Synthesis And Characterization Of High-crystalline Nickel-iron Layered Double Hydroxides

Layered double hydroxides(LDHs),also well-known as hydrotalcite-like compounds or anionic clays,exhibit specific physical and chemical features such as the tunable control of intralayer composition,anionic-exchange in interlayer space,memory effect,and micro-or macro-porous properties ect.,which result in their potential applications in separation,adsorption, catalysis,biological sensing,drug delivery,and so on.In recent years,the research on the synthesis of LDHs with high crystallinity becomes very important in order to enlarge their applications in the functional layered materials.The co-precipitation method is a common and effective synthetic technology for the fabrication of LDHs because it has some advantages such as a wide application,various synthesized products, and the fabrication of pillar-LDHs etc.However,the supersaturation solution and high nucleation rate make the synthesis of LDHs with high crystallinity and well-defined shapes difficult during the co-precipitation process.It has been found that the homogeneous precipitation technology shows a possibility for the synthesis of LDHs with high crystallinity and well-defined hexagonal shapes,in which the slow hydrolysis of urea or hexamethylenetetramine(HMT) can be used to decrease the nucleation rate and optimize the synthesis condition and make the synthesis of M²⁺-Al³⁺ LDHs possible.The homogeneous precipitation technology is mainly used to the synthesis of M²⁺-Al³⁺ LDHs,while the synthesis for the transition metals LDHs such as Ni²⁺-Fe³⁺,Co²⁺-Cr³⁺ etc.with high crystallinity and well-defined shapes is very difficult due to the need of the amphoteric nature of Al³⁺.Up to now,the synthesis work of the transition metals LDHs such as Ni²⁺-Fe³⁺,Co²⁺-Cr³⁺ etc. with high crystallinity and well-defined shapes has not been found,which restricts their applications in electronic,magnetic and optical properties.On the basis of the M²⁺-Fe³⁺ LDHs research,a new complexing-agent-assisted homogeneous precipitation technology was successfully developed to synthesize M²⁺-Fe³⁺ LDHs materials with high crystallinity and well-defined hexagonal shapes.By using this method,Ni²⁺-Fe³⁺ LDHs with different molar ratio,high crystallinity and the hexagonal shapes were fabricated,and its formation mechanism was discussed.This research work is not only favorable for the synthsis of the other transition metals LDHs,but also for the fabrication of Ni²⁺-Fe³⁺ nanosheets by a delamination technology.The main works consist of four parts:(1) Study on the new synthesis method of Ni²⁺-Fe³⁺ LDHs with high crystallinity and hexagonal shaps Ni²⁺-Fe³⁺ LDHs material with high crystallinity and the hexagonal shaps was hydrothermally synthesized by using urea as hydrolysis agent,sodium fluoride(NaF),sodium tartrate (C₄H₄O₆Na₂·2H₂O) and trisodium citrate(C₆H₅Na₃O₇·H₂O) as chelating reagent.The optimization conditions of Ni²⁺-Fe³⁺ LDHs material with high crystallinity and the hexagonal shaps were urea as hydrolysis agent,trisodium citrateas chelating reagent,and the hydrothermal temperature of 150℃for 2 days.The effect of the chelating reagent kind the added amount of trisodium citrate,reaction temperature and reaction time on the crystallinity and shape of the obtained LDHs materials was systematically studied.(2) Effect of Ni/Fe molar ratios on the crystyallinity and shape of Ni²⁺-Fe³⁺ LDHsThe effect of Ni/Fe molar ratios(2.4,3 and 4) on the crystallinity and shape of Ni²⁺-Fe³⁺ LDHs was studied in this chapter.The experimental results show that Ni²⁺-Fe³⁺ LDHs in formula of [Ni_0.75Fe_0.25(OH)₂](CO₃)_0.125·0.5H₂O and[Ni_0.8Fe_0.2(OH)₂](CO₃)_0.1·0.54H₂O with high crystallinity and well-defined hexagonal shapes were hydrothermally synthesized at 150℃for 2 days when the molar ratios of Ni(NO₃)₂·6H₂O,Fe(NO₃)₃·9H₂O,CO(NH₂)₂ and C₆H₅Na₃O₇·H₂O were 3:1:7:0.05 and 4:1:9:0.05,respectively,while Ni²⁺-Fe³⁺ LDHs material with Ni/Fe molar ratio of 2.4 with high crystallinity and well-defined hexagonal shapes was not obtained at 150℃for 2 days.(3) Study on the formation mechanism of Ni²⁺-Fe³⁺ LDHs materials with high crystallinity and well-defined hexagonal shapeOn the basis of the difference of Ni²⁺-Fe³⁺ LDHs materials obtained by the traditional homogeneous precipitation technology and the complexing-agent-assisted homogeneous precipitation new method,a formation mechanism of Ni²⁺-Fe³⁺ LDHs materials with high crystallinity and well-defined hexagonal shape was presented.The experimental results show that the trisodium citrate plays a key role in the formation process of Ni²⁺-Fe³⁺ LDHs materials.One reason is that TSC makes the pH deposition range of Fe(OH)₃ increase due to the chelation of Fe³⁺ ions with TSC to[Fe(C₆H₅O₇)₂]^5- when pH＞8,and the another is that the existation of[Fe(C₆H₅O₇)₂] ^5- ions makes the translation process of Fe(OH)₃ with Ni²⁺ ions change,and causing a directional growth along a direction of Ni²⁺-Fe³⁺ LDHs materials.(4) Study on physical-chemical properties of Ni²⁺-Fe³⁺ LDHs materials with high crystallinity and well-defined hexagonal shapesBy using a NaCl-HCl mixed solution exchange method,Ni²⁺-Fe³⁺ CO₃^2- LDHs materials were translated to Ni²⁺-Fe³⁺ Cl^- - LDHs with high crystallinity.Then Ni²⁺-Fe³⁺ Cl^- - LDHs with high crystallinity was acted as a precursor,Ni²⁺-Fe³⁺-ClO₄^- LDHs materials with high crystallinity were obtained by treating the precursor with 2.5mol/L NaClO₄ solution. The thermal decomposition behaviors for material[Ni_0.75Fe_0.25(OH)₂](CO₃)_0.125·0.5H₂O form 100℃to 900℃was investigated.Its layered structures could be remained to 200℃,and the layered structure collapsed between 200℃and 300℃to NiFe oxide.A nickel-iron spinel phase was obtained when the calcination temperature reached to 600℃.The physical adsorption-desorption properties of the obtained materials calcined at 300℃,400℃,and 500℃, respectively,have been investigated.The results show that the obtained material caicined at 300℃had the largest BET surface area,and the BET surface area of the obtained materials decreased and the average pore in size increased with the increase of the calcined temperature.The BET surface area was little relation with the Ni/Fe molar ratio.The materials obtained at different stages were characterized by XRD,SEM,TEM,SAED, TGA-DSC,and atomic absorption analyses.