen evolution reaction (HER). Accordingly, MoO₂ /C delivers a high reversible capacity of 940 mAh g^-1 at 0.1 A g^-1 and 775 mAh g^-1 at 1 A g^-1 with good rate capability and long cycle performance. Moreover, Mo₂ C/C also exhibits an enhanced electrocatalytic performance with a low overpotential for HER in both acidic and alkaline conditions, as well as remarkable stability.

Novel Dual-Ion Hybrid Supercapacitor Based on a NiCo2O4 Nanowire Cathode and MoO2-C Nanofilm Anode

Cobalt/nickel-based compounds have been extensively used as cathode (positive electrode) materials in alkaline electrolyte for hybrid supercapacitors (HSCs). In these HSCs, however, the anodes (negative electrodes) are almost carbon-based materials that exhibit limited capacitance, leading to relatively low energy density of the device. Herein, we report a novel dual-ion HSC concept, that is, utilizing anion and cation in the electrolyte, respectively, by the two electrodes for charge storage, to promote the device's performance. Based on this, it is possible to exploit cation-consumed metal oxide as a capacitive anode to couple with a cobalt/nickel oxide cathode. As a demonstration, a 1.8 V MoO₂-C/LiOH electrolyte/NiCo₂O₄ HSC device is established. In such a design, NiCo₂O₄ cathode and MoO₂-C anode react with OH^- and Li⁺, respectively, to store energy. With the benefits from enhanced kinetics in NiCo₂O₄ nanowire array (direct electron transport pathway and sufficient electrolyte/ion penetration) and increased stability and electrical conductivity in carbon-encapsulated MoO₂ nanofilm, our device delivers a high capacitance (94.9 F g^-1), high energy density and power density (41.8 Wh kg^-1 and 19922.2 W kg^-1), long cycling stability >3000 times, and good rate capability (∼3.3 s charging/discharging with 43.6% capacitance retention). The dual-ion charge storage concept will stimulate great interest in the design of high-performing all-oxide hybrid electric energy storage systems.

Hierarchical MoO2/Mo2C/C Hybrid Nanowires as High-Rate and Long-Life Anodes for Lithium-Ion Batteries

Hierarchical MoO2/Mo2C/C hybrid nanowires (MoO2/Mo2C/C HNWs) have been fabricated through facile calcination of Mo3O10(C6H5NH3)2·2H2O nanowires which serve as both precursors and self-templates. In the MoO2/Mo2C/C HNWs, nanoparticles dispersed in the nanowires are beneficial for Li(+) transportation due to the decreased diffusion paths. Moreover, hybridization with Mo2C and carbon facilitates the electron transfer and increases the structural stability without sacrifice of capacity. As anode materials for lithium-ion batteries, the MoO2/Mo2C/C HNWs exhibit a reversible capacity of 950 mA h g(-1) after 320 cycles at a current density of 200 mA g(-1). Even when cycled at 2000 mA g(-1), they maintained a reversible capacity of 602 mA h g(-1) after 500 cycles. By incorporation of Mo2C and C with MoO2, the MoO2/Mo2C/C HNWs show high-rate capability and long cycle life and can be a promising candidate for lithium-ion battery anodes.