Morphology-controlled Preparationof Co-doped BiOCl And Their Photocatalytic Performanecein Degradationof Aqueous Phenolic Pollutants

Sustainable photocatalytic technology has emerged as an ideal technology in environmental pollution remediation due to its directly harvest energy from solar light,efficient degradation of organic pollutants at room temperature and no secondary pollution.Although the conventional photocatalyst,titanium dioxide?Ti O₂?,shows superstrong photooxidizing ability,nontoxic and low cost,it suffers from critical limitations of poor sunlight utilization and fast recombination probability of the photogenerated charge carriers.In the search for novel and alternative photocatalysts to Ti O₂and focus on the photocatalytic performance in degradation of organic pollutants under sunlight/visible-light is therefore elicited great attention.Bismuth oxyhalides?Bi OX,X=Cl,Br and I?with unique layered structure and internal static electric fields,which promoted separation efficiency of the photogenerated charge carriers,is the promising candidate of new generation alternative photocatalysts to Ti O₂.As for BiOCl,it has shown superstrong photoactivity in degradation of organic pollutants.However,BiOCl possesses a wide band gap and its sunlight utilization efficiency is very low,which severely hinders its visible-light performance.Aiming at to develop novel visible-light-driven BiOCl-based photocatalysts for efficient degradation of organic pollutants,in the present work,we focus on the design of various modified BiOCl materials with transition metal cobalt?Co?doping as well as different micro-morphology,including 2D nanosheets?Co-BiOCl NSs?and 3D sphere-like microflowers?Co-BiOCl MFs?.Their morphologies,phase and chemical structures,optical absorption properties and electrochemical behaviors were well-characterized.To evaluate the visible-light photocatalytic performance as well as reusability and stability of as-prepared Co-BiOCl NSs and Co-BiOCl MFs,two emerging phenolic derivatives without visible-light photosensitive effect,acetaminophen?APAP?and methylparaben?MPB?,are chosen as the target pollutants.The present studies find out that the Co doping levels and morphological characteristics influence the visible-light photocatalytic performance of as-preapred Co-BiOCl heterojunctions.Meanwhile,the reaction mechanism of visible-light photocatalytic degradation of phenolic pollutants over the Co-BiOCl heterojunctions is tentatively put forward,and the degradation pathway of phenolic pollutants are revealed.The main results of the present work are as follows.1. Two dimensional?2D?Co-doped BiOCl nanosheet?Co-BiOCl NSs?were facilely fabricated via one-step additive-free hydrothermal treatment method by uing water and glycol as the mixing solvent,Bi?NO₃?₃·5H₂O as a Bi source and1-hexadecyl-3-methylimidazolium chloride(C₁₆[Mim]Cl)as a Cl source.In this preparation system,the molar ratio of Co-to-Bi was 1:146.The Co-BiOCl NSs exhibit well-defined tetragonal crystal phase with?001?facet exposed dominantly,and the estimated particle size is in the range of?100-150 nm???100-150 nm?,and the estimated thickness is 15 nm.The BET surface area of Co-BiOCl NSs is 11.5 m²g^-1.The doped Co element is on the surface or in the shallow lattice of the BiOCl host crystal.Their intrinsic optical absorption occurred at 200-360 nm,and Co-BiOCl NSs had a weak absorption in visible light region.Under the visible-light?400 nm<?<680 nm?irradiation,the Co-BiOCl NSs displays significantly enhanced photocatalytic ability towards degradation of aqueous APAP and MPB.Additionally,the degradation performance of APAP is still remained after four recycling runs.The good catalytic reusability and stability of the Co-BiOCl NSs is attributed to Co interacts with BiOCl via chemical reaction in which doped Co element is tightly bonded on the surface or in the shallow lattice of the BiOCl host crystal.2. A series of three dimensional?3D?monodispersed Co-doped BiOCl sphere-like microflowers?Co-BiOCl MFs?were facilely fabricated via one-step polyvinyl pyrrolidone?PVP?-directed solvothermal treatment method by using glycol as the solvent,Bi?NO₃?₃·5H₂O as a Bi source and C₁₆[Mim]Cl as a Cl source.In the above preparation system,the molar ratio of Co-to-Bi was 1:225,1:187 and 1:135,respectively.The Co-BiOCl MFs exhibit well-defined tetragonal crystal phase with?110?facet exposed dominantly.The estimated particle size and BET surface area of as-prepared Co-BiOCl MFs is in the range of 800-1000 nm and 57.8-61.5 m²g^-1,respectively.The doped-Co element is on the surface or in the shallow lattice of the BiOCl host crystal.Co-BiOCl MFs exhibit intrinsic optical absorption in the region of200-390 nm;meanwhile they have a weak absorption in visible-light region.Moreover,the light-harvesting ability improved as compared with 2D Co-BiOCl NSs.Through photocatalytic degradation of aqueous PNP,APAP and MPB under visible-light?400 nm<?<680 nm?irradiation,the photocatalytic activities of various Co-BiOCl MFs heterojunctions are compared.It shows that doping Co in BiOCl crystal lattice can significantly enhance the photocatalytic activity of BiOCl,and Co doping levels significantly affect its photocatalytic activity.Furthermore,3D Co-BiOCl MFs exhibits remarkably higher visible-light photocatalytic performance toward phenolic pollutants as compared with 2D Co-BiOCl NSs at similar Co doping level.For the most photocatalytically active Co-BiOCl MFs?the molar ratio of Co-to-Bi was 1:187?,its apparent rate constant is 4.72,2.20 and 1.97 times higher than that of BiOCl NSs,Co-BiOCl NSs and BiOCl MFs,respectively,for photodegradation of aqueous APAP;while the value is 3.59,2.69 and 2.06 times higher than that of BiOCl NSs,Co-BiOCl NSs and BiOCl MFs,respectively,for photodegradation of aqueous MPB.Additionally,as-prepared Co-BiOCl MFs also show excellent catalytic stability and reusability,and after four consecutive photocatalytic degradation runs,the activity loss is neglectable,attributing to chemical bonding Co within BiOCl crystal lattice.3. Based on the photoelectrochemical experiments,free radical and hole scavenging experiments as well as crystal phase structure,the visible-light photocatalytic degradation mechanism of Co-BiOCl NSs and Co-BiOCl MFs are proposed.It shows that the doped Co element is on the surface or in the shallow lattice of the BiOCl host crystal,which generates a doping energy level in the band gap of BiOCl and thus the electron transition can be well-regulated without changing the energy band structures.Accordingly,doping BiOCl with Co can improve the light harvesting ability of BiOCl and extending the light absorption in visible-light region,as a consequence,plentiful photogenerated charge carriers are generated and participate in target photocatalytic reactions,which plays the dominated role to enhance the inherent photocatalytic activity of BiOCl.In addition,the morphology,porosity properties and exposed facet also significantly influence the photocatalytic activity.3D Co-BiOCl MFs with the advantages including larger surface BET areas,higher light harvesting ability and proper exposed facets are also benefit to this excellent photocatalytic activity towards the degradation of phenolic pollutants as compared with 2D Co-BiOCl NSs.Finally,the pathway of visible-light photocatalytic degradation of APAP and MPB over the Co-BiOCl MFs is tentatively put forward based on the identified intermediates yielded during the photodegradation process.