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A Facile Synthesis for Studying the Effect of Molar Concentration on Structural, Morphological and Optical Behaviour of MoO3 Thin Films using Jet Nebulizer Spray Pyrolysis Technique for n-MoO3/p-Si Photodiode Application
Abstract
In this study, molybdenum trioxide (MoO3) thin films were coated with various Mo concentrations 1, 2, 3 and 4 wt.% referred as L1, L2, L3 and L4 respectively on to the silica substrate calcined at 450 ºC by a facile, rapid cost-effective and a custom-made jet nebulizer spray pyrolysis (JNSP) technique. The concentration impact on the structural, morphological and compositional parameters were examined by XRD, FESEM, EDX, FTIR and Raman studies. The XRD peaks confirmed the presence of the orthorhombic phase of MoO3 (α-MoO3) in the prepared films. FESEM images depicted their nanorod structured morphology. EDX spectra confirm the presence of elements such as oxygen (O) and molybdenum (Mo). Molecular fingerprint of the samples were diagnosed by probing their molecular vibrations through the Raman Spectroscopy. Vibrational frequencies of the bonds in the samples was investigated by FTIR. The I-V measurements were analyzed in light and dark conditions for the n-MoO3/p-Si junction diode.
... Marnadu et al. [27], studied Cu/Ce-WO 3 /p-Si (MIS) UV photodetector with high photosensitivity (P S = 17509.62%). Lavanya et al. [28], investigated the effect of various molybdenum concentrations on the properties of pure MoO 3 thin films using a jet nebulizer spray pyrolysis technique for n-MoO 3 /p-Si photodiode and found poor rectification behavior in dark conditions with a saturation current density of 5.41 × 10 − 5 (A cm − 2 ). Apart from Park et al. [29], who investigated infrared (IR) Si photoelectric devices using reactive sputtering, we found no evidence for fabricating and studying IR photodetection properties of the n-MoO x /p-Si structure using the thermal oxidation method. ...
For the frst time, the molybdenum oxide (MoO3) thin flms were deposited on a p-type Si substrate to create a
heterojunction infrared (IR) photodiode via the simple thermal oxidation technique, while the oxygen flow rate
was varied from 20 to 80 sccm. The XRD diffractograms confrmed the formation of an orthorhombic structure
(α-MoO3 phase) in all the synthesized layers, consistent with their Raman spectra. The FESEM images showed
that the prepared samples have similar flat flakes on top and nano-spherical particles underneath with a
thickness of 220 ± 20 nm. The current density-voltage (J-V) measurements showed that with increasing O2 flow
rate, the overall rectifcation ratio (IF/IR) of the samples is increased, with a maximum of 43.60 for the one
fabricated at 60 sccm O2 flow rate. Also compared to other layers, this sample with the highest oxygen vacancies,
i.e., MoO2.84, showed this property led this sample to achieve the highest photoresponse ratio (ILight/IDark) of 17
to the 835 nm IR radiation.
The reaction kinetics of hydrogen evolution reaction (HER) is largely determined by balancing the Volmer step in alkaline media. Bifunctionality as a proposed strategy can divide the work of water dissociation and intermediates (OH* and H*) adsorption/desorption. However, sluggish OH* desorption plagues water re‐adsorption, which leads to poisoning effects of active sites. Some active sites may even directly act as spectators and do not participate in the reaction. Furthermore, the activity comparison under approximate nanostructure between bifunctional effect and single‐exposed active sites is not fully understood. Here, a facile three‐step strategy is adopted to successfully grow molybdenum disulfide (MoS2) on cobalt‐containing nitrogen‐doped carbon nanotubes (Co‐NCNTs), forming obvious dual active domains. The active sites on domains of Co‐NCNTs and MoS2 and the tuned electronic structure at the heterointerface trigger the bifunctional effect to balance the Volmer step and improve the catalytic activity. The HER driven by the bifunctional effect can significantly optimize the Gibbs free energy of water dissociation and hydrogen adsorption, resulting in fast reaction kinetics and superior catalytic performance. As a result, the Co‐NCNTs/MoS2 catalyst outperforms other HER electrocatalysts with low overpotential (58 and 84 mV at 10 mA cm⁻² in alkaline and neutral conditions, respectively), exceptional stability, and negligible degradation.
Complex transition-metal oxides (TMOs) are critical materials for cutting-edge electronics and energy-related technologies, on the basis of their intriguing properties including ferroelectricity, magnetism, superconductivity, (photo- and electro-) catalytic activity, ionic conductivity, etc. These properties are fundamentally determined by the partially occupied TM d orbitals and the corresponding local coordination environments, which are sensitive to defects (or impurities), compositions, grain boundaries, surface and interfaces, etc. Recently, motivated by the advance in thin film epitaxy techniques, complex oxide research community has shown great interests in controlling defects for enhanced or even unprecedented functional properties. In this review, we provide an overview on recent progress in tuning the functional properties of TMOs thin films via defect engineering. We begin with a brief introduction to the defect chemistry of TMOs, including types of defects and their effects on local atomic structure, electron configurations and electronic structure, etc. We then review recent research efforts in engineering defects in TMOs for novel functionalities, such as ferroelectricity, magnetism, multi-ferroelectricity and dielectricity, two-dimensional electron/hole gas, metal-insulator transitions, resistive switching, ionic conductivity, photo-electrocatalysis, etc. We also provide insights into understanding the defect-structure-property relationship from the perspective of electronic structure. Finally, challenges and perspectives on control of defects for design of novel devices are discussed.
Effects of different reaction parameters in the hydrothermal synthesis of molybdenum oxides (MoO3) were investigated and monoclinic (β-) MoO3 was prepared hydrothermally for the first time. Various temperatures (90/210 °C, and as a novelty 240 °C) and durations (3/6 h) were used. At 240 °C, cetyltrimethylammonium bromide (CTAB) and CrCl3 additives were also tested. Both the reaction temperatures and durations played a significant role in the formation of the products. At 90 °C, h-MoO3 was obtained, while at 240 °C the orthorhombic (α-) MoO3 formed with hexagonal rod-like and nanofibrous morphology, respectively. The phase transformation between these two phases was observed at 210 °C. At this temperature, the 3 h reaction time resulted in the mixture of h- and α-MoO3, but 6 h led to pure α-MoO3. With CTAB the product was bare o-MoO3, however, when CrCl3 was applied, pure metastable m-MoO3 formed with the well-crystallized nanosheet morphology. The gas sensing of the MoO3 polymorphs was tested to H2, which was the first such gas sensing study in the case of m-WO3. Monoclinic MoO3 was found to be more sensitive in H2 sensing than o-MoO3. This initial gas sensing study indicates that m-MoO3 has promising gas sensing properties and this MoO3 polymorph is promising to be studied in detail in the future.
The present research article is about the visible light photocatalytic degradation of Methylene Blue (MB) by aqueous heterogeneous medium containing orthorhombic phase of nanocrystalline (NCs) molybdenum oxide (MoO3). The two different oxidation forms of molybdenum oxides formed at annealing temperatures of 90 °C and 400 °C are Mo17O47 and α-MoO3 nanocrystals which are found to exhibit good photocatalytic activity. In the present work, a simple conventional wet chemical method has been used to synthesize molybdenum oxide nanoparticles (NPs) by combining ammonium heptamolybdate tetrahydrate (AHMT) with capped sodium dodecyl sulfate (SDS) and ethanol solution. The orthorhombic phase is present in the samples annealed at 90 °C, 200 °C, 300 °C, and 400 °C, respectively, and it is found that the orthorhombic phase is a highly stable phase in both Mo17O47 and α-MoO3. The photocatalytic activity of the synthesized samples is estimated by using MB degradation. The photocatalytic behaviors of the synthesized Mo17O47 and α-MoO3 nanostructures have been studied using the color degradation of MB. It is found that Mo17O47 and α-MoO3 with nanorods like structure has the potential to degrade the MB dye and for a time of 90 min, the degradation efficiency of Mo17O47 and MoO3 are 56.15% and 95.78%, respectively.
A low-cost hydrothermal method has been employed to synthesize phase pure α-MoO3 and MoO3/rGO nanocomposite electrodes with novel structures for high-performance supercapacitors. Both nanocomposites exhibited orthorhombic layered structure. Pure MoO3 exhibited nanorod-like morphology, whereas MoO3/rGO nanocomposite showed flower-like structure. The TEM studies reveal that the GO reduced to rGO and wrapped to the MoO3 nanorods. The pure MoO3 nanorods electrode demonstrated a specific capacitance of 331 F/g at a current density of 1 A/g. The MoO3/rGO nanocomposite electrode with unique flower-like structure showed enhanced electrochemical performance with a specific capacitance of 486 F/g at a current density of 1 A/g with 92% capacity retention even after 1000 discharge cycles.
Undoped and Cu doped cobalt oxide thin films with (x= 1, 3, 5, 7, and 9)% were prepared by spray pyrolysis at substrate temperature (350)°C. Structural and optoelectronic properties of thin films were studied by X-ray diffraction XRD, Atomic Force Microscope AFM, current- voltage (I-V) and capacitance- voltage (C-V) measurement. X-ray diffraction results revealed that all films consist of single Co3O4 phase with preferred orientation (111) and cubic spinel structure. AFM micrographs indicated that the grain size decrease with increasing Cu doping. Current transport properties of the p-Cu:Co3O4/n-Si heterojunctions are investigated by current–voltage measurements in dark and illumination with different incident power. The ideality factor were found to be strongly depending on Cu doping, there is an increase in the ideality factor with increasing Cu doping. In part of C-V measurements, built-in potential value were found to decrease with increasing of Cu. The higher spectral responsivity of the junction was 54 A/W at (600-700) nm wavelength for copper ratio 9%.
Exploring new nanowaveguide materials and structures is of great scientific interest and technological significance for optical and photonic applications. In this work, high-quality single-crystal MoO3 nanoribbons (NRs) are synthesized and used for optical guiding. External light sources are efficiently launched into the single MoO3 NRs using silica fiber tapers. It is found that single MoO3 NRs are as good nanowaveguides with loss optical losses (typically less than 0.1 dB/μm) and broadband optical guiding in the visible/near-infrared region. Single MoO3 NRs have good Raman gains that are comparable to those of semiconductor nanowaveguides, but the second harmonic generation efficiencies are about 4 orders less than those of semiconductor nanowaveguides. And also no any third-order nonlinear optical effects are observed at high pump power. A hybrid Fabry-Pérot cavity containing an active CdSe nanowire and a passive MoO3 NR is also demonstrated, and the ability of coupling light from other active nanostructures and fluorescent liquid solutions has been further demonstrated. These optical properties make single MoO3 NRs attractive building blocks as elements and interconnects in miniaturized photonic circuitries and devices.
The biomedical, chemical, drug and food industries have great demand of an alcohol sensor with high sensitivity and selectivity which can operate at low temperature. In this study, CuO thin films were deposited on well cleaned glass substrates by spray pyrolysis technique for 0.1, 0.2 and 0.3 molar concentrations of copper chloride precursor solutions and at a substrate temperature of 300°C and at an air pressure of 0.4 kg cm-2. X-ray diffraction patterns of the prepared films revealed the formation of CuO thin films having monoclinic structure with a grain size ranging from 35 to 54 nm. The microstrain and dislocation densities are found to decrease with increase in molar concentration. The surface morphology and the elemental composition of the deposited samples were analyzed using SEM and EDAX. The change in resistivity of the films was studied with respect to the change in temperature using two probe method. From Laser Raman spectral studies, the characteristic Raman peaks of the prepared samples were observed. The ethanol gas sensing properties of the prepared films were studied for 100 and 200 ppm ethanol vapour.
This study investigated the use of ethylene glycol to form alpha-MoO3 (molybdenum trioxide) from ammonium molybdate tetrahydrate at various sintering temperatures for 1 h. During the sintering process, the morphologies of the constituents were observed using scanning electron microscopy (SEM), and Fourier transform infrared (FTIR) spectroscopy was used to explain the reaction process. In this work, the results obtained using X-ray photoelectron spectroscopy (XRD) demonstrated that, when the molybdenum trioxide powder was treated thermally at 300 degrees C, the material exhibited crystallinity. The peaks were indexed to correspond with the (110), (040), (021), (111), and (060) crystallographic planes, and the lattice parameters of a, b, and c were about 3.961, 13.876, and 3.969 angstrom. Using these observations, we confirmed that orthorhombic alpha-MoO3 was formed for sintering temperatures from 300 to 700 degrees C. Pattern images were obtained by the selected area electron diffraction pattern (SAED) technique, and the d distance of the high resolution transmission electron microscopy (HRTEM) images were almost 0.39 and 0.36 nm, and the Mo 3d(5/2), Mo 3d(3/2), and O 1s of X-ray photoelectron spectroscopy (XPS) were located at 233.76, 237.03, and 532.19 eV, which also demonstrated that alpha-MoO3 powder had been synthesized.
MoO3 nanostructures have been grown in thin film form on five different substrates by RF magnetron sputtering and subsequent annealing; non-aligned nanorods, aligned nanorods, bundled nanowires, vertical nanorods and nanoslabs are formed respectively on the glass, quartz, wafer, alumina and sapphire substrates. The nanostructures formed on these substrates are characterized by AFM, SEM, GIXRD, XPS, micro-Raman, diffuse reflectance and photoluminescence spectroscopy. A detailed growth model for morphology alteration with respect to substrates has been discussed by considering various aspects such as surface roughness, lattice parameters and the thermal expansion coefficient, of both substrates and MoO3. The present study developed a strategy for the choice of substrates to materialize different types MoO3 nanostructures for future thin film applications. The gas sensing tests point towards using these MoO3 nanostructures as principal detection elements in gas sensors.
Orthorhombic α-MoO3 microplates were produced from (NH4)6Mo7O24 · 4H2O solid powder by a 900 W microwave plasma for 40, 50, and 60 min. Phase, morphologies, and vibration modes were characterized by X-ray diffraction (XRD), selected area electron diffraction (SAED), scanning electron microscopy (SEM), and Raman and Fourier transform infrared (FTIR) spectroscopy. Sixty min processing resulted in the best crystallization of the α-MoO3 phase, with photoluminescence (PL) in a wavelength range of 430–440 nm.
Two gas-phase catalytic cycles for the two-electron oxidation of primary and secondary alcohols were detected by multistage mass spectrometry experiments. A binuclear dimolybdate center [Mo(2)O(6)(OCHR(2))](-) acts as the catalyst in both these cycles. The first cycle proceeds via three steps: (1) reaction of [Mo(2)O(6)(OH)](-) with alcohol R(2)HCOH and elimination of water to form [Mo(2)O(6)(OCHR(2))](-); (2) oxidation of the alkoxo ligand and its elimination as aldehyde or ketone in the rate-determining step; and (3) regeneration of the catalyst via oxidation by nitromethane. Step 2 does not occur at room temperature and requires the use of collisional activation to proceed. The second cycle is similar but differs in the order of reaction with alcohol and nitromethane. The nature of each of these reactions was probed by kinetic measurements and by variation of the substrate alcohols (structure and isotope labeling). The role of the binuclear molybdenum center was assessed by examination of the relative reactivities of the mononuclear [MO(3)(OH)](-) and binuclear [M(2)O(6)(OH)](-) ions (M = Cr, Mo, W). The molybdenum and tungsten binuclear centers [M(2)O(6)(OH)](-) (M = Mo, W) were reactive toward alcohol but the chromium center [Cr(2)O(6)(OH)](-) was not. This is consistent with the expected order of basicity of the hydroxo ligand in these species. The chromium and molybdenum centers [M(2)O(6)(OCHR(2))](-) (M = Cr, Mo) oxidized the alkoxo ligand to aldehyde, while the tungsten center [W(2)O(6)(OCHR(2))](-) did not, instead preferring the non-redox elimination of alkene. This is consistent with the expected order of oxidizing power of the anions. Each of the mononuclear anions [MO(3)(OH)](-) (M = Cr, Mo, W) was inert to reaction with methanol, highlighting the importance of the second MoO(3) unit in these catalytic cycles. Only the dimolybdate center has the mix of properties that allow it to participate in each of the three steps of the two catalytic cycles. The three reactions of these cycles are equivalent to the three essential steps proposed to occur in the industrial oxidation of gaseous methanol to formaldehyde at 300-400 degrees C over solid-state catalysts based upon molybdenum(VI)-trioxide. The new gas-phase catalytic data is compared with those for the heterogeneous process.
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.
We report an inorganic aqueous solution route for pure molybdenum trioxide (MoO3) and zirconium oxide composite (MoO3-ZrO2) insulating layer of Cu/MoO3-ZrO2/p-Si metal-insulator-semiconductor structured Schottky barrier diodes. These composite films were coated by Jet Nebulizer Spray Pyrolysis techniques with a substrate temperature of 500 °C and analyzed using X-ray diffraction, Field emission scanning electron microscope, DC electrical conductivity and I–V characterization. These MoO3-ZrO2 films exhibited different crystal structures. Field emission scanning electron microscopes images displayed plate-like structure with improved grain size. The presence of Zr, Mo and O atoms were confirmed by energy dispersive x-ray spectrum. Optical studies shows the maximum absorption and higher optical badgap for (15 wt%) of Zr in MoO3. DC electrical studies recorded the maximum activation energy with minimum conductivity for Zr based composite films. Current - Voltage measurements were analyzed in both dark and illuminated conditions for Cu/MoO3-ZrO2/p-Si Schottky Barrier diodes. Minimum ideality factor (2.98) and maximum barrier height (0.664 eV) was obtained for 15 wt% of Zr composite thin films.
Crystalline molybdenum oxide nanoparticles (MoO3 NPs) were prepared by laser ablation synthesis in solution (LASiS). The resulting MoO3 NPs are water suspended with average size of 23 nm. Subsequently, in order to produce hole injection layers for solar cells, these nanoparticles were processed as thin films onto indium tin oxide (ITO)/glass substrate using ultrasonic spray deposition, which allows fast and uniform deposition in large areas with controllable thickness and low roughness; the water is removed by heating the substrate during the processing. Moreover, scanning electronic microscopy images pointed out that the bottom of the films is mainly composed of small nanoparticles. Thereafter, the optimized glass/ITO/MoO3NPs/PTB7:PC71BM/Ca/Al solar cells displayed open circuit voltage (Voc) of 0.75 V, short circuit current density (Jsc) of 13 mA/cm2, fill factor (FF) of 58% and power conversion efficiency of 5.7% under AM1.5 illumination, presenting increased stability when compared with devices having polymeric hole transporting layer. Since LASiS method does not require the use of organic precursors/solvents, it is a green route to produce MoO3 NPs. In addition, the ultrasonic spray deposition is a versatile method to achieve homogeneous and transparent thin films from water suspended nanoparticles. The organic solar cell response pointed out the potential use of these procedures to produce hole injection layers for photovoltaic devices.
Pristine and Cobalt (Co)-doped MoO3 nanofilms were synthesized on glass substrates using the spray pyrolysis method. The nanometric pristine MoO3 films were prepared from the 10⁻² M.L⁻¹ solution of ammonium molybdate tetrahydrate [(NH4)6Mo7O24,4H2O] in distilled water. Co-doping at 0.5, 0.75 and 1% was achieved by adding cobalt (II) chloride hexahydrate (Cl2CoH12O6) in the pristine solution. The structure and the morphology of the films were investigated by means of X-ray diffraction and atomic force microscopy: two pronounced (020) and (040) peaks corresponding to the orthorhombic structure phase of α-MoO3 were detected. The AFM observations revealed the formation of micro-plates, parallel to the surface plane, with a roughness ranging from 33 nm to 54 nm. Optical properties were investigated through reflectance, transmittance and photoluminescence measurements. The optical band gap, the Urbach energy and the refractive index were deduced from these measurements. The presence of oxygen vacancies was revealed from the interband transitions in the blue and green domains. Co-doped MoO3 nanofilms showed ferromagnetic behavior. The photocatalytic degradation of an aqueous solution of methylene blue (MB) under UV irradiation, in the presence of Co-MoO3 nanomfilms, has been carried out using UV–vis spectrometery: the intensity of the absorption peak recorded at 660 nm decreased with the increase of the UV-illumination time while the color of the initial MB solution was drastically waned.
A novel Cu-MoO3/g-C3N4 hybrid nano composite was successfully synthesized by a two step strategy of one-pot pyrolysis followed by impregnation method. The structural, phase, morphology and electronic environment of MoO3, g-C3N4 and Cu in the composite has been determined by various characterization methods. The oxygen vacancies of MoO3 have been ascertained by UV-DRS, Raman and XPS analysis. The formation of heterostructure has been characterised by electrochemical measurement. The photo catalytic performance of the composite was investigated by water reduction reaction and reduction of an important inorganic pollutant Cr (VI). In presence of Cu NPs as cocatalyst, the H2 evolution of MoO3/g-C3N4 hybrid was found to be 652 μmol/h with an apparent energy conversion efficiency of 13.46 %, and Cr (VI) was reduced up to 95% by using citric acid as a hole scavenger. The remarkably enhanced photo catalytic performance was attributed to the combined effect of the double Z-scheme mechanism and defective MoO3. In situ formation process of MoO3/g-C3N4 hybrid follows a direct Z-scheme charge transfer by generating a great number of defects at the solid-solid interface, similar to that of a conductor and offer low electric resistance. Whereas, loading Cu NPs builds up an indirect Z-scheme charge transfer to establish double Z-scheme charge transfer mechanism. This hybrid material produces a photocurrent density of 12.1 mA cm-2 in good agreement with the photo catalytic activity. This work highlights the facilitation effect of MoO3 due to oxygen vacancy and charge transfer through double Z-scheme mechanism to opens up a new window in the field of 2D nanostructured photo catalytic materials.
The properties and applications of molybdenum oxides are reviewed in depth. Molybdenum is found in various oxide stoichiometries, which have been employed for different high-value research and commercial applications. The great chemical and physical characteristics of molybdenum oxides make them versatile and highly tunable for incorporation in optical, electronic, catalytic, bio, and energy systems. Variations in the oxidation states allow manipulation of the crystal structure, morphology, oxygen vacancies, and dopants, to control and engineer electronic states. Despite this overwhelming functionality and potential, a definitive resource on molybdenum oxide is still unavailable. The aim here is to provide such a resource, while presenting an insightful outlook into future prospective applications for molybdenum oxides.
The mosaic films made of TiO2 and hexagonal MoO3 nanoparticles not only demonstrate high activity in the reactions of photooxidation of the adsorbed organics in air conditions but also store the photoinduced charge as the result of MoO3 reduction by photoelectrons injected from titania which behaves as a photogenerating component. The charge accumulated via this light-driven reduction is spent under dark conditions in the reaction of molecular oxygen reduction yielding peroxide species. As the result, TiO2/h-MoO3 nanocomposite films retain oxidation ability for ca. 4 h after UV illumination. This photocatalytic material opens fresh avenues in fabrication of self-sterilizing coatings capable to generate reactive oxygen species not only under actinic illumination but also under dark conditions.
In this work, the synthesis of (MoO3)1−x(V2O5)x thin films (0 < x < 1) was made by the Chemical Spray Pyrolysis technique. However, an easy method for the growth with a good crystallization of thin films has been used. The depositions were done at 423 K substrate temperature on the cleaned glass substrates. The structural investigation conducted by X-ray diffraction showed that (MoO3)1−x(V2O5)x thin films was polycrystalline. The optical measurements were done with a UV-visible-NIR spectrophotometer in the wavelength range from 300 to 2500 nm. The optical properties and the electrical conductivities were dependent with the composition. The variation of the optical band gaps of each composition was found in the range of 2.44–3.35 eV. The characterization of no degenerate semiconductor was found and the absorption and dispersion process were described by polaronic hopping. The electrical conductivity increased with the increase of composition x from 2.15 × 10−6 Ω−1 cm−1 until 1.09 × 10−1 Ω−1 cm−1.
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.
Epitaxial cuprite (Cu2O) and tenorite (CuO) films are grown on (001)-oriented MgO single crystal by reactive cross-beam pulsed-laser deposition assisted by radio-frequency plasma discharge in oxygen atmosphere. X-ray diffraction (XRD) shows that the films are single phased and well crystallized. The Cu2O films grow according to two directions with their (200) and (110) planes parallel to the (001) MgO basal plane, while the CuO films show a single main axis growth along the [111] direction. The precise in-plane orientations on the (001) MgO plane are determined by XRD measurements in asymmetric mode. As the cuprite phase is cubic, the (200) Cu2O crystallites grow according to a simple cube-on-cube growth mode, i.e. [100]Cu2O‖[100]MgO relationships, while the (110) crystallites display the [100]Cu2O‖[110]MgO in-plane orientation. Despite its large differences in structure and crystallographic symmetry with the cubic MgO substrate, the monoclinic CuO phase grows with the (111) planes parallel to the (001) MgO planes and the [1¯10]CuO∥[110]MgO in-plane orientations are observed. This last feature is explained by the low lattice misfit between the film and the substrate in this orientation, the large atomic density of the (111) CuO plane, and the minimizing of interface energy by electrostatic interactions between the (001) MgO and (111) CuO planes.
One-dimensional nanostructures of orthorhombic molybdenum trioxide (α-MoO3) have been synthesized in the forms of ribbons or rods via acidification under hydrothermal conditions at 140−200 °C. The reaction path has been revealed with our kinetic investigations, which shows the following sequence: (i) from the starting compound (NH4)6Mo7O24·4H2O to (ii) formation of intermediate compound ((NH4)2O)0.0866·MoO3·0.231H2O, and then to (iii) final α-MoO3 nanoribbons or nanorods in 100% phase purity. The optimal growth temperature is in the range of 170−180 °C under the current experimental settings. At higher reaction temperatures, this transformation can be accelerated, but with poorer crystal morphological homogeneity. It has been found that the dimensions of these rectangular nanorods are about 50 nm in thickness, 150−300 nm (mean value at 200 nm) in width, and a few tens of micrometers in length. The crystal morphology can be further altered with inorganic salts such as NaNO3, KNO3, Mg(NO3)2, and Al(NO3)3. Using an H2S/H2 stream, the above-prepared α-MoO3 nanorods can be converted completely to 2H−MoS2 nanorods at 600 °C. The original rodlike morphology is well-retained, although the aspect ratio of the oxide template is reduced upon the sulfidation treatment.