MoO3 Crystals Research Articles

Unveiling the structure-performance of MoO3/La2O2CO3 for ultra-deep aerobic oxidative desulfurization of diesel

The production of clean fuel via the aerobic oxidative desulfurization (ODS) technique is a potential substitute due to its moderate reaction conditions and outstanding catalytic efficiency. However, the relationship between the catalyst structure and the catalytic performance for sulfur-containing compounds with high steric hindrance effect in this process remains unclear hitherto. Herein, a series of MoO3/La2O2CO3 catalysts with various MoO3 particle sizes were carefully designed and prepared through a facile citric acid-assisted hydrothermal strategy. The structures of the loaded Mo-based catalysts were analyzed by series characterization methods, and their structure-performance relationships were investigated for catalytic ODS of 4,6-dimethyldibenzothiophene (4,6-DMDBT). The results revealed that the particle size of MoO3 significantly influenced the redox properties, reactive oxygen species, and ODS performances of MoO3/La2O2CO3 catalysts. With the decrease of MoO3 crystal size from 15.8 nm to 5.3 nm, the ODS performances of MoO3/La2O2CO3 catalysts considerably increased. Among them, MoO3/La2O2CO3-3 exhibited the best ODS activity with complete 4,6-DMDBT conversion within 2.5 h at 130 °C because of its appropriate MoO3 size and the lowest apparent activation energy (Ea). Nevertheless, further downsizing the crystal size of MoO3 to 2.8 nm resulted in a significant increase in the Ea, which dramatically suppressed the catalytic ODS performance of the corresponding MoO3/La2O2CO3-4 catalyst instead. It therefore reveals for the first time that there is a balance between the geometry and the catalytic efficiency of MoO3/La2O2CO3 catalysts for catalytic ODS of 4,6-DMDBT with high steric hindrance effect, which could provide insight for developing advanced ODS catalysts in the future.

Anisotropy of photocatalytic properties of MoO3/TiO2–SiO2 core–shell polycrystalline composites

In this work, the effect of the preferred orientation of MoO3 crystals in polycrystalline MoO3/TiO2–SiO2 composites on their optical properties and photocatalytic activities in the decomposition reaction of organic dyes was investigated. The anisotropy of the photocatalytic properties was shown. The MoO3/TiO2–SiO2 core–shell spherical composites were prepared using TOKEM-320Y anion exchange resin as a template by the combined method, including template and sol–gel methods. The XRD, XPS, EDS, SEM, Raman, IR, DRS, N2 sorption, and PL techniques were used to characterize the elemental and phase compositions, textural and optical properties of the MoO3/TiO2–SiO2 spherical particles. The MoO3/TiO2–SiO2 composites prepared at 600 °C were characterized by the lowest anisotropy coefficient (1.23) of the crystals growth rate in the plane (l00) relative to the crystals growth rate in the plane (lll) and the highest concentration of oxygen vacancies. High temperature treatment of MoO3/TiO2–SiO2 samples up to 700 °C leads to a change in the degree of their texture, which is due to the growth of the anisotropy coefficient up to 1.38 and crystallites in the (l00) plane, which was confirmed by increasing the Lotgering factor. The content of Mo5+ in the surface and subsurface layers of such composites is the highest. The MoO3/TiO2 − SiO2 composites prepared at 600 °C exhibited a much higher photocatalytic activity (the rate constant is 0.0319 min-1). This work demonstrates that the predominant orientation of MoO3 crystals in the (l00) plane negatively affects the oxygen defect (the value decreases to 0.0486) and MoO3/TiO2 − SiO2 polycrystalline composites photocatalytic activity.

Modified hot plate method for synthesizing MoO3 nanoplates

Molybdenum trioxide (MoO3 ) with layered structures adopts exotic physical features, which has evoked an extensive interest in electronic and photoelectronic devices. Here, we report a low-cost, simplehandle, atmospheric-pressure, and rapid-synthesis technique for growing large-scale MoO3 crystals, i.e., a modified hot plate method. The growth rate and morphology of the MoO3 crystals were well controlled by changing source temperatures and substrates. Complementary measurements, including optical microscope, atomic force microscope, X-ray diffraction, Raman spectroscope, and scanning near-field optical microscope, were used to investigate the structural and physical properties. The results reveal that large-scale MoO3 crystals with well-defined crystallinity have been obtained. Meanwhile, surface hyperbolic phonon polaritons on as-prepared MoO3 crystal planes have also been observed, which may provide an attractive insight into nanoelectronic and nanophotonic devices.

Investigation of Ni, Co, and Zn promoters on Mo/HY modified zeolite for developing an efficient bimetallic catalyst for oxidative desulfurization of dibenzothiophene

Bimetallic Mo-M (M = Ni, Co, and Zn)/RHY supported catalysts were synthesized for oxidative desulfurization (ODS) of dibenzithiophene (DBT). Recrystallized HY zeolite (RHY) included a superior meso-porous structure (RHY) which is attractive for bulky dibenzothiophene (DBT) elimination. The facilities of M promoters on the Mo/RHY features and activity were investigated. Despite similar ionic radius, different electronegativity of Ni, Co, and Zn transition metals caused to different interaction with Mo and RHY zeolite. The results demonstrated that Ni influenced negatively on dispersion of Mo species over the support, while Co and Zn improved the dispersion. Ni interacted readily with framework Al, leading to extraction of Al, collapse of the zeolite structure, and more octahedral and crystalline Mo species. The formation of MoO3 crystals was inhibited by Zn and Co addition leading to improvement of Mo dispersion. Moreover, Zn- and Co-doped Mo/RHY exhibited higher Lewis acidic sites. The ODS activity decreased in the order of MoZn > MoCo > Mo > MoNi. UV–vis results approved that the most of Mo in the Zn doped sample was non-framework type which can well disperse over the external surface. Meanwhile, Co addition enhanced the tetrahedral Mo species incorporated into the zeolite framework. Allowing more exposed high active Mo species for ODS reaction by Zn promoter, the catalytic performance was boosted. The best catalyst, MoZn/RHY, exhibited 100% DBT removal (t = 30 min, T = 40 °C, catalyst/fuel = 10 g/L, O/S = 10, VSolvent/VOil = 1/2, DBT = 1000 ppm) and a high reusability in 4 consecutive recycles. The highest removal of indole as a nitrogen compound was also achieved by the introduced MoZn/RHY catalyst.

MoO3 structures transition from nanoflowers to nanorods and their sensing performances

Morphology transformation and crystal growth strategies of metal oxide semiconductors are still extensively studied in material science recently, because the morphology and crystallinity significantly affect the physicochemical characteristics of metal oxide nanomaterials. However, understanding the morphology changes of α-MoO3 induced by annealing is still a challenge. Herein, the nanostructure transition of α-MoO3 induced by the annealing temperature is carefully investigated via the XRD and SEM methods. It can be found that crystallization is highly dependent on the annealing temperature. Interestingly, the MoO3 nanoflowers can change into nanosheets at 500 °C. Afterward, the nanosheets turned into microrods with the increase in annealing temperature due to the continuous growth of MoO3 crystal. On the other hand, the sensing performances of various MoO3 nanostructures are studied toward ethanol gas. Compared to the MoO3 nanoflowers and microrods, the MoO3 nanosheets-based sensor exhibits superior sensing performance to ethanol, and the maximum response value is 8.06.

MoO3 crystal facets modulation by doping heteroatom Fe from polyoxometalate for quasi-industrial oxygen evolution reaction

The widespread implementation of water splitting urgently requires low-cost, effective and stable electrocatalysts for the industrial oxygen evolution reaction (OER) that can reach a large current density (>500 mA cm−2). Herein, we report a facile solvothermal strategy to fabricate a binder-free electrode of Fe-S-NiMoO4/MoO3 with nanorods array uniformly grown on a nickel-foam (NF). The Anderson-type polyoxometalate (NH4)3[FeMo6O24H6]·7H2O as the precursor has enabled an accurate and controllable Fe-doping in transition metal oxides (TMO). The combination of experiments and density functional theory (DFT) calculations reveal that Fe-doping is conductive to regulate the crystal plane of MoO3, modulate the electronic structure and improve the conductivity. Consequently, the Fe-S-NiMoO4/MoO3@NF electrode can yield 500 mA cm−2 at an overpotential of 271 mV and work steadily over 100 h under 50 °C that can meet the requirements of the industrial water electrolysis. This study provides a practical approach to design electrocatalysts for future industrial applications.

Substrate-mediated hyperbolic phonon polaritons in MoO3.

Hyperbolic phonon polaritons (HPhPs) are hybrid excitations of light and coherent lattice vibrations that exist in strongly optically anisotropic media, including two-dimensional materials (e.g., MoO3). These polaritons propagate through the material's volume with long lifetimes, enabling novel mid-infrared nanophotonic applications by compressing light to sub-diffractional dimensions. Here, the dispersion relations and HPhP lifetimes (up to ≈12 ps) in single-crystalline α-MoO3 are determined by Fourier analysis of real-space, nanoscale-resolution polariton images obtained with the photothermal induced resonance (PTIR) technique. Measurements of MoO3 crystals deposited on periodic gratings show longer HPhPs propagation lengths and lifetimes (≈2×), and lower optical compressions, in suspended regions compared with regions in direct contact with the substrate. Additionally, PTIR data reveal MoO3 subsurface defects, which have a negligible effect on HPhP propagation, as well as polymeric contaminants localized under parts of the MoO3 crystals, which are derived from sample preparation. This work highlights the ability to engineer substrate-defined nanophotonic structures from layered anisotropic materials.

Nickel decorated MoO3 single crystal microflakes with multi-site functionality for enhanced hydrogen evolution reaction

Role of hybrid material with metal-oxide interface has been explored by coating 2 nm nickel on α-MoO3 single crystals for hydrogen evolution reaction (HER). The investigated aspects reveal that α-MoO3/Ni hybrid exhibits a remarkable performance in HER showing +6 mV onset potential and 37 mV overpotential at 10 mA/cm2 current density along with Tafel slope of 47 mV/dec. The single crystalline-stepped CVD-grown MoO3 microflakes having the advantage of higher hydrogen binding energy of Ni exhibits the enhanced catalytic performance due to strong electronic coupling at the metal-oxide interface and hydrogen spill over effect. Similar hybrid material composed of Cu-MoO3 does show improvement but not as good as Ni-MoO3. A decrease of ~36% is observed in the overpotential for Ni-coated MoO3 compared to pure MoO3 crystals indicating the positive contribution of Ni-coating. The hybrid Ni-MoO3 shows the new route to develop alternate transition metal oxide-based hybrid catalyst towards production of hydrogen fuel.

The Rise of Twist-Optics.

Two-dimensional materials stacked with a relative twist angle have been the focus point of tremendous interest in the condensed matter community. In the last four months, a theoretical prediction and four independent experimental works have demonstrated the tremendous impact these results can have on the nanophotonics community: the phonon polariton dispersion of twisted stacks of MoO3 crystals can range from hyperbolic to elliptic, depending on the twist angle and thicknesses. In this Perspective, we provide a fast introduction to this exciting new topic and explain the importance of these findings.

Recycling Molybdenum Oxides from Waste Molybdenum Disilicides: Oxidation Experimental Study and Photocatalytic Properties

To recycle elemental molybdenum from waste molybdenum disilicide (MoSi2) heating elements, the MoSi2 was first disintegrated to MoO3 and SiO2 powders in air at a pest oxidation temperature of 500 °C. X-ray diffraction (XRD) patterns confirmed the completion of the pest oxidation reaction. The mixture of MoO3 and SiO2 powders were heated to 950 °C in a tube furnace to evaporate MoO3, and the XRD patterns of the residue showed that only SiO2 was left in the crucible, confirming that the MoO3 was removed through thermal evaporation. The collected MoO3 crystals had a striped morphology. Photocatalytic performance of MoO3 showed superior activity in comparison with commercial MoO3 and P25 for the degradation of methylene blue under visible light irradiation. The photocatalytic degradation activity of MoO3 synthesized by thermal evaporation at 950 °C was 99.25% in 60 min.

Comparison of γ and δ-Al2O3 supported CoMo catalysts in the hydrodesulfurization of straight-run gas oil

The effect of two different crystal species of alumina on hydrodesulfurization activity of the corresponding CoMo catalysts was studied. Cylindrical extruded alumina with two different crystal structures i.e. γ-Al2O3 and δ-Al2O3 was prepared using boehmite and nitric acid as a peptizing agent by calcination at 550 ˚C and 900 ˚C, respectively. The Al2O3 support were impregnated with 9 wt.% of Mo and 2 wt.% Co via incipient wetness impregnation method. The CoMo/Al2O3 catalysts were used for hydrodesulfurization (HDS) and hydrodenitrification (HDN) of Iranian straight-run gas oil (ISRGO). The supports and catalysts were characterized by nitrogen adsorption-desorption isotherm, XRD, UV-vis-DRS, TPD, TPR and ICP-OES. The HDS activity of CoMo/γ-Al2O3 catalyst was higher than that of CoMo/δ-Al2O3 and was found to be 95.74%. This result was due to the formation of larger CoMoO4 and MoO3 crystals in CoMo/δ-Al2O3 catalyst which reduces the active metal phase dispersion and the performance of the catalyst. The HDS activity of CoMo/γ-Al2O3 catalyst was remarkable as the metal content of the catalyst was low. The HDN activity of CoMo/γ-Al2O3 was also about 66%.

One-step synthesis of centimeter-size alpha-MoO3 with single crystallinity

By utilizing the convenient physical vapor deposition procedure, a simple and low-cost synthesis route is developed to fabricate high-quality orthorhombic MoO3 crystals with single crystallinity and significant size (bigger than 10 mm × 10 mm2). A combined characterization techniques of atomic force microscopy, X-ray photoelectron spectroscopy, synchrotron-based X-ray diffraction and Raman spectroscopy confirm the high quality of α-MoO3 crystal from the point view of both electronic and physical structures. Compared to previous reports in literature, we demonstrate herein a rather simplified one-step approach with only the MoO3 powder needed. In the end, corresponding electrochromic performance has also been tested which reveals high transmittance and remarkable optical properties which may broadens the potential applications of MoO3 single crystal in nanosensors, nanoelectronics and so on.

Quantitative determination of oxygen defects, surface lewis acidity, and catalytic properties of mesoporous MoO3/SBA-15 catalysts

A set of MoO3/SBA–15 mesoporous catalysts were characterized with a variety of spectroscopic techniques and their crystalline structures were refined with Rietveld method. Oxygen defect concentration, crystallite size, phase composition, surface acidity, mesoporous regularity, and textural properties were reported. Both α–MoO3 and β–MoO3 phases coexisted but α–MoO3 was predominated. Oxygen defects were created in the orthorhombic structure and its concentration decreased from 3.08% for the 20 wt%MoO3/SBA-15 to 0.55% for the 25 wt%MoO3/SBA-15. All the MoO3/SBA–15 catalysts chiefly contained a big number of Lewis acid sites originating from oxygen defects in MoO3 crystals. In the absence of formic acid, the oxidation of 4,6-dibenzothiophene (4,6–DMDBT) in a model diesel was almost proportional to the number of Lewis acid sites. In the presence of formic acid, 4,6–DMDBT oxidation was significantly affected by the formation of surface peroxometallic complex and Lewis acidity. Formic acid addition could improve the ODS efficiency by promoting peroxometallic complex formation and enhancing oxidant stability. Under the optimal reaction condition using the best 15 and 20 wt%MoO3/SBA-15 catalysts, more than 99% 4,6–DMDBT could be removed at 70 °C within 30 min. This work confirmed that 4,6–DMDBT oxidation is a texture and particle size sensitive and Lewis acidity dependent reaction. This work also shows that crystalline structure refinement combination with experiments can gain new insights in the design of heterogeneous nanocatalysts and help to better understand the catalytic behavior in the oxidative desulfurization reactions.

Two-dimensional MoO3 via a top-down chemical thinning route

Two-dimensional MoO3 is a versatile planar material, whose properties can be readily tuned, rendering it anywhere from a wide bandgap semiconductor to semi-metallic. This makes it a desirable candidate for a wide range of applications. However, to utilise its full potential, a repeatable process to produce high-quality two-dimensional (2D) crystals is yet to be reported. Here, we report a wet chemical etching process to controllably thin down bulk crystals of MoO3. This process does not result in any structural or compositional changes, while retaining the desirable intrinsic electronic properties of the material. Field effect transistors based on post-etched crystals exhibit switching ratios of over three orders of magnitude. As such, the proposed thinning process opens pathways to exploit the exotic properties of planar MoO3 to create versatile 2D systems.

Volumetric flame synthesis of mixed tungsten–molybdenum oxide nanostructures

Mixed-metal (Mo/W) oxide nanostructures were synthesized volumetrically in the oxygen-rich zone of a counter-flow diffusion flame. High purity Mo and W wires were used to generate Mo and W oxide vapors that are transformed into mixed-metal oxide nanostructures while transported by the gas flow in the upward direction toward the stagnation plane. The Mo wire was inserted in a lower position within the flame volume than the W wire. Samples of generated nanomaterials collected thermophoretically from the flame volume at various axial positions allowed tracing of the particle evolution. The following synthesis stages were identified. MoO3 structures are first formed in the lower part of the flame. As they are carried by the gas flow, W oxide vapors deposit on the surfaces of the early formed MoO3 nanostructures. W atoms diffuse at the interfaces of the MoO3 structures to form mixed W–Mo oxide nanoparticles. Fully grown structures, collected near the stagnation plane (SP), are nanocubes exhibiting well-defined edges with sizes less than 100nm. Energy-dispersive X-ray spectroscopy elemental mapping revealed that the mature structures have uniform distribution of W, Mo, and O atoms. The measured lattice spacing showed expanded lattice distance of MoO3 which may be attributed to the incorporation of W atoms in the MoO3 crystal and formation of W–Mo oxide structures.

Growth and physical properties of large MoO3 single crystals

Molybdenum trioxide (MoO3) is an interesting material for smart windows, electrodes for batteries, and gas sensors. So far, it is known that the growth of large single crystal of MoO3 is elusive because of the rapid changing of activation energy at 650 °C. Small MoO3 crystals can be made by sublimation. In this study, we successfully grew large MoO3 single crystals through the modified vertical Bridgman method, which can prevent the sublimation of MoO3. Similar structural and optical properties were measured like those of nano/micro-crystalline MoO3. This study can provide evidence to contribute to the creation of large single crystals, which have a lower sublimation temperature than melting temperature.

Centimeter-Scale Synthesis of Ultrathin Layered MoO3 by van der Waals Epitaxy

We report on the large-scale synthesis of highly oriented ultrathin MoO3 layers using a simple and low-cost atmospheric pressure by van der Waals epitaxy growth on muscovite mica substrates. By this method we are able to synthetize high quality centimeter-scale MoO3 crystals with thicknesses ranging from 1.4 nm (two layers) up to a few nanometers. The crystals can be easily transferred to an arbitrary substrate (such as SiO2) by a deterministic transfer method and extensively characterized to demonstrate the high quality of the resulting crystal. We also study the electronic band structure of the material by density functional theory calculations. Interestingly, the calculations demonstrate that bulk MoO3 has a rather weak electronic interlayer interaction and thus it presents a monolayer-like band structure. Finally, we demonstrate the potential of this synthesis method for optoelectronic applications by fabricating large-area field-effect devices (10 micrometers by 110 micrometers in lateral dimensions), finding responsivities of 30 mA/W for a laser power density of 13 mW/cm2 in the UV region of the spectrum and also as an electron acceptor in a MoS2-based field-effect transistor.

Synthesis of α-MoO3nanobelts with preferred orientation and good photochromic performance

Stable phase α-MoO3 nanobelts with preferred orientation were successfully evolved on transforming from metastable phase h-MoO3via an aqueous approach. H+ ions play a key role in the nuclei formation and crystal growth processes. Variation of the Mo : H+ molar ratio led to MoO3 crystals with different polymorphs. Oriented α-MoO3 nanobelts along the [001] direction were achieved under a Mo : H+ ratio of 1 : 9, which is relative to a heterogeneous nucleation process on the surface of metastable phase h-MoO3. The α-MoO3 nanobelts along the [100] direction were obtained under a Mo : H+ ratio of 1 : 10; homogeneous nucleation dominated the synthesis without the presentation of h-MoO3 microrods. The α-MoO3 nanobelts with distinguishing crystalline growth orientations exhibited different photochromic behaviors.

Metastable h-MoO3 and stable α-MoO3 microstructures: controllable synthesis, growth mechanism and their enhanced photocatalytic activity

Metastable h-MoO3 hexagonal prisms have been fabricated through a simple green ultrasonic-assisted chemical route. After calcination at 440 °C for 2 h, the thermodynamically stable α-MoO3 nanoplate-based rods have been achieved through a process of in situ phase transformation. The as-synthesised products have been characterised by powder X-ray diffraction, field emission scanning electron microscopy, Fourier transform infrared spectrum, UV–vis diffuse reflection spectroscopy and photoluminescence spectroscopy. The phase transformation from metastable h-MoO3 to stable α-MoO3 is observed at 419 °C according to the differential scanning calorimetry results. The possible growth mechanism of MoO3 crystals has been proposed based on the experimental results. The prepared two kinds of MoO3 samples both display higher photocatalytic performance for degrading rhodamine B compared to that of commercial MoO3. In the present system, α-MoO3 nanoplate-based rods exhibit slightly higher degradation activity than h-MoO3 hexagonal prisms, which is possibly due to its smaller band gap energy, smaller scale of nanoplate, better adsorption capacity and lower recombination of photogenerated electrons and holes.

Influence of MoO3(110) crystalline plane on its self-charging photoelectrochemical properties.

Nanocrystalline molybdenum oxide (α-MoO3) thin films with iso-oriented crystalline layers were synthesised by the anodisation of Mo foils. Upon band-gap excitation using light illumination, α-MoO3 generates excited electrons for reductive reactions and stores some of the excited electrons in its layered crystalline structure via alkali cation intercalation. These stored electrons can be subsequently discharged from α-MoO3 to allow reductive reactions to continue to occur under non-illuminated conditions. The modulation of water concentrations in the organic/aqueous anodisation electrolytes readily produces α-MoO3 crystals with high degree of (kk0) crystallographic orientation. Moreover, these (kk0)-oriented MoO3 crystals exhibit well-developed {hk0} and {0k0} crystal facets. In this paper, we show the benefits of producing α-MoO3 thin films with defined crystal facets and an iso-oriented layered structure for in situ storing of excited charges. α-MoO3 crystals with dominant (kk0) planes can achieve fast charging and a strong balance between charge release for immediate exploitation under illuminated conditions and charge storage for subsequent utilisation in dark. In comparison, α-MoO3 crystals with dominant (0k0) planes show a preference for excited charge storage.