Study On Energy Band Structure Regulation Of Bismuth-Based Oxide And Its Photocatalytic Hydrogen Production Performance

With the depletion of fossil energy and the increasing environmental pollution,the development of clean and reliable renewable energy technologies is imminent.Photocatalytic water splitting technology for hydrogen production has attracted more and more attention from researchers because of its high reusability and environmental friendliness.The core of photocatalytic water splitting for hydrogen production lies in the development of photocatalysts.Among many photocatalysts,bismuth-based oxide semiconductor materials are considered to have great application potential because of their excellent visible light responsiveness.However,the conduction band position of common bismuth-based oxides is usually low,which makes them unable to meet the requirements of H⁺/H₂ hydrogen production potential.Therefore,it is necessary and meaningful to adjust the energy band structure of bismuth-based oxides to meet the requirements of photocatalytic water splitting for hydrogen production.In this paper,the energy band structures of three bismuth-based oxides were regulated by constructing solid solutions,and the performances and mechanisms of photocatalytic water splitting for hydrogen production were studied.The main research contents are as follows:（1）The Y element was introduced into the β-BiNbO₄ semiconductor material for the first time by a solid-state method,and a new type of solid solution material Bi_0.5Y_0.5NbO₄ was successfully synthesized and used for photocatalytic water splitting for hydrogen production.The introduction of Y element didn’t change the original crystal structure ofβ-BiNbO₄,which still was the triclinic phase.Bi_0.5Y_0.5NbO₄ has excellent light absorption properties,and its absorption band edge is about 400nm.According to the first-principle calculation results,the introduction of Y element will elevated the CBM position of the original BiNbO₄ and increase its band gap,which is consistent with the experimental data.The band gap of Bi_0.5Y_0.5NbO₄ is about 3.26 eV,the conduction band minimum energy（CBM）is about-0.26 eV,and the valence band maximum energy（VBM）is about 3 eV,its energy band structure meets the redox potential of photocatalytic water splitting for hydrogen production.The photocatalytic reduction of Bi_0.5Y_0.5NbO₄ is enhanced due to the elevated position of CBM and the increased band gap.Therefore,the photocatalytic hydrogen production performance of Bi_0.5Y_0.5NbO₄ is much better than that of β-BiNbO₄,and its photocatalytic H₂ generation rate(3.35 μmol h^-1)is 6.1 times that of β-BiNbO₄(0.55μmol h^-1).（2）A series of Bi_3-xY_xO₄Cl solid solution materials were successfully synthesized by a solid-state method,and they were used for photocatalytic water splitting for hydrogen production for the first time.The synthesized Bi_3-xY_xO₄Cl solid solution materials are Sillen-Aurivillius type bismuth-based oxides with layered structure.Bi_3-xY_xO₄Cl is a photocatalyst with visible light responsiveness,and its light absorption region can occupy the entire visible region of the solar spectrum.As the value of Bi/Y increases gradually（from 0.5 to 5）,the absorption band edge of Bi_3-xY_xO₄Cl gradually moves from about 500 nm to 740 nm,and the band gap gradually decreases from 2.6 eV to 2 eV.According to the first-principle calculation results,the band gap of Bi_3-xY_xO₄Cl will gradually increase with the increase of Y element doping,which is consistent with the experimental results.It is worth noting that the CBM of Bi_3-xY_xO₄Cl always satisfies the hydrogen production potential of H⁺/H₂,which makes all Bi_3-xY_xO₄Cl sample materials have the ability of photocatalytic water splitting for hydrogen production,and the photocatalytic water splitting for hydrogen production performance decreases with increasing Bi/Y value.Under visible light irradiation,the optimal H₂ generation rate of Bi_3-xY_xO₄Cl can reach 164.4 μmol h^-1.（3）A new Bi₄MO₈Cl-type solid solution material Bi₄SbO₈Cl was synthesized for the first time by a solid-state method and used for photocatalytic water splitting for hydrogen production.Bi₄SbO₈Cl is also a Sillen-Aurivillius bismuth-based oxide with a layered structure,and its structure is-[Bi₂O₂]²⁺-[SbO₄]^3--[Bi₂O₂]²⁺-[Cl]^--.Compared with Bi₄NbO₈Cl,another typical Bi₄MO₈Cl-type material,it is found that when the main group element Sb replaces the transition element Nb,the original s-p-d hybridization in the energy band structure of Bi₄NbO₈Cl will be replaced by the s-p hybridization in Bi₄SbO₈Cl,which enhances the inherent hybridization of Bi 6s and O 2p orbitals in Bi₄MO₈Cl-type materials,thereby increasing the band gap and raising the CBM position.Bi₄SbO₈Cl is a photocatalyst with visible light responsiveness,and its absorption band edge is about 490 nm.The band gap of Bi₄SbO₈Cl is about 2.53 eV,the CBM is -0.33 eV,and the VBM is 2.2 eV,its energy band structure meets the redox potential of photocatalytic water splitting for hydrogen production.Under visible light irradiation,the H₂ generation rate of Bi₄SbO₈Cl reaches 33.34 μmol h^-1,while Bi₄NbO₈Cl has almost no performance.The enhanced photocatalytic performance of Bi₄SbO₈Cl can be attributed to its higher mobility of photogenerated carriers and stronger reduction ability.