Preparation And Photocatalytic Properties For Hydrogen Production Of Double Perovskites And Aurivillius Phase Materials With Narrow Band Gaps

Nowadays,environmental pollution and energy crisis is getting much worse than ever before in this world.Under this circumstance,it is imperative to develop an alternative clean energy for our sustainable future.Photocatalytic water splitting for hydrogen production has been widely regarded as a viable solution to tack with problems mentioned before.The technical kernel of photocatalysis highly depends on synthesized semiconductors.Nevertheless,majority of semiconductors only respond to ultraviolet rays owing to their intrinsic band gaps,which strictly circumscribe the practical application in contemporary society.Therefore,development of excellent semiconductors that can respond to visible light and possess chemical stability is the current research orientation.Thanks to the structural simplicity and component diversity of perovskites,which make it easier to regulate the physical and chemical properties of perovskites.This research is mainly focusing on improving photocatalytic performance of perovskite semiconductors by controlling crystal structure,micromorphology,light absorption,and chemical components on surface.Furthermore,the mechanism of enhanced photocatalytic activity is also investigated in our study.The experimental systems of this paper are listed below:In the first part,a series of transition metal cations（Cr,Fe,Ni）doped double perovskites Ba₂InNbO₆ with visible light adsorption were synthesized to achieve the target of water splitting for hydrogen evolution under visible light irradiation.Pristine Ba₂InNbO₆ is a kind of favorable ultraviolet light responded semiconductor with a band gap～3.1 eV.This material has an intrinsic wide band gap nature,which is detrimental to harness sufficient visible light.Hence,we elected to accommodate transition metal cations（Cr,Fe,Ni）into Ba₂InNbO₆to alter its band structure and were looking forward to realize high efficient visible light photocatalytic activity as well as investigated the impacts of doping elements on physical and chemical properties such as crystal structure,micro morphology and surface state.The results showed that transition metal cations can be successfully introduced into the In site of double perovskite Ba₂InNbO₆and broaden the adsorption range of visible light.Besides,the introduction of transition metal could shrink unit cell,enlarge the particle size,and improve the surface hydrophilia greatly.Compared to Ba₂InNbO₆,transition metal ion-doped samples display an improved photocatalytic activity of hydrogen evolution.Among those prepared materials,Ba₂In_0.9Fe_0.1NbO₆had the best photocatalytic performance under both ultraviolet light irradiation（λ≥250 nm）and visible light irradiation（λ≥420 nm）,gave a highest H₂ production of 158.06μmol·g^-1and a highest H₂ evolution rate of 31.42μmol·g^-1·h^-1corresponding to an apparent quantum efficiency of 0.16%respectively.In the second part,a bunch of Ba₂In_1-xFe_xNbO₆（x=0,0.1,0.3,0.5,0.7）with abilities of utilizing visible light were prepared to accomplish the goal of H₂ production under visible irradiation.In the last chapter,we optimized the doping element with different kind of transition metals and found out that doping Fe in into In site is capable of decreasing the amount of defects which act as a recombination centers,reducing band gaps and extending the light absorption range from ultraviolet light to visible light so as to realize the high performance of photocatalytic water splitting for hydrogen production.In this chapter,Therefore,we modulated the doping level of Fe to further optimize the photocatalytic activity of this material.Our results showed that the unit cell shrank,agglomeration occurred,light adsorption ability got strengthened and surface hydrophilia changed with the doping level increased progressively.What’s more,the photocatalytic activity of doped samples was more outstanding than those of undoped ones remarkably,and every doped sample acquired a better visible photocatalytic activity.When x=0.1,Ba₂In_0.9Fe_0.1NbO₆gave a best photocatalytic activity.However,the photocatalytic activity was getting worse with the doping level increased.The enhanced photocatalytic activity was mainly from deduced band gap after doping,which provided an excellent ability of light adsorption.But when the doping level became larger,with the shrunken specific surface area of samples and the decreased number of active sites came the worse photocatalytic activity.In the third part,a variety of Aurivillius phase materials Bi₅Ti₃CrO₁₅with visible light response were synthesized via hydrothermal method with different reaction periods for the purpose of achieving photocatalytic hydrogen evolution under visible light irradiation.Meantime,conventional solid-state method was also applied to prepare target material for contrast.Compared with solid-state method,the advantage of hydrothermal method could control the morphology of those materials and ensure a high Brunauer-Emmett-Teller surface.All the materials synthesized via hydrothermal reaction possessed an average band gap of 2.45 eV and an average adsorption edge of600 nm as well.Among all prepared materials,the highest activity was found in HT-48,where～2261μmol·g^-1 of H₂ was produced under visible light（λ≥250 nm）irradiation within 3 hours and～61.83μmol·g^-1·h^-1of H₂ evolution rate was also detected under visible light（λ≥420 nm）irradiation.In the light of BET analysis,the smallest band gap and large surface area of HT-48 played a pivotal role in providing an extra driving force and an increasing number of active sites for improving photocatalytic activity.