Synthesis And Properties Of Nickel Sulfide In Choline Chloride-ethylene Glycol Deep Eutectic Solven

Nickel sulfides exist in the form of Ni₉S₈,Ni₇S₆,Ni₆S₅,Ni₃S₂,α-Ni S,β-Ni S,Ni S_1.03,Ni₃S₄,Ni S₂ due to their unique electronic structures,and their excellent electrical,magnetic,and catalytic properties have attracted extensive attention.At present,nickel sulfides have been successfully synthesized by electrochemical synthesis,chemical vapor deposition,solid-phase,and hydrothermal/solvent thermal methods.However,the traditional methods suffer from high reaction temperature,complicated operation,poor safety,and the product phase and morphology are not easily controlled.Therefore,it is particularly important to seek a new method for the synthesis of nickel sulfides in a simple and safe way.The discovery of deep eutectic solvents（DESs）has provided a new pathway for the synthesis of nickel sulfides.As a green solvent,DESs have become a hot research topic in green chemical synthesis due to their simple synthesis,low vapor pressure,wide source of raw materials and environmental friendliness,etc.DESs can be used as reaction solvents in material synthesis to synthesize functional materials with different morphological characteristics for different applications.In this paper,a new process for controllable synthesis of nickel sulfides was proposed based on the choline chloride-ethylene glycol（Ch Cl-EG）DES with low vapor pressure and good thermal stability,which is green,mild reaction conditions,simple process and low requirements for equipment.The possible reaction mechanism was proposed,and the adsorption and electrocatalytic properties of nickel sulfides were studied.The main research results are as follows.（1）At atmospheric pressure,Ch Cl-EG DES was used as the reaction solvent,Na₂S₂O₃·5H₂O as the sulfur source,and a series of Ni X_n(X=SO₄^2-,Cl^-,CH₃COO^-,NO₃^-,n=1 or 2)·y H₂O（y=4,6）nickel salts as the nickel sources,and powdered nickel sulfides with different morphological phase compositions of Ni₃S₄、β-Ni S/Ni₃S₄、α-Ni S/β-Ni S/Ni₃S₄、Ni S₂/Ni₃S₄ were synthesized at 120℃.The reaction mechanism for the synthesis of nickel sulfides using Na₂S₂O₃·5H₂O as the sulfur source was proposed by characterizing the property changes of the system before and after the reaction,and testing the SO₄^2-concentration and p H value in the solution.In the system,Ni（II）reacts with S₂O₃^2-and H₂O to form a-Ni S,β-Ni S,Ni₃S₄,Ni S₂,and produces SO₄^2-and H⁺.（2）With the aid of ethylenediaminetetraacetic acid（H₄Y）,Na₂S₂O₃·5H₂O was used as the sulfur source and Ni X_n·y H₂O as the nickel sources,the spherical Ni S₂ was synthesized.The effect of H₄Y concentration on the physical phases of the synthesized products was investigated.The experimental results showed when Ni X_n·y H₂O was Ni SO₄·6H₂O,Ni Cl₂·6H₂O,Ni（CH₃COO）₂·4H₂O,Ni（NO₃）₂·6H₂O,respectively,Ni S₂ could be synthesized at the corresponding H₄Y concentrations of0.06 M,0.05 M,0.07 M and 0.03 M.It is shown that the H₄Y-assisted synthesis of Ni S₂ proceeds in two steps.In the first step,Ni（II）in the solution undergoes a coordination reaction with H₄Y to form[Ni Y]^2-,and the H⁺ionized by H₄Y reacts with S₂O₃^2-to form S,SO₂ and H₂O.In the second step,Ni（II）in solution that does not participate in the coordination reacts with S,SO₂ and H₂O to form Ni S₂,SO₄^2-and H⁺.Then Ni₃S₂/NF was synthesized using nickel foam（NF）as the substrate,Na₂S₂O₃·5H₂O as the sulfur source and Ni X_n·y H₂O as the nickel sources,and the effect of reaction temperature on the physical phase and morphology of the products was investigated using Ni SO₄·6H₂O as the nickel source.（3）Ni S₂ was obtained by synthesis at 120℃using sulfur powder（S⁰）as the sulfur source and Ni X_n·y H₂O as the nickel sources.Ni SO₄·6H₂O and Ni（CH₃COO）₂·4H₂O were used as the nickel sources,Ni S₂ powders with good crystallinity of submicron solid spherical,grape bunch and cracked ellipsoidal shape were synthesized.When the nickel source was Ni Cl₂·6H₂O,the product was the mixture of Ni S₂ and elemental S in irregular polyhedral shape.Ni S₂ was not synthesized with Ni（NO₃）₂·6H₂O as the nickel source by adjusting the experimental parameters such as reaction temperature,time and reactant concentration ratio.The mechanism of Ni S₂synthesis using S⁰ as the sulfur source was proposed by testing the solution SO₄^2-concentration,p H value and characterizing the change of the reaction system properties.Dissolved S⁰ is subjected to disproportionation reaction with Ni（II）and H₂O,and S⁰ is reduced to S₂^2-combined with Ni（II）to form Ni S₂,while S⁰ is oxidized to SO₄^2-and H⁺is produced.（4）The adsorption performance of the synthesized series of powdered nickel sulfides on rhodamine B（Rh B）solution was investigated.The experimental results showed that the best adsorption performance of the synthesized Ni S₂ was achieved under the conditions of 0.1 M Ni（CH₃COO）₂·4H₂O and 0.2 M S⁰ as reactants.By examining the effects of the initial p H,oscillation time,adsorption temperature and adsorbent dosage on the adsorption of Rh B by Ni S₂,the saturation adsorption capacity of Ni S₂ on Rh B was obtained as 33.52 mg·g^-1.The adsorption kinetic fitting data indicates that the adsorption process is consistent with pseudo-second-order kinetic model,and the isotherm fitting data indicates that the adsorption is consistent with Freundlich model.The adsorption mechanism of Ni S₂ on Rh B is electrostatic attraction.（5）The electrocatalytic performance of the synthesized Ni₃S₂/NF was investigated.The Ni₃S₂/NF synthesized with Ni SO₄·6H₂O as the reactant was optimal at a current density of 10 m A·cm^-2 under alkaline environment,had the electrocatalytic performance with overpotentials of hydrogen precipitation reaction（HER）and oxygen precipitation reaction（OER）were 147 m V and 326 m V,respectively.The electrocatalytic performance of Ni₃S₂/NF synthesized under different temperature conditions was investigated.The results showed that the Ni₃S₂/NF（Ni₃S₂/NF-60）synthesized at 60℃had the best activity,with HER and OER overpotentials of 115 m V and 287 m V,respectively.The overall water splitting system was assembled with Ni₃S₂/NF-60 as cathode and anode,respectively,and the voltage was 1.66 V at a current density of 10 m A·cm^-2.After a long stability test of110 h,the voltage decreased to 1.64 V,which showed high catalytic activity and excellent stability.