Controllable Synthesis,Defect Engineering And Photocatalytic Performance Of TiO₂ Nanomaterials With Exposed{001} Facets

Photocatalytic hydrogen production and pollutant degradation have been research hotspots in recent years.With a suitable energy band structure,TiO₂ is regarded as the benchmark material in photocatalytic reactions,and makes hydrogen evolution and pollutant degradation thermodynamically possible.We synthesized a series of morphology-controlled and defect-engineered titanium based nanomaterials.The physico-chemical properties are investigated,and the relationship between material structure and performance is well revealed.The results of the research are summarized as follows.1.A series of titanium based nanocrystals was synthesized under solvothermal conditions,and the effects of the amount of HF on the structure evolution were revealed.Our results show that anatase TiO₂ nanoparticles with exposed{101}facets are synthesized without HF.After addition of a small amount of HF,the sheet-shaped TiO₂ is formed with exposed{001}facets.The as-prepared sample exhibits enhanced photocatalytic performance because of the increased charge separation efficiency,which depends on the surface heterosructure generated between{101}and{001}facets.With more HF,the as-prepared sheet-stacking sample is formed which contains TiO₂ and TiOF₂ phases at both surface and bulk regions,and shows the decreased activity.When we further increase the amount of HF,the F^-ion totally moves into the crystal lattice and?NH₄?_0.3TiO_1.1F_2.1 is formed.Though the as-prepared sample shows a low photocatalytic activity because of the improper band gap structure,it could be used as the precursor for the synthesis of N,F doped titanium based semiconductors.2.We chose TiO₂ with exposed{001}facets as the model catalyst to study the effects of defect distribution?surface and bulk?on photocatalytic hydrogen evolution.We found that only surface oxygen vacancies?V_o's?and Ti³⁺centers in TiO₂ can be induced by hydrogen spillover enhanced hydrogenation,whereas the generation of bulk V_o's and Ti³⁺species depends on the thermal treatment in nitrogen.Both the surface and bulk defects in TiO₂ could promote the separation of electron-hole pairs,enhance the light absorption,and increase the donor density.Compared with bulk defects,the surface defects are more favorable for the improvement of the separation efficiency of photo-generated electrons and holes.In addition,TiO₂ with surface defects displays much higher photocatalytic activity than TiO₂ with only bulk defects.The presence of surface and bulk defects in TiO₂ can not change the valence band maximum,but determines the conduction band minimum.Surface defects in TiO₂induce a tail of conduction band located above the H⁺/H₂ redox potential,which benefits the photocatalytic performance.However,bulk defects in TiO₂ generate a band tail below the H⁺/H₂ potential,which inhibits hydrogen production.The enhanced photocatalytic activity of hydrogenated TiO₂ with surface defects results from the favorable band gap structure which is propitious to photocatalytic hydrogen evolution.