Preparation Of Ag/SBA-15(MCM-41) Nanocomposites Using A Supercritical Fluid Deposition Method And Basic Research Of The Process

The composites that prepared by deposited metal particles into the channels of the substrate have widely used in catalytic,optical and electrochemical,et al.Using the conventional methods,the control over the metal uptake,particle size and distribution in the composites remain the major problems.So in this paper we used supercritical fluid deposition to prepare supported nanocomposites.In this method,we use mesoporous materials as substrate,using the special properties of the supercritical fluid,the agglomeration of particles and collapse of the substrates in other ways can be successfully solved.With the restriction of the substrate channels,the obtained metal particles are amall and well dispersed,the nanowires are continuous.The main work of this paper is as following:(1) Using SBA-15 as substrate,AgNO₃ as precursor,ethanol or ethylene glycol as cosolvent,Ag/SBA-15 nanocomposites are synthesized at 20-25MPa,30℃,50℃and 80℃. Through the experiments,we can see that the preparation of composites using supercritical fluid deposition is stable and repeatable;the trend of formation of nanowires is better with increased temperature when the deposition temperature from 30～80℃;with ethanol as cosolvent,when the deposition pressure is higher than 8MPa,the precursor can deposited into the channels of the substrate;it is disadvantage for the preparation of composites with quikly depressurized;Ag nanowires can be synthesized with less deposition time and lower deposition pressure when using ethanol and ethylene glycol as colsolvent.(2) Ag/SBA-15 nanocomposites were also synthesized using the method of impregnation and using nitrogen as substitute for carbon dioxide.When using impregnation,Ag nanowires /SBA-15 nanocomposites were obtained after dried and reduction of the AgNO₃/SBA-15 composites,the size of the nanowire can from 100nm to 600nm.When using nitrogen as substitute for carbon dioxide,13mL ethanol as cosolvent,we can synthesize Ag nanowire /SBA-15 nanocomposites.In the deposition conditions to prepare nanocomposites,nitrogen is also in its supercritical state.(3) Using MCM-41 as substrate,AgNO₃ as precursor,ethanol and ethylene glycol as cosolvent,Ag/MCM-41 nanocomposites are synthesized at 20-25MPa and 50℃.In our experiments,using 4.5mL ethanol and 0.5mL ethylene glycol as cosolvent,the mass ratio of substrate to precursor is 3:1,Ag nanowires/MCM-41 composites are obtained at 50℃and deposition time of 3h.Ag nanowires are continuous and follow the curvature of nanoscale channels.The Ag content in the composites is 6.40wt%.When the deposition time prolong to 6h,the Ag content in the composites can achieve to 10.02wt%.As well as we know,there are no reports on the synthesis ofAg nanowires in the channels of MCM-41,this also shown the advantage of using supercritical fluid deposition to prepare nanocomposites.(4) In this work,we also investigated the thermodynamics of the solubility,phase equilibria,and the dynamics of the diffusion of precursor in the channels of the substrate in the supercritical fluid depositon process.Using ethanol and ethylene glycol as cosolvent, AgNO₃ can disolved in supercritieal carbon dioxide.We can control the uptake and morphology of the nanophase in the composites through investigation of the adsorption and diffusion of precursor in the supercritical fluid deposition process.