| With the development of industrialization and the rapid growth of population in modern society,environmental pollution and energy shortage have become the two major global issues facing the sustainable development of human beings.Because of its rich,accessible,clean and inexhaustible property,solar energy is widely concerned by many researchers.Semiconductor photocatalysis can directly convert solar energy into chemical energy that can be stored and transported?such as hydrogen and methanol,etc?,or it can be used as energy source to trigger photocatalytic redox reaction and then to treat undegradable organic or inorganic pollutants in the environment.Transition metal selenides have already attracted much attention in terms of their unique electronic band structure,catalase-like feature,stably physical and chemical properties and abundant earth reserves,which makes it a kind of promising photocatalytic materials.However,the intrinsic bottlenecks of single-component semiconductor photocatalyst have still seriously retarded its practical application,such as weak solar light harvesting ability,low photogenerated charge carriers separation efficiency,sluggish interfacial reaction kinetics and poor selectivity.Therefore,how to rationally design and develop novel photocatalytic systems,precisely regulate the structure composition and microstructure of photocatalysts,and optimize surface reactive sites in order to achieve high activity and selectivity of photocatalytic reaction process are still the most core issue in the current photocatalytic technology.In this paper,transition metal selenides?FeSe2 and NiSe2?,graphitic carbon nitride?g-C3N4?and red phosphorus?RP?are served as primary catalysts,two-dimensional-two dimensional?2D-2D?lateral heterostructure,2D/2D inter-plane heterostructure and 0D/3D point-plane heterostructure photocatalytic system have been successfully synthesized by a series of different prepared method,such as morphology control and heterojunction construction.This thesis aims to explore the effect of photocatalyst morphology,heterojunction combination,photogenerated electron/hole separation and transportation path at their heterojunction interface for the activity and stability of heterojunction photocatalyst,and then to reveal the controlling steps of interfacial reaction rate during the photocatalytic reaction process.Finally,it will provide some inspiring research ideas for the design and preparation of facile,high-efficient and good-stable photocatalytic system.The main contents of this dissertation are summarized as follow:?1?Preparation of transition metal selenides?MSe2,M=Fe and Ni?and their photocatalytic performance studyBoth novel FeSe2 and NiSe2 semiconductor photocatalytic material were successfully synthesized by a facile,fast and similar hot-injection method.The results of investigation show that FeSe2 sample has wrinkled lamellar morphology with an average diameter ranging from 50-200 nm,and Ni Se2 crystals appear relatively regular 0D nanoparticle morphology with an average size of 20 nm,respectively.In the presence of 0.15/0.35mol/L Na2S/Na2SO3 aqueous solution as a sacrificial reagent and without any noble metal cocatalyst?such as Pt,Au and Ag etc.?,single FeSe2 and NiSe2 exhibit relatively high H2evolution rate of 985.7?mol·g-1·h-11 and 1004.5?mol·g-1·h-1,respectively.The photocatalytic cyclic H2 generation experiments show that there are no obvious changes in the H2 generation rate and crystal structure after consecutive 12 h reaction process,indicating moderate photocatalytic stability.?2?Construction of FeS-FeSe2 2D-2D lateral heterojunctions and their photocatalytic performance studyThe rational structure design of semiconductor heterojunction is an effective method to control interfacial charge generation,separation and catalytic reaction pathway.The FeS-FeSe2 2D-2D lateral heterostructures were prepared via low temperature rapid nucleation method followed by in situ hot-injection selenization treatment.The crystalline structure,optical property and photocatalytic H2 activity of FeS-FeSe2 heterojunction photocatalyst are detailedly investigated in order to analyze the internal relationship of the microstructure and catalytic performance.The origin of the outstanding photocatalytic performance are carried out based on photoelectrochemical characterizations.The FeS-FeSe2 composite photocatalyst shows a significant enhancement of H2 evolution rate of 2071.1?mol·g-1·h-1,which is approximately 4.69 and 2.05 times higher than that of pristine FeS and FeSe2 semiconductor,respectively.A series of photocatalytic mechanistic studies,such as transient photocurrent response,electrochemical impedance spectroscopy,hole injection efficiency,radical trapping and hydrogen peroxide analysis,clearly demonstrate that the enhanced photocatalytic H2 generation performance could be attributed to increased carrier density,longer electron lifetime and higher hole injection efficiency as well as a stepwise two-electron/two-step reduction pathway.?3?Construction of FeSe2/g-C3N4 2D/2D inter-plane heterojunctions and their photocatalytic performance studyTwo-dimensional layered heterostructures have attracted much attention in the field of photocatalysis owing to their excellent optoelectronic and interfacial transport properties.The FeSe2/g-C3N4 2D/2D inter-plane heterostructures had been synthesized by successive high temperature calcination and in-situ hot injection routes.The application of the inter-plane heterostructures to constructing nanocomposite and photocatalytic water splitting are explored in detail.The resulting 15%FeSe2/g-C3N4 composite sample shows the highest photocatalytic H2 evolution activity(1655.6?mol·g-1·h-1)in Na2S/Na2SO3solution,being nearly 2.65 and 1.73 times higher than that of pure g-C3N4 and FeSe2sample,respectively.The results of photochemical and photoelectrochemical analysis illustrate that the composite photocatalyst has strong visible light trapping ability and efficient separation and transportation of photocarriers.The formation of?OH and H2O2generated during the process of photocatalytic reaction can be detected by active oxygen radical trapping experiment,EPR analysis and colorimetric method,which indicates that the photogenerated e-will reduce O2 to generate H2O2,followed by H2O2 decomposing into?OH over the FeSe2/g-C3N4 surface via a stepwise two-electron,two-step reduction procedure.Such significantly enhanced photocatalytic activity can be resulting from efficient photocarriers mobility,acceleration of H2O2 decomposition and the formed 2D heterojunction interfacial contact between g-C3N4 and FeSe2 nanosheets.?4?Construction of NiSe2/red phosphorus 0D/3D point-plane heterojunctions and their photocatalytic performance studyThe construction of 3D porous microstructure is also one of the effective ways to improve the photocatalytic performance of single semiconductor.The NiSe2/RP 0D/3D heterostructure had been successfully fabricated by a facial hydrothermal treatment to obtain porous honeycomb-like nano RP followed by NiSe2 nanoparticle in-situ depositing on the surface of nano RP.The 5.4%NiSe2/RP composite sample shows superior H2generation performance(1968.8?mol·g-1·h-1),which is 2.32 and 1.90 times than that of pure RP and NiSe2,respectively.The enhanced H2 evolution activity can be owing to improved light trapping ability,3D porous microstructure,intimate contact interface,type-II heterojunction establishment and a successive two-electron/two-step reduction process.This work provides an illuminating insight for the rational exploration and design of potentially photocatalytic system with 0D/3D integrated nanoarchitecture and multi-step electron transfer process for efficiently realizing solar energy capture and conversion. |
PDF Full Text Request