Supercritical Co <sub> 2 </ Sub> Metal Oxide Nanopowder Application Research

As typical n-type semiconductors, Zinc oxide and Tin oxide can exhibit unique electrical/optical characteristics compared to bulk materials, which have been widely used as catalyst, gas sensors, solar cells and electric/optical materials. As other nanostructure materials investigation, the research of metal oxide semiconductors is firstly focused on synthesis methods, including improvement of nanostructure materials preparation methods, experiments on different precursors and investigations on synthesis conditions and parameters.Although chemical synthesis method of nanostructure materials has advantages on mass production, problem of aggregation is still unresolved, which results from chemical reaction nucleation and crystal growth to washing and drying of gelatins and calcinations of powders. The problem of products pollution occurring in the preparation process is not yet to be solved. Particle design is a major development of supercritical carbon dioxide since it exhibits higher diffusivity, lower viscosity, tunable solvency, lower supercritical temperature, environmentally friendly property etc. As a result, much attention has been attracted and numerous investigations have been made. Up to day, applications mainly in pharmaceuticals, explosives and polymers have been investigated and particles size is in the range of micrometers in diameter. Synthesis of metal oxides by using supercritical carbon dioxide as a solvent has not been reported. In this work, supercritical carbon dioxide anti-solvent precipitation technology (SAS) is used to produce zinc oxide nano-powders and supercritical carbon dioxide drying is used to produce tin dioxide nano-particles and attractive results were gotten. The results give a fundamental to further investigation of powders synthesis by using supercritical carbon dioxide.1. Nanoparticles of zinc acetate were produced via batch SAS process with the raw zinc acetate precursor at 40 ℃, 150bar, which mean particle size is about 35nm. The products were analyzed by TEM, XRD and FTIR. The XRD patterns of the raw and the processed precursors show that the structure changes from a typical crystal toamorphous. FTIR patterns show bands position shifts and intensity changes and the processed precursor is the mixture of acetate and carbonate salts. Based on experiments, we found that temperature, pressure, pressure increase rate and CO2 flow route are the main factors influencing the results.Nanoparticles of ZnO were prepared via thermal decomposition of zinc acetate precursor processed by the batch SAS. The TG-TGA curves confirm the thermal behavior of the processed precursor, which indicates the decomposition steps are different to the raw precursor. The XRD patterns reveal the crystal temperature of ZnO derived from the processed precursor is lower than from the raw precursor. The FTIR measurements reveal that no residual organic compounds exist and pure hexagonal wurtzite crystallite is produced. The further analysis reveals that ZnO particle size and size distribution are determined by the SAS process. So, control of the batch SAS process is the key.2. Firstly, a conventional method is used to synthesis tin hydroxide precipitation by mixture SnCU solution with the cationic surfactant CTAB (cetyltrimethylammonium bromide) and ammonia, under vigorous stirring. The products are aged at ambient temperature for a short time, then, repeatedly centrifugal separation after each washing with distilled water and a selected organic solvent (DMSO) to remove surfactant, water and Cl"'. The selection of the organic solvent is a key in order to carry the supercritical CO2 drying.Dried Sn(OH)4 nanoparticles were produced after the supercritical CO2 drying of the washed precipitation at 40°C 150bar, which particle size range is 3-25nm. XRD patterns reveal that crystallization is incompletely. FTIR spectra indicate that the products are pure. The results indicate the washing and the supercritical CO2 drying process is feasible.The dried Sn(OH)2 powders are calcined at 400, 600 and 800°C to ensure it completely convert into tin dioxide. XRD patterns show that phase pure rutile SnO2 is formed. No other phases are detected. The mean sizes of SnO2 powders are ~~7nnu ~ 10nm^P~12nm in diameter at 400, 600 and 800 °C respectively. Both temperatures and times of calcinations influence crystallization degree and size in diameter. Poorlycrystallized rutile S11O2 is obtained at low calcinations temperatures. The result is same for a short calcining time. FTIR patterns show bands position and intensity depend on the powder size.Both the principles of supercritical CO2 anti-solvent precipitation and supercritical CO2 drying processes are complex. In addition to the theory of crystallography, phase behavior, mass-transfer kinetics as well as hydrodynamics considerations are all involved in the processes. The further online monitoring and theoretical investigation are needed.