Synthesis Of TiO₂ Hierarchical Spheres-based Nanocomposites And Their Photocatalytic Performance

In the past few decades,TiO₂ materials have been intensively studied and used as water treatment and water-splitting photocatalysts due to their easy available,low cost,non-toxicity and high chemical stability.However,the photocatalysis application of TiO₂ is restricted due to the high recombination rate of photogenerated carriers and the wide band-gap of anatase TiO₂（3.2eV）,which causes TiO₂ to exhibit fundamental absorption in the UV region,covering only 4%of the sunlight spectrum.Thus,Extending TiO₂ absorption to the visible light range and improving the separation of photogenerated carriers are much importance.In this thesis,we has been devoted to the modification of TiO₂ to expand the light absorption range and improve charge separation,including controlled by tuning morphologies,crystalline,structures of TiO₂,depositing with noble metal nanoparticles（Au,Ag and Pt）,combining with Ag₃PO₄ strategies.The structure and catalytic mechanism of light microscopic composites were carried out a detailed characterization and in-depth discussions,the results obtained are as follows:（1）We via a facile one-pot solvothermal method to synthesis of TiO₂ Hierarchical spheres（THS）.The as-prepared THS with large specific surface area are sel-assembled form ultrathin TiO₂nanosheets with thickness of 3-5 nm and show a uniform spherical morphology with an average size of 500-700 nm.However,the as-prepared light yellow THS exhibit inferior photocatalytic activity for hydrogen evolution form water splitting due to the poor crystallization of TiO₂ and the existence of oxygen vacancies.Significantly,a subsequent thermal treatment improves the crystallinity of THS,reduces the oxygen vacancies,and thereby enhances the photocatalytic performance.It demonstrates that the sample annealed at 550^oC（THS550）exhibits the highest photocatalytic activity,about 5 times higher thanthat of commercial TiO₂ nanoparticles（C-TiO₂）.Moreover,the THS550 sampleloaded with 1 wt%Pt exhibits an hydrogen evolution rate as high as 17.9 mmol h^-1g^-1,and the corresponding apparent quantum efficiency has been determined to be 28.46%under 350 nm light irradiation.（2）A synergistic strategy involving oxygen vacancy generation and noble-metal deposition is developed to improve the photocatalytic performance of TiO₂ hierarchical spheres（THS425）through a redox reaction between the THS with oxygen vacancies and metal salt precursors.Noble-metal NPs（Ag,Au and Pt）are uniformly deposited on the defective THS425 surface in the absence of any reducing agents or stabilizing ligands.The resulting THS425-M（M=Ag,Au and Pt）shows high-performance photocatalytic hydrogen production.The results show that the oxygen vacancy creation obviously enhance the visible-light absorption of THS425 and the noble-metal deposition can effectively improve charge-separation of THS425.In particular,THS425-Pt shows the highest photoactivity and reached 13.16 mmol h^-1g^-1,which can be attributed to the more efficient charge-carrier separation of THS425-Pt than THS425-Au and THS425-Ag,the formation mechanism have been investigated.（3）In this section,a new Ag₃PO₄/TiO₂ hierarchical spheres（THS550）heterojunction structure was designed in order to achieve the facilitation of the separation of photo-induced holes and electrons via the band match between Ag₃PO₄ and THS550,the improvement of the stability of Ag₃PO₄,and increasing the range of absorbed light frequencies.Ag₃PO₄/THS550heterojunctions was systhesised by the in-situ deposition of Ag₃PO₄ nanoparticles onto THS550with large specific surface area.The morphology and structure of the Ag₃PO₄/THS550 were characterized.The photocatalytic abilities of the Ag₃PO₄/THS550 for the degradation of Methylene blue（MB）were then investigated and the photochemical stability of the Ag₃PO₄ was also determined.Finally,a possible mechanism for the photocatalytic degradation of MB is presented.