Kinetics and Mechanism for the Reaction of Hexafluoroacetylacetone with CuO in Supercritical Carbon Dioxide

A kinetic model and mechanism were developed for the heterogeneous chelation reaction of thin CuO films with hexafluoroacetylacetone (hfacH) in supercritical CO2. This reaction has relevance for processing nanoscale structures and, more importantly, serves as a model system to tune the reaction behavior of solids using supercritical fluids. Precise control over reaction conditions enabled accurate etching rates to be measured as a function of both temperature [(53.5-88.4) +/- 0.5 degrees C] and hfacH concentration (0.3-10.9 mM), yielding an apparent activation energy of 70.2 +/- 4.1 kJ/mol and an order of approximately 0.6 with respect to hfacH. X-ray photoelectron spectroscopy and scanning electron microscopy were used to characterize the CuO surface, and a maximum etching rate of 24.5 +/- 3.1 angstrom/min was obtained. Solvation forces between hfacH and the dense CO2 permitted material removal at temperatures more than 100 degrees C lower than that of the analogous gas-phase process. In the low concentration regime, the etching reaction was modeled with a three-step Langmuir-Hinshelwood mechanism. Small amounts of excess water nearly doubled the reaction rate through the proposed formation of a hydrogen-bonded hfacH complex in solution. Further increases in the hfacH concentration up to 27.5 mM caused a shift to first-order kinetics and an adsorption-limited or Rideal-Eley mechanism. These results demonstrate that relatively modest increases in concentration can prompt a heterogeneous reaction in supercritical CO2 to switch from a mechanism most commonly associated with a low-flux gas to one emblematic of a high-flux liquid.