In-situ Rational Synthesis Of BiOCl-based Photocatalysts And Their Applications In Photocatalytic Fuel Cells

Semiconductor photocatalysis is considered as one of the most promising technologies for water treatment due to its environmental and mild reaction conditions.The photogenerated electrons can be transmitted to the{001}facets of BiOCl(BOC)in the action of internal electric field(IEF),which accelerates carrier separation efficiency.Oxygen vacancies(OVs)are induced on the{001}facets of BOC.BOC modified by g-C₃N₄(CN)enhances light absorption.However,the disordered growth and loose contact restrict their photocatalytic activity.Cl atoms of g-C₃N₄-Cl(CNCl)have strong electronegativity and strong electron-absorption,which causes uneven charge distribution and can capture positive ions.Thus,CNCl can work as the"core"for in-situ growth.The in-situ growth of BOC/CNCl heterojunctions has not been reported.To improve light absorption of BOC in practical application,high efficient photocatalysts were obtained by means of IEF control,heterostructure construction and solid solution formation in the work,based on the theory of IEF and energy band structure.A series of BiOCl-based photocatalysts with efficient visible-light photocatalytic activity were designed and prepared,including BOC with regulating{001}crystal facet exposure,in-situ growth of heterogeneous BiOCl/g-C₃N₄-Cl(BOC/CNCl)and BiOCl_xI₁-x/g-C₃N₄-Cl(BOC_xI₁-x/CNCl).BOCI/CNCl/r GH photoanode is subsequently fabricated with graphene hydrogel(r GH)as the carrier,which overcame the difficulty in recovery of powder photocatalyst and solved the problem of instability of immobilization of anode materials.The main research contents are listed as follows:(1)Through adjusting the{001}facet percentage of BOC,IEF and concentration of oxygen vacancy were enhanced,which thus improved the photocatalytic activity.A series of BOC materials with different exposed crystal faces were obtained by simple room temperature precipitation method.The exposure of BOC-1 and BOC-2 and BOC-3 with{001}crystal planes is 65%,70%and 76%,respectively.DFT calculations and EPR results showed that the larger{001}exposure of BOC not only possessed more oxygen vacancies site,resulting in the adsorption of more O₂ and the formation of·O₂^-.Meanwhile,the increased charge density improved the IEF intensity,which is conducive to the separation efficiency of photogenerated carriers.BOC-3 exhibited the highest photocatalytic activity for TCH,and its photodegradation kinetic constant of BOC-3(0.022min^-1)is 1.4 times that of BOC-1(0.016 min-1).(2)A composite photocatalyst BiOCl/g-C₃N₄-Cl(BOC/CNCl)is obtained by introducing CNCl into the preparation of BOC system.Accordingly,the mechanism of in-situ growth is that Bi³⁺is easily adsorbed on the CNCl surface because the zeta potential of CNCl is negative.BOC/CNCl generated stronger IEF compared with BOC/CN,which enhanced the separation of photogenerated carriers and improved photocatalytic activity.The removal rate of BOC/CNCl increased by 40%than BOC/CN.Thus,a direct Z-scheme is proposed,in which,holes and electrons were concentrated on the valence band of BOC and the conduction band of CNCl,respectively,resulting in stronger redox activity.BOC/CNCl-3 showed the highest photocatalytic activity,and the removal rate of TCH and Cr(VI)were 90%and 99%,respectively after 60 minutes of visible light irradiation.(3)The BiOCl_xI_1-x solid solution formed by the introduction of I showed strong visible light absorption ability.Composite photocatalyst BiOCl_xI_1-x/g-C₃N₄-Cl(BOC_xI_1-x/CNCl)is synthesized by adjusting the precursor ratio of chlorine and iodine source to modify BOC/CNCl.The formation mechanism of composite photocatalyst BOC_xI_1-x/CNCl is the same as that of BOC/CNCl.The strong IEF between BOC_xI_1-xand CNCl is conducive to the separation of photogenerated carriers.The removal rate of TCH by BOC_0.75I_0.25/CNCl increased by 45%,while that of Cr(VI)decreased by 21%compared with BOC/CNCl.The introduction of I source broadened the light absorption region,thereby improving the removal efficiency of TCH.However,in p-n heterojunction BOC_0.75I_0.25/CNCl,the reduction reaction occurred in the conduction band of BOC_0.75I_0.25,which is more positive than CNCl,resulting in a decrease in the removal rate of Cr(VI).(4)BOCI/CNCl/r GH photoanode is obtained by heating in water bath.In this process,BOCI/CNCl is loaded on the r GH grid.The short-circuit current and maximum power density of BOCI/CNCl/r GH PFC were 5.1 and 1.2 times higher than that of BOCI/CNCl/FTO PFC,respectively,which is ascribed to low resistance of r GH and strong light absorption.BOCI/CNCl/r GH PFC showed superior photocatalytic activity.The removal rate of TCH and Cr(VI)increased by 29%and 32%respectively,compared with that of BOCI/CNCl/FTO.Notably,the removal efficiency of BOCI/CNCl/FTO PFC in TCH+Cr(VI)co-existence system is higher than that of isolation TCH or Cr(VI)system,and the removal rate of TCH and Cr(VI)increased to87%and 85%,respectively.In this work,IEF intensity is improved by regulating exposed facet and in-situ growth strategies of heterostructure,so as to speed up the separation efficiency of carriers and promote the photocatalytic activity of BOC-based photocatalyst.It can provide theoretical basis and new strategies for the design and preparation of other similar photocatalyst with high photocatalytic performance.