Growth and characterization of molybdenum oxide thin films prepared by photochemical metal–organic deposition (PMOD)

Molybdenum oxide thin films have been successfully prepared by direct UV irradiation of amorphous films of a molybdenum dioxide acetylacetonate complex on Si(1 0 0) substrates. Photodeposited films were characterized by X-ray photoelectron spectroscopy (XPS) and X-ray diffraction (XRD) and the surface morphology examined by Atomic Force Microscopy (AFM). It was found that as-photodeposited films are uniform and smooth, with thickness of 350 nm, with rms surface roughness of 28 nm and contain non-stoichiometric oxides (MoO_3−x). The results of XRD analysis showed that post-annealing of the films in air at 450 °C transforms the sub-oxides to α-MoO₃ phase with a much rougher surface morphology (rms = 144 nm). The as-photodeposited MoO_3−x films are amorphous, and exhibit better optical quality than annealed films.     

Deposition and dielectric characterization of strontium and tantalum-based oxide and oxynitride perovskite thin films

We have synthesized the composition x = 0.01 of the (Sr_1-xLa_x)₂(Ta_1-xTi_x)₂O₇ solid solution, mixing the ferroelectric perovskite phases Sr₂Ta₂O₇ and La₂Ti₂O₇. Related oxide and oxynitride materials have been produced as thin films by magnetron radio frequency sputtering. Reactive sputter deposition was conducted at 750 °C under a 75 vol.% (Ar) + 25 vol.% (N₂,O₂) mixture. An oxygen-free plasma leads to the deposition of an oxynitride film (Sr_0.99La_0.01) (Ta_0.99Ti_0.01)O₂N, characterized by a band gap E_g = 2.30 eV and a preferential (001) epitaxial growth on (001) SrTiO₃ substrate. Its dielectric constant and loss tangent are respectively Epsilon' = 60 (at 1 kHz) and tanDelta = 62.5 × 10⁻³. In oxygen-rich conditions (vol.%N₂ ≤ 15%), (110) epitaxial (Sr_0.99La_0.01)₂(Ta_0.99Ti_0.01)₂O₇ oxides films are deposited, associated to a larger band gap value (E_g = 4.55 eV). The oxide films permittivity varies from 45 to 25 (at 1 kHz) in correlation with the decrease in crystalline orientation; measured losses are lower than 5.10⁻³. For 20 ≤ vol.% N₂ ≤ 24.55, the films are poorly crystallized, leading to very low permittivities (minimum Epsilon' = 3). A correlation between the dielectric losses and the presence of an oxynitride phase in the samples is highlighted.     

Defect structures in the brannerite-type vanadates: VI. Preparation and study of Co1−x∅xV2−2xMo2xO6 and phase diagram of the ternary CoO-V2O5-MoO3 system

The phase diagram of the ternary CoO-V₂O₅-MoO₃ system and in particular its T-CoV₂O₆-MoO₃ slice have been determined with DTA and X-ray phase analysis. CoV₂O₆ crystallizes in two modifications: a low-temperature γ-form of unknown structure and a high-temperature α-form of brannerite-type structure. The transition temperature is 660-665°C. The γ ? α transformations are very slow and the α-polymorph may be easily frozen. On doping with MoO₃, a solid solution is formed that is described by the formula Co_1?x?_xV_2?2xMo_2xO₆. Above x = 0.02 the α-type structure is stabilized. The x_max equals 0.22 at the eutectic temperature of 620°C and 0.20 at room temperature. Other features of the phase diagram, including its division into the natural subdiagrams and three ternary eutectics, are described in detail. X-Ray data are listed for α-CoV₂O₆ and for the solid solution having x = 0.20. On doping with MoO₃ the monoclinic lattice dilates primarily in the direction of the b-axis.     

Preparation and Characterization of Molybdenum Oxide Thin Films by Sol-Gel Process

This paper presents a new sol-gel process to prepare molybdenum oxide thin films. A molybdenum acetylacetonate sol was prepared by employing the system CH₃COCH₂COCH₃/MoO₃/C₆H₅CH₃/HOCH₂CH₂OCH₃. A molybdenum acetylacetonate gel was prepared by addition of aqueous NH₃. Thermal gravimetry (TG) and differential thermal analyses (DTA) of the gel suggested that crystallization of MoO₃ occurs in a 140 K temperature range around 508°C. MoO₃ films were prepared on fused silica, Si (111) and Al₂O₃ (012) substrates by annealing spin coating films of the sol in oxygen environment at 508°C. X-ray diffraction (XRD) showed that all films crystallize in -MoO₃ structure, and crystallites on fused silica substrate are arbitrarily oriented while those on Si (111) and Al₂O₃(012) substrates oriented in the (010) direction. SEM investigations showed that MoO₃ grains of all films are randomly distributed, with a longitudinal dimension of about 1–5 m and the film thickness is about 1 m.     

Liquid phase deposition synthesis of hexagonal molybdenum trioxide thin films

Hexagonal molybdenum trioxide thin films with good crystallinity and high purity have been fabricated by the liquid phase deposition (LPD) technique using molybdic acid (H₂MoO₄) dissolved in 2.82% hydrofluoric acid (HF) and H₃BO₃ as precursors. The crystal was found to belong to a hexagonal hydrate system MoO₃.nH₂O (n~0.56). The unit cell lattice parameters are a=10.651 Å, c=3.725 Å and V=365.997 Å³. Scanning electron microscope (SEM) images of the as-deposited samples showed well-shaped hexagonal rods nuclei that grew and where the amount increased with increase in reaction time. X-ray photon electron spectroscopy (XPS) spectra showed a Gaussian shape of the doublet of Mo 3d core level, indicating the presence of Mo⁶⁺ oxidation state in the deposited films. The deposited films exhibited an electrochromic behavior by lithium intercalation and deintercalation, which resulted in coloration and bleaching of the film. Upon dehydration at about 450 °C, the hexagonal MoO₃.nH₂O was transformed into the thermodynamically stable orthorhombic phase.     

X-ray Photoelectron Spectroscopy and Time-Of-Flight Secondary Ion Mass Spectroscopy studies of electrodeposited molybdenum oxysulfide cathodes for lithium and lithium-ion microbatteries

Thin-film molybdenum oxysulfide cathodes for lithium and lithium–ion microbatteries were fabricated by a simple electrodeposition method. According to Scanning Electron Microscopy (SEM) data, the deposition parameters affect the morphology of the cathodes. X-ray diffraction (XRD) tests indicated that the sub-micron-thick molybdenum oxysulfide films are amorphous or form too small crystallites to give rise to detectable X-ray diffraction peaks. A variety of poly-ion clusters containing both oxygen and sulfur (like MoOS, MoO₂S and MoS₂O and others) detected by TOF-SIMS tests unambiguously indicates the formation of molybdenum oxysulfide compounds, and not a mixture of oxides and sulfides, during electrodeposition. The sulfur-to-oxygen ratio in the bulk of the deposit is about 1.76 and does not depend much on the electrodeposition parameters. XPS studies reveal that electrodeposition in unbuffered solutions produces deposits with high oxygen and low sulfur content, as compared with cathodes deposited in buffered solutions. Potentiostatic, as compared to galvanostatic deposition, is followed by the formation of cathode films with slightly higher sulfur and lower oxygen content at the same pH. An increase in the pH of electrolyte solutions from 8 to 9.5 slightly reduces sulfur content, but appreciably increases oxygen concentration. Charge–discharge overpotential of Li/hybrid polymer electrolyte microbatteries is lower in sulfur-rich MoO_xS_y cathodes.     

Hydrothermal synthesis of one-dimensional α-MoO3 nanomaterials and its unique sensing mechanism for ethanol

In gas sensor applications, the availability of highly sensitive and rapid response/recovery detector for ethanol gas is sparse. One-dimensional orthogonal crystalline molybdenum trioxide nanomaterials were synthesized by an economical and environmentally friendly hydrothermal method. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and energy spectroscopy (EDS) were used to investigate the structure and morphology of the nanometer materials. The relevant characterization shows that nanobelts are highly crystalline layered structures with a width of about 200 nm and a length of a few micrometers. The synthesized ethanol gas sensors based on α-MoO₃ semiconductor material show the highest response at 350 °C. Gas sensitivity tests indicated that α-MoO₃ nanobelts respond well to 50 ～ 600 ppm ethanol at optimal operating temperatures. The selectivity test among various reducing gases shows that the sensor responds better to ethanol compared to other gases such as xylene, NO₂, CO, and H₂ gases. This excellent sensing performance is attributed to the unique sensing mechanism formed in the layered MoO₃ nanobelts through the catalytic reaction between ethanol and MoO₃ lattice oxygen and adsorbed oxygen. The sensing mechanism of the co-catalytic effect of lattice oxygen and adsorbed oxygen on ethanol is also discussed in depth.     

Glass-ceramic nanocomposites in the [(1 − x)PbO–xBaO]–Na2O–Nb2O5–SiO2 system: Crystallization and dielectric performance

The crystallization process, microstructure and dielectric properties of [(1 − x)PbO–xBaO]–Na₂O–Nb₂O₅–SiO₂ (PBNNS) (0 ≤ x ≤ 1) glass-ceramics prepared by controlled crystallization were investigated. The crystallization strategies for acquiring nano-crystallized PBNNS glass-ceramics were monitored by differential thermal analysis (DTA). X-ray diffraction (XRD) analysis revealed a major crystal phase transition in PBNNS glass matrix as the crystallization temperature increased. At low temperatures (700–750 °C), the major crystal phases precipitating in the glass matrix are identified as Pb₂Nb₂O₇ for x = 0, Ba₂NaNb₅O₁₅ for x = 1 and their solid solution for 0.2 ≤ x ≤ 0.8; while at higher temperatures (≥850 °C), heat treatment produces different crystalline phases, PbNb₂O₆ and NaNbO₃ for x = 0, Ba₂NaNb₅O₁₅ and NaNbO₃ for x = 1, and the solid solution of these three phases for 0.2 ≤ x ≤ 0.8. Corresponding to the result of phase transition, microstructural observation proves increasing crystallite sizes with increasing temperature of heat treatment. At different crystallization temperatures, the dielectric properties of the [(1 − x)PbO–xBaO]–Na₂O–Nb₂O₅–SiO₂ glass-ceramics show a strong dependence on the chemical composition x. At low temperatures (700–750 °C), a maximum of the dielectric constant of the PBNNS glass-ceramic is found for the composition x = 0.6; while at higher crystallization temperatures (≥850 °C), the dielectric constants of all samples (0 ≤ x ≤ 1) exhibit decreasing values with increasing x.     

Acetalization of glycerol using mesoporous MoO3/SiO2 solid acid catalyst

Acetalization of glycerol with various aldehydes has been carried out using mesoporous MoO₃/SiO₂ as a solid acid catalyst. A series of MoO₃/SiO₂ catalysts with varying MoO₃ loadings (1–20 mol%) were prepared by sol–gel technique using ethyl silicate-40 and ammonium heptamolybdate as silica and molybdenum source respectively. The sol–gel derived samples were calcined at 500 °C and characterized using various physicochemical characterization techniques. The XRD of the calcined samples showed the formation of amorphous phase up to 10 mol% MoO₃ loading and at higher loading of crystalline α-MoO₃ on amorphous silica support. TEM analyses of the materials showed the uniform distribution of MoO₃ nanoparticles on amorphous silica support. Raman spectroscopy showed the formation of silicomolybdic acid at low Mo loading and a mixture of α-MoO₃ and polymolybdate species at high Mo loadings. Moreover the Raman spectra of intermediate loading samples also suggest the presence of β-MoO₃. Acetalization of glycerol with benzaldehyde was carried out using series of MoO₃/SiO₂ catalysts with varying MoO₃ loadings (1–20 mol%). Among the series, MoO₃/SiO₂ with 20 mol% MoO₃ loadings was found to be the most active catalyst in acetalization under mild conditions. Maximum conversion of benzaldehyde (72%) was obtained in 8 h at 100 °C with 60% selectivity for the six-membered acetal using 20% MoO₃/SiO₂. Interestingly with substituted benzaldehydes under same reaction conditions the conversion of aldehydes decreased with increase in selectivity for six-membered acetals. These results indicate the potential of this catalyst for the acetalization of glycerol for an environmentally benign process.     

The phase diagram of V<Subscript>2</Subscript>O<Subscript>5</Subscript>-MoO<Subscript>3</Subscript>-Ag<Subscript>2</Subscript>O system

The results concerning the synthesis, structure and thermal properties of V₂O₅-MoO₃-Ag₂O samples in the molybdenum rich region of ternary system are presented in the form of quasi-binary systems: β-AgVO₃-β-Ag₂MoO₄, AgVMoO₆-MoO₃, AgVMoO₆-Ag₂Mo₄O₁₃, AgVMoO₆-Ag₂Mo₂O₇, AgVMoO₆-β-Ag₂MoO₄ and also of the system in which at V₂O₅/MoO₃ molar ratio 3:7 the content of Ag₂O was variable. The ternary phase AgVMoO₆ was not described earlier in the literature.     

Electrochemical deposition of molybdenum oxide into films of poly(3,4-ethylenedioxythiophene) conducting polymer on glassy carbon substrates

Electrochemical deposition of molybdenum oxides from molybdate-containing solutions onto glassy carbon electrodes and the same electrodes coated with a film of poly(3,4-ethylenedioxythiophene) conducting polymer was studied. The morphology of the deposited molybdenum oxides was examined by scanning electron microscopy. The method of X-ray photoelectron spectroscopy was used to determine the state of molybdenum in surface molybdenum oxides and a conclusion was made that the oxide MoO₂ is mostly present.     

Structural and Mössbauer characterization of the ball milled Fex(Cr2O3)1−x system

The Fe_x(Cr₂O₃)_1?x system, with 0.10 ≤ X ≤ 0.80, was mechanically processed for 24 h in a high-energy ball-mill. In order to examine the possible formation of iron–chromium oxides and alloys, the milled samples were, later, thermally annealed in inert (argon) and reducing (hydrogen) atmospheres. The as-milled and annealed products were characterized by X-ray diffraction, Mössbauer spectroscopy, transmission electron microscopy and magnetization. The as-milled samples showed the formation of an Fe_1+YCr_2?YO_4?δ nanostructured and disordered spinel phase, the α₁-Fe(Cr) and α₂-Cr(Fe) solid solutions and the presence of non-exhausted precursors. For the samples annealed in inert atmosphere, the chromite (FeCr₂O₄) formation and the recrystallization of the precursors were verified. The hydrogen treated samples revealed the reduction of the spinel phase, with the phase separation of the chromia phase and retention of the Fe–Cr solid solutions. All the samples, either as-milled or annealed, presented the magnetization versus applied field curves typical for superparamagnetic systems.     

The oxidation of toluene on transition metal oxides

The oxidation of toluene on pure vanadium and molybdenum oxides was found to follow independent paths; it was benzene ring oxidation on V₂O₅ and side chain oxidation on MoO₃. On mixed xV₂O₅ · yMoO₃ oxides, the main reaction was the addition at the double bond preferably positioned meta rather than one-electron oxidation.     

Spectroscopic,mechanical and magnetic characterization of some bismuth borate glasses containing gadolinium ions

The ultrasonic parameters, the optical parameters along with the IR spectroscopy and magnetic susceptibility studies have been employed to explore the role of Gd₂O₃ in the structure of the glasses xGd₂O₃–60B₂O₃–10MoO₃–(30-x)Bi₂O₃, with 0 ≤ x ≤ 7 mol %. IR analysis indicates that Gd₂O₃ is preferentially incorporated into the borate network-forming BO₄ units. It is assumed that Bi₂O₃ and MoO₃ enter the structure as modifiers in the form of BiO₆ and MoO₆ only. The compositional dependence of the mechanical and the optical parameters are interpreted in terms of the transformation of the structural units BO₃ into BO₄, the increase in the number of bridging oxygen atoms, and the substitution of high bond strength Gd–O, in the place of low bond strength Bi–O bond. The results of the magnetic susceptibility reveal the paramagnetic behavior as described by the Curie-Weiss law and indicating the presence of weak antiferromagnetic exchange interactions between Gd³⁺ ions. The magnetic entropy change of the glasses was determined according to the temperature and magnetic field dependence of magnetization.     

Synthesis and electrochemical characteristics of xLi2MnO3–(1−x)Li(Mn0.375Ni0.375Co0.25)O2 (0.0 ≤ x ≤ 1.0) thin film cathode for lithium rechargeable micro batteries

We have compared the structure, microstructure, and electrochemical characteristics of xLi₂MnO₃–(1−x)Li(Mn_0.375Ni_0.375Co_0.25)O₂ (0.0 ≤ x ≤ 1.0) thin films with their bulk cathode laminate counterparts of identical compositions. Pure Li(Mn_0.375Ni_0.375Co_0.25)O₂ as well as the synthesized composite films partially transform into cubic spinel structure during charge–discharge cycling. In contrast, such layered to spinel phase transformation has only been identified in bulk cathode laminates with x ≥ 0.75. At a current density 0.05 mAcm⁻², the discharge capacity of Li(Mn_0.375Ni_0.375Co_0.25)O₂ thin film was measured to be ∼60 μAhcm⁻². The discharge capacity (∼217 μAhcm⁻²) was markedly improved in x∼0.5 composite thin film. The capacity retention after 20 charge discharge cycles are improved in composite films; however, their capacity fading could not be eliminated completely.     

Band gap bowing of nanocrystalline Zn(1−x)CaxO thin films for blue and ultraviolet optoelectronic applications

Alloying materials having different band gaps is a tool to tailor the optical energy gaps of semiconducting materials. In the present study, the effect of alloying ZnO with CaO was investigated. Thin films of Zn_(1−x)Ca_xO (0 ≤ x ≤ 0.20) were deposited on glass substrates by spray pyrolysis technique. All the films possessed nanocrystalline grains and crystallinity deteriorated with increase in Ca²⁺ substitution level. Elemental composition analysis confirmed the presence of Ca in the samples. Films showed good optical transmission in the visible and near infrared region and the absorption edge blue-shifted with Ca²⁺ substitution. Optical energy gap enhanced by 9.89% upon 20% Ca²⁺ substitution. Photoluminescence analysis also confirmed band gap broadening with mesovalent cation substitution.     

Controllable thermal expansion and phase transition in Yb2−xCrxMo3O12

The thermal expansion and phase transition of solid solutions Yb_2?xCr_xMo₃O₁₂ have been investigated by X-ray powder diffraction and differential thermal analysis. The XRD patterns and the results of Rietveld refinement of Yb_2?xCr_xMo₃O₁₂ indicate that the solid solution limit was in the composition range of 0.0 ≤ x ≤ 0.4 and 1.7 ≤ x ≤ 2.0. Yb_2?xCr_xMo₃O₁₂ (0.0 ≤ x ≤ 0.4) has an orthorhombic structure and exhibits negative thermal expansion between 200 °C and 800 °C. Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) crystallizes in monoclinic below the phase transition and above, transforms to orthorhombic. Both monoclinic and orthorhombic compounds Yb_2?xCr_xMo₃O₁₂ (1.7 ≤ x ≤ 2.0) present positive thermal expansion. Orthorhombic Yb_2?xCr_xMo₃O₁₂ exhibit anisotropic thermal expansion with the contraction of a and c axes, and the linear thermal expansion coefficients range from negative to positive with increasing chromium content. Partial substitution of Yb³⁺ for Cr³⁺ exhibits depressed monoclinic to orthorhombic phase transition.     

Hierarchically structured MoO2/dopamine-derived carbon spheres as intercalation electrodes for lithium-ion batteries

A hydrogen peroxide initiated sol-gel process involving molybdenum transformation in the presence of dopamine (Dopa) hydrochloride excess produced the metastable precipitate composed of polydopamine (PDopa) spheres coated with Dopa preintercalated molybdenum oxide, (Dopa)_xMoO_y@PDopa. The hydrothermal treatment (HT) of the (Dopa)_xMoO_y@PDopa precursor resulted in the simultaneous carbonization of Dopa and molybdenum reduction generating MoO₂ nanoplatelets distributed and confined on the surface of the Dopa-derived carbon matrix (HT-MoO₂/C). The consecutive annealing (An) of the HT-MoO₂/C sample at 600 °C under Ar atmosphere led to the formation of MoO₂ with increased Mo oxidation state and improved structural stability (AnHT-MoO₂/C). Annealing had also further facilitated interaction between the molybdenum-derived and Dopa-derived components resulting in the modification of the carbon matrix confirmed by Raman spectroscopy. Morphology of both materials is best described as Dopa-derived carbon spheres decorated with MoO₂ nanoplatelets. These integrated metal oxide and carbon structures were tested as electrodes for lithium-ion batteries in the potential window that corresponds to the intercalation mechanism of charge storage. The AnHT-MoO₂/C electrode showed enhanced electrochemical activity, with an initial specific discharge capacity of 260 mAh/g and capacity retention of 67% after 50 cycles, compared to the HT-MoO₂/C electrode which exhibited an initial specific discharge capacity of 235 mAh g^?1 and capacity retention of 47% after 50 cycles. The rate capability experiments revealed that the capacity of 93 mAh/g and 120 mAh/g was delivered by the HT-MoO₂/C and AnHT-MoO₂/C electrodes, respectively, when the current density was increased to 100 mA/g. The improved specific capacity, electrochemical stability, and rate capability achieved after annealing were attributed to higher crystallinity of MoO₂, increased oxidation state of Mo, and formation of the tighter MoO₂/carbon contact accompanied by the annealing assisted interaction between MoO₂ and Dopa-derived carbon.     

Electrochromic properties of MoO<Subscript>3</Subscript> thin films derived by a sol–gel process

Molybdenum trioxide (MoO₃) films were deposited on ITO/Glass substrates by the sol–gel method using a spin-coating technique and heat treated at various temperatures under different ambient atmosphere. Effects of the process parameters on the electrochromic properties of MoO₃ films were studied using cyclic voltammetry (CV) in a propylene carbonate (PC) non-aqueous solution containing 1 M lithium perchlorate (LiClO₄). Electrochromic MoO₃ film on lithium intercalation was investigated by in-situ transmittance measurement during the CV process. The MoO₃ films showed reversible recharge ability on Li⁺/e⁻ intercalation/deintercalation. Experimental results revealed that the heat-treatment temperature, the ambient atmosphere, and the thickness will have the string influence on the electrochromic properties of MoO₃ thin films. X-ray diffraction (XRD) results show that the amorphous MoO₃ films can be obtained with the heat-treatment temperature below 300 °C in O₂ ambient atmosphere. The optimum electrochromic MoO₃ film, with a thickness of 130 nm, exhibits a maximum transmittance variation (ΔT%) of 30.9%, an optical density change (ΔOD) of 0.213, an intercalation charge (Q) of 8.47 mC/cm², an insertion coefficient x in Li _x MoO₃ was 0.21 and a coloration efficiency (η) of 25.1 cm²/C between the colored and bleached states at a wavelength (λ) of 550 nm.     

Synthesis,characterization, and dielectric properties of β-Gd<Subscript>2</Subscript>(MoO<Subscript>4</Subscript>)<Subscript>3</Subscript> thin films prepared by chemical solution deposition

A chemical solution was employed for deposition of gadolinium molybdate [β-Gd₂(MoO₄)₃] thin films. Gadolinium acetylacetonate hydrate {[CH₃COCH = C(O–)CH₃]₃Gd·xH₂O}, molybdenum isopropoxide {Mo[OCH(CH₃)₂]₅}, and acetylacetone were used in synthesis of this molybdate. Thermal gravimetry and differential scanning calorimetry suggested that crystallization of β-Gd₂(MoO₄)₃ occurs at around 480 °C. Phase-pure, orthorhombic β-Gd₂(MoO₄)₃ films were deposited on Pt/Ti/SiO₂/Si(100) substrates. β-Gd₂(MoO₄)₃ films crystallized at 750 °C showed a strong (00l) preferred orientation. The film dielectric constant measured was 10~14 and the dielectric loss was less than 3%. There was no marked signature in the permittivity at the bulk Curie temperature, approximately 159 °C.