Amorphous Molybdenum Research Articles

Colloidal synthetic methods of amorphous molybdenum phosphide nanoparticles for hydrogen evolution reaction catalysts

Transition metal phosphides (TMPs) have recently emerged as promising hydrogen evolution reaction (HER) catalytic alternatives to platinum. Among them, molybdenum phosphide (MoP) has attracted extensive attention due to its high electrical conductivity, good stability, and Pt-like electronic structure; however, there is no systematic comparison of its different colloidal synthetic routes. This study systematically compares two colloidal synthetic methods, one-pot and two-step, for amorphous MoP and the associated morphological changes during their reaction time. The amorphous MoP nanoparticles synthesized via the two-step method within 4 h exhibited the highest HER performance with an overpotential of 177 mV in 0.50 M H2SO4 for a current density of −10 mA cm−2; this might be due to their highly developed Mo-P bondings revealed by X-ray photoelectron spectroscopy analysis. Thus, this work demonstrates that the HER catalytic performance of MoP can be significantly influenced by its synthetic method and reaction time.

Development of highly active hydrogen evolution reaction (HER) catalysts composed of reduced graphene oxide and amorphous molybdenum sulfides derived from (NH4)2MoOmS4-m (m = 0, 1, and 2)

Photochemically synthesized catalysts using (NH4)2MoOmS4-m (m = 0, 1, and 2) as raw materials were studied for the purpose of hydrogen evolution reaction (HER) from H2O. Each starting material is found to generate a composite of MoSx/RGO covered by MoO3 layer, where RGO is reduced graphene oxide. XRD, TEM, XPS, and elemental analysis of the resulting MoSx/RGO reveals that MoSx exists in an amorphous phase and is composed of Mo(IV)Sx (x≈3). The HER activity of each prepared composite is estimated by η10 (the overpotential at 10 mA cm−2 current density), Ts (the Tafel slope) from linear sweep voltammetry (LSV), and turnover frequency (TOF). The composites from m = 0, 1, and 2 show η10: 198, 206, and 297 mV vs. RHE, Ts: 49.1, 46.9, and 89.1 mV dec-1, and TOF: 0.16, 0.20, and 0.10 s−1, respectively. From the viewpoint of Ts and TOF, the composites from m = 0 and 1 display a remarkably efficient activity, but they show the decline of HER performance with the electrochemical reaction time and cycle. The composite from m = 2, however, shows less activity than the others. The origins of the prominent efficiency of the composites from m = 0 and 1 are predicted as follows: (1) Photoirradiation of the starting materials on RGO leads to a selective and confined formation of amorphous Mo(IV)Sx, which is constructed, intriguingly, by abundant reactive bridging S22- ligands. (2) The HER efficiency is related to the presence of the Mo(IV) centers and the bridging S22- ligands. The Ts data suggest that the catalytic HER proceeds by a Volmer–Heyrovsky pathway.

Molybdenum sulphide modified chelating resin for toxic metal adsorption from acid mine wastewater

Acid mine wastewater from mining industries leads to serious environmental problems due to its acidic nature and high levels of hazardous heavy metal ions. Thus, it is important to develop novel adsorbent materials for selective removing heavy metal ions. In this work, an amorphous molybdenum sulphide functionalized dipicolyamine exchanger were prepared and ultilised to scavenge toxic heavy metal ions from both synthetic and real acid mine waste water (copper mining industry). This new exchanger combines the advantages of exceptional heavy metal affinities from molybdenum sulphide and engineered macroporous structure from dipicolylamine resin. The results of batch and fixed-bed column tests suggest high selectivity towards heavy metals, fast adsorption kinetics, good reusability and excellent adsorption capacities in the sequence of Hg(II)≫Cd(II) > Cu(II). The metal adsorption mechanism has been demonstrated by X-ray Photoelectron Spectroscopy (XPS). Our findings support the potential practical application of this new material for scavenging heavy metal ions in mining wastewater.

Synthesizing High-Capacity Oxyfluoride Conversion Anodes by Direct Fluorination of Molybdenum Dioxide (MoO2 ).

High-capacity metal oxide conversion anodes for lithium-ion batteries (LIBs) are primarily limited by their poor reversibility and cycling stability. In this study, a promising approach has been developed to improve the electrochemical performance of a MoO2 anode by direct fluorination of the prelithiated MoO2 . The fluorinated anode contains a mixture of crystalline MoO2 and amorphous molybdenum oxyfluoride phases, as determined from a suite of characterization methods including X-ray diffraction, Raman spectroscopy, and X-ray photoelectron spectroscopy, and scanning transmission electron microscopy. Electrochemical measurements indicate that fluorination facilitates the conversion reaction kinetics, which leads to increased capacity, higher coulombic efficiency, and better cycling stability as compared to the nonfluorinated samples. These results suggest that fluorination after prelithiation not only favors formation of the oxyfluoride phase but also improves the lithium-ion diffusivity and reversibility of the conversion reaction, making it an attractive approach to address the problems of conversion electrodes. These findings provide a new route to design high-capacity negative electrodes for LIBs.

Formation of Effective Electrocatalysts of Hydrogen Evolution MoSx > 2 by Pulsed Laser Ablation Assisted by the Deposition of Mo Nanoparticles

The mechanisms of film formation during pulsed laser ablation of a MoS2 target were studied. The conditions for the deposition of laser erosion plume were determined. This made it possible to obtain coatings with a porous structure consisting of round Mo nanoparticles coated with a thin shell of amorphous molybdenum sulfide MoSx > 2. Due to its hybrid structure, the MoSx > 2/Mo nanomaterial can be effectively used for electro- and photocatalysis of water splitting. The MoSx > 2/Mo films deposited on a glassy carbon substrate are characterized by good current transport and high active surface area. When the film thickness increased due to the increased deposition time, the overvoltage of hydrogen evolution in an acid solution monotonically decreased to 142.5 mV, which was necessary in order to obtain current density of hydrogen evolution of 10 mA/cm2. The catalyst loading was 230 μg/cm2. Further increase in the loading did not significantly reduce the overvoltage. The results indicate that it is promising to use Mo nanoparticles as an ultrafine support for the catalytic nanolayers of amorphous molybdenum sulfide.

Amorphous superconducting nanowire single-photon detectors integrated with nanophotonic waveguides

Future applications in integrated quantum photonics will require large numbers of efficient, fast, and low-noise single-photon counters. Superconducting nanowire single-photon detectors made from amorphous material systems are best suited to meet these demands, but the integration with nanophotonic circuits has remained a challenge. Here, we show how amorphous molybdenum silicide (MoSi) nanowires are integrated with nanophotonic silicon nitride waveguides in traveling wave geometry. We found a saturated on-chip detection efficiency of (73 ± 10) % for telecom wavelength photons and the sub-10 Hz dark count rate at a temperature of 2.1 K, which allows for operation in robust, compact, and economic cryogenic systems. Applications requiring fast counting will benefit from the sub-5 ns recovery times of our devices that we combine with 135 ps timing accuracy. Achieving this performance with waveguide-integrated amorphous superconductors is an important step toward enabling high yield fabrication of competitive single-photon detectors on a large variety of nanophotonic material systems.

Extraordinary lithium storage capacity and lithiation mechanism of partially amorphous molybdenum sulfide on chemically exfoliated graphene

Molybdenum sulfides have been researched actively for application in electrochemical energy storages and conversions. For molybdenum sulfides, modifications in stoichiometry and crystal structure can lead to enhancements in their electro-chemical properties. However, even though performance of crystalline molybdenum disulfide-based lithium ion battery (LIB) anodes have been investigated extensively, amorphous molybdenum sulfide-based anodes still need to be researched. Herein we report specific capacity as high as 1872 mAh/g observed from non-stoichiometric partially amorphous molybdenum sulfide formed on chemically exfoliated graphene (MoSx-rGO) as a LIB anode. According to our analysis, amorphous part consisting [Mo3S13]2- building blocks contains an excess amount of sulfur as bridging, shared, or terminal S22−, which contribute significantly in enhancing the capacity of the anode. In depth electrochemical investigations and analysis suggest that MoSx-rGO undergoes a different lithiation mechanism from crystalline MoS2, and consequently exhibits such high capacity with excellent stability at least up to 200 cycles and a decreased c-rate dependence.

Disorder‐Induced Transport Peculiarities in Amorphous Molybdenum Silicide Thin Films

Disorder‐induced localization as a kernel of metal–insulator transition (MIT) in amorphous materials has been widely studied over the past few decades. Herein, the effect of disorder on the electronic transport behavior in the prototypical amorphous molybdenum silicide thin films is reported. The metallic conduction with negative temperature coefficient of resistivity (TCR) is demonstrated in amorphous molybdenum silicide thin films. Also, the surface disorder can induce the discrepancy in sheet resistance of amorphous molybdenum silicide thin films. This work clarifies the effect of disorder on the electronic transport and provides an alternative strategy to tune the electronic transport behavior of amorphous materials through engineering the surface disorder.

Synthesis, properties, and applications of 2D amorphous inorganic materials

In the last decade, the research on two-dimensional (2D) materials has drawn a lot of interest from the aspects of both fundamental study and practical application. The atomic-scale thickness and unique layered structure make the materials in this family exhibit a number of distinct optical and electrical properties from their bulk counterparts. Previous studies have mainly focused on the crystalline 2D candidates. Recently, the highly disordered form of 2D materials, such as amorphous 2D materials, is emerging to attract increasing attention since it has shown great potential for applications in various fields. Here, we give a perspective on the recent progress in 2D amorphous inorganic materials. First, the synthesis techniques and process of 2D amorphous materials and their hybrid structure are given. Furthermore, the intriguing properties and applications in electronic, optoelectronic, and energy fields from typical 2D amorphous inorganic materials, including amorphous graphene, amorphous molybdenum disulfide, amorphous boron nitride, and amorphous black phosphorus, are introduced. Furthermore, the advantages and mechanisms of 2D amorphous materials are illustrated, pointing out the application scope of this material group. Finally, the underlying challenges of amorphous inorganic 2D materials are briefly outlined and some future outlooks are suggested.

Amorphous molybdenum sulfide monolayer nanosheets for highly efficient electrocatalytic hydrogen evolution

Monolayer amorphous molybdenum sulfide (ML-a-MoSx) nanosheets were synthesized via a space-confined strategy, and their electrocatalytic activity was evaluated towards the hydrogen evolution reaction. The so-obtained ML-a-MoSx, with a quite high monolayer ratio of ~ 93%, shows a polymeric chain or network structure consisting of trinuclear [Mo3] clusters, with abundant Mo defects. ML-a-MoSx exhibits much higher activity in comparison with bulk a-MoSx (B-a-MoSx), and is superior among the Mo-S electrocatalysts reported to date. In particular, ML-a-MoSx can deliver a high current density of 400 mA cm−2 at an overpotential of 230 mV vs. RHE, which is similar to the state-of-the-art catalyst commercial 20% Pt/C. The Tafel slope of ML-a-MoSx remains almost unchanged with an increase in overpotential, very conducive to practical applications. The higher electrocatalytic activity of ML-a-MoSx than B-a-MoSx results from its larger specific surface area, higher active site density, stronger conductivity, and faster turnover frequency. The active sites of ML-a-MoSx are identified as Mo defects. This is the first report on the fabrication and catalytic activity of ML-a-MoSx. This work provides an effective route for large-scale fabrication of ML-a-MoSx and contributes to the understanding of the origin of its excellent electrocatalytic activity.

Engineering Anion Resin based Amorphous Molybdenum Sulphide Composite for Treatment of Authentic Acid Mine Drainage

Abstract Acid mine drainage (AMD) is an acidic effluent containing many toxic heavy metal ions (e.g. Cu2+, Cd2+, Hg2+) in mining industry, leading to serious environmental issues such as natural soil and aquatic pollution that threats the whole ecosystem and biosafety. Conventional neutralization and precipitation process cannot effectively remove heavy metal ions due to the risk of secondary contamination. Herein, we report an engineered amorphous molybdenum sulphide composite for selective removal of heavy metal ions from other co-existing ions in AMD solution. This composite was prepared by a facile ion exchange reaction, in which tetrathiomolybdate (MoS42-) anions are firmly bonded to strong anion exchange resins by replacing chloride ions. The obtained composite was used to remove heavy metal ions from both synthetic and authentic AMD solution. The results of batch and fixed-bed column tests suggest high selectivity towards heavy metals, fast adsorption kinetics, good reusability and excellent adsorption capacities in the order of Hg(II)>>Pb(II)>Cu(II)>Cd(II). The adsorption data are fitted well by Langmuir model, indicating the single-layer adsorption mechanism. The theoretical adsorption capacities calculated by Langmuir model are 259.0 mg/g for Cu(II), 204.1 mg/g for Cd(II), 495.0 mg/g for Pb(II) and 1538.4 mg/g for Hg(II). The interaction between metal ions (Cu, Cd, Pb, Hg) and MoS42- anions are demonstrated by the formation of Mo-S-metal (Cu, Cd, Pb, Hg) bonding (the red shift of S2p peak in XPS spectra). Our results support the potential practical application of this new material for scavenging heavy metal ions in AMD wastewater.

Polarization Processes in Thin Layers of Amorphous MoS2 Obtained by RF Magnetron Sputtering

The polarization processes in thin layers of amorphous molybdenum disulfide MoS2 are studied by dielectric spectroscopy techniques. The process of dipole-relaxation polarization is observed. The microscopic parameters of the system are calculated, and the relaxation time of the dipole-polarization process, as well as the activation energies Ea and Eσ of the relaxation process and conductivity, respectively, are determined. The fact that the two activation energies are close to each other suggests that the processes of relaxation and charge transport are driven by the same mechanism.

Origin of the Superior Electrochemical Performance of Amorphous-Phase Conversion-Reaction-Based Electrode Materials for Na-Ion Batteries: Formation of a Bicontinuous Metal Network

The realization of conversion type electrode materials in Na-ion batteries (NIBs) has been hindered due to the nucleation property of the active material. During the sodiation, the transition metal (TM) cations reduce to the metallic state, and the respective anions react with the sodium ions. As a result, the metal particles are surrounded by the matrix of the insulating sodium compound, resulting in loss of electrical contact among the TM particles. Here, an amorphous molybdenum sulfide (a-MoSx) electrode is made highly reversible by suppressing TM particle growth via elongating the cation diffusion pathway. Because of the long distance among Mo atoms in a-MoSx, the growth of Mo nuclei is limited. This leads to more frequent nucleation and formation of smaller particles (3-5 nm in diameter). Since the smaller particles have a larger surface area than the bigger particles, the electrical contacts among Mo particles are clearly retained. The a-MoSx anode for NIBs demonstrates a high capacity and excellent cycling retention. This work establishes that the amorphous structure enhances the reversibility and cycling stability of conversion-reaction-based electrodes by elongating the diffusion pathway of the metal ions.

Gold nanorod/molybdenum sulfide core–shell nanostructures synthesized by a photo-induced reduction process

Coupling of plasmonic nanostructures and semiconductors gives promising hybrid nanostructures that can be used in different applications such as photosensing and energy conversion. In this report, we describe an approach for fabricating a new hybrid material by coupling a gold nanorod (Au NR) core and amorphous molybdenum sulfide (MoSx) shell. The Au NR/MoSx core–shell structure is achieved by exploiting the hot electrons generated in the plasmonic excitation of Au NRs to drive the reduction of [MoS4]2−, which is pre-adsorbed on the Au NR surface, producing a thin MoSx layer. This approach allows us to control the thickness of the MoSx coating layer on the Au NR surface. The resultant Au NR/MoSx hybrid is characterized by absorption spectroscopy, scanning electron microscopy, transmission electron microscopy, energy-dispersive x-ray spectroscopy elemental mapping, x-ray diffraction and x-ray photoelectron spectroscopy.

Reactive Pulsed Laser Deposition of Clustered-Type MoSx (x ~ 2, 3, and 4) Films and Their Solid Lubricant Properties at Low Temperature.

We studied the tribological properties of amorphous molybdenum sulfide (MoSx) thin-film coatings during sliding friction in an oxidizing environment at a low temperature (−100 °C). To obtain films with different sulfur contents (x ~ 2, 3, and 4), we used reactive pulsed laser deposition, where laser ablation of the Mo target was performed in H2S at various pressures. The lowest coefficient of friction (0.08) was observed during tribo-testing of the MoS3 coating. This coating had good ductility and low wear; the wear of a steel counterbody was minimal. The MoS2 coating had the best wear resistance, due to the tribo-film adhering well to the coating in the wear track. Tribo-modification of the MoS2 coating, however, caused a higher coefficient of friction (0.16) and the most intensive wear of the counterbody. The MoS4 coating had inferior tribological properties. This study explored the mechanisms of possible tribo-chemical changes and structural rearrangements in MoSx coatings upon contact with a counterbody when exposed to oxygen and water. The properties of the tribo-film and the efficiency of its transfer onto the coating and/or the counterbody largely depended on local atomic packing of the nanoclusters that formed the structure of the amorphous MoSx films.

Self-supported MoSx/V2O3 heterostructures as efficient hybrid catalysts for hydrogen evolution reaction

Earth-abundant and low-cost hydrogen evolution reaction (HER) electrocatalysts represent a future direction for achieving sustainable hydrogen energy production. Low-cost amorphous molybdenum sulfides (MoSx), with their highly active HER activity, have emerged as outstanding catalysts for electrochemical hydrogen production. Herein, we report the development of a synergetic amorphous MoSx hybrid catalysts on V2O3 with optimized HER activity of MoSx. Our synthetic and structural characterization shows that MoSx distributes on V2O3 uniformly. HER-inert V2O3 provides a highly electrochemically active surface area for HER and promotes electron transport. The obtained hybrid MoSx/V2O3/CC catalyst exhibits a low overpotential of 146 mV at 10 mA cm−2 toward HER under acidic conditions, which is comparable with the current advanced catalysts, and high stability with no significant changes over 10 h of electrolysis. The density functional theory calculations also demonstrate that the interface of V2O3 and MoSx helps to improve the conductivity.

Anchoring acetylene black on graphene for growing of molybdenum disulfide as high performance electrocatalysts in hydrogen evolution reaction

A series of Molybdenum sulfide/acetylene black-reduced graphene oxide composites (MoSx/AB-rGO) was prepared by a facile one-step solvothermal method. The morphology characterizations confirmed that the presence of AB could effectively prevent the reduced graphene oxide (rGO) from restacking in the solvothermal process, providing the abundant anchoring area for the MoS2 generated during the solvothermal process. For the as prepared MoS2/AB-rGO catalysts, a large number of amorphous molybdenum sulfide (a-MoS2) on the surface of rGO and low-crystallization molybdenum sulfide (1c-MoS2) on the surface of AB provide sufficient active sites for hydrogen evolution reaction (HER). HER tests of MoS2/AB-rGO proved that the dual function of AB, both augmenting the active sites and elevating the conductivity, could be a well-founded account for its excellent HER activity. In contrast, MoS2/rGO synthesized without AB showed severe agglomeration and poor internal conductivity on account of interlamellar van der Waals’s force. MoS2/AB-rGO(55) exhibits high HER performance including a low onset potential (−100 mV) and a large current density (10.0 mA/cm2 at η = 142 mV), indicating it is a promising electrocatalyst for HER.

Cover Feature: Facile Fabrication of Amorphous Molybdenum Oxide as a Sensitive and Stable SERS Substrate under Redox Treatment (Chem. Eur. J. 12/2020)

The amorphous MoO3−x nanosheet exhibits more sensitive and stable surface-enhanced Raman spectroscopy (SERS) activity than the crystalline counterpart. The signal is stable under repeated oxygen insertion/extraction condition, which allows the recycling of the substrate through H2O2-based Fenton-like reaction. More importantly, the interference-free operando monitoring of the photocatalytic p-nitrothiophenol reduction by loaded Pt nanoclusters can be accurately attained under UV irradiation by using methanol as the hole capturer. More information can be found in the Full Paper by J. L. Zhang, L. Z. Wang, et al. on page 2653.

Facile Fabrication of Amorphous Molybdenum Oxide as a Sensitive and Stable SERS Substrate under Redox Treatment.

Amorphous MoO3-x nanosheets were fabricated by the room-temperature oxidation of molybdenum powder with H2 O2 , followed by light-irradiation reduction in methanol. When applied as a substrate for surface-enhanced Raman spectroscopy (SERS), these nanosheets exhibit higher sensitivity than the crystalline counterpart for a wide range of analytes. Moreover, the SERS activity remains stable on repeated oxygen insertion/extraction. In contrast, the performance of crystalline MoO3-x continuously deteriorates on successive redox treatments. This unique SERS activity allows the recycling of the substrate through an H2 O2 -based Fenton-like reaction. More importantly, the non-invasive SERS was unprecedentedly used for the self-diagnosis of amorphous MoO3-x as a more selective photocatalyst than its crystalline counterpart.

Amorphous MoSx embedded within edges of modified graphite as fast-charging anode material for rechargeable batteries

Anode materials with high Li storage capacity and fast charging/discharging characteristics are necessary to produce high-performance lithium-ion batteries (LIBs) for future clean electric and hybrid vehicles. Several researchers have suggested that nanomaterials with excellent electrochemical performances have the potential to overcome the limitations of current LIBs. However, owing to their complex synthetic fabrication methods, low tap density, and poor first coulombic efficiency, they are not favorable as commercial anode materials. In this study, we introduce a straightforward strategy to fabricate anode materials based on commercially suitable graphite to satisfy the requirements of future LIBs. Graphite was modified by ball-milling and mild oxidation, which led to an increase in the number of exposed edges and anchoring sites between the graphite and the amorphous molybdenum sulfide (MoSx). MoSx, which is an important component of several high capacity materials, was stable under a high C-rate condition owing to its oxygen functional groups. The MoSx@B-rGtO electrode exhibited excellent specific capacity (1239 and 403 mAh g−1 at current densities of 0.13 and 2.60 A g−1, respectively) and cycling stability (1016 mAh g−1 after 100 cycles). Therefore, we believe that our graphite-based anode material can be used as a potential LIB electrode for future electric automobiles.