Preparation Of Hexagonal Tungsten Trioxide And Photocatalytic Properties

WO3 is a kind of important semiconductor material with excellent performance and has a wide variety of applications in degradation of organic pollutions, photodecomposition of water and other aspects. However, the performance improvement of WO3 depends on its special structure and morphology. Therefore, it's very important to study the effects of different factors on the crystal structure, morphology and photocatalytic activity of WO3.Hydrothermal method was used in this paper to investigate the influence on the structures and morphologies of WO3 when using the alkali metal cation oxalate as additives. The results show that nanorods structure hexagonal WO3(h-WO3)were synthesized when the monovalent cations are Li+ and Na+;while K+ and NH4+ induce the formation of h-WO3 nanowires which gathered into mesh structure with porous; besides, nanoplate structures h-W03 were obtained when using Ru+, Cs+ oxalate as additives; however, the addition of H2C2O4 leads to the formation of monoclinic WO3 nanoplates. It is found that the morphology of WO3 changed from 1D nanowires structures to 2D nanoplates structures with the increase of alkali metal cation radius. With the increase of the electronegativity of alkali metal cations, the WO3 crystals grew more completely and induced the crystal structure of WO3 transformed from hexagonal to monoclinic. In addition, we prepared unusual morphologies of needles and acetabuliform structures WO3 by adding EDTA-2Na and L(+)-sodium tartaric as additives respectively.In this paper, the photocatalytic degradation of methylene blue(MB) solution was used as model to investigate the photocatalytic properties of as-prepared WO3. We firstly investigated the influence of crystal structure on the photocatalytic properties of WO3, the results indicate that hexagonal WO3 exhibited higher photocatalytic activity than monoclinic WO3. The degradation rate of MB after 2h UV irradiation is over 30.0% with the h-WO3 as catalyst. While the degradation rate of MB is only 21.4% with monoclinic WO3 as catalyst. Further more, the effect of different morphologies on the photocatalytic properties of h-WO3 was also discussed. It was found that the WO3 prepared with K2C2O4 as additive shows the highest photocatalytic activity due to the network nanowires structure with high specific surface area of 93.73 m/g. Meanwhile, the results indicate that the degradation rate of MB with nanowires WO3 as catalyst is about 44.5%, higher than 32.0% when using the nanoplates WO3 as catalyst.In addition, the influence of molecule chirality on photocatalytic performance of products was also investigated by addition of tartaric acid. The results show that WO3 prepared with L(+)-tartaric acid as additive shows better photodegradation activity than that of WO3 synthesized by adding D(+)-tartaric acid.Finally, the impact of hydrothermal temperature and reaction time on the preparation of hexagonal WO3 was studied and reached the following conclusions:the WO3 crystal grew more perfect and bigger with increasing hydrothermal temperature, while showed lower photodegradation efficiency of MB. Meanwhile, longer reaction time contributed to the further growth of WO3 crystal, but weakened the performance of photocatalytic degradation of MB.