Fabrication Of Tungsten Trioxide-Based Composite Materials And Their Photocatalytic Performance

The problems of energy shortage and environmental pollution that the world is facing today call for the development and utilization of new clean energy sources.Although the conversion of water into H₂ over direct Z-scheme semiconducting materials is considered to be a promising method for H₂ production,the construction of efficient and stable direct Z-scheme photocatalytic systems is still a challenge in this field.For this reason,several WO₃-based nanocomposites have been fabricated,the effects of composition,crystal phase,microstructure and interface controlling on the photoabsorption,Z-scheme charge transfer mechanism,photogenerated charge separation efficiency and photocatalytic H₂ production activity of those composite materials were investigated.The main research contents,experimental phenomena and conclusions are summarized as follows:1.Ag₂S/WO₃ heterojunctions were fabricated by decorating Ag₂S nanoparticles（NPs）on the hydrothermally prepared hexagonal WO₃（h-WO₃）nanorods（NRs）via a precipitation process,whereby h-WO₃ NRs are 100-200 nm in diameter and 1-5?m in length,and the Ag₂S NPs deposited on h-WO₃ NRs are 5-50 nm in diameter and form close contact with WO₃.All Ag₂S/WO₃ heterojunctions with various composite ratios showed better photocatalytic H₂generation activities than the single Ag₂S NPs,and the 10.0wt%Ag₂S/WO₃composite exhibited relatively good stability and the best photocatalytic H₂ generation activity(32.9μmol h^-1),which is～4 times higher than that of the Ag₂S NPs(7.97μmol h^-1).The corresponding results demonstrate that the WO₃ NRs as support can not only effectively avoid the aggregation of Ag₂S NPs,but also promote the charge transfer between WO₃ and Ag₂S to follow the Z-scheme mechanism,which can improve the charge separation efficiency and avoid the Ag₂S NPs’photocorrosion.In addition,the small sized Ag₂S NPs not only facilitate the rapid transfer of their photogenerated charge carriers,but also enhance the separation and transfer efficiency of the photogenerated charge in the composites by combining with Z-scheme charge transfer mechanism.The present results provide a reference for the microstructure optimization of direct Z-scheme photocatalysts and the application of narrow bandgap semiconductors in the field of photocatalysis.2.WS₂/WO₃ heterojunctions were fabricated by partially sulfurizing the hydrothermally prepared hydrated WO₃ nanosheets（NSs）into hexagonal WS₂（2H-WS₂）through an in situ sulfurization process.During the sulfurization process,the orthorhombic hydrated WO₃ NSs were transformed into monoclinic WO₃（m-WO₃）NSs,and ultra-thin 2H-WS₂ layers with a thickness of only～4.2 nm were formed on m-WO₃ NSs.The WS₂/WO₃ heterojunctions with different composite ratios by adjusting the sulfurization degree showed better photocatalytic H₂ production activities than the single ultra-thin 2H-WS₂.Among them,the WS₂/WO₃heterojunction with an optimal ratio delivered a relatively good long-term stability and a H₂production activity of 20.4μmol h^-1,which was about 5 times of that of the ultra-thin 2H-WS₂(4.05μmol h^-1).The ultra-thin 2H-WS₂ obtained through the in situ sulfurization process greatly shortens the migration distance of the photogenerated charge,and the"S-W-O"transition layer formed between 2H-WS₂ and m-WO₃ makes the two phases in close contact,which is then conducive to the interfacial transfer of the photogenerated charge.Besides,the Z-scheme charge transfer mechanism between m-WO₃ and 2H-WS₂ can not only significantly reduce the recombination rate of the photogenerated charge,but also effectively avoid the photocorrosion of 2H-WS₂ by photogenerated holes.The results show that the in situ sulfurization method is feasible and advantageous in the construction of direct Z-scheme heterojunction,which opens up a new way for the construction of novel and efficient Z-scheme photocatalytic system.3.Hierarchically structured h-WO₃ films composed by numerous bundles parallel to the FTO substrate were hydrothermally synthesized in the absence of directing agent on the FTO substrate coated with WO₃ seed layer.The bundles are formed by orderly stacking of h-WO₃NRs.The research results demonstrate that h-WO₃ NRs can grow along the[001]direction and stack along the[200]direction.The Na⁺ions of Na₂WO₄ solution can inhibit the stacking of h-WO₃ nanorods along the[200]direction,while promote their growing along the[001]direction,and thus adjusting the Na₂WO₄ concentration can induce the bundle structure transforming between flat and fusiform shapes,which ultimately affects the surface roughness,pore size and specific surface area of the hierarchically structured h-WO₃ films.In addition,the crystal phase,morphology and orientation of WO₃ films can also be precisely controlled by adjusting the hydrothermal temperature,hydrothermal solution’s p H value and WO₃ seed layer.Among which,the WO₃ film fabricated with flat-shaped bundles has rougher surface and larger pore structure,which can not only provide larger specific surface area,but also improve the light absorption and utilization ability of the h-WO₃ films,and therefore showing the best photocatalytic O₂ production performance(54.1μmol h^-1 cm^-2).The present results lay a solid theoretical foundation for the preparation of WO₃-based photocatalytic films,and also have guiding significance for controllable fabrication of hierarchical structured films of WO₃ and other metal oxides.4.g-C₃N₄/WO₃ photocatalytic films were prepared by calcining g-C₃N₄ nanosheets（CN NSs）and hierarchically structured h-WO₃ films mentioned above.In which,the hierarchically structured h-WO₃ films are formed by flat-shaped bundles,and the particles formed by folded CN NSs are embedded in the gaps of those WO₃ bundles.When the calcination temperature is higher than 400?C,the WO₃ film gradually transforms from h-WO₃ to m-WO₃,causing the formation of h-WO₃/m-WO₃ heterophase junctions.In addition,the CN NSs proportion in g-C₃N₄/WO₃ photocatalytic films can be controlled by adjusting the calcination temperature.Among those g-C₃N₄/WO₃ photocatalytic films,the film obtained at 450?C exhibited the best H₂ production activity(28.5μmol h^-1)under visible light（λ?400 nm）illumination,which is about 2.8 times of that of the Pt-loaded CN NSs film(10.5μmol h^-1).The mosaic structure of those g-C₃N₄/WO₃ photocatalytic films can not only make full use of the specific surface area of the hierarchically structured WO₃ films,but also form intimate contact between CN NSs and WO₃,which is conducive to the interfacial charge transfer.In addition,the charge transfer between CN NSs and WO₃ follows the Z-scheme mechanism,which can significantly improve the charge separation/migration efficiency,and the formed WO₃ heterophase junctions can promote the pre-separation of the photogenerated charge in the WO₃ component,which then strengthens the Z-scheme charge transfer mechanism of the g-C₃N₄/WO₃photocatalytic films.These results demonstrate that the modification of WO₃ plays an important role in improving the photocatalytic performance of WO₃-based Z-scheme composite materials,which provides a technical reference for the design and construction of direct Z-scheme photocatalytic films,it will also promote the practical application of photocatalytic systems.