Salt-Assisted Synthesis Of Two-Dimensional Materials For Energy Conversion And Storage

Two-dimensional（2D）materials are widely fascinating to the scientific community and have been studied extensively in the fields of condensed matter physics,chemistry,nanotechnology and material science in recent years.Owing to their compelling physical,chemical electronic and optical properties,2D materials show impressive performance in field-effect transistors,sensors,catalysis,supercapacitors,batteries,ferroelectricity,and thermoelectricity.Most of these applications are based on the special properties of 2D structure,which require the capabilities for the preparation of 2D materials with desired size,crystal phase,thickness,composition,defect and surface property.The synthesis method is the most important restrictive factor for the development of 2D materials.Various methods have been developed including the micromechanical cleavage,liquid exfoliation,CVD,wet-chemical syntheses and so on.In detail,micromechanical cleavage and CVD aim at synthesizing high quality 2D materials,and liquid exfoliation and wet-chemical method aim at large-scale synthesis of 2D materials.However,it is still a challenge to obtain high quality2D materials by a method with low-cost and large-yield.To get that goal,this method should own high reaction temperature,fast reaction process,high reaction efficiency and possibility of reaction scale-up.Therefore,new reaction medium and mechanism should be introduced.This doctoral dissertation presents the salt-assisted method for synthesizing 2D materials,including salt-templated method and molten salt method.Salt-templated method is based on the solid state of salts,and molten salt method is mainly based on the liquid state of salts.We focus on large-scale synthesizing various high quality 2D layered,nonlayered,amorphous materials via salt-templated method.Moreover,we could enhance the production efficiency of 2D materials via molten salt method.These results are described as following:1.Salt-templated synthesis of layered and amorphous two-dimensional Nb₂O₅ for Li-ion capacitors.Firstly,we treat the surface of salts as plane to provide reaction space.The Nb precursors（NbCl₅）are coated on the surface of water-soluble salt（K₂SO₄）template forming2D amorphous Nb₂O₅ by hydrolysis.The product could be obtained after removing K₂SO₄template.After annealing the amorphous Nb₂O₅ coated K₂SO₄ particle under the air atmosphere and washing out the template,we can get 2D crystalline Nb₂O₅.Then we construct two electrodes for Li-ion capacitors based on these two kinds of Nb₂O₅.Both crystalline and amorphous Nb₂O₅ performed high capacitance due to the 2D structure.Owing to the Li ion could not only diffuse into bilayer spacing of amorphous Nb₂O₅ electrode but also transfer into layer,the capacitance of 2D amorphous Nb₂O₅ can reach to 463 F/g（1111C/g）at 2 mV/s,which is about twice of 2D crystalline Nb₂O₅（262 F/g,628 C/g）.The amorphous layer structure could also weaken the grain-boundary scattering which may reduce transmittance.Based on the lower grain-boundary scattering,the 2D amorphous Nb₂O₅ thin film with higher transmittance could be assembled a transparent Li ion capacitor with an active carbon cathode.2.Salt-templated synthesis of nonlayered 2D single crystalline transition metal phosphides（TMPs）.Owing to the nonlayered structure,we hypothesize that the lateral growth of phosphides on the salt surfaces is guided by the salt crystal geometry and promoted by lattice matching.The growth perpendicular to the salt surface is limited by precursor depletion.We demonstrate various 2D single crystalline TMPs(Co₂P,MoP₂,Ni₁₂P₅ and WP₂)synthesized by salt-templated method based on the lattice match mechanism.Especially,the2D TMPs are single crystalline and show well-defined exposed crystal facets,such as（^<sub>1^<sub>30）facet for Co₂P,（010）facet for MoP₂,（010）facet for Ni₁₂P₅ and（001）facet for WP₂.Further,the as-synthesized 2D Co₂P exhibited efficient electrocatalytic ability for HER with an overpotential of 41 mV at 10 mA/cm and a Tafel slope of 35 mV/dec in the 0.5 M H₂SO₄solution,which shows much higher performance than Co₂P nanoparticles.3.Synthesis of 2D transition metal chalcogenides via molten salt method（MSM）.We first refitted the tube furnace to meet the synthetic requirement of inert atmosphere,adding precursors at high temperature and fast cooling.By introducing ammonium molybdate or ammonium tungstate into sulfur-containing or selenium-containing molten salts at high temperature,we could obtain various 2D transition metal chalcogenides（MoS₂,WS₂,MoSe₂and WSe₂）with 2H phase.The synthesis mechanism and condition are discussed in detail in this system,and the productivity of MoS₂ is up to 67%in 1 minute’s reaction at 700℃.Additionally,we coated the obtained 2D MoS₂ on the sponge to construct photothermal electrode for water evaporation.High water evaporation efficiency（90.58%）can be achieved based on the high average solar spectrum absorption of MoS₂/sponge hybrid electrode（82.3%）.4.Controllable synthesis of 2D K⁺-intercalated MnO₂ by using molten salt method（MSM）for energy harvesting from organics degradation.MnSO₄ precursor was introduced into potassium nitrate（molten salt）to carry out large-scale synthesis of 2D K⁺-intercalated MnO₂.We build an energy harvesting galvanic cell by using a Pt foil as an anode and 2D K⁺-intercalated MnO₂ as a cathode,which combines both dye degradation in waste water and the energy harvesting during the degradation process.Owing to the galvanic effect,this cell could accelerate the dye degradation rate.Different kinds of organics could be degraded and produce energy in this cell with a long-lasting current and a stable open-circuit voltage（0.45V）.