Design,Preparation Of Composite Photocatalyts Based On Energy Band Engineering And Its Nitrogen Fixation Performances

With the rapid development of human society,people are facing severe challenges in energy and environment issues.As a renewable and clean energy solar energy is the basis for the sustainable development of human society.Therefore,how to efficiently convert and store solar energy is an important topic of scientific research in the 21st century.As a cutting-edge technology semiconductor photocatalysis provides us with an ideal method of solar energy conversion and storage.The persistent in-depth study of semiconductor photocatalysis materials promotes the continuous progress of photocatalysis technology.As a typical polymer semiconductor,the band gap of carbon nitride（CN）is 2.7 eV.It has suitable conduction band and valence band positions,large specific surface area and good stability.CN has become a semiconductor photocatalytic material attracting much attention.However,CN has a series of problems,such as weak reactivity and high recombination rate of photogenerated electrons and holes,which greatly limit the further improvement of photocatalytic efficiency of CN.Therefore,this paper takes energy band engineering as the guiding ideology,optimizes the spatial behavior of photogenerated carriers in CN by designing and constructing different energy band systems and conducts in-depth research on its photocatalytic mechanism.The main research work includes four parts:1.p-n heterojunction Cu₂O/CN composite photocatalytic material was designed and prepared based on energy band engineering theory,which improved the performance of photocatalytic nitrogen fixation and studied its energy band structure and mechanism of photocatalytic nitrogen fixation.The p-n heterojunction was constructed by electrochemical deposition.The difference of charge concentration and electron affinity between Cu₂O and CN semiconductor interfaces resulted in the rearrangement of charges at the interface,forming a built-in electric field and accelerating the relative movement of photogenerated carriers.In addition,the difference of the interface work function of the p-n heterojunction accelerated the separation of photocarriers and improved the efficiency of charge separation and transmission.SEM and TEM images showed that Cu₂O nanospheres uniformly grew on the surface of CN nanosheets.XRD and XPS characterization proved the successful preparation of composites and Motschottky test proved the successful construction of p-n junction.The formation of p-n heterojunction promoted the efficient separation of photocarriers and improved the photocatalytic nitrogen fixation performance of the materials.Cu₂O/CN exhibited excellent photocatalytic nitrogen fixation performance(10 μmol·h^-1),with a quantum efficiency of 0.57%at 400 nm and the yield of NH₃ remained unchanged after continuous reaction for more than 20 hours.Through the in-depth study of its band structure,it was confirmed that p-n heterojunction can effectively inhibit the recombination of photogenerated electrons and holes,which will provide guidance for the subsequent design of improved nitrogen reduction photocatalyst.2.A band matching WO₃/B-CN composite photocatalytic material was designed and prepared.By adjusting the doping ratio of B element,the space control of CN band position was realized,so as to better match the band of WO₃ and further improved the performance of photocatalytic nitrogen fixation.Z-scheme photocatalytic heterojunction was constructed by solid-phase calcination method.The construction of Z-scheme photocatalytic system avoided the reduction of REDOX barrier caused by p-n heterojunction.Meanwhile,B element was used to regulate the energy band of CN and achieved a good match between the energy band of CN and WO₃.The carrier transport barrier in Z-scheme photocatalytic system was reduced.The structure and morphology characterization proved that the composite was successfully prepared,the ESR test proved the successful construction of Z-scheme system and the variation rule of sample energy band was explored by UV-vis spectroscopy and Motschottky test.The WO₃/B-CN photocatalytic system effectively improved the separation efficiency of photocarriers and prolonged their lives.The quantum efficiency of photocatalytic nitrogen fixation at 400 nm was 0.71%and the yield of NH₃ under visible light was 450.94μmol·g^-1·h^-1.Moreover,the yield of NH₃ remained unchanged after continuous testing for more than 20 hours.The change of CN band with the introduction of B was deeply studied,and the good band matching between B-CN and WO₃ reduced the carrier transport resistance,which will have important guiding significance for the design of efficient photocatalyst in photocatalytic nitrogen fixation.3.The P-CN/BCN composite photocatalytic material was designed and prepared.Through reasonable regulation of P and B elements,the energy band matching between the components of the composite material was realized,the separation of photocarriers was accelerated,and the performance of photocatalytic nitrogen fixation was improved.Z-scheme P-CN/BCN composite photocatalytic materials were prepared by calcination method.Energy band matching and REDOX ability coordination were realized by adjusting the composition of composite materials.The doping of P element increased the conduction band position of CN,enhanced its reducing ability,changed the content of B element in BCN,lowered the valence band position of BCN,and improved its oxidation ability.The Z-scheme system of P-CN and BCN was constructed by recombination to take advantage of the strong reducing ability of conduction band of P-CN and the strong oxidation ability of valence band of BCN.Therefore,the composite photocatalytic materials not only had high REDOX ability,but also accelerated the transfer of electrons and holes,prolonged the life of the carriers.SEM and TEM images showed that both P-CN and BCN were gauze,XRD and XPS proved the successful preparation of the material and ESR proved the successful construction of Z-scheme system.The variation rule of sample energy band was explored by UV-vis spectroscopy and Mottschottky test.By regulating P and B elements,the active sites of the composite were increased and the reactivity of the composite was improved.The yield of NH₃ under P-CN/BCN visible light was 460.52 μmol·g^-1·h^-1.4.The α-Fe₂O₃/Ti₃C₂/CN composite photocatalytic materials were designed and prepared.The energy band at the interface between semiconductor and metal was bent by the difference of work function between semiconductor materials and metal phase materials and the internal Shottky heterojunction was constructed,which reduced the electron transport resistance in Z-scheme system and further accelerated the separation and transfer of electrons and holes.The performance of photocatalytic nitrogen fixation was improved.The Z-schemeα-Fe₂O₃/Ti₃C₂/CN composite photocatalytic materials were prepared by hydrothermal method combined with solid-phase calcination method.The Fermi energy levels of α-Fe₂O₃5 Ti₃C₂ and CN were investigated by first principles.The formation mechanism of the Schttky heterojunctions was mainly due to the difference of the work function of each component of the composite.The difference of the work function leaded to the bending of the semiconductor band edge,which reduced the transmission barrier of electrons in Z-scheme system,effectively ensured the fast conduction of photogenerated carriers and prolonged the life of carriers.The morphology characterization showed that α-Fe₂O₃ was evenly distributed on the surface of Ti₃C₂ modified CN.The structure characterization proved the successful preparation of the composite material,and ESR proved the successful construction of Z-scheme system.The construction of the internal Schottky heterojunction promoted the transfer of photocarriers and improved the lifetime of the separation efficiency.The composite photocatalyst showed excellent photocatalytic nitrogen fixing performance(460.52μmol·g^-1·h^-1)and quantum efficiency（0.80%at 380nm）.After 20 hours of continuous testing,the yield of NH₃ stayed the same.This study has important guiding significance for the design and preparation of efficient photocatalysts.