Synthesis And Evaluation Of Novel Visible-light Phtotocatalysts

Recently the pollution of environment has become more and more serious,which threatens the existence of human beings.In order to solve this problem,many researchers contribute their work to protect the environment.The way by which the poisonous substance is decompounded has become a new research hotspot.Nontoxic nano TiO₂ has high stabilization and high ability of photocatalysis,at the same time,it is effective only under ultraviolet irradiation(λ＜387.5nm).Sunlight is comprised of less than 5% ultraviolet light,so it is important to develop new photocatalyst to extend the absorbed wavelength range into the visible region,where less expensive light sources exit.In recent years,many research groups have focused on the binary metal oxides with different crystal structure to find a more effective photocatalyst by improving the efficiency of extending the photo-responding range.Bismuth tungsten oxide,is the simplest member of the Aurivillius family of layered perovskites,which are structurally comprised of alternating perovskite-like slab of WO₄^2- and[Bi₂O₂]²⁺ layers.It has been found that Bi₂WO₆ samples possess some interesting applications,such as:a well-known ferroelectric material,an oxygen ion conductor,a substance for the photocatalytic decomposition of organic contaminants,and soon.Among all these properties,it is attractive to us that Bi₂WO₆ can be used as avisible-light photo catalyst to degrade the organic compounds.The microwave hydrothermal method supplies rapid heating,faster kinetics, homogeneity,higher yield,better reproducibility,and is economical and energy saver. We have synthesized bismuth tungsten oxide nanocrystals by microwave hydrothermal method,it just cost 1 hour to get perfect crystal at low temperature(180℃),whereas common hydrothermal method cost tens hours.Two kinds of bismuth tungsten oxide nanocrystals(Bi₂WO₆ and Bi_3.84W_0.16O_6.24)were prepared by microwave hydrothermal method for the first time. We studied the effects of temperature,time of microwave hydrothermal and pH value on structure and photocatalysis of Bi₂WO₆.The morphology modulation of nanocrystals synthesized with precursor suspension's pH varied from 0.25 to 10.05 was studied.As the precursor suspension was in acidity,Bi₂WO₆ nano flakes were observed through SEM and TEM,which aggregated like cabbages and flowers.As the pH value reaches 9,morphology of the whole material is mainly constructed by nano brick like of Bi₂WO₆ crystals and ball-like nano crystals of Bi_3.84W_0.16O_6.24 phase.As the pH value reaches above 10,the Bi_3.84W_0.16O_6.24 phase is the dominate phase,The Bi₂WO₆ phase degenerated to amorphous ball-like nano crystals dotting on Bi_3.84W_0.16O_6.24 octahedron crystals.The growth mechanism of bismuth tungsten oxide is proposed.The different morphology of bismuth tungsten oxide nano materials result from the different solubility of (WO₄)^2- and(Bi₂O₂)²⁺ in precursor suspensions with various pH.The optical properties of Bi₂WO₆ samples were measured using UV/Vis spectroscopy.The UV-Vis reveals that the absorption Edge of Bi₂WO₆ nano crystals is 420nm which shift to short band.The bandgap(Eg) obtained by extrapolation of the plots are 2.95 eV.It is well known that the bandgap energy of semiconductor nanoparticles increases with the decrease of grain size.In this work,a blue shift in the bandgap(from 2.68 of Bi₂WO₆ powder to 2.95 eV of nano flakes) transition is observed.The photodegradation efficiencies of Rhodamine B(RhB) were evaluated by the different photocatalysts under visible-light illumination(λ＞400 nm).As pH values differ from 0.25 to1.98,the photocatalyst activity of the samples was higher than others.As pH value increased from 4,the higher the pH value is,the lower the photocatalyst activity of the sample performed.It is mainly attributed to the increasing of samples' thickness, which is consistent with the XRD and TEM results.As the pH value of precursor suspensions reached to alkalescence,degradation rate of RhB was extremely slow because of the appearance of Bi_3.84W_0.16O_6.24 nanocrystals.The photocatalytic evaluation reveals that Bi₂WO₆ nano crystals obtained under different conditions exhibit different photocatalytic activities which depend on pH value of precursor suspensions.Two kinds of bismuth molybdenum oxide nanocrystals were prepared by microwave hydrothermal method.The morphology modulation of nanocrystals synthesized precursor suspension with different pH value was studied.As the precursor suspension was in strong acidity(pH=0.22),the crystallinity of like crystals was perfect and the morphology of Bi₂MoO₆ nano flakes observed from SEM.As the precursor suspension was in medium acidity,(pH value vary from 1.67 to 5.46),the thickness of Bi₂MoO₆ nano flakes increased.As the pH value was neuter,a second crystal phase of Bi_3.84Mo_0.16O_6.24 appeared together with Bi₂MoO₆ thick nanoflakes.While the pH value of precursor reached to 12.97 The Bi_3.84Mo_0.16O_6.24 ball-like nanocrystals with good morphology were observed,the size of crystals is several hundreds nanometers.The UV-Vis of samples reveals that the absorption Edge of Bi₂MoO₆ nano crystals is 464nm.The bandgap(Eg) obtained is 2.67 eV.These results also indicate that all these Bi₂MoO₆ samples have a bandgap suitable for photocatalytic decomposition of organic contaminants under visible-light irradiation.The photodegradation efficiencies of methylene blue were evaluated by the different photocatalysts under visible-light illumination(λ＞400 nm).As pH value was in strong acidity,the higher the pH value is,the higher the photocatalyst activity of the sample performed.It is mainly attributed to a better crystalline phase.As the pH value reached to alkalescence,degradation rate of methylene blue was extremely slow because of the appearance of Bi_3.84Mo_0.16O_6.24 nanocrystals.Three kinds of Ag-based oxide(Ag₂CrO₄,Ag₂MoO₄ and Ag₂WO₄) nanocrystals were prepared by microwave hydrothermal method.The UV-Vis of samples reveals that the absorption Edge of Ag₂CrO₄ nano crystals is 681nm,the bandgap(Eg) obtained is 1.82 eV.The result indicates that we can extend the absorbed wavelength range into the visible region by using Ag₂CrO₄ nano crystals.The bandgap(Eg) of Ag₂MoO₄ is 3.1 eV and Ag₂WO₄ is 3.0 eV.The photodegradation efficiencies of methylene blue were evaluated by the different photocatalysts under visible-light illumination(λ＞400 nm). The photocatalytic evaluation reveals that the photocatalyst activity of the Ag₂CrO₄ is higher than Ag₂MoO₄ and Ag₂WO₄ corresponding to the bandgap value.