During the process of nanostructured WO3 synthesis it is found that citric acid content, system pH value before hydrothermal synthesis, hydrothermal reaction temperature and hydrothermal reaction time will have great influence on the product, therefore the following will focus on several influence factors affecting the phase and morphology of nanostructured WO3.
1.0 Effect of citric acid content on nanostructured WO3
The morphology of WO3 nanostructures prepared with different citric acid content is significantly different, with the increase of citric acid content in the solution, the diffraction peak intensity is stronger and stronger, it shows that the crystallinity of the synthesized samples is higher and higher, so citric acid content has little effect on the phase composition of tungsten trioxide. According to the test result the content of citric acid in the initial solution can not only control the nanostructure and size of nanostructured tungsten trioxide, but control their preferred selection. When there is no citric acid but sodium sulfate in the initial solution, a large number of WO3 nanoplates are found, only when the citric acid exists WO3 nanorods can grow up gradualy from the nanosheets. It has been pointed out that the interaction between organic acids and crystal surface plays a decisive role in controlling the morphology and growth direction of the obtained samples.
2.0 Effect of pH on nanostructured tungsten trioxide
It can be seen from the XRD pattern that when the pH is from 1 to 1.5, all are tungsten trioxide, when the pH increases to 2.5, other phases appear. Therefore, the pH value has great influence on the phase component of tungsten trioxide. When the pH is 1.5, the crystallinity of tungsten trioxide is the highest. When the pH value is low, a large number of H+ appears in the hydrothermal solution, a large amount of supersaturated tungsten acid is produced, the supersaturated tungsten acid decomposes rapidly under the hydrothermal condition, a large number of tungsten trioxide crystal nuclei is generated, then the concentration of tungsten acid in the solution decreases, the growth of crystal nuclei is slowed down, WO3 crystal crystallinity is decreased. When the pH value gets high, the supersaturated tungsten acid decreases, the number of WO3 crystal nuclei generated lessens, which promotes the subsequent growth of the crystal nucleus, and finally make the particle size larger and the agglomeration more serious. Therefore, pH=1.5 is the best optimal hydrothermal synthesis conditions good for the generation, also best for producing and orderly growth of crystal nuclei.
3.0 Effect of hydrothermal reaction temperature on nanostructured WO3
Scanning electron microscope (SEM) image shows that with the increase of hydrothermal temperature nanostructure WO3 gradually grows from irregular nanowire into thin and long nanorod with regular morphology. However, the regularity of the rod structure will be destroyed to a certain extent if the temperature is too high, which means that the hydrothermal reaction temperature plays an important role in the producing of WO3 nanorods, according to the experiment, it can be determined that the optimal temperature of the hydrothermal reaction is 180℃.
4.0 Effect of hydrothermal reaction time on nanostructured WO3
The SEM image of hydrothermal reaction time shows that the hydrothermal reaction time of 1d and 2d has little effect on the morphology of WO3 nanorods, and the morphology of the nanorods is uniform. However, when the hydrothermal reaction time was 3d, the nanorod gets shorter and thicker, meanwhile some other morphology appears, it means that heterophase will be generated if the reaction time was too long. Therefore, the best reaction time is 1d considering time and energy saving.
