In-situ Synthesis Of Bismuth-based Heterojunctions And Thier Photocatalytic Performance

Along with the high speed development of our country’s economy,the human’s standard of living increases universally;however,we still face energy deficiency and environmental pollution problems.In recent years,semiconductor photocatalytic oxidation technology has great application prospects in the aspect of environmental purification and energy conversion because of its high efficiency,cheap,environmentally friendly and sustainable utilization.However,most traditional photocatalysts have wide band gaps,which can only absorb the ultraviolet portion of solar energy.Therefore,developing visible-light-driven photocatalysts with low cost and high-efficiency has become the focus and difficulty in the field of photocatalysis.Firstly,bismuth-rich Bi₃O₄Br photocatalyst has been synthesized by ionic liquid self-combustion,which photocatalytic activity was surveyed by degradation of dye methyl orange（MO）and photocatalytic reduction of CO₂ under visible-light irradiation.The as-prepared photocatalysts were characterized by X-ray diffraction（XRD）,Transmission electron microscopy（TEM）,Scanning electron microscopy（SEM）,UV-Vis diffuse reflectance（DRS）,and Surface photovoltage spectroscopy（SPV）.The results show that Bi₃O₄Br exhibits higher photocatalytic ability than pure Bi OBr,which can be attributed to that Bi₃O₄Br has a good absorption performance in the visible light region and the lift of Bi₃O₄Br conduction band,thus improving the reduction power of photo-induced electrons.The photocurrent and electrochemical impedance tests prove excellent charge separation ability of the as-prepared Bi₃O₄Br.Secondly,Bi₃O₄Br/α-Bi₂O₃ heterojunctions with different proportions were prepared by adjusting the tetrabutyl ammonium bromide（TBAB）,which exhibit higher photocatalytic degradation ability for MO and phenol than pure Bi₃O₄Br andα-Bi₂O₃.The pseudo-first-order rate constant of heterojunction containing 61 wt%Bi₃O₄Br and 39%α-Bi₂O₃ is approximately 11.6 and 5.2 times that of the pure Bi₂O₃ and Bi₃O₄Br in degradation of MO,respectively.Work function test and scavenger experiments display that holes play key role for pollutant degradation and the position of holes onα-Bi₂O₃ is lowered after the combination of Bi₃O₄Br and α-Bi₂O₃.However,photoelectrochemical measurements show that separation efficiencies of photo-generated electrons and holes are decreased for heterojunction.The enhanced photocatalytic activity over Bi₃O₄Br/α-Bi₂O₃ heterojunction can be attributed to the position decline ofα-Bi₂O₃ valence band.In this work,the concept that valence band position plays more important role than charge separation efficiency in constructing heterojunctions is proposed for the first time.Finally,Bi₂Al₄O₉/β-Bi₂O₃ heterojunctions were synthesized through ionic liquid self-combustion,which exhibit the higher photocatalytic activity for degradating MO under visible light irradiation.Meanwhile,the as-synthesized samples have high selectivity for photocatalytic reduction of CO₂ under the simulated sunlight.The as-prepared photocatalysts were characterized by XPS,SPV,photoelectrochemical test.The result indicate that Bi₂Al₄O₉ andβ-Bi₂O₃ are bound by chemical interaction,which favors the effective separation and transportation process of photo-induced charge carriers.