Preparation Of Two-Dimensional Layered Transition Metal Chalcogenides And Nonoporous Graphenes For Electrocatalytic Application

Two-dimensional(2D)nanomaterials are a new class of sheet-like nanomaterials.Although their researches can be traced back to decades ago,the landmark event that set off a wave of nanomaterial research is the success preparation of graphene stripped from graphite with Scotch tape by Novoselov,Geim,and their colleagues in 2004.The electrons of 2D nanomaterials are confined to 2D space,and therefore have unique,unprecedented physical,electronic,and chemical properties.Since then,2D nanometers have received extensive and in-depth researches in the fields of condensed matter physics,materials science,chemistry,and nanoscience.On the other hand,traditional fossil are increasingly depleted,new sustainable and clean energy sources are urgently needed to be explored.Hydrogen with the highest mass energy density and oxygen as an important energy source in fuel cell and recyclable metal-air batteries attracte worldwide inverstigating interest.This article takes transition metal chalcogenides and nanoporous graphenes as research objects.The new preparation methods are studied to synthesize target materials.Then,the chemical composition,phase structure,and optical properties of the materials are analyzed.Finally,the relationship between the elemental composition,phase structure,and edge defect sites of materials and electrocatalysis including electrocatalytic water reduction(also known as hydrogen evolution reaction,HER),water oxidation(also called oxygen evolution reaction,OER),and over all water splitting were thoroughly explored and studied.The main research contents and results are as follows:1.2H phase molybdenum disulfide(hexagonal symmetry,two layers per repeat unit,trigonal prismatic coordination,2H MoS₂)with lower Gibbs free energy of hydrogen adsorption is considered to have potential HER catalyst because of its unique physical and stable chemical properties and earth-aboundent resources.However,the basal plane of MoS₂ is considered to be inactive to hinder its further applications.Therefore,how to obtain 2H MoS₂ with rich defect sites has become a new problem for researchers.Herein,a new mass-scalable and facile intercalation method for the fabrication of multiple products,including pore-rich monolayer MoS₂(PR MoS₂)and MoS₂ quantum dots(MoS₂ QDs),has been developed via the acetone vapor etching of bulk MoS₂.The obtained monolayer MoS₂ QDs with a narrow lateral size distribution(average size:2.5 nm)present excitation-independent photoluminescence emission.Furthermore,the PR MoS₂ shows a significantly enhanced HER electrocatalytic activity and good stability in 0.5 M H₂SO₄ solution with a small overpotential of 241 m V at a current density of 10 m A cm^-2.These results demonstrate that the as-prepared PR MoS₂ is very promising for the application in HER.2.The metallic distorted/undistorted(1T'/T)phase of TMCs has the effects of lowering the charge transfer resistance,reducing the overpotential,and increasing the activity of basal plane,which is more benefit for the electrocatalytic HER activity than the 2H phase.Researches on multi-component alloy of TMCs have attracted wide attention due to the synergistic effects of different atoms incorporation.However,large-scale preparation of 1T'/T phase alloy under mild condition is still a challenge.Here,we obtained 1T'/T phase dominated tenary alloy MoS_1.24Se_0.76 via a simple hydrothermal synthesis strategy,and MoSe₂,MoSe₂@MoS₂,MoS₂,MoS₂@MoSe₂were also synthesized for investigating the relationship between electrocatalytic performance and chemical composition or structure.After comprehensive characterization and analysis,the attractive ternary alloy of MoS_1.24Se_0.76 was confirmed through the ultraviolet visible spectrum.Compared with binary MoSe₂ and MoSe₂ or ternary heterojunction MoSe₂@MoS₂ and MoS₂@MoSe₂,the ternary alloy MoS_1.24Se_0.76presents superior HER catalytic activity with a small overpotential of108 m V for the electrocatalytic current density of j=-10 m A cm^-2,and a Tafel slope of 55 m V dec^-1.3.The electrocatalytic OWS is currently regarded as an efficient method for reservation and conversion of sustainable energy sources.However,the development of efficient bifunctional TMCs electrocatalysts are still in its infancy and remain an attractive and difficult task.On the one hand,the 1T'/T phase molybdenum chalcogenides possess unique properties to realize HER,but they lack the OER active center.On the other hand,Co is a good electrocatalytic center of OER.Herein,Co-doped 1T'/T phase dominated MoS_1+XSe_1+Y alloy(Co-MoS_1+XSe_1+Y)nanosheets electrocatalysts were synthesized by a one-step facial hydrothermal method.Importantly,the as-prepared catalysts Co-MoS_1+XSe_1+Y with the atomic ratio of Co:Mo=1:5(Co-MoS_1+XSe_1+Y1-5)shows superior electrochemical activity,achieving current density(|j|)of 10 m A cm^-2 at small overpotentials of 119 m V and 280 m V for HER and OER in 1.0 M KOH,respectively.Moreover,applying the bifunctional electrode in an alkaline water electrolytic cell allows a high-efficiency electrocatalyst with only 1.60 V at j=10 m A cm^-2,which could be practical in electrocatalytic overall water splitting reaction devices.4.Although 2D nanoporous graphene(NPGs)and their analogs have been extensively studied in the field of electrocatalytic OER,the influence of the edge sites of 2D NPGs on the electrocatalytic performance has not been thoroughly explored.One of the main reasons is that it is difficult to synthesize a large number of exact carbon edge sites 2D NPGs.Two types of crystalline 2D NPGs with exact carbon edge sites and pore size of 6.3?(labeled as NPG-1)and mesopore of 2.3 nm(labeled as NPG-2),respectively,were successfully synthesized by decarboxylation reaction with Na OH as catalyst in a hydrothermal system.Scanning tunneling microscope(STM)results directly reveal the porous structure and the different pore size of the two kinds of NPG with hexagonal honeycomb crystal lattice.Moreover,NPG-1 and NPG-2 exhibit relatively good performance in non-doped electrocatalytic oxygen evolution reaction(OER).This work paves a new way for the controllable synthesis of a large amount 2D NPG with different pore size.