Controllable Syntheses Of Surface-defect Induced Ti-based Catalysts And Its Applications In Environmental Catalysis

As a clean and efficient energy,hydrogen has been considered to be a promising alternative for fossil fuels.The full use of solar energy for photocatalytic hydrogen evolution is of great significance in the production of hydrogen energy.As a semiconductor,TiO₂ has been wildly used as a photocatalyst,owing to its great stability,high catalytic efficiency,mild reaction conditions and wide applicability.When the surface structure of TiO₂ is defective,these defects can not only provide active sites for photocatalytic reactions,but also trap photo-generated electrons or holes to promote the separation of photo-induced carriers.Herein,TiO₂ nanoparticles that were fabricated using supercritical water?sc-H2O?in a continuous hydrothermal flow system?CHFS?were used as precursors for the syntheses of defective sc-NiO-TiO₂-N₂ and C₃N₄-Ni/TiO₂-N₂ hybrids?followed by a thermo-reduction process?.The synthetic process and photocatalytic mechanism for hydrogen evolution were explored to provide experimental and theoretical basis for the development and application of highly efficient defective TiO₂ based photocatalysts.Defective sc-NiO-TiO₂-N₂ with high full-spectrum photocatalytic activity was fabricated using aforementioned combined route,where organic species were first bounded onto the surface of sc-TiO₂ in CHFS,followed by a relatively low temperature ageing in N₂ to remove the organics.Such a removal process could attract oxygen molecules from the TiO₂ surface,leading to the generation of oxygen vacancies at surface.The NiO was introduced as a thermocatalyst to promote the full oxidation of these organics,which thereby resulted in more oxygen vacancies at the surface.During the thermo-process,the NiO also formed intimate heterojunctions with host-catalysts to further drive the photocatalytic hydrogen evolution.The generated surface oxygen vacancies were found to induce a series of impurity energy levels within the valence band maximum?VBM?and conduction band minimum?CBM?of TiO₂,which narrowed the electron transmission gap in the TiO₂ and acted as active sites for the reaction between adsorbed H2O and photoinduced trapped electrons to produce H2.The hydrogen evolution rate for the developed sc-NiO-TiO₂-N₂ defective heterojunctions?NiO:TiO₂= 3:10?in a sacrificial system was measured at ca.1.41 mmol/g/h,which was 21.7-folds higher than that of pure sc-TiO₂.The C₃N₄-Ni/TiO₂-N₂ with high visible-light photocatalytic activity was made using a similar combined route.In the preparation of sc-Ni/TiO₂ precursor,Ni entered the lattice of TiO₂,which led to the introduction of a series of impurity energy levels and thereby promoted the electronic transmission in the catalyst.The small particle size of sc-Ni/TiO₂?ranging from 3 to 10 nm?revealed the features of semiconductor quantum dots?SQDs?.Under visible light irradiation,these quantum dots provided more active sites for photocatalytic hydrogen evolution,and formed heterojunctions with C₃N₄,increasing the visible light absorption of the catalyst.When irradiated by visible light?>400 nm?with Pt as co-catalyst,the photo-induced electrons could easily transfer from C₃N₄ to sc-Ni/TiO₂ and Pt⁰,leaving the majority of holes on the C₃N₄ sheets.When the mass ratio of C₃N₄:sc-Ni/TiO₂ was 2:1,the photocatalytic activity of C₃N₄-Ni/TiO₂-N₂ for hydrogen evolution was as high as 1183?mol/g/h,which was 11.7-folds higher than that of pure C₃N₄.