Preparation And Properties Of Defect Pyrochlore Oxides

In recent years,the large consumption of fossil fuels in the global industrialization leads to the severe energy and environmental problems.Therefore,the application of semiconductor-based photocatalysts in environmental purification and solar energy conversion has become hot topics for researchers.Various semiconductor photocatalysts including metal oxides,nitrogen oxides,sulfides and organometallic complexes have been widely studied.Among them,defect pyrochlore oxides have recently attracted much interest owing to its unique structure,good chemical stability and thermal stability.Moreover,its compositional flexibility has a huge impact on its electron/hole mobility which could realize by introducing a wide range of ions into the defect pyrochlore lattice.Defect pyrochlore oxides have a vast application in the photocatalytic organic degradation and water splitting.In this paper,we modified the defect pyrochlore oxide KNbWO6·H2O by ion exchange,constructing heterogeneous junctions and thin-film preparation.Some scientific issues such as narrow absorbance range,poor charge separation efficiency,and difficulty in recovery have been improved.The photocatalytic activity is enhanced for reduction of CO₂ and degradation of organic dyes.And the catalyst is easy to recycle and reuse.On the other hand,we studied the metal ions exchange capacity of KNbWO₆·H₂O in solution,and characterized the ion-exchangeable products.The major results are as follows:1.We use a two-step solvothermal method to prepare the defect pyrochlore structure oxide KNbWO₆·H₂O.And Sn²⁺ions are doped into the crystals by a facile ion exchange process at room temperature to obtain KNbWO₆·H₂O:xSn²⁺.Compared with KNbWO₆·H₂O,the products after ion exchange still maintain the defect pyrochlore structure with no obvious change in morphology.XPS results show that the oxidation state of Sn²⁺have no change during the ion exchange process.KNbWO₆·H₂O:xSn²⁺exhibits an effective photocatalytic activity for converting chemical stable CO₂ to CO and CH₄.The KNbWO₆·H₂O:0.208Sn²⁺(the highest Sn²⁺incorporation)exhibits the highest photocatalytic performance with a production of CH₄(7.5?mol h^-1 g^-1),CO(12.5?mol h^-1 g^-1)and O₂(20.0?mol h^-1 g^-1).The promoted photocatalytic activity can be attributed to the extended visible light absorption,enhanced charge separation and the improved CO₂ adsorption after Sn²⁺doping.2.The KNbWO₆·H₂O:0.208Sn²⁺was supported on g-C₃N₄ via mixing and sintering.XRD results showed that the phases of KNbWO₆·H₂O:0.208Sn²⁺and g-C3N4 did not change during the mixing process.The diffraction peaks of g-C₃N₄-supportedKNbWO₆·H₂O:0.208Sn²⁺werethesuperpositionof KNbWO₆·H₂O:0.208Sn²⁺and g-C₃N₄.The results of TEM confirmed that a tight interfacewasformedbetweenKNbWO₆·H₂O:0.208Sn²⁺andg-C₃N₄.g-C₃N₄-supported KNbWO₆·H₂O:0.208Sn²⁺show higher photocatalytic activity than KNbWO₆·H₂O:0.208Sn²⁺,g-C₃N₄,or simple mixtures of them.Within 5 h of the xenon lamp irradiation,the methylene blue degradation efficiency of?g-C₃N₄?_0.5(KNbWO₆·H₂O:0.208Sn²⁺)_0.5.5 and?g-C₃N₄?_0.25(KNbWO₆·H₂O:0.208Sn²⁺)_0.75can reach over 90%.The results of PL spectra show that the g-C₃N₄-supported KNbWO₆·H₂O:0.208Sn²⁺greatly inhibited the recombination of carriers and improved the photocatalytic performance.3.The sol-gel method was used to synthesize the defect pyrochlore structure oxide KNbWO₆·H₂O.This method not only opens up a new synthesis route,but also can facilitate the fomation of pyrochlore structural oxide thin film.We discussed the influence of reaction conditions on the synthesis,such as the pH value,the amount of solvent,sintering temperature and so on.And the optimized synthetic parameters were confirmed.In addition,a spin-coating method can be used to attach precursor on quartz substrate and obtain a KNbWO₆·H₂O film after sintering.The Sn²⁺ion exchange reaction can still occur on the obtained film,the product shows a good photocatalytic activity.Compared with the powder photocatalyst,the film material simplifies the recycling and reuse procedures.4.We used KNbWO₆·H₂O for the ion exchange reaction in a mixed solution of Fe³⁺,Cr³⁺,Co²⁺,Ni²⁺,Cu²⁺,Zn²⁺and Mn²⁺.It is found that KNbWO₆·H₂O preferentially exchanges Fe³⁺in the mixed solution.The exchange process conforms to the Langmuir adsorption model.The exchange efficiency can reach more than 90%.After the Fe³⁺exchange is completed,KNbWO₆·H₂O continues to exchange toxic Cr³⁺ions.In addition,KNbWO₆·H₂O can occur ion exchange reaction with Ag⁺.All ion-exchanged products have no change in phase and morphology.Compared with KNbWO6·H2O,the photocatalytic activity of organic dye degradation has been improved after ion exchange.