Non-stoichiometric Molybdenum Oxides Research Articles

Synthesis and photothermal performance of non-stoichiometric molybdenum oxide (MoO3-x) prepared by gamma radiation.

This work reports the interaction of γ-rays with MoO3 in several solvents to obtain non-stoichiometric (sub-oxide) MoO3-x through a one-pot synthesis. The effect of different doses of γ-radiation (30-90 kGy) with different protic solvents (water, N,N-dimethylformamide and formic acid) was investigated. Structural modifications were characterized by X-ray diffractometry and scanning electron microscopy while optical properties were investigated by UV-Vis spectroscopy. The analysis of the highly intense (020), (040) and (060) diffraction peaks suggests that there is a reduction in water and formic acid allowing the formation of MoO3-x nanosheets. Additionally, the photothermal response of MoO3-x obtained under different conditions was characterized, and a possible mechanism to explain the interaction of γ-rays with solvents and the oxide structure by oxidative/reductive processes in the solution was proposed.

Photo-thermo catalytic selective oxidation of cyclohexane by In-situ prepared nonstoichiometric Molybdenum oxide and Silver-palladium alloy composite

The highly selective oxidation of cyclohexane to cyclohexanone and cyclohexanol (KA oil) is one of the most challenging issues in the chemical industry. However, the difficulty in attaining high selectivity and high conversion rate in parallel for the existing catalysts limits its practical application. In this paper, a novel photo-thermo synergistic catalyst was reported for the aerobic oxidation of cyclohexane. The uniform blue MoO3-x nanowires with small diameter stabilized by polyvinyl pyrrolidone (PVP) were synthesized by a hydrothermal method, and a series of MoO3-x-AgPd composite materials of different proportions were prepared by an in-situ reduction process. The morphology, crystalline structure, surface chemical bonding, photoelectrochemical properties of MoO3-x-AgPd composites are thoroughly characterized. The MoO3-x-AgPd composites present significantly increased catalytic performance than MoO3-x nanowires in the photo-thermo synergistic catalytic oxidation of cyclohexane under dry air. The high conversion rate of 11.3% with the KA oil selectivity of 99.0% was achieved by the MoO3-x-Ag20Pd20 composites under photo-thermo catalytic process at 120 ℃, which is 1.5 times of that by MoO3-x nanowires. Under photo-thermo catalytic process, a high cyclohexane conversion rate similar to that of higher temperature thermal catalysis can be obtained at lower reaction temperature, and more cyclohexanol can be produced with a ketone to alcohol (K/A) ratio of 0.254. The significantly enhanced catalytic activity can be attributed to the effective charge transfer in the AgPd alloy nanoparticles, the optimized band gap structure, the suppressed charge recombination, and the promoted photo-thermo synergetic catalytic effect. This work provides a new reference scheme for the design and preparation of high-efficiency photo-thermo catalysts for the selective oxidation of cyclohexane.

Fabrication of graphitic carbon Nitride/Nonstoichiometric molybdenum oxide nanorod composite with the nonmetal plasma enhanced photocatalytic hydrogen evolution activity

The extended light absorption and the prevented charge recombination are crucial for the graphitic carbon nitride (g-C3N4) based photocatalytic materials. Herein, nonstoichiometric molybdenum oxide (MoO3-x) nanorods with oxygen vacancies were synthesized by a hydrothermal method with trace amount of oleylamine, and the Z-scheme two-dimentional (2D)/one-dimentional (1D) g-C3N4/MoO3-x composites were prepared by a facile electrostatic assembling approach. The blue MoO3-x nanorods with oxygen vacancies are loaded uniformly on the g-C3N4 nanosheets. The g-C3N4/MoO3-x composite materials exhibit strong absorption in the visible and near-infrared light regions, and the improved charge separation efficiency through the Z-scheme charge transfer mechanism. The g-C3N4/MoO3-x composite presents a significantly improved photocatalytic hydrogen generation activity with good cycling stability compared with sonicated g-C3N4 nanosheets. The best hydrogen generation activity of 209.2 μmol·h−1 under solar light irradiation and the highest apparent quantum efficiency of 4.4% irradiated at 365 nm are obtained by the g-C3N4/MoO3-x composite with a mass percent of 27.5%, which is 2.63 times of g-C3N4. The weight ratios and the content of oxygen vacancies in the small-size MoO3-x nanorods have a significant influence on the photocatalytic hydrogen performance. Moreover, effective photocatalytic overall water splitting can be achieved with the H2 and O2 evolution rates of 0.755 and 0.368 μmol∙h−1 by the g-C3N4/MoO3-x composite. The novel g-C3N4/MoO3-x composite will have broad prospects in the field of photocatalytic applications.

СІНТЕЗ І ФОТОКАТАЛІТИЧНІ ВЛАСТИВОСТІ КОМПОЗИЦІЙНИХ ПОКРИТТІВ НА ОСНОВІ КОБАЛЬТУ З ТУГОПЛАВКИМИ КОМПОНЕНТАМИ

The possibility of electrosynthesis and control of the surface composition and morphology of the electrolytic cobalt coatings with refractory metals by varying the parameters of electrolysis has been proved. It was found that oxygen and carbon are included in the composition of the coatings as well as the main components, thus such systems can be considered as composite. The coatings deposited by pulsed current can be considered as composite materials the oxide phase for which is formed directly in the electrode process as an intermediate of incomplete reduction of tungstates and hydrolysis of zirconium (IV) salts. The topography of the films is distinguished by the presence of elliptical and spherical grains with crystallite sizes of 80 - 180 nm. On the surface of the coatings, there are hills (large grains) with a diameter of 1 - 3 μm. The fractal dimension of the surface is 2.77, which indicates the 3D mechanism of crystal growth during the formation of coatings. In terms of phase composition, composites are predominantly amorphous materials that contain nanocrystalline cobalt and the intermetallic compound Co3W and Zr3Co. The study of the morphology and topography of the composite coatings surface, as well as its quantitative and phase composition, indicates the possibility of photocatalytic activity of the Co-Mo-WOх, Co-Mo-ZrO2 and Co-W-ZrO2 coatings. Investigation of the photodegradation of the azo dye methyl orange found that the efficiency of MO removal from the solution was 24%, 18%, and 10% for 30 min of ultraviolet irradiation in the presence of Co-Mo-WOх, Co-Mo-ZrO2 and Co-W-ZrO2 on composite coatings, respectively. The higher photoactivity of Co-Mo-WOx composite coatings can be explained by the presence of non-stoichiometric molybdenum and tungsten oxides.

Plasma-liquid synthesis of MoOx and WO3 as potential photocatalysts.

Plasmas in contact with liquids represent a green chemistry method for the synthesis of metal oxides. In this work, underwater plasma was used for the synthesis of molybdenum and tungsten oxides. The obtained samples were analyzed by various techniques. Results showed that underwater plasma with Mo electrodes allows obtaining non-stoichiometric molybdenum oxide (MoOx). In the case of tungsten electrodes, monoclinic WO3 was formed. The synthesized oxides have a wide band gap (3.21 eV for MoOx and 3.27 eV for WO3). The photocatalytic and sorption activities of the synthesized oxides towards the decomposition of cationic and anionic dyes (Methylene Blue, Rhodamine B, and Reactive Red 6C) were studied. MoOx shows excellent photocatalytic performance under UV and visible light irradiation. The photocatalytic activity of WO3 under visible light is less than that under UV irradiation.

Assembly of nonstoichiometric molybdenum oxide on Si as p-n junction photocathode for enhanced hydrogen evolution

We propose the utilization of a compositionally tuned MoOx catalyst on Si photocathode for the enhanced hydrogen production and improved charge separation. The efficient MoOx catalyst stoichiometry on Si photocathode was optimized by controlling the deposition gas environment and the target during RF magnetron sputtering. The optimized MoOx stoichiometry on Si photocathode exhibited one of the highest photocurrent density of ∼-30 mA cm−2 at -0.5 V vs RHE under AM 1.5 G illumination. Importantly, the deposition of MoOx on p-Si photocathode causes a significant cathodic shift in the overpotential, indicating the catalytic effect of MoOx. The valence band edge and electrochemical studies were used to analyze the band edge positions with respect to water redox potentials and the formation of p-n junction system. The results of this work demonstrate the advantage of employing a sulfur-free and oxygen-deficient MoOx catalyst on p-Si in p-n junction configuration for the efficient PEC water splitting process.

MALDI-TOF Mass Spectrometry of Nanosized MoO2. Structure and Relative Stability of Isomers of Lower Molybdenum Oxide Cations

MALDI-TOF was used to study molybdenum dioxide (MoO2) containing a nanosized fraction. The composition of cationic clusters of nonstoichiometric lower molybdenum oxides in the gas phase was determined, and the thermodynamic stabilities and configurations of isomers were calculated for selected symmetric molecular structures and for cations MoSO 8 + and Mo5O 9 + . Molecular orbital analysis was performed for two trigonal-bipyramidal clusters Mo5O8 and Mo5O9. Changes in molybdenum–molybdenum interatomic distances in going from MoO 8 + and Mo5O 9 + cations to neutral clusters are discussed.

The study of the tribological, thermal and physical properties of phenylone C-2 based composites containing nonstoichiometric molybdenum and tungsten oxides

The given paper lays bare the preparation and study of multifunctional tribocomposites with matrix based on Phenylone C-2 and the antifriction fillers. Besides, it contains a comparative evaluation of physical, mechanical, tribological and thermophysical properties of multifunctional tribocomposites to assess the effect of nanodimensional fillers.

Symmetric cage structures of isomers of nonstoichiometric lower molybdenum oxides

Molybdenum dioxide (Mo2) synthesized through thermal decomposition of the [MoO2(i-C3H7NHO)2] complex and containing a nanosized fraction has been studied by MALDI-TOF. The composition of clusters of lower nonstoichiometric molybdenum oxide cations in the gas phase has been determined, and theoretical calculations of the thermodynamic stability and structure of the isomers of the most symmetric structures have been performed. Molecular orbitals of the cationic and neutral clusters Mo5O8 and Mo5O9 have been examined, and the aromaticity of the Mo-Mo bonds in metallacycles has been demonstrated.

Photoinduced conversion of carbon dioxide and water molecules to methanol on the surface of molybdenum oxide MoO x (x < 2)

X-ray and UV photoelectron spectroscopic data are used to demonstrate that, when pulsed laser light with a photon energy of 6.4 eV acts on the surface of nonstoichiometric molybdenum oxide MoO x (x < 2), methanol is effectively formed from adsorbed molecules of carbon dioxide and water. The processes in which CO2 and H2O molecules are adsorbed on substrate surface defects and their bonds are activated, enhanced under the effect of photons, should be regarded as the key factors.

MOLYBDENUM TRIOXIDE THIN FILMS DOPED WITH GOLD NANOPARTICLES GROWN BY A SEQUENTIAL METHODOLOGY: PHOTOCHEMICAL METAL-ORGANIC DEPOSITION (PMOD) AND DC-MAGNETRON SPUTTERING

Gold nanoparticles (AuNPs) were deposited by DC-magnetron sputtering onto molybdenum trioxide (MoO3) thin films grown by Photochemical Metal- Organic Deposition (PMOD) on Si(100) and borosilicate glass substrates. The chemical, optical and morphology properties of the films were studied by UV/ Vis Spectroscopy, Scanning Electron Microscopy (SEM), X-Ray Photoelectron Spectroscopy (XPS), and X-Ray Diffraction (XRD). SEM revealed that AuNPs formed after 5 s of sputtering. AuNPs are spherical and have both an average diameter of 18 nm and a relatively narrow size distribution. As the deposition time increases, larger structures are formed by an aggregation of AuNPs. XPS studies of the AuNP/MoO3 films on Si(100) showed the presence of Mo(VI) and Mo(V), which indicated that the films were primarily non-stoichiometric molybdenum oxides. The occurrence of oxygen vacancies in the substrate play an important role to stabilize the AuNPs.

Synthesis and characterization of thin molybdenum oxide films prepared from molybdenum dioxo tropolonate precursors by photochemical metal-organic deposition (PMOD) and its evaluation as ammonia gas sensors

Amorphous non-stoichiometric molybdenum oxide thin films have been successfully prepared by direct UV irradiation of amorphous films of molybdenum dioxo tropolonate complexes on Si(100) substrates. Photodeposited films were characterized by X-ray photoelectron spectroscopy (XPS) and the surface morphology examined by Atomic Force Microscopy (AFM). It was found that as-photodeposited films are uniform with rms surface roughness of 231nm and contain non-stoichiometric oxides (MoO3−x). The results of XRD analysis showed that post-annealing of the films in air at 350°C transforms the sub-oxides to α-MoO3 phase with a much smoother surface morphology (rms=164nm). Both, the as-photodeposited and annealed MoO3−x films exhibit good optical quality with transparency in the visible region better than 80%. Gas sensing tests reveal that annealed MoO3−x films exhibit a better response than as-deposited films towards 50ppm ammonia at an operating temperature of 350°C.

MoOx thin films deposited by magnetron sputtering as an anode for aqueous micro-supercapacitors

In order to examine the potential application of non-stoichiometric molybdenum oxide as anode materials for aqueous micro-supercapacitors, conductive MoOx films (2  x  2.3) deposited via RF magnetron sputtering at different temperatures were systematically studied for composition, structure and electrochemical properties in an aqueous solution of Li2SO4. The MoOx (x ≈ 2.3) film deposited at 150 °C exhibited a higher areal capacitance (31 mF cm−2 measured at 5 mV s−1), best rate capability and excellent stability at potentials below −0.1 V versus saturated calomel electrode, compared to the films deposited at room temperature and at higher temperatures. These superior properties were attributed to the multi-valence composition and mixed-phase microstructure, i.e., the coexistence of MoO2 nanocrystals and amorphous MoOx (2.3 < x  3). A mechanism combining Mo(IV) oxidation/reduction on the hydrated MoO2 grain surfaces and cation intercalation/extrusion is proposed to illustrate the pseudo-capacitive process.

Mechanochemical preparation of vanadium- and molybdenum-containing catalysts II: The effect of mechanochemical activation of vanadium pentoxide + ammonium dimolybdate compositions on chemical and phase transformations

We studied the effect of mechanochemical treatment (MCT) of V2O5 + (NH4)2Mo2O7 compositions (V: Mo = 0.7: 0.3) in ethanol, water, and air on the physicochemical properties of the compositions. X-ray powder diffraction, differential thermal analysis (DTA), and IR spectroscopy showed that mechanochemical treatment in water or ethanol does not change the phase state of vanadium pentoxide. (NH4)2Mo2O7 partially decomposes during MCT to yield nonstoichiometric molybdenum oxides. MCT in water leads to the complete decomposition of (NH4)2Mo2O7, and the nonstoichiometric molybdenum oxides that have been formed in 60–120 min are segregated into a molybdenum phase during further treatment for 240–360 min. During such a treatment, V2O5 first forms V2O5 · nH2O intercalation compounds, which then react with ammonia during long-term treatment to form ammonium hexavanadate (AHV).

Electrolytic molybdenum oxides in lithium batteries

Nonstoichiometric molybdenum oxides (e-MoxOy) were synthesized by cathodic reduction of aqueous ammonium and sodium molybdate solutions. Surface morphology of electrolytic (e) deposits, the chemical composition, crystal lattice structure, and the characteristics of electrochemical Li+ intercalation for such synthesized oxides were determined by the cation composition of molybdate solution and the conditions of deposit annealing. The electrochemical intercalation of Li+ ions in these Mo-oxides was investigated in thin-layer “ballast-free” electrodes, as a pasted соmposite cathode in lithium batteries, and as an anode in lithium-ion batteries, with liquid organic and polymer electrolytes. The reversible discharge capacity of e-Mo4O11 synthesized from ammonium molybdate electrolyte in thin-layer ballast-free electrodes can exceed 225 mAh g−1 for more than 170 cycles.

Anodic oxidation of free nitric oxide at gold electrodes modified by a film of trans-[Ru(III)(NH 3) 4(SO 4)4pic] + and molybdenum oxide

Gold surfaces were modified with an electrochemically deposited layer of non-stoichiometric molybdenum oxides. At these surfaces, trans-[Ru(III)(NH 3) 4(SO 4)4pic] + complex was incorporated in a controlled way by cycling the potential consecutively in the range +0.50 to −0.25 V at pH 2.6. Very reproducible voltammetric curves corresponding to the electrochemical process of the ruthenium complex were obtained, confirming the immobilisation of the material into the molybdenum oxide film. The anodic oxidation of nitric oxide (NO) at pH 7.4 was investigated at the modified electrode containing the molybdenum oxide+ trans-[Ru(III)(NH 3) 4(SO 4)4pic] + complex and an enhancement in the current response was observed compared with the signal at a bare electrode. The rate for NO electrochemical oxidation was dependent on the amount of catalytic ruthenium sites dispersed into the molybdenum oxide film. A linear relationship between current signals measured by square wave voltammetry and NO concentration was obtained in the 0–10 μM range. The applicability of the modified electrode as a sensor for real-time NO monitoring was also demonstrated.

Interaction of oxygen with Mo(100), Mo(110), and Mo(111) surfaces. RHEED and AES analyses of the molybdenum oxide nucleation and growth

A study of the nucleation and growth of MoO 3 generated by interaction of oxygen with Mo(100), Mo(110), and Mo(111) single crystalline surfaces is investigated at high oxygen pressure (10 4 Pa) and low temperature (620 to 820 K). The results of RHEED and AES analyses prove that under these oxidation conditions, MoO 3 nucleates directly from the metal without intermediate formation of MoO 2 or nonstoichiometric molybdenum oxide such as Mo 4O 11. The structure and orientation of MoO 3 nuclei are characterized in relation with the parent molybdenum surface. On the Mo(110) and Mo(111) surfaces, which are faceting, the nucleation and growth of MoO 3 takes place by successive structural steps. On the Mo(100) surface, epitaxial relations induce the formation of MoO 3(010) plates oriented onto the Mo surface, which stabilizes and makes passive the Mo(100) surface. The features of the nucleation and growth of MoO 3 are interpreted as linked respectively to structural relations of the metal-oxide interface and to preferential diffusion paths of oxygen through MoO 3.

Superficial oxidation of molybdenum at high pressure and low temperature: RHEED and AES analyses of the molybdenum oxide formation

Numerous studies relate to the interaction of the molybdenum surface with oxygen at low pressure and high temperature. They give results about oxygen chemisorption, surface facetting and the epitaxial formation of MoO2 crystallites. This work deals with the interaction of Mo(100), Mo(110) and Mo(111) surfaces with oxygen at high pressure (104 Pa) and low temperature (620–820 K). RHEED and AES analyses results prove that, in these oxidation conditions: MoO2 and non-stoichiometric molybdenum oxide such as Mo4O11 are not evidenced in any of the molybdenum oxidation steps. The MoO3 phase nucleates directly from any Mo surface. The structure and orientation of MoO3 nuclei are characterized in relation with the parent molybdenum surface. Some MoO3 nucleation and growth mechanisms are proposed, involving the specific properties of the MoO3 phase and the structure of molybdenum surfaces.

Thermal decomposition of molybdenum(V) thioglycolate 2-hydrate

The thermal decomposition of molybdenum(V) thioglycolate, having the composition [Mo 2(L) 5(H 2O) 2] (H 2L = CH 2SHCOOH, thioglycolic acid), has been studied in argon, air and oxygen atmospheres in the 300–900 K region by TG and DTA methods. The results indicate a simultaneous gradual dissociation of ligand along with the removal of water molecules, without exhibiting any distinct steps in the TG curve. It is interesting to find that the compound decomposes in argon atmosphere to amorphous Mo metal as an end product at about 700 K, which was identified from its X-ray photoelectron and EPR spectra (conduction electrons), and XRD. The end product obtained in air or oxygen atmospheres appears to be a non-stoichiometric molybdenum oxide. A scheme for the thermal dissociation of the compound in argon atmosphere is discussed by combining the TG and DTA data.

Onset of oxidation of the non-stoichiometric molybdenum oxide Mo 18O 52(100) surface at low temperatures: A study by RHEED

Mo 18O 52(100) surface oxidation was investigated at constant temperature (670 K) under an oxygen pressure of about 3 × 10 3 Pa for increasing time up to a few tens of hours. Three oxidation stages were evidenced by RHEED analysis and interpreted by associating RHEED results and SEM observations as a three steps oxidation mechanism: the first step concerns chemisorption of oxygen onto CS plane boundaries, the second one leads to an epitaxial growth of crystallites within the MoO 3 structure including randomly spaced CS planes, the third oxidation step proceeds from a sublimation phenomenom and leads to the spread of epitaxial MoO 3 films on the whole surface. The proposed oxidation mechanism involves mobility of the CS planes, hyperreactivity of the boundary of the layer structure and also increasing mobility of (MoO 3) n species in equilibrium on the oxide surface; these fundamental features of the reaction paths in molybdenum oxides broaden the knowledge of non-stoichiometric oxides of transition metals and are particularly related with the catalytic behaviour of MoO 3.