Investigation On Synthesis And Photocatalytic Performance Of Bi₄NbO₈Cl-based Materials

The environmental pollution caused by global industrial development,has become one of the primary challenges for human society.Semiconductor photocatalysts can utilize solar energy to oxidation organic pollutants and transfer into non-toxic and harmless small molecules.Thus,semiconductor photocatalysis is considered as a promising and widely used technology to eliminate organic pollutants.Among the well-known Bi-based oxychlorides,Bi₄NbO₈Cl is a Sillén-Aurivillius hybrid perovskite and has been reported for its outstanding visible-light-response ability.The presence of strong internal electric fields produced by the layered structure is believed to enable the effective separation and fast migration of the photoinduced electron-hole pairs.Moreover,the lone-pair distortion of Bi 6s orbital in the Bi₄NbO₈Cl semiconductor may cause the hybridization of O 2p and Bi 6s orbitals in the valence band,which leaded to a more positive valence band position and improved oxidation ability of the photocatalyst.In this thesis,the pure phase Bi₄NbO₈Cl was first prepared by the solution combustion method and solid-state method.Then,noble metal?Ag?loading and semiconductor coupling strategies were used for extending the optical response range and improving the separation and migration efficiency of carriers.The as-prepared photocatalysts were used in the field of environmental purification,such as decomposition of organic pollutants and reduction of heavy metal ions in water.The main works are as follows:?1?Bi₄NbO₈Cl was prepared via the solution combustion method,and then we synthesized Ag/Bi₄NbO₈Cl photocatalyst by the photo-deposition method.The results of TEM and XPS display that metallic Ag nanoparticles??10 nm?are uniformly distributed on the surface of Bi₄NbO₈Cl.Methyl orange?MO?,methylene blue?MB?,Rhodamine B?RhB?,and Cr?VI?were selected as the target pollutants to investigate the photocatalytic activities under simulated-sunlight irradiation,and the Ag modified photocatalysts exhibited better degradation performance for Rh B.Among the series of Ag/Bi₄NbO₈Cl composites based on mass ratio of metal Ag?Ag:Bi₄NbO₈Cl=1%,3%,5%?,the 3%Ag/Bi₄NbO₈Cl sample processes best photocatalytic degradation activity of RhB?degradation rate up to almost 99%within 3 h?,and also shows excellent reusability.The enhancement of photocatalytic performance of 3%Ag/Bi₄NbO₈Cl could be ascribed to the surface plasmon resonance?SPR?and electron traps by Ag nanoparticles.Therefore the synergetic effect can strengthen the visible-light absorption ability,suppress the recombination of photo-generated electrons and holes,and promote the migration rate of charge carriers.?2?Bi₄NbO₈Cl and NiTiO₃ were prepared by solution combustion method and sol-gel method,respectively.Then,NiTiO₃/Bi₄NbO₈Cl heterostructure photocatalysts were successfully synthesized by mechanical mixing method which includes grinding,ultrasonic and calcination.The crystal phase,morphology,optical properties,and energy band structures of all samples were characterized by XRD,XPS,SEM,TEM,UV-Vis DRS,and Mott-Schottky tests.The results show that NiTiO₃ particles uniformly distribute around the surface of Bi₄NbO₈Cl,and close interfacial contacts are formed.Among all the samples,10%NiTiO₃/Bi₄NbO₈Cl composite exhibit the best photocatalytic performance for decomposition of RhB.The enhanced photocatalytic can be ascribed to formation of the intimate interfacial contacts and type-II band alignment between NiTiO₃ and Bi₄NbO₈Cl,which resulted in rapid separation and transportation of photoinduced charge carriers.The mechanism of the improved photocatalytic performance were also proved by photoluminescence spectra?PL?,time-resolved decay spectra?TR-PL?,photocurrent and electrochemical impedance spectroscopy?EIS?.?3?Bi₄NbO₈Cl photocatalyst was first synthesized by the ball-milling calcination method.Then,Bi₄NbO₈Cl and NbCl₅ were dispersed in a mixture of ethanol and acetic acid,and the mixture underwent the one-pot hydrothermal process.The ternary Bi₄NbO₈Cl/BiOCl/Nb₂O₅ composites were fabricated where ultrafine Nb₂O₅ and BiOCl nanoparticles were in-situ grown and loaded on the surface of Bi₄NbO₈Cl.Based on the TEM and SEM observation and phase composition analysis?XRD?FTIR and XPS?,an in-situ hydrothermal-induced formation mechanism for ternary Bi₄NbO₈Cl/BiOCl/Nb₂O₅ could be proposed.The exfoliated[Bi₂O₂]²⁺layers in the Bi₄NbO₈Cl combine with plenty of dissociated Cl^-to form BiOCl nanoparticles,and Nb₂O₅ was formed via NbCl₅ precursor.An improved photocatalytic activity towards RhB removal by 10%Bi₄NbO₈Cl/BiOCl/Nb₂O₅ is attributed to the suitable energy levels to form a cascade structure,which supress the recombination of photoinduced electron-hole pairs,increase the fluorescence lifetime of charge carriers,and strength the oxidation ability,subsequently enhance the photocatalytic efficiency.