Chloride Molten Salt Synthesis And Properties Of Lanthanide And Molybdenum Borides

Lanthanide borides are rich in composition and novel in crystal structure.However,there is still a lack of research on the chemical properties and applications of lanthanide borides with nanostructures due to the challenge in synthesis in comparison to other types of lanthanide compounds.The case for molybdenum borides is somewhat different since there have been some progresses on their applications in catalysis in spite of their low activity in many cases and difficulty in synthesis due to the small stoichiometric difference among borides and presence of multiple crystalline phases.Molten salt synthesis is a simple and versatile route which favors the phase and size controllable preparation of metal boride nanoparticles.In this thesis,a series of lanthanide and molybdenum boride with nanostructures were synthesized in a molten salt environment.The potential applications of these borides in organic dye treatment and electrocatalytic hydrogen evolution as well as their magnetic properties were explored.The main results are as follows:（1）Synthesis of CeB₆ and GdB₆ and their application in organic dye treatment.Synthetic organic dyes with complex and stable structure are widely used.Efficient treatment of the industrial wastewater that contains these dyes is therefore of great importance due to its hazard to environment and human health.Adsorption and catalytic degradation are widely studied chemical methods for the removal of dye contaminants from wastewater,and a variety of lanthanide materials have shown outstanding adsorption or catalytic properties.However,there is limited information regarding the application of lanthanide borides for this purpose.Herein,lanthanide hexaborides,CeB₆ and GdB₆,were synthesized via the reactions of lanthanide fluorides and NaBH4 in the LiCl-KCl melt and the performance of synthesized borides for the removal of azo and cationic dyes[Congo red（CR）and methylene blue（MB）]was evaluated.The results show that homogeneously dispersed CeB₆ nanoparticles exhibit great performances for the removal of CR,which has not been investigated before.The CR removal process follows pseudo-second-order kinetics,and a maximum removal capacity of 775.6 mg/g was obtained in a solution with an initial concentration of 800 mg/L.The mechanism study shows that the CR removal process is composed of adsorption and photocatalytic degradation.Hydroxyl（·OH）radicals are the major active species involved in photocatalytic degradation while superoxide（·O2-）radicals and holes（h+）play minor roles as evidenced by the radical quenching and ESR results.GdB₆ synthesized in molten salt possesses cubic-shaped structures with an average size of 100 nm and BET surface area of 51.5 m2/g.During the evaluation of MB removal performance,reduction of MB to colorless leucomethylene on the surface of GdB₆ was observed for the first time,which makes it possible to provide a more reliable evaluation on the capability of GdB₆ in the treatment of MB.In spite of the reduction,MB decolorization mainly results from the adsorption on the surface of GdB₆.The adsorption behavior follows pseudo-second-order kinetics and Freundlich isotherm model.An adsorption capacity of 3108 mg/g with an adsorption ratio of 77.7%was obtained in a solution with an initial concentration of 2000 mg/L,and the performance remained unchanged after six adsorption-desorption cycles.This work demonstrates the potential applications of lanthanide boride nanocrystals in the remediation of wastewater containing dye contaminants.（2）Synthesis of β-MoB₂ and α-MoB₂ and their electrocatalytic hydrogen evolution properties.Hydrogen（H2）is an ideal clean energy carrier,and water splitting via electrocatalytic hydrogen evolution reaction（HER）is considered as a promising route for H2 production,however,the kinetics of the reaction are slow due to high energy barrier.Therefore,the development of efficient,economical and stable catalysts is the key to the current research of electrocatalytic HER.Molybdenum borides are potential water splitting electrocatalysts,but their synthesis has always been difficult.Herein,β-MoB₂ and α-MoB₂ with nanosheet morphology were successfully synthesized for the first time in the LiCl-KCl molten salt with MoCl3 and B powders as well as MoCl5 and NaBH4 as precursors,respectively.Such β-MoB₂ nanosheets with a high BET surface area of 48 m2/g exhibit a promising HER activity in acidic medium with an overpotential of 187 mV at a current density of 10 mA/cm2 and a Tafel slope of 49 mV/decade,which is much better than the known bulk β-MoB₂ and even close to the results of α-MoB₂ that is commonly recognized as the best molybdenum boride electrocatalyst for HER.The high HER activity of β-MoB₂ nanosheets results from the high surface area that allows more active sites to be exposed,which compensates the disadvantage arising from the intrinsic structure of β-MoB₂.As for α-MoB₂ nanosheets,a remarkable HER performance in alkaline electrolytes was observed,delivering a low overpotential of 124 mV at a current density of 10 mA/cm2 and a small Tafel slope of 61 mV/decade.In acidic electrolytes,the overpotential at 10 mA/cm2 is as low as 141 mV,which is better than the known bulk and nanoα-MoB₂.Such HER performance benefits from the nanosheet structure that enables the exposure of the more active planes as a result of a high BET surface area of 43 m2/g.In addition,the α-MoB₂ nanosheets possess an outstanding stability in both acidic and alkaline media,showing negligible current density drop even after 3000 cycles and more than 24 h of potentiostatic operation.The above results not only provide a facile method for the construction of molybdenum diboride nanocrystals,but also demonstrate the possibility of improving the electrocatalytic activity of metal borides by nanostructuring.（3）Synthesis and magnetic properties of SmB4 and SmB₆.Samarium borides exhibit unique electrical and magnetic properties that have potential applications in a number of fields.However,it is still experimental difficulity to obtain crystalline samarium borides with controllable stoichiometry and morphology by simple one-pot methods.Herein,we report the one-pot synthesis of two samarium boride phases without impurities via the reaction between samarium halide and NaBH4 in molten LiCl-KCl.The stoichiometry and morphology were controlled by varying the precursor ratio and reaction time.The crystalline SmB4 possesses rod-shaped structure with diameters around 60-90 nm,and nanocubes with sizes around 50100 nm were obtained for SmB₆.On the basis of the XRD and SEM results,the SmB4 nanorods obtained at a Sm/B ratio of 1:3 evolve from the SmB4 namofilms,while formation of the SmB₆ nanocubes at a Sm/B ratio of 1:10 is mediated by SmB4.The transition from rod-shaped SmB4 to cubic SmB₆ most likely results from the destruction of the SmB4 nanocrystal upon partial reduction of Sm3+to Sm2+ in the presence of excess NaBH4,which is consistent with the change in oxidation state of samarium from XANES analysis.The SmB4 synthesized in molten salt is paramagnetic.The SmB₆ nanocubes possess paramagnetic property under high magnetic field,and the linear relationship between magnetization and magnetic field is slightly deviated under low magnetic field that is similar to those obtained by other methods,which demonstrates the facile synthesis of lanthanide borides with controllable stoichiometry and morphology can be achieved by using the molten salt method under mild conditions.