The Preparation Of Z-scheme BCN/TiO₂ Heterostru-Cture And Its Visible-light-driven Photocatalytic Hydrogen Production

Titanium oxide（TiO₂）is a well-known photocatalytic semiconductor owing to its strong oxidizing ability,nontoxicity,high chemical stability,and low cost.However,its poor photocatalysis performance is severely insu cient for practical applications due to the extremely low utilization ratio of solar energy caused by the large band gap,the rapid recombination of photogenerated electron-hole（e^--h⁺）pairs.The synthesis of a composite of TiO₂ with other semiconductors to form heterostructure material was demonstrated to be useful in improving the photocatalytic activity of TiO₂-based materials,owing to an e ective separation of electron–hole charge pairs and also decreasing the rate of their recombination.The two-dimensional layered material h-BCN is a new type of semiconductor with tuneable band gap energies between graphene（zero bandgap）and h-BN（bandgap=5.5 eV）,which have the same atomic structure and share many similar properties.When the content of carbon doping is optimal,the energy band structure of BCN is similar to that of g-C₃N₄.Namely,the band gap of BCN is about 2.72 eV,which can absorb visible light up to 460 nm.Furthermore,the CB potential（-1.24 eV vs.NHE）of BCN is extremely negative,so photogenerated electrons should have high reduction ability.However,the potential of the photogenerated holes in the VB of BCN is inadequate to oxidize OH-to hydroxyl radicals（E（OH-/·OH）=2.80 eV）.Coupling BCN with TiO₂ is a good strategy for improved photocatalytic performance by the effective separation of photogenerated electron-hole pairs,due to the more negative conduction band（CB）potentials of BCN and more positive valance band（VB）potentials of TiO₂ in Z-scheme photocatalysts.In this paper,BCN-TiO₂ nanocomposites were constructed by calcination method or ball milling method using a hexagonal boron carbon nitride（BCN）semiconductor and nanotubular titanic acid（NTA）as precursors.The physical,chemical and optical properties of the resulted composites were thoroughly characterized with XRD,TEM,FT-IR,UV-vis,BET,PL,XPS,and so on.The effect of the ratio of two semiconductors,surface oxygen vacancies,TiO₂ crystalline and different preparation methods of BCN-TiO₂ nanocomposites on the visible-light photocatalytic activity of H₂ production were studied in detail.The details are as follows:（1）BCN-TiO₂ nanocomposites are obtained by a simple calcination method using a hexagonal boron carbon nitride（BCN）semiconductor and nanotubular titanic acid（NTA）as precursors.The effect of the ratio of two semiconductors on the visible photocatalytic activity of the BCN-TiO₂ semiconductor was studied.The morphology and structure characterization results indicate that TiO₂ nanoparticles were uniformly dispersed on the surface of two-dimensional layered material BCN after high temperature calcination.Compared with single TiO₂,BCN-TiO₂ composite semiconductor exhibits obvious visible light absorption and enhanced photocurrent response.The content of BCN has obvious effect on the photocatalytic activity of BCN-TiO₂ nanocomposites.When the content of BCN in BCN-TiO₂nanocomposites is 4%,the photocatalytic activity of hydrogen production is the highest（19.72μmol/g/h）,which is 2 times and 3 times than that of single TiO₂ and BCN,respectively.A direct solid-state Z-scheme mechanism for the enhanced photocatalytic activity of BCN-TiO₂ was confirmed by the fluorescence hydroxyl capture experiment.Therefore,both high oxidation ability of novel TiO₂ and high reduction ability of BCN can be utilized for BCN-TiO₂ nanocomposites by Z-scheme charge transfer.So the visible light photocatalytic activity for H₂ production of BCN-TiO₂ nanocomposites has been improved significantly.（2）All solid-stated Z-scheme BCN-TiO₂ heterostructrues with surface oxygen vacancy layer as the contact interface are fabricated by NaBH₄-reudction and ball milling.The effect of surface oxygen vacancy layer of BCN-TiO₂ binary heterostructrues on the charge carrier transfer and photocatalytic hydrogen evolution was thoroughly investigated.The surface oxygen vacancies of BCN-TiO_2-x nanocomposites were confirmed by means of ESR,XPS and TEM.UV-vis diffuse reflection spectra results reveal that surface oxygen vacancy can extend the light absorption from the UV to visible-light region for BCN-TiO₂heterostructrues.The BCN-TiO_2-x heterostructrues exhibited a higher visible-light photocatalytic efficiency,about 3 times higher than that of BCN-TiO₂ nanocomposites.The steady-state/time-resolved photoluminescence spectra exhibit the increased charge carrier lifetime,the improved charge carrier separation efficiency and the strengthened direct Z-scheme charge transfer process for BCN-TiO_2-x-x heterostructrues.Namely,the photocatalytic activity of hydrogen evolution can be greatly promoted by constructing the surface oxygen vacancy layer due to the rapid transfer of photogenerated electron and hole.Moreover,the circulation experiments showed that the BCN-TiO_2-x heterostructrues possess high stability for its practical application.（3）BCN-TiO₂ was prepared by high temperature calcination method with BCN and Nanotube Titanic Acid as precursors.The effect of calcination temperature on the performance of hydrogen production by visible light catalysis of TiO₂ and composite semiconductors was investigated.The results of XRD show that the TiO₂ is anatase phase in the range of 450-650°C,and the rutile phase appears at 750°C,and the TiO₂ in the compound is mainly rutile at 850°C.The results of photocatalytic hydrogen production show that the transformation of crystal type has a great influence on the activity of hydrogen production by visible light,and the 10%BCN-TiO₂ complex obtained at 750°C has the highest visible photocatalytic activity of hydrogen production,which is about 68.54μmol/g/h。The photocurrent time curve shows that compared with the single TiO₂（A+R）,the BCN-TiO₂（A+R）photocurrent increased obviously after the composite of BCN,indicating that the composite semiconductor BCN-TiO₂（A+R）has a higher electron hole separation efficiency,which is beneficial to the improvement of the hydrogen production performance of visible light catalysis.