The Structure And Property Regulation Of Carbon Nitride-based Photocatalysts For Photocatalytic Water Splitting

Faced with the two major global problems:environmental polutions and fossil energy depletion that need to be solved,the photocatalytic technology to transform the inexhaustible solar energy into hydrogen energy is a new way with broad research potential and far-reaching significance.Since Fujishima and Honda have discovered that the TiO2electrodes under the irradiation could produce hydrogen by water photodecomposition in 1972,semiconductor-based photocatalytic decomposition of water has been widely studied.So far,more than 150 kinds of photocatalysts for producing H2 from water have been reported.However,semiconductors usually suffer from low light absorption efficiency,low carriers separation efficiency,and bad stability in cyclic test,which greatly inhibit the progress of photolytic water hydrogen production technology and practical industrial application.Carbon Nitride,a visible light responsive organic conjugated 2D semiconductor material with a band gap of 2.7 eV,has certain absorption to visible light.With the high stability in high temperature?strong acid?alkali conditions,Carbon Nitride has become the hot material in the field of solar energy conversion and utilization.The unique optical functional and responsive unit of carbon nitride is a tricyclic triazine(C6N7 unit).And the covalent bonds between C6N7 ring along(001)two-dimensional plane lays a foundation for design of functional recombination,recutting and reconstruction,and the feasibility also plays an important role in the improvement of synergistic splitting water.Surrounding by the tactics of 2D planar heterojunction,active sites modulation and surface functional groups modification to solve the problem of insufficient light absorption and low photogenerated carries utilization,carbon nitride-based photocatalysts were used for the study of structural regulation and H2 production in this dissertation.In this paper,a systematic summary to explore the functional structure design and structure-activity relationship and catalytic mechanism of carbon nitride have been made.And for the faced bottlenecks,we proposed the improvements in structures and performance.With the in-depth analysis of 2D conjugated polymer in energy problems and challenges water splitting,the optimal design and controllable preparation provides the new thinking for acquiring high efficiency,stable and cheap photocatalyst.The specific research of this paper is as follows:1.Research on the water splitting properties of two-dimensional planar heterojunction(Cring)-C3N4Based on the preconditions that C6N7 ring,the core component for light response in two-dimensional plane possesses the realization of functional cutting and splicing.We use the "C-N" covalent bond to join heptazine unit and graphite fragment in the plane.In this work,2D planar heterojunction(Cring)-C3N4 achieves the high hydrogen production with the rate of 370 umol h-1g-1.The visible light quantum efficiency of fully decomposed water was greatly improved.The new concept of planar heterostructure was proposed to regulate the fast separation and transport of photo-generated carriers.The intrinsic mechanism of fast separation and migration of photo-generated electrons and holes in two-dimensional heterogeneous structure is studied by means of X-ray absorption spectroscopy and electrochemical methods.At the same time,confirmed by the synchronous radiation XAFS technology that the embedding carbon ring into the carbon nitride plane by conjugate bonds to form a two-dimensional heterostructure,the internal electric field of the local micro-region heterogeneity interface was used to rapidly drive the separation of photo-generated electrons and hole,which greatly improved the visible light quantum efficiency for overal water spliting.This research has enriched people's understanding and directivity of the functional design between two-dimensional heterogeneous microstructures and properties,and provided new ideas and structural design to construct a new kind of materials for solar energy conversion.2.Study on the performance of splitting water based on effect of regulation and engineering by platinum single atom within carbon nitride planeAchieving hydrogen production in pure water during the whole photocatalytic processes has been considered to be an effective clean strategy for storing renewable solar energy to solve the energy crises and environmental problems.Two-dimensional carbon nitride has been widely considered as a potential catalyst for hydrogen production.However,in general,pure g-C3Na material synthesized by the thermal polymerization of organic precursors that contain C,N elements could not achieve full pyrolyzation,because the valence band maximum of the pure materials is more positive than the oxidation potential of water,and the lack of effective thermodynamic drive making the production of oxygen in aqueous solutions is skimpy.Based on the problems above,the current task is the regulation the infrastructure of the pure carbon nitride material.So that the conduction and valence band of carbon nitride semiconductor simultaneously across both sides of H+/H2 and H2O/O2 redox potential could achieve the goal of realizing whole water splitting.Therefore,the work in this chapter is aim at modifying the valence band structure of carbon nitride semiconductor by using the effect of monatomic Pt's high valence state restriction,so that the material achieve the spontaneous total disosociation performance in pure water.In this chapter,the synthesized Pt?-C3N4 realizes photo hydrolysis of hydrogen with prominent H2 production rate up to?140 ?mol g-1 h-1,which is more than 10 times better than Pt nanoparticle-C3N4 material(15 umol h-1g-1).The notable quantum yield at 420 nm is 1.5%,The characterization and theoretical calculation results of related electronic structure indicated that the pt?-C3N4 valence band maxmum moves down by 0.26 eV compared with the unmodified g-C3N4 resulted from the hybridization of Pt? 5d orbital and N 2p orbital,which enabled the valence band to produce oxygen by leading the reaction to thermodynamic dominance and greatly promoted the generation of oxygen in the process of whole water splitting.This work provides an extended idea to regulate and engineer energy level semiconductor structure for realizing spontaneous complete solution in pure water.3.Study on the H2 production in hydroxylated carbon nitrideIdeally,the photo-generated electrons in the conduction band of conventional semiconductor in the water system are used to drive the reductive reaction to produce H2,while the holes in the valence band are consumed for the other half reaction to produce oxidative production of O2.More often than not,the actual reactions happened in the aqueous solution are that electrons from the conduction band and holes of valence band are easily suffered from recombination.And photo-generated electrons are not only used for reducing hydrogen reaction and valence band holes are not only used for oxidizing oxygen generation,which are not ideally agreed with mechanism described in the concept above.In fact,more complex mechanisms occured in the range of redox potential from the energy of semiconductor band.For example,the range of band potential in pure g-C3N4 was observed[-0.88?1.90eV],which is not only crossed the potentials required for photocatalytic H2 and O2 in the water,but also crossed the electrode potential to produce H2O2.In general,some catalysts,such as Fe2O3 and WO3,that have been widely used in water splitting systems have been found to produce H2O2 under illumination.H2O2 which exists in the photolytic water system has been classified as poisonous species.Particularly,hydrogen peroxide is more toxic to reducible metal sulfide semiconductors.On the other hand,on the favorable condition of thermodynamics,when some oxygen containing groups of superoxide(O2·-),hydrogen peroxide(H2O2)and hydroxyl free radical(HO·)have been produced and make the catalyst poisoned at the same time.What's,more,the catalyst is easy to undergo self-oxidation reaction in the photolytic water system with the attacking of oxygen containing groups,which results in degeneration and inactivation.The strong oxidizing property of H2O2 oxidizes the sulfur element(-2)in metal sulfide into elemental sulfur,thus making the metal sulfide catalytic 'deactivation.In addition,semiconductors which were used for photolytic water could be easily deactivated by the strong oxidation and corrosion of photo-generated holes.Causing the disturbence to the whole water splitting systems,the competitive consumption of the optical carriers required for water splitting process is also the crux of the fact that hydrogen and oxygen producing ratio in pure water system is often not theoretical result of 2:1.Based on the analysis of the classical structure of carbon nitride based semiconductor,this study effectively improved the adsorption potential barrier of oxygen groups by hydroxylation modification on the surface,and inhibited the recombination and non-faraday consumption of surface photo-generated holes.The water splitting activity of the carbon nitride based materials was ssignificantly increased.The synthetic g-C3N3.5(O0.5H0.5)has superior performance of hydrogen production in pure water,reaching the H2 production rate up to 947.7 umol h-1g-1.Also,we hoped the conceptual analysis embodied in this work guiding the process of experimental modification that could optimize the strategy and provide a new reference path for solar energy sustainable conversion into hydrogen energy.