Synthesis Of Defective TiO₂-based Heterojunction Photocatalysts And Evaluation Of Degradation Mechanism Of Antibiotics

In recent years,a large quantity of antibiotics have been discharged into the environment due to their unreasonable use and lower removal efficiency.As an emerging contaminant in the environment,direct hazard and resistance genes induced from antibiotics pose potential threat to human and ecological health.Heterogeneous photocatalysis based on semiconductors is a green and economic advanced oxidation technique,which can realize solar-to-fuel conversion to effectively degrade organic pollutants in water.Owing to low cost,chemical stability,nontoxicity,and high catalytic activity,titanium oxide(TiO₂)has the most prospectively market application,however,the weak light-harvesting capacity and easy recombination of photoinduced carriers greatly restrict its actual large-scale application.Accordingly,this paper proposes a strategy for tuning defect component and intensity of built-in electric field in heterojunction by different doping methods(self-doping,metal doping and metal-nonmetal codoping),to synergistically enhance the optical response range and separation efficiency of electron-hole pairs,meanwhile the relationship between the improved photocatalytic activity and surface defect,energy band position,electronic structure,and interfacial interaction of composite were systematically investigated.Firstly,from the point view of oxygen defect engineering,highly stable oxygen vacancies/Ti³⁺self-doped CSs/TiO_2–x@g-C₃N₄heterojunction photocatalysts were prepared by a mild solvothermal approach.During the synthesis process,TiO_2–xnanodots were evenly loaded onto CSs with intimate chemically bonded(Ti-O-C)interfaces,while g-C₃N₄were tightly coated on the surface of TiO_2–x@CSs,with the purpose of preventing the oxidation of the directly exposed Ti³⁺.The results of both XPS and EPR clarify the presence of Ti³⁺/oxygen vacancies,which not only increase the light absorption but also reduce the charge recombination of TiO₂.The enhanced specific surface area of the heterojunction can effectively shorten the distance of mass transfer and promote the adsorption of reactant molecules.Under either visible,UV or simulated sunlight exposure,CSs/TiO_2–x@g-C₃N₄showed preferable ciprofloxacin(CIP)photodegradation than that of the binary photocatalysts and pristine components.Analogously,such ternary heterojunction still exhibited excellent and broad-spectrum photooxidation ability towards norfloxacin(NOR),oxytetracycline hydrochloride(OTTCH),and tetracycline(TC).The enhanced photocatalytic performance can be ascribed to the synergistic effects of the introduced Ti³⁺/oxygen vacancies and the vectorial electron transfer of g-C₃N₄?TiO_2–x?CSs.Then,all-solid-state Z-scheme V-TiO₂@1T/2H-MoS₂ photocatalysts with V doping were fabricated via an in-situ hydrothermal method,in which 2H phase of Mo S₂acts as a photosensitiser in 1T/2H-Mo S₂cocatalyst,while the conductive metallic 1T phase of Mo S₂serves as an electron mediator.V-doped TiO₂with surface defect and edge-rich ultrathin nanostructures of 1T/2H-Mo S₂are beneficial for generating reactive sites.The results of FTIR and contact-angle test showed more hydroxyl groups on the heterojunction,suggesting that the hydrophilic antibiotic molecules are prone to be adsorbed on its surface.For OTTCH,VTM-5%(the mass ratio of 1T/2H-Mo S₂and V-TiO₂in composite)exhibited the best degradation rate(92.4%)and mineralization rate(65.8%)of TOC within 80 min under visible light irradiation,and 84.9%of CIP were decomposed after irradiation for 120 min.On the basis of energy band position and interfacial electron density distribution calculated by Density functional theory(DFT),1T/2H-Mo S₂and V-TiO₂with negative-charge and positive-charge,respectively,result in a driving force of an built-in electric field,thereby the electrons would be accumulated on the negative conduction band of V-TiO₂,which is helpful for activating oxygen species to form active radicals.The constructed all-solid-state Z-scheme heterostructure can also protect 1T phase of Mo S₂from photocorrosion and phase transformation,giving rise to good catalytic activity and cyclic stability.At last,direct Z-scheme V/N-TiO₂/Bi₃TaO₇ photocatalysts were successfully obtained based on codoping of V and N using a facile solvothermal route.Ammonium metavanadate was used as the source of V,followed by thermal treatment in flowing NH₃to achieve N doping species.The codoping of V and N not only dramatically narrowed the band gap of TiO₂,but also shallowly captured the electrons to prolong the lifetime of free carriers.In particular,in situ grown of Bi₃Ta O₇nanoparticles on V/N-TiO₂possess small size effect,and interfacial interactions between two moieties are capable of modulating the local electronic structure for synergistically enhanced photodegradation of quinolone antibiotics(CIP and NOR)and tetracycline antibiotics(OTTCH and TC).According to the results of XPS,the transfer of interfacial electrons was found from V/N-TiO₂to Bi₃Ta O₇,and the formed built-in electronic field led to a direct Z-scheme structure,rather than type-II,as confirmed by the generated radicals during the degradation process and band structure from DFT.In addition,it can be concluded that open-ring reactions and cleavages of the central carbon might be the two main reaction routes for OTTCH degradation according to the primary intermediates identified by GC-MS.