Microwave Solvothermal Synthesis Characterization Tungsten Oxide Nanorods Engineering Essay

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A simple and efficient method enables to synthesize nanorods WO3 with high quantity by microwave-solvothermal. Tungsten oxide is an important transition metal oxide material, which possesses some unique properties such as electrochromic, opochromic, and gaschromic properties. Using microwave assisted solvothermal method, where the advantages of both microwave and solvothermal methods, to synthesize 1-D tungsten oxide such as nanorods. The synthesis of tungsten oxide nanorods has been investigated under microwave-solvothermal conditions at 240°C, and uses WCl6 and poly(ethylene glycol) (PEG) 20000 as a precursor and dispersant in 20% ethanol. The average diameter of WO3 nanorods were 80-100 nm and 3 μm long. The crystallized products were characterized by X-ray diffraction (XRD), thermo gravimetric analysis (TGA), scanning electron microscopy (SEM), and transmission electron microscopy (TEM).

In the nanostructured materials, one-dimensional (1-D) structured, such as nanorods, nanowires, nanobelt and nanotubes have great meaning since nanomaterials has recently brought to light by reveal new physical, chemical or optical properties that corresponding bulk materials do not usually possess. Among the various transition metals semiconducting oxides are of much importance due to their wide band gap feature (2.6eV) electrochromic, optochromic, and gaschromic properties . Moreover, because of their high surface areas, are promising candidates for a vast range of applications including positive electrode materials for lithium-ion batteries. Furthermore, tungsten oxides have been used to make flat panel displays, photoelectrochromic "smart" windows, optical modulation devices, writing-reading-erasing optical devices, semiconductor gas sensors, photo catalysts, humidity and temperature sensors, and so forth. Especially, one-dimensional tungsten oxides have been of special interest due to their lower dimensionality and superior properties. Recently, nanostructure tungsten oxide materials have attracted more attention in physics, chemistry and materials areas for their interesting electronic properties such as superconductivity and charge carrying abilities.

Many efforts have been dedicated to research of the synthesis of semiconductor nanowires, nanorods, and nanobelts including vapor-liquid-solid (VLS), plasma torch methods, laser-assisted catalytic growth, chemical vapor deposition (CVD), heat treatment and wet chemical reactions in the past few years. Generally tungsten oxide nanorods were synthesized by heat treatment of tungsten metal, such as, heating a tungsten foil, partly covered by a SiO2 plate, in an Ar atmosphere at 1600 °C.

The solvothermal method is an efficacy low-temperature method and environmental friendly in the wet chemical reaction. Furthermore, 1-D tungsten oxide have been synthesized by solvothermal reactions of tungsten-containing precursors such as WCl6, W(CO)6 and WCl4 in different solvents that are usually alcohol, water and cyclohexanol in few literatures. However, this method usually needs few hours to reaction at low temperatures, but this also brings about energy waste simultaneously. The solvothermal method is combined with microwave irradiation leads to higher heating rates because microwave was able to trigger dipole rotation and ionic conduction thus results in a rapid heating ramp. Since 1986 the development of microwave synthesis technology, microwave technology has been extensively used in chemical synthesis and chemical analysis due to microwave energy has rapid heating and can be heated for a temperature control effectively. This method has some advantages such as heating throughout the media, fast reaction, high yield, excellent reproducibility, narrow particle distribution, high purity and energy transformation efficiently.

In this study, we report here the synthesis and characterization of 1-D WO3 nanostructured from WCl6 as precursor, using a fast, efficient and simple microwave solvothermal method with dispersant PEG 20000. The process also enables to control the morphology of tungsten oxide by changing the solvent concentration.

Experimental

The precursor material used in this study was WCl6 (ACROS). The microwave solvothermal system for synthesis performed in a generated device (Multiwave 3000, Anton Paar). This system operates at a maximum output power of 1400 W, and power can be regulated from 0 to 1400 W. Power also can be controlled by both pressure as well as temperature to maximum of 40 bars and 240°C. The microwave frequency set at 2.45 GHz generally, and also the same as domestic microwave oven. The syntheses were occurred in double-walled digestion vessels which have an inner liner and cover made up of Teflon PFA and an outer high strength vessel made of ceramics. In a typical synthesis, 0.05 M of WCl6 was dissolved in 100 ml of different concentration ethanol solution which dissolved 0.3g PEG 20000 (BDH) as the dispersant. The mixture was placed in the microwave solvothermal system. A microwave solvothermal treatment of resultant mixture was executed in 200°C for 15 mins. All the reaction temperatures were attained within about 3 min. Then a great amount of yellow precipitate produced. After cooling to room temperature, the precipitate was centrifuged, washed with distilled water, absolute ethanol and acetone in sequence and dried in air at room temperature. The obtained products were collected for characterizations by XRD, SEM, TEM, and TGA.

XRD:

Determination of phase purity and identification of the samples were performed using X-ray Diffraction (XRD) studies using a PANalytical X'pert PRO powder diffractometer system with Cu-Kα radiation (45kV, 40mA) with 0.02° step and 1-s time over the range 15° < 2θ < 85°.

TGA:

The thermal analysis was performed using a TA instruments Q50 thermogravimetry-differential thermal analysis with and a heating rate of 20℃/min.

SEM:

The structure, morphology, and chemical composition of the as-synthesized and thermally processed WO3 products were then characterized by using a field emission scanning electron microscope (SEM) equipped with energy-dispersive x-ray spectroscopy.

TEM:

The size and morphology of tungsten oxide nanorods were determined by using a Hitachi model H-7500 high-resolution transmission electron microscope, with a tungsten filament at an accelerating voltage of 120 kV.

Results and Discussion

Figure 1 presents typical XRD data of the WO3 by the microwave solvothermal in different ethanol concentration. XRD peak intensity of the (200) plane was relatively higher than that of other planes, especially at 20% ethanol (a) was more obvious. As the concentration increased, crystallinity contrary increasingly reduced. This denotes that the nanorods grow along the [200] direction. It should be noted that the overall intensities of the diffraction spots at using 95% ethanol are weak. All the diffraction peaks are assigned well to the know structures of monoclinic WO3, with β= 89.930∘, a = 0.7274 nm, b = 0.7501 nm, c= 0.3824 nm (JCPDS No. 75-2072). No extraneous diffraction peaks are found in the pattern. The result indicates the monoclinic WO3 nanorods obtained by the microwave induced solvothermal process.

We may indicate by Fig 2 shows SEM of WO3 structure at different ethanol concentration. When the solvent was 95% ethanol (fig 2a) showed the structure almost was plate-like. The structure began to break plate into rods, but most of the main structure still in plate when the ethanol concentration reduced to 80% (fig 2b). With the reduction in ethanol concentration to the 40% (fig 2d), the rods structure was more obvious. At 20% ethanol (fig 2e), structure the formation of nanorods in evidence. The diameter of nanorods about 80 nm, while the length of about 2 μm, and quite uniform size distribution. As a result of fig 2, the structure will be effected by ethanol concentration, and the highest aspect ratio of nanorods was more than 30 when concentration down to 20%. As can be seen, at lower ethanol concentration, thin and long bundles composed of numbers of nanorods can be obtained, while larger and shorter bundles or even block-shape products were produced with increasing ethanol concentration. Here, we believe that low ethanol concentration contributed to the growth of tungsten oxide nanorods. The higher ethanol concentration can lead to short nanorods into the in shorter and thicker bundles due to agglomeration finally. Fig 3 shows the SEM for no added dispersant, nanorods particle size distribution more messy. These particles relatively large, about 400 200 nm sized, irregular plate-like particles. This means that the role of dispersant is to make the particles could not aggregate into plate structure when synthesis of WO3. More interesting of increasing with the WCl6 concentration to the 0.1M, the reaction time will also increase, but its morphology no significant change (Fig 4).

Figure 5 shows the TEM and high-resolution TEM image of the nanorods, and the corresponding selected area electron diffraction pattern is given in the inset. The measured mean diameter of nanorods on the basis of the TEM investigation was about 20-80 nm, and the length of the nanorods was up to several microns by figure 2. Therefore, the nanorods reached a high aspect ratio of more than 30. From the figure 5(a), we reaffirm that the growth direction of the nanorods is along [200]. The measured lattice spacing from the HRTEM micrograph is about 0.380 nm. The tungsten oxide nanorods also showed the phase of monoclinic WO3 (c = 0.3824 nm), which was confirmed by XRD results. The shape of a crystal is determined by the difference in the relative growth rates of the individual crystal planes and the resulting particles are anisotropic in shape under certain supersaturation.

In order to insightful understanding of the WO3 nanorods thermal analysis investigations were performed by using TGA and DTA (differential thermal analysis) in figure 6. The TGA result shows only a small weight loss of 3% before 450℃, which includes a possible loss of water and organic material absorbed from the air and during washing. After 450℃, the DTA curve is close to a straight line and little weight change occurs in the TGA plot, showing that the WO3 is stable from 450 ℃ to 680 ℃. There was no weight gain from room temperature to 680℃ is observed, indicating no any oxidation reaction, probably from other sub-stoichiometric tungsten oxides, such as WO2.9, WO2.92, and W18O49, to WO3.

Conclusion

In summary, tungsten oxide nanorods were synthesized by microwave solvothermal method with tungsten hexachloride (WCl6) as the precursor and PEG 20000 as the dispersant. With decreasing concentration of ethanol, gradually morphological evolution can be observed. The morphological became sharp straighter and smaller in aspect ratio. The resulting tungsten oxide nanorods are 800-100 nm diameter, up to 3μm long and have high quantity. This method is easy and efficient to produce large quantity of nanorods of WO3.

Fig. 1. X-ray diffraction profiles of the tungsten oxide nanorods synthesized at different ethanol concentrations: (a) 20%, (b) 40%, (c) 60%, (d) 80%, and (e) 95%.

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Fig. 2. SEM images of WO3 synthesized at different ethanol concentrations of (a) 95%, (b) 80%, (c) 60%, (d) 40%, (e) 20%.

Fig. 3. SEM images of tungsten oxide nanorods without dispersant.

Fig. 4. SEM images of tungsten oxide nanorods synthesized at a concentration of 0.1M.

(a)

(b)

(200)

(200)

Fig. 5. TEM images (a) and corresponding high-resolution atomic images of a WO3 nanorod. Selected area electron diffraction pattern shows inset.

DTA (%。C)

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402

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308

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010

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Fig . 6. TGA/DTA thermogram of WO3 nanorods synthesized by microwave solvothermal system

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