Design,Synthesis And Photocatalytic Activity Of Tantalum-oxide-based Materials

To break through the dependence on traditional non-renewable fuels and meet the requirements of managing environmental pollution in a green way,increasing attentions have been paid to the promising photocatalytic technology.Compared other technologies,photocatalysis is clean,efficient and free from secondary pollution.It is pointed out that catalyst is the key factor for governing the photocatalytic activity,therefore,searching for cheap,efficient and stable photocatalysts has become the primary task of researchers.Crystalline catalysts have been widely studied for decades,but still do not meet the requirements of commercial application,due to the following two shortcomings:?1?less surface active sites,and?2?lattice mismatch at the crystalline heterojunction interface,which may be a barrier for photogenerated carrier transport.Compared with crystalline materials,amorphous materials are rich in active sites and energy band structures.Moreover,the electrons in tail states and valence band of amorphous materials can be excited under illumination.Additionally,the isotropic characteristics of amorphous materials also provide the possibility for the rapid transportation of photogenerated carriers.These features of amorphous materials are all conducive to photocatalysis,the investigations in present thesis are focused on amorphous materials.Take tantalum oxide materials as example,the relationship between crystallinity and photocatalytic activity was studied firstly.It is found that amorphous?a-TaO_x?exhibit excellent photocatalytic hydrogen production activity comparingto crystalline?c-Ta₂O₅?.Then,amorphous/amorphous homojunction and amorphous/crystalline heterojunctions were designed to further study their photocatalytic properties of amorphous materials that were anchored on different crystallinity materials.The contents mainly including the following four aspects:1.The effect of crystallinity on photocatalytic hydrogen production.a-TaO_x nanoparticles with a cubic-like shape were synthesized via the direct evaporation process.The photocatalytic hydrogen evolution rate of a-TaO_x was 17 times higher than that of c-Ta₂O₅,and the activity remained unchanged even after six runs,indicating its excellent stability of a-TaO_x.In addition,a-TaO_x owns a wider light response range compared to the c-Ta₂O₅,which may stem from the excitation of tail-state electrons,a typical feature for amorphous materials.This work provides a new direction for the design and synthesis of highly efficient and stable photocatalysts.2.Design and synthesis of amorphous/amorphous homojunctions.Based on the above study,amorphous tantalum oxide could be considered as a promising photocatalytic candidate.Amorphous tantalum oxyhydroxide homojunction TaO_x?OH?_y NPs/SP was designed and synthesized to further improve the photocatalytic activity of amorphous materials.In this TaO_x?OH?_y NPs/SP homojunction TaO_x?OH?_y nanoparticles?NPs?is anchored in situ on the surface of TaO_x?OH?_y sphere?SP?.The results of surface photovoltage?SPV?,transient photovoltage?TPV?and photoluminescence?PL?spectra show that the amorphous homogeneous structure can promote the separation of photogenerated charges.This work initiated the study of amorphous homojuctions and proved that amorphous homogunction has a higher photocatalytic efficiency compared to the single one?TaO_x?OH?_y NPs or TaO_x?OH?_y SP?.3.An investigation of amorphous/crystalline heterojunction loaded on g-C₃N₄.Amorphous Ta₂O₅ modified TaC?TaC_x O_y?was in situ anchored on g-C₃N₄ for the first time to obtain a composite TaC_x O_y/g-C₃N₄ catalyst,which then was applied to photocatalytic reduction of the poisonous Cr⁶⁺.Because of introducing trace TaC_x O_y into the synthetic system,the original smooth surface of g-C₃N₄ was turned into rough scaly,and the thickness of TaC_x O_y/g-C₃N₄ was reduced to expose more active sites.TaC_x O_y has metal-like properties to show good conductivity and large work function.These features of TaC_x O_y could induce the transfer of photogenerated electrons from g-C₃N₄ to TaC_x O_y and thus promote the separation of photogenerated charges.Compared with previous reported g-C₃N₄-based photocatalysts,the obtained TaC_x O_y/g-C₃N₄ exhibits a higher photocatalytic activity,and reduces Cr⁶⁺completely in 35 minutes???420 nm?.4.Design and synthesis of amorphous/crystalline heterojunction.Amorphous TaO_x nanoparticles?about 3 nm?were firstly anchored on defective g-C₃N₄?DCN?to construct amorphous/crystalline heterojunction,and applied to photocatalytic hydrogen production.The results show that the introduction of defective nitrogen not only improves the absorption of visible light,but also enhances the interaction between TaOx and DCN.The addition of TaO_x promotes the separation and transfer of photogenerated charges.When the loading of TaO_x is 0.05 g,the photocatalytic hydrogen evolution rate reaches the maximum(1783?mol h^-1 g^-1),which is about 3.7 times that of DCN???420 nm?.