Preparation, Growth Mechanism And Performance Study Of Molybdenum And Tungsten Oxide Nanomaterials

The solar energy is being used in decomposition of organic matter throughoxidation, reduction of heavy metal ions, killing bacteria and eliminating odors owingto their distinct advantages of that the surface of the semiconductor oxide material canbe activated by the light. Compared with traditional methods, semiconductorphotocatalysis technology is a low-power technology with mild conditions, simpleoperation and wide application, difficult to form secondary pollution, cyclic andrepetitive use. According to the experts, photocatalytic oxidation technology is one ofthe best ways for future wastewater treatment. And the development of thesemiconductor photocatalyst driving by visible light is particularly interesting. Sincemolybdenum oxide, tungsten oxide is an n-type metal oxide semiconductor, they havewidely used in electrically photochromic devices, photochromic devices, the field ofinformation storage, optical devices, sensors, field emitter and catalytic. In this article,several important molybdenum oxide and tungsten oxide have been prepared byhydrothermal method and ultrasonic-assisted chemical. The as-synthesized productswere characterized by powder X-ray diffraction (XRD), field emission scanningelectron microscopy (FESEM), transmission electron microscopy (TEM) and Fouriertransform infrared spectrum (FT-IR). The effects of synthesis parameters onmorphology and structure of nanomaterials were clarified. Their formationmechanisms were discussed and their potential applications were also studied.1. Monodisperse and uniform hierarchical MoO2microspheres have beensuccessfully fabricated through a novel one-pot hydrothermal reduction route. Theresults indicated that the MoO2microspheres with the size of1.5to3.5μm areassembled by tiny nanoflakes or nanoparticles. Comparative experimental resultsreveal that the parameters of synthetic conditions, such as the adding amount ofreducing agent C6H8O7(CA) and mineralizer Na2CO3, greatly affect the growth ofmonodisperse MoO2microspheres. The orientated aggregation combining withOstwald ripening growth mechanism of the MoO2microspheres was proposed, basedon the evolution of the structure and the morphology with the prolongation of reactiontime. Photocatalytic experiment indicated that the prepared MoO2microspheresexhibited higher photocatalytic activity for the degradation of Rhodamine B (RhB) in the presence of H2O2under tungsten lamp irradiation.2. Hierarchical MoO2hollow microspheres with monoclinic phase have beensuccessfully fabricated on a large scale through a convenient hydrothermal reductiveprocess. The hollow spheres, having sizes of1-2μm and shell thicknesses of150-300nm, are composed of subunits (nanospheres or nanoplates) which are actuallyself-aggregated by hundreds of tiny primary nanoparticles. BET nitrogen adsorptionreveals the MoO2hollow microspheres have mesoporous shell with pore size of5-20nm. The BET specific surface area of the product was measured to be37.7m2/g.Center-hollowing evolution process induced by Ostwald ripening was proposed toexplain the formation mechanism of hierarchical MoO2hollow microspheres. The resultof the adsorption experimental indicate that the synthesized MoO2hollow microspheresexhibit fast adsorption rate and high removal of Cr (VI) from water, which has potentialapplication in wastewater treatment especially for corrosive acidic water. Moreover, theprepared hierarchical MoO2hollow microspheres, having larger band gap, highersurface area and mesoporous structure, would greatly widen its range of applications invarious fields, such as catalysis, sensors and energy storage devices, etc.3. A mild hydrothermal route has been successfully designed to controllablyprepare orthorhombic α-MoO3nanobelts and monoclinic MoO2microaxletreesrespectively by adjusting the dosage of (NH4)6Mo7O24·4H2O (AHM). The as-preparedα-MoO3nanobelts, with widths of100-400nm and lengths up to30-40μm, grow along[001] direction. The as-obtained MoO2microaxletrees are assembled by countlessnanolaths with the thickness of80-150nm. The band gap energies (Eg) of the obtainedα-MoO3nanobelts and MoO2microaxletrees are calculated to be2.90and3.72eV,respectively. The chemical reaction processes for the formation of MoOx(MoO3andMoO2) phases are investigated based on the experimental phenomena. The possiblegrowth mechanisms are also discussed.4. A simple and green hydrothermal route has been presented to synthesize3DMoO2nanoplate-based microspheres,1D MoO3nanobelts and nanowires. Theexperimental results reveal that the dosage of hydrochloric acid (HCl) seriouslyinfluences the phase and morphology of molybdenum oxides. According to theUV-visible absorption spectrum, the band gap Eg of MoO2microspheres is calculatedto be3.92eV, and that of MoO3nanobelts and nanowires is estimated to be3.06eV.5. phase-and morphology-controlled MoO3crystals (metastable h-MoO3 hexagonal prisms and stable α-MoO3nanoplate-based rods) have been successfullyfabricated through a facile ultrasound-assisted chemical route. The optical band gap Egvalues of h-MoO3and α-MoO3are estimated to be2.98eV and2.84eV, respectively.The phase transformation from metastable h-MoO3to stable α-MoO3is observed at419oC. A possible growth mechanism for the MoO3crystals has been proposed on thebasis of experimental results. The obtained h-MoO3and α-MoO3powders displayedhigher photocatalytic performance for degrading Rhodamine B (Rh B) compared tocommercial MoO3. In addition, the controlled synthesis of h-MoO3and α-MoO3crystals with special morphology would be valuable for further investigation ofpotential applications in other fields, such as lithium battery, photochromic and opticaldevices.6. Monoclinic WO3nanosheets have been prepared on a large scale via one-pothydrothermal reduction technique. The length and thickness of these nanosheets are70～170nm and15～70μm, respectively. The optical band gap investigation showedthat the Eg value of WO3is2.62eV. The influence of the temperature and the growthmechanism were investigated in detail. The monoclinic WO3nanosheets show anexcellent photocatalytic activity for degradation of RhB. This facile route could bepotentially applied to rationally design other high performance metal oxides.7. An ultrasonic-assisted chemical synthesis route has been used to prepareorthorhombic WO3·H2O nanosheets. Monoclinic WO3nanosheets were successfullyobtained by calcining orthorhombic WO3·H2O nanosheets. The obtained WO3nanosheets remain maintaining the morphology of precursor. The thickness anddiameter ranges of the WO3nanosheets are8～25nm and300～900nm, respectively.Photocatalytic experiments reveal that monoclinic WO3powders own a highphotocatalytic activity for degradation of MB, which indicates that these WO3samplesmay be expected to be used in the photocatalytic, wastewater treatment, environmentalimprovement and other fields.