Design Of Defect Engineering In Titanium Dioxide Nanosheets For Photocatalytic Degradation

Due to the rapid development of manufacturing and pharmaceutical industries,antibiotics and dyes in sewage have attracted widespread attention in the field of public health.Commercial water disinfection methods,such as physical adsorption,biodegradation,have unavoidable side effects,which increase treatment cost and environmental problems.Photocatalytic technology uses renewable energy(solar energy)and low-cost semiconductor materials to generate highly reactive oxidative species(ROS)(such as superoxide radicals(·O2-)and hydroxyl radicals(·OH))to degrade pollutants in sewage,showing unique advantages and prominent application prospects in water treatment.In this paper,we designed a defect engineering strategy,which using anatase titanium dioxide(TiO2)as a model photocatalyst to explore the superiority of oxygen vacancies(Vo)and adjusting its structure to promote the production of more ROS and improve the performance of photocatalytic degradation in sewage.The overall results were the following conclusions:(1)A simple one-step hydrothermal synthesis method was used to prepare TiO2 samples rich in Vo with tetrabutyl titanate(Ti(OBu)4)as a titanium precursor and hydrofluoric acid(HF)as a shape controlling agent.The morphology and structure of the sample were characterized as anatase TiO2 nanosheets(denoted as XF-TiO2,X represents the amount of HF added,X=0.8ml,1.0ml,1.2ml,1.4ml);Raman,EPR,and XPS were used to prove the existence of Vo in TiO2,in addition,compared with commercially pure anatase TiO2(denoted as Bulk-TiO2),Vo in XF-TiO2 were more abundant;Combining UV-DRS?PL and photoelectrochemical testing characterization results and photocatalytic degradation performance of organic dyes-Rhodamine B(RhB)of XF-TiO2 proved Vo could increase the light absorption of TiO2,capture electrons(e-)to inhibit the recombination of photo-generated e-and holes(h+)in the photocatalytic process of TiO2 and activate O2 adsorbed on the TiO2 to generate·O2-,which promoted the degradation of organic dyes.(2)Optimizing Vo-surrounding metal sites to engineer the defect structure of TiO2 nanosheets in order to enhance the photocatalytic activity.0%-TiO2(also called 1.2F-TiO2)were doped with transition metals Fe,Co,Cr,Mn to synthesize 2.5M-TiO2(M=Fe,Co,Cr,Mn),respectively.XRD?ICP-AES?UV-DRS were used to characterize the samples:metals were well doped into TiO2 in the form of metal ions;compared with Bulk-TiO2 and 0%-TiO2,the light absorption capacity of 2.5M-TiO2 was significantly enhanced,even extending to the visible light region;and the photocatalytic degradation performance of RhB was significantly improved,among them,2.5Fe-TiO2 had the best photocatalytic degradation performance.(3)Based on the above research results,the amount of Fe metal sites around Vo was further optimized to engineer the defect structure of TiO2 nanosheets to enhance ROS production for boosting photodegradation.0%-TiO2 were doped with different amounts of Fe ions to synthesize X%-TiO2,where X%is the Fe/Ti molar ratio in percentage and X%=1.5%,2.5%,3.5%,4.5%);STEM?XPS?EPR were used to characterize the uniform distribution of Fe in X%-TiO2 and the existence form of Fe was the coexistence of Fe3+/Fe2+ions.PL?UV-DRS and photoelectrochemical testing were used to characterize the optical properties of X%-TiO2:Fe doping was verified to improve light absorption properties of anatase TiO2,shorten the band gap(Eg)of TiO2,promote the separation and transfer of photo-generated e-and reduce the recombination rate of photo-generated e-and h+.The photocatalytic degradation ability of X%-TiO2 were tested under light,and the optimal Fe doping amount was 3.5%.3.5%-TiO2 could photocatalytically degrade 99.53%of RhB in just 20 minutes and the degradation rate(0.3073 min-1)was 134 times and 27 times than Bulk-TiO2 and 0%-TiO2,respectively and was superior to most of reported photocatalysts.;Further extending to the degradation of antibiotics,the 3.5%-TiO2 could photocatalytically degrade 99.53%of tetracycline hydrochloride(TC-HCl)with degradation rate(0.3073 min-1)in 60 minutes under light,which was faster than 0%-TiO2.Through the capture experiment of ROS and in situ electron paramagnetic resonance,it was found that ROS in the photocatalytic reaction were ·O2-and h+.At last,the possible photocatalytic mechanisms were proposed:under light conditions,the photo-generated e-of 3.5%-TiO2 rich in Vo and with Fe doping were excited from VB to CB,leaving h+on the VB;Because of the defective energy level between VB and CB,both Vo and Fe3+ could capture e-to inhibit photo-generated e-and h+ recombination;O2 molecules adsorbed on the surface of 3.5%-TiO2 were activated by e-from Vo and/or Fe2+ to converted into ·O2-;·O2-and h+attacked to destroy the molecular structure of RhB and TC-HCl for degradation;finally,through the Fenton reaction,part of Fe2+was oxidized to Fe3+ by hydrogen peroxide(H2O2)or O2,realizing the cycle of photocatalytic degradation process.This article provides an in-depth understanding of defect-promoted ROS,and proposes a practical design strategy for photocatalysts,which can be optimized to be widely used in photooxidation.