Enhanced Mechanisms Of The Carrier Separation In Bi-based Photocatalysts For Converting Solar Energy Into Hydrogen Energy From Suspension System

Due to photocatalytic hydrogen evolution that could fulfill the target of converting solar energy into hydrogen directly without the consumption of other energies,it has received much attention from researchers.The properties of simple reaction techniques and easy-scaling up also made this procedure popular.The limitation,however,was the low efficiency of the system,which constraints its industriousness applications.The efficiency has a close relationship with the absorption ability of the catalysts,the separation efficiency of the photocarriers as well as the redox reaction rate.Considering the mentioned problems,our work was mainly concentrated on promoting the photocatalytic H₂ activity of the visible-light-responsible bismuth-based photocatalysts.The methods can be summarized as follows:constructing the“junction”engineering（included heterojunction and Mott-Schottky junction）,modulating the internal electrical field,and establishing the solid solution structure.Furthermore,enhanced mechanisms of photocatalytic via the charge separation were also revealed in this work.Detailed works were concluded as follows:Firstly,the Bi₄MO₈X（M=Ta,Nb;X=Cl,Br）materials,which possess the[Bi₂O₂]²⁺layered structure,were prepared successfully via the solid-state method.It has similar structures and showed the photocatalytic hydrogen production ability under visible light irradiation.The materials exhibited different performances can be ascribed to the[Bi₂O₂]²⁺structures with the lower twisting degree,which affected the photogenerated internal electrical field.Considering the mutual restriction relationship between light absorption ability and thermodynamical property,Bi₄Nb O₈Cl was selected as the primary catalyst.Y source was introduced to improve its photocatalytic reduction property.Simultaneously,the separation efficiency of photocarriers could be enhanced by the formation of Z-type heterojunction construction,which improved the photocatalytic efficiency synergistically.In conclusion,Bi Y₃Nb O₈Cl showed a rate of 169.0μmol/h,17 times that of Bi₄Nb O₈Cl,with a result of 10.1μmol/h.Although the visible light responsible BiVO₄ with a structure of BiO₆octahedral has a positive CB position for hydrogen evolution,it has a suitable VB position,which could afford the requirement of the overall water splitting thermodynamically.This work prepared the MOF-derived Bi_0.5Y_0.5VO₄ to introduce the Y source by controlling the calcined time.The role of the Y promoted the crystalline transformation and the formation of a solid solution structure,which made the Bi_0.5Y_0.5VO₄ fulfill the overall water splitting.The hydrogen evolution property exhibited the rule of“volcano peaks”.Results revealed that samples with a calcined time10 hours showed the best photocatalytic overall water splitting performance with a hydrogen evolution rate of 124.2μmol/h,much higher than the rate achieved by the tetragonal phase Bi_0.5Y_0.5VO₄,11.7μmol/h.Enhanced mechanisms could be ascribed to the improved separation efficiency of photocarriers and light absorption ability as well as the thermodynamical property of the materials.Subsequently,introducing the Y source via the hydrothermal method,m-BiVO₄ coupled with Bi_0.5Y_0.5VO₄ composites(H-Bi_0.5Y_0.5VO₄)with a morphology of nano-dodecahedron has been prepared.The occurrence of m-BiVO₄ could be attributed to the inhibition of the crystalline transformation.Results showed that the enhanced twisting degree in hetero-Bi_0.5Y_0.5VO₄(H-Bi_0.5Y_0.5VO₄),which reconstructed the internal electric field and thus promoting the photogenerated carriers migrating directionally to form the direct Z-scheme heterojunction,should be responsible for the improvement of the photocatalytic performances.Furthermore,it also contributed to the morphology edge formation,which could be another critical factor for improving hydrogen production efficiency.Photocatalytic results illustrated that H-Bi_0.5Y_0.5VO₄ exhibited a performance of 103.2μmol/h,15 times that of H-Bi_0.5Y_0.5VO₄,which showed the pure tetragonal phase after prepared by hydrothermal method.Metallic bismuth also played an essential role in the photocatalytic hydrogen evolution due to its whole spectrum absorption ability.However,the single metallic Bi could not accomplish the procedure of photocatalytic hydrogen evolution.In this section,Bi-PCN materials were prepared via the hydrothermal method,which applied PCN to the semiconductor materials in the Mott-Schottky junction.Photocatalytic experiment results showed that the composites exhibited a superior performance than both pure PCN and nano metallic bismuth spheres.Furthermore,the formation of the Mott-Schottky junction improved the efficiency of photocarriers’separation.The rate reaches 36.0μmol/h under visible light,three times that of the pure PCN samples.Mechanisms and hydrogen evolution factors were investigated in the synergetic system.The results showed that the competition relationship between H⁺and O₂ in the reaction with photoelectrons should play a more critical role.In consequence,in this work,the“junction”engineering,modulating the internal electric field and the solid solution band energy strategy was constructed to solve the efficiency bottlenecks in photocatalytic hydrogen evolution,which will be beneficial for improving the photocatalytic efficiency.It will be helpful for the construction of the bismuth-based materials applied for photocatalytic hydrogen production.