| Supercritical fluid deposition is a new effective method for synthesizing supported nanocomposites with green solvent scCO2, taking the advantage of scCO2’s feature, such as environmental benign, zero surface tension and easily separation of production from solvent by reducing the operating pressure. In this work, Ag nanoparticles and nanowires were successfully deposited into micropores of two mesoporous silica substrates, including SBA-15and KIT-6by using inexpensive AgNO3as precursor and suitable co-solvents. The work focused on studying the influence of different co-solvents on the morphology of nanophase, as well as the effects of temperature, pressure and deposition time on the morphology, size distribution and metal loading. The main results are as follows:(1) Co-solvent was found to influence the deposition results significantly, morphology of the nanophase in all the samples prepared using the dual co-solvent of ethanol and ethylene glycol is nanowire while there were both nanoparticles and nanowires in the samples prepared using only ethanol as co-solvent. Furthermore, it is interesting to note that adding a little ethylene glycol in the CO2-EtOH system could significantly reduce the deposition time compared with ethanol as co-solvent. Thus, a series of experiments have been carried out to study the mechanism of action of ethylene glycol to the ethanol-scCO2system. The results showed that (1) the ethylene glycol added improved the mass transfer ability of AgNO3by increasing the solubility of AgNO3in it.(2) The small amount of Ag reduced by ethylene glycol in the deposition process played the role of "seed crystal" to induce to the formation of nanowire.(3) The small amount of Ag reduced by ethylene glycol in the deposition process possess the property of autocatalysis to catalyze the reduction reaction of AgNO3, leading to the decrease of the AgNO3concentration in the channel and inducing rapid non-equilibrium sorption to shorten the deposition time while deposit a large amount of AgNO3into the nano-channels.(2) Based on the research of mechanism of action of ethylene glycol, Ag/SBA-15nanocomposites were prepared with various co-solvents, such as ethanol+water, ethanol+dilute HNO3and dilute HNO3. The influences of deposition temperature, deposition pressure, deposition time and the amount of co-colvent on the morphology of nanophase, particle size, size distribution and loading amount were systematically investigated in different co-solvent systems. The results for different systems were different. It is worth mentioning some interesting results. The metal loading achieved20.19wt%when using5mL ethanol and0.5mL water as co-solvent in1h. However, when using5mL of ethanol and0.5mL of8.7wt%HNO3as co-solvent, only0.72mmol of HNO3would result in the silver morphology changing from highly dispersed nanoparticles to spindly nanowires, indicating that HNO3plays an important role in the formation of Ag nanowire in the channels of SBA-15. Further investigation on the mechanism of action was needed.(3) The influence of deposition time and dosage of co-solvent on metal loading and morphology were studied when KIT-6with three dimensionals pore network structure was used as substrate while the mixture of ethanol and ethylene glycol as co-solvent. The results showed that the metal loading amount increased when extended the deposition time and tend to achieve balance after6h, which was much longer than that of SBA-15system. In addition, the best result was achieved with only dilute HNO3as co-solvent when the deposition time was3h while the metal loadings amount reached18.45wt%. |
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