Preparation Of Nb₃O₇F-based Heterojunctions And Their Hydrogen Evolution Properties

Semiconductor photocatalysis technique for water splitting is an important method of solar energy conversion and utilization,which has great potential in solving energy and environment problems.The key issue of photocatalytic hydrogen evolution is to develop new photocatalytic materials with wide visible light response,high photogenerated electron transport rate and efficient photogenerated carrier separation.Nb₃O₇F has attracted wide attention because of its suitable hydrogen evolution potential,good photostability and thermal stability.However,its wide band gap and fast photogenerated carrier recombination of Nb₃O₇F have severely restricted the development of Nb₃O₇F in the field of photocatalytic hydrogen production.In this thesis,the band gap of Nb₃O₇F is adjusted by constructing semiconductor heterojunction.Consequently,the photogenerated electrons and holes are effectively separated,finally obtaining highly efficient Nb₃O₇F-based photocatalyst.The main research contents are as follows:（1）Preparation of Nb₃O₇F/C₃N₄ heterojunctions and their photocatalytic performance.In this chapter,the heterojunction was successfully constructed between Nb₃O₇F and C₃N₄ by thermal polymerization.The effect of heterojunction on morphology,phase composition,energy band structure and photocatalytic performance of Nb₃O₇F was studied in detail.The results showed that after the formation of Nb₃O₇F/C₃N₄ heterojunction,the phase composition and morphology of the Nb₃O₇F did not change.Compared with pure Nb₃O₇F,the band gap of Nb₃O₇F/C₃N₄photocatalytic material was obviously narrowed from 3.18 e V to 2.64 e V.Platinum was used as co-catalyst,photocatalytic hydrogen evolution performance was improved obviously under simulated sunlight irradiation.Nb₃O₇F/C₃N₄-0.1 catalysts showed the highest hydrogen evolution rate of 1242.1μmol h^-1 g^-1,which was about 2.4 times of that of pure Nb₃O₇F and 2.9 times of that of pure C₃N₄,respectively.Electrochemical impedance and transient photocurrent confirmed that Nb₃O₇F/C₃N₄ heterojunction could significantly reduce the transfer resistance,enhance the migration ability of photogenerated charges,and improve the separation efficiency of photogenerated electron/hole pairs.（2）Preparation of ReS₂/Nb₃O₇F heterojunctions and their photocatalytic performance.In this chapter,ReS₂/Nb₃O₇F heterojunctions were synthesized by one-step hydrothermal method.The phase composition,morphology,light absorption ability and photocatalytic performance of the final samples were studied in detail.The results showed that ReS₂ did not affect the phase structure and morphology of Nb₃O₇F,and a few layers of ReS₂ nanosheets closely adhered to the surface of Nb₃O₇F nanosheets to form a heterogeneous structure.The visible-light absorption of ReS₂/Nb₃O₇F heterojunctions was widened markedly,and its hydrogen evolution efficiency was also enhanced,compared to pure Nb₃O₇F.ReS₂/Nb₃O₇F-4 showed the best performance.Under simulated sunlight irradiation,the photocatalytic hydrogen evolution rate of ReS₂/Nb₃O₇F-4 was 54.86μmol h^-1 g^-1,about 25.4 times that of pure NOF.Further study found that ReS₂/Nb₃O₇F heterojunctions significantly accelerated the transport of photogenerated charges and inhibited the recombination of photogenerated electrons/hole pairs,which could be attributed to the main reason for the enhancement of photocatalytic hydrogen evolution performance.（3）Preparation of Cu₂O/NaNbO₃/Nb₃O₇F ternary heterojunctions and their photocatalytic performance.In this chapter,Cu₂O/NaNbO₃/Nb₃O₇F heterojunctions were successfully prepared by chemical bath method,The phase composition,morphology,spectrum absorption and photocatalytic performance of the final samples were studied in detail.The results showed that part of Nb₃O₇F was etched by NaOH to form NaNbO₃,and the surface of the smooth Nb₃O₇F nanosheets became rough,which could expose more active site and be befenicial to Cu₂O loading.The formation of Cu₂O/NaNbO₃/Nb₃O₇F heterojunction obviously contributed to the enhancement the light absorption.Moreover,the band gaps were narrowed from 3.18 e V to 2.96 e V.The photocatalytic hydrogen evolution performance was also much higher than that of pure Nb₃O₇F.Cu₂O/NaNbO₃/Nb₃O₇F-2 showed the best photocatalytic performance.Under the irradiation of simulated sunlight,the photocatalytic hydrogen evolution rate of ReS₂/Nb₃O₇F-2 was 45.55μmol h^-1 g^-1,which was about 21 times that of pure Nb₃O₇F.Further study found that Cu₂O/NaNbO₃Nb₃O₇F heterojunction significantly improved the migration of photogenerated charge,enhanced the separation efficiency of photogenerated electron/hole pairs,and eventually led to the improvement of photocatalytic hydrogen production efficiency.There are 41 pictures,4 tables and 121 references in this thesis.