Study On The Preparation, Photocatalysis And Adsorption Properties Of Two New TiO₂(B)-based Semiconductor Materials

It is necessary to solve the energy shortage and environment pollution as the essential problems to guarantee world stability and sustainable development.Semiconductor-based photocatalysis,a perfect idea to fuel production.and environment-pollution solution,is becoming an important scientific technology with sunlight-driven key reaction including water splitting to hydrogen evolution,organic pollutant photodegradation and CO2 reduction to fuel production.It is well known that the application of traditional photcatalysts TiO2 and ZnO was limited by their wide band gaps and low separative efficiency of photogenerated electron-hole pairs.Therefore,controlling and optimizing the microstructure is an effective methods to adjust the photocatalyst bands for solar energy utilization.Additionally,development df new semiconductors has become a top-priority method to release the limitation from the traditional photcatalysts.Herein,two new TiO2-based semiconductors,Nao.9Mgo.45Ti3.55O8 and Na2Fe2Ti6O16,were prepared for the first time via hydrothermal method.The main results were summarized as follows:1.A new-type of semiconductor Na0.9Mg0.45Ti3.55O8:preparation,characterization and photocatalysisUltrapure Na0.9Mg0.45Ti3.55O8 nanosheets with dominant {0 0 1} facets were synthesized via a simple hydrothermal method for the first time.UV-DRS result demonstrated that the semiconductor was a direct bandgap semiconductor with a bandgap of about 3.55 eV.The photocatalytic activity of the Na0.9Mg0.45Ti3.55O8 nanosheets was evaluated by photodegrading MB under UV-visible light irradiation.NMTO exhibits higher photodegradative ability in comparison with the raw TiO2 photocatalyst due to its direct bandgap and appropriate band.The photogenerated holes are mainly responsible for MB photodegradation according to the trapping experiments.2.Interface role in the enhanced photocatalytic activity of TiO2-Na0.9Mg0.45Ti3.55O8 nanoheterojunctionHeterojunction TiO2-Na0.9Mg0.45Ti3.55O8 photocatalyst were synthesized via a simple hydrothermal method.The anatase TiO2 nanoparticles were loaded onto the NMTO sheets in different connections,forming perfect nano-heterojunctions with a well matched lattice,which was confirmed by the spin results.Additionally,hall effect analysis indicates a n-type semiconductor of Na0.9Mg0.45Ti3.55O8 and formation of a n-n type TiO2-Na0.9Mg0.45Ti3.55O8 heterojunction.And the heterojunctions enjoy a high photocatalytic activity for RhB degradation in comparison with pure materials TiO2 and Na0.9Mg0.45Ti3.55O8 due to its effective separation of photogenerated electron-hole pairs under simulated sunlight irradiation.The aggravated changes in surface potential across the material interface,detected by SPM under illumination,indicate that the electrons accumulate at interface and holes remain in the valence bands of TiO2 and Na0.9Mg0.45Ti3.55O8.The interface states play a key role in inhibiting the recombination of the photogenerated electronhole pairs by capturing electrons on the dangling bonds,resulting in the enhanced photocatalytic activity.The trapping experiments revealed that photogenerated holes and superoxide radicals are mainly responsible for MB photodegradation.3.Novel magnetic semiconductor Na2Fe2Ti6O16:synthesis,double absorption and strong adsorptive abilityHighly pure Na2Fe2Ti6O16 with long hexagonal nanosheets was synthesized via a simple hydrothermal method.The growth mechanism was investigated via XRD,SEM and Ostwald ripening process,as well as Hartman-Perdok method.UV-DRS result demonstrated that the semiconductor was a direct bandgap semiconductor with a double absorption band gap of about 2.53 and 2.20 eV,extending visible-light adsorption and being applied in visible-light harvesting.As a multifunctional material,Na2Fe2Ti6O16 possesses the saturation magnetization of 0.34 emu g-1 at room temperature to favour magnetic separation from aqueous solution for cyclic utilization.The double absorption and magnetic property originate from the d-d transitions of Fe3+ under the effect of crystal field.The magnetic Na2Fe2Ti6O16 nanosheets enjoy excellent removal capability toward methylene blue after several recycles,and absolute ethanol is its desorbent.The adsorption process is consistent with the pseudo-second-order kinetics model.The equilibrium isotherm meets the Langmuir isotherm with a maximum adsorption capacity of 165.8 mg g-1 at an equilibrium.4.Na2Fe2Ti6O16 as a hybrid cocatalyst on g-C3N4 for enhanced photocatalytic hydrogen evolution under visible lightNovel semiconductor Na2Fe2Ti6O16,synthesized via a simple hydrothermal method,possesses excellent visible-light havesting but shows poor photocatalysis.The heterojunction Na2Fe2Ti6O16/g-C3N4 was prepared via loading Na2Fe2Ti6O16 as cocatalyst onto g-C3N4.The SEM and TEM results indicated that smooth and tight heterostructures with a well matched lattice between Na2Fe2Ti6O16 and g-C3N4 were formed.In addition,infrared spectrum analysis revealed the stable covalent chemical bonds between g-C3N4 and Na2Fe2Ti6O16 instead of a simple physical attachment,which is consistent with the above SEM and TEM results.Mott-schottky plots of Na2Fe2Ti6O16 show a positive slope,indicating the typical behavior of n-type semiconductor,and the verified conduction band potential was similar to that of theoretical calculation.The n-n type heterojunctions Na2Fe2Ti6O16/g-C3N4 enjoy enhanced photocatalytic activity in water splitting for hydrogen evolution in comparison with pure materials g-C3N4 under visible light irradiation.Based on above results,schematic diagram of heterojunction was established to reveal the separation of photogenerated electron-hole pairs and transfer of photoexcited charge carriers.Finally,the reasonable photocatalytic mechanism of Na2Fe2Ti6O16/g-C3N4 heterojunction was proposed and analysized in detail.