Study On The Hydrothermal Synthesis And Photocatalytic Properties Of Bi₄Ti₃O₁₂ Nanosheets Derived From K₂Ti₆O₁₃ Nanofibers

Bismuth titanate(Bi₄Ti₃O₁₂)is a typical bismuth-layered perov-skite-like ferroelectric material,which not only has excellent ferroelectric properties,but also has good visible-light photocatalytic activity.Nanosheets and their self-assembled two-dimensional nanostructures exhibit more excellent photocatalytic performance than zero-dimensional and one-dimensional nanoparticles because they can absorb more solar energy and generate more photogenerated carriers.In this paper,the photocatalysis technology and preparation of Bi₄Ti₃O₁₂2 nanostructures and their application in the field of photocatalysis are reviewed in detail first.Secondly,based on the previously prepared layered structure K₂Ti₆O₁₃ nanofibers,hydrothermal preparation of two Bi₄Ti₃O₁₂ single crystal nanosheets with different light absorption properties is realized by selecting different bi source precursors.Then it was applied to the degradation of rhodamine B?RhB?and studied the visible light photocatalytic activity.In addition,Bi₄Ti₃O₁₂ nanosheets were successfully coated with Bi₂WO₆nanosheets by a two-step hydrothermal method,and the two-phase heterojunction composite visible light catalytic modification was explored.The following main research results have been achieved:?1?The previously prepared layered structure K₂Ti₆O₁₃ nanofibers were mixed with bismuth nitrate or bismuth ammonium citrate.Under the action of the mineralizer KOH,no external surface modifier was introduced.After hydrothermal treatment,facets were successfully achieved.It is the synthesis of Bi₄Ti₃O₁₂ single crystal nanosheets with?010?crystal plane,thickness less than 10 nm,and size up to20?m.In the hydrothermal process,due to Bi³⁺replacing K⁺in the layered K₂Ti₆O₁₃nanofibers,the TiO₆ octahedral layered peeling is caused.The strong binding of bismuth nitrate and ammonium bismuth citrate to the nitrite and ammonium citrate ions in solution and Bi³⁺on the bismuth-rich?010?crystal plane play a key role in the successful synthesis of Bi₄Ti₃O₁₂ single crystal nanosheets without the addition of surface modifiers.?2?Due to the different hydrothermal environments,Bi₄Ti₃O₁₂ single crystal nanosheets synthesized using bismuth nitrate and bismuth ammonium citrate as bismuth sources exhibit different light absorption and photocatalytic properties.Bi₄Ti₃O₁₂ single crystal nanosheets synthesized using Bismuth nitrate as the Bi source are pale yellow,with an optical band gap of 2.41 eV.After loading Pt nanocrystals,they promote the separation of photogenerated electrons and holes,and exhibit excellent visible light catalytic activity in the degradation experiment of RhB,the photodegradation reaction constant is increased by an order of magnitude,from0.0048 min^-1 to 0.0149 min^-1.However,the use of bismuth ammonium citrate as the Bi source resulted in the hydrothermal reaction proceeding in a reducing atmosphere.The synthesized Bi₄Ti₃O₁₂ single crystal nanosheets were black and gray,a large amount of Ti³⁺was present in the crystal lattice,and the optical band gap was 2.01 eV.Although the Bi₄Ti₃O₁₂ nanosheets synthesized using ammonium bismuth citrate have better visible light absorption,due to the large amount of Ti³⁺and oxygen vacancies,they act as trapping centers to accelerate the recombination of photogenerated electrons and holes.The photodegradation reaction constant of this nanosheets is0.00613 min^-1,which visible light catalytic activity did not produce a greater improvement.?3?Epitaxial deposition of Bi₂WO₆ on Bi₄Ti₃O₁₂ single crystal nanosheets with a Bi-rich outer surface was successfully achieved by a two-step hydrothermal method under weakly alkaline conditions,and Bi₄Ti₃O₁₂@Bi₂WO₆ heterojunction composite two-dimensional structure was obtained.Due to the more excellent visible light absorption of Bi₂WO₆ and the promotion effect of heterojunction recombination on the separation of photogenerated electrons and holes,the obtained Bi₄Ti₃O₁₂@Bi₂WO₆ heterojunction composite two-dimensional nanostructure exhibits more excellent visible light catalytic performance than pure Bi₂WO₆ nanosheets,and the absolute value of the reaction constant was increased from 0.028min^-1 to0.04min^-1.