MoO3 Single Crystals Research Articles

Synthesis of Large-Sized van der Waals Layered MoO3 Single Crystals with Improved Dielectric Performance.

The applications of two-dimensional semiconductors strictly require the reliable integration of ultrathin high-κ dielectric materials on the semiconductor surface to enable fine gate control and low power consumption. As layered oxide materials, MoO3 can be potentially used as a high-κ two-dimensional material with a larger bandgap and high electron affinity. In this work, relying on the oxidization of molybdenum chlorides, we have synthesized α-MoO3 single crystals, which can be easily exfoliated into flakes with thicknesses of a few nanometers and sizes of hundreds of micrometers and fine thermal stability. Based on measurement results of conventional metal/insulator/metal devices and graphene based dual-gate devices, the as-received MoO3 nanosheets exhibit improved dielectric performance, including high dielectric constants and competitive breakdown field strength. Our work demonstrates that MoO3 with improved crystalline quality is a promising candidate for dielectric materials with a large gate capacitance in future electronics based on two-dimensional materials.

Porous single-crystalline molybdenum nitride enhances electroreduction of nitrogen to ammonia

The industrial ammonia synthesis reaction has the disadvantage of large energy consumption; thus, the electrochemical reduction method of ammonia synthesis characterized by its clean nature and environmental protectiveness has received extensive attention. Molybdenum nitride is a commonly used electrocatalyst for ammonia synthesis, and its Faraday efficiency is low, which may be due to many internal grain boundaries and few active sites. In this work, we grow microscale porous Mo2N single crystals and polycrystalline Mo2N from non-porous MoO3 single crystals. Porous molybdenum nitride materials facilitate charge transport in grain boundaries due to their single-crystal nature and enhance the catalytic properties of ammonia synthesis reactions. Compared with polycrystalline Mo2N, the porous Mo2N single crystal shows better performance, with a high NH4+ yield of 272.56 μg h−1 mg−1 and a Faradaic efficiency of 7.3%. In addition, the porous Mo2N single crystal exhibits superior long-term stability with little attenuation after 16 h electrolysis reaction. It provides a new method for the catalyst of ammonia synthesis.

Growth of porous single-crystalline molybdenum nitrides microcubes with enhanced electrocatalysis performance

Porous single crystal has the characteristics of long-range order, continuous lattice and large specific surface area, which could reduce energy losses and keep high activity and stability in electrochemical systems. Here, we grow porous single-crystalline and polycrystalline molybdenum nitrides microcubes from MoO3 single crystals. These porous microcubes show superior HER and OER performance. The overpotential of Mo2N porous single crystal microcubes is only 73.13 mV at a current density of 10 mA cm−2, which is 150.53 mV, 192.76 mV and 255.87 mV lower than that of MoN single crystal, Mo2N polycrystal and MoN polycrystal, respectively. The advantages of Mo2N porous single crystals in electrocatalytic properties are also reflected in OER.

Reductive-annealing-induced changes in Mo valence states on the surfaces of MoO3 single crystals and their high temperature transport

Reduction of MoO3 in extreme reducing condition is a way to achieve Mo metal. However, effect of less extreme reductive-annealing, where it allows to keep the crystal structure, on physical and chemical properties of MoO3 has not been well-studied. In this work, we studied the evolution of Mo valence state during reductive annealing and its effect on high temperature transport. We found the formation of oxygen vacancies on surface of MoO3 single crystals at the low temperature, which is evidenced by increase of Mo5+ and color change. In addition, formation of Mo4+ was at the elevated temperature. For understanding the relation between bulk conductivity and Mo valence state, real-time impedance spectroscopy is employed. Use of two different gases makes it possible to distinguish impedance responses of MoO3 from those of reduced MoO3-x. Also, from time-dependent impedance measurements, we observed the evolution of transport behaviour by evolution of Mo valence state.

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.

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.

Ultrasonic properties near 50 K of the quasi-one-dimensional conductorsA0.30MoO3 (A = K, Rb) andRb0.30(Mo1−xVx)O3

The charge density wave (CDW) nonlinear conductivity of the blue bronzesA0.30MoO3 (A = K, Rb) shows two different regimes depending on the temperature: a strongly damped CDW motion above∼50 K and a CDW motion with almost no damping below∼50 K.In a search for an elastic signature of this CDW behaviour, we performed ultrasonic measurementson A0.30MoO3 single crystals in the temperature range 4–300 K. InRb0.30MoO3,at T∼50 K, upon cooling, a large increase of the sound velocity for the longitudinal mode measuredalong the , [102] and b directions is observed. The ultrasonic attenuation coefficient shows anincrease down to 50 K followed by a plateau. Similar results are found inK0.30MoO3. In V-dopedsamples, Rb0.30(Mo1−xVx)O3 (x = 0.4%) the anomaly broadens and is shifted towards higher temperatures. The results are discussedin relation to the changes in the CDW rigidity, disorder and dielectric response. Ascenario based on a glass transition for the CDW superstructure is proposed.

The Second Threshold Field of Charge-Density-Wave ConductorRb0.3MoO3 in High Temperature Range

The switching and threshold properties of quasi-one-dimensionalcharge-density-wave conductor rubidium blue bronzeRb0.3MoO3 single crystals are investigated in acomparative high and large temperature range. Beyond the limittemperature 50 K of Littlewood's theory, even up to about 100 K, typicalsharp switching to negative or zero differential resistance is observedin E–I characteristic curves. Correspondingly, an obvious switchingbetween two conducting states, from a lowly conducting state to ahighly conducting state, is observed in the I–E characteristiccurves in the same temperature range. Temperature dependence of thesecond threshold field ET2 accompanied by this kind of highfield switching behaviour is firstly obtained. These new observationsare discussed in the mechanism of the current inhomogeneity andredistribution due to the existence of transverse energy barrierssuggested by Zhang et al. [Solid State Commun. 85 (1993) 121]

Optical properties and electronic structures oflayered MoO3 single crystals

Reflectivity spectra of layered MoO3 single crystals have been measured at6 K in a wide energy range up to 25 eV. The spectra exhibit remarkableanisotropy for the polarization E∥c and E∥a. Optical constants have been derived through theKramers-Kronig analysis. The fundamental absorption tail located below 3.7 eVdisplays an exponential dependence on photon energy for both polarizations. Itis found that two luminescence bands peaking at 2.93 and 3.58 eV arestimulated under the excitation with photons in the fundamental absorptionregion. Their decay behaviours are also examined at 8 K. With use of thediscrete variational Xα (DV Xα) method, the electronic structureof an embedded [MoO4] cluster has been calculated for betterunderstanding of the optical properties of MoO3. The calculated results arefavourably compared with the experimental data.

Generation of Double-Layer Steps on (010) Surface of Orthorhombic MoO3 via Chemical Etching at Room Temperature

Using the methods of atomic force microscopy, X-ray diffraction and Fourier transform infrared spectroscopy, chemical etching of the (010) surface of flux-grown orthorhombic MoO3 (α-MoO3) single crystals has been investigated in detail. The (010) surface was etched repeatedly in a 0.08 M NaOH aqueous solution, and its topographies were recorded to reveal the evolution patterns upon the etching time. It is found that the “molecular steps” at a height of a single double-layer of MoO3 (6.9 A, or 1/2 bo) can be generated on the surface of (010) in the basic solution. A “layer-by-layer” etching mechanism proposed previously has been confirmed in this model catalyst fabrication. The observed etch pit volumes and multiple-step formation have also been addressed. Although they are created at room temperature, the surface steps/edges are chemically and thermally stable in a common operating temperature range of 350−400 °C. The fabrication/modification of future catalysts via chemical etching is anticipated, since ...

A STUDY ON THE PEIERLS TRANSITIONS OF BLUE BRONZE K0.3MoO3 SINGLE CRYSTAL

K0.3MoO3 single crystals with the size of 2cm×1.5cm×1cm have been grown by electrolytic reduction of K2CO3-MoO3 melt with a K2CO3-to-MoO3 mole ratio of 1 to 4.32. The temperature dependence of normalized resistivity R(T)/R(290K)-T of a K0.3MoO3 single crystal has been determined. It is found that the semiconductor gap of the single crystal is 1320K(0.11eV). The threshold electric field ET is ０.1２９Ｖ／ｃｍ derived from the data of nonlinear conductivity at T=77K．ＤＳＣ study of the samples shows that the K0.3MoO3 single crystal undertakes a new Peierls phase transition around 240K and that the phase transition is of first order. The Δｃp-Ｔ curve shows that the phase transition around T=180K is a combination of a second order and a first.

ChemInform Abstract: Surface Phases Formed by Sulfidation of MoO3 Single Crystals: Characterization and Catalytic Activity in Hydrodesulfurization

Abstract(HDS).

ESR investigations of defects in MoO3 single crystals

We have investigated ESR centres in MoO3 single crystals as a function of the different methods of crystal growth. The best-investigated centre involves two Mo ions and one hydrogen nucleus.

ESR and optical absorption of rhenium in MoO3

From MoO3 single crystals doped with Re the authors observed ESR to be attributed to Re6+ (5d1). The spectra are described by the following spin-Hamiltonian parameters: ga=1.617+or-0.002, gb=1.606+or-0.002, gc=1.667+or-0.002; Aa=330+or-5)*10-4 cm-1, Ab=687+or-5)*10-4 cm-1, Ac=(370+or-5)*10-4 cm-1. Quadrupole interaction parameters, determined from forbidden lines, are P=60*10-4 cm-1, eta =0.21. The same specimens show broad optical absorption bands for E//c and E//a. This absorption correlates with the Re content of the crystals, but not to the Re6+ ESR intensity. The authors tend to ascribe it to Re6+-Vo or Re5+-Vo, Vo being an oxygen vacancy.

Response of the Electrical Resistance of Molybdenum Oxide Single Crystals to Hydrogen in Air

The electrical resistance of MoO3 single crystals activated with platinum was measured in air and in air containing 1% molar hydrogen concentration over the temperature region of 500–700 °K. The electrical resistance of the single crystal in air at 653 °K was about 108 Ω and decreased by a factor of 30 when exposed to the hydrogen atmosphere. These results strongly suggest that MoO3 may be used to monitor hydrogen concentrations where possible hazardous concentrations may occur.

Charge Compensation of Impurities and Electrical Properties of MoO3

AbstractAn investigation was made of the EPR spectra of MoO3 single crystals grown in air and in argon. In the crystals grown in air and not specially doped no EPR signal was found. For crystals grown in argon a spectrum is observed due to electrons localized on two different sites of the lattice. This spectrum shows a hfs from a nuclear spin I = 1/2. The spin‐Hamiltonian constants are: S = 1/2, I = 5/2 gx = 1.942, gx = 1.953, gx = 1.878, Ax = 32 G, Ay = 69 G. The operator of the hyperfine interaction is of the form equation image The splitting of the transitions with ma ≠ mb is equal to 1/3 A. The splitting by the nuclear spin I = 1/2 is nearly isotropic 3 to 4 G. The same spectrum is observed in air‐grown crystals after irradiation with a quartz‐mercury lamp. In copper doped crystals grown in argon the copper is found to be in the state 3d10. After heating the crystal in air the Cu2+ (3d9) spectrum is observed. The constants of the spin Hamiltonian are: S = 1/2, I = 3/2 gx = 2.087, gy = 2.054, gz = 2.416, Ax = 6 G, Ay = 22.5 G, Az = 93.5 G, Qx = 2.9 G, Qy = 3.0 G, Qz = 5.9 G. Suppositions are made about a reason for the unstability of the electrical conductivity of MoO3 according to the data obtained.