Catalyzed Synthesis Of α-Fe₂O₃ Nanoparticles In The Microreactor

Nanosizedα-Fe₂O₃ particles have been widely applied in many aspects,such as catalysis, antisepsis,pigments and magnetic recording materials.Optical property,electrical property and magnetic property of nanomaterals can only be shown on the macroscopic level when the size of nanomaterals are uniform.Therefore,the preparation of monodisperse materials with uniform size is the research hotspot in current material science.Based on the investigation of iron oxides,our group successfully developed a new method of preparing nanosizedα-Fe₂O₃ particles by catalytic phase transformation in solution. This method has such outstanding characteristics as fast rate of transformation,simple equipment,low cost etc.But the nucleation and growth process of nanosizedα-Fe₂O₃ are difficult to control in solution so that the particle distribution of nanosized hematite is wide, andα-Fe₂O₃ nanoparticle with diameter of 10-20 nm nearly cannot be prepared in solution.On the basis of the previous work of our laboratory,we studied the preparation of nanosizedα-Fe₂O₃ particles in the microreactor.The results showed that when trace Fe(â…¡) ion was added to the reaction system,the catalytic phase transformation mechanism was applied to microreactor and nanosizedα-Fe₂O₃ particles could be directly prepared in the microreactor. The product prepared by above method was not in need of high-temperature calcining.At the same time,the above method could maintain a great many of advantages,such as simple equipment,homogeneous grain size distribution,controllable particle size etc.The main research aspects are stated as follows:(1) In this paper,Pseudo-ternary phase diagrams in microemulsion systems of cetyltrimethyl ammonium bromide(CTAB),n-butanol,n-octane and water(or FeCl₂-FeCl₃ mixture solution,or NaOH solution) were presented.The influences of concentration of solution,pH value as well as n_Fe(â…¡)/n_Fe(â…¢) value on microemulsion areas were also considered. The results showed that stable microemulsion phase regions didn't change with pH value between 7 and 13,and would gradually decrease with the increase of concentration of solution,and this system was insensitive for n_Fe(â…¡)/n_Fe(â…¢) value.(2) The effects of trace Fe(â…¡) ion on the transformation of ferrihydrite toα-Fe₂O₃ in microreactor were studied.The results showed that Fe(â…¡) ion could be directly added to each microreactor by preparing mixed microemulsion.In the microreactor,trace Fe(â…¡) ion had obvious catalytic role in the transformation of ferrihydrite at initial pH 5～9,and the rate of phase transformation was speeded up with the increace of n_Fe(â…¡)/n_Fe(â…¢) value.When n_Fe(â…¡)/n_Fe(â…¢)≥0.12,α-FeOOH impurity was produced.A high Fe(â…¢) concentration and a fast heating rate favored the transformation of ferrihydrite and the formation of hematite.(3) The optimal conditions for the transformation of ferrihydrite toα-Fe₂O₃ were found. That is,C_Fe(â…¢)=0.3mol/L,n_Fe(â…¡)/n_Fe(â…¢)=0.08,pH=7,t=4.5h,and the heating rate was 7.2℃·min^-1.(4) The effects of various factors on the size and morphology of nanosizedα-Fe₂O₃ were studied in detail.The size of hematite nano-particles could be controlled by controlling reaction conditions.The results showed that the increase ofωvalue and initial concentration favored the formation of larger grains.With the increase of mC₄H₉OH/mCTAB value or the formation temperature of precursor,the size of nanosizedα-Fe₂O₃ particles firstly decreased then increased.With the increase of mC₈H₁₈/mCTAB value,the size of nanosizedα-Fe₂O₃ particles gradually decreased.The size of nanosizedα-Fe₂O₃ particles was insensitive for pH value.