Molybdenum dichalcogenide nanotube arrays for hydrogen-evolution-reaction catalysis: Synergistic effects of sulfur and selenium in a core-shell tube wall

The present work shows the growth and conversion of self-organized anodic Mo-oxide nanotube arrays to core-shell structures consisting of a conducting molybdenum sub-oxide core and a shell of Mo-Se/S – this structure is then investigated for electrochemical hydrogen evolution catalysis. To form the core-shell tubes, we first anneal MoO₃ nanotube arrays under vacuum conditions, to induce reduction to MoO₂. Subsequently these oxide tubes are thermally sulfurized and selenized resulting in dichalcogenide@sub-oxide structures. Under optimized conditions, the mixed dichalcogenide (selenized and sulfurized) tube walls on the conductive oxide core lead to a synergistic beneficial effect for the electrocatalytic H₂ generation from H₂SO₄ solution.     

Large‐scale Two‐dimensional MoSx Catalyst Prepared under Mild Conditions for Enhancing Electrocatalytic Hydrogen Evolution Reaction

As an electrocatalyst with abundant resources and great potential, molybdenum disulfide is regarded as one of the most likely alternatives to expensive noble‐metals catalysts. However, it is still a challenge to achieve large scale production of few‐layer MoS₂ with enhancing activity of electrocatalytic hydrogen reaction at ambient conditions. Herein, we developed a simple environmentally friendly two‐step method, which included intercalation reaction and a subsequent electrochemical reduction reaction for mass preparation of defect‐rich desulfurized MoS_x (D?MoS_x) nanosheets with plentiful sulfur vacancies. The ratio of sulfur‐molybdenum atoms can be adjusted from 2 : 1 to 1.4 : 1 by regulating the desulfurization voltage. It was found that the HER catalytic activity of the D?MoS_x was enhanced compared with that of pristine MoS₂ (P?MoS₂), the current density of D?MoS_x (desulfurization at ?1.0 V) at ?0.3 V versus RHE was about 169% of the P?MoS₂, and the Tafel slope decreased to 136 mV dec^?1. This method can be widely applied to large‐scale preparation of other two‐dimensional materials.     

Hybrid Fibers Made of Molybdenum Disulfide,Reduced Graphene Oxide,and Multi‐Walled Carbon Nanotubes for Solid‐State,Flexible, Asymmetric Supercapacitors

One of challenges existing in fiber‐based supercapacitors is how to achieve high energy density without compromising their rate stability. Owing to their unique physical, electronic, and electrochemical properties, two‐dimensional (2D) nanomaterials, e.g., molybdenum disulfide (MoS₂) and graphene, have attracted increasing research interest and been utilized as electrode materials in energy‐related applications. Herein, by incorporating MoS₂ and reduced graphene oxide (rGO) nanosheets into a well‐aligned multi‐walled carbon nanotube (MWCNT) sheet followed by twisting, MoS₂‐rGO/MWCNT and rGO/MWCNT fibers are fabricated, which can be used as the anode and cathode, respectively, for solid‐state, flexible, asymmetric supercapacitors. This fiber‐based asymmetric supercapacitor can operate in a wide potential window of 1.4 V with high Coulombic efficiency, good rate and cycling stability, and improved energy density.     

Hybrid Fibers Made of Molybdenum Disulfide,Reduced Graphene Oxide,and Multi‐Walled Carbon Nanotubes for Solid‐State,Flexible, Asymmetric Supercapacitors

One of challenges existing in fiber‐based supercapacitors is how to achieve high energy density without compromising their rate stability. Owing to their unique physical, electronic, and electrochemical properties, two‐dimensional (2D) nanomaterials, e.g., molybdenum disulfide (MoS₂) and graphene, have attracted increasing research interest and been utilized as electrode materials in energy‐related applications. Herein, by incorporating MoS₂ and reduced graphene oxide (rGO) nanosheets into a well‐aligned multi‐walled carbon nanotube (MWCNT) sheet followed by twisting, MoS₂‐rGO/MWCNT and rGO/MWCNT fibers are fabricated, which can be used as the anode and cathode, respectively, for solid‐state, flexible, asymmetric supercapacitors. This fiber‐based asymmetric supercapacitor can operate in a wide potential window of 1.4 V with high Coulombic efficiency, good rate and cycling stability, and improved energy density.     

Two‐Dimensional Layered Heterostructures Synthesized from Core–Shell Nanowires

Controlled stacking of different two‐dimensional (2D) atomic layers will greatly expand the family of 2D materials and broaden their applications. A novel approach for synthesizing MoS₂/WS₂ heterostructures by chemical vapor deposition has been developed. The successful synthesis of pristine MoS₂/WS₂ heterostructures is attributed to using core–shell WO_3?x/MoO_3?x nanowires as a precursor, which naturally ensures the sequential growth of MoS₂ and WS₂. The obtained heterostructures exhibited high crystallinity, strong interlayer interaction, and high mobility, suggesting their promising applications in nanoelectronics. The stacking orientations of the two layers were also explored from both experimental and theoretical aspects. It is elucidated that the rational design of precursors can accurately control the growth of high‐quality 2D heterostructures. Moreover, this simple approach opens up a new way for creating various novel 2D heterostructures by using a large variety of heteronanomaterials as precursors.     

Interpretation of the difference of optimal Mo density in MoS<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and MoS<Subscript>2</Subscript>-TiO<Subscript>2</Subscript> HDS catalysts

The first part of this paper deals with the morphology of the MoS₂ phase and its oxide precursor, the MoO₃ phase, mainly from a geometrical point of view. After giving a brief review of the literature describing the structure of these compounds, Mo densities in both phases were calculated along various crystallographic planes. Further, using structural models recently proposed by others, Mo densities in MoS₂ were also calculated in the case of an epitactic growth on γ-Al₂O₃ and TiO₂ model surfaces. Then, the calculated Mo densities were compared with experimental results (Mo density when HDS activity is maximal) previously obtained for catalysts constituted of MoS₂ supported on a low SSA TiO₂, a high SSA TiO₂ and a conventional γ-alumina. It was suggested that either on alumina or titania the MoS₂ phase is growing as (100) MoS₂ planes. However, while on the alumina the optimal MoS₂ phase might be constituted of dispersed MoS₂ slabs covering only a part of the alumina surface (2.9–3.9 Mo atoms/nm²), on titania the optimal MoS₂ phase might be constituted of a uniform MoS₂ monolayer (5.2 atoms/nm² for the high SSA titania, which is equal to the Mo density of a perfect MoS₂ (100) plane). This difference may originate in the creation of a 'TiMoS' phase enhancing the S atoms mobility over Mo/TiO₂-sulfided catalysts. Indeed, while in the case of a γ-alumina carrier the active sites (labile S atoms) are located on the edge of MoS₂ slabs making the ratio Mo_edge/Mo_total a crucial parameter for the catalytic performances, in the case of a titania carrier the labile sulfur atoms might be statistically distributed all over the TiMoS active phase. Further, the higher Mo density observed over the high SSA titania (5.2 atoms/nm²) when compared to that over the low SSA titania (4.2 atoms/nm²) was supposedly due to the pH-swing method advantageously used to prepare the former carrier. Indeed, this method allows giving a solid with enhanced mechanical properties providing a good stability to the derived catalysts under experimental conditions. In addition, this TiO₂ carrier exhibits a great homogeneity, with a surface structure substantially uniform, which might be adequate for a long-range growth of (100) MoS₂ slabs.     

Solution preparation of the amorphous molybdenum oxysulfide MoOS2 and its use for catalysis

Acid condensation of aqueous MoO₂S₂²⁻ anion yields amorphous MoOS₂ oxysulfide. This compound possesses tubular morphology and when freshly precipitated is soluble in polar organics such as acetone and ethanol. The ensemble of characterizations (IR, UV-visible, EXAFS spectroscopy) suggests that it contains cyclic or short linear oligomers of neutral molybdenum (V) oxysulfide MoOS₂ core. Thermal decomposition of MoOS₂ under inert atmosphere leads to the formation of a mixture of MoO₂ and MoS₂ phases. Promotion of MoOS₂ with cobalt followed by sulfidation leads to highly active HDS catalysts.     

Enhanced Photocatalytic Performance of ZnO Nanorods Coupled by Two‐Dimensional α‐MoO3 Nanoflakes under UV and Visible Light Irradiation

We exploit the utilization of two‐dimensional (2D) molybdenum oxide nanoflakes as a co‐catalyst for ZnO nanorods (NRs) to enhance their photocatalytic performance. The 2D nanoflakes of orthorhombic α‐MoO₃ were synthesized through a sonication‐aided exfoliation technique. The 2D MoO₃ nanoflakes can be further converted to substoichiometric quasi‐metallic MoO_3?x by using UV irradiation. Subsequently, 1D–2D MoO₃/ZnO NR and MoO_3?x/ZnO NR composite photocatalysts have been successfully synthesized. The photocatalytic performances of the novel nanosystems in the decomposition of methylene blue are studied by using UV‐ and visible‐illumination setup. The incorporated 2D nanoflakes show a positive influence on the photocatalytic activity of the ZnO. The obtained rate constant values follow the order of pristine ZnO NR<MoO₃/ZnO NR<MoO_3?x/ZnO NR composites. The enhancement of the photocatalytic efficiency can be ascribed to a fast charge carrier separation and transport within the heterojunctions of the MoO₃/ZnO NRs. In particular, the best photocatalytic performance of the MoO_3?x/ZnO NR composite can be additionally attributed to a quasi‐metallic conductivity and substoichiometry‐induced mid‐gap states, which extend the light absorption range. A tentative photocatalytic degradation mechanism was proposed. The strategy presented in this work not only demonstrates that coupling with nanoscale molybdenum oxide nanoflakes is a promising approach to significantly enhance the photocatalytic activity of ZnO but also hints at new type of composite catalyst with extended applications in energy conversion and environmental purification.     

Hierarchical MoS2/polyaniline binary hybrids with high performance for improving fire safety of epoxy resin

Here we report a facile strategy to fabricate phosphoric acid doped polyaniline/molybdenum disulfide (PANI/MoS₂) hybrids as high-performance nanofillers in epoxy (EP) resin for the first time. In situ growth of PANI on the surface of two-dimensional MoS₂ template resulted in the uniform dispersion and strong interfacial adhesion of PANI/MoS₂ hybrids within EP matrix, which can be confirmed by the obvious increase (13.5°C) in glass transition temperature (T_g) of EP composites. The MoS₂ nanosheets also acted as a critical component to generate synergistic effect with PANI on reducing the fire hazards of EP resin. It resulted in a remarkable removal of flammable decomposed products and a considerable reduction of toxic CO yield. The dramatical decreases in real-time smoke density and total smoke production, and high-graphitized char layer in condensed phase were obtained for EP composite with 5 wt% PANI/MoS₂ hybrids. The multiple synergistic effects (synergistic dispersion and synergistic char formation) are believed to be the primary source for these obvious enhancements of properties of EP composites. This facile strategy may achieve the potential application of functionalized MoS₂ in polymeric nanocomposites.     

CO2‐Assisted Fabrication of Two‐Dimensional Amorphous Molybdenum Oxide Nanosheets for Enhanced Plasmon Resonances

As a remarkable class of plasmonic materials, two dimensional (2D) semiconductor compounds have attracted attention owing to their controlled manipulation of plasmon resonances in the visible light spectrum, which outperforms conventional noble metals. However, tuning of plasmonic resonances for 2D semiconductors remains challenging. Herein, we design a novel method to obtain amorphous molybdenum oxide (MoO₃) nanosheets, in which it combines the oxidation of MoS₂ and subsequent supercritical CO₂‐treatment, which is a crucial step for the achievement of amorphous structure of MoO₃. Upon illumination, hydrogen‐doped MoO₃ exhibits tuned surface plasmon resonances in the visible and near‐IR regions. Moreover, a unique behavior of the amorphous MoO₃ nanosheets has been found in an optical biosensing system; there is an optimum plasmon resonance after incubation with different BSA concentrations, suggesting a tunable plasmonic device in the near future.     

Polyoxometalate derived p-n heterojunction for optimized reaction interface and improved HER

MoS₂ is a typical electrocatalyst for hydrogen evolution reaction (HER), but the HER activity is spoilt by intensive adsorption towards H*, which requires further improvement. For n-type MoS₂, the construction of p-n heterojunction with p-type MoO₃ can reverse this situation, because inner electronic field in p-n heterojunction can facilitate H* desorption. Based on this hypothesis, p-n heterojunction is built between MoS₂ and MoO₃ with polyoxometalate compound as precursor. The obtained MoO₃/MoS₂ exhibits excellent HER activity, which only requires 68 mV to obtain 10 mA/cm². With MoO₃/MoS₂ as cathode material and Zn slice as anode, Zn-H⁺ battery is assembled. Its open circuit voltage achieves 1.11 V with short circuit current 151.4 mA/cm². The peak power density of this Zn-H⁺ battery reaches 47.6 mW/cm². When discharge at 10 mA/cm², the specific capacity and energy density reach 728 mAh/g and 759 Wh/kg. In this process, H₂ production rate of Zn-H⁺ battery achieves 364 μmol/h with Faradic efficiency 97.8%. It realizes H₂ production and electricity generation simultaneously.     

Nickel Hydroxide Nanoparticle Activated Semi‐metallic TiO2 Nanotube Arrays for Non‐enzymatic Glucose Sensing

Semi‐metallic TiO₂ nanotube arrays (TiO_xC_y NTs) have been decorated uniformly with Ni(OH)₂ nanoparticles without the aid of a polymer binder. The resulting hybrid nanotube arrays exhibit excellent catalytic activity towards non‐enzymatic glucose electro‐oxidation. The anodic current density of the glucose oxidation is significantly improved compared with traditional TiO₂ nanotubes decorated with Ni(OH)₂. Moreover, the Ni(OH)₂/TiO_xC_y NT‐based electrode shows a fast response, high sensitivity, wide linear range, good selectivity and stability towards glucose electro‐oxidation, and thus provides a promising and cost‐effective sensing platform for non‐enzymatic glucose detection.     

Synthesis,structure and properties of eight novel molybdenum(VI) complexes of the types: [MoO2LD] and [{MoO2L}2D] (L = thiosemicarbazonato ligand,D = N-donor molecule)

Eight new molybdenum(VI) complexes with 4-(diethylamino)salicylaldehyde and 2-hydroxy-3-methoxybenzaldehyde thiosemicarbazones have been prepared. They were characterized as mononuclear [MoO₂LD] or dinuclear [{MoO₂L}₂D] complexes. In all the compounds the MoO₂²⁺ core is coordinated by a tridentate ONS thiosemicarbazonato ligand and by the N-donor molecule (imidazole, pyridine or γ-picoline). All the complexes were characterized by chemical analysis, IR spectroscopy and thermogravimetry. Three of the mononuclear complexes, dioxo[(2-hydroxy-3-methoxybenzaldehyde thiosemicarbazonato)(pyridine)]molybdenum(VI), dioxo[(2-hydroxy-3-methoxybenzaldehyde thiosemicarbazonato)(γ-picoline)]molybdenum(VI) and dioxo[(2-hydroxy-3-methoxybenzaldehyde thiosemicarbazonato)(imidazole)]molybdenum(VI) were also characterized by single-crystal X-ray structural analysis. A spectrophotometric method for the determination of molybdenum based on extraction of ion-pairs formed by the cationic surfactant and the [MoO(SCN)₄]⁻ anion is described.     

Synthesis,Characterization, and Crystal Structure of Mononuclear and Dinuclear Dioxomolybdenum(VI) Complexes with Tridentate Schiff‐base Ligands. Part 2

Mononuclear [MoO₂LD], and dinuclear [MoO₂L]₂ or [MoO₂L]₂ · D dixomolybdenum(VI) complexes have been prepared by the reaction of tridentate Schiff‐base ligands L with [MoO₂(acac)₂]. The Schiff‐base ligands have been synthesized from salicylaldehyde ( 1 , 1a , 1c , 1d ), 2‐hydroxy‐1‐naphthaldehyde ( 2 , 2c ) and 2‐hydroxy‐3‐methoxybenzaldehyde ( 3a , 3b , 3c , 3d , 3e ) with 2‐amino‐p‐cresol. All prepared complexes consist of cis‐MoO₂²⁺core coordinated by Schiff‐base ligand through two deprotonated hydroxyl groups and one imino nitrogen atom. The usual octahedral coordination around the molybdenum atoms is completed by the neutral ligand D (methanol, ethanol, dimethyl sulfoxide, imidazole or 4, 4′‐bipyridine). All compounds were characterized by elemental analyses, IR spectroscopy and some of them by X‐ray crystallography ( 1a , 2c , 3a , 3b , 3c and 3e ).     

Investigation on the Growth Mechanism of Cu2MoS4 Nanotube,Nanoplate and its use as a Catalyst for Hydrogen Evolution in Water

Cu₂MoS₄ is a ternary transition‐metal sulfide that shows great potential in the field of energy conversion and storage, namely catalytic H₂ evolution in water and Li‐, Na‐ or Mg‐ion battery. In this work, we report on a growth mechanism of the single‐crystalline Cu₂MoS₄ nanotube from (NH₄)₂MoS₄ salt and Cu₂O nanoparticle. By probing the nature and morphology of solid products generated in function of reaction conditions we find that the crystalline Cu(NH₄)MoS₄ nanorod is first generated at ambient conditions. The nanorod is then converted into Cu₂MoS₄ nanotube under hydrothermal treatment due to the Kirkendall effect or a selective etching of the Cu₂MoS₄ core. Extending the hydrothermal treatment causes a collapse of nanotube generating Cu₂MoS₄ nanoplate. The catalytic activities of these sulfides are investigated. The Cu₂MoS₄ shows superior catalytic activity to that of Cu(NH₄)MoS₄. Catalytic performance of the former largely depends on its morphology. The nanoplate shows superior catalytic activity to the nanotube, thanks to its higher specific electrochemical surface area.     

Spectral and thermogravimetric studies of oxohalobis(4-morpholinyldithiocarbamato)molybdenum(V) complexes

Summary Molybdenum(V) complexes [MoOX(4-Morphdtc)₂] (X=F, Cl, Br or I; 4-Morphdtc = 4-morpholinyldithiocarbamate) have been prepared from molybdenum trioxide using hydrazine hydrohalides as reducing agents. The magnetic moment values atca. 1.65 B.M. and e.p.r. studies indicated that the complexes are mononuclear and that molybdenum is in + 5 oxidation state. The i.r. spectral bands at 1500 and 960 cm^–1 suggest that the dithiocarbamate acts as a univalent bidentate ligand and the bands at 930 cm^–1 confirms the presence of a MoO³⁺ moiety in the complex. The low conductivity values for the complexes inN, N-dimethylformamide indicate that the complexes are non-ionic. The [MoOX(4-Morphdtc)₂] complexes (X=F, Cl or Br) decompose in the first step by the loss of one dithiocarbamate group, whereas in [MoOI(4-Morphdtc)₂] the iodine atom is lost in the first step. The second and third steps lead to the formation of MoS₃ and MoO₃, respectively. A six coordinate structure around Mo with an oxo, halo and two dithiocarbamate groups is proposed.     

Growth of carbon nanotubes from butadiene on a Fe-Mo-Al2O3 catalyst

The MoO₃-Fe₂O₃-Al₂O₃ catalysts were prepared from metal nitrates using a coprecipitation method. It was found that the modification of an alumina-iron catalyst with molybdenum oxide resulted in the formation of a solid solution based on hematite, in which a portion of iron ions was replaced by aluminum and molybdenum ions. The MoO₃-Fe₂O₃-Al₂O₃ catalyst was reduced with a reaction mixture at 700°C. Under the action of 1,3-butadiene diluted with hydrogen, the solid solution based on hematite was initially converted into magnetite and then into an Fe-Mo alloy. The modification of an alumina-iron catalyst with molybdenum oxide considerably changed its properties in the course of carbon nanotube formation. As the Mo content was increased, the yield of carbon nanotubes passed through a maximum. The optimum catalyst was 6.5% MoO₃–55% Fe₂O₃-Al₂O₃. The addition of small amounts of MoO₃ (to 6.5 wt %) to the aluminairon catalyst increased the dispersity and modified the properties of active metal particles: because of the formation of an Fe-Mo alloy, the rate of growth decreased but the stability of carbon nanotube growth and the yield of the nanotubes increased. A further increase in the molybdenum content decreased the yield because molybdenum is inactive in the test process.     

Selective modification of two-dimensional MoS2 nanosheets by polymer grafting

We reported on the direct creation of polymer brushes on two-dimensional molybdenum disulfide via the formation of C-S bond by UV-induced photopolymerization. The functionalization can be manipulated in forming polymer grafts on one side or both sides of the nanosheets.     

Improved electrode fabrication method to enhance performance and stability of MoS2-based lithium-ion battery anode

Choice of binder and the electrode-making process play a pivotal role in the electrochemical performance of MoS₂, when used as lithium-ion battery anode. In this work, MoS₂ nanorods are prepared by gas phase synthesis method using molybdenum trioxide (MoO₃) nanobelts and sulfur as starting materials. It has been observed that by tuning the reaction conditions, morphology and yield of the final product can be controlled. Carboxymethyl cellulose (CMC) is used as binder to fabricate the MoS₂ electrode, and its electrochemical performance is tested against Li/Li⁺. The performance of electrode can be further improved by incorporating heat treatment to the active material and conductive carbon mixture prior to electrode fabrication. The electrochemical data shows that the optimum temperature for heat treatment is 700 °C. In the current report, we would like to elucidate a detailed study based on electrode fabrication process and their impact on the electrochemical performance.     

2D/2D h‐BN/N‐doped MoS2 Heterostructure Catalyst with Enhanced Peroxidase‐like Performance for Visual Colorimetric Determination of H2O2

Recently, nanozymes have attracted extensive attention because of their advantages of combining nanomaterials with enzymes. Herein, hexagonal boron nitride (h‐BN) and nitride‐doped molybdenum disulfide (N?MoS₂) nano‐composites (h‐BN/N?MoS₂) were synthesized by facile and cost‐effective liquid exfoliation with a solvothermal method in nontoxic ethanol solution. The results show that h‐BN, as a co‐catalyst, can not only dope into the lattice of MoS₂ but also form a heterogeneous structure with MoS₂NSs. It expanded the layer spacing and specific surface area of MoS₂NSs, which was beneficial to the contact between the catalyst and the substrate, and resulted in a synergistic enhancement of the catalytic activity of hydrogen peroxide (H₂O₂) with MoS₂. A colorimetric determination platform of h‐BN/N?MoS₂‐TMB‐H₂O₂ was constructed. It exhibited a wide linear range of 1–1000 μM with a low limit of detection (LOD) of 0.4 μM under optimal conditions, high sensitivity and stability, as well as good reliability (99.4–110.0%) in practice, making the measurement system more widely applicable.1. Introduction