Design,Synthesis And Photoelectrochemical Properties Of Atomically Thin Two-dimensional Layers

Global warming is closely related to the use of fossil fuels.With the rapid development of human society,fossil fuels still account for 80%of global energy consumption at present,and the demand for energy is increasing rapidly.Thus,energy consumption based on fossil fuels is unsustainable.The fundamental way to solve the energy crisis and greenhouse effect is to develop new renewable energy,build global energy network,accelerate the replacement of green energy and electricity,develop and utilize green energy in a large-scale efficiently.In order to more effective and more environmentally friendly energy utilization,the development of solar energy,hydrogen energy and hydrocarbon fuel through CO2 reduction is highly desirable.Among the majority of developed electrochemical and photoelectrochemical water splitting catalysts and CO2 reduction semiconductor catalysts,atomic-thickness two-dimension layers is considered to be a good candidate,due to its unique atomic structure,electronic structure,and the defect structure introduced by the absence of atoms in the lattice.In this dissertation,the research object is atomic-thickness two-dimensional layers,the correlation between macro function?photoreduction of CO2,electrochemical and photoelectrochemical water splitting?and microstructure?atomic structure,electronic structure and defect structure?as the main line,aiming to reveal the structure-properties correlations,deepen the essence understanding,provide new idea,new method and new material system for function-oriented structure design and controllable preparation of atomic-thickness two-dimensional layers.This dissertation focus on design model,controllable fabrication and characterization of atomic-thickness two-dimensional layers catalysts,gives an indepth research on electrochemical and photoelectrochemical water splitting and CO2 reduction based on the understanding of structure-properties correlations ofatomic-thickness two-dimensional layers catalysts from the atomic scale.The main contents of this dissertation include the following aspects:1.Atomically-thin oxide-based semiconductors are proposed as an excellent platform to promote solar CO2 conversion by affording abundant catalytically active sites,increased two-dimensional conductivity,and superior structural stability.We proposed a lamellar hybrid intermediate strategy for successfully synthesizing single-unit-cell Bi2WO6 layers,taking advantage of an intermediate precursor of lamellar Bi-oleate complex.CO2 adsorption isotherms and VV/Vis diffuse reflectance spectra reveal that the ultra-large surface area endows Bi2WO6 atomic-layers with 3-times higher CO2 adsorption capacity and larger photoabsorption relative to bulk Bi2WO6.Time-resolved fluorescence emission spectra disclose that the single-unit-cell thickness helps to increase the carriers lifetime from 14.7 to 83.2 ns.DFT calculations demonstrate the increased DOS at the conduction band edge,while the vast majority of charge density concentrates on the surface,which benefits the two-dimensional conductivity as verified by their temperature-dependent resistivities.As a result,the single-unit-cell Bo2WO6 layers achieve a superior methanol formation rate.This study may open new opportunities for designing highly efficient and robust solar-driven CO2 conversion catalysts.2.The bottleneck in water electrolysis lies in the kinetically sluggish oxygen evolution reaction?OER?.Herein,conceptually new metallic non-metal atomic layers are proposed to overcome this drawback.Herein,clean and freestanding single-unit-cell thick CoSe₂ sheets with non-layered orthorhombic structure have been successfully synthesized through heating the artificially fabricated lamellar hybrid CoSe₂-DETA?DETA=diethylenetriamine?intermediate.The metallic character of orthorhombic CoSe₂ atomic layers,verified by DFT calculations and temperature-dependent resistivities,allows fast oxygen evolution kinetics with a lowered overpotential of 0.27 V.The single-unit-cell thickness means 66.7%of the Co2+ions are exposed on the surface and serve as the catalytically active sites.The lowered Co2+ coordination number down to 1.3 and 2.6,gives a lower Tafel slope of 64 mV dec-1 and higher turnover frequency of 745 h-1.Thus,the single-unit-cell CoSe₂ sheets have around 2 and 4.5 times higher catalytic activity compared with the lamellar CoSe₂-DETA hybrid and bulk CoSe₂.3.Thickness-dependent ultrathin p-type semiconductor sheets models are constructed to study the relationship between the thickness and the photoelectrochemical water splitting.Herein,free-floating ultrathin SnO sheets with different thicknesses were successfully synthesized via a convenient liquid exfoliation strategy,with efforts to disclose the thickness-dependent solar water splitting efficiency in p-type semiconductors.The thinner thickness and larger surface area afford a higher fraction of surface atoms to serve as active sites,while the calculated increased density of states near the Fermi level ensures rapid carrier transport/separation efficiency along the two-dimensional conducting paths of the thinner SnO sheets.As expected,the 3 nm thick SnO sheet-based photocathode shows an incident photon-to-current conversion efficiency of up to 20.1%at 300 nm,remarkably higher than 10.7%and 4.2%for the 5.4 nm thick SnO sheet-and bulk SnO-based electrodes.This work discusses the thickness-dependent solar water splitting efficiency in ultrathin p-type semiconductor sheets,thus opening new opportunities in the field of solar cells and photocatalysts.