Controllable Synthesis Of Metal/Metal Oxide Nano-composites In Supercritical CO₂and Investigation Of The Underlying Mechanism

Metal and Metal Oxide nanocomposites play important role in the fields of catalysis, storing of information, medicine, photoelectron etc. The properties of nanocomposites mainly depend on the size and size distribution, the morphology and composition of the nanophase, thus the controllable synthesis of nanocomposites is very important. The traditional impregnation method is simple but it has the disadvantages of time consuming and poor dispersion of nanophase.Supercritical fluid deposition (SCFD) method has attracted growing attentions in recent years. This method involves the dissolution of metal precursor in supercritical fluids (SCFs), the diffusion and impregnation of SCF solution into the pores of the substrates and adsorption of metal precursor onto the walls of the channels, followed by thermal or chemical reduction after depressurization. The lack of surface tension of SCFs accelerates the penetration and diffusion of precursors into the nanoscale pores compared to liquid solvent. This paper was based on this method and the details are shown in the following:(1) Well-defined Ag nanowires were obtained in SBA-15at a short deposition time of5minutes in supercritical CO2when adding a small amount of ethylene glycol as co-solvent. Ethylene glycol played a key role in dictating the nanowire morphology. Through detailed investigation of the whole deposition process, including the dissolution of precursor assisted by the co-solvent, the diffusion properties of the precursor into the nano-scale channels of the substrate, and the interaction between the precursor and the substrate, we found that a small amount of precursors were reduced into Ag0by ethylene glycol at the initial stage of the deposition. The resultant Ag0then acted as nuclei leading to a non-equilibrium sorptionof the precursor. The rapid adsorption of the precursor onto the substrate resulted in a high loading of Ag in a short deposition time and the formation of nanowire morphology.(2) In previous reports, organic solvents were usually used as co-solvents in supercritical fluid technique. In this study, dilute acids were introduced into supercritical CO2as co-solvent for the first time to improve the adsorption and dispersion of metal precursors on the substrate. Super-highly dispersed Co3O4nanoparticles were confined in the nanochannels of SBA-15and KIT-6after deposition and calcinations when adding a small amount of nitric acid as co-solvent. Furthermore, the morphologies of the nanostructure were quite different when using different acids. Only Co3O4nanoparticles were obtained when adding nitric acid in supercritical CO2-ethanol solution whereas nanowires were found instead when applying hydrochloric acid. It indicated that H+favored the adsorption and dispersion of the precursors on the substrates and Cl-played a key role in dictating the nanowire morphology. (3) The phase behavior of multi system in the reactor is the thermodynamics of preparing nanocomposites using SCFD process. The expansion of the CO2-cosolvent solution(which was called CO2expanded liquid) containing inorganic salts at different temperatures and pressures were investigated trough a view cell. It was found that the expansion for the CO2expanded liquid containing inorganic salts increased with the pressure and decreased with temperature just like the system without inorganic salts did. However, the maximum expansion of the system containing inorganic salts was much lower that the system without any salts. In addition, it was found that tower temperature and higher pressure were beneficial to the phase separation. The fluid was composed of three phases including inorganic salt-rich phase, organic solvent-rich phase and CO2-rich phase.(4) Willow like Mn3O4nanoplates@graphene nanocomposites were synthesized using graphene instead of graphene oxide as initial materials. After that, MnCl2·4H2O ultrasmall nanoparticles with diameter of1-3nm were supported on graphene using MnCl2·4H2O as precursor, supercritical CO2as solvent and methanol as co-solvent. It was found that the specific capacitance of the Mnch·4H2O ultrasmall nanoparticles@graphene was twice that of pure graphene and the capacitance retention ratio at large scan rate was as high as that of pure graphene.