Ultrathin Transition Metal Oxides And Chalcogenides:Design And Their Applications In New Energy Conversion

The increasingly severe environmental problems have prompted us to seek environmentally friendly and renewable energy sources to gradually replace the traditional fossil resources.Among them,it is extremely meaningful to reduce carbon dioxide(CO2)and water(H2O)into fuels,such as CO and H2,by semiconductor electro/photocatalysts,which is sustainable and green,holding great promise to solve the energy crisis and environmental problems.In this dissertation,the controllable synthesis of ultrathin transition metal oxides and chalcogenides were realized by designing and developing appropriate strategies,such as liquid-liquid interface-mediated strategy.In order to boost their performance in new energy conversion,the electronic structures of ultrathin transition metal oxides and chalcogenides were precisely manipulated through alloying,coordination environment modulation and single-site doping.The main contents of this dissertation are as follows:1.Partially delocalized charge in ternary TMDs alloys boosts CO2 electro reduction into syngas:it would be extremely meaningful,if the syngas could be efficiently produced by environment-friendly means such as reduction of CO2 and H2O mixture by electrocatalysts,since it could not only decrease dependency on the fossil fuels but also reduce the amount of carbon dioxide in atmosphere.Besides,ternary TMDs alloy often exhibits boosted electrocatalytic performances relative to its two pristine binary TMDs,and in-depth understanding on the underlying reasons is still an open question.In this work,MoSeS alloy monolayers,MoS2 monolayers and MoSe2 monolayers were successfully synthesized via a novel liquid-liquid interface-mediated strategy.DFT calculations illustrate MoSeS alloy monolayers have increased DOS at the conduction band,which allows for faster electron transfer ability confirmed by their lowest work function and smallest charge-transfer resistance.Meanwhile,synchrotron radiation XAFS manifests the partially delocalized charge of Mo atoms,fairly agreeing with the optimized model,which not only benefits for stabilizing*COOH intermediate verified by its most negative formation energy,but also facilitates the rate-limiting CO desorption step by lowering the overlap between Mo d-orbital and C p-orbital in*CO,confirmed by the lowest CO desorption temperature/potential on MoSeS alloy monolayers.As a result,MoSeS alloy monolayers exhibited the most excellent performances to electroreduction CO2 into syngas.This work discloses how the partially delocalized charge in ternary TMDs alloys accelerates electrocatalytic performances at atomic level,opening new horizons for manipulating CO2 electroreduction properties.2.Visible-light-driven overall water splitting boosted by tetrahedrally-coordinated blende CoO atomic layers:overall water splitting into H2 and O2 on semiconductor photocatalysts is a fascinating and sustainable strategy for solar hydrogen generation.However,very few single photocatalyst is able to directly split pure water into H2 and O2 by using visible light irradiation,while their photoconversion efficiencies still remain extremely low,which may be ascribed to the sluggish kinetics of charge carrier transfer and surface redox reactions.Herein,two types of semiconductor ultrathin layers with the same element and the similar thicknesses are first designed to uncover how the different atomic arrangements influence the water splitting efficiency thermodynamically and kinetically.As an example,tetrahedrally-coordinated blende CoO atomic layers and octahedrally-coordinated rocksalt CoO atomic layers with nearly the same thicknesses were successfully fabricated for the first time.The blende CoO atomic layers have a smaller Eg and abundant d-d internal transition feature relative to the rocksalt CoO atomic layers,which ensure their enhanced visible-light harvesting ability.Density-functional-theory calculations disclosed that the states density near the Fermi level for both the CoO atomic layers mainly came from Co atoms,suggesting the Co atoms may serve as the catalytically active sites,while the higher Bader charge for Co atoms of the blende CoO atomic layers favored the transfer and separation of charge carriers,supported by PL spectra and time-resolved fluorescence emission decay spectra.In situ FT-IR spectra and Gibbs free energy calculations unveiled the*OOH dissociation step was the rate-determining step,where the blende CoO atomic layers had the smaller*OOH dissociation energy,profiting from their higher Bader charge and stronger steric effect,manifested by their elongated Co-OOH bonds.As a result,the blende CoO atomic layers exhibited boosted visible-light-driven overall water splitting with H2 and o2 evolution rates of 4.43 and 2.63 ?mol g-1 h-1,which were roughly 3.7 times higher than those of the rocksalt CoO atomic layers.Briefly,this work shows promise for designing high efficiency overall water-splitting photocatalysts.3.Photocatalytic CO2 reduction boosted by single-site Co doped Ga2O3 atomic layers:it is extremely meaningful to reduce carbon dioxide(CO2)and water(H2O)into high-value fuels by solar energy,which not only can store the renewable and green solar energy in the form of chemical energy,but also reduce the content of CO2 in the atmosphere.In this work,single-site doped Ga2O3 atomic layers were constructed by inorganic-organic hybrid intermediate strategy.XRD,Raman,HRTEM and AFM etc.manifested the formation Ga2O3 atomic layers.Meanwhile,synchrotron radiation XAFS showed that the doped Co atoms possessed similar coordination environment with the Ga atoms,and no Co-Co bonds were observed,indicating that Ga atoms were substituted by isolated Co atoms.In situ EPR and in situ FT-IR detected the formation of*COOH,which is one of the key intermediates during the CO2 reduction processes.DFT calculations revealed that the formation of COOH*was the rate-limiting step,while single-site doped Co atoms could reduce the formation energy of*COOH intermediate.The single-site doped Co atoms would cause the charge redistribution of the neighboring Ga atoms,and charge would shift to the Co side,which helped to stabilize the*COOH.As a result,single-site Co doped ?-Ga2O3 atomic layers exhibit enhanced CO2 photoreduction activity.This work demonstrates single-site doped atoms could lead to charge redistribution of the neighboring atoms,which accounts for the boosted photocatalytic activities,providing a new idea for the design of catalysts with better CO2 photoreduction performance.