Study On Preparation,Modification And Photocatalytic Activity Of Bismuth-based Photocatalysts With Layered Structure

Photocatalytic technology shows broad application prospects in solving energy and environmental problems.Bismuth-based semiconductor photocatalyst has become a research hot spot in photocatalysis field for its favourable visible light response,simple preparation methods,and low cost.Bismuthyl carbonate（（Bi O）₂CO₃）with unique layered structure is a kind of photocatalyst with good application prospect.However,the broad band gap limits its effective utilization of visible light.Considering that（Bi O）₂CO₃has the layered structure similar to bismuth oxyhalides,it is possible to introduce iodide ions（I^-）or bromide ions（Br^-）into the layered structure of（Bi O）₂CO₃to adjust its energy band structure and improve the photocatalytic performance under visible light.Bismuth oxyhalides（Bi OX,X=I,Br,Cl）and bismuth-rich bismuth oxyhalides（Bi_xO_yX_z,x,y,z are not 1 at the same time）are highly anisotropic layered structure semiconductors with adjustable band gaps,inherent internal electric fields,and adjustable oxygen vacancies.The photocatalytic activity can be improved by changing the stoichiometric ratio to adjust the band gap and constructing heterojunction to avoid the rapid recombination of carriers.In this study,the band gaps and microstructure of layered（Bi O）₂CO₃and Bi_xO_yX_zwere adjusted to improve their photocatalytic activity under visible light by means of ion doping,heterostructure construction and change of stoichiometric ratio.The phase structure and its transformation,morphology and microstructure,surface area and pore structure,surface element state,photocatalytic performance and mechanism,and the transfer of photogenerated carriers of as-prepared samples were analyzed by various characterization methods,combined with the results of photocatalysis and electrochemistry tests.The main research contents of this paper are as follows:（1）Iˉions doped（Bi O）₂CO₃was prepared by a facile and rapid solid-state chemical reaction method.The as-prepared samples exhibited excellent visible light photocatalytic activity for rhodamine B removal.Furthermore,the samples also exhibited favourable visible light photocatalytic activity for the removal of methylene blue,crystal violet and salicylic acid.The research on the photocatalytic mechanism indicated that the doping of I^-ions decreased the particle size and band gap simultaneously,which resulted in an increase in the exposed active centers and visible light absorption range,thus enhancing visible light photoactivity.（2）Bi and Bi₂S₃are commonly used to improve the photocatalytic performance of bismuth-based photocatalysts because of surface plasmon resonance effect and visible light absorption efficiency,respectively.Nevertheless,the modified efficiency between Bi and Bi₂S₃has never been compared.In addition,Bi and Bi₂S₃modified semiconductors were usually prepared via two-steps routes,where a semiconductor was firstly prepared,then reduced by a reducing reagent like Na BH₄or ethylene glycol and sulfurized by thiourea,sodium sulfide or thiacetamide.Herein,a facile one-step solid state reaction route was used to prepare Bi（Bi₂S₃）/Bi OCl photocatalysts at room temperature through the reaction between Bi（NO₃）₃·5H₂O and KCl followed by sulfur sources.More importantly,these sulfur sources were not only acted as sulfurization reagents,but also showed reducibility to different extent.Among these sulfur sources,thiourea possessed the strongest reducibility,whereas sodium sulfide presented the strongest sulfurization ability.The investigation on the photocatalytic activity of these composites for rhodamine B and malachite green removal under visible light irradiation shows that Bi-coupled Bi OCl exhibited the highest removal efficiency,which further demonstrates the important role of surface plasmon resonance effect of Bi.（3）Highly crystallized Bi₅O₇I nanorod and Bi OI nanosheet was prepared in urea solution by a simple precipitation method at room temperature.The photocatalytic activity of as-prepared samples for rhodamine B was studied under visible light irradiation.The influence of the preparation conditions（p H,urea dosage and reaction time）on the photocatalytic performance for the samples was studied.The results show that Bi₅O₇I has the best photocatalytic performance under the condition of p H value at about 13,urea content at 7 g and reaction time at 12 h.（4）A series of bismuth oxyiodides were fabricated by grinding-assistant calcining in the presence of thiourea,including Bi₄O₅I₂/Bi OI heterojunction,Bi₄O₅I₂,Bi₄O₅I₂/Bi₅O₇I heterojunction,and Bi₅O₇I.The grinding-induced mechanical force could increase the reaction rates and thiourea could prohibit crystal particles from growing due to anchoring effect.The combined effect of grinding and thiourea could decrease the temperature of phase transformation for bismuth oxyiodides.Among those,Bi₄O₅I₂/Bi₅O₇I heterojunction containing uniform flower-like microspheres assembled by ultra-thin nanosheets exhibited the highest photocatalytic activity and favourable stability for the removal of antibiotic tetracycline under visible light irradiation.This work could provide a good reference for the design of bismuth-rich bismuth oxyhalides heterojunction with excellent photocatalytic performance.