MoO3 Nanocrystals Research Articles

Incorporation of insufficiently vulcanized MoO3 nanocrystals into the hydrothermally grown MoS2/ZnS heterostructures for highly efficient photocatalytic applications

Engineering the multiple heterojunctions of MoS2-based materials provides an important and effective strategy to improve photocatalytic activity. In this work, the insufficiently vulcanized MoO3 intermediates had been incorporated into MoS2/ZnS nanostructured composites using a facile hydrothermal method. The analysis of scanning electron microscopy (SEM), X-ray diffraction (XRD), transmission electron microscopy (TEM) and X-ray photoelectron spectroscopy (XPS) shows that the multiple heterojunctions were formed among the sphalerite ZnS, molybdenum oxide (MoO3) and MoS2 nanocrystals. Different heterogeneous microstructures among nanosheets, nanoparticles and nanobelts were aggregated through changing the mole fraction of the molybdenum in the mixture precursors. Photocatalytic results confirm that the relatively low ratio of Mo+4/Mo+6 in the ternary composites can significantly improve the catalytic activity for efficiently degrading the organic dye due to the synergistic effect of the multiple heterojunctions. The degradation efficiency achieves 97.9 % of methylene blue (20 mg/L) in 40 min using the photocatalyst dosage of 10 mg. A possible photocatalytic mechanism is also proposed.

Non-stationary electrochemical synthesis of flexible binder-free hybrid electrode materials for supercapacitors

The rapid development of flexible energy storage devices, such as supercapacitors, requires the creation of appropriate electrode materials with high capacitance and cycling stability. In this work, we propose a new method for the synthesis of hybrid electrode materials by depositing transition metal oxides on carbon fabrics using non-stationary electrolysis. An amorphous deposit with inclusions of Mo4O11, MoO3, MoO2, Mo8O23, Mo9O26, V2O5, and MnO2 nanocrystals was obtained on the carbon surface. The hybrid was used as a binder-free electrode and had a capacitance of 498 mF∙cm–2 in 2 M KOH electrolyte at a current density of 4 mA∙cm–2. This value is comparable to or even higher than the values reported for similar materials. A long-term test of the two-electrode system showed a capacitance retention of 62% over 10,000 cycles. The power density and energy density of the hybrid electrode are 15∙10–4 W∙cm–2 and 6∙10–7 Wh∙cm–2, respectively. The synthesized metal oxide/carbon hybrids are promising electrode materials for symmetrical supercapacitors with alkaline electrolytes.

Thermal decomposition synthesis of cylindrical rod-like MoO3 and irregular sphere-like Ag2MoO4 nanocrystals for accelerating photocatalytic degradation of industrial reactive dyes and biosensing application

This paper reports the thermal decomposition method to prepare a facile, one-pot synthesis of cylindrical rod-like MoO3 and irregular sphere-like Ag2MoO4 nanocrystals. The characteristic properties of MoO3 and Ag2MoO4 nanocrystals were investigated by XRD, FESEM, HRTEM, EDS, IR, UV-Vis, PL, EIS, XPS, and CV techniques. Ag2MoO4 nanocrystals result in an optical band gap to redshift, assessed by Tauc plots of the ultraviolet-visible (UV–vis) spectra. When compared to other MWCNTs modified and unmodified CPEs for the detection of the 5-Flououracil drug, the MoO3/MWCNTs/CPE showed excellent electrocatalytic activity in terms of a larger anodic peak current and an inferior peak potential. Ag2MoO4 nanocrystals shown in an excellent photocatalytic degradation efficiency of 91.39% and 57.89% for the Cibacron Deep Red C-D (CR) and Cibacron Orange C-RN (CO) dyes as compared to MoO3 as evaluated by UV–visible spectrophotometric analysis in 150 mins under 100 W visible-light irradiation. Electrochemical impedance spectroscopy has shown that the improved separation and transport of photogenerated electron-hole pairs account for Ag2MoO4’s greater efficiency than MoO3. The effects of experimental parameters for the photocatalytic degradation of dyes, including initial dye concentration of 10 mg/L, catalyst dose 75 mg, pH 7, and catalyst recycling, have been investigated to optimize reaction conditions. The primary active species were obtained to be photogenerated holes (h+) and superoxide radical anions (O2•‾). The cyclic photocatalytic degradation of CR and CO testified to Ag2MoO4’s good extent of reusability. The details of the electrochemical and photocatalytic mechanisms were then provided. Also, liquid chromatography-mass spectra (LC-MS) were used to find out the photocatalytic degradation intermediate products.

Material Science for catalysis: Quo vadis?

Material Science for catalysis: Quo vadis?

Regulation of Active Oxygen Species by Grain Boundaries to Optimize Reaction Paths toward Aerobic Oxidations

Aerobic oxidation by using molecular oxygen (O2) as the oxidant is highly attractive, in which activating O2 to reactive oxygen species (ROS) is a prerequisite. Although some progress has been achieved in regulating ROS by heterogeneous catalysts, the strategies to efficiently control ROS in aerobic oxidation are still urgently desired. Herein, grain boundaries (GBs) in metal oxides are discovered to be able to facilely regulate ROS. Impressively, MoO3 nanocrystals with high density of GBs (MoO3‐600) deliver a mass activity of 83 mmol g‐1 h‐1 in aerobic oxidation of benzyl alcohol, 7 and 8 times as high as that of MoO3 nanoparticles without GBs and Pt/C, respectively. In addition, the selectivity of benzoic acid is 100% during whole reaction process over MoO3‐600. Mechanistic studies reveal that the oxygen atoms at GBs in MoO3‐600 are highly active to form ∙OH radicals with the generation of oxygen vacancies, while the oxygen vacancies are replenished by O2. The reaction path directly contributes to the excellent catalytic performance.

Controlled growth of α-MoO3 nanostructures with enhanced optical and electrochemical properties without capping agents

Size and shape controlled crystalline α-MoO3 nanostructures were synthesized through modified hydrothermal method without using any capping agents. Variation of concentration of oxidizing agent (H2O2) governed the morphology of the resulted nanostructures. FESEM and TEM techniques confirmed the unique morphological pattern with uniform size distribution without any impurity. XRD data revealed the crystalline α -MoO3 nanocrystals having size of ~35–37 nm. α-MoO3 nanomaterial showed the diffusion controlled electrochemical response. The band gap energy was evaluated to be ~4.0 eV by UV–Vis absorption spectra. PL spectra showed the unique emission pattern due to polyvalent Mo cation and oxygen vacancies present in α-MoO3 nanocrystals. Analysis of Nyquist plot and Cyclic Voltagrams revealed the lower resistivity and enhanced capacitance values of synthesized materials. Synthesized α-MoO3 nanocrystals may have potential applications in energy storage devices and optoelectronic devices due to their good structural, optical and enhanced electrochemical properties.

Large Scale Process for Low Crystalline MoO₃-Carbon Composite Microspheres Prepared by One-Step Spray Pyrolysis for Anodes in Lithium-Ion Batteries.

This paper introduces a large-scale and facile method for synthesizing low crystalline MoO3/carbon composite microspheres, in which MoO3 nanocrystals are distributed homogeneously in the amorphous carbon matrix, directly by a one-step spray pyrolysis. The MoO3/carbon composite microspheres with mean diameters of 0.7 µm were directly formed from one droplet by a series of drying, decomposition, and crystalizing inside the hot-wall reactor within six seconds. The MoO3/carbon composite microspheres had high specific discharge capacities of 811 mA h g−1 after 100 cycles, even at a high current density of 1.0 A g−1 when applied as anode materials for lithium-ion batteries. The MoO3/carbon composite microspheres had final discharge capacities of 999, 875, 716, and 467 mA h g−1 at current densities of 0.5, 1.5, 3.0, and 5.0 A g−1, respectively. MoO3/carbon composite microspheres provide better Li-ion storage than do bare MoO3 powders because of their high structural stability and electrical conductivity.

Friction between van der Waals Solids during Lattice Directed Sliding.

Nanometer-scale crystals of the two-dimensional oxide molybdenum trioxide (MoO3) were formed atop the transition metal dichalcogenides MoS2 and MoSe2. The MoO3 nanocrystals are partially commensurate with the dichalcogenide substrates, being aligned only along one of the substrate's crystallographic axes. These nanocrystals can be slid only along the aligned direction and maintain their alignment with the substrate during motion. Using an AFM probe to oscillate the nanocrystals, it was found that the lateral force required to move them increased linearly with nanocrystal area. The slope of this curve, the interfacial shear strength, was significantly lower than for macroscale systems. It also depended strongly on the duration and the velocity of sliding of the crystal, suggesting a thermal activation model for the system. Finally, it was found that lower commensuration between the nanocrystal and the substrate increased the interfacial shear, a trend opposite that predicted theoretically.

Hydrothermally Synthesized h-MoO3 and α-MoO3 Nanocrystals: New Findings on Crystal-Structure-Dependent Charge Transport

The charge transfer characteristics of metastable-phase hexagonal molybdenum oxide (h-MoO3) and stable-phase orthorhombic MoO3 (α-MoO3) nanocrystals have been investigated for the first time using impedance spectroscopy. The results imply that the metastable phase h-MoO3 displays a 550-fold increase (at 150 °C) in the electrical conductivity relative to the stable phase α-MoO3. The conductivity also increases as the temperature increases from 130 to 170 °C, whereby analysis shows a thermal activation energy (Ea) of ∼0.42 eV. The investigation clearly identifies that the presence of intercalated ammonium ions (NH4+) and crystal water molecules (H2O) in the internal structure of h-MoO3 plays a vital role in enhancing the charge transfer characteristics and showing an ionic conductive nature. Before the impedance investigations, the h-MoO3 and α-MoO3 nanocrystals were successfully synthesized through a wet-chemical process. Here, a controlled one-step hydrothermal route was adopted to synthesize stable-phase...

Large-Scale Production of MoO3-Reduced Graphene Oxide Powders with Superior Lithium Storage Properties by Spray-Drying Process

A MoO3-reduced graphene oxide (rGO) composite powder with spherical shape is prepared by a simple spray-drying process by applying a water-soluble metal salt. Ammonium molybdate–GO composite powders prepared by spray drying turn into a MoO3–rGO powder by heat treatment at 300°C under air atmosphere. The MoO3 nanocrystals are uniformly dispersed in a crumpled spherical rGO structure. MoO3 and MoO3–C powders, both with spherical particles, are also prepared by the same process for comparison of their electrochemical properties. The discharge capacities of the MoO3, MoO3–C, and MoO3–rGO powders after 100 cycles at a current density of 500mAg−1 are 506, 738, and 1115mAhg−1, respectively, and their corresponding capacity retentions measured from the second cycle are 53, 71, and 92%, respectively. The rGO layers improve the structural stability and electric conductivity of the MoO3–rGO composite powder. Therefore, the MoO3–rGO powder shows superior electrochemical properties as compared with the MoO3 and MoO3–C powders prepared by the same preparation procedure.

Ultrathin MoO3 nanocrystalsself-assembled on graphene nanosheets via oxygen bonding as supercapacitor electrodes of high capacitance and long cycle life

Ultrathin MoO3 nanocrystals were assembled on 3D graphene oxide frameworks via a hydrothermal reaction forming a layered structure by oxygen-bonding interactions at the interface. The structure and morphology of the resulting MoO3-GAs hybrids were characterized by a range of experimental tools including atomic force microscopy, scanning electron microscopy, transmission electron microscopy, X-ray photoelectron spectroscopy, infrared and Raman spectroscopy. Because of abundant exposed active sites of the ultrathin MoO3 and rapid ion diffusion and electron transport of 3D graphene frameworks, the resulting MoO3-GAs hybrids possess the highest specific capacitances and excellent cycling stability in both aqueous (527Fg−1 at the current density of 1.0Ag−1, 100% retention after 10,000 cycles) and solid electrolytes (373Fg–1 at 1.0Ag−1, 100% retention after 5,000 cycles) among leading literature results of similar systems.

Vacuum Topotactic Conversion Route to Mesoporous Orthorhombic MoO3 Nanowire Bundles with Enhanced Electrochemical Performance

The growth of mesoporous bundles composed of orthorhombic MoO3 nanowires with diameters ranging from 10 to 30 nm and lengths of up to 2 μm by topotactic chemical transformation from triclinic α-MoO3·H2O nanorods under vacuum condition at 260 °C is achieved. During the process of vacuum topotactic transformation, the nanorod frameworks of the precursor α-MoO3·H2O can be preserved. The crystal structures, molecular structures, morphologies, and growth behavior of the precursory, intermediate and final products are characterized using powder X-ray diffraction (PXRD), Raman spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), and selected-area electron diffraction (SAED). Detailed studies of the mechanism of the mesoporous MoO3 nanowire bundles formation indicate topotactic nucleation and oriented growth of the well-organized orthorhombic MoO3 nanowires inside the nanorod frameworks. MoO3 nanocrystals prefer [001] epitaxial growth direction of triclinic α-MoO3·H2O nanorods...

Crumpled Graphene–Molybdenum Oxide Composite Powders: Preparation and Application in Lithium‐Ion Batteries

Crumpled graphene-MoO2 composite powders are directly prepared by means of spray pyrolysis and from a stable graphene oxide colloidal solution in the presence of Mo ions. The crumpled graphene-MoO2 composite powders are transformed into MoO3 -based composite powders after post-treatment at 300 °C. The transmission electron microscopy and dot-mapping images of the post-treatment composite powders show uniform distribution of MoO3 nanocrystals in the crumpled graphene powders. The two typical D and G bands of graphene are observed at 1350 and 1590 cm(-1) , respectively, in the Raman spectrum of the graphene-MoO3 composite. In addition, the crumpled graphene-MoO3 powders exhibit superior electrochemical behavior compared to that of pure MoO3 as an anode material for lithium-ion batteries. The initial discharge capacities of the graphene-MoO3 composite and bare MoO3 powders at a current density of 2 A g(-1) are 1490 and 1225 mA h g(-1) , respectively. The capacity retention of the graphene-MoO3 composite is 87 % after the first cycle, whereas that of bare MoO3 is 47 %, as measured after 100 cycles. The reversible discharge capacity of the graphene-MoO3 composite decreases slightly from 1228 to 845 mA h g(-1) as the current density increases from 0.5 to 3 A g(-1) .

The Effect of Substrate Temperature on the Optical Properties of MoO3 Nano-crystals Prepared Using Spray Pyrolysis

In this study, thin films of molybdenum oxide were prepared by spray pyrolysis technique on glass substrates. The influence of substrate temperature on their crystallographic structure, surface morphology, and optical properties was studied. The formation of a MoO3 film on the substrate was confirmed through XRD analysis. Furthermore, the presence of the two phases α and β in each of the films was evident. The percentage of phase α varied from 55 % for the film deposited at 200 °C up to 97 % for the film deposited at 400 °C. According to SEM images, MoO3 films have a sponge-type structure on the order of nanometers. Both the optical gap and the refraction index strongly depend on substrate temperature. The optical gap decreases from 3.63 eV for the film deposited at 150 °C up to 3.30 eV for the one prepared at 400 °C. On the contrary, the refraction index measured at 800 nm increases from 1.54 up to 1.61 for the films prepared at 150 °C and 400 °C, respectively.

Self-assembly and photoluminescence of molybdenum oxide nanoparticles

We report on the synthesis of self-assembled hillock shaped MoO3 nanoparticles on thin films exhibiting intense photoluminescence (PL) by RF magnetron sputtering and subsequent oxidation. MoO3 nanocrystals of size ∼29 nm are self-assembled into uniform nanoparticles with diameter ∼174 nm. The mechanism of the intense PL behaviour from MoO3 nanoparticles is investigated and systematically discussed. The films exhibit two bands; a near-band-edge UV emission and a defect related deep level visible emission. The enhancement in PL intensity with annealing is not only by the improvement in crystallinity and grain size but also by the increase in the rms surface roughness and porosity of the films. The PL intensity is thermally activated with activation energy 1.07 and 0.87 eV respectively for the UV and visible emissions. The UV band exhibits a blue shift according to the band gap with increasing post-annealing temperatures, which suggests the possibility to tune the UV photoluminescence band by varying the oxidation temperature.

Electron microscopy characterization of hexagonal molybdenum trioxide (MoO3) nanorods

A simple low-temperature chemical method to produce metastable hexagonal phase molybdenum oxide (h-MoO3) nanorods and their excellent structural characteristics has been demonstrated. In this approach, h-MoO3 samples were prepared by precipitating molybdenum oxide from an ammonium paramolybdate solution by the adding nitric acid. The structure of h-MoO3 nanorods was examined in detail using high-resolution scanning electron microscopy (SEM) and high-resolution transmission electron microscopy (HR-TEM). The SEM data reveal that the nanorod h-MoO3 crystals have the shape of straight hexagonal rods with an aspect ratio ∼60. The HR-TEM results confirm the hexagonal structure of the MoO3 nanocrystals. Computations of the observed TEM data along with x-ray diffraction pattern further confirm the stability of the nanostructure of h-MoO3 rods.

Load-Independent Friction: MoO3 Nanocrystal Lubricants

The behavior of the model lubricant system molybdenum disulfide (MoS2) has been characterized as a function of thermal oxidation on the atomic to nanometer scales with scanning tunneling microscopy (STM) and atomic force microscopy (AFM) in ultrahigh vacuum (UHV). STM studies of single-crystal MoS2 surfaces showed that the initial stages of thermal oxidation produce atomic-scale pits and then molybdenum oxide (MoO3) nanocrystals 1−5 nm in diameter. The densities of pits and MoO3 nanocrystals and the size of MoO3 nanocrystals increased with increasing oxidation time. UHV AFM friction measurements made on these same samples exhibited a direct relationship between defect density and friction for short oxidation times. After longer oxidation times when well-defined MoO3 nanocrystals were present, load-independent friction was observed. A model, which involves adhesion of MoO3 nanocrystals to the AFM probe tip end, has been developed to explain this first observation of load-independent friction. The implications of these results are discussed.

Nanotribology and Nanofabrication of MoO3 Structures by Atomic Force Microscopy

Atomic force microscopy was used to characterize the sliding of molybdenum oxide (MoO3) nanocrystals on single-crystal molybdenum disulfide (MoS2) surfaces. Highly anisotropic friction was observed whereby MoO3 nanocrystals moved only along specific directions of the MoS2 surface lattice. The energy per unit area to move the MoO3 nanocrystals along their preferred sliding direction was an order of magnitude less than required to slide macroscopic MoS2-bearing contacts. This extreme friction anisotropy was exploited to fabricate multicomponent MoO3 nanostructures. These reversibly interlocking structures could serve as the basis for devices such as mechanical logic gates.