Synthesis of Nanostructured Materials by Etching in Supercritical Carbon Dioxide.

Supercritical CO2 (scCO2) is emerging as a viable and environmentally sustainable platform for nanomaterials synthesis due to its tunable solvent properties combined with low surface tension and viscosity, which allow rapid, non-destructive wetting within small features. However, to advance the utility of this fluid, a more thorough understanding of surface chemistry at high pressures is needed. In this study, the behavior of reactive solids in scCO2 was examined by etching thin dielectric, metal, and alloy films to determine the fundamental mechanisms controlling the reactions. Models were developed to describe the etching processes and to benchmark scCO 2 with conventional methods. Dielectric SiOxNy films were etched with an HF/pyridine complex dissolved in scCO2. The anhydrous etching process resulted in formation of a residual (NH 4)2SiF6 layer that limited transport of reactants to the film and caused a drop in reaction order. Partial removal of the salt was accomplished by sublimation under vacuum. Etching of thin CuO films with hexafluoroacetylacetone (hfacH) in scCO2 was studied and found to occur via a 3-step Langmuir-Hinshelwood reaction sequence. The kinetic model showed that lower scCO2 densities favored hfacH adsorption on the CuO surface and that scCO2 solvation forces lowered the activation barrier for the rate-limiting step. Adding up to 10x the molar ratio of pure H2O to hfacH nearly doubled the etching rate through formation of a hydrogen-bonded hfacH complex. Both bulk and thin film AgCu alloys were selectively etched in scCO 2 to generate nanoporous Ag structures. As Cu was preferentially removed through selective oxidation and chelation, the Ag atoms conglomerated into successively larger clusters similar to mechanisms reported in aqueous phase dealloying. Supercritical dealloying was observed at Cu compositions below typical parting limits, suggesting enhanced fluid transport in the evolving pores. When using in situ oxidation, the etching reaction was limited by decomposition of H2O2. Inverse space image analysis of samples with initial phase domain sizes between 250--1000 nm showed that below a threshold of approximately 500 nm, the dealloyed feature size mimicked the starting microstructure. Larger phase domains prohibited surface diffusion of Ag between phases producing a mixture of large and small Ag nanostructures.