Design,Structure And Light Drive Catalytic Performance Of 2D/2D Nanosheet Heterojunctions Based On Layered Double Hydroxides

In order to solve the problems caused by energy shortage and the three wastes generated from the use of fossil energy,considerable efforts are made worldwide on new sustainable high-quality energy and beautiful homes.The nature uses clean and evenly distributed solar energy to achieve the storage of chemical energy,creating basic living conditions for the reproduction of living things.Imitating natural photosynthesis has always been the focus of multidisciplinary research in chemistry and biology.The core of artificial photosynthesis is the need to find high-performance catalysts to achieve high solar energy conversion efficiency.Layered double hydroxides(LDHs)are a kind of economical anionic materials,which possess unique two-dimensional structure,adjustable metal element composition and species,and uniform height distribution of elements.It meets the basic requirements for constructing highly dispersed 2D/2D nanosheet heterojunctions,which have developmental potenial in the fields of light,heat and electricity.The main contents and results of this dissertation are as followed:1.Based on the characteristics of high dispersion uniformity of metal elements,two-dimensional ultrathin ZnxCd1-xS nanosheet was fabricated by topotactic sulfurization process using trimetallic ZnCdAl LDHs as precursor and thiourea as sulfur source.The band gap of ZnxCd1-xS nanosheets can be adjusted by modulating Zn/Cd molar ratio in ZnCdAl-LDH precursors.With the decrease of Zn content,the photoreduction ability of ZnxCd1-xS nanosheets decreased.By contrast,Zn0.67Cd0.33S exhibits the best visible light photocatalytic activity,which is attributed to its ability to maintain a certain light absorption capacity and a high flat band potential.The exposed(111)ZB or(002)WZ polar faces of ZnxCd1-xS nanosheets are beneficial to reduce the photogenerated carriers recombination efficiency,which can be attributed to well distribution of electrons and holes which are completely delocalized on metal/sulfide ions.This work proposes a strategy for preparing two-dimensional ZnxCd1-xS nanosheets from LDHs precursors,which is a simple,low-cost and environmentally friendly process.2.2D/2D ZnO/ZnxCd1-xS single crystal nanosheet heterojunction was prepared by in-situ topological sulfurization/oxidative pyrolysis.The ZnO/ZnxCd1-xS-4 single crystal nanosheet heterojunction exhibited excellent photocatalytic activity(38.93 mmol·h-1·g-1)under visible light irradiation,and the quantum efficiency reached 40.97%at 420 nm.The in-situ topological sulfurization/oxidation pyrolysis process demonstrates the mechanism of ZnO/ZnxCd1-xS heterojunction formed in single crystal nanosheets.With the increase of topological sulfurization temperature(200-500?),the crystallinity of Zn0.67Cd0.33S become better,which is favorable for the exposedness of the surface catalytic active sites.XRD proved the coexistence of two crystalhydrogen phases of ZnO and ZnxCd1-xS in the catalyst with different the topological oxidation time(2-6 h).The longer time for oxidation,the more ZnO is obtained.The formation of ZnO constructed with ZnxCd1-xS into 2D/2D ZnO/ZnxCd1-xS single crystal nanosheet heterojunction,which increases the visible light absorption of the nanosheet and promotes the separation of photogenerated carriers.The ESR experiment confirmed that the 2D/2D ZnO/ZnxCd1-xS single crystal nanosheet is a direct Z-Scheme structure,which makes the catalyst possess very low carrier recombination rate and high redox ability.3.The Cu2S/Zn0.67Cd0.33S(CZCS)nanosheet heterojunction was prepared by in-situ topological sulfurization under hydrothermal conditions using CuZnCdAl-LDH as precursor.The sulfurization process imparts a 2D/2D CZCS atomic in-plane heterojunction structure that broadens the light absorption range of the catalyst.Through activity evaluation,it can be found that CZCS-20 has excellent photocatalytic hydrogen production activity(15.27 mmol·h-1·g-1),which can be attributed to 2D/2D atomic in-plane heterojunction,dense interfacial structure and a short charge transfer distance.In the hexagonal nanosheets,Cu2S/Zn0.67Cd0.33S has a matching crystal orientation,which makes it easy to form an intraplane heterojunction structure.A short distance promoted the separation and migration of photogenerated electron-holes,besides,a suitable flat band potential can easily induce surface reduction reaction.In terms of excellent photocatalytic performance and light stability,the 2D/2D CZCS in-plane nanosheet heterojunction is a promising photocatalyst.4.MgFe-LDH nanosheet with small particle size was prepared by the colloid mill method created in our lab.The topological decomposition product based on MgFe-LDH maintains high dispersibility for divalent and trivalent elements.A 2D/2D CN/MgFe-X nanosheet heterojunction was constructed by a mixed calcination method of MgFe-MMO and melamine precursor.The MgFe-MMO content can be adjusted to regulate the thermal growth process of g-C3N4,which promotes the formation of nano-scale g-C3N4 with higher specific surface area and more active sites.Narrow band gap MgFe-MMO(-2.08 eV)can sensitize g-C3N4 and enhance the visible light absorbance of nano-g-C3N4.MgFe-MMO and g-C3N4 form a 2D/2D interplanar nanosheet heterojunction,which promotes the separation of electron-hole pairs on the surface of g-C3N4.Under visible light irradiation,the optimal photocatalytic hydrogen production activity of 2D/2D CN/MgFe-X nanosheet heterojunction is 6.64 times better than the bulk phase g-C3N4.The synergistic effect between nano-g-C3N4 and MgFe-MMO enhances the hydrogen production efficiency of the catalyst.This work provides new insights into the design of 2D/2D nanosheet heterojunction photocatalysts based on nano-g-C3N4.